J. Phys. Chem. 1994,98, 2735-2138
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Monoparticulate Layer and Langmuir-Blodgett-Type Multiparticulate Layers of Size-Quantized Cadmium Sulfide Clusters: A Colloid-Chemical Approach to Superlattice Construction N. A. Kotov, F. C. Meldrum, C. Wu, and J. H. Fendler' Department of Chemistry, Syracuse University, Syracuse, New York 13244-41 00 Received: November 19, 1993; In Final Form: January 25, 1994"
Monoparticulate layers of dodecylbenzenesulfonicacid-stabilized, 34.0-, 29.4-, and 26.5-&diameter CdS clusters have been formed a t air-water interfaces in a Langmuir film balance. Surface pressure vs surface area isotherms and Brewster angle microscopy demonstrated that increasing surface pressure resulted in transitions from well separated to well-compressed monoparticulate layers and, ultimately, to multiparticulate layers. CdS particles with diameters of 34.0,29.4, and 26.5 A formed films with critical areas per particle (conceptually analogous to headgroup areas) of 1100, 800, and 650 A2,respectively, which corresponded well to the values calculated by assuming hexagonal close packing of hard spheres (887, 749, and 608 A2). Sequential transfer to solid substrates, by the Langmuir-Blodgett (LB) technique, led to the formation of multiparticulate layers of sizequantized CdS clusters. Absorbances of CdS in the LB film were found to increase linearly with the number of layers transferred, substantiating the uniformity of deposition. The position of the absorption edge was invariant between the CdS particles in chloroform solution and in LB films. Fluorescence maxima of the CdS particles, however, shifted from 510 nm in chloroform to 455 nm in LB films.
Introduction The arrangement of semiconductor nanoparticles,with defined interparticle distances, as single monolayers on solid substrates is an essential requirement for the construction of quantum-dotbased advanced materials. Currently available technologies are based on atom-by-atom and/or molecule-by-molecule formation of semiconductors in ultrahigh vacuum.' As part of our overall interest in developing a "wet" colloid-chemical approach to advanced materials,2 we have launched investigations into the formation of monoparticulate layers of semiconductors at airwater interfaces and into their transfer onto solid substrates. Monolayer formation at air-water interfaces from simple surfactants has matured into an established ~ c i e n c e . More ~~~ recently, analogous formation of monoparticulate layers from polystyrene microspheres,~*3 silylated glass beads," and organoclay complexes's has been described. We report here the construction of well-behaved monoparticulate layers of CdS nanoclustersat air-water interfaces. CdS nanocrystalliteswere synthesizedin the presenceof a surfactant stabilizer in controllable sizes and size distributions. The hydrophobic surfactant coating permitted the spreading of CdS particles on the water surface in a film balance in a highly reproducible manner. Transfer of the monoparticulate layers of CdS clusters to solid supports resulted in films whose thicknesses were proportional to the number of layers deposited. The physical properties of the nanoparticulate CdS films on solid supports were characterized by absorption and fluorescence spectroscopy and by transmission electron microscopy (TEM).
Experimeatpl Section Cadmium sulfide particles were synthesized using a protocol which was slightly modified from that of Weller et a1.16 Cd(C10)2.6H20 and dodecylbenzenesulfonic acid, sodium salt (DBS), were dissolved in 100mL of l-propanol to concentrations of 8.8 X 10-3 and 4.0 X 10-3 M, respectively. The stoichiometric quantity of H2S (4.4 mL) was then injected into the solution in four aliquots of 1 mL, and the preparation was permitted to ripen for 3 days. The solid CdS product was then isolated by extraction of the 1-propanol using rotary evaporation at 55 OC and was Abstract published in Advance ACS Abstracts, March 1, 1994.
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redissolved in 50 mL of chloroform. Any residual white precipitate, probably resulting from partial decomposition of the DBS," was removed by centrifugation. The resulting yellow solution was totally clear and exhibited a sharp absorption band onset, which confirmed the narrow size distribution of the CdS particles.16 The mean diameter of the crystallites was estimated fromthepitionof the absorption edge,18J9andtheconcentration of the solution, with respect to the particles themselves, was determined on this basis. This preparation constituted the basic stock solution from which monolayers were spread. The chloroform solution of CdS was stable over a time period of months when stored in the dark at 2 OC; dissolution of the semiconductor particles occurred upon exposure to daylight, as demonstrated by a blue shift in the absorption spectrum. Consequently, all CdS monolayers were prepared in the dark. Three CdS dispersions with average sizes of 34.0, 29.4, and 26.5 A were utilized in the experiments described here. Surface pressure isotherms were investigated using a Lauda Langmuir balarlce. Fifty to 250 pL of the stock solution of CdS particles was measured in a Hamilton syringe and spread on a water subphase by locating the syringe tip 1 mm from the water surface; the droplets had a tendency to sink when released from a greater height. The spreading solvent was allowed to evaporate for 15 min prior to compression. LB films were prepared by compressing the monoparticulate layer to a surface pressure of 30-35 mN/m and allowing 5 min for equilibration. A slide of either glass, silanized glass, or quartz was then immersed and subsequently extracted from the subphase at a rate of 10 mm/ min. The slide was paused, between changes in the dipping direction, for 5 s in the solution phase and for 9.5 min in the air. Brewster angle microscopy was performed over a homemade Teflon trough fitted with a Wilhelmy-typesurfacepressure sensor. Absorptionspectra of the stock solutions and LB films were taken using a Hewlett-Packard HP8452A diode array spectrometer. Fluorescence measurements were carried out using a Tracor Northern TH6500 rapid scan spectrophotometer coupled with SPEX 1681monochromators in the excitation and emission paths. An excitation wavelength of 380 nm was used. The fluorescence measurements of the LB films were made with the slide oriented at 122O to the excitation light, so as to minimize interference of the reflected excitation light with the fluorescence. 0 1994 American Chemical Society
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Figure 1. Surface pressure us surface area isotherms for CdS monoparticulate layers. Curves I, 2, and 3 correspond to the spreading of 34.0-, 29.4-, and 26.5-A-diameter CdSparticles. Arrows marktheplaces on curve 1 where TEM samples were taken. .
