Масштабируемые методы нанесения пленок из композитов на основе нанокристаллов CsPbBr3 с высокоэффективной фотолюминесценцией тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Полушкин Артём Сергеевич

• Полушкин Артём Сергеевич
• кандидат наук
• 2024

Оглавление диссертации кандидат наук Полушкин Артём Сергеевич

Реферат

Synopsis

Введение

Глава 1. Литературный обзор

1.1 История исследования галогенидных перовскитов

1.2 Свойства галогенидных перовскитов

1.3 Методы стабилизации перовскитов

1.4 Методы нанесения тонких плёнок из раствора

1.4.1 Литье

1.4.2 Центрифугирование

1.4.3 Нанесение ножевым ракелем

1.4.4 Горизонтальная щелевая экструзия

1.4.5 Рулонная технология

1.5 Примеры светоизлучаюгцих устройств на основе перовскитов

1.5.1 Перовскитный светодиод

1.5.2 Перовскитная электрохимическая ячейка

1.5.3 Перовскитный светодиод с помощью экструзии

Глава 2. Методы синтеза и исследования

2.1 Методы изготовления тонких плёнок и устройств на их основе

2.1.1 Изготовление перовскитных растворов

2.1.2 Подготовка подложек

2.1.3 Нанесение плёнок методом центрифугирования

2.1.4 Нанесение плёнок методом горизонтальной щелевой экструзии

2.1.5 Синтез перовскитных нанокристаллов

2.1.6 Термическое напыление плёнок

2.1.7 Создание электрода из жидкого метала

2.2 Исследование полученных плёнок

2.2.1 Измерение толщин плёнок

2.2.2 Исследование морфологии плёнок

2.2.3 Исследование состава плёнок

2.3 Исследование оптических свойств

2.3.1 Измерение квантового выхода фотолюминесценции

2.3.2 Исследование затухания фотолюминесценции

2.3.3 Исследование двухфотонной люминесценции

2.4 Измерение характеристик светоизлучаюгцих электрохимических ячеек

Глава 3. Фотолюминесценция композитных плёнок из СбРЬВгз

3.1 Синтез композитных плёнок

3.2 Измерение люминесцентных свойств

3.3 Исследование рекомбинации носителей заряда

3.4 Заключение главы

Глава 4. Двухфотонная люминесценция нанокристаллов

СбРЬВГз

4.1 Синтез перовскитных композитных структур

4.1.1 Синтез перовскитных нанокристаллов

4.1.2 Синтез перовскитных нанолистов

4.1.3 Синтез поликристаллических плёнок

4.1.4 Синтез композитных плёнок

4.1.5 Синтез нанокристаллов СйРЬВгз в сферах СаС03

4.2 Двухфотонная люминесценция в плёнках на основе нанокристаллов СйРЬВгз

4.3 Двухфотонная люминесценция в нанокристаллах СйРЬВг3 синтезированных в сферах СаС03

4.4 Визуализация инфракрасного лазерного излучения

4.5 Заключение главы

Глава 5. Светоизлучающие электрохимические ячейки на

основе СбРЬВгз

5.1 Подбор параметров нанесения

5.2 Исследование морфологии плёнок

5.3 Светоизлучающая электрохимическая ячейка

5.4 Заключение главы...........................ИЗ

Заключение

Список сокращений

Список рисунков

Список таблиц

Список литературы

Благодарности

Приложение А. Основные публикации автора по теме

диссертации

Реферат

Введение диссертации (часть автореферата) на тему «Масштабируемые методы нанесения пленок из композитов на основе нанокристаллов CsPbBr3 с высокоэффективной фотолюминесценцией»

Общая характеристика диссертации Актуальность

Свинцово-галогенидные перовскиты привлекают значительное внимание исследователей всего мира как перспективный класс полупроводниковых материалов, обладающий уникальными характеристиками, что делает их востребованными в области оптоэлектроники, солнечной энергетики и фотоники [1—6]. Высокие коэффициенты поглощения, оптимальные значения ширины запрещенной зоны и доступность технологий производства позволяют создавать высокоэффективные солнечные элементы [7; 8] и фотодетекторы [9; 10]. Высокий квантовый выход фотолюминесценции (КВФЛ) и регулируемая ширина запрещенной зоны, зависящая от химического состава, открывают возможности для использования перовскитов в производстве светодиодов [11; 12]. Высокий показатель преломления, наличие экситона при комнатной температуре и высокий коэффициент оптического усиления делают перовскиты перспективными для создания фотонных структур и лазеров [13; 14]. Также следует отметить наличие выраженных нелинейных эффектов в перовскитах [15; 16], что позволяет использовать их для преобразования инфракрасного (ИК) света в видимый [17; 18]. Перовскиты могут быть использованы для создания транзисторов [19; 20], люминофоров [21—23], сцинтилляторов [24; 25] и газовых сенсоров [26; 27]. Не менее важным является устойчивость характеристик перовскитов к образованию дефектов [28; 29], что позволяет применять печатные технологии для производства устройств на основе перовскитов [30; 31].

Однако в настоящее время большинство описанных устройств разрабатываются в лабораторных условиях и не соответствуют требованиям индустриального применения ввиду ограниченности размеров и скорости производства, обусловленных используемыми методами. Для успешного внедрения перовскитных материалов в современные технологические процессы,

отвечающие требованиям индустрии, необходимо проведение дополнительных исследований, направленных на масштабирование устройств и разработку методов производства, адаптированных для промышленного использования [32; 33]. Наиболее распространённым методом создания тонких плёнок перовскита является центрифугирование [34; 35]. Впрочем, данный метод не является масштабируемым и не подходит для промышленного применения [36]. Альтернативные методы печати, такие как горизонтальная щелевая экструзия (ГЩЭ), нанесение ножевым ракелем, струйная печать и распыление [31; 37; 38], демонстрируют больший потенциал для коммерциализации. Тем не менее, несмотря на то, что данные технологии могут быть использованы для изготовления перовскитных солнечных элементов, их применение для создания светодиодов остаётся недостаточно исследованным [37]. Методы нанесения ножевым ракелем и горизонтальной щелевой экструзией являются наиболее перспективными, поскольку с их помощью были успешно изготовлены светодиоды различных цветов [39—43], однако параметры нанесения данными методами требуют дополнительной оптимизации.

Первые созданные перовскитовые светодиоды были изготовлены на основе перовскита смешанного анионного состава СН3ЫН3РЬХ3 (X = С1, Вг, I или их соединения) и излучали в ближнем инфракрасном, красном и зелёном диапазонах [44]. Тем не менее, полностью неорганические свинцово-галогенидные перовскиты (СвРЬХ3) демонстрируют значительно лучшую стабильность по сравнению с органо-неорганическими аналогами и показывают многообещающие результаты в качестве электролюминесцентных материалов [28; 45; 46]. Особенно выделяется перовскит СвРЬВг3 и нанокристаллы (НК) на его основе, которые демонстрируют наивысший квантовый выход фотолюминесценции [47; 48]. Следует отметить, что перовскитовые светодиоды представляют собой многослойные тонкоплёночные устройства, а изготовление таких многослойных структур усложняется с ростом числа наносимых слоёв. С этой точки зрения, однослойные светоизлучающие электрохимические ячейки (СЭЯ) представляют собой более простые объекты для исследования и разработки масштабируемых методов производства. Кроме того, они демонстрируют значительно более высокую яркость электролюминесценции [49; 50].

Таким образом, разработка масштабируемых методов создания плёнок па основе нанокристаллов СйРЬВг3 с высоким квантовым выходом

фотолюминесценции позволит создать светоизлучающие устройства, перспективные для коммерческого применения.

Цель исследования: разработка масштабируемых методов нанесения люминесцентных плёнок из композитов на основе нанокристаллов и микрокристаллов перовскита CsPbBr3 с высоким квантовым выходом для создания светоизлучаюгцих устройств на их основе.

Научные задачи поставленные для достижения этой цели:

— Синтез композитных пленок на основе перовскита CsPbBr3 и полиэтиленоксида (ПЭО) с использованием методов центрифугирования и горизонтальной щелевой экструзии.

— Изучение влияния различных параметров нанесения и составов используемых растворов на качество полученных пленок, а также оценка влияния процесса сушки горячим воздухом в ходе нанесения.

— Исследование фотолюминесценции (ФЛ) синтезированных пленок методами флюоресцентной и время-разрешенной спектроскопии.

3

— Разработка светоизлучающих электрохимических ячеек на основе синтезированных композитных пленок.

— Измерение вольт-амперных и вольт-яркостных характеристик созданных светоизлучающих электрохимических ячеек.

Методы исследования. Формирование тонких плёнок осуществлялось посредством методов центрифугирования и горизонтальной щелевой экструзии. Для создания металлического электрода применялся метод термического осаждения слоёв в вакууме. Анализ морфологии и толщины полученных плёнок проводился с использованием стилусного профилометра KLA Тепсог Р-7. Дополнительно, плёнки исследовались посредством флюоресцентного микроскопа Carl Zeiss Axio Imager A2m, интегрированного с ртутной лампой Osram HBO 100. Для детального изучения морфологии плёнок применялась сканирующая электронная микроскопия. Исследование оптических свойств плёнок осуществлялось методами флюоресцентной и времяразрешённой спектроскопии. КВФЛ измерялся волоконным спектрометром с интегрирующей сферой после накачки лазерным диодом с длиной волны 405 нм. Для получения

и

вольт-амперной характеристики СЭЯ использовался источник-измеритель Keithley 2400. Измерение яркости СЭЯ осуществлялось с помощью спектрорадиометра Instrument Systems CAS 120 с телескопическим оптическим зондом ТОР 150.

Основные положения, выносимые на защиту:

1. Композитные плёнки на основе перовскита CsPbBr3 и полиэтиленоксида с соотношением по массе 4:1, созданные методом горизонтальной щелевой экструзии в условиях воздушной атмосферы, имеют от 6 до 8 раз меньший коэффициент безызлучательной рекомбинации, рассчитанный по результатам время-разрешённой фотолюминесцентной спектроскопии с фемтосекундной лазерной накачкой с длиной волны 405 нм в диапазоне плотностей мощности от 0,1 до 2 мкДж см-2, чем плёнки, созданные с добавлением обработки горячим воздухом в процесс нанесения, за счёт образования меньшего количества безызлучательных дефектов.

