Molybdenum Oxide Thin Films Research Articles

Fabrication of poly(methyl methacrylate)-MoS2/graphene heterostructure for memory device application

Combination of two dimensional graphene and semi-conducting molybdenum disulfide (MoS2) is of great interest for various electronic device applications. Here, we demonstrate fabrication of a hybridized structure with the chemical vapor deposited graphene and MoS2 crystals to configure a memory device. Elongated hexagonal and rhombus shaped MoS2 crystals are synthesized by sulfurization of thermally evaporated molybdenum oxide (MoO3) thin film. Scanning transmission electron microscope studies reveal atomic level structure of the synthesized high quality MoS2 crystals. In the prospect of a memory device fabrication, poly(methyl methacrylate) (PMMA) is used as an insulating dielectric material as well as a supporting layer to transfer the MoS2 crystals. In the fabricated device, PMMA-MoS2 and graphene layers act as the functional and electrode materials, respectively. Distinctive bistable electrical switching and nonvolatile rewritable memory effect is observed in the fabricated PMMA-MoS2/graphene heterostructure. The developed material system and demonstrated memory device fabrication can be significant for next generation data storage applications.

Electrical Switching in Thin Film Structures Based on Molybdenum Oxides

We report on the experimental study of electrical instabilities in thin film structures on the basis of molybdenum oxides. Thin films of molybdenum oxide are obtained by thermal vacuum evaporation and anodic oxidation. The results of X-ray structural analysis, investigation of optical and electrical properties, are presented. It is shown that the initial vacuum-deposited oxide represents amorphous MoO3. In the MOM (metal-oxide-metal) structures with Mo oxide films obtained by the two methods, the effect of electrical switching with an S-shaped current-voltage characteristic is found. We put forward a hypothesis according to which the switching mechanism is associated with the development of electrical instability caused by the insulator-to-metal transition in Mo8O23. The switching channel, comprising this lower valence oxide, emerges in the initial film during the process of electrical forming of the MOM structure. The obtained results indicate the possibility of application of these structures in oxide micro- and nanoelectronics as electronic switches and other electronic devices.

Improved lifetimes of organic solar cells with solution‐processed molybdenum oxide anode‐modifying layers

AbstractWe report on the use and stability of solution‐processed molybdenum oxide (sMoOx) thin films as anode‐modifying layers to replace conventionally used poly(3,4‐ethyldioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) layers in poly(3‐hexylthiophene):[6,6]‐phenyl C61 butyric acid methyl ester (P3HT/PC61BM) bulk heterojunction organic solar cells. Our results show that while as prepared devices using the two anode‐modifying layers possess similar performances, the sMoOx devices exhibit a staggering 20‐fold stability improvement in its performance half‐life compared with PEDOT/PSS devices, ~3400 h versus ~150 h, respectively. A further comparison of the stability between encapsulated and unencapsulated devices demonstrates the necessity for protection from atmospheric moisture and oxygen but again highlights the lucrative nature of sMoOx as a protective anode‐modifying layer compared with PEDOT/PSS even under ambient conditions. Copyright © 2014 John Wiley & Sons, Ltd.

Temperature-resistant high-infrared transmittance indium molybdenum oxide thin films as an intermediate window layer for multi-junction photovoltaics

For optimal performance, the intermediate window layer in multijunction photovoltaics should transmit as much light as possible to guarantee maximum device efficiency. In this work, we demonstrate that indium molybdenum oxide (IMO) is a more suitable intermediate layer, compared to indium tin oxide (ITO), as it would absorb significantly less infrared light with comparable electrical conductivity once integrated into a multijunction solar cell. In fact, we show that IMO optoelectronic properties are virtually unchanged by the typical thermal budgets used in solar absorber deposition processes used in low-cost high-performance multijunction photovoltaics (e.g. CuInGaSe2). Specifically, IMO and ITO thin films were reactively sputtered onto glass substrates at 150°C, then subjected to a vacuum annealing process (550°C, 2h) identical to that of co-evaporated copper gallium diselenide (CGSe), a candidate material for the top absorber in multijunction cells. We found that annealing substantially reduces the infrared transmittance of ITO starting at 900nm, reducing by 2.5% per 100nm, while IMO only started experiencing a reduction at 1400nm and decaying more slowly at 1.6% per 100nm. Furthermore, the resistivity of IMO was comparable to that of ITO after annealing. The resilience of IMO to such high temperature processes show that it has potential to enhance the performance of multijunction devices.

