SCIENCE OFADVANCED MATERIALS

Editor-in-Chief Dr Ahmad Umar

AMERICANSCIENTIFICPUBLISHERS

VOLUME 2 bull NUMBER 4 DECEMBER 2010wwwaspbscomsam

A Special Issue on

Advanced Engineering MaterialsGUEST EDITORS Chunhui Yang and Pengjian Zuo

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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Science ofAdvanced Materials

Vol 2 572ndash577 2010

One Step Growth of ZnO Nano-Tetrapods bySimple Thermal Evaporation Process

Structural and Optical Properties

Aurangzeb Khan1lowast Saima N Khan1 and Wojciech M Jadwisienczak21Biomimetic Nanoscience and Nanoscale Technology (BNNT) Initiative Chemistry and Bio-ChemistryOhio University Athens OH 45701 and Department of Physics University of Peshawar Peshawar

Khyber-Pukhtoonkhwa Pakistan2School of Electrical Engineering and Computer Science Ohio University Athens OH 45701 USA

Large scale zinc Oxide (ZnO) tetrapod structures were synthesized in a tube furnace via simpleone-step thermal evaporation method by heating metallic Zn powders at 900plusmn50 C The as grownmaterials were characterized by X-ray diffractometery (XRD) Scanning and Transmission electronmicroscopy Raman-scattering (RS) and Photoluminescence (PL) spectroscopy Room-temperaturePL spectrum exhibits strong near band edge (NBE) emission at sim380 nm (UV) and defects relatedbroad blue-green band emission at sim520 nm Raman spectroscopic analysis shows that thesetetrapods have good crystalline quality It is believed that this one-step growth will pave an easyway to large scale production of ZnO tetrapod structures for useful applications in nanotechnology

Keywords Tetrapod Photoluminescence Raman Spectroscopy Zinc Oxide

1 INTRODUCTION

Considerable attention has been given to synthe-size quasi-one-dimensional semiconductor nanostructures(nanowires nanorods and nanobelts) recently due to theirpossible application as building blocks for nano-scaledelectronic and photonic devices1ndash4 Among several semi-conducting materials ZnO is one of the most importantmaterials It is recognized as a promising photonic materialin the blue-UV region due to its direct band-gap (337 eV)at room temperature and large exciton binding energy(sim60 meV) The strong exciton binding energy (greaterthan the thermal energy at room temperature sim26 meV)can cinch an efficient exciton emission at room tempera-ture under low excitation energyDue to its various properties and versatile applications

various ZnO nanostructures have been synthesized andreported in the literature1ndash15 Among various ZnO nano-structures tetrapod-like ZnO structure has recently gainedattention due to their unique morphologies and propertieseg it can be used as nano-composite to enhance physicalproperties and sharp tips of ZnO tetrapods can be used asan electron field emission source or scanning probe5ndash7 Thegrowth mechanism for ZnO tetrapods is under debate espe-cially such characteristics of the seed nucleus as shape

lowastAuthor to whom correspondence should be addressed

geometry and crystallographic structure58ndash15 It is not clearhow the morphologies of tetrapod nanostructures is con-trolled and whether different tetrapod ZnO nanostructurescan be controlled in synthesis under suitable reactionconditions16 To achieve the growth of tetrapod struc-tures with good control many efforts have been made toachieve this kind of structures and understand its growthmechanism58ndash101214ndash1517ndash18

In this article we report a simple one step growthof tetrapod structures of ZnO at a large scale with-out any catalyst via a simple low cost thermal evapora-tion and oxidation technique The synthesized tetrapodswere characterized in detail in terms of their structuraland optical properties To examine elaborately about theRaman-scattering properties of as-grown tetrapod struc-tures Raman analysis for various parts of a single tetra-pod structures has been reported and presented in thismanuscript Finally a plausible growth mechanism has beenpresented based on the previously published articles on thesame topic

2 EXPERIMETAL DETAILS

One step growth of tetrapod ZnO structures was doneby simple thermal evaporation process using metallic zincpowder In a typical reaction process metallic Zn powderwas kept in quartz boat (self made by cutting the 1 inch

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

Scheme 1 Typical schematic for the reaction setup used for the growthof ZnO tetrapod structures

diameter tube along its axis) and then placed in a quartztube (OD of 3 inch and 48 inch long) fixed in the resis-tively heated furnace (Scheme 1) Si(100) substrates wereused as substrates and placed adjacent to the source mate-rial The furnace was heated up and synthesis was per-formed at temperature range of 900 Cplusmn50 C During thereaction the quartz tube was open on both the sides andthe experiment was performed at atmospheric pressure andambient air After desired reaction time the furnace wasallowed to cool at room-temperature After the reactionthe ZnO nanomaterials were deposited on whole Si(100)substratesThe synthesized products were characterized in terms

of their structural and optical properties Structural proper-ties were examined by using scanning electron microscope(SEM) [JOEL 6400] transmission electron microscope(TEM) [JOEL 1010] and X-ray diffractometer (XRD)[Rigaku Geigerflex 2000 Watts] with Cu K (154 Aring)as the incident radiation The micro-Raman spectra wereacquired using a Scanning Optical Microscope imag-ing system (WiTECH -SNOM) integrated with Ramansystem and with 100X objective lense with a laser spotsize of sim500 nm The photoluminescence (PL) characteri-zation of ZnO tetradpods was conducted using He-Cd laserwith 325 nm excitation wavelength A full detail of thesetup is described elsewhere19

3 RESULTS AND DISCUSSION

31 Detailed Structural Properties of As-GrownZnO Nano-Tetrapods

The morphologies of the synthesized ZnO structures wasexamined by using scanning electron microscopy (SEM)and shown in Figure 1 Figure 1 depicts the SEM micro-graphs of the as-grown tetrapod structures Fig 1(a) showsthe low-magnification and (b) demonstrates the high-resolution SEM images of as-grown ZnO tetrapod struc-tures It is clear from the SEM images that the as-growntetrapod structures have four needle shaped tetrahedrallyarranged prongs connected at the center junction and

(a)

(b)

