Polymer-assisted Deposition Method Research Articles

Room-temperature ferromagnetic semiconductor Fe-doped β-Ga2O3 thin films with high saturation magnetization and low coercivity.

Emergent ferromagnetism in β-Ga2O3 with an ultra-wide bandwidth and high electrical breakdown strength offers exciting opportunities for fabricating robust spintronic devices. One pertinent obstacle in the material has been the low saturation magnetization, which precludes its practical application in magnetic devices. In this work, large-scale Fe-doped β-Ga2O3 diluted magnetic semiconductor (DMS) films are synthesized using a polymer-assisted deposition method, and the effect of Fe doping on their structural and magnetic properties is investigated. Remarkably, the optimal sample exhibits a high saturation magnetization (70 emu cm-3 at 300 K), much larger than those in previously reported stable oxide DMS films, as well as a low coercivity (12 Oe at 300 K). Further analysis shows that our samples manifest a typical bound magnetic polaron (BMP) model and the high saturation magnetization originates from the strong ferromagnetic coupling between the BMPs which is enhanced by Ga vacancies. The Fe-doped β-Ga2O3 thin films with high saturation magnetization and low coercivity may provide a promising platform for related semiconductor spintronics.

Enhanced Ferromagnetism Induced by Chemical Doping and Epitaxial Strain in La0.8Sr0.2CoO3 Films

The perovskite LaCoO3 and La0.8Sr0.2CoO3 thin films were synthesized successfully by a polymer-assisted deposition method. The structural characterization and thermal decomposition measurements indicate that appropriately increasing the annealing temperature is helpful in improving the film crystallinity. Compared with LaCoO3 films, the Curie temperature is enhanced to a higher temperature in La0.8Sr0.2CoO3 films, which is attributed to the changes in both the valence state and spin state induced by the chemical doping. In addition, a broad transition temperature region is observed in La0.8Sr0.2CoO3 films, revealing the existence of inhomogeneous ground states in this system.

The role of epitaxial strain on photoconductive performance in LaNiO3 epitaxial thin films

In this work, LaNiO3 epitaxial thin films on (001) single-crystal LaAlO3 and SrTiO3 substrates were deposited by polymer assisted deposition method, respectively. Additionally, LaNiO3 polycrystalline film was coated on ceramic substrate using LaNiO3 powder obtained from precursor solution in polymer assisted deposition. Due to the difference of lattice mismatch between LaNiO3 and single-crystal substrates, LaNiO3 thin films suffer different epitaxial strain, resulting in distortion of NiO6 octahedrons which increases with the intensity of strain. Such phenomena lead to the decrease of the field splitting energy of NiO6 octahedron and the band gap value of LaNiO3, further increasing the charge transition and enhancing the visible-light photoconductivity. The existence of epitaxial strain brings more Ni2+ and adsorbed oxygen into perovskite type LaNiO3, introduces more oxygen vacancy defects, reduces the complex rate of photo induced electron-hole pairs, and thereby improves the photoelectric activity of LaNiO3. This work provides a feasible method to control the structure distortion and photoconductivity effect of all-inorganic perovskite.

Visible-light photoelectric performance of RMnO3 (R = La, Pr and Nd) epitaxial films with structural distortion

All-inorganic perovskite has aroused extensive attention as the light absorption layer of solar cells due to the high photoelectric conversion efficiency and chemical stability. In this work, RMnO3 (RMO, R = La, Pr and Nd)/(001) LaAlO3 (LAO) epitaxial films were prepared by the polymer assisted deposition method. All of the RMO films were grown epitaxially with tetragonal distortion (like Jahn-Taller distortion) of MnO6 octahedron. Owing to lattice mismatch between RMO and LAO, the tetragonal distortion of MnO6 octahedron increases when the radius of trivalent R ions increases. This reduces the crystal field splitting energy of MnO6 octahedron, resulting in a reduction in the band gap and enhancement of the visible-light photoelectric performance. It also reveals that the surface-adsorbed oxygen plays a crucial role in the photoelectric process of the RMO epitaxial films. More surface-adsorbed oxygen, existing in RMO films with larger R ions, can enhance the double exchange of electrons in Mn3+ - O2− - Mn4+, thus preventing the recombination of photogenerated electron-hole pair and improving the photoelectric performance. These results provide a channel to tune the structure and photoelectric performance of perovskite manganese oxide.

