Laboratory of Multifunctional Molecular Nanofilms at IIT Kanpur, India

Current research interest

The rich diversity of the molecules can be exploited in studying the “proof-of-principle” of the proto-type systems for which the molecules need to be confined in space and time to have better control over the molecular dynamics. We aim at surface engineering using small molecules towards the development of varied techniques for the fabrication of molecular electronic devices. Such platforms are exploited or the understanding of current-voltage (I-V) response as functions of temperature, molecular layer thickness, structures, functionalities, and more importantly electrode compositions. We utilize various grafting methods, electrochemical reduction is one of them. This technique yields covalent bond formation between substrate and target molecules. Recent advances in this area reveal that conducting sp2 carbon electrodes can challenge conventional metallic electrodes in the field of "Molecular Electronics" (ME). Functional organic molecules such as aryl diazonium can easily be electrochemically grafted on carbon , and metallic electrodes with desired thickness, and compositions.

Our primary research lies on the "molecules as circuit elements" in understanding and mimicking CMOS functions. 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.

We use different surface analysis tools including UV-Vis-NIR spectrophotometer, FT-IR and electro-analytical techniques (DC, and AC measurements), AFM, XPS, SEM, electrical measurements, polarized microscope, PXRD, etc. We have made all the facilities needing to fabricate and electrical measurements of nanoscale molecular junctions.

Latest research from the Group

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)– Tris(phenanthroline)-Based Molecular Assemblies (Nano Lett. 2023)

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)–tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4–16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current–voltage (j–V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm–1). The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10–16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole–Frenkel charge conduction, showing a clear “molecular signature” in the nanometric junctions [Gupta et al., Nano Lett. 2023, 23, 10998].

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Coexistence of Electrochromism and Bipolar Nonvolatile Memory in a Single Viologen (ACS Appl. Mater. Interfaces 2023, 51527)

Viologens are fascinating redox-active organic compounds that have been widely explored in electrochromic devices (ECDs). However, the combination of electrochromic and resistive random-access memory in a single viologen remains unexplored. We report the coexistence of bistate electrochromic and single-resistor (1R) memory functions in a novel viologen. A high-performance electrochromic function is achieved by combining viologen (BzV2+2PF6) with polythiophene (P3HT), enabling a “push–pull” electronic effect due to the efficient intermolecular charge transfer in response to an applied bias. The ECDs show high coloration efficiency (ca. 1150 ± 10 cm2 C–1), subsecond switching time, good cycle stability (>103 switching cycles), and low-bias operation (±1.5 V). The ECDs require low power for switching the color states (55 μW cm–2 for magenta and 141 μW cm–2 for blue color). The random-access memory devices (p+2-Si/BzV2+2PF6/Al) exhibit distinct low and high resistive states with an ON/OFF ratio of ∼103, bipolar and nonvolatile characteristics that manifest good performances, and “Write”–“Read”–“Erase” (WRE) functions. The charge conduction mechanism of the RRAM device is elucidated by the Poole–Frenkel model where SET and RESET states arise at a low transition voltage (VT = ±1.7 V). Device statistics and performance parameters for both electrochromic and memory devices are compared with the literature data. Our findings on electrochromism and nonvolatile memory originated in the same viologen could boost the development of multifunctional, smart, wearable, flexible, and low-cost optoelectronic devices. [Parashar et al., ACS Appl. Mater. Interfaces 2023, 51527].

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films (Angewandte Chemie International Edition 2023)

To understand broadband molecular spintronics, pyrene oligomer film (≈20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ≈1.02×1018 m−2, a value comparable to the conventional heavy metals. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics. [Gupta et al., Angewandte Chemie International Edition. 2023, 23, 10998].

Flexible molecular electrochromic devices run by low-cost commercial cells (Advanced Optical Materials 2023)

The present era has seen tremendous demands for low-cost electrochromic materials for visible-region multicolor display technology, paper-based, flexible, and wearable electronic devices, smart windows, and optoelectronic applications. Towards this goal, we fabricate large-scale, high-yield and robust polyelectrochromic devices fabricated on rigid to flexible ITO substrates comprising novel anthracene containing viologen, (1,1″-bis(anthracen-9-ylmethyl)-[4,4″-bipyridine]-1,1'-diium bromide, abbreviated as AnV2+), and polythiophene (P3HT). Interestingly, the devices show three states of reversible visible color in response to the applied bias, sub-second to second switching time (0.7 s/1.6 s), high coloration efficiency (484 cm2/C), and longer cycling stability up to 9,000 s (3,000 switching cycles). Introduction of the anthracene moieties to viologen inhibits the formation of an undesired dimer of cation radicals in response to the applied bias, otherwise the device's color-switching would be hampered when the bias polarity is reversed. The fabricated electrochromic devices are tested with commercially available low-cost cells to perform—a unique approach toward practical applications. This work shows CMOS compatibility and can pave the way for developing cost-effective flexible and wearable electrochromic devices. [Parashar et al., Adv. Optical Mater. 2023, 2202920].

Nanoscale molecular rectifiers (Nature Reviews Chemistry 2023, 106)

The use of molecules bridged between two electrodes as a stable rectifier is an important goal in molecular electronics. Until recently, however, and despite extensive experimental and theoretical work, many aspects of our fundamental understanding and practical challenges have remained unresolved and prevented the realization of such devices. Recent advances in custom-designed molecular systems with rectification ratios exceeding 105 have now made these systems potentially competitive with existing silicon-based devices. Here, we provide an overview and critical analysis of recent progress in molecular rectification within single molecules, self-assembled monolayers, molecular multilayers, heterostructures, and metal–organic frameworks and coordination polymers. Examples of conceptually important and best-performing systems are discussed, alongside their rectification mechanisms. We present an outlook for the field, as well as prospects for the commercialization of molecular rectifiers. [Gupta et al., Nature Reviews Chemistry 7, 106, 2023].