Article pubs.acs.org/Langmuir
Hierarchically Structured Porous Cadmium Selenide Polycrystals Using Polystyrene Bilayer Templates Jin Young Park,† Nicholas R. Hendricks, and Kenneth R. Carter* Polymer Science and Engineering Department, University of MassachusettsAmherst, Conte Center for Polymer Research, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.
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INTRODUCTION II−VI semiconductors (CdSe, CdS, and CdTe) are an attractive class of materials for active layers in optoelectronic devices because of their suitable work function and acceptable transparency in the visible light range.1 In particular, CdSe, with a high refractive index of ∼2.5 and a direct band gap of 1.74 eV, is an interesting semiconductor and has attracted much interest for potential use in applications such as solar cells and photoelectrochemical cells.2−5 In terms of its use in photovoltaic (PV) applications, control of the crystal structure and morphology of CdSe nanocrystals has been found to be important. A number of studies have reported improved power conversion efficiencies of bulk heterojuction photovoltaic devices based on CdSe nanorods instead of spherical CdSe nanoparticles as the electron acceptor material.6−10 By segregating the inorganic nanoparticle-conjugated polymer blends into vertical rods, the interfacial area between the acceptor and the donor material can be increased, providing extended pathways for holes and electrons to reach the respective electrodes, thus improving the photovoltaic cell performance.11 A variety of oriented semiconductors can be electrochemically synthesized within the free area of a porous template. Preformed porous templates are often sputtered with gold or other metals on one side of the membrane which serve as the electrode for the electrochemical growth of the semiconductors.12−15 The pores in the template guide the formation of the semiconductors, and the orientation of the semiconductor nanorod growth is dictated by the orientation of the pores. In particular, the successful electrochemical deposition of CdSe has been accomplished from specific templates with a © 2012 American Chemical Society
variety of shapes, sizes, and structures such as nanowires, pillars, or nanopores.3,16−19 Furthermore, porous templates themselves can be used for various potential applications such as transparent electrodes in organic photovoltaics,20 antireflection and light trapping in photovoltaics,21 and fuel cells.22 Braun and co-workers reported the use of ordered colloidal assemblies, so-called colloidal lithography, to template the electrodeposition of CdSe.23 They used multilayers of packed colloidal spheres to form 3D microperiodic porous electrodeposited layers. In that work and a follow-up report,24 the colloidal template consisting of Si particles was assembled on an indium tin oxide (ITO) surface through evaporative processes followed by sintering. The substrates were then used to grow CdS or CdSe through electrochemical deposition followed by removal of the Si template by acid etching with HF. Later work by Yeo and co-workers demonstrated that polystyrene (PS) could be vertically deposited on ITO to yield monolayer (2D) and multilayer (3D) colloidal assemblies which were used to create porous 2D and 3D CdSe layers via a similar process.25 In these bottom-filling electrochemical methods, control of size-tunable nano/micropores is mostly achieved by varying electrochemical factors (electrolyte concentration, electrodeposition time, applied potential/current, scan rate, and scan range) and the sizes of self-assembled colloids. However, manipulation of these parameters gives limited ability to simultaneously tune both the sizes and shapes of the porous Received: May 22, 2012 Revised: August 13, 2012 Published: August 17, 2012 13149
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Figure 1. (a) UV−vis spectra of CdSe polycrystallites (1, the as-made film; 2, the annealed film at 500 °C for 1.5 h; 3, the annealed porous CdSe film made using 500 nm PS colloid arrays) and (b) X-ray diffraction spectra of all the CdSe thin films. (PS, Mw= 280 000) was purchased from Scientific Polymer Products, Inc. Customized ITO-coated glass (thickness: 1450 ± 100 Å; resistivity: 20 ± 2 ohms/cm2) was purchased from Thin Film Devices Inc. Preparation of PDMS Replica Molds. Two PDMS replica molds (line) [mold #1: period (P) = 1600 nm, width (W) = 400 nm, height (H) = 220 nm; mold #2: period (P) = 810 nm, width (W) = 410 nm, height (H) = 180 nm] and two PDMS replica molds (twisted ellipsoidal and circular shapes) were prepared as follows: (1) Silicone elastomer base and curing agent (Sylgard 184, Dow Corning Co.) with a ratio of 10:1 were well mixed, degassed