Epitaxial growth and characterization of high quality Bi2O2Se thin films on SrTiO3 substrates by pulsed laser deposition.

Recently, Bi2O2Se was revealed as a promising two-dimensional (2D) semiconductor for next generation electronics, due to its moderate bandgap size, high electron mobility and pronounced ambient stability. Meanwhile, it has been predicted that high-quality Bi2O2Se-related heterostructures may possess exotic physical phenomena, such as piezoelectricity and topological superconductivity. Herein, we report the first successful heteroepitaxial growth of Bi2O2Se films on SrTiO3 substrates via pulsed laser deposition (PLD) method. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate. The growth mode transition to three-dimensional (3D) island from quasi-2D layer of the heteroepitaxial Bi2O2Se films on SrTiO3 (001) substrates is observed as prolonging deposition time of films. The maximum value of electron mobility reaches 160 cm2 V-1 s-1 at room temperature in a 70 nm thick film. The thickness dependent mobility provides evidence that interface-scattering is likely to be the limiting factor for the relatively low electron mobility at low temperature, implying that the interface engineering as an effective method to tune the low temperature electron mobility. Our work suggests the epitaxial Bi2O2Se films grown by PLD are promising for both fundamental study and practical applications.

Persistent surface states with diminishing gap in MnBi2Te4/Bi2Te3 superlattice antiferromagnetic topological insulator.

Magnetic topological quantum materials (TQMs) provide a fertile ground for the emergence of fascinating topological magneto-electric effects. Recently, the discovery of intrinsic antiferromagnetic (AFM) topological insulator MnBi2Te4 that could realize quantized anomalous Hall effect and axion insulator phase ignited intensive study on this family of TQM compounds. Here, we investigated the AFM compound MnBi4Te7 where Bi2Te3 and MnBi2Te4 layers alternate to form a superlattice. Using spatial- and angle-resolved photoemission spectroscopy, we identified ubiquitous (albeit termination dependent) topological electronic structures from both Bi2Te3 and MnBi2Te4 terminations. Unexpectedly, while the bulk bands show strong temperature dependence correlated with the AFM transition, the topological surface states with a diminishing gap show negligible temperature dependence across the AFM transition. Together with the results of its sister compound MnBi2Te4, we illustrate important aspects of electronic structures and the effect of magnetic ordering in this family of magnetic TQMs.

Synthesis of High-Quality Graphene and Hexagonal Boron Nitride Monolayer In-Plane Heterostructure on Cu-Ni Alloy.

Graphene/hexagonal boron nitride (h-BN) monolayer in-plane heterostructure offers a novel material platform for both fundamental research and device applications. To obtain such a heterostructure in high quality via controllable synthetic approaches is still challenging. In this work, in-plane epitaxy of graphene/h-BN heterostructure is demonstrated on Cu-Ni substrates. The introduction of nickel to copper substrate not only enhances the capability of decomposing polyaminoborane residues but also promotes graphene growth via isothermal segregation. On the alloy surface partially covered by h-BN, graphene is found to nucleate at the corners of the as-formed h-BN grains, and the high growth rate for graphene minimizes the damage of graphene-growth process on h-BN lattice. As a result, high-quality graphene/h-BN in-plane heterostructure with epitaxial relationship can be formed, which is supported by extensive characterizations. Photodetector device applications are demonstrated based on the in-plane heterostructure. The success will have important impact on future research and applications based on this unique material platform.

Oriented graphene nanoribbons embedded in hexagonal boron nitride trenches.

Graphene nanoribbons (GNRs) are ultra-narrow strips of graphene that have the potential to be used in high-performance graphene-based semiconductor electronics. However, controlled growth of GNRs on dielectric substrates remains a challenge. Here, we report the successful growth of GNRs directly on hexagonal boron nitride substrates with smooth edges and controllable widths using chemical vapour deposition. The approach is based on a type of template growth that allows for the in-plane epitaxy of mono-layered GNRs in nano-trenches on hexagonal boron nitride with edges following a zigzag direction. The embedded GNR channels show excellent electronic properties, even at room temperature. Such in-plane hetero-integration of GNRs, which is compatible with integrated circuit processing, creates a gapped channel with a width of a few benzene rings, enabling the development of digital integrated circuitry based on GNRs.

C3 N-A 2D Crystalline, Hole-Free, Tunable-Narrow-Bandgap Semiconductor with Ferromagnetic Properties.

Graphene has initiated intensive research efforts on 2D crystalline materials due to its extraordinary set of properties and the resulting host of possible applications. Here the authors report on the controllable large-scale synthesis of C3 N, a 2D crystalline, hole-free extension of graphene, its structural characterization, and some of its unique properties. C3 N is fabricated by polymerization of 2,3-diaminophenazine. It consists of a 2D honeycomb lattice with a homogeneous distribution of nitrogen atoms, where both N and C atoms show a D6h -symmetry. C3 N is a semiconductor with an indirect bandgap of 0.39 eV that can be tuned to cover the entire visible range by fabrication of quantum dots with different diameters. Back-gated field-effect transistors made of single-layer C3 N display an on-off current ratio reaching 5.5 × 1010 . Surprisingly, C3 N exhibits a ferromagnetic order at low temperatures (<96 K) when doped with hydrogen. This new member of the graphene family opens the door for both fundamental basic research and possible future applications.

