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Homogeneously Alloyed CdSe1-xSx Quantum Dots (0 #x # 1): an Efficient Synthesis for Full Optical Tunability

Tangi Aubert, Marco Cirillo, Stijn Flamee, Rik Van Deun, Holger Lange, Christian Thomsen, and Zeger HensChem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401019t • Publication Date (Web): 15 May 2013

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Homogeneously Alloyed CdSe1-xSx Quantum Dots (0 ≤ x ≤ 1): an Efficient Synthesis for Full Optical Tunability

Tangi Aubert,†,‡,* Marco Cirillo,†,‡ Stijn Flamée,†,‡ Rik Van Deun,# Holger Lange,$,§ Christian Thomsen,$ Zeger Hens†,‡,*

† Physics and Chemistry of Nanostructures,

‡ Center for Nano- and Biophotonics (NB-Photonics) and

# L

3 – Lumi-

nescent Lanthanide Lab, f-element coordination chemistry, Ghent University, Belgium $ Institut für Festkörperphysik, TU Berlin, Germany

KEYWORDS: nanocrystals, solid solution, alloy, Raman.

Over the last 20 years, semiconductor quantum dots (QDs) have attracted wide interest as versatile lumines-cent materials, finding numerous applications in fields ranging from photonics to biotechnology.1-4 Tuning the emission color of QDs is usually done by varying their size. However, to meet the constantly increasing demands of their applications, a decoupling of QD size and emis-sion color is needed. This is for example the case for LEDs that benefit from close packed QD layers, or for multicol-or bioimaging.2, 5 To provide same-size-different-color QDs, syntheses of alloyed QDs (allQDs) have been re-cently developed.6 Alloying provides an additional degree of freedom as it allows for a fine tuning of the emission wavelength by varying the QD composition rather than the size. In particular, II-VI alloys such as Cd(Se,Te), Cd(Se,S), Cd(Te,S) or (Zn,Cd)Se have been investigated,6-

10 while (Zn,Cd)(Se,S) proved to offer emission wave-length tunability from 440 to 650 nm.11 Next to wave-length tunability, allQDs were also reported to have high photoluminescence quantum yield (PLQY),12 and low blinking rates.13, 14

A major challenge in the synthesis of colloidal allQDs is to obtain a random distribution of the elements over the nanocrystal. Indeed, having true solid solution QDs con-stitutes a prerequisite to investigate the unique optical properties of these novel nanomaterials. According to the literature, this requires a balancing of the reactivity of the different precursors.15 To demonstrate homogeneous alloying, electron-microscopy techniques are often com-bined with elemental analyses.11, 16 However, these meth-ods are only reliable for larger QDs and can only address a small number of nanocrystals. Alternative, ensemble level methods that apply to small QDs as well are therefore still needed. In the case of II-VI allQDs, most reported synthe-ses make use of tri-octylphosphine (TOP) based chalco-gen precursors, which are oxidation sensitive and toxic. An alternative approach is based on precursors with a low reactivity, resulting in a low yield reaction where the size of the QDs can only be controlled by the reaction time.11

Here, we report on a new, fast and phosphine-free syn-thesis of homogeneously CdSe1-xSx allQDs (0 ≤ x ≤ 1) and we introduce Raman spectroscopy as a tool to analyze

homogeneous alloying in colloidal QDs. The synthesis makes use of the hot injection of a heterogeneous disper-sion of Se in ODE, which we recently reported and pa-tented for making several kinds of metal selenide QDs.17-19 We find that the reactivity of this heterogeneous ODE-Se toward the Cd precursor matches that of S homogeneous-ly dissolved in ODE. Tracking the frequency and the width of the longitudinal optical phonon resonance as a function of composition using Raman spectroscopy, we show that combining both precursors yields homogene-ously CdSe1-xSx allQDs. Importantly, the synthesis reaches close to full yield in no more than 5 minutes, even at high solid loading, and the size of the QDs at the end of the reaction can be reproducibly controlled by using carbox-ylic acids of different length. Growth of a CdS shell around CdSe1-xSx allQDs can raise the PLQY to 40% or more, yet the higher the sulfur content, the lower the PLQY increase, an effect we attribute to an enhanced hole delocalization.

