NANO LETTERS
Nanowire Array Gratings with ZnO Combs
2005 Vol. 5, No. 4 723-727
Zheng Wei Pan,*,†,‡ Shannon M. Mahurin,§ Sheng Dai,§ and Douglas H. Lowndes† Condensed Matter Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Department of Materials Science and Engineering, UniVersity of Tennessee, KnoxVille, Tennessee 37996, and Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received January 26, 2005; Revised Manuscript Received February 17, 2005
ABSTRACT Diffraction gratings are mainly manufactured by mechanical ruling, interference lithography, or resin replication, which generally require expensive equipment, complicated procedures, and a stable environment. We describe the controlled growth of self-organized microscale ZnO comb gratings by a simple one-step thermal evaporation and condensation method. The ZnO combs consist of an array of very uniform, perfectly aligned, evenly spaced and long single-crystalline ZnO nanowires or nanobelts with periods in the range of 0.2 to 2 µm. Diffraction experiments show that the ZnO combs can function as a tiny three-beam divider that may find applications in miniaturized integrated optics such as three-beam optical pickup systems.
Diffraction gratings, a simple pattern constituting periodic lines and spaces, are manufactured either classically with the use of a ruling engine by burnishing grooves with a diamond stylus1-5 or holographically with the use of interference fringes generated at the intersection of two coherent beams of light.3-9 These techniques generally require sophisticated and expensive equipment, complicated and precisely controlled fabrication processes, and an extremely stable and clean fabricating environment. To make gratings commercially available, a high fidelity yet complicated replication technique3-5,10,11 was subsequently developed by resin casting of the ruled and holographic master gratings onto a substrate, and the replicas thus fabricated are now widely used in fields as diverse as spectroscopy, metrology, and integrated optics. Other techniques that involve lithography and/or etching, such as electron-beam lithography,12 imprint lithography13 and photolithography,14 were also developed. Self-organization, a growth phenomenon often observed in natural organisms15 such as sea urchins and ferns, represents a straightforward and simple way to obtain aligned nanowire arrays compared with the traditional top-down approaches based on lithography16 and the recent bottomup methodologies using nanowires as the building blocks.17,18 As a successful example of self-organization, microscale comblike structures consisting of ZnO nanowire arrays were synthesized recently by several groups.19-23 These unique * Corresponding author. E-mail:
[email protected]. † Condensed Matter Sciences Division, Oak Ridge National Laboratory. ‡ University of Tennessee. § Chemical Sciences Division, Oak Ridge National Laboratory. 10.1021/nl050165b CCC: $30.25 Published on Web 03/01/2005
© 2005 American Chemical Society
ZnO combs, with parallel nanowires distributed at one or both sides of a stem, resemble the screw-wire-wound prototype gratings3-5 fabricated by American astronomer David Rittenhouse in 1785, suggesting that the ZnO combs might be used as a diffraction grating especially for working in the transmission regime. However, no diffraction experiments were attempted so far on the ZnO combs. In fact, most of the combs prepared previously were not suitable for use as a grating due to the short wire length (usually less than 5 µm) and nonuniform spacing between the wires. In this article we report the controlled growth of selforganized tiny ZnO transmission gratings by a simple onestep “thermal evaporation and condensation” method, which has been widely used to grow various inorganic nanowires.24-26 The gratings resemble a microscale comb and consist of a periodic array of single-crystalline ZnO nanowires with periods in the range of 0.2 to 2 µm. The gratings are freestanding without supporting substrates and resins and have a symmetrical groove profile with atomically smooth sidewalls. Diffraction experiments show that the ZnO comb gratings can function as a beam divider that may find applications in miniaturized integrated optics such as threebeam optical pickup systems.4,27,28 The ZnO combs were synthesized in a tube furnace system25 by thermal evaporation of commercial ZnO powder at 1350-1450 °C under a precisely controlled condition. Briefly, about 2 g of commercial ZnO powder (Alfa Aesar) was placed at the center of an alumina tube that was inserted in a horizontal tube furnace, where the temperature, pressure, gas flow rate, and evaporation time can be precisely
Figure 1. SEM images of ZnO combs formed by evaporating ZnO powder at 1400 °C for 30 min. (a) Low-magnification SEM image of ZnO combs. The white arrows highlight the positions of nanowire arrays. (b) Enlarged image of the box area in (a), showing comblike structure made of periodic array of nanowires. (c) High-magnification SEM image of the box area in (b), showing aligned nanowires 150 nm in diameter at a spacing of 150 nm. (d) An array of about 200 nanowires 120 nm in diameter at a spacing of 120 nm. The inset shows a high-magnification image of this nanowire array. (e) Aligned nanowires 100 nm in diameter at a spacing of 200 nm. (f) Parallel nanowires 100 nm in diameter at a spacing of 250 nm.
