Nov 18, 2011
Band excitation: a new approach for SPM operation
Force-based SPM techniques based on single-frequency sinusoidal excitation and detection are fundamentally limited in the information they provide. Band excitation (BE) offers a universal modulation method in SPM beyond classical lock-in and phase-locked loop detection that enables quantitative and reliable studies of dissipative and conservative phenomena for mechanical, electrical, electromechanical, magnetic, and thermal imaging and spectroscopies.
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.
About the author
Stephen Jesse and Sergei Kalinin are research staff members at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory. Oak Ridge National Laboratory is the Department of Energy's largest science and energy laboratory. ORNL has a staff of more than 4,800 and annually hosts approximately 3,000 guest researchers who spend two weeks or longer in Oak Ridge. As an international leader in a range of scientific areas that support the Department of Energy's mission, ORNL has six major mission roles: neutron science, energy, high-performance computing, systems biology, materials science at the nanoscale, and national security. ORNL's leadership role in the nation's energy future includes hosting the Center for Nanophase Materials Sciences-one of the five Department of Energy Nanoscale Science Research Centers, which serve as user facilities for interdisciplinary research at the nanoscale supported by the DOE Office of Science.