Characterization of fluorescent quantum dots via molecular modeling approach

Abstract

Unique characteristics of fluorescent materials have led to many applications in life, consequently, been subject to rigorous study as one-of-kind object in bio-imaging fields. However, as is the fast growing field, qualitative scheme of applications has been mainly sought with short of quantitative understanding. We resort to molecular simulation method in studying those respects. Our ultimate system of interest is a bi-metallic solid, CdSe, of which shape follows quantum dot such as colloidal particle in surfactant medium. To study the actual system, we adopt the model system of hard-sphere with point-charge to conduct the preliminary research, which involves application of correct Ewald-sum calculation. Upon finding a right passage to implement the Ewald-sum, different sizes of charged hard-spheres, which mimics CdSe, are to be investigated to collect qualitative figures of realistic system. This procedure is particularly important to reveal size and inner structures of quantum dots, which affect the fluorescence. We determine and compare the thermodynamic properties of mono-and divacancies in the face-centered-cubic and hexagonal-close-packed hard-sphere crystals via a modified grand-canonical ensemble. Widom-like particle insertion was employed to estimate the free energy of formation of mono-and divacancy and the results are supported by an alternative approach, which quantifies the entropy gain of the neighbor particles. In hcp crystal, we found a strong anisotropy in the orientational distribution of vacancies and observe an 8-fold increase in the number of divacancies in the hexagonal plane compared to the one in the out-of-plane at highest density of interest. This phenomenon is induced by the different arrangement and behavior of the shared nearest neighbor particles, which are located at the same distance from each vacant site in divacancy. The effect of divacancies on the free energy is to reduce that of the hcp crystal relative to the fcc by around 7×10-6kBT at melting.