Nanoscale View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
PAPER
Cite this: Nanoscale, 2014, 6, 3861
View Journal | View Issue
Novel synthetic methodology for controlling the orientation of zinc oxide nanowires grown on silicon oxide substrates† Jinhyun Cho,a Najah Salleh,b Carlos Blanco,c Sungwoo Yang,d Chul-Jin Lee,e Young-Woo Kim,f Jungsang Kim*a and Jie Liu*d This study presents a simple method to reproducibly obtain well-aligned vertical ZnO nanowire arrays on silicon oxide (SiOx) substrates using seed crystals made from a mixture of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2) solution. In comparison, high levels of OH concentration obtained using NaOH or KOH solutions lead to incorporation of Na or K atoms into the seed crystals, destroying the c-axis alignment of the seeds and resulting in the growth of misaligned nanowires. The use of NH4OH eliminates the metallic impurities and ensures aligned nanowire growth in a wide range of OH concentrations in the seed solution. The difference of crystalline orientations between NH4OHand NaOH-based seeds is directly observed by lattice-resolved images and electron diffraction patterns
Received 18th July 2013 Accepted 17th January 2014
using a transmission electron microscope (TEM). This study obviously suggests that metallic impurities incorporated into the ZnO nanocrystal seeds are one of the factors that generates the misaligned ZnO
DOI: 10.1039/c3nr03694d www.rsc.org/nanoscale
nanowires. This method also enables the use of silicon oxide substrates for the growth of vertically aligned nanowires, making ZnO nanostructures compatible with widely used silicon fabrication technology.
Zinc oxide (ZnO) nanowires (wurtzite crystal structure as shown in Fig. 1a)1 are one-dimensional (1-D) nanostructures that are emerging as important materials in many applications, such as electrical generators, dye-sensitized solar cells (DSSCs) and light emitting diodes (LEDs), due to their distinct optical and electrical properties.2–12 The properties that distinguish ZnO nanowires from other nanomaterials in these applications include high exciton binding energy (60 meV), piezoelectricity and a wide direct band gap of 3.37 eV that can be tuned from 3.0 to 4.5 eV in ZnxMg1xO or Zn1xCdxO alloys.6,7,10,13–16 Despite these promising characteristics, fabrication of ZnO nanoscale devices on a commercial scale remains a challenge due to the difficulty in uniform growth of the material with high throughput and
a
Department of Electrical and Computer Engineering, Fitzpatrick Institute for Photonics, Duke University, Durham, North Carolina, 27708, USA. E-mail: [email protected]
b
Department of Chemistry, North Carolina Central University, Durham, North Carolina, 27707, USA
c Department of Physics and Department of Chemistry, Purdue University, West Lafayette, Indiana, 47907, USA d
Department of Chemistry, French Family Science Center, Duke University, Durham, North Carolina, 27708, USA. E-mail: [email protected]
e
Department of Biochemistry, Duke University, Durham, North Carolina, 27708, USA
f
Department of Automotive Engineering, Hoseo University, Asan, Chungcheongnam-do, 336-795, Republic of Korea † Electronic supplementary information (ESI) available: Additional SEM images, photographs of seed solution and XRD peaks and XPS. See DOI: 10.1039/c3nr03694d
This journal is © The Royal Society of Chemistry 2014
low cost.17 The successful creation of effective and efficient nanoscale devices made using ZnO nanowires hinges on the production of well-aligned vertical ZnO nanowire arrays, which can be exploited to create optoelectronic devices.15,18,19 It has been shown that the performance of LEDs and solar cells are signicantly inuenced by the directionality, uniformity, length and spacing of the nanowire arrays.9,11,20 The challenge in growing well-aligned vertical nanowires is attributed to either lattice mismatch between ZnO and the substrate or accumulated strain in the material during the growth process, which typically results in randomly oriented nanowires.19,21 Vertically aligned ZnO nanowires are normally obtained on single crystalline substrates, such as ZnO, sapphire (Al2O3) and galliumnitride (GaN).15,17,22,23,31 The diameter and position of nanowires can be determined by patterns of catalysts using e-beam or laser interference lithography.17,23,24,32 The drawbacks of this approach include the requirement of specialized and costly substrates, and the presence of catalyst impurities in nanowires potentially impacting electrical and optical performance.23 Homoepitaxy methods using ZnO nanocrystalline seeds coated on the surface of silicon (Si) substrates provide a more favorable approach in terms of cost and versatility. Previous reports suggest synthetic methods to generate ZnO nanocrystal seeds to initiate the growth, deriving them from either zinc acetate combined with either sodium hydroxide (NaOH) or potassium hydroxide (KOH), or simply using zinc acetate alone.20,25,26 Although there is successful growth of vertically aligned nanowires,20 the consistency of the alignment obtained
Nanoscale, 2014, 6, 3861–3867 | 3861
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
Fig. 1 (a)1 The representative planes of the ZnO wurtzite structure. Left figure shows the +c-plane (001) and the c-plane (001) and the right figure shows the m-plane (100) and a-plane (110). (b)29 Schematic of the CVD system. (c)30 Schematic of a TEM grid made of a thin silicon dioxide membrane.
using these synthetic methodologies remains a challenge. Some groups have successfully obtained vertically aligned ZnO nanowires while others have not.3,20,23,25,27 We have noticed that in earlier reports, the concentration of NaOH was altered in order to control the size and the crystalline properties of the seed particles.28 In our own experiments, we have noticed that vertically aligned nanowires are consistently obtained at low levels of NaOH concentration, but insoluble Zn(OH)2 particles aggregate on the substrate leading to uneven surface coverage (see ESI, Fig. S3†). At higher levels of NaOH concentration, the Zn(OH)2 particles fully dissolve forming zincate ions (Zn(OH)42), but subsequent growth results in randomly oriented nanowires. In this paper, we explore the mechanism that leads to the misorientation, and demonstrate an alternative approach to the seed formation that reliably results in vertically aligned nanowires with consistent coverage. As zincate ions dry on the surface, Na (or K) is incorporated into the seed material to maintain charge neutrality. These metallic impurities tend to disorient the crystal axis of the ZnO seeds on the surface upon annealing. When we replaced the base needed for the formation of ZnO nanocrystalline seeds with ammonium hydroxide
3862 | Nanoscale, 2014, 6, 3861–3867
Paper
(NH4OH), well-aligned vertical ZnO nanowire arrays with high surface coverage were obtained consistently at all levels of NH4OH concentration. The existence of metallic impurities on nanocrystalline seeds made using NaOH- or KOH-based solution was conrmed by X-ray photoelectron spectroscopy (XPS). By analyzing the lattice-resolved images and electron diffraction patterns using a transmission electron microscope (TEM), we conrm that the NH4OH-based seed solution results in seed layers whose c-axis crystal orientation is aligned to the substrate normal over a wide concentration range, unlike the seeds resulting from NaOH and KOH solutions. ZnO nanowires were grown on an amorphous substrate (a silicon dioxide layer on top of a silicon substrate) using nanometer-scale crystalline ZnO seeds on the substrate. The ZnO seeds were synthesized from the seed solution prepared by mixing 0.01 M zinc acetate dihydrate [Fluka, assay [99.5%] with various concentrations (0.03 M, 0.06 M and 0.09 M) of either sodium hydroxide (NaOH) [Fisher Scientics, assay [98.6%], potassium hydroxide (KOH) [Fisher Scientics, assay [85.9%], or ammonium hydroxide (NH4OH) [Fisher Scientic, assay ¼29.5%, 14.8 normality] dissolved in methanol [OmniSolv, assay ¼99.9%]. For control experiments, 0.06 M or 0.27 M sodium acetate (CH3COONa) [Alfa Aesar, anhydrous, assay ¼99%], 0.06 M sodium nitrate (NaNO3) [Acros Organics, assay ¼99+%] and 0.09 M sodium chloride (NaCl) [Fisher Scientic, assay >99%] were also added in above seed solutions to control the Na concentration in the solution without changing the OH concentration. The mixture was thoroughly stirred at 60 C for 2 hours in a water bath.20,25 A silicon substrate with a thin silicon oxide layer was coated with 6–7 droplets of the prepared