In the three decades since scanning probe microscopy (SPM) methods have entered the scientific arena, they have become one of the main tools of nanoscale science and technology by offering the capability for imaging topography, magnetic, electrical, and mechanical properties on the nanometre scale. Of extreme interest for applied and fundamental science alike are the measurements of energy losses and dissipation on the nanoscale. This information is required to understand the factors limiting the efficiency of materials and devices, with applications ranging from fundamental physical science to efficient energy transport, generation, and storage.

Resonant system dynamics

Classical SPM methods based on lock-in or phase-locked loop detection are not well suited to the measurement of localized energy dissipation. This limitation follows from the fundamental operational principle of modern SPMs; that is, the use of a sinusoidal excitation signal. In the Fourier domain, this corresponds to a single frequency. Hence, the measured response of the system is convoluted with the much stronger effect of the probe, cabling, and electronics. This limitation can be overcome by measuring full response-frequency curve and by detecting minute changes in resonance peak width as the SPM probe tip approaches the surface and scans along it. However, this requires time-consuming scanning of the frequencies at each spatial point. An alternative way to interpret this limitation is to note that the resonant system dynamics (that is, the probe interacting with the surface) requires at least three independent parameters to describe it (amplitude, resonant frequency, and quality factor), whereas standard SPM detection schemes allow only two to be determined. The assumption of constant driving force implicitly used in dissipation analysis is inapplicable to techniques with voltage or thermal excitation, and leads to qualitative errors in techniques with acoustic excitation due to large frequency dispersion of the piezoactuator transfer function. Correspondingly, there is a virtual absence of SPM studies of dissipative phenomena on the nanoscale.

Full spectral response

Band Excitation overcomes an intrinsic limitation of single-frequency SPM modes based on lock-in and phase-locked-loop detection by detecting the full spectral response at each pixel. The use of a digitally synthesized band excitation signal allows responses to be detected at multiple frequencies in parallel, giving rapid acquisition of full amplitude-frequency response curves in the time corresponding to a single point measurement in standard SPM methods. Band excitation thus provides a new approach for SPM operation alternative to traditional lock-in and phase-locked-loop methods. This enables unambiguous and cross-talk-free probing of local energy losses and dissipation, and can be implemented for virtually all ambient and liquid SPM methods.

Additional information can be found in J. Phys. D. Appl. Phys. 44 464006.

•  For more on this theme, check out the special issue of Journal of Physics D: Applied Physics celebrating the 30th anniversary of the invention of the scanning tunelling microscope - three decades of scanning tunnelling microscopy that changed the course of surface science.