Electronic Structure and Optical Properties of Pristine and Modi¯ed Diamondoids vorgelegt von Diplom-Physiker Lasse Landt Berlin von der FakultÄatII - Mathematik und Naturwissenschaften der Technischen UniversitÄatBerlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften - Dr. rer. nat. - genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Dr. Mario DÄahne Berichter/Gutachter: Prof. Dr. Thomas MÄoller Berichter/Gutachter: PD Dr. Uwe Hergenhahn Tag der wissenschaftlichen Aussprache: 15. November 2010 Berlin 2010 D 83 ii Abstract In this work the optical properties of diamondoids, a new form of perfectly size- and shape-selected, neutral, and hydrogen-passivated diamond nanocrystals, are investi- gated. The absorption and luminescence properties are studied as a function of size and shape and the optical gap of the investigated diamondoid species has been determined. The shape is found to dominate the optical response of the diamondoid outweighing size e®ects in the investigated size range. According to their growth scheme and their absorption behavior the diamondoids are categorized as 1D, 2D and 3D nanodiamond structures. The tetrahedral C26H32 cluster is identi¯ed as the smallest diamond nanos- tructure to exhibit bulk-like absorption behavior. Further, diamondoids are shown to exhibit photoluminescence in the ultraviolet spectral region. The spectra for eight dia- mondoids of di®erent sizes and shapes have been recorded. The photoluminescence is spectrally broad and only little size-dependent. A spectral structure is observed and a careful analysis allows for a tentative assignment to di®erent vibrational modes. Quan- tum chemical electronic structure calculations and group theoretical consideration have been employed to facilitate the interpretation of the experimental data. In a second part of the thesis, surface and bulk modi¯ed diamondoid structures of di®erent sizes with either a thiol functional group or an oxygen inclusion are investi- gated to determine the influence of targeted chemical modi¯cation on the electronic structure. The two di®erent modi¯cations are found to lead to fundamentally di®erent e®ects. The thiol functional group induces an impurity state that dominates the optical properties and leads to a loss of the size dependence of the optical gap for structures up to 30 carbon atoms. Oxygen inclusion strongly influences the optical response but a size-dependence of the optical gap persists. Both modi¯cations are found to quench the UV photoluminescence of pristine diamondoids. The present data, taken on atomically de¯ned diamond clusters in the gas phase, re- veal for the ¯rst time the exact interdependence of the optical response of diamondoids with each of several di®erent structural parameters, such as size, shape and surface functionalization. iii iv Kurzfassung Im Rahmen dieser Arbeit wurden die optischen Eigenschaften von Diamantoiden, einer neuen Art grÄo¼en-und formselektierter, neutraler und wassersto®passivierter Diamant- nanokristalle, untersucht. Es wurde die genaue AbhÄangigkeit der Absorptions- und Lumineszenzeigenschaften von der GrÄo¼eund der Form der Diamantcluster bestimmt. Es zeigt sich, dass die Form im untersuchten GrÄo¼enbereich einen stÄarkeren Einfluss auf das Absorptionsverhalten von Diamantoiden hat als die GrÄo¼e. Die untersuchten Diamantoide wurden, ihrer Struktur und ihren charakteristischen Absorptionsmerk- malen folgend, in ein-, zwei, und dreidimensionale Diamantnanostrukturen unterteilt. Es wurde gezeigt, dass das Absorptionsverhalten des tetraedrischen C26H32 Clusters dem von makroskopischem Diamant sehr nahe kommt und dieser somit als kleinster Nanodiamant angesehen werden kann. Au¼erdem wurde in dieser Arbeit erstmalig die Photolumineszenz von Diamantoiden nachgewiesen. Die Spektren acht verschiedener Diamantoide wurden aufgenommen, welche zeigen, dass die Emission von Diaman- toiden im ultravioletten Spektralbereich liegt, energetisch sehr breit ist und nur eine geringe GrÄo¼enabhÄangigkeit aufweist. Die Photolumineszenzspektren wurden einge- hend analysiert und ein mÄoglicher ErklÄarungsansatzÄuber Vibrationsanregungen wird aufgezeigt. Quantenchemische Berechnungen der elektronischen Struktur sowie grup- pentheoretische UberlegungenÄ wurden zur Interpretation der experimentellen Daten herangezogen. Im zweiten Teil der Arbeit wird der Einfluss von Modi¯zierungen der Oberfl¨ache und des Kohlensto®gerÄustsauf die optischen Eigenschaften der Diamantoide untersucht. Dies geschieht an den Beispielen der Oberfl¨achenfunktionalisierung mit einer Thiol- gruppe sowie des Austauschs eines Kohlensto®s durch ein Sauersto®atom. Die zwei verschiedenen Modi¯zierungen unterscheiden sich in ihren Auswirkungen auf die elek- tronische Struktur fundamental