Enhanced gas sensing response for 2D α-MoO3 layers: thickness-dependent changes in defect concentration, surface oxygen adsorption, and metal-metal oxide contact

In this study, α-MoO3 two-dimensional (2D) layers and thin films were synthesized using pulsed laser deposition technique. X-ray diffraction and Raman spectroscopy confirm the formation of anisotropic α-MoO3. Atomic Force Microscopy images show the evolution of morphology from 2D layers to oriented crystallite growth with increase in film thickness. Temperature and gas concentration dependent, gas sensing response has been observed to be quite different in 2D layers in comparison to thin film sample. 2D α-MoO3 layers (∼ 6 nm) show higher response of about 25 % at a lower temperature (100 °C); They exhibit lower detection limits (up to 100 ppb) and selectivity towards NO2 gas. Also, 2D layers show p-type gas sensing response and nonlinear current-voltage (I–V) characteristics of metal- metal oxide junctions, in complete contrast to thin film sample, which shows an n-type gas sensing response and linear I–V characteristics. The results have been explained on the basis of lower oxygen defect concentration, enhanced oxygen adsorption at the surface, and metal- α-MoO3 contact favoring hole conduction.acceptedVersio

Charge Transport in 2D MoS2, WS2, and MoS2–WS2 Heterojunction-Based Field-Effect Transistors: Role of Ambipolarity

Electrical and optical characteristics of few-layered (3–4 L) chemical vapor deposition (CVD) grown MoS2, WS2, and MoS2–WS2 heterostructure-based back-gated field-effect transistor (FET) devices have herein been studied. The structure, stoichiometry, and work function of the two-dimensional (2D) materials that comprise the channel region have been comprehensively characterized. The MoS2 device exhibits a unipolar n-type behavior with a high field-effect ON/OFF ratio (>103) and a low subthreshold swing of 668 mV/decade at room temperature. WS2 and MoS2–WS2 heterostructure devices exhibit gate driven ambipolarity due to chemically active defect sites, offering precise control on the carrier type necessary for realization of logic devices. Record-high room-temperature electron mobility (19 cm2/V.s) exhibited by the MoS2–WS2 heterostructure device displays an improved electrical performance of almost one order of magnitude higher than already existing 2D devices. The prototype of a 2D complementary metal–oxide–semiconductor (CMOS) logic inverter switch integrating high electronic and optical responses of the MoS2–WS2 heterostructure junction owing to ambipolar FET operation has been demonstrated. The achieved results encompassing superior photoabsorption, atomically thin thickness, and high performance indices suggest that soft 2D heterostructure devices may open a new paradigm in artificial retinal implants and photoelectronics.acceptedVersio

Error Events Due to Island Size Variations in Bit Patterned Media

Control of the variations of island properties is one of the key challenges in fabricating Bit-Patterned Media for future storage systems. The presence on any variation in the size and position of an island has a detrimental effect on the ability to recover recorded data, particularly in the case of variation in island size. By analyzing error events when island size variations are present we have identified that these are more likely to be single-bit in nature. To understand the origins of these error events we have investigated the size and magnetization state of islands in the vicinity where a single-bit error event is encountered. It is shown that these error events occur due to particular combinations of island size and magnetization state for the three islands investigated. In every case the central island, from which the data bit is recovered in error, is small compared to the nominal island size. These results show that size variations must be controlled in the fabrication process in order to maximize the bit-error-rate performance of the read channel

Bending machine for testing reliability of flexible electronics

A novel bending machine has been designed and tested. It enables flexible electronics to be subjected to repeated bending with constant radius and tension. In-situ electrical characterization can give accurate analysis of lifetime distributions if sufficiently many samples are ran to failure, allowing reliability prediction models to be developed. Four sets of test samples with different combinations of substrate, routing, interconnect technology and components were examined. A poor level of reliability was observed when using anisotropic conductive paste to form interconnects, whereas a significantly higher level of reliability was observed when using a bismuth-tin solder paste. The assembly of larger components resulted in shortened time to failure, whereas increasing the bending radius prolonged the observed lifetimes.acceptedVersio

High-Rate Epitaxial Growth of Silicon Using Electron Beam Evaporation at High Temperatures

