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Physica E 31 (2006) 180–186

www.elsevier.com/locate/physe

Effect of ratios of Cd:Se in CdSe nanoparticles on optical edge shifts andphotoluminescence properties

Himani Sharmaa, Shailesh N. Sharmab, Gurmeet Singha, S.M. Shivaprasadb,�

aDepartment of Chemistry, University of Delhi, Delhi 110 007, IndiabElectronic Materials Division, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110 012, India

Received 7 October 2005; received in revised form 5 December 2005; accepted 12 December 2005

Available online 20 February 2006

Abstract

This paper addresses the issue related to morphology of CdSe nanoparticles capped with organic molecules. Semiconducting CdSe

nanoparticles of 5–16 nm are synthesized using CdO precursor, capped with trioctyl phosphine (TOP)/trioctyl phosphine oxide (TOPO)

using different starting precursor ratios of Cd:Se. At an optimum ratio of Cd/Se-2:1, highly luminescent and small sized (�5 nm)

nanoparticles are obtained. At other Cd/Se precursor ratios (0.5:1, 1:1, 3:1) larger particles are formed with varying photoluminescence

(PL) intensity and optical absorption (UV–VIS). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to

determine the crystallinity and stoichiometry of the system, respectively. It is shown that the blue shifts of the optical absorption edge

concurrent with the CdSe nanocrystal size reduction, for sizes measured by XRD with respect to the bulk semiconductor, agree perfectly

with the strong quantum confinement model. The optical edge shifts are significantly higher for CdSe nanocrystallite as measured by

transmission electron microscopy (TEM) than the theoretical prediction based on the strong quantum confinement model. This is

understood on the basis of agglomeration effects as observed by TEM for CdSe nanocrystallites. The nano-sized CdSe growth island thus

formed comprises of several TOP/TOPO passivated nanocrystals.

r 2006 Elsevier B.V. All rights reserved.

PACS: 78.67.Bf; 81.07.�b; 68.37.Lp; 79.60.�i

Keywords: CdSe nanoparticles; Transmission electron microscopy; Quantum confinement; Photoluminescence

1. Introduction

The emergence of colloidal semiconductor nanocrystalshave generated great fundamental and technical interest inrecent years, and represent one of the most rapidlydeveloping areas of current semiconductor research [1–3].The size-dependent emission property, particularly forCdSe nanocrystals, renders it indispensable in the field ofnanocrystal-based emitters such as light-emitting diodes[4], photovoltaic devices [5], lasers [6], biomedical tags [7],etc. For this reason, the control of the photoluminescence(PL) properties by controlling the size of the semiconduc-

e front matter r 2006 Elsevier B.V. All rights reserved.

yse.2005.12.154

ing author. Tel.: +9111 25742610x2403;

26938/25726952.

ess: prasad@mail.nplindia.ernet.in (S.M. Shivaprasad).

tor nanocrystals has been a major goal in the synthesis ofcolloidal semiconductor nanocrystals.Synthesis of high-quality semiconductor nanocrystals,

particularly CdSe, with nearly monodispersed size andshape, are of importance in electronic and biologicalapplications [8]. Since Murray et al. [9] reported thesynthesis of high quality cadmium chalcogenides nano-crystals using dimethyl cadmium (Cd(CH3)2) as the Cd-precursor, the synthesis of CdSe nanocrystals using this asCd precursor has been well developed. However, CdOprecursor has been preferred over dimethyl cadmium whichis highly toxic, pyrophoric, expensive, unstable at roomtemperatures and explosive at elevated temperatures [9].Capping of CdSe colloids formed at a high temperature(300 1C) with trioctyl phosphine oxide (TOPO) facilitatessize stabilization against extended particle growth [10].Moreover this method has yielded size-controlled CdSe

ARTICLE IN PRESSH. Sharma et al. / Physica E 31 (2006) 180–186 181

nanocrystallites with nearly monodispersed size and shapedistributions.

