The structural, electronic and optical properties of four α-Se-based heterostructures with hyperbolic characteristics.

The physical properties and potential applications of two-dimensional (2D) materials can be effectively modulated and enriched by constructing van der Waals heterostructures (VDWHs) with two or more 2D monolayer materials. In this work, by using first-principles calculations based on density functional theory (DFT), we have systematically investigated the structural, electronic and optical properties of four α-Se-based VDWHs, that is, α-Se/Ca(OH)2, α-Se/GaSe, α-Se/h-BN and α-Se/MoS2 VDWHs. The results show that both the band alignment and band gap of these four VDWHs can be effectively modulated by interlayer coupling, biaxial strain and an external electric field. Compared with interlayer coupling and biaxial strain, the external electric field can modulate the electronic properties of these VDWHs more significantly, which makes them exhibit more rich electronic properties. Interestingly, the optical property calculations revealed that both α-Se and the four α-Se-based VDWHs have intrinsic hyperbolic properties. In addition, compared with the individual components, the optical absorption of these four VDWHs in the visible and ultraviolet light regions is significantly enhanced. These results enrich the properties of selenene and selenene-based VDWHs and extend their potential applications in electronic and optoelectronic devices.

Formation of stable polonium monolayers with tunable semiconducting properties driven by strong quantum size effects.

Elemental two-dimensional (2D) materials have attracted extraordinary interest compared with other 2D materials over the past few years. Fifteen elements from group IIIA to VIA have been discussed experimentally or theoretically for the formation of 2D monolayers, and the remaining few elements still need to be identified. Here, using first-principles calculations within density functional theory (DFT) and ab initio molecular dynamics simulations (AIMDs), we demonstrated that polonium can form stable 2D monolayers (MLs) with a 1T-MoS2-like structure. The band structure calculations revealed that polonium monolayers possess strong semiconducting properties with a band gap of ∼0.9 eV, and such semiconducting properties can well sustain up to a thickness of 4 MLs with a bandgap of ∼0.1 eV. We also found that polonium monolayers can be achieved through a spontaneous phase transition of ultrathin films with magic thicknesses, resulting in a weaker van der Waals interaction of ∼32 meV Å-2 between each three atomic layers. Also, the underlying physics comes from layered Peierls-like distortion driven by strong quantum size effects. Based on these intriguing findings, a suitable substrate on which the polonium monolayer can be grown through an epitaxial growth technique is proposed for further experiments. Our work not only extends completely the puzzle of elemental 2D monolayer materials from group IIIA to VIA, but also presents a new formation mechanism of 2D materials beyond the database of bulk materials with layered van der Waals interactions.

Spontaneous Formation of 1D Pattern in Monolayer VSe2 with Dispersive Adsorption of Pt Atoms for HER Catalysis.

Creation of functional patterns in two-dimensional (2D) materials provides opportunities to extend their potential for applications. Transition-metal dichalcogenides (TMDCs) are suitable 2D materials for pattern generation because of properties including alterable polymorphic phases, easy chalcogen-vacancy formation, metal-atom insertion, and alloying. Such patterning can be used for selective functionalization. Here we report the spontaneous formation of long-range, well-ordered 1D patterns in monolayer vanadium diselenide (VSe2) by a single annealing stage during growth. Atomic-resolution images in real space combined with density-functional-theory (DFT) calculations reveal the 1D features of patterned VSe2. Further experimental characterization of the intermediate states in the growth process confirm the spontaneous formation of the 1D pattern by annealing-induced Se-deficient linear defects. The 1D pattern can be reversibly transformed to homogenous VSe2 monolayer by reintroducing Se atoms. Moreover, additional experiments demonstrate that a dispersive deposition of Pt atoms along the 1D structures of patterned VSe2 is achieved, while DFT calculations find that their catalytic activity for hydrogen evolution reaction (HER) is as good as that of Pt surfaces. The formation of long-range, well-ordered 1D patterns not only demonstrates an effective way of dimension modulation in 2D materials but also enriches the potential of intrinsically patterned 2D materials for promising catalytic activities.

