Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubeswere either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures

Geohegan, David

Ivanov, Ilia

Kayastha, Vijaya

Mensah, Samuel

Yap, Yoke Khin

English

Michigan Technological University

Michigan Technological University Digital Commons @ Michigan Tech Department of Physics Publications Department of Physics 3-13-2007 Formation of single crystalline ZnO nanotubes without catalysts and templates Samuel Mensah Michigan Technological University Vijaya Kayastha Michigan Technological University Ilia Ivanov Oak Ridge National Laboratory David Geohegan Oak Ridge National Laboratory Yoke Khin Yap Michigan Technological University Follow this and additional works at: https://digitalcommons.mtu.edu/physics-fp  Part of the Physics Commons Recommended Citation Mensah, S., Kayastha, V., Ivanov, I., Geohegan, D., & Yap, Y. K. (2007). Formation of single crystalline ZnO nanotubes without catalysts and templates. Applied Physics Letters, 90(11), 113108-1-113108-3. http://dx.doi.org/10.1063/1.2714186 Retrieved from: https://digitalcommons.mtu.edu/physics-fp/304 Follow this and additional works at: https://digitalcommons.mtu.edu/physics-fp  Part of the Physics Commons Appl. Phys. Lett. 90, 113108 (2007); https://doi.org/10.1063/1.2714186 90, 113108© 2007 American Institute of Physics.Formation of single crystalline ZnOnanotubes without catalysts and templatesCite as: Appl. Phys. Lett. 90, 113108 (2007); https://doi.org/10.1063/1.2714186Submitted: 21 November 2006 . Accepted: 10 January 2007 . Published Online: 13 March 2007Samuel L. Mensah, Vijaya K. Kayastha, Ilia N. Ivanov, David B. Geohegan, and Yoke Khin YapARTICLES YOU MAY BE INTERESTED INGrowth of Crystal WhiskersThe Journal of Chemical Physics 37, 428 (1962); https://doi.org/10.1063/1.1701338A comprehensive review of ZnO materials and devicesJournal of Applied Physics 98, 041301 (2005); https://doi.org/10.1063/1.1992666Optical properties of the ZnO nanotubes synthesized via vapor phase growthApplied Physics Letters 83, 1689 (2003); https://doi.org/10.1063/1.1605808Formation of single crystalline ZnO nanotubes without catalystsand templatesSamuel L. Mensah and Vijaya K. KayasthaDepartment of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton,Michigan 49931Ilia N. Ivanov and David B. GeoheganCondensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831Yoke Khin YapaDepartment of Physics, Michigan Technological University, 1400 Townsend Drive, Houghton,Michigan 49931Received 21 November 2006; accepted 10 January 2007; published online 13 March 2007Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy bandgaps, and hydrophilic natures for various innovative applications. These nanotubes were eithergrown by templates or multistep processes with uncontrollable crystallinity. Here the authors showthat single crystal ZnO nanotubes can be directly grown on planar substrates without using catalystsand templates. These results are guided by the theory of nucleation and the vapor-solid crystalgrowth mechanism, which is applicable for transforming other nanowires or nanorods intonanotubular structures. © 2007 American Institute of Physics. DOI: 10.1063/1.2714186Oxide and nitride nanotubes are unique for their wideenergy band gaps and hydrophilic natures, which are notavailable from carbon nanotubes. GaN,1 silica,2 ZnO,3,4 andTiO2 Ref. 5 nanotubes have been synthesized by usingmultistep processes with templates or hydrothermal tech-niques. Some exciting applications including photochemicalcells,5 nanofluidic transistors,6 and DNA sensors7 have beendemonstrated with the use of these nanotubes. Among thesematerials, ZnO nanotubes have generated significant interestsdue to their multifunctional properties. Various ZnO nano-structures have been reported including nanowires8,9 andnanobelts10 for their electronic,11,12 optics,13 mechanical,14and piezoelectric15 applications. ZnO nanotubes are expectedto accomplish the uses of other ZnO nanostructures for ap-plications at the cutting edge of nanoscale. These nanostruc-tures will enable unique electronic, optical, photochemical,nanofluidic, biological, and chemical applications.Crystal quality of these oxide and nitride nanotubes isimportant for their optimum physical properties and perfor-mance. Thus it is desired to establish basic understanding onthe growth of single crystalline ZnO nanotubes using theconventional thermal chemical vapor deposition CVD tech-nique. Based on the theory of nucleation and vapor-solidcrystal growth, we have experimentally proven the directgrowth of single crystalline ZnO nanotubes without the useof multiple processes, catalysts, or templates. According tothe theory of nucleation, the probability of nuclei formationis given by16,17 PN=A exp−2 /k2T2 In , where A is aconstant,  is the surface energy, −1 is the