Chapter 1shodhganga.inflibnet.ac.in/bitstream/10603/36545/5/chapter 1.pdf · synthesis. Moreover, absorption spectral analysis may provide an insight into band structure, band gap

50

Chapter 1

Synthesis and opto-physical

characterization of CdTe

quantum dots

Synthesis and opto-physical characterization of CdTe quantum dots

Chapter 1

51

1.1. INTRODUCTION

1.1.1. Opto-physical properties of QDs

In recent years, smart nanomaterials such as QDs have gained a lot of

interest because of their unique spectral properties (Bruchez et al., 1998;

Aoyagi and Kudo, 2005). A brief description on QDs has been given in the

literature review (Literature review, Section 4). QDs exhibit size dependent

opto-physical and electronic properties due to inherent quantum confinement

effects of excited electrons and their corresponding holes called excitons

(Alivisatos, 1996). As a result, QDs behave differently in contrast to their bulk

counterparts. Quantum confinement effect is a phenomenon of widening of

bandgap energy as the size of the material shrunken to nano scale. Due to

this quantum confinement effect the movement of electrons in QDs are

confined in all three spatial dimensions and hence are called as zero

dimensional semiconductor materials (Hornyak et al., 2008). QDs exhibit

discrete conduction bands resulting in emission of light due to radiative

relaxation when an excited electron returns to the valence band. When an

electron absorbs energy from an electromagnetic wave, it reaches an excited

state and tends to return to its ground state by releasing absorbed energy.

This process of relaxation of an electron from excited state to its ground state

is termed as fluorescence (Wiedemann, 1888; Valeur and Berberan-Santos,

2011). Another feature of quantum confinement effect in QDs is the energy

band gap which is the distance between valence band and conduction band.

The band gap in QDs is size dependent and is inversely proportional wherein

band gap of the material increases as the size of the QD decreases. This size

dependent property of band gap in QDs allows engineering the QD size to


Chapter 1

52

restrict emission frequencies. Thus, emission wavelength in QDs shifts

towards higher wavelength as the size of the particle increases (Prasad,

2004). In fact, gradual increase in the molar extinction coefficient towards

shorter wavelengths is an important feature for biological applications allowing

simultaneous excitation of multicolor QDs with a single wavelength.

QDs exhibit a typical Gaussian fluorescence curve indicating

polydispersity of the material. Hence, Gaussian curves with smaller Full Width

at Half Maximum (FWHM) suggests samples with narrower QD size

distributions (Galian et al., 2009). Therefore, fluorescence in QDs may be

designed based on applications by engineering their crystal size during

synthesis. Moreover, absorption spectral analysis may provide an insight into

band structure, band gap and in turn on the quantum confinement effects in

QDs. Although intrinsic energy states are determined by the material used,

band gap energy is significantly directed by size dependent quantum

confinement in QDs. Therefore, it is possible to synthesize QDs of the same

material to emit at different wavelengths by restricting their sizes.

Surface passivation or capping is a critical factor to be addressed for

having photostability in QDs. In general, phosphenes (Manna et al., 2000) and

mercaptans (Rogach et al., 2007) are the most widely used capping ligands.

Alongside, an issue of hydrophilic or hydrophobic nature of the capping

material has to be considered while selecting an appropriate ligand in order to

have biocompatible QDs for biosensing applications.


Chapter 1

53

1.1.2. Synthesis of QDs

QDs can be synthesized either by chemical methods or by physical

methods. Typical chemical method of synthesis includes conversion of

precursors to achieve nanoparticle growth through nucleation process. This

occurs by the combination of solute atoms or molecules to reach a critical

size. During the course, parameters such as temperature, stabilizers,

precursor‟s concentration and ratios of anionic to cationic species along with

nature of the solvent are critically monitored to synthesize QDs of desired

size, shape and composition. Hydrothermal synthesis process (Yang et al.,

2008), sol-gel process (Lin et al., 2005), microemulsion process (Darbandi et

al., 2005), hot-solution decomposition process (Murray et al., 1993) and

microwave synthesis process (Qian et al., 2005) are some of the common

synthesis procedures to name just a few by this approach.

In contrary to above, physical methods of QDs synthesis generally

begins with formation of layers in an atom-by-atom addition and elemental

growth. Molecular beam epitaxy (MBE), Physical vapor deposition (PVD) and

Chemical vapor deposition (CVD) are some of the important methods in this

category. MBE uses deposition of overlayers for elemental growth on a

heated substrate under ultra-high vacuum (Jiao et al., 2006), PVD is done by

forming elemental layers through condensation of vapors produced by thermal

evaporation or sputtering (Burda et al., 2005) and in CVD QDs are self-

assembled on a thin film (Lobo et al., 1998).


