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and were found to grow in preference to other compositions.

Based on first-principle calculations corecage structure was

proposed with puckered (CdSe)28 cages with (CdSe)56 cores.

They were proposed to have a diameter of 1.5 nm by atomic force

microscopy and displayed sharp excitonic peak at 415 nm and

smaller peaks at 382 nm and 352 nm. Magic-size nanocrystals

reported by Bowerset. al.8 showed sharp band-edge absorption

at 414 nm corresponding to size of 1.5 nm. The emission spectra

lacked the sharp band gap PL but displayed broad deep-trap PL.Similarly, families of magic-sizes of CdSe have been reported

recently by Ouyang et al.9 which possess only sharp band-gap

emissions. These have been reported to exist along with regular

quantum dots and have been assigned size on the basis of the

absorptions and diffused ordered NMR spectroscopy. In

another study the structureproperty relationship for CdSe

cluster upto sizes of 2 nm has been carried out and it has been

reported17 that for sizes greater than or equal to (CdSe)14, sharp

spectra are observed, because the PL due to excitons (which is

inherently sharp) is predominant. On the other hand broad

spectra are observed for sizes smaller than (CdSe)14. It can be

inferred from the examples cited above on stable CdSe clusters

that the absorption bands in all the cases are characteristicoccurring at isolated positions but the emission bands are far

from characteristic. There is also a certain difficulty with regard

to assigning the crystal structure as either zinc-blende or wurt-

zite. However, recently researchers18 have assigned them to be

tentatively wurtzite albeit by the use of phosphine ligands and by

adopting high temperature pyrolytical methods.

Other methods for obtaining magic-size nanocrystals have

described the use of etchants wherein larger crystals grown have

been etched to smaller ones by using amines.19 However, magic-

crystals formed without the use of etchants have also been repor-

ted.20,21 Low temperature organometallic preparations have also

yielded magic-crystals both by injection22 and non-injection

methods.9 Low temperature inverse wateroil micellular have beenreported.16 Most of the organometallic methods still advocate the

use of phosphine reagents such as trioctylphosphine (TOP)/TOP-

oxide, amines such as hexadecylamine (HDA) for etching or

otherwise, and the use of 1-octadecene for preparation of nano-

crystals withsuperior optical properties.They warrantextreme care

while handling due to high toxicities of reagents and are also diffi-

cult to reproduce dueto complexity of procedures which sometimes

require quenching of reactions within seconds for obtaining parti-

cles of sizes

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2.3 Preparation of magic-size CdSe using oleic acid and

diphenyl ether (DPE)

In a typical experiment oleic acid (17 ml, 53.5 mmol) was heated

to 150 C in a 250 ml flask under argon atmosphere. To this was

added cadmium acetate (3.14 g, 11.75 mmol) and stirred for

5 min for dissolution followed by the addition of diphenyl ether

(45 ml) at 150 C. Following the drop in temperature the reaction

was re-heated for

15 min for it to re-attain 150

C. Cyclo-hexeno-1,2,3-selenadiazole (1 g, 5.35 mmol) dissolved in 5 ml of

DPE was added to the above via syringe. Colour change from

yellow to deep orange was noticed and the reaction was moni-

tored by UV-Vis spectroscopy using aliquots. The reaction was

heated for a maximum of 6 hours after which heating was

stopped and the reaction mixture was cooled to 60 C.

Isolation and purification. Ethanol (50 ml) was added and

orange-yellow precipitates thus obtained were centrifuged. The

solid was separated by decantation, washed with ethanol (50 ml)

and re-centrifuged. The process was repeated 23 times. The free

flowing solids obtained after filtration were dried at 60 C. Yield:

1.8 g (capped CdSe with organics).

3. Results and discussion

When cadmium acetate and oleic acid are heated together in

diphenyl ether in the temperature between 100 and 150 C and

further with cyclo-hexeno-1,2,3-selenadiazole dissolved in the

same solvent reacted (Scheme 1), the solution darkens from pale-

yellow to orange indicating the formation of CdSe nanocrystals.

The thermolysis of the selenadiazole provides reactive selenium

and/or 1,4-diselenine25 that reacts with the cadmium oleate

generated in situ by the reaction of cadmium acetate and oleic

acid. Either of the above decomposition products of 1,2,3-sele-

nadiazole may form an intermediate single-source precursor

complex with cadmium oleate which decomposes to form magic-

size CdSe nano-crystals under the defined conditions outlined

herein.

