Взаимодействие лазерного излучения с гибкими металл-органическими каркасами: структурная модификация и нелинейно оптический отклик тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Мезенов Юрий Анатольевич

• Мезенов Юрий Анатольевич
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Мезенов Юрий Анатольевич

РЕФЕРАТ

SYNOPSIS

INTRODUCTION

CHAPTER 1. Metal-organic frameworks as materials for nonlinear optics

1.1 Second harmonic generation in MOFs

1.2 Third harmonic generation in MOFs

1.3 Lasing in MOFs

1.4 Nonlinear absorption in MOFs

1.5 Photoluminescence in MOFs

CHAPTER 2. Materials and Methods

2.1 The experimental optical measurements

2.1.1 Setup for femtosecond laser ablation

2.1.2 Setup for Raman and Photoluminescence optical measurements

2.1.3 Setup for Second harmonic generation

2.2 The chemical synthesizes of metal-organic frameworks and its composites

2.2.1 Model objects for the creation MOF-based nanoparticles with nonlinear optical response

2.2.2 Synthesizes of one- and two-dimensional MOFs as precursors for MOF-based carbon nanospheres

2.2.3 Synthesis of polymer composites ZIF-8-PMMA

2.2.4 Synthesizes of the series of M-BTC MOFs

2.2.5 Creation of amorphous MOF-based composites

2.3 Additional post-synthetic and post-ablation characterization

2.3.1 Single crystal X-ray diffraction structure analysis

2.3.2 Characterization by electronic microscopy

2.3.3 Size analysis based on TEM data

2.3.4 Johnson-Mehl-Avrami-Kolmogorov modelling

2.4. Quantum chemical calculations

CHAPTER 3. Non-linear optical response of MOF based nanoparticles

3.1. Model objects of metal-organic frameworks

3.2. Non-linear optically active MOF-based nanoparticles

3.3 The effect of the electronic structure of the initial MOF on MOF-based nanoparticles with non-linear optical effects

CHAPTER 4. Linear optical response of MOF based carbon nanoparticles

4.1 Creation of MOF-based carbon nanoparticles by femtosecond laser ablation

4.2 Linear optical properties of MOF-based carbon nanoparticles

CHAPTER 5. Non-linear optical response of MOF-based composite

5.1. Creation of a polymer MOF-based composite

5.2. Non-linear optical properties of MOF-based composite and its stability under laser irradiation

CHAPTER 6. Luminescence of MOF based amorphous nanocomposite

6.1. Model objects for the creation of MOF-based amorphous nanocomposite

6.2. The creation of MOF-based amorphous nanocomposites

6.3. The effect of the electronic structure on the chemical composition of MOF-based amorphous nanocomposites

6.4 Optical properties of MOF-based amorphous nanocomposites

CONCLUSIONS

ACKNOWLEDGMENTS

LIST OF ACRONYMS AND SYMBOLS

BIBLIOGRAPHY

APPENDIX A. Main journal papers

Введение диссертации (часть автореферата) на тему «Взаимодействие лазерного излучения с гибкими металл-органическими каркасами: структурная модификация и нелинейно оптический отклик»

РЕФЕРАТ Общая характеристика диссертации

Актуальность темы. С момента появления первых лазеров, нелинейная оптика заняла важную роль в жизни человечества и получила большой импульс развития. В частности, этому способствовало изучение и открытие различных нелинейно-оптических эффектов. Николаас Блумберген был первым исследователем, который систематизировал теоретические основы большей части нелинейно-оптических процессов в своей монографии "Нелинейная оптика" в 1965 году [1]. С тех пор, лазеры стали движущей силой для исследования новых нелинейно-оптических эффектов, а материалы, обладающие нелинейно-оптическими свойствами, вносят значительный вклад в развитие этой научной и прикладной области.

Существующие неорганические материалы занимают особое место в списке кристаллов для нелинейно-оптических применений, и особенно для генерации второй гармоники за счет отсутствия центра инверсии. К ним относятся такие материалы, как Ва(В02>2, КЯЮз, LiЮз, BiBзO6, КТЮРО4, КН2РО4, LiBзO5 и ряд неорганических перовскитов [2]. Несмотря на их широкое использование в лазерных системах в настоящее время, они имеют ряд недостатков. Эти материалы часто состоят из редких элементов, не являются биосовместимыми, а процесс их синтеза в форме крупных кристаллов для лазерных применений является довольно дорогостоящим. Таким образом, поиск новых, более дешевых и безопасных материалов для нелинейной оптики остро стоит на повестке дня. В связи с этим, подход через комбинацию металлических и органических строительных элементов может быть чрезвычайно многообещающим способом поиска новых альтернативных материалов для нелинейной оптики.

Существующие геологические исследования [3] показывают, что наиболее распространенными металлическими элементами в земной коре являются такие элементы, как хром, марганец, железо, кобальт, никель, медь, цинк и др. Чаще всего они представляют собой руду и их добыча экономически выгодна. Более того, эти элементы являются микроэлементами, биосовместимыми и нетоксичными, и играют решающую роль в биологических процессах не только для людей, но и для всех живых существ на Земле [4]. Существуют также исследования, посвященные важности каждого металлического элемента для высокотехнологичной промышленности в экономике страны. Было установлено, что наиболее важными для российской высокотехнологичной экономики являются такие металлы как литий, германий, скандий, ниобий, иттрий, рений и др. Все эти элементы имеют чрезвычайно высокий уровень риска добычи и доставки для высокотехнологичной российской экономики. Уровень риска доставки включает в себя множество факторов, таких как дефицит сырья, геополитическая нестабильность, разнообразие сырья, доступность критических технологий для производства,

кластеризация и переработка производства критических металлов для стран экспорта и импорта. Однако существуют элементы с низким уровнем рисков -медь, кобальт, никель, хром, марганец и т.д. [5].

С точки зрения органической части потенциально новых нелинейно-оптических материалов, их синтез идет через процесс переработки нефти, включающий в себя важную область в мировой экономике и промышленном производстве, так как конечные продукты представляют собой большое количество органических соединений, которые участвуют в повседневных процессах жизнедеятельности человека. Это также относится к конечным нефтехимическим продуктам, которые могут быть использованы в качестве прекурсоров для производства различных органических молекул для дальнейшего применения в химической промышленности. В настоящее время в базе данных Chemical Universe GDB-17 насчитывается более 160 миллионов органических соединений [6]. В этом смысле сочетание доступных в России металлов с органическими компонентами является довольно интригующей областью для поиска новых альтернативных материалов для нелинейной оптики.

Одним из таких перспективных материалов является металл-органический каркас (МОК). Металл-органические каркасы - это химические соединения, которые обладают уникальной комбинацией таких интересных свойства, как высокая кристалличность, иерархичность, гибкость и органо-неорганическая природа. Благодаря этому, металл-органические каркасы были совсем недавно выделены в абсолютно новый класс химических соединений. Их внутренняя структурная организация обеспечивается благодаря координационным взаимодействиям между ионами металлов с органическими лигандами. Высокая кристалличность сама по себе позволяет предсказать с помощью пространственной группы, какие нелинейные эффекты мы можем наблюдать для МОК. Структурная иерархия МОК позволяет рационально подходить к синтезу различных структур, надстроек и топологий в металлорганических каркасах с различными механическими воздействиями. Гибкость - еще одно важное свойство, позволяющее интегрировать МОК в гибкие устройства. Органическая и неорганическая природа обеспечивают эффективное и усиленное взаимодействие оптическиого излучения с МОК. Это делает металлорганические каркасы весьма перспективными материалами для различных научных применений - от биофотоники до лазерной физики [7-9]. Помимо структурного разнообразия и отсутствие центра инверсии, высокая структурная стабильность во внешних условиях открывают возможности для этих соединений использования в качестве активных материалов для нелинейной оптики [10-15].

Недавно было обнаружено, что металлорганические каркасы могут так же составить конкуренцию коммерческим неорганическим материалам за счет демонстрации впечатляющих эффективностей нелинейно оптического преобразования: генерация второй оптической гармоники, генерация третьей

оптической гармоники, высокий нелинейно-оптический коэффициент поглощения и лазирование [16-19]. Однако в этой области все еще остаются два вопроса. Первый связан с тем фактом, что при использовании наноразмерных МОК в качестве активных материалов не всегда можно наблюдать нелинейно-оптические свойства из-за того, что структура может изменяться (потеря нецентросимметричности), а свойства наноразмерных материалов отличаются от их крупногабаритных аналогов. И второе - для металлорганических каркасов, работающих работают в течение длительного периода времени, их нелинейно-оптические свойства не всегда стабильны [20-22].

Соответственно, будущее металлорганических каркасов как нелинейно-оптических материалов заключается в следующем: получение наноразмерных оптических материалов на основе МОК с возможностью высокой стабильности их оптической активности.

Диссертация посвящена новаторским экспериментальным исследованиям взаимодействия металлорганических каркасов и лазерного облучения для создания оптически активных наночастиц МОК, а также исследованиям оптической стабильности (генерации второй гармоники и люминесценции) композитов на основе МОК. В качестве объектов исследования рассматриваются следующие соединения и композиты:

1) Трехмерные взаимопроникающие и гибкие металлорганические каркасы с длинными и гибкими органическими лигандами 4,4'-стилбендикрабоксилат ^с) и 1,4-ди(1Н-имидазол-1-ил)бутан ^тВ) или 1,6-ди(1Н-имидазол-1-ил)гексан ^т^) и ионами кадмия, которые использовались для создания наноразмерных частиц МОК со сложной внутренней структурой, полученных при фемтосекундном лазерном облучении:

- Трехмерный МОК [Cd(sdc)(dimb)]•DMF был синтезирован коллегами из института неорганической химии им. А. В. Николаева СО РАН [107]. Это соединение обладает жесткой структурой и имеет шестикратную взаимо проросшую алмазную топологию, а также моноклинную и центросимметричную пространственную группу Р2/с. Химически, данный МОК содержит ионы кадмия (II), которые связаны с 1,4-ди(1Н-имидазол-1-ил)бутан и 4,4'-стильбендикрабоксилатным органическими лигандами.

- Трехмерный МОК [Cd(dmf)(sdc)(dimh)]•DMF также был синтезирован нашими коллегами [107] и имеет восьмикратно взаимопроросшую структуру ромбовидной топологии и нецентросимметричную орторомбическую пространственную группу Р2Л2ь Химически, данное соединение содержит ионы кадмия (II), которые связаны с 4,4'-стильбенедикрабоксилатным и 1,6-ди(1Н-имидазол-1-ил)гексановым органическими лигандами.