The Nature of Bonding in Bulk Tellurium Composed of One-Dimensional Helical Chains.

Bulk tellurium (Te) is composed of one-dimensional (1D) helical chains which have been considered to be coupled by van der Waals (vdW) interactions. However, on the basis of first-principles density functional theory calculations, we here propose a different bonding nature between neighboring chains: i.e., helical chains made of normal covalent bonds are connected together by coordinate covalent bonds. It is revealed that the lone pairs of electrons of Te atoms participate in forming coordinate covalent bonds between neighboring chains, where each Te atom behaves as both an electron donor to neighboring chains and an electron acceptor from neighboring chains. This ligand-metal-like bonding nature in bulk Te results in the same order of bulk moduli along the directions parallel and perpendicular to the chains, contrasting with the large anisotropy of bulk moduli in vdW crystals. We further find that the electron effective masses parallel and perpendicular to the chains are almost the same as each other, consistent with the observed nearly isotropic electrical resistivity. It is thus demonstrated that the normal/coordinate covalent bonds parallel/perpendicular to the chains in bulk Te lead to a minor anisotropy in structural and transport properties.

Even-odd oscillation of bandgaps in GeP3 nanoribbons and a tunable 1D lateral homogenous heterojunction.

GeP3 is a new kind of IV-V two dimensional material that has been predicted very recently. Here, we have theoretically explored the electronic properties of GeP3 nanoribbons (NRs) by employing first-principles calculations within density functional theory. We find that, unlike other monolayer materials, the bandgaps of armchair GeP3 NRs exhibit a strong even-odd oscillation as a function of nanoribbon width and such oscillations can remain intensive even when the width reaches up to 9 nm. The underlying physics of such oscillation originates from both the parity-dependent geometric symmetry and quantum size effects. Furthermore, we also find that suitable chemical decoration at the nanoribbon edge, e.g., by passivating hydrogen atoms, effectively tunes the indirect band gap into a direct one, making these nanoribbons potentially important for photovoltaic applications. Based on the strong bandgap oscillating nature of GeP3 NRs, we have conceptually designed a lateral homogenous heterojunction, constructed by GeP3 nanoribbons with different widths, which has shown a type-II band alignment beneficial for photo-detector applications.

Multivalency-Driven Formation of Te-Based Monolayer Materials: A Combined First-Principles and Experimental study.

Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable 1T-MoS_{2}-like (α-Te), and metastable tetragonal (ß-Te) and 2H-MoS_{2}-like (γ-Te) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the α-Te phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the α- and ß-Te phases possess electron and hole mobilities much higher than MoS_{2}. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.

Quantum effect enhanced magnetism of C-doped phosphorene nanoribbons: first-principles calculations.

Manipulating magnetism of low-dimensional materials is of great importance for their practical applications. Here, using first-principles calculations, we report a systematic investigation of the magnetic properties of C-doped H saturated zigzag phosphorene nanoribbons (H-ZPNRs), which are rather different from those of 2D periodic systems due to the quantum size effect. First of all, we observed a greatly enhanced magnetic moment locating mainly on the C atom and also slightly on its surrounding P atoms. Our results also indicated a strong dependence of the magnetic moment of the C atom on its location, which decays from the edge to the center site of the nanoribbons with an odd-even oscillating behavior originating from Friedel oscillation in low-dimensional materials. As for the C atom on a specific location, its magnetic moment decreases gradually with increasing width of H-ZPNRs, degenerating to the 2D case. What is more, we found that both the magnitude and the oscillating behavior of the magnetic moment on the C atom can be tuned by the edge saturation atoms. In addition, the case of two C atoms co-doping H-ZPNRs was also studied, showing non-magnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) states depending on the locations of the two C atoms. Our findings suggest a plausible route for manipulating magnetism of the sp element doped H-ZPNRs, which are expected to have potential applications in spintronics.

Anomalous doping effect in black phosphorene using first-principles calculations.

