VOLUME 102, NUMBER 19, MAY 7, 1998
© Copyright 1998 by the American Chemical Society
LETTERS Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals Margaret A. Hines and Philippe Guyot-Sionnest* James Franck Institute, UniVersity of Chicago, Chicago, Illinois 60637 ReceiVed: February 5, 1998
Relatively monodisperse, highly luminescent zinc blende ZnSe nanocrystals are synthesized in a hexadecylamine/trioctylphosphine coordination solvent. The controlled growth process affords tunable sample sizes. Pure band-edge fluorescence size-tunable between 2.8 and 3.4 eV is obtained at room temperature with quantum yields between 20% and 50% relative to Stilbene 420.
Introduction Semiconductor nanocrystals have gained the attention of the scientific community during the past decade with their unique size-tunable optical properties.1,2 At present, synthetic procedures have been developed for a few II-VI3-5 and III-V6-10 materials. Yet, none of these systems currently exhibit intense UV-blue luminescence. ZnSe, with a room temperature bulk band gap of 2.7 eV (460 nm), has long been a material of choice for blue diode lasers. Nanocrystals of ZnSe have been produced by means of an arrested precipitation technique, but these earlier reports only provide optical absorption spectra with no luminescence information.11 More recently, ZnSe quantum dots prepared from a supersaturated glass solution were reported but without any optical spectra.12 A report of doped ZnSe in glass prepared by a sol-gel process does provide optical spectra for particles ranging in size between 2.8 and 9.5 nm, but the samples appear to have large size distributions and contain defected particles causing broad heavily red-shifted emission.13 ZnSe has been also used as a capping material on CdSe nanocrystals for surface passivation with marginal luminescence enhancement results14 compared to ZnS.15 To our knowledge, there is no published report detailing the production of ZnSe nanocrystal colloids via an organometallic synthetic route. CdSe has so far been the most studied nanocrystal colloid system. With the controlled growth of particles of diameters between 1.7 and 12 nm, their size-dependent absorption edge
covers the entire visible spectrum. However, the small CdSe nanocrystals with an edge in the UV-blue remain very difficult to passivate and exhibit low quantum yield and broad emission. This limits CdSe in the blue and justifies development of other materials. In this paper we present a procedure for the production of highly luminescent ZnSe nanocrystals from organometallic precursors nucleated and slowly grown in hot alkylamine coordinating solvent. The results of the characterization by room-temperature optical absorption and luminescence, elemental analysis, powder X-ray diffraction (PXRD), and highresolution transmission electron microscopy (HRTEM) are presented and discussed. Experimental Section All reagents were used as received without any further purification. Tri-n-octylphosphine (TOP), selenium shot, hexadecylamine, and anhydrous solvents of methanol, chloroform, hexane, and octane were purchased from Aldrich; diethylzinc was purchased from Alfa Aesar, Inc. Using standard airless techniques, 15 mL of hexadecylamine (HDA) was dried and degassed under vacuum at 125 °C for several hours. The HDA was then heated to 310 °C under 1 atm of Ar. Et2Zn (0.8 mmol) was added to 1 mL of 1M TOPSe solution (previously prepared by dissolving Se shot into TOP), all of which was diluted with an additional 4 mL of TOP. The Zn/Se in TOP solution was injected into the hot HDA. The
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3656 J. Phys. Chem. B, Vol. 102, No. 19, 1998 nanocrystals were then grown at 270 °C while monitoring the evolution of the optical absorption spectrum. During the growth stage, portions of the reaction were removed by syringe and transferred to vials containing anhydrous butanol. All of the nanocrystal/BuOH mixtures were taken into the glovebox for precipitation/isolation procedures. Once isolated the nanocrystals were readily dispersed in a variety of organic solvents including CCl4, CHCl3, THF, toluene, and aliphatic hydrocarbons such as hexane, heptane, and octane. It should be noted that the luminescence is most stable for nanocrystal dispersions in the reverse order of that solvent list and showed little degradation in octane over weeks. The room-temperature optical absorption spectra were measured with dilute hexane solutions of the nanocrystals in 1-cm quartz cuvettes using a Hewlett-Packard 8453 diode array spectrometer. The luminescence spectra were measured with a Perkin-Elmer LS50 spectrometer using 90° collection at 2.5 nm resolution. The relative quantum yield was determined by comparison of the integrated emission with Stilbene 420 (Lambda Physik laser dye) in methanol at an excitation wavelength of 325 nm. The elemental analysis was performed by inductively coupled plasma for Zn, Se, and P and combustion for N (Chemisar Laboratories Inc.). Samples were thoroughly washed in MeOH several times to remove residual HDA and TOP then dried in a vacuum at 100 °C for 24 h prior to analysis. The X-ray diffraction patterns were collected on a Philips Norelco Powder Diffractometer with a Cu X-ray source operated at 40 kV and 20 mA. Nanocrystal powders were smeared onto glass slides for analysis. A background of the clean glass slide was collected and subtracted from the nanocrystal patterns. The nanocrystals were imaged with a Hitachi HF-2000 cold field emission transmission electron microscope. The samples were prepared by dropping dilute hexane solutions of the nanocrystals onto thin holey carbon-coated copper grids. The micrographs were taken at a magnification of 500 × K.
