Sublimation-based wafer-scale monolayer WS2 formation via self-limited thinning of few-layer WS2

Atomically-thin monolayer WS2 is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS2 also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS2, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS2 by applying the in situ self-limited thinning of multilayer WS2 formed by sulfurization of WOx films. Through a pulsed supply of sulfur precursor vapor under a continuous H2 flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS2 to the monolayer limit without damaging the remaining bottom WS2 monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS2 top layers as opposed to a stable bottom monolayer WS2 on sapphire above a vacuum sublimation temperature of WS2. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2nd layer WS2 domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS2 on the wafer-scale.

Liquid-Metal-Printed Ultrathin Oxides for Atomically Smooth 2D Material Heterostructures

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOTs), excellent gate control, and low leakage currents. Here, large-area liquid-metal-printed ultrathin Ga₂O₃ dielectrics for 2D electronics and optoelectronics are reported. The atomically smooth Ga₂O₃/WS₂ interfaces enabled by the conformal nature of liquid metal printing are directly visualized. Atomic layer deposition compatibility with high-k Ga₂O₃/HfO₂ top-gate dielectric stacks on a chemical-vapor-deposition-grown monolayer WS₂ is demonstrated, achieving EOTs of ∼1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultrascaled low-power logic circuits. These results show that liquid-metal-printed oxides can bridge a crucial gap in dielectric integration of 2D materials for next-generation nanoelectronics.

Gate-Defined Quantum Confinement in CVD 2D WS2

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS₂ single- and bilayer devices before and after atomic layer deposition (ALD) of HfO₂ . This isolates the influence of HfO₂ deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS₂ and ALD-grown HfO₂ . The high dielectric constant and low leakage current enabled by HfO₂ allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts

Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer length measurements are performed on chemical vapor deposition grown single-layer and bilayer WS₂ devices with indium alloy contacts. The devices exhibit low contact resistances and Schottky barrier heights (∼10 kΩ μm at 3 K and 1.7 meV). Efficient carrier injection enables high carrier mobilities (∼190 cm² V^-1 s^-1) and observation of resonant tunnelling. Density functional theory calculations provide insights into quantum transport and properties of the WS₂-indium interface. Our results reveal significant advances toward high-performance WS₂ devices using indium alloy contacts.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides

The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Employing a Bifunctional Molybdate Precursor To Grow the Highly Crystalline MoS2 for High-Performance Field-Effect Transistors

Growth of the large-sized and high-quality MoS₂ single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS₂ contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS₂ is yet far below the expected theoretical values, implying that the MoS₂ crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS₂ FETs with room-temperature electron mobility up to ∼90 cm² V^-1 s^-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS₂ crystals (edge length up to ∼260 μm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS₂ single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS₂ crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS₂ sheds light on its potential in low-voltage and self-powered electronics.

Modification of Vapor Phase Concentrations in MoS2 Growth Using a NiO Foam Barrier

Single-layer molybdenum disulfide (MoS₂) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS₂ films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS₂ on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO₃, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin-orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline and well-oriented MoS₂ monolayer.

Thermally Stable Mesoporous Perovskite Solar Cells Incorporating Low-Temperature Processed Graphene/Polymer Electron Transporting Layer

In the short time since its discovery, perovskite solar cells (PSCs) have attained high power conversion efficiency but their lack of thermal stability remains a barrier to commercialization. Among the experimentally accessible parameter spaces for optimizing performance, identifying an electron transport layer (ETL) that forms a thermally stable interface with perovskite and which is solution-processable at low-temperature will certainly be advantageous. Herein, we developed a mesoporous graphene/polymer composite with these advantages when used as ETL in CH₃NH₃PbI₃ PSCs, and a high efficiency of 13.8% under AM 1.5G solar illumination could be obtained. Due to the high heat transmission coefficient and low isoelectric point of mesoporous graphene-based ETL, the PSC device enjoys good chemical and thermal stability. Our work demonstrates that the mesoporous graphene-based scaffold is a promising ETL candidate for high performance and thermally stable PSCs.

Chemically doped radial junction characteristics in silicon nanowires

We evaluate the boron (B) and phosphorus (P) core-surface codoped radial p-n junction characteristics in silicon nanowires (SiNWs) using density functional theory calculations. We find that the formation of radial p-n junction is energetically favorable. The stability depends on the diameter of SiNWs and the dopant concentration. Generally, a higher concentration of B-P pair dopants results in a more stable nanowire. More importantly, we predict that the radial p-n junction can evolve into a Schottky-like junction in relatively highly doped SiNWs when the diameter increases, attributing to the change of the core p-doping characteristic, that is, the core p-junction becomes metallic, while the n-junction near the surface remains semiconducting. The interfacial contact between the junctions is found to be the key for such change. Our calculated results support an experimental observation in SiNW solar cells.

