Nb Doping Concentration Research Articles

Donor Defect Induced Ferromagnetism in Nb‐Doped ZnO Thin Films Grown by RF Magnetron Sputtering

The effect of Nb doping concentration (0, 1, 2, and 4 at. %) on donor defect‐induced ferromagnetism in ZnO thin films is investigated. The films are deposited on Si(111) substrates utilizing the radio frequency magnetron sputtering. X‐ray diffraction pattern unveils that the films show a pronounced orientation along the (002) direction. The relative intensities of defect‐related bands with that of the ultraviolet band from photoluminescence (PL) spectra show that 2 at. % Nb doping results in a greater number of donor defects (Zni+ and VO+) in the ZnO lattice. The parameters extracted from the electron paramagnetic resonance spectra follow a similar trend. The results from vibrating sample magnetometer measurement indicate that pure ZnO displays diamagnetic nature, whereas Nb‐doped ZnO exhibits a ferromagnetic nature. The saturation magnetization value is found highest for 2 at. % Nb doping, which correlates with the presence of a greater number of donor defects, as supported by the PL and electron paramagnetic resonance results. Images obtained from atomic force microscopy show that the surface roughness of the ZnO thin film reduces upon Nb doping. X‐ray photoelectron spectroscopy validates that Nb is doped in 2 at. % Nb‐doped ZnO thin film with Nb oxidation state of +5.

Effect of Nb Doping on the Electrical Contact Properties of AgNi Contact Materials

AgNi contact materials have received widespread attention with the acceleration of the process of replacing AgCdO contact materials. However, the practical applications of AgNi contact materials are limited due to its disadvantage of poor resistance to melting welding. Firstly, following the first principles of the density functional theory, we simulated and tested an interfacial model of AgNi doped with varying amounts of Nb. Next, we fabricated AgNi electrical contact materials. Subsequently, we conducted electrical contact tests. Finally, the impact of Nb doping on the arc erosion behavior of AgNi electrical contact materials was analyzed. The results indicate that, with an increase in Nb doping content, the electrical contact performance and the degree of arc erosion exhibit a trend of initially decreasing and then increasing, which aligns with the simulation results. The mean values of arc energy, arc duration, and welding force for the material doped with 4.55% Nb were 181.02 mJ, 9.43 mS, and 38.45 cN, respectively. Moreover, the anode is more responsive to changes in Nb content compared to the cathode. The introduction of Nb enhances the viscosity of the molten pool in the AgNi electrical contact. Furthermore, the mechanisms of grain boundary strengthening and solid solution strengthening by Nb improve the weld performance resistance of the contact.

Enhancing thermoelectric performance of n-type AgBi3S5 through synergistically optimizing the effective mass and carrier mobility

AgBi3S5 is a new n-type thermoelectric material that is environmentally friendly and composed of elements of earth-abundant, non-toxic and high performance-cost ratio. This compound features an intrinsically low thermal conductivity derived from its complex monoclinic structure. However, the terrible electrical transport properties greatly limited the improvement of thermoelectric performance. Most previous studies considered that carrier concentration is the main reason for low electrical conductivity and focused on improving carrier concentration by aliovalent ion doping. In this work, we found that the critical parameter that restricts the electric transport performance of AgBi3S5 was the extremely low carrier mobility instead of the carrier concentration. According to the Pisarenko relationships and density functional theory calculations, Nb doping can sharpen the conduction band of AgBi3S5, which contributes to reducing the effective mass and improving the carrier mobility. With a further increase of the Nb doping content, the conduction band convergence can enlarge the effective mass and preserve the carrier mobility. Combined with the decrease in lattice thermal conductivity due to the intensive phone scattering, a maximum ZT value of ∼0.50 at 773 K was achieved in Ag0.97Nb0.03Bi3S5, which was ∼109.6% higher than that of pure AgBi3S5. This work will stimulate the new exploration of high-performance thermoelectric materials in ternary metal sulfides.

First-principles investigation on structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2

The structural and photoelectric properties of (Nb, Ta) codoped anatase TiO2 were studied by first-principles calculations based on density functional theory and the Hubbard U correction. The results demonstrated that hybridized states composed of Ta 5d and Nb 4d orbitals at the bottom of the conduction band enabled the Fermi level to enter the conduction band and facilitated (Nb, Ta) codoped anatase TiO2 to exhibit n-type metallic characteristics, and the conductivity of Ti0·875Nb0·0625Ta0·0625O2(I) is approximately 3.2 times that of Ti0·9375Nb0·0625O2. Noted that both conductivity and mobility decreased with the increase of the distance between impurity Nb and Ta atoms and the Nb doping concentration. Moreover, the calculated results of optical properties presented the shifts of the absorption edges towards the ultraviolet region, and (Nb, Ta) codoped anatase TiO2 possessed the high transmittance thanks to its low absorption and reflection of visible light.