Fig 1 Typical (a) low and (b) high-magnification SEM images of as-grown ZnO tetrapods grown via simple thermal evaporation process

angled 120 to each other The length of the prongs ofthe tetrapod-like structures is a few micrometers and about150ndash300 nm in width with sharper tips It can be clearlyseen that the prongs of the tetrapod are gradually narrow-ing outwards as well as having step (s) towards the tip endand the tips of the tetrapods are also connected to eachother as well Figure 2 shows SEM images along with itscartoon drawing shown in Figures 2(c) and (d) depictingthe angles between any two adjacent prongs of 120 Theoutward prong in Figure 2(b) is colored darker for under-standing and also shown in Figure 2(d)In order to see the elemental compositions and crys-

tallographic structure of the as-grown product EDX andXRD were performed Figure 3(a) shows the EDX spec-trum of the as-grown tetrapod-like ZnO structures TheEDX spectrum peaks are related to oxygen at sim545 eVand Zn at 1040 eV 8607 eV and 9532 eV confirmingthat the synthesized products are purely ZnO without anyimpurity Figure 3(b) represents the XRD spectrum of theas grown tetrapods Peaks at 312 339 357 47 56 and623 degrees of angle are attributed to hexagonal wurtziteZnO crystal structure with the lattice constants a= 32 nmand c = 52 nm (ICDD PDF card 00-003-0888)20ndash22 con-firming that the grown structures are pure wurtzite hexag-onal ZnO

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(c)

(b)

(d)

Fig 2 SEM images and the corresponding cartoon diagrams of nano-tetrapod structures (a) and the top view and its (b) cartoon illustrationwhile (c) SEM side view with its (d) cartoon illustration

The morphology and crystallinity of as-grown ZnOtetrapods were further investigated using transmissionelectron microscopy (TEM) Figure 4(a) shows the low-magnification TEM image of as-grown ZnO tetrapodstructures which exhibit full consistency with the observed

(a)

(b)

Fig 3 (a) Typical X-ray diffraction pattern and (b) EDS spectrum ofas-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

Fig 4 Typical (a) low-magnification and (b) high-resolution TEMimages of as-grown ZnO nano-tetrapods synthesized by simple thermalevaporation process Inset of (a) exhibits the corresponding SAED patternof as-grown nano-tetrapods

SEM images The inset in Figure 4(a) is the selected areaelectron diffraction (SAED) pattern of a single prong ofthe tetrapod The SAED pattern clearly shows bright spotsconfirming the well-crystallinity and wurtzite hexagonal

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

phase for the as-grown ZnO tetrapods Figure 4(b) showsthe HRTEM image of a portion of the ZnO tetrapod withthe well-aligned lattice fringes The distance of 052 nmbetween the parallel planes along the ZnO nanowire axiscorresponds to a d-spacing of the (0001)-planes TheFigure 4(c) shows the line brightness contrast scan alongthe c-axis from the HRTEM image The separation of thealternating peaks in the line scan is 052 nm which is ingood agreement with the lattice constant of wurtzite ZnOalong the c-axisRegarding the growth of ZnO tetrapod structures sev-

eral groups have explained the growth mechanism of thetetrapod nanostructures Different growth mechanisms arefound in the literature explaining the formation of ZnOtetrapod structures13 The first mechanism explained byShiojiri et al23 is based on the assumption that a zinc-blende phase core at the center of the wurtzite ZnOtetrapods exists The second mechanism is proposed byFujii and Iwanaga et al17 They proposed building an octa-hedral multiple twin structure from a multiple inversion-twin embryo and suggested that the octahedral multipleinversion-twin formed first The third mechanism is pro-posed by Nishio et al14 which suggests that the growthof ZnO tetrapods is coming from wurtzite-phased ZnOmultiple twins induced in a zinc-blende phase structurednucleus and that the zinc-blende nucleus exists only in thehigh-temperature tetrapods which will degenerate to mul-tiple twins at room temperature Recently Ding et al11

observed directly the zinc-blende structure core in theinitial formation of wurtzite tetrapods of ZnO and henceconfirmed the zinc-blende core in the nucleation of theZnO tetrapodsIn our synthesized tetrapods we also believed that zinc-

blende (ZB) structure core in the initial formation ofwurtzite tetrapods of ZnO and then secondly the octahe-dral multiple twin structure is building upon followed bythe ZB-type nucleus which only exists at high-temperaturein the tetrapods and degenerate to multiple twins whencooled down to room temperature Our observation is con-sistent with the existing reported literature1423ndash24

32 Detailed Optical Properties of As-GrownZnO Nano-Tetrapods

Figure 5 shows the Raman-scattering spectrum of the as-grown ZnO tetrapods measure with 532 nm laser light(NdYAG) laser as an excitation source The Ramansignals are very sensitive to the crystal structures and thedefects in the nanostructures ZnO has wurtzite hexago-nal phase belongs to the C4

6v space group with two for-mula unit per primitive cell where all the atoms occu-pying the C3v sites Group theory predicts eight sets ofzone centre optical phonons where A1 and E1 modes arepolar and split into transverse optical (A1T and E1T andlongitudinal-optical (A1L and E1L phonons while the E2

Fig 5 Typical Raman-scattering spectra obtained the four legs of asingle tetrapod The inset is Raman Scan filtered image in the range431ndash446 cmminus1

mode consists of two modes of low and high-frequencyphonons (E2L and E2H are Raman-active2225 The insetin the spectra is the filtered (431ndash446 cmminus1 image col-lected in the Raman mode The different identical spectracollected from the four prongs of a single ZnO tetrapodstructure labeled as a b c and d (outward) which suggestthat they have identical wurtzite structuresThe main dominant sharp peak labeled as E2 at

437 cmminus1 was observed and is known as Raman-activeoptical phonon mode which is the characteristic ofwurtzite hexagonal phase ZnO24 The peak at sim98 cmminus1

corresponds to E2 (Low) and the peak at 339 cmminus1 cor-respond to the second order Raman spectrum originatingfrom zone-boundary phonons 3E2HndashE2L and the peak at388 cmminus1 can be labeled as A1T The peak at 521 cmminus1

is coming from Si substrate The higher in intensity andsharp peak of the E2 mode peak at 437 cmminus1 shows that theas-grown ZnO tetrapods are of wurtzite hexagonal phasewith good crystal qualityFigure 6 shows the room-temperature and low-

temperature photoluminescence spectra of the as-growntetrapods measured with the excitation wavelength of325 nm (He-Cd laser) Figure 6(a) demonstrates the roomtemperature PL spectrum of the as-grown tetrapod struc-tures which shows two peaks the intense peak on the leftcentered at sim380 nm is known as the near band edgeemission peak and the wide peak on the right centeredat sim515 nm26 The NBE peak at 380 nm has full widthat half maximum (FWHM) of sim9 nm and considered tobe due to the free excitons recombination via an excitonndashexciton collision2227 while the wide band in the blue-green (510 nm) with a FWHM of sim100 nm may bedue to defects in the lattice either due to oxygen or zincvacancies or interstitials and their complexes present inZnO28 This green emission from tetrapod-structured ZnOhas been widely studied12ndash13151829ndash31 Figure 6(b) showsthe low-temperature PL spectrum of as-grown tetrapods

