Robust Luttinger Liquid State of 1D Dirac Fermions in a Van der Waals System Nb9Si4Te18.

We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate 1D Dirac Fermions. A ternary van der Waals material Nb9Si4Te18 incorporates in-plane NbTe2 chains, which produce a 1D Dirac band crossing Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains is found to be substantially suppressed at Fermi energy, which follows a power law with a universal temperature scaling, hallmarking a TLL state. The obtained Luttinger parameter of ∼0.15 indicates a strong electron-electron interaction. The TLL behavior is found to be robust against atomic-scale defects, which might be related to the Dirac electron nature. These findings, combined with the tunability of the compound and the merit of a van der Waals material, offer a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.

Phase transition-induced band edge engineering of BiVO4 to split pure water under visible light.

Through phase transition-induced band edge engineering by dual doping with In and Mo, a new greenish BiVO4 (Bi1-XInXV1-XMoXO4) is developed that has a larger band gap energy than the usual yellow scheelite monoclinic BiVO4 as well as a higher (more negative) conduction band than H(+)/H2 potential [0 VRHE (reversible hydrogen electrode) at pH 7]. Hence, it can extract H2 from pure water by visible light-driven overall water splitting without using any sacrificial reagents. The density functional theory calculation indicates that In(3+)/Mo(6+) dual doping triggers partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening.

Highly selective solar-driven methanol from CO2 by a photocatalyst/biocatalyst integrated system.

The successful development of a photocatalyst/biocatalyst integrated system that carries out selective methanol production from CO2 is reported herein. The fine-tuned system was derived from a judicious combination of graphene-based visible light active photocatalyst (CCG-IP) and sequentially coupled enzymes. The covalent attachment of isatin-porphyrin (IP) chromophore to chemically converted graphene (CCG) afforded newly developed CCG-IP photocatalyst for this research endeavor. The current work represents a new benchmark for carrying out highly selective methanol formation from CO2 in an environmentally benign manner.

Thermoelectric imaging of structural disorder in epitaxial graphene.

Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes to diffuse along the temperature gradient inducing a thermoelectric voltage. Here we show that local thermoelectric measurements can yield high-sensitivity imaging of structural disorder on the atomic and nanometre scales. The thermopower measurement acts to amplify the variations in the local density of states at the Fermi level, giving high differential contrast in thermoelectric signals. Using this imaging technique, we uncovered point defects in the first layer of epitaxial graphene, which generate soliton-like domain-wall line patterns separating regions of the different interlayer stacking of the second graphene layer.

Dual Higgs modes entangled into a soliton lattice in CuTe.

Recently discovered Higgs particle is a key element in the standard model of elementary particles and its analogue in materials, massive Higgs mode, has elucidated intriguing collective phenomena in a wide range of materials with spontaneous symmetry breaking such as antiferromagnets, cold atoms, superconductors, superfluids, and charge density waves (CDW). As a straightforward extension beyond the standard model, multiple Higgs particles have been considered theoretically but not yet for Higgs modes. Here, we report the real-space observations, which suggest two Higgs modes coupled together with a soliton lattice in a solid. Our scanning tunneling microscopy reveals the 1D CDW state of an anisotropic transition metal monochalcogenide crystal CuTe is composed of two distinct but degenerate CDW structures by the layer inversion symmetry broken. More importantly, the amplitudes of each CDW structure oscillate in an out-of-phase fashion to result in a regular array of alternating domains with repeating phase-shift domain walls. This unusual finding is explained by the extra degeneracy in CDWs within the standard Landau theory of the free energy. The multiple and entangled Higgs modes demonstrate how novel collective modes can emerge in systems with distinct symmetries broken simultaneously.

A photocatalyst-enzyme coupled artificial photosynthesis system for solar energy in production of formic acid from CO2.

The photocatalyst-enzyme coupled system for artificial photosynthesis process is one of the most promising methods of solar energy conversion for the synthesis of organic chemicals or fuel. Here we report the synthesis of a novel graphene-based visible light active photocatalyst which covalently bonded the chromophore, such as multianthraquinone substituted porphyrin with the chemically converted graphene as a photocatalyst of the artificial photosynthesis system for an efficient photosynthetic production of formic acid from CO(2). The results not only show a benchmark example of the graphene-based material used as a photocatalyst in general artificial photosynthesis but also the benchmark example of the selective production system of solar chemicals/solar fuel directly from CO(2).

Photoconductivity and enhanced memory effects in hybrid C60-graphene transistors.

