Surface redox induced bipolar switching of transition metal oxide films examined by scanning probe microscopy

The bipolar resistive switching mechanisms of a p-type NiO film and n-type TiO₂ film were examined using local probe-based measurements. Scanning probe-based current–voltage (I–V) sweeps and surface potential/current maps obtained after the application of dc bias suggested that resistive switching is caused mainly by the surface redox reactions involving oxygen ions at the tip/oxide interface. This explanation can be applied generally to both p-type and n-type conducting resistive switching films. The contribution of oxygen migration to resistive switching was also observed indirectly, but only in the cases where the tip was in (quasi-) Ohmic contact with the oxide.     

Anomalous vortex dynamics in

We report on measurements of current–voltage (I–V) characteristics for YNi₂B₂C single crystals with weak pinning in various fields at 7.6 K. We find nonmonotonic, N-shaped I–V curves in a certain field region deep in the vortex solid phase. This behavior is anomalous, since there exists an intermediate I region where flow voltage V shows a decrease with increasing I (a driving force). While the exact nature remains unknown, this phenomenon suggests vortex motion (driving I) induced pinning.     

Resistance-switching properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films with Ag–Al alloy top electrodes

A novel Ag–Al alloy electrode has been prepared on the La_0.67Ca_0.33MnO₃ (LCMO) film grown by pulsed laser deposition, with the aim to improve its resistance-switching properties. Nonlinear, asymmetric, and hysteretic current–voltage characteristics and reversible polarity-dependent switching properties are achieved in the Ag–Al alloy/LCMO/Pt structure. Detailed current–voltage characteristics analysis indicates that the resistance-switching behavior can be well explained by the mechanism of trap-controlled space charge limited conduction at the Ag–Al alloy/LCMO interface. The LCMO film with an Ag–Al alloy top electrode exhibits much better resistance-switching properties than that with an Al top electrode, including the shorter switching time and more stable switching process, demonstrating that the Ag–Al alloy electrode is a promising electrode materials of manganite films for resistance random access memory applications.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Effect of sorbitol doping in PEDOT:PSS on the electrical performance of organic photovoltaic devices

Organic photovoltaic cells have important advantages, such as low cost and mechanical flexibility. The conducting polymer poly(3,4 ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as an interfacial layer or a polymer electrode in polymer electronic devices, such as photovoltaic devices and light-emitting diodes. In this report, we discuss the direct current (DC) conductivity of PEDOT:PSS films containing various weight ratios of sorbitol dopant. The work function is shown to steadily decrease with increasing dopant content. With different dopant contents, illuminated current–voltage photovoltaic characteristics were observed. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed that the work function of the PEDOT:PSS was affected by its sorbitol content. The morphologies of the doped PEDOT:PSS films were characterized by atomic force microscopy (AFM). For the device fabrication, we made organic photovoltaic cells by a spin-coating process and Al deposition by thermal evaporation. The sorbitol dopant is able to improve the efficiency of the device.     

掺杂PEDOT ∶PSS对聚合物太阳能电池性能影响的研究

研究了掺杂后poly(3,4-ethylene dioxythiophene):poly(styrenesulphonic acid)(PEDOT ∶PSS)电导率的变化以及掺杂PEDOT ∶PSS薄膜对聚合物太阳能电池器件性能的影响. 实验发现,向PEDOT ∶PSS中掺入极性溶剂二甲基亚砜(DMSO)明显提高了薄膜的电导率,掺杂后的电导率最大值达到1.25 S/cm,比未掺杂时提高了3个数量级. 将掺杂的PEDOT ∶PSS薄膜作为缓冲层应用于聚合物电池 (ITO/PEDOT ∶PSS/P3HT ∶PCBM/LiF/Al) 中,发现高电导率的PEDOT ∶PSS降低了器件的串联电阻,增加了器件的短路电流,从而提高了器件的性能. 最好的聚合物太阳能电池在100 mW/cm²的光照下,开路电压(V_oc)为0.63 V,短路电流密度(J_sc)为11.09 mA·cm^-2,填充因子(FF)为63.7%,能量转换效率(η)达到4.45%. 关键词： PEDOT ∶PSS 电导率 聚合物太阳能电池 能量转换效率     

