原子力显微镜(AFM)原理

Contact mode is the most basic mode of Atomic Force Microscopy for measuring topography. In this mode, the cantilever scans while applying a constant force onto the surface of the sample. As the tip approaches close to the sample, the tip eventually contacts the surface. Once engaged, the cantilever bends upwards proportional to the amount of applied imaging force. As the tip passes over higher sample features, the cantilever will bend and deflect. The deflected laser spot on the position sensitive photo detector will move due to the change in contact force. The feedback loop responds by moving the Z scanner to restore the initial cantilever deflection to keep the applied imaging force constant. By tracking the displacement of the Z scanner, you can ultimately determine the surface topography of the sample.

In this technique, the cantilever oscillates just above the surface as it scans. A precise, high-speed feedback loop prevents the cantilever tip from crashing into the surface, keeping the tip sharp and leaving the surface untouched. As the tip approaches the sample surface, the oscillation amplitude of the cantilever decreases. By using the feedback loop to correct for these amplitude deviations, one can generate an image of the surface topography.

In this alternative technique to non-contact mode, the cantilever again oscillates just above the surface, but at a much higher amplitude of oscillation. The bigger oscillation makes the deflection signal large enough for the control circuit, and hence an easier control for topography feedback. It produces modest AFM results but blunts the tip’s sharpness at a higher rate, ultimately speeding up the loss of its imaging resolution.

EFM-DC utilizes a lock-in amplifier to study the electrical properties and topography of a sample surface in a single scan, unlike the two scans required by standard EFM. Here, the cantilever is biased with an AC current different than the resonance of the cantilever. The oscillation component of the PSPD signal is extracted by the lock-in amplifier, resulting in the EFM signal.

Electrostatic Force Microscopy (EFM) probes ferroelectric regions of a sample surface with a conductive cantilever. An EFM image is the result of two separate scans: one scan probes the topography, while in the other the cantilever is raised away from the surface to the region where long-range, electrostatic force begins to dominate. In this electrostatic domain, the attractive and repulsive deflections of the cantilever correspond to regions of positive and negative charge on a sample surface. EFM gives users an image that couples topography with the electrical properties of a nanoscale region.

The conductivity of the sample can be measured while scanning in contact mode by using a current amplifier to measure current flow between a conductive tip and an electrically-biased sample. Regions of high conductivity on the sample surface allow current to pass through easily, while regions of low conductivity will have a lower current due to local changes in resistance. In addition, detailed electrical properties of the surface at a specific point can be measured, which is called IV Spectroscopy. A plot of the measured tip current (I) as a function of the applied bias voltage (V) reveals current-voltage behavior at individual contact points of a sample. Conductive AFM yields both the topography and the current map of a sample surface simultaneously.

After bringing the cantilever into contact with the sample, a voltage bias is applied between the two. This current is measured while the bias changes, giving insight into the detailed electrical characteristics of the surface at that specific point. A collection of these data points can be used to generate an image of the electrical properties of the surface.

Scanning Capacitance Microscopy (SCM) is used to characterize a sample surface by recording local changes in capacitance between the surface and a metal probe. The tip-sample capacitance can be probed by modulating carriers with a bias containing AC and DC components. An amplifier is used to measure the capacitance sensor output with a high signal-to-noise ratio. The magnitude of the SCM output (dC/dV) signal is a function of carrier density or dopant concentration.

In Kelvin Probe Force Microscopy (KPFM), the AFM operates in non-contact mode while a conductive cantilever, oscillated at its fundamental resonant frequency, laterally scans over the sample surface. The resulting electrostatic signal provides information related to surface potential and the capacitance gradient. The topographic data is taken by controlling the force between the tip and the sample.

SSRM is a mode mainly used to study the electrical properties of semiconductor cross-sections, which can be measured in a vacuum to prevent oxidation after removing the oxide film on the sample. The conductive cantilever is brought into contact with the sample, and a voltage is applied between the cantilever and the sample. The cantilever strongly presses the sample surface, penetrate the oxide layer on the surface, then makes ohmic contacts. It comprehensively measures electrical properties such as current and resistance, as well as topography. The part with a high resistance does not allow current to flow easily, and the part with a low resistance allows for easy current flow. These electrical characteristics are observed through current and resistance images.

