39

CHAPTER 2

EFFECT OF MICROWAVE-IRRADIATION ON CdTe

NANOCRYSTALS CAPPED BY MERCAPTOACETIC ACID

SYNTHESIZED BY WET CHEMICAL ROUTE

2.1 INTRODUCTION

Semiconductor nanocrystals (NCs) show unique size-dependent

optical properties and are currently of utmost importance both for basic

research in physics, chemistry, biology, and colloidal science (Medintz et al

2005, Peng and Peng 2002, Peng et al 2000, Coe et al 2002) and for

application in biomedical labeling, light emitting diodes (LEDs), solar cells,

lasers, and other fields (Colvin et al 1994, Steckel et al 2003). CdTe

nanostructures are among the best-studied semiconductor quantum systems.

CdTe nanoparticles (NPs) synthesized in organic systems are passivated by

long chain organic surfactants such as trioctylphosphine oxide (TOPO)

(McGinley et al 2002) and alkylamines. Nanocrystals soluble in an aqueous

medium have been investigated for their potential applications, because of

their many advantages such as compatibility with biomaterials and low

toxicity (Gaponik et al 2002). Recently, researchers have studied numerous

methods to synthesize water-soluble semiconductor nanoparticles, which can

be obtained mainly by two different methods. The first way is to replace the

surface-capping ligands on the particles prepared by the TOPO

(trioctylphosphine oxide) method with water-soluble thiols or a silica shell

(Mattoussi et al 2000, Aldana et al 2001). But this method brings about

another problem. After the substitution of the surface-capping molecules by

40

hydrophilic molecules, the nanocrystals photoluminescence decreases

markedly (Talapin et al 2002). Another choice is to synthesize semiconductor

nanoparticles directly in aqueous solution using water-soluble stabilizers such

as thiols. One of the most successful cases was the thiocarboxylic acid-capped

CdTe nanocrystals prepared through wet chemical synthesis route

(Abd El-sadek et al 2009).

Microwave irradiation is an attractive method for synthesis of

nanocrystals. The synthesis of nanocrystals by microwave irradiation was first

introduced by the Kotov group (Correa-Duarte et al 1998). Some studies

showed that the synthesis of nanocrystals by microwave irradiation was

generally quite faster, simpler, and very energy efficient as compared to

conventional hydrothermal synthesis (Grisaru et al 2003). Recently,

researchers have reported a new method for rapid synthesis of high-quality

CdTe QDs and alloy CdSe-CdS nanocrystals using microwave irradiation

with a controllable temperature (Li et al 2005 a, Qian et al 2005). Water is an

excellent solvent for microwave-assisted synthesis since its tan

0.127. The aqueous solvent will be rapidly heated above 100 °C in a sealed

vessel by microwave irradiation. At the elevated temperature, the synthesis

rate will be improved dramatically, and some reactions that cannot occur at

room temperature will rapidly take place. The combination of using water as a

solvent to synthesize nanocrystals and applying microwave irradiation as an

efficient heating source is a very desirable way to make the synthesis of

nanocrystals cleaner and more environment-friendly (Leadbeater 2005).

On the other hand, microwave dielectric heating is fast emerging as

a widely accepted new processing technology. With comparison to

conventional thermal techniques, microwave dielectric heating has three

dominating merits: (a) temperature can be rapidly raised due to high

utilization factor of microwave energy, and the kinetics of the reaction rate are

41

increased by 1-2 orders of magnitude, (b) thermal gradient effects can be

effectively reduced due to the volumetric heating of microwaves, which is

favorable for realizing homogeneous heating and producing a more uniform

product formation, and (c) reaction selectivity is enhanced since different

kinds of substances have a varied dipole constant (Michael et al 1991, Galema

1997). Therefore, microwave methodology has been widely used in various

fields including plasma and analytical chemistry, chemical catalysis, and

organic reactions (Hueso et al 2005). Besides these, it is a rapidly developing

area of research, which has attracted much attention as a new method for

preparing nanoscale inorganic compounds (Brooks et al 2005). It was also

demonstrated that high-quality quantum dots could be successfully prepared

by microwave dielectric heating (Gerbec et al 2005). In addition, CdTe NCs,

whose Photo-Luminescence Quantum Yield (PLQY) was increased to 60%

under optimum conditions, were obtained by microwave irradiation in the

aqueous phase (Li et al 2005 a).

