Nitrogen Acceptor Research Articles

Neutral nitrogen acceptors in ZnO: The 67Zn hyperfine interactions

Electron paramagnetic resonance (EPR) is used to characterize the 67Zn hyperfine interactions associated with neutral nitrogen acceptors in zinc oxide. Data are obtained from an n-type bulk crystal grown by the seeded chemical vapor transport method. Singly ionized nitrogen acceptors (N−) initially present in the crystal are converted to their paramagnetic neutral charge state (N0) during exposure at low temperature to 442 or 633 nm laser light. The EPR signals from these N0 acceptors are best observed near 5 K. Nitrogen substitutes for oxygen ions and has four nearest-neighbor cations. The zinc ion along the [0001] direction is referred to as an axial neighbor and the three equivalent zinc ions in the basal plane are referred to as nonaxial neighbors. For axial neighbors, the 67Zn hyperfine parameters are A‖ = 37.0 MHz and A⊥ = 8.4 MHz with the unique direction being [0001]. For nonaxial neighbors, the 67Zn parameters are A1 = 14.5 MHz, A2 = 18.3 MHz, and A3 = 20.5 MHz with A3 along a [101¯0] direction (i.e., in the basal plane toward the nitrogen) and A2 along the [0001] direction. These 67Zn results and the related 14N hyperfine parameters provide information about the distribution of unpaired spin density at substitutional neutral nitrogen acceptors in ZnO.

Synthesis and Conformational Study of Model Peptides Containing N‐Substituted 3‐Aminoazetidine‐3‐carboxylic Acids

AbstractModel peptides containing N‐substituted 3‐aminoazetidine‐3‐carboxylic acids have been synthesized to investigate the conformational features of these Cα‐tetrasubstituted amino acids. The target peptides were designed and prepared by classical synthetic methods in solution, exploiting the convenient N‐substituted 3‐azidoazetidine‐3‐carboxylic acids as precursors. The conformational preferences of the newly synthesized peptides were investigated in solution by IR and NMR spectroscopy. It was observed that the 3‐aminoazetidine‐3‐carboxylic acid moiety is likely a β‐turn inducer. In addition, an interesting main‐chain‐to‐side‐chain hydrogen bond that forms a six‐membered pseudo‐cycle was detected. It connects the nitrogen (acceptor) of the azetidine ring to the amide NH (donor) of the immediately following residue. This unexpected hydrogen bond increases the number of conformational options offered by N‐substituted 3‐aminoazetidine‐3‐carboxylic acids when designing foldamers with new and predictable 3D structures.

硼对氮掺杂的p型ZnO薄膜的影响

A stable and repeatable p-type ZnO is the key to realize the practical applications of photoelectric devices. Nitrogen has been considered as a possible acceptor dopant for p-type ZnO for a long time. However, the low solubility of nitrogen acceptor at oxygen site in ZnO is considered to be one of the main obstacles for p-type ZnO. In this paper, B/N co-doped p-type ZnO thin films were grown on sapphire substrate by plasma-assisted molecular beam epitaxy. The effect of boron on the nitrogen doping was studied by comparing with the single nitrogen doped thin films. X-ray photoelectron spectroscopy demonstrated there exists boron and B-N bond in the ZnO thin film. Compared to the single N-doped sample, B/N codoped samples present much higher carrier density. And the ZnO:(B, N) thin films show stable p-type conduction in two years. It is attributed to the increase of solubility of nitrogen acceptor. It benefits from the strong B-N bonding and weak donor character of boron at zinc site. Boron will not bring strong compensation for acceptors. It suggests that boron is a good co-doping candidate for nitrogen doped ZnO thin films.

Evidence for the carbon–nitrogen complex in ZnO nanostructures with very high nitrogen doping

This work presents positive experimental evidence for the formation of a carbon-nitrogen complex in ZnO, which was theoretically predicted previously. A very high nitrogen content up to ~8 at% can be doped into ZnO nanostructures via the formation of a carbon-nitrogen complex, which in turn suppresses the formation of a nitrogen acceptor.