In a typical synthesis, CdSe1-xSx allQDs are synthesized by injecting a mixture of heterogeneous ODE-Se and homogeneous ODE-S solutions under ambient conditions in a reaction mixture made by dissolving CdO in ODE using myristic acid and kept at 270 °C. After 5 minutes of reaction at 260°C, the reaction was quenched, the QDs were purified by precipitation and centrifugation and the ligands were exchanged for oleic acid (see Supporting Information (SI), paragraph 1.1). Figure 1a shows the (normalized) yield development – as obtained by UV-Vis analysis of weighted aliquots – for syntheses where only ODE-Se or ODE-S are used as precursors, yielding pure CdSe and pure CdS, respectively. It follows that even if the ODE-Se precursor has a higher initial rate, both reac-tions have an overall comparable development, reaching more than 92% of the respective final amount of moles after 5 minutes. One can thus expect that simply mixing the Se and S precursor in the reaction may lead to a ho-mogeneous alloying of these two elements. Thus, CdSe1-

xSx QDs were synthesized over the entire composition range (0 ≤ x ≤ 1) and systematically analyzed by X-ray fluorescence (XRF) and energy dispersive X-ray spectros-copy (EDX) to determine the actual S/Se ratio of the

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Figure 1. (a) Normalized amount of CdSe and CdS in moles as a function of the reaction time. (b) TEM micrograph of CdSe0.5S0.5 QDs. (c) Raman spectra and (d) LO frequency and FWHM of CdSe1-xSx QDs depending on the sulfur content.

samples. We find that the obtained S/Se ratio was always equivalent to the S/Se ratio of the solution injected in the reaction (see SI, Figure S1).XRF was also used to deter-mine the amount of unreacted precursors, from which we obtain reaction yields higher than 90%. As shown in Fig-ure 1b for the case of CdSe0.5S0.5 QDs, the synthesis results in quasi spherical and monodisperse QDs. Although it appears that the size of the QDs slightly decreases when increasing the sulfur content (see SI, Figure S2), all the sizes were centered around 3 nm with a size dispersion always below 10%. X-ray diffraction (XRD) analyses showed that the synthesized CdSe1-xSx QDs have a zinc blende structure as previously reported for CdSe QDs synthesized with the same procedure (see SI, Figure S3).17 In addition, the lattice parameter of these QDs decreases linearly with the amount of sulfur according to Vegard’s law,20 a first indication of homogeneous alloying.

To further confirm the homogeneity of the CdSe1-xSx al-loying, we used Raman spectroscopy since the depend-ence of the phonon frequencies on the reduced atomic mass of the atoms in the unit cell allows extracting the average lattice configuration. We fitted the fundamental Raman bands with two Lorentzian functions to account for the longitudinal optical phonon (LO) and surface optical phonon, while using a second order polynomial to fit the background.21 Figure 1c represents a set of back-ground-corrected spectra, normalized to the intensity of the LO related band (see SI, Figure S4 for larger figure, and Figure S5 for the fits to the LO-related band). Clearly, changing the S/Se ratio shifts the Raman bands, where incorporating the lighter S leads to an approximately linear shift of the LO frequency to higher energy (Figure 1d). Since the formation of minority domains would result in independent peaks at the CdSe or CdS frequency, this strongly supports a homogeneous distribution of Se and S atoms over the QD. Additionally, new bands appear in

Figure 2. Normalized (a) absorption and (b) emission spectra of CdSe1-xSx QDs with different sulfur content (see SI, Figure S6 for additional spectra).

Figure 3. (a) PLQY and (b) confinement energy of CdSe1-

xSx/CdS QDs depending on the number of CdS layers (see SI, paragraph 3.2 for determination of the confinement energy).

the Raman spectra at frequencies between the CdSe and CdS LO frequencies. This reflects the breaking of the translational invariance of the lattice in a random alloy, which lifts the q=0 Raman selection rule and thereby making previously forbidden vibrations afar from the LO phonon frequency Raman active.22-24 Additionally, non-longitudinal alloy vibrations incorporating both anions could be Raman active and result in additional peaks, as observed for CdSe0.5S0.5 alloys and confirmed by DFT cal-culations.21 These effects agree with published studies on alloyed semiconductors and isotope mixtures. Raman measurements of the zone-center optical mode in silicon with various isotopic compositions shows a linear fre-quency-mass dependence, whereas disorder-induced features appear.25 The same is valid for alloyed semicon-ductors, such as Ga1-xAlxAs/GaAs,22 ZnxCd1-xSe23 and InAl-GaN.14 Homogeneous alloying is further confirmed by the overall increase of the peak full width at half maximum (FWHM) with respect to the pure compounds, except for the CdSe0.5S0.5 LO peak. This indicates a very homogenous distribution of S and Se for this composition, whereas other S/Se ratios result in an increased asymmetry and a subsequent increase of the FWHM, in agreement with the expectations for homogeneous alloys.24, 26 Size variations generally also result in a broadened Raman peak due to a varying phonon confinement. We expect the contribute of this to be small for our samples due to weak confine-ment and rather narrow distributions. For the lowest S content (x=0.2), we find a LO frequency close to that of CdSe QDs. Here, a partial segregation of Se and S cannot be fully excluded, which could actually be due to the initially higher reactivity of the Se precursor as seen in Figure 1a.

