Scanning Probe Microscopy

Scanning probe microscopy (SPM) scans a fine tip close to a surface and measures the tunneling current (STM) or force (SFM), based on many possible tip-surface interactions. STM provides atomic resolution imaging, or the local electronic structure (spectroscopy) as a function of bias voltage, and is also used to manipulate adsorbed atoms on a clean surface. STM operates in two modes— constant current or height—and requires a conducting specimen. SFM uses a cantilever (force sensor) to measure short range (< 1 nm) chemical, and a variety of long-range (< 100 nm) forces, depending on the tip and the specimen; a conducting specimen is not required. In static mode, the tip height is controlled to maintain a constant force, and measure surface topography. In dynamic mode, changes in the vibrational properties of the cantilever are measured using frequency, amplitude, or phase modulation as feedback to control the tip-surface distance and form the image. Dynamic imaging includes contact and non-contact modes, but intermittent contact or tapping mode is common. SPMs measure properties (optical, acoustic, conductance, electrochemical, capacitance, thermal, magnetic, etc.) using appropriate tips, and find applications in the physical and life sciences. They are also used for nanoscale lithography.