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
Trang 1V 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
Trang 2208 K c C u o n g et a l / VN U J o u rn a l o f Science M athem atics - P h ysics 25 (2009) 2 0 7 -2 Ị Ị
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;
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
Trang 3K 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
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
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
C d S e F F C s t r u c t u r e
( 2 2 0 )
A, (311)
’Mv
10 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
Trang 4210 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
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 h e auth o rs express the sincere thanks to the N A F O S T E D u n d er Grant num ber
o f 103.03.93.09 and M inisterial-level project o f M O E T for the financial support
Trang 5K 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 2 11
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