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.

MoS2 /Polymer Heterostructures Enabling Stable Resistive Switching and Multistate Randomness.

Resistive random-access memories (ReRAMs) based on transition metal dichalcogenide layers are promising physical sources for random number generation (RNG). However, most ReRAM devices undergo performance degradation from cycle to cycle, which makes preserving a normal probability distribution during operation a challenging task. Here, ReRAM devices with excellent stability are reported by using a MoS2 /polymer heterostructure as active layer. The stability enhancement manifests in outstanding cumulative probabilities for both high- and low-resistivity states of the memory cells. Moreover, the intrinsic values of the high-resistivity state are found to be an excellent source of randomness as suggested by a Chi-square test. It is demonstrated that one of these cells alone can generate ten distinct random states, in contrast to the four conventional binary cells that would be required for an equivalent number of states. This work unravels a scalable interface engineering process for the production of high-performance ReRAM devices, and sheds light on their promising application as reliable RNGs for enhanced cybersecurity in the big data era.

A Single-Step Route to Single-Crystal Molybdenum Disulphide (MoS2) Monolayer domains.

We report a simple, single-cycle synthetic method for forming highly-crystalline, micron-sized monolayer domains of phase-pure MoS2. This method combines liquid chemistry with discrete, layer-by-layer deposition from a novel Mo precursor. Single-crystalline MoS2 with domain sizes up to 100 µm have been obtained and characterised by optical and electron microscopy as well as Raman and photoluminescence spectroscopy.

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.

Enhanced Photocarrier Generation with Selectable Wavelengths by M-Decorated-CuInS2 Nanocrystals (M = Au and Pt) Synthesized in a Single Surfactant Process on MoS2 Bilayers.

A facile approach for the synthesis of Au- and Pt-decorated CuInS2 nanocrystals (CIS NCs) as sensitizer materials on the top of MoS2 bilayers is demonstrated. A single surfactant (oleylamine) is used to prepare such heterostructured noble metal decorated CIS NCs from the pristine CIS. Such a feasible way to synthesize heterostructured noble metal decorated CIS NCs from the single surfactant can stimulate the development of the functionalized heterostructured NCs in large scale for practical applications such as solar cells and photodetectors. Photodetectors based on MoS2 bilayers with the synthesized nanocrystals display enhanced photocurrent, almost 20-40 times higher responsivity and the On/Off ratio is enlarged one order of magnitude compared with the pristine MoS2 bilayers-based photodetectors. Remarkably, by using Pt- or Au-decorated CIS NCs, the photocurrent enhancement of MoS2 photodetectors can be tuned between blue (405 nm) to green (532 nm). The strategy described here acts as a perspective to significantly improve the performance of MoS2 -based photodetectors with the controllable absorption wavelengths in the visible light range, showing the feasibility of the possible color detection.

Phase-Engineered PtSe2 -Layered Films by a Plasma-Assisted Selenization Process toward All PtSe2 -Based Field Effect Transistor to Highly Sensitive, Flexible, and Wide-Spectrum Photoresponse Photodetectors.

The formation of PtSe2 -layered films is reported in a large area by the direct plasma-assisted selenization of Pt films at a low temperature, where temperatures, as low as 100 °C at the applied plasma power of 400 W can be achieved. As the thickness of the Pt film exceeds 5 nm, the PtSe2 -layered film (five monolayers) exhibits a metallic behavior. A clear p-type semiconducting behavior of the PtSe2 -layered film (≈trilayers) is observed with the average field effective mobility of 0.7 cm2 V-1 s-1 from back-gated transistor measurements as the thickness of the Pt film reaches below 2.5 nm. A full PtSe2 field effect transistor is demonstrated where the thinner PtSe2 , exhibiting a semiconducting behavior, is used as the channel material, and the thicker PtSe2 , exhibiting a metallic behavior, is used as an electrode, yielding an ohmic contact. Furthermore, photodetectors using a few PtSe2 -layered films as an adsorption layer synthesized at the low temperature on a flexible substrate exhibit a wide range of absorption and photoresponse with the highest photocurrent of 9 µA under the laser wavelength of 408 nm. In addition, the device can maintain a high photoresponse under a large bending stress and 1000 bending cycles.

Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process.

