(CdSe)ZnS Core-Shell Quantum Dots:Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites
B.O.Dabbousi,†J.Rodriguez-Viejo,‡F.V.Mikulec,†J.R.Heine,§H.Mattoussi,§R.Ober,⊥
K.F.Jensen,‡,§and M.G.Bawendi*,†
Departments of Chemistry,Chemical Engineering,and Materials Science and Engineering,
Massachusetts Institute of Technology,77Massachusetts A V e.,Cambridge,Massachusetts02139,and
Laboratoire de Physique de la Matie`re Condense´e,Colle`ge de France,11Place Marcellin Berthelot,
75231Paris Cedex05,France
Recei V ed:March27,1997;In Final Form:June26,1997X
We report a synthesis of highly luminescent(CdSe)ZnS composite quantum dots with CdSe cores ranging in
结合律
diameter from23to55Å.The narrow photoluminescence(fwhm e40nm)from these composite dots
spans most of the visible spectrum from blue through red with quantum yields of30-50%at room temperature.
We characterize these materials using a range of optical and structural techniques.Optical absorption and
photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots.
We use a combination of wavelength dispersive X-ray spectroscopy,X-ray photoelectron spectroscopy,small
and wide angle X-ray scattering,and transmission electron microscopy to analyze the composite dots and
determine their chemical composition,average size,size distribution,shape,and internal structure.Using a
simple effective mass theory,we model the energy shift for the first excited state for(CdSe)ZnS and(CdSe)-
CdS dots with varying shell thickness.Finally,we characterize the growth of ZnS on CdSe cores as locally
epitaxial and determine how the structure of the ZnS shell influences the photoluminescence properties.
I.Introduction
Semiconductor nanocrystallites(quantum dots)whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter.1Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size.Conse-quently,both the optical absorption and emission of quantum dots shift to the blue(higher energies)as the size of the dots gets smaller.Although nanocrystallites have not yet completed their evolution into bulk solids,structural studies indicate that they retain the bulk crystal structure and lattice parameter.2 Recent advances in the synthesis of highly monodisperse nanocrystallites3-5have paved the way for
numerous spectro-scopic studies6-11assigning the quantum dot electronic states and mapping out their evolution as a function of size.
Core-shell type composite quantum dots exhibit novel properties making them attractive from both an experimental and a practical point of view.12-19Overcoating nanocrystallites with higher band gap inorganic materials has been shown to improve the photoluminescence quantum yields by passivating surface nonradiative recombination sites.Particles passivated with inorganic shell structures are more robust than organically passivated dots and have greater tolerance to processing conditions necessary for incorporation into solid state structures. Some examples of core-shell quantum dot structures reported earlier include CdS on CdSe and CdSe on CdS,12ZnS grown on CdS,13ZnS on CdSe and the inverse structure,14CdS/HgS/ CdS quantum dot quantum wells,15ZnSe overcoated CdSe,16 and SiO2on Si.17,18Recently,Hines and Guyot-Sionnest reported making(CdSe)ZnS nanocrystallites whose room tem-perature fluorescence quantum yield was50%.19
This paper describes the synthesis and characterization of a series of room-temperature high quantum yield(30%-50%) core-shell(CdSe)ZnS nanocrystallites with narrow band edge luminescence spanning most of the visible spectrum from470 to625nm.These particles are produced
using a two-step synthesis that is a modification of the methods of Danek et al.16 and Hines et al.19ZnS overcoated dots are characterized spectroscopically and structurally using a variety of techniques. The optical absorption and photoluminescence spectra of the composite dots are measured,and the lowest energy optical transition is modeled using a simplified theoretical approach. Wavelength dispersive X-ray spectroscopy and X-ray photo-electron spectroscopy are used to determine the elemental and spatial composition of ZnS overcoated dots.Small-angle X-ray scattering in solution and in polymer films and high-resolution transmission electron microscopy measurements help to deter-mine the size,shape,and size distribution of the composite dots. Finally,the internal structure of the composite quantum dots and the lattice parameters of the core and shell are determined using wide-angle X-ray scattering.