2) Металлорганические каркасы с одно- и двумерной структурой с длинными и гибкими органическими лигандами 1,2-ди(4-пиридил)этилена (Ьре) и 2,2'-бипиридина (Ыру) и ионами меди, которые использовались для создания полых углеродных наночастиц при фемтосекундном лазерном облучении:

- Одномерный гибкий МОК [Си(Ьре)(КОз)2]^МРп-РгОН, который был синтезирован в соответствии с сольвотермическим методом медленной диффузии. Это соединение кристаллизуется в триклинной кристаллической системе Р1 и содержит ионы меди, органические лиганды 1,2-ди(4-пиридил)этилена (Ьре) и анион КО3- в структуре.

- Двумерный более жесткий МОК [Си(Ыру)2(ОМР)(Шз)][КОз]^МР также был синтезирован с использованием сольвотермического метода медленной диффузии. Это соединение имеет пространственную группу Р21/п с моноклинной кристаллической системой и содержит в структуре две молекулы 2,2'-бипиридина (Ыру), одну молекулу DMF и анион КО3-.

3) Полимерный композит на основе 7^-8 и поли(метилметакрилата) (7Ш-8-РММА) с нелинейно оптическим откликом был синтезирован в соответствии с полимеризационным ультрафиолетовым синтезом с использованием исходного мономера метилметакрилат и коммерчески доступного металлорганического каркаса 7Ш-8, который состоит из тетраэдров 7п, координированых с 2-метилимидазольными лигандами, и имеет цеолитоподобную топологию с нецентросимметричной кубической структурой 1-43т.

4) Серия трехмерных металлорганических каркасов М-ВТС (где М = Си2+, №2+, Со2+, Fe3+), состоящих из органического лиганда бензол-1,3,5-трикарбоксилата (ВТС) и ионов металлов, которые использовались для создания аморфных композитов на основе МОК со стабильными люминесцентными свойствами.

- МОК №-ВТС [№з(ВТС)2-12ШО] и Со-ВТС [Соз(ВТС)2-12ШО] обладают аналогичной кристаллической структуру с моноклинной системой и пространственной группой С2, где атомы Ni и Со имеют отдельные октаэдрические положения, а структура состоит из зигзагообразных цепей, построенных из двух симметричных неэквивалентных звеньев тетра-аква М(П) и ВТС (бензол-1,3,5-трикарбоновых) лигандов.

- МОК Со-ВТС* [Col2(BTC)4(DMF)l5(NOз)], который кристаллизуется в ромбическую сингонию Р212121. Атомы кобальта создают две координационных сферы: первая координационная сфера состоит из двух атомов кобальта, образующих хорошо сформированные и искаженные октаэдры, и имеющая связи с тремя лигандами ВТС, двумя молекулами DMF и одним нитрат анионом (КОз-); вторая координационная сфера содержит один атом кобальта в октаэдрической позиции со связями с тремя лигандами ВТС и тремя молекулами DMF.

- МОК Fe-BTC ^езОН(Н2О)2О[(СбНз)-(га2)з]214.5ШО) кристаллизуется в кубической сингонии FdЗ т. Три атома железа занимают октаэдрические позиции

и содержат один общий атом кислорода. Формируемые октаэдры обладают похожими структурой и связями с атомами кислорода от органического лиганда ВТС.

-МОК Си-ВТС [Сщ(ВТС)2-3ШО] (НКШТ-1) кристаллизуется в кубической сингонии FmЗ т. Атомы меди образуют две отдельных координационные сферы в виде удлиненных прямоугольных пирамид. Каждый атом меди в структуре формирует четыре связи с атомами кислороды от органического лиганда ВТС и одну связь с молекулой воды.

В данной работе исследуется взаимодействие лазерного излучения с перестраиваемыми металлорганическими каркасами с различной топологией, влияние их структурных особенностей на морфологию полученных наноразмерных частиц МОК, которые демонстрируют линейный и нелинейный оптический отклик, а также исследуется стабильность оптических свойств полимерных и аморфных композиционных материалов на основе МОК.

Целью диссертации является создание оптически активных наноразмерных частиц на основе металлорганических каркасов, а также создание композитов на основе МОК со стабильными оптическими свойствами.

Научные задачи:

1. Разработка наноразмерных частиц с нелинейно оптическим откликом из взаимопроникающих гибких трехмерных металлорганических каркасов на основе органических лигандов sdc и dimh/dimB с ионами кадмия с использованием холодной инфракрасной фемтосекундной лазерной абляции.

2. Разработка наноразмерных углеродных частиц с линейным оптическим откликом из гибких одномерных и двумерных металлорганических каркасов на основе лигандов ВРЕ и BPY с ионами меди путем воздействия инфракрасными фемтосекундными лазерными импульсами.

3. Разработка полимерного композита с нелинейно оптическим откликом на основе поли(метилметакрилата) с кристаллами нецентросимметричного металл-органического каркаса 7^-8 и исследование его временной стабильности генерации второй оптической гармоники.

4. Разработка аморфных нанокомпозитов с люминесцентными свойствами с использованием серии металлорганических каркасов на основе лиганда ВТС с различными ионами металлов (Си2+, Со2+, №2+, Fe3+) при электронном облучении.

Научные положения, выносимые на защиту:

1. Сферические металл-органические частицы типа ядро-оболочка

диаметром от 100 до 1000 нм, полученные из металл-органических каркасов

с помощью инфракрасной фемтосекундной лазерной абляции в холодном режиме с плотностью энергии 10 кДж см-2, демонстрируют 10-5 эффективность генерации второй оптической гармоники.

2. Полые углеродные кристаллические наночастицы с диаметром от 100 до 600 нм, полученные из металл-органических каркасов с помощью инфракрасной фемтосекундной лазерной абляции в горячем режиме с плотностю энергии в 0,03 Дж см-2, демонстрируют резонансный оптический отклик, преимущественно обусловленный возбуждением мультипольных моментов низших порядков с локализацией магнитной составляющей света в полости.

3. Полимерный композит, состоящий из поли(метилметакрилата) с включениями микрокристаллов нецентросимметричного металл-органического каркаса 7Ш-8, демонстрирует стабильный во времени сигнал второй гармоники в течение 2 часов без видимой структурной деформации образца при инфракрасной накачке фемтосекундными лазерными импульсами в диапазоне плотности энергий от 1,6 мДж см-2 до 1,3 кДж см-2.

4. Аморфные нанокомпозиты, состоящие из органической матрицы и металлических или оксидных наночастиц, полученные из исходного серии металл-органических каркасов М-ВТС (где М = Си2+, Со2+, №2+; ВТС = 1,3,5-бензендикарбоновая кислота) в процессе фазового превращения под электронным облучением, демонстрируют люминесцентные свойства при накачке когерентным излучение на длине волны 350 нм в течение четырех месяцев.

Научная новизна работы:

1. Нелинейно-оптические процессы генерации второй оптической гармоники и трехфотонной люминесценции происходят благодаря уникальным морфологическим особенностям полученных наночастиц, которые представляют собой аморфные металлоорганические частицы или хорошо организованные структуры с металл-оксидным дендритным ядром и аморфной органической оболочкой.

2. Морфология наноразмерных нелинейно-оптических сферических частиц, полученных методом лазерной абляции, обусловлена специфической делокализацией молекулярных орбиталей в исходных металлоорганических каркасах по структурообразующим цепям и определенным звеньям органического лиганда.

3. Переход от одномерной к двумерной топологии металл-органических каркасов на основе ионов меди с лигандами ВРЕ или BPY приводит к образованию высоко кристаллических полых углеродных сфер и полусфер, соответственно,

демонстрирующих линейный оптический отклик, после взаимодействия с инфракрасными фемтосекундными лазерными импульсами (150 фс, 1050 нм).

4. Структурная стабильность полученного полимерного композита на основе полиметилметакрилата с включениями микрокристаллов нецентросимметричного МОК ZIF-8 при накачке инфракрасным лазером превосходит все известные неорганические соединения, такие как золото, кремний, стекло.

5. Динамика формирования композитов из МОК на основе аморфной органической матрицы и наночастиц металлов или оксидов хорошо описывается моделью JMAK.

6. Химический состав образующихся наночастиц в аморфной органической матрице определяется координационной сферой исходного металлоорганического каркаса.

Научная и практическая значимость работы заключается в следующих важных аспектах.

Первый - получение оптически активных, в том числе с нелинейно-оптической активностью, наноразмерных частиц из металлорганических каркасов с использованием высокоинтенсивных лазерных источников. Второй -исследование стабильности оптических свойств композиционных материалов на основе металлорганических каркасов. Описанные подходы к получению оптически активных наноразмерных частиц на основе МОК и композиционных материалов на основе МОК со стабильными во времени оптическими свойствами решают поставленные вопросы, которые еще не были решены в отношении металлорганических каркасов, а именно: 1) получение оптически активных наноразмерных частиц из металлорганических каркасов; 2) достижение стабильность оптической активности в металлорганических каркасах. Кроме того, фундаментальные процессы получения определенных композиционных материалов на основе МОК были подкреплены фундаментальными и теоретическими расчетами и выводами.

Апробация работы. Основные результаты диссертации были представлены на Международной конференции по метаматериалам и нанофотонике METANANO 2019-2021 (два устных выступления и один стендовый доклад); Международной конференции Mendeleev 2019 в Санкт-Петербурге (устный доклад); и 4-й Европейской конференции по металлоорганическим каркасам и пористым полимерам EUROMOF 2021 (стендовый доклад).

Достоверность научных достижений были подтверждены передовыми экспериментальными методами исследования, которые уже были апробированы и широко используются. Кроме того, представленные обоснования полученных

результатов имеют хорошую корреляцию с используемыми вычислительными методами.

Публикации. Основные научные результаты, которые представлены в данной кандидатской диссертации, были подробно описаны в 7 статьях (1 обзор и 6 оригинальных статей), индексируемых в таких научных базах данных, как Scopus и Web of Science, из которых 6 статей из перечня журналов Q1, 1 статья из Q2.

Вклад автора. Данная кандидатская диссертация содержит результаты научных исследований, которые были проведены лично или в сотрудничестве с коллегами. Вклад автора заключается в следующем: в обработке полученных оптических и структурных экспериментальных данных; в большинстве случаев синтезировании исходных модельных объектов; обработка большого объема информации; обобщении полученных результатов в общую мировоззренческую картину механизмов тех процессов, которые наблюдались при взаимодействии лазерного излучения с объектами исследования.

