Charge Transfer Transition Energies Research Articles

Exploring the chemical physics in tetraphenylporphyrin-N-methylfulleropyrrolidine supramolecular complex by spectroscopic investigations and quantum chemical calculations

The interaction between N-methylfulleropyrrolidine (A) and 5,10,15,20-tetraphenylporphyrin, i.e., D, is studied in solution. Absorption spectrophotometric study reveals formation of charge transfer (CT) absorption band for the DA complex from which CT transition energy (hνCT) is estimated to be 1.673 eV. Quenching of fluorescence of D in presence of A is utilized for determination of the binding constant of the DA complex as 13780 dm3⋅mol−1. DFT calculations generate both geometry optimized and electronic structures of the DA complex and the estimated HOMO-LUMO energy gap, i.e., 1.683 eV, correlates excellently well with the experimentally obtained value of hνCT. Proton NMR studies provide substantial support in favour of strong binding between D and A and provide a clear picture regarding orientation of bound guest (here A) with host (here D). The DA system reported in present work would certainly provide very good perspective for the construction of fulleropyrrolidine based photochromic devices in near future.

High performance and illumination stable In2O3 nanofibers-based field effect transistors by doping praseodymium

Although different kinds of one dimensional (1D) metal oxide nanofibers (MO-NFs) based field effect transistors (FETs) have been developed in recent years, their intrinsic deficiency in illumination instability are not addressed yet, which further prevented their application in practical life. In this work, due to the outstanding performance of suppressing oxygen vacancies and the blue light down-conversion with low charge transfer transition energy, praseodymium (Pr) is demonstrated an ideal candidate to dope into In2O3 NFs for both transferring the depletion mode to the enhancement mode and enhancing the illumination stability of the FETs. The optima In2O3 NFs-based FET was achieved by doping with 3 mol% Pr and exhibited an excellent electrical performance with a carrier mobility of 6.92 cm2V−1s−1, an on/off current ratio of ∼5.4 × 107, threshold voltage of 5.2 V, and subthreshold swing of 0.37 V/dec. Moreover, the FETs exhibited good illumination stability under 3600 s negative gate bias stress test with illumination (NBIS). Finally, the FET was also used as a switch to bright the LED in this work, demonstrating its great promise of Pr-doped In2O3 NFs in future one-dimensional electronic devices.

First-principles study of Ti-doped sapphire. I. Formation and optical transition properties of titanium pairs

Titanium sapphire is one of the most important laser crystals suitable for widely tunable and ultrashort pulsed lasers with high gain and high power outputs, but its performance is limited by the residual infrared absorption at the operation wavelength region of the laser. Although studies have been made over decades in improving the laser performance and the solutions to eliminate this residual absorption, there still remain some controversies for the binding tendency and charge-transfer transition energy of Ti related pairs, which is supposed to be the culprit of this residual absorption. In this paper, we clarify that previously predicted strong binding tendencies in ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$ pairs should be artificial and are blamed on the intrinsic delocalization error in general approximate density functionals. We show that such errors can be eliminated by Hubbard $U$ corrected generalized-gradient-approximation method with $4\ensuremath{\le}U\ensuremath{\le}5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ or hybrid density functionals such like HSE06 and PBE0, which approximately satisfy the generalized Koopmans' condition. Our calculations reveal that the equilibrium geometry and electronic structure of ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$ pairs can be ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$-type or ${\mathrm{Ti}}^{3.5+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3.5+}$-type configurations with very small energy differences ($\ensuremath{\lesssim}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$), and both of them have residual infrared absorption, which are predicted to be about three orders of magnitude stronger in oscillator strength per defect than the pump absorption of ${\mathrm{Ti}}^{3+}$ dopants. Regarding ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ pairs, it is shown that they do not contribute to the residual infrared absorption in the wavelength range of laser operation, but their charge-transfer transitions can explain the residual UV band at 270 nm and $E$ band at 400 nm in the absorption spectra of Ti:${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ crystals. Furthermore, the charge-transfer transition energies and Stokes shift of ${\mathrm{Ti}}^{4+}$ and ionization energies for ${\mathrm{Ti}}^{3+}$ are also well interpreted by our calculations. The calculation method developed together with the predicted optical properties forms the basis for exploration on eliminating the residual infrared absorption in titanium sapphire.

