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Structural and optical properties of ZnO whiskers grown on ZnO-coated silicon substrates by non-catalytic thermal evaporation process
ARTICLE IN PRESS Physica E 42 (2010) 1928–1933
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Structural and optical properties of ZnO whiskers grown on ZnO-coated silicon substrates by non-catalytic thermal evaporation process Libing Feng, Aihua Liu n, Yuying Ma, Mei Liu, Baoyuan Man College of Physics and Electronics, Shandong Normal University, Jinan 250014, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 26 December 2009 Received in revised form 26 February 2010 Accepted 26 February 2010 Available online 4 March 2010
Well-crystallized with excellent optical properties, tetrapod-like and multipod-like ZnO whiskers were successfully synthesized by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation method. Firstly, the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD. The ZnO whiskers grew on ZnO-coated Si(1 1 1) substrates by the simple thermal evaporation of the metallic zinc powder at 800 1C in the air without any catalysts or additives. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers. Also it can further advance the growth of the ZnO whiskers accordingly. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Fourier transform infrared spectrum. The results showed that the legs of the ZnO whiskers were highly crystalline with the wurtzite hexagonal structure phase, grown along the [0 0 0 1] in the c-axis direction. Room-temperature photoluminescence spectrum of the assynthesized whiskers showed UV (390 nm) and green (517 nm) emission, respectively. In addition, the possible growth mechanism of ZnO whiskers is also discussed based on the experimental results. & 2010 Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanostructure Crystal growth Photoluminescence
1. Introduction Semiconductor nanostructures have been particularly attractive because of interest in investigating their fundamental physical properties and their potential applications in electronic and optoelectronic devices [1]. Among the various kinds of nanostructures, II–VI compound semiconductors nanostructures have attracted much attention because of their many potential applications for optoelectronic devices operating in the shortwavelength region [2]. Among these, zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 mV), is one of the most important promising compound semiconductor materials for its unique optical and electronic nanodevice applications as a promising candidate for field emission display [3], chemical sensors [4], UV light-emitting diodes (LEDs) and laser diodes (LDs) [5,6]. Moreover, ZnO is also nontoxic, bio-safe, and possibly biocompatible and hereby can be directly used for biomedical applications without additional coating [7]. ZnO has a stable noncentral symmetric wurtzite crystal structure, spontaneous surface polarization characteristics [8] and the ability to grow in a large variety of self-organized nanostructures, which make it one of the richest ranges of
n
Corresponding author. Tel.: + 86 531 86182531; fax: + 86 531 86180804. E-mail address: [email protected] (A. Liu).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.02.030
morphologies among the entire family of wide band gap semiconductors nanostructures. So, many works have been focused on preparation of diverse morphologies of ZnO nanostructures. Up to now, various morphologies ZnO nanostructures have been synthesized, such as nanowires [9], nanoneedles [10], nanorods [11], nanobelts [12], nanotube [13], nanopencils [14], nanofibers [15], nanosprings [16], nanocages [17], whiskers [18] and so on. Among them, the ZnO whiskers with a high surface-tovolume ratio received extensive attention and expected to display significant properties. Various approaches have been developed to fabricate the ZnO whiskers such as metal organic chemical vapor deposition (MOCVD) [19], aqueous solution deposition [20], vapor–liquid–solid (VLS)-assisted method [21], hydrothermal method [22], wet chemical route [23], direct thermal evaporation of metal oxide and carbon powers [24], etc. However, most of them have some drawbacks, involving long reaction time, toxic templates and exotic metal catalysts, and the outcomes are poor in purity in the products, which may influence some applications of the ZnO whiskers. In this paper, tetrapod-like and multipod-like ZnO whiskers were synthesized by the simple thermal evaporation of the high purity Zn powders onto a quartz tube at a temperature of 800 1C under air atmosphere without introducing any catalysts or other carrier gases approach. The morphological differences between the tetrapod-like whiskers and the multipod-like whiskers might be a consequence of the different in the size of the nucleus. To our knowledge, there also have not been reported using this means in
ARTICLE IN PRESS L. Feng et al. / Physica E 42 (2010) 1928–1933
the literature. Specially, in our experiment, a layer of ZnO films is firstly pre-deposited on the Si (1 1 1) substrates by pulsed laser deposition (PLD) before the thermal evaporating process. The predeposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers and also promote the growth of the ZnO whiskers effectively. The as-synthesized products are pure, single-crystalline ZnO whiskers. There is also some discussion on its growth mechanism.
