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Surface Chemistry Studies of (CdSe)ZnS Quantum Dots at the Air-Water Interface Xiaojun Ji, Chengshan Wang, Jianmin Xu, Jiayin Zheng, Kerim M. Gatta´s-Asfura, and Roger M. Leblanc* Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431 Received February 4, 2005. In Final Form: March 8, 2005 Trioctylphosphine oxide- (TOPO-) capped (CdSe)ZnS quantum dots (QDs) were prepared through a stepwise synthesis. The surface chemistry behavior of the QDs at the air-water interface was carefully examined by various physical measurements. The surface pressure-area isotherm of the Langmuir film of the QDs gave an average diameter of 4.4 nm, which matched very well with the value determined by transmission electron microscopy (TEM) measurements if the thickness of the TOPO cap was counted. The stability of the Langmuir film of the QDs was tested by two different methods, compression/decompression cycling and kinetic measurements, both of which indicated that TOPO-capped (CdSe)ZnS QDs can form stable Langmuir films at the air-water interface. Epifluorescence microscopy revealed the two-dimensional aggregation of the QDs in Langmuir films during the early stage of the compression process. However, at high surface pressures, the Langmuir film of QDs was more homogeneous and was capable of being deposited on a hydrophobic quartz slide by the Langmuir-Blodgett (LB) film technique. Photoluminescence (PL) spectroscopy was utilized to characterize the LB films. The PL intensity of the LB film of QDs at the first emission maximum was found to increase linearly with increasing number of layers deposited onto the hydrophobic quartz slide, which implied a homogeneous deposition of the Langmuir film of QDs at surface pressures greater than 20 mN‚m-1.
Introduction
* To whom correspondence should be addressed: tel (305) 2842194; fax (305) 284-6367; e-mail
[email protected].
In recent years, luminescent QDs have been successfully attached to dendrimers, proteins, sugars, and other biologically active agents.8 Among the various types of applications, (CdSe)ZnS QDs have been studied most thoroughly in cell biology. However, the consideration of QD behavior at biological interfaces, e.g., cells and biomembranes, is one of the most important issues requiring further study prior to implementing an application. Currently, a few studies have been conducted regarding surface chemistry properties of quantum dots that mimic biological interfaces by using Langmuir and Langmuir-Blodgett (LB) film techniques.9 In this study, (CdSe)ZnS quantum dots with a TOPO cap were synthesized and purified. Because of the hydrophobic long-chain moieties of TOPO, stable (CdSe)ZnS QD monolayers were prepared at the air-water interface by the Langmuir film technique. The surface pressure- and surface potential-area isotherms of pure (CdSe)ZnS QD monolayers were obtained, and in situ UVvis spectra at different surface pressures were recorded. The in situ epifluorescence microscopic technique was used to study the topography of (CdSe)ZnS QD monolayers at different surface pressures. The photoluminescence prop-
(1) (a) Chestnoy, N.; Brus, L. E. J. Chem. Phys. 1986, 85, 22372242. (b) Alivisatos, A. P. Science 1996, 271, 933-937. (2) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506-1508. (3) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314-317. (4) Harrison, M. T.; Kershaw, S. V.; Burt, M. G.; Rogach, A. L.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. Pure Appl. Chem. 2000, 72, 295-307. (5) (a) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (b) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 12, 1759-1762. (6) (a) Mews, A.; Eychmu¨ller, A.; Giersig, M.; Schoos, D.; Weller, H. J. Phys. Chem. 1994, 98, 934-941. (b) Alivisatos, A. P. Science 1996, 271, 933-937. (c) Hines, M. A.; Guyot-Sionnest, P. J. J. Phys. Chem. 1996, 100, 468-471. (d) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475.