under vacuum, and poured onto the master molds in a plastic Petri dish. (2) Thermal curing was performed at a temperature of 70 °C for 4 h in an oven. (3) After demolding, the PDMS stamps were stored in a clean Petri dish prior to usage. Polystyrene Bilayered Templates. With PDMS soft imprint lithography, PS patterns (lines, ellipsoidal features, and circular features) were patterned on ITO substrates (area: 1.5 × 1.5 cm2). Using a 4 wt % polystyrene solution in PGMEA, spin-coating was performed within 3500−5000 rpm for 5 s. After spin-coating, a patterned PDMS mold was immediately placed on the thin PS film at specific surface pressure for 1 min. After demolding, line transfer was successfully accomplished over a large area. The residual layer in the PS template was removed by inductively coupled plasma reactive ion etching (ICP-RIE) (optimized etching parameters: O2 = 49 sccm, pressure = 200 mTorr, RIE power = 90 W, ICP power = 20 W, and etching time = 18−24 s). To fabricate 2D PS particle arrays on the PS prepatterned substrates, a spin-coating method, as previously reported, was used (500−1000 rpm for 3 min for all of PS colloids).26−28 To prevent formation of an imperfect array, a reorganization step is required at the air−water interface. Several droplets of SDS solution (2 wt % in H2O) were spread on the water surface in a plastic Petri dish, and the spincoated film was carefully floated on the water. It must be noted that addition of SDS (200 μL) is required to make the incomplete hexagonal structure of PS colloid array highly close-packed. Sequentially, more SDS solution (200−300 μL) was added to make the monolayer coalescent until the monolayer was in a solid state at the air−water interface. The solid-state monolayer was then transferred on PS prepatterned ITO substrates. Potentiostatic Electrochemical Deposition. The electrochemical synthesis was carried out in a three-electrode cell using a potentiostat (CH Instruments) to electrodeposit CdSe on interstitial voids of the PS bilayered templates. A platinum wire and an Ag/AgCl electrode were used as counter and reference electrode, respectively. An aqueous solution of 0.25 M CdSO4 and 0.25 mM SeO2 was used as the supporting electrolyte (pH = 2.5). The pH was controlled with H2SO4. A constant potential (potentiostatic method) of −0.655 V versus Ag/AgCl was applied on the working electrode for various electrodeposition times. Then, the electrodeposited film was immersed in toluene to remove the PS template.
structures. In many cases, one simply selects a specific colloid size and lives with the resulting 2D/3D structure. Therefore, development of new templating methods will enable improved directed control in the fabrication of shape-tuned porous structures as well as unconventional device architectures. Our goal is to create hierarchically patterned porous CdSe layers, where two types of lithographic processes would be used in tandem to yield valuable new nano- and microstructures. We hypothesized that colloidal arrays deposited on top of nanopatterned surfaces would yield new hierarchically patterned CdSe layers with the additional benefit of a rich and diverse collection of previously unavailable patterns. Design parameters for this hierarchically patterned methodology include colloid size as well as the geometry of the underlying pattern. The underlying patterns can be formed by traditional top-down lithographic processes, in this case nanoimprint lithography (NIL), and the top layer uses the self-assembly of colloidal particles to create the nanoporous surface of the CdSe. Accordingly, we describe our new method for the fabrication of size/shape-tunable nano/microporous CdSe thin films via potentiostatic electrochemical deposition of hierarchically templated colloids. We use PS bilayered templates consisting of a microscale PS pattern as a bottom layer and a PS colloidal array as a top layer. Various PS bottom layer patterns (lines, ellipsoids, and circles) are prepared by NIL followed by a reactive ion etching process. Colloidal self-assembly is used to make highly packed PS colloidal arrays which serve as the top layer of the bilayered template. Electrochemical growth of CdSe crystals in the voids of the PS bilayered templates followed by a removal of templates results in 2D hexagonal porous CdSe structures. This simple and effective technique provides a route toward a variety of unconventional porous CdSe films, which arise from the structure of the PS bottom template. This patterning takes advantage of two simple processing steps, one top down directed patterning followed by a quick and controllable self-assembly step, allowing for new ways of manipulating geometries potentially suitable for electronics and optoelectronic devices.
