Search

Photoisomerization, aggregation, and UV-visible absorbance of cis and trans azo-polyurea thin films and solution are investigated. Hydrogen bonding and changes in excitation energies render cis azo-PU more stable in solution than in the film state.

Inorganic drugs are capable of tight interactions with proteins through coordination towards aminoacidic residues, and this feature is recognized as a key aspect for their pharmacological action. However, the "protein metalation process" is exploitable for solving the phase problem and structural resolution. In fact, the use of inorganic drugs bearing specific metal centers and ligands capable to drive the binding towards the desired portions of the protein target could represent a very intriguing and fruitful strategy. In this context, a theoretical approach may further contribute to solve protein structures and their refinement. Here, we delineate the main features of a reliable experimental-theoretical integrated approach, based on the use of metallodrugs, for protein structures solving.

Photo-ionization induced ultrafast electron dynamics is considered as a precursor to the slower nuclear dynamics associated with molecular dissociation. Here, using ab initio multielectron wave-packet propagation method, we study the overall many-electron dynamics, triggered by the ionization of outer-valence orbitals of different tautomers of a prototype molecule with more than one symmetry element. From the time evolution of the initially created averaged hole density of each system, we identify distinctly different charge dynamics response in the tautomers. We observe that keto form shows charge migration direction away from the nitrogen bonded with tautomeric hydrogen, while in enol - away from oxygen bonded to tautomeric hydrogen. Additionally, the dynamics following ionization of molecular orbitals of different symmetry reveal that a' orbitals show fast and highly delocalized charge in comparison to a" symmetry. These observations indicate why different tautomers respond differently to an XUV ionization, and might explain the subsequent different fragmentation pathways. An experimental schematics allowing detection and reconstruction of such charge dynamics is also proposed. Although the present study uses a simple, prototypical bio-relevant molecule, it reveals the explicit role of molecular symmetry and tautomerism in the ionization-triggered charge migration that controls many ultrafast physical, chemical, and biological processes, making tautomeric forms a promising tool of molecular design for desired charge migration.

In the present study, density function theory (DFT) tool is used to compute structural, electronic and optical properties for hexagonal-boron-nitride (h-BN) nanosheet, perfect and defected (one carbon atom remove) graphene (G)/boron-nitride nanoisland. The basis set 6-31G is utilizing in a present study with hybrid function (B3LYP). Relaxation calculation is showing that all bond length between atoms are agreement with theoretical and experimental measurements, also the surface of all systems was remain in plane form. Energy gap calculation shows that a h-BN nanosheet has insulator nature. Doping h-BN by carbon (C) and defected (by remove one carbon atom) the energy gap is modifying and reduce from (6.006) to (3.464 and 1.569) eV, respectively. In the other hand, doped and defected mechanism enhance the conductivity of h-BN nanosheet. UV-Visible calculation is showing that h-BN absorbing in the ultra-violet region, doped and defected nanoisland were absorbed light in the visible region of electromagnetic radiation and shifted to long wavelength (Low energy). Fourier transformation-infrared radiation (FT-IR) calculations are showing that all active group are appearing like (C-C), (C-B), (C-N), (B-H) and (N-H) and approximately agreement with experimental measurements. Light harvesting efficiency (LHE) calculation is concluding that h-BN/G nanoisland have high ability to harvest light compared with h-BN nanosheet and defected h-BN/G nanoisland. Results show that higher occupied molecular orbital (HOMO) is being distributed above minimum conduction band (CBM) of titanium dioxide (TiO2), and lower unoccupied molecular orbital (LUMO) is being populate below iodine/ tri-iodine (I/I3) electrolyte. Meanwhile, all nanostructure is having ability to generate pair electron-hole. Finally, results show that the measurements are indicating that it is possible to apply the composites under study in the field of photonic devices especially in solar cell field.

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular alignment and orientation (AnO). In this work, we examine the critical role of different pulse parameters on the field-free AnO dynamics of the CH3F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal AnO for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distributions, the origin of specific AnO dynamics was addressed. We could identify two qualitatively different AnO behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of ⟨cos2θ⟩=0.843 andorientation of⟨cos(θ)⟩=0.886 for CH3F molecule at 2K using single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance AnO in laboratory conditions for advanced applications.

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular "alignment" and "orientation" (AO). In this work, we examine the critical role of different pulse parameters on the field-free AO dynamics of the CH[Formula: see text]F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal AO for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific AO dynamics was addressed. We could identify two qualitatively different AO behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of [Formula: see text] and orientation of [Formula: see text] for CH[Formula: see text]F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance AO in laboratory conditions for advanced applications.

