Touru Sumiya

Amperometric determination of choline with enzyme immobilized in a hybrid mesoporous membrane.

Choline sensor is successfully prepared by using immobilized enzyme, i.e., choline oxidase (ChOx) within a hybrid mesoporous membrane with 12 nm pore diameter (F127M). The measurement was based on the detection of hydrogen peroxide, which is the co-product of the enzymatic choline oxidation. The determination range and the response time are 5.0-800 microM and approximately 2 min, respectively. The sensor is very stable compared to the native enzyme sensor and 85% of the initial response was maintained even after storage for 80 days. These results indicate that ChOx is successfully immobilized and well stabilized, and at the same time, enzyme reaction proceeds efficiently. Such ability of hybrid mesoporous membrane F127M suggests great promise for effective immobilization of enzyme useful for electrochemical biosensors.

Amperometric l-lactate biosensor based on screen-printed carbon electrode containing cobalt phthalocyanine, coated with lactate oxidase-mesoporous silica conjugate layer

A novel amperometric biosensor for the measurement of L-lactate has been developed. The device comprises a screen-printed carbon electrode containing cobalt phthalocyanine (CoPC-SPCE), coated with lactate oxidase (LOD) that is immobilized in mesoporous silica (FSM8.0) using a polymer matrix of denatured polyvinyl alcohol; a Nafion layer on the electrode surface acts as a barrier to interferents. The sampling unit attached to the SPCE requires only a small sample volume of 100 μL for each measurement. The measurement of l-lactate is based on the signal produced by hydrogen peroxide, the product of the enzymatic reaction. The behavior of the biosensor, LOD-FSM8.0/Naf/CoPC-SPCE, was examined in terms of pH, applied potential, sensitivity and operational range, selectivity, and storage stability. The sensor showed an optimum response at a pH of 7.4 and an applied potential of +450 mV. The determination range and the response time for L-lactate were 18.3 μM to 1.5 mM and approximately 90s, respectively. In addition, the sensor exhibited high selectivity for L-lactate and was quite stable in storage, showing no noticeable change in its initial response after being stored for over 9 months. These results indicate that our method provides a simple, cost-effective, high-performance biosensor for l-lactate.

Initial growth stage of CaF2 on Si(111)-7 × 7 studied by high temperature UHV-STM

The initial stages of CaF 2 growth on Si(111) at a high substrate temperature have been studied, in situ, with an ultra high vacuum scanning tunneling microscope (UHV-STM) from submonolayer range up to a monolayer. The STM images directly demonstrate that the initial growth mode changes from a three-dimensional island formation to a wetting heteroepitaxial layer growth with increasing substrate temperature. At a substrate temperature of about 470°C, islands of characteristic shape, with steps arranged in the [110] direction, is observed on the Si(111) surface. This island formation initially occurs both at steps and on the flats of Si terraces. At a higher temperature of around 680°C, a submonolayer grows epitaxially from Si step edges, exhibiting a well- ordered row-like structure along the [110] direction. The empty state image of this row-like structure has a 3 X 1 periodicity at 680°C. Further deposition of CaF 2 results in covering the Si surface uniformly with the heteroepitaxial monolayers.

Initial growth stages of CaF2 on Si(111) investigated by scanning tunneling microscopy

Abstract The initial stages of calcium fluoride (CaF2) growth on Si(111)-(7×7) have been studied using ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) from the submonolayer range up to two monolayers (ML: 1 ML CaF2=7.84×1014 cm−2). The STM images directly indicate that the initial growth mode changes from an island formation at around 640°C to the growth of a wetting row-like region above about 750°C. At a substrate temperature of around 640°C, the islands of characteristic shapes, with steps arranged in the [110] directions of the substrate, are formed initially both at steps and on the flats of Si terraces. The island is a CaF layer that has a (1×1) periodicity. At a higher temperature of around 750°C, the region that has a well-ordered row structure along the [110] directions are observed only at Si step edges. The LEED pattern indicates that the row-like region has a (3×1) periodicity, and XPS measurements show that the row-like region still has a Ca:F stoichiometry of 1:1. Based on the coverage of the deposited CaF2 molecules, the row-like region is a reconstructed layer induced by CaF adsorption. Furthermore, in situ STM measurements of the initial growth stages of CaF2 have been performed at 700°C and 800°C. The results clearly show that the first row-like region and the second layer grown on the first region have different growth modes.