seed solution, then rinsed with clean methanol several times to ensure deposition of zinc hydroxide (Zn(OH)2) on the substrate. The drop-coated substrate was subsequently annealed at 350 C for 20 minutes to decompose the zinc hydroxide into ZnO and water.20,25 This synthesis procedure created ZnO crystalline seeds, which were used as a template for growth of ZnO nanowires via chemical vapor transport in an experimental setup as shown in Fig. 1b.29 In the growth process, equal amounts of ZnO powder [Acros Organics, 99.5+%] and graphite powder [Alfa Aesar, crystalline, 300 mesh, assay 99%] (0.6 g) were ground and placed in an alumina boat located in the middle of the tube furnace [Lindberg Blue M, TF55035A], where the substrate is located downstream in the quartz tube (inner diameter: 2.54 cm, length: 90 cm). The furnace temperature was ramped to 920 C in approximately 15 minutes and maintained for 30 minutes under argon (50 sccm) and oxygen (1 sccm) ow. Aer the growth process, the characteristics of the nanowire were investigated using scanning electron microscopy (SEM, FEI XL 30 SEM-FEG), X-ray diffraction (XRD, Panalytical X'Pert PRO MRD HR X-ray Diffraction System) and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Axis Ultra). The analysis of seed alignment was conducted using bright eld imaging and diffraction contrast mode using a transmission electron microscope (TEM, FEI Tecnai G2 Twin). Samples observed under a TEM were grown on a special TEM grid comprised of a thin silicon dioxide membrane on a silicon nitride mesh as shown in Fig. 1c.30
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
Fig. 2a shows vertical nanowires grown from the ZnO seeds synthesized using 0.03 M sodium hydroxide (NaOH) combined with 0.01 M zinc acetate (Zn(O2CCH3)2). One can conrm that supplying hydroxide ions (OH) to zinc acetate (Zn(O2CCH3)2) plays a critical role in the formation of ZnO nanocrystalline seeds, particularly in comparison with the nanostructures grown from seeds made using zinc acetate (Zn(O2CCH3)2) alone (ESI, Fig. S1†). The hydroxide ions (OH) react with zinc acetate (Zn(O2CCH3)2) resulting in zinc hydroxide (Zn(OH)2), which is insoluble in methanol and forms a suspension of a micronscale precipitate in the solvent (ESI, Fig. S2a†). These neutral Zn(OH)2 particles of varying size in the seed solution is deposited on the substrate surface and reduces to ZnO nanocrystal seeds upon annealing with the c-axis alignment normal to the substrate, leading to vertically aligned ZnO nanowire growth as shown in Fig. 2a. However, we oen observed non-uniform ZnO nanowire growth originating from large white precipitates in seed solution adsorbed onto the substrate (ESI, Fig. S3†). At higher concentration of NaOH, more hydroxide ions (OH) bind to zinc hydroxide (Zn(OH)2) to form zincate ions (Zn(OH)42) which are soluble in methanol and the mixture turns transparent (ESI, Fig. S2b†). The transparent seed solution prepared in this way leads to uniform and high density seeds on the substrate, devoid of any clumps originating from the precipitates. However, increasing the sodium concentration led to the growth of misaligned nanowires as shown in Fig. 2b and c. The misalignment is attributed to incorporation of metallic Na atoms into the ZnO seed crystals (see below). Similar behavior was observed when NaOH is replaced with KOH. To solve this problem, we turned to ammonium hydroxide (NH4OH), an analogue of NaOH in which the sodium ion is (Na+) substituted with an ammonium ion (NH4+). Since the ammonium ion (NH4+) will decompose under heat and leave the substrate in the form of ammonia (NH3), we can eliminate the incorporation of metallic impurities into the seed crystals that might impede the seed alignment. This approach yielded immediate improvement in the orientation of ZnO nanowires at all concentrations
Nanoscale
of NH4OH in the seed solution. Fig. 2d–f, show vertically aligned uniform nanowire arrays grown from ZnO seeds synthesized using 0.03 M, 0.06 M and 0.09 M NH4OH combined with 0.01 zinc acetate (Zn(O2CCH3)2) in methanol, respectively. In contrast to the creation of misaligned nanowires at high NaOH concentration, vertical alignment of nanowire arrays were preserved regardless of the concentration of NH4OH. These experiments show that NH4OH yields a complete ZnO seed layer without incorporating metallic impurities, maintaining the vertical alignment of nanowires grown from these seed crystals. In addition to the SEM images, the vertical alignment of nanowires is also conrmed through X-ray diffraction (XRD) patterns (ESI, Fig. S4†). The samples grown from NaOH-derived seeds, except 0.03 M, show different peaks originating from all crystal surfaces of ZnO, indicating the non-uniform crystal orientation of ZnO nanowires on the substrate. The samples grown from NH4OH-derived seeds present a high intensity peak at (002) and (004), indicating the c-axis of the ZnO crystal is aligned perpendicular to the substrate, and no other crystal orientations exist in the substrate plane. We attempted to directly observe the crystal orientation of the seeds using the XRD technique, but the thin seed samples did not provide enough X-ray diffraction signals for unambiguous measurement of the crystal planes. In order to verify the crystal orientation of the ZnO seed layer as a function of the concentration of different bases, we prepared the ZnO seeds on a TEM grid made of a thin silicon dioxide membrane, as shown in Fig. 1c,30 (Ted Pella, Inc. 18 nm thickness, 0.5 0.5 mm window) according to the procedure identical to that used in the nanowire growth. Fig. 3a shows one of the ZnO nanocrystalline seeds synthesized using the combined solution of 0.06 M NH4OH with 0.01 M zinc acetate. The lattice constant of the seed crystal in the substrate plane can be directly measured from such images. For example, Fig. 3a shows a TEM image of a seed particle showing a lattice ˚ corresponding to the separation of (110) constant of 2.83 A planes. We collected random samples of 30 such seed images
Fig. 2 SEM images of ZnO nanowire arrays grown from (a) 0.03 M, (b) 0.06 M and (c) 0.09 M NaOH-derived seeds; (d) 0.03 M, (e) 0.06 M and (f) 0.09 M NH4OH-derived seeds. Images were taken 52 tilt with respect to the substrate.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 3861–3867 | 3863
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
Paper
Fig. 4 (a) Selected area electron diffraction (SAED) image of 0.09 M
(a) TEM image of 0.06 M NH4OH-derived seeds grown on the SiO2membrane of a TEM grid. The inset in the upper-right hand corner represents the top-view of the c-plane of the ZnO wurtzite structure. In the figure, labeled lattice directions on the seed image imply the direction of the c-axis perpendicular to the seed layer. (b) and (c) represent histograms of the crystal orientation distribution of ZnO nanocrystalline seeds, counted based on the lattice spacing on seed images acquired by TEM. In both figures, blue, green and red bars indicate the amount of seeds generated from 0.03 M, 0.06 M and 0.09 M of (a) NaOH and (b) NH4OH, combined with 0.01 M zinc acetate, respectively. Fig. 3
from each sample, and plotted the histogram of the observed lattice orientation as a function of the base type and concentration. Fig. 3b and c show such histograms of seed samples prepared with NaOH and NH4OH solutions, respectively. The observed histogram shows signicant preferences of seeds to orient their (001) planes parallel to the substrate surface when the concentration of NaOH is low (0.03 M), but this preference is largely lost when the NaOH concentration is increased to over 0.06 M. In contrast, the seed crystals orient preferentially, with (001) planes parallel to the surface, on the substrates in the cases of NH4OH-based solution regardless of the concentration. A similar conclusion is drawn from the comparison of selected area electron diffraction (SAED) images shown in Fig. 4. SAED looks at far-eld diffraction patterns of electron beams
3864 | Nanoscale, 2014, 6, 3861–3867
NaOH-derived seeds containing a (002) ring; right upper inset shows a zoomed-out view and the bottom left inset shows three shoulders corresponding to the (101), (002) and (100) planes in terms of the gray value. (b) Selected area electron diffraction (SEAD) image of 0.09 M NH4OH-derived seeds missing a (002) ring, confirming the c-axis alignment of seeds; the right upper inset shows a zoomed-out view and the bottom left inset shows one shoulder and a high peak corresponding to the (101) and (100) planes in terms of the gray value.
