Search

We fabricated large-area atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) by sulfurization at relatively low temperatures. The obtained MoS2 sheets are polycrystalline (~10&ndash, 20 nm single-crystal domain size) with areas of up to 300 &times, 300 &micro, m2, 2&ndash, 4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements.

Liquid marbles are entities made of a small amount of liquid encapsulated by micro/nano scale particles. The functionality of liquid marbles depends on both the core liquid and particle coating. In this work, functional silver nanoparticle (Ag NPs)/oleic acid liquid marbles are developed using Ag NPs to encapsulate micro-sized oleic acid liquid spheres, which is achieved via sonication. We demonstrate that both the size of the encapsulated spheres and the order of the Ag NPs coating are a function of the sonication time. These micro-sized liquid marbles are investigated for sensing an organic thiolate as a model material. Their capacity for increasing solar light absorption and energy conversion into heat is also studied. The liquid marbles generated after sonication for 60 min show the strongest Raman peak signals which originate from the thiolate. They also demonstrate the highest solar energy absorption. We ascribe these enhancements to the increased surface area of the Ag NPs/oleic acid spheres and the optimum order of Ag NPs to produce the strongest surface enhanced Raman scattering. The liquid marbles can be used for the development of future biosensors and solar-thermal absorbers.

We present gas-sensing performance of two dimensional tungsten oxide nanosheets (2D WO3-1) with extraordinary selectivity and sensitivity to NO 2 . The nanosheets were synthesized using wet chemical synthesis. Material characterizations including transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD) were carried out to ascertain morphology and compositional properties showing that as sensitized nanosheets are substoichiometric. The developed sensor's performance was compared to that of the full stoichiometric based sensor, showing a substantial difference in response factor as well as reaction kinetics in response to NO 2 gas. The WO 3−X based sensor showed a large response to NO 2 at low parts per billion (ppb) which is at least 3 times greater than that of the stoichiometric sensor. Both stoichiometric and substoichiometric based sensors are selectively responsive to NO 2 and have a lower detection limit of less than 20 ppb.

A novel synthesis approach for the fabrication of silver nanoparticles and platelets using MoS2 nanosheets has been investigated herein, which is based on the direct photo excitation of the semiconductor bandgap in the presence of silver ions. The silver ions were effectively reduced to silver metal and a range of silver–MoS2 heterostructures were synthesised, depending on illumination time. Short light exposure led to the formation of silver nanoparticles on the MoS2 sheets. Longer illumination time led to a templated growth mechanism and the formation of large silver nanoplatelets, fully encapsulating the MoS2 templates. These silver nanoplatelets were found to self-assemble into large micrometre sized silver nanodendrites/nanobranches which were effectively used as a surface enhanced Raman spectroscopy platform.

Quasi-two-dimensional (Q2D) Nb2O5 nanosheets with thickness of approximately 15-25 nm were fabricated by combined sol-gel/exfoliation method. Obtained Nb2O5 nanosheets were chemically intercalated with H+ in order to improve their electrical conductivity. Then, their morphology and electrical properties were investigated at the nanoscale level with a combination of conductive atomic force microscopy (C-AFM), current sensing force spectroscopy atomic force microscopy (CSFS-AFM, or PeakForce TUNA), scanning tunneling microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman techniques. This allows us to probe the electrical properties of 10-20-nm Nb2O5 nanosheets as well as the interfaces between the substrate and the fabricated Nb2O5 nanocrystals providing direct comparison between the morphology and the electrical properties at the nanometer scale. It was also proven that chemical H+ intercalation can improve conductivity of Q2D Nb2O5 nanostructures.

For the first time, we have integrated nano-platelet Pd/WO3 films, which are formed using an acid etching method, with optical fibers to develop a hydrogen gas (H2) sensors. The tungsten films were firstly RF sputtered on top of optical fiber tips, and subsequently etched in 1.5 M nitric acid to produce tungstite films. After annealing at 300 and 480 °C, these films were coated with approximately 25 A thick Pd layers. The films were characterized using SEM, GADDS, EDX, and both Raman and UV–vis–NIR spectroscopy. H2 testing measurements were conducted when the sensors were interacting with 0.06–1% H2 in synthetic air at an optimum temperature of 100 °C. It was found that the highly crystalline nano-platelet Pd/WO3 film based optical fiber H2 sensors show large reflectance responses in the NIR wavelength of 6% and 12% in the presence of 0.06% and 1% H2, respectively.

