Спектрально перестраиваемая лазерная генерация в свинцово-галогенидных перовскитных нитевидных микро- и нанокристаллах тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Маркина Дарья Игоревна

• Маркина Дарья Игоревна
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Маркина Дарья Игоревна

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Synopsis

Introduction

Chapter 1. Literature review

1.1 Lead halide perovskites as the novel material platform

1.1.1 Structural and electronic properties

1.1.2 Optical properties

1.2 Spontaneous and stimulated emission in perovskite micro- and nanocavities

1.2.1 Introduction to emission mechanisms in semiconductors

1.2.2 Properties of stimulated emission in micro- and nanocavities

1.2.3 Stimulated emission in perovskite micro- and nanocavities

1.3 Improvement of perovskite nanolaser photophysical properties

1.4 Band gap rearrangement approaches

1.5 Gas sensing

1.5.1 Chemical sensing principles

1.5.2 Physical sensing principles

1.6 Conclusion on chapter

Chapter 2. Experimental methods

2.1 CsPbX3 micro- and nanostructures fabrication

2.1.1 Micro- and nanocrystals

2.1.2 Thin films

2.2 Integration of lead-halide perovskite micro- and nanowires with low-refractive-index nanostructured substrates of different morphology

2.2.1 Fabrication of the nanostructured indium-tin-oxide

substrates and integration with CsPbBr3 NWs

2.2.2 Fabrication of island-like aluminum oxide (Al2O3) substrates

and integration with CsPbBr3 NWs

2.3 Chemical vapor anion exchange (CVAE) process

2.4 Microscopy

2.5 Scanning electron microscopy (SEM) and atomic force microscopy (AFM)

2.5.1 Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), fast Fourier transform (FFT)

2.5.2 Optical microscopy

2.6 X-ray diffraction (XRD) and X-ray photoelectron spectroscopy

(XPS) and Energy Dispersive X-Ray Analysis (EDX)

2.7 Spectroscopy

2.7.1 Dark-field spectroscopy

2.7.2 Time-resolved photoluminescence spectroscopy

2.7.3 Laser spectroscopy

2.8 Numerical modeling

2.9 Conclusion on chapter

Chapter 3. Improvement of photophysical properties of

lead-halide perovskite micro- and nanowires on nanostructured substrates

3.1 Structural properties of CsPbBr3 nanowires

3.2 Optical properties of CsPbBr3 nanowires on regular indium-tin

oxide and glass substrates

3.3 Experimental investigation of optical performance of OsPbBr3 nanolasers on nanostructured indium-tin oxide substrate

3.4 Numerical simulation of CsPbBr3 nanowire cavity eigenmodes on different substrates

3.5 Conclusion on chapter

Chapter 4. Spectral control of the lead-halide perovskite

nanowires emission

4.1 Spectral rearrangement of nanowires emission by direct synthesis approach

4.2 Spectral rearrangement of nanowires emission by means of heterophase anion exchange

4.2.1 Broadband spontaneous and stimulated emission tuning via exposure to hydrogen halides vapor

4.2.2 Conclusion on chapter

Chapter 5. Optical sensing by CsPbBr3 micro- and nanowires

5.1 Structural and optical characterisation of the initial CsPbBr3 nanowire lasers

5.2 Optical detection of the low-concentrated hydrogen chloride vapors

at ambient conditions

5.3 Physical mechanism of the hydrogen chloride vapor detection

5.3.1 Experimental investigation on gas detection mechanism

5.3.2 Theoretical modeling

5.4 Conclusion on chapter

Conclusion

References

Acknowledgements

Appendix A. Texts of key publications

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Общая характеристика диссертации

Введение диссертации (часть автореферата) на тему «Спектрально перестраиваемая лазерная генерация в свинцово-галогенидных перовскитных нитевидных микро- и нанокристаллах»

Актуальность темы

На сегодняшний день производительность и быстродействие электронных интегральных схем подходит к пределу своих возможностей. Ограниченность электронных устройств рождает необходимость в переходе к оптической платформе, использующей фотон в качестве переносчика единицы информации. Оптоэлектронные и нанофотонные устройства позволят осуществить большой шаг в развитии вычислительной техники и передачи больших объемов данных.

Для создания оптических чипов в первую очередь необходимы миниатюрные источники фотонов - нанолазеры. Эффективность работы нанолазеров во многом зависит от выбора материала усиливающей среды. Классические полупроводниковые материалы, способные генерировать лазерное излучение, такие как СаЛэ и СаК, требуют дорогих и

высокотехнологичных процессов фабрикации, в частности эпитаксии. В отличие от них новый класс полупроводниковых материалов - перовскиты на основе галогенидов свинца - не только способны формировать резонаторы правильной формы в процессе быстрого и дешевого синтеза, но и отличаются выдающимися оптическими свойствами, успешно удовлетворяя потребности современной оптоэлектроники и фотоники.

Свинцово-галогенидные перовскиты - экситонные материалы, в силу чего для них характерна высокоэффективная фотолюминесценция при комнатной температуре [1], высокая подвижность носителей заряда [2], а также достаточно высокие значения оптического усиления (^104 см-1 [3-5]) и показателя преломления (2-2.5), что обеспечивает наличие высокодобротных оптических резонансов в микро- и наноструктурах [6].

Кроме того, миниатюрные источники лазерного излучения могут выступать в качестве высокочувствительных оптических сенсоров таких вредных для человека веществ как галогениды водорода. Для определения концентрации галогенидов водорода в воздухе существуют различные коммерчески доступные электрохимические анализаторы, которые однако

имеют высокую погрешность и ограниченный диапазон детектируемой концентрации газа. Более чувствительные оптические сенсоры на основе фотонных кристаллов и микрорезонаторов требуют сложного высокотехнологичного производства и зачастую неприменимы для детектирования паров галогенидов водорода. Перовскитные же нано-и микроструктуры способны вступать в реакцию анионного обмена из паровой фазы [7; 8] с образованием смешанно-галогенидных соединений АВ(Х,Х')з или полностью замещенных гомогалогенидов АВХ'3 перовскита, которые имеют отличную от исходного состава АВХ3 ширину запрещенной зоны и коэффициент преломления, и следовательно иные спектральные характеристики. Подобная in situ спектральная настройка оптического отклика, проявляющаяся в изменении спектрального положения спонтанного или лазерного излучения [7-12], позволяет создавать высокочувствительные оптические сенсоры аналитов в паровой фазе.

Диссертационная работа посвящена экспериментальному исследованию и численному моделированию фотофизических свойств перовскитных нитевидных микро- и нанокристаллов (ННК), демонстрирующих лазерную генерацию, улучшению их оптических свойств за счет интеграции с наноструктурированными подложками, а также созданию и исследованию новых высокочувствительных оптических сенсоров галогенидов водорода на основе свинцово-галогенидных перовскитных нанолазеров.

Целью диссертационной работы является экспериментальное и теоретическое исследование спектральных свойств перовскитных нано- и микролазеров, управления этими свойствами посредством анионного обмена из паровой фазы и интеграции с наноструктурированными подложками, а также исследование возможных применений перовскитных свинцово-галогенидных нанолазеров в области оптического газового сенсинга.

Задачи работы

1. Экспериментальное исследование и чиленное моделирование оптических и структурных свойств микро- и нанолазеров состава CsPbBr3 на подложках из оксида индия-олова или стекла, а именно: генерация спонтанного и вынужденного излучения, кристаллическая фаза и морфология.

2. Экспериментальное и теоретическое исследование влияния наноструктурированной подложки с низким показателем преломления на модовый состав, добротность и порог генерации перовскитных нитевидных микро- и нанокристаллов состава СэРЬБг3.

3. Экспериментальное исследование фотофизических и структурных свойств смешанно-галогенидных микролазеров составов СэРЬБг2С1 и СбРЬБг1.511.5. Исследование температурной зависимости фотоинициированной фазовой нестабильности в тонких пленках и нитевидных нанокристаллах состава С8РЬБг15115.

4. Разработка, экспериментальное и теоретическое исследование высокочувствительных сенсоров паров галогенидов водорода на основе перовскитных микро- и нанолазеров.

Основные положения, выносимые на защиту:

1. Нитевидные микро- и нанокристаллы состава СэРЬБг3, поддерживающие резонансы Фабри-Перо и помещенные на наноструктурированный слой оксида индия-олова с эффективным показателем преломления пе// = 1.15, демонстрируют примерно двукратное снижение порога лазерной генерации с оптической накачкой и четырехкратное уменьшение ширины моды генерации на полувысоте по сравнению с аналогичными структурами на поверхности компактного слоя оксида индия-олова

с показателем преломления п = 1.9 за счет улучшенного в 8 раз удержания оптического поля и увеличения добротности резонансов Фабри-Перо.

2. Подавление фотоиндуцированной фазовой неустойчивости, вызванной миграцией фотоактивированных ионов галогенидов, в нитевидных микро- и нанокристаллах перовскита смешанного галогенидного состава происходит при 250 К, что на 50 К выше, чем в поликристаллических пленках аналогичного состава за счет снижения скорости дефект-опосредованной анионной диффузии.

3. Одиночный СэРЬБг3 нитевидный нанокристалл на наноструктурированной подложке Л1203 работает как оптический сенсор паров хлороводорода. Принцип детектирования основан на спектральном сдвиге лазерных мод, генерируемых нитевидным нанокристаллом, при воздействии широкого диапазона концентраций паров от 5 до 500 миллионных

долей под фотовозбуждением. Спектральный сдвиг мод лазерной генерации обусловлен образованием смешанной галогенидной оболочки с показателем преломления, отличным от показателя преломления ядра.

Научная новизна

1. Экспериментально показаны рекордно малые ширины линий лазерной генерации для класса перовскитных CsPbBr3 микро- и нанолазеров, поддерживающих резонансы Фабри-Перо, за счет интеграции с наноструктурированными подложками оксида индия-олова.

2. Впервые проведено сравнительное исследование подавления фотоиндуцированной фазовой нестабильности в монокристаллическом нитевидном нанокристалле и поликристаллической пленке одинакового химического состава CsPbBr15I15.

3. Впервые экспериментально показано детектирование паров хлороводорода (HCl) при концентрациях от 5 до 500 миллионных долей (мд) перовскитными нанолазерами состава CsPbBr3 на наноструктурированной Al2O3 подложке с островковой поверхностной морфологией.

Теоретическая и практическая значимость диссертационной работы заключается в комплексном исследовании, разработке методов модификации и улучшения фотофизических свойств перовскитных свинцово-галогенидных микро- и нанолазеров, поддерживающих резонансы Фабри-Перо, а также создания и исследования оптических сенсоров на основе полученных данных. Результаты исследования могут быть применены в дальнейшей разработке прототипов фотонных интегральных схем и ультра чувствительных газовых сенсоров.

Достоверность результатов работы основана на использовании многочисленных современных методов исследований, обеспечивающих высокую воспроизводимость полученных данных. Результаты комплексного экспериментального

исследования согласуются с результатами численного моделирования и не противоречат данным литературных источников. Полученные результаты

прошли апробацию на всероссийских и международных конференциях и опубликованы в реферируемых журналах.

Апробация результатов работы. Основные результаты работы были представлены и обсуждались на всероссийских и международных конференциях и школах:

1. XX Всероссийская молодежная конференция по физике полупроводников и наноструктур, полупроводниковой опто- и наноэлектронике, Санкт-Петербург, 26-30 ноября 2018

2. IV International Conference on Metamaterials and Nanophotonics, METANANO 2019, Санкт-Петербург, 15 - 19 июля 2019

3. International Conference on Nanophotonics and Micro/Nanooptics, NANOP 2019, Мюнхен, 4 - 6 сентября 2019

4. XXI Всероссийская молодежная конференция по физике полупроводников и наноструктур, полупроводниковой опто- и наноэлектронике, Санкт-Петербург, 25-29 ноября 2019

5. School on Advanced Light-Emitting and Optical Materials SLALOM 2019, Санкт-Петербург, 12-13 декабря 2019

6. V International Conference on Metamaterials and Nanophotonics, METANANO 2020, Тбилиси, 14 - 18 сентября 2020

7. School on Advanced Light-Emitting and Optical Materials SLALOM 2021, Владивосток, 28 - 30 июня 2021

8. VI International Conference on Metamaterials and Nanophotonics, METANANO 2021, online, 13 - 17 сентября 2021

9. 20th International Conference Laser Optics ICLO 2022, Санкт-Петербург, 20 - 24 июня 2022

10. International Conference on Emerging Light Emitting Materials (EM-LEM22), Лимассол, 3-5 Октября 2022

Личный вклад автора заключается в проведении экспериментов по оптической микроскопии и спектроскопии, создании экспериментальной установки для газового сенсинга, обработке экспериментальных данных, численном моделировании собственных мод резонатора в различных условиях. Автор участвовал в разработке метода создания образцов, их фабрикации, и последующей интеграции с наноструктурированными подложками оксида

индия-олова, а также непосредственной постановке целей и задач исследования и подготовке научных статей.

