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ELF calculations and COHP analyses confirm that the Cl–Mn bonds in the Cs2AgInCl6:Mn2+ system exhibit more ionic characteristics and stronger bond strengths, which accounts for the blue-shift of the PL peak of Mn ions from Cs2AgInCl6 to Cs2NaBiCl6.

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

Composition engineering plays an important role in generating novel properties and decreasing the lead (Pb) toxicity for halide perovskite materials. To find out the modulation effect introduced by the composition engineering, namely

Double perovskite A

To understand the light enhanced moisture degradation of CH3NH3PbI3 (MAPbI3) material, we carry out a series of first principles calculations and find the decisive role played by the giant photostriction effect. The photostriction effect in MAPbI3 material changes the charge distribution and thus the bonding interactions between atoms. This effect enhances the adsorption and penetration of H2O molecules nearby the MAPbI3 surface after the above band gap illumination, which is ascribed to the alternation of bonding interactions between MA molecules, H2O molecules and PbI networks. Owing to the straightening of the PbI networks induced by the photostriction effect, the band gap of MAPbI3 decreases, leading to more photon absorption and thus self-accelerated photostriction processes. Hence the photostriction effect could accelerate the moisture decomposition of MAPbI3 perovskite under illumination. Therefore, to develop perovskite solar cells with strong moisture resistance, an effective strategy is to develop perovskite materials with no or a minor photostriction effect.

Here, we choose (C4H9NH3)2PbI4 to represent the two-dimensional layered hybrid organic–inorganic perovskites and investigate its excellent moisture resistance by theoretical analyses. (C4H9NH3)2PbI4 adsorbs H2O molecules easily on all surfaces. However, the most energetic surface, i.e., C4H9NH3-terminated surface, provides an energy barrier of 3.46 eV to prevent the penetration of H2O molecules, which is 1 order higher than the barrier in MAPbI3 surface. This obstacle for the invasive H2O molecule comes from the methyl/methylene radicals of C4H9NH3 groups. These methyl/methylene radicals are chemically inactive and only generate repulsive interactions with the H2O molecule, leading to the outstanding moisture resistance of (C4H9NH3)2PbI4. Herein, we propose a practical strategy to obtain good moisture-resistant perovskite solar cell materials, i.e., modulate the shape and symmetry of organic molecules in perovskites to let as many methyl radicals as possible prevent the penetration of adsorbed H2O molecul...

Motivated by extensive investigations on the In2O3-based multicomponent transparent conducting oxides (TCOs), we want to know the carrier transport properties in these systems and how they evolve with adjusting of geometric structure and constituent elements. Guided by the empirical minimum effective mass physics, we select nine impurities M (M = Sn, Zn, Cd, Hg, Mn, Ni, Co, Ga and Ge) to tune the effective mass (m∗) of In2O3-based TCOs. By means of systematic theoretical investigations, we find that in impurity M isolated or corporately doped In2O3 materials, the substitutional position for dopant M is determined by the charge state. Codoping Sn and other eight impurities into In2O3 (to form IMTO) is an effective strategy to reduce the indium content in In2O3-based TCOs materials. For the recognized n-type carrier contributor, namely, VO defect, it induce spatial localization and orbital localization effects in electronic structures and thus enlarge m∗, which are caused by the large lattice distortion and complicated orbital wavefunction hybridizations. However, for IMTO:VO (M = Zn, Ga and Hg) systems, both localization effects are weakened, leading to a decrease of m∗. It even reduces by half value for In-Hg-Sn-O:VO systems. This systematic work in In2O3-based materials may be helpful in designing of a new range of high performance TCOs.

The lead-free double perovskite Cs2AgInCl6 is a potential candidate for LEDs, the photoluminescence performance of which is reinforced greatly by Mn doping. Here, we analyzed the geometric, electronic and photoluminescence properties of Mn-doped Cs2AgInCl6 by means of first-principle calculations. We found that in the interior of Cs2AgInCl6, the Mn dopant formed defect complexes by substituting an Ag atom and generating an Ag vacancy (MnAgVAg) owing to the charge balance and the weak distortion of the metal octahedra. The MnAgVAg defect introduced two defect bands in the forbidden gap, which was contributed predominantly by the 3d orbitals of the Mn2+ ions. The electron transition of the Mn2+ ions from the first excited state to the ground state, i.e., from 4T1 to 6A1 states, gives rise to the PL spectrum that is lower than the bandgap. Therefore, we show that the Mn dopant indeed reinforces the PL performance of Cs2AgInCl6 greatly and is beneficial for its use as an LED material.

