Optical transitions in polarized CdSe, Cd Se ∕ Zn Se , and Cd Se ∕ Cd S ∕ Zn S quantum dots dispersed in various polar solvents Ung Thi Dieu Thuy, Nguyen Quang Liem, Do Xuan Thanh, Myriam Protière, and Peter Reiss Citation: Applied Physics Letters 91, 241908 (2007); doi: 10.1063/1.2822399 View online: http://dx.doi.org/10.1063/1.2822399 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Homogeneous and inhomogeneous sources of optical transition broadening in room temperature CdSe/ZnS nanocrystal quantum dots Appl Phys Lett 105, 143105 (2014); 10.1063/1.4897347 Optical study of strongly coupled CdSe quantum dots J Vac Sci Technol B 28, C3D17 (2010); 10.1116/1.3290748 Voltage-dependent electroluminescence from colloidal Cd Se ∕ Zn S quantum dots Appl Phys Lett 91, 243114 (2007); 10.1063/1.2824397 Spin polarization of self-assembled CdSe quantum dots in ZnMnSe Appl Phys Lett 83, 4604 (2003); 10.1063/1.1630381 Transitions in ZnS and CdSe quantum dots and wave-function symmetry J Chem Phys 118, 5937 (2003); 10.1063/1.1557178 This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 129.127.200.132 On: Wed, 10 Dec 2014 18:00:39 APPLIED PHYSICS LETTERS 91, 241908 ͑2007͒ Optical transitions in polarized CdSe, CdSe/ ZnSe, and CdSe/ CdS / ZnS quantum dots dispersed in various polar solvents Ung Thi Dieu Thuy, Nguyen Quang Liem,a͒ and Do Xuan Thanh Institute of Materials Science (IMS), Vietnamese Academy of Science and Technology (VAST), 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Myriam Protière and Peter Reiss DRT/LITEN/DTNM/L2 T and DSM/DRFMC/SPrAM (UMR 5819 CEA–CNRS–UJF 1)/LEMOH CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France ͑Received September 2007; accepted 16 November 2007; published online 12 December 2007͒ The optical transitions in ensembles of colloidal CdSe-based quantum dots ͑QDs͒ have been systematically studied as a function of the net QDs’ polarity/polarization and of the solvent’s polarity While the general trend observed for all QD systems dispersed in different solvents is similar, the spectral shifts are more pronounced in core QDs than in core/shell structures Our results can be rationalized by taking account of the electric field experienced by the QDs that results from their effective polarization in solvents of different polarities ͑quantum confined Stark effect͒ as well as from the effect of the external dielectric environment ͑solvatochromatic effect͒ © 2007 American Institute of Physics ͓DOI: 10.1063/1.2822399͔ Various QDs of II-VI and III-V semiconductors have been studied intensively in the past decade for their promising applications in biological labeling1–3 and optoelectronic devices.4–8 CdSe-based core and core/shell QDs have been prepared by different methods using organometallic dimethylcadmium9–11 or, more recently, nonpyrophoric CdO or cadmium carboxylate precursors.12–14 High-quality CdSe QDs are characterized by their narrow photoluminescence ͑PL͒ spectra and a sharp excitonic absorption peak.9–14 We note that different solvents have been used to disperse hydrophobic colloidal CdSe QDs capped with long alkyl chain containing organic surfactants In some reports, nonpolar solvents such as n-hexane were used,11 while elsewhere more polar solvents such as toluene,13,15 chloroform,16 or even n-butanol9 were applied This observation stands out against the fact that the solubility parameter of QDs in solvents of significantly different polarities cannot be the same The growth of larger band gap semiconductor shells on the surface of CdSe QDs is an established method for increasing their PL quantum yield ͑QY͒ from some percent to values in the range of 50%–85% The inorganic shell, consisting, for example, of ZnSe,14 ZnS,10,11 or of a ZnSe/ ZnS15 or CdS / ZnS double epilayer,17 efficiently passivates fast nonradiative decay channels originating from surface states.18,19 However, core/shell systems still exhibit PL QY variations upon dispersion in solvents of different polarities Another phenomenon is at the origin of this behavior, namely, the quantum confined Stark effect, occurring on the nanometer scale with the contributions of the electric field induced by polarization of QDs, ligand molecules, polarity of solvent, and surface states.20 This effective electric field on the QDs could imprints clearly on the optical transitions with the shifts of the PL and absorption spectra For colloidal CdSe QDs in solvents with different dielectric constants, one should also take account of solvatochromism which induces redshift of the absorption spectrum.21 a͒ Author to whom correspondence should be addressed Also at: College of Technology, Hanoi National University, 144 Xuan Thuy, Hanoi, Vietnam Electronic mail: liemnq@ims.vast.ac.vn The aim of this letter is to address in detail the spectral shifts caused by the Stark effect and solvatochromatic effect on CdSe-based QDs capped with organic ligands and dispersed in solvents of different polarities The peak shifts in the PL and absorption spectra taken from the polarized ligands capped CdSe QDs as a function of the solvent’s polarity and