Rhombic Coulomb diamonds are clearly observed in a chemically anchored Au nanoparticle single-electron transistor. The stability diagrams show stable Coulomb blockade phenomena and agree with the theoretical curve calculated using the orthodox model. The resistances and capacitances of the double-barrier tunneling junctions between the source electrode and the Au core (R1 and C1, respectively), and those between the Au core and the drain electrode (R2 and C2, respectively), are evaluated as 4.5 MΩ, 1.4 aF, 4.8 MΩ, and 1.3 aF, respectively. This is determined by fitting the theoretical curve against the experimental Coulomb staircases. Two-methylene-group short octanedithiols (C8S2) in a C8S2/hexanethiol (C6S) mixed self-assembled monolayer is concluded to chemically anchor the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes even when the Au nanoparticle is protected by decanethiol (C10S). This is because the R1 value is identical to that of R2 and corresponds to the tunneling resistances of the octanedithiol chemically bonded with the Au core and the Au electrodes. The dependence of the Coulomb diamond shapes on the tunneling resistance ratio (R1/R2) is also discussed, especially in the case of the rhombic Coulomb diamonds. Rhombic Coulomb diamonds result from chemical anchoring of the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes.

In this review, we describe recent progress made in the study of nanoparticles characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Basic principles of STM measurements and single-electron tunneling phenomena through a single NP are summarized. We highlight the results of electrical and photonic properties on NPs studied by STM and STS. Because nanoparticles are single-digit nanometre in diameter, a single-electron transport on individual nanoparticles such as Coulomb blockade and resonant tunneling through discrete energy levels are investigated. Photon-emission from NPs is also introduced based on STM measurements. Novel single-nanoparticle functions such as stochastic blinking and one-write erasing behaviours are presented. This review provides an overview of nanoparticle characterization methods based on STM and STS that include the detailed understanding of the electrical and photonics properties of nanoparticles.

A simultaneous fabrication process of multiple nanogap electrodes at desired gap separations by the technique of molecular ruler electroless gold plating (MoREP) is reported. Initial gold nanogap electrodes with a gap separation of 22 nm were immersed into MoREP solutions consisting of chloroauric acid, surfactant molecules of alkyltrimethylammonium bromide (CnTAB, n = 12–18) and ascorbic acid as a reducing agent. The electroless plating locally self-terminates between the gap when the surfactant molecules physisorbed to the surface of one electrode interdigitate with the ones of the opposite electrode. The mean nanogap separation correlated with the alkyl chain length, and can be controlled between 2.5 ± 0.6 and 3.3 ± 0.8 nm by choosing the alkyl chain length of the surfactant molecules (C12–C18). A double-gate single-electron transistor (SET) was chemically assembled by introducing chemically a synthesized gold nanoparticle into the MoREP nanogap electrodes, and showed stable Coulomb diamonds under application of both gate voltages.

Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their applications toward single molecule-based devices requires a fundamental knowledge of single molecular conductance between tribenzosubporphines and the STM metal tip. We utilized a tungsten (W) STM tip to investigate the electronic properties of B-(5-mercaptopentoxy)tribenzosubporphine 1 at the single molecular level. B-(5-mercaptopentoxy)-tribenzosubporphine 1 was anchored to the Au(111) surface via reaction with 1-heptanethiol linkers that were preorganized as a self-assembled monolayer (C7S SAM) on the Au(111) substrate. This arrangement ensured that 1 was electronically decoupled from the metal surface. Differential conductance (dI/dV – V) measurements with the bare W tip exhibited a broad gap region of low conductance and three distinct responses at 2.4,–1.3, and −2.1 V. Bias-voltage-dependent STM imaging of 1 at 65 K displayed a triangle shape at −2.1 < V < −1.3 V and a circle shape at V < −2.1 V, reflecting its HOMO and HOMO–1, respectively. In addition, different conductance behaviors were reproducibly observed, which has been ascribed to the adsorption of a tribenzosubporphine-cation on the W tip. When using a W tip doped with preadsorbed tribenzosubporphine-cation, negative differential resistance (NDR) phenomena were clearly observed in a reproducible manner with a peak-to-valley ratio of 2.6, a value confirmed by spatial mapping conductance measurements. Collectively, the observed NDR phenomena have been attributed to effective molecular resonant tunneling between a neutral tribenzosubporphine anchored to the metal surface and a tribenzosubporphine cation adsorbed on a W tip.

Robust nanogap electrodes for nanodevices with a separation of 3.0 ± 1.7 nm were simultaneously mass-produced at a yield of 90% by a combination of electron beam lithography (EBL) and electroless gold plating (EGP). Nanogap electrodes demonstrated their robustness as they maintained their structure unchanged up to temperatures of 170 °C, during the isotropic oxygen plasma ashing removal of the amorphous carbon overlayer resulting from scanning electron microscopy observations, therefore maintaining their surface reactivity for EGP and formation of a self-assembled monolayer. A gold layer grows over the electrode surface during EGP, narrowing the separation between the electrodes; growth stops around 3 nm due to a self-termination phenomenon. This is the main factor in the high yield and reproducibility of the EGP process because it prevents contact between the electrodes. A 90% yield is achieved by also controlling the etching and physisorption of gold clusters, which is accomplished by reduction of triiodide ions and heat treatment of the EGP solution, respectively. A mixed self-assembled monolayer of octanethiol and decanedithiol can be formed at the surface of the nanogap electrodes after the oxygen plasma treatment, and decanethiol-protected Au nanoparticles were chemisorbed between the self-terminated nanogap electrodes via decanedithiol. Chemically assembled single-electron transistors based on the nanogap electrodes exhibit ideal, stable, and reproducible Coulomb diamonds.

