Gang Meng

Cellulose Nanofiber Paper as an Ultra Flexible Nonvolatile Memory

On the development of flexible electronics, a highly flexible nonvolatile memory, which is an important circuit component for the portability, is necessary. However, the flexibility of existing nonvolatile memory has been limited, e.g. the smallest radius into which can be bent has been millimeters range, due to the difficulty in maintaining memory properties while bending. Here we propose the ultra flexible resistive nonvolatile memory using Ag-decorated cellulose nanofiber paper (CNP). The Ag-decorated CNP devices showed the stable nonvolatile memory effects with 6 orders of ON/OFF resistance ratio and the small standard deviation of switching voltage distribution. The memory performance of CNP devices can be maintained without any degradation when being bent down to the radius of 350 μm, which is the smallest value compared to those of existing any flexible nonvolatile memories. Thus the present device using abundant and mechanically flexible CNP offers a highly flexible nonvolatile memory for portable flexible electronics.

Ultrafast and Wide Range Analysis of DNA Molecules Using Rigid Network Structure of Solid Nanowires

Analyzing sizes of DNA via electrophoresis using a gel has played an important role in the recent, rapid progress of biology and biotechnology. Although analyzing DNA over a wide range of sizes in a short time is desired, no existing electrophoresis methods have been able to fully satisfy these two requirements. Here we propose a novel method using a rigid 3D network structure composed of solid nanowires within a microchannel. This rigid network structure enables analysis of DNA under applied DC electric fields for a large DNA size range (100 bp–166 kbp) within 13 s, which are much wider and faster conditions than those of any existing methods. The network density is readily varied for the targeted DNA size range by tailoring the number of cycles of the nanowire growth only at the desired spatial position within the microchannel. The rigid dense 3D network structure with spatial density control plays an important role in determining the capability for analyzing DNA. Since the present method allows the spatial location and density of the nanostructure within the microchannels to be defined, this unique controllability offers a new strategy to develop an analytical method not only for DNA but also for other biological molecules.

Impact of Preferential Indium Nucleation on Electrical Conductivity of Vapor–Liquid–Solid Grown Indium–Tin Oxide Nanowires

Highly conductive and transparent indium-tin oxide (ITO) single-crystalline nanowires, formed by the vapor-liquid-solid (VLS) method, hold great promise for various nanoscale device applications. However, increasing an electrical conductivity of VLS grown ITO nanowires is still a challenging issue due to the intrinsic difficulty in controlling complex material transports of the VLS process. Here, we demonstrate a crucial role of preferential indium nucleation on the electrical conductivity of VLS grown ITO nanowires using gold catalysts. In spite of the fact that the vapor pressure of tin is lower than that of indium, we found that the indium concentration within the nanowires was always higher than the nominal composition. The VLS growth of ITO through gold catalysts significantly differs from ITO film formations due to the emergence of preferential indium nucleation only at a liquid-solid interface. Furthermore, we demonstrate that the averaged resistivity of ITO nanowires can be decreased down to 2.1 × 10(-4) Ω cm, which is the lowest compared with values previously reported, via intentionally increasing the tin concentration within the nanowires.

Prominent Thermodynamical Interaction with Surroundings on Nanoscale Memristive Switching of Metal Oxides

This study demonstrates the effect of surroundings on a memristive switching at nanoscale by utilizing an open top planar-type device. NiO(x) and CoO(x) planar-type devices have exhibited a memristive behavior under atmospheric pressure, whereas TiO(2-x) planar-type devices did not show a memristive switching even under the same surroundings. A memristive behavior of TiO(2-x) planar-type devices has emerged when reducing an ambient pressure and/or employing a SiO(2) passivation layer. These results reveal that a thermodynamical interaction with surroundings critically determines the occurrence of memristive switching via varying a stability of nonstoichiometry. Since this effect tends to be more significant for smaller devices with larger specific surface area, tailoring the surrounding effect by an appropriate passivation will be essential for high density devices.

Carrier type dependence on spatial asymmetry of unipolar resistive switching of metal oxides

We report a carrier type dependence on the spatial asymmetry of unipolar resistive switching for various metal oxides, including NiOx, CoOx, TiO2−x, YSZ, and SnO2−x. n-type oxides show a unipolar resistive switching at the anode side whereas p-type oxides switch at the cathode side. During the forming process, the electrical conduction path of p-type oxides extends from the anode to cathode while that of n-type oxides forms from the cathode to anode. The carrier type of switching oxide layer critically determines the spatial inhomogeneity of unipolar resistive switching during the forming process possibly triggered via the oxygen ion drift.

