Nipun Gupta

@article{ISI:000865428500001,
title = {Temperature-Dependent Coherent Tunneling across Graphene-Ferritin Biomolecular Junctions},
author = {Nipun Kumar Gupta and Senthil Kumar Karuppannan and Rupali Reddy Pasula and Ayelet Vilan and Jens Martin and Wentao Xu and Esther Maria May and Andrew R Pike and Hippolyte P A G Astier and Teddy Salim and Sierin Lim and Christian A Nijhuis},
doi = {10.1021/acsami.2c11263},
times_cited = {0},
issn = {1944-8244},
year = {2022},
date = {2022-09-23},
journal = {ACS APPLIED MATERIALS & INTERFACES},
volume = {14},
number = {39},
pages = {44665-44675},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {Understanding the mechanisms of charge transport (CT) across biomolecules in solid-state devices is imperative to realize biomolecular electronic devices in a predictive manner. Although it is well-accepted that biomolecule-electrode interactions play an essential role, it is often overlooked. This paper reveals the prominent role of graphene interfaces with Fe-storing proteins in the net CT across their tunnel junctions. Here, ferritin (AfFtn-AA) is adsorbed on the graphene by noncovalent amine-graphene interactions confirmed with Raman spectros-copy. In contrast to junctions with metal electrodes, graphene has a vanishing density of states toward its intrinsic Fermi level ( "Dirac point "), which increases away from the Fermi level. Therefore, the amount of charge carriers is highly sensitive to temperature and electrostatic charging (induced doping), as deduced from a detailed analysis of CT as a function of temperature and iron loading. Remarkably, the temperature dependence can be fully explained within the coherent tunneling regime due to excitation of hot carriers. Graphene is not only demonstrated as an alternative platform to study CT across biomolecular tunnel junctions, but it also opens rich possibilities in employing interface electrostatics in tuning CT behavior.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000853565300001,
title = {The Role of Structural Order in the Mechanism of Charge Transport across Tunnel Junctions with Various Iron-Storing Proteins},
author = {Nipun Kumar Gupta and Naofumi Okamoto and Senthil Kumar Karuppannan and Rupali Reddy Pasula and Zhang Ziyu and Dong-Chen Qi and Sierin Lim and Masakazu Nakamura and Christian A Nijhuis},
doi = {10.1002/smll.202203338},
times_cited = {0},
issn = {1613-6810},
year = {2022},
date = {2022-09-14},
journal = {SMALL},
volume = {18},
number = {42},
publisher = {WILEY-V C H VERLAG GMBH},
address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY},
abstract = {In biomolecular electronics, the role of structural order in charge transport (CT) is poorly understood. It has been reported that the metal oxide cores of protein cages (e.g., iron oxide and ferrihydrite nanoparticles (NPs) present in ferritin and E2-LFtn, which is E2 protein engineered with an iron-binding sequence) play an important role in the mechanism of CT. At the same time, the NP core also plays a major role in the structural integrity of the proteins. This paper describes the role of structural order in CT across tunnel junctions by comparing three iron-storing proteins. They are (1) DNA binding protein from starved cells (Dps, diameter ( null ) = 9 nm); (2) engineered archaeal ferritin (AfFtn-AA, null = 12 nm); and (3) engineered E2 of pyruvate dehydrogenase enzyme complex (E2-LFtn, null = 25 nm). Both holo-Dps and apo-Dps proteins undergo CT by coherent tunneling because their globular architecture and relative structural stability provide a coherent conduction pathway. In contrast, apo-AfFtn-AA forms a disordered structure across which charges have to tunnel incoherently, but holo-AfFtn-AA retains its globular structure and supports coherent tunneling. The large E2-LFtn always forms disordered structures across which charges incoherently tunnel regardless of the presence of the NP core. These findings highlight the importance of structural order in the mechanism of CT across biomolecular tunnel junctions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000750743100028,
title = {Role of Order in the Mechanism of Charge Transport across Single-Stranded and Double-Stranded DNA Monolayers in Tunnel Junctions},
author = {Nipun Kumar Gupta and Edward A Wilkinson and Senthil Kumar Karuppannan and Lily Bailey and Ayelet Vilan and Ziyu Zhang and Dong-Chen Qi and Anton Tadich and Eimer M Tuite and Andrew R Pike and James H R Tucker and Christian A Nijhuis},
doi = {10.1021/jacs.1c09549},
times_cited = {0},
issn = {0002-7863},
year = {2021},
date = {2021-12-08},
journal = {JOURNAL OF THE AMERICAN CHEMICAL SOCIETY},
volume = {143},
number = {48},
pages = {20309-20319},
publisher = {AMER CHEMICAL SOC},
address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
abstract = {Deoxyribonucleic acid (DNA) has been hypothesized to act as a molecular wire due to the presence of an extended p-stack between base pairs, but the factors that are detrimental in the mechanism of charge transport (CT) across tunnel junctions with DNA are still unclear. Here we systematically investigate CT across dense DNA monolayers in large-area biomolecular tunnel junctions to determine when intrachain or interchain CT dominates and under which conditions the mechanism of CT becomes thermally activated. In our junctions, double-stranded DNA (dsDNA) is 30-fold more conductive than single-stranded DNA (ssDNA). The main reason for this large change in conductivity is that dsDNA forms ordered monolayers where intrachain tunneling dominates, resulting in high CT rates. By varying the temperature T and the length of the DNA fragments in the junctions, which determines the tunneling distance, we reveal a complex interplay between T, the length of DNA, and structural order on the mechanism of charge transport. Both the increase in the tunneling distance and the decrease in structural order result in a change in the mechanism of CT from coherent tunneling to incoherent tunneling (hopping). Our results highlight the importance of the interplay between structural order, tunneling distance, and temperature on the CT mechanism across DNA in molecular junctions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000688514000024,
title = {Switching of the mechanism of charge transport induced by phase transitions in tunnel junctions with large biomolecular cages},
author = {Nipun Kumar Gupta and Rupali Reddy Pasula and Senthil Kumar Karuppannan and Zhang Ziyu and Anton Tadich and Bruce Cowie and Dong-Chen Qi and Peter Bencok and Sierin Lim and Christian A Nijhuis},
doi = {10.1039/d0tc05773h},
times_cited = {0},
issn = {2050-7526},
year = {2021},
date = {2021-09-07},
journal = {JOURNAL OF MATERIALS CHEMISTRY C},
volume = {9},
number = {33},
pages = {10768-10776},
publisher = {ROYAL SOC CHEMISTRY},
address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND},
abstract = {Tunnel junctions based on Fe storing globular proteins are an interesting class of biomolecular tunnel junctions due to their tunable Fe ion loading, symmetrical structure and thermal stability, and are therefore attractive to study the mechanisms of charge transport (CT) at the molecular level. This paper describes a temperature-induced change in the CT mechanism across junctions with large globular (similar to 25 nm in diameter) E2-proteins bioengineered with Fe-binding peptides from ferritin (E2-LFtn) to mineralise Fe ions in the form of iron oxide nanoparticles (NPs) inside the protein's cavity. The iron oxide NPs provide accessible energy states that support high CT rates and shallow activation barