Langmuir 1999, 15, 6845-6850
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Self-Assembly of Monolayers of Cadmium Selenide Nanocrystals with Dual Color Emission Chia-Chun Chen,* Chao-Pei Yet, Hai-Ning Wang, and Chi-Yau Chao Department of Chemistry, National Chung-Cheng University, Ming-Hsiung, Chia-Yi 621, Taiwan Received February 17, 1999. In Final Form: May 14, 1999 We report here a simple and low-temperature method to induce the patterning of monolayers of semiconductor nanocrystals with dual color emission. We have used the patterned self-assembled monolayers (SAMs) of alkanethiolates on gold as a template to assemble two specific size CdSe nanocrystals into regular two-dimensional arrays. The nanocrystal arrays on each region of SAMs are organized spontaneously on the basis of different interactions between specific size nanocrystals and alkanethiolates of SAMs. As characterized by scanning probe microscopy, scanning electron microscopy, and optical spectroscopy, our results show that small and specific size nanocrystals can be assembled into large monolayer arrays with micrometer-scale features. Our method provides a new way to assemble semiconductor nanocrystals and should be useful for the fabrication of optical and electronic devices composed of nanocrystals.
Introduction Many fundamental physical and chemical behaviors of semiconductor nanocrystals are dominated by quantum size effects.1,2 In particular, their optical and electronic properties have shown markedly size-dependent characteristics.3-5 Of recent, the successful development in the syntheses of semiconductor nanocrystals with narrow size distribution has facilitated the characterization of their physical and chemical properties and thus has brought great opportunity for the construction of a new class of optical and electronic devices. For example, a lightemitting diode has been fabricated using a junction of n-type CdSe nanocrystal and p-type semiconducting polymer (p-paraphenylenevinylene) layers.6,7 The emitting colors of the diode ranging from green to red can be finetuned by the sizes of the deposited nanocrystals and applied voltages. This successful example of employing size-dependent characteristics of the nanocrystals into a useful device has opened a wide imagination of constructing other possible devices, among which the fabrication of field emission displays, photovoltaics, switches, electronic storage systems, and sensors is of particular perspective.8,9 One critical step associated with the device fabrication of nanocrystals is the control of spatial positioning of the nanocrystals. The construction of nanocrystals into one-, two-, or three-dimensional arrays as well as networks is * To whom correspondence should be addressed. (1) For a recent review, see: Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Chen, C.-C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398. (3) Rossetti, R.; Nakahara, S.; Brus, L. E. J. Chem. Phys. 1983, 79, 1086. (4) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovis, I. G.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (5) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802. (6) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (7) Dabbousi, B. O.; Bawendi, M. G.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (8) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (9) Markovich, G.; Leff, D. V.; Chung, S. W.; Soyez, H. M.; Dunn, B.; Heath, J. R.; Appl. Phys. Lett. 1997, 70, 3107.