Harvesting, storing and utilising solar energy using MoO3 : modulating structural distortion through pH adjustment.

Nanostructured molybdenum oxide (α-MoO3 ) thin film photoelectrodes were synthesised by anodisation. Upon band gap-excitation by light illumination, α-MoO3 is able to store a portion of the excited charges in its layered structure with the simultaneous intercalation of alkali cations. The stored electrons can be discharged from α-MoO3 for utilisation under dark conditions, and α-MoO3 is able to recharge itself with successive illuminations to behave as a 'self-photo-rechargeable' alkali-ion battery. The alteration of the anodisation pH allowed the crystal structure and oxygen vacancy concentrations of α-MoO3 to be modulated to achieve (i) a distorted MoO6 octahedra for enhanced charge separation and storage, (ii) a layered structure with a greater exposed (010) crystal face for rich and reversible ion intercalation and (iii) a highly crystalline thin film that suppresses electron-hole pair recombination. Overall, the larger MoO6 octahedral distortion in α-MoO3 at a higher pH favours charge storage, whereas smaller octahedral distortion at a lower pH leads to anodic photocurrent enhancement.

Effect of formaldehyde on the photochromic properties of ordered molybdenum oxide thin films produced by hydrothermal process

Molybdenum oxide thin films are prepared with different volume ratios of formaldehyde solution and sodium molybdate solution ranging from 0 to 0.5:10 via hydrothermal method. The results characterized from XRD and TEM demonstrate that all of the as-prepared films are MoO3 hexagonal structures. The SEM images reveal that the surface of the film formed without formaldehyde consists of hexagonal prisms, and the surface morphology of the film to be composed of fibration structures when the volume ratio of formaldehyde solution and sodium molybdate solution is 0.1:10. When increasing the volume ratio of formaldehyde solution and sodium molybdate solution, the surface morphologies of the film transformation to flower-like structures are observed. It is found that the higher the content of formaldehyde, the more complete the flower-like structures. The most ordered flower-like molybdenum oxide film which has the best photochromic performance exhibits a maximum color difference value of 8.921 when the volume ratio of formaldehyde solution and sodium molybdate solution is 0.3:10.

Synthesis and characterization of thin molybdenum oxide films prepared from molybdenum dioxo tropolonate precursors by photochemical metal-organic deposition (PMOD) and its evaluation as ammonia gas sensors

Amorphous non-stoichiometric molybdenum oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of molybdenum dioxo tropolonate complexes on Si(100) substrates. Photodeposited films were characterized by X-ray photoelectron spectroscopy (XPS) and the surface morphology examined by Atomic Force Microscopy (AFM). It was found that as-photodeposited films are uniform with rms surface roughness of 231nm and contain non-stoichiometric oxides (MoO3−x). The results of XRD analysis showed that post-annealing of the films in air at 350°C transforms the sub-oxides to α-MoO3 phase with a much smoother surface morphology (rms=164nm). Both, the as-photodeposited and annealed MoO3−x films exhibit good optical quality with transparency in the visible region better than 80%. Gas sensing tests reveal that annealed MoO3−x films exhibit a better response than as-deposited films towards 50ppm ammonia at an operating temperature of 350°C.

