Friction on a single MoS2 nanotube

Friction was measured on a single molybdenum disulfide (MoS₂) nanotube and on a single MoS₂ nano-onion for the first time. We used atomic force microscopy (AFM) operating in ultra-high vacuum at room temperature. The average coefficient of friction between the AFM tip and MoS₂ nanotubes was found considerably below the corresponding values obtained from an air-cleaved MoS₂ single crystal or graphite. We revealed a nontrivial dependency of friction on interaction strength between the nanotube and the underlying substrate. Friction on detached or weakly supported nanotubes by the substrate was several times smaller (0.023 ± 0.005) than that on well-supported nanotubes (0.08 ± 0.02). We propose an explanation of a quarter of a century old phenomena of higher friction found for intracrystalline (0.06) than for intercrystalline slip (0.025) in MoS₂. Friction test on a single MoS₂ nano-onion revealed a combined gliding-rolling process.PACS, 62.20, 61.46.Fg, 68.37 Ps     

Fabrication and investigation of the optoelectrical properties of MoS2/CdS heterojunction solar cells

Molybdenum disulfide (MoS₂)/cadmium sulfide (CdS) heterojunction solar cells were successfully synthesized via chemical bath deposition (CBD) and chemical vapor deposition (CVD). The as-grown CdS film on a fluorine tin oxide (FTO) substrate deposited by CBD is continuous and compact. The MoS₂ film deposited by CVD is homogeneous and continuous, with a uniform color and a thickness of approximately 10 nm. The optical absorption range of the MoS₂/CdS heterojunction covers the visible and near-infrared spectral regions of 350 to 800 nm, which is beneficial for the improvement of solar cell efficiency. Moreover, the MoS₂/CdS solar cell exhibits good current-voltage (I-V) characteristics and pronounced photovoltaic behavior, with an open-circuit voltage of 0.66 V and a short-circuit current density of 0.227 × 10^-6 A/cm², comparable to the results obtained from other MoS₂-based solar cells. This research is critical to investigate more efficient and stable solar cells based on graphene-like materials in the future.     

Fabrication and electrical properties of MoS2 nanodisc-based back-gated field effect transistors

Two-dimensional (2D) molybdenum disulfide (MoS₂) is an attractive alternative semiconductor material for next-generation low-power nanoelectronic applications, due to its special structure and large bandgap. Here, we report the fabrication of large-area MoS₂ nanodiscs and their incorporation into back-gated field effect transistors (FETs) whose electrical properties we characterize. The MoS₂ nanodiscs, fabricated via chemical vapor deposition (CVD), are homogeneous and continuous, and their thickness of around 5 nm is equal to a few layers of MoS₂. In addition, we find that the MoS₂ nanodisc-based back-gated field effect transistors with nickel electrodes achieve very high performance. The transistors exhibit an on/off current ratio of up to 1.9 × 10⁵, and a maximum transconductance of up to 27 μS (5.4 μS/μm). Moreover, their mobility is as high as 368 cm²/Vs. Furthermore, the transistors have good output characteristics and can be easily modulated by the back gate. The electrical properties of the MoS₂ nanodisc transistors are better than or comparable to those values extracted from single and multilayer MoS₂ FETs.     

Comparison of resistive switching characteristics using copper and aluminum electrodes on GeOx/W cross-point memories

Comparison of resistive switching memory characteristics using copper (Cu) and aluminum (Al) electrodes on GeO_x/W cross-points has been reported under low current compliances (CCs) of 1 nA to 50 μA. The cross-point memory devices are observed by high-resolution transmission electron microscopy (HRTEM). Improved memory characteristics are observed for the Cu/GeO_x/W structures as compared to the Al/GeO_x/W cross-points owing to AlO_x formation at the Al/GeO_x interface. The RESET current increases with the increase of the CCs varying from 1 nA to 50 μA for the Cu electrode devices, while the RESET current is high (>1 mA) and independent of CCs varying from 1 nA to 500 μA for the Al electrode devices. An extra formation voltage is needed for the Al/GeO_x/W devices, while a low operation voltage of ±2 V is needed for the Cu/GeO_x/W cross-point devices. Repeatable bipolar resistive switching characteristics of the Cu/GeO_x/W cross-point memory devices are observed with CC varying from 1 nA to 50 μA, and unipolar resistive switching is observed with CC >100 μA. High resistance ratios of 10² to 10⁴ for the bipolar mode (CCs of 1 nA to 50 μA) and approximately 10⁸ for the unipolar mode are obtained for the Cu/GeO_x/W cross-points. In addition, repeatable switching cycles and data retention of 10³ s are observed under a low current of 1 nA for future low-power, high-density, nonvolatile, nanoscale memory applications.     