The use of large excess of oleic acid proves an effective cap or

surfactant for obtaining particles below 2 nm. The use of

diphenyl ether as solvent ensures easy work-up and temperature

homogeneity throughout the reaction flask. The reaction

produces CdSe magic-size nanocrystals at temperatures

(#150 C). The type of the particle obtained is found to be

greatly dependent on the temperature at which the reaction is

carried out apart from ratio of reactants used.

3.1 Optical spectral features

Formation of magic-size clusters was monitored by UV-Vis

spectroscopy (Fig. 1A) using aliquots taken out during the

synthesis. Within minutes of the addition of the Se-precursor two

well-defined absorptions appear at 371 nm and 392 nm. With

time these peaks become sharper but persist at the same position.

The doublets have been assigned to electronic transitions 1S(e)

1S3/2

(h); 1S(e)2S3/2

(h) with energy separation of

180 meVbetween the two transitions.27 The size assignment based on peak

at 392 nm as per reported observations9 is 1.5 nm. The pho-

toluminescence (PL) emission spectrum (Fig. 1B) shows two

features first a sharp peak at 405 nm (3.06 eV) assigned as band

gap emission and second a broad band (430 to 680 nm)

assigned as the deep trap emission based on the behaviour of

semiconductor nanoclusters.6 The band gap emission is narrow;

FWHM 30 nm and is only slightly Stokes shifted (13 nm)

from the absorption value. The band gap is the first electronically

allowed transition and it involves the metal and chalcogen atoms.

The deep trap emission is, however, substantially red shifted

from the absorption onset (typically by 0.5 eV). The large Stokes

shift of this luminescence band indicates that the equilibriumgeometry in the excited state is distorted from the ground state

due to radiative recombination of localized surface trapped

charge carriers (arising due to surface defects).

Further it has been reported8 that long lifetime of this state

(deep trap) is due to alternate energy dissipation modes

(via coupling of vibrational modes of ligand to surface atoms

when they are of comparable size) or collision relaxation makes it

appear broad. The fact that no absorption can be detected in the

spectral region of the PL, clearly indicates that this emission

electronic transition is forbidden. These features are consistent

with deep-trap emission in nanoclusters where HOMOLUMO

gap involves forbidden transitions of surface states.6

It is interesting to note that reported work on magic-size CdSegrown pyrolytically,8 do not show band-edge PL and families of

CdSe magic-size NCs synthesized by a non-injection method do

not show deep-trap PL9 whereas the nanocrystals obtained in the

Scheme 1 Formation of CdSe MSNCs using 1,2,3-selenadiazoles.

Fig. 1 Room temperature optical spectra: (A) UV-Vis absorption

showing evolution of magic-size CdSe and (B) superimposed UV-Vis and

photoluminescence (lex 350 nm) of magic-size CdSe.

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present synthesis show both PL emissions i.e. sharp as well as

broad. Our samples show white-light emission (Fig. 2) as well as

less balanced yellow light emission from different samples

depending on relative intensity ratios of band-gap PL and deep

trap PL. The quantum efficiency of >10% from our measure-

ment26 using anthracene as reference seems to be very attractive

as against the reported efficiency of 15%.9

3.2 Effect of varying ratios of reactants and surfactant on

optical properties

It is well established that for the nucleation and growth of the

nanocrystal, temperature is one of the critical factors which

determines optimal growth conditions as well as product yield.

To study this, the preparation of CdSe by the use of cyclo-hex-

eno-1,2,3-selenadiazoles was carried out at different tempera-

tures ranging from 100 C to 250 C. Based on the TGA it was

found that 150 C is the optimum decomposition temperature of

the selenadiazole. It was found that in general lower tempera-

tures are more suitable for the formation of MSNCs. However,

at a particular temperature, the ratios in which reactants andsurfactants or capping agents are employed, can be used as

efficient parameters for exercising control over the type of

particle obtained. The results of carrying out the reaction at

150 C are summarised in Table 1. For a ratio of 1 : 1 : 20 of

selenadiazole (selenium source) to cadmium acetate (metal

source) to oleic acid (surfactant) mixtures of quantum dots and

MSNCs are produced. For either lower capping agent ratio (1 :

1 : 10) or high metal source ratio i.e. (1 : 2 : 20) or (1 : 2 : 10)

magic-size nanocrystals are produced exclusively and reliably.

Temperatures lower than 150 C results in incomplete reaction

with MSNCs being the only product, however, reactions at

180 C lead to formation of quantum dots.29 Large excesses of

oleic acid with respect to cadmium acetate may lead to completeconversion of cadmium acetate to cadmium-oleate which may

decompose at high temperatures to release Cd ions. Similarly, the

selenadiazole also decomposes at high temperature to release

highly reactive Se. When the ratio of cadmium precursor to

selenium precursor is more than unity then an excess of Cd ions

may be responsible for stabilizing the magic-size nuclei formed.

