Исследование структурно-люминесцентных свойств синтетических микро- и наноалмазов методами рамановской и люминесцентной спектроскопии тема диссертации и автореферата по ВАК РФ 01.04.05, кандидат наук Рунова Марианна Васильевна

• Рунова Марианна Васильевна
• кандидат наук
• 2019

Оглавление диссертации кандидат наук Рунова Марианна Васильевна

ОГЛАВЛЕНИЕ

РЕФЕРАТ

SYNOPSIS

ВВЕДЕНИЕ

I. ОСНОВНАЯ ЧАСТЬ

ГЛАВА 2. АНАЛИТИЧЕСКИЙ ОБЗОР НАУЧНО-ТЕХНИЧЕСКОЙ

ЛИТЕРАТУРЫ ПО ТЕМЕ ДИССЕРТАЦИИ

1.1 Методики синтеза люминесцентных нано и микроалмазов

с NV- и SiV центрами окраски

1.1.1 HPHT и ДС отт^з алмазных частиц

1.1.2 Формирование люминесцирующих центров окраски в 54 кристаллической решетке алмаза

1.1.3 Оптимизация люминесцентных параметров NV- и SiV центров в 56 синтетических нано- и микроалмазах

ГЛАВА 2. ТЕХНИКА И МЕТОДЫ ИССЛЕДОВАНИЯ СТРУКТУРНО-

ЛЮМИНЕСЦЕНТНЫХ СВОЙСТВ НАНО И МИКРОАЛМАЗОВ С NV-И SiV ЦЕНТРАМИ ОКРАСКИ

2.1 Введение

2.2 Определение морфологии нано и микроалмазов с использованием 60 техники сканирующей и просвечивающей электронной микроскопии

2.3 Определение концентрации замещающих атомов азота NS 62 и NV- центров окраски в кристаллической решетке алмаза

методом электронного парамагнитного резонанса

2.4 Определение структуры алмазных частиц и 64 спектрально-кинетических параметров люминесценции центров

окраски с использованием методов микрорамановского и микролюминесцентного анализа

2.5 Выводы к главе

ГЛАВА 3. ИССЛЕДОВАНИЯ СТРУКТУРНО-ЛЮМИНЕСЦЕНТНЫХ 69 СВОЙСТВ ЧАСТИЦ АЛМАЗОВ С ЛЮМИНЕСЦИРУЮЩИМИ ЦЕНТРАМИ ОКРАСКИ

3.1 Введение

3.2 Синтез образцов HPHT-алмазов с внедренными NV- центрами

3.3 Особенности методов микрорамановской и микролюминесцентной 71 спектроскопии при исследовании HPHT-алмазов с внедренными

центрами

3.4 Влияние содержания замещающих атомов азота, N и дозы 72 облучения электронами на интенсивность ФЛ центров и разупорядочение кристаллической решетки НРНТ-алмазов.

3.5 Исследование процессов, контролирующих спектрально- 78 кинетические параметры ФЛ центров в синтетических НРНТ-алмазах.

3.5 Выводы по главе

ГЛАВА 4. ИССЛЕДОВАНИЕ СТРУКТУРНО-ЛЮМИНЕСЦЕНТНЫХ

СВОЙСТВ НАНОЧАСТИЦ АЛМАЗОВ С ЛЮМИНЕСЦИРУЮЩИМИ SiV ЦЕНТРАМИ ОКРАСКИ

4.1 Введение

4.2 Синтез и первичная характеризация образцов ДС-алмазов с SiV 90 центрами окраски

4.3 Особенности методов микрорамановской и микролюминесцентной 94 спектроскопии при исследовании ДС-алмазов с люминесцирующими

SiV центрами

4.4 Исследование структурно-люминесцентных параметров образцов 97 ДС-наноалмазов с различным размером поликристаллов

4.4.1 Спектры люминесценции и рамановского рассеяния образцов 97 ДС-наноалмазов с различным размером поликристаллов

4.4.2 Механизмы, определяющие зависимость люминесценции SiV 101 центров в ДС-наноалмазах от размеров поликристаллов

4.4.3 Выводы к главе 4 106 ЗАКЛЮЧЕНИЕ 108 СПИСОК СОКРАЩЕНИЙ И УСЛОВНЫХ ОБОЗНАЧЕНИЙ 110 СПИСОК ЛИТЕРАТУРЫ 112 ПРИЛОЖЕНИЕ А (Обязательное) Оттиски статей

РЕФЕРАТ

Введение диссертации (часть автореферата) на тему «Исследование структурно-люминесцентных свойств синтетических микро- и наноалмазов методами рамановской и люминесцентной спектроскопии»

I. ОБЩАЯ ХАРАКТЕРИСТИКА ДИССЕРТАЦИИ 1. Актуальность темы

Предметом настоящей диссертационной работы являются исследования физических механизмов, контролирующих люминесцентные параметры синтетических алмазных нано и микрочастиц с внедренными люминесцирующими центрами окраски.

Люминесцирующие алмазные наночастицы являются одним из представителей семейства наноуглеродных материалов, включающих в себя луковичные нанографиты, углеродные точки, нанотрубки, графены, которые благодаря уникальной комбинации механических, электрических и оптических свойств, привлекают пристальное внимание в качестве исходных «строительных блоков» для создания новых наноматериалов для функциональных элементов нанофотоники. Исследования, выполненные в настоящей работе направлены на выяснение взаимосвязи структурных и оптических параметров нано- и микрочастиц синтетических алмазов, с внедренными люминесцирующими центрами окраски типа азот-вакансия (ЫУ) и кремний-вакансия (31У), что является весьма актуальными для понимания процессов, формирующих люминесцентные свойства алмазных частиц, важных для их использования в оптоэлектронике, нанофотонике и биоимиджинге, включая однофотонные источники излучения для квантового компьютинга, криптографии и телепортации. Данная тематика соответствуют приоритетному направлению развития науки, технологий и техники в Российской Федерации «Индустрия наносистем».

В общих чертах известно, что реализация оптимальных люминесцентных характеристик алмазных нано- и микрочастиц определяется не только концентрацией центров окраски в кристаллической решетке но и характеристиками внутренней кристаллической структур алмазной матрицы, а в случае нанокристаллов и структурой их приповерхностного слоя, которые могут значительно меняться в зависимости

от условий их синтеза, внедрения примесных атомов и вакансий, а также дополнительной обработки. Поэтому информация о влиянии структуры наноалмазов на спектральные и кинетические параметры люминесценции центров окраски является ключевой для оптимизации условий их изготовления для использования в качестве элементов оптоэлектроники и нанофотоники с улучшенными функциональными параметрами.

В настоящее время, одним из наиболее информативных методов изучения структуры наноуглеродных материалов, включая наличие структурных дефектов разного типа является рамановская спектроскопия. Использование техники микро-рамановского рассеяния, которая дает возможность одновременной регистрации рамановского рассеяния и люминесценции от одного и того же объема исследуемой наноструктуры, позволяет установить однозначную связь между параметрами кристаллической структуры и люминесценции. Таким образом, совокупность методов микрорамановской и микролюминесцентной спектроскопии, используемая в данной работе с привлечением данных электронной микроскопии и спектроскопии электронного парамагнитного резонанса является адекватным методом достижения поставленной в диссертационной работе цели. 2. Цель работы:

Выяснение факторов и закономерностей, определяющих параметры фотолюминесценции (ФЛ) отрицательно заряженных центров окраски азот-вакансия (NV-) и кремний-вакансия (SiV), формируемых в нано- и микрокристаллах синтетических алмазов, полученных в результате статического синтеза при высоком давлении и температуре (High Pressure High Temperature, HPHT) и динамического синтеза (ДС), путем сравнительного анализа их спектров люминесценции и рамановского рассеяния с использованием данных электронной микроскопии и электронного парамагнитного резонанса (ЭПР).

3. Задачи работы:

1. Исследовать влияние концентрации замещающих атомов азота и дозы облучения электронным пучком, создающим вакансии в кристаллической решетке НРНТ-алмазов, на интенсивность фотолюминесценции NV- центров окраски и качество кристаллической структуры алмазов.

2. Исследовать влияние концентрации NV- центров окраски в НРНТ алмазах на дефектность кристаллической структуры алмазов, интенсивность и кинетику фотолюминесценции центров.

3. Исследовать люминесцентные свойства центров в алмазных нанокристаллах, формирующих поликристаллические частицы разных размеров в результате динамического синтеза с последующим измельчением и разделением на фракции поликристаллов различных размеров.

4. Научная новизна работы:

Впервые путем сравнительного анализа спектров фотолюминесценции и рамановского рассеяния синтетических нано- и микро алмазов с внедренными NV- и SiV центрами окраски, с использованием данных электронной микроскопии и ЭПР, показано, что:

1. Интенсивность фотолюминесценции NV- центров окраски в НРНТ-алмазах ограничена возникновением связанных с азотом дефектов кристалла и появлением разупорядоченной углеродной структуры.

2. Интенсивность фотолюминесценции NV- центров в НРНТ-алмазах определяется конкуренцией между ее ростом с образованием МУ^ центров и тушением их люминесценции за счет безызлучательной рекомбинации фотовозбуждённых центров с участием структурных дефектов кристаллической решетки.

3. В алмазах динамического синтеза с коротким временем (100 мкс) возможно образование люминесцирующих SiV центров окраски, демонстрирующих при комнатной температуре узкую (10-12 нм) интенсивную бесфононную полосу люминесценции на 738 нм.

4. Интенсивность люминесценции SiV центров в нанокристаллах ДС-

алмазов, образующих в результате синтеза поликристаллические агломераты, определяется конкуренцией между дезактивацией SiV-центров дефектами решетки нанокристаллов и дезактивацией центрами безызлучательной рекомбинации в объеме межкристаллитных слоев поликристаллической частицы.

5. Теоретическая и практическая значимость работы:

В результате выполнения работы показана возможность оптимизации параметров НРНТ и динамического синтеза люминесцирующих алмазов для достижения максимальных интенсивностей люминесценции внедренных в них NV- и SiV центров окраски. Показано, что интенсивность люминесценции NV- центров в HPHT-алмазах может быть максимизирована путем соответствующего выбора концентрации легирования азотом, которая существенно не нарушает кристаллическую решетку алмаза, а в случае ДС-алмазов имеет место оптимальный размер поликристаллических алмазных частиц, что позволяет оптимизировать параметры помола и фракционирования полученных в результате детонационного синтеза поликристаллических алмазных частиц микронного размера.

6. Положения, выносимые на защиту:

1. Максимальная интенсивность фотолюминесценции NV- центров в НРНТ-алмазах ограничивается прежде всего наличием дефектов кристаллической решетки, связанных с внедрением атомов азота, и появлением разупорядоченной углеродной структуры, через которую возможна безызлучательная рекомбинации фотолюминесценции.

2. Интенсивность фотолюминесценции NV- центров в НРНТ-алмазах определяется конкуренцией между ее ростом при образовании центров и тушением ФЛ за счет безызлучательной рекомбинации центров с участием дефектов, вызванных атомами азота в кристаллической решетке.

3. В ДС-наноалмазах возможно образование люминесцирующих SiV центров окраски с интенсивной и бесфононной полосой люминесценции на 738 нм.

4. Интенсивность люминесценции SiV центров в алмазных нанокристаллах, образующих поликристаллическую частицу, зависит от размеров поликристалла, что объясняется конкуренцией между дезактивацией SiV-центров внутренними дефектами наноалмазов и дезактивацией из-за безызлучательной рекомбинации на дефектах межкристаллитных слоев поликристаллической частицы, имеющих разнонаправленные размерные зависимости.

7. Апробация работы:

Основные результаты работы докладывались и обсуждались на семинарах Университета ИТМО, а также на международных и всероссийских конференциях:

1. 2016 г. Международный оптический конгресс "Оптика - XXI век". СПб НИУ ИТМО, г. Санкт-Петербург, Россия, 17-21 октября 2016.

