Electron Donor States Research Articles

Self-modulation doping effect in the high-mobility layered semiconductor Bi2O2Se

Recently an air-stable layered semiconductor Bi2O2Se was discovered to exhibit an ultrahigh mobility in transistors fabricated with its thin layers. In this work, we explored the mechanism that induces the high mobility and distinguishes Bi2O2Se from other semiconductors. We found that the electron donor states lie above the lowest conduction band. Thus, electrons get spontaneously ionized from donor sites (e.g., Se vacancies) without involving the thermal activation, different from the donor ionization in conventional semiconductors. Consequently, the resistance decreases as reducing the temperature as observed in our measurement, which is similar to a metal but contrasts to a usual semiconductor. Furthermore, the electron conduction channels locate spatially away from ionized donor defects (Se vacancies) in different van der Waals layers. Such a spatial separation can strongly suppress the scattering caused by donor sites and subsequently increase the electron mobility, especially at the low temperature. We call this high-mobility mechanism self-modulation doping, i.e. the modulation doping spontaneously happening in a single-phase material without requiring a heterojunction. Our work paves a way to design novel high-mobility semiconductors with layered materials.

Electronic states and optical properties of single donor in GaN conical quantum dot with spherical edge

In this paper we present a theoretical investigation of quantum confinement effects on the electron and single donor states in GaN conical quantum dot with spherical edge. In the framework of the effective mass approximation, the Schrödinger equations of electron and donor have been solved analytically in an infinite potential barrier model. Our calculations show that the energies of electron and donor impurity are affected by the two characteristic parameters of the structure which are the angle Ω and the radial dimension R. We show that, despite the fact that the reduction of the two parameters Ω and R leads to the same confinement effects, the energy remains very sensitive to the variation of the radial part than the variation of the angular part. The analysis of the photoionization cross-section corresponding to optical transitions between the conduction band and the first donor energy level shows clearly that the reduction of the radius R causes a shift in resonance peaks towards the high energies. On the other hand, the optical transitions between 1s−1p, 1p−1d and 1p−2s show that the increment of the conical aperture Ω (or reduction of R) implies a displacement of the excitation energy to higher energies.

Mediating Reductive Charge Shift Reactions in Electron Transport Chains.

We report the synthesis of a full-fledged family of covalent electron donor-acceptor1-acceptor2 conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn(II)Porphyrins or Zn(II)Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C60 and C70. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C60•- toward C70•-. This strategy helps minimize energy losses in the reductive, short-range charge shift from C60 to C70. At the forefront of our investigations are excited-state dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet-triplet energy transfer and charge transfer.

Excited States of Light-Harvesting Systems Based on Fullerene/Graphene Oxide and Porphyrin/Smaragdyrin

In this work, we present a theoretical study at the density functional theory (DFT) level and time-dependent DFT of the ground and singlet excited states of electron donor–acceptor complexes formed by porphyrin (TPP)/smaragdyrin (TPOS) (expanded porphyrin), as light-harvesting systems, and fullerene (C60)/graphene oxide (GO), as acceptor nanocarbon structure. We investigate the effect of the nanocarbon on UV–vis electronic absorption of porphyrin/smaragdyrin, using the functionals B3LYP, PBE, M06, and wB97XD. The results showed the lowest deviation of the Q-band for the functional M06 (0.01–0.02 eV). Electronic absorption spectra calculated for the complexes with M06 predict that (a) graphene oxide increases the intensity of the Soret band, (b) fullerene produces a red-shift of the Q bands (4 nm) with respect to graphene oxide, and (c) smaragdyrin causes a red-shift of Q (27–48 nm) and Soret (37 nm) absorption bands as compared to porphyrin. We also investigate the effect of the nanocarbon structure on th...

