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Electronic structure and optical properties of pristine and modified diamondoids [Elektronische Ressource] / vorgelegt von Lasse Landt

160 pages
Electronic Structure and OpticalProperties of Pristine and ModifledDiamondoidsvorgelegt vonDiplom-PhysikerLasse LandtBerlinvon der Fakult˜at II - Mathematik und Naturwissenschaftender Technischen Universit˜at Berlinzur Erlangung des akademischen GradesDoktor der Naturwissenschaften- Dr. rer. nat. -genehmigte DissertationPromotionsausschuss:Vorsitzender: Prof. Dr. Mario D˜ahneBerichter/Gutachter: Prof. Dr. Thomas M˜ollerBerichhter: PD Dr. Uwe HergenhahnTag der wissenschaftlichen Aussprache: 15. November 2010Berlin 2010D 83iiAbstractIn this work the optical properties of diamondoids, a new form of perfectly size- andshape-selected, neutral, and hydrogen-passivated diamond nanocrystals, are investi-gated. Theabsorptionandluminescencepropertiesarestudiedasafunctionofsizeandshape and the optical gap of the investigated diamondoid species has been determined.The shape is found to dominate the optical response of the diamondoid outweighingsize efiects in the investigated size range. According to their growth scheme and theirabsorption behavior the diamondoids are categorized as 1D, 2D and 3D nanodiamondstructures. The tetrahedral C H cluster is identifled as the smallest diamond nanos-26 32tructure to exhibit bulk-like absorption behavior. Further, diamondoids are shown toexhibit photoluminescence in the ultraviolet spectral region. The spectra for eight dia-mondoids of difierent sizes and shapes have been recorded.
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Electronic Structure and Optical
Properties of Pristine and Modifled
Diamondoids
vorgelegt von
Diplom-Physiker
Lasse Landt
Berlin
von der Fakult˜at II - Mathematik und Naturwissenschaften
der Technischen Universit˜at Berlin
zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften
- Dr. rer. nat. -
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. Mario D˜ahne
Berichter/Gutachter: Prof. Dr. Thomas M˜oller
Berichhter: PD Dr. Uwe Hergenhahn
Tag der wissenschaftlichen Aussprache: 15. November 2010
Berlin 2010
D 83iiAbstract
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. Theabsorptionandluminescencepropertiesarestudiedasafunctionofsizeand
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 efiects 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 C H cluster is identifled as the smallest diamond nanos-26 32
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 difierent sizes and shapes have been recorded. The photoluminescence is
spectrally broad and only little size-dependent. A spectral structure is observed and a
carefulanalysis allows for a tentativeassignment to difierent vibrational modes. Quan-
tumchemicalelectronicstructurecalculationsandgrouptheoreticalconsiderationhave
been employed to facilitate the interpretation of the experimental data.
In a second part of the thesis, surface and bulk modifled diamondoid structures of
difierent sizes with either a thiol functional group or an oxygen inclusion are investi-
gated to determine the in uence of targeted chemical modiflcation on the electronic
structure. The two difierent modiflcations are found to lead to fundamentally difierent
efiects. Thethiolfunctionalgroupinducesanimpuritystatethatdominatestheoptical
properties and leads to a loss of the size dependence of the optical gap for structures
up to 30 carbon atoms. Oxygen inclusion strongly in uences the optical response but
a size-dependence of the optical gap persists. Both modiflcations are found to quench
the UV photoluminescence of pristine diamondoids.
The present data, taken on atomically deflned diamond clusters in the gas phase, re-
veal for the flrst time the exact interdependence of the optical response of diamondoids
with each of several difierent structural parameters, such as size, shape and surface
functionalization.
iiiivKurzfassung
ImRahmendieserArbeitwurdendieoptischenEigenschaftenvonDiamantoiden, einer
neuenArtgr˜o…en-undformselektierter,neutralerundwasserstofipassivierterDiamant-
nanokristalle, untersucht. Es wurde die genaue Abh˜angigkeit der Absorptions- und
Lumineszenzeigenschaften von der Gr˜o…e und der Form der Diamantcluster bestimmt.
