Biomedical Engineering Reference
In-Depth Information
by which surface chemistry may influence the optical and dynamic properties of
QDs. Correspondingly, PL characteristics for colloidal QDs in solution are subject
to various dynamic processes which are related to QD interface properties: (1)
the adsorption of spurious molecules [
28
,
29
], (2) attachment and detachment of
protective electrostatically bound [
43
,
44
], chemisorbed [
39
,
40
] or chelating [
35
]
ligands, and (3) the anchoring of functionalized dye molecules [
23
,
36
-
38
,
45
]or
protein complexes [
46
,
47
] to QD surfaces in hybrid nanoassemblies. In general,
a surface reconstruction due to “mobile traps” [
48
] can be invoked by any of the
ligands mentioned. Typically, changes in solvent or surface-bound ligands have
been found to affect these surface traps and thereby influence steady-state [
49
-
53
]
and time-resolved [
54
,
55
] PL of QDs. It has also been theoretically shown that
surface states split off from the band edge of the valence and conduction band in
case of non-passivated QDs [
56
]. In case of “QD-organic ligand” nanoassemblies
the formation of OD-ligand delocalized (hybridized) surface states may take place
which are mostly inactive in absorption spectra but have significantly influence
on QD PL dynamics [
57
,
58
]. Thus, the kind and concentration of ligands in
nanoassemblies may control the optical energies of surface states and their influence
on the optical properties of QDs which is important upon analysis of QD PL
quenching mechanisms.
Basically there are several routes to realize “QD-Dye” nanoassemblies in liquid
solutions and polymeric matrices in the form of (1) blends [
59
], (2) QD-polymer-
dye composites [
60
,
61
], and (3) self-assembled QD-Dye assemblies via functional
groups [
36
,
38
,
45
,
62
,
63
]. With respect to formation of self-assembled “QD-
Dye” nanoassemblies in the liquid phase (besides attachment/detachment of dye
molecules and the presence/formation of various surface trap states mentioned
above) the competitive interplay of dye molecule attachment and capping ligand
exchange dynamics [e.g., tri-
n
-octyl phosphine oxide (TOPO) or long chain amines
(AM)] [
28
-
34
,
64
,
65
] is of essential importance both from fundamental principles
and from possible applications. In addition, this competitive interplay dynamic
is influenced by temperature [
66
] and the dielectric properties of the solvent
[
67
]. In the past a few studies have been devoted to the elucidation of thiol-
type ligand exchange at relative low concentrations between one and ten relative
to the QD concentration [
33
,
68
]. Recently, “QD-Dye” nanoassembly formation
has been studied under the conditions of extremely low concentrations of QDs
and dye molecules and a microscopic description of QD PL quenching has been
presented [
64
,
65
]. In contrast to the number of bulk measurements on large
ensembles of QDs, there have been far fewer studies on the effects of surface ligands
on the photoluminescence of CdSe QDs at the single-nanocrystal level focusing
predominantly on photoluminescence intermittency (or blinking) investigations
[
69
-
73
,
124
].
However, a better understanding of the effects of ligand binding on single
QD PL is critical to interprete existing ligand binding data derived from solution
photoluminescence measurements. In this respect, the combination of bulk and
single molecule/single nanoassembly experiments [
74
,
75
] is a tool to precisely
identify the interaction of exactly one QD with one dye molecule leading to a
microscopic understanding of the formation (including ligand dynamics) and related
