Design of Core-Shell Quantum Dots-3D WS2 Nanowall Hybrid Nanostructures with High-Performance Bifunctional Sensing Applications.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WOx into three-dimensional (3D) WS2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core-shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS2 nanowalls further improves the performance of WS2-based photodetector devices, including a photocurrent enhancement of 320-470% and shorter response time. The significant results of the core-shell QD-WS2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core-shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core-shell QDs and the absorption of WS2. Besides, outstanding NO2 gas-sensing performance of core-shell QDs/WS2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p-n junctions generated by p-type 3D WS2 nanowalls and n-type core-shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core-shell QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.

Flexible Photocatalytic Paper with Cu2O and Ag Nanoparticle-Decorated ZnO Nanorods for Visible Light Photodegradation of Organic Dye.

We report on the fabrication of flexible photocatalytic paper comprised of Cu2O and Ag nanoparticle (NP)-decorated ZnO nanorods (NRs) and its application in visible light photodegradation of organic dye. ZnO NRs are first grown on a kraft paper substrate using a hydrothermal method. The NRs are subsequently decorated with Cu2O, Ag, or both NPs formed by photoreduction processes. Scanning electron microscopy and X-ray diffraction analysis confirm the crystallinity of ZnO NRs. Transmission electron microscopy analysis confirms the compositions of the two types of NPs. Four different types of photocatalytic papers with a size of 10 × 10 cm2 are prepared and used to degrade a 10-µM and 100-mL rhodamine B solution. The paper with Cu2O and Ag NP-co-decorated ZnO NRs has the best efficiency with first-order kinetic constants of 0.017 and 0.041 min-1 under the illumination of a halogen lamp and direct sunlight, respectively. The performance of the photocatalytic paper compares well with other substrate-supported ZnO nanocomposite photocatalysts. With the advantages of flexibility, light weight, nontoxicity, low cost, and ease of fabrication, the photocatalytic paper has good potential for visible light photocatalysis.

An indoor light-activated 3D cone-shaped MoS2 bilayer-based NO gas sensor with PPb-level detection at room-temperature.

Utilization of light to boost the performance of gas sensors allows us to operate sensor devices at room temperature. Here, we, for the first time, demonstrated an indoor light-activated 3D cone-shaped MoS2 bilayer-based NO gas sensor with ppb-level detection operated at room-temperature. Large-area cone-shaped (CS)-MoS2 bilayers were grown by depositing 2 nm-thick MoO3 layers on a 2'' three-dimensional (3D) cone-patterned sapphire substrate (CPSS) followed by a sulfurization process via chemical vapor deposition. Because the exposed area of MoS2 bilayers is increased by 30%, the CS-MoS2 gas sensor (GS) demonstrated excellent performance with a response of ∼470% and a fast response time of ∼25 s after exposure to 1 ppm of NO gas illuminated by ultraviolet (UV) light with a wavelength of 365 nm. Such extraordinary performance at room temperature is attributed to the enhanced light absorption because of the light scattering effect caused by the 3D configuration and photo-desorption induced by UV illumination. For NO concentrations ranging from 2 ppm down to 0.06 ppm, the CS-MoS2 GS demonstrated a stable sensing behavior with a high response and fast response time (470% and 25 s at 2 ppm NO) because of the light absorption enhanced by the 3D structure and photo-desorption under constant UV illumination. The CS-MoS2 GS exhibits a high sensitivity (∼189.2 R% ppm-1), allowing the detection of NO gas at 0.06 ppm in 130 s. In addition, the 3D cone-shaped structure prolonged the presence of sulfur vapor around MoO3, allowing MoO3 to react with sulfur completely. Furthermore, the CS-MoS2 GS using an indoor lighting to detect NO gas at room temperature was demonstrated for the first time where the CS-MoS2 GS exhibits a stable cycling behavior with a high response (165% at 1 ppm NO) in 50 s; for concentration as low as ∼0.06 ppm, the response of ∼75% in 150 s can be achieved.

NO gas sensing at room temperature using single titanium oxide nanodot sensors created by atomic force microscopy nanolithography.

In this work, the fabrication of single titanium oxide nanodot (ND) resistive sensors for NO gas sensing at room temperature is reported. Two atomic force microscopy nanolithography methods, nanomachining and nano-oxidation, are employed. A single titanium nanowire (NW) is created first along with contact electrodes and a single titanium oxide ND is subsequently produced in the NW. Gas sensing is realized by the photo-activation and the photo-recovery approaches. It is found that a sensor with a smaller ND has better performance than a larger one. A response of 31%, a response time of 91 s, and a recovery time of 184 s have been achieved at a concentration of 10 ppm for a ND with a size of around 80 nm. The present work demonstrates the potential application of single metal oxide NDs for gas sensing with a performance that is comparable with that of metal oxide nanowire gas sensors.

Creating optical near-field orbital angular momentum in a gold metasurface.

Nanocavities inscribed in a gold thin film are optimized and designed to form a metasurface. We demonstrate both numerically and experimentally the creation of surface plasmon (SP) vortex carrying orbital angular momentum in the metasurface under linearly polarized optical excitation that carries no optical angular momentum. Moreover, depending on the orientation of the exciting linearly polarized light, we show that the metasurface is capable of providing dynamic switching between SP vortex formation or SP subwavelength focusing. The resulting SP intensities are experimentally measured using a near-field scanning optical microscope and are found in excellent quantitative agreements as compared to the numerical results.

Fabrication of metal nanostructures by atomic force microscopy nanomachining and related applications.

In this work, the fabrication of metal nanostructures by a combination of atomic force microscopy nanomachining on a thin polymer resist, metal coating and lift-off is presented. Nanodots with sizes down to 20 nm and nanowires with widths ranging between 40 and 100 nm have been successfully created by nanoindenting and nanoscratching. The results exemplify the feasibility and effectiveness of the present technique as an alternative to e-beam lithography. The localized surface plasmon resonance properties of the fabricated nanostructures are characterized. The chemical sensing capability of a single nanowire based on resistance increase is also demonstrated.

Single-crystalline zinc oxide nanowires as photoanode material for dye-sensitized solar cells.

This study reports the use of single-crystalline and well-aligned ZnO nanowires as photoanode material for dye-sensitized solar cells. The ZnO nanowires are grown on fluorine-doped tin oxide coated glass substrates without catalysts by thermal evaporation. In spite of low roughness factors of around 25 for the nanowire photoanodes, the fabricated solar cells yield power conversion efficiencies of around 1.3% under AM 1.5G (100 mW cm-2) illumination. Moreover, fill factors of around 0.5 have been achieved and are relatively high when compared with reported values from ZnO nanowire photoanodes. The results reveal the advantage of using single-crystalline nanowires as photoanode material and provide clues for the advancement of nanowire based dye-sensitized solar cells.

Fabrication of a single metal nanowire connected with dissimilar metal electrodes and its application to chemical sensing.

We report a convenient method for the fabrication of a single metal nanowire connected with dissimilar metal electrodes and its application to chemical sensing. The method is based on a combination of atomic force microscopy nanomachining and conventional photolithography. The success of this integrated approach is confirmed by the linear current-voltage behavior of the created nanowires and comparable resistivities with those reported previously. The chemical sensing capability is demonstrated by the selective binding of a self-assembled monolayer onto a single Au nanowire connected with Ti electrodes and the subsequent resistance increase due to increased surface scattering effects after adsorption. It is found that the resistance increases by around 9% after the complete coverage of either octadecanethiol or dodecanethiol molecules onto a 20 nm thick Au nanowire. A theoretical explanation for the relationship between the resistance increase and the alkanethiol concentration is also given.