N-doping Concentration Research Articles

Effect of the N-doping concentration on the formation of the wide carrot defect in 4H-SiC homoepitaxial layer grown by trichlorosilane (TCS) as silicon precursor

The effect of epilayer growth system, growth rate and N-doping concentration on the formation of the wide carrot defects in 4H-SiC homoepitaxial layer was investigated in this work. We found that the wide carrot defect formed on 4H-SiC epilayer only with low N-doping concentration of 5 × 1014 cm−3 grown by the SiHCl3+C2H4+H2 (TCS) system, which is different with the previous reports on the SiH4+C3H8+H2 growth system that can form such defect regardless of N-doping concentration. Besides, such wide carrot defect was never found on high speed growth of epilayer surface grown by TCS growth system. The morphology and structure of wide carrot defects were characterized by optical microscope (OM), micro-photoluminescence (PL) spectroscopy, atomic force microscope (AFM) and molten KOH etching. We speculate that the amount of wide carrot defects is related with the basal plane dislocations (BPDs) density which depends on the epitaxial growth system, growth rate of epilayer and the N-doping concentration. The formation mechanism and elimination methods of these wide carrot defects were proposed.

Vis-IR black nano-silicon produced by wet femtosecond-laser nanotexturing/hyperdoping and nanosecond-laser annealing

Compared to conventional oven-based annealing in ambient air of a silicon surface layer nanotextured and hyperdoped in CS2 fluid by femtosecond laser (black nano-silicon), low-intensity nanosecond laser annealing facilitated its delicate recrystallization, preserving quasi-regular nanoscale topography and high n-doping concentration by sulfur donor impurities. The retained and recrystallized topography of the laser-annealed nano-silicon with a few-percent doping level exhibits broadband optical absorption in the visible−near-infrared (vis−NIR) range, rendering this optical material promising for photovoltaic and IR-photonic applications.

Coupling field optimization to improve the thermal transport of Gr/h-BN heterostructure

We perform a numerical evaluation on the multi-physical field coupling effect to improve the interfacial thermal conductivity (ITC) of Gr/h-BN (Graphene/Hexagonal boron nitride) planar heterostructure. The model of Gr/h-BN planar heterostructure (C-BN bonded configuration) is built to investigate the effect of the multifactor coupling effect of different temperatures, N-doping concentration, and single vacancy concentration on the ITC. Finally, the Box-Behnken orthogonal model is established to determine the optimal value of ITC under the coupling effect of multi-physical fields by response surface analysis, and the optimal value of ITC is 14.456 GW m−2 K−1 when the temperature is 673 K, the single vacancy is 2.57 %, and the N-doping is 18.9 %. It implies that the proper coupling configuration is found effective in improving the interfacial thermal conductivity of the Gr/h-BN heterostructure. This work is hoped to help promote the thermal reliability design of graphene heterogeneous interface for related micro/nano semiconductor devices.

Bamboo-like Carbon Nanotubes with the Theoretical Upper-Limit N-Doping Concentration for Advanced Nonradical Oxidation

Bamboo-like Carbon Nanotubes with the Theoretical Upper-Limit N-Doping Concentration for Advanced Nonradical Oxidation

