Influence Of Nitrogen Doping Research Articles

Band Structure Engineering: Insights from Defects, Band Gap, and Electron Mobility, from Study of Magnesium Tantalate

Anion doping of semiconductors with nitrogen is a strategy often adopted to narrow the band gap of semiconductors and increase the range of light absorption. However, the influence of nitrogen doping on the electron mobility in the semiconductor is not fully understood and characterized. In this work, we used magnesium tantalate MgTa2O6 as a model system and hybrid density-functional theory calculations to characterize the mobility of electrons using the small polaron model in the presence of nitrogen-doping defects as well as oxygen-vacancy defects. We found that electron mobility is not significantly affected when MgTa2O6 is doped with a molar ratio N/O of ∼2%. However, in the presence of oxygen vacancies combined with nitrogen doping with the same molar ratio N/O of ∼2%, the barrier to electron hopping in the vicinity of the defects is much lower than that in pristine MgTa2O6 and in MgTa2O6 with oxygen-vacancy defects only. These results suggest that nitrogen doping combined with anion vacancy not only...

Influence of nitrogen doping on properties of NiO films

Undoped and nitrogen (N) doped nickel oxide (NiO) thin films were deposited onto glass substrates using a simplified spray pyrolysis technique with two different doping levels of N (10 and 20 at-%). The effects of N doping level on the structural, electrical and optical properties were studied and reported. From the structural studies, it is found that the preferential orientation changed from (111) to (200) for higher concentrations. The optical studies revealed that the average optical transmittance and optical band gap decreased (from 3·5 to 3·4 eV) due to N doping. The electrical resistivity decreased by one order (from 3·84E+03 Ωcm to 5·38E+02 Ωcm) in the case of higher doping of N. The defects generated by N doping were analysed using the photoluminescence studies. The size of the grains decreased due to N doping as seen from SEM studies.

Influence of Nitrogen Doping on Device Operation for TiO₂-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices.

Solid-state dye-sensitized solar cells (ssDSSC) constitute a major approach to photovoltaic energy conversion with efficiencies over 8% reported thanks to the rational design of efficient porous metal oxide electrodes, organic chromophores, and hole transporters. Among the various strategies used to push the performance ahead, doping of the nanocrystalline titanium dioxide (TiO2) electrode is regularly proposed to extend the photo-activity of the materials into the visible range. However, although various beneficial effects for device performance have been observed in the literature, they remain strongly dependent on the method used for the production of the metal oxide, and the influence of nitrogen atoms on charge kinetics remains unclear. To shed light on this open question, we synthesized a set of N-doped TiO2 nanopowders with various nitrogen contents, and exploited them for the fabrication of ssDSSC. Particularly, we carefully analyzed the localization of the dopants using X-ray photo-electron spectroscopy (XPS) and monitored their influence on the photo-induced charge kinetics probed both at the material and device levels. We demonstrate a strong correlation between the kinetics of photo-induced charge carriers probed both at the level of the nanopowders and at the level of working solar cells, illustrating a direct transposition of the photo-physic properties from materials to devices.

Effect of nitrogen doping on structural and optical properties of ZnO nanoparticles

Influence of nitrogen doping on structural and optical properties of ZnO nanoparticles has been studied. Undoped and N doped ZnO nanoparticles were synthesized via chemical precipitation approach. The prepared samples were characterized through X-ray diffraction (XRD), Transmission electron microscopy (TEM) equipped with Energy dispersive X-ray (EDAX) spectroscopy, UV–visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy (µRS). Wurtzite phase of undoped as well as 0.5–10% N doped ZnO nanoparticles was confirmed through characteristic XRD patterns. The particle size expansion due to N incorporation in ZnO was further revealed by TEM and EDAX analysis where 11nm size undoped and 18–22nm size 0.5–10% N doped ZnO (N:ZnO) nanoparticles without any impurity were ascertained. Slight blue-shift in band gap energy, as observed in our case, symbolized weak quantum confinement of the prepared nanoparticles. The alterations in vibrational modes of ZnO due to N incorporation, remarkably H substituting at O site and subsequently causing the passivation in N:ZnO nanoparticles, were detected through FTIR analysis. Finally, the effect of the nano-size of crystallite and gradual prominence of N into ZnO lattice due to increase of N doping concentration in prepared nanoparticles was meticulously expatiated though µRS analysis.

