Single-electron Charging Research Articles

Probing the Orbital Levels of Engineered Fullerenic Molecules from a Nonvolatile Memory Cell

ABSTRACTThe Coulomb blockade behavior was observed for both C60-PCBM and C70-PCBM at room temperature utilizing a nonvolatile memory cell fabricated through a liquid-transfer process. Room-temperature and low-temperature (10K) electrical characterizations verified the blockade effect was originated from both molecular energy levels and single electron charging energy. Molecular orbital energy was extracted and shown good agreement with the literature [1].The successful integration and operation of this hybrid structure signified a strong potential for molecule-based electronic device design.

Semiconducting Two‐Dimensional Graphene Nanoconstriction Arrays

The fabrication and characterization of two-dimensional nanoconstriction arrays consisting of sub-20-nm graphene constrictions and interconnecting graphene islands are reported. The arrays are fabricated in a scalable top-down fashion using self-assembled close-packed polystyrene nanospheres as lithographic templates and characterized using electron microscopy, Raman spectroscopy, and charge transport measurements. At room temperature, the arrays behave as semiconductors with a field-effect conductance modulation of up to 450 and charge mobilities of ~1 cm(2) V(-1) s(-1) . The effective bandgap of the arrays scales inversely with the nanoconstriction width, indicating that its magnitude is determined by quantum confinement in the constrictions. At low temperatures, the arrays act as semiconductors, with increasing ON/OFF conductance modulation up to ∼1000, and simultaneously act as 2D arrays of coupled Coulomb islands affected by single-electron charging events. The high conductance modulation of these nanopatterned graphene materials, combined with the scalability of the patterning approach is expected to impact thin film, flexible, and transparent semiconductor electronics.

Simulations of silicon nanocrystals embedded in oxide for nanoelectronic applications

AbstractThe purpose of this paper is to report the results of a study on single electron charging of a nanocrystal floating gate in a metal‐oxide‐semiconductor transistor. Our model is based on the geometrical and physical parameters of hypothetically spherical nanocrystals embedded in oxide (nanocrystal radius, distance between nanocrystals, oxide thickness, bulk band structure). This model can be numerically solved through a two‐dimensional finite element approach.The channel and gate electrodes are described through energy subbands, and the silicon nanocrystal through discrete energy levels. Two types of tunnel transport are considered here: indirect tunneling from the channel to the nanocrystal and then from the nanocrystal to the gate, and direct tunneling from the channel to the gate. We show that the latter type can be neglected for the current range of nanocrystal densities used for memory applications (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge exchange of multiply charged fluorine and lithium ions with Ne atoms

The charge exchange of multiply charged fluorine and lithium ions with Ne atoms in a gas jet was recorded from the line spectra in the 125 — 350 Å range, which arise from the radiative decay of the excited states of Li II — III and F III — VIII ions populated in the charge exchange. In the F III — VI ion spectra obtained from the plasma — gas interaction region we observed transitions from levels with an open (partly filled or completely open) 2s-shell, which may be populated in sequential events of single-electron charge exchange as well as in double- and many-electron charge exchange. Partial cross sections were calculated for the single-electron charge exchange of F VIII ions with Ne atoms. By way of Hartree — Fock energy level calculations it was possible to reveal resonances between the ground states of k-fold (k = 1 — 4) ionised donor atoms and the states of k-fold excited multiply charged fluorine ion resulting from the k-fold charge exchange of F VIII ions. These resonances may be responsible for the relatively large cross sections of double- and many-electron charge exchange

Interface defect-assisted single electron charging (and discharging) dynamics in Ge nanocrystals memories

The charging and discharging dynamics of Ge nanocrystal memories is measured and compared with a realistic quantum mechanical model that is able to reproduce qualitatively the overall device behavior. Quantitatively, the charging (discharging) dynamics is faster (slower) than predicted by calculations. To explain the discrepancies, we propose the quantum confined nanocrystal states are responsible for collecting the incoming electrons, but some of them are captured by defects in the nanocrystal surface. The potential created by the filled defects modify the spatial distribution of the nanocrystal wave functions, enhancing their penetration in the tunneling oxide and increasing the incoming transition rates. In the discharging process, the electrons confined in the nanocrystal states escape initially, while the ones in the defects have to be thermally excited to the nanocrystals states in order to tunnel out, slowing down the escape of the last few electrons.

