Formation Of Donor-acceptor Pairs Research Articles

Metal-Loaded Synthetic Melanin via Oxidative Polymerization of Neurotransmitter Norepinephrine Exhibiting High Photothermal Conversion.

Polydopamine (PDA)-derived melanin-like materials exhibit significant photothermal conversion owing to their broad-spectrum light absorption. However, their low near-infrared (NIR) absorption and inadequate hydrophilicity compromise their utilization of solar energy. Herein, we developed metal-loaded poly(norepinephrine) nanoparticles (PNE NPs) by predoping metal ions (Fe3+, Mn3+, Co2+, Ca2+, Ga3+, and Mg2+) with norepinephrine, a neuron-derived biomimetic molecule, to address the limitations of PDA. The chelation between catechol and metal ions induces a ligand-to-metal charge transfer (LMCT) through the formation of donor-acceptor pairs, modulating the light absorption behavior and reducing the band gap. Under 1 sun illumination, the Fe-loaded PNE coated wood evaporator achieved a high seawater evaporation rate and efficiency of 1.75 kg m-2 h-1 and 92.4%, respectively, owing to the superior hydrophilicity and photothermal performance of PNE. Therefore, this study offers a comprehensive exploration of the role of metal ions in enhancing the photothermal properties of synthetic melanins.

Pyroelectric properties of the wide-gap semiconductor CdS in the low-temperature region

Spontaneous polarization, thermally stimulated conductivity and depolarization are comprehensively studied in the range from 4.2 to 300 K on nonstoichiometric n-type CdS crystals grown from the gas phase in an argon atmosphere at T = 1450 K. The objects of study are initial samples and samples polarized by a weak electric field at T = 4.2 K. Sample polarization results in a decrease in the conductivity σ33 due to restructurization of the entire energy level spectrum associated with the formation of donor-acceptor pairs. The latter processes also contribute to the temperature dependences of the spontaneous polarization and the pyroelectric effect, characterized by the formation of anomalies below 15 K and the formation of thermoelectret. The role of an uncontrollable oxygen impurity in the formation of CdS cationic conductivity above 270 K, associated with the decay of a fraction of donor-acceptor pairs, is discussed. In the temperature range from 20 to 250 K, the pyroelectric coefficient and spontaneous polarization are independent of external influences within experimental error; at T = 200 K, they are ΔPs = −(6.1 ± 0.2) × 10−4 C/m2 and γs = −(4.1 ± 0.3) × 10−5 C/m2 K.

Effect of optical and thermal stimulation on GaAs photosensitivity

We have studied the effect of continuous band-gap photoexcitation intensity on the extrinsic photosensitivity of structures based on semi-insulating GaAs with an epitaxial GaAs〈S〉 layer (n = 7 × 1015 cm−3). The results demonstrate that combined photoexcitation leads to sensitization of the structure in the low-energy (hν < 0.76 eV) region of the impurity photoconductivity spectrum owing to arsenic vacancies, isolated or involved in the formation of donor-acceptor pairs, which act as donors. The observed increase in photosensitivity in the spectral range hν < 0.76 eV is due to the increase in the concentration of (D0-A0)0* excited pairs. Weakly photoactive trapping centers (which are commonly studied by thermally stimulated conductivity measurements) can be identified by analyzing the temperature dependence of impurity photoconductivity due to deep centers at Ec − (0.95–1.15) eV.

Effect of halogens on the formation and properties of the porous silicon layers

The method of atomic-force microscopy is used to study the morphology of the surface of porous silicon layers formed on the p-Si substrate and obtained by anodic etching in an electrolyte with addition of free halogens (bromine, iodine) and potassium halogenides (KCl, KI). It is established that the presence of halogens in the electrolyte is conducive to formation of large pores with the diameter as large as 150 nm. The mechanism of an increase in the pore sizes with involvement of halogens is related to an increase in the concentration of free holes due to formation of donor-acceptor pairs in the case of adsorption of halogens on the silicon surface.

