Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption versus Intercalation Ajinkya Puntambekar,† Qi Wang,† Lauren Miller,† Nicholas Smieszek,† and Vidhya Chakrapani*,†,‡ †
Howard P. Isermann Department of Chemical and Biological Engineering and ‡Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *
ABSTRACT: Effects of electrochemical charging of quantum dots (QDs) have been reported previously, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this work, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e−, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. The nature of charging mechanisms is discussed using the results of combined photoluminescence, radiative lifetime, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics, and other devices. KEYWORDS: quantum dots, electrochemical charging, electrochromism, lifetime, adsorption, intercalation
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such as smart displays, optical sensors, and photovoltaic devices. Modulation of electrical properties through charging has been exploited for development of electrochemically gated field-effect transistors, where controlled injection of charge carriers can be used for controlling the source−drain channel conductance.1,3,8−11 Electrochemically gated transistors based on CdSe/CdS have demonstrated on−off current ratios as high as 108 for devices operating under low drain (0.5 V) and gate bias (