Profound changes observed in optical properties ofnanocrystal quantum dots, especially in direct gap II–VIsemiconductors have been a subject of intensive study dueto possible photonic applications [11–13]. CdSe has a largeexciton Bohr diameter �11.2 nm, and therefore thissemiconductor offers the possibility of studying quantumconfinement effects in higher cluster size regimes [14]. Theeffect of band structure on optical absorption edge shiftsdue to quantum size effect is described theoretically by theeffective mass approximation (EMA) on the basis of twoconfinement regimes [15] depending on whether thenanocrystallite size (Rnc) to Bohr exciton diameter (RB)ratios (Rnc/RB) are p2 or X4. The former case representsstrong confinement in which electron and hole areindependently confined when the CdSe nanocrystals(average radius Rnc) are much smaller than RB. Theexciton energy on the size of the nanocrystals is given by[15]

EðRÞ ¼ EbulkðCdSeÞ þ2h2

mp

Rnc

� �2

, (1)

where m is the effective mass of electron–hole pair of massesme and mh given by

1

m¼

1

meþ

1

mh.

In the weak confinement regime, when CdSe nanocrystalsize is greater than 4RB exciton within the nanocrystals,show translational motion confinement due to strongCoulomb interaction. The excitonic energy is given by theabove relation except that the effective mass of electron–hole (m) is replaced by the total mass of the exciton M ¼

me þmh and represented by

EðRÞ ¼ EbulkðCdSeÞ þ2h2

M

pRnc

� �2

. (2)

The II–VI semiconductors such as CdS and CdSe as wellas some of the III–V semiconductors such as GaAs whichhave relatively large values of Bohr diameter are found tobe suitable systems for strong quantum confinement effect[14]. There are several theoretical efforts to account for theblue shift in the energy gap of a semiconductor as afunction of particle size [16–18]. Many researchers haveemployed the EMA to understand the size-dependent bandgap of low-dimensional systems because of its simplicity[15]. The other treatments that are used to account for theband gap of nanoparticles include empirical tight-bindingmethod (ETBM) [16,17], effective pseudo-potential method(EPM) [18] etc. However, each of these methods has certainlimitations and it is mainly the EMA method which iswidely used to explain the quantum confinement effects insemiconductor nanocrystals on the basis of strong andweak quantum confinement regimes [15].

Generally certain threshold value of the crystallite sizes isimperative in order to observe quantum confinement

effects. This paper reports on the quantum size effectsobserved in CdSe nanocrystals prepared by the chemicalroute using trioctyl phosphine (TOP)/TOPO as the cappingagent. In this report, a study of the systematic variation inCdSe crystallite size by changing the Cd to Se precursorratio has been carried out and its effect on the shift of theoptical absorption edge has been investigated. It has beenobserved that certain critical ratio of Cd/Se is essential forthe realization of highly luminescent, monodispersive andstable CdSe nanocrystals. Optical absorption spectroscopicstudies show that edge shifts are strongly correlated tocrystallite sizes indicating strong confinement effects inCdSe nanocrystals passivated by TOP/TOPO groups.X-ray diffraction (XRD) and X-ray photoelectron spectro-scopy (XPS) are also used to ascertain the crystallinity andstoichiometry of the system respectively. It has beencommonly observed that colloidal nanoparticles in solidstate have the tendency to agglomerate, therefore thedispersion media plays an important role in the realizationof an uniform and homogeneous distribution of nanopar-ticles. However, for its possible application in solid statedevices, it is imperative to have an even distribution ofisolated nanoparticles, a condition which cannot berealized when the nanoparticles are in clustered oragglomerated state. Therefore, in this study, we havefocused on this problem of agglomeration in CdSenanoparticles by means of transmission electron micro-scopy (TEM) studies and correlated the results with PL,XRD and XPS studies. It has been observed that crystallitesizes of CdSe nanocrystals as measured by opticalabsorption and XRD techniques are in good agreementwith each other and they differ from TEM measurementsdue to the problem of agglomeration. In light of thetheoretical model for strong and weak quantum confine-ment, we will discuss our experimental results.

