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The reversible displacement current was generated from a single monolayer of l -α-dioleoyllecithin (DOL) formed on the water surface by regulated application of repeated decrease and increase of the DOL monolayer covering-area with the aid of a movable barrier. The behavior of the surface pressure—area isotherm of the DOL monolayer was reversible along with the increase and decrease of the monolayer-covering area. In addition, the reversible change in the surface micro-structure of a DOL monolayer system was visualized at various two-dimensional molecular densities by using a new replica technique coupled with transmission electron microscopy. We concluded that the displacement current measurement and the replica technique are useful for a better understanding of the relationship between the function and structural change of monolayers.

We have studied random telegraph signals (RTSs) in a chemically assembled single-electron transistor (SET) at temperatures as low as 300 mK. The RTSs in the chemically assembled SET were investigated by measuring the source–drain current, using a histogram of the RTS dwell time, and calculating the power spectrum density of the drain current–time characteristics. It was found that the dwell time of the RTS was dependent on the drain voltage of the SET, but was independent of the gate voltage. Considering the spatial structure of the chemically assembled SET, the origin of the RTS is attributed to the trapped charges on an alkanethiol-protected Au nanoparticle positioned near the SET. These results are important as they will help to realize stable chemically assembled SETs in practical applications.

In the study, we set our motivation to consider the mechanism to explain such an interesting phenomena particularly happened at the ultra low voltage situation. At first we gathered a set of SE and BSE images simultaneously at ultra low voltage condition from various kinds of specimen. Second, we compared the SE and BSE image to investigate the difference. A simulation results by CASINO [3] was also applied for the theoretical consideration. In the study the recent cold FE-SEM (Hitachi SU8000) is used. The SEM is offering the SE/BSE filtering capability even at ultra low voltage condition as shown in Fig.1.

INTEREST in the behaviour of Langmuir–Blodgett (LB) films1 is motivated by possible applications that include molecular electronics, biosensors and modelling of biological membranes. We recently2–5 reported an electrical technique that allows molecular motions in LB films to be probed by measuring the Maxwell displacement current6 generated across electrodes above and below the film: this current results from changes in the orientation of the molecular dipoles. Here we use the technique to study the effect of photoisomerization in an LB film consisting of an azobenzene derivative, which can be switched photochemically between cis and trans isomers. Films of this sort are of particular interest because of their possible use in information-storage devices7–11. Successive isomerizations induced by irradiation with ultraviolet and visible light produced transient displacement-current pulses between the electrodes. We anticipate that this technique will become widely used for dynamical studies of LB films.

A new scanning probe method for evaluating the local carrier concentration of semiconductors is presented. The distance between a semiconductor surface and a vibrating tip is changed sinusoidally, and is adjusted to as small as a few nm across which tunneling current IT flows between the semiconductor surface and the vibrating tip. Displacement current ID due to the change in electric flux from the semiconductor surface to the vibrating tip flows periodically in accordance with the vibration of the tip. IT also flows when the distance becomes minimum. Both IT and ID flow simultaneously when a d.c. tip voltage V is applied, and these two currents are separated by using a two-phase lock-in amplifier. ID-V curve is analyzed by taking into account two-dimensional electric flux profile under the tip. In the tip voltage range within which majority carriers are accumulated, ID-V curve is extrapolated by a straight line. On the contrary, the slope of the curve decreases as the tip voltage is reduced from the voltage range of the accumulation, indicating that the majority carriers are depleted at the center of the Si surface. The theoretical ID-V curves are in good agreement with the measurement in the tip voltage region where the majority carriers are both accumulated and depleted. The local carrier concentration is able to be determined by makingthe theoretical ID-V curve fit the experimental results without contacting the semiconductor surface.

The surface topography and the distribution of stray magnetic flux density of Fe70Co30 are observed simultaneously by using scanning Lorentz force microscopy (SLFM). The SLFM images of the magnetic domain of Fe70Co30 have been observed repeatedly, and they were found to be unaffected by the surface roughness and friction force. As compared to the magnetic force microscopy image, it can be inferred that small stray magnetic flux loops were detected near the sample surface in the SLFM image.