Структура и объем диссертации. Данная кандидатская диссертация содержит введение, шесть глав, заключение и список использованной литературы. Общий объем данной кандидатской диссертации составляет 195 страниц, включая библиографию со 129 ссылками. Работа содержит 78 рисунков и 4 таблицы, которые размещены внутри глав.

ОСНОВНОЕ СОДЕРЖАНИЕ РАБОТЫ

Во введении в рамках данной кандидатской диссертации излагается актуальность научных исследований, определяются основные цели и задачи, формулируются научная новизна, практическая значимость работы, и научные положения, выносимые на защиту.

В первой главе рассматриваются металлорганические каркасы как класс перспективных материалов, в которых можно наблюдать когерентные и некогерентные нелинейно-оптические процессы, такие как генерация второй и третьей гармоник, лазерное излучение, нелинейное поглощение и фотолюминесценция, а также проводится их сравнение с существующими неорганическими материалами, которые широко используются в коммерческой нелинейной оптике.

Во второй главе представлены основные экспериментальные методы, которые использовались для изучения и получения сложных наночастиц на основе МОК, а также синтеза большинства исходных соединений МОК и сложных полимерных композитов на их основе. Здесь также описаны установки, используемые для абляции наночастиц из кристаллов МОК под действием инфракрасного фемтосекундного лазера, и оптические эксперименты.

Третья глава посвящена созданию наноразмерных наночастиц из МОК с помощью метода холодной фемтосекундной лазерной абляции, которые обладают нелинейно оптическим откликом.

В качестве модельных объектов были использованы два взаимопроникающих и гибких трехмерных МОК [73]. Соединение 1 обладает шестикратно взаимопроросшую жесткую и плотную структуру алмазной топологии ^а) с формулой [Cd(sdc)(dimb)]•DMF или C29HзlCdN5O5 (DMF = диметилформамид), и который кристаллизуется в моноклинной и центросимметричной пространственной группе Р2/с. Соединение 2 обладает восьмикратно взаимопроросшую структуру алмазной с формулой [Cd(dmf)(sdc)(dimh)]•DMF или C59H6зCd2N9O9, и которое кристаллизуется в нецентросимметричной пространственной группе Р2Л21.

Оптическая характеризация этих соединений показала, что кристаллы соединений 1 и 2 демонстрируют такие нелинейно-оптические эффекты, как генерация второй гармоники (SHG) и трехфотонная люминесценция (3PL). В соединение 1, были зарегистрированы испускаемые фотоны при 525 нм, а также была обнаружена генерация второй оптической гармоники с квадратичным наклоном 1,85±0,15 при плотности энергии лазера -0,7-7,5 мДж см-2 (Рисунок 1 а). Различные уровни квадратичного наклона в этом случае зависят от симметрии кристаллической структуры, а также и слабого и сильного взаимодействия внутри структуры. Спектр трехфотонной люминесценции этого соединения находится в диапазоне от 2,2 до 3,1 эВ с кубическим наклоном 2,7±0,2 от интенсивности возбуждения. Вторая оптическая гармоника в нецентросимметричном соединении 2 была обнаружена с квадратичным наклоном 1,8±0,2 в широком диапазоне плотности энергии лазера от 0,2 до 6 мДж см-2 (Рисунок 2Ь). Различные уровни квадратичного наклона в этом случае также зависят от симметрии. Спектр трехфотонной люминесценции этого соединения имеет тот же диапазон, что и у соединения 1 (от 2,2 до 3,1 эВ) с наклоном 3,0±0,1 от интенсивности возбуждения.

Более того, при различных плотностях энергии фемтосекундного лазерного излучения эти соединения проявляют себя по-разному. До 7,5 мДж/см2 соединения 1 и 2 имеют стабильные сигналы SHG и 3PL с квадратичным и кубическим наклонами соответственно. При воздействии от 7,5 до 9,5 МДж/см2 наклоны SHG и 3PL изменяются с кубической на квадратичную зависимость и линейную функцию (Рисунок 2). Более высокая мощность лазера (>11,5 МДж/см2) полностью сжигает кристаллы МОК. Исходя из этого, для получения наночастиц из соединения 1 и 2 путем фемтосекундного лазерного облучения были выбраны диапазоны от 10,5 до 11,5 МДж/см2 и от 7 до 8,8 МДж/см2, соответственно, для постсинтетической лазерной абляции наночастиц МОК.

Optical Image SHG/3PL Image SHG/3PL Spectra

ЖмОпдШ (ПЛ1)

Рисунок 1 - Характеристика исходных МОК. Оптические изображения, изображения SHG/3PL, а также спектры SHG/3PL с соответствующим квадратичным и кубическим наклоном (вставка) для а) соединения 1, б) соединения 2. Масштабные полосы, 10 и 5 мкм для оптического и SHG/3PL

изображений, соответственно

а

с. 8

300 400 500 600 700 300 400 500 600 700

Wavelcngth (nm) Waveleogth (nm)

Рисунок 2 - (а, б) Спектры SHG и 3PL для монокристаллов соединений МОК 1 и 2, соответственно, при различной плотности энергии лазера

Для соединения 1 и 2 использовался фемтосекундный лазер при 10 и 8 мДж/см2, соответственно, в качестве наиболее подходящих значений для постсинтетической лазерной абляции наночастиц МОК с лучшей морфологией и формой. Полученные наночастицы были охарактеризованы с помощью сканирующего электронного микроскопа (SEM) и просвечивающего электронного микроскопа (TEM). Было обнаружено, что фемтосекундная лазерная абляция соединения 1 на 10 мДж см-2 приводит к образованию четко определенных сферических частиц с ращмерами от 100 до 1000 нм. Соединение 2 при 8 МДж/см2 превращается в частицы с неправильной структурой и формой с размерами 8003800 нм.

TEM анализ соединения 1 выявил наличие двух типов частиц с различной морфологией. Первый тип - это аморфизованные капли МОК с одинаковым химическим составом и однородным распределением элементов. Второй тип - это частицы сферической формы: их внутренняя структура выглядит как структура типа ядро-оболочка. Оболочка представляет собой аморфизованную органическую фазу, что было подтверждено элементным анализом, рамановской спектроскопией и красным смещением спектров 3PL. Ядро при этом имеет более сложную морфологию и может быть описано как дендритоподобная фаза оксида металла, что было подтверждено на основе анализа TEM и энергодисперсионной рентгеновской спектроскопии (EDX). О такой сложной морфологии производных второго типа с использованием процесса абляции металлорганических каркасов ранее не сообщалось.

Для соединения 2 были обнаружены только производные неправильной формы и аморфная фаза МОК, подтвержденная TEM и красным смещением 3PL. Присутствие и распределение атомов кадмия в полученных наночастицах также было подтверждено анализом EDX в камере TEM.

Список литературы диссертационного исследования кандидат наук Мезенов Юрий Анатольевич, 2022 год

й 90 -

I

(Л

80

10 15 20 25 30 Time (min)

r

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Figure 3. Ultrafast laser melting of MOFs. a) Scheme of interaction of MOF single crystal with 150 fs 1050 nm laser pulses with 80 MHz repetition rate, b) Four regimes (nonperturbed I, transition II, melting III, and burning IV regimes) of the interaction of MOFs with fs laser radiation with different fluence. As an example, the dependence ofSHC intensity (in arbitrary units) and melting efficiency (M.E.) is shown for compound 1 demonstrating the step-like melting. Corresponding images of melted and burned single crystals are shown. Scale bar, 10 pm. For compounds 2 and 3 see Figure S8 in the Supporting Information, c) Raman scattering for single crystal of 1 before and after irradiation by 6 mj cm"2, and for single derivatives of 1. d) Stability of SHC signal from single crystal of! irradiated by 6 mj cm-2 for 30 min.

compound 3 to 1. Moreover, the estimated value of =3% for all compounds corresponds well to the values of 3-5% obtained for other materials with melting points (Bi, M.P. 280 °C and InSb, M.P. 530 °C) close to MOFs used in this study (M.P. at =400 °C).!131 The crystalline semiconductors with higher melting point (>940 °C) require more than 6% of exited valence electrons (Figure lb). We can argue that the presence of number of relatively weak bonds, which are involved in the formation of crystal structurer of MOFs, is also responsible for a decreased value of exited electrons needed to weaken these bonds and soften the MOF lattice.

Statistical investigation of the interaction of 40 single crystals of 1, 2, and 3 with fs pulses of different fluence was also performed (Figure 3b). We reveal that compound 1 is more rigid to the increasing fluence than compound 3, while compound 2 was intermediate in terms of stability. The explanation of this effect (i.e., different fluence thresholds for ultrafast melting) can be given with regard to the analysis of ultrafast dynamics of the interpenetrated structure of such MOFs with flexible building blocks, as well as thorough analysis of metal-nitrogen (metal-oxide) strength.!10"' However, we assume that increase

of the crystal density from =1.2 to 1.5 g cm-3 and decrease in geometrical length of weakly interacting flexible alkali-chained ligands in a row from 3 to 1 limit the compounds' degree of freedom to react on light and their ability to change their conformation. Therefore, the more rigid compound demonstrates higher fluence threshold for melting by fs light. Concerning the thermal stability under the slow heating (see thermogravi-metric analysis, TGA, Figure S6, Supporting Information),!13' there is no evidenced link between TGA data and the threshold for ultrafast melting.