Структура и объем диссертации Диссертация состоит из введения, пяти глав и заключения. Полный объем диссертационной работы составляет 169 страниц, включая библиографический список из 176 наименований. Работа содержит 70 рисунков и 2 таблицы.

Публикации. Основные результаты по теме диссертации изложены в 5 публикациях, индексируемых в базах данных Scopus и Web of Science.

ОСНОВНОЕ СОДЕРЖАНИЕ РАБОТЫ

Во введении настоящей диссертации сформулированы цель, задачи и основные выносимые на защиту положения. Обоснованы актуальность, достоверность и значимость работы, а также описаны новизна и личный вклад автора.

В первой главе представлен обзор литературных источников, посвященный структурным и оптическим свойствам свинцово-галогенидных перовскитов, а также детальному рассмотрению механизмов спонтанного и вынужденного излучения в перовскитных микро- и нанорезонаторах. В главе также рассматриваются существующие методы изменения оптических свойств таких структур и в том числе механизмы управления шириной запрещенной зоны и эмиссии в свинцово-галогенидных перовскитах. В заключительном разделе первой главы диссертации описаны основные принципы детектирования аналитов из газовой фазы и приведен обзор последних работ в данной области.

Во второй главе приведены экспериментальные методики получения и исследования образцов, используемые в работе. Описан разработанный метод высаливания из капель прекурсора перовскита нитевидных микро-и нанокристаллов состава СэРЬБг3 и его модификации для получения структур других гомогалогенидных и смешанно галогенидных составов

(Рисунок 1). Описаны методы фабрикации наноструктурированных подложек для последующей интеграции их с перовскитными ННК. Приведено описание экспериментальной реализации процесса анионного обмена из паровой фазы для исследования изменения ширины запрещенной зоны и длины волны эмиссии перовскита. Представлено описание всех структурных и оптических методов исследования, использованных в работе, приведены схемы установок, в том числе разработанной автором для экспериментального исследования физических принципов детектирования паров галогенидов водорода.

Рисунок 1 — Схема метода получения нитевидных микро- и нанокристаллов состава СэРЬБг3 посредством высаливания из капель прекурсора перовскита

Третья глава диссертационной работы посвящена экспериментальному исследованию структурных и оптических свойств нитевидных микро- и нанокристаллов состава СэРЬБг3 на подложках из стекла и оксиде индия-олова, а также исследованию метода улучшения свойств лазерной генерации в таких структурах при интеграции с наноструктурированными подложками с низким показателем преломления.

В первой части главы приведены результаты структурных исследований СэРЬБг3 ННК (Рисунок 2а). Плоская прямоугольная или усеченная пирамидальная форма торцов ННК, а также отсутствие значительных поверхностных дефектов показаны посредством сканирующей электронной микроскопии (СЭМ) (Рисунок 2Ь). Высокое кристаллическое качество и малое количество дефектов в объеме ННК подтверждаются данными просвечивающей электронной микроскопии высокого разрешения и быстрого

Фурье преобразования, что показано на рисунке 2c. С помощью метода рентгеновской дифракции показано, что монокристаллические ННК состава CsPbBr3 имеют орторомбическую кристаллическую фазу (Рисунок 2d, e). Кроме того, данные структурных методов исследования подтверждают низкую шероховатость поверхности (<2 нм), что важно для высокой эффективности лазерной генерации.

Вторая часть главы посвящена комплексному исследованию оптических свойств полученных CsPbBr3 ННК (Рисунок 3a). Показано, что перовскитные нитевидные нанокристаллы способны эффективно генерировать спонтанную и вынужденную фотолюминесценцию (ФЛ). Пик спонтанной ФЛ как правило расположен на длине волны 525 нм при ширине линии на полувысоте (FWHM) равной 17 нм (Рисунок 3b, красная кривая (i)). Спектры пропускания ННК при возбуждении сфокусированным на торце ННК лазерным пучком (Рисунок 3b, красная кривая (ii)) и при пропускания белого света (Рисунок 3b, синяя кривая (iv)) подтверждают способность ННК поддерживать резонансы типа Фабри-Перо. Эффективность генерации спонтанной ФЛ количественно выражается в значении квантового выхода фотолюминесценции (КВ ФЛ), который экспериментально можно оценить из кривых затухания ФЛ от времени, т.е. динамики носителей заряда. Используя ABC модель [13; 14] для аппроксимации кинетики затухания ФЛ, показано, что при возбуждении с плотностью энергии ниже порога лазерной генерации сигнал ФЛ затухает по моноэкспоненциальному закону с постоянной времени 7 нс (Рисунок 3c, синяя кривая) и по закону бимолекулярной рекомбинации с постоянной времени 1.1 нс при возбуждении с плотностью энергии выше порога генерации (Рисунок 3c, красная кривая). Значение КВ ФЛ достигает 41.7% для плотности генерируемых носителей равной 3-1015 см-3 и увеличивается до 83.4% при плотности носителей 6-1016 см-3.

Более подробно изучена лазерная генерация в CsPbBr3 ННК при оптическом возбуждении с фемтосекундными импульсами (220 фс) с длиной волны 405 нм. При увеличении плотности падающей энергии до порогового значения наблюдается появление узких линий лазерной генерации с шириной на уровне полувысоты менее 1 нм. Количество линий зависит от геометрических параметров резонатора ННК. Спектр лазерной генерации ННК длиной 11 мкм состоит из четырех пиков (Рисунок 3d). Полученные линии соответствуют модам Фабри-Перо резонатора с наибольшими сечениями эмиссии в области

5 nm

(004) '

S (002) *

(H2> (220>

(d) 5000

</) Cl

^ 2500 </) с 0

as-grown NWs + NPLs + bulk 1 *тЛ................--л-------------- J

single NWs (002) (110) (°04) \i _.................... 4 1(220) 1.

(020) (qo2)/11°) orth CsPbBr3 (200) (004)i I i ■ ii i i i 1 (220)

(Ю0) (110> cubic CsPbBr3 I (200) . 1

15

20

30

35

14.5 15.0

15.5 30.0 20 (°)

30.5 31.0

25 20 (°)

Рисунок 2 — (а) Фотография СэРЬВгз ННК на подложке из оксида индия-олова, (1ТО) в режиме светлого поля. (Ь) СЭМ-изображение нитевидного нанокристалла состава СэРЬВгз с усеченными пирамидальными торцами под скользящим углом 45°. Вставка: увеличенное изображение торца. (с) Изображения просвечивающей электронной микроскопии высокого разрешения и быстрого Фурье преобразования того же ННК вдоль оси [001]. (с!) Спектры рентгеновской дифракции образца в разных режимах измерения. Красный спектр демонстрирует особенности характерные для СэРЬВгз нитевидных и пластинчатых микро и нанокристаллов, а также для поликристаллического осадка в режиме сбора сигнала с большой области образца при скользящем падении луча, синий спектр содержит пики, относящиеся к СэРЬВгз ННК, при сборе сигнала с малой области, сфокусированной на единичной ННК, в геометрии 6 — 6, розовая и зеленая кривые - эталонные спектры орторомбической и кубической фаз перовскита, СэРЬВгз. (е) Рентгенограммы высокого разрешения, соответствующие одиночному ННК в геометрии 6 — 6

оптического усиления материала, что экспериментально подтверждается спектрами пропускания ННК (Рисунок ЗЬ, синяя кривая).

Зависимость интенсивности ФЛ от падающей плотности энергии возбуждения имеет Б-образный характер (Рисунок Зе, синяя кривая),

демонстрируя порог лазерной генерации в момент начала значительного роста зависимости, что совпадает с резким сужением ширины линии от 17 до 0.28 им вблизи порогового значения равного 25 мкДж-см~2 (Рисунок Зе, красная кривая).

......, Ц . о 1.6 Ц_|/СГТ12 ■ 1 ■ ц о32^/ст2 .

: | Ч "" .............. ц„ —т :

' 1 ^Ч*. Т = 7 ПБ

! | Т = 1.1 ПЯ^ВЬ^.. : 1 г :

0 2 4 6 8 10

"Пте (пб)

■р>р1И

а = 1боо

1

510 520 530 \Л/ауе1епдИп (пт)

540

550

525 550 575 \Л/ауе1епдИп (пт)

20 25 30 Р1иепсе (^и/ст2)

35 40

Рисунок 3 — (а) Микрофотография нитевидных нанокристаллов состава СбРЫЗгз в флуоресцентном режиме. (Ь) Спектры: (1) спонтанной фотолюминесценции при равномерном возбуждении ННК, (11) модулированной спонтанной фотолюминесценции при возбуждении сфокусированным пучком с торцевой грани, (ш) пропускания белого света в режиме темнопольной спектроскопии, (с) Кинетика затухания фотолюминесценции ННК СэРЬВгз при плотности энергии возбуждения ниже (синяя кривая) и выше (красная кривая) порога лазерной генерации, (с!) Спектры спонтанной и вынужденной генерации при различных плотностях энергии возбуждения для ННК длиной 11 мкм. (е) Интенсивность фотолюминесценции и ширина линии на уровне полувысоты как функция плотности энергии возбуждения

Одной из ключевых характеристик лазерной генерации является добротность лазерных мод, которая экспериментально определяется как

Q=A/AA, где A — длина волны линии лазерной генерации, ДА — ее ширина на половине максимальной высоты. Показано, что для исследуемых ННК значение добротности мод лазерной генерации лежит в диапазоне Q = 1017-6166 при порогах генерации от 13 мкДж-см-2. В рамках исследования посредством численного моделирования была продемонстрирована связь между добротностью собственных мод резонатора и формой торцевой грани ННК. Форма торца ННК в виде усеченной пирамиды обеспечивает меньшую эффективность вывода излучения из резонатора, т.е. меньшие радиационные потери, что описывается уравнением:

1 - 1 + 1 + 1 (1) Q cav Qrad Qabs Q sea

где QCav - добротность резонатора, Qrad - радиационные потери на излучение или утечку, Qabs - нерадиационные потери, обусловленные поглощением излучения материалом резонатора и окружающей его средой и Qsca - потери на рассеяние на неровностях поверхности и объемных дефектах. Таким образом, демонстрируемые высокие значения добротности лазерных мод ННК связаны с добротностями мод резонатора согласно уравнению Шавлова-Таунса под редакцией Лакса [15]:

А Р П2 А2

^ — _А — 1 out^cavА (2)

4las — ÓA — 2п2 hc2 (2)

где Qias - добротность лазерной моды, Qcav - добротность собственной

моды резонатора, Pout - выходная мощность нанолазера, А - центральная

длина волны лазерной линии, h - постоянная Планка, c - скорость света

в вакууме. Согласно этому уравнению, добротность лазерной моды ННК

становится максимальной за счет высокого кристаллического качества, малой

шероховатости поверхности и формы торцевых граней ННК.

Третья часть главы посвящена исследованию улучшения оптических

свойств полученных ННК CsPbBr3 при интеграции с подложками с низким

показателем преломления. Помимо модификации формы торцевых граней

ННК, уменьшить радиационне потери можно за счет уменьшенния утечки

оптического поля из резонатора в подложку с низким диэлектрическим

контрастом по отношению к материалу нанолазера. Тогда решением задачи

оптимизации оптических свойств перовскитных микро- и нанолазеров является

их интеграция с наноструктурированными подложками из оксида индия-олова

(NS ITO) с эффективным показателем преломления neff=1.15.

