Multi-band optical resonance of all-dielectric metasurfaces toward high-performance ultraviolet sensing.

All-dielectric sensors featuring low-loss resonances have been proposed instead of plasmonic-based sensors. However, reported dielectric-based sensors generally work in the visible and near-infrared regions and detect the intensity variation of resonant modes because the electromagnetic energy is mainly confined inside dielectric nanoparticles. It is a challenge to adjust the hotspots from the inside to the surface of the all-dielectric metasurface. In this study, highly uniform Si3N4 all-dielectric metasurfaces have been successfully fabricated as sensing platforms by utilizing nanosphere self-assembly and plasma enhanced chemical vapor deposition techniques. Experimental and simulated results demonstrate that proposed Si3N4 all-dielectric metasurfaces exhibit multiple optical resonant modes in the ultraviolet and visible wavelength and present distinct field-confinement in the gaps of nanoparticles. The hotspots have been successfully adjusted to the surface of Si3N4 nanoparticles. Delightedly, Si3N4 all-dielectric metasurfaces show characteristic wavelength shifts with variation of the refractive index, and the sensitivity can reach 707 nm per RIU for trace detection as sensing substrates. Proposed Si3N4 all-dielectric metasurfaces are promising to act as high-sensitive sensing substrates in the ultraviolet and visible wavelength with the ease of high-throughput fabrication.

Single-layered phase-change metasurfaces achieving efficient wavefront manipulation and reversible chiral transmission.

Efficient control of the phase and polarization of light is of significant importance in modern optics and photonics. However, traditional methods are often accompanied with cascaded and bulky designs that cannot fulfill the ongoing demand for further integrations. Here, a single-layered metasurface composed of nonvolatile phase-change material Ge2Sb2Se4Te1 (GSST) is proposed with tunable spin-orbit interactions in subwavelength scale. According to the spin-dependent destructive or constructive interference, asymmetric transmission for circularly polarized incidence (extinction ratio > 8:1) can be achieved when GSST is in an amorphous state. Moreover, when GSST changes to crystalline state, reversed chiral transmission (extinction ratio > 12:1) can be observed due to the existence of intrinsic chirality. In addition, as the average cross-polarized transmitted amplitude is larger than 85%, arbitrary wavefront manipulations can be achieved in both states simultaneously based on the theory of Pancharatnam-Berry phase. As a proof of concept, several functional metasurface devices are designed and characterized to further demonstrate the validation of our design methodology. It is believed that these multifunctional devices with ultrahigh compactness are promising for various applications including chiroptical spectroscopy, EM communication, chiral imaging, and information encryption.

Multistate Nonvolatile Metamirrors with Tunable Optical Chirality.

Compared with conventional mirrors that behave as isotropic electromagnetic (EM) reflectors, metamirrors composed of periodically aligned artificial meta-atoms exhibit increased degrees of freedom for EM manipulations. However, the functionality of most metamirrors is fixed by design, and how to achieve active EM control is still elusive. Here, we propose a multistate metamirror based on the nonvolatile phase change material Ge2Sb2Te5 (GST) with four distinct functionalities that can be realized in the infrared region by exploiting the temperature-activated phase transition. When varying the crystallinity of GST, the metamirror has the capability to perform as a right-handed circular polarization chiral mirror, a narrowband achiral mirror, a left-handed circular polarization chiral mirror, or a broadband achiral mirror, respectively. The inner physics is further explained by the construction or cancellation of extrinsic two-dimensional chirality. As a proof of concept, experimental verification is carried out and the measured results agree well with their simulated counterparts. Such a multifunctional tunable metamirror could address a wide range of applications from sensing and spectroscopy to analytical chemistry and imaging.

Wafer-Scale Highly Oriented Monolayer MoS2 with Large Domain Sizes.

Two-dimensional molybdenum disulfide (MoS2) is an emergent semiconductor with great potential in next-generation scaled-up electronics, but the production of high-quality monolayer MoS2 wafers still remains a challenge. Here, we report an epitaxy route toward 4 in. monolayer MoS2 wafers with highly oriented and large domains on sapphire. Benefiting from a multisource design for our chemical vapor deposition setup and the optimization of the growth process, we successfully realized material uniformity across the entire 4 in. wafer and greater than 100 µm domain size on average. These monolayers exhibit the best electronic quality ever reported, as evidenced from our spectroscopic and transport characterizations. Our work moves a step closer to practical applications of monolayer MoS2.

Boundary activated hydrogen evolution reaction on monolayer MoS2.

Recently, monolayer molybdenum disulphide (MoS2) has emerged as a promising and non-precious electrocatalyst for hydrogen evolution reaction. However, its performance is largely limited by the low density and poor reactivity of active sites within its basal plane. Here, we report that domain boundaries in the basal plane of monolayer MoS2 can greatly enhance its hydrogen evolution reaction performance by serving as active sites. Two types of effective domain boundaries, the 2H-2H domain boundaries and the 2H-1T phase boundaries, were investigated. Superior hydrogen evolution reaction catalytic activity, long-term stability and universality in both acidic and alkaline conditions were achieved based on a multi-hierarchy design of these two types of domain boundaries. We further demonstrate that such superior catalysts are feasible at a large scale by applying this multi-hierarchy design of domain boundaries to wafer-scale monolayer MoS2 films.

