Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 189

band are responsible for luminescence. 50-59 For Au NCs that comprise
a Au core of 28 atoms and a GSH layer of 16 molecules [i.e.,
Au 28 (SG) 16 ], the two visible luminescence wavelengths correspond
to an excitation of the d band electrons into the sp conduction band
(interband transition) and to radiative recombination after being
excited at 2.48 eV (500 nm) (Fig. 9.2A,B). 59 Because the visible
luminescence is independent of the excitation wavelength between
3.10 and 2.07 eV (400 and 600 nm), the high energy luminescence
band corresponds to the recombination of the excited electron from
higher excited states in the sp band with the hole in the lower-lying
d band (interband transition). The low energy luminescence band is
assigned to the relaxed radiative recombination across the HOMO-
LUMO gap at 1.3 eV within the sp conduction band (intraband
transition) (Fig. 9.2B). To make it easier to understand the transitions
for the luminescence bands, let us consider well-known transitions
in luminescence: two luminescence bands corresponding to the two
radiative transitions from the lowest excited singlet state (S 1 , referred
to as luorescence) and the lowest excited triplet state (T 1 , referred
to as phosphorescence). 59 Accordingly, the two luminescence bands
served for Au 28 (SG) 16 can be assigned to the singlet (luorescence)
and triplet (phosphorescence) excited states of the molecule-type
Au NC (Fig. 9.2C).
(B)
Figure 9.2 (A) Luminescence spectrum of Au 28 (SG) 16 NCs in D 2 O. (B) Solid
state model for the origin of the two luminescence bands. (C)
Molecular model for the origin of the two luminescence bands.
Reprinted with permission from Ref. 59.