Были рассмотрены ННК близких геометрических размеров на наноструктурированном 1ТО (ННК 1) и на компактном слое 1ТО с показателем преломления п=1.9 (ННК 2) (Рисунок 4а, Ь). Показано, что при накачке фемтосекундными импульсами с плотностью энергии меньше порога лазерной генерации пик спонтанной ФЛ для ННК на обеих подложках расположен на длине волны около 526 нм при значении FWHM = 16 нм. При возбуждении с плотностью энергии выше пороговой оба ННК демонстрируют выраженную лазерную генерацию. Однако, для ННК 1 на N8 1ТО характерны вдвое меньший порог лазерной генерации (Рисунок 4с, ^ и в 2-4 раза большие значения добротностей, достигая рекордного значения Q = 7860 для нанолазеров типа Фабри-Перо (Рисунок 4е, £).

Fluence (|jJ cm"2) Wavelength (

Рисунок 4 — (a, b) СЭМ-изображения ННК 1 и ННК 2 на наноструктурированном и компактном ITO соответственно (масштабная линейка - 1 мкм). (c, d) Зависимости интегральной интенсивности ФЛ и значения FWHM от плотности энергии падающего возбуждения для ННК 1 и ННК 2 соответственно. (e, f) Анализ добротностей мод лазерной генерации

ННК на разных подложках

Кроме того, лазерные линии для ННК 1 и ННК 2 имеют разную форму. Моды лазерной генерации в случае ННК на компактном 1ТО имеют две компоненты в отличие от ННК на N8 1ТО. Эти компоненты соответствуют оптическим модам с разной поляризацией из-за наличия

подложки с показателем преломления, отличным от перовскита, что нарушает симметрию и снимает вырождение мод, чего не происходит в случае ННК 1 на наноструктурированном 1ТО. Данная особенность, как и улучшенные оптические свойства, связана с повышенным диэлектрическим контрастом между резонатором и окружающей средой, что дополнительно выражается в более богатом модовом составе ННК 1 и является следствием подавления утечки оптического поля из резонатора.

Заключительная часть главы содержит результаты численного моделирования, представляющие подтверждение экспериментальных результатов. Добротности и распределения компонент электромагнитного поля собственных мод в ННК в зависимости от показателя преломления подложки рассчитаны для структур размерами 3.5 х 0.4 х 0.4 мкм (Рисунок 5а-с). Для подтверждения ключевой роли подложки был рассчитан фактор удержания поля в резонаторе, как ц = WNwсец, где WNW это усредненная по времени полная энергия внутри резонатора, WCe// — усредненная по времени полная энергия во всей расчетной ячейке. Показано, что ННК на слое компактного 1ТО демонстрируют слабое удержание поля п=10% , в то время использование стеклянной подложки с показателем преломления п=1.5 приводит к увеличению значения до 56%, а осаждение нанолазеров на поверхность наноструктурированного 1ТО дает рекордное значение п=82%.

Добротности моделируемых собственных мод рассчитываются согласно выражению = Де(ш)/2|/т(ш)|, где ш — резонансная частота. В

результате при уменьшении показателя преломления подложки с 1.9 до 1.15, демонстрируется значительное увеличение добротности собственных мод резонатора с 100 до 350 (Рисунок 5^.

Таким образом, результаты моделирования подтверждают предположение об увеличении добротности мод генерации за счет интеграции с подложкой с низким показателем преломления и как следствие улучшенного удержания оптического поля.

Рисунок 5 — Пространственное распределение компоненты Ег оптического поля моды ТМ22 в ННК СэРЬБг3 размером 3.5 х 0.4х 0.4 мкм на подложках с разными показателями преломления: (а) 1.15, (Ь) 1.5 и (с) 1.9. Вставка: распределение модуля магнитного поля |Н| ННК в поперечном сечении. (^ Добротность собственной моды резонатора в зависимости от показателя

преломления подложки

В четвертой главе диссертационной работы исследованы подходы к управлению спектрального положения спонтанной и вынужденной эмиссии в свинцово-галогенидных перовскитных микро- и нанорезонаторах.

Известно, что замещение анионов в кристаллической решетке перовскита вызывает изменение ширины запрещенной зоны (Е3). Для гомогалогенидных составов величина Ед меняется от 3.0 [16] до 2.39 [17] и 1.73 эВ [18] для хлора, брома и йода соответственно. Тонкое изменение запрещенной зоны и, как следствие, спектров излучения может быть достигнута путем получения стехиометрических смесей галогенидов. Например, положение пика ФЛ для составов СзРЬ(11-жБгж), где 0 ^ х ^ 1 зависит от соотношения галогенов и принимает значения от 530 до 700 нм [19-21].

Существует два основных подхода к получению наноструктур гомогалогенидных и смешанных галогенидных составов и, как следствие, перестройке ширины запрещенной зоны и спектрального положения эмиссии: прямой синтез или анионный обмен из паровой фазы.

Первый раздел главы описывает результаты применения прямых методов растворной химии для получения ННК смешанного и гомогалогенидного состава с заданным положением пика фотолюминесценции. Подробно описание метода представлено в главе 2. Показано, что для перовскитных ННК состава СэРЬБг2С1 спонтанное и стимулированное излучение ведут себя аналогично ННК гомогалогенидного состава СэРЬБг3, описанным в третьей главе, но демонстрируют спектральный сдвиг в область коротких длин волн. Однако, описываемый подход сложно применить для точного изменения ширины запрещенной зоны перовскита.

Список литературы диссертационного исследования кандидат наук Маркина Дарья Игоревна, 2022 год

О 20 -

10

Е с

05 С

О

-С

о

ч—

О

(Л

о CL

о -

200

400 Time (s)

600

800

О 20 40 60 80 100

Shell thickness (d2, nm)

Рисунок 10 — (а) Временная динамика распространения границы смешанно-галогенидной оболочки с 10% содержанием ионов С1~ в кристалле перовскита под воздействием различных концентраций газообразного HCl. (b) Численно полученная зависимость спектрального сдвига АЛ от толщины оболочки нитевидного нанокристалла для различных значений поперечного

сечения

Таким образом, показана возможность использования свинцово-галогенидных перовскитных нанолазеров в качестве высокочувствительных сенсоров паров галогенидов водорода в атмосфере.

Выводы по основным результатам диссертации:

1. Свинцово-галогенидные перовскитные нитевидные микро- и нанокристаллы состава СэРЬБг3, полученные оригинальным методом высаливания из капель прекурсора перовскита, демонстрируют высокодобротное когерентное лазерное излучение с добротностями Q = 1017-6166 при низких порогах оптической накачки от 13 мкДж-см-2 за счет высокого кристаллического качества, низкой шероховатости поверхности и усеченно-пирамидальной формы торцевых граней, обеспечивающей меньшую эффективность вывода излучения из резонатора и как следствие меньшие радиационные потери.

2. Интеграция перовскитных свинцово-галогенидных нитевидных микро- и нанокристаллов состава СэРЬБг3, генерирующих лазерное излучение, с наноструктурированными подложками оксида индия-олова обеспечивает улучшенное удержание оптического поля в нанорезонаторе за счет увеличения диэлектрического контраста между перовскитной активной средой и подложкой с низким эффективным показателем преломления (neff = 1.15). Вследствие чего экспериментально продемонстрированы рекордные значения добротностей лазерной генерации перовскитных нанолазеров типа Фабри-Перо Q = 7860.

3. Изменение стехиометрического соотношения анионов галогенов в структуре смешанно-галогенидных перовскитных нитевидных кристаллов, происходящее в результате их прямого получения растворным методом, а также гетерофазного анионного обмена между насыщенными парами галогенидов водорода и нанокристаллами состава СэРЬБг3, вызывает значительную спектральную перестройку эмиссии перовскита. Впервые проведено сравнительное исследование подавления фотоиндуцированной фазовой нестабильности в монокристаллическом нитевидном нанокристалле и поликристаллической пленке одинакового химического состава СзРЬБг^б^.б. Показано, что монокристаллические нитевидные нанокристаллы обладают повышенной устойчивостью к фотоиндуцированной фазовой нестабильности, её подавление происходит при более высокой температуре по сравнению с поликристаллическими пленками. Данный эффект объясняется дефект-опосредованной диффузией

в перовскитах и означает, что исходно монокристаллические перовскитные нитевидные микро- и нанокристаллы после процедуры анионного обмена содержат меньшее количество дефектных состояний.

4. Впервые экспериментально показано детектирование паров хлороводорода (HCl) в широком диапазоне концентраций от 5 до 500 миллионных долей перовскитными нанолазерами состава CsPbBr3 на островковой подложке оксида алюминия. В процессе взаимодействия молекул хлороводорода с поверхностью перовскитных нитевидных нанокристаллов происходит формирование оболочки смешанного галогенидного состава CsPb(Cl,Br)3. Наличие оболочки с меньшим показателем преломления по отношению к бромному ядру вызывает сдвиг линий лазерной генерации в коротковолновую область спектра, зависящий от концентрации паров хлороводорода и поперечных размеров нитевидного нанокристалла. Область линейного отклика перовскитных наноструктур позволяет определять концентрацию аналита в воздухе.

Основные публикации по теме диссертации:

[A1] Markina D. I., Pushkarev A. P., Shishkin I.I., Komissarenko F.E, Beresten-nikov A. S., Pavluchenko A.S., Smirnova I. P., Markov L.K., Vengris M., Zakhidov A.A., Makarov S. V. Perovskite nanowire lasers on low-refractive-index conductive substrate for high-Q and low-threshold operation //Nanophotonics, 2020, vol. 9, no. 12, pp. 3977-3984.

[A2] Markina D. I., Tiguntseva E.Yu., Pushkarev A.P., Samsonov M.A., Vengris M., Munkhbat B., Shegai T., Hix G.B., Zakhidov A.A., Makarov S.V. Photophysical properties of halide perovskite CsPb (Bri-xIx)3 thin films and nanowires //Journal of Luminescence, 2020, vol. 220, pp. 116985.

[A3] Pushkarev A. P., Korolev V.I., Markina D.I., Komissarenko F.E., Naujokaitis A., Drabavicius A., Pakstas V., Franckevicius M., Khubezhov S.A., Sannikov D.A., Zasedatelev A.V., Lagoudakis P.G., Zakhidov A.A., Makarov S.V. A few-minute synthesis of CsPbBr3 nanolasers with a high quality factor by spraying at ambient conditions //ACS applied materials & interfaces, 2018., vol. 11, no. 1, pp.

1040-1048.

[A4] Markina D. I., Pushkarev A.P., Shishkin I.I., Zuev D.A., Makarov S.V. Rapid synthesis and optical properties of CsPbBr2Cl perovskite nanolasers //Journal of Physics: Conference Series. - IOP Publishing, 2020, vol. 1461, no. 1, pp. 012091.

[A5] Markina D. I., Pushkarev A. P., Makarov S. V. Theoretical study of perovskite nanowires optical response to hydrogen halides vapor exposure //Journal of Physics: Conference Series. - IOP Publishing, 2021, vol. 2015, no. 1, pp. 012087.

Synopsis

General thesis summary

Relevance

Nowadays the performance and speed of integrated electronic circuits are approaching the limit of their capabilities. The limitation of electronic devices gives rise to the need for a transition to an optical platform that uses a photon as a carrier of an information unit. Optoelectronic and nanophotonic devices can stimulate a big step in the development of computer technology and the transmission of large amounts of data.

To create optical chips, first of all, miniature photon sources - nanolasers - are needed. The efficiency of the nanolasers operation largely depends on the choice of the material of the amplification medium. Convenient semiconductor materials capable of laser generation, such as Si, GaAs and GaN, require expensive and hightech manufacturing processes, in particular epitaxy. In contrast the new class of semiconductor materials - lead halide perovskites - are not only able to form regular shape resonators in the process of fast and cheap synthesis, but also distinguished by outstanding optical properties, successfully meeting the needs of modern optoelectronics and photonics.

Lead-halide perovskites are excitonic materials, due to which they demonstrate highly efficient photoluminescence at ambient conditions [1], high mobility of charge carriers [2], rather high values of optical gain (^104 cm-1 [3-5]) and the refractive index (2"2.5), which provides the presence of high-Q optical resonances in micro-and nanostructures [6].

