Localized Donor States Research Articles

Enhanced electron–phonon coupling by delocalizing phonon states for desirable interlayer transfer of excited charges in MoSSe/WS2 heterobilayer

Regulating the interlayer transfer of excited charges is challenging but crucial for high-efficiency photoelectric conversion devices based on semiconductor heterojunction. In this work, the interlayer transfer and recombination of excited charges are investigated for the heterobilayers based on monolayer MoSSe and WS2 by combining density functional theory calculation with nonadiabatic molecular dynamics simulation. Our results reveal that the heterobilayers possess type-II band alignments and the interface engineering from S–Se to S–S stacking configuration reverses the spatial distribution of valence and conduction bands from MoSSe and WS2 to WS2 and MoSSe layers, respectively, which produces interlayer transfer of excited charges in opposite direction. The interface engineering also causes the delocalization of out-of-plane phonon states from the WS2 layer to both WS2 and MoSSe layers. This delocalized character in S–S configuration facilitates the simultaneous coupling of out-of-plane phonon states with the localized donor and acceptor electronic states, accelerates the motion of interface atoms, and reduces the band energy differences, which synergistically promote interlayer transfer of excited charges. As a result, the interlayer transfer of excited charges in S–S configuration is faster than that in S–Se configuration. Our investigation demonstrates that delocalizing phonon states through interface engineering can regulate electron–phonon coupling and interlayer transfer of excited charges.

The barrier capacitance of n-SnS2/n-CdIn2Te4 heterojunction

The results of the study of the electrical characteristics of the n-SnS2/n-CdIn2Te4, obtained by a method of deposition over optical contact, have been presented. It is shown that the current-voltage characteristics and the capacitance-voltage (C-V) characteristics are typical for an abrupt heterojunction. The frequency dependence of the C-V characteristic is established, namely, with increasing frequency of alternating voltage, the barrier capacitance decreases, which is due to the presence of intrinsic structural defects and related localized donor states in the band gap of CdIn2Te4. The obtained results are explained by the frequency-dependent recharging processes of deep donor centers.

Luminescence and solar cell from ligand-free colloidal AgInS2 nanocrystals

Ligand-free AgInS2 nanocrystals display significant defect-related sub-band gap photo-absorption in addition to the excitonic absorption. The post-synthesis annealing of the nanocrystal dispersion at 150 °C reduces the intrinsic defect density, and also increases the nanocrystal size (5.8 nm) thereby reducing surface related defects. The AgInS2 nanocrystals exhibit a broad emission spectra, with a large Stokes shift compared to the excitonic absorption, which is attributed to two radiative electron–hole recombination processes: the first one involves the localized defect-state and the delocalized valence/conduction band with a lifetime of ~20 ns, and the second one involves two localized donor and acceptor states with a lifetime >250 ns. The faster decay dominates the higher energy end of the emission spectrum. Quantum dot sensitized solar cells using these ligand-free AgInS2 nanocrystals exhibit a maximum energy conversion efficiency of 0.8% and an open circuit voltage of 0.45 V. The device performance is better compared to previous AgInS2 nanocrystal based solar cells, due to both the use of the ligand-free nanocrystals that improve the charge transport and the post-synthesis annealing which partially removes the mid-gap defect states.

Hopping transport through defect-induced localized states in molybdenum disulphide

Molybdenum disulphide is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. Here we report a study of transport in few-layer molybdenum disulphide, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulphur vacancies exist in molybdenum disulphide, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbour hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of molybdenum disulphide.

Enhanced Field Emission Properties of Electrochemically Synthesized Self-Aligned Nitrogen-Doped TiO2 Nanotube Array Thin Films

Vertically aligned TiO2-δ and TiO2–3yN2y nanotube array thin films were synthesized electrochemically in a single step. N-Doping at different levels was achieved by using different concentrations of urea, as nitrogen precursor, in the electrolyte. The GIXRD and Raman studies revealed that the nanotube arrays are of anatase phase. The field emission measurements showed low turn-on fields of 10, 9.21, and 6.54 V/μm at 3.88, 4.12, and 4.74 at% of N-doping, respectively, implying enhanced field emission upon N-doping as well as an increase in electron emission with increase in N-doping. The electron emission current was found to be stable for a period of 5 h. The high electron emission current and low threshold field on N-doping is due to the presence of localized donor states below the conduction band minimum, as a result of oxygen vacancies, created by N-doping, and introduction of localized N 2p states above the valence band minimum by the doped nitrogen .