2. 2017 г. 8th International Conference on Nanomaterials: Research & Application (NANOCON 2016, Brno, Czech Republic, EU, October 19th - 21st 2017.

3. 2017 г. 13th International Conference Advanced Carbon Nano Structure, ACNS'2017 Saint-Petersburg, Russia July 3-7, 2017.

4. 2018 г. Международная конференция "Фундаментальные проблемы оптики - 2018, ФПО-2018, СПб НИУ ИТМО, г. Санкт-Петербург, Россия, 15-18 октября 2018.

8. Достоверность полученных результатов:

Достоверность результатов и обоснованность выводов не вызывает сомнений, поскольку обеспечивается использованием современной техники оптической спектроскопии и проверенных экспериментальных методов исследования структурно-физических свойств низкоразмерных систем, интерпретацией полученных экспериментальных данных в рамках современных физических представлений о процессах, формирующих люминесцентные отклики кристаллов с центрами окраски, а также положительными экспертными оценками рецензентов научных журналов, в

которых опубликованы результаты работы и их презентациями на всероссийских и международных конференциях.

9. Внедрение результатов работы:

Материалы диссертационной работы используются при реализации в Университете ИТМО образовательной программы магистратуры - Физика и технология наноструктур (направление подготовки Фотоника и оптоинформатика), а также при выполнении проектов в рамках государственных заданий, грантов РФФИ и Правительства Санкт-Петербурга, аналитических ведомственных программ Министерства образования и науки РФ.

10. Публикации по теме работы:

Основное содержание диссертации опубликовано в 4 статьях, из них 4 публикации в изданиях, индексируемых в базах цитирования Web of Science и/или Scopus.

11. Личный вклад автора:

Содержание диссертации и основные положения, выносимые на защиту, отражают персональный вклад автора. Вклад автора заключался в непосредственном выполнении основной части экспериментальных исследований, написании и редактировании статей и тезисов докладов. Обсуждение результатов и подготовка к публикации полученных результатов проводилась совместно с соавторами, причем вклад диссертанта был определяющим. Общая постановка целей и задач исследования диссертационной работы проведена совместно с научным руководителем работы.

12. Структура и объем диссертации

Диссертация состоит из введения, четырех глав и заключения. Общий объем диссертации - 120 страниц, включая 24 рисунка, 3 таблицы и библиографию, содержащую 81 наименование.

II. ОСНОВНОЕ СОДЕРЖАНИЕ РАБОТЫ

1. Во введении обоснована актуальность темы диссертационной работы, направленной на выяснение взаимосвязи структурных и оптических параметров нано- и микрочастиц синтетических алмазов, люминесцирующих за счет внедрения центров окраски типа азот-вакансия (КУ-) и кремний-вакансия (31У), важной для оптимизации технологии синтеза люминесцирующих алмазных частиц. Отмечена перспективность использования люминесцирующих алмазов в нанофотонике в качестве стабильных однофотонных источников света для оптического квантового компьютинга, криптографии и телепортации, а для медико-биологических применений и поскольку они при яркой люминесценции обладают высокой стабильностью физико-химических параметров, что, в сочетании с биосовместимостью и поверхностной биофункционализацией, делает их идеальными зондами для люминесцентной визуализации и маркировки внутриклеточных структур.

2. В первой главе диссертации приведен аналитический обзор литературы, содержащий общие сведения о методах синтеза нано- и микроалмазов с люминесцирующими NV- и SiV центрами окраски, их физических свойствах и применении. Проанализированы актуальные направления научных исследований структурно-люминесцентных свойств люминесцентных алмазов. Отмечено, что имеет место недостаток информации о механизмах, контролирующих спектрально-кинетические параметры люминесценции центров окраски в синтетических алмазах разного типа, что не позволяет оптимизировать их оптические параметры.

3. Вторая глава диссертации посвящена оптическим методам и технике исследования структурных и люминесцентных свойств нано- и микроалмазов с КУ- и SiV центрами окраски, используемым в диссертационной работе. Особенностью предложенного экспериментального подхода является одновременная регистрация спектров люминесценции и рамановского

рассеяния от одного и того же объема исследуемой наноструктуры с дифракционным пространственным разрешением с использованием спектрометра микро-рамановского рассеяния "inVia" Renishaw, Англия, оборудованного конфокальным микроскопом, что позволяет установить однозначное соответствие между параметрами кристаллической структуры и люминесценции. Для исследования кинетики люминесценции использовался спектрометр MicroTime100, PicoQuant, Германия с опцией измерения времени затухания люминесценции и построения FLIM (Fluorescence-lifetime imaging microscopy) изображений. Сканирующий электронный микроскоп "Merlin", Zeiss, Германия использовался для контроля размеров исследуемых образцов.

Образцы для исследования и данные о деталях синтеза, морфологии нанокристаллов ДС-алмазов, о содержании замещающих атомов азота (NS) и NV- центров в HPHT-алмазах, полученные с привлечением просвечивающей электронной микроскопии (ПЭМ) и спектроскопии ЭПР, были предоставлены изготовителями образцов. Таким образом, используемая в работе совокупность оптических методов микро-рамановской и микролюминесцентной спектроскопии, наряду с привлечением данных ЭМ и ЭПР, позволила достичь цели диссертационной работы.

4. В третьей главе диссертации приведены результаты исследования структурно-люминесцентных свойств микрочастиц HPHT-алмазов с внедренными NV- центрами окраски. Приведены сравнительные данные о степени влияния числа замещающих атомов азота, NS и дозы облучения (F) пучком электронов на интенсивность ФЛ (PL - photoluminescence) NV-центров и разупорядочение кристаллической решетки HPHT-алмазов. Для исследования были изготовлены 4 образца, с сильно отличающейся концентрацией NS (LN - Low (низкая) и HN - High (высокая)) и дозой облучения: LF - низкая и HF - высокая. На рисунке 1 приведены спектры ФЛ (а) и рамановского рассеяния (б) исследуемых образцов, анализ которых

показал, что концентрация азота в большей степени, чем доза облучения контролирует интенсивность люминесценции и образование дефектов алмазной кристаллической решетки, включая появление разупорядоченной углеродной структуры. Наличие последней следует из появления в рамановском спектре характерных полос разупорядоченного углерода (1475 см-1 и 1545 см-1), интенсивность которых максимальна для образца с высокой концентрацией атомов азота (ИМ). Таким образом, качественный сравнительный анализ спектральных параметров люминесценции и рамановского рассеяния показал, что простое увеличение концентрации N5 в ИРИТ-алмазах не приводит к соответствующему увеличению интенсивности люминесценции МУ- центров, что объясняется возникновением индуцируемых азотом дефектов кристаллов и появлением разупорядоченной углеродной структуры, через которые возможна безызлучательная рекомбинации фотовозбужденных центров.

Рисунок 1 - (а) спектры вторичного свечения образцов ИРИТ-алмазов, содержащие бесфононные полосы (2РЬ) нейтральных (МУ0) и отрицательно заряженных (МУ-) центров окраски на 575 нм и 637 нм, соответственно, а также широкие полосы ФЛ центров с участием фононов. Слева внизу отмечена область рамановских спектров; (б) рамановские спектры образцов; показаны характерные полосы алмаза (1332 см-1) и

аморфного углерода (1475 см-1 и 1545 см-1)

На следующем этапе работы был проведен сравнительный количественный анализ спектральных и кинетических параметров ФЛ и рамановского рассеяния 4-х образцов синтетических НРНТ-алмазов с различной концентрацией как замещающих атомов азота N5, так и образовавшихся NV- центров, концентрации которых измерялись независимо

с использованием техники ЭПР. Во всех образцах при формировании NV-

18

центров использована ЬБ (низкая) доза облучения (плотность потока 7 х 10

_ 2

е /см ), которая, как было определено в предыдущих экспериментах, не приводила к заметной разупорядоченности алмазной кристаллической решетки. В то же время, концентрация N5 заметно менялась: 50, 90, 250 и 600 ррт (образцы #1, #2, #3 и #4, соответственно). Отметим, что концентрация N5 в 150-250 ррт соответствует уровню концентрации азота в НРНТ-алмазах, получаемых при "стандартных" условиях синтеза, а большая или меньшая концентрации получаются при использованием при синтезе неорганических азотсодержащих добавок или геттеров азота. Поскольку было показано, что интенсивность фотолюминесценции NV- центров определяется прежде количеством связанных с азотом дефектов кристалла, было логично выбрать для исследования образцы с концентрацией N5 существенно меньшей "стандартной".

На рисунке 2 приведено СЭМ изображение микрокристалов исследуемых НРНТ-алмазов, со средними размерами ~ 2 мкм, от которых были получены спектры ФЛ и рамановского рассеяния при фокусировании возбуждающего излучения на каждый из микрокристаллов в пятно с диаметром ~ 0.7 мкм.

Рисунок 2 - СЭМ изображение исследуемых HPHT алмазов. Спектры возбуждаются и регистрируются из области 0.7 мкм частиц, помеченных шестиугольником. Метка - 2 мкм. Сканирующий электронный микроскоп Merlin, Zeiss

На Рис. 3 приведены спектры ФЛ НРНТ-алмазов с различной концентрацией

NS, содержащие бесфононные полосы (zero phonon lines, ZPL) нейтральных

(NV0) и отрицательно заряженных (NV-) центров окраски азот-вакансия на

575 нм и 637 нм, соответственно, а также их широкие полосы ФЛ с участием

фононов с максимумами примерно на 620 нм и 700 нм.

с

15

.Q

го 10

от с CD

Ns content, ppm.

ZPL NV" -#1 50

637 nnyv^ , -#2 90

V —#3 250

ZPL NV° .Л ! #4 600 .

575 nm f i ' J \ fj — _ ii ^^ ' i - jrJ\ ■

1

600 700 800

Wavelength, nm

Рисунок 3 - Спектры ФЛ (PL) 4-х образцов НРНТ-алмазов с различным содержанием NS (50, 90, 250 и 600 ppm), демонстрирующие бесфононные полосы (ZPL) нейтральных (NV0) и отрицательно заряженных (NV-) центров окраски на 575 нм и 637 нм, соответственно, а также широкие полосы ФЛ центров с участием фононов. Длина волны возбуждающего

излучения - 488 нм

Было найдено, что максимальная интенсивность ФЛ NV- центров наблюдается при концентрациях атомов азота в несколько раз меньших, чем

в случае стандартного КРНТ синтеза, а повышение концентрации N5, как это обычно предлагается, приводит не к повышению, а к уменьшению интенсивности ФЛ. Для ответа на вопрос о том, не связано ли это с возможным уменьшением эффективности образования люминесцирующих NV- центров с увеличением концентрации N5, были получены зависимости интенсивности ФЛ и концентрации NV- центров от концентрации N5 в диапазоне 50-600 ррт. Эти зависимости приведены на Рис 4, где видно, что увеличение концентрации N5 от 50 ррт до 90-100 ррт приводит к практически параллельному росту как концентрации КУ- центров, так и интенсивности их ФЛ. Дальнейшее увеличение концентрации N5 приводит к росту числа NV- центров, однако интенсивность их люминесценции резко уменьшается, образуя максимум в диапазоне 90-100 ррт. О таком квазирезонансном возрастании интенсивности ФЛ МУ- при концентрациях N5, намного меньших обычно используемым при стандартном синтезе НРНТ-алмазов, ранее не сообщалось. Наблюдаемое уменьшение интенсивности ФЛ NV- центров с увеличением их числа указывает на появление центров тушения их люминесценции с увеличением концентрации замещающих атомов азота.