Optical properties, band structures, and phase transition of UO2+x epitaxial films deposited by polymer-assisted deposition

Optical properties of the UO2+x film deposited by a polymer-assisted deposition method have been investigated by spectroscopic ellipsometry (SE). This epitaxial film contains at least two kinds of uranium oxides of U3O8 and UO3, and the O/U ratio is 2.74, which is confirmed by x-ray diffraction (XRD) and scanning Auger microscopy methods. By investigating the optical constants, the bandgaps of U3O8 and UO3 are determined as 2.3 and 1.0 eV, respectively, and 80% of the epitaxial film is U3O8 and 20% is UO3. The speciation signatures from the XRD and band structures show that the UO2+x epitaxial film reduced to U3O8 with the heating treatment at 480 K in a vacuum while oxidized to UO3 at 650 K. This work demonstrates a useful tool for studying the optical properties, band structures, and phase transition of uranium oxide film by SE.

Strain-controlled oxygen content and the cationic electronegativity in LaBaCo2O5.5+δ thin films

LaBaCo2O5.5+δ (LBCO) films were grown on (001)-oriented SrTiO3 (STO), (LaAlO3)0.3(SrAlTaO6)0.7 (LSAT), and LaAlO3 (LAO) substrates by a polymer assisted deposition method. The effects of strain induced by lattice mismatch on magnetic and electrical properties of LBCO films were studied. It is found that both the conductivity and ferromagnetic (FM) transition temperature of LBCO films show an abnormally ascending order of STO < LAO < LSAT. It is suggested that there are three main factors essentially to affect the physical properties of LBCO films, i.e., the oxygen content, cationic ordering, and cationic electronegativity. The tensile strain induced-decrease of oxygen content and lowest nanoscale ordering of cations will result in the decrease of FM interactions and conductivity and the increase of antiferromagnetic interactions. Besides, the difference of the cationic electronegativities can lead to the different shrinkages of Co3+–O and Co4+–O bonds when strain is applied, especially under compressive strain, resulting in the changes in the cobalt spin states and a charge transfer between Co3+ and Co4+ ions, altering the physical properties of LBCO films. The combination and competition of the functions of the oxygen content, cationic ordering, and the cationic electronegativity lead to an abnormal change of the conductivity and magnetization for LBCO films with substrates. This provides a new way to understand the function of electronegativity in the strain-controlled physical properties of the films.

Low-Temperature Selective Catalytic Reduction of NO x with NH3 over Mn-Ce Composites Synthesized by Polymer-Assisted Deposition.

The MnxCey binary catalysts with a three-dimensional network structure were successfully prepared via a polymer-assisted deposition method using ethylenediaminetetraacetic acid and polyethyleneimine as complexing agents. The developed pore structure could facilitate the gas diffusion and accelerate the catalytic reaction for NH3 selective catalytic reduction (SCR). Moreover, the addition of Ce is beneficial for the exposure of active sites on the catalyst surface and increases the adsorption of the NH3 and NO species. Therefore, the Mn1Ce1 catalyst exhibits the best catalytic activity for NOx removal with a conversion rate of 97% at 180 °C, superior water resistance, and favorable stability. The SCR reaction over the Mn1Ce1 catalyst takes place through the E–R pathway, which is confirmed by the in situ diffuse reflectance Fourier transform analysis. This work explores a new strategy to fabricate multimetal catalysts and optimize the structure of catalysts.

Durable Tungsten Carbide Support for Pt-Based Fuel Cells Cathodes.

