Laser-Ablated Red Phosphorus on Carbon Nanotube Film for Accelerating Polysulfide Conversion toward High-Performance and Flexible Lithium-Sulfur Batteries.

The use of a conducting interlayer between separator and cathode is one of the most promising methods to trap lithium polysulfides (LiPSs) for enhancing the performance of lithium-sulfur (Li-S) batteries. Red phosphorus nanoparticles (RPEN )-coated carbon nanotube (CNT) film (RPEN @CF) is reported herein as a novel interlayer for Li-S batteries, which shows strong chemisorption of LiPSs, good flexibility, and excellent electric conductivity. A pulsed laser ablation method is engaged for the ultrafast production of RPEN of uniform morphology, which are deposited on the CNT film by a direct spinning method. The RPEN @CF interlayer provides pathways for effective Li+ and electron transfer and strong chemical interaction with LiPSs. The S/RPEN @CF electrode shows a superior specific capacity of 782.3 mAh g-1 (3 C-rate) and good cycling performances (769.5 mAh g-1 after 500 cycles at 1 C-rate). Density functional theory calculations reveal that the morphology and dispersibility of RPEN are crucial in enhancing Li+ and electron transfer kinetics and effective trap of LiPSs. This work demonstrates the possibility of using the RPEN @CF interlayer for the enhanced electrochemical performances of Li-S batteries and other flexible energy storage devices.

Indium-contacted van der Waals gap tunneling spectroscopy for van der Waals layered materials.

The electrical phase transition in van der Waals (vdW) layered materials such as transition-metal dichalcogenides and Bi2Sr2CaCu2O8+x (Bi-2212) high-temperature superconductor has been explored using various techniques, including scanning tunneling and photoemission spectroscopies, and measurements of electrical resistance as a function of temperature. In this study, we develop one useful method to elucidate the electrical phases in vdW layered materials: indium (In)-contacted vdW tunneling spectroscopy for 1T-TaS2, Bi-2212 and 2H-MoS2. We utilized the vdW gap formed at an In/vdW material interface as a tunnel barrier for tunneling spectroscopy. For strongly correlated electron systems such as 1T-TaS2 and Bi-2212, pronounced gap features corresponding to the Mott and superconducting gaps were respectively observed at T = 4 K. We observed a gate dependence of the amplitude of the superconducting gap, which has potential applications in a gate-tunable superconducting device with a SiO2/Si substrate. For In/10 nm-thick 2H-MoS2 devices, differential conductance shoulders at bias voltages of approximately ± 0.45 V were observed, which were attributed to the semiconducting gap. These results show that In-contacted vdW gap tunneling spectroscopy in a fashion of field-effect transistor provides feasible and reliable ways to investigate electronic structures of vdW materials.

Screening of Electrolyte Complexes Formed Between Dextran Sulfate and Amine Structures of Small-Molecule Drugs.

Formation of an electrolyte complex using the electrostatic interactions between a polyanionic polymer and a cationic drug is a simple and efficient method of preparing a colloidal drug carrier system. Dextran sulfate, with a negatively charged sulfate group, was reacted in an acetate buffer solution of pH 3 with positively charged 1° amine, 2° amine, 3° amine, piperazine, and piperidine structures from 24 small-molecule drugs. The electrolyte complex was formed from 15 drugs, 63% of those tested. The tendency to form the electrolyte complex was in the order of piperazine and piperidine >3° amine >>2° amine. The drugs with the 1° amine structure failed to form an electrolyte complex. The mean particle sizes were in the range of 50-740 nm, and most of them showed a submicron colloidal dispersion of <400 nm. Regarding drug encapsulation efficiency (%), 11 drugs with piperazine, piperidine, and 3° amine structures showed 60-98% efficiency, which was fairly high. The results suggest that directly forming the electrolyte complex with dextran sulfate yields promising structural attributes as a submicron colloidal drug carrier system.

Adsorption of Acetonitrile on Si(111)-(7 × 7).

The adsorption of acetonitrile (CH3CN) on Si(111)-(7 × 7) at a room temperature has been investigated using scanning tunneling microscopy (STM) and first-principles calculations. The site-specific information on adsorption enables us to understand the site-by-site and step-by-step adsorption mechanism. From theoretical simulations, the most stable configuration of CH3CN on Si(111)-(7 × 7) is found to be a molecularly chemisorbed CH3CN with the carbon and nitrogen atoms of CN bonded to the rest atom and adatom on the Si surface, respectively. Some chemisorption-induced features in the STM topographic image are assigned based on the theoretical calculations.