focused on a specic area of the TEM membrane grid. Fig. 4a corresponds to the seed layers prepared using NaOH solution. The diffraction rings show three peaks, corresponding to the crystal planes (100), (002) and (101) lying in the plane of the substrate membrane. The inset shows the cross-section of rings showing the three peaks. Fig. 4b shows a similar diffraction of the seed layers prepared using a NH4OH solution. In this case, the predominant diffraction ring corresponds to the (100) plane and the (002) peak is missing, indicating that most of the seeds have their c-axis alignment perpendicular to the substrate plane. The relative strength of these three peaks is consistent with the statistics of the lattice constants measured directly in Fig. 3. We attribute the incorporation of metallic Na (or K) impurities in the ZnO seed crystals as the main source that leads to the misalignment. X-ray photoelectron spectroscopy (XPS) was
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
performed on SiO2 substrates coated with ZnO seeds to directly detect the presence of metallic Na impurities. Fig. S5a–c† show the core-level spectra of sodium (Na) in XPS. While 0.03 M NaOH-based seeds on the substrate do not show any peak during regional scanning in XPS, ZnO seeds on the substrate derived from 0.06 M and 0.09 M NaOH solutions show the sodium 1s peak, indicating the presence of sodium impurities and their role in the subsequent misalignment of nanowires. Table S1† shows the atomic concentrations of Na in XPS corresponding to Fig. S5a–c.† The correlation between detected Na levels in the seed and the orientation of seed crystals and subsequent alignment of nanowires are strong indicators that the impurities disorient the crystalline ZnO seeds. To understand the mechanism of Na incorporation into the seeds, we performed control experiments varying the Na concentration in the seed solution while holding the OH concentration constant. Fig. 5 shows the SEM images of vertically aligned ZnO nanowires grown from the nanocrystalline seeds synthesized using 0.01 M zinc acetate (Zn(O2CCH3)2) combined with (a) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium acetate (CH3COONa), (b) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium nitrate (NaNO3), (c) 0.09 M ammonium hydroxide (NH4OH) and 0.09 M sodium chloride (NaCl), and (d) 0.09 M ammonium hydroxide (NH4OH) and 0.27 M sodium acetate (CH3COONa), respectively. Although the total level of sodium cations (Na+) in the seed solution is equivalent to 0.09 M in Fig. 5a–c and much higher in (d), we consistently see well-aligned nanowires in all of these samples. Furthermore, XPS measurement conrmed that no sodium component was
Nanoscale
detected in the seed crystals on the surface of the substrate in these samples (two of them as shown in Fig. S5d and e†). These experiments indicate that high levels of Na concentration alone do not lead to Na incorporation into the seeds. We measured the concentration of OH in various seed solutions using a conventional pH meter (although the solvent is methanol in our experiments). The OH concentration increases dramatically for NaOH-based solutions as the NaOH concentration is increased from 0.03 M to 0.06 M (Table S2†), consistent with the fact that NaOH is a strong base. For the NH4OH-based solutions, the OH concentration remains relatively low at all levels of NH4OH concentration (consistent with the fact that NH4OH is a weak base). Addition of high concentrations of sodium chloride and sodium acetate to 0.09 M NH4OH-based seed solution did lead to neither sodium incorporation nor misoriented nanowires, strongly indicating that metallic impurity incorporation is driven by high OH concentration. At low levels of hydroxide ions (OH) in the seed solutions, the neutral zinc hydroxide particles (Zn(OH)2) do not interact with sodium cations (Na+) as they adsorb onto the substrate and all sodium cations are washed away. Thus, zinc hydroxide (Zn(OH)2) particles on the surface produce ZnO nanocrystalline seeds upon annealing with c-axis alignment normal to the substrate, leading to vertically aligned nanowires. On the other hand, high levels of OH concentration lead to zincate ions (Zn(OH)42) in the seed solution. As these ions adsorb onto the surface, they must combine with available cations to maintain charge neutrality as the solution dries on the surface of the
SEM images of ZnO nanowire arrays grown from seeds synthesized using 0.01 M zinc acetate combined with (a) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium acetate (CH3COONa), (b) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium nitrate (NaNO3), (c) 0.09 M ammonium hydroxide (NH4OH) and 0.09 M sodium chloride (NaCl), (d) 0.09 M ammonium hydroxide (NH4OH) and 0.27 M sodium acetate (CH3COONa), respectively. Images were taken 60 tilt with respect to the substrate. Fig. 5
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 3861–3867 | 3865
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
substrate. This leads to the formation of sodium zincate (Na2Zn(OH)4) particles on the surface which, upon annealing, lead to ZnO seeds with a substantial amount of Na impurities incorporated in them. The crystal orientation of the ZnO seeds is randomized in this process, leading to misaligned nanowires aer the growth process. Through this study, we have successfully synthesized vertically aligned ZnO nanowires on silicon oxide substrates using seed crystals from a mixture of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2) solution. We also provide a mechanism of the creation of misaligned nanocrystalline seeds arising from the metallic (Na or K) impurity incorporation in the case of high OH concentration in the seed solution. The seed orientations studied by TEM indicate that NH4OH-derived seeds lead to c-axis alignment normal to the substrate. From these studies, we conclude that (1) pure ZnO nanocrystals prefer to orient on a at substrates with their c-axis perpendicular to the surface and (2) impurities incorporated into the ZnO seed crystals at high NaOH (or KOH) concentrations destroy the preferred orientation of the nanocrystal seeds and result in the growth of misaligned nanowires. These ndings enhance our understanding on the control of growth orientation of nanowires on various substrates and enable the growth of vertically aligned nanowires on silicon oxide substrates, making the ZnO nanostructures compatible with widely used Si fabrication technology.
Acknowledgements This work was supported by grants from NSF (ECCS-0925587). The authors also thank the service from Duke SMIF. JGS acknowledges the support from a DOD SMART fellowship.
