nanospheres, polyhedral cages, tetra-pods, needles, tipped nanorods, nanowires and other ... materials is relatively easy to grow and bio-safe to handle.
Mater. Res. Soc. Symp. Proc. Vol. 900E © 2006 Materials Research Society
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Synthesis and luminescence properties of novel ZnO nanostructures: micro and nanospheres, polyhedral cages, tetra-pods, needles, tipped nanorods, nanowires and other “microphone–shaped” structures. Aurangzeb Khan1, Wojciech M. Jadwisienczak2 and Martin E Kordesch1 1
Department of Physics and Astronomy and CMSS Program, Ohio University, Athens, OH, 45701, USA. 2 School of Electrical Engineering and Computer Science, Ohio University, Athens OH 45701. ABSTRACT Novel ZnO nanostructures such as hollow nanospheres, nano-cages, nanoneedles, tetrapods, nanowires, aligned nanorods and nanotubes are synthesized via thermal evaporation of ZnO and graphite powder mixtures in reduced oxygen atmosphere in the presence of argon and nitrogen flows. The ZnO nanostructures, especially nanospheres, have a unique shape and are hollow inside with walls densely decorated with aligned nanowires. Photoluminescence of synthesized ZnO structures measured at 300 K exhibits a strong near band edge peak at ~380 nm and deep level green band centered at ~550 nm. Fabricated ZnO structures can be studied for various applications in optoelectronics and sensors. INTRODUCTION The syntheses of semiconducting nanostructures such as nanowires, nanorods and nanobelts have attracted much attention due to their morphology dependent properties and potential applications in nano-devices and optoelectronics [1-3]. The ZnO semiconductor is an important II-VI compound with wide band gap (3.37 eV at 300 K), good chemical stability, low-lasing threshold and high excitonic binding energy of 60 meV [4]. Also, ZnO in contrast to III-N materials is relatively easy to grow and bio-safe to handle. The synthesis of ZnO nanostructures have been widely studied with various growth techniques including physical and chemical vapor deposition, sputtering and laser ablation [5-9]. The combined thermal evaporation and vapor transport method is the one most frequently used due to the high yield, easy scalability and low cost [2,4]. ZnO materials synthesized in those ways have shown variety of morphologies affecting the optical, semiconducting, and piezoelectric properties [10, 11]. So far, the various applications of ZnO nanomaterials such as biosensors, UV detectors, nanocantilevers and nanoresonators are under way. Different shaped ZnO nanomaterials can be obtained by varying the deposition conditions in relatively ease way. However, precise control over ZnO nanomaterials synthesis remains a big challenge. Here, we report results of the synthesis and luminescence properties of various ZnO morphological structures grown in a tube furnace via the thermal evaporation and vapor transport method. In particular, novel highly symmetrical ZnO micro and nano spheres were fabricated in a two step grow process. These structures having unique shape and high volume-to-surface ratio might be attractive for biosensors or other applications. EXPERIMENTAL ZnO nanostructures were synthesized via combined thermal evaporation and vapor transport methods. A mixture of ZnO powder (99.0 % min and -325 mesh) and graphite powder ( 99.0 % and -300 mesh) was loaded into a quartz cylindrical crucible with one end open and transported
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to a quartz tube (OD of 1 or 3 inch and 4 feet long) placed inside of a resistively heated furnace for synthesis. For some experiments the Zn metallic powder was also used. The synthesis temperature and duration time were changed between 600-1100 oC and 20-60 minutes, respectively. The products of synthesis were collected on Si (100) substrates (and neighboring areas) placed downstream outside of the furnace hot zone in the quartz tube so that a sufficient temperature gradient could be achieved. High purity Ar, N2 and O2 gases were used at 10-30 sccm to transport vapors between the reaction and deposition zones. The morphologies of the as grown materials were characterized using an energy dispersive X-ray spectroscopy (EDX) system combined with the scanning electron microscope (SEM) [JOEL JSM 5300], transmission electron microscope (TEM) [JOEL 1010], and the