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EXPERIMENTAL SECTION
Materials. Polystyrene latex beads (0.2, 0.5, 0.75, and 1.0 μm in diameter, 2.5% solids (w/v) aqueous suspension) were purchased from Alfa Aesar Corp. and used without any further purification. These particles contain a slight anionic charge from the sulfate ester. Cadmium sulfate (CdSO4), selenium oxide (SeO2), sodium dodecyl sulfate (SDS), and propylene glycol monomethyl ether acetate (PGMEA) were purchased from Sigma-Aldrich Co., and polystyrene 13150
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Scheme 1. Schematic Illustrating the Nanopore Formation Process: (a) NIL of PS Features on ITO-Coated Glass; (b) Primary Electrodeposition of CdSe; (c) Preparation of PS Colloidal Array; (d) Conformal Deposition of PS Colloidal Array on Substrate; (e) Secondary Electrodeposition of CdSe; and (f) Dissolution of PS Templates To Yield Porous CdSe
Characterization. Atomic force microscopy (AFM, Digital Instruments, Dimension 3000/3100) was used to investigate surface morphologies and surface analysis. AFM measurements were carried out with a piezoscanner at room temperature. Commercially available tapping mode tips (Veeco, phosphorus n-doped Si, f 0: 312−342 kHz) were used on cantilevers with a resonance frequency in the range of 290−410 kHz. All images (AFM topography, tapping mode) were flattened, filtered, and analyzed by using SPIP software (Scanning Probe Image Processor). All data in the dimensions of all the patterns were collected and averaged by at least 10 measurements via line profilometry of AFM images. For SEM images, a JSM-6700F SEM was used to investigate the surface topographies of the porous CdSe microstructures. For optical properties of the porous CdSe matrix, ultraviolet−visible (UV−vis) spectroscopy (Shimadzu UV 3600) was used. Crystal structure of electrochemically deposited porous CdSe was investigated by X-ray diffraction (PANalytical B.V.) with Cu Kα radiation (λ = 1.5418 Å).
Figure 1b shows X-ray diffraction patterns obtained from three electrodeposited CdSe thin films (nonannealed, annealed, and annealed microporous films). The peaks corresponding to the (111) and (220) planes at 25° and 43° were obtained for all of the CdSe films, indicating that the crystals have a cubic structure. This also confirms that the framework around the micropores consists of cubic CdSe crystals with (111) preferred orientation. Using the width of the diffraction peaks at halfmaximum (fwhm), the crystallite sizes for the CdSe nanoparticles were calculated to be 32 nm (nonannealed film) and 43−45 nm (annealed porous and planar film) based on the Scherrer equation D = κλ/β cos(θ), where κ is the geometrical factor, λ is the X-ray wavelength (1.542 Å), β is the line broadening at half the maximum intensity in radians, and θ is the Bragg angle.32 Next we explored the use of optimized PS bilayered templates for electrodeposition to fabricate porous CdSe structures and to understand the crystal growth process in the designed templates. In order to achieve this, the electrochemical crystal growth was immediately performed after templating each PS layer so that the mechanism of formation of nanocrystalline CdSe in each template layer could be assessed. Scheme 1 illustrates the fabrication steps used to make hierarchicaly highly ordered 2D nano/microporous CdSe lines. The process consists of four main steps: (a) Using NIL, PS line patterns were formed by imprinting patterned PDMS molds into a PS film (4 wt % in propylene glycol monomethyl ether acetate, PGMEA) that was spin-coated onto an ITO-coated glass substrate, and then any residual material between the lines (scum layer) was removed with a reactive ion etch (RIE) O2 plasma. (b) CdSe polycrystals were electrochemically grown (potentiostatic method) from the exposed ITO surface defined by the imprinted PS features. The electrochemical deposition conditions were optimized so that the height of the grown CdSe was almost identical to that of the templated PS lines. (c) A hexagonally packed 2D PS colloidal monolayer was prepared and (d) conformal placement on top of the patterned PS/CdSe layer. (e) A second electrochemical deposition was employed to grow CdSe vertically from the CdSe underlayer within the interstitial area of the top PS colloid array layer. The open shapes of the nano/microporous CdSe structures were tunable by adjusting electrochemical deposition time and size of PS colloid. Using NIL, PS lines (W = 780 nm, P = 1450 nm, and H = ∼137 nm) were first patterned on ITO-coated glass
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RESULTS AND DISCUSSION In order to establish the ideal experimental parameters, layers of planar CdSe crystals electrodeposited from ITO surfaces were first thoroughly investigated with regards to optical properties and surface topography (see Supporting Information for a detailed description of deposition conditions). Next, CdSe deposition templated by a 2D colloidal coating of PS particles was explored. In both cases conditions for the deposition of CdSe thickness ranging from 130 to 160 nm were optimized. Details of the colloid array formation for the porous films are also described in the Experimental Section and Supporting Information (Figure S2). The absorption properties of the homogeneous planar and porous CdSe thin films were studied using UV−vis spectroscopy. Figure 1a shows the CdSe thin films have broadband absorption at an optical wavelength from 445 to 745 nm. After annealing at 150 °C for 1.5 h, the absorbance develops a shoulder at 720 nm, consistent with previously reported data.3 Interestingly, the porous film derived from the PS colloidal array (d = 500 nm) shows dips in absorption at the wavelength of 618 and 675 nm due to interference and scattering in the visible region, measured at normal incidence (θ = 90°). A similar phenomenon has been reported on nanostructured porous metal films using reflection spectra.29 In addition, the optical band gap of all the deposited CdSe thin film is 1.65 eV, identical to the reported experimental value obtained by extrapolating a line to the hυ axis on a plot of (αhυ)2 vs hυ.30,31 13151
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Table 1. Dimensions of Nano/Microporous CdSe Line Patterns Fabricated via Electrochemical Synthesis on PS Bilayered Templates Obtained from a Combination of Soft- and Colloidal-Lithography Procedures sample no.
