Distant Donor-acceptor Research Articles

Fast Charge Separation in Distant Donor-Acceptor Dyads Driven by Relaxation of a Hot Excited State.

A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (β) while discussing anomalous CS trends.

Detailed photoluminescence study of Cu2Ge(SSe)3 microcrystals

We present a detailed temperature and laser power dependent photoluminescence (PL) study of Cu2Ge(S0.4Se0.6)3 microcrystals. At T = 20 K, two relatively narrow PL peaks were detected at about 1.16 eV (peak No. 1) and 1.12 eV (peak No. 2) and a weak, broad PL band was detected at about 0.82 eV (peak No. 3). The temperature and laser power dependencies indicate that at T = 20 K, the properties of PL peak Nos. 1 and 2 can be explained by the distant donor–acceptor (DA) pair model where a donor defect has a depth of ED ≈ 20 meV and ED ≈ 60 meV for peak Nos. 1 and 2, respectively. The depth of acceptor defects was 57 and 76 meV for peak Nos. 1 and 2, respectively. At around T = 90 K, the DA pair recombination of peak No. 1 gradually starts to transform into the conduction band–acceptor recombination, but peak No. 2 shows a DA pair recombination even at room temperature. The estimated bandgap energy of this compound at room temperature was Eg = 1.225 eV.

Broad-band photoluminescence of donor–acceptor pairs in tetrahedrite Cu10Cd2Sb4S13 microcrystals

We present temperature and laser power dependent photoluminescence (PL) study of Cd substituted tetrahedrite Cu10Cd2Sb4S13 microcrystals. At T = 10 K a single broad, asymmetric and structureless PL band was detected at about 1.08 eV. The temperature and laser power dependencies indicate that the properties of PL emission can be explained by the distant donor–acceptor (DA) pair model, where a donor defect has a depth of E D ≈ 30 meV and an acceptor defect E A = 88 ± 6 meV. It was shown that the shape of the DA pair band could be effectively described using statistical distribution of donor–acceptor defects, recombination probability of DA pairs with different spatial separation, relatively strong electron–phonon coupling and occupation probabilities of donor and acceptor defects. At T = 200 K the DA pair recombination gradually starts to transform into conduction band-acceptor recombination.

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (TD) AgCd acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P3/2 (Г8) and 2S3/2 (Г8) states is clearly observed for AgCd centers located at a short distance (5–7 nm) from a hydrogen-like donor (ClTe). This splitting results from the reduction of the TD symmetry taking place when the acceptor is a member of a donor–acceptor pair. For the Cl-related complex acceptor with an activation energy of ∼121 meV (A-center), the energies of eight excited states are measured. It is shown that this defect produces low-symmetry central-cell correction responsible for the strong splitting of S-like TD shells. The energy spectrum of the Bi-related shallow acceptor with an activation energy of ∼36 meV is measured as well. The spectrum obtained differs drastically from the hydrogen-like set of levels, which indicates the existence of repulsive low-symmetry perturbation of the hydrogen-like Coulomb potential. It is also shown that the spectra of selectively excited PL recorded for a macroscopic ensemble of distant donor–acceptor pairs allow one to detect the low symmetry of acceptors of a given type caused by their complex nature or by the Jahn–Teller distortion. This method does not require any additional (external) field and is applicable to acceptors in diverse zinc-blende compound semiconductors.

Optical Properties of Heavily Mg-Doped p-GaN Films Prepared by Reactive Ion Etching

The effect of the damage caused by reactive ion etching on the optical properties of heavily Mg-doped p-type GaN (p-GaN) was investigated in this study. After etching, we found that the blue luminescence (BL) is redshifted and the intensity of the redshifted BL is markedly increased at room temperature, which are attributed to the formation of the lattice disorder resulting in the domination of distant donor–acceptor pairs (DAPs) and the formation of more nitrogen-vacancy-related defects resulting in more recombination centers D responsible for DAP transition.

