High-Speed and Low-Energy Nitride Memristors Choi, Byung Joon; Torrezan, Antonio C.; Strachan, John Paul ...
Advanced functional materials,
August 2, 2016, Letnik:
26, Številka:
29
Journal Article
Recenzirano
Odprti dostop
High‐performance memristors based on AlN films have been demonstrated, which exhibit ultrafast ON/OFF switching times (≈85 ps for microdevices with waveguide) and relatively low switching current ...(≈15 μA for 50 nm devices). Physical characterizations are carried out to understand the device switching mechanism, and rationalize speed and energy performance. The formation of an Al‐rich conduction channel through the AlN layer is revealed. The motion of positively charged nitrogen vacancies is likely responsible for the observed switching.
Ultrafast switching of an AlN memristor: ON switching is acheived using an 85 ps positive voltage pulse, and OFF switching using an 85 ps negative voltage pulse on the Al electrode of a Pt/AlN/Al memristor stack. A relatively low switching current (≈15 μA for 50 nm devices) has also been demonstrated in these memristors based on AlN films. The formation of an Al‐rich conduction channel through the AlN layer is revealed.
Fundamental phenomena governing the tribological mechanisms in sputter deposited amorphous MoS2/Sb2O3/Au nanocomposite coatings are reported. In dry environments the nanocomposite has the same low ...friction coefficient as pure MoS2 (a arrow right 40.007). However, unlike pure MoS2 coatings, which wear through in air (50% relative humidity), the composite coatings showed minimal wear, with wear factors of a arrow right 41.2-1.4A-10aˆ ; '7 mm3 Nma degree 1 in both dry nitrogen and air. The coatings exhibited non-Amontonian friction behavior, with the friction coefficient decreasing with increasing Hertzian contact stress. Cross-sectional transmission electron microscopy of wear surfaces revealed that frictional contact resulted in an amorphous to crystalline transformation in MoS2 with 2H-basal (0002) planes aligned parallel to the direction of sliding. In air the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0002)-oriented basal planes of MoS2, resulting in predominantly self-mated "basal on basal" interfacial sliding and, thus, low friction and wear.
This paper describes the synthesis, structure, and tribological behavior of nanocomposite tungsten disulphide (WS
2) solid lubricant films grown by atomic layer deposition. A new catalytic route, ...incorporating a diethyl zinc catalyst, was established to promote the adsorption and growth of WS
2. The films were grown down to 8
nm in thickness by sequential exposures of WF
6 and H
2S gases in a viscous flow reactor on Si, SiO
2, stainless steel, and polycrystalline Si and electroplated Ni microelectromechanical systems structures. Films were studied by cross-sectional transmission electron microscopy (XTEM) with Automated eXpert Spectral Image Analysis (AXSIA) software for X-ray spectral images and X-ray diffraction to determine the coating conformality and crystallinity. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (0
0
2) basal planes. Regardless of orientation to the substrate surface, these basal planes when sheared imparted low friction with a steady-state friction coefficient as low as 0.008 to 50,000 cycles in a dry nitrogen environment. The formation of smooth transfer films during wear provided low interfacial shear stresses during sliding thus achieving low friction and wear. The XTEM combined with AXSIA of the wear tracks identified this mechanism and the effects of vapor phase reaction by-product etching on insulating and native polycrystalline Si and Ni surfaces.
This paper describes the friction and wear behavior of a Co-Cr alloy sliding on a Ta-W alloy. Measurements were performed in a pin-on-flat configuration with a hemispherically tipped Co-base alloy ...pin sliding on a Ta-W alloy flat from ambient to 430 degreesC. Focused ion beam-scanning electron microscopy (FIB-SEM) and cross-sectional transmission electron microscopy (TEM) were used to identify the friction-induced changes to the chemistry and crystal structure in the subsurface regions of wear tracks. During sliding contact, transfer of material varied as a function of the test temperature, either from pin-to-flat, flat-to-pin, or both, resulting in either wear loss and/or volume gain. Friction coefficients ( mu ) and wear rates also varied as a function of test temperature. The lowest friction coefficient ( mu =0.25) and wear rate (1 x 10 super(-4) mm super(3)/N m) were observed at 430 degreesC in argon atmosphere. This was attributed to the formation of a Co-base metal oxide layer (glaze), predominantly (Co, Cr)O with Rocksalt crystal structure, on the pin surface. Part of this oxide film transferred to the wear track on Ta-W, providing a self-mated oxide-on-oxide contact. Once the oxide glaze is formed, it is able to provide friction reduction for the entire temperature range of this study, ambient to 430 degreesC. The results of this study indicate that glazing the surfaces of Haynes alloys with continuous layers of cobalt chrome oxide prior to wear could protect the cladded surfaces from damage.
