Anisotropic Spectroscopy and Electrical Properties of 2D ReS2(1-x) Se2x Alloys with Distorted 1T Structure.

2D black phosphorus (BP) and rhenium dichalcogenides (ReX2 , X = S, Se) possess intrinsic in-plane anisotropic physical properties arising from their low crystal lattice symmetry, which has inspired their novel applications in electronics, photonics, and optoelectronics. Different from BP with poor environmental stability, ReX2 has low-symmetry distorted 1T structures with excellent stability. In ReX2 , the electronic structure is weakly dependent on layer numbers, which restricts their property tunability and device applications. Here, the properties are tuned, such as optical bandgap, Raman anisotropy, and electrical transport, by alloying 2D ReS2 and ReSe2 . Photoluminescence emission energy of ReS2(1-x) Se2x monolayers (x from 0 to 1 with a step of 0.1) can be continuously tuned ranging from 1.62 to 1.31 eV. Polarization behavior of Raman modes, such as ReS2 -like peak at 212 cm-1 , shifts as the composition changes. Anisotropic electrical property is maintained in ReS2(1-x) Se2x with high electron mobility along b-axis for all compositions of ReS2(1-x) Se2x .

Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit.

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

Temperature-dependent photoluminescence emission and Raman scattering from Mo1-x W x S2 monolayers.

2D transition metal dichalcogenide (TMD) alloys with tunable band gaps have recently gained wide interest due to their potential applications in future nanoelectronics and optoelectronics. Here, we report the temperature-dependent photoluminescence (PL) and Raman spectra of Mo1-x W x S2 monolayers with W composition x = 0, 0.29, 0.53, 0.66 and 1 in the temperature range 93-493 K. We observed a linear temperature dependence of PL emission energy and Raman frequency. The PL intensity is enhanced at high temperature (>393 K). The temperature coefficients are negative for both PL and Raman bands, which may result from anharmonicity, thermal expansion and composition disorder.

Thickness-dependent electrical conductivities and ohmic contacts in transition metal dichalcogenides multilayers.

We report on the observation of the substantial thickness (t)-dependent electrical conductivity (σ) at a wide thickness range for an MoSe2 layer semiconductor. The conductivity increases for more than two orders of magnitude from 4.6 to 1500 Ω(-1) cm(-1) with a decrease in thickness from 2700 to 6 nm. The conductivity was found to follow a nearly linear relationship with the reciprocal thickness, i.e. σ ∝ 1/t. The temperature-dependent conductivity measurements also show that the MoSe2 multilayers have much lower activation energies at 3.5-8.5 meV than those (36-38 meV) of their bulk counterparts, indicating the different origins of the majority carrier. These results imply the presence of higher surface conductivity or carrier surface accumulation in this layer crystal. The fabrication of ohmic contacts for the MoSe2 layer nanocrystals using the focused-ion beam (FIB) technique was also demonstrated. This study provides a new understanding which is crucial for the development of flexible electronic devices and transparent conducting materials using ultrathin dichalcogenide layer materials.

Effects of ion insertion on cycling performance of miniaturized electrochemical capacitor of carbon nanotubes array.

Capacity degradation and ion insertion of a miniaturized electrochemical capacitor are studied using ionic liquid [EMI] [TFSI] as the electrolyte. This capacitor is featured with two comb-like electrodes of vertical carbon nanotubes, ∼70 µm in height and 20 µm in interelectrode gap. We quantify the levels of ion insertion damage with Raman spectroscopy after the electrode experiences 120 consecutive voltammetric cycles to various potential limits. Distinct structural damage emerges due to [EMI] when the negative potential reaches -1.7 V, and those due to [TFSI] arise when the positive potential reaches 1.7 V vs. RHE. Judging from the peak broadenings, [EMI] is more detrimental than [TFSI]. When the voltage window ΔU is set as less than or equal to 2.8 V, both electrode potentials are within the two intercalation limits, little or no decay is observed in 10(4) charge/discharge cycles. When ΔU is 3.4 V, the positive potential exceeds the upper limit, but the negative potential stays within the lower limit, the cell capacitance decreases moderately. When ΔU increases to 3.8 V, both electrodes suffer from damages because of exceeding the intercalation limits. And the cell capacitance decreases substantially, even leading to a premature failure.

The study of optical band edge property of bismuth oxide nanowires α-Bi2O3.

The α-phase Bi(2)O(3) (α-Bi(2)O(3)) is a crucial and potential visiblelight photocatalyst material needless of intentional doping on accommodating band gap. The understanding on fundamental optical property of α-Bi(2)O(3) is important for its extended applications. In this study, bismuth oxide nanowires with diameters from tens to hundreds nm have been grown by vapor transport method driven with vapor-liquid-solid mechanism on Si substrate. High-resolution transmission electron microscopy and Raman measurement confirm α phase of monoclinic structure for the as-grown nanowires. The axial direction for the as-grown nanowires was along < 122 >. The band-edge structure of α-Bi(2)O(3) has been probed experimentally by thermoreflectance (TR) spectroscopy. The direct band gap was determined accurately to be 2.91 eV at 300 K. Temperaturedependent TR measurements of 30-300 K were carried out to evaluate temperature-energy shift and line-width broadening effect for the band edge of α-Bi(2)O(3) thin-film nanowires. Photoluminescence (PL) experiments at 30 and 300 K were carried out to identify band-edge emission as well as defect luminescence for the α-Bi(2)O(3) nanowires. On the basis of experimental analyses of TR and PL, optical characteristics of direct band edge of α-Bi(2)O(3) nanowires have thus been realized.

