Article pubs.acs.org/cm
Epitaxial Growth of Three-Dimensionally Mesostructured SingleCrystalline Cu2O via Templated Electrodeposition Jinwoo Kim,†,¶ Ha Seong Kim,†,¶ Jun Hee Choi,†,§,¶ Hyeongtag Jeon,‡ Yohan Yoon,∥ Jinyun Liu,† Jea-Gun Park,⊥ and Paul V. Braun*,† †
Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States ‡ Department of Materials Science and Engineering, Hanyang University, 133-791 Seoul, Korea § Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 443-803, Korea ∥ Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ Department of Electronics and Computer Engineering, Hanyang University, 133-791 Seoul, Korea S Supporting Information *
ABSTRACT: Significant efforts have been made over the past few decades to realize functional three-dimensional photonic crystal devices including zero-threshold lasers, waveguides, light-emitting diodes (LEDs), and solar cells; however, progress has been limited because of difficulties in creating three-dimensional photonic crystals from single-crystal materials. Most have been formed from polycrystalline materials and thus have generally exhibited poor electronic properties. Realization of materials containing complex three-dimensional mesostructures, in single-crystal form, remains a significant challenge. Here, we demonstrate the epitaxial growth of three-dimensionally mesostructured Cu2O by bottom-up electrodeposition through a 3D silica colloidal template. Not only is the templated Cu2O single crystal, but when the electrodeposition continues past the colloidal template, the crystallinity of the overlying solid Cu2O appears to be improved because the template blocks threading dislocations, resulting in substantial reductions in the dislocation density of the overlying solid Cu2O.
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INTRODUCTION Despite significant efforts over the past few decades to realize functional 3D photonic crystal devices including zero-threshold lasers, waveguides, light-emitting diodes (LEDs), and solar cells, progress has been limited at least in part because of difficulties in creating three-dimensional photonic crystals from single-crystal materials with the requisite low defect density. Most 3D photonic crystals have been formed from polycrystalline materials and thus have exhibited poor electronic properties. Realization of materials containing complex 3D mesostructures, in single-crystal form, remains a significant challenge. This is in stark contrast to the many successes in the epitaxial growth of single-crystal films of a wide set of chemistries for electronic and optical applications.1−4 Many groups have reported 3D mesostructured amorphous and polycrystalline materials;1,4−6 however, only two materials, GaAs6 and GaN,7 have been 3D mesostructured in singlecrystal form, both as inverse opals, using a selective area epitaxy chemical vapor deposition (CVD) process. While the GaAs structure was demonstrated to be optoelectronically active, Xray analysis revealed it still contained stacking faults and other defects. Furthermore, both the GaAs and GaN were grown via expensive and slow gas-phase deposition procedures. Expansion of the materials “toolbox” to include additional singlecrystalline materials that can be grown in a 3D mesostructured form, further reductions in defect density, and more efficient © XXXX American Chemical Society
growth procedures are all important to enable future functional 3D mesostructured devices. A diverse set of materials have been fabricated in 3D mesostructured form by approaches including layer-by-layer micromanipulation, direct ink deposition, and wafer bonding.8−12 Many materials have also been templated into 3D mesostructured form through colloidal templating, including titania,13,14 germanium,15 tungsten,16 selenium,17 CdSe,18 and calcite19 via dry processes (e.g., CVD15 and atomic layer deposition (ALD)16), melt imbibing,17 wet chemistries (e.g., sol−gel and precipitation13,18,19), and ceramic technique.20 However, except for the aforementioned GaAs and GaN examples, the infilled material was polycrystalline or amorphous. Here, for the first time, we demonstrate the epitaxial growth of 3D mesostructured Cu2O by bottom-up filling via electrodeposition through a 3D silica colloidal template. The nature of epitaxy was not only preserved during the growth through the complex geometry of template, but analysis showed that, when the electrodeposition continues past the colloidal template, the template improved the crystallinity of the overlying solid Cu2O by blocking threading dislocations. Cuprous oxide (Cu2O) is of particular interest as a wellReceived: September 22, 2014 Revised: November 29, 2014
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Figure 1. Schematic illustrating key steps for epitaxial electrodeposition of Cu2O through a colloidal crystal template. (a) Epitaxial growth of pregrown Cu2O on Si (001) via electrodeposition. (b) Self-assembly of a silica colloidal crystal on the Cu2O layer. (c) Electrodeposition of singlecrystal Cu2O into and through the silica colloidal template. Electrodeposition of Epitaxial Cu2O. Electrodeposition of Cu2O was performed using a conventional three-electrode setup.31 The three electrode cell was submerged into a silicone oil bath to heat the solution to 65 °C. Once this temperature was reached, the pH was readjusted to 9.0 by addition of sodium hydroxide. A 1 cm × 3 cm piece of p-type silicon (001) wafer (WRS Materials, 0.005−0.025 Ω cm resistivity) was treated with 1 min dip in 5% hydrofluoric acid (HF), 15 min dip in boiling Millipore water, and another 5% HF dip for 20 s prior to submerging the substrate in the 65 °C electrolyte for 1 min. The prepared substrate was used as the working electrode. Epitaxial Cu2O film was then deposited by applying −0.45 V vs standard calomel electrode reference electrode at 65 °C to the silicon substrate. A 2 cm × 4 cm platinum (Pt) foil served as the counterelectrode. Fabrication of the Silica Opal Template. Synthesized silica colloidal nanoparticles were assembled as previously described.32 Synthesized silica nanoparticles were dispersed in 200-proof ethanol to prepare a ∼3 wt % suspension. After drying the Cu2O coated silicon substrate with air, each substrate was placed at a 30° angle in a 20 mL glass scintillation vial containing the colloidal suspension at 33−37 °C for growth of the 3D colloidal template on the substrate. Fabrication of the Polystyrene Opal Template. Carboxylated polystyrene (PS) colloids (400 or 500 nm diameter) were dispersed in Millipure water to prepare an ∼4 wt % suspension. After rising and drying the bare silicon or Cu2O coated silicon substrate with air, each substrate was placed at a 30° angle in a 20 mL glass scintillation vial containing the colloidal suspension at 55 °C. As the water evaporated, the colloidal crystal grew on the substrate. Cu2O was then epitaxially electrodeposited through the 500 nm PS opal template following the procedure outlined above. Finally, the PS colloids were dissolved with tetrahydrofuran (THF) to form the 3D mesostructured Cu2O crystal. Optical Characterization. Reflectance spectra were collected from an ∼250 μm diameter spot on the sample using a Vertex 70 FTIR and a Bruker Hyperion microscope. The reflectance from 1.0− 1.8 μm was collected using a 10× glass microscope objective (numerical aperture = 0.25), a quartz beam splitter, and a liquid nitrogen-cooled InSb detector. Photoluminescence (PL) measurements were performed using a confocal Raman microscope (Nanophoton Raman 11) using 532 nm excitation, a 100× objective (∼1.0 μm diameter spot size), and a Si detector at room temperature. Raman spectra were collected using the same objective at room temperature with a resolution of 1 cm−1 using the same laser at a power of 2 mW.
known nontoxic and earth-abundant p-type semiconductor with a band gap of 2.17 eV. It has been suggested for applications in photocatalysis, photovoltaics, gas sensing, and lithium-ion batteries as an anode.21−23 For many of these applications, in particular those where photoexcited species are important, a single-crystalline material is expected to have lower rates of recombination and, thus, better properties.21−24 Singlecrystalline materials also generally exhibit better motilities due to lower rates of grain boundary scattering. Of particular relevance for this study, as Switzer and co-workers first demonstrated, Cu2O can be epitaxially electrochemically grown on single-crystalline substrates,25−29 providing the starting point for the work presented here.
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EXPERIMENTAL SECTION
Synthesis of 400 nm Silica Nanoparticles. A modified Stöber method was utilized to prepare 400 nm diameter silica colloids.30 Briefly, 10.6 g of Millipore water (18.2 MΩ resistivity), 3.1 g of ammonium hydroxide solution (28−30% NH3, Macron Chemicals, lot no. K36036), and 100 mL of 200-proof ethanol (Decon Laboratories lot no. 165211) were added to a 1000 mL round-bottom flask and stirred slowly for 30 min. After mixing for 30 min, 5.25 g of tetraethyl orthosilicate (TEOS, Sigma-Aldrich lot no. BHBV4940) was added to the mixture. The flask was capped loosely, and the contents were stirred at 350 rpm for 24 h. After 24 h, roughly spherical, ∼300 nm diameter SiO2 colloids were formed. The polydispersity of these colloids was reduced and the diameter increased by a series of growth steps. Each SiO2 growth step was performed by adding 25% of the initial reactants every 12 h until a desired average diameter of 400 nm was reached. The colloids were purified by employing a series of centrifugations in ethanol. The remaining solvent was evaporated, and the particles were heat-treated at 600 °C for 10 h to densify the particles. Electrolyte Preparation. An aqueous solution of 0.4 M copper sulfate pentahydrate (CuSO4·5H2O, Fisher Scientific, lot no. 110549) and 3 M lactate ion (85% lactic acid solution, Sigma-Aldrich, lot no. KMBK2646 V) solution was prepared in 200 mL portions. To a 200 mL glass container, ∼75 g of 85% lactic acid solution was poured. Afterward, Millipore water (18.2 MΩ resistivity) was added until the 150 mL mark. The bottle containing the acidic solution was submerged into an ice bath, and ∼17.5 g of sodium hydroxide (Fisher Scientific) was slowly added while being stirred. The solution was left in the ice bath to cool the heat released from the exothermic reaction of acid and base and returned to room temperature. Twenty g of CuSO4·5H2O was added prior to adding additional Millipore water until a total volume of 200 mL was reached. Prior to electrodeposition, the pH of the electrolyte was adjusted to 9.0 at 65 °C using small amounts of NaOH and/or lactic acid solutions.