Investigation of atomic and electronic properties of 2D-MoS2/3D-GaN mixed-dimensional heterostructures.

We have performed density functional theory calculations to study the effects caused by the interfacial structure between 2D-MoS2 and 3D-GaN. Two different surface terminations of GaN are considered: Ga-terminated (0001) (Ga-GaN) and N-terminated ([Formula: see text]) (N-GaN) configurations. We confirm that Rashba spin splitting occurs in band structure of MoS2 on GaN. We also find that the surface states of GaN move to the deep position in band structure in the MoS2/Ga-GaN case, while the surface states of GaN are hybridized with MoS2 near the Fermi level for the MoS2/N-GaN case. Furthermore, we investigate the variation in electronic structure of MoS2/GaN heterostructures depending on the number of MoS2 layers. Especially, the top layer MoS2 of the 2L-MoS2/GaN structures shows both n-type and p-type properties depending on the GaN surface termination. As a result, we suggest that the electrical characteristics of the 2D/3D heterostructures could be controlled by the surface terminations of substrate materials.

Remote homoepitaxy of ZnO microrods across graphene layers.

Two-dimensional atomic layered materials (2d-ALMs) are emerging candidates for use as epitaxial seed substrates for transferrable epilayers. However, the micrometer-sized domains of 2d-ALMs preclude their practical use in epitaxy because they cause crystallographically in-plane disordering of the overlayer. Ultrathin graphene can penetrate the electric dipole momentum from an underlying crystal layer to the graphene surface, which then drives it to crystallize the overlayer during the initial growth stage, thus resulting in substantial energy saving. This study demonstrates the remote homoepitaxy of ZnO microrods (MRs) on ZnO substrates across graphene layers via a hydrothermal method. Despite the presence of poly-domain graphene in between the ZnO substrate and ZnO MRs, the MRs were epitaxially grown on a- and c-plane ZnO substrates, whose in-plane alignments were homogeneous within the wafer's size. Transmission electron microscopy revealed a homoepitaxial relationship between the overlayer MRs and the substrate. Density-functional theory calculations suggested that the charge redistribution occurring near graphene induces the electric dipole formation, so the attracted adatoms led to the formation of the remote homoepitaxial overlayer. Due to a strong potential field caused by long-range charge transfer given from the substrate, even the use of bi-layer and tri-layer graphene resulted in remote homoepitaxial ZnO MRs. The effects of substrate crystal planes were also theoretically and empirically investigated. The ability of graphene, which can be released from the mother substrate without covalent bonds, was utilized to transfer the overlayer MR arrays. This method opens a way for producing well aligned, transferrable epitaxial nano/microstructure arrays while regenerating the substrate for cost-saving device manufacturing.

Mixed-dimensional 2D/3D heterojunctions between MoS2 and Si(100).

For utilization of two-dimensional (2D) materials as electronic devices, their mixed-dimensional heterostructures with three-dimensional (3D) materials are receiving much attention. In this study, we have investigated the atomic and electronic structures of the 2D/3D heterojunction between MoS2 and Si(100) using density functional theory calculations; especially, we focus on the contact behavior dependence on the interfacial structures of heterojunctions by considering two types of surface termination of Si(100) surfaces. Calculations show that MoS2 and clean Si(100) form an almost n-type ohmic contact with a very small Schottky barrier height (SBH) due to strong covalent bonds between them, and that the contact between MoS2 and H-covered Si(100) makes a p-n heterojunction with weak van der Waals interactions. Such a difference in contact behaviors can be explained by different electric dipole formation at the heterojunction interfaces. Overall, it is concluded that contact properties can be varied depending on the interfacial structures of 2D(MoS2)/3D(Si) semiconductor heterojunctions.

Visualizing Degradation of Black Phosphorus Using Liquid Crystals.

Black Phosphorus (BP) is an excellent material from the post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge for its practical and long-term usage. The optical visualization of the oxidized BP is the key and the foremost step for its successful passivation from the ambience. Here, we have conducted a systematic study of the oxidation of the BP and developed a technique to optically identify the oxidation of the BP using Liquid Crystal (LC). It is interesting to note that we found that the rapid oxidation of the thin layers of the BP makes them disappear and can be envisaged by using the alignment of the LC. The molecular dynamics simulations also proved the preferential alignment of the LC on the oxidized BP. We believe that this simple technique will be effective in passivation efforts of the BP, and will enable it for exploitation of its properties in the field of electronics.