necessary to fabricate, for examples, a single or serial nanocrystal switch (1D) or a field emission display (2D or 3D). From a microscopic aspect, the key issue of the assembly of nanocrystals is to control relevant internanocrystal and nanocrystal-substrate interactions. These interactions mainly incorporate covalent interactions, i.e., chemical bonding, and noncovalent interactions, such as hydrophobic, ionic, van der Waal forces, and specific hydrogen bonding. Recent studies have shown that the nanocrystals with a modification of surface-capping molecules can selectively attach to a substrate via a covalent interaction.10-12 One remarkable example is that specific size CdSe-CdS or CdSe-ZnS core-shell nanocrystals with strong emitting color were successfully attached to biological substances such as mouse cells, proteins, DNA, or viruses.10,11 In these cases, the spatial positioning of nanocrystals is achieved using bifunctional molecules to form covalent bonding specifically between the nanocrystals and biological substances, and the nanocrystals can served as fluorescent probes in biological staining and diagnostics. Another intriguing example is that a single CdSe nanocrystal was chemically bonded on the tips of two very small gold electrodes.12 Here, a linear alkanedithiol molecule is used as a connecting medium. Besides linking individual nanocrystals with a defined object using bifunctional molecules, several approaches have been made using a template to assemble nanocrystals into a regular two-dimensional structure. A specially designed substrate such as a patterned SAMs on a gold surface,13-17 a physically lithographic silicon oxide,18 or an electrically etched membrane19,20 has served as a (10) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (11) Warren, C. W.; Nie, S. Science 1998, 281, 2016. (12) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; Mceuen, P. L. Nature 1997, 389, 699. (13) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209. (14) Palacin, S.; Hidber, P. C.; Bourgoin, J. P.; Miramond, C.; Fermon, C.; Whitesides, G. M. Chem. Mater. 1996, 8, 1316. (15) Yang H.; Coombs, N.; Ozin, G. A. Adv. Mater. 1997, 9, 811. (16) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S.-H.; Peng, X. G.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (17) Hu, K.; Brust, M.; Bard, A. J. Chem. Mater. 1998, 10, 1160. (18) Liu, J.; Barnard, J. C.; Seeger, K.; Palmer, R. E. Appl. Phys. Lett. 1998, 73, 2030.
10.1021/la990165p CCC: $15.00 © 1999 American Chemical Society Published on Web 07/07/1999
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template. In these cases, the selective growth or assembly of the nanocrystals into a two-dimensional structure is achieved mainly by invoking different noncovalent interactions, for example hydrophobic or hydrophilic interaction but not a covalent bonding, between the nanocrystals and template. Among those approaches, Whitesides and co-workers have introduced a relatively simple and inexpensive method to fabricate a template. They utilized a microcontact printing technique (µCP) to generate patterned SAMs of alkanethiolates on the gold surface,21,22 which were then used as a template to assemble the materials such as metal nanoparticles, biological substances,23 polymers,24,25 and organic liquids26 into two-dimensional patterns in micrometer scale. In these experiments, the materials are organized spontaneously on patterned SAMs according to different noncovalent interactions between those materials and alkanethiolates of the patterned SAMs. Taking account of size-dependent optical and electronic properties of semiconductor nanocrystals, how to assemble the nanocrystals into two-dimensional arrays according to their sizes is a technical challenge. However, only utilizing the patterned SAMs as a template is insufficient to create distinct spatial positioning of two or more specific size nanocrystals. In addition, it is necessary to prepare different size nanocrystals with distinct chemical properties, for example, with different wettabilities, and thus, selective interactions between nanocrystals and substrates can be generated. The wettabilities of the nanocrystals are mainly determined by the functionality of the surfactants that are passivated on the nanocrystal surface. Recently, through a simple reaction to exchange the surfactants with different functionality, specific size nanocrystals with distinct wettabilities were prepared.27 Such an achievement makes the assembly of specific size nanocrystals onto the patterned SAMs feasible. In this paper, we describe a simple method to induce the patterning of two-dimensional arrays with two sizeselected CdSe nanocrystals by controlling hydrophobic or hydrophilic interactions between nanocrystals and patterned SAMs. We have prepared the SAMs patterned with distinct hydrophobic and hydrophilic regions and also two types of CdSe nanocrystals with contrast wettability. In our experiment, the nanocrystal patterns on each region of the SAMs are organized spontaneously according to hydrophilic or hydrophobic interaction between the surfactants of nanocrystals and alkanethiolates of SAMs. To control the light-emission property of the resulting nanocrystal patterns, we have taken advantage of the sizedependent emission spectra of nanocrystals.1 On the basis of this principle, we have synthesized CdSe nanocrystals of which the particle sizes vary from 30 to 80 Å in diameter with corresponding optical emission ranging from green to red colors. Thus, the nanocrystal patterns with distinct dual emitting colors are prepared. Monolayers of CdSe (19) Foss, C. A.; Hornzak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 96, 7497. (20) Hoyer, P. Langmuir 1996, 12, 1411. (21) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (22) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600. (23) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (24) Xia, Y. N.; Kim, E.; Whitesides, G. M. Chem. Mater. 1996, 8, 1558. (25) Zhao, X. M.; Xia, Y. N.; Schueller, O. J. A.; Qin, D.; Whitesides, G. M. Sens. Actuators, A 1998, 65, 209. (26) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 2790. (27) Peng, X. G.; Wilson, T. E.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1997, 36, 145.
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nanocrystals have been successfully formed on the surface of SAMs, as characterized by scanning electron microscopy (SEM) and scanning probe microscopy (SPM). The results suggest that our assembling method can selectively deposit CdSe nanocrystals of a certain particle size as well as control the growth of monolayers of the nanocrystals. Experimental Section Syntheses of CdSe Nanocrystals Capped with Hydrophobic and Hydrophilic Surfactants. (a) Hydrophobic Surfactant-Capped Nanocrystals. The CdSe nanocrystals were synthesized using a modified method derived from previous reports.28,29 Dimethyl cadmium (Cd(CH3)2, 99%), selenium (99.999%), tri-n-octylphosphine oxide (TOPO, 99%), tributylphosphine (TBP), and 4-mercaptobenzoic acid were used as received. Anhydrous methanol, toluene, and hexane were purified by distillation in argon before use. Prior to the reaction, a stock solution was prepared as the following. The desired amount of Cd(CH3)2 was added to a solution of Se powder dissolved in TBP at room temperature in a drybox. The molar ratio of cadmium to selenium was varied from 1.1/1 to 1.4/1. The Se concentration was varied between 0.2 (mol/kg) and 0.33 (mol/kg). Certain amounts of TOPO (2-8 g) (mp 52-55 °C) were degassed by purging with argon in a three-neck flask which was then heated to ∼360 °C under an argon flow on a Schlenk line. Approximately 1 mL of the stock solution was quickly injected into the TOPO solution while the solution was kept stirring. Right after the injection of the stock solution to the clear TOPO solution, a significant color change from orange to red, depending on the sizes of the resulting nanocrystals, was observed, accompanied by a ∼50 °C drop of the temperature. Following the injection, the heating mantle was removed from the flask and the solution allowed to cool to ambient temperature. Anhydrous methanol (30-50 mL) was then injected into the solution to precipitate the nanocrystal solid. Evaporation of methanol under argon afforded a dried product (typically 10 mg). The solid was then dissolved in either toluene or n-hexane for further characterization. Under the reaction condition described above, the average size of resulting nanocrystals is generally in the range of 30-50 Å in diameter. For nanocrystals of larger sizes, the heating mantle was not removed after injection. Instead, at a steady temperature between 300 and 310 °C, a second or more injecting of stock solution (∼0.25-0.5 mL) was made to obtain an average particle size of >80 Å in diameter. The average particle sizes were monitored by an UV-vis spectrometer during the reaction. (b) Hydrophilic Surfactant-Capped Nanocrystals. The nanocrystals (10 mg) from part a and 4-mercaptobenzoic acid (20 mg) were added into methanol (20 mL) under an argon atmosphere. A certain amount (8 mL, 0.1 N) of tetramethylammonium hydroxide in propanol was gradually added into the nanocrystal solution. The solution was then heated at 57 °C for 6-12 h until it became optically clear. Afterward, THF was added to precipitate out the nanocrystal solid followed by the repeated dissolution in methanol. The resulting solid was dissolved in common hydrophilic solvents such as methanol and water for optical measurements. Previous experimental data of X-ray photoemission spectroscopy measurements have suggested that the surfaces of CdSe nanocrystals in part a were capped with a layer of TOPO.30 Due to the alkyl chain, TOPO-capped CdSe nanocrystals usually have great solubility in common hydrophobic solvents such as toluene and hexane. To prepare hydrophilic surfactant-capped CdSe nanocrystals, the TOPO layer on CdSe nanocrystals was substituted by a layer of 4-mercaptobenzoic acid via the surface exchanging reaction as we described above and the resulting nanocrystals was soluble in hydrophilic solvents. To simplify the discussion, hereafter, we name TOPO- and 4-mercaptobenzoic (28) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (29) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (30) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109.