Photoinduced Proton-Coupled Electron Transfer of Hydrogen-Bonded p-Nitrophenylphenol–Methylamine Complex in Solution

Proton-coupled electron transfer can occur through concerted (electron-proton transfer, EPT) or sequential mechanisms, but this distinction becomes less well-defined for photoinduced reactions. These issues have been examined with transient absorption experiments on a hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine. These experiments revealed two spectroscopically distinct states: the higher-energy excited state was interpreted to be a conventional intramolecular charge transfer (ICT) state within the p-nitrophenylphenol, whereas the lower-energy state was interpreted to be an ICT-EPT state, where photoexcitation resulted in both ICT and the shifting of electronic density corresponding to effective proton transfer from the phenol to the amine. In the present work, the singlet excited states of the hydrogen-bonded p-nitrophenylphenol-methylamine complex in 1,2-dichloroethane are studied with time-dependent density functional theory and higher-level ab initio methods. The calculations suggest that the ππ* state, which is the S(1) state at the Franck-Condon geometry, corresponds to the state denoted ICT-EPT in the experimental analysis, whereas the nπ* state, which is the S(2) state at this geometry, likely corresponds to the state denoted ICT in the experimental analysis. According to the calculations, the ππ* state has charge-transfer character, as well as a change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the nitrogen acceptor, consistent with proton transfer. The nπ* state has little charge-transfer character, as well as negligible change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the oxygen donor. The calculations also provide evidence of an avoided crossing between these two states located energetically close to the Franck-Condon point. These calculations provide the foundation for future nonadiabatic molecular dynamics studies of the relaxation process.

Electron paramagnetic resonance and photo-electron paramagnetic resonance investigation on the recharging of the substitutional nitrogen acceptor in ZnO

We investigated the substitutional nitrogen center in ZnO single crystals by electron paramagnetic resonance (EPR) and photo-EPR spectroscopy. Aside the three principle hyperfine lines due to the interaction of the N0 (2p5) electron spin with the nitrogen nucleus (I = 1, natural abundance 99.6%), we identify additional satellite lines which arise from ΔmS = ±1 and ΔmI = ±1, ±2 transitions becoming allowed due to quadrupole interaction. The quadrupole coupling constant e2qQ/h is determined to −5.9 MHz with an asymmetry parameter of η = 0.05. These values are somewhat different from those obtained for the nitrogen center in ZnO powders, but are closer to the theoretical calculations of Gallino et al. We further carefully investigated the photon induced recharging of the N centers. We determine the energy hυ required for the process NO− + hυ → NO0 + ecb− to 2.1 ± 0.05 eV, the dependence of the EPR signal intensity on the illumination time shows a mono-exponential behavior which gives evidence that a direct ionization process is monitored.

Optical signatures of nitrogen acceptors in ZnO

We report on the optical properties of nitrogen acceptor-doped ZnO epilayers in the medium and high doping regimes using temperature and excitation power-dependent, as well as time-resolved photoluminescence experiments. The epilayers were doped with ammonia during homoepitaxial growth on ZnO single-crystal substrates with different surface polarities. Significant differences in the optical characteristics of the epilayers are observed between growth on nonpolar $a$-plane, polar $c$-plane Zn-face substrates and polar $c$-plane O-face substrates, which demonstrates different incorporation of the nitrogen acceptor depending on the substrate polarity. The incorporation of nitrogen into the ZnO films ranges between 10${}^{19}$ and 10${}^{21}$ cm${}^{\ensuremath{-}3}$ as determined by secondary ion mass spectrometry. Within this doping range the samples change from lightly compensated to highly doped compensated. We discuss the unique photoluminescence features of nitrogen-doped ZnO epilayers within the concept of shallow donor-acceptor-pair recombinations and at the highest doping level by the appearance of potential fluctuations.