A simple synthetic route for synthesis of different sizes of CdTe

nanocrystals in aqueous system upon addition of Mercaptoacetic Acid (MAA)

[Thioglycolic Acid (TGA)] (Figure 2.1) as a stabilizer at room temperature

was discussed. The effect of nanocrystal size on the absorption bands and

photoluminescence emission are also investigated. Also, initial pH value of

the precursors solution was changed to study the effect of pH value on the

optical properties of CdTe nanocrystals. Microwave irradiation method was

extended to the synthesis of water-soluble CdTe nanocrystals in aqueous

phase. The effect of microwave irradiation on synthesis of CdTe nanocrystals

was studied. A series of nanocrystals with different sizes and optical

properties were prepared within ten minutes, which was several times faster

than conventional aqueous synthesis.

42

Figure 2.1 Chemical structure of Thioglycolic acid (TGA)

2.2 PREPARATION OF THIOCARBOXYLIC ACIDSTABILIZED CdTe NANOCRYSTALS USING Te (VI) PRECURSOR

2.2.1 Introduction

Colloidal nanoparticles have been investigated for many years, and it has been reported that thiolates are effective stabilizer agents for preparation of nanometer-sized particles (Thangadurai et al 2008, Rogach 2000). Mercaptoacetic Acid (MAA) is a kind of thiolate that has been used in preparation and assembly of CdS (Wageh and Badr 2008) and CdTe (Rogach et al 2007, DeMenezes et al 2005, Abd El-sadek et al 2009) nanoparticles as the stabilizer agent. Mercaptoacetic Acid (MAA) is a bifunctional molecule, which contains both thiols and carboxylic acid groups. Both carboxylic acids and thiols are known to be absorbed on metal surface, forming well-packed monolayers (Han et al 1999, Tao 1993). CdTe nanoparticles were prepared in aqueous solution with H2Te or NaHTe as a source for tellurium (DeMenezeset al 2005, Gaponik et al 2002) in the literature. Al2Te3 lumps were used as

43

source for the generation of H2Te and are transported to the reaction chamber with a slow nitrogen flow to synthesis the precursors. These precursors are then converted to CdTe nanocrystals by refluxing the reaction mixture at 100 C. The refluxing time controls the size of the particles. When sodium hydrogen tellurite (NaHTe) was used as source, it was synthesized with suitable reaction of sodium borohydrate with tellurium in water. The reaction was carried out in an ice cooled reacting system and proper care should be taken to avoid the excess pressure from resulting hydrogen. In order to overcome, these cumbersome processes to get the tellurium source, efforts were made to use potassium tellurite as tellurium source in the present work(Abd El-sadek et al 2010).

Potassium tellurite was used as tellurium source to prepare different sizes of CdTe nanoparticles by wet chemical route and microwave-assisted method with different pH values as 7.4, 10.5 and 11.2 and stabilized with Mercaptoacetic Acid (MAA). A detailed experimental procedure is briefly in described in the following paragraphs.

2.2.2 Synthesis Procedure All chemicals used were of the highest purity available. De-ionized

water obtained from a Millipore Milli-Q Plus purification system was used in all experiments. A chemical and microwave-assisted method has been used to synthesize CdTe nanoparticles (Gaponik et al 2002, DeMenezes et al 2005, Li et al 2005 a, Li et al 2006) with modifications in the choice of the chemicals.