Рисунок 4 - Интегральная интенсивность ФЛ (РЬ) и концентрация КУ- центров как функция концентрации N в НРНГ-алмазах

Полученные данные позволили сделать вывод о том, что интенсивность

люминесценции центров КУ- определяется конкуренцией между ее ростом

вследствие образования центров и ее тушением из-за безызлучательной

рекомбинации фотовозбужденных центров с участием структурных дефектов в кристаллической решетке алмаза, индуцированных атомами N5. Другими словами, интенсивность люминесценции отрицательно заряженных центров окраски азот-вакансия КУ- в HPHT-алмазах определяется не столько концентрацией центров, но, прежде всего, качеством кристаллографической структуры алмаза, которая при оптимальной концентрации вакансий определяется N5. Поэтому интенсивность люминесценции КУ- центров уменьшается, несмотря на увеличение их концентрации при увеличении числа атомов N5.

Этот вывод подтверждается данными о дефектности кристаллической структуры исследованных микрокристаллов алмаза и о наличии каналов безызлучательной дезактивации фотовозбужденных КУ- центров, которые были получены с помощью рамановской спектроскопии и кинетики затухания ФЛ NV- центров. На Рис. 5 приведены зависимости ширины характерной рамановской линии алмаза 1332 см-1 и времени затухания люминесценции центров в исследуемых образцах от N5.

Рисунок 5 - (а) Рамановская линия алмаза 1332 см-1 образцов #1 и #4 с самыми низкими и самыми высокими концентрациями N5. На вставке показана зависимость ширины полосы

от концентрации N5; (Ь) Интенсивность и время затухания ФЛ (РЬ) КУ- центров в зависимости от концентрации N в исследуемых образцах алмазов. На вставке показана

кривые затухания люминесценции образцов

Наблюдаемое пороговое уширение рамановской линии при концентрации N5 более 90 ррт указывает на начало формирования структурных дефектов.

Одновременное уменьшение времени жизни люминесценции и ее интенсивности свидетельствует о возникновении каналов безызлучательной релаксации фотовозбужденных центров, связанных с появлением дефектов, индуцированных азотом в кристаллической решетке алмаза.

Таким образом, впервые показано, что увеличение N5 в ИРИТ-алмазах приводит к возникновению связанных с азотом дефектов кристалла и появлением разупорядоченной углеродной структуры, через которые идет безызлучательная рекомбинация фотовозбужденных NV- центров. Показано, что интенсивность ФЛ NV- центров в ИРИТ-алмазах определяется конкуренцией между её ростом при образовании центров и их тушением за счет безызлучательной рекомбинации с участием структурных дефектов, индуцированных наличием атомов азота в кристаллической решетке. При этом максимальная интенсивность ФЛ NV- центров наблюдается при концентрациях атомов азота в несколько раз меньших, чем в случае стандартного HPHT синтеза, когда количество дефектов кристаллической структуры достаточно мало.

5. В четвертой главе диссертации приведены результаты исследования структурно-люминесцентных свойств наночастиц ДС-алмазов с люминесцирующими SiV центрами окраски

В результате динамического синтеза формируются поликристаллические частицы микронных размеров, состоящие из нанокристаллов кубического алмаза с размерами порядка 10-15 нм каждый. Хаотически ориентированные нанокристаллы тесно связаны друг с другом посредством различных углеродных фрагментов на границах между нанокристаллами, образуя межкристаллической слой толщиной 1-2 нм. Для практического использования микрокристаллы, обычно содержащие люминесцирующие NV- центры, подвергаются измельчению с последующим разделением на фракции с разным средним размером (2М) в диапазоне 251000 нм. Каждая из фракций содержит субмикронные поликристаллические

люминесцирующие алмазные наночастицы с колоколообразным распределением по размерам.

Нами было обнаружено, что в спектре люминесценции фракций ДС -алмазов, при синтезе которых использовались кремнийсодержащие материалы, наблюдается узкая (10-12 нм) интенсивная бесфононная полоса люминесценции на 738 нм, принадлежащая люминесцирующим SiV центрам окраски, интенсивность которой сильно зависела от размера поликристаллов ZM. Спектрально узкая люминесценция SiV центров весьма привлекательна для многих приложений нанофотоники, и, в частности, биологических, поскольку попадает в спектрально прозрачную область биологических тканей. Поэтому задача установления механизмов контролирующих интенсивность люминесценции ДС-алмазов с SiV центрами окраски являлась актуальной.

Методами рамановской и люминесцентной конфокальной микроскопии было исследовано 9 образцов фракций ДС-алмазов, с разными ZM. На Рис. 6 схематически показан процесс измельчения поликристаллов и список исследуемых образцов с диапазоном размеров и средним размером поликристаллов ZM.

Sample Size range (цт) ZM (nm)

25 50 75 90 180 210 350 750 1000

Рисунок 6 - Схема процесса измельчения поликристаллов и список исследуемых образцов фракций ДС-алмазов с диапазоном размеров и средним размером ZM

На рисунке 7 на примере фракции S5 показан типичный спектр вторичной

эмиссии образца, содержащий наряду с широкими полосами люминесценции

NV центров узкую полосу ФЛ SiV центров на 738 нм и рамановский спектр

образца.

Рисунок 7 - Спектр вторичной эмиссии поликристаллической алмазной фракции S5 (Zm=180 нм) при возбуждении излучением 488 нм. Показана деконволюция спектра ФЛ на

полосы гауссовой формы. Окружность показывает область рамановского спектра. Вставка: сравнение спектров, возбуждаемых при 488 нм (красная линия) и 514,5 нм (синяя линия), показывающее сдвиг рамановских полос с длиной волны возбуждения

Деконволюция полученного спектра с использованием контуров гауссовой формы позволяет выделить полосу ФЛ SiV-центров и спектр рамановского рассеяния образца в виде, достаточном для количественного анализа. На рисунке 8 приведен репрезентативный набор полос ФЛ SiV-центров для фракций поликристаллов с разным средним размером ZM (Рис. 8а), а также зависимость интенсивности ФЛ SiV-центров от ZM, демонстрирующую квазирезонансное возрастание интенсивности при ZM около 180 нм (Рис. 8б).

Список литературы диссертационного исследования кандидат наук Рунова Марианна Васильевна, 2019 год

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ПРИЛОЖЕНИЕ А (обязательное) Оттиски статей

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Cite this: RSC Adv., 2016, 6, 51783

Received 11th April 2016 Accepted 20th May 2016

DOI: 10.1039/c6ra09317e

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Size-dependent Raman and SiV-center luminescence in polycrystalline nanodiamonds produced by shock wave synthesis

K. V. Bogdanov,a V. Yu. Osipov,b M. V. Zhukovskaya,a C. Jentgens,c F. Treussart,d T. Hayashi,e K. Takai,f A. V. Fedorova and A. V. Baranov*a

Size-dependent structural and luminescent properties of the diamond polycrystals produced by shock wave synthesis followed by grinding and separation into fractions of different polycrystal median sizes (25-1000 nm) are studied by comparative Raman and luminescence spectroscopy. The intense 738 nm narrow band luminescence of the SiV-centers are observed for all fractions. The SiV luminescence intensity has a maximum at the median size of about 180 nm that is controlled by competition between deactivation of the SiV-centers by defects in the diamond nanocrystal lattice and that controlled by nonradiative recombination centers in the volume of the intergranular layers.

Introduction

It is well-known that synthetic diamonds of different shapes and sizes can be produced by different methods, including CVD techniques and static synthesis at high pressure (up to 7 GPa) and high temperature (up to 2200 °C) which have found broad applications in the industry including nanophotonics.1 One of most effective methods of large-scale synthesis of diamond powder is the shock wave synthesis of diamond developed several decades ago by DuPont in the US.2-5 In this method, the direct conversion of graphite into a diamond-containing phase takes place for a very short period of time (z 200 ms) during the detonation of explosive material outside a metal tube filled with powder graphite.3'6 The pressure and temperature during the process may be as high as ~50 GPa and 1100 °C, respectively. Such high dynamic loads are responsible, inter alia, for the sintering of diamond nanocrystals into polycrystalline particles. An important feature of this synthesis is the use of metal powder with high thermal conductivity (copper as a rule), which does not form carbides, and is mechanically mixed with a precursor - graphite powder. Copper transmits well hydrostatic pressure in the fluid state at high pressures and temperatures and provides effective heat dissipation after synthesis,

aITMO University, Saint-Petersburg 197101, Russia. E-mail: [e-mail] bIoffe Physical-Technical Institute, Saint-Petersburg 194021, Russia. E-mail: osipov@ mail.ioffe.ru

cMicrodiamant AG, Lengwil CH-8574, Switzerland

dLaboratoire Aimé Cotton, CNRS, Univ. Paris-Sud, ENS Cachan, Universite Paris Saclay, 91405 Orsay, France

eFaculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

fDepartment of Chemical Science and Technology, Hosei University, 3-7-2, Kajino, Koganei, Tokyo 184-8584, Japan

thereby preventing reverse graphitization of the nanodiamonds formed.5

Diamond powders manufactured by this technique using a thick-walled steel pipe driver are produced currently, in particular by Microdiamant USA, and commercially distributed by L. M. Van Moppes & Sons SA (Geneva, Switzerland) under the trade name Super Syndia™ SSX,7 and earlier (1976) were known under the trade mark Mypolex™ by E. I. Du Pont De Nemours.5 According to the manufacturer, after initial chemical treatment and deep removal of non-diamond phase, the product is a powder of polycrystalline diamond particles of micron size (10-60 mm) consisting of individual diamond nanocrystals with sizes not exceeding 20-25 nm each.7 Differently oriented diamond nanocrystals are tightly bound together by means of the spliced edges of the crystal lattices and by covalent bonds of shorter atomic groups existing on the grain boundaries. It is also assumed that the particles of diamond polycrystals may contain, in addition to cubic diamonds, up to 50 wt% of hexagonal diamonds including lonsdaleite,8 which may be formed inside the intergranular layers of thickness of up to several lattice constants that exist at the interface of cubic diamond nanocrystals.9 The presence of a dense "sintered" polycrystalline structure makes this material similar to so-called "Carbonado" and other granular diamond-containing materials of meteoric origin.2'10'11 By grinding and fractionation of the as-synthesized polycrystalline diamonds, powders of polycrystals with smaller sizes (25-1000 nm) suitable for finishing the polishing of hard materials can be obtained. Fig. 1 illustrates schematically the fragmentation of a large as-synthesized polycrystalline diamond particle into smaller particles during milling.

It has been noted that the rapid synthesis and direct conversion of graphite to diamond in air results in the

grinding

Fig. 1 Schematic representation of the fragmentation of a large as-synthesized polycrystalline diamond particle into smaller particles during milling. The intergranular layers between the diamond nanocrystals are shown by solid black lines.

formation of fairly large numbers of defects (dangling C-C bonds and vacancies) and in the presence of impurity atoms in the crystalline lattice of polycrystalline diamond produced by the synthesis method mentioned above. This leads to the appearance of NV- and SiV-centers, for example, which can be identified by electron paramagnetic resonance12 and luminescence spectroscopy. The presence of luminescent NV-centers in diamond polycrystals of meteoric origin (carbonado) has been reported.13 Other impurities in the charge used for shock-wave synthesis (Si, Cr, or Cu) may also incorporate into the diamond lattice as substitutional defects, leading to the appearance of luminescen t color centers such as SiV. It was proposed that the intergranular layers between the diamond nanocrystals are the major locations of paramagnetic defects and various color centers. Some parameters of the nanodiamond synthesis by shock wave and of the subsequent chemical treatments may influence the content in luminescent NV- or SiV-centers. To investigate such dependencies, structural characterization of the nanodiamond powders and diagnostics of luminescence centers content are instrumental. To this aim, Raman and luminescence spectroscopy are particularly adequate as they permit the quick and contactless investigation of the samples without disturbing the structure of the constituent materials. Moreover, these techniques are already widely used to determine the structural features of different nanocarbon materials, including diamond14-22 and graphite23-27 nanoparticles obtained by different methods.