Synthesis, characterization and dual-band electrochromic properties of Nb-doped WO3 films

• Nb doped WO 3 film were synthesized by a facile sol gel dip coating method. • Nb doped WO 3 film shown independent modulation property in visible and NIR regions. • Nb doped WO 3 film has LSPR and redox electrochromic processes. • The film shown “bleached and warm” - “bleached and cool” - “colored and cool” modes. Nb-doped WO 3 films were prepared on FTO substrates by a facile sol–gel dip coating method. The influence of Nb doping on the microstructure and electrochromic characteristics of the films have been explored. The results indicate that the pure WO 3 film can achieve a synchronizing modulation of visible and near-infrared light bands by suppling voltage. However, the Nb-doped WO 3 films exhibit an ability of independent and selective transmittance modulation in both bands. Moreover, the Nb-doped WO 3 films undergo two electrochromic processes at different voltages, which are local surface plasmon resonance (LSPR) electrochromic process and electrochemical redox electrochromic process, respectively. These two electrochromic processes exhibit a unique photoelectric conversion behavior, that is, the reversible “bleached and warm” - “bleached and cool” - “colored and cool” modes. Additionally, with the increase of Nb doping content, the LSPR electrochromic effect becomes stronger. The high LSPR enhances the electrochemical redox electrochromic performance of the Nb-doped WO 3 films, enabling them to exhibit better electrochromic performance in the visible and near-infrared bands.

Nb-doped TiO2 mesoporous films with improved electrochemical and electrochromic energy storage performance

The development of high-quality cathode material is of important value to the research of bifunctional electrochromic energy storage device. Therefore, Nb-doped TiO2 mesoporous film as the high-performance cathode was prepared via a facile sol-gel technology successfully. The Nb doping content has great effects on the crystallinity and chemical state of TiO2 films. Compared to the pure TiO2 film, the optimum Nb-doped TiO2 film exhibits the excellent optical-electrochemical performance including highest large-rate capability, largest specific capacity (62.4 mAh/g), short switching time (3.4 s/4.5 s) and large transmittance modulation (50.7%). These data indicate that Nb-doped TiO2 mesoporous films possess the important application prospect.

Effect of Nb doping on the structural, optical, and photocatalytic properties of SrTiO3 nanopowder synthesized by sol-gel auto combustion technique

ABSTRACT Metal oxide photocatalyst is a promising wastewater treatment process due to its simplicity, high efficiency, and low cost, as well as being environmentally friendly and non-energy consuming. The perovskite structure material, SrTi1− x Nb x O3 (STNO) with x = 0, 0.01, 0.03, and 0.05, was successfully synthesized by the sol-gel auto-combustion method. Samples with a pure phase and cubic perovskite structure were obtained. The Rietveld refinement results showed that the lattice parameter and unit cell density increased as the Nb-doping level increased. The particle size of the STNO decreases with increasing Nb doping content. Photoluminescence was excited at 327 nm, producing violet, green, and blue emissions, while the Nb content asserts an insignificant effect on the bandgap (Eg ). The Nyquist plot and Mott-Schottky analysis were used to determine the photoelectrode performance of the STNO samples. The photocatalytic activity of STNO on the decolorization of methylene blue (MB) under UV irradiation increased with increasing Nb content and optimization at x = 0.05, corresponding to the results obtained from photoluminescence, Eg , Nyquist plot, and Mott-Schottky analyses.

Effects of Nb concentration on Nb-doped anatase TiO2: DFT + U calculations

The crystal structure, formation energy, electronic structure, electrical properties and optical properties of anatase TiO2 with various Nb concentrations were studied by first-principles calculations based on density functional theory (DFT) and the Hubbard U correction. Firstly, the crystal structures of TiO2 with various Nb concentrations were optimized successfully. The higher concentrations of Nb facilitate the synthesis of Nb-doped TiO2 systems in an O-rich environment. Furthermore, the band structures and density of states prove that the electronic structure of Nb-doped TiO2 is mainly transformed by the hybridization of Ti 3d, Nb 4d and O 2p states. Next, broadening of the valence band and occupied states at the bottom of the conduction band enhance the conductivity of Nb-doped TiO2, and the conductivity reaches the maximum value at the high Nb-doping concentration of 6.25 at.%. Last but not least, the calculated optical properties show that the absorption edge of Nb-doped TiO2 produces an blue shift. TiO2 with Nb-doping concentration below 14.58 at.% exhibits the high transmittance in the visible region.