Resistance switching effect in LaAlO3/Nb-doped SrTiO3 heterostructure

The authors report on the fabrication and electronic transport property of LaAlO₃/Nb-doped SrTiO₃ heterostructure. The current–voltage curves of this heterostructure show hysteresis and a remarkable resistance switching behavior, which increase dramatically with decreasing temperature. Multiresistance states were realized by voltage pulses with different amplitudes and polarities and the ratio of the electrical pulse induced resistance change is larger than 10⁴. More interestingly, the relaxation of junction current after switching follows the Curie–von Schweidler law J∝t ⁻ⁿ with an exponential increase of n with temperature. The results were discussed in terms of the trap-controlled space charge limited conduction process via defects near the interface of the heterostructure.     

Formation of Schottky-type metal/SrTiO3 junctions and their resistive properties

Motivated by the successful use of strontium titanate with different doping metals for memory cells on the basis of resistive switching and the recent findings on the major importance of oxygen vacancy redistribution in this compound, the present work shows the possibility of a non-volatile resistance change memory based on vacancy-doped SrTiO₃. The formation of corresponding metal/SrTiO_3−δ junctions (δ>0) in an electric field will be discussed as well as the switching between ohmic and Schottky-type contact behavior. A notable hysteresis in the current–voltage characteristics is used to carry out Write, Read, and Erase operations exemplifying the memory cell properties of such junctions. But whereas the electric field-induced formation of Schottky-type junctions is explainable by oxygen vacancy redistribution, the resistive switching needs to be discussed in terms of vacancies serving as electron trap states at the metal/oxide interface.     

Electric-field induced ion-leveraged metal–insulator transition in conducting polymer-based field effect devices

The field effect devices prepared completely from conducting polymers, especially poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS), were studied. Normally in a conductive “on” state, the transistor-like device has a transition to a substantially less conductive “off” state at an applied positive gate voltage, typically ∼15–25 V. The current ratio I_off/I_on can exceed 10⁻⁴ at room temperature. We have found that the field effect is strongly temperature dependent and is substantially reduced upon decreasing the temperature by only a 10 °C. This loss of current reduction upon application of a gate voltage is not due to the temperature dependence of the electrical conductivity of polymers of which the devices are made. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage, though with an increased activation energy, T₀. We suggest that the conducting polymer is near the metal–insulator transition and that the field effect in the device is related to the electric field modulating this transition in the region underneath the gate electrode. The transition is controlled and leveraged by ion motion. The time dynamics of the current with the gate modulation strongly supports our conjecture. We demonstrate the generality of the phenomena by presenting similar results for devices fabricated from the conducting polypyrrole doped with Cl.     

Effect of PVAc dispersal into PVA-NH4SCN polymer electrolyte

The present paper deals with ion transport studies on a new proton conducting composite polymer electrolyte — (PVA_x:NH₄SCN)_y:PVAc system. Complexation and morphology of the composite electrolyte films are discussed on the basis of X-ray diffraction and differential scanning calorimetry data. Coulometry and transient ionic current measurements revealed charge transport through protons. The maximum ion conductivity was found to be 7.4·10⁻⁴ S·cm⁻¹ for the composition: x=0.15, y=0.12. The observed conductivity behaviour is correlated to the morphology of the films. The temperature dependence of the electrical conductivity exhibits Arrhenius characteristics in two different temperature ranges separated by a plateau region related to morphological changes occurring in the electrolyte.     