In addition, miniature sources of laser emission can act as highly sensitive optical sensors of harmful to human health hydrogen halides. There are various commercially available electrochemical analyzers suitable to determine the concentration of hydrogen halide in air, however, have a high error and a limited range of detectable vapor concentrations. More responsive optical sensors based on photonic crystals and microcavities require complex high-tech production and are often not applicable for hydrogen halide vapors detection. Perovskite nano- and microstructures are capable of entering into an anion exchange reaction from the vapor phase [7;

8] with the formation of mixed halide compounds AB(X,X')3 or completely substituted perovskite homohalides ABX'3, which band gap and refractive index and hence other spectral characteristics are different from the initial ABX3 composition. Such in situ spectral tuning of the optical response, which manifests itself in a change in the spectral position of spontaneous or laser emission [7-12], makes it possible to create highly sensitive optical sensors for analytes in the vapor phase.

The dissertation is devoted to experimental study and numerical simulation of the photophysical properties of perovskite micro- and nanowires (NWs) demonstrating laser generation, improvement of their optical properties via the integration with nanostructured substrates, as well as the creation and study of new highly sensitive optical sensors of hydrogen halide based on lead halide perovskite nanolasers.

The goal of the dissertation is the experimental and theoretical study of the spectral properties of perovskite nano- and microlasers, the control of these properties via the anion exchange from the vapor phase and integration with nanostructured substrates, and the study of possible applications of perovskite lead halide nanolasers in the field of optical gas sensing.

In order to achieve the goal in the framework of the thesis, the following objectives have been established:

1. Experimental study and numerical simulation of optical and structural properties of CsPbBr3 micro- and nanolasers on indium-tin oxide or glass substrates, namely: generation of spontaneous and stimulated emission, crystal phase and morphology.

2. Experimental and theoretical study of the effect of a nanostructured substrate with a low refractive index on the mode composition, quality factor, and generation threshold of perovskite CsPbBr3 micro- and nanowires.

3. Experimental study of photophysical and structural properties of mixed halide microlasers of CsPbBr2Cl and CsPbBr15I15 compositions. Investigation of the temperature dependence of photoinduced phase instability in thin films and nanowires of CsPbBri.5Ii.5 composition.

4. Development, experimental and theoretical study of highly sensitive hydrogen halide vapor sensors based on perovskite micro- and nanolasers.

Statements that are presented for defense

1. CsPbBr3 nanowires supporting Fabry-Pérot resonances and placed on a nanostructured indium-tin oxide layer with an effective refractive index of neff = 1.15 exhibit approximately a two-fold decrease of optically pumped laser generation threshold and a four-fold decrease of the lasing mode spectral width at half maximum in comparison with similar structures on the surface of a compact indium-tin oxide layer with refractive index of n = 1.9 because of the 8 times improved optical field confinement and increase of quality factor of the Fabry-Perot resonances.

2. The suppression of photoinduced phase instability caused by the migration of photoactivated halide ions in cesium lead mixed-halide perovskite micro-and nanowires occurs at 250 K that is 50 K higher then in polycrystalline films of similar composition due to the reduction of the rate of defect-assisted anion diffusion.

3. A single CsPbBr3 nanowire on a nanostructured Al2O3 substrate works as an optical sensor for vapors of hydrogen chloride. Detection principle is based on a spectral shift of lasing modes generated by the nanowire exposed to a wide range of vapor concentration from 5 to 500 parts per million (ppm) upon the photoexcitation. The spectral shift of the lasing modes is invoked by the mixed-halide shell formation with refractive index different from that of the core.

The novelty of research

1. Record-breaking low linewidths of laser generation modes for a class of perovskite CsPbBr3 micro- and nanolasers supporting Fabry-Perot resonances due to integration with nanostructured indium-tin oxide substrates are experimentally shown.

2. A comparative study of the suppression of photoinduced phase instability in a single-crystal nanowire and a polycrystalline film of the same CsPbBr15I15 chemical composition has been carried out for the first time.

3. For the first time, the detection of hydrogen chloride (HCl) vapors at concentrations from 5 to 500 ppm by CsPbBr3 perovskite nanolasers on a nanostructured Al2O3 substrate with an island-like surface morphology has been experimentally shown.

The theoretical and practical significance of the dissertation work lies in a comprehensive study, development of methods for modifying and improving the pho-tophysical properties of perovskite lead halide micro- and nanolasers that support Fabry-Perot resonances, as well as the creation and study of optical sensors based on the obtained data. The results of the study can be applied in the further development of prototypes of photonic integrated circuits and ultra-sensitive gas sensors.

The accuracy of the work results is based on the use of numerous modern research methods that ensure high reproducibility of the obtained data. The results of a comprehensive experimental study are consistent with the results of numerical simulation and do not contradict the literature data. The obtained results were tested at all-Russian and international conferences and published in peer-reviewed journals.

Approbation of research results Key research results were presented and discussed at the following conferences:

1. XX All-Russian Youth Conference on Physics of Semiconductors and Nanos-tructures, Semiconductor Opto- and Nanoelectronics, St. Petersburg, 26-30 November 2018

2. IV International Conference on Metamaterials and Nanophotonics, METANANO 2019, St. Petersburg, 15 - 19 July 2019

3. International Conference on Nanophotonics and Micro/Nanooptics, NANOP 2019, Munich, 4 - 6 September 2019

4. XXI All-Russian Youth Conference on Physics of Semiconductors and Nanostructures, Semiconductor Opto- and Nanoelectronics, St. Petersburg, 25-29 November 2019

5. School on Advanced Light-Emitting and Optical Materials SLALOM 2019, St. Petersburg, 12 - 13 December 2019

6. V International Conference on Metamaterials and Nanophotonics, METANANO 2020, Tbilisi, 14 - 18 September 2020

7. School on Advanced Light-Emitting and Optical Materials SLALOM 2021, Vladivostok, 28 - 30 June 2021

8. VI International Conference on Metamaterials and Nanophotonics, METANANO 2021, online, 13 - 17 September 2021

9. 20th International Conference Laser Optics ICLO 2022, St. Petersburg, 20 - 24 June 2022

10. International Conference on Emerging Light Emitting Materials (EM-LEM22), Limassol, 3-5 October 2022

Personal contribution of the author consists in carrying out experiments on optical microscopy and spectroscopy, creating an experimental setup for gas sensing, processing experimental data, and numerically simulating resonator eigenmodes under various conditions. The author participated in the development of a method for samples synthesis, their fabrication, and subsequent integration with nanostructured indium-tin oxide substrates, as well as the setting of research goals and objectives and the preparation of scientific articles.

Thesis structure and number of pages. The thesis consists of an introduction, five chapters and a conclusion. The full volume of the thesis is 169 pages, including a bibliographic list of 176 titles.The work contains 70 figures and 2 tables.

Publications Key results of research are described in five publications indexed in the Scopus and Web of Science databases.

MAIN CONTENT OF THE WORK

In the introduction of the dissertation, the goal, objectives and statements submitted for defense are formulated. The relevance, reliability and significance of the work are substantiated, as well as the novelty and personal contribution of the author are described.

The first chapter presents a review of the literature devoted to the structural and optical properties of lead halide perovskites, as well as a detailed consideration of the mechanisms of spontaneous and stimulated emission in perovskite micro-and nanocavities. The chapter also discusses the existing methods for changing the optical properties of such structures, including the mechanisms for controlling the band gap and emission in lead halide perovskites. The final section of the first chapter of the dissertation describes the basic principles of the detection of analytes from the gas phase and provides an overview of recent work in this field.

The second chapter presents experimental methods for obtaining and investigating samples. The developed method for salting out CsPbBr3 NWs from the droplets of perovskite precursor and its modification to obtain structures of other homo and mixed halide compositions is described (Figure 1). Methods for nanostruc-tured substrates fabrication for their subsequent integration with perovskite NWs are described. A description of the experimental implementation of the process of the anion exchange from the vapor phase to study the change of the perovskite band gap and emission wavelength is given. A description of all the structural and optical research methods used in the work is presented, schemes of installations are given, including the one developed by the author for the experimental study of the physical principles of hydrogen halide vapors detection.

Figure 1 — Scheme of the method for CsPbBr3 NWs fabrication by salting out from

the droplets of the perovskite precursor

Third chapter of the dissertation is devoted to the experimental study of the structural and optical properties CsPbBr3 micro- and nanowires on glass and indium-tin oxide substrates, as well as to the study of a method for improving the laser generation performance in such structures via integration with nanostructured substrates with a low refractive index.

The first part of the chapter presents the results of structural studies of CsPbBr3 NWs (Figure 2a). The flat rectangular or truncated pyramidal shape of the NW ends and the absence of significant surface defects are demonstrated by scanning electron microscopy (SEM) (Figure 2b). The high crystalline quality and low number of defects in the NW value are confirmed by high-resolution transmission

electron microscopy and fast Fourier transform data, which is shown in Figure 2c. The orthorhombic crystal phase of the single CsPbBr3 NWs is revealed by means of the X-ray diffraction method (Figure 2d, e). In addition, the data, of structural methods confirm the low surface roughness (<2 nm), which is important for the high efficiency of laser generation.

5 nm

(004) '

S ((J02) * (220>

(d) 5000

</> Cl

^ 2500 </> c 0

as-grown NWs + NPLs + bulk 1 1—A...............—A------------- J

single NWs (002) (110) (°04) \I _.................. \ 1(220) 1.

(020) (002),|(110) orth CsPbBr3 (200) (004)i I i ■ ii i i ■ 1 (220)

doo) <110> cubic CsPbBr3 I (200) . 1

15

20

35

14.5 15.0

15.5 30.0 30.5 20 (°)

31.0

25 30

20 (°)

Figure 2 — (a) Photograph of CsPbBr3 NWs on an indium tin oxide (ITO) substrate in bright field regime, (b) SEM image of a CsPbBr3 NW with truncated pyramidal ends at a grazing angle of 45°. Insert: Enlarged image of the end facet, (c) High--resolution transmission electron microscopy and fast Fourier transform images of the same NW along the [001] axis, (cl) X-ray diffraction spectra of the sample in different measurement modes. The red spectrum demonstrates the typical features of CsPbBr3 micro- and na.nowires and plates, as well as of polycrystalline bulk material, when collecting a signal from a large area, of the sample in a grazing incidence regime, the blue spectrum contains peaks related to CsPbBr3 NWs, when collecting a signal from small area focused on a single NW in the 6-6 geometry, finally the pink and green curves are the reference spectra of the orthorhombic and cubic phases of CsPbBr3 perovskite. (e) High-resolution X-ray diffraction patterns corresponding

to a single NW collected in the 6-6 geometry

The second part of the chapter is devoted to a comprehensive study of the optical properties of the obtained CsPbBr3 NWs (Figure 3a). It is shown that perovskite NWs are capable of efficient spontaneous and stimulated photoluminescence (PL) generation. The spontaneous PL peak is typically located at a wavelength of 525 nm with a full width at half maximum (FWHM) of 17 nm (Figure 3b, red curve (i)). The transmission spectrum of NWs excited by a focused on the end facet laser beam (Figure 3b, red curve (ii)) and the white light transmission spectrum (Figure 3b, blue curve (iv)) confirm the ability of NWs to support Fabry-Perot type resonances. The efficiency of spontaneous PL generation is quantitatively expressed in terms of the photoluminescence quantum yield (PL QY), which can be experimentally estimated from the PL decay curves versus time, i.e., charge carrier dynamics. Using the ABC model [13; 14] to approximate the PL decay kinetics, it is shown that upon the excitation with energy fluence below the lasing threshold the PL signal decays according to a monoexponential law with a time constant of 7 ns (Figure 3c, blue curve) and according to the law of bimolecular recombination with a time constant of 1.1 ns upon the excitation above the generation threshold (Figure 3c, red curve). The PL QY value reaches 41.7% for the density of generated carriers equal to 3-1015 cm-3 and increases to 83.4% for the carrier density of 6-1016 cm-3.

Laser generation in CsPbBr3 NW under femtosecond (220 fs) optical excitation with 405 nm wavelength is studied in more detail. As the incident fluence increases to the threshold value, the appearance of narrow lasing lines with a FWHM less than 1 nm is observed. The number of lines depends on the geometrical parameters of the NW cavity. The lasing spectrum of NWs with a length of 11 |am consists of four peaks (Figure 3d). The obtained lines correspond to the Fabry-Perot modes of the resonator with the largest emission cross sections in the optical gain region of the material, which is experimentally confirmed by the NW transmission spectra (Figure 3b, blue curve).