2. Experimental Tetrapod-like and multipod-like ZnO nanowhiskers were synthesized by two steps. Firstly, a thin layer of ZnO films was deposited on Si(1 1 1)substrates by PLD. The PLD apparatus and the experimental methods have been described in detail elsewhere [25]. High-purity zinc oxide (99.99%) with a diameter of 60 mm was used as a target. The deposition chamber was evacuated by a turbomolecular pump yielding typical base pressures of 1.1 10 4 Pa. The Q-switched Nd:YAG laser (l ¼1064 nm) was used to ablate the zinc oxide target. The laser energy, the repetition rate and the oxygen background pressure were set to be 100 mJ, 10 Hz and 1.33 Pa, respectively. The substrates are Si(1 1 1) substrates and the distance between the target and the substrates is 40 mm. The deposition time is 1 min and the substrate temperature is 400 1C. The film thickness was measured to be approximately 40 nm. Then, both the ZnO films by PLD and the high purity metallic Zn powder (99.99%) were inserted into the conventional horizontal tube furnace (L4513II-2/QWZ) together. The Zn powder as the source material was placed in a high-temperature region of a quartz boat covered by another two-end opened quartz cap, while the ZnO-coated Si(1 1 1) substrates prepared in at the first step were placed in the low temperature region of the same boat. After heating the furnace to 800 1C at a rate of 50–100 1C/min, the quartz boat loaded with the substrates and the source material were pushed into the center constant temperature region of the furnace. Meanwhile, two side of the furnace was closed. The reaction time was 20 min. After the reaction process, the reaction system was dragged out from the furnace immediately. As a result, it also can be seen clearly that the color of the products in the boat changed from yellow to white immediately. Finally, the white products like cotton were obtained on the whole surface of the substrates and in the quartz boat. In addition, under the same condition except the nonexistence of ZnO films on the substrates, we have done a contrast experiment. The structural properties and morphology of as-synthesized products were characterized and analyzed by X-ray diffraction (XRD, A Rigaku D/max-rB X-ray diffraction meter with Cu Kaline), scanning electron microscopy (SEM, Hitachi S-570), transmission electron microscopy (TEM, Hitachi H-800), high resolution transmission electron microscopy (HRTEM, JEOL JEM2100, 200 kV) and Fourier transform infrared spectrum (FTIR, TENSOR27). The room-temperature photoluminescence spectrum (PL) of the products was measured using Edinburgh Instruments FLS920 steady-state fluorescence spectrometer (UK) with Xe lamp as the excitation light source (with a wavelength of 325 nm).
3. Results and discussion The typical morphologies of the as-synthesized products were observed by SEM as shown in Fig. 1(a)–(c) with different magnifications. Fig. 1(a) shows that the high yield accumulated ZnO whiskers were formed and distributed randomly on the surfaces of the substrates. It can be seen that most of the whiskers
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exhibited a tetrapod-like morphology. And also we found a few multipod-like ZnO whiskers on the substrates. From Fig. 1(b) and (c), it is clearly shown that the whiskers have a straight, needlelike shape. The legs were connected to a center junction which can be regarded as a central nucleus. The central nucleus of the multipod-like ZnO whiskers is bigger than those of the tetrapodlike whiskers. Besides the differences in the nucleus sizes, the essence of the legs of the tetrapod-like whiskers and those of the multipod-like whiskers are the same, which implies that they have the same growth mechanism [26]. The results indicates that the morphological differences between the tetrapod-like whiskers and the multipod-like whiskers might be a consequence of the different in the size of the nucleus. Most of the whiskers have a rod-like root and exhibit a sharp tip. The root diameters of the whisker are in the range 300–700 nm, while the tips have an average diameter about 130 nm. The legs of the ZnO whiskers are up to several micrometers in length, and each leg is tapered off from the center to its end. Fig. 1(d) shows the SEM image of the ZnO whiskers in the contrast experiment. It is found that that only a few tetrapod-like whiskers grow on the substrates. Comparing the differences in experimental conditions, we considered that the ZnO particles coated on the substrate by PLD may probably act as the nucleating sites for embryos [27], which provide growing sites for the later ZnO crystal nuclei. So, the pre-deposited ZnO films by PLD on the substrates can promote the growth of the ZnO whiskers effectively. Fig. 1(e) shows the SEM image of the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD. It can be seen that many uniformly distributed micro-sized ZnO particles were coated on the Si substrates. To determine the crystallinity and crystal phases of the as-synthesized ZnO whiskers, XRD patterns were measured with Cu Ka radiation, and are shown in Fig. 2. All the diffraction peaks in Fig. 2 can be well indexed to the hexagonal wurtzite phase of ZnO with lattice constants of a ¼0.3249 nm and c ¼0.5205 nm, which are consistent with the values in the standard card (JCPDS 05-0664). The results indicates that the products consisted of pure phase and no characteristic peaks were observed for other impurities. The strong intensity and narrow width of the ZnO diffraction peaks also reveal that the resulting ZnO whiskers are of high purity and good crystallinity. Further structural and elemental analyses of the ZnO whiskers were investigated by TEM, HRTEM and selected area electron diffraction (SAED) patterns. Figs. 3(a) and (b) shows a typical low magnification TEM images of the tetrapod-like ZnO whiskers and the tips of the ZnO whiskers. It can be shown that the grown whiskers have smooth and clean surfaces throughout their legs. The diameter of the legs decreases from the root to the sharp tip, forming the needle-like shape. The root diameters of the whisker are in the range 300–700 nm, while the tips have an average diameter about 130 nm, in agreement with the results from SEM observations. The SAED pattern (inset in Fig. 3(a) and (b)) obtained from the root and the tip of the leg of the ZnO whisker can be indexed as a hexagonal structure, indicating that the legs of the as-synthesized ZnO whiskers are single crystalline. Fig. 3(c) shows the HRTEM lattice image of the ZnO whiskers. The clear lattice fringes confirmed that the synthesized whiskers are single crystal. In the image, the spacing of 0.26 nm between adjacent lattice planes corresponds to the distance between two adjacent (0 0 0 2) crystal planes. It confirmed that the growth direction for each leg of the ZnO whiskers is along the [0 0 0 1] in the c-axis direction. Furthermore, the lattice is very perfect, which reveals that the whiskers have a high-quality crystal lattice. The products were mixed with potassium bromide (KBr) in the ratio of 1:100. The background spectrum recorded using KBr was subtracted from the products spectrum.
