Nanofabrication of Materials with a Scanning Tunneling Microscope
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Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
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being etched by Fig. 1 STM image of graphite in air after the application of a positive bias voltage over 2.5 V. Tunneling current: I nA; bias voltage: 400mV.
Fig. 2 Relationship between the threshold voltage for etching the surface and the binding energy of the material.
The nanofabrication procedure is as follows; the tip is brought into tunneling with the flat sample surface and the bias voltage is raised several volts from the imaging bias voltage. The same area is then imaged to detect modification, and this procedure is repeated, increasing bias voltage by 0.1 V each time, until the surface is modified under the tip, which defines the threshold voltage, Vt, as shown in Fig. 1. RESULTS The relationship between the Vt and the binding energies of materials (standard enthalpy of formation [8]) is shown in Fig. 2. Vt's are independent of the measured tunneling current between 0. 1 and 50 nA and the duration time of the high voltage between 10 ms and 1 sec, which indicates that this process is not thermally activated. Vt values have a linear dependence on the binding energy and this comparison leads us to attribute the mechanism to the local sublimation of surface atoms induced by tunneling electron energy (SITE). In contrast, the etching in air occurs at lower energy than in UHV. For example, C (graphite) is etched at 7.2 V in UHV, while in air, it decreases down to 2.3 V, as also shown in Fig. 2. Neither oxygen nor nitrogen atmosphere affects this process [9], and oxidation by adsorbed water should lower Vt in air. Hence, the mechanism is ascribed to chemical reaction induced by tunneling electrons (CRITE). The etching processes in both environments are shown below. in UHV; in air;,
C (graphite) + 7.43 eV -, C (gas) C (graphite) + H2 0 (liquid) + 1.82 eV -, CO (gas) + H2 (gas)
(1) (2)
It is thought that the Vt for C decreases from 7.2 V in UHV to 2.3 V in air, because reaction energy changes from sublimation (binding) energy of 7.43 eV in the process (1) to the oxidation energy of 1.82 eV in the process (2). Hence, when the binding energy is replaced by oxidation energy, the Vt in air can be plotted on the same line in Fig. 2 and the mechanism in air is also understood systematically. In order to verify this mechanism, two kinds of experiments were performed. Firstly we measured Vt's of gold and Bi 2 Sr 2 CaCu2 Ox, which are not oxidized by water in air. Their Vt's in UHV are expected to correspond to the Vt's in air, since oxidation does not occur even in air. The process was as follows: in UHV and air-, Au (solid) + 3.79 eV -b Au (gas) 192
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