Current-induced Changes Research Articles

Electrical current-induced structural changes and chemical functionalization of carbon nanotubes

We demonstrate that high current densities, combined with air exposure can slice, weld, and chemically functionalize multiwalled carbon nanotubes (CNTs) with carboxyl and allyl moieties, and alter the electrical properties. The conductance of thin film assemblies of CNTs increases by 150%, indicating that the increase in the number of low-resistance pathways caused by CNT junction formation offsets the conductance decrease expected from defect creation, surface functionalization, and fissure. Such welded high-conductance CNT networks of functionalized CNTs could be useful for device and sensor applications, and may serve as high mechanical toughness mat fillers that are amenable to integration with nanocomposite matrices.

Mechanisms for current-induced conductivity changes in a conducting polymer

A layer of polyethylene dioxythiophene:polystyrene sulfonic acid (PEDT:PSS) spun onto the surface of an inorganic semiconductor forms a highly asymmetric rectifying junction when a small current is applied and can be permanently open circuited with application of a high current density. This allows the polymer/semiconductor junction to function as a write-once-read-many-times memory element. We use x-ray photoelectron spectroscopy and temperature induced conductivity measurements to study the morphological and chemical changes responsible for the large current-induced conductivity changes. It is found that by applying a large current to the organic-inorganic semiconductor rectifying heterojunction structure Au∕PEDT:PSS∕Si, the ratio of PEDT+ to PSS− near the interface changes due to phase segregation in the presence of both high electric field (>105V∕cm) and temperature. This leads to a decrease in film conductivity by up to six orders of magnitude from its value in the conductive state.

Dopaminergic modulation of long-lasting direct current-induced cortical excitability changes in the human motor cortex

Dopaminergic mechanisms participate in N-methyl-D-aspartate (NMDA) receptor-dependent neuroplasticity, as animal experiments have shown. This may be similar in humans, where dopamine influences learning and memory. We tested the role of dopamine in human cortical neuroplasticity. Changes of excitability were induced by transcranial direct current stimulation (tDCS). D2 receptor blocking by sulpiride abolished the induction of after-effects nearly completely. D1 activation alone in the presence of D2 receptor blocking induced by co-administration of sulpiride and pergolide did not re-establish the excitability changes induced by tDCS. This suggests that D2 receptors play a major supporting role in inducing neuroplasticity in the human motor cortex. Enhancement of D2 and, to a lesser degree, D1 receptors by pergolide consolidated tDCS-generated excitability diminution until the morning after stimulation. The readiest explanation for this pattern of results is that D2 receptor activation has a consolidation-enhancing effect on tDCS-induced changes of excitability in the human cortex. The results of this study underscore the importance of the dopaminergic system for human neuroplasticity, suggest a first pharmacological add-on mechanism to prolong the excitability-diminishing effects of cathodal tDCS for up to 24 h after stimulation, and thus render the application of tDCS practicable in diseases displaying enhanced cortical excitability, e.g. migraine and epilepsy.

A comparative study of the electron- and hole-doped compositions of single crystalline Nd1−xCaxMnO3 ([formula omitted] and 0.4) by Brillouin scattering

In order to understand the differences between the nature of the electron- and the hole-doped compositions of rare earth manganates, properties of single crystals of Nd1−xCaxMnO3 with x=0.6 and 0.4 have been investigated. In the electrical resistivity measurements, the electron-doped (x=0.6) composition shows no effect of magnetic field unlike the hole-doped (x=0.4) composition, but both exhibit similar current-induced changes. Brillouin scattering studies show that the x=0.6 and 0.4 compositions show softening of the modes on cooling the samples towards charge-ordering transitions. The strength of coupling between the eg orbitals and the elastic strain is considerably larger in the hole-doped composition containing a larger population of Mn3+ ions.

The current-induced changes of resistivity and Young's modulus in the charge ordered La 0.25Ca 0.75MnO 3 manganite

We have investigated the influence of electric current on the charge-order (CO) state in La 0.25Ca 0.75MnO 3 manganite by means of in-situ measurements of the temperature dependence of Young ’s modulus E ( T ) and resistivity ρ ( T ) . We also measured simultaneously some magnetic properties. E ( T ) exhibits a minimum that may be due to the competition between short-range ferromagnetic (FM) and short-range aniferromagnetic (AFM) order in the temperature region between AFM transition T N and the charge ordering transition temperature T CO . Both E and ρ are obviously lowered with increasing current at low temperatures. The correlation of these changes in E and ρ is discussed and the resistivity drops induced by the current is suggested to originate from the lattice variation.

