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Flexible Photonic Crystal Fabricated by Two-Dimensional Free-Standing ZnO Nanomesh Arrays Ming Fu,*,†,‡ Ji Zhou,‡ and Jianhui Yu† Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong UniVersity, Beijing 100044, People’s Republic of China, and State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: April 25, 2010
A photonic band gap is well recorded for electrodeposited free-standing ZnO nanomesh arrays with controllable directions and a high deposition density. The symmetry-dependent photonic property of free-standing ZnO nanomesh arrays, which were fabricated on a colloidal crystal template, is different from that of conventional inverse opals. The prominent magnification-sensitive reflection peak of the nanomeshes is studied from microscopic spectra. A reflection stop band with a broader width is recorded at a shorter wavelength for the nanomesh arrays compared with that for inverse opals fabricated by the same colloidal crystal template. The photoluminescence of ZnO nanomeshes is partly suppressed at the wavelength of the photonic band gap. The reflection band is also very sensitive to the current velocity when water current flows parallel to the surface of the substrate covered with nanomesh arrays. The reflection peaks turn blue as the current velocity increases. 1. Introduction Photonic crystals are spatially periodic structures constructed from alternating regions of dielectric materials for the control of light.1-3 Aside from the lithography method, the colloidal crystal template method is proven to be a simple and efficient approach for fabricating three-dimensional (3D) photonic crystals with easily controlled periodicities.4-7 However, only inverse opal structures or nanobowl array films are made by the colloidal crystal template method. In fact, single-layer nanobowl arrays are inverse opal structures, in which the thickness of the porous structures is controlled to be less than the radius of the microsphere.8,9 Therefore, the symmetry of fabricated structures using the colloidal crystal template method is only determined by the shape of the template, which restricts the diversity of the photonic crystal properties. Although the fill ratio of the secondary materials, the thickness of the inverse opal wall, or the shape of the interconnected structures can alter the width of the photonic band gap in photonic crystals,10 the effects are limited compared with the symmetrical alternation of the whole structure. Therefore, the use of a face-centered cubic (fcc) colloidal crystal template to fabricate structures other than inverse opals is necessary. It is helpful in preparing heterostructures of photonic crystals,11 which consist of inverse opals and structures with other symmetries on a single templated substrate. In fact, aside from inverse opals, several kinds of porous structures can be fabricated via the colloidal crystal template, including shaped nanoparticles,12-14 mesoscopic rings,15 calcite crystal shaped structures,16,17 nanopillar arrays,18 hierarchical pore structures,19-21 and free-standing nanomeshes.22,23 Among these, the template-induced directional-growth mechanism23 is effective in fabricating nanomeshes with controlled directions and patterns. To measure the reflection stop band of * To whom correspondence should be addressed. E-mail:
[email protected]. † Beijing Jiaotong University. ‡ Tsinghua University.
the nanomesh, fabrication of nanomesh arrays with uniform patterns and directions is necessary. However, the reflection stop band of the reported nanomeshes22,23 is difficult to record because of low deposition density. Therefore, this study investigates other ZnO nanomeshes with higher densities formed by different electrochemical routes, which might show a prominent photonic reflection band. An important photonic band gap property different from inverse opal structures is exhibited because of the different symmetries of the fabricated freestanding nanomeshes. Furthermore, the nanoscale 2D sheetlike shape makes the structure more flexible than the 3D macroporous structure. The symmetry of the nanomesh structure is more sensitive to outside forces. The current-velocity-sensitive property of nanomeshes under water current is also studied because the photonic properties are different from those of the inverse opals. 2. Experimental Methods Colloidal crystal templates were fabricated by a vertical deposition method at 50 °C and 30% humidity from 0.5 vol % monodispersed polystyrene colloidal microspheres onto a 1 cm ×5 cm glass substrate covered with indium tin oxide (ITO). Several kinds of colloidal crystals were synthesized by monodispersed microspheres with different diameters ranging from 180 to 430 nm. The ITO substrates were placed into a 15 mL weighing bottle filled with suspensions of microspheres kept at an inclination angle of 80° to the horizontal plane. A threeelectrode system (CHI660A Electrochemical Workstation) was used for the electrochemical deposition of ZnO nanomeshes. Colloidal crystal-covered ITO substrates were used as the electrodeposition working electrode, while the saturated calomel electrode worked as the reference electrode. The free-standing 2D nanomeshes (nanosheets with ordered pore periodicities) were electrodeposited at -1.1 V in a solution of 0.05 M Zn(NO3)2 and 0.06 M KCl for 1 h at 70 °C by a colloidal crystal template without surfactant. In the electrodeposition system, the
10.1021/jp9113283 2010 American Chemical Society Published on Web 05/03/2010
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Figure 1. SEM images of the free-standing nanomeshes fabricated in (a) 0.05 M Zn(NO3)2 and 0.06 M KCl solution at 70 °C and in (b) 0.1 M Zn(NO3)2 solution at 5 °C.
