S o l-g e l synthesis and particle size characterization o f CdSe Quantum dots

V N U J o u rn a l o f S cience, M ath em a tics - Physics 25 (2009) 207- 211

S o l-g e l synthesis and particle size characterization o f CdSe Quantum dots

K h o n g C a t C u o n g ' , T r i n h D u e T h i e n ' , P h a m T h u N g a ' , N g u y e n V a n M i n h ' , N g u y e n V a n H u n g *

^ ỉỉa n o i N ational U niversity'of Education, lĩ ó X u a n Thuy Road, C auG iay, Hanoi, Vietnam hìstituỉe o f Síateriaỉs Science. H oang Quoc Viet Road, Hanoi, Vietnam

R eceived 9 N o v e m b e r 2009; rcceivcd in revised form 24 N o v e m b e r 2009

A b s t r a c t . In this article, we report on the preparation o f C d S e q u a n tu m dots (Q D s) by sol-gel m eth o d and their optical properties. T he average size o f Q D s is also estim ated b y using various ways, such as the S c h e r e r ’s formula. T h e E f f r o s - B r u s - K a y a n u m a 's theoretical expression, T E M ctc. I hc TILM im ages o f sam ples show that the m ean sizes o f Q D s are 4 nm. T he m ean sizes o f Q D s arc sm aller than that o f other m ethods and arranging from 2 to 3.6 nm.

1. I n t r o d u c t i o n

S ize-dcpendcnt optoelectronic properties o f C dS e q u an tu m dots (Q D s) m ake them ideal candidates for tunable absorbers and em itters in application, such as nanoscale electronics, laser technology, and biological iluorcsccnt labeling.

I'hc properties o f Q D s are strongly influenced not only by the com po sitio n and structure o f the b u t ali>u b y Ihc p i c p a i a l i u i i I c c lim q u c . I l i c b a n d - c d g c ciiiibbiOĩi OÍ C d b c Ụ D s in a s t r o n g l y confincd regim e has been generally altributed lo electron transitions from the highest occupied to the lowest n o n -o ccu p ied m olecu la r orbital [1]. Therefore, there exist m any m eth o d s that have been applied to synthesize C d S e q u a n tu m dot. A variety o f m ethods has b een em ployed to synthesize sem iconductor n anorods in rcccnl years. 'Fhcse m ethods include the hot coordination solvents method using tri-//-octylphosphinc oxide (T O P O ) and trioctylphosphinc (T O P ) [2], ihc hydrotherm al or solvothcrmal m ethod [2,3] and the m icelle or reverse m icclle m ethod [3]. T h e electrical and optical properties o f nanoparticlcs arc affcctcd by the chcm istry involved in their synthesis. Bottom -up approaches such as those using surfactants or miccllcs as the regulating agents are very cffcctive for the synthesis o f o nc-dim cnsional nanostructure because o f their high efficiency, controllability, simplicily and versatility. H ydrotherm al techniques have b een w idely applied for the synthesis o f conventional and advanced materials. The advantages o f this m ethod include the relatively low temperature required for processing, the possibility o f controlling particle m orphology and the good crystalline o f the products. P eng et al. first em ployed a m m o n iu m as a com p letin g agent for cadm ium ions to synthesize c a d m iu m selenide (C dS e) nanocrystals using the h y d ro th e n n a l m ethod. T h e y found that at 140

°c,

C d S c with a m ixed m orphology o f branch-shaped fractals and nanorods w as produced,

Corresponding author. E-mail: hungnvsp@yahoo.com.

207

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and at 180 the products were m ainly C dS c nanorods [4]. C hen ct al. used a cationic s u rfa c ta n t cetyltrimethyl am m o n iu m bro m id e (C T A B ) via a hydrotherm al m ethod at 180 ° c to synthesize CdSe nanorods. T h ey found that the concentration o f C T A B is a key p aram eter in the control o f nanoparticlc m orphology [5]. H ow ever, to investigate the size effect w e need the sam ple w ith hom ogeneous distribution in particle size. B esides that, the h y drothennal m ethod requires the long lim e reaction and its distribution in particle size is in the broadening range.

In this article, w e report on the preparation o f C dS e quan tu m dots (Q D s) by sol-gel method and investigate their optical properties. This is a n ew route to get C d S e Q D s and also very econom ic. We also estim ate the average size o f Q D s by using various ways, such as ửie S c h c r c r 's formula, The E f f r o s - B r u s - K a y a n u m a ’s theoretical experssion, T E M etc.

2. E xp erim en t

The m ethod used to prepare Q D s C dS e w as presented in previous p aper [6]. T he crystalline processes h appened from 1 to 15 minutes, and Q D s C dS e w ere dispersed in toluen solvent.

Pow der X -ray diffraction (X R D ) patterns w ere recorded using a D 5005 (Siem ens) X-ray diffractom eter using CuA^a radiation {X = 0.15406 nm). T ransm ission electron m icro sco p y (TEM ) was e a rn e d out using a m icroscope. U ltraviolet-visible (U V - v is ) absorption spcctra o f the nanoparticlcs were recorded by using a Jasco V 6 7 0 spectrophotom eter.

