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We propose and demonstrate a new principle for sensing. Impact of light, radiation, charge particle and gas molecular etc. can induce the change of the surface work function of a solid. This will result in a profound effect to a device’s output, such as current and voltage. Such an effect is in particular outstanding in a nanodevice using van der Waals stacked two-dimensional (2D) atomic crystals. In this paper, a study of a gas sensing device relying on such a principle is presented. Van der Waals bipolar junction transistors (V2D-BJT) based on vertically stacked MoS2/WSe2/MoS2 and WSe2/MoS2/WSe2 heterostructures are separately fabricated to detect NH3 and NO2 gas. Kelvin probe force microscope measurement shows that the work function change at the overlapped region of MoS2/WSe2/MoS2 heterostructure upon exposure to 50 ppm NH3 gas is 80 meV. In addition, the MoS2/WSe2/MoS2 V2D-BJT manifest superior sensing performances in detecting NH3, while the WSe2/MoS2/WSe2 V2D-BJT exhibited superior NO2 sensing properties in terms of high sensitivity (18), low power dissipation (12 nW), fast response (26 s) and recovery (14 s) time. We demonstrate that the NH3 and NO2 gas can be selectively detected by the V2D-BJT through changing the van der Waals heterostructure stacking sequences.

The adaptability of field emitters in oxidizing environment is an important issue for practical applications. Herein, the effect of ozone exposure on the field emission property of molybdenum (Mo) nanowalls was studied. Results indicate that Mo nanowall cold cathode shows strong resistance to high doses of atomic oxygen exposure. The immunity of Mo nanowalls to oxidizing environment was attributed to the protection of surface amorphous layer. Mo nanowall cold cathode could be a promising candidate for electron source used in oxidizing environment.

In this paper, we report the degradation of gasochromic coloration performance in Pt-coated WO 3 (Pt/WO 3 ) nanowire films when the samples were exposed in air, due to the poisoning of Pt nanoparticles catalyst. The poisoned Pt nanoparticles can be reactivated by annealing in air, suggesting that the degradation originated from the absorption of certain gas species. X-ray photoelectron spectroscopy (XPS) was applied to investigate the gas desorption from poisoned Pt/WO 3 nanowire films during the annealing process. Based on the XPS results, a model involving chemical reactions and physical processes on the surface of Pt/WO 3 nanowires during annealing was developed to explain the degradation phenomenon.

Self-heating effect can be applied to develop gas sensors avoiding use of external heaters. We report here a prototype self-heated hydrogen gas sensor based on Pt-coated W 18 O 49 nanowire networks. Such a sensor has been shown to have promising performance, e.g., high sensitivity (able to detect down to 50 ppm of H 2 ), good selectivity (poor response to ethanol, CH 4 , CO, and C 3 H 8 ), and low power consumption of 30–60 mW at 6 V compatible to a portable device. Coaxial cable model and percolation theory are applied to explain the gas sensing effect of the Pt-coated W 18 O 49 nanowire networks.

Being a typical Kondo topological insulator, samarium hexaboride (SmB6) attracts much interest in condensed physics and material sciences in recent years. In comparison with their bulk counterparts, SmB6 nanostructures have more abundant surface electron states due to larger specific surface area, which are believed as idea platforms for studying surface quantum properties and physic mechanism. Through chemical vapor deposition (CVD), SmB6 nanobelt and nanowire films were respectively prepared on Si substrate. Both SmB6 nanowires and nanobelts are proven as the cubic single crystals, and their growth directions are, along [100] and [110], respectively. Field emission (FE) results show that SmB6 nanobelts have a turn-on field of 3.24 V/μm and their maximum current density arrives at 466.16 µA/cm 2, which are better than SmB6 nanowires. Considering that SmB6 nanostructures have lower electron affinity, higher electron conductivity and more abundant surface states, they are regarded as excellent cold cathode nanomaterials if their FE performances can be further improved.

H2 sensors able to operate at room temperature are very important for safe detection of H2 leakage. We report the large electrical response to H2 of Pt-coated WO3 (Pt−WO3) nanowire films without the need of using an external heater. More important, hydrogen sensing processes have been investigated under various conditions, including in air, vacuum filled with pure gas, and a mixture of H2 and other gases. This is carried out with both electrical and optical methods. The evidence shows that hydrogen will inject into the nanowire and create oxygen vacancy, in addition to reducing adsorbed oxygen at the surface, as is often recognized. It is experimentally demonstrated that the increase of electrical conductivity resulting from the reaction with hydrogen is hampered by coadsorption of O2, while N2 has no such effect. A model has been developed to combine these new findings to give a clearer understanding of the mechanism responsible for H2 sensing behavior.

The gasochromic effect has important applications in energy savings, for example, in a smart window. This study reveals evidence of the existence of localized water molecules in colored WO3 nanowir...

The mechanical resonance behavior of a ZnO nanowire/nanorod at ambient condition has been studied under optical microscope by cutting its length using focused ion beam microscopy. Nanobalance using a ZnO nanowire as the cantilever has been demonstrated for measuring the mass in the order of pico-grams in working atmosphere (see optical microscopy images). The measurement limit of the balance is estimated to be ∼1 pg. The technique demonstrated here has potential for commercial applications in general laboratories, especially for measuring the mass of wet biological cells or species.