Composition engineering plays an important role in generating novel properties and decreasing the lead (Pb) toxicity for halide perovskite materials. To find out the modulation effect introduced by the composition engineering, namely, B'-site co-metal ions, in (MA)2AgBi1-xSbxBr6 systems with various Bi/Sb ratios of x=0, 0.25, 0.75, 1.00, series of theoretical simulations and analyses are carried out. For the (MA)2AgBi1-xSbxBr6 systems, the Goldschmidt tolerance factor t and the octahedral factor μ indicate that all samples are in a standard double perovskite structure with alternating AgBr6 and Bi/SbBr6 octahedra. The calculated electronic structures show that the band gap of (MA)2AgBi1-xSbxBr6 decreases with the increase of Sb content, but the indirect band gaps are maintained for all samples. By analyses of the imaginary part ɛ2(ω) of dielectric function and the absorption spectra, we find that all (MA)2AgBi1-xSbxBr6 systems show absorption in the visible-light region. All these results indicate that the composition engineering adopted in this paper is an effective strategy to modulate the optical properties of (MA)2AgBi1-xSbxBr6 systems and may open a new way to put it into applications in the fields of solar cells and other optoelectronic devices.

The magnetism induced by Carbon (C) in SnO 2 surfaces are investigated by first principle calculations. The results show that C substitution at the outmost surface oxygen sites can induce magnetism in (110), (001) and (101) surfaces of SnO 2 . (110) surface is the most stable surface and the magnetism in which is stronger than that in other two surfaces, indicating that it is (110), but not other surfaces provides the main contribution to the surface magnetism of C-doped SnO 2 (SnO 2 :C). The magnetic moments predominantly come from C-2 p orbitals, which arise from the crystal field transformation induced by the loss of coordinated atoms and the destroy of the local symmetry, and is enhanced by the local lattice distortion due to the Jahn–Teller effect. In all three surface slabs, the magnetism decays when C dopants are deeper from the outmost surfaces and disappears eventually. This work provides more rational understanding to the observed magnetism in SnO 2 :C materials than ever.

The main purpose of this work is to make clear that in the newly synthesized multicomponent materials IZTO, namely, Sn/Zn cosubstituted In2O3, how can these cosubstituted dopants interact with each other and affect the electronic properties. We carry out a series of systematic Density Functional Theory calculations. Compared with ITO materials, the existence of Zn2+ in IZTO is important for the control of the electronic properties. Although there is large lattice distortion in the local structure around Zn2+ due to the Jahn–Teller effect, it releases distortions induced by other defects and thus reduces the formation energy of cosubstitution of the Zn/Sn impurity pair in In2O3. The VO defect prefers to form in the ZnO6 octahedron for the breaking of the Zn–O bond is easier than the Sn–O bond, which generates free electrons. In all IZTO configurations with and without defects, the bottom of conduction bands is comprised mainly of cation-s and O-p orbitals, which constructs a three dimensional network for charge transport. The bandstructure renormalization effect exerted by Zn atoms narrows the band gap. The existence of Zn atoms, on the one hand, weakens the spatial localization and orbital localization effects induced by defects and on the other hand strengthens the charge transport ability. These results indicate that the multicomponent semiconductor IZTO is an excellent candidate for the low indium content TCOs. Our work is useful to control the physical properties of IZTO experimentally, and is also helpful in understanding other multicomponent TCO materials.

To analyze the controversial conclusions on the magnetism of C-doped SnO2 (SnO2:C) bulk materials between theoretical calculations and experimental observations, we propose the critical role of the charge states of defects in the geometric structures and magnetism, and carry out a series of first principle calculations. By changing the charge states, we can influence Bader charge distributions and atomic orbital occupancies in bulk SnO2:C systems, which consequently conduct magnetism. In all charged SnO2:C supercells, C-2px/py/pz electron occupancies are significantly changed by the charge self-regulation, and thus they make the C-2p orbitals spin polarized, which contribute to the dominant magnetic moment of the system. When the concentration of C dopant in the SnO2 supercell increases, the charge redistribution assigns extra electrons averagely to each dopant, and thus effectively modulates the magnetism. These findings provide an experimentally viable way for controlling the magnetism in these systems.

