Zintl phase compounds as thermoelectric materials

alt text The Zintl-phase compounds are composed of electropositive cations and covalently bonded anionic networks formed from more electronegative elements. By first principles density functional theory and Boltzmann transport calculations, we report the excellent thermoelectric properties of Zintl phase compounds ACd2Sb2 (where, A = Ca, Ba, Sr). The calculated electronic structures of these compounds show charge carrier pockets and heavy light bands near the band edge, which lead to a large power factor. Furthermore, the strong anharmonicity in these compounds, results in very low lattice thermal conductivity. The combination of low thermal conductivity and the excellent transport properties give a high ZT value of 1.4–1.9 in CaCd2Sb2 and BaCd2Sb2 at moderate p and n-type doping.

Pt poisoning free CO oxidation bimetallic catalyst

alt text Pt bimetallic systems have extensively exhibited better catalytic activities and lower CO poisoning, however, complete elimination of Pt poisoning is still unresolved. We report for the first time that for Pt3Co/MgO(100), the preferred CO adsorption site inverts to Co from Pt. This inversion results from the better availability of empty anti-bonding d-states of Co atom in Pt3Co than those of the Pt atoms. This shall effectively address the issue of Pt-degradation by CO. Furthermore, Mars van Krevelan mechanism of CO oxidation on Pt3Co/Li-doped MgO(100), low reaction barrier of 0.11 eV predicts good reaction kinetics. Li-doped MgO(100) has a spontaneous activation of oxygen.

Anode Materials for Li-ion Batteries

alt text Finding a better anode material, having high specific capacity compared to the commercially available graphite (372 mAh/g) anode is the biggest challenge. While silicon has the highest theoretical capacity of 4200 mAh/g, the large volume expansion during charging/discharging prevents it from practical use. In order to protect Si anode, carbonaceous materials such as graphene or fullerene have been experimentally used to cover making it as artificial Solid-Electrolyte Interface (SEI). The success of this approach depends on achieving faster Li kinetics in such materials. We study the kinetics of Li-diffusion in pristine, defected, doped and doped defected mono-layer graphene structures. We find that Li prefers to diffuse over the basal plane of the sheet compared to penetrating through it. With optimum adsorption energy for Li and low diffusion barrier, the B doped mono-vacancy structures turn out to be better anode materials for LIBs.

Effect of pressure on 2D materials

alt text Transition metal dichalcogenides (TMDs)TMDs undergo a reversible semiconductor to metal (S-M) transition when an applied pressure exceeds a critical value. The metallization arises from the overlap of the valance and conduction bands owing to sulphur-sulphur interactions as the interlayer spacing reduces. We further extended this study to different monolayer MoS2 polytypes. Under hydrostatic pressure, the band gap of monolayer MoS2 increases by 11.7 %. Our results indicate that interlayer interaction plays an important role in semiconductor to metal transition. Due to the absence of interlayer interaction, the S-M transition for monolayer MoS2 occurs at a very high pressure of 69 GPa. .

Designing metallacarborane based room temperature hydrogen storage media

alt text Metallacarboranes are promising towards realizing room temperature hydrogen storage media because of the presence of both transition metal and carbon atoms. We have unraveled the underlying principles of designing an efficient metallacarborane based hydrogen storage media. The storage capacity of hydrogen depends upon the number of available transition metal d-orbitals, number of carbons, and dopant atoms in the cluster. These factors control the amount of charge transfer from metal to the cluster, thereby affecting the number of adsorbed hydrogen molecules. This correlation between the charge transfer and storage capacity is general in nature, and can be applied to designing efficient hydrogen storage systems.

Semiconductor to metal transition in bilayer phosphorene under normal compressive strain

alt text Phosphorene has recently garnered interest due to its high p-type mobility and layer-dependent direct band gap. However, the possibility of tuning its properties through strain was never explored. We have shown a semiconductor to metal (S-M) transition under normal compressive (NC) strain. From an applications perspective, transport properties as a function of NC strain were studied. Interestingly, the order of magnitude of the mobilities did not change significantly under strain, implying that transport properties are preserved. Such tunability in the electronic structure whilst preserving the transport properties makes phosphorene a suitable candidate in nanoelectronics applications.

Pentahexoctite: a new two dimensional allotrope of carbon

alt text The success of graphene has motivated the search for newer forms of 2D materials. This resulted into finding of various 2D allotropes such as graphyne, graphdiyne, pentaheptite and haeckelites. On the other hand, sheets consisting of 5–7 defects such as, pentaheptite and haeckelite are formed by the Stone-Wales transformation of bonds. Here, we find ‘‘pentahexoctite’’, a sheet consisting of a network of pentagons, hexagons and octagons. The mechanical strength of the sheet is comparable to graphene. Electronic structure shows this sheet to be metallic with a combination of flat and dispersive bands at the Fermi level. The sheet can be rolled into stable nanotubes with chirality-dependent mechanical and electronic properties. These novel properties of the pentahexoctite sheet make it a potential candidate for nanoelectronics devices.

Vacancy mediated clipping of multi-layered graphene

alt text Defects in bi-layer graphene (both AA- and AB-stacked) interact forming inter-layer covalent bonds, giving rise to two-dimensional (2D) clipped structures, without explicit use of functional groups. These clipped structures can be transformed into one-dimensional (1D) double wall nanotubes (DWCNT) or multi-layered three dimensional (3D) bulk structures. These clipped structures show good mechanical strength due to covalent bonding between multi-layers. Clipping also provides a unique way to simultaneously harness the conductivity of both walls of a double wall nanotube through covalently bonded scattering junctions. With additional conducting channels and improved mechanical stability, these clipped structures can lead to a myriad of applications in novel devices.