Common scanning probe modes Basic mode: surface morphology Contact mode Non-contact Tapping mode Secondary modes: Surface potential Capacitance derivative and actual Capacitance Surface conductivity Scanning gate Magnetic force microscopy Thermal conductivity and temperature Trivia: AFM was discovered after the inventor of STM (Gerd Binnig and Heinrich Rohrer were awarded Nobel Prize in1986) wanted to develop a technique for mapping non-conductive samples.

Scanning probe microscopy Atomic force microscopy (AFM) and associated modes: Applicable for conductive and non- conductive samples. Not as high resolution as STM. Best resolution possible in conductive AFM. Scanning tunneling microscopy (STM): Only applicable for conductive samples. True atomic resolution possible.

Picture of a typical AFM machine A larger view of the system A close-up of the scanning chamber

Parts of an AFM system A microcantilever to transduce changes in physical parameters A probe tip to obtain nanoscale resolution A transduction method to quantify the change in deflection or the oscillation amplitude of the cantilever Environment control chamber Noise cancellation schemes The sample with proper connections

Contact mode AFM schematic Basic idea: In contact mode, the probe tip is in physical contact with the sample, within few Å from the surface, and operates in the repulsive region of the inter-atomic force curve (explained later). In contact mode the cantilever deflection is measured to track the surface morphology using a feedback loop.

Non-contact mode AFM schematic Basic idea: In non-contact mode, the probe tip oscillates in resonance frequency with an amplitude of several tens of nm, and operates in the attractive region of the inter-atomic force curve (explained later). In non-contact mode the oscillation amplitude of the cantilever is measured to track the surface morphology by a feed back loop. Laser deflection from cantilever sensed with 4 quadrant photo-diode

Tapping mode (Intermittent contact mode) This mode was developed (patented by Veeco Instruments) to take care of some of the problems of both the contact and intermittent contact modes. Specifically, it takes care of the lateral force problem of the contact mode, and the “water layer sticking” problem of the non-contact mode. In this mode the tip intermittently comes in contact with the sample and thus can provide much better imaging results, especially in liquid environments.

Basics of AFM modes Amplitude Frequency d Change in amplitude Curve 2 (near) Amplitude Curve 1(far) d Frequency Non-contact mode amplitude vs. frequency curve for the tip, far from and near the surface For van der Waals force region of the F-d curve, the slope dF/dz is positive. Hence 0 decreases as the tip comes closer to the surface, i.e. the curve shifts left

Associated modes with contact and non-contact /Intermittent contact modes Scanning gate (Contact) Scanning capacitance and capacitance derivative (Contact) Scanning conductivity (Contact) Scanning thermal conductivity and temperature (non-contact) Magnetic force microscopy (non-contact) Scanning Kelvin probe (non-contact)