PS colloid [nm]
electrodeposition time [s]
CdSe line width [nm]
height [nm]a
total height [nm]b
CdSe pore diameter [nm]
1 2 3
1000 500 200
500 250 150
∼980 ∼980 ∼980
60 ± 6 50 ± 4 55 ± 3
209 ± 7 112 ± 3 80 ± 4
443 ± 17 195−200 117−120
a
Distance from the ITO substrate to the bottom of each pore (the actual height of CdSe patterned lines). bDistance from the ITO substrate to the small spandrels of the CdSe film.
Figure 2. Formation of nano/microporous CdSe patterned strips after being grown cathodically on a second PS colloid template arranged on alternating PS/CdSe line patterns using potentiostatic method, followed by removing the PS bilayered templates. For all samples the PS line template dimensions were W = 780 nm, P = 1450 nm, and H = 140 nm and PS colloid array: (a) and (b) d = 1000 nm, (c) and (d) d = 500 nm, and (e) SEM and (f) AFM d = 200 nm.
lines. AFM and SEM micrographs in Figure 2e,f show nanoporous CdSe lines obtained from 200 nm PS colloids used as a second template. After the film growth on the colloidal template, there was no significant change observed in the surface topography until the PS template was removed. The resulting structure was a result of the thickness of the material deposited in the interstitial voids during the short period of the electrochemical deposition (150 s) which was much thinner than the height of PS colloid arrays (t ≪ dps). After the removal of the PS templates (line and particles) with toluene, the 2D porous array (d = ∼120 nm and t = 55 ± 3 nm) with a constant pore-to-pore distance was formed. The extraction process of the hexagonal close-packed array of PS colloids did not disrupt the regularity of the resultant porous CdSe array film. The center-to-center distance (200 nm) between each pore was almost identical to the diameter of the PS colloid template. The total thickness of the thin CdSe film, including small concave triangular gaps (spandrels) among the holes, was found to be ∼89−93 nm. The film growth appears to be below the radius of the sphere template (d < 200 nm), and the open size of the pore was found to be controllable. Table 1 (above) summarizes the dimensions of the CdSe porous line patterns, measured
substrates. After etching and electrochemical deposition of CdSe, the observed height difference between the PS lines and CdSe lines slightly varied (±13−16 nm) depending on the exact height of the etched patterned PS film and electrodeposition conditions. A series of three PS colloids (PS1000: 1000 nm; PS500: 500 nm; PS200: 200 nm in diameter; see Table 1) were arranged on these PS/CdSe patterned substrates. The electrochemical deposition of CdSe thin films was reinitiated around these templated colloids, nucleating from the underlying CdSe lines and filling in the voids. As shown in Figure 2a−d, the resulting microporous CdSe array was hexagonally structured on the underlying CdSe patterns. In Figure 2, filling defects were observed, as outlined by white dotted rectangles. These microperiodic pores obtained from the bilayered templates using PS1000 and PS500 colloid arrays, respectively, do not have any pinhole defects in the bottom of the pore, which are generally observed on 2D/3D porous structures fabricated using only colloidal lithography. The top-open shapes and sizes of the pores on the CdSe stripes can be controlled with the size of PS colloids and electrochemical deposition time. In addition, side-open porous structures were observed in areas where the PS colloids were collocated in the intermediate zones between the PS and CdSe 13152