Molecular switching in phospholipid and azobenzene mixed monolayers on a water surface was examined by Maxwell displacement current (MDC) measurements coupled with surface–pressure measurements. MDC and thermally stimulated current (TSC) measuring techniques also examined molecular switching in these monolayers on a solid substrate. It was found that the molecular aggregation of pure azobenzene monolayers in both Langmuir (L) and Langmuir–Blodgett (LB) films is reduced by the introduction of phospholipids possessing unsaturated long alkyl chains (DOLPC) or saturated long alkyl chains (DPPC), and that the cis–trans photoisomerization in a mixed monolayer is accelerated. However the contribution of the two kinds of lipids in mixed monolayers was found to be different. The temperature dependence of charge induced across these mixed LB films, due to photoisomerization, corresponded well with that of 4-octyl-4′-(5-carboxyl-pentamethyleneoxy)-azobenzene (8A5H) LB films. Finally it was concluded that the cis-state 8A5H molecules, in monolayers, return to the trans-state during heating.

The electrical properties of Au nanoparticles protected by thiol-functionalized oligo(phenylene-ethynylene) (OPE), 1,4-bis-(3-mercapto-phenylethynyl)benzene have been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM and scanning electron microscopy images of chemisorbed OPE-protected Au nanoparticles on Au(111) surface were similar, and the densities were almost identical. OPE-protected Au nanoparticles exhibited stochastic conductance switching behaviors, and current-voltage (I–V) and log I–V characteristics by STS at 100 K showed Coulomb blockade behaviors. The charging energy of Au nanoparticles was as high as 0.57 eV when the core diameter was 2.1 nm. Our results are significant for single-electron transistor memory applications.

In this letter, we describe resistive switching and gap narrowing using nanogap electrodes fabricated by electroless gold plating. Resistive switching was observed using these nanogap electrodes, and gap narrowing was caused by applying low bias voltages that were below the operation voltages for resistance switching. The results indicate that nanogap electrodes with small gap widths could lower the voltage of the narrowing operation. Moreover, because the electroless gold plating method is a very promising technique for the mass production of nanogaps, this method is useful for application to nanogap devices.

Coulomb blockade behaviors in individual Au nanoparticles of 2 nm core diameter in double-barrier structures have been studied by means of noncontact atomic force spectroscopy (NC-AFS) at room temperature. The Au nanoparticles with a 1-decanethiol ligand were chemisorbed by 1,10-decanedithiol molecules of a mixed 1-octanethiol/1,10-decanedithiol self-assembled monolayer coated on a Au(111) surface; these particles were observed through NC-AFS. NC-AFS measurements of the cantilever frequency shift-sample voltage (Δf-V(S)) curves were sequentially conducted on three Au nanoparticles under the same experimental conditions; the Δf-V(S) curves were found to deviate from the parabolic (Δf(N)) curve in the cases where no extra charge existed on the Au core. The experimental Δf(CB)(=Δf-Δf(N)) and Δf(CB)/V curves agree well with the theoretical curves obtained using a golden-rule calculation and the same parabolic parameters. All the results, through NC-AFS, suggest Coulomb blockade behaviors in the Au nanoparticles at room temperature.

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Single molecular memory operation was observed on a porphyrin derivative by scanning tunneling microscopy at room temperature. A porphyrin derivative with four disulfide groups was chemically synthesized and chemisorbed on a Au(111) surface. Coulomb blockade behaviors and switching behaviors in current-voltage (I-V) characteristics were observed on a single porphyrin derivative by scanning tunneling spectroscopy. Based on the switching behaviors, the memory operation of electrical conductance in the porphyrin derivative was demonstrated by applying a programmed pulse sequence with an on/off ratio of 2.9 at room temperature.

Single-electron transistors have the potential to become next-generation nanodevices and sensors owing to their small size, low power consumption, and high charge sensitivity, where the charging energy of the devices is the most important parameter determining the operational temperature. Here, we have demonstrated that the charging energy of single-electron transistors can be controlled (48 ± 4 meV) by adopting electroless gold plating to make separation-defined nanogap electrodes and employing size-controlled chemically synthesized Au nanoparticles (5.2 ± 0.5 nm) as a Coulomb island. At this charging energy, the devices can be operated up to 160 K with on/off current ratio of 60%.

Au nanoparticle single-electron transistors with metal-bridged top-gates and nanogap electrodes were fabricated using two consecutive electron beam lithography and electroless Au plating steps. The metal-bridged top-gate electrodes were suspended above electroless Au plated nanogap electrodes. Au nanoparticles (5.2 nm in diameter) were chemisorbed between the nanogap electrodes after top-gate fabrication. Clear Coulomb diamonds were observed at 9 K. The gate capacitance Cg of the top-gate electrodes was 99 zF, which is 10 times larger than that of a similar device with only side-gate electrodes.

