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Electronic Properties of Transition Metal Doped Silicon Clusters [Elektronische Ressource] / Jochen Rittmann. Betreuer: Thomas Möller

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Electronic Properties ofTransition Metal DopedSilicon Clustersvorgelegt vonDiplom-PhysikerJochen Rittmannaus Duisburgvon 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: Prof. Dr. Thomas M¨ ollerBerichter: Prof. Dr. Bernd von IssendorffTag der wissenschaftlichen Aussprache: 20.09.2011Berlin 2011D 83AbstractThe interaction of a transition metal atom with a silicon cluster is investigated tounderstand the stabilizing effect responsible for silicon cage formation in transi-tion metal doped silicon clusters. The analysis was done using VUV spectroscopyand element specific x-ray spectroscopy. Partial ion yield analysis allows observ-ing resonant excitation and direct photoionization channels separately. First ex-+perimental indications for the predicted high symmetry [1,2] of VSi are found.16In addition, studies on different transition metal dopant atoms in this specificsilicon cage show that deviation from electronic shell closure seems to affect thesilicon cage more strongly than the local electronic structure at the transitionmetal dopant. Furthermore, the size of transition metal doped silicon clustersshows a strong influence on the localization of electrons and position of valencelevels.

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Published 01 January 2011
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Electronic Properties of
Transition Metal Doped
Silicon Clusters
vorgelegt von
Diplom-Physiker
Jochen Rittmann
aus Duisburg
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: Prof. Dr. Thomas M¨ oller
Berichter: Prof. Dr. Bernd von Issendorff
Tag der wissenschaftlichen Aussprache: 20.09.2011
Berlin 2011
D 83Abstract
The interaction of a transition metal atom with a silicon cluster is investigated to
understand the stabilizing effect responsible for silicon cage formation in transi-
tion metal doped silicon clusters. The analysis was done using VUV spectroscopy
and element specific x-ray spectroscopy. Partial ion yield analysis allows observ-
ing resonant excitation and direct photoionization channels separately. First ex-
+
perimental indications for the predicted high symmetry [1,2] of VSi are found.
16
In addition, studies on different transition metal dopant atoms in this specific
silicon cage show that deviation from electronic shell closure seems to affect the
silicon cage more strongly than the local electronic structure at the transition
metal dopant. Furthermore, the size of transition metal doped silicon clusters
shows a strong influence on the localization of electrons and position of valence
levels. The latter is studied using a novel analysis method based on the com-
bination of VUV and x-ray spectroscopy. Determination of the HOMO-LUMO
gap for a wide range of cluster sizes show good agreement with theoretical pre-
dictions. An enhanced HOMO-LUMO gap of (1.9± 0.2) eV is observed in case
+
of the highly symmetric VSi , which can be understood in terms of a spherical
16
potential model [1,3].Kurzfassung
¨
In dieser Arbeit wird die Wechselwirkung zwischen einem Ubergangsmetallatom
und einem Siliziumcluster untersucht. Die Untersuchungsmethoden beinhalten
VUV Spektroskopie und elementspezifische R¨ ontgenspektroskopie. Durch die
Analyse der partiellen Ionenausbeute k¨ onnen die resonante Anregung und die di-
rekte Photoionisation eines Clusters getrennt betrachtet werden. Erstmalig wer-
+
den experimentelle Indizien fur¨ die vorhergesagte hohe Symmetrie von VSi [1,
16
2] gezeigt. Untersuchungen an diesem Siliziumk¨ afig mit Titan und Chrom als
Dotieratom zeigen, dass die Dotierung einen gr¨ oßeren Einfluss auf die Struktur des
¨
K¨ afigs hat als auf die lokale elektronische Struktur des Ubergangsmetallatoms.
¨
Des Weiteren konnte die gr¨ oßenabh¨ angige Anderung der Lokalisierung von Valenz-
elektronen n¨ aher bestimmt werden. Der in einem erstmalig eingesetzten Ver-
fahren bestimmte Abstand von dem h¨ ochsten besetzten und dem niedrigsten
unbesetzten Molekulorbital¨ (HOMO-LUMO gap) in Abh¨ angigkeit der Clustergr¨ oße
¨
zeigt eine gute Ubereinstimmung mit theoretischen Berechnungen desselben. Eine
+
besondere Stellung hat hierbei das hochsymmetrische VSi mit einem gemesse-
16
nen HOMO-LUMO gap von (1.9± 0.2) eV.iiContents
1 Theoretical Background of X-ray Absorption Spectroscopy 3
1.1 X-Ray Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1 Direct Photo Ionization . . . . . . . . . . . . . . . . . . . 4
1.1.2 Resonant X-ray Absorption . . . . . . . . . . . . . . . . . 7
1.1.3 XAS on Particles in the Gas Phase . . . . . . . . . . . . . 8
1.1.4 Auger Decay . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 Density Functional Theory Calculations . . . . . . . . . . . . . . . 11
2 State of the Art 13
2.1 Geometric Structures of Transition Metal Doped Silicon Clusters . 14
2.2 Reactivity of Transition Metal Doped Silicon Clusters . . . . . . . 16
2.3 Electronic Structure of Transition Metal Doped Silicon Clusters . 18
2.4 Cluster Assembled Materials . . . . . . . . . . . . . . . . . . . . . 25
3 Experimental Setup 27
3.1 Cluster Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Hexapole Ion Guide / Reaction Cell . . . . . . . . . . . . . . . . . 31
3.3 Mass Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4 Ion Lenses and First 90° Deflector . . . . . . . . . . . . . . . . . . 35
3.5 Ion Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.6 Second 90° Bender and Ion Lenses . . . . . . . . . . . . . . . . . . 36
3.7 Mass Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.7.1 Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . 39