When this ratio of cadmium : selenium is 2 : 1 then it might just

be sufficient to prevent the dissociation of magic-size nuclei to

form regular quantum dots. At the same time maintaining the

temperature of 150 C is a critical factor that ensures formation

of thermodynamically controlled stable nuclei but prevents their

growth to regular quantum dots. If, however, the oleic acid is not

in large excess then the incomplete displacement of acetate from

cadmium salt may lead to the formation of an intermediate

species Cd(OOCCH3)x(OOC(CH2)nCH3)2x which may not

release Cd ions readily leading to an Se-rich environment which

may again be responsible for stabilizing the magic-nuclei. The

effect may be related perhaps to differences of feed rates of

reactive Se and Cd which may in-turn be related to the rate of

decomposition of precursors. Surfaces enriched in Cd or Se or

other such defects may perhaps be leading to unique photo-

physical properties which show good consistency when obtained

by different methods (Fig. 3). It is further observed that the

cadmium rich environment leads to slight red-shift in the emis-

sion maximum of the MSNCs. The Cd-rich environment may

cause further defects in the nanocrystals thus creating additional

deep trap states resulting in the red-shifts.

Fig. 2 White light emission from (A) magic size CdSe and (B) solvent

alone.

Table 1 Effect of varying ratios of reactants and surfactant on forma-tion of magic-size CdSe at 150 C in diphenyl ether

Ratios of reactants

Product obtainedSelenadiazole Cadmium acetate Oleic acid

1 1 20 Quantum dots + MSNCs1 1 10 MSNCs1 2 20 MSNCs

1 2 10 MSNCs

Fig. 3 Formation of magic-size CdSe at 150C using different ratios of

selenium source, cadmium source and surfactant: (A) absorption spectra

and (B) emission spectra.

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3.3 XRD diffraction

The X-ray diffraction pattern (Fig. 4A) indicates cubic structure

of CdSe.28 The five main diffraction peaks identified as (111),

(200), (220), (311) and (400) show that the conformation is

convincingly, essentially zinc-blende. Interesting differences are

observed in the intensity of peaks in comparison to CdSe

quantum dots29 obtained by similar choice of reagents but at

higher temperatures. The intensity of (220) is enhanced at theexpense of the (111) and (331) planes. We suspect that this could

be due to orientation or elongation along (220) due to forma-

tion of distorted structures as a result of the closed-shell

configuration. Further, diffraction peaks (102) and (103) char-

acteristic for wurtzite (hexagonal) are clearly absent. Our

observations comply with theoretical calculations30 and experi-

mental observations31 on several Cd(S/Se) cluster sizes where it

was predicted that stable CdSe clusters exist with a zinc blende

conformation rather than the wurtzite structure. The XRD

observations from different batches are consistent (Fig. 4B) and

can be used reliably to differentiate our magic-size nanocrystals

from quantum dots.

3.4 XPS

The elemental composition and surface properties can be best

understood by XPS analysis. The binding energy value of 54.9 eV

corresponds to Se 3d. (Fig. 5A). There is no evidence of oxidation

due to absence of additional peak for SeO2indicating the effec-

tive capping due to the use of oleic acid.

In addition the 3d doublet with binding energies 405.6 eV (Cd

3d

5

) and 412.4 eV(Cd3d

3

) match with expected binding energies

32

for CdObond formationshowingthe chemisorption and binding

of oleic acid to Cd via carboxylate. (Fig. 5B). The symmetric

feature of the O 1s peak at 532 eV indicates the presence of two

symmetric oxygen atoms in the carboxylate (COO) moiety.

(Fig. 5C). This is further indicated by the FTIR spectra.

3.5 IR

To understand the adsorption mechanism of oleic acid on the

surface of CdSe nanoparticles, the FTIR spectra (Fig. 6) were

Fig. 4 (A) Comparison of powder X-ray diffraction of CdSe: (a) Qdots

and (b) MSNCs showing zinc-blende crystallites; (B) MSNCs formed at

150 C using different ratio surfactants.

Fig. 5 XPS spectra of magic-size CdSe (A) Se 3d (core level) (B) Cd 3d

(core level) and (C) O 1s (core level).