Chapter 1

54

1.1.3. Aqueous synthesis of QDs

Synthesis of QDs with biocompatibility is the primary requirement for

any biosensing applications. In this direction, aqueous method of synthesis

has gained much attention in recent years and remains the best approach

aimed at biosensing techniques. Surface coating ligands employed in

aqueous phase synthesis render biocompatibility to QDs that enable them to

be directly conjugated with biomolecules. Therefore, researchers have

focused their attention on coating characteristics to obtain QDs with high

quantum yield, narrow size distribution and better fluorescence. Out of several

existing methods of synthesis (Samina et al., 2006; Hines and Guyot-

Sionnest, 1996; Rosetti and Brus, 1982), currently non-aqueous and aqueous

phase synthesis strategies are being commonly adopted to synthesize QDs

with high quality. Non-aqueous phase method results in hydrophobic QDs

(Murray et al., 1993; Qu and Peng, 2002) due to the pyrolysis of

organometallic precursors in organic solvents at high temperature. These

QDs need post synthesis surface modification with hydrophilic surface ligands

to attain biocompatibility which often result in decreased fluorescence

(Bruchez et al., 1998; Mattoussi et al., 2000). Alternatively, QD synthesis in

aqueous phase is a direct approach to produce water-soluble QDs without the

need for further modification (Yang et al., 2008). Hence, organic capping

materials with thiol moieties are being frequently used for surface attachment

where terminal polar head group facilitate hydrophilic interactions and

bioconjugation to other molecules (Bruchez et al., 1998; Gerion et al., 2001;

Mattoussi et al., 2000). Although hydrothermal method has been used for the

synthesis of a variety of QD nanoparticles (Gaponik et al., 2002; Gao et al.,


Chapter 1

55

1998; Zhong et al., 2006) such as CdS, CdSe, CdTe, ZnS, ZnSe and HgTe,

focus has been given on synthesis and application of CdTe QD in our work.

Thus, engineering the size and opto-physical characterization of QDs

are very much essential for their applications in biosensing and bioanalytical

methods. In this chapter, a facile route for the synthesis of CdTe QD using

water-soluble thiols as stabilizing agents at a low temperature (100 °C) has

been discussed. Further, studies on engineering the morphology and

structure of CdTe QD by adjusting precursor and ligand compositions have

also discussed in detail.

1.2. EXPERIMENTAL

1.2.1. Materials

Cadmium acetate dihydrate [Cd(CH3COO)2.2H2O], Tellurium powder,

Sodium borohydride (NaBH4), 3-Mercaptopropionic acid (MPA), Propionic

acid (PA), 2-Mercaptoethylamine hydrochloride (MEA), 2-Mercaptoethanol

(ME), Potassium bromide (KBr), Rhodamine-6G were procured from Sigma-

Aldrich India Pvt. Ltd. Bangalore, India. Dialysis membranes having 6-8 kDa

molecular weight cut off was procured from Spectra/Por, USA. Amicon

bioseparation filters were procured from Millipore (India) Pvt. Ltd., Bangalore,

India. All reagents used were of analytical grade and acquired from standard

suppliers.

1.2.2. Instruments

The instruments used were UV-Vis Spectrophotometer (UV-1601,

Shimadzu, Japan), Spectrofluorophotometer (RF-5301 PC, Shimadzu,


Chapter 1

56

Japan), Atomic Force Microscopy (AFM, Molecular Imaging, USA),

Transmission Electron Microscopy (TEM, Jeol 2100, USA), 400-mesh carbon

grid from Pacific Grid Technology (San Francisco, CA 94111, USA). X-ray

diffraction (XRD) was carried out using Desktop X-ray diffractometer (Rigaku,

Miniflex-II). Fourier Transform Infrared Spectrometer (FT-IR, Nicolet 5700,

Thermo Electron India PVT. Ltd. Pune, India).

1.2.3. Synthesis and functionalization of colloidal CdTe QDs

CdTe QDs were synthesized according to Eychmuller and Rogach,

(2000), Li et al. (2007). In brief, 0.02 M of Cd(CH3COO)2.2H2O was dissolved

in 25 mL of argon saturated double distilled water. The solution was reacted

with 0.05 M of MPA to obtain a molar ratio of 1:2.5 between Cd2+ and MPA

respectively followed by bubbling argon gas for 30 minutes and preserved till

further use.

1.2.4. Synthesis of sodium hydrogen telluride (NaHTe)

NaHTe was produced in argon saturated aqueous solution by reducing

0.01 M Te powder in presence of 0.03 M NaBH4. The reaction mixture was

incubated for 120 minutes at room temperature.

1.2.5. Growth of CdTe nanoparticles

Finally, NaHTe produced was added dropwise to argon saturated Cd-

MPA mixture followed by adjusting the pH to 11 ± 0.5 using 1N NaOH. The

molar ratio of Cd2+:MPA:HTe- was fixed at 1:2.5:0.5 for initial experiment.

Further, the resulting mixture was subjected to refluxing at 99 ± 1 °C for 150


Chapter 1

57

minutes separately under continuous argon flow to control the size of the

CdTe nanoparticles (Fig 1.1). CdTe nanoparticles produced were precipitated

using absolute ethanol and further, centrifuged at 8000 RPM repeatedly for 3

times to obtain the crystals.

Fig. 1.1 Setup for quantum dot synthesis

(a) Magnetic stirrer with heating coil, (b) Oil bath, (c) Sample injection

port, (d) Water condenser, (e) Water outlet, (f) Water inlet, (g) Argon

cylinder

Both absorption (Fig. 1.2) and fluorescence (Fig. 1.3) spectra were

recorded for CdTe QDs. CdTe QDs were excited at 350 nm and FWHM was

monitored.


