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The scanning tunnelling microscope (STM) developed by Dr Gerd Binnigand his colleagues in 1981 at the IBM Zurich Research Laboratory, Ruschlikon, Switzerland, is the first instrument capable of directly obtaining three-dimensional (3D) images of solid surfaces with atomic

resolution. STMs have been used for the formation of nano-features by localized heating or by inducing chemical reactions under the STM tip and nano-machining. AFMs have been used for nano-fabrication and nano-machining. STMs and AFMs are used at extreme magnifications ranging from 103 to 109 in the x-, y- and z-directions for imaging macro to atomic dimensions with high resolution information and for spectroscopy. These instruments can be used in any environment such as ambient air, various gases, liquids, vacuum, at low temperatures (lower than about 100 K) and high temperatures.

The principle of electron tunnelling was proposed by Giaever. He envisioned that if a potential difference is applied to two metals separated by a thin insulating film, a current will flow because of the ability of electrons to penetrate a potential barrier. Although small, there is a finite probability for tunnelling. To be able to measure a tunnelling current, the two metals must be spaced no more than 10 nm apart. The tunnelling current is highly sensitive to the separation distance between tip and sample; it decreases exponentially with increase in tip– sample separation distance. This factor is crucial in ensuring the excellent vertical resolution of STM (less than 0.1 nm). Tunnelling current decreases by a factor of 2 as the separation is increased by 0.2 nm. Very high lateral resolution depends upon sharp tips. Binnig and his group overcame two key obstacles for damping external vibrations and for moving the tunnelling probe in close proximity to the sample. Their instrument is called the scanning tunnelling microscope (STM). Today’s STMs can be used in the ambient environment for atomic-scale images of surfaces.

The principle of the STM is straightforward. A sharp metal tip (one electrode of the tunnel junction) is brought close enough (0.3–1 nm) to the surface to be investigated (the second electrode), such that, at a convenient operating voltage (10 mV–1 V), the tunnelling current varies from 0.2 to 10 nA, which is measurable. The tip is scanned over a surface at a distance of 0.3–1 nm, while the tunnelling current between it and the surface is measured. In STM, a conductive tip placed above the surface of a sample moves on the sample surface with its height being adjusted continuously to keep the tunnelling current constant. The tip position is monitored to map the surface topography of the sample. Figure 5.11 schematically depicts an STM structure. A sharp tip of tungsten or PtIr alloy is mounted on a three-dimensional positioning stage. The tip movement above the sample surface in three dimensions is controlled by piezoelectric arrays. The distance between the tip and the sample is around 0.2 and 0.6 nm, which generates a tunnelling current of about 0.1–l0 nA. The spacial resolution is about 0.01 nm in the x- and y- directions and about 0.002 nm in the z-direction, which leads to a true atomic resolution in three dimensions.

5.6.1 Modes of operation

STM is commonly operated in two modes. One of them is constant current imaging, in which a constant current is maintained between the sample and tip. During the movement of the tip over the sample surface, the vertical position of the tip is changed to maintain a constant separation between the two. Since the tunnelling current is sensitive to distance, constant current imaging will provide excellent surface topographic contrast of the surface atom contours. In contrast to the constant current mode, constant tip position results in variations

in tunnelling current due to changes in tip separation distance brought about by the 3D topographic features of the surface atoms. In the constant current mode, the contrast is related to electron charge density profiles, while faster scan rates are possible in the constant height mode. STMwas first developed by Binnig and Rohrer in 1982, and its atomic-scale resolution was first demonstrated on an image of silicon. After etching the oxide with an HF solution, the (111)silicon wafer was immediately transferred to the STM in a UHV chamber. Repeated heating to 900°C in a vacuum not exceeding 3 × 10–8 Pa resulted in effective sublimation of the SiO layer grown during the transfer, resulting in a clean surface. Only unidirectional scans were recorded to avoid the non-linear effects of the scanning piezoelectric drives.

