MoO3 Films Research Articles

Mo nitrides and carbonitrides via metallic phase transition of MoO3 films using ammonium salt precursors in chemical vapor deposition

Transition-metal-based nitrides and carbides have been widely studied for various applications because of their excellent mechanical, electrical, and catalytic properties. Although outstanding performances of Mo nitride and carbonitride synthesized by chemical reaction methods have been reported recently for catalytic applications, a metallic phase transition method for pre-deposited MoO3 precursor films, which is suitable for application to Si or metallic substrates, has rarely been reported. Herein, we investigate Mo nitride and carbonitride thin films synthesized via nitriding and carbonitriding of MoO3 films using two types of ammonium salt precursors, urea and ammonium carbonate (AC), in the chemical vapor deposition (CVD) process. The phase-changed crystalline films of Mo carbonitride via the urea reaction and Mo nitride via the AC reaction were determined to be MoC0.5N0.5 and MoN by X-ray diffractometry, respectively. Nevertheless, amorphous states of carbons and their related compounds, unreacted residual MoO3, and incompletely reacted MoO2 remained in the films, as determined by other thin film analyses. When the residual MoO2 was minimal, the films exhibited the lowest electrical resistivity, i.e., ∼10−4 Ω·cm for Mo carbonitride and ∼10−3 Ω·cm for Mo nitride, implying an optimum metallic phase transition. Our results pave the way for the nitriding and carbonitriding of transition metal oxides using ammonium salt precursors for a one-step reaction in the CVD process.

Efficiency improvement of molybdenum oxide doped with graphene oxide thin films solar cells processed by spray pyrolysis technique

The selective hole nature of MoOx is described by its high-level work function, which creates band alignment at the MoO3-Si heterointerface that turns the surface to inversion. However, its low conductivity and carrier concentration hinder the material predicted theoretical efficiency. Attempts to enhance the performance of the MoO3 was made by doping graphene oxides (GO) using the spray pyrolysis technique. The GO doping concentrations were varied in the range of 0–3 wt %. The properties of the pure MoO3 and chemically GO-doped MoO3 films were analyzed, and the efficiency variations and stability of the pure and GO-doped MoO3/n-Si solar cells were presented. The films doped with 2 % GO showed a higher concentration of 7.41 × 1019 cm−3 and a low resistivity of 40.16 Ωcm. This attributed to the improvement of the solar cell's parameters with, Voc = 484.34 mV, Jsc = 10.12 mA/cm2, FF = 66.21 % and η = 3.26 %.

Hydrogen sensing by RGB investigation of gasochromic coloration of MoO3/Pd-coated polyester fabrics

Recently, colorimetric sensors capable of detecting explosive and toxic molecules have attracted much attention for applications in health and environmental monitoring. Here, we demonstrate the detection of hydrogen gas using gasochromic fabrics and evaluate their performance with a commercial RGB color sensor module. For this purpose, first a thin Pd film was sputter deposited onto the polyester fabric and then amorphous MoO3 films (several microns) were deposited by RF reactive magnetrons in various O2/Ar ratios (from 2.5 to 12.5 O2). Field emission scanning electron microscope (FESEM) identified the morphological changes of the surface at each coating step and showed that the fibrous structure of the fabrics was preserved after coating of the films, which is suitable for better gas adsorption. Exposure to hydrogen caused the obtained MoO3/Pd/polyester samples to turn blue-gray at room temperature and the UV–Vis spectra showed an increase in optical absorption. Their sensitivity was evaluated in the presence of different H2 concentrations from 250 to 10,000 ppm by recording the time variation of color difference (RGB Euclidean distance). A significant correlation was found between absorption spectrum and RGB analysis with respect to oxygen content. The color variation showed a linear behavior with respect to hydrogen concentration for all samples. Response and recovery times, reversibility and reproducibility in the presence of hydrogen gas were also evaluated using the RGB readout data. Overall, the RGB module optimally depicts the sensing characteristics of the gasochromic fabrics in terms of reversibility, sensitivity and reproducibility, which may contribute to their commercial development.

