Mass Spectrometric Developments and a Study of Lithium Doped Silicon and Germanium Clusters.

Clusters, consisting of a few up to several thousands of atoms, exhibit size-dependent physical and chemical properties linking the nanoscale to bulk matter in a non-trivial way. The careful design of clusters of two different elements (binary clusters) allows the synthesis of particularly stable species with tailored properties that eventually could be used as building blocks for novel nanomaterials. However, the continuation of our experimental research of binary clusters in the gas phase often is hampered by the limited mass resolution of the mass spectrometer. Beside the investigation of lithium doped silicon and germanium clusters a novel mass spectrometer has been designed and constructed, dividing this thesis into two distinct parts:Part I: High resolution tandem time-of-flight mass spectrometerOne of the problems with the mass spectrometric investigation of binary clusters is the need of a resolution high enough to identify clusters unambiguously. Therefore a novel instrument was designed and constructed in the framework of this thesis. It consists of a binary cluster source and a high resolution tandem time-of-flight mass spectrometer (HRTTOF). Analytical calculations and extensive simulations were performed to understand and optimize the mass resolution of the system. The orthogonal extraction optic implements a new design with incorporated deflection which offers an extended mass range for simultaneous detection and which leaves the vertical velocity component of the incoming ions unaltered. The TOF chamber contains two dual-stage reflectrons and a chevron-type multichannel plate (MCP) detector. The detector operates in positive or negative ion mode with up to 5 kV post acceleration and can be coupled to a 0.25 ns time-to-digital convertor (TDC), ensuring high resolution in the full mass range of the clusters produced. Due to the tandem design an intermediate time focal point can be accessed, which can be employed for mass selected photofragmentation spectroscopy. A mass resolution close to 10 000 and a mass selectivity of 1/400 has been demonstrated for small silicon clusters. Peak analysis software and a new deconvolution method were developed to improve data analysis.Part II: Properties of lithium doped silicon and germanium clustersPhotofragmentation spectroscopy shows that germanium and silicon agglomerates are composed of stable building blocks with six to eleven atoms. These building blocks are dianionic deltahedra, reminiscent of borane and carborane, which follow Wade s rules rather than the spherical shell model for metals. The units can be stacked, functionalized with organo-metallic groups, or doped with interstitial atoms to tailor the chemical and physical properties of what can be conceived as promising nanomaterials. We report on a combined experimental and theoretical study of the geometric and electronic structure, stability, and ionization potentials of lithium doped silicon and germanium clusters. Mass spectra for doped silicon and doped germanium reveal a striking similarity and show enhanced abundances for particular sizes that relate to the building blocks of larger agglomerates. The structures of those stable sizes are further studied with density functional theory. The stability of bare and doped silicon and germanium is determined both by electronic and geometric motifs, which are strongly intertwined. We confirm the high stability of Ge52-, Si52-, Ge94- and Si94- units, previously synthesized in solution, in the gas phase by analysis of mass abundance spectra of lithium doped silicon and germanium cations after laser fragmentation. Together with neutral Si7, Ge7, Si10 and Ge10, which are favoured in dissociation experiments, they form a set of highly stable structures. Each structure has a deltahedral geometry with highly regular bond lengths and an associated optimal charge state. There is a clear distinction between the electronic s-band and p-band in these clusters. A new model is proposed to explain the stability of