Binding Characteristics of Surface Ligands on PbSe QDs and Impact on Electrical Conductivity
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1113-F03-09
Binding Characteristics of Surface Ligands on PbSe QDs and Impact on Electrical Conductivity Won Jin Kim1, Sung Jin Kim1,2, Jangwon Seo1, Y. Sahoo1, A. N. Cartwright1,2, Kwang-Sup Lee1,4 and Paras N. Prasad1,2,3 1 Institute for Lasers, Photonics and Biophotonics, 2Department of Chemistry, and 3Department of Electrical Engineering, University at Buffalo, Buffalo, New York 14260, USA. 4 Department of Advanced Materials, Hannam University, Daejeon 305-811, Korea.
ABSTRACT We report the binding and conductivity characteristics of PbSe nanocrystal quantum dots which have different ligands on the surface. The PbSe nanocrystal quantum dots were surface functionalized using chemical treatment. The results of post-treatment analysis show that the PbSe surface can be successfully functionalized with various ligands based on thiol- and aminemolecules (e.g., oleic acid, dodecanethiol and butylamine). Conductivity measurements performed using a metal-semiconductor-metal structure indicate that there is an increase of conductivity as the length of the ligand connecting the quantum dots is reduced. INTRODUCTION Nanocrystal quantum dots (QDs) and their composites have attracted significant research efforts over the past two decades for their use in optoelectronic devices such as visible and infrared light-emitting diodes[1, 2], photodetectors,[3-5] field-effect transistors,[6, 7] photovoltaic cells,[8, 9] and organic photorefractives [10]. QD-based devices can combine tailorable photoabsorption with mechanical flexibility. The solubility of QDs in organic solvents or water based liquids allows QD-based devices to be fabricated using simple wet processes (such as spin casting, drop casting, inkjet printing and spray methods). In addition, the absorption band-gap of semiconductor QDs can be modified by varying the size of the nanocrystal, due to quantum effects at the nanoscale. Specifically, the bandgap of the QDs can be tuned to energies exceeding the band gap energy of the bulk constituent semiconductor by reducing the size of the QD. In this way, inclusion of inorganic nanocrystal QDs as optical sensitizers enables not only the enhancement of charge generation and conduction efficiency,[11] but also the broadening of the photoresponse spectrum and the ease of integration into a variety of matrices. However, pure QD devices typically suffer from low charge mobility and low conductivity due to potential barriers between the nanocrystals, originating from the organic ligands resident on the surface of QDs. There have been attempts in the past to improve properties of QD-based devices, where the original bulky insulating ligand has been exchanged by a shorter one to increase conductivity. For example, CdSe QDs have been treated with pyridine to replace trioctylphosphine oxide.[9] Moreover, if the ligand is exchanged by one carrying certain chemical functionalities a variety of chemical surface modifications of QDs becomes possible, which opens a facile way to approach optoelectronic applications such as QD photovoltaic
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