Asymmetric Current-voltage Characteristics Research Articles

Molecular rectification: asymmetric current–voltage curves from self-assembled monolayers of a donor–(π-bridge)–acceptor dye

Thioacetic acid S-(10-{4-[2-cyano-2-(4-dicyanomethylenecyclohexa-2,5-dienylidene)ethylidene]-4H-quinolin-1-yl}decyl) ester (1a) forms self-assembled monolayers on gold with loss of the acetyl group, the thickness and contact area being 2.25 ± 0.05 nm and 0.36 ± 0.03 nm2 molecule−1 respectively. The films exhibit asymmetric current–voltage characteristics, which have been unambiguously assigned to molecular rectification.

Molecular rectification: self-assembled monolayers of a donor–(π-bridge)–acceptor chromophore connected via a truncated Au–S–(CH2)3bridge

Bis-[1-{(3-propyl)-4-(2-(4-dimethylaminophenyl)vinyl)quinolinium}]disulfide diodide interacts with gold to form self-assembled monolayers, the thickness and contact area being 2.25 ± 0.05 nm and 0.30 ± 0.03 nm2 molecule−1 respectively. The films exhibit asymmetric current–voltage characteristics with a rectification ratio of ca. 12 at ±1 V. These become symmetrical when the intramolecular charge-transfer axis is suppressed by protonation but the electrical asymmetry is restored when the films are exposed to base. Molecular rectification is intrinsic to the bridged donor–acceptor combination.

Bistable molecular conductors with a field-switchable dipole group

A class of bistable "stator-rotor" molecules is proposed, where a stationary bridge (stator) connects the two electrodes and facilitates electron transport between them. The rotor part, which has a large dipole moment, is attached to an atom of the stator via a single sigma bond. Hydrogen bonds formed between the rotor and stator make the symmetric orientation of the dipole unstable. The rotor has two potential minima with equal energy for rotation about the sigma bond. The dipole orientation, which determines the conduction state of the molecule, can be switched by an external electric field that changes the relative energy of the two potential minima. Both orientation of the rotor correspond to asymmetric current-voltage characteristics that are the reverse of each other, so they are distinguishable electrically. Such bistable stator-rotor molecules could potentially be used as parts of molecular electronic devices.

InAs/GaAs quantum dot infrared photodetector (QDIP) with double Al/sub 0.3/Ga/sub 0.7/As blocking barriers

The ten stacked self-assembled InAs/GaAs quantum dot infrared photodetectors (QDIP) with different Al/sub 0.3/Ga/sub 0.7/As barrier widths and growth temperatures were prepared. Asymmetric current-voltage (I-V) characteristics and 2/spl sim/7.5 /spl mu/m detection window were observed. Peak responsivity of 84 mA/W at -0.4 V and peak specific detectivity of 2.5/spl times/10/sup 9/ cm-Hz/sup 1/2//W at zero bias were observed at 50 K. The characteristics of polarization insensitivity over the incident light and the high background photocurrent suggest that the self-assembled QDIP can be operated at higher temperature (/spl sim/250 K) under normal incidence condition in contrast to quantum well infrared photodetector (QWIP).

Molecular rectification: characterisation of a dye sandwiched between gold electrodes

Langmuir–Blodgett (LB) films of the dye, 4-{2-[4-(N,N-dibutylaminophenyl]vinyl}-N-octadecylquinolinium octadecyl sulfate, are non-centrosymmetric and exhibit quadratic enhancement of the second-harmonic intensity with the number of layers when corrected for absorbance. The optimum effective susceptibility at 1.064 μm, monolayer thickness and refractive indices at the fundamental and harmonic frequencies are χ(2)eff = 80 pm V−1, d = 2.46 ± 0.05 nm, nω = 1.51 ± 0.02 and n2ω = 1.65 ± 0.02 respectively. In addition, metal/(LB film)/metal devices of the dye exhibit asymmetric current–voltage characteristics and, under forward bias, the current increases exponentially with the electric field. The behaviour is attributed to molecular rectification, the assignment being unambiguous for oxide-free gold electrodes.