2.3. Ultrafast Melting: The Derivatives

Since the regime III has been shown to be the most suitable for producing the particles (Figure 3b), we use fluences of 10, 8, and 7 mj cm-2 to produce MOF derivatives from compounds 1, 2, and 3, respectively (Figure 4a). To carry out both electron microscopy and optical characterization, the derivatives have been printed on carbon grid and gold substrate. Scanning electron microscopy (SEM, FEI Quanta 200) is used to analyze the

Figure 5. TEM analysis, a) HAADF (high angle annular dark field) and b) bright field TEM micrographs of MOF derivatives from compound 1. c,g-j) Three zoom in images ofthe core-shell derivatives, d-f) EDX analysis reveals the elemental ratio, and line scan confirms the core-shell structure, k) Bright field TEM image ofthe amorphous phase with l,m) corresponding elemental analysis and n) line scan representing the homogeneous element redistribution over the shape. Scale bars: a,b) 1 Jim, c) 80 nm, g-j) 200 nm, and k) 200 nm.

www.afm-journal.de

the derivatives obtained especially from 1 have three key features to be considered for application in nonlinear nanopho-tonics: metal-organic composition, which is important for efficient photoluminescence and enhanced optical response; nonlinear optical response of initial MOFs; and morphology (core-shell structure, size, and shape) which can drive the interaction of light with wavelengths from ultraviolet to IR range.I71-15! Therefore, we investigate the interaction of light with single derivatives by linear and nonlinear optical micro-spectroscopy (Figure 6). Concerning nonlinear optical response, we reveal that 3PL spectra of the derivatives obtained from compounds 1 and 2 are redshifted in comparison to that of initial MOFs (Figure 6a,c). This can be explained by amorphized state of the organic components.!16' In contrast, derivatives from 3 do not demonstrate any photoluminescence signal that can be explained either by low signal of 3PL or complete destruction of organic components. The signals of SHG are also preserved for derivatives from 1 and 2 due to the well-developed surface,'1'•'I distorted organic-inorganic structure,I9al or light-induced broken symmetry.'93-14"'

It is worth comparing the efficiency of SHG from the nanometer scale derivatives obtained from centrosymmetric crystal 1 with that of nanoparticles with (Si) and without (BaTi03) centrosymmetric space group (Figure S10, Supporting Information). Interestingly, we detect SHG signal from the derivatives, that is just 2 orders of magnitude smaller than that for BaTi03 nanoparticleslI5fl of the same diameter (430 nm). Compared to centrosymmetric and optically resonant Si nanoparticle with the diameter of 275 nm, for which the well-developed surface of the nanoparticle can be the reason of optical nonlinearity,'15d' MOF derivatives demonstrate the efficiency of the same order of magnitude. The smaller or bigger Si nanoparticles generate second harmonics with efficiency 1 order of magnitude less (Figure S10, Supporting Information).

Moreover, the ratio of intensities of SHG and 3PL for derivatives and corresponding MOFs (1) is different: SHGj„iv/3P Lderiv ~ 5*SHGMOF/3PLMOF. This effect can be explained by resonant interaction of spherical derivatives of diameter from 100 to 1000 nm and typical refractive index of 2.5 (generally for organic-inorganic materials) with light: high-order Mie-type electromagnetic modes'15' with high quality factor g (Q = Xq/AX, where A^ is the central wavelength of emission and AA is the full width at half maxima) can be optically generated inside such kind of particles supporting an enhancement of intrinsic optical emission.'15dl We reveal these modes by optical spectroscopy (Figure 6b) from single derivatives in the dark-field geometry (Figure S2, Supporting Information). Indeed, SHG signal with high quality (Q = 138) from derivatives overlaps with high order and same high quality Mie modes,l1Sc_el which significantly enhance SHG. Concerning 3PL, this broad spectrum with low quality (Q = 4) cannot be significantly affected by the same Mie modes. This situation is for derivatives from 2, where the low quality of Mie-type modes cannot contribute to enhancement of nonlinear optical response.

Based on these results, one can conclude that the more complicated the morphology of MOF derivatives we have (Figure 5c,g,i), the more efficient linear and nonlinear optical response can be observed. This also raises a question of potential application of amorphous MOFs and the derivatives

in photonics. Indeed, recent reports of MOF derivatives application were focused on catalysis and energy storage. In this regard, our study is one of the first demonstration of developed optical synthesis of active nanophotonics components based on metal-organic frameworks.

3. Conclusion

For the first time the ultrafast melting of 3D interpenetrated MOFs with flexible alkali-chained ligands by femtosecond IR laser pulses was demonstrated as an effective way to produce unique nanometer scale derivatives with complex morphology and enhanced nonlinear optical response. Structurally, the products of this nonequilibrium process can be categorized as complex derivatives representing well-organized spheres with metal-oxide dendrite core and amorphous organic (noncarbon) shell or as amorphous metal-organic species. An ability to produce such derivatives and the complexity of their morphology are a result of a tradeoff between the crystal density, ligand length and flexibility, electronic structure, and thermal stability of the starting MOF compounds. The analysis of ultrafast melting also reveals that the denser structure with shorter flexible ligand, the higher the fluence threshold for ultrafast melting of such MOFs. Moreover, the delocaliza-tion of electronic density contributes to the process of laser-induced melting and results to complexity of the morphology of MOF derivatives. We also demonstrated an enhanced second harmonic generation and three-photon luminescence due to resonant interaction of the 100-1000 nm spherical derivatives with light. The presented data clearly indicate a vast potential of ultrafast melting of MOF for producing new generation of materials for applications in nonlinear nanophotonics and highlight a general methodology for the research on ultrafast melting of other metal-organic framework complexes and related porous materials.

Supporting Information

Supporting Information is available from the Wiley Online Library from the author.

Acknowledgements

N.K.K. and S.B. contributed equally to this work. Authors thank M. O. Barsukova for providing the samples of MOF compounds for the research. V.A.M., N.K.K., and Y.A.M. acknowledge support from the Russian Foundation for Basic Research (Project No. 18-32-20089 mol_a_ved) for optical investigation of MOF derivatives and the Russian Science Foundation (Projects No. 19-79-10241 and 17-19-01637) for investigation the ultrafast melting of MOFs and their composition, respectively. E.P. acknowledges support from the Ministry of education and Science of the Russian Federation (Project 11.1706.2017/4.6). D.S. and E.P. thank NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) for providing for providing access to the SurfSARA supercomputer facilities. S.B., J.C., and T.B. benefited from Programme Hubert Curien-Kolmogorov and would like to thank Dr.-lng. P. Boulet (Institut Jean Lamour, CC X-Gamma) for XRD measurements. AN. acknowledges financial support by the Government of the Russian Federation through the ITMO Fellowship and Professorship program.

Conflict of Interest

The authors declare no conflict of interest.

Keywords

derivatives, femtosecond laser, metal-organic framework, nanophotonics, ultrafast melting

Received: October 8, 2019 Revised: November 1 3, 2019 Published online:

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Adv. Funct. Mater. 2019,1908292

1908292 (11 ofll)

) 2019 WILEY-VCH Verlag GmbH & Co. KG a A Weinheim

Angewandte Chemie International Edition

COMMUNICATION

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Wl LEY-VCH

hydrated HKUST-1 single crystal in air within the visible range by NIR laser pulses to decrease its transmittance by 2 order of magnitude. This allowed us to turn off and on the light (530 nm wavelength) passing through this crystal by increasing/decreasing the fluence of NIR laser (Figure 5a, inset). The rate of switching was 27 s~1, while the relaxation process was slower (0.52 s~1). It should be noted, that the maximal change in transmittance (2 order of magnitude) is achieved at a wavelength of 530 nm (Figure S10), while the transmittance at UV and NIR ranges remain the same. Herein, the higher rates of switching (up to 40 s~1) correspond to 550-600 nm (Figure S7a) that makes the wavelength of 550 nm to be optimal to observe efficient and fast (34 s"1) switching. At second, we have achieved the modulation of initial PL intensity recently demonstrated for photochromic MOFs1211. Indeed, the dehydration of HKUST-1 results in significant changes of electronic transitions (Figure 4). Therefore, It is expected to observe the changes of photoemission properties. For this, we have excited PL by different wavelengths as 450 and 530 nm (6 ps, 60 MHz repetition rate, 45 mJ/cm2). As one can see in Figure 5a, the intensity of PL excited by 530 nm is five time higher due to increased absorption at this wavelength (Figure 1). Herein, the increase of intensity of additional NIR radiation (150 fs, 1050 nm) results in decreasing of PL intensity (Figure 5b) by 30%. Intriguingly, the same PL excited by 450 nm can be increased by this additional NIR radiation up to 20%. To the best of our knowledge, such optical modulation have been demonstrated for the first time in photochromic-free metal-organic frameworks.

In summary we demonstrate a highly reversible absorption band shift from 2.1 eV to 2.5 eV of HKUST-1 with tens to hundreds of S"1 rates due to dehydration of single crystals induced by near-infrared (800 and 1050 nm) ultrafast (fs and ps) laser pulses. The process associated with reversible shrinking of [CU2C4O8] cages of HKUST-1 was confirmed by in-situ Raman spectroscopy, XRD analyzes and DFT calculation. This effect can be applied for communication devices as MOF-based optical switch operating at kHz rate which allows one to turn off/on the visible light passing through the single crystal and modulate the intensity of initial photoluminescence by NIR laser pulses.

Acknowledgements

Authors thank Prof. Thierry Belmonte and Dr. Pascal Boulet for fruitful discussions and association with XRD. Authors also acknowledge Magnetic Resonance Research Centre and Centre for Chemistry Educational Centre of the Research Park of Saint Petersburg State University and Dr. Anton S. Mazur for providing 13C NMR MAS spectra. V.A.M., N.K.K, and Y.A.M. acknowledge financial support from the Russian Science Foundation (Project No. 19-79-10241) for all-optical experiments. D.P and E.A.P acknowledge financial support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725686). The authors thank the Netherlands Organization for Scientific Research (NWO) for the access to SURFsara computational facilities. V.D. and AN. acknowledge financial support by the Government of the Russian Federation through the ITMO Fellowship and Professorship program. A.N. also thanks PHC program for XRD research.

Keywords: metal-organic frameworks, photoswitching, HKUST-1, all-optical switch

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This article is protected by copyright. All rights reserved

Nanoscale

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Received 12th March2019. Accepted 12th April 2019

DOI: 10.1039/c9nr02167a rscli/nanoscale

Leila R. Mingabudinova.í3 Anastasiia S. Zalogina,Jb Andrei A. Krasitin, < b Margarita I. Petrova,b Pavel Trofimov, b Yuri A. Mezenov,b Evgeniy V. Ubyivovk, &c Peter Lonnecke,d Alexandre Nominé,b Jaafar Ghanbaja,6 Thierry Belmonte e and Valentin A. Milichko©*b

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Using a hybrid approach involving a slow diffusion method to synthesize ID and 2D MOFs followed by their treatment with femtosecond infrared laser radiation, we generated 100-600 nm well-defined hollow spheres and hemispheres of graphite. This ultra-fast technique extends the library of shapes of crystalline MOF derivatives appropriate for all-dielectric nanophotonics.