Families of instruments based on STMs and AFMs, called scanning probe microscopes (SPMs), have been developed for various applications of scientific and industrial interest. These include STM, AFM, FFM (or LFM), scanning electrostatic force microscopy (SEFM), scanning force acoustic microscopy (SFAM) or atomic force acoustic microscopy (AFAM), scanning magnetic microscopy (SMM) or magnetic force microscopy (MFM), scanning near field optical microscopy (SNOM), scanning thermal microscopy (SThM), scanning electrochemical microscopy (SEcM), scanning Kelvin Probe microscopy (SKPM), scanning chemical potential microscopy (SCPM), scanning ion conductance microscopy (SICM), and scanning capacitance microscopy (SCM). Families of instruments that measure forces (e.g., AFM, FFM, SEFM, SFAM and SMM) are also referred to as scanning force microscopy (SFM). Although these instruments offer atomic resolution and are ideal for basic research, they are

used for cutting-edge industrial applications which do not require atomic resolution. There are a number of commercial STMs available in the market.

The STM operates in both constant height and constant current modes depending on a parameter selection in the control panel. In the constant current mode, the feedback gains are set high, the tunnelling tip closely tracks the sample surface, and the variation in tip height required to maintain constant tunnelling current is measured by the change in the voltage applied to the piezo tube. In the constant height mode, the feedback gains are set low, the tip remains at a nearly constant height as it sweeps over the sample surface, and the tunnelling current is imaged.

5.6.2 STM configuration

Physically, the STM consists of three main parts: the head, which houses the piezoelectric tube scanner for three-dimensional motion of the tip and the pre-amplifier circuit (FET input amplifier) mounted on top of the head for the tunnelling current, the base on which the sample is mounted, and the base support, which supports the base and head. The base accommodates samples up to 10 mm × 20 mm and 10 mm in thickness. Scan sizes available for the STM are 0.7 μm (for atomic resolution), 12 μm, 75 μm and 125 μm square. The three-dimensional movement of the tip is controlled by the scanning head. The removable head consists of a piezo tube scanner, about 12.7 mm in diameter, mounted into an invar shell used to minimize vertical thermal drifts because of a good thermal match between the piezotube and the invar. The piezo tube has separate electrodes for x, y and z,movementwhich are driven by separate drive circuits.

The electrode configuration provides x- and y- motions that are perpendicular to each other, minimises horizontal and vertical coupling, and provides good sensitivity. The vertical motion of the tube is controlled by the Z electrode, which is driven by the feedback loop. The x- and y-scanning motions are each controlled by two electrodes, which are driven by voltages of the same magnitude, but opposite signs. These electrodes are called −Y, −X, +Y, and +X. The tip holder is a stainless steel tube with a 300 μm inner diameter for 250 μm diameter tips, mounted in ceramic in order to keep the mass on the end of the tube low. The tip is mounted either on the front edge of the tube (to keep mounting mass low and resonant frequency high) or the centre of the tube for large range scanners, namely 75 μm and 125 μm (to preserve the symmetry of the scanning). This commercial STM accepts any tip with a 250 μm diameter shaft. The piezo tube requires X–Y calibration, which is carried out by imaging an appropriate calibration standard. Cleaved graphite is used for the small-scan length head while two-dimensional grids (a gold-plated ruling) can be used for longer range heads.

Characterization of monolayers of nanoparticles using many characterization techniques, like SEM or TEM, is difficult due to the limitations of the resolution offered by the techniques used. Scanning probe techniques have made it possible to investigate the surfaces of materials at the atomic level. In addition, with scanning tunnelling spectroscopy, the electronic structure of surfaces can be studied at molecular level. Though several techniques are used to study such functionalized metal nanoparticles, STM/STS is a unique tool because it enables the topographic and local electronic properties of these films to be investigated with atomic

and near-atomic resolution. Two main properties which can be studied with tunnelling spectroscopy on nanoparticles are density of states on isolated nanoparticles and Coulomb blockade and Coulomb staircase phenomena due to charging of the nanoparticles.