2. Нанокристаллы CsPbBr3, синтезированные в порах сфер карбоната кальция со средним размером пор в 14,4 нм, имеют от 2 до 3 раз большую интенсивность фотолюминесценции, вызванную двухфотонным поглощением инфракрасного лазерного излучения длиной волны 800 нм, чем нанокристаллы, синтезированные в сферах со средним размером пор 3,5 нм, за счёт большего соотношения объема к поверхности у нанокристаллов в более крупных порах, приводящего к увеличению квантового выхода фотолюминесценции.

3. Добавление обработки горячим воздухом в процесс нанесения методом горизонтальной щелевой экструзии в условиях воздушной

атмосферы поликристаллической плёнки композита на основе

3

по массе 4:1 приводит к увеличению на три порядка яркости (с 3 -2

3

аналогичной ячейкой, полученной без добавления данной обработки, за счёт сокращения дрейфового пути носителей заряда благодаря уменьшению размеров кристаллитов композитной плёнки.

Научная новизна. Впервые была исследована двухфотонная люминесценция в нанокристаллах СйРЬВгз, синтезированных в сферах карбоната кальция микронного и субмикронного размера. А также продемонстрирована электролюминесценция в светоизлучаюгцих

электрохимических ячейках из композитных плёнок на основе перовскита з

условиях воздушной атмосферы.

Научная значимость работы заключается в комплексном исследовании

однофотонной люминесценции в плёнках из композита на основе перовскита з

Также исследована двухфотонная ФЛ в композитных плёнках на основе

з

з

плёнок на эффективность работы светоизлучаюгцих электрохимических ячеек. Измерены вольт-амперные и вольт-яркостные характеристики перовскитных СЭЯ, изготовленных методом горизонтальной щелевой экструзии и центрифугирования в условиях воздушной атмосферы с последующей обработкой горячим воздухом и без неё.

Практическая значимость полученных результатов заключается в

разработке масштабируемого метода горизонтальной щелевой экструзии

з

в воздушной среде. Эти плёнки могут быть использованы в дальнейшем для создания индустриальных установок по производству светоизлучающих

устройств на основе перовскита. Кроме того, была продемонстрирована

з

визуализации инфракрасного лазерного излучения и проведено практическое сравнение с коммерческими аналогами, которые сравнимы по характеристикам с полученными экспериментальными образцами.

Достоверность полученных результатов подтверждается согласованностью экспериментальных и теоретических исследований, а также отсутствием противоречий с данными, представленными в литературных источниках. Измерения проводились на сертифицированном оборудовании с

использованием современных методов, обеспечивающих высокую степень воспроизводимости результатов. Полученные результаты были апробированы на международных конференциях и опубликованы в международных рецензируемых журналах, индексируемых в базах данных Scopus и Web of Science.

Апробация работы. Результаты диссертации были представлены на следующих международных и всероссийских конференциях, школах и воркшопах:

1. Международная конференция METAN ANO 2021: VI International Conference on Metamaterials and Nanophotonics, 13-17.09.2021, Тбилиси, Грузия.

2. Всероссийская конференция XI конгресс молодых учёных, 04-08.04.2022, Санкт-Петербург, Россия

3. Международная школа SLALOM: School on Advanced Light-Emitting and Optical Materials 2022, 30.11-02.12.2022, Санкт-Петербург, Россия

4. Международная школа METANANO Summer School on Nanophotonics and Advanced materials, 16-18.08.2023, Циндао, Китай

5. Международный воркшоп Wokshop on Nanophotonics: fundamentals, materials, designs, and applications, 18-20.04.2024, Циндао, Китай

6. Международный воркшоп Wokshop on Nanotechnology for optics and optoelectronics, 26-28.04.2024, Циндао, Китай

Личный вклад автора. Все представленные в диссертационном исследовании результаты были получены соискателем лично или при его непосредственном участии. Соискатель самостоятельно осуществил все оптические измерения, включая исследования двухфотонной люминесценции, время-разрешающей спектроскопии, а также измерения квантового выхода фотолюминесценции. Были проведены измерения вольт-амперных и вольт-яркостных характеристик разработанных устройств, а также измерения толщин и морфологии плёнок с использованием профилометра и оптического микроскопа. Кроме того, соискатель принимал участие в постановке задач для синтеза нанокристаллов и осуществлял самостоятельный синтез тонких плёнок на их основе. В рамках исследования были выполнены численные расчёты, а также обработка и интерпретация полученных результатов. Автором были

подготовлены рукописи всех пяти публикаций, на которых основывается диссертация, что подтверждается его первым авторством в четырёх из них. Содержание диссертации и научные положения, выносимые на защиту, полностью отражают личный вклад соискателя в данное исследование.

Объём и структура работы. Диссертация состоит из введения, 5 глав, заключения и 1 приложения. Полный объём диссертации составляет 171 страницу, включая 54 рисунка и 4 таблицы. Список литературы содержит 111 наименований.

Основное содержание работы

В главе 1 представлен обзор литературных источников, посвященных теме диссертации. Рассмотрена история исследования свинцово-галогенидных перовскитов, их основные свойства, преимущества и недостатки. Описаны наиболее значимые результаты в области создания оптоэлектронных устройств на базе галоген иди ых перовскитов. Проанализированы последние достижения в производстве светоизлучающих тонкоплёночных устройств с использованием перовскитов. Также рассмотрены методы стабилизации перовскитов и улучшения их фотолюминесцентных характеристик, а также методы нанесения тонких плёнок, их преимущества, недостатки и способность к масштабированию.

Глава 2 посвящена описанию всех использованных методов синтеза и исследования, включая описание применяемого оборудования. Рассматриваются химические методы создания растворов для печати тонких плёнок, непосредственные методы печати плёнок и методы их характеризации. Представлен обзор методов исследования толщины и морфологии полученных образцов, а также методы измерения фотолюминесцентных свойств, КВФЛ и время-разрешённой спектроскопии. В данной главе также рассматриваются методы создания светоизлучающих устройств на основе перовскитов и методы измерения вольт-амперных и волы-яркостиых характеристик полученных устройств.

В главе 3 проводится исследование фотолюминесценции (ФЛ) композитных плёнок на основе перовскита СбРЬВгз и полимера полиэтиленоксида (ПЭО), полученных различными методами печати в условиях воздушной атмосферы. Качество ФЛ полученных плёнок оценивается посредством измерения КВФЛ, спектров ФЛ и время-разрешёшюй спектроскопии. Особое внимание уделяется изучению рекомбинации носителей заряда в этих плёнках с использованием АВС-модели. В качестве методов нанесения применяются центрифугирование с сушкой и без сушки горячим воздухом, а также горизонтальная щелевая экструзия (ГЩЭ) с сушкой и без сушки горячим воздухом. Схемы данных методов нанесения с сушкой горячим воздухом приведены на рисунке 1.

Вращение

Рисунок 1 — Схема нанесения композитной плёнки из перовскита СбРЬВгз а) методом ГЩЭ с параллельной сушкой горячим воздухом и б) методом центрифугирования с параллельной сушкой горячим воздухом [51].

Для оценки качества фотолюминесценции плёнок измеряются КВФЛ и затухание ФЛ во времени. Результаты этих измерений приведены на рисунке 2. Установлено, что плёнки, полученные без сушки воздухом, демонстрируют лучшие характеристики, включая более высокий квантовый выход и большее время жизни носителей заряда. Для более точного определения излучательных характеристик плёнок в главе разработана модель на основе АВС-модели [52 54], описывающая рекомбинацию носителей зарядов. Концентрация носителей в плёнке, при отсутствии Оже-рекомбинации, описывается по следующей формуле:

Ащ exp(-At)

n(t) =

(1)

А + Вщ(1 - ехр(-АЪ)) где п(р) - концентрация электронов, А - коэффициент безызлучательной рекомбинации, В - коэффициент излучательной рекомбинации, п0 -концентрация электронов в начальный момент времени.

Рисунок 2 — (а) Затухание ФЛ при накачке в 0,1 мкДж см-2 и (б) зависимость КВФЛ от мощности накачки измеренная экспериментально (соединённые линиями точки [51]) и рассчитанная теоретически (звездочки [55]) пленок СбРЬВгз, полученных центрифугированием и ГЩЭ с сушкой горячим воздухом

и без.

На основе результатов время-разрешённой спектроскопии ФЛ при различных мощностях накачки рассчитываются коэффициенты безызлучательной и излучательной рекомбинации плёнок из формулы (1). Значение В в данном приближении со ставило 9.82 • 10-10 см3 с-1. Теоретические расчёты качественно согласуются с экспериментальными данными. Наилучшие характеристики наблюдаются у образцов, изготовленных без сушки. Кроме

коэффициентов рекомбинации, из затухания ФЛ определяется КВФЛ. Теоретические значения КВФЛ не совпадают с экспериментально измеренными (рис. 26), что связано с заполнением безызлучательных дефектов. При измерении КВФЛ использовался непрерывный лазер, тогда как для измерения затухания ФЛ применялся фемтосекундный лазер, что вносит разницу в концентрацию дефектов, участвующих в рекомбинации. Тем не менее, представленная модель успешно предсказывает качество фотолюминесценции плёнок на основе результатов время-разрешённой спектроскопии.

В главе 4 исследуется ФЛ вызванная двухфотонным поглощением в композитных плёнках на основе нанокристаллов (НК) СвРЬВг3. Проводится сравнительный анализ эффективности преобразования инфракрасного (ПК) лазерного излучения в видимый свет между различными образцами и коммерческими аналогами — визуализаторами ПК лазерного излучения. Рассматриваются плёнки на основе перовскитных НК СвРЬВг3, синтезированные методом горячей инжекции в полимерной матрице полидиметилсилоксана (ПДМС) и без неё. Также изучаются плёнки из нанолистов (НЛ) СвРЬВг3, полученные методом лиганд-опосредованного осаждения, и поликристаллическая плёнка из СвРЬВг3.

Исследуется зависимость интенсивности двухфотонной ФЛ от плотности энергии лазера накачки. Коэффициент наклона этой зависимости в двойном логарифмическом масштабе позволяет определить, какой процесс вносит больший вклад в двухфотонную люминесценцию: межзонный переход или экситонная аннигиляция.

Наилучшие результаты преобразования ИК лазерного излучения в видимый свет наблюдаются в поликристаллической плёнке. На основе этих плёнок были созданы визуализаторы И К лазерного излучения, и их эффективность визуализации сравнена с коммерческими лазерными карточками [56]. На рисунке 3 представлена визуализация ИК лазерного излучения длиной волны 1000 нм данными визуализаторами без дополнительной фокусировки.