S-shaped current–voltage characteristics of polymer composite films containing graphene and graphene oxide particles

The resistive switching effects in composite films containing polyfunctional polymers, such as derivatives of carbazole (PVK), fluorene (PFD), and polyvinyl chloride (PVC), and also graphene particles (Gr) and graphene oxide (GO), the concentration of which in the polymer matrices varied in the range from 1 to 3 wt % corresponding to the percolation threshold in such systems, have been studied. The analysis of the elemental composition of the investigated composites by means of X-ray photoelectron spectroscopy have shown that the oxidation degree of Gr in GO is about 9 to 10%. It has been established that a sharp conductivity jump characterized by S-shaped current-voltage curves and the presence of their hysteresis occurs upon applying a voltage pulse to the Au/PVK (PFD; PVC): Gr (GO)/ITO/PET structures, where ITO is indium tin oxide, and PET is poly(ethylene terephthalate), with the switching time, t, in the range from 1 to 30 μs. The observed effects are attributed to the influence of redox reactions taking place on the Gr and GO particles enclosed in the polymer matrix, and the additional influence of thermomechanical properties of the polymer constituent of the matrix.

     

Enhanced bipolar resistive switching of HfO2 with a Ti interlayer

The characteristics of resistive switching of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks on SiO₂/Si substrates were investigated and compared to TiN/HfO₂/Pt/Ti stacks in order to study Ti interlayer effects on resistive switching. The Ti interlayers were deposited in situ during the reactive sputtering of HfO₂ films. The current–voltage measurements showed that the Ti interlayers enhanced the memory window but reduced the endurance of SET/RESET operations. The energy filtered images by TEM showed asymmetric oxygen accumulation at the Ti/HfO _x interfaces. Subsequent heat treatment improved the endurance of SET/RESET operation of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks.     

Electrical and thermal properties of Ge40S60 films

Films of the composition Ge₄₀S₆₀ have been studied in the temperature range of 313–423 K for electrical conductivity, and 293–373 K for thermal conductivity. The dc conductivity results indicate a single value activation energy of 0.863 eV for the conductivity in the applied temperature range. The thermal conductivity coefficient increases linearly with temperature at a thickness of d=0.311 cm. It was found that the investigated samples show a memory effect. The threshold switching voltage was found to increase linearly with film thickness. Moreover, the threshold voltage decreases exponentially with temperature. The data are analysed using a thermal model for the switching process.     

AC conductivity and relaxation behavior in ion conducting polymer nanocomposite

We report the ac conductivity and relaxation behavior analysis for a heterogeneous polymer–clay nanocomposite (PNC) having composition (polyacrylonitrile)₈LiCF₃SO₃ + x wt.% dodecylamine modified montmorillonite. Charge transport behavior in an ionically conducting PNC has been analyzed systematically and correlated with the macroscopic parameters like polymer glass transition temperature and available free mobile charge carriers. Intercalation of cation coordinated polymer into the nanometric clay channels has been confirmed by high-resolution transmission electron microscopy. The electrical properties of the intercalated PNC films have been studied using complex impedance/admittance spectroscopy. Excellent correlation of relaxation behavior with polymer glass transition temperature (T _g) confirmed the objectives of the work. An analysis of dielectric relaxation indicates that PNC films are lossy when compared with polymer–salt film. This result is a direct outcome of faster ion dynamics leading to strong electrode polarization effect due to the accumulation of charge carriers at the interface.     

Photovoltaic activity in a ZnSe/PEO–chitosan blend electrolyte junction

Thin films of ZnSe and PEO–chitosan blend polymer doped with NH₄I and iodine crystals were prepared to form the two sides of a semiconductor electrolyte junction. ZnSe was electrodeposited on indium tin oxide (ITO) conducting glass. The polymer is a blend of 50 wt% chitosan and 50 wt% polyethylene oxide. The polymer blend was complexed with ammonium iodide (NH₄I), and some iodine crystals were added to the polymer–NH₄I solution to provide the I⁻/I³⁻redox couple. The room temperature ionic conductivity of the polymer electrolyte is 4.32 × 10⁻⁶ S/cm. The polymer film was sandwiched between the ZnSe semiconductor and an ITO glass to form a ZnSe/polymer electrolyte/ITO photovoltaic cell. The open circuit voltage (V _oc) of the fabricated cells ranges between 200 to 400 mV and the short circuit current between 7 to 10 μA.     

