Giant Carrier Mobility in a Room-Temperature Ferromagnetic VSi2N4 Monolayer

Using density functional theory (DFT), we investigate that two possible phases of VSi2N4 (VSN) may be realized, one called the "H phase" corresponding to what is known from calculation and herein the other new "T phase" being stabilized by a biaxial tensile strain of 3%. Significantly, the H phase is predicted to display a giant carrier mobility of 1 × 106 cm2 V-1 s-1, which exceeds that for most 2D magnetic materials, with a Curie temperature (TC) exceeding room temperature and a band gap of 2.01 eV at the K point. Following the H-T phase transition, the direct band gap shifts to the Γ point and increases to 2.59 eV. The Monte Carlo (MC) simulations also indicate that TC of the T phase VSN can be effectively modulated by strain, reaching room temperature under a biaxial strain of -4%. These results show that VSN should be a promising functional material for future nanoelectronics.

Calculating and analyzing time delay in zigzag graphene nanoscrolls based complementary metal-oxide-semiconductors

Graphene Nano Scrolls (GNSs) and Zigzag graphene nanoscrolls (ZGNSs) are semi-one-dimensional materials with exceptional electrical and optical properties, making them attractive to be used in nanoelectronics and complementary metal-oxide-semiconductor (CMOS) technology. With in CMOS device technology, time delay is a crucial issue in the design and implementation of CMOS based ZGNSs. Current paper focus is on ZGNSs application in the channel area of metal-oxide-semiconductor field-effect transistors (MOSFETs) in CMOS technology. We studied analytically, the importance of different parameters on time delay reduction, resulting in faster switching and higher frequency in integrated circuits (ICs). The results of this research demonstrates that, the ZGNS-based CMOS proves considerable variations in the current due to the geometrical parameters, such as chirality number, channel length, and nanoscroll length which can be engineered to produce faster ICs.

Chemically sprayed CdO: Cr thin films for formaldehyde gas detection and optoelectronic applications

Chromium (Cr) doped CdO films are chemically sprayed and are characterized by their optical, electrical, structural, and microstructural characteristics. The thickness of the films is determined by spectroscopic ellipsometry. The cubic crystal structure with a superior growth along (111) plane of the spray-deposited films is confirmed from the powder X-ray diffraction (XRD) analysis. XRD studies also suggested that some of the Cd²⁺ ions were substituted by Cr³⁺ ions, and the solubility of Cr in CdO is minimal, to be around ∼0.75 wt%. The analysis by atomic force microscopy shows uniform distribution of grains throughout the surface, whose roughness is varied from 33 to 13.9 nm concerning Cr-doping concentration. The microstructures from the field emission scanning electron microscope reveal a smooth surface. The elemental composition is examined using an energy dispersive spectroscope. The micro-Raman studies carried out in room temperature endorse the presence of metal oxide (Cd-O) bond vibrations. Transmittance spectra are obtained using UV-vis-NIR spectrophotometer, and the band gap values are estimated from the absorption coefficient. The films show high optical transmittance (>75%) in vis-NIR region. A maximum optical band gap of 2.35 eV is obtained from 1.0 wt% Cr-doping. The electrical measurement (Hall analysis) confirmed the degeneracy nature and n-type semi-conductivity. The carrier density, carrier mobility, and dc-conductivity are increased for higher Cr-dopant percentage. High mobility (85 cm²V^-1s^-1) is observed for 0.75 wt% Cr-doping. The 0.75 wt% Cr-doping show a remarkable response to formaldehyde gas (74.39%).

Ni and O co-modified MoS2 as universal SERS substrate for the detection of different kinds of substances