First-principles study of the influence of Nb doping on the electronic structure and optoelectronic properties of β-Ga2O3

The electronic structure and optoelectronic properties of Nb-doped β-Ga2O3 were investigated by using plane wave ultrasoft pseudopotential generalized gradient approximation +U (GGA + U) method based on density functional theory. Four supercell models, namely, Ga23O36Nb1, Ga32O48, Ga39O60Nb1, and Ga47O72Nb1, were constructed. Results show that the formation energy of the doped system is lower and has higher stability under Ga-rich conditions than in O-rich conditions. With the increment in Nb doping concentration, the formation energy and lattice constant gradually increase, the stability of the system gradually decreases, and the band gap of the system gradually narrows. As the doping concentration increase, the absorption intensity increases. Furthermore, the increasing doping concentration accelerates the separation of holes and electrons, prolongs the carrier lifetime, and enhances conductivity. These conclusions provide a certain theoretical basis for the wide application of β-Ga2O3.

Nb-doped VO2 thin films with enhanced thermal sensing performance for uncooled infrared detection

VO2 thin films are widely studied materials for uncooled infrared detection because of their high temperature coefficient of resistance. In this research, Nb-doped VO2 films were synthesized with the expectation that Nb doping would reduce the metal-insulator-transition temperature via electron doping and strain effects caused by the relatively large ionic radius of Nb, thus improving the resistance temperature coefficient of the doped films. The doped films displayed superior thermal sensing abilities compared with pure VO2 films. As the amount of Nb doping increased, the thermal hysteresis and resistivity of the doped films decreased, while the transition temperature range widened. When the Nb-doping content reached 10%, the temperature coefficient of resistance reached -3.2%/K, while the metal-insulator transition temperature dropped to 52.2 °C. The high performance of the doped films proved that the strategy of Nb doping in VO2 was promising for thermal-sensing applications.

Photoelectrochemical properties of butane flame-treated niobium-doped hematite thin films grown by the liquid-phase deposition method

In this study, thin films of undoped and Nb-doped hematite were grown on FTO-coated glass substrates via the liquid-phase deposition method as photoanodes for PEC water splitting applications. The samples were then annealed and flame-treated for 90 s with a butane flame. At all doping concentrations, the thinnest films (~200 nm) yielded the best properties. FESEM images showed that film morphology is not dependent on Nb-doping concentration. Compared with the undoped hematite film, the optimum Nb-doped sample showed an approximately five-fold increase in photocurrent density at 1.6 V vs. RHE and a ~0.3 eV decrease in energy band gap value. The charge carrier density value of the optimum Nb-doped sample was shown to increase from 1.38 × 1017 to 3.54 × 1018 cm-3 after flame treatment due to the introduction of oxygen vacancies. The number of times the reducing butane flame is applied was shown to affect the PEC performance, both positively and negatively, giving rise to an approximately sixteen-fold increase in photocurrent density at 1.6 V vs. RHE in comparison with the untreated sample after four times of application. Butane flame treatment was also shown to facilitate the process of charge transfer in Nb-doped hematite and its interface with the electrolyte.

Novel niobium-doped titanium oxide towards electrochemical destruction of forever chemicals

Electrochemical advanced oxidative processes (EAOP) are a promising route to destroy recalcitrant organic contaminants such as per- and polyfluoroalkyl substances (PFAS) in drinking water. Central to EAOP are catalysis-induced reactive free radicals for breaking the carbon fluorine bonds in PFAS. Generating these reactive species electrochemically at electrodes provides an advantage over other oxidation processes that rely on chemicals or other harsh conditions. Herein, we report on the performance of niobium (Nb) doped rutile titanium oxide (TiO2) as a novel EAOP catalytic material, combining theoretical modeling with experimental synthesis and characterization. Calculations based on density functional theory are used to predict the overpotential for oxygen evolution at these candidate electrodes, which must be high in order to oxidize PFAS. The results indicate a non-monotonic trend in which Nb doping below 6.25 at.% is expected to reduce performance relative to TiO2, while higher concentrations up to 12.5 at.% lead to increased performance, approaching that of state-of-the-art Magnéli Ti4O7. TiO2 samples were synthesized with Nb doping concentration at 10 at.%, heat treated at temperatures from 800 to 1100 °C, and found to exhibit high oxidative stability and high generation of reactive oxygen free radical species. The capability of Nb-doped TiO2 to destroy two common species of PFAS in challenge water was tested, and moderate reduction by ~ 30% was observed, comparable to that of Ti4O7 using a simple three-electrode configuration. We conclude that Nb-doped TiO2 is a promising alternative EAOP catalytic material with increased activity towards generating reactive oxygen species and warrants further development for electrochemically destroying PFAS contaminants.