(7) (a) Ebenstein, Y.; Mokari, T.; Banin, U. J. Phys. Chem. B 2004, 108, 93-99. (b) Ding, S. Y.; Jones, M.; Tucker, M. P.; Nedeljkovic, J. M.; Wall, J.; Simon, M. N.; Rumbles, G.; Himmel, M. E. Nano Lett. 2003, 3, 1581-1585. (c) Walker, G. W.; Sundar, V. C.; Rudzinski, C. M.; Wun, A. W.; Bawendi, M. G.; Nocera, D. G. Appl. Phys. Lett. 2003, 83, 3555-3557. (d) Goldman, E. R.; Balighian, E. D.; Mattoussi, H.; Kuno, M. K.; Mauro, J. M.; Tran, P. T.; Anderson, G. P. J. Am. Chem. Soc. 2002, 124, 6378-6382. (8) (a) Chan, W. C. W.; Nieb, S. M. Science 1998, 281, 2016-2018. (b) Cordero, S. R.; Carson, P. J.; Estabrook, R. A.; Strouse, G. F.; Buratto, S. K. J. Phys. Chem. B 2000, 104, 12137-12124. (c) Wu, X. Y.; Liu, H. J.; Liu, J. Q.; Haley, K. N.; Treadway, J. A.; Larson, J. P.; Ge, N. F.; Peale, F.; Bruchez, M. P. Nat. Biotechnol. 2003, 21, 41-46. (d) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800-803. (9) (a) Sui, G.; Orbulescu, J.; Ji, X.; Gatta´s-Asfura, K. M.; Leblanc, R. M.; Micic, M. J. J. Clust. Sci. 2003, 14, 123-133. (b) Lowman, G. M.; Nelson, S. L.; Graves, S. M.; Strouse, G. F.; Buratto, S. K. Langmuir 2004, 20, 2057-2059.
Quantum dots of II-VI semiconductors (CdS, CdSe, and CdTe) in the size range of 1∼12 nm have attracted great interest in both fundamental research and technical applications in recent years.1 Due to their tunable sizedependent emission with high photoluminescence quantum yields, broad excitation spectra, and narrow emission bandwidths, the semiconductor quantum dots have been intensively investigated in versatile applications, including thin-film light-emitting devices (LEDs),2 low-threshold lasers,3 optical amplifier media for telecommunication networks, and biological labels.4,5 It has also been proved that overcoating the quantum dots with higher band gap inorganic semiconductor materials can substantially increase the photoluminescence quantum yields and, especially, the chemical stability and photostability by passivating surface nonradiative recombination sites.6 Consequently, the core-shell type quantum dots, such as (CdSe)ZnS QDs, have been widely used in both optoelectronic and biological applications.7
10.1021/la050327j CCC: $30.25 © 2005 American Chemical Society Published on Web 04/22/2005
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erty of the QD LB films was also examined at various surface pressures. Experimental Section Materials. Cadmium oxide (CdO), selenium (Se), trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), hexamethyldisilathiane [(TMS)2S], and 2-mercaptoacetic acid were purchased from Sigma-Aldrich (St. Louis, MO). Tetradecylphosphonic acid (TDPA) was obtained from Alfa Aesar (Ward Hill, MA). Diethylzinc (ZnEt2, 15 wt % solution in hexane) was obtained from Acros Organics (Morris Plains, NJ). All the starting materials for the synthesis and surface modification of (CdSe)ZnS coreshell quantum dots were used without purification. Synthesis of (CdSe)ZnS Quantum Dots. The (CdSe)ZnS core-shell QDs capped with trioctylphosphine oxide (TOPO) ligand were prepared through a stepwise procedure as described elsewhere.6c The precursor of (CdSe)ZnS core-shell QDs, nearly monodispersed TOPO-capped CdSe quantum dots, was synthesized by the method reported previously by Peng and Peng.10 Cadmium oxide (CdO) and tetradecylphosphonic acid (TDPA) were used as the Cd precursor and the trioctylphosphine selenide as the Se precursor. The CdSe QDs were formed by pyrolysis of the Cd and Se precursors in a coordinating solvent, trioctylphosphine oxide (TOPO), at high temperature (250∼270 °C). The QDs were collected as powders by size-selective precipitation11 with methanol followed by drying under vacuum. The average size of the CdSe/TOPO QDs was determined by UV-vis absorption spectroscopy of its chloroform solution.12 An approximate particle concentration of CdSe/TOPO QD solutions can also be determined via UV-vis spectroscopy.4 By knowing the mass of CdSe/TOPO QDs used when preparing the solution and the total volume of