The effect of charge transfer transition on the photostability of lanthanide-doped indium oxide thin-film transistors

Amorphous oxide semiconductors are promising for their use in thin-film transistor (TFT) devices due to their high carrier mobility and large-area uniformity. However, their commercialization is limited by the negative gate bias stress experienced under continuous light illumination. Here, we report an approach to improve the negative bias illumination stress (NBIS) stability of amorphous oxide semiconductors TFTs by using lanthanide-doped indium oxide semiconductors as the channel layer. The effect of different lanthanide dopants on performances of solution-processed Ln:In2O3 TFTs are investigated. All lanthanides exhibit strong suppression of oxygen vacancy, which shift the Von from −13.5 V of pure In2O3 TFT to −1~1 V of Ln:In2O3 TFTs (except Ce). However, only Pr:In2O3 and Tb:In2O3 TFTs exhibit much better NBIS stability with same ΔVon of −3.0 V, compared to much higher ΔVon of −7.9~−15.6 V for other Ln:In2O3 TFTs. Our comprehensive study reveals that praseodymium and terbium act as a blue light down-conversion medium with low charge transfer transition energy for lowing photosensitivity of oxide semiconductors.

Theoretical and experimental examinations to substantiate the structure of coal derived asphaltene by formation of complex with o-chloranil

The structure of coal derived asphaltene (CDA) proposed by Mullins et al has been corroborated through theoretical and experimental examination by the virtue of its electron donor property. Here, o-chloranil has been used as an electron acceptor. Investigation of o-chloranil/CDA complex in gas phase has been performed by density functional theory (DFT) methods. The Charge transfer (CT) transition energy of the complex in gas phase was calculated by the time-dependent density function theory (TD-DFT) using the ground state optimized geometry at the respective levels of theory. For this, the three semi empirical methods AM1, PM3 and PM3MM were computed under the MPW1PW91, B3PW91, PBE1PBE, BPV86 and RCIS formalism. It is found that the two formalisms MPW1PW91 and PBEPBE under PM3 and PM3MM reproduced values close to the experimental one. However, all of them provide lower charge transfer (CT) transition energy values than that has been experimentally determined. Experimentally, FTIR spectra of the donor, acceptor and the complex further supported the understanding of complex formation and demonstrated close similarity with the theoretically determined CDA frequencies. These findings support the validity of the CDA structure to an extent.

Interaction of Water at the Hydrophobic Interface of Alkyl Group of Alcohol with p‐Nitro‐Aniline Charge Transfer State

AbstractThe evolution of photo‐induced intra molecular charge transfer (ICT) transition energy of p‐nitro‐aniline (PNA), with respect to the mole fraction of alcohol in alcohol water mixture has been used to study, the interaction of hydrophobic hydration ‐shell of linear alcohols with PNA‐ICT state. The linear alcohol ranges from methanol to propanol. The plot of absorption energy of PNA charge transfer transition versus mole fraction of alcohol shows the following features. At lower mole fraction of alcohol, the absorption energy of PNA versus mole‐fraction of alcohol plot at first, decreases below the absorption energy of PNA recorded in pure water and alcohol, goes through a minimum and then increases. Similar behaviour is obtained even after subtracting the contribution from dielectric non‐ideality. The behaviour at lower mole fraction thus indicates the modification of specific interaction of PNA with water due to the presence of alcohol. Further, the absorption energy minimum shifts towards lower mole fraction of alcohol as the chain length of alcohol increases. These results, thus reveals the interaction of PNA with the modified water structure at the hydrophobic interface of alkyl group and supports the current understanding of hydrophobic hydration. To our knowledge, this study for the first time reveals that, hydration shell around hydrophobic group has higher ability to stabilize the PNA‐ICT state by hydrogen bonding interaction compared to bulk water.

Boosting Efficiency of Near‐Infrared Organic Light‐Emitting Diodes with Os(II)‐Based Pyrazinyl Azolate Emitters

AbstractTremendous effort has been devoted to developing novel near‐infrared (NIR) emitters and to improving the performance of NIR organic light‐emitting diodes (OLEDs). Os(II) complexes are known to be an important class of NIR electroluminescent materials. However, the highest external quantum efficiency achieved so far for Os(II)‐based NIR OLEDs with an emission peak wavelength exceeding 700 nm is still lower than 3%. A new series of Os(II) complexes (1–4) based on functional pyrazinyl azolate chelates and dimethyl(phenyl)phosphane ancillaries is presented. The reduced metal‐to‐ligand charge transfer (MLCT) transition energy gap of pyrazinyl units in the excited states results in efficient NIR emission for this class of metal complexes. Consequently, NIR OLEDs based on 1–4 show excellent device performance, among which complex 4 with a triazolate fragment gives superior performance with maximum external quantum efficiency of 11.5% at peak wavelength of 710 nm, which represent the best Os(II)‐based NIR‐emitting OLEDs with peak maxima exceeding 700 nm.