Direct synthesis of ethanol via CO2 hydrogenation using supported gold catalysts.

An efficient titania supported Au nanocluster (NC) has been prepared for the direct synthesis of useful EtOH from CO2 and H2. The unique creation of an excellent synergistic effect between Au NCs and the underlying TiO2 support, especially the anatase crystal phase with abundant oxygen vacancies, can achieve the high performance for EtOH synthesis under moderate and practical conditions.

Lithium-ion-based solid electrolyte tuning of the carrier density in graphene.

We have developed a technique to tune the carrier density in graphene using a lithium-ion-based solid electrolyte. We demonstrate that the solid electrolyte can be used as both a substrate to support graphene and a back gate. It can induce a change in the carrier density as large as 1 × 1014 cm-2, which is much larger than that induced with oxide-film dielectrics, and it is comparable with that induced by liquid electrolytes. Gate modulation of the carrier density is still visible at 150 K, which is lower than the glass transition temperature of most liquid gating electrolytes.

Controllable Edge Oxidation and Bubbling Exfoliation Enable the Fabrication of High Quality Water Dispersible Graphene.

Despite significant progresses made on mass production of chemically exfoliated graphene, the quality, cost and environmental friendliness remain major challenges for its market penetration. Here, we present a fast and green exfoliation strategy for large scale production of high quality water dispersible few layer graphene through a controllable edge oxidation and localized gas bubbling process. Mild edge oxidation guarantees that the pristine sp2 lattice is largely intact and the edges are functionalized with hydrophilic groups, giving rise to high conductivity and good water dispersibility at the same time. The aqueous concentration can be as high as 5.0 mg mL-1, which is an order of magnitude higher than previously reports. The water soluble graphene can be directly spray-coated on various substrates, and the back-gated field effect transistor give hole and electron mobility of ~496 and ~676 cm2 V-1 s-1, respectively. These results achieved are expected to expedite various applications of graphene.

A combined method for synthesis of superconducting Cu doped Bi2Se3.

We present a two-step technique for the synthesis of superconducting CuxBi2Se3. Cu0.15Bi2Se3 single crystals were synthesized using the melt-growth method. Although these samples are non-superconducting, they can be employed to generate high quality superconducting samples if used as precursors in the following electrochemical synthesis step. Samples made from Cu0.15Bi2Se3 reliably exhibit zero-resistance even under the non-optimal quenching condition, while samples made from pristine Bi2Se3 require fine tuning of the quenching conditions to achieve similar performance. Moreover, under the optimal quenching condition, the average superconducting shielding fraction was still lower in the samples made from pristine Bi2Se3 than in the samples made from Cu0.15Bi2Se3. These results suggest that the pre-doped Cu atoms facilitate the formation of a superconducting percolation network. We also discuss the useful clues that we gathered about the locations of Cu dopants that are responsible for superconductivity.

Copper-Vapor-Assisted Rapid Synthesis of Large AB-Stacked Bilayer Graphene Domains on Cu-Ni Alloy.

The synergic effects of Cu85Ni15 and the copper vapor evaporated from copper foil enabled the fast growth of a ≈300 µm bilayer graphene in ≈10 minutes. The copper vapor reduces the growth rate of the first graphene layer while the carbon dissolved in the alloy boosts the growth of the subsequently developed second graphene layer with an AB-stacking order.

Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu-Ni alloys.

Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications. At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimetre-sized single-crystalline graphene domains in ∼12 h, by suppressing nucleation events on the growth substrate. Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve into a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an ∼1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates an isothermal segregation mechanism that greatly expedites the growth rate. This approach may also prove effective for the synthesis of wafer-scale single-crystalline monolayers of other two-dimensional materials.

Silane-catalysed fast growth of large single-crystalline graphene on hexagonal boron nitride.

The direct growth of high-quality, large single-crystalline domains of graphene on a dielectric substrate is of vital importance for applications in electronics and optoelectronics. Traditionally, graphene domains grown on dielectrics are typically only ~1 µm with a growth rate of ~1 nm min(-1) or less, the main reason is the lack of a catalyst. Here we show that silane, serving as a gaseous catalyst, is able to boost the graphene growth rate to ~1 µm min(-1), thereby promoting graphene domains up to 20 µm in size to be synthesized via chemical vapour deposition (CVD) on hexagonal boron nitride (h-BN). Hall measurements show that the mobility of the sample reaches 20,000 cm(2) V(-1) s(-1) at room temperature, which is among the best for CVD-grown graphene. Combining the advantages of both catalytic CVD and the ultra-flat dielectric substrate, gaseous catalyst-assisted CVD paves the way for synthesizing high-quality graphene for device applications while avoiding the transfer process.

Urea-assisted aqueous exfoliation of graphite for obtaining high-quality graphene.