References 1 S. Choi, Spectroscopic ellipsometry of group-III adatom kinetics on III-nitride semiconductor surfaces, PhD dissertation, Department of Physics of Duke University, Durham, NC, USA, 2007. 2 M.-T. Chen, M.-P. Lu, Y.-J. Wu, J. Song, C.-Y. Lee, M.-Y. Lu, Y.-C. Chang, L.-J. Chou, Z. L. Wang and L.-J. Chen, Near UV LEDs Made with In Situ Doped p–n Homojunction ZnO Nanowire Arrays, Nano Lett., 2010, 10(11), 4387–4393. 3 M. Law, L. E. Greene, J. C. Johnson, R. Saykally and P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater., 2005, 4(6), 455–459. 4 A. Nadarajah, R. C. Word, J. Meiss and R. K¨ onenkamp, Flexible Inorganic Nanowire Light-Emitting Diode, Nano Lett., 2008, 8(2), 534–537. 5 R. K¨ onenkamp, R. C. Word and M. Godinez, Ultraviolet Electroluminescence from ZnO/Polymer Heterojunction Light-Emitting Diodes, Nano Lett., 2005, 5(10), 2005–2008. 6 X. Wang, J. Song, J. Liu and Z. L. Wang, Direct-Current Nanogenerator Driven by Ultrasonic Waves, Science, 2007, 316(5821), 102–105. 7 Z. L. Wang and J. Song, Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays, Science, 2006, 312(5771), 242–246.
3866 | Nanoscale, 2014, 6, 3861–3867
Paper
8 Y. Wei, C. Xu, S. Xu, C. Li, W. Wu and Z. L. Wang, Planar Waveguide—Nanowire Integrated Three-Dimensional DyeSensitized Solar Cells, Nano Lett., 2010, 10(6), 2092– 2096. 9 B. Weintraub, Y. Wei and Z. L. Wang, Optical Fiber/Nanowire Hybrid Structures for Efficient Three-Dimensional DyeSensitized Solar Cells, Angew. Chem., Int. Ed., 2009, 48(47), 8981–8985. 10 S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang and Z. L. Wang, Selfpowered nanowire devices, Nat. Nanotechnol., 2010, 5(5), 366–373. 11 S. Xu, C. Xu, Y. Liu, Y. Hu, R. Yang, Q. Yang, J.-H. Ryou, H. J. Kim, Z. Lochner, S. Choi, R. Dupuis and Z. L. Wang, Ordered Nanowire Array Blue/Near-UV Light Emitting Diodes, Adv. Mater., 2010, 22(42), 4749–4753. 12 X.-M. Zhang, M.-Y. Lu, Y. Zhang, L.-J. Chen and Z. L. Wang, Fabrication of a High-Brightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film, Adv. Mater., 2009, 21(27), 2767–2770. 13 J. Cho, Q. Lin, S. Yang, J. G. Simmons, Y. Cheng, E. Lin, J. Yang, J. V. Foreman, H. O. Everitt, W. Yang, J. Kim and J. Liu, Sulfur-doped zinc oxide (ZnO) Nanostars: Synthesis and simulation of growth mechanism, Nano Res., 2012, 5(1), 20–26. 14 J. V. Foreman, J. Li, H. Peng, S. Choi, H. O. Everitt and J. Liu, Time-Resolved Investigation of Bright Visible Wavelength Luminescence from Sulfur-Doped ZnO Nanowires and Micropowders, Nano Lett., 2006, 6(6), 1126–1130. 15 M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science, 2001, 292, 1897– 1899. 16 A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma and M. Kawasaki, Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO, Nat. Mater., 2005, 4(1), 42–46. 17 Y. Wei, W. Wu, R. Guo, D. Yuan, S. Das and Z. L. Wang, Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays, Nano Lett., 2010, 10(9), 3414–3419. 18 J. C. Johnson, H.-J. Choi, K. P. Knutsen, R. D. Schaller, P. Yang and R. J. Saykally, Single gallium nitride nanowire lasers, Nat. Mater., 2002, 1(2), 106–110. 19 M. I. B. Utama, Z. Peng, R. Chen, B. Peng, X. Xu, Y. Dong, L. M. Wong, S. Wang, H. Sun and Q. Xiong, Vertically Aligned Cadmium Chalcogenide Nanowire Arrays on Muscovite Mica: A Demonstration of Epitaxial Growth Strategy, Nano Lett., 2010, 11(8), 3051–3057. 20 L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai and P. Yang, General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds, Nano Lett., 2005, 5(7), 1231–1236. 21 M. C. Newton, S. J. Leake, R. Harder and I. K. Robinson, Three-dimensional imaging of strain in a single ZnO nanorod, Nat. Mater., 2010, 9, 120–124.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
22 X. Wang, J. Song, P. Li, J. H. Ryou, R. D. Dupuis, C. J. Summers and Z. L. Wang, Growth of Uniformly Aligned ZnO Nanowire Heterojunction Arrays on GaN, AlN, and Al0.5Ga0.5N Substrates, J. Am. Chem. Soc., 2005, 127(21), 7920–7923. 23 S. Xu, Y. Wei, M. Kirkham, J. Liu, W. Mai, D. Davidovic, R. L. Snyder and Z. L. Wang, Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low Temperature without Catalyst, J. Am. Chem. Soc., 2008, 130(45), 14958–14959. 24 Y.-J. Kim, C.-H. Lee, Y. J. Hong, G.-C. Yi, S. S. Kim and H. Cheong, Controlled selective growth of ZnO nanorod and microrod arrays on Si substrates by a wet chemical method, Appl. Phys. Lett., 2006, 89(16), 163128. 25 L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. Zhang, R. J. Saykally and P. Yang, Low-Temperature Wafer-Scale Production of ZnO Nanowire Arrays, Angew. Chem., Int. Ed., 2003, 42(26), 3031–3034. 26 C. Pacholski, A. Kornowski and H. Weller, Self-Assembly of ZnO: From Nanodots to Nanorods, Angew. Chem., Int. Ed., 2002, 41(7), 1188–1191.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
27 M.-P. Lu, J. Song, M.-Y. Lu, M.-T. Chen, Y. Gao, L.-J. Chen and Z. L. Wang, Piezoelectric Nanogenerator Using p-Type ZnO Nanowire Arrays, Nano Lett., 2009, 9(3), 1223–1227. 28 S. S. Alias, A. B. Ismail and A. A. Mohamad, Effect of pH on ZnO nanoparticle properties synthesized by sol–gel centrifugation, J. Alloys Compd., 2010, 499, 231–237. 29 D. Yu, T. Trad, J. T. McLeskey, V. Craciun and C. R. Taylor, ZnO Nanowires Synthesized by Vapor Phase Transport Deposition on Transparent Oxide Substrates, Nanoscale Res. Lett., 2010, 5(8), 1333–1339. 30 http://www.tedpella.com/grids_html/silicon-dioxide-details.htm. 31 H. Fan, F. Fleischer, W. Lee, K. Nielsch, R. Scholz, M. Zacharias, U. G¨ osele, A. Dadgar and A. Krost, Patterned Growth of Aligned ZnO Nanowire Arrays on Sapphire and GaN Layers, Superlattices Microstruct., 2004, 36, 95–105. 32 D. S. Kim, R. Ji, H. J. Fan, F. Bertram, R. Scholz, A. Dadgar, K. Nielsch, A. Krost, J. Christen, U. G¨ osele and M. Zacharias, Laser-Interference Lithography Tailored for Highly Symmetrically Arranged ZnO Nanowire Arrays, Small, 2007, 3(1), 76–80.