Functionalization of Single-Layer MoS2 Honeycomb Structures

Based on first-principles plane-wave calculations, we studied the functionalization of the two-dimensional single-layer MoS2 structure through adatom adsorption and vacancy defect creation. Minimum-energy adsorption sites were determined for 16 different adatoms, each of which gives rise to diverse properties. Bare, single-layer MoS2, which is normally a nonmagnetic, direct-band-gap semiconductor, attains a net magnetic moment upon adsorption of specific transition-metal atoms, as well as silicon and germanium atoms. The localized donor and acceptor states in the band gap expand the utilization of MoS2 in nanoelectronics and spintronics. Specific adatoms, such as C and O, attain significant excess charge upon adsorption onto single-layer MoS2, which might be useful for tribological applications. Each MoS2 triple vacancy created in a single layer of MoS2 gives rise to a net magnetic moment, whereas other vacancy defects related to Mo and S atoms do not influence the nonmagnetic ground state. The present results are also relevant for the surface of graphitic MoS2.

Localized donor states in axially symmetrical heterostructures

We present a general method for calculating the energy spectrum of donors confined in heterostructures with axial symmetry in the presence of magnetic and electric fields applied along the symmetry axis. The donor’s wave functions are chosen as a product of the Slater orbitals and an envelope function that is a solution of a one-dimensional differential equation, which we derive starting from Schrödinger’s variational principle. We calculate the energies of the ground and some excited states of a donor confined in multiple quantum wells and a nanowire superlattice as functions of the donor position, electric and magnetic fields for structures with different numbers and widths of the wells and the barriers. Our method could be applicable to a variety of complex quantum-confined semiconductor structures for which more rigorous approaches require extensive numerical calculations.

Terahertz photoluminescence from GaAs doped with shallow donors at interband excitation

We report on the observation of efficient generation of terahertz radiation at continuous-wave interband excitation of n-GaAs at low temperatures. The radiative transitions, accompanying relaxation and trapping of photoexcited electrons to localized donor states or to empty states in impurity subband, lead to the emission of terahertz photons with a relatively high external quantum yield up to 0.3%.

Low Temperature Photoluminescence Characteristics of Chemically Synthesized Indium Doped Zinc Oxide Nanostructures

Photoluminescence (PL) emission and excitation (EPL) spectra of un-doped and indium (1%) doped 1D zinc oxide nanostructures are studied at different temperatures. The nanostructures reveal a blue emission band attributed to localized donor states. Indium doping enhances the blue emission. While at low temperatures (< 50 K) PL spectra are dominated by the emission attributed to the recombination of excitons bound to neutral donors (D(0),X), at higher temperatures (>100 K), defect related emissions in the visible range dominate over the excitonic emission. Temperature dependence measurements on the doped sample reveal that (D(0),X) emission energies obey the Varshni's formula with fitting constants alpha = 8.4 +/- 0.3 x 10(-4) eV/K and beta = 650 +/- 40 K. The (D(0),X) emission intensity decays exponentially with temperature.

Towards identification of localized donor states in InN

Transport studies of as-grown and proton-irradiated n-InN have been performed aiming at verification of the nature of localized donor states resonant with the InN conduction band. These resonant donor states (RDS) show a clear contribution to the electrical conduction in low electron concentration InN epitaxial layers. We used proton irradiation to increase the number of incorporated native point defects of donor character in InN layers. Then, the performed studies of pressure dependence of the Hall electron concentration clearly show no increase in the number of RDS in samples exposed to irradiation in spite of the increase in the conducting electron concentration.