Influence of Nitrogen doping on the Catalytic Activity of Ni-incorporated Carbon Nanofibers for Alkaline Direct Methanol Fuel Cells

In this study, the influence of nitrogen doping on the electrocatalytic activity of carbon nanofibers incorporated with nickel nanoparticles toward methanol oxidation is introduced. The modified carbon nanofibers have been synthesized from calcination of electrospun nanofiber mats composed of nickel acetate tetrahydrate, poly(vinyl alcohol) and urea in argon atmosphere at 750°C. The utilized physicochemical characterizations indicated that the proposed strategy leads to form carbon nanofibers having nickel nanoparticles and doped by nitrogen. Formation of pure nickel can be attributed to the abnormal decomposition of the acetate anion which leads to form strong reducing gases (CO and H2), the formed nickel catalyzes graphitization of the utilized polymer to form carbon nanofibers. Investigation of the electrocatalytic activity indicated that nitrogen doping strongly enhances the oxidation process of methanol as the current density increases from 52.5 to 198.5mA/cm2 when the urea content in the original electrospun solution was 4 wt% urea. Moreover, the nanofibrous morphology exhibits distinct impact on the electrocatalytic activity. Also, nitrogen-doping enhanced the stability of the introduced Ni-based electrocatalyst.

Synthesis of high carbon content microspheres using 2-step microwave carbonization, and the influence of nitrogen doping on catalytic activity

In the present work, nitrogen-containing carbon particles were synthesized by a two-step microwave carbonization. Nitrogen was introduced into the carbon particles via the addition of urea solution during microwave-assisted hydrothermal carbonization. The nitrogen content was easily controlled by varying the urea concentration. Carbon microspheres of over 90% carbon content by atomic ratio were obtained using the microwave-assisted hydrothermal carbonization of glucose solutions, followed by the secondary microwave treatment of the dried samples. The effects of further microwave irradiation on the oxygen reduction reaction catalytic activity of nitrogen containing carbon particles were studied. This study showed that microwave-assisted hydrothermal carbonization, followed by additional microwave treatment can produce oxygen reduction reaction catalytic active carbon materials which contain nitrogen as catalytic active sites in conducting hexagonal layers of carbon structures. It is meaningful that carbon materials are obtained from environmental friendly starting materials biomass derivatives, under mild reaction condition.

Influence of nitrogen doping on carbon nanotubes towards the structure, composition and oxygen reduction reaction

Nitrogen-doped carbon nanotubes (NCNTs) as noble-metal free catalysts were synthesised via chemical vapour deposition using iron (II) phthalocyanine as a metal catalyst for growth of the nanotubes. The synthesis process was performed in one step in a tube furnace using different chemical precursors (aniline, diethylamine and ethylenediamine) as nitrogen sources. The NCNT samples were physically characterised using scanning electron microscopy, transmission electronic microscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry measurements were conducted to investigate the oxygen reduction reaction (ORR), and they showed that the electrocatalytic activities depend on the structural and morphological changes in the NCNTs. The results showed that NCNTs synthesised from ethylenediamine precursors exhibit an ORR scheme proceed via indirect four electron transfer process in acidic media, which implies that these NCNTs are a candidate for serving as the cathodic catalyst in PEMFCs. The highly active NCNTs possess unique characteristics, including a high concentration of surface defects with high pyridinic-N and pyridinic-N-oxides configurations that serve as active sites for ORR activity in acidic media.