Impurity absorption and luminescence of CuGaSe2 crystals

A comparison of the experimental and calculated absorption spectra of CuGaSe2 crystals revealed the existence of two acceptor levels with ionization energies of 66 and 167 meV in the samples under study. It was found that the luminescence spectra of CuGaSe2 measured at 77 K exhibit four impurity-band transitions with impurity ionization energies of 66, 99, 136, and 190 meV. An analysis of the temperature dependence of the luminescence intensity in the temperature range 11–77 K revealed a band peaking at 1.671 eV due to the radiation of donor-acceptor pairs. The calculated sum of the ionization energies of the impurities responsible for the formation of donor-acceptor pairs and the temperature dependence of the relative intensities of impurity-band emission were used to determine the ionization energies of the corresponding donor and acceptor.

Interaction of nitrogen dioxide molecules with the surface of silicon nanocrystals in porous silicon layers

The methods of infrared absorption spectroscopy and electron paramagnetic resonance are used for studying the effect of adsorption of NO2 molecules, which are strong acceptors of electrons, on the electronic and optical properties of silicon nanocrystals in mesoporous silicon layers. It is found that the concentration of free charge carriers (holes) in silicon nanocrystals, which exhibits a nonmonotonic dependence on the NO2 pressure, sharply increases in the presence of these molecules. At the same time, a monotonic increase in the concentration of dangling silicon bonds (Pb1 centers) is observed. A microscopic model proposed for explaining this effect presumes the formation of donor-acceptor pairs P+b1-(NO2)− on the surface of nanocrystals, which ensure an increase in the hole concentration in nanocrystals, as well as Pb1 centers, which are hole-trapping centers. The proposed model successfully explains a substantial increase in photoconductivity (by two or three orders of magnitude) in the layers of porous silicon in the presence of NO2 molecules; the increment in the concentration of free charge carriers is detected within an order of magnitude of this quantity. The results can be used in designing electronic and luminescence devices based on silicon nanocrystals.

Low temperature studies in LiF:Mg,Cu,P.

Despite extensive investigations carried out in LiF:Mg,Cu,P, the nature of the emission centre is not clearly understood. Results of X ray excited emission in this material at room temperature and at 16 K are presented to obtain more data that can throw light on the emission characteristics of this material. At room temperature only a single emission peak is seen around 390 nm but at 16 K the emission appears to consist of more than one emission band. The emission spectra could be fitted with three bands with peaks at 332, 385 and 447 nm. The X ray excited emission at 16 K after annealing at 573 K for 5 min suppresses the 332 nm emission but enhances 447 nm emission. Moreover, annealing at 573 K greatly reduces the emission intensity, which signifies that the luminescent centres are also destroyed in this process. Temperature-dependent X ray excited fluorescence below room temperature provides evidence of the existence of shallow traps, which give rise to a thermoluminescence peak around 130 K. On the basis of the present investigations it is proposed that the complex nature of the emission may be related to the formation of donor-acceptor pairs in this material. The possible nature of this donor-acceptor complex is discussed.

Low-temperature time-resolved photoluminescence in InGaN/GaN quantum wells

The results of the investigation of low-temperature time-resolved photoluminescence in undoped and Si-doped In0.2Ga0.8N/GaN structures, which contain 12 quantum wells of width 60 A separated by barriers of width 60 A, are reported. The structures were grown by the MOCVD technique on sapphire substrates. The photoluminescence properties observed are explained by the manifestation of two-dimensional donor-acceptor recombination. These properties are the high-energy shift of the peak upon increasing the pumping intensity, a low-energy shift with increasing delay time, and a power law of luminescence decay of the t -γ type. The estimates of the total binding energy for donor and acceptor centers are given. This energy is 340 and 250 meV for Si-doped and undoped quantum wells, respectively. The role of the mosaic structure, which is typical for Group III hexagonal nitrides, is discussed as a factor favorable for the formation of donor-acceptor pairs.

Native Point Defect Interactions in ZGP Crystals under Influence of e-Beam Irradiation.