Ultra-thin Solution-based coating of Molybdenum Oxide on Multiwall Carbon Nanotubes for High-performance Supercapacitor Electrodes

Uniform and conformal coating of ultrathin molybdenum oxide (MoO3) thin film onto conducting MWCNTs was successfully synthesized through a facile, nontoxic and generally applicable precipitation method, followed by a simple heat treatment. The ultrathin MoO3 coating enables a fast and reversible redox reaction which improves the specific capacitance by utilizing the maximum number of active sites for the redox reaction, while the high porosity of the MWCNTs facilitates ion migration in the electrolyte and shorten the ion diffusion path. The ultrathin MoO3 coated MWCNTs electrodes show a very high specific capacitance of 1145 Fg−1 in 2M Na2SO4 aqueous solution when 5nm thick MoO3 was considered alone despite the low weight percentage of the MoO3 (16wt%). Furthermore, the ultrathin MoO3 coated MWCNTs supercapacitor electrodes exhibited excellent cycling performance of>97% capacitance retention over 1000 cycles.

Influence of O2 – plasma ambience and growth temperature on the oxidation of Mo-metal and volatilization of oxides

In this communication, we investigated role of the oxygen plasma on the oxidation of Mo strip surface (source of Mo) and volatilization of formed oxide species. Molybdenum oxide thin films are deposited on Si substrate simultaneously by using the volatilized oxide species as vapor source. A special setup is designed to create stable discharge oxygen plasma. Both the Mo strip surface and oxide films deposited on Si substrates at different temperatures are characterized independently. The presence of well indexed and intense peaks corresponding to MoO3 and Mo8O23 oxide phases in x-ray diffraction (XRD) patterns of metals strips, heated at different temperatures in oxygen plasma ambience has been considered as proof of oxidation of Mo-strip surface. The reduction in peaks intensity with increase in metal strip temperature indicates high rate of oxidation and oxide volatilization. It is observed that film formation on silicon substrate could not take place without oxygen plasma even at 500ºC. The scanning electron microscopy (SEM) studies of Mo strips as well as the films deposited on Si substrates, at different temperatures are showing the modifications in surface morphology and agreed with the XRD results. Both Raman and FTIR observations of Mo strip are also in consonance with the XRD and SEM findings. The occurrence of oxidation of Mo metal and volatilization of oxide species at this low temperature 300ºC is reported for the first time.

Characterization of reactively sputtered molybdenum oxide films for solar cell application

Molybdenum oxide (MoO3) thin films were prepared via Radio Frequency (RF) sputtering at different ambient composition and post-deposition annealing. The effects on the structural, optical, and surface properties of the deposited films were investigated. The ambient oxygen concentration O2/(O2 + Ar) was varied from 10% to 100% at 10 mTorr. Post deposition anneals were performed in Ar at 300–500 °C. The films were analyzed using glancing incidence x-ray diffraction (GIXRD), UV/Vis/NIR spectrophotometry, and x-ray photoelectron spectroscopy (XPS). As-deposited films have amorphous structures, independent of the oxygen partial pressure. Annealing at 300 °C in air resulted in crystallization of the molybdenum oxide films to the monoclinic β-MoO3 phase. Samples annealed at 400 and 500 °C were identified as pure orthorhombic α-MoO3 phase with (020) preferred orientation. High resolution XPS studies showed the presence of Mo6+ (MoO3) and Mo5+ (Mo4O11) oxidation states at the surface of as deposited and low temperature (300 °C) annealed films, and the Mo6+ to Mo5+ did not change much with deposition oxygen partial pressure. Annealing at 400–500 °C suppressed the oxygen deficiency at the surface, resulting in films with composition close to stoichiometric phases. UV/Vis/NIR spectrophotometry revealed that all films have a high optical transmittance (>80%) in the visible range, followed by a steep drop at λ ≈ 400 nm indicating a strong absorption due to band-to-band transition. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and the bandgaps in the range of 2.6 eV to 2.9 eV were obtained. Annealing at 300 °C had a negligible effect on the optical properties of the MoO3 films, but samples annealed at 400 °C and 500 °C exhibited wider bandgaps within the range of 3.1–3.4 eV.