Quantum Dynamics of Photoinduced Electron-Transfer Reactions in Dye−Semiconductor Systems: First-Principles Description and Application to Coumarin 343−TiO2

A method to describe the quantum dynamics of photoinduced heterogeneous electron-transfer processes at dye−semiconductor interfaces is proposed. The method is based on a model Hamiltonian, the parameters of which are determined by first-principles electronic structure calculations and a partitioning scheme to define localized donor and acceptor states as well as donor−acceptor coupling matrix elements. On the basis of this modeling procedure, accurate quantum dynamical simulations are performed employing the multilayer multiconfiguration time-dependent Hartree method. As a representative example, applications to coumarin 343 adsorbed on titanium oxide nanoparticles are presented. The results of the simulations show that the ultrafast electron-injection process in this system is accompanied by electronic coherence effects, which are partially quenched due to electronic−nuclear coupling.

Role of localized donor states in transport and photoluminescence of InN revealed by hydrostatic pressure studies

AbstractPressure‐dependent transport and photoluminescence (PL) measurements were performed on InN. In the transport measurements on the sample with a low electron concentration (ne lower than about 1018 cm–3) a Localized Donor State (LDS) was seen to cross the Fermi level at elevated pressure magnitude. It was accompanied by the transfer of electrons from the Conduction Band to the LDS and a significant increase in electron mobility. In a sample with a higher concentration of electrons no transport anomaly was observed. Application of pressure helps to confirm that the disputed photoluminescence mechanism in InN consists in band‐to‐band recombination. No evidence of the participation of this LDS in radiative recombination processes was found in the pressure‐dependent PL measurements. On the contrary, the dependence of the PL intensity on excitation power suggested the involvement of the LDS in nonradiative recombination processes. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High pressure studies of radiative recombination mechanisms in InN

AbstractReported hydrostatic pressure measurements of Hall electron concentration and Hall mobility of InN suggest the presence of a localized donor state in the conduction band. Since localized‐state emission is rather inefficient, the involvement of such a localized state would be highly inadvantageous for device applications. Pressure‐dependent photoluminescence (PL) experiments of InN epilayers show PL pressure coefficients dE E/dp of 30.7 ± 2.0 meV/GPa and 21.4 ± 3.0 meV/GPa for InN epilayers with electron concentration of ∼5 × 1018 cm–3 and ∼5 × 1017 cm–3, respectively. These values are comparable with the InN band gap pressure coefficient dE G/dp of ∼25–30 meV/GPa (indicating regular band‐to‐band emission), and differ significantly from the value of 5–10 meV/GPa that should be expected in the case of localized‐state emission. Therefore, in the presented experiments no evidence of the involvement of localized donor states in radiative recombination processes in InN is found. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Localized donor state above the conduction band minimum in InN revealed by high pressure and temperature transport experiments

We have studied the pressure and temperature dependences of Hall electron concentration and mobility in degenerated, not intentionally doped InN samples. The results obtained for a whole set of samples with electron concentrations from 5.4 x 10 17 cm -3 to 3.3 x 10 18 cm -3 consistently revealed the existence of a localized donor type state resonant with the conduction band. The concentration of this state is of the same order in all studied samples and not exceeds 10 18 cm -3 . Population of this state determines electrical properties of InN samples with low carrier concentration. This donor state is not the main source of conducting electrons in these not intentionally doped samples and can be entirely populated and hidden in samples with high electron concentration exceeding about 3 x 10 18 cm -3 .

Chemical Functionalization of Carbon Nanotubes by Carboxyl Groups on Stone-Wales Defects: A Density Functional Theory Study

Chemical functionalization of carbon nanotubes with Stone-Wales (SW) defects by carboxyl (COOH) groups is investigated by density functional calculations. Due to the localized donor states induced by the SW defect, the binding of the COOH group with the defective carbon nanotube is stronger than that with the perfect one. A quasi-tetrahedral bonding configuration of carbon atoms, indicating sp3 hybrid bonding, is formed in the adsorption site. The charge distribution analysis shows that, in comparison with benzoic acid, the localized or delocalized pi states on the nanotube would affect the polarities of chemical bonds of the COOH group without losing the acidity. Furthermore, it is found that the double-adsorption system (two COOH groups are respectively adsorbed on two individual carbon atoms of the SW defect) is more energetically favorable than the monoadsorption one. The adsorption of COOH groups leads to a significant change of the electronic states around the Fermi level, which is advantageous for the electrical conductivity. The functionalization by introducing functional groups on the topological defects provides a pathway for applications of carbon nanotubes in chemical sensors and nanobioelectronics.

