α-MoO3 Thin Films Research Articles

Highly sensitive and low-temperature triethylamine sensor based on in situ growth of hierarchical α-MoO3 flowers

To challenge high working temperature of present triethylamine (TEA) sensor, well-defined hierarchical α-MoO3 flowers were in situ grown on Al2O3 tubes through a simple solvothermal method. The α-MoO3 flowers were about 3 μm in diameter consisted of overlapping nanosheets with average thickness of ∼30 nm, growing radially from the center of the flowers. The α-MoO3 flowers in situ grew on Al2O3 tubes to constitute the thin film sensors, which exhibited improved gas-sensing performance to TEA compared to the thick film sensors painted with the powder of α-MoO3 produced in the same reaction system. The average response value of the α-MoO3 thin film sensors to 10 ppm TEA was 243.5 at relatively low operating temperature of 133 °C, which was 4.5 times as high as that of the thick film sensors (54.2). And the detection limit of the thin film sensors to TEA is further decreased to 0.001 ppm compared to the α-MoO3 thick film ones. The superior TEA-sensing capability should be attributed to the hierarchical nanostructure grown in situ that supplies more active sites for gas molecule adsorption and electron transfer. The synthesized α-MoO3 flowers with hierarchical nanostructure are promising material for trace TEA detection in practical application.

Hyperbolic phonon polaritons-induced photonic spin Hall effect in an α -MoO3 thin film

Photonic spin Hall effect (PSHE) can be achieved by using the patterned structures or metal/dielectric multilayers in the nanophotonic systems; however, the complicated structures of these devices hinder their further applications. Herein, we demonstrate that highly directional PSHE can be realized through the excitation of hyperbolic phonon polaritons (HPPs) in a comparably simple architecture based on an anisotropic α-MoO3 thin film. It is shown that the propagation of the HPP modes of α-MoO3 in the reststrahlen (RS) bands exhibits topological transitions between open hyperbola and closed ellipse in both real space and momentum space (k-space) due to the extreme in-plane anisotropy. Specifically, larger dispersion angle possesses larger figure of merit (FoM), and high k mode of HPPs exhibits robust propagation properties at the maximum dispersion angle. Spin-selected propagation with asymmetric ratio of intensity equal to ±0.94 can be realized by changing the handedness of the dipole emitters. By changing the incident wavelengths of the RS bands, the propagation angle of the HPP modes can be dynamically tuned in wide angular and wavelength ranges.

Broadband transmissive polarization rotator by gradiently twisted α -MoO3

Polarization engineering has been proven to enhance the capabilities of light manipulation and thus facilitate the development of integrated photonic devices. In this study, we introduce a polarization rotator based on a gradiently twisted α-MoO3 thin film, which works in the mid-infrared range and functions in a transmission mode. To be specific, the proposed device is constructed by gradiently twisted α-MoO3 multilayers with a subwavelength thickness of only 5 μm, namely, one-third of the working wavelength. Our analytical calculation demonstrates the efficacy of this subwavelength thin film rotator in converting a linearly polarized wave into its orthogonal counterpart, thanks to its chiral nature. The twisted α-MoO3 multilayers exhibit the capability to significantly manipulate dispersion characteristics while maintaining low optical losses, thereby enabling a wide bandwidth exceeding 2.5 THz with a polarization ratio surpassing 17 dB. Moreover, the operational frequency can be adjusted across a 3.4 THz range by altering the incident angle of the incident waves. This adaptable design, characterized by its polarization versatility, can be customized to suit practical applications within wireless communication, radar systems, optical switching, and imaging technologies.