controlled. A long alumina substrate was placed downstream to collect the products. The ZnO powder was then heated to 1350-1450 °C in flowing Ar at a flow rate of 50-200 sccm (standard cubic centimeters per minute) and a pressure of 50-150 Torr for 0.5-3 h. During evaporation, comblike ZnO structures were grown only in the region with temperature ranging from ∼1050 to 1250 °C, regardless of the evaporation temperature used, which is significantly higher than the reported temperature of 500-900 °C (refs 19-23). Systematic growth experiments show that the formation and quality of the ZnO combs are sensitive to the growth parameters. For example, the yield and dimension of the combs increase significantly with the growth time. Figure 1 shows typical scanning electron microscopy (SEM) images of the ZnO combs formed by evaporating ZnO powder at 1400 °C for 30 min. Many thick ZnO ribbons with widths of 5-30 µm and lengths up to 500 µm were grown on the alumina substrate surface (Figure 1a). The ribbons have a triangular cross-section with dense nanowire arrays (indicated by white arrows) growing on the thinner edges, forming a comblike structure. The widths of the nanowire arrays are up to 100 µm and the heights are 1025 µm. High-magnification SEM images (Figure 1b-f) show that each array usually contains several tens to several hundreds of very straight, perfectly aligned and evenly spaced nanowires with almost constant diameter and spacing. About one-third of the combs have nanowire diameter equal to the 724
Figure 2. TEM and high-resolution TEM images of ZnO combs formed by evaporating ZnO powder at 1400 °C for 30 min. (a) TEM image of a ZnO comb. The bottom inset is an electron diffraction pattern of the entire comb recorded along the [21h1h0] zone axis. The top inset shows the growth front of a nanowire. (b-d) Atomic resolution TEM images recorded in different regions of the comb structure in (a).
spacing (Figure 1c and 1d), while the rest have nanowire diameter smaller than the spacing (Figure 1e and 1f). The diameters of the nanowires generally range from 100 to 150 nm and the spacings are 100-250 nm, which correspond to a periodicity of 200-400 nm and a wire density of 25005000 wires/mm. Figure 1 also shows that the interfaces between the nanowire arrays and the comb ribbons are not flat, but rather rough; the spaces between the nanowires are filled to different levels, with height differences up to several micrometers (Figure 1b-d). This indicates that while the nanowires are growing the comb ribbons are also widening at a slower rate by planar filling of the intervals along the nanowire growth direction. The presence of parallel wrinkles on the upper part of the ribbon surfaces (Figure 1b and 1d), which are apparently the traces of the planar filling, strongly supports this growth phenomenon. To better understand the growth of this unique comb structure, we studied the microstructures of the ZnO combs using a transmission electron microscope (TEM). Figure 2a is a TEM image of part of a ZnO comb, displaying parallel, straight, uniform nanowires growing perpendicularly from one edge of the comb ribbon. The electron diffraction pattern (lower inset in Figure 2a) recorded on the entire comb structure shows that the entire comb is a single crystal with nanowires growing along [0001] and comb ribbon growing along [011h0]. Figure 2b-d shows high-resolution TEM images taken in different regions on this comb (see Figure 2a). Such images clearly show the (0001) atomic planes (separation, 0.52 nm) perpendicular to the nanowire axis and parallel to the comb ribbon long axis, confirming 〈0001〉 and 〈011h0〉 to be the preferred growth directions for the wurtzite ZnO nanowires and ribbons, respectively, as well as the single-crystalline nature of the entire comb structure. The Nano Lett., Vol. 5, No. 4, 2005
Figure 3. SEM images of ZnO combs formed by evaporating ZnO powder at 1400 °C for 2 h. (a) Low-magnification SEM image of ZnO combs. (b) High-magnification SEM image of a comb made of a periodic array of rectangular ZnO nanobelts ∼400 nm in width at a spacing of ∼700 nm. The inset shows the growth fronts of two nanobelts displaying rectangular cross section. Scale bar, 1 µm. (c) An array of nanobelts ∼280 nm in width at a spacing of ∼250 nm. The upper right inset shows the growth front of one rectangular nanobelt. Scale bar, 500 nm. The lower left inset is a SEM image of the stem of a comb. Scale bar, 10 µm. (d) Aligned nanobelts ∼500 nm in width at a spacing of ∼300 nm. The inset is the growth front of a nanobelt. Scale bar, 500 nm.