α- and β-Phase MoO3 are synthesized using an electrodeposition method on fluorine-doped tin oxide (FTO) glass substrates from sodium-molybdate (Na2MoO4) solutions. We show that it is possible to obtain both α- and β-MoO3 by manipulating the cyclic voltammetry (CV) parameters during electrodeposition. Raman spectroscopy, X-ray diffraction, and scanning electron microscopy indicate that the applied potential range and sweep rate are strongly influential on the phase obtained and the surface morphology of the electrodeposited thin films. Gasochromic measurements were carried out on the annealed samples by exposing them to H2 gas. It was revealed that α-MoO3 thin films provided better response to H2 interaction than β-MoO3 films did. Additionally, porous films provided significantly larger responses than smooth films.

Polypyrrole (PPy) nanofibers loaded with either palladium (II) acetate (PPy/Pd(O2CCH3)2) or palladium (II) nitrate (PPy/Pd(NO3)2) were characterized by TEM, SEM, EDX, Raman and FTIR techniques prior to their use as sensing materials for QCM based mercury vapor sensors. The QCM based sensor loaded with PPy/Pd(O2CCH3)2 nanofibers was observed to perform better over that of the PPy/Pd(NO3)2 loaded QCM and is postulated to be due to the lack of Pd salt reduction in the PPy/Pd(O2CCH3)2 matrix. The developed sensor was observed to operate with a response precision of ±5% over a 16 h testing period, and can withstand the presence of different levels of humidity at an operating temperature of 28 °C. Furthermore, the PPy/Pd(O2CCH3)2 based sensor was found to have a response time (t90) of ∼45 min towards a Hg vapor concentration of 10.55 mg/m3 balanced in either dry nitrogen or in streams containing up to 38% RH at near room temperature (28 °C). The stability of the developed sensor over 50 h of continuous testing towards Hg vapor under varying humidity levels indicates that a PPy/Pd(O2CCH3)2 thin layer is a good choice for near room temperature Hg vapor sensing applications.

Highly dispersed gold nanoparticles (AuNPs) supported on nitrogen-doped multiwalled carbon nanotubes (NCNTs) were synthesized using an electrochemical method. Size of AuNPs and their dispersion profile were tuned in a facile manner by controlling the applied potential and deposition time. The structure of the films was characterized using SEM, HRTEM, XRD and Raman spectroscopy. Electron microscopy revealed that the nanoparticles were deposited homogeneously on the outer surface of the NCNTs in high density. The average dimensions of the AuNPs could be tuned by varying the applied voltage and the duration of the electrochemical process. The XRD patterns confirmed the presence of metallic fcc gold along with disordered graphitic phases. The Raman spectroscopy showed that NCNTs possess 3–5 at.% nitrogen with a heterogeneous distribution profile, which is consistent with nitrogen incorporation in both graphitic and pyridinic moieties within the carbon sp 2 network of disordered NCNTs. SERS enhancement was also observed for AuNPs/NCNTs prepared at 10 V/20 min condition. Conductometric gas sensors were developed from the thin films of AuNPs/NCNTs nanocomposites and evaluated towards H 2 gas. The sensors were exposed to different concentrations of H 2 in synthetic air at room temperature. The highest sensitivity of 75% was measured towards 1% H 2 when average size of AuNPs on NCNTs is about 4–6 nm.

It is known that the unique layered structure of orthorhombic MoO3 (α-MoO3) facilitates the interaction with H2 gas molecules and that the surface-to-volume ratios of the crystallites play an important role in the process. MoO3 was deposited on a wide variety of transparent substrates using thermal evaporation in order to alter the surface-to-volume ratios of the crystallites. In situ Raman spectroscopy was employed to investigate the interaction between MoO3 and 1% H2 in both N2 and synthetic air environments, while incorporating Pd as a catalyst at room temperature. This study confirmed that the layered MoO3 with a high surface-to-volume ratio facilitated the H2 gas interaction. The Raman spectroscopy studies revealed that the H+ ions mainly interacted with the doubly coordinated oxygen atoms and caused the crystal transformation from the original α-MoO3 into the mixed structure of hydrogen molybdenum bronze and substoichiometric MoO3, eventually forming oxygen vacancies and water. It was also found tha...

