We demonstrate a facile and effective approach to significantly improve the photoluminescence of bulk MoS2 via laser thinning followed by gold particle decoration. Upon laser thinning of exfoliated bulk MoS2, photoluminescence emerges from the laser-thinned region. After further treatment with an AuCl3 solution, gold particles self-assemble on the laser-thinned region and thick edges, further increasing the fluorescence of bulk MoS2 28 times and the Raman response 3 times. Such fluorescence enhancement can be attributed to both surface plasmon resonance and p-type doping induced by gold particles. The combination of laser thinning and AuCl3 treatment enables the functionalization of bulk MoS2 for optoelectronic applications. It can also provide a viable strategy for mask-free and area-selective p-type doping on single MoS2 flakes.
Beyond graphene, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their potential in next-generation nanoelectronics and optoelectronics. Nevertheless, grain boundaries are ubiquitous in large-area as-grown TMD materials and would significantly affect their band structure, electrical transport, and optical properties. Therefore, the characterization of grain boundaries is essential for engineering the properties and optimizing the growth in TMD materials. Although the existence of boundaries can be measured using scanning tunneling microscopy, transmission electron microscopy, or nonlinear optical microscopy, a universal, convenient, and accurate method to detect boundaries with a twist angle over a large scale is still lacking. Herein, we report a high-throughput method using mild hot H2O etching to visualize grain boundaries of TMDs under an optical microscope, while ensuring that the method is nearly noninvasive to grain domains. This technique utilizes the reactivity difference between stable grain domains and defective grain boundaries and the mild etching capacity of hot water vapor. As grain boundaries of two domains with twist angles have defective lines, this method enables to visualize all types of grain boundaries unambiguously. Moreover, the characterization is based on an optical microscope and therefore naturally of a large scale. We further demonstrate the successful application of this method to other TMD materials such as MoS2 and WSe2. Our technique facilitates the large-area characterization of grain boundaries and will accelerate the controllable growth of large single-crystal TMDs.
Because of the coupling between semiconducting and piezoelectric properties in wurtzite materials, strain-induced piezo-charges can tune the charge transport across the interface or junction, which is referred to as the piezotronic effect. For devices whose dimension is much smaller than the mean free path of carriers (such as a single atomic layer of MoS2), ballistic transport occurs. In this study, transport in the monolayer MoS2 piezotronic transistor is studied by presenting analytical solutions for two-dimensional (2D) MoS2. Furthermore, a numerical simulation for guiding future 2D piezotronic nanodevice design is presented.
In-plane symmetry is an important contributor to the physical properties of two-dimensional layered materials, as well as atomically thin heterojunctions. Here, we demonstrate anisotropic/isotropic van der Waals (vdW) heterostructures of ReS2 and MoS2 monolayers, where interlayer coupling interactions and charge separation were observed by in situ Raman-photoluminescence spectroscopy, electrical, and photoelectrical measurements. We believe that these results could be helpful for understanding the fundamental physics of atomically thin vdW heterostructures and creating novel electronic and optoelectronic devices.
The size and density of Ag nanoparticles on n-layer MoS2 exhibit thicknessdependent behavior. The size and density of these particles increased and decreased, respectively, with increasing layer number (n) of n-layer MoS2. Furthermore, the surface-enhanced Raman scattering (SERS) of Ag on this substrate was observed. The enhancement factor of this scattering varied with the thickness of MoS2. The mechanisms governing the aforementioned thickness dependences are proposed and discussed.
Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.