Probing Transient Hoogsteen Hydrogen Bonds in Canonical Duplex DNA Using NMR Relaxation Dispersion and Single-Atom Substitution

Nucleic acids transiently morph into alternative conformations that can be difficult to characterize at the atomic level by conventional methods because they exist for too little time and in too little abundance. We recently reported evidence for transient Hoogsteen (HG) base pairs in canonical B-DNA based on NMR carbon relaxation dispersion. While the carbon chemical shifts measured for the transient state were consistent with a syn orientation for the purine base, as expected for A(syn)•T(anti) and G(syn)•C(+)(anti) HG base pairing, HG type hydrogen bonding could only be inferred indirectly. Here, we develop two independent approaches for directly probing transient changes in N-H···N hydrogen bonds and apply them to the characterization of transient Hoogsteen type hydrogen bonds in canonical duplex DNA. The first approach takes advantage of the strong dependence of the imino nitrogen chemical shift on hydrogen bonding and involves measurement of R(1ρ) relaxation dispersion for the hydrogen-bond donor imino nitrogens in G and T residues. In the second approach, we assess the consequence of substituting the hydrogen-bond acceptor nitrogen (N7) with a carbon (C7H7) on both carbon and nitrogen relaxation dispersion data. Together, these data allow us to obtain direct evidence for transient Hoogsteen base pairs that are stabilized by N-H···N type hydrogen bonds in canonical duplex DNA. The methods introduced here greatly expand the utility of NMR in the structural characterization of transient states in nucleic acids.

Carrier Control in Polycrystalline ZnO:Ga Thin Films via Nitrogen Implantation

The electrical characteristics of gallium-doped zinc oxide (ZnO:Ga) thin films prepared by rf diode sputtering were altered via nitrogen implantation by performing two implants at 40 keV and 80 keV with doses of 1×1015 and 1×1016 cm−2 to achieve a p-type semiconductor. An implantation of 1×1015 cm−2 N+-ions yielded a p-type with hole concentrations 1017–1018 cm−3 in some as-implanted samples. The films annealed at temperatures above 200°C in O2 and above 400°C in N2 were n-type with electron concentrations 1017–1020 cm−3. The higher nitrogen concentration (confirmed by SRIM and SIMS), in the films implanted with a 1×1016 cm−2 dose, resulted in lower electron concentrations, respectively, higher resistivity, due to compensation of donors by nitrogen acceptors. The electron concentrations ratio n(1×1015)/n(1×1016) decreases with increasing annealing temperature. Hall measurements showed that 1×1016 cm−2 N-implanted films became p-type after low temperature annealing in O2 at 200°C and in N2 at 400°C with hole concentrations of 3.2×1017 cm−3 and 1.6×1019 cm−3, respectively. Nitrogen-implanted ZnO:Ga films showed a c-axes preferred orientation of the crystallites. Annealing is shown to increase the average transmittance (>80%) of the films and to cause bandgap widening (3.19–3.3 eV).

Acceptor Dopants in Bulk and Nanoscale ZnO

ABSTRACTZinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nano-lasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, recent theoretical and experimental work suggests that nitrogen is a deep acceptor and will not lead to p-type conductivity. In nitrogen-doped samples, a red photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the 1.4-1.5 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. In our model, the 0.46 eV level is shallow with respect to the surface valence band, raising the possibility of surface hole conduction.

Crystallography Aided by Atomic Core‐Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Brønsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.

Kinemage of action – Proposed reaction mechanism of glutamate-1-semialdehyde aminomutase at an atomic level

Glutamate-1-semialdehyde aminomutase (GSAM), a key enzyme in tetrapyrrole cofactor biosynthesis, performs a unique transamination on a single substrate. The substrate, glutamate-1-semialdehyde (GSA), undergoes a reaction that exchanges the position of an amine and a carbonyl group to produce 5-aminolevulinic acid (ALA). This transamination reaction is unique in the fact that is does not require an external cofactor to act as a nitrogen donor or acceptor in this transamination reaction. One of the other remarkable features of the catalytic mechanism is the release free in the enzyme active site of the intermediate 4,5-diaminovaleric acid (DAVA). The action of a gating loop prevents the escape of DAVA from the active site. In a MD simulation approach, using snapshots provided by X-ray crystallography and protein crystal absorption spectrometry data, the individual catalytic steps in this unique intramolecular transamination have been elucidated.