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

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Copyright copy 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Science ofAdvanced Materials

Vol 2 572ndash577 2010

One Step Growth of ZnO Nano-Tetrapods bySimple Thermal Evaporation Process

Structural and Optical Properties

Aurangzeb Khan1lowast Saima N Khan1 and Wojciech M Jadwisienczak21Biomimetic Nanoscience and Nanoscale Technology (BNNT) Initiative Chemistry and Bio-ChemistryOhio University Athens OH 45701 and Department of Physics University of Peshawar Peshawar

Khyber-Pukhtoonkhwa Pakistan2School of Electrical Engineering and Computer Science Ohio University Athens OH 45701 USA

Large scale zinc Oxide (ZnO) tetrapod structures were synthesized in a tube furnace via simpleone-step thermal evaporation method by heating metallic Zn powders at 900plusmn50 C The as grownmaterials were characterized by X-ray diffractometery (XRD) Scanning and Transmission electronmicroscopy Raman-scattering (RS) and Photoluminescence (PL) spectroscopy Room-temperaturePL spectrum exhibits strong near band edge (NBE) emission at sim380 nm (UV) and defects relatedbroad blue-green band emission at sim520 nm Raman spectroscopic analysis shows that thesetetrapods have good crystalline quality It is believed that this one-step growth will pave an easyway to large scale production of ZnO tetrapod structures for useful applications in nanotechnology

Keywords Tetrapod Photoluminescence Raman Spectroscopy Zinc Oxide

1 INTRODUCTION

Considerable attention has been given to synthe-size quasi-one-dimensional semiconductor nanostructures(nanowires nanorods and nanobelts) recently due to theirpossible application as building blocks for nano-scaledelectronic and photonic devices1ndash4 Among several semi-conducting materials ZnO is one of the most importantmaterials It is recognized as a promising photonic materialin the blue-UV region due to its direct band-gap (337 eV)at room temperature and large exciton binding energy(sim60 meV) The strong exciton binding energy (greaterthan the thermal energy at room temperature sim26 meV)can cinch an efficient exciton emission at room tempera-ture under low excitation energyDue to its various properties and versatile applications

various ZnO nanostructures have been synthesized andreported in the literature1ndash15 Among various ZnO nano-structures tetrapod-like ZnO structure has recently gainedattention due to their unique morphologies and propertieseg it can be used as nano-composite to enhance physicalproperties and sharp tips of ZnO tetrapods can be used asan electron field emission source or scanning probe5ndash7 Thegrowth mechanism for ZnO tetrapods is under debate espe-cially such characteristics of the seed nucleus as shape

lowastAuthor to whom correspondence should be addressed

geometry and crystallographic structure58ndash15 It is not clearhow the morphologies of tetrapod nanostructures is con-trolled and whether different tetrapod ZnO nanostructurescan be controlled in synthesis under suitable reactionconditions16 To achieve the growth of tetrapod struc-tures with good control many efforts have been made toachieve this kind of structures and understand its growthmechanism58ndash101214ndash1517ndash18

In this article we report a simple one step growthof tetrapod structures of ZnO at a large scale with-out any catalyst via a simple low cost thermal evapora-tion and oxidation technique The synthesized tetrapodswere characterized in detail in terms of their structuraland optical properties To examine elaborately about theRaman-scattering properties of as-grown tetrapod struc-tures Raman analysis for various parts of a single tetra-pod structures has been reported and presented in thismanuscript Finally a plausible growth mechanism has beenpresented based on the previously published articles on thesame topic

2 EXPERIMETAL DETAILS

One step growth of tetrapod ZnO structures was doneby simple thermal evaporation process using metallic zincpowder In a typical reaction process metallic Zn powderwas kept in quartz boat (self made by cutting the 1 inch

572 Sci Adv Mater 2010 Vol 2 No 4 1947-293520102572006 doi101166sam20101127

Delivered by Ingenta toGuest User

IP 2121384716Tue 17 Aug 2010 120806

RESEARCH

ARTIC

LE

Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

Scheme 1 Typical schematic for the reaction setup used for the growthof ZnO tetrapod structures

diameter tube along its axis) and then placed in a quartztube (OD of 3 inch and 48 inch long) fixed in the resis-tively heated furnace (Scheme 1) Si(100) substrates wereused as substrates and placed adjacent to the source mate-rial The furnace was heated up and synthesis was per-formed at temperature range of 900 Cplusmn50 C During thereaction the quartz tube was open on both the sides andthe experiment was performed at atmospheric pressure andambient air After desired reaction time the furnace wasallowed to cool at room-temperature After the reactionthe ZnO nanomaterials were deposited on whole Si(100)substratesThe synthesized products were characterized in terms

of their structural and optical properties Structural proper-ties were examined by using scanning electron microscope(SEM) [JOEL 6400] transmission electron microscope(TEM) [JOEL 1010] and X-ray diffractometer (XRD)[Rigaku Geigerflex 2000 Watts] with Cu K (154 Aring)as the incident radiation The micro-Raman spectra wereacquired using a Scanning Optical Microscope imag-ing system (WiTECH -SNOM) integrated with Ramansystem and with 100X objective lense with a laser spotsize of sim500 nm The photoluminescence (PL) characteri-zation of ZnO tetradpods was conducted using He-Cd laserwith 325 nm excitation wavelength A full detail of thesetup is described elsewhere19

3 RESULTS AND DISCUSSION

31 Detailed Structural Properties of As-GrownZnO Nano-Tetrapods

The morphologies of the synthesized ZnO structures wasexamined by using scanning electron microscopy (SEM)and shown in Figure 1 Figure 1 depicts the SEM micro-graphs of the as-grown tetrapod structures Fig 1(a) showsthe low-magnification and (b) demonstrates the high-resolution SEM images of as-grown ZnO tetrapod struc-tures It is clear from the SEM images that the as-growntetrapod structures have four needle shaped tetrahedrallyarranged prongs connected at the center junction and

(a)

(b)