the stock solution, the “molecular” weight of CdSe/ TOPO quantum dots could be estimated. ZnEt2 and [(TMS)2S] were used as Zn and S precursors for the ZnS cap. The amounts of Zn and S precursors needed to grow a ZnS shell of desired thickness for each CdSe sample were determined as follows: First, the average size of the CdSe core was estimated from UV-vis data.12 Next, the mole number of ZnS necessary to form a shell per mole of CdSe quantum dots was calculated on the basis of the volume of individual CdSe QDs and the desired thickness of the shell. It was assumed that the QD core and shell were spherical. The bulk density of ZnS was used in the calculations. Then, the amount of ZnS for one synthesis was calculated by knowing the mass of CdSe/TOPO QDs used and the “molecular” weight of CdSe/TOPO QDs estimated previously from UV-vis measurements. Finally, the amounts of ZnEt2 and (TMS)2S were calculated from the amount of the ZnS needed in the synthesis. For example, if the CdSe core has an average size of 2.48 nm and a “molecular” weight of 32 000 Da, the amount of ZnS needed for a 1 nm shell for each single particle will be 1.599 × 10-19 g. Thus, by knowing the amount of CdSe QDs, the amount of ZnEt2 and (TMS)2S were calculated. The common coating procedure was carried out by adding a ZnEt2 and (TMS)2S mixture solution dropwise into a coordination solution (TOPO/TOP as solvent) of CdSe QDs at high temperature under argon atmosphere.6c Usually, the temperature during coating was lower than that used for growing the CdSe nanocrystals to avoid compromising the integrity of the native cores. The (CdSe)ZnS core-shell QDs were collected as a powder by precipitation, washed with methanol, and dried under vacuum. Photophysical Measurements. A Perkin-Elmer UV/vis/NIR spectrometer Lambda 900 (Perkin-Elmer, Inc., Wellesley, MA) was used to obtain the UV-vis absorption spectrum of the solutions with a 1 cm optical path length quartz cuvette. A Spex Fluorolog 1680 spectrophotometer (SPEX Industries, Edison, NJ) was used to measure the emission spectrum of solutions. All samples were placed in a quartz cuvette with 1 cm optical path length and excited at 350 nm. (10) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2000, 122, 1270012706. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (12) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854-2860.
Ji et al. TEM Micrographs. Transmission electron microscopy was performed on a Philips EM300, 100 kV microscope with eucentric goniometer stage ((45° sample tilt) and attached Link Analytical AN10000 X-ray analyzer. Sample preparation consisted of dropcoating the QDs onto carbon-coated copper grids and air-dried. Estimated average particle size and size distribution were performed by the Scion Image software (version 4.0.4)13 from the raw TEM micrographs. Epifluorescence Microscopy. An epifluorescence microscope (Olympus IX-FLA, Melville, NY) was used to obtain all the luminescent images of the monolayer at the air-water interface under different surface pressures. The Langmuir films were prepared on a Kibron minitrough (5.9 cm × 19.5 cm, Kibron Inc., Helsinki, Finland) with a quartz window in the middle. The UV light source and lens system of the microscope were all fixed under this quartz window. An Optronic Magnafire TM CCD camera was used to detect the fluorescence emission from the floating monolayer at the air-water interface and transfer the digital images into the computer. Surface Chemistry Measurements of (CdSe)ZnS monolayer. The surface chemistry experiments were performed in a clean room class 1000, with a constant temperature of 20.0 ( 0.5 °C and a relative humidity of 50% ( 1%. The surface chemistry measurements at the air-water interface were taken on a KSV minitrough (Model 2000, KSV Instruments Ltd., Helsinki, Finland) with dimensions of 7.5 cm × 30 cm. Two computercontrolled symmetrically movable barriers were employed to regulate the surface area. Surface pressure was measured by the Wilhelmy method, and the