Description:

In scanning probe microscopy (SPM, commonly AFM), the interaction of a stylus probe and sample surface is quantified and mapped across the sample. The probe or "tip" is of nanometer-scale sharpness, and the standard image is 3D surface topography at resolution approaching the atomic or molecular scale. The tip is attached to a microfabricated cantilever of low spring constant. Property-sensitive imaging modes are performed simultaneous to topographic imaging. Gaseous or liquid media, plus sample temperature, can be controlled. Tip chemistry can be modified for controlled studies of probe-sample interaction.

Equipment:

Environmental Scanning Probe Microscope (SPM Station 4), Molecular Imaging PicoScan /PicoPlus with auxiliary PicoStat and Witec Digital Pulsed Force Mode electronics, closed-loop scanning and Pico IC vibration isolation chamber with video access, in Windows XP environment. Sample temperature can be varied from 20 to 200&degree; C; humidity can be varied from 3 to 90 % RH. Special imaging capabilities include friction force, vertical force modulation with phase measurement, AC ("tapping") mode / non-contact with (true) phase measurement, electrostatic/magnetic force, current sensing, flex grid (like force volume except fewer curves at custom XY locations), and volume spectroscopy (like force volume except more grid locations and interlaced with conventional imaging), and under controlled electrochemical environment. Both acoustic and magnetic (MAC) cantilever excitations are available for dynamic modes, magnetic being optimal for in-fluid imaging. Digital pulsed force mode enables high-resolution mapping of adhesion, stiffness and energy dissipation during break of tip-sample contact, and more because full approach-retract curves can be captured at each pixel location (GB data files; DVD writer). Maximum lateral scan size is 100 µm.

Accessories:

• Optical access. Ability to acquire digital video image.
• Liquid cells: open liquid cell including custom deep-well attachment for larger objects (developed for contact lens studies).
• BNC breakout box.
• Closed loop scanner.
• Mobile LabView workstation with Virtual Bench and 5 MHz data acquisition, and Windows NT environment. Can be attached via BNC interface for custom measurements in LabView. Virtual (computer-based) instruments include digital multimeter, oscilloscope, spectrum analyzer, function generator, and strip chart recorder. These capabilities can be used to determine, for example, the cantilever spring constant, or track high-frequency cantilever oscillations. A custom-developed program for analyzing cantilever oscillation following tip-sample separation, to assess energy dissipation in breaking contact, is available.
• Custom adder circuit box for signal manipulation, for example to ramp set point during imaging
• Digital pulsed force mode (adhesion and stiffness imaging plus full spectroscopic mode, i.e. GB data files).
• Infinitesima ActuResonance controller for manipulating the or quality factor (Q) of cantilever resonance.
• Mathematica and ICAdams available for quantitative analysis and modeling of force-distance data and dynamic tip-sample interactions, in order to extract sample storage/loss moduli, surface energy, etc.
• SPIP available for broad suite of image (e.g. grain size) and force curve (e.g. freely jointed chain model) analysis applications. Freeware programs WSXM and Gwyddion also available.
• XYZ manipulator to attach SiO2 or polystyrene microspheres to tipless cantilevers
• Plasma chamber and metal evaporator for chemical modification of tips by users. Tip modification services available, using silane linkage procedures customized by staff member Dr. Jinping Dong.

Applications:

• Sensitive to the following properties: surface chemistry, storage/loss modulus, hardness, interfacial energy, crystallinity, polarization, magnetization, surface charge, and local work function (surface potential).
• Applicable to conductive and non-conductive samples.
• Samples can be imaged in air or in liquid media in all modes of operation.
• Can measure tribological response versus load, scan velocity, temperature, relative humidity.

Capabilities:

• No sample pre-treatment needed and imaging can be performed in air.
• Sample can be conductive or nonconductive, hard or soft.
• Maximum scan size 150x150 µm laterally and 8 µm vertically
• Positioning resolution is 0.1 nm laterally and 0.01 nm vertically; imaging lateral resolution depends on sample/tip characteristics (adhesive contact mechanics) and typically is of order 1-10 nm.