In addition to having higher efficiencies,ZnS overcoated particles are more robust than organically passivated dots and potentially more useful for optoelectronic device structures. Electroluminescent devices(LED’s)incorporating(CdSe)ZnS dots into heterostructure organic/semiconductor nanocrystallite light-emitting devices may show greater stability.20Thin films incorporating(CdSe)ZnS dots into a matrix of ZnS using electrospray organometallic chemical vapor deposition(ES-OMCVD)demonstrate more than2orders of magnitude improvement in the PL quantum yields(∼10%)
relative to identical structures based on bare CdSe dots.21In addition,these structures exhibit cathodoluminescence21upon excitation with high-energy electrons and may potentially be useful in the
*To whom correspondence should be addressed.
†Department of Chemistry,MIT.
‡Department of Chemical Engineering,MIT.
保利民爆
济南科技有限公司§Department of Materials Science and Engineering,MIT. ⊥Colle`ge de France.
X Abstract published in Ad V ance ACS Abstracts,September1,1997.9463
J.Phys.Chem.B1997,101,9463-9475
S1089-5647(97)01091-2CCC:$14.00©1997American Chemical Society
春江花月夜教案
production of alternating current thin film electroluminescent devices(ACTFELD).
II.Experimental Section
Materials.Trioctylphosphine oxide(TOPO,90%pure)and trioctylphosphine(TOP,95%pure)were obtained from Strem and Fluka,respectively.Dimethylcadmium(CdMe2)and di-ethylzinc(ZnEt2)were purchased from Alfa and Fluka,respec-tively,and both materials were filtered separately through a0.2µm filter in an inert atmosphere box.Trioctylphosphine selenide was prepared by dissolving0.1mol of Se shot in100mL of TOP,thus producing a1M solution of TOPSe.Hexamethyl-disilathiane((TMS)2S)was used as purchased from Aldrich. HPLC grade n-hexane,methanol,pyridine,and1-butanol were purchased from EM Sciences.
Synthesis of Composite Quantum Dots.(CdSe)ZnS.Nearly monodisperse CdSe quantum dots ranging from23to55Åin diameter were synthesized via the pyrolysis of the organome-tallic precursors,dimethylcadmium and trioctylphosphine se-lenide,in a coordinating solvent,trioctylphosphine oxide (TOPO),as described previously.3The precursors were injected at temperatures ranging from340to360°C,and the initially formed small(d)23Å)dots were grown at temperatures between290and300°C.The dots were collected as powders using size-selective precipitation3with methanol and then redispersed in hexane.
A flask containing5g of TOPO was heated to190°C under vacuum for several hours and then cooled to60°C after which 0.5mL of trioctylphosphine(TOP)was added.Roughly0.1-0.4µmol of CdSe dots dispersed in hexane was transferred into the reaction vessel via syringe,and the solvent was pumped off.
Diethylzinc(ZnEt2)and hexamethyldisilathiane((TMS)2S) were used as the Zn and S precursors.The amounts of Zn and S precursors needed to grow a ZnS shell of desired thickness for each CdSe sample were determined as follows:First,the average radius of the CdSe dots was estimated from TEM or SAXS measurements.Next,the ratio of ZnS to CdSe necessary to form a shell of desired thickness was calculated based on the ratio of the shell volume to that of the core assuming a spherical core and shell and taking into account the bulk lattice parameters of CdSe and ZnS.For larger particles the ratio of Zn to Cd necessary to achieve the same thickness shell is less than for the smaller dots.The actual amount of ZnS that grows onto the CdSe cores was generally less than the amount added due to incomplete reaction of the precursors and to loss of some material on the walls of the flask during the addition. Equimolar amounts of the precursors were dissolved in2-4 mL of TOP inside an inert atmosphere glovebox.The precursor solution was loaded into a syringe and transferred to an addition funnel attached to the reaction flask.The reaction flask containing CdSe do
ts dispersed in TOPO and TOP was heated under an atmosphere of N2.The temperature at which the precursors were added ranged from140°C for23Ådiameter dots to220°C for55Ådiameter dots.22When the desired temperature was reached,the Zn and S precursors were added dropwise to the vigorously stirring reaction mixture over a period of5-10min.