Es zeigt sich, dass die Form im untersuchten Gr˜o…enbereich einen st˜arkeren Ein uss
auf das Absorptionsverhalten von Diamantoiden hat als die Gr˜o…e. 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 C H Clusters26 32
dem von makroskopischem Diamant sehr nahe kommt und dieser somit als kleinster
Nanodiamant angesehen werden kann. Au…erdem 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 Gr˜o…enabh˜angigkeit aufweist. Die Photolumineszenzspektren wurden einge-
hend analysiert und ein m˜oglicher Erkl˜arungsansatz ub˜ er Vibrationsanregungen wird
aufgezeigt. Quantenchemische Berechnungen der elektronischen Struktur sowie grup-
˜pentheoretische Uberlegungen wurden zur Interpretation der experimentellen Daten
herangezogen.
Im zweiten Teil der Arbeit wird der Ein uss von Modiflzierungen der Ober ˜ache und
des Kohlenstofigerusts˜ auf die optischen Eigenschaften der Diamantoide untersucht.
Dies geschieht an den Beispielen der Ober ˜achenfunktionalisierung mit einer Thiol-
gruppe sowie des Austauschs eines Kohlenstofis durch ein Sauerstofiatom. Die zwei
verschiedenen Modiflzierungen unterscheiden sich in ihren Auswirkungen auf die elek-
tronische Struktur fundamental voneinander. Die Thiolgruppe induziert ein St˜orstel-
lenniveau, das die optischen Eigenschaften komplett dominiert und zu einem Verlust
der Gr˜o…enabh˜angigkeit der Bandluc˜ ke fur˜ Cluster mit weniger als 30 Atomen fuhrt.˜
Das Einfugen˜ eine Sauerstofiatoms in den Kohlenstofik˜aflg beein usst die optischen
Eigenschaften in geringerem Ma…e und erh˜alt die Gr˜o…enabh˜angigkeit der Bandluc˜ ke.
Beide Modiflzierungen unterdruc˜ ken die in reinen Diamantoiden vorhandene Photolu-
mineszenz.
Die in dieser Arbeit vorgestellten Untersuchungen an atomar deflnierten Diamantclus-
tern in der Gasphase zeigen erstmals den genauen Zusammenhang zwischen den op-
tischen Eigenschaften und verschiedenen strukturellen Parametern, wie Gr˜o…e, Form
und Funktionalisierung.
vviContents
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 Difierent 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 Deflnes The Smallest Diamond . . . . . . . . . . 60
vii5.1.3 Evolution of the Optical Gap . . . . . . . . . . . . . . . . . . . . 62
5.2 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6 Results & Discussion - Part II: Modifled 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
viiiChapter 1
Introduction
Diamondoids constitute a series of perfectly deflned, 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-
deflned 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
knowledgeofeachdiamondoid’sstructureafiordsunprecedentedopportunities, suchas
investigating size and shape efiects in nanocrystals or the in uence of single impurity
atoms.
Typical investigations on neutral clusters and nanocrystals sufier from poorly deflned
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, undeflned 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 deflned, interactionless,
neutral particles. These are the same boundary conditions of typical theoretical elec-
tronic structure investigations. The results of this study therefore provide unrestricted
andfornanocrystalsunprecedentedcomparabilityofexperimentalandtheoreticaldata.
The goal of the present study is to determine with atomic precision the in uence of
difierent parameters that deflne the electronic and optical properties of diamondoids.
The investigated parameters are particle size, particle shape, and chemical modiflca-
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 difierent sizes and shapes are investigated to determine the
in uence of size and shape on the optical properties. Diamondoids with surface func-
tional groups and bulk-substituted diamondoids with incorporated impurity atoms are
12 Chapter 1. Introduction
studied to learn about the impact of difierent kinds of chemical modiflcations on the
electronic and optical properties of diamondoids. Quantum chemical calculations and
group theoretical methods are used to help interpret the results.
In chapter2 diamondoids are introduced in detail and the previous studies on their
electronic structure are brie y reviewed. The theoretical and experimental methods
that are used in this work are presented in chapters3 and 4, respectively. Chapter5
contains the results on the optical properties of pristine diamondoids and in chapter6
the results for modifled diamondoids are discussed. The flnal chapter summarizes the
most important flndings of this thesis and gives an outlook on possible interesting
developments in the fleld.

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