Electrical and optical enhancements of single-layer semiconducting materials such as transition metal dichalcogenides have recently been studied to achieve sensitive properties via external treatments, such as the formation of organic/inorganic protecting layers on field-effect transistors (FETs), thermal annealing, and nano-dot doping of sensors and detectors. Here, we propose a new analytical approach to electrical and optical enhancement through a passivation process using atomic layer deposition (ALD), and demonstrate a synthesized MoS2 monolayer incorporated with Al atoms in an Al2O3 passivation layer. The incorporated Al atoms in the MoS2 monolayer are clearly observed by spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and TEM-energy-dispersive X-ray spectroscopy results. We demonstrate that the chemically incorporated FETs exhibit highly enhanced mobilities of approximately 3.7 cm2·V?1·s?1, forty times greater than that of as-synthesized MoS2, with a three-fold improvement in the photoluminescence properties.
Recent studies have indicated that two-dimensional (2D) MoS2 exhibits low in-plane and inter-plane thermal conductivities. This poses a significant challenge to heat management in MoS2-based electronic devices. To address this challenge, we have designed MoS2-graphene interfaces that fully utilize graphene, a 2D material that exhibits very high thermal conductivity. First, we performed ab initio atomistic simulations to understand bonding and structural stability at the interfaces. The interfaces that we designed, which were connected via strong covalent bonds between Mo and C atoms, were energetically stable. We then performed molecular dynamics simulations to investigate interfacial thermal conductance in these materials. Surprisingly, the interfacial thermal conductance was high and comparable to those of covalently bonded graphene-metal interfaces. Importantly, each interfacial Mo–C bond served as an independent thermal channel, enabling modulation of the interfacial thermal conductance by controlling the Mo vacancy concentration at the interface. The present work provides a viable heat management strategy for MoS2-based electronic devices.
Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS2 sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS2 nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS2 nanosheets using freeze-dried silk fibroin powders. Compared to MoS2 dispersion in the absence of silk fibroin powder, sonicated MoS2 dispersions with silk fibroin powder (MoS2/Silk dispersion) show noticeably higher exfoliated MoS2 nanosheet yields, with suspended MoS2 concentrations in MoS2/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL–1, respectively. The MoS2 concentration in the MoS2/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS2 nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS2 nanosheets spin-cast from MoS2 /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS2 crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS2 nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS2 nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS2 nanosheets contents.
Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS2@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS2 nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm2) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS2 and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.
Bismuth telluride (Bi2Te3) is one of the most important commercial thermoelectric materials. In recent years, the discovery of topologically protected surface states in Bi chalcogenides has paved the way for their application in nanoelectronics. Determination of the fracture toughness plays a crucial role for the potential application of topological insulators in flexible electronics and nanoelectromechanical devices. Using depth-sensing nanoindentation tests, we investigated for the first time the fracture toughness of bulk single crystals of Bi2Te3 topological insulators, grown using the Bridgman-Stockbarger method. Our results highlight one of the possible pitfalls of the technology based on topological insulators.
The clinical translation of many inorganic nanomaterials is severely hampered by toxicity issues because of the long-term retention of these nanomaterials in the body. In this study, we developed a bio-clearable theranostic agent based on ultra-small MoS2 nanodots, which were synthesized by a facile bottom-up approach through one-step solvothermal decomposition of ammonium tetrathiomolybdate. After modification by glutathione (GSH), the obtained MoS2-GSH nanodots exhibited sub-10-nm hydrodynamic diameters without aggregation in various physiological buffers. Without showing appreciable in vitro toxicity, such MoS2-GSH nanodots with strong near-infrared (NIR) absorbance could induce remarkable photothermal ablation of cancer cells. Upon intravenous (i.v.) injection, efficient tumor accumulation of MoS2-GSH nanodots was observed by photoacoustic imaging, and further confirmed by analysis of the biodistribution of Mo. Notably, the MoS2-GSH nanodots, in contrast to conventional MoS2 nanoflakes with larger sizes, showed rather efficient body clearance via urine, where the majority of the injected dose was cleared within just seven days. Photothermal ablation of tumors on mice was then realized with the MoS2-GSH nanodots, achieving excellent therapeutic efficacy. This study presents a new type of ultra-small nanoparticle with efficient tumor homing/treatment abilities, as well as rapid body clearance behavior, making it promising for cancer theranostics without long-term toxicity concerns.