In the present work, optical spectroscopy methods (Raman and luminescence analysis) were employed for investigating polycrystalline diamond powders obtained by shock wave synthesis and subjected to grinding followed by separation into fractions of different polycrystal median size (ZM) in the 25-1000 nm range.

Experimental

Materials

We have investigated powders of polycrystalline diamonds produced by shock wave synthesis. Polycrystals composed of tightly connected differently oriented diamond nanocrystals with mean sizes of 10-15 nm were formed by merging the

boundary areas of the crystal lattices of adjacent nanocrystals and short covalent bonds of atomic groups of different carbon fragments present on the boundaries between the nanocrystals.

Polycrystalline nanodiamonds of the Super Syndia™ SSX series were provided by Van Moppes & Sons (Geneva, Switzerland).7 This nanodiamonds were treated by mild low-energy stainless steel bead milling of large starting as-synthesized polycrystalline microdiamonds (ash content < 0.1 wt%) obtained after shock wave synthesis and extensive acid cleaning (including aqua regia and HF) of the synthesized product. The raw graphite material used as a precursor for such synthesis contained 97.5 wt% of carbon, 0.5 wt% of moisture, and approximately 2 wt% of ash, where the content of SiO2 is around 0.8-1 wt%. The SiO2 present in graphite is the origin of the silicon present in the synthesized diamond product. According to ICP elemental analysis the graphite content of other elements is as follows: Fe - 9000 ppm, Si - 4870 ppm, Ca - 790 ppm, Al -260 ppm, Cu - 170 ppm, Mg - 150 ppm, Ni - 50 ppm, Ti - 30 ppm, Mn - 30 ppm. The average content of Si over batches in raw polycrystalline diamond powder is ~100 ppm as measured by XRF, although Si cannot be considered as a characteristic contaminant of products of the SSX series. The goal of the standard bead milling was to produce polycrystalline microdiamonds with selected size ranges (varying in the wide range from <1-2 to 10-20 mm) by avoiding loss as fine diamond dust (both submicro- and nano-diamonds). The composition of the steel milling beads includes iron (~70%), Cr (17.0-19.5%), Ni (8.0-10.5%), Mn (#2%), Si (#1%), and other remaining elements.

The studied set of polycrystalline diamond powders with a median size of diamond polycrystals from 1000 to 25 nanometers was obtained by step-by-step fractionation of once milled starting powder. Size fractionation of the submicron dust was accomplished by centrifugation or sedimentation in water the micron fraction with median particle size 1-2 microns. This process gave nine Super Syndia SSX samples (S1-S9), each containing submicron polycrystalline diamond nanoparticles with a bell-shaped size distribution. The nanoparticle size ranges of the samples and corresponding median particle size (ZM) is presented in Table 1.

Each fraction of Super Syndia SSX nanodiamonds was subsequently extra-purified in acid mixture in autoclave at

Table 1 Size range of the Super Syndia SSX samples obtained by size fractionation (see text) and corresponding median particle size (ZM) according to the Van Moppes & Sons Product Catalog7

Sample Size range (mm) Zm (nm)

S1 0-0.05 25

S2 0-0.10 50

S3 0-0.15 75

S4 0-0.20 90

S5 0-0.35 180

S6 0-0.50 210

S7 0.25-0.5 350

S8 0.5-1.00 750

S9 0.75-1.25 1000

~160 °C for removal traces of metals and graphitic phase and then washed in rinsing water several times. According the XRF data, the main pollutants in the extra-purified SSX nanodiamonds among transition metals are Fe (50-170 ppm) and Cu (6-17 ppm). The content of silicon in the six finest fractions of the samples (S1-S6) is on the 5-6 ppm level, while the content of other metal pollutants (except alkaline metals) is below 1-3 ppm.28

Characteristic HRTEM images of the as-fabricated SSX0.05 polycrystalline diamond fraction (smallest grade powder) are shown in Fig. 2. JEOL JEM-2100F equipped with CEOS Cs corrector operated at 80 keV was used for TEM observation, and Gatan Digital Micrograph 1.71 was used for FFT and image analysis. Diamond nanocrystal domain was analyzed by using the method similar to ref. 29, which uses the FFT spots to determine the domains. The crystalline lattice of cubic diamond crystallites together with a few twinning boundaries are clearly seen on the images. No evident signs of lonsdaleite or other hexagonal-type diamond phases were seen in the HRTEM images, but there may exist such a phase in very small amount, in intergranular layers.

The X-ray diffraction (XRD) pattern of the S1 sample obtained with a Smart Lab X-ray Diffractometer (Rigaku Co., Japan) using CuKa radiation (l = 1.541 A) is shown in Fig. 3. The (111), (220) and (311) reflections from cubic diamond phase together with weak (002) reflection from graphite phase are clearly seen. Weak shoulders on the left and the right sides of the (111) peak come from the stacking faults and/or twinning boundaries inside the diamond lattice as shown in ref. 28. The X-ray coherent scattering size for cubic diamond crystallites obtained on the base of analysis (220) and (311) reflections was estimated as ~7-8 nm. This value matches well with the size of individual crystallites of cubic diamonds observed in the high resolution TEM images.

Methods

A micro-Raman spectrometer "inVia" (Renishaw, UK) equipped with a Leica microscope and a CCD detector cooled to —70 °C

.o

s—

ro

S1

Cu Ka (111)cd purified

V (002) g /

(220}c d

A (311)c d (400)t d

5 25 45 65 85 105 125

20(deg.)

Fig. 3 Wide angle XRD pattern of the S1 sample. The (111), (220), (311), and (400) reflections from cubic diamond (d) phase and (002) from graphite phase (g) are shown. The CuKa radiation at 1.541 A was used.

was used for the acquisition of Raman spectra of the samples in backscattering geometry and with a spectral resolution of

2 cm—1. The Raman modes were excited by radiation of 488 nm and 514 nm from an argon-ion laser with power lower than 20 W cm—2 on the sample surface. A standard scheme of focusing of the laser beam with a 50 x and NA = 0.75 microscope objective allows to collect scattered light from a spot of ~2 mm diameter. However, in this study the recording of the Raman spectra was performed with use of a new technique for focusing the laser radiation and collection of the scattered light, the StreamLine™ Plus (Renishaw, UK). This technique significantly reduces the incident light power density on the sample due to its focus on a ~2 x 60 mm2 stripe. At the same time, there is no loss of measurement sensitivity since the whole matrix of the CCD camera is used to record the spectrum. The Stream-Line™ Plus technique allows not only to avoid the influence of sample heating on the parameters of the Raman and luminescence

(a) (b) (c)

Fig. 2 (a)-(c) Characteristic HRTEM images of the S1 polycrystalline diamond fraction. Areas shaded in different colors correspond to the individual crystallites with different orientation. Scale bars of 4 nm are shown.

spectra but also obtaining averaged spectra from a sufficiently large volume of the sample, which is important for samples with micron-scale structure heterogeneities. Luminescence spectra of the samples were obtained with the "inVia" spectrometer together with Raman spectra, with one scan recording Raman and luminescence spectra from exactly the same portion of the sample. For a correct comparison of the intensities of Raman and luminescence bands the spectra were normalized to the spectral sensitivity of the spectrometer measured with a black-body radiation unit. All measurements were done at room temperature.

The samples for the optical measurements were prepared as follows. Powders of polycrystal diamond particles of different fractions were pressed into cylinders 5 mm in length and 2 mm in diameter using a special Renishaw sealing press. Raman and luminescence spectra were recorded from the end of cylinder.

The studied fractions of polycrystalline diamond particles of various sizes are powders of color varying from light gray to black due to the presence of various amounts of amorphous carbon formed during grinding. The presence of amorphous carbon leads to different absorption of incident light as well as Raman and luminescence spectra, which impedes the quantitative comparison of their intensities and, therefore, of the concentration of different allotropic forms of carbon and luminescence centers. For the quantitative comparison of the emission intensities of the various factions, potassium nitrate (KNO3) powder was added to diamond powders in order to use the intensity of the KNO3 characteristic Raman band at ~1049 cm-1 as a reference. Firstly, KNO3 was mixed homogeneously to diamond in a weight ratio of 5 : 1 (KNO3 : diamond), and then the mixture was pressed into cylinders as described above, yielding a weakly absorbing solid solution of diamond polycrystal particles in KNO3 with volume ratio of ~1 : 8.5.

Results

A typical example of the secondary emission spectrum of the sample S5 (ZM ~ 180 nm) in the 500-800 nm spectral range is shown in Fig. 4. The spectrum was excited by 488 nm radiation. The spectrum shows photoluminescence (PL) and Raman bands observed in the spectra of all fractions of polycrystalline diamonds. The inset in the figure shows the comparison of this spectrum with the one obtained by excitation at 514.5 nm wavelength, that allows to clearly distinguish (i) a broad band luminescent background, (ii) a narrow PL line at 738 nm and (iii) Raman bands. An analogous luminescence broadband background is usually observed in secondary emission spectra of diamond particles produced both by detonation techniques17 and CVD methods18'19'26 and is attributed to a broad orange-to-red PL caused by NV centers and H3 centers at wavelengths around 520-530 nm in distorted diamond lattices. The 738 nm narrow line observed in the spectra of all fractions is attributed to PL from SiV colour centers formed by the intercalation of silicon atoms into the crystal lattice of diamond particles.19'21'22

Fig. 4 shows also an example of the deconvolution of the broad luminescence background. This deconvolution was performed for the spectra of all samples studied and used for

Fig. 4 Spectrum of secondary emission of the S5 polycrystalline diamond fraction (ZM = 180 nm) for excitation wavelength of 488 nm. The deconvolution of the broad PL background by Gaussians is shown. The pink circle shows the region of the first order Raman spectrum of the sample. Inset: comparison of the spectra excited at 488 nm (red line) and 514.5 nm (blue line). The constancy of the PL bands and shift of the Raman bands with excitation wavelength is marked by vertical dotted lines.

subtracting the luminescence background, a procedure necessary for the quantitative comparison of the Raman and SiV luminescence band intensities.

An example of the spectrum after subtraction for the S4 fraction is shown in Fig. 5a. By construction, this subtracted spectrum only contains (i) the Raman lines from the nano-diamond sample, and (ii) the 738 nm narrow line attributed to PL from SiV colour centers.

Fig. 5b depicts the 738 nm PL band for different fractions of the polycrystalline diamond particles after the normalization of its maximum to KNO3 1049 cm-1 Raman line. Fig. 5c shows the dependence of 738 nm PL band intensity on ZM, exhibiting a maximum for fraction S5 (ZM = 180 nm).

Fig. 6a displays a representative set of Raman spectra (in the 1100-2000 cm-1 range) of samples of diamond polycrystals of different median size containing the main features of Raman scattering. Like for Fig. 5b, the spectra were obtained by first subtracting the continuous luminescence background from the raw spectra, and then normalize the intensity by the one of KNO3.

Raman band at 1049 cm-1. Since the Raman intensity drops with decreasing size of the diamond polycrystals, we were not able to obtain Raman spectrum with for the sample S1 (ZM = 25 nm) due to a too low signal-to-noise ratio.

Raman spectra of different samples show the same set of bands. The diamond Raman band at about 1330 cm-1 as well as well-known Raman bands of disordered nanocarbon struc-tures23-27 dominate all spectra. The Raman bands of diamond phase, marked as DIA, are comparable in intensity to the other lines in the spectrum. However, keeping in mind that the Raman cross-section for diamond at 488 nm excitation wavelength is smaller by more than one order of magnitude than that for sp2 carbon nanostructures we can conclude that the structure of the studied samples is dominated by the diamond

Fig. 5 (a) An example of the spectrum after PL background subtraction for the S4 fraction; (b) representative set of Gaussian fits of SiV-center PL bands at 738 nm for different fractions of polycrystalline diamonds after normalization to KNO3 Raman line intensity. (c) Size dependence of the integral intensity of the SiV-center luminescence band. Excitation laser wavelength: 488 nm.

