V N U Jo u rn a l o f Science, M athem atics - Physics 25 (2009) 207- 211 S ol-gel 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 in 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, lĩó X u a n Thuy Road, C auG iay, Hanoi, Vietnam hìstituỉe o f Síateriaỉs Science Hoang Quoc Viet Road, Hanoi, Vietnam Received 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 dSe q uan tum dots (Q D s) by sol-gel m ethod and their optical properties The average size o f Q D s is also estim ated by using various ways, such as the S c h e r e r ’s formula T he E ff ro s - B r u s - K a y a n u m a 's theoretical expression, TEM ctc I hc TILM images o f sam ples show that the m ean sizes o f Q D s are nm The m ean sizes o f QD s arc sm aller than that o f other m ethods and arranging from to 3.6 nm I n tr o d u c t io n Size-dcpendcnt optoelectronic properties o f CdSe quantum dots (Q D s) make 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 position 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 Ic c lim q u c Ilic b a n d - c d g c ciiiibbiOĩi OÍ C d b c Ụ D s in a s tro n g ly confincd regime has been generally altributed lo electron transitions from the highest occupied to the lowest no n-occupied m olecular orbital [1] Therefore, there exist m any m ethods that have been applied to synthesize C dS e quantum dot A variety o f m ethods has been em ployed to synthesize sem iconductor nanorods in rcccnl years 'Fhcse methods include the hot coordination solvents method using tri-//-octylphosphinc oxide (TO P O ) and trioctylphosphinc (TO P ) [2], ihc hydrothermal or solvothcrmal m ethod [2,3] and the micelle or reverse micclle 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 onc-dim cnsional nanostructure because o f their high efficiency, controllability, simplicily and versatility H ydrotherm al techniques have been 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 Peng et al first em ployed am m onium as a com pleting agent for cadm ium ions to synthesize cad m ium selenide (CdSe) nanocrystals using the hy droth en nal method T hey found that at 140 °c, CdS c with a m ixed m orphology o f branch-shaped fractals and nanorods was produced, Corresponding author E-mail: hungnvsp@yahoo.com 207 208 K c C uong et a l / VN U Jo u rn a l o f Science M athem atics - P hysics 25 (2009) 20 -2 Ị Ị and at 180 the products were mainly CdSc nanorods [4] Chen ct al used a cationic su rfacta n t cetyltrimethyl am m onium brom ide (C T A B ) via a hydrothermal m ethod at 180 ° c to synthesize CdSe nanorods T hey found that the concentration o f C T A B is a key param eter in the control o f nanoparticlc m orphology [5] H ow ever, to investigate the size effect w e need the sam ple with hom ogeneous distribution in particle size Besides that, the hydrothennal method requires the long lime reaction and its distribution in particle size is in the broadening range In this article, w e report on the preparation o f CdSe quantum dots (Q D s) by sol-gel method and investigate their optical properties This is a new route to get C dS e Q D s and also very economic We also estimate 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 ro s -B ru s-K a y a n u m a ’s theoretical experssion, T E M etc E xperim ent The m ethod used to prepare Q D s CdSe w as presented in previous paper [ ] The crystalline processes happened from to 15 minutes, and Q D s CdSe w ere dispersed in toluen solvent Pow der X-ray diffraction (X R D ) patterns were recorded using a D 5005 (Siemens) X-ray diffractometer using CuA^a radiation {X = 0.15406 nm) Transm ission electron m icroscopy (TEM ) was earned 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 670 spectrophotometer Result and discussion To calculate the particle size o f Q D s we use some following models: + Using absorption spectra to estimate the m ean sizes o f QDs: The E ffros, B ru s and K a y a n u m a ’s theoretical expression shows the relation between mean si/i’ and speciiic param eters of Q D s [7J; E W = E , + ji' r ; - ,2 R * (1) a ; w here Eg(a) is the e ffectiv e b an d gap o f Q D s w ith radius o f a, the band gap Eg, B o h r cxciton radius 3b and B ohr exciton energy R* are the specific param eters o f bulk material F ro m absorption spectra, w e can d e te rm in e the Eg(a) o f Q D s, h en ce can estim ate 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 on the analysis it has been expressed the experimental formula to estim ate the mean sizes o f Q D s C dSe as: Z) = 1,6122 X 10"’ - , X 10^ Ắ' +1,6242 X - 'A ' - ,4 7 ^ + 41,57 (2) where, D (nm ) is the size o f a given nanocrystal