Room-Temperature Sol–Gel Derived Molybdenum Oxide Thin Films for Efficient and Stable Solution-Processed Organic Light-Emitting Diodes

Molybdenum oxide (MoO3) thin films were prepared by sol-gel methods at room temperature from the precursors of MoO3 powder mixing into NH3 or H2O2 solution and then directly treated by UV-ozone instead of widely used high-temperature annealing. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) characteristics demonstrated that the room-temperature sol-gel derived MoO3 thin films exhibited excellent uniformity, unchanged chemical structure, and high work function. For the first time, the novel solution-processed MoO3 thin films were successfully applied as the hole injection layers (HILs) for solution-processed organic light-emitting diodes (OLEDs). The efficiencies of the resulting OLEDs were comparable or even higher than that of the device using PEDOT:PSS as the HIL. More importantly, the lifetimes of the solution-processed OLEDs are improved by nearly 2 orders of magnitude. This study should provide a potential approach to develop low-cost, high-performance, and long-lifetime OLEDs for practical applications.

Vapor-deposited hydrogenated and oxygen-deficient molybdenum oxide thin films for application in organic optoelectronics

Vapor-deposited molybdenum oxide films are used as low resistance anode interfacial layers in applications such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). A versatile method for the vapor deposition of molybdenum oxide layers is presented, which offers the control of the oxygen stoichiometry of the deposited films and their doping with hydrogen. The possibility of tuning the electronic structure of the deposited molybdenum oxides is also investigated by controlling oxygen deficiency and hydrogenation (the incorporation of hydrogen within the molybdenum oxide's lattice). To take advantage of the altered electronic properties of the non-stoichiometric Mo oxides, we embedded them as anode interfacial layers in organic optoelectronic devices. Large improvement in the operational characteristics of both electroluminescent devices and bulk heterojunction solar cells was achieved and correlated with the oxygen deficiency and the hydrogen content of the Mo oxides.

Enhanced bulk conductivity and bipolar transport in mixtures of MoOx and organic hole transport materials

We study the conductivity of thin films of molybdenum oxide (MoOx) mixed with an organic hole transport material, such as N,N′-bis(naphthalen-1-yl)-N,N′-bis (phenyl)benzidine or 4′,4″-tri(N-carbazolyl)triphenylamine, in lateral test devices. Contrary to previous reports, the conductivity of the mixture is found to exceed that of neat MoOx, exhibiting ~5 orders of magnitude higher conductivity in comparison to the neat films. Studies also show that the mixing enhances both hole and electron transport. The higher conductivity may be attributed to a higher concentration of “free” carriers in the mixture, as a result of the formation of a charge transfer complex between the MoOx and the hole transport material. The findings shed light on the potential of hybrid composites of inorganic and organic materials in realizing enhanced conductivity.

Structure induced conductivity enhancement in metal-doped molybdenum oxide thin films

We study doped molybdenum oxide (MoO3) thin films as thickness-tunable transport layers in organic photovoltaics. In an illustrative device, we show that the introduction of thick pristine MoO3 layers is unfavorable for device operation due to an increased series resistance, but this can be overcome by doping the transport layer with small amounts of metal. However, as opposed to a conventional doping mechanism, we ascribe the doping-induced conductivity enhancement to originate from an improved crystallinity of MoO3. This latter effect appears to be an example of metal induced crystallization. These flat, transparent, and conductive MoO3 layers are ideal candidates for high work function transport layers in several architectures of organic or other thin film optoelectronic devices and we demonstrate this by its use in tandem organic photovoltaics.

