Proximal/Local Probe Microscopes

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Proximal/Local Probe Microscopes Description

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With proximal/local probes the interaction of a stylus probe (aka needle, tip, indenter) and sample surface enables the vertical location (measurement of height) of a surface while simultaneously probing other material properties. Thus under lateral scanning these instruments can produce images or maps of heights (surface topography) as well as properties (“response maps”). There are different categories or constructs of proximal/local probes, including nanoindenters and scanning probe microscopes (SPM). The CharFac has several SPM’s including (i) an ultrahigh vacuum scanning tunneling microscope (UHV STM) wherein the tip-sample interaction is manifest as a tunneling current (requiring electrical conduction through the sample as a whole); and (ii) six atomic force microscopes (AFM), one of which includes infrared spectroscopy and imaging (AFM IR).  In all SPMs the probe or tip is of nanometer-scale sharpness (radii of curvature on the order of 10 nm in most cases), and the image resolution can approach the atomic or molecular scale in certain cases. UHV STM can produce true atomic resolution.

In AFM the tip is attached to a microfabricated cantilever of user-selected spring constant (~0.001 - 100 N/m). The appropriate spring constant for one’s application depends on the mode of operation, the environment and the sample characteristics. Vertical and lateral force is measured by reflecting a red low-power laser off the top face of the cantilever into a split photodiode array (2x2). (Flexural or torsional bending of the cantilever produces changes in the vertically or laterally split photodiode output signals as the laser spot incrementally displaces.) Forces can be gauged under tip-sample contact or out of contact (i.e., van der Waals or electrostatic/magnetic); thereby a wide variety of properties can be interrogated (mechanical, electro/magnetic with metal tips). Gaseous or liquid media environments (including in vitro/vivo), plus sample temperature, can be controlled (with two of our AFMs). For dynamic mechanical or rheological studies one can additionally vary rate, whether via the frequency of normal or lateral tip oscillations or via the shear velocity during scanning (the latter up to 7 decades of rate). Tip chemistry can be altered for controlled studies of probe-sample interaction (e.g., with CharFac plasma or UV/ozone cleaners, and/or via published methods for attaching organic monolayers of hydrophobic/hydrophilic terminus, or biomolecules; or simply by purchasing tips that are precoated as such). Alternatively one can work with colloidal microspheres attached to cantilevers to spread out the force to reduce contact pressure (e.g., on gels), or to provide a well-determined probe radius of curvature (to better quantify surface energy, elastic modulus, shear stiffness, etc., and with controlled chemistry probe: hydrophobic/hydrophilic or coated with polymer).

In AFM IR one additionally pulses a tunable infrared laser at the tip-sample interface; the detection of absorption (at different IR wave numbers) is confined to the tip-sample interaction zone and thus the spatial resolution can reach 10’s of nm (whereas a conventional IR microscope has ~10 micron resolution). One commonly performs single-point spectroscopy (i.e., at a specified location in an AFM image) or collects an image at a selected wave number. Hyperspectral mapping in 1D or 2D (i.e., a programmable 1D or 2D array of complete spectra) also can be accomplished, albeit requiring considerable time for 2D (hours).

With a nanoindenter (Bruker/Hysitron Triboindenter) the stylus is blunter and typically diamond. The force transducer can access a much higher range of forces such that hard materials can be plastically indented or scratched. The force transducer is also more specifically designed for purely vertical force control than an AFM cantilever/tip construct (which can produce parasitic lateral movement or lateral stress under flexural cantilever bending).

In summary one can access the following material properties: surface roughness, surface atomic structure, surface chemistry, storage/loss modulus, hardness, coefficient of friction, interfacial energy (tip/sample Hamaker constant), crystalline vs amorphous content, electric/magnetic polarizability, capacitance, surface charge, local work function (surface potential), glass transition/melting temperatures, thermal conductivity