The interest in two-dimensional and layered materials continues to expand, driven by the compelling properties of individual atomic layers that can be stacked and/or twisted into synthetic heterostructures. The plethora of electronic properties as well as the emergence of many different quasiparticles, including plasmons, polaritons, trions, and excitons with large, tunable binding energies that all can be controlled and modulated through electrical means, has given rise to many device applications. In addition, these materials exhibit both room-temperature spin and valley polarization, magnetism, superconductivity, piezoelectricity that are intricately dependent on the composition, crystal structure, stacking, twist angle, layer number, and phases of these materials. Initial results on graphene exfoliated from single bulk crystals motivated the development of wide-area, high purity synthesis and heterojunctions with atomically clean interfaces. Now by opening this design space to new synthetic two-dimensional materials “beyond graphene,” it is possible to explore uncharted opportunities in designing novel heterostructures for electrically tunable devices. To fully reveal the emerging functionalities and opportunities of these atomically thin materials in practical applications, this review highlights several representative and noteworthy research directions in the use of electrical means to tune these aforementioned physical and structural properties, with an emphasis on discussing major applications of beyond graphene 2D materials in tunable devices in recent years and an outlook of what is to come in the next decade.

Scandium nitride (ScN) has recently attracted much attention for its potential applications in thermoelectric energy conversion, as a semiconductor in epitaxial metal/semiconductor superlattices, as a substrate for GaN growth, and alloying it with AlN for 5G technology. This study was undertaken to better understand its stoichiometry and electronic structure. ScN (100) single crystals 2 mm thick were grown on a single crystal tungsten (100) substrate by a physical vapor transport method over a temperature range of 1900–2000 °C and a pressure of 20 Torr. The core level spectra of Sc 2p_3/2,1/2 and N 1s were obtained by x-ray photoelectron spectroscopy (XPS). The XPS core levels were shifted by 1.1 eV toward higher values as the [Sc]:[N] ratio varied from 1.4 at 1900 °C to ∼1.0 at 2000 °C due to the higher binding energies in stoichiometric ScN. Angle-resolved photoemission spectroscopy measurements confirmed that ScN has an indirect bandgap of ∼1.2 eV.

Broadband, energy-efficient optoelectronic synaptic devices are essential for neuromorphic computing systems and embodied intelligent perception. Here, we present a plasmon-enhanced optoelectronic synapse based on an Ag nanoparticle-Sb2Te3/FTO (fluorine-doped tin oxide) heterojunction. The incorporation of Ag and FTO enhances light absorption in Sb2Te3 across a broad spectral range, while the localized surface plasmon resonance effect of the Ag nanoparticles and the p-Sb2Te3/n-FTO heterojunction improves the generation efficiency and subsequent separation and transport of photocarriers, resulting in a significantly enhanced photocurrent response. The device exhibits tunable photo-responsive memristive behavior and multilevel optoelectronic synaptic plasticity even at a low voltage of 1 V. Furthermore, we demonstrate compressed image encoding and recognition using two artificial neural networks, achieving an average accuracy of ∼95%. This work not only provides a strategy for designing high-performance, broadband, low-power optoelectronic synapses but also offers a viable device foundation for image compression encoding and intelligent visual perception.