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Mao et al. Chem Synth 2023;3:26 https://dx.doi.org/10.20517/cs.2022.41 Page 7 of 33

DOPING STRATEGY
The carrier density is a major parameter that determines the LSPR properties [66-68] . Therefore, the LSPR
feature could be manipulated in a controllable manner with shapes, size, and surface properties that
determined the distribution of free charge carriers or other strategies to tune the density of the carriers,
including the light or electronic doping. We will discuss these details in the following paragraph.

Electronic doping
Electronic doping is an effective method for the dynamic regulation of plasmonic materials [69-71] . For
example, Ou et al. reported an electrochemical lithiation process for forming the doped plasmonic Cu Se
2-x
nanocrystals with dynamic and reversible operation . As described in Figure 3A, the valence state of Se was
[70]
reduced from intermediate states to -2 after the electron injection operation. On top of this, the hole
concentration was dramatically decreased, which made the Fermi energy level of NCs change from the
initial value of -0.82 eV to -0.39 eV. In contrast, during the discharged process, the Fermi energy level drops,
and the band gap returns to the initial level of -0.82 eV, as shown in Figure 3B and C.

Photo-induced LSPR modulation
Cu S reveals strong LSPR features within the near-infrared (NIR) region (800-1800 nm), which are
2-x
determined by the concentration of the copper vacancy generated through the self-doping method. Using a
photo-reduction process [70-72] to manipulate the free hole density, researchers developed a simple strategy for
fabricating plasmonic Cu S NCs. Alam et al. have successfully modulated the LSPR feature of Cu S NCs
2-x
2-x
through the photochemically generated radicals MV +•[73] . The electron transfer is reversible through the
changed procedure, as shown in the infrared LSPR absorption spectra presented in Figure 4A and B. The
possibility of controlling LSPR by electron donors generated by photochemical protocols can be further
stretched to create optical windows in the infrared region. Additionally, it is also possible to achieve the
LSPR properties of Cu S NCs with the assistance of a semiconductor in a photoreduction reaction.
2-x
Interestingly, when the Fermi level underwent equilibration with electron transfer, there was an increase
compared to the unequilibrated state, leading to a reduction of the NIR LSPR feature, as demonstrated in
Figure 4C. The reversibility of electron transfer through LSPR response is probed and shown in Figure 4D.

Morphology and surface ligands effect for LSPR
Different shapes of the synthesized nanomaterials will lead to distinct LSPR features and generate
interesting optical properties. The LSPR feature in the near-infrared region has been confirmed in self-
doped Cu E (E = S, Se and Te) semiconductor NCs due to the existence of free holes related to Cu
2-x
vacancies in the lattice [74-77] .

To date, plasmonic NCs have been developed by changing the composition, morphology, or surface ligands
of NCs [68,78,79] . Li et al. exhibited a strategy to synthesize high-quality copper telluride nano-cubes,
nanoplates, and nanorods. Figure 5A-C showed the morphology of the CuTe NCs through the high-
resolution transmission electron microscope (HRTEM) micrographs obtained. The corresponding
absorption spectra of copper telluride with different shapes were shown in Figure 5D. In the spectra
obtained from CuTe nano-cubes, a strong absorption band centered at 900 nm and associated with LSPR is
clearly observed. At the same time, weak absorption intensity in UV-Vis-NIR spectra and almost vanishing
NIR absorption characteristics were observed in CuTe nanoplates and nanorods, respectively. This may be
caused by the small size of the fine nanorods not supporting detectable plasma . The synthesis procedure
[80]
involved the reaction of a copper salt with trioctylphosphine telluride precursor in the presence of lithium
bis-(trimethylsilyl)amide and oleylamine (OAm).