Efficient photo-induced intramolecular charge transfer (ICT) from donor to acceptor in dye molecules is the functional basis and key property in the working of dye-sensitized solar cell (DSSC). To understand the ICT process in photo-excited dye molecules, we analyse the electronic properties and structural parameters of a chosen set of experimentally synthesized donor-acceptor (D-A) and donor-$\pi$--spacer-acceptor (D-$\pi$-A) type dye molecules in their ground, excited and cationic states. The correlation between structural modification and charge redistribution in different parts of the molecule helps to identify the extent of $\pi$-conjugation and spatial rearrangement of electron density localization along the molecular skeleton. We find that prominent twisting of several groups and resulting molecular bond rearrangements in larger parts of the molecule promotes efficient donor to acceptor ICT, such as in D-A type, ADEKA1 and C275 dyes. Thus based on modest computation of structural and electronic properties of dye molecules in their respective ground, excited and cationic states, we identify the desired structural changes that facilitate tunable intramolecular charge transfer to highlight a simple and direct prescription to screen out probable efficient dye molecules among many samples. Motivated by recent experimental evidence of capturing the structural view of the excited-state charge transfer in molecules, we provide a fresh outlook towards predictive and systematic computational screening and design of dye molecules, complementing parallel experimental approaches in the development of state-of-the-art DSSC.

Attoscience is the emerging field that accesses the fastest electronic processes occurring at the atomic and molecular length scales with attosecond (1 as = 1 0 − 18 s) time resolution having wide ranging physical, chemical, material science and biological applications. The quintessential and one of the most fundamental processes in this domain is the generation of phase locked XUV attosecond pulses. The theoretical approach to understand the process incorporates a fully quantum or semi classical or relativistic description of coherent charge dynamics in intense ultrashort electromagnetic fields driving a quantum system (an atom, a molecule, solid band gap materials or surface plasmas). Modelling of such physical and dynamical systems in science and also in many other branches often leads to equations represented in terms of complex multi-dimensional integrals. These integrals can often be solved using the stationary phase approximation, which leads to a series of equations identifying the points in the multi-dimensional space, having most significant contributions in their evaluation. These points are usually indicated as saddle points. The description of the dynamics of quantum mechanical or relativistic systems that results from such an approach enables near to classical physics intuitive perceptions of the processes under investigation. Thus, the saddle point methods are very powerful and valuable general theoretical tools to obtain asymptotic expressions of such solutions and help also to gain physical insights on the underlying phenomena. Such techniques developed in the past have been adapted to study the emission of as pulses by different physical systems and have been widely employed in calculating and estimating the response of matter to intense electromagnetic pulses on ultrafast time scales. Here we provide an extensive disposition of the saddle point approaches unifying their ubiquitous applications within the domain of attoscience valid for simple atomic to more complex condensed matter systems undergoing ultrafast dynamics and present current trends and advancements in the field. In this review we would delineate the methodology, present a synthesis of seminal works and describe the state of the art applications. Finally we also address ultrashort time dynamics of novel materials that have gained much attention recently, namely lower dimensional material systems and micro-plasma systems.

The laser-induced alignment-and-orientation (A&O) dynamics of the prolate symmetric top CH$_3$X (X = F, Cl, Br, I) molecules is investigated, with particular emphasis on the effect of halogen substitution on the rotational constants, dipole moments, and polarizabilities of these species, as these quantities determine the A&O dynamics. Insight into possible control schemes for preferred A&O dynamics of halogenated molecules and best practices for A&O simulations are provided, as well. It is shown that for accurate A&O-dynamics simulations it is necessary to employ large basis sets and high levels of electron correlation when computing the rotational constants, dipole moments, and polarizabilities. The benchmark-quality values of these molecular parameters, corresponding to the equilibrium, as well as the vibrationally averaged structures are obtained with the help of the focal-point analysis (FPA) technique and explicit electronic-structure computations utilizing the gold-standard CCSD(T) approach, basis sets up to quintuple-zeta quality, core-correlation contributions and, in particular, relativistic effects for CH$_3$Br and CH$_3$I. It is shown that the different A&O behavior of the CH$_3$X molecules in the optical regime is mostly caused by the differences in their polarizability anisotropy, in other terms, the size of the halogen atom. In contrast, the A&O dynamics of the CH$_3$X series induced by an intense few-cycle THz pulse is mostly governed by changes in the rotational constants, due to the similar dipole moments of the CH$_3$X molecules. The A&O dynamics is most sensitive to the $B$ rotational constant: even the difference between its equilibrium and vibrationally-averaged values results in noticeably different A&O dynamics. The contribution of rotational states having different symmetry, weighted by nuclear-spin statistics, to the A&O dynamics is also studied.

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {SrZnO}_2$$\end{document}SrZnO2 (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vacancies on near edge structure.

