A hybrid quantum-classical theory for predicting terahertz charge-transfer plasmons in metal nanoparticles on graphene

Metal nanoparticle (NP) complexes lying on a single-layer graphene surface are studied with a developed original hybrid quantum-classical theory using the Finite Element Method (FEM) that is computationally cheap. Our theory is based on the motivated assumption that the carrier charge density in the doped graphene does not vary significantly during the plasmon oscillations. Charge transfer plasmon (CTP) frequencies, eigenvectors, quality factors, energy loss in the NPs and in graphene, and the absorption power are aspects that are theoretically studied and numerically calculated. It is shown the CTP frequencies reside in the terahertz range and can be represented as a product of two factors: the Fermi level of graphene and the geometry of the NP complex. The energy losses in the NPs are predicted to be inversely dependent on the radius R of the nanoparticle, while the loss in graphene is proportional to R and the interparticle distance. The CTP quality factors are predicted to be in the range ∼10-100. The absorption power under CTP excitation is proportional to the scalar product of the CTP dipole moment and the external electromagnetic field. The developed theory makes it possible to simulate different properties of CTPs 3-4 orders of magnitude faster compared to the original FEM or the finite-difference time domain method, providing possibilities for predicting the plasmonic properties of very large systems for different applications.

Charge-transfer plasmons with narrow conductive molecular bridges: A quantum-classical theory

We analyze a new type of plasmon system arising from small metal nanoparticles linked by narrow conductive molecular bridges. In contrast to the well-known charge-transfer plasmons, the bridge in these systems consists only of a narrow conductive molecule or polymer in which the electrons move in a ballistic mode, showing quantum effects. The plasmonic system is studied by an original hybrid quantum-classical model accounting for the quantum effects, with the main parameters obtained from first-principles density functional theory simulations. We have derived a general analytical expression for the modified frequency of the plasmons and have shown that its frequency lies in the near-infrared (IR) region and strongly depends on the conductivity of the molecule, on the nanoparticle-molecule interface, and on the size of the system. As illustrated, we explored the plasmons in a system consisting of two small gold nanoparticles linked by a conjugated polyacetylene molecule terminated by sulfur atoms. It is argued that applications of this novel type of plasmon may have wide ramifications in the areas of chemical sensing and IR deep tissue imaging.

[Modification of the surface of bioresorbed polymer matrices by laser treatment]

The effect of laser irradiation on the properties of the surface of film matrices obtained from the bioresorbed polymer polyhydroxybuturate has been studied. To determine the spectral region of the polymer optimal for the effective action of radiation on electron molecular bonds, theoretical investigations have been performed, which have shown that, for modifying the surface of matrices obtained from polyhydroxybuturate, it is expedient to use a vacuum laser at a wavelength of 160 nm. Using the laser regime of irradiation of matrices at a radiation power from 3.0 to 30 Wt, a series of films with modified surface, from roughnesses to perforations, have been obtained. The microstructure and properties of the film surface depending on the mode of irradiation have been examined, and conditions have been found under which the contact marginal angles of moistening of films with water can be decreased to 50 degrees (compared with 76-80 degrees in starting products). Thus, the conditions of laser treatment of polyhydroxybuturate matrices have been theoretically substantiated and experimentally realized that provide a beneficial effect on the properties of the surface without destroying the structure of the material.

The Role of Histidine in the Ligand-Bonding Capacity of the Hemoglobin Gene

The atomic and electronic structures of heme complexes with His, Gly, and Cys residues (Heme-His, Heme-Gly, and Heme-Cys) in the fifth coordination position of the Fe atom and with oxygen and nitrogen oxide molecules in the sixth Fe position were studied by the semiempirical quantum-chemical method PM3. A comparative analysis of internuclear distances showed that the strength of chemical bonding between the ligand molecules (oxygen and nitrogen oxide) is greater for Heme-Cys than for Heme-His and Heme-Gly complexes. Consequently, the strengthening of the chemical bond of the oxygen (or nitrogen oxide) molecule with Heme-Cys substantially weakens the chemical bond in the ligand molecule. The Mulliken population analysis showed that the electronic density of ligand (oxygen or nitrogen oxide) p-orbitals is transferred to the d-orbitals of the Fe atom, whose charge, calculated according to the Mulliken analysis, formally becomes negative. In the Heme-His complex with oxygen, this charge is substantially greater than in the complex with NO, and the oxygen molecule becomes polarized. No oxygen polarization is observed in the Heme-Cys complex, and the electron density (judging from the change in the Fe charge) is transferred to the coordinated sulfur atom. This is also characteristic of Heme-Cys complexes with nitrogen oxide. An analysis of charges on the atoms indicates that the character of chemical bonding of the oxygen molecule in Heme-Cys and Heme-Gly complexes is similar and basically differs from that in the case of the Heme-His complex. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2004, vol. 30, no. 2; see also http://www.maik.ru.

[Structure and electronic properties of heme complexes with various ligands]

The electronic and atomic structures, and the molecular dynamics of the atomic structure at 310 K of a set of heme complexes with His and Gly amino acids in the 5th coordination position and some ligands (O2, NO) in the 6th position were studied by ab initio (3-21G basis set) and semiempirical (PM3) quantum chemistry methods and the method of molecular dynamics. It was shown that the type of coordination of the imidazole ring influences the constant of chemical bonding of molecular oxygen of the complexes. On the other hand, NO and O2 molecules have different transinfluence on the ligand in the 5th coordination position. It was shown that temperature affects profoundly the atomic and electronic structures of the complexes, the tightness of chemical bonding and their reactivity.

[Electronic structure of hemoglobin's heme complexes with nitric oxide and dynamics of atomic base under physiological temperature]

The comparative study of atomic and electronic structure of hem complexes of hemoglobin with molecular oxygen and nitric oxide has been performed by semiempirical quantum chemical PM3 method. It has been shown that the length of chemical bonding in oxygen molecule coordinated with hem increases by 0.046 A and the length of chemical bonding in nitrogen oxide coordinated with hem increases by 0.064 A in comparison with pure substances. This fact indicates that chemical bonding between nitric oxide and Fe atom of hem is stronger that one with oxygen molecule. Analysis of charge of the molecules indicates that NO bounded with Fe by covalent chemical bonding and oxygen molecule bounded with Fe by weak dipole interaction. Atomic orbitals of ligand atoms in oxygen complex play small part in high occupied (HOMO) and low vacant (LVMO) molecular orbitals in comparison with HOMO and LVMO of complex with NO. In the last one unpaired electron of NO molecule moves from ligand to d-orbitals of Fe atom and creates d7-configuration. Molecular dynamics simulation under physiological temperature (310 K) indicates visible difference in atomic and electronic structure of the complexes in comparison with ones under low (from 77 up to 0 K) temperatures.