Extinction Coefficient Modulation of MoO3 Films Doped with Plasmonic Nanoparticles: From an Effective Medium Theory Description.

This work focused on the application of the effective medium theory to describe the extinction coefficient (Qext) in molybdenum trioxide (MoO3) doped with different kinds of plasmonic nanoparticles, such as silver (Ag), gold (Au), and copper (Cu). Usually, in studies of these materials, it is normal to analyze the transmission or absorption spectra. However, the effect of this type or size of nanoparticles on the spectra is not as remarkable as the effect that is found by analyzing the Qext of MoO3. It was shown that the β-phase of MoO3 enhanced the intensity response of the Qext when compared to the α-phase of MoO3. With a nanoparticle size of 5 nm, the Ag-doped MoO3 was the configuration that presents the best response in Qext. On the other hand, Cu nanoparticles with a radius of 20 nm embedded in MoO3 was the configuration that presented intensities in Qext similar to the cases of Au and Ag nanoparticles. Therefore, implementing the effective medium theory can serve as a guide for experimental researchers for the application of these materials as an absorbing layer in photovoltaic cells.

Evolution of fractal dimension in pulsed laser deposited MoO3 film with ablation time and annealing temperature

Evolution of fractal dimension in pulsed laser deposited MoO3 film with ablation time and annealing temperature

MoO3 films grown on stepped sapphire (0001) by molecular beam epitaxy

MoO3 films were grown on stepped c-plane sapphire substrates by molecular beam epitaxy using MoO3 vapor from a conventional Knudsen cell. Stepped sapphire (0001) substrates were prepared by ex situ annealing at 1100–1300 °C in dry air. Step bunching typically resulted in multistepped surfaces with wide atomically smooth terraces. Ex situ annealing at 1100 °C followed by in vacuo annealing at 700 °C provided clean substrates for growth. Ultrathin films were grown at 450 °C via a self-limiting process that represents a balance between the incident MoO3 flux and the desorption flux. Elongated bilayer islands (0.7-nm thick) were formed on sapphire (0001) terraces. Monocrystalline α-MoO3 (010) thin films [(010)α-MoO3∥(0001)sapphire] were grown at 450 °C using a higher incident MoO3 flux and characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and cross-sectional transmission electron microscopy. The step-terrace surface morphology of the monocrystalline films strongly suggests multilayer growth.

Facile preparation of anodized MoO3−x films and their boosted photocatalytic activity

Flexible MoO3−x films were prepared by anodic oxidation followed by annealing and liquid-phase reduction. The prepared samples were characterized by XRD, Raman spectra, SEM, UV–visible spectrophotometer, XPS, and EPR. The anodized amorphous MoO3 was transferred to the orthorhombic phase during the annealing. With the increase in annealing temperature, the MoO3 films’ photocatalytic activity increased. After liquid-phase reduction, defects, including oxygen vacancies, were introduced. The defect introduction enhanced the visible light absorption and increased the charge transfer efficiency to varying degrees. Compared with the as-annealed MoO3 films, the as-reduced MoO3 films’ photocatalytic activity rose by nearly four times at most. With the increase in reduction time, the bulk defect concentration increased. However, the surface defect concentration reached the maximum when the reduction time came to 2 h. The most excellent photocatalytic activity corresponded to the largest surface defect concentration, suggesting surface defects improved the photocatalytic activity. On the other hand, the bulk defects had the opposite effect. The photogenerated holes played a crucial role in the photocatalytic degradation of MB. Anodic oxidation followed by annealing and liquid-phase reduction is a facile and effective way to prepare MoO3 films with high photocatalytic activity.