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of $$\hbox {SrZnO}_2$$ SrZnO 2 (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vacancies on near edge structure.

Among the different synthesis approaches to colloidal nanocrystals, a recently developed toolkit is represented by cation exchange reactions, where the use of template nanocrystals gives access to materials that would be hardly attainable via direct synthesis. Besides, postsynthetic treatments, such as thermally activated solid-state reactions, represent a further flourishing route to promote finely controlled cation exchange. Here, we report that, upon in situ heating in a transmission electron microscope, Cu2Se or Cu nanocrystals deposited on an amorphous solid substrate undergo partial loss of Cu atoms, which are then engaged in local cation exchange reactions with Cu “acceptor” phases represented by rod- and wire-shaped CdSe nanocrystals. This thermal treatment slowly transforms the initial CdSe nanocrystals into Cu2–xSe nanocrystals, through the complete sublimation of Cd and the partial sublimation of Se atoms. Both Cu “donor” and “acceptor” particles were not always in direct contact with each othe...

Using concepts of defects in graphene lattices (see image) a new class of semiconducting carbon nanotubes is predicted, with an unusual one-dimensional electronic structure. These CNTs are of comparable stability to known carbon nanostructures, have similar mechanical properties, and may be useful in applications based on a combination of electronic transport properties.

We determine atomic and electronic structures of arm-chair and zigzag boron nitride nanotubes (BN-NTs) of different diameters using first-principles pseudopotential-based density functional theory calculations. We find that the structure of BN-NTs in bundled form is slightly different from that of the isolated BN-NTs, reflecting on the inter-tube interactions. Effects of carbon doping on the electronic structure of (5,5) and (5,0) BN-NTs are determined: carbon substitution either at B-site, being energetically very stable, or at N-site can yield magnetically polarized semiconducting state, whereas carbon substitution at neighbouring B and N sites yields a non-magnetic insulating structure.

We determine atomic and electronic structures of boron- and/or nitrogen-doped carbon nanotubes (CNTs) and carbon-doped boron nitride nanotubes (BN-NTs) of armchair and zigzag types using first-principles pseudopotential-based density functional theory calculations. For comparison, we also determine the atomic and electronic structures of two-dimensional honeycomb lattices of carbon (graphene) and boron nitride. Although carbon doping at either the B or N site in BN-NTs results in anti-ferromagnetically ordered semiconducting state, B or N doping in CNTs gives a simple shift in the Fermi energy and a nonmagnetic state.

BCN nanotube brushes have been obtained by the high temperature reaction of amorphous carbon nanotube (a-CNT) brushes with a mixture of boric acid and urea. The a-CNT brushes themselves were obtained by the pyrolysis of glucose in a polycarbonate membrane. The BCN nanotubes have been characterized by EELS, XPS, electron microscopy, Raman spectroscopy and other techniques. The composition of these nanotubes is found to be BC4N. The nanotubes, which are stable up to 900 °C, are insulating and nonmagnetic. They exhibit a selective uptake of CO2 up to 23.5 wt%. In order to understand the structure and properties, we have carried out first-principles density functional theory based calculations on (6,0), (6,6) and (8,0) nanotubes with the composition BC4N. While (8,0) BC4N nanotubes exhibit a semiconducting gap, the (6,0) BC4N nanotube remains metallic if ordered BN bonds are present in all the six-membered rings. The (6,6) BC4N nanotubes, however, exhibit a small semiconducting gap unlike the carbon nanotubes. The most stable structure is predicted to be the one where BN3 and NB3 units connected by a B–N bond are present in the graphite matrix, the structure with ordered B–N bonds in the six-membered rings of graphite being less stable. In the former structure, (6,0) nanotubes also exhibit a gap. The calculations predict BC4N nanotubes to be overall nonmagnetic, as is indeed observed.