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Fig. 1. (a)–(c) SEM images of surface morphology of the ZnO whiskers at different magnifications. (d) The tetrapod ZnO whiskers in the contrast experiment. (e) SEM image of the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD.
Fig. 4 exhibits FTIR spectrum of the ZnO whiskers. The absorption region from 400 to 2000 cm 1 generally represents the finger print region of the materials, which are unique in characteristics. The spectrum contains four prominent absorption bands around 433.96, 548.46, 1384.07 and 1633.85 cm 1. The 433.96 cm 1correspond to the telescopic vibration absorption of the Zn–O bond of the as-synthesized wurtzite ZnO whiskers [28]. The absorption band around 548.46 cm 1 is the typical reference spectra of the pure ZnO powders often shown in Ref. [29]. The two bands at 1384.07 and 1633.85 cm 1 are associated with Zn–O stretching vibration in wurtzite hexagonal type ZnO crystal [30]. No other peak is observed in the spectrum, which confirms that
the synthesized products are ZnO alone. The result is consistent with that of the XRD analysis of the products. Based on our observations, a possible growth mechanism of the ZnO whiskers was proposed which is different from the conventional vapor–liquid–solid (VLS) growth mechanism [31] because no metal catalyst was employed during the growth process. The possible growth mechanism of the ZnO whiskers is a vapor–solid (VS) growth mechanism [32]. The key of VS mechanism is nucleation and growth of ZnO whiskers. In the light of the theory of the crystal nucleation, the products can be affected by the condition of the substrate surface [33]. From Fig. 1(e), it can be seen that many uniformly distributed micro-sized ZnO particles
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Fig. 2. X-ray diffraction pattern of the ZnO whiskers synthesized at 800 1C.
were coated on the Si substrates by PLD. We considered that these ZnO particles may probably act as the nucleating sites for embryos, which provide growing sites for the later ZnO crystal nuclei. Since Zn evaporation is a violent breakout process at the high temperature of 800 1C, the zinc vapor (the boiling point of Zn is 907 1C) is produced from the source material. While in our experiments, the amount of oxygen is limited and some suboxides (ZnOx, xo1) also formed accordingly. Because the Zn and Zn-suboxides (ZnOx, xo1) have a low melting temperature (approximately 419 1C) [34], Zn and ZnOx should be in vapor phases at the beginning of our experiment. Due to the air circulation, the Zn (g) or ZnOx (g) were transported and then deposited on the above-mentioned sites resulting in the nucleation of the ZnO. Since the substrates located at a relatively low temperature, Zn or ZnOx vapors should condense into liquid Zn (l) or ZnOx (l) droplets located on the surface of substrates. These droplets continued to absorb oxygen vapor around and then crystallized to form ZnO nuclei. Some of these droplets migrated and merged on the substrate surface, forming some large liquid droplet. Then the incorporated large liquid droplet also continued to absorb oxygen vapor around and formed more larger ZnO nuclei. In our experiment, the nucleation plays an important role in the growth of the ZnO whiskers. According to the octa-twin nucleus model [35], ZnO nuclei formed in the atmosphere are octahedral nuclei [36]. The octahedral nucleus was considered to lead to the formation of the tetrapod-like ZnO whiskers [37]. Moreover, owing to the sizes of these incorporated ZnO nuclei were larger than the previously formed nuclei, the insufficient oxygen and the high temperature will make these incorporated nuclei accumulate to a multiple facets probability. It finally led to the formation of the polyhedral ZnO nuclei rather than octahedral nuclei. For the larger ZnO polyhedral nuclei, the growth process would lead to the formation of the multipod-like whiskers. From the SEM (Fig. 1(e)) of the ZnO in the contrast experiment, we can see that only a few tetrapod-like whiskers grow on the substrate. It indicates that the formation of the high yield accumulated ZnO whiskers grown on the ZnO-coated substrates, while only a few tetrapod-like whiskers grown on the substrate without ZnOcoated. So, the pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers effectively. Also it can further advance the growth of the ZnO whiskers accordingly. Based on the principle of the lowest surface energy, the polyhedral and the octahedral nuclei will be exposed at the crystal surfaces with the lowest energy [35]. Then the ZnO whisker sprouted from these nuclei surfaces and grew along the
Fig. 3. (a) A typical low magnification TEM images of the tetrapod-like ZnO whiskers and the corresponding SADE pattern obtained from the root of the single whisker (inset). (b) A TEM images of the tips of the ZnO whiskers and the corresponding SADE pattern obtained from the tips of the single whisker (inset). (c) The corresponding HRTEM image of a single leg of the ZnO whiskers.