Chapter 1

58

0

0.2

0.4

0.6

0.8

1

1.2

1.4

350 400 450 500 550 600 650 700

Ab

so

rba

nce

Wavelength in nm

1 2 3

4

5

Fig. 1.2 Absorption spectra of CdTe QDs. (1) CdTe516, (2) CdTe523,

(3) CdTe557, (4) CdTe576 and (5) CdTe601

0

100

200

300

400

500

450 500 550 600 650

RF

U

Wavelength in nm

1

2

3

4 5

Fig. 1.3 Fluorescence spectra of CdTe QDs. (1) CdTe516, (2) CdTe523,

(3) CdTe557, (4) CdTe576 and (5) CdTe601


Chapter 1

59

1.2.6. Studies on reaction time on nanoparticles growth

The reaction mixture having molar ratio of Cd2+:MPA:HTe- fixed at

1:2.5:0.5 was subjected to prolonged refluxing with the above mentioned

reaction conditions. Te powder was reduced at room temperature as

mentioned previously. Sample was drawn at regular intervals of 30 minutes

from 0 hrs to 6 hrs. Both absorption and fluorescence spectral changes were

recorded for each sample. FWHM with respect to reaction time was

monitored.

1.2.7. Studies on effect of ligand concentration

CdTe QDs were synthesized at different molar ratios of precursors to

probe the effect of capping material (MPA, MEA, PA and ME) on nanoparticle

growth. Initially, molar concentration of MPA was varied (0.02, 0.03, 0.04,

0.05, 0.06, 0.07, 0.08 M) in presence of 0.02 M of Cd(CH3COO)2.2H2O and

0.01 M of Te. The effective molar ratio of Cd2+:MPA:HTe- were 1:1:0.5,

1:1.5:0.5, 1:2:0.5, 1:2.5:0.5, 1:3:0.5, 1:3.5:0.5 and1:4:0.5. Te was reduced at

room temperature in presence of 0.03 M NaBH4 by incubating for 120

minutes. Further, the reaction mixtures were subjected to refluxing for 120

minutes at 99 ± 1 °C separately under continuous argon flow. Further, MEA,

ME and PA were also tested as capping materials with the above mentioned

reaction conditions. The molar ratio of Cd2+:R:HTe- was fixed at 1:2.5:0.5

where „R‟ is the respective capping material.


Chapter 1

60

1.2.8. Effect of pH on QD synthesis

CdTe QDs were synthesized at various pH to probe the influence of

hydrogen ion concentration on nanoparticle growth. The molar ratio of

Cd2+:MPA:HTe- was fixed at 1:2.5:0.5 and pH of the reaction mixtures were

adjusted to 4.0, 5.5, 6.5, 7.0, 8.0, 9.0, 10.0, 11.5, 12.0 ± 0.2 in separate

experiments prior to refluxing. The refluxing conditions remained same as

mentioned in previous section. Finally, CdTe nanoparticles of various sizes

having emission maximum at 516 nm, 523 nm, 557 nm, 576 nm and 601 nm

were synthesized at different refluxing conditions by adjusting pH to 9 ± 0.2 in

separate experiments. CdTe nanoparticles produced were precipitated with

absolute ethanol as mentioned earlier. Photo-absorption and fluorescence

spectra were recorded.

1.2.9. Photophysical characterization of QDs by absorption and

fluorescence profiles

Absorption spectra were taken for CdTe nanoparticles of various sizes

(516 nm, 523 nm, 557 nm, 576 nm and 601 nm) and respective first

absorption peak was recorded. Sizes of QDs were determined according to

equation 1.1 given by Donega and Koole, (2009).

E (CdTe) = Eg () + 1/(ad2+bd+c) (1.1)

Where E (CdTe) and Eg () are the band gap energies (electron Volt, eV) for

CdTe synthesized and bulk respectively. a, b and c are constants (0.137, 0

and 0.206 respectively) for CdTe QDs and d is the diameter of QD in nm.


Chapter 1

61

Size of QDs was also determined by equation 1.2 given by Kayanuma,

(1986 and 1988), equation 1.3 given by Yu et al. (2003) and Pu et al. (2006)

∆E = E-Eg + Ry = (ћ22/2r2) - (1.786е2/εCdTer) + 0.752Ry (1.2)

d = (9.8127 x 10-7)3 - (1.7147 x 10-3)2 + 1.0064 - 194.84 (1.3)

where is the wavelength of the first excitonic absorption peak in nm, Eg =

1.606 eV is the band gap energy ( in eV) for CdTe bulk and E is the band gap

energy of CdTe synthesized, ћ is reduced plank‟s constant, = 0.0774m0 is

the reduced mass of an electron mass m*e = 0.096m0 and a hole mass m*h =

0.4m0, m0 is electron mass, r is the radius of the dot, е is the charge of an

electron, εCdTe = 7.1 is the dielectric constant and Ry = 10 meV is the exciton

Rydberg energy (Masumoto and Sonobe, 1997). The band gap energy of

CdTe QDs was calculated using absorption spectra. Molar extinction

coefficient (ε) for all the QDs was calculated according to Yu et al. (2003) and

Pu et al. (2006) as given below:

ε = 3450 (Eg) (d)2.4 (1.4)

Fluorescence spectra were taken for all QDs exciting at their respective

first excitonic peak (Fig. 1.3). FWHM and effective Stokes shift were recorded.

Fluorescence efficiency (quantum yield, QY) of QDs was determined by

comparative method according to Williams et al. (1983). For this Rhodamine

6G was employed as standard considering its QY as 95% in absolute ethanol

at room temperature (Crosby and Demas, 1971; Kubin and Fletcher, 1982).

