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J. Funct. Biomater. 2022, 13, 27 11 of 36 which causes the weakening of the van der Waals force between the layers [54]. In most cases, hydrogen gas is produced due to the interaction of intercalated ions with water, which can help to separate the adjacent layers [47]. Finally, high-yield 2D nanosheets can be obtained after the centrifugation process. Bath or probe ultrasonication produces 2DNMs at the scale of 10 or 100 mg, while more scales are needed to move towards com- mercialization [27]. Another recently emerging method for producing 2DNMs is the use of shear force- assisted exfoliation, which can be scaled up to the industrial level. The shear force appa- ratus has a rotating blade constituting a rotor and a stator (Figure 8c). In this apparatus the rotating blade can produce high shear rates in liquid containing layered bulk crystals [43]. The fabrication process of 2DNMs using a shear exfoliation method includes: (i) ro- tation of the rotor to create a strong pressure gradient that directs the dispersion from the bottom to the center of the container; (ii) the centrifugal force to move the particles to the gap between the rotor and the stator (particles are milled here); (iii) shear exfoliation of particles due to the high-speed rotation and then a push out of the small holes in the stator; and (iv) draw the materials into the container to maintain the mixing cycle. Based on a high-shear rotor-stator mixer, Woomer et al. used shear-assisted exfoliation to exfoliate BP bulk crystals into large-scale of few-layer nanosheets [55]. They have shown that the band gap of BP increased from 0.33 ± 0.02 eV in bulk to 1.88 ± 0.24 eV in bilayers, a range that is larger than any other 2D material. In recent years, chemical vapor deposition (CVD) has emerged as the most promising bottom-up approach to prepare 2DNMs. In this technique, the vapor-phase active precur- sors can react or decompose on the desired substrate at high temperature and low pres- sures to form large-scale ultrathin 2DNMs [56]. It is worth pointing out that precursors, substrates, catalysts, temperature, and atmosphere play a key role in the quality of the final products in this technique. Compared to other methods, CVD produces 2DNMs with the highest level of control, high purity and crystal quality and the least defects [56]. For instance, Lee et al. successfully prepared single-layer MoS2 films onto amorphous SiO2 substrates by the CVD process with MoO3 and S powders as the reactants at 650 °C. Their findings reveal that the on-off current ratio exceeds 107, and the mobility is up to 1.2 cm2/V·s, which is comparable to an exfoliated MoS2 monolayer fabricated without high k- dielectrics [57]. 4. General Properties of 2DNMs 4.1. Physical Properties The reduced dimensionality of 2DNMs results in unique characteristics that make them different from their bulk counterparts, yet similar to each other. Since the surface of 2DNMs plays a crucial role in their properties, manipulating the surface enables fine tun- ing of different properties in different fields of research and interest such as lubricants, thermoelectronics, catalysis, energy storage application, FET devices and specifically bio- medical applications [58]. In fact, the exclusive structure of 2DNMs can give rise to unique characteristics such as high surface area, high modulus and strength which play a key role in biomedical applications. 2DNMs stemming from restriction of bulk components to grow in a certain direction result in their interesting physicochemical properties. While preserving their large longi- tudinal dimension, 2DNMs can still maintain their atomic thickness which leads to ultra- high specific surface area and high ratio of exposed atom on a surface. Such a high surface area per volume of 2DNMs makes them different from conventional nanomaterials with potential biomedical applications (e.g., drug delivery and nucleic acid carriers) [59,60]. The ultra-thinness of 2DNMs combined with their intrinsic strong in-plane covalent bonds also plays an important role in the flexibility of these materials. Within a layer of 2DNMs, atoms are tightly bound to each other by covalent or ionic bonds which results in their high strength [61].PDF Image | Nanomaterials beyond Graphene for Biomedical Applications
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