VNƯ Journal o f Science, Mathematics - Physics 24 (2008) 24-29 Photoluminescence of ZnO nano-tetrapods N guyen T hi Thuc Hien*, D oan M anh H a, N go X u an D ai, N g u y en T h i T hu H uong D e p a rtm e n t o f Physics, C ollege o f S cien ce, V N U 334 N guyen Trai, Thanh X uan, H anoi, V ietnam R eceived 29 N o v em b er 2007; receiv ed in re v ise d fo rm 13 M arch 2008 A b s tr a c t T he ZnO nano-tetrapods have b ee n p re p are d b y the e v a p o tio n o f m etallic Z n p o w d e r at the tem perature range o f 700°C - 950"c in air T he in flu en c e o f e x p e rim e n ta l co n d itio n s, su ch as the ev ap o ratio n tem p eratu re and the substrate tem p eratu re, on the m o rp h o lo g y , th e size an d the p h o to lu m in escen ce (P L ) w as investigated T he X R D p a tte rn sh o w ed th a t the n a n o -te a p o d s w ere Z n O w ith hexagonal structure T he branch size o f the tetra p o d s w as ab o u t tw o hundreds n an o m eters T he ph o to lu m in escen ce spectrum m easu red at ro o m te m p e tu re co n sists o f tw o b an d s, a n a o w ban d at 380 nm (U V band) and a w ide b a n d aro u n d 0 n m (g re e n b a n d (G B )) T he in ten sity o f each band, the ratio o f the peak in ten sity o f Ư V b a n d to the G B b a n d and the GB b an d p o sition depend on the experim ent conditions H o w e v er, the p e a k p o sitio n o f the Ư V b an d w as n o t changed In tro d u c tio n Zinc oxide is one o f the m ost efficient wide band-gap phosphor m aterials so it is atfracting more attention in the field o f optoelectronics and photonic devices It has a large exciton binding energy (60 meV) w hich allows ƯV lasing action to occur even at room tem perature [1] In ZnO, oxygen vacancies exhibit an efficient green emission ZnO is also one o f gas-sensing m aterials due to advantageous features, such as high sensitivity to am bient conditions, low cost and simplicity in fabrication [2] Recently, the grow th and elucidation o f the properties o f w ell-defined nanoscale m aterials are critical to efforts directed towards understanding the fundam ental physics o f nanostructures, creating nanosừuctured m aterials, and developing n ew nanotechnologies [3] In this regard, one-dim entional ZnO , such as nanowires, nanotubes, nanorods have great potential to address basic issues in applications In this report we present the influence o f the preparation conditions on the photolum inescence o f ZnO nano-tetrapods prepared by evaporation method Experim ental ZnO nano-teừapods have been prepared by simple evaporation method The evaporation precursor was Zn powders The system consists o f a horizontal tube furnace and a quartz tube (Fig 1) The Zn powder was placed at the sealed end o f the tube The first Si subsừate was placed 1.0 cm far from the Zn source The place o f the Zn source is in the centrum o f the ilim ace The system was performed in air The Corresponding author E-mail: hienntt@vnu.edu.vn 24 Nguyen Thi Time ỉỉien et a! / VNU Journal o f Science, Mathematics - Physics 24 (2008) 24-29 evaporation temperature (the temperature o f the Zn source) was changed from 700°c to -6 After the evaporation, white fluffy products formed on the Si substrates 25 950°c and hold for Quartz tube Fumacc F i g l T h e evaporation system The synthesized products w ere characterized using scanning electron m icroscopy (SEM , JEOLJSM 5410 LV) X -ray diffraction (X RD -D 5005) The photolum inescence (PL) and excitation photolum inescence (PLE) m easurem ents w ere caưied out on the FL3-22 specfrophotom eter with a 450 w Xe lamp R esults a n d d iscu ssio n The XRD pattern show n in Fig indicated that the products deposited on the substrate are ZnO crystals w ith hexagonal w urtzite structure and there is no Zn peak 500- 400- 300- s - 30 40 -ứicla (dcgs) Fig XRD pattern o f ZnO nanoteừapods 60 26 Nguyetĩ Thi Thuc Hien et al / VNU Journal o f Science, Mathematics - Physics 24 (2008) 24-29 The m orphology o f as-prepared products was characterized by SEM The results showed