SAMs of Cadmium Selenide Nanocrystals
Figure 1. A schematic illustration for the assembly of two specific size nanocrystals capped with different surfactants onto patterned SAMs. The substrate on the top represents patterned SAMs with hydrophobic (light area) and hydrophilic (dark area) regions formed separately by microcontact printing technique. The arrow indicates subsequent depositions of monolayers of hydrophilic nanocrystals (small unfilled circle) capped with 4-mercaptobenzoic acid and hydrophobic nanocrystals (large filled circle) capped with TOPO onto selective regions of patterned SAMs. Dimensions of the nanocrystals, surfactants and monolayers are not to scale. acid-capped CdSe nanocrystals as hydrophobic and hydrophilic nanocrystals, respectively. Patterned SAMs by a Microcontact Printing Technique. All chemicals were reagent grade and used as received. Solvents, 1-dodecanethiol (HS(CH2)11CH3, 98%), and 16-mercaptohexadecanoic acid (HS(CH2)15COOH, 90%) were purchased from Aldrich. Gold (99.999%) and chromium (99.99%) were obtained from Alfa Chemicals. The stamps were prepared from poly(dimethylsiloxane) (PDMS: Sylgard 184, Dow Corning). Silicon substrates were semiconductor grade silicon wafers (4 in. in diameter). The substrate was cleaned by a standard treatment in piranha solution (30% H2O2/70% H2SO4). A chromium layer of 50 Å thickness that served as an adhesion promoter was first deposited on the silicon substrate, followed by depositing 2000 Å of gold on the top of chromium layer. Unless used right after the evaporation, the substrate was normally cleaned by ozone plasma and washed by ethanol and deionized water. Methods for fabrication of stamps and for preparation of patterned SAMs on gold have been elaborated.31 In brief, the stamps were inked with an ethanolic solution of HS(CH2)15COOH (1-10 mM) and then placed in contact with the surface of a gold substrate for ∼30 s. The substrate was then immersed in a solution of CH3(CH2)11SH for 5 min to fill up the remaining areas with the saturated alkanethiol. The treated sample was rinsed with ethanol (∼50 mL), dried under a stream of nitrogen, and stored in a vacuum container. The patterned SAMs of hydrophobic and hydrophilic regions were characterized using a lateral force microscope (LFM). Assembly of CdSe Nanocrystals. A schematic illustration is shown in Figure 1 to describe a general procedure of depositing CdSe nanocrystals onto a patterned SAMs. Hydrophilic CdSe nanocrystals of specific particle size were dissolved in methanol in a vial where the gold substrate with patterned SAMs was immersed into the nanocrystal solution and allowed to remain (31) Xia, Y. N.; Whitesides, G. M. Langmuir 1997, 13, 2059.