Physical Origin of Color Changes in Lutetium Hydride under Pressure

Recently, near-ambient superconductivity was claimed in nitrogen-doped lutetium hydride (LuH3–δ N ε ). Unfortunately, all follow-up research still cannot find superconductivity signs in successfully synthesized lutetium dihydride (LuH2) and N-doped LuH2±x N y . However, a similar intriguing observation was the pressure-induced color changes (from blue to pink and subsequent red). The physical understanding of its origin and the correlation between the color, crystal structure, and chemical composition of Lu–H–N is still lacking. In this work, we systematically investigated the optical properties of LuH2 and LuH3, and the effects of hydrogen vacancies and nitrogen doping using the first-principles calculations by considering both interband and intraband contributions. Our results demonstrate that the evolution of reflectivity peaks near blue and red light, which is driven by changes in the band gap and Fermi velocity of free electrons, resulting in the blue-to-red color change under pressure. In contrast, LuH3 exhibits gray and no color change up to 50 GPa. Furthermore, we investigated the effects of hydrogen vacancies and nitrogen doping on its optical properties. Hydrogen vacancies can significantly decrease the pressure of blue-to-red color change in LuH2 but do not have a noticeable effect on the color of LuH3. The N-doped LuH2 with the substitution of a hydrogen atom at the tetrahedral position maintains the color change when the N-doping concentration is low. As the doping level increases, this trend becomes less obvious, while other N-doped structures do not show a blue-to-red color change. Our results can clarify the origin of the experimental observed blue-to-red color change in lutetium hydride and also provide a further understanding of the potential N-doped lutetium dihydride.

DFT simulation of structure stability and nitrogen oxide adsorption for nitrogen and oxygen co-modified carbon nanotubes

Nitrogen (N) and oxygen (O) co-modified carbon nanotubes (ONCNTs) have demonstrated potential applications in adsorption and catalysis processes, because the oxygen-containing groups and N-doping atoms can provide abundant surface binding sites. However, their structural stabilities and adsorption interactions under different co-modified levels remain to be explored. Herein, for the first time, the density functional theory approach was applied to quantitatively characterize the structural morphologies and thermodynamic stabilities for a series of ONCNTs. Our findings indicate that co-modification levels can significantly affect their structures and stabilities. An increase in the N-doping concentration largely decreases the thermodynamic stability of ONCNTs, whereas the oxygen-containing functional groups can enhance the stability. A suitable co-modification level was proposed, at which the ONCNTs could maintain stable structural morphology. Meanwhile, the interaction energies and electronic mechanisms between the ONCNTs and nitrogen oxides (NO and NO2) were comprehensively investigated. The degree of oxidation is the main factor determining the interaction and the stronger interaction generally occurs at the 20–40 % oxidation degrees, wherein both chemical and physical interactions are exhibited for NO and only physical interaction for NO2. Our simulation results could be important and valuable to the design and application of the modified CNTs.

Mechanistic insights into hydrogen production from formic acid catalyzed by Pd@N-doped graphene: The role of the nitrogen dopant

The catalytic decomposition of formic acid (HCOOH) is a crucial process for hydrogen production technologies. Herein, periodic density functional theory (DFT) calculations were employed to explore the effect of N-doping on the decomposition of formic acid. We designed a series of single Pd-atoms deposited in the single vacancy of N-doped graphene sheets, namely Pd-DGr, Pd–N1Gr, Pd–N2Gr, and Pd–N3Gr, as the proposed catalysts. Our findings show that H2 production from HCOOH dehydrogenation on these surfaces proceeds via the formate (HCOO) pathway (Path-I) rather than the carboxylate (COOH) pathway (Path-II). Furthermore, the Pd–N3Gr catalyst shows the greatest catalytic reactivity toward HCOOH dehydrogenation via Path-I, requiring an activation energy (Ea) of 0.38 eV.On the other hand, the undesirable dehydration of HCOOH to carbon monoxide (CO) through COOH (Path-IIIA) or formyl (HCO) (Path-IIIB) intermediates is unlikely to occur on Pd–N3Gr due to a large activation energy. We found that the active species on the catalyst surface increased with N-doping concentration. Additionally, microkinetic simulations of the HCOOH decomposition on these surfaces confirmed the high activity and selectivity of the Pd–N3Gr catalyst toward HCOOH dehydrogenation (Path-I). These calculated results highlight that the Pd–N3Gr catalyst is a promising candidate for the formic acid decomposition reaction to yield hydrogen.