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In terms of optical properties, all the CdSe1-xSx QDs showed well defined and narrow absorption and emission peaks (Figures 2a and 2b). Both show a non-linear blueshift with increasing sulfur content, reflecting the concomitant change of the band gap. The non-linear dependence of the band gap on composition is well-known for alloyed materials, with a quadratic relation reported for bulk CdSe1-xSx.

27 After synthesis, the CdSe1-xSx

QDs typically showed a PLQY below 10%. Apart from the band gap shift, increasing the sulfur content also raises the trap emission at longer wavelength, which is com-monly found for CdS QDs28 Similar to pure CdSe QDs synthesized with the same protocol,17 we found that the size of CdSe1-xSx QDs with given composition decreases when increasing the chain length of the carboxylic acid (see SI, Figure S7). As this results in a blueshift of the absorption and emission features (see SI, Figure S8), the synthesis method introduced here makes size and compo-sition two independent variables to tune the optical prop-erties of the allQDs synthesized, while keeping a high reaction yield.

To further improve the PLQY of these QDs, up to 7 lay-ers of CdS were grown on CdSe1-xSx QDs through a succes-sive ion layer adsorption and reaction procedure (SILAR, see SI, paragraph 1.2).29 All the synthesized core-shell CdSe1-xSx/CdS QDs were quasi-spherical and monodis-perse, with average sizes in agreement with the theoreti-cal number of CdS layers grown and showing well defined and narrow absorption and emission spectra (see SI, Fig-ures S9 and S10). Both the absorption and emission peaks shift to the red with the growth of the CdS shell, reflect-ing a decrease in the confinement energy. As expected, the PLQY of the CdSe1-xSx/CdS QDs increases with the growth of the first 3 CdS layers (Figure 3a) and then de-creases with further growth of thicker CdS shells, proba-bly reflecting the introduction of strain and defects in the heterostructures.21 However, the initial increase of the PLQY is less pronounced with S-rich alloyed cores. Since it concurs with an enhanced reduction of the confine-ment energy (Figure 3b), we attribute this to an enhanced delocalization of the hole that results from a reduced valence band offset between core and shell with increas-ing S content of the core (see SI).

In conclusion, we report on a new, fast and phosphine-free method for the synthesis of CdSe1-xSx allQDs with 0≤x≤1. The approach makes use of a heterogeneous ODE-Se precursor which shows similar reactivity towards the Cd precursor as a homogeneous ODE-S precursor. As confirmed by Raman spectroscopy, the synthesized CdSe1-

xSx QDs consist of random solid solutions, whose optical properties are composition-dependent. In addition, the emission wavelength can also be adjusted by changing the size of the QDs, which is readily achieved using carboxylic acids with a different chain length in the synthesis. In this way, the synthesis proposed here offers two independent parameters – size and composition – to tune the optical properties of QDs synthesized at close to full yield. We also showed that for CdSe1-xSx/CdS core-shell QDs, the efficiency of the shell in confining the exciton in the core

decreases when increasing the sulfur content, most likely due to a smaller interfacial valence band offset.

ASSOCIATED CONTENT

Supporting Information. Full synthesis and analytical details, additional XRF, EDX, XRD, TEM, PL, UV-vis absorp-tion, Raman analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

* E-mails: zeger.hens@ugent.be, tangi.aubert@ugent.be

Present Addresses § Department of Physics, Columbia University, United States.

ACKNOWLEDGMENT

The authors acknowledge BelSPo (IAP 7.35, photonics@be), FWO-Vlaanderen (project G.0760.12), German Research Foundation (project LA 2901/1-1), Max Kade Foundation, Hercules Foundation (project AUGE/09/024 “Advanced Luminescence Setup”) for financial support. Mathieu Pas-turel (UR1) is acknowledged for his help on XRD analyses.

REFERENCES

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