В рамках исследования также рассматривались перовскитные НК СвРЬВг3, синтезированные в порах сфер из карбоната кальция двух различных размеров: субмикронного и микронного. Изменение размера сфер приводило к варьированию размеров пор внутри них, что, в свою очередь, влияло на размеры синтезированных перовскитных кристаллов. В данной главе

Рисунок 3 Визуализация несфокусированного ИК лазера с длиной волны 1000 нм с помощью (а) поликристаллической плёнки CsPbBr3, (б) плёнки, состоящей из НЛ CsPbBr3, (в) плёнки, состоящей из НК CsPbBr3 на стекле, (г) пленки, состоящей из НК CsPbBr3 в матрице ПДМС на стекле, и (д) на рассеивающей подложке, (е) НК CsPbBr3 в сферах СаС03 [57; 58].

рассматривается процесс преобразования света с повышением частоты в указанных НК в зависимости от условий их синтеза, размера кристаллов и добавления иттербия. Особое внимание уделяется исследованию зависимости интенсивности двухфотонной люминесценции от длины волны накачки и плотности энергии накачки (рис. 4). Кроме того, на основе ватеритных сфер с перовскитными НК был разработан визуализатор 14К лазерного излучения, не уступающий коммерческим аналогам (рис. Зе).

В рамках главы 5 проводится анализ оптимальных параметров нанесения композитных плёнок CsPbBr3 методом горизонтальной щелевой экструзии (ГЩЭ) в условиях воздушной атмосферы. Изучается влияние параметров нанесения, составов используемых растворов и использование обработки горячим воздухом в процессе нанесения на шероховатость, морфологию и размеры кристаллитов получаемых плёнок. После определения оптимальных условий, композитные плёнки, созданные методом ГЩЭ, сравниваются с аналогичными плёнками, полученными методом центрифугирования с параллельной сушкой воздухом или без неё. Для оценки состава и

Рисунок 4 Измерения преобразования ИК излучения в видимое для субмикронных и микронных сфер СаС03 с перовскитными НК. а) и б) Зависимость интенсивности двухфотонной ФЛ от длины волны возбуждения, в) и г) Интенсивность двухфотонной ФЛ для разных образцов при одинаковой интенсивности возбуждения при лазерном облучении с длиной волны 800 нм. д) и е) Зависимость интенсивности двухфотонной ФЛ от плотности энергии

возбуждения при длине волны 800 нм [58].

кристалличности плёнок применяется рентгеноструктурный анализ (ХЕШ), а морфология исследуется с использованием флюоресцентной оптической и сканирующей электронной микроскопии (СЭМ). На рисунке 5 представлено СЭМ-изображение поверхностей всех четырёх образцов, исследованных в главе.

Исследование морфологии показало, что, несмотря на то что добавление обработки горячим воздухом в процесс нанесения поликристаллических плёнок ухудшает их фотолюминесцентные свойства, она значительно улучшает шероховатость, уменьшает размеры кристаллитов и количество отверстий в плёнке.

Все четыре исследованных метода нанесения из плёнок были использованы для создания светоизлучающие электрохимические ячейки (СЭЯ). Для верхнего электрода использовались два различных материала: жидкий 1пСа, который создавался вручную и нивелировал влияние шероховатости плёнок на характеристики СЭЯ, и твёрдый алюминиевый электрод, который создавался методом термического осаждения. У полученных

Рисунок 5 СЭМ-изображения поверхности плёнок перовскита, полученных различными методами синтеза: а) методом центрифугирования с последующим отжигом на горячей плитке, б) центрифугированием с одновременной сушкой горячим воздухом, в) ГЩЭ с последующим отжигом на горячей плитке, г) ГЩЭ с сушкой горячим воздухом с помощью воздушного ножа в процессе нанесения.

Все шкалы равны 2 мкм [51].

СЭЯ были измерены вольт-амперные и вольт-яркостные характеристики, результаты которых представлены на рисунке 6. СЭЯ, созданные с помощью ГЩЭ, показывают куда более качественные результаты в сравнении с устройствами, полученными центрифугированием. Кроме того, сушка воздухом в процессе нанесения позволяет значительно улучшить морфологию плёнок, что приводит к увеличению яркости устройств при использовании алюминиевого электрода. Максимальная яркость СЭЯ составила 8300 кд м-2 с использованием 1пСа электрода и 2900 кд м-2 с более коммерчески применимым алюминиевым электродом.

Рисунок 6 а) Вольт-амперные и б) вольт-яркостные характеристики устройств, полученные всеми четырьмя способами нанесения с использованием верхнего электрода из 1пСа. На вставках: а) схема устройства, б) фотография работы устройства, в) Вольт-амперные и i1) вольт-яркостные характеристики устройств, полученные ГЩЭ с сушкой на воздухе и без с верхним А1 электродом. Вставки: в) схема устройства, г) фотография работы

устройства [51].

Публикации автора по теме диссертации

Основные результаты по теме диссертации изложены в 5 публикациях. Из них 4 опубликовано в изданиях, индексируемых в базе цитирования Scopus. Список всех публикаций автора по теме диссертации:

1. Polushkin A., Zelenkov L., Khmelevskaia D., Markina D., Rogach A., Makarov S. Semitransparent visualizers of infrared lasers based on per-ovskite quantum dots // Journal of Physics: Conference Series. T. 2015.

IOP Publishing. 2021. C. 012112.

2. Polushkin A., Cherevkov S., Gets D., Zelenkov L., Makarov S. UpCon-version Films and Polymer Matrices with СнРЬВгЗ Perovskite Micro and

Nanostructures // Bulletin of the Russian Academy of Sciences: Physics. _ 2022. - T. 86, № 1. - SI79 S182.

3. Peltek 0. 0., Talianov P. M.. Krylova A., Polushkin A. S., Anastasova E. I., Mikushina D. D., Gets D., Zelenkov L. E., Khubezhov S., Pushkarev A., Zyuzin M.. Makarov S. Ligand-free template-assisted synthesis of stable perovskite nanocrystals with near-unity photoluminescence quantum yield within the pores of vaterite spheres // Nanoscale. — 2023. — T. 15, № 16. _ c. 7482 7492.

4. Polushkin A., Danilovskiy E., Sapozhnikova E., Kuzmenko N., Pushkarev A., Makarov S. Morphological and structural defect optimization in CsPb-Br3 nanoparticle films for light-emitting electrochemical cells // Photonics and Nanostructures-Fundamentals and Applications. — 2024. — T. 58. — C. 101232.

5. Полушкин А. С., Махров С. В. Рекомбинация носителей заряда в пленках CsPbBr3 с высокой квантовой эффективностью фотолюминесценции // Журнал технической физики. — 2024. — Т. 94, № 10. - С. 1633.

Synopsis

General thesis summary Relevance of the chosen topic

Lead halide perovskites attract significant attention from researchers around the world as a prospective class of semiconductor materials with unique characteristics, which makes them suituble in the field of optoelectronics, solar energy and photonics [1—6]. Significant absorption coefficient, optimal band gap values and the availability of production technologies make it possible to create highly efficient solar cells [7; 8] and photo detectors [9; 10]. The high photoluminescence quantum yield (PLQY) and the tunable band gap, depending on the chemical composition, open up opportunities using perovskites in the production of light-emitting diodes (LEDs) [11; 12]. The large refractive index, denominated exciton at room temperature and the high optical gain make perovskites promising for the creation of photonic structures and lasers [13; 14]. It should also be noted that there are high degree of optical nonlinearity in perovskites [15; 16], which allows to use them for up-conversion of infrared (IR) light [17; 18]. Perovskites can be used to create transistors [19; 20], luminophors [21—23], scintillators [24; 25] and gas sensors [26; 27]. Equally important is defect tolerance of perovskites [28; 29], which allows the use of printing technologies for the production of devices based on perovskites [30; 31].

However, currently most of the described devices are being developed in the laboratory and do not meet the requirements of industrial applications due to the limited size and speed of production because of the methods used. For the successful implementation of perovskite materials into modern technological processes that meet the requirements of the industry, additional research is needed aimed at scal-ing-up devices and developing production methods adapted for industrial use [32; 33]. The most common method of fabrication thin films of perovskite is spin-coating [34; 35]. However, this method is not scalable and is not suitable for industrial applications [36]. Alternative printing methods such as slot-die coating, blade-coating, inkjet printing and spray-coating [31; 37; 38], demonstrate greater potential for

commercialization. Nevertheless, despite the fact that these technologies can be used to manufacture perovskite solar cells, their application for fabrication perovskite LEDs remains insufficiently investigated [37]. Blade-coating and slot-die coating are the most promising methods, since LEDs of various colors have been successfully manufactured with their help [39—43], however, the deposition parameters of these methods require additional optimization.

The first perovskite LEDs were created from mixed anionic composition perovskite CHsNHsPbXs (X = CI, Br, I or their compounds) and were illuminate in the near infrared, red and green ranges [44]. Nevertheless, completely inorganic lead halide perovskites (CsPbX3) demonstrate significantly better stability compared to organo-inorganic analogues and show promising results as electroluminescent materials [28; 45; 46]. Especially noteworthy CsPbBr3 perovskite and nanocrystals (NC) based on it demonstrate the highest quantum yield of photoluminescence [47; 48]. It should be noted that perovskite LEDs are multilayer thin-film devices, and the manufacture of such multilayer structures becomes more complicated with the increase in the number of layers applied. From this point of view, single-layer light-emitting electrochemical cells (LECs) represent simpler objects for research and development of scalable production methods. In addition, they demonstrate significantly higher electroluminescence brightness [49; 50].

Список литературы диссертационного исследования кандидат наук Полушкин Артём Сергеевич, 2024 год

Список литературы

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Благодарности

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Semitransparent visualizers of infrared lasers based on perovskite quantum dots

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6th International Conference on Metamaterials and Nanophotonics METANANO 2021 IOP Publishing

Journal of Physics: Conference Series 2015 (2021) 012112 doi:10.1088/1742-6596/2015/1/012112

Semitransparent visualizers of infrared lasers based on perovskite quantum dots

A S Polushkin1, L E Zelenkov1, D Khmelevskaia1, DI Markina1, A L Rogach2 and S V Makarov1

department of Physics and Engineering, ITMO University, St. Petersburg 197101, Russia 2Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong SAR, P. R. China

E-mail: artem.polushkin@metalab. if mo. ru s,makarov@metalab. ifmo.ru

Abstract. These days halide perovskite is a very popular material to be applied both in photovoltaics and photonics due to its unique properties and low-cost methods of fabrication. High photoluminescence quantum yield and good nonlinear coefficients of perovskite material allow achieving multiphoton absorption in perovskite. Encapsulation of perovskite quantum dots in polymer matrix enables to prevent interaction with the environment and increase the material lifetime. In this paper, we present a semitransparent visualizer of infrared lasers working on two-photon absorption in halide perovskite nanocrystals encapsulated in the polymer matrix.