surfaces of the nanowires and ribbons are clean, atomically sharp, and without any amorphous sheathed phase. No stacking faults or other structural defects are observed at the ribbon-wire corner (Figure 2b) and the ribbon surface (Figure 2d). Finally, TEM investigations on the growth fronts of the nanowires show that each nanowire has a clean and atomically flat end surface (upper inset in Figure 2a). By simply increasing the growth time, we were able to control the dimensions of the combs and even the shape of the associated nanowires. Figure 3 shows SEM images of the ZnO combs formed by evaporating ZnO powder at 1400 °C for 2 h. After 2 h growth, large ZnO combs with lengths up to 1 mm and widths of 20-80 µm were obtained in a high yield (Figure 3a). Distinct from the 30-minute combs shown in Figure 1, the 2-hour combs are mainly made of periodic arrays of straight ZnO nanobelts25 that have a rectangular cross section, as shown in the insets in Figure 3b-d. Each nanobelt has a uniform width and thickness, and the typical widths and thicknesses are in the range of 200500 nm and 50-150 nm, respectively. The spacing between the nanobelts varies in a wide range from 0.2 to 1.5 µm; however, for an individual comb the nanobelts on it are evenly spaced (Figure 3b-d) and such uniformity can extend to up to 100 µm wide. Correspondingly, the 2-hour-combs have a periodicity of 0.5-2 µm and a belt density of 5002000 belts/mm. The lengths of the belts range from 10 to 50 µm. As shown in Figure 3b-d, three kinds of stem morphologies can be distinguished for the 2-hour combs: the first is a long, wide ribbon with a rectangular cross section (Figure 3b); the second is a thick whisker with a rhombic cross section (Figure 3c and lower left inset); and the third is a thin, wide ribbon with thickness the same as its associated nanobelts (Figure 3d). The former two have Nano Lett., Vol. 5, No. 4, 2005
parallel wrinkles on their surfaces, while the third one has a very smooth surface showing no evidence of planar filling. TEM and electron diffraction show that the nanobelts grow along [0001] and are enclosed by ((21h1h0) top/bottom facets and ((011h0) side facets. The one-step growth of the unique ZnO comblike structures by simple evaporation of ZnO powder without the presence of a catalyst is a spontaneous and self-organized process. Although the synthesis procedure is very simple, the formation and assembly processes are complex and precisely self-controlled since they involve: (1) the growth of the comb ribbon along [011h0] by vapor-deposition;25 (2) the nucleation and growth of evenly spaced nanowire or nanobelt arrays along [0001] on one edge of the comb ribbon; and (3) the planar filling that makes the comb ribbon widen and thicken.20,21 Several speculations such as supersaturation19,20 and polarization22 were proposed to account for the formation of these remarkable ZnO combs, but they could not explain the following three essential growth phenomena: (1) Why and how do the nanowires nucleate and grow periodically? (2) For an individual comb, why do the tens to hundreds of nanowires on it grow exactly in the same diameter, direction and even to the same length? (3) Why do nanowire arrays grow only from one edge of the comb ribbon for our combs? Much experimental and theoretical work is needed to answer these questions. The results presented above demonstrate the direct fabrication of ZnO combs made of an array of fine, long, parallel, and equally spaced nanowires (or nanobelts) by a one-step high temperature synthesis process. These nanowire arrays resemble in structure the standard diffraction gratings that are widely used today. It is therefore natural to use these microscale ZnO combs as tiny diffraction gratings especially for working in the transmission regime. Indeed, these ZnO combs possess several unique inherent features that make them good transmission gratings. First, the combs are selfsupported ZnO crystals embedded in air without supporting glass substrates and resins. This not only eliminates the light loss and internal reflection caused by the substrates and resins but also extends the use of the comb gratings to some spectral