We describe the observation of photoconductivity and enhanced memory effects in graphene devices functionalized with clusters of alkylated C(60) molecules. The alkylated C(60) clusters were adsorbed on chemical vapor deposition-grown graphene devices from an aprotic medium. The resulting alkylated C(60)-graphene hybrid devices showed reproducible photoconductive behavior originating from the electron-accepting nature of the C(60) molecules. Significantly enhanced gate hysteresis was observed upon illumination with visible light, thereby enabling the use of C(60)-graphene hybrid devices in three-terminal photo-memory applications.

Resolving microscopic interfaces in Si(1-x)Ge(x) alloy nanowire devices.

We have fabricated Si(1-x)Ge(x) alloy nanowire devices with Ni and Ni/Au electrodes. The electrical transport characteristics of the alloy nanowires depended strongly on the annealing temperature and contact metals. Ni/Au-contacted devices annealed at 400 degrees C showed p-type transistor behavior as well as a resistance switching effect, while no switching was observed from Ni-contacted alloy nanowire devices. To identify the origin of such a hysteretic resistance switching effect, we constructed nanowire devices on a 40 nm Si(3)N(4) membrane. Transmission electron microscopy analysis combined with electrical transport measurements revealed that devices contacted with Ni/Au, and thereby showing resistance switching, have Au atoms right next to the alloy nanowire.

Synthesis of silver sulfide nanoparticles and their photodetector applications.

Silver sulfide nanoparticles (Ag2S NPs) are currently being explored as infrared active nanomaterials that can provide environmentally stable alternatives to heavy metals such as lead. In this paper, we describe the novel synthesis of Ag2S NPs by using a sonochemistry method and the fabrication of photodetector devices through the integration of Ag2S NPs atop a graphene sheet. We have also synthesized Li-doped Ag2S NPs that exhibited a significantly enhanced photodetector sensitivity via their enhanced absorption ability in the UV-NIR region. First-principles calculations based on a density functional theory formalism indicated that Li-doping produced a dramatic enhancement of NIR photoluminescence of the Ag2S NPs. Finally, high-performance photodetectors based on CVD graphene and Ag2S NPs were demonstrated and investigated; the hybrid photodetectors based on Ag2S NPs and Li-doped Ag2S NPs exhibited a photoresponse of 2723.2 and 4146.0 A W-1 respectively under a light exposure of 0.89 mW cm-2 at 550 nm. Our novel approach represents a promising and effective method for the synthesis of eco-friendly semiconducting NPs for photoelectric devices.

Detection of tumor markers using single-walled carbon nanotube field effect transistors.

We have developed a biosensor capable of detecting carcinoembryonic antigen (CEA) markers using single-walled carbon nanotube field effect transistors (SWNT-FETs). These SWNT-FETs were fabricated using nanotubes produced by a patterned catalyst growth technique, where the top contact electrodes were generated using conventional photolithography. For biosensor applications, SU-8 negative photoresist patterns were used as an insulation layer. CEA antibodies were employed as recognition elements to specific tumor markers, and were successfully immobilized on the sides of a single-walled carbon nanotube using CDI-Tween 20 linking molecules. The binding of tumor markers to these antibody-functionalized SWNT-FETs was then monitored continuously during exposure to dilute CEA solutions. The observed sharp decrease in conductance demonstrates the possibility of realizing highly sensitive, label-free SWNT-FET-based tumor sensors.

Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be(2+) as co-dopant.

For ex-situ co-doping methods, sintering at high temperatures enables rapid diffusion of Sn(4+) and Be(2+) dopants into hematite (α-Fe2O3) lattices, without altering the nanorod morphology or damaging their crystallinity. Sn/Be co-doping results in a remarkable enhancement in photocurrent (1.7 mA/cm(2)) compared to pristine α-Fe2O3 (0.7 mA/cm(2)), and Sn(4+) mono-doped α-Fe2O3 photoanodes (1.0 mA/cm(2)). From first-principles calculations, we found that Sn(4+) doping induced a shallow donor level below the conduction band minimum, which does not contribute to increase electrical conductivity and photocurrent because of its localized nature. Additionally, Sn(4+)-doping induce local micro-strain and a decreased Fe-O bond ordering. When Be(2+) was co-doped with Sn(4+)-doped α-Fe2O3 photoanodes, the conduction band recovered its original state, without localized impurities peaks, also a reduction in micro-strain and increased Fe-O bond ordering is observed. Also the sequence in which the ex-situ co-doping is carried out is very crucial, as Be/Sn co-doping sequence induces many under-coordinated O atoms resulting in a higher micro-strain and lower charge separation efficiency resulting undesired electron recombination. Here, we perform a detailed systematic characterization using XRD, FESEM, XPS and comprehensive electrochemical and photoelectrochemical studies, along with sophisticated synchrotron diffraction studies and extended X-ray absorption fine structure.