Miniature asymmetric ultracapacitor of patterned carbon nanotubes and hydrous ruthenium dioxide.

A symmetric ultracapacitor CNT_CNT and an asymmetric ultracapacitor CNT_hRuO(2) of mini size have been prepared with patterned carbon nanotubes (CNT) and hydrous ruthenium dioxide. Galvanostatic charge/discharge results indicate that CNT_hRuO(2) is the superior one in both power and energy densities. In a potential window 2.0 V, the CNT_hRuO(2) cell displays an energy density of 24.0 W h kg(-1) at a power density of 22.9 kW kg(-1). Its power density can be raised to 41.1 kW kg(-1) at the expense of the energy density, which drops to 6.8 W h kg(-1). On the other hand, CNT_CNT performs at a lower level, delivering 5.2 W h kg(-1) at 5.5 kW kg(-1). The favorable charge/discharge performance of CNT_hRuO(2) is attributed to hydrous RuO(2), whose pseudocapacitance drives the other electrode of the vertical CNT array to work harder and makes more use of its double-layer capacitance. The analysis of individual electrode capacitance indicates that the high capacitance of hRuO(2) also causes a disproportion in voltage partition, which restricts the low limit of cycling current in an extended potential window. On energy cycling, CNT_hRuO(2) demonstrates sufficient stability in 10,000 cycles, after an initial 13% drop in capacitance.

Anomalous structural phase transition properties in ReSe2 and Au-doped ReSe2.

The high pressure induced phase transition in rhenium diselenides (ReSe(2)) and gold-doped rhenium diselenides (Au-ReSe(2)) at ambient temperature have been investigated using angular-dispersive x-ray diffraction (ADXRD) under high pressure up to around 10.50 and 9.98 GPa, respectively. In situ ADXRD measurements found that the phase transition pressures of ReSe(2) and Au-ReSe(2) began at 9.98 and 8.52 GPa, respectively. Compressibilities analysis shows the relationship of along c-axis > along a-axis > along b-axis. The linear compressibility of the pressure dependence of α, ß, and γ of ReSe(2) shows that a phase transition can be related to a counterclockwise rotational trend of the selenium atoms around the chain of Re(4) atoms during the decrease of the c-axis distance by a combination of stresses due to the bending effect of α and stretching effect of ß. The cause of the reduction of the phase transition pressure of Au-ReSe(2) is attributed mainly to a structural distortion as evidenced by the observation of a weak clockwise rotational trend of Se atoms around the chain of Re(4) atoms in the pressure range 3.99-4.99 GPa which subsequently reversed to counterclockwise rotation under higher pressure.

A nanostructured electrode of IrOx foil on the carbon nanotubes for supercapacitors.

IrO(x) nanofoils (IrO(x)NF) of high surface area are sputtered on multi-wall carbon nanotubes (CNT) in the preparation of a structured electrode on a stainless steel (SUS) substrate for supercapacitor applications. This IrO(x)/CNT/SUS electrode is featured with intriguing IrO(x) curved foils of 2-3 nm in thickness and 400-500 nm in height, grown on top of the vertically aligned CNT film with a tube diameter of ∼ 40 nm. These nanofoils are moderately oxidized during reactive sputtering and appeared translucent under the electron microscope. Detailed structural analysis shows that they are comprised of contiguous grains of iridium metal, iridium dioxide, and glassy iridium oxide. Considerable Raman line broadening is also evidenced for the attributed nanosized iridium oxides. Two capacitive properties of the electrode are significantly enhanced with addition of the curved IrO(x) foils. First, IrO(x)NF reduces the electrode Ohmic resistance, which was measured at 3.5 Ω cm(2) for the CNT/SUS and 2.5 Ω cm(2) for IrO(x)NF/CNT/SUS using impedance spectroscopy. Second, IrO(x)NF raises the electrode capacitance from 17.7 F g(-1) (CNT/SUS) to 317 F g(-1) (IrO(x)/CNT/SUS), measured with cyclic voltammetry. This notable increase is further confirmed by the galvanostatic charge/discharge experiment, measuring 370 F g(-1) after 2000 uninterrupted cycles between - 1.0 and 0.0 V (versus Ag/AgCl).

Photoconductivities in MoS2 Nanoflake Photoconductors.