2. Experimental

The synthesis of CdSe nanoparticles were carried out bythe chemical route using TOP/TOPO capping method,whose details can be found elsewhere [9]. The materialsused were of the purest quality commercially available andused as received. CdO (Aldrich, 99.99% purity), tetradecylphosphonic acid (TDPA) (Alfa Aesar, 98% purity), TOPO(Aldrich, 99% purity), TOP (Aldrich, 90% purity) andselenium powder (Aldrich, 99.5% purity) were used.Absorption spectra were recorded using Shimadzu 3101spectrometer. The PL was measured using a homeassembled system consisting of a two stage monochroma-tor, a photomultiplier tube (PMT) with a lock-in amplifierfor PL detection, and an Ar+ ion laser operating at 488 nmand 5mW (corresponding to 0.125Wcm�2) for excitationin all the measurements. XRD studies were done by using aSiemens D500 X-ray diffractometer (CuKa �1.5406 A).XPS measurements were performed in an ultra-highvacuum chamber (PHI 1257) with a base pressure of�4� 10�10 Torr. The XPS spectrometer is equipped with a

ARTICLE IN PRESSH. Sharma et al. / Physica E 31 (2006) 180–186182

high-resolution hemispherical electron analyzer (279.4mmdiameter with 25meV resolution) and a MgKa (hn ¼1253:6 eV) X-ray excitation source. All XPS data werecorrected for electrostatic charging using the XPS lines ofC 1 s as internal reference energy.

Fig. 1. TEM of CdSe showing the agglomeration of CdSe nanocrystals

with different precursor Cd/Se ratios. (a) 1:1 and (b) 3:1.

3. Results and discussion

Size-quantization effects markedly influence electronic andoptical properties of semiconductor materials. In a chemicalprecipitation method, a variety of approaches have beenconsidered to arrest the growth of particles of desired size[19]. We employed TOPO capping to arrest the growth ofCdSe particles. A change in the ratio of Cd:Se during theprecipitation conditions produced particles of differentdiameters. Different Cd/Se ratios were explored since lowerCd/Se ratios result in the large size particle regime with highquantum yield, whereas, particles produced using higher Cd/Se ratios were found to be of smaller size [20].

Fig. 1(a) shows a typical TEM image of the as-synthesized CdSe nanocrystals corresponding to Cd/Se-1:1 system. As shown in Fig. 1(a), the particles are found tobe spherical and highly agglomerated with an average sizeof �45 nm. A careful analysis of Fig. 1 reveals that everysphere is a secondary particle aggregated by small CdSenanocrystals, due to their extremely small dimensions andhigh surface energy. Thus, a typical nanosize CdSe growthisland as observed by TEM in Fig. 1, could, infact be acluster of several smaller nanocrystals each surrounded andsurface passivated by TOP/TOPO groups. It is difficult todetermine individual particle size since the particles areclustered together. Fig. 1(b) shows TEM image of CdSe-3:1system exhibiting an average particle size of �50 nm. Here,as in the case of CdSe-1:1 system, agglomeration effects areprominent. However, in CdSe-1:1 system, the particlesshow a higher necking tendency as compared to CdSe-3:1system. Moreover, CdSe-1:1 system does not show anyclustered nanoparticles of smaller size unlike in the CdSe-3:1 system where a considerable number of small clusters(size �30 nm) are visible. As shown in Fig. 1, from thenature of aggregation of particles as we move from CdSe-1:1 to 3:1 system, indicates that the surface passivation hasimproved thus underlining the importance of higher Cd/Seprecursor ratio. The crystallite sizes of CdSe nanocrystals

Table 1

Summary of optical properties of CdSe systems with different precursor Cd/Se ratios where Rnc (TEM), Rnc(XRD) and Rnc (optical) are crystallite sizes as

measured by TEM, XRD and UV-VIS absorption respectively

System (Cd/Se) Energy gap,

Eg (eV)

Optical edge

shifts, DEsh (eV)

Rnc (TEM)

(nm)

Rnc (XRD)

(nm)

Rexp(optical)

(nm)

Rc/Rb (TEM) Rc/Rb (XRD) Rc/Rb

(optical)

0.5:1 1.85 0.07 43.0 14.12 16.33 3.84 1.26 1.46

1:1 2.18 0.43 45.0 6.90 6.54 4.02 0.62 0.58

2:1 2.33 0.59 20.8 5.29 5.64 1.86 0.47 0.50

3:1 2.23 0.49 50.0 6.28 6.16 4.46 0.56 0.55

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0 100 200 300 400 500 600

(c)

(b)

(a)

O

Cd

C

Se P

Co

un

ts (

a.u

.)

Binding energy(eV)

Fig. 3. XPS survey spectra of CdSe nanocrystals with different precursor

Cd/Se ratios. (a) 1:1, (b) 2:1, (c) 3:1. The curves are displaced vertically for

clarity.

