Real-time device-scale imaging of conducting filament dynamics in resistive switching materials

ReRAM is a compelling candidate for next-generation non-volatile memory owing to its various advantages. However, fluctuation of operation parameters are critical weakness occurring failures in 'reading' and 'writing' operations. To enhance the stability, it is important to understand the mechanism of the devices. Although numerous studies have been conducted using AFM or TEM, the understanding of the device operation is still limited due to the destructive nature and/or limited imaging range of the previous methods. Here, we propose a new hybrid device composed of ReRAM and LED enabling us to monitor the conducting filament (CF) configuration on the device scale during resistive switching. We directly observe the change in CF configuration across the whole device area through light emission from our hybrid device. In contrast to former studies, we found that minor CFs were formed earlier than major CF contributing to the resistive switching. Moreover, we investigated the substitution of a stressed major CF with a fresh minor CF when large fluctuation of operation voltage appeared after more than 50 times of resistive switching in atmospheric condition. Our results present an advancement in the understanding of ReRAM operation mechanism, and a step toward stabilizing the fluctuations in ReRAM switching parameters.

Electrical characterization of benzenedithiolate molecular electronic devices with graphene electrodes on rigid and flexible substrates

We investigated the electrical characteristics of molecular electronic devices consisting of benzenedithiolate self-assembled monolayers and a graphene electrode. We used the multilayer graphene electrode as a protective interlayer to prevent filamentary path formation during the evaporation of the top electrode in the vertical metal-molecule-metal junction structure. The devices were fabricated both on a rigid SiO2/Si substrate and on a flexible poly(ethylene terephthalate) substrate. Using these devices, we investigated the basic charge transport characteristics of benzenedithiolate molecular junctions in length- and temperature-dependent analyses. Additionally, the reliability of the electrical characteristics of the flexible benzenedithiolate molecular devices was investigated under various mechanical bending conditions, such as different bending radii, repeated bending cycles, and a retention test under bending. We also observed the inelastic electron tunneling spectra of our fabricated graphene-electrode molecular devices. Based on the results, we verified that benzenedithiolate molecules participate in charge transport, serving as an active tunneling barrier in solid-state graphene-electrode molecular junctions.

Microtube Light-Emitting Diode Arrays with Metal Cores

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

Emission color-tuned light-emitting diode microarrays of nonpolar In(x)Ga(1-x)N/GaN multishell nanotube heterostructures

Integration of nanostructure lighting source arrays with well-defined emission wavelengths is of great importance for optoelectronic integrated monolithic circuitry. We report on the fabrication and optical properties of GaN-based p-n junction multishell nanotube microarrays with composition-modulated nonpolar m-plane InxGa1-xN/GaN multiple quantum wells (MQWs) integrated on c-sapphire or Si substrates. The emission wavelengths were controlled in the visible spectral range of green to violet by varying the indium mole fraction of the InxGa1-xN MQWs in the range 0.13 ≤ x ≤ 0.36. Homogeneous emission from the entire area of the nanotube LED arrays was achieved via the formation of MQWs with uniform QW widths and composition by heteroepitaxy on the well-ordered nanotube arrays. Importantly, the wavelength-invariant electroluminescence emission was observed above a turn-on of 3.0 V because both the quantum-confinement Stark effect and band filling were suppressed due to the lack of spontaneous inherent electric field in the m-plane nanotube nonpolar MQWs. The method of fabricating the multishell nanotube LED microarrays with controlled emission colors has potential applications in monolithic nonpolar photonic and optoelectronic devices on commonly used c-sapphire and Si substrates.

Statistical Analysis of Electrical Properties of Octanemonothiol versus Octanedithol in PEDOT:PSS-Electrode Molecular Junctions

We fabricated a large number of octanemonothiol (C8) and octanedithol (DC8) molecular electronic devices with

PEDOT

PSS (3,4-ethylenedioxythiophene) interlayer and performed a statistical analysis on the electronic properties of these devices. From the analysis, we obtained the Gaussian plot of histograms of Log10 (current density (J)) and several statistical estimates such as arithmetic mean, median, Gaussian mean, arithmetic standard deviation, adjusted absolute median deviation, and Gaussian standard deviation. We determined the current density-voltage (J-V) characteristics from the statistically representative data for C8 and DC8 devices and found that the conductivity of C8 is higher than that of DC8 by a factor of ~10. The difference of the conductivity of C8 and DC8 devices is attributed to the difference of the contact properties between the C8 and DC8 PEDOT:PSS-interlayer molecular junctions.

Stimulated emission features of bound excitons in ZnO nanotubes

We have investigated the detailed features of photoluminescence (PL) in vertically aligned ZnO nanotube (NT) arrays as a function of temperature, pumping power, and experimental geometries. In samples with different wall thickness (15 or 60 nm), the temperature-dependent PL energy followed the Varshni's formula whose fitting parameters were found to be rather close to zero-dimensional case in the 15 nm-thick NTs with much larger intensity. In reflective geometry with circular excitation beam shape, the emission gradually evolved from spontaneous to stimulated regime, inferred from amplitude and line-width variation. On the other hand, in the edge-emission geometry with needle-like excitation shape, the interaction length dependence was directly traced by using an adjustable slit.