The dependence of the PL intensity on the incident fluence has an S-shaped behavior (Figure 3e, blue curve), revealing the lasing threshold by the significant growth of the intensity, which coincides with a sharp narrowing of the line width from 17 to 0.28 nm near the threshold value of 25 ^J-cm-2 (Figure 3e, red curve).

One of the key characteristics of laser generation is the quality factor of laser modes, which is experimentally determined as Q=A/AA, where A is the wavelength of the laser mode, AA is its full width at half maximum. It is shown that for the NWs under study the quality factor of lasing modes lies in the range of Q = 1017-6166

, «r—ol.6 uJ/cm2 o32 uJ/cm2 ,

: 1 X -fit :

T = 7 ns

if x = 1.1 ns^ifrh^

0 2 4 6 8 10

Time (ns)

"-P<Pth Q=1600

P = Pth

P>Pth

........

500 510 520 530 540 550 Wavelength (nm)

525 550 575 Wavelength (nm)

15 20 25 30 35 40

Fluence (|jJ/cm2)

Figure 3 — (a) Microphotograph of CsPbBr3 NW in fluorescent mode, (b) Spectra of (i) spontaneous photoluminescence under uniform excitation of NWs, (ii) modulated spontaneous photoluminescence under excitation by a focused beam at the end facet, (iii) white light transmission in the dark-field regime, (c) Photoluminescence decay kinetics of CsPbBr3 NWs at an excitation fluence below (blue curve) and above (red curve) the lasing threshold, (cl) Spontaneous and stimulated emission spectra at different excitation fluences for NW with the length of 11 |xm. (e) Photoluminescence intensity and FWHM as a function of excitation fluence

with the lasing thresholds above 13 |xJ-cm-2. In the frameworks of the study, the relationship between the quality factor of the cavity eigenmocles and the shape of the NW end facet is demonstrated by means of the numerical simulation. A truncated pyramidal shape of the NW end facet provides a lower efficiency of emission outcou-pling from the cavity, i.e. lower radiation losses, which is described by the equation:

1

Q<

l

+

l

+

l

(4)

Qrad Qabs Q sea

where Qcav is the quality factor of the cavity, Qrad is the radiative loss clue to emission or leakage, Qabs is the non-radiative loss clue to the absorption of the

emission by the cavity material and its environment, and Q sca - loss due to scattering on surface irregularities and bulk defects. Thus, the demonstrated high Q-factors of the NW laser modes are related to the Q-factors of the rcavity modes according to the Schawlow-Townes equation corrected by Lax [15]:

A PC)2 A

_ A _ 1 out^cavA (5)

_ SA _ 2n2he2 (5)

where Qias is the Q factor of the laser mode, Qcaw is the Q factor of the cavity

eigenmode, Pout is the output power of the nanolaser, A is the central wavelength of

the laser line, h is Planck constant, c is the speed of light in vacuum. According to

this equation, the quality factor of the NW laser mode becomes maximum due to the

high crystalline quality, low surface roughness, and the shape of the NW end facets.

The third part of the chapter is devoted to the study of the improvement

of the optical properties of the obtained CsPbBr3 NWs via the integration with

substrates with a low refractive index. Besides modifying the shape of the NW end

facets, radiation losses can be decreased by reducing the leakage of the optical field

from the cavity into the substrate with a low dielectric contrast with respect to

the nanolaser material. Thus the problem of optimizing the optical properties of

perovskite micro- and nanolasers is solved by their integration with nanostructured

indium-tin oxide (NS ITO) substrates with an effective refractive index of neff=1.15.

NWs of similar geometric dimensions on nanostructured ITO (NW1) and on a

compact ITO layer with a refractive index of n=1.9 (NW2) are considered (Figure 4a,

b). It is shown that, upon the femtosecond pulses excitation with fluence below the

lasing threshold, the spontaneous PL peak of NWs on both substrates is located at

a wavelength of about 526 nm with FWHM = 16 nm. Upon excitation with fluence

above the threshold, both NWs demonstrate pronounced lasing. However, NW 1

placed on NS ITO demonstrate twofold lower laser generation threshold (Figure 4c,

d) and 2-4 times higher Q-factors reaching a record value Q = 7860 for Fabry-Perot

nanolasers (Figure 4e, f). Moreover, the laser lines of NW1 and NW2 have different

shapes. The lasing modes in the case of NW2 on compact ITO have two components,

in contrast to NW1 on NS ITO. These components correspond to the optical modes

with different polarizations due to the presence of a substrate with refractive index

different from the perovskite, which breaks the symmetry and and takes off the

degeneracy of the modes, what does not happen in the case of NW1 on a NS ITO

substrate. This feature, as well as improved optical properties, is associated with

an increased dielectric contrast between the cavity and the environment, which

additionally results in a richer mode composition of NW1 and is a consequence of the suppression of optical field leakage from the cavity.

Figure 4 — (a, b) SEM images of NW 1 and NW 2 on nanostructured and compact ITO, respectively (scale bar, 1 ym). (c, d) Dependences of the integral PL intensity and FWHM on the excitation fluence for NW 1 and NW 2, respectively. (e, f) Analysis of Q-factors of NW lasing modes on different substrates

The final part of the chapter contains the results of numerical simulations, which provide confirmation of the experimental results. The quality factors and distributions of the components of the electromagnetic field of eigenmodes in NWs depending on the refractive index of the substrate are calculated for structures with dimensions of 3.5x0.4x0.4 |xm (Figure 5a-c). To confirm the key role of the substrate, the field confinement factor in the cavity was calculated as n=Wnw/Wcea, where W^w is the time-averaged total energy inside the cavity, Wcea is the time-averaged total energy in the entire computational cell. It is shown that NWs on a layer of compact ITO demonstrate weak field confinement of n=10% , while the use of a glass substrate with a refractive index of n=1.5 leads to an increase of the n value up to 56%, and the deposition of nanolasers on the surface of nanostructured ITO gives a record value of n=82%.

The quality factors of the simulated eigenmodes are calculated according to the expression Qcaw=Re(w)/2|Im(w)|, where w is the resonant frequency. As a result, when the substrate refractive index decreases from 1.9 to 1.15, a significant

Figure 5 — Spatial distribution of the Ez component of the optical field of the TM22 mode in CsPbBr3 NWs with 3.5x0.4x0.4 ym dimensions on substrates with different refractive indices: (a) 1.15, (b) 1.5 and (c) 1.9. Insert: distribution of the magnetic field modulus |H| in cross section of NW. (d) Q-factor of the cavity eigenmode as a

function of the substrate refractive index

increase in the quality factor of the cavity eigenmodes from 100 to 350 is demonstrated (Figure 5d).

Thus, the simulation results confirm the assumption of an increase in the quality factor of the lasing modes due to integration with a substrate with a low refractive index and, as a consequence, an 8 times improved confinement of the optical field.

The fourth chapter of the dissertation, approaches to controlling the spectral position of spontaneous and stimulated emission in lead-halide perovskite micro-and nanocavities are studied.

It is known that the substitution of anions in the perovskite crystal lattice causes a change in the band gap (Eff). For homohalide compositions the value of Eg varies from 3.0 [16] to 2.39 [17] and 1.73 eV [18] for chlorine, bromine, and

iodine, respectively. A subtle change in the band gap and, as a consequence, the emission spectra can be achieved by obtaining stoichiometric mixtures of halides. For example, the position of the PL peak for the compositions of CsPb(I1-xBrx), where 0 ^ x ^ 1 depends on the halogen ratio and takes values from 530 to 700 nm [19-21].

There are two main approaches to obtaining nanostructures of homohalide and mixed halide compositions and, as a consequence, to tuning the band gap and emission spectral position: direct synthesis or anion exchange from the vapor phase.

The first section of the chapter describes the results of using direct solution chemistry methods to obtain NWs of mixed and homohalide composition with a given position of the photoluminescence peak. A detailed description of the method is presented in Chapter 2. It is shown that for perovskite CsPbBr2Cl NWs, spontaneous and stimulated emission behave similarly to CsPbBr3 homohalide NWs described in Chapter 3, but demonstrate a spectral shift to shorter wavelengths. However, the described approach is difficult to apply for accurate changing of the perovskite band gap.

More precise control of the emission spectral position can be achieved by anion exchange from the vapor phase. In the second part of the chapter, to study the influence of this effect, the comparison of two types of perovskite structures with the initial CsPbBr3 composition is provide. Thin films with a thickness of 80-82 nm and a roughness of 3-4 nm and NWs on a glass substrate are compared. The study of the optical properties of homohalide NWs showed similar results as in Chapter 3. The typical spontaneous PL peak of the initial polycrystalline CsPbBr3 film occurs at the same wavelength as in the case of single-crystal NWs (Figure 6a, c), however, laser generation is not observed in the films.

Exposure to hydrogen iodide (HI) vapor at a concentration of 1000 ppm followed by annealing at 150°C causes a spectral shift of the PL peak to the long wavelength region by ^100 nm in the case of a thin film, which corresponds to a decrease in the band gap to ^2 eV and change in stoichiometric composition close to CsPbBr15I15 [22]. In the case of NW, the position of the spontaneous PL peak depends on the NW cross section due to the surface nature of the anion exchange process. The non-uniformity of substitution of halogen anions in the volume of NW is more noticeable at large cross-sections, while at small cross-sections it gives an almost uniform result. However, both thin films and NWs after the exposure to highly concentrated HI vapor demonstrate the effect of photoinduced phase instability upon excitation with fluence of 0.1 mW^m-2 at ambient conditions. The

effect is expressed in the appearance of additional PL peaks, which indicates the separation of crystalline phases [23]. In the case of thin films, the appearance of two PL peaks at wavelengths of 624 and 690 nm is observed. It is shown that as the temperature decreases, the phase instability in polycrystalline CsPbBr15I15 films disappears at 200 K (Figure 6b). NWs of the same mixed halide composition show similar photoinduced phase instability with two PL peaks at wavelengths around 625 and 670 nm at room temperature (Figure 6d). It was found that the phase instability suppression in NWs occurs at a higher temperature of 250 K (Figure 6d). The observed difference is explained by the fact that the diffusion of anions in lead halide perovskites is defect-assisted [24] and directly depends on the diffusion coefficient of vacancies, expressed as:

E

D = Doexp(-) (6)

where D0 is the diffusion constant at T^ to, Ea is the diffusion activation energy, and k is the Boltzmann constant. In addition, the concentration of vacancies also obviously affects the rate of anion separation according to Fick's second law. This means that the initially single-crystal perovskite NWs after the anion exchange procedure contain a smaller number of defect states than polycrystalline thin films, which increases the resistance of NWs to photoinduced phase instability.

After exposure to highly concentrated HI vapor, no laser generation is observed in the structures under consideration.

The last part of the chapter presents the results of experiments on changing the spectral characteristics of perovskite NWs while maintaining the laser generation after anion exchange procedure.

Three NWs of CsPb(Br,Cl) 3 and CsPb(Br,I)3 compositions, obtained by anion exchange from the vapor phase via the exposure to 1500-2000 ppm of hydrogen chloride or hydrogen iodine followed by annealing at 100°C are considered. It is shown that laser generation is preserved, however, NWs of mixed halide compositions demonstrate higher generation thresholds and lower Q factors of the dominant laser line compared to the initial composition, as well as lower temporal stability, which is associated with the generation of defects in the crystal lattice during anion exchange process (Figure 7). It is assumed that the retention of laser generation is associated with a decrease in the annealing temperature and, as a consequence, with a lower rate of vacancy generation. Additionally, it was shown that CsPb(Cl,Br)3 NWs have higher structural stability compared to CsPb(Br,I)3 analogs.

Figure 6 — Spontaneous photoluminescence spectra at different temperatures for (a) a CsPbBr3 thin film and (c) NWs of the same composition in comparison with (b, d) the same dependences for CsPbBr15I15 samples after the exposure to HI. Insert: microphotographs of NWs (c) before and (d) after the process of anion exchange in the vapor phase in the fluorescent mode, (c) SEM image of the studied NW before

exposure to HI vapor

Thus, optical properties of mixed halide nanolasers are inferior to nanolasers of homohalide compositions CsPbBr3 and CsPbCl3. However, mixed halide nanolasers are capable of demonstrating significant lasing performance. This explains the choice homohalide composition NWs, capable of transformation into mixed-halide phases with retention of laser generation, as a platform for optical sensing of low concentrations of hydrogen halide vapors.