Fig. 6 (a) Representative set of Raman spectra of the polycrystalline diamond fractions S2-S9 for excitation wavelength 488 nm. (b) Size dependence of the diamond Raman peak intensity (/DIA). (c) Ratio of the intensities of amorphous carbon (/A) and diamond Raman bands (/A//DIA).

phase. Other bands in the spectra indicate the presence of disordered forms of carbon compounds. The spectra shown in Fig. 6a exhibit bands at ~1350 cm-1 (D), ~1500 cm-1 (A), ~1587 cm-1 (G), and ~1625 cm-1 (D0) which are characteristic for disordered nanocarbon.23-25'27 The presence of the G-band in the spectra points out to the presence of sp2-bonded C atoms. The D and D0 bands correspond to the breathing vibration of aromatic rings in the carbon network and their intensities are proportional to the degree of structural disorder in graphite-like structures.26'27 The broad A band at about 1500 cm-1 is usually assigned to different kinds of amorphous carbon structures, including short atomic groups on different carbon fragments,30'31 and its intensity can serve as an indication of the relative content of amorphous carbon in the samples.27'32

A simple comparison of the Raman spectra of the fractions shows a general tendency of increased intensity of graphite-related bands with respect to diamond bands with decreasing size of the polycrystalline particles. A more detailed analysis allows finding other important differences. In particular, more pronounced size-dependence is observed for the DIA-band intensity (Fig. 6b), which remains virtually unchanged when ZM decreases from 1000 to 180 nm but drops about 3 times with further reduction of the particle size down to 50 nm. Analogous size-dependence is also observed for the amorphous carbon band intensity (1A) that results in an about

5-fold reduction of the ratio of intensities of amorphous carbon and diamond Raman bands (/A//DIA) in the same range of the particle size.

Discussion

According to the Microdiamant AG data5'7'28 the as-synthesized polycrystalline diamond particles consist of cubic diamond nanocrystals of size about 10-15 nm with 1-2 nm thick intergranular layers of less ordered carbon material between the diamond nanocrystals. The relative volume of the intergranular layers may be estimated to about 10% of that of the poly-crystalline diamond particle. Mechanical fragmentation of the as-synthesized diamond polycrystals into particles of smaller sizes destructs these intergranular layers and, probably, damages the diamond nanocrystals. It is expected that this does not increase the number of the luminescent centers like SiV- or NV-centers formed at shock wave induced conversion of graphite to diamond. However, we observed unexpected ZM dependence of the SiV PL accompanied by changes of the Raman spectra of the fractions. These changes in Raman and PL spectra can be explained in the framework of the following scenario.

There are two diamond crystal phases in the samples: cubic diamond in the nanocrystals and hexagonal diamond, lonsda-leite, located in the intergranular layers. The DIA Raman band

integral intensity reflects the content of diamond crystal phases in the form of cubic diamond in the nanocrystals and hexagonal diamond, lonsdaleite, in the intergranular layers.9 Since the relative volume of the intergranular layers represents only ~10%, this band can be assigned mainly to the Raman signal from cubic diamond nanocrystals. However, some evidence for the presence of lonsdaleite in the intergranular layers comes from the up-shift of the DIA band from 1328 cm-1 to 1331.2 cm-1 observed with decreasing ZM from 1000 to 50 nm. Indeed, the Raman shift of the lonsdaleite (~1325 cm-1)33 is smaller than that of cubic diamond (1332 cm-1) and the DIA band may consist of overlapping diamond and lonsdaleite bands unresolved spectrally in our experiment. Then, a reduction of volume of the intergranular layers containing lonsdaleite with decreasing ZM will result in a decrease of the lonsdaleite Raman signal and an up-shift of the DIA band. At the same time, the removal of lonsdaleite cannot explain the about three-fold reduction of the DIA band intensity observed for fractions from S5 to S2 (Fig. 6b) since its content in the fractions does not exceed ~10%. This threshold reduction of the DIA band intensity indicates the increasing presence of damaged cubic diamond nanocrystals in the fractions of ZM smaller than 180 nm. Correspondently, the content of disordered amorphous carbon structures in the fractions increases proportionally, resulting in an about five-fold reduction of the ratio of intensities of amorphous carbon and diamond Raman bands (1A/1DIA), as it shown in Fig. 6c.

As a results of the analysis of the Raman spectra of the fractions of diamond polycrystals of different ZM we can conclude that S9-S5 fractions (ZM decreasing from 1000 nm to 180 nm, respectively) consist of polycrystals of as-synthesized 10-15 nm cubic diamond nanocrystals with a relatively small number of defects. These fractions have intergranular layers probably containing lonsdaleite and some amount of disordered carbon structures due, in particular, to the destruction of the intergranular layers. In this range of polycrystal size we have not found evidence of the damage of diamond nanocrystals, but the damage of the intergranular layers with the increase of amorphous carbon content has been observed. For the fractions from S5-S2, where ZM is smaller than 180 nm, a strong decrease of the intensity of the DIA band indicates the onset of damage of the cubic diamond nanocrystals with the appearance of disordered carbon structures, which increases with decreasing ZM.

The size dependences of the SiV-center luminescence band at 738 nm shown in Fig. 5c differ from that of the DIA band intensity (Fig. 6b) by the presence of a clear maximum at ZM = 180 nm. Fig. 5c shows that decreasing the nanocrystal size from 1000 nm to 180 nm (S9-S5 samples) leads to an increase of SiV luminescence intensity. As mentioned earlier, the size decrease is accompanied by a decrease of volume of intergranular layers. Therefore, SiV PL intensity increase suggests that the SiV-centers are localized mainly in the diamond nanocrystals, and not in the intergranular layers. This conclusion is supported by the parallel decrease of the SiV-center PL intensity and the DIA band Raman intensity for the S5-S2 series, where increasing damage of the nanocrystals takes place. It has been shown that for CVD diamonds with luminescent SiV-centers an increasing

number of defects inside the crystals leads to a parallel decrease in the diamond Raman and SiV-centers PL intensities.19 The latter was associated to the formation of recombination centers, opening non-radiative decay channels in disordered regions. At the same time, the decrease of the PL intensity upon increase of size from 180 nm to 1000 nm (S5-S9 series) requires additional assumptions. Since the most evident change in the structure in samples when moving from fraction S5 to fraction S9 is an increase in the volume fraction of the intergranular layers, it is natural to assume that nonradiative recombination centers simply exist there. Then, increasing the intergranular layer volume in the S5-S9 series leads to a decrease of the SiV-center PL intensity. Therefore, the maximum observed in the size dependence of the PL intensity should be the result of competition between processes of deactivation of the luminescent SiV-centers due to the formation of defects in the cubic diamond nanocrystals and due to the reduction of nonradiative recombination centers in the volume of the intergranular layers. Typically, efficient non-radiative recombination requires a nanometer range proximity of luminescent and recombination centers. The quenching of SiV-center PL by recombination centers located in intergranular layers observed in our experiments indicates that a large number of the SiV centers is located near the surface of the diamond nanocrystals of 10-15 nm primary size. The nature of the recombination centers is not well established yet and the size dependence of the PL decay time should be analyzed for a better understanding of the PL quenching mechanism. Nevertheless, we have shown that there exists an optimum median size of the diamond polycrystals produced by shock wave synthesis for which a maximum intensity of the SiV-center luminescence takes place.

Let us note that silicon is an embedded substitutional impurity in polycrystalline diamond which appears unintentionally in the diamond lattice during the shock wave synthesis. This is because precursor graphite used for synthesis contains silica on the 0.8-1 wt% level. Thus, explosive-induced graphite-to-diamond phase transformation of such silica containing graphite seems very effective for making diamond polycrystals doped by silicon and having vacancies inside the crystallites at the same time. To, the best of our knowledge, apart the so-called meteoritic diamonds, it is the first report of the presence of SiV-color centers in diamonds produced by dynamic synthesis with 100 ms short synthesis time. It means that this method may be used for further intentional doping of diamond polycrystals by substitutional silicon for the purpose of enrichment the diamond polycrystals in SiV-centers. The use of special silicon-containing additives to precursor graphite in the solid, liquid or even gas form (instead of silica) should promote the doping and make the technology of obtaining polycrystalline diamonds with SiV more efficient and controllable. We envision a 10-30 fold increase of SiV concentration in polycrystalline diamonds by means of optimization the type of silicon-containing additive to graphite.

Conclusion

Raman and luminescence spectroscopy were used for investigating polycrystalline diamond powders produced by shock

wave synthesis followed by grinding and separation into fractions of different polycrystal median size in the range 25-1000 nm. The TEM data showed that the diamond polycrystals consist of 10-15 nm nanocrystals with thin (2-3 nm) intergranular layers of partially ordered carbon material between the nanocrystals. A distinctive feature of the diamond powders studied is the presence of a narrow intense PL band of SiV centers at 738 nm. It is the first case of SiV-color centers found in diamonds produced by short duration dynamic synthesis. The analysis of the size dependencies of Raman bands showed that in the 1000-180 nm median size range the diamond poly-crystals consist mainly of 10-15 nm cubic diamond nano-crystals with a fairly good crystalline structure and intergranular layers which, most probably, contain lonsdaleite and disordered carbon material. When the median size decreases from 180 nm to 25 nm, the damaged diamond nanocrystals fraction grows up, along with an increasing amount of disordered carbon structures.

As for the luminescent SiV-centers, related to silicon impurity incorporated as substitutional impurity in the diamond lattice during the shock wave synthesis, our data allows to conclude that the majority of these centers are located inside the diamond nanocrystals. SiV PL intensity depends on the polycrystalline powder mean size and has a maximum at size z180 nm. The latter is the result of a competition between two SiV non-radiative decay channels with opposite size dependencies. One channel was tentatively associated to the formation of crystalline defects inside the 10-15 nm primary size nanocrystals upon size reduction of the polycrystalline powder. The concentration of such defects increases when the poly-crystal size decrease. The other non-radiative channel, was tentatively related to the presence of recombination centers in the volume of the intergranular layers. The concentration of these recombination centers is expected to decrease when the polycrystal size decreases, hence the observed SiV PL maximum at a compromised polycrystal size of 180 nm. The nature of the recombination centers is not established yet and requires further investigations.

Acknowledgements

Authors express their gratitude to Dr Jean-Paul Boudou (CNRS, France) for his help in chemical purification of nanodiamond samples and Christophe Bangerter from L. M. Van Moppes & Sons SA (Geneva, Switzerland) for help in elemental analysis and sample supply. K. T. was supported by JSPS KAKENHI Grant No. 26107532. V. Y. O. was supported by the Russian Scientific Foundation (project N 14-13-00795). K. V. B. and A. V. B. acknowledge financial support from the Ministry of Education and Science of the Russian Federation, Government Assignment No. 3.109.2014/K.

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THE JOURNAL OF

PHYSICAL CHEMISTRY

Article

pubs.acs.org/JPCC

Does Progressive Nitrogen Doping Intensify Negatively Charged Nitrogen Vacancy Emission from e-Beam-Irradiated Ib Type High-Pressure-High-Temperature Diamonds?

