An accurate and numerically efficient method for the calculation of intermolecular Coulomb couplings between charge densities of electronic states and between transition densities of electronic ...excitations is presented. The coupling of transition densities yields the Förster type excitation energy transfer coupling, and from the charge density coupling, a shift in molecular excitation energies results. Starting from an ab initio calculation of the charge and transition densities, atomic partial charges are determined such as to fit the resulting electrostatic potentials of the different states and the transition. The different intermolecular couplings are then obtained from the Coulomb couplings between the respective atomic partial charges. The excitation energy transfer couplings obtained in the present TrEsp (transition charge from electrostatic potential) method are compared with couplings obtained from the simple point-dipole and extended dipole approximations and with those from the ab initio transition density cube method of Krüger, Scholes, and Fleming. The present method is of the same accuracy as the latter but computationally more efficient. The method is applied to study strongly coupled pigments in the light-harvesting complexes of green sulfur bacteria (FMO), purple bacteria (LH2), and higher plants (LHC-II) and the “special pairs” of bacterial reaction centers and reaction centers of photosystems I and II. For the pigment dimers in the antennae, it is found that the mutual orientation of the pigments is optimized for maximum excitonic coupling. A driving force for this orientation is the Coulomb coupling between ground-state charge densities. In the case of excitonic couplings in the “special pairs”, a breakdown of the point-dipole approximation is found for all three reaction centers, but the extended dipole approximation works surprisingly well, if the extent of the transition dipole is chosen larger than assumed previously. For the “special pairs”, a large shift in local transition energies is found due to charge density coupling.
Abstract
The stability of the molecular self-assembled monolayers (SAMs) is of vital importance to the performance of the molecular electronics and their integration to the future electronics ...devices. Here we study the effect of electron irradiation-induced cross-linking on the stability of self-assembled monolayer of aromatic 5,5′-bis(mercaptomethyl)-2,2′-bipyridine BPD; HS-CH
2
-(C
5
H
3
N)
2
-CH
2
-SH on Au (111) single crystal surface. As a refence, we also study the properties of SAMs of electron saturated 1-dodecanethiol C12; CH
3
-(CH
2
)
11
-SH molecules. The stability of the considered SAMs before and after electron-irradiation is studied using low energy Ar
+
cluster depth profiling monitored by recording the X-ray photoelectron spectroscopy (XPS) core level spectra and the UV-photoelectron spectroscopy (UPS) in the valance band range. The results indicate a stronger mechanical stability of BPD SAMs than the C12 SAMs. The stability of BPD SAMs enhances further after electron irradiation due to intermolecular cross-linking, whereas the electron irradiation results in deterioration of C12 molecules due to the saturated nature of the molecules. The depth profiling time of the cross-linked BPD SAM is more than 4 and 8 times longer than the profiling time obtained for pristine and BPD and C12 SAMs, respectively. The UPS results are supported by density functional theory calculations, which show qualitative agreement with the experiment and enable us to interpret the features in the XPS spectra during the etching process for structural characterization. The obtained results offer helpful options to estimate the structural stability of SAMs which is a key factor for the fabrication of molecular devices.
The hybrid organic-inorganic lead halide perovskite materials have emerged as remarkable materials for photovoltaic applications. Their strengths include good electric transport properties in spite ...of the disorder inherent in them. Motivated by this observation, we analyze the effects of disorder on the energy eigenstates of a tight-binding model of these materials. In particular, we analyze the spatial extension of the energy eigenstates, which is quantified by the inverse participation ratio. This parameter exhibits a tendency, and possibly a phase transition, to localization as the on-site energy disorder strength is increased. However, we argue that the disorder in the lead halide perovskites corresponds to a point in the regime of highly delocalized states. Our results also suggest that the electronic states of mixed-halide materials tend to be more localized than those of pure materials, which suggests a weaker tendency to form extended bonding states in the mixed-halide materials and is therefore not favourable for halide mixing.
Density functional theory in combination with the nonequilibrium Green's function formalism is used to study the electronic transport properties of MAPbI3, MASnI3 and mixed-... perovskites ...(MA-methylammonium). The largest electronic transport is obtained for tin-halide sample due to the delocalization of electronic states in the system. The mixed sample also shows improved transport properties as compared to the lead-halide system. In addition, tin-based perovskites are less sensitive to the spin-orbit interactions, whereas the electronic transport properties of MAPbI3 are strongly affected by the relativistic effects. These findings indicate the possibility of enhancing charge carrier transport in organometallic perovskites by metal atom mixing.
Display omitted
•Effect of surface termination on the optoelectronic properties of TiC2 is studied.•Fluorinated, oxidized and hydroxylated surfaces are considered.•Partial charges are ...calculated.•Absorption of the system increases by surface passivation.•Electronic transport reduces considerably due to the termination.