Effect of the crystallinity of MOCVD-grown ZnO:N on the diffusion of impurities

The solubility of nitrogen in ZnO films grown at various temperatures by metalorganic chemical vapor deposition, using NO as both oxygen and nitrogen sources, was investigated. ZnO films were grown at 270, 310 and 370 °C, and subsequently annealed in oxygen at either 700 or 850 °C. In addition to nitrogen, hydrogen and carbon were also present in the samples. The presence of these impurities is partially explained by the formation of complexes such as (CN) O and CH x ( x=1,2,3). Post-growth annealing performed on the samples suggests that the out-diffusion of such complexes is strongly influenced by the crystallinity of the films. The high porosity of films grown at temperatures ≤310 °C is favorable for the diffusion of the complexes, which results in a more efficient thermal activation of nitrogen acceptors in ZnO:N.

Electrical and Point Defect Properties of TiO2 Nanotubes Fabricated by Electrochemical Anodization

Pure and N-doped titanium oxide nanotubes (TiO2nt) were manufactured by anodization of sputtered Ti thin films on a Si (100) substrate. Both solid state contacts and electrolyte contacts were used to investigate the properties of TiO2nt. Mott−Schottky analysis gave a flat-band potential Efb = −0.57 V vs Ag/AgCl for pure TiO2nt and Efb = −0.22 V vs Ag/AgCl for N-doped TiO2nt. The charge carrier density was ND = 6.7 × 1020 cm−3 for pure TiO2nt and ND = 3.9 × 1020 for N-doped TiO2nt. This corresponds to 1.1% oxide ion vacancies in pure TiO2nt. In nitrogen-doped TiO2nt, the decrease of donor density would correspond to 0.47% of vacancies being occupied by nitrogen acceptors. This investigation also allowed estimation of the apparent diffusion coefficient of H+ in TiO2nt. Following the Randles−Sevcik method, the effective proton diffusion coefficient in TiO2nt is (2 ± 1)·10−11 cm2 s−1 while in N-doped TiO2nt it is (4 ± 1)·10−11 cm2 s−1. Using the Warburg diffusion element determined by electrochemical impedance spectroscopy, the proton diffusion coefficient is (2 ± 1)·10−11 cm2 s−1 for pure TiO2nt while for nitrogen-doped TiO2nt a value of (7 ± 3)·10−11 cm2 s−1 is found. These values are consistent with those of the Randles−Sevcik method.

Statistical descriptors to measure the effectiveness of hydrogen bonding groups and an example of ether oxygen

To quantify the effectiveness of hydrogen-bonding donors and acceptors for forming interactions, we here introduce the concept of statistical competitiveness, defined in terms of three numerical parameters namely, absolute, relative and exclusive statistical competitiveness. The concept can be applied to evaluate the effectiveness of a donor or acceptor in any chemical context. We have evaluated the efficacy of ether oxygen for hydrogen-bonding with respect to carbonyl oxygen and nitrogen acceptors. The quantification in terms of statistical parameters reveals a striking result that the population of hydrogen-bonding groups predominately dictates their effectiveness irrespective of their acidic or basic strengths. The quantification of the competitiveness serves to estimate better the outcome of crystal structure prediction and engineering methodologies, and molecular docking.

Nitrogen defects from NH3in rare-earth sesquioxides and ZrO2

Effects of nitrogen defects on the electrical properties of RE(2)O(3) (RE = Nd, Gd, Er, Y) and ZrO(2) have been investigated by equilibration in ammonia (NH(3)) atmospheres in the temperature range 1000-1200 °C. The electrical conductivity in ammonia corresponded to that in H(2)-Ar mixtures of similar pO(2). However, upon replacing ammonia with an inert gas, the conductivity increases abruptly, typically one order of magnitude, before gradually returning to its equilibrium value. A defect model based on dissolution and dissociation of effectively neutral imide defects substituting oxide ions, NH, is proposed to describe this behavior. Conductivity measurements are interpreted in terms of nitrogen acceptors which are passivated by protons in the presence of H(2)(g), and subsequently compensated by positive charge carriers in an inert atmosphere as out-diffusion of hydrogen leaves an effective acceptor, N. In the case of Y(2)O(3), a NH concentration of 0.7 mol% was estimated from quantification of the nitrogen and hydrogen contents of a sample quenched in NH(3).