Selective excitation of the yellow luminescence of GaN

The yellow luminescence of n-type GaN has been studied with selective excitation using a combination of Ar+-ion and dye lasers. Narrower structures whose peak energies follow the excitation photon energy over the width of the yellow luminescence have been observed. Unlike the yellow luminescence excited by the above band gap excitations, these fine structures exhibit thermal activated quenching behavior. We propose that these fine structures are due to emission occurring at complexes of shallow donors and deep acceptors which can be resonantly excited by photons with energies below the band gap. The activation energy deduced from their intensity is that for delocalization of electrons out of the complexes. Our results therefore suggest that there is more than one recombination channel (usually assumed to be due to distant donor–acceptor pairs) to the yellow luminescence in GaN.

Selective excitation and thermal quenching of the yellow luminescence of GaN

We report the observation of narrower structures in the yellow luminescence of bulk and thin-film n-type GaN, using the technique of selective excitation. These fine structures exhibit thermal quenching associated with an activated behavior. We attribute these fine structures to phonons and electronic excitations of a shallow donor-deep acceptor complex, and determine its activation energy for delocalization. Our results suggest that in addition to distant donor-acceptor pairs, the yellow luminescence can also involve emission complexes of shallow donors and deep acceptors.

Electron tunneling in biological molecules

Abstract

Behavior of 2.8- and 3.2-eV photoluminescence bands in Mg-doped GaN at different temperatures and excitation densities

The blue and ultraviolet photoluminescence bands in Mg-doped GaN have been investigated over a wide range of temperatures and excitation intensities. Redshifts of the bands were observed with increasing temperature. The bands underwent a blueshift with increased excitation density. The observed shifts of the 3.2 eV band are explained by a potential fluctuation model for a compensated semiconductor. In contrast, the shifts of the 2.8 eV band are essentially related to saturation of luminescence from distant donor-acceptor pairs responsible for this emission. Thermal quenching of the 2.8 eV luminescence band was observed at high temperatures with an activation energy of 0.3--0.4 eV. It is attributed to thermal release of trapped electrons from a deep donor state.

Origin of defect-related photoluminescence bands in doped and nominally undoped GaN

The efficient room-temperature photoluminescence bands of wurtzite GaN, which are peaked in the red (1.8 eV), the yellow (2.2 eV), and the blue (2.8 eV) spectral range, have been studied as a function of doping (species and concentration) and excitation power density (PD). It is shown that the yellow and the blue band are induced by Si and Mg doping, respectively, while codoping with Si and Mg generates the red band. At high-doping levels, the yellow and the blue band reveal strong peak shifts to higher energy with increasing PD providing very strong evidence for their distant donor-acceptor (DA) pair recombination character. The deep centers involved in DA recombination having electrical activity opposite to that of the shallow level of the dopant, are suggested to arise from self-compensation and to be vacancy-dopant associates. Self-compensation is found to be weak in the case of Si doping, but significant for Mg doping. A recombination model is presented, which accounts for the essential properties of all three bands in deliberately doped GaN. These results also suggest that the yellow and the blue bands in nominally undoped GaN arise from distant DA pairs involving residual Si and Mg impurities, respectively, as well as their respective vacancy associates.

Synergetic electron transfer in molecular electronic and photosynthetic mechanisms

The flow of energy may be enhanced in non-linear systems, when the order is increased through export of entropy, facilitated by autocatalytic (feedback) loops. This principle has been applied to derive a far-from-equilibrium ‘self-organized’ mechanism of electron transfer as well as a synergetic multi-electron transfer. A key requirement is the functioning of feedback loops within the macromolecules or their environment, which can transfer small amounts of energy required for self-organization. Possible macromolecular mechanisms are discussed, by which an active friction force can be exerted on electrons during transfer. A possible active role of intervening protein structure in contrast to the passive role discussed for conventional distant donor acceptor coupling is investigated. Evidence for self-organized electron transfer in photosynthetic structures is discussed. The last step in oxygen evolution is considered to be a synergetic multi-electron transfer reaction, with oscillating behavior being a consequence of it. Quasi-symmetric molecular geometries (reaction centers, manganese complex) are considered as mechanistically essential elements facilitating symmetry breaking mechanisms.