ZnO deposits can be fabricated with a free-standing nanosheet shape on the bare ITO substrates. When electrodeposited on the colloidal crystal-covered ITO glass, the free-standing nanosheet crystal habit is preserved. ZnO grew in the interstices of the colloidal crystal template, and nanomeshes formed after the colloidal microspheres were removed by calcination at 450 °C for 2 h. The morphology of ZnO nanomeshes were characterized by scanning electron microscopy (SEM, JEOL, JSM-6301F). The microscopic reflection spectra were characterized by a fiber optic spectrometer (Ocean Optic USB2000 or Acton InSpectrum spectrograph with embedded CCD InSpectrum 150) aided by a microscope (Zeiss Z1m or Leica DMR). The fiber of the spectrometer was placed in the position of the beam path via a C-mount after the detecting light passed through the objective lens. Light was introduced by a focusing lens with the same magnification as the eye lens. The reflection spectra were finally obtained from the reflection intensity contrast between the sample and the reflecting mirror. Photoluminescence (PL) was measured by a Raman spectrometer with a He-Cd laser (325 nm). A square glass boat connected with two water pipes was used below the objective lens to measure the microscopic reflection spectra via different current velocities. The ITO glass covered with nanomesh arrays was adhered to the glass boat. The local average velocity was estimated from the current flux and the local section area. 3. Results and Discussion The morphology of the free-standing (the structure has only one of its margins coming in contact with the substrate to sustain its shape in air) zinc oxide 2D nanomeshes is shown in Figure 1a. The nanomeshes are all grown along the {111} plane family of the colloidal crystal template. The ZnO also forms nanomeshes along the {111} plane family of the colloidal crystals when deposited22 at -1 V in 0.1 M Zn(NO3)2 solution at 5 °C for 8 h (Figure 1b). As the (111) crystal face of the colloidal crystals is parallel to the ITO substrate, the nanomeshes fabricated in both cases have a 70.5° angle to the substrates. The intersection lines (the bottom edges are connected with the substrate) of the nanomeshes along the substrate have 60, 120, or 180° between each other due to the plane relationship of the {111} plane family of the colloidal crystals. The pores in the nanomeshes are all two-dimensional hexagonal structures. However, the structures deposited in KCl solution, as shown in Figure 1a, exhibit higher nanomesh densities than the ones formed in pure Zn(NO3)2 solution at 5 °C, as shown in Figure
1b. The electric current density of the electrochemical process in KCl solution is at least 20 times higher compared with that in pure Zn(NO3)2 solution. The uniform symmetry of all nanomeshes is helpful in exhibiting a reflection band along the normal direction of the substrate. The reflection band can easily be recorded when the deposition density of the nanomesh arrays is high (Figure 1a). No measurable reflection band is observed for the nanomeshes deposited in pure Zn(NO3)2 solution at 5 °C. The following optical results involve samples that were deposited in the Zn(NO3)2 and KCl solutions. Figure 2a shows the optical image of the nanomeshes on the substrate. The SEM image of the nanomeshes in large areas according to the magnification of the optical image is shown in Figure 2b. Free-standing nanomesh arrays cover the whole surface of the substrate. Many blue rods can be observed in the optical image. Each blue rod consists of several nanomeshes with parallel geometries. Several parallel nanomeshes are always formed in adjacent areas, as shown in Figure 1a. Nearby parallel nanomeshes have a facial angle of 70.5° relative to the surface of the substrate. The homogeneous direction and symmetry of the parallel nanomeshes result in the uniform reflection stop band with a blue color in a rod shape. The direction of the rods in the optical image has a 6-fold symmetry. This 6-fold symmetry is in accordance with the directions of the intersecting lines of the nanomeshes on the substrate. Meanwhile, the top edges of the nanomeshes are always along the three directions (drawn in white arrows in Figure 2b). The direction of the intersecting lines of each nanomesh on the substrate is also parallel to the direction of its top edge. When all nanomeshes are grown along the {111} plane family of the colloidal crystal template,22 the intersecting lines between the nanomeshes and the substrate manifest a 6-fold symmetry. The reflection spectra of the nanomeshes along the normal direction of the substrate are shown in Figure 3. Reflection peaks at 482, 474, and 446 nm for Nanomeshes 265 (nanomeshes fabricated by colloidal crystals with microspheres 265 nm in diameter) are observed when the magnifications of the objective lens are 10, 20, and 50×, respectively. The width of the reflection peaks also increases as the magnification of the objective lens increases. As the detecting light is absolutely vertical to the substrate, the 2D hexagonal pore arrays in the nanomeshes have a 19.5° angle to the incident light. As the magnification of the objective lens increases, the detecting light on the sample tends to converge from the original parallel incidence. The relationship between the magnification of the objective lens and the incident
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Figure 2. (a) Optical image of the nanomeshes on the substrate. (b) SEM image of the nanomeshes according to the magnification of the optical image.
