Semiconductors are the basis of many vital technologies such as electronics, computing, communications, optoelectronics, and sensing. Modern semiconductor technology can trace its origins to the invention of the point contact transistor in 1947. This demonstration paved the way for the development of discrete and integrated semiconductor devices and circuits that has helped to build a modern society where semiconductors are ubiquitous components of everyday life. A key property that determines the semiconductor electrical and optical properties is the bandgap. Beyond graphene, recently discovered two-dimensional (2D) materials possess semiconducting bandgaps ranging from the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides, to the ultraviolet in hexagonal boron nitride. In particular, these 2D materials were demonstrated to exhibit highly tunable bandgaps, achieved via the control of layers number, heterostructuring, strain engineering, chemical doping, alloying, intercalation, substrate engineering, as well as an external electric field. We provide a review of the basic physical principles of these various techniques on the engineering of quasi-particle and optical bandgaps, their bandgap tunability, potentials and limitations in practical realization in future 2D device technologies.

We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II and III heterojunctions; an enticing possibility being atomic heterostructures where the constituents monolayers have band edges at the zone center, i.e. Γ valley. Our study, which includes the Group IV and III-V compound monolayer materials, Group V elemental monolayer materials, transition metal dichalcogenides (TMD) and transition metal trichalcogenides (TMT) reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using firstprinciples density functional calculations, we present the type of the heterojunction for 903 different possible combination of these 2D materials which establishes a periodic table of heterojunctions.

In integrated photonics, specific wavelengths are preferred such as 1550 nm due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, layered two-dimensional (2D) materials bear scientific and technologicallyrelevant properties such as electrostatic tunability and strong light-matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with 2D transition metal dichalcogenide (TMDCs) materials due to their large optical bandgap. Here, we demonstrate a MoTe2-based photodetector featuring strong photoresponse (responsivity = 0.5 A/W) operating at 1550 nm on silicon micro ring resonator enabled by strain engineering of the transition-metal-dichalcogenide film. We show that an induced tensile strain of ~4% reduces the bandgap of MoTe2, resulting in large photo-response in the telecommunication wavelength, in otherwise photo-inactive medium when unstrained. Unlike Graphene-based photodetectors relying on a gapless band structure, this semiconductor-2D material detector shows a ~100X improved dark current enabling an efficient noise-equivalent power of just 90 pW/Hz 0.5 . Such strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe2) phase, metallic monoclinic (1T′) MoTe2 phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T′ MoTe2 homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe2 grow out of a polycrystalline 1T′-MoTe2 matrix. We further demonstrate the operation of MoTe2 FETs made on these in situ-grown lateral homojunctions with 1T′ contacts. The use of a 1T′ phase as electrodes in MoTe2 FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T′ electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T′ interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe2 homojunction FETs for use in memory and logic circuity applications.

Controlled doping of the p-type 2D material tungsten diselenide, done with niobium substitution for tungsten on the crystal lattice, can tune 2D transistor characteristics.

We investigate excitons in stacked transition metal dichalcogenide (TMDC) layers under perpendicularly applied electric field, herein MoSe2/WSe2 van der Waals heterostructures (vdWH). Band structures are obtained with density functional theory (DFT), along with electron and hole wave functions in conduction and valence bands, respectively. A minimal continuum model, parametrized by the DFT results, is presented, allowing for calculation of the excitonic states. Although the type-II nature of the heterostructure leads to fully charge separated inter-layer exciton on the ground states, our results show that moderate values of electric field produce more evenly distributed wave functions along the vdWH, namely, hybrid inter/intra layer exciton states, where both the inter-layer exciton binding energy and, most notably, its oscillator strength are enhanced.PACS numbers:

We show that when two Heusler alloys are layered in the [001], [110], or [111] directions for various thicknesses to form a superlattice, the Slater-Pauling rule may still be satisfied and the resulting superlattice is often half-metallic with gaps comparable to or larger than those of its constituents. In addition, uniaxial magnetocrystalline anisotropy is induced because of the differences in the electronic structure of the two Heuslers in the superlattice. Various full-full, full-half, and half-half Heusler superlattices are studied, and potential half-metallic superlattices with perpendicular magnetocrystalline anisotropy are identified.

Based on first-principles density functional calculations, we show that SnS and SnSe layers can form mechanically rigid heterostructures with the constituent puckered or buckled monolayers. Due to the strong interlayer coupling, the electronic wavefunctions of the conduction and valence band edges are delocalized across the heterostructure. The resultant bandgap of the heterostructures reside in the infrared region. With strain engineering, the heterostructure bandgap undergoes transition from indirect to direct in the puckered phase. Our results show that there is a direct correlation between the electronic wavefunction and the mechanical rigidity of the layered heterostructure.PACS numbers: