ELSEVIER
`
`Journal of Crystal Growth 145 (1994) 714—720
`
`______________________
`
`CRYSTAL
`GROWTH
`
`Preparation of Il—VI quantum dot composites by electrospray
`organometallic chemical vapor deposition
`M. Danek a, K.F. Jensen b,* C.B. Murray a M.G. Bawendi a
`Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
`h Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
`
`Abstract
`
`New thin film composites consisting of a ZnSe matrix and CdSe nanocrystals (NCs) have been prepared by a
`novel technique combining electrospray and organometallic chemical vapor deposition (OMCVD). CdSe NCs,
`synthesized in solution by controlled growth of CdSe nuclei, were derivatized with pyridine or overcoated with a thin
`ZnSe layer. The derivatized NCs were dispersed in a pyridine/ acetonitrile mixture and transferred into the growth
`zone of an OMCVD reactor using an electrospray. The transferred NCs were co-deposited with ZnSe grown by
`OMCVD from hydrogen selenide and diethyl zinc at temperatures ranging from 150 to 250°C.The absorption and
`emission spectra of the composites show characteristic transitions of the NCs. The emission wavelength can be tuned
`by selecting the size of the NCs. A pre-formed ZnSe passivation layer, also synthesized in solution, improves thermal
`stability of the NCs during the co-deposition, and enhances the photoluminescence emission efficiency of the
`composites. The elemental composition and microstructure of the materials are probed by Auger electron spec-
`troscopy, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy.
`
`1. Introduction
`
`(NCs) possess
`Semiconductor nanocrystals
`unique optical properties due to the quantum
`confinement of
`the electronic exciton [1—31.
`Semiconductor quantum dot composites, the ma-
`terials incorporating semiconductor NCs in a thin
`film semiconductor matrix, have promising appli-
`cations in optoelectronics, e.g., in light emitting
`devices and optical switches. The choice of a
`
`_______
`
`* Corresponding author.
`
`semiconductor matrix provides significant advan-
`tages over the use of conventional glass matrices.
`The composites with the semiconductor matrix
`can be more easily integrated into electronic and
`optoelectronic devices and the conductivity of the
`matrix can be controlled by doping. The semicon-
`ductor matrix is expected to stabilize the NCs and
`provide for integration of
`these materials into
`optoelectronic device
`structures. Metalorganic
`vapor phase epitaxy (MOVPE) techniques, rou-
`tinely used for fabricating quantum well struc-
`tures with quantum confinement between layers,
`have been adapted to the fabrication of quantum
`wires [41and dots [51.The growth of these struc-
`tures makes use of varying growth rates along
`
`0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
`SSDI 0022-0248(94)00340-R
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`1 of 7
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`EXHIBIT 1006
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`M Danek et aL /Journal of Crystal Growth 145 (1994) 714—720
`
`715
`
`different crystallographic planes exposed in elec-
`tron beam lithography defined mask structures
`[5].