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RESULTS AND DISCUSSION Growth of 3D Mesostructured Cu2O. The epitaxial templated electrodeposition of 3D mesostructured Cu2O was performed as outlined in Figure 1 (for details, see Experimental Section). About 500 nm of [001]-oriented Cu2O was then grown by electrodeposition (Figure 1a). This “pregrown” layer B
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observed in the SEM image of the initial Cu2O growth on the Si (001) substrate (Supporting Information, Figure S4). Determination of Epitaxial Growth. Infiltration of Cu2O into a template is only evidence of bottom-up filling, not epitaxy. The nature of epitaxy of ∼15 μm thick 3D mesostructured Cu2O was quantitatively verified using 2θ/ω X-ray diffraction (XRD) and X-ray pole figures (Figure 2c and d). As shown in Figure 2c, a 2θ/ω scan probes the out-of-plane lattice spacing between the 3D mesostructured crystal and the substrate. Aside from the (004) peak of the silicon substrate, the 2θ/ω scan shows two distinct peaks corresponding to the (002) and the (004) orientation of the Cu2O crystal, indicating that only (001) planes of the 3D mesostructured Cu2O are parallel to the substrate surface even within the complex geometry of the template. The lattice parameter of the Cu2O is determined to be a = 0.425 nm, in agreement with the bulk lattice parameter of 0.4252 nm (JCPDS file no. 03-0898), indicating that the mesostructured Cu2O is relaxed. The inplane orientation of templated 3D mesostructured Cu2O was confirmed by the X-ray pole figure. Figure 2d shows a (111) pole figure of the 3D mesostructured Cu2O, obtained by setting 2θ = 53.37° for the (111) planes of Cu2O. Four sharp peaks are observed, each one at ψ =54.71° versus the substrate normal and 90° with respect to each other, suggesting that the Cu2O layer is single-crystalline, or at least does not contain large-angle grain boundaries. The appearances of four symmetric (111) poles indicate the layer has a ⟨001⟩ direction parallel to the [001] direction of the Si substrate constituting the substrate normal.27−29,36 Figure 3a shows azimuthal scans of the 3D mesostructured Cu2O, obtained by tilting the sample to ψ = 54.74 and selecting the Cu2O (111) planes at 2θ = 36.45 and the Si (111) planes at 2θ = 28.47. Parts b and c of Figure 3 illustrate the possible epitaxial relationship of the 3D mesostructured Cu2O with the Si substrate. Cu atoms are blue, O atoms are red, and Si atoms are green. The alignment of the Cu2O with the Si substrate was obtained by an XRD azimuthal scan of 3D mesostructured Cu2O (Figure 3a) and resulted in a 3D mesostructured Cu2O (001)[100]//Si (001)[110]. The structure of Cu2O is cubic with lattice parameter of 0.4270 nm (space group Pn3m), and the structure of Si is diamond cubic with lattice parameter of 0.5431 nm (space group: Fd3m).25−29 Cu2O can be considered as two interpenetrating lattices: a face-centered cubic (fcc) lattice occupied by the Cu atoms and a body-centered cubic (bcc) lattice occupied by O atoms. The Si also consists of two interpenetrating fcc lattices, both of which are occupied by Si atoms. Despite large apparent lattice mismatch (LM) = (aSi − aCu2O)/acu2O = (0.5431 − 0.4270 nm)/0.4270 nm = 27.2%, we observed that the LM of the 3D mesostructured Cu2O can be reduced to −10.1% by a 45° in-plane rotation of 3D mesostructured Cu2O lattice relative to that of Si around [001] as illustrated in Figure 3b and c (the crystallographic relationship of Cu2O (001)[100]//Si (001)[110]). Study of initial heteroepitaxial growth of the Cu2O layer on Si (001) was conducted because this layer (Figure 1a) plays an important role in the epitaxial growth of Cu2O through the colloidal template. Once sufficiently thick, this layer provides a uniform [001] oriented Cu2O onto which first the colloidal template, and then additional Cu2O, is grown. XRD was performed on 100, 200, and 400 nm thick Cu2O layers grown on the Si (001) substrate. SEM images of each film are shown in Figure 4a, and 2θ/ω scans of each sample are shown in Figure 4b and c. As shown in Figure 4a, Cu2O first exhibits a
provided a well-aligned [001]-oriented Cu2O epitaxial layer. A self-assembled opal was then grown on the substrate from 400 nm diameter silica colloids (Figure 1b). Finally, Cu2O was epitaxially electrodeposited through and above the silica opal template (Figure 1c). If the pregrown solid [001]-oriented layer was not used, the templated Cu2O was polycrystalline. As expected based on previous work,25−29 we confirmed that, under these growth conditions when a thick solid Cu2O film was electrodeposited, it also grew epitaxially (Supporting Information, Figure S1). The scanning electron microscope (SEM) cross-sectional image in Figure 2a shows a representative 3D mesostructured
Figure 2. Characterization of 3D mesostructured epitaxially grown Cu2O. (a) Cross-sectional SEM of Cu2O epitaxially grown through a template formed from 400 nm diameter silica colloids. Inset shows the Cu2O completely filling the complex geometry of the template. (b) Cross-sectional SEM at the interface of Cu2O and the Si substrate. The initial Cu2O seeds at the substrate−Cu2O interface are outlined by dotted lines. (c) 2θ/ω X-ray diffraction scan of ∼15 μm thick 3D mesostructured epitaxial Cu2O. (d) Pole figure (111) X-ray diffraction measurement of the same Cu2O crystal. Four sharp peaks are observed at ψ = 54.71°. The scale rings are in 30° increments.