Molecular rectification: dipole reversal in a cationic donor–(π-bridge)–acceptor dye

The cationic donor–(π-bridge)–acceptor dye, E-4-[(N-alkyl-5,6,7,8-tetrahydroisoquinolinium-5-ylidene)methyl]-N,N-dibutylaniline octadecyl sulfate is a molecular rectifier. Its Langmuir–Blodgett films exhibit asymmetric current–voltage characteristics but the dodecyl and octadecyl analogues behave differently. Rectification occurs in opposite quadrants of the I–V plot as a consequence of anion-induced dipole reversal: the aromatic form (D–π–A+–CnH2n + 1) exists when the negatively charged sulfate is adjacent to the heterocycle and the quinonoid form (D+πA′–CnH2n + 1 where A′ is the donor) when it is adjacent to the amino group. This is supported by the fact that mixed monolayers of the two dyes do not exhibit second-harmonic generation whereas films of the individual dyes have high second-order susceptibilities (e.g.χ(2)zzz = 100 pm V−1 at 1.064 µm for the dodecyl analogue). The monolayer structure is non-centrosymmetric and, therefore, the nonlinear optical behaviour can only be interpreted as above. Theoretical modelling has also verified the transition as the counterion is relocated. MNDO, AM1 and PM3 calculations indicate changes of ca. 0.08 A in the C–C bonds of the central bridging unit as well as the exocyclic C–N bond. It is the first time that anion-induced dipole reversal has been observed.

Quantum coherent transport versus diode-like effect in semiconductor-free metal/insulator structure

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current–voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al2O3 barriers.

Molecular rectification using a gold/(LB film)/gold structure

Langmuir–Blodgett (LB) films of 5-(4-dibutylaminobenzylidene)-2-octadecyl-5,6,7,8-tetrahydroisoquinolinium octadecylsulfate, co-deposited in a 1 ∶ 1 ratio with octadecanoic acid and sandwiched between gold electrodes, have shown asymmetric current–voltage characteristics with an optimum rectification ratio of 70 at ±1 V; the use of noble metal electrodes provides unambiguous evidence of molecular rectification.

Josephson and quasiparticle currents in tunneling junctions between partially dielectrized (partially gapped) superconductors with spin density waves

The current-voltage characteristics (CVC) are calculated for the Josephson, interference, and quasiparticle components of the current through a tunneling junction formed by two superconductors with spin density waves (SDW). The treatment is based on the model of partial dielectrization (gapping) of the Fermi surface and the assumption of pinning of the spin density waves. The following particular cases are studied in detail: asymmetric SDW superconductor-ordinary superconductor junctions and symmetric junctions between two identical SDW superconductors. The positions and nature of the singularities in the CVC are determined. For a symmetric contact the possibility of the existence of asymmetric CVC’s is predicted. The calculations are in qualitative agreement with the experimentally observed behavior of the CVC’s of tunneling junctions and microcontacts containing the SDW superconductor with heavy fermions URu2Si2.

Asymmetric current-voltage characteristics in type-II superconductors.

We report asymmetric current-voltage characteristics in superconducting films in a parallel magnetic field. The critical current, ${\mathit{I}}_{\mathit{c}}$, is found to differ by as much as 30% for negative and positive currents, and beyond ${\mathit{I}}_{\mathit{c}}$, the magnitude of the voltage is also different for positive and negative currents, \ensuremath{\Vert}V(+I)\ensuremath{\Vert}\ensuremath{\ne}\ensuremath{\Vert}V(-I)\ensuremath{\Vert}. The critical current difference \ensuremath{\Vert}${\mathit{I}}_{\mathit{c}+}$\ensuremath{\Vert}-\ensuremath{\Vert}${\mathit{I}}_{\mathit{c}\mathrm{\ensuremath{-}}}$\ensuremath{\Vert} has complicated behavior, changing sign as temperature and magnetic field are varied. We propose a model based on the Bean-Livingston surface barrier and inhomogeneous bulk pinning that accounts for all observed behavior.