After the addition was complete,the mixture was cooled to 90°C and left stirring for several hours.A5mL aliquot of butanol was added to the mixture to prevent the TOPO from solidifying upon cooling to room temperature.The overcoated particles were stored in their growth solution to ensure that the surface of the dots remained passivated with TOPO.They were later recovered in powder form by precipitating with methanol and redispersed into a variety of solvents including hexane, chloroform,toluene,THF,and pyridine.
(CdSe)CdS.Cadmium selenide nanocrystallites with diam-eters between33.5and35Åwere overcoated with CdS to varying thickness using the same basic procedure as that outlined for the ZnS overcoating.The CdS precursors used were Me2-Cd and(TMS)2S.The precursor solution was dripped into the reaction vessel containing the dots at a temperature of180°C and a rate of∼1mL/min.The solution became noticeably darker as the overcoat precursors were added.Absorption spectra taken just after addition of precursors showed a significant shift in the abs
orption peak to the red.To store these samples,it was necessary to add equal amounts of hexane and butanol since the butanol by itself appeared to flocculate the particles.
Optical Characterization.UV-vis absorption spectra were acquired on an HP8452diode array spectrophotometer.Dilute solutions of dots in hexane were placed in1cm quartz cuvettes, and their absorption and corresponding fluorescence were measured.The photoluminescence spectra were taken on a SPEX Fluorolog-2spectrometer in front face collection mode. The room-temperature quantum yields were determined by comparing the integrated emission of the dots in solution to the emission of a solution of rhodamine590or rhodamine640 of identical optical density at the excitation wavelength. Wavelength Dispersive X-ray Spectroscopy.A JEOL SEM 733electron microprobe operated at15kV was used to determine the chemical composition of the composite quantum dots using wavelength dispersive X-ray(WDS)spectroscopy. One micrometer thick films of(CdSe)ZnS quantum dots were cast from concentrated pyridine solutions onto Si(100)wafers, and after the solvent had completely evaporated the films were coated with a thin layer of amorphous carbon to prevent charging.
X-ray Photoelectron Spectroscopy.XPS was performed using a Physical Electronics5200C spectrometer equipped with a dual X-ray anode(Mg and Al)and a concentric hemispherical analyzer(
CHA).Data were obtained with Mg K R radiation (1253.6eV)at300W(15keV,20mA).Survey scans were collected over the range0-1100eV with a179eV pass energy detection,corresponding to a resolution of2eV.Close-up scans were collected on the peaks of interest for the different elements with a71.5eV pass energy detection and a resolution of1eV.
A base pressure of10-8Torr was maintained during the experiments.All samples were exchanged with pyridine and spin-cast onto Si substrates,forming a thin film several monolayers thick.
Transmission Electron Microscopy.A Topcon EM002B transmission electron microscope(TEM)was operated at200 kV to obtain high-resolution images of individual quantum dots. An objective aperture was used to selectively image the(100), (002),and(101)wurtzite lattice planes.The samples were prepared by placing one drop of a dilute solution of dots in octane onto a copper grid supporting a thin film of amorphous carbon and then wicking off the remaining solvent after30s.
A second thin layer of amorphous carbon was evaporated onto the samples in order to minimize charging and reduce damage to the particles caused by the electron beam.
