Axial Higgs mode detected by quantum pathway interference in RTe3.

The observation of the Higgs boson solidified the standard model of particle physics. However, explanations of anomalies (for example, dark matter) rely on further symmetry breaking, calling for an undiscovered axial Higgs mode1. The Higgs mode was also seen in magnetic, superconducting and charge density wave (CDW) systems2,3. Uncovering the vector properties of a low-energy mode is challenging, and requires going beyond typical spectroscopic or scattering techniques. Here we discover an axial Higgs mode in the CDW system RTe3 using the interference of quantum pathways. In RTe3 (R = La, Gd), the electronic ordering couples bands of equal or different angular momenta4-6. As such, the Raman scattering tensor associated with the Higgs mode contains both symmetric and antisymmetric components, which are excited via two distinct but degenerate pathways. This leads to constructive or destructive interference of these pathways, depending on the choice of the incident and Raman-scattered light polarization. The qualitative behaviour of the Raman spectra is well captured by an appropriate tight-binding model, including an axial Higgs mode. Elucidation of the antisymmetric component is direct evidence that the Higgs mode contains an axial vector representation (that is, a pseudo-angular momentum) and hints that the CDW is unconventional. Thus, we provide a means for measuring quantum properties of collective modes without resorting to extreme experimental conditions.

Two-Dimensional Violet Phosphorus P11: A Large Band Gap Phosphorus Allotrope.

The discovery of novel large band gap two-dimensional (2D) materials with good stability and high carrier mobility will innovate the next generation of electronics and optoelectronics. A new allotrope of 2D violet phosphorus P11 was synthesized via a salt flux method in the presence of bismuth. Millimeter-sized crystals of violet-P11 were collected after removing the salt flux with DI water. From single-crystal X-ray diffraction, the crystal structure of violet-P11 was determined to be in the monoclinic space group C2/c (no. 15) with unit cell parameters of a = 9.166(6) Å, b = 9.121(6) Å, c = 21.803(14)Å, ß = 97.638(17)°, and a unit cell volume of 1807(2) Å3. The structure differences between violet-P11, violet-P21, and fibrous-P21 are discussed. The violet-P11 crystals can be mechanically exfoliated down to a few layers (∼6 nm). Photoluminescence and Raman measurements reveal the thickness-dependent nature of violet-P11, and exfoliated violet-P11 flakes were stable in ambient air for at least 1 h, exhibiting moderate ambient stability. The bulk violet-P11 crystals exhibit excellent stability, being stable in ambient air for many days. The optical band gap of violet-P11 bulk crystals is 2.0(1) eV measured by UV-Vis and electron energy-loss spectroscopy measurements, in agreement with density functional theory calculations which predict that violet-P11 is a direct band gap semiconductor with band gaps of 1.8 and 1.9 eV for bulk and monolayer, respectively, and with a high carrier mobility. This band gap is the largest among the known single-element 2D layered bulk crystals and thus attractive for various optoelectronic devices.

Wafer-Scale Fabrication of 2D Nanostructures via Thermomechanical Nanomolding.

With shrinking dimensions in integrated circuits, sensors, and functional devices, there is a pressing need to develop nanofabrication techniques with simultaneous control of morphology, microstructure, and material composition over wafer length scales. Current techniques are largely unable to meet all these conditions, suffering from poor control of morphology and defect structure or requiring extensive optimization or post-processing to achieve desired nanostructures. Recently, thermomechanical nanomolding (TMNM) has been shown to yield single-crystalline, high aspect ratio nanowires of metals, alloys, and intermetallics over wafer-scale distances. Here, TMNM is extended for wafer-scale fabrication of 2D nanostructures. Using In, Al, and Cu, nanomold nanoribbons with widths < 50 nm, depths ≈0.5-1 µm and lengths ≈7 mm into Si trenches at conditions compatible is successfully with back end of line processing . Through SEM cross-section imaging and 4D-STEM grain orientation maps, it is shown that the grain size of the bulk feedstock is transferred to the nanomolded structures up to and including single crystal Cu. Based on the retained microstructures of molded 2D Cu, the deformation mechanism during molding for 2D TMNM is discussed.

