Lujie Huang

High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity

The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the atomic, sub-nanometre length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technology. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high electrical carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technologically relevant scale without film transfer. In addition, their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. Here we report the preparation of high-mobility 4-inch wafer-scale films of monolayer molybdenum disulphide (MoS2) and tungsten disulphide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal–organic chemical vapour deposition technique, and show high electrical performance, including an electron mobility of 30 cm2 V−1 s−1 at room temperature and 114 cm2 V−1 s−1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.

Coherent, atomically thin transition-metal dichalcogenide superlattices with engineered strain

Coherent strained superlattices Two-dimensional superlattices represent the atomic-thickness limit of heterostructures that enable technologies such as strain-engineered multiferroics and quantum-cascade lasers. Xie et al. were able to produce monolayer superlattices of transition metal dichalcogenides (WS2 and WSe2) with full lattice coherence, despite a 4% lattice mismatch. They used a modulated metal-organic chemical vapor deposition process that precisely controlled each precursor. Furthermore, the authors could strain-engineer the optical properties of the superlattices to observe out-of-plane rippling. Science, this issue p. 1131 Omnidirectional epitaxy produced superlattices with strain-engineered optical properties and mechanical deformations. Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically thin superlattices in which different transition metal dichalcogenide monolayers—despite large lattice mismatches—are repeated and laterally integrated without dislocations within the monolayer plane. Grown by an omnidirectional epitaxy, these superlattices display fully matched lattice constants across heterointerfaces while maintaining an isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 millielectron volts. We present theoretical models to explain this coherent growth and the energetic interplay governing the ripple formation in these strained monolayers. Such coherent superlattices provide building blocks with targeted functionalities at the atomically thin limit.

Ultrafast microscopy captures the dynamics of bound excitons in twisted bilayer van der Waals materials

Stacking and twisting 2D van der Walls (vdW) materials can create unique electronic properties that are not accessible in a single sheet of material. When two sheets of van der Waals material such as graphene are stacked in an off-axis angle, in a twisted bilayer graphene (tBLG) configuration, electronic properties are modified from interlayer orbital hybridization effects. For instance, in tBLG we can access both massless and massive chiral quasiparticles characteristics of graphene and bilayer graphene, as well as angle tunable optical resonances that are not present in graphene or bilayer graphene. In addition, first principle simulation predicts that upon optical resonant excitation of tBLG, bound exciton formation is a possibility due to cancelation of exciton-continuum coupling from anti-symmetric superposition of degenerate resonant transitions. In order to study possible bound exciton formation, we map out the electronic structure of single grain tBLG using multi-photon transient absorption microscopy. Surprisingly, upon resonant optical excitations, tBLG shows enhanced transient response with longer carrier compared to AB stacked bilayer graphene. Further, we find that the origin of this unexpected optical response can be best explained by the presence of a lower lying bound exciton state predicted by recent theoretical simulations. This suggests that tBLG is a novel 2D hybrid material that enables the creation of both strongly-bound excitons along-side highly-conductive continuum states. Recently, the family of 2D vdW materials has grown appreciably. As such, there are countless possibilities for stacking and twisting 2D vDw materials to produce similar interlayer electronic states for next generation optoelectronics.