Figure 3. Microscopic reflection spectra of the nanomeshes and inverse opals fabricated by a colloidal crystal with microspheres 265 nm in diameter.
light is shown in the inset of Figure 3. Therefore, the angle between the incident light and the nanomeshes does not remain at a fixed value of 70.5°. It increases or decreases accordingly. Because the direction distributions of the intersecting lines of the nanomeshes along the substrate are equiprobable, the changes in the angles are also distributed uniformly at around 19.5°. The change in the reflection band can be understood by calculating using Translight software. The blue shift and the broadening of the reflection peaks are both induced by the deviation of the detecting light from the normal axis of the substrate. The stop band of the pores in the nanomeshes has a blue shift when the angles between the light and the nanomeshes increase. In a reverse circumstance, a red shift of the stop band is exhibited. According to the simulation (Supporting Information, Part II), the blue shift is stronger than the red shift when the angle deviates from 19.5° with the same increment. Hence, the deviation of the incident light from the normal axis of the substrate results in the broadening of the reflection peak. Simultaneously, the center of the reflection peak turns blue. The microscopic reflection spectrum obtained with a low-magnification objective lens more reliably represents the stop band of the nanomesh along the direction normal to the substrate. The reflection spectrum of the inverse opal structures fabricated by the same colloidal crystal template is represented by the dotted line in Figure 3. A photonic stop band with a broader width is measured at a shorter wavelength for the nanomesh arrays compared with the inverse opals fabricated by the same colloidal crystal template. The difference in position
Figure 4. Photoluminescence spectra (lines with solid symbols) and reflection spectra (lines with hollow symbols) of Nanomeshes 265 (lines with star symbols) and Nanomeshes 198 (lines with square symbols).
and shape of the reflection band is induced by the different symmetries between the free-standing nanomesh arrays and the inverse opals. Figure 4 shows the PL spectra and reflection spectra of the ZnO nanomesh structures fabricated by colloidal crystals with different microsphere diameters. The lines with solid symbols are the PL spectra. The photonic band gaps of these structures exhibited in the reflection spectra are represented by lines with hollow symbols. Nanomeshes 198 (star symbols) and Nanomeshes 265 (square symbols) have a stop band centered at about 400 (achieved by calculation with experimental data of nanomeshes with larger pore diameters due to the limited measuring range of the microscopic spectrum system) and 530 nm, respectively. The PL of the nanomeshes is excited by a UV laser at 325 nm. An exciton luminescent band at about 385 nm and an impurity broad luminescent band at about 550-750 nm are shown in the PL spectra. With regard to the effects of the photonic band gap on the PL of the nanomeshes, the intensities of the impurity luminescent band are considered to be equal to that of the exciton luminescence. As the center of the photonic band gap of Nanomeshes 198 is almost superposed with the exciton luminescence of ZnO, the intensity of the exciton luminescence for Nanomeshes 198 is much lower than that for Nanomeshes 265. The stop band centered at about 400 nm inhibits the exciton luminescence of ZnO in the structure.
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Figure 5. (a) Simulated reflection spectra of Nanomeshes 235 in relation to different water current velocities (0, 8, 10, and 12 m/s). (b) Measured reflection spectra of Nanomeshes 235 in relation to different water current velocities. (c, d) Relationship between current velocity and the wavelength of the centroid of the (c) simulated and (d) measured reflection peaks.
