ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2009, том 83, № 9, с. 1687-1690
^ STRUCTURE OF MATTER ^^^^^^^^^^
AND QUANTUM CHEMISTRY
УДК 539.192
OPTICAL PROPERTIES OF CdSe AND CdSe/ZnS QUANTUM DOTS DISPERSED IN SOLVENTS OF DIFFERENT POLARITY
© 2009 N. D. Abazovic, J. Z. Kuljanin-Jakovljevic and M. I. Comor
Vinca Institute of Nuclear Sciences, 11001 Belgrade, PO Box 522, Serbia e-mail: mirjanac@vinca.rs
Abstract — Original organic capping TOPO/TOP groups of CdSe and CdSe/ZnS quantum dots (QDs), from mother solution were replaced with 2-mercaptoethanol, which was chosen as model compound, in order to achieve water solubility. Obtained water dispersions of CdSe and CdSe/ZnS QDs were characterized by UV/VIS absorption and luminescence techniques. Luminescence measurements revealed that bare cores are very sensitive to surface capping, transfer into water diminished emission intensity. Core/shell, CdSe/ZnS, QDs are much more resistant to changes of the capping and solvent, and significant part of emission intensity was preserved in water.
INTRODUCTION
One of the major goals in cell biology is realizing complex interactions between biomolecules. In these studies, organic dyes are commonly used for fluorescent labeling of biomolecules what enables following of reaction pathways and mechanism [1]. However, this approach has significant limitations, as organic fluorophores undergo fast photobleaching and have narrow excitation spectra. Furthermore, they often exhibit broad emission spectra with red tailing and therefore are limited in applications involving long-term imaging and multicolor detection.
In the past decade semiconductor quantum dots (QDs) have emerged as principal candidates in replacing organic fluorophores in biological labeling. Major advantage of QDs is possibility of tuning emission spectral range, especially position of max intensity, by changing their size, due to quantization effect — optical and electronic properties are critically influenced by carrier confinement (electron/hole). A broad absorption (excitation) spectrum permits excitation of mixed QDs populations at a single wavelength [1, 2].
Up to now, II—VI group semiconductors have exhibited the most suitable characteristics for biolabeling applications. Beside this, it has been shown that overcoating QDs with higher band gap inorganic materials improves the photoluminescence quantum yields by passivating surface nonradiative recombination sites.
The organic ligand bound to the surface of colloidal QDs plays an important role in determining their electronic and optical properties. A mixture of trio-ctylphosphine and trioctylphosphine oxide (TOP/TOPO) is the prototypical ligand system for most II—VI semiconductor systems, such as CdSe. These ligands provide colloidal stability in common organic solvents such as hexane and chloroform. However, main request for usage of QDs for biological
labeling is to enable their solubility in water. There are three major ways to achieve this objective: a) by exchanging surface ligand with thiol-derived silane — performing surface silanization, b) by coating the surface with amphiphilic polymers or c) by exchanging the hydrophobic surfactant (TOP/TOPO) with bi-functional molecules [3, 4]. Most often, mercaptocar-boxylic acids have been used for this purpose. Mercap-tocarboxylic acids have mercapto (-SH) group at one side, which can be easily anchored to surface metal ion, and carboxylic (-COOH) group at the other end which enables water solubility of QDs.
Because the final size of QD and its shell is important parameter which determinates possible application, in process of transfer of QDs in aqueous media great concern should be given to both: initial size of synthesized QDs and size/length of chosen bifunc-tional molecule.
In this article, we report a study of the optical properties of CdSe and CdSe/ZnS QDs, that has been synthesized in noncoordinating organic solvent (TOP/TOPO) at high temperature, dispersed in nonpolar solvent and water. Water solubility have been achieved by substitution of surface attached TOPO by 2-mecraptoethanol (Scheme) which has been chosen as model compound due to its short hydrocarbon chain. We show that capping of QDs with 2-mecrapto-ethanol readily produces stable water dispersions.
EXPERIMENTAL
All reagents were commercial products of highest purity available (p.a.).
Synthesis of CdSe QDs. Highly crystalline CdSe QDs were synthesized by method previously described by Peng and Peng 5]. Briefly, CdO, TOPO and HPA were loaded in 25 ml flask. The mixture was heated to
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ABAZOVIC et al.
CH3(CH2)7)3-P TOP
'CdSeN Se Cd—O=
P-((CH2)7CH3)3 TOPO
'CdSe^
CH3(CH2)7)3-P—Se Cd-SH(CH2)2OH TOP 2-mercaptoethanol
CH3(CH2)7)3P-TOP
. (CdSe);
S (CdSe)Zn-O=P((CH2)7CH3)3 ZnS.