Owing to the synergistic combination of organic and inorganic building blocks providing chemically active nanoporous structures with exceptional mechanics,1 metal-organic frameworks are establishing their position as an important class of highly versatile and tuneable crystalline materials for diverse applications.2-4 Their unconventional property of softness provides provides the structural changes under different stimuli such as pressure, temperature, electric and magnetic fields and light.4-6 Utilizing these stimuli at an extreme regime (high temperatures, strong electric fields, etc.) yields the MOF structure transformation to the amorphous phase' and then to the derivative compound:8-10 amorphous carbon nanostructures, nanocrystalline graphite, metal oxides, chalcogenides, phosphides, carbides and pure metallic nanoclusters. Utilizing MOFs as precursors for the fabrication of nanoparticles with a regular structure (e.g., graphite) and complex shape (hollow spheres, cubes, yolk-shell, etc.) is essential due to key reasons: the MOF crystal itself represents the template for its derivatives

"Physics and Chemistry of Nanostructures Group, Ghent University, B-9000 Gent, Belgium

bDepartment of Nanophotonics and Metamaterials, ITMO University, St. Petersburg,

197101, Russia E-mail: [e-mail]

cSt Petersburg State University, St. Petersburg 199034, Russia

dInstitut für Anorganische Chemie, Universität Leipzig 04103 Leipzig, Germany

'Université de Lorraine, Institut Jean Lamour, UMR CNRS 7198, Nancy F-54011,

France

fElectronic supplementary information (ESI) available: Extended Fig. S1-S16, tables, calculation results, MOF structures and CIF files. See DOI: 10.1039/ C9nr02167a

Î These authors contributed equally.

with well-defined shapes;8-10 the relatively high temperature stability of MOFs allows their transformation to derivatives avoiding the thermal decomposition of pure organic components prior to completion of the transformation process;8 and finally, the regular structure of MOFs yields a uniform distribution and ordering of carbon which is not possible by simple mixing of the carbon precursors.10 However, the existing techniques for fabrication of the MOF derivatives are slow (from tens of minutes to hours) and energy expensive, as well as the limited number of derivative types and shapes constrain the fields of their application by energy storage9-11 and catalysis.12 On the other hand, the wide variety of complex topologies and mesoscale architectures available among the several hundreds of chained (ID) and layered (2D) MOFs reported so far (see the Cambridge Ciystallographic Data Centre) renders these materials unique starting points for fabricating previously implausible nanostructures for nanophotonics and light manipulation.

Here we report a general top-down strategy that enables the fabrication of a new type of MOF derivatives for all-dielectric nanophotonics,13 - nanometer-scale well-defined hollow spheres and hemispheres, whose shells are ciystalline carbon (graphite). We utilize MOFs synthesized via reticular chemistry14 as the template material. We then investigate for the first time the use of infrared femtosecond laser pulses as the means for providing the external stimulus necessary for reassembling the complex structure of MOF single crystals into three-dimensional (3D) crystalline hollow carbon structures (c-HCS). The structures obtained have diameters ranging from 100 to 600 nm and demonstrate the resonant light-matter interaction - the polarization-dependent scattering of light due to the generation of specific Mie-type modes.

At first, the bulk ID MOF crystalline precursors were synthesized by a slow diffusion method following a modified procedure15 (for details, see ESIf) that allows controlled growth of the coordination polymer from a bidentate 1,2-di(4-pyridy 1) ethylene (BPE) ligand and Cu(N0,)2.6(H20) metal precursor.

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complex of SEM and TEM analyses proves that up to 40% of all the nanostructured produced are c-HCSs, and the remaining 60% are amorphous nanoparticles and nanometre to micrometre shards of MOFs.

Besides the conventional application of carbon structures in energy storage,9'10 the current 3D structures hold a great potential for all-dielectric nanophotonics,11'29 where the functional elements should strictly fulfill the following requirements: (i) a high refractive index necessary for a sufficient optical contrast with an ambient medium (air), (ii) specific shape to enable the generation of different optical modes as well as (iii) perfect crystallinity to suppress non-radiative decay of light energy at optical resonances. Indeed, crystalline carbon (graphite) has a relatively high refractive index of 2.6-3.0 within the whole visible range.'" The optical characterization of single c-HCSs in dark field geometry with polarization resolution was performed with a commercial confocal spectrometer, Horiba LabRam (Fig. 3G, H and ESI, S3f). The measurements revealed the colour selectivity in scattering from the blue to red regions of the visible range, depending on the light polarization. This can be explained by the generation of different optical modes (Mie-type) inside the spheres (ESI, Fig. S16t). Intriguingly, due to the hollow shape of c-HCSs, the electric field of the magnetic Mie-type modes should be localized inside the shell," while the magnetic part of light is concentrated in a cavity. Such a behaviour can only be manifested in hollow nanostructures and may find optical and even medical application. Previously such an effect has only been observed for all-dielectric silicon cylinders'1 rendering the current c-HCSs as the first example of a magnetic resonant structure being easy to be fabricated. This provides a unique opportunity for the enhancement and efficient interaction of the magnetic component of light with matter inside the cavity.

In summary, we demonstrate an ultra-fast laser-assisted technique to fabricate new types of MOF derivatives -100-600 nm well-defined hollow spheres and hemispheres, whose shells are crystalline carbon (graphite). This technique overcomes the requirements of specific gas atmosphere, additional precursors and high-energy consumption to heat the compound; this significantly simplifies the synthesis which is important for real-world application. Intriguingly, the technique also allows us to extend the library of shapes and size of ciystalline MOF derivatives. The nanostructures obtained possess unique shapes and crystallinities, and consequently demonstrate polarization-dependent light scattering due to the generation of different optical modes inside important for all-dielectric nanophotonics and light manipulation.

Author contributions

L. R. M. synthesized the initial MOFs. A. S. Z. and P. T. fabricated MOF derivatives. A. A. K. performed SEM analysis and analysed the structures. E. V. U. performed TEM analysis. P. L. performed single ciystal X-ray diffraction

analysis. A. N., J. G. and T. B. performed HRTEM and EDX analysis. L. R. M., A. A. K., M. I. P., Y. A. M., A. N., T. B. and V. A. M. analysed the results and wrote the paper. V. A. M. conceived and supervised the work.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to thank Prof. E. A. Pidko (TU Delft) for fruitful discussions, Dr S. A. Sapchenko (Nikolaev Institute of Inorganic Chemistry) for kindly providing rigid 3D MOFs, Dr A. V. Vinogradov (ITMO University) for support in reticular synthesis, Prof. Kristof Van Hecke (Ghent University) for ciys-tallographic support, Dr D. A. Zuev (ITMO University) for assistance in laser ablation, Dr S. V. Makarov (ITMO University) for discussion on optical scattering, and F. E. Komissarenko (ITMO University) for preliminary SEM analysis. This work is a part of a joint Russian-French project "Hybrid photonics nanodevices" and is supported by the Ministiy of Science and Higher Education of the Russian Federation (project No 14.587.21.0050 with unique identification RFMEFI58718X0050). ITMO University is acknowledged for providing access to the unique scientific instruments (AliTT-NT TrIOS) for optical experiments.

Notes and references

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Communication

Polymer Matrix Incorporated with ZIF-8 for Application in Nonlinear Optics

Yuri A. Mezenov Nikita K. Kulachenkov Andrei N. Vankin 1 , Sergey S. Rzhevskiy1, Pavel V. Alekseevskiy 1 , Venera D. Gilemkhanova 1, Semyon V. Bachinin 1/ Vyacheslav Dyachuk 1,2 and Valentin A. Milichko 1'3'*

1 Department of Physics and Engineering, ITMO University, 197101 St. Petersburg, Russia; [e-mail] (Y.A.M.); [e-mail] (N.K.K.); [e-mail] (A.N.Y.); [e-mail] (S.S.R.); [e-mail] (PV.A.); [e-mail] (V.D.G.); [e-mail] (S.V.B.); [e-mail] (V.D.)

2 A.V. Zhirmunsky National Scientific Center of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, 690041 Vladivostok, Russia

3 Institut Jean Lamour, Université de Lorraine, UMR CNRS 7198, F-54011 Nancy, France * Correspondence: [e-mail]

Abstract: Polymers with embedded metal-organic frameworks (MOFs) have been of interest in research for advanced applications in gas separation, catalysis and sensing due to their high porosity and chemical selectivity. In this study, we utilize specific MOFs with high thermal stability and non-centrosymmetric crystal structures (zeolitic imidazolate framework, ZIF-8) in order to give an example of MOF-polymer composite applications in nonlinear optics. The synthesized MOF-based polymethyl methacrylate (PMMA) composite (ZIF-8-PMMA) demonstrates the possibility of the visualization of near-infrared laser beams in the research lab. The resulting ZIF-8-PMMA composite is exposed to a laser under extreme conditions and exhibits enhanced operating limits, much higher than that of the widely used inorganic materials in optics. Overall, our findings support the utilization of MOFs for synthesis of functional composites for optical application.

Keywords: metal-organic frameworks; polymers; composite; nonlinear optics; second-harmonic generation

1. Introduction

Metal-organic frameworks (MOFs) represent a new class of crystalline materials consisting of metal-containing nodes connected by multitopic organic linkers. MOFs stand out due to their unconventional hierarchical structure, high porosity and flexibility, which turn them into prospective materials for many applications in chemistry [1], biology [2], and physics [3]. The features of the crystal structure of specific MOFs, such as structural diversity [4], the lack of an inversion center (non-centrosymmetric) [5], and high resistance to heating [6] and humidity [7], open up new perspectives for using them as active materials for nonlinear optical devices [8-10]. Moreover, the fabrication of MOF thin films and the integration with polymers [11] can trigger the development of new chemical technology [12] for the fabrication of active optical elements compatible with industrial devices and optical metrology.

The fabrication of composite materials, integrating MOF micro crystals with polymeric binders, has recently been investigated in relation to their diverse chemical applications [11,13-19]. The resulting thin films and membranes, with a thickness of hundreds of nanometers to microns, exhibit functional

Nanomaterials 2020,10,1036; doi:10.3390/nanol0061036 www.mdpi.com/joumal/nanomaterials

Received: 9 April 2020; Accepted: 25 May 2020; Published: 28 May 2020

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properties for advanced gas storage and separation. Generally, MOF compounds such as ZIF-8, UiO-66, MIL-101, MIL-53, and HKUST-1 and others [11] are incorporated into thermoplastic polymer matrices such as poly(methyl methacrylate) (PMMA), polysulfone (PSF), polydimethylsiloxane (PDMS) or polyvinylidene difluoride (PVDF). There, the homogenous distribution of MOF microcrystals with unperturbed crystallinity can be achieved for MOF loadings ranging from 2.5 up to 87 mass. From a physical point of view, the latter allows one to successfully employ the specific physical properties of MOFs, such as the generation of high optical harmonics and structural resistance under extreme conditions, in nonlinear optical applications. Indeed, highly intense ultraviolet (UV) and infrared (IR) laser radiation can damage the structure of organic nonlinear optical materials, limiting device operation cycles. In this sense, MOFs can be considered as competitive materials for nonlinear optics.