1. Introduction

Today high-power infrared lasers are used in many parts of human life, e. g., in medicine, in science, and for cutting and engraving metals. Infrared light is invisible for human eyes; as a result, different laser visualizers are used for visualizing the laser beam path. However, currently available visualizers possess some problems. Several of them work only in a narrow spectral range and are not applicable for any lasers. Other visualizers are "necessary to charge the active region with visible light" which means that they lose brightness of luminescence under the high power laser which makes them not very convenient to be used. Moreover, powerful laser emission can burn and destroy a visualizer. We have decided to develop an infrared laser visualizer that works in a wide spectral range, needs no "charging" and semitransparent which allows it to be used without laser beam path interruption.

After halide perovskite potential for solar cells was demonstrated, they attracted a lot of attention [1]. In addition, perovskite also shows some opportunities for the advanced nanophotonic device design due to its unique optical properties [2, 3]. Halide perovskites possess exciton at room temperature, a high refractive index, great optical gain, and large nonlinear coefficients. Perovskite quantum dots (QDs) synthesized by simple chemical methods can have a quantum yield up to 90% [4]. Furthermore, most laser visualizers are opaque, but active material can be used in a transparent polymer membrane for light up-conversion [5]. However, perovskite material can be damaged by water and oxygen, yet an encapsulation of perovskite QDs in a polymer matrix can protect them from harmful environment [6].

In this work, we synthesize CsPbBr3 nanocrystals (NCs) encapsulated in polydimethylsiloxane (PDMS) matrix on a glass substrate and use it for infrared light up-conversion.

© I Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution K^HH of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1

6th International Conference on Metamaterials and Nanophotonics METANANO 2021 IOP Publishing

Journal of Physics: Conference Series 2015 (2021) 012112 doi:10.1088/1742-6596/2015/1/012112

Wavelength (nm) Wavelength (nm)

Figure 1. a) Photoluminescence and b) absorption spectrum of perovskite nanocrystals encapsulated in PDMS matrix

2. Results and discussion

CsPbBr3 NCs were synthesized according to standard protocol [7, 8]. Perovskite NCs were separated by centrifugation and redispersed in toluene forming colloidally stable solutions. Dow Corning Sylgard 184 was used as an encapsulation matrix. A film curing of the resulting composite on a glass substrate was made by heating.

We have investigated obtained samples on two-photon absorption and photoluminescence (PL). Continuous PL from QDs in the polymer matrix was excited by a 365 nm UV mercury lamp and collected by Axio Imager A2m (Carl Zeiss) microscope with 50x objectives (Carl Zeiss EC Epiplan-NEOFLUAR). An optical fiber spectrometer (Ocean Optics QE Pro) connected with the microscope in a fluorescent regime was used for recording PL spectra. Fig. 1 represents obtained PL spectra and absorption spectra of perovskite NCs in the polymer matrix in the near-IR range. obtained by Shimadzu UV-3600 Plus UV-VIS-NIR Spectrophotometer. Peaks in fig. 1b correspond to the absorption of PDMS. A central wavelength of PL is 512 nm with full width at half maximum (FWHM) 18 nm. Photoluminescence quantum yield of perovskite QDs in PDMS was measured using Labsphere integrated sphere and appeared to be around 12%.

Figure 2. a) Up-conversion luminescence of perovskite quantum dots in PDMS b) dependence of luminescence integral intensity on pump fluence

6th International Conference on Metamaterials and Nanophotonics METANANO 2021 IOP Publishing

Journal of Physics: Conference Series 2015 (2021) 012112 doi:10.1088/1742-6596/2015/1/012112

Figure 3. Visualization of IR laser beam a, b) 800 nm, c) 900 m, d) 1000 nm

Further, we investigated IR to visible light up-conversion. Pharos PH2-SP-20W-2mJ single-unit integrated femtosecond laser system and high power optical parametric amplifier Orpheus-FFor were used for excitation. A Laser beam was focused on the sample by 10x objective (Mitutoyo M Plan APO NIR, NA = 0.26) through the glass substrate and light was collected by 50x objective (Mitutoyo M Plan APO, NA = 0.55). Spectra were obtained by using the imaging spectrograph Andor Kymera 328i with CCD camera Andor iDus. Fig. 2a presents the luminescence spectrum of perovskite QDs in PDMS under the 1000 nm laser with pules duration 150 fs and repetition rate 100 kHz. The peak at 500 nm corresponds to excitation laser go to spectrometer due to no enough filtering. Fig. 2b shows the dependence of PL integral intensity on pump fluency in a log-log scale. A slope in fig. 2b equals two, which corresponds to the quadratic dependence, proving two-photon luminescence.

Moreover, we tested visualization of the infrared laser beam by our samples without any additional focusing. Fig. 3 demonstrates the luminescence under 800, 900, and 1000 nm laser with approximate beam diameter 2 mm. The visualizer is semitransparent and passes more than 50% of light. The sample also luminesces stably with a constant brightness under IR laser excitation and is not damaged by a 4 W laser beam.

In the future, we are planning to reveal the dependence of polymer on up-conversion by comparing samples with different polymers, and to study the dependence of polymer with QDs layer thickness on visualization quality. Creating a non-transparent sample can increase the efficiency of up-conversion by laser beam scattering but lose the semitransparency advantage. Another essential improvement of the visualizers can be the changing CsPbBr3 NCs on CsPbI3 NCs, which have a wider bandgap, that can increase spectral range and efficiency of up-conversion. However, CsPbI3 NCs possess a lower quantum yield and less stability.

3. Conclusion

It has been demonstrated that CsPbBr3 nanocrystals encapsulated in PDMS matrix have photoluminescence on 512 nm with FWHM 18 nm, and support IR to visible light up-conversion. We have also revealed that the dependence of integral intensity on excitation fluence is quadratic. Moreover, perovskite QDs in the polymer matrix can be used as semitransparent infrared laser beam visualizers.

6th International Conference on Metamaterials and Nanophotonics METANANO 2021 IOP Publishing

Journal of Physics: Conference Series 2015 (2021) 012112 doi:10.1088/1742-6596/2015/1/012112

4. Acknowledgments

This work is supported by the Russian Science Foundation (project #17-73-20336). References

[1] Green M A, Ho-Baillie A and Snaith H J 2014 Nature photonics 8 506-514

[2] Makarov S, Furasova A, Tiguntseva E, Hemmetter A, Berestennikov A, Pushkarev A, Zakhidov A and Kivshar Y 2019

Advanced optical materials 7 1800784

[3] Berestennikov A S, Voroshilov P M, Makarov S V and Kivshar Y S 2019 Applied Physics Reviews 6 031307

[4] Huang H, Bodnarchuk M I, Kershaw S V, Kovalenko M V and Rogach A L 2017 ACS energy letters 2 2071-2083

[5] Fedorov V V, Bolshakov A, Sergaeva O, Neplokh V, Markina D, Bruyere S, Saerens G, Petrov M I, Grange R, Timofeeva

M et al. 2020 ACS nano 14 10624-10632

[6] Raja S N, Bekenstein Y, Koc M A, Fischer S, Zhang D, Lin L, Ritchie R O, Yang P and Alivisatos A P 2016 ACS applied

materials & interfaces 8 35523-35533

[7] Protesescu L, Yakunin S, Bodnarchuk M I, Krieg F, Caputo R, Hendon C H, Yang R X, Walsh A and Kovalenko M V 2015

Nano letters 15 3692-3696

[8] Huang H, Chen B, Wang Z, Hung T F, Susha A S, Zhong H and Rogach A L 2016 Chemical science 7 5699-5703

ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2022, Vol. 86, Suppl. 1, pp. S179-S182. ©Allerton Press, Inc., 2022.

Up-Conversion Films and Polymer Matrices with CsPbBr3 Perovskite Micro and Nanostructures

A. S. Polushkin"- *, S. A. Cherevkov", D. S. Gets", L. E. Zelenkov", and S. V. Makarov"

a ITMO University, St. Petersburg, 197101 Russia *e-mail: [e-mail] Received September 20, 2022; revised October 14, 2022; accepted October 20, 2022

Abstract—Up-conversion is a process that can be used for infrared laser visualization. Lead halide perovskite has a fine opportunity to be employed for up-conversion. Halide perovskite has not only good photovoltaic and optoelectronic properties but also good photonic properties. Simple chemical fabrication methods and unique properties make perovskites cheap and attractive materials for different applications. In this article, we show different perovskite structures that possess up-conversion of near-infrared laser emission to visible green light.

Keywords: halide perovskites, up-conversion, nanocrystals, nonlinear optics, IR visualization DOI: 10.3103/S1062873822700642

INTRODUCTION

Lead halide perovskites attract a lot of attention in the science world after they show good opportunities for solar cell and light-emitting diode fabrication [1,2]. Nevertheless, perovskite shows good photonic properties, such as a high refractive index, big optical gain, excitonic states at room temperature, and great nonlinear coefficients [3]. Also, simple chemical methods of fabrication and bandgap engineering make perovskite a very good semiconductor material for different applications, like lasers, scintillators, and sensors [4—7].

Visualization of infrared laser emission, invisible to the human eye, can be very useful for any laser beam manipulation in optical laboratories all over the world and other places using infrared lasers. Up-conversion of near-infrared light to visible can be achieved in perovskites through two-photon absorption.

All-inorganic CsPbBr3 perovskite has bright green luminescence and is much more stable compared to hybrid perovskite like CH3NH3PbBr3 [8]. It was shown that CsPbBr3 nanocrystals (NCs) can have a photoluminescence quantum yield (PLQY) more than 90% [9]. However, not only perovskite NCs can have bright luminescence [6].

In this work, we demonstrate up-conversion in perovskite films made of different CsPbBr3 structures with various methods of fabrication. The thick poly-crystalline film, film made of perovskite NCs and film of perovskite nanosheets (NSs) are compared. We show the potential for using obtained films as near-infrared laser beam visualizers.

SYNTHESIS

All sample was made on the glass substrate. Before film deposition substrates were consistently washed in an ultrasonic bath with soapy water, deionized water, acetone, and isopropanol. Then isopropanol remains were removed from the glass surface by air gun.