Compact Super Electron-Donor to Monolayer MoS2.

The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS2, achieving a carrier density of 1.10 ± 0.37 × 1014 cm-2 at optimal functionalization conditions; the achieved carrier density is much higher than those by other OEDs such as benzyl viologen and an OED based on 4,4'-bipyridine. This impressive doping power is attributed to the compact size of Me-OED, which leads to high surface coverage on MoS2. To confirm, we study tBu-OED, which has an identical reduction potential to Me-OED but is significantly larger. Using field-effect transistor measurements and spectroscopic characterization, we estimate the doping powers of Me- and tBu-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, in good agreement with calculations. Our results demonstrate that the small size of Me-OED is critical to maximizing the surface coverage and molecular interactions with MoS2, enabling us to achieve unprecedented doping of MoS2.

Synergizing Electron and Heat Flows in Photocatalyst for Direct Conversion of Captured CO2.

We report a ternary hybrid photocatalyst architecture with tailored interfaces that boost the utilization of solar energy for photochemical CO2 reduction by synergizing electron and heat flows in the photocatalyst. The photocatalyst comprises cobalt phthalocyanine (CoPc) molecules assembled on multiwalled carbon nanotubes (CNTs) that are decorated with nearly monodispersed cadmium sulfide quantum dots (CdS QDs). The CdS QDs absorb visible light and generate electron-hole pairs. The CNTs rapidly transfer the photogenerated electrons from CdS to CoPc. The CoPc molecules then selectively reduce CO2 to CO. The interfacial dynamics and catalytic behavior are clearly revealed by time-resolved and in situ vibrational spectroscopies. In addition to serving as electron highways, the black body property of the CNT component can create local photothermal heating to activate amine-captured CO2 , namely carbamates, for direct photochemical conversion without additional energy input.

General Nanomolding of Ordered Phases.

Large-scale, controlled fabrication of ordered phases is challenging at the nanoscale, yet highly demanded as their well-ordered structure and chemistry is the key for advanced functionality. Here, we demonstrate a general nanomolding process of ordered phases based on atomic diffusion. Resulting nanowires are single crystals and maintain their composition and structure throughout their length, which we explain by a self-ordering process originating from their narrow Gibbs free energy. The versatility, control, and precision of this thermomechanical nanomolding method of ordered phases provides new insights into single crystal growth and suggest itself as a technology to enable wide spread usage for nanoscale and quantum devices.

Ultrathin dendrimer-graphene oxide composite film for stable cycling lithium-sulfur batteries.

Lithium-sulfur batteries (Li-S batteries) have attracted intense interest because of their high specific capacity and low cost, although they are still hindered by severe capacity loss upon cycling caused by the soluble lithium polysulfide intermediates. Although many structure innovations at the material and device levels have been explored for the ultimate goal of realizing long cycle life of Li-S batteries, it remains a major challenge to achieve stable cycling while avoiding energy and power density compromises caused by the introduction of significant dead weight/volume and increased electrochemical resistance. Here we introduce an ultrathin composite film consisting of naphthalimide-functionalized poly(amidoamine) dendrimers and graphene oxide nanosheets as a cycling stabilizer. Combining the dendrimer structure that can confine polysulfide intermediates chemically and physically together with the graphene oxide that renders the film robust and thin (<1% of the thickness of the active sulfur layer), the composite film is designed to enable stable cycling of sulfur cathodes without compromising the energy and power densities. Our sulfur electrodes coated with the composite film exhibit very good cycling stability, together with high sulfur content, large areal capacity, and improved power rate.

A Highly Efficient All-Solid-State Lithium/Electrolyte Interface Induced by an Energetic Reaction.