Work function recovery of air exposed molybdenum oxide thin films

We report substantial work function (WF) recovery of air exposed molybdenum oxide thin films with vacuum annealing. We observed a sharp reduction in the MoOx WF (from 6.8 eV to 5.6 eV) as well as a very thin layer of oxygen rich adsorbate on the MoOx film after an hour of air exposure. The WF of the exposed MoOx film started to gradually recover with increasing annealing temperature in vacuum, and the saturation in the WF recovery was observed at 450 °C with WF ∼6.4 eV. We further studied the interface formation between the annealed MoOx and copper phthalocyanine (CuPc). The highest occupied molecular orbital (HOMO) level of CuPc was observed to be almost pinned to the Fermi level, strongly suggesting the possibility of efficient hole injection with the vacuum annealed MoOx film.

The Effect of Substrate Temperature on the Optical Properties of MoO3 Nano-crystals Prepared Using Spray Pyrolysis

In this study, thin films of molybdenum oxide were prepared by spray pyrolysis technique on glass substrates. The influence of substrate temperature on their crystallographic structure, surface morphology, and optical properties was studied. The formation of a MoO3 film on the substrate was confirmed through XRD analysis. Furthermore, the presence of the two phases α and β in each of the films was evident. The percentage of phase α varied from 55 % for the film deposited at 200 °C up to 97 % for the film deposited at 400 °C. According to SEM images, MoO3 films have a sponge-type structure on the order of nanometers. Both the optical gap and the refraction index strongly depend on substrate temperature. The optical gap decreases from 3.63 eV for the film deposited at 150 °C up to 3.30 eV for the one prepared at 400 °C. On the contrary, the refraction index measured at 800 nm increases from 1.54 up to 1.61 for the films prepared at 150 °C and 400 °C, respectively.

Effect of oxygen partial pressure and Fe doping on growth and properties of metallic and insulating molybdenum oxide thin films

We report the effect of oxygen partial pressure (OPP) and 5% Fe doping on the structural, electrical, and magnetic properties of MoOx thin films on c-Al2O3 substrate prepared by pulsed laser deposition technique. Detailed analyses of the structural properties suggest that the grown phase of molybdenum oxides and its orientation strongly depend on the OPP as well as Fe doping. Undoped and Fe doped α-MoO3 films formed at 350 mTorr OPP value show insulating character, where as MoO2 films formed at lower OPP values reveal metallic behavior. Resistivity minima are observed in Fe doped MoO2 films, which could be due to weak localization effect or Kondo scattering of the conduction electrons from the Fe impurities. Interestingly, all the Fe doped molybdenum oxide films show magnetic hysteresis at room temperature irrespective of their insulating (MoO3 phase) or metallic (MoO2 phase) behavior.

Improvement of electrical and optical properties of molybdenum oxide thin films by ultralow pressure sputtering method

In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7 × 10−1 to 6.7 × 10−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450 °C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ω cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7 × 10−2 Pa) and O2 content of 40%, had an atomic ratio of O to Mo ≈ 2.85 and was highly transparent and conductive: the transmittance in the visible wavelength range of 400–500 nm was about 73% and the resistivity was 1.05 × 10−3 Ω cm. This result is superior to those of MoOx films epitaxially grown by the pulse laser deposition method.

Preparation and Characterization of Transparent Conducting Molybdenum Oxide Thin Films

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Deposition of Transparent Indium Molybdenum Oxide Thin Films and the Application for Organic Solar Cells

In this study, we systematically investigate the effects of post-annealing on the structural, electrical, and optical characteristics of indium molybdenum oxide (IMO) films. The incorporation of IMO films into organic solar cells (OSCs) is further studied. The optimum content of Mo is 2.36 wt %. Furthermore, a significant improvement in crystallinity, surface morphology, electrical resistivity, and optical transmittance is observed after thermal annealing. The lowest resistivity is obtained with post annealing at 300 °C for 140 s. The OSC utilizing the IMO electrode shows an efficiency of up to 3.77%, which is higher than that of the OSC utilizing the ITO electrode. The diffusion length of the carriers in the OSC is approximately 10 nm. Thus, the higher efficiency can be explained by the smoother surface morphology of the post annealed IMO electrode. These results indicate that the novel IMO films with superior material properties have enormous potential applications for solar cells.