In an effort to develop durable, corrosion-resistant catalyst support materials for polymer electrolyte fuel cells, modified polymer-assisted deposition method was used to synthesize tungsten carbide (WC, WC1-x), which was later used as a support material for Pt-based oxygen reduction reaction catalyst, as an alternative for the corrosion-susceptible, carbon supports. The Pt-deposited tungsten carbide's corrosion-resistance, oxygen reduction reaction electrocatalysis, and durability were studied and compared to that of Pt/C. Bulk free carbon was found to be absent from the ceramic matrix which had particle size in the range of 2-25 nm. Tungsten carbide support appears to enhance the oxygen reduction activity on Pt, showing an increase in mass activity (nearly 2-fold at 0.85 V vs RHE) and specific activity (more than 7 times higher), alongside decrease in overpotential, in comparison to Pt/C. A significant increase in durability was also observed with the tungsten carbide-based system.

Tear resistant Tyvek/Ag/poly(3,4-ethylenedioxythiophene): Polystyrene sulfonate (PEDOT:PSS)/carbon nanotubes electrodes for flexible high-performance supercapacitors

• Tyvek paper substrate applied in the supercapacitors for the first time. • High flexibility and excellent tear resistance of the supercapacitors. • Higher power density and energy density compared with CNTs-related and PEDOT:PSS-based supercapacitors. The current information era has witnessed the fast development of flexible high-performance energy storage devices for portable and wearable smart electronics. Here, tough supercapacitor with high flexibility and tear resistance based on Ag-coated Tyvek/PEDOT:PSS/carbon nanotubes (Tyvek/Ag/PCNTs) composite electrodes has been well designed and fabricated for the first time via a facile and scalable method. In the supercapacitor, Ag-coated Tyvek substrate roles as the current collector through a polymer-assisted metal deposition method while the treatment of sulfuric acid on the electrode contributes to the removal of insulating PSS part and the increase of the crystallinity of the active materials. Due to the tough and flexible substrate and the increased conductivity of the electrodes, the supercapacitor exhibits excellent stability and rate capacity as well as brilliant mechanical strength and flexibility. The prepared supercapacitor can exhibit large specific mass capacitance (138.7 F/g) and specific volume capacitance (544.2 F/cm 3 ) at the scan rate of 50 mV/s. As far as we can concern, the Tyvek/Ag/PCNTs-based supercapacitor owns higher energy density or power density than any other CNTs-based or PEDOT-related supercapacitors. In addition, after 1000 bending cycles, the capacitance of the supercapacitor can still reach to 91.2% of the initial value. This work will help to enlarge the study in the Tyvek-based supercapacitors for the flexible and tough energy storage devices.

Single Source Precursor for PAD-LaMnO3 Thin Films

A new lanthanum and manganese ethylenediaminetetraacetate (EDTA) coordination polymer (EDTA4− = [(CH2N)2(CH2-COOH)(CH2COO)4]) was synthesized from La(NO3)3 and Mn(NO3)2 reagents, ethylenediaminetetraacetic acid, and water at room temperature. The structure of the new compound formed, [{La2Mn3(EDTA)3(H2O)11}·12H2O]n, was determined by the single crystal X-ray diffraction technique. The synthesis and characterization of the La(III) and Mn(II) coordination complex, characterized by FTIR spectroscopy, thermogravimetry, and differential thermal analysis of the complex, are envisaged. X-ray crystal structure determination indicates that seven- and four-coordinate modes between La(III)/Mn(Π) and H4EDTA exist. [{La2Mn3(EDTA)3(H2O)11}·12H2O]n crystallizes in the monoclinic space group C2 with unit cell parameters of a = 16.1227(17) Å, b = 14.8049(16) Å, c = 14.8736(16) Å, and β = 116.107(2)°. Using this precursor, LaMnO3 (LMO) epitaxial thin films were grown by the polymer-assisted deposition (PAD) method on (100)SrTiO3 (STO) single crystalline substrates at a temperature of 900 °C. The LMO crystallized films exhibit a (001)LMO/(001)STO out-of-plane epitaxial relationship and a smooth surface morphology.

