Enhanced ripening behavior of Mg-doped CdSe quantum dots
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Tae Geun Kim Department of Electronic Engineering, Korea University, Seoul 136-713, South Korea (Received 6 November 2007; accepted 24 March 2008)
Pure CdSe and Mg-doped CdSe nanocrystal quantum dots were synthesized into the zinc-blende structure at a low temperature by the inverse micelle technique using paraffin oil and oleic acid as surface capping agents. The ripening behavior of the nanocrystals was monitored using the red shift in ultraviolet (UV)-visible light absorption peaks, and their size variation was estimated using the so-called, quantum confinement theory. The Lifshitz–Slyozov–Wagner (LSW) kinetics analyses were performed based on the variation in size according to the ripening temperature and time period. The activation energy (Q) and reaction rate constant (Ko) were determined for the ripening reaction using Arrhenius-type plots. The kinetics analyses reveal that the volume diffusion through the liquid-phase solution is the governing mechanism for the ripening of both nanocrystals. The Mg-doped CdSe nanocrystals showed enhanced ripening kinetics due to the low activation energy for the volume diffusion.
I. INTRODUCTION
Colloidal semiconductor nanocrystal quantum dots have attracted a great deal of interest because of their size-tunable photoemission characteristics, which originate from the quantum confinement effect of both the electrons and holes in all three dimensions, leading to an increase in the effective energy band gaps of the nanocrystals.1–4 Among the colloidal semiconductor nanocrystals, CdSe has shown almost full-range visible light emission within a reasonable size range, and thus it has been intensively studied for applications in lightemitting diodes, lasers, and biological labels.5–9 In addition to the size variation, doping of impurities such as Mn and Mg is another simple way to control the light emission wavelength,10,11 since MnSe (3.4 eV) and MgSe (3.60 eV) have higher-energy band gaps compared with CdSe (1.74 eV).12,13 Recently, Norris and coworkers14 have proposed that impurity incorporation into II-VI semiconductors is possible only when they are in the zinc-blende structure that possesses (001) facets with very high binding energy. It has been shown that at the initial nucleation and growth stage, the size of nanocrystals is primarily determined by the precursor concentration, capping ligand and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0238 1916
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 14 Mar 2015
precursor anion types, and temperature.15,16 However, after nucleation and growth, ripening dominates between the nanoparticles. The final size of nanocrystals is thus determined by the temperature and the time period of ripening, and so is the color of the emitting light.17,18 Thus, the precise control of ripening is a critical issue not only for the applications of CdSe, but also for those of doped CdSe nanocrystals. Although the ripening mechanism of doped nanocrystals must
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