Transmissionelectron microscopy wascarriedout using a JEOL ZOOOEX electron microscopeoperatingat 120 kV. Samples were supported on carbon-coated, Formvar-covered, 400-mesh copper TEM grids and were prepared by dipping a grid vertically into the subphase and then raising it horizontally through the monolayer. Results and Discussion
The ability of the prepared CdS particles to form monoparticulate layers on water derives from their hydrophobic character.
Letters Binding of the DBS molecules to the particles via the charged headgroups in a reverse-micellar conformation effectively yields particles coated with the surfactant chains; 100-250 surfactant molecules can be calculated to be adsorbed on a CdS cluster, based on the assumption of optimum close packing of the headgroups. Additionally, the adsorbed surfactants preclude further growth by such mechanisms as Ostwald ripening. The surface pressure (r)us surface area ( A )isotherms corresponding to monoparticulate layers of 34.0-A CdS particles on water can be seen to closely approach the classical compression curves of gases and ideal monolayers (Figure 1, curve'l). Comparing the hehavioroftheparticulatefilmstothisidealcase,theCdSparticles in region I may be well separated, their interaction being predominately repulsive in nature (2D gas). It was demonstrated by using Brewster angle microscopy (BAM) that the monolayer is entirely uniform in the millimeter scale in this region (Figure 2a). Theuniformityofthefilm wasmaintainedupncompression until a surface pressure of 16-20 mN/m was reached, at which stage the CdS monoparticulate layer was supposed to be quite tightlypacked (region 11). Yet, further compression (region 111) caused the CdS particles to overcome their intrinsic repulsion barrier toformquiterigid, stable,and icelikeagglomerates(Figure Zc). In this pressureregime,theCdS crystals graduallyunderwent a phase transition to rigid crystalline agglomerates; areas of monoparticulate layer and thicker agglomerated film coexist. Region IV represents the final stages of compression and corresponds to an almost uniform, thick film of CdS particles which was shown, by BAM, to be immobile (Figure 2d). On the basis of the intensity of the Brewster angle reflection and the surface area per particle at the onset of region IV, it can be estimated that the film is 4-5 particles thick (approximately 150
Flgure 2. Brewster angle images of the monoparticulate layers formed from 34.0-A-diameter CdS particles. A = 1300, 1000,600, and 400 A'/particle for a, b, c, and d, respectivcly. The scale bar = 1.00 mm.
The Journal of Physical Chemistry. Vol. 98, No. I I . 1994 2131
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a
b
C
Figure 3. TEM images of the monoparticulate layers formed by 34.0-A-diameter CdS particles. A = 1650, 1070, and 740 A’lparticle for a. b, and c, respectively. The bar = 50 nm.
A) at this stage. It is to be noted that unbound surfactants do notconstitutea significantproportionoftbe monolayers. Indeed, not only did DES fail to form insoluble monolayers on water, hut a 5-fold increase of thesurfactant concentration in the CdS stock solution increased the resultant surface pressure by only 2-3 mN/m. *-A curves were obtained for CdS dispersions with mean particle sizes of 34.0,29.4, and 26.5 A and showed critical areas per particle (corresponding to the concept of headgroup areas in surfactant mono1ayers)of 1100,800, and650Az(Figure I,curves 1,2,and3, respcctively). Calculationoftheseareas, by assuming hexagonal close packing of hard spheres, yielded values of 887, 749, and 608 A2. That the experimentally obtained values were 7-10% greater than the theoretical ones can be attributed to the presence of adsorbed surfactants on the particle surfaces and at the interface. A change in particle size was also manifested in the shape of the isotherms. Reducing the crystal diameter from 34.0 to 26.5 &resulted in a contraction of region I11 to a small shoulder on the isotherm (Figure 1, curve 3) and to a reduced tendency for the CdS particles to form thick aggregates. Transmission electron microscope images of the monolayer (Figure 3) were recorded at three different surface pressures (arrowed on the isotherm in Figure 1). Compression of the monoparticulatelayer resulted in an increase in the particledensity spread on the surface (Figure 3a-c) and in the generation of more closely packed structures. The efficiency of LE film formation strongly depended on the monolayer surface pressure. In general, the deposition of LB films is inefficient at surface pressures below 20 mNJm;V the present system did not prove to bean exception. Transfer ratios of only 0.1 or less were obtained upon transferring CdS nanoparticulate clusters to substrates at 15 mN/m surface pressure. However, transfer ratiosof0.88 wereachieved at surface pressures of 32 mN/m using either hydrophobic or hydrophilic substrates. Under this condition, optical absorbances of CdS in the LB film were found to increase linearly with the number of layers transferred (Figure 4). substantiating the uniformity of deposition.
