Srimanta Pakhira

Dirac cone in two dimensional bilayer graphene by intercalation with V, Nb, and Ta transition metals

Bilayer graphene (BLG) is a semiconductor whose band gap and properties can be tuned by various methods such as doping or applying gate voltage. Here, we show how to tune electronic properties of BLG by intercalation of transition metal (TM) atoms between two monolayer graphene (MLG) using a novel dispersion-corrected first-principle density functional theory (DFT) approach. We intercalated V, Nb, and Ta atoms between two MLG. We found that the symmetry, the spin, and the concentration of TM atoms in BLG-intercalated materials are the important parameters to control and to obtain a Dirac cone in their band structures. Our study reveals that the BLG intercalated with one vanadium (V) atom, BLG-1V, has a Dirac cone at the K-point. In all the cases, the present DFT calculations show that the 2pz sub-shells of C atoms in graphene and the 3dyz sub-shells of the TM atoms provide the electron density near the Fermi energy level (EF) which controls the material properties. Thus, we show that out-of-plane atoms can influence in-plane electronic densities in BLG and enumerate the conditions necessary to control the Dirac point. This study offers insight into the physical properties of 2D BLG intercalated materials and presents a new strategy for controlling the electronic properties of BLG through TM intercalation by varying the concentration and spin arrangement of the metals resulting in various conducting properties, which include: metal, semi-metal and semiconducting states.

Achieving Fast and Efficient K+ Intercalation on Ultrathin Graphene Electrodes Modified by a Li+ Based Solid-Electrolyte Interphase

Advancing beyond Li-ion batteries requires translating the beneficial characteristics of Li+ electrodes to attractive, yet incipient, candidates such as those based on K+ intercalation. Here, we use ultrathin few-layer graphene (FLG) electrodes as a model interface to show a dramatic enhancement of K+ intercalation performance through a simple conditioning of the solid-electrolyte interphase (SEI) in a Li+ containing electrolyte. Unlike the substantial plating occurring in K+ containing electrolytes, we found that a Li+ based SEI enabled efficient K+ intercalation with discrete staging-type phase transitions observed via cyclic voltammetry at scan rates up to 100 mVs-1 and confirmed as ion-intercalation processes through in situ Raman spectroscopy. The resulting interface yielded fast charge-discharge rates up to ∼360C (1C is fully discharge in 1 h) and remarkable long-term cycling stability at 10C for 1000 cycles. This SEI promoted the transport of K+ as verified via mass spectrometric depth profiling. This work introduces a convenient strategy for improving the performance of ion intercalation electrodes toward a practical K-ion battery and FLG electrodes as a powerful analytical platform for evaluating fundamental aspects of ion intercalation.

Modulating Electrocatalysis on Graphene Heterostructures: Physically Impermeable Yet Electronically Transparent Electrodes

The electronic properties and extreme thinness of graphene make it an attractive platform for exploring electrochemical interactions across dissimilar environments. Here, we report on the systematic tuning of the electrocatalytic activity toward the oxygen reduction reaction (ORR) via heterostructures formed by graphene modified with a metal underlayer and an adlayer consisting of a molecular catalyst. Systematic voltammetric testing and electrochemical imaging of patterned electrodes allowed us to confidently probe modifications on the ORR mechanisms and overpotential. We found that the surface configuration largely determined the ORR mechanism, with adlayers of porphyrin molecular catalysts displaying a higher activity for the 2e- pathway than the bare basal plane of graphene. Surprisingly, however, the underlayer material contributed substantially to lower the activation potential for the ORR in the order Pt > Au > SiO x, strongly suggesting the involvement of the solution-excluded metal on the reaction. Computational investigations suggest that ORR enhancements originate from permeation of metal d-subshell electrons through the graphene layer. In addition, these physically impermeable but electronically transparent electrodes displayed tolerance to cyanide poisoning and stability toward long-term cycling, highlighting graphene as an effective protection layer of noble metal while enabling electrochemical interactions. This work has implications in the mechanistic understanding of 2D materials and core-shell-type heterostructures for electrocatalytic reactions.