Cu2O crystal grown through and above a template formed from 400 nm diameter silica colloids. The inset of Figure 2a provides a higher magnification image of the interface between the templated and overgrown Cu2O layer. The structure of the 400 nm silica colloidal template on the solid Cu2O layer was confirmed by SEM before Cu2 O infilling (Supporting Information, Figure S2). As shown in the inset of Figure 2a and Figure S3, Supporting Information, Cu2O successfully grows through the complex geometry of the template. Initial bottom-up growth of Cu2O begins in the [001] direction from the Si (001) substrate surface; however, to grow around the silica template, the growth front propagates in various directions as observed in the SEM image of the partially infilling of a template, where the growth front moves off-normal to propagate around the template (Supporting Information, Figure S3). Figure 2b reveals that the Cu2O layer initiates as three-dimensional islands (outlined by dotted lines), with diameters ranging from about 10 to 50 nm. Growth then continues through the nucleation and coalescence of adsorbate “islands”, following Volmer−Weber or Stranski−Karastanov growth modes.33,34 Growth and coarsening of these islands results in a rough surface on the substrate,33−35 such as that C
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Figure 4. Initial growth of the heteroepitaxial Cu2O layer on the Si (001) substrate. (a) SEM images of (i) 100, (ii) 200, and (iii) 400 nm thick heteroepitaxially grown Cu2O on Si. Initially the Cu2O surface exhibits rough three-dimensional pyramid shapes (i), which condense into a continuous phase as the thickness increases (ii and iii). (b, c) High-resolution 2θ/ω X-ray scans focusing on the Cu2O (111), (002), and (004) ranges for 100, 200, and 400 nm thick Cu2O films.
(111) peak and strong (002) and (004) peaks are visible. The ratio of the intensity of the (002) to (111) peak is now ∼485, suggesting that the film becomes a [001]-oriented Cu2O crystal significantly before the film is 400 nm thick. There may still be some degree of mosaicity in the Cu2O film; however, as shown in the various X-ray pole figures (e.g., Figure 2d), this does not lead to polycrystallinity in the templated Cu2O. If Cu2O is grown directly through a colloidal template on a Si (001) substrate, the resulting Cu2O is polycrystalline. In this experiment, a template was self-assembled from 400 nm diameter PS colloids directly on a Si (001) substrate (Figure S5a and b, Supporting Information). The native SiO2 was then etched by 5% HF, followed by “electroless” Cu2O seed deposition for 60 s. After Cu2O electrodepostion, a wellordered Cu2O inverse opal structure was observed after the PS colloids were dissolved using tetrahydrofuran (Figure S5c, Supporting Information). A 2θ/ω scan on the Cu2O inverse opal appears to show only a distinct Si (004) peak and negligible Cu2O peaks (Figure S5d, Supporting Information). After zooming in (Figure S5e, Supporting Information), Cu2O (110), (111), (002), and (220) peaks are observed, indicating that the Cu2O is polycrystalline. The direct contact of the PS particle with the Si substrate and the small pore size (∼146 nm) formed by three touching 400 nm diameter colloids only 200 nm above the substrate might prevent the initial Cu2O seeds from merging and growing into an epitaxially oriented Cu2O layer. It may also be possible that incomplete native SiO2 removal by post HF cleaning due to the complex geometry of the template inhibits the initial nucleation. Regardless, it is clear that the pregrown Cu2O layer plays an important role in achieving 3D mesostructured single-crystal Cu2O. In addition, Cu2O directly grown using a silica opal (400 nm diameter colloids) without a seed layer is also polycrystalline, supporting the hypothesis that the pregrown seed layer plays a key role in forming opal-templated single-crystalline Cu2O. Figure 5 shows cross-sectional high-resolution transmission electron microscope (HR-TEM) and selective area diffraction (SAD) patterns collected from various regions of the 3D mesostructured Cu2O. As shown in Figure 5a, the HR-TEM analysis was carried out at three different regions: (i) bottom of silica template (region a), (ii) middle region of the templated
Figure 3. Epitaxial relationship of 3D mesostructured Cu2O. (a) Azimuthal XRD scans of (111) peaks of 3D mesostructured Cu2O (ψ = 54.74 and 2θ = 36.45) and the Si (111) peaks (ψ = 54.74 and 2θ = 28.47). (b, c) Schematic illustrations of possible epitaxial relationship between 3D mesostructured Cu2O and the Si substrate. Cu, O, and Si atoms are blue, red, and green, respectively. The solid lines indicate proposed coincidence unit cells.