The internal transitions of Be acceptors confined in the center of GaAs/AlAs multiple quantum wells are investigated by Raman and photoluminescence (PL) spectra. A series of Be δ-doped GaAs/AlAs multiple quantum wells with doping at the well center and well widths ranging from 30 to 200 A were grown on (100) GaAs substrates by molecular beam epitaxy. Both the Raman and PL spectra were measured at 4 and 20 K, respectively. The confined longitudinal optical (LO) modes, and transitions of Be acceptors from the 1S3/2(Γ6) ground state to 2S3/2(Γ6) first excited state were clearly observed in Raman spectra. Two-hole transitions of the acceptor-bound excitons from the 1S3/2(Γ6) ground state to the 2S3/2(Γ6) first excited state were also observed. It is found that the experimental results in the Raman spectra are close to those measured in the PL experiments.

Ce-doped silica films with various Ce concentrations were prepared via ion beam sputtering (IBS) and ion implantation. The samples containing 1.46 at.% Ce were annealed at various temperatures from 500 to 1100 °C in air ambient and a separate sample with the same Ce concentration was annealed at 1100 °C in nitrogen gas. The Ce-related photoluminescence (PL) was observed for the samples and found to be sensitive to the Ce concentrations, annealing conditions and fabricating processes. The PL intensity increases with increasing Ce concentrations from 0.09 to 0.58 at.%, and the concentration-induced quenching effect occurred as the Ce concentrations reached 1.46 at.%. Also, the PL intensity is enhanced as the samples were annealed at various temperatures from 500 to 900 °C in air ambient, then decreases remarkably as the annealing temperature reached 1100 °C. In addition, compared with the sample annealed in air, the PL intensity can be enhanced for the sample annealed in nitrogen gas. Distinctive PL spectra were found for the samples prepared via different processes.

The doping is one of important means in the semiconductor manufacturing techniques, by which the optical and electric properties of semiconductor materials can be significantly improved. The doping level and energy level structure of dopants have a great influence on the operating performances of micro-electronic devices. Beryllium is one of acceptors, which is frequently used to be doped in GaAs bulk, because it is very stable with respect to diffusion at higher temperatures. Therefore, it is significant for the application to optoelectronic devices that the energy-state structure of Be acceptors in GaAs bulk can be investigated in detail. The sample GaAs:Be used in experiment is a 5-μm-thick epitaxial single layer doped uniformly by Be acceptors with a doping level of 2 × 1016 cm–3, and grown by molecular beam epitaxy on 450-μm-thick semi-insulating (100) GaAs substrates in a VG V80 H reactor equipped with all solid sources. The transitions between the energy states of Be acceptors are studied experimentally by different spectroscopy techniques. The far-infrared absorption experiments are performed by using a Fourier-transform spectrometer equipped with a tungsten light source and a multilayer wide band beam splitter. Prior to the absorption spectrum measurement, the sample is thinned, polished and wedged to approximately a 5° angle to suppress optical interference between the front and back faces. Then, the sample is placed into the cryostat with liquid helium (4.2 K). The photoluminescence and Raman spectra are also measured at 4.2 K by a Renishaw Raman imaging microscope. The optical excitation to the sample is provided by an argon-ion laser with a wavelength of 514.5 nm, and the excited power is typically 5 mW. The odd-parity transitions from the Be acceptor ground state 1S3/2Γ8 to three excited states, i.e. 2P3/2Γ8, 2P5/2Γ8 and 2P5/2Γ7 are clearly observed in the far-infrared absorption spectra, then the respective transition energy values are obtained, which are in excellent agreement with the experimental results reported previously. In the photoluminescence spectrum, the emission peak labelled two holetransition, originating from the two-hole transition of recombination of the neutral-accptor bound excitons, is seen obviously, thus the energy of the even-parity transition between 1S3/2Γ8 and 2S3/2Γ8 states is found indirectly. Furthermore, in the Raman spectrum measured, the transition peak between 1S3/2Γ8 and 2S3/2Γ8 states is well resolved, and the transition energy between them is gained directly. By comparison, the transition energy values gained directly and indirectly are found to be consistent with each other.