Population dynamics and dephasing of excitons and electron-hole pairs in polytype wurtzite/zinc-blende InP nanowires

We investigated the dephasing and population dynamics of electron-hole (e-h) pairs and excitons in vapor-liquid-solid grown polytype wurtzite/zinc-blende (WZ/ZB) InP nanowires (NWs) using heterodyne four-wave mixing (HFWM) in three-beam configuration at temperatures from 80 to 270 K. The photon energy of the 100 fs excitation pulses was varied to predominantly excite either mobile excitons and e-h pairs or indirect WZ/ZB excitons. The population dynamics reveal a multiexponential decay with time constants ranging over six orders of magnitude. The dynamics has been interpreted by a coupled rate equation model which considers WZ and ZB electron states, donor electron states, and band bending trapping holes to the surface. The model reproduces the essential features of the experimentally observed dynamics at different excitation energies, fluences, and lattice temperatures. Intraband thermalization is reached within 5--50 ps, after which the nonradiative recombination dominates the dynamics. Notably, the screenable surface band bending results in long-lived spatially separated carriers, causing a photogenerated, spatially separated electron and hole density which is dominating the long-lived dynamics.

Donor Impurity-Related Optical Absorption in GaAs Elliptic-Shaped Quantum Dots

The conduction band and electron-donor impurity states in elliptic-shaped GaAs quantum dots under the effect of an externally applied electric field are calculated within the effective mass and adiabatic approximations using two different numerical approaches: a spectral scheme and the finite element method. The resulting energies and wave functions become the basic information needed to evaluate the interstate optical absorption in the system, which is reported as a function of the geometry, the electric field strength, and the temperature.

Nanoflower structures of ZnO–SnO2 arranged on nanoporous Al2O3 for sensing ethanol and methanol gases

A compound sensing structure based on nanoflower-like ZnO---SnO2 coated on the top of nanoporous anodic aluminum oxide (AAO) layer to form ZnO---SnO2---AAO composite as a gas sensing structure is developed for detecting ethanol and methanol simultaneously. The ZnO---SnO2 powders were synthesized by hydrothermal synthesis method. The SnO2 was grown densely and uniformly that has unique loose and porous nano structures to increase the accessible surface area for improving gas diffusion and mass transport. A solenoid-type heater was designed to surround the sensing column for controlling operation temperature to promote gas sensitivity. The ethanol and methanol responses of ZnO---SnO2---AAO are high because the porous AAO greatly improves the specific adsorption surface area, and the ZnO produces more electron donor states or oxygen vacancies to enhance oxygen adsorption as well as the heterojunction of SnO2 and ZnO grains offers access to facile electronic interaction to enhance the surface reaction between adsorbed oxygen and current-carrying electrons. By analyzing a series of temperature-varied cyclic tests, the optimal operation temperatures of 180 °C for sensing ethanol and 260 °C for sensing methanol were obtained to have the strongest output responses. The developed ZnO---SnO2---AAO gas sensing composites include the advantages: simple fabrication process, low cost, high specific surface area and porosity, rapid detection, high sensitivity, stable stability, and good repeatability.

Understanding How Isotopes Affect Charge Transfer in P3HT/PCBM: A Quantum Trajectory-Electronic Structure Study with Nonlinear Quantum Corrections.

The experimentally observed effect of selective deuterium substitution on the open circuit voltage for a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM; Nat. Commun. 2014, 5, 3180) is explored using a 221-atom model of a polymer-wrapped PCBM molecule. The protonic and deuteronic wave functions for the H/D isotopologues of the hexyl side chains are described within a quantum trajectory/electronic structure approach where the dynamics is performed with newly developed nonlinear corrections to the quantum forces, necessary to describe the nuclear wave functions; the classical forces are generated with a density functional tight binding method. The resulting protonic and deuteronic time-dependent wave functions are used to assess the effects of isotopic substitution (deuteration) on the energy gaps relevant to the charge transfer for the donor and acceptor electronic states. While the isotope effect on the electronic energy levels is found negligible, the quantum-induced fluctuations of the energy gap between the charge transfer and charge separated states due to nuclear wave functions may account for experimental trends by promoting charge transfer in P3HT:PCBM and increasing charge recombination on the donor in the deuterium substituted P3HT:PCBM.