Nanoscale, 2014, 6, 3861–3867 | 3867
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
PAPER
Cite this: Nanoscale, 2014, 6, 3861
View Journal | View Issue
Novel synthetic methodology for controlling the orientation of zinc oxide nanowires grown on silicon oxide substrates† Jinhyun Cho,a Najah Salleh,b Carlos Blanco,c Sungwoo Yang,d Chul-Jin Lee,e Young-Woo Kim,f Jungsang Kim*a and Jie Liu*d This study presents a simple method to reproducibly obtain well-aligned vertical ZnO nanowire arrays on silicon oxide (SiOx) substrates using seed crystals made from a mixture of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2) solution. In comparison, high levels of OH concentration obtained using NaOH or KOH solutions lead to incorporation of Na or K atoms into the seed crystals, destroying the c-axis alignment of the seeds and resulting in the growth of misaligned nanowires. The use of NH4OH eliminates the metallic impurities and ensures aligned nanowire growth in a wide range of OH concentrations in the seed solution. The difference of crystalline orientations between NH4OHand NaOH-based seeds is directly observed by lattice-resolved images and electron diffraction patterns
Received 18th July 2013 Accepted 17th January 2014
using a transmission electron microscope (TEM). This study obviously suggests that metallic impurities incorporated into the ZnO nanocrystal seeds are one of the factors that generates the misaligned ZnO
DOI: 10.1039/c3nr03694d www.rsc.org/nanoscale
nanowires. This method also enables the use of silicon oxide substrates for the growth of vertically aligned nanowires, making ZnO nanostructures compatible with widely used silicon fabrication technology.
Zinc oxide (ZnO) nanowires (wurtzite crystal structure as shown in Fig. 1a)1 are one-dimensional (1-D) nanostructures that are emerging as important materials in many applications, such as electrical generators, dye-sensitized solar cells (DSSCs) and light emitting diodes (LEDs), due to their distinct optical and electrical properties.2–12 The properties that distinguish ZnO nanowires from other nanomaterials in these applications include high exciton binding energy (60 meV), piezoelectricity and a wide direct band gap of 3.37 eV that can be tuned from 3.0 to 4.5 eV in ZnxMg1xO or Zn1xCdxO alloys.6,7,10,13–16 Despite these promising characteristics, fabrication of ZnO nanoscale devices on a commercial scale remains a challenge due to the difficulty in uniform growth of the material with high throughput and
a
Department of Electrical and Computer Engineering, Fitzpatrick Institute for Photonics, Duke University, Durham, North Carolina, 27708, USA. E-mail: [email protected]
b
Department of Chemistry, North Carolina Central University, Durham, North Carolina, 27707, USA
c Department of Physics and Department of Chemistry, Purdue University, West Lafayette, Indiana, 47907, USA d
Department of Chemistry, French Family Science Center, Duke University, Durham, North Carolina, 27708, USA. E-mail: [email protected]
e
Department of Biochemistry, Duke University, Durham, North Carolina, 27708, USA
f
Department of Automotive Engineering, Hoseo University, Asan, Chungcheongnam-do, 336-795, Republic of Korea † Electronic supplementary information (ESI) available: Additional SEM images, photographs of seed solution and XRD peaks and XPS. See DOI: 10.1039/c3nr03694d
This journal is © The Royal Society of Chemistry 2014
low cost.17 The successful creation of effective and efficient nanoscale devices made using ZnO nanowires hinges on the production of well-aligned vertical ZnO nanowire arrays, which can be exploited to create optoelectronic devices.15,18,19 It has been shown that the performance of LEDs and solar cells are signicantly inuenced by the directionality, uniformity, length and spacing of the nanowire arrays.9,11,20 The challenge in growing well-aligned vertical nanowires is attributed to either lattice mismatch between ZnO and the substrate or accumulated strain in the material during the growth process, which typically results in randomly oriented nanowires.19,21 Vertically aligned ZnO nanowires are normally obtained on single crystalline substrates, such as ZnO, sapphire (Al2O3) and galliumnitride (GaN).15,17,22,23,31 The diameter and position of nanowires can be determined by patterns of catalysts using e-beam or laser interference lithography.17,23,24,32 The drawbacks of this approach include the requirement of specialized and costly substrates, and the presence of catalyst impurities in nanowires potentially impacting electrical and optical performance.23 Homoepitaxy methods using ZnO nanocrystalline seeds coated on the surface of silicon (Si) substrates provide a more favorable approach in terms of cost and versatility. Previous reports suggest synthetic methods to generate ZnO nanocrystal seeds to initiate the growth, deriving them from either zinc acetate combined with either sodium hydroxide (NaOH) or potassium hydroxide (KOH), or simply using zinc acetate alone.20,25,26 Although there is successful growth of vertically aligned nanowires,20 the consistency of the alignment obtained
Nanoscale, 2014, 6, 3861–3867 | 3861
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
Fig. 1 (a)1 The representative planes of the ZnO wurtzite structure. Left figure shows the +c-plane (001) and the c-plane (001) and the right figure shows the m-plane (100) and a-plane (110). (b)29 Schematic of the CVD system. (c)30 Schematic of a TEM grid made of a thin silicon dioxide membrane.
using these synthetic methodologies remains a challenge. Some groups have successfully obtained vertically aligned ZnO nanowires while others have not.3,20,23,25,27 We have noticed that in earlier reports, the concentration of NaOH was altered in order to control the size and the crystalline properties of the seed particles.28 In our own experiments, we have noticed that vertically aligned nanowires are consistently obtained at low levels of NaOH concentration, but insoluble Zn(OH)2 particles aggregate on the substrate leading to uneven surface coverage (see ESI, Fig. S3†). At higher levels of NaOH concentration, the Zn(OH)2 particles fully dissolve forming zincate ions (Zn(OH)42), but subsequent growth results in randomly oriented nanowires. In this paper, we explore the mechanism that leads to the misorientation, and demonstrate an alternative approach to the seed formation that reliably results in vertically aligned nanowires with consistent coverage. As zincate ions dry on the surface, Na (or K) is incorporated into the seed material to maintain charge neutrality. These metallic impurities tend to disorient the crystal axis of the ZnO seeds on the surface upon annealing. When we replaced the base needed for the formation of ZnO nanocrystalline seeds with ammonium hydroxide
3862 | Nanoscale, 2014, 6, 3861–3867
Paper
(NH4OH), well-aligned vertical ZnO nanowire arrays with high surface coverage were obtained consistently at all levels of NH4OH concentration. The existence of metallic impurities on nanocrystalline seeds made using NaOH- or KOH-based solution was conrmed by X-ray photoelectron spectroscopy (XPS). By analyzing the lattice-resolved images and electron diffraction patterns using a transmission electron microscope (TEM), we conrm that the NH4OH-based seed solution results in seed layers whose c-axis crystal orientation is aligned to the substrate normal over a wide concentration range, unlike the seeds resulting from NaOH and KOH solutions. ZnO nanowires were grown on an amorphous substrate (a silicon dioxide layer on top of a silicon substrate) using nanometer-scale crystalline ZnO seeds on the substrate. The ZnO seeds were synthesized from the seed solution prepared by mixing 0.01 M zinc acetate dihydrate [Fluka, assay [99.5%] with various concentrations (0.03 M, 0.06 M and 0.09 M) of either sodium hydroxide (NaOH) [Fisher Scientics, assay [98.6%], potassium hydroxide (KOH) [Fisher Scientics, assay [85.9%], or ammonium hydroxide (NH4OH) [Fisher Scientic, assay ¼29.5%, 14.8 normality] dissolved in methanol [OmniSolv, assay ¼99.9%]. For control experiments, 0.06 M or 0.27 M sodium acetate (CH3COONa) [Alfa Aesar, anhydrous, assay ¼99%], 0.06 M sodium nitrate (NaNO3) [Acros Organics, assay ¼99+%] and 0.09 M sodium chloride (NaCl) [Fisher Scientic, assay >99%] were also added in above seed solutions to control the Na concentration in the solution without changing the OH concentration. The mixture was thoroughly stirred at 60 C for 2 hours in a water bath.20,25 A silicon substrate with a thin silicon oxide layer was coated with 6–7 droplets