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Next, the PS colloid arrays were deposited onto substrates with patterned PS prior to CdSe deposition (T1 and T2) (see Scheme 2). This presented a slightly different type of bilayered template than those described in Scheme 1. In the latter case, a PS bilayered template is directly on the ITO with no thin CdSe layer. A one-stage electrodeposition of CdSe from the exposed ITO was then performed. After removal of the PS bilayered template, a new series of porous CdSe films were fabricated. The films exhibit a unique microporous CdSe structures due to the crystal growth inhibited on the bottom template. As shown in Figure 3c, the trench width among porous CdSe stripes was narrower than that in the CdSe lines in the inset of Figure 3c (rescaled optical micrograph of Figure 3a). This significant difference was due to the unconventional crystal growth preferentially overfilling the PS trenches and electrodepositing CdSe within the PS colloid array and sequentially lateral growth across the top of the imprinted PS line patterns. In addition, two distinctive features appeared on the porous CdSe lines, in which a zigzag porous structure was formed on each CdSe stripe at region I, whereas the pores were arranged along the lines at region II. This moiré-like interference pattern is mainly attributed to the different orientations of two different PS colloidal crystal domains on top of the PS striped template. The optical micrograph (Figure 3e) confirms that full or half pores appeared along the electrochemically patterned CdSe stripes as shown in the region II of Figure 3c. A longer electrochemical deposition time (1500 s) led to thicker microporous CdSe film growth that exhibited two distinguishable structures (closed and side-open pores) (see Supporting Information Figure S6). The side-open pores could be generated in areas where the colloids were placed in between the PS pattern and trench. In Figure 3d, with the bilayered template, T2, the trenches among the porous CdSe stripes were extremely narrow (red arrows) or disappear due to discontinuous interconnection of the strips in the process of the excessive crystal growth over the relatively smaller PS striped template than that used in Figure 3c. The radii of the porous CdSe structure were between ∼512 nm (dmin) (region III) and ∼590 nm (dmax) (region IV) depending on either partial or full occupation of PS colloids on the trenches. More clearly, it is observed that the radius of the porous component increases sequentially along the white arrow
from AFM micrographs in Figure 2f and Supporting Information (Figure S5). The study of electrodeposition using the colloidal template described above allowed the investigation into the effects of the geometry of the bottom template on the resulting CdSe growth patterns. Next, a variety of PS bilayered templates were fabricated. Table 2 summarizes all the dimensions of the PS Table 2. Dimensions of All the PS Bilayered Templates Prepared by Soft Imprint Lithography and Colloidal Lithography PS bottom template [nm] PS bilayered templates T1 T2 T3 T4 a
stripes (I) stripes (II) twisted ellipsoidal features circular features
width (W)
period (P)
height (H)
PS top colloidal template [nm]a
780 550 1000
1450 800 1250
140 131 75−80
1000 750 750
750
1500
6
750
The diameter of the PS colloids.
bilayered templates used. We demonstrated that the electrodepostion on these as-prepared PS bilayered templates can create more interesting, unconventional microporous structures via control of a CdSe crystal growth pathway. In the next set of experiments, two different types of bilayered templates with line-patterned bottom layers templates T1 (780 nm wide) and T2 (550 nm wide) were used for the deposition of patterned CdSe (for complete dimensions of the T1 and T2 patterns, see Table 2). First, electrodeposition of CdSe from the PS line-patterned ITO substrates with no PS colloids was performed using the same line patterns as those in T1 and T2. After deposition, the PS lines were dissolved leaving lithographically defined CdSe lines. As shown in Figures 3a and 3b, these CdSe crystal stripes were precisely grown from the lithographically selected areas on the conducting substrates. The height of each CdSe stripe is about 90 and 73 nm, respectively.