supersaturation,= p / p0, p is the pressure of vapor, p0 is the equilibriumvapor pressure of the condensed phase at that temperature, kis the Boltzmann constant, and T is the temperature inKelvin. In addition, the binding energy of an ionic growthspecies and generally for nonionic growth species too on asubstrate is given by18 E=q2 /a, where  is a numericalfactor which depends on the site of the deposition, a is thelattice spacing, and q is the charge of the ion. These ionsprefer to condense on locations with the maximum numberof nearest neighbors. Thus for a growth surface with a step asshown in Fig. 1a, ions prefer to condense at sites 6, 5, 1, 4,2, and 3, according to sequence since 651423.18 Thus an atomically flat layer will always spreadfrom the edges under conditions with sufficient surface en-ergy.For a flat two-dimensional 2D growth surface withoutsteps, only sites 1, 2, and 3 exist. In this case, higher bindingenergies at the edges sites 1 and 2 will enable selectivecondensation of growth species at the edges. For ZnO, it iswell known that the growth rate along the c axis is relativelyfaster.8,9 At decreased growth temperatures, the nucleationprobability PN and the surface migration will be suppressed.Thus if the growth temperatures are low enough to suppressmigration along the 2D surface of the c plane but highenough to sustain the growth along the c axis, the condensa-tion and growth will be limited at the edges of the c plane ofZnO, as shown in Fig. 1b. Under these conditions, thegrowth of ZnO nanotubes along the c axis will be possible.However, if steps or vacancies are found in locations beyondthe edges, nucleation will also take place at these sites asshown in Fig. 1c due to their higher  values than those atthe edges.aAuthor to whom correspondence should be addressed; electronic mail:ykyap@mtu.eduFIG. 1. Color online a Nucleation sites on a surface with a step. bPreferential condensation of growth species at the c-plane edges of a ZnOnanorod. c Addition nucleation beyond the edges when steps and vacanciesare found.APPLIED PHYSICS LETTERS 90, 113108 20070003-6951/2007/9011/113108/3/$23.00 © 2007 American Institute of Physics90, 113108-1Based on this model, we have conducted a series of ex-periments to first start the growth of the c surfaces of ZnOnanorods and then decrease the growth temperatures for theformation of the ZnO nanotubes. The growth was performedin a horizontal furnace consisting of a quartz tube vacuumchamber. A smaller quartz tube 60 cm long and 2 cm indiameter containing the precursor materials and the sub-strates was placed within the vacuum chamber. A mixture ofZnO 0.2 g and graphite 0.1 g powder in an alumina boatwas used as the precursor materials. These are placed at theclosed end of the smaller quartz tube, as shown in Fig. 2a.A series of oxidized silicon substrates 1–4 was then placeddownstream from the mixture in the small quartz tube. Thesmall tube was then inserted into the vacuum chamber suchthat the closed end is at the center of the furnace. The tem-perature of the furnace was raised to 1100 °C. At 350 °C,oxygen gas was introduced into the furnace at a flow rate of40 SCCM SCCM denotes cubic centimeter per minute atSTP. The temperature was held at 1100 °C for 30 min andturned off to allow cooling to 600–700 °C in 30 min.Then experiments are terminated by cooling the system toRT by opening the heating panel of the furnace. According toa calibration experiment, when the furnace is heated at1100 °C, the temperatures of substrates 1–4 are about 700,600, 500, and 420 °C, respectively. These temperatures willbe lower as the furnace temperatures decrease from 1100 °C.After the growth, all samples were examined with scanningelectron microscopy SEM. In all experiments, no coatingwas formed on substrate 1. ZnO Nanorods with sharp tipswere grown on substrates 2 and 3, as shown in Figs. 2b and2c, respectively. We detected ZnO nanotubes only on sub-strates 4, as shown in Fig. 2d. The diameters of these nano-tubes are between 80 and 500 nm near the opened tips.The appearance of these ZnO nanotubes is highlighted inFig. 3. Some of these nanotubes have single tubular channel,as shown in Figs. 3a and 3b, which is as predicted bytheory. As shown in Figs. 3c and 3d, some of these nano-tubes have multiple channels. The formation of these multi-channel nanotubes is also expected by the nucleation modeldiscussed earlier. During the nucleation and growth, steps orvacancies could be formed at locations beyond the edges, asshown in Fig. 1c. Especially at suppressed growth tempera-tures, the formation of steps and vacancies will induce theformation of addition islands at these locations. These is-lands will tend to merge with the edges so as to reduce theirsurface energy larger island, lower surface energy.18 Theseprocesses are accompanied by the growth along the c