Langmuir, Vol. 15, No. 20, 1999 6847 for 10 min. After removal from the solution, the substrate was quickly dried by a stream of nitrogen. For consecutive deposition of hydrophobic CdSe nanocrystals of another specific particle size, the hydrophobic nanocrystals were dissolved completely in a small amount of toluene and then diluted by acetonitrile in a vial. The gold substrate with patterned hydrophilic CdSe nanocrystals was immersed into the hydrophobic nanocrystal solution and incubated for 5 min. Afterward, the substrate was dried under a stream of nitrogen for further characterization. Thermal Stability of the Patterned SAMs Deposited with Nanocrystals. Samples of the patterned hydrophilic nanocrystals on SAMs were placed in a clean quartz tube under an argon flow and heated in an oven. The oven temperature was increased at a rate of 3 °C/min to reach the set point. The sample was kept at this temperature for 20 min and subsequently cooled to room temperature at the same rate for further characterization using SEM and SPM. Instrumentation. Optical absorption and emission spectra of CdSe nanocrystal solutions at room temperature were obtained by a commercial UV-vis spectrometer (Hewlett-Packard, 8453A) and emission spectrometer (Hitachi, F-4500), respectively. The average size and size distribution of some samples were confirmed by transmission electron microscopy (TEM; Zeiss, 10C, and Hitachi, Fe-200) operated at 80 and 200 KV, respectively. Samples for TEM measurements were prepared by depositing an aliquot of nanocrystal solution in toluene onto either an amorphous carbon film or a porous carbon film on a Cu grid. An SEM (Hitachi, S2400) operated at 15 KV was used to obtain the surface topography of patterned nanocrystals on SAMs. Scanning probe microscopy (SPM) was performed with a Topometrix TMX 2000 microscope (Mountain View, CA). The probe of the microscope is a cantilever with pyramidal tips coated with a silicon carbide (Topometric, contact 1520 and HRF noncontact 1650). All SPM images were collected in the forward part of the scan at a constant rate across a surface. Various scanning rates (∼1-150 mm/s) were selected, depending on the size of the scanned region. The SPM can be operated in several modes based on various research purposes. Lateral force microscopy (LFM) was used specifically to distinguish two chemically distinct monolayer regions. Topographical information of the samples was measured using atomic force microscopy (AFM) under a contact mode. The data of force modulation microscope (FMM) was obtained simultaneously with AFM images. A chopped He-Cd laser (442 nm, Kimmon, IK Series) was used as an excitation source to acquire emission spectra of patterned nanocrystal monolayers on SAMs. The emission signal was collected by a single monochromator (Acton, SpectraPro150) and detected by a lock-in amplifier coupled (Stanford Instrument, SR830) with a photomultiplier tube (Hamamatsu, R928).
Results and Discussion Characterization of Hydrophobic and Hydrophilic CdSe Nanocrystals. Figure 2 presents a typical example of the evolution of room-temperature absorption and emission spectra as a function of particle sizes of TOPO-capped CdSe nanocrystals dissolved in toluene. The average sizes depicted in Figure 2 were derived from the exciton energy in the absorption spectra on the basis of a comparison to the calculated spectra of previous reports.28,29 The average sizes and size distribution of these samples were further confirmed using TEM. The TEM images showed good crystallinity and a spherical shape in general with an average size distribution of σ ≈ 5-10%. The average sizes calculated from the TEM images agree well with those obtained from the absorption spectra. Both absorption and emission spectra shown in Figure 2 exhibit the quantum size effects. Each absorption spectrum indicates a shift of the exciton peak position in the range of 516-575 nm. For all these nanocrystals, the edges of the absorption band shift dramatically relative to the corresponding band gap (716 nm). Also, the emission
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Figure 2. Room-temperature UV-vis absorption (solid line, left) and emission spectra (dashed line, right) of four different diameters of hydrophobic CdSe nanocrystals. Photoluminescence spectra were taken by exciting at 450 nm. The average diameters of the nanocrystals are (a) 25 Å, (b) 30 Å, (c) 38 Å, and (d) 42 Å, which were deduced from their corresponding exciton energy in the absorption spectra.
Figure 3. Lateral force microscopy image of the patterned SAMs formed by µCP. The bright regions represent -S(CH2)11CH3, and dark regions, -S(CH2)15COOH. Each square in the image is 10 µm × 10 µm in size. The image was recorded as the tip moved from left to right.
