Electrochemical reaction of pInSe electrodes

Surface structure changes of p-InSe were monitored by AFM with atomic resolution when the potential was scanned from negative (-1.0 V) to positive (+0.2 V) and back again at a rate of 15 mV/s. Figure 1 shows the AFM images obtained sequentially when the potential was cycled [16]. All images shown in Fig. 1 were obtained while the tip was scanned from the left to the right. It took 11 s to capture one image and therefore the potential differs by 165 mV between the top and the bottom of each image. In relatively negative potential region (less than -0.5 V), the atomic arrangement was clearly resolved (Fig. 1(a)). The image is essentially the same as those obtained in air [17] and in water [16] and is in good agreement with the structure expected from the crystallographic structure. This result suggests that the surface is stable under these conditions. When the potential became more positive than -0.5 V, the atomic image became less clear (Fig. 1(b)). The more positive the potential was, the larger was the non-atomically resolved area (Fig. 1(c) and (d)). The atomic image was completely lost at potentials more positive than 0 V (Fig. 1(d)). The atomic image was partly recovered when the potential was scanned back to a potential more negative than -0.7 V (Fig. 1(e)) and was completely recovered if the potential was kept at -0.8 V for 1 min (Fig. 1(f)).

Fig. 1. (a) - (e) Sequentially obtained AFM images of p-InSe in 10 mM Na2S04 while the potential was scanned. Potential regions where images were taken are shown in the figure, (f) The image was taken after the potential was kept for 1 min at -0.8 V.

Fig. 1. (a) - (e) Sequentially obtained AFM images of p-InSe in 10 mM Na2S04 while the potential was scanned. Potential regions where images were taken are shown in the figure, (f) The image was taken after the potential was kept for 1 min at -0.8 V.

It is known that the anodic process at InSe is [18]:

Since InOH2+ is soluble but Se is insoluble in the solution, Se is left on the InSe surface and forms an amorphous layer upon oxidation. This should explain why the atomic image was lost at positive potentials. The deposited Se is removed reductively in cathodic potential region by [ 19]:

when the Se layer is not too thick, and the atomically ordered van der Waals face appears again.

3.2 Atomically resolved structure of GaAs(lOO) surfaces in electrolyte solutions

Since a GaAs surface is covered with native oxide, atomic arrangement is not observed in air. When an electrode was immersed in electrolyte solution and a cathodic potential was applied, the oxide layer seemed to be reduced electrochemically as atomic arrangement was observed at the p-GaAs electrode, as shown in Fig. 2 [20], The top view (Fig. 2(a)) and the Fourier spectrum (Fig. 2(b)) show that the atomic structure is of nearly four-fold symmetry with a nearest neighbor distance of about 0.42 ± 0.04 nm. This result shows that the surface has a GaAs(100)-(lxl) structure [21]. This is in contrast with the fact that a GaAs(lOO) surface forms a reconstructed structure such as

Fig. 2. (a) An atomically resolved AFM image of the p-GaAs electrode surface in 10 mM HC1 obtained at 0 V vs. Ag/AgCl. (b) Two-dimensional Fourier spectrum of the image.

c(2 x 8), (2 x 4), and c(4 x 4) in UHVconditions because of the existence of the dangling bond in (1 x 1) structures. In HC1 solution, the atomic images are clearer near the open circuit potential than in more cathodic potential regions. XPS shows that the Ga/As ratio is higher and the CI peak is stronger for samples which were removed from the cell after a positive potential was applied than at samples without anodic treatment

Thus, we speculate that the topmost atoms are Ga-terminated with CI and the (1 x 1) structure is stabilized by the CI termination.

In H2SO4 solution, however, atomic images were obtained more clearly at the potentials close to that of H2 evolution than at the potentials near the open-circuit potential [22]. Figure 3 shows a cyclic voltammogram and atomically resolved AFM images at the various potential regions of p-GaAs in H2SO4 solution. As the electrode potential became closer to the open-circuit potential, the atomic arrangement became less clear. This result suggests that the SO42" ion does not stabilize the dangling bonds on the GaAs(lOO) surface. In the cathodic potential region where the atomically ordered structure was observed, the GaAs(100)-(l x 1) structure seems to be stabilized as a result of termination with H^ ions.

The atomically ordered surface structure of (100)-(1 x 1) was observed also at n-type GaAs(lOO) electrodes.

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