axisand thus transform the single channel nanotubes to multi-channel structures.Obviously, the diameters of these ZnO nanotubes re-sembled those of the ZnO nanorods. Since the substrate sur-faces are smooth as inspected by SEM Hitachi S-4700, reso-lution of 2.1 nm at 1 kV, the diameters of these nanotubesand nanorods 80–600 nm are unlikely to be determinedby surface steps or roughness of the substrates. We anticipatethat the diameters of the ZnO nanotubes can be controlled bythe growth conditions including the partial vapor pressuresand the growth temperatures.Figure 4a shows the typical x-ray diffraction spectra ofthese nanotube samples. These spectra can be indexed fordiffractions from the 002, 101, 102, and 103 planes ofwurtzite crystals with lattice constants a=0.325 mm and c=0.521 nm, which are corresponding to ZnO. The predomi-FIG. 2. a Experimental setup in a double-tube horizontal furnace. Appear-ance of ZnO nanorods on substrates b 2, c 3, and ZnO nanotubes on dsubstrate 4. The scale bars are 6 m.FIG. 3. Appearance of various ZnO nanotubes. The scale bars are 300 nm.FIG. 4. a X-ray diffraction, b TEM, and c higy resolution TEM of aZnO nanotubes. The arrows representing the tubular axes of thesenanotubes.113108-2 Mensah et al. Appl. Phys. Lett. 90, 113108 2007nated 002 peak indicates that these ZnO nanotubes arepreferentially grown in the 0001 direction since these nano-tubes are partially vertically aligned on the substrates. The101, 102, and 103 peaks may be induced by the imper-fect vertical alignment as well as other trace nanostructures.We have further verified that these ZnO nanotubes are grownalong the 0001 axis by transmission electron microscopyTEM. Figure 4b shows the hollow tubular tip of a ZnOnanotube at low resolution TEM. A high resolution TEMimage Fig. 4c indicates that these ZnO nanotubes aresingle crystalline with the growth direction arrows alignedto the c axis of ZnO with lattice spacing of 0.526 nm.These ZnO nanotubes are then analyzed by photolumi-nescence PL. A predominant emission peak at 380 nm isobserved and corresponds to the near-band-edge emission ofZnO attributed to the recombination of the free excitonsFig. 5a. A weak broad emission at 600 nm is attributedto intraband defect levels including the singly ionized oxy-gen vacancy in ZnO. For example, emission of 520 nm isgenerated from the recombination of a photogenerated holewith an electron occupying the oxygen vacancy.19 Ramanspectra of these ZnO nanotubes as excited by a HeNe laserwavelength=632.8 nm indicate phonon frequencies at 331,380, 438, and 581 cm−1, which are corresponding to theE2H-E2L, A1T, E2H, and E1L phonon modes of ZnORefs. 20 and 21 Fig. 5b. The detected PL and Ramansignals indicate that the ZnO nanotubes maintain the bandstructures and crystalline structures of the bulk wurtzite ZnOcrystals.In summary, we demonstrate a promising route of grow-ing single crystalline ZnO nanotubes without catalysts andtemplates by conventional thermal CVD technique. TheseZnO nanotubes were grown on the c surfaces of ZnO nano-rods, as predicted by the theory of nucleation and vapor-solidcrystals growth. In principle, the height of these nanorodbases can be minimized so that nanotubes with longer tubu-lar section can be formed.One of the authors Y.K.Y. thanks support from the U.S.Department of Army Grant No. W911NF-04-1-0029,through the City College of New York and the Center forNanophase Materials Sciences sponsored by the Division ofMaterials Sciences and Engineering, U.S. Department of En-ergy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. This work is in part supported by NationalScience Foundation CAREER award DMR 0447555.1J. Goldberger, R. He, Y. Zhang, S. Lee, H. Yan, H. J. Choi, and P. Yang,Nature London 422, 599 2003.2R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, and P. Yang, J. Am. Chem.Soc. 125, 5254 2003.3Y. Sun, G. M. Fuge, N. A. Fox, D. J. Riley, and M. N. R. Ashfold,Adv. Mater. Weinheim, Ger. 17, 2477 2005.4G. S. Wu, T. Xie, X. Y. Yuan, Y. Li, L. Yang, Y. H. Xiao, and L. D. Zhang,Solid State Commun. 134, 485 2005.5G. K. Mor, K. Shankar, M. Paulose, O. K. 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Phys. 37, 428 1962.18Laboratory Manual on Crystal Growth, edited by I. Tarjan and M. MatraiAkadémiai Kiadó, Budapest, 1972, pp. 29–30.19K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, andB. E. Gnade, J. Appl. Phys. 79, 7983 1996.20T. C. Damen, S. P. S. Porto, and B. Tell, Phys. Rev. 142, 570 1966.21R. P. Wang, G. Xu, and P. Jin, Phys. Rev. B 69, 113303 2004.FIG. 5. a Raman scattering and b PL spectra of ZnO nanotubes.113108-3 Mensah et al. Appl. Phys. Lett. 90, 113108 2007

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Formation of single crystalline ZnO nanotubes without catalysts and templates

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