Tunneling Distance Research Articles

Ab initio study of the electrochemical H2SO4/Pt(111) interface

The co-adsorption of H(3)O(+) and/or H(2)O on SO(4)(2-)/Pt(111) has been studied on the basis of DFT calculations. Several combinations of co-adsorbates have been considered in order to determine the most stable configuration. We find an adlayer of H(3)O(+) coadsorbed with SO(4)(2-)/Pt(111) having either none or one additional H(2)O molecule. When including the effects of an external electrode potential the nature of the adlayer is modified. This is also reflected in the behavior of the electrostatic potential normal to the surface, which should be directly comparable to future distance tunneling spectroscopy measurements.

Local capacitance analysis using a modified deep level spectrometer

Abstract A bimorph-based xz scanner and an amplifier, increasing the capacitance and current measurement sensitivities 250-times and 1000-times, respectively, have been built into the cryostat of a deep level transient spectrometer. The setup renders point by point local capacitance-voltage (C–V) at 1 MHz and dc currentvoltage (I–V) measurements using a sharp tip placed into tunnelling distance from the surface of analysed semiconductor sample. The C–V measurements revealed a strong dependence on the probe/sample separation, ranging from MOS-type at small tunnelling currents to Schottky-type at currents exceeding approximately 10 pA. Marked hysteresis was observed, indicating changes of surface state occupancy. These slow states are otherwise hardly detected, since they are absent in semiconductor/metal contacts and in MOS structures they would become mostly passivated interface states. The setup enables calibrated, fixed zero level capacitance measurement. The stray capacitance is the dominant component of the measured capacitance but it can be easily discarded.

A model to describe the humplike feature observed in the accumulation branch of capacitance-voltage characteristics of MOS capacitors with oxide-hosted silicon nanoparticles

A model is proposed for description of a humplike feature that was previously observed in the lowfrequency capacitance-voltage (C-U) characteristics of MOS capacitors with oxide layers containing silicon nanoparticles. The formation of this feature is related to variation (at the frequency of measurements) of a charge accumulated in the oxide due to injection of charge carriers from electrodes and their migration by tunneling through oxide along linear chains formed by Si nanoparticles (with a certain spread of tunneling distances in the chain). The adequacy of the model is illustrated with the simple example of metal-oxidesilicon (MOS) capacitor with a planar array of Si nanoparticles in the oxide and a monopolar injection of holes from the accumulation layer of the p-type semiconductor. A C-U curve measured on an MOS capacitor with amorphous Si clusters is presented, which closely resembles curves with a two-hump feature predicted by the proposed model.

A new photoelectrochemical biosensors based on DNA conformational changes and isothermal circular strand-displacement polymerization reaction

We report a strategy for the transduction of DNA hybridization into a readily detectable photoelectrochemical signal by means of a conformational change analogous to electrochemical DNA (E-DNA) approach. To demonstrate the effect of distance change for photosensitizer to the surface of electrode on the change of photocurrent, photosensitizer Ru(bpy)2(dcbpy)2+ tagged DNA stem–loop structures were self-assembled onto a nanogold modified ITO electrode. Hybridization induced a large conformational change in DNA structure, which in turn significantly altered the electron-transfer tunneling distance between the electrode and photosensitizer. The resulting change in photocurrent was proportional to the concentration of DNA in the range of 1.0×10−10–8.0×10−9M. In order to improve the sensitivity of the photoelectrochemical biosensor, an amplified detection method based on isothermal strand displacement polymerization reaction was employed. With multiple rounds of isothermal strand replication, which led to strand displacement and constituted consecutive signal amplification, a detection limit of 9.4×10−14M target DNA was achieved.

A quantum mechanical treatment of low frequency noise in high-K NMOS transistors with ultra-thin gate dielectrics

Our paper presents a quantum mechanical treatment of low-frequency noise in scaled NMOS transistors to extend the “unified” noise model and includes remote Coulomb scattering and surface roughness – the latter is a new consideration in the theory. Our experimental work focuses on scaled NMOS devices with a composite dielectric consisting of a 0.5nm SiO2 covered with a high-K, 1.6nm HfO2 with a metal gate. In the past, Coulomb scattering was assumed to arise from trapping centers located at the Si–SiO2 interface; however, this cannot give rise to a 1/f noise spectrum. We model remote Coulomb scattering into the dielectric film as traps in these films easily lie within a tunneling distance from the interface. This approach explains the decrease in the Coulomb scattering parameter (α) as a function of gate voltage. In addition, we introduce surface roughness scattering through fluctuations in the normal electric field due to fluctuations in the free carrier density with a surface scattering parameter (β) proportional to the SPICE surface roughness parameter, θS. Good agreement is obtained between our model and experimental results for both IDS–VGS and the power spectral density, SId, characteristics in very strong inversion region where surface quantization of the 2D subbands is strong.