Letters
Figure 1. Room-temperature optical absorption and luminescence spectra of ZnSe nanocrystals. The upper two sets of spectra correspond to 60 Å (uppermost) and 43 Å (next down).
Discussion: The synthesis of ZnSe should not differ much from that of CdSe on the basis of the similarity of the organometallic precursors. For CdSe, the TOP/TOPO combination is extremely successful, where TOP binds preferentially to the Se and TOPO binds to Cd. However, the TOP/TOPO coordination solvent combination fails for ZnSe. The reaction results in extremes when using varying ratios of TOP:TOPO. Either the dispersed nanocrystals are so small that they cannot be isolated by standard solvent/nonsolvent precipitation techniques or they precipitate out of the growth solution probably as large aggregates. The difficulties arise from TOPO binding too strongly to Zn, and TOP too weakly. Amines are a logical choice for ligands of intermediate strength. They are slightly weaker bases than phospine oxide. Pyridine is known to provide stable capping through the N atom,3 but its low boiling point suggests limitations as a growth solvent. Long-chain alkylamines have much higher boiling points (hexadecylamine, bp 330 °C). In addition, the less sterically hindered amine creates a larger capping density, which probably increases the surface passivation. As such, dodecylamine has successfully been used to cap the surface of CdSe/ CdS core/shell nanocrystals.16 The chain length of the amine cap must affect the solubility of the nanocrystals and its colloidal stability, while the reaction temperature affects the growth rate and the annealing of defects. We have tried several alkyl lengths to optimize the reaction. When ZnSe is synthesized in octylamine at 150 °C, the
Figure 2. Powder X-ray diffraction patterns of ZnSe nanocrystals approximately 43 Å (top) and 60 Å (bottom) in diameter.
nanocrystals precipitate out of the reaction solvent once they grow to a size with a corresponding first absorption peak at 350 nm. In dodecylamine, larger ZnSe nanocrystals are successfully grown at 200 °C, but the growth is slow with bottlenecks requiring subsequent injections of reagents. Hexadecylamine is a far superior growth solvent. The high growth temperature leads to fairly rapid growth while retaining good monodispersitvity. In addition, hexadecylamine is miscible with appropriate precipitation solvent/nonsolvent pairs. Optical absorption and luminescence spectra of ZnSe nanocrystals synthesized in HDA are shown in Figure 1. The absorption edges are shifted to higher energies from the bulk band gap of ZnSe. The appearance of at least two resolved features, most notably the first exciton peak, indicates that there is control of the size distribution. Most dramatically, the luminescence is purely from the band edge without any evidence of trap emission, and the quantum yields vary from 20 to 50% relative to Stilbene 420. We propose that these excellent luminescence characteristics are due to the high growth tem-
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J. Phys. Chem. B, Vol. 102, No. 19, 1998 3657 In summary, we have established a new synthesis of relatively monodisperse and colloidal ZnSe nanocrystals that provides highly luminescent UV-blue nanocrystal materials. In addition, bulk ZnSe has cubic crystal structure while CdSe is wurtzite, and comparison of the two materials will shed light onto the relationship between structure and electronic properties of the nanocrystals.17,18 Investigations are underway to further stabilize and enhance the photoluminescence by growing a semiconductor capping layer onto the ZnSe as previously done for CdSe15,16,19 as well as to obtain a deeper understanding of their physical, optical, and electronic properties. Acknowledgment. This work made use of the MRSEC Shared Facilities supported by the National Science Foundation under Award No. DMR-9400379. We thank Wen-An Chiou for the HRTEM work, which was conducted at the Materials Research Center of Northwestern University using central facilities supported by the MRSEC Award No. DMR-9632472. We thank Joseph Pluth for the PXRD data. P.G.S. gratefully acknowledges a fellowship from the Alfred P. Sloan Foundation. References and Notes
Figure 3. HRTEM image of ZnSe nanocrystals with an average diameter size of 60 Å. The size bar represents 10 nm.
perature resulting in minimal crystalline defects and to the efficient capping by the combination HDA/TOP. The results of the elemental analysis indicate that Zn and Se are nearly 1:1 in stoichiometry. The analysis also reveals that a large portion of phosphorus is present among the nanocrystals, which can be accounted for by estimating that one-third of the organic surface caps are TOP and the rest amines. The PXRD patterns, Figure 2, indicate that the ZnSe nanocrystals are cubic as expected from the bulk crystal structure. The peaks are characteristically broader for the smaller sized nanocrystals (top), but both patterns are centered on the bulk crystal lines with no other features. The high degree of crystallinity can also be seen by HRTEM, as pictured in Figure 3. The two largest samples, 60 and 43 Å, corresponding to the two uppermost sets of optical spectra in Figure 1, were analyzed for size information. From these images we determined that the colloids are nearly spherical in shape with aspect ratios of 1.1 ( 0.1 and 10% standard deviations in the average diameters.
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