The solution for the polycrystalline film was made in a glovebox with an N2 atmosphere. 0.4 M (mol/L) CsPbBr3 solution in DMSO was created by mixing CsBr and PbBr2 salts. On a clean glass substrate after oxygen plasma treatment, CsPbBr3 films were spin-coated in 120 s at 1500 rpm inside the glovebox. Annealing of films was made in two steps, at first films were vacuum dried for 1 min, then annealed on the hot plate at 50°C for 5 min. Further, two films were connected surface to surface and were pressed on the Harry Gestigkeit PZ 28-3TD high-temperature titanium hotplate by 250 g weight. Films were heated up to 500°C in 15 min, then taken out from the hot plate. Thus, a thick film was formed on the top glass. The thickness of the obtained film was 500—1000 nm.

CsPbBr3 nanosheets were fabricated by ligand-assisted reprecipitation method [6]. Typically, the Cs precursor and PbBr2 precursor were prepared separately, and the reaction was initiated by injecting the latter into the former in a molar ratio of 4 : 1. First, the Cs precursor solution was prepared by dissolving 32 mg of cesium acetate in 1 mL of 1-propanol in a 20 mL vial under stirring in the air at room temperature, followed by the addition of 6 mL of hexane and 2 mL of 1-propanol. Second, PbBr2 precursor solution was prepared by dissolving 245 mg of PbBr2 into a

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(a)

Polycrystalline film 1000 nm excitation

0

500

6.7 mJ cm-2 11.1 mJ cm-2 15.6 mJ cm-2 20.1 mJ cm-2 24.5 mJ cm-2 28.0 mJ cm-2 33.4 mJ cm-2

20 000 18 000 16 000 14 000 12000 10 000 8000 6000 4000 2000 0

(b)

Nanocrytals film 1000 nm excitation

510 520 530 540 550 Wavelength, nm

560 570 580

6.7 mJ cm-2 11.1 mJ cm-2 15.6 mJ cm-2 20.1 mJ cm-2 24.5 mJ cm-2 28.0 mJ cm-2 33.4 mJ cm-2

470 480 490 500 510 520 530 540 550 560 570 Wavelength, nm

Fig. 1. Spectra of two-photon photoluminescence of (a) CsPbBr3 polycrystalline film, and (b) film consisting of CsPbBr3 nano-crystals under 1000 nm laser excitation at different fluence.

mixture solution of 0.45 mL of 1-propanol, 0.45 mL of octanoic acid, and 0.45 mL of octylamine at 90°C in the air under vigorous stirring. Third, the hot PbBr2 precursor was injected into Cs precursor swiftly under vigorous stirring at room temperature. The system turned green immediately, and the reaction was completed in 2 min. The CsPbBr3 NSs were isolated by centrifugation at 12 000 rpm. Then NSs were washed from organic remains by toluene with acetone and centrifugation again at 12000 rpm. Further, perovskite nanosheets were dispersed into 2 mL of toluene and spin-coated (1500 rpm for 2 min) on a clean, not plasma-treated glass substrate. The thickness of the obtained film was approximately 500 nm.

CsPbBr3 NCs were synthesized following Prote-sescu et al. [9]. Cs2CO3 (0.407 g) was loaded into a 100 mL 2-neck flask along with octadecene (20 mL) and oleic acid (1.25 mL), dried for 1 h at 120°C, and then heated under N2 to 150°C. 1-octadecene (15 mL) and PbBr2 (0.207 g) were loaded into a 25 mL 3-neck flask and dried under vacuum for 1 h at 120°C. Oleyl-amine (1.5 mL) and oleic acid (1.5 mL) were injected at 120°C under N2. After complete dissolution of a PbBr2 salt, the temperature was raised to 180°C and Cs oleate solution (1.5 mL, prepared as described above) was quickly injected and, 5 s later, the reaction mixture was cooled by the ice-water bath. The crude solution was cooled down with a water bath, and perovskite NCs were separated by centrifugation (12000 rpm for 10 min) and redispersed in toluene forming colloidally stable solutions. Separation and redispersion were repeated twice. Obtained CsPbBr3 NCs were spin-coated at 1500 rpm for 2 min on a clean oxygen plasma-treated glass substrate. The thickness of the obtained film was around 500 nm.

Besides, the CsPbBr3 nanocrystals—polydimethyl-siloxane (PDMS) composite film was fabricated. To prepare composite films toluene solution of perovskite NCs with 3 mg/mL concentration was used. Dow Corning Sylgard 184 was used as an encapsulation matrix. A solution of nanocrystals was added to a mixture of components A and B of Sylgard 184 in a ratio of 5 : 1, respectively. The resulting mixture was homogenized manually. Film curing of the resulting composite on the clean glass substrate was carried out at 80°C for 60 min.

OPTICAL MEASURMENTS

Pharos PH2-SP-10W-2mJ single-unit integrated femtosecond 1030 nm laser system with high-power optical parametric amplifier Orpheus-F for wavelength shifting were used to investigate two-photon luminescence in obtained perovskite films. Spectra of two-photon photoluminescence (PL) were obtained by Andor Kymera 328i spectrometer with CCD camera Andor iDus. An infrared laser was focused on samples from behind by *10 objective (Mitutoyo M Plan APO NIR, NA = 0.26), and the signal was collected by x50 objective (Mitutoyo M Plan APO, NA = 0.55). Figure 1 shows spectra of two-photon luminescence of polycrystalline film (a) and film consisting of NCs (b). The central wavelength of PL of the polycrystalline film was 537 nm with full width at half maximum (FWHM) 17 nm, for the film made of NCs was 515 nm with FWHM 20 nm. Excitation has a 1000 nm wavelength with a 100 kHz repetition rate of 150 femtoseconds pulses. The dependence of the maximum intensity of PL on excitation fluence was estimated from obtained spectra. Figure 2 shows this dependence in

UP-CONVERSION FILMS AND POLYMER MATRICES S181

Fig. 2. Dependence of maximum intensity on excitation fluence in log-log scale with a linear fit for (a) CsPbB^ polycrystalline film, and (b) film consisting of CsPbBr3 nanocrystals.

Fig. 3. Up-conversion of non focused 1000 nm laser by (a) CsPbBr3 polycrystalline film, (b) film consisting of CsPbBr3 nanosheets, (c) film consisting of CsPbBr3 nanocrystals on glass, (d) film consisting of CsPbBr3 nanocrystals in PDMS matrix on glass, and (e) on scattering substrate.

the log-log scale, the slope for the polycrystalline film is 2.37, and for NCs film is 1.85.

Besides, the up-conversion of films was investigated without focusing a laser beam at a different wavelength from 800 to 1000 nm. The diameter of the

laser beam was 2 mm. Figure 3 shows up-conversion in different films without excitation focusing at 1000 nm. The efficiency of up-conversion by different films was compared by estimating the minimal power of 800 nm laser excitation then up-conversion can be noticed by

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Table 1. Comparison of 800 nm laser beam visualization by different CsPbBr3 films. Minimal power of laser excitation at which the eye can notice up-conversion in the film

Film Polycrystalline film NSs film NCs film NCs with PDMS film on glass NCs with PDMS on DVD

Minimal power, mW 0.5 12 5 5 2

the eye without any focusing. Table 1 shows the results of the comparison. Also, the influence of substrate was studied. CsPbBr3 NCs in the PDMS matrix were deposited on a scattering substrate (DVD with metallization). Turn out that up-conversion can be noticed by the eye better on scattering substrate due to a bigger area of laser-matter interaction.

CONCLUSIONS

Halide perovskite shows itself as good material for up-conversion of near-infrared laser in the range 800— 1000 nm. Different CsPbBr3 perovskite films were compared, and the polycrystalline film turns out to have the best performance. The central wavelength of two-photon photoluminescence was 536 nm with FWHM 17 nm. The slope of intensity on excitation fluence dependence in the log-log scale was 2.36.

FUNDING

This work was supported by the Russian Science Foundation (project no. 21-73-20189).

CONFLICT OF INTEREST The authors declare that they have no conflicts of interest.

REFERENCES

1. Hodes, G., Science, 2013, vol. 342, p. 317. https://doi.org/10.1126/science.1245473

2. Stranks, S.D. and Snaith, H.J., Nat. Nanotehnol., 2015, vol. 10, p. 391.

https://doi.org/10.1038/nnano.2015.90

3. Makarov, S., Furasova, A., Tiguntseva, E., Hemmet-ter, A., Berestennikov, A., Pushkarev, A., Zakhidov, A., and Kivshar, Y., Adv. Opt. Mater, 2019, vol. 7, 1800784. https://doi.org/10.1002/adom.201800784

4. Tiguntseva, E., Koshelev, K., Furasova, A., Tonkaev, P., Mikhailovskii, V., Ushakova, E.V., Baranov, D.G., Shegai, T., Zakhidov, A.A., Kivshar, Y., and Makarov, S.V., ACSNano, 2020, vol. 14, p. 8149. https://doi.org/10.1021/acsnano.0c01468

5. Pushkarev, A.P., Korolev, V.I., Markina, D.I., Komis-sarenko, F.E., Naujokaitis, A., Drabavicius, A., Paks-tas, V., Franckevicius, M., Khubezhov, S.A., San-nikov, D.A., Zasedatelev, A.V., Lagoudakis, P.G., Zakhidov, A.A., and Makarov, S.V., ACSAppl. Mater. Interfaces, 2019, vol. 11, p. 1040. https://doi.org/10.1021/acsami.8b17396

6. Zhang, Y., Sun, R., Ou, X., Fu, K., Chen, Q., Ding, Y., L.-Xu, J., Liu, L., Han, Y., Malko, A.V., Liu, X., Yang, H., Bakr, O.M., Liu, H., and Mohammed, O.F., ACS Nano, 2019, vol. 13, p. 2520. https://doi.org/10.1021/acsnano.8b09484

7. Xu, W., Li, F., Cai, Z., Wang, Y., Luo, F., and Chen, X., J. Mater. Chem. C, 2016, vol. 4, p. 9651. https://doi.org/10.1039/C6TC01075J

8. Ullah, S., Wang, J., Yang, P., Liu, L., Yang, S.E., Xia, T., Guo, H., and Chen, Y., Mater. Adv., 2020, vol. 2, p. 646.

https://doi.org/10.1039/D0MA00866D

9. Protesescu, L., Yakunin, S., Bodnarchuk, M.I., Krieg, F., Caputo, R., Hendon, C.H., Yang, R.X., Walsh, A., and Kovalenko, M.V., Nano Lett, 2015, vol. 15, p. 3692. https://doi.org/10.1021/nl5048779

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Ligand-free template-assisted synthesis of stable perovskite nanocrystals with near-unity photoluminescence quantum yield within the pores of vaterite spheresf

Oleksii O. Peltek.J3 Pavel M. Talianov.J3 Anna Krylova,3 Artem S. Polushkin, a Elizaveta I. Anastasova,b Daria D. Mikushina,3 Dmitri Gets, a Lev E. Zelenkov, a Soslan Khubezhov,3 Anatoly Pushkarev,3 Mikhail V. Zyuzin *a and Sergey V. Makarov *a c

Received 13th January 2023, Accepted 1st March 2023

DOI: 10.1039/d3nr00214d rsc.li/nanoscale

Ligand-free methods for the synthesis of halide perovskite nanocrystals are of great interest because of their excellent performance in optoelectronics and photonics. In addition, template-assisted synthesis methods have become a powerful tool for the fabrication of environmentally stable and bright nanocrys-tals. Here we develop a novel approach for the facile ligand-free template-assisted fabrication of perovskite nanocrystals with a near-unity absolute quantum yield, which involves CaCO3 vaterite micro- and submicrospheres as templates. We show that the optical properties of the obtained nanocrystals are affected not mainly by the template morphology, but strongly depend on the concentration of precursor solutions, anion and cation ratio, as well as on adding defect-passivating rare-earth dopants. The optimized samples are further tested as infrared radiation visualizers exhibiting promising characteristics comparable to those that are commercially available.

