Facile and Versatile Functionalization of Two‐Dimensional Carbon Nitrides by Design: Magnetism/Multiferroicity, Valleytronics, and Photovoltaics

Abstract

Ab initio calculation evidence has shown that two-dimensional (2D) carbon nitrides may enable "facile functionalization" when a domain of carbon nitride is wetted by a solution of metal halides with mobile cations/anions. During the wetting process, each cavity can be functionalized by a unit of metal halide. Compared with prevailing functionalization or doping strategies through either high-temperature diffusion of source ions or ion implantation by using accelerators, such a room-temperature "wet-lab" functionalization approach is more facile and efficient. The wet-lab functionalization not only can facilitate isolation of the 2D monolayer, but also, with applying different metal halides, enable various new and desirable properties for broad applications, e.g., 2D magnetism and 2D ferroelectricity with high piezoelectric coefficient. The latter can be implemented in spin-independent valleytronics for non-volatile electrical manipulations. Notably, tunable bandgaps, ranging from 1.0 to 2.5 eV, can be realized by controlling the metal-halide functionalization density, while the separation of electrons/holes can be facilitated by the ferroelectric polarizations and heterostructure band alignments. Moreover, multifunctional domains like P/N doped or magnetic/ ferroelectric domains can be selectively constructed through such solutionprocessed functionalization with different halides, followed by seamless integration into a single sheet of carbon nitride, akin to the P/N channels in silicon wafers.

Constructing Stable and Potentially High-Performance Hybrid Organic-Inorganic Perovskites with “Unstable” Cations

“…With the reorientations of these polar cations in (H 5 O 2 )SnI 3 as marked by the circles shown in Figure 3(c), where the blue arrows denote the polarization direction of cations and the whole crystal, a ferroelectric polarization of 22.7 μC/cm 2 would emerge, much higher than the previously predicted polarization value of MAPbI 3 [24]. Theoretically, ferroelectricity may benefit pho-tovoltaics because high photovoltage can be induced by polarization, while photogenerated electrons and holes can be separated by a built-in electric field [25,26]. Moreover, combination of a high-mobility narrow-bandgap semiconductor and nonvolatile memory is also desirable [27,28].…”

Two dimensional ferroelectrics: Candidate for controllable physical and chemical applications

“…The stable in‐plane spontaneous polarization in atomic‐thick SnTe down to a one‐unit cell was observed experimentally. The applications like nonvolatile ferroelectric random‐access memory device, 46 valleytronics, and photovoltaics 34 were thus proposed by taking advantage of the ferroelectricity. The successful synthesis and direct demonstration of ferroelectricity in the atomic thick SnTe relived the research enthusiasm of real 2D ferroelectrics.…”

“…Theoretical simulations have predicted the existence of ferroelectricity in stable 2D layered materials, including intrinsic in‐plane FE such as group IV monochalcogenides with the formula MX (M = Ge, Sn; X = S, Se) 22,23 , 2H and 1T‐VSe 2 , 24,25 charge doped CrI 3 monolayer 26 and atomically thick SnTe film 27–29 . The 2D materials with out‐of‐plane FE include but not limit to the distorted 1T‐MoTe 2 , 30 MoS 2 31,32 monolayers, chemical functionalized phosphorene, 33 graphene analogues (silicene, germanene, and stanene), carbon nitrides, 34 2D In 2 Se 3 , 35–37 monolayer AgBiP 2 Se 6 38 and CuInP 2 S(Se) 6 (CIPS), 39–41 and so on. Especially, the successful synthesis and experimental demonstrations of monolayer SnTe, In 2 Se 3 and CIPS have paved the solid foundations for the exploration of unique physical and chemical properties in 2D ferroelectrics.…”

A family of ionic supersalts with covalent-like directionality and unconventional multiferroicity

“…However, we note that supersalts may not necessarily adopt the nonpolar structures for most ionic systems: compared with isotropic single atoms, the anisotropic geometry of superatoms may give rise to breaking of centro-symmetry. If such breaking can induce switchable polarization, highly ionic ferroelectrics might be formed via facile and solution-processed 28 fabrications, offering a route for large-scale production that cannot be applied to conventional ferroelectrics. This, in turn, can find promising applications in a broad range of fields including nonvolatile memories, sensors and actuators, nonlinear optical materials, etc.…”