Strong Charge Localization Research Articles

Synthesis of Double Trivalent Perovskite Quantum Dots Cs3BiSbX9 (X=Cl, Br, I) for Efficient CO2 Photoreduction Performance.

Non-toxic Bi halides have great potential in the field of CO2 photoreduction, but strong charge localization limits their charge separation and transfer. In this study, a series of Cs3BiSbX9 (X=Cl, Br, I) perovskite quantum dots (PQDs) are synthesized by antisolvent recrystallization at room temperature, in which Cs3BiSbBr9 PQDs has high selectivity (94.51%) and yield (15.32µmolg-1h-1) of CO2 to CO. In situ DRIFTS and theoretical calculations suggest that the surface charge can be tailored by halogen modulation, allowing for the customization of intermediate species. The Bi─Br─Sb symmetric charge distribution induced by the halogen Br promotes the formation of b─HCOO and reduces the reaction energy barrier of the rate-limiting step, while the weak electronegativity of Cl and the high electronegativity of I leads to m─HCOO and ─COOH production, which are detrimental to CO generation. This work provides new insights into the design of halide alloy perovskites for CO2 photoreduction.

Ultrabright Light Emission Properties of All-Inorganic and Hybrid Organic–Inorganic Copper(I) Halides

All-inorganic and hybrid copper(I) halides have recently emerged as candidate optical materials due to their extremely high light emission efficiencies, nontoxic and earth-abundant elemental compositions, and low-cost solution processability. Originally inspired and motivated by the research on the halide perovskites family, the development of copper(I) halide light emitters has been following its unique path. In this perspective, we discuss the distinct low-dimensional crystal structures of all-inorganic halides, which enable strong charge localization and formation of room-temperature self-trapped excitonic (STE) states, giving rise to largely Stokes-shifted light emission with near-unity quantum yields. Due to their unique electronic structures, all-inorganic copper halides are predominantly blue emitters with unusually weak tunability of their optical properties (e.g., halide substitution has a minimal impact on their photoluminescence properties). These shortcomings paved the way for the exploration of more robust and diverse families of hybrid organic–inorganic copper(I) halides, which are discussed next. Our discussions of crystal and electronic structures and optical properties of all-inorganic and hybrid Cu(I) halides are complemented by the reported uses of these materials in optoelectronic applications. Finally, we identify remaining fundamental questions, knowledge gaps, and practical challenges and outline several paths forward for addressing these issues, including through new materials discovery and in-depth studies of photophysics of Cu(I) halides and derivative materials.

Electron structure and exciton energy level of HgI2, CH3NH3HgI3 and (CH3NH3)2HgI4

The electronic structures and exciton properties of HgI2, CH3NH3HgI3 and (CH3NH3)2HgI4 are reported. Compared with HgI2, organic-inorganic hybrids CH3NH3HgI3 and (CH3NH3)2HgI4 have lower dimension and higher band gap, which show significant excitonic properties. The effective masses of electrons and holes of these three materials were predicted by their energy band structures. Their exciton binding energy at room temperature therein was calculated to be 13.77, 166.81, and 256.00 meV, respectively. HgI2 has low exciton binding energy that cannot exist stably, while CH3NH3HgI3 and (CH3NH3)2HgI4 have strong charge localization and high exciton binding energy. In addition, the UV–vis diffuse reflectance spectra reveal the presence of exciton absorption in CH3NH3HgI3 and (CH3NH3)2HgI4, and the exciton energy levels is confirmed by combining the diffuse reflectance spectra with its exciton binding energy. Importantly, the exciton luminescence peaks of CH3NH3HgI3 and (CH3NH3)2HgI4 were observed at 468 nm and 419 nm in their photoluminescence spectra at room temperature, respectively. This indicates that CH3NH3HgI3 and (CH3NH3)2HgI4 have strong exciton properties and are potential candidates for high-light-yield scintillators and light-emitter applications.