`An alternative general strategy for synthesizing
`semiconductor quantum dot
`(QD) composites
`would be co-deposition of preformed semicon-
`ductor NCs and semiconductor host matrix con-
`stituents. Gas phase synthesis of
`Ill—V com-
`pound semiconductor NCs, with a perspective of
`incorporating these particles into semiconductor
`matrices, has been demonstrated using laser abla-
`tion and exploding wire techniques, and homoge-
`neous nucleation from organornetallic precursors
`[6—81.These in situ nucleation techniques provide
`a convenient route to the NC materials with the
`size of the NCs below 10—20 nm, but control of
`the nucleation processes is rather difficult. The
`size distribution of the NCs, prepared in this way,
`is broad, typically comparable to the size of the
`particles. This results in a severe inhomogeneous
`broadening of the optical transitions and the dis-
`crete nature of these transitions, due to the quan-
`turn size effects,
`is smeared out. In addition, the
`stoichiornetry and crystallinity of the NCs is diffi-
`cult to control. The resulting defects are known
`to be non-radiative recombination centers and
`thus limit the usefulness of the material,
`We have developed a novel technique for syn-
`thesizing Il—VI QD composites, combining elec-
`trospray and organometallic chemical vapor de-
`position (OMCVD) [91.In this technique, CdSe
`NCs of high optical quality are synthetized sepa-
`rately using a well-developed solution technique
`[101.The NCs of selected size are dispersed in a
`acetonitrile/ pyridine mixture
`and transferred
`into the growth zone of an OMCVD reactor by
`employing an electrospray [lii. Finally, the NCs
`are co-deposited with a ZnSe matrix grown by
`OMCVD. The wide band gap ZnSe matrix (‘-s 2.7
`eV at room temperature) has the advantage of
`allowing observation of the lowest energy optical
`transitions of
`the CdSe NCs of sizes down to
`2.0 nm. In this report we present optical and
`structural characterizations of new ZnSe/CdSe
`QD composites. Our preliminary growth experi-
`ments with the e!ectrospray-OMCVD technique
`have pointed to a critical effect of the NC—matrix
`interface defects on the photoluminescence (PL)
`
`properties of the materials. In order to control
`these defects, we have explored passivation of the
`NCs surface with a ZnSe overlayer grown by
`using a solution procedure.
`
`2. Experimental procedure
`
`The NCs were synthetized by controlled homo-
`geneous nucleation of CdSe in a trioctyiphos-
`phine/ trioctylphosphineoxide (TOP/TOPO) so-
`lution using trioctyiphosphine selenide (TOPSe)
`and dimethylcadmium as the precursors [101.Se-
`lective precipitation of the reaction mixture with
`n-butanol or methanol was used to isolate nearly
`3.5 and -~ 5.0 nm.
`monodispersed NCs of sizes
`The NCs dispersed in hexanes showed a narrow
`absorption bands corresponding to the lowest is-
`is transition at 560 and 608 nrn, respectively. The
`TOP/TOPO surface functionality (cap) was ex-
`changed for pyridine by repeated dispersion of
`the NCs in pyridine and precipitation by hexanes
`[11]. The pyridine cap was selected to ensure
`compatibility of the NCs surface with OMCVD
`chemistry for the ZnSe matrix. Wright et a!. [121
`have shown that the presence of pyridine irn-
`proved the morphology of OMCVD ZnSe, pre-
`sumably by blocking prereactions, while not lead-
`ing to any unintentional C or N incorporation.
`The CdSe NCs passivated with ZnSe were
`prepared by controlled overgrowth of ZnSe on
`‘~ 3.3 nm CdSe nuclei
`in a TOP solution at
`temperatures of 210—220°Cusing TOPSe and di-
`ethyizinc (DEZn) as the ZnSe forming reagents
`in a four-fold excess. The re-growth yielded corn-
`posite NCs approximately 5—6 nm, as determined
`by transmission electron microscopy (TEM). U!-
`trahigh resolution TEM images showed a high
`degree of crystallinity of the composite NCs with
`relatively few structural defects. The overall
`ZnSe/CdSe stoichiometry of
`the samples was
`determined by X-ray fluorescence spectroscopy
`to be
`‘~ 4. This number corresponded to the
`stoichiometry of the reaction mixture. The passi-
`vated NCs showed the is—is absorption transi-
`tion at 560 nm, compared to 554 nm for the
`parent CdSe NCs. The surface of the passivated
`NCs was finally derivatized with pyridine.
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`M. Danek et al. /Journal of Crystal Growth 145 (1994) 714—720
`
`I
`
`— I.
`
`I~I L~---Thermocouple
`
`the !iquid-gas interface (Taylor cone) and emis-
`sion of a microscopic stream of aerosol particles.