Three samples of GaAs/AlAs multiple-quantum wells with different quantum well widths and δ-doped with Be acceptors at the well center were grown on (100) GaAs substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4–50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical (LO) phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.

Ce-doped silica films with different Ce concentrations were prepared by ion-beam sputtering and ion implantation. The films containing 1.2 at% Ce were annealed at temperatures from 500 °C to 1200 °C in air ambient, and were annealed in different ambient at 1100 °C. Ce-related photoluminescence was observed in films sensitive to the Ce concentrations, annealing temperatures and the annealing ambient. The peak intensity of the photoluminescence band is approximately linear with Ce concentrations. Also, the photoluminescence intensity is dependent on the annealing temperatures and reaches its highest value after annealing at 700 °C. In addition, the experimental results show that compared with the annealing in an air ambient, the photoluminescence intensity can be enhanced with nitrogen gas. There would be no obvious change for the photoluminescence position or shape.

The material of methylammonium lead iodide, CH3NH3PbI3 (MAPbI3), has shown significant promise in solar cell applications. A way to infer the microscopic scattering mechanism(s) in MAPbI3 is through the measured temperature dependence of carrier mobility. To this end, how does the carrier effective mass depend on temperature, m* = m*(T), is a useful information since the mobility is a function of m*. By atomistic first principles, we report the calculated m*(T) due to the thermal expansion of MAPbI3 materials, in the experimentally relevant range of 130 K to room temperature. The calculated results suggest m* = m*(T) to be linear in T. The increase of m* versus temperature is predominantly due to the expansion of the longitudinal atomic spacing that weakens the s/p hybridization between the I/Pb atoms.

A series of GaAs/AlAs multiple-quantum wells doped with Be is grown by molecular beam epitaxy. The photoluminescence spectra are measured at 4, 20, 40, 80, 120, and 200 K, respectively. The recombination transition emission of heavy-hole and light-hole free excitons is clearly observed and the transition energies are measured with different quantum well widths. In addition, a theoretical model of excitonic states in the quantum wells is used, in which the symmetry of the component of the exciton wave function representing the relative motion is allowed to vary between the two- and three-dimensional limits. Then, within the effective mass and envelope function approximation, the recombination transition energies of the heavy- and light-hole excitons in GaAs/AlAs multiple-quantum wells are calculated each as a function of quantum well width by the shooting method and variational principle with two variational parameters. The results show that the excitons are neither 2D nor 3D like, but are in between in character and that the theoretical calculation is in good agreement with the experimental results.

As transparent conducting oxides (TCOs), In2O3 in the high pressure phase attracts extensive research interests. Because physical properties are determined by the geometric structures, we investigate the electronic and optical properties of Zn/Sn codoped In2O3 materials (IZTO) being modulated by the bixbyite/corundum phase transition via Density Functional Theory calculations. For IZTO in high pressure phase, i.e. corundum phase, Sn/Zn dopant pair tends to form face-sharing ZnO6 and SnO6 octahedrons. The radius differences between Zn2+/Sn4+ dopants and In3+ host cations make Jahn-Teller effect occur and IZTO transform from bixbyite to corundum phase under a slight higher pressure than that of pure In2O3. Although Zn/Sn cosubstitution of In ions may increase the free carrier effective mass m * near the band edge, when IZTO crystal transforms to corundum phase, the more dense packing structure results in stronger cation s-orbital overlaps than in bixbyite phase, which makes m * recover to a smaller value. In addition, corundum IZTO has a larger indirect band gap and a high dopant solubility. So these investigations may open a new way to search for TCOs materials with low indium content.

A Be δ-doped GaAs/AlAs three quantum wells emitter in the terahertz range is fabricated and electroluminescence is investigated. An electroluminescence peak centered at 23.4 meV with a full width at half maximum of 4.2 meV is observed under a bias of 2 V at low temperature (T=4.5 K). The emission peak is attributed to the 2p to 1s internal transitions of the Be acceptors in the center of δ-doped GaAs quantum well. The current-voltage characteristics of the device measured at different temperatures demonstrate a strong negative differential resistance and temperature dependence.