Large-area polycrystalline α-MoO3 thin films for IR photonics

In recent years, the excitation of surface phonon polaritons (SPhPs) in van der Waals materials received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide (α-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic material due to its ability to support SPhPs for three orthogonal directions at different wavelength bands (range 10–20 μm). Here, we report on the fabrication, structural, morphological, and optical IR characterization of large-area (over 1 cm2 size) α-MoO3 polycrystalline film deposited on fused silica substrates by pulsed laser deposition. Due to the random grain distribution, the thin film does not display any optical anisotropy at normal incidence. However, the proposed fabrication method allows us to achieve a single α-phase, preserving the typical strong dispersion related to the phononic response of α-MoO3 flakes. Remarkable spectral properties of interest for IR photonics applications are reported. For instance, a polarization-tunable reflection peak at 1006 cm−1 with a dynamic range of ΔR = 0.3 and a resonance Q-factor as high as 53 is observed at 45° angle of incidence. Additionally, we report the fulfillment of an impedance matching condition with the SiO2 substrate leading to a polarization-independent almost perfect absorption condition (R < 0.01) at 972 cm−1 which is maintained for a broad angle of incidence. In this framework our findings appear extremely promising for the further development of mid-IR lithography-free, scalable films, for efficient and large-scale sensors, filters, thermal emitters, and label-free biochemical sensing devices operating in the free space, using far-field detection setups.

Role of thermal heat-treatment to achieve a highly polycrystalline and compact α-MoO3 thin films

Role of thermal heat-treatment to achieve a highly polycrystalline and compact α-MoO3 thin films

Sputter-grown two-dimensional α-MoO3 thin films: Microstructure dependence on growth conditions

Van der Waals epitaxy is an emerging technology for transferring freestanding films grown at high temperatures onto arbitrary substrates using two-dimensional epitaxial layers. This study investigates the microstructure and growth of α-MoO3 thin films prepared via sputtering. Under a narrow deposition temperature range, epitaxial α-MoO3 thin films were successfully grown on SrTiO3 substrates. The films were characterized by a flat surface morphology, corresponding to (0k0) planes, without undergoing reaction with the substrates. To examine the growth behavior and surface morphological variation of the α-MoO3 thin films under various process parameters, we modified the Ar:O2 ratio, working pressure, sputtering power, and crystal structure of the substrates. Transmittance electron microscopy confirmed the epitaxial growth of the α-MoO3 thin films on (001)-oriented SrTiO3 substrates with a two-dimensional layer structure. Finally, we demonstrate the morphological evolution of the α-MoO3 thin films etched in heated water as a function of soaking time. The etching results suggest that the α-MoO3 thin films are promising sacrificial layers for the transfer of large-scale epitaxial thin films onto flexible substrates. External control of the strain states of transferred freestanding thin films by bending or stretching provides new perspectives for the design of flexible or wearable electronic devices.

Impact of thickness on optoelectronic properties of α-MoO3 film photodetectors: Integrating first-principles calculations with experimental analysis

This study focused on investigating the optoelectronic properties of molybdenum trioxide (α-MoO3) thin films using the atomic layer deposition (ALD) technique through different cycle numbers and theoretical investigation. Initial band gap calculations using standard DFT with GGA-PBE resulted in a value of 1.19 eV, which deviated significantly from experimental measurements. The GGA + U method with Hubbard U corrections was applied for the first time to improve the accuracy. This refinement led to a more precise band gap value of 3.09 eV, closely matching previously reported experimental data. The electronic parameters of the α-MoO3 photodetector, such as ideality factor (n), barrier height (Φ0), and series resistance (Rs), were analyzed using the thermionic emission theory and confirmed by Cheung and Nord's methods. The results demonstrated that the sample deposited with 100 pulses exhibited higher photodetector performance under UV illumination, despite having a lower Rs.

Molybdenum precursor delivery approaches in atomic layer deposition of α-MoO3.

In this research work, we present a study on time-sequenced plasma-enhanced atomic layer deposition (PE-ALD) processes towards the achievement of high-quality α-MoO3 thin films which are suitable for exfoliation. In particular, a conventional precursor injection method along with a boosted precursor delivery approach are discussed and analysed. In the latter, the proposed gas supply mechanism ensures a large number of deposited Mo atoms per unit of time, which, along with a proper thermal energy, leads to high-quality and oriented orthorhombic α-MoO3 films. The proposed boosted approach is also compared with post growth annealing steps, resulting in more effective achievement of a highly oriented orthorhombic α-MoO3 phase and less time consumption.

