Optical Properties of Colloidal MoS 2
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Optical Properties of Colloidal MoS2 G. A. Wagoner and P. D. Persans Physics Department,Rensselaer Polytechnic Institute, 110 Eighth Street. Troy, NY 12180-3590
A. F. Ruppert Exxon Research and Engineering Company. Route 22 East.Annandale. NJ 08801 We report measurements and analysis of the optical transmission spectra of colloidal 2H-MoS2, prepared by ultrasonic shattering of synthetic crystals in N-Methyl Formamide. We used a combination of TEM, X-ray diffraction and dynamic light scattering to characterize particle shape and size. For a typical sample, the particles are platelets which have an average thickness of 50 A and a diamncter of 500 A. Particle dielectric functions, deduced from both dilute colloids and thin films prepared by flocculation, differ significantly from bulk values. Calculations show that the average extinction coefficient is affected by particle aspect ratio, but this can not explain all the features of the data.
Specifically, we observe a suppression of the absorbance peaks in the band edge exciton region between 1.7 and 2.1 eV, and increased absorbance below 1.7 eV. We suggest that the particles do not possess bulk-like dielectric properties and that this is due to defects. Introduction
The electronic and optical properties of nanoparticles deviate significantly from those of their bulk constituents due to the considerable influence of surface effects. In particular, electron confinement, increased defect density, and surface state perturbations may substantially alter the electronic structure of these microscopic structures. Therefore, a meaningful characterization of nanoparticle composites requires an accurate determination of surface geometry. Furthermore, because the optical properties of a material are intimately connected with its electronic structure, spectroscopic techniques can serve to probe of these effects. Previously, we showed how particle geometry can dramatically affect the observed optical spectra of colloidal and related systems, even if the dielectric function of the particle is bulk-like., Herein, we apply classical electromagnetic scattering theory to model the optical transmission spectra of low density colloidal molybdenum disulfide. Our results indicate, that using bulk optical constants, we can estimate average particle shape, and in conjunction with average particle size data obtained from TEM and X-ray diffraction analysis, it is possible to determine particle volume fraction. In addition, a further comparison between modeling results with spectroscopic data can indicate the appropriateness of characterizing particle dielectric properties using those of the bulk. Specifically, in the case of colloidal MoS 2 , we find that the particles exhibit increased absorbance below the direct edge and suppressed direct edge exciton resonance. We attribute both of these effects to and increased defect density, introduced during sample preparation, which reduces band filling, provides mid-gap states, and causes free carrier screening thereby 2 reducing the coulombic bindi
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