Small-Angle X-ray Scattering(SAXS)in Polymer Films. Small-angle X-ray scattering(SAXS)samples were prepared using either poly(vinyl butyral)(PVB)or a phosphine-func-tionalized diblock copolymer
[methyltetracyclododecene]300-[norbornene-CH2O(CH2)5P(oct)2]20,abbreviated as(MTD300P20), as the matrix.23Approximately5mg of nanocrystallites of dispersed in1mL of toluene,added to0.5mL of a solution containing10wt%PVB in toluene,concentrated under vacuum to give a viscous solution,and then cast onto a silicon wafer. The procedure is the same for MTD300P20,except THF is used
9464J.Phys.Chem.B,Vol.101,No.46,1997Dabbousi et al.
as the solvent for both nanocrystallites and polymer.The resulting∼200µm thick film is clear to slightly opaque.X-ray diffraction spectra were collected on a Rigaku300Rotaflex diffractometer operating in the Bragg configuration using Cu K R radiation.The accelerating voltage was set at60kV with a300mA flux.Scatter and diffraction slits of1/6°and a0.3 mm collection slit were used.
Small-Angle X-ray Scattering in Dilute Solutions.The X-ray source was a rotating copper anode operated at40kV and25mA.The apparent point source(electron beam irradiated area on the anode)was about10-2mm2.The beam was collimated onto a position sensitive detector,PSPE(ELPHYSE).
A thin slit,placed before the filter,selects a beam with the dimensions of3×0.3mm2on the detector.Th
e position sensitive linear detector has a useful length of50mm,placed at a distance D)370mm from the detector.The spatial resolution on the detector is200µm.This setup allows a continuous scan of scattering wavevectors between6×10-3 and0.40Å-1,with a resolution of about3×10-3Å-1.
The samples used were quartz capillary tubes with about1 mm optical path,filled with the desired dispersion,and then flame-sealed after filling.The intensity from the reference,I ref, is collected first,and then the intensity from the sample,I s.The intensity used in the data analysis is the difference:I)I s-I ref.
Wide-Angle X-ray Scattering(WAXS).The wide-angle X-ray powder diffraction patterns were measured on the same setup as the SAXS in polymer dispersions.The TOPO/TOP capped nanocrystals were precipitated with methanol and exchanged with pyridine.The samples were prepared by dropping a heavily concentrated solution of nanocrystals dispersed in pyridine onto silicon wafers.A slow evaporation of the pyridine leads to the formation of glassy thin films which were used for the diffraction experiments.
III.Results and Analysis
A.Synthesis of Core-Shell Composite Quantum Dots. We use a two-step synthetic procedure similar
to that of Danek et al.16and Hines et al.19to produce(CdSe)ZnS core-shell quantum dots.In the first step we synthesize nearly mono-disperse CdSe nanocrystallites ranging in size from23to55Åvia a high-temperature colloidal growth followed by size selective precipitation.3These dots are referred to as“bare”dots in the remainder of the text,although their outermost surface is passivated with organic TOPO/TOP capping groups. Next,we overcoat the CdSe particles in TOPO by adding the Zn and S precursors at intermediate temperatures.22The resulting composite particles are also passivated with TOPO/ TOP on their outermost surface.
The temperature at which the dots are overcoated is very critical.At higher temperatures the CdSe seeds begin to grow via Ostwald ripening,and their size distribution deteriorates, leading to broader spectral line widths.Overcoating the particles at relatively low temperatures could lead to incomplete decom-position of the precursors or to reduced crystallinity of the ZnS shell.An ideal growth temperature is determined independently for each CdSe core size to ensure that the size distribution of the cores remains constant and that shells with a high degree of crystallinity are formed.22
The concentration of the ZnS precursor solution and the rate at which it is added are also critical.Slow addition of the precursors at low concentrations ensures that most of the ZnS grows heterogeneously onto existing CdSe nuclei instead of undergoing homogeneous nucleation.This pro
bably does not eliminate the formation of small ZnS particles completely so a final purification step in which the overcoated dots are subjected to size selective precipitation provides further assurance that mainly(CdSe)ZnS particles are present in the final powders.
