Published on Web Date: March 11, 2010

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Organic Surfactant-Controlled Composition of theSurfaces of CdSe Quantum DotsAdam J. Morris-Cohen, Matthew T. Frederick, G. Daniel Lilly, Eric A. McArthur, andEmily A. Weiss*

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113

ABSTRACT The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots(QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinatingsolvent increases from 1.2:1 for QDswith radius Rg 3.3 nm to 6.5:1 for R=1.9 nm,as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. Thedependence of Cd/Se on R fits a geometric model that describes the QDs as CdSecores with Cd/Se=1:1 encapsulated by a shell of Cd-organic complexes. Use of99%TOPOas the coordinating solvent producesQDswith Cd/Se≈1:1 for all valuesof R, and use of 99% TOPO “doped” with n-octylphosphonic acid (OPA), animpurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. Theseresults imply that Cd enrichment of the QDs is driven by tight-binding Cd2þ-alkylphosphonate complexes that stabilize the interface between the polar CdSecore and the organic medium.

SECTION Nanoparticles and Nanostructures

T his Letter describes a study of the ratio of Cd to Se (Cd/Se)within colloidal CdSe quantum dots (QDs) synthe-sizedwith trioctylphosphineoxide (TOPO) as the coor-

dinating solvent, as a function of the radius, R, of the QD.Using inductively coupled plasma atomic emission spectro-scopy (ICP-AES) and several electronmicroscopy techniques,we recently measured a value of Cd/Se of 3.4:1 for QDssynthesized with the common combination of technical-grade (90%) TOPO, trioctylphosphine selenide (TOPSe),and hexadecylamine (HDA) as surfactants.1 This value ofCd/Se is higher than the range of values (1:1 to 1.8:1) incolloidal CdSe QDs reported in the literature.2-8 Here, we useICP-AES to investigate the mechanism of the observed Cdenrichment by determining its dependence on R and on thesurfactants present in the synthetic reaction mixture. Weshow that (i) Cd/Se for QDs synthesized with 90% TOPO,which contains several previously identified alkylphosphonic/phinic acid impurities,9,10 increases from 1.2:1 for QDs withRg3.3 nm to 6.5:1 forR=1.9 nm.Amodel that describes theQDs as CdSe cores encapsulated by a shell of Cd complexesfits the dependence of Cd/Se onR. (ii)We can control Cd/Se inthe QDs by adjusting the concentration of phosphonic/phinicacids in the reaction mixture; use of 99% TOPO as thecoordinating solvent produces QDs with Cd/Se ≈ 1:1 for allvalues of R, and use of 99% TOPO “doped”with n-octylphos-phonic acid (OPA), an impurity in 90% TOPO, produces QDswith values of Cd/Se up to 1.5:1.

We synthesized CdSe QDs using both cadmium stearate(CdSt2) and a mixture of CdO and stearic acid as Cd pre-cursors in separate syntheses.7 We took between 6 and 120.5 mL aliquots from the reaction mixture over the course of

20 min and immediately added each aliquot to 1.5 mL ofroom-temperature chloroform to arrest growth of the QDs.The Supporting Information contains details of this synthesisand the subsequent purification. Following digestion ofthe samples in aqua regia (1:3 by volume, HNO3/HCl), wemeasured the Cd/Se of the aliquots of QDs with ICP-AES.

Figure 1 shows the set of sizes, as determined fromabsorptionmeasurements calibrated by TEM (see SupportingInformation), of QDs that we prepared with 90% TOPO andtheir measured Cd/Se ratios (black and green); these QDs areCd-enriched whether CdSt2 is added directly to the reactionmixture or is produced in situ by reacting CdO with stearicacid. The largest QDs (R g 3.3 nm) have Cd/Se ≈ 1.2:1; thisvalue increases to 6.5:1 for R=1.9 nm. The Cd-enriched QDshave excellent optical properties; they have absorption spec-tra with well-defined fine structure (see Supporting Informa-tion), narrow photoluminescence peaks, quantum yields of10-40%, and no detectable deep trap emission.1

What is the mechanism for Cd enrichment in these QDs?Previous work has shown that Cd/Se is not sensitive to theratios of Cd and Se precursors in the reaction mixture.7 Wehypothesized that since Cd enrichment translates into a netpositive charge on each QD, this enrichment is driven by thepresence of negatively charged (X-type) alkylphosphonates,the tight-binding, deprotonated form of acid impurities in90% TOPO.1,11 We tested this hypothesis by replacing 90%TOPO, which contains, by 31P NMR,∼0.1% OPA and∼0.5%