of the prepared seed solution, then rinsed with clean methanol several times to ensure deposition of zinc hydroxide (Zn(OH)2) on the substrate. The drop-coated substrate was subsequently annealed at 350 C for 20 minutes to decompose the zinc hydroxide into ZnO and water.20,25 This synthesis procedure created ZnO crystalline seeds, which were used as a template for growth of ZnO nanowires via chemical vapor transport in an experimental setup as shown in Fig. 1b.29 In the growth process, equal amounts of ZnO powder [Acros Organics, 99.5+%] and graphite powder [Alfa Aesar, crystalline, 300 mesh, assay 99%] (0.6 g) were ground and placed in an alumina boat located in the middle of the tube furnace [Lindberg Blue M, TF55035A], where the substrate is located downstream in the quartz tube (inner diameter: 2.54 cm, length: 90 cm). The furnace temperature was ramped to 920 C in approximately 15 minutes and maintained for 30 minutes under argon (50 sccm) and oxygen (1 sccm) ow. Aer the growth process, the characteristics of the nanowire were investigated using scanning electron microscopy (SEM, FEI XL 30 SEM-FEG), X-ray diffraction (XRD, Panalytical X'Pert PRO MRD HR X-ray Diffraction System) and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Axis Ultra). The analysis of seed alignment was conducted using bright eld imaging and diffraction contrast mode using a transmission electron microscope (TEM, FEI Tecnai G2 Twin). Samples observed under a TEM were grown on a special TEM grid comprised of a thin silicon dioxide membrane on a silicon nitride mesh as shown in Fig. 1c.30
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
Fig. 2a shows vertical nanowires grown from the ZnO seeds synthesized using 0.03 M sodium hydroxide (NaOH) combined with 0.01 M zinc acetate (Zn(O2CCH3)2). One can conrm that supplying hydroxide ions (OH) to zinc acetate (Zn(O2CCH3)2) plays a critical role in the formation of ZnO nanocrystalline seeds, particularly in comparison with the nanostructures grown from seeds made using zinc acetate (Zn(O2CCH3)2) alone (ESI, Fig. S1†). The hydroxide ions (OH) react with zinc acetate (Zn(O2CCH3)2) resulting in zinc hydroxide (Zn(OH)2), which is insoluble in methanol and forms a suspension of a micronscale precipitate in the solvent (ESI, Fig. S2a†). These neutral Zn(OH)2 particles of varying size in the seed solution is deposited on the substrate surface and reduces to ZnO nanocrystal seeds upon annealing with the c-axis alignment normal to the substrate, leading to vertically aligned ZnO nanowire growth as shown in Fig. 2a. However, we oen observed non-uniform ZnO nanowire growth originating from large white precipitates in seed solution adsorbed onto the substrate (ESI, Fig. S3†). At higher concentration of NaOH, more hydroxide ions (OH) bind to zinc hydroxide (Zn(OH)2) to form zincate ions (Zn(OH)42) which are soluble in methanol and the mixture turns transparent (ESI, Fig. S2b†). The transparent seed solution prepared in this way leads to uniform and high density seeds on the substrate, devoid of any clumps originating from the precipitates. However, increasing the sodium concentration led to the growth of misaligned nanowires as shown in Fig. 2b and c. The misalignment is attributed to incorporation of metallic Na atoms into the ZnO seed crystals (see below). Similar behavior was observed when NaOH is replaced with KOH. To solve this problem, we turned to ammonium hydroxide (NH4OH), an analogue of NaOH in which the sodium ion is (Na+) substituted with an ammonium ion (NH4+). Since the ammonium ion (NH4+) will decompose under heat and leave the substrate in the form of ammonia (NH3), we can eliminate the incorporation of metallic impurities into the seed crystals that might impede the seed alignment. This approach yielded immediate improvement in the orientation of ZnO nanowires at all concentrations
Nanoscale
of NH4OH in the seed solution. Fig. 2d–f, show vertically aligned uniform nanowire arrays grown from ZnO seeds synthesized using 0.03 M, 0.06 M and 0.09 M NH4OH combined with 0.01 zinc acetate (Zn(O2CCH3)2) in methanol, respectively. In contrast to the creation of misaligned nanowires at high NaOH concentration, vertical alignment of nanowire arrays were preserved regardless of the concentration of NH4OH. These experiments show that NH4OH yields a complete ZnO seed layer without incorporating metallic impurities, maintaining the vertical alignment of nanowires grown from these seed crystals. In addition to the SEM images, the vertical alignment of nanowires is also conrmed through X-ray diffraction (XRD) patterns (ESI, Fig. S4†). The samples grown from NaOH-derived seeds, except 0.03 M, show different peaks originating from all crystal surfaces of ZnO, indicating the non-uniform crystal orientation of ZnO nanowires on the substrate. The samples grown from NH4OH-derived seeds present a high intensity peak at (002) and (004), indicating the c-axis of the ZnO crystal is aligned perpendicular to the substrate, and no other crystal orientations exist in the substrate plane. We attempted to directly observe the crystal orientation of the seeds using the XRD technique, but the thin seed samples did not provide enough X-ray diffraction signals for unambiguous measurement of the crystal planes. In order to verify the crystal orientation of the ZnO seed layer as a function of the concentration of different bases, we prepared the ZnO seeds on a TEM grid made of a thin silicon dioxide membrane, as shown in Fig. 1c,30 (Ted Pella, Inc. 18 nm thickness, 0.5 0.5 mm window) according to the procedure identical to that used in the nanowire growth. Fig. 3a shows one of the ZnO nanocrystalline seeds synthesized using the combined solution of 0.06 M NH4OH with 0.01 M zinc acetate. The lattice constant of the seed crystal in the substrate plane can be directly measured from such images. For example, Fig. 3a shows a TEM image of a seed particle showing a lattice ˚ corresponding to the separation of (110) constant of 2.83 A planes. We collected random samples of 30 such seed images
Fig. 2 SEM images of ZnO nanowire arrays grown from (a) 0.03 M, (b) 0.06 M and (c) 0.09 M NaOH-derived seeds; (d) 0.03 M, (e) 0.06 M and (f) 0.09 M NH4OH-derived seeds. Images were taken 52 tilt with respect to the substrate.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 3861–3867 | 3863
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
Paper
Fig. 4 (a) Selected area electron diffraction (SAED) image of 0.09 M
(a) TEM image of 0.06 M NH4OH-derived seeds grown on the SiO2membrane of a TEM grid. The inset in the upper-right hand corner represents the top-view of the c-plane of the ZnO wurtzite structure. In the figure, labeled lattice directions on the seed image imply the direction of the c-axis perpendicular to the seed layer. (b) and (c) represent histograms of the crystal orientation distribution of ZnO nanocrystalline seeds, counted based on the lattice spacing on seed images acquired by TEM. In both figures, blue, green and red bars indicate the amount of seeds generated from 0.03 M, 0.06 M and 0.09 M of (a) NaOH and (b) NH4OH, combined with 0.01 M zinc acetate, respectively. Fig. 3
from each sample, and plotted the histogram of the observed lattice orientation as a function of the base type and concentration. Fig. 3b and c show such histograms of seed samples prepared with NaOH and NH4OH solutions, respectively. The observed histogram shows signicant preferences of seeds to orient their (001) planes parallel to the substrate surface when the concentration of NaOH is low (0.03 M), but this preference is largely lost when the NaOH concentration is increased to over 0.06 M. In contrast, the seed crystals orient preferentially, with (001) planes parallel to the surface, on the substrates in the cases of NH4OH-based solution regardless of the concentration. A similar conclusion is drawn from the comparison of selected area electron diffraction (SAED) images shown in Fig. 4. SAED looks at far-eld diffraction patterns of electron beams
3864 | Nanoscale, 2014, 6, 3861–3867
NaOH-derived seeds containing a (002) ring; right upper inset shows a zoomed-out view and the bottom left inset shows three shoulders corresponding to the (101), (002) and (100) planes in terms of the gray value. (b) Selected area electron diffraction (SEAD) image of 0.09 M NH4OH-derived seeds missing a (002) ring, confirming the c-axis alignment of seeds; the right upper inset shows a zoomed-out view and the bottom left inset shows one shoulder and a high peak corresponding to the (101) and (100) planes in terms of the gray value.