Fig 1 Typical (a) low and (b) high-magnification SEM images of as-grown ZnO tetrapods grown via simple thermal evaporation process

angled 120 to each other The length of the prongs ofthe tetrapod-like structures is a few micrometers and about150ndash300 nm in width with sharper tips It can be clearlyseen that the prongs of the tetrapod are gradually narrow-ing outwards as well as having step (s) towards the tip endand the tips of the tetrapods are also connected to eachother as well Figure 2 shows SEM images along with itscartoon drawing shown in Figures 2(c) and (d) depictingthe angles between any two adjacent prongs of 120 Theoutward prong in Figure 2(b) is colored darker for under-standing and also shown in Figure 2(d)In order to see the elemental compositions and crys-

tallographic structure of the as-grown product EDX andXRD were performed Figure 3(a) shows the EDX spec-trum of the as-grown tetrapod-like ZnO structures TheEDX spectrum peaks are related to oxygen at sim545 eVand Zn at 1040 eV 8607 eV and 9532 eV confirmingthat the synthesized products are purely ZnO without anyimpurity Figure 3(b) represents the XRD spectrum of theas grown tetrapods Peaks at 312 339 357 47 56 and623 degrees of angle are attributed to hexagonal wurtziteZnO crystal structure with the lattice constants a= 32 nmand c = 52 nm (ICDD PDF card 00-003-0888)20ndash22 con-firming that the grown structures are pure wurtzite hexag-onal ZnO

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(c)

(b)

(d)

Fig 2 SEM images and the corresponding cartoon diagrams of nano-tetrapod structures (a) and the top view and its (b) cartoon illustrationwhile (c) SEM side view with its (d) cartoon illustration

The morphology and crystallinity of as-grown ZnOtetrapods were further investigated using transmissionelectron microscopy (TEM) Figure 4(a) shows the low-magnification TEM image of as-grown ZnO tetrapodstructures which exhibit full consistency with the observed

(a)

(b)

Fig 3 (a) Typical X-ray diffraction pattern and (b) EDS spectrum ofas-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

Fig 4 Typical (a) low-magnification and (b) high-resolution TEMimages of as-grown ZnO nano-tetrapods synthesized by simple thermalevaporation process Inset of (a) exhibits the corresponding SAED patternof as-grown nano-tetrapods

SEM images The inset in Figure 4(a) is the selected areaelectron diffraction (SAED) pattern of a single prong ofthe tetrapod The SAED pattern clearly shows bright spotsconfirming the well-crystallinity and wurtzite hexagonal

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

phase for the as-grown ZnO tetrapods Figure 4(b) showsthe HRTEM image of a portion of the ZnO tetrapod withthe well-aligned lattice fringes The distance of 052 nmbetween the parallel planes along the ZnO nanowire axiscorresponds to a d-spacing of the (0001)-planes TheFigure 4(c) shows the line brightness contrast scan alongthe c-axis from the HRTEM image The separation of thealternating peaks in the line scan is 052 nm which is ingood agreement with the lattice constant of wurtzite ZnOalong the c-axisRegarding the growth of ZnO tetrapod structures sev-

eral groups have explained the growth mechanism of thetetrapod nanostructures Different growth mechanisms arefound in the literature explaining the formation of ZnOtetrapod structures13 The first mechanism explained byShiojiri et al23 is based on the assumption that a zinc-blende phase core at the center of the wurtzite ZnOtetrapods exists The second mechanism is proposed byFujii and Iwanaga et al17 They proposed building an octa-hedral multiple twin structure from a multiple inversion-twin embryo and suggested that the octahedral multipleinversion-twin formed first The third mechanism is pro-posed by Nishio et al14 which suggests that the growthof ZnO tetrapods is coming from wurtzite-phased ZnOmultiple twins induced in a zinc-blende phase structurednucleus and that the zinc-blende nucleus exists only in thehigh-temperature tetrapods which will degenerate to mul-tiple twins at room temperature Recently Ding et al11

observed directly the zinc-blende structure core in theinitial formation of wurtzite tetrapods of ZnO and henceconfirmed the zinc-blende core in the nucleation of theZnO tetrapodsIn our synthesized tetrapods we also believed that zinc-

blende (ZB) structure core in the initial formation ofwurtzite tetrapods of ZnO and then secondly the octahe-dral multiple twin structure is building upon followed bythe ZB-type nucleus which only exists at high-temperaturein the tetrapods and degenerate to multiple twins whencooled down to room temperature Our observation is con-sistent with the existing reported literature1423ndash24

32 Detailed Optical Properties of As-GrownZnO Nano-Tetrapods

Figure 5 shows the Raman-scattering spectrum of the as-grown ZnO tetrapods measure with 532 nm laser light(NdYAG) laser as an excitation source The Ramansignals are very sensitive to the crystal structures and thedefects in the nanostructures ZnO has wurtzite hexago-nal phase belongs to the C4

6v space group with two for-mula unit per primitive cell where all the atoms occu-pying the C3v sites Group theory predicts eight sets ofzone centre optical phonons where A1 and E1 modes arepolar and split into transverse optical (A1T and E1T andlongitudinal-optical (A1L and E1L phonons while the E2

Fig 5 Typical Raman-scattering spectra obtained the four legs of asingle tetrapod The inset is Raman Scan filtered image in the range431ndash446 cmminus1

mode consists of two modes of low and high-frequencyphonons (E2L and E2H are Raman-active2225 The insetin the spectra is the filtered (431ndash446 cmminus1 image col-lected in the Raman mode The different identical spectracollected from the four prongs of a single ZnO tetrapodstructure labeled as a b c and d (outward) which suggestthat they have identical wurtzite structuresThe main dominant sharp peak labeled as E2 at

437 cmminus1 was observed and is known as Raman-activeoptical phonon mode which is the characteristic ofwurtzite hexagonal phase ZnO24 The peak at sim98 cmminus1

corresponds to E2 (Low) and the peak at 339 cmminus1 cor-respond to the second order Raman spectrum originatingfrom zone-boundary phonons 3E2HndashE2L and the peak at388 cmminus1 can be labeled as A1T The peak at 521 cmminus1

is coming from Si substrate The higher in intensity andsharp peak of the E2 mode peak at 437 cmminus1 shows that theas-grown ZnO tetrapods are of wurtzite hexagonal phasewith good crystal qualityFigure 6 shows the room-temperature and low-

temperature photoluminescence spectra of the as-growntetrapods measured with the excitation wavelength of325 nm (He-Cd laser) Figure 6(a) demonstrates the roomtemperature PL spectrum of the as-grown tetrapod struc-tures which shows two peaks the intense peak on the leftcentered at sim380 nm is known as the near band edgeemission peak and the wide peak on the right centeredat sim515 nm26 The NBE peak at 380 nm has full widthat half maximum (FWHM) of sim9 nm and considered tobe due to the free excitons recombination via an excitonndashexciton collision2227 while the wide band in the blue-green (510 nm) with a FWHM of sim100 nm may bedue to defects in the lattice either due to oxygen or zincvacancies or interstitials and their complexes present inZnO28 This green emission from tetrapod-structured ZnOhas been widely studied12ndash13151829ndash31 Figure 6(b) showsthe low-temperature PL spectrum of as-grown tetrapods

