Enhancing optical characteristics of mediator-assisted wafer-scale MoS2and WS2on h-BN.

Two-dimensional (2D) materials and their heterostructures exhibit intriguing optoelectronic properties; thus, they are good platforms for exploring fundamental research and further facilitating real device applications. The key is to preserve the high quality and intrinsic properties of 2D materials and their heterojunction interface even in production scale during the transfer and assembly process so as to apply in semiconductor manufacturing field. In this study, we successfully adopted a wet transfer existing method to separate mediator-assisted wafer-scale from SiO2/Si growing wafer for the first time with intermediate annealing to fabricate wafer-scale MoS2/h-BN and WS2/h-BN heterostructures on a SiO2/Si wafer. Interestingly, the high-quality wafer-scale 2D material heterostructure optical properties were enhanced and confirmed by Raman and photoluminescence spectroscopy. Our approach can be applied to other 2D materials and expedite mass production for industrial applications.

The international glycan repository GlyTouCan version 3.0.

Glycans serve important roles in signaling events and cell-cell communication, and they are recognized by lectins, viruses and bacteria, playing a variety of roles in many biological processes. However, there was no system to organize the plethora of glycan-related data in the literature. Thus GlyTouCan (https://glytoucan.org) was developed as the international glycan repository, allowing researchers to assign accession numbers to glycans. This also aided in the integration of glycan data across various databases. GlyTouCan assigns accession numbers to glycans which are defined as sets of monosaccharides, which may or may not be characterized with linkage information. GlyTouCan was developed to be able to recognize any level of ambiguity in glycans and uniquely assign accession numbers to each of them, regardless of the input text format. In this manuscript, we describe the latest update to GlyTouCan in version 3.0, its usage, and plans for future development.

Enhanced Performance of WS2 Field-Effect Transistor through Mono and Bilayer h-BN Tunneling Contacts.

Transition metal dichalcogenides (TMDs) are of great interest owing to their unique properties. However, TMD materials face two major challenges that limit their practical applications: contact resistance and surface contamination. Herein, a strategy to overcome these problems by inserting a monolayer of hexagonal boron nitride (h-BN) at the chromium (Cr) and tungsten disulfide (WS2 ) interface is introduced. Electrical behaviors of direct metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) contacts with mono- and bilayer h-BN in a four-layer WS2 field-effect transistor (FET) are evaluated under vacuum from 77 to 300 K. The performance of the MIS contacts differs based on the metal work function when using Cr and indium (In). The contact resistance is significantly reduced by approximately ten times with MIS contacts compared with that for MS contacts. An electron mobility up to ≈115 cm2  V-1  s-1 at 300 K is achieved with the insertion of monolayer h-BN, which is approximately ten times higher than that with MS contacts. The mobility and contact resistance enhancement are attributed to Schottky barrier reduction when h-BN is introduced between Cr and WS2 . The dependence of the tunneling mechanisms on the h-BN thickness is investigated by extracting the tunneling barrier parameters.

Self-Forming p-n Junction Diode Realized with WSe2 Surface and Edge Dual Contacts.

Owing to their practical applications, two-dimensional semiconductor p-n diodes have attracted enormous attention. Over the past decade, various methods, such as chemical doping, heterojunction structures, and metallization using metals with different work functions, have been reported for fabrication of such devices. In this study, a lateral p-n junction diode is formed in tungsten diselenide (WSe2 ) using a combination of edge and surface contacts. The appearance of amorphous tungsten oxide at etched WSe2 , and the formation of a junction near the edge contact, are verified by high-resolution transmission electron microscopy. The device demonstrates high on/off ratio for both the edge and surface contacts, with respective values of 107 and 108 . The diode can achieve extremely high mobility of up to 168 cm2 V-1 s-1 and a rectification ratio of 103 . The ideality factor is 1.11 at a back gate voltage VG   = 60 V at 300 K. The devices with encapsulation of hexagonal boron nitride exhibit good stability to atmospheric exposure, over a measured period of 2 months. In addition, the architecture of the contacts, which is based on a single-channel device, should be advantageous for the implementation of more complicated applications such as inverters, photodetectors, and light-emitting diodes.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure.

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e2/h in addition to quarter plateaus such as 0.25 × 2e2/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

Investigation of plant leaf-derived graphene quantum dot clusters via magnetic force microscopy.

Magnetic force microscopy (MFM) is utilized to characterize the magnetic moment in nanostructured plant leaf-derived graphene quantum dot clusters (GQDCs). The MFM signal reveals that the magnetic response of the GQDCs depends on the height and width of the GQDCs. However, individual GQDs, and smaller clusters with widths of less than 20 nm, have not shown any observable magnetic signal. Importantly, experimental analyses suggest that the magnetic signal of GQDCs distributed in a plane can be effectively detected at room temperature. These results could pave the way for future graphene-based magnetic storage media and spin manipulation quantum devices.

High mobility field-effect transistors based on MoS2crystals grown by the flux method.

Two-dimensional (2D) molybdenum disulphide (MoS2) transition metal dichalcogenides (TMDs) have great potential for use in optical and electronic device applications; however, the performance of MoS2is limited by its crystal quality, which serves as a measure of the defects and grain boundaries in the grown material. Therefore, the high-quality growth of MoS2crystals continues to be a critical issue. In this context, we propose the formation of high-quality MoS2crystals via the flux method. The resulting electrical properties demonstrate the significant impact of crystal morphology on the performance of MoS2field-effect transistors. MoS2made with a relatively higher concentration of sulphur (a molar ratio of 2.2) and at a cooling rate of 2.5 °C h-1yielded good quality and optimally sized crystals. The room-temperature and low-temperature (77 K) electrical transport properties of MoS2field-effect transistors (FETs) were studied in detail, with and without the use of a hexagonal boron nitride (h-BN) dielectric to address the mobility degradation issue due to scattering at the SiO2/2D material interface. A maximum field-effect mobility of 113 cm2V-1s-1was achieved at 77 K for the MoS2/h-BN FET following high-quality crystal formation by the flux method. Our results confirm the achievement of large-scale high-quality crystal growth with reduced defect density using the flux method and are key to achieving higher mobility in MoS2FET devices in parallel with commercially accessible MoS2crystals.

Investigation of laser-induced-metal phase of MoTe2 and its contact property via scanning gate microscopy.

Although semiconductor to metal phase transformation of MoTe2 by high-density laser irradiation of more than 0.3 MW cm-2 has been reported, we reveal that the laser-induced-metal (LIM) phase is not the 1T' structure derived by a polymorphic-structural phase transition but consists instead of semi-metallic Te induced by photo-thermal decomposition of MoTe2. The technique is used to fabricate a field effect transistor with a Pd/2H-MoTe2/LIM structure having an asymmetric metallic contact, and its contact properties are studied via scanning gate microscopy. We confirm that a Schottky barrier (a diffusion potential) is always formed at the Pd/2H-MoTe2 boundary and obstacles a carrier transport while an Ohmic contact is realized at the 2H-MoTe2/LIM phase junction for both n- and p-type carriers.

GlyTouCan 1.0--The international glycan structure repository.

Glycans are known as the third major class of biopolymers, next to DNA and proteins. They cover the surfaces of many cells, serving as the 'face' of cells, whereby other biomolecules and viruses interact. The structure of glycans, however, differs greatly from DNA and proteins in that they are branched, as opposed to linear sequences of amino acids or nucleotides. Therefore, the storage of glycan information in databases, let alone their curation, has been a difficult problem. This has caused many duplicated efforts when integration is attempted between different databases, making an international repository for glycan structures, where unique accession numbers are assigned to every identified glycan structure, necessary. As such, an international team of developers and glycobiologists have collaborated to develop this repository, called GlyTouCan and is available at http://glytoucan.org/, to provide a centralized resource for depositing glycan structures, compositions and topologies, and to retrieve accession numbers for each of these registered entries. This will thus enable researchers to reference glycan structures simply by accession number, as opposed to by chemical structure, which has been a burden to integrate glycomics databases in the past.