ABSTRACTOptical absorption in the defect-related region of highly efficient non-linear ZnGeP2 crystals under e-beam irradiation and post-irradiation anneals has been investigated.Partially irreversible changes of the absorption were found in the spectral range 0.9–2.5 μm (0.5–1.3 eV) after irradiation and subsequent low-temperature anneals. Data obtained do not support the vacancy model for ZnGeP2 absorption in the 0.5–1.3 eV range.The least squares fit for the parameters of the theoretical dependence of optical absorption cross-section to the experimentally measured ZnGeP2 optical absorption coefficient spectra show that the defect-related absorption in 0.5–1.3 eV region is caused by deep donor levels with energy position E=Ev+(0.85–0.90) eV.Significant changes in the energy spectrum of the dominant optically active centers have been observed under influence of e-beam irradiation and post-irradiation anneals.Based on the optical absorption measurements obtained for as-grown, annealed and e-irradiated ZnGeP2 crystals, a model of point defect interactions has been proposed. This takes into account both the reversible interactions, such as the formation of donor-acceptor pairs, and the irreversible interactions of a quasi-chemical type.The behavior of the energy spectrum of the optically active defects is discussed in terms of the modes of interaction between the initial point defects and those generated by irradiation. The analysis performed showed that the best agreement with experimental data is reached when it is assumed that optical defect-related absorption in the 0.5–1.3 eV range related mainly to the disordering defect in the cation sublattice of ZnGeP2, namely, to atoms of Ge substituting for Zn.Defect concentration profiles created by e-irradiation in ZGP crystals of different thickness were calculated. The optimum conditions for providing a uniform defect distribution with depth in irradiated ZnGeP2 samples were determined.The optimal e-beam irradiation fluences, giving maximum ZnGeP2 enlightenment, allowed us to reduce the defect-related absorption down to a value of 0.01 cm-1 at 2 μm.

Spectral shift of photoluminescence bands of the (SiC)1−x (AlN)x epitaxial films due to laser annealing

The effect of laser annealing on the photoluminescence properties of (SiC)1−x (AlN)x epitaxial films was studied. It was proposed that annealing causes the displacement of the Al and N atoms from their lattice sites and the formation of AlSi-NC donor-acceptor pairs acting as the luminescence centers. According to this model, the increase in the annealing time is accompanied by the formation of donor-acceptor pairs with the shortest interatomic distances at the expense of associations of the distant defects and by a shift of the respective photoluminescence band to the high-energy spectral region.

PHOTOLUMINESCENCE STUDIES IN CuInxGa1-xSe2 FILMS PREPARED BY GRAPHITE BOX ANNEALING OF In/Ga/Cu/Se STACKED ELEMENTAL LAYERS

CuIn x Ga 1-x Se 2 films were synthesized by graphite box annealing of In/Ga/Cu/Se stacked elemental layers deposited by thermal evaporation onto sodalime glass substrates. Photoluminescence (PL) studies were carried out in near stoichiometric and copper-rich films with Ga/(In+Ga) ratios varying between 0.24–0.29. The PL spectra were dominated by peaks at ~1.19 eV and 0.87 eV with a shoulder at ~0.84 eV. There was also a peak around 1.3 eV which could be assigned to the absorption from the split-off band. The transitions at ~1.19 eV and 0.87 eV were due to excitonic transition and donor-acceptor pair formation respectively.

Physics of Neutral-to-Ionic Phase Transition in Organic Charge Transfer Semiconducting Compounds

An uncommon excitonic instability takes place in some exotic semiconducting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transforma tion, observed in some organic charge-transfer complexes, originate from intraand inter-chain cooperative effects between structurally relaxed charge-transfer excitations. This electronic-structural phase transition manifests itself by a change of the degree of charge-transfer and a dimerization distortion with the formation of donor-acceptor pairs along the stacking axis in the I phase. Thermal charge-transfer excitations associated with the formation of I strings along N chains are at the heart of the mechanism of this phase transition. These relaxed electronic excitations, which are an intrinsic feature of low-dimensional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer excitons. Precise structural studies on the prototype compound, tetrathiafulvalene-p-chloranil allow to highlight the respective pole taken by the ionicity and the dimerization. Symmetry and thermodynamics analysis of the N-I transition, based on recent determination of the pressure-temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-transfer excitations, following a phase diagram analogous to the solid—liquid—gas one.

Energy transfer from colour centres to the dopant in alkali halides

Energy transfer from F centres to the dopant ions in photostimulated processes is examined. It was found that in alkali halide systems the radiation defects are disposed in near vicinity of the dopants. Recombination of defects also takes place to a limited extent near the dopant with a fast and high-yield energy transfer to the latter. Role of unrelaxed H centres in the formation of donor-acceptor pairs is discussed. Some special exciton energy transfer mechanisms are analysed as well.