Switching behavior of microcrystalline silicon deposited by hot-wire chemical vapor deposition

We study current-induced changes in electrical and structural properties of a p-type microcrystalline silicon (μc-Si) films sandwiched between two metal contacts. After switching, the resistance decreases from above 1MΩ to below 1kΩ. The voltage–current characteristics show a saturated voltage region where irreversible changes occur gradually. In this region, transient temperature measurements show that film temperature rises considerably. The resistance of the switched state depends strongly on device area, suggesting that structural changes responsible for switching involve a large fraction of the device area. Micro-Raman studies show that film crystallinity increases during switching. This increase in crystallinity may indicate that the electrical switching process involves the formation of percolation paths through an amorphous incubation layer formed during film growth. This switching mechanism is quite different from the metallic filament formation observed in amorphous silicon switches.

Current driven resistance changes in low resistance x area magnetic tunnel junctions with ultra-thin Al-Ox barriers

Current induced resistance changes were investigated in magnetic tunnel junctions with ultrathin Al-Ox barriers. The nonuniformity of the insulator induced a strong coupling between the two magnetic electrodes and no magnetoresistance. However, the current-voltage (I–V) characteristics at low bias voltages were consistent with a tunnellike behavior. At larger bias voltages, they showed an abrupt change of slope that was reversible for an opposite voltage polarity. The resistance versus current (R–I) curves exhibited reversible resistance changes that reached over 100%. We interpret this as controlled electromigration in local nanoconstrictions of the barrier.

Current-induced phase control in charged-ordered Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 crystals

Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced insulator–metal (IM) transitions at low temperatures. In addition, the charge-ordering transition temperature, Tco, decreases with increasing current. The electroresistive ratio, defined as ρ0.5/ρI where ρ0.5 is the resistivity at a current of 0.5 mA and ρI the resistivity at a given applied current, I, varies markedly with temperature and the value of I. Thermal hysteresis observed in Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the IM transition indicates that the transition is first order. The current-induced changes are comparable to those induced by magnetic fields, and accordingly the IM transition in Pr0.6Ca0.4MnO3 is associated with a larger drop in resistivity.

Manipulation of phosphene thresholds by transcranial direct current stimulation in man.

Transcranial direct current stimulation (tDCS) can modulate the excitability of the human motor cortex, as revealed by the amplitude of the motor-evoked potentials (MEP). The aim of our study has been to produce localized changes of cerebral excitability of the visual cortex in the intact human by weak anodal and cathodal stimulation. For quantification of current-induced excitability changes, we measured phosphene threshold (PT) using short trains of 5-Hz transcranial magnetic stimulation (TMS) pulses in nine healthy subjects before, immediately after, 10 min, and 20 min after the end of tDCS. PTs are suggested as representative values of visual cortex excitability changes. Reduced PT was detected immediately and 10 min after the end of anodal stimulation, while cathodal stimulation resulted in an opposite effect. Our results show that tDCS elicits a transient, reversible excitability alteration of the visual cortex, thus representing a promising tool for neuroplasticity research.

Sodium channel function and the excitability of human cutaneous afferents during ischaemia.

The changes in excitability of cutaneous afferents in the median nerve of healthy subjects were compared during 13 min of ischaemia and during 13 min continuous depolarizing DC. In addition, intermittent polarizing currents were used to compensate for or to accentuate the threshold change produced by ischaemia. Measurements were made alternately of the ischaemic (or current-induced) changes in threshold, refractoriness and, in some experiments, supernormality. The strength-duration time constant (tau(SD)) was calculated from the thresholds to test stimuli of different duration. During ischaemia for 13 min, the threshold decreased steadily by 34 % over the initial 8 min, reached a plateau and increased slightly over the final few minutes. However, with continuous depolarizing DC, the threshold decreased linearly with the applied current, by 55 % with strong current ramps. Intermittent injection of hyperpolarizing DC was used to compensate for the ischaemic threshold change, but the compensating current increased progressively and did not reach a plateau as had occurred with the ischaemic threshold change. During ischaemia, tau(SD) increased to a plateau, following the threshold more closely than the current required to compensate for threshold. Refractoriness, on the other hand, increased more steeply than the applied compensating current. There were similar discrepancies in the relationships of tau(SD) and refractoriness to supernormality. The smaller-than-expected threshold change during ischaemia could result from limitations on the change in excitability produced by ischaemic metabolites acting on the gating and/or permeability of Na(+) channels. Intermittent depolarizing DC was applied during the ischaemic depolarization to determine whether it would reduce or accentuate the discrepancies noted during ischaemia alone. The extent of the threshold change was greater than with ischaemia alone, and there was a greater change in tau(SD) and a proportionately smaller change in refractoriness. It is concluded that ischaemia produces factors that can block Na(+) channels and/or alter their gating. Without these processes, the ischaemic change in threshold would be much greater than that actually recorded, probably sufficient to produce prominent ectopic impulse activity.