three-dimensional pyramidal morphology (Figure 4a.i) and grows into islands of the condensed phase as the thickness increases (Figure 4a.ii and a.iii). 2θ/ω X-ray scans (Figure 4b and c) reveal heteroepitaxial Cu2O orientation for each sample. The 100 nm thick Cu2O film exhibits a strong (111) peak and weak (002) and (004) peaks, indicating that the [111] direction initially dominates. As the film thickness increases to 200 nm, the Cu2O (111) remains visible; however, the Cu2O (002) and (004) peaks become significantly stronger. The ratio of the intensity of the (002) to (111) peaks increases from 0.45 to 8.96 as the film thickness increases from 100 to 200 nm, indicating that the [001]-oriented Cu2O is emerging in the 200 nm thick film. For 400 nm thick film, there is no distinct Cu2O D
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Figure 6. Characterization of crystallinity of solid and 3D mesostructured Cu2O. (a) X-ray rocking curves for solid and 3D mesostructured Cu2O. The values of full width at half-maximum (fwhm) for solid Cu2O and 3D mesostructured Cu2O are 4.1° and 2.8°, respectively. (b, c) High-resolution 2θ/ω scans focusing on the Cu2O (002) and (004) ranges for 8 μm thick solid and 3D mesostructured Cu2O. 2θ/ω scans of both samples reveal a narrower fwhm in the 3D mesostructured Cu2O compared to that of the solid sample.
Figure 5. HR-TEM and SAD of 3D mesostructured epitaxial Cu2O. (a) Schematic illustration indicating location of the TEM samples. (b− d) HR-TEM and SAD of these three regions confirm the singlecrystalline nature of the 3D mesostructured Cu2O. Low-angle grain boundaries, outlined by red dotted circles, appear to decrease going upward through the template (regions a and b). No distinct defects are observed in the overgrown Cu2O crystal (region c). SAD (insets) confirm that Cu2O maintains epitaxy during growth through the complex geometry of the template. (e) Blocking of threading dislocations by the silica template was observed.
approximately the same thickness of 8 μm, are shown in parts b and c of Figure 6, respectively. High-resolution 2θ/ω scans reveal the fwhm of 3D mesostructured Cu2O to be 0.55° and 0.91° for the Cu2O (002) and (004) peaks, respectively. In comparison, the fwhm of solid sample was found to be 0.78° and 1.23° for the (002) and (004) Cu2O, respectively. The fwhm of the Si (004) peak (2θ = 69.21°) is 0.51° under our experimental conditions, providing an approximate measure of the instrument function. On the basis of the decrease in fwhm of both rocking curves and 2θ/ω scans, we suspect that the 3D mesostructuring at least does not increase the density of defects in the epitaxially electrodeposited Cu2O and may even serve to slightly improve the epitaxial alignment of the Cu2O. Optical Properties. The optical properties of the 3D mesostructured Cu2O photonic crystal were probed by collecting reflectance measurements using a Fourier transform infrared spectrometer microscope and via photoluminescence spectroscopy. Because silica colloids could not be removed from the Cu2O by HF solution without damaging the Cu2O, the 3D mesostructured Cu2O photonic crystal used for optical measurements was grown using 500 nm PS colloids, followed by removal of the PS colloids with THF. Reflectance spectra were collected from various regions on a 1 cm × 1.5 cm Cu2O inverse opal photonic crystal. Figure 7 presents typical normalincidence reflectance spectra and finite difference time domain (FDTD) reflectance simulations of the PS template and the 3D mesostructured Cu2O photonic crystal, SEM images of the top surface of the PS colloid templated 3D mesostructured Cu2O photonic crystal, and XRD 2θ/ω scan and pole figure of 3D mesostructured Cu2O inverse opal. In the reflectance spectra of the PS template (Figure 7a, red line), a peak was observed at ∼1.14 μm. Once the PS template is inverted into Cu2O, the reflectance red-shifts to ∼1.36 μm (Figure 7a, black line). Fabry−Perot interference fringes are observed as a wavy background in the structure. The reflectance intensity of the 3D