Electron-paramagnetic-resonance identification of transition-metal (Cr,Fe)-shallow-impurity (S,Zn) pairs in GaP.

Three Fe- or Cr-related EPR spectra are observed in liquid-encapsulated-Czochralski-grown GaP. The spectra in n-type GaP:Fe:S and GaP:Cr:S show trigonal symmetry. Their analysis suggests that the spectra are caused by the donor-acceptor nearest-neighbor pair defects [${\mathrm{Fe}}_{\mathrm{Ga}}^{+}$-${\mathrm{S}}_{\mathrm{P}}$] and [${\mathrm{Cr}}_{\mathrm{Ga}}^{+}$-${\mathrm{S}}_{\mathrm{P}}$]. In p-type GaP:Cr:Zn the spectrum reveals a Cr center of monoclinic symmetry. This center is identified as a [${\mathrm{Cr}}_{\mathrm{Ga}}^{3+}$-${\mathrm{Zn}}_{\mathrm{Ga}}$] pair. The concentration of the three pair defects shows that Coulomb attraction is responsible for the donor-acceptor pair formation.

Donor-Acceptor Pairs in Silicon

ABSTRACTStable forms of donor-acceptor pairs in silicon arise from the interaction between negatively charged acceptors and 3d transition element donors, which, occupying interstitial sites, have high mobilities. The atomic and electronic structure of these pairs can be studied in detail by magnetic resonance. The symmetry of centers as observed in the resonance experiments gives strict constraints on the applicable atomic models. The spin of centers is related to their charge state. Hyperfine interactions frequently lead to a specific structure in the spectra, which characterizes the chemical identity of atomic components of the pair. Deep level transient spectroscopy provides a way of sensitive observation of electrically active impurities: to determine their concentrations and associated electronic levels. It allows the study of the kinetics of the pair formation process and thermally induced dissociation; binding energies can be determined. Transformation of pairs as a result of illumination was also observed. Several of the pairs can exist in geometrically different atomic configurations leading to the phenomenon of bi- or multi-stability. In the paper the donor-acceptor pair formation process is illustrated by examples involving the substitutional double acceptor zinc, for which new data became recently available.

Electrical characterization of gallium planar-doped ZnSe grown by molecular-beam epitaxy

We report electrical characterization of a series of ZnSe samples which are planar doped by a new approach to doping involving periodic deposition of sheets of Ga on Zn- or Se-rich surfaces. For samples planar doped on Zn-rich surfaces, the mobility could be described by ionized-impurity scattering and polar optical-phonon scattering mechanisms, while planar-doped samples on Se-rich surfaces and uniformly doped samples require additional scattering mechanisms to describe their mobilities. The latter two cases give higher mobilities than the first, a result which is in conflict with the fact that the latter have higher total ionized-impurity concentrations and compensation. These results of higher mobilities along with higher total ionized-impurity concentrations are interpreted as evidence of donor-acceptor pair formation.

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor-acceptor pairs (DAP) with a maximum at about 4200 Å has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to E D = = 0.045−0.065 eV and E A = 0.155−0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 Å) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton-phonon interaction (LO − 18.7 meV) has been determined.

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor—acceptor pairs (DAP) with a maximum at about 4200 A has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to ED = = 0.045–0.065 eV and EA = 0.155–0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 A) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton—phonon interaction (LO − 18.7 meV) has been determined.

The influence of donor-acceptor pair formation and space charge on the fabrication of p-n junctions by diffusion : A. V. Shelley and R. H. Tredgold. Solid-St. Electron., Vol. 13 (1970), p. 1219

The influence of donor-acceptor pair formation and space charge on the fabrication of p-n junctions by diffusion : A. V. Shelley and R. H. Tredgold. Solid-St. Electron., Vol. 13 (1970), p. 1219

The influence of donor-acceptor pair formation and space charge on the fabrication of p- n junctions by diffusion

The combined effects of donor-acceptor pair formation and space charge on the fabrication of p- n junctions by diffusion are considered. Pair formation is described by a non-equilibrium second-order, rate equation and the space charge field by Zaromb's approximation. It is shown that the diffusion of a singularly ionized impurity of one type, into a semiconductor uniformly doped with an impurity of the opposite type can, as the result of pair formation, produce relatively sharp, asymmetric junction profiles at considerable junction depths.