Enhanced Long-Term Stability of Crystalline Nickel-Boride (Ni23B6) Electrocatalyst by Encapsulation with Hexagonal Boron Nitride.

Nickel boride catalysts show great potential as low-cost and efficient alternatives to noble-metal catalysts in acidic media; however, synthesizing and isolating a specific phase and composition of nickel boride is nontrivial, and issues persist in their long-term stability as electrocatalysts. Here, a single-crystal nickel boride, Ni23B6, is reported which exhibits high electrocatalytic activity for the hydrogen evolution reaction (HER) in an acidic solution, and that its poor long-term stability can be overcome via encapsulation by single-crystal trilayer hexagonal boron nitride (hBN) film. Interestingly, hBN-covered Ni23B6 on a Ni substrate shows an identical overpotential of 52 mV at a current density of 10 mA cm-2 to that of bare Ni23B6. This phenomenon indicates that the single-crystalline hBN layer is catalytically transparent and does not obstruct HER activation. The hBN/Ni23B6/Ni has remarkable long-term stability with negligible changes to its polarization curves for 2000 cycles, whereas the Ni23B6/Ni shows significant degradation after 650 cycles. Furthermore, chronoamperometric measurements indicate that stability is preserved for >20 h. Long-term stability tests also reveal that the surface morphology and chemical structure of the hBN/Ni23B6/Ni electrode remain preserved. This work provides a model for the practical design of robust and durable electrochemical catalysts through the use of hBN encapsulation.

Unusual Raman Enhancement Effect of Ultrathin Copper Sulfide.

In surface-enhanced Raman spectroscopy (SERS), 2D materials are explored as substrates owing to their chemical stability and reproducibility. However, they exhibit lower enhancement factors (EFs) compared to noble metal-based SERS substrates. This study demonstrates the application of ultrathin covellite copper sulfide (CuS) as a cost-effective SERS substrate with a high EF value of 7.2 × 104 . The CuS substrate is readily synthesized by sulfurizing a Cu thin film at room temperature, exhibiting a Raman signal enhancement comparable to that of an Au noble metal substrate of similar thickness. Furthermore, computational simulations using the density functional theory are employed and time-resolved photoluminescence measurements are performed to investigate the enhancement mechanisms. The results indicate that polar covalent bonds (Cu─S) and strong interlayer interactions in the ultrathin CuS substrate increase the probability of charge transfer between the analyte molecules and the CuS surface, thereby producing enhanced SERS signals. The CuS SERS substrate demonstrates the selective detection of various dye molecules, including rhodamine 6G, methylene blue, and safranine O. Furthermore, the simplicity of CuS synthesis facilitates large-scale production of SERS substrates with high spatial uniformity, exhibiting a signal variation of less than 5% on a 4-inch wafer.

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production.

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a ß-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

2D Materials in the Display Industry: Status and Prospects.

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

P-type electrical contacts for 2D transition-metal dichalcogenides.

Digital logic circuits are based on complementary pairs of n- and p-type field effect transistors (FETs) via complementary metal oxide semiconductor technology. In three-dimensional (3D) or bulk semiconductors, substitutional doping of acceptor or donor impurities is used to achieve p- and n-type FETs. However, the controllable p-type doping of low-dimensional semiconductors such as two-dimensional (2D) transition-metal dichalcogenides (TMDs) has proved to be challenging. Although it is possible to achieve high-quality, low-resistance n-type van der Waals (vdW) contacts on 2D TMDs1-5, obtaining p-type devices by evaporating high-work-function metals onto 2D TMDs has not been realized so far. Here we report high-performance p-type devices on single- and few-layered molybdenum disulfide and tungsten diselenide based on industry-compatible electron beam evaporation of high-work-function metals such as palladium and platinum. Using atomic resolution imaging and spectroscopy, we demonstrate near-ideal vdW interfaces without chemical interactions between the 2D TMDs and 3D metals. Electronic transport measurements reveal that the Fermi level is unpinned and p-type FETs based on vdW contacts exhibit low contact resistance of 3.3 kΩ µm, high mobility values of approximately 190 cm2 V-1 s-1 at room temperature, saturation currents in excess of 10-5 A µm-1 and an on/off ratio of 107. We also demonstrate an ultra-thin photovoltaic cell based on n- and p-type vdW contacts with an open circuit voltage of 0.6 V and a power conversion efficiency of 0.82%.

Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111).

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene1-3, hexagonal boron nitride (hBN)4-6 and transition metal dichalcogenides7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni23B6 interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni23B6 and between Ni23B6 and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO2 (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO2 substrate in MoS2 FETs. Our results demonstrate high-quality single-crystal  multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN.

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics.

Radio-frequency-transmitting hexagonal boron nitride-based anti- and de-icing heating system.

Anti- and de-icing heating systems are used to both prevent the accumulation of ice and to remove it and thus avoid damage. Typically, anti- and de-icing heating systems employ carbon-based materials, metal frames, and bulky ceramic structures. These structures generally lead to the loss of radio-frequency (RF) signals and are also relatively heavy. Therefore, RF equipment such as radar domes (radomes) and antennas require anti- and de-icing systems with high RF transmittance for normal operation. In this work, we fabricated a fluorine-doped tin oxide (FTO) wave pattern covered with hexagonal boron nitride (h-BN) layers (i.e., an h-BN/FTO wave pattern) on a glass substrate for use as an RF-transmitting heating system for anti- and de-icing. The FTO wave pattern and h-BN layer act as the heating element and heat spreader, respectively. The h-BN layer showed a transmittance of approximately 90% for RF waves on glass (X band: 8.2-12.4 GHz) (the 10% loss was attributable to the glass substrate). The differences in the temperatures of the FTO-patterned and non-patterned areas for the h-BN(3.6 nm)/FTO and FTO wave pattern were 19.3 and 25.5 °C, respectively. This means that the h-BN layer improved the heat-spreading performance by 6.2 °C. Furthermore, a de-icing test was performed using the h-BN(3.6 nm)/FTO wave pattern by applying a voltage of 40 V at -20 °C. The ice on the non-patterned area melted within 1 min while that on the FTO-patterned area melted within 30 s. These results suggest that the fabricated h-BN(3.6 nm)/FTO wave pattern for RF-transmitting heating systems is suitable for use with the radomes of drones, unmanned aerial vehicles, aircraft, and spaceships in extremely cold environments.

Ultralow-dielectric-constant amorphous boron nitride.

Decrease in processing speed due to increased resistance and capacitance delay is a major obstacle for the down-scaling of electronics1-3. Minimizing the dimensions of interconnects (metal wires that connect different electronic components on a chip) is crucial for the miniaturization of devices. Interconnects are isolated from each other by non-conducting (dielectric) layers. So far, research has mostly focused on decreasing the resistance of scaled interconnects because integration of dielectrics using low-temperature deposition processes compatible with complementary metal-oxide-semiconductors is technically challenging. Interconnect isolation materials must have low relative dielectric constants (κ values), serve as diffusion barriers against the migration of metal into semiconductors, and be thermally, chemically and mechanically stable. Specifically, the International Roadmap for Devices and Systems recommends4 the development of dielectrics with κ values of less than 2 by 2028. Existing low-κ materials (such as silicon oxide derivatives, organic compounds and aerogels) have κ values greater than 2 and poor thermo-mechanical properties5. Here we report three-nanometre-thick amorphous boron nitride films with ultralow κ values of 1.78 and 1.16 (close to that of air, κ = 1) at operation frequencies of 100 kilohertz and 1 megahertz, respectively. The films are mechanically and electrically robust, with a breakdown strength of 7.3 megavolts per centimetre, which exceeds requirements. Cross-sectional imaging reveals that amorphous boron nitride prevents the diffusion of cobalt atoms into silicon under very harsh conditions, in contrast to reference barriers. Our results demonstrate that amorphous boron nitride has excellent low-κ dielectric characteristics for high-performance electronics.

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe2/WSe2 van der Waals Heterostructures.

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe2/WSe2 vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe2 and WSe2 layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe2 and WSe2.

Hexagonal Boron Nitride/Au Substrate for Manipulating Surface Plasmon and Enhancing Capability of Surface-Enhanced Raman Spectroscopy.

We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices.

Catalytic Conversion of Hexagonal Boron Nitride to Graphene for In-Plane Heterostructures.

Heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications in 2D materials. Although several methods have been developed to produce this material through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we report herein a new chemical route in which the Pt substrate plays a catalytic role. We propose that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate.