Abstract:

Atomic Force Microscopyʹs (AFM) operating principle rests on the measurement of small forces, ideally at a large bandwidth. The measurement of small forces is difficult, and frequency modulation force microscopy transforms the daunting challenge of precise force measurements to the measurement of frequencies. Of all physical observables, time and frequency can be measured at the greatest precision, therefore frequency modulation force microscopy is a superb experimental method. The oscillation frequency of a cantilever and its variation due to the action of tip‐sample force gradients determine the spatial and force resolution of the microscope. An analysis of the factors that determine the spatial resolution of the force microscope led to the conclusion that optimal results occur when the oscillation amplitude of the cantilever in dynamic AFM has a magnitude similar to the range of the forces at play. When the cantilever is to oscillate with small amplitudes in a stable fashion under the influence of the field of the chemical bonding forces present between tip and sample, cantilevers with a stiffness of roughly 1kN/m in stead of the commonly used 40 N/m are required. These stiff sensors allow operation at sub Angstrom amplitudes. Small amplitude operation allows ʺsubatomicʺ resolution ‐ the imaging of features within single atoms. Small amplitude AFM often exceeds scanning tunneling microscopy in spatial resolution. The introduction of the qPlus sensor, a cantilever based on a quartz tuning fork, facilitates simultaneous STM and AFM imaging. Recently, various STM experiments have been repeated where the STM tip was replaced by a qPlus sensor and in the forces that act during imaging were recorded in addition to the tunneling current. In a collaboration with the IBM Low Temperature STM group in Almaden, San Jose, we measured the forces that act during atomic manipulation and together with Gerhard Meyer and coworkers at IBM Rüschlikon, we measured the charge state of individual atoms by AFM. Recently, the Meyer group has imaged pentacene and other organic molecules with unprecedented resolution using the qPlus sensor.
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