focused on a specic area of the TEM membrane grid. Fig. 4a corresponds to the seed layers prepared using NaOH solution. The diffraction rings show three peaks, corresponding to the crystal planes (100), (002) and (101) lying in the plane of the substrate membrane. The inset shows the cross-section of rings showing the three peaks. Fig. 4b shows a similar diffraction of the seed layers prepared using a NH4OH solution. In this case, the predominant diffraction ring corresponds to the (100) plane and the (002) peak is missing, indicating that most of the seeds have their c-axis alignment perpendicular to the substrate plane. The relative strength of these three peaks is consistent with the statistics of the lattice constants measured directly in Fig. 3. We attribute the incorporation of metallic Na (or K) impurities in the ZnO seed crystals as the main source that leads to the misalignment. X-ray photoelectron spectroscopy (XPS) was
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
performed on SiO2 substrates coated with ZnO seeds to directly detect the presence of metallic Na impurities. Fig. S5a–c† show the core-level spectra of sodium (Na) in XPS. While 0.03 M NaOH-based seeds on the substrate do not show any peak during regional scanning in XPS, ZnO seeds on the substrate derived from 0.06 M and 0.09 M NaOH solutions show the sodium 1s peak, indicating the presence of sodium impurities and their role in the subsequent misalignment of nanowires. Table S1† shows the atomic concentrations of Na in XPS corresponding to Fig. S5a–c.† The correlation between detected Na levels in the seed and the orientation of seed crystals and subsequent alignment of nanowires are strong indicators that the impurities disorient the crystalline ZnO seeds. To understand the mechanism of Na incorporation into the seeds, we performed control experiments varying the Na concentration in the seed solution while holding the OH concentration constant. Fig. 5 shows the SEM images of vertically aligned ZnO nanowires grown from the nanocrystalline seeds synthesized using 0.01 M zinc acetate (Zn(O2CCH3)2) combined with (a) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium acetate (CH3COONa), (b) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium nitrate (NaNO3), (c) 0.09 M ammonium hydroxide (NH4OH) and 0.09 M sodium chloride (NaCl), and (d) 0.09 M ammonium hydroxide (NH4OH) and 0.27 M sodium acetate (CH3COONa), respectively. Although the total level of sodium cations (Na+) in the seed solution is equivalent to 0.09 M in Fig. 5a–c and much higher in (d), we consistently see well-aligned nanowires in all of these samples. Furthermore, XPS measurement conrmed that no sodium component was
Nanoscale
detected in the seed crystals on the surface of the substrate in these samples (two of them as shown in Fig. S5d and e†). These experiments indicate that high levels of Na concentration alone do not lead to Na incorporation into the seeds. We measured the concentration of OH in various seed solutions using a conventional pH meter (although the solvent is methanol in our experiments). The OH concentration increases dramatically for NaOH-based solutions as the NaOH concentration is increased from 0.03 M to 0.06 M (Table S2†), consistent with the fact that NaOH is a strong base. For the NH4OH-based solutions, the OH concentration remains relatively low at all levels of NH4OH concentration (consistent with the fact that NH4OH is a weak base). Addition of high concentrations of sodium chloride and sodium acetate to 0.09 M NH4OH-based seed solution did lead to neither sodium incorporation nor misoriented nanowires, strongly indicating that metallic impurity incorporation is driven by high OH concentration. At low levels of hydroxide ions (OH) in the seed solutions, the neutral zinc hydroxide particles (Zn(OH)2) do not interact with sodium cations (Na+) as they adsorb onto the substrate and all sodium cations are washed away. Thus, zinc hydroxide (Zn(OH)2) particles on the surface produce ZnO nanocrystalline seeds upon annealing with c-axis alignment normal to the substrate, leading to vertically aligned nanowires. On the other hand, high levels of OH concentration lead to zincate ions (Zn(OH)42) in the seed solution. As these ions adsorb onto the surface, they must combine with available cations to maintain charge neutrality as the solution dries on the surface of the
SEM images of ZnO nanowire arrays grown from seeds synthesized using 0.01 M zinc acetate combined with (a) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium acetate (CH3COONa), (b) 0.03 M sodium hydroxide (NaOH) and 0.06 M sodium nitrate (NaNO3), (c) 0.09 M ammonium hydroxide (NH4OH) and 0.09 M sodium chloride (NaCl), (d) 0.09 M ammonium hydroxide (NH4OH) and 0.27 M sodium acetate (CH3COONa), respectively. Images were taken 60 tilt with respect to the substrate. Fig. 5
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 3861–3867 | 3865
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Nanoscale
substrate. This leads to the formation of sodium zincate (Na2Zn(OH)4) particles on the surface which, upon annealing, lead to ZnO seeds with a substantial amount of Na impurities incorporated in them. The crystal orientation of the ZnO seeds is randomized in this process, leading to misaligned nanowires aer the growth process. Through this study, we have successfully synthesized vertically aligned ZnO nanowires on silicon oxide substrates using seed crystals from a mixture of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2) solution. We also provide a mechanism of the creation of misaligned nanocrystalline seeds arising from the metallic (Na or K) impurity incorporation in the case of high OH concentration in the seed solution. The seed orientations studied by TEM indicate that NH4OH-derived seeds lead to c-axis alignment normal to the substrate. From these studies, we conclude that (1) pure ZnO nanocrystals prefer to orient on a at substrates with their c-axis perpendicular to the surface and (2) impurities incorporated into the ZnO seed crystals at high NaOH (or KOH) concentrations destroy the preferred orientation of the nanocrystal seeds and result in the growth of misaligned nanowires. These ndings enhance our understanding on the control of growth orientation of nanowires on various substrates and enable the growth of vertically aligned nanowires on silicon oxide substrates, making the ZnO nanostructures compatible with widely used Si fabrication technology.
Acknowledgements This work was supported by grants from NSF (ECCS-0925587). The authors also thank the service from Duke SMIF. JGS acknowledges the support from a DOD SMART fellowship.