[0 0 0 1] direction because the surface energy of the (0 0 0 2) facet is the lowest. On the other hand, it is known that ZnO crystal has the wurtzite polar crystal structure which has a hexagonal unit cell with space group P6mc. The oxygen anions and Zn cations form a tetrahedral unit. The structure of the ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2 and Zn2 + , stacked alternatively along the c-axis. The inherent asymmetry along the c-axis leads to the anisotropic growth of the ZnO crystallites [38]. The formation of the ZnO whisker crystals is attributed to the difference in the growth rate of the various crystal facets. According to the ZnO
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results from the radiative recombination of a photo-generated hole with an electron occupying of oxygen vacancy (Vo) [41]. At higher temperature the numbers of molten Zn atoms increases, combined with oxygen to form ZnO. To maintain the growth of whiskers there should be a balance between the melting rate of Zn and the growth rate of ZnO. So, higher temperature requires more oxygen for the growth of the ZnO whiskers. But the constant flow rate of oxygen even at higher temperature leads to oxygen deficiency. In our experiment, because ZnO whiskers are fabricated at 800 1C, a quantity of oxygen vacancies can also easily be produced. Thus, the green emission at 517 nm would be a result of the existence of the oxygen vacancies in the ZnO whiskers. Therefore, it is appropriate to conclude that there are a number of oxygen vacancies in ZnO whiskers, and some defects could exist in the ZnO nuclei. Therefore, we expect that the ZnO whiskers with strengthened green light emission would be a promising material for applications in optoelectronic nanodevices.
Fig. 4. FTIR spectrum of the ZnO whiskers.
4. Conclusions In summary, the ZnO whiskers was synthesized on ZnO-coated Si(1 1 1) substrate through catalyst-free thermal evaporation in a tube furnace at 800 1C. The pre-deposited ZnO films by PLD on the substrate can promote the growth of the ZnO whiskers effectively. The growth of the multipod-like ZnO whiskers mainly depends on the size of the formed ZnO nuclei. XRD, SEM and HRTEM show that the whiskers exhibited a single-crystalline wurtzite hexagonal structure and preferentially oriented in the [0 0 0 1] direction. The growth mechanism of ZnO whiskers can be explained by a VS growth mechanism. Room temperature PL spectra of the ZnO nanowires shows a strong UV emission band at about 390 nm and a green emission band around 517 nm, which was ascribed to the near band-edge emission and the deep-level emission, respectively. We believe that the green-light emission property of the ZnO whiskers may open up new opportunities for fabricating the optoelectronic nanodevices, such as LED and LD.
Acknowledgements
Fig. 5. Room temperature PL spectrum of the ZnO whiskers.