Cu2O, between several silica colloidal particles (region b), and (iii) overgrown layer on top of the template (region c). HRTEM images and SAD (insets) shown in Figure 5b−d confirm that Cu2O crystal maintains the epitaxial nature through the complex template geometry, which agrees with the epitaxial relationship observed by XRD. Small-grain boundaries (or dislocations), which are outlined by dotted circles, were observed at the bottom of the template (Figure 5b). However, the density of those defects decreases moving up through the complex template geometry (Figure 5c), and no distinct defects were observed in the overgrown (solid) Cu2O (Figure 5d). Both vertical and lateral growth occur simultaneously during the bottom-up epitaxial infiltration of the silica template. Lateral overgrowth has been commonly used in semiconductor heteroepitaxy for substantial reduction in dislocation density in the areas of lateral growth of epitaxial layer,37,38 and we suspect that the continuous lateral overgrowth as the Cu2O infills the template also serves to reduce the defect density. Additionally, blocking of threading dislocations by the template (Figure 5e) was observed. It is noteworthy that 3D silica template layers improve the crystallinity of the Cu2O layer not only via lateral growth around the colloids comprising the silica template but also due to blocking of threading dislocations. To further study the effect of 3D silica template on epitaxial growth, Figure 6a exhibits the rocking curves of Cu2O (002) from solid and 3D mesostructured Cu2O. The full width at halfmaximum (fwhm) of the solid Cu2O (002) is 4.1°, and the fwhm of the 3D mesostructured Cu2O is 2.8°. XRD performed on solid and 3D mesostructured Cu2O samples, which had E
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Growth Mechanism. Figure 8 presents schematic illustrations outlining the proposed mechanism for the epitaxial
Figure 7. (a) Measured and (b) FDTD simulated optical spectra obtained from the 3D mesostructured Cu2O photonic crystal and the PS opal template. (c) Plan view SEM images of the 3D mesostructured Cu2O photonic crystals used for the optical measurements. The inset shows a higher magnification image of the 3D mesostructured Cu2O photonic crystal. (d) 2θ/ω X-ray diffraction scan of 3D mesostructured Cu2O photonic crystal. Inset shows a pole figure (111) X-ray diffraction measurement of the same Cu2O photonic crystal.
mesostructured Cu2O photonic crystal ranged from ∼0.42− 0.45 across the sample. The measured peak positions match well with FDTD simulations (Figure 7b), which assume the Cu2O forms an inverse face-centered cubic lattice of closepacked 500 nm voids. The measured reflectances are less than the calculated intensity, which is to be expected given there are defects in the template, and the experimental samples have some degree of surface roughness. However, we note the simulated and experimentally observed position of the reflectance peak from the Cu2O structure match, providing evidence for complete inversion of the PS opal with void-free or nearly void-free Cu2O. To confirm the mesostructured Cu2O grown using the PS template grew epitaxially, X-ray 2θ/ω and pole figures were collected. Aside from the (002) and (004) peaks of the silicon substrate, the scan shows two distinct peaks corresponding to the (002) and the (004) orientations of the Cu2O inverse opal, indicating that only (001) planes of the 3D mesostructured Cu2O inverse opal are parallel to the substrate surface (Figure 7d). As shown in the inset of Figure 7d, the inplane orientation of 3D mesostructured Cu2O inverse opal was confirmed by the X-ray pole figure. Raman and photoluminescence (PL) collected from ∼2 μm thick single-crystalline 3D mesostructured Cu2O (grown using a seed layer) and polycrystalline 3D mesostructured Cu2O (without seed layer) samples are shown in Figure S6 (see Supporting Information for details). XRD 2θ/ω scans and a pole figure confirmed the single-crystalline and polycrystalline nature of the samples (Figure S6a and b, Supporting Information). The single-crystalline 3D mesostructured Cu2O exhibits sharp Raman modes, while the polycrystalline sample does not exhibit any Raman peaks (Figure S6c). The singlecrystalline 3D mesostructured Cu2O exhibits significant excitonic PL, while the polycrystalline sample exhibits only a broad weak PL (Figure S6d), indicating reduced nonradiative recombination in the epitaxally grown material. Both the Raman and PL data indicate that the defect density in the epitaxially grown samples is significantly lower than that in the polycrystalline samples.