Simple fabrication of N-doped mesoporous TiO2 nanorods with the enhanced visible light photocatalytic activity

N-doped mesoporous TiO₂ nanorods were fabricated by a modified and facile sol–gel approach without any templates. Ammonium nitrate was used as a raw source of N dopants, which could produce a lot of gasses such as N₂, NO₂, and H₂O in the process of heating samples. These gasses were proved to be vitally important to form the special mesoporous structure. The samples were characterized by the powder X-ray diffraction, X-ray photoelectron spectrometer, nitrogen adsorption isotherms, scanning electron microscopy, transmission electron microscopy, and UV-visible absorption spectra. The average length and the cross section diameter of the as-prepared samples were ca. 1.5 μm and ca. 80 nm, respectively. The photocatalytic activity was evaluated by photodegradation of methylene blue (MB) in aqueous solution. The N-doped mesoporous TiO₂ nanorods showed an excellent photocatalytic activity, which may be attributed to the enlarged surface area (106.4 m² g^-1) and the narrowed band gap (2.05 eV). Besides, the rod-like photocatalyst was found to be easy to recycle.     

Microstructure and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition

Cuprous oxide (Cu₂O) thin films were prepared by using electrodeposition technique at different applied potentials (−0.1, −0.3, −0.5, −0.7, and −0.9 V) and were annealed in vacuum at a temperature of 100°C for 1 h. Microstructure and optical properties of these films have been investigated by X-ray diffractometer (XRD), field-emission scanning electron microscope (SEM), UV-visible (vis) spectrophotometer, and fluorescence spectrophotometer. The morphology of these films varies obviously at different applied potentials. Analyses from these characterizations have confirmed that these films are composed of regular, well-faceted, polyhedral crystallites. UV–vis absorption spectra measurements have shown apparent shift in optical band gap from 1.69 to 2.03 eV as the applied potential becomes more cathodic. The emission of FL spectra at 603 nm may be assigned as the near band-edge emission.     

Enhanced resistive switching phenomena using low-positive-voltage format and self-compliance IrOx/GdOx/W cross-point memories

Enhanced resistive switching phenomena of IrO_x/GdO_x/W cross-point memory devices have been observed as compared to the via-hole devices. The as-deposited Gd₂O₃ films with a thickness of approximately 15 nm show polycrystalline that is observed using high-resolution transmission electron microscope. Via-hole memory device shows bipolar resistive switching phenomena with a large formation voltage of -6.4 V and high operation current of >1 mA, while the cross-point memory device shows also bipolar resistive switching with low-voltage format of +2 V and self-compliance operation current of <300 μA. Switching mechanism is based on the formation and rupture of conducting filament at the IrO_x/GdO_x interface, owing to oxygen ion migration. The oxygen-rich GdO_x layer formation at the IrO_x/GdO_x interface will also help control the resistive switching characteristics. This cross-point memory device has also Repeatable 100 DC switching cycles, narrow distribution of LRS/HRS, excellent pulse endurance of >10,000 in every cycle, and good data retention of >10⁴ s. This memory device has great potential for future nanoscale high-density non-volatile memory applications.     

Synthesis and high sensing properties of a single Pd-doped SnO2 nanoribbon

Monocrystal SnO₂ and Pd-SnO₂ nanoribbons have been successfully synthesized by thermal evaporation, and novel ethanol sensors based on a single Pd-SnO₂ nanoribbon and a single SnO₂ nanoribbon were fabricated. The sensing properties of SnO₂ nanoribbon (SnO₂ NB) and Pd-doped SnO₂ nanoribbon (Pd-SnO₂ NB) sensors were investigated. The results indicated that the SnO₂ NB showed a high sensitivity to ethanol and the Pd-SnO₂ NB has a much higher sensitivity of 4.3 at 1,000 ppm of ethanol at 230°C, which is the highest sensitivity for a SnO₂-based NB. Pd-SnO₂ NB can detect ethanol in a wide range of concentration (1 ~ 1,000 ppm) with a relatively quick response (recovery) time of 8 s (9 s) at a temperature from 100°C to 300°C. In the meantime, the sensing capabilities of the Pd-SnO₂ NB under 1 ppm of ethanol at 230°C will help to promote the sensitivity of a single nanoribbon sensor. Excellent performances of such a sensor make it a promising candidate for a device design toward ever-shrinking dimensions because a single nanoribbon device is easily integrated in the electronic devices.     

