Haiyang Peng

Electrode dependence of resistive switching in Mn-doped ZnO: Filamentary versus interfacial mechanisms

We carry out a comparative study on resistive switching in Mn-doped ZnO thin films; samples grown on Pt and Si show unipolar and bipolar switching behaviors, respectively. Fittings of the current-voltage curves and area dependence of the device resistance reveal the filamentary conduction in Pt/Mn:ZnO/Pt. On the other hand, the interfacial effect dominates in Pt/Mn:ZnO/Si, and its low resistance state exponentially relaxes toward the high resistance state in contrast to the good data retention in Pt/Mn:ZnO/Pt. Our results suggest that selecting electrodes dictates the resistive switching mechanism presumably by affecting the migration dynamics of oxygen vacancies.

Bound magnetic polarons and p-d exchange interaction in ferromagnetic insulating Cu-doped ZnO

A systematic study on the magnetic and electrical transport properties of single-phase wurtzite Zn1−xCuxO is performed. Efros variable range hopping dominates the conduction, which is accompanied by a ferromagnetic order up to 700 K for x>1%. Both the first-principles calculations and Cu/Al co-doping experiments suggest that the spontaneous spin polarization originates from the p-d exchange interaction between O 2p and Cu 3d orbitals. Furthermore, our results are consistent with the scenario that the intrinsic ferromagnetism is established through indirect interactions between bound magnetic polarons mediated by magnetic impurities.

Room temperature ferromagnetism in partially hydrogenated epitaxial graphene

We report room temperature ferromagnetism in partially hydrogenated epitaxial graphene grown on 4HSiC(0001). The presence of ferromagnetism was confirmed by superconducting quantum interference devices measurements. Synchrotron-based near-edge x-ray absorption fine structure and high resolution electron energy loss spectroscopy measurements have been used to investigate the hydrogenation mechanism on the epitaxial graphene and the origin of room temperature ferromagnetism. The partial hydrogenation induces the formation of unpaired electrons in graphene, which together with the remnant delocalized π bonding network, can explain the observed ferromagnetism in partially hydrogenated epitaxial graphene.

Nonvolatile resistive switching in spinel ZnMn2O4 and ilmenite ZnMnO3

We report that spinel ZnMn2O4 and ilmenite ZnMnO3 show excellent unipolar resistive switching behaviors, with ON/OFF ratios larger than 104. For both oxides, retention of more than 10 h and good endurance are achieved. Conduction of the OFF state is dominated by the space-charge-limited conduction mechanism, while the Ohmic behavior dictates the ON state, which suggests a filamentary conduction mechanism. Our study introduces two promising materials candidates for nonvolatile resistive random access memory devices, and furthermore it suggests that formation and rupture of conducting filaments are universal in certain ternary oxides even though they may possess distinct crystalline structures.

Complementary Charge Trapping and Ionic Migration in Resistive Switching of Rare-Earth Manganite TbMnO3

Perovskite rare-earth manganites like TbMnO₃ exhibit rich magnetic and electric phases, providing opportunities for next-generation multifunctional devices. Here, we report the nonvolatile bipolar switching of resistance and capacitance in TbMnO₃ thin films grown on conducting Nb:SrTiO₃ substrates. The device shows an ON/OFF resistance ratio of ∼1 × 10⁴, and the resistive switching is accompanied by a frequency-dependent capacitance switching. Detailed analysis of the conduction mechanisms reveals that the migration of oxygen vacancies and the charge trapping/detrapping at the heterojunction interface play important and complementary roles in the switching behaviors. Our results suggest that both electronic and ionic processes should be considered in order to elucidate the conduction mechanisms and the switching behaviors in such heterostructures made of complex oxides.

Morphology-controlled synthesis and a comparative study of the physical properties of SnO2 nanostructures: from ultrathin nanowires to ultrawide nanobelts.