Urea-assisted aqueous exfoliation of graphite was found to be more efficient than exfoliation in N,N-dimethylformamide (DMF), and high-quality graphene was obtained with a yield up to 2.4%. The mechanism in which a primary amine facilitates aqueous exfoliation was proposed and experimentally validated, which may inspire new strategies for efficient liquid exfoliation.

Synthesis of large single-crystal hexagonal boron nitride grains on Cu-Ni alloy.

Hexagonal boron nitride (h-BN) has attracted significant attention because of its superior properties as well as its potential as an ideal dielectric layer for graphene-based devices. The h-BN films obtained via chemical vapour deposition in earlier reports are always polycrystalline with small grains because of high nucleation density on substrates. Here we report the successful synthesis of large single-crystal h-BN grains on rational designed Cu-Ni alloy foils. It is found that the nucleation density can be greatly reduced to 60 per mm(2) by optimizing Ni ratio in substrates. The strategy enables the growth of single-crystal h-BN grains up to 7,500 µm(2), approximately two orders larger than that in previous reports. This work not only provides valuable information for understanding h-BN nucleation and growth mechanisms, but also gives an effective alternative to exfoliated h-BN as a high-quality dielectric layer for large-scale nanoelectronic applications.

High-Tc superconductivity in ultrathin Bi2Sr2CaCu2O(8+x) down to half-unit-cell thickness by protection with graphene.

High-Tc superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi2Sr2CaCu2O(8+x) (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here, we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure. More interestingly, although the critical temperature remains essentially the same with reduced thickness of Bi2212, the slope of the normal state T-linear resistivity varies by a factor of 4-5, and the sheet resistance increases by three orders of magnitude, indicating a surprising decoupling of the normal state resistance and superconductivity. The developed technique is versatile, applicable to investigate other two-dimensional (2D) superconducting materials.

Superconducting nanowire single photon detector with on-chip bandpass filter.

Dark count rate is one of the key parameters limiting the performance of the superconducting nanowire single photon detector (SNSPD). We have designed a multi-layer film bandpass filter that can be integrated onto the SNSPD to suppress the dark counts contributed by the stray light and blackbody radiation of the fiber. The bandpass filter is composed of 16 SiO(2)/Si bilayers deposited onto the backside of a thermally oxidized Si substrate. The substrate shows an excellent bandpass filter effect and provides a high transmittance of 88% at the central wavelength of the pass band, which is the same as that of the bare substrate. The SNSPDs fabricated on the substrate integrated with the bandpass filter show conspicuous wavelength-sensitive detection efficiency. The background dark count rate is reduced by two orders of magnitude to sub-Hz compared with the conventional SNSPD (a few tens of Hz). The detector exhibits a system detection efficiency of 56% at DCR of 1 Hz, with the measured minimal noise equivalent power reaching 2.0 × 10(-19) W/Hz(1/2).

Raman enhancement by graphene-Ga2O3 2D bilayer film.

2D ß-Ga2O3 flakes on a continuous 2D graphene film were prepared by a one-step chemical vapor deposition on liquid gallium surface. The composite was characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that Ga2O3 flakes grew on the surface of graphene film during the cooling process. In particular, tenfold enhancement of graphene Raman scattering signal was detected on Ga2O3 flakes, and XPS indicates the C-O bonding between graphene and Ga2O3. The mechanism of Raman enhancement was discussed. The 2D Ga2O3-2D graphene structure may possess potential applications.

Effect of local structure distortion on superconductivity in Mg- and F-codoped LaOBiS2.

La1-xMgxO1-2xF2xBiS2 (x = 0.1-0.35) were synthesized, and their superconductive properties were investigated. The superconducting transition temperature (Tc) increased below the codoping level (x ≤ 0.25). La1-xMgxOBiS2 (x = 0-0.2) and La1-xMgxO0.6F0.4BiS2 (x = 0.1-0.3) were further prepared to explore the effect of Mg(2+). We found that the introduction of Mg(2+) and F(-) leads to local structure distortion. Larger distortion is beneficial for superconductivity in LaOBiS2, which was further confirmed by the results in La1-xCaxO1-2xF2xBiS2 (x = 0.2, 0.3).

Core-shell nanostructured "black" rutile titania as excellent catalyst for hydrogen production enhanced by sulfur doping.

Modification of rutile titanium dioxide (TiO2) for hydrogen generation and water cleaning is a grand challenge due to the chemical inertness of rutile, while such inertness is a desired merit for its stability in photoelectrochemical applications. Herein, we report an innovative two-step method to prepare a core-shell nanostructured S-doped rutile TiO2 (R'-TiO2-S). This modified black rutile TiO2 sample exhibits remarkably enhanced absorption in visible and near-infrared regions and efficient charge separation and transport. As a result, the unique sulfide surface (TiO(2-x):S) boosts the photocatalytic water cleaning and water splitting with a steady solar hydrogen production rate of 0.258 mmol h(-1) g(-1). The black titania is also an excellent photoelectrochemical electrode exhibiting a high solar-to-hydrogen conversion efficiency of 1.67%. The sulfided surface shell is proved to be an effective strategy for enhancing solar light absorption and photoelectric conversion.