Geometric, Electronic and Magnetic Properties of Transition Metal Doped Silicon Clusters, and Development of a Magnetic Deflection Setup

The aim of the work presented in this thesis is to provide deeper understanding on the geometric, electronic, and magnetic properties of gas phase clusters as a function of size and composition using a combination of the spectroscopic techniques and density functional theory (DFT) calculations. The thesis is divided into four parts. A review of recent research on the structural identification of doped silicon clusters is given in Part I. In Part II, the geometric structures of transition metal (TM = Ag, Au, and Co) doped silicon clusters are investigated by infrared (IR) action spectroscopy and DFT calculations. Nonlinear effects in IR action spectroscopy are investigated by joint experimental and dedicated modeling studies in Part III. In Part IV, recent progress on the development of a magnetic deflection setup at the KU Leuven is presented.

The interest in silicon clusters was triggered by the ongoing trend towards further miniaturization in microelectronics. An important discovery was the observation of unique luminescence properties in silicon nanostructures, where the emission wavelength can be tuned by changing the particle size, making them very attractive for a variety of applications. However, bare silicon clusters are not well suitable as building blocks for nanoassembled materials, as they have chemically reactive dangling bonds. A possible approach to overcome this deficiency is to add transition metal dopant atoms to the silicon clusters. Different dopant elements have different atomic structures, therefore one would expect that they will have disparate effects on the silicon clusters. In Part I, we review recent research on the structural identification of isolated doped silicon clusters by combining state-of-the-art experiments (chemical probe mass spectrometric methods, IR action spectroscopy, photoelectron spectroscopy, and x-ray absorption spectroscopy) and computational modelling using the density functional theory formalism. The growth mechanisms of the doped silicon clusters are described with particular emphasis on the formation of endohedral cages. Besides geometric structures, also electronic and magnetic properties of the clusters are commented on.

In Part II, the geometric structures of cationic SinAg+ (n = 6−15), SinAu+ (n = 2−11, 14, 15), SinCo+ (n = 5−8) and SinCo2+ (n = 8−12) clusters were investigated by infrared multiple photon dissociation spectroscopy (IR-MPD), and those of neutral SinCo (n = 10−12) by infrared ultraviolet two-color ionization (IR−UV2CI), both in combination with DFT computations. The influence of the different coinage metal dopant atoms (Cu, Ag, and Au) on the structures of Sin+/0 clusters is systematically investigated. The coinage metal atoms favor adsorption on bare Sin+ clusters rather than taking substitutional positions, which is in sharp contrast with silicon clusters doped by transition metals atoms like V, Nb and Mn, where substitution is preferable. On the other hand, there are slight differences among the growth mechanisms of the different coinage metal doped Sin+ clusters, which can be attributed to the different atomic radii of the dopants, the different d occupancy of the dopant atoms, and the different charge transfer.

Different from the coinage metal doped silicon cluster, small SinCo+ (n = 5−8) clusters have exohedral structures in which the Co atom substitutes an atom of bare Sin+1+ clusters, while retaining a high local magnetic moment. Large SinCo (n = 10−12) clusters were found to have endohedral caged structures. Electronic structure analysis indicates strong hybridization between the Co dopant atom and the silicon host, which quenches the local magnetic moment of the encapsulated Co atom. In the doubly doped SinCo2+ clusters, the second Co atom is adsorbed to the singly doped counterparts and for n ≥ 9 one of the Co atoms is encapsulated by a silicon cage. Computational analysis of the electronic and magnetic properties of the identified isomers indicates a distance dependent magnetic coupling between the Co atoms in SinCo2+.

For structural identification, the experimental IR spectra are normally compared to computed linear harmonic absorption spectra, although both IR-MPD and IR-UV2CI spectroscopy involve complex processes, such as sequential photon absorption, vibrational energy redistribution, and cluster dissociation. With the objective to obtain fundamental insight about the mechanisms of the IR action spectroscopy, the non-linear effects are investigated by joining experiment and dedicated modelling in Part III. IR spectra of neutral silicon clusters are recorded by IR−UV2CI and cationic vanadium oxide clusters are studied either by dissociation of O2 or Xe messenger atoms following infrared multiple photon absorption. In the simulation, vibrational anharmonicities, laser interaction through photon absorption and stimulated emission, as well as the relevant ionization or dissociation rates, have been taken into account. Comparison of the measured and calculated spectra illustrates how non-linear effects, such as the absorption of multiple infrared photons, energy redistribution, and statistical dissociation or ionization, affect the spectroscopic features.

The aim of Part IV is to further develop and characterize a recently constructed Stern-Gerlach magnetic deflection setup and to use this setup to study the magnetism of binary clusters, with the objective to build up fundamental knowledge on the magnetic interactions between the different types of atoms and on the interplay of geometric structure and magnetism. The Stern-Gerlach magnetic deflection system consists of a cryogenic laser vaporization cluster source, a beam chopper, a dual-stage collimator system, a Stern-Gerlach magnet, a drift tube, a reflectron time-of-flight mass spectrometer and a position sensitive detector. Implemented on a dual-target dual-laser vaporization source, the cryogenic system allows to control the temperature of the clusters, which influences the deflection behavior since the magnetism of clusters depends on cluster temperature. The cluster velocity is measured by the chopper wheel. Two collimators define a beam shape of 0.3×3.0 mm2. The collimated beam is deflected in the magnet and has a free flight before entering the extraction. A magnet with inhomogeneous magnetic field deflects clusters that have a time-averaged non-zero magnetic moment. Information on cluster size distribution and deflection can be measured simultaneously by the position-sensitive detector installed in the high resolution time-of-flight mass spectrometer. Recent progress made with the setup is discussed in Part IV. In particular, a cryocooler is installed to cool the cluster source and tested. It is mounted to the nozzle and allows to cool it down to 20 K within 50 min. In combination with a temperature controller, it allows to vary the temperature from 20 K to room temperature. Furthermore, the Rabi-type magnet, which is used to deflect the cluster beam, is calibrated by Al and Y atoms. At last, we show the magnetic deflection measurement of small Con (n = 7−21) clusters. The magnetic moment of Con clusters, as derived from the deflected beam profiles, is compared with literature data.  Finally the error margins on the measured magnetic moment are discussed and further improvement are proposed.