In the wet chemical method, the precursor, CdCl22.5H2O (0.046 g) was dissolved in 250 mL of de-ionized water in the presence of 0.01 M of Mercaptoacetic Acid (MAA) and the pH of the solution was adjusted to different pH values (7.4, 10.5, and 11.2) using 1 M KOH. Occasionally, the solution can remain slightly turbid at this stage because of the incomplete solubility of Cd thiolate complexes, but this does not influence the further

44

synthesis. The solution was placed in a three-necked flask fitted with a septum and valves as shown in Figure 2.2 and was deaerated by N2 bubbling for 30 min (Solution A). Solution B contains 0.063 g of potassium tellurite dissolved in 250 mL of de-ionized water in the presence of 0.01 M ofMercaptoacetic Acid (MAA) (Te:RSH is 1:10). An equal volume of solution A and B are mixed together under continuous (vigorous) stirring. The solution was stirred for two hours at room temperature then CdTe precursors were formed at this stage. The molar ratio of Cd2+/MAA/Te2- was set as 1:10:1.

Figure 2.2 Experimental setup for the synthesis of CdTe semiconductor nanocrystals, (R represents a carboxylic acid group)

In the microwave assisted method, an equal volume of solution

A and B are mixed together and allowed to stir for 30 min, then CdTe precursor solution was injected into the exclusive vitreous vessel with a volume of 50 ml, and reacted in microwave furnace (GMS 19 A Microwave Oven as shown in Figure 2.3) for 5 minutes. In general, CdTe precursor occupied 1/6 volume of the vessel, and air filled the rest of the space. The reactions were operated at 700 W power in a cyclic mode (on: 15 s, off 6 s) to avoid intense boiling of the solution. The reaction was stopped by removing the vessel from the microwave furnace and cooling naturally to room temperature. The size and optical properties of the CdTe nanoparticles were

45

controlled by controlling the microwave irradiation time and the concentration of Cd2+, Te2- and MAA.

Figure 2.3 GMS 19 A Microwave Oven for the synthesis of CdTe

nanoparticles

The method of size selective precipitation was used to isolate the

nanoparticles. Isopropyl alcohol was added to the solution of the crude

products until the large particles started to precipitate. The mixtures were

stirred at room temperature, after addition of the nonsolvent, and the

supernatant and precipitate were, then, separated by centrifugation. This

procedure was repeated several times. After isolation, all nanocrystals were

purified by reprecipitation and washed three times with acetone or alcohol.

The wet precipitates were dried and stored as free standing powders. Six

samples were prepared having different sizes (Table 2.1), hereafter called (S1,

S2, S3, S4, S5, and S6).

46

Table 2.1 Sample name, Preparation method, pH values, Absorption edges and Scherrer sizes of CdTe nanoparticles capped with Mercaptoacetic Acid

Samples Preparation method pH Scherrer size (nm)

Absorption edge (nm)

S1 Wet chemical route 7.4 8 495S2 Wet chemical route 10.5 10 509S3 Wet chemical route 11.2 12 530S4 Microwave method 7.4 15 540S5 Microwave method 10.5 17 562S6 Microwave method 11.2 20 574

2.3 STRUCTURAL CHARACTERIZATION OF CdTe NANOCRYSTALS SYNTHESIZED BY WET CHEMICAL AND MICROWAVE-ASSISTED METHOD

2.3.1 X-ray Diffraction

X-ray powder diffraction analysis was carried out with a Philips

X’Pert PRO diffractometer with CuK ( = 0.15406 nm) radiation. Thiol-

stabilized CdTe nanoparticles belonging to the cubic (zincblende, see Figure

2.4) crystalline phase were formed in aqueous solution by reaction of Cd2+

and Te2- ions in the presence of thiols as stabilizers. The X-ray diffraction

patterns for CdTe nanocrystals (pH = 7.4, 10.5 and 11.2 for two different

preparation methods) with different sizes are shown in Figure 2.5. The

positions of the diffraction peaks matched the cubic phase of CdTe

(zincblende phase) and CdTe samples exhibit four peaks that can be assigned

to (200), (220), (222) and (400) diffractions corresponding to cubic

zincblende structure. These peaks are Scherrer broadened due to finite

crystalline size. The crystallite size, DXRD, was further estimated from XRD

patterns by determining the full-width-half-maximum (FWHM) of

characteristic peak (200), and utilizing the Debye-Scherrer equation for

determining the radius of nanoparticles (spherical particles) (Equation 1.3)

