CommunicationSimulation, synthesis and optical properties of cadmium telluride (CdTe) semiconductor nanoparticles
Introduction
The current interest in nanotechnology and nanoscience arises from the fact that nanoparticles exhibit electrical, optical, electronic, and magnetic properties, that are substantially different compared to those of their bulk state [1] and that are considered useful in a variety of technological applications. Those properties include unique luminescence characteristics, broad as well as continuous absorption spectra, narrow emission spectra, and high stability under photon exposure. Altogether, the aforementioned optoelectronic properties [2,3] of semiconductor quantum dots (QDs), can be understood employing quantum mechanics arguments, and density functional theory (DFT) is considered suitable to simulate the optical absorption and electronic transitions observed in quantum dots. In part due to their high quantum efficiency and slow exciton recombination rate [4,5], CdTe QDs have attracted extensive interest not only for elucidating physical chemistry phenomena [6], but also because they are considered well suited for sensors, photo electrochemical (PEC) solar cells, optoelectronic devices, bio-labelling, bio-imaging and gamma ray detectors, among other applications [[7], [8], [9], [10], [11], [12], [13]]. Moreover, CdTe is a material presently employed in the fabrication of solid state devices, including light emitting diodes (LEDs), light amplifiers, high efficiency thin film transistors, lasers, gas sensors, photo-detectors, large-screen liquid crystal display and photoluminescent devices [8,[12], [13], [14]]. Thus, in view of the relevance and interest in CdTe QDs in particular, and QDs in general, demonstrating and disseminating predictive computational models would enable scientists and technologists to explore novel applications and reduce their development time. To this end, we discuss the optical absorption of CdTe QDs of different sizes and compare the observed results with simulations and calculations employing a Thomas–Fermi–Dirac–Weizsacker density functional theory (DFT) model. The optoelectronic properties of CdTe QDs can be controlled through changes in size, geometry, or composition [[15], [16], [17]] and, since their structural and electronic properties are sensitive to modifications of atomic configuration, impurities, and dopants [[18], [19], [20]], it is necessary to understand the underlying phenomena employing quantum mechanics principles [21,22]. Cadmium telluride (CdTe) semiconductor nanoparticles were synthesized using a water-based approach, and their size was observed to increase monotonically with the reaction time. The photoluminescence and UV–visible absorption spectra exhibited an absorption maxima in the UV regime that shifted to shorter wavelengths for smaller nanoparticles. In addition to particle size and shape, optical absorption is attributed to the electronic structure in terms of an inter-band excitation of electrons from the valence band of Cd atoms to the higher energy levels of the conduction band of Te atoms [23]. Since CdTe QDs have a discrete band gap with a strong dependence on particle size, shape, and lattice structure, a quantum theory treatment is required to understand the transitions between the electronic states to explain the optical properties of the nanoparticles considered. The calculated absorption maxima employing the DFT model discussed herein, matches the measured absorption maxima reasonably well, corroborating that the optical absorption of semiconductor QDs can be properly delineated employing quantum mechanics.
Section snippets
Materials and procedure
A number of techniques have been reported to synthesize CdTe QDs including thermal decomposition, solvothermal, sonochemical, microwave irradiation [[24], [25], [26], [27], [28], [29]], sol-gel [30,31], microemulsion [32], and gamma irradiation [33,34]. However, the water-based approach described herein was found to be repeatable and relatively straightforward. To this end, analytical grade cadmium acetate dehydrate (Cd(CH3COO)2·2H2O (99.5%)), thioglycolic acid (TGA, 90%), potassium tellurite (K
Experimental QD properties
The absorption and photoluminescence spectra of the synthesized CdTe QDs refluxed at various intervals of time are exhibited in Fig. 1, Fig. 2. The results indicate that by extending the refluxing time, the absorption and photoluminescent (PL) spectra of CdTe QDs are shifted to longer wavelengths. This reflects that a size increase as a consequence of quantum confinement effects in the CdTe semiconductor nanoparticles. Therefore, the size of CdTe QDs could be controlled through tailoring the
Conclusions
A density functional theory (DFT) model has been employed to determine the absorption spectra of CdTe quantum dots, and compared to the values observed experimentally. The theoretical and experimental absorption spectra were also employed to extract the corresponding band gap energies, and the Eg values were in agreement within 15.7%. It is anticipated that the approach described herein will permit design engineers to tailor quantum dot optoelectronic responses for a variety of prospective
CRediT authorship contribution statement
Elham Gharibshahi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization, Project administration.
Declaration of competing interest
The author declares that there is no conflict of interest.
Acknowledgments
We thank the U.S Army Research Office and the Air Force Office of Scientific Research (AFOSR), for the financial support provided for this project (Grant W911NF-15-1-0297; PI: Dr. Arturo Ayon). The cooperation and encouragement of the staff of the MEMS Research Laboratory at UTSA is also appreciated.
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