Electronic structures of supported nanometer-size clusters using field emission energy analyzer

Mong-Ea Lin, Purdue University

Abstract

The discrete energy states of a nanometer-size gold cluster have been measured using energy-resolved field emission microscopy. The clusters were prepared in a multiple expansion cluster source (MECS) which is capable of producing nanometer-size clusters with a narrow size distribution. An individual cluster was deposited on a tungsten field emission tip which was then transferred under vacuum into a UHV field emission chamber. A 127$\sp\circ$ differential energy analyzer with 80 meV resolution was used to measure the energy distribution of electrons emitted from the individual cluster. Several peaks are observed in the field emission energy distributions. A simple model will be presented to explain the structure observed and relate the observed peaks to discrete energy levels of the Au cluster. As a next example of such 'single cluster' experiments, we will discuss the evidence for electron emission from the first quantum charge state of a metal cluster. This charge state is located at an energy e$\sp2$/2C above the highest filled electronic state of the neutral cluster and becomes populated at high rates of electron emission. We will also investigate the electrons emission rates of cluster and tungsten tips. The emission rates of cluster tips are not as stable as those of tungsten tip and do not obey Poisson distributions. The reduced $\chi\sp2$ test shows two different geometrical structures of a small cluster and indicates quasimelting of this cluster. Another interesting topic to study is electron emission from the newly discovered C$\sb{60}$ molecule. C$\sb{60}$ dust has been heated near a tungsten tip situated in an ultra high vacuum chamber. The rate of cluster deposition was found to greatly increase when the tip was biased positive with respect to the oven potential. These experiments have shown that with care, individual C$\sb{60}$ can be deposited on a field emitter and studied using field emission microscopy techniques. Size estimates of the field emission image indicate that electrons from a single C$\sb{60}$ are being observed. Current-voltage characteristics allow estimates of the effective tunnel barrier for electron emission and provide information about the electron states in the supported C$\sb{60}$ cluster. Furthermore, the field emission energy distributions of C$\sb{60}$ show a two-level splitting of the HOMO $h\sb{u}$ level under high electric field.

Degree

Ph.D.

Advisors

Reifenberger, Purdue University.

Subject Area

Condensation

COinS