EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative

OPEN SUBJECT AREAS: ATOMIC FORCE MICROSCOPY IMAGING TECHNIQUES Received 23 September 2013 Accepted 8 November 2013 Published 27 November 2013 Correspondence and requests for materials should be addressed to S.L. (samuele_lilliu@ hotmail.it) EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative S. Lilliu1,2, C. Maragliano2, M. Hampton1, M. Elliott1, M. Stefancich2, M. Chiesa2, M. S. Dahlem2 & J. E. Macdonald1 1 School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF243AA, United Kingdom, 2Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates. We report a simple technique for mapping Electrostatic Force Microscopy (EFM) bias sweep data into 2D images. The method allows simultaneous probing, in the same scanning area, of the contact potential difference and the second derivative of the capacitance between tip and sample, along with the height information. The only required equipment consists of a microscope with lift-mode EFM capable of phase shift detection. We designate this approach as Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM). An open-source MATLAB Graphical User Interface (GUI) for images acquisition, processing and analysis has been developed. The technique is tested with Indium Tin Oxide (ITO) and with poly(3-hexylthiophene) (P3HT) nanowires for organic transistor applications. A tomic Force Microscopy (AFM) has been extensively used to measure electrical properties of a sample at the nanoscale. In 1988, Y. Martin et al.1 used an AFM tip to measure the electric force between tip and sample. In this technique, established later as Electrostatic Force Microscopy (EFM), a conductive AFM tip is electrically biased against a grounded sample and the derivative of the electrostatic force is probed. EFM has been used to measure the local electrostatic properties across surfaces in a variety of applications such as thin film transistors2–5, solar cells6,7, carbon nanotubes8, DNA9, and surfaces in general10. Different EFM modes have been developed over the years: Scanning Capacitance Microscopy (SCM), which measures the local capacitance11; Kelvin Probe Force Microscopy (KPFM), which measures the contact potential difference VCPD between the tip and the sample12,13; as well as other techniques14–16. Each one of the different scanning modes is suitable for probing specific information, requiring several scans of the same area, if more than one measurement is needed. This task is often challenging since it is not straightforward to guarantee that the same area is probed after switching between different scanning modes. Considering the potential of EFM-based techniques, it would be a clear advantage if a microscope could simultaneously extract all, or at least some, of the needed information in a single scanning process. Apart from the works of Riedel et al.17–19, this challenge has been mainly addressed in AFM research using complex and costly external circuitry, such as multiple lock-in amplifiers that are able to extract information at multiple frequencies20. The two-step EFM mode, also known as lift-mode EFM-phase, allows simultaneous reconstruction of the topography of the surface and the electrostatic force field between the tip and the sample. In the first line scan, the topography is obtained in tapping mode; in the second line scan, the electrostatic interaction is probed at larger distance while biasing the tip or the sample21,22. In lift-mode EFM-phase, the possibility of simultaneously acquiring different information about the same area of a sample represents a considerable advantage. A step forward in the direction of increasing-throughput techniques would be a method able to probe, in a single process and over the same area, different electrical parameters of the sample. These include the tip-sample contact potential difference VCPD, which includes information on the sample work function, and the second derivative of the capacitance, which could be further processed in order to obtain a map of the sample dielectric constant17–19,23. Recently, Riedel et al. developed a method for obtaining the local dielectric permittivity of thin films from EFMphase scans and the Equivalent Charge Method (ECM)17–19. By following a similar approach, we present a technique based on lift-mode EFM-phase, which allows simultaneous probing of the second derivative of the capacitance and the contact potential difference VCPD between the tip and the sample, along with the surface SCIENTIFIC REPORTS | 3 : 3352 | DOI: 10.1038/srep03352 1 www.nature.com/scientificreports topography. This information is acquired over the same scanning area, without the need of switching into a different scanning mode. We designate this approach as the Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM) technique. SPP-EFM thus allows extracting information analogous to what can be acquired with SCM and KPFM, but in a single scan. As in the case of Riedel’s technique, SPP-EFM is a low-cost technique since it requires only a microscope able to operate in lift-mode EFM-phase. Contrary to other multi-parameters extraction approaches, SPP-EFM does not require additional equipment (such as external lock-in amplifiers). The method described here can be integrated with the Equivalent Charge Method (ECM)17–19 for the quantification of the local dielectric permittivity. The SPP-EFM technique described here is based on EFM-phase and EFM-sweep. The basics of these two techniques are here outlined. Lift-mode EFM-phase is an AFM-based technique sensitive to the sample electrical properties. It allows imaging the electrostatic morphology of the sample on a relatively large scale (e.g. 1 mm). Liftmode EFM-phase consists of two steps. The surface topography is first determined by a line scan in AFM tapping mode. During this step, both the cantilever and the sample electric grounded. An EFM line scan at a certain height above the surface is then performed. The cantilever is set to electric ground and the sample is biased. The tipsample force gradient is probed by monitoring the following phase shift24:     e ðjvÞ {arg½W ðjvÞ jv~vn DQ~ arg W ð1Þ e ðjvÞ is the frequency response of the cantilever under the where W action of an external force field fS, W(jv) is the frequency response of the free cantilever, and vn is the cantilever natural frequency (see supporting information). To a first approximation, DQ can be written as: DQ~{ Q dfS ðz Þ z~z0 K dz j ð2Þ where Q is the cantilever quality factor, K is the cantilever elastic constant, z is the tip-cantilever distance, and z0 is the cantilever equilibrium position. Equation (2) is only valid under the following assumptions: (i) the cantilever is approximated as a monodimensional Linear Time-Invariant (LTI) system, and (ii) K=Q? dfS ðz Þ=dz jz0 (see supporting information). The tip-sample electrosta (...truncated)