Room temperature oxygen exchange and diffusion in nanometer-thick ZrO2 and MoO3 films

The diffusion of oxygen in thin films of ZrO2 and MoO3 was investigated with atomic 18O as a tracer using low energy ion scattering sputter depth profiling. 3 nm amorphous and 20 nm polycrystalline films were prepared by reactive magnetron sputtering and exposed to atomic oxygen species at room temperature. Exposure results in a fast diffusion of oxygen to a limited depth of ~1 nm and ~2.5 nm for ZrO2 and MoO3, respectively, and surface exchange limited to a maximum of 65% to 75%. The influence of the crystalline structure of the films on exchange and diffusion was negligible. We propose that the transport of oxygen in oxides at room temperature is dominated by a field-induced drift, generated by the chemisorption of reactive oxygen species. The maximum penetration of oxygen is limited by the oxide space charge region, determined by the oxide electrical properties. We applied a drift–diffusion model to extract values of surface potential and kinetic parameters of oxygen exchange and diffusion. The developed experimental analysis and modelling suggest that the electric field and consequent distribution of charged species are the main factors governing exchange rates and species diffusion in an oxide thin film at room temperature.

Thermal diffusivity downscaling of molybdenum oxide thin film through annealing temperature-induced nano-lamelle formation: a photothermal beam deflection study

The present work proposes a method of downscaling the thermal diffusivity (α) of MoO3 thin films through annealing temperature-induced nano-lamelle formation. The thermal diffusivity modification of the MoO3 films, prepared by the doctor blade method, is investigated by the sensitive transverse photothermal beam deflection technique. The X-ray diffraction analysis confirms the structural phase transformation from monoclinic to orthorhombic in the films annealed from 300 to 450 °C. The thermal induced anisotropy of the film is evident from the variation of the morphology index and texture coefficient. The field emission scanning electron microscopic analysis unveils the morphology modifications from blocks to the nano-lamelle structure with layers of average thickness ~ 77 nm. The thermal diffusivity measurement reveals a 53% reduction upon annealing the film to 450 °C. The drastic reduction is achieved through the annealing temperature-induced nano-lamelle formation and the phase transformation from monoclinic to orthorhombic in the MoO3 films.

Tailoring the structure of MoS2 using ball-milled MoO3 powders: hexagonal, triangular, and fullerene-like shapes

Single and few-layered MoS2 materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO3 films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS2 nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS2 materials using MoO3 powders previously exposed to a high-energy ball milling treatment by the salt-assisted CVD method. The MoO3 powders milled for 30, 120, and 300 min were used to synthesize sample MoS2-30, MoS2-120, and MoS2-300, respectively. We found morphologies mainly of hexagons (MoS2-30), triangles (MoS2-120), and fullerenes (MoS2-300). The MoS2 nanostructures and MoO3 powders were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray diffraction, and thermogravimetric analysis. It was found that MoO3 milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS2 crystals and a large number of milled MoO3-x + NaCl promote greater nucleation sites for the formation of MoS2, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ∼30–600 nm with 1–12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS2 samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.

Speckle Interferometric Investigation of Argon Pressure-Induced Surface Roughness Modifications in RF-Sputtered MoO3 Film

Speckle Interferometric Investigation of Argon Pressure-Induced Surface Roughness Modifications in RF-Sputtered MoO3 Film

Study of the Processes of the Formation of Nanoscale MoO3 Films by Thermal Oxidation and Ion Bombardment

Nanoscale MoO3 films are obtained on the surface of a Mo single crystal by means of thermal oxidation and ion implantation. The optimal modes of ion implantation (the partial oxygen pressure, the ion energy and dose, and the annealing temperature) are determined to form homogeneous MoO3/Mo films with thicknesses ranging from 30 to 100 A and with good stoichiometry. It is established that, for low doses of oxygen ions (D ≤ 1016 cm–2), nanoscale MoO3 phases form on the Mo-crystal surface. The dependence of the degree of Mo surface coverage by MoO3 cluster phases on the ion dose is determined. Films with a thickness of ~100 A are obtained by successively implanting $${\text{O}}_{2}^{ + }$$ ions with energies of 5, 3, and 1 keV. The composition, the electronic structure, and the emission and optical properties of the nanoscale phases and MoO3 films are studied using a complex of methods (Auger- and photoelectron spectroscopy, secondary electron emission, and secondary-ion mass spectroscopy).

