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Page 2 of 20 Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33

INTRODUCTION
Flexible and wearable electronics capable of continuous monitoring of physiological signals , minimally
[1-5]
invasive interrogation of neural circuitry [6-10] , and performing diagnostic and therapeutic treatment [11-15] are
offering unprecedented value to society and our everyday lives [16-20] . Functional materials that are responsive
to stimuli (such as photons [21-25] , stress [26-30] , and chemicals [31-34] ) are the core part of flexible and wearable
electronics [35-37] . The incorporation of semiconducting materials with excellent electrical and optical
properties into flexible and wearable electronics enables innovative smart devices and systems with powerful
potential in personalized healthcare, precision medical surgery and large-area energy harvesting and
storage [38-40] . Selenium (Se) is a trace element of group VI with a direct bandgap at room temperature . Due
[41]
to its unique band structure and crystal structure, Se, in its bulky form, shows attractive optical, electrical,
electronic and piezoelectric properties opening up interesting applications in photodetection and solar
energy harvesting [42-44] . Compared with typical semiconductors in electronic industry (such as silicon and
germanium) that require sophisticated micro- and nanofabrication conditions, Se can be physically,
thermally or chemically processed in simple, low-cost and robust manners, which makes its integration with
flexible and wearable electronics straightforward, thereby enabling versatile devices and applications [45,46] .

Thanks to the remarkable progress in nanofabrication and nanotechnology, bulky Se can be scaled down to
tunable nanostructures ranging from zero-dimensional (0D) nanoparticles to two-dimensional (2D)
nanosheets. Benefiting from the “nano-size” effect, Se nanomaterials exhibit distinct physical and chemical
properties that cannot be found in their bulky counterparts, featuring improved mechanical, electrical,
thermal, optical and piezoelectric performance [47-49] . Compared with other functional semiconducting
[50]
nanomaterials such as Si nanowires, Ge nanowires and ZnSe nanowires employed in flexible electronics ,
Se nanomaterials show the distinctive advantage of processability. They can be easily integrated with
traditional conductors and semiconductors over the flexible, bendable, stretchable, and highly curved
substrates to fabricate innovative functional devices in the form factors of two-dimensional large-area films
and patches as well as one-dimensional indefinitely long fibers as we elaborated in the section of
“APPLICATIONS OF SELENIUM NANOMATERIALS IN FLEXIBLE AND WEARABLE
ELECTRONICS”. This allows for the development of a new generation of Se nanomaterial-based flexible
and wearable electronics. This mini review highlights the recent advances in Se nanomaterials enabled
flexible and wearable electronics. We will first briefly review the typical morphologies and crystal structure
of Se nanomaterials that directly determine their properties and subsequent applications. We will then
present the synthesis and fabrication methodologies of Se nanostructures, namely the “bottom-up” and
“top-down” strategies that have been widely exploited in the nanofabrication field. In the next section, we
will discuss the physical properties of Se nanomaterials, focusing on optical, electrical, optoelectrical,
photovoltaic, and piezoelectric behaviors that are of great importance for their functional applications in
flexible and wearable electronics. Finally, we will discuss how Se nanomaterials can be integrated with
substrates that are flexible, bendable, large-area and highly curved and how these innovative devices open
up interesting opportunities in photosensing, mechanical deformation sensing, physiological sensing and
energy storage. The review concludes with some key remaining challenges and perspectives that will be of
value for future advances and breakthroughs in the field.

MORPHOLOGIES AND CRYSTAL STRUCTURES OF SELENIUM NANOMATERIALS
The morphologies, sizes and crystal structure of materials determine their properties and applications. We
begin with a brief introduction to the morphologies and structure of Se nanomaterials.

Generally, nanostructured Se can be categorized into zero-dimensional (0D), one-dimensional (1D), and
two-dimensional (2D) form factors, as shown by the microscopic images in Figure 1 and Table 1. 0D Se