High-pressure response of vibrational properties of b-AsxP1-x:in situRaman studies.

The structural evolution of black arsenic-phosphorous (b-AsxP1-x) alloys with varying arsenic concentrations was investigated under hydrostatic pressure usingin situRaman spectroscopy. High-pressure experiments were conducted using a diamond anvil cell, which revealed pressure-induced shifts in vibrational modes associated with P-P bonds (A1g,A2g,B2g), As-As bonds (A1g,A2g,B2g), and As-P bonds in b-AsxP1-xalloys. Two distinct pressure regimes were observed. In the first regime (region I), all vibrational modes exhibited a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In the second regime (region II), As0.4P0.6and As0.6P0.4alloys displayed a linear blueshift (or negligible change in some modes) at a reduced rate, suggesting local structural reorganization with less compression on the bonds. Notably, the alloy with the highest As concentration, As0.8P0.2, exhibited anomalous behavior in the second pressure regime, with a downward shift observed in all As-As and As-P Raman modes (and some P-P modes). Interestingly, the emergence of new peaks corresponding to theEgmode andA1gmode of the gray-As phase was observed in this pressure range, indicating compressive strain-induced structural changes. The anomalous change in region II confirms the formation of a new local structure, characterized by elongation of the P-P, As-As, and As-P bonds along the zigzag direction within the b-AsxP1-xphase, possibly near the grain boundary. Additionally, a gray-As phase undergoes compressive structural changes. This study underscores the significance of pressure in inducing structural transformations and exploring novel phases in two-dimensional materials, including b-AsxP1-xalloys.

Ultra-Narrow Phosphorene Nanoribbons Produced by Facile Electrochemical Process.

Phosphorene nanoribbons (PNRs) have inspired strong research interests to explore their exciting properties that are associated with the unique two-dimensional (2D) structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several important properties of PNRs, the production of these structures with narrow widths is still a great challenge. Here, a facile and straightforward approach to synthesize PNRs via an electrochemical process that utilize the anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction, is reported. The produced PNRs display widths of good uniformity (10.3 ± 3.8 nm) observed by high-resolution transmission electron microscopy, and the suppressed B2g vibrational mode from Raman spectroscopy results. More interestingly, when used in field-effect transistors, synthesized bundles exhibit the n-type behavior, which is dramatically different from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined widths, paving the way toward the development of phosphorene and other highly anisotropic nanoribbon materials for high-quality electronic applications.

Synthesis of Fluorine-Doped Lithium Argyrodite Solid Electrolytes for Solid-State Lithium Metal Batteries.

Solid-state lithium metal batteries (SSLMBs) that utilize novel solid electrolytes (SEs) have garnered much attention because of their potential to yield safe and high-energy-density batteries. Sulfide-based argyrodite-class SEs are an attractive option because of their impressive ionic conductivity. Recent studies have shown that LiF at the interface between Li and SE enhances electrochemical stability. However, the synthesis of F-doped argyrodites has remained challenging because of the high temperatures used in the state-of-the-art solid-state synthesis methods. In this work, for the first time, we report F-doped Li5+yPS5Fy argyrodites with a tunable doping content and dual dopants (F-/Cl- and F-/Br-) that were synthesized through a solvent-based approach. Among all compositions, Li6PS5F0.5Cl0.5 exhibits the highest Li-ion conductivity of 3.5 × 10-4 S cm-1 at room temperature (RT). Furthermore, Li symmetric cells using Li6PS5F0.5Cl0.5 show the best cycling performance among the tested cells. X-ray photoelectron spectroscopy and ab initio molecular dynamics simulations revealed that the enhanced interfacial stability of Li6PS5F0.5Cl0.5 SE against Li metal can be attributed to the formation of a stable solid electrolyte interphase (SEI)-containing conductive species (Li3P), alongside LiCl and LiF. These findings open new opportunities to develop high-performance SSLMBs using a novel class of F-doped argyrodite electrolytes.

Visualization of Solid-State Synthesis for Chalcogenide Na Superionic Conductors by in-situ Neutron Diffraction.