STRUCTURAL, OPTICAL AND DIELECTRIC PERFORMANCE OF MOLYBDENUM TRIOXIDE THIN FILMS SANDWICHED WITH INDIUM SHEETS

In this work, we report the enhancements in the structural, optical and dielectric properties of molybdenum trioxide that are achieved by insertion of 50 and 100 nm thick indium sheets between layers of MoO3. The films which are coated onto ultrasonically glass substrates under a vacuum pressure of 10-5 mbar exhibited metal induced crystallization process upon insertion of indium sheets. Optically, indium sheets tuned the transmittance and reflectance, significantly, increased the absorption coefficient values and formed interbands in the band gap of MoO3. The energy band gap decreased with increasing indium sheets thickness. On the other hand, the insertion of indium layers into the structure of MoO3 is observed to improve the dielectric response of these films to values that nominate them for used in thin film transistor technology. In the same context, the analyses of the optical conductivity which are carried out with the help of Drude-Lorentz approach have shown that the presence of indium sheets can increase the optical conductivity and enhance the plasmon frequency and free charge carrier density of MoO3. The plasmon frequency is tuned in the range of 1.68-7.16 GHz making MoO3 films attractive for plasmonic applications.

Detection and analysis of structural changes in MoO3/Ag heterostructure by ultrafast transient absorption spectroscopy

Ultrafast transient absorption spectroscopy (UFTS) is a widely used technique to determine the change in optical parameters on laser irradiation and can highlight the pathways and the lifetime of decay of the charge carriers. Here, UFTS was used to detect the structural changes occurring in a MoO3/Ag heterostructure because of dewetting and dealloying. A comparative study of transient absorption behavior of samples exposed to different temperatures indicated a change in phase and crystallinity of the film. Later through GI-XRD, it was concluded that the MoO3 film has become crystalline at 350 °C along α-(110) plane. A wide plasmonic band in UFTS data of dealloyed sample followed by FESEM showed randomly distributed Ag-Mo nanoporous clusters (315 nm - 6 μm). Tuning of surface plasmon resonance (~19 nm) was seen with etching time. This study proves UFTS's suitability for correlating the structural change in a film or heterostructure of films with spectroscopic results.

Structural, optical, and electrical properties of spray-pyrolyzed MoO3 thin films by varying precursor molarity, as hole-selective contact for silicon-based heterojunction devices

Sub-stoichiometric molybdenum oxide (MoO3) films are recurrently deposited on crystalline silicon (c-Si)-based devices by thermal evaporation. This technique necessitates high temperature, which offers limited control of oxide stoichiometry and therefore affects the control of hole-transport mechanism. In this work, the effect of precursor molarity on the properties of deposited MoO3 films by spray pyrolysis at a substrate temperature of 300 °C was investigated. The molarity was varied in a range of 0.05 M to 0.2 M with 0.05 M increment. The films were characterized by analyzing their structural, morphological/composition, and electrical properties. X-ray diffraction findings revealed the growth of orthorhombic lamellar crystal structure with (0k0) preferred orientation. Field emission scanning electron microscopes images displayed nanobelts-like structure with improved grain size as the molarity increases. The typical diode parameters, such as ideality factor (n) and barrier height (ϕB), are determined. At the same time, the quantitative analysis of the carrier transport mechanism of MoO3/n-Si heterojunction was conducted by I–V characteristics curve. The decrease in barrier heights and the subsequent increase in the ideality factor were observed by increasing precursor molarity. Similarly, when the junction exposed to illumination, both the parameters were found to decrease.

RF Sputtered MoO3 Thin Film on Si (100) for Gas Sensing Applications

Molybdenum Trioxide (MoO3) films are grown on Si(100) substrates by reactive RF magnetron sputtering in plasma containing a mixture of Argon and Oxygen, using a pure Molybdenum target. In this paper, we report the deposition of (MoO3) films on Si(100) substrates under varying gas flow (O2 + Ar gas) (20 sccm to 30 sccm with the duration of deposition~ 1hr) by RF reactive magnetron sputtering at room temperature. To get crystalline MoO3 films annealing in O2 environment at 500 °C for 4 h is done. Phase formation and orientation of the film is characterized by Glancing incidence X-ray diffraction (GIXRD). The identification of the orthorhombic MoO3 phase is investigated by XRD and Raman spectroscopy. Raman lines at 819 cm-1 and 995 cm-1 are due to the (A1g, B1g) symmetric stretching (Mo-O–Mo) bond and asymmetric stretching band (Mo=O) respectively. Surface morphology and cross-sectional image of the deposited thin films were investigated by FE-SEM image. UV-Visible reflectance and cross-sectional FE-SEM image confirm the thickness of the MoO3 films with oxygen-rich and oxygen deficient phase formation occur. Reverse leakage current density of 20 sccm 1hr sample is low (1×10-6 mA/cm2) as compared to 30 sccm 1hr sample (1×10-3 mA/cm2). The higher leakage is due to crack formation during the ex-situ annealing of MoO3 films. This MoO3 films can be used in Gas sensing and switching devices.

