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The Raman spectra of several locally stable structures of the graphene oxide (GO) have been simulated by ab initio calculations. Compared to graphite, the G band of GO is broadened and blueshifted due to the emergence of a series of new Raman peaks. The Raman intensities and positions of the D and G bands depend sensitively on the local atomic configurations. In addition to the normal epoxy and hydroxyl groups, other oxidation groups such as epoxy pairs are also studied. Epoxy pairs induce large blueshift of G band with respect to that of the graphite. [ABSTRACT FROM AUTHOR]

β-FeSi2, a semiconductor material made of two of the most earth-abundant elements, has important applications in thermoelectrics, photovoltaics and optoelectronics owing to its attractive properties such as suitable band gap and air stability over a wide temperature range. While point defects always play a vital role in semiconductor materials, only sporadic studies have been dedicated to the defects in β-FeSi2. Here, using first-principles calculations we systematically investigate the intrinsic point defects in β-FeSi2. Our results reveal that the formation energies of the intrinsic defects in β-FeSi2 are high enough to prevent them from forming in a significant concentration under thermal equilibrium growth conditions. As a possible kinetic process generating intrinsic defects, we study the α-to-β phase transition of FeSi2. We find that the phase transition is a slow process occurring on the time scale of an hour. Incomplete phase transition may lead to kinetically formed intrinsic defects. We further calculate the activation energies of the intrinsic defects and show that the experimentally observed conductivity of pure β-FeSi2 should be a result of unintentional doping. Possible extrinsic impurities that may lead to n-type and p-type conductivity and their activation energies are calculated, which are in good agreement with available experiments. Our results provide guidance for optimizing the doping strategy of β-FeSi2 for device applications. [ABSTRACT FROM AUTHOR]

Defect formation energy governs the thermodynamics of a specific dopant within the host material. Here, we introduce an approach to decomposing the defect formation energy into intuitive components, each representing a distinct physical step in the process of defect formation. Through this approach, we illustrate that adhering solely to conventional criteria, such as ionic radius, may overlook potential dopants. Taking β-FeSi2, a promising high-temperature thermoelectric material, as an example, we demonstrate that non-intuitive chemical interactions can play a more significant role in lowering the defect formation energy. As a result, Ir on Fe site is found to exhibit unexpected low defect formation energy among the 26 candidate dopants and has been employed in experiment to enhance the thermoelectric figure of merit of n-type β-FeSi2. The understanding gained from this work could be of general interest for addressing the doping limit issue for other potential thermoelectric materials. [ABSTRACT FROM AUTHOR]

Using density functional theory calculation we investigate the carbon doping of anatase TiO2, a technique widely studied for visible-light driven water splitting. By a detailed analysis of the thermodynamics of C defects in TiO2, we show that any significant concentration of C dopants in the TiO2 lattice must be a result of non-equilibrium doping, which emphasizes the importance of kinetics stabilized C defects. Based on the band gaps calculated using hybrid density functionals, we exclude the possibility of C occupying Ti lattice sites or interstitial sites to enhance visible-light absorption of TiO2, as extensively discussed in the literature. Also, the recently proposed defect with a CO species occupying two O sites yields a too small band gap for water splitting. Two defects that can effectively reduce the band gap for the water splitting application are identified to be: (1) the CO–VO complex, i.e., a C substituting for O (CO) paired with an O vacancy (VO) and (2) the (C2)2O complex with a C dimer (C2) occupying two neighboring O vacancies. Compared with the CO–VO complex, (C2)2O exhibits strong binding (greater than 2.5 eV) between the two C atoms, which could significantly enhance its kinetic stability to survive from high temperature annealing. With a reduced band gap of about 1.4 eV, carbon dimers could be ideal for kinetic doping of anatase TiO2 to enhance its visible-light activity in photocatalytic reactions. Molecular doping using C2H2 or C2H4 as C precursors has been proposed to introduce the carbon dimers into TiO2. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally a chemical codoping approach that would simultaneously narrow the band gap and control the band edge positions of TiO2 semiconductors. It is shown that a sequential doping scheme with nitrogen (N) leading the way, followed by phosphorus (P), is crucial for the incorporation of both N and P into the anion sites. Various characterization techniques confirm the formation of the N–P bonds, and as a consequence of chemical codoping, the band gap of TiO2 is reduced from 3.2 eV to 1.8 eV. The realization of chemical codoping could be an important step forward in improving the general performance of electronic and optoelectronic materials and devices. [ABSTRACT FROM AUTHOR]

