Acceptor Energy Research Articles

Modeling the effect of SiO2 additions and cooling rate on the electrical behavior of donor-acceptor codoped positive temperature coefficient thermistors

The resistivity-temperature [ρ(T)] characteristics of two series of (Ba,Ca)TiO3 based positive temperature coefficient (PTC) of resistance thermistors, prepared with different levels of SiO2 addition and cooling rates, have been analyzed in the context of the double Schottky barrier model. A significant improvement in fit, compared to earlier studies, was achieved between the modeled and experimental ρ(T) behavior over the full temperature range of the PTC transition through the incorporation of experimentally obtained permittivity data, coupled with an optimized acceptor energy spread (0.40±0.05eV). The permittivity below the Curie temperature (TC) increased slowly when either the SiO2 content or cooling rate was increased, but above TC the permittivities were almost unaffected. The bulk resistivity and average grain size were affected only slightly by increases in SiO2 addition or cooling rate, but the gradient of the ρ(T) curve and peak resistivity gradually decreased due to a reduction in surface state density from 1.1×1014to5.9×1013cm−2 at an acceptor energy depth of 1.57–1.60eV. A reduction in room temperature resistivity with increasing SiO2 content and cooling rate was consistent with a concomitant reduction in the ratio of residual surface charge unit density to permittivity.

Influence of the energy of H-bond protons on the electron transfer rate in photosynthetic reaction centers

We present here a theoretical interpretation of the temperature dependence of the rate of dark recombination between a primary quinone (QA) and a bacteriochlorophyll dimer in the reaction center of Rhodobacter sphaeroides. We were able to describe qualitatively the nonmonotonous character of this dependence using the energy of interaction between an excess electron and H-bond protons. We considered a molecular model of QA and two reaction center fragments that make H-bonds with QA: His(M219) and Asn(M259)-Ala(M260). We used the two-center approach with regard for electron-phonon interaction in order to calculate the characteristic time of electron tunneling during the recombination reaction. The energy of the phonon emitted/ absorbed during the electron tunneling was determined by the relative shift of donor and acceptor energy levels, the detuning of levels. The detuning was shown to depend on temperature nonmonotonously for H-bonds with double-well potential energy surface. The characteristic time (or the reaction rate) depended on temperature parametrically. The computed dependence was in qualitative agreement with the experimental one.

Non-Gaussian statistics of binding/unbinding events and the energetics of electron transfer reactions

We describe a model of electron transfer reactions affected by local binding to the donor or acceptor sites of a particle in equilibrium with the solution. The statistics of fluctuations of the donor–acceptor energy gap caused by binding/unbinding events are non-Gaussian, and the resulting free energy surfaces of electron transfer are non-parabolic. The band-width of the charge-transfer optical transition is predicted to pass through a maximum as a function of the concentration of binding particles in the solution. The breakdown of the link between the Stokes shift and the reorganization energy of electron transfer is also predicted by the model.

Mg acceptor energy levels inAlxInyGa1−x−yNquaternary alloys: An approach to overcome thep-type doping bottleneck in nitrides

The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.

Electronic structures of wurtzite ZnO, BeO, MgO and p-type doping in Zn1−xYxO (Y = Mg, Be)

Using the density function theory within the generalized gradient approximation, the band structures of wurtzite ZnO, BeO and MgO have been calculated. The effective-mass parameters are fitted using the calculated eigenvalues. The Dresselhaus spin–orbit effect appears in the k[100] direction, and is zero in the high symmetry direction k[001]. The orderings of valence band split by the crystal-field and spin–orbit coupling in wurtzite ZnO, BeO and MgO are identified by analyzing the wave function characters calculated by projecting the wave functions onto p-state in the spherical harmonics. For wurtzite ZnO, the ordering of valence band is still Γ7>Γ9>Γ7 due to the negative spin–orbit coupling splitting energy and the positive crystal-field splitting energy. Thus, the Thomas’ conclusion is confirmed. For wurtzite BeO and MgO, although their orderings of valence bands are Γ7>Γ9>Γ7 too, the origins of their orderings are different from that of wurtzite ZnO. Zn1−xYxO (Y=Mg, Be) doped with N and P atoms have been studied using first-principles method. The calculated results show that N atom doped in Zn1−xBexO has more shallow acceptor energy level with increasing the concentration of Be atom.

