AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Principle of operation of the scanning tunnelling microscope (STM).
Full specification wording
Electron microscopes (A-level only): Principle of operation of the scanning tunnelling microscope (STM).
An STM uses a very sharp metal tip brought extremely close to a conducting surface, typically within about a nanometre.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An STM uses a very sharp metal tip brought extremely close to a conducting surface, typically within about a nanometre. A small potential difference is applied between tip and surface. Electrons can tunnel across the gap, producing a tunnelling current that depends very strongly on the gap width. As the tip is scanned across the surface, a feedback system adjusts its height to keep the tunnelling current constant, or the current is measured at constant height. The recorded height or current variations build up a map of the surface at atomic resolution. The surface must be electrically conducting or semiconducting, and the instrument is often operated in vacuum or controlled atmosphere to keep the tip and surface clean.
What earns the marks
- A sharp conducting tip is brought very close to a conducting or semiconducting surface without touching it.
- A small potential difference is applied between tip and surface so that electrons can tunnel across the gap.
- The tunnelling current depends very strongly on the width of the gap, so tiny changes in distance produce measurable current changes.
- A feedback mechanism adjusts the tip height to keep the tunnelling current constant as the tip scans the surface, or the current is recorded at constant height.
- The resulting height or current data are processed to produce an atomic-resolution image of the surface.
- The sample must be electrically conducting or semiconducting, and clean conditions are needed for reliable operation.
Common mistakes and how to put them right
Thinking the tip touches the surface.
Put it right: Electrons tunnel across a small gap without contact.
Believing the STM works by electron transmission through the sample.
Put it right: It detects a tunnelling current across a vacuum gap between tip and surface.
Assuming any material can be imaged.
Put it right: The surface must conduct or semiconduct, so insulators are unsuitable.
Exam tips
- State clearly that the tunnelling current changes very rapidly with gap distance, which gives the technique its sensitivity.
- Describe the feedback loop that maintains constant current and explain how this generates the image.
- Mention the need for a conducting surface and clean conditions when explaining limitations.
Try the lesson
The start of the Electron microscopes (A-level only): Principle of operation of the scanning tunnelling microscope (STM). lesson, made from checked teaching notes for this specification point.
Have a go first
In a scanning tunnelling microscope, what is detected as the tip is moved across a conducting surface?
Listen
An STM uses a very sharp metal tip brought extremely close to a conducting surface, typically within about a nanometre. A small potential difference is applied between tip and surface. Electrons can tunnel across the gap, producing a tunnelling current that depends very strongly on the gap width. As the tip is scanned across the surface, a feedback system adjusts its height to keep the tunnelling current constant, or the current is measured at constant height. The recorded height or current variations build up a map of the surface at atomic resolution. The surface must be electrically conducting or semiconducting, and the instrument is often operated in vacuum or controlled atmosphere to keep the tip and surface clean. A strong answer earns marks for these points. A sharp conducting tip is brought very close to a conducting or semiconducting surface without touching it. A small potential difference is applied between tip and surface so that electrons can tunnel across the gap. The tunnelling current depends very strongly on the width of the gap, so tiny changes in distance produce measurable current changes. A feedback mechanism adjusts the tip height to keep the tunnelling current constant as the tip scans the surface, or the current is recorded at constant height. A common slip: Thinking the tip touches the surface. Instead, electrons tunnel across a small gap without contact. Exam tip: State clearly that the tunnelling current changes very rapidly with gap distance, which gives the technique its sensitivity.
Key words
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Quantum tunnelling
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Atomic-resolution surface imaging
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Feedback control systems
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Electrical conductivity of samples
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See every definition
- Quantum tunnelling
- Electrons can cross the gap between a very sharp metal tip and a conducting surface when a small potential difference is applied.
- Atomic-resolution surface imaging
- Recorded height or current variations build up a map of the surface at atomic resolution.
- Feedback control systems
- A feedback system adjusts the tip height to keep the tunnelling current constant as the tip is scanned across the surface.
- Electrical conductivity of samples
- The surface must be electrically conducting or semiconducting, so insulators are unsuitable.
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