Structural properties, thicknesses, and qualities of plutonium oxide thin films prepared by polymer assisted deposition

Phase purity, morphology, and defect structures on plutonium oxide surfaces likely influence reactivity of plutonium, and high quality PuO2 crystalline substrates are needed to probe the effects of these properties on a model plutonium oxide surface. Indeed, different thin film thicknesses are required for conducting the range of state-of-the-art measurements available for studying these surfaces. In this Article, modification of a polymer-assisted deposition method is employed to grow various film thicknesses of PuO2 on yttria stabilized zirconia (YSZ) substrates, and measurements are carried out to characterize these films. Phase purity and epitaxial qualities of the films are assessed using thin film X-ray diffraction analysis, and the first Grazing Incidence Extended X-ray Absorption Fine Structure measurements carried out on a PuO2 thin film are reported, providing information about the short-range order of the lattice structure. Elemental purity is measured via X-ray photoelectron spectroscopy. Thicknesses and densities of the films are evaluated using neutron reflectivity, revealing that phase-pure PuO2 thin films of high epitaxial quality are formed. Micro-Raman spectroscopy suggests development of structural agglomerates of PuO2 on the surfaces, likely resulting from defects in the lattice match between the PuO2 film and the YSZ substrate.

Terahertz transmission properties of vanadium dioxide films deposited on gold grating structure with different periods

Vanadium dioxide (VO2) is a typical thermal induced phase transition material, exhibiting a transition from metallic phase at high temperature to insulating phase at low temperature, which is also accompanied by a conductivity change of over several orders of magnitude. The transition property makes VO2 prominent to achieve an effective degree of control of terahertz (THz) wave. In this paper, composite films consisting of metal grating with different periods and VO2 film were prepared by polymer assisted deposition method. Although the conductivity change of VO2 films deposited on gold grating structure across phase transition was declined to about two orders of magnitude, the amplitude modulation depth of THz of the composite films can still reach a high value. Furthermore, it was found that the THz modulation depth was related with the grating period. According to theoretical simulation, the fluctuation height of VO2 films, caused by metal grating structure during growth, can be used to regulate THz wave. These results demonstrate an economic and unsophisticated method to fabricate VO2 films with thickness fluctuation structure and then tune the THz waves.

An ultrathin MoSe2 photodetector with near-perfect absorption

An ultrathin near-perfect MoSe2 absorber working in the visible regime is demonstrated theoretically and experimentally, and it consists of a MoSe2/Au bi-layer film. The polymer-assisted deposition method is used to synthesize MoSe2 films, which can reduce the roughness and thus improve the film absorption. Simulation results show that the absorption of the absorber with 22 nm MoSe2 reaches to larger than 90% between 628.5 nm and 718 nm with a peak value up to 99.5% at 686 nm. Moreover, the measured absorption also shows near-perfect absorption of this simple absorber. Finally, an ultrathin photodetector is fabricated based on this perfect absorber and shows on/off reproducibility and remarkable photocurrent, which is three orders of magnitude higher than the dark current.

Polymer-Assisted Deposition of Gallium Oxide for Thin-Film Transistor Applications.

We report the fabrication of gallium oxide (GaOx) thin films by a novel polymer-assisted deposition (PAD) method. The influence and mechanism of postannealing temperature (200-800 °C) on the formation and properties of GaOx thin films are investigated by complementary characterization analyses. The results indicate that solution-deposited GaOx experiences the elimination of organic residuals as well as the transformation of amorphous GaOx to crystalline GaOx with the increase in annealing temperature. High-quality GaOx could be achieved with a smooth surface, wide band gap, and decent dielectric performance. Moreover, the solution-processed In2O3 thin-film transistors based on optimized GaOx dielectrics demonstrate outstanding electrical performance, including a low operating voltage of 5 V, a mobility of 3.09 cm2 V-1 s-1, an on/off current ratio of 1.8 × 105, and a subthreshold swing of 0.18 V dec-1. Our study suggests that GaOx achieved by PAD shows great potential for further low-cost and high-performance optoelectronic applications.