3. Result and discussion

T o calculate the particle size o f Q D s we use some following models:

+ U sing absorption spectra to estim ate the m ean sizes o f QDs:

T he E ffros, B ru s a n d K a y a n u m a ’s theoretical expression show s the relation betw een mean s i/i’

and speciiic param eters of Q D s [7J;

E . W = E , +

ji

'

r

;

- 0 , 2 4 8 R * (1)

a ;

w h ere Eg(a) is the e ffe c tiv e b a n d g a p o f Q D s w ith ra d iu s o f a, the band gap Eg, B o h r cxciton radius 3b and B ohr exciton en ergy R* are the specific param eters o f bulk m aterial. F ro m absorption spectra, w e can d e te rm in e the Eg(a) o f Q D s, h e n c e can e s tim a te the m e a n size o f Q D s.

From this formula, the standard curve and m easured absorption spectra, w e can estim ate the mean size o f QDs.

Based o n the analysis it has b een expressed the experim ental form ula to estim ate the mean sizes o f Q D s C dS e as:

Z) = 1,6122 X 10"’ - 2 , 6 5 7 5 X 10^ Ắ' +1,6242 X 1 0 - 'A ' - 0 ,4 2 7 7 ^ + 41,57 (2) where, D (nm ) is the size o f a given nanocrystal sample, and ^ n m ) is the w avele n g th o f the first excitonic absorption p eak o f the corresponding sample.

+ T he second one, we estim ate the m ean size o f Q D s by the S c h e r e r ’s formula [8]:

r = -...;; (3)

D c o s O

K c. CỉíOtìịĩ eỉ a l / VNU Jo u rn a l o f Science, M ath en u itics - P hysics 25 (2009) 2 07-2 Ỉ I 209

' V “ - t . w here, i:) (rad) is the h a lf w idth at halt m axim um

o f the XIU) peak; X- the X-ray difTraction o wavelength (with radiation C u K a: À = 1,5406 A );

0 : Ih c d i i i 'r a c l i o n a l a n g l e a n d k - c o n s t a n t (k

- 0,9).

The strucUire and m orphologies o f the CdS e nanoparticlcs w ere characterized using transm ission electron m icroscopy. The niorphoiogics o f the CciSc nanoparliclcs were mainly alTcclcd by ihc C d :S c ratio, the rcaclion tem perature and lime. F’roiii VEM im age on fig.

1, \vc can sec C d S e Q D s are d is p e rs e d in loluen solvent a n d have the sphcrical s hapes wilh the m ean d ia m e te r o f a b o u t 4 nni.

A typical X R I) pattern from the prepared CdSe nanoparticlcs and the positions o f the X-

ray peaks tor CdSe

\ cc

are shown in rig. 2. A ll the diffraction peaks from the CdSe nanoparticles are consistent witli the w urtzilc structure o f C dS e with m easured lattice constants o f a = 6.1 Ả (this can be com pared to the lattice constants o f a = 6,077 A from J C P D S file No. 19-0191). The sharp diffraction peaks a h o indicate lliat the products are highly crystalline. X R D analysis revealed no impurities such as Se and ScO^ in the sample.

As cxpcclcd, the widtli o f the difTraction peaks is considerably broadened and can be determined easily because tlic s i/e eifcct is exliibited vcr)' d e a r . By using the S cherrer formula, u e can calculate ihe mean sizes o f llie C J S e Q D s from the peak width at half-m axim um . Parliclc sizes obtained from the width o l'th e (1 1 1) d iiiraction are depleted in the tabic 1.

Tahlo 1 P an icle s i/e o f ‘inniple*^ with reaction time s atui li) min

e 3 4 ' 4 8 8 .8 K U ^ X 1 8 0 K S 0 n a Fig. 1. TEM image ofCdSe QDs.

Sam ples 20 D (radian) cosO d (nm)

5 Iiiinutos 25,381 0,0697 0,97555 2,039

10 minutes 25,402 0,06806 0,97553 2,088

These results .'ihow that llie panicle s i/c s arc about 2 nm.

When tlic c r\s la llin c time increased iVoiri 5 to 10 minutes, the peaks bccom e b roadening but not v er\ considerably.

The mean s i/c s o f sam ples obta ned from X R I) patterns arc smaller than those o f these sam ples obla ncd iVom I HM image. In this m etlod. we did not elim inate ihc sysUni standard error. In addition, in tiis X R I) niclliod, the mean sizes are obtained from all the structural layers o f samples

C d S e F F C s t r u c t u r e

10 20

(2 2 0)

A,

⁽³¹¹⁾

’Mv

10 m i n

V ' 5 m i n

30 40 50

2 T h e t a ( d e g r e e )

60 70

F-'ig. 2. XRD patterns ofQ D s CdSe with crystalline times o f 5 and 10 minutes.