Gradient absorption was taken in the range 0, 0.02, 0.04, 0.06, 0.08 and 0.10

for both standard and QD having identical optical density at the respective

excitation wavelength (λex 528 nm for Rhodamine 6G and λex 452 nm, 469 nm,


Chapter 1

62

501 nm, 509 nm and 547 nm for CdTe516, CdTe523, CdTe557, CdTe576 and

CdTe601 respectively). A graph of integrated fluorescence against absorption

was plotted (n=5) to determine the gradient „m‟ for both standard (Fig. 1.4)

and QD (Fig. 1.5) separately.

y = 286644x R² = 0.9992

0

5000

10000

15000

20000

25000

30000

35000

0 0.02 0.04 0.06 0.08 0.1 0.12

Inte

gra

ted

flu

ore

sce

nc

e

Absorbance of Rhodamine 6G at 528 nm

Fig. 1.4 Standard graph of Rhodamine 6G integrated fluorescence

y = 8542.3x R² = 0.9979

y = 76214x R² = 0.9921

y = 100634x R² = 0.9988

y = 26550x R² = 0.9955

y = 67855x R² = 0.9982

0

2000

4000

6000

8000

10000

12000

0 0.02 0.04 0.06 0.08 0.1

Inte

gra

ted

flu

ore

sc

en

ce

Absorbance of QDs at first excitonic point

CdTe516

CdTe523

CdTe557

CdTe576

CdTe601

Fig. 1.5 Standard graph of QDs integrated fluorescence


Chapter 1

63

Further, fluorescence QY () of QDs were calculated according to the

following equation:

X = ST (GradX/GradST)(2X/2

ST) (1.5)

In this equation, the subscripts ST and X denote standard and QD

respectively, Grad was the gradient obtained from the plot and was the

refractive index of the solvent used, methanol for Rhodamine 6G and water

for QDs at their excitonic wavelengths according to El-Kashef, (2000); Daimon

and Masumura, (2007) respectively (Table 1.1).

Table 1.1 Opto-physical properties of CdTe QDs

Sl

No.

CdTe

QDs

(em in

nm)

First

excitonic

peak

(nm)

Band

Gap

(eV)

Particle

Sizea

(nm)

Molar

extinction

coefficientb

(/M cm) x 104

Stokes

shift

(nm)

FWHM

(nm)

QY

(%)c

1 516 452 2.74 2.20 6.2716 64 64 2.812

2 523 469 2.64 2.35 7.0792 54 45 25.078

3 557 501 2.47 2.61 8.5202 56 60 33.046

4 576 509 2.43 2.70 9.0928 67 65 8.710

5 601 547 2.26 3.06 11.4198 54 80 22.234

a is calculated according to Donega and Koole, (2009), b is calculated

according to Pu et al. (2006) and Yu et al. (2003), c is calculated according to

Williams et al. (1983) using Rhodamine 6G as standard considering its QY as

95% in absolute ethanol at room temperature


Chapter 1

64

1.2.10. TEM, AFM and XRD studies

Morphology of CdTe557 was studied using TEM AFM and XRD. TEM

was carried out by placing a drop of colloidal solution dried on a 400-mesh

carbon copper grid at an acceleration voltage at 200 kV. An AFM picture was

taken in tapping mode on a molecular imaging system. AFM picture was

scanned over an area of 5 x 5 m on a mica slide spread with CdTe557 using a

cantilever of 4-8 m thickness and a typical length of 125-225 m at a

resonant frequency of 190-300 kHz. Powder X-Ray diffraction (XRD) was

measured on the dried powder sprinkled over glass slide in a Rigaku Miniflex

X-ray diffractometer using rotating anode coupled with Cu-Kα line (λ= 1.54 Å)

operating at 30 kV output voltage. The 2θ scanning range was from -3° to

+145°. The diffractograms were recorded between angles 6° and 80°.

1.2.11. Disposal of QDs

CdTe QDs were disposed after experiments by treating them in 10%

potassium dichromate solution prepared in concentrated sulfuric acid as a

general technique proposed by International Agency for Research on Cancer

(IARC) for carcinogens. The solutions were kept for 2 days before draining

them in running water (Montesano et al., 1979).


Chapter 1

65

1.3. RESULTS AND DISCUSSION

1.3.1. Synthesis and functionalization of colloidal CdTe QDs

1.3.1.1. Growth of CdTe QD nanoparticles

CdTe QDs were synthesized in aqueous phase according to a general

procedure of reacting transition metal with thiol-based stabilizer in presence of

a chalcogen source. Generally nanoparticle growth depends on molar

concentrations of precursors, capping material, pH and temperature of the

medium. Initially at high temperature, the chemical transformation of

precursors into monomers results in supersaturation. This in turn results in

nanoparticle growth with the nucleation process followed by slower growth on

the existing nuclei. Temperature being a critical factor allows rearrangement

and annealing of atoms. During this process called “focusing”, initially all

monomers will turn into smaller particles. As the monomer concentration

drops below the critical concentration for nucleation, materials can only be

added to the existing nuclei. At this distinct growth stage of “Ostwald ripening”

smaller nanoparticles undergo dissolution and atoms are re-deposited on

larger particles called as “defocusing” resulting in increase in the average

nanoparticle size over time (Ge et al., 2008).