`
`=cI~~~ The aerosol was carried in a stream of hydrogen,
`
`issued from a 300 ~m (ID) coaxial capillary,
`through the grounded counter-electrode into the
`growth zone of the OMCVD reactor. Under these
`conditions
`the electrospray generated aerosol
`evaporates and undergoes a breakup due to its
`Coulombic instability [11,13,141. This evapora-
`tion—breakup mechanism is expected to yield
`submicron sized droplets containing only a small
`number of the NCs, depending on the actual
`concentration of the NCs in the partially evapo-
`rated dispersion. Evaporation of the remaining
`solvent and dispersion of individual NCs into the
`gas phase are assumed to be the last steps in the
`electrospray transfer of the NCs into the gas-
`phase [11,131.
`The transferred NCs were combined with hy-
`drogen selenide (20—110 /.Lmol/min) and DEZn
`(1—10 JLmol/min), the matrix forming precursors,
`in the mixing zone at
`the reactor
`inlet. The
`ZnSe/CdSe(QDs) films of thickness 0.5—5 ~m
`were deposited on degreased glass substrates at
`temperatures between 150 and 250°Cand a reac-
`tor pressure of 600 Torr. At
`the end of the
`growth procedure, the delivery of the NC disper-
`sion into the electrospray head was discontinued
`and the films were covered with a <0.3 j.~mtop
`layer of ZnSe.
`The concentration of Cd in the ZnSe/CdSe
`QD composites was determined by X-ray fluores-
`cence analysis on a spectrometer equipped with a
`Diano X-ray generator (Cr-anode) and HNU Sys-
`tem Si : Li detector. Elemental depth profiles were
`measured with a Perkin-Elmer Auger scanning
`microprobe (Model 660). The films were sput-
`tered with Ar~ ions. X-ray diffraction patterns
`were acquired on a Rigaku 300 rotating anode
`diffractometer in the i9—2@ configuration using
`Cu Ka radiation. Room temperature photolumi-
`nescence spectra were collected on a SPEX Fluo-
`rolog 2 spectrometer. UV/Vis absorption spectra
`were measured on a Hewlett-Packard 8452 diode
`array spectrometer. The samples for TEM and
`STEM were prepared by deposition of thin films
`of the composites on Ni—carbon TEM grids. The
`TEM images were obtained on a Topcon EMOO2B
`
`A
`~T
`— i
`U.
`i
`H2Se/H2
`
`.--
`
`Heater
`Substrate
`
`Reactor inlet
`
`~DEZn/H2
`
`*
`
`=
`
`Ring counter
`electrode (ground)
`
`-
`
`-
`
`- Capillary electrode
`(+5 kV)
`
`HVpower +
`
`H2
`
`CdSe dispersion
`Fig. 1. Schematic drawing of electrosprayOMCVD reactor.
`
`The solution of NCs to be used in the deposi-
`tion process was made in the following manner.
`The CdSe NCs, with or without the ZnSe over-
`layer and capped with pyridine, were dispersed in
`anhydrous pyridine under nitrogen and the dis-
`persion was diluted with anhydrous acetonitrile
`on a vacuum line. The pyridine/ acetonitrile ratio
`was optimized at 1:2 to achieve stability of the
`dispersion and steady operation of the electro-
`spray under the OMCVD growth conditions. The
`concentration of
`the NCs was adjusted in the
`range of 1—4 mg/rn!.
`The growth of the CdSe—ZnSe composites was
`carried out in a tubular, up-flow OMCVD reactor
`equipped with an external resistive heater (Fig.
`1). The dispersion of the NCs was delivered at
`flow rates of ‘-~ 15 /Ll/rnin into an electrospray
`head consisting of a stainless steel capillary (100
`~.tm ID and 200 ~m OD) and a 6 mm ring
`counter electrode supported by a Teflon holder.
`The dispersion flowed into the capillary electrode
`maintained at 4.0—4.5 kV of positive polarity. The
`high local electric field on the capillary tip (‘— 106
`V/m) caused the characteristic deformation of
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`M. Danek et aL/Journal of Crystal Growth 145 (1994) 714—720
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`717
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`electron microscope operating at an acceleration
`voltage of 200 kV.
`
`3. Results and discussion
`
`X-ray diffraction showed that growth at 150°C
`yielded materials with an amorphous matrix, while
`the films grown above 200°Cwere polycrystalline,
`with preferred orientation along the cubic (111)
`direction.