Here, we report the synthesis of MOF-based PMMA composites that allow for the visualization of near-infrared laser radiation under extreme conditions in the research lab. This is possible due to the stable second-harmonic generation (SHG) of non-centrosymmetric MOFs dispersed inside the optically transparent polymer matrix under highly intense laser pumping. We confirm the reliability of the composite under such irradiative conditions and regimes, which cause damage to inorganic materials, such as gold and silicon. Our findings support this approach for the preparation of functional MOF-based PMMA composites and significantly contribute to the field of nonlinear optics.

2. Materials and Methods

As a model MOF with a non-centrosymmetric structure, we utilized the commercial compound ZIF-8 (Sigma-Aldrich, Ludwigshafen, Germany), a member of a widespread zeolitic imidazolate framework subclass of MOFs [20]. This compound consists of a Zn tetrahedra coordinated with 2-methylimidazolate linker units, which results in a zeolite-like sod topology. ZIF-8 demonstrates a strong intrinsic SHG signal due to its non-centrosymmetric cubic I-43m space group symmetry [21]. The value of the second order nonlinear optical coefficient for ZIF-8 is also relatively high, compared to commercial inorganic crystals such as potassium dihydrogen phosphate (K.DP) and other non-centrosymmetric MOFs [9,21]. Moreover, ZIF-8 is transparent in the visible and near-infrared ranges and is remarkably stable, both thermally [22] and chemically. Indeed, ZIF-8 remains SHG-active upon heating up to 450 °C, when decomposition occurs [21], while structural deformations start at ~300 in air [22,23]. The average size of used crystals was optically estimated to be several hundred nm, allowing us to detect a SHG signal [21 ].

As a polymer, we selected PMMA, an optically transparent compound with a high resistance to heating of up to 150 °C [24]. For the synthesis of ZIF-8-PMMA composites, we used a solution based on methyl methacrylate (Sigma-Aldrich) as the main monomer, ethylene glycol dimethacrylate (Sigma-Aldrich) as a stitch reagent to create the 3D structure of polymethylmethacrylate, and 2-hydroxy-2-methylpropiophenone (Sigma-Aldrich) as a photoinitiator for UV polymerization in the ratio of 1:0.5:0.02, respectively. The powder weight of ZIF-8 was calculated as 4, 8 and 12 mass % depending on the weight of the monomer for each experiment. Before the process of polymerization under UV, a sufficient quantity of ZIF-8 was successfully dispersed in the main suspension within 30 s under sonication. The final suspension was exposed to UV irradiation for 10 min to create composites with different quantities of ZIF-8 in a matrix of polymethyl methacrylate. The resulting ZIF-8-PMMA composites have the form of pellets 1 inch in diameter and 2 mm in thickness (Figure la).

For the optical experiments, the pellets of ZIF-8 powder and ZIF-8-PMMA composite were placed on a glass substrate. The SHG spectra measurements in air were performed on a self-made confocal microscope setup (Figure lb). The excitation was implemented via Mitutoyo M Plan APO MR lOx objective (numerical aperture NA = 0.26) by Yb3+ laser (TeMa, Avesta Project, Troitsk, Russia) with a 1047 nm central wavelength and 150 fs pulse duration, operating at different regimes—an 80 MHz and a 1 kHz pulse repetition rates. The integral power of the laser varied from 0 to 500 mW (80 MHz) and 12 mW (1 kHz), allowing us to observe laser damaging (dewetting) or the ablation of inorganic solids like gold and silicon [25]. The transmission signal was collected by Mitutoyo HR

NIR 50x objective (numerical aperture NA = 0.65) and then analyzed by a confocal spectrometer, HORIBA Labram (Longjumeau Cedex, France) with a water-cooling Andor DU 420A-OE 325 CCD (charge-coupled device) camera and 150-g/mm diffraction grating. The diameters of the area of excitation and the detection of the SHG signal were 5 and 1 (im, respectively. Optical images of the samples, irradiated by white light, and SHG signals from the pellets were obtained by a commercial CCD camera in reflection mode (Figure lb).

In order to estimate the damage threshold of the ZIF-8-PMMA composite upon IR laser irradiation under different regimes, we compared the behavior of the composite with a 50-nm-thick Au film on the glass substrate and bulk crystalline silicon under similar extreme radiative conditions. For this purpose, the laser beam a with varied integral power and pulse repetition rate was focused on the surface of films and composites using a Mitutoyo M Plan APO NIR lOx objective (numerical aperture NA = 0.26); the imaging of the results of this light-matter interaction were performed by CCD (Figure lb).

515 520 525 530 535 30 40 50 60 70 80 90

Wavelength, nm Power. mW

Figure 1. (a) Schematic illustration of ZIF-8- polymethyl methacrylate (PMMA) composite synthesis, (b) Optical scheme of imaging of the composite and analysis of SHG intensity. PP - pulse picker to produce 1 kHz pulse repetition rate; filter is used to tune the integral power, (c) Initial second-harmonic generation (SHG) signal of ZIF-8 powder excited by IR laser radiation (1047 nm central wavelength, 150 fs pulse duration, 50 mW, 80 MHz repetition rate) and corresponding SHG power dependence with a quadratic 2.02 ± 0.05 slope (d).

3. Results and Discussion

One of the key aspects of optical metrology in most research laboratories is measurement of laser beam parameters (wavelength, power and pulse energy, profile, coherence, etc.) and their adjustment. Most laser systems allow one to operate with coherent pulsed (ns, ps, and fs) and continuous wave (cw) radiation from the deep UV to IR regions. In this case, the optical elements of the optical systems can vary depending on the radiation parameters. Alignment systems, such as laser viewing cards [26], can be respectively universal, however. These cards are made of plastic with a photosensitive area. This area of each card makes it easy to determine the location of the UV, visible or IR laser beam and its focal point. However, the thermal effect of laser radiation of high intensity can lead to the destruction or burning of the surface layer. In this sense, utilizing PMMA embedded with the stable and non-centrosymmetric structures of MOFs can be crucial.

Figure la represents the resulting PMMA composites containing ZIF-8 microcrystals with different mass concentrations (4% to 12%). It should be noted that an increase in the mass fraction of ZIF-8 makes it possible to achieve a more uniform distribution of microcrystals within PMMA volume. Moreover, it allows for a more detailed visualization of the IR laser beam due to intense SHG signal and diffusion scattering between ZIF-8 microcrystals. The pellets of the composite were polished in order to form smooth surfaces and remove residual ZIF-8 powder.

Figure lc,d represents SHG spectra for ZIF-8 powder. Since second-harmonic generation is a process of coherently emitting doubled frequencies (1/Aj + 1/Aj = I/A2), where A\ and A2 are the wavelengths of excitation (1047 nm) and emission, we detected an SHG signal at 523.5 nm. A quadratic slope (2.02 ± 0.05) for SHG intensity is also observed within the laser power, ranging from 30 to 90 mW (Figure Id).

In the second stage, we analyzed the ZIF-8-PMMA composite with 12 mass % of MOF crystals for the visualization of the laser beam with high intensity. The integral laser powers were chosen to be 10 and 100 mW for 1 kHz and 80 MHz pulse repetition rates, respectively. Under such extreme conditions, inorganic materials such as gold film (Figure 2a) and bulk silicon (Figure 2b) are completely destroyed. The laser beam causes the surface of gold to melt and makes silicon ablate due to high absorption coefficients [25]. This usually occurs at temperatures slightly lower than the melting point of the material. Such radiation conditions are also critical for organic polymers [27]. However, both the ZIF-8 powder and ZIF-8-PMMA composite demonstrate an intense SHG signal without visible damaging (Figure 2d-f). As mentioned above, ZIF-8-PMMA with a higher concentration of MOF crystals allows for a more detailed visualization of the laser beam profile (Figure 2f). Here, the more inhomogeneous the structure is, the more inhomogeneous the obtained SHG signal is (Figure 2e). Moreover, we irradiated ZIF-8 for 1 min by unfocused IR laser radiation with 500 mW integral power (80 MHz repetition rate), which corresponds to the power needed for damaging of several glasses and optical lenses [28]. Surprisingly, unchanged optical images of the microcrystals and SHG signal were detected.

Figure 2. (a) Au film (50-nm thick) and (b) bulk crystalline silicon substrates damaged by laser radiation (1047 nm central wavelength, 150 fs pulse duration, 10 mW, 1 kHz repetition rate). Scale bars (a and b), 50 nm. Operation at 80 MHz pulse repetition rate results to more dramatic damaging of Au film and silicon, (c) Optical image of ZIF-8 powder with corresponding SHG signal (d) upon irradiation with an integral power of 100 mW (80 MHz). Scale bars (c and d), 20 |xm. (e,f) Optical images of SHG for the ZIF-8-PMMA composite with 4 and 12% of ZIF-8. Scale bars (e and f), 50 fim. The optical images of SHG from ZIF-8-PMMA composite at 1 kHz pulse repetition rate are similar to (e,f).

