Elsevier

Chemical Physics Letters

Volume 297, Issues 1–2, 20 November 1998, Pages 133-140
Chemical Physics Letters

Evolution of electronic structure as a function of size in gallium phosphide semiconductor clusters

https://doi.org/10.1016/S0009-2614(98)01104-XGet rights and content

Abstract

Anion photoelectron spectra have been taken of GaxPy clusters at a photodetachment wavelength of 266 nm (4.657 eV). Clusters of varying stoichiometry with up to 18 atoms have been investigated. We obtain electron affinities and vertical detachment energies to the ground and low-lying excited states of the neutral clusters. Photoelectron spectra of clusters with 3–5 atoms are compared to previously reported ab initio calculations. Trends in the electron affinities and excitation energies for the larger clusters are discussed.

Introduction

Understanding the spectroscopic and thermodynamic properties of semiconducting particles as a function of size has become increasingly important in the search for technological advances in the fabrication of smaller and faster electronic devices. This has motivated the study of semiconductors in several size regimes. Semiconductor nanocrystals, particles typically in the size range of 103 or more atoms, have been the focus of much research in recent years 1, 2, 3, 4, 5, 6. The spectroscopy of these species, particularly quantum confinement effects, can generally be understood in terms of perturbations to the macroscopic material related to their finite size. Molecular clusters in the range of 10–100 atoms present additional challenges, as they are generally too small to be considered as perturbed macroscopic material but too large to treat with the standard spectroscopic and theoretical methods that work so well for small molecules. It is therefore of considerable interest to determine how the structure and spectroscopy of molecular clusters evolve with size. In this Letter we address this issue for gallium phosphide clusters by photoelectron spectroscopy of size-selected cluster anions of various stoichiometries with up to 18 atoms.

Photoelectron spectroscopy of anions has been an important tool for studying size-dependent properties of clusters, as it combines mass-selectivity with moderate spectral resolution. The photoelectron spectra of homonuclear semiconductor clusters such as Sin7, 8, 9, 10, 11 and Gen8, 12 are well-characterized. Heteronuclear clusters present additional complexity since both size and stoichiometry can be varied. The first systematic study of heteronuclear semiconductor clusters was performed on GaxAsy clusters by Smalley and co-workers 13, 14. Wang et al. [15] and Kaya and co-workers [11] have measured photoelectron spectra of SixOy and SixCy clusters, respectively.

We have previously reported photoelectron and zero electron kinetic energy (ZEKE) spectra of small InxPy clusters (x, y≤4) 16, 17. This work and the photodissociation studies by Kolenbrander and Mandich [18] on neutral InP clusters have shown that the spacing between the ground and first excited electronic states in the neutral clusters was close to the bulk InP band gap of 1.344 eV (300 K). InP is a direct gap semiconductor, while GaP has an indirect band gap of 2.272 eV (300 K). If these significant differences in the bulk electronic structure are reflected in the molecular cluster size regime, one would expect rather different photoelectron spectra for small GaP and InP clusters; this was a major motivation for the work presented here. More generally, we wish to compare our results to previous experimental and theoretical investigations in the hope of understanding the electronic structure of these clusters and how they extrapolate to the bulk properties.

Only four experimental studies of gallium phosphide clusters in any size range have been published. Nozik and co-workers [19] have taken the optical absorption spectra of passivated nanocrystals in the 20–30 Å size regime. Stucky and co-workers [20] measured spectra of GaP clusters in a similar size range produced in the cavities of zeolite Y. Both observed quantum confinement effects, but found that the optical spectra anomalously extended to the red of the indirect gap in bulk GaP. In the small molecule regime, Weltner and co-workers [21] have recorded the IR absorption spectrum of GaP, Ga2P, and GaP2 trapped in a 4 K argon matrix. From these spectra they were able to obtain isotope specific vibrational fundamentals for these species. Huang et al. [22] have measured mass spectra of GaP cluster anions produced by laser ablation and observed a tendency for the clusters to have more gallium than phosphorus atoms.

Theoretical investigations of GaP clusters have been carried out by Andreoni [23], Ramakrishna and co-workers 24, 25, and Feng and Balasubramanian 26, 27, 28, 29. Andreoni used the Car–Parrinello molecular dynamics method to study the structures, stability, and melting of small stoichiometric GaAs, GaP, and AlAs clusters. Ramakrishna and co-workers have used an empirical pseudopotential method to investigate the electronic states of GaP nanocrystals ranging from 5–100 Å in radius. These calculations predict the nanocrystals to change from an indirect to a direct band gap semiconductor for clusters smaller than ∼20 Å, a prediction which has yet to be confirmed experimentally. Feng and Balasubramanian have performed ab initio calculations on a series of GaxPy clusters with five or fewer atoms 26, 27, 28, 29. They calculated ground and excited state geometries and energies at the complete active space self-consistent field (CASSCF) and multi-reference singles-doubles configuration interaction (MRSDCI) levels of theory.

The spectra reported here represent the first study of the electronic structure of gallium phosphide in the molecular cluster size regime. We obtain electron affinities (EAs) and vertical detachment energies (VDEs) to the ground and low-lying electronic states of the neutral clusters. In general, the photoelectron spectra of GaxPy clusters are quite similar to those of InxPy clusters 17, 30. Photoelectron spectra of the smallest clusters can be directly compared to the ab initio calculations of Feng and Balasubramanian in order to assign the observed electronic transitions. Trends in the larger clusters are also discussed; the most notable of these is that the EAs of stoichiometric (x=y) clusters with 10 or more atoms appear to extrapolate smoothly to the bulk value.

Section snippets

Experimental

The anion photoelectron spectrometer used in this study has been described in detail previously 31, 32. Cluster anions were generated in a laser ablation/pulsed molecular beam source. A rotating and translating single crystal disc of GaP (Crystallode) was ablated with second harmonic (532 nm) of a pulsed Nd:YAG laser. The laser pulses were typically 5.0–7.5 mJ pulse−1 before focussing on the target with a 1 m lens. Any ablated material is caught up in a supersonic beam of argon expanded through

Results

Fig. 1 shows the mass spectrum of GaxPy cluster anions produced by the laser ablation source; it represents a composite of three different mass spectra, each taken under different source conditions to optimize a particular mass range. The spectrum is composed of bunches of peaks separated in mass by a Ga atom. Each bunch of peaks is composed of 2–3 different cluster stoichiometries, as shown in the inset. Each cluster stoichiometry is represented by several peaks due to the natural isotope

Smaller clusters

Assignment of electronic features in the gallium phosphide photoelectron spectra for clusters with 3–5 atoms is possible through comparison with the calculations of Feng and Balasubramanian at the CASSCF and MRSDCI levels of theory. For GaP2 Feng and Balasubramanian find a 2B2 ground state with C2v symmetry and a ∠P–Ga–P bond angle of 43.9°, indicating strong P–P bonding. The low-lying 2A1 and 2B1 excited states, also with C2v symmetry, are predicted to have term energies of 1.07 and 2.33 eV,

Larger clusters

Fig. 4 shows a plot of EA vs. the number of atoms for clusters with various stoichiometries. The EAs are plotted as a function of N−1/3, where N is the total number of atoms. Clusters with an even number of atoms are shown with solid shapes and odd clusters are represented by open shapes. In general, for a particular stoichiometry GaxPx+y, the EA increases with x. Moreover, for clusters with approximately the same number of atoms, odd clusters have higher EAs than even clusters, consistent with

Acknowledgements

This research is supported by the National Science Foundation under Grant No. DMR-9521805. KRA gratefully acknowledges a postdoctoral fellowship from the Swiss National Science Foundation.

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