Surface-enhanced Raman scattering (SERS) has attracted extensive attention as an ultrasensitive detection method. However, the poor biocompatibility and expensive synthesis cost of noble metal SERS substrates have become non-negligible factors that limit the development of SERS technology. Metal chalcogenide semiconductors as an alternative to noble metal SERS substrates can avoid these disadvantages, but the enhancement effect is lower than that of noble metal substrates. Here, we report a method to co-modify MoS₂ by Ni and O, which improves the carrier concentration and mobility of MoS₂. The SERS effect of the modified MoS₂ is comparable to that of noble metals. We found that the improved SERS performance of MoS₂ can be attributed to the following two factors: strong interfacial dipole-dipole interaction and efficient charge transfer effect. During the doping process, the incorporation of Ni and O enhances the polarity and carrier concentration of MoS₂, enhances the interfacial interaction of MoS₂, and provides a basis for charge transfer. During the annealing process, the introduction of O atoms into the S defects reduces the internal defects of doped MoS₂, improves the carrier mobility, and promotes the efficient charge transfer effect of MoS₂. The final modified MoS₂ as a SERS substrate realizes low-concentration detection of bilirubin, cytochrome C, and trichlorfon. This provides promising guidance for the practical inspection of metal chalcogenide semiconductor substrates.

Strain-tuned mechanical, electronic, and optoelectronic properties of two-dimensional transition metal sulfides ZrS2: a first-principles study

Two-dimensional semiconductor material zirconium disulfide (ZrS₂) monolayer is a new promising material with good prospects for nanoscale applications. Recently, a new zirconium disulfide (ZrS₂) monolayer with a space group of 59_Pmmn has been successfully predicted. Using first-principles calculations, this new monolayer ZrS₂ structure is obtained with stable indirect bandgaps of 0.65 eV and 1.46 eV at the DFT-PBE (HSE06) functional levels, respectively. Strain engineering studies on the ZrS₂ monolayer show effective bandgap modulation. The bandgap shows a nearly linear regularity from narrow to wide under strain (ranged from - 6 to + 8%). Young's modulus of elasticity of ZrS₂ along the tensile directions (x-axis and y-axis) is 83.63 (N/m) and 63.61 (N/m) with Poisson's ratios of 0.09 and 0.07, respectively. The results of carrier mobility show that the electron mobility along the y-axis can reach 1.32 × 10³ cm² V^-1 s^-1. Besides, the order of magnitude of the light absorption coefficient in the ultraviolet spectral region is calculated to reach 2.0 × 10⁵ cm^-1 for ZrS₂ monolayers. Moreover, the bandgap and band edge position of Pmmn-ZrS₂ can satisfy the redox potentials of photocatalytic water splitting by strain regulating. The results indicate that the new two-dimensional Pmmn-ZrS₂ monolayer is a potential material for photovoltaic devices and photocatalytic water decomposition.

SnxPy Monolayers: a New Type of Two-Dimensional Materials with High Stability, Carrier Mobility, and Magnetic Properties

Searching for two-dimensional (2D) group V materials with ferromagnetism, elastic anisotropy, and carrier mobility and tunable band structure is one key to developing constantly developing nanodevices. The 2D monolayers SnxPy with x/y (1/1, 1/2, 1/3, and so on) coordination number are studied based on the particle-swarm optimization technique combined with the density functional theory optimization. Its thermal stability can be confirmed by molecular dynamics at 70K and 300K, indicating that the novel 2D materials have a stable existence. The electronic band structures of four stable structures suggest that all the monolayers of SnxPy are fully adjustable and flexible tunable band gaps semiconductors under the biaxial strain. The monolayer of P[Formula: see text]m-SnP2 with unique valence band structure can go from nonmagnetic to ferromagnetic by the hole doping because of the "Stoner criterion," and Pmc21-SnP2 is a direct-like gap semiconductor with in-plane elastic anisotropy to possess a high electron mobility as high as 800 cm2V-1 s-1 along the kb direction, which is much higher than that of MoS2 (∼ 200 cm2V-1 s-1). The optical absorption peak of the material is in the ultraviolet region. These discoveries expand the potential applications of the emerging field of 2D SnxPy structures in nanoelectronics.