Chalcogenide superionic sodium (Na) conductors have great potential as solid electrolytes (SEs) in all-solid-state Na batteries with advantages of high energy density, safety, and cost effectiveness. The crystal structures and ionically conductive properties of solid Na-ion conductors are strongly influenced by synthetic approaches and processing parameters. Thus, understanding the synthesis process is essential to control the structures and phases and to obtain Na-ion conductors with desirable properties. Thanks to the high-flux and deep-penetrating time-of-flight neutron diffraction (ND), in-situ experiments were able to track real-time structural changes of two chalcogenide SEs (Na3 SbS4 and Na3 SbS3.5 Se0.5 ) during the solid-state synthesis. For these two conductors, the ND results revealed a fast one-step reaction for the synthesis and the molten process when heating up, and the recrystallization as well as the cubic-to-tetragonal phase transition up on cooling. Moreover, Se-doping was found to influence the reaction temperatures, lattice parameter, and structure stability based on neutron experimental observations and theoretical simulation. This work presents a detailed structural study using in-situ ND technology for the solid synthesis process of chalcogenide Na-ion conductors, beneficial for the design and synthesis of new solid-state conductors.

Chemical Vapor Transport Route toward Black Phosphorus Nanobelts and Nanoribbons.

Chemical vapor transport (CVT) method is widely used for bulk black phosphorus (BP) fabrication. In this work, we demonstrate that CVT provides a route for the fabrication of BP nanoribbons and nanobelts. This method consists of a two-step procedure, including initial BP column growth using the CVT technique, followed by ultrasonic treatment and centrifugation. The obtained nanostructures preserve BP column dimensions, forming ultralong ribbon-like structures with the length to the width aspect ratio of up to 500. Computational modeling of the growth mechanism of a BP flake is also presented in support of the observed columnar growth. Calculation of the average energy of the molecule in the asymmetric flakes shows that the growth of the structure in the zigzag direction is more energetically favorable than in the armchair direction.

Insight the process of hydrazine gas adsorption on layered WS2: a first principle study.

The process of hydrazine gas adsorption on layered WS2 has been systematically studied from first principle calculations. Our results demonstrate that this adsorption process is exothermic, and hydrazine molecules are physically adsorbed. The layer-dependent adsorption energy and interlayer separation induced by van der Waals interaction exerted by hydrazine molecules lead to the difficulty in desorbing hydrazine molecules from layered WS2 as the number of layers increases. The most interesting finding is the emergence of localized impurity states below the Fermi level upon the hydrazine adsorption, irrespective of the number of WS2 layers, resulting in a significant effect on the band structures and subsequently changing its electrical conductivity. Furthermore, a layer-dependent small charge transfer occurs between hydrazine and layered WS2, leading to a charge redistribution and considerable polarization in the adsorbed systems. The existence of defects and the humidity, on the other hand, influences the sensitivity of layered WS2 to the hydrazine adsorption. Obtained results show that a perfectly layered WS2 might be a promising candidate as an efficient nanosensor to detect such toxic gas in dry environment.

Stable and Flexible Sulfide Composite Electrolyte for High-Performance Solid-State Lithium Batteries.

Sulfide-based lithium (Li)-ion conductors represent one of the most popular solid electrolytes (SEs) for solid-state Li metal batteries (SSLMBs) with high safety. However, the commercial application of sulfide SEs is significantly limited by their chemical instability in air and electrochemical instability with electrode materials (metallic Li anode and oxide cathodes). To address these difficulties, here, we design and successfully demonstrate a novel sulfide-incorporated composite electrolyte (SCE) through the combination of inorganic sulfide Li argyrodite (Li7PS6) with poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) polymer. In this composite structure, Li7PS6 is embedded in PVDF-HFP polymer matrix, making the SCE flexible and air-stable and achieve great chemical and electrochemical stability. Meanwhile, the presence of sulfide facilitates Li-ion transport in SCE, leading to a superior room-temperature ionic conductivity of 1.1 × 10-4 S cm-1. Using the SCE with enhanced stability while maintaining high conductivity, Li||Li symmetric cells achieved stable cycling up to 1000 h at 0.2 mA cm-2. In addition, LiFePO4 (LFP)||SCE||Li cells can deliver an impressive specific capacity of 160 mAh g-1 over 150 cycles. These features indicate that Li7PS6/PVDF-HFP SCE is a promising candidate to contribute to the practical development of SSLMBs.

Strain-induced vibrational properties of few layer black phosphorus and MoTe2 via Raman spectroscopy.

