Optical and AFM study of electrostatically assembled films of CdS and ZnS colloid nanoparticles

Abstract

This work presents a cost-effective technology for the formation of nanostructured semiconductor materials for tunable light emitting devices. CdS and ZnS semiconducting colloid nanoparticles coated with organic shell, containing either SO 3 or NH 3 + groups, were deposited as thin films using the technique of electrostatic self-assembly. The films produced were characterized with UV-vis spectroscopy, spectroscopic ellipsometry, and AFM. UV-vis spectra show a substantial blue shift of the main absorption band of both CdS and ZnS containing films with respect to the bulk materials, due to the phenomenon of quantum confinement. The nanoparticles' radius of 1.8 nm, evaluated from the spectral shift, corresponds well to the film thickness obtained by ellipsometry. AFM shows the formation of larger aggregates of nanoparticles on solid surfaces.

Electrostatically self-assembled films containing II–VI semiconductor nanoparticles: Optical and electrical properties

“…The radius of semiconductor clusters of about 2 nm, evaluated from the spectral data, corresponds reasonably well to the thickness values obtained from ellipsometry measurements [1]. However the atomic force microscopy (AFM) study of the film morphology reveals the aggregation of electrically charged CdS and ZnS nanoparticles, which can only happen by intercalation with oppositely charged polyelectrolyte layers [1].…”

“…CdS and ZnS colloid nanoparticles were synthesized as described in [1]. The films of CdS and ZnS nanoparticles were deposited layer-by-layer on solid substrates (glass slides coated with either Chromium-Gold or ITO layers) using intermediate layers of PAH and PSS [20].…”

“…In our previous work [1], we used aqueousphase synthesis of colloid nanoparticles, similar to the route described in [2], and deposited layers of colloid nanoparticles using the technique of electrostatic self-assembly via intermediate polyelectrolyte layers of Poly-allylamine Hydrochloride (PAH) and Poly-styrene-sulfonate Sodium salt (PSS). Comparing to other chemical (or wet) techniques for the preparation and deposition of II-VI semiconductor nanoparticles, such as Langmuir-Blodgett (LB) film method [3,4] and Electrochemical Deposition [5,6], the current combination of colloids' synthesis and electrostatic self-assembly of colloid nanoparticles [7][8][9][10][11][12] is particularly attractive.…”

Optical characterization of SnO2:F films by spectroscopic ellipsometry

“…where R p and R s are the complex reflection coefficients for the light polarized parallel (p) and perpendicular (s) to the plane of incidence respectively [50,55,[57][58][59][60][61][62][63][64]. The fundamental equation for the complex reflectance ratio ρ is also described as follows:…”

“…In order to extract useful information about a sample (thicknesses and optical constants of the layers) the experimental data are compared with the data generated using a model which describes the structure of the sample and its optical response [55,57,61].…”