3.7.2 Simulation of the Particles Flight Path . . . . . . . . . . . 41
3.8 Synchrotron Facility . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.8.1 Basic Principles of Synchrotron Radiation . . . . . . . . . 43
3.8.2 Electron Source and Microtron . . . . . . . . . . . . . . . 44
3.8.3 Synchrotron . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.8.4 Storage Ring and Undulators . . . . . . . . . . . . . . . . 45
iii3.8.5 Beamlines U49/2-PGM1, U125/2-SGM, and U125/2-NIM 48
3.9 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.10 Time Line of Major Setup Modifications . . . . . . . . . . . . . . 51
4 Data Analysis 53
4.1 Mass Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 Ion Yield Spectra and Photon Flux Normalization . . . . . . . . . 54
4.2.1 Handling of Dropped Points in Ion Yield Spectra . . . . . 59
4.2.2 Derivatives of Ion Yield Spectra . . . . . . . . . . . . . . . 59
5 Reactivity of Doped Silicon Clusters 61
5.1 Reactivity of Vanadium Doped Silicon Clusters . . . . . . . . . . 62
+
5.1.1 Stability of Singly Vanadium Doped Silicon Clusters VSi 65n
+
5.1.2 Stability of Doubly V Doped Silicon V Si 66
2 n
5.1.3 Pressure Dependence of the Stability S . . . . . . . . . . . 66
5.2 Exohedral-Endohedral Transition of Sc, Ti, V, Cr, and Mn Doped
Silicon Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.3 Application of Selective Etching . . . . . . . . . . . . . . . . . . . 70
6 Ion Yield Spectroscopy at the Transition Metal Dopant 2p Edge 71
6.1 Excursion: Evaluation of Ion Yield Spectroscopy at the Metal L
3,2
Edge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.2 Experimental Results and General Discussion . . . . . . . . . . . 73
6.3 Detailed Analysis of Selected Transition Metal Doped Silicon Clus-
ters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
+ + +
6.3.1 Highly Symmetric VSi and Equal Sized TiSi and CrSi 86
16 16 16
6.3.2 Comparison of Silicon and Germanium Cage Structures . . 93
7 Ion Yield Spectroscopy at the Silicon Cage 2p Edge 97
7.1 Effect of the Generation Channel of Product Ions on the PIY Spectra 98
7.2 Resonant Excitation and Photoionization Edges . . . . . . . . . . 107
7.2.1 Resonant Partial Ion Yield - Excitation Energy and Elec-
tron Screening . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2.2 Direct Photoionization Efficiency - Chemical Shift of Silicon
2p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
7.2.3 Silicon 2p Spin Orbit Splitting and Relative Chemical Shift
Compared to Geometric Structures . . . . . . . . . . . . . 115
iv8 Ion Yield Spectroscopy at the Clusters Valence Orbitals 127
8.1 Photoionization Efficiency . . . . . . . . . . . . . . . . . . . . . . 128
+
8.2 Direct Threshold of MSi Clusters . . . . . . . . 129n
9 HOMO-LUMO Gap 135
9.1 Determination of the HOMO-LUMO Gap by Ion Yield Spectroscopy
in the VUV and at the Silicon 2p Edge . . . . . . . . . . . . . . . 137
9.2 HOMO-LUMO Gaps of Transition Metal Doped Silicon Clusters . 139
+
9.2.1 Theoretical Model for the HOMO-LUMO Gap in VSi . . 142
16
+ +
9.2.2 HOMO and LUMO in TiSi and CrSi . . . . . . . . . . . 143n n
9.3 Level Spacing Between Valence Orbitals . . . . . . . . . . . . . . 145
10 Conclusion 151
A Supplementary Information 165
+
A.1 Energy Dependent PIY of ScSi at the Metal L Edge . . . . . . 165
3,2n
A.2 Unidentified Product Ion Generation Channels in the VUV Energy
Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
vviIntroduction
Atomic clusters are ideal systems to analyze fundamental properties of matter
on a microscopic scale. They can be tailored in size and composition to address
a specific question. Systematic manipulation of one or more properties of a free
cluster, as for example its size, allows to monitor the response of the whole system
and therefore to determine the microscopic origin of electronic structure, mag-
netism, and reactivity or other features on a highly defined unperturbed system.
Transition metal doped silicon clusters are therefore the ideal system to analyze
the interaction between a semiconducting material and a metal. Like metal dop-
ing drastically changes the properties of a semiconducting bulk, silicon clusters
show a complete change in their geometry and stability upon interaction with a
transition metal atom [2, 4]. While a stabilization of the silicon cluster through
the metal atom was observed experimentally for specific transition metal doped
silicon clusters [5], theoretical results showed that highly symmetric silicon cage
structures were formed around the metal atom [2, 6]. The high symmetry of
certain sizes and compositions of systems is predicted to result in a high degener-
acy of electronic levels, thus allowing further stabilization by potential electronic
shell closure and high HOMO-LUMO gaps. This interesting stabilizing interac-
tion of transition metal atoms on silicon clusters gave rise to many theoretical
publications on this topic, predicting that the optimal cage structures occur if
− +
sixteen silicon atoms encapsulate a Sc , Ti, or V atom [1,2]. On the other hand,
experimental studies analyzing the binding mechanism and electronic structure
responsible for the reshaping of the silicon cluster are sparse. No experiment has
been performed so far to determine the geometric structure of the silicon cages.
In order to understand the interaction and binding mechanism between the
transition metal and the silicon cluster, experimental techniques probing global
properties of the cluster are of limited use. The role of either the metal or the sil-
icon cage in this interaction has to be probed separately with an element specific
method. The method best suited is x-ray spectroscopy, which is therefore applied
in this thesis. This element specific method allows to analyze the electronic struc-
1