Surface modification of AISI 316 steel by α-MoO3 thin films grown using cathodic cage plasma deposition

In recent years, the MoO3 in different morphologies and phases have attracted the attention due to their physical and chemical properties, mainly in the form of thin films. In this paper, we present the growth of thin films of molybdenum trioxide on AISI 316 steel using the cathodic cage plasma deposition (CCPD) technique at low temperature, in fluctuating potential and with variation of the gas flow, argon and oxygen. Our study revealed, for the first time that the films deposited under 50:50 (50 sccm Ar/50 sccm O2) and 80:20 (80 sccm Ar/20 sccm O2) gases composition crystallized in the orthorhombic phase (α-MoO3), without secondary phases and with a thickness of 5 μm and 1 μm, respectively. The 80:20 sample a higher roughness and consequently a larger grain. Vickers microhardness measurements show that low temperature plasma deposition maximizes the surface hardness of AISI 316 austenitic stainless steel (80:20–545 HV/50:50–465 HV/AISI 316–187 HV) with excellent adhesion, classified as HF1, with no cracks or detachment regions, which indicates a good level of plasticity of the films.

Lithography-free tunable absorber at visible region via one-dimensional photonic crystals consisting of an α-MoO3 layer.

Flexible control of light absorption within the lithography-free nanostructure is crucial for many polarization-dependent optical devices. Herein, we demonstrated that the lithography-free tunable absorber (LTA) can be realized by using two one-dimensional (1D) photonic crystals (PCs) consisting of an α-MoO3 layer at visible region. The two 1D PCs have different bulk band properties, and the topological interface state-induced light absorption enhancement of α-MoO3 can be realized as the α-MoO3 thin film is inserted at the interface between the two 1D PCs. The resonant cavity model is proposed to evaluate the anisotropic absorption performances of the LTA, and the results are in good agreement with those of the transfer matrix method (TMM). The absorption efficiency of the LTA can be tailored by the number of the period of the two PCs, and the larger peak absorption is the direct consequence of the larger field enhancement factor (FEF) within the α-MoO3 layer. In addition, near-perfect absorption can be achieved as the LTA is operated at the over-coupled resonance. By varying the polarization angle, the absorption channels can be selected and the reflection response can be effectively modulated due to the excellent in-plane anisotropy of α-MoO3.

Quasi-two-dimensional α-molybdenum oxide thin film prepared by magnetron sputtering for neuromorphic computing.

Two-dimensional (2D) layered materials have attracted intensive attention in recent years due to their rich physical properties, and shown great promise due to their low power consumption and high integration density in integrated electronics. However, mostly limited to mechanical exfoliation, large scale preparation of the 2D materials for application is still challenging. Herein, quasi-2D α-molybdenum oxide (α-MoO3) thin film with an area larger than 100 cm2 was fabricated by magnetron sputtering, which is compatible with modern semiconductor industry. An all-solid-state synaptic transistor based on this α-MoO3 thin film is designed and fabricated. Interestingly, by proton intercalation/deintercalation, the α-MoO3 channel shows a reversible conductance modulation of about four orders. Several indispensable synaptic behaviors, such as potentiation/depression and short-term/long-term plasticity, are successfully demonstrated in this synaptic device. In addition, multilevel data storage has been achieved. Supervised pattern recognition with high recognition accuracy is demonstrated in a three-layer artificial neural network constructed on this α-MoO3 based synaptic transistor. This work can pave the way for large scale production of the α-MoO3 thin film for practical application in intelligent devices.

Thermal conductivity across the van der Waals layers of α-MoO3 thin films composed of mosaic domains with in-plane 90° rotations

α-Molybdenum trioxide (α-MoO3) is a typical two-dimensional metal oxide material, in which the covalently bonded layers are bonded by van der Waals forces. Herein, the thermal conductivities across the van der Waals layers of α-MoO3 thin films with nominal thicknesses of 50, 100, and 125 nm were investigated. The α-MoO3 thin films were fabricated on the (100) plane of a single-crystalline SrTiO3 substrate heated up to 400 °C by DC reactive magnetron sputtering using a Mo metal target. The b-axis-oriented α-MoO3 thin films epitaxially grown on the SrTiO3 (100) plane were confirmed by x-ray diffraction and x-ray pole figure analyses. Electron diffraction patterns and plane-view transmission electron micrographs revealed that the α-MoO3 thin films were composed of mosaic domains with a diameter of ∼2 nm, with each domain rotated in-plane by 90° with respect to the neighboring one. The mean thermal conductivity across the van der Waals layers of the three α-MoO3 thin films was evaluated to be 1.2 ± 0.3 W m−1 K−1, which is comparable to the reported thermal conductivities of layered cobalt oxide thin films. The reduced thermal conductivity is mainly due to phonon scattering at domain boundaries lying in the in-plane direction.

