Coulomb Blockade Phenomena Research Articles

Properties of Thiol End‐Capped and Iodine‐Doped Sexithiophene Disulfide Semiconducting Polymers Bridging Nanogap Gold Electrodes

Doped photoresponsive nanodevices. A thiol end-capped and iodine-doped sexithiophene disulfide polymer is used to bridge nanogap gold electrodes via an in-situ oxidation, doping, and self-assembly method. Temperature-dependent semiconducting and photoresponsive nanodevices are formed and Coulomb-blockade phenomena are observed in these organic molecule-based nanodevices.

Coulomb interaction and transient charging of excited states in open nanosystems

We obtain and analyze the effect of electron-electron Coulomb interaction on the time dependent current flowing through a mesoscopic system connected to biased semi-infinite leads. We assume the contact is gradually switched on in time and we calculate the time dependent reduced density operator of the sample using the generalized master equation. The many-electron states (MES) of the isolated sample are derived with the exact diagonalization method. The chemical potentials of the two leads create a bias window which determines which MES are relevant to the charging and discharging of the sample and to the currents, during the transient or steady states. We discuss the contribution of the MES with fixed number of electrons N and we find that in the transient regime there are excited states more active than the ground state even for N=1. This is a dynamical signature of the Coulomb blockade phenomenon. We discuss numerical results for three sample models: short 1D chain, 2D lattice, and 2D parabolic quantum wire.

Hole-Trapping Mechanism and SILC of Dual-Layer nc-ITO Embedded ZrHfO High-k Nonvolatile Memories

MOS capacitors containing singe- and dual-layer nc-ITO embedded ZrHfO high-k gate dielectric films were fabricated and characterized for nonvolatile memory functions and reliability. The addition of the second nc-ITO into the high-k dielectric enhanced the memory function of the devices, e.g., a larger memory window, more sensitive to the gate bias voltage, and longer charge retention time. From the SILC test, the dual-layer nc-ITO sample also showed more pronounced Coulomb blockade phenomena than the single nc-ITO embedded sample, which is consistent with the hole-trapping characteristics. The novel charge storage characteristics of the dual-layer nc-ITO embedded high-k make it suitable for future memory applications.

Spin-dependent transport through interacting graphene armchair nanoribbons

We investigate spin effects in transport across fully interacting, finite-size graphene armchair nanoribbons (ACNs) contacted to collinearly spin-polarized leads. In such systems, the presence of short-range Coulomb interaction between bulk states and states localized at the ribbon ends leads to novel spin-dependent phenomena. Specifically, the total spin of the low-energy many-body states is conserved during tunneling but that of the bulk and end states is not. As a consequence, in the single-electron regime, dominated by Coulomb blockade phenomena, we find pronounced negative differential conductance features for ACNs contacted to parallel polarized leads. These features are, however, absent in an anti-parallel contact configuration, which in turn leads, within a certain gate and bias voltage region, to a negative tunneling magneto-resistance. Moreover, we analyze the changes in the transport characteristics under the influence of an external magnetic field.

Single-electron tunneling and Coulomb blockade in carbon-based quantum dots

Single-electron tunneling (SET) and Coulomb blockade (CB) phenomena have been widely observed in nanoscaled electronics and have received intense attention around the world. In the past few years, we have studied SET in carbon nanotube fragments and fullerenes by applying the so-called “Orthodox” theory [28]. As outlined in this review article, we investigated the single-electron charging and discharging process via current-voltage characteristics, gate effect, and electronic structure-related factors. Because the investigated geometric structures are three-dimensionally confined, resulting in a discrete spectrum of energy levels resembling the property of quantum dots, we evidenced the CB and Coulomb staircases in these structures. These nanostructures are sufficiently small that introducing even a single electron is sufficient to dramatically change the transport properties as a result of the charging energy associated with this extra electron. We found that the Coulomb staircases occur in the I–V characteristics only when the width of the left barrier junction is smaller than that of the right barrier junction. In this case, the transmission coefficient of the emitter junction is larger than that of the collector junction; also, occupied levels enter the bias window, thereby enhancing the tunneling extensively.

Nonvolatile Memory Characteristics with Embedded High Density Ruthenium Nanocrystals

Ruthenium (Ru) nanocrystals (NCs) embedded in SiO2 gate stacks are formed by rapid thermal annealing for the whole gate stacks and embedded in the memory structure, which is compatible with conventional CMOS technology. The devices exhibit a substantial and clockwise hysteresis in capacitance-voltage measurement. The Ru NCs exhibit high density (2 × 1012 cm−2), small size (2–4nm) and good uniformity both in spatial distribution and morphology. The charging and long-term retention performances are explained by the Coulomb Blockade phenomena and the asymmetric electron tunnel barrier between the Ru NCs and the Si substrate, respectively.