Carrier recombination in SrTiO3 single crystals: impacts of crystal faces and Nb doping

We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.

Wafer-Scale Synthesis of WS2 Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition.

Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS2 films and in situ Nb-doped p-type WS2 films were synthesized through atomic layer deposition (ALD) on 8-inch α-Al2O3/Si wafers, 2-inch sapphire, and 1 cm2 GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS2 n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS2 n-FET are as high as 105 and 6.85 cm2 V−1 s−1, respectively. In WS2 p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm2 V−1 s−1, respectively. The p-n structure based on n- and p- type WS2 films was proved with a 104 rectifying ratio. The realization of controllable in situ Nb-doped WS2 films paved a way for fabricating wafer-scale complementary WS2 FETs.

Effects of Nb-doping on the mechanical properties and high-temperature steam oxidation of annealing FeCrAl fuel cladding alloys

Abstract FeCrAl alloys have been developed as one of the promising candidates of accident tolerant fuel (ATF) cladding materials for light water reactor (LWR) to replace zirconium-based alloys. In this work, different contents of Nb (0.5, 1.0, and 1.5 wt% Nb) are doped into the Fe-13.5Cr-4.5Al–2Mo (in wt.%) alloy to develop three Nb-doping FeCrAl alloys and the Nb-free alloy is fabricated for comparison. Effects of Nb-doping content on the mechanical properties (at room temperature and 800 °C) and high-temperature (1200 °C) steam oxidation (HTSO) were investigated. Although annealing temperature increased, the grain size decreased with the increase of Nb content. The volume fraction of Fe2Nb-type Laves precipitation increased with the increment of Nb content. The strength of FeCrAl alloys at room temperature and 800 °C were both enhanced by Nb-doping. At room-temperature, Nb-doping improved the mechanical properties through grain matrix strengthening, precipitation strengthening, grain refining strengthening, and dislocation strengthening, according to the theoretical calculation analysis. On the other hand, the precipitation of the Fe2Nb phase and the solution of Nb collectively improved the yield strength of the FeCrAl alloy at 800 °C. The weight gain rate and the spallation of Al2O3 scales in high-temperature (1200 °C) steam oxidation both increased with the increment of Nb content. Discontinuous Nb oxides at the interface of Al2O3 scales and matrix formed in the Nb-doping FeCrAl alloys, which may be caused by the limited Nb content and/or Nb oxides (Nb2O5) reacted with Al2O3. Besides, Nb oxides also enhanced the stress of Al2O3 scales, resulting in the promotion of cracks and the peeling of Al2O3 scales. Last but not least, 0.5 wt% Nb content was advising for FeCrAl alloys by considering the balance of mechanical properties and HTSO.

Effects of Nb doping on switching-voltage stability of zinc oxide thin films

Nb-doped ZnO (NbxZn1−xO, NZO) thin films with various Nb additions (x = 0, 0.2, 0.5, and 0.8 at. %) were deposited on Pt/TiO2/SiO2/Si substrates by radio frequency magnetron sputtering. The Nb doping concentration was found to affect the microstructure, the number of oxygen vacancies, and work function of the Pt/NZO/Pt structures. Among the various devices, the film with 0.5 at. % Nb addition showed a better switching-voltage stability [i.e., the optimal coefficient of variation (Cv) for reset (7.02%) and set (2.73%) operations, respectively], a high endurance (∼1000 cycles), and lower reset (0.57 V) and set (1.83 V) voltages due to a larger number of oxygen vacancies and a lower work function. In general, the results show that the present NZO thin films are promising candidates for stable and low power-consumption resistive random access memory applications.

Palladium-niobium bimetallic doped organosilica membranes for H2/CO2 separation

Abstract In this work, high-performance palladium-niobium doped 1,2-bis(triethoxysilyl)ethane (Pd–Nb-BTESE) membranes were prepared for H2/CO2 separation by using the sol-gel method. Nb was first incorporated into the organosilica networks in the form of Nb–O–Si covalent bonds, while Pd nanoparticles were successfully embedded into the networks with size of 5–10 nm. Based on the synergetic effects of Pd–Nb doping, the as-prepared membranes showed more appropriate structures for H2/CO2 separation. By controlling the same doping amount of Nb and changing Pd doping contents (n (Pd): n(Si) = 0.05–0.4), we found that the Pd–Nb-BTESE membrane with higher Pd doping content showed higher H2/CO2 permselectivity and stronger H2 adsorption capacity. The H2/CO2 separation performance of all the prepared Pd–Nb-BTESE membranes surpass the upper bound 2008. The membrane with n (Pd): n (Si) of 0.4 showed a superior H2/CO2 permselectivity of 107 and a high H2 permeance of 1.12 × 10−7 mol m−2 s−1·Pa−1. Our findings provide novel insights into tailoring and functionalizing organosilica membranes through bimetallic doping for practical applications.

