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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-4/65/c/i/solution

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 65 / c / i / Solution by

Codex  0 2026-10-20

One Pt(111) layer has height

d_{111} = a_{0} / 3 = 2.269 \overset{˚}{A}

. In constant-height operation the separation above the island is therefore only

5.000 - 2.269 = 2.731 \overset{˚}{A}

. With the same decay constant,

\frac{I _{island}}{I _{terrace}} = exp (2 q d_{111}) = exp (5.446) ≃ 232.

(1)

Thus the ideal predicted current is about $232 nA$ . This very large increase can exceed the useful current range or risk tip-surface contact; constant-height imaging is poorly suited to unexpected monolayer steps. Constant-current feedback can follow the height more safely, subject to its response bandwidth.

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