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

Scheme 1 Typical schematic for the reaction setup used for the growthof ZnO tetrapod structures

diameter tube along its axis) and then placed in a quartztube (OD of 3 inch and 48 inch long) fixed in the resis-tively heated furnace (Scheme 1) Si(100) substrates wereused as substrates and placed adjacent to the source mate-rial The furnace was heated up and synthesis was per-formed at temperature range of 900 Cplusmn50 C During thereaction the quartz tube was open on both the sides andthe experiment was performed at atmospheric pressure andambient air After desired reaction time the furnace wasallowed to cool at room-temperature After the reactionthe ZnO nanomaterials were deposited on whole Si(100)substratesThe synthesized products were characterized in terms

of their structural and optical properties Structural proper-ties were examined by using scanning electron microscope(SEM) [JOEL 6400] transmission electron microscope(TEM) [JOEL 1010] and X-ray diffractometer (XRD)[Rigaku Geigerflex 2000 Watts] with Cu K (154 Aring)as the incident radiation The micro-Raman spectra wereacquired using a Scanning Optical Microscope imag-ing system (WiTECH -SNOM) integrated with Ramansystem and with 100X objective lense with a laser spotsize of sim500 nm The photoluminescence (PL) characteri-zation of ZnO tetradpods was conducted using He-Cd laserwith 325 nm excitation wavelength A full detail of thesetup is described elsewhere19

3 RESULTS AND DISCUSSION

31 Detailed Structural Properties of As-GrownZnO Nano-Tetrapods

The morphologies of the synthesized ZnO structures wasexamined by using scanning electron microscopy (SEM)and shown in Figure 1 Figure 1 depicts the SEM micro-graphs of the as-grown tetrapod structures Fig 1(a) showsthe low-magnification and (b) demonstrates the high-resolution SEM images of as-grown ZnO tetrapod struc-tures It is clear from the SEM images that the as-growntetrapod structures have four needle shaped tetrahedrallyarranged prongs connected at the center junction and

(a)

(b)

Fig 1 Typical (a) low and (b) high-magnification SEM images of as-grown ZnO tetrapods grown via simple thermal evaporation process

angled 120 to each other The length of the prongs ofthe tetrapod-like structures is a few micrometers and about150ndash300 nm in width with sharper tips It can be clearlyseen that the prongs of the tetrapod are gradually narrow-ing outwards as well as having step (s) towards the tip endand the tips of the tetrapods are also connected to eachother as well Figure 2 shows SEM images along with itscartoon drawing shown in Figures 2(c) and (d) depictingthe angles between any two adjacent prongs of 120 Theoutward prong in Figure 2(b) is colored darker for under-standing and also shown in Figure 2(d)In order to see the elemental compositions and crys-

tallographic structure of the as-grown product EDX andXRD were performed Figure 3(a) shows the EDX spec-trum of the as-grown tetrapod-like ZnO structures TheEDX spectrum peaks are related to oxygen at sim545 eVand Zn at 1040 eV 8607 eV and 9532 eV confirmingthat the synthesized products are purely ZnO without anyimpurity Figure 3(b) represents the XRD spectrum of theas grown tetrapods Peaks at 312 339 357 47 56 and623 degrees of angle are attributed to hexagonal wurtziteZnO crystal structure with the lattice constants a= 32 nmand c = 52 nm (ICDD PDF card 00-003-0888)20ndash22 con-firming that the grown structures are pure wurtzite hexag-onal ZnO

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(c)

(b)

(d)

Fig 2 SEM images and the corresponding cartoon diagrams of nano-tetrapod structures (a) and the top view and its (b) cartoon illustrationwhile (c) SEM side view with its (d) cartoon illustration

The morphology and crystallinity of as-grown ZnOtetrapods were further investigated using transmissionelectron microscopy (TEM) Figure 4(a) shows the low-magnification TEM image of as-grown ZnO tetrapodstructures which exhibit full consistency with the observed

(a)

(b)

Fig 3 (a) Typical X-ray diffraction pattern and (b) EDS spectrum ofas-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

Fig 4 Typical (a) low-magnification and (b) high-resolution TEMimages of as-grown ZnO nano-tetrapods synthesized by simple thermalevaporation process Inset of (a) exhibits the corresponding SAED patternof as-grown nano-tetrapods

SEM images The inset in Figure 4(a) is the selected areaelectron diffraction (SAED) pattern of a single prong ofthe tetrapod The SAED pattern clearly shows bright spotsconfirming the well-crystallinity and wurtzite hexagonal

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

phase for the as-grown ZnO tetrapods Figure 4(b) showsthe HRTEM image of a portion of the ZnO tetrapod withthe well-aligned lattice fringes The distance of 052 nmbetween the parallel planes along the ZnO nanowire axiscorresponds to a d-spacing of the (0001)-planes TheFigure 4(c) shows the line brightness contrast scan alongthe c-axis from the HRTEM image The separation of thealternating peaks in the line scan is 052 nm which is ingood agreement with the lattice constant of wurtzite ZnOalong the c-axisRegarding the growth of ZnO tetrapod structures sev-

eral groups have explained the growth mechanism of thetetrapod nanostructures Different growth mechanisms arefound in the literature explaining the formation of ZnOtetrapod structures13 The first mechanism explained byShiojiri et al23 is based on the assumption that a zinc-blende phase core at the center of the wurtzite ZnOtetrapods exists The second mechanism is proposed byFujii and Iwanaga et al17 They proposed building an octa-hedral multiple twin structure from a multiple inversion-twin embryo and suggested that the octahedral multipleinversion-twin formed first The third mechanism is pro-posed by Nishio et al14 which suggests that the growthof ZnO tetrapods is coming from wurtzite-phased ZnOmultiple twins induced in a zinc-blende phase structurednucleus and that the zinc-blende nucleus exists only in thehigh-temperature tetrapods which will degenerate to mul-tiple twins at room temperature Recently Ding et al11

observed directly the zinc-blende structure core in theinitial formation of wurtzite tetrapods of ZnO and henceconfirmed the zinc-blende core in the nucleation of theZnO tetrapodsIn our synthesized tetrapods we also believed that zinc-

blende (ZB) structure core in the initial formation ofwurtzite tetrapods of ZnO and then secondly the octahe-dral multiple twin structure is building upon followed bythe ZB-type nucleus which only exists at high-temperaturein the tetrapods and degenerate to multiple twins whencooled down to room temperature Our observation is con-sistent with the existing reported literature1423ndash24