First-principles study of silicon nanowire approaching the bulk limit

First-principles density functional theory calculations on hydrogenated silicon nanowires (SiNWs) with diameters up to 7.3 nm are carried out for comparing to experimentally relevant SiNWs and evaluating its radial doping profiles. We show that the direct band gap nature of both the small diameter (110) and (100) SiNWs fades when the diameter reaches beyond about 4 nm, where the difference of direct and indirect band gaps are close, within the experimental measurement uncertainty of ±0.1 eV, suggesting the diameter size where the gap nature transition starts. In addition, we reveal that core-surface boron (B) codoped SiNW forms more preferably at large diameter than that of the surface-surface codoped one, attributing to the lower energy configuration raised by the core B dopant at large diameter SiNW. More importantly, the diameter for such a preferential transition increases as the doping concentration decreases. Our results rationalize photoluminescent measurements and radial doping distributions of SiNWs.

Amino acid sequence of rabbit skeletal muscle myosin. 50-kDa fragment of the heavy chain

The amino acid sequence of the 50-kDa fragment that is released by limited tryptic digestion of the head portion of rabbit skeletal muscle myosin was determined by analysis and alignment of sets of peptides generated by digestion of the fragment at arginine or methionine residues. This fragment contains residues 205-636 of the myosin heavy chain; among the residues of particular interest in this fragment are N epsilon-trimethyllysine, one of four methyl-amino acids in myosin, and Ser-324, which is photoaffinity labeled by an ATP analogue (Mahmood, R., Elzinga, M., and Yount, R. G. (1989) Biochemistry 28, 3989-3995). Combination of this sequence with those of the 23- and 20-kDa fragments yields an 809-residue sequence that constitutes most of the heavy chain of chymotryptic S-1 of this myosin.

The sequence of the NH2-terminal 204-residue fragment of the heavy chain of rabbit skeletal muscle myosin

The amino acid sequence of the Mr = 23,000 peptide that is generated by limited tryptic digestion of rabbit skeletal muscle heavy meromyosin was determined. This fragment represents the NH2-terminal 204 residues of the heavy chain of myosin, and its sequence is (formula; see text) This peptide contains the lysine residue (Lys 83) whose reaction with 2,4,6,-trinitrobenzenesulfonate alters the enzymatic activity of myosin, as well as two types of methylated lysines. Position 34 is occupied by epsilon-N-monomethyllysine and lysine in an approximate 60:40 ratio, while position 129 is fully occupied by one of the two epsilon-N-trimethyllysines in myosin heavy chain. There is evidence suggesting that this fragment contains residues that contribute to the ATP-binding site of myosin; the sequence surrounding the epsilon-N-trimethyllysine is devoid of charges and could form a hydrophobic pocket for binding of the adenine moiety while the epsilon-N-trimethyllysine, which carries a positive charge at all pH values, could bind an ATP phosphate group.

Cloned mRNA sequences for two types of embryonic myosin heavy chains from chick skeletal muscle. I. DNA and derived amino acid sequence of light meromyosin

Two myosin heavy chain cDNA clones (251 and 110), constructed from chick embryonic skeletal muscle mRNA, were subjected to extensive DNA sequence analysis. A complete description of the DNA sequence of clone 251 was obtained. This 1.5-kilobase pair cDNA sequence specified the COOH-terminal 439 amino acids of the myosin heavy chain, and included the entire 3' nontranslated region. The translated and 3' nontranslated sequences were purine- (64%) and AT-(71%) rich, respectively. The derived amino acid sequence of clone 251 correlated well with sequences obtained by direct amino acid sequencing of adult rabbit back muscle myosin heavy chain protein (87% homology), as well as with cloned myosin heavy chain sequences from other species. Comparison of clone 251 with a partial DNA sequence of clone 110 revealed significant structural differences both in the translated, and 3' nontranslated regions. This data indicates that these two clones represent two distinct myosin heavy chain genes. The protein sequence specified by clone 251 corresponds to the light meromyosin portion of the myosin heavy chain rod. These sequences, like other myosin heavy chain rod sequences, are alpha-helical and exhibit 7- and 28-residue periodicities in the linear distribution of nonpolar, and basic and acidic amino acids, respectively.