Application of Single Electron Devices Utilizing Stochastic Dynamics

Single electron devices utilizing the Coulomb blockade phenomenon have attractive features such as extreme low power consumption, one by one electron flow controllability, small device size, etc. However, besides promising applications such as the current standard and charge detection, it is not easy to apply the single electron devices to conventional computational tasks due to its stochastic operation and low amplification capability. Therefore, it is important for us to consider suitable applications of single electron devices. In this paper, we show three applications such as a noise generator, a stochastic neural network, and a charge detector employing stochastic resonance. Trough these applications, we see the advantages of single electron devices and study the direction of applications.

Interaction-induced strong localization in quantum dots

We argue that Coulomb blockade phenomena are a useful probe of the crossover to strong correlation in quantum dots. Through calculations at low density using variational and diffusion quantum Monte Carlo (up to ${r}_{s}\ensuremath{\sim}55$), we find that the addition energy shows a clear progression from features associated with shell structure to those caused by commensurability of a Wigner crystal. This crossover (which occurs near ${r}_{s}\ensuremath{\sim}20$ for spin-polarized electrons) is, then, a signature of interaction-driven localization. As the addition energy is directly measurable in Coulomb blockade conductance experiments, this provides a direct probe of localization in the low density electron gas.

Effect of ammonia adsorption on the electrical characteristics of mesoporous silicon

A detailed study of the current-voltage curves of mesoporous silicon (mesoPS) has been carried out for various dosages of ammonia, employing two different measurement configurations. The gas adsorption strongly modifies the electrical characteristics of the material, both affecting their shape and varying the conductivity values. In particular, it is found that the conductivity gap, arising from a collective Coulomb blockade phenomenon, and the electrical anisotropy of mesoPS can be gradually cancelled by NH3 molecules. Moreover, a nonmonotonic behavior of the conductivity with respect to the gas pressure is observed, in analogy with previous infrared spectroscopy results. From the analysis of such experimental findings, we propose an interaction mechanism between the mesoPS surface and ammonia, involving the screening of trapped positive charges through the dipole moment of NH3 molecules.

Electronic transport in Ta2O5 resistive switch

The authors examined the electronic transport of a solid electrolyte resistive switch. Using element analysis and the temperature dependence of its electronic transport, they deduced that the conductive path is composed of Cu metal precipitated in the solid electrolyte film by an electrochemical reaction. Furthermore, they observed Coulomb blockade phenomena at 4K when the switch was in the off state. Their observations and experimental results suggest that the metallic conductive path consists of metallic islands separated by tunneling barriers and that switching between the on and off states originates from modulation in the tunneling barriers.

High density Ru nanocrystal deposition for nonvolatile memory applications

Arrays of Ru nanocrystals 1–4nm in diameter are deposited via a hybrid chemical vapor deposition/atomic layer deposition reaction. The nanocrystal density is found to depend sensitively on the nucleating surface. A maximum density of (7–8)×1012cm−2 is achieved on Al2O3. Incorporation of these nanocrystals in floating-gate memory cells results in C-V curves that exhibit large, counterclockwise hysteresis. Leakage current analysis reveals Coulomb blockade phenomena, Frenkel-Poole emission, and space-charge-limited conduction. This analysis allows for the determination of nanocrystal size and connectivity. Charge storage converges to approximately 50% of the maximum value after two days. The corresponding loss mechanisms are discussed.

On modelling and characterization of single electron transistor

As the nanotechnology rose to the surface, single electron transistor (SET) was invented. In contrast to the well-known response of MOS current, SET current has peaks at certain gate voltages, which disappears at other gate voltages. The SET has promised to be valuable in many applications for its high speed and low power consumption. First, a comparison was drawn between different models of SET based on the orthodox theory. Such theory explains electron transport from source to drain, employing free energies, tunnel rates and coulomb blockade phenomenon, in addition to quantizing electron tunnelling. Afterwards, a simplified model was proposed to account for unnecessary lengthy calculation processes, resulting from the large number of states assumed for simulation. The proposed PSPICE simplified model was confirmed by comparing its results to the results of the available models, and it was found to agree well with them. Taking much less runtime than the available models, the proposed model can easily be used to simulate SET-based integrated circuits on PSPICE.

Room-temperature observation of a Coulomb blockade phenomenon in aluminum nanodots fabricated by an electrochemical process

An aluminum nanodot was self-organized between two electrodes using the anodization process of an aluminum microelectrode of 3μm in width. The authors observed a clear Coulomb staircase with a very large Coulomb energy of about 2eV at room temperature. This very large Coulomb energy is attributed to the device structure which depends strongly on the aluminum nanodot formation mechanism. The authors’ results indicate that a single electron transistor operating at room temperature can be fabricated at an appropriate position using both bottom-up and top-down processes.

Electronic collective transport in disordered array of C49-phase TiSi2 nanocrystals in Si

We have studied the longitudinal electronic collective transport properties in a disordered array of TiSi2 nanocrystals (with surface density of 1012cm−2) embedded in Si polycrystalline matrix as a function of temperature. The system is characterized by a high degree of disorder compared to the standard disordered nanocrystal array usually studied in the literature. Despite of this fundamental difference, we demonstrate that the theoretical models used to describe the collective electronic transport in standard systems are adequate to describe the electrical behavior of such a “nonstandard” system. In particular, we show that two different conduction regimes, separated by a crossover temperature T*, exist: at T<T* the collective electronic transport is characterized by a Coulomb blockade phenomenon (with a positive threshold voltage) and a scaling behavior characteristic of a two-dimensional transport. Above T*, at low field, a thermally activated conduction mechanism is evident, and at high field the col...