regions where substrates and resins cannot transmit.5 Second, for the combs made of periodic ZnO nanobelts, such as those shown in Figure 3b-d, the nanobelts as well as the grooves between the nanobelts have a rectangular cross section. Such combs resemble the lamellar (rectangular-profile) gratings that are used extensively as beam dividers for optical disk readers, where three beams (one zero-order and two firstorder beams) are used.4,27 The advantages of lamellar gratings over sinusoidal and blazed (triangular-profile) gratings lie in their ability to suppress higher diffraction orders from the second and to produce a markedly lower level of stray light, thus providing higher efficiency. Third, the combs consist of perfectly aligned single-crystalline ZnO nanowires with the same [0001] growth direction and atomically sharp surfaces. Especially, the groove walls of the lamellar combs are enclosed exactly by {011h0} facets, and these walls are essentially atomically flat and parallel over long distances (up to 50 µm for our combs). Such smooth sidewalls can 725
Figure 4. Schematic diagram of the experimental setup for diffraction measurement. A ZnO comb grating is inserted into the light path as an independent optical component.
prevent scattering loss of the incident light.29 Fourth, depending on growth time, our ZnO combs have periods ranging widely from 0.2 to 2 µm, which correspond to a wire density of 500-5,000 wires/mm. In principle, these combs can be used over a wide spectral range even though most transmission gratings have had their use limited to the visible spectrum (limited by the transmittance of the substrates and resins) and the groove frequencies for standard transmission gratings seldom exceed 600 gr/mm (efficiency drops off rapidly at higher groove frequencies).4 Finally, the combs are “seeable and touchable” microscale free-standing entities, which can be easily manipulated using a sharp tweezers under an optical microscope without damaging the perfect alignment of the nanowires. Most importantly, the combs can be regarded as ready-made optical components that can be used as-grown in desired system architectures such as the diffraction measurement system shown in Figure 4. In Figure 4, a Nikon TE2000 inverted microscope operating in epi-illumination mode was utilized to focus the incident laser light to an approximately 2-5 µm spot. An Ar ion laser was used to acquire diffraction data at 514.5 nm (green light) while a He-Ne laser was employed for diffraction at 632.8 nm (red light). The laser entered the microscope through the back port and was directed onto the ZnO combs, which were supported on a glass cover slip, by a 60× dry objective. The use of the microscope permitted accurate alignment of the laser spot on the nanowire array of the comb. A screen was positioned a known distance above the sample stage to observe the diffracted beam and to allow for measurement of the separation between the central and diffracted spots. Since the incident light is perpendicular to the comb surface, the diffraction behavior of the comb gratings is then determined by the simplified grating equation4 mλ ) dsinθ, where m is the diffraction order, λ the wavelength of the incident light, d the period of the comb, and θ diffraction angle with respect to the grating normal. Before measurement, the combs were imaged in a SEM to obtain dimensions. 726
Figure 5. Digital camera images of diffraction patterns generated from ZnO comb gratings under the irradiation of monochromatic laser beams. (a) Diffraction pattern recorded on a comb grating with a period of ∼1350 nm using a green laser (514.5 nm) as the incident light. The inset shows the SEM image of the corresponding ZnO comb. Scale bar, 5 µm. (b) Diffraction pattern generated from the same comb as that in (a) using a red laser (632.8 nm) as the incident light. (c) Diffraction pattern recorded on a comb grating with a period of ∼700 nm using a green laser (514.5 nm) as the incident light. The inset is the SEM image of the corresponding ZnO comb. Scale bar, 1 µm. (d) Diffraction pattern generated from the same comb as that in (c) using a red laser (632.8 nm) as the incident light.