GlyTouCan: an accessible glycan structure repository.

Rapid and continued growth in the generation of glycomic data has revealed the need for enhanced development of basic infrastructure for presenting and interpreting these datasets in a manner that engages the broader biomedical research community. Early in their growth, the genomic and proteomic fields implemented mechanisms for assigning unique gene and protein identifiers that were essential for organizing data presentation and for enhancing bioinformatic approaches to extracting knowledge. Similar unique identifiers are currently absent from glycomic data. In order to facilitate continued growth and expanded accessibility of glycomic data, the authors strongly encourage the glycomics community to coordinate the submission of their glycan structures to the GlyTouCan Repository and to make use of GlyTouCan identifiers in their communications and publications. The authors also deeply encourage journals to recommend a submission workflow in which submitted publications utilize GlyTouCan identifiers as a standard reference for explicitly describing glycan structures cited in manuscripts.

WURCS 2.0 Update To Encapsulate Ambiguous Carbohydrate Structures.

Accurate representation of structural ambiguity is important for storing carbohydrate structures containing varying levels of ambiguity in the literature and databases. Although many representations for carbohydrates have been developed in the past, a generalized but discrete representation format did not exist. We had previously developed the Web3 Unique Representation of Carbohydrate Structures (WURCS) in an attempt to define a generalizable and unique linear representation for carbohydrate structures. However, it lacked sufficient rules to uniquely describe ambiguous structures. In this work, we updated WURCS to handle such ambiguous monosaccharide structures. In particular, to handle structural ambiguity around (potential) carbonyl groups incidental to the carbohydrate analysis, we defined a representation of backbone carbons containing atomic-level ambiguity. As a result, we show that WURCS 2.0 can represent a wider variety of carbohydrate structures containing ambiguous monosaccharides, such as those whose ring closure is undefined or whose anomeric information is only known. This new format provides a representation of carbohydrates that was not possible before, and it is currently being used by the International Glycan Structure Repository GlyTouCan.

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Raman shift and electrical properties of MoS2 bilayer on boron nitride substrate.

We have fabricated a bilayer molybdenum disulphide (MoS2) transistor on boron nitride (BN) substrate and performed Raman spectroscopy and electrical measurements with this device. The characteristic Raman peaks show an upshift about 2.5 cm(-1) with the layer lying on BN, and a narrower line width in comparison with those on a SiO2 substrate. The device has a maximum drain current larger than 1 µA and a high current on/off ratio of greater than 10(8). In the temperature range of 100 K-293 K, the two terminal gate effect mobility and the carrier density do not change significantly with temperature. Results of the Raman and electrical measurements reveal that BN is a suitable substrate for atomic layer electrical devices.

UniCarb-DB: An MS/MS Experimental Glycomic Fragmentation Database.

Glycosylation is a unique posttranslational modification that dynamically shapes the surface of cells. Glycans attached to proteins or lipids in a cell or tissue are studied as a whole and collectively designated as a glycome. UniCarb-DB is a glycomic spectral library of tandem mass spectrometry (MS/MS) fragment data. The current version of the database consists of over 1500 entries and over 1000 unique structures. Each entry contains parent ion information with associated MS/MS spectra, metadata about the original publication, experimental conditions, and biological origin. Each structure is also associated with the GlyTouCan glycan structure repository allowing easy access to other glycomic resources. The database can be directly utilized by mass spectrometry (MS) experimentalists through the conversion of data generated by MS into structural information. Flexible online search tools along with a downloadable version of the database are easily incorporated in either commercial or open-access MS software. This chapter highlights UniCarb-DB online search tool to browse differences of isomeric structures between spectra, a peak matching search between user-generated MS/MS spectra and spectra stored in UniCarb-DB and more advanced MS tools for combined quantitative and qualitative glycomics.