Photoconductivities in molybdenum disulfide (MoS2) layered nanostructures with two-hexagonal crystalline structure prepared by mechanical exfoliation were investigated. The photoconductor-type MoS2 nanoflakes exhibit remarkable photoresponse under the above bandgap excitation wavelength of 532 nm at different optical intensity. The photocurrent responsivity and photoconductive gain of nanoflakes can reach, respectively, 30 AW(-1) and 103 at the intensity of 50 Wm(-2), which are several orders of magnitude higher than those of their bulk counterparts. The vacuum-enhanced photocurrent and power-independent responsivity/gain indicate a surface-controlled photoconduction mechanism in the MoS2 nanomaterial.

Correction: Composition-dependent Raman modes of Mo1-xWxS2 monolayer alloys.

Correction for 'Composition-dependent Raman modes of Mo1-xWxS2 monolayer alloys' by Yanfeng Chen et al., Nanoscale, 2014, 6, 2833-2839.

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal-semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 Ω(-) (1) cm(-) (1), with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

The interactome of soybean GmWRKY53 using yeast 2-hybrid library screening to saturation.

Soybean GmWRKY53 functions in both biotic and abiotic stress signaling. Using GmWRKY53 as a bait yeast 2-hybrid library screening to saturation isolated multiple independent fragments for many interacting proteins, enabling delineation of minimal interacting domains and computation of a confidence score. Multiple independent clones coding for the LATE ELONGATED HYPOCOTYL clock protein GmLCL2 (MYB114) were isolated and the binding site for GmWRKY53 was mapped to 90 amino acids separate from the MYB domain. This suggests a direct input from the clock on GmWRKY53 activity. The GmWRKY53-interacting proteins also included 3 water stress-inducible AP2/ERF transcription factors. One of these (Glyma03g26310) is one of the most strongly water stress induced genes in soybean roots, suggesting that GmWRKY53/ERF complexes regulate water stress responses.

Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy.

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Three-fold rotational defects in two-dimensional transition metal dichalcogenides.

As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal-chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8-5-5-8 membered rings. First-principles calculations provide insights into the evolution of rotational defects and show that they give rise to p-type doping and local magnetic moments, but weakly affect mechanical characteristics of transition metal dichalcogenides. Thus, controllable introduction of rotational defects can be used to engineer the properties of these materials.

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors.

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼10(7)) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

Electrical transport properties of single-layer WS2.

We report on the fabrication of field-effect transistors based on single layers and bilayers of the semiconductor WS2 and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long in situ annealing drastically improves the contact transparency, allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with a high room-temperature on/off current ratio of ∼10(6). They show clear metallic behavior at high charge carrier densities and mobilities as high as ∼140 cm(2)/(V s) at low temperatures (above 300 cm(2)/(V s) in the case of bilayers). In the insulating regime, the devices exhibit variable-range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS2, comparable to those of single-layer MoS2 and WSe2, together with its strong spin-orbit coupling, make it interesting for future applications in electronic, optical, and valleytronic devices.

Atomic mechanism of the semiconducting-to-metallic phase transition in single-layered MoS2.

Phase transitions can be used to alter the properties of a material without adding any additional atoms and are therefore of significant technological value. In a solid, phase transitions involve collective atomic displacements, but such atomic processes have so far only been investigated using macroscopic approaches. Here, we show that in situ scanning transmission electron microscopy can be used to follow the structural transformation between semiconducting (2H) and metallic (1T) phases in single-layered MoS2, with atomic resolution. The 2H/1T phase transition involves gliding atomic planes of sulphur and/or molybdenum and requires an intermediate phase (α-phase) as a precursor. The migration of two kinds of boundaries (ß- and γ-boundaries) is also found to be responsible for the growth of the second phase. Furthermore, we show that areas of the 1T phase can be controllably grown in a layer of the 2H phase using an electron beam.

Properties of individual dopant atoms in single-layer MoS2: atomic structure, migration, and enhanced reactivity.

Single-layered MoS2 doped with Re (n-type) and Au (p-type) are investigated by in situ scanning transmission electron microscopy. Re atoms substituting Mo sites enhance the local chemical affinity, evidenced by agglomeration of other dopant/impurity atoms. Au atoms exist as adatoms and show larger mobility under the electron beam. These behaviors are consistent with density functional theory calculations.

Composition-dependent Raman modes of Mo(1-x)W(x)S2 monolayer alloys.

Two-dimensional (2D) transition-metal dichalcogenide alloys with tunable band gaps have promising applications in nanoelectronics and optoelectronics. Characterization of structures of 2D alloys, such as composition and atom mixing, is of fundamental importance to their applications. Here, we have conducted systematic Raman spectroscopic studies on Mo1-xWxS2 monolayers (0 ≤x≤ 1). First-order Raman modes and second-order Raman modes have been observed in the range of 100-480 cm(-1) in the 2D alloys. The out-of-plane A1' modes and in-plane E' modes showed one-mode and two-mode behaviors, respectively. The broadening of A1' and E' modes in the alloys has been observed. The disorder-related Raman peaks at ∼360 cm(-1) were only observed in the 2D alloys but not in the two end materials. Modified random-element-isodisplacement (MREI) model has been adopted to successfully predict mode behaviors of A1' and E' modes in the monolayer alloys. Further, composition-dependent A1' and E' frequencies can be well fitted by the MREI model, giving composition-dependent force constants.