Electric-field-induced migration of oxygen ions in epitaxial metallic oxide films: Non-Debye relaxation and1/fnoise

We have investigated the kinetics of current-induced change of resistance and conductivity noise in thin epitaxial metallic films of ${\mathrm{LaNiO}}_{3}.$ The resistance of the film changes at a very low current (threshold current density ${J}_{\mathrm{th}}\ensuremath{\sim}{10}^{3}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2}).$ We find that the time dependence associated with the change of resistance shows a stretched-exponential-type dependence at lower temperature. Above a certain temperature scale ${T}^{*}(\ensuremath{\approx}350\mathrm{K}),$ this crosses over to a creep-type behavior. At $T\ensuremath{\sim}{T}^{*},$ the time scale shows a drastic drop in the magnitude, and a long-range diffusion sets in, which leads to an increase in the conductivity noise. The phenomenon is like a ``glass-transition'' in the random lattice of oxygen ions. We observe that the stretched exponential relaxation function, as obtained from time dependence of resistivity change, can explain the spectral structure as well as the temperature dependence of the low-frequency conductivity noise. The frequency and temperature dependence of noise could clearly identify the various processes, which had been seen in the current-stressing experiments carried out in the time domain. This establishes a quantitative link between the dynamics of current-induced resistivity changes and the conductivity noise. Both the phenomena are direct consequences of the low-frequency dynamics associated with the migration of the oxygen ions. Though done in the specific context of oxide films (used in oxide electronics), this observation has a generic aspect, and the treatments developed here can be used for establishing a quantitative link between electromigration current stressing and the conductivity noise in other metallic interconnects as well.

Point-contact studies of current-controlled domain switching in magnetic multilayers

We present measurements demonstrating current-induced magnetic domain switching, and also other magnetic excitations, in point-contact devices containing alternating ferromagnetic (F) and noble metal (N) layers, for perpendicular currents ∼109 A/cm2. For F/N/F trilayers in which one F layer is much thinner than the other, we can controllably switch the magnetic moments in the two F layers parallel with a current bias of one sign, and switch them antiparallel with a reversed current. For thicker magnetic films, and for thin films in the presence of a saturating magnetic field, we observe nonhysteretic current-induced changes in resistance, which can be understood as current-induced spin-wave excitations. These observations are in agreement with a model of current-induced magnetic reorientations caused by local exchange forces between conduction electrons and the magnetic moments.

Nanoconstriction microscopy of the giant magnetoresistance in cobalt/copper spin valves

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

Energy Dependence of Defects in a-Si:H Solar Cells During Degradation and Annealing Processes

ABSTRACTIn this work we report on the effect of current-induced degradation and annealing on p-i-n amorphous silicon solar cells. Current-voltage curves and capacitance measurements under forward bias have been used to monitor the current-induced changes as a function of time. We found that the recovery rate increases with the annealing current, while the stabilized value of efficiency decreases. Comparison of short circuit current and capacitance evolution suggests that defect kinetics in the electronic gap occurs in a different way during degradation and annealing. This behavior can be modeled assuming a faster annealing of defects closest to the extended band and a slower annealing of mid-gap defects.

Dynamic drive of semiconductor injection lasers compensating thermal drift

Pulse-driven injection lasers exhibit generally high tuning rates, which are thermally produced. To bring about high precision resolution power in spectroscopic application, the tuning rate should be reduced. In this paper, it is proposed to do this by current-induced changes in the refractive index of the laser resonator.

Effects of nonuniform current injection on the spectral dynamics of multisection distributed feedback lasers

The effects of nonuniform current injection on the linewidth enhancement factor (α) and the adiabatic chirp in a multielectrode distributed feedback laser are measured through a FM/AM modulation technique. This technique measures the adiabatic chirp due solely to carrier density effects and excludes the contributions of current induced temperature changes. The effects are shown to be a function of the pumping ratio between two active sections in the laser. The adiabatic chirp is enhanced by as much as a factor of 3 and the α parameter is reduced from a value of approximately 4 to 2.

Barium- or quinine-induced depolarization activates K+, Na+ and cationic conductances in frog proximal tubular cells.