A novel approach for preparation of europium (III)-doped yttrium oxide (Y2O3:Eu3+) nanoparticles by microemulsion–microwave heating was reported in this paper. Y2O3:Eu3+ nanoparticles were successfully obtained by microwave-heating the as-prepared particles of yttrium hydroxide and europium hydroxide, formed in a reaction between Y(NO3)3–Eu(NO3)3 and NH3·H2O in the reverse microemulsion composed of Triton X-100, n-hexanol, cyclohexane and water. Field-emission scanning electron microscopy (FE-SEM) and powder X-ray diffraction (XRD) results indicated that the resultant nanoparticles showed a narrow size distribution, small size (20–30 nm) and spherical shape. The ratios of Eu to Y in the products, determined by energy dispersive spectroscopy (EDS), were in good agreement with those of the feed. The nanoparticles emitted much strong red light at 611 nm under UV excitation, especially around 255 nm. Furthermore, this novel method required a very short heating-time and the energy consumption was much smaller compared with conventional heating ways.

A novel organic ligand, 6-paramethylaniline carbonyl 2-pyridine carboxylic acid (Hp-MAP), and the corresponding europium complex, tris(6-paramethylaniline carbonyl 2-pyridine carboxylato) europium(III) (Eu-p-MAP) have been designed and synthesized. The results showed that Eu-p-MAP was a conjugated complex, emitting remarkably strong red luminescence. The lifetimes of 5 D 0 of Eu 3+ in the complex were examined using time-resolved spectroscopic analysis; the lifetime values were 0.72±0.01 ms for 1.0×10 −5 mol l −1 Eu(p-MAP) 3 ethanol solution and 1.16±0.01 ms for solid of Eu(p-MAP) 3 , respectively. Thermogravimetric analysis showed the europium complex had good thermal stability.

Silica-based composite materials incorporated with sodium tris(pyridine dicarboxylato) europate(III), SiO2 : Na3Eu(DPA)3, where H2DPA is 2,6-pyridine dicarboxylic acid, were prepared by a sol-gel method. Photoluminescent properties of SiO2 : Na3Eu(DPA)3 composite were studied after heat treatment between room temperature and 600 °C. The strong red emission of SiO2 : Na3Eu(DPA)3 was intensified by heat-treatment under appropriate conditions. The thermal stability of Na3Eu(DPA)3·9H2O and its silica-based composite was examined. The results demonstrate that SiO2 : Na3Eu(DPA)3 composite material possesses very good thermal stability with intense visible fluorescence, and may be applied as a potential luminescent material.

A terbium(III) complex, Na 3 Tb(DPA) 3 ·9H 2 O, where H 2 DPA is 2,6-dicarboxy pyridine acid, has been synthesized and incorporated into silica gel by sol-gel method to obtain a composite material, SiO 2 :Na 3 Tb(DPA) 3 . Photoluminescent properties of SiO 2 :Na 3 Tb(DPA) 3 composite were studied after heat treatments between room temperature and 600°C. The strong green emission of the composite material was intensified by heat-treatment under appropriate conditions. The effects of the dopant concentration and heat-treatment on the luminescence for the composite material have been examined. The results indicate that SiO 2 :Na 3 Tb(DPA) 3 composite material possesses high thermal stability with intense green fluorescence, and may be applied as a potential luminescent material.

Interest in silicon field emitters for flat screen display and other applications has continued to grow very rapidly over the past several years. To obtain stable field emission from silicon emitters, diamond and DLC coatings are being considered widely. In this paper we report results from a series of experiments to assess the influence of a thin layer of amorphous diamond coating on ultra sharp single crystal silicon field emitter arrays.

The performance of a solid-state gas sensor is characterized by its sensitivity, stability, and selectivity. The working principle relies on modulation of electrical conductivity due to surface oxidation reduction caused by gas exposure. Because only the surface layer is affected by such reactions, the sensitivity is strongly dependent on the surface-to-volume ratio of the material used. This purpose has been pursued by synthesizing layers with a porous morphology to enhance the material surface area. Porosity is enhanced by means of the thick film synthesis approach typically adopted in the gas sensing field. Such high porosity is not easy to achieve by thin film approach. Another approach largely used in the field is the rheotaxial growth and its thermal oxidation RGTO method, which allows synthesizing a porous thin film mono

The photoluminescence spectra of WO3−x nanowire networks, WO3 nanowires networks, and the bulk phase WO3 powder have been investigated under the continuous wave excitation light at 320nm. Two broad band emissions in spectrum of WO3−x nanowire networks are observed including the ultraviolet-visible (UV-vis) emission centered at 395nm and the blue emission centered at 467nm. The authors demonstrate that the blue emission in WO3−x nanowire networks is due to band-band transition of the bulk phase WO3 and the UV-vis emission is attributed to the state of oxygen vacancies, which are characterized by using scanning electron microscopy and x-ray photoelectron spectroscopy. The UV-vis emission exhibits a strong dependence on the excitation wavelength, while the blue emission is independent of the excitation wavelength. The UV-vis emission is discussed in configuration-coordinate diagram of a localized state of oxygen vacancies. In particular, this state of defect is a high-lying resonant state in the conduction band of WO3.

Tungsten trioxide (WO3) nanowires were synthesized by thermal evaporation of tungsten powder in two steps: tungsten suboxide (WO3−x) nanowires were synthesized, and then oxidized in O2 ambient and transformed into WO3 nanowires. Raman spectroscopy was applied to study the thermochromic phase transition of one-dimensional WO3 nanowires. From the temperature dependence of the characteristic mode at 33cm−1 in WO3, the phase transition temperature was determined. It was found that the phase transition of WO3 nanowires was reversible and the phase transition temperatures were even lower than that of WO3 nanopowder.

Large-area, quasialigned nanotips of tungsten oxide have been grown by a two-step high-temperature, catalyst-free, physical evaporation deposition process. The tungsten oxide nanotips are single crystalline with growth direction of [010]. The tungsten oxide nanotips exhibit excellent field-emission properties with a low threshold field (for an emission current density of 10mA∕cm2) ∼4.37MV∕m and uniform emission from the entire arrays, as well as high time stability. These results make tungsten oxide nanotip arrays a competitive candidate for field-emission displays.