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Page 12 of 20 Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33

and piezoelectric properties. This section summarizes and discusses the fabrication and application of Se
nanomaterials-based flexible or wearable electronics, including flexible and wearable sensors for
photedetection, mechanical deformation sensing and physiological sensing, and flexible batteries.

Flexible and wearable sensors for biomedicine and healthcare
Selenium is an intriguing optoelectronic material, and its bulk form has been used for a myriad of
optoelectronic applications in xerography, ultrasensitive imaging, chemical detection and photodiodes [86-89] .
Selenium nanowires with a trigonal phase offer more appealing optoelectronic properties over their bulky
form [90,91] . The integration of trigonal selenium nanowires into flexible devices is bringing novel
opportunities for smart sensing, surgical tools and optogenetics [92,93] . The iterative thermal drawing
discussed in the previous section enabled ultralong Se nanowire array to align along the fiber length.
Interfacing the nanowires at the two extremities of the fiber with external electrodes formed a fiber-based
[70]
photodetector that was sensitive to visible light . However, the in-fiber selenium exhibited either an
amorphous or a polycrystalline structure. The disorganized atomic arrangement and crystal defects like
grain boundaries act as recombination centers for charge carriers, hindering the charge collection and
reducing the photosensitivity and photoresponsivity. Yan et al. have recently devised a novel method that
combines thermal drawing and sonochemical synthesis to create flexible fiber-based optoelectronic devices
[39]
made of single-crystal semiconducting nanowires [Figure 6A] . The bulk in-fiber Se underwent a
transformation into a dense array of nanowires through the manipulation of the anisotropy in the surface
energy of the crystal’s crystal planes. The growth of single crystal nanowires from the amorphous bulk in 1-
propanol resulted in the formation of in-fiber optoelectronic devices with built-in electrodes in intimate
contact. These fiber devices showcased exceptional optical and optoelectronic performances, including high
photoresponsivity and photosensitivity, low dark current, low noise-equivalent power, and ultrafast
response speed, which rivaled many wafer-based devices. Notably, this innovative approach enabled high-
throughput integration of nanowires into devices on an ultra-large scale, eliminating the need for intricate
clean room contacting procedures. This was demonstrated through the growth of high-performance
nanowire-based devices along the fiber length. The integration of multiple Se nanowire-based
photodetectors and an optical fiber resulted in a hybrid fiber [Figure 6B] . The unique capability of this
[70]
technology for fluorescent bioimaging based on the single multimodal fiber exhibiting simultaneous
efficient optical guidance and excellent photodetection performance was demonstrated. The outstanding
performance of the Se nanowires was analyzed to uncover the underlying mechanism. Ultrafast transient
absorption spectroscopy, nanosecond flash photolysis, and time-resolved terahertz spectroscopy were used
to study the charge carrier dynamics and mobility of Se nanowire meshes [11,31] . These noninvasive, contact-
free methods uncovered a picosecond lifetime for free carriers and a microsecond lifetime for trapped
carriers, both limited by trap-assisted recombination. Additionally, a high free carrier mobility of
-1 -1
2
approximately 3.0 cm V s was discovered.
Due to their flexibility, small cross-section and high aspect ratio, fiber-shaped selenium nanowire
optoelectronics are particularly useful for minimally invasive bioimaging, and remote and distributed
photodetection. In addition to the fiber form factor, the selenium nanomaterials can also be integrated into
large-area planar devices. Luo et al. reported fabrication of a thin film-based photodetector with full
transparency and flexibility, in which the individual Se nanobelts were deposited on a flexible polyethylene
terephthalate (PET) matrix [Figure 6C] . A number of characteristics were demonstrated to be impressive,
[94]
including excellent transparency and flexibility, high light sensitivity, and stable response under various
bending conditions.