Effect on nitrogen acceptor as Mg is alloyed into ZnO

Our Raman measurement indicates that the intensity of the peaks (510 and 645cm−1) related to nitrogen concentration is enhanced in MgZnO compared with that in ZnO. Using first-principles band structure methods, we calculated the formation energy and transition energy level for nitrogen acceptor in ZnO and random MgxZn1−xO (with x=0.25) alloy. Our calculations show that the incorporation of nitrogen can be enhanced as Mg is alloyed into ZnO, which agrees with our experiments. The acceptor energy level deeper in the alloy ascribes to the downward shift of the valence-band maximum edge in the presence of magnesium.

The influence of cooling rate and SiO 2 additions on the grain boundary structure of Mn-doped PTC thermistors

BaTiO 3 based positive temperature coefficient of resistance (PTC) thermistors were prepared with 0, 1.0, 2.0 and 3.0 at.% SiO 2 additions. The effects of these SiO 2 additions and cooling rate variations on microstructural development and bulk electrical performance are discussed in the context of the double Schottky barrier (DSB) model. An increase in SiO 2 content increased the proportion of triple junctions containing BaTiSi 2O 8, but reduced the density slightly, without affecting the grain size. The bulk charge carrier density was unaffected either by SiO 2 level or changes to the cooling rate. Adding SiO 2 or increasing the post sintering cooling rate decreased ρ 25 and ρ max, and increased Tρ max. These effects were attributed to a reduction in activated surface state density, while maintaining a fixed acceptor energy depth. By assuming a single acceptor state energy level, satisfactory agreement between theory and experimental PTC behaviour was only obtained at Tρ max. Agreement was extended over a wider range of temperatures when the acceptor states were considered to be spread over a broader energy interval.

Using phosphorescence as a fingerprint for the Hope and other blue diamonds

Sixty-seven natural blue diamonds, including the two largest such gemstones known (the Hope and the Blue Heart), were probed by ultraviolet radiation, and their luminescence was analyzed using a novel spectrometer system. Prior to this study, the fiery red phosphorescence of the Hope Diamond was regarded as quite rare compared to greenish-blue phosphorescence. However, our results demonstrated that virtually all blue diamonds phosphoresce at 660 nm (orange-red) but that this emission often is obscured by a concomitant luminescence at 500 nm (green-blue). Although both bands were nearly always present, the relative intensities of these emissions and their decay kinetics varied dramatically. Consequently, phosphorescence analysis provides a method to discriminate among individual blue diamonds. Treated and synthetic blue diamonds showed behavior distinct from natural stones. Temperature-dependent phosphorescence revealed that the 660 nm emission has an activation energy of 0.4 eV, close to the 0.37 eV acceptor energy for boron, suggesting that the phosphorescence is caused by donor-acceptor pair recombination.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

First-principles study of color centers in PbMoO4 crystals

Electronic structures of PbMoO4 crystals containing Mn ion impurities located at Pb2+ sites are studied within the framework of the fully relativistic self-consistent Direc–Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that Mn2+ ions have donor energy levers in the forbidden band, which may correspond to the yellowish color absorption band of PbMoO4 as-grown in air. The new-formed Mn3+ ions have acceptor energy levers in the forbidden band, which may relate to the photochromic effect in PbMoO4 crystal.

Modeling the resistance-temperature characteristic of a positive temperature coefficient thermistor, using experimentally determined permittivity data

The resistivity-temperature [ρ(T)] and current-voltage [j(V)] characteristics of a (Ba,Ca)TiO3 based donor/acceptor codoped positive temperature coefficient of resistivity thermistor have been analyzed in the context of the double Schottky barrier (DSB) model, using experimentally measured permittivity data. The best fit between the modeled and experimental ρ(T) behavior occurred for a broad acceptor energy fluctuation width (Δ=0.35eV). Modeling the ρ(T) behavior below the Curie temperature required a nonvanishing surface charge, consistent with a residual potential barrier. j(V) characteristics showed satisfactory agreement with the DSB model at low applied voltages per grain boundary (Vgb⩽0.13–0.22eV).

