arXiv subjects

Srimanta Pakhira

Publications and source records attributed to Srimanta Pakhira.

22 records · Page 2Linked to original sources

arXiv2017

Iron Intercalation in Covalent-Organic Frameworks: A Promising Approach for Semiconductors

Covalent-organic frameworks (COFs) are intriguing platforms for designing functional molecular materials. Here, we present a computational study based on van der Waals dispersion-corrected hybrid density functional theory (DFT-D) to design boroxine-linked and triazine-linked COFs intercalated with Fe. Keeping the original $P-6m2$ symmetry of the pristine COF (COF-Fe-0), we have computationally designed seven new COFs by intercalating Fe atoms between two organic layers. The equilibrium structures and electronic properties of both the pristine and Fe-intercalated COF materials are investigated here. We predict that the electronic properties of COFs can be fine tuned by adding Fe atoms between two organic layers in their structures. Our calculations show that these new intercalated-COFs are promising semiconductors. The effect of Fe atoms on the electronic band structures and density of states (DOSs) has also been investigated using the aforementioned DFT-D method. The contribution of the $d$-subshell electron density of the Fe atoms plays an important role in improving the semiconductor properties of these new materials. These intercalated-COFs provide a new strategy to create semi-conducting materials within a rigid porous network in a highly controlled and predictable manner.

cond-mat.mtrl-sci↗

arXiv2017

Low temperature synthesis of heterostructures of transition metal dichalcogenide alloys (WxMo1-xS2) and graphene with superior catalytic performance for hydrogen evolution

Large-area ($\sim$cm$^2$) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide(TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature (300 $^\circ$C). In particular, we synthesized stacked graphene and W$_x$Mo$_{1-x}$S$_2$ alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec$^{-1}$ and 96 mV onset potential (at current density of 10 mA cm$^{-2}$) when the heterostructure alloy is annealed at 300 $^o$C. These results indicate that heterostructure formed by graphene and W$_{0.4}$Mo$_{0.6}$S$_2$ alloys are far more efficient than WS$_2$ and MoS$_2$ by at least a factor of two, and it is superior than any other reported TMD system. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e. the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the W$_x$Mo$_{1-x}$S$_2$ alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H$_2$, with the lowest energy barrier occurring for W$_{0.4}$Mo$_{0.6}$S$_2$ alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies of creation of point defects, vacancies and edges. The synthesis of graphene/W$_{0.4}$Mo$_{0.6}$S$_2$ produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.

cond-mat.mtrl-sci↗

arXiv2017

Iron Intercalated Covalent-Organic Frameworks: First Crystalline Porous Thermoelectric Materials

Covalent-organic frameworks (COFs) are intriguing platforms for designing functional molecular materials. Here, we present a computational study based on van der Waals dispersion-corrected hybrid density functional theory calculations to analyze the material properties of boroxine-linked and triazine-linked intercalated-COFs. The effect of Fe atoms on the electronic band structures near the Fermi energy level of the intercalated-COFs have been investigated. The density of states (DOSs) computations have been performed to analyze the material properties of these kind of intercalated-COFs. We predict that COFs's electronic properties can be fine tuned by adding Fe atoms between two organic layers in their structures. The new COFs are predicted to be thermoelectric materials. These intercalated-COFs provide a new strategy to create thermoelectric materials within a rigid porous network in a highly controlled and predictable manner.

cond-mat.mtrl-sci↗

arXiv2017

An Alternative Strategy to Control the Electronic Properties of Bilayer Graphene: Semi-metal to Metal Transition and a 2D Material with Dirac Cone

The AB-stacked bilayer graphene (BLG) is a pure semiconductor whose band gap and properties can be tuned by various methods such as doping or applying gate voltage. Here we show an alternative method to control the electronic properties of BLG; intercalation of transition metal atoms between the two monolayers graphene (MLG). A theoretical investigation has been performed on two-dimensional MLG, BLG, BLG-intercalated nanostructured materials, all of which are energetically stable. Our study reveals that only MLG and BLG-intercalated with one Vanadium (V) atom (BLG-1V) materials have a Dirac Cone at the K-point. This study reveals a new strategy to control the material properties of BLG to exhibit various behaviors, including: metallic, semi-metallic, and semiconducting by varying the concentration and spin arrangement of the V atoms in BLG. In all the cases, the present DFT calculations show that the 2p$_z$ sub-shells of C atoms in graphene and the 3d$_{yz}$ sub-shells of the V atoms provide the electron density near the Fermi level controlling the material properties of the BLG-intercalated materials. Thus we prove that out-of-plane atoms can influence in plane electronic densities in BLG.

cond-mat.mtrl-sci↗