`The average Cd concentration in the compos-
`ites was typically 1.5 at%, although composites
`with Cd levels up to 25 at% could be prepared by
`placing the substrate close to the counter elec-
`trode and reducing the matrix growth rate. The
`Auger electron spectroscopy (AES) depth profile
`analysis showed a uniform distribution of Cd and
`Zn throughout the composite layer. Concentra-
`tions of oxygen, carbon, phosphorus, and nitrogen
`were below the detection level of the Auger mi-
`croprobe.
`The most prominent properties of the compos-
`ites are their absorption and emission character-
`istics. The is—is absorption band of the NCs is
`clearly visible in the absorption spectra of the
`composites with a high concentration of Cd (Fig.
`2). Observation of the NCs absorption band on
`less dense samples in a transmission configura-
`
`C
`
`-ja.
`w
`
`-
`
`650
`630
`610
`590
`570
`550
`530
`Wavelength (nmj
`Fig. 3. Room temperature PL spectra of ZnSe/CdSe OD
`composites grownat 150°Cwith two different sizes of the NCs
`grown at 150°Con glass. The PL was excited at 470 nm: (a)
`NC size
`3.5 nm, (b) NC size -~5.0 nm. The parent NCs
`absorbed in hexanes at 560 and 608 nm, respectively.
`
`tion is complicated by scattering from the film
`roughness and by a presence of interference
`fringes. The room temperature PL spectra of the
`composites are shown in Fig. 3 for the two NC
`3.5 and ‘~ 5.0 nm. The emission wave-
`sizes,
`lengths, 568 and 6i6 nm, respectively, are approx-
`imately at the same position as for the parent
`NCs dispersed in organic solvents. PL excitation
`spectra confirmed that the emission is excited by
`direct absorption of the light in the NCs. At
`
`2.50
`
`2.00
`
`~
`
`1.50
`
`J1.00
`
`0.50
`
`Ce
`
`-J
`0.
`
`0.00
`450
`
`500
`
`600
`550
`Wavelength Inmi
`Fig. 2. Room temperature absorption spectrumof ZnSe/CdSe
`QD composite with
`3.5 nm CdSe NCs grown at 240°Con
`glass. The absorption maximum of the parent NCs in n-hexane
`was at 560 nm. The concentration of Cd was
`25 at%.
`
`650
`
`700
`
`530
`
`550
`
`610
`590
`570
`Wavelength (nm]
`Fig. 4. The effect of the surface passivation of the CdSe NCs
`on the FL intensity: (a) composite grown at 150°Cfrom NCs
`capped with pyridine; (b) composite grown at 150°Cfrom the
`NCs capped with ZnSe layer.
`
`630
`
`650
`
`4 of 7
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`M. Danek et al. /Journal of Crystal Growth 145 (1994) 714—720
`
`470 nm
`excitation wavelengths shorter than
`the PL efficiency sharply drops, which is consis-
`tent with a strong absorption of the light in the
`ZnSe matrix. The relative PL efficiency strongly
`depends on the growth temperature. The highest
`
`PL efficiency was observed on samples prepared
`at 150°C,while the samples grown above 200°C
`exhibited a marginal or non-detectable PL.
`Incorporation of the CdSe NCs passivated with
`a ZnSe layer into the thin ZnSe matrix had a
`
`CdSe
`
`CdSe
`
`________
`
`5.Onm
`
`~.Onrn~
`
`~nm~
`
`Fig. 5. Bright field TEM images of ZnSc/CdSe QD composite with
`
`5.0 nm NCs deposited on a C/Ni TEM grid at 250°C.
`
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`M. Danek et al. /Journal of Crystal Growth 145 (1994) 714—720
`
`719
`
`pronounced effect on the PL efficiency (Fig. 4).