Since QDs are synthesized from organometallic precursors, they have

no intrinsic aqueous solubility. The native coordinating organic ligands

(capping agents) on the surface of QDs must either be exchanged or

functionalized with a ligand that can impart both solubility and potential

bioconjugation sites (Sapsford, et al., 2006). Ligands those form a capping

layer on the surface of the QDs must be biocompatible as biomolecules and

most of biological reactions except reactions involving lipids are highly


Chapter 1

66

hydrophilic in nature. Therefore, MPA was used to stabilize the QD that impart

biocompatibility and necessary functional carboxylic group for bioconjugation

in aqueous medium. Initial Cadmium-thiol complex formation can be

explained from the following equations. Sodium borohydride being a strong

reductant reduces Te (equation 1.6).

2Te + 4NaBH4 + 7H2O 2NaHTe + Na2B4O7 + 14H2 (1.6)

R-SH + OH- SR- + H2O (1.7)

Cd2+ + 2SR- Cd(SR)2 (1.8)

Cd(SR)2 + HTe- CdTe + SR- (1.9)

During nanoparticle growth, thiolated ligands (R-SH) reversibly adsorb

on to the surfaces of nanoparticles forming a capping layer. This stabilizes the

nanoparticle size mediating their growth. Cd2+ ion reacts with thiolate ion

(equation 1.7) to form a complex (equation 1.8), which strongly depends on

the pH of the solution. The complex formation is more favorable at basic pH

and is insoluble in acidic conditions as complexes may exist in a polymer

state as reported by Gao et al. (1998). Thus, cadmium-thiol complex at high

pH values further reacts with reduced tellurium ion promoting the growth rate

of CdTe nanoparticles (equation 1.9) (Zou et al., 2008).

1.3.1.2. Coordination chemistry

Generally surface atoms which are decisive in the creation of highly

luminescent nanoparticle bears all structural defects as they differ from the

core atoms in coordination number and charge state. Therefore, capping

agents containing hetero-atoms with the lone pair of electron are added to


Chapter 1

67

stabilize the nanoparticles preventing them from aggregation. On the other

hand, presence of an excess of uncoordinated Te atoms with free valencies

diminishes luminescence. There are two possibilities of coordination for MPA

with Cd2+. MPA forms complex with Cd2+ either by losing their -COOH proton

or -SH proton binding to the same Cd2+ atom or two adjacent atoms (Zhang et

al., 2006). MPA forms 6-membered planar ring if both S and one of the

oxygen of the carboxylate bind to the same Cd2+ atom. On the other hand, it

may form 8-membered ring if it coordinates with two adjacent Cd2+ atoms.

Acar et al. (2009) has reported that thermodynamically most stable form is

MPA coordinating with two adjacent Cd2+ atoms. However, thiol coordinated

QDs possessing free -COOH functional group is the most preferred state

aiming at bioconjugation studies.

1.3.1.3. Characterization of functionalized CdTe557

CdTe557 was analyzed by FTIR spectroscopy equipped with KBr

detector and KBr beam splitter. The FTIR spectrum showed an intense

characteristic broad band at 3385 cm-1 corresponding to -OH stretching

vibration, strong stretching vibration at 1566.4 cm-1 and medium stretching

vibration for carboxyl group at 1404.6 cm-1. Disappearance of characteristic

absorptions for -SH thiols, generally found at 2550-2600 cm-1, infers thiol

coordinated CdTe QD formation. This confirms free carboxylic functional

moiety in CdTe QD and coordination from thiol moiety of MPA. This also

infers that –OH group is not involved in the coordination and is free to render

biocompatibility to CdTe QD (Fig. 1.6).


Chapter 1

68

712.0

900.0

934.6

957.7

1213.6

1308.9

1404.6

1566.4

1621.7

1660.7

1918.0

2228.7

3385.0

CdTe 557

10

20

30

40

50

60

70

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

%Tra

nsm

itta

nce

Fig. 1.6 FTIR spectrum revealing thiol coordinated CdTe557 nanoparticle

having free -COOH moiety

A typical XRD pattern for the MPA-coated CdTe557 was shown in Fig.

1.7. The powder XRD profile shows characteristic broad peaks of Zinc blende

cubic CdTe due to small size of the nanoparticles (Zou et al., 2008). The

reflections could be indexed to the (111), (220) and (311) planes of Zinc

blende cubic CdTe (Chen and Yan, 2009; Duan et al., 2009).

Fig. 1.7 Typical XRD pattern of MPA-coated CdTe557


Chapter 1

69

Masumoto and Sonobe, (1997) reported that due to valence band

degeneracy, Zinc-blende CdTe QD has got complicated excited quantized

levels as the conduction band is made of the s orbital of Cd and valence band

is made of the p orbital of Te. Temperature being a critical factor during

nanoparticle growth determines the crystalline structure formation and thus,

synthesis at low temperature always favors Zinc blende structure (Jose et al.,

2004). However, it appears that bulk CdTe is known only in the Zinc blende

form under ambient conditions possessing tetrahedral coordination of every

atom (Ratcliffe et al., 2006).

1.3.2. Optical characterization of CdTe QDs

Photo-absorption and fluorescence studies revealed the synthesis of

CdTe QDs. CdTe QDs with different emission peaks were synthesized under

conditions of varying pH, different ratios of precursors and at different reaction

time. Photo-absorption and fluorescence properties of these CdTe QDs

revealed the effects of above parameters on their surface modification and

crystal growth. Photo-absorption spectrum of QDs appeared as a series of

overlapping peaks revealing the multiple energy states in these particles that

allow the possibility of excitation at shorter wavelengths (Fig. 1.2).