Improvement in both sensor's characteristics and antifouling resistance has been achieved by utilising Cu2O as a dopant to the sub-micron-sized RuO2 sensing electrode (SE) of the water quality sensors. 20 mol% Cu2O-doped RuO2-SE has exhibited a linear response to dissolved oxygen (DO) between 0.5 and 8.0 ppm at various temperatures with the sensitivity slope of −46 mV/decade. This paper describes the structural properties and characteristics of thin-film Cu2O-doped RuO2-SE subjected to a field trial in sewerage environment for three months. Selectivity measurements revealed that the presence of PO43−, SO42−, Ca2+, Mg2+, Li+, NO3−, F−, Na+, K+ and Cl− in the test solution had no significant effect on the sensor's emf. Long-term stability test in harsh sewerage environment demonstrated that the Cu2O-doped RuO2-SE possesses improved antifouling resistance compared to RuO2-SE. Morphology and crystalline structure of Cu2O-doped RuO2-SE were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) technique and Raman spectroscopy.

Here we report on the synthesis of a graphene/polyaniline (PANI) nanocomposite and its application in the development of a hydrogen (H2) gas sensor. Using a chemical synthetic route, graphene was prepared and ultrasonicated with a mixture of aniline monomer and ammonium persulfate to form PANI on its surface. The developed material was characterized by scanning electron microscopy (SEM), transmission electron microscopy, Raman spectroscopy, and X-ray photoemission spectroscopy. The SEM study revealed that the PANI in the composite has a nanofibrillar morphology. We investigated the H2 gas sensing performance of this material and compare it with that of the sensors based on only graphene sheets and PANI nanofibers. We found that the graphene/PANI nanocomposite-based device sensitivity is 16.57% toward 1% of H2 gas, which is much larger than the sensitivities of sensors based on only graphene sheets and PANI nanofibers.

Atomically thin WO3 is fabricated using a three-step process, involving wet-chemical synthesis of hydrated-WO3, mechanical exfoliation of fundamental layers and dehydration by annealing at 300 °C. Atomic force microscopy reveals that the minimum resolvable thickness of the hydrated flakes to be ∼1.4 nm, which corresponds to the unit-cell height. The subsequent annealing temperature determines the degree of dehydration and thereby the crystallographic structure of the resultant WO3. The hydrated and dehydrated flakes are characterized by Raman spectroscopy to reveal spectroscopic fingerprints of flake thickness. Thin flakes are also shown to have higher propensity for lithium intercalation, which is also evidenced in Raman planar peak intensity.

Pt/graphene nanosheet/SiC based devices are fabricated and characterized and their performances toward hydrogen gas are investigated. The graphene nanosheets are synthesized via the reduction of spray-coated graphite oxide deposited onto SiC substrates. Raman and X-ray photoelectron spectroscopies indicate incomplete reduction of the graphite oxide, resulting in partially oxidized graphene nanosheet layers of less than 10 nm thickness. The effects of interfaces on the nonlinear behavior of the Pt/graphene and graphene/SiC junctions are investigated. Current−voltage measurements of the sensors toward 1% hydrogen in synthetic air gas mixture at various temperatures ranging up to 100 °C are performed. From the dynamic response, a voltage shift of ∼100 mV is recorded for 1% hydrogen at a constant current bias of 1 mA at 100 °C.

Plasmonic biosensors based on noble metals generally suffer from low sensitivities if the perturbation of refractive-index in the ambient is not significant. By contrast, the features of degenerately doped semiconductors offer new dimensions for plasmonic biosensing, by allowing charge-based detection. Here, this concept is demonstrated in plasmonic hydrogen doped molybdenum oxides (H x MoO 3 ), with the morphology of 2D nanodisks, using a representative enzymatic glucose sensing model. Based on the ultrahigh capacity of the molybdenum oxide nanodisks for accommodating H + , the plasmon resonance wavelengths of H x MoO 3 are shifted into visible-near-infrared wavelengths. These plasmonic features alter significantly as a function of the intercalated H + concentration. The facile H + deintercalation out of H x MoO 3 provides an exceptional sensitivity and fast kinetics to charge perturbations during enzymatic oxidation. The optimum sensing response is found at H 1.55 MoO 3 , achieving a detection limit of 2 × 10 -9 m at 410 nm, even when the biosensing platform is adapted into a light-emitting diode-photodetector setup. The performance is superior in comparison to all previously reported plasmonic enzymatic glucose sensors, providing a great opportunity in developing high performance biosensors.

This paper describes the formation of self-organized nanopores in thin films of titanium prepared using a Filtered Cathodic Vacuum Arc (FCVA) deposition system. The post-deposition anodization was performed using 0.5% (wt) NH 4 F in ethylene glycol and an aqueous based solution containing 0.5% (wt) NH 4 F and 1 M (NH 4 ) 2 SO 4 electrolytes. Homogenously distributed nanopores with dimensions in the range of 10 to 20 nm were obtained. Nanoporous TiO 2 thin films were obtained after annealing the anodized samples at 600 °C for 4 h. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize these nanoporous films. Raman measurements revealed that the rutile TiO 2 polymorph dominates these structures along with imperfect titanium oxidation resulting in the formation of defect structures, particularly when aqueous electrolyte was used for the anodization.