that the morphology o f all products was tetrapods and depends on the evaporation tem perature the substrate tem perature (exactly, the subsữate temperature zone) and the evaporation tim e In this study the evaporation tim e w as 60 Fig shows the SEM images o f the tetrapods w hen the evaporation temperatures w ere 700“c and 800°c It IS seen from Fig (a) that at 700°c, the tetrapods started to form but the lengths w ere short At 800”c the lengths were longer (Fig.3 (b)) a) b) Fig SE M im ages o f Z nO nano-tetrapods evaporated at (a) 0 ° c and (b) 800°c When the evaporation temperature was 900°c, the products have two types depending on the substrate temperature W e denote the products with higher substrate temperature (next to the Zn source and far from the open end o f the tube (F ig.l) by sample A and the products with low er substrate tem perature zone (far from the Zn source, next to the open end) by sample B The results showed that A has bigger size with a tip shape and B has a rod shape with a smaller size (about 200 nm) Fig S E M im ag es o f Z nO nano-tetrapods evaporated at 950 °C: (a) sam ple A , (b) sam ple B Nguyen Thi Thuc Hien et aỉ / VNU Journal o f Science, Mathematics - Physics 24 (2008) 24-29 27 This behavior was also for the evaporation temperature o f 950°c In Fig are SEM im ages o f the sample evaporated at 950*^0 From the SEM images it is expected that our synthesis was based on the thermal evaporation o f Zn powders without the presence o f catalyst, therefore the ZnO teữapods were formed by oxidation o f evaporated zinc vapor in gas phase This grow th w as governed by a vapor-solid (V S) process [4] For investigation o f the optical properties o f ZnO nano-tetrapods, the PL and PLE spectra were m easured at room tem perature Fig shows the PL spectra for ZnO nano-teừapods evaporated at 900°c The excitation w avelength is 335 nm It is seen that the PL specừa consist o f a strong, n aư o w peak in a near u v region (380nm) (UV band) and a broad peak o f 495 nm in the visible region (green band (GB)) T he u v emission peak is due to the exciton recom bination and the broad peak at 495 nm is attributed to defect levels in the band- gap, e.g the radiative recom bination o f a photogenerated hole with an electron trapped by the oxygen vacancy [5] Wavelength (nm) Fig PL specữa of ZnO nano-tetrapods evaporated at 900 °c, under 335 nm excitation The substrate temperature zone is about (a) 800 ®c, (b) 700 '’c, (c) 600 °c It is seen from Fig that, the lower subsừate tem perature is, the low er PL intensities for both bands are, but the ratio o f the peak intensities o f u v band to that o f the green band (UV/GB) increases, e.g 1/1, 2/1 and 3/1 for a, b and c, respectively Besides, the peak positions o f the two bands w ere unchanged w hen the substrate tem perature was changed The decrease o f PL peak intensities w ith the substrate tem perature in Fig is probably concerned w ith the crystallization o f ZnO tetrapods W hen the subsữate temperature increases, the higher crystallization o f nano-teừapods occurs and this leads to the increase o f the PL intensity The increase o f the ratio U V /G B is due to the increase o f the oxygen concentration o f the subsừates closer to the open end o f the quartz tube, e.g the decrease o f oxygen vacancies results in the decrease o f the GB intensity 28 Nguyen Thi Thuc Hien et al / VNU Journal o f Science Mathematics - Physics 24 (2008) 24-29 In Fig and are PL spectra o f ZnO nano-teữapods deposited at tw o fixed substrate tem perature (sam ples A and B) and were evaporated at 700 “c and 800 ”c A com parison o f PL spectra in Fig 5, and illusừates that, beside the sim ilar behavior, such as the ratio UV/GB o f the sample A is sm aller than that o f B, the difference betw een them is the red shift o f green band when the substrate tem perature decreases (e.g when the substrate is m ore far from the Zn source and closer to the open end o f the quartz tube) The change o f the ratio UV/GB can be explained like above W