B.Optical Characterization.The synthesis presented above produces ZnS overcoated dots with a range of core and shell sizes.Figure1shows the absorption spectra of CdSe dots ranging from23to55Åin diameter before(dashed lines)and after(solid lines)overcoating with1-2monolayers of ZnS. The definition of a monolayer here is a shell of ZnS that measures3.1Å(the distance between consecutive planes along the[002]axis in bulk wurtzite ZnS)along the major axis of the prolate-shaped dots.We observe a small shift in the absorption spectra to the red(lower energies)after overcoating due to partial leakage of the exciton into the ZnS matrix.This red shift is more pronounced in smaller dots where the leakage of the exciton into the ZnS shell has a more dramatic effect on the confinement energies of the charge carriers.Figure2shows the room-temperature photoluminescence spectra(PL)of these Figure 1.Absorption spectra for bare(dashed lines)and1-2 monolayer ZnS overcoated(solid lines)CdSe dots with diameters measuring(a)23,(b)42,(c)48,and(d)55Å.The absorption spectra for the(CdSe)ZnS dots are broader and slightly red-shifted from their respective bare dot spectra.
Figure2.Photoluminescence(PL)spectra for bare(dashed lines)and ZnS overcoated(solid lines)dots with the following core sizes:(a) 23,(b)42,(c)48,and(d)55Åin diameter.The PL spectra for the overcoated dots are much more intense owing to their higher quantum yields:(a)40,(b)50,(c)35,and(d)30.
(CdSe)ZnS Core-Shell Quantum Dots J.Phys.Chem.B,Vol.101,No.46,19979465
same samples before (dashed lines)and after (solid lines)overcoating with ZnS.The PL quantum yield increases from 5to 15%for bare dots to values ranging from 30to 50%for dots passivated with ZnS.In smaller CdSe dots the surface-to-volume ratio is very high,and the PL for TOPO capped dots is dominated by broad deep trap emission due to incomplete surface passivation.Overcoating with ZnS suppresses deep trap emission by passivating most of the vacancies and trap sites on the crystallite surface,resulting in PL which is dominated by band-edge recombination.
Figure 3(color photograph)displays the wide spectral range of luminescence from (CdSe)ZnS composite quantum dots.The photograph shows six different samples of ZnS overcoated CdSe dots dispersed in dilute hexane solutions and placed in identical quartz cuvettes.The samples are irradiated with 365nm ultraviolet light from a UV lamp in order to observe lumines-cence from all the
solutions at once.As the size of the CdSe core increases,the color of the luminescence shows a continuous progression from blue through green,yellow,orange,to red.In the smallest sizes of TOPO capped dots the color of the PL is normally dominated by broad deep trap emission and appears as faint white light.After overcoating the samples with ZnS the deep trap emission is nearly eliminated,giving rise to intense blue band-edge fluorescence.
To understand the effect of ZnS passivation on the optical and structural properties of CdSe dots,we synthesized a large quantity of ∼40Ådiameter CdSe dots.We divided this sample into multiple fractions and added varying amounts of Zn and S precursors to each fraction at identical temperatures and addition times.The result was a series of samples with similar CdSe cores but with varying ZnS shell thickness.Figure 4shows the progression of the absorption spectrum for these samples with ZnS coverages of approximately 0(bare TOPO capped CdSe),0.65,1.3,2.6,and 5.3monolayers.(See beginning of this section for definition of number of monolayers.)The spectra reflect a constant area under the lowest energy 1S 3/2-1S e absorption peak (constant oscillator strength)for the samples with varying ZnS coverage.As the thickness of the ZnS shell increases,there is a shift in the 1S 3/2-1S e absorption to the red,
reflecting an increased leakage of the exciton into the shell,as well as a broadening of the absorption
peak,indicating a distribution of shell thickness.The left-hand side of Figure 4shows an increased absorption in the ultraviolet with increasing ZnS coverage as a result of direct absorption into the higher band gap ZnS shell.
The evolution of the PL for the same ∼40Ådiameter dots with ZnS coverage is displayed in Figure 5.As the coverage of ZnS on the CdSe surface increases,we see a dramatic increase in the fluorescence quantum yield followed by a steady
decline
Figure 3.Color photograph demonstrating the wide spectral range of bright fluorescence from different size samples of (CdSe)ZnS.Their PL peaks occur at (going from left to right)470,480,520,560,594,and 620nm (quartz cuvettes courtesy of Spectrocell Inc.,photography by F.Frankel).
Figure 4.Absorption spectra for a series of ZnS overcoated samples grown on identical 42Å(10%CdSe seed particles.The samples displayed have the following coverage:(a)bare TOPO capped,(b)0.65monolayers,(c)1.3monolayers,(d)2.6monolayers,and (e)5.3monolayers (see definition for monolayers in text).The right-hand side shows the long wavelength region of the absorption spectra showing the lowest energy optical transitions.The spectra demonstrate an increased red-shift with thicker ZnS shells as well as a broadening of the first peak as a result of increased polydispersity.The left-hand side highlights the ultraviolet region of the spectra showing an increased absorption at higher energies with increasing coverage due to direct absorption into the ZnS shell.
9466J.Phys.Chem.B,Vol.101,No.46,1997Dabbousi et al.
after∼1.3monolayers of ZnS.The spectra are red-shifted (slightly more than the shift in the absorptio
n spectra)and show
an increased broadening at higher coverage.The inset to Figure 5charts the evolution of the quantum yield for these dots as a function of the ZnS shell thickness.For this particular sample the quantum yield starts at15%for the bare TOPO capped CdSe dots and increases with the addition of ZnS,approaching a maximum value of50%at approximately∼1.3monolayer coverage.At higher coverage the quantum yield begins to decrease steadily until it reaches a value of30%at about∼5 monolayer coverage.In the following sections we explain the trends in PL quantum yield based on the structural characteriza-tion of ZnS overcoated samples.
C.Structural Characterization.Wa V elength Dispersi V e X-ray Spectroscopy.We analyze the elemental composition of the ZnS overcoated samples using wavelength dispersive X-ray spectroscopy(WDS).This method provides a quantitative analysis of the elemental composition with an uncertainty of less than(5%.We focus on obtaining a Zn/Cd ratio for the ZnS overcoated samples of interest.Analysis of the series of samples with a∼40Ådiameter core and varying ZnS coverage gives the Zn/Cd ratios which appear in Table1.The WDS analysis confirms that the Zn-to-Cd ratio in the composite dots increases as more ZnS is added.We also use this technique to measure the Se/Cd ratio in the bare dots.We consistently measure a Se/Cd ratio of∼0.8-0.9/1,indicating Cd-rich n
anoparticles.
X-ray Photoelectron Spectroscopy.Multiple samples of ∼33and∼40Ådiameter CdSe quantum dots overcoated with variable amounts of ZnS were examined by XPS.Figure6shows the survey spectra of∼40Ådiameter bare dots and of
the same sample overcoated with∼1.3monolayers of ZnS.The
presence of C and O comes mainly from atmospheric contami-
nation during the brief exposure of the samples to air(typically
around15min).The positions of both C and O lines correspond
to standard values for adsorbed species,showing the absence
of significant charging.24As expected,we detect XPS lines
from Zn and S in addition to the Cd and Se lines.Although
the samples were exchanged with pyridine before the XPS
measurements,small amounts of phosphorus could be detected
on both the bare and ZnS overcoated CdSe dots,indicating the
presence of residual TOPO/TOP molecules bound to Cd or Zn
on the nanocrystal surfaces.25The relative concentrations of
Cd and Se are calculated by dividing the area of the XPS lines
by their respective sensitivity factors.24In the case of nano-
crystals the sensitivity factor must be corrected by the integral振动器
∫0d e-z/λd z to account for the similarity between the size of the nanocrystals and the escape depths of the electrons.26The
integral must be evaluated over a sphere to obtain the Se/Cd
ratios in CdSe dots.In the bare CdSe nanocrystals the Se/Cd
ratio was around0.87,corresponding to46%Se and54%Cd.