Influence of N-doped concentration on the electronic properties and quantum capacitance of Hf2CO2 MXene

The electronic properties and quantum capacitance of pristine Hf2CO2 (PH) and N-doped Hf2CO2 systems are investigated using density functional theory (DFT). All the doped systems have the metallic characteristics. The redshift of C-p, Hf-d, O-p, and Hf-p states in valence band gradually increases with the increasing N-doping concentration. The quantum capacitance and the electrode type of Hf2CO2 monolayer can be effectively modulated by N-doping. PH and PH systems with N-doped concentration of 78% are potential cathode materials, while PH systems with N-doped concentration of 11%, 22%, 33%, 44%, 56%, and 67% are potential anode materials, PH systems with 89% and 100% N-doped concentration are suitable for symmetric electrode materials in aqueous system. The wide voltage changes the electrode type of PH system with N-doped concentration of 78%, 89% and 100%. The electrode type of N-doping MXene with mixed terminations are further explored.

Effects of N-doped concentration in graphene on CO2 adsorption

Solving the problem of the environment related to CO2 emission is an urgent and challenging task to prevent the escalation of climate change, which is due to increasing energy demand from fossil fuels and industrial activities. Using graphene to capture CO2 gas is of great interest to mitigate global warming, where CO2 capture by doped graphene has been shown to significantly improve the CO2 adsorption capacity compared to that of pure graphene. In particular, N-doped graphene is a unique structure that was suggested for CO2 capture by the adsorption phenomenon. However, there is no research available to clarify nitrogen doping concentration in graphene on CO2 adsorption. Therefore, this work has been devoted to elucidating the problem using the density functional theory method considering van der Waals interaction. We showed that increasing the doping content of nitrogen by 3.1, 6.3, 9.4, and 12.5% will increase the CO2 adsorption energy, monotonically for N content below 9.4% and significantly for 12.5%. The most favorable adsorption configuration of the CO2 molecule should be parallel to the surface of the N-doped graphene. The structure of the substrate at 12.5% N distorted upon the CO2 adsorption; therefore, the substrate is more active for CO2 capture. The physical meaning underlying the interaction of the N@graphene system is physisorption, which is due to the contribution of the N pz state and the CO2 pz state along the surface normal of the substrate. Increasing the N-doping content shifts down the unoccupied states of the N pz orbital in the conduction band across the Fermi level to the valence band. Hence, the population of the occupied state at the Fermi level increases as increasing the N-doping concentration, leading to the stronger interaction of the CO2 molecule with the N@graphene substrate. The results should be useful for the rational design of suitable N@graphene substrates for CO2 capture and storage applications.

First-principles insights into thermoelectric properties of topological nontrivial semimetal LiAuTe material

Structural, electronic and thermoelectric properties of LiAuTe ternary compound are studied using density functional theory (DFT) and semi-classical Boltzmann transport theory. The cubic -phase (space group F3m) is predicted to be ground state structure with a significant energy difference compared to honeycomb structure (space group P63mmc). The mechanical and dynamical stability of the -phase is confirmed by calculating the elastic constants and phonon dispersion frequencies. At equilibrium lattice, with and without spin–orbit coupling, the LiAuTe compound band structure calculations show an s-p band inversion at Γ point, leading to a topological nontrivial semimetal phase. Thermoelectric parameters, such as Seebeck coefficient (S), electrical conductivity (σ), electronic (κ e ) and lattice (κ L ) thermal conductivities are computed. Electrons and holes relaxation times (τ) are also predicted. Hence, LiAuTe compound exhibits a low κ L value of 1.76 W mK−1 at room temperature which decreases with temperature increasing. At 900 K, κ L falls to 0.58 W mK−1 leading to a maximum ZT value of 0.52 at optimized n-doping concentration of 2.5 × 1020 cm−3. The present study reveals that LiAuTe compound is a suitable candidate for thermoelectric applications and will open new horizons for further researches on similar types of topological thermoelectric materials with better ZT.

