J. Phys. Chem. 1993,97, 9161-9170
9767
Stabilizer-Mediated Photoluminescence Quenching in Quantum-Confined Cadmium Sulfide Clusters Robin R. Chandler and Jeffery L. Coffer’ Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129 Received: April 26, 1993; In Final Form: July 1 , 1993’
In this account the influence of charge possessed by the stabilizing medium on the steady-state quenching behavior of quantum confined cadmium sulfide (Q-CdS) semiconductorclusters is reported. Q-cluster stabilizers such as the inverse micelle/hexametaphosphate (HMP) system and thiophenol caps, which afford an anionic solution/stabilizer interface, permit quenching of the integrated trap photoluminescence (PL) intensity (Amx between 550 and 650 nm) up to -45% (HMP) or 54% (thiophenol) by the cation methyl viologen ( M V ) . However, minimal quenching is observed upon addition of the anion iodide (I-), and virtually no quenching is observed following the addition of neutral molecules (amines and ketones). This is in stark contrast to Q-CdS clusters stabilized by macrocyclic aminocalixarene stabilizer molecules, which give rise to a cationic layer (-NR2H+) at the solution/stabilizer interface. Integrated PL from aminocalixarene stabilized Q-CdS clusters can be quenched up to 70% by the addition of I-; however, negligible quenching is observed upon the addition amines, MV2+, or ketones. These differences in Q-cluster photoluminescence behavior are discussed in terms of possible electrostatic interactions of the various quenchers with the cationic/anionic Q-CdS charge layers.
-
Introduction Quantum-confined semiconductor (Q-SC’s) possess a hybrid of molecular and bulk characteristics and exhibit a size-dependent bandgap that increases as the particle size becomes smaller.’ For the 11-VI material CdS, the onset of behavior consistent with quantum confinement is observed in clusters with diameters of 60 A or less.2 The synthesis of Q clusters is plagued by the tendency of the extremely small particles to rapidly agglomerate and precipitate out of solution or to grow to diameters which are well outside the regime of quantum confinement. For this reason, the synthesisof quantum-size semiconductorclusters necessitates the use of stabilizing media to prevent agglomeration. Many stabilizers which are commonly employed in Q-SC synthesis, including inverse micelles,’ polymers? glasses,S zeolites: thiols,’ and Langmuir-Blodgett filmss afford an “anionic charge layer” at the solution-stabilizer interface. That is, the macromolecular stabilizer intimately interacting with the cluster surface possesses extensive negative charge emanating toward the surrounding medium. For example, Figure l a illustrates the anionic environment composed of SO3-and PO3- ions surrounding Q-CdS in a w = 10 inverse micellef hexametaphosphate stabilizer. In contrast to these “anionic” stabilizers, we have recently described a stabilizer system which gives rise to a cationic charge layer at aminocalixarene molthe solution-stabilizer i n t e r f a ~ e .These ~ ecules are macrocyclic ”baskets” with positively charged amino groups on their upper rim. A schematic of Q-CdS stabilized by aminocalixarene molecules is depicted in Figure 1b. Traditional synthetic routes produce clusters which often initially possess many surface defects, for the nature of the interaction between the stabilizer and the surface is poorly understood. One focus of research in our laboratories has been “mapping” out the surface of 11-VI Q-SC’s, predominantly the paradigm of Q-cluster materials, Q-CdS. Due to their small diameters, the surface area to volume ratio in Q-SC’s is very large and up to one-third of the total atoms in the cluster can reside at the surface.1° For this reason, physical and chemical perturbations of the surface atoms have a profound effect on the physical characteristics of the cluster. It has been shown that “titration” of the cluster surface with small molecules gives clues
* To whom correspondence should be addressed.