`At a growth temperature of 150°Cthe PL effi-
`ciency increased by more than one order of mag-
`nitude, compared to the PL for the composites
`grown from CdSe NCs terminated onlywith pyri-
`dine. Moreover, the PL was easily measurable
`even on the composites grown at 250°C. This
`result is consistent with the observation of the
`“activated” emission from colloids containing
`semiconductor particles overcoated with a layer
`of a wider band-gap material, e.g., ZnS on CdSe
`[151 and CdS on HgS [16,17]. This behavior may
`be rationalized as the passivation layer removing
`the electronic traps for non-radiative recombina-
`tion of the exciton.
`The microstructure of the composites has been
`probed by
`transmission electron microscopy
`(TEM) with films deposited on a C/Ni TEM grid
`at 150 and 250°C(thickness below 0.1 ~.Lm).The
`TEM study revealed that the composites contain
`both individual and agglomerated NCs. High res-
`olution TEM images and selected area electron
`diffraction confirmed that the matrix grown at
`150°Cis largely amorphous, while the film grown
`at 250°Ccontains sphalerite and, to a lesser ex-
`tent, wurtzite grains. The fringes of
`the (100)
`wurtzite lattice planes with the characteristic 6-
`fold symmetry have been used for the calibration
`of the lateral scale on the high resolution TEM
`pictures. The CdSe NCs have been identified on
`these images on the basis of the characteristic
`spacing of the (002) CdSe wurtzite planes (d002 =
`0.351 nm). Examples of an individual CdSe NC
`buried in the ZnSe matrix are shown in Fig.
`The image of the crystalline interface between
`the CdSe NC and ZnSe matrix reveals a presence
`of structural defects on the NC—matrix interface.
`These defects become even more severe on the
`interface of agglomerated NCs.
`The emission of the agglomerated NCs should
`be particularly susceptible to elevated growth
`temperature because of the close proximity of the
`particles. The enhancement of the PL efficiency
`of the composite with the overcoated NC clearly
`demonstrate that the effect of the interface im-
`perfections and agglomeration may be controlled
`by isolating the CdSe particles with a layer of a
`wider band-gap material,
`
`~,
`
`4. Conclusion
`
`New Il—VI QD composites, incorporating high
`quality CdSe NCs in a thin film ZnSe matrix,
`have been prepared by electrospray OMCVD.
`The growth experiments with two different sizes
`of the NCs have proven that the optical proper-
`ties of these materials can be finely adjusted by
`selecting the NC size. Although amorphous ZnSe
`appears to be more effective in stabilization of
`the NC surface than polycrystalline ZnSe,
`the
`crystalline matrix is desirable for applications due
`to its higher stability. The optical properties of
`the QDs can be preserved at the temperatures
`necessary for the growth of ZnSe by overcoating
`the CdSe particles with a ZnSe layer before the
`co-deposition. The available energy window for
`this combination of the NCs and matrix is ‘~ 1.8—
`2.5 eV. The emissivity of these materials can be
`dramatically enhanced by
`incorporating NCs
`overcoated with a wider band-gap material, as
`demonstrated with the CdSe/ZnSe particles.
`Overcoating allows not only passivation of the
`CdSe interface, but also has the potential
`for
`control of
`the minimum separation between
`neighboring NCs. Characterization of
`the de-
`tailed structure of
`the overcoated NCs is in
`progress.
`The electrospray OMCVD possesses the flexi-
`bility of the OMCVD process and various semi-
`conductor matrices can be selected for the QD
`composites. In addition, tremendous progress has
`been made in solution synthesis of NCs based on
`materials other than Il—VI
`semiconductors [18—
`21]. Thus, electrospray OMCVD has great poten-
`tial for the preparation of a large variety of new
`QD composites. These materials are attractive
`not only from the scientific point of view, but also
`from the standpoint of constructing novel quan-
`turn confinement devices.
`
`Acknowledgments
`
`We thank Mr. M. Frongillo for his assistance
`with the TEM imaging. The financial support of
`the National Science Foundation (CHE-89-14953,
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`M Danek et aL/Journal of Crystal Growth 145 (1994) 714—720
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`DMR-90-22933, and DMR-90-23162) is gratefully
`acknowledged.
`
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