therefore as a function of the number of remaining surface ligands have been measured and compared Furthermore, particular attention has been paid to polarization effects not only of bare CdSe QDs but also of core/shell ͓CdSe/ ZnSe ͑CS͔͒ and core/double-shell QDs ͓CdSe/ CdS / ZnS ͑CSS͔͒ Finally, we propose a model, which describes qualitatively the dispersibility of QDs in solvents of varying polarity Monodisperse CdSe and CdSe/ ZnSe QDs were prepared following the procedure described in Ref 14 The synthesis of CdSe/ CdS / ZnS CSS QDs was carried out using exclusively air-stable metal precursors, namely, cadmium ͑zinc͒ ethylxanthate and stearate.22 Depending on the number of purification cycles ͑vide infra͒, the samples were dispersed in n-hexane, toluene, chloroform, n-butanol, or water, which have polarities ͑dielectric constants͒ of ͑2͒, 0.37 ͑2.4͒, 1.01 ͑4.8͒, 1.66 ͑18͒, and 1.85 ͑80͒, respectively Absorption spectra were recorded in the cm cells using a Cary 5000 UV-vis-NIR spectrometer In PL measurement, several excitation sources were used such as 370 or 460 nm light-emitting diode ͑LEDs͒ The excitation power density was weak, about 100 W / cm2 in all cases The PL signals were dispersed by a 0.6 m grating monochromator ͑JobinYvon HRD1͒ and then detected by a thermoelectric cooled Si charge-coupled device ͑CCD͒ camera ͑Hamamatsu͒ As-prepared core, CS, or CSS QDs were separated from the reaction medium by precipitation with methanol, followed by centrifugation, resulting samples denominated 0-cleaning-cycle QDs Depending on the reaction conditions, the surface of these QDs was passivated by different types of molecular ligands such as trioctylphosphine ͑TOPO͒, hexadecylamine ͑HDA͒, or stearate Consequently, the QDs are soluble in rather nonpolar solvents ͑n-hexane and toluene͒, hardly soluble in chloroform, and insoluble in n-butanol or This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 0003-6951/2007/91͑24͒/241908/3/$23.00 91, 241908-1 © 2007 American Institute of Physics 129.127.200.132 On: Wed, 10 Dec 2014 18:00:39 241908-2 Appl Phys Lett 91, 241908 ͑2007͒ Thuy et al FIG ͑Color online͒ PL and absorption spectra of as synthesized CdSe QDs in hexane ͑a͒ and the absorption ͑Abs͒ and PL peak positions as functions of the solvent and the number of cleaning cycles ͑b͒ In the inset, Abs ͑PL͒ ͑2͒ means absorption ͑PL͒ spectra for the one-͑two͒ cleaning-cycle QDs The data points for Abs ͑PL͒ corresponding to the QDs in water are not shown because these one-cleaning-cycle QDs are not dispersible in water cleaned CdSe QDs are located at lower energy This is diwater The PL QY from as-synthesized CdSe QDs is typirectly resulting from the electric dipoles’ cancellation becally rather high ͑40%͒ tween the net polarization of the QD-ligands complex and Upon the use of a well-defined purification procedure to the polarity of the solvent The resulting polarization is supremove organic surfactant molecules from the QDs surface, posed to affect the optical transitions inside the QD in a the QDs could be dispersed in polar solvents such as chlorosimilar way as the Stark effect In single CdSe QDs, the form, n-butanol, and even water due to the enhanced net Stark shift of absorption induced by intentionally applying polarity of the QDs that induced by complex of QDs polaran external electric field was studied by Empedocles and ization and surfactant molecules The applied procedure conBawendi.23 sisted in the dispersion of ϳ5 mg of the zero-cleaning-cycle In order to study the Stark shifts without interference QDs in ml of toluene, followed by an ultrasound treatment from surface states, the PL and absorption spectra for well͑55 kHz, min͒ and centrifugation ͑6000 rpm, min͒ to passivated two-cleaning-cycle CdSe/ CdS / ZnS CSS QDs in eliminate any nonliquid residual Subsequently, ml of different solvents were measured Figure shows the specmethanol were added, resulting in the flocculation of the tral shifts in different solvents that follow a similar trend as QDs, which were collected by centrifugation ͑12000 rpm, those of the CdSe QDs ͑Fig 1͒: The spectral shift in chloro5 min͒ In the following, the obtained samples are denomiform is minimum, corresponding to the least electric field nated one-cleaning-cycle QDs and repeating the described experienced by the QDs For the solvents of lower and procedure led to two-cleaning-cycle QDs From the practical higher polarities, the peaks were shifted to lower energy, point of view, it is important to note that with increasing indicating a stronger Stark effect Once again, we attribute number of cleaning cycles QDs, they can be dispersed in the observed behavior to the net electric field applied to the solvents of increasing polarity We attribute this behavior to QDs However, as compared to the bare CdSe QDs, the obthe fact that the net polarity of the inorganic part of the QDs served spectral shifts of CSS ones are much less pronounced becomes more evident upon successive surface ligand reThis can be explained by the contribution of the shell thickmoval As a result, the one-cleaning-cycle QDs could