Prediction of charge transfer transition energies of the molecular complexes of PMDA with a series of methylbenzenes by TDDFT

Prediction of charge transfer transition energies of the molecular complexes of PMDA with a series of methylbenzenes by TDDFT

Resonance Raman and UV–vis characterization of charge transfer complexes of TCNQ and aromatic amines

Although the molecular charge transfer complexes formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) and several different donors have been widely investigated in the past, in this paper it is shown for the first time the vibrational spectroscopic characterization of the complexes formed by TCNQ and the simplest series of substituted anilines in solution. The UV–vis spectra indicate the formation of TCNQ complexes stabilized by two aromatic amines in a sandwich π type complex. It was observed that the charge transfer transition energies of the complexes follow a linear correlation with the ionization potential of the amines. The resonance Raman spectra revealed, by the analysis of the ν(CC) and ν(CN) stretching modes of TCNQ, that the complexes keep their neutral character in the ground electronic state, and not as a TCNQ2− dianion species as reported before. The observed enhancement of the TCNQ bands as the amine donor capacity increases, confirmed the charge transfer nature of the electronic transitions. The DFT and TDDFT results were obtained and the theoretical Raman and electronic spectra data supported the experimental findings. The results of the model systems presented herein can contribute to a deep understanding of the photoinduced charge transfer process in complexes of TCNQ, which is a key step in the designing of organic molecular devices.

Control of Electronic Structure and Conductivity in Two-Dimensional Metal-Semiquinoid Frameworks of Titanium, Vanadium, and Chromium.

The isostructural, two-dimensional metal-organic frameworks (H2NMe2)2M2(Cl2dhbq)3 (M = Ti, V; Cl2dhbqn- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) and (H2NMe2)1.5Cr2(dhbq)3 (dhbqn- = deprotonated 2,5-dihydroxybenzoquinone) are synthesized and investigated by spectroscopic, magnetic, and electrochemical methods. The three frameworks exhibit substantial differences in their electronic structures, and the bulk electronic conductivities of these phases correlate with the extent of delocalization observed via UV-vis-NIR and IR spectroscopies. Notably, substantial metal-ligand covalency in the vanadium phase results in the quenching of ligand-based spins, the observation of simultaneous metal- and ligand-based redox processes, and a high electronic conductivity of 0.45 S/cm. A molecular orbital analysis of these materials and a previously reported iron congener suggests that the differences in conductivity can be explained by correlating the metal-ligand energy alignment with the energy of intervalence charge-transfer transitions, which should determine the barrier to charge hopping in the mixed-valence frameworks.

Photocarrier generation from interlayer charge-transfer transitions in WS2-graphene heterostructures.

Efficient interfacial carrier generation in van der Waals heterostructures is critical for their electronic and optoelectronic applications. We demonstrate broadband photocarrier generation in WS2-graphene heterostructures by imaging interlayer coupling-dependent charge generation using ultrafast transient absorption microscopy. Interlayer charge-transfer (CT) transitions and hot carrier injection from graphene allow carrier generation by excitation as low as 0.8 eV below the WS2 bandgap. The experimentally determined interlayer CT transition energies are consistent with those predicted from the first-principles band structure calculation. CT interactions also lead to additional carrier generation in the visible spectral range in the heterostructures compared to that in the single-layer WS2 alone. The lifetime of the charge-separated states is measured to be ~1 ps. These results suggest that interlayer interactions make graphene-two-dimensional semiconductor heterostructures very attractive for photovoltaic and photodetector applications because of the combined benefits of high carrier mobility and enhanced broadband photocarrier generation.

