Effects of molecular adsorption on optical losses of Ag (111) surface. A. V. Gavrilenko, ∗ C. S. McKinney, and V. I. Gavrilenko Center for Materials Research, Norfolk State University, 700 Park Ave, Norfolk, VA 23504 The first principles density functional theory (DFT) is applied to study effects of molecular ad- sorption on optical losses of silver (111) surface. The ground states of the systems including water, methanol, and ethanol molecules adsorbed on Ag (111) surface were obtained by the total energy minimization method within the local density approximation (LDA). Optical functions were calcu- lated within the Random Phase Approximation (RPA) approach. Contribution of the surface states to optical losses was studied by calculations of the dielectric function of bare Ag (111) surface. Substantial modifications of the real and imaginary parts of the dielectric functions spectra in the near infrared and visible spectral regions, caused by surface states and molecular adsorption, were obtained. The results are discussed in comparison with available experimental data. In the present work, we study physical and chemical processes affecting optical losses of silver surfaces that substantially contribute to the optics of metamaterials (the artificially fabricated nano-materials $^{9}$). The adsorp- tion of water, ethanol, and methanol on silver nano- films is studied by first principle density functional theory (DFT). These systems are widely used in modern nan- otechnology and nano-physics $^{2,3,9}$. Contribution of the surface states to optical losses is studied by calculations of the imaginary part of the dielectric function for metal- lic slabs of different thickness. We obtained substantial modifications of the optical function of metallic nano- slabs in the near infrared and visible spectral regions that are discussed in comparison with literature. The effects of metallic surface states and molecular ad- sorption on the atomic surface structure and electronic properties of transition metals is extensively discussed in literature (see e. g. Refs. 1–4 and references therein). The silver based metallic nanostructures are actively stud- ied for numerous applications of modern nanotechnol- ogy such as nanophotonics and nanoplasmonics $^{5}$. It has been demonstrated before that electronic surface states on metallic surfaces substantially contribute to the bond- ing of physisorbed atoms and molecules 4 and to their op- tical absorption spectra $^{5,6}$. The semi-classical approach developed by Persson 6 using Drude model and modified by Pinchuk et al. 5 clearly demonstrated increasingly im- portant contributions of the surfaces states on optical ab- sorption spectra of metallic nanoparticles with descrease of their size. Typical metallic nanoparticles are much bigger in size than the molecules studied. Consequently the interac- tions of small molecules with the nano-particles can be studied using widely used model of the surface physics: molecule on an atomic slab $^{10}$. Equilibrium atomic struc- tures of silver surfaces with organic molecules are ob- tained from the total energy minimization method within DFT using ab initio (gradient-corrected) functionals $^{11,12}$. The basis functions are given numerically as values on an atomic-centered spherical-polar mesh, rather than as analytical functions (i. e. Gaussian orbitals), as imple- mented in the DMol 3 package $^{13}$. We employ the super- cell approach to model the surface; our unit cell is a slab consisting of seven monolayers of Ag (111) − √ 3 × √ 3, of which the top three layers were allowed to relax. A Monkhorst Pack mesh of 6 × 6 × 1 k -points was used to sample the Brillouin zone. Explicit tests were performed to verify that the results were converged for the param- eters given above. Reduction of optical losses in nanostructured materi- als is one of the most important issues affecting success- ful engineering of different optoelectronics photonics de- vices. Optical losses of materials are determined through the imaginary part of the dielectric function ( Im ( ε )) $^{7}$. Though losses could be compensated through external gain, another possibility is the modification of electronic structure of metallic nanoparticles by chemical adsorp- tion on the surfaces. However, this approach requires detailed understanding of electronic processes accompa- nied by chemical adsorption on a microscopic scale. Recently using the first principle modeling we pre- dicted the effect of the surface states on the dielec- tric function spectra ( ε ( ω )) of the nanometer-thick silver films $^{8}$. It has been shown that a decrease of the silver- slab thicknesses resulted in substantial increase of the Im ( ε ) due to the surface state contributions. Indepen- dently, these predictions have been confirmed experimen- tally by Drachev et al. 9 by simultaneous measurements of reflectance and transmittance of differently shaped sil- ver nanostructures. They show that the dramatic in- crease of Im [ ε ( ω )] for both TE and TM light polariza- tions is due to the decrease of characteristic dimensions of the silver-based nanostructures; agreeing well with our predictions $^{8}$. The electronic structure and optical properties of the system were then calculated using ab initio norm- conserving (NC) pseudopotentials (PP), as implemented in the CASTEP package $^{13}$. The eigen value problem was solved using the first principle PP method with NC-PP $^{14}$. Convergence of the results has been carefully checked by arXiv:1001.0182v1 [cond-mat.mtrl-sci] 31 Dec 2009 II. METHOD I. INTRODUCTION