sensitivity of the Wilhelmy plate was known to be (0.02 mN‚m-1. The Langmuir trough was equipped with a quartz window (located in the center of the trough) for in situ spectroscopic measurements. The in situ UVvis absorption spectrum of the Langmuir monolayer was recorded on an HP spectrophotometer model 8452A, which was placed on a rail in close proximity to the quartz window of the KSV trough. The in situ fluorescence spectrum of the Langmuir monolayer was measured on a Spex Fluorolog 1680 spectrometer (SPEX Industries, Edison, NJ). An optical fiber probe was placed 1 mm above the water surface so that the excitation light from the fluorescence spectrometer and the emission light from the Langmuir monolayer can be transmitted through this optical fiber. The LB film was prepared via Y-type deposition on hydrophobic quartz slides by use of a µ-S trough (Kibron Inc. Fin 00171, Helsinki, Finland) with an area of 115 cm2 (5.9 cm × 19.5 cm). This trough had a centrally located cavity for LB film deposition. The trough was supplied with a Wilhelmy plate wire probe, which had a sensitivity of (0.02 mN‚m-1. Each trough in our clean room is equipped for a particular application, including spectroscopy measurements (KSV), LB deposition (Kibron), and epifluorescence microscopy (Kibron). This is the reason three different troughs were utilized in this study. One advantage of the KSV over the Kibron may be that its trough area is bigger. However, all three troughs have similar if not the same sensitivity for the surface chemistry studies presented herein. For preparation of the QD LB films, the quartz slides were cleaned by first dipping them in an ultrasonic bath containing a detergent solution for 20 min in order to remove the impurities from the surface. An additional 30 min of sonication in deionized water was conducted to eliminate detergent residues. This was followed by drying the slides in an oven for 1 h. Activation of the quartz slide to make it hydrophobic was carried out by dipping the clean slides into 0.4 g/L octadecyltrichlorosilane (OTS) solution in cyclohexane for 30 min (with stirring). The hydrophobic slides were sonicated in cyclohexane to remove the excess OTS and stored under cyclohexane solvent prior to use. The monolayer was transferred onto the surface of the hydrophobic quartz slides by the LB film deposition technique. The subphase for the preparation of LB film was the same as that for the Langmuir film preparation. The deposition ratio was near unity, and 5 layers were deposited on the slide at the specified surface pressure. (13) The Scion Image software is free for downloading and available at the website: http://www.scioncorp.com/frames/fr_scion_products.htm.
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Figure 3. Surface pressure- and surface potential-area isotherms of (CdSe)ZnS/TOPO QDs.
Figure 1. (a) TEM image and (b) size distribution chart of (CdSe)ZnS/TOPO quantum dots.
Figure 2. UV-Vis absorption and photoluminescence spectra of (CdSe)ZnS/TOPO quantum dots in CHCl3 (4.4 × 10-6 M). λex ) 350 nm.
Results and Discussion Synthesis of (CdSe)ZnS QDs. In this study, the stepwise synthesis procedure was adopted, and the UVvis spectroscopy of QD solutions indicated an average CdSe core size or diameter of 2.9 nm.12 The amounts of Zn(Et)2 and (TMS)2S were then calculated for deposition of a 0.5 nm thick ZnS shell on the QDs. Figure 1 shows the TEM micrograph and corresponding size distribution chart of (CdSe)ZnS/TOPO QDs. The calculated average size of (CdSe)ZnS/TOPO QDs was 3.7 nm. The QD size and the thickness of the TOPO surface cap helped to explain the limiting nanoparticle area of the QDs as derived from the surface pressure-area isotherm. The UV-vis absorption and photoluminescence spectra of (CdSe)ZnS/TOPO QD solutions are shown in Figure 2. The first electronic transition peak revealed a core size of 2.9 nm according to the literature.12 The PL emission peak was found to be at 571 nm with a fwhm of 35 nm, which indicated a very small size distribution. Surface Pressure- and Surface Potential-Area Isotherms of (CdSe)ZnS/TOPO QD Monolayers. The three alkyl side chains of the surface TOPO molecules make the QDs soluble in many nonpolar solvents such as chloroform, hexane, and cyclohexane. Besides that, the large dipole moment derived from the phosphorus-oxygen bond14 makes it possible for TOPO-capped QDs to form a stable monolayer at the air-water interface. The surface pressure- and surface potential-area isotherms give the most important characteristics of the
