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Scanning Tunneling Microscope

Key idea: See how a scanning tunnelling microscope (STM) uses electron tunnelling current to map conducting surfaces with near-atomic resolution (optional enrichment for A Level Physics).

  • A-level H2 Physics topic extensions
On this page

Learning objectives

  • Explore potential-barrier transmission, tunnelling and X-ray production and spectra beyond the H2 syllabus.
Optional / Enrichment (9478 scope)

STM is an application of tunnelling and is not required by the 9478 quantum learning outcomes. Treat this page as enrichment that connects the tunnelling idea to a real device.

1. Definitions (Must Know)

A. Scanning tunnelling microscope (STM)

A scanning tunnelling microscope (STM) is a non-optical microscope that uses electron tunnelling between a sharp metallic tip and a conducting surface to map the surface at near-atomic scale.

B. Tunnelling current

The tunnelling current is the small current that flows because electrons can tunnel across the tiny gap between tip and surface.

C. Feedback control (constant-current mode)

In constant-current mode, a feedback circuit adjusts the tip height to keep the tunnelling current roughly constant while scanning. The recorded tip position produces a map influenced by both surface height and the availability of local electronic states; it is not a literal picture of hard atomic spheres.

2. Key Ideas (What Earns Marks)

  • STM relies on tunnelling: electrons cross a classically forbidden gap.
  • The tunnelling current depends extremely strongly on the tip–sample separation, so very small height changes produce measurable current changes (high resolution).
  • STM requires a conducting path (tip and sample need to conduct).

3. Detailed Explanations

A conducting tip scans sideways above a conducting sample. A bias voltage produces tunnelling current across the tiny gap. An amplifier and feedback controller move the tip vertically to maintain the set current, and the recorded tip height forms a map influenced by surface height and local electronic states.
In constant-current mode, feedback converts current changes into vertical tip corrections. The resulting contrast contains topographic and electronic information.

A. Why STM can “see” atoms

Quantum tunnelling probability falls very rapidly as the gap increases. That means the current changes a lot when the tip moves by a tiny amount, allowing very fine height sensitivity.

B. A typical constant-current workflow

  1. Set a bias voltage between tip and sample.
  2. Bring the tip close enough for a measurable tunnelling current.
  3. Scan sideways while a feedback loop adjusts the tip height to keep current constant.
  4. Plot the recorded tip height against lateral position. Interpret this constant-current contrast as a combination of surface topography and local electronic structure.

C. Limitations

STM needs a conducting surface. Insulating contamination layers (e.g. oxides) can suppress tunnelling current and reduce image quality.

4. Common Mistakes

  • Thinking STM uses light (it is not an optical microscope).
  • Saying electrons “jump because they have enough energy” (the point is tunnelling when energy is insufficient).
  • Forgetting the need for conductivity (no reliable tunnelling current if the surface is insulating).

5. Exam Tips

  • If asked “why is resolution high?”, answer with “tunnelling current depends extremely strongly on tip–sample separation.”
  • If asked for limitations, lead with “requires conducting surface / clean surface”.
  • Use the key link back to quantum content: the signal exists because tunnelling probability is non-zero.

6. Worked Examples

Modelled example 1

Trend question

Core

Problem

In an STM, tip–sample separation increases slightly while bias voltage stays fixed. What happens to tunnelling current, and why?
Study the worked solution

  1. Map the gap to a barrier

    Method

    The vacuum gap acts as the tunnelling-barrier width.

    Reason

    Electrons cross a classically forbidden separation.

    Working

    L≡ d_gap

  2. Apply the decay trend

    Method

    A larger gap strongly reduces transmission probability.

    Reason

    The wavefunction decays farther before reaching the sample.

    Working

    T ∝ e^(-2κ d)

  3. Map probability to current

    Method

    Tunnelling current decreases strongly, often approximately exponentially.

    Reason

    Fewer electrons tunnel per unit time at fixed bias.

    Working

    d↑ ⇒ Iₜᵤₙₙₑₗ↓

Common misconception 2

Why feedback is needed

Find and correct the mistake

Learner claim

A learner says constant-current feedback keeps the tip at a fixed height and the recorded output is a literal map of hard atomic spheres. Diagnose both parts.

Try this before viewing the solution

What the controller holds approximately constant

View solution step by step

  1. State the controlled quantity

    Method

    The feedback holds tunnelling current approximately constant.

    Reason

    Current changes sharply with gap size.

    Working

    Iₜᵤₙₙₑₗ ≈ Iₛₑₜ

  2. State the actuator

    Method

    The controller adjusts tip height while scanning sideways.

    Reason

    Moving vertically restores the target separation-sensitive current.

    Working

    zₜᵢₚ varies

  3. Interpret the map

    Method

    Recorded height reflects both topography and local electronic states.

    Reason

    Tunnelling current depends on electronic-state availability as well as separation.

    Working

    contrast = topographic + electronic influences

7. Mind Stretchers

Mind stretcher 1: Constant-current vs constant-height modeExtension

Which mode is safer for rough surfaces: constant-current or constant-height?

Show Answer

Constant-current is safer for rough surfaces because the tip height adjusts to avoid crashing into the sample. Constant-height can be faster but risks collision if the surface height varies too much.

8. Optional (Enrichment)

A. Historical note

Scanning tunnelling microscopy became a major surface-analysis technique in the 1980s and was recognised with the 1986 Nobel Prize in Physics.

Continue with the next resource in this course.

Course and syllabus information
Course
A-level H2 Physics topic extensions
Syllabus scope
Beyond the syllabus
Edition
A-level H2 Physics topic extensions