Table 2

Determination of sample composition (at%) for CdSe systems with

different precursor Cd/Se ratios

System: Cd/Se Cd (at%) Se (at%) O (at%) P (at%) Cd/Se

0.5:1 9.9 10.4 55.4 23.3 0.96

1:1 11.6 11.8 39.8 36.5 0.99

2:1 18.0 10.4 39.7 32.0 1.73

3:1 12.6 7.4 32.2 6.3 1.70

H. Sharma et al. / Physica E 31 (2006) 180–186 183

as measured by TEM, XRD and optical absorption atdifferent Cd-to-Se precursor ratios are summarized inTable 1.

Fig. 2 (curves a–c) shows the XRD pattern of the CdSesystem corresponding to different Cd:Se ratios (1:1–3:1).From the XRD patterns, a considerable broadening, indi-cating nanocrystallite dimensions, were obtained (Fig. 2).As evident from Fig. 2 (curves a–c), the peak at lowerangles is the envelope of several reflections which providesfurther confirmation for the narrow size distribution of theparticles. The average sizes of the CdSe nanocrystallitesestimated from the full-width at half-maximum (FWHM)of diffraction peaks according to the Scherrer’s formula½d ¼ ðklÞ=b cos y� [20] (where k�1.0 for spherical particlesas visible from TEM) were found to be as 6.9, 5.3and 6.3 nm for CdSe ratio 1:1, 2:1 and 3:1, respectively(Table 1). The diffraction pattern peaks were indexed to the(1 0 1), (1 0 2) and (1 1 1) planes confirming the wurtzitephase of CdSe [22] (Fig. 2). No impurity phase is detectedby XRD. Thus there is a discrepancy in sizes as estimatedfrom TEM and XRD data. This indicates that the colloidsare made by the agglomeration of many smaller individualCdSe crystallites and thus it is difficult to determine theindividual crystallite sizes by TEM. TEM measurements ingeneral are not able to differentiate particles in clustered,aggregated or coalesced form unless special preparationtechniques or high-resolution TEM is used, whereas XRDmeasures planes of single crystals and gives average size ofindividual particles [21].

In order to further examine the chemical composition,stoichiometric and bonding changes in nanocrystallineCdSe system with varied Cd/Se ratios (1:1–3:1), the XPSsurvey spectra of CdSe nanocrystals were obtained andare shown in Fig. 3. The XPS results are summarized inTable 2. As shown in Fig. 3 (curves a–c), the presence of Cd3d5/2 peak at �405 eV indicates that Cd exists either inmetallic form (i.e., unreacted Cd) or in CdSe form [23]. It isinteresting to note that with the increase in Cd/Se ratio

10 20 30 40 50 60 70

100

200

300

400

500

600

700

800

(102

)

(101

)

(111

)

(c)

(b)

(a)

Inte

nsi

ty (

a.u

.)

2θ (degree)

Fig. 2. XRD patterns of CdSe nanocrystals with different precursor Cd/

Se ratios. (a) 1:1, (b) 2:1, (c) 3:1.

from 1:1 to 3:1, the intensity of the Cd-core levels increaseswhile the broadness of the Cd-core levels, i.e., FWHMremains the same, which indicates that there is no effect ofany oxidation in our CdSe system. Moreover, from Fig. 3,no splitting of the Cd 3d doublet was found which indicatesthat only Cd (from CdSe) is present on the surface and byXPS there is no evidence for CdO formation in our CdSesystem. Similarly, from Fig. 3 (curves a–c), the presence ofSe 3d binding energy at �55.5 eV further confirms thepresence of CdSe phase only [23]. From Fig. 3 (curves a-c),the signatures of C corresponding to XPS signal (�286 eV),Cd Auger signal (�405 eV), O XPS signal (531 eV) and P(�133 eV) [24] are also indicated for CdSe system varyingfrom 1:1 to 3:1 respectively. The presence of C, P and Ocomes from TOP/TOPO group indicating the presence ofTOP/TOPO molecules as a cap layer, passivating the CdSenanocrystals. Since XPS core-level studies confirm theabsence of CdO, hydroxide or selenium oxide, high purityof CdSe of the nanoparticles is evident. The areas under thepeaks of Cd and Se were measured in order to calculate theCd:Se ratio and were found as �0.96, 0.99, 1.73 and 1.70,respectively, for CdSe systems 0.5:1, 1:1, 2:1 and 3:1. Thedetails summarized in Table 2 show that Cd/Se ratios asobtained from the quantification of the XPS peaks, in