ZnCdO2 monolayer - A complex 2D structure of ZnO and CdO monolayers for photocatalytic water splitting driven by visible-light

ZnO monolayer possesses band structure matching the conditions of water splitting for hydrogen generation but cannot well response to the visible light, while CdO one, contrariwise, have obvious optical absorption in the visible light range but no satisfactory band edges for the water splitting to produce hydrogen. Here, we predict a two-dimensional ZnCdO₂ structure comprising of ZnO and CdO ones to achieve their strengths. The band structures, optical properties, carrier mobility, and the strain engineering for ZnCdO₂, ZnO and CdO monolayers are investigated by using the first-principles hybridization functional calculations. The results demonstrate that the two-dimensional ZnCdO₂ structure is a promising candidate for water splitting to produce hydrogen. All the structures show a direct band energy gap and the character remains unchanged under the considered biaxial strains. All the conduction band minimums are suitable for water splitting reaction even under the -4% to +4% strain. Moreover, the valence band maximum of ZnCdO₂ monolayer matches the conditions of the water-splitting reaction under the -2% to +4% strain. Interestingly, the unsatisfactory valence band maximum of CdO monolayer can be overcome by strain larger than +2%. As expected, the enhanced optical absorption in the visible light range is observed for the ZnCdO₂ monolayer. Additionally, the mobilities of the hole and the electron are significantly different for the three monolayers, implying that the low recombination ratio of the photogenerated carrier pairs is available, which is also beneficial for the photocatalytic performance. Therefore, ZnCdO₂ monolayer and CdO monolayer (with tensile strain larger than 2%) is a promising candidate for the water splitting to produce hydrogen under the irradiation of the solar light.

Theoretical Study on Carrier Mobility of Hydrogenated Graphene/Hexagonal Boron-Nitride Heterobilayer

Hydrogenated graphene (HG)/hexagonal boron nitride (h-BN) heterobilayer is an ideal structure for the high-performance field effect transistor. In this paper, the carrier mobilities of HG/h-BN heterobilayer are investigated based on the first-principles calculations by considering the influence of stacking pattern between HG and h-BN, hydrogen coverage and hydrogenation pattern. With the same hydrogenation pattern, the electron mobility monotonously decreases when the hydrogen coverage increases. With the same hydrogen coverage, different hydrogenation patterns lead to significant changes of mobility. For 25% and 6.25% HGs, the μe (ΓK) of 25% pattern I is 8985.85 cm2/(V s) and of 6.25% pattern I is 23,470.98 cm2/(V s), which are much higher than other patterns. Meanwhile, the h-BN substrate affects the hole mobilities significantly, but it has limit influences on the electron mobilities. The hole mobilities of stacking patterns I and II are close to that of HG monolayer, but much lower than that of stacking patterns III and IV.

Toward High Carrier Mobility and Low Contact Resistance: Laser Cleaning of PMMA Residues on Graphene Surfaces

ABSTRACT

Poly(methyl methacrylate) (PMMA) is widely used for graphene transfer and device fabrication. However, it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance. How to eliminate contamination and restore clean surfaces of graphene is still highly demanded. In this paper, we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure. Under proper laser conditions, PMMA residues can be substantially reduced without introducing defects to the underlying graphene. Furthermore, by applying this laser cleaning technique to the channel and contacts of graphene field-effect transistors (GFETs), higher carrier mobility as well as lower contact resistance can be realized. This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.

GRAPHICAL ABSTRACT

Optimizing pentacene thin-film transistor performance: Temperature and surface condition induced layer growth modification

In this work we present in situ electrical and surface analytical, as well as ex situ atomic force microscopy (AFM) studies on temperature and surface condition induced pentacene layer growth modifications, leading to the selection of optimized deposition conditions and entailing performance improvements. We prepared p++-silicon/silicon dioxide bottom-gate, gold bottom-contact transistor samples and evaluated the pentacene layer growth for three different surface conditions (sputtered, sputtered + carbon and unsputtered + carbon) at sample temperatures during deposition of 200 K, 300 K and 350 K. The AFM investigations focused on the gold contacts, the silicon dioxide channel region and the highly critical transition area. Evaluations of coverage dependent saturation mobilities, threshold voltages and corresponding AFM analysis were able to confirm that the first 3-4 full monolayers contribute to the majority of charge transport within the channel region. At high temperatures and on sputtered surfaces uniform layer formation in the contact-channel transition area is limited by dewetting, leading to the formation of trenches and the partial development of double layer islands within the channel region instead of full wetting layers. By combining the advantages of an initial high temperature deposition (well-ordered islands in the channel) and a subsequent low temperature deposition (continuous film formation for low contact resistance) we were able to prepare very thin (8 ML) pentacene transistors of comparably high mobility.