In the final fifth chapter, the principles of optical sensing of hydrogen halide vapors by CsPbBr3 perovskite lead-halide nanolasers are studied. The possibility of detecting hydrogen chloride (HCl) vapors in a wide range of concentrations from 5 to 500 ppm is shown.

The first part of the chapter is devoted to the experimental study of the mechanism of hydrogen chloride vapors detection. CsPbBr3 NWs are obtained on an island-like aluminum oxide (Al2O3) substrate according to the method described in Chapter 2. The resulting NWs lie on Al2O3 islands of submicron size, which ensures uniform access of the detected analyte to the entire NW surface. The data of high-resolution transmission electron microscopy, X-ray powder diffraction, and optical microscopy in the dark field mode confirm the high crystal quality of NWs, which are capable of serving as optically active Fabry-Perot cavities for high-Q laser generation. The photoluminescence spectrum measured for an ensemble of NWs consists of one exciton peak centered at 524 nm with a FWHM of 17 nm. NWs of 7-15 |xm length demonstrate high-quality (Q = 1800-3000) multimode laser generation when excited with fluence above 17-32 yJ-cm-2.

Experiments on the in situ detection of HCl vapors at room temperature are carried out inside a sealed gas cell (Figure 8a). When vapors of different concentrations pass through the cell, three patterns of PL time evolution are observed

495 640

Wavelength (nm)

Figure 7 — Laser generation spectra of CsPbBri.5Ii.5 (NW 1), CsPbClBr2 (NW 2), and CsPbBrI2 (NW 3) nanowires after the anion exchange procedure in vapor phase when exposed to hydrogen chloride or hydrogen iodide vapor at a concentration of

1500-2000 ppm followed by annealing at 100°C

(Figure 8b_d). All dependences demonstrate a spectral shift of lasing lines towards short wavelengths and have a typical linear part. The time-dependent rate of linear shift of the laser mode |AA|/At depends on both the HCl concentration and the NW cross section. It is shown that for NWs with close cross-sectional dimensions, the higher the concentration of HCl vapor, the greater the blueshift of the laser mode for the same time lapse of interaction with the analyte.

It was found that the anion exchange reaction from the vapor phase at room temperature without subsequent annealing modifies the surface of CsPbBr3 NWs and leads to the formation of a thin shell of the mixed halide composition CsPb(Cl,Br)3. The resulting shell with a lower refractive index than the CsPBbr3 tribromide core reduces the spatial confinement of the eigenmode field in the cavity and provokes an increase in the eigenfrequency to preserve modes spatial localization in the core-shell cavity. It is also shown that laser modes change their intensity during the anion exchange process by passing through the region of maximum optical gain, which, in turn, does not change its spectral position (Figure 8i). The stationar behavior of the gain region is confirmed by the absence of the spontaneous photoluminescence spectral shift (Picture 8f).

It is experimentally demonstrated that the rate of linear spectral shift |AA|/At is directly proportional to the concentration of HCl with a constant of 4.8-10_5 nm-s-1 •ppm-1 (purple squares in Figure 8g). It is noted that thin NWs are more sensitive to the formation and expansion of a mixed halide shell due to a larger ratio of the shell volume to the core one.

The results of X-ray photoelectron spectroscopy (XPS), as well as in situ X-ray diffraction, energy dispersive X-ray spectroscopy (EDX), and fluorescence microscopy confirm the formation of a mixed halide shell. It is shown that when exposed to 250 ppm HCl vapor at room temperature for 5 minutes, a quantitative evaluation of the spatial distribution of halogens gives an exponential decline (growth) of the chlorine (bromine) content from 20% (80%) on the surface to 0% (100%) at a depth of 35 nm (Figure 9a). In-situ X-ray diffraction patterns from an ensemble of microcrystals exposed to 3000 ppm HCl vapor demonstrate a decrease in the intensity of peaks from the (004) and (220) crystal planes with exposure time due to a decrease of the core volume. The appearance of a diffuse shoulder at large 26 angles (Figure 9b) indicates the formation of a mixed halide shell with an uneven depth distribution of Cl_ and Br_ ions. The EDX results visualize the distribution of Cs, Pb, Cl and Br ions in the microstructure after exposure to the same concen-

Figure 8 — (a) Illustration of the scheme and mechanism of optical detection of HCl vapors by the CsPbBr3 NW in a sealed gas cell. (b-d) Time evolution maps of spontaneous and laser emission for (b) 500, (c) 250, and (d) 5 ppm hydrogen chloride vapor concentrations in a gas cell. (e) Narrow-band lasing spectra at 200 and 640 seconds of exposure to 250 ppm hydrogen chloride to visualize the change in mode intensity as it passes through the region of maximum optical gain. (f) Spectral shifts |AA| of spontaneous and laser emission when exposed to 250 ppm HCl depending on the exposure time. (g) The linear shift rate of laser lines as a function of hydrogen chloride concentration in air for nanolasers with different cross sections

tration of HCl for 15 minutes. It is shown that the crystal facets are enriched with chloride ions as the bromine concentration decreases (Figure 9c, d). In addition, the presence of mixed halide shell after the exposure to 5000 ppm HCl vapor for 15 minutes is confirmed by a noticeable blueshift of the PL peak, as well as a decrease in its intensity after transfer of excitons and free charge carriers from the shell to the core with a smaller band gap (Figure 9e).

A theoretical simulation of the temporal dynamics of the mixed halide shell formation is carried out at various concentrations of hydrogen halide vapor based on the diffusion of halogen ions. It is shown that the position of the shell boundary with a 10% content of chlorine ions under the influence of a low concentration of HCl undergoes a significant onset lag, followed by a linear dependence of the shell thickness growth with time. While exposure to high concentrations of HCl leads to the rapid formation of a shell, the thickness of which depends on time initially according to a non-linear law, turning into a linear dependence after 300 s of exposure (Figure 10a). The observed behavior is consistent with the results of

Wavelength [nm)

Figure 9 — (a) XPS results of depth profiling of the same microcrystal, showing the change in the content of Cl- (Br-) ions with etch depth. (b) In situ X-ray diffraction patterns of microcrystals of the initial CsPbBr3 composition exposed to 3000 ppm HCl vapor for various time lapses. Inset: Enlarged part of the spectrum in a logarithmic scale showing the formation of a mixed halide shell with a non-uniform depth distribution of Cl- and Br- ions. (c) EDX profile of the elements distribution in the same crystal along the dotted yellow line in the SEM image. (d) SEM image of the microcrystal after exposure to 3000 ppm HCl vapor for 15 min and EDX maps of the spatial distribution of Cs, Pb, Cl and Br in the crystal. (e) PL spectra collected from the end facet and center of NW exposed to 5000 ppm HCl vapor for 15 min. Inset: Enlarged plot in a logarithmic scale, showing a weak signal in the blue region of the spectrum, corresponding to shell emission

optical experiments. Numerical simulation of the spectral position of the eigenmocles of the core-shell type NWs showed that the expansion of the shell does not affect the localization of the eigenmode, but causes a change in the eigenfrequency expressed in a significant blueshift depending on the thickness of the shell (Figure 10b). The model adequately describes the magnitude of the experimentally observed linear blueshift of laser modes for a shell thickness not exceeding 20 nm.

(a) ? 30

c 03

O 20 .c o

o

o

4—

c

o

Figure 10 — (a) Temporal dynamics of propagation of mixed halicle shell boundary with 10% content of Cl~ ions under the influence of various concentrations of HC1 vapor, (b) Numerically obtained dependence of the spectral shift AA on the thickness of the NW shell for various values of its cross section dimensions

Thus, the possibility of using lead halicle perovskite na.nolasers as highly sensitive sensors of hydrogen halicle vapor in the atmosphere has been shown.

1 1 1 1 1 — 50 ppm 1 1

- —1 — 75 ppm -

100 ppm

— 175 ppm

250 ppm ,

375 ppm

- — 500 ppm -

-9—^ i , i , 1

200

(D)

400 600 Time (s)

800

i 1 i 1 i 1 r —«— d-|= 400 nm —«— d-|= 500 nm 20 - EH n e genmode

E

c

<< <

— 10

0 20 40 60 80 100

Shell thickness (d2, nm)

Conclusions on the main results of the thesis:

1. Lead halide perovskite micro- and nanowires of CsPbBr3 composition obtained by an original method of salting out from perovskite precursor droplets demonstrate high-Q coherent laser emission with quality factors of 1017-6166 at low optical pumping thresholds from 13 ^J-cm_2 due to the high crystalline quality, low surface roughness and truncated-pyramidal shape of the end facets, which provides a lower efficiency of emission outcoupling from the cavity and, as a result, lower radiation losses.

2. Integration of CsPbBr3 perovskite lead halide micro- and nanowires generating laser emission with nanostructured indium-tin oxide substrates provides improved confinement of the optical field in the nanocavity due to an increase in the dielectric contrast between the perovskite active medium and the substrate with a low effective refractive index (ne// = 1.15). As a result, record values of Q-factors of Fabry-Perot type perovskite nanolasers achieving Q = 7860 have been experimentally demonstrated.

3. The change in the stoichiometric ratio of halogen anions in the structure of mixed halide perovskite micro- and nanowires, which occurs as a result of their direct preparation by the solution method, as well as heterophase anion exchange between saturated hydrogen halide vapor and CsPbBr3 nanocrystals, causes a significant spectral rearrangement of the perovskite emission. A comparative study of the suppression of photoinduced phase instability in a single-crystal nanowire and a polycrystalline thin film of the same chemical composition CsPbBr1.5I1.5 is carried out for the first time. It is shown that single-crystal nanowire has an increased resistance to photoin-duced phase instability, and its suppression occurs at a higher temperature compared to polycrystalline thin film. This effect is explained by defect-assisted diffusion in perovskites and means that the initially single-crystal perovskite micro- and nanowires after the anion exchange procedure contain a smaller number of defect states.

4. The detection of hydrogen chloride (HCl) vapors in a wide range of concentrations from 5 to 500 ppm by CsPbBr3 perovskite nanolasers on an aluminum oxide island-like substrate has been experimentally shown for the first time. During the interaction of hydrogen chloride molecules with the surface of perovskite nanowire, a shell of mixed halide composition CsPb(Cl,Br)3 is formed. The presence of a shell with a lower refractive

index relative to the bromine core causes a blueshift of the laser generation lines, which depends on the hydrogen chloride vapor concentration and the cross-sectional dimensions of the nanowire. The linear response regime of perovskite nanostructures makes it possible to determine the analyte concentration in air.

Main publications on the topic of the thesis:

[A1] Markina D. I., Pushkarev A. P., Shishkin I.I., Komissarenko F.E, Beresten-nikov A. S., Pavluchenko A.S., Smirnova I. P., Markov L.K., Vengris M., Zakhidov A.A., Makarov S. V. Perovskite nanowire lasers on low-refractive-index conductive substrate for high-Q and low-threshold operation //Nanophotonics, 2020, vol. 9, no. 12, pp. 3977-3984.

[A2] Markina D. I., Tiguntseva E.Yu., Pushkarev A.P., Samsonov M.A., Vengris M., Munkhbat B., Shegai T., Hix G.B., Zakhidov A.A., Makarov S.V. Photophysical properties of halide perovskite CsPb (Br1-XIX)3 thin films and nanowires //Journal of Luminescence, 2020, vol. 220, pp. 116985.

[A3] Pushkarev A. P., Korolev V.I., Markina D.I., Komissarenko F.E., Naujokaitis A., Drabavicius A., Pakstas V., Franckevicius M., Khubezhov S.A., Sannikov D.A., Zasedatelev A.V., Lagoudakis P.G., Zakhidov A.A., Makarov S.V. A few-minute synthesis of CsPbBr3 nanolasers with a high quality factor by spraying at ambient conditions //ACS applied materials & interfaces, 2018., vol. 11, no. 1, pp. 1040-1048.

[A4] Markina D. I., Pushkarev A.P., Shishkin I.I., Zuev D.A., Makarov S.V. Rapid synthesis and optical properties of CsPbBr2Cl perovskite nanolasers //Journal of Physics: Conference Series. - IOP Publishing, 2020, vol. 1461, no. 1, pp. 012091.

