Graphene: Materials to devices (invited)

Despite recent progress in understanding geometric structure, electronic structure, and transport properties in a graphene device (GD), role of point defects, edges, traps in a GD or a gate insulator has been poorly defined. We have studied electronic and geometric structures of these defects using scanning probe microscopy and try to link those with the transport properties of the GD. We perform scanning gate microscopy study to understand the local carrier scattering. It was found that geometric corrugations, defects and edges directly influence the local transport current. This observation is linked directly with a proposed scattering model based on macroscopic transport measurements. We suggest that dangling bonds in insulator-material SiO₂ mainly used in GDs produce charge puddles and they work as scattering centers.     

Modeling of failure mode of laser welds in lap-shear specimens of HSLA steel sheets

Failure mode of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel sheets is investigated in this paper. The experiments for laser welds in lap-shear specimens under quasi-static loading conditions are briefly reviewed first. The experimental results showed that the laser welds failed in a ductile necking/shear failure mode and the ductile failure was initiated at a distance away from the crack tip near the boundary of the base metal and heat affected zone. In order to understand the failure mode of these welds, finite element analyses under plane strain conditions were conducted to identify the effects of the different plastic behaviors of the base metal, heat affected zone, and weld zone as well as the weld geometry on the ductile failure. The results of the reference finite element analysis based on the homogenous material model show that the failure mode is most likely to be a middle surface shear failure mode in the weld. The results of the finite element analysis based on the multi-zone non-homogeneous material models show that the higher effective stress–plastic strain curves of the weld and heat affected zones and the geometry of the weld protrusion result in the necking/shear failure mode in the load carrying sheet. The results of another finite element analysis based on the non-homogeneous material model and the Gurson yield function for porous materials indicate that the consideration of void nucleation and growth is necessary to identify the ductile failure initiation site that matches well with the experimental observations. Finally, the results of this investigation indicate that the failure mode of the welds should be examined carefully and the necking/shear failure mode needs to be considered for development of failure or separation criteria for welds under more complex loading conditions.     

Characterization of nanostructures of ZnO and ZnMnO films deposited by successive ionic layer adsorption and reaction method

ZnO and ZnMnO thin films were obtained by the successive ionic layer adsorption and reaction (SILAR) method. All thin films were deposited on glass microscope slide. A precursor solution of 0.1 M of ZnCl₂ complexed with ammonium hydroxide and water close to boiling point (92 °C) as a second solution was used for the ZnO films. An uncomplexed bath comprised of 0.1 M ZnCl₂, 0.1 M MnCl_2, and a second solution of 0.1 ml of NH₄OH with water close to boiling point was used for the ZnMnO films. The film samples were deposited by the SILAR method and annealed at 200 °C for 15 min. These samples were characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (EDS), and Atomic Force Microscope. Atomic absorption was used to determine quantitatively the amount of Mn incorporated into the films. According to the XRD patterns these films were polycrystalline with wurtzite hexagonal structure. The morphology of the ZnO films constituted by rice-like and flower-like structures changed significantly to nanosheet structures with the Mn incorporation. The Mn inclusion in a ZnO structure was less than 4% according to the results from EDS, XRD, and atomic absorption.     

Transparent Ga and Zn co-doped In2O3 electrode prepared by co-sputtering of Ga:In2O3 and Zn:In2O3 targets at room temperature

This study examined the characteristics of Ga:In₂O₃ (IGO) co-sputtered Zn:In₂O₃ (IZO) films prepared by dual target direct current (DC) magnetron sputtering at room temperature in a pure Ar atmosphere for transparent electrodes in IGZO-based TFTs. Electrical, optical, structural and surface properties of Ga and Zn co-doped In₂O₃ (IGZO) electrodes were investigated as a function of IGO and IZO target DC power during the co-sputtering process. Unlike semiconducting InGaZnO₄ films, which were widely used as a channel layer in the oxide TFTs, the co-sputtered IGZO films showed a high transmittance (91.84%) and low resistivity (4.1 × 10^− 4 Ω cm) at optimized DC power of the IGO and IZO targets, due to low atomic percent of Ga and Zn elements. Furthermore, the IGO co-sputtered IZO films showed a very smooth and featureless surface and an amorphous structure regardless of the IGO and IZO DC power due to the room temperature sputtering process. This indicates that co-sputtered IGZO films are a promising S/D electrode in the IGZO-based TFTs due to their low resistivity, high transmittance and same elements with channel InGaZnO₄ layer.     

Gas sensing characteristics of WO3 nanoplates prepared by acidification method

WO₃⋅ H₂O nanoplates were prepared by the acidification of Na₂WO₄? 2H₂O and converted into monoclinic WO₃ nanoplates by heat treatment. The sizes, morphologies and preferred orientation of the WO₃ nanoplates could be controlled by manipulating the acidity of the solution used for the acidification reaction. All of the WO₃ nanoplates showed the selective detection of NO₂ in the presence of other reducing gases, such as C₂H₅OH, CH₃COCH₃, CO, C₃H₈, and H₂. The gas response, selectivity, and response speed were optimized by varying the morphology of the sensing materials and operation temperature. The WO₃ nanoplates with a mean edge size of 192 nm showed the most rapid gas response along with a high response and selectivity to NO₂ when operated at 300 °C.     