Rapid Epitaxial Growth of GdBa2Cu3O7-σ Films by Dilute Co Doping

The effect of dilute Co doping on the epitaxial growth of GdBCO films was investigated by self-developed fluorine-free polymer-assisted metal-organic deposition (PA-MOD) method. Under the same sintering heat treatment, dilute Co doping dramatically improved the crystallinity, microstructure, and critical current density (Jc) of GdBCO films. High Jc of 4.0 MA/cm2 at 77 K and self-field was obtained in Co-doped film after only sintering for 30 min, which is eight times of the Jc for the undoped film. Also, Co-doped GdBCO film sintered for as short as 20 min still possesses a high Jc of 1.5 MA/cm2, whereas the Jc is 0 for the similarly sintered pure film. These results suggest that dilute Co doping facilitates the epitaxial growth of the high-performance GdBCO film.

Tunable superconductivity in parent cuprate Pr2CuO4±δ thin films**Project supported by the National Key Basic Research Program of China (Grant Nos. 2015CB921000, 2016YFA0300301, 2017YFA0303003, and 2018YFB0704100), the National Natural Science Foundation of China (Grant Nos. 11674374 and 11474338), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH008), the Strategic Priority Research Program of the

We studied the role of oxygen in Pr2CuO4±δ thin films fabricated by the polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr2CuO4±δ samples were systematically investigated. It was found that with decreasing oxygen content, the low-temperature Hall coefficient (RH) and magnetoresistance changed from negative to positive, similar to those with the increase of Ce-doped concentration in CexCuO4 (R=La, Nd, Pr, Sm, Eu). In addition, we observed that the dependence of the superconducting critical temperature Tc with RH for the Pr2−xCexCuO4 perfectly overlapped with that of Pr2CuO4±δ. These findings point to the fact that the doped electrons induced by the oxygen removal are responsible for the superconductivity of the -phase parent compounds.

Polycarbonate Assisted Bi-Walled Multiscale Cathode for High-Performance Solid Oxide Fuel Cells (SOFCs) at 500 °C

As an environmentally friendly future power source, the Solid Oxide Fuel Cells (SOFCs) have several attractive advantages such as high efficiency of direct conversion from chemical energy to electric, reduced over-potential losses and an application of non-precious metal catalyst for electrodes due to high operating temperature (> 800 °C). In contrast, this high operating temperature of SOFCs results in main issues including higher system cost and rapid cell degradation rate which limits practicality of SOFCs for commercial applications such as portable device. The most recently, the electrolyte material, Er0.4Bi1.6O3 (ESB) with exceptionally high ionic conductivity (0.027 S/cm at 500 °C) was developed to lower the operating temperature. However, their applications are limited by inter-diffusion reaction with cobalt-based perovskite catalyst such as La0.6Sr0.4CoO3- δ (LSC), known as a conventional and highly active for materials for oxygen reduction reaction and severe weakness in reducing atmosphere. In this study, we suggest a novel architecturing method to utilize ESB along with LSC for superior electrochemical performance. For this, we used Ce0.9Gd0.1O1.95 (CGO) and ESB as dense electrolyte and barrier electrolyte to prevent current leakage through CGO. Anode support with a thickness of 1 mm was fabricated by tape casting method. CGO and ESB were deposited on the anode support by screen printing and polymer-assisted electrospray deposition (PA-ESD) method, respectively. To achieve the compatibility of ESB with LSC, we suggested bi-walled nanotube structure cast by polycarbonate (PC) template. Ln-doped ceria (Ln: Ga or Sm) sol spread on the PC template inner surface enter into the cylindrical hole of PC due to the capillary force. After the gelation, the Ln-doped ceria sol converges to the wall surface to form a perforated cylinder. The same procedure was repeated with LSC sol, resulting in bi-walled nanotube after sintering. In this case, Ln-doped ceria not only blocks the reaction pathway between the LSC and the ESB but can lead to improved performance from extended reaction area obtained from the multiscale structure. In conclusion, our study shows that the novel architecturing method opens the possibility of expanding the range of materials selection for low-temperature SOFCs.