The gas sensing properties of graphene-like nano-sheets deposited on 36° YX lithium tantalate (LiTaO3) surface acoustic wave (SAW) transducers are reported. The thin graphene-like nano-sheets were produced via the reduction of graphite oxide which was deposited on SAW interdigitated transducers (IDTs). Their sensing performance was assessed towards hydrogen (H2) and carbon monoxide (CO) in a synthetic air carrier gas at room temperature (25 °C) and 40 °C. Raman and X-ray photoelectron spectroscopy (XPS) revealed that the deposited graphite oxide (GO) was not completely reduced creating small, graphitic nanocrystals ∼2.7 nm in size. © 2008 Elsevier B.V.

Incorporating Raman micro-spectroscopy into microfluidic units provides certain opportunities for in situ monitoring of bio-systems at the micro scale. Specifically, it allows for the observation of exchange, release and uptake of biochemical components within and in proximity of microorganisms in the highly controlled liquid phase environments of microfluidics. However, such systems are still in infancy and their capabilities need to be explored further. In this paper, the Escherichia coli (E. coli)/glucose bio-system is used for assessing a Raman/microfluidics unit. Two scenarios were used in the observations: the first that includes nutrient rich broth, ideal for E. coli growth, and the second using simplified deionised water.

Raman microscopy systems are becoming increasingly widespread and accessible for characterising chemical species. Microfluidic systems are also progressively finding their way into real world applications. Therefore, it is anticipated that the integration of Raman systems with microfluidics will become increasingly attractive and practical. This review aims to provide an overview of Raman microscopy-microfluidics integrated systems for researchers who are actively interested in utilising these tools. The fundamental principles and application strengths of Raman microscopy are discussed in the context of microfluidics. Various configurations of microfluidics that incorporate Raman microscopy methods are presented, with applications highlighted. Data analysis methods are discussed, with a focus on assisting the interpretation of Raman-microfluidics data from complex samples. Finally, possible future directions of Raman-microfluidic systems are presented.

Raman microscopy systems are becoming increasingly widespread and accessible for characterising chemical species. Microfluidic systems are also progressively finding their way into real world applications. Therefore, it is anticipated that the integration of Raman systems with microfluidics will become increasingly attractive and practical. This review aims to provide an overview of Raman microscopy-microfluidics integrated systems for researchers who are actively interested in utilising these tools. The fundamental principles and application strengths of Raman microscopy are discussed in the context of microfluidics. Various configurations of microfluidics that incorporate Raman microscopy methods are presented, with applications highlighted. Data analysis methods are discussed, with a focus on assisting the interpretation of Raman-microfluidics data from complex samples. Finally, possible future directions of Raman-microfluidic systems are presented.

The fundamental mechanism proposed to explain surface-enhanced Raman scattering (SERS) relies on electromagnetic field enhancement at optical frequencies. In this work, we demonstrate the use of microfabricated, silver nanotextured electrode pairs to study, in situ, the influence of low frequency (5 mHz to 1 kHz) oscillating electric fields on the SERS spectra of thiophenol. This applied electric field is shown to affect SERS peak intensities and influence specific vibrational modes of the analyte. The applied electric field perturbs the polar analyte, thereby altering the scattering cross section. Peaks related to the sulfurous bond which binds the molecule to the silver nanotexture exhibit strong and distinguishable responses to the applied field, due to varying bending and stretching mechanics. Density functional theory simulations are used to qualitatively verify the experimental observations. Our experimental and simulation results demonstrate that the SERS spectral changes relate to electric field induced molecular reorientation, with dependence on applied field strength and frequency. This demonstration creates new opportunities for external dynamic tuning and multivariate control of SERS measurements.

Two dimensional molybdenum disulfide (MoS(2)) has recently become of interest to semiconductor and optic industries. However, the current methods for its synthesis require harsh environments that are not compatible with standard fabrication processes. We report on a facile synthesis method of layered MoS(2) using a thermal evaporation technique, which requires modest conditions. In this process, a mixture of MoS(2) and molybdenum dioxide (MoO(2)) is produced by evaporating sulfur powder and molybdenum trioxide (MoO(3)) nano-particles simultaneously. Further annealing in a sulfur-rich environment transforms majority of the excess MoO(2) into layered MoS(2). The deposited MoS(2) is then mechanically exfoliated into minimum resolvable atomically thin layers, which are characterized using micro-Raman spectroscopy and atomic force microscopy. Furthermore Raman spectroscopy is employed to determine the effect of electrochemical lithium ion exposure on atomically thin layers of MoS(2).