1.3.2.1. Studies on surface stabilizing ligand (capping agents) on QD

synthesis

Growth of QD may be controlled by tailoring the ratio of the

concentration of precursors to that of stabilizing ligands. Generally high

stabilizer concentration initially favors the formation of small nuclei and thus


Chapter 1

70

smaller particles. Fig. 1.8 and 1.9 show the absorption and fluorescence

spectra of CdTe QDs synthesized at different molar ratio of MPA respectively.

Presence of first excitonic peak at lower wavelength indicates quantum

confinement effect in these particles.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

350 400 450 500 550 600 650 700

Ab

so

rba

nce

Wavelength in nm

7 6

5

4

3

2

1

Fig. 1.8 Effect of capping agent (MPA) concentration on photo-absorption

spectra of CdTe QDs. (1) 0.07 M, (2) 0.08 M, (3) 0.04M, (4) 0.06

M, (5) 0.05 M, (6) 0.03 M and (7) 0.02 M

0

100

200

300

400

500

600

700

800

900

1000

400 450 500 550 600 650

RF

U

Wavelength in nm

1

2

3

4

5

6

Fig. 1.9 Effect of capping agent (MPA) concentration on fluorescence

spectra of CdTe QDs. (1) 0.02 M, (2) 0.03 M, (3) 0.04M, (4) 0.05

M, (5) 0.06 M and (6) 0.07 M


Chapter 1

71

It was observed that effective molar ratios of 1:2:0.5, 1:2.5:0.5 and

1:3:0.5 (Cd2+:MPA:HTe-) resulted in CdTe QDs with better fluorescence

quantum yield. Fluorescence was stronger for QD synthesized at 1:2.5:0.5

followed by 1:2:0.5, 1:3:0.5 respectively and weaker with gradual increase in

the concentration of MPA. This could be due to an unfavorable environment

created by surface crowding for surface construction and passivization. On

the other hand, molar ratio of Cd2+:MPA below 1:2 fails to synthesize QDs.

Reason could be surface defects resulting in non radiative couplings, as lower

concentrations of MPA were not sufficient for proper surface coverage (Acar

et al., 2009). In case of MEA, better fluorescence was obtained with effective

molar ratios of 1:1.5:0.5 followed by 1:1:0.5. However, QDs synthesized with

ME resulted in poor fluorescence and with PA there was no detectable

fluorescence (Fig. 1.10).

0

200

400

600

800

1000

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

RF

U

molar Concentration

MPA

ME

MEA

Fig. 1.10 Effect of capping agent concentration on fluorescence intensity

(RFU) of CdTe QDs


Chapter 1

72

1.3.2.2. Influence of reaction time on QD synthesis

Temperature is one of the critical parameter during QD synthesis as it

influences the reaction rate. QD size increases with increasing reaction time

at high temperature due to increase in the rate of addition of precursors

(Cadmium and Tellurium ions) in to the existing nuclei. In general,

temperature determines the critical size for the stable primary particles (Acar

et al., 2009). Therefore, temperature must be high enough to allow the

rearrangement and annealing of atoms during surface modification of primary

particles. Further, time of refluxing allows QDs growth based on Ostwald

ripening that reflects on photoluminescent properties of QD. Fig. 1.11 and Fig.

1.12 shows the evolutional fluorescence peak position of CdTe QDs

synthesized at 100 C at different refluxing period.

0

50

100

150

200

250

300

450 500 550 600 650

RF

U

Wavelength in nm

0 min

15 min

30 min

60 min

90 min

120 min

150 min

180 min

210 min

240 min

270 min

300 min

330 min

360 min

Fig. 1.11 Effect of reaction time on fluorescence spectra and homogeneity of

CdTe QDs


Chapter 1

73

45

50

55

60

65

70

75

80

0

50

100

150

200

250

300

0 50 100 150 200 250 300 350 400

FW

HM

RF

U

Reaction time in minutes

RFU

FWHM

Fig. 1.12 Effect of reaction time on fluorescence intensity and FWHM of

CdTe QD

There was a shift in the emission peak towards longer wavelength with

prolonged refluxing period. Initial emission peak was observed at 522 nm after

15 minutes of refluxing that shifted to 633 nm after 360 minutes. Initially, there

was a gradual increase in the luminescence intensity up to 120 minutes of

refluxing that started decreasing slowly with continuous refluxing (Fig. 1.11).

Moreover, FWHM that determines the homogeneity of QD growth was narrow

up to 90 minutes and increased gradually with refluxing time (Table 1.2).


Chapter 1

74

Table 1.2 Effect of refluxing time on CdTe QD growth and its optical

properties

Refluxing

time

(in minutes)

ex em RFU FWHM

(nm)

Stokes

shift (nm)

15 470 522 68.505 51 52

30 480 529 121.055 49 49

60 520 543 190.178 50 23

90 520 555 242.987 49 35

120 510 560 270.123 51 50

150 520 570 262.732 57 50

180 520 583 230.965 65 63

210 520 597 211.919 69 77

240 520 601 208.999 71 81

270 530 611 195.784 73 81

300 530 621 191.882 74 91

330 530 627 171.821 77 97

360 530 633 158.203 77 103

It was observed that there was a visible change in the emission color of

CdTe QDs (Fig. 1.13) that was ascribed to the electronic structure of

nanoparticle resulting in a direct correlation between crystal size and band

gap energy. It was reported by Acar et al. (2009) that surface adsorption-

desorption might cause surface defects in nanoparticle resulting in


Chapter 1

75

nonradiative combination of electron and the hole. Initial increase in the

fluorescence could be due to generation of particles with small size resulting

in increased electron transfer rates with better surface coordination (Acar et

al., 2009; Li et al. (2007).