Direct reduction of metal oxides into a few transition metal dichalcogenide (TMDCs) monolayers has been recently explored as an alternative method for large area and uniform deposition. However, not many studies have addressed the characteristics and requirement of the metal oxides into TMDCs by the selenization/sulfurization processes, yielding a wide range of outstanding properties to poor electrical characteristics with nonuniform films. The large difference implies that the process is yet not fully understood. In particular, the selenization/sulfurization at low temperature leads to poor crystallinity films with poor electrical performance, hindering its practical development. A common approach to improve the quality of the selenized/sulfurized films is by further increasing the process temperature, thus requiring additional transfer in order to explore the electrical properties. Here, we show that by finely tuning the quality of the predeposited oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relationship between the role of selecting different metal oxides prepared by e-beam evaporation and reactive sputtering and their oxygen deficiency/vacancy leading to quality influence of TMDCs was investigated in detail. Because of its outstanding physical properties, the formation of tungsten diselenide (WSe2) from the reduction of tungsten oxide (WO x) was chosen as a model for proof of concept. By optimizing the process parameters and the selection of metal oxides, layered WSe2 films with controlled atomic thickness can be demonstrated. Interestingly, the domain size and electrical properties of the layered WSe2 films are highly affected by the quality of the metal oxides, for which the layered WSe2 film with small domains exhibits a metallic behavior and the layered WSe2 films with larger domains provides clear semiconducting behavior. Finally, an 8'' wafer scale-layered WSe2 film was demonstrated, giving a step forward in the development of 2D TMDC electronics in the industry.

Thermally Strained Band Gap Engineering of Transition-Metal Dichalcogenide Bilayers with Enhanced Light-Matter Interaction toward Excellent Photodetectors.

Integration of strain engineering of two-dimensional (2D) materials in order to enhance device performance is still a challenge. Here, we successfully demonstrated the thermally strained band gap engineering of transition-metal dichalcogenide bilayers by different thermal expansion coefficients between 2D materials and patterned sapphire structures, where MoS2 bilayers were chosen as the demonstrated materials. In particular, a blue shift in the band gap of the MoS2 bilayers can be tunable, displaying an extraordinary capability to drive electrons toward the electrode under the smaller driven bias, and the results were confirmed by simulation. A model to explain the thermal strain in the MoS2 bilayers during the synthesis was proposed, which enables us to precisely predict the band gap-shifted behaviors on patterned sapphire structures with different angles. Furthermore, photodetectors with enhancement of 286% and 897% based on the strained MoS2 on cone- and pyramid-patterned sapphire substrates were demonstrated, respectively.

Wafer Scale Phase-Engineered 1T- and 2H-MoSe2 /Mo Core-Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction.

The necessity for new sources for greener and cleaner energy production to replace the existing ones has been increasingly growing in recent years. Of those new sources, the hydrogen evolution reaction has a large potential. In this work, for the first time, MoSe2 /Mo core-shell 3D-hierarchical nanostructures are created, which are derived from the Mo 3D-hierarchical nanostructures through a low-temperature plasma-assisted selenization process with controlled shapes grown by a glancing angle deposition system.

Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors.

Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS2) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS2) and tungsten disulfide (WS2), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS2. By controlling the sulfur diffusion, we are able to synthesize continuous MS2 layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photosensing devices fabricated on the synthesized 2D WS2 without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process.

Direct formation of large-scale multi-layered germanene on Si substrate.

Germanene layers with lonsdaleite structure has been synthesized from a SiGe thin film for the first time using a N2 plasma-assisted process in this investigation. Multi-layered germanene can be directly observed, and the derived lattice parameters are nearly consistent with the theoretical results. Furthermore, large-scale multi-layered germanene has also been demonstrated for applications.

Plasma-Assisted Synthesis of High-Mobility Atomically Layered Violet Phosphorus.

Two-dimensional layered materials such as graphene, transition metal dichalcogenides, and black phosphorus have demonstrated outstanding properties due to electron confinement as the thickness is reduced to atomic scale. Among the phosphorus allotropes, black phosphorus, and violet phosphorus possess layer structure with the potential to be scaled down to atomically thin film. For the first time, the plasma-assisted synthesis of atomically layered violet phosphorus has been achieved. Material characterization supports the formation of violet phosphorus/InN over InP substrate where the layer structure of violet phosphorus is clearly observed. The identification of the crystal structure and lattice constant ratifies the formation of violet phosphorus indeed. The critical concept of this synthesis method is the selective reaction induced by different variations of Gibbs free energy (ΔG) of reactions. Besides, the Hall mobility of the violet phosphorus on the InP substrate greatly increases over the theoretical values of InP bulk material without much reduction in the carrier concentration, suggesting that the mobility enhancement results from the violet phosphorus layers. Furthermore, this study demonstrates a low-cost technique with high compatibility to synthesize the high-mobility atomically layered violet phosphorus and open the space for the study of the fundamental properties of this intriguing material as a new member of the fast growing family of 2D crystals.

Ultrafast and low temperature synthesis of highly crystalline and patternable few-layers tungsten diselenide by laser irradiation assisted selenization process.

Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe2 on an insulator via laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO3 film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe2, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe2 concept was demonstrated by patterning the WO3 film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe2 back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO2/P(+)Si substrate with extracted field effect mobility of ∼0.2 cm(2) V(-1) s(-1). Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe2 from other metal oxides such as MO3 film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs.

Ultrafast Graphene Growth on Insulators via Metal-Catalyzed Crystallization by a Laser Irradiation Process: From Laser Selection, Thickness Control to Direct Patterned Graphene Utilizing Controlled Layer Segregation Process.

Despite the vast progress in chemical vapor deposition (CVD) graphene grown on metals, the transfer process is still a major bottleneck, being not devoid of wrinkles and polymer residues. In this paper, a structure is introduced to directly synthesize few layer graphene on insulating substrates by a laser irradiation heating process. The segregation of graphene layers can be manipulated by tuning the metal layer thickness and laser power at different scanning rates. Graphene deposition and submicrometer patterning resolution can be achieved by patterning the intermediate metal layer using standard lithography methods in order to overcome the scalability issue regardless the resolution of the laser beam. The systematic analysis of the process based on the formation of carbon microchannels by the laser irradiation process can be extended to several materials, thicknesses, and methods. Furthermore, hole and electron mobilities of 500 and 950 cm(2) V(-1) s(-1) are measured. The laser-synthesized graphene is a step forward along the direct synthesis route for graphene on insulators that meets the criteria for photonics and electronics.

Large-scale and patternable graphene: direct transformation of amorphous carbon film into graphene/graphite on insulators via Cu mediation engineering and its application to all-carbon based devices.

Chemical vapour deposition of graphene was the preferred way to synthesize graphene for multiple applications. However, several problems related to transfer processes, such as wrinkles, cleanness and scratches, have limited its application at the industrial scale. Intense research was triggered into developing alternative synthesis methods to directly deposit graphene on insulators at low cost with high uniformity and large area. In this work, we demonstrate a new concept to directly achieve growth of graphene on non-metal substrates. By exposing an amorphous carbon (a-C) film in Cu gaseous molecules after annealing at 850 °C, the carbon (a-C) film surprisingly undergoes a noticeable transformation to crystalline graphene. Furthermore, the thickness of graphene could be controlled, depending on the thickness of the pre-deposited a-C film. The transformation mechanism was investigated and explained in detail. This approach enables development of a one-step process to fabricate electrical devices made of all carbon material, highlighting the uniqueness of the novel approach for developing graphene electronic devices. Interestingly, the carbon electrodes made directly on the graphene layer by our approach offer a good ohmic contact compared with the Schottky barriers usually observed on graphene devices using metals as electrodes.

Scalable graphene synthesised by plasma-assisted selective reaction on silicon carbide for device applications.

Graphene, a two-dimensional material with honeycomb arrays of carbon atoms, has shown outstanding physical properties that make it a promising candidate material for a variety of electronic applications. To date, several issues related to the material synthesis and device fabrication need to be overcome. Despite the fact that large-area graphene films synthesised by chemical vapour deposition (CVD) can be grown with relatively few defects, the required transfer process creates wrinkles and polymer residues that greatly reduce its performance in device applications. Graphene synthesised on silicon carbide (SiC) has shown outstanding mobility and has been successfully used to develop ultra-high frequency transistors; however, this fabrication method is limited due to the use of costly ultra-high vacuum (UHV) equipment that can reach temperatures over 1500 °C. Here, we show a simple and novel approach to synthesise graphene on SiC substrates that greatly reduces the temperature and vacuum requirements and allows the use of equipment commonly used in the semiconductor processing industry. In this work, we used plasma treatment followed by annealing in order to obtain large-scale graphene films from bulk SiC. After exposure to N2 plasma, the annealing process promotes the reaction of nitrogen ions with Si and the simultaneous condensation of C on the surface of SiC. Eventually, a uniform, large-scale, n-type graphene film with remarkable transport behaviour on the SiC wafer is achieved. Furthermore, graphene field effect transistors (FETs) with high carrier mobilities on SiC were also demonstrated in this study.

Direct assessment of the mechanical modulus of graphene co-doped with low concentrations of boron-nitrogen by a non-contact approach.

Boron and nitrogen co-doping has been shown to be an effective way to induce a band gap in graphene for electrical applications but only a few theoretical studies have been done to understand the elastic and mechanical properties of the modified graphene. Until now, no experimental assessment of the mechanical modulus of boron-nitrogen-doped graphene (BNG) has been reported in the literature. Here, we demonstrate a novel non-contact approach to determine the in-plane stiffness of BNG at low BN concentrations. The in-plane stiffness of BNG with 2 at% BN concentration was estimated to be about 309 N m(-1), which is lower than that of pristine graphene, in good agreement with some theoretical studies. Moreover, we correlated the conductivity of BNG with induced strain and found the BNG to be more sensitive than pristine graphene in response to externally applied strain. This result indicates that BNG is a more suitable material than graphene for strain sensor applications.