peaks shift from 542 to 595 nm as the nanocrystal sizes increase from 25 to 42 Å in diameter. The sharp absorption features and narrow emission line widths suggest narrow size distribution of the samples. In the visible and nearinfrared ranges, a single emission peak of each sample was detected. The relative emission quantum yield of the samples with respect that of Rohodamine 6G at room temperature was estimated to be 7-10%. The average sizes and size distribution of 4-mercaptobenzoic acid capped CdSe nanocrystals were determined using UV-vis spectroscopy. We have found that their absorption features were very similar to that of the TOPOcapped nanocrystals before the surface exchanging reaction. The similarity suggested that substituting 4-mercaptobenzoic acid for TOPO on the CdSe nanocrystal surface affects negligibly the size of individual crystal core. Lateral Force Images of the Patterned SAMs on Gold. Figure 3 shows the LFM image of the patterned SAMs on a gold substrate before the deposition of CdSe nanocrystals. The LFM image shows contrast of regular square-shape patterns. The contrast reflects the different regions of patterned SAMs possessing two chemically distinct monolayers fabricated by µCP. Here, the darker
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regions correspond to the surface with SAMs of -S(CH2)15COOH, while the brighter regions reflect the surface with SAMs of -S(CH2)11CH3. The patterned SAMs were made using alkanethiolates that have different lengths (C12 vs C16) and terminal functionality (CH3 vs COOH). Hence, the contrast in LFM image could be attributed to different functionality and/ or height between regions of hydrophilic and hydrophobic SAMs. Previous studies have shown that the contrast of patterned SAMs are mainly caused from different frictional forces between tips and terminating groups but not from the different height of alkanethiolates.32,33 Since the difference in height between regions terminated by CH3 and COOH is very small (1 h) resulting in the formation of multilayers on the patterned SAMs. FMM Measurements of the Patterned Nanocrystal Monolayers on SAMs. We have used FMM to
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characterize SAMs deposited with monolayers of CdSe nanocrystals capped with hydrophilic surfactants. We found that the contrast patterns of the FMM image were similar to those observed in Figure 4A,B. The patterns in the FMM image suggested that hydrophilic regions of the patterned SAMs were filled entirely with hydrophilic CdSe nanocrystals. More importantly, the force modulation data revealed that the hydrophilic regions with nanocrystal deposition were stiffer than the hydrophobic regions without the nanocrystal deposition. Because the alkanethiolates of hydrophobic SAMs and the surfactants capped on hydrophilic nanocrystals possess terminated functional groups of CH3 and COOH, respectively, the force modulation results of the stiffness actually reflect the difference in the interactions of FMM tip with respect to CH3 and COOH. However, at this stage, it is difficult to obtain an image of the detailed arrangements of the surfactants on the nanocrystal surface using FMM or AFM. Therefore, here we only report qualitative results of the relative stiffness between nanocrystals and SAMs. We have also used FMM to characterize patterned SAMs deposited with both hydrophobic and hydrophilic CdSe nanocrystals. The FMM image displayed a contrast similar to that of the sample deposited with only hydrophilic nanocrystals. We found that the stiffness of hydrophilic nanocrystals was greater than that of hydrophobic nanocrystals, reflecting different interactions of tip with respect to the two different surfactants, 4-mercaptobenzoic acid and TOPO. Because similar images were observed in both samples, i.e., the one with only hydrophilic CdSe pattern and the one with both hydrophilic and hydrophobic CdSe pattern, the formation of monolayers of both types of nanocrystals cannot be directly proven by the FMM measurement. However, the optical emission measurements described in the next section will provide direct evidence for the successful deposition of both nanocrystals. During this study, we noticed that the success of the experiment relies on two critical procedures. One is the order of immersion, putting the patterned SAMs into hydrophilic nanocrystal solution followed by the hydrophobic solution, cannot be altered. We found that reversing deposition order may generate irregular aggregates or nanocrystal multilayers on SAMs. The other is the use of mixed solvent of toluene and acetonitrile to dissolve hydrophobic CdSe nanocrystals. According to our experience, monolayers of nanocrystals were not obtained in the neat toluene solvent. Optical Emission Spectrum of the Patterned Hydrophilic and Hydrophobic Nanocrystals. Figure 5 shows the optical emission spectrum of the patterned SAMs deposited with nanocrystals of two specific sizes capped with hydrophobic and hydrophilic surfactants, respectively. The observation of dual emission peaks confirms the successful deposition of both types of nanocrystals. The wavelengths of the emission maxima were the same as those in solution phase, suggesting that average sizes of nanocrystals remain intact after the deposition. The relatively lower intensity of hydrophilic nanocrystals could be attributed to less deposited areas of hydrophilic nanocrystals on SAMs or to less passivation on the surface of hydrophilic nanocrystals.37,38 The successful assembly of two different size nanocrystals with dual color emission has given us an idea to further fabricate a device which can have multiple color emissions including blue, green, and red. Currently, we (37) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468. (38) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019.