Donor and acceptor energies for muonium in GaAs

Donor and acceptor energies for muonium in GaAs

Chemical impurity effects on transport in polymer transistors

Exposure of unencapsulated bottom gate polythiophene thin film transistors (TFTs) to several organic and inorganic volatile chemicals induces large changes in the TFT properties. Various neutral molecules cause the mobility of the TFT to decrease and enhance the bias stress effect, with the changes being reversible upon annealing. The effect on the mobility and stress are characteristically different for each compound and are attributed to different preferred locations in the host. Some molecules cause electronic doping of the semiconductor with corresponding changes in threshold voltage. In the case of the donor, ammonia, a gate-induced incorporation rate is observed. The distribution of acceptor energies of the recently found ozone acceptor is analyzed.

Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N 2 and O 2

Zinc oxide (ZnO) thin films prepared on sapphire substrates by employing a pulsed laser deposition (PLD) have been annealed in vacuum, N 2, and O 2 ambient at annealing temperature 600 °C. X-ray diffraction (XRD) and atomic force microscopy (AFM) observation show that the annealed thin films possess (0 0 0 2) textured feature and form better crystal grains with a large size. X-ray photoelectron spectroscopy (XPS) and Raman analysis show the defects of ZnO thin films annealed in different ambient is distinctly different. The film annealed in vacuum possesses large oxygen vacancies (V o) and Zn interstitials (Zn i); while large surface defects exist in film annealed in N 2 ambient. The concentrations of the intrinsic and extrinsic defects are the lowest in ZnO thin film annealed in oxygen gas. Photoluminescence (PL) spectra also reveal that the ultraviolet (UV) emission is the best for thin film annealed in O 2. The blue emission (2.66 eV) is ascribed to the electronic transition from the donor energy level of Zn interstitials to acceptor energy level of Zn vacancies rather than the O vacancy.

Population dynamics of the 3F4 and 3H4 levels in highly-doped Tm3+:ZB(L)AN glasses

Population dynamics of the 3F4 and 3H4 levels in Tm3+ doped ZB(L)AN glasses was studied for Tm3+ concentrations from 0.5 to 12mol%. Fluorescence waveforms from these levels were measured at 1.8μm (3F4) and 800nm (3H4) with both direct and indirect pumping. Decay from the 3F4 level was found to be exponential with non-radiative decay rates proportional to the square of the Tm concentration. This indicated a process of energy migration by diffusion within the excited Tm3+ ions followed by quenching at sites to which the ions could migrate. The decay of the directly pumped 3H4 level exhibited both exponential and non-exponential behavior depending on the concentration. For the lowest concentration (0.5mol%) the decay was exponential, but at concentrations of 1, 2, 4 and 6mol% the decay waveforms were distinctly non-exponential. The non-exponential waveforms could be fitted by the Yokota–Tanimoto model for diffusion of excited donors and dipole–dipole interactions with acceptors. This model produced values for CDD and CDA, the donor–donor and donor–acceptor energy transfer parameters, respectively. At the higher concentrations (8, 10, 12mol%) the waveforms were exponential with decay rates from which the cross-relaxation parameter for the process 3H4, 3H6→3F4, 3F4 was obtained. When the 3F4 level is pumped at 1660nm, the decay of the 3H4 level confirmed the influence of the up-conversion energy transfer process 3F4, 3F4→3H4, 3H6.

Blue Luminescent Center and Ultraviolet-Emission Dependence of ZnO Films Prepared by Pulsed Laser Deposition

ZnO thin films were grown on sapphire substrates at a growth temperature of 400 °C by Nd:yttrium–aluminum–garnet (Nd:YAG 1064 nm) pulsed-laser deposition. The effects of oxygen pressure on the structure, stoichiometric composition and optical properties of ZnO thin films were investigated in detail. X-ray diffraction (XRD) shows that the crystal quality is highest for the ZnO film grown at a pressure of 10 mTorr. Optical transmittance, electronic properties and Raman spectra show the increase in the stoichiometric composition ratio of ZnO films with increasing oxygen pressure owing to the reduction in the number of oxygen vacancies (VO) and Zn interstitials (Zni). Photoluminescence (PL) spectra reveal that the UV-emission properties have greater dependence on the stoichiometriy of ZnO films than on the crystal quality. A blue-light emission peak at about 465 nm (Eg=2.66 eV) was observed in the PL spectra of ZnO thin films fabricated at various oxygen pressures. This emission peak was ascribed to the electronic transition from the donor energy level of the Zn interstitials to the acceptor energy level of the Zn vacancies (VZn) rather than the oxygen vacancies.

