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Atomic force microscopy (AFM) can compare a molecule in different charge states and, in a separate high-speed mode, record topographic movies of biomolecules in motion. In charge-state experiments, a sharp tip transfers electrons to or from a molecule adsorbed on a surface. The resulting change in electrostatic force shifts the cantilever’s resonance response, allowing researchers to identify charging events and image the molecule under controlled charge conditions.
That is not the same as photographing a free molecule or directly recording its electron cloud. AFM measures tip–sample forces. Structural conclusions come from calibrated force or frequency-shift contrast, chemical knowledge and often complementary measurements such as scanning tunnelling microscopy (STM) or Kelvin-probe force microscopy.
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What “AFM captures changing structures” actually means
The phrase describes two related but distinct experiments:
- Charge-state-resolved imaging: a surface-adsorbed molecule is deliberately switched between charge states, then scanned under each condition to compare its apparent structure and properties.
- High-speed AFM (HS-AFM): an instrument repeatedly scans a biomolecule on a surface, producing a time sequence of topographic images that can show conformational changes, binding, stepping or other motion.
Both use the AFM family of instruments, but they answer different questions. Charge-state work asks how adding or removing an electron changes a molecule’s geometry, bonding or reactivity. HS-AFM asks how a biomolecule’s surface shape changes over time. A movie from HS-AFM is not a direct movie of electron redistribution, and a charge-state image is not automatically evidence that a molecule is moving in real time.
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How an AFM detects a molecule
An AFM uses a flexible cantilever with a nanosharp tip. The tip is brought close to a sample while the cantilever is driven or deflected. Attractive and repulsive interactions between tip and sample alter the cantilever’s deflection, oscillation amplitude, phase or resonance frequency. A feedback loop adjusts the vertical position so the tip follows a selected interaction signal as it scans laterally.
In frequency-modulation AFM, the important observable is often the change in resonance frequency, written as a frequency shift. The shift depends on the gradient of the tip–sample force, so variations in chemical bonding, electrostatic fields and local height can all contribute to contrast. A functionalized tip can make that contrast chemically informative: a carbon-monoxide (CO) molecule attached to the apex is used in molecular-resolution studies because its flexible end responds sensitively to the contours and chemical structure beneath it.
The image is therefore a map of a measured interaction under defined conditions. Apparent bright and dark features do not have a single universal meaning; they must be interpreted with the tip, bias, oscillation amplitude, sample preparation and theoretical model in mind.
How charge-state AFM experiments work
1. Place a single molecule on a controlled surface
Researchers first deposit individual molecules on a conducting substrate, usually with an insulating film between the molecule and the metal. The insulating layer is important because it suppresses uncontrolled electron tunnelling between the adsorbed molecule and the supporting metal. It gives the experiment a better-defined starting state while still permitting the microscope tip to interact electrically with the molecule.
This is a specialized, often low-temperature scanning-probe experiment. The molecule must remain sufficiently stable on the surface, the film must be uniform enough to control coupling, and the tip must approach without displacing or chemically damaging the molecule.
2. Use the tip bias to transfer an electron
A voltage (tip bias) is applied between the tip and the sample. At an appropriate bias, an electron can move between the tip and the molecule. Depending on the polarity and energy alignment, the molecule may be reduced by gaining an electron or oxidized by losing one. The electric field also changes the electrostatic interaction felt by the tip.
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Researchers can sweep the bias while monitoring the cantilever response. A discrete step in the frequency-shift signal is a described signature of a charging event: the molecule has moved from one charge state to another, changing the force gradient at the tip.
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Once a transition is located, the experiment can be operated at a bias where the desired charge state is stable. The tip then scans the molecule to acquire an image. Repeating the scan at another controlled charge state provides a like-for-like comparison, provided the tip condition and scan parameters are kept sufficiently consistent.
4. Compare structural and electronic contrast
With a CO-functionalized tip, AFM can resolve structural features of individual molecules at very high spatial detail. Researchers compare bond-like features, atom positions, apparent height and other contrast changes between charge states. The same transition can also alter electronic and chemical properties, including charge transfer, bond rearrangements and the preferred reaction pathway.
A contrast difference is evidence that the tip–molecule interaction changed. It supports a structural interpretation only when other causes—such as a changed tip, a different adsorption geometry, drift or a varying electrostatic background—have been excluded or modelled. AFM does not display “positive” or “negative” charge as a camera would.