Local engineering of topological phase in monolayer MoS2.

Monolayer transition metal dichalcogenides (TMDCs) with the 1T' structure are a new class of large-gap two-dimensional (2D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1T' phase is metastable compared to the 2H phase for most of 2D TMDCs, among which the 1T' phase is least favored in monolayer MoS2. Here we report a clean and controllable technique to locally induce nanometer-sized 1T' phase in monolayer 2H-MoS2 via a weak Argon-plasma treatment, resulting in topological phase boundaries of high density. We found that the stabilization of 1T' phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy (STS) clearly reveals a spin-orbit band gap (~60 meV) and topologically protected in-gap states residing at the 1T'-2H phase boundary, which are corroborated by density-functional theory (DFT) calculations. The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation.

Twist angle-dependent conductivities across MoS2/graphene heterojunctions.

Van der Waals heterostructures stacked from different two-dimensional materials offer a unique platform for addressing many fundamental physics and construction of advanced devices. Twist angle between the two individual layers plays a crucial role in tuning the heterostructure properties. Here we report the experimental investigation of the twist angle-dependent conductivities in MoS2/graphene van der Waals heterojunctions. We found that the vertical conductivity of the heterojunction can be tuned by ∼5 times under different twist configurations, and the highest/lowest conductivity occurs at a twist angle of 0°/30°. Density functional theory simulations suggest that this conductivity change originates from the transmission coefficient difference in the heterojunctions with different twist angles. Our work provides a guidance in using the MoS2/graphene heterojunction for electronics, especially on reducing the contact resistance in MoS2 devices as well as other TMDCs devices contacted by graphene.

Wafer-Scale Growth and Transfer of Highly-Oriented Monolayer MoS2 Continuous Films.

Large scale epitaxial growth and transfer of monolayer MoS2 has attracted great attention in recent years. Here, we report the wafer-scale epitaxial growth of highly oriented continuous and uniform monolayer MoS2 films on single-crystalline sapphire wafers by chemical vapor deposition (CVD) method. The epitaxial film is of high quality and stitched by many 0°, 60° domains and 60°-domain boundaries. Moreover, such wafer-scale monolayer MoS2 films can be transferred and stacked by a simple stamp-transfer process, and the substrate is reusable for subsequent growth. Our progress would facilitate the scalable fabrication of various electronic, valleytronic, and optoelectronic devices for practical applications.

Argon Plasma Induced Phase Transition in Monolayer MoS2.

In this work, we report a facile, clean, controllable and scalable phase engineering technique for monolayer MoS2. We found that weak Ar-plasma bombardment can locally induce 2H→1T phase transition in monolayer MoS2 to form mosaic structures. These 2H→1T phase transitions are stabilized by point defects (single S-vacancies) and the sizes of induced 1T domains are typically a few nanometers, as revealed by scanning tunneling microscopy measurements. On the basis of a selected-area phase patterning process, we fabricated MoS2 FETs inducing 1T phase transition within the metal contact areas, which exhibit substantially improved device performances. Our results open up a new route for phase engineering in monolayer MoS2 and other transition metal dichalcogenide (TMD) materials.

Precisely Aligned Monolayer MoS2 Epitaxially Grown on h-BN basal Plane.

Control of the precise lattice alignment of monolayer molybdenum disulfide (MoS2 ) on hexagonal boron nitride (h-BN) is important for both fundamental and applied studies of this heterostructure but remains elusive. The growth of precisely aligned MoS2 domains on the basal plane of h-BN by a low-pressure chemical vapor deposition technique is reported. Only relative rotation angles of 0° or 60° between MoS2 and h-BN basal plane are present. Domains with same orientation stitch and form single-crystal, domains with different orientations stitch and from mirror grain boundaries. In this way, the grain boundary is minimized and a continuous film stitched by these two types of domains with only mirror grain boundaries is obtained. This growth strategy is also applicable to other 2D materials growth.

Patterned Peeling 2D MoS2 off the Substrate.

The performance of two-dimensional (2D) MoS2 devices depends largely on the quality of the MoS2 itself. Existing fabrication process for 2D MoS2 relies on lithography and etching. However, it is extremely difficult to achieve clean patterns without any contaminations or passivations. Here we report a peel-off pattering of MoS2 films on substrates based on a proper interface engineering. The peel-off process utilizes the strong adhesion between gold and MoS2 and removes the MoS2 film contact with gold directly, leading to clean MoS2 pattern generation without residuals. Significantly improved electrical performances including high mobility ∼17.1 ± 8.3 cm(2)/(V s) and on/off ratio ∼5.6 ± 3.6 × 10(6) were achieved. Such clean fabrication technique paves a way to high quality MoS2 devices for various electrical and optical applications.

Rolling Up a Monolayer MoS2 Sheet.