Silver sulfide in monoclinic phase (α-Ag2S) has attracted significant attention owing to its metal-like ductility and promising thermoelectric properties near room temperature. However, first-principles studies on this material by density functional theory calculations have been challenging as both the symmetry and atomic structure of α-Ag2S predicted from such calculations are inconsistent with experimental findings. Here, we propose that a dynamical approach is imperative for correctly describing the structure of α-Ag2S. The approach is based on a combination of ab initio molecular dynamics simulation and deliberately chosen density functional considering both proper treatment of the van der Waals interaction and on-site Coulomb interaction. The obtained lattice parameters and atomic site occupations of α-Ag2S are in good agreement with experimental data. A stable phonon spectrum at room temperature can be obtained from this structure, which also yields a bandgap in accord with experimental measurements. The dynamical approach thus paves the way for studying this important ductile semiconductor in not only thermoelectric but also optoelectronic applications. [ABSTRACT FROM AUTHOR]

BaZrS 3 is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of 1.7–1.8 eV, a very strong light-matter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many fundamental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of BaZrS 3 thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of 1019-1020 cm−3. Depending on the processing temperature, the Hall mobility ranges from 2.1 to 13.7 cm2/Vs. The absorption coefficient is > 105 cm−1 at photon energy >1.97 eV. Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that BaZrS 3 is a promising candidate, these results potentially unleash the family of chalcogenide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes. Image 1 • The first fabrication of high quality chalcogenide perovskite BaZrS 3 thin films, opening the door for device applications. • Exceptionally high absorption coefficient higher than 105 cm-1 at photon energy greater than 1.97 eV. • n-type doping with carrier mobility greater than 10 cm2/Vs. • Shallow defect level. [ABSTRACT FROM AUTHOR]

Correction for 'Thermodynamics, kinetics and electronic properties of point defects in β-FeSi2' by Jun Chai et al., Phys. Chem. Chem. Phys., 2019, 21, 10497–10504. [ABSTRACT FROM AUTHOR]

As soluble catalysts, redox mediators can reduce the high charging overpotential of lithium-oxygen batteries by providing sufficient liquid-solid interface for lithium peroxide decomposition. However, the redox mediators usually introduce undesirable reactions. In particular, the so-called "shuttle effect" leads to the loss of both the redox mediators and electrical energy efficiency. In this study, an organic compound, triethylsulfonium iodide, is found to act bifunctionally as both a redox mediator and a solid electrolyte interphase-forming agent for lithium-oxygen batteries. During charging, the organic iodide exhibits comparable lithium peroxide-oxidizing capability with inorganic iodides. Meanwhile, it in situ generates an interfacial layer on lithium anode via reductive ethyl detaching and the subsequent oxidation. This layer prevents the lithium anode from reacting with the redox mediators and allows efficient lithium-ion transfer leading to dendrite-free lithium anode. Significantly improved cycling performance has been achieved by the bifunctional organic iodide redox mediator. To fulfill the great promise of Li-O2 batteries, the high charge overpotential is a major challenge that has to be addressed. Here the authors introduce triethylsulfonium iodide as a redox mediator as well as an enabler of a protective layer on Li anode, leading to notable electrochemical performance. [ABSTRACT FROM AUTHOR]

Highlights from the article: In article number 1900052, Saifeng Zhang, Jun Wang, and co-workers investigate the superior nonlinear optical performance and ultrafast dynamics of layered PtSe SB 2 sb and signatures of the transition from semiconductor to semimetal. The evolution of nonlinear absorption and carrier dynamics with increasing layer thickness demonstrate the PtSe SB 2 sb 's promising potential in nanophotonic devices, such as in infrared detectors, optical switches, and saturable absorbers.