The energetic chemical reaction between Zn(NO3 )2 and Li is used to create a solid-state interface between Li metal and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) electrolyte. This interlayer, composed of Zn, ZnLix alloy, Li3 N, Li2 O, and other species, possesses strong affinities with both Li metal and LLZTO and affords highly efficient conductive pathways for Li+ transport through the interface. The unique structure and properties of the interlayer lead to Li metal anodes with longer cycle life, higher efficiency, and better safety compared to the current best Li metal electrodes operating in liquid electrolytes while retaining comparable capacity, rate, and overpotential. All-solid-state Li||Li cells can operate at very demanding current-capacity conditions of 4 mA cm-2 -8 mAh cm-2 . Thousands of hours of continuous cycling are achieved at Coulombic efficiency >99.5 % without dendrite formation or side reactions with the electrolyte.

Unveiling the Interfacial Effects for Enhanced Hydrogen Evolution Reaction on MoS2 /WTe2 Hybrid Structures.

Using the MoS2 -WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2 . Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large-area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2 . The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large-area heterojunctions. Based on this understanding, a MoS2 /WTe2 hybrid catalyst is fabricated with an HER overpotential of -140 mV at 10 mA cm-2 , a Tafel slope of 40 mV dec-1 , and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.

Dual Tuning of Ni-Co-A (A = P, Se, O) Nanosheets by Anion Substitution and Holey Engineering for Efficient Hydrogen Evolution.

Seeking earth-abundant electrocatalysts with high efficiency and durability has become the frontier of energy conversion research. Mixed-transition-metal (MTM)-based electrocatalysts, owing to the desirable electrical conductivity, synergistic effect of bimetal atoms, and structural stability, have recently emerged as new-generation hydrogen evolution reaction (HER) electrocatalysts. However, the correlation between anion species and their intrinsic electrocatalytic properties in MTM-based electrocatalysts is still not well understood. Here we present a novel approach to tuning the anion-dependent electrocatalytic characteristics in MTM-based catalyst for HER, using holey Ni/Co-based phosphides/selenides/oxides (Ni-Co-A, A = P, Se, O) as the model materials. The electrochemical results, combined with the electrical conductivity measurement and DFT calculation, reveal that P substitution could modulate the electron configuration, lower the hydrogen adsorption energy, and facilitate the desorption of hydrogen on the active sites in Ni-Co-A holey nanostructures, resulting in superior HER catalytic activity. Accordingly we fabricate the NCP holey nanosheet electrocatalyst for HER with an ultralow onset overpotential of nearly zero, an overpotential of 58 mV, and long-term durability, along with an applied potential of 1.56 V to boost overall water splitting at 10 mA cm-2, among the best electrocatalysts reported for non-noble-metal catalysts to date. This work not only presents a deeper understanding of the intrinsic HER electrocatalytic properties for MTM-based electrocatalyst with various anion species but also offers new insights to better design efficient and durable water-splitting electrocatalysts.

Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage.

Two-dimensional (2D) energy materials have shown the promising electrochemical characteristics for lithium ion storage. However, the decreased active surfaces and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO4) nanosheets as a model material for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol and tetrahydrofuran, the sodium ion transport in VOPO4 nanosheets has been significantly improved. In addition to advanced characterization including X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption fine structure to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO4 host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO4 nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO4 nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO4 nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg2+ and Al3+.

Strong Metal-Phosphide Interactions in Core-Shell Geometry for Enhanced Electrocatalysis.

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal-phosphide core-shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron-iron phosphide (Fe@FeP) core-shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53-110 mV to reach catalytic current densities of 10-100 mA cm-2.

Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction.

The ability to intercalate guest species into the van der Waals gap of 2D layered materials affords the opportunity to engineer the electronic structures for a variety of applications. Here we demonstrate the continuous tuning of layer vertically aligned MoS2 nanofilms through electrochemical intercalation of Li(+) ions. By scanning the Li intercalation potential from high to low, we have gained control of multiple important material properties in a continuous manner, including tuning the oxidation state of Mo, the transition of semiconducting 2H to metallic 1T phase, and expanding the van der Waals gap until exfoliation. Using such nanofilms after different degree of Li intercalation, we show the significant improvement of the hydrogen evolution reaction activity. A strong correlation between such tunable material properties and hydrogen evolution reaction activity is established. This work provides an intriguing and effective approach on tuning electronic structures for optimizing the catalytic activity.

Revealing Surface States in In-Doped SnTe Nanoplates with Low Bulk Mobility.