Bandgap bowing in Ni1−xMgxO alloy

It is crucial to understand the compositional dependence of the bandgap of the Ni1–xMgxO alloy as it is a promising bandgap-tunable material for ultraviolet (UV) and deep-UV photodetectors. However, the dependence of the bandgap of this material on the Mg content, x, is still a subject of debate, as the experimentally measured optical bandgap of Ni1–xMgxO is 2–3 eV lower than that of pure MgO (7.77 eV), in contrast to that predicted by conventional bandgap bowing theories. Here, we demonstrate that the Ni1−xMgxO alloy has two bandgaps: (i) the Ni-3d bandgap Eg3d, i.e., the bandgap between the valence band (O 2p) and Ni-3d(eg) bands, where the charge-transfer (CT) transition energy weakly depends on the Mg content and (ii) the alloy bandgap EgNi1−xMgxO, i.e., the bandgap between the valence band and the conduction band of the Ni1−xMgxO alloy, which is related to the Ni 4s/Mg 3s states; the alloy bandgap obeys the conventional bandgap bowing model. The Ni1−xMgxO absorption spectra at low Mg contents are difficult to deconvolute because the electronic bands originating from the Ni-3d bands and the alloy conduction band overlap. The band structure described above elucidates the anomalous characteristics of the bandgap of Ni1–xMgxO, i.e., most of the optical bandgap values reported so far are smaller than the expected value corresponding to the transition from the valence band to the alloy conduction band because they were evaluated based on the absorption due to the O 2p-Ni 3d(eg) CT transition.

Systematic First-Principles Calculations of Charge Transfer Transitions of Rare Earth Ions in Oxides Using Simple Cubic Model Cluster

[Introduction] It is important for trap levels to capture carriers for persistent phosphors, and the properties depend on the positions of the trap levels within the band gap [1]. It is necessary to estimate the positions of impurity levels which work as trap levels for material design of the persistent phosphors. As one of the ways, the position of the impurity level within the band gap can be estimated by calculating the charge transfer transition energy from the valence band (Ligand to Metal Charge Transfer: LMCT). However, calculation of the LMCT based on quantum theory is exceedingly difficult in d electron system like transition metal and f electron system like rare earth ion because the calculation requires to consider many-electron effect, symmetry and relativistic effect such as spin-orbit interaction. Recently, we successfully reproduced the LMCT energies and their experimental trend of transition metal ions in α-Al2O3 by performing configuration interaction (CI) calculation using the DVME method [2]. From these results, we expect that the relativistic DVME method can be also applied to rare earth ions. In this study, as a first approach, in order to obtain the LMCT energy for various trivalent RE ions in REO8 13- cubic model cluster, we performed first-principles calculations using the DVME method and constructed the theoretical energy diagram of the rare earth ions, and compared it with the reported LMCT energies in YPO4 [3] and YAG [4]. [Computational Method] We constructed a 9-atom cluster consisting of the central RE ion and the 8 oxygen ions with Oh symmetry (cubic) and 2.4 Å bond length. We considered the ground state configuration 4f n 2p48 and one-electron excitation configuration 4f n +12p47, where n is the number of 4f electrons. In order to improve the computational accuracy, the configuration dependent correction (CDC) was considered. The LMCT energy was calculated as the transition energy from the top of the valence band corresponding to the highest one of the MOs mainly consisting of oxygen 2p orbital to the MO mainly consisting of RE 4f orbital based on the MO calculation using the DV-Xα method and the CI calculation using the DVME method. [Result] The theoretical LMCT energies calculated with CDC are shown as the diagram in Fig. 1 together with the diagram for the reported values in YPO4 and YAG. The figure shows that the reported trend, especially around Eu to Gd, was successfully reproduced by first-principles calculations. The overestimation for Ho to Tm is probably due to the insufficient consideration of electron correlation. [1] J. Ueda, et al,. Appl. Phys. Lett. 104, 101904, (2014). [2] K. Ogasawara, et al., Phys. Rev. B, 64, 115413, (2001). [3] E. Nakazawa, J. Lumin. 100, 89, (2002). [4] P. Dorenbos, J. Lumin. 134, 310, (2013). Figure 1

The vacuum referred electron binding energies in the [formula omitted] and [formula omitted] states of [formula omitted] and [formula omitted] in inorganic compounds

An overview of the spectroscopic data on Tl+ and Pb2+ in 37 and 126 different inorganic compounds, respectively, is presented. Using the metal-to-metal charge transfer and A-band transition energies, the electron binding energies in the S01 ground state and P13 excited state of Tl+ and Pb2+ were determined relative to the vacuum level. By constructing vacuum referred binding energy (VRBE) diagrams that display the energy levels of the Tl+ and Pb2+ activator ion together with that of the host compound, insight in the luminescent properties of these activated compounds are gained. The obtained VRBEs of the electron in the S01 and P13 states of Tl+ and Pb2+ are compared to those of Bi3+.