The demand for an ultrabroad optical material with a bandgap tunable from zero to at least 1–2 eV has been one of the driving forces for exploring new 2D materials since the emergence of graphene, transition metal dichalcogenides, and black phosphorus. As an ultra‐broadband 2D material with energy bandgap ranging from 0 to 1.2 eV, PtSe2 shows much better air stability than its analogue, black phosphorous. In this work, the superior nonlinear optical performance and ultrafast dynamics of layered PtSe2, and signatures of the transition from semiconductor to semimetal are systematically studied. Combining with rate equations, first‐principles calculation, and electrical measurements, a comprehensive understanding about the evolution of nonlinear absorption and carrier dynamics with increasing layer thickness is provided, indicating its promising potential in nanophotonic devices such as infrared detectors, optical switches, and saturable absorbers. [ABSTRACT FROM AUTHOR]

Zinc halides have been less explored even though they are a promising lead‐free family with the merits of environmental friendliness and low cost. Here, the organic–inorganic hybrid zinc halides are screened with aromatic molecules containing amino groups (MXDA: m‐Xylylenediamine, ABI: 2‐Aminobenzimidazole, pipda: 2‐piperidylmethylamine, p‐MPEA: p‐Methylphenethylamine), and six new compounds are discovered: (MXDA)ZnCl4 (1), (ABI)2ZnCl4 (2), (pipda)2H2OZnCl2 (3), [(pipda)4Zn2Br4]·H2O (4), (p‐MPEA)2ZnCl4 (5), and (p‐MPEA)2ZnBr4 (6). The discovered compounds exhibit diverse efficient light emissions (blue, green, yellow, and ultraviolet) under ultraviolet light irradiation. By doping Sb and Mn at the Zn site, (MXDA)ZnCl4 exhibits different colors of light under excitation light of different wavelengths, so it is successfully used for anti‐counterfeiting. (MXDA)Zn0.91Mn0.09Cl4 is also found to be an efficient X‐ray scintillator for X‐ray imaging. Among the as‐explored new phases, a maximum photoluminescent quantum yield of 82.60% is realized, enabling the utilization for white light‐emitting diodes. The results suggest that the zinc halides are a new lead‐free family with superior optoelectronic properties, which can be used for multiple applications. [ABSTRACT FROM AUTHOR]

SnO 2 is one of the most interesting oxide semiconductors due to its wide band gap, good transparency, high thermal/chemical resistances, and low cost. As-grown SnO 2 usually exhibits n -type conductivity with high carrier concentrations. Similar to the case of ZnO, it is difficult to dope SnO 2 into p -type. Consequently, applications of SnO 2 for optoelectronic as well as electronic devices are limited. In principle, substitution of group-III elements, including Al, Ga, and In, for Sn atom in SnO 2 could give a hole carrier resulting in a p -type conductivity. Based on the HSE functional calculations (Varley et al., 2009) [13], it has been reported that these dopants are shallow acceptor defects. However, the calculations with PBE0 functional (Scanlon and Watson, 2012) [14] showed that these dopants are deep acceptors. In this work, Ga-doped SnO 2 are revisited by using HSE functional. We found that Ga Sn defect is indeed a deep acceptor indicating that Ga Sn can serve only as a compensating acceptor defect in SnO 2 . Hoping to make the acceptor level shallower, we further study the effect of (compressive) strain on the acceptor level associated with Ga Sn defect by alloying SnO 2 with Si atoms (Si x Sn 1− x O 2 alloy where x ~ 0.17). Our results showed that even with the application of strain, the acceptor level remains too deep to be useful. [ABSTRACT FROM AUTHOR]