Electric charging/discharging characteristics of super capacitor,using de-alloying and anodic oxidized Ti-Ni-Si amorphous alloy ribbons

Charging/discharging behaviors of de-alloyed and anodic oxidized Ti-Ni-Si amorphous alloy ribbons were measured as a function of current between 10 pA and 100 mA, using galvanostatic charge/discharging method. In sharp contrast to conventional electric double layer capacitor (EDLC), discharging behaviors for voltage under constant currents of 1, 10 and 100 mA after 1.8 ks charging at 100 mA show parabolic decrease, demonstrating direct electric storage without solvents. The supercapacitors, devices that store electric charge on their amorphous TiO_2-x surfaces that contain many 70-nm sized cavities, show the Ragone plot which locates at lower energy density region near the 2nd cells, and RC constant of 800 s (at 1 mHz), which is 157,000 times larger than that (5 ms) in EDLC.     

Optical properties of TiO2 nanoceramic films as a function of N–Al co-doping

The N-doped TiO₂, Al-doped TiO₂ and N–Al co-doped TiO₂ films had the similar structure to that of the TiO₂ film and corresponded to nanocrystalline anatase. By N and/or Al doping, the TiO₂ film became more stoichiometric and the nanocrystallinity was enhanced, especially N–Al co-doping. The optical transmission of N–Al co-doped TiO₂ film was the highest because of the lowest surface roughness and the lowest porosity. The nonlinear refractive index of N–Al co-doped TiO₂ film on the glass substrate was measured by Moiré deflectometry, and was of the order of 10⁻⁸ cm² W⁻¹. By N–Al co-doping, the TiO₂ nanoceramic film exhibited highest linear refractive index, lowest stress and lowest stress-optical coefficient.     

Hydrothermal Synthesis of β-Nb2ZnO6 Nanoparticles for Photocatalytic Degradation of Methyl Orange and Cytotoxicity Study

β-Nb₂ZnO₆ nanoparticles were synthesized by a hydrothermal process and calcined at two temperatures, 500 °C and 700 °C, and assigned as A and B, respectively. X-ray diffraction, together with transmission electron microscopy, revealed that the β-Nb₂ZnO₆ nanoparticles calcined at 700 °C (B) were more crystalline than the β-Nb₂ZnO₆ calcined at 500 °C (A) with both types of nanoparticles having an average size of approximately 100 nm. The physiochemical, photocatalytic, and cytotoxic activities of both types of β-Nb₂ZnO₆ nanoparticles (A and B) were examined. Interestingly, the photodegradation of methyl orange, used as a standard for environmental pollutants, was faster in the presence of the β-Nb₂ZnO₆ nanoparticles calcined at 500 °C (A) than in the presence of those calcined at 700 °C (B). Moreover, the cytotoxicity was evaluated against different types of cancer cells and the results indicated that both types of β-Nb₂ZnO₆ nanoparticles (A and B) exhibited high cytotoxicity against MCF-7 and HCT116 cells but low cytotoxicity against HeLa cells after 24 and 48 h of treatment. Overall, both products expressed similar EC₅₀ values on tested cell lines and high cytotoxicity after 72 h of treatment. As a photocatalyst, β-Nb₂ZnO₆ nanoparticles (A) could be utilized in different applications including the purification of the environment and water from specific pollutants. Further biological studies are required to determine the other potential impacts of utilizing β-Nb₂ZnO₆ nanoparticles in the biomedical application field.     

Growth and optical properties of large-scale MoS2 films with different thickness

The synthesis of large-scale molybdenum disulfide (MoS₂) with high quality is highly desirable for the promising applications in flexible optoelectronic devices. Here, we report a feasible one-step chemical vapor deposition (CVD) synthesis of continuous MoS₂ films with different layer-number via adjusting the growth temperature in the range of 740–800 °C. Influences of the annealing treatments at diverse temperature ranging from 300 to 500 °C on Raman and PL spectra of the monolayer MoS₂ film grown at 780 °C are reported. PL characterization shows that the PL emission of film annealed at 400 °C exhibits highest intensity with a blue-shift in comparison to that of the pristine film grown at 780 °C. The PL fluctuation of the MoS₂ film annealed at 400 °C is mainly originated from the high crystalline quality and strain-release. This study sheds a light on growth and performance optimization of the large-area two-dimensional transition metal dichalcogenides films.     

