2D Materials Beyond Graphene

Graphene undoubtedly revolutionized condensed matter physics introducing a new class of 2D materials that, beyond the fascinating feature of being atomically thick, have shown so far an incredible plethora of electronic, optical, magnetic, and topological properties, hardly findable elsewhere. Therefore, 2D materials beyond graphene are a valuable platform where one can study multiple aspects of solid-state physics with an outlook on the technological applications in which they have already demonstrated disruptive advancements, and this is what this Special Collection is about. Among the many fields in which 2D materials beyond graphene are expected to provide advancements, electronics is likely the most important one. Indeed, many of these 2D materials were investigated in light of the well-known absence of a sizable bandgap in graphene. In the Review article [pssr.202500133], Jung et al. discussed the gate dielectric strategies for field-effect transistors (FETs) based on 2D materials as conductive channels. The Authors summarized the main requirements for dielectric materials in 2D devices and classified such materials in layered (e.g. hBN) and nonlayered (e.g. CaF2) categories. The dielectric integration techniques based on growth, transfer, and interface engineering methods were thereby commented focusing on limitations and advantages. The conventional atomic layer deposition used for gate dielectrics does not work for 2D materials because of their dangling bond-free surface, and therefore modified strategies like plasma pretreatment or seeding-layer deposition might be introduced to favor the presence of nucleation sites. Alternatively, the transfer of predeposited dielectric layers (onto sacrificial substrates) or layer-by-layer oxidation of 2D materials to obtain complete conversion into their corresponding oxides (analogously to SiO2 in conventional electronics) can be implemented as well. As a more specific case, in [pssr.202500200], Dadashnia et al. summed up the experimental methods and applications of thin fluoride films. Indeed, the exploitation of 2D materials as channels in FETs requires suitable gate dielectrics to accomplish a full downscaling of the devices. The deposition methods, including molecular beam epitaxy, thermal evaporation, magnetron sputtering, and the strategies adopted to obtain smooth layers with low density of defects, are thus reviewed. The applications involving the integration of these dielectrics in devices such as FETs and photodetectors were discussed, highlighting particularly the improvements in the figures of merit. Both Reviews spotlight the most recent outcomes in the field of 2D dielectrics, thus paving the way for full 2D electronics. In this framework, the fabrication of FETs with α-MoO3 as active channels reported in [pssr.202500104] by Pereira et al. shows the important role played by thermal treatments to tune the electrical properties by controlling the oxygen vacancies. n-type FETs were fabricated by exfoliating bulk MoO3 crystal and depositing contacts in a bottom-gate configuration. With this geometry and after annealing in vacuum, the formation of oxygen vacancies gives rise to an increase of the free carriers and consequently of the electrical conductivity. The Authors found a working trade-off between the too resistive as-fabricated and the vacuum annealed devices by performing annealing in air. Moreover, when modulating the channel resistance with gate voltage, anticlockwise hysteresis is observed and related to capture and release of electrons in intrinsic defects or traps. These results are therefore promising for the development of 2D materials-based FETs with enhanced performances and specifically with the chance to modulate the electrical conductivity via surface defects engineering. Interface engineering is a viable route to modify the properties of materials and, for the 2D case, where the surface coincides with the bulk, enables the manipulation of these properties at the atomic level. In [pssr.202500124] by Araujo et al., the chemical vapor-deposited MoSe2 was engineered by two approaches involving surface fluorination by XeF2 and nitrogen implantation. In the former case, mild p-type doping could be induced, while in the latter, a marked n-type doping was obtained. Interestingly, these implantations did not affect the 2H semiconducting phase that remains largely intact. These outcomes turn out to be promising for tailoring the electronic properties of 2D semiconductors with the chance to create an on-demand doping profile for the fabrication of in-plane diodes. Ohno et al., in [pssr.202500186], explored the formation of germanene on the Ag(100) surface aiming at understating the formation of incommensurate interface at variance with the most studied commensurate interfaces obtained on (111)-oriented substrates like Ag(111). The growth of Ge films was achieved by atomic segregation epitaxy of Ge atoms from