AFM Overview

Description:

In scanning probe microscopy (SPM), commonly termed atomic force microscopy (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 a 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.

The Characterization Facility has six scanning probe microscopes:

• Bruker Nanoscope V Multimode 8 (SPM1) with peakforce QNM and Intermodulation AFM
• Bruker Nanoscope V Multimode 8 (SPM2) with peakforce QNM and Intermodulation AFM
• Bruker Nanoscope V Dimension ICON with peakforce QNM
• Keysight 5500 environmental (SPM3) with optional inverted light microscope
• Keysight 5500 environmental (SPM4) with optional digital pulsed force mode
• Bruker/Anasys NanoIR3

Accesories and special features (all SPM's except where indicated)

• Optical access.  Ability to acquire a digital video image.  Optional inverted light microscope on SPM 3.
• BNC breakout; may interface with the following three items as well as temperature and humidity signals
• Custom adder circuit box for signal manipulation, for example, to ramp setpoint during imaging (e.g., for friction-load characterization; only SPM 3-4)
• LabView station with virtual instruments: oscilloscope, digital signal analyzer, function generator (only SPM 3-4)
• Witec Pulsed Force Mode for adhesion and stiffness imaging (Digital version only on SPM 4, to capture entire force curves with the second computer).
• Bruker PeakForce Tapping on SPM 1-2. SPM 2 additionally has QNM capability for adhesion and stiffness imaging.
• Optional closed-loop scanners (nPoint on SPM 1-2; Agilent on SPM 4)
• MAC III box for multifrequency excitation, three lock-in amplifiers, Q control, contact resonance, and single-pass EFM/KFM (only on SPM 4)
• Magnetic AC mode (only on SPM 3-4 and requires magnetically coated maclevers)
• Closed (SPM 1-2) and open (SPM 3-4) liquid cells
• Sample heating (to 250° C) and Peltier (-30 to +35° C) stages (only SPM 4)
• Humidity control (1-95% on SPM 3, 0.1-95% on SPM 4)
• Cryo-microtome for cross-sectioning samples.  Double-D clamp for mounting cross-sectioned samples for AFM.
• 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 using silane linkage or gold coating/thiolate chemistry
• Special software:
  •  Mathematica and ICAdams 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 for a broad suite of image (e.g. grain size) and force curve (e.g. freely jointed chain model) analysis applications.  Also freeware programs WSXM and Gwyddion.
  • SPManalysis for custom histogram generation (multi-region, variable bin size), X-offsetted image subtraction (e.g., friction loop), ramped-signal image analysis (average each image over X lines, plot vs. Y meaning time), and myriad image and histogram generation from "force volume" data (Bruker data type).

Applications:

• Sensitive to the following properties: surface chemistry, storage/loss modulus, hardness, interfacial energy (Hamaker constant), crystallinity, polarization, magnetization, surface charge, and local work function (surface potential).
• 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.
• 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.