the (lithium doped) silicon and germanium clusters. In short, the shell closing in the former determines the geometry, while a shell closing in the latter determines the optimal charge state for the stable structures. Splitting this thesis into two separate parts risks to imply that technological innovation and fundamental cluster science are distinct topics. This is strongly contradicted by the daily reality in the research lab. That both topics are intertwined can be illustrated by the new instrumentation that was readily developed for the ionization energy measurements, while the new mass spectrometer was build with the complicated isotope patterns and interesting fragmentation patterns of silicon and germanium clusters in mind.Summary i Samenvatting v Nomenclature vii Table of contents ix Part I: High resolution tandem time-of-flight mass spectrometer 1 1 Design 3 1.1 Introduction 3 1.1.1 Objectives 3 1.1.2 A need for higher resolution 4 1.1.3 A need for mass selective experiments 6 1.1.4 Mass spectrometers 6 1.2 How to achieve enhanced resolution? 9 1.2.1 Resolution in one dimension: an analytical model 9 1.2.2 Corrections to the analytical model 17 1.2.3 Resolution in an ideal three dimensional system: simulations 19 1.2.4 Corrections to the ideal case 25 1.3 Conclusion 28 1.3.1 Predicting the resolution 28 1.3.2 Design of the HRTTOF 31 2 Realization 33 2.1 Introduction 33 2.2 Vacuum housing 35 2.2.1 Vacuum chambers 35 2.2.2 Support Tables 36 2.2.3 Vacuum pumps and pressure measurement 36 2.3 Cluster production 38 2.3.1 Laser ablation cluster source 38 2.3.2 Cluster beam characterization 43 2.4 Time-of-flight Mass Spectrometer 50 2.4.1 Extraction optics 50 2.4.2 Reflectrons 55 2.4.3 Mass selected photofragmentation 57 2.5 Data acquisition system 59 2.5.1 Efficiency 59 2.5.2 MCP detector 59 2.5.3 Pre-amplifier 61 2.5.4 Data collection 62 2.5.5 Data analysis 62 3 Results 65 3.1 Mass range 65 3.2 High resolution 66 3.3 Mass selection 67 3.3.1 Mass selection 67 3.3.2 Mass selected photofragmentation of cobalt clusters 68 3.4 Discussion and outlook 69 Part II: Stability of (lithium doped) silicon and germanium clusters 71 4 Structures of silicon and germanium clusters 73 4.1 Experimental characterization methods 73 4.1.1 Laser spectroscopy 73 4.1.2 Photoelectron spectroscopy 74 4.1.3 Photoionization efficiency 76 4.1.4 Ion mobility measurements 79 4.1.5 Mass selected dissociation experiments 80 4.2 Sizes and structures 81 4.2.1 Charge dependence of small silicon and germanium clusters 81 4.2.2 Elongated structures 86 4.3 Metal doped silicon and germanium clusters 89 4.3.1 Sodium doped silicon clusters 89 4.3.2 Lithium doped germanium clusters 91 4.3.3 Coinage metal doped silicon and germanium clusters 93 4.4 Nanomaterials 94 4.4.1 Nanorods 94 4.4.2 Organic ligands 96 5 Experimental investigation of lithium doped silicon and germanium clusters 97 5.1 Ge1Lim and Si1Lim: filling of atomic orbitals 97 5.1.1 Lithium doped germanium monomer 97 5.1.2 Lithium doped silicon monomer 103 5.2 Ionization energies 105 5.2.1 Experimental method and data evaluation 105 5.2.2 Ionization energies of lithium doped silicon clusters 108 5.2.3 Ionization energies of lithium doped germanium clusters 112 5.2.4 Discussion 117 5.3 Photofragmentation of lithium doped silicon and germanium clusters 119 5.3.1 Photofragmentation of lithium doped germanium clusters 120 5.3.2 Photofragmentation of silicon clusters 122 6 A shell model for silicon and germanium clusters 125 6.1 Shell models for metal clusters 126 6.2 Chemical stability rules 127 6.3 Stability of lithium doped semiconductor species 130 6.3.1 Growth mechanism 130 6.3.2 Identification of stable systems 132 6.3.3 Overview of some stable systems 134 6.4 Stability model for semiconductor clusters 137 7 General conclusions and perspectives 141 Appendix 149 A.1 Analytical formulas 149 A.2 Numerical calculations 155 A.3 Custom vacuum components 158 A.4 Overview of cluster structures 159 A.5 Overview of cluster DOS 164 References 165 Curriculum vitae 177 List of publications 179 Acknowledgments 181nrpages: 182status: publishe