voneinander. Die Thiolgruppe induziert ein StÄorstel- lenniveau, das die optischen Eigenschaften komplett dominiert und zu einem Verlust der GrÄo¼enabhÄangigkeit der BandlÄucke fÄurCluster mit weniger als 30 Atomen fÄuhrt. Das EinfÄugeneine Sauersto®atoms in den Kohlensto®kÄa¯gbeeinflusst die optischen Eigenschaften in geringerem Ma¼e und erhÄaltdie GrÄo¼enabhÄangigkeit der BandlÄucke. Beide Modi¯zierungen unterdrÄucken die in reinen Diamantoiden vorhandene Photolu- mineszenz. Die in dieser Arbeit vorgestellten Untersuchungen an atomar de¯nierten Diamantclus- tern in der Gasphase zeigen erstmals den genauen Zusammenhang zwischen den op- tischen Eigenschaften und verschiedenen strukturellen Parametern, wie GrÄo¼e,Form und Funktionalisierung. v vi Contents 1 Introduction 1 2 Diamondoids 3 2.1 History . 5 2.2 Structure & Nomenclature . 7 2.3 Present Understanding of the Electronic Structure . 9 2.4 Diamondoids From Di®erent Perspectives . 13 3 Theoretical Background and Computational Methods 19 3.1 The Electronic Structure of Matter . 19 3.2 Molecular Orbital Theory . 24 3.3 Group Theoretical Aspects . 29 4 Experimental Methods 35 4.1 Diamondoid Samples . 35 4.2 Multi-Purpose Gas Cell . 36 4.3 Synchrotron Radiation . 41 4.4 Optical Absorption Spectroscopy . 42 4.5 Photoluminescence Spectroscopy . 45 4.6 Ultraviolet Photoelectron Spectroscopy . 49 5 Results & Discussion - Part I: Pristine Diamondoids 53 5.1 Optical Absorption . 53 5.1.1 Nanodiamonds in 1D, 2D, and 3D . 55 5.1.2 Shape Not Size De¯nes The Smallest Diamond . 60 vii 5.1.3 Evolution of the Optical Gap . 62 5.2 Photoluminescence . 79 6 Results & Discussion - Part II: Modi¯ed Diamondoids 97 6.1 Adamantane-1-Thiol - The Whole Picture . 98 6.2 Larger Diamondoid Thiols . 106 6.3 Bulk-substituted Diamondoids . 117 7 Summary & Outlook 125 Appendix 128 A Character Tables and Selection Rules 131 B List of Publications 137 List of Tables 141 Bibliography 141 viii Chapter 1 Introduction Diamondoids constitute a series of perfectly de¯ned, hydrogen-passivated nanodia- monds. These diamond nanocrystals have become available in various sizes and shapes through their recent isolation from petroleum [1]. Diamondoids always contain a well- de¯ned number of diamond cage units and can therefore be regarded as a series of diamond clusters that, in the macroscopic limit, converges against bulk diamond. Diamondoids are interesting from various perspectives: With respect to diamond they represent miniaturization in the ultimate, molecular size limit; diamondoids possess technologically interesting properties such as negative electron a±nity [2] and are, among other things, used as seeds in CVD diamond growth [3]. The chemical func- tionalization of diamondoids [4] has paved the way for new diamondoid devices [2] and opens new possibilities for tailoring diamondoid properties. Further, the precise knowledge of each diamondoid's structure a®ords unprecedented opportunities, such as investigating size and shape e®ects in nanocrystals or the influence of single impurity atoms. Typical investigations on neutral clusters and nanocrystals su®er from poorly de¯ned experimental parameters. Especially when studying the optical properties, which pro- hibits the use of charged particles, a size distribution of the investigated samples is inevitable. Further experimental shortcomings usually include an unknown surface reconstruction, unde¯ned particle shape and interactions of the sample with its envi- ronment. The present gas phase investigations on diamondoids allow one to get rid of these experimental nuisances and produce data of atomically de¯ned, interactionless, neutral particles. These are the same boundary conditions of typical theoretical elec- tronic structure investigations. The results of this study therefore provide unrestricted and for nanocrystals unprecedented comparability of experimental and theoretical data. The goal of the present study is to determine with atomic precision the influence of di®erent parameters that de¯ne the electronic and optical properties of diamondoids. The investigated parameters are particle size, particle shape, and chemical modi¯ca- tion. For this purpose a gas cell setup is developed that facilitates synchrotron-based absorption and photoluminescence measurements on diamondoids in the gas phase. Pristine diamondoids of di®erent sizes and shapes are investigated to determine the influence of size and shape on the optical properties. Diamondoids with surface func- tional groups and bulk-substituted diamondoids with incorporated impurity atoms are 1 2 Chapter 1. Introduction studied to learn about the impact of di®erent kinds of chemical modi¯cations on the electronic and optical properties of diamondoids. Quantum chemical calculations and