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studied. All samples of MSNCs showed identical spectra with

3% cm1 variation in the peak positions. The major peaks were

assigned on the basis of typical IR spectrum of pure oleic acid33

given in parenthesis. The broad feature between 3405 and

3390 cm1 (3500 and 2500 cm1) was undoubtedly due to the

OH stretch of the carboxylic acid. The two sharp bands at 2917

(2925) cm1 and 2850 (2854) cm1 can be attributed to the

asymmetric and symmetric CH2, stretch, respectively and are

shifted to lower frequency compared to oleic acid indicating thatthe hydrocarbon particles in the monolayer surrounding the

CdSe particles are in close-packed crystalline state. The C]O

stretch observed in IR spectrum of oleic acid alone (1700

1730 cm1) is absent in the CdSe particles. Instead well defined

bands 1528 cm1 (s) characteristic of the asymmetric nas and

1435, 1405 cm1 (COO) characteristic of the symmetric ns(COO) stretch are observed. This shows that the oleic acid is

chemisorbed as a carboxylate onto the CdSe quantum dots.

Further the wavenumber separation D of120 cm1 seems to

indicate chelating bidentate mode of interaction of carboxylate

with metal atom rather than monodentate or bridging bidentate.

The observation of both OH stretch and C]O stretch of oleic

acid indicates that the adsorption of oleic acid is not confined tosingle mode and/or monolayer. Both protonated and deproto-

nated oleic acid (viabidentate mode) seem to be present akin to

a CdSe cluster capped with oleic acid. The protonated oleic acid

may be interacting weaklyvia its double bond. The bands in the

region 682 cm1 and 719 cm1 can be assigned out of plane OH

bend and CH2rocking and are again shifted to lower frequency

compared to oleic acid. The rocking frequency of different

samples shows maximum deviation. The FTIR data together

with XPS is strongly indicative of the bidendate mode of bonding

of carboxylate group to metal in which both oxygens are

equivalent rather than monodendate bond with inequivalent

oxygen atoms. In that case the IR spectrum would have dis-

played strong band at 17001730 cm

1 and the O 1s XP spectrumwould have shown another peak around 532 eV. Whereas sele-

nadiazoles are known to decompose thermally to produce

alkynes or a 1,4-diselenin compounds25 the absence of IR bands

in the region 1500 cm1 indicate that there is no surface capping

of alkyne or contamination of the diselenin in the final product.

3.6 TEM

Though the use of TEM for identification of particles below 2 nm

is challenging, the TEM micrographs (Fig. 7A) of dried and re-

dispersed CdSe were collected and the figure showed onion like

patterns at certain places with certain degree of elongation which

is also indicated by XRD (220 plane) analysis. Further the

HRTEM (Fig. 7B) clearly shows the existence of lattice fringes

and sub nm scale particles in the atomic planes which may be

point defects. The concentric rings in the selected area electron

diffraction (SAED) are considered due to zinc blende crystal

structure and the presence of these rings confirm that the mate-

rial is highly crystalline.

Conclusions

Organoselenium compounds such as 1,2,3-selenadiazoles can

function as low temperature sources of reactive selenium for the

preparation of metal selenide (CdSe) nanocrystals. Fine tuning

the process by maintaining moderate temperatures (100150 C)

and varying the ratios in which reactants and capping agents are

used gives thermodynamically stabilized magic-size CdSe

nanocrystals. The above results have led us to define the differ-

entiating conditions which lead exclusively to formation of

magic-size nanocrystals. The process can be easily scaled up to

give gram quantities of material. The above results furtherdemonstrate a method which gives exclusively zinc-blende

nanocrystals with the avoidance of TOP or similar phosphorus

ligands. The emission properties of these particles (12 nm) (on

the basis of UV-absorptions) are dominated by surface

phenomenon which produces the broad emissions due to surface

defects. IR and XPS indicate that the oleic acid carboxylate

seems to be binding via a bidentate mode. The powder XRD

shows that the material is clearly zinc-blende (cubic). Visualiza-

tion of the particles by TEM showed the material is highly

crystalline. We are currently exploring the utility of this method

for the preparation of other metal selenide nanocrystals.Fig. 6 Comparison of FTIR spectra of magic-size CdSe formed at

150 C using different ratios of reactants.

Fig. 7 (A) TEM with inset selected area electron diffraction (SAED) of

magic-size CdSe formedat 150 C using the ratio of1 : 2 : 10of Sesource:

Cd source : oleic acid; (B) HRTEM lattice imaging with inset lattice

fringes.

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Acknowledgements

This work was supported by DST Govt. of India (Grant no. SR/

S1/PC-17/2006). We thank Executive Director C-MET, Dr AK

Viswanath, Dr CVVV Satyanarayana for fruitful discussion and

Prof. BR Mehta for permission to use TEM and HRTEM facility

of Unit on Nanoscience and Nanotechnology Initiative at IIT

Delhi (Project No. SR/S5/NM-22/2004) of DST Govt. of India.

Notes and references

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