Coulomb blockade sensors based on nanostructured mesoporous silicon

Mesoporous silicon (mesoPS) is a nanosponge where Si nanocrystals are interconnected forming a disordered 3D array. The electronic characteristics of this material are particularly interesting, due to some intriguing effects, such as a huge increase of conductivity, reversible insulator-to-metal transition and n- or p-type doping of the nanocrystals, exhibited in presence of donor or acceptor molecules like NH 3 and NO 2. Here we report on the observation of a sharp conductance gap, which can be ascribed to Coulomb blockade phenomena. Moreover, we show that the width of the gap can be tuned by NO 2 molecules, so that the fabrication of highly sensitive threshold sensors is possible. Our results suggest that electrochemical etching of heavily doped Si can be used as a simple self-assembly technique for the production of Si nanocrystal arrays and for the fabrication of sensitive nanosensors.

Nanoscale fabrication of metal particles and wires using mesoporous silica templates: Electronic properties and catalytic performances in PROX reaction

Pt nanowires and nanoparticles were selectively synthesized in mesoporous silica templates FSM-16 and HMM. The wires and particles were characterized by physicochemical methods. Extracted Pt nanoparticles on HOPG show Coulomb blockade phenomena in STM/STS measurements. Pt nanowires and -particles in FSM-16 shows high catalytic activity and selectivity in preferential oxidation (PROX) of CO in H2.

Theoretical investigation of negative differential conductance regime of silicon nanocrystal single-electron devices

The current-voltage characteristics of metal-insulator-Si quantum dot (QD)-insulator-metal structures are numerically simulated to investigate the design and the possible applications of single-electron devices taking advantage of Coulomb-blockade phenomenon. The simulation technique is based on a physical description of the devices and only requires fundamental quantities of the system but no fitting parameter. One of the originality of this work lies in the accurate calculation of tunneling rates by a perturbation method, which allows us to properly include the effect of a bias voltage on the wave functions in the QD. As a consequence, we show that the bias influence on the wave function may lead to negative-differential-conductance effects depending on the design of the structure.

Spin torque in a nanomagnet coupled to noncollinear ferromagnetic electrodes

We present a detailed theory of the spin torque phenomena in an ultrasmall nanomagnet coupled to noncollinear ferromagnetic electrodes through tunneling junctions. This model system can be described by a simple microscopic model which captures many physical effects characteristic of spintronics: tunneling magnetoresistance, intrinsic and transport-induced magnetic relaxation, current-induced magnetization reversal, and spin accumulation. Treating on the same footing the magnetic and transport degrees of freedom, we arrive at a closed equation for the time evolution of the magnetization. This equation is very close to the Landau-Lifshitz-Gilbert equation used in spin valve structures. We discuss how the presence of the Coulomb blockade phenomena and the discretization of the one-body spectrum gives some additional features to the current-induced spin torque. The dynamic induced by the coupling to the electrodes can be viewed either as a spin torque or as a relaxation process. In addition to the possibility of stabilizing uniform spin precession states, we find that the system is highly hysteretic: up to three different magnetic states can be simultaneously stable in one region of the parameter space (magnetic field and bias voltage). We also discuss how the magnetoresistance can be used to provide additional information on the nonequilibrium peaks present in the nanomagnet spectroscopy experiments. This paper expands the results presented in a previous Letter [Phys. Rev. Lett. 94, 247206 (2005)].

Numerical study of turnstile operation in random-multidot-channel field-effect transistor

We have numerically studied the single-charge transfer operation in two-dimensional (2D) random-multidot-channel field-effect transistors (FETs) using orthodox theory of the Coulomb blockade phenomenon. The randomness of the multidot structure is reflected in the gate capacitance (Cg) in the equivalent circuit, embodying the dot-size disorder of the realistic devices developed in our laboratory. It was found that “turnstile operation” meaning that individual electron is transferred one by one from the source to the drain with a cycle of an alternating gate voltage can be performed in both random and homogeneous 2D multidot-channel FETs, although their equivalent circuits are significantly different from the ordinary four-junction turnstile device. By increasing the Cg randomness, some devices show that the average gate and drain bias condition (Vg0,Vd) which allows the turnstile operation is more relaxed. Consequently, the random-multidot-channel FET can work as a single-electron turnstile device.

Electron transport through the electromechanical quantum dots devices

In the nanometer structure, the island with discrete energy spectrum should be considered. The transport characteristics of an electromechanical quantum dots device at zero temperature are investigated by using Monte Carlo method. An indirect and effective method is applied to estimate the trend of the current curves, by analyzing the average electrostatic forces. The current–voltage curves show the Coulomb blockade phenomena, which is the result of the interaction between discrete levels and the island vibration.