easier ness ͑1.5 nm͒ that reduces the influence of the solvent’s pobe dispersed in chloroform, which is a polar solvent with a polarity of 1, than in n-hexane or in toluene After two cycles larity on the polarized CdSe core CdSe/ ZnSe QDs of comof cleaning, all kinds of CdSe and ZnSe- or parable shell thickness exhibited a very similar behavior as CdS / ZnS-capped CdSe QDs could readily be dispersed in the CSS QDs chloroform We noticed that our two-cleaning-cycle QDs Figure shows a model proposed for the net polarization could easily be dispersed in n-butanol, methanol, and even in from the QD-ligands complex in solvent In fact, in CdSe water upon a short ͑5 min͒ application of ultrasound This is QDs built up from different kinds of elements ͑Cd and Se͒, the number of the surface atoms is large as compared to the an important characteristic applicable to various experiments volume atoms Depending on the termination and stoichiomwhere water-soluble QDs are needed, without necessity to etry at the QD surface, an electric dipole moment, or charge, ligand exchange ͑e.g., with mercaptocarboxylic acids͒ polarization corresponding to the QD results.20,24,25 Several Along with the cleaning process comprising the partial papers reported on the permanent dipole moment of nonmeremoval of surface ligands from the QDs, we observed distallic nanoparticles ͑ZnSe and CdSe͒ that always exists even tinct shifts of the spectral features in both PL and absorption if they crystallized in the highly symmetric zinc-blende spectra Figure summarizes the observed shifts for CdSe structure.24,26 Ligand molecules of surfactant type could act QDs in different solvents after one and two cleaning cycles as a neutralization buffer for the QD’s polarity The neutralThe PL and absorption peaks for cleaned CdSe QDs dispersization rate depends on the dipole moment ͑polarization͒ of ing in chloroform are located at the shortest wavelength In the QD and on the number of ligand molecules on its surboth kinds of solvents with higher or lower polarity with This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: face Depending on the number of cleaning cycles and hence respect to chloroform, the PL and absorption peaks from 129.127.200.132 On: Wed, 10 Dec 2014 18:00:39 241908-3 Appl Phys Lett 91, 241908 ͑2007͒ Thuy et al FIG ͑Color online͒ PL and absorption spectra of the CdSe/ CdS / ZnS CSS QDs in hexane ͑a͒ and the absorption ͑Abs͒ and PL peak positions after two cleaning cycles in solvents of different polarity ͑b͒ the average number of ligand molecules per QD, the net polarization varies that induces the shift of the QD energy levels due to essentially the Stark effect The Stark shift observed in the present experiment is very large, namely, about 60 meV in the case of CdSe QDs in hexane with respect to those in chloroform, or at least of 30 meV in the case of CdSe/ CdS / ZnS CSS QDs in a similar comparison These shifts are very large as compared to the solvatochromatic shift that originates from the effect of the external dielectric environment, which is usually below 10 meV.21 In conclusion, we have studied the PL and absorption behaviors of CdSe, CdSe/ ZnSe, and CdSe/ CdS / ZnS QDs dispersed in solvents of different polarities Our experiments revealed that all types of QDs possess a natural polarization that make them dispersible in polar organic solvents and even in water upon partial removal of the organic surface ligands by an appropriate cleaning procedure Due to the net polarization of the QDs and ligands complex, Stark effect induced shifts of the PL and absorption spectra as a function of the solvent’s polarity took place These effects occurred on CdSe QDs without inorganic shell and, to a lower extent, on those with single ͑CdSe/ ZnSe͒ and double shells ͑CdSe/ CdS / ZnS͒ The authors thank Professor N.V Hieu and professor L.V Hong for helpful discussions and L.Q Phuong for technical assistance The Basic Research Programme in Natural Science ͑MOST Vietnam͒ and the Materials Science Direction ͑VAST͒ are gratefully acknowledged for financial support M Han, X Gao, J Z Su, and S Nie, Nat Biotechnol 19, 631 ͑2001͒ A P Alivisatos, Nat Biotechnol 22, 47 ͑2004͒ I L Medintz, H T Uyeda, E R Goldman, and H Mattoussi, Nat Mater 4, 435 ͑2005͒ S Coe, W K 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indicated in the article Reuse of AIP content is subject4310 to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: ͑2004͒ respectively 129.127.200.132 On: Wed, 10 Dec 2014 18:00:39 ... ͑2007͒ Optical transitions in polarized CdSe, CdSe/ ZnSe, and CdSe/ CdS / ZnS quantum dots dispersed in various polar solvents Ung Thi Dieu Thuy, Nguyen Quang Liem,a͒ and Do Xuan Thanh Institute... CdSe/ ZnSe, and CdSe/ CdS / ZnS QDs dispersed in solvents of different polarities Our experiments revealed that all types of QDs possess a natural polarization that make them dispersible in polar. .. QDs in solvents of varying polarity Monodisperse CdSe and CdSe/ ZnSe QDs were prepared following the procedure described in Ref 14 The synthesis of CdSe/ CdS / ZnS CSS QDs was carried out using