First-principles studies of SnS2 nanotubes: a potential semiconductor nanowire.

First principles calculations are used to predict the stability and electronic structures of SnS(2) nanotubes. Optimization of several structures and their corresponding strain energies confirm the stability of SnS(2) nanotube structures. Band structure calculations show that SnS(2) nanotubes could have moderate band gaps regardless of their chirality. It suggests that SnS(2) nanotubes would be well-suited to use as semiconductor wires in nanoelectronic devices if they are synthesized. Adsorption of NH(3) onto SnS(2) is also investigated and discussed with regard to potential sensor application.

Drawing circuits with carbon nanotubes: scratch-induced graphoepitaxial growth of carbon nanotubes on amorphous silicon oxide substrates.

Controlling the orientations of nanomaterials on arbitrary substrates is crucial for the development of practical applications based on such materials. The aligned epitaxial growth of single-walled carbon nanotubes (SWNTs) on specific crystallographic planes in single crystalline sapphire or quartz has been demonstrated; however, these substrates are unsuitable for large scale electronic device applications and tend to be quite expensive. Here, we report a scalable method based on graphoepitaxy for the aligned growth of SWNTs on conventional SiO2/Si substrates. The "scratches" generated by polishing were found to feature altered atomic organizations that are similar to the atomic alignments found in vicinal crystalline substrates. The linear and circular scratch lines could promote the oriented growth of SWNTs through the chemical interactions between the C atoms in SWNT and the Si adatoms in the scratches. The method presented has the potential to be used to prepare complex geometrical patterns of SWNTs by 'drawing' circuits using SWNTs without the need for state-of-the-art equipment or complicated lithographic processes.

Unconventional pore and defect generation in molybdenum disulfide: application in high-rate lithium-ion batteries and the hydrogen evolution reaction.

A 2H-MoS2 (H=hexagonal) ultrathin nanomesh with high defect generation and large porosity is demonstrated to improving electrochemical performance, including in lithium-ion batteries (LIBs) and the hydrogen evolution reaction (HER), with the aid of a 3D reduced graphene oxide (RGO) scaffold as fast electron and ion channels. The 3D defect-rich MoS2 nanomesh/RGO foam (Dr-MoS2 Nm/RGO) can be easily obtained through a one-pot cobalt acetate/graphene oxide (GO) co-assisted hydrothermal reaction, in which GO, cobalt and acetate ions are co-morphology-controlling agents and defect inducers. As an anode material for LIBs, Dr-MoS2 Nm/RGO has only a 9% capacity decay at a 10 C discharge rate versus 0.2 C with stable cyclability at the optimized composition (5 wt% RGO to MoS2 and 2 mol% Co to Mo), and significantly achieves 810 mA h g(-1) at a high current density of 9.46 A g(-1) over at least 150 cycles. Moreover, Dr-MoS2 Nm/RGO exhibits superior activity for the HER with an overpotential as low as 80 mV and a Tafel slope of about 36 mV per decade. In contrast to the MoS2 nanosheet/RGO (MoS2 Ns/RGO), which is synthesized in the absence of cobalt ions, Dr-MoS2 Nm/RGO provides high interconnectivity for efficient lithium-ion transport, and rich defects as electrochemically active sites. DFT is used to prove the existence of rich defects due to anion replacement to become a Co-Mo-S atomic structure, releasing inert basal planes to active sites.

Structurally and electronically designed TiO2Nx nanofibers for lithium rechargeable batteries.

The morphology and electronic structure of metal oxides, including TiO(2) on the nanoscale, definitely determine their electronic or electrochemical properties, especially those relevant to application in energy devices. For this purpose, a concept for controlling the morphology and electrical conductivity in TiO(2), based on tuning by electrospinning, is proposed. We found that the 1D TiO(2) nanofibers surprisingly gave higher cyclic retention than 0D nanopowder, and nitrogen doping in the form of TiO(2)N(x) also caused further improvement. This is due to higher conductivity and faster Li(+) diffusion, as confirmed by electrochemical impedance spectra. Our findings provide an effective and scalable solution for energy storage efficiency.