Scalable graphite/copper bishell composite for high-performance interconnects.

We present the fabrication and characterizations of novel electrical interconnect test lines made of a Cu/graphite bishell composite with the graphite cap layer grown by electron cyclotron resonance chemical vapor deposition. Through this technique, conformal multilayer graphene can be formed on the predeposited Cu interconnects under CMOS-friendly conditions. The low-temperature (400 °C) deposition also renders the process unlimitedly scalable. The graphite layer can boost the current-carrying capacity of the composite structure to 10(8) A/cm(2), more than an order of magnitude higher than that of bare metal lines, and reduces resistivity of fine test lines by ∼10%. Raman measurements reveal that physical breakdown occurs at ∼680-720 °C. Modeling the current vs voltage curves up to breakdown shows that the maximum current density of the composites is limited by self-heating of the graphite, suggesting the strong roles of phonon scattering at high fields and highlighting the significance of a metal counterpart for enhanced thermal dissipation.

Síndrome de hipotensión endocraneana: ¿una cefalea pospunción dural? / Intracranial hypotension syndrome: A post dural puncture headache?

El síndrome de hipotensión endocraneana (SHE) es una patología causada por el descenso del cerebro debido a fuga de líquido cefalorraquídeo a partir de lesiones durales diagnósticas, terapéuticas o espontáneas. Tanto la fisiopatología como el enfoque clínico y terapéutico son similares a la cefalea pospunción dural, siendo esta ultima considerada como una forma leve del SHE. Se describen 2 pacientes con cefalea ortostática y alteraciones neurológicas severas luego de anestesia epidural y espinal que fueron diagnosticados y tratados como cefalea pospunción dural, pero que por su evolución anormal debieron recibir atención adicional. El SHE constituye una complicación seria que puede llevar al deterioro clínico y a la muerte, motivo por el cual requiere de un abordaje integral sobre sus factores desencadenantes, cuadro clínico, métodos diagnósticos, fisiopatología y manejo.

The intracranial hypotension syndrome (IHS) is a disorder caused by brain descent due to a CSF leak resulting from diagnostic, therapeutic or spontaneous lesions. The pathophysiology, the clinical and the therapeutic approach are similar as in post dural puncture headache, the latter being considered a mild form of IHS. This paper describes two patients with orthostatic headache and severe neurological involvement after epidural and spinal anesthesia, diagnosed and treated as post dural puncture headache, but who required additional care because of their abnormal course. IHS is a serious complication that may result in clinical decline and death; consequently, it requires a comprehensive approach to the various triggering factors, the clinical picture, diagnostic methods, pathophysiology and management.

Guía para la intubación con fibrobroncoscopio en un Hospital Universitario / Guidelines for intubation under fiberoptic bronchoscopy in a University Hospital

Introducción: El recurso del fibrobroncoscopio como instrumento para la intubación traqueal es relativamente reciente en nuestro medio. Su disponibilidad es cada vez mayor y por ello los anestesiólogos deben entrenarse suficientemente en el uso de este equipo. Conocer y dominar una técnica, respetando cada uno de sus pasos, es el primer paso para la realización exitosa de un procedimiento. Objetivo: En la presente guía se describe la técnica utilizada en para la intubación con Intubaciónfibrobroncoscopio en el Hospital Universitario del Valle Evaristo García. Metodología: El artículo se basa en la revisión de la literatura, la experiencia de los autores y un foro de discusión. Resultados: Se presenta la guía para la intubación con fibrobroncoscopio en el Hospital Universitario del Valle Evaristo García. Se describe la técnica para la construcción de una máscara para la ventilación del paciente durante el procedimiento de intubación traqueal con la utilización del fibrobroncoscopio.

Introduction: The use of fiberoptic bronchoscopy as a tool for tracheal intubation is relatively new in our setting. Its availability has been increasing, and hence the need for anesthetists to receive adequate training in the use of this device. Knowledge and mastery of this technique, following all the necessary steps, is the starting point for the successful performance of a procedure. Objective: These guidelines describe the technique for intubation using the fiberoptic bronchoscope at the Evaristo García University Hospital of the Colombian Department of Valle. Methodology: This paper is based on a review of the literature, the authors' experience, and a discussion forum. Results: Presentation of the guidelines for intubation using fiber optic bronchoscope at the Evaristo García University Hospital. Description of the technique for constructing a mask for patient ventilation during tracheal intubation under fiberoptic bronchoscopy.