Absorption Spectra Of Monolayers Research Articles

Ab initio and semiempirical modeling of excitons and trions in monolayer TiS3

We explore the electronic and the optical properties of monolayer ${\mathrm{TiS}}_{3}$, which shows in-plane anisotropy and is composed of a chain-like structure along one of the lattice directions. Together with its robust direct band gap, which changes very slightly with stacking order and with the thickness of the sample, the anisotropic physical properties of ${\mathrm{TiS}}_{3}$ make the material very attractive for various device applications. In this study, we present a detailed investigation on the effect of the crystal anisotropy on the excitons and the trions of the ${\mathrm{TiS}}_{3}$ monolayer. We use many-body perturbation theory to calculate the absorption spectrum of anisotropic ${\mathrm{TiS}}_{3}$ monolayer by solving the Bethe-Salpeter equation. In parallel, we implement and use a Wannier-Mott model for the excitons that takes into account the anisotropic effective masses and Coulomb screening, which are obtained from ab initio calculations. This model is then extended for the investigation of trion states of monolayer ${\mathrm{TiS}}_{3}$. Our calculations indicate that the absorption spectrum of monolayer ${\mathrm{TiS}}_{3}$ drastically depends on the polarization of the incoming light, which excites different excitons with distinct binding energies. In addition, the binding energies of positively and the negatively charged trions are observed to be distinct and they exhibit an anisotropic probability density distribution.

Polyene antibiotic amphotericin B in monomolecular layers: spectrophotometric and scanning force microscopic analysis

Monolayers of amphotericin B (AmB) and monolayers composed of AmB and dipalmitoylphosphatidylcholine (DPPC) were formed at the argon–water interface and deposited on a solid support by means of the Langmuir–Blodgett technique. The hypsochromic shift observed in the absorption spectra of monolayers is indicative of aggregated structures of AmB. The exciton splitting theory allowed us to calculate the distance between neighboring molecules in the aggregates as 7.8 Å. Scanning force microscopy of the AmB monolayers revealed the formation of a homogeneous monolayer composed of molecules separated by a distance of 6–8 Å. Microscopy also reveals the formation of cylindrical structures of AmB with a diameter close to 17 Å (internal diameter close to 6 Å) in the monolayers containing additionally 10 mol% DPPC.

Characterization of π-conjugated monolayers of oligo(phenylenevinylene) derivatives by absorption spectroscopy and fluorescence microscopy

Monolayer formation of oligo(phenylenevinylene) derivatives, methyl 4-(2-(4-(2-phenylethenyl)phenyl)ethenyl)benzoate (I) and N,N-dimethyl-4-(2-(4-(2-phenylethenyl)phenyl)ethenyl)benzamide (II), was demonstrated and confirmed by surface pressure-area isotherms and fluorescence microscopy. Stable monolayers were formed in the case of 4-(2-(4-(2-(4-methoxyphenyl)ethenyl)phenyl)ethenyl)benzyltrimethylammonium bromide (III) only through formation of a polyion complex at the interface. Fluorescence microscopy revealed island formation even at large molecular areas in all cases. Absorption spectra of monolayers (I) and (II) were much blue-shifted relative to those in solution, suggesting parallel stacking of the π-conjugated molecules. Mixed monolayers of (I) with stearic acid or dioctadecyldimethylammonium bromide were phase-separated, as indicated by absorption spectra. However, the domain size of the latter mixture was smaller than the resolution of fluorescence microscopy.