crystal growth mode, the growth rate along the [0 0 0 1] direction is faster than any other directions [39]. So the growth along the [0 0 0 1] direction is a dominated growth facet compared to other growth facets. Therefore, the ZnO whisker are finally formed, growing along the easiest direction of the type [0 0 0 1]. In our synthesized process, the ZnO whisker exhibit a preferential growth in the [0 0 0 1] direction which was also corresponded to the HRTEM and SAED patterns. Fig. 5 shows the measurement of PL spectra at room temperature of the products. It can be observed that the as-synthesized products show two emission peaks, which consist of a strong ultraviolet (UV) emission band located at 390 nm, a green emission band centered at 517 nm, which correspond to the ultraviolet (UV) emission and green emission, respectively. The UV emission is originated from the exitonic recombination corresponding to the near band-edge emission of band gap ZnO [40]. Furthermore, we observed a green light emission peak at 517 nm, commonly referred to a deep-level or trap-state emission. The visible green band is attributed to the singly oxygen vacancy in the ZnO whiskers and the emission
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Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Structural and optical properties of ZnO whiskers grown on ZnO-coated silicon substrates by non-catalytic thermal evaporation process Libing Feng, Aihua Liu n, Yuying Ma, Mei Liu, Baoyuan Man College of Physics and Electronics, Shandong Normal University, Jinan 250014, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 26 December 2009 Received in revised form 26 February 2010 Accepted 26 February 2010 Available online 4 March 2010
Well-crystallized with excellent optical properties, tetrapod-like and multipod-like ZnO whiskers were successfully synthesized by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation method. Firstly, the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD. The ZnO whiskers grew on ZnO-coated Si(1 1 1) substrates by the simple thermal evaporation of the metallic zinc powder at 800 1C in the air without any catalysts or additives. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers. Also it can further advance the growth of the ZnO whiskers accordingly. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Fourier transform infrared spectrum. The results showed that the legs of the ZnO whiskers were highly crystalline with the wurtzite hexagonal structure phase, grown along the [0 0 0 1] in the c-axis direction. Room-temperature photoluminescence spectrum of the assynthesized whiskers showed UV (390 nm) and green (517 nm) emission, respectively. In addition, the possible growth mechanism of ZnO whiskers is also discussed based on the experimental results. & 2010 Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanostructure Crystal growth Photoluminescence
1. Introduction Semiconductor nanostructures have been particularly attractive because of interest in investigating their fundamental physical properties and their potential applications in electronic and optoelectronic devices [1]. Among the various kinds of nanostructures, II–VI compound semiconductors nanostructures have attracted much attention because of their many potential applications for optoelectronic devices operating in the shortwavelength region [2]. Among these, zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 mV), is one of the most important promising compound semiconductor materials for its unique optical and electronic nanodevice applications as a promising candidate for field emission display [3], chemical sensors [4], UV light-emitting diodes (LEDs) and laser diodes (LDs) [5,6]. Moreover, ZnO is also nontoxic, bio-safe, and possibly biocompatible and hereby can be directly used for biomedical applications without additional coating [7]. ZnO has a stable noncentral symmetric wurtzite crystal structure, spontaneous surface polarization characteristics [8] and the ability to grow in a large variety of self-organized nanostructures, which make it one of the richest ranges of
n
Corresponding author. Tel.: + 86 531 86182531; fax: + 86 531 86180804. E-mail address: [email protected] (A. Liu).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.02.030
morphologies among the entire family of wide band gap semiconductors nanostructures. So, many works have been focused on preparation of diverse morphologies of ZnO nanostructures. Up to now, various morphologies ZnO nanostructures have been synthesized, such as nanowires [9], nanoneedles [10], nanorods [11], nanobelts [12], nanotube [13], nanopencils [14], nanofibers [15], nanosprings [16], nanocages [17], whiskers [18] and so on. Among them, the ZnO whiskers with a high surface-tovolume ratio received extensive attention and expected to display significant properties. Various approaches have been developed to fabricate the ZnO whiskers such as metal organic chemical vapor deposition (MOCVD) [19], aqueous solution deposition [20], vapor–liquid–solid (VLS)-assisted method [21], hydrothermal method [22], wet chemical route [23], direct thermal evaporation of metal oxide and carbon powers [24], etc. However, most of them have some drawbacks, involving long reaction time, toxic templates and exotic metal catalysts, and the outcomes are poor in purity in the products, which may influence some applications of the ZnO whiskers. In this paper, tetrapod-like and multipod-like ZnO whiskers were synthesized by the simple thermal evaporation of the high purity Zn powders onto a quartz tube at a temperature of 800 1C under air atmosphere without introducing any catalysts or other carrier gases approach. The morphological differences between the tetrapod-like whiskers and the multipod-like whiskers might be a consequence of the different in the size of the nucleus. To our knowledge, there also have not been reported using this means in
ARTICLE IN PRESS L. Feng et al. / Physica E 42 (2010) 1928–1933
the literature. Specially, in our experiment, a layer of ZnO films is firstly pre-deposited on the Si (1 1 1) substrates by pulsed laser deposition (PLD) before the thermal evaporating process. The predeposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers and also promote the growth of the ZnO whiskers effectively. The as-synthesized products are pure, single-crystalline ZnO whiskers. There is also some discussion on its growth mechanism.
