STM tip

In summary, we have shown that the stability of binding sites, diffusion and rotational barriers can be extracted straightforwardly from real time variable temperature STM experiments. Moreover, we 

HAROLD J.W. ZANDVLIET, GEERT BROCKS AND BENE POELSEMA 

For the elementary group IV semiconductor (001) surfaces there are in principle 3 plausible adsorption sites for ad-dimers on-top of the substrate rows with the dimer bond of the ad-dimer either parallel (A configuration) or perpendicular (B configuration) to the substrate dimer bonds and a third adsorption site in the trough with the dimer bond of the ad-dimer aligned parallel to the substrate dimer bonds (C configuration). For Ge/Ge(0 01) and Ge/Si(0 01) ad-dimers is also some intermediate configuration on-top of the substrate dimer rows labeled A/B. Whether this is really a stable adsorption site or just due to the fact that the ad-dimer rapidly rotates back and forth from A to B and vice versa is not yet settled. 

For the system Si/Si(0 01) the B configuration is the lowest in energy followed by A and C, whereas for the system Ge/Ge(00 1) C is energetically the most favorable adsorption site followed by B. In the case of the heteroepitaxial systems is not straightforward to extract which dimer adsorption site is energetically the most favorable. The main reason for this is that intermixing of atoms originating from the ad-dimer and the substrate can occur, even at temperatures as low as room temperature. 

For the system Si/Si(0 0 1) is rotational mode of the on-top ad-dimer is active at room temperature. Activation barriers and prefactors for this rotation process are presented. For the system Ge/Si(0 0 1) another rotation mode is found. Mixed Si-Ge dimers rock back and forth between the B configuration (actually a 180° rotation). The activation barrier and attempt frequency for this rocking process are given too. 

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Fig. VIII-2. Scanning tunneling microscopy images illustrating the capabilities of the technique (a) a 10-nm-square scan of a silicon(lll) crystal showing defects and terraces from Ref. 21 (b) the surface of an Ag-Au alloy electrode being electrochemically roughened at 0.2 V and 2 and 42 min after reaching 0.70 V (from Ref. 22) (c) an island of CO molecules on a platinum surface formed by sliding the molecules along the surface with the STM tip (from Ref. 41).

One of the more interesting new areas of surface science involves manipulation of adsorbates with the tip of an STM. This allows for the fonuation of artificial structures on a surface at the atomic level. In fact, STM tips are being investigated for possible use m lithography as part of the production of very small features on microcomputer chips [74]. 

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

The molecular-level observation of electrochemical processes is another unique application of STM [53, 54]. There are a number of experimental difficulties involved in perfonning electrochemistry with a STM tip and substrate, although many of these have been essentially overcome in the last few years. 

Fig. 4. Atom manipulation by the scanning tunneling microscope (STM). Once the STM tip has located the adsorbate atom, the tip is lowered such that the attractive interaction between the tip and the adsorbate is sufficient to keep the adsorbate "tethered" to the tip. The tip is then moved to the desired location on the surface and withdrawn, leaving the adsorbate atom bound to the surface at a new location. The figure schematically depicts the use of this process in the formation of a "quantum corral" of 48 Fe atoms arranged in a circle of about 14.3 nm diameter on a Cu(lll) surface at 4 K.

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

Figure 8 Spectroscopic study of GaAs(110). With a positive voltage on the STM tip, the left-hand image represents As atoms, while the corresponding negative tip voltage on the right shows Ga atoms. (Courtesy of Y. Yang and J.H. Weaver, University of Minnesota)...

STM tips will last for a day or so in ultrahigh vacuum. Most ultrahigh-vacuum STM systems provide storj e for several tips so the chamber does not have to be vented just to change tips. In air, tips will oxidize more rapidly, but changing tips is a simple process. 

Another class of anifacts occurs when scanning vertical or undercut features. As the tip approaches a vertical surface, the side wall may encounter the feature before the end of the tip does. The resulting imj e will appear to contain a discontinuous shift. Changing the angle of the tip with respect to the sample s surface can minimize the problem. Side wall imj ing also occurs in STM, but less frequently since an STM tip has a higher aspect ratio than that of an SFM tip. 

The tips used for STM experiments should be sharp and stable. Chemical stability can be achieved by using a noble metal. Mechanical rigidity can be reached by short wires. Alloys of Pt and Ir are frequently used for fabrication of STM tips. They can be produced in a surprisingly simple way just by cutting a metal wire with conventional cutting tools. Because of their high chemical stability, such Pt/Ir tips are well... 

The most arresting development is the use of an STM tip, manipulated to move both laterally and vertically, to shepherd individual atoms across a crystal surface to generate features of predeterminate shapes an atom can be contacted, lifted, transported and redeposited under visual control. This was first demonstrated at... 

Two working modes are used for the STM first, the constant height-mode, in which the recorded signal is the tunneling current versus the position of the tip over the sample, and the initial height of the STM tip with respect to the sample surface is kept constant (Fig. 22(a)). In the constant currentmode, a controller keeps the measured tunneling current constant. In order to do that, the distance between tip and sample must be adjusted to the surface structure and to the local electron density of the probed sample via a feedback loop (Fig. 22(b)). 

FIG. 16 Current versus bias voltage for a CdS nanoparticle on the end of an STM tip. The CdS particles were formed by exposing a bilayer of cadmium arachidate on the STM tip to H2S gas. The other conducting surface is a highly oriented pyrolytic graphite electrode. The inset is a plot of differential conductance versus the bias voltage. (Reproduced with permission from Ref. 202. Copyright 1996 National Academy of Sciences, U. S. A.)... 

As the second step, the STM tip was locked over the desired particle, feedback was temporally switched off, and voltage-current (V-I) characteristics were measured. The typical trend of the V-I characteristics is shown in Figure 29. Current steps are clearly observable in the presented curve, indicating that the single-electron junction was formed. It is worth mentioning that the characteristics observed in areas without particles demonstrate a normal tunneling behavior (see Fig. 30). 

It is possible to generate holes on the surface of the substrate through the application of very short negative voltage pulses to the STM tip. This procedure only succeeds using highly concentrated electrolytes. 

As pointed out above, an STM tip can be used to nucleate and grow single clusters. In this type of experiment, cluster deposition on a STM tip is achieved when it is retracted about 10 to 20 run from the substrate surface. Under these conditions, where the feedback loop is disabled, absence of mechanical contact between the tip and the substrate in ensured. Then a positive potential pulse is applied to the tip, the metal deposited on it is dissolved, and it diffuses toward the substrate surface, where a nucleus develops and grows to yield a cluster, typically 20 nm wide. 

Pettinger, B., Picardi, G., Schuster, R. and Ertl, G. (2002) Surface-enhanced and STM-Tip-enhanced Raman spectroscopy at metal surfaces. Single Mol., 5-6, 285—294. 

STM Tip-enhanced photoluminescence from porphyrin film. Surf. Sci., 601, 3601-3604. 

Figure 12. A ligand-protected Pt309 nanocluster between an STM tip and an Au (111) surface to determine the current-voltage characteristics.

One of the junctions, which is between the ground plane and the metal particle is mechanically fixed, while another one which is between the particle and the STM tip is adjustable. 

Figure 3. SE two-junction system consisting of a scanning trm-neling microscope (STM) tip and a metallic nanoclusters as a central electrode on a ground plane.

Figure 4. A single ligand stabilized Ptsog-cluster between STM tip and an Au( 1 11) facet. The junction between the cluster and the substrate is built up by the ligand shell.

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