This value agrees with the WDS results.
We use the Auger parameter,defined as the difference in
binding energy between the photoelectron and Auger peaks,to
identify the nature of the bond in the different samples.24This
difference can be accurately determined because static charge
corrections cancel.The Auger parameter of Cd in the bare and
TABLE1:Summary of the Results Obtained from WDS,TEM,SAXS,and WAXS Detailing the Zn/Cd Ratio,Average Size, Size Distribution,and Aspect Ratio for a Series of(CdSe)ZnS Samples with a∼40ÅDiameter CdSe Cores and Varying ZnS Coverage
ZnS coverage
(TEM)measd TEM size measd average
aspect ratio
calcd size(SAXS
in polymer)
measd Zn/Cd双缩脲
ratio(WDS)
calcd Zn/Cd ratio
(SAXS in polymer)
calcd Zn/Cd ratio
(WAXS)
bare39Å(8.2% 1.1242Å(10%
0.65monolayers43Å(11% 1.1646Å(13%0.460.580.7
1.3monolayers47Å(10% 1.1650Å(18% 1.50 1.32 1.4
2.6monolayers55Å(13% 1.23
3.60 2.9 5.3monolayers72Å(19% 1.23 6.80 6.8 Figure5.PL spectra for a series of ZnS overcoated dots with42(
10%Ådiameter CdSe cores.The spectra are for(a)0,(b)0.65,(c)
1.3,(d)
2.6,and(e)5.3monolayers ZnS coverage.The position of the
maximum in the PL spectrum shifts to the red,and the spectrum
broadens with increasing ZnS coverage.(inset)The PL quantum yield
is charted as a function of ZnS coverage.The PL intensity increases
with the addition of ZnS reaching,50%at∼1.3monolayers,and then declines steadily at higher coverage.The line is simply a guide to the eye.Figure6.(A)Survey spectra of(a)∼40Ådiameter bare CdSe dots and(b)the same dots overcoated with ZnS showing the photoelectron and Auger transitio
ns from the different elements present in the quantum dots.(B)Enlargement of the low-energy side of the survey spectra, emphasizing the transitions with low binding energy.
(CdSe)ZnS Core-Shell Quantum Dots J.Phys.Chem.B,Vol.101,No.46,19979467
我们这一代人的困惑overcoated samples is466.8(0.2eV and corresponds exactly to the expected value for bulk CdSe.In the case of ZnS the Auger parameter for Zn in the1.3and2.6monolayer ZnS samples is757.5eV,which is also very close to the expected value of758.0eV.
The degree of passivation of the CdSe surface with ZnS is examined by exposing the nanocrystal surface to air for extended periods of time and studying the evolution of the Se peak. The oxidation of CdSe quantum dots leads to the formation of a selenium oxide peak at higher energies than the main Se peak.27Figure7shows the formation of a SeO2peak at59eV after an80h exposure to air in both the bare,TOPO capped, CdSe and0.65monolayer ZnS overcoated samples.These results indicate that in the0.65monolayer samples the ZnS shell does not completely surround the CdSe nanocrystals,and there are still Se sites at the surface that are susceptible to oxidation. In samples with an estimated coverage of∼1.3monolayers ZnS or more the oxide peak does not appear even after prolonged exposure to air,indicating that the CdSe surface is possibly protected by a continuou
s ZnS shell.After exposure to air for 16h,the bare CdSe nanocrystals display a selenium oxide peak which represents13%of the total Se signal,and the Se/Cd ratio decreases to0.77,corresponding to43%Se and57%Cd.The same sample after80h exposure to air had a ratio of Se/Cd of 0.37(28%Se and72%Cd),and the SeO2peak area was22% of the total Se signal.For a∼40Ådiameter sample,34%of the atoms are at the surface which means that in the sample measured most of the surface Se has been desorbed from the surface after80h.In the samples with more than 1.3 monolayers of ZnS coverage no change in the Se/Cd ratio was detected even after exposure to air for80h.Although no Cd-(O)peak appears after similar exposure to air,the Cd Auger parameter shifts from466.8eV for bare unoxidized CdSe to 467.5eV for particles exposed to air for80h.The Auger parameter for the1.3and2.6monolayer coverage samples remains the same even after prolonged exposure to air. Another method to probe the spatial location of the ZnS relative to the CdSe core is obtained by comparing the ratios of the XPS and Auger intensities of the Cd photoelectrons for bare and overcoated samples.14,28The depth dependence of the observed intensity for the Auger and XPS photoemitted electrons is