The individual transport of electrons through a chemisorbed Au nanodot is observed in accordance with a nanomechanical vibration of the Au nanodot on a cantilever at 86 MHz; the experimental setup consists of a scanning tunneling microscopy probe/vacuum/chemisorbed Au nanodot/cantilever. In the tunneling current-distance characteristics, a constant current of ef [where f is an eigenfrequency of the cantilever (86 MHz)] is observed as a plateau over a distance of 0.35 nm; this plateau is five times wider than that observed in the case of physisorbed Au nanodots. Coulomb blockade electron shuttle devices with chemisorbed Au nanodots are one of the candidates for current standard devices.

Sub-2-nm-size basic ligand Au nanoparticles were chemically synthesized and chemisorbed on an acidic self-assembled monolayer/Au(111) substrate by acid–base interaction. Coulomb blockade behaviors with clear Coulomb gaps were observed in current–voltage (I–V) and log I–V curves of the chemisorbed Au nanoparticles by scanning tunneling spectroscopy at room temperature. By fitting the measured I(V) and log I(V) to a Coulomb blockade model, we estimated the charging energy of one electron on the Au nanoparticles to be 10 times greater than the thermal energy k T; the tunneling resistance of the Au core–Au(111) surface was evaluated to be 3.5 GΩ ±15%.

We report the fabrication of nickel nanospaced electrodes by electroplating and electromigration for nanoelectronic devices. Using a conventional electrochemical cell, nanogaps can be obtained by controlling the plating time alone and after a careful optimization of electrodeposition parameters such as electrolyte bath, applied potential, cleaning, etc. During the process, the gap width decreases exponentially with time until the electrode gaps are completely bridged. Once the bridge is formed, the ex situ electromigration technique can reopen the nanogap. When the gap is ∼1 nm, tunneling current–voltage characterization shows asymmetry which can be corrected by an external magnetic field. This suggests that charge transfer in the nickel electrodes depends on the orientation of magnetic moments. (Some figures in this article are in colour only in the electronic version)

Coulomb diamonds were clearly observed on single-electron transistors (SETs) fabricated by bottom-up processes of electroless plating of Au nanogap electrodes and chemisorption of a Au nanoparticle at 80 K. In the drain current–drain voltage characteristics, Coulomb staircases were modulated by the side gate voltage. Tunneling resistances and source/drain/gate capacitances of the SET were evaluated by fitting the theoretical Coulomb staircase determined on the basis of the full orthodox theory in a double-barrier tunneling junction to the experimental results of Coulomb blockade under the application of side gate voltages. The theoretical results for the Coulomb diamond are in good agreement with the experimental results.

Alkanethiol-protected Au nanoparticles have been chemically immobilized on Au(111) substrates by alkanedithiol and alkanethiol mixed self-assembled monolayers (SAMs). Octanethiol SAMs on Au(111) have been used as host matrices for molecular insertion by 1,8-octanedithiol (C8S2) and 1,10-decanedithiol (C10S2). The dithiol molecules in the mixed SAMs and the Au nanoparticles on the mixed SAMs were observed by scanning probe microscopy. The densities of C8S2 and C10S2 mixed SAMs were controlled by varying the dithiol concentration from 0.5 to 50 mM. The densities of Au nanoparticles were almost equal to those of the dithiol molecules, indicating that the Au nanoparticle chemisorbed through the protruding sulfur of the dithiol molecules displaced the protecting group of the Au core. The chemisorbed Au nanoparticles were stable for scanning tunneling spectroscopy (STS) measurements. The resistance of the chemisorbed nanoparticles between the Au core and the Au(111) substrate is discussed.

We report a simple and high yield method for fabricating multiple nanogaps simultaneously by an electroless gold plating technique using electroless gold plating solution which consists of common medical liquid of iodine tincture and L(+)-ascorbic acid (vitamin C). The distance between the gold electrodes (33nm in average) on the SiO2∕Si substrate was decreased by selective deposition of gold onto the surface of the gold electrodes. By electroless gold plating, we fabricated nanogaps below 5nm in width with a 41% process yield. We also demonstrated the Coulomb blockade effect in octanethiol(C8)-protected Au nanoparticles by using such a fabricated nanogap.

Transport of electrons through a Au nanodot has been observed under a nanomechanical vibration of a Au nanodot on cantilever that consists of scanning tunneling microscopy probe/vacuum/Au nanodot/cantilever. In the probe tunneling current-distance characteristics, a constant probe current of 2ef has been observed as a plateau region, where f is an eigenfrequency of the cantilever of 86MHz. The authors discuss this quantized tunneling current in relation to one by one single-electron transport per cycle of operation in nanomechanical Coulomb blockade shuttle.

