From KPFM measurement, we obtain contact potential difference

(CPD) between a metallic AFM tip and a sample which is denoted as V CPD. V CPD can be defined as Equation (1) and identical as the work function difference between the tip and the sample if there Nutlin-3a concentration are no defect states on the surface of the sample. If the tip approaches to the sample surface, electrostatic force is getting stronger between the tip and the sample surface. When the tip is close enough to the sample surface, Fermi levels of the tip and the sample will be aligned and become equilibrium state but the vacuum levels are not the same [23]. The external bias DC voltage (V DC) nullifies V CPD as shown in Figure  1a. A Pt/Ir-coated tip was used for C-AFM and KPFM (Nano sensor). The surface potential and topography were determined under a non-contact mode by applying AC voltage with amplitude of 1 V (peak to peak) and frequency of 70 kHz to get clear images and sufficient sensitivity. The AC voltage will lead to an oscillating force to the tip. The feedback loop adjusted the DC potential to nullify the V CPD component by applied DC bias to the tip, so we can obtain the two-dimensional surface potential image. The topography images were obtained by

using the noncontact mode at a resonant frequency of the probe of about 73.84 kHz. The scanning rate was with 0.5 Hz to minimize topological signal and samples were not damaged performing these selleckchem tuclazepam measurements. A lock-in 17DMAG cost amplifier was operated with a sensitivity of the 100 mV/nA. Figure 1 Schematic illustration of (a) Kelvin probe force microscopy and (b) conductive atomic force microscopy. (1) Current maps were obtained at contact mode with applying external constant voltage 0.2 V on the samples in a 5 × 5 μm2 scanning areas shown in Figure  1b. The Mo layer is used for back contact which was connected to a metal-coated conducting probe that is ground. Silver paste was used for the electrical contact for this measurement. A contact force of 1 nN was applied onto

a probe for the scanning area and the scanning time was set at 500 ms for each line to acquire a local current map measurement. Local current maps can be measured simultaneously together between sample and tips. The AFM laser has the wavelength of 633 nm (E = 1.95 eV) is above the band gap of CZTSSe films (E = 1.0 to 1.1 eV). Thus, the photon energy is greater than the band gap of the CZTSSe layer, the power of laser is low which does not affect photo-current contribution significantly. Considering local current and surface potential results, we can identify local electrical properties such as GBs of the CZTSSe thin film by comparing the images of the topography with that of the surface potential and current maps. Results and discussion A typical device characteristic of the CZTSSe samples that are studied in this paper is summarized in Table  1.