Scanning probe microscopy includes the techniques of Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM). The facilities in the School of Materials also includes a nanoscale infrared spectrometer which incorporates an IR laser with contact-mode AFM.
In AFM, sharpened tips are scanned across a surface to measure topography with picometre height resolution and typically nanometre lateral resolution (depending on the scan size). The technique became possible with the advent of semiconductor fabrication techniques which enable cantilevers to be made with low spring constants so that the surface and probe suffer only minimal damage. Nowadays a variety of operational modes are combined with topology measurements, including magnetic and electrical measurements.
The Multimode8 is a versatile scanning probe microscope capable of both STM and AFM. The microscope is capable of operating in air, liquids and under electrochemical and some environmental control.
The AFM may be operated in tapping mode, where a large amplitude oscillation is applied to the tip as it moves above the surface, and changes in topography alter the amplitude of oscillation. This mode results in less damage to tip and surface than contact mode, and also has the advantage that surface debris is less likely to damage the tip. Measuring changes in the frequency of oscillation, known as phase imaging, may reveal mechanical properties of the surface.
The microscope may be operated using Bruker's patented ScanAsyst mode (where AFM feedback loop paraemeters are automatically optimized), Magnetic AFM, PeakForce tapping, Quantum NanoMechanical AFM (QNM), Kelvin Probe Microscopy (SKPM) for measuring surface potential, and the instrument has the PeakForce TUNA (electrical properties) add-on module.
Operating the AFM in contact mode enables use of lateral force and force modulation mode. In lateral force mode the surface is scanned perpendicular to the cantilever so that changes in frictional forces cause a twisting of the cantilever. In force modulation mode a small oscillation is applied to the tip so that the mechanical properties of the surface can be measured. This is best performed in liquid to prevent the surface tension of adsorbed water layers masking changes due to the mechanical proerties of the surface.
The NanoIR2 (Anasys Instruments) enables nanoscale infrared spectroscopy with contact-mode atomic force microscopy. This offers an unique opportunity to map chemical state information with the spatial precision provided by AFM. A pulsed infrared laser (an optical parametric oscillator) providing tunable infared light from 900 to 3000 cm-1 is shone onto the surface, causing a temporal oscillation measured by the AFM cantilever (a ringdown), and after Fourier transforming, the magnitude of this can be realted to the infrared absorption. The instrument may measure IR spectra at a location on the surface, or AFM maps with associated infared absorption at a specified wavelength.
The instrument also includes a nano thermal analysis (nanoTA) function which allows the sample temperature to be ramped locally through an AFM probe in order to measure and map thermal transitions and other thermal properties.
The Dimension 3100 is an atomic force microscope capable of operating in contact mode (including lateral force mode), and tapping mode, including phase imaging and electrical modes including Kelvin Probe microscopy. The microscope can accomodate large samples and enables fast and easy movement across the surface, which is important for non-homogeneous samples or samples prepared using drop-casting or spin-coating on substrates, where identification of a suitable measurement area can sometimes be challenging.
This AFM is generally faster and easier to operate that the Multimode8, because tip engagement is automated after the user finds the focal plane of the tip and the surface using the inbuilt camera (with x10 objective). However, it does not have as high topology sensitivity as the Multimode8.
The Contour-GT White Light Interferometer is an optical microscope that also measures topology. We have 2.5X, 10X, 50X and 115X objectives, the latter of which allows us to somewhat approach the lateral resolution offered by AFM (when combined with a secondary internal x2 focussing lens). The instrument is fast and easy to use, and allows for the measurement of surface roughness, volume analysis, and many other metrology parameters. This instrument is used for a variety of different applications, including measurement of substrate thicknesses, crater heights, but also for quick measurements of samples that haven't been measured with AFM before, to ensure there are no large (>> a few microns) features on the surface which wouldn't be measurable with AFM (and indeed which would lead to AFM probes being damaged).