Columnar nitrogen-doped ZnO nanostructured thin films obtained through atomic layer deposition

The present study was aimed to develop nitrogen-doped nanostructured ZnO thin films. These films were produced in a sequential procedure involving the atomic layer deposition technique, and a hydrothermal process supported by microwave heating. Employing the atomic layer deposition technique, through self-limited reactions of diethylzinc (DEZn) and H2O, carried out at 3.29 × 10−4 atm and 190 °C, a high-quality ZnO seed was grown on a Si (100) substrate, producing a textured film. In a second stage, columnar ZnO nanostructures were grown perpendicularly oriented to the silicon substrate on those films, using a solvothermal process in a microwave heating facility, employing Zn(NO3)2 as zinc precursor, while hexamethylenetetramine (HMTA) was used to produce the bridging of Zn2+ ions. The consequence of N-doping concentration on the physicochemical properties of ZnO thin films was studied. The manufactured films were structurally analyzed by scanning electron microscopy and x-ray diffraction. Also, x-ray photoelectron spectroscopy, Raman, and UV–vis spectroscopies were used to provide further insight on the effect of nitrogen doping. The N-doped films displayed textured wurtzite-like structures that changes their preferential growth from the (002) to the (100) crystallographic plane, apparently promoted by the increase of nitrogen precursor. It is also shown that nitrogen-doped films undergo a reduction in their bandgap, compared to ZnO. The methodology presented here provides a viable way to perform high-quality N-ZnO nanostructured thin films.

Precise synthesis of N-doped graphitic carbon via chemical vapor deposition to unravel the dopant functions on potassium storage toward practical K-ion batteries

Nitrogen doped carbon is a burgeoning anode candidate for potassium-ion battery (PIBs) owing to its outstanding attributes. It is imperative to grasp further insight into specific effects of different nitrogen dopants in carbon anode toward advanced K-ion storage. However, the prevailing fabrication method is plagued by the fact that considerable variations in the total N-doping concentration occur in the course of regulating the type of nitrogen dopants, incapable of distinguishing the certain roles of them under similar conditions. Herein, throughout the precise preparation of high edge-N doped carbon (HENC) and high graphitic-N doped carbon (HGNC) harnessing basically identical N-doping levels (5.78 at.% for HENC; 5.07 at.% for HGNC) via chemical vapor deposition route, the effects of edge-N and graphitic-N in the carbon anode on K-ion storage are revisited, offering guidance into the design of low-cost and high-performance PIB systems.

Ab initio study of oxygen evolution reaction and hydrogen evolution reaction via water splitting on pure and nitrogen-doped graphene surface

Water electrolysis is widely considered to be a realistic technology for large, highly purified hydrogen production. In recent years, electrocatalytic water splitting achieved by graphene hybrids has received wide recognition; additionally, nitrogen (N)-doped carbon materials have shown promising catalytic activity for oxygen-reduction reactions (ORR) as metal-free alternatives to platinum. In an effort to elucidate the underlying molecular mechanism that drives such high catalytic activity, density functional theory (DFT) calculations were applied to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) on pristine and N-doped graphene surfaces. A wide portfolio of investigations was carried out on pristine, 1.7% N-doped, and 3.3% N-doped graphene systems including water adsorption energy calculations, charge density differences calculations, the density of states (DOS) and band structure comparisons. The defect formation energy of 3.3% N-doped graphene for different configurations has been carried out to find the most stable configuration. Lastly, the nudged elastic band (NEB) theory was utilized to estimate the reaction coordinate and minimum energy path (MEP) at each stage of the reaction. By modulating N-doping concentration on the graphene layer, the MEP of the reaction coordinate was calculated. It was found that the barrier potential of energetic elementary steps decreases with increasing N-doping concentration. Finally, our results reveal that the primary reasons that drive an efficient water-splitting reaction include the nitrogen species’ intrinsic electronegativity together with a shift of the Fermi level towards the conduction band, thus making the system more conductive.