Alexander I. Shames,*^® Vladimir Yu. Osipov,*^ Kirill V. Bogdanov,§ Alexander V. Baranov,§ Marianna V. Zhukovskaya,§ Adam Dalis," Suresh S. Vagarali," and Arfaan Rampersaud^

^Department of Physics, Ben-Gurion University of the Negev, Be'er-Sheva, P.O. Box 653, 8410501, Israel ^Ioffe Physical-Technical Institute, Polytechnicheskaya 26, St. Petersburg, 194021, Russia §ITMO University, Kronverksky Propeckt 49, St. Petersburg, 197101, Russia "Sandvik Hyperion, 6325 Huntley Road, P.O. Box 568, Worthington, Ohio 43085, United States ^Columbus Nanoworks Inc., 1507 Chambers Road, Columbus, Ohio 43212, United States

* Supporting Information

Low Nitrogen Content High Nitrogen Content

g = 4.27 A "Forbidden"

J \ line

ABSTRACT: Micron-sized samples of Ib type high-pressure-high-temperature diamonds synthesized with low and high substitutional nitrogen content and high energy e-beam irradiated to form luminescent negatively charged nitrogen-vacancy (NV-) centers are studied by X-band electron paramagnetic resonance (EPR), photoluminescence (PL), and Raman techniques. High nitrogen doping leads to the appearance of paramagnetic centers characterized by strong interactions between unpaired spins of substitutional nitrogen defects. Actual concentrations of paramagnetic substitutional nitrogen and NV- centers were obtained by EPR. The intensity of the PL emission from NV- centers was analyzed as a function of the content of NV- centers. We report

that the NV- PL intensity is controlled by both the content of NV- centers and the presence of nitrogen-related crystal defects/ imperfections. Increasing the nitrogen content increases the structural imperfections, which are responsible for the appearance of additional nonradiative recombination centers and significant intensification of PL quenching. It is suggested that PL intensity may be optimized by the appropriate choice of nitrogen doping and irradiation fluence.

g = 4.27 A "Forbidden"

line

156 158 55 h?

500 |im ■ Itfà**

e-beam fluence 7x1018 e /cm2

1. INTRODUCTION

Diamonds containing negatively charged nitrogen-vacancy (NV-) color centers are within the scope of recent scientific and technological interests as prospective molecular photon emitters for quantum telecommunication or qubits in quantum computing.^2 The fairly long spin coherence time of their electron spin-triplet ground state at room temperature makes NV- centers attractive objects for both research and technological applications.3,4 In addition, NV- centers may be effectively used as precise optically detected magnetic resonance (ODMR) sensors in magnetometry of superweak fields, single molecular nuclear magnetic resonance (NMR), and super-high-resolution NMR imaging (MRI) as well as single-spin electron paramagnetic resonance techniques. Submicron- and nanosized NV- centers containing diamonds are under development as sensors for monitoring disease, as drug delivery vehicles for the treatment of cancer, and, at the cellular and subcellular levels, as unique biomarkers for the early detection of cancer and cardiovascular and neurological diseases.5-8 The features of NV- center diamonds that are important for their use in biological systems include their

remarkable photostability during persistent and intense optical pumping, absence of blinking and degradation, large Stokes shift, and their biocompatibility.9'10 Typical NV- emission within the spectral band over 600 nm lies close to the window of transparency for biological tissues and, thus, may be effectively used for optical biolabeling of living objects including subcutaneous viscera visualization.

The nitrogen-vacancy (NV) center is an impurity within the diamond lattice in which a nitrogen atom is adjacent to a vacancy. Nitrogen is the most common inherent, substitutional impurity in diamond crystals of both natural and artificial origins, whereas single (nonaggregated) vacancies are induced purposefully. Usually this involves bombarding pristine diamond crystallites with high energy (up to 10 MeV) electron or proton beams to create vacancies within the diamond lattice. Irradiated diamonds are then annealed at 850-900 °C for several hours to mobilize the vacancies, which are captured by

Received: December 21, 2016 Revised: February 13, 2017 Published: February 14, 2017

ACS Publications © 2017 American Chemical Society

5232

substitutional nitrogen centers, resulting in stable NV- centers. Additional details of NV- fabrication for both micron-size and nanometer-sized diamond crystals have been described in refs 11 and 12. The electron paramagnetic resonance (EPR) properties of NV- centers in diamonds, from various origins, were first described by van Wyk and co-workers13 and since have been designated as W15 paramagnetic centers. Such a center has electronic spin S =1, and its EPR spectra are characterized by a set of multiple lines that correspond to so-called "allowed" (AMS = 1) and "forbidden" (AMS = 2) microwave transitions between Zeeman levels of triplet paramagnetic centers.

Practical applications of NV- center diamonds, especially for biological imaging, demand optimization of the intensity of NV-related photoluminescence (PL). The simplest way of creating very bright fluorescent diamonds is to just increase the substitutional nitrogen content within a diamond to increase the NV- density. Such an increase in the concentration of NV-centers within a diamond should lead to significant increases in the intensity of its optical fluorescent emission. However, increasing the substitutional nitrogen content in diamond crystal also causes crystal imperfectness due to formation of undesirable (for the aforementioned application) defects of Atype (close N—N pairs), B-type (4N-V complexes), H3 centers (N—V—N in a neutral charge state), clusterized nitrogen (tens of nitrogen atoms accumulated within nanoareas of ~3—4 nm sizes), etc.14-16 All these defects, together with vacancy clusters, which form from the accumulation of single vacancies during irradiation, milling and other treatments,15 may work as effective quenchers of useful PL emission from NV- centers.

Fabricating fluorescent diamonds that are exceptionally bright due to high NV- center densities is not trivial, and simply increasing substitutional nitrogen density might not, a priori, lead to desirable increases in NV- emission. Indeed, one opposing process that might accompany any increase in NV-density is an increase in the types of fluorescence quenching processes. As a rule, defects located a few tens of nanometers from the emitting center (NV- in this case) effectively quench PL by several mechanisms. Quenching mechanisms could include capture of photoexcited electrons by different defects in the crystal lattice and other nonradiative channels of energy dissipation, such as Foster resonant energy transfer (FRET).17'18 Defects within the diamond crystal lattice that quench PL can include intergrain boundaries, dislocations, multivacancies complexes, microhollows, and a crystallite surface. All these defects increase local inhomogeneities and degradation of the crystal's optical properties.

Quenching mechanisms that modulate NV- emission intensity still have not been reliably proven. The observation of a plateau-like maximum in NV- emission on reaching the nitrogen content around 150-200 ppm has been reported.19 However, there are still no reliable data on the dependence of the emission intensity on the content of NV- centers responsible for such an emission in micron-sized crystals.

Synthetic diamond crystals manufactured by high-pressure-high-temperature (HPHT) techniques usually contain ppm levels of nitrogen which are unintentionally adsorbed from the atmosphere by the micrographite precursors and metal catalysts used for HPHT synthesis. Nitrogen concentrations higher than atmospheric levels can be achieved by adding additional inorganic nitrogen-containing additives {like sodium azide (NaN3) or other azides and nitrides [P3N5, BaN3, Ba(N3)2,

Fe3N]} or even additives of organic type (like C3N6H6) to the precursors.20-22 Thus, it is fairly simple to manufacture synthetic microcrystals containing up to 2000 ppm of nitrogen or higher.23,24 However, usually these crystals are morphologically defective, having opaque, black, or dark green colors, characterized by strong optical absorption, and, thus, are useless for optical applications, although fabrication of transparent green and dark green crystals of acceptable quality is also possible in principle.21 Nitrogen content in diamond crystals may be checked by means of EPR ("isolated" neutral C-centers)25 and IR (A-, B-, and C-centers) spectroscopies.23,24 The EPR technique is especially useful for studying crystallites of high structural and optical quality with low (up to 300 ppm) C-center content, regardless of the actual crystallite size (from tens of nanometers to millimeters).2^26 This is because the EPR pattern of substitutional nitrogen in the diamond lattice has a unique triplet structure related to the hyperfine interaction of the magnetic moment of 14N nuclei with spin 1/2 of the unpaired electron orbital of nitrogen.

In this work, we present our EPR and FTIR analysis of NV--center-containing diamonds characterized by different levels of nitrogen within the microcrystals. Microcrystals were prepared by HPHT synthesis and irradiated with high-energy e-beams for different times. We show that these samples vary by NV-emission intensity, defectiveness of the crystal structure, and actual content of NV- center estimated by EPR. Our results show that optimization of the nitrogen content to create higher NV- density and PL emission needs to take into account the increasing density of the surrounding cluster defects affecting the integral PL intensity.

2. EXPERIMENTAL SECTION

2.1. Samples' Preparation. Micron-sized diamonds were created from carbon precursors by a tightly controlled HPHT process. While the detailed procedure is proprietary, the overall HPHT process is published in several reports; see, for instance, ref 15 and references therein. Briefly, an HPHT capsule is filled with commercial graphite as the carbon precursor, diamond "seeds", and a catalyst that lowers the temperature and pressure conditions needed for diamond growth. The HPHT process begins when pressure and temperature within the capsule are increased to between 5 and 6 GPa and between 1300 and 1600 °C, respectively, so that the catalyst melts. The graphite dissolves in the catalyst, and crystallizes on the diamond seeds, thus forming new diamond. The catalyst and unconverted graphite are removed by means of concentrated acid treatment, leaving behind diamond crystals containing a nominal level of nitrogen. The amount of nitrogen in the diamond crystals is controlled by doping the catalyst with inorganic nitrogen-containing substances in low and high concentrations.

Following the synthesis, two microcrystalline diamond batches, differing by doped nitrogen content, were characterized using optical microscopy image analysis and FTIR spectra. Diamond crystals, from the HPHT press, varied in size between about 200 and 400 ^m, as measured by a commercially available optical image analyzer.27 The diamond crystals were mostly cuboctahedral in shape with low levels of catalyst inclusions.

IR spectra of the low-nitrogen-content sample (designated as LN) and the high-nitrogen-content sample (designated as HN) are shown in Figure 1. Bands at 1130 and 1280 cm-1 are related to absorption due to C- and A-centers, respectively.14 The small, sharp peak at 1344 cm-1 is directly related to isolated C-

5233

Figure 1. FTIR spectra of two microcrystalline diamond batches (5% diamond crystals in KBr): black trace, LN; red trace, HN.

defects in a neutral charge state (substitutional nitrogen N0), additionally indicating the presence of C-centers in these diamond samples. At the same time, the characteristic absorption line at 1332 cm-1 related to substitutional nitrogen in the positively charged state (N+) is not observed in both FTIR spectra. This means that recharging of centers according to the scheme 2N0 ^ N+ + N- does not practically occur in the pristine HN sample.14,19,2°

A wide absorption band in the range from 1700 to 2340 cm-1 is due to two-phonon absorption of IR in the diamond lattice. The band comprises two peaks at ~2033 and ~2160 cm-1 and a dip at 2120 cm-1. The depth of that dip

characterizes two-phonon absorption in bulk diamond.19,20 This spectral feature may be considered as a universal characteristic in various types of bulk diamonds. The integral intensities and other general features of this broad band are independent of starting nitrogen content. In contrast to the two-phonon absorption band, the intensities of the 1130 and 1280 cm-1 bands in LN and HN strongly differ. On the basis of

these data, the content of C-defects in HN is at least two times higher than that in LN.

Using an empirical formula proposed for fine (particle size >5 y«m) diamond crystals,19 we estimate the concentrations of C-centers as 340 ± 50 ppm in LN and 650 ± 50 ppm in HN. Details of the estimation technique may be found in the Supporting Information (SI) (see eq 1 and Figure S1). It is notable that the intensities of the sharp absorption line at 1344 cm-1 (due to N0) differ about two times. On the basis of the analysis of the ~1280 cm-1 band intensity,16 we estimate the Acenter content in LN is below 0.8%, whereas in HN it is ~3.5%.

Following their initial FTIR characterization, diamonds were milled to between 5 and 20 j«m and then extensively cleaned in concentrated acid. The LN and HN diamond samples were spread across a 774 cm2 stainless steel plate and irradiated with a 5 MeV electron beam (25 mA) applied for 16 h. On the basis of capturing at least 60% of the total electrons within the area, we estimated the fluence to be approximately 7.0 X 1018 e-/ cm2. In some cases, the irradiation was for a shorter period (5 h) and the estimated fluence was calculated to be 2.2 X 1018

e-/cm2.