Direct photocurrent generation from nitrogen doped TiO2 electrodes in solid-state dye-sensitized solar cells: Towards optically-active metal oxides for photovoltaic applications

Nitrogen-doped titanium dioxide (TiO2) is considered as a promising photocatalytic material due to its optical absorption extended in the visible region compared to pure TiO2. In the field of photovoltaic applications, dye-sensitized solar cells based on N-doped nanocrystalline titania electrodes have demonstrated improved performance due to the beneficial effects of nitrogen on the electronic and optical properties of TiO2. In this context, we report on the influence of nitrogen doping on the performance of solid-state dye-sensitized solar cells, starting from TiO2 and N-TiO2 nanocrystals synthesized by laser pyrolysis. Using an integrated approach based on experimental and theoretical investigations, the relationship between the local electronic features of the starting metal oxide materials and device operation is described. We demonstrate that the short-circuit current density of the solar cells based on an N-doped TiO2 electrode increases by more than 10% compared to that of pure anatase. This improvement is clearly associated with the extended absorption of the doped electrode, suggesting that alternative charge generation mechanisms occur in the cells in addition to the conventional dye absorption. Computer simulations on isolated nanoclusters, as well as electron paramagnetic resonance (EPR) experiments, confirm that nitrogen atoms in the presence of oxygen vacancies can explain the introduction of additional energy states near the valence band of TiO2. Surface states associated with nitroxide radicals are also suggested to act as charge traps under illumination. These aspects confirm the strong potentialities of optically-active metal oxides for photovoltaic applications.

Hydrogen Storage in High Surface Area Carbons: Experimental Demonstration of the Effects of Nitrogen Doping

The influence of nitrogen doping on the hydrogen uptake and storage capacity of high surface area carbon materials is presented in this report. To generate suitable study materials, we have exploited the relationship between synthesis conditions and textural properties of zeolite-templated carbons to generate a range of high surface area carbons with similar pore size distribution but which are either N-doped or N-free. For N-doped carbons, the nitrogen content was kept within a narrow range of between 4.7 and 7.7 wt %. The carbon materials, irrespective of whether they were doped or not, exhibited high surface area (1900-3700 m(2)/g) and pore volume (0.99 and 1.88 cm(3)/g), a micropore surface area of 1500-2800 m(2)/g, and a micropore volume of 0.65-1.24 cm(3)/g. The hydrogen uptake varied between 4.1 and 6.9 wt %. We present experimental data that indicates that the effect of N-doping on hydrogen uptake is only apparent when related to the surface area and pore volume associated with micropores rather than total porosity. Furthermore, by considering the isosteric heat of hydrogen adsorption and excess hydrogen uptake on N-free or N-doped carbons, it is shown that N-doping can be beneficial at lower coverage (low hydrogen uptake) but is detrimental at higher coverage (higher hydrogen uptake). The findings are consistent with previous theoretical predictions on the effect of N-doping of carbon on hydrogen uptake. The findings, therefore, add new insights that are useful for the development of carbon materials with enhanced hydrogen storage capacity.

Effect of Nitrogen Concentration on Capacitance, Density of States, Electronic Conductivity, and Morphology of N-Doped Carbon Nanotube Electrodes

Heteroatom doping (e.g., boron and nitrogen) of graphitic carbon lattices affects various physicochemical properties of sp2 carbon materials. The influence of nitrogen doping in carbon nanotubes (N-CNTs) on their electrochemical and electrical properties such as the differential capacitance, density of states at the Fermi level (D(EF)), bulk conductivity, and work function is presented. Studies were performed on free-standing N-CNTs electrode mats to understand the intrinsic physicochemical properties of the material without relying on the secondary influence of another conductive support. N-Doping levels ranging from 0 to 7.4 atom % N were examined, and electrochemical impedance spectroscopy (EIS) was used to evaluate the differential capacitance and to estimate the effective density of states, D(EF). X-ray photoelectron spectroscopy (XPS) and Raman microscopy were used to assess the compositional and structural properties as a function of nitrogen doping. XPS N1s spectra show three principle types of ni...