A Simple Multichromophore Design for Energy Transfer in Distyrylbenzenes with Pyrene Pendants

A set of Don --> Acc <-- Don multichromophores was synthesized based on a simple design with two pyrene donors (absorbing antennae) connected to a bis-1,4-(3,4,5-trimethoxystyryl)benzene core acceptor using various flexible, nonconjugated tethers. Excitation of the pyrene donors at 276 nm in solution yields near exclusive emission from the core chromophore at 445-450 nm, with energy transfer efficiencies up to 92%, far better than achieved with simple mixtures. The simple tethering design imposes a high "local" concentration of the pyrene near the acceptor core unit that is maintained even at very low multichromophore concentrations. Solvent effects on absorption and emission spectra are very small, except in cases where a pi-conjugating O-C=O moiety of the tethering group is directly attached to the core chromophore, rather than being placed in the middle of the tether. Energy transfer in the systems is effective due to good donor-acceptor energy matching. The optimal energy transfer efficiency was achieved using an eight-atom flexible linker.

Study of energy transfer in gadolinium—gallium garnet crystals doped with Yb3+ and Ho3+ ions

The luminescence kinetics of Yb3+ donors and Ho3+ acceptors is quantitatively studied in gallium—gadolinium garnet (GGG) crystals doped with Yb3+ and Ho3+ ions. It is shown that the sensitisation of transitions in Ho3+ ions occurs due to migration-accelerated (hopping) energy transfer. The microparameters of donor—donor energy transfer are determined at 300 and 77 K. The microparameters of donor—acceptor energy transfer are found at the same temperatures at the first stage of successive sensitisation (resulting in the population of the 5I6 state of Ho3+ ions) and at the second stage of successive sensitisation of the 5S2, 5F4→5I8 transition in Ho3+ ions. At the second stage of sensitisation, the values of the microparameter of reverse energy transfer are also determined. The possibility of obtaining lasing at sensitised transitions in Ho3+ ions in Yb3+:Ho3+:GGG crystals pumped into the absorption band of Yb3+ ions is discussed.

Effect of boron on the superconducting transition of heavily doped diamond

We have performed electronic structure calculations by means of density functional theory combining plane-wave method with cluster method for the heavily boron-doped diamond. The results for an isolated boron center with different doping levels indicate an acceptor energy level apart from the top of the valence band, and the results for the heavily boron-doped diamond, considering the interaction of boron centers, show the impurity levels mixed with valence band edge and the Fermi level located in the valence band. Consequently, the results support a mechanism of the type of metal–superconductor transition and indicate the critical temperature T c should be related to the B concentration.

Ultrafast fluorescence resonance energy transfer in a micelle

Ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) to rhodamine 6G (R6G) is studied in a neutral PEO(20)-PPO(70)-PEO(20) triblock copolymer (P123) micelle and an anionic micelle (sodium dodecyl sulfate, SDS) using a femtosecond up-conversion setup. Time constants of FRET were determined from the rise time of the acceptor emission. It is shown that a micelle increases efficiency of FRET by holding the donor and the acceptor at a close distance (intramicellar FRET) and also by tuning the donor and acceptor energies. It is demonstrated that in the P123 micelle, intramicellar FRET (i.e., donor and acceptor in same micelle) occurs in 1.2 and 24 ps. In SDS micelle, there are two ultrafast components (0.7 and 13 ps) corresponding to intramicellar FRET. The role of diffusion is found to be minor in the ultrafast components of FRET. We also detected a much longer component (1000 ps) for intramicellar FRET in the larger P123 micelle.