Electron scattering effects at physisorbed hydrogen molecules on break-junction electrodes and nanowires formation in hydrogen environment

Physisorption of hydrogen molecules on the surface of gold and other coinage metals has been studied using distance tunneling spectroscopy. We have observed that the distance dependence of the tunnel current (resistance) displays a strong N-shaped deviation from exponential behavior. Such deviations are difficult to explain within the Tersoff–Hamann approximation. We suggest the scattering of tunneling electrons by H2 molecules as an origin for the observed effect. We have found that this phenomenon is also common for strongly adsorbed organic molecules with a single anchoring group. Pulling Au, Cu and Pt nanowires at 22 K in hydrogen environment shows that the break-junction electrodes are still connected through hydrogen–metal monoatomic chains down to very low conductance values of 10−4–10−6 G0.

An Electrical Rectifier Based on Au Nanoparticle Array Fabricated Using Direct‐Write Electron Beam Lithography

AbstractClose‐packed arrays of Au nanoparticles are produced in patterned regions by electron beam (e‐beam) lithography using a highly sensitive direct–write resist, N+AuCl4−(C8H17)4Br. While the e–beam causes dewetting of the resist to nucleate Au nanoparticles, the following step of thermolysis aids particle growth and removal of the organic part. Thus formed arrays contain Au nanoparticles. Such arrays are patterned into ≈10 μm wide stripes between Au contact pads on SiO2/Si substrates to realize electrical rectification. Under forward bias, the device exhibits a threshold voltage of +4.3 V and a high current rectification ratio of 3 × 105, which are stable over many repetitive measurements. The threshold voltage of the rectifier can be reduced by applying an electric stress or by varying the electron dosage used for array formation. The nanoparticle rectifier element could be transferred onto flexible substrates such as PDMS, where the nanoparticle coupling is influenced by swelling of the substrate. Obviously, the nanoparticle size, shape, and the spacing in array are all important for the rectifier device performance. Based on the electrical measurements the mechanism of rectification is found to be due to switching of electrical conduction with applied bias, from short–distance tunneling to F–N type tunneling followed by transient filament formation.

Electrical transport properties of nanostructured ferromagnetic perovskite oxides La0.67Ca0.33MnO3 and La0.5Sr0.5CoO3 at low temperatures (5 K ⩾ T ⩾ 0.3 K) and high magnetic field

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La0.67Ca0.33MnO3 (LCMO) (with size down to 15 nm) and La0.5Sr0.5CoO3 (LSCO) (with size down to 35 nm) in the temperature range 0.3–5 K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I–V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (ΦB). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magneto-resistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the nonlinear I–V data in this temperature range, and the I–V data were found to be strongly dependent on magnetic field. We made the important observation that a gap-like feature (with magnitude ∼EC, the Coulomb charging energy) shows up in the conductance g(V ) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ⩽ 0.7 K). The gap closes when the magnetic field and temperature are increased.

In situ STM, AFM and DTS study of the interface 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate/Au(1 1 1)

The Au(111)/1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIm]FAP) interface is examined using in situ scanning tunnelling microscopy (STM), atomic force microscopy (AFM) and distance tunnelling spectroscopy (DTS). Both in situ STM and AFM results reveal that multiple ionic liquid (IL) interfacial layers form at the Au(111) electrode interface. The applied potential determines whether cations (cathodic regime) or anions (anodic regime) are preferentially adsorbed to the substrate, with stronger near surface layering detected at higher surface potentials. DTS measurements suggest that the height of the tunnelling barrier between the STM tip and the gold substrate was 1.4eV at open circuit potential (ocp). The IL double layer structure elucidated is not consistent with Stern–Gouy–Chapman model as per aqueous electrolyte systems due to the absence of a diffuse layer. Instead a capacitor-like structure is present with the potential decaying sinusoidally over the oscillatory ion arrangements.