Influence of nitrogen doping on the sputter-deposited WO3 films

Tungsten oxide films were produced using reactive rf magnetron sputtering. In this work, nitrogen doping was used to modify structural and, optical properties of the material in the presence of two inert gases (argon and helium). Substituting helium gas with argon results in a decrease in the particle sizes and thus affects the band gap values. Bandgap values were obtained over the range of 2.43 to 3.01eV via incorporating oxygen–nitrogen–argon/helium mixtures in the gas ambient. It was also observed that the atomic mass of the sputtering gas plays a major role for changing the primary crystallite size as well as the surface morphology and texture.

Cathodoluminescence characterization of a nitrogen-doped homoepitaxial diamond thin film

Strong modification of the optical spectra is apparent in nitrogen-doped chemical vapor deposited (CVD) diamonds. Nitrogen-vacancy (NV) defects in CVD diamond are effectively created by nitrogen doping with a high concentration of the gaseous phase. In particular, the 575 nm center and 637 nm centers are enhanced in intensity at a N doping level of around 1018 at./cm3, while nitrogen addition during CVD growth leads to quenching of both the exciton and H3 centers. The influence of nitrogen doping on the exciton and NV defect states in a homoepitaxial CVD diamond thin film was investigated by high-resolution cathodoluminescence experiment. In addition, Raman experiments were performed to detect the internal stress. The results show that the exciton and nitrogen-related defect emission spectra underwent a shift of the peak position to a longer wavelength by nitrogen doping. The characteristic Raman peak of diamond at 1332 cm−1 showed a shift toward lower wave numbers with increasing nitrogen incorporation.

Influence of nitrogen doping on the radial breathing mode in carbon nanotubes

The influence of nitrogen doping in semiconducting carbon nanotubes is investigated by first-principles calculations considering a wide variety of substitution sites and concentrations. The frequency of the radial breathing mode of a (8,0) nanotube is calculated using density-functional theory and frozen-phonon approximation for different doping concentrations, substitution sites, and vacancies. The results are compared to a one-dimensional first-neighbor spring constant model using experimental parameters. We estimate the effect of doping in Raman spectra by examining the electronic band structure of doped carbon nanotubes.

Influence of nitrogen doping on growth rate and texture evolution of chemical vapor deposition diamond films

Chemical vapor deposition (CVD) diamond films were prepared using a variation in nitrogen addition into the gas source admixture by a direct current CVD method. The influence of nitrogen addition on the crystallographic texture and grain shape evolution in heteroepitaxial polycrystalline diamond films was investigated using high-resolution electron backscattering diffraction and x-ray diffraction. The analysis reveals that an addition of 1.5% N2 to the CH4 gas flow leads to a strong enhancement in a {110} fiber texture. The phenomenon is discussed in terms of a competitive growth selection mechanism.

Influence Of Growth Conditions on Irradiation Induced Defects in 4H-SiC

Nitrogen doped 4H-SiC epitaxial layers grown by hot-wall chemical vapor deposition were investigated by Deep Level Transient Spectroscopy after irradiation with 6 MeV electrons at room temperature. This study is focusing on the influence of nitrogen doping and C/Si ratio on the behaviour of the Z1,2 and EH6,7 levels which occur in already as-grown material but are substantially enhanced by electron and ion irradiation. It was found that both the Z1,2 and EH6,7 concentrations increase with both the nitrogen doping and the C/Si ratio. However, while the Z1,2 concentration increases during post-irradiation thermal treatment the opposite holds for the EH6,7 level especially in silicon rich samples. On the basis of these results, the influence of carbon and nitrogen on the formation of the Z1,2 complex is reconfirmed and a possible identity of the EH6,7 defect is discussed.

Influence of nitrogen doping on thermal stability of fluorinated amorphous carbon thin films

Abstract Nitrogen doping fluorinated amorphous carbon ( a-C: F) films were deposited using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and annealed in Ar environment in order to investigate their thermal stability. Surface morphology and the thickness of the films before and after annealing were characterized by AFM and ellipsometer. Raman spectra and FTIR were used to analyze the chemical structure of the films. The results show that the surface of the films becomes more homogeneous either by the addition of N 2 or after annealing. Deposition rate of the films increases a little at first and then decreases sharply with the increase of N 2 source gas flux. It is also found that the fraction of aromatic rings structure increases and the thermal stability of the films is strengthened with the increase of N 2 flux. Nitrogen doping is a feasible approach to improve the thermal stability of a-C: F films.