We studied and compared the effect of tensile strain on the Raman spectra of black phosphorus (BP) and molybdenum ditelluride (MoTe2) crystals by using a simple custom strain device. In-situ Raman spectroscopy on BP revealed clear red shifting of all three phonon modes, A1 g, B2g and A2 g, under tensile stress. From our theoretical analyses, we found that such red shifting strongly depends on the direction of the strain exerted on the system even within the elastic deformation limit (i.e. strain ≤ 1 %). In particular, calculated results for the strain along the armchair direction are consistent with our experimental data, confirming that the strain applied to the sample acts effectively along the armchair direction. In a comparative study, we found that the effect of strain on the Raman shifting is larger for BP than that for MoTe2, presumably due to the smaller Young's modulus of BP. We also see a remarkable resemblance between donor-type intercalation induced vibrational properties and tensile stress-induced vibrational properties in BP. We anticipate that our method of in-situ Raman spectroscopy can be an effective tool that can allow observation of strain effect directly which is critical for future flexible electronic devices.

Gas adsorption and light interaction mechanism in phosphorene-based field-effect transistors.

Phosphorene-based field effect transistor (FET) structures were fabricated to study the gas- and photo-detection properties of phosphorene. The interplay between device performance and environmental conditions was probed and analyzed using in situ transport measurements. The device structures were exposed to different chemical and light environments to understand how they perform under different external stimuli. For the gas/molecule detection studies, inert (Ar), as well as, oxidizing (N2O), and reducing (H2 and also N2H4) agents were selected. The FET structure was exposed to these different gases, and the effect of each gas on the device resistance was measured. The study showed varying response towards different molecules. Specifically, no significant resistance change was observed upon exposure to Ar, while H2 and N2H4 were found to decrease the resistance and N2O had the opposite effect resulting in an increase in resistance. This work is the first demonstration for the detection of N2H2 and N2O using a phosphorene-based system. These phosphorene-based FET structures were also found to be sensitive to light exposure. When such structure was irradiated with light, the current modulation was lost. The observed resistance changes can be explained as a result of the modulation of the Schottky barrier at the phosphorene-electrical contact interface due to the adsorbed molecules and charge transfer, and/or photo-induced carrier generation. The results were consistent with the transfer characteristics of Vdsvs. Vg.

Bilayer phosphorene under high pressure: in situ Raman spectroscopy.

In this study, bilayer phosphorene samples were subjected to high pressure using a Diamond Anvil Cell (DAC) and their vibrational properties were studied via in situ Raman spectroscopy. Systematic shifting in the Raman frequency of A1g, B2g, and A2g modes was observed and theoretical calculations were performed to understand the relationship between the strain and the vibrational properties. The changes in the vibration modes under high pressure are found to reflect the deformation in the structure and its stiffness. Firstly, the study shows a substantial pressure-induced enhancement of the interactions between atoms for the out-plane mode A1g, mainly due to the directional nature of the lone pair of electrons and charge transfer. However, these interactions and the observed blue shift of the A1g Raman peak are much weaker than those in bulk black phosphorous. Secondly, while a significant enhancement of the atomic interactions due to bond length change is also observed for the in-plane mode B2g along the zigzag direction, there is almost negligible effect on the in-plane mode A2g along the armchair direction. The results add to the knowledge on mechanical properties and strain engineering in phosphorene towards novel functionalities and applications of this intriguing two-dimensional (2D) material.

Graphene-WS2 heterostructures by a lithography free method: their electrical properties.

We have developed a lithography free technique for the fabrication of two-dimensional (2D) material based heterostructures. We fabricated graphene-WS2 heterostructured devices using a transmission electron microscope grid as a shadow mask and their electrical transport characteristics were studied by electrical and magneto transport measurements. Graphene was directly deposited on a Si/SiO2 substrate by radio frequency plasma enhanced chemical vapor deposition. WS2 was synthesized by first depositing WO3 followed by sulfurization. The temperature dependence of the resistance and magnetoresistance are measured for graphene, WS2, and graphene-WS2 heterostructure. At low temperatures, the transport is found to follow the variable-range hopping (VRH) process, where logarithmic R exhibits a T -1/3 temperature dependence, an evidence for the 2D Mott VRH transport. The measured low-field magnetoresistance also exhibits a quadratic magnetic field dependence ∼B 2, consistent with the 2D Mott VRH transport.

Mesoporous TiO2 coating on carbon-sulfur cathode for high capacity Li-sulfur battery.