Electron tunneling in proteins: role of the intervening medium

Analysis of electron-transfer (ET) kinetics data obtained from experiments on Ru-modified proteins (cytochrome c, azurin, myoglobin) reveals that distant donor-acceptor electronic couplings depend upon the secondary structure of the intervening polypeptide matrix. Rates of Fe2+→Ru3+ ET reactions in cytochrome c decay exponentially with tunneling-pathway length (decay constant 0.73 A–1); these rates also decay exponentially with Ru-Fe distance (decay constant 1.1 A–1). In azurin, a β-sheet protein, Cu+→Ru3+ rates exhibit an exponential Cu-Ru distance dependence with a decay constant of 1.1 A–1. Comparison of distant couplings in azurin and myoglobin suggests that hydrogen bonds are better mediators across β sheets than through α helices.

Light Generating Carrier Recombination and Impurities in Wurtzite GaN/Al2O3Grown by MOCVD

ABSTRACTWe have studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) un-doped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. From the PL data for free excitons an accurate value of the free A-type exciton binding energy and a more accurate estimate for the hole effective mass is deduced. The localization energies of the Mg and the Zn neutral acceptor bound excitons are found to be in good agreement with Haynes’ rule. A sharp emission line, assigned to free electron recombination at a 116 meV shallow acceptor, together with three additional weak zero-phonon-lines (ZPLs), assigned to distant donor-acceptor (DA) pairs, are reported for the first time. The chemical nature of this acceptor and that of three residual donors, inferred from the DA pair ZPLs, is discussed. The effects of strain in thin GaN layers on a dissimilar substrate like sapphire are emphasized with respect to the energetic position of narrow PL lines. The ODMR data obtained for undoped, Mg-doped and Zn-doped GaN layers provide insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors donotbehave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core.

31P electron-nuclear double resonance of the PIn antisite in InP:Zn detected via luminescence.

Optically detected electron-nuclear double resonance (ODENDOR) has been observed via photoluminescence from the first-neighbor $^{31}\mathrm{P}$ shell of the phosphorus antisite in zinc-doped InP. Analysis of the ENDOR data confirms a tetrahedral arrangement of $^{31}\mathrm{P}$ nuclei. The hyperfine interaction for each of these nuclei is axial with \ensuremath{\Vert}${\mathit{A}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\Vert}/h=368.0\ifmmode\pm\else\textpm\fi{}0.5 MHz and \ensuremath{\Vert}${\mathit{A}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\Vert}/h=247.8\ifmmode\pm\else\textpm\fi{}0.5 MHz. These parameters are slightly different from those reported by Jeon et al. [Phys. Rev. B 36, 1324 (1987)]. A shift of the ENDOR frequencies correlated with a change in the central nuclear-spin state has also been observed. We have been able to account for this shift with a perturbation treatment in which the electronic spin and central nuclear spin are treated exactly and a neighboring nuclear spin provides the perturbation. The best ENDOR signals are obtained with low optical-excitation-power density (\ensuremath{\sim}0.1 W/${\mathrm{cm}}^{2}$) and low microwave modulation frequency (17 Hz). These conditions emphasize the contributions to the optically detected magnetic-resonance signal from distant donor-acceptor pairs.

Optically detected magnetic resonance of the zinc vacancy in ZnS

Optical detection of magnetic resonance (ODMR) is reported for the single negative charge state, V Zn −, of the isolated zinc vacancy in ZnS. Produced by 2.5 MeV electron irradiation, it is detected in a distant donor-acceptor (DA) pair luminescent band at 570 nm in which the vacancy acts as the acceptor. Excitation and emission spectral dependences of the V Zn − ODMR signals are analyzed in terms of a configurational coordinate model. We conclude that the double acceptor level (V Zn =/V Zn −) is located ∼1.1 eV from the valence band edge and that the trigonal Jahn-Teller relaxation energy for the V Zn − state is ∼0.5 eV.

Radiative recombination of donor-acceptor pairs in ZnTe

Lifetime of the first excited state of donor-acceptor pairs has been measured in ZnTe as a function of the donor-acceptor distance. The measured lifetime for the radiative recombinations of donor-acceptor pairs agrees well with the calculated one in which the central cell correction for the 1s state of the acceptors is taken into account. It has been found that the lifetime depends on the impurity concentration. The concentration dependence is discussed in connection with the non-exponential decay of the luminescence observed for more distant donor-acceptor pairs.