Introduction

Halide perovskites are a class of semiconductors, which have recently attracted a lot of attention due to their excellent photophysical and optoelectronic properties.1'2 These materials are successfully used in optoelectronics,3 nanopho-tonics4 and photovoltaics,5 in particular, for the development of light-emitting diodes,6 lasers,7 scintillators,8 solar cells,9 and photodetectors.10 The most commonly used laboratory methods of perovskite nanocrystal (PNC) synthesis are hot-injection11-13 and ligand-assisted reprecipitation.14 These methods heavily rely on the use of various ligands to control the shape and size of PNCs.15 Most of the approaches to PNC synthesis utilize various organic ligands, such as oleylamine, oleic acid or trioctylphosphine.16 The use of ligands helps to

"School of Physics and Engineering, ITMO University, Lomonosova 9, St. Petersburg 191002, Russian Federation. E-mail: [e-mail], [e-mail]

bInternational Institute "Solution Chemistry of Advanced Materials and Technologies", ITMO University, St. Petersburg, 197101, Russian Federation cQingdao Innovation and Development Center, Harbin Engineering University, Qingdao 266000, Shandong, China

f Electronic supplementary information (ESI) available. See DOI: https://doi.org/ 10.1039/d3nr00214d J Equal contribution.

obtain monodisperse particles, control PNC growth, and passi-vate the defects, which results in a higher brightness and photoluminescence quantum yield (PLQY). Furthermore, various organic ligands such as zwitterionic ligands or organic acids were shown to significantly enhance the stability of PNCs.17-21 However, these organic components lead to the worse performance of PNCs, when it comes to the manufacturing of various optoelectronic devices, where charge carrier transport through the ligands is limited.

Therefore, ligand-free methods of PNC synthesis are of great interest, and include the solvothermal reaction,22 post-

23 27 ultrasonication,28 microwave-assisted chemical vapor deposition, microfluidic

treatment, synthesis,29-31 approach,32 mechanochemical method and others.33

However,

these methods are either time-consuming,34 sensitive to atmospheric conditions,35 or demanding expensive equipment.36 Among these approaches, it is necessary to highlight a promising PNC synthesis technique that has attracted a lot of attention - the template-assisted method.37-40 According to this method, the size of the nanocrystals is sterically limited by their growth within the templates without the use of any ligands. For instance, mesoporous silica39 was studied as a template for bismuth-halide double-perovskite nanocrystal growth with the composition Cs2AgBiBr6, where three kinds of templates with different intrinsic forms were used (cubes,

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rods, and wires), which results in different geometries of nano-crystals leading to changes in the band gap energy. In the other work, a template approach was realized with a polydi-methylsiloxane template to fabricate highly aligned 2D layered perovskite one-dimensional micro-wire arrays.38 The obtained structures possessed excellent uniformity, high-quality crystal-linity, and high structural anisotropy, which allowed the fabrication of high-performance polarization-sensitive photo-detectors. Overall, there are a number of various templates that are used to synthesize miscellaneous ligand-free perovs-kite nano- and microstructures. These templates include titania,41 polystyrene,42 metal-organic frameworks,43 and mesoporous silica44 that are utilized for the fabrication of per-ovskite nanostructures with different dimensionality, such as 1D-quantum dots, 2D-nanorods or nanowires and 3D-bulk materials. However, to the best of our knowledge, there is no study which conducts an in-depth investigation of how the variation of synthesis parameters influences the resulting optical performances of PNCs obtained by the templateassisted synthesis approach.

In this work, we develop a new facile ligand-free templateassisted approach to PNC fabrication with near-unity absolute quantum yield and extensively study how various synthesis parameters affect their optical properties. We show that the optical properties of the obtained PNCs are affected not only by the template morphology, they depend more on the concentration of precursor solutions, anion, and cation ratio, as well

as on adding defect-passivating dopants. As templates for PNC preparation, we utilize CaCO3 vaterite micro- and submicro-spheres. Vaterite is one of the polymorphic modifications of calcium carbonate that has a highly porous surface with the pore size lying in the range of tens of nanometers.45 PNCs are synthesized in the pores of vaterite spheres as the solvent evaporates, thus making the growth of PNCs confined to the pore size. We thoroughly study how different synthesis parameters such as perovskite concentrations, precursor ratios, template size, surface area, pore volume, and addition of dopants affect the optical properties of the obtained PNCs. Additionally, we carry out the comprehensive structural and optical characterization of the synthesized PNCs. Furthermore, the developed synthetic approach is not only facile and easily tunable, but also does not depend on conditions such as an air- and moisture-free atmosphere, which is confirmed by testing for 30 days. Moreover, we modify PNCs with Yb3+ and measure their PLQY and up-conversion properties. Finally, we discuss the potential of the obtained PNCs for use as infrared radiation visualizers, showing their excellent characteristics comparable to those available on the market.

Results and discussion

In this work, vaterite CaCO3 particles were utilized as a template material for the synthesis of PNCs (Fig. 1). For this, we

Fig. 1 (A) Schematic illustration of a synthetic route of perovskite nanocrystals (PNCs) obtained in CaCO3 templates and deposited onto a substrate. (B) The influence of various synthesis parameters, such as the amount of perovskite precursors, the ratio between CsBr and PbBr2, and the amount of Yb used in synthesis on the optical properties of the obtained PNCs.

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chose two sizes of CaCO3 templates with the same chemical composition but with different morphologies, which allowed us to study the influence of template parameters on the resulting properties of PNCs to a greater extent. Submicro (Sub) and micrometric (Mic) sized particles of CaCO3 were obtained as described elsewhere.46 The vaterite spheres were soaked in a mixture of perovskite precursors (CsBr and PbBr2) dissolved in DMSO. Afterwards, the suspension of CaCO3 particles (Sub and Mic) was then deposited on a glass substrate via spin coating. As a result of DMSO evaporation CsPbBr3 PNCs were crystallized in pores of CaCO3.

According to SEM analysis (Fig. 2), the average sizes of CaCO3 submicrospheres (Sub) and microspheres (Mic) were 0.4 ± 0.2 |im and 4.7 ± 2.0 |im (Fig. 2B), respectively. A scanning electron microscope (SEM) with a backscattered electron detector showed the presence of PNCs (Fig. 2C and D) embedded into vaterite spheres and energy-dispersive X-ray (EDX) analysis confirmed the presence of carbon (C), lead (Pb) and bromine (Br) (Fig. 2E). Moreover, the Brunauer-Emmett-Teller (BET) method was performed in order to evaluate the pore sizes (3.5 and 14.4 nm for Sub and Mic, respectively) of CaCO3 (Fig. 2F). We assume that the pore size is one of the crucial parameters

that manage the behavior of PNC formation. As we suppose, a change in the optical properties of the obtained PNCs occurs due to the decrease of PNC sizes that is caused by the alteration of synthesis parameters such as variation of the added amount of perovskite precursors and the sizes of pores. To this end, the surface area (39.2 and 13.6 m2 g-1 for Sub and Mic, respectively) and pore volume (0.138 and 0.062 cm3 g-1 for Sub and Mic, respectively) were measured.

Optimization of PNCs obtained in the pores of CaCO3 templates was achieved by the alteration of the synthesis parameters displayed in Table S1.f First, the amount of the added precursors (PbBr2 and CsBr) for the synthesis was changed with a constant amount of CaCO3 templates. The amount of precursors used was limited by two factors: on the one hand, the use of less than 0.6 |imol CsPbBr3 precursors led to an extreme decrease of the PL intensity. On the other hand, we did not use more than 6.3 |imol CsPbBr3, since this caused the growth of PNCs on the surface of the glass substrate instead of CaCO3 spheres. The PL spectra of the resulting PNCs obtained in the pores of the CaCO3 templates were measured. The highest wavelength maxima of PL spectra correspond to 516.8 nm for Sub and 518.4 for Mic spheres in the case of the

S o Q

j • ^

Sub

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0.2 0.4 0.6 0,8 Size (Mm)

c Sub

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Mic

500 nm

! - I

D Sub + CsPbBr3 200 nm

S Mic + CsPbBr3

/ 500 nm

F Sub CaCOj Mic CaC03

Pore volume 0.138 (cm3/g)

Fig. 2 Characterization of Sub and Mic. (A) Representative scanning electron microscopy (SEM) images of CaCO3 spheres (Sub and Mic). (B) Size distributions of CaCO3 spheres as derived from SEM images. (C) Representative SEM images of single Sub and Mic. (D) Representative SEM images of single Sub and Mic with PNCs using a backscattered electron detector. (E) Elemental mapping of CaCO3 spheres. (F) Brunauer-Emmett-Teller (BET) analysis of Sub and Mic.