Single-electron capture in collisions of proton beams with molecules of biological interest

Single-electron charge exchange from simple molecular targets of biological interest to fast ion beams is investigated using the continuum-distorted-wave-eikonal-initial-state model. Thus, this model is applied for electron capture from molecules with many orbitals in the fundamental state. The role played by the representation of the initial state is analyzed and contributions from different molecular orbitals to total cross sections are calculated. The results are shown to be in very good agreement with existing experimental data for collision energies larger than a few tens of keV.

Single Electron Charging in Optically Active Nanowire Quantum Dots

We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot.

Correlating the Nanostructure and Electronic Properties of InAs Nanowires

The electronic properties and nanostructure of InAs nanowires are correlated by creating multiple field effect transistors (FETs) on nanowires grown to have low and high defect density segments. 4.2 K carrier mobilities are approximately 4x larger in the nominally defect free segments of the wire. We also find that dark field optical intensity is correlated with the mobility, suggesting a simple route for selecting wires with a low defect density. At low temperatures, FETs fabricated on high defect density segments of InAs nanowires showed transport properties consistent with single electron charging, even on devices with low resistance ohmic contacts. The charging energies obtained suggest quantum dot formation at defects in the wires. These results reinforce the importance of controlling the defect density in order to produce high quality electrical and optical devices using InAs nanowires.

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.

Preparation of Mg-Yb alloy film by electrolysis in the molten LiCl-KCl-YbCl 3 system at low temperature

The electrochemical behavior of Yb 3+ and electrodeposition of Mg-Yb alloy film at solid magnesium cathode in the molten LiCl-KCl-YbCl 3 (2 wt.%) system at 773 K was investigated. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to explore the deposition mechanism of Yb. The reduction process of Yb 3+ is stepwise reactions which are single-electron and double-electron reversible charge transfer reactions. The speed control step was a diffusion-controlled step which is caused by concentration polarization. The microstructures of Mg-Yb alloy film were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and EPMA area analysis. A very thin Mg 2Yb alloy film (∼200 nm) was formed by potentiostatic electrolysis at −1.85 V (vs. Ag/AgCl) for 12 h. A much thicker Mg 2Yb alloy film (∼450 μm) was obtained at −2.50 V (vs. Ag/AgCl) for 2.5 h. The corrosion resistance of magnesium can be enhanced by electrochemical formation of Mg-Yb alloy film on its surface.

Cluster state entanglement of charge qubits

The cluster state is a special, highly entangled quantum state that provides a universal resource on which to perform measurement-based quantum computation. In this paper, we propose a scheme for the generation of such entanglement in a solid-state medium by the suppression of resonant tunneling of a ballistic electron by a nearby single-electron charge qubit. To investigate the viability of this scheme for the creation of cluster states, we performed numerical simulations and studied the entanglement interaction in detail. We show that high levels of entanglement can be achieved, with an average concurrence of 0.968.

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.

Electronic transport in silicon nanocrystals and nanochains

Si nanocrystals and nanochains, prepared by material synthesis, provide a means to define nanoscale devices using growth rather than lithographic techniques. Electronic transport in thin films of Si nanocrystals is influenced strongly by single-electron charging and quantum-confinement effects, and by the grain boundary regions between nanocrystals. This paper reviews electronic transport mechanisms in Si nanocrystal materials. These include thermionic emission of electrons across grain boundaries, space charge limited current, hopping transport, and single-electron charging effects. The fabrication of single-electron devices in Si nanocrystal thin films and nanochains is considered, particularly with regards to their operation at room temperature.

Single electron charging in deterministically positioned InAs/InP quantum dots

We demonstrate precise control of electron charging within a single deterministically positioned InAs/InP quantum dot emitting in the telecommunications band around 1500 nm. Photolumine-scence emission as a function of vertical electric field is used to monitor the electron number within the dot. From Stark shift spectroscopy, we extract strength and orientation for the built-in dipole moment that suggests a uniform InAs dot composition and a configuration in which the electron lies above the hole at zero electric field. The scalable gating technology we employ to electrically contact individual prepositioned quantum dots promises arrays of initialized single spins that can be used for fiber-based quantum information applications.

Escape dynamics of a few electrons in a single-electron ratchet using silicon nanowire metal-oxide-semiconductor field-effect transistor

Transport dynamics of a few electrons in a quantum dot are investigated in a single-electron ratchet using silicon nanowire metal-oxide-semiconductor field-effect transistors. Time-resolved measurements in a nanosecond regime are carried out to determine the escape times of the first, second, and third electrons from the quantum dot originally containing three electrons. The escape time strongly depends on the number of electrons due to the single-electron charging effect in the quantum dot, which makes it possible to achieve selective ejection of a desired number of electrons.

