Scanning Tunneling Microscopy

Abstract

Scanning tunneling microscopy (STM) is a technique for the determination of the structure of surfaces, with spatial resolution on the angstrom scale. In an oversimplified description, STM allows imaging of the surface of conductive materials down to the atomic level. The technique is based on the measurement and control of the current of electrons tunneling between a sharp stylus (hereafter called the tip) and the sample surface. During imaging, the tip is separated a few angstroms (3 to 10 Å) from the surface, while it is being rastered with the help of piezoelectric transducers. The area scanned is tens to thousands of angstroms on a side. STM has the advantage over other microscopies that its operation does not require any special environmental conditions. Imaging can be performed in vacuum, at high pressures (including air), and in liquids. The choice of the environment is determined mostly by the requirements of the sample and its surface condition. Acquisition of STM images is relatively simple with modern instruments. Most of the experimental time is spent on sample preparation and tip conditioning in the appropriate environment (ultrahigh vacuum, electrochemical cell, or controlled atmosphere). Tip conditioning and noise reduction can sometimes add considerable time, depending on factors such as the nature of the experiment and reactivity of the sample. This time depends also on the type of information being sought. In considering the ultimate resolution obtainable with the STM, it is important to consider the role of tip geometry. On flat parts of the sample, where one tip atom is responsible for the tunneling, STM reaches its maximum performance. On rough surfaces, however, the tunneling point on the tip apex changes as it moves over sharp corners and changing slopes. The image is then a convolution of the tip and surface shapes, and the final resolution is determined by the tip radius and profile. STM is thus not the best technique to probe, for example, highly corrugated a patterned surfaces, and porous materials.

Automated Tip Conditioning for Scanning Tunneling Spectroscopy

“…10−12 In most cases, STS measurements are performed in ultrahigh vacuum (UHV) and low temperature (4 K) to minimize external disturbances. Initial tip preparation 13−15 and continuous in situ tip conditioning 15,16 are often required throughout the characterization of target molecules and materials. A common way to prepare STM tips is to repetitively poke the tip into welldefined and bare substrates, i.e., coinage metals or silicon, to remove contaminations and in the process coat the tip with a thin layer of substrate atoms.…”

“…Scanning tunneling microscopy (STM) techniques − and associated spectroscopic (STS) methods, − such as d I /d V point spectroscopy, have been widely used to measure electronic structures and local density of states (LDOS) of single molecules and materials with unprecedented spatial and energy resolutions. − However, the quality of d I /d V spectra highly depends on the shape of the probe tips, and atomically sharp tips with stable, well-defined apex structures are required for obtaining reliable spectra. − In most cases, STS measurements are performed in ultrahigh vacuum (UHV) and low temperature (4 K) to minimize external disturbances. Initial tip preparation − and continuous in situ tip conditioning , are often required throughout the characterization of target molecules and materials. A common way to prepare STM tips is to repetitively poke the tip into well-defined and bare substrates, i.e., coinage metals or silicon, to remove contaminations and in the process coat the tip with a thin layer of substrate atoms. − The d I /d V spectrum of the substrate will then be used as a reference to determine whether the tip is suitable for STS experiments.…”