Multi-shelled CoFe2O4 hollow microspheres with a tunable number of layers (1–4) were successfully synthesized via a facile one-step method using cyclodextrin as a template, followed by calcination. The structural features, including the shell number and shell porosity, were controlled by adjusting the synthesis parameters to produce hollow spheres with excellent capacity and durability. This is a straightforward and general strategy for fabricating metal oxide or bimetallic metal oxide hollow microspheres with a tunable number of shells.
Two-dimensional (2D) materials have received significant attention due to their unique physical properties and potential applications in electronics and optoelectronics. Recent studies have demonstrated that exfoliated PdSe2, a layered transition metal dichalcogenide (TMD), exhibits ambipolar field-effect transistor (FET) behavior with notable performance and good air stability, and thus serves as an emerging candidate for 2D electronics. Here, we report the growth of bilayer PdSe2 on a graphene-SiC(0001) substrate by molecular beam epitaxy (MBE). A bandgap of 1.15 ± 0.07 eV was revealed by scanning tunneling spectroscopy (STS). Moreover, a bandgap shift of 0.2 eV was observed in PdSe2 layers grown on monolayer graphene as compared to those grown on bilayer graphene. The realization of nanoscale electronic junctions with atomically sharp boundaries in 2D PdSe2 implies the possibility of tuning its electronic or optoelectronic properties. In addition, on top of the PdSe2 bilayers, PdSe2 nanoribbons and stacks of nanoribbons with a fixed orientation have been fabricated. The bottom-up fabrication of low-dimensional PdSe2 structures is expected to enable substantial exploration of its potential applications.
MoS2 has attracted a lot of interest in the field of lithium-ion storage as an anode material owing to its high capacity and two-dimensional (2D)-layer structure. However, its electrochemical properties, such as rate capability and cycling stability, are usually limited by its low conductivity, volume variation, and polysulfide dissolution during lithiation/delithiation cycling. Here, a designed two-layer carbon-coated MoS2/carbon nanofiber (MoS2/C/C fiber) hybrid electrode with a double-layer carbon coating was achieved by a facile hydrothermal and subsequent electrospinning method. The double carbon layer (inner amorphous carbon and outer carbon fiber) shells could efficiently increase the electron conductivity, prevent the aggregation of MoS2 flakes, and limit the volume change and polysulfide loss during long-term cycling. The as-prepared MoS2/C/C fiber electrode exhibited a high capacity of up to 1,275 mAh/g at a current density of 0.2 A/g, 85.0% first cycle Coulombic efficiency, and significantly increased rate capability and cycling stability. These results demonstrate the potential applications of MoS2/C/C fiber hybrid material for energy storage and may open up a new avenue for improving electrode energy storage performance by fabricating hybrid nanofiber electrode materials with double-layer carbon coatings.
Catalytic hydrogenation is an important process in the chemical industry. Traditional catalysts require the effective cleavage of hydrogen molecules on the metal-catalyst surface, which is difficult to achieve with non-noble metal catalysts. In this work, we report a new hydrogenation method based on water/proton reduction, which is completely different from the catalytic cleavage of hydrogen molecules. Active hydrogen species and photo-generated electrons can be directly applied to the hydrogenation process with Cu1.94S-Zn0.23Cd0.77S semiconductor heterojunction nanorods. Nitrobenzene, with a variety of substituent groups, can be efficiently reduced to the corresponding aniline without the addition of hydrogen gas. This is a novel and direct pathway for hydrogenation using non-noble metal catalysts.