Control of Single-Electron Charging of Metallic Nanoparticles onto Amorphous Silicon Surface

Sequential single-electron charging of iron oxide nanoparticles encapsulated in oleic acid/oleyl amine envelope and deposited by the Langmuir-Blodgett technique onto Pt electrode covered with undoped hydrogenated amorphous silicon film is reported. Single-electron charging (so-called quantized double-layer charging) of nanoparticles is detected by cyclic voltammetry as current peaks and the charging effect can be switched on/off by the electric field in the surface region induced by the excess of negative/positive charged defect states in the amorphous silicon layer. The particular charge states in amorphous silicon are created by the simultaneous application of a suitable bias voltage and illumination before the measurement. The influence of charged states on the electric field in the surface region is evaluated by the finite element method. The single-electron charging is analyzed by the standard quantized double layer model as well as two weak-link junctions model. Both approaches are in accordance with experiment and confirm single-electron charging by tunnelling process at room temperature. This experiment illustrates the possibility of the creation of a voltage-controlled capacitor for nanotechnology.

Gated-charge force microscopy for imaging a surface-acoustic-wave-induced charge in a depleted one-dimensional channel

Gated-charge force microscopy (GCFM) generates images of subsurface induced charge with single-electron charge resolution and zero sensitivity to background charge, for the noninvasive study of confined quantum electronic systems. By gating the radio-frequency signal applied to transducers, which generate surface-acoustic waves (SAWs), we use GCFM to generate images of a GaAs depleted one-dimensional channel and provide insight to SAW-induced transport. Unexpected SAW-induced subsurface charge was imaged at points of high potential gradient. This suggests that electrons depopulating a SAW minimum due to a high potential gradient rapidly lose energy and interact with following SAW minima.

Database for inelastic collisions of sodium atoms with electrons, protons, and multiply charged ions

The available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d, and 4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed. In addition to existing data, electron-impact cross sections, for both excitation and ionization, have been calculated using the convergent close-coupling approach. In the case of proton-impact cross section, the database was enlarged by new atomic-orbital close-coupling calculations. Both electron-impact and proton-impact processes include excitation from the ground state and between excited states ( n = 3–5). For electron-impact, ionization from all states is also considered. In the case of proton-impact electron loss, cross sections (the sum of ionization and single-electron charge transfer) are given. Well-established analytical formulae used to fit cross sections, published by Wutte et al. and Schweinzer et al. for collisions with lithium atoms, were adapted to sodium. The “recommended cross sections” for the processes considered have been critically evaluated and fitted using the adapted analytical formulae. For each inelastic process the fit parameters determined are tabulated. We also present the assessed data in graphical form. The criteria for comprehensively evaluating the accuracy of the experimental data, theoretical calculations, and procedures used in determining the recommended cross sections are discussed.

Charge noise in single-electron transistors and charge qubits may be caused by metallic grains

We report on measurements of low frequency noise in a single electron transistor from a few Hz up to 10 MHz. Measurements were done for different bias and gate voltages, which allows us to separate noise contributions from different noise sources. We find a 1/f noise spectrum with two Lorentzians superimposed. The cut-off frequency of one of the Lorentzians varies systematically with the potential of the SET island. Our data is consistent with two single-charge fluctuators situated close to the tunnel barrier. We suggest that these are due to random charging of aluminum grains, each acting as a single electron box with tunnel coupling to one of the leads and capacitively coupled to the SET island. We are able to fit the data to our model and extract parameters for the fluctuators.

Resistance Fluctuations and Aharonov–Bohm-Type Oscillations in Antidot Arrays in the Quantum Hall Regime

Resistance fluctuation phenomenon in antidot lattices in the quantum Hall regime are studied. Magnetoresistance of finite antidot array systems in the quantum Hall plateau transition regime exhibits two types of oscillatory effect. One is the aperiodic resistance fluctuations (RFs) and the other is the Aharonov-Bohm (AB)-type oscillations. Their dependences on the magnetic field and the gate voltage are quite distinct. While the aperiodic RFs are attributed to the complex evolution of the conducting network of compressible channels, the AB-type oscillations are interpreted in terms of edge states formed around individual antidots. The self-consistent screening effect is important for the both phenomenon, whereas, the single electron charging effect plays a minor role in the present case.