Figure 8. Schematic illustrations and corresponding SEMs at the various stages of epitaxial growth of the 3D mesostructured Cu2O. (a) Heteroepitaxially nucleated Cu2O columns. (b) Geometrical selection of (001) orientation during electrodeposition. (c) Coalescence by vertical and lateral growth. (d) Bottom-up filling Cu2O into template. 3D silica template layers improve the crystallinity of the Cu2O layer by lateral growth through the silica template layers (purple arrows) and by blocking threading dislocations (dotted red lines) (e) The final 3D mesostructured epitaxial single-crystalline Cu2O containing an embedded silica template. Red dotted lines indicate threading dislocations. Cross-hatching represents the tendency for improvement in the Cu2O crystalline order going upward from the substrate.
growth of 3D mesostructured Cu2O (left column) and corresponding SEM images (right column). First, a “columnar epilayer”, i.e., a layer that has the same single-crystalline information as that of the substrate and a columnar morphology, was grown. The growth steps of the columnar epilayer are as follows: (i) Prior to electrodeposition, Cu2+ ions in the electrolyte are reduced by the exposed Si and agglomerate with O ions to produce epitaxial Cu2O nuclei (seeds) on Si. (ii) Electrodeposition of Cu2O proceeds on the Cu2O nuclei, forming and growing columns of Cu2O (Figure 8a). The crystallinity of the Cu2O is improved in this stage. (iii) Geometrical selection of the Cu2O columns appears to occur. F
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Chemistry of Materials Each Cu2O column grows along [001], from the nuclei (Figure 8b). However, some columns lack a heteroepitaxial relationship with the substrate, making their [001] random. The columns emerging at the growth front have the proper order because [001]-oriented columns grow most rapidly, starting formation of a film with its [001] parallel to the surface normal. (iv) The [001]-oriented columns preferentially grow larger by both lateral and vertical growth, covering the misoriented islands. Finally, the [001]-oriented columns coalesce, resulting in a well-aligned [001]-oriented Cu2O layer (Figure 8c). After formation of the silica opal template on the “pregrown” Cu2O layer, Cu2O was electrodeposited through the complex internal morphology of the template. The 500 nm thick pregrown columnar epilayer provides the highly oriented substrate onto which the templated Cu2O grows, effectively transferring the single-crystalline nature of the Si substrate into the templated Cu2O mesostructure. Inside the template, the Cu2O crystallinity appears to be further enhanced through repeated lateral and vertical growth (Figure 8d). This enhancement may be related to selective area growth techniques, which have been actively utilized to reduce threading dislocation densities. In such systems, most of the threading dislocations are blocking by a growth mask.38 Here, the opal template appears to serve as a 3D growth mask, which serves to block threading dislocations. As self-assembled opals typically have ABA or ABC stacking, there is no straight path normal to the substrate, and thus, there are multiple opportunities to block threading dislocations as the Cu2O grows (Figure 8e).
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REFERENCES
(1) Braun, P. V. Chem. Mater. 2014, 26, 277−286. (2) Braun, P. V.; Wiltzius, P. Nature 199, 402, 603−604. (3) Stein, A.; Wilson, B. E.; Rudisill, S. G. Chem. Soc. Rev. 2013, 42, 2763−2803. (4) Lousse, V.; Fan, S. Opt. Express 2006, 14, 866−878. (5) Graugnard, E.; Chawla, V.; Lorang, D.; Summers, C. J. Appl. Phys. Lett. 2006, 89, 211102. (6) Nelson, E. C.; Dias, N.; Bassett, K.; Dunham, S.; Verma, V.; Miyake, M.; Wiltzius, P.; Rogers, J.; Coleman, J.; Li, X.; Braun, P. V. Nat. Mater. 2011, 10, 676−681. (7) Subramania, G.; Li, Q. M.; Lee, Y. J.; Figiel, J. J.; Wang, G. T.; Fischer, A. J. Nano Lett. 2011, 11, 4591−4596. (8) Noda, S.; Tomoda, K.; Yamamoto, N.; Chutinan, A. Science 2000, 289, 604−606. (9) Aoki, K.; Miyazaki, H. T.; Hirayama, H.; Inoshita, K.; Baba, T.; Sakoda, K.; Shinya, N.; Aoyagi, Y. Nat. Mater. 2003, 2, 117−121. (10) Subramania, G.; Lee, Y. J.; Fischer, A. J.; Luk, T. S.; Brinker, C. J.; Dunphy, D. Appl. Phys. Lett. 2009, 95, 151101. (11) Duoss, E. B.; Twardowski, M.; Lewis, J. A. Adv. Mater. 2007, 19, 3485−3489. (12) Garcia-Santamaria, F.; Xu, M. J.; Lousse, V.; Fan, S. H.; Braun, P. V.; Lewis, J. A. Adv. Mater. 