Atomic layer deposition of TiO 2− xN x thin films for photocatalytic applications

Titanium dioxide (TiO 2) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradiation. Doping TiO 2 with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO 2 films were grown by atomic layer deposition (ALD) using TiCl 4, NH 3 and water as precursors. All growth experiments were done at 500 °C. The films were characterized by XRD, XPS, SEM and UV–vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degradation of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination.

Influence of nitrogen doping on the properties of 4H–SiC single crystals grown by physical vapor transport

Investigations of heavily doped 4H–SiC single crystals grown by physical vapor transport show that already a few percent of nitrogen in the argon atmosphere result in one order of magnitude higher values of nitrogen chemically incorporated in the crystals compared to the usual background level. At our growth conditions the crystals seem to be saturated with nitrogen at a concentration of about 4×1019cm−3. The chemical nitrogen concentration generally exceeds the net donor concentration. In the photoluminescence spectra of oxidised wafers measured at room temperature the intensity of the dominating bandgap emission (394nm) decreases whereas the intensity of a band at about 500nm increases with increasing nitrogen concentration above 2×1019cm−3. At that concentration also the wafer warp starts to enhance. For these samples, scanning electron microscopy combined with cathodoluminescence indicates a strongly enhanced planar defect formation. Furthermore, their appearance can be correlated to the strong anisotropy of the resistivity measured by Hall effect which is only found for heavily nitrogen-doped substrates after thermal treatment. The heavy doping seems to be a necessary but not sufficient precondition for the formation of planar defects. Investigations of oxidised as well as of argon-annealed PVT layers have demonstrated that an additional thermal treatment is required. The distribution of the planar defects on {1010} cleavage planes is discussed.

Influence of nitrogen doping on different properties of a-C:H

Amorphous hydrogenated carbon (a-C:H) exhibits outstanding properties such as high hardness, low mechanical wear and friction, high thermal conductivity, etc. These properties are irreversibly altered above 400 °C. Doping the a-C:H films with nitrogen revealed the possibility to continuously tune properties such as internal stress, hardness, electrical conductivity and surface energy. X-ray photoelectron spectroscopy has been used to analyze the chemical and structural modifications caused by the introduction of nitrogen into the films. An increased onset temperature of the thermal C-H bond decomposition in a-C:H films up to 600 °C (in vacuum) is observed. Hardness measurements on N-doped samples show an increase in thermal stability of the films, however, without ever reaching the hardness values obtained from the undoped a-C:H film.

Investigation of the high-field conductivity and dielectric strength of nitrogen containing polycrystalline diamond films

The influence of nitrogen doping on the electrical properties of polycrystalline diamond films has been studied. The films were prepared in a microwave plasma chemical vapor deposition process using a H2/CH4/N2 gas mixture. The CH4 concentration was held constant at 0.5% and the nitrogen to carbon atomic ratio was varied between 0.01 and 0.2. The phase purities, surface morphologies, and the nitrogen contents of the films were analyzed by Raman spectroscopy, scanning electron microscopy, and secondary-ion mass spectroscopy, respectively. From current-voltage characteristics at field strengths up to 106 V cm−1 and in the temperature range between 300 and 800 K the conductivity was determined. The dielectric strength was obtained from the breakdown voltage measured using a voltage ramping rate between 50 and 100 V s−1. A minimum in high field and high-temperature conductivity and a maximum in dielectric strength was found for the samples prepared with a nitrogen to carbon atomic ratio of 0.02. Compared with nitrogen free samples the conductivity is lowered by more than three orders of magnitude and the dielectric strength is enhanced by a factor of two. The results will be discussed in terms of compensation of acceptor states by nitrogen donors and structural changes of the films.