ARTICLE IN PRESSH. Sharma et al. / Physica E 31 (2006) 180–186184

general, increases with increase in Cd/Se precursor ratiofrom 1:1 to 3:1 using standard elemental sensitivity factors[24]. From Table 2, the discrepancy in the values of Cd/Sefrom the precursor ratio and quantified values from XPScould be attributed either to the undissociated CdO speciesduring the synthesis of CdSe nanocrystals or to the loweratomic sensitivity factor values of Se XPS signal [24] orboth. These factors thus may be responsible for the highermolar ratio of Cd to Se (�1.73) as shown in Table 2 forCdSe-2:1 system which could infact be close to unityindicating stoichiometric CdSe, as evident from its stablephotoluminescent properties.

Fig. 4 (curves a–d) show absorption spectra correspond-ing to CdSe system with precursor Cd/Se ratio varyingfrom 0.5:1 to 3:1. With the increase in Cd/Se ratio from0.5:1 to 3:1, well-defined absorption peaks are observedand is accompanied by a gradual shift in absorption edgeto the longer wavelength side (red shift) due to the increasein the CdSe nanocrystal size in accordance with quantumconfinement effects [14]. For these quantized nanoparticles,as can be seen from Fig. 4 (curves a–d), the shift in theedges of the absorption band from 670 to 534 nm indicatesan increased band gap in the range, Eg �1.85–2.32 eV,relative to the bulk band gap of �1.74 eV of bulk CdSe[25]. From the blue shift of the absorption edge, weestimated the particle size [9] of CdSe nanoparticles as16.33, 6.54, 5.64 and 6.16 nm for Cd:Se ratios 0.5:1, 1:1, 2:1and 3:1, respectively. Here, it is noteworthy that sizes ofCdSe nanocrystals (Rnc) as estimated from optical absorp-tion and XRD measurements are much lower than thecorresponding ones obtained from TEM measurements(Table 1). As discussed earlier, this again suggests that itcould be due to the agglomeration effect usually observedfor colloidal nanoparticles in the powder form (in theabsence of any solvent) while carrying out TEM measure-

400 450 500 550 600 650 7000.0

0.1

0.2

0.3

0.4

0.5

(b)

(c)

(d)

(a)

Ab

sorb

ance

(a.

u)

Wavelength (nm)

Fig. 4. Absorption spectra of CdSe nanocrystals with different precursor

Cd/Se ratios. (a) 0.5:1, (b) 1:1, (c) 2:1, (d) 3:1.

ments, whereas UV–VIS absorption measurements werecarried out for CdSe nanocrystals suspended in toluenesolvent. The sharp absorption features particularly for5.64 nm CdSe sample (Cd:Se-2:1)(Fig. 4, curve d) suggestshigh monodispersity while that for the remaining CdSesamples suggests polydispersity in the 1:1 case and largerparticles at 3:1, as evident from their relative broadabsorption features (Fig. 4, curves b and d).Fig. 5 shows the PL spectra for CdSe systems with

precursor Cd/Se ratio varying from 0.5:1 to 3:1 respec-tively. At lower Cd/Se ratio �0.5:1, the PL spectrum isquite broad and its intensity is relatively weak (Fig. 5, curvea) thus indicating that the crystallite dimensions are largeenough for quantum confinement effect to hold good. Withincrease in Cd/Se ratio from 0.5:1 to 2:1, the PL intensityincreases and the spectra becomes narrow, particularly forCd/Se -2:1 system. From the inset in Fig. 5, the PL peakwidth decreases with increase in Cd/Se precursor ratiofrom 0.5:1 to 2:1 and thereafter, it increases marginally.The narrow PL emission of Cd:Se-2:1 system with FWHM�36.23 nm, confirms good monodispersity and indicatesband-edge emission. Thus, at an optimum Cd:Se ratio as2:1, the fact that the PL emission maximum lies close to itsabsorption-onset (Fig. 4, curve d) with least FWHM value,indicates that the PL emission arises as a result of directrecombination between LUMO and HOMO chargecarriers, thus implying efficient passivation by TOP/TOPOgroups in this case. However, beyond Cd/Se -2:1 system,the PL peak shifts from 510 to 540 nm with a concurrentdrop in PL intensity (Fig. 5, curves c and d) as thenanocrystal size increases from 5.64 to 6.16 nm (Table 1).From Fig. 5, the presence of broad PL band emission atl4600 nm, particularly for 0.5:1, 1:1 and 3:1 CdSe systems,indicates the presence of surface states [26]. These surfacestates, as also observed by others, have been attributed to