32 Detailed Optical Properties of As-GrownZnO Nano-Tetrapods

Figure 5 shows the Raman-scattering spectrum of the as-grown ZnO tetrapods measure with 532 nm laser light(NdYAG) laser as an excitation source The Ramansignals are very sensitive to the crystal structures and thedefects in the nanostructures ZnO has wurtzite hexago-nal phase belongs to the C4

6v space group with two for-mula unit per primitive cell where all the atoms occu-pying the C3v sites Group theory predicts eight sets ofzone centre optical phonons where A1 and E1 modes arepolar and split into transverse optical (A1T and E1T andlongitudinal-optical (A1L and E1L phonons while the E2

Fig 5 Typical Raman-scattering spectra obtained the four legs of asingle tetrapod The inset is Raman Scan filtered image in the range431ndash446 cmminus1

mode consists of two modes of low and high-frequencyphonons (E2L and E2H are Raman-active2225 The insetin the spectra is the filtered (431ndash446 cmminus1 image col-lected in the Raman mode The different identical spectracollected from the four prongs of a single ZnO tetrapodstructure labeled as a b c and d (outward) which suggestthat they have identical wurtzite structuresThe main dominant sharp peak labeled as E2 at

437 cmminus1 was observed and is known as Raman-activeoptical phonon mode which is the characteristic ofwurtzite hexagonal phase ZnO24 The peak at sim98 cmminus1

corresponds to E2 (Low) and the peak at 339 cmminus1 cor-respond to the second order Raman spectrum originatingfrom zone-boundary phonons 3E2HndashE2L and the peak at388 cmminus1 can be labeled as A1T The peak at 521 cmminus1

is coming from Si substrate The higher in intensity andsharp peak of the E2 mode peak at 437 cmminus1 shows that theas-grown ZnO tetrapods are of wurtzite hexagonal phasewith good crystal qualityFigure 6 shows the room-temperature and low-

temperature photoluminescence spectra of the as-growntetrapods measured with the excitation wavelength of325 nm (He-Cd laser) Figure 6(a) demonstrates the roomtemperature PL spectrum of the as-grown tetrapod struc-tures which shows two peaks the intense peak on the leftcentered at sim380 nm is known as the near band edgeemission peak and the wide peak on the right centeredat sim515 nm26 The NBE peak at 380 nm has full widthat half maximum (FWHM) of sim9 nm and considered tobe due to the free excitons recombination via an excitonndashexciton collision2227 while the wide band in the blue-green (510 nm) with a FWHM of sim100 nm may bedue to defects in the lattice either due to oxygen or zincvacancies or interstitials and their complexes present inZnO28 This green emission from tetrapod-structured ZnOhas been widely studied12ndash13151829ndash31 Figure 6(b) showsthe low-temperature PL spectrum of as-grown tetrapods

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

576 Sci Adv Mater 2 572ndash577 2010

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(c)

(b)

(d)

Fig 2 SEM images and the corresponding cartoon diagrams of nano-tetrapod structures (a) and the top view and its (b) cartoon illustrationwhile (c) SEM side view with its (d) cartoon illustration

The morphology and crystallinity of as-grown ZnOtetrapods were further investigated using transmissionelectron microscopy (TEM) Figure 4(a) shows the low-magnification TEM image of as-grown ZnO tetrapodstructures which exhibit full consistency with the observed

(a)

(b)

Fig 3 (a) Typical X-ray diffraction pattern and (b) EDS spectrum ofas-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

Fig 4 Typical (a) low-magnification and (b) high-resolution TEMimages of as-grown ZnO nano-tetrapods synthesized by simple thermalevaporation process Inset of (a) exhibits the corresponding SAED patternof as-grown nano-tetrapods

SEM images The inset in Figure 4(a) is the selected areaelectron diffraction (SAED) pattern of a single prong ofthe tetrapod The SAED pattern clearly shows bright spotsconfirming the well-crystallinity and wurtzite hexagonal

574 Sci Adv Mater 2 572ndash577 2010

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

phase for the as-grown ZnO tetrapods Figure 4(b) showsthe HRTEM image of a portion of the ZnO tetrapod withthe well-aligned lattice fringes The distance of 052 nmbetween the parallel planes along the ZnO nanowire axiscorresponds to a d-spacing of the (0001)-planes TheFigure 4(c) shows the line brightness contrast scan alongthe c-axis from the HRTEM image The separation of thealternating peaks in the line scan is 052 nm which is ingood agreement with the lattice constant of wurtzite ZnOalong the c-axisRegarding the growth of ZnO tetrapod structures sev-

eral groups have explained the growth mechanism of thetetrapod nanostructures Different growth mechanisms arefound in the literature explaining the formation of ZnOtetrapod structures13 The first mechanism explained byShiojiri et al23 is based on the assumption that a zinc-blende phase core at the center of the wurtzite ZnOtetrapods exists The second mechanism is proposed byFujii and Iwanaga et al17 They proposed building an octa-hedral multiple twin structure from a multiple inversion-twin embryo and suggested that the octahedral multipleinversion-twin formed first The third mechanism is pro-posed by Nishio et al14 which suggests that the growthof ZnO tetrapods is coming from wurtzite-phased ZnOmultiple twins induced in a zinc-blende phase structurednucleus and that the zinc-blende nucleus exists only in thehigh-temperature tetrapods which will degenerate to mul-tiple twins at room temperature Recently Ding et al11

observed directly the zinc-blende structure core in theinitial formation of wurtzite tetrapods of ZnO and henceconfirmed the zinc-blende core in the nucleation of theZnO tetrapodsIn our synthesized tetrapods we also believed that zinc-

blende (ZB) structure core in the initial formation ofwurtzite tetrapods of ZnO and then secondly the octahe-dral multiple twin structure is building upon followed bythe ZB-type nucleus which only exists at high-temperaturein the tetrapods and degenerate to multiple twins whencooled down to room temperature Our observation is con-sistent with the existing reported literature1423ndash24

32 Detailed Optical Properties of As-GrownZnO Nano-Tetrapods

Figure 5 shows the Raman-scattering spectrum of the as-grown ZnO tetrapods measure with 532 nm laser light(NdYAG) laser as an excitation source The Ramansignals are very sensitive to the crystal structures and thedefects in the nanostructures ZnO has wurtzite hexago-nal phase belongs to the C4

6v space group with two for-mula unit per primitive cell where all the atoms occu-pying the C3v sites Group theory predicts eight sets ofzone centre optical phonons where A1 and E1 modes arepolar and split into transverse optical (A1T and E1T andlongitudinal-optical (A1L and E1L phonons while the E2