Systematic First-Principles Calculations of Charge Transfer Transitions of Transition Metal Ions in α-Al2O3 and Optimization of the Computational Condition

[Introduction] Persistent phosphors are materials showing continuous luminescence for long durations from several minutes to hours after optical excitation has ceased. The materials have carrier traps that play the most important role in the persistent luminescence process. Ueda et al. of Kyoto University developed a blue-light-induced persistent luminescence material Ce3+ -Cr3+:Y3Al2Ga3O12 by doping Cr into Ce3+:Y3Al2Ga3O12, where the 3d orbitals of Cr3+ work as trap centers to capture electrons[1]. The persistent luminescence properties are dominated by the position of the energy levels of the trap centers in the band gap and therefore can be controlled by changing the doped transition metal (TM) ion. Generally, it is difficult to calculate the band gap energy by first-principles calculations based on the cluster method due to the poor reproduction of the conduction band. Accordingly, the transition energy from the impurity level to the conduction band cannot be calculated directly. However, the position of the impurity level within the band gap can be estimated by calculating the charge transfer transition energy from the valence band (Ligand to Metal Charge Transfer: LMCT). In this study, in order to obtain the LMCT energy for various trivalent TM ions in α-Al2O3, we performed first-principles calculations using the DVME method[2]based on configuration interaction (CI) method and constructed the theoretical energy diagram of the transition metal 3d levels within the band gap. By comparing the theoretical diagram with the available experimental LMCT energies in detail, the optimized computational condition was also proposed. [Computational Method] We constructed a 7-atom cluster with C3 symmetry consisting of the central aluminum ion and the first-neighbor oxygen ions from the crystal data of α-Al2O3 [3]and then we substituted various trivalent TM (Sc, Ti, V, Cr, Mn, Fe, Co) ions for the central aluminum ion. The effective Madelung potential was considered by setting point charges on the atomic sites around the cluster. In order to improve the computational accuracy, the lattice relaxation (LR) caused by the substitution of TM for aluminum was considered by estimating the change of the bond length based on the Shannon’s crystal radii, and the configuration dependent correction (CDC) was also considered. The LMCT energy was calculated as the transition energy from the top of the valence band corresponding to the highest one of the MOs mainly consisting of oxygen 2p orbital to the MO mainly consisting of TM 3d orbital based on the MO calculation using the DV-Xα method and the CI calculation using the DVME method. [Result] The theoretical LMCT energies calculated with LR and CDC are shown as the diagram in Fig. 1 together with the diagram for the experimental values[4-6]. The figure shows that the experimental trend was successfully reproduced by first-principles calculations. The remaining underestimation for Sc and Ti is probably due to the overestimation of the lattice relaxation arising from the simple approximation. On the other hand, the overestimation for Fe is probably due to the insufficient consideration of electron correlation. [References] [1] J. Ueda, et al,. Appl. Phys. Lett. 104, 101904, (2014). [2] K. Ogasawara, et al., Phys. Rev. B, 64, 115413, (2001). [3] E. N. Maslen, et al., Acta Crystallogr. Sect. B, 49, 973-980 (1993). [4] B. R. Namozov, et al., Physics of the Solid State, 40, 599-600 (1998). [5] H. H. Tippins, Phys. Rev. B, 1, 126-135 (1970). [6] D. S. McClure, J. Phys. Chem., 36, 2757-2779 (1962). Figure 1

(Invited) Computational Design of Lanthanide-Doped Optical Materials By a Combined DFT and Crystal-Field Approach: Recent Developments and Applications