Dielectrophoresis (DEP) utilizing a curved microelectrode pattern was developed and integrated with a Raman spectroscopy system. The electrodes were patterned on a Raman transparent quartz substrate, and integrated with a microfluidic channel in poly-dimethylsiloxane (PDMS). This integrated system can be efficiently used for the determination of suspended particles type and the direct mapping of their spatial concentrations. It will be demonstrated that the integration of Raman mapping with dielectrophoretically controlled WO3 particles can be used for studying suspended particles in situ.

a b s t r a c t Nanocrystalline CuO-CuxFe3−xO4 thin films were developed using a radio­frequency sputtering method followed by a thermal oxidation process. Thin films were deposited applying two very different condi­ tions by varying the argon pressure and the target­to­substrate distance. Structural, microstructural and gas­sensing characteristics were performed using grazing incidence X­ray diffraction (GXRD), Raman spectroscopy, atomic force microscopy (AFM), X­ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrical measurements. Their sensing properties were examined using hydrogen gas in dry synthetic air. The shortest response and recovery times were observed between 280 and 300 ◦C independently of the deposition conditions.

The formation of MoO(3) sheets of nanoscale thickness is described. They are made from several fundamental sheets of orthorhombic alpha-MoO(3), which can be processed in large quantities via a low cost synthesis route that combines thermal evaporation and mechanical exfoliation. These fundamental sheets consist of double-layers of linked distorted MoO(6) octahedra. Atomic force microscopy (AFM) measurements show that the minimum resolvable thickness of these sheets is 1.4 nm which is equivalent to the thickness of two double-layers within one unit cell of the alpha-MoO(3) crystal.

There have been many investigations to reveal the nature of the hydrogen gas and ZnO nanopowder interaction at elevated temperatures, while at present no conclusive description of such an interaction has been confidently reported. In this work, we demonstrate that a hydroxyl group is formed during this interaction, depending on size and relative crystallinity of nanopowders. Our in situ Raman spectroscopy investigations show that the interaction directly affects the intensity of the Raman signal at 483 cm(-1), relative to the peak at 519 cm(-1). Ex situ x-ray diffraction (XRD) and infrared spectroscopy also show extra peaks at 44° and 1618 cm(-1), respectively, after hydrogenation. These peaks were all identified as surface hydroxyl groups, which can be related to the formation of water on the ZnO nanopowder surfaces.

A microfluidic dielectrophoresis platform consisting of curved microelectrodes was developed and integrated with a Raman spectroscopy system. The electrodes were patterned on a quartz substrate, which has insignificant Raman response, and integrated with a microfluidic channel that was imprinted in poly-dimethylsiloxane (PDMS). We will show that this novel integrated system can be efficiently used for the determination of suspended particle types and the direct mapping of their spatial concentrations. We will also illustrate the system's unique advantages over conventional optical systems. Nanoparticles of tungsten trioxide (WO(3)) and polystyrene were used in the investigations, as they are Raman active and can be homogeneously suspended in water.

It is well known that WO(3) interacts efficiently with H(2) gas in the presence of noble metals (such as Pd, Pt and Au) at elevated temperatures, changing its optical behaviors; and that its crystallinity plays an important role in these interactions. For the first time, we investigated the in situ Raman spectra changes of WO(3) films of different crystal phases, while incorporating Pd catalysts, at elevated temperatures in the presence of H(2). The Pd/WO(3) films were prepared using RF sputtering and subsequently annealed at 300, 400 and 500 °C in air in order to alter the dominant crystal phase. The films were then characterized using SEM, XRD, XPS, and both UV-VIS and Raman spectroscopy. In order to fundamentally study the process, the measurements were conducted when films were interacting with 1% H(2) in synthetic air at elevated sample temperatures (20, 60, 100 and 140 °C). We suggest that the changes of Raman spectra under such conditions to be mainly a function of the crystal phase, transforming from monoclinic to a mix phase of monoclinic and orthorhombic achieved via increasing the annealing temperature. The as-deposited sample consistently shows similar Raman spectra responses at different operating conditions upon H(2) exposure. However, increasing the annealing temperature to 500 °C tunes the optimum H(2) response operating temperature to 60 °C.