2D Monoelements. Группа авторов
cell, due to its matched energy level with CH3NH3PbI3. Comparing with the HTL-free device, both of the highest short-circuit current density (Jsc) and external quantum efficiency (EQE) of antimonene-HTL device were increased by 30%.
In addition to generating shear force via pre-grinding antimony crystals before the LPE, shear force can also be obtained in the process of LPE by using rotating blades mixers. Gusmão et al. obtained arsenene, antimonene, and bismuthene exfoliated nanosheets by the surfactant-assisted LPE method under the action of shear force generated by rotating household kitchen blenders [22]. As the transmission electron microscopy (TEM) shown in Figure 2.4f, the morphology of exfoliated antimonene (SbSE) nanosheets consisted of shapes with defined angles and nanostripes because the preferential cleavage of antimony crystals was easy to occur in different crystallographic directions. The size distribution of antimonene nanosheets exhibited a broad range from 100 nm to 900 nm and the maxima value appeared at around 200 nm. Then, the high-resolution TEM (HRTEM) was employed to determine the structure of antimonene (Figure 2.4g). The Raman intensity of SbSE was lower than that of bulk antimony (Sbbulk), and the Raman peak shifted to higher frequency owing to partial oxidation (Figure 2.4h). The bonding states of Sbbulk and SbSE were also investigated by the high-resolution X-ray photoelectron spectra (XPS), it can be seen that both the element phase (Sb 3d5/2, 3d3/2) and partial oxidation phase (Sb2O3) existed in the SbSE (Figure 2.4i). As a pnictogen, SbSE can be used for the electrochemical detection of ascorbic acid due to its high electron transfer properties and low onset oxidation potential. Also, SbSE can be considered as a wide-pH-range catalyst for the hydrogen evolution reaction (HER).
2.3.3 Epitaxial Growth
Epitaxial growth is a method that a crystalline layer can be directly deposited onto a crystalline substrate [23]. This method is widely used in the preparation of high-quality crystalline 2D materials with large capacity. At present, van der Waals epitaxy (vdWE) and molecular beam epitaxy (MBE) are explored to grow few- or even single-layer antimonene on different crystalline substrates.
Traditional epitaxial growth requires very similar lattices to realize the matching between the substrate and the epitaxial layer. Koma et al. found that the vdWE proceeded with the van der Waals force can almost avoid the limitation of the substrate and can easily grow various layered materials on the substrates without dangling bonds [24]. Ji et al. first used the vdWE method to synthesize few-layer β-antimonene monocrystalline polygons on different substrates and highlighted their atomic structure and ambient stability [25]. The synthesis process was carried out in a two-zone tube furnace for 60 min, where antimony powders in the T1 zone was heated up to 660°C to generate antimony vapor and the substrates (fluorophlogopite mica, silicon, sapphire) were placed in the downstream T2 zone (380°C) (Figure 2.5a). Antimony vapor was carried from T1 zone to T2 zone by Ar/H2 (70%/30%) mixed gas and then deposited on the substrates to grow various antimonene polygons. The absence of dangling bonds and low migration energy barrier of antimony atoms on mica substrate are beneficial to a fast growth of antimonene on mica. From the optical microscope images shown in Figure 2.5b, it can be seen that few-layer antimonene polygons exhibit various shapes, including triangles, hexagons, rhombus, and trapezoids with lateral sizes about 5–10 μm. The thicknesses of these polygons are down to 4 nm, while the thinnest one is found to be 1 nm, corresponding to a monolayer antimonene. The HRTEM image of a typical antimonene polygon is shown in Figure 2.5c, it is extracted that the synthesized antimonene belongs to the rhombohedral structure, namely, β-antimonene. The Raman spectra of antimonene polygons also show a thickness dependence, in which the Raman peaks move to the higher frequency region with the decrease of thickness (Figure 2.5d). By comparing optical microscopy, AFM, Raman spectroscopy, and XPS results of antimonene before and after one-month aging, it is observed that few-layer antimonene show outstanding stability in ambient condition. Moreover, the synthesized antimonene polygons exhibited high electrical conductivity up to 1.6×104 S m−1 and good transparency in the visible range.
Figure 2.5 (a) Schematic diagram of the synthesis process of antimonene by the vdWE method. (b) Optical microscope images of few-layer antimonene polygons with various shapes. The scale bar is 5 μm. (c) HRTEM image of a typical antimonene polygon. Inset: selected area electron diffraction (SAED) pattern along the [001] axis. The scale bar is 2 nm. (d) Raman spectra of antimonene polygons with different thicknesses. (e) Scanning electron microscopy (SEM) image of 10-nm thick antimonene on an as-grown graphene substrate. (f) Atomic models of the epitaxial alignments between Sb (0001) and graphene in three morphs of Sb. (a–d) Reproduced with permission [25]. Copyright 2016, Nature Publishing Group. (e, f) Reproduced with permission [26]. Copyright 2018, American Chemical Society.
In particular, there are no dangling bonds in 2D materials, thus the 2D materials can also be employed as the substrates to grow antimonene. Sun et al. grew a series of antimonene (Sb) with various morphs on two types of single-crystalline graphene substrates by the vdWE method [26]. One of the substrates was the as-grown graphene on Cu (111)/c-sapphire via chemical vapor deposition (CVD), and another was the transferred graphene on SiO2/Si. On the as-grown graphene, triangle Sb islands (up-pointing and down-pointing triangles) were grown in Volmer-Weber (VW) modes with height of ~17.5 ± 0.7 nm, while Sb sheets were grown in Frank-van der Merve (FM) modes with height of ~4.5 ± 0.7 nm (Figure 2.5e). The growth of Sb sheets was probably caused by the remote epitaxy between Sb and underlying Cu. The epitaxial alignments between Sb (0001) and graphene were different in various morphs of Sb, i.e., Sb [1i00]∥graphene [10] and [10i0]∥graphene [10] for up-pointing and down-pointing Sb islands, [2ii0]∥graphene [10] for Sb sheets (Figure 2.5f). In contrast, only VW growth of Sb islands was found on the transferred graphene due to the absence of Cu. In addition, high-quality Sb thin films were also grown on both substrates, and these two films showed no significant difference. The epitaxial alignment in Sb thin films was the same as that in Sb islands. In another work, β-antimonene was also grown on WSe2 substrate to form the Sb/WSe2 heterostructure through van der Waals interactions [27].
The earliest MBE growth of antimonene was reported by T. Lei et al. in 2016 [28]. In this work, bilayer antimonene was grown smoothly on 3D topologically insulated Bi2Te3 (111) and Sb2Te3 (111) substrates with small lattice mismatch. The clear low-energy electron diffraction (LEED) pattern indicated the 1 × 1 periodicity of the antimonene/Sb2Te3 (111) surface, showing that high-quality epitaxial antimonene was formed. The surface states of MBE grown antimonene were probed by angle-resolved photoemission spectroscopy (ARPES), and it showed similar electronic structures on the surfaces of two substrates,