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J. Phys. Chem. 1996, 100, 468-471
Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals Margaret A. Hines and Philippe Guyot-Sionnest* James Franck Institute, The UniVersity of Chicago, Chicago, Illinois 60637 ReceiVed: August 10, 1995; In Final Form: October 16, 1995X
We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method. X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27-30 Å diameter with a ZnS capping 6 ( 3 Å thick. The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps. The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature.
Introduction Semiconductor nanocrystals exhibit interesting size-tunable optical properties due to the confinement of the electronic wave functions. Over the past decade, much progress has been made in the synthesis and characterization of monodisperse nanocrystals of a wide variety of semiconductors, such as II-VI,1,2 various chalcogenides,2 Si,3,4 and GaAs.5-7 In particular, IIVI materials have received much attention and a synthesis has been recently developed for CdSe that leads to an unprecedented degree of monodispersivity and crystalline order,8 allowing detailed investigations of the size-dependent optical absorption.9 The high surface-to-volume ratio of small nanocrystals suggests that the surface properties should have significant effects on their structural and optical properties. While surfaces capped by various organic or inorganic layers appear to influence only mildly the absorption characteristics, it is well-known that the emission efficiency, spectrum, and time evolution are very strongly affected by the surface. This is generally understood as being due to the presence of gap surface states arising from surface nonstoichiometry, unsaturated bonds, etc. Control of the surface is in particular the key to highly luminescent nanocrystals. Organically capped nanocrystals have already a quantum yield of ∼10% at room temperature reaching nearly 100% at low temperatures8,9 but at the expense of very long (microseconds) fluorescence lifetimes. These nanocrystals do not yet have a perfectly passivated surface. They exhibit some red-shifted luminescence and complex decay of the excited state.10,11 Inorganic capping provides an alternative. Layered and composite semiconductor nanocrystals have already been studied by several groups. Systems studied have been (CdS)Cd(OH)2,12,13 (CdSe)ZnS,14 (CdSe)ZnSe,15,16 and ((CdS)HgS)Cd(OH)2.17,18 Choosing the relative bandgap positions leads to enhanced charge transfer or improved luminescence.2 Ideally, epitaxial encasing would be achieved although the similar dielectric constants and bonding characteristics introduce additional difficulties to maintain size control or prevent alloying. Prior to this work, colloidal CdS capped with the inorganic capping Cd(OH)2 displayed the highest roomtemperature quantum yield (20%).12 At low temperature, the quantum yield increases to 80%, and the fluorescence lifetime does not increase much.13 These favorable characteristics have, in particular, allowed us to obtain single-cluster fluorescence spectra in a previous experiment.19 CdSe synthesized by the inverse micelle method has also been successfully capped with X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0468$12.00/0
ZnS14 and ZnSe.15 The ZnS-capped CdSe exhibit enhanced band-edge luminescence and an order of magnitude increase of the quantum yield.14 ZnSe-capped CdSe has not been discussed for its luminescence properties, but structural studies confirmed the shell-core structure.15 Thin film composites of CdSe nanocrystals in a ZnSe matrix have also been made by a combination of electrospray and organometallic chemical vapor deposition,16 and enhanced luminescence has been reported. With the excellent size control that can now be achieved for CdSe nanocrystals, the capping of a size-tunable lower bandgap core nanocrystal with a higher bandgap shell is an attractive possibility that could lead to nanocrystals with improved luminescence, higher stability (protected from the surrounding matrix), and yet electrically connected (better than with an organic capping layer). The (CdSe)ZnS nanocrystals that are described here are a step in this direction with the highest luminescence quantum yield yet achieved while at the same time preserving a good degree of monodispersivity. We describe the synthesis and provide the results of characterization by room-temperature optical absorption and luminescence, fluorescence decay, transmission electron microscopy, and X-ray photoelectron spectroscopy. Experimental Section The nanocrystals were synthesized by using modifications of previously reported methods.8,20 All reagents were used as purchased with no additional purification. Tri-n-octylphosphine [TOP], tri-n-octylphosphine oxide [TOPO], selenium shot, 1 M dimethylzinc [Me2Zn] in heptane, and anhydrous methanol and chloroform were purchased from Aldrich. Bis(trimethylsilyl) sulfide[(TMS)2S] and dimethylcadmium [Me2Cd] were purchased from Fluka and Organometallics Inc., respectively. Stock solutions of Cd and Se were prepared in a N2-filled drybox by dissolving 0.2 g (0.0025 mol) of Se in 4.5 mL of TOP. Me2Cd (0.25 mL, 0.0035 mol) was added to the TOPSe and diluted with 19.5 mL of TOP. The Zn and S stock solution was similarly prepared with 0.52 mL of (TMS)2S (0.0025 mol) in 4.5 mL of TOP, adding 3.5 mL of Me2Zn solution (0.0035 mol), and diluting with 16 mL of TOP. The synthesis was performed by the following method: 12.5 g of TOPO was heated to 200 °C under vacuum, at which temperature it was dried and degassed for approximately 20 min. The temperature was then raised to 350 °C under approximately 1 atm of Ar. Once the temperature had stabilized, 0.7 mL (0.07 mmol Se, 0.1 mmol Cd) of Cd/Se/TOP stock solution was injected into the reaction flask, and © 1996 American Chemical Society
Letters the heat removed. The reaction mixture was allowed to cool to approximately 310 °C, and a small aliquot was extracted for characterization of the initial CdSe nanocrystals. When the temperature reached 300 °C the Zn/S/TOP solution was injected in five 0.55 mL portions at approximately 20 s intervals. A total mole ratio of injected reagents was 1:4 Cd/Se:Zn/S. Upon cooling the reaction mixture was stirred at 100 °C for 1 h. The nanocrystals were purified by precipitation with anhydrous methanol. The precipitate was collected by centrifuging and subsequently washed three times with anhydrous methanol to rinse any residual TOPO. The nanocrystals were then dispersed in 10 mL of anhydrous chloroform. The solution was centrifuged to remove any residual debris and unreacted reagents. Dilutions of the concentrated nanocrystal solution were used for room-temperature optical characterization. The absorption spectra were compared to those in the literature6 to estimate the size. The absorption spectra were acquired on a PerkinElmer UV/vis spectrometer. The luminescence spectra were acquired at right angle on a SPEX Fluorolog spectrometer. The quantum yield was measured by comparison of the integrated emission of rhodamine 560 (exciton) in ethanol at an excitation of 470 nm with collection between 480 and 850 nm. Samples for X-ray photoelectron spectroscopy were prepared using gold films (2000 Å) on silicon wafers with a thin (50 Å) Cr adhesion layer that were placed into 5 mM 1,6-hexanedithiol in EtOH for 2 h.20 The films were removed, rinsed thoroughly with EtOH, dried with Ar, and placed into dilute solutions of nanocrystals in CHCl3 for 12 h. The samples were once again rinsed and dried. The spectra were acquired on a Physical Electronics 5000 LS ESCA System using an Al KR X-ray source. The TEM images of the nanocrystals were acquired on a Philips CM electron microscope operating at 200 kV. Dilute solutions of the nanocrystals in CHCl3 were dropped onto 50 Å thick carbon coated copper grids (400 mesh) with the excess solution immediately wicked away. Results and Discussion Synthesis of nearly monodisperse nanocrystals is achieved by using supersaturation for sudden nucleation and subsequent growth.8 In this work, we followed more closely a variant20 from the original method,8 which produces smaller amounts of material with a faster turnover but still nearly monodisperse CdSe nanocrystals with well-defined absorption features for small sizes (