Scanning Ion Conductance Microscopy (SICM) utilizes a pipette with a nanometer-scale aperture. The ion current between the inside and the outside of the pipette in aqueous solution changes depending on the distance from the nanopipette tip to the surface. The topography is acquired by using a feedback controller to hold the ion current constant.

A force-distance curve is acquired by bringing the cantilever tip into contact with the sample surface. The shapes of specific regions of force-distance curves offer insight into different mechanical properties, such as adhesion, Young’s modulus, etc.

As much as EFM couples a topography scan with a separate scan for electrical properties, Magnetic Force Microscopy (MFM) combines a topography scan with a separate scan for magnetic properties. MFM features a contact AFM scan to obtain the topography, and a scan farther from the surface to probe long-range magnetic force. In this magnetic force domain, deflections of the magnetized cantilever correspond to regions of magnetization on the sample surface.

In Force Modulation Microscopy (FMM), the cantilever is oscillated while it is scanned across the sample surface. The oscillation amplitude of the cantilever changes depending on the local hardness of the surface, which is reflected in the PSPD signal. The amplitude change of the PSPD signal, which is used to calculate surface hardness, is extracted by an internal lock-in amplifier.

While more traditional AFM techniques focus on vertical deflections of the cantilever to image the surface topography, lateral force microscopy (LFM) instead focuses on torsional deflections as the cantilever scans across the surface. The amount the cantilever twists as the tip is dragged across a sample surface provides useful insight into the frictional force and adhesion properties of the sample.

Nanoindentation inspects the local hardness of a sample material by indenting an AFM tip into the sample surface and performing in-situ imaging of the indented surface. Hardness and elasticity are acquired by analyzing the loading and unloading curves of the indentation.

Here, the cantilever is used to intentionally modify the sample surface via mechanical and/or electrical means. To mechanically alter a surface, a specialized, robust cantilever gouges the surface with excessive force. To electrically alter a surface, a cantilever with a high bias is used to oxidize local surface regions.

In Near-Field Scanning Optical Microscopy (NSOM), a topography scan is performed with a specialized cantilever featuring a nanoscale aperture. During the scan, a laser is directed through the aperture to excite the sample surface, while a photon counter is used to detect the optical response of each excited region. The resulting image combines the topography of the surface with an optical image.

Like NSOM measurement, TERS measures the optical signal from a sample surface with an excitation laser. Here, the laser comes in from the side, taking advantage of tip enhancement effect. Moreover, a spectrophotometer is used instead of a photon counter, allowing users to acquire local Raman spectra to probe the local chemical composition of their sample surface.

In order to measure the thermal properties of a sample surface, a contact AFM scan is performed using a cantilever with temperature-dependent resistivity. Any changes in the tip resistance during the scan are recorded and correlated into a thermal image of the sample surface.

Electrochemical AFM (EC-AFM) is an optional mode that monitors changes at the sample surface due to oxidation-reduction (redox) reactions. A potentiostat is combined with the AFM to control the voltage potentials and measure current between a working electrode (sample), a reference electrode, and a counter electrode immersed in a liquid electrolyte. Cyclic voltammetry is an EC technique to study redox behavior by sweeping the working electrode potential relative to the reference electrode and measuring the current response in the electrochemical cell. A cyclic voltammogram CV shows the cathodic and anodic peak currents during the forward and backward sweeps between switching potentials. During CV sweeps, electron transfer processes occur at the working electrode and analyte molecules can undergo oxidation and reduction at the electrode surface. The AFM can image the surface morphology of the working electrode at fixed electrode potentials along a CV curve or continuously monitor changes in the surface morphology in real-time during multiple CV cycles. EC-AFM can provide information into many electrochemical processes like deposition, corrosion, and electron transfer mechanisms, as well as provide insight into material design for sensors, catalysts, and battery/energy cell applications.