In this paper, a meso-porous TiO2 (titania) coating is shown to effectively protect a carbon-sulfur composite cathode from polysulfide dissolution. The cathode consisted of a sulfur impregnated carbon support coated with a few microns thick mesoporous titania layer. The carbon-sulfur cathode is made using activated carbon powder (ACP) derived from biomass. The mesoporous titania coated carbon-sulfur cathodes exhibit a retention capacity after 100 cycles at C/3 rate (433 mA g -1) and stabilized at a capacity around 980 mA h g-1. The electrochemical impedance spectroscopy (EIS) of the sulfur cathodes suggests that the charge transfer resistance at the anode, (R act) is stable for the titania coated sulfur electrode in comparison to a continuous increase in R act for the uncoated electrode implying mitigation of polysulfide shuttling for the protected cathode. Stability in the cyclic voltammetry (CV) data for the first 5 cycles further confirms the polysulfide containment in the titania coated cathode while the uncoated sulfur electrode shows significant irreversibility in the CV with considerable shifting of the voltage peak positions. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies confirm the adsorption of soluble polysulfides by mesoporous titania.

The prospects of phosphorene as an anode material for high-performance lithium-ion batteries: a fundamental study.

To completely understand lithium adsorption, diffusion, and capacity on the surface of phosphorene and, therefore, the prospects of phosphorene as an anode material for high-performance lithium-ion batteries (LIBs), we carried out density-functional-theory calculations and studied the lithium adsorption energy landscape, the lithium diffusion mobility, the lithium intercalation, and the lithium capacity of phosphorene. We also carried out, for the very first time, experimental measurement of the lithium capacity of phosphorene. Our calculations show that the lithium diffusion mobility along the zigzag direction in the valley of phosphorene was about 7 to 11 orders of magnitude faster than that along the other directions, indicating its ultrafast and anisotropic diffusivity. The lithium intercalation in phosphorene was studied by considering various Li n P16 configurations (n = 1-16) including single-side and double-side adsorptions. We found that phosphorene could accommodate up to a ratio of one Li per P atom (i.e. Li16P16). In particular, we found that, even at a high Li concentration (e.g. x = 1 in Li x P), there was no lithium clustering, and the structure of phosphorene (when fractured) is reversible during lithium intercalation. The theoretical value of the lithium capacity for a monolayer phosphorene is predicted to be above 433 mAh g-1, depending on whether Li atoms are adsorbed on the single side or the double side of phosphorene. Our experimental measurement of the lithium capacity for few-layer phosphorene networks shows a reversible stable value of ∼453 mAh g-1 even after 50 cycles. Our results clearly show that phosphorene, compared to graphene and other two-dimensional materials, has great promise as a novel anode material for high-performance LIBs.

Efficient hydrogen evolution in transition metal dichalcogenides via a simple one-step hydrazine reaction.

Hydrogen evolution reaction is catalysed efficiently with precious metals, such as platinum; however, transition metal dichalcogenides have recently emerged as a promising class of materials for electrocatalysis, but these materials still have low activity and durability when compared with precious metals. Here we report a simple one-step scalable approach, where MoOx/MoS2 core-shell nanowires and molybdenum disulfide sheets are exposed to dilute aqueous hydrazine at room temperature, which results in marked improvement in electrocatalytic performance. The nanowires exhibit ∼100 mV improvement in overpotential following exposure to dilute hydrazine, while also showing a 10-fold increase in current density and a significant change in Tafel slope. In situ electrical, gate-dependent measurements and spectroscopic investigations reveal that hydrazine acts as an electron dopant in molybdenum disulfide, increasing its conductivity, while also reducing the MoOx core in the core-shell nanowires, which leads to improved electrocatalytic performance.

Synergy between graphene and Au nanoparticles (heterojunction) towards quenching, improving Raman signal, and UV light sensing.

Here, we developed a simple method for obtaining a heterojunction composed of graphene (G) and surfactant-coated Au nanoparticles (NPs) to measure film conductivity and surface enhanced Raman scattering (SERS). Monolayer G is obtained by chemical vapor deposition (CVD) and transferred via poly(methyl methacrylate) (PMMA) to microfabricated Au electrodes, glass, and silicon. Post-synthesis treatments of G with PMMA and ozone (O3) showed 1 and 6 orders of magnitude decrease in film conductivity, respectively. The heterojunction formation with Au NPs had no major effect on G conductivity. In this work is demonstrated that G quenches more than 90% of the combined photoluminescence and fluorescence of Au NPs and Rhodamine B (RhB), respectively. Signal quenching permitted quantitative analysis of SERS of RhB on various substrates including as-transferred graphene, oxidized graphene (OG), and the heterojunction. While G is mainly responsible for quenching photoluminescence and fluorescence, ∼3 orders of magnitude increase SERS activity for RhB was accomplished by the heterojunction. Finally, we wanted to correlate changes in film current during UV light sensing experiments. We found striking differences in the sensing profiles at different UV energies.