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patterns were destroyed completely when they were heated above 500 °C as indicated by SEM images. We are currently investigating the change of detailed packing structures of nanocrystal monolayers at different temperatures using SPM, in combination with absorption and emission spectra after being thermally activated. The resulting absorption spectra reveal that the aggregation and consequent growth in size of nanocrystals occur when the sample was heated over 200 °C. Conclusions
Figure 5. Emission spectra of patterned hydrophilic and hydrophobic nanocrystals on SAMs. The excitation energy (442 nm) was kept as low as 2 mJ in order to prevent the sample from heating, and the size of the laser spot was much larger than that of each nanocrystal pattern (10 µm × 10 µm). A red shift of the emission maximum was observed upon applying a high power excitation. The sizes of the hydrophilic and hydrophobic nanocrystals are 27 and 42 Å in diameter, respectively, which were derived from the exciton energy of the absorption spectra.
are able to prepare various sizes of CdS and CdSe nanocrystals capped with either hydrophobic or hydrophilic surfactants. Different size CdS nanocrystals have the emission from green to blue color, and different size CdSe nanocrystals have the emission from green to red color. Therefore, the assembly of three specific size CdS and CdSe nanocrystals into a micrometer-scale pattern becomes feasible using our assembling method. Thermal Stability of the Patterned CdSe Nanocrystal Monolayers on SAMs. Samples of the patterned hydrophilic nanocrystals were heated at 200, 300, and 500 °C to test their thermal stability. Previous studies have shown that desorption of alkanethiolates of SAMs on a gold substrate occurs over 200 °C.39 The SEM images of our samples showed that the contrast patterns still existed when the sample was heated at 200 and 300 °C for 20 min. This result implies that CdSe nanocrystal arrays still remain on the gold substrate under conditions where SAMs may be thermally desorbed. The nanocrystal (39) Ulman, A. Chem. Rev. 1996, 96, 1533-1554.
This paper reports a simple method to organize a twodimensional array of the CdSe nanocrystals. By control of the wetting pattern of a gold surface by µCP, the controlled of the deposition of the different size nanocrystals can be achieved. The studies of the patterned nanocrystals by SEM and SPM measurements provide new information regarding the surface of self-assembled nanocrystals. Our results have shown that small and specific sizes of crystals can be assembled to large, mesoscopic structures with micrometer-scale features. Our method could be applied to the fabrication of other types of optical and electronic materials. For example, adding another layer of conducting materials such as conducting polymers on the top of the patterned nanocrystals may allow the fabrication of a light-emitting device with micrometer scale. In comparison to other deposition techniques such as chemical vapor deposition, our method renders simpler and lower temperature procedures. As a result, our method provides an alternative route to fabricate some advanced optical materials of current interest in which a lowtemperature requirement is critical during the fabrication process. Acknowledgment. We thank Dr. Yu-Tai Tao and Dr. Kannaiyan Pandian at the Institute of Chemistry, Academic Sinica, for technical support and helpful discussion. We thank Prof. Bi-Tai Chou for comments on this manuscript. This work was supported by the National Science Council and Chinese Petroleum Corporation in Taiwan, Order No. NSC 87-2114-M-194-010 and NSC 88-CPC-M-194-004. LA990165P
