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International Journal of Nanoscience Vol. 8, Nos. 1 & 2 (2009) 119–122 c World Scientific Publishing Company
HIGHLY ORDERED MICRO-ARRAYS OF CdSe QUANTUM DOTS ON CHEMICALLY PATTERNED SUBSTRATES LI ZHANG, HUA-YAN SI, HUA XU and HAO-LI ZHANG∗ State Key Laboratory of Applied Organic Chemistry College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, P. R. China ∗
[email protected] YU-QING XIONG National Key Laboratory of Surface Engineering Lanzhou Institute of Physics, Lanzhou 730000, P. R. China
Highly ordered arrays consisting uniform fluorescent cadmium selenide (CdSe) quantum dots (QDs) ring or dot structures were obtained by self-assembly of QDs on chemically patterned substrates. In this method, Au substrates with alternating hydrophobic and hydrophilic square patterns are firstly fabricated by microcontact printing, which allows water droplets to condense on the hydrophilic regions to provide two-dimensional template arrays. The CdSe QDs are then assembled at the liquid/liquid interfaces to give uniform micro or nanostructures. The shape and size of the rings and dots can be tailored by controlling the relative evaporation speed of the water and the organic solvents. The obtained nanostructures have ideal topography to avoid substrate-induced fluorescence quenching. Keywords: Microcontact printing; CdSe; quantum dots; micro-arrays.
selective photoactivation.8 However, the controlled deposition of nanoparticles onto specific regions of surfaces with high selectivity and easy operation still poses a significant challenge. Microcontact printing (µCP) has been extensively applied in micro- and nanostructuring technologies due to many advantages including fast, simple, inexpensive operation, adaptable to large areas, and suitable for arbitrary surfaces. Since µCP is based on a pattern transfer from topographic featured PDMS stamp to substrate via conformal contact, the patterning resolution is normally restricted by the feature size on the stamp. However, by combining with other techniques, much higher resolution could be obtained.9 In this work, a method combining µCP with self-assembly of nanoparticles at liquid/liquid interface is applied
1. Introduction Patterning of functional materials into twodimensional arrays is very important for many photonic and electronic applications, such as multiple color light emitting diodes (LEDs), field-emission displays, and multichannel chemical sensors. Therefore, the schemes of positioning functional materials in predetermined areas are of great scientific and technological interest.1 Highly fluorescent cadmium selenide (CdSe) quantum dots (QDs) are currently of great interest due to their unique sizedependent optical properties,2 and wide applications in bioimaging,3 LEDs, and many photonic and optoelectronic devices.4 Various methods have been applied in order to manipulate the organizational geometry of QDs, including photolithography,5 selective dewetting,6 capillary organization,7 and 119
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to produce highly ordered array patterns with feature size smaller than that on the stamps. In this method, µCP is used to create chemical patterns on substrates, which induce selective condensation of water droplets. Then the QDs are assembled to the liquid/liquid interface10 to produce uniform ring or dot structures that are much smaller than the chemical patterns. This method represents a simple, convenient, and versatile approach for fabricating arrays of CdSe QDs aggregates of various patterns without the aid of advanced lithographical tools.
2. Experimental Section 2.1. Materials Gold substrates were prepared by sputtering onto the silicon wafers whose surfaces had been primed with thin layers of chromium. Octadecanethiol (ODT), 11-mercaptoundecanoic acid (MUA) were purchased from Sigma-Aldrich. Poly(dimethylsiloxane) (PDMS) elastomer kits (Sylgard 184) were purchased from Dow Coring (Midland, MI). Trioctylohosphine oxide (TOPO) capped CdSe QDs with different sizes (3 and 4.5 nm) were synthesized using a one-pot synthesis as reported previously.11
2.2. Fabrication of CdSe QDs arrays Scheme 1 illustrated the procedure for the fabrication of ordered two-dimensional (2D) CdSe
Scheme 1. Schematic of the process steps for the fabrication of CdSe QDs arrays on patterned gold substrates with hydrophilic and hydrophobic SAMs. (a) clean gold substrate; (b) patterned ODT (hydrophobic) and MUA (hydrophilic) SAMs on gold substrate; (c) water vapor condense on hydrophilic area to form droplets; (d) the substrate with water droplets is dipped into the solution of CdSe QDs; (e) water and organic solvents evaporate from the substrate surface; (f) CdSe QD ring array; (g) CdSe QD dot array.
QD arrays. PDMS stamps with 2D arrays of 16 or 32 µm square patterns were used to print hydrophobic SAMs of ODT (1 mM in ethanol) onto the gold substrate. Then the patterned substrates were immerged into an ethanol solution of MUA (1 mM) for 2 h to form hydrophobic and hydrophilic alternating arrays. Subsequently, the substrates were rinsed with a copious amount of deionized water, dried in a stream of flowing Argon, and finally placed onto a glass-slide in an atmosphere with controlled humidity to nucleate, grow, and freeze individual water droplet on the hydrophilic squares. This process was monitored by optical microscopy. Water droplet array template on patterned substrate was dipped into a chloroform or toluene solution of CdSe QDs and then dried in air to give the final patterns.