An in Situ STM and DTS Study of the Extremely Pure [EMIM]FAP/Au(111) Interface

Herein the structure of the interfacial layer between the air- and water-stable ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM]FAP) and Au(111) is investigated using in situ scanning tunneling microscopy (STM), distance tunneling spectroscopy (DTS) and cyclic voltammetry (CV) measurements. The in situ STM measurements reveal that structured interfacial layers can be probed in both cathodic and anodic regimes at the IL/Au(111) interface. The structure of these layers is dependent on the applied electrode potential, the number of subsequent STM scans and the scan rate. Furthermore, first DTS results show that the tunneling barrier during the 1st STM scan does not seem to change significantly in the cathodic potential regime between the ocp (-0.2 V) and -2.0 V.

Molecular Length, Monolayer Density, and Charge Transport: Lessons from Al–AlOx/Alkyl–Phosphonate/Hg Junctions

A combined electronic transport-structure characterization of self-assembled monolayers (MLs) of alkyl-phosphonate (AP) chains on Al-AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that "long" APs (C14 and C16) form much denser MLs than do "short" APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed "short" AP MLs attenuate the current relatively more efficiently than those with densely packed, "long" ones. Furthermore, "long" AP ML junctions showed strong bias variation of the length decay coefficient, β, while for "short" AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed.

Incorporation of electron tunnelling phenomenon into 3D Monte Carlo simulation of electrical percolation in graphite nanoplatelet composites

The percolation threshold problem in insulating polymers filled with exfoliated conductive graphite nanoplatelets (GNPs) is re-examined in this 3D Monte Carlo simulation study. GNPs are modelled as solid discs wrapped by electrically conductive layers of certain thickness which represent half of the electron tunnelling distance. Two scenarios of ‘impenetrable’ and ‘penetrable’ GNPs are implemented in the simulations. The percolation thresholds for both scenarios are plotted versus the electron tunnelling distance for various GNP thicknesses. The assumption of successful dispersion and exfoliation, and the incorporation of the electron tunnelling phenomenon in the impenetrable simulations suggest that the simulated percolation thresholds are lower bounds for any experimental study. Finally, the simulation results are discussed and compared with other experimental studies.

Transportation Matching Design of Earth Pressure Balance Shield in Medium Distance Tunnel

Aiming at medium distance transportation of excavated spoil and constructional components in the tunnelling process, this paper combines with a water-conveyance tunnel engineering and analyzes the influence factors that affect the construction efficiency of earth pressure balance shield(EPB shield), and then works out an available scheme based on that the transport link should well match with shield tunnelling. The field condition indicates that the equipment lectotype, marshaling and transport organization in this scheme guarantee the transport capacity and continuity for tunnelling, which ensures construction progress. The idea in this design can provide similar engineering with reference.

Marcus–Hush–Chidsey theory of electron transfer to and from species bound at a non-uniform electrode surface: Theory and experiment

Two simple models of electrode surface inhomogeneity based on Marcus–Hush theory are considered; a distribution in formal potentials and a distribution in electron tunnelling distances. Cyclic voltammetry simulated using these models is compared with that simulated using Marcus–Hush theory for a flat, uniform and homogeneous electrode surface, with the two models of surface inhomogeneity yielding broadened peaks with decreased peak-currents. An edge-plane pyrolytic graphite electrode is covalently modified with ferrocene via ‘click’ chemistry and the resulting voltammetry compared with each of the three previously considered models. The distribution of formal potentials is seen to fit the experimental data most closely.

Electronic properties associated with conformational changes in azobenzene-derivative molecular junctions

The electronic properties of azobenzene-derivative ([4-(phenylazo)phenoxy]hexane-1-thiol) molecular junctions were studied in terms of their molecular configurations with vertical device structure as solid-state device platform. This molecule has two distinct molecular configurations (trans- and cis-isomer) depending on the wavelength of irradiating light, which converts from more thermodynamically stable trans-isomer to cis-isomer under UV exposure (∼365 nm) and reversible photoisomerization of cis-isomer to trans-isomer under visible light (400–500 nm). The two states showed that the conductance of cis-isomer (compact form) was higher than that of trans-isomer (extended form). From the temperature-variable electrical characterization, the main charge conduction mechanism for the two isomers was found to be tunneling. And, from the transition voltage spectroscopy analysis and ultraviolet photoelectron spectroscopy measurement, the origin of such result can be explained by reduction of molecular tunneling distance between two isomers.