47

(Guinier 1963). The calculated nanocrystallite sizes from equation 1.3 are

shown in Table 2.1. It was observed that the (200) peak shift to lower

scattering angles as the nanoparticles size and pH value are increased. This

behavior was attributed to an isotropic lattice contraction of a few percent due

to surface tension during surface reconstruction in the growth of nanocrystals

(Zhang et al 2002). The degree of crystallinity was better with tellurite source,

compared with the nanocrystals synthesized with NaHTe solution (without

refluxing) (Gaponik et al 2002).

Figure 2.4 The zinc blende structure of CdTe

Figure 2.5 Powder X-ray diffraction pattern of CdTe nanoparticles,

(S1, S2, S3, S4, S5, and S6)

48

2.3.2 Energy-Dispersive X-ray Analysis (EDAX)

The as-prepared powders of nanocrystals were characterized by elemental analysis using EDS Noran System Six (Thermo Electron

Corporation). The EDAX (energy-dispersive X-ray Analysis) results indicate the presence of Cd, Te and S in the CdTe nanoparticles (Figure 2.6). The S peak was attributed to the stabilizer (Mercaptoacetic Acid).

Figure 2.6 EDAX spectrum of CdTe nanoparticles, (S1)

2.3.3 Transmission Electron Microscopy (TEM) Analysis

Transmission electron microscopy (TEM) provides valuable information about the size, shape, and the distribution of the CdTe

nanocrystal samples. Transmission electron microscopy (TEM) was performed by a Philips CM 20 (accelerating voltage of 200 kV) in order to

analyze the size and structure of the resulted samples. Figure 2.7 shows the TEM image, selected area electron diffraction (SAED) pattern and the distribution size diagram of the CdTe nanoparticles (Sample S1). The particles were separately dispered on a Cu grid and transmission electron microscopy

(TEM) investigations of MAA-capped nanoparticles revealed crystalline, free-standing and approximately spherical particles. The SAED image of a selected area displays rings which identify the crystalline structure of the

49

CdTe nanoparticles. The size distribution of as-prepared CdTe nanocrystals was determined by measuring more than 100 particles, which also reveals the

good monodispersity of the sample. Estimates of particle diameters based on the XRD data of MAA-capped CdTe particles suggested a particle diameter of 8 nm (S1 Table.1.), using Debye-Scherrer equation, slightly larger than the average particle size calculated by TEM (7.5 nm). Since the Scherrer method

actually measure the coherence length of the X-rays, any crystal imperfections will cause the calculated size to be smaller than the true size. In our case, the fact that XRD sizes are close to the TEM sizes implies that the samples are crystalline, in the (200) directions.

Figure 2.7 The TEM image, SAED pattern and size dispersion histogram of the CdTe nanoparticles, (S1)

2.4 OPTICAL PROERTIES OF CdTe NANOPARTICLES

SYNTHESIZED BY WET CHEMICAL AND MICROWAVE-

ASSISTED METHOD

2.4.1 Absorption Spectra

The quantum size effect on the optical absorption spectra is well

known for semiconductor nanoparticles. The decrease in particle size shifts

the absorption edge towards the lower wavelength of the electromagnetic

spectrum as the band gap energy of the semiconductors increases. Room

3 4 5 6 7 8 9 10 110

4

8

12

16

20

24

Diameter (nm)