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.

Synthesis and electrochemical characterization of pseudocapacitive α-MoO3 thin film as transparent electrode material in optoelectronic and energy storage devices

Molybdenum oxide thin film was deposited on indium tin oxide (ITO) coated glass via a simple and cost-effective approach. Some surface and electrochemical properties of the film were studied. Characteristic peaks from x-ray diffraction patterns indicated a well crystalline orthorhombic structure of MoO3 thin film. Raman spectroscopy revealed Mo–O band bending in the range of wave number below 600 cm−1 and stretching vibration modes in the higher range. Optical characterization showed that the film exhibited strong absorption in the ultraviolet (UV) and high optical transparency across visible light regions. Energy band gap was estimated between 3.36 and 3.44 eV while Urbach energy lied between 0.42 and 0.93 eV. Electrochemical performance of the film was investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The results showed that the film yielded relatively high specific capacitance of 220 F g−1 and areal capacity of 4.94 mA h cm−2. In addition, access to more active sites with minimum ion diffusion length of the MoO3 electrode enhanced the redox behavior leading to superior rate capability and excellent long-term cycle stability on the substrate. This study presents a cost effective means of preparing nanoporous MoO3 thin film from chemical reagents. It reveals charge transport capability of MoO3 structure for optoelectronic and energy storage devices.

Major role of intrinsic defects in optical and electrical properties adopting a highly defect-controlled photoconductivity in spin-coated α-MoO3 thin films gained after UV illumination

Spin-coated MoO3 thin films were subsequently subjected to later thermal treatment (LTT) at different temperatures. The x-ray diffraction (XRD) results corroborated that the produced films crystallise in their α-phase with layer structure featured by preferential orientations along the (0k0) planes, and it was also revealed that the thermal energy gained by the later heat treatment plays a major role in enhancing crystallinity enlarging crystallite size. The optical spectroscopic analysis showed that in the visible and near-infrared regions, the average transmission of the film samples remarkably increases with increasing the LTT temperature, whereas the films prepared at 625 °C exhibits an average optical transmission of 79.92%. The optical bandgaps of the film samples were calculated to be of comparable values to the bulk one of α-MoO3 when increasing the LTT temperature from 375 to 525 °C, but it was found to be little greater than the bulk value by further increase in the LTT temperature. The DC electrical results revealed that raising the LTT temperature significantly enhances the electrical resistivity of the film samples, chiefly over the low working-temperatures. These results ascertained the realization of more than one conduction mechanism with different activation energies for the same film, and ramarkable upswings in activation energies were observed by increasing the LTT temperature. The photoconductivity (PC) analysis indicated the occurrence of various trapping processes associated with different photoexcitation energies. The PC analysis also corroborated that the highly resistive thin films exhibit much greater sensitivity to UV illumination compared with the remaining films, whereas the film prepared at the LTT temperature 625 °C presents the higher illumination current at the steady state condition exceeding the dark current value by a factor of 66.01.

A highly transparent humidity sensor with fast response speed based on α-MoO3 thin films.

Metal oxide based humidity sensors are important indicators in environmental monitoring. However, most of them are non-transparent and have a long response time, which cannot meet the application of real-time humidity sensing in transparent electronics. Here, we report a metal oxide humidity sensor based on chemically synthesized molybdenum oxide (α-MoO3) thin films. By a green reaction in an ice water bath, the stable precursor containing nanocrystalline colloids was obtained. Molybdenum oxide films with controllable morphology were fabricated through one-step spin coating. The α-MoO3 based humidity sensor exhibits extremely high transparency (85%) in the visible region and has short response and recovery times (0.97 and 12.11 s). In addition, it also shows high sensitivity, good logarithmic linearity and selectivity in a wide relative humidity range of 11% to 95%. The mechanism of humidity sensing was further studied by complex impedance spectroscopy. This novel metal oxide humidity sensor combined with high transparency and fast response speed is expected to broaden the application ranges of humidity sensors.