sample, and ^ n m ) is the w avelength o f the first excitonic absorption p eak o f the corresponding sample + The second one, we estimate the m ean size o f Q D s by the S c h e re r’s formula [ ]: r = - ;; D cosO (3) K c CỉíOtìịĩ eỉ a l / VNU Jo u rn a l o f Science, M athenuitics - P hysics 25 (2009) 207-2 Ỉ I 209 where, i:) (rad) is the half width at halt maximum o f the XIU) peak; X- the X-ray difTraction o wavelength (with radiation C uK a: À = 1,5406 A ); : 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 ta n t (k - 0,9) The strucUire and m orphologies o f the CdSe nanoparticlcs were characterized using ' V “ -t transm ission electron microscopy The niorphoiogics o f the CciSc nanoparliclcs were mainly alTcclcd by ihc C d :S c ratio, the rcaclion temperature and lime F’roiii VEM im age on fig , \vc can sec C d S e Q D s are disp ersed in e '4 8 KU^ X 180K S0na loluen solvent and have the sphcrical shapes wilh the mean d ia m e te r o f about nni Fig TEM image ofCdSe QDs A typical X R I) pattern from the prepared CdSe nanoparticlcs and the positions o f the Xray peaks tor CdSe \ cc are shown in rig A ll the diffraction peaks from the CdSe nanoparticles are consistent witli the wurtzilc structure o f C dSe with m easured lattice constants o f a = 6.1 Ả (this can be compared to the lattice constants o f a = 6,077 A from JC 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 Scherrer 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 ol'th e ( 1 ) diiiraction are depleted in the tabic Tahlo Panicle si/e o f ‘inniple*^ with reaction time s atui li) Samples Iiiinutos 10 minutes D (radian) 20 25,381 25,402 These results 'ihow that llie panicle si/cs arc about nm When tlic cr\slallin c time increased iVoiri to 10 minutes, the peaks bccome broadening but not ver\ considerably The mean s i/c s o f samples obta ned from X R I) patterns arc smaller than those o f these samples obla ncd iVom I HM image In this m etlod we did not elim inate ihc sysUni standard error In addition, in tiis X RI) niclliod, the mean sizes are obtained from all the structural layers of samples 0,0697 0,06806 d (nm) cosO 0,97555 0,97553 2,039 2,088 C d S e F F C stru ctu re (220) A, ’Mv 10 20 30 (311) 10 mi n V' 40 50 60 70 Theta (degree) F-'ig XRD patterns ofQ D s CdSe with crystalline times o f and 10 minutes 210 K ( ' ('ĩỉonii eí aỉ \'N U Jo u rn a l o f Science, h iathem atics - P hysics 25 (2009) ( r - ỉ ỉ In i h c o t l i c r iii c t liio d s , there are some other objccts which arc involved in the shell o f C dS e cores so it may be larger Fig shows absorption spectra, o f the prepared C dSc QDs it can be seen from F'ig that with increasing grow ing lime, the redshift o f the spectra can be clearly observed and optical absorption in the visible region due to CdSe Q Ds is demonstrated The average diameters o f the C dSe Q D s for each growth time interval is estimated using the cffcctivc mass approximation giving diameters ranging from nm to nm These values are com parable to those obtained by TEM and by the w av e le n g th o f the first excitonic a bsorption peak (T ab le 2) The deviation o f the peaks in absorption spectra is about 50 nm, w hich may be due lo the difference o f surface states o f these QDs It is also thought that the strong intensity from the CdSe Q D s can be attributed to their high crystallinity W a v e l e n g t h (n m ) [9], which is in good agreem ent with the X RD Fig U V -v is absorption spectra o f CdSe Q D s with patterns discussed earlier and the presence o f good various crystalline times surface states on the QDs 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 tlie d i a m e t e r s o f c r y s t a l COICS Table The parameters o f the C dSe Q D s vs grow ing lime Ratio Cd:Se 1:8 (2 T ) Nam e CdSe CdSe CdSe CdSe crystalline The m ean diam eter o f T he mean diam eter o f The wavelength o f the time (m inute) first absorption excitonic C dSe Q D s using formula C dSe Q D s using peak (nm) (l)(n m ) formula (2) (nm ) 520 2.2 2.6 557 2.5 3.2 10 56! 2.5 3.3 15 574 2.6 3.6 Sum m ary In summary, 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 The m orphologies o f the prepared nanoparticlcs can be controlled by the reaction time, the am ount o f Cd:Se ratio and the reaction temperature A ck n ow led gem en ts The authors express the sincere thanks to the N A F O S T E D under Grant number o f 103.03.93.09 and Ministerial-level project o f M O E T for the financial support K c C uong ct a I / VNU J o u rn a l o f Science, M athem atics - P hysics 25 (2009) -2 ! 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