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s

о Q

Fig. 3 Optical characterization of Sub and Mic. (A) Representative fluorescence microscopy images of CsPbBr3 obtained in CaCO3 spheres. Scale bar corresponds to 20 ^m. (B) Typical PL spectra of CsPbBr3 obtained in CaCO3 templates (Sub and Mic). (C) Evolution of the maximum PL peaks of CsPbBr3 obtained in CaCO3 templates at different amounts of added perovskite precursors in mg. (D) Evolution of maximum PL peaks of CsPbBr3 obtained in CaCO3 templates at different added ratios of PbBr2 : CsBr.

addition of 6.3 |imol CsPbBr3 precursors for template-assisted synthesis (Fig. 3A). However, the decrease of the used amount of perovskite precursors results in a blue-shift of the PL spectra of the obtained CsPbBr3 for about 9.3 nm in the case of Sub and 6.9 nm in the case of Mic spheres (Fig. 3B and C). This can be attributed to the decrease of the PNC size, since the bandgap is directly related to the size of PNCs. A decrease of the nanocrystal size causes the widening of the bandgap, which leads to the shifting of the PL wavelength to the blue region due to the quantum confinement effect.47'48 It should be noted that the presence of Cl- ions can potentially affect the PL properties of CsPbBr3 PNCs. Thus, we used CaBr2 instead of CaCl2 which is commonly used for vaterite synthesis to avoid chloride ion contamination.45

The blue-shift of PL spectra is similar for two types of matrix spheres: Sub and Mic. The maximum peaks of PL spectra wavelengths vary from 508 to 517 nm for CsPbBr3 (Sub) PNCs and from 512 to 517 nm for CsPbBr3 (Mic) PNCs. As can be seen, the addition of the same concentration of the perovs-kite precursor solution to the Mic and Sub spheres results in a higher decrease of the PL wavelength in the case of Sub particles. For example, the addition of 3.3 |imol of CsPbBr3 precursors results in peak wavelengths of 514.4 nm for Sub and 516.6 nm for Mic vaterite spheres (Fig. 3C). Overall, the addition of the same amount of CsPbBr3 precursors leads to shorter PL wavelengths in the case of Sub spheres. Since we add the same concentration of the perovskite precursor to

both Sub and Mic particles, but the Mic spheres have bigger pores that lead to higher amounts of precursor molecules in a single pore of Mic (Fig. 2F). This results in bigger crystals grown within the larger pores, and thus we observe a smaller PL shift in the case of Mic particles. Therefore, it can be seen that tuning of the size and spectral properties of PNCs can be achieved not only by the variation of the template pore size, but also through the variation of the added amount of perovs-kite precursor.

Our next step was to analyze the influence of the PbBr2 : CsBr ratio on the optical performances of PNCs. We have varied the molar ratio between PbBr2 and CsBr from 1: 1.0 to 1: 1.5 (Table S2f) and measured how it affected the intensity and peak wavelength of PL. Interestingly, we have noticed that this change in ratio does not affect either the PL peak wavelength (Fig. 3D) or the PL intensity (Fig. S1f). Thus, the main contributing factor to the blue shift was the overall amount of perovskite precursors added to the vaterite spheres and the morphology of the spheres themselves.

Furthermore, we have investigated the influence of synthesis conditions on the PLQY. According to the performed measurements (Table S3f and Fig. 4), the PLQY of CsPbBr3 PNCs obtained in CaCO3 templates increases as we increase the CsBr amount and the ratio of PbBr2 : CsBr changes from 1 : 1.0 to 1 : 1.5 in the molar ratio for both types of spheres (Sub and Mic). Additionally, the PLQY increases with the rise of the amount of perovskite precursors added to the synthesis.

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Fig. 4 PLQY of Sub and Mic loaded with PNCs with and without the addition of Yb. From top to bottom: Sub, Sub with the addition of Yb (6 mol%), Mic, Mic with the addition of Yb (6 mol%). For the samples without the addition of Yb ratios of PbBr2 : CsBr varied from 1: 1 to 1: 1.5. For the samples with the addition of Yb, the ratio PbBr2 : CsBr was chosen as 1: 1.15 and the concentrations of added Yb varied from 0 to 6 mol%.

Generally, the addition of rare-earth elements, in particular Yb3+, results in an increase of the near-infrared PLQY of PNCs due to the quantum cutting effect.49-51 The electrons in the conduction band and the holes in the valence band reach the intermediate defect states, and then the energy is transferred to Yb3+ ions.52 Furthermore, Yb3+ is able to substitute the Pb2+ ions, and since the atomic radius of Yb3+ is smaller than that of Pb2+ this results in a decrease in the interplanar distance in the crystal lattice. Combined with the increase in the bonding

energy this makes Yb3+-doped PNCs more resistant to external conditions. Moreover, the doping of PNCs with lanthanides results in a higher chance of oxidation of Pb0 defects to Pb2+, thereby increasing perovskite stability and emission efficiency.53 For this reason, to investigate the influence of the Yb3+ ions on the PLQY of fabricated CsPbBr3 PNCs, different amounts of Yb3+ (0-6 mol%) were added during the synthesis (Table S4f and Fig. 4). The presence of Yb3+ in Mic with the obtained PNCs was verified using ICP-AES (Table S5f).

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According to the obtained data, the PLQY increased by 20-30% in the case of the addition of Yb3+ to PNCs obtained at a ratio of PbBr2: CsBr = 1:1.5. PNCs obtained by the addition of perovskite precursors from 4 to 5.1 |imol with the addition of 4-6 mol% Yb3+ for Sub showed a PLQY close to 100%. The difference in the PLQY of PNCs synthesized with the addition of 4 mol% or 6 mol% of Yb3+ is not significant for Sub and Mic proving that at this added amount of Yb3+ the PLQY reaches its saturation level. Therefore, we can conclude that this amount of Yb3+ shows the saturation level. It can be noticed that the PLQY for Mic spheres after the addition of Yb3+ starts to decrease as the laser power density rises. We assume that this can be attributed to the ability of Yb3+ to pas-sivate the defects in the formed PNCs, and thus the PLQY graph starts to form a plateau. However, with the further increase of the laser power density, the Auger recombination begins to prevail, which leads to a decrease in the PLQY.54

Upon achieving the maximal PLQY of CsPbBr3 PNCs obtained in CaCO3, we investigated their structural properties. For instance, X-ray diffraction (XRD) analysis of a sample with the highest ratio of PbBr2 : CsBr (1: 1.5) indicates the formation of an orthorhombic perovskite phase (group Pbnm)55

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and the vaterite phase of calcium carbonate CaCO3 (Fig. 5A).45 The obtained diffractogram contains intensive diffraction peaks corresponding to the crystallographic planes (002), (110), (020), (200), (004), and (220) of the orthorhombic CsPbBr3 phase, along with the ones assigned to the planes (020), (120), (211) of the vaterite CaCO3 phase. However, we also can observe an intensive signal at 29.35°, which is attributed to the plane (213) of the tetragonal CsPb2Br5 phase. The formation of the tetragonal phase can be attributed to the peculiarities of the template synthesis approach and the difference in solubility of CsBr and PbBr2 in polar solvents.56'57 For instance, CsBr has high water solubility and easily can be washed from CsPbBr3, whereas the CsPb2Br5 tetragonal phase demonstrates low solubility in water and other polar solvents, and can completely or partially coat the orthorhombic CsPbBr3 phase during the CsBr washing process.565 Furthermore, the presence of the orthorhombic CsPbBr3 phase is additionally confirmed by data from high-resolution transmission electron microscopy. Microphotographs show PNCs at the edge of CaCO3 spheres, in only regions that can be pierced through (Fig. S3f). A distance of 0.59 nm between the lattice planes corresponds to the orthorhombic CsPbBr3 phase.

S o Q

otthorhoinbic CsPbBij (•) I IJ! .. i o § i i

tetragonal CaPbiBisi«) <.,11 £ 1 ,i i.

valeriEe ! ♦) 2 I 5

CaCOs w/ ♦ perovskites .. : h ill ¿1 WM - ^

B1 J-.' :_; c*Hcir, JB,. A. [—«mi m BHCO, Wl J r w

Ki MS jsi 3!; m Binding Enflrgy <tVl

20 25 30 28 (cleg)

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B

Before etching After etching a _ 01s Ca2[i Cts PMF7 _J ^JL Br3d

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800 700 600 500 400 300 200 100 0

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Fig. 5 Structural characterization of Mic with fabricated PNCs in CaCO3 templates. (A) XRD spectra of Mic with fabricated PNCs in CaCO3 templates and the corresponding reference spectra of CsPbBr3, CsPb2Br6, and vaterite. (B) XPS spectra of Mic with fabricated PNCs in CaCO3 templates zoomed of different regions. (C) XPS spectra of Mic with fabricated PNCs in CaCO3 templates before and after etching.

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Moreover, the Moiré fringe was detected with a size of nearly 2.2 nm. It can be explained by the overlapping of the parallel lattice planes of CsPbBr3 or/and CsPb2Br5 phases.58 Thus, CsPbBr3 orthorhombic PNCs protected with tetragonal CsPb2Br5 can be obtained. Due to the low solubility of CsPb2Br5 in water this phase may serve as a protection layer against humidity in the atmosphere.

Furthermore, it should be noted that despite the presence of the CsPb2Br5 phase in the CaCO3 phase we did not observe a shift in the PL peak wavelength (Fig. 3D). Thus, we assume that the amount of non-perovskite phases present is relatively small. If this was not the case, then a part of the precursor material would have been converted into a non-perovskite material, which resulted in a decrease of the size of the obtained PNCs and the consequent blue shift. Therefore, the increase of CsBr compared to PbBr2 does not lead to the formation of a CsPb2Br5 matrix, but of a thin shell on the surface of CsPbBr3 PNCs within the pores of CaCO3. We assume that the presence of this shell leads to the passivation of surface defects of the obtained PNCs. This idea is further supported by PLQY measurements which demonstrate that the samples with higher amounts of CsBr in the precursor solution had higher PLQYs (Fig. 4). Furthermore, we have investigated the single-photon PL intensity dependence on the excitation fluence for Mic samples (Fig. S2f). The slope y of the dependence on a log-log scale is shown in the plots and it corresponds to the coefficient in I ~ F (I - PL intensity, F - excitation fluence). As we increase the amount of CsBr the slope changes from 0.41 to 0.95, which we link to the passivation of surface defects due to the formation of CsPb2Br5.

An X-ray photoelectron spectroscopy (XPS) study was carried out to investigate the chemical composition of the synthesized samples of Mic with PNCs (1: 1.5 molar ratio). Fig. 5B shows the XPS spectra obtained from the surface of the freshly prepared sample (red line) and after ion etching (green line) to investigate PNCs within CaCO3. The primary structure of the XPS spectra that corresponds to the freshly prepared Mic with PNCs (red line) contains core photoemission lines C 1s (~285 eV), O 1s (~532 eV), Ca 2p (~349.5 eV), and Cs 3d (~726 eV). However, after ion etching (green line), the photoelectron lines Pb 4f (~140 eV) and Br 3d (~70 eV) appear in the spectrum. XPS spectra show that there are no observable signals from PNCs on the surface of Mic (before etching), and hence, we can conclude that PNCs on the surface of Mic were not formed. To understand the chemical state of the elements and determine the stoichiometry of the formed Mic loaded with PNC structures, the high-resolution spectra of the C 1s, O 1s, Ca 2p, Cs 3d, Pb 4f, and Br 3d photoelectron lines were obtained after ion etching (Fig. 5B). In particular, the analysis of the high-resolution spectrum of Ca 2p and C 1s photoelectron lines (Fig. 5C) allows us to conclude that the majority of the synthesized material stoichiometrically is calcium carbonate CaCO3 and its defects are in the form of hydrocarbonates Ca(HCO3)2.59 A significant amount of CO3 corresponding to 290 eV in the high-resolution spectrum of the C 1s core line also indicates the presence of other carbonates in the sample.