Evaluation of the permeability of formaldehyde and water through a permeation tube for the preparation of an accurate formaldehyde reference gas mixture.

A system for continuous generation and analysis of formaldehyde (HCHO) in a nitrogen gas mixture prepared using a permeation method was fabricated in order to evaluate the permeability of HCHO and water (H2O) through a permeation tube. Specifically, the mass balance of HCHO and H2O through a permeation tube was evaluated using the system. The results indicated that the mass loss in the permeation tube accounted for the amount of HCHO and H2O measured using a spectrometer. The permeability of HCHO was calculated by subtracting the mass loss of H2O from the permeation tube per unit of time as determined from the mass balance results. The calculated permeability of HCHO was 75.7 ± 3.4 mg min(-1) (k = 2) for the HCHO gas mixture prepared by the permeation method using a permeation tube filled with paraformaldehyde that was vacuum-dried at 95 °C. The calculated permeability agreed with the permeability obtained using the dinitrophenylhydrazine-derivatization method (72.7 ± 4.4 mg min(-1) (k = 2)) within the level of uncertainty. This technique, in which the mass loss of H2O from the permeation tube is subtracted, can therefore provide a reference gas mixture with an accurate HCHO concentration using the permeation method.

Using optical vortex to control the chirality of twisted metal nanostructures.

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.

Application of post-column reaction gas chromatography with a single reference gas for offshore air and gas seeped from the seafloor samples.

Ambient air commonly contains carbon dioxide at concentrations greater than 400 µmol mol-1 and methane at ~ 2000 nmol mol-1; non-methane hydrocarbons are also widespread in the atmosphere at much lower concentrations. For quantification of various carbon-containing compounds in typical analytical instrument, corresponding number of reference materials are required. Therefore, the development of a method that uses a single reference material applicable to air monitoring is desired. Here, we examined a post-column reaction system combined with a gas chromatograph equipped with a flame ionization detector (FID), which involves oxidation and reduction processes after separation. To determine various carbon-containing gases by post-column reaction gas chromatography with FID (GC-r-FID) using a single reference, it is necessary to confirm a good linearity of the response with carbon concentrations originating from various carbon-containing gases. When mixtures of carbon-containing gases at three different concentrations and the calibration curve of the FID response with the concentration converted into methane were used, a single linear calibration curve (correlation coefficient > 0.9999, 18 points) was obtained over four orders of magnitudes (to ~ 5000 µmol mol-1 as methane). The applicability of GC-r-FID was confirmed by determining carbon-containing gases in air and gas seeped from the seafloor samples. Because the results were comparable to those obtained by conventional GC-FID and GC-thermal conductivity detector, typically GC-r-FID with a single reference gas should be suitable for air monitoring.

Doping-Free High-Performance Photovoltaic Effect in a WSe2 Lateral p-n Homojunction Formed by Contact Engineering.

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs p-n diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral p-n homojunction photovoltaic. By combining surface and edge contacts for p-n diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.

Signatures of hot carriers and hot phonons in the re-entrant metallic and semiconducting states of Moiré-gapped graphene.

Stacking of graphene with hexagonal boron nitride (h-BN) can dramatically modify its bands from their usual linear form, opening a series of narrow minigaps that are separated by wider minibands. While the resulting spectrum offers strong potential for use in functional (opto)electronic devices, a proper understanding of the dynamics of hot carriers in these bands is a prerequisite for such applications. In this work, we therefore apply a strategy of rapid electrical pulsing to drive carriers in graphene/h-BN heterostructures deep into the dissipative limit of strong electron-phonon coupling. By using electrical gating to move the chemical potential through the "Moiré bands", we demonstrate a cyclical evolution between metallic and semiconducting states. This behavior is captured in a self-consistent model of non-equilibrium transport that considers the competition of electrically driven inter-band tunneling and hot-carrier scattering by strongly non-equilibrium phonons. Overall, our results demonstrate how a treatment of the dynamics of both hot carriers and hot phonons is essential to understanding the properties of functional graphene superlattices.