400 500 600 700 8000

5

10

15

20

(d)

(c)(b)

(a)

No

rmal

ized

PL

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

0.5 1.0 1.5 2.0 2.5 3.0406080

100120140160

PL

peak

wid

th (

nm)

Cd/Se

Fig. 5. PL spectra of CdSe nanocrystals with different precursor Cd/Se

ratios. (a) 0.5:1, (b) 1:1, (c) 2:1, (d) 3:1. The inset in the figure shows a plot

between PL peak width and Cd/Se precursor ratio.

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

0.1

0.2

0.3

0.4

0.5

0.6

(a)

(b)

1/Rnc2 (X 10-2nm-2)

Sh

ift

in C

dS

e ab

sorp

tio

n e

dg

e (e

V)

Weak Confinement

XRD dataTEM data

Strong Confinement

Fig. 6. Theoretical blue shifts in the CdSe absorption edge as a function of

inverse squared average CdSe nanocrystal size corresponding to (a) weak

confinement and (b) strong confinement. Experimentally observed blue

shifts for different samples with different nanocrystalline sizes as measured

from TEM and XRD analysis are shown as curve (.) and (’)

respectively.

H. Sharma et al. / Physica E 31 (2006) 180–186 185

Se-unsaturated bonds and these sites serve as traps andprohibit radiative recombination of electrons and holes[26]. It is known that TOPO forms a strong complex withCd via the oxygen lone pair and since oxygen is veryelectronegative, TOPO does not introduce hole traps [27].The narrow PL emission of Cd:Se-2:1 system indicatesperfect passivation effect of CdSe nanocrystals by TOP/TOPO groups. For both the CdSe-0.5:1 and 1:1 systems,the relative number of Cd atoms may be insufficient forefficient capping effect by TOP/TOPO groups. However,beyond a critical Cd/Se42:1, in the 3:1 case, presence ofextra Cd atoms may render the number of TOP/TOPOgroups insufficient and so do not electronically passivateCdSe nanocrystallites. This is evident from Table 2, whereas compared to 2:1 CdSe system, both 1:1 and 3:1 CdSesystems exhibit lower concentration of Cd (at%) andfurther for CdSe-3:1 system, concentration of P (at%) issignificantly lower as compared to the correspondingvalues for 1:1 and 2:1 CdSe systems, thus indicating lowerTOP/TOPO passivation in the former as compared to thelatter. All these factors may be responsible for the presenceof surface states at l4600 nm in the PL spectra for the 1:1and 3:1 CdSe systems.

From absorbance and PL results, it is clear that thereexists a reasonable correlation between the change in sizeof CdSe nanocrystals and shifts in optical band gap. FromTable 1, the shift DEg ¼ (Eg(nanocrystal CdSe)�Eg(bulkCdSe)) is largest 0.59 for CdSe-2:1 system which exhibitslowest crystallite size from both XRD (�5.3 nm) andoptical absorption data (�5.64 nm). From Table 1, it isevident that the band edge shift reduces to a minimum(�0.07 eV) with increase in CdSe nanocrystal dimensions[(�14.12 nm from XRD) and 16.33 nm from opticalabsorption data)] for CdSe:0.5:1 system. The blue shift inthe band edge from bulk CdSe values with the reduction incrystallite size is attributed to the effects of quantumconfinement on the electronic band structure of CdSe [14].