Design of two-dimensional carbon-nitride structures by tuning the nitrogen concentration

Nitrogen-doped graphene (NG) has attracted increasing attention because its properties are significantly different to pristine graphene, making it useful for various applications in physics, chemistry, biology, and materials science. However, the NGs that can currently be fabricated using most experimental methods always have low N concentrations and a mixture of N dopants, which limits the desirable physical and chemical properties. In this work, first principles calculations combined with the local particle-swarm optimization algorithm method were applied to explore possible stable structures of 2D carbon nitrides (C1−xNx) with various C/N ratios. It is predicted that C1−xNx structures with low N-doping concentration contain both graphitic and pyridinic N based on their calculated formation energies, which explains the experimentally observed coexistence of graphitic and pyridinic N in NG. However, pyridinic N is predominant in C1−xNx when the N concentration is above 0.25. In addition, C1−xNx structures with low N-doping concentration were found to have considerably lower formation energies than those with a high N concentration, which means synthesized NGs with low N-doping concentration are favorable. Moreover, we found the restrictions of mixed doping and low N concentration can be circumvented by using different C and N feedstocks, and by growing NG at lower temperatures.

Nonresonant Polarized Raman Spectra Calculations of Nitrogen-Doped Single-Walled Carbon Nanotubes: Diameter, Chirality, and Doping Concentration Effects.

Raman spectra of nitrogen-doped single-walled carbon nanotubes are calculated using the spectral moment's method combined with the bond polarizability model. The influence of the nanotube diameter and chirality is investigated. We also address the important question of the effect of the N-doping concentration, and we propose an equation to estimate the doping concentration from the knowledge of the tube diameter and the frequency of the radial breathing mode.

Nitrogen-doped hierarchical porous carbons prepared via freeze-drying assisted carbonization for high-performance supercapacitors

Functionalized carbon electrodes with hierarchical porous structure play an essential role in the fabrication of high-performance supercapacitors. Here, we report a novel freeze-drying assisted carbonization approach for the preparation of nitrogen-doped hierarchically porous carbons (NHPCs). Using urea as both the pore-forming agent and the nitrogen source, and sodium alginate (SA) as the carbon precursor, we are able to modulate the porosity and N-doping concentration of the NHPCs by simply altering the carbonization temperature and the dosage of urea. The prepared NHPCs exhibit specific surface area as high as 1179 m2 g−1 with 70% micropore and high N- and O-doping concentration of 6.7 wt% and 13.8 wt%, respectively. Carbon electrodes based on these NHPCs show a high gravimetric capacitance (305 F g−1 at 1 A g−1) and an excellent rate ability (retaining 73.8% at 20 A g−1) in three-electrode systems. The NHPCs symmetric supercapacitor delivers excellent energy density of up to 23.8 W h kg−1 as well as long-term electrochemical stability (95.8% capacitance retention over 10, 000 cycles). The abundance of the raw materials combined with the simple fabrication procedure makes the SA-derived NHPCs as promising electrode materials for the next generation advanced supercapacitors.

One-step synthesis of tunable nitrogen-doped graphene from graphene oxide and its high performance field emission properties

Simultaneous reduction, repairing and doping of graphene oxide has been realized by the chemical vapor deposition method, using acetonitrile as a nitrogen source. A step-wise increase of acetonitrile partial pressure from 15 to 90 Pa results in nitrogen doped graphene (NG) with gradually tuning N-doping concentration from 0.38 to 0.66 at% and systematically rising graphite-N ratio from 23.25 to 45.39%, which in turn modulates field emission performance and enhance the stability. Raman spectroscopy and X-ray photoelectron spectroscopy suggest pyrrolic-N and pyridinic-N doping bring defects, while defects decrease as N-doping concentration increases due to the rising of graphite-N ratio. Proper defects may increase emission site density and N-doping can reduce work function. When N-doping concentration is controlled at 0.42 at%, NG exhibits the considerable decreasing of turn-on field from 3.35 to 2.18 V/μm at the emission current of 10 μA/cm2 and increasing of field enhancement factor from 1835 to 2967. It also reveals a good field emission stability with no degradation, which is superior to pristine reduced GO emitters. It is suggested the NG with tuning concentration of three type nitrogen emitter is a widely candidate for various field emission devices and applications.

