Atomic Force Microscope Study of Ag-Conduct Polymer Hybrid Films: Evidence for Light-Induced Charge Separation

Scanning Kelvin probe microscopy (SKPM), electrostatic force microscopy (EFM) are used to study the microscopic processes of the photo-induced charge separation at the interface of Ag and conductive polymers, i.e., poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-bʹ]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and poly(3-hexylthiophene-2,5-diyl) (P3HT). They are also widely used in order to directly observe the charge distribution and dynamic changes at the interfaces in nanostructures, owing to their high sensitivity. Using SKPM, it is proved that the charge of the photo-induced polymer PCPDTBT is transferred to Ag nanoparticles (NPs). The surface charge of the Ag-induced NPs is quantified while using EFM, and it is determined that the charge is injected into the polymer P3HT from the Ag NPs. We expect that this technology will provide guidance to facilitate the separation and transfer of the interfacial charges in the composite material systems and it will be applicable to various photovoltaic material systems.


Atomic Force Microscopy (AFM), Scanning Kelvin Probe Microscopy (SKPM), and Electrostatic Force Microscopy (EFM)
All of the AFM-based Experiments were performed in ambient condition using an MFP-3D AFM (MFP-3D-Stand Alone, Asylum Research, Abingdon-on-Thames, UK), and Pt/Cr coated silicon tips (Multi75E-G Budget sensor, Asylum Research, Abingdon-on-Thames , UK) with a spring constant of k ~ 2-5 N/m，a resonance frequency at ~ 60-70 KHz, a tip radius of curvature is 30 nm were used in imaging. The measurement of AFM is carried out at approximately the same temperature (24 °C-28 °C) and humidity (33%-38%). A scan rate of ~0.5-1.0 Hz is used in order to evaluate the light-induced composite film surface potential change of the sample.
SKPM and EFM measurements were both carried out in the two-pass manner. The first pass was used to determine the topography of the surface, and it was done exactly like a standard tapping mode scan line in AFM. The second pass was done by retracing the topography at a constant distance in z direction.
In SKPM and EFM ( Figure S1), the tip and the sample interaction could be treated as a parallel plate capacitor, and then the electrostatic force was proportional to the square of the applied voltage. Assuming the voltage between the sample and the probe, and the equivalent capacitance C, the total energy of the system is: Taking z as the distance from the sample surface at the normal direction, the electrostatic force on the tip could be expressed as: Where F is the electrostatic force acting on the tip, V is the potential, C is the capacitance, and z is the distance from the tip to the sample [1,2]. Figure S1. A scheme of light-modulated EFM and SKPM.
In SKPM, an alternating current (AC) voltage and a tunable direct current (DC) voltage were applied between the tip and the substrate during the second pass. The DC voltage was adjusted at each point to cancel the force at the AC frequency and, accordingly, the value was recorded as the potential difference.
In case of SKPM, the voltage between the tip and the sample is: Where, V(t) is the tip potential, VDC and VAC are the DC and AC voltage (a frequency of ) applied between the tip and the substrate, respectively, and VS is the surface potential of the sample. The vertical electrostatic force is a summary of terms at three frequencies, DC,  and 2, as indicated in Equation (2). According to Equation (2), at the frequency , if the VDC was adjusted to zero the force signal, then it should be equal to the surface potential (VS) of the sample. In SKPM, the VDC is actively adjusted to cancel the signal from the Lock-in amplifier at the frequency  and recorded as the surface potential while imaging [3,4]. In SKPM measurement, an offset bias voltage is applied between the sample and needle tip to counteract the electrostatic force between the tip and samples. In EFM (Figure 1), only a constant DC voltage was applied, and the phase shift of the tip was used to measure the electrostatic force between the tip and the sample. As the tip was raised up a distance from the sample, the major part of force on the cantilever was the long-range tip-sample interactions, e.g., electrostatic force [5,6].