Variable-color light-emitting diodes using GaN microdonut arrays

Microdonut-shaped GaN/Inx Ga1-x N light-emitting diode (LED) microarrays are fabricated for variable-color emitters. The figure shows clearly donut-shaped light emission from all the individual microdonut LEDs. Furthermore, microdonut LEDs exhibit spatially-resolved blue and green EL colors, which can be tuned by either controlling the external bias voltage or changing the size of the microdonut LED.

Controlled growth of inorganic nanorod arrays using graphene nanodot seed layers

We report the density- and size-controlled growth of zinc oxide (ZnO) nanorod arrays on arbitrary substrates using reduced graphene oxide (rGO) nanodot arrays. For the controlled growth of the ZnO nanorod arrays, rGO nanodot arrays with tunable density and size were designed using a monolayer of diblock copolymer micelles and oxygen plasma etching. While the diameter and number density of the ZnO nanorods were readily determined by those of the rGO nanodots, the length of the ZnO nanorods was easily controlled by changing the growth time. x-ray diffraction and electron microscopy confirmed that the vertically well-aligned ZnO nanorod arrays were heteroepitaxially grown on the rGO nanodots. Strong, sharp near-band-edge emission peaks with no carbon-related peak were observed in the photoluminescence spectra, implying that the ZnO nanostructures grown on the rGO nanodots were of high optical quality and without carbon contamination. Our approach provides a general and rational route for heteroepitaxial growth of high-quality inorganic materials with tunable number density, size, and spatial arrangement on arbitrary substrates using rGO nanodot arrays.

High-resolution observation of nucleation and growth behavior of nanomaterials using a graphene template

By using graphene as an electron beam-transparent substrate for both nanomaterial growth and transmission electron microscopy (TEM) measurements, we investigate initial growth behavior of nanomaterials. The direct growth and imaging method using graphene facilitate atomic-resolution imaging of nanomaterials at the very early stage of growth. This enables the observation of the transition in crystal structure of ZnO nuclei and the formation of various defects during nanomaterial growth.

Geometry-induced dislocations in coaxial heterostructural nanotubes

Highly localized dislocations in GaN/ZnO hetero-nanostructures are generated from the residual strain field by lattice mismatches at two interfaces: between the substrate and hetero-nanostructures, and between the ZnO core and GaN shell. The local strain field is measured using tranmission electron microscopy, and the relationship between the nanostructure morphology and the highly localized dislocations is analyzed by a finite element method.

Epitaxial GaN microdisk lasers grown on graphene microdots

Direct epitaxial growth of inorganic compound semiconductors on lattice-matched single-crystal substrates has provided an important way to fabricate light sources for various applications including lighting, displays and optical communications. Nevertheless, unconventional substrates such as silicon, amorphous glass, plastics, and metals must be used for emerging optoelectronic applications, such as high-speed photonic circuitry and flexible displays. However, high-quality film growth requires good matching of lattice constants and thermal expansion coefficients between the film and the supporting substrate. This restricts monolithic fabrication of optoelectronic devices on unconventional substrates. Here, we describe methods to grow high-quality gallium nitride (GaN) microdisks on amorphous silicon oxide layers formed on silicon using micropatterned graphene films as a nucleation layer. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots with intermediate ZnO nanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaN microdisk with a Q-factor of 1200 was observed at room temperature.

Metal-lined semiconductor nanotubes for surface plasmon-mediated luminescence enhancement

Highly efficient solid-state light-emitting devices require semiconductor architectures equipped with high quantum efficiency and integratability on conductive substrates. Surface plasmon (SP)-mediated luminescence enhancement has been considered as one of the most promising solutions, because SP resonance can greatly improve the radiative recombination rate and be achieved using metal entities compatible with the electrode fabrication process. Nevertheless, metal/semiconductor heterostructures have had several fabrication-compatible issues due to metal as a potential contaminant of the semiconductor. We present here a simple fabrication scheme for a metal-lined semiconductor nanotube heterostructure, in which a metal layer is selectively formed on the inner wall of the semiconductor nanotube. The Ag-lining process in a ZnO nanotube resulted in 7.5-fold enhancement of the photoluminescence intensity at 11 K. This SP fabrication technique looks promising for highly efficient solid-state lighting based on semiconductor nanostructures without detrimental effects.

Orientation-dependent local structural properties of Zn(1-x)Mg(x)O nanorods studied by extended X-ray absorption fine structure

The orientation-dependent structural properties of Zn(1-x)Mg(x)O nanorods with different Mg concentrations were investigated quantitatively using polarization-dependent extended X-ray absorption fine structure (EXAFS) measurements at the Zn K edge. Vertically-aligned Zn(1-x)Mg(x)O nanorods were synthesized on Si substrates using catalyst free metal organic chemical vapor deposition. Polarization-dependent EXAFS measurements showed that Mg ions mainly occupied the Zn sites of the nanorods. EXAFS revealed that the distance between Zn-Mg pairs in all directions is - 0.2 angstroms shorter than that of Zn-Zn pairs and that there is a substantial amount of disorder in the Mg sites of the nanorods, independent of Mg concentrations.