Thus, the analysis of the high-resolution spectra of the Cs 3d5 photoelectron line (Fig. 5C) revealed the presence of two components corresponding to the chemical bonds of caesium with bromine Cs-Br (724.8 eV) and with oxygen Cs-O (725.3 eV), which can be assigned to the formation of perovskite CsPbBr3 and carbonate caesium Cs2CO3.60,61 This explains the presence of the Cs 3d photoelectron signal in the spectrum before ion etching. Apparently, more active caesium enters into a chemical reaction with calcium carbonate on the surface of Mic forming defects in it. It should be noted that the absence of diffraction lines (Fig. 5A) corresponding to the crystalline phases of Ca(HCO3)2 and Cs2CO3 indicates their miniscule amounts, which are formed in the form of amorphous layers at the Mic and PNC interfaces. A detailed examination of the high-resolution spectra of Br 3d and Pb 4f7 (Fig. 5C) shows that the photoelectron lines of bromine and lead have components corresponding to Br-Pb (68.8 eV), Br-O (69.7 eV) and Pb-Br (138.8 eV), Pb-O (137.2 eV), respectively.62-64 The presence of the component with a Pb-O bond in the spectrum of Pb 4f can be eventually the result of the interaction of perovs-kite with calcium bicarbonate, which leads to the degradation of CsPbBr3 followed by recrystallization in CsPb2Br5, as evidenced by the results of XRD analysis (Fig. 5A). The presence of CsPbBr3 PNCs in the Mic is confirmed not only by the XRD method, but also by the quantitative calculation of the elements Cs, Pb, and Br from the components of the highresolution spectra Cs-Br(3d), Pb-Br(4f7), and Br-Pb(3d) respectively, which gives the stoichiometric ratio of Cs : Pb : Br that is equal to 1 : 1 : 3 (Fig. 5C). In addition, it is worth noting that the signal of the C-C bond at 285 eV can be attributed to trace ethanol appearing after purification of the final product.

Additionally, the up-conversion of infrared (IR) radiation to green light was measured for different samples (Sub and Mic with ratios of Cs : Pb equal to 1 : 1.0, 1 : 1.3, and 1 : 1.5 either with or without Yb3+ 6 mol% doping). As a light source for pulsed laser photoexcitation we used a tunable femtosecond laser with an optical parametric amplifier (pulse duration 220 fs, wavelength range 400-2100 nm), the laser beam of which was focused into a spot with a diameter of 120 |im (Fig. S4f).

According to the obtained data, Yb3+-doping of PNCs did not significantly increase the upconversion in the near-IR region. Fig. 6A and B shows the dependence of the two-photon PL intensity normalized on a maximum value of the excitation wavelength. The absence of additional peaks for Yb3+-doped samples indicates that Yb3+ only passivates defects in the per-ovskite structure and does not embed in the lattice. The up-conversion efficiency shows the same dependence on the PbBr2 : CsBr ratio as the PLQY, and the growth of the CsBr concentration leads to an increase in the upconversion efficiency (Fig. 6C and D). The addition of Yb3+ also increases the up-conversion efficiency of CsPbBr3 PNCs obtained in CaCO3 due to defect passivation. The up-conversion was measured under 800 nm laser excitation. The two-photon PL intensity dependence on excitation fluence is presented in Fig. 6E and F. The slope of the dependence on a log-log scale is given in tables on the plots and it corresponds to the coefficient in I ~ F (I -

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Fig. 6 IR to visible light up-conversion measurements for Sub and Mic CaCO3 with PNCs. (A and B) Normalized intensity of two-photon PL dependence on the excitation wavelength. (C and D) The intensity of two-photon PL for different samples at the same excitation fluence under 800 nm laser irradiation. (E and F) Two-photon PL intensity dependence on excitation fluence at 800 nm wavelength. (G and H) PLQY of visualizers prepared from Sub and Mic CaCO3 with PNCs measured right after the synthesis and after 30 days. (J) A digital image of the prepared visualizer under IR irradiation (wavelength 1000 nm).

PL intensity, F - excitation fluence). Generally, for the two-photon processes in perovskite nanocrystals the coefficient is around 2 due to the domination of exciton recombination, while for the three-photon processes the coefficient is around 3.54 PNCs with about 10 nm sizes which are not aggregated with each other possess an average slope close to 2. In our experiments, two-photon PL from the Sub and Mic spheres has an average slope close to 2, however, slopes for Sub spheres are slightly greater than for Mic spheres (Fig. 6E and F). This effect can be explained by the shift of the excitonic absorbance peak to a shorter wavelength due to quantum confinement. In the case of Sub spheres particles with smaller sizes contribute not only to two-photon absorption, but also to three-photon absorption which results in higher slopes. Moreover, the addition of Yb3+ leads to defect passivation, which in turn increases the slope values.65

Furthermore, the PNCs obtained via reported template synthesis show great air and humid stability. The PLQY of CaCO3 with PNCs was studied directly after the synthesis of PNCs and after 30 days of storing these spheres in open air (Fig. 6G and H). As can be seen, the PLQY did not change with time, and

thus, it can be assumed that they are stable in open air. We attribute this effect due to both properties of CaCO3 and the presence of the CsPb2Br5 phase. Moreover, the developed synthesis approach does not require the use of dry reagents. Therefore, we believe that this is an important step towards facilitating perovskite synthesis, which generally requires air-and water-free conditions.

Generally, this work is dedicated to the investigation of limitations and possibilities of the template-assisted approach for the manufacturing of PNCs. Therefore, it is important to highlight several drawbacks of this approach when it comes to the real-life application of this method for the manufacturing of optoelectronic devices. The broad emission profile of the obtained PNCs may decrease the efficiency of devices created based on PNCs synthesized using this method. Furthermore, it is necessary to point out that the PNCs that are obtained using the template-assisted approach remain embedded within the matrix of the template. We believe that this is the main limiting factor for the application of the developed approach in device manufacturing. Thus, we assume that these problems might be solved using another type of template, for example,

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porous films of aluminum or titanium oxide, which we plan to investigate in our further research. Nonetheless, the templateassisted approach can still be utilized for applications that rely mainly on the optical properties of PNCs.

To demonstrate the practical application of the developed template-assisted approach based on CaCO3 spheres, we created an infrared (IR) laser visualizer (Fig. 6J). For this Mic CaCO3 with CsPbBr3 PNCs (1:1.5 and Yb 6 mol%) was used. Briefly, the spheres with perovskites were removed from the glass substrate using toluene and then concentrated via cen-trifugation. Afterwards, a concentrated suspension of CaCO3 with PNCs in a single drop was applied on the surface of a new glass substrate to obtain a thick layer of PNCs to increase the light-matter interaction volume. The produced visualizers were investigated at a wavelength of 1000 nm for femtosecond laser beam visualization without any focusing and showed performance comparable to the commercial laser viewing cards. The minimum fluence at which the visualizer operates was around 10 nJ cm-2. For the NIR laser viewing card that does not require charging with light, the minimum fluence equals 9 nJ cm-2, and for the viewing IR card made of a slow-fading phosphor material that requires charging with room light, the minimum fluence is 3 nJ cm-2.

Conclusion

We have developed a new approach for the manufacturing of PNCs, which does not require the use of any kinds of organic ligands and allows us to obtain perovskites with nearly 100% absolute PLQY. Generally, this work offers an in-depth study of template-assisted synthesis of CsPbBr3 PNCs in CaCO3 particles of different sizes. We have utilized the obtained optimized samples for an up-converting IR visualizer operating due to strong PL induced by two-photon absorption and exhibiting performance compatible with commercially available analogues. Finally, the synthesized PNCs protected by CaCO3 templates demonstrated not only excellent optical properties but also 30 days of stability, which, we believe, paves the way for efficient photonic devices for down- and up-conversion of light frequency.

Materials and methods

Synthesis and structural characterization of CsPbBr3 PNCs in CaCO3 spheres

CaCO3 submicrospheres and microspheres (Sub and Mic) were synthesized in a co-precipitation reaction by mixing CaBr2 and Na2CO3 and further used as templates. Afterwards, CsPbBr3 PNCs were obtained via a template-assisted method in CaCO3 spheres in simultaneous processes of CsPbBr3 precursor solution (CsBr and PbBr2 in DMSO) evaporation and PNC crystallization. Furthermore, CsPbBr3 NCs were used to coat glass substrates by spin coating. In the template-assisted synthesis concentrations of perovskite precursors, the ratios of PbBr2: CsBr

and CaCO3 template parameters, such as pore size and surface area were varied. The optical properties of the obtained CsPbBr3 in CaCO3 spheres such as PL spectra and photoluminescence quantum yield (PLQY) were further investigated. The obtained CsPbBr3 PNCs in CaCO3 spheres were comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and energy-dispersive X-ray (EDX), and X-ray photoelectron (XRP) spectroscopy. To evaluate the pore sizes, intrinsic surface areas and pore volumes of CaCO3 spheres Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements were made. To confirm the presence of Yb3+ in the obtained samples, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was utilized. The detailed description of the synthetic route and the full data set are presented in the ESI.f

Optical measurements

PLQY measurements were carried out using an integrating sphere (Labsphere), a continuous-wave laser as an excitation source (405 nm), and a QE Pro fiber spectrometer. Power attenuation was performed with filter wheels equipped with absorptive ND filters FW212 (Thorlabs).

The upconversion PL measurements were performed using a custom 10 W Pharos single-unit integrated femtosecond laser system and a high power optical parametric amplifier Orpheus-F as an excitation source. The excitation femtosecond laser was focused on a sample from behind with a 10x objective. The signal was collected using a 10x objective. The signal collected through the long-pass filter was guided to an Andor Kymera 328i imaging spectrograph with an Andor iDus CCD camera. A detailed description of the optical measurements and the full data set are presented in the ESI.f

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (Project 075-152021-589) and the Russian Science Foundation (Project No. 2173-20189). The work was partially done in ITMO Core Facility Center "Nanotechnologies".

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