We developed an analytical model of an organic field-effect transistor (OFET) based on the gradual channel approximation with electric field (E)-dependent mobility µ=µ0 exp (β√E). The theoretical drain current–drain voltage curves are in good agreement with the measurements of pentacene OFETs, and the prefactor mobility µ0 at temperatures ranging from 233 to 303 K was evaluated. The electric-field- and temperature-dependent carrier transports are discussed independently in terms of µ0 on the basis of this model.

We examined the orientational transition process of p-pentyl-p'-cyano-biphenyl (5CB) molecules in liquid-crystal (LC) cells triggered by the conformational change of the surface azobenzene monolayer on polyimide (PI) Langmuir-Blodgett (LB) films, by measuring the optical transmittance and the electrical capacitance simultaneously. The experimental results were analyzed using a continuum theory describing the free energy of LC cells. It was shown that the motion of 5CB molecules triggered by the surface azobenzene monolayer on PI LB films was ruled by the continuum theory. The transmittance and the capacitance of the LC cells with 1-layer PI LB films were changed reversibly by the alternating irradiation of ultraviolet (UV) and visible light, whereas those of LC cells with 5- and 11-layer PI LB layers were not changed reversibly. These results were discussed, taking into account the difference in the anchoring strength of PI LB films.

A new simultaneous measuring system for tunneling current and surface potential has been developed. The distance (d) between a sample and an electrochemically etched tungsten tip is changed sinusoidally with high precision. The tunneling current flows periodically in accordance with the vibration of the tip when d becomes as small as a few nm. The surface potential is measured using the principle of the Kelvin method in which the displacement current due to the presence of surface potential and the change in capacitance difference between the tip and the sample is detected. Simultaneous measurement of the tunneling current and the displacement current has been attained by utilizing their phase difference in tip vibration. The method for determining the tip radius and d is also presented.

The dielectric relaxation phenomena of a 4-cyano-4′-5-alkyl-biphenyl (5CB) monolayer at the air-water interface have been investigated. Maxwell displacement current (MDC; I)-area per molecule (:A) isotherms and surface pressure (:π)–A isotherms were measured at several monolayer compression ratios. The dipole moment (:m Z )–A isotherm and dm Z / dt–A isotherm were evaluated from the I–A isotherm. It was found that in the range (range 2) where the surface pressure is almost zero, m Z tends to increase at wide A as the compression ratio becomes small, whereas m Z –A isotherms coincide for various compression ratios in the range where the surface pressure increases by monolayer compression. These results indicate that the compression process of a 5CB monolayer in range 2 is a non-equilibrium process. Relaxation time (:τ)–A isotherm and dipole moment at equilibrium state (:m Z eq)–A isotherm were evaluated for the first time using the equation d(m Z –m Z eq)/ dt=-(m Z –m Z eq)/τ, which represents the dielectric relaxation process.

The optical molecular switching in single monolayers containing azobenzene derivatives deposited on solid substrates due to photo-induced cis-trans isomerization was detected by using a displacement current-measuring technique. The reversible displacement current generation was found to be sustained by the alternative applications of UV light and visible light, whereas the current generation was never initiated by the successive irradiation of UV or visible light. Finally, we concluded that the technique developed here will be used in future molecular electronic technologies.

We developed a new current-measuring technique to replace the conventional surface potential method. In this technique, two electrodes parallel to the water surface are used. One electrode (electrode 2) is placed in the water while the other (electrode 1) is suspended in the air. Both electrodes are grounded. Investigation of the physical properties of fatty acid monolayers at the air-water interface was carried out by using the technique, and the vertical component μz of the dipole moment of the acid molecules was determined. From the investigation, we found that the behaviour of molecules at the edge of the trough plays a very important role in the current measurement. Finally, it was concluded that the dipole moment of the acids determined by the present current-measuring technique, when electrode 1 was electrically shielded, is in good agreement with that determined by the conventional surface potential method.

Dielectric relaxation phenomena of 4-cyano-4′-n-alkyl-biphenyl (nCB; n = 5 or 10) monolayers at the air‐water interface have been investigated by measuring Maxwell displacement current (MDC; I)-area per molecule (A) isotherms and surface pressure (p)‐A isotherms at several monolayer compression rates. It is found that I‐A isotherms of 5CB monolayer clearly depend on molecular compression rates (a) in the range (range 2) where the surface pressure is almost zero, and that in contrast, I‐A isotherms of 10CB monolayer fluctuate in range 2. These results indicate that the monolayer compression of 5CB in range 2 is a non-equilibrium process. © 1998 Elsevier Science S.A. All rights reserved

Surface potentials of phthalocyanine derivative Langmuir–Blodgett (LB) films on metal electrodes were studied. Two kinds of phthalocyanine derivatives (C6PcCu and CuttbPc) were examined as a function of the number of deposited layers. The distributions of spatial excess charge density and electronic density of states at the metal/film interface were determined. The difference in the electrostatic phenomena observed in these two phthalocyanine LB films was attributed to the difference in the density of surface states of these two films.