It is worth noting that decreasing the integral laser power (from 100 to 90 mW) reduces the brightness of the SHG picture by -25%; however, it provides the significantly longer stability of the signal. Indeed, we have analyzed the operating time under extreme laser conditions—regimes of 100 mW, 80 MHz and 10 mW, 1 kHz. Figure 3a shows that the SHG intensity for a ZIF-8-PMMA composite irradiated with a 1 kHz repetition rate remains slightly constant (error of 10%) for 2 h. In this case, the heating effect of laser exposure does not affect the optical properties of the composite as it does for gold and silicon (Figure 2a,b). In contrast, upon irradiation by laser pulses with 80 MHz repetition rates, the SHG intensity decreases over time both for ZIF-8 powder and ZIF-8-PMMA composite. The intensity drops by 30% (for ZIF-8 powder) and 85% (for ZIF-8-PMMA) within 2 h of continuous exposure to the laser radiation. Further analysis of SHG signal intensity over time shows that the output of the signal reached a plateau. The behavior of ZIF-8 here can be explained by laser-induced heating: the inducing defects or thermodynamically possible rotation of linkers irreversibly create local centers of centrosymmetry [21,29]. Compared to ZIF-8 powder, the ZIF-8-PMMA composite demonstrates a higher decrease in SHG intensity. One possible explanation for such behavior is the laser-induced heating of the polymer matrix itself with the resultant damage to the irradiated area of the composite. Taking into account the fact that PMMA [24] demonstrates less thermal stability compared to ZIF-8 [22], the stronger decrease in the SHG signal for the composite can be attributed to the additional local deformations of the polymer during heating. This has also been confirmed by the analysis of the thermal stability of SHG intensity for the ZIF-8-PMMA composite (Figure 3b) during heating on the Peltier element. The decrease in the SHG intensity, excited by 50 mW at an 80 MHz repetition rate, starts at 80-90 °C. This temperature is more likely to affect the structure of the polymer with structural resistance to heating up to 150 °C [24] than MOF itself (300 °C [22,23], respectively).

a

b

♦ ZIF-8 PMMA (80 MHz) O ZIF-8 PMMA (1 kHz)

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♦

0.0 - ♦ ZIF-8 PMMA

0.0

0 30 60 90 120 150 Time, min

20 40 60 80 100 Temperature,°C

Figure 3. (a) Evolution of the SHG intensity for ZIF-8 powder and ZIF-8-PMMA composite upon laser radiation (1047 nm central wavelength, 150 fs pulse duration, 80 MHz and 1 kHz repetition rate) with an integral power of 100 mW (80 MHz) and 10 mW (1 kHz), (b) The thermal stability of SHG signal, excited by 50 mW at an 80 MHz repetition rate, for the ZIF-8-PMMA composite analyzed by heating on the Peltier element.

4. Conclusions

In this study, we used specific MOFs (ZIF-8) with high thermal stability and non-centrosymmetric crystal structures to demonstrate of their applications in nonlinear optics. We synthesized an MOF-based polymer composite for the visualization of IR-pulsed laser radiation with different regimes (80 MHz and 1 kHz pulse repetition rates) and high integral power. The resulting ZIF-8-PMMA composite demonstrates a damage threshold much higher than that of widely used materials in optics, like gold and silicon. Herein, the composite demonstrates a more stable operation under the laser regime of 1 kHz for 2 h, while the laser regime of 80 MHz induces heating, with a negative effect on the optical properties of the composite. Overall, these findings support our approach to functional composite preparation and contribute to nonlinear optics research.

Author Contributions: V.A.M. designed the project. Y.A.M. and V.D. contributed to the synthesis of composites. N.K.K., Y.A.M. and A.N.Y. performed the optical measurements. S.S.R., P.V.A., V.D.G., and S.V.B. contributed to the analysis of the experimental data. All authors have read and agreed to the published version of the manuscript.

Funding: This work is a part of joint Russian-French project, "Hybrid photonics nanodevices", and is supported by The Ministry of Science and Higher Education of the Russian Federation (project No 14.587.21.0050 with unique identification RFMEFI58718X0050). ITMO University is acknowledged for providing access to the unique scientific instruments (AIST-NT TrIOS) for optical experiments.

Conflicts of Interest: The authors declare no conflict of interest.

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Contents lists available at ScienceDirect

ELSEVIER

Photonics and Nanostructures - Fundamentals and Applications

journal homepage: www.elsevier.com/locate/photonics

Probing the dynamics of Cu nanoparticle growth inside metal-organic frameworks upon electron beam irradiation

Yuri A. Mezenov'1'1, Stéphanie Bruyère1'1, Nikita K. Kulachenkov'1, Andrei N. Yankin ', Sergey S. Rzhevskiya, Pavel V. Alekseevskiy1, Venera D. Gilemkhanovaa, Semyon V. Bachinin", Vyacheslav Dyachuka'L, Andrei A. Krasilina,d, Julien Zollinger1', Thierry Belmonte1', Alexandre Nominé ',b'*, Valentin A. Milichkoa,b'*

'' Department of Physics and Engineering, ITMO University, Kronverksky Pr. 49, SL Petersburg, 197101, Russia b Université de Lorraine, CNRS, UL, F-S4000, Nancy, France

' A V. Zhirmunsky National Scientific Cotter of Marine Biology, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, 690041, Russia d loffe Institute, Politekhnkheskaya 26, SL Petersburg, 194021, Russia

H)

ARTICLE INFO

ABSTRACT

Keywords:

Metal-organic framework Nanoparticles Crystal growth Electron microscopy

Metal-organic frameworks (MOFs) represent a unique platform for fabrication of nanoparticles (NPs) of diverse composition and crystallinity. The growth of NPs from constituent parts of MOFs is usually initiated by external stimuli such as temperature, light and electron irradiation. Herein, the kinetics and NP growth mechanisms remain unexplored. Here, we utilized electron irradiation to initiate the nucleation and growth of crystalline Cu NPs of tunable size from several nanometers to hundreds of nanometers inside MOF as a precursor. Simultaneously, the process of the NPs growth, captured in real time using transmission electron microscope, demonstrates the evolution of their size, shape and spatial distribution. We also analyze the NP growth by the classical kinetic theory taking into account a phase transformation. Our results contribute to crystal engineering and developing of functional MOF-based nanocomposites.

1. Introduction

Metal-organic frameworks (MOFs) hold a special place in material science and crystal engineering [1]. Over the last two decades, such materials have attracted increasing attention of researchers in chemistry, biology and physics [2-4]. Belonging to a class of porous coordination polymers, MOFs consist of metal-containing nodes connected by multi-topic organic linkers. As a result, they are characterized by hierarchical structure with high porosity, chemical versatility and flexibility, which allow them to demonstrate advanced adsorption, catalytic, mechanical and radiative properties [2-7].

The development of synthetic approaches and post-processing methods brings MOFs closer to commercial applications [8]. On the one hand, the compatibility of MOFs with chemical vapour deposition technology [9], plasma synthesis [10], and thin film growth [11] supports their integration into microelectronic devices. On die other hand, the post-processing of MOFs by heat [12], light [13,14] and electron irradiation [15-18] highlights their application as optical glasses and

active optical elements in nanophotonics. Indeed, such post-processing initiates the deformation of framework backbone and breaking of chemical bonds [19] leading to formation of amorphous glassy compounds [12], nanocomposites [20] and nanoparticles (NPs) with different optial properties [13,14,21]. Herein, compared to investigating mechanisms of nucleation and growth of initial MOFs [1] and different inorganic NPs [22-24], the kinetics of formation of NPs from constituent parts of MOFs remain unexplored.

In this work, we report on exposure of MOFs to focused electron beam (e-beam) of transmission electron microscope (TEM, Fig. la) to initiate the nucleation and growth of crystalline metallic NPs with tunable size from ~2 nm to -100 nm. The process of the NPs growth, captured in real time, demonstrates the evolution of their size, shape and spatial distribution and can be analyzed by classical kinetic theory taking into account the phase transformation [23,24].

"Corresponding authors at: Department of Physics and Engineering, ITMO University, Kronverksky Pr. 49, St. Petersburg, 197101, Russia.

E-mail addresses: [e-mail] (A. Nominé), [e-mail] (V.A. Milichko). 1 Equal contribution.

https://doi.Org/10.1016/j.photonics.2020.100832

Received 8 May 2020; Received in revised form 10 June 2020; Accepted 14 June 2020

Available online 20 June 2020

1569-4410/ © 2020 Elsevier B.V. All rights reserved.

Fig. 1. (a) Schematic illustration of exposure of HKUST-1 single crystal to focused electron beam in TEM chamber for initiation and controll the growth of Cu NPs. (b,c) Optical image and atomic force microscopy (AFM) micrograph of HKUST-1 single crystal. Scale bars, 20 ^im and 10 |im. Inset (c): AFM cross section of single crystal representing the heigth/length ratio as 1/4,6. (d-g) Bright field TEM micrographs illustrating the dynamics of the NP growth inside HKUST-1 under magnification of 60 k. Scale bar, 200 nm. Corresponding video is in SI, video 1. (h-k) Dynamics of the NP growth inside HKUST-1 under magnification of 80 k. Scale bar, 100 nm. Corresponding video is in SI, video 2.

2. Results and discussion

As a model compound, we have utilized the commercially available HKUST-1, Basolite® C 300 produced by BASF (Fig. lb). This is highly porous three-dimensional compound, Cu3BTC2, composed of Cu (II) ions and benzene-1,3,5-tricarboxylate (BTC) linkers [25]. HKUST-1 is also a representative class of MOFs that are easy to grow on different surfaces following a layer-by-layer approach [26], which makes it promising for applied research. Moreover, the compound has been recently investigated to reveal the mechanism of electron-induced decomposition and the reason for NPs formation [15-18].

The composition of HKUST-1 and its cubic space group Fm3m have been confirmed by Raman spectroscopy and powder X-ray diffraction, respectively, in a recent article of ours [27]. TEM experiments on initiation and simultaneous analysis of the NPs growth have been carried out using MET - ACCEL ARM 200 F operating at 200 kV (emission 15 ^lA and various current density). For this, the powder of HKUST-1 has been placed on carbon TEM grids. Fig. ld-k represents distinctive TEM micrographs captured in time with defferent magnification (60 and 80 k). In contrast to previous results [28,29], demonstrating that MOFs begin to lose their crystallinity when the cumulative electron dose reached 10-20 e~ A-2 with 300 kV-accelerated electron beam, the pores of HKUST-1 are not observable under our conditions. This is due to possible instability of the crystal structure under vacuum and electron irradiation with a higher electron density.

Comprehensive analysis of in situ e-beam initiation of the NPs

growth inside the HKUST-1 single crystals and the dynamics of the

growth reveal the following:

(i) Highly crystalline NPs can grow inside HKUST-1 under focused e-beam (Fig. 2). Energy dispersive X-ray (EDX) analysis with a JEOL Centurio, 1 sr detector, shows the exclusive presence of Cu within the NPs after electron irradiation (Fig. 3). This observation is confirmed by the fact that measured interplanar distances (< dhki > = 0.207 ± 0.004 nm) are in excellent agreement with the (111) interplanar distance of Cu (dni = 0.2087 nm). Based on the previous results on the effect of e-beam on the chemical bounding of MOFs [15-18] and similar reason for growth of Pt and Au NPs from organometallic precursors [30,31], the possibility of the growth of the NPs from the constituent parts of HKUST-1 is due to breaking of bonds between copper and organic part Moreover, the growth of pure Cu NPs instead of its oxide form is explained by a lower oxidation potential of Cu and relatively weak Cu—COO bond energy, which promote the metallic state [15].

