Photoemission spectra of deuterated silicon clusters: experiment and theory

We compare experimentally measured and ab initio computed photoelectron spectra of negatively charged deuterated silicon clusters (SimD − n , 4 ≤ m ≤ 10, 0 ≤ n ≤ 2) produced in a plasma environment. Based on this comparison, we discuss the kinetics and thermodynamics of the cluster formation and the effect of deuterium on the geometrical and electronic structure of the clusters. PACS. 36.40.-c Atomic and molecular clusters – 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals Silicon is undoubtedly the quintessential electronic material [1]. The backbone of the modern electronics industry is, of course, single-crystalline Si. However, many commercial and laboratory devices rely on other forms of silicon — poly-crystalline Si, amorphous Si, porous Si, quantumdot Si, and more. It is therefore natural that much work has been aimed at examining, and perhaps utilizing, the unique properties that Si nano-clusters exhibit. This effort spans over 15 years — from the pioneering experimental and theoretical work of Smalley et al. [2] and Raghavachari et al. [3], respectively, to the present day (Refs. [4–10] are but a few recent examples). Surfaces of single-crystalline Si are often passivated with hydrogen [11]. Hydrogen is a simple and effective means for such passivation because by bonding to surface Si atoms it removes dangling bonds, maintains the tetrahedral coordination of Si atoms found in the bulk, and prevents surface dimerization. Consequently, it also removes the surface electronic states typically associated with the bare Si surface. For the same reasons, hydrogen passivation is often used in the preparation of Si quantum dots [12], and surface hydrogen atoms are often added to theoretical calculations involving Si as a means of suppressing surface effects [13]. In light of the beneficial effects of hydrogen adsorption on macroscopic Si surfaces, it is natural to ask how hydrogen adsorption affects the structural and electronic properties of Si nano-clusters. In this article, we take a a Present address: Department of Materials and Interfaces, Weizmann Institue of Science, Rehovoth 76100, Israel. e-mail: [email protected] step towards answering this question, by comparing experimentally measured and ab initio computed results for the photoelectron spectroscopy (PES) of small, negatively charged silicon clusters capped with one or two atoms of a hydrogen isotope — deuterium. Negatively charged SimDn clusters (4 ≤ m ≤ 10, 0 ≤ n ≤ 2, D denoting deuterium), were produced using a pulsed arc cluster ion source [14,15]. Bulk silicon was vaporized in a pulsed arc, with the formed hot plasma flushed by He carrier gas to an extender, into which molecular deuterium was introduced for the generation of deuterated clusters. Clusters were cooled down to approximately room temperature and flushed into vacuum at the extender exit, with no further annealing. Negatively charged clusters were mass-selected in a time-offlight mass spectrometer and their photoelectron spectra were recorded using a “magnetic bottle” time-of-flight electron spectrometer. All theoretical calculations were based on solving the Kohn-Sham DFT equations within the local density approximation (LDA), using the higher-order finite-difference pseudopotential method [16]. Low-energy metastable isomers of the clusters were obtained via simulated annealing based on Langevin molecular dynamics [17]. Theoretical photoelectron spectra were computed within the constant matrix approximation, i.e., crosssectional effects on the computed density of states (DOS) curves were neglected. The computed PES curves were broadened by averaging the DOS over several picoseconds of isothermal molecular dynamics and then convoluted with a Gaussian to facilitate comparison to experiment.