Photoluminescence of ZnO nano-tetrapods

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VNƯ Journal o f Science, Mathematics - Physics 24 (2008) 24-29

Photoluminescence of ZnO nano-tetrapods

N g u y en T hi Thuc Hien*, D oan M anh H a, N g o X u a n D ai, N g u y e n T h i T hu H u o n g

D e p a rtm e n t o f P hysics, C o lle g e o f S cien c e, V N U 3 3 4 N g u yen Trai, Thanh X u a n , H a n o i, V ietnam

R ece iv ed 29 N o v e m b e r 2007; re ceiv ed in re v is e d fo rm 13 M a rc h 2 0 0 8

A b s tr a c t. T he Z nO nano-tetrapods have b e e n p re p a re d b y the e v a p o ra tio n o f m e ta llic Z n p o w d e r at th e tem p eratu re ran g e o f 700°C - 950"c in air. T he in flu e n c e o f e x p e rim e n ta l c o n d itio n s, su c h as the ev a p o ratio n tem p eratu re an d the su b strate te m p e ra tu re , on th e m o rp h o lo g y , th e size a n d the p h o to lu m in esc en ce (P L ) w as investigated. T h e X R D p a tte r n sh o w e d th a t th e n a n o - te ừ a p o d s w ere Z n O w ith h ex ag o n al stru ctu re. T he b ran ch size o f th e te tra p o d s w as a b o u t tw o h u n d red s n an o m ete rs. T h e p h o to lu m in e sc e n c e sp ec tru m m e a su re d at ro o m te m p e ra tu re co n sists o f tw o b an d s, a n a ư o w b an d at 3 8 0 n m (U V b and) and a w id e b a n d a ro u n d 5 0 0 n m (g re e n b a n d (G B )).

T h e in ten sity o f each b an d , th e ratio o f the p e a k in te n sity o f Ư V b a n d to th e G B b a n d and the GB b a n d p o sitio n d ep en d on th e ex p e rim en t conditions. H o w e v e r, th e p e a k p o s itio n o f the Ư V b an d w as n o t changed.

1. In tro d u c tio n

Zinc oxide is one o f the m ost efficient wide band-gap ph o sp h o r 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 p eratu re [1]. In ZnO, oxygen vacancies exhibit an efficient green em ission. ZnO is also one o f g as-sensing m aterials due to advantageous features, such as high sensitivity to am bient conditions, low cost and sim plicity in fabrication [2]. Recently, the grow th and elucidation o f the pro p erties o f w ell-defined nanoscale m aterials are critical to efforts directed towards u n d erstan d in g the fundam ental physics o f nanostructures, creating nanosừ uctured m aterials, and dev elo p in g n ew nanotechnologies [3]. In this regard, one-dim entional ZnO , such as nanow ires, 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.

2. E xperim 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 perform ed in air. The Corresponding author. E-mail: [email protected]

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Nguyen Thi Time ỉỉien et a!. / VNU Journal o f Science, Mathematics - Physics 24 (2008) 24-29 25

evaporation temperature (the temperature o f the Zn source) was changed from

700°c

to

950°c

and hold for 2 -6 0 min. After the evaporation, white fluffy products formed on the Si substrates.

Fumacc Quartz tube

F i g . l . T h e ev a p o ratio n system .

The synthesized products w ere characterized using scanning electron m icroscopy (SEM , JEO L- JSM 5410 LV) X -ray diffraction (X R D -D 5005). The photolum inescence (PL) and excitation photolum inescence (P L E ) m easurem ents w ere caư ied out on the FL3-22 specfrophotom eter w ith a

450 w

X e lamp.

3. R esu lts a n d d isc u ssio n

T he XRD p attern show n in Fig. 2 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-

0-

s

30 40

2-ứicla (dcgs)

60

Fig. 2. XRD pattern o f ZnO nanoteừapods.

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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 show ed that the m orphology o f all products was tetrapods and depends on the evaporation tem perature the substrate tem perature (exactly, the subsữate tem perature zone) and the evaporation tim e. In this study the evaporation tim e w as 60 min. Fig. 3 shows the SEM images o f the tetrapods w hen the evaporation tem peratures w ere

700“c

and

800°c.

It IS seen from Fig. 3 (a) that at

700°c,

the tetrapods started to form but the lengths w ere short. A t

800”c

the lengths w ere longer (Fig.3 (b)).

a) b)

Fig. 3. S E M im ages o f Z nO nan o -tetrap o d s ev a p o rated at (a) 7 0 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 tem perature (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 sm aller size (about 200 nm).

Fig. 4. S E M im ag es o f Z nO n an o -tetrap o d s ev a p o rated at 950 °C: (a) sam p le A , (b) sam p le B .

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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 tem perature o f

950°c.

In Fig. 4 are SEM im ages o f the sample evaporated at 950*^0. From the SEM im ages it is expected that our synthesis was based on the thermal evaporation o f Zn pow ders w ithout the presence o f catalyst, therefore the ZnO teữapods w ere 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 PL E spectra were m easured at room tem perature. Fig. 5 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

em ission peak is due to the exciton recom bination and the broad p eak at 495 nm is attributed to defect levels in the band- gap, e.g. the radiative recom bination o f a photogenerated hole w ith an electron trapped by the oxygen vacancy [5].

Wavelength (nm)

Fig. 5. 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. 5 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. B esides, 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. 5 is probably concerned w ith the crystallization o f ZnO tetrapods. W hen the subsữate tem perature 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.

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In Fig. 6 and 7 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, 6 and 7 illusừates that, beside the sim ilar behavior, such as the ratio UV/GB o f the sam ple 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 U V /G B can be explained like above.

28 Nguyen Thi Thuc Hien et al. / VNU Journal o f Science. Mathematics - Physics 24 (2008) 24-29

W avelength (nm)

Fig. 6. P L s p e c ừ a o f Z n O nan o -tetrap o d s ev ap o rated at 800 ‘’C: (a) S am ple A , (b) sam ple B.

496 nm 8.7x10* -

8.⁰x^{1 0}" - 7.3x10*- 6.7x10® -

6.⁰x^{1 0}®- 5.3x10* -

d 4.7x10^-

>> 4,0x10*-

-4->

• pH

w 3,3x10' -

s

0) 2.7x10*-

2.⁰x^{1 0}*-

h H 1.3x10"- 6.7x10* - 0.0 1

350 400 450 500 550 600 650

W av elen gth (nm)

Fig.7. P L sp e c tra o f Z nO nano -tefrap o d s evap o rated at 700 "C; (a) S am p le A , (b) sam ple B,

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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, 6] 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 sam ples 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, w hich 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, 6 and 7 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) w ere from the products deposited at the same place in the furnace. This m eans that the higher evaporation tem perature IS, the m ore 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 w ith 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

4. C onclusions

ZnO nano-tetrapods w ere 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. T he 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 ck n o w led g em en ts. A uthors o f this paper w ould like to thank to the C enter for M aterials Science (CM S), H anoi U niversity o f Science for perm ission to use PL equipm ent. Thanks to H oang D ue Anh and N guyen Q uang H oa in CM S for the results o f SEM and XRD. This w ork is supported by N atural Science R esearch Program (Project QT-07-16) o f V ietnam N ational U niversity, H anoi (V M J).

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