x-ray diffractometer (XRD) [Rigaku Geigerflex, 2000 Watts] with Cu Kα (1.54 Å) as the incident radiation, respectively. The room temperature photoluminescence (PL) characterization of ZnO composites was conducted using the Xenon arc lamp based fluorescence spectrometer and He-Cd laser implemented into the setup described elsewhere [12]. RESULTS AND DISCUSSIONS Figure 1 depicts SEM images of three-dimensional (3D) ZnO nanostructures. The first two images (Fig.1(a) and (b)) are the SEM micrographs of ZnO microspheres. These nanospheres are hollow inside and their walls are densely decorated with nanowhiskers. The synthesis of the hollow ZnO nanospheres was done in two steps. Initially the ZnO/graphite powder mixture is heated up to 1000 °C to reduce ZnO to Zn vapors. Upon condensation of Zn vapor, the metallic Zn spheres are formed. Then the Zn spheres are annealed at ~600 oC in air for 20-30 min to convert to ZnO structure. During the Zn sphere oxidization process the inner core of the Zn spheres evaporates making them hollow and at the same time breaking the Zn spheres at one side (see Fig.1 (a),(b)).We found that Zn spheres collected on Si substrate kept at 1000 °C produced different 3D ZnO micro and nanostructures. Figure.1(c), (d) and (e) shows examples of ZnO microspheres made of uniform nanorods, sharp-tip nanorods and wide-tip nanorods, respectively. It is apparent that ZnO nanorods originate from a common center and are connected in a 3D isotropic fashion. Figure 1(f) shows ZnO donut like structure made of nanowhiskers grown at ~700 °C in O2/Ar ambient. There are also visible some not well defined ZnO structures lying on the ZnO donut structure. The Fig.1(g) shows microphone-like structures made of small ZnO microcrystals after heating the ZnO/graphite powder mixture in air at 1000 oC for 30 minutes. Using the same experimental conditions as for the nanowires and nanorods ZnO structures and reducing the growth process temperature to 950 oC we were able to synthesize the ZnO polygons and hexagon shown in Fig.1 (h) and (i), respectively. These structures are almost substrate independent. Figure 2 shows examples of 2D ZnO structures fabricated on Si (100) substrate placed on the top of the crucible loaded with ZnO/graphite powder mixture and kept at 1000-1100 oC for 30 minutes in O2/Ar ambient. The Fig.2(a) is a low magnification SEM image of the rectangular envelope like structures whereas the Fig.2(b) is a relatively high magnification image
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Figure1. SEM images of ZnO nanostructures grown in tube furnace under different conditions. Figure shows microspheres (a) and (b); 3D flower-like structures made of uniform nanorods (c), sharp tips nanowhiskers (d) and wide tip nanorods (e); donut-like structure made of nanorods (f); microphone-like structures made of micro crystals (g); polygons (h) and hexagon (i). The bar scale in all images is 10 µm except (g) and (i) for which it is 100 µm and 2µm, respectively.
of the same ZnO structure. The Fig.2(b) shows that this structure is composed of stacking small irregular ZnO layers. In this experiment we have also observed that the large area of 1 inch OD quartz tube was covered with ZnO plate like structures all along the substrate position (see Fig.2 (c) and (d). Figure 3 illustrates SEM images of 1D ZnO nanostructures deposited on the Si(100) substrate from the ZnO/graphite powder mixture. Zn powders alone are also used in some experiments. It must be noted that the XRD and EDX were done on all the ZnO samples presented here (figures not shown) confirming the ZnO crystallographic structure. The XRD confirms that all the ZnO samples are hexagonal structures (a = b = 3.249 Å and c = 5.206 Å, ICDD, card # 00-003-0888). Fig.3(a), (b) show the ZnO nanowires produced from the ZnO/graphite powder synthesized at ~750 °C in air for 30-40 minutes. The diameters of ZnO nanowires are ~100 nm and ~30 nm, respectively. The nanowires in Fig.3(b) have a “chef’s hat” shape cap like ends. The SEM side view of ZnO aligned nanowires forest deposited on the inverted Si (100) template placed on the top of the crucible is shown in the Fig.3 (c). It is noteworthy that experiments performed at temperatures ~600 oC by evaporating Zn powders in O ambient without Si substrate cap produced large ZnO tetrapods decorating the quartz tube in the growth zone.
