Scanning Probe Microscopy

Introduction

Scanning probe microscopes map surfaces by moving a sharp tip across them. Atomic force microscopy, AFM , uses mechanical interactions and can measure topography on many insulating or conducting samples. Scanning tunneling microscopy, STM , relies on electron tunneling and can resolve electronic structure on suitable conducting surfaces. Both can reveal local variation hidden in an ensemble measurement, but neither gives a simple photograph independent of the tip or feedback settings.

Core explanation

An AFM cantilever carries a sharp tip. Forces between tip and sample deflect the cantilever or change its oscillation, depending on operating mode. Feedback adjusts the tip or sample height to maintain a chosen interaction signal. The resulting map is often interpreted as topography. For a rigid particle on a flat support, measured height can be robust, although adsorbed layers and compression of soft material alter it. Lateral width is more vulnerable to tip convolution : a finite-radius tip touches the sides of a small object before its apex passes directly over it, so the apparent object looks wider.

The magnitude of this lateral error depends on tip geometry and feature shape. A tip worn during scanning can change apparent widths within one experiment. Narrow trenches can appear too narrow or even inaccessible. Tip calibration against a known standard, use of sharper tips and comparison with another imaging method can reduce uncertainty. It is incorrect to assume that every apparent 20 nm wide object truly has a 20 nm lateral dimension.

AFM can also measure local mechanical, adhesive or electrical responses with specialized modes. A force–distance curve may probe how a surface deforms or how strongly the tip adheres. Conductive AFM maps current through a tip contact. These measurements depend on contact area, tip coating, applied load, humidity and substrate. A bright “stiffness” or “current” map is a model-dependent property map, not an independent composition map.

STM places a conductive tip very close to a conducting or semiconducting sample. Electrons tunnel across the tiny gap when a bias is applied. The current is highly sensitive to distance but also to the local electronic density of states. The feedback-controlled tip height needed to hold current constant mixes geometric height with electronic contrast. An atom that appears high in an STM image may not physically protrude by the full displayed amount; its electronic states may tunnel more effectively at the chosen bias.

Bias-dependent STM spectroscopy can probe local electronic features. It can help distinguish a defect state from clean surface states, but interpretation requires a suitable tip, stable surface and knowledge of tunneling conditions. An insulating nanoparticle on an insulating substrate is generally not a straightforward STM target, whereas AFM may image it. Metal nanoparticles on conductive supports can be accessible to both, though their interaction with the support may influence the signal.

Both methods are local. A scan may cover a small region and require a flat, stable specimen. Adsorbed water, contamination, thermal drift, vibration and tip changes can create artifacts. Repeated scanning at different directions, checking multiple spots and comparing with electron microscopy or spectroscopy increase confidence. For particle-size distributions, sample selection still matters just as in TEM.

AFM can image some samples in liquid, allowing measurement in a closer-to-use environment than a dried grid. Yet the probe may push weakly bound particles or deform a soft ligand layer. A measured height change after adding salt could reflect swelling, collapse or tip force rather than core growth. Control scan force and compare reversible conditions before making a structural claim.

Step-by-step reasoning

Define whether the question concerns geometry, local electrical states or mechanics. Choose AFM for height and force-based maps, or STM when tunneling into a suitable conductive surface is informative. Calibrate lateral and vertical scales and document tip, mode, setpoint and environment. Scan multiple fields and, if possible, reverse direction or change tips to test artifacts. Interpret AFM width with tip shape and STM apparent height with electronic contrast in mind.

Visual explanation

Draw a rounded AFM tip passing over a narrow nanoparticle: its center follows a broad path even though the particle is narrow, while its maximum height remains near the true top. Beside it draw an STM tip with a tunneling-current arrow and two surface sites of equal geometric height but different local electronic response. The feedback trace rises differently over the two sites.

Real-world analogy

Tracing a narrow groove with a thick pencil gives an outline determined partly by the pencil tip. A thinner pencil follows the groove more accurately. AFM lateral dimensions face a similar finite-tip problem. STM adds another layer: the “pencil” responds to electronic ease of tunneling as well as physical distance.

Real-world example

A lab deposits nanocrystals on a flat substrate and reports AFM heights around 5 nm but widths around 15 nm. TEM of the same batch shows approximately 6 nm core widths. Tip broadening can explain much of the AFM lateral discrepancy. The AFM heights still help identify single layers or stacked particles, provided ligands and substrate offsets are considered.

Why?

Scanning probe methods connect local structure to local behavior: height, adhesion, conductance and electronic states can be mapped at nanoscale positions. Their value lies in controlled interpretation of the probe interaction. Recognizing artifacts keeps a useful local measurement from becoming a false size or chemical assignment.

Common misconception

“AFM is a camera that records the exact outline of a nanoparticle” is false because a finite tip broadens lateral features. “STM height is purely physical height” is also false; tunneling current depends on electronic states and bias. Both modes require calibration and complementary evidence when geometry and electronics might be confused.

Worked example

A rigid nanoparticle deposited on a flat substrate has an AFM peak height of 8 nm and apparent width of 22 nm. TEM gives a projected diameter near 9 nm. The 8 nm height and 9 nm TEM diameter are compatible with a roughly equiaxed core, while the 22 nm AFM width is likely inflated by tip shape. Do not average 8, 9 and 22 into a single “particle size”; they are different observables with different errors.

Quick check

1. Which AFM dimension is generally more directly traceable for an isolated rigid particle on a flat substrate: height or lateral width? Answer: Height is generally less directly affected by finite tip radius than lateral width, though sample layers and deformation still matter.

Exam focus

State the measured signal in AFM and STM and the sample requirements of each. Explain tip convolution for lateral AFM size and local density-of-states contrast for STM. A strong answer identifies a suitable control, such as a calibrated tip or comparison with TEM.

Advanced insight

Dynamic AFM modes can distinguish some conservative and dissipative tip–sample interactions, but interpreting them quantitatively requires a force model and careful calibration. In STM, changing bias can change contrast or reveal states at different energies. A conductive AFM current map additionally depends on contact resistance, so a low-current pixel is not automatically a chemically insulating phase.

Summary

Scanning probe microscopy maps nanoscale surfaces through a tip interaction. AFM is versatile for topography and local forces; STM probes tunneling on conductive specimens. Tip shape, feedback, environment and electronic response affect the image. Report settings and use complementary methods before equating displayed feature width or height with a complete physical structure.

Practice questions

1. Why does an AFM tip often overestimate a narrow particle's lateral width? Answer: The finite tip contacts the feature's sides before its apex is centered, broadening the recorded outline. 2. Why can two atoms at the same geometric height have different STM contrast? Answer: Their local electronic states can support different tunneling currents at the applied bias. 3. Why might AFM be preferred over STM for an insulating nanoparticle on an insulating substrate? Answer: AFM senses force and does not require a continuous tunneling-current path through the sample. 4. Name two variables that should accompany a reported scanning-probe image. Answer: Tip type and setpoint are important; mode, bias, scan force and environment may also be needed.

Sources: NIST AFM measurement and tip-geometry guidance; NIST primary study of higher-order tip effects; Primary nanoscale tip-convolution study.