Photoluminescence (PL) of transition metal dichalcogenides (TMDs) can be engineered by controlling the density of defects, which provide active sites for electron-hole recombination, either radiatively or non-radiatively. However, the implantation of defects by external stimulation, such as uniaxial tension and irradiation, tends to introduce local damages or structural non-homogeneity, which greatly degrades their luminescence properties and impede their applicability in constructing optoelectronic devices. In this paper, we present a strategy to introduce a controllable level of defects into the MoS2 monolayers by adding a hydrogen flow during the chemical vapor deposition, without sacrificing their luminescence characteristics. The density of the defect is controlled directly by the concentration of hydrogen. For an appropriate hydrogen flux, the monolayer MoS2 sheets have three times stronger PL emission at the excitonic transitions, compared with those samples with nearly perfect crystalline structure. The defect-bounded exciton transitions at lower energies arising in the defective samples and are maximized when the total PL is the strongest. However, the B exciton, exhibits a monotonic decline as the defect density increases. The Raman spectra of the defective MoS2 reveal a redshift (blueshift) of the in-plane (out-of-plane) vibration modes as the hydrogen flux increases. All the evidence indicates that the generated defects are in the form of sulfur vacancies. This study renders the high-throughput synthesis of defective MoS2 possible for catalysis or light emitting applications.
Conducting polymers generally show high specific capacitance but suffer from poor rate capability and rapid capacitance decay, which greatly limits their practical applications in supercapacitor electrodes. To this end, many studies have focused on improving the overall capacitive performance by synthesizing nanostructured conducting polymers or by depositing a range of coatings to increase the active surface area exposed to the electrolyte and enhance the charge transport efficiency and structural stability. Despite this, simultaneously achieving high specific capacitance, good rate performance, and long cycle life remains a considerable challenge. Among the various two-dimensional (2D) layered materials, octahedral (1T) phase molybdenum disulfide (MoS2) nanosheets have high electrical conductivity, large specific surface areas, and unique surface chemical characteristics, making them an interesting substrate for the controlled growth of nanostructured conducting polymers. This paper reports the rational synthesis of carbon shell-coated polyaniline (PANI) grown on 1T MoS2 monolayers (MoS2/PANI@C). The composite electrode comprised of MoS2/PANI@C with a ~3 nm carbon shell exhibited a remarkable specific capacitance of up to 678 F·g–1 (1 mV·s–1), superior capacity retention of 80% after 10,000 cycles and good rate performance (81% at 10 mV·s–1) due to the multiple synergic effects between the PANI nanostructure and 1T MoS2 substrates as well as protection by the uniform thin carbon shell. These properties are comparable to the best overall capacitive performance achieved for conducting polymers-based supercapacitor electrodes reported thus far.
Hexagonal ultrathin WO3 nano-ribbons (HUWNRs) of subnanometer thicknesses, 2–5 nm widths, and lengths of up to several micrometers were prepared by a solvothermal method. The as-prepared HUWNRs grow along the [001] direction, and the main exposed facet is the (120) crystal plane. The HUWNRs exhibit good electrochemical performance as an anode material in lithium ion batteries because of their unique structure. It is believed that these unique materials may be applied in many fields.
Edge structures are highly relevant to the electronic, magnetic, and catalytic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) and their one-dimensional (1D) counterparts, i.e., nanoribbons, and should be precisely tailored for the desired application. In this work, we report the formation of novel Mo6S6 nanowire (NW)-terminated edges in monolayer molybdenum disulfide (MoS2) via an e–beam irradiation process combined with high temperature heating. The atomic structures of the NW-terminated edges and the dynamic formation process were observed experimentally using scanning transmission electron microscopy. Further analysis showed that the NW-terminated edge could be formed on both the Mo-zigzag (ZZ) edge and S-ZZ edge and could exhibit a stability superior to that of the pristine ZZ and armchair (AC) edges. In addition, analogous edge structures could also be formed in MoS2 nanoribbons and other TMD materials such as MoxW1?xSe2. We believe that these novel edge structures may impart novel properties to the 2D and 1D TMD materials and provide new opportunities for their applications in catalytic, spintronic, and electronic devices.