[A5] Markina D. I., Pushkarev A. P., Makarov S. V. Theoretical study of perovskite nanowires optical response to hydrogen halides vapor exposure //Journal of Physics: Conference Series. - IOP Publishing, 2021, vol. 2015, no. 1, pp. 012087.

Introduction

Relevance

Nowadays the performance and speed of integrated electronic circuits are approaching the limit of their capabilities. The limitation of electronic devices gives rise to the need for a transition to an optical platform that uses a photon as a carrier of an information unit. Optoelectronic and nanophotonic devices can stimulate a big step in the development of computer technology and the transmission of large amounts of data.

To create optical chips, first of all, miniature photon sources - nanolasers - are needed. The efficiency of the nanolasers operation largely depends on the choice of the material of the amplification medium. Convenient semiconductor materials capable of laser generation, such as Si, GaAs and GaN, require expensive and hightech manufacturing processes, in particular epitaxy. In contrast the new class of semiconductor materials - lead halide perovskites - are not only able to form regular shape resonators in the process of fast and cheap synthesis, but also distinguished by outstanding optical properties, successfully meeting the needs of modern optoelectronics and photonics.

Lead-halide perovskites are excitonic materials, due to which they demonstrate highly efficient photoluminescence at ambient conditions [1], high mobility of charge carriers [2], rather high values of optical gain (^104 cm-1 [3-5]) and the refractive index (2-2.5), which provides the presence of high-Q optical resonances in micro-and nanostructures [6].

In addition, miniature sources of laser emission can act as highly sensitive optical sensors of harmful to human health hydrogen halides. There are various commercially available electrochemical analyzers suitable to determine the concentration of hydrogen halide in air, however, have a high error and a limited range of detectable vapor concentrations. More responsive optical sensors based on photonic crystals and microcavities require complex high-tech production and are often not applicable for hydrogen halide vapors detection. Perovskite nano- and microstructures are capable of entering into an anion exchange reaction from the vapor phase [7; 8] with the formation of mixed halide compounds AB(X,X')3 or completely substituted perovskite homohalides ABX'3, which band gap and refractive index and hence other spectral characteristics are different from the initial ABX3 composition. Such

in situ spectral tuning of the optical response, which manifests itself in a change in the spectral position of spontaneous or laser emission [7-12], makes it possible to create highly sensitive optical sensors for analytes in the vapor phase.

The dissertation is devoted to experimental study and numerical simulation of the photophysical properties of perovskite micro- and nanowires (NWs) demonstrating laser generation, improvement of their optical properties via the integration with nanostructured substrates, as well as the creation and study of new highly sensitive optical sensors of hydrogen halide based on lead halide perovskite nanolasers.

The goal of the dissertation is the experimental and theoretical study of the spectral properties of perovskite nano- and microlasers, the control of these properties via the anion exchange from the vapor phase and integration with nanostructured substrates, and the study of possible applications of perovskite lead halide nanolasers in the field of optical gas sensing.

In order to achieve the goal in the framework of the thesis, the following objectives have been established:

1. Experimental study and numerical simulation of optical and structural properties of CsPbBr3 micro- and nanolasers on indium-tin oxide or glass substrates, namely: generation of spontaneous and stimulated emission, crystal phase and morphology.

2. Experimental and theoretical study of the effect of a nanostructured substrate with a low refractive index on the mode composition, quality factor, and generation threshold of perovskite CsPbBr3 micro- and nanowires.

3. Experimental study of photophysical and structural properties of mixed halide microlasers of CsPbBr2Cl and CsPbBr15I15 compositions. Investigation of the temperature dependence of photoinduced phase instability in thin films and nanowires of CsPbBr1.5I1.5 composition.

4. Development, experimental and theoretical study of highly sensitive hydrogen halide vapor sensors based on perovskite micro- and nanolasers.

Statements that are presented for defense

1. CsPbBr3 nanowires supporting Fabry-Perot resonances and placed on a nanostructured indium-tin oxide layer with an effective refractive index of neff = 1.15 exhibit approximately a two-fold decrease of optically pumped

laser generation threshold and a four-fold decrease of the lasing mode spectral width at half maximum in comparison with similar structures on the surface of a compact indium-tin oxide layer with refractive index of n = 1.9 because of the 8 times improved optical field confinement and increase of quality factor of the Fabry-Perot resonances.

2. The suppression of photoinduced phase instability caused by the migration of photoactivated halide ions in cesium lead mixed-halide perovskite micro-and nanowires occurs at 250 K that is 50 K higher then in polycrystalline films of similar composition due to the reduction of the rate of defect-assisted anion diffusion.

3. A single CsPbBr3 nanowire on a nanostructured Al2O3 substrate works as an optical sensor for vapors of hydrogen chloride. Detection principle is based on a spectral shift of lasing modes generated by the nanowire exposed to a wide range of vapor concentration from 5 to 500 parts per million (ppm) upon the photoexcitation. The spectral shift of the lasing modes is invoked by the mixed-halide shell formation with refractive index different from that of the core.

The novelty of research

1. Record-breaking low linewidths of laser generation modes for a class of perovskite CsPbBr3 micro- and nanolasers supporting Fabry-Perot resonances due to integration with nanostructured indium-tin oxide substrates are experimentally shown.

2. A comparative study of the suppression of photoinduced phase instability in a single-crystal nanowire and a polycrystalline film of the same CsPbBr15I15 chemical composition has been carried out for the first time.

3. For the first time, the detection of hydrogen chloride (HCl) vapors at concentrations from 5 to 500 ppm by CsPbBr3 perovskite nanolasers on a nanostructured Al2O3 substrate with an island-like surface morphology has been experimentally shown.

The theoretical and practical significance of the dissertation work lies in a comprehensive study, development of methods for modifying and improving the pho-tophysical properties of perovskite lead halide micro- and nanolasers that support Fabry-Perot resonances, as well as the creation and study of optical sensors based

on the obtained data. The results of the study can be applied in the further development of prototypes of photonic integrated circuits and ultra-sensitive gas sensors.

The accuracy of the work results is based on the use of numerous modern research methods that ensure high reproducibility of the obtained data. The results of a comprehensive experimental study are consistent with the results of numerical simulation and do not contradict the literature data. The obtained results were tested at all-Russian and international conferences and published in peer-reviewed journals.

Approbation of research results Key research results were presented and discussed at the following conferences:

1. XX All-Russian Youth Conference on Physics of Semiconductors and Nanos-tructures, Semiconductor Opto- and Nanoelectronics, St. Petersburg, 26-30 November 2018

2. IV International Conference on Metamaterials and Nanophotonics, METANANO 2019, St. Petersburg, 15 - 19 July 2019

3. International Conference on Nanophotonics and Micro/Nanooptics, NANOP 2019, Munich, 4 - 6 September 2019

4. XXI All-Russian Youth Conference on Physics of Semiconductors and Nanostructures, Semiconductor Opto- and Nanoelectronics, St. Petersburg, 25-29 November 2019

5. School on Advanced Light-Emitting and Optical Materials SLALOM 2019, St. Petersburg, 12 - 13 December 2019

6. V International Conference on Metamaterials and Nanophotonics, METANANO 2020, Tbilisi, 14 - 18 September 2020

7. School on Advanced Light-Emitting and Optical Materials SLALOM 2021, Vladivostok, 28 - 30 June 2021

8. VI International Conference on Metamaterials and Nanophotonics, METANANO 2021, online, 13 - 17 September 2021

9. 20th International Conference Laser Optics ICLO 2022, St. Petersburg, 20 - 24 June 2022

10. International Conference on Emerging Light Emitting Materials (EM-LEM22), Limassol, 3-5 October 2022

Personal contribution of the author consists in carrying out experiments on optical microscopy and spectroscopy, creating an experimental setup for gas sensing, processing experimental data, and numerically simulating resonator eigenmodes under various conditions. The author participated in the development of a method for samples synthesis, their fabrication, and subsequent integration with nanostructured indium-tin oxide substrates, as well as the setting of research goals and objectives and the preparation of scientific articles.

Thesis structure and number of pages. The thesis consists of an introduction, five chapters and a conclusion. The full volume of the thesis is 169 pages, including a bibliographic list of 176 titles.The work contains 70 figures and 2 tables.

Publications Key results of research are described in five publications indexed in the Scopus and Web of Science databases.

Chapter 1. Literature review

1.1 Lead halide perovskites as the novel material platform

In the last decade perovskite materials have generated a surge of research interest due to the possibility of their use as a material platform for nanophotonics and optoelectronics applications. In particular, the first breakthrough works on perovskite photovoltaics [25; 26] that showed high-efficient solar cells turned the vector of research towards this new class of semiconductors. The combination of unique structural and optical properties with cost-effective and simple manufacturing approaches makes lead halide perovskites promising candidates for replacing conventional semiconductors in such devices as LEDs [27], photodiodes [28], photonic integrated circuits for fast information processing [29], optical sensors [30], nanolasers [31-34] etc.

1.1.1 Structural and electronic properties

The first mention of the perovskite structure, found in the form of a CaCO3 mineral, dates back to 1839 [35]. Nowadays artificially synthesized perovskites are described by a general chemical formula of ABX3, where A and B are positively charged cations, X is an anion of halogen (Cl, Br, I). Usually A is relatively large cation of formamidinium (FA, NH2CHNH+), methylammonium (MA, CH3NH+) or cesium ion (Cs+), B is smaller divalent one such as Pb2+ or Sn2+ in the frames of this work lead ions are considered. A typical perovskite crystal lattice is a network of (BX6)4- octahedra with B ions in the center, common anions and A cations located between them (Figure 1.1a). However, the type of symmetry of the structure is determined from the ratio of the radii of the A and X ions, expressed in the Goldschmidt tolerance factor

t = (1.1)

V2(rB + rx)

where r denotes the radius of the corresponding ion. If t > 1.02, then the structure is characterized by tetragonal or hexagonal symmetry, due to the fact that the A ion is too large or the B ion is too small (Figure 1a). If t < 0.89, then the structure has orthorhombic symmetry, since the A ions are too small to accommodate the B ions in the voids (Figure 1.1b). The values in the intermediate range show that the A and B ions are of ideal size providing a cubic type of lattice [36] (Figure 1.1c).

a Orthorhombic b Tetragonal c Cubic

a a a

a^b^c a = b ^ c a = b = c

0 A cation 4 B cation 41 X anion

Figure 1.1 — Crystal structures of different crystal lattice types of ABX3 perovskites

Speaking about the influence of crystal structure to the electronic properties it should be noted that the band structure is mostly determined by the largest angle of the metal-halide-metal bond that could be controlled by the (BX6)4- octahedra tilt [37]. This octahedra tilting is affected by the Coulomb and steric interactions with A-site cation thus influencing the near band edge electronic structure. The type of A-cation largely determines the perovskite structure and stabilizes crystal lattice unit [38], although its electron orbitals are not directly involved in band gap (Eg) formation. The edge of the valence band is resulted from the hybridization of p electron orbitals of the halide anion and 6s orbitals of B cation like lead. The conduction band minimum is formed by 6p electron orbitals of the same B cation and p-orbitals of X anion [39; 40] (Figure 1.2).

Figure 1.2 — The band structure of ABX3 perovskite. The bottom of the conduction band and the top of the valence band (green lines) are built of s and p orbitals belonging to lead and halide ions that result in a direct band gap [39]

1.1.2 Optical properties

Optical properties of metal lead halide perovskites strongly depend on the type of the anion X-, which largely contributes to the electronic structure of perovskite. The lead-halogen bond strength weakens in accordance with halogens electronegativity. This allows to tune the wavelength of perovskites emission in the wide visible range from red to blue edge of the spectrum by the substitution of halogen atoms for Cl, Br, I or their mixture.

To further investigate optical properties of lead halide perovskites it is important to understand the nature of their emission. Plenty of studies confirm the excitonic nature of the emission in perovskites and give a wide range of exciton binding energies in organic-inorganic perovskites of various compositions from 5 to 40 meV [41-47]. The results of studies of all-inorganic perovskites where cesium plays the role of the A cation are more clear. The introduction of an inorganic cation increases the binding energy to ^75, 40 and 20 meV in CsPbCl3, CsPbBr3 and CsPbI3 perovskite compositions, respectively [48; 49]. Such large values indicate the existence of excitons at room temperature and lead to a narrow photolumi-

nescence (PL) line (Figure 1.3) and high values of its quantum yield compared to conventional semiconductor counterparts.