2. Experimental Tetrapod-like and multipod-like ZnO nanowhiskers were synthesized by two steps. Firstly, a thin layer of ZnO films was deposited on Si(1 1 1)substrates by PLD. The PLD apparatus and the experimental methods have been described in detail elsewhere [25]. High-purity zinc oxide (99.99%) with a diameter of 60 mm was used as a target. The deposition chamber was evacuated by a turbomolecular pump yielding typical base pressures of 1.1 10 4 Pa. The Q-switched Nd:YAG laser (l ¼1064 nm) was used to ablate the zinc oxide target. The laser energy, the repetition rate and the oxygen background pressure were set to be 100 mJ, 10 Hz and 1.33 Pa, respectively. The substrates are Si(1 1 1) substrates and the distance between the target and the substrates is 40 mm. The deposition time is 1 min and the substrate temperature is 400 1C. The film thickness was measured to be approximately 40 nm. Then, both the ZnO films by PLD and the high purity metallic Zn powder (99.99%) were inserted into the conventional horizontal tube furnace (L4513II-2/QWZ) together. The Zn powder as the source material was placed in a high-temperature region of a quartz boat covered by another two-end opened quartz cap, while the ZnO-coated Si(1 1 1) substrates prepared in at the first step were placed in the low temperature region of the same boat. After heating the furnace to 800 1C at a rate of 50–100 1C/min, the quartz boat loaded with the substrates and the source material were pushed into the center constant temperature region of the furnace. Meanwhile, two side of the furnace was closed. The reaction time was 20 min. After the reaction process, the reaction system was dragged out from the furnace immediately. As a result, it also can be seen clearly that the color of the products in the boat changed from yellow to white immediately. Finally, the white products like cotton were obtained on the whole surface of the substrates and in the quartz boat. In addition, under the same condition except the nonexistence of ZnO films on the substrates, we have done a contrast experiment. The structural properties and morphology of as-synthesized products were characterized and analyzed by X-ray diffraction (XRD, A Rigaku D/max-rB X-ray diffraction meter with Cu Kaline), scanning electron microscopy (SEM, Hitachi S-570), transmission electron microscopy (TEM, Hitachi H-800), high resolution transmission electron microscopy (HRTEM, JEOL JEM2100, 200 kV) and Fourier transform infrared spectrum (FTIR, TENSOR27). The room-temperature photoluminescence spectrum (PL) of the products was measured using Edinburgh Instruments FLS920 steady-state fluorescence spectrometer (UK) with Xe lamp as the excitation light source (with a wavelength of 325 nm).
3. Results and discussion The typical morphologies of the as-synthesized products were observed by SEM as shown in Fig. 1(a)–(c) with different magnifications. Fig. 1(a) shows that the high yield accumulated ZnO whiskers were formed and distributed randomly on the surfaces of the substrates. It can be seen that most of the whiskers
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exhibited a tetrapod-like morphology. And also we found a few multipod-like ZnO whiskers on the substrates. From Fig. 1(b) and (c), it is clearly shown that the whiskers have a straight, needlelike shape. The legs were connected to a center junction which can be regarded as a central nucleus. The central nucleus of the multipod-like ZnO whiskers is bigger than those of the tetrapodlike whiskers. Besides the differences in the nucleus sizes, the essence of the legs of the tetrapod-like whiskers and those of the multipod-like whiskers are the same, which implies that they have the same growth mechanism [26]. The results indicates that the morphological differences between the tetrapod-like whiskers and the multipod-like whiskers might be a consequence of the different in the size of the nucleus. Most of the whiskers have a rod-like root and exhibit a sharp tip. The root diameters of the whisker are in the range 300–700 nm, while the tips have an average diameter about 130 nm. The legs of the ZnO whiskers are up to several micrometers in length, and each leg is tapered off from the center to its end. Fig. 1(d) shows the SEM image of the ZnO whiskers in the contrast experiment. It is found that that only a few tetrapod-like whiskers grow on the substrates. Comparing the differences in experimental conditions, we considered that the ZnO particles coated on the substrate by PLD may probably act as the nucleating sites for embryos [27], which provide growing sites for the later ZnO crystal nuclei. So, the pre-deposited ZnO films by PLD on the substrates can promote the growth of the ZnO whiskers effectively. Fig. 1(e) shows the SEM image of the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD. It can be seen that many uniformly distributed micro-sized ZnO particles were coated on the Si substrates. To determine the crystallinity and crystal phases of the as-synthesized ZnO whiskers, XRD patterns were measured with Cu Ka radiation, and are shown in Fig. 2. All the diffraction peaks in Fig. 2 can be well indexed to the hexagonal wurtzite phase of ZnO with lattice constants of a ¼0.3249 nm and c ¼0.5205 nm, which are consistent with the values in the standard card (JCPDS 05-0664). The results indicates that the products consisted of pure phase and no characteristic peaks were observed for other impurities. The strong intensity and narrow width of the ZnO diffraction peaks also reveal that the resulting ZnO whiskers are of high purity and good crystallinity. Further structural and elemental analyses of the ZnO whiskers were investigated by TEM, HRTEM and selected area electron diffraction (SAED) patterns. Figs. 3(a) and (b) shows a typical low magnification TEM images of the tetrapod-like ZnO whiskers and the tips of the ZnO whiskers. It can be shown that the grown whiskers have smooth and clean surfaces throughout their legs. The diameter of the legs decreases from the root to the sharp tip, forming the needle-like shape. The root diameters of the whisker are in the range 300–700 nm, while the tips have an average diameter about 130 nm, in agreement with the results from SEM observations. The SAED pattern (inset in Fig. 3(a) and (b)) obtained from the root and the tip of the leg of the ZnO whisker can be indexed as a hexagonal structure, indicating that the legs of the as-synthesized ZnO whiskers are single crystalline. Fig. 3(c) shows the HRTEM lattice image of the ZnO whiskers. The clear lattice fringes confirmed that the synthesized whiskers are single crystal. In the image, the spacing of 0.26 nm between adjacent lattice planes corresponds to the distance between two adjacent (0 0 0 2) crystal planes. It confirmed that the growth direction for each leg of the ZnO whiskers is along the [0 0 0 1] in the c-axis direction. Furthermore, the lattice is very perfect, which reveals that the whiskers have a high-quality crystal lattice. The products were mixed with potassium bromide (KBr) in the ratio of 1:100. The background spectrum recorded using KBr was subtracted from the products spectrum.