Patterned structures fabricated on chalcogenide phase-change thin films by laser direct writing

In laser direct writing technology, the pattern is usually written in a photoresist. In this work, we use the chalcogenide phase change thin films as the laser direct writing materials, and patterned structures with different shapes and sizes were directly written with different laser wavelengths. Compared with traditional photoresist materials, the patterned structures can be directly formed in the chalcogenide phase change thin films without developing and etching procedures, and also can be directly written with different laser wavelengths. By tuning the laser parameters precisely, patterned structures with different sizes and shapes could be obtained as well. The analysis indicates that the formation mechanism of the patterned structure is mainly due to the volume expansion caused by material vaporization and the interior of the patterned structure is hollow with some solid leavings, and the chalcogenide phase change thin films are very good candidate materials for patterned structure formation.     

Substrate- and oxidation-induced roughness of individual terraces of pentacene thin films

The roughness at the surface of individual pentacene terraces on naturally oxidized silicon wafers was investigated with scanning force microscopy as function of film thickness (one to five layers) and sample exposure to ambient air. For pristine samples, the root-mean-square roughness on individual (001) pentacene terraces was 0.18 nm and varied by less than 0.02 nm between monolayer terraces and terraces in the fifth layer. Storing samples in air and ambient light led to a substantial increase of the roughness, which for terraces up to the third layer became 0.24 nm after four weeks. For fourth layer terraces, the roughness increased less, and terraces in the fifth layer exhibited no significant roughness increase. We explain the roughness increase by photo-oxidation of pentacene, particularly strong within the first layer, which is supported by the appearance of grain boundary widening with storage time. The observation that layers beyond the third one from the substrate are less affected by photo-oxidation (smaller terrace roughness) is likely due to better structural perfection in layers farther from the substrate, which reduces the effective cross-section of molecules for oxidation. These results indicate that native silicon oxide does not allow for the immediate formation of structurally perfect pentacene films in the range of one to three layers, which will reduce charge carrier mobility in pentacene thin film transistors. Thicker pentacene layers can protect underlying layers against oxidation.     

Optimization of silicon nanocrystals growth process by low pressure chemical vapor deposition for non-volatile memory application

The processes of silicon nanocrystals (Si-NCs) growth on both SiO₂ and Si₃N₄ substrates by low pressure chemical vapor deposition have been systematically investigated. A two-step process was adopted for Si-NCs growth: nucleation at a high temperature (580-600 °C) and growth at a low temperature (550 °C). By adjusting the pre-deposition waiting time and deposition time, the density, size and uniformity can be effectively controlled. Compared to the growth of Si-NCs on SiO₂, the coalescence speed of Si-NCs on Si₃N₄ is faster. Uniform Si-NCs with a high density of 1.02 × 10¹² cm^− 2 and 1.14 × 10¹² cm^− 2 have been obtained on SiO₂ and Si₃N₄, respectively. Finally, a Si-NCs-based memory structure with a 2.1 V memory window was demonstrated.     

Microstructural and frictional control of diamond-like carbon films deposited on acrylic rubber by plasma assisted chemical vapor deposition

In this paper we concentrate on the microstructure of diamond-like carbon films prepared by plasma assisted chemical vapor deposition on acrylic rubber. The temperature variation produced by the ion impingement during plasma cleaning and subsequent film deposition was monitored and controlled as a function of bias voltage and treatment time. Its influence during film growth on the appearance of patterns of cracks and wrinkles, caused by the thermal stresses is evaluated. Different growth modes are proposed in order to explain the smaller patch sizes observed at negative variations of temperature. The coefficient of friction (CoF) of the samples is measured using a pin-on-disk tribometer in non-lubricated conditions. Much lower CoF values than unprotected rubber are seen, which can be correlated with the observed patch size.     

Highly conductive, transparent flexible films based on open rings of multi-walled carbon nanotubes

Open rings of multi-walled carbon nanotubes were stacked to form porous networks on a poly(ethylene terephthalate) substrate to form a flexible conducting film (MWCNT-PET) with good electrical conductivity and transparency by a combination of ultrasonic atomization and spin-coating technique. To enhance the electric flexibility, we spin-coated a cast film of poly(vinyl alcohol) onto the MWCNT-PET substrate, which then underwent a thermo-compression process. Field-emission scanning electron microscopy of the cross-sectional morphology illustrates that the film has a robust network with a thickness of ~ 175 nm, and it remarkably exhibits a sheet resistance of approximately 370 Ω/sq with ~ 77% transmittance at 550 nm even after 500 bending cycles. This electrical conductivity is much superior to that of other MWCNT-based transparent flexible films.