MoS2 nanoscrolls are formed by argon plasma treatment on monolayer MoS2 sheet. The nanoscale scroll formation is attributed to the partial removal of top sulfur layer in MoS2 during the argon plasma treatment process. This convenient, solvent-free, and high-yielding nanoscroll formation technique is also feasible for other 2D transition metal dichalcogenides.

Observation of Strong Interlayer Coupling in MoS2/WS2 Heterostructures.

Epitaxial growth of A-A and A-B stacking MoS2 on WS2 via a two-step chemical vapor deposition method is reported. These epitaxial heterostructures show an atomic clean interface and a strong interlayer coupling, as evidenced by systematic characterization. Low-frequency Raman breathing and shear modes are observed in commensurate stacking bilayers for the first time; these can serve as persuasive fingerprints for interfacial quality and stacking configurations.

(2,2'-Bipyridine-κN,N'){N-[2-oxido-5-(phenyl-diazen-yl)benzyl-idene-κO]glycinato-κN,O}copper(II).

In the title compound, [Cu(C(15)H(11)N(3)O(3))(C(10)H(8)N(2))], the Cu(II) atom is five-coordinated in a distorted square-pyramidal CuN(3)O(2) geometry. The basal positions are occupied by three donor atoms from the tridentate Schiff base ligand and by one N atom from the 2,2'-bipyridine ligand. The axial position is occupied by the other N atom of the 2,2'-bipyridine ligand. The crystal structure is consolidated by weak C-H⋯O hydrogen bonds. In addition, π-π inter-actions between adjacent pyridine rings (centroid-centroid distances = 3.238 and 3.313 Å) may also stabilize the crystal packing.

Drug releasing behavior of hybrid micelles containing polypeptide triblock copolymer.

We report a new type of hybrid polymeric micelles for drug delivery applications. These micelles consist of PLGA (PLGA: poly(l-glutamic acid)) and PEG (PEG: polyethylene glycol) mixed corona chains. In acidic condition, PLGA undergoes a transformation from water-soluble random coils to water-insoluble alpha-helix, leading to microphase separation in micelle coronas and formation of PEG channels. These channels connect the inner core and the outer milieu, accelerating the diffusion of drugs from micelles. The micelles were prepared through a co-micellization of PLGA-b-PPO-b-PLGA (PPO: poly(propylene oxide)) and PEG-b-PPO in water. During the self-assembly, the PPO blocks of both block copolymers aggregated into cores that were surrounded by mixed corona chains of PLGA and PEG blocks. We confirmed this structure by using a number of characterization techniques including nuclear magnetic resonance spectroscopy, zeta potential, circular dichroism, and dynamic light scattering. We also performed molecular dynamics (MD) simulations to verify the models of the hybrid micelle structure. One advantage of the hybrid micelles as drug carriers is their tunable release rate without sacrificing colloidal stability. The rate can be tuned by either micelle structures such as the composition of the mixture or external parameters such as pH.

The pH-controlled dual-drug release from mesoporous bioactive glass/polypeptide graft copolymer nanomicelle composites.

Dual-drug delivery systems are investigated for combined therapy with drugs having distinct therapeutic effects. However, the majority of current dual-drug delivery systems are designed for simultaneous release of two different drugs; the release of each individual drug cannot be controlled. In this study, we have demonstrated a novel dual-drug delivery system based on mesoporous bioactive glass/polypeptide graft copolymer nanomicelle composites. Water-soluble gentamicin and fat-soluble naproxen were used as model drugs in the study of this system. A pH-controlled release of individual drugs was achieved by the predominant release of gentamicin from mesoporous bioactive glass in an acid environment and fast release of naproxen in an alkaline environment from polypeptide nanomicelles. Our results suggest that the mesoporous bioactive glass/PBLG-g-PEG nanomicelle composites can be used as a dual-drug delivery system, and that the individual drug release can be controlled by the pH of the surrounding environment.

[Study on structures of blattela germanica allergen, Bla g 2 expressed by eukaryotic and prokaryotic vectors using fluorescence and CD spectra].

The recombinant allergen, Bla g 2, was expressed by prokaryotic vector E. coli and eukaryotic vector P. Pastoris. The different structures and configurations of the Bla g 2 from E. coli and P. Pastoris were studied by fluorescence and circular dichroism. The secondary structures of Bla g 2 in solution, and the composition besides the type of its tertiary structure were proposed. These studies help understand the differences between prokaryotic and eukaryotic expression systems, reveal the relationship between the structure and the function of Bla g 2, and improve the production of this significant allergen.

[Study of fluorescence spectra of blattela germanica allergen, derenatured and renatured Bla g 2].

Recombinant proteins extracted from inclusion body remain in denaturation status. Renaturation in vitro after initial purification is a key step of downstream processing. A common method of renaturation of recombinant proteins is the dilution method. With Bla g 2 as a model protein, the conformational changes of denatured and renatured Bla g 2 were investigated by applying fluorescence spectra. The effects of different urea concentrations, different SDS concentrations and different pH on the fluorescence intensity of renatured protein were also investigated. The reasons for these were studied with the knowledge of molecular structure.