210 K ( ' ('ĩỉo n ii eí aỉ. \'N U J o u rn a l o f Science, h ia th em a tics - P hysics 25 (2009) 2 ( r - 2 ỉ ỉ

i n v o l v e d d i f f r a c t i o n p r o c c s s , s o tliC r e s u l t s a r c t lie d i a m e t e r s o f c r y s t a l COICS. In i h c o t l i c r i i i c t l i i o d s ,

there are some other objccts w hich arc involved in the shell o f C d S e cores so it may be larger.

Fig. 3 show s absorption spectra, o f the prepared C dS c QDs. it can be seen from F'ig. 3 that with increasing g row ing lime, the redshift o f the spectra can be clearly observed and optical absorption in the visible region du e to C dS e Q D s is dem onstrated. T he average diam eters o f the C dS e Q D s for each grow th tim e interval is estimated using the cffcctivc m ass approxim ation giving diam eters ranging from 2 . 2 nm to 2 . 6 nm.

These values are co m parable to those obtained by T E M and by the w a v e le n g th o f the first e x c ito n ic a b s o rp tio n peak ( T a b le 2).

The deviation o f the peaks in absorption spectra is about 50 nm, w hich m ay be due lo the difference o f surface states o f these QDs. It is also thought that the strong intensity from the C dSe Q D s can be attributed to their high crystallinity [9], which is in good agree m en t with the X RD patterns discussed earlier and the presence o f good surface states on the Q Ds.

W a v e l e n g t h (n m )

Fig. 3. U V - v is absorption spectra o f C dSe Q D s with various crystalline times.

T able 2. T he parameters o f the C dS e Q D s vs. gro w in g lime

Ratio N am e crystalline T he wavelength o f the The m ean diam eter o f T h e mean d iam eter o f Cd:Se time (m inute) first absorption excitonic C dSe Q D s using formula C d S e Q D s using

peak (nm) ( l ) ( n m ) formula (2) (nm )

CdSe 1 520 2.2 2.6

1 : 8 CdSe 5 557 2.5 3.2

( 2 6 0 T ) CdSe 10 56! 2.5 3.3

CdSe 15 574 2.6 3.6

S u m m ary

In sum m ary, Q D s o f C d S e with a diam eter o f 2.2 - 2.6 nm have been successfully synthesized through a novel m ethod at a relative low temperature. T he m orphologies o f the prepared nanoparticlcs can be controlled by the reaction time, the am ount o f C d:S e ratio and the reaction temperature.

A ck n o w le d g em en ts. T he authors express the sincere thanks to the N A F O S T E D under Grant num ber o f 103.03.93.09 and M inisterial-level project o f M O E T for the financial support.

K .c . C u o n g ct a I. / VNU J o u rn a l o f Science, M a th em a tic s - P h ysics 25 (2009) 2 0 7 -2 ! I 211

R eferen ces

[1] lỉk im o v .7 . L iw u n 70 (1996) h P.D. Pcrsans, Au I'u. Y J . \Vu, M. l.cvis. J, D pi So c A m 6 ( 1 9 8 9 ) 818-; V. JungnickcK i'. IIcnncbcrgcr, J ỉ.u n u n ^0 (1 9 9 6 ) 238; T. Aral, K. M atsuishi, J Luntifi 70 (1*>96) 281; M, Kuno, J.K. Lee, B .o . Dabbousi, F . v M ik ulcs, M.G. U a w c n d i . a c /ie m rh y s. 1 0 6 ( 1 9 9 7 ) 9 8 6 9 .

[2] X.G. Pcnii, L. Manna, \v.[). Yanti, J, Wickham, \~. Schcr, A. Kadavanii'h, A-P. Alivisatos, S a iu r e ‘iOA (2000) 59.

13] L.r. Xi. Y M . I,ani, J ('oỉỉouỉ InicrfiUV Set 316 {2007) 771; M MaiilarJ, s. Giorgio, M.p. Pilcni, Adv. Mater. 14 ( 2 0 0 2 )1 0 8 4 .

[4] Q Peng. V J . Dong, / X. Deng, Y.D. Li, ỉn o rịĩ C h a n 41 (20U2) 5249.

[5] M ,n . Chen, L. G ao, J. A m C eram . Soc. 88 (2005) 1643.

[6 | K hong Cat Cuoni*. T n n h Due Thicn, P ham Van Hai, N guyen Phi Hung, Bui Thi P h u o n g 'Ilianh, N guyen \^an Hung, Pham Thu Nga, V'u [)uc Chinh, Vu 'Ihi Hontĩ íỉanh, Synthesis C d S c quantunidols and determine its SI/C from optical spcclra, A d va n c e s in O p tic s P h o to fitcs S p ec tro sc o p y ổí A p p iic a tto n s V (2 0 0 8 ) 517.

[7] S. V.Gaponenko, Optical Ề^ropcrties o f Semiconductor Nanocrysíaỉs, Cambridge University Press, 1998.

[8] Lc C o n g D uong, The sírư c íu ra ỉ a n a ly ze b y A>m', Publishin g H ouse for Science and T ec h n o lo g y ỉla n o i, 1984.

[9] R. Vcnugopal, p'l. Lin, c . c Liu, Y.T. C h e n , / Am. Chan. Soc 127 (2005) 1 1262-