Received Date: February 17, 2010Accepted Date: March 5, 2010

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mono-n-octylphosphinic acid (MOPA), with (i) 99% TOPO,which does not contain detectable concentrations of alkylphos-phonic or phosphinic acid impurities,10 and (ii) a mixture of99% TOPO and 1.3 mol % OPA, to which MOPA oxidizesunder simulated reaction conditions (see Supporting Infor-mation). We chose OPA because we and others1,11,12 haveshown through 31P NMR of the QDs that the only phospho-nate ligands bound to QDs synthesized with 90% TOPO areOPA and its self-condensation product, P0-P0-(di-n-octyl)dihydrogen pyrophosphonic acid (PPA). The QDs synthe-sized with 99% TOPO (Figure 1, red) have Cd/Se=1.00 (0.06:1 (average and standard deviation of 17 samples); thisratio does not depend on the size of the QD. The Cd/Se of theQDs synthesized with 99% TOPO “doped” with OPA alsoincreases as R decreases, but only to 1.5:1 at R=1.9 nm(Figure 1, blue). The Cd enrichment on the surfaces of CdSeQDs is therefore correlated with the presence of alkylphos-phonic acid surfactants in the reaction mixture. The diffe-rence between the values of Cd/Se that we measured for theQDs preparedwith 90%TOPO (∼1.2:1-6.5:1) and the valuesof Cd/Se reported in refs 1-7 (1:1-1.8:1) probably arisesbecause the syntheses in those reports use either 99% TOPO,90% TOPO that was distilled prior to use, or no TOPO at all.

We note that Se is known to volatilize during digestion inaqua regia, and this volatilization may lead to artificially lowconcentrations of Se in samplesmeasured by ICP.We showedin a previous publication,1 however, that we obtain the sameCd/Se ratios (3.2(0.7) forQDs (with λabs=540nm), whetherwe use 45 �C or room-temperature aqua regia ormeasure Cd/Se using energy-dispersive X-ray spectroscopy (EDS) withinscanning electron microscopes (SEM), transmission electronmicroscopes (TEM), or scanning transmission electronmicro-scopes (STEM). Preparation of samples for SEM, TEM, andSTEM involves only drop-casting of QDs from solution, and itis very improbable that exposure toUHVconditionswithin themicroscopes would result in depletion of Se that is reprodu-cible, both from sample to sample and between EDS and ICPmeasurements. In addition, ICP-AES and EDS measure-ments on a powder sample of bulk CdSe yielded, as expected,

Cd/Se=1:1. On the basis of the reproducibility of measuredCd/Se ratios using different sample preparations and differenttechniques and, evenmore convincingly, upon the disappear-ance of Cd enrichment upon switching from 90% TOPO to99% TOPO in the reaction mixture, we conclude that ourobserved Cd/Se ratios are not artifacts of Se volatilization.

The correlation between the presence of alkylphospho-nates and Cd enrichment can be explained by the strength ofthe bond between Cd2þ and a phosphonic acid; whether theacid is protonated or deprotonated, this bond is calculated tobe the strongest ligand-QD bond in this system.13,14 TheCd2þ-OPAcomplex is therefore themost effective surfactantfor stabilizing the high-energy interface between the polar QDand the nonpolar organic solvent. We suspect that when OPAis present in the reactionmixture, the system is driven to formCd-enriched surfaces in order to minimize the free energy ofthe QD-organic interface, and when OPA is absent, that is,when 99% TOPO is used as the coordinating solvent, severaltypes of interfaces with similar free energies, such asCd2þ-HDA, Cd2þ-TOPO, and Se2--TOP, compete to formsurfaces with Cd/Se ≈ 1:1. We note that we could not repro-duce the values of Cd/Se for 90% TOPO QDs by addingincreasing concentrations (>1.3%) of OPA to 99% TOPO.This result is not surprising as others have shown that otherimpurities in 90% TOPO besides OPA mediate the growthdynamics of CdSe QDs.10

We obtained evidence that, as asserted previously,2,3,6 theCd enrichment of the QDs occurs at their surfaces by etchingthe surfaces of the QDs using an excess of OPA (500:1 OPA/QD). ExposingpurifiedCdSeQDs to this concentrationofOPAfor 2 h shifted the band edge absorption peak of QDswithR=2.0 nm from529nm to 525nm; this shift indicates a 0.05 nmreduction in the average diameter of theQDs in the ensemble.Concurrent with the shift in the absorption peak, etching thesurface resulted in a decrease in the value of Cd/Se from2.9:1to 1.7:1; see Supporting Information. The relatively smallchange in excitonic radius coupled with the large decreasein Cd/Se upon etching suggests that (i) the cadmium enrich-ment is localized on the surface of the QD and (ii) the excitondoes not fully extend into the cadmium-enriched surfaceregion that was depleted by etching.

We derived an expression for the dependence of Cd/Seon R (fit lines in Figure 1) by modeling the QD as a core withCd/Se=1:1 surrounded by a Se-free shell (of thickness A) ofligand-stabilized Cd2þ ions. Within this model, the number ofatoms of a certain element in a given region of the QD (eithercore or shell) is the product of the number density, F, of theelement in that region and the volume of the region, V. Then,Cd/Se for a QD is given by eq 1, where FSe,shell=0, and for aCdSe wurtzite crystal, FCd,core=FSe,core=15.4 nm-3.