Using a current-measuring technique, we investigated the abrupt change of a current observed at the liquid-to-solid phase transition in a current-area (I-A) isotherm of a fatty acid monolayer at the air/water interface. The change occurred when an electrode suspended in air was not electrically shielded. It was found both experimentally and theoretically that the behavior of the monolayer at the edge of the trough plays a very important role in the abrupt change described above.

Large polysilane spherulites have been observed. The spherulites were prepared by controlling the removal rate of solvents from poly(n-butyl-n-pentylsilane) under a xylene atmosphere. The diameter of the spherulites was greater than 0.1 mm. The structure of the spherulites was compared with that of the polysilane powder by polarized microscopy, SEM, DSC, and x-ray diffractometry. A high photoluminescence quantum yield of 0.3 was obtained for the spherulites.

On the basis of the rotational Brownian motion equation, we have analyzed the dielectric relaxation phenomena in Langmuir films, assuming that the orientational distribution of the constituent rod-like polar molecules is ruled by Boltzmann statistics. In this report, an equivalent circuit model representing the transient relaxation process by monolayer compression is presented, and the dielectric relaxation time τ of the monolayers is obtained in a generalized form. Finally, we examine the generation of Maxwell displacement current (MDC) across a 4-cyano-4′-8-alkyl-biphenyl (8CB) Langmuir film at the onset of the polar orientational phase by monolayer compression on the basis of the analysis developed here.

Anomalous transition of the dispersion parameter α was observed for the first time in poly( n-butyl- n-pentylsilane) (PBPS). Time-of-flight photocurrent (I) of PBPS was measured. The photocurrent transient was dispersive: I∝t -(1-α). The dispersion parameter α dropped around the glass transition temperature T g with increasing temperature, and became temperature-independent above T g. The mobility was described within the framework of a formalism based on disorder due to Bässler and coworkers. Both the diagonal disorder parameter σ, the variance of the density of states, and the off-diagonal disorder parameter Σ, the degree of positional disorder, decreased around T g. Nevertheless the ratio of the off-diagonal disorder parameter Σ to the diagonal disorder parameter σ was found to increase above the transition temperature. This anomalous transition is rationalized by the assumption that the off-diagonal disorder dominates charge transport in PBPS above T g.

Photoisomerization in monolayers of a novel azobenzene compound, azobenzene dendrimer, was investigated for the first time by means of the absorption spectrum and Maxwell displacement current (MDC) technique. According to the absorption spectrum, trans-to-cis conversion ratio was estimated to be approximately 10% for the third generation of azobenzene dendrimer deposited onto a glass substrate. Temperature-dependent induced charge with trans-cis isomerization was also measured by means of MDC technique, and we found that the thermal isomerization process must be taken into account for theoretical analysis of trans-to-cis photoisomerization. The temperature dependence was theoretically treated in terms of the three-state model that takes into account the stimulated thermal isomerization. As a result, we could obtain a ground state energy barrier of 0.93 eV in cis-to-trans isomerization for the first-generation of the azobenzene dendrimer (AZ-G1), and the value is in good agreement with the previous results obtained from the molecular orbital (MO) calculation which suggests that thermal cis-to-trans isomerization in AZ-G1 passes through the inversion process.

In this paper, we provide an approach for determining the dielectric relaxation time of Langmuir films by using a whole curve of Maxwell-displacement-currents (MDCs), and examine the dielectric relaxation time of 4-cyano-4'-alkyl-biphenyl (5CB) monolayers at the air-water interface.

A whole-curve method using the Maxwell displacement current (MDC) is presented for determining the dielectric relaxation time τ of floating monolayers on the water surface. The dielectric relaxation time of 4-cyano-4'-alkyl-biphenyle (5CB) monolayers was examined. It was revealed that the dielectric relaxation time τ of 5CB monolayers depends on the molecular area, and shows a good agreement with the theoretical relaxation time in the isotropic polar orientational phase, which was calculated using a rodlike columnar molecule model.

Using the Maxwell-displacement-current (MDC) measurement, the piezoelectric property of monolayers is examined. The equation representing the piezoelectric coefficient is derived in connection with the MDC. The piezoelectric coefficient of 4-cyano-4'-alkyl-biphenyle (5CB) monolayers was determined on the basis of the derived equation.