Self-activated luminescence in zinc-sulfo-selenide crystals

The orange luminescence at 2.15eV of As-grown ZnS 0.52Se 0.48crystals has been studied. The peak energy, halfwidth and Gaussian band shape exhibit a temperature dependence which is similar to that of the self-activated (SA) luminescence of ZnS at 2.66 eVand to that of ZnSe at 2.03 eV, and which can be described satisfactorily by a configurational coordinate model. However, optically detected magnetic resonance experiments at 1.8 K show that the luminescence is due to the recombination of an electron thermally released from a donor state with a hole trapped at an A-center acceptor state. The temperature dependence of the thermoluminescence spectra above 77 K and their spectral shift under additional IR exposure are also consistent with radiative recombination involving distant donor-acceptor pairs.

Radiative Recombination in Cadmium Sulfide

Luminescent centers involving Ag impurities were introduced into CdS single crystals through doping with 109Cd radioisotopes. Thus, the Ag concentration increases with time as more 109Cd decays. This enables a study of photoluminescence versus Ag concentration in a given crystal without changing the concentrations of other impurities.A new emission band at 5600 Å results in addition to the 6100 Å band present in Ag-doped crystals using conventional techniques. This new emission is quenched with increasing Ag concentration at high concentrations. Also concentration quenching by the Ag impurities occurs for the green edge, and the bound-exciton emissions I1, and I2. The quenching is explained by assuming a donor–acceptor recombination process.The new emission probably arises from the recombination of a bound electron with a bound hole at a distant donor–acceptor pair, with Ag as the acceptor. The acceptor role of Ag is supported by electrical conductivity measurements on 109Cd-doped crystals. Estimates are obtained for the acceptor binding energy, the donor concentrations, and the separations of pairs responsible for the new 5600 Å emission and the green-edge emission. The 6100 Å emission is attributed to Ag closely associated with other impurities. These conclusions are verified by our temperature annealing and quenching experiments.

Interpair energy transfer and donor-acceptor dipairs

The thermoluminescence and related properties of ZnS:Cu, In single crystals are described. The red thermoluminescence which is not accompanied by thermally-stimulated conductivity is interpreted as phonon-assisted energy transfer from distant donor-acceptor pairs to associated pairs. The spectral shifts and structure in the red thermoluminescence are attributed to transfer to different associated pairs. The model for the complete system can be described as a dipair. The theory of square planar dipairs is reviewed, and then the trapezoidal dipairs which are believed responsible for the thermoluminescence are considered. The statistical distribution of the trapezoidal dipairs is evaluated approximately, and their electronic energy levels determined. Several mechanisms of transfer are analyzed. From theoretical analyses we tentatively conclude that the most probable mechanism involves thermally-populating excited states of the distant pair followed by sequential tunneling of the electron and hole to the associated pair where radiative recombination occurs. Although the model and mechanisms have not been proven and require further experimental tests and rigorous analyses, they do provide from elementary analyses at least semiquantitative explanations of the thermoluminescent and related properties of ZnS:Cu, In crystals.

Pair Spectra and "Edge Emission" in Zinc Selenide

We have observed a large number of sharp lines in photoluminescence spectra of ZnSe at 4.2\ifmmode^\circ\else\textdegree\fi{}K, leading to a broad band at lower energy. The peak energy of this band increases from \ensuremath{\sim}2.70 to \ensuremath{\sim}2.72 eV with large increase in excitation intensity. The energies and intensities of the sharp lines are consistent with recombinations between distant donor-acceptor pairs. This is the first report of such a spectrum from a II-VI compound or any direct gap semiconductor. Two high-energy pair spectra (HEPS) occur with similar patterns of sharp lines mutually displaced by 2.0\ifmmode\pm\else\textpm\fi{}0.2 meV. A lower-energy pair spectrum (LEPS), with peak near 2.69 eV, also often occurs in ZnSe. Sharp lines have not been resolved in the LEPS. A high-energy series of broad bands, whose properties suggest free-to-bound recombinations, is readily seen in association with the LEPS, but not with the HEPS. The thermal-quenching rate is greater for the LEPS than for the HEPS. This difference in behavior can be understood if the donor and acceptor in the HEPS have comparable ionization energies, while these energies differ considerably in the LEPS. The identities of the donors and acceptors are unknown.