Effect of Molybdenum Disulfide Layer on Surface Plasmon Resonance Biosensor for the Detection of Bacteria

In this study, a molybdenum disulfide (MoS₂) based surface plasmon resonance (SPR) biosensor is proposed. The reflectance curves for the proposed SPR biosensor are analyzed and compared with the graphene based and the conventional SPR biosensors. It is observed that the performance parameters of the proposed biosensor- sensitivity, detection accuracy, and the quality factor are enhanced by the utilization of the adsorption property of MoS₂ for monolayer and bi-layer MoS₂. Also, the effect of increasing the number of layers of MoS₂ on the reflectance curve is analyzed and compared.     

A nontoxic and low-cost hydrothermal route for synthesis of hierarchical Cu2ZnSnS4 particles

We explore a facile and nontoxic hydrothermal route for synthesis of a Cu₂ZnSnS₄ nanocrystalline material by using l-cysteine as the sulfur source and ethylenediaminetetraacetic acid (EDTA) as the complexing agent. The effects of the amount of EDTA, the mole ratio of the three metal ions, and the hydrothermal temperature and time on the phase composition of the obtained product have been systematically investigated. The addition of EDTA and an excessive dose of ZnCl₂ in the hydrothermal reaction system favor the generation of kesterite Cu₂ZnSnS₄. Pure kesterite Cu₂ZnSnS₄ has been synthesized at 180°C for 12 h from the reaction system containing 2 mmol of EDTA at 2:2:1 of Cu/Zn/Sn. It is confirmed by Raman spectroscopy that those binary and ternary phases are absent in the kesterite Cu₂ZnSnS₄ product. The kesterite Cu₂ZnSnS₄ material synthesized by the hydrothermal process consists of flower-like particles with 250 to 400 nm in size. It is revealed that the flower-like particles are assembled from single-crystal Cu₂ZnSnS₄ nanoflakes with ca. 20 nm in size. The band gap of the Cu₂ZnSnS₄ nanocrystalline material is estimated to be 1.55 eV. The films fabricated from the hierarchical Cu₂ZnSnS₄ particles exhibit fast photocurrent responses under intermittent visible-light irradiation, implying that they show potentials for use in solar cells and photocatalysis.     

Ferromagnetism and optical properties of La1???x Al x FeO3 nanopowders

La_{1 − x} Al _x FeO₃ (x = 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) nanopowders were prepared by polymerization complex method. All prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and UV-vis spectrophotometry (UV-vis). The magnetic properties were investigated using a vibrating sample magnetometer (VSM). The X-ray results of all samples show the formation of an orthorhombic phase with the second phase of α-Fe₂O₃ in doped samples. The crystallite sizes of nanoparticles decreased with increasing Al content, and they are found to be in the range of 58.45 ± 5.90 to 15.58 ± 4.64 nm. SEM and TEM images show the agglomeration of nanoparticles with average particle size in the range of 60 to 75 nm. The FT-IR spectra confirm the presence of metal oxygen bonds of O-Fe-O and Fe-O in the FeO₆ octahedra. The UV-vis spectra show strong absorption peaks at approximately 285 nm, and the calculated optical band gaps are found to be in the range of 2.05 to 2.09 eV with increasing Al content. The M-H loop of the pure sample is antiferromagnetic, whereas those of the doped samples tend to be ferromagnetic with increasing Al content. The magnetization, remanent magnetization, and coercive field of the Al-doped sample with x = 0.5 are enhanced to 1.665 emu/g, 0.623 emu/g, and 4,087.0 Oe, respectively.     

Low-temperature growth of highly crystalline β-Ga2O3 nanowires by solid-source chemical vapor deposition

Growing Ga₂O₃ dielectric materials at a moderately low temperature is important for the further development of high-mobility III-V semiconductor-based nanoelectronics. Here, β-Ga₂O₃ nanowires are successfully synthesized at a relatively low temperature of 610°C by solid-source chemical vapor deposition employing GaAs powders as the source material, which is in a distinct contrast to the typical synthesis temperature of above 1,000°C as reported by other methods. In this work, the prepared β-Ga₂O₃ nanowires are mainly composed of Ga and O elements with an atomic ratio of approximately 2:3. Importantly, they are highly crystalline in the monoclinic structure with varied growth orientations in low-index planes. The bandgap of the β-Ga₂O₃ nanowires is determined to be 251 nm (approximately 4.94 eV), in good accordance with the literature. Also, electrical characterization reveals that the individual nanowire has a resistivity of up to 8.5 × 10⁷ Ω cm, when fabricated in the configuration of parallel arrays, further indicating the promise of growing these highly insulating Ga₂O₃ materials in this III-V nanowire-compatible growth condition.