Method to determine the interface’s fractal dimensions of metal-semiconductor electric contacts from their static instrumental characteristics

With the concept of fractal surfaces, the influence of the relief of the interface (roughness) and the heterogeneity of the potential barrier on the behavior of the current–voltage and capacity–voltage characteristics of the metal–semiconductor electric contacts with the Schottky barrier has been studied. The necessary and satisfactory conditions for the accurate relative measurements of the surface relief have been found with the mathematical apparatus of the Hausdorff–Besikovitsch fractional dimensionality 2 D _f3. It is shown that due to the fractal geometry the relative change of the real area of the interface of the metal–semiconductor contacts with the Schottky barrier is proportional to the ratio of their linear dimensions in the 4-D_f power. This is considerably slower than the change of the dimensions of the topological areas of their contact windows. The method to determine the fractal dimensionality of the real interface of the metal–semiconductor electric contacts with the Schottky barrier from the current–voltage and capacity–voltage characteristics has been developed.     

Mechanisms controlling the efficiency of polymer solar cells

To improve the efficiency of polymer solar cells, it is vital to understand which mechanisms control the current–voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters are very informative for analyzing losses. We report on the current–voltage characteristics and the external photogeneration quantum yield of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al as well as of ITO/PEDOT:PSS/P3HT:PCBM/Al devices investigated in the broad temperature range 120–325 K under variable illumination, between 0.02 and 100 mW/cm². We discuss the recombination on traps and the low mobility of charge carriers caused by poor morphology of active layers as possible mechanisms limiting the efficiency of these devices. PACS 73.50.P; 73.61.P; 72.80.R     

X-ray dosimetric characteristics of CdIn2S4〈Cu〉 single crystals

The effect of doping CdIn₂S₄ single crystals by copper (3 mol %) on their X-ray dosimetric characteristics is investigated. It is found that the characteristic X-ray conductivity of CdIn₂S₄〈Cu〉 single crystals increases 3–16 times compared with undoped CdIn₂S₄ at effective radiation hardness V _a = 25−50 keV and dose rate E = 0.75−78.05 R/min. Moreover, the persistence of the crystal characteristics completely disappears and the supply voltage of a CdIn₂S₄〈Cu〉 X-ray detector decreases fivefold. The dependence of the steady X-ray-induced current in CdIn₂S₄〈Cu〉 on the X-ray dose is described as ΔI _{E, 0} ∝ E ^α, where 0.6 ≤ α ≤ 1.8.     

Synthesis and studies of new plasticized PVP: NaClO3 electrolyte system for battery applications

A new thin film sodium ion conducting plasticized polymer electrolyte based on poly(vinyl pyrrolidone) (PVP) complexed with NaClO₃ salt systems was prepared by the solution-cast method. The interaction of NaClO₃ salt with PVP was confirmed by Infrared (IR) study. Charge transport of these polymer electrolytes is due to ions, which was confirmed by Wagner’s polarization method. From the conductivity measurements, the highest conductivity value 6.71×10⁻⁵ S/cm was observed for the composition PVP:PEG:NaClO₃(30:60:10) at room temperature 35 °C. The redox behaviour and good reversibility of the plasiticized electrolytes are confirmed by electrochemical techniques. Electrochemical cell studies of these polymer electrolytes were analyzed from their discharge characteristics. The open-circuit voltage (OCV) and short-circuit current (SCC) were found to in the range of 2.52 V to 2.36 V and 760 μA to 1040 μA, respectively.     

Resistive switching effect in thin-film heterojunctions based on electron-doped Nd2 ? xCexCuO4 ? y superconductor

Thin-film heterojunctions Nd_{2 − x} Ce _x CuO_{4 − y} /Ag were obtained. The bipolar effect of resistive switching in these heterostructures was detected and investigated. X-ray diffraction data indicate the presence of a second phase in thin films; along with the basic phase Nd_{2 − x} Ce _x CuO_{4 − y} , it affects the behavior of the interface of investigated heterojunctions and leads to an alteration of the type of conductivity. The threshold frequency of alternating voltage at which the resistive switching effect is observed in heterojunctions was detected.