Effect of Substrate Temperature on the Growth of Molybdenum Trioxide Thin Films

Molybdenum oxide is one of the most important inorganic material, which exhibits several phases, such as MoO3, MoO2, Mo4O11, Mo5O14, etc. Among them, molybdenum trioxide (MoO3) can crystallize in various phases, such as orthorhombic, monoclinic etc., which makes it useful for possible applications in chemical, electrical and electrochemical industries. In this work, MoO3 films were obtained by pulsed laser deposition by varying the substrate temperature from room temperature to 400°C. The thin films were deposited on fine cleaned glass substrates coated with fluorine tin oxide under a pressure of 10–5 mbar. X-ray diffraction patterns display two polymorphic phases of MoO3 (α and β), but no other phases are observed, and the structure changes from orthorhombic to monoclinic. The substrate temperature strongly influences the structure and surface topography. Morphological studies show the surface homogeneity, crack-free, layered structure and crystallinity of the films. The band gap of the obtained MoO3 thin films increases from 3.0 to 3.39 eV with increasing substrate temperature from room temperature to 400°C.

Effect of Vanadium Doping on the Catalytic Activity of MoO₂ Thin Films in Hydrogen Evolution Reaction.

Recently, Mo-based materials have emerged as promising candidates to replace Pt catalysts for hydrogen evolution reaction. Among these, MoO₂ holds a great promise due to its metallic conductivity. Here, we investigate the effect of V doping on the hydrogen evolution reaction activity of MoO₂ thin films. V doped MoO₂ thin films with different dopant concentrations were prepared on conductive Si substrates by co-sputtering followed by thermal sulfurization. The electrochemical measurements revealed that V doping has an adverse effect on the hydrogen evolution reaction performance of MoO₂ films, attributed to the p-type doping nature of V. This study gives insight into the effect of dopant type on the catalytic activity of hydrogen evolution reaction catalysts and provides a hint for the correct selection of dopant elements to enhance the activity of the existing catalyst materials.

Mist chemical vapor deposition of MoO2 thin films

Molybdenum dioxide (MoO2), which exhibits both high chemical stability and high electrical conductivity, has been studied for various applications, e.g. as a photocatalyst, an active material for secondary batteries, and the capacitor electrode of dynamic random access memory devices. MoO2 films are mainly fabricated by vacuum processes, which incur a high manufacturing cost. In this study, we fabricated MoO2 thin films by mist chemical vapor deposition. Since this method does not require vacuum or a volatile precursor, the processing cost is reduced. Examination of the films deposited at various positions in the furnace at different furnace temperatures revealed that smooth MoO2 films composed of densely packed crystal grains with a constant thickness of 200 nm can be obtained over a wide area of >30 cm2 at a furnace temperature of ~480 °C. The films showed low electrical resistivity of 2 × 10−3 Ω cm. Thus, this study provides a low-cost method for the fabrication of MoO2 thin films.

Branched nanorods-built MoO3 film with excellent electrochemical performance for energy storage

Branched nanorods-built MoO3 film (E-500) is fabricated by etching and followed oxidation of Mo foil. However, directly oxidation of Mo foil results in nanoparticle-based films. It is confirmed that the conductivity and electrochemical performance of nanorods-built MoO3 film are much higher than that of nanoparticle-based MoO3 film. More specifically, E-500 exhibits 116.7 mF cm−2 even at 20 mA cm−2.