This paper describes the evolution of friction-induced microstructures underneath the wear surfaces of nickel single crystals, and their role in influencing the steady state coefficient of friction. ...Using a custom built tribometer, friction measurements were made on {001} and {011} crystal faces along several crystallographic directions in unidirectional linear mode with a Si3N4 ball. Cross-sections of wear scars were prepared by focused ion beam (FIB) microscopy, and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyze the evolution of crystallographic texture and recrystallization. The formation of a thin (10–15 nm) ultra-nanocrystalline layer in the {011} orientation resulted in significant reductions in the coefficient of friction for that configuration, from the initial maximum of 0.64 to a steady state value of 0.32, after approximately 500 cycles of linear, unidirectional sliding. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provides an estimate of the characteristic grain size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material suggests a mechanism for friction reduction. Model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to friction provide a phenomenological description of the lubrication due to ultra-nanocrystalline surface microstructures.
•A novel combination of electron microscopy and tribotesting techniques were utilized.•Friction measurements on single crystal surfaces along specific crystallographic directions was demonstrated.•Subsurface characterization provided insights into the relationships between microstructure and friction.•A model was proposed to help understand the relationships between plasicity, microstructure, and friction.•The model predictions agree well with the available observations and measurements.
Nanocrystalline Ni films with 20–100
nm size grains exhibited either of two distinct friction behaviors,
μ
∼
0.30–0.35 or
μ
∼
0.6–0.7, depending upon the contact pressure and sliding speed. ...Friction-induced changes to grain structure were analyzed by cross-sectional transmission electron microscopy of wear surfaces. Formation of stable ultrafine nanocrystalline layers with 2–10
nm size grains underneath the wear surface may be responsible for the observed friction transitions, possibly due to a transition from traditional dislocation plasticity to deformation controlled by grain boundaries.
Titanium oxide/oxynitride coatings were created on the polished surface of commercially pure, grade 2 titanium substrates by irradiating samples in air using a nanosecond-pulsed, infrared (1064nm) ...fiber laser. Coatings consist of three distinct layers, including a thin TiO2 rutile cap, a TiO middle layer, and an inhomogeneous bottom layer that is composed of TiOxN1−x and possibly oxygen-intercalated phases such as Ti6O. The combined thickness of TiO2 and TiO layers was varied from ~10 to 120nm by increasing the accumulated laser fluence. Laser-grown coatings exhibit different colors, which vary with oxide thickness. The observed color is attributed to the interference of incident white light reflected from the upper and lower boundaries of the TiO2 capping layer.
•Laser-stimulated surface oxidation of titanium forms color markings.•Oxide thickness, microstructure, phase and composition are characterized.•Reflectance and chromaticity are determined.•Substrate structure and composition are examined.
The synthesis of Au–MoS2 nanocomposite thin films and the evolution of their structures during film growth, in situ transmission electron microscopy (TEM) heating, and sliding contact were ...investigated. TEM revealed that the films deposited at ambient (room) temperature (RT) consisted of 2–4 nm size Au particles in a matrix of MoS2. With increasing growth temperatures, the nanocomposite film exhibited structural changes: the Au nanoparticles coarsened by diffusion-driven Ostwald ripening to 5–10 nm size and the MoS2 basal planes encapsulated the Au nanoparticles thereby forming a solid Au-core MoS2 structure. However, when the RT deposited film was heated inside the TEM, the highly ordered MoS2 basal planes did not encapsulate the Au, suggesting that MoS2 surface diffusivity during film growth is different than MoS2 bulk diffusion. Increases in MoS2 crystallinity and coarsening of Au nanoparticles (up to 10 nm at 600 °C) were observed during in situ TEM heating of the RT deposited film. Sliding contact during friction and wear tests resulted in a pressure-induced reorientation of MoS2 basal planes parallel to the sliding direction. The subsurface coarsened Au nanoparticles also provide an underlying load support allowing shear of surface MoS2 basal planes.
We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET ...was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal–liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3
wt.% S, compared to 0.5 or 9
wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.
Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3
wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.
Phosphate inclusions in EET show Δ
17O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150
km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal–liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.
Nanosecond-pulsed, infrared (1064nm) laser irradiation was used to create metal oxide coatings on the surface of polished stainless steel 304L austenite for application as color markings and unique ...tags/identifiers. By rastering a Gaussian-shaped, focused laser beam across a specimen in air, continuous metal oxide coatings were grown to thicknesses in the range of 20 to ~500nm. Oxide coating thickness generally increased with laser fluence. However, for large accumulated fluences in excess of ~600–800J/cm2, oxide growth was affected by evaporation and particle ejection resulting in a decreased coating thickness. Transmission electron microscopy and X-ray diffraction revealed that oxide coatings developed a polycrystalline, spinel structure having a lattice constant=8.4Å (consistent with MnCr2O4 and Fe3O4). Pulsed laser irradiation and oxide growth modified the composition of stainless steel substrates by reducing the Cr and Mn concentration within the melt zone. The reflectance and chromaticity of laser-fabricated oxide coatings were characterized using spectrophotometry. These optical properties are described in the context of measured oxide thicknesses.
► Laser-stimulated surface oxidation of stainless steel ► Formation of color markings ► Characterization of oxide thickness, microstructure, phase and composition ► Optical properties including reflectance and chromaticity ► Correlation of optical properties with oxide thickness
PDF