MoO(3-x) nanowire arrays as stable and high-capacity anodes for lithium ion batteries.

In this study, vertical nanowire arrays of MoO(3-x) grown on metallic substrates with diameters of ~90 nm show high-capacity retention of ~630 mAhg(-1) for up to 20 cycles at 50 mAg(-1) current density. Particularly, they exhibit a capacity retention of ~500 mAhg(-1) in the voltage window of 0.7-0.1 V, much higher than the theoretical capacity of graphite. In addition, 10 nm Si-coated MoO(3-x) nanowire arrays have shown a capacity retention of ~780 mAhg(-1), indicating that hybrid materials are the next generation materials for lithium ion batteries.

Graphene nanoribbons exfoliated from graphite surface dislocation bands by electrostatic force.

We have developed a novel technique to produce long and narrow graphene ribbons with smooth edges. This technique is free of any chemical treatments and involves a combination of two steps: (i) creation of surface dislocation ribbons by high velocity clusters impacting the graphite surface and (ii) electrostatic transferring of the dislocation ribbons to a desired substrate. The width of the ribbons can be controlled by varying the impact velocity of a cluster jet stream from a gas jet impactor. The electrical transport properties were investigated on the ribbons in field effect transistor (FET) configuration. The p-type behavior observed under ambient conditions was found to be reversed upon annealing at 180 degrees C in a vacuum of 10( - 7) Torr. Charge transfer effects were observed when the degassed graphene was exposed to N(2)O and NH(3).

Preferential growth of single-walled carbon nanotubes with metallic conductivity.

Single-walled carbon nanotubes can be classified as either metallic or semiconducting, depending on their conductivity, which is determined by their chirality. Existing synthesis methods cannot controllably grow nanotubes with a specific type of conductivity. By varying the noble gas ambient during thermal annealing of the catalyst, and in combination with oxidative and reductive species, we altered the fraction of tubes with metallic conductivity from one-third of the population to a maximum of 91%. In situ transmission electron microscopy studies reveal that this variation leads to differences in both morphology and coarsening behavior of the nanoparticles that we used to nucleate nanotubes. These catalyst rearrangements demonstrate that there are correlations between catalyst morphology and resulting nanotube electronic structure and indicate that chiral-selective growth may be possible.

Electrostatic deposition of graphene in a gaseous environment: a deterministic route for synthesizing rolled graphenes?

The synthesis of single-wall carbon nanotubes of desired diameters and chiralities is critical to the design of nanoscale electronic devices with desired properties. The existing methods are based on self-assembly, therefore lacking control over the diameters and chiralities. The present work reports a direct route for rolling graphene. Specifically, we found that the electrostatic deposition of graphene yielded: (i) flat graphene layers under high vacuum (10(-7) Torr), (ii) completely scrolled graphene under hydrogen atmosphere, (iii) partially scrolled graphene under nitrogen atmosphere, and (iv) no scrolling for helium atmospheres. Our study shows that the application of the electrostatic field facilitates the rolling of graphene sheets exposed to appropriate gases and allows the rolling of any size of graphene. The technique proposed here, in conjunction with a technique that produces graphene nanoribbons of uniform widths, will have significant impact on the development of carbon nanotube based devices. Furthermore, the present technique may be applied to obtain tubes/scrolls of other layered materials.

Hybrid tin oxide nanowires as stable and high capacity anodes for Li-ion batteries.

In this report, we present a simple and generic concept involving metal nanoclusters supported on metal oxide nanowires as stable and high capacity anode materials for Li-ion batteries. Specifically, SnO(2) nanowires covered with Sn nanoclusters exhibited an exceptional capacity of >800 mAhg(-1) over hundred cycles with a low capacity fading of less than 1% per cycle. Post lithiation analyses after 100 cycles show little morphological degradation of the hybrid nanowires. The observed, enhanced stability with high capacity retention is explained with the following: (a) the spacing between Sn nanoclusters on SnO(2) nanowires allowed the volume expansion during Li alloying and dealloying; (b) high available surface area of Sn nanoclusters for Li alloying and dealloying; and (c) the presence of Sn nanoclusters on SnO(2) allowed reversible reaction between Sn and Li(2)O to produce both Sn and SnO phases.