References 1 S. Choi, Spectroscopic ellipsometry of group-III adatom kinetics on III-nitride semiconductor surfaces, PhD dissertation, Department of Physics of Duke University, Durham, NC, USA, 2007. 2 M.-T. Chen, M.-P. Lu, Y.-J. Wu, J. Song, C.-Y. Lee, M.-Y. Lu, Y.-C. Chang, L.-J. Chou, Z. L. Wang and L.-J. Chen, Near UV LEDs Made with In Situ Doped p–n Homojunction ZnO Nanowire Arrays, Nano Lett., 2010, 10(11), 4387–4393. 3 M. Law, L. E. Greene, J. C. Johnson, R. Saykally and P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater., 2005, 4(6), 455–459. 4 A. Nadarajah, R. C. Word, J. Meiss and R. K¨ onenkamp, Flexible Inorganic Nanowire Light-Emitting Diode, Nano Lett., 2008, 8(2), 534–537. 5 R. K¨ onenkamp, R. C. Word and M. Godinez, Ultraviolet Electroluminescence from ZnO/Polymer Heterojunction Light-Emitting Diodes, Nano Lett., 2005, 5(10), 2005–2008. 6 X. Wang, J. Song, J. Liu and Z. L. Wang, Direct-Current Nanogenerator Driven by Ultrasonic Waves, Science, 2007, 316(5821), 102–105. 7 Z. L. Wang and J. Song, Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays, Science, 2006, 312(5771), 242–246.
3866 | Nanoscale, 2014, 6, 3861–3867
Paper
8 Y. Wei, C. Xu, S. Xu, C. Li, W. Wu and Z. L. Wang, Planar Waveguide—Nanowire Integrated Three-Dimensional DyeSensitized Solar Cells, Nano Lett., 2010, 10(6), 2092– 2096. 9 B. Weintraub, Y. Wei and Z. L. Wang, Optical Fiber/Nanowire Hybrid Structures for Efficient Three-Dimensional DyeSensitized Solar Cells, Angew. Chem., Int. Ed., 2009, 48(47), 8981–8985. 10 S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang and Z. L. Wang, Selfpowered nanowire devices, Nat. Nanotechnol., 2010, 5(5), 366–373. 11 S. Xu, C. Xu, Y. Liu, Y. Hu, R. Yang, Q. Yang, J.-H. Ryou, H. J. Kim, Z. Lochner, S. Choi, R. Dupuis and Z. L. Wang, Ordered Nanowire Array Blue/Near-UV Light Emitting Diodes, Adv. Mater., 2010, 22(42), 4749–4753. 12 X.-M. Zhang, M.-Y. Lu, Y. Zhang, L.-J. Chen and Z. L. Wang, Fabrication of a High-Brightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film, Adv. Mater., 2009, 21(27), 2767–2770. 13 J. Cho, Q. Lin, S. Yang, J. G. Simmons, Y. Cheng, E. Lin, J. Yang, J. V. Foreman, H. O. Everitt, W. Yang, J. Kim and J. Liu, Sulfur-doped zinc oxide (ZnO) Nanostars: Synthesis and simulation of growth mechanism, Nano Res., 2012, 5(1), 20–26. 14 J. V. Foreman, J. Li, H. Peng, S. Choi, H. O. Everitt and J. Liu, Time-Resolved Investigation of Bright Visible Wavelength Luminescence from Sulfur-Doped ZnO Nanowires and Micropowders, Nano Lett., 2006, 6(6), 1126–1130. 15 M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Room-temperature ultraviolet nanowire nanolasers, Science, 2001, 292, 1897– 1899. 16 A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma and M. Kawasaki, Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO, Nat. Mater., 2005, 4(1), 42–46. 17 Y. Wei, W. Wu, R. Guo, D. Yuan, S. Das and Z. L. Wang, Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays, Nano Lett., 2010, 10(9), 3414–3419. 18 J. C. Johnson, H.-J. Choi, K. P. Knutsen, R. D. Schaller, P. Yang and R. J. Saykally, Single gallium nitride nanowire lasers, Nat. Mater., 2002, 1(2), 106–110. 19 M. I. B. Utama, Z. Peng, R. Chen, B. Peng, X. Xu, Y. Dong, L. M. Wong, S. Wang, H. Sun and Q. Xiong, Vertically Aligned Cadmium Chalcogenide Nanowire Arrays on Muscovite Mica: A Demonstration of Epitaxial Growth Strategy, Nano Lett., 2010, 11(8), 3051–3057. 20 L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somorjai and P. Yang, General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds, Nano Lett., 2005, 5(7), 1231–1236. 21 M. C. Newton, S. J. Leake, R. Harder and I. K. Robinson, Three-dimensional imaging of strain in a single ZnO nanorod, Nat. Mater., 2010, 9, 120–124.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 28 January 2014. Downloaded by University of Groningen on 08/05/2014 19:32:23.
Paper
22 X. Wang, J. Song, P. Li, J. H. Ryou, R. D. Dupuis, C. J. Summers and Z. L. Wang, Growth of Uniformly Aligned ZnO Nanowire Heterojunction Arrays on GaN, AlN, and Al0.5Ga0.5N Substrates, J. Am. Chem. Soc., 2005, 127(21), 7920–7923. 23 S. Xu, Y. Wei, M. Kirkham, J. Liu, W. Mai, D. Davidovic, R. L. Snyder and Z. L. Wang, Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low Temperature without Catalyst, J. Am. Chem. Soc., 2008, 130(45), 14958–14959. 24 Y.-J. Kim, C.-H. Lee, Y. J. Hong, G.-C. Yi, S. S. Kim and H. Cheong, Controlled selective growth of ZnO nanorod and microrod arrays on Si substrates by a wet chemical method, Appl. Phys. Lett., 2006, 89(16), 163128. 25 L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. Zhang, R. J. Saykally and P. Yang, Low-Temperature Wafer-Scale Production of ZnO Nanowire Arrays, Angew. Chem., Int. Ed., 2003, 42(26), 3031–3034. 26 C. Pacholski, A. Kornowski and H. Weller, Self-Assembly of ZnO: From Nanodots to Nanorods, Angew. Chem., Int. Ed., 2002, 41(7), 1188–1191.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
27 M.-P. Lu, J. Song, M.-Y. Lu, M.-T. Chen, Y. Gao, L.-J. Chen and Z. L. Wang, Piezoelectric Nanogenerator Using p-Type ZnO Nanowire Arrays, Nano Lett., 2009, 9(3), 1223–1227. 28 S. S. Alias, A. B. Ismail and A. A. Mohamad, Effect of pH on ZnO nanoparticle properties synthesized by sol–gel centrifugation, J. Alloys Compd., 2010, 499, 231–237. 29 D. Yu, T. Trad, J. T. McLeskey, V. Craciun and C. R. Taylor, ZnO Nanowires Synthesized by Vapor Phase Transport Deposition on Transparent Oxide Substrates, Nanoscale Res. Lett., 2010, 5(8), 1333–1339. 30 http://www.tedpella.com/grids_html/silicon-dioxide-details.htm. 31 H. Fan, F. Fleischer, W. Lee, K. Nielsch, R. Scholz, M. Zacharias, U. G¨ osele, A. Dadgar and A. Krost, Patterned Growth of Aligned ZnO Nanowire Arrays on Sapphire and GaN Layers, Superlattices Microstruct., 2004, 36, 95–105. 32 D. S. Kim, R. Ji, H. J. Fan, F. Bertram, R. Scholz, A. Dadgar, K. Nielsch, A. Krost, J. Christen, U. G¨ osele and M. Zacharias, Laser-Interference Lithography Tailored for Highly Symmetrically Arranged ZnO Nanowire Arrays, Small, 2007, 3(1), 76–80.
Nanoscale, 2014, 6, 3861–3867 | 3867
