TIPS potentials

FIG. 3 (a) Block schematic of the typical instrumentation for SECM with an amperometric UME tip. The tip position may be controlled with various micropositioners, as outlined in the text. The tip potential is applied, with respect to a reference electrode, using a potential programmer, and the current is measured with a simple amplifier device. The tip position may be viewed using a video microscope, (b) Schematic of the submarine UME configuration, which facilitates interfacial electrochemical measurements when the phase containing the UME is more dense than the second phase. In this case, the glass capillary is attached to suitable micropositioners and electrical contact is made via the insulated copper wire shown. 

Figure 5.4 Electric circuit for in situ STM, which allows sample and tip potentials to be controlled independent from each other.

Figure 2.29 Tip current as a function of the potential of an etched n-GaAs electrode in 2 M HC104. Tip potential 0 V vs. Ag/AgCI. Preset tunnelling current 6nA. From Uosaki and...

In addition, a bipotentiostat is used to control the tip potential with respect to the surface and independent of control of the surface potential with respect to the reference electrode. The tip potential E, is given by E, = Eg + E , where Eg is the bias potential that generates the tnnneling current between tip and surface, and E (a vital variable not typical of other applications of STM and AFM) is the potential of the surface relative to the reference electrode. 

Fig. 5. Simulated corrugation of Fe(100) surface with three different tungsten tips (left), and simulation of scans of the same surface if the surface states are quenched due to the STM tip potentials (right). The quenching of surface states due to the tip in the very close distance regime leads to a corrugation reversal, which is actually observed in the experiments.

This detailed picture of the movement of the atom during manipulation was achieved with the aid of simulations [6]. The atom moves in a local potential minimum on the surface. This potential is the sum of the surface potential and the tip potential. The surface potential can be expressed by the migration barrier while the tip potential describes the direct interaction via chemical or electrostatic forces. The local potential minimum is not identical with the adsorption site, in the limit of close tip-atom separation this minimum always resides below the tip resulting in the sliding mode. The atom is slowly pushed/pulled by the tip out of the adsorption site until it jumps into the next local potential minimum. The jump to the next potential minimum proceeds on a timescale of picoseconds [7,8] whereas typical tip speeds are of the order of 0.5-2.5nm/s. 

For the finite heterogeneous kinetics at the tip and diffusion-controlled mediator regeneration at the substrate, an approximate equation (22) was recently obtained for IT as a function of tip potential, E, and L [51]... 

The SECM can be used to measure the ET kinetics either at the tip or at the substrate electrode. In the former case, the tip is positioned in a close proximity of a conductive substrate (d < a). The substrate potential is kept at a constant and sufficiently positive (or negative) value to ensure the diffusion-controlled regeneration of the mediator at its surface. The tip potential is swept linearly to obtain a steady-state voltammogram. The kinetic parameters (k°, a) and the formal potential value can be obtained by fitting such a voltammogram to the theory [Eq. (22)]. A high value of the mass transfer coefficient (m) is achieved under steady-state conditions when d rate constants (k° > 1 cm-1 s) were measured with micrometersized SECM tips [92-94]. 

When an ionic single crystal is immersed in solution, the surrounding solution becomes saturated with respect to the substrate ions, so, initially the system is at equilibrium and there is no net dissolution or growth. With the UME positioned close to the substrate, the tip potential is stepped from a value where no electrochemical reactions occur to one where the electrolysis of one type of the lattice ion occurs at a diffusion controlled rate. This process creates a local undersaturation at the crystal-solution interface, perturbs the interfacial equilibrium, and provides the driving force for the dissolution reaction. The perturbation mode can be employed to initiate, and quantitatively monitor, dissolution reactions, providing unequivocal information on the kinetics and mechanism of the process. 

The SG/TC mode of SECM was also applied by Martin et al. [86] to study the oxidation of DMPPD. The generator was a 2-mm2 substrate electrode, and the collector was a 25-pm diameter Pt disk electrode. The substrate potential was stepped from 0 V versus Ag quasi reference electrode, where no Faradic process took place, to +500 mV, where the oxidation of DMPPD was diffusion controlled. The tip potential was held at 0 V, at which the oxidized form of DMPPD could be reduced at a diffusion controlled rate. After the tip-substrate separation was found from the positive feedback current-distance curve, the rate constant was obtained from the current transient at the tip. The feedback and SG/TC modes were also used to study the reduction of... 

If the tunnel junction of Fig. 1 a is simply immersed in an electrolyte, the polarization between the tip and the sample will promote an electrolysis. A bi-potentiostat is necessary to ensure real tunneling between the sample and the tip. Such a device, classically used in electrochemistry, enables to split the tunnel junction into two sol-id/liquid interfaces, independently polarized against a reference of potential (Fig. 1 b). Using this configuration, also referred to as the four-electrode configuration and introduced very early by several groups, it is possible to avoid any electrochemical transfer between the sample and the tip [25,26]. The reference potential is an electrode whose potential is well defined and constant with respect to the vacuum level. The sample is biased against the reference electrode to monitor reactions at the surface, just as in a classical electrochemical cell. The tip potential is adjusted... 

In vacuum, scanning tunneling spectroscopy (STS) has already opened new possibilities of studying the energy distribution of states at an atomic level. Owing to the necessity of scanning the tip potential over several volts, true spectroscopy has still not... 

This technique, thus named by Carlsson et al. [78], consists in recording the tip current as a function of the potential of the sample. TCV can be applied with the regulation loop of the tunnel current active or not. Though the terminology tip current voltammetry is rather confusing, because the tip potential is fixed, we use it in the following to conform with published work. 

The discussion above shows that TCV can be reasonably interpreted in the framework of known electronic states. The direct determination of interface states from TCV seems difficult because the dependence of (7° with the tip potential [78, 81] finds no simple explanation within a simple one-dimensional energy diagram. Tip-induced local modifications of the band diagram of the semiconductor may exist (see Sec. 4.2.3) which complicates the determination of energy levels. The experimental dependence of [7° on the pre-history of the electrode [78, 81] stem probably from... 

We refer here to techniques where the tip potential is scanned within limits of low faradaic currents. The tip insulation is critical. The results presented below are preliminary observations made by our group. No other results have been published, to our knowledge. 

In-situ SPV measurements seem possible with minor modifications (1) the tip potential (versus the reference) is set at a value close to the rest potential of the semiconductor in darkness (this must be compatible with the electrochemical response of the tip), and (2) the tip current is quenched by adjusting the sample voltage (versus the reference) with the second feedback system. With p-type materials the method seems more obvious than with n-type specimens, since illumination promotes surface electrons. At n-type materials SPV measurements will induce corrosion since holes are driven to the interface. If absolute measurements of the SPV seem difficult, because they depend on the adjustment of the tip potential, differential measurements appear accessible to experiment. 

Figure 6.27 Demonstration of a local deposition and selective removal of Cu clusters on Au(lll) in the system Au(lll)/0.05 M H2SO4 + 0.1 mM CUSO4 at T = 298 K [6.188]. (a) local deposition of copper clusters and (b) selective removal of a cluster by applying a very positive tip potential Ef = 500 mV vs. Cu/Cu. ...

Nemst equilibrium potential of Me/Me electrode standard potential of Me/Me electrode potential of zero charge substrate potential tip potential... 

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