Photochromic Materials: Photochromic Response and Steady‐State Photoconductivity of Fibrous‐Reticulated α‐MoO3 Thin Films Prepared by Modified Spray Pyrolysis Technique (Crystal Research and Technology 10/2019)

Crystal Research and TechnologyVolume 54, Issue 10 1970028 Cover PictureFree Access Photochromic Materials: Photochromic Response and Steady-State Photoconductivity of Fibrous-Reticulated α-MoO3 Thin Films Prepared by Modified Spray Pyrolysis Technique (Crystal Research and Technology 10/2019) First published: 07 October 2019 https://doi.org/10.1002/crat.201970028AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Molybdenum oxide thin films show considerable changes in their physical and chemical properties after UV exposure. In article number 1900122, Aly M. Badr and co-workers use a modified spray pyrolysis technique for preparing α-MoO3 thin films. The thickness effect on the structural, morphological, optical, electrical, and photo-electrical properties of the films is studied. The UV exposure time effect as well as the thickness effect on the photochromic response of the samples are investigated and discussed based on the particle size analysis. Additionally, the photosensitivity is elucidated, and the mechanism of the recombination processes in these films is determined. Volume54, Issue10October 20191970028 RelatedInformation

Plasmonic fiber optic hydrogen sensor using oxygen defects in nanostructured molybdenum trioxide film

Hydrogen is one of the most promising candidates for fulfilling the next energy demands in transportation, aerospace, heating, and power generation. Due to its highly explosive nature, hydrogen leakage sensors are considered a critical industrial need. We propose a room-temperature, high-sensitivity hydrogen sensor using oxygen defect-induced plasmonic features. The proposed sensing probe utilizes nanostructured α-MoO3 thin film as the sensing material in which free carriers and plasmonic properties are induced in response to hydrogen exposure. A notable blue spectral shift of 70.6nm is observed in response to hydrogen gas exposure from 150ppm to 2000ppm, which confirms the sensor's capability for efficient detection of low hydrogen concentrations. The sensor's sensitivity, linearity, and reversibility are experimentally investigated through a simple optical setup.

Facile synthesis, characterization, and photoluminescence of lamellar α-MoO3 thin films

In this research, molybdenum trioxide (MoO3) thin films were coated on glass substrates using the spray pyrolysis technique. 0.05 M ammonium heptamolybdate tetrahydrate was used as a precursor and deionized water as a solvent. Four different samples were synthesized by varying carrier gas pressure during the spray process. The morphological, structural and optical properties of thin films were investigated. XRD results revealed the formation of α-MoO3 phase and the best crystalline quality was found for films grown at the carrier gas pressure of 2.0 bar, which exhibited a strong peak at (0 2 0). The result of XRD analysis was further confirmed by Raman spectroscopy which showed the formation of α-MoO3 thin films with the most intense peak at 818 cm−1. SEM images showed that MoO3 films have lamellar morphology. UV–Vis spectroscopy results showed that the sample prepared under the carrier gas pressure of 2.5 bar has the highest optical absorption. Bandgaps of samples were calculated to be in the range of about 3.48–3.66 eV. Synthesized samples show intense photoluminescence emission at room temperature with the four prominent peaks located at 345, 396, 418 and 436 nm in accordance with the degree of crystallinity.

The influence of photointercalaction and photochromism effects on the photocatalytic properties of electrochemically obtained maze-like MoO3 microstructures

Molybdenum oxide (α-MoO3) thin films with oriented crystalline facets were synthesised by anodization of Mo foils. The obtained samples were exposed to UV–Vis illumination in aqueous electrolytes providing different cations. The morphology and structure of modified samples were investigated. The effect of photointercalated alkali metal cations (Li+, Na+, K+) on optical and structural properties was studied using UV–vis and Raman spectroscopies, respectively. The observed energy band gap narrowing caused by the photochromic effect is found to affect the photocatalytic properties of the intercalated oxide. The photoactivity of obtained samples was tested during the photocatalytic process of methylene blue decomposition in the presence of 0.1 M Li2SO4, Na2SO4, K2SO4. After 2 h of continuous illumination, the photodecomposition efficiency of MB in the presence of K2SO4, Na2SO4 and Li2SO4 was 67%, 69% and 76%, respectively, whereas when no inorganic salt had been dissolved it reached only 57%.