monolayer properties. The surface pressure-area isotherm of (CdSe)ZnS/TOPO QDs is shown in Figure 3 and three distinct phases are well displayed. The film collapsed at a surface pressure of 45 mN‚m-1, indicating that the (CdSe)ZnS/TOPO QDs can form a stable Langmuir film at the air-water interface. This is similar to other amphiphilic molecules such as stearic acid, whose Langmuir film usually collapses at a surface pressure between 40 and 45 mN‚m-1. The limiting nanoparticle area of the QDs was obtained by extrapolating the linear part of the isotherm to zero surface pressure, 1500 Å2. As discussed before, the average particle diameter obtained from TEM measurement was 3.7 nm. The TOPO cap forms a monolayer at the surface of nanocrystals.15 The thickness of a TOPO monolayer was reported to be 0.7 nm as measured on metal surfaces.14b Then, the diameter of the TOPO-capped CdSe(ZnS) QDs should be (2 × 0.7) + 3.7 or 5.1 nm. If assuming spherical QDs, an average diameter of 4.4 nm for a single QD was calculated from the limiting particle area of the QD Langmuir films. Obviously, at the air-water interface, the TOPO moieties were compressed and resulted in a smaller observed particle size due to interdigitation of the TOPO tails (Scheme 1). The surface potential-area isotherm measures the dipole moment changes during compression of the monolayer at the air-water interface. This isotherm for the QDs is shown in Figure 3. When the average particle area was larger than 2250 Å2, the QDs randomly existed at the air-water interface. As a result, the total surface potential contribution of QDs was zero. When the monolayer was compressed, the hydrophobic part of the QDs began to stretch out of the water and the change of the dipole moment resulted in a rapid increase of surface potential up to 220 mV, which corresponded to the lifting point of the surface pressure. When the surface area of the QDs in the monolayer was smaller than 1750 Å2, the hydrophobic moieties of TOPO-capped QDs were lifted up into the air and the monolayer was in a liquid condensed phase. From this point, a slight increase of the surface potential was observed although the change of surface pressure was still significant. This may be due to reorientation of the nanoparticle in order to maximize exposure of the hydrophobic TOPO moiety to the air. As the surface potential reached the maximum, the surface pressurearea isotherm showed the collapse of the monolayer. When the QDs were compressed at the air-water interface, the surface potential started to increase prior to an increase (14) (a) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226-13239. (b) Jiang, J.; Krauss, T. D.; Brus, L. E. J. Phys. Chem. B 2000, 104, 1193611941. (15) Taylor, J.; Kippeny, T.; Rosenthal, S. J. J. Cluster Sci. 2001, 12, 571-582.
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Scheme 1. Size of QDs Obtained from TEM Micrographs and Surface Pressure-Area Isotherma
Figure 5. Changes in the surface area per QD at the airwater interface as a function of time and at a constant surface pressure of 30 mN‚m-1.
a (a) Reported thickness of TOPO monolayer (0.7 nm) at metal surface. (b) Model of TOPO capped QDs. The TEM measure gave a diameter of bare QD of 3.7 nm because the TOPO is nonconductive; the diameter of TOPO-capped QDs was (2 × 0.7) + 3.7 nm ) 5.1 nm. (c) At the air-water interface, the TOPO moieties were compressed and resulted in a smaller diameter (4.4 nm) due to the interdigitation of the TOPO tails.