Visible and near-infrared luminescent properties of Pr3+ doped strontium molybdate thin films by a facile polymer-assisted deposition process

Quartz substrate supported Praseodymium (Pr) doped strontium molybdate (SrMoO4) thin films with good uniformity and outstanding fluorescent properties are successfully fabricated via a facile polymer-assisted deposition (PAD) method. In combination with the strong chelating effect of water-soluble polymer on metal cations, the free cations without chelating are effectively ruled out but the remaining chelating metal cations are employed for the highly uniform and accurate stoichiometry luminescent SrMoO4: Pr thin films, layer-by-layer mounting on the common quartz substrates. More importantly, the excellent release of stress from polymer during the growth process of epitaxial thin film can effectively overcome the mismatch between thin film and common quartz substrate and then guarantee the quality of thin film. Under the ultraviolet (UV) light excitation, the samples show high luminescence intensity both in visible and near-infrared (NIR) regions peaked at 646 nm and 1037 nm, mainly ascribing to the transitions of 3P0 → 3F2 and 1G4 → 3H4 of Pr3+ ions. The luminescent properties can be tailored by optimizing the number of spin-coated layers and doping concentrations. The maximum emission occurs at 4 mol% of Pr3+ dopant, and it exhibits an impressive high photoluminescence quantum yield (QY) of up to 86.12%. These results evidently demonstrate the present PAD method is a useful prototype for preparing high performance luminescent thin films even on the cheap quartz substrate.

Structural and Optical Investigations of Quasi-Single Crystal Eu3+-Doped BaWO4 Thin Films.

Scheelite-structure tungstates with unique structural features and excellent luminescence possess promising applications, such as light-emitting diodes (LEDs), scintillators, and displays. The controllable growth of high quality and uniform composition thin films mounted on cheap substrates is a key factor to realize the above commercial applications, however, which is also a big challenge due to the difficult stress release stemming from intrinsic lattice mismatches. Here, we employed the simple and composition-controlled polymer-assisted deposition (PAD) method to successfully obtain a series of high quality and well-proportioned BaWO4:Eu3+ (BWOE) thin films with red emission. Screening out the unbound freedom metal ions bypoly(ether imide)(PEI) and ethylene diamine tetraacetic acid (EDTA), the firmly bound metal ions (Ba2+, W6+ and Eu3+) in polymer solution were applied to accurately control the chemical composition and effectively governed the release of stress during the growth process of BWOE thin films. Furthermore, XRD, SEM and EDS mapping detections evidently authenticated the quasi-single crystallinity, uniform morphology and well-distributed composition of as-grown thin films. Additionally, excited by 250 nm light,these thin films could efficiently produce the red emission, peaked at around 612 nm originated from the 5 D0 → 7 F2 transition of Eu3+. Moreover, the optimal doping concentration of thin films was confirmed to be 9% and corresponding Commission International de l'Eclairage (CIE) chromaticity coordinate was (0.618, 0.365), which evidently implied the excellent color rending index. Therefore, this work highlights the rather superior PAD method to prepare uniform and high-quality BWOE thin films, which can be expanded toward the other photoelectric devices including white lighted-emitting diodes, scintillators, displays, and photoelectric detectors.

Substrate-dependent structural and CO sensing properties of LaCoO3 epitaxial films

LaCoO3 thin films were grown on different (0 0 1) oriented LaAlO3, SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 by the polymer assisted deposition method, respectively. All the LaCoO3 thin films are in epitaxial growth on these substrates, with tetragonal distortion of CoO6 octahedrons. Due to different in-plane lattice mismatch, the LaCoO3 film on LaAlO3 has the largest tetragonal distortion of CoO6 octahedrons while the film grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 has the smallest tetragonal distortion. The relative contents of the surface absorbed oxygen species are found to increase for the LaCoO3 epitaxial films grown on (0 0 1) oriented (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3 and LaAlO3 substrates, sequentially. The film sensors exhibit good CO sensing properties at 150 °C, and the LaCoO3 film on LaAlO3 shows the highest response but the film on (LaAlO3)0.3(Sr2AlTaO6)0.7 shows the lowest. It reveals that the larger degree of Jahn-Teller-like tetragonal distortion of CoO6 octahedrons may greatly improve the surface absorbing and catalytic abilities, corresponding to more excellent CO sensing performance. The present study suggests that the formation of epitaxial films is an efficient methodology for controlling the octahedral distortion and thereby improving the gas sensing performance of perovskite transition metal oxides.