Non-Condon nonequilibrium Fermi’s golden rule rates from the linearized semiclassical method

The nonequilibrium Fermi’s golden rule describes the transition between a photoexcited bright donor electronic state and a dark acceptor electronic state, when the nuclear degrees of freedom start out in a nonequilibrium state. In a previous paper [X. Sun and E. Geva, J. Chem. Theory Comput. 12, 2926 (2016)], we proposed a new expression for the nonequilibrium Fermi’s golden rule within the framework of the linearized semiclassical approximation and based on the Condon approximation, according to which the electronic coupling between donor and acceptor is assumed constant. In this paper we propose a more general expression, which is applicable to the case of non-Condon electronic coupling. We test the accuracy of the new non-Condon nonequilibrium Fermi’s golden rule linearized semiclassical expression on a model where the donor and acceptor potential energy surfaces are parabolic and identical except for shifts in the equilibrium energy and geometry, and the coupling between them is linear in the nuclear coordinates. Since non-Condon effects may or may not give rise to conical intersections, both possibilities are examined by considering the following: (1) A modified Garg-Onuchic-Ambegaokar model for charge transfer in the condensed phase, where the donor-acceptor coupling is linear in the primary-mode coordinate, and for which non-Condon effects do not give rise to a conical intersection; (2) the linear vibronic coupling model for electronic transitions in gas phase molecules, where non-Condon effects give rise to conical intersections. We also present a comprehensive comparison between the linearized semiclassical expression and a progression of more approximate expressions, in both normal and inverted regions, and over a wide range of initial nonequilibrium states, temperatures, and frictions.

Nonequilibrium Fermi's Golden Rule Charge Transfer Rates via the Linearized Semiclassical Method.

Nonequilibrium Fermi's golden rule (NE-FGR) describes the transition between a photoexcited bright donor electronic state and a dark acceptor electronic state when the nuclear degrees of freedom start out in a nonequilibrium state. In this article, we derive a new expression for NE-FGR within the framework of the linearized semiclassical approximation. The new expression opens the door for applications of NE-FGR in complex condensed-phase molecular systems described in terms of anharmonic force fields. We show that the linearized semiclassical expression for NE-FGR yields the exact fully quantum-mechanical result for the canonical Marcus model, where the coupling between donor and acceptor is assumed constant (the Condon approximation) and the donor and acceptor potential energy surfaces are parabolic and identical except for a shift in the equilibrium energy and geometry. For this model, we also present a comprehensive comparison between the linearized semiclassical expression and a hierarchy of more approximate expressions, in both normal and inverted regions and over a wide range of initial nonequilibrium states, temperatures, and frictions.

Polysilane-Wrapped Carbon and Boron-Nitride Nanotubes: Effects of B or P Doping on Electron Transport

We perform a comprehensive study of effects of wrapping either undoped or doped polysilane (PSi) around the outer surface of a carbon nanotube (CNT) or a boron-nitride nanotube (BNNT) using density functional theory and nonequilibrium Green’s function calculations. For CNT, because the wrapping of either undoped PSi or B-doped PSi has little effect on the electronic band structure near the Fermi surface Ef, the conductivity of the wrapped CNT is still dominated by the CNT π state. This behavior is also confirmed by using the two-probe device model system with a unit cell of undoped or B-doped PSi-wrapped CNT sandwiched between two Au electrodes. For P-doped PSi/CNT, the P dopant can introduce electron donor state in the valence band. However, such a P-dopant effect is still suppressed and the conductivity is still controlled by the CNT π state based on the two-probe device computation. Contrary to CNT, the PSi-wrapped BNNT can markedly influence the band structure of the BNNT. The wrapping of either undop...

Electrical current through individual pairs of phosphorus donor atoms and silicon dangling bonds.

Nuclear spins of phosphorus [P] donor atoms in crystalline silicon are among the most coherent qubits found in nature. For their utilization in scalable quantum computers, distinct donor electron wavefunctions must be controlled and probed through electrical coupling by application of either highly localized electric fields or spin-selective currents. Due to the strong modulation of the P-donor wavefunction by the silicon lattice, such electrical coupling requires atomic spatial accuracy. Here, the spatially controlled application of electrical current through individual pairs of phosphorus donor electron states in crystalline silicon and silicon dangling bond states at the crystalline silicon (100) surface is demonstrated using a high‐resolution scanning probe microscope operated under ultra‐high vacuum and at a temperature of 4.3K. The observed pairs of electron states display qualitatively reproducible current-voltage characteristics with a monotonous increase and intermediate current plateaus.