TOPO
RESULTS AND DISCUSSION
The absorption spectra of CdSe and CdSe/ZnS QDs are presented in Fig. 1A and B. The positions of the absorption threshold give us band gap energy of CdSe colloidal particles; CdSe bare cores have Eg ~ 2.2 eV and CdSe/ZnS core/shell particles have Eg ~ 2.1 eV (bulk Eg = 1.79 eV). Also, overcoating of CdSe QDs with shell of ZnS led to changes in shape and position of peak in absorption spectrum (Figure, a). First exciton absorption peak moved from 543 to 553 nm and its FWHM increased. We used empirical formula proposed by Yu et al. [7], that describes connection between diameter of the CdSe particles and wavelength of the their first absorption peak to calculate diameter of the bare and capped CdSe:
D = ( 1.6122 x 10-9)V - (2.6575 x 10-6+ + ( 1.6242 x 10-3)^2 - (0.4277)X + 41.57.
; (Cds^z
CH3(CH2)7)3P-S CSe)Znj-SH(CH2)2OH TOP V ZnS^^2-mercaptoethanol
Scheme. Exchange of hydrophobic surfactant on surface of CdSe and CdSe/ZnS QDs.
300—320°C under Ar flow. Selenium stock solution was injected in the mixture at 270°C and left nanoc-rystals to grow for 4 minutes.
Synthesis of CdSe/ZnS core/shell QDs. Roughly, 0.1 ^mol of CdSe QDs dispersed in hexane was transferred into reaction flask with dry TOPO and solvent was pumped out. Then, temperature of the solution was raised to 160°C, and precursors of Zn and S dissolved in TOP were added dropwise over a period of 5 minutes. Concentrations of the precursors were adjusted to be enough for 3 monolayers of ZnS in the schell. The overcoated particles were stored in their mother solution during the night to ensure that the surface of the dots remained passivated with TOPO [6].
Extraction of QDs in water. Equal volumes of CdSe or CdSe/ZnS QDS dispersed in hexane and 0.01 M aqueous solution of 2-mercaptoethanol (pH 8—9) were mixed. Two separated layers were formed, hexane (colored) layer was on the top. After slight shaking, QDs were transferred in aqueous (bottom) layer, and it was used as is for optical measurements.
Optical characterization. Measurements of the absorption spectra were carried out on an Evolution 600 UV—Vis spectrophotometer (Thermo Scientific). The photoluminescence spectra were recorded on Perkin-Elmer LS—45 spectrofluorimeter. Quantum yields of emission were calculated using Tris[2,2'-bipyridyl] Ruthenium(II)chloride as standard.
(1)
Here D (nm) is the size of the CdSe cores and X (nm) is the wavelength of the first absorption peak of the corresponding sample. Calculated diameters of bare and capped CdSe QDs were about 2.9 and 3.1 nm, respectively. Since ZnS is transparent in visible part of the spectrum, change of diameter indicated that CdSe QDs sizes seem to be increased and their size distribution is wider in comparison to bare CdSe QDs. Applied temperature for overcoating of CdSe cores with ZnS is too low for Oswald ripening of CdSe QDs. Observed change of diameter can be explained by partial leakage of the excitons from CdSe into the ZnS matrix. This effect is more pronounced in smaller CdSe QDs where the leakage of the exciton into the ZnS shell has a more dramatic effect on the confinement energies of the charge carriers [6]. The difference between diameters of CdSe in bare and core/shell nano-particles is about 2 A, that correspond to homogenous shell of1 A through which exciton freely leak, just 30% of ZnS monolayer (the definition of a monolayer is a shell of ZnS that measures 3.1 A along the major axis of the dots). Exchange of capping agent of bare CdSe QDs and their extraction in water didn't affect shape of absorption spectrum (Figure, a and b). Absorption spectrum of CdSe/ZnS QDs lost its original shape and dispersion became turbid (Figure, a and b), but colloidal solution retain stability.
Changes in emission spectra (Figure, c and d) are more prominent. Emission intensity of core/shell QDs increased 10 times (QY= 4.2%) compared to bare CdSe and emission peak moved from 564 to 582 nm. Exchange of capping agent and extraction in water didn't affect shape of emission spectrum just its intensity. Unfortunately, extraction of bare CdSe QDs in water led to disappearance of characteristic emission peak, Figure c (in chloroform QY ~ 0.42%). CdSe/ZnS QDs extracted in water retain a measurable part of their original emission (QY = 2.55%). Reduc-
OPTICAL PROPERTIES OF CdSe AND CdSe/ZnS
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Absorbance
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Absorption spectrum of (1) CdSe and (2) CdSe/ZnS QDs dispersed in (a) chloroform and (b) water; emission spectrum (A,exp = = 450 nm) of (c) CdSe and (d) CdSe/ZnS QDs dispersed in (1) chloroform and (2) water.
tion of emission QYof CdSe and CdSe/ZnS QDs upon capping their surface with 2-mercaptoethanol could be explained through mechanism of formation of disulfide bonds. As Kloepfer et al. [8] have proposed in their study of MAA capped CdSe QDs, surface bonded mercapto-compound can scavenge photocreated hole, which subsequently leads to cleavage of Cd — S bond and formation of mercapto-ion. So-formed ion reacts with neighboring mercapto-molecule and between them dis
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