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Fig. 1. (a)–(c) SEM images of surface morphology of the ZnO whiskers at different magnifications. (d) The tetrapod ZnO whiskers in the contrast experiment. (e) SEM image of the ZnO films were pre-deposited on Si(1 1 1) substrates by PLD.
Fig. 4 exhibits FTIR spectrum of the ZnO whiskers. The absorption region from 400 to 2000 cm 1 generally represents the finger print region of the materials, which are unique in characteristics. The spectrum contains four prominent absorption bands around 433.96, 548.46, 1384.07 and 1633.85 cm 1. The 433.96 cm 1correspond to the telescopic vibration absorption of the Zn–O bond of the as-synthesized wurtzite ZnO whiskers [28]. The absorption band around 548.46 cm 1 is the typical reference spectra of the pure ZnO powders often shown in Ref. [29]. The two bands at 1384.07 and 1633.85 cm 1 are associated with Zn–O stretching vibration in wurtzite hexagonal type ZnO crystal [30]. No other peak is observed in the spectrum, which confirms that
the synthesized products are ZnO alone. The result is consistent with that of the XRD analysis of the products. Based on our observations, a possible growth mechanism of the ZnO whiskers was proposed which is different from the conventional vapor–liquid–solid (VLS) growth mechanism [31] because no metal catalyst was employed during the growth process. The possible growth mechanism of the ZnO whiskers is a vapor–solid (VS) growth mechanism [32]. The key of VS mechanism is nucleation and growth of ZnO whiskers. In the light of the theory of the crystal nucleation, the products can be affected by the condition of the substrate surface [33]. From Fig. 1(e), it can be seen that many uniformly distributed micro-sized ZnO particles
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Fig. 2. X-ray diffraction pattern of the ZnO whiskers synthesized at 800 1C.
were coated on the Si substrates by PLD. We considered that these ZnO particles may probably act as the nucleating sites for embryos, which provide growing sites for the later ZnO crystal nuclei. Since Zn evaporation is a violent breakout process at the high temperature of 800 1C, the zinc vapor (the boiling point of Zn is 907 1C) is produced from the source material. While in our experiments, the amount of oxygen is limited and some suboxides (ZnOx, xo1) also formed accordingly. Because the Zn and Zn-suboxides (ZnOx, xo1) have a low melting temperature (approximately 419 1C) [34], Zn and ZnOx should be in vapor phases at the beginning of our experiment. Due to the air circulation, the Zn (g) or ZnOx (g) were transported and then deposited on the above-mentioned sites resulting in the nucleation of the ZnO. Since the substrates located at a relatively low temperature, Zn or ZnOx vapors should condense into liquid Zn (l) or ZnOx (l) droplets located on the surface of substrates. These droplets continued to absorb oxygen vapor around and then crystallized to form ZnO nuclei. Some of these droplets migrated and merged on the substrate surface, forming some large liquid droplet. Then the incorporated large liquid droplet also continued to absorb oxygen vapor around and formed more larger ZnO nuclei. In our experiment, the nucleation plays an important role in the growth of the ZnO whiskers. According to the octa-twin nucleus model [35], ZnO nuclei formed in the atmosphere are octahedral nuclei [36]. The octahedral nucleus was considered to lead to the formation of the tetrapod-like ZnO whiskers [37]. Moreover, owing to the sizes of these incorporated ZnO nuclei were larger than the previously formed nuclei, the insufficient oxygen and the high temperature will make these incorporated nuclei accumulate to a multiple facets probability. It finally led to the formation of the polyhedral ZnO nuclei rather than octahedral nuclei. For the larger ZnO polyhedral nuclei, the growth process would lead to the formation of the multipod-like whiskers. From the SEM (Fig. 1(e)) of the ZnO in the contrast experiment, we can see that only a few tetrapod-like whiskers grow on the substrate. It indicates that the formation of the high yield accumulated ZnO whiskers grown on the ZnO-coated substrates, while only a few tetrapod-like whiskers grown on the substrate without ZnOcoated. So, the pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO whiskers effectively. Also it can further advance the growth of the ZnO whiskers accordingly. Based on the principle of the lowest surface energy, the polyhedral and the octahedral nuclei will be exposed at the crystal surfaces with the lowest energy [35]. Then the ZnO whisker sprouted from these nuclei surfaces and grew along the
Fig. 3. (a) A typical low magnification TEM images of the tetrapod-like ZnO whiskers and the corresponding SADE pattern obtained from the root of the single whisker (inset). (b) A TEM images of the tips of the ZnO whiskers and the corresponding SADE pattern obtained from the tips of the single whisker (inset). (c) The corresponding HRTEM image of a single leg of the ZnO whiskers.