Following irradiation, the diamonds were annealed in an inert gas atmosphere at 800 °C for 6 h and then air oxidized at 450 °C for 1 h. At this point, the diamond sample appeared as a light purple powder. These irradiated and annealed samples are designated as follows: low nitrogen content/low fluence irradiation, LNLF; high nitrogen content/low fluence irradiation, HNLF; low nitrogen content/high fluence irradiation, LNHF; and high nitrogen content/high fluence irradiation, HNHF.

Figure 2 shows optical images of fabricated fluorescent microcrystals after milling. The fluorescence images were taken on a Nikon Eclipse Ti S epifluorescence inverted microscope using a standard Texas Red filter cube (excitation 540-580 nm, dichroic 595 nm, emission 600-660 nm). This is a standard filter cube that is able to capture most of the emission from the NV- center. The LN and HN microcrystalline powder samples,

LNLF

500 nm

X ■HÉfc**

HNLF

500 nm

HNHF

i-1 500 nm

Figure 2. Fluorescence from NV- center diamonds prepared under low-nitrogen (LN) and high-nitrogen (HN) conditions and then irradiated with electrons to achieve low electron fluences (LF) and high electron fluences (HF). Images were collected on a Nikon TiS epifluorescence microscope using a Texas Red filter cube. The isolated particles of micron size are clearly seen on some low-intensity images. All images were collected under identical conditions (10 ms exposure using a 10X objective).

5234

Figure 3. (a, b) g = 2.00 region RT EPR spectra of the micron-sized diamond samples: black traces, e-beam irradiated by 2.2 X 1018 e /cm2; red traces, e-beam irradiated by 7.0 X 1018 e-/cm2; low N content samples (LNLF, LNHF), V = 9.466 GHz (a); high N content samples (HNLF, HNHF), V = 9.465 GHz (b). Spectra were recorded under the same experimental conditions: nonsaturating microwave power level Pmw = 20 ^W, 100 kHz magnetic field modulation with the amplitude Amod = 0.02 mT, receiver gain RG = 2 X 104. (c, d) Deconvolution of EPR spectra in part b: black trace, experimental spectrum; red trace, spectrum of so-called "isolated" P1 obtained by subtraction of broad Lorentzian line; green trace, simulated spectrum of broad Lorentzian line; LNLF (c) and LNHF (d). (e, f) Half-field (g = 4.00) RT EPR spectra: black traces, e-beam irradiated by 2.2 X 1018 e-/cm2; red traces, e-beam irradiated by 7.0 X 1018 e-/cm2; (e) LNLF, LNHF, RG = 2 X 105, V = 9.466 GHz; (f) HNLF, HNHF, RG = 2 X 106, V = 9.465 GHz. Spectra were recorded under the same experimental conditions (except RG): Pmw = 100 ^W, Amod = 0.1 mT, number of coherent acquisitions n = 25. All intensities are normalized per unit mass; spectra are shifted vertically for better presentation in parts e and f.

which were irradiated with different e-beam fluences, demonstrate very different NV- center emission intensities. Some fluorescence speckling was observed in the dark images (for example, the HNHF image in Figure 2), representing the emission of a few individual diamond crystallites of micrometer sizes. The brightest emission is seen in the diamonds that contained low levels of nitrogen and received the largest e-beam fluences. The diamonds having the dimmest fluorescence were those that contained high levels of nitrogen and received a low e-beam fluence. It is notable that diamonds that contained low nitrogen levels and that received a low e-beam fluence showed brighter fluorescence than those that had high nitrogen levels and received large e-beam fluence. On the basis of the results shown in Figure 2, increasing electron fluence appears to increase fluorescence intensity, presumably by increasing the actual number of NV- color centers. However, a high nitrogen concentration does not increase the fluorescence.

2.2. EPR Measurements. Continuous wave X-band (v = 9.4 GHz) EPR measurements on quasi-polycrystalline samples were carried out using a Bruker EMX-220 spectrometer equipped with Agilent 53150A frequency counter at room temperature (RT) (T ~ 295 K). Precise determination of g-factors (for spin S = V2 species) and densities of paramagnetic centers (Ns) were done by comparison with the reference sample, a well-purified detonation ND powder with g = 2.0028(2) and Ns = 6.3 X 1019 spins/g.28 Evaluation of the electron spin-lattice and spin-spin relaxation times (TSLe and TSSe, correspondingly) was done by analysis of the saturation dependencies of the peak-to-peak intensities of the multi-component central g = 2.00 EPR line following the technique described in ref 29. Spectra processing and simulation were

done using Bruker's WIN-EPR/SimFonia and OriginLab software.

2.3. PL and Raman Measurements. PL and Raman characterization of nitrogen-doped and irradiated diamond samples was performed using an "inVia" Renishaw micro-Raman spectrometer. The spectrometer was equipped by a thermoelecrically cooled CCD detector and 3000 mm-1 diffraction grating allowing 0.3 cm-1 spectral resolution of Raman and PL spectra excited by 457.9 and 488 nm laser wavelengths. For quantitative comparison of emission of different diamond samples, both diamond Raman and NV-center PL spectra were normalized to the spectral sensitivity of the spectrometer and to the 1332 cm-1 diamond Raman band intensity. The Raman band measurement normalizes the PL response and provides assurance that the measured PL emission comes from the equal volume of samples. Thus, any observed difference in the PL intensity can be related solely to the change in concentration of the NV centers in the diamond crystal lattice.

Since the PL intensities are significantly higher than the Raman bands, two acquisition modes were used. First, an overall Raman/PL spectrum was recorded with a relatively short accumulation time (30 s). Second, the part of the spectrum comprising Raman bands were recorded with an accumulation time of 3000 s to provide a more reliable measurement of band intensities. The width of the 1332 cm-Raman band for different diamond crystals was used to estimate the degree of crystal lattice disorder and was measured with the 457.9 nm excitation line, where the profile of the Raman band can be measured with more accuracy. All measurements were done at room temperature.

Table 1. EPR, PL, and Raman Spectroscopy Data

sample irradiation fluence (e-/cm2) total S = V2 content (ppm)" "isolated" Pi content (ppm)" magnetic impurities content (arb units/mg)1. AHpp (mT) NV^ content (ppm)" ZPL and PSB integrated intensity (X10~2) fwhm of 1332 cm 1 Raman band (cm-1)

LNLF 2.2 X 1018 135 108 621 0.10 ± 0.02 0.9 460 ± 50 3.4 ± 0.3

LNHF 7.0 X 1018 103 86 2523 0.27 ± 0.02 3.4 3540 ± 350 3.6 ± 0.3

HNLF 2.2 X 1018 580 116 1038 0.43 ± 0.02 0.4 80 ± 10 4.2 ± 0.3

HNHF 7.0 X 1018 538 215 4025 0.73 ± 0.02 4.0 865 ± 85 4.2 ± 0.3

"Error in spin concentration determination does not exceed ±15% bError in spin concentration determination does not exceed ±30%

3. RESULTS AND DISCUSSION

The general features of RT EPR spectra obtained from diamonds irradiated at low- and high-electron fluences are consistent with those reported previously by us30 (see comparison of samples MD and FMD). Each general view spectrum (SI, Figure S2) contains several groups of EPR signals that have been successfully recognized and reliably assigned:30 (i) intense signals in the region of g = 2.00 due to PI (also called C-centers or №, substitutional nitrogen in a neutral charge state) and other paramagnetic defects with S = V2; (ii) intense broad lines in the g ~ 4 region attributed to technologically induced unwanted ferro- and paramagnetic impurities; (iii) well-distinguishable narrow signals within the half-field (HF) region (g = 4.00); and ( iv) weak satellite signals symmetrically located at distances ~50 and ~100 mT at low-and high-field regions of the g = 2.00 signals. Signals of both iii and iv groups were attributed to so-called "forbidden" AMs = 2 and "allowed" AMs = 1 microwave transitions in polycrystalline patterns of triplet (S = l) paramagnetic defects, induced by the e-beam irradiation/annealing technique (see ref 30 and references therein).

Figure 3a,b shows representative RT EPR spectra within the g = 2.00 region for all samples under study. Spectra of LNLF and LNHF in Figure 3a demonstrate the prevailing contribution of "isolated" (or, better, weakly interacting, distant) PI centers with S = V2, nuclear spin of 14N 1=1 and spin—Hamiltonian (SH) parameters determined by SimFonia simulations: giso = 2.0024(2), Azz = 4.07(2) mT, A^ = Ayy = 2.93(2) mT, individual line widths AHppLorentz = 0.10(2) mT (LNLF) and 0.27(2) mT (LNHF). Deconvolution of the experimental spectra in Figure 3a (SI, Figure S3) reveals the presence of another weak singlet Lorentzian-like signal with g = 2.0024 and AHpp ~ 0.5 mT (LNLF) and ~0.8 mT (LNHF).

In contrast to the LN samples, EPR spectra of the HN samples show clearly observed superposition of two distinguishable types of signals: (i) broadened PI patterns, which may be described by the aforementioned SH parameters except AHppLorentz = 0.43(2) mT (HNLF) and 0.73(2) mT (HNHF), and (II) intense singlet Lorentzian-like line with the same g-factor and AHpp ~ 2.46(5) mT. Deconvolutions of HNLF and HNHF EPR spectra are presented in Figure 3c,d.

The most prominent features observed in the EPR spectra of all irradiated/annealed samples is the appearance of new, fluence-dependent EPR signals. Thus, the EPR spectra of e-beam-irradiated fluorescent micron-sized diamonds reveal clear characteristic half-filled signals with g = 4.26(l) attributed to triplet (S = 1) NV" centers30'31 (see Figure 3e,f). The HF EPR spectrum of LNLF shows a weak but detectable signal with experimental g = 4.27(1) and AHpp ~ 0.25(2) mT (Figure 3e, black trace). The 3-fold increase of the irradiation fluence causes a significant growth of the g = 4.27 line peak intensity as

well as almost 2-fold broadening of the g = 4.27 line, AHpp ~ 0.44(2) mT. Simulations of experimental polycrystalline patterns of "allowed" transitions (SI, Figure S4) provide SH parameters giso = 2.003(l), D = 958(5) X 10"4 cm"1, andE ~ 0, which are in a good agreement with data previously reported for NV~ (or W15) triplet centers; see, for instance, ref 13 and references therein.

Table 1 shows the changes in the content and composition of S = V2 defects and magnetic impurities as a function of both initial nitrogen content and applied high-energy e-beam fluence. Here the total concentrations S = V2 defects are obtained by double integration of g = 2.00 signals, whereas the concentrations of "isolated" PI centers were determined as the differences between the aforementioned values and double-integrated singlet signals obtained by subtraction of SymFonia-simulated broad Lorentzian signals from the corresponding experimental spectra. It is clearly shown that for the LNLF sample 80% of S = V, defects are of the conventional PI origin. After a high fluence irradiation (LNHF), both the total content of S = V2 defects as well as the PI content decrease, whereas the ratio between PI and other S = V2 defects remains practically the same. Sample HNLF contains much higher content of paramagnetic species, with only 20% of them due to non- (or weakly) interacting PI centers. Similar to low-nitrogen samples, the high fluence irradiation sample (HNHF) causes reduction of the total content of paramagnetic species but surprisingly increases PI content almost twice.

Previously,30 we reported that the integral intensity of the so-called "characteristic g = 4.26 line" is proportional to the content of NV~ centers measured by double integration of the entire polycrystalline triplet pattern. This allows straightforward quantification of the fluence-dependent content of NV~ triplets. The quantification was done by comparison of the integral intensity of the g = 4.26(l) line in the FMD sample with known NV" content (5.4 X 1017 spin/g; see Table 2 in ref 30) with the intensities of the corresponding lines in EPR spectra of samples under study. Low-fluence irradiation induces in both LNLF and HNLF samples very few NV~ centers, below 1 ppm. Higher irradiation fluence creates ~4 ppm of NV~ centers, independently off the initial nitrogen content.