Using the first-principles density functional theory, we study the effect of surface functionalization on the structural and optoelectronic properties of recently proposed quasi-two-dimensional material TiC2 T. Zhao, S. Zhang, Y. Guo, Q. Wang, Nanoscale 8 (2016) 233. Hydrogenated, fluorinated, oxidized and hydroxylated surfaces are considered. Significant changes in the lattice parameters and partial charge distributions are found due to the surface termination. Direct contribution of the adatoms to the system density of states near the Fermi level is obtained, which has a major impact on the optoelectronic properties of the material. For example, surface termination results in larger absorption in the visible range of the spectrum. The electronic transport is also affected by the surface functionalization: the current in the system can be reduced by an order of magnitude. These findings indicate the importance of the effects of surface passivation on optoelectronic properties of this quasi-2D material.
The conductance G(E) (in comparison to the maximum value at Δ∊=0.1,Gmax) as a function of energy E (in units of t). Panel (a) shows the results for the simple cubic structure, while Panel (b) shows ...the results for the perovskite structure. The red solid line corresponds to the case Δ∊=0.1, and the green dashed line corresponds to the case Δ∊=1. For both structures, when Δ∊ increases by a factor of 10, G(E) drops by a factor of about 10 for all values of E. In both panels the DOS is shown as the black dotted line, and for the perovskite case the DOS is multiplied by 5 to make it easier to view on the same scale as G(E). For the simple cubic structure the conductance closely follows the DOS. For the perovskite structure, the conductance behaves quite differently from the DOS, with the strongest peak in the conductance occurring near the top of the band. Display omitted
We consider a disordered tight-binding model for particle transport in a periodic lattice. We present a parameter that quantifies the effective hopping strength between the single-particle energy eigenstates in neighbouring supercells that each contains a relatively large number of basic unit cells. We investigate the properties of this parameter in the analysis of long-range transport, in particular the suppression of conductivity as a result of disorder. We perform the analysis for two different crystal structures: simple cubic and perovskite. In each simulation we generate two disordered supercells and treat them as neighbouring supercells in the bulk of the material. Because the two supercells have different disorder patterns, the energies and wave functions are not perfectly matched at the boundary between the supercells, resulting in reduced effective coupling between them. We compare these results for the effective hopping strength with those obtained using the inverse participation ratio, which quantifies the spatial extension of the energy eigenstates. We show that there is a close, though not perfect, correspondence between the disorder effects as reflected in the effective hopping strength and in the inverse participation ratio, and hence the new parameter gives additional information in the study of quantum transport in tight-binding models.
Time-local and time-nonlocal theories are used in combination with optical spectroscopy to characterize the water-soluble chlorophyll binding protein complex (WSCP) from cauliflower. The recombinant ...cauliflower WSCP complexes reconstituted with either chlorophyll b (Chl b) or Chl a/Chl b mixtures are characterized by absorption spectroscopy at 77 and 298 K and circular dichroism at 298 K. On the basis of the analysis of these spectra and spectra reported for recombinant WSCP reconstituted with Chl a only (Hughes, J. L.; Razeghifard, R.; Logue, M.; Oakley, A.; Wydrzynski, T.; Krausz, E. J. Am. Chem. Soc. U.S.A. 2006, 128, 3649), the “open-sandwich” model proposed for the structure of the pigment dimer is refined. Our calculations show that, for a reasonable description of the data, a reduction of the angle between pigment planes from 60° of the original model to about 30° is required when exciton relaxation-induced lifetime broadening is included in the analysis of optical spectra. The temperature dependence of the absorption spectrum is found to provide a unique test for the two non-Markovian theories of optical spectra. Based on our data and the 1.7 K spectra of Hughes et al. (2006), the time-local partial ordering prescription theory is shown to describe the experimental results over the whole temperature range between 1.7 K and room temperature, whereas the alternative time-nonlocal chronological ordering prescription theory fails at high temperatures. Modified-Redfield theory predicts sub-100 fs exciton relaxation times for the homodimers and a 450 fs time constant in the heterodimers. Whereas the simpler Redfield theory gives a similar time constant for the homodimers, the one for the heterodimers deviates strongly in the two theories. The difference is explained by multivibrational quanta transitions in the protein which are neglected in Redfield theory.
Synopsis We show that the dynamical electron correlation suppresses the dipole field of incoming photon while inducing a strong attractive force at the spectral region of the giant plasmon resonance ...of C60. A signature of this force is seen in the form of a binding well in the imaginary part of the induced potential. This attraction can significantly influence the emerging photoelectron's temporal delay behavior.
Large CO2 uptake on a monolayer of CaO Berdiyorov, G R; Neek-Amal, M; Hussein, IA ...
Journal of materials chemistry. A, Materials for energy and sustainability,
2017, Letnik:
5, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Density functional theory calculations are used to study gas adsorption properties of a recently synthesized CaO monolayer, which is found to be thermodynamically stable in its buckled form. Due to ...its topology and strong interaction with the CO2 molecules, this material possesses a remarkably high CO2 uptake capacity ( similar to 0.4 g CO2 per g adsorbent). The CaO + CO2 system shows excellent thermal stability (up to 1000 K). Moreover, the material is highly selective towards CO2 against other major greenhouse gases such as CH4 and N2O. These advantages make this material a very promising candidate for CO2 capture and storage applications.
PDF