In the last decades, lanthanide activated luminescent materials for lighting and display applications have been explored and optimized mainly by the cumbersome and costly trial-and-error experimental approach. Nowadays, the quick development of lanthanide spectroscopy theory together with an increase of the computational resources have made it possible to propose a systematical calculation strategy to understand the physics picture behind experiment and build up the link between the structural and optical properties of lanthanide-doped materials. It is well known that density functional theory (DFT) calculations employing the single-particle ground-state picture can provide very sufficiently accurate structural relaxation and the chemical trends that determine the emission energies and the luminescence mechanisms of optical dopants [1], whereas crystal-field (CF) theory calculations can very well reproduce experimentally observed optical spectra, which involve transitions between multi-electronic energy states of lanthanide ions [2]. Therefore, a combined theoretical scheme of modern DFT and conventional CF models has been proposed to give full play to their advantages [3]. In this talk, we will demonstrate how such a combined theoretical scheme supports the recent experimental studies of lanthanide phosphors. Ce3+ and Eu2+ ions will be paid more attention to due to their applications for white LED phosphors. The real reason resulting in the red or blue shift of the lowest 5d-4f emissions of Eu2+ or Ce3+ ions [4] will be analyzed by considering the combined effects of electronic structure and Stokes shift. Moreover, the misunderstanding about the position of the lowest 4f65d energy level in the 4f-5d excitation spectra of Eu2+ ions will be clarified, and thus the Stokes shift can be properly reproduced and further fed into understanding the electron-phonon coupling effect of Eu2+ ions with host lattices [5]. The successful application to the solid solution phosphors Ca2(Al1-x Mg x )(Al1-x Si1+x )O7: Eu2+will be shown in order to highlight the role of the theoretical work in the discovery of novel lanthanide phosphors [6]. Final, the recent developments and future challenges of the combined theoretical scheme will be given and commented, which include the first-principles structural determination of the 5d excited states of lanthanide ions and reproduction of the lowest charge transfer transition energy and its Stokes shift. Acknowledgements: The work is financially supported by National Natural Science Foundation of China (Grant No. 11204393), Natural Science Foundation Project of Chongqing (Grant No. CSTC2014JCYJA50034) and National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No. 201410617001).

Performance of the M06 family of functionals in predicting the charge transfer transition energies of molecular complexes of TCNE with a series of methylated indoles

Ground state intermolecular interactions in the molecular complexes of TCNE with a series of methylated indoles have been predicted with almost equal efficiency by the B3LYP and the M06 family of functionals. However, TDDFT calculation with the B3LYP functional failed to find any charge transfer (CT) absorption band of the complexes while each of the functionals M06, M06-2X, M06-HF and M06-L succeeded; the calculated CT transition energies vary with the calculated vertical ionization potentials of the indoles, complying with Mulliken’s theory of charge transfer complexes and are fairly close to reported experimental values.

Performance of the M06 family of functionals in prediction of the charge transfer transition energies of the naphthalene–TCNE and pyrene–TCNE molecular complexes

The B3LYP and the M06 family of functionals were found to be of nearly equal efficiency for predicting the 13C and 15N NMR chemical shifts (which are ground state properties) of molecular complexes of TCNE with naphthalene and pyrene. But the B3LYP functional could not find any charge transfer (CT) absorption band of the complexes. The M06, M06-2X, M06-HF and M06-L functionals could find two CT absorption bands in CCl4 medium by a TDDFT calculation under the PCM formalism for solvation; the calculated CT transition energies are comparable with the reported experimental values, M06-2X and M06-L agreeing best with experiment.

Effect of Excited-State Structural Relaxation on Midgap Excitations in Co2+-Doped ZnO Quantum Dots

The unique midgap excited states of Co2+-doped ZnO quantum dots have given rise to new applications in photocatalysis, sensing, magneto-optics, and magnetoelectronics. However, the electronic characteristics of these midgap transitions are not fully understood, and the uncertain interplay between these transitions has led to disagreement in the literature. In this work, midgap excited states of Co2+-doped ZnO quantum dots are analyzed using linear response time-dependent density functional theory and the effective mass theory with a focus on the geometry relaxation in the donor-type photoionization excited state. Relaxation of the excited-state geometry lowers the charge-transfer transition energy to be in the vicinity of the prominent spin-allowed 4A2 → 4T1 Co2+ d–d transition that gives this material its characteristic color. For large quantum dots, the excited-state population distribution between this Co2+ d–d excited state and the charge-transfer excited state can be tuned by thermal energy, resultin...

Thickness-dependent optical properties in compressively strained BiFeO3/LaAlO3 films grown by pulsed laser deposition

Bismuth ferrite (BiFeO3) films with various thicknesses were epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The X-ray diffraction and Raman scattering spectra reveal that the films were highly (1 1¯ 1) oriented with the single phase. With increasing the thickness, the compressive strain decreases and the strain ratios between the film and bulk crystal are evaluated to be 1.75, 1.57, and 1. Moreover, the compressive strain induces band gap shrinkage from 2.7 to 2.65eV, while the charge transfer transition energy increases from 3.5 to 4.1eV. It could be due to the shift of O 2p states and the variation of local Fe3+ crystal field.