Figure 5a shows the diffraction pattern generated from a lamellar comb (inset in Figure 5a) with a period of ∼1350 nm (∼430 nm in width at a spacing of ∼920 nm; belt density, ∼740 belts/mm) using an Ar ion laser (green light at 514.5 nm) as the incident light. Clearly, the incident laser beam is diffracted into one zero-order central beam and two firstorder side beams with approximately equal intensity, showing that the ZnO comb functions as a three-beam divider. The theoretical angle of the first-order diffraction calculated using the grating equation for this comb is 22.4°, while the measured angle, calculated basing on the distance between Nano Lett., Vol. 5, No. 4, 2005
the screen and comb and the separation between the central and diffracted spots, is ∼21.3°. Considering errors in distance measurements, these two values match very well. By switching the incident light to a longer wavelength we are able to obtain a well-separated three-beam pattern on the same comb, as is shown in Figure 5b in which a He-Ne laser (red light at 632.8 nm) was used as the light source. Correspondingly, the theoretical and measured diffraction angles increase to 27.9° and 27.3°, respectively. We also studied the diffraction effect of the comb gratings with higher wire density. Figures 5c and 5d show the diffraction patterns generated from a finer comb (inset in Figure 5c) with a period of ∼700 nm (∼300 nm in width at a spacing of ∼400 nm; belt density, ∼1420 belts/mm) using a green and a red laser as the light source, respectively. The measured diffraction angles of this 700-nm-period comb for both green and red lights are about double of those obtained from the 1350nm-period comb (Figure 5a and 5b), which are coincident with the prediction from the grating equation. (Note that the distance between the screen and comb for the images in Figure 5c and 5d is about one-third of that for the images in Figure 5a and 5b). These results demonstrate that ZnO combs, prepared directly by a simple one-step thermal evaporation and condensation method, exhibit a strong diffraction effect and function as an excellent microscale beam divider. This behavior might help them find applications in miniaturized integrated optics such as three-beam optical pickup systems,4,27 as the millions of transmission gratings (made as plastic film replicas on a glass substrate) do in almost every CD-player, where they serve as beam dividers required for keeping reading heads in focus and on track. To reach such applications, however, work is still needed on both synthesis and diffraction behavior measurements. For synthesis, it is desirable to grow nanobelt arrays with 50% duty cycle (the belt-to-groove width ratio) and controllable belt thickness to reach the maximum diffraction efficiency and to control the ratio of zero- to first-order transmission,4 respectively. For the diffraction behavior, it is necessary to calculate and measure the diffraction efficiencies of the comb gratings with different wire densities and shapes over a wide spectral range. Nevertheless, our results clearly demonstrate the capability of direct growth of ready-made complex optical components that can be used as-grown by a simple “thermal evaporation and condensation” nanowire growth technique. Finally, the ZnO combs can also be used as a mask to prepare metal (such as Au and Fe) gratings by evaporation of related metals. Acknowledgment. This research was sponsored by the Office of Basic Energy Science of the U.S. Department of
Nano Lett., Vol. 5, No. 4, 2005
Energy (DOE) and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL). The research was carried out at the ORNL, managed by UT-Battelle, LLC, for the U.S. DOE under contract No. DE-AC05-00OR22725. We thank the ORNL SHaRE Collaborative Research Center for the use of their electron microscope facilities. Z.W.P. is grateful to Dr. Zurong Dai for helpful discussions. References (1) (2) (3) (4) (5) (6) (7) (8) (9)
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