Wavelength (nm) 680 640 600 560 520 480

1.9 2.1 2.3 2,5 2.7

Photon Energy (eV)

Figure 1.3 — Photoluminescence lineshape for different light-emitting materials: Rhodamine 6G (black curve), CdSe/CdS-ZnS core/shell nanocrystals (blue curve), and CsPbBr3 nanocrystals (red line). Inset: image of light-emitting device (LED) obtained by covering an UV-LED with CsPbBr3 nanocrystals [50]

One of the most crucial parameters for optical description of the material is a refractive index (n). Typical wavelength dependence of n undergoes a sharp increase at the wavelength corresponding to exciton state as well as growth of losses expressed in the extinction coefficient (k) right before excitonic peak (Figure 1.4 a,b). Also the crystal morphology largely influences the refractive index. It is important to mention that lead halide perovskites provide a significant dielectric contrast due to the relatively high refractive index comparing to the commonly used SiO2 and most of polymers, but at the same time low enough to provide contrast with Si and GaAs, that make perovskite structures promising for the resonant nanophotonics.

Another useful property of lead halide perovskites is high absorption coefficient up to 2-105 cm-1 [10; 51; 52] at above-exciton energies (see Figure 1.4), leading to the efficient photoluminescence.

Figure 1.4 — ((a, b) Refractive index (n) and extinction coefficient (k) of the CsPbBr3 thin film, uncertainty of the measurement is shown using shadow banc! [53]. (c)

Absorption coefficient of different perovskite thin film composition [52]

1.2 Spontaneous and stimulated emission in perovskite micro- and

nanocavities

1.2.1 Introduction to emission mechanisms in semiconductors

To describe the phenomena, of spontaneous and stimulated emission in the simplest case one needs to consider a, two-level energy system (atom). Without an external stimuli the most of the charge carriers (electrons) are located at the lowest energy level Ei and the other levels (E2 and beyond) have a, minimum occupancy or free. External excitation with the energy greater than (E2 - Ei) can induce transition to the excited state E2 with the probability expressed in terms of the charge carriers density (or population) at the certain energy state Ni and corresponding Einstein coefficient (Bi2). Moreover, it is proportional to the external irradiation density:

^ =-Bl2NiP(y) (1-2)

where p(v) is the flux (energy density) of the incident light. This process is called an absorption.

After the excitation, while no additional external influence is applied, the atom should spontaneously relax from the high-energy state, converting the excess of energy into electromagnetic radiation - photon with the frequency v = {Eo — E\)/h.

This process is known as spontaneous emission (SE). Its probability is expressed in a similar way, however, the Einstein coefficient for a spontaneous radiative transition (A2i) is determined only by the properties of a particular transition.

dN2 dt

= -A21N2

(1.3)

Spontaneous emission is not temporal or spatially coherent. Although, it should be mentioned that such transition may not cause the formation of the photon, but happen through a non-radiative channel.

Alternatively, if the already excited system is perturbed by an electric field with the energy of hv = (E2 — E{), it can generate photon of the same energy (frequency) and phase, amplifying external field. This is a stimulated emission process. The similarity of the frequency, phase, polarization and direction of propagation of the initial and resulting photons allows us to assert that stimulated emission is coherent with incident light (Figure 1.5). The rate (probability) of the process can be written as the same rate for the absorption using the corresponding Einstein coefficient (B\2 = B2\):

dN2 dt

= -B21 N2p(v)

(1.4)

Figure 1.5 — Absorption, spontaneous and stimulated emission mechanisms

However, it is worth remembering that when radiation passes through a medium, optical absorption also occurs, which decreases the intensity of radiation. Thus, stimulated generation of new carriers must exceed the absorption. In the state of thermodynamic equilibrium, absorption predominates. Therefore, we come to a parameter that plays an important role in the achievement of stimulated emission generation - optical gain of the material that reflects the ability of the material to amplify the input power. Emission flux density dI changes due to both stimulated emission and absorption processes in the material layer dz according to the following equation (Figure 1.6a):

dT I

— = (N2 - Nl)B2i-hv dz c

(1.5)

By solving this equation we obtain a law similar to the Bouguer-Lambert-Beer

one:

I = Ioe+gl (1.6)

where g = (N2—1)B21 ^r = (N2 — N1)u21 is an optical gain, where a21 is the stimulated emission cross-section. If N2 > N1 then the medium is amplifying (called active medium). The condition of N2 > N1 is so called 'population inversion'.

Experimental methods of the material gain estimation include various approaches such as stripe-length method [54; 55], transmission method, etc [56; 57].

Although, besides the high optical gain values outweighing losses and population inversion in order to turn the amplifier into a laser generator, it is necessary to provide positive feedback by placing the active medium in the cavity (Figure 1.6b). Then the rate equations 1.3, 1.4 will transform to

N2 = -N2B21P - A12N2 + N1B12P + G

(1.7)

where G is the excitation rate.

Figure 1.6 — (a) Optical amplification schematic mechanism. (b) Scheme of the laser generation in the active medium placed in the external resonator

In the process of spontaneous emission in the active medium, generated photons are not emitted in the same direction, but a small part of the photons emitted along the resonator axis will retain in the resonator and undergo multiple reflection propagating back and forth along the axis. As a result, the amplification of the output emission as well as losses due to absorption, scattering in the amplifying

medium, and transmission through mirrors are observed. For steady-state laser operation, it is necessary to maintain a compensation of the losses by gain, i.e. constant output intensity, then the expression 1.6 is modified as:

Io = Ioexp[2(uth — a)LRiR2\ (1.8)

where ath = a — ) is the threshold gain coefficient, L is the active

medium length and Ri, R2 are the reflection coefficient of the resonator facet mirrors.

Next step is to move from consideration of systems consisting of individual atoms or ions to conventional bulk (3D) direct bandgap semiconductors, where the maximum of the valence band (VB) and the minimum of the conduction band (CB) are characterized by the same wave vector (k). The indirect bandgap semiconductors won't be discussed here because in them the wave vectors of the VB maximum and the CB minimum do not coincide and therefore the excitation with the consequent radiative recombination of the carriers require a change of crystal momentum through the involvement of the third particle (phonon) that leads to lower emission efficiency.

Incident energy dependent optical gain of the semiconductor material through the recombination of free carriers is expressed as [58]:

g(hv) = a(hv)(fc (hv) — fv (hv)) (1.9)

where a(hv) is the absorption coefficient, fcf) is the probability that the minimum (maximum) states of the CB (VB) bands would be occupied. In the case if fc(hv) > fv(hv) then the material exhibits optical gain, or in the opposite case material loss supremes the gain.

The probability of Ec(hv) and (hv) states occupation mentioned in Eq 1.7 are conformed the following Fermi distributions:

fAhv) = 1 + eXP((Ec(l) — Efc)/kT (U0)

lv (hy) = 1 + exp((Ev (hv) — Efv )/kT (U1)

where E/c and Efv are quasi Fermi levels of CV and VB. A quasi Fermi level unlike a regular Fermi level is used to describe the population of charge carriers (electrons) in the CB and VB separately out of equilibrium when excited by the external stimuli (voltage, light). When the incident light is absorbed by the semiconductor material it results in some changes in population of electrons (nc) in the

CB and holes (pw) in the VB with the shift of the quasi Fermi levels E/c and Efv responsively [58]. These shifts are expressed as (Figure 1.7a):

rp

Efc = Ec + kTF-2( ^) (1.12)

Efv = Ev — kTF—1 (^) (1.13)

where F1/2 (u) = r^iT 1+e(a;-u) is the inverse function of the Fermi integral of 1/2 order, k is the Boltzmann constant, T - the temperature, Ec and

Ew are the minimum (maximum) energy levels of the CB (VB) and Nc (Nv) is the

density of states in the CB (VB) (Eq. 1.14).

The density of states (DOS) is the number of states with a particular energy that electrons can occupy, or according to the definition is the number of electron states per unit volume per unit energy D(E) = ^^ = b(E — E(hi)),

where N is the number of countable energy levels, V is a volume of the system, k is the momentum. It is important to mention here that the spatial dimension of the system significantly affects the DOS expression since in semiconductors the free motion of carriers can be limited to two (quantum wall), one (quantum wire), and zero (quantum dot) spatial dimensions. For the bulk 3D semiconductor NC(NW) can be written as

NciNv) = Y^K^ ?/2 (1.14)

where m*(m)j) is the electron (hole) effective mass and h - reduced Planck constant.

Strictly speaking the condition fc(hv) > fv(hv) in Eq.(1.7) is the population inversion condition which is necessary to ensure the stimulation emission process. Therefore to reveal the mechanism of the stimulated emission one needs to equiv-alently present the band diagram in the form of a four-level system depicted on the Figure 1.7b. Here, G = I/hvpa(hvp), where I is an excitation fluence, is the excitation rate, levels n0 and n1 are the bottom and the top of the CB, and levels n2 and n3 are the bottom and of the VB, correspondingly, T1, t2 and Tr are the carrier lifetimes at the corresponding energy levels (t1, t2 « Tr).

The main processes that occur in a semiconductor material after its excitation can be roughly divided into (a) radiation and (b) non-radiation:

Figure 1.7 — (a) Band diagram of a 3D direct bandgap semiconductor. (b) Scheme of the free carriers recombination mechanisms in a four-level system

(a1) Radiative recombination (spontaneous emission) with the rate Rsp = Bnc = Bpv, where B is the bimolecular recombination coefficient, nc is the number of photogenerated electrons in the CB and pw is the number of holes in the VB.

(a2) Stimulated emission with the rate Rst.

(b1) Nonradiative recombination via defects or trap states. The rate of this process is Rtr = Anc = Apv, where A is the non-radiative recombination coefficient.

(b2) Auger recombination is a three particle process, when at the high incident fluences the excess energy and momentum of the electron and the hole are transferred to the additional particle. The rate of the process is Raug = Cn2cpv = Cncpl, where C is the Auger recombination coefficient.

Assuming that the lifetime of the carriers relaxed from level nc to n is much longer then the fast recombination to nc and n0 (t , Tc « Tr) and that the number of generated electrons in the CB is equal to the number of the holes left in VB, one can consider simplificated two-level system [59]. To investigate the steady-state as well as dynamic behavior of such system the rate equation is written:

n

— = G - Rtr (n) - Raug - Rst{n)S

(1.15)

or

n,

s

-f = G - Anc - Bnc - Cnc - - a(fc - fv )

(1.16)

where S is the density of generated photons reabsorbed f > fc) or emitted with the amplification provided by the optical gain (fc > fv).

It is also important to know the rate equation for the generated photons:

j = — - + rE fi(n)Rsp(n) + TE Rst(n)S (1.17)

t Tp

where tp is the photon lifetime and TE is the energy confinement factor.

It is necessary to consider that RS:P(n) includes the contribution of all cavity modes as well as the radiation into the free space continuum of modes and it is Ke-lated to the single-mode spontaneous emission rate RsP;k(n) though the spontaneous emission coupling factor which is literally the fraction of spontaneous emission coupled into a certain cavity mode in regard to all cavity modes [60].

Rsp,k = fi(n)Rsp(n) (1.18)

|3 factor appears to be one of the most important parameters for the effective laser generation.

The first laser used ruby as an active medium and generated coherent emission in the IR spectral region were demonstrated in 1960 [61]. Then in 1962 the solid-state laser diode was invented that marked the first steps towards laser miniaturization, the size of the diode shrinked by 1000 times from meter to millimeter scale [62]. Further significant reduction of the laser sizes allowed them to reach tens of micrometers in size when vertical-cavity surface-emitting laser (VCSEL) were created by H. Soda and coathours [63]. The tendency to the miniaturization continues to this day according to the needs of modern photonics, many micro- and nanoscale laser designs have been created, such as micro-pillars/disks [64; 65] and spheres [66] supporting whispering gallery modes (WGM), micro- and nanowires supporting Fabry-Perot modes [67; 68], 2D photonic crystals [69], etc. The great contribution to the investigation of stimulated emission in semiconductors including low-dimensional structures was done by Z. I. Alferov [70].