Nonlinear Current-Voltage Characteristics of Double-Layered ZnO/R<I><SUB>x</SUB></I>O<I><SUB>y</SUB></I> (R=La, Ce, Pr, Nd) Sputtered Films

The I-V characteristics of double-layered films with ZnO/R x O y (R = La, Ce, Pr, Nd) structures fabricated by RF sputtering technique were studied. Any double-layered films show asymmetric and nonlinear current-voltage (I-V) characteristics. Especially ZnO/Pr 6 O 11 film exhibits good varistor properties (nonlinear coefficient, α = 30 and breakdown voltage, V b = 12.5 V). Nonlinear I-V characteristics are originated from Schottky barrier of 0.8 eV at the interface of ZnO/R x O y films

Tunnel diode with asymmetric spacer layers for use as microwave detector

A novel single-barrier, intraband tunnel diodes with asymmetric spacer layers has been designed and fabricated. The structural asymmetry leads to asymmetric current-voltage characteristics and makes the diodes suitable for use as zero-bias microwave detectors. The voltage sensitivity as 9.375 GHz shows a weak temperature dependence (typically less than 1 dB variation over − 40° –+ 80°C, compared with 3 dB for a zero-bias Schottky diode). The maximum microwave power handling is also superior to a germanium back diode (1 dB rolloff typically at + 10 dBm compared with –10 dBm for the Ge back diode) and similar to a zero-bias Schottky diode.

Asymmetric resonant tunnelling diodes as microwave detectors

Multilayer resonant tunnelling diodes with asymmetric current-voltage characteristics and the first measurements of such devices as microwave detectors are reported. Their performance is compared with that of conventional detectors in terms of the transfer function and its temperature dependence. In particular a variation of output voltage as low as ±0.3dB over −40°C to +80°C was found.

Electrical Properties of Au/Polyimide/Squarylium-Arachidic Acid Junctions Fabricated by the Langmuir-Blodgett Technique

We fabricated metal-insulator-semiconductor (MIS) junctions having the structure of a Au/polyimide (PI)/squarylium dyearachidic acid (SQ-C20) mixed system by the Langmuir-Blodgett (LB) technique, and then examined the electrical properties of the junctions. It was found that a PI LB film becomes a good electrical insulator even when the number of deposited layers is 5. From the capacitance-voltage (C-V) measurement, it was found that a (SQ-C20) multilayered film is depleted at the interface between the PI layer and the (SQ-C20) layer even when biasing voltage was zero millivolt because positive excess charges are displaced from the gate-Au electrode to the PI layer as deposited. The polyimide layer incorporated in the junctions makes a significant contribution to the electrical conduction, and asymmetric current-voltage (I-V) characteristics were obtained for the junctions.

Interface roughness and asymmetric current-voltage characteristics in resonant tunnelling

The current plotted against voltage for a resonant tunnelling device having one rough interface has been calculated with first-order perturbation theory. The roughness gives rise to a small asymmetry in the characteristics for the two possible directions of the current. Other possible reasons for such observed asymmetries are also discussed.

Fabrication and investigation of asymmetric current-voltage characteristics of a metal/Langmuir–Blodgett monolayer/metal structure

Metal/Langmuir–Blodgett monolayer/metal devices, where the monolayer consists of the dodecyloxyphenylcarbamate of 2-bromo, 5(2′-hydroxyethoxy) tetracyanoquinodimethan, a donor-σ-acceptor molecular system, have been constructed, and dc current-voltage (I-V) characteristics of these devices were recorded at room temperature. For biases of ±20 mV, the I-V characteristics are linear, changing to a nonlinear form for higher voltages. This nonlinear characteristic exhibited a ln(I)∝V1/4 dependence for both positive and negative voltages up to ±1 V. For positive voltages >+2.0 V for as-prepared devices and >+1.5 V for annealed devices, a ln(I)∝V3 dependence was observed, revealing a very large increase in current for small voltage changes. Such behavior was not observed for corresponding negative voltages, indicating rectification across a distance approximated by the length of an individual molecule.

Rectifying characteristics of Mg|(C16H33-Q3CNQ LB film)|Pt structures

The Mg|(LB monolayer)|Pt structures of Z-β-(1-hexadecyl-4-quinolinium)-α-cyano-4-styryldicyanomethanide (C16H33–Q3CNQ) show asymmetric current–voltage characteristics; the behaviour is attributed to the organic monolayer although whether it is due to the presence of the permanent dipole moment or molecular rectification is unclear.