Page 73 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 59

sources of selenide in trioctylphosphine oxide (TOPO) used as solvent
and capping molecule. The mixture is heated at 230–260 C. It is best to
prepare samples over a period of a few hours of steady growth by modu-
lating the temperature in response to changes in the size distribution as
estimated from the absorption spectra of aliquots removed at regular
intervals. Temperature is lowered in response to a spreading of size dis-
tribution and increased when growth appears to stop. Using this method,
a series of sizes of CdSe nanocrystals ranging from 1.5 to 11.5 nm in
diameter can be obtained (Figure 3.21). These particles, capped with

(a) (b)

115Å

Absorbance (arbitrary units) 37Å 65Å
83Å
51Å
43Å

23Å
–19Å 32Å
–16Å
–12Å
300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
(c)
Figure 3.21 TEM image of (a) well-dispersed CdSe nanoparticles of around
3.5 nm in mean diameter (bar: 20 nm); (b) CdSe crystallite 8 nm in size show-
ing stacking faults in the (002) direction (bar 5 nm); (c) room temperature opti-
cal absorption spectra of CdSe nanocrystallites dispersed in hexane and ranging
in size from 1.2 to 11.5 nm [7].