(ii) The growth process begins mainly on the MOF's surface, and then propagates throughout the whole volume (SI, video 3). It should be noted that full conversion of MOF to the nanocomposite can be observed after several minutes (Fig. 4a and b). Herein, the time threshold of the detection of small NPs (5 and 11 s for 60 and 80k magnification) in TEM is observed (Fig. 5a and b), which confirms the relatively slow free Cu atom movement process to form a nucleus of 1 - 3 nm in diameter with other free atoms.

Fig. 3. (a,b) STEM-EDX map of HKUST-1 before focused e-beam irradiation confirming the homogeneous redistribution of the elements (C, Cu, O). Scale bar, 300 nm. (c,d) STEM-EDX map of HKUST-1 after focused e-beam irradiation. Cu NPs are represented in Red. Scale bar, 250 nm. Overlapping of Cu and O is represented in (b,d) demonstrating homogenity and inhomogenoty of Cu redistribution, respectively.

0 10 20 30 40 50

70 80 90 100 110 120 130

Time S

.............. < ><

/ ~ 1 r T | T T -JMAK model ^ exp data

20 30 40 50 60 70 Time, s

90 100 110 120 130

Fig. 5. (a,b) Analysis of the dynamics of the NP growth (size and the number of NPs) under focused e-beam irradiation with magnification of (a) 60 k and (b) 80 k. Irradiation started at t = 0 s, while the NPs with a minimal size of 2.5 nm have been detected by TEM in 11 and 5 s, respectively. (c,d) Time evolution of the Cu NP volume under e-beam irradiation with magnification of (c) 60 k and (d) 80 k. JMAK model (Eq. (1)) represents the red curve, while experimental data (the volume of growing NPs) are represented in squares. Dashed lines are the error of the JMAK model based on the correct analysis of the volume of MOF and the NPs.

(vi) Finally, the shape of the growing NP can also vary due to coalescence (Fig. 4e and f) [24]. As a result, spherical NPs with —100 nm in diameter can be observed. In turn, smaller NPs demonstrate well-defined faceted shape that is commonly encountered for crystalline Cu NPs [32]. Since the larger NPs come from the assembling of several smaller NPs, they are more likely to be poly-crystalline and therefore closer to an overall spherical shape. Smaller NPs are more likely to be single crystals and therefore the growth is more likely to follow lower energy directions.

Based on experimental results, we then model the kinetic of phase transformation. The nucleation and growth can be modelled by either the Gualtieri or Johnson-Mehl-Avrami-Kolmogorov (JMAK) models. Van Vleet et al. have demonstrated that the Gualtieri model is more appropriate for the prediction of MOF nucleation and growth [1]. However, in the present case, the phase transformation corresponds to a solid - solid one (MOF Cu NPs + glassy phase). Moreover, by separating the nucleation and the growth process, the Gualtieri model requires input parameters of both steps. In the present case, the determination of nucleation input parameters is challenging due to detecting of the NPs with a minimum diameter of — 2 nm (Fig. 5a and b). Hence, we model the growth of Cu NPs using classical growth theory with JMAK equation [23,33]:

VNP(t) = V^l - exp(-Ktn)) (1)

where V,wp is the volume of Cu NP formed, vf^ the maximum volume of Cu that can be formed, K and n the JMAK coefficients [23]. The first JMAK coefficient K is expressed by:

K ■■

7rNrs 3

(2)

with N the nucleation rate (number of new nuclei per unit of volume per unit of time) and r the linear growth rate. The later can be estimated from the experimental data as 0.22 nm s-1 and 1.25 nm s_1 for magnification of 60k and 80k, respectively (Figs. 5a and b, and SI). N is estimated by the following equation [34]:

where d is the NP size (Fig. 5a and b). The parameter n accounts for the dimensionality of the transformation, the nucleation and the growth type (interface or diffusion limited). If the nucleation rate is nonzero and time-independent, n = m + 1, where m is equal to the dimensionality of the phase transformation for the interface-limited growth and one-half of the dimensionality for diffusion-limited growth [33]. Therefore, n has been chosen as equal to 4. This choice is first sustained by relatively small size of the NPs as compared to MOF crystal thickness varying from 0.1-10 (im (atomic force microscopy analysis, Fig. lc), allowing a three dimensional growth of the crystals. Moreover, the evolution of the growth rate over time is linear (Fig. 5a and b), while parabolic evolution is expected for a diffusion-driven transformation. Rather than a diffusive process, the nucleation and growth of the NPs is attributed to breaking of the chemical bonding by the e-beam, leading to free Cu atoms precipitate into the NPs. This is supported by the fact that some NPs are facetted (Fig. 2d) which tends to show that, while Cu is growing, attachment kinetics is the limiting process. This can be illustrated by the JMAK model predicting a transformation time of 60 s and 25 s for 60 k and 80 k magnification, respectively (Fig. 5c and d). Therefore, it is possible to tune the quantity of Cu NPs before this time. Once the transformation is completed, the exposure time allows one to control the NP number and size thanks to the coalescence phenomenon. Fig. 5c and d also demonstrates the comparison of the experimental data and JMAK modelling of the time evolution of the NPs volume inside MOF, Eq. (1). As in case of the complicated process of growth of the crystals [23], one can observe a good agreement between the growth theory and the experimental results. This allows us to speculate on relatively classical microscopic-level processes of the crystalline NP growth inside MOF from its constituent parts due to breaking of the bonds between metal ions and organic linkers.

3. Conclusion

In summary, we experimentally analyzed and modeled the growth of crystalline Cu NPs of tunable size from ~ 2 nm to ~ 100 nm from the constituent parts of MOFs. For this, we utilized focused electron beam to initiate the nucleation and growth of NPs inside HKUST-1. The

process of growth, captured in real time using transmission electron microscope, demonstrated the evolution of NP size, shape and spatial distribution. The experimental results are in a good agreement with the classical kinetic theory of the NP growth using JMAK model taking into account a phase transformation. Our results contribute to crystal engineering and developing of functional MOF-based nanocomposites.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work is a part of joint Russian-French project "Hybrid photonics nanodevices" and is supported by The Ministry of Science and Higher Education of the Russian Federation (project No 14.587.21.0050 with unique identification RFMEFI58718XX0050). ITMO University is acknowledged for providing access to the unique scientific instruments (AIST-NT TrIOS) for AFM experiments.

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.photonics.2020. 100832.

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Declaration of Competing Interest

Acknowledgements

Appendix A. Supplementary data

References

Inorganic Chemistry

pubs.acs.org/IC ^-Ti

Insights into Solid-To-Solid Transformation of MOF Amorphous Phases

Yuri A. Mezenov," Stephanie Bruyere, Andrei Krasilin, Ekaterina Khrapova, Semyon V. Bachinin, Pavel V. Alekseevskiy, Sergei Shipiloskikh, Pascal Boulet, Sebastien Hupont, Alexandre Nomine, Brigitte Vigolo, Alexander S. Novikov, Thierry Belmonte, and Valentin A. Milichko*

Cite This: Inorg. Chem. 2022,61,13992-14003

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ABSTRACT: Metal—organic frameworks (MOFs) have been recently explored as crystalline solids for conversion into amorphous phases demonstrating non-specific mechanical, catalytic, and optical properties. The real-time control of such structural transformations and their outcomes still remain a challenge. Here, we use in situ high-resolution transmission electron microscopy with 0.01 s time resolution to explore non-thermal (electron induced) amorphization of a MOF single crystal, followed by transformation into an amorphous nanomaterial. By comparing a series of M-BTC (M: Fe3+, CoJt, Co2+, Ni2*, and Cu2+; BTC: 1,3,5-benzentricarboxylic acid), we demonstrate that the topology of a metal cluster of the parent MOFs determines the rate of formation and the chemistry of the resulting phases containing an intact ligand and metal or metal oxide nanoparticles. Confocal Raman and photoluminescence spectroscopies further confirm the integrity of the BTC ligand and coordination bond breaking, while high-resolution

imaging with chemical and structural analysis over time allows for tracking the dynamics of solid-to-solid transformations. The revealed relationship between the initial and resulting structures and the stability of the obtained phase and its photoluminescence over time contribute to the design of new amorphous MOF-based optical nanomaterials.

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rystalline metal—organic frameworks (MOFs) appeared to be promising solids for application as gas sorbents,1 separators,2 and new active elements in opto- and microelectronic dcvices. The highly ordered porous structure of MOFs provides an efficient and lossless propagation of mechanical,'' thermal, and electromagnetic energy/ as well as molecules" and charged particles along the structure,10 which underlies the operation of the above devices. Meanwhile, the transformation of the parent crystal structure into various amorphous and liquid phases11-15 allows for designing new MOF-based materials for which a long-range order plays a minor role. Such amorphous (glassy and non-glassy MOFs) and liquid counterparts can be obtained generally by heating before the decomposition temperature, followed by fast or slow cooling. Herein, the organic ligands remain intact, and the coordination binding between them and metal clusters is partially retained. In contrast, overcoming the decomposition temperature leads to the formation of amorphous materials' -1" which consist of fully or partially decomposed ligands, carbon, and a wide range of nanoparticles. Herein, no structural integrity and coordination bonds are observed.

The resulting amorphous phases demonstrate highly promising mechanical, catalytic and optical properties,1, 21 ~2, being nonspecific for their crystalline analogues. Nevertheless, in spite of the great potential of such phases, the dynamics of their

transformation and the effect of parent MOF chemistry on the result yet remain unclear. Very recently, Rasmus S.K. Madsen and colleagues 1 investigated the glass formation mechanism in ZIF-62, utilizing ultrahigh-field 6 Zn solid-state nuclear magnetic resonance spectroscopy. The authors revealed that the two sites (the less and more distorted tetrahedron Zn—N4) have been transferred into a single tetrahedral site upon melting and vitrification, related to the reconstruction of the Zn—N bond. The melting mechanism for ZIF series has been investigated by F.X. Coudert and colleagues2, with the use of in situ variable temperature X-ray and ex-situ neutron pair distribution function experiments. The authors confirmed that during melting, bond breaking and reorientation between Zn2+ and imidazolate occur from the undercoordinated Zn2+, producing the disordered structures. A similar in situ X-ray pair distribution function technique has been also utilized to probe the mechanism through which MOFs transform into nanocomposites during pyrolysis.2" Zhihengyu Chen et aL

Received: June 8, 2022 Published: August 24, 2022