Controlled synthesis of one-dimensional materials, such as nanowires and nanobelts, is of vital importance for achieving the desired properties and fabricating functional devices. We report a systematic investigation of the vapor transport growth of one-dimensional SnO(2) nanostructures, aiming to achieve precise morphology control. SnO(2) nanowires are obtained when SnO(2) mixed with graphite is used as the source material; adding TiO(2) into the source reliably leads to the formation of nanobelts. Ti-induced modification of crystal surface energy is proposed to be the origin of the morphology change. In addition, control of the lateral dimensions of both SnO(2) nanowires (from approximately 15 to approximately 115 nm in diameter) and nanobelts (from approximately 30 nm to approximately 2 microm in width) is achieved by adjusting the growth conditions. The physical properties of SnO(2) nanowires and nanobelts are further characterized and compared using room temperature photoluminescence, resonant Raman scattering, and field emission measurements.

Nanoscale resistive switching and filamentary conduction in NiO thin films

We fabricate regular arrays of nanoelectrodes on NiO thin films via nanosphere lithography and directly probe the nanoscale resistive switching using a conductive atomic force microscope. The unipolar resistive switching is consistent with the conducting filament formation/rupture mechanism, and the switching power is as low as 10−9 W. We find that only about half of devices are switchable, and the Monte Carlo simulation suggests strong correlations between the switching reliability, the electrode size, and the filament dimension and density.

Enhanced Electrical Conductivity of Individual Conducting Polymer Nanobelts

One-dimensional (1D) conducting polymer nanostructures (wires, fibers, belts, tubes etc.) have attracted growing interest due to their unique properties, such as efficient charge transport, higher energy-storage density, and enhanced chemical sensitivity, which make them more advantageous than their bulk counterparts.[1–3] Furthermore, the ease of bandgap tunability, high mechanical flexibility, and light weight, together with their biocompatibility make 1D conducting polymers excellent complements to 1D inorganic nanostructures, especially useful in microelectronics and nanoelectronics.[3–5] However, so far most current applications related to 1D conducting polymer nanostructures have been based on bundles rather than individual structures, which limits their potential.[3,6] One reason for this is the easy aggregation of 1D polymer nanostructures in solution, while dispersed polymer nanostructures are a prerequisite for applications based on individual nanowires.[3] Another is the difficulty of manipulating and positioning 1D conducting polymer in nanodevices because the lithography processing (e.g., electron-beam (e-beam) and focused ion beam) inevitably deteriorates the conducting polymer’s properties.[7,8] In addition, it should be noted that 1D conducting polymer nanostructures with improved electrical conductivity will enhance many applications, such as light-emitting diodes, fieldeffect transistors, sensors, and electronic circuit boards,[9–11] but few examples of conducting polymer nanostructures have succeeded in achieving an electrical conductivity in excess of

Concurrent nonvolatile resistance and capacitance switching in LaAlO3

We report on the correlated nonvolatile resistance and capacitance switching in Pt/LaAlO3/Nb:SrTiO3 heterostructures. The pristine devices show the typical characteristics of a Schottky junction; however, after forming, a reverse bias switches the device into a low resistance and high capacitance state while a forward bias drives it into a high resistance and low capacitance state. Our experiments suggest that both the formation of conducting filaments and the modulation of interface barrier contribute to the resistance switching. Oxygen vacancies play critical roles in determining the switching characteristics and can be controlled in the process of device fabrication.

High sensitivity low field magnetically gated resistive switching in CoFe2O4/La0.66Sr0.34MnO3 heterostructure

The phenomenon of resistive switching (RS) has been demonstrated in several non-magnetic and some magnetic oxide systems, however the “magnetic” aspect of magnetic oxides has not been emphasized especially in terms of low field tunability. In our work, we examined the CoFe2O4/La0.66Sr0.34MnO3 all-magnetic oxide interface system for RS and discovered a very sharp (bipolar) transition at room temperature that can be gated with high sensitivity by low magnetic fields (∼0–100 mT). By using a number of characterizations, we show that this is an interface effect, which may open up interesting directions for manipulation of the RS phenomenon.