Joydeep Bhattacharjee

Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures

The first synthesis and characterization of [9]-, [12]-, and [18]cycloparaphenylene was demonstrated utilizing a novel aromatization reaction. We refer to these fascinating structures as “carbon nanohoops” due to their structural similarity to carbon nanotubes. Additionally, we have utilized computational methods to understand the unique properties of these fully conjugated macrocycles.

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions

Carbon nanotube heterojunctions (HJs), which seamlessly connect nanotubes of different chiral structure using a small number of atomic-scale defects, represent the ultimate scaling of electronic interfaces. Here we report the first electrical transport measurements on a HJ formed between semiconducting and metallic nanotubes of known chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD) with approximately 60 meV charging energy and approximately 75 meV level spacing. A detailed atomistic and electronic model of the HJ enables the identification of specific defect arrangements that lead to the QD behavior consistent with the experiment.

Activation of Graphenic Carbon Due to Substitutional Doping by Nitrogen: Mechanistic Understanding from First Principles.

Nitrogen-doped graphene and carbon nanotubes are popularly in focus as metal-free electrocatalysts for oxygen reduction reactions (ORR) central to fuel cells. N-doped CNTs have been also reported to chemisorb mutually, promising a route to their robust predetermined assembly into devices and mechanical reinforcements. We propose from first principles a common mechanistic understanding of these two aspects pointing further to a generic chemical activation of carbon atoms due to substitution by nitrogen in experimentally observed configurations. Wannier-function based orbital resolved study of mechanisms suggests increase in C-N bond-orders in attempt to retain π-conjugation among carbon atoms, causing mechanical stress and loss of charge neutrality of nitrogen and carbon atoms, which remedially facilitate chemical activation of N-coordinated C atoms, enhancing sharply with increasing coordination to N and proximity to zigzag edges. Activated C atoms facilitate covalent adsorption of radicals in general, diradicals like O2 relevant to ORR, and also other similarly activated C atoms, leading to self-assembly of graphenic nanostructures while remaining inert to ordinary graphenic C atoms.

Synthesis and characterization of robust three-dimensional chiral metal sulfates

Two chiral three-dimensional metal sulfates of the compositions [NH4]8[Mn8(SO4)12], (1) and [NH4]8[Mg8(SO4)12], (2) of the langbeinites family have been synthesized under hydro/solvothermal conditions and studied using first principles within the framework of density functional theory (DFT). Both the compounds 1 and 2 are isostructural and crystallize in the cubic chiral space group P2(1)3. In the presence of NH4+ cations, the frameworks of 1 and 2 have three-dimensional structures formed by the corner sharing of metal octahedra and sulfate tetrahedra through M–O–S linkages, which leads to a pinwheel arrangement of the metal octahedra surrounded by six sulfate tetrahedra. Both 1 and 2 are observed to retain their structures upon the thermal decomposition of ammonium ion, resulting into the compositions [H]8[Mn8(SO4)12] and [H]8[Mg8(SO4)12], respectively. The magnetic measurement of 1 shows a paramagnetic behaviour and the hydrogen adsorption of 1 and 2 shows 0.45 and 0.7 wt%, respectively at 77 K. The proton conductivity of 1 under various relative humidities (RH) shows 2.6 × 10−7 S cm−1 at RH = 80% and reaches to 3.1 × 10−4 S cm−1 at RH = 90%. First-principles calculations suggest structural evolution along with the retention of framework structures of similar stabilities by both the compounds after de-ammoniation.

Bias induced ferromagnetism and half-metallicity in graphene nano-ribbons

Towards spin selective electronics made of three coordinated carbon atoms, here we computationally propose robust and reversibly bias driven evolution of pristine undoped graphene nano-ribbons(GNR) into ferromagnetic-semiconductor, metal or a half metal, irrespective of their edge configurations. The evolution is a result of a rare ferromagnetic(FM) order emerging among nearest neighbouring(n-n) sites, in positively biased regions in their in-homogeneous bias unit-cells, in attempt to cooperatively minimise on-site Coulomb repulsion and kinetic energy, while maximising localization of electrons at the positively biased sites. The phenomenon appears to be a general property of in-homogeneously biased Coulomb correlated bipartite systems. Consequences are particularly rich in zigzag edged graphene nano-ribbons(ZGNR) due to the contest of bias driven n-n FM order and the inter-edge antiferromagnetic order inherent to ZGNRs, leading to systematic closing of gap for one of the spins, amounting to bias controlled unmissable opening of window for FM-semiconducting and half-metallic transport.