PACS

77.55.D; 61.46.Km; 78.40.Fy     

Self-compliance RRAM characteristics using a novel W/TaO x /TiN structure

Self-compliance resistive random access memory (RRAM) characteristics using a W/TaO _x /TiN structure are reported for the first time. A high-resolution transmission electron microscope (HRTEM) image shows an amorphous TaO _x layer with a thickness of 7 nm. A thin layer of TiO _x N _y with a thickness of 3 nm is formed at the TaO _x /TiN interface, owing to the oxygen accumulation nature of Ti. This memory device shows 100 consecutive switching cycles with excellent uniformity, 100 randomly picked device-to-device good uniformity, and program/erase endurance of >10³ cycles. It is observed that the 0.6-μm devices show better switching uniformity as compared to the 4-μm devices, which is due to the thinner tungsten (W) electrode as well as higher series resistance. The oxygen-rich TaO _x layer at the W/TaO _x interface also plays an important role in getting self-compliance resistive switching phenomena and non-linear current-voltage (I-V) curve at low resistance state (LRS). Switching mechanism is attributed to the formation and rupture of oxygen vacancy conducting path in the TaO _x switching material. The memory device also exhibits long read endurance of >10⁶ cycles. It is found that after 400,000 cycles, the high resistance state (HRS) is decreased, which may be due to some defects creation (or oxygen moves away) by frequent stress on the switching material. Good data retention of >10⁴ s is also obtained.     

Structures and performance of Pd–Mo–K/Al2O3 catalysts used for mixed alcohol synthesis from synthesis gas

The structures of the palladium-modified Mo–K/Al₂O₃ catalyst samples were characterized by the XRD, LRS, and EXAFS techniques and correlated to the catalytic properties of the samples for alcohol synthesis from synthesis gas. It is found that in the oxidic palladium-modified samples a strong interaction of the palladium modifier with the supported K–Mo–O species occurs. This interaction leads to a decrease in the size of the molybdenum species and stabilization of the cationic palladium species on the samples during sulfidation. Upon sulfidation, the sulfided molybdenum species in the palladium-free sample is mainly present as large patches of MoS₂-like slabs with their basal sulfur planes interacting with the support surface. With the modification of palladium to the samples, the supported MoS₂-like species becomes highly dispersed as revealed by the decrease in the average size of the sulfided molybdenum species. The interaction of the palladium species with the molybdenum component may cause the basal planes of the MoS₂-like species to become oriented perpendicular to the support surface due to favorable bonding of the MoS₂ edge planes to the support through Mo–O–Al bonds. In comparison with the sulfided Pd-free sample, the properties of the Pd-modified samples for alcohol synthesis from synthesis gas are much improved, which most probably results from the synergic interaction of the palladium with the molybdenum species that gives rise to the appearance of the active sites.     

Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field

Using first-principles calculations, we investigate the adsorption of various gas molecules (H₂, O₂, H₂O, NH₃, NO, NO₂, and CO) on monolayer MoS₂. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS₂ surface and act as charge acceptors for the monolayer, except NH₃ which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS₂ can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS₂ as a potential material for gas sensing application.     

Edge defect-assisted synthesis of chemical vapor deposited bilayer molybdenum disulfide

The synthesis of two-dimensional molybdenum disulfide (MoS₂) has invoked much research interest in recent years. In this work, chemical vapor deposition (CVD) was employed to grow a novel molybdenum disulfide. The synthesis reaction was operated in an inert gas environment, with the sulfur precursor placed in the upstream zone of a double zone tube furnace, and the molybdenum trioxide (MoO₃) precursor placed in a small tube in the downstream zone. A piece of glass was used as the substrate, which was placed beside the small tube. We intentionally built a high concentration of molybdenum precursor above the substrate. The specific morphology hinged highly on the concentration of molybdenum. Abundant molybdenum atoms adsorbed on the edges of the bottom MoS₂ and formed defects, which promoted the nucleation of the top layer MoS₂ and preferentially grew from the edges. This result provides an innovative method to controllably synthesize novel bilayer molybdenum disulfide by a one-step method of chemical vapor deposition.