Carbon Soot Polymer Nanocomposites (CSPNCs): Production, Surface Morphological, Glass Transition Temperature Phenomenon and Optical Properties

Abstract

The scanning tunneling microscope (STM) was invented by G. Binnig and H. Rohrer in 1982 and they were subsequently awarded the Nobel Prize for Physics in 1986. From an experimental standpoint, the basic idea is as follows: a fine metal tip is brought close to a surface (typically to within one nanometer) and the current flowing between tip and surface is measured when a voltage is applied across the gap. According to classical physics, as there is no contact between the tip and the surface, no current can flow (open circuit). But according to quantum mechanics, if the distance between two electrodes (here, the tip and surface) is small enough, a current can in fact flow across the gap between the tip and the surface. This is the so-called tunnel effect, which has given its name to the microscope based upon it. The tunnel effect, a purely quantum phenomenon, was first hypothesised in 1927. A particle such as the electron, described by its wave function, has a nonzero probability of penetrating a barrier, although this would be forbidden in classical mechanics. As a consequence, the electron can actually cross a barrier which separates two classically allowed regions. The tunneling probability, i.e., the probability that an electron will pass from one electrode to the other across the barrier, decreases exponentially with the width of the barrier. The tunnel effect can therefore only be observed for narrow barriers, of the order of the nanometer. Theory shows that the current detected is related to the chemical nature of the opposing surfaces, and this on the atomic scale. The microscope is based on a combination of two factors: controlled approach of a metal tip towards a conducting surface, using piezoelectric tubes, and a high-performance anti-vibration system. The piezoelectric tubes have extension coefficients of the order of a few Å/volt and can thus ensure very accurate movements of the tip (bonded onto a piezoelectric ceramic) relative to the fixed surface by applying very low voltages (a few volts). Binnig and Rohrer demonstrated their invention using a conducting sample and a rather fine conducting tip, which acted as a local probe when brought within a few angstrom units of the surface. With tip-surface voltages of the order of 1mV to 4 V, tunneling currents of between 0.1 nA and 10 nA were observed. Varying the tip-surface distance established the exponential character of the current as a function of the separation.