Molecular Design Features for Charge Transport in Nonconjugated Radical Polymers

Conducting polymers based on open-shell radical moieties exhibit potentially advantageous processing, stability, and optical attributes compared with conventional doped conjugated polymers. Despite their ascendance, reported radical conductors have been based almost exclusively on (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), which raises fundamental questions regarding the ultimate limits of charge transport in these materials and whether some of the deficiencies exhibited by contemporary materials are due to the choice of radical chemistry. To address these questions, we have performed a density functional theory (DFT) study of the charge transfer characteristics of a broad range of open-shell chemistries relevant to radical conductors, including p-type, n-type, and ambipolar open-shell chemistries. We observe that far from being representative, TEMPO exhibits anomalously high reorganization energies due to strong charge localization. This, in turn, limits charge transfer in TEMPO compared with more delocalized open-shell species. By comprehensively mapping the dependence of charge transfer on radical-radical orientation, we have also identified a large mismatch between the conformations that are favored by intermolecular interactions and the conformations that maximize charge transfer in all of the open-shell chemistries investigated. These results suggest that significant opportunities exist to exploit directing interactions to promote charge transport in radical polymers.

High pressure induced structural, elastic and electronic properties of Calcium Chalcogenides CaX (X = S, Se and Te) via first-principles calculations

We present an ab initio study of the structural, elastic and electronic properties of CaX (X=S, Se and Te) compounds. In order to describe the properties of these materials rather well, the calculations were based on the DFT theory with generalized gradient approximation (GGA). In particular, our results for the pressure phase transition, elastic stiffness and band structures are in good agreement with the available experimental and theoretical results. We also presented the pressure dependence for all parameters. The generalized stability criteria show that CaSe and CaTe to be mechanically stable at pressures up to the transition pressure. The electronic band structure calculations suggest that these compounds are semiconductors at 0GPa, in agreement with literature data. We discuss the pressure effect on the band gaps and the metallization phenomena. We investigated the bonding character of CaX in terms of electronic charge density and found out that the strong charge localization around the anion side.

Chiral Conducting Salts of Nickel Dithiolene Complexes

Conducting and chiral [Ni(dmit)(2)] dithiolene salts were obtained by electrocrystallization of the radical [n-Bu(4)N][Ni(dmit)(2)] salt in the presence of chiral, enantiopure trimethylammonium cations. Three different cations were investigated, namely, (R)-Ph(Me)HC*-NMe(3)(+), (S)-((t)Bu)(Me)HC*-NMe(3)(+), and (S)-(1-Napht)MeHC*-NMe(3)(+), noted (R)-1, (S)-2, and (S)-3. Salts of 1:3 stoichiometry were obtained with (R)-1 and (S)-2, formulated as [(R)-1][Ni(dmit)(2)](3) and [(S)-2][Ni(dmit)(2)](3)·(CH(3)CN)(2). They both crystallize in the P2(1)2(1)2(1) chiral space group, with three crystallographically independent complexes exhibiting different oxidation degrees. Another salt with 2:5 stoichiometry was isolated with (S)-3. The semiconducting character of the three salts (σ(RT) = 20-30 × 10(-3) S cm(-1)) finds its origin in a strong electron localization, favored by the large number of crystallographically independent [Ni(dmit)(2)] complexes in these chiral structures and their association into weakly interacting dimeric or trimeric motifs. Racemic salts with the same cations, obtained only with difficulties with the tert-butyl-containing (rac)-2 cation, afforded similar trimerized structures. The observed unusual stoichiometry and strong charge localization is tentatively assigned to the size and anisotropic charge distribution of the cations.

Metal to insulator transition in Mn substituted underdoped NdBa2Cu3O7−y: Evidence of strong charge localization

We have synthesized underdoped NdBa2Cu3O7−y (NBCO) and NdBa2Cu3−xMnxO7−y (x=0.1, 0.2, and 0.3) samples. The analysis of the lattice parameters has been done by using the X-ray diffraction (XRD) method. Using the Scanning Electron Microscope (SEM) the granular nature as well as the intergranular networks has been studied. The Energy Dispersive X-ray (EDX) and Rutherford Backscattering Spectroscopy (RBS) studies confirm the substitution of Mn in the Cu-sites. The transport measurements in several undoped and Mn-substituted NBCO samples have been carried out. We have observed an indication of the metal to insulator transition as a result of the strong charge localization induced by Mn substitution. The applicability of various conductivity equations has been verified for comparison. Estimations of the activation energy and localization length have been carried out and discussed.