Fig. 1.13 Effect of reaction time on CdTe QD growth resulting in shift in

coloration and emission wavelength towards red end of

electromagnetic spectrum

However, desorption of MPA from QD surface might have left

uncoordinated sites associated with Te atoms that has resulted in dangling

bond, which act as hole traps. Thus, due to these surface defects CdTe QD

emission was red shifted resulting in increased FWHM and loss of QY (Byrne

et al., 2006). Stokes shift is also an important characteristic feature while

determining the optical properties of QDs. Stokes shift in QDs generally

depends on the thickness of surface capping material and quantum

confinement effect that influences the absorption and the emission


Chapter 1

76

wavelengths. Stokes shift is an additional observation due to frequency shifts

during phonon emission that couples the fluorescence and absorption spectral

profiles. Prolonged reaction time resulted in increased Stokes shift suggesting

efficient electron-phonon coupling (Ipatova et al., 2001).

1.3.2.3. Effect of pH on QD synthesis

Nanoparticle growth also depends on pH values of the reaction

medium as it influences the nucleation and stability of primary particles. Fig.

1.14 and Fig. 1.15 displayed the impact of pH on the photoabsorption and

fluorescence properties of MPA capped CdTe QDs respectively. Cadmium

ions react with MPA to form a complex whose solubility and stability depends

on pH of the reaction medium. Initially, Cd2+-MPA complex formed a white

precipitate that was insoluble at lower pH. The complex found soluble above

pH 4.0 but growth of CdTe QD was observed in presence of sodium hydrogen

telluride only above pH 5.5. Fluorescence also strongly depends on the pH

value of the reaction medium due to the possibility of structural changes on

the surface. First excitonic peak and fluorescence was observed for CdTe

QDs synthesized above pH 5.5 ± 0.2 with MPA. Intense excitonic peak and

fluorescence was observed for QDs synthesized at pH 9 ± 0.2.


Chapter 1

77

0

0.2

0.4

0.6

0.8

1

400 450 500 550 600 650 700

Ab

so

rban

ce

Wavelength in nm

pH 5.5

pH 6.5

pH 7

pH 8

pH 9

pH 10

pH 11.5

pH 12

Fig. 1.14 Effect of pH on photo-absorption spectra of MPA capped CdTe QDs

0

100

200

300

400

500

600

700

450 500 550 600 650

RF

U

Wavelength in nm

1

2

3

4

5

6

7

8

Fig. 15 Effect of pH on fluorescence spectra of MPA capped CdTe QDs.

(1) pH 5.5, (2) pH 6.5, (3) pH 7, (4) pH 8, (5) pH 9 (6) pH 10, (7) pH

11.5 and (8) pH 12


Chapter 1

78

The wavelength at which both excitonic peak and fluorescence

observed were almost narrower up to pH 9 ± 0.2 and shifted towards longer

wavelength gradually after pH 9 ± 0.2. CdTe QDs synthesized at pH 11.5 ±

0.2 also showed intense fluorescence but QDs synthesized at pH 9 ± 0.2

were found to be more homogenous as FWHM was lesser compared to QDs

synthesized at pH 11.5 ± 0.2 (Fig. 1.16 and Fig. 1.17).

450

470

490

510

530

550

570

590

610

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

5 6 7 8 9 10 11 12 13

Wa

ve

len

gth

in

nm

Ab

so

rba

nc

e

pH

AbsWavelength

Fig. 1.16 Effect of pH on absorption at first excitonic peak of MPA capped

CdTe QDs

35

40

45

50

55

60

65

70

75

80

0

100

200

300

400

500

600

700

2 3 4 5 6 7 8 9 10 11 12 13

FW

HM

RF

U

pH

RFUFWHM

Fig. 1.17 Effect of pH on homogeneity (FWHM) of MPA capped CdTe QDs


Chapter 1

79

Since QD was synthesized at basic pH coordination bond can occur

only through thiol moiety as carboxylic acid do not ionize to carboxylate ion at

basic pH due to its low pK values (Gao et al., 1998). It was reported that the

coordination between the carboxyl groups and cadmium ions can effectively

improve the photoluminescence (PL) efficiency of the CdTe nanoparticles by

diminishing the nonradiative channel for electron-hole recombination (Zhang

et al., 2003). However, coordination between the carboxyl groups and

cadmium ions are not favorable for bioconjugation applications. Hence, to

retain free carboxylic acid functional moiety necessary for bioconjugation

applications CdTe QD has been synthesized at basic pH. The concentration

of the cadmium-thiol complexes dramatically decreases as the pH of the

solution decreases from a neutral to acidic range releasing free thiols and

cadmium ions. Thus, excess MPA together with cadmium ions will form a

shell around the surface of CdTe particles at acidic pH. Thus, increase in

FWHM and diminished fluorescence suggests the influence of pH on

nanoparticle surface.

Studies conducted with other capping agents such as MEA and ME

exhibited difference in their optimum pH for QD synthesis. In case of MEA,

maximum fluorescence yield was observed in the range of pH 5-6. However,

stability of MEA capped QDs was not as good as MPA capped QDs which

was in accordance with the earlier report by Gaponik et al. (2002). Whereas,

fluorescence yield of ME capped QDs was poor (Fig. 1.10). Each type of

capping agents has its own advantages and disadvantages, however

considering the future bioconjugation work MPA was chosen as an ideal

capping agent.