Three-dimensional Nanowire Structures for Ultra-Fast Separation of DNA, Protein and RNA Molecules

Separation and analysis of biomolecules represent crucial processes for biological and biomedical engineering development; however, separation resolution and speed for biomolecules analysis still require improvements. To achieve separation and analysis of biomolecules in a short time, the use of highly-ordered nanostructures fabricated by top-down or bottom-up approaches have been proposed. Here, we reported on the use of three-dimensional (3D) nanowire structures embedded in microchannels fabricated by a bottom-up approach for ultrafast separation of small biomolecules, such as DNA, protein, and RNA molecules. The 3D nanowire structures could analyze a mixture of DNA molecules (50–1000 bp) within 50 s, a mixture of protein molecules (20–340 kDa) within 5 s, and a mixture of RNA molecules (100–1000 bases) within 25 s. And, we could observe the electrophoretic mobility difference of biomolecules as a function of molecular size in the 3D nanowire structures. Since the present methodology allows users to control the pore size of sieving materials by varying the number of cycles for nanowire growth, the 3D nanowire structures have a good potential for use as alternatives for other sieving materials.

A flux induced crystal phase transition in the vapor–liquid–solid growth of indium-tin oxide nanowires

Single crystalline metal oxide nanowires formed via a vapor-liquid-solid (VLS) route provide a platform not only for studying fundamental nanoscale properties but also for exploring novel device applications. Although the crystal phase variation of metal oxides, which exhibits a variety of physical properties, is an interesting feature compared with conventional semiconductors, it has been difficult to control the crystal phase of metal oxides during the VLS nanowire growth. Here we show that a material flux critically determines the crystal phase of indium-tin oxide nanowires grown via the VLS route, although thermodynamical parameters, such as temperature and pressure, were previously believed to determine the crystal phase. The crystal phases of indium-tin oxide nanowires varied from the rutile structures (SnO2), the metastable fluorite structures (InxSnyO3.5) and the bixbyite structures (Sn-doped In2O3) when only the material flux was varied within an order of magnitude. This trend can be interpreted in terms of the material flux dependence of crystal phases (rutile SnO2 and bixbyite In2O3) on the critical nucleation at the liquid-solid (LS) interface. Thus, precisely controlling the material flux, which has been underestimated for VLS nanowire growths, allows us to design the crystal phase and properties in the VLS nanowire growth of multicomponent metal oxides.

Facile and scalable patterning of sublithographic scale uniform nanowires by ultra-thin AAO free-standing membrane

Creating sublithographic scale uniform nanowires for large area is an important issue for nanowire-based various device applications. Although anodic aluminium oxide (AAO) membrane is a promising technique, existing attached AAO membrane mask methods have not been able to fabricate such small nanowires for large areas due to difficulties on transferring a thin membrane. Here we demonstrate a facile and scalable methodology to fabricate sublithographic scale uniform-sized nanowires by introducing ultra-thin AAO free-standing membrane with a rigid Al frame and a hydrophilic contacting method. The present method allows us to fabricate sub 20 nm nanowires with the standard deviations of 1.1 nm via defining the size and spacing of metal catalysts for nanowire growth. We also show the scalability of the present attached AAO membrane mask method for a 4-inch scale wafer.

Modulation of thermoelectric power factor via radial dopant inhomogeneity in B-doped Si nanowires.

We demonstrate a modulation of thermoelectric power factor via a radial dopant inhomogeneity in B-doped Si nanowires. These nanowires grown via vapor-liquid-solid (VLS) method were naturally composed of a heavily doped outer shell layer and a lightly doped inner core. The thermopower measurements for a single nanowire demonstrated that the power factor values were higher than those of homogeneously B-doped Si nanowires. The field effect measurements revealed the enhancement of hole mobility for these VLS grown B-doped Si nanowires due to the modulation doping effect. This mobility enhancement increases overall electrical conductivity of nanowires without decreasing the Seebeck coefficient value, resulting in the increase of thermoelectric power factor. In addition, we found that tailoring the surface dopant distribution by introducing surface δ-doping can further increase the power factor value. Thus, intentionally tailoring radial dopant inhomogeneity promises a way to modulate the thermoelectric power factor of semiconductor nanowires.

Nanoscale Size-Selective Deposition of Nanowires by Micrometer Scale Hydrophilic Patterns

Controlling the post-growth assembly of nanowires is an important challenge in the development of functional bottom-up devices. Although various methods have been developed for the controlled assembly of nanowires, it is still a challenging issue to align selectively heterogeneous nanowires at desired spatial positions on the substrate. Here we report a size selective deposition and sequential alignment of nanowires by utilizing micrometer scale hydrophilic/hydrophobic patterned substrate. Nanowires dispersed within oil were preferentially deposited only at a water/oil interface onto the hydrophilic patterns. The diameter size of deposited nanowires was strongly limited by the width of hydrophilic patterns, exhibiting the nanoscale size selectivity of nanowires deposited onto micrometer scale hydrophilic patterns. Such size selectivity was due to the nanoscale height variation of a water layer formed onto the micrometer scale hydrophilic patterns. We successfully demonstrated the sequential alignment of different sized nanowires on the same substrate by applying this size selective phenomenon.