Study of Metal/Semiconductor Interface in Nanostructures

Abstract

The fabrication of metal/semiconductor (M/S) hybrid nanostructures (HNS) opens new opportunities for designing multifunctional materials with properties that can not be obtained in the bulk phase. The electronic structure at the interface of M/S contact on the nanoscale are fundamental and intriguing problems since understanding the interfacial electronic structure of such nanocontacts is important for usage in nanoelectronic devices and other applications in the photovoltaic systems.

In this thesis, we investigated the electronic band structure at the interface of Cd-chalcogenide (CdS, CdSe and CdTe) NPs in contact with Au tip and their effect on the exciton absorption the and charge separation efficiency because this study has a great impact on the field of plasmonics for improved photovoltaic solar cells.

First, the high resolution transmission electron microscope (HRTEM) images of Au tipped CdX (X=S, Se and Te) hybrid nanostructures (HNSs) were performed before and after the Au deposition to obtain a clear information about the structure and the size of the nanoparticles (NPs). Second, we used UHV scanning tunneling microscope (STM) in the topographic mode to obtain the dimensions of Au tipped Cd-chalcogenide HNSs before and after the Au deposition and also in the scanning tunneling spectroscopy (STS) mode to get detailed information about the band bending at the interface of such HNSs, and then we compared the STS results to a model of the energy band diagrams of these HNSs. That is because STM is a unique technique for studying the electronic structure at the interface between a metal and a semiconductor in the nanoregime. The STS results show that: 1. The measured sizes using STM images are in agreement with those obtained from the HRTEM images. 2. The STS spectra at the interface of Au tipped CdS and CdSe HNSs show a negative shift relative to the STS spectra away from the interface. These shifts are due to the positive Fermi level shift of CdS and CdSe NPs for equilibration with the Fermi level of the Au tip, causing a down band bending towards the interface by the same amount in each case. The amounts of the STS shifts are confirmed by the model of the energy band diagram of the Au tipped CdS and CdSe HNSs respectively. These results are a direct observation of the electron accumulation at such interfaces. 3. The positive shift of the STS spectrum at the interface of Au tipped CdTe HNS relative to the STS spectrum far from the interface is due to the negative shift of the Fermi level of CdTe NP for equilibration with that of the Au tip. This shift leads to an upward band bending in CdTe towards the interface by the same amount, agreeing with that obtained from the model of the energy band diagram of Au tipped CdTe HNS. This is a direct observation of electron depletion at the interface of Au tipped CdTe HNS.