Eika Tsunemi

Multi-Probe Atomic Force Microscopy Using Piezoelectric Cantilevers

We developed a multi-probe atomic force microscopy (AFM) system using piezoelectric thin film (PZT) cantilevers. The use of self-sensing cantilevers with integrated deflection sensors as probes markedly reduced complexity in the ordinary AFM setup. Address-patterned samples having microfabricated x–y coordinate patterns, fabricated by electron beam lithography, were developed as well. These samples allow us to evaluate the relative distance between the probes by the comparison of the images obtained. Although the minimum distance between these probes was 126 µm using the original cantilevers, it was reduced to 9.2 µm by using the PZT cantilevers modified by a focused ion beam. Furthermore, we found that the interaction forces between the cantilevers were detected by determining the change in the amplitude of each cantilever.

Development of dual-probe atomic force microscopy system using optical beam deflection sensors with obliquely incident laser beams.

We developed a dual-probe (DP) atomic force microscopy (AFM) system that has two independently controlled probes. The deflection of each cantilever is measured by the optical beam deflection (OBD) method. In order to keep a large space over the two probes for an objective lens with a large numerical aperture, we employed the OBD sensors with obliquely incident laser beams. In this paper, we describe the details of our developed DP-AFM system, including analysis of the sensitivity of the OBD sensor for detection of the cantilever deflection. We also describe a method to eliminate the crosstalk caused by the vertical translation of the cantilever. In addition, we demonstrate simultaneous topographic imaging of a test sample by the two probes and surface potential measurement on an α-sexithiophene (α-6T) thin film by one probe while electrical charges were injected by the other probe.

Multi-Probe Atomic Force Microscopy with Optical Beam Deflection Method

We developed a multi-probe atomic force microscope (AFM) having two AFM cantilevers independently controlled using the optical beam deflection method. We succeeded in simultaneously obtaining images with two independent probes by frequency modulation (FM) detection method. To evaluate the distance between the AFM tips of the cantilevers, we used a new the address-patterned sample, which was also developed for this study. The images obtained show that the distance between the probes was 2 µm. The development of the multi-probe AFM opens a wide variety of applications in the present nanoscience and engineering field.

Influence of grain boundary on electrical properties of organic crystalline grains investigated by dual-probe atomic force microscopy

We performed electrical transport measurements on α-sexithiophene crystalline grains using a dual-probe atomic force microscopy system having two independently controlled cantilever probes. The field-effect transistor characteristics were measured by varying the distances between the two probes brought in contact with the surface of the grains. It was clearly shown by the transfer line method that the grain boundary is the dominant factor limiting the electrical properties of organic thin films. Moreover, the hole transport across the grain boundary was found to be more affected by the oxygen hole doping than that within the crystalline grain.

Visualization of anisotropic conductance in polydiacetylene crystal by dual-probe frequency-modulation atomic force microscopy/Kelvin-probe force microscopy

The authors recently developed a dual-probe atomic force microscope (DP-AFM) system as a powerful measurement or fabrication tool in the fields of nanoelectronics and nanobiology. In this study, they performed frequency-modulation Kelvin-probe force microscopy (FM-KFM) experiments on a polydiacetylene (PDA) single crystal using the DP-AFM system. While a bias voltage was locally applied to the PDA surface with one probe, the surface potential on the surrounding area was mapped with the other probe by FM-KFM. The surface potential images showed anisotropic distributions, which are explained by the anisotropic conductance of the PDA crystal due to the quasi-one-dimensional electronic band structure along the diacetylene main chain. They also discuss the mechanisms of charge injection from a probe to the PDA crystal and the difference in the anisotropic conductance ratio for electrons and holes.

Development of multi-environment dual-probe atomic force microscopy system using optical beam deflection sensors with vertically incident laser beams.

We developed a dual-probe atomic force microscopy (DP-AFM) system with two cantilever probes that can be operated in various environments such as in air, vacuum, and liquid. The system employs the optical beam deflection method for measuring the deflection of each cantilever mounted on a probe scanner. The cantilever probes mounted on the probe scanners are attached to inertia sliders, which allow independent control of the probe positions. We constructed three types of probe scanners (tube, shear-piezo, and tripod types) and characterized their performance. We demonstrated AFM imaging in ambient air, vacuum, and ultrapure water, and also performed electrical measurement and pick-up manipulation of a Au nanorod using the DP-AFM system.

Investigations of Local Electrical Properties of Pentacene Thin Films by Dual-Probe Atomic Force Microscopy

We performed local electrical transport measurement on single grains of a pentacene thin film using a lab-built dual-probe atomic force microscopy (DP-AFM) system. We brought two conducting cantilever tips in contact with a single grain and successfully measured the p-type field-effect characteristics. Moreover, we investigated the effect of contact resistance on the measured characteristics by performing a series of transport measurements while varying tip distance. The contact resistance and hole mobility of the channel region were estimated as 3.1 GΩ and 2.7×10-2 cm2 V-1 s-1, respectively. The results demonstrate the applicability of the DP-AFM system to the nanometer-scale transport measurement of molecules.

Visualization of Charge Injection Processes in Polydiacetylene Thin Film Grains by Dual-Probe Atomic Force Microscopy

We have recently developed a dual-probe atomic force microscopy (DP-AFM) system having two AFM cantilever probes whose positions can be independently controlled with a precision on the order of ten nanometers. In this study, we visualized charge injection processes in polydiacetylene thin film grains using the DP-AFM system. One of the two probes was used to apply bias voltages to the grains while the other probe was used to obtain surface potential images of the grains. The surface potential of the polymerized grain changed according to the applied bias voltages. The potential profiles at the edge of the grain were not sharp when negative voltages were applied to the grain, which suggests the existence of an electron injection barrier at the interface. Moreover, it is suggested from the same experiment conducted in vacuum that the p-type semiconductor characteristics of the polydiacetylene is due to the O2 hole doping.