Non-Enzymatic Electrochemical Detection of Mercury Ions on Graphene Quantum Dot-Based Electrodes

This study adopts an efficient method to electrochemically detect Hg2+ ions in aqueous solutions using graphene quantum dot (GQD) based electrodes. Previous studies reported that GQDs are capable of detecting metal ions (e.g., Hg2+ and Fe2+) at ultralow concentrations, showing an obvious fluorescence quenching in the presence of metal ions in aqueous solutions. However, there are few studies focusing on electrochemical detection of metal ions using GQD-based electrodes. The electrochemical detection technique is a convenient and simple method to accurately estimate real concentration of the contaminants based on the change of peak current density or ohmic resistance. In this study, GQDs were prepared by a hydrothermal technique using organic compound as precursor. The as-prepared GQDs with an average particle size of 5 nm were functionalized by surface oxygen groups, including carbonyl, carboxyl, and hydroxyl groups attached to basal planes and edges of sp 2-domain graphene. The non-enzymatic electrochemical detection of Hg2+ ions on GQD-based electrodes was carried out using cyclic voltammetry within a potential range between 0 and 1.7 V vs. Ag/AgCl. Experimental results showed that the reduction of peak current at 0.35 V is proportional to the concentration of Hg2+ ions. The detection limit of mercury ions by GQD-based electrode can reach as low as 0.1 mg/L. Accordingly, the unique design of GQD-based electrode displays the feasibility in application to electrochemical detection of environmentally pollution micro-inspection. Future work on how N-doping and dopant concentration on the sensitivity and selectivity of GQD-based electrodes is on-going.

Enhanced Photocatalytic Performance of Nitrogen-Doped TiO2 Nanotube Arrays Using a Simple Annealing Process

Nitrogen-doped TiO2 nanotube arrays (N-TNAs) were successfully fabricated by a simple thermal annealing process in ambient N2 gas at 450 °C for 3 h. TNAs with modified morphologies were prepared by a two-step anodization using an aqueous NH4F/ethylene glycol solution. The N-doping concentration (0–9.47 at %) can be varied by controlling N2 gas flow rates between 0 and 500 cc/min during the annealing process. Photocatalytic performance of as-prepared TNAs and N-TNAs was studied by monitoring the methylene blue degradation under visible light (λ ≥ 400 nm) illumination at 120 mW·cm−2. N-TNAs exhibited appreciably enhanced photocatalytic activity as compared to TNAs. The reaction rate constant for N-TNAs (9.47 at % N) reached 0.26 h−1, which was a 125% improvement over that of TNAs (0.115 h−1). The significant enhanced photocatalytic activity of N-TNAs over TNAs is attributed to the synergistic effects of (1) a reduced band gap associated with the introduction of N-doping states to serve as carrier reservoir, and (2) a reduced electron‒hole recombination rate.

Nitrogen-Doped ZnO Film Fabricated Via Rapid Low-Temperature Atomic Layer Deposition for High-Performance ZnON Transistors

High-performance nitrogen-doped ZnO (ZnON)-based thin-film transistors (TFTs) were fabricated by atomic layer deposition (ALD) with a rapid purging time of only 5 s at a temperature as low as 150 °C. It is the first time to report ALD ZnON TFT using NH3 as N source. It is found that ZnON TFT with a N: Zn atomic ratio of 1:19 (ZnON 1:19) exhibited excellent properties, such as lower subthreshold swing of 0.47 V/decade and smaller $\Delta {V}_{\textsf {th}}$ of 0.71 V under the temperature stress from 25 to 105 °C. The ${I}_{ \mathrm{\scriptscriptstyle ON}}$ and ${I}_{ \mathrm{\scriptscriptstyle OFF}} $ decreased as N-doping concentration increased, and ZnON 1:19 TFT presented a high ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of 1.75 $\times \,\,10^{\textsf {7}}$ . The density of state was calculated by temperature stress. Combining with the X-ray photoelectron spectroscopy analysis, we built a model to explain the reaction mechanism that the moderate amount of N doping could significantly suppress the creation of oxygen defects.