The quantum size effects in thin films of CdSe, CdTenanocrystallites in a glass matrix and TiO2/SiO2 have beenreported earlier [11,28]. The quantum effect becomesapparent when the semiconductor crystal size is smallerthan the bulk Bohr diameter of the exciton RB. For CdSe,this value is �11.2 nm [14]. In both cases of quantumconfinement (strong and weak), the excitonic shift in theenergy change scales linearly with the inverse square of theCdSe nanocrystal size (1/Rnc

2 ), where Rnc is the average sizeof the CdSe crystallites as determined by XRD, TEM andoptical studies (Table 1). Theoretical dependence intransition energy on the two extreme confinement situa-tions on the inverse squared average CdSe crystal size hasbeen analyzed in Fig. 6. The translational reduced mass ofthe exciton given by M ¼ me þmh ¼ 0:13 mo+0.45 mo ¼

0:58 mo and exciton reduced mass by m ¼ 0:1008mo, wheremo is the rest mass of electron [28]. The bulk Bohr diameterof exciton in CdSe is �11.2 nm.Except in the case of CdSe-2:1 system, where average size of CdSe nanocrystal (asestimated from TEM) to Bohr diameter ratio is less than 2,

in most other cases this falls between 3.8 and 4.5 (Table 1).As shown in Table 1, the ratio of the crystallite size andBohr diameter (o2) as obtained from XRD and opticalstudies indicates strong quantum confinement effect(Fig. 6). In Fig. 6(curve a), the measured absorption edgeshifts for CdSe systems for crystallite sizes measured byTEM are larger than those predicted by the strongconfinement model (Fig. 6, curve (.)). Also the shift inCdSe absorption edge for CdSe nanocrystallites, whosesizes are calculated from XRD almost coincides with thetheoretical data of strong quantum confinement model(Fig. 6). As evident from TEM results (Fig. 1), whereagglomeration effect was observed and the individualparticle size was difficult to be determined, we notice thatlarger crystallite size and hence higher optical absorptionedge shifts. The dashed line in Fig. 6 correlates the size ofnanocrystals determined experimentally from TEM andthat expected on the basis of strong confinement regime.The agglomeration effect gives rise to larger absorptionedge blue shifts experimentally observed in Fig. 6 and thisexplains the discrepancy in the crystallite size as measuredfrom TEM and optical absorption/XRD measurements.A cluster containing passivated CdSe nanocrystals presentsa situation where differences that exist between surfaceatoms and those within the nanocrystals is large in favor ofsurface atoms. Vast interface between CdSe nanocrystalsand TOP/TOPO passivating medium will have a pro-nounced effect on optical and electrical properties. Thiscluster of CdSe nanocrystals could result in overestimationof their size. Taking this into account, the band edge shiftsfor CdSe system with precursor ratio varying from 0.5:1 to3:1 observed here agree well with the strong quantum

ARTICLE IN PRESSH. Sharma et al. / Physica E 31 (2006) 180–186186

confinement regime (Fig. 6). Further, alloying effect withoxygen as also observed for CdTe nanocrystallites [29]when oxygen enters substitutionally in Cd sites in the CdSelattice could also shift the band gap to higher energies andadd to the quantum-confinement effect to give the largerabsorption edge blue shifts experimentally observed.However, from XPS results, formation of cadmium andselenium-related oxides was not observed and we can ruleout this possibility at present. However, a detailedinvestigation of TEM and XPS results is underway andwill be reported shortly.

4. Conclusions

Synthesis of highly luminescent and quantized CdSenanoparticles was carried out successfully by the chemicalroute using single precursor CdO and TOP/TOPO as anefficient capping agent. XRD and XPS confirm thecrystallinity and bonding of the CdSe system, respectively.Optical absorption behavior of CdSe nanocrystals werestrongly modulated, depending on the Cd/Se precursorratio and/or size of CdSe nanocrystallites. The energy gapshifts of CdSe nanocrystallites from the bulk CdSe showlinear relation with inverse square of nanocrystal size inaccordance with Strong Quantum Confinement theory. Incase of crystallite sizes as determined from XRD studies,the optical absorption band edge shift coincides with thetheoretical values as predicted from the model. However, asignificant deviation observed for crystallite sizes deter-mined by TEM is attributed to the agglomeration effects ofCdSe nanocrystallites. Agglomeration effect is responsiblefor difference in sizes as measured by TEM and opticalabsorption/XRD measurements for CdSe nanocrystallites.A detailed investigation of the means to minimize theagglomeration effect of CdSe nanocrystallites for itspossible application in solid state devices is in progress.

Acknowledgments

We thank Director NPL for the encouragement toperform this work. The financial assistance from Depart-ment of Science and Technology, New Delhi, is gratefullyacknowledged. Dr. Shukla (USIC, Delhi University) andMr. Manjeet (AIIMS, New Delhi) are acknowledged fortheir help in XRD and TEM measurements.

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