**3. Results and discussion**

The colloidal suspensions of Cd-Se quantum dots of increasing size from left to right are shown in Figure 1.

Fig. 1. Colloidal suspensions of Cd-Se quantum dots

The samples viewed in ambient light vary from green-yellow to orange-red. These changes in color which have been noted by other workers1, 10 are attributed to the increasing size of the Cd-Se nano-crystals.

Figure 2 presents the calculated diameter of nanocrystal with time1. The diameter of nanocrystals increases with increasing time. As the nano-crystal size increases, the energy of the first excited state decreases.

The Cd-Se nano-crystals stay suspended in solution and cannot be filtered out. The oleic acid acts as a surfactant, binding to the exterior of the crystal lattice and allowing for the

UV-Vis spectrophotometer (Perkin Elmer Lambda 950) was used for spectroscopic measurements. The scan speed was 11.54 nm /min, integration time was 0.52 s and the data interval was 0.10 nm. The hotplate was Labcongo (115 V, 12 A) and the heating was set at level 3. All chemicals used were bought from Sigma-Aldrich and were of analytical grade. 60 mg of Se, 10 cm3 of 1-octadecene and 0.8 cm3 of trioctylphospine were mixed together in a round-bottomed flask. The solution was then continuously stirred with a magnetic stirrer on a hot-plate and warmed for a few minutes in a fume-hood. Separately, 26 mg of CdO was added to a 25 cm3 round-bottomed flask and clamped in a heating mantle. 1.2 cm3 of acid and 20 cm3 of octadecene were added and mixed together. The solution was heated until CdO dissolved. The CdO solution was then sub-divided into 5 Erlenmeyer flasks each containing 4 cm3 of the stock solution. 0.5 cm3 of Se stock solution was then transferred into the CdO solution with pipette. The samples were heated for 50 s, 60 s, 70 s, 80 s and 120 s,

The colloidal suspensions of Cd-Se quantum dots of increasing size from left to right are

The samples viewed in ambient light vary from green-yellow to orange-red. These changes in color which have been noted by other workers1, 10 are attributed to the increasing size of

Figure 2 presents the calculated diameter of nanocrystal with time1. The diameter of nanocrystals increases with increasing time. As the nano-crystal size increases, the energy of

The Cd-Se nano-crystals stay suspended in solution and cannot be filtered out. The oleic acid acts as a surfactant, binding to the exterior of the crystal lattice and allowing for the

**2. Experimental** 

respectively.

shown in Figure 1.

the Cd-Se nano-crystals.

the first excited state decreases.

**3. Results and discussion** 

Fig. 1. Colloidal suspensions of Cd-Se quantum dots

crystal to remain soluble in the octadecene10. The diameter of the nanocrystal was calculated using Kippeny1 method and was found to be in the range found by other workers10. The Cd-Se crystal growth has been found to be temperature dependent. Transmission electron microscope (TEM) measurements of Cd-Se nanocrystals by others suggest that such wavelengths correspond to 2- 4 nm diameter crystals10 with at most a few hundred atoms.

Fig. 2. The effect of particle size growth with time

Figure 3 presents ground state peak wavelengths as a function of reaction time. As the reaction progresses, the peak wavelength decreases. As nanocrystals grow, it has been suggested that their peak emission quickly approaches the band gap of bulk Cd-Se (730 nm).

The observable peak maximum shifts from violet to green with increasing crystal size. The absorption shows peak maxima with additional absorption at lower wavelengths due to the starting materials and oleic acid polymerization. Heating oleic acid and octadecene alone yields increasing visible absorption at increasing wavelengths over time as the effects of oleic acid polymerization become noticeable.

Figure 4 presents UV-Vis spectra of Cd-Se colloidal nanocrystals. The scan range was between 400 nm to 600 nm. The maximum peak shifted toward the longer wavelength. This observation is expected because as the crystal size increases, the energy absorbed or emitted decreases. The sample heated for only 50 sec did not show any peak.

Synthesis and Characterization of CdSe Quantum Dots by UV-Vis Spectroscopy 85

2. The interior of the nanocrystal consists of uniform medium and the excited electron and

3. The potential energy outside the radius R is infinite—the radius R defines the confining

The solution to the spherical Schrödinger equation leads to the energy of the exciton—

1 1 1.8

*ex k e h CdSe k h e e S <sup>E</sup> R m m RR R*

 

The first term is the kinetic energy and the second term the Coulomb potential attractive

At small R the predominant term is the first term (because of the inverse square R

We can therefore use the first term to approximate R the radius of the nanoparticles as

The energy needed to create the first peak – corresponding to the peak position in the

2 2 2 2

 

1 1 0.78

*e h <sup>h</sup> <sup>E</sup> eV R m m* Using h as Planck's constant ; the electron effective mass me = 0.13 mass of a free electron

0 1

*k*

The energy then converts to 2.48 eV (using the well known conversion formula 1.24

The energy gap of bulk CdSe corresponds to 730 nm (0.73 μm ) and is 1.70 eV [1,10]

1 1 1.8

2 2

*ex k e h CdSe k h e e S <sup>E</sup> R m m RR R*

 

8 4

8 *ex*

1 1 (0.5) 0.5

).

8 4

spectra is *u g ex EEE* this energy corresponds to 500 nm from Figure 4.

2 2 2 2

 

0 1

*k*

1*m* for

1. The nanocrystal is spherical with a radius R.

2

energy; and the third term is the polarization energy. **b. Using the first term to calculate the exciton energy** 

dependence since R<1 for a simple example <sup>2</sup>

This leads to the exciton energy of 0.78 eV

2

and the first term alone as the approximation for small R

and mh equals 0.45 times the free electron mass R can then be calculated to be the following:

hole pair.

electron hole pair as1:

follows:

photon energy to eV.

Using the formula:

boundary of the box.

Fig. 3. The change of wavelength with reaction time

Fig. 4. UV-Vis spectra of Cd-Se colloidal suspension
