Single-electron Charging Phenomena Research Articles

Measuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope

We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage $(IV)$ tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, $(61.4\ifmmode\pm\else\textpm\fi{}2.4)\text{ }\text{meV}$, and analysis of the DBTJ, show trap states $\ensuremath{\sim}1.09\text{ }\text{eV}$ below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.

Influence of Linker Molecules on Charge Transport through Self-Assembled Single-Nanoparticle Devices

We investigate electrical characteristics of single-electron electrode/nanoisland/electrode devices formed by alkanedithiol assisted self-assembly. Contrary to predictions of the orthodox model for double tunnel junction devices, we find a significant ( approximately fivefold) discrepancy in single-electron charging energies determined by Coulomb blockade (CB) voltage thresholds in current-voltage measurements versus those determined by an Arrhenius analysis of conductance in the CB region. The energies do, however, scale with particle sizes, consistent with single-electron charging phenomena. We propose that the discrepancy is caused by a multibarrier junction potential that leads to a voltage divider effect. Temperature and voltage dependent conductance measurements performed outside the blockade region are consistent with this picture. We simulated our data using a suitably modified orthodox model.

Nanosilicon for single-electron devices

This paper presents a brief overview of the physics of nanosilicon materials for single-electron device applications. We study how a nanosilicon grain and a discrete grain boundary work as a charging island and a tunnel barrier by using a point-contact transistor, which features an extremely short and narrow channel. Single-electron charging phenomena are investigated by comparing as-prepared devices and various oxidized devices. The optimization of grain and grain-boundary structural parameters is discussed for improving the Coulomb blockade characteristics and realizing room temperature device operation.

Quantum-effect and single-electron devices

In this paper, we review the current status of nanoelectronic devices based on quantum effects such as quantization of motion and interference, and those based on single electron charging phenomena in ultrasmall structures. In the first part, we discuss wave-behavior in quantum semiconductor structures, and several device structures based on quantum waveguide behavior such as stub tuners, Y-branches, and quantum ratchets. Discussion is also given of proposals for use of interference phenomena in quantum computing followed by the issue of quantum decoherence which ultimately limits utilization of quantum effects. In the second part, we discuss single electron effects such as Coulomb blockade, and associated devices such as the single electron transistor and single electron charge pumps. This is followed by an overview of some recent work focusing on Si based single electron structures. We conclude with a discussion of proposals and realizations for single-electron circuits and architectures including single electron memories, single electron logic, and single electron cellular nonlinear networks.

Single-Electron Charging Phenomena in Nano/Polycrystalline Silicon Point Contact Transistors

Single-Electron Charging Phenomena in Nano/Polycrystalline Silicon Point Contact Transistors

Room-temperature single-electron charging phenomena in large-area nanocrystal memory obtained by low-energy ion beam synthesis

We investigated the dependence of implantation dose on the charge storage characteristics of large-area n-channel metal–oxide–semiconductor field-effect transistors with 1-keV Si+-implanted gate oxides. Gate bias and time-dependent source–drain current measurements are reported. Devices implanted with 1×1016 cm−2 Si dose exhibit a continuous (trap-like) charge storage process under both static and dynamic conditions. In contrast, for 2×1016 cm−2 implanted devices, electrons are stored in Si nanocrystals in discrete units at low gate voltages, as revealed by a periodic staircase plateau of the source–drain current with a low gate voltage sweep rate, and the step-like decrease of the time-dependent monitoring of the channel current. These observations of room-temperature single-electron storage effects support the pursuit of large-area devices operating on the basis of Coulomb blockade phenomena.

Tunneling and optical spectroscopy of semiconductor nanocrystals.

The use of a combination of tunneling and optical spectroscopy to investigate the size and shape-dependent level structure and single-electron charging phenomena in semiconductor nanocrystals is reviewed. The artificial atom character of semiconductor nanocrystal quantum dots is manifested in both the discrete level structure and in the charging multiplicity of the single-electron tunneling data, revealing s and p atomic-like states. Such states can be directly imaged using scanning tunneling microscopy, providing the extent and symmetry of the envelope wavefunctions. A detailed description of the effect of the tunneling geometry on the single-electron tunneling spectra is presented. Correlation of the optical and tunneling data allows for the assignment of the level spectrum. The generality of this powerful combination is further demonstrated in the study of quantum rods that manifest the transition from zero-dimensional quantum dots to one-dimensional quantum wires.

Temperature-induced change from p to n conduction in metallofullerene nanotube peapods

Metallofullerene nanotube peapods were prepared by introducing Dy@C82 into the interior space of single-walled carbon nanotubes. Transport measurements show that the Dy@C82 molecules function as electron donors and transfer charge to the carbon nanotube host. The amount of charge transferred varies with the temperature. At room temperature, the doped nanotube shows p-type behavior as seen from the response to a back gate. As the temperature decreases, the conductance becomes n type and at T<215 K metallic behavior is observed, indicating the degenerate state by doping. Below about 75 K, single-electron charging phenomena dominate the transport and show irregular Coulomb blockade oscillation, implying that the insertion of Dy@C82 splits the tube into a series of several quantum dots.

Single-electron charging phenomena in silicon nanopillars with and without silicon nitride tunnel barriers

Electron transport in silicon nanopillars has been studied for pillars with zero, one, or two silicon nitride barrier layers of 2 nm thickness. Evidence of Coulomb blockade is presented and the role of the silicon nitride layers is discussed. Wide zero current regions are observed for some devices with two silicon nitride tunnel barriers and these are attributed to the formation of fully depleted quantum dots.

Molecular-dynamics study of single-electron charging in semiconductor wires.

A molecular-dynamics technique is applied to single-electron charging effects in semiconductor wires, and the impact of strong electron-electron correlation on the conductance is investigated. Because of the relatively low electron density in semiconductors compared to a metal, the screening length is comparable to the sample size, which requires a treatment beyond the conventional Coulomb-blockade argument using macroscopic capacitance. Based on the molecular-dynamics method, most features of the periodic conductance oscillation in the double-barrier system are reproduced, and the feasibility of this technique in single-electron charging phenomena is demonstrated. Experimental observation of an activation energy smaller than the threshold energy of the nonlinear conductance, which the normal Coulomb-blockade model cannot explain, is reproduced in the present approach. This effect is due to the strong microscopic correlation, so that this is essential to describe accurately the single-electron charging effects in semiconductor systems.