Probing quantum confinement within single core-multishell nanowires

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.

Repeatable switching of the bending direction of ZnO nanoneedles by ion beams

We studied the ion beam bending of ZnO nanoneedles to find the dependence of their bending direction on the ion beam energy and to clarify the bending mechanism. Through gallium focused ion beam (FIB) bending, the stems of the nanoneedles were found to be bent to the direction of the ion beam source for ion beam energies of 30 keV whereas they were bent in the opposite direction at ion energies lower than 20 keV. We found for the first time that the bending direction of ZnO nanoneedles could be changed by repeated switching of the ion beam energy between lower and higher energy levels, and that the thin tip parts of the nanoneedles were bent toward to the ion beam source like the higher energy bending mode during the process of lower energy bending below 20 keV. Through high resolution transmission electron microscopy (HRTEM) observations of the microstructure of a nanoneedle, bent by 30 keV higher energy ion beams, based on the atomic scale, we found that more edge dislocations were created in the rear side, deeper than the central plane of the nanoneedle, than the front side and that each edge dislocation added an extra lattice plane in this region. These observations clearly showed that the bent nanoneedles were plastically deformed by the edge dislocations created by the ion beams.

Tunable catalytic alloying eliminates stacking faults in compound semiconductor nanowires

Planar defects in compound (III-V and II-VI) semiconductor nanowires (NWs), such as twin and stacking faults, are universally formed during the catalytic NW growth, and they detrimentally act as static disorders against coherent electron transport and light emissions. Here we report a simple synthetic route for planar-defect free II-VI NWs by tunable alloying, i.e. Cd(1-x)Zn(x)Te NWs (0 ≤ x ≤ 1). It is discovered that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates interfacial instability during the catalytic growth by the surface energy minimization and forms homogeneous zinc blende crystals as opposed to unwanted zinc blende/wurtzite mixtures. As a direct consequence of the tunable alloying, we demonstrated that intrinsic energy band gap modulation in Cd(1-x)Zn(x)Te NWs can exploit the tunable spectral and temporal responses in light detection and emission in the full visible range.

Exciton scattering mechanism in a single semiconducting MgZnO nanorod

Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution. This study was performed using temperature-dependent cathodoluminescence and time-resolved photoluminescence spectroscopies. The exciton diffusion length in the MgZnO nanorod decreased from 100 to 70 nm with increasing temperature in the range of 5 and 80 K. The results obtained for the temperature dependence of exciton diffusion length and luminescence lifetime revealed that the dominant exciton scattering mechanism in MgZnO nanorod is exciton-phonon assisted piezoelectric field scattering.

Catalyst-free metal-organic chemical vapor deposition growth of InN nanorods

We demonstrated the successful growth of catalyst-free InN nanorods on (0001) Al2O3 substrates using metal-organic chemical vapor deposition. Morphological evolution was significantly affected by growth temperature. At 710 degrees C, complete InN nanorods with typical diameters of 150 nm and length of approximately 3.5 microm were grown with hexagonal facets. theta-2theta X-ray diffraction measurement shows that (0002) InN nanorods grown on (0001) Al2O3 substrates were vertically aligned along c-axis. In addition, high resolution transmission electron microscopy indicates the spacing of the (0001) lattice planes is 0.28 nm, which is very close to that of bulk InN. The electron diffraction patterns also revealed that the InN nanorods are single crystalline with a growth direction along (0001) with (10-10) facets.

Microstructures of GaN thin films grown on graphene layers

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Inorganic nanostructures grown on graphene layers

This article presents a review of current research activities on the hybrid heterostructures of inorganic nanostructures grown directly on graphene layers, which can be categorized primarily as zero-dimensional nanoparticles; one-dimensional nanorods, nanowires, and nanotubes; and two-dimensional nanowalls. For the hybrid structures, the nanostructures exhibit excellent material characteristics including high carrier mobility and radiative recombination rate as well as long-term stability while graphene films show good optical transparency, mechanical flexibility, and electrical conductivity. Accordingly, the versatile and fascinating properties of the nanostructures grown on graphene layers make it possible to fabricate high-performance optoelectronic and electronic devices even in transferable, flexible, or stretchable forms. Here, we review preparation methods and possible device applications of the hybrid structures consisting of various types of inorganic nanostructures grown on graphene layers.

Hydrothermally grown ZnO nanostructures on few-layer graphene sheets

This study describes the hydrothermal growth of ZnO nanostructures on few-layer graphene sheets and their optical and structural properties. The ZnO nanostructures were grown on graphene sheets of a few layers thick (few-layer graphene) without a seed layer. By changing the hydrothermal growth parameters, including temperature, reagent concentration and pH value of the solution, we readily controlled the dimensions, density and morphology of the ZnO nanostructures. More importantly, single-crystalline ZnO nanostructures grew directly on graphene, as determined by transmission electron microscopy. In addition, from the photoluminescence and cathodoluminescence spectra, strong near-band-edge emission was observed without any deep-level emission, indicating that the ZnO nanostructures grown on few-layer graphene were of high optical quality.