Experimental Evidence for Nitrogen as a Deep Acceptor in ZnO

ABSTRACTWhile zinc oxide is a promising material for blue and UV solid-state lighting devices, the lack of p-type doping has prevented ZnO from becoming a dominant material for optoelectronic applications. Over the past decade, numerous reports have claimed that nitrogen is a viable p-type dopant in ZnO. However, recent calculations by Lyons, Janotti, and Van de Walle [Appl. Phys. Lett. 95, 252105 (2009)] suggest that nitrogen is a deep acceptor. In our work, we performed photoluminescence (PL) measurements on bulk, single crystal ZnO grown by chemical vapor transport. Nitrogen doping was achieved by growing in ammonia. In prior work at room temperature, we observed a broad PL band at ∼1.7 eV, with an excitation threshold of ∼2.2 eV, consistent with the calculated configuration-coordinate diagram. In the present work, at liquid-helium temperatures, the PL emission increases in intensity and red-shifts by ∼0.2 eV. A peak is observed at 3.267 eV, which we tentatively attribute to an exciton bound to a nitrogen acceptor. Our experimental results indicate that nitrogen is indeed a deep acceptor and cannot be used to produce p-type ZnO.

Mechanism of proton transfer in the strong OHN intermolecular hydrogen bond

The structure of the complex of 3,5-dinitrobenzoaic acid with 3,5-dimethylpyridine was studied by neutron diffraction at 330, 300, 270, 240, 210, 180, 150, 120, 90, 60, and 30 K. The O⋯H bond length gradually changes from 1.403(10) A at 300 K to 1.424(4) A at 30 K. The proton shifts in the hydrogen bridge towards the acceptor nitrogen atom. Temperature-dependent changes in the strong OHN hydrogen bond are used to discuss the proton transfer mechanism.

Synthesis of p-type zinc oxide films by plasma-assisted pulsed laser deposition

Heavily acceptor doped zinc oxide (ZnO) films were deposited on quartz substrates by plasma-assisted pulsed laser deposition (PA-PLD) using a non-sintered target heavily doped with phosphorus or copper and radio frequency induction-coupled nitrogen or oxygen plasma (RF-ICP). The p-type ZnO layer was achieved by a nitrogen acceptor dopant using the technique of plasma-assisted nitrogen (PA-N) pulsed laser deposition. Photoluminescence spectra showed a peak from phosphorus- or copper-bound excitons at about 380 nm and a broad, green defect-related band occurring at about 550 nm. Transmission spectra showed a blue shift of the near-band-edge wavelength and a worsening of transmission by heavily copper-doped zinc oxide.

Structure and stability of N–H complexes in single-crystal ZnO

Zinc oxide (ZnO) is semiconductor with a wide band gap of 3.4 eV. It continues to gain more attention not only for its versatile use in industry but also its potential for further application in electronics, optics, spintronics, and transparent circuits. Many of these applications require p-type ZnO. Nitrogen substituting for oxygen is a possible acceptor for such applications. In this paper, we report a study of nitrogen-hydrogen (N–H) complexes grown into single-crystal ZnO, using seeded chemical vapor transport in an ammonia ambient. An infrared (IR) absorption peak arising from N–H complexes was observed at 3150.6 cm−1 at liquid-helium temperatures. The assignment of this peak was confirmed by nitrogen and hydrogen isotope substitution. Polarized IR spectroscopy shows that the N–H dipole is oriented at an angle ∼114° to the c axis, in agreement with previous first-principles calculations. To probe the stability of the N–H complexes, samples were annealed in air, oxygen, and argon. Samples annealed in oxygen at 725 °C showed a significant increase in resistivity, due to outdiffusion of hydrogen and compensation by nitrogen acceptors.