Yoshinori Suganuma

A hybrid scanning tunneling–atomic force microscope operable in air

We describe a hybrid scanning tunneling–atomic force microscope (STM–AFM) capable of measuring current and force simultaneously under ambient conditions. In order to reduce meniscus forces, the microscope uses a sharp STM tip as a probe and an AFM cantilever as a sample substrate. This improvement allows use of correspondingly flexible cantilevers enhancing force detection sensitivity. Using the STM–AFM, we have been able to explore a number of phenomena that can occur in nanometer scale tunnel junctions in air, including a correlation between hysteretic changes in contact potential and rapid increases in current at large bias voltages.

Discrete electron forces in a nanoparticle-tunnel junction system

According to the “orthodox” model for single electron tunneling, sudden changes in current–voltage characteristics of nanoparticle (NP)-tunnel junction (TJ) systems [“Coulomb blockade” (CB) and “Coulomb staircase” (CS) phenomena] arise fundamentally due to charge quantization. We have embedded NPs (∼2.5 nm in diameter) in the TJ of a hybrid scanning tunneling-atomic force microscope and have simultaneously measured current and forces generated in the system. We discuss an application to micromechanical switching actuated by single electrons. We also show that CB and CS phenomena are in fact associated with steplike changes in force, directly confirming the discrete charge nature of the phenomena.

Microfabricated, silicon devices with nanowells and nanogap electrodes: a platform for dielectric spectroscopy with silane-tunable response

Combining the advantages of nanogap devices and impedance spectroscopy can potentially provide a platform for dielectric spectroscopy with widely ranging applications—from fundamental studies at the nanoscale and surfaces to label free and selective sensors. The present study characterizes the impedance response of a microfabricated, silicon-based device with a large array of nanowells surrounded by annular, nanogap detection regions. Device impedance is measured versus frequency over 5 orders in a variety of organic solvents with dielectric constants ranging over 2 orders. The study finds two key results. First, an equivalent R/C circuit model is found to compare favorably with device impedance response over these wide ranges of parameters. Importantly, the model correlates with structure of the nanogap device, which suggests that such a structure-impedance response approach can help guide modeling of other devices geometries. Second, the model points to—and data confirm—correlation between nanogap device response and dielectric constant of materials in the nanogaps, particularly at low frequencies. In addition, the correlation is significantly modified by robust, silane functionalization of the devices due to a large surface-to-volume ratio of the nanogaps. These results demonstrate that nanogap impedance spectroscopy using microfabricated/silanized silicon devices is a robust and versatile platform for dielectric spectroscopy of materials on the nanoscale and on surfaces.

Evaluation of a low cell constant conductance detector for detection of charged species in high-performance liquid chromatography

The present study evaluates high performance liquid chromatography (HPLC) detection based on a commercial conductance detector with a low cell constant of 0.005 cm−1 and a volume of 2 μL and a photodiode array UV-vis detector typically used in HPLC. When detecting a static NaCl solution, the conductance detector yields a limit of detection (LOD, 3 × noise) for NaCl of 39 parts per trillion. When flowing methanol through both conductance and UV-vis detectors and injecting benzoic acid/methanol, the signal-to-noise (S/N) ratio of the conductance detector is 8-fold higher than that of the UV-vis at its optimal wavelength. Using an HPLC with a C-18 column, flowing a 75 : 25 water : methanol solution, and using an acetylsalicylic acid (ASA)/water solution, the conductance detector yielded an ∼18-fold higher S/N ratio. It was found that HPLC system noise reduces the S/N ratio of the conductance detectors. The conductance detector detected non-chromophoric species generated by atmospheric CO2 as well as by decomposition of ASA. Conductance chromatograms yielded ASA peak heights and areas that varied linearly with the ASA concentration from 0.5 ppm to 10 ppm with linear correlation coefficients exceeding 0.999. In view of the sensitivity of conductance detection, its potential application as a sensitive tool for cleaning assessments for pharmaceutical equipment was confirmed by dispersing 50 μg of ASA on a 2′′ × 2′′ stainless steel sheet, swabbing the surface, dissolving the collected material in water and injecting the solution into the HPLC. The results in this study demonstrate that low cell constant conductance detection can be remarkably sensitive to ionized/charged species and thereby has potential to serve as an analytical tool for this important class of molecules.