2.3. Characterization of CdSe patterns Fluorescence images were acquired using BA2500I fluorescence microscope. Atomic force microscopy (AFM) images were collected using VISTA-MPX (Vecco) in contact mode. Optical microscopy images were taken using a XQT-2 optical microscope (in reflection mode).
3. Results and Discussion In this work, the chemical patterns on the gold substrate fabricated by µCP had a chessboardlike 2D array, i.e. both of the hydrophobic and hydrophilic SAMs on the gold substrate are squares. Therefore, when water condensed on the substrate, the initial droplets are squares corresponding to the underlaid hydrophilic SAMs. The positions of water droplets reflect the lateral pattern of varying wettability on the gold substrate. These square-like water droplets are gradually changed to hemisphere-like water droplets when the substrates are “dried” in a less humidity atmosphere before being dipped into the QDs solution. This drying step allows a controlled evaporation of water from the substrate. Figure 1 shows the results of CdSe QD rings prepared on the hydrophilic regions in patterned substrates using chloroform as solvent. These images represent the level of perfection, order that could be routinely achieved using our method. From the fluorescence microscopy images, one can see almost all the CdSe QDs aggregate into rings. The areas between the rings and the interior of the rings
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Fig. 1. Fluorescence microscopy images of CdSe QD ring array formed on patterned gold substrate with chloroform as the solvent. The diameters of CdSe QD are (a) 3 nm (green) and (b) 4.5 nm (orange) (color online).
are nearly free from fluorescent CdSe QDs. The rings are well ordered with uniform sizes over an area of a few millimeters. There are a few defects in the patterns shown in Fig. 1(a), which could be attributed to the coalescence of two water droplets. The fluorescence color is determined by the sizes of the QDs. The formation of the ring structures can be understood by the following mechanism. Firstly, the CdSe QDs spontaneously adsorb onto the liquid/liquid interface.12 Under our condition, because chloroform evaporates faster than water, the shrinking of the chloroform will drive more QDs toward the chloroform/water interface. Since the QDs are insoluble in water, after the evaporation of the chloroform, the QDs will not move with the retraction of water droplet. Therefore, after all the liquids evaporate away, QD ring arrays were deposited on the substrates replicating the shapes of the initial chloroform/water interface. When toluene is used as the solvent for the CdSe QDs, the resulted pattern is strongly dependent on the thickness of the toluene layer. As shown in Fig. 2, when the toluene layer is relatively thick, dot array is obtained. In contrast, when the toluene layer is thin, ring array is produced (Figs. 2(c) and 2(d)). Owing to its low vapor pressure, toluene evaporates from the surface in a comparable rate with water. When the toluene layer is relatively thick, it covers the substrate long enough for all the water to evaporate away from the surface. During this process, with the continuously reducing size of the water droplet, the CdSe QDs are driven toward the center of each square along the water/toluene interface. Eventually, the move of the water/toluene/substrate contact line condenses all
Fig. 2. (a) Fluorescence microscopy image of CdSe QD dot array formed on patterned substrate. (b) AFM image topography of CdSe QD dot array. (c) Fluorescence microscopy images of CdSe QD ring array. (c) Optical microscopy images of CdSe QD ring array. Toluene was used as the solvent of CdSe QD in all the cases.
the QDs into a compact dot (Figs. 2(a) and 2(b)). The diameter of CdSe QD dot is about 3 µm, much smaller than the 16 µm size of each hydrophilic area. However, if the toluene film is very thin, the toluene layer evaporates slightly faster than the water droplet. In this situation, CdSe QD ring array can be obtained, which is similar to that of the chloroform (Figs. 2(c) and 2(d)). The absences of ring patterns in Fig. 2(d) are attributed to defects of the chemical patterns on the gold substrate patterned using defective PDMS stamp. SEM measurement reveals that the rings generally have a width of 200–300 nm. AFM data show that the height of the rings and dots are within the range of 40–110 nm. The fluorescence of QDs deposited on metal substrates frequently suffered strong quenching effect from the substrate. All the QD patterns in this work are strongly fluorescent suggesting that the topography is ideal to avoid substrate-induced quenching effect.
4. Conclusion In summary, highly ordered 2D arrays of CdSe QDs have been fabricated by self-assembly of CdSe QDs on patterned gold substrates. This method combines selective condensation of water droplet template on chemically patterned substrate and self-assembly of nanoparticles at the liquid/liquid interface. Both dot and ring arrays can be obtained
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by adjusting the experimental conditions, such as solvent type, relative amount of the two liquids on the substrate. Directly assembly of nanoparticles into ordered micro-array may open new avenues of technology through the controlled fabrication of nanoscopic materials with unique optical, magnetic, and electronic properties.
Acknowledgments The authors are grateful to the financial support from the program for New Century Excellent Talents in University (NCET), NSFC (20503011, 20621091), and MOE (SRFDP. 20050730007, 106152).
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