Chapter 1

80

1.3.3. Size characterization of CdTe QDs by absorption and

fluorescence profiles

The average particle size of CdTe QDs was determined from the

absorption spectral 1s-1s electronic transition (Table 1.1). Size of each CdTe

QD was calculated based on the empirical relation between size (diameter in

nm) and respective band gap energy in eV given by Donega and Koole,

(2009). Here energy of the wavelength (in eV) at first excitonic peak which is

the lowest excited energy state or band edge absorption of respective CdTe

QDs was considered as band gap energy. Size was also calculated based on

the relation between band gap energy of CdTe bulk, CdTe synthesized, size

in radius, reduced mass of electron/hole and exciton Rydberg energy, which

accounts for spatial correlation between the excited electron and hole

according to equation 2 given by Kayanuma, (1986 and 1988). Donega and

Koole, (2009) have reported that equation 2 fails to quantitatively describe the

size dependence because the effective Coulomb interaction and the self-

polarisation energies are not properly evaluated. However, equation

adequately describes the transition energy shifts in the strong confinement

regime. Yu et al. (2003) has reported another polynomial relation between

wavelength of the first excitonic peak (in nm) and size (diameter in nm) of

CdTe QD that was also used to calculate the size of QDs synthesized. With

respect to the above there was a shift in the first excitonic peak and

fluorescence towards higher wavelength with decrease in band gap energy

suggesting quantum confinement effect. More the band gap, higher will be the

energy required for electrons to cross the energy barrier to attain excited

conduction band region. Therefore, band edge absorption will be at lower


Chapter 1

81

wavelength and particles will be smaller. On the other hand, smaller the band

gap lesser will be the energy requirement for electrons and band edge

absorption will be at higher wavelength suggesting bigger particles. This is

because larger the QD size, energy levels will be more and are more closely

spaced allowing the QD to absorb photons containing less energy. The

particle size of CdTe QD synthesized were inversely proportional with band

gap energy (Table 1.1). Thus, shift in the photo-absorption and fluorescence

peak were correlated to the size of the material. This in turn depends on the

band gap energy, which is determined by the quantum confinement

contribution of the synthesized particle (Donega and Koole, 2009).

AFM and TEM revealed morphological characteristics and surface

topological information of the CdTe surface structure. The height variation plot

across wide-area AFM image showed even-sized particles (Fig. 1.18).

Fig. 1.18 Atomic force micrograph of CdTe557


Chapter 1

82

TEM has further confirmed the size and distribution of CdTe557 and

CdTe601 that was calculated by mathematical equations based on

observations of optical characteristics (Table 1.1). Both QDs appeared as

spherical crystalline particles with a narrow size range of 3 ± 0.2 nm and well

dispersed in aqueous medium without any aggregation (Fig. 1.19).

20 nm

20 nm

Fig. 1.19 Transmission electron micrograph showing CdTe557 (a & c),

CdTe601 (b) appeared as spherical crystalline particles with a

narrow size range of 3 ± 0.2 nm and well dispersed in aqueous

medium

1.3.4. FWHM and QY of CdTe QDs

FWHM that determines the homogeneity and purity of CdTe QDs

synthesized was in the range of 45 nm to 80 nm. Fluorescence efficiency

(QY) was also calculated in comparison with Rhodamine 6G. CdTe557 whose

band gap was 2.47 eV had a calculated QY of 33.046% and FWHM of 60 nm.


Chapter 1

83

Therefore, CdTe557 was selected for further work on bioconjugation studies.

Although, FWHM for CdTe QDs emitting at 516 nm and 576 nm were in the

expected range their QY were significantly low. However, QY of CdTe523

(25.078%) and CdTe601 was satisfactory (22.234%) but FWHM of CdTe601

was considerably higher (80 nm). The average size of CdTe601 was 3.06 nm

as calculated (Table 1.1).

1.4. CONCLUSIONS

In brief, aqueous route of synthesis provided biocompatibility for QDs

aimed at biosensing applications. CdTe QDs were synthesized at an effective

molar concentration of 1:2.5:0.5 respectively for Cd2+:MPA:HTe- at pH 9 ± 0.2.

CdTe557 which was approximately 2.61 nm in size having band gap of 2.47 eV

unveiled fluorescence quantum yield (QY) up to 33% with a narrow spectral

distribution. The powder X-ray diffraction profile elucidated characteristic

broad peaks of zinc blende cubic CdTe nanoparticles with 2.5-3 nm average

crystallite size having regular spherical morphology as revealed by

transmission electron microscopy (TEM). Infrared spectroscopy confirmed

disappearance of characteristic absorptions for -SH thiols inferring thiol

coordinated CdTe557 nanoparticles. Systematic investigations on photo-

absorption and fluorescence studies revealed the effects of varying pH, molar

ratios of precursors, nature of the surface ligands, temperature and reaction

time playing a critical role on surface modification and crystal growth of CdTe

QDs with diverse size having better luminescent properties. Thus, engineering

the size and opto-physical characterizations of QDs are very much essential

for their applications in biosensing and bioanalytical methods.


Chapter 1

84

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Chapter 1shodhganga.inflibnet.ac.in/bitstream/10603/36545/5/chapter 1.pdf · synthesis. Moreover, absorption spectral analysis may provide an insight into band structure, band gap

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