Probing Electronic Properties of Nanostructures in Pb/Si(111) and Nano-Au Wires by Scanning Tunneling Microscopy

• Main Authors: Wen-Bin Jian, 簡紋濱
• Other Authors: Tien T. Tsong
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/81872358016689450028

博士 === 國立臺灣大學 === 物理學研究所 === 90 === Tailored materials, stacking by nano-scale structures, with their electronic properties have aroused scientific research and exploration. The fundamental rules governing these low-dimensional materials interest physicists to study experimentally and theoretically. Quantum mechanics, wholly contradicted to our classical view points, play the most important role among rules on the nano-scale. Examples are interference effects of electronic waves, phase coherence, ballistic transport, and instability of low dimensional electronic systems etc. In this work, utilizing atomic-scale precision and variable-temperature technique of scanning tunneling microscopy, we have explored electronic properties of nano-Au wires, two-dimensional crystalline Pb islands, and structure phases of Pb adsorbates on Si(111) surfaces. We try to understand quantum-world-limited electronic disciplines that play in nano-scale materials. The first portion of this work is to study conductance quantization of nano-Au wires in quantum-point-contact regime. The nanostructures, nano-Au necks, are developed in the tip and sample. The conductance varied as a function of separation length of the tip and sample and we try to draw a relation between the average geometry of the nanostructure and the quantized conductance. We give a trend of their effects on conductance jumps and conductance distribution in the conductance histogram. The conductance jumps are downward-shifted from ideally quantized value of $2e^2/h$ ($G_0$). We also have measured the nano-structure's deformation speed and its effects on deteriorating the quantized conductances. A scanning tunneling microscope with a low-frequency-modulation voltage applied on Z piezo is used to study conductance quantization of nano-Au necks at ambient pressure and at room temperature. Conductance up to 40 multiples of $G_0$ can be repeatedly generated by pressing and pulling a Au tip against a Au sample with a sinusoidal distance modulation. By applying a symmetric double-cone model, only one parameter, the cone angle, is needed to describe the overall geometry of the contact. A conductance histogram in the low conductance range is then tabulated as a function of the cone angle. The quantized value of the conductance is found to decrease from the idealized value, $nG_0$, by an amount which increases with the angle of the cone. The distribution of the first conductance peak is analyzed to study the modulation speed. The peak width broadens as the speed is increased from 400 to 1000 \AA/s. The second portion of this work is trying to measure electronic charge density on surfaces of nanostructures. Isolated nanostructures, two-dimensional crystalline Pb islands with a height of $\ \sim1$ nm, can be developed by deposition of Pb on Si(111) surfaces. Electrons are confined and they transport ballistically in the out-of-plane direction. Any potential variation, in the Pb/Si metal-semiconductor interface, may generate electronic wave interference and result in excess charge density on surfaces of Pb islands. Atomically flat surfaces of two-dimensional islands are formed by deposition of Pb atoms on the incommensurate-phased Pb/Si(111) substrate at low temperatures. Atop these two-dimensional islands we have observed two types of periodic patterns, classified into Type I and II, with the same basis length of 3.85 nm. Type I islands emerge obviously a periodically pattern on odd number layers and dim a complement pattern on even number layers. On the contrary, Type II islands appear clear and the same periodically pattern on even number layers. Patterns appearing on surfaces of Type I and II islands are reversely dependent bi-layer oscillations as the growth is continued layer-by-layer. Electronic patterns with their oscillations may originate in electron scattering at the Pb/Si metal-semiconductor interface. Considering quantum size effects which suppress any small roughness in two-dimensional islands and leave vacancies at the buried interface, a simple model that electrons scattered by interface vacancies produce charge density oscillations in the out-of-plane direction is given to describe experimental data concordantly. The last portion of this work is to study structure phase transition on surfaces of Pb/Si(111). Instability of low-dimensional electronic system inducing structure phase transition is studied. Different structure phases developed in the Pb/Si(111) system have been imaged in atomic scale to clarify the (Pb atoms) coverage dependence and to find out the electronic effects contributing to the structure phase transition. There are various phases that can be realized in the Pb/Si(111) system. The structure phases listed from low to high coverage are $\gamma$, $\alpha$, $1\times1$, hexagonal incommensurate (HIC), and stripped incommensurate (SIC) phases. In addition, the $1\times1$ phase undergoes a structure phase transition to a $\sqrt{7}\times\sqrt{3}$ phase at temperatures lower than 250\ K. We confirm the same coverage of Pb atoms on surfaces among $1\times1$, low temperature $\sqrt{7}\times\sqrt{3}$, and HIC phases. We also find that more Pb atoms are required to form the SIC phase. To study intermediate state between $1\times1$ and HIC phases, we have confirmed the mechanism of an electronic origin, appearing hexagonal structures centering on single-atom defects which can be observed in empty state STM images. The electronic origins result in redistribution of single-atom defects and pin down the unstable $1\times1$ phase to form the HIC phase at room temperature.