First-principles elastic and bonding properties of barium chalcogenides

First-principles calculations have been used to investigate the elastic and the bonding properties of barium chalcogenides BaS, BaSe, and BaTe at equilibrium as well as high pressures using full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used local density approximation (LDA) with and without generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. A numerical first-principles calculation of the elastic constants was used to calculate C 11, C 12, and C 44 for these compounds. As the experimental elastic constants are not available; hence our results were only compared with the available theoretical values obtained at equilibrium volume. Additionally, the nature of the chemical bond in these compounds was analyzed. We investigated the bonding character of BaS, BaSe, and BaTe in terms of electronic charge density and found out that the strong charge localization around the anion side let these compounds to behave as the most ionic ones among the alkaline-earth chalcogenides family.

Cd-Doped LaMnO<sub>3</sub> Manganites Prepared by the Sol-Gel Technique

To study the phase diagram of low Cd doped LaMnO3 manganite a structural, electrical and magnetic characterization was performed. In the studied composition range (0 < x < 10%) our results show that La1¯xC dkM nO3 compounds have a Rhombohedric symmetry with a unit cell volume smoothly decreasing with increasing Cd. A magnetic and transport study showed that Cd addition up to x ≤ 0.06 favors ferromagnetism via Double-Exchange (DE) interaction. Above this concentration strong charge localization occurs with a progressive suppression of DE and a corresponding increase of the antiferromagnetic interactions.

Local electronic states in the topmost surface layer probed by metastable atom electron spectroscopy: N2adsorbed and condensed onNi(111)

Electron emission spectra obtained by thermal collisions of ${\mathrm{He}}^{*}{(2}^{3}S)$ metastable atoms with ${\mathrm{N}}_{2}$ molecules on a Ni(111) surface in the chemisorbed, physisorbed, and condensed phases were measured together with the gas-phase spectrum. In the gas-phase collision where the metastable atoms approach randomly oriented ${\mathrm{N}}_{2}$ molecules, Penning ionization rates for the $3{\ensuremath{\sigma}}_{g}$ and $2{\ensuremath{\sigma}}_{u}$ states are almost identical reflecting their spatial extent, while that for the $1{\ensuremath{\pi}}_{u}$ state is rather small. In the saturated overlayer at \ensuremath{\sim}50 K where the ${\mathrm{N}}_{2}$ molecules are chemisorbed with the molecular axes perpendicular to the surface, the $3{\ensuremath{\sigma}}_{g}$-derived state is drastically enhanced relative to the $2{\ensuremath{\sigma}}_{u}$-derived state. This indicates that, upon chemisorption, two \ensuremath{\sigma} states are strongly modified in space by mixing with each other to yield a strong charge localization, i.e., an outer N atom in the $3{\ensuremath{\sigma}}_{g}$-derived state gains considerable charge whereas an outer N atom in the $2{\ensuremath{\sigma}}_{u}$-derived state loses it. The orbital mixing was confirmed by crystal orbital overlap population obtained using density functional theory within a generalized gradient approximation. Our spectra at \ensuremath{\sim}20 K show that the physisorbed ${\mathrm{N}}_{2}$ molecules are located on the chemisorbed species with a parallel orientation in the monolayer region and then condense to form a multilayer. We also found a characteristic band shift and narrowing with increasing layer thickness and these findings are discussed in terms of two final-state relaxation effects.

Local electron distribution of N2 adsorbed on a Ni(111) surface probed by metastable impact electron spectroscopy

Electron emission spectra resulting from thermal collisions of He*(23S) metastable atoms with N2 on a Ni(111) surface in the physisorbed, chemisorbed, and condensed phases were measured as well as the gas-phase spectrum. The ionization cross sections for the 3σg-, 1πu-, and 2σu-derived states depend on the orientation of N2 with respect to the metastable beam, reflecting the spatial electron distribution. Our data also show that upon chemisorption the 3σg- and 2σu-derived states are modified by mixing with each other to yield a strong charge localization. This finding indicates that, even if the metal contribution is small in the chemisorbed states, it plays a crucial role in the topology of the adsorbate wave functions.