Resistive switching characteristics of Pt/Nb:SrTiO3/LaNiO3 heterostructure

Herein, Pt/Nb:SrTiO3/LaNiO3(Pt/NSTO/LNO) heterostructures are prepared by the sol–gel method, and the resistive switching (RS) characteristics of NSTO films are studied under different preparation conditions and Nb-doping content. It is shown from SEM images that the grain size of NSTO decreases with the increase of Nb-doping content. It is observed from electrical tests and analysis that the RS phenomenon of the Pt/NSTO/LNO structure can be attributed to the migration of internal oxygen vacancies, which are formed during the material preparation. J–V curves of Pt/NSTO/LNO, prepared under different heat treatment atmospheres, exhibit that pure nitrogen atmosphere results in optimal RS performance. In addition, the RS performance has shown an increasing trend with Nb content up to a threshold limit, followed by a gradual decrease. Furthermore, the RS performance of Pt/NSTO/LNO, prepared at different heat treatment temperatures, was investigated. The results demonstrate that the hysteresis window of RS for the Pt/NSTO/LNO structure first increased with increasing temperature, followed by a decrease. Moreover, the optimal RS performance is achieved at the heat treatment temperature of 700 °C.

Thermal stability study of niobium doped SnO2 thin film for transparent conducting oxide application

Abstract The temperature-dependent electrical property of niobium (Nb) doped SnO2 (NTO) films were investigated for optoelectronic applications. Temperature-dependent electrical properties of the films show two different conduction processes between 200 °C and 450 °C, as calculated by Arrhenius approaches. In the metal oxide system increase in temperature may be possible to reduce oxygen formation energy, this leads to create more oxygen vacancies. The conductivity of NTO film increased up to 250 °C due to thermally generated free carrier concentration. The electrical conductivity decreased beyond 250 °C due to decreased free carrier mobility because of rapid defect formation at higher temperatures. Generally, free carrier collision, charge carrier absorption by native oxygen, and ionized impurity scattering are predominant factors which affect the mobility of free carrier at different temperature ranges. More importantly, the NTO film deposited at 1.5 wt % of Nb doping concentration in SnO2 system shows better optical transmittance and electrical resistivity than other films. The maximum resistivity of 6.45 × 10−4 Ω cm, sheet resistance of 25.94 Ω/□, optical transmittance of 82%, and thermal stability of 250 °C indicates the suitability of NTO thin films for potential applications in optoelectronic devices.

Improved high rate performance and cycle stability for LiNi0.8Co0.2O2 by doping of the high valence state ion Nb5+ into Li+ sites

High rate performance has been a challenging issue for LiNi0.8Co0.2O2 material. Elemental doping is a very effective method that has been used to maintain the structure of cathode materials with high stability and improve the high rate performance. Encouraged by previous research and considering the shortcomings of LiNi0.8Co0.2O2, materials with a composition of Li1-xNbxNi0.8Co0.2O2 (x = 0, 0.01, 0.03) were prepared by co-precipitation and the solid phase sintering method. The structure and electrochemical performance were studied in detail. The results from structural analysis suggested that the doping element was successfully doped into LiNi0.8Co0.2O2. Electrochemical measurements suggested that high rate capacities led to distinct improvements for a moderate Nb-doping content. Specifically, the initial capacities delivered by LiNi0.8Co0.2O2 and Li0.99Nb0.01Ni0.8Co0.2O2 increased from 97 to 156 mAh/g at 25 °C and 62.1 to 144.7 mAh/g at 50 °C at a rate of 5 C. In addition, the results from differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis demonstrated that the Nb-doped LiNi0.8Co02O2 had a higher thermal stability in the charged state compared to the un-doped material. Therefore, the Li+ sites in LiNi0.8Co0.2O2 were partially substituted by the high valence element Nb, which can lower Li/Ni mixing and polarization, accelerate the migration rate of Li+ and stabilize the structure of the cathode material, thus improving the high rate performance and cycling stability.