1. Frog proximal tubular cells were fused into giant cells. We measured membrane potential (Vm), its changes (delta Vm), and current-induced voltage changes (delta psi) in single cells, during control and experimental states. Each cell served as its own control. 2. In the presence of a physiological Ringer solution, the transference number for potassium (tK) was 0.50. Barium (3 mM) reduced membrane conductance (Gm) by 50%; low-Cl- solutions and low-Na+ solutions also diminished Gm, by 52 and 30%, respectively. The association of barium and low-NaCl solutions decreased Gm to approximately 38% of control, indicating that the impermeant substitute of a physiological ion may interact with other pathways; alternatively, blockade of steady-state conductances may activate physiologically silent processes. 3. In an attempt to enhance the contribution of the partial K+ conductance (GK) to Gm, fused cells were exposed to low-Cl- solutions, containing in addition 0.1 mM-methazolamide, to inhibit the rheogenic Na(+)-HCO3-symport, and 1 microM-amiloride, to block Na+ conductance (GNa). tK went up to 0.83. 4. The high tK preparation was challenged with barium (3 mM) or quinine (Quin, 1 mM). These blockers produced large depolarizations (approximately 60 mV), however, although Gm decreased along early- and mid-depolarization, Gm plateaued and eventually it increased with larger and larger depolarization. 5. Depolarization-associated increase in Gm reflects activation of other conductances. These are Na+, cationic, and K+ conductance(s) poorly sensitive to quinine or barium. In the presence of Ba(2+)- or Quin-induced depolarization, injection of depolarizing current produces delayed increase in conductance. 6. Depolarization-induced activation of cationic conductance (Gcat) and GNa results in enlargement of the K+ electrochemical potential difference, to about 70 mV; this difference allows recycling of K+ ions outwards, since a GK is still detected and may contribute up to 38% of the total conductance.

Direct evidence for a basolateral membrane Cl- conductance in toad retinal pigment epithelium.

There is now evidence that a Cl- conductance on the basal membrane of the retinal pigment epithelium (RPE) is involved in the generation of both the fast oscillation and the light peak of the direct-current electroretinogram as well as being critical for transepithelial fluid and salt movement. In the present study, we characterized the basolateral membrane Cl- conductance of an in vitro preparation of toad RPE-choroid using conventional and Cl(-)-selective microelectrodes. Under control conditions, the potential across the apical (Vap) and basal (Vba) membranes averaged -60 +/- 2 and -45 +/- 2 mV, respectively (n = 40). Intracellular Cl- activity (aiCl = 20 +/- 1 mM) was distributed above equilibrium across both membranes, consistent with active accumulation of Cl-. A sixfold decrease in Cl- in the basal bath depolarized Vba by 12 +/- 1 mV (n = 17) and increased the apparent basal membrane resistance. By sequential measurement of aiCl and subepithelial Cl- activity during a step decrease in basal Cl-, we constructed the change in Cl- equilibrium potential (ECl) across the basal membrane. Estimation of the change in basal membrane electromotive force during the change in ECl gave an average value for the Cl- transference number (TCl) of 0.45. Further evidence for a Cl- conductance was obtained by measuring changes in aiCl induced by transepithelial current. Depolarizing Vba elevated aiCl, whereas hyperpolarizing Vba had the opposite effect, consistent with conductive Cl- movement across the basal membrane. Both the amplitude of the Cl- diffusion potential and the current-induced changes in aiCl were reduced by basal perfusion with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (250-500 microM), a blocker of Cl- channels in some epithelia.

A method to increase the sensitivity of temperature measurements of current-carrying microelectronic components

The spatially resolved measurement of current-induced temperature changes in current-carrying microelectronic components is described theoretically and experimentally. A method to increase the sensitivity of such measurements in comparison with thermoreflectance is explained.

Electrotonic current spread in colonic smooth muscle.

Current-induced changes in the membrane potential (electrotonic potentials) were measured intracellularly. The electrotonic potentials were seen to decay exponentially over many cells, suggesting electrotonic current spread. The characteristics of the electrotonic current spread were used to determine passive membrane properties of both circular and longitudinal muscle cells of human and dog colon. Electrotonic current spread was first determined along the long axes of the cells. The space constant of the circular muscle of human colon was 2.14 mm and that of the longitudinal muscle was 1.63 mm. The space constants for the dog colon were similar. The value for the time constant of dog colon circular muscle was 160 ms, whereas much higher time constants, averaging between 500 and 800 ms, were recorded from dog longitudinal muscle and both human colon muscle layers. These data suggest good electrotonic coupling in all tissues studied, along the long axes of the cells. They further suggest a relatively high membrane resistance and junctional resistance in the longitudinal muscle. Electrotonic coupling along the short axes of circular muscle cells, along the long axis of the colon, was studied in the dog. The space constant was 0.43 mm, suggesting a relatively high resistance to current flow along the short axes of the cells. In addition, along the short axes of the cells from the submucosa to the myenteric plexus side (i.e., in radial direction) a gradient was observed in resting membrane potential, slow-wave amplitude, and rate of rise of the slow-wave upstroke.(ABSTRACT TRUNCATED AT 250 WORDS)