Surface reconstruction in CaF2/Si(001) investigated by scanning tunneling microscopy

Abstract Scanning tunneling microscopy (STM) has been used to study the initial growth stages of calcium fluoride (CaF 2 ) on Si(001)-(2 × 1) surfaces in situ. At high coverage, the deposited CaF 2 molecules nucleate into small hemispherical islands at room temperature, which are of the order of 1 to 3 nm in diameter and cover the Si(001) surface homogeneously. Annealing this surface at 520°C produces a drastic change in the surface morphology. Due to the increased mobility of the deposited CaF 2 molecules, they rearrange into long and narrow islands typically of a few nm width and whose length is often more than four times the width. The long, narrow islands grow three-dimensionally, and their long axes are aligned along [110] or [11¯0] of the substrate Si lattice. Increasing the substrate temperature to about 610°C induces a shape transition of the islands from long and narrow to two-dimensional layers. The STM image shows that the layers consist of rows aligned with respect to the underlying Si(001) lattice. Considering the coverage of the deposited CaF 2 molecules, the row-like layers are not reconstructed Si(001) surfaces induced by CaF 2 adsorption, but are layers that are formed initially in the CaF 2 heteroepitaxial growth on a Si(001) surface. STM images clearly show that the terraces are completely covered with row-like layers at 670°C. The formation of the row-like layers can be attributed to the chemical reaction between CaF 2 and Si. The perpendicular spacing between adjacent rows varies from 0.8 to 2.4 nm. Thus, it is suggested that the inhomogeneous row-like layers will prevent growth of CaF 2 films with high crystalline quality on Si(001).

Single-Crystalline Nanogap Electrodes: Enhancing the Nanowire-Breakdown Process with a Gaseous Environment

A method for fabricating single-crystalline nanogaps on Si substrates was developed. Polycrystalline Pt nanowires on Si substrates were broken down by current flow under various gaseous environments. The crystal structure of the nanogap electrode was evaluated using scanning electron microscopy and transmission electron microscopy. Nanogap electrodes sandwiched between Pt-large-crystal-grains were obtained by the breakdown of the wire in an O(2) or H(2) atmosphere. These nanogap electrodes show intense spots in the electron diffraction pattern. The diffraction pattern corresponds to Pt (111), indicating that single-crystal grains are grown by the electrical wire breakdown process in an O(2) or H(2) atmosphere. The Pt wires that have (111)-texture and coherent boundaries can be considered ideal as interconnectors for single molecular electronics. The simple method for fabrication of a single-crystalline nanogap is one of the first steps toward standard nanogap electrodes for single molecular instruments and opens the door to future research on physical phenomena in nanospaces.

Schottky barrier inhomogeneity at Au/Si(111) interfaces investigated using ultrahigh-vacuum ballistic electron emission microscopy

Abstract Ultrahigh-vacuum ballistic electron emission microscopy (UHV-BEEM) has been used to study the electron transport across Au/n-type Si(111) interfaces. A BEEM image revealed that an Au film deposited at about 130°C included regions in which the BEEM current was significantly reduced. The ballistic transmissivity across the Au/Si(111) interface was found to be markedly reduced in these regions. Post-annealing of the sample at 300°C in UHV resulted in the absence of ballistic transmissivity throughout the sample. This indicated that the AuSi alloy formed at the interface scattered the ballistic electrons strongly. We attribute the appearance of the regions to the formation of the AuSi alloy at the Au/Si interface. We further demonstrate that the growth of Au on a clean Si(111) surface took place in a layer-by-layer fashion at room temperature. The BEEM measurements indicated the formation of a homogeneous interface without regions with reduced ballistic transmissivity.