Photocurrent generation by direct electron transfer using photosynthetic reactioncentres

Photosynthetic reaction centres (RCs) convert light into separated charges with nearlyperfect quantum efficiency, and have been used to generate photocurrent. Previous workhas shown that electron tunnelling rates between redox centres in proteins dependexponentially on the tunnelling distance. In this work the RC from Rhodobactersphaeroides was genetically modified with the aim of achieving the shortest tunnellingdistances yet demonstrated between the RC’s electron-accepting P site and underlyinggraphite and gold electrodes, and between the electron donor Q site and graphiteelectrodes. Opposite charges are carried to counter electrodes using mobile mediators, as indye-sensitised solar cells. Native RCs are bound to graphite surfaces throughN-(1-pyrene)iodoacetamide. Although the linker’s length is only 4 Å, the electrontransfer pathway between the Q electron donor site on the RC and the electrodesurface is still too large for current to be significant. A mutant version with theelectron acceptor P side close to the graphite surface produced currents of 15 nA cm−2 upon illumination. Direct binding of RCs to a gold surface is shown, resulting in currents of 5 nA cm−2. In both cases the current was unaffected by mediator concentration but increased withillumination, suggesting that direct electron transfer was achieved. The engineering of anRC to achieve direct electron transfer will help with long term efforts to demonstrateRC-based photovoltaic devices.

Edge Plane Pyrolytic Graphite Electrode Covalently Modified with 2‐Anthraquinonyl Groups: Theory and Experiment

An edge plane pyrolitic graphite (EPPG) electrode was modified by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N(2)(+)BF(4)(-)), giving an EPPG-AQ2-modified electrode of a surface coverage below a monolayer. Cyclic voltammograms simulated using Marcus-Hush theory for 2e(-) process assuming a uniform surface gave unrealistically low values of reorganisation energies, λ, for both electron transfer steps. Subsequently, two models of surface inhomogeneity based on Marcus-Hush theory were investigated: a distribution of formal potentials, E', and a distribution of electron tunneling distances, r(0). The simulation of cyclic voltammograms involving the distribution of formal potentials showed a better fit than the simulation with the distribution of tunneling distances. Importantly the reorganization energies used for the simulation of E' distribution were similar to the literature values for adsorbed species.

A Tunnel FET for $V_{DD}$ Scaling Below 0.6 V With a CMOS-Comparable Performance

We propose a modified structure of tunnel field-effect transistor (TFET), called the sandwich tunnel barrier FET (STBFET). STBFET has a large tunneling cross-sectional area with a tunneling distance of ~2 nm. An orientation-dependent nonlocal band-to-band tunneling (BTBT) model was employed to investigate the device characteristics. The feasibility of the STBFET realization using a complementary metal-oxide-semiconductor-compatible process flow has been shown using advanced process calibration with Monte Carlo implantation. STBFET gives a high ION, exceeding 1 mA/μm at IOFF of 0.1 pA/μm with a subthreshold swing below 40 mV/dec. The device also shows better static and dynamic performances for sub-1-V operations. STBFET shows a very good drain current saturation, which is investigated using an ab initio physics-based BTBT model. Furthermore, the simulated ION improvement is validated through analytical calculations. We have also investigated the physical root cause of the large voltage overshoot of TFET inverters. The previously reported impact of Miller capacitance is shown to be of lower importance; the space-charge buildup and its relaxation at the channel drain junction are shown to be the dominant effect of large voltage overshoot of TFETs. The STBFET are shown to have negligible voltage overshoots compared with conventional TFETs.

Analysis of Reverse Leakage Current and Breakdown Voltage in GaN and InGaN/GaN Schottky Barriers

A study of the reverse-leakage-current mechanisms in metal-organic-chemical-vapor-deposition (MOCVD)-grown GaN Schottky-barrier diodes is presented. An analysis is carried out of the characteristics of GaN Schottky diodes as well as of diodes with an InGaN surface layer to suppress the reverse leakage current and increase the breakdown voltage. The experimental results of the diodes with InGaN surface layers showed a ~ 40-V breakdown voltage increase and a significant leakage-current reduction under high reverse bias, in comparison with the design with GaN only. Such improvements are attributed to the reduced surface electric field and the increased electron tunneling distance induced by the polarization charges at the InGaN/GaN interface. We also report the effect of a high-pressure (near atmospheric pressure) MOCVD growth technique of the GaN buffer layer to further improve the leakage current and breakdown voltage.

Tunable Control over the Ionization State of Single Mn Acceptors in GaAs with Defect-Induced Band Bending

A scanning tunneling microscope was used to study the ionization of single Mn acceptors in GaAs(110). The ionization state switches when the GaAs valence band is bent across a Mn acceptor level. This produces a ringlike feature in STM images, whose diameter depends on the tunneling conditions and distance to charged arsenic vacancies. By varying the latter, we could tune the ionization switching, as well as quantify the contributions from tip- and vacancy-induced band bending.