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Figure 2. SEM pictures of the 2D ZnO structures. Figure shows ZnO rectangular envelope-like structures (a), (b); 2D ZnO plate-like structures (c), (d). The bar scale in Fig.2 (a), (c), (d) is 100 µm and 10 µm in Fig.2 (b), respectively.
We have also observed in the same experiment that the ZnO material resembling cotton wool was synthesized on the Si substrate kept in the growth zone of quartz tube. An example of the ZnO wool like structure is shown in the Fig.3(d). In addition to the ZnO structures already described we have observed different ZnO nanowires structures deposited on the same Si substrate at 800-1000 °C growth zone temperature in air. The Fig.3 (e), (f) and (g) show examples of hexagonal ZnO nanowires with one sharp end (pencil like) (Fig.3(e)), a chef hat at one end (Fig.3(f)) and nanorods having several branches attached to one end (Fig.3(g)), respectively. Furthermore, the ZnO nanotubes shown in Fig.3(h),(i) were synthesized from ZnO/graphite powder at 800-1000 °C in air. The ZnO tubes like structures were collected on the Si substrate and have the length of tens of micrometers and diameter of several micrometers. Luminescence studies were conducted at room temperature for all ZnO samples and results are shown in Fig.4. ZnO samples ware excited above band gap and collective global luminescence was recorded for a given ZnO nanostructure. In general, all characterized ZnO materials exhibit two bands, one centered at ~380 nm and second between 450-600 nm, respectively. The spectral position of the peak at 380 nm corresponds to the near band edge emission (NBE) [13], whereas the blue-green emission band peaking at ~500 nm is typically attributed to the nonstoichiometric composition of ZnO defects (O and Zn vacancies or interstitials and their complexes) [14,15] and also likely to surface states [10,14,16]. It is seen in Fig.4 that the nanowires and nanorods (both aligned and random) have intense NBE relative to the broad green-blue band whereas for the microspheres and tetrapod structures the UV emission is weaker. The microspheres and tetrapods were synthesized by direct heating from Zn phase at ~600 oC for time duration shorter than for other structures reported here. Therefore, it is possible that these structures have higher density of structural defects responsible for blue-green emission.
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Figure 3. SEM images of 1D ZnO nanostructures. Figure shows straight ZnO nanowires (a) and straight ZnO nanowires with cap head on it (b); ZnO aligned nanorods (c); ZnO tetrapod structures (d); ZnO hexagonal nanorods with sharp edges (e); ZnO nanorods with cap heads (f); ZnO nanorods with multi branches at the top (g); ZnO nanotubes (h), (i). The bar scales are 10 µm,((a),(d), (h), (i)), 3 µm ((b), (e), (f)) and 2 µm ((c),(g)), respectively.
Furthermore, the synthesis of ZnO nanomaterials via the thermal evaporation and vapor transport method results frequently in formation of nanowires and nanorods, which have stronger UV emission comparing with microspheres and tetrapods perhaps due to smaller number of structural defects. Also, it should be mention here that the EDX analysis did not reveal any intrinsic impurities which might be responsible for observed visible emission. The influence of the surface states on observed luminescence is more than ever possible if one considers the huge surface-to-volume ratio of fabricated ZnO microspheres. The FWHM of the NBE peak is almost the same for all structures while the FWHM of blue-green band varies among ZnO nanostructures. Due to the complexity of the microscopic detail, there is still much speculation as regards the explanation of the luminescence emission of ZnO nanostructure. Further work is ongoing. CONCLUSION In summary, we demonstrated the synthesis of ZnO micro and nanostructures having
different morphologies. Novel ZnO nanostructures such as hollow nanospheres, polygons, various types of nanorods were observed. An intensive PL emission around 380 nm was observed for most of synthesized ZnO materials. The intensity of the blue-green emission band depends on the growth conditions and as it was demonstrated can be changed in controllable
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way. Novel ZnO micro and nanostructures presented here are expected to possess unique properties useful in optoelectronics and sensor technologies.
Figure 4. Room temperatures PL spectra of ZnO nanostructures. PL of ZnO nanopowder (a); ZnO hallow nanospheres (b); ZnO nanorods (c); ZnO nanowires (d) and aligned nanowires (e); ZnO tetrapods (f).
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