Kenichi Umeda

Impact of UV/O3 treatment on solution-processed amorphous InGaZnO4 thin-film transistors

Ultraviolet–ozone (UV/O3) treatment was adopted to the fabrication of solution-processed amorphous In–Ga–Zn–O thin-film transistors (TFTs), with metal composition of In:Ga:Zn = 1:1:1 represented by InGaZnO4. By applying UV/O3 treatment In–Ga–Zn–O gel films, their condensation was notably enhanced through decomposition of organic- and hydrogen-based elements, which drastically improved the quality of the amorphous InGaZnO4 films. As a result, high TFT performance, with values of on/off ratio, 108; subthreshold swing, 150 mV/decade; threshold voltage, 9.2 V; and field-effect mobility, 5.1 cm2 V−1 s−1, was achieved.

High-Resolution Frequency-Modulation Atomic Force Microscopy in Liquids Using Electrostatic Excitation Method

We developed a novel method to drive the cantilever oscillation for frequency modulation atomic force microscopy (FM-AFM) in liquid environments using electrostatic excitation. The cantilever with a gold backside coating was vibrated by applying an oscillating bias voltage between the cantilever backside and an optically transparent electrode used as a liquid cell window. The frequency spectrum of the oscillation shows a simple resonance curve without spurious peaks. The method does not require electrical conductivity of samples at all. In fact, both muscovite mica and potassium chloride surfaces in aqueous solutions were successfully imaged on an atomic scale.

Modification of a commercial atomic force microscopy for low-noise, high-resolution frequency-modulation imaging in liquid environment

A key issue for high-resolution frequency-modulation atomic force microscopy imaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensor noise density from initially 100 fm/√Hz to around 10 fm/√Hz after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions.

Practical aspects of Kelvin-probe force microscopy at solid/liquid interfaces in various liquid media

The distributions of surface charges or surface potentials on biological molecules and electrodes are directly related to various biological functions and ionic adsorptions, respectively. Electrostatic force microscopy and Kelvin-probe force microscopy (KFM) are useful scanning probe techniques that can map local surface charges and potentials. Here, we report the measurement and analysis of the electrostatic and capacitive forces on the cantilever tip induced by application of an alternating voltage in order to discuss the feasibility of measuring the surface charge or potential distribution at solid/liquid interfaces in various liquid media. The results presented here suggest that a nanometer-scale surface charge or potential measurement by the conventional voltage modulation techniques is only possible under ambient conditions and in a non-polar medium and is difficult in an aqueous solution. Practically, the electrostatic force versus dc voltage curve in water does not include the minimum, which is us...

Analysis of capacitive force acting on a cantilever tip at solid/liquid interfaces

Dielectric properties of biomolecules or biomembranes are directly related to their structures and biological activities. Capacitance force microscopy based on the cantilever deflection detection is a useful scanning probe technique that can map local dielectric constant. Here we report measurements and analysis of the capacitive force acting on a cantilever tip at solid/liquid interfaces induced by application of an alternating voltage to explore the feasibility of the measurements of local dielectric constant by the voltage modulation technique in aqueous solutions. The results presented here suggest that the local dielectric constant measurements by the conventional voltage modulation technique are basically possible even in polar liquid media. However, the cantilever deflection is not only induced by the electrostatic force, but also by the surface stress, which does not include the local dielectric information. Moreover, since the voltage applied between the tip and sample are divided by the electric...

Direct actuation of cantilever in aqueous solutions by electrostatic force using high-frequency electric fields

We recently developed a method to directly actuate a cantilever in aqueous solutions by electrostatic force [K.-I. Umeda et al., Appl. Phys. Express 3, 065205 (2010)]. However, the cantilever was actuated by surface stress in a low frequency regime. We solved this problem by applying amplitude-modulated high-frequency electric fields, which actuates the cantilever solely by electrostatic force. The time variations of the self-oscillation frequency of a cantilever and the Au(111) surface images by frequency-modulation atomic force microscopy using acoustic, photothermal, and the improved electrostatic actuation methods were compared, which demonstrates the advantages of the last method such as stability and simplicity in instrumentation.

Molecular-scale quantitative charge density measurement of biological molecule by frequency modulation atomic force microscopy in aqueous solutions

Surface charge distributions on biological molecules in aqueous solutions are essential for the interactions between biomolecules, such as DNA condensation, antibody-antigen interactions, and enzyme reactions. There has been a significant demand for a molecular-scale charge density measurement technique for better understanding such interactions. In this paper, we present the local electric double layer (EDL) force measurements on DNA molecules in aqueous solutions using frequency modulation atomic force microscopy (FM-AFM) with a three-dimensional force mapping technique. The EDL forces measured in a 100 mM KCl solution well agreed with the theoretical EDL forces calculated using reasonable parameters, suggesting that FM-AFM can be used for molecular-scale quantitative charge density measurements on biological molecules especially in a highly concentrated electrolyte.

All solution-processed amorphous oxide thin-film transistors using UV/O3 treatment

In the fabrication of amorphous oxide thin-film transistors (TFTs) by all-solution process, an ultraviolet–ozone (UV/O3) treatment and solution materials were adopted. By applying the UV/O3 treatment for solution-processed In2−xGaxZnO4 channel layers, enhancement of TFT characteristics was achieved. In particular, the most appropriate metal composition for the In2−xGaxZnO4 system with UV/O3 treatment was found to be x = 1.0. In addition to the channel layers, solution-processed LaNiO3, Bi–Nb–O/La–Ta–O stacked layer, and ITO films were formed as the gate electrode, gate insulator, and source and drain electrodes, respectively, for TFT fabrication. Using UV/O3 treatment and solution materials, all-solution-processed amorphous oxide TFTs were successfully fabricated, and superior TFT properties, including an on–off current ratio of 107, a threshold voltage of 1.6 V, a subthreshold swing of 200 mV/decade, and a field-effect mobility of 0.49 cm2 V−1 s−1, were achieved.