where J0is the X-ray flux,N(z)i is the number of i atoms,σi is the absorption cross section for atoms i,Y i,n is the emission quantum yield of Auger or XPS for atoms i,F(KE)is the energy-dependent instr
ument response function,andλ(KE)is the energy-dependent escape depth.Taking the ratio of the intensities of the XPS and Auger lines from the same atom,Cd or Zn,it is possible to eliminate the X-ray flux,number of atoms, and absorption cross sections from the intensity equations for the Auger and the primary X-ray photoelectrons.The value of the intensity ratio I)i overcoated(Cd)/i bare(Cd),where i)i XPS-(Cd)/i Auger(Cd),is only a function of the relative escape depths of the electrons.Therefore,due to the smaller escape depths of the Cd Auger electrons in both ZnS(13.2Å)and CdSe(10Å)compared to the Cd XPS photoelectron(23.7Åin ZnS and 15Åin CdSe),the intensity I should increase with the amount of ZnS on the CdSe surface.Calculated values of1.28and 1.60for the0.65and2.6monolayer,respectively,confirm the growth of ZnS on the surface of the CdSe dots. Transmission Electron Microscopy.High-resolution TEM allows us to qualitatively probe the internal structure of the composite quantum dots and determine the average size,size distribution,and aspect ratio of overcoated particles as a function of ZnS coverage.We image the series of(CdSe)ZnS samples described earlier.Figure8shows two dots from that series, one with(A)no ZnS overcoating(bare)and one with(B)2.6 monolayers of ZnS.The particles in the micrographs show well-resolved lattice fringes with a measured lattice spacing in the bare dots similar to bulk CdSe.For the2.6monolayer sample these lattice fringes are continuous throughout the entire particle; the growth of the ZnS shell appears to be epitaxial.A well-defined interface between C
dSe core and ZnS shell was not observed in any of the samples,although the“bending”of the lattice fringes in Figure8B s the lower third of this particle is slightly askew compared with the upper part s may be suggestive of some sort of strain in the material.This bending is somewhat anomalous,however,as the lattice fringes in most particles were straight.Some patchy growth is observed for the highest coverage samples,giving rise to misshapen particles,but we do not observe discrete nucleation of tethered ZnS particles on the surface of existing CdSe particles.We analyze over150 crystallites in each sample to obtain statistical values for the length of the major axis,the aspect ratio,and the distribution of lengths and aspect ratios for all the samples.Figure9shows histograms of size distributions and aspect ratio from these same samples.This figure shows the measured histograms for(A)
Figure7.X-ray photoelectron spectra highlighting the Se3d core transitions from∼40Åbare and ZnS overcoated CdSe dots:(a)bare CdSe,(b)0.65monolayers,(c)1.3monolayers,and(d)2.6monolayers of ZnS.The peak at59eV indicates the formation of selenium oxide upon exposure to air when surface selenium atoms are
exposed.Figure8.Transmission electron micrographs of(A)one“bare”CdSe nanocrystallite and(B)one CdSe nanocrystallite with a2.6monolayer ZnS shell.
I)J
N(z)
i
σ
i
Y
i,n
F(KE)e-z/λ(KE)(1)
9468J.Phys.Chem.B,Vol.101,No.46,1997Dabbousi et al.
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