Nature of the special-pair radical cation in bacterial photosynthesis

Primary charge separation in bacterial photosynthesis occurs at the “special pair,” a bacteriochlorophyll dimer that, on optical excitation, ejects an electron to become the special-pair radical cation. Understanding the nature of this species is important to both the charge separation process itself and details of subsequent steps including charge recombination. Electron spin resonance (ESR)-based studies have led to the conclusion that the positive charge is delocalized over both bacteriochlorophyll monomers, the degree of delocalization being affected by site-directed mutagenesis. However, Breton et al. have observed charge-transfer electronic absorption spectra (centred at ca. 2500 cm−1), which, when interpreted using standard electron-transfer theory, indicate strong charge localization on just one of the bacteriochlorophylls. We present a complex computational strategy aimed at resolving this issue through vibronic coupling analysis of the high- and low-resolution spectra using a priori computed vibrational analyses, quantum chemical calculation of the strengths of a variety of key interactions, quantum mechanical/molecular mechanical (QM/MM) calculation of the structure of over 20 mutant reaction centers, calculation and interpretation of the observed midpoint potentials, analyses relevant to ESR and related spectroscopies, etc. The current state of progress is described, leading to a consistent picture that includes almost all available experimental data. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1224–1243, 2000

Structural Aspects of the Bechgaard Salts and Related Compounds

Structural aspects of the Bechgaard salts (TMTSF) 2 X and related 2:1 salts, mostly based on the sulfur donors TMTTF and BCPTTF as well as on the hybrid TMDTDSF donor, are considered in relationship with their physical properties. The basic structure of these salts is reviewed with a special emphasize on the dimerization of the organic stack and on the disorder of the anions (X). Then the density waves instabilities driven by the donor stacks are discussed. It is shown in particular that (TMTSF)2PF6 exhibits a mixed 2k F CDW-SDW ground state. The 4k F charge localization phenomena and the 2k F instability of the organic stacks are described in relationship with the structural dimerization and the importance of Coulomb interactions. In the strong charge localization case the spin Peierls instabilities of (TMTTF)2PF6 and of (BCPTTF)2PF6 are more particularly detailed. General aspects of the anion ordering transitions exhibited by salts with non centrosymmetrical anions are pointed out as well as the influence of such transitions on the electronic properties. Finally the relevance of these structural features is briefly discussed by comparing the Bechgaard salts series' of quasi-one dimensional conductors to the family of two dimensional (BEDTTTF) 2 X conductors.

An AM1 semiempirical molecular orbital investigation of the group 14 [1.1.1] propellanes and bicyclo[1.1.1]pentanes

AM1 semiempirical calculations were performed on the group 14 [1.1.1]propellanes and bicyclo[1.1.1]pentanes. For the carbon and tin propellanes, the results were in agreement with several previous theoretical and experimental studies, predicting substantial bonding interaction between the bridgehead atoms in the case of carbon and none in the case of tin. Calculations on the silicon propellane were in sharp disagreement with ab initio calculations using a variety of basis sets. Both geometry and singlet-triplet splitting were incorrectly reproduced. The silicon propellane calculations were repeated using the HF/6-31G* basis set, and the singlet-triplet order was in agreement with the previous work. The germanium results show very strong charge localization. We suggest a possible reason for this observation.

Electronic band structure of the (GaAs)1/(InAs)1 (111) superlattice.

We present a first-principles calculation of the electronic band structure of (GaAs${)}_{1}$/(InAs${)}_{1}$ (111) superlattice, performed by using ab initio norm-conserving pseudopotentials. The folding in the smaller Brillouin zone and the new symmetry properties of the electronic states have been thoroughly investigated by a comparison with the band structure of the corresponding disordered alloy treated within the virtual-crystal approximation. The interactions between the folded states giving rise to energy splittings and electronic charge localization on given layers have been analyzed at the high-symmetry points. The effects of the new ordering potential, introduced by differentiating the two cations in the virtual crystal, and of the strain potential---arising when the superlattice is subjected to a lateral strain on the (111) planes---are separately investigated. We find a strong charge localization in the first two conduction states at the \ensuremath{\Gamma} point and a considerable minimum-band-gap reduction in the new ordered structure with respect to the virtual crystal. The strength of this localization and the fundamental band gap depend on the superlattice orientation and on the strain condition.