Indium (In) doping in topological crystalline insulator SnTe induces superconductivity, making In-doped SnTe a candidate for a topological superconductor. SnTe nanostructures offer well-defined nanoscale morphology and high surface-to-volume ratios to enhance surface effects. Here, we study In-doped SnTe nanoplates, In(x)Sn(1-x)Te, with x ranging from 0 to 0.1 and show they superconduct. More importantly, we show that In doping reduces the bulk mobility of In(x)Sn(1-x)Te such that the surface states are revealed in magnetotransport despite the high bulk carrier density. This is manifested by two-dimensional linear magnetoresistance in high magnetic fields, which is independent of temperature up to 10 K. Aging experiments show that the linear magnetoresistance is sensitive to ambient conditions, further confirming its surface origin. We also show that the weak antilocalization observed in In(x)Sn(1-x)Te nanoplates is a bulk effect. Thus, we show that nanostructures and reducing the bulk mobility are effective strategies to reveal the surface states and test for topological superconductors.

Highly conductive single-walled carbon nanotube thin film preparation by direct alignment on substrates from water dispersions.

A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant attention, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. We present a method that uniquely produces a noncorrosive ink that can be directly applied to a device in situ, which is different from previous fabrication techniques. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g., superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with a rodlike morphology as measured by light scattering. The Mayer rod technique is used to prepare urea-SWNT, highly aligned films (two-dimensional nematic order parameter of 0.6, 5 µm spot size, via polarized Raman) with resistance values as low as 15-1700 Ω/sq in a transmittance range of 2-80% at 550 nm. These values compete with the best literature values for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

One-dimensional helical transport in topological insulator nanowire interferometers.

The discovery of three-dimensional (3D) topological insulators opens a gateway to generate unusual phases and particles made of the helical surface electrons, proposing new applications using unusual spin nature. Demonstration of the helical electron transport is a crucial step to both physics and device applications of topological insulators. Topological insulator nanowires, of which spin-textured surface electrons form 1D band manipulated by enclosed magnetic flux, offer a unique nanoscale platform to realize quantum transport of spin-momentum locking nature. Here, we report an observation of a topologically protected 1D mode of surface electrons in topological insulator nanowires existing at only two values of half magnetic quantum flux (±h/2e) due to a spin Berry's phase (π). The helical 1D mode is robust against disorder but fragile against a perpendicular magnetic field breaking-time-reversal symmetry. This result demonstrates a device with robust and easily accessible 1D helical electronic states from 3D topological insulators, a unique nanoscale electronic system to study topological phenomena.

Metal seed layer thickness-induced transition from vertical to horizontal growth of MoS2 and WS2.

Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, that is, horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for optoelectronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications. In this Letter, we investigate the transition of vertical-to-horizontal growth directions in 2D molybdenum (or tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (>∼ 3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties.

Synthesis of SnTe nanoplates with {100} and {111} surfaces.

SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morphology of SnTe such that only desired crystallographic surfaces are present. Here, we grow SnTe nanostructures using vapor-liquid-solid and vapor-solid growth mechanisms. Previously, SnTe nanowires and nanocrystals have been grown [Saghir et al. Cryst. Growth Des. 2014, 14, 2009-2013; Safdar et al. Cryst. Growth Des. 2014, 14, 2502-2509; Safdar et al. Nano Lett. 2013, 13, 5344-5349; Li et al. Nano Lett. 2013, 13, 5443-5448]. In this report, we demonstrate the synthesis of SnTe nanoplates with lateral dimensions spanning tens of micrometers and thicknesses of a few hundred nanometers. The top and bottom surfaces are either (100) or (111), maximizing topological surface states on these surfaces. Magnetotransport on these SnTe nanoplates shows a high bulk carrier density, consistent with bulk SnTe crystals arising due to defects such as Sn vacancies. In addition, we observe a structural phase transition in these nanoplates from the high-temperature rock salt to a low-temperature rhombohedral structure. For nanoplates with a very high carrier density, we observe a slight upturn in resistance at low temperatures, indicating electron-electron interactions.