Aaditya Manjanath

Semiconductor to metal transition in bilayer phosphorene under normal compressive strain

Phosphorene, a two-dimensional analog of black phosphorous, has been a subject of immense interest recently, due to its high carrier mobilities and a tunable bandgap. So far, tunability has been predicted to be obtained with very high compressive/tensile in-plane strains, and vertical electric field, which are difficult to achieve experimentally. Here, we show using density functional theory based calculations the possibility of tuning electronic properties by applying normal compressive strain in bilayer phosphorene. A complete and fully reversible semiconductor to metal transition has been observed at [Formula: see text] strain, which can be easily realized experimentally. Furthermore, a direct to indirect bandgap transition has also been observed at [Formula: see text] strain, which is a signature of unique band-gap modulation pattern in this material. The absence of negative frequencies in phonon spectra as a function of strain demonstrates the structural integrity of the sheets at relatively higher strain range. The carrier mobilities and effective masses also do not change significantly as a function of strain, keeping the transport properties nearly unchanged. This inherent ease of tunability of electronic properties without affecting the excellent transport properties of phosphorene sheets is expected to pave way for further fundamental research leading to phosphorene-based multi-physics devices.

pentahexoctite: A new two-dimensional allotrope of carbon

The ability of carbon to exist in many forms across dimensions has spawned search in exploring newer allotropes consisting of either, different networks of polygons or rings. While research on various 3D phases of carbon has been extensive, 2D allotropes formed from stable rings are yet to be unearthed. Here, we report a new sp2 hybridized two-dimensional allotrope consisting of continuous 5-6-8 rings of carbon atoms, named as “pentahexoctite”. The absence of unstable modes in the phonon spectra ensures the stability of the planar sheet. Furthermore, this sheet has mechanical strength comparable to graphene. Electronically, the sheet is metallic with direction-dependent flat and dispersive bands at the Fermi level ensuring highly anisotropic transport properties. This sheet serves as a precursor for stable 1D nanotubes with chirality-dependent electronic and mechanical properties. With these unique properties, this sheet becomes another exciting addition to the family of robust novel 2D allotropes of carbon.

Diffusive nature of thermal transport in stanene

Using the phonon Boltzmann transport formalism and density functional theory based calculations, we show that stanene has a low thermal conductivity. For a sample size of 1 × 1 μm(2) (L × W), the lattice thermal conductivities along the zigzag and armchair directions are 10.83 W m(-1) K(-1) and 9.2 W m(-1) K(-1) respectively, at room temperature, indicating anisotropy in thermal transport. The low values of thermal conductivity are due to large anharmonicity in the crystal resulting in high Grüneisen parameters, and low group velocities. The room temperature effective phonon mean free path is found to be around 17 nm indicating that the thermal transport in stanene is completely diffusive in nature. Furthermore, our study reveals the relative importance of the contributing phonon branches and that, at very low temperatures, the contribution to lattice thermal conductivity comes from the flexural acoustic (ZA) branch and at higher temperatures it is dominated by the longitudinal acoustic (LA) branch. We also show that the lattice thermal conductivity of stanene can further be reduced by tuning the sample size and creating rough surfaces at the edges. Such tunability of lattice thermal conductivity in stanene suggests its applications in thermoelectric devices.

Negative differential resistance in armchair silicene nanoribbons

Due to dimensional confinement of carriers and non-trivial changes in the electronic structure, novel tunable transport properties manifest in nanoscale materials. Here, we report using first-principles density functional theory and non-equilibrium Greens function formalism, the occurrence of negative differential resistance (NDR) in armchair silicene nanoribbons (ASNRs). Interestingly, NDR manifests only in pristine [Formula: see text] ASNRs, where [Formula: see text]. We show that the origin of such a novel transport phenomenon lies in the bias-induced changes in the density of states of this particular family of nanoribbons. With increasing width of the nanoribbons belonging to this family, the peak-to-valley ratios of current decrease due to the increase in the number of sub-bands leading to a reduction in NDR. NDR is possible not only in [Formula: see text] ASNRs, but also in mixed configurations of armchair and zigzag silicene nanoribbons. This universality of NDR along with its unprecedented width-induced tunability can be useful for silicene-based low-power logic and memory applications.

Simultaneous tunability of the electronic and phononic gaps in SnS2 under normal compressive strain

Controlled variation of the electronic properties of. two-dimensional (2D) materials by applying strain has emerged as a promising way to design materials for customized applications. Using density functional theory (DFT) calculations, we show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS2 exhibit a reversible semiconductor to metal (S-M) transition with applied strain. For bilayer (2L) SnS2, the S-Mtransition occurs at the strain values of 17%,-26%, and -24% under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve the S-Mtransition in SnS2 under NC strain is much higher than for MoS2. From a stability viewpoint, SnS2 becomes unstable at very low strain values on applying BC (-6.5%) and BT strains (4.9%), while it is stable even up to the transition point (-24%) in the case of NC strain. In addition to the reversible tuning of the electronic properties of SnS2, we also show tunability in the phononic band gap of SnS2, which increases with applied NC strain. This gap increases three times faster than for MoS2. This simultaneous tunability of SnS2 at the electronic and phononic levels with strain, makes it a potential candidate in field effect transistors (FETs) and sensors as well as frequency filter applications.

Strain-induced phenomena in layered materials

Using first principles density functional theory (DFT), we investigate the effect of normal compressive strain on the bilayers of MoS2, SnS2, and their van der Waals heterostructure. These materials and the corresponding heterostructure show a universal phenomenon of reversible semiconductor-metal (S-M) transition under applied strain. Most interestingly, a van der Waals heterostructure of MoS2 and SnS2 is found to have an effective direct band gap of 0.71 eV at Γ-point. This inherent ease of tunability of electronic properties of these materials by applying strain or heterostructuring is expected to pave way for further fundamental research leading to multi-physics devices.