Temperature-Dependent and Gate-Tunable Rectification in a Black Phosphorus/WS2 van der Waals Heterojunction Diode.

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS2. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 104, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

Controlled Electrochemical Intercalation of Graphene/h-BN van der Waals Heterostructures.

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electrochemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new heterointerface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density >5 × 1013 cm2 with mobility >103 cm2/(V s) in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.

Van der Waals heterojunction diode composed of WS2 flake placed on p-type Si substrate.

P-N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p-n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p-n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p-n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

In situ hybridization of carbon nanotubes with bacterial cellulose for three-dimensional hybrid bioscaffolds.

Carbon nanotubes (CNTs) have shown great potential in biomedical fields. However, in vivo applications of CNTs for regenerative medicine have been hampered by difficulties associated with the fabrication of three-dimensional (3D) scaffolds of CNTs due to CNTs' nano-scale nature. In this study, we devised a new method for biosynthesis of CNT-based 3D scaffold by in situ hybridizing CNTs with bacterial cellulose (BC), which has a structure ideal for tissue-engineering scaffolds. This was achieved simply by culturing Gluconacetobacter xylinus, BC-synthesizing bacteria, in medium containing CNTs. However, pristine CNTs aggregated in medium, which hampers homogeneous hybridization of CNTs with BC scaffolds, and the binding energy between hydrophobic pristine CNTs and hydrophilic BC was too small for the hybridization to occur. To overcome these problems, an amphiphilic comb-like polymer (APCLP) was adsorbed on CNTs. Unlike CNT-coated BC scaffolds (CNT-BC-Imm) formed by immersing 3D BC scaffolds in CNT solution, the APCLP-adsorbed CNT-BC hybrid scaffold (CNT-BC-Syn) showed homogeneously distributed CNTs throughout the 3D microporous structure of BC. Importantly, in contrast to CNT-BC-Imm scaffolds, CNT-BC-Syn scaffolds showed excellent osteoconductivity and osteoinductivity that led to high bone regeneration efficacy. This strategy may open a new avenue for development of 3D biofunctional scaffolds for regenerative medicine.

Rhizosphere microbial activity during phytoremediation of diesel-contaminated soil.

To know microbial activity and diesel-removal efficiency influencing through plant roots, we examined the effect of the rhizosphere on phytoremediation of diesel-contaminated soils by alfalfa (Medicago sativa L.). Pots were treated with and without diesel and allowed to stabilize for 7 weeks, at which time four experimental/control groups were prepared: (1) planted diesel-contaminated soil, (2) unplanted diesel-contaminated soil, (3) planted uncontaminated soil, and (4) unplanted uncontaminated soil. Samples of rhizosphere and bulk soils were separately taken from all planted pots. After 7 weeks of alfalfa growth from seeds, the removal efficiencies in rhizosphere and bulk soil samples were 82.5% and 36.5 approximately 59.4%, respectively. The total microbial activity was highest in diesel-contaminated rhizosphere soils. Significantly more culturable soil bacteria and hydrocarbon-degraders were found in diesel-contaminated rhizosphere soil versus unplanted and uncontaminated bulk soil, with a greater increase seen in hydrocarbon-degraders (172-fold) versus general soil bacteria (14-fold). DGGE (Denaturing Gel Gradient Electrophoresis) analysis revealed that the bacterial community structure was most highly influenced by the combined presence of diesel contamination and plant roots (39.13% similarity compared to the control), but that diesel contamination alone had a higher influence (42.31% similarity compared to the control) than the rhizosphere (50.00% similarity compared to the control). Sequence analysis and BLAST searches revealed that all samples were dominated by members of alpha -, gamma -, delta - and epsilon -proteobacteria, and Chloroflexi. The rhizosphere samples additionally contained novel dominant members of alpha -proteobacteria and Cytophaga-Flexibacter-Bacteroides, while the diesel samples contained additional dominant alpha -proteobacteria and the rhizosphere plus diesel samples contained other epsilon -proteobacteria. Collectively, these findings indicate that the presence of plant roots (i.e., a rhizosphere) had a greater effect on bacterial activity in diesel contamination than did the absence of diesel contamination, whereas diesel contamination had a greater effect on bacterial community structure. These novel findings provide new insight into the mechanisms of phytoremediation.