5 SWCNTs
Single-walled carbon nanotube (SWCNTs) can be pictured as a single graphene
sheet rolled up seamlessly into a nanocylinder with its diameter ranging from less
than 1 nm to a few nanometers. SWCNTs have unique intrinsic physical and
chemical properties such as Raman scattering cross sections and UV/visible/NIR
absorption [27, 29, 81]. SWCNTs have shown potential for biological and medical
applications because of their intrinsic ability to load both targeting ligands and
chemotherapy drugs, in vitro and in vivo. Because of van Hove transitions across
bandgaps, semiconducting SWCNTs demonstrate intrinsic fluorescence in the
NIR-II window, which are ideal as biological probes because of the inherently low
autofluorescence and large Stokes shift between the excitation and emission bands,
which allows excitation in the biological transparency window near 800 nm while
further reducing the background effects of autofluorescence and scattering.
5.1 Mechanism of NIR-II Emission
Due to the quantum confinement along the transverse direction of a single carbon
nanotube, which can be considered as a quasi-one-dimensional nanomaterial,
SWCNTs feature very sharp maxima of electronic density of states (DOS) called
van Hove singularities in their energy band diagrams. The energies of van Hove
maxima of SWCNTs in a band diagram are mainly dependent on the diameter of the
nanotube and the chiral angle at which the specific nanotube is rolled up from a
single graphene sheet with 2D honeycomb structure (Fig. 20a) [82]. The band
diagram of a semiconducting SWCNT shown in Fig. 20b show great detail to
fully understand the NIR-II photoluminescence process, which can be described in
three consecutive steps:
First, an SWCNT absorbs a photon with the photon energy matching the bandgap
of the E 22 transition, which is the difference in energy between the second valence
band V 2 and the second conduction band C 2 . The absorption of this excitation
photon leads to the excitation of an electron, leaving a hole behind. The excited
electron and the remaining hole form a bound pair called an exciton that is held by
the exciton binding energy.
Second, the electron and the hole undergo rapid nonradiative relaxations to the
lowest level in both valence and conduction bands.
Lastly, the bound pair of electron and hole travel together along the length of the
nanotube through diffusion until they recombine radiatively to give off a fluorescence photon with its energy corresponding to the bandgap across E 11 minus the
exciton energy.
Advancements of Second Near-Infrared Biological Window Fluorophores:. . .
109
Single-walled carbon nanotube (SWCNTs) can be pictured as a single graphene
sheet rolled up seamlessly into a nanocylinder with its diameter ranging from less
than 1 nm to a few nanometers. SWCNTs have unique intrinsic physical and
chemical properties such as Raman scattering cross sections and UV/visible/NIR
absorption [27, 29, 81]. SWCNTs have shown potential for biological and medical
applications because of their intrinsic ability to load both targeting ligands and
chemotherapy drugs, in vitro and in vivo. Because of van Hove transitions across
bandgaps, semiconducting SWCNTs demonstrate intrinsic fluorescence in the
NIR-II window, which are ideal as biological probes because of the inherently low
autofluorescence and large Stokes shift between the excitation and emission bands,
which allows excitation in the biological transparency window near 800 nm while
further reducing the background effects of autofluorescence and scattering.
5.1 Mechanism of NIR-II Emission
Due to the quantum confinement along the transverse direction of a single carbon
nanotube, which can be considered as a quasi-one-dimensional nanomaterial,
SWCNTs feature very sharp maxima of electronic density of states (DOS) called
van Hove singularities in their energy band diagrams. The energies of van Hove
maxima of SWCNTs in a band diagram are mainly dependent on the diameter of the
nanotube and the chiral angle at which the specific nanotube is rolled up from a
single graphene sheet with 2D honeycomb structure (Fig. 20a) [82]. The band
diagram of a semiconducting SWCNT shown in Fig. 20b show great detail to
fully understand the NIR-II photoluminescence process, which can be described in
three consecutive steps:
First, an SWCNT absorbs a photon with the photon energy matching the bandgap
of the E 22 transition, which is the difference in energy between the second valence
band V 2 and the second conduction band C 2 . The absorption of this excitation
photon leads to the excitation of an electron, leaving a hole behind. The excited
electron and the remaining hole form a bound pair called an exciton that is held by
the exciton binding energy.
Second, the electron and the hole undergo rapid nonradiative relaxations to the
lowest level in both valence and conduction bands.
Lastly, the bound pair of electron and hole travel together along the length of the
nanotube through diffusion until they recombine radiatively to give off a fluorescence photon with its energy corresponding to the bandgap across E 11 minus the
exciton energy.
Advancements of Second Near-Infrared Biological Window Fluorophores:. . .
109