with no apparent defects, and the height of the Sb island is around 0.78 nm (Figure 2.6g). The atomic-resolution STM image (Figure 2.6h) revealed a reconstructed √2 √2 lattice of α-antimonene with lattice constants of 0.62 × 0.63 nm. Monolayer α-antimonene possesses a linearly dispersed metallic band crossing the Fermi level, resulting in a high electrical conductivity. Large-scale multilayer antimonene was also grown by controlling the amount of Sb, which showed ultrahigh stability in air.
In the meantime, MBE was employed to grow antimonene on different metal substrates as well, including Ge (111), Ag (111), Pd (111), Cu (110), and Cu (111) [33–36]. Niu et al. fabricated single-crystalline monolayer antimonene on Cu (111) and Cu (110) substrates via MBE [36]. When Sb atoms were deposited on two substrates at low coverage, surface alloys were first formed, including √3 × √3 R30°-Sb on Cu (111) and c (2 × 2)-Sb on Cu (110). After increasing the coverage to 1 monolayer followed by postannealing, two atomic types of monolayer antimonene were then formed. Schematic of epitaxial growth of Sb atoms on Cu (111) substrates was shown in Figure 2.6i, where Sb atoms were evaporated from a Knudsen cell onto clean Cu (111) surfaces held at room temperature in an UHV chamber (2 × 10−10 mbar). The topographic STM image of 0.5 monolayer antimonene illustrated that Cu (111) surface was covered by short-range ordered single Sb atoms and their small clusters (Figure 2.6j). After annealing at 700 K, a variety of patches with dark holes (missing atoms) and black lines (misalignment of neighboring domains) were generated (Figure 2.6k). When increasing the coverage and postannealing, well-ordered monolayer antimonene was formed, showing a buckled honeycomb lattice and a larger lattice constant than the free-standing antimonene because of a 7.5% tensile strain (Figure 2.6l). But unlike the result of Cu (111) surface, threefold symmetric antimonene was formed on the twofold symmetric c (2 × 2) Sb-Cu (110) surface, inducing a 6.8% compressive strain. The generation of strain also caused the changes of band gaps of antimonene on two substrates.
2.3.4 Other Methods
In addition to the above three common synthesis methods, there are other methods can also be used to synthesize few- or multilayer antimonene nanosheets, nanoribbons, flakes, or Sb films [37–40]. Few-layer β-antimonene nanosheets were successfully prepared by the solution-phase synthesis method, where SbCl3 solutions as the Sb source were reduced by oleylamine [37]. In this anisotropic growth, both dodecylthiol (DDT) and halide ions played an important role in the formation of β-antimonene. The synthesized β-antimonene nanosheets were single-crystalline with lateral size of 0.5–1.5 μm and thickness of 5–30 nm. Besides, multilayer antimonene nanoribbons were synthesized by the plasma-assisted process, where InSb (001) was selected as the substrates and provided Sb source for the formation of antimonene [38]. During the growth process, the indium at the surface of InSb occurred preferentially the nitridation under the action of N2 plasma, which induced simultaneously the condensation of Sb atoms to form multilayer antimonene. The band gap of multilayer antimonene was opened because of the quantum confinement effect and the turbostratic stacking, then producing the orange light emission (610 nm). Like other 2D materials, antimonene flakes can also be grown through the CVD on SiO2 substrates [39]. Sb powders were used as the Sb source, and Ar gas was the carrier gas. The growth of antimonene flakes began at 600°C and maintained for 10 min. The obtained flakes showed various shapes, including nanoribbon, hexagon, and trapezoid. The antimonene flakes were very stable in air even when heated by a hot plate below 250°C. Moreover, ultrathin Sb films were alternatively grown on the topological insulators (such as Bi2Te2Se and Bi2Se3) by a thermal effusion cell [40, 41].
2.4 Applications of Antimonene
2.4.1 Nonlinear Optics
As an excellent nonlinear absorption material, antimonene has broadband nonlinear optical response, high photothermal efficiency, strong Kerr nonlinearity, low saturable intensity, high two-photon absorption coefficient, and large cross-section, offering the potential nonlinear optical applications in all-solid-state lasers, fiber lasers, optical switchers, optical modulators, and optical thresholders [42–47].
Wang et al. first reported the passively Q-switched Nd3+ solid-state lasers using few-layer antimonene peeled off by the LPE as the saturable absorber (SA) [42]. With antimonene/quartz as the passively Q-switcher, the laser operations were realized at 946, 1,064 nm in Nd:YAG crystal with pulse widths of 209, 129 ns and peak powers of 1.48, 1.77 W, while the laser emission of Nd:YVO4 crystal generated at 1,342 nm with pulse width and peak power of 48 ns and 28.17 W, respectively. By fitting the saturable absorption results of nanosecond laser pulses, the saturable intensities of antimonene were obtained to be 0.43 MW/cm2 at 532 nm and 0.53 MW/cm2 at 1064 nm. In this work, the 1,342 nm Nd:YVO4 laser achieved the best results, including the shortest pulse width (48 ns), the highest peak power (28.17 W), and the largest single pulse energy (1.36 μJ). The following drawbacks often occurred in passive Q-switching, including the degradation, failed saturable absorber, and inaccurate modification of repetition rate. To overcome the drawbacks in passive Q-switching, Wang et al. devised an actively Q-switched fiber laser with an antimonene-based all-optical modulator (AOM) as a Q-switcher [43]. The antimonene nanosheets fabricated by the LPE were deposited onto a 10 μm microfiber, which was then employed to devise an AOM with a fiber-type Michelsom interferometer (MI) due to the obvious photothermal effect of antimonene. The antimonene-based AOM exhibited a large phase modulation capacity with a conversion efficiency of 0.049 π mW−1, and at the same time, it achieved the intensity modulation with large modulation depth of 25 dB. Also, this AOM showed a long-term stability with only 8.2% decline of phase conversion efficiency after 1 month (Figure 2.7a). The obtained active Q-switching pulses possessed a microsecond duration (the rise time constant of 3.2 ms) and tunable repetition rate ranging from 0.96 to 6.64 kHz (Figure 2.7b).
Using the strong Kerr nonlinearity of antimonene, Song et al. devised a new type of optical device based on few-layer antimonene (FLA)-decorated microfiber, which was operated both as an all-optical Kerr switcher and an all-optical wavelength converter [46]. The FLA-based Kerr switcher featured a high extinction ratio of 12 dB with a long-time stability, which could be used to realize the process of controlling light by light in optical communication systems (Figure 2.7c). In addition, by taking advantage of the four-wave mixing (FWM) effect, the designed FWM-based wavelength converter achieved a high conversion efficiency of 63 dB and converted efficiently the modulated radio frequency (RF) signals to the sidebands with a maximum frequency of ~18 GHz, which was a vital part in the optical signal processing (Figure 2.7d). In another work, Song et al. also employed FLA-decorated microfiber as an all-optical pulse thresholder to effectively suppress the noise in the transmission system, by which the signal to noise ratio (SNR) was largely improved (~10 dB) [47].
Figure 2.7 (a) Phase