Ideal discrete energy levels in synthesized Au nanoparticles (6.2 ± 0.8 nm) for a chemically assembled single-electron transistor (SET) are demonstrated at 300 mK. The spatial structure of the double-gate SET is determined by two gate and drain voltages dependence of the stability diagram, and electron transport to the Coulomb box of a single, nearby Coulomb island of Au nanoparticles is detected by the SET. The SET exhibits discrete energy levels, and the excited energy level spacing of the Coulomb island is evaluated as 0.73 meV, which well corresponds to the expected theoretical value. The discrete energy levels show magnetic field evolution with the Zeeman effect and dependence on the odd–even electron number of a single Au nanoparticle.Keywords: Au nanoparticle; chemical assembly; discrete energy level; parity; single-electron transistor; Zeeman effect

Double-gate single-electron transistors (SETs) were fabricated by chemical assembling using electroless gold-plated nanogap electrodes and chemisorbed chemically synthesized gold nanoparticles. The fabricated SET showed periodic and stable Coulomb oscillations under application of voltages of both gates. The sole SET also exhibited all two-input logic operations—XOR, XNOR, NAND, OR, NOR, and AND—with an on/off ratio of 102. This demonstrates the potential of chemical assembling to give highly stable SETs exhibiting all logic operations.Keywords: bottom-up process; logic operation; nanogap electrode; nanoparticle; self-assembly; single-electron transistor

The interface trap level in top-contact pentacene thin-film transistors (TFTs) is obtained by displacement current measurement (DCM) during device operation at temperatures ranging from 260 to 340 K. The carrier injection voltage at the source electrode (VinjVinj) was measured from the displacement current and found to be independent of temperature, suggesting an Ohmic contact at the Au source electrode/pentacene interface. On the contrary, the threshold voltage (VthVth) depended on the temperature. The interface trap level at the pentacene/gate dielectric interface was estimated as 36 meV from the temperature dependence of the interface trap density. By using the DCM method, it becomes possible to demonstrate the carrier injection and transport properties of top-contact pentacene TFTs.

We identified the orientation of individual Lu@C82 molecules on alkanethiol self-assembled monolayers (SAMs) by scanning tunneling microscopy (STM) at a molecular resolution. STM images of Lu@C82 on alkanethiol SAMs at 65 K showed a striped structure corresponding to the molecular orbitals of the Lu@C82 molecule, suggesting that thermal rotation of Lu@C82 on alkanethiol SAMs is prevented at 65 K. By comparing these molecular-resolution STM images with Kohn−Sham molecular orbitals of Lu@C82 calculated by density functional theory (DFT), we identified the molecular orientation of Lu@C82. Spatial mapping of the differential conductance on individual Lu@C82 molecules revealed that the local conductivity within a molecule became large around the Lu atom at a negative sample bias voltage. From spatial mapping of the differential conductance measurements, we also evaluated the HOMO−LUMO gap of Lu@C82 to be 0.47 eV. From the results of the spatial mapping of the differential conductance and DFT calculations, the locally high conductivity around the Lu atom was attributed to the HOMO-2 level orbital concentrated on the Lu atom and its six nearest C atoms at 0.055 eV below the HOMO level. We demonstrated changes in the molecular orientation of Lu@C82 by applying a high electric field (about 1 × 107 V/cm) with a large tunneling current (1.5 nA).

We measured the surface potential of 1,10-decanedithiol (C10S2) molecules inserted into 1-octanethiol (C8S) self-assembled monolayers (SAMs) using Kelvin-probe force microscopy (KFM) with noncontact atomic force microscopy (NC-AFM) under ultrahigh-vacuum (UHV) conditions. C8S SAMs on Au(111) were used as host matrices for C10S2 molecular insertion. Molecular insertion was used to orient the inserted C10S2 molecules to form a Au(111) surface−thiol bond at one end, whereas the thiol termini at the other end protruded from the C8S SAMs. The histograms of the surface potential images of the mixed C10S2:C8S SAMs exhibited two Gaussian peaks; however, the histograms of the surface potential images of the C8S SAMs exhibited only one peak. The surface fractions of C10S2 molecules in the mixed C10S2:C8S SAMs were evaluated from the Gaussian peaks. The surface potential of the inserted C10S2 molecules was 11 mV higher than that of the host C8S SAMs. The dipole moment difference between C10S2 molecules and C8S molecules was evaluated as 16 mD.

In this review, we describe recent progress made in the study of nanoparticles characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Basic principles of STM measurements and single-electron tunneling phenomena through a single NP are summarized. We highlight the results of electrical and photonic properties on NPs studied by STM and STS. Because nanoparticles are single-digit nanometre in diameter, a single-electron transport on individual nanoparticles such as Coulomb blockade and resonant tunneling through discrete energy levels are investigated. Photon-emission from NPs is also introduced based on STM measurements. Novel single-nanoparticle functions such as stochastic blinking and one-write erasing behaviours are presented. This review provides an overview of nanoparticle characterization methods based on STM and STS that include the detailed understanding of the electrical and photonics properties of nanoparticles.