The nanoscale 2D structure is more flexible than the 3D inverse opal structure. Therefore, the reflection stop band of the 2D structure is more sensitive to transverse forces. Bending of the nanomeshes is easily induced when the flow direction of the water current is parallel to the substrate. The reflection spectra of the nanomesh arrays are sensitive to current velocity. The simulated and measured reflection spectra of the nanomeshes in the presence of water current are shown in Figure 5a,b, respectively. The deflected angle of the nanomesh was first calculated in a simulation study (eq 1). The detailed origin of the deflected angle is discussed in the Supporting Information, Part III
θ(rad) ≈ -4 × 1018U3y5 sin β/(m8s3)
(1)
where θ is the deflected angle of the nanomesh, β is the angle between the direction of the water current and the intersection line of the nanomesh on the substrate, y is the height of the investigating point, and U is the current velocity. Because the intersection lines of the nanomeshes on the substrate have a 6-fold symmetry, β is selected to be 0, 60, 120, 180, 240, and 300° to simplify the simulation. As the top layers of the ordered structure reflect most of the light in the wavelength of the stop band, the value of θ for the top five periodicities of the nanomeshes is calculated by eq 1. Figure 5a simulates the reflection spectra of Nanomeshes 235 when the current velocity is 0, 8, 10, 12 m/s. As the plane arrangements of the nanomeshes grown along the {111} plane family of the colloidal crystals are equiprobable, the same ratios of angles increase or decrease due to the water current. By accumulating the reflection intensities of all nanomeshes with different original directions, the reflection peak becomes broader as a result of the increase in current velocity. The center of the reflection band also turns blue. The centroids of the reflection peak in relation to different current velocities are given in Figure 5c. The change of the centroids in the reflection spectra is especially sensitive at high current velocities. Figure 5b gives the measured reflection spectra of Nanomeshes 235 in relation to different current velocities. The difference in position between the measured reflection peak and the calculated peak is probably due to the uncertain aspect ratio of the nanomesh. The relationship between the position of the peak centroid and the current velocity is shown in Figure 5d. Though the change in the three reflection peaks is very small, the peak centroid blue shifts as the current velocity increases from 0 to 8 m/s. As depicted in the calculated results in Figure
5c, the blue shift of the reflection peak is more obvious when the current velocity is higher than 8 m/s. However, the blue shift of the centroid was still recorded in the reflection spectra when the current velocity is lower than 8 m/s. (The current velocity cannot be increased beyond 8 m/s in our experimental system.) Further, the reflection spectra in relation to different water current velocities were measured at different areas of the sample. (The reflection spectra were measured at a fixed area of the sample when the current velocity is controlled to be 0, 4, and 8 m/s in sequence. Another area of the sample was then fixed under the objective lens for measuring the reflection spectra.) Though the positions of the reflection peaks in different areas are not the same due to the differences of aspect ratios in the nanomesh, the blue shift of the peak centroids with the increasing current velocities is found in each measured area. In contrast, no blue shifts appear for the reflection peaks of ZnO inverse opals. The physical origin of the current-velocity-sensitive blue shift is the same as the aforementioned changes of the magnificationsensitive microscopic reflection peaks (Figure 3). The broadening and blue shift of the reflection peak by the microscopic spectra are induced by a change in the incident angle on the substrate. The broadening and blue shift of the reflection peak in relation to the water current are brought by a change in the direction of the nanomesh. In both cases, the angles between the nanomeshes and the incident light vary. 4. Conclusions Free-standing 2D nanomeshes deposited on ITO substrates show prominent stop bands when they are grown along the {111} plane family of colloidal crystals. The reflection stop band is very sensitive to the direction of the incident angle and the stress conditions of the nanomeshes. These novel structures fabricated by the colloidal crystal template method have a different symmetry compared with inverse opal structures, showing potential for further photonic crystal applications. Acknowledgment. This work was supported by the National Science Foundation of China under Grant Nos. 10774087, 60825407, 60877025, and 50902008. Supporting Information Available: Setup in Translight when calculating nanomeshes, calculated reflection stop band of the nanomeshes with different incident angles, calculated deflected angle of nanomeshes under water current. This material is available free of charge via the Internet at http://pubs.acs.org.
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