0.40 0.20
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n nn
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Figure 4. Plot of absorption edge heights of CdS against number of
layers transferred to a glass substrate.
The optical properties of LE films,prepared from CdS particles, are illustrated in Figure 5 . The position of the absorption edge was found to he invariant between the CdS particles inchloroform solution and in LB films, indicating that the formation of monoparticulatelayendidnotaltertheCdScrystalsizeorinduce cluster coalescence. In contrast, the fluorescence properties of the CdS particles were sensitive to the environment. The chloroform-dispersedCdS particlesshowed an emission maximum at 510 nm,a wavelength corresponding to electron-hole recombination in traps lying 0.2-1.0 eV below the conduction band edge. Formation of LE films from the CdS clusters resulted in a blue shift in the emission maximum to 455 nm (Figure 9, which is attributable toexcitonicrecombinationor to theemission from very shallow traps. This behavior may be rationalized by assuming that the traps, responsible for fluorescence in the bulk solution, are principally formed via solvation of the surface Cd or S atoms. Desolvation of particles in the LE films removes the trap blocks, thereby reducing the number of conduction bandsurface trap transitions, increasing the efficiency of excitonic
2738 The Journal of Physical Chemistry, Vol. 98, No. 11, 1994
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Acknowledgment. Support of this research by a grant from the National Science Foundation is gratefully acknowledged. References and Notes (1) NiinistB, L.;Leskela, M. Thin Solid Films 1993, 225, 130-135. Suntola, T. Thin Solid Films 1992, 216, 84-89. Capasso, F. Thin Solid Films 1992, 216, 59-67. (2) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Springer-Verlag: Heidelberg, Germany, in press. (3) Ulman, A. An Introduction to Ultrathin Organic Films from Lungmuir-Blodgett to Self-Assembly;Academic Press: San Diego, CA, 199 1. (4) Lungmuir-Blodgett Films; Roberts, G.,Ed.; Plenum Press: New
0 m m 4 300
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York, 1990. (5) (6) (7) (8) (9) (10)
WAVELENGTH. nm Figure 5. Optical properties of the original CdS dispersion and LB films prepared from it. Curves 1 and 2 show absorption and fluorescence of the CdS dispersion in chloroform. Curves 3 and 4 show absorption and fluorescence of 15 layers of LB film deposited at 32 mN/m.
recombinations, and, thus, resulting in a blue shift of the fluorescence band. To the best of our knowledge, the present report represents the first description of monoparticulate film formation at water-air interfaces from size-quantized semiconductor clusters and the successful layer-by-layer construction of LB films from them. Formation, characterization, and utilization of monoparticulate films, prepared from structurally different size-quantized semiconductor, magnetic, and metallic particles, will be the subject of subsequent communications from our laboratories.
Pieranski, P. Phys. Rev. Left. 1980,45 (7), 569. Robinson, D. J.; Earnshaw, J. C. Phys. Rev. A. 1992,46 (4). 2045. Robinson, D. J.; Earnshaw, J. C. Phys. Rev. A . 1992,46 (4), 2055. Robinson, D. J.; Earnshaw, J. C. Phys. Rev. A . 1992,46 (4), 2065. Robinson, D. J.; Earnshaw, J. C. Lungmuir 1993, 9, 1436. Amstrong, A. J.; Mockler, R.C.;O'Sullivan, W. J. J.Phys.: Condens,
Matter 1989, 1 , 1707. (1 1) O n d a , G. Phys. Rev. Lett. 1985, 55 (2), 226. (12) Doroszkowski, A.; Lambourne, R. J . Polym. Sci.: Part C 1971,34, 253. (13) Sheppard, E.; Tcheurekdjian, N. J. Colloid Interface Sci. 1968, 28 (3/4\. , ,, 481. (14) H6rvBlgy, Z.; Nemeth, S.; Fendler, J. H. Colloids Surf. A: Physicochem. Eng. Aspects 1993, 71, 327. (15) Kotov, N. S.;Fendler, J. H.; Tomb&, E.; DtkBny, I. Manuscript
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