الفهرس 
Nanotechnology
Physics of NanoparticlesOnly 14 pages are availabe for public view
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Abstract
In this work, our main interest is investigating the electronic structure at the interface between a metal and a semiconductor in the nanoregime, and its effect on the exciton absorption and the charge separation efficiency. Au tipped Cd-chalcogenide (CdS, CdSe and CdTe) hybrid nanostructures (HNSs) are specially designed for this study because the interface in such nanostructures can be probed by scanning tunneling microscope (STM).
STM was used for studying the electronic structure at the interface of Au tipped CdX (X= S, Se and Te) because STM is a unique technique for the interfacial study of such contacts on the nanoscale. It can be operated in a topographic mode to obtain the dimensions of Au tipped Cd-chalcogenide hybrid nanostructures (HNSs), and also in the scanning tunneling spectroscopy (STS) mode to get detailed information about the interfacial band bending in such HNSs. In addition, the high resolution transmission electron microscope (HRTEM) was performed to get clear information about the structure and the dimension of these HNSs. Furthermore, the ultra visible - visible (UV-vis) optical absorption was recorded for these HNS to detect the effect of the Au tip deposition on the exciton absorption of such HNSs.
CdX (X=S, Se and Te) HNS were prepared by organo-metallic chemical method, and then Au tipped CdX HNSs were synthesized by phase transfer chemical methods. Their structure and size before and after the Au formation were determined using STM in the topographic mode as well as HRTEM.
In STS mode, we used an asymmetrical configuration (conducting substrate - Cd-chalcogenide nanoparticle (NP) - vacuum gap - scanning probe), in which the change of the voltage bias leads to an equal change of the probe Fermi level relative to the energy levels of the Cd-chalcogenide NP, while the Fermi level of the conducting substrate remains unchanged. Therefore, the probe Fermi level scans over the energy levels of the Cd-chalcogenide NP since there are no linker molecules between the substrate and the NP. At a positive sample bias, the onset of the differential of the current relative to the voltage bias (dI/dV>0) represents the resonant tunneling from the probe to the NP via the lowest unoccupied molecular orbitals (LUMO) of the Cd-chalcogenide NP. At a negative sample bias, the onset of (dI/dV>0) indicates the resonant tunneling from the NP to the probe via the highest occupied molecular orbitals (HOMO) of the Cd-chalcogenide NP. Thus, the band gap of a Cd-chalcogenide NP can be measured directly from the region of zero conductance around 0 V sample bias of the STS tunneling spectrum. The fine structures of electron and hole levels are hard to be detected in the STS at room temperature.
The STS at every point on a bare Cd-chalcogenide NP shows a symmetrical tunneling spectrum around 0 V bias (Fermi level position in STS) with a width at zero conductance equal to the Cd-chalcogenide NP band gap, on the assumption that such NP is an intrinsic SC. On the other hand, the STS of Au tipped Cd-chalcogenide hybrid NP shows different forms around 0 V bias, according to the position of scanning probe in the STS measurement. At the Au tip position, the tunneling spectrum has a narrow width at zero conductance, indicating the Coulomb blockade due to the metallic surface. When the scanning probe is at the Cd-chalcogenide position away from the Au tip, a symmetrical spectrum around 0 V bias is expected. However, the STS at the position of the probe on the Au edge with the Cd-chalcogenide NP show an asymmetrical tunneling spectrum around 0 V bias due to shifts of the conduction band edge〖 E〗_C and the valence band edge〖 E〗_V. Such shifts indicate the band bending at the interface of Au tipped Cd-chalcogenide HNSs.
In the case of Au tipped CdS and CdSe HNSs, the negative shift of the band edges of the STS at the interface is an indication of the positive Fermi level shift, causing a downward band bending towards the interface, which indicates the electron accumulation at the interface of these HNSs. However, the positive shift of the STS at the interface of Au tipped CdTe is an indication of the negative Fermi level shift, leading to an upward band bending towards the interface, indicating electron depletion at the interface. The values of the measured band bending were compared by a model of the energy band diagrams of these HNS.
The UV-vis absorption spectra of Au tipped CdX (X= S, Se and Te) HNSs show localized surface plasmon (LSP) peaks, which is an indication of the Au tip formation on the Cd-chalcogenide quantum dots (QDs), and also gives the exciton peak of the Cd-chalcogenide QD. By applying the effective mass approximation (EMA) model, it is possible to calculate the average size of the Cd-chalcogenide QD to compare with those obtained directly using STM and HRTEM images. Furthermore, A bleaching feature is observed of the exciton peaks of Au tipped CdS and CdSe HNSs that indicates inefficient charge separation resulting from electron accumulation at their interfaces as mentioned in the STS measurements. However, an exciton enhancement feature, indicating efficient charge separation, is observed for Au tipped CdTe and explained as a result of the electron depletion at its interface as mentioned in STS measurements.
from these studies, it can be concluded that the interfacial electronic structure of Au tipped Cd-chalcogenide HNSs has a great impact on the exciton absorption and the charge separation efficiency, which are two important parameters for choosing suitable active materials for solar cells. Therefore, the study of the electronic structure at the interface of such HNSs is important in the area of plasmonics for improved photovoltaic devices.