Current research interest
Inorganic, organic or inorganic-organic hybrid materials own intriguing phenomena such as conductivity, magnetism, optics to name a few which are not fully understood yet. The rich diversity of the molecules can be exploited in studying “proof-of-principle” of the proto- type systems for which the molecules need to be confined in space and time in order to have a better control over the molecular dynamics. Till date, well explored molecular assemblies are thiolated derivatives adsorbed mostly on Au substrate. However, Au-S bonds suffer due to its reactivity, instability over time and thus not suitable for devices for real world applications. To overcome this issue, covalent based monolayers are attractive alternative, as it forms stable covalent bond formation between substrate and target molecules. Recent advances in this area reveals that conducting sp2 carbon electrode can challenge conventional metallic electrode in the field of "Molecular Electronics" (ME). Functional organic molecules such as aryl diazonium can easily be electrochemically grafted on carbon electrode with desired thickness. Carbon-carbon (substrate-molecule) bond is highly thermally stable and thus suitable for low to high temperature range study under some other stimuli.
Our primary research would be focused on "engineering of synthesized molecules as circuit elements'. We are interested in creating Carbon-Molecule-Carbon (CMC) stacking configuration a fascinating alternative of metal-molecule-metal (MMM) based molecular junctions. Charge-transport phenomena would be studied under external stimuli such as magnetic field, solvents, light to see modulation of conductance. Molecules with an appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are fruitful for efficient charge injection to/from electrode. Our group will be actively in search of these molecules for ME studies.
Laboratory synthesized molecules and metal/molecule interfaces would be characterized by full battery techniques such as NMR, UV-Vis, ESI-MS, FTIR and electrochemistry, electrical measurements, AFM, TEM, SEM, PXRD, Raman spectroscopy, ESI, etc.
Charge-transport in organic molecular junctions (research carried out at University of Alberta, Canada):
Mobile ions containing molecular junctions sandwiched between sp2 conducting carbon electrodes were studied for solvent mediated charge-transport behavior. We observed internal potential profile and electric field which controls electronic properties drastically changed in presence of polar solvent media over dry condition. Molecular junctions consisting of ~8.5 nm thick layers of fluorene (FL) oligomer oriented in parallel between conducting contacts didn't show any effect of the solvent. In this case, the potential profile is assumed mostly linear in the simplest cases, but can be affected by internal dipoles, charge polarization, and electronic coupling between the contacts and the molecular layer. However, mobile ions (Li+) containing benzoic acid (BA) electrochemically grafted on predefined FL-layer on flat, conducting carbon electrode showed highly modulation in conductance when measured in acetonitrile (ACN) vapor in a vacuum chamber (Janis probe station). Transient experiments revealed that conductance changes occur on a microsecond- millisecond time scale, and are accompanied by significant alteration of device impedance and temperature dependence. A single molecular junction containing lithium benzoate could be reversibly transformed from symmetric current-voltage behavior to a rectifier by repetitive bias scans [Mondal et al., J. Am. Chem. Soc., 2018, 7239].
Spin-dependent electrochemistry in chiral molecules (research executed at Weizmann Institute of Science, Israel:
At WIS, we have studied “spin dependent electrochemistry of chiral molecular films embedded on ferromagnetic electrodes”. Earlier studies showed that an electrochemical charge-transfer process can be affected by an external magnetic field. However, no report was available whether a chiral electrode can exhibit spin-selectivity. In order to accomplish the spin selectivity effect, spin polarized electrons need to be injected into target molecules. According to the "spintronics", a specific spin can be injected from a ferromagnetic electrode by applying an external magnetic field. Recent experimental studies by Ron and his team showed that molecules can act as electron spin filters which means transmission of electrons through chiral molecules depends on the electrons’ spin orientation; this effect is referred to as chirality induced spin selectivity (CISS). Thus, CISS effect can be studied electrochemically by attaching chiral molecules onto a ferromagnetic electrode. Ferromganetic materials have several advantages, however, due to its extremely reactivity in aqueous solution which oxidize the substrate [Mondal et al., Acc. Chem. Res., 2016, 2560]. Thus, FMs need a special care, particularly during electrochemical studies. We have developed several intriguing methods to coat the ferromagnetic electrodes with biomolecules such as amino acids, oligopeptides, redox proteins, chiral polymers, and oligopeptide-CdSe NPs without affecting/damazing their structures assembled on the substrate. Freshly deposited Ni electrode [in a Clean Room of class 1000] was either in situ electrochemically coated by chiral conducting polymers or an ultrathin Au over layer for studying spin-dependent electrchemistry. Interestingly, we have observed reverse spin-polarization when chirality of the molecules were reversed (for eg., L-cysteine to D-cysteine) and also reasonable spin-polarization was obtained with redox-protein such as cytochrome c [Mondal et al., ACS Nano, 2015, 3377]. In addition to the magnetic field dependent electrochemical studies, solid state magnetoresistance by applying an external magnetic field was also performed to ensure spin-selectivity presence in chiral conducting polymer [Mondal et al., Adv. Mater., 2015,1924].
Spin-dependent photoluminescence study in helical oligopeptide-CdSe NPs assembly showed that SPIN UP direction quenches the PL intensity over SPIN DOWN measurements. We observed nearly 30% PL quenching, while achirla-CdSe NPs assemblies didn't show detectable alternation [Mondal et al., Nano Lett., 2016, 2806].