Full counting statistics of vibrationally assisted electronic conduction: Transport and fluctuations of thermoelectric efficiency

We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double quantum dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic- a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second-order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the non-equilibrium Green's function approach (harmonic mode) and a kinetic quantum master equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property.

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin-Copper Corrole Donor-Acceptor Conjugates.

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10(10) s(-1) and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.

The impacts of electronic state hybridization on the binding energy of single phosphorus donor electrons in extremely downscaled silicon nanostructures

We present the density functional theory calculations of the binding energy of the Phosphorus (P) donor electrons in extremely downscaled single P-doped Silicon (Si) nanorods. In past studies, the binding energy of donor electrons was evaluated for the Si nanostructures as the difference between the ionization energy for the single P-doped Si nanostructures and the electron affinity for the un-doped Si nanostructures. This definition does not take into account the strong interaction of donor electron states and Si electron states explicitly at the conductive states and results in a monotonous increase in the binding energy by reducing the nanostructure's dimensions. In this paper, we introduce a new approach to evaluate the binding energy of donor electrons by combining the projected density of states (PDOS) analysis and three-dimensional analysis of associated electron wavefunctions. This enables us to clarify a gradual change of the spatial distribution of the 3D electron wavefunctions (3DWFs) from the donor electron ground state, which is fully localized around the P donor site to the first conductive state, which spreads over the outer Si nanorods contributing to current conduction. We found that the energy of the first conductive state is capped near the top of the atomistic effective potential at the donor site with respect to the surrounding Si atoms in nanorods smaller than about 27 a0. This results in the binding energy of approximately 1.5 eV, which is virtually independent on the nanorod's dimensions. This fact signifies a good tolerance of the binding energy, which governs the operating temperature of the single dopant-based transistors in practice. We also conducted the computationally heavy transmission calculations of the single P-doped Si nanorods connected to the source and drain electrodes. The calculated transmission spectra are discussed in comparison with the atomistic effective potential distributions and the PDOS-3DWFs method.

Synthesis and spectroscopic properties of a soluble semiconducting porphyrin polymer.

A semiconducting porphyrin polymer that is solution processable and soluble in organic solvents has been synthesized, and its spectroscopic and electrochemical properties have been investigated. The polymer consists of diarylporphyrin units that are linked at meso-positions by aminophenyl groups, thus making the porphyrin rings an integral part of the polymer backbone. Hexyl chains on two of the aryl groups impart solubility. The porphyrin units interact only weakly in the ground electronic state. Excitation produces a local excited state that rapidly evolves into a state with charge-transfer character (CT) involving the amino nitrogen and the porphyrin macrocycle. Singlet excitation energy is transferred between porphyrin units in the chain with a time constant of ca. 210 ps. The final CT state has a lifetime of several nanoseconds, and the first oxidation of the polymer occurs at ca. 0.58 V vs. SCE. These properties make the polymer a suitable potential excited state electron donor to a variety of fullerenes or other acceptor species, suggesting that the polymer may find use in organic photovoltaics, sensors, and similar applications.

Charge separation across P3HT/carbon nanotube interface: First-principles calculations of electronic structures

Electronic structures and charge redistribution between P3HT and (10,2)/(6,5) carbon nanotubes (CNT) are investigated by density functional theory calculations. The simulations show that electron is transferred from flat P3HT to (10,2) CNT while hole is transferred from (6,5) CNT to wrapped P3HT due to the different work functions of these materials. The large built-in potential can compete to exciton binding energy, leading to efficient charge separation across the type-II photovoltaic heterojunctions. Electron transfer faster than hole is expected because the electron donor state is much more delocalization, creating larger donor–acceptor coupling, which provides critical insights of organic photovoltaic solar cells.

Metal impurities in crystallographic voids of beta-rhombohedral boron lattice: Binding energies and electron levels

Applying quasi-classical approach, the binding energies and electron levels of metal impurities (Li, Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf, Ta, and Re) introduced into crystallographic voids of types A, D and E in the beta-rhombohedral boron lattice are calculated. Binding energies are estimated as ∼1 eV–60 eV per bond. The most of the obtained metal–boron bond lengths are very close to the mean radii of voids in the undoped crystal. Relatively light impurities (from Li to Cu) are found to form donor electron states directly inside the conduction band, i.e., they cause metallization of the material being introduced at sufficiently high concentrations. Heavy impurities (from Zr to Re) form shallow or deep donor levels inside the band gap or even valence band depending on dopants and voids of accommodation.