2007, 19, 1567−1570. (13) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802−804. (14) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538−804. (15) Miguez, H.; Meseguer, F.; López, C.; Holgado, M.; Andreasen, G.; Mifsud, A.; Fornés, V. Langmuir 2000, 16, 4405−4408. (16) Arpin, K. A.; Losego, M. D.; Cloud, A. N.; Ning, H.; Mallek, J.; Sergeant, N. P.; Zhu, L.; Yu, Z.; Kalanyan, B.; Parsons, G. N.; Girolami, G. S.; Abelson, J. R.; Fan, S.; Braun, P. V. Nat. Commun. 2013, 4, 2630−2647. (17) Braun, P. V.; Zehner, R. W.; White, C. A.; Weldon, M. K.; Kloc, C.; Patel, S. S.; Wiltzius, P. Adv. Mater. 2001, 13, 721−724. (18) Braun, P. V.; Wiltzius, P. Adv. Mater. 2001, 13, 482−485. (19) Hetherington, N. B. J.; Kulak, A. N.; Sheard, K.; Meldrum, F. C. Langmuir 2006, 22, 1955−1958. (20) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho, K.-M. Appl. Phys. Lett. 1999, 74, 3933. (21) Zhang, H. G.; Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. Adv. Funct. Mater. 2007, 17, 2766−2771. (22) Briskman, R. N. Sol. Energy Mater. Sol. Cells 1992, 27, 361−368. (23) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Taracon, J. M. Nature 2000, 407, 496−499. (24) Machlin, E. S. Materials Science in Microelectronics; Elsevier: New York, 2005. (25) Poizot, P.; Hung, C.-J.; Nikiforov, M. P.; Bohannan, E. W.; Switzer, J. A. Electrochem. Solid-State Lett. 2003, 6, C21−25. (26) Zhou, Y. C.; Switzer, J. A. Mater. Res. Innovations 1998, 2, 22− 27. (27) Zhou, Y.; Switzer, J. A. Scr. Mater. 1998, 38, 1731−1738. (28) Switzer, J. A.; Hung, C.-J.; Huang, L.-Y.; Miller, F. S.; Zhou, Y.; Raub, E. R.; Shumsky, M. G.; Bohannan, E. W. J. Mater. Res. 1998, 13, 909−916. (29) Oba, F.; Ernst, F.; Yu, Y.; Liu, R.; Kothari, H. M.; Swizter, J. A. J. Am. Ceram. Soc. 2005, 88, 253−270.
CONCLUSION In summary, we have developed a novel electrodepositionbased bottom-up filling approach to fabricate 3D mesostructured single-crystalline Cu2O. The nature of epitaxy of 3D mesostructured Cu2O was not only preserved during the growth through the complex geometry of the template, but the colloidal template appears to actually improve the crystallinity of the overlying solid Cu2O by blocking threading dislocations. This work indicates that electrodeposition of single-crystal materials should be included in the list of techniques to create low defect density mesostructured solids and is the first example of a low defect density mesostructured solid grown in solution. This approach may be applicable for a range of materials of interest for functional optoelectronics, thermoelectrics, and photocatalysis, and the results here will hopefully stimulate work on such systems. ASSOCIATED CONTENT
S Supporting Information *
Characterization of epitaxial growth of solid Cu2O on Si (001), and mesostructured Cu2O grown without a pre-grown Cu2O layer. Raman and photoluminescence spectroscopy of both single and polycrystalline mesostructured Cu2O. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award no. DE-FG02-07ER46471 (colloidal template formation), and the Air Force Office of Scientific Research (AFOSR) MURI FA9550-08-1-0407 (electrodeposition and physical characterization). We thank Seung Wook Beak of Hanyang University for TEM sample preparation and Dr. Julio Soares and Dr. Mauro Sardela of the University of Illinois for experimental assistance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ¶
These authors contributed equally.
Notes
The authors declare no competing financial interest. G
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(30) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62−69. (31) Miyake, M.; Chen, Y.-C.; Braun, P. V.; Wiltzius, P. Adv. Mater. 2009, 21, 3012−3015. (32) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132−2140. (33) Venables, J. Introduction to Surface and Thin Film Processes; Cambridge University Press: Cambridge, U.K., 2000. (34) Pimpinelli, A.; Villai, J. Physics of Crystal Growth; Cambridge University Press: Cambridge, U.K., 1998. (35) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M. Rep. Prog. Phys. 1984, 47, 399−459. (36) Switzer, J. A.; Liu, R.; Bohannan, E. W.; Ernst, F. J. Phys. Chem. B 2002, 106, 12369−12372. (37) Zheleva, T. S.; Nam, O.-H.; Bremser, M. D.; Davis, R. F. Appl. Phys. Lett. 1997, 71, 2472−2474. (38) Nam, O.-H.; Bremser, M. D.; Zheleva, T. S.; Davis, R. F. Appl. Phys. Lett. 1997, 71, 2638−3640.
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