Negative resistance behavior and reproducible resistance switching were found in polycrystalline NiO films deposited by dc magnetron reactive sputtering methods. Oxygen to argon gas ratio during deposition was critical in deciding the detailed switching characteristics of either bi-stable memory switching or mono-stable threshold switching. Both metallic nickel defects and nickel vacancies influenced the negative resistance and the switching characteristics. We obtained a distribution of low resistance values which were dependent on the compliance current of high-to-low resistance switching. At 200°C, the low-resistance state kept its initial resistance value while the high-resistance state reached 85% of its initial resistance value after 5×105s. We suggested that the negative resistance and the switching mechanism could be described by electron conduction related to metallic nickel defect states existing in deep levels and by small-polaron hole hopping conduction.

Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now it has been difficult to fabricate nanoscale devices consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.

The demand for non-volatile memory technologies that offer high speed, high storage density and low power consumption has stimulated extensive research into new functional materials and device physics. [1][2][3][4][5] Nano-ferronic devices based on multiferroic/ferroelectric materials have been emerging as nextgeneration nano-electronics, which deal with the interplay between ferroic orders (e.g. ferroelectricity and ferromagnetism) and electronic transport on the nanoscale. [ 6 ] Recent investigations into various multiferroic/ferroelectric materials have revealed remarkable polarization dependent electronic transport properties, which include signifi cant electroresistance effects in a switchable ferroelectric diode [7][8][9][10][11] and multiferroic/ferroelectric tunnel junctions (M/FTJs) [12][13][14][15][16][17] and intriguing charge conduction in ferroelectric domain/walls. [ 18 , 19 ] These conduction properties can be utilized for fast and non-destructive readout in emergent non-volatile memories such as resistive random access memory (RRAM) and memristor. [ 20 ] Especially, ferroelectric-resistive memories based on ferroelectric diode and tunnel junctions have demonstrated that it is possible to achieve high resistive ON/OFF ratio, high speed and low write power with a high reproducibility by controlling ferroelectric polarization. In a switchable ferroelectric diode, the Schottky-to-Ohmic contacts, forming at the interfaces between metal electrodes and semiconducting ferroelectric thin fi lms, are reversibly modulated by the polarization fl ipping which gives rise to rectifi cation direction switching . [8][9][10][11] The tunnel junctions with ultrathin ferroelectric barrier yield a giant tunnel electroresistance effect resulting from the change of asymmetric tunnel barrier heights controlled by ferroelectric polarization direction. However, multiferroic/ferroelectric nano-structures such as nano-islands and nano-wires have not yet been exploited for ferroelectric-resistive memories, although large storage capacity, lower power consumption and high reliability are expected for such nano-structures. They also provide an effective way to understand and manipulate the ferroelectric-resistive switching, piezoelectricity, polarization and domain structures on the nanoscale. On the other hand, the fabrication of ferroelectric nano-structures through bottom-up approach is crucial to realizing high-performance of nano-ferronic devices since top-down approach may induce serious deterioration in ferroelectric nano-structures. [ 21 , 22 ] Here, we explored the local charge conductions and their coupling with ferroelectric polarization in highly oriented ferroelectric BiFeO 3 (BFO) nano-islands array by using conductive atomic force microscopy (CAFM) and piezoresponse force microscopy (PFM). We observed a switchable diode effect in BFO nano-islands grown on SrRuO 3 /SrTiO 3 (SRO/STO) substrate, which showed the direct correlation between rectifi cation and ferroelectric polarization directions. The rectifi cation...

A one-bit cell of a general nonvolatile memory consists of a memory element and a switch element. Several memory elements have been tried given that any bistable states, that is, two charging states, two spin states, or two resistance states, can be used for a memory element. On the other hand, silicon-based transistors have been the most popularly used switch element. However, silicon-based transistors do not conform to high-density, nonvolatile memories with three-dimensional (3D) stack structures due to their high processing temperatures and the difficulty of growing high-quality epitaxial silicon over metals. Here, we show a low-temperaturegrown oxide diode, Pt/p-NiO x /n-TiO x /Pt, applied as a switch element for high-density, nonvolatile memories. The diode exhibits good rectifying characteristics at room temperature: a rectifying ratio of 10 5 at ± 3 V, a forward current density of up to ∼ 5×10 3 A cm -2 , an ideality factor of 4.3, and a turn-on voltage of 2 V. Furthermore, we verify its ability to allow and deny access to the Pt/NiO/Pt memory element with two stable resistance states. Under the forward-bias condition, we could access the memory element and change the resistance state, although access was denied under the reverse bias condition. This one-diode/one-resistor (1D/1R) structure could be a promising building block for high-density, nonvolatile random-access memories with 3D stack structures.

Conductivity switching phenomena controlled by external voltages have been investigated for various NiO films deposited by dc reactive sputtering methods. Pt∕NiO∕Pt capacitor structures with top electrodes of different diameters have showed increasing off-state current with the diameter of a top electrode and nearly the same on-state current independent of the diameter. Local conductivity switching behaviors have been observed in a series structure consisting of two Pt∕NiO∕Pt capacitors with different resistance values. By reasoning out conductivity switching mechanisms from the switching characteristics and introducing multilayers consisting of NiO layers with different resistance values, we have reduced the reset current by two orders of magnitude.