Electron Transfer in Rhodobacter sphaeroides Reaction Centers Containing Zn-Bacteriochlorophylls: A Hole-Burning Study

Nonresonant and resonant transient, photochemical hole-burned (HB) spectra are presented for primary electron donor states of a novel bacterial reaction center (Zn-RC) of Rhodobacter sphaeroides, containing six Zn-bacteriochlorophylls (Zn-BChls). A "Zn-β-RC" in which the Zn-BChl in the bacteriopheophytin (BPhe)-binding site on the A side (H(A)) has the Zn penta-coordinated, was also studied. The fifth ligand comes from a histidine introduced by site-directed mutagenesis. Formation of the P(+)Q(A)(-) state was observed in both types of RC, although under identical experimental conditions a significantly deeper P(-) band (corresponding to the lower-energy, special pair, excitonic component) was revealed in the Zn-RC. Assuming a similar lifetime of the P(+)Q(A)(-) state, the quantum yield of P(+)Q(A)(-) formation decreased by ~60% in the Zn-β-RC (compared to the Zn-RC), as was seen in a comparison of analogous (Mg) BChl-containing wild type and β-RCs of Rb. sphaeroides [Kirmaier et al. Science1991, 251, 922]. However, the average (weakly frequency-dependent) low-temperature electron transfer (ET) rates of the Zn-RC and Zn-β-RC (measured from zero phonon holes in resonant transient HB spectra) were both ~1 ps and similar to a rate previously measured in the Rb. sphaeroides native RC [Johnson et al. J. Phys. Chem. 1989, 93, 5953]. Electron transfer rates observed in this work on the Zn-RC yielded a P870* decay rate in good agreement with recent room-temperature, time-domain data [Lin et al. Proc. Natl. Acad. Sci. 2009, 106, 8537]. A lack of correlation observed between the holes near 810 and 883 nm, accounting for electrochromically induced shifts of the Zn-BChl transitions in the B(A,B) and H(A,B) binding sites, produced by formation of the P(+)BHQ(A)(-) state, indicates that the 810 nm bleach does not correspond to the P(+) (upper excitonic component of the dimer) band and is mostly contributed to by a shift of the B(B) absorption band. ZPH-action spectra indicated inhomogeneous broadening (Γ(inh)) of ~110 cm(-1) (Zn-RC) and ~130 cm(-1) (Zn-β-RC). Experimentally determined Γ(inh) decreased the number of variables in theoretical fits of the absorption and frequency-dependent shapes of resonant HB spectra, leading to more reliable Huang-Rhys factors for both low-frequency phonons and a pseudolocalized phonon, ω(SP), often referred to as the special pair marker mode.

Electron Donor–Acceptor Interactions with Flanking Purines Influence the Efficiency of Thymine Photodimerization

Quantum yields for thymine photodimerization (Φ(TT)) have been determined for a series of short DNA single-strand and base-paired hairpin structures possessing a single thymine-thymine step with flanking purines. Values of Φ(TT) are strongly dependent upon the oxidation potential of the flanking purine, decreasing in the order: inosine > adenine > guanine > deazaguanine. The dependence of Φ(TT) on the ionization potential of the flanking purine is more pronounced when the purine of lower oxidation potential is located at the 5'- versus 3'-position in either a single strand or a hairpin. Molecular dynamics simulations for hairpin structures indicate that the TT step is π-stacked with both the 5' and 3' purine, but that there is little π-stacking with either purine in single-strand structures. The observation of moderately intense long-wavelength UV absorption features for hairpins having 5'-Z or G flanking purines suggests that excitation of ground state donor-acceptor complexes may account for more extensive quenching of dimerization by 5'- versus 3'-purines. The "purine effect" on Φ(TT) is attributed to a combination of ground state conformation, ground state electron donor-acceptor interactions, and excited state exciplex formation.