Fig 5 Typical Raman-scattering spectra obtained the four legs of asingle tetrapod The inset is Raman Scan filtered image in the range431ndash446 cmminus1

mode consists of two modes of low and high-frequencyphonons (E2L and E2H are Raman-active2225 The insetin the spectra is the filtered (431ndash446 cmminus1 image col-lected in the Raman mode The different identical spectracollected from the four prongs of a single ZnO tetrapodstructure labeled as a b c and d (outward) which suggestthat they have identical wurtzite structuresThe main dominant sharp peak labeled as E2 at

437 cmminus1 was observed and is known as Raman-activeoptical phonon mode which is the characteristic ofwurtzite hexagonal phase ZnO24 The peak at sim98 cmminus1

corresponds to E2 (Low) and the peak at 339 cmminus1 cor-respond to the second order Raman spectrum originatingfrom zone-boundary phonons 3E2HndashE2L and the peak at388 cmminus1 can be labeled as A1T The peak at 521 cmminus1

is coming from Si substrate The higher in intensity andsharp peak of the E2 mode peak at 437 cmminus1 shows that theas-grown ZnO tetrapods are of wurtzite hexagonal phasewith good crystal qualityFigure 6 shows the room-temperature and low-

temperature photoluminescence spectra of the as-growntetrapods measured with the excitation wavelength of325 nm (He-Cd laser) Figure 6(a) demonstrates the roomtemperature PL spectrum of the as-grown tetrapod struc-tures which shows two peaks the intense peak on the leftcentered at sim380 nm is known as the near band edgeemission peak and the wide peak on the right centeredat sim515 nm26 The NBE peak at 380 nm has full widthat half maximum (FWHM) of sim9 nm and considered tobe due to the free excitons recombination via an excitonndashexciton collision2227 while the wide band in the blue-green (510 nm) with a FWHM of sim100 nm may bedue to defects in the lattice either due to oxygen or zincvacancies or interstitials and their complexes present inZnO28 This green emission from tetrapod-structured ZnOhas been widely studied12ndash13151829ndash31 Figure 6(b) showsthe low-temperature PL spectrum of as-grown tetrapods

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RESEARCH

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

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Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

phase for the as-grown ZnO tetrapods Figure 4(b) showsthe HRTEM image of a portion of the ZnO tetrapod withthe well-aligned lattice fringes The distance of 052 nmbetween the parallel planes along the ZnO nanowire axiscorresponds to a d-spacing of the (0001)-planes TheFigure 4(c) shows the line brightness contrast scan alongthe c-axis from the HRTEM image The separation of thealternating peaks in the line scan is 052 nm which is ingood agreement with the lattice constant of wurtzite ZnOalong the c-axisRegarding the growth of ZnO tetrapod structures sev-

eral groups have explained the growth mechanism of thetetrapod nanostructures Different growth mechanisms arefound in the literature explaining the formation of ZnOtetrapod structures13 The first mechanism explained byShiojiri et al23 is based on the assumption that a zinc-blende phase core at the center of the wurtzite ZnOtetrapods exists The second mechanism is proposed byFujii and Iwanaga et al17 They proposed building an octa-hedral multiple twin structure from a multiple inversion-twin embryo and suggested that the octahedral multipleinversion-twin formed first The third mechanism is pro-posed by Nishio et al14 which suggests that the growthof ZnO tetrapods is coming from wurtzite-phased ZnOmultiple twins induced in a zinc-blende phase structurednucleus and that the zinc-blende nucleus exists only in thehigh-temperature tetrapods which will degenerate to mul-tiple twins at room temperature Recently Ding et al11

observed directly the zinc-blende structure core in theinitial formation of wurtzite tetrapods of ZnO and henceconfirmed the zinc-blende core in the nucleation of theZnO tetrapodsIn our synthesized tetrapods we also believed that zinc-

blende (ZB) structure core in the initial formation ofwurtzite tetrapods of ZnO and then secondly the octahe-dral multiple twin structure is building upon followed bythe ZB-type nucleus which only exists at high-temperaturein the tetrapods and degenerate to multiple twins whencooled down to room temperature Our observation is con-sistent with the existing reported literature1423ndash24

32 Detailed Optical Properties of As-GrownZnO Nano-Tetrapods

Figure 5 shows the Raman-scattering spectrum of the as-grown ZnO tetrapods measure with 532 nm laser light(NdYAG) laser as an excitation source The Ramansignals are very sensitive to the crystal structures and thedefects in the nanostructures ZnO has wurtzite hexago-nal phase belongs to the C4

6v space group with two for-mula unit per primitive cell where all the atoms occu-pying the C3v sites Group theory predicts eight sets ofzone centre optical phonons where A1 and E1 modes arepolar and split into transverse optical (A1T and E1T andlongitudinal-optical (A1L and E1L phonons while the E2

Fig 5 Typical Raman-scattering spectra obtained the four legs of asingle tetrapod The inset is Raman Scan filtered image in the range431ndash446 cmminus1

mode consists of two modes of low and high-frequencyphonons (E2L and E2H are Raman-active2225 The insetin the spectra is the filtered (431ndash446 cmminus1 image col-lected in the Raman mode The different identical spectracollected from the four prongs of a single ZnO tetrapodstructure labeled as a b c and d (outward) which suggestthat they have identical wurtzite structuresThe main dominant sharp peak labeled as E2 at

437 cmminus1 was observed and is known as Raman-activeoptical phonon mode which is the characteristic ofwurtzite hexagonal phase ZnO24 The peak at sim98 cmminus1

corresponds to E2 (Low) and the peak at 339 cmminus1 cor-respond to the second order Raman spectrum originatingfrom zone-boundary phonons 3E2HndashE2L and the peak at388 cmminus1 can be labeled as A1T The peak at 521 cmminus1

is coming from Si substrate The higher in intensity andsharp peak of the E2 mode peak at 437 cmminus1 shows that theas-grown ZnO tetrapods are of wurtzite hexagonal phasewith good crystal qualityFigure 6 shows the room-temperature and low-

temperature photoluminescence spectra of the as-growntetrapods measured with the excitation wavelength of325 nm (He-Cd laser) Figure 6(a) demonstrates the roomtemperature PL spectrum of the as-grown tetrapod struc-tures which shows two peaks the intense peak on the leftcentered at sim380 nm is known as the near band edgeemission peak and the wide peak on the right centeredat sim515 nm26 The NBE peak at 380 nm has full widthat half maximum (FWHM) of sim9 nm and considered tobe due to the free excitons recombination via an excitonndashexciton collision2227 while the wide band in the blue-green (510 nm) with a FWHM of sim100 nm may bedue to defects in the lattice either due to oxygen or zincvacancies or interstitials and their complexes present inZnO28 This green emission from tetrapod-structured ZnOhas been widely studied12ndash13151829ndash31 Figure 6(b) showsthe low-temperature PL spectrum of as-grown tetrapods