Ballistic electron emission microscopy studies on Au/CaF2/n-Si(111) heterostructures

Electron transport phenomena across Au/CaF2/n-Si (111) heterostructures, in which calcium fluoride (CaF2) [about two monolayers (ML)] was introduced into the interface at room temperature (RT), 550 °C, and 700 °C, have been studied by ballistic electron emission microscopy (BEEM) and ballistic electron emission spectroscopy (BEES). Not only the Au growth but also the electron transport properties strongly depend upon the growth temperatures of CaF2 intralayers. In the case of CaF2 growth at RT, CaF2 molecules will exist on the surface of the 50 ML Au/2 ML CaF2(RT)/n-Si (111) sample. BEES clearly shows that the Schottky barrier of the intermixed layer on Si (111) is about 1.06 V which is higher than the value of 0.73 V for Au/Si (111). At 550 and 700 °C, thin, flat Au islands, each about 0.15 nm thick, grow in stacks on the CaF2 layer. The threshold voltage of the BEEM current for an insulating CaF2 intralayer, which is about 3.58 V, is obtained only in the sample in which CaF2 was deposited at 700 °C. Fur...

Ballistic electron emission microscopy studies of Au/CaF2/n-Si(111) interfaces

Abstract We have performed ultrahigh-vacuum (UHV) ballistic electron emission microscopy (BEEM) measurements on Au/CaF2/n-Si(111) into which calcium fluoride (CaF2) (about 2 monolayers (ML)) was introduced between Au and Si. A BEEM image clearly shows that a CaF2 intralayer deposited at 700°C induces the coexistence of two terrace types, each with a different BEEM I–V spectrum shape. A typical threshold voltage of the BEEM current for one type is about 0.75 V. In contrast, the other type shows a threshold voltage of about 3.6 V, which is much higher than that of the first type. Furthermore, the BEEM current on the second type is significantly reduced and saturates above approximately 6 V. Ca distributions measured by Auger electron spectroscopy (AES) strongly suggest an inhomogeneous distribution of CaF2 coverage on 2 ML CaF2 (deposited at 700°C)/n-Si(111); there are two types of Si terraces, each of which has a different CaF2 coverage. Based on the AES measurements, we attribute the coexistence of the two terrace types in the BEEM image to the different degrees of coverage of the CaF2 intralayers. The second type of terrace has a Au/2 ML CaF2/1 ML CaF/Si(111) heterostructure. A 2 ML CaF2/1 ML CaF is an insulating intralayer which induces the threshold voltage of 3.6 V and the saturation of the BEEM current. In contrast, the first type has a Au/1 ML CaF/Si(111) heterostructure which has the threshold voltage of 0.75 V.

Scanning Tunneling Microscopy Study of Initial Growth of CaF2 and BaF2 on Si(111)

Scanning tunneling microscopy (STM) has been used to investigate nucleation and initial growth in the heteroepitaxies of calcium fluoride ( CaF2) and barium fluoride ( BaF2) on Si(111) surfaces in situ. The fluoride depositions and the STM measurements are performed at a substrate temperature of about 400° C. STM images clearly show that a BaF2-deposited surface has a different morphology from that of CaF2-deposited surface. Preferential nucleation and island growth of BaF2 only occur at steps and domain boundaries on a Si(111)-7×7 reconstructed surface. On the other hand, CaF2 islands nucleate not only at steps and domain boundaries but also in domain-boundary-free regions of a Si(111) surface. We attribute the difference in the morphologies to the much higher mobility and diffusion length of a BaF2 molecule in comparison to those of a CaF2 molecule on a Si(111) surface at 400° C. We also report the first STM measurement of a (CaF2+BaF2)-coexisting surface at 480° C.