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Page 8 of 20 Dang et al. Chem Synth 2023;3:14 https://dx.doi.org/10.20517/cs.2022.33

rod that was sealed in a thermoplastic matrix, the first draw scaled down the lateral feature size of Se to a
few millimeters. Assembling a bundle of first-drawn fibers and embedding them into a second polymer
matrix that was re-drawn reduced the feature size to a few micrometers. A third step finally enabled
nanowires and nanotubes with diameters of a few tens of nanometers [Figure 3C (ii)-(iii)]. The
nanostructures are radially and axially uniform, axially parallel and indefinitely long, unachieved with any
other approaches. Most strikingly, it is very easy to handle these nanostructures and interface them with
external electrodes forming flexible electronic devices.

2D Se nanomaterials
2D nanomaterials have attracted exponentially increased academic attention since graphene was discovered.
The unique physicochemical properties of 2D nanomaterials make them suitable for use in optoelectronics,
electronics, energy storage, and catalysts. Due to Se’s intrinsic anisotropic atomic structure, the fabrication
of 2D Se nanomaterials remains challenging as opposed to the 0D and 1D Se nanomaterials.

Physical vapor deposition (PVD), a thin-film coating process in which solid materials are vaporized under
vacuum conditions and then deposited onto a substrate, has been considered as a promising technique for
2D Se nanomaterials fabrication. Se nanoflakes featured with thicknesses of 5 nm have been fabricated on a
Si wafer substrate via depositing Se vapor generated from bulky Se powders [Figure 4A (i)] . The
[57]
nanoflakes were irregularly shaped and zigzagged [Figure 4A (ii)]. Structural characterization reveals the
helical atomic chains as well as lattice fringe spacing of typical trigonal structure [Figure 4A (iii)]. Chemical
vapor transport (CVT) has also been emerging as one of the promising techniques for 2D Se nanomaterials
fabrication . CVT utilizes reversible chemical reactions to trigger crystal growth in different directions at
[71]
different temperatures. The source materials are easy to obtain, and chemical reactions can be controlled.
More importantly, the reactant composition can be easily tailored, thus enabling control over the
characteristics of the targeted 2D Se nanomaterials. With this technique, Se nanosheets with typical trigonal
crystals have been fabricated [Figure 4B (i)]. The thickness of the monolayer is 0.75 nm
[Figure 4B (ii)-(iii)] .
[71]
However, stringent conditions such as ultrahigh vacuum and high temperature frustrate the large-scale
production and potential applications of those 2D Se nanomaterials. Exploring more facile and green
strategies and techniques is the mainstream direction for future development. Due to their strong in-plane
chemical covalent bonds and weak out-of-plane van der Waals interactions, 2D Se nanomaterials can be
readily fabricated by the well-established “top-down” methods, such as exfoliating from Se bulk materials in
liquid phase. However, it is important to note that the processing temperature should be maintained at 0-10
℃ in an ice bath, which can prevent the serious oxidation of Se nanostructures. Se nanosheets with ultrathin
nanolayered structure have thus been fabricated in an isopropanol solution with probe sonication and
centrifugation [Figure 4C (i)] . Under a low centrifugation speed, Se nanosheets showed average
[58]
thicknesses of between 10 and 27 nm, whereas ultrathin features with average thicknesses of 3-6 nm were
observed at high centrifugation speeds. TEM characterization revealed the lattice fringe spacings of 0.38,
0.22, and 0.29 nm, which correspond to the crystal plane spacings of (100), (110), and (101) planes
[Figure 4C (ii)-(iii)].

PROPERTIES OF SELENIUM NANOMATERIALS
As shown in Table 1, several unique properties of Se nanomaterials, including optical, optoelectronic and
piezoelectric properties, and the morphologies and structure-properties relationship are briefly discussed in
this section.