Potential 2D Materials with Phase Transitions: Structure, Synthesis, and Device Applications

Abstract

bandgap can overcome the short channel effect, and then can scale down transistors to sub-10 nm [13,14] and even sub-5 nm [10,[15][16][17][18] to extend the Moore's law. Over last decades, typical 2D semiconductors, such as MoS 2 and WSe 2 , have been intensively studied. [19][20][21][22] On the other hand, 2D materials with versatile phase-transition properties offer new opportunities in both fundamental research and future device applications. In the last few years, a lot of exciting experiments have been conducted on this subfamily of 2D materials in the monolayer limit, such as charge density wave (CDW) 1T-TaS 2 , [23][24][25][26] superconducting (SC) 2H-NbSe 2 , [27][28][29] ferromagnetic (FM) CrI 3 , [12,30,31] and Cr 2 Ge 2 Te 6 . [11,32] CDW is a phenomenon of forming a standing-wave-like electron density usually accompanied with a lattice distortion and a bandgap opening due to electronphonon interaction. [33] SC is a phenomenon of zero electrical resistance and expulsion of magnetic flux fields also originated from electron-phonon interaction. [34] Magnetism and electric polarization stem from spontaneous spin/dipole ordering, respectively. [35,36] Parallel and antiparallel spins constitute long-range ferromagnetic (FM) and antiferromagnetic (AFM) ordering, respectively. Similarly, parallel and antiparallel dipoles form ferroelectric (FE) and antiferroelectric (AFE) ordering, respectively. Van der Waals layered materials can show a lot of novel properties, especially in the atomic thickness limit. This includes the follow ing terms: (1) The odd/even layer effect may emerge due to weak interlayer coupling. For example, FM or FE ordering may appear in the odd layers, while AFM or AFE ordering may appear in the even layers; (2) Due to confinement effect in the out-of-plane direction, new ordering structures may appear;(3) The properties can be easily tuned by strain, external fields, interface interaction, and so on. Toward fundamental property measurement and device applications, the challenge remains, such as predicting/discovering new 2D phase-transition materials, controlling the chemical synthesis, overcoming the poor air stability, and so on. Here, a literature survey has been carried out to search possible 2D phase-transition materials. Recent progress on the chemical synthesis and property investigation of some obtained monolayers has been reviewed, including CDW 1T-TaS 2 monolayers, FM CrI 3 mono layers, and so on.Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few-layers and then become a large and important subfamily of two-dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long-range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase-transition materials have vast potential for use in new-concept and functional devices. Here, potential 2D phase-transit...

Controllable nonlinear optical properties of different-sized iron phosphorus trichalcogenide (FePS3) nanosheets

“…Near 157 cm −1 , there is a highly Raman active counterpart of an infrared active Eu-type mode vibration originating from the (P 2 S 6 ) 4− unit. The A 1g (1) and A 1g (2) vibrations are at approximately 380 and 248 cm −1 , respectively, which are caused by the out-of-plane phonon modes, while the two in-plane vibrations (E g (1) and E g (2) ) are at approximately 279 and 227 cm −1 , respectively. These results are consistent with previous studies on the Raman vibration modes of the FePS 3 [34].…”

“…After exfoliation, Figure 2h shows there is an absence of the Eu-type mode, suggesting the presence of only few-and single-layer FePS 3 nanosheets [6]. The sharp features of the A 1g (1) and the other Raman vibration modes in the Raman spectra indict that the FePS 3 nanosheets have retained a stable structure [30]. Furthermore, an obvious distinction between the bulk and exfoliated materials is the Raman shift of the iron cation vibration (C), which is also indicative of the success of the exfoliation process [16,35].…”

“…Raman spectroscopy is a powerful and widely used technique for characterizing the structure of van der Waals-stacked materials and for revealing differences between bulk materials and the corresponding exfoliated nanosheets. The bulk FePS 3 has a monoclinic structure with C2 symmetry which displays five Raman active modes attributed to vibrations of the iron cations (C Raman active modes) and (P 2 S 6 ) 4− bipyramid structures with D 3d symmetry (A 1g (1) , A 1g (2) and E g (1) , E g (2) Raman active modes), as shown in Figure 2h. Near 157 cm −1 , there is a highly Raman active counterpart of an infrared active Eu-type mode vibration originating from the (P 2 S 6 ) 4− unit.…”

“…Metal phosphorus trichalcogenides (MPTs) have drawn considerable interest in recent years due to their excellent magnetic, electronic, and optical properties for applications in catalysis, energy storage, and optoelectronic devices [1][2][3][4][5][6][7][8]. MPTs have a general structure of MPX 3 , where M stands for a transition metal (Fe, Co, Ni, Cr, Mn, etc.)…”

Phase engineering of two-dimensional transition metal dichalcogenides

“…On the basis of close connection among TMD structures, stability and properties, it is vital to exploit effective pathways toward the specific phases. Therefore, phase engineering for TMDs has gained wide attention, which can open up a new territory for modulating the structure and properties of TMDs [30,31].…”