Figure 4. Multiple compression/decompression cycles of the (CdSe)ZnS/TOPO QD Langmuir film at the surface pressure of 30 mN‚m-1.
in the surface pressure. This is a common response when both curves are compared. The surface pressure increased considerably when the QDs started to interact among them. However, the film could be compressed with minimum particle-particle interactions as during the gas phase, resulting in a higher surface potential. This also explains why, at the solid phase of the isotherm, the surface potential curve changed its slope. Stability of QD Monolayer. The stability of the QD monolayer at the air-water interface was examined by two different methods, compression/decompression cycling and kinetic measurements. Figure 4 shows the compression/decompression cycles of the QD monolayer at a surface pressure of 30 mN‚m-1. A decrease of about 250 Å2 in the apparent limiting nanoparticle area was observed after the first compression/ decompression cycle. Then, a hysteresis behavior of the isotherm was observed for the successive six compression/ decompression cycles that followed, which indicated longterm stability of the Langmuir film of the QDs. Figure 5 shows the kinetic measurements of the QD Langmuir film at the air-water interface. The QD monolayer was compressed to a surface pressure of 30 mN‚m-1 and held constant at this value for a period of 40 min. The area per nanoparticle decreased to about 77
Figure 6. In situ (a) UV-vis absorption and (b) photoluminescence spectra of the Langmuir film of the (CdSe)ZnS QDs at different surface pressures. The inset in panel a presents the absorbance at 553 nm as a function of the surface pressure. The inset in panel b presents the PL intensity of QDs at emission maximum (568 nm) as a function of the surface pressure. Linear fit of the data sets are also presented in both insets. The error bar represents the standard deviation of the data set.
Å2‚particle-1 over the 40-min period. This value corresponds to 15 mN‚m-1, the QD Langmuir film was almost homogeneous as seen from the epifluorescence images (Figure 7e,f). As shown before, both the first absorption peak and PL intensity of the QD Langmuir film displayed poor linearity (R ) 0.90 and 0.95, respectively) with increased surface pressure. In addition, the increasing trends of both absorbance and PL intensity are of little significance (Figure 6). As we can see, when the surface pressure was smaller than 15 mN‚m-1 (Figure 7a-c), the QD Langmuir film was composed of discrete domains. So it can be concluded that the observed results of the UV-vis and PL measurements (Figure 6) were due to the heterogeneous topography of the QD Langmuir film at low surface pressures (0∼15 mN‚m-1). Photoluminescence Study of Langmuir-Blodgett Films of (CdSe)ZnS/TOPO QDs. LB films were prepared at three surface pressures, namely, 15, 25, and 35 mN‚m-1.
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ratio (barrier area swept/deposited area on the slide) was close to 1.0, indicating that the slide was completely coated by the QD monolayer. In this study, five layers were deposited at three different surface pressures. Figure 8 shows the photoluminescence spectra of LB films of (CdSe)ZnS QDs deposited at three different surface pressures. The photoluminescent emission maximum of the QDs at 573 nm for the LB film was identical within experimental error to the value of QDs in chloroform solution (λmax ) 571 nm). In all three cases, the PL intensity at 573 nm increased linearly with increasing number of layers, which indicated that homogeneous Langmuir films were deposited. Conclusion
Figure 8. Photoluminescence spectra of LB films deposited at surface pressures of (a) 15, (b) 25, and (c) 35 mN‚m-1. The insets show the PL intensity at emission maximum (573 nm) as a function of the number of layers.
This choice was based on the epifluorescence measurements, which showed homogeneity in the topography of Langmuir films for surface pressure g15 mN‚m-1. The (CdSe)ZnS QD Langmuir film was deposited onto a chemically treated hydrophobic quartz substrate. The silanized quartz slide was suspended above the monolayer and gradually lowered vertically into the subphase at a deposition rate of 1.8 mm‚min-1. Because hydrophobic quartz slides were used, the dipping of the slides through the QD monolayers resulted in adsorption of hydrophobic moieties of the QDs, that is, TOPO. The calculated transfer
The surface chemistry of (CdSe)ZnS/TOPO QDs was thoroughly investigated. The surface pressure-area isotherm of the QD Langmuir film gave a limiting nanoparticle area of 1500 Å2, which was larger than the average size obtained from TEM images of the QDs (diameter 3.7 nm). The observed difference of size is due to the thickness of the insulating moieties or TOPO capping. The stability of the QD Langmuir film was studied through two different methods, compression/decompression cycling and kinetic measurements. The results of both experiments showed long-term stability of the QDs at a surface pressure of 30 mN‚m-1. The topographic images obtained from the epifluorescence microscope revealed that the QD Langmuir films were not homogeneous at low surface pressure (