What changes when a molecule gains or loses charge?
Electrons occupy molecular orbitals that also influence bonding. Adding or removing an electron can therefore change bond orders, bond lengths, angles and the balance between competing conformations. In a reactive molecule, the new charge distribution can make one site more susceptible to attack or facilitate a bond rearrangement. In a larger molecule, a charge change may alter how it bends, twists or couples to a neighbouring molecule.
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- Structural: geometry, conformation, bond lengths and adsorption orientation.
- Electronic: orbital energies, local electrostatic potential and charge-transfer behaviour.
- Chemical: reaction barriers, preferred pathways and the stability of intermediates or products.
AFM can map force-related consequences of these changes. To assign an electronic mechanism, researchers often combine AFM with spectroscopy, STM or Kelvin-probe force microscopy, which provide complementary information about electronic states or local potential.
How to read a charging signal without over-interpreting it
A bias sweep that produces a frequency-shift step is a useful event marker, not a complete molecular explanation. The step indicates that the tip–sample interaction changed abruptly at a particular electrical condition. The working model may be electron transfer to or from the molecule, but the assignment should be checked against the polarity, repeatability, tip position and expected energy levels.
Several observations strengthen a charge-state interpretation:
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minute- The step occurs repeatedly near the same bias when the tip is positioned over the molecule.
- The transition shifts or disappears when the tip is moved away from the molecule.
- Images acquired on opposite sides of the transition show reproducible contrast differences.
- The observed geometry agrees with calculations or with independent STM or Kelvin-probe measurements.
- The molecule remains in the same adsorption site instead of being displaced during the voltage sweep.
None of these checks turns AFM into a direct charge camera. They make the inferred charge-state model more credible by connecting the electrical transition, force response and molecular structure.
Why a CO-functionalized tip matters
At ordinary imaging scales, the AFM tip’s own shape can blur a molecule. Attaching a CO molecule to the tip apex creates a well-defined, chemically inert end that can bend slightly as it passes over different parts of the sample. This improves lateral resolution and makes intramolecular features easier to distinguish.
Tip functionalization is also a source of risk. A CO tip can be picked up, moved or reconfigured during a scan. A changed apex alters the image even if the sample has not changed. Researchers therefore inspect tip performance on reference features, monitor scan stability and repeat images before assigning a new contrast pattern to a molecular charge transition.
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AFM charge-state imaging versus high-speed AFM movies
| Reader’s question | Charge-state-resolved AFM | High-speed AFM |
|---|---|---|
| What is varied? | Electron number on a molecule, controlled through tip bias | Time; the instrument scans repeatedly to follow a biomolecule |
| Typical sample | An individual molecule adsorbed on an insulating film over a metal | An individual biomolecule attached or adsorbed on a surface |
| Main output | Images and frequency-shift changes associated with different charge states | A sequence of surface-topography frames |
| Primary question | How does charge affect geometry, interactions or reactivity? | How does the molecule’s visible surface shape change over time? |
| Key controls | Bias, tip functionalization, insulating layer, temperature and scan conditions | Scan rate, feedback, tip force, sample attachment and environmental stability |
| What it does not directly provide | A photograph of electron density or proof that every contrast change is structural | A direct movie of internal electronic motion or every atomic rearrangement |
The 2024 Annual Review account of HS-AFM emphasizes that the technique acquires topographical information. Its movies can show a biomolecule’s changing outline or height distribution, but internal events that do not alter the surface topography may remain invisible.
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What high-speed AFM can show
HS-AFM reduces the time between successive scans enough to follow dynamic biomolecular behaviour. Depending on the sample and setup, a sequence may reveal a protein changing conformation, a motor protein stepping, a filament assembling or disassembling, or a complex binding and releasing a partner. Each frame is still a force-based topographic measurement, and the probe must be gentle enough not to arrest or reshape the motion it is meant to observe.
Interpreting an HS-AFM movie requires separating three effects:
- Real molecular motion: a reproducible change that persists across frames and trajectories.
- Tip or feedback artefacts: streaks, duplicated features, sudden height jumps or lag caused by an aggressive setpoint or scan rate.
- Surface and attachment effects: a molecule may move differently when tethered to a substrate than it would in solution.