Estimation of electron spin—lattice relaxation time for S = V2 paramagnetic species was done on the central g = 2.0024 line by analyzing microwave saturation dependences measured on all samples (SI, Figure S5). Following aforementioned spectra' deconvolutions, best fittings were obtained, suggesting two groups of S = V, spins contributing to this central line: slow relaxing spins and fast relaxing spins. We found that TSLe for the slow relaxing spins, which is associated with "isolated" PI, significantly depends on the initial nitrogen content: 20 ¡us in LNLF and LNHF vs ~4 fis in HNLF and HNHF (SI, Figure S6). Irradiation hardly affects TSLe for these spins. Spin—lattice relaxation for the fast relaxing spins (associated with non-Pi

centers providing broader singlet lines) appears to be more sensitive to both nitrogen content and electron fluence. Thus, TSLe values for LNLF, LNHF, HNLF, and HNHF are 0.6, 0.3, 0.24, and 0.18 ^s, respectively (SI, Figure S6). The relative contributions of slow and fast relaxing spins, obtained from saturation curves, agrees well with the corresponding values obtained by EPR spectra' deconvolution.

Independent data on the effects of initial nitrogen content and irradiation fluences on the amount of NV- centers was obtained from the comparative analysis of PL and Raman spectra of the samples under study. Figure 4a presents the

Figure 4. (a) The diamond Raman bands and NV0 and NV- centers' PL bands of the samples with different nitrogen content and e-beam fluences. The rectangle in the lower left corner points out the region of the Raman scattering shown as a close-up in panel b. (b) Normalized Raman spectra of the same samples; wavenumbers of the Raman bands are shown. Excitation wavelength 488 nm.

diamond Raman bands and NV0 and NV- centers' PL bands of the samples with different nitrogen content and e-beam fluences, excited at 488 nm radiation. The spectra are dominated by the zero-phonon line (ZPL) due to NV- centers (637 nm) and their phonon sideband (PSB) at 640-800 nm.32 In our analysis, the integrated PL intensity of NV- centers was used. It was found that both nitrogen content and e-beam fluence affect the NV- center PL. Integrated intensities of ZPL and their PSB are presented in Table 1.

The rectangle in the lower left corner in Figure 4a points out the region of the Raman scattering, while Figure 4b shows normalized Raman spectra of the samples in more detail. The appearance of the 1475 and 1545 cm-1 broad bands in the Raman spectra of LNHF and HNHF samples together with the 1332 cm-1 diamond Raman band indicates formation of a certain amount of amorphous sp2-hybridized carbons upon irradiation of samples.33 The widths of the 1332 cm-1 Raman band of samples with different treatments, measured with 457.9 nm excitation, are presented in Table 1. Correct analysis of the evolution of EPR spectral patterns, occurring due to initial nitrogen content and fluences, must distinguish between external and intrinsic factors that may affect the shape and widths of the EPR lines observed. Thus, a first look at the EPR spectra in Figure 3a,b and the values of the line widths in Table 1 indicates that both starting nitrogen content and increasing irradiation fluence cause P1 line broadening. However, such a conclusion seems to be misleading. Indeed, one can easily see that line broadening in each pair of samples perfectly correlates

with the actual level of magnetic impurities, which is 4 times higher in high fluence irradiated samples (Table 1). Most likely, irradiation induces additional contamination of diamond samples that is proportional to the fluence. This amount of magnetic contamination is responsible for line broadening and corresponding reduction of peak intensities of P1-related signals, observed in each pair of samples (Figures 3a,b). Thus, irradiated samples require additional thorough purification to remove these contaminants.

Considering technologically induced magnetic impurities as an extrinsic factor, it is possible that increasing electron beam fluence (within the fluence range under study) might not significantly affect actual intrinsic SH parameters of P1 centers. On the other hand, when comparing a pair of samples irradiated by the same fluence (LNLF vs HNLF and LNHF vs HNHF), one can find that the initial nitrogen content does affect the EPR line width independently of impurities. This broadening cannot be explained by strengthening exchange and dipole-dipole interactions between the lone P1 centers. For instance, P1 contents in LNLF and HNLF are practically the same, whereas AHpp increases a factor 4. An insignificant difference in the level of impurities (which is low in both samples) also cannot explain this broadening. The main intrinsic factor that may be responsible for the broadening effect is formation in the high-N samples of other S = 1/2 centers, manifesting in the appearance of an intense singlet broad line. Just these paramagnetic species are the main factor responsible for the broadening of P1 EPR lines in high-N samples.

The observed EPR signals could be interpreted more accurately if extrinsic factors are set aside. Main S = 1/2 paramagnetic centers in LNLF sample consist of ~80% P1 and ~20% other defects induced by both the sample's micronization and irradiation. High-fluence irradiation does not significantly change either the content or distribution of S = 1/2 defects. In contrast, EPR spectra of the HN samples suggest at least two well-distinguished types of N-related paramagnetic defects. Thus, along with lone, weakly interacting nitrogen substitution defects (P1), other clusters of defects provide a structureless, broad Lorentzian-like EPR line. There is also a high inhomogeneous distribution of N-related defects in both HNLF and HNHF samples. HNLF sample contains only 20% P1, and most of the S = 1/2 defects seems to be distributed in the diamond lattice as magnetic entities with strongly interacting individual spins.

The EPR spectrum of HNHF (see Figure 3b and data in Table 1) reveals a surprising effect: that is, a high irradiation fluence causes not only reduction of the total amount of S = 1/2 defects (which is observed for low N samples as well) but a change in the actual distribution of these defects. Upon irradiation, the P1 content increased by a factor 2, whereas the content of defects responsible for the broad component decreased correspondingly. It is possible that, at higher irradiation fluences, magnetically concentrated N-related entities are destroyed and subsequently enrich the diamond with "isolated" P1.

Half-field g = 4.27(1) EPR lines observed in all investigated samples reliably indicate the presence of luminescent NV-centers. Since all these samples underwent irradiation and annealing processing, the appearance of these defects (NV-color centers) is a result of the corresponding treatment. Since the actual amount of NV- centers is obtained by double integration of the g = 4.27(1) EPR lines, well-observed changes

in their line widths (Figures c,d) may be neglected. Most likely, broadening of this line in HNHF has the same origin as broadening of P1 EPR lines in the same sample, i.e., the magnetic interaction of NV- defects with impurities. Low irradiation fluence (2.2 X 1018 e-/cm2) leads to relatively low densities of NV- centers: ~1 ppm in LNLF and ~0.4 ppm in HNLF. Increasing the electron beam fluence causes significant increases in the number of NV- centers. Thus, the LNHF sample shows an almost 4-fold increase of the NV- content ( 3.4 ppm), whereas the HNHF sample provides a 10-fold increase (4 ppm).

Comparison of PL intensities for different samples (see Table 1) indicates a fairly complicated dependence of NV-emission characteristics on nitrogen content and irradiation fluence. It may be that the PL intensity of these color centers in the diamond lattice is controlled by both the number of NV-centers within a crystal and the nonradiative dissipation of optically excited electron states due to neighboring crystal defects.34'35

The combined use of PL and Raman data may provide valuable information on the formation of crystal defects. We have shown that additional defects may be induced by HPHT synthesis with a nitrogen-containing inorganic additive and treatment, and that these defects have an impact on the NV-luminescence intensity. On increasing the nitrogen content in low-fluence irradiated samples LNLF and HNLF, the essential decrease in PL intensity is observed. Here it is worth mentioning that this decrease is more pronounced than just a simple reduction of the content of NV- centers detected by EPR, 5.8 vs 2.3. Such an effect may be attributed to the PL quenching through the point-like crystal defects (A-, B-centers, etc.) and extended imperfections arising due to the excessive nitrogen doping.

The presence of defects in the samples is clearly seen from the analysis of the 1332 cm-1 Raman bandwidth (fwhm). Even at low nitrogen content, substitutional nitrogen impurities disturb the crystalline lattice and induce in the diamond crystal structure some additional disorder. This disorder causes broadening of the diamond Raman band (3.4 cm-1) compared to that in defect-free bulk diamond crystals (1.6 cm-1).36 Further growth of the substitutional nitrogen content in HNLF sample (above 500 ppm) increases the density of diamond lattice pointlike defects and extended imperfections, which is accompanied by additional broadening of the diamond Raman band (4.2 cm-1) and pronounced reduction of PL intensity.

As expected, higher e-beam irradiation fluence causes formation of an increasing density of vacancies and, correspondently, an increasing number of NV- centers, which is manifested in an increase of NV- PL intensities in both LNHF and HNHF samples. The fact that the PL intensity in LNHF is substantially stronger than that in HNHF shows that NV- emission characteristics in high-fluence irradiated samples are also controlled by vacancies, despite the efficiency of luminescence quenchers being enhanced with growth of the nitrogen content. On the other hand, the real increase of the PL intensity on increasing irradiation fluence in the HN series is ~1.4 times stronger than in LN one. Since the actual amounts of substitutional nitrogen in both LN and HN samples are much higher than resulting amounts of NV- centers, it is reasonable to propose that the vacancies' densities in LNLF and HNLF are different.

High-fluence irradiation of the samples does not lead to noticeable broadening of the 1332 cm-1 diamond Raman band.

This indicates that the appearance of additional vacancies (as evidenced by the increase in both PL and g = 4.27 EPR signal intensities) does not lead to formation of additional defects in the diamond lattice (the latter are responsible for quenching of luminescence). At the same time, high-fluence irradiation of the samples under study results in formation of some amount of sp2-hybridized amorphous carbon, which is evidenced by the appearance of broad bands at 1475 and 1545 cm-1 in Raman spectra (see Figure 4b).33 However, we found no evidence that amorphous carbon species lead to noticeable luminescence quenching, in contrast with defects introduced by substitutional nitrogen.

Therefore, the observed luminescence intensity due to NV-centers created by nitrogen doping and e-beam irradiation in Ib type HPHT diamonds is actually determined by the competition between the NV- centers content (number of elementary light emitters) and the presence of crystal defects/ imperfections responsible for the PL quenching. Thus, the PL intensity may be optimized by the appropriate choice of nitrogen doping and irradiation fluence. A decrease of the nitrogen content is preferable due to the fact that it reduces the density of structural defects and lattice imperfections that are responsible for the nonradiative relaxation of the optically excited electron state of NV- centers. Increases of PL intensity are also expected at higher e-beam fluence. Although such an increase is accompanied by formation of the sp2-hybridized amorphous carbon, the latter does not strongly affect the diamond crystal structure and, correspondingly, the NV-emission intensity.

4. CONCLUSIONS

Samples of Ib type HPHT diamonds synthesized with low and high substitutional nitrogen content and e-beam-irradiated to form luminescent NV- centers were studied by EPR and optical (PL and Raman) techniques. The intensity of the PL emission from NV- centers is analyzed as a function of the content of NV- centers directly obtained by EPR in samples with different contents of substitutional nitrogen ions and irradiated by low and high e-beam fluence. High nitrogen doping leads to the appearance of paramagnetic entities characterized by strong interactions between substitutional nitrogen defects. It is found that the NV- PL intensity is controlled not solely by the content of NV- centers but, to a large extent, by the presence of nitrogen-related crystal defects. Increasing nitrogen content during HPHT synthesis correspondingly increases this structural imperfection and, thus, is responsible for the appearance of an additional nonradiative recombination channel via the defects contributing to intensification of PL quenching. It is suggested that PL intensity may be optimized by the appropriate choice of nitrogen doping and irradiation fluence.

■ ASSOCIATED CONTENT Q Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12827.