Counts Fitting Data

50

300 350 400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

S1 S2 S3 S4 S5 S6

W avelength (nm)

temperature UV-Visible absorption spectra were measured using a Cary 5E

UV-VIS-NIR spectrophotometer. The absorbance (A) was measured as the

logarithm of the transmission (T =I/Io) with water as a reference. The room

temperature normalized absorption spectra of the CdTe nanoparticles

synthesized by wet chemical route and microwave-assisted method at

different pH values are shown in Figure 2.8. The absorption spectra are

characterized by well defined absorption edges and the absorption peak can be

assigned to excitonic transitions between the lowest electron state (1S1/2) and

hole states (1S3/2). It is clear from the experimental spectra that there is a large

shift of the absorption edge of the nanocrystallites towards shorter wavelength

(higher energy), systematically increasing with decreasing crystallite size and

shifting from the bulk that appears approximately at 826 nm (1.5 eV) (Table 2.1)

(Rajh et al 1993, Abd El-sadek et al 2010), indicating the strong quantum

confinement effects and the effect of pH value, where the absorption edge was

significantly blueshifted as the pH value decreases for two preparation

methods.

Figure 2.8 Absorption spectra for CdTe nanoparticles, (S1, S2, S3, S4, S5,

and S6)

51

2.4.2 Photoluminescence (PL) Spectra

Experimentally, pH-dependent of PL spectra was observed from the colloidal CdTe nanoparticles stabilized by mercaptocarboxylic acids (Gao et al 1998). Photoluminescence measurements were performed at room temperature using JY Fluorolog-3-11 spectrofluorometer. Normalized absorption spectra of the CdTe nanoparticles synthesized by wet chemical and microwave-assisted method at different pH values are shown in Fig. 2.9, at the excitation wavelength exc=350 nm. Generally, the photoluminescence,

for different sizes of CdTe nanocrystals consists of band edge and surface state emissions. The PL peaks were gradually enhanced with increasing pH of precursor solutions from 7.4 to 11.2 for wet chemical and microwave-assisted preparation method. Previous reports (Guo et al 2005, Zhang et al 2003 a), also indicate strong dependence of PL on the pH of solution. The static PLspectra of CdTe nanoparticles (Figure 2.9) show only a sharp band edge luminescence at 493 nm (2.52 eV), 506 nm (2.45 eV), 511 nm (2.43 eV), 517 nm (2.4 eV), 526 nm (2.36 eV), and 537 nm (2.31 eV) for S1, S2, S3, S4, S5, and S6, respectively and close to the absorption maximum, attributed to radiative recombination of electron hole pairs after their relaxation to lowest quantum dot level (the band-edge emission). The corresponding FWHM (the full width at half maximum) of the band-edge luminescence was from 50 nm to 83 nm, which indicate the narrow size distribution of the as-prepared CdTe nanoparticles (Li et al 2005 a). The sharp and narrow emission peaks indicate that they have desirable dispensability, uniformity and good photoluminescenceproperties (Wu et al 2003).

Figures 2.10 and 2.11 shows the Stokes shift against the nanoparticles size for CdTe samples synthesized by wet chemical and microwave-assisted method, respectively. The Stokes shift increases with decreasing size of particles. This increase in the Stokes shift is ascribed to the larger coupling of the electron hole pairs to LO phonons in the emitting state with decreasing size of the nanocrystals.

52

7 8 9 10 11 12 1355

60

65

70

75

80

85

90

Size (nm)

pH = 7.4

pH = 10.5

pH = 11.2

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

W av elen gth (n m )

S1 S2 S3 S4S5S6

Figure 2.9 Photoluminescence spectra for CdTe nanoparticles,

(S1, S2, S3, S4, S5, and S6)

Figure 2.10 Variation of Stokes shift with the size of the nanoparticles

(for S1, S2, and S3)

53

14 15 16 17 18 19 20 2170

75

80

85

90

Size (nm)

pH = 7.4

pH = 10.5

pH = 11.2

Figure 2.11 Variation of Stokes shift with the size of the nanoparticles

(for S4, S5, and S6)