* Abstract published in Advance ACS Abstracrs, September 1, 1993. 0022-3654f 93f 2097-9161%04.O0 f0
P = CH3
Figure 1. Illustrationsof the interaction of stabilizingmedia with Q-CdS clusters. (a) Inverse micellc/hexametaphosphatc-stabilized Q-CIS (b) p [(Dimethylamino)methyl]calix[61arene-stabilized Q-CdS.
as to the nature of surface defects.11-13In our surface titration studies, we have found that the choice of stabilizer exerts a significant influence on the degree to which the Q-SC cluster interacts with small molecular titrants.13 In this work, we specifically examine quantum-confined semiconductor clusters stabilized by three distinctly different stabilizers: (1) aminocalixarene molecules, (2) w = 10 inverse micelles in the presence of hexametaphosphate polymer (HMP), and (3) thiophenol capping-reagents. The ability of these stabilizersto mediate quenching of Q-CdS trap photoluminescence (PL) (A, between 550 and 650 nm) by several PL modifying agents (halide ions, methylviologen(MV2+),amines, and ketones) is compared via the Stern-Volmer model. Experimental Section Semiconductor Cluster Synthesis. w = 10 Micelle/ HMP Q-CdS (w = IO-QCdS): A solution of w = 10 = [H20]/[AOT] micelles in the presence of hexametaphosphate14 (HMP, sodium salt) was prepared by first dissolvingHMPin deoxygenated water at a concentrationof 1 mM. The water/HMP solutionwas added to a solutionof AOT (dioctyl sulfosuccinate, sodium salt, Aldrich) in 2,2,4-trimethylpentane (“isooctane”, Aldrich, 99+%) so that [HzO]/[AOT] = 10. Freshly prepared 1 M Cd(C104)~6H20 (10.0 pL, Johnson-Matthey, electronic grade) was added via syringe to micelle solution, and the mixture was stirred until completely homogeneous. Freshly prepared 1 M NazS (10.0 pL, 98%, Aldrich) was added dropwise with stirring, and then the 0 1993 American Chemical Society
9768
Chandler and Coffer
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
TABLE I: Stem-Volmer Constant (&v) Values for Q-CdS Prepared with Three Different Stabilizers upon the Addition of I-, MV*+, Acetone, and Tripropylamine Q-CdS stabilizer IMV2+ acetone tripropylamine -195 to 325 4 5 0 to 260 1040 to 3500 5160 to 12200 w = 10 micelle/HMP thiophenol capping reagent -1 15 to 163 13300 to 23200 -282 to 113 -357 to -100 325 to 335 1078 to 5050 9400 to 37500 590 to 2300 aminocalix[6]arene a The maximum range of values obtained for a given stabilizer are listed with respect to a given quencher. flask was shaken vigorously. The solutions turned yellow immediately and fully developed in color after stirring for 30 min. Thiophenol-cappedQ-CdS (SPh-QCdS): Thiophenol-capped Q-CdS clusters were prepared according to the method of Wang and Herronls utilizing 100 mL of 0.1 M Cd(C2Hs02)~2H20 (Baker) in a methanol/acetonitrile mixed solvent. Na2S (50 mL, 0.1 M) in water/methanol was added to the cadmium acetate solution followed by 50 mL of 0.2 M thiophenol(97%, Aldrich) in acetonitrile (S:SPh = 1:2). A pale yellow solid formed which was filtered under nitrogen and extracted into acetonitrile. This method produced a yellow solution which was evaporated to dryness. The resulting yellow solid was stored in a Schlenk tube and was dissolved in acetonitrile as needed. Aminocalixarene-stabilizedQ-CdS (Calix-QCdS):"Type 111" aminocalixarene-stabilizedQ-CdS ([CdS] / [aminocalixarene] = 4: 1) in acetonitrile was prepared according to a literaturemethod9 employing p- [(dimethylamino)methyl]~alix[6]arene~~ as the stabilizer. The aminocalixarene was dissolved in degassed acetonitrile (Aldrich, anhydrous, 99+%) by sonication in the presence of 1 mM Cd(C104)2.H20. An equimolar amount of H2S (Aldrich, 99.5+%) was injected into the headspace above the solution and the flask was shaken vigorously for 30 s. A bright yellow color formed immediately. PL Titration Experiments. PL Titrant solutions: Solutions (0.01 M) of each PL-modifying titrant in acetonitrile (or isooctane, depending on the Q-CdS cluster solvent) were prepared using tripropylamine, dibutylamine (Eastman Kodak); triethylamine, n-butylamine (Mallinckrodt, Organic Reagent; amines