CdSe

¼ VcoreðFCd;coreÞþVshellðFCd;shellÞVcoreðFSe;coreÞ

¼ 1þ R3 -ðR-AÞ3ðR-AÞ3

!B

FCd;coreð1Þ

ExpressingVcore andVshell in termsofRand the shell thicknessas A yields the second equality in eq 1, where B=FCd,shell.

Figure 1. (A) ICP-AES-measured Cd/Se ratios within QDs synthe-sized with CdSt2 and 90% TOPO (black), CdO, stearic acid, and90% TOPO (green), CdSt2 and 99% TOPO (red), and CdSt2 and99% TOPO plus 1.3 mol % OPA (blue), as a function of radius, R.The black lines are fits of the data to eq 1. (B) TEM images of QDssynthesized with 90% TOPO (R = 3.3 ( 0.3 nm) (top) and with99% TOPO (R = 2.1 ( 0.2 nm) (bottom). Scale bars represent20 nm in the large images and 5 nm in the insets.

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Cd/Sef1 asRf¥, aswewould expect sinceVshell,Vcore forlarge R. There exist several options to fit the plot of Cd/Seversus R using eq 1. If we assume that both A and B areconstant with R, then fitting the data in Figure 1 to eq 1 leadsto an unphysical value of A (=1.4 nm). If we assume thatshell thickness A is constant with R and shell density B varieswith R (since it is reasonable that the geometry and thereforepacking density, B, of Cd-OPA complexes at the surfaceshould depend on the radius of curvature of the QD), thenwe can derive a function B(R) that, when inserted into eq 1,results in the fit lines shown in Figure 1. The SupportingInformation contains details of the fitting procedure usedto derive B(R). For A=0.5 nm (approximately 1.5 times theinternuclear distance between two Cd2þ ions in a dinuclearorganometallic complex15), the value of B=FCd,shell that weobtain for R=2.7 nm, for example, is 10 nm-3 for the 90%TOPOQDs and 3 nm-3 for the 99%TOPOþ 1.3%OPAQDs.We emphasize that it is probable that both A and B arechanging with R; therefore, although this model presents amathematical justification for the core-shell picture of Cd-enriched QDs, it does not provide quantitative informationabout the nanoscopic structure of the Cd-OPA shell.

Figure 2A summarizes our proposed model for the rela-tionship between the ligands present in the reaction mixtureand the resulting Cd/Se ratio of the QDs. Those QDs with aCd/Se ratio of ∼1:1 result from synthesis without alkylphos-phonates, that is, with 99% TOPO, and therefore are coordi-nated primarily to L-type (neutral) surfactant molecules, suchas HDA, and a small number of X-type stearate ligands.1 Thisassertion is confirmed by Figure 2B, a plot of the number ofphosphorus-containing ligands per unit surface area of theQDs synthesized with 99% TOPO (red), as calculated fromP/Cd measured with ICP-AES.1 In contrast, the surfaces ofCd-enriched QDs are saturated with alkylphosphonates(Figure 2B, black and green), assuming a footprint of 0.075nm2 per alkylphosphonate ligand.

Trioctylphosphine oxide is a widely used reagent in colloi-dal QD syntheses. This work demonstrates that both theoverall elemental ratios within semiconductor QDs and thechemical composition of their surfaces can be controlled by

changing the concentrations of charged surfactants in TOPO,and in the reaction mixture in general. The presence of tight-binding alkylphosphonates during the synthesis of colloidalCdSe QDs yields QDs that are increasingly Cd-enriched asR decreases. These QDs can be modeled as stoichiometric(Cd/Se= 1:1) cores capped with disordered shells of Cd-alkylphosphonate complexes. Since the cadmium enrich-ment is on the surface of the QDs, the degree of enrichmentwill affect experiments and applications that involve thechemistry and electronic properties of these surfaces. Strate-gies like those presented here for synthetic manipulation ofthenanoscopicorganic/inorganic interfacesof semiconductorQDs are therefore useful for controlling interfacial processessuch as charge and energy transfer and further chemicalfunctionalization of QDs with polymers and biomolecules.We are currently investigating the influence of other X-typeligands on the growth and elemental composition of QDsand exploring methods to directly probe the surface ofCd-enrichedQDs in order to optimize themodel presented hereand further test our mechanistic hypothesis for Cd enrichment.

SUPPORTING INFORMATION AVAILABLE Details of syn-thesis, purification, and characterization of QDs and TOPO, calibra-tion curve to determine R from λabs, derivation of B(R), etching, andFigures S1-S10. This material is available free of charge via theInternet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author:*To whom correspondence should be addressed. E-mail: e-weiss@northwestern.edu.

ACKNOWLEDGMENT This material is based upon work suppor-ted under a NSF Graduate Research Fellowship (A.J.M.-C.). G.D.L. issupported as part of the Nonequilibrium Energy Research Center(NERC), an Energy Frontier Research Center funded by the U.S.Department of Energy, Office of Science, Office of Basic EnergySciences (DE-SC0000989).

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