[0 0 0 1] direction because the surface energy of the (0 0 0 2) facet is the lowest. On the other hand, it is known that ZnO crystal has the wurtzite polar crystal structure which has a hexagonal unit cell with space group P6mc. The oxygen anions and Zn cations form a tetrahedral unit. The structure of the ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2 and Zn2 + , stacked alternatively along the c-axis. The inherent asymmetry along the c-axis leads to the anisotropic growth of the ZnO crystallites [38]. The formation of the ZnO whisker crystals is attributed to the difference in the growth rate of the various crystal facets. According to the ZnO
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results from the radiative recombination of a photo-generated hole with an electron occupying of oxygen vacancy (Vo) [41]. At higher temperature the numbers of molten Zn atoms increases, combined with oxygen to form ZnO. To maintain the growth of whiskers there should be a balance between the melting rate of Zn and the growth rate of ZnO. So, higher temperature requires more oxygen for the growth of the ZnO whiskers. But the constant flow rate of oxygen even at higher temperature leads to oxygen deficiency. In our experiment, because ZnO whiskers are fabricated at 800 1C, a quantity of oxygen vacancies can also easily be produced. Thus, the green emission at 517 nm would be a result of the existence of the oxygen vacancies in the ZnO whiskers. Therefore, it is appropriate to conclude that there are a number of oxygen vacancies in ZnO whiskers, and some defects could exist in the ZnO nuclei. Therefore, we expect that the ZnO whiskers with strengthened green light emission would be a promising material for applications in optoelectronic nanodevices.
Fig. 4. FTIR spectrum of the ZnO whiskers.
4. Conclusions In summary, the ZnO whiskers was synthesized on ZnO-coated Si(1 1 1) substrate through catalyst-free thermal evaporation in a tube furnace at 800 1C. The pre-deposited ZnO films by PLD on the substrate can promote the growth of the ZnO whiskers effectively. The growth of the multipod-like ZnO whiskers mainly depends on the size of the formed ZnO nuclei. XRD, SEM and HRTEM show that the whiskers exhibited a single-crystalline wurtzite hexagonal structure and preferentially oriented in the [0 0 0 1] direction. The growth mechanism of ZnO whiskers can be explained by a VS growth mechanism. Room temperature PL spectra of the ZnO nanowires shows a strong UV emission band at about 390 nm and a green emission band around 517 nm, which was ascribed to the near band-edge emission and the deep-level emission, respectively. We believe that the green-light emission property of the ZnO whiskers may open up new opportunities for fabricating the optoelectronic nanodevices, such as LED and LD.
Acknowledgements
Fig. 5. Room temperature PL spectrum of the ZnO whiskers.
crystal growth mode, the growth rate along the [0 0 0 1] direction is faster than any other directions [39]. So the growth along the [0 0 0 1] direction is a dominated growth facet compared to other growth facets. Therefore, the ZnO whisker are finally formed, growing along the easiest direction of the type [0 0 0 1]. In our synthesized process, the ZnO whisker exhibit a preferential growth in the [0 0 0 1] direction which was also corresponded to the HRTEM and SAED patterns. Fig. 5 shows the measurement of PL spectra at room temperature of the products. It can be observed that the as-synthesized products show two emission peaks, which consist of a strong ultraviolet (UV) emission band located at 390 nm, a green emission band centered at 517 nm, which correspond to the ultraviolet (UV) emission and green emission, respectively. The UV emission is originated from the exitonic recombination corresponding to the near band-edge emission of band gap ZnO [40]. Furthermore, we observed a green light emission peak at 517 nm, commonly referred to a deep-level or trap-state emission. The visible green band is attributed to the singly oxygen vacancy in the ZnO whiskers and the emission
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