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Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33 Page 11 of 20

Moreover, Se nanoflakes presented a centrosymmetric structure possessing a spontaneous in-plane
-1
-10
ferroelectric polarization of about 2.68 × 10 cm per layer, which is favorable in nanoscale electronic
devices. First-principles calculations revealed that two-dimensional layered Se structure exhibited two
gapped semi-Dirac cones in the square Brillouin zone, indicating that they were topological insulators with
nontrivial topological properties . The Brillouin zone had Dirac-cone-like dispersion at P1. However, these
[61]
band distributions showed significant anisotropy at the band contours around P1 [Figure 5C (i)-(iii)]. A
density-functional-theory method was conducted to calculate the total and orbital projected densities of
states of the trigonal Se . As shown in Figure 5D, the lowest valence band ranging from -15.9 to -9.5 eV
[73]
can be found in the VB1 region. VB2/VB3 and CB1 regions can also be observed in upper valence bands
and lower conduction bands, respectively. A great deal of potential for the development and application of
trigonal Se in linear electro-optic devices has thus been suggested.

Taking advantages of high photoconductivity, electrical anisotropy, and nonlinear optical properties, Se
nanomaterials show great potential in optoelectrical applications [73-77] . Figure 5E shows the optoelectronic
[57]
property of a phototransistor based on Se nanosheets . The current-gate voltage curves show a gate
tunability, where a photocurrent of up to 54 nA is generated even at a low illumination power of 0.21 mW
cm . This result indicates an excellent photoresponsivity of Se nanosheets. Control over the grain size of Se
-2
can further increase the optoelectronic performance. For example, a laser-based annealing approach
enabled a polycrystalline domain where the grain size was a few micrometers in the direction perpendicular
to the electric field while the grain size was much smaller in the direction of illumination, which led to a
[77]
super-sensitive photodetector with performance on par with some planar nanoscale devices [Figure 5F] .
Besides 2D and 3D Se nanomaterials, 0D Se nanomaterials, such as Se quantum dots, also show excellent
optoelectronic properties. As shown by Figure 5G, the photocurrent is detected in Se quantum dots under
[78]
an applied bias voltage of 0.6 V . Notably, the potential gradient generated within Se quantum dots
promoted the separation of electrons and holes, thus enabling a higher photocurrent density (1.80 μA cm )
-2
than photodetectors based on Se nanosheets or Se nanowires.

Since its deployment in solar cells about 140 years ago, Se has been acting as a promising semiconducting
material for the fabrication of photovoltaic devices [79,80] . Modulating crystalline structure can be an approach
to improving solar cell efficiency. The relationship between solar cell performance and Se crystalline
[81]
structure was investigated by Hadar et al. . The presence of small crystal grains in Se’s structure, as shown
in Figure 5H, contributed to the low short-circuit current density (J ), which could be explained by the low
sc
orientation order of the crystals. In addition, another efficient strategy to improve high solar cell efficiency
is incorporating Se with other metal elements, such as germanium (Ge), copper (Cu), and indium (In) [82-84] .

Piezoelectric properties are also discovered in Se nanomaterials. Se nanowires exhibit strong piezoelectric
properties due to the anisotropic crystal structure of the trigonal phase [Figure 5I] . When longitudinal
[85]
stress is applied along the x-axis, the Se atoms undergo internal displacement and the electronic charge is
displaced against the Se cores, resulting in piezoelectric polarization parallel to the x-axis. When Se
nanowires are stretched (compressed) vertically, piezoelectric polarization pointing upward (downward) is
generated [Figure 5I (i)]. As a result, short-circuit current (I ) is generated by compressing the Se nanowires
sc
along the alignment direction, showing a good perspective for piezoelectric nanogenerators [Figure 5I (ii)].

APPLICATIONS OF SELENIUM NANOMATERIALS IN FLEXIBLE AND WEARABLE
ELECTRONICS
Several recent studies have demonstrated that Se nanomaterials represent an intriguing class of functional
materials in flexible and wearable electronics because of their anisotropic structure, quantum confinement
effects, large surface areas, and interesting optical, electrical, optoelectronic, electrochemical, photovoltaic,