HS-AFM and charge-state imaging can be complementary in a broad research program, but they are not interchangeable modes. One prioritizes temporal topography; the other prioritizes controlled electrical state and molecular-scale structural comparison.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical workflow for a charge-state measurement
The exact instrument interface differs by laboratory, but the experimental logic follows a consistent sequence:
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- Stabilize the microscope: allow temperature, vibration isolation, laser alignment and drift to settle. Charge-state measurements are sensitive to small changes in tip height and electrical noise.
- Approach and locate: bring the prepared tip toward the surface using a conservative setpoint, locate an isolated molecule and record a reference scan.
- Verify the tip: check that the CO-functionalized apex produces the expected resolution and has not picked up contamination.
- Sweep bias locally: position the tip over the molecule, vary the bias and record the frequency-shift response. Mark reproducible steps as candidate charging transitions.
- Image below and above the transition: acquire scans at stable biases that correspond to the selected charge states, keeping other settings as comparable as practical.
- Repeat and cross-check: return to the reference condition, test another molecule or tip position, and compare with STM, Kelvin-probe force microscopy or calculations when available.
If the molecule moves, the tip changes, or the frequency shift drifts during this sequence, the images cannot be treated as a clean charge-state comparison. The appropriate response is to re-establish a stable tip and sample rather than forcing an interpretation.
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Common failure modes and how researchers diagnose them
“The image changed, so the molecule changed shape.”
Not necessarily. A changed tip apex, altered tip height, electrostatic background or adsorption site can produce new contrast. Repeating the scan with the same tip, checking a reference feature and correlating the change with a reproducible charging step helps distinguish a structural transition from an imaging artefact.
“The bias sweep shows no clear step.”
The molecule may be too strongly coupled to the substrate, the insulating film may be unsuitable, the bias range may not reach the relevant transition, or electrical noise may mask a small signal. A different molecule, tip position or sample preparation can be more informative than simply increasing the voltage, which risks molecular motion or damage.
“The molecule disappears during scanning.”
High lateral forces, an unstable adsorption site or an accidental tip–molecule contact can displace it. Lowering the interaction setpoint, reducing scan speed, using a more stable adsorbate and approaching from a clean area are typical corrective measures. The lost molecule should not be used for a before-and-after structural claim.
“The HS-AFM movie contains streaks or jumps.”
Those features often indicate feedback lag, excessive tip force, mechanical vibration or a scan rate that is too high for the chosen control settings. Compare multiple trajectories and adjust the feedback and force before assigning the feature to biomolecular dynamics.
What AFM can and cannot establish
Strong conclusions when controls agree
- A discrete change in the force-related signal can mark a molecular charge transition under the tested bias and surface conditions.
- Reproducible, charge-dependent intramolecular contrast can support a model in which geometry or bonding differs between charge states.
- HS-AFM can reveal time-dependent changes in the topography of an individual surface-bound biomolecule.
Claims that require additional evidence
- The exact number or location of electrons cannot be read directly from an AFM image alone.
- A contrast change does not uniquely identify a bond rearrangement without controls or modelling.
- A surface-bound molecule’s behaviour should not automatically be generalized to a free molecule in solution or to other substrates.
- An HS-AFM frame cannot show electronic properties or internal atomic motion that do not affect the measured surface topography.
The 2017 Nature Reviews Chemistry review by Niko Pavliček and Leo Gross summarized the broader promise of functionalized-tip AFM: “The molecular structures of reactants, intermediates and products are revealed at unprecedented resolution by using atomic force microscopy (AFM) and a suitably functionalized tip.” The qualification is essential: the resolution and interpretation depend on the functionalized tip, sample preparation, operating conditions and the model used to connect force contrast with molecular structure.
Why these experiments matter
Molecular charge is not merely a label. It can determine whether a bond forms or breaks, which reaction pathway is accessible and how a molecule interacts with its environment. Conventional ensemble measurements average over many molecules and charge states. A controlled AFM experiment can instead connect an electrical transition to the structure of one adsorbed molecule.
HS-AFM addresses a different limitation: many biomolecular mechanisms are transient and heterogeneous. Recording successive topographic frames lets researchers separate distinct pathways and relate a conformational change to a visible event, while retaining the caveat that the microscope measures the molecule’s surface shape.
Together, these approaches show why “seeing a molecule” is not one operation. AFM is a force-sensing platform whose tip, bias, surface, timing and feedback determine which aspect of molecular behaviour becomes measurable.
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