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One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

576 Sci Adv Mater 2 572ndash577 2010

Delivered by Ingenta toGuest User

IP 2121384716Tue 17 Aug 2010 120806

RESEARCH

ARTIC

LE

One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties Khan et al

(a)

(b)

Fig 6 Typical (a) room-temperature photoluminescence (PL) spectrumand (b) low-temperature PL spectrum obtained at 20 K of the as-grown ZnO nano-tetrapods synthesized by simple thermal evaporationprocess

at 20 K measured with the excitation wavelength of325 nm (He-Cd laser) The peak at 36986 nm (sim336 eV)is known as the exciton bound to neutral donor-bound(D0X) and the peak originated at 37626 nm (sim330 eV)is known as donorndashacceptor recombination peak (DAP)There is another peak visible at 38606 nm (sim320 eV)and is labeled as the first-order LO phonon replica ofthe main bound exciton line usually falls at 329 eVrespectively

4 CONCLUSION

In summary one step large scale non-catalytic growthof ZnO tetrapod structures was demonstrated The grownZnO tetrapod structures were characterized in detail interms of their structural and optical properties From thedetailed structural characterizations it is confirmed that thegrown tetrapods are well crystalline and exhibiting typi-cal wurtzite crystal structure Moreover Raman scatteringanalysis of a single tetrapod from three of its branchesand the center exhibit strong E2 peak at sim437 cmminus1

reveal good crystalline quality Room temperature PLstudy shows a sharp intense NBE peak at sim380 nm anda broad deep level band centered sim520 nm It is believed

that this one-step growth will pave an easy way to largescale fabrication of ZnO tetrapod structures for usefulapplications in nanotechnology

Acknowledgments We are thankful to Dr Richardsonand Dr Martin E Kordesch for their support suggestionand using their laboratory facilities

References and Notes

1 A Dakhlaoui M Jendoubi L S Smiri A Kanaev and N JouiniJ Cryst Growth 311 3989 (2009)

2 (a) Y W Heo D P Norton L C Tien Y Kwon B S Kang F RenS J Pearton and J R LaRoche Materials Science amp EngineeringR-Reports 47 1 (2004) (b) R Wahab Y S Kim D S Lee J MSeo and H S Shin Sci Adv Mater 2 35 (2010)

3 Y Hu J F Chen X Xue T W Li and Y Xie Inorg Chem44 7280 (2005)

4 (a) A Umar B Karunagaran E K Suh and Y B Hahn Nanotech-nology 17 4072 (2006) (b) L Irimpan V P N Nampoori andP Radhakrishnan Sci Adv Mater 2 117 (2010)

5 N Hongsith T Chairuangsri T Phaechamud and S Choopun SolidState Commun 149 1184 (2009)

6 K Yu Y Zhang R Xu S Ouyang D Li L Luo Z ZhuJ Ma S Xie S Han and H Geng Mater Lett 59 1866(2005)

7 K Zheng H Shen J Li D Sun G Chen K Hou C Li andW Lei Vacuum 83 261 (2008)

8 (a) C Ronning N G Shang I Gerhards H Hofsass and M SeibtJ Appl Phys 98 034307 (2005) (b) S K Mohanta D C Kim BH Kong H K Cho W Liu and S Tripathy Sci Adv Mater 2 64(2010)

9 M Fujii H Iwanaga M Ichihara and S Takeuchi J Cryst Growth128 1095 (1993)

10 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

11 Y Ding Z L Wang T J Sun and J S Qiu Appl Phys Lett90 15 (2007)

12 F Q He and Y P Zhao J Phys D-Appl Phys 39 2105(2006)

13 J Y Li H Y Peng J Liu and H O Everitt Eur J Inorg Chem20 3172 (2008)

14 (a) K Nishio T Isshiki M Kitano and M Shiojiri Philos MagA-Phys Condens Matter Struct Defect Mech Prop 76 889 (1997)(b) H Zhang N Du B Chen D Li and D Yang Sci Adv Mater1 13 (2009)

15 C Zollfrank C R Rambo M Batentschuk and P Greil J MaterSci 42 6325 (2007)

16 Z G Chen A Ni F Li H T Cong H M Cheng and G Q LuChem Phys Lett 434 301 (2007)

17 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

18 M N Jung S Y Ha S H Park M Yang H S Kim W H LeeT Yao and J H Chang Physica E-Low-Dimensional Systems ampNanostructures 31 187 (2006)

19 H J Lozykowski W M Jadwisienczak and I Brown J Appl Phys88 210 (2000)

20 U Ozgur I A Ya C Liu A Teke M A Reshchikov S DoganV Avrutin S J Cho and H Morkoc J Appl Phys 98 041301(2005)

21 A Khan W M Jadwisienczak H J Lozykowski and M EKordesch Physica E 39 258 (2007)

22 A Khan and M E Kordesch Mater Lett 62 230 (2008)23 M Shiojiri and C Kaito J Cryst Growth 52 173 (1981)

576 Sci Adv Mater 2 572ndash577 2010

Delivered by Ingenta toGuest User

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RESEARCH

ARTIC

LE

Khan et al One Step Growth of ZnO Nano-Tetrapods by Simple Thermal Evaporation Process Structural and Optical Properties

24 H Iwanaga M Fujii and S Takeuchi J Cryst Growth 134 275(1993)

25 A Umar and Y B Hahn Cryst Growth and Design 8 2741(2008)

26 N O Korsunska L V Borkovska B M Bulakh L YKhomenkova V I Kushnirenko and I V Markevich J Lumin102ndash103 733 (2003)

27 B Kumar H Gong S Y Chow S Tripathy and Y Hua ApplPhys Lett 89 7 (2006)

28 K Vanheusden W L Warren C H Seager D R Tallant J AVoigt and B E Gnade J Appl Phys 79 7983 (1996)

29 W D Yu X M Li X D Gao P S Qiu W X Cheng and A LDing Appl Phys A-Mater Sci Process 79 453 (2004)

30 W D Yu X M Li and X D Gao Appl Phys Lett 84 2658(2004)

31 A B Djurisic W C H Choy V A L Roy Y H Leung C YKwong K W Cheah T K G Rao W K Chan H T Lui andC Surya Adv Funct Mater 14 856 (2004)

Received 1 April 2010 Accepted 19 May 2010

Sci Adv Mater 2 572ndash577 2010 577