This paper describes the high-rate (~1.5 μm/min) growth of Si films on Si supporting substrates with (100) crystallographic orientation at 600 °C, 800 °C, and 1000 °C in a vacuum environment of ~1 × 10−5 mbar using electron beam (e-beam) evaporation. The microstructure, crystallinity, and conductivity of such films were investigated. It was established that fully crystalline (Raman spectroscopy, EBSD) and stress-free epi-Si layers with a thickness of approximately 50 µm can be fabricated at 1000 °C, while at 600 °C and 800 °C, some poly-Si inclusions were observed using Raman spectroscopy. Hall effect measurements showed that epi-Si layers deposited at 1000 °C had resistivity, carrier concentration, and mobility comparable to those obtained for c-Si wafers fabricated through ingot growth and wafering using the same solar grade Si feedstock used for the e-beam depositions. The dislocation densities were determined to be ∼2 × 107 cm−2 and ∼5 × 106 cm−2 at 800 and 1000 °C, respectively, using Secco etch. The results highlight the potential of e-beam evaporation as a promising and cost-effective alternative to conventional CVD for the growth of epi-Si layers and, potentially, epi-Si wafers. Some of the remaining technical challenges of this deposition technology are briefly indicated and discussed.publishedVersio

High-Performance and Ultralow-Noise Two-Dimensional Heterostructure Field-Effect Transistors with One-Dimensional Electrical Contacts

Two-dimensional heterostructure field-effect transistors (2D-HFETs) with one-dimensional electrical contacts to atomically thin channels have recently shown great device performance, such as reduced contact resistance, leading to ballistic transport and enhanced carrier mobility. While a number of low-frequency noise studies exists on bare graphene devices supported on silicon dioxide gate insulators with surface contacts, such studies in heterostructure devices comprising epitaxial graphene on hexagonal boron nitride (hBN) with edge contacts are extremely limited. In this article, we present a systematic, temperature-dependent study of electrical transport and low-frequency noise in edge-contacted high-mobility HFET with a single atomic-layer graphene channel encapsulated by hBN and demonstrate ultralow noise with a Hooge parameter of ≈10–5. By combining measurements and modeling based on underlying microscopic scattering mechanisms caused by charge carriers and phonons, we directly correlate the high-performance, temperature-dependent transport behavior of this device with the noise characteristics. Our study provides a pathway towards engineering low-noise graphene-based high-performance 2D-FETs with one-dimensional edge contacts for applications such as digital electronics and chemical/biological sensing.acceptedVersio

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surrounding a − Si matrix by combining scanning transmission electron microscopy imaging, electron energy loss spectroscopy and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, simulated results found that the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmon energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further.publishedVersio

Tuning thermoelectric properties of Sb2_2Te3_3-AgSbTe2_2 nanocomposite thin film -- synergy of band engineering and heat transport modulation

The present study demonstrates a large enhancement in the Seebeck coefficient and ultralow thermal conductivity (TE) in Sb$_2$Te$_3$-AgSbTe$_2$ nanocomposite thin film. The addition of Ag leads to the in-situ formation of AgSbTe$_2$ secondary phase nanoaggregates in the Sb$_2$Te$_3$ matrix during the growth resulting in a large Seebeck coefficient and reduction of the thermal conductivity. A series of samples with different amounts of minor AgSbTe$_2$ phases are prepared to optimize the TE performance of Sb$_2$Te$_3$ thin films. Based on the experimental and theoretical evidence, it is concluded that a small concentration of Ag promotes the band flattening and induces a sharp resonate-like state deep inside the valence band of Sb$_2$Te$_3$, concurrently modifying the density of states (DOS) of the composite sample. In addition, the electrical potential barrier introduced by the band offset between the host TE matrix and the secondary phases promotes strong energy-dependent carrier scattering in the composite sample, which is also responsible for enhanced TE performance. A contemporary approach based on scanning thermal microscopy is performed to experimentally obtain thermal conductivity values of both the in-plane and cross-plane directions, showing a reduced in-plane thermal conductivity value by ~ 58% upon incorporating the AgSbTe$_2$ phase in the Sb$_2$Te$_3$ matrix. Benefitting from the synergistic manipulation of electrical and thermal transport, a large ZT value of 2.2 is achieved at 375 K. The present study indicates the importance of a combined effect of band structure modification and energy-dependent charge carrier scattering along with reduced thermal conductivity for enhancing TE properties

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surrounding aSi matrix by combining scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmons energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further

Vertical Field Effect Transistor based on Graphene-WS2 Heterostructures for flexible and transparent electronics

The celebrated electronic properties of graphene have opened way for materials just one-atom-thick to be used in the post-silicon electronic era. An important milestone was the creation of heterostructures based on graphene and other two-dimensional (2D) crystals, which can be assembled in 3D stacks with atomic layer precision. These layered structures have already led to a range of fascinating physical phenomena, and also have been used in demonstrating a prototype field effect tunnelling transistor - a candidate for post-CMOS technology. The range of possible materials which could be incorporated into such stacks is very large. Indeed, there are many other materials where layers are linked by weak van der Waals forces, which can be exfoliated and combined together to create novel highly-tailored heterostructures. Here we describe a new generation of field effect vertical tunnelling transistors where 2D tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or CVD-grown graphene. Our devices have unprecedented current modulation exceeding one million at room temperature and can also operate on transparent and flexible substrates