avelength (nm) Fig P L sp e c a o f Z n O nano-tetrapods evaporated at 800 ‘’C: (a) S am ple A, (b) sam ple B 8.7x10* x1 " - 496 nm 7.3x10*6.7x10® x đd >> -4-> pH w s ) 5.3x10* 4.7x10^4,0x10*3,3x10' 2.7x10* x1 *- hH 1.3x10"6.7x10* 0.0 350 400 450 500 550 600 650 W avelength (nm) Fig.7 P L sp ectra o f Z nO nano-tefrapods evaporated at 700 "C; (a) S am ple A , (b) sam ple B, Nguyen Thi Time Hien et a! / VNU Journal o f Science, Mathematics - Physics 24 (2008) 24-29 29 The red shift o f the green band can be explained as follows: A fter several reports [5, ] the green band (500 nm) is attributed to the oxygen vacancies and the yellow band (about 600 nm) is due to zinc vacancies As m entioned above, the samples A are next the Zn source and far from the open end o f the quartz tube so it has a lack o f oxygen (rich Zn) It is opposite w ith sam ple B, which was producted at the place far from the Zn source and next the open end o f the quartz tube So these products have enough o f oxygen but lack o f zinc It is seen from Fig 5, and that the GB o f 576 nm (evaporation tem perature is 700°C), 550 nm (evaporation tem perature is 800°C) and 500 nm (evaporation tem perature is 900 °C) were from the products deposited at the same place in the furnace This m eans that the higher evaporation temperature IS, the more blue shift o f green band is It is due to the m ore Zn vapor can be evaporated when the evaporation tem perature is higher This leads to most Zn for the products o f ZnO tefrapods evaporated at 900°c, w hen the defect is mainly oxygen vacancies The GB is the superim position o f two bands resulting from oxygen vacancies and zinc vacancies A com parison o f our PL specfrum with other studies showed that the peak positions o f 495 nm and around 570 nm o f our study are similar to the PL o f ZnO nano-tetrapods nanorods reported by 7-9 C onclusions ZnO nano-tetrapods were successfully synthesized by the therm al evaporation o f Zn pow ders without a catalyst The m orphology o f the tetrapods depends on the subsfrate tem perature The size o f the tetrapods w as about 200 nm The photolum inescence spectrum has tw o bands, a u v band at 380 nm and a green band around 560 nm depending on the evaporation and the substrate tem perature The green band is the superim position o f two bands, related with oxygen and zinc vacancies, respectively A cknow ledgem ents A uthors o f this paper w ould like to thank to the Center for M aterials Science (CM S), Hanoi U niversity o f Science for permission to use PL equipm ent Thanks to Hoang Due Anh and Nguyen Quang H oa in CMS for the results o f SEM and XRD This w ork is supported by Natural Science R esearch Program (Project QT-07-16) o f Vietnam National U niversity, H anoi (V M J) References [1] Y.K Park, J Inhan, M.G Kwak, H Yang, S.H Yu, w s Cho, J Lumin 78 (1998) 87 [2] Ni Bendahan, R Boulmani, J.L Seguin, K Aguir, Sensor Actuator B 100 (2004) 320 [3] Snokc, Science 273 (1996) 1351 [4] Yang, C.M Liebcr, J Mater Res 12 (1997) 2981 [5] K Vanhcusden, W.L Warren, C.H Seager, D.R Tallan, J.A Voigt, B.E Gnade, J Appl Phys 79 (1996) 7983, [6] J Zhong, A.H Kitai, p Mascher, J Electrochem Soc 140 (1993) 3644 [7] Y Dai, Y Zhang, Q X Li, c w Nam, Chem Phys Lett 358 (2002) 83 [8] A.L Roy, A.B Djurisic, W.K Chan, J Gao, H.F Liu, c Surya, Appl Phys Lett 83 (2003) 141 [9] Hong Jin Fan, Roland Scholz, Florian M Kolb, Magrit Zachariar, Appl Phys Lett 85 (2004) 4142 ... specữa of ZnO nano-tetrapods evaporated at 900 °c, under 335 nm excitation The substrate temperature zone is about (a) 800 ®c, (b) 700 '’c, (c) 600 °c It is seen from Fig that, the lower subsừate... investigation o f the optical properties o f ZnO nano-tetrapods, the PL and PLE spectra were m easured at room tem perature Fig shows the PL spectra for ZnO nano-teừapods evaporated at 900°c... tetrapods and depends on the evaporation tem perature the substrate tem perature (exactly, the subsữate temperature zone) and the evaporation tim e In this study the evaporation tim e w as 60 Fig shows