2.5 SPECTRAL STUDIES OF CdTe NANOPARTICLES

SYNTHESIZED BY WET CHEMICAL AND MICROWAVE-

ASSISTED METHOD

2.5.1 FT-IR Spectra

The adsorption of thiols on surface of CdTe nanoparticles were

investigated by FT-IR spectral analysis using a Perkin Elmer FT-IR

spectrometer within the range of 500-4000 cm-1. The FT-IR spectrum of the

Mercaptoacetic Acid (MAA) capped CdTe nanoparticles is shown in Figure

2.12 and the infrared absorption bands of CdTe capped with Mercaptoacetic

Acid (MAA) are shown in Table 2.2.

54

Table 2.2 indicates that the broad peaks in the range from 3435 cm-1 to

3441 cm-1 are present in each spectrum, may be due to the effect of –OH

stretching vibration. The band around 1630 cm-1 present in S4, S5, and S6, may

arise from the bending vibrations modes of water which could have been

adsorbed during preparation or from the atmosphere. The active mode peaks

of CH2 in the capped layer are shifted to lower frequency with respect to that

of Mercaptoacetic Acid (MAA), which nearly occurs at 2940 cm-1 (Wageh

2007).

Figure 2.12 FT-IR spectrum of CdTe nanoparticles capped with

Mercaptoacetic Acid (MAA), (S1, S2, S3, S4, S5, and S6)

55

Table 2.2 Assignment of IR spectra recorded for CdTe nanoparticles

Frequency wavenumber (cm-1) Functional Group/ AssignmentSamples

S1 S2 S3 S4 S5 S6

675 592 635 655 680 681 (C-S)

1227 1230 1229 1223 1223 1225 ((C-O) stretching vibration)

1375 1385 1398 1384 1382 1380 s(COO-)

1582 1596 1587 1572 1564 1566 as(COO-)

2927 2928 2928 2928 2926 2927 (CH2)

3437 3435 3440 3438 3441 3439 OH stretching vibration

The shift in CH2 vibrations to smaller frequency indicates that the

surfactant molecules in the adsorbed state are affected by the field of solid

surface. It is important to note that the S-H vibrations band located around

2580 cm-1 in the spectrum of Mercaptoacetic Acid (MAA) (Colthup et al

1990 a) are disappeared completely in the IR spectrum of CdTe nanoparticles,

while the vibration band of C=O around 1717 cm-1 in the spectrum of

Mercaptoacetic Acid (MAA) shifts to 1582 cm-1 for S1 (Figure 2.12 and Table

2.2). This result implies that the Mercaptoacetic Acid (MAA) was chemically

modified on the surface of CdTe nanoparticles and the COOH was in a state

of COO-, making it possible to use the surface-modified CdTe nanoparticles

as an analogous polyanionic compounds in the molecular deposition process.

On the other hand, the appearance of the bands at 1582 cm-1 and 1375 cm-1

which are attributed to asymmetric as(COO-) and symmetric s(COO-)

vibration, respectively (Colthup et al 1990 b) for the CdTe nanoparticles

indicate that Mercaptoacetic Acid (MAA) is chemisorbed as carboxylate onto

Cd ions on CdTe nanoparticles surface. The carboxylic acid is known to be

absorbed on metal as carboxylate after deprotonation (Han et al 1999, Tao

1993).

56

2.5.2 Raman Spectra

Raman spectra (Figure 2.13 a,b) were recorded on the powdered

samples of CdTe nanoparticles prepared at different pH values by using

Raman Systems-R 3000. Raman bands of CdTe capped with Mercaptoacetic

Acid (MAA) are shown in Table 2.3.