The binding strength of the carboxylic acid group (-COOH) with different divalent metal ions displays considerable variation in arachidic acid (AA) thin films. It is considered that in AA thin films the metal ions straddle the hydrophilic regions of the stacked bilayers of AA molecules via formation of carboxylates. In this study first the uptake of different divalent cations in films of AA is estimated by atomic absorption spectroscopy (AAS). Through the amount of cation uptake, it is found that the strength of binding of different cations varies as Ca 2+ > Co 2+ > Pb 2+ > Cd 2+ . Variation in the binding strength of different ions is also manifested in experiments where AA thin films are exposed to metal ion mixtures. The higher binding strength of AA with certain metal ions when exposed individually, as well as the preference over the other metal ions when exposed to mixtures, reveal some interesting deviation from the expected behavior based on considerations of ionic radii. For example, Pb 2+ is always found to bind to AA much more strongly than Cd 2+ even though the latter has smaller ionic radius, indicating that other factors also play an important role in governing the binding strength trends apart from the effects of ionic radii. Then, to get a more meaningful knowledge regarding the binding capability, first-principles calculations based on density functional theory have been applied to study the interaction of different cations with the simplest carboxylic acid, acetic acid, that can result in formation of metal diacetates. Their electronic and molecular structures, cohesive energies, and stiffness of the local potential energy well at the cation (M) site are determined and attempts are made to understand the diversity in geometry and the properties of binding of different metal ions with -COOH group. We find that the calculated M-O bond energies depend sensitively on the chemistry of M atom and follow the experimentally observed trends quite accurately. The trends in M-O bond energies and hence the total M-acetate binding energy trends can actually be related to their molecular structures that fall into different categories: Ca and Cd have tetrahedral coordination; Fe, Ni, and Co exhibit planar 4-fold coordination; and Pb is off-centered from the planar structure (forming pyramidal structure) due to its stereochemically active lone pair of electrons.

Carbon Nanotubes based sensors are gaining popularity due to their high selectivity, sensitivity, fast response and recovery time, low operating temperatures and low power consumption. However, due to a strong sp2 carbon-carbon bonding within CNTs, the interaction between defect free CNT walls and gas molecules had been expected to be relatively weak and consequently, the electronic transport properties of the nanotubes are insensitive to the exposure of CNTs to various gas molecules.

We present first-principles pseudopotential-based density functional theory (DFT) calculation of structures, full phonon dispersions and thermal properties of armchair single wall armchair carbon nanotubes (SWCNTs) in the isolated and bundle forms. Comparison between the properties of isolated and bundled nanotubes is used to estimate the intertube interaction. We determine correlation between vibrational modes of a graphene sheet and of the nanotube to understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum. The radial breathing mode hardens with increasing diameter (or decreasing curvature). We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.

We present a comparative study of atomic and electronic structures, Young's modulus, and optical conductivity of trigonal bulk tellurium and its nanowires of different diameters using first-principles density-functional theory calculations based on pseudopotentials and a generalized gradient approximation to exchange-correlation energy. Bulk trigonal Te consists of parallel helices of Te stacked on a triangular lattice in a plane perpendicular to the helical axis. In Te nanowires, interhelical distances change by about 1% of their bulk value due to rearrangement of atoms at the surfaces. Due to a decreased overlap between electronic states of neighboring helices, band gap of nanowires increases with reducing diameter, whereas their Young's modulus correspondingly decreases. Optical conductivity of nanowires depends on their diameter and exhibits a marked anisotropy due to quantum confinement of electronic states in the plane perpendicular to their axis.

Electronic structures of faulted nanosystems are of particular technological relevance because realistic large scale synthesis of nanostructures inevitably leads to defects of one form or the other. In this work, we determine the atomic and electronic structures of carbon nanotubes (CNTs) with two of the major types of defects using first-principles pseudopotential-based density functional theory calculations: (i) substitution with other atoms, and (ii) defects formed by removal of C atoms in the otherwise perfect honeycomb graphene network. We find that doping with B or N atom at different carbon sites of a CNT gives a simple shift in the Fermi energy and a nonmagnetic state. We evaluate the energy cost associated with the formation of nanotube interconnects by introducing a pair of heptagon and pentagon at the junction of two nanotubes and discuss how different defects can lead to band structure control. Further, a line defect for faulted CNTs is discussed for CNTs, which exhibit stronger one-dimensiona...

We have studied the structural, elastic, and optical properties of selenium nanowires, as well as bulk selenium, by performing first-principles density functional theory calculations. The nanowires are structurally similar to bulk trigonal Se, in that they consist of hexagonal arrays of helices, though there is a slight structural rearrangement in response to the finite size of the nanowires. These small structural changes result in Young's modulus decreasing slightly for progressively thinner nanowires. However, there is a significant effect on electronic structure and optical properties. The thinner the nanowire, the greater the band gap, and the greater the anisotropy in optical conductivity. The latter is due to the effects of finite size being much more marked for the case where the electric field is polarized perpendicular to the helical axis, than in the case where the polarization is parallel to c. For the case of bulk Se, we obtain good agreement with experimental data on the structure, elastic constants, and dielectric function.

The authors use first-principles pseudopotential-based density functional theory calculations of phonon dispersions to determine the size dependence of thermal properties of armchair single wall carbon nanotubes (SWCNTs), such as their negative thermal expansion and specific heat. While the specific heat is found to depend rather weakly on the diameter of SWCNTs, their negative thermal expansion behavior determined within a quasiharmonic approximation (QHA) exhibits a relatively strong dependence on the diameter. They identify the low energy vibrational modes that are responsible for the negative thermal expansion.