Figure 3. SEM micrographs of CdSe lines electrodeposited from only PS line templates [(a) in T1 and (b) in T2]; microperiodic porous CdSe lines electrodeposited from PS strip/colloid bilayered templates [(c) T1 and (d) T2]; (e) optical micrograph of the region II in (c). 13153
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Scheme 2. Schematic Illustrating the Nanopore Formation Process Variant: (a) NIL of PS Features on ITO-Coated Glass and Preparation of PS Colloidal Array; (b) Conformal Deposition of PS Colloidal Array on Substrate; (c) Electrodeposition of CdSe; and (d) Dissolution of PS Templates To Yield Porous CdSe
component was larger than the size of the templated ellipsoidal feature due to the excessive growth of the CdSe crystals overfilling the limited volume of the feature during the electrodeposition. The twisted ellipsoidal CdSe crystal arrays comprise various sizes and shapes of microporous structures depending on the location of the arranged colloids on each 75− 80 nm deep feature. Each patterned ellipsoidal component was observed to possess 2−6 micropores, and the diameter of the biggest pore formed was close to 750 nm, which was identical to the diameter of the PS colloid used. By tracing the arrangement of the colloids, as shown in the white dashed circles in Figure 4c, it can be seen that the CdSe porous structures arise from a hexagonal structured array of PS colloids. However, once another template patterned with smaller twisted ellipsoids was used, significantly different features were observed. It was similar to conventional CdSe porous structures grown from a 2D PS colloidal monolayer due to a dimensional matching (almost identical diameter) and a pattern type (hexagonal structure) of the bottom features and top colloids (see Supporting Information Figure S7). Thus, dimensional intercorrelations between PS sublayered templates and colloids were both an important parameter and controllable to enable nano/microstructuring in these porous thin films. Finally, a PS bilayer comprised of a circular bottom template and colloidal (d = 750 nm) arrays, T4, was studied. Considering the PS template with circular features, as shown in Figure 5a, the resulting patterned components of the CdSe porous film were interconnected to one another due to the overgrowth along the walls of the holes. Each CdSe pattern component has one to three top-open pores with different diameters (within 346−577 nm) depending on the precise location of the colloidal array on top of the underlying circular pattern. The majority of the pattern components have only one pore with a maximum diameter of ∼577 nm arranging toward a specific direction (indicated by a white arrow). This structure likely occurred when a PS colloid was situated on the center of a circular feature on the bottom template. The diameter matching of the circular feature and the colloid increases the possibility of one-to-one stacking on the bilayered template. In contrast, two to three pores per CdSe pattern component are observed in other areas (indicated by a red arrow in Figure 5b) due to the stacking of the corresponding number of the colloids on top of each circular feature. The template features of the PS bottom layer dominate the formation of the overall porous CdSe structures, and the geometric structure of the stacked bilayered template controls the size and shape of the pores. In conclusion, we have demonstrated that the unconventional 2D nano/microporous semiconducting materials can be fabricated from PS bilayered templates through electrochemical deposition and lithography. We have shown that any arbitrary template can give rise to porous geometry. The variety of the
direction since the location of the PS colloids on the bottom template contributes to the distinct change in height, thus reflecting a variable range of pore sizes between dmax and dmin. Therefore, the electrochemical control of the porous CdSe structures highly depends on geometric shapes of PS bilayered templates. Pattern variation by changing the geometry of the bottom template was investigated next. A PS bilayer-templated film, T3, consisting of twisted ellipsoidal template and colloidal array was used to template the electrochemical deposition of CdSe nanocrystals (Figure 4a). The crystals grew from the ellipsoidal features on the ITO substrate, avoiding the PS colloids arranged on top of the bottom template. After the template removal, Figures 4b,c demonstrate that each pattern
Figure 4. (a) AFM micrograph of a PS bottom template with twisted ellipsoidal features and (b, c) SEM micrographs of microporous CdSe crystals formed via electrochemical growth on the PS ellipsoidal feature/colloid bilayered template (T3), followed by the removal of PS by dissolution with toluene. 13154
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Award DE-SC0001087. We further thank the National Science Foundation for facility support through the Center for Hierarchical Manufacturing at UMass, CMMI-1025020.
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Figure 5. SEM images of (a) a PS bottom template with circular features and (b) microporous CdSe crystals grown cathodically on the PS circular feature/colloid bilayered template (T4).
types of the porous structures produced can mainly be attributed to be the unique combination of the initial bottom templates and the resulting geometric structures of stacked PS bilayered templates. Considering that both of these determining factors are controllable (bottom template geometry and colloid size), it is expected that this novel templating method can be used as a new control route to develop various nano/ microporous structures of semiconductors, even in conducting polymeric materials. Furthermore, beyond providing a new avenue in colloidal lithography, this method can be used to effectively control and tailor bulk optical properties (e.g., reflectivity and transmission) for light trapping in solar cell applications. We feel this area is a new direction for advanced patterning. While we note that we introduce this work with relatively simple sets of patterns, undoubtedly theory and modeling approaches that have not yet been exploited will add the richness and complexity of patterns that can be prepared by this powerful new method.
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ASSOCIATED CONTENT
S Supporting Information *
Additional experimental data and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Department of Renewable Energy, Jungwon University, Chungcheongbuk-do, 367-805, South Korea.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported in part by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science Polymer-Based Materials for Harvesting Solar Energy at UMass, an Energy Frontier Research Center (EFRC) under 13155
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