Figure 2.13a The whole range Raman spectra of CdTe nanoparticles,

(S1, S2, S3, S4, S5, and S6)

57

Figure 2.13b Raman spectra (for 1750-500 cm-1 expanded region) of

CdTe nanoparticles, (S1, S2, S3, S4, S5, and S6)

Table 2.3 Assignment of Raman spectra recorded for CdTe nanoparticles

Frequency wavenumber (cm-1) Functional Group/

AssignmentSamples

S1 S2 S3 S4 S5 S6

724 744 782 751.6 803 808 (C-S) 1450.4 1357.4 1461.9 1470.6 1455.9 1467.9 s(COO-) 1657.6 1603 1650.5 1610.7 1644.7 1657.6 as(COO-)

3329 3330 3328 3329 3330 3330 OH stretching vibration

Sharp bands occur at 3330 cm-1, for all the samples of CdTe

nanoparticles prepared with different pH values. They are assigned to the -OH

stretching vibrations. The interaction between the carboxylate and the metal

58

atom can be explained based on the previous studies of carboxylates

(Nakamoto 1997). These interactions were categorized as four types:

monodentate, bridging bidentate, chelating bidentate, and ionic interaction.

The wave number separation, , between the as(COO-) and s(COO-) bands

can be used to diagnose the type of the interaction between the carboxylate

and the metal atom. The largest (200-320 cm-1) was corresponding to the

monodentate interaction and the smallest ( 110 cm-1) was for the chelating

bidentate. The medium range (140-190 cm-1) was for the bridging bidentate.

In the present work, the values of and the type of the interaction between the

carboxylate and the metal ion for IR and Raman peaks are shown in Table 2.4.

Table 2.4 The values of and the type of the interaction between the

carboxylate and the metal ion

Samples values fromIR peaks

values from Raman peaks

The interaction of carboxylate head

and Cd ions

S1 (1582-1375)

= 207 cm-1

(1657.5-1450.4)

= 207.1 cm-1

The monodentate Interaction

S2 (1596-1385)

= 211 cm-1

(1603-1357.4)

= 245.6 cm-1

The monodentate Interaction

S3 (1587-1398)

= 189 cm-1

(1650.5-1461.9)

= 188.6 cm-1

The bridging Bidentate

S4 (1572-1384)

= 188 cm-1

(1610.7-1470.6)

= 139.9 cm-1

The bridging Bidentate

S5 (1564-1382)

= 182 cm-1

(1644.7-1455.9)

= 188.8 cm-1

The bridging Bidentate

S6 (1566-1380)

= 186 cm-1

(1657.6-1467.9)

= 189.7 cm-1

The bridging Bidentate

59

Table 2.4 indicates that the type of the interaction between the

carboxylate and the metal ion change from the monodentate interaction to the

bridging bidentate when the size of nanocrystals increases from 8 to 20 nm.

2.6 CONCLUSION

CdTe nanocrystals were synthesized through a simple wet chemical

route and microwave-assisted method using Mercaptoacetic acid (MAA) as

capping agent and water as solvent. Direct source of potassium tellurite was

used as the source for tellurium, instead of H2Te or NaHTe, which were

synthesized by a cumbersome process. Synthesized CdTe were characterized

by UV-Vis absorption, fluorescence spectroscopy, X-ray diffraction, EDAX,

Raman and Infrared Spectroscopy. The size-quantization effect together with

the surface chemistry was determined using the optical properties of the

nanoparticles. Reasonably strong room-temperature luminescence with a

maximum value depending on the particle size and pH value was observed in

the visible spectral range. In the microwave, the growth rate of the

nanocrystals was greatly enhanced, which is the important prerequisite for

reducing the amount of surface defects in nanocrystals. Both absorption and

PL spectra exhibit “quantum size effects”. The absorption edge shifts from

495 to 574 nm when the particle size increases from 8 to 20 nm with

increasing pH of precursor solutions from 7.4 to 11.2 and correspondingly, PL

peak position shifts from 493 to 537 nm. From the FT-IR and Raman Spectra

it is concluded that the nanoparticles were adsorbed by the –SH groups and the

free carboxylic acid group exists as carboxylate ion and make the capped

particles soluble in water. Also, Raman spectra showed that both thiolate and

carboxylate can bind to Cd ions on the surface of nanoparticles.