Resonant localized donor state above the conduction band minimum in InN

We have studied the pressure dependence of Hall electron concentration and mobility in degenerated, not intentionally doped InN samples. The results obtained for a whole set of samples with electron concentrations from 5.4×1017cm−3 to 3.3×1018cm−3 consistently reveal the existence of a localized donor type state, resonant with the conduction band. Its energy position is estimated to be about 80–90 meV above the bottom of the conduction band. This donor state is not the only source of electrons in these not intentionally doped samples and can be entirely populated and hidden in samples with very high electron concentrations.

Electron transfer in proteins: nonorthogonal projections onto donor-acceptor subspace of the Hilbert space.

We develop nonorthogonal projectors, called Löwdin projectors, to construct an effective donor-acceptor system composed of localized donor (D) and acceptor (A) states of a long-distance electron transfer problem. When these states have a nonvanishing overlap with the bridge states these projectors are non-Hermitian and there are various possible effective two-level systems that can be built. We show how these can be constructed directly from the Schrödinger or Dyson equation projected onto the D-A subspace of the Hilbert space and explore these equations to determine the connection between Hamiltonian and Green function partitioning. We illustrate the use of these effective two-level systems in estimating the electron transfer rate in the context of a simple electron transfer model.

The Intrinsic Formation Of Localized Electronic States At [001] Tilt Grain Boundaries In YBa2Cu3O7 And Their Effect On Jc

Abstract Grain boundaries have long been known to have a deleterious effect on the superconducting critical current that can be carried by YBa2Cu3O7-δ. Recent theoretical analyses have proposed that the origin of this behavior may be band bending, which results in the depletion of charge carriers at the grain boundaries. For this to occur in these p-type superconductors there must, by definition, be a high density of localized donor states in the boundary plane. Here we describe a structural feature intrinsic to all [001] tilt boundaries that may be the origin of these localized states. Direct atomic-resolution images of asymmetric [001] tilt grain boundary structures have been obtained using the Z-contrast imaging technique. The grain boundaries are observed to be composed of distinct structural units, as shown in figure 1. Within these structural units, a 2×1 reconstruction of the CuO columns is seen to occur. This reconstruction, which is caused by the constraint imposed on the structure by the sizes of the component atoms, leads to effective oxygen vacancies in the grain boundary plane (figure 2). Further oxygen annealing of the boundary cannot fill these vacancies as there is no space in the structure for more oxygen atoms.

Donor states in tunnel-coupled quantum wells

We study the energy spectrum of the impurity states in tunnel-coupled double quantum wells for Coulomb and short-range donor potentials. We calculate the impurity contribution and the density of states and detect the transformation of a localized donor state into a resonant state when the binding energy of the donor in an isolated quantum well is less than the separation of the energy levels of the double quantum wells. In the opposite case, where the binding energy is greater than the level separation, there is tunneling repulsion between adjacent impurity levels, with the degree of degeneracy of the levels changing when there is tunneling mixing of the ground and excited impurity states from different wells. Resonant states emerge in an asymmetric double quantum well, while in a symmetric double quantum well the impurity level at the barrier’s center proves to be localized even against the background of the continuum. The calculations are based on a general expression for the impurity contribution to the density of states in terms of a 2-by-2 matrix Green’s function, i.e., only a pair of tunnel-coupled levels of the double quantum wells is taken into account. For an impurity with a short-range potential, we derive a matrix generalization of the Koster-Slater solution, while the impurity with a Coulomb potential is analyzed by using the approximation of a narrow resonance and close arrangement of the repulsive levels.

Investigating the Relationship Between Atomic Structure and Carrier Depletion at [001] tilt Grain Boundaries in Ybco

AbstractRecent theoretical analyses [1] have suggested that the origin of the reduced Jc at grain boundaries in high-Tc superconductors may be band bending, which results in the depletion of mobile charge carriers at the boundaries. For this to occur in these p-type superconductors there must, by definition, be a high density of localized donor states in the boundary plane. Here we report a “structural unit” analysis of [0011 tilt grain boundaries in YBa2Cu 3O7-δ (YBCO) that indicates there may be a simple structural origin for such localized donor states.