Ag(100) thin films grown on Ge(100). Depending on the annealing temperature during the segregation process, different Ge superstructures covering the Ag(100) surface were reported. Interestingly, at 460°C, a striped phase was observed, while at higher temperatures (540°C) a hexagonal phase, consistent with the germanene formation, was gained as confirmed by low-energy electron diffraction and scanning tunneling microscopy investigations. Ag(100) thus turns out to be a versatile platform to investigate low-dimensional Ge structures. In article [pssr.202500432], Pianetti et al. investigated the growth by molecular beam epitaxy on Si(111) (even Sb-passivated) and stabilization of gallium telluride, a post-transition metal monochalcogenide with a stable monoclinic form and a metastable hexagonal polymorph. By combining structural, morphological, spectroscopical characterizations, it is possible to understand that the growth of h-GaTe is mainly ruled by Ga flux provided that Te is supplied in excess because of its volatility. The growth rate was enhanced at low temperature (375°C) and the Sb–Si(111) surface-modified nucleation dynamics without affecting the final morphology. A 2D to 3D transition was observed after a critical number of layers, that was not attributable to strain relaxation but to changes in the wetting conditions. Annealing demonstrated that thermal stability of h-GaTe occurs up to 250°C before undergoing amorphization and transformation into an optically active phase. These outcomes demonstrate that a balanced interplay between growth kinetics, wetting, and phase stability in metastable layered 2D materials occurs and careful attention must be paid to the microscopical mechanisms responsible for the phase transformations. The topological properties of matter have recently gained increasing interest. Even in this field, 2D materials show promising and nontrivial properties. Article [pssr.202500181] by Gong et al. investigated the dual topological orders in Ti4FeBi2. In the topological matter framework, two typical orders are topological insulators and topological semimetals. A topological insulator is characterized by a gapped bulk band structure with gapless Dirac-like topological surface states, while a topological semimetal shows a gapless bulk band structure with a Dirac point and nonclosed topological surface states termed Fermi arcs. The Ti4FeBi2 crystal was grown by self-flux method and cleaved on the (110) plane showing Fe chains and representing a quasi-1D system. Supported by theoretical calculations, the angle-resolved photoemission spectroscopy investigation evidenced the presence of bulk Dirac cones and a weak signal that can be related to Fermi arcs whose detection was particularly difficult because of the overwhelming bulk bands. These findings are encouraging for the development of hybrid topological orders materials with promising multiple topological states for multifunctional spintronic applications. The exploitation of 2D materials in applications requires conducting aging studies, aiming at assessing their modifications when integrated, for instance, into a device. Article [pssr.202500372] sheds light on the temporal evolution of the Ti3C2Txcompound. Ti3C2Tx belongs to the MXenes family with interesting potential in optoelectronics. By comparing fresh and aged (after 1 year) samples, using multiple characterization techniques, including diffraction and high-resolution spectroscopies, Osama et al. found that, even if a minor oxidation occurs, the conductivity of the aged sample dropped by one order of magnitude compared to the as-grown sample. These findings further corroborate the importance of postsynthesis treatments on 2D materials to mitigate oxidation and preserve electrical performance when integrated into devices. In summary, all these studies exhaustively illustrate the wealth of properties and applications of 2D materials beyond graphene. Current research is now in a mature stage to consider the full 2D electronics concept in which electronic devices are assembled by 2D layers only. Concomitantly, the development of peculiar growth protocols enables the tuning of the 2D materials’ crystallographic phases or electronic properties. Meanwhile emerging studies on the topological properties show that, even in this field, 2D materials might revolutionize our understanding of condensed matter physics. C.G. and C.M. acknowledge financial support of the PRIN projects EMPEROR (grant number 20225L4EBJ and CUP B53D23008560006) and DESIGN (grant number 2022EE8KH9 and CUP B53D23008770006), respectively. These projects received funding from the Italian programme for Research Projects of outstanding National Interest (PRIN) and Next Generation EU under the National Recovery and Resilience Plan (NRRP), Mission 04 Component 2 Investment 3.1 and 1.3. [pssr.202500133] [pssr.202500200] [pssr.202500104] [pssr.202500124] [pssr.202500186] [pssr.202500432] [pssr.202500181] [pssr.202500372]