were distilled prior to use); 2-butanone, pinacolone (methyl tert-butyl ketone, Aldrich, 99.5+%, HPLC grade); 2-pentanone (Matheson, Coleman and Bell); acetone (American Scientific Products); KI (Kodak, reagent grade); KBr (Harshaw Chemical Co., IR quality); KC1 (Aldrich). Methylviologen, hexafluorophosphate V ( P F ~ ) ~by] mixing , 1 equiv salt, C ~ ~ H I ~ N ~ P ~ F ~ ~ [ Mwasprepared of C I ~ H I ~ N ~ C ~(MVClyH20, YH~O Aldrich) with 2 equiv of (NH4+)(PF6-) (99.5%, Johnson Matthey) in HzO. The white precipitate was filtered and recrystallized from acetone/CHzC12. Anal.: %C = 30.59 (30.27 calc); %H = 2.89 (2.96 calc); %N = 5.86 (5.88 calc); %F = 48.06 (47.88 calc), %P = 13.60 (13.01 calc) . Titration of Q-CdS: PL measurements were made on each colloid sample after the successive addition of small aliquots (160pL) of PL titrant. "Saturation" was reached when the addition of titrant ceased to change the integrated photoluminescence intensity between 520 nm and 700 nm (trap PL region). It should be noted that there is no detectable evidence for either particle corrosion or aggregation (via absorption spectroscopy) for any of the Q-CdS/stabilizer systems studied in this work on the timescale of a given quenching experiment as a consequence of quencher addition (1 h). It should also be pointed out that the addition of these particular quenchers to saturation neither shortens nor lengthens the period of stability of the Q-CdS particles studied here. Apparatus. A b s o r p t i o n and p h ot o l u m in escence measurements: Absorption measurements were made using an HP 8452A diode array spectrophotometer. Steady-state PL measurements were recorded using a Spex Fluorolog-2 0.22-m double spectrometer at an excitation wavelength of 370 (400 nm
550
600
650
550
600
650
450
533
617
700
Wavelength (nm) Figure 2. Photoluminescencequenchingupon the addition of I- to Q-CdS. w = 10 inverse micelle/hexametaphosphate Q-CdS: (i) 0.0 mM I-; (ii) 0.017 mM I-; (iii) 0.033 mM I-; (iv) 0.066 mM I-. Aminocalixarenestabilized Q-CdS: (i) 0.0 mM I-; (ii) 0.017 mM I-; (iii) 0.033 mM I-; (iv) 0.066 mM I-. Thiophenol-capped Q-CdS: (i) 0.0 mM I-; (ii) 0.010 mM I-; (iii) 0.017 mM I-; (iv) 0.132 mM I-.
for SPh-QCdS). All emission spectra were corrected for fluctuations in photomultiplier tube response. Results Titrationof Q-CdSwith PGMadifying Solutions. The average absorption threshold for the three types of Q-CdS employed in this study (aminocalixarene-stabilized CdS, inverse micelle/ hexametaphosphate-stabilized CdS, and thiophenol-stabilized CdS) was 485 nm in each case, consistent with an average particle size of 55 A.IC'9 Each cluster preparation also resulted in comparableoptical densities, reflecting comparable concentrations of Q-CdS in each solution. The luminescence spectrum for each type of cluster displaysdominant trap photoluminescence between 550 and 650 nm. A Stern-Volmer analysis was used to compare the quenching efficiencies of each PL titrant. The Stern-Volmer equation is given simply as Zo/Z = Ksv[C], where 10= initial integrated PL intensity, Z = integrated PL intensity after each addition of titrant, [C] = concentration of titrant, and K ~ =vStern-Volmer constant. KSVvalues for the addition of each titrant solution to each type of Q-CdS are given in Table I. Alternatively, one can view the effect of titrant addition in termsof percent quenching, calculated as
S quenching (SQ) = (I,,, - Zdn)/ZmaX (1) where I,,,,, = initial integrated PL intensity and Z~,, = integrated PL intensity at saturation. Titration with halide ions (Z-, B r , Cl-): Figure 2 illustrates the quenching effect that I- has on trap photoluminescence from Q-CdS. The average percent quenching (WQ) resulting from Iaddition to aminocalixarene stabilized QCdS was 70%, while the average KSVvalue was 19 500 (Table I). In contrast, I- addition to w = 10 micelle/HMP-stabilized Q-CdS (w = ratio of H20 to surfactant) resulted in an average %Q of only 20% with a corresponding average K ~ of v 2500, while I- addition to SPhQCdS resulted in virtually no PL quenching (