Using first-principles density functional theory calculations, we investigate the geometries, electronic structures, and thermodynamic stabilities of substitutionally doped phosphorene sheets with group III, IV, V, and VI elements. We find that the electronic properties of phosphorene are drastically modified by the number of valence electrons in dopant atoms. The dopants with an even number of valence electrons enable the doped phosphorenes to have a metallic feature, while the dopants with an odd number of valence electrons retain a semiconducting feature. This even-odd oscillating behavior is attributed to the peculiar bonding characteristics of phosphorene and the strong hybridization of sp orbitals between dopants and phosphorene. Furthermore, the calculated formation energies of various substitutional dopants in phosphorene show that such doped systems can be thermodynamically stable. These results propose an intriguing route to tune the transport properties of electronic and photoelectronic devices based on phosphorene.

Atmospheric phthalate esters in a multi-function area of Hangzhou: Temporal variation, gas/particle phase distribution, and population exposure risk.

Phthalate esters (PAEs) are prevalent in both indoor and outdoor environments. However, there are relatively few studies on phthalate contamination in the air of multi-function areas. Experiments were conducted to analyze the concentrations of 14 distinct PAEs in outdoor air in the college town of Hangzhou throughout both the warm and cold seasons. Correlation and principal component analyses were performed to investigate the influence and source factors of PAEs. This study also focused on the relationship between the gas/particle partition coefficient Kp and temperature, as well as the application of the gas/particle partition model. The risk of exposure to PAEs via inhalation was predicted for four groups of the general population: toddlers, adolescents, adults, and older adults. The results indicated that the concentration levels of Σ14PAEs in outdoor air were 1573 ng/m3 in the gaseous phase and 126 ng/m3 in the particulate phase. Additionally, this study indicated three primary sources of PAEs: indoor diffuse sources, industrial emission sources, and building construction sources. The gas/particle partitioning of PAEs also revealed that low-molecular-weight PAEs are more prevalent in gas, whereas high-molecular-weight PAEs are more predominant in the particle phase. A health risk analysis revealed high estimations of daily intakes (EDI) for toddlers and adolescents and high lifetime average daily doses (LADD) for older adults. This study establishes a solid foundation for formulating scientific and effective air pollution control measures by analyzing the characteristics and assessing the health risks of PAEs.

Rectangular-Phase Tellurene on Ni(111) from Monolayer Films to Periodic Striped Patterns.

As an emerging member of monoelemental two-dimensional (2D) materials, 2D tellurium (tellurene) has recently attracted intensive attention due to its polymorphism arising from the multivalent nature and fascinating properties such as wide-range band gaps, high carrier mobilities, etc. Herein, we predict the formation of a rectangular-phase tellurene on Ni(111) by first-principles density functional theory (DFT) calculations and realize its direct syntheses and characterizations by molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM). We reveal that the monolayer rectangular tellurene and underlying Ni(111) substrate are strongly coupled, along with good lattice registry along two mutually perpendicular directions, which serves as the key driving force for the tellurene formation. We also uncover the unique morphological transitions of Te/Ni(111) from rectangular tellurene monolayer, to uniform periodic striped patterns at the second layer, and then to thick striped patterns. This work should offer valuable insights for the substrate-mediated syntheses of monoelemental 2D materials, thus propelling their phase engineering and intriguing property explorations.

Randomized Controlled Trial of Modified Nasobiliary Fixation and Drainage Technique.

Objectives: We investigated the clinical efficacy of a modified nasobiliary fixation and drainage technique which was designed in an attempt to reduce unplanned extubation and tube blockage and improve bile drainage and the comfort of catheterized patients. Methods: From January 2019 to December 2020, 230 patients receiving Endoscopic nasobiliary drainage (ENBD) during hospitalization were recruited to this study. Participants were randomly allocated to 2 groups by using the block randomization method: in the control group: the conventional method of nasobiliary fixation was adopted after surgery; in the test group: intraoperative annular cutting of nasobiliary tubes was performed and the exposed catheter length was standardized. The modified "tube-nose-ear" three-step technique was performed after surgery. The clinical efficacy of a modified nasobiliary fixation and drainage technique was evaluated and compared between the test group and the control group. Results: The rate of unplanned extubation and incidence of complications were significantly lower in the test group than the control group. In addition, the rate of bilirubin decrease after drainage was higher in the test group. Patient discomfort during catheterization was also significantly reduced using the modified technique (P < 0.05). Conclusions: The modified technique of nasobiliary fixation and drainage technique can significantly reduce unplanned extubation and nasobiliary tube blockage after ENBD, facilitate biliary drainage, and improve patient comfort. This technique warrants wider application in clinical practice.

Experimental Realization of Atomic Monolayer Si9 C15.

Monolayer Six Cy constitutes an important family of 2D materials that is predicted to feature a honeycomb structure and appreciable bandgaps. However, due to its binary chemical nature and the lack of bulk polymorphs with a layered structure, the fabrication of such materials has so far been challenging. Here, the synthesis of atomic monolayer Si9 C15 on Ru (0001) and Rh(111) substrates is reported. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and density functional theory (DFT) calculations is used to infer that the 2D lattice of Si9 C15 is a buckled honeycomb structure. Monolayer Si9 C15 shows semiconducting behavior with a bandgap of ≈1.9 eV. Remarkably, the Si9 C15 lattice remains intact after exposure to ambient conditions, indicating good air stability. The present work expands the 2D-materials library and provides a promising platform for future studies in nanoelectronics and nanophotonics.

Pollution characteristics, sources, and health risk of atmospheric phthalate esters in a multi-function area of Hangzhou, China.

Phthalate esters (PAEs) are widely used in the manufacturing of plastics, and their demands have grown rapidly, especially in China, which will lead to much more environmental pollution of PAEs. In this study, fourteen common PAEs in ambient air were investigated during non-typhoon and typhoon seasons in a mixed multi-functional area of Hangzhou, China. The average concentrations of ∑14 PAEs in gaseous and PM2.5-bound phase (G-PAEs and P-PAEs) were 2317 ng/m3 and 128 ng/m3 during sampling period, while the mean concentrations of total PAEs in non-typhoon and typhoon seasons were 2412 ng/m3 and 2183 ng/m3, respectively. Bis(2-ethylhexyl)phthalate (DEHP) was the most abundant one, averagely accounting for 63.2% of G-PAEs and 88.3% of P-PAEs. Relative humidity showed a significant negative correlation with short-chain PAE (r = - 0.479, P < 0.01) and long-chain PAE (r = - 0.305, P < 0.05) concentrations in non-typhoon and typhoon seasons, and O3 could degrade G-PAEs through photoreaction. Source identification by the positive matrix factorization model and conditional probability function indicated that P-PAEs were mainly from the release from indoor environment (43%), PVC source (34%), construction source (12%), and industry source (11%). Air mass transport from both inland and oceans affected the PAE pollution in non-typhoon season, while its long-range transport from oceans took an important role in typhoon season. The daily inhalation intakes of PAEs for infants, teenagers, and adults were estimated, which showed that infants experienced the highest exposure risk.

Randomized controlled trial of PosiFlow regulator with infusion clamp for reducing intravenous catheter plugging.

BACKGROUND: Intravenous (IV) catheter insertion may cause adverse reactions. It is necessary to investigate optimal methods for reducing blood return and extending the usage time of IV catheter by existing technologies. We investigated the efficacy of the PosiFlow regulator with an infusion clip for preventing IV catheter plugging due to venous blood return. METHODS: From January 2017 to August 2017, 960 patients requiring IV catheterization were recruited and by using the block randomization method randomly allocated to 4 groups (n=240 in each group): a control group receiving IV catheter with infusion clip; experimental group A receiving IV catheter without infusion clip; experimental group B receiving an IV catheter with the PosiFlow regulator; and experimental group C receiving an IV catheter with a PosiFlow regulator and infusion clip. The rates of catheter plugging and blood return were compared among groups. RESULTS: In different reasons for needle pulling, The rates of catheter plugging and blood return in experimental group C (Received IV catheterization with an infusion clip and PosiFlow regulato) were the lowest among all groups (P<0.05). In different puncture sites, forearm puncture further reduced IV catheter plugging in all groups compared to other puncture sites. CONCLUSIONS: Catheter plugging and blood return can be significantly reduced by combining the PosiFlow regulator and an infusion clip, resulting in reduced medical costs and patient distress from catheter reinsertion.

Author Correction: Universal mechanical exfoliation of large-area 2D crystals.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Universal mechanical exfoliation of large-area 2D crystals.

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.

Strain Tunable Bandgap and High Carrier Mobility in SiAs and SiAs2 Monolayers from First-Principles Studies.

Searching for new stable free-standing atomically thin two-dimensional (2D) materials is of great interest in the fundamental and practical aspects of contemporary material sciences. Recently, the synthesis of layered SiAs single crystals has been realized, which indicates that their few layer structure can be mechanically exfoliated. Performing a first-principles density functional theory calculations, we proposed two dynamically and thermodynamically stable semiconducting SiAs and SiAs2 monolayers. Band structure calculation reveals that both of them exhibit indirect band gaps and an indirect to direct band even to metal transition are found by application of strain. Moreover, we find that SiAs and SiAs2 monolayers possess much higher carrier mobility than MoS2 and display anisotropic transportation like the black phosphorene, rendering them potential application in optoelectronics. Our works pave a new route at nanoscale for novel functionalities of optical devices.

Red phosphorus in its two-dimensional limit: novel clathrates with varying band gaps and superior chemical stabilities.

First-principles calculations within density functional theory reveal the preferred structures of red phosphorus in the two-dimensional (2D) limit to be porous with intriguing structural, electronic, and chemical properties. These few-atomic-layer structures are stabilized as novel 2D clathrates with tunable pore sizes and varying semiconducting band gaps, labelled as V-Hex, P-Monoclinic, P-Hex, and V-Tetr in descending energetic stabilities. The cohesive energies of the 2D clathrates are all substantially higher than that of white phosphorus. More strikingly, the V-Hex structure is energetically as stable as black phosphorene, but possesses distinctly superior chemical stability when exposed to O2 due to the presence of a much higher activation barrier against chemisorption. We also exploit the salient properties of these 2D clathrates for their important application potentials, including serving as effective elemental photocatalysts for visible-light-driven water splitting, and as a new class of sieves for molecular separation and DNA sequencing.

Stable GaSe-Like Phosphorus Carbide Monolayer with Tunable Electronic and Optical Properties from Ab Initio Calculations.

On the basis of density functional theory (DFT) calculations, we propose a stable two-dimensional (2D) monolayer phosphorus carbide (PC) with a GaSe-like structure, which has intriguing electronic and optical properties. Our calculated results show that this 2D monolayer structure is more stable than the other allotropes predicted by Tománek et al. [Nano Lett., 2016, 16, 3247⁻3252]. More importantly, this structure exhibits superb optical absorption, which can be mainly attributed to its direct band gap of 2.65 eV. The band edge alignments indicate that the 2D PC monolayer structure can be a promising candidate for photocatalytic water splitting. Furthermore, we found that strain is an effective method used to tune the electronic structures varying from direct to indirect band-gap semiconductor or even to metal. In addition, the introduction of one carbon vacancy in such a 2D PC structure can induce a magnetic moment of 1.22 µB. Our findings add a new member to the 2D material family and provide a promising candidate for optoelectronic devices in the future.

Epitaxially grown monolayer VSe2: an air-stable magnetic two-dimensional material with low work function at edges.

Recent experimental breakthroughs open up new opportunities for magnetism in few-atomic-layer two-dimensional (2D) materials, which makes fabrication of new magnetic 2D materials a fascinating issue. Here, we report the growth of monolayer VSe2 by molecular beam epitaxy (MBE) method. Electronic properties measurements by scanning tunneling spectroscopy (STS) method revealed that the as-grown monolayer VSe2 has magnetic characteristic peaks in its electronic density of states and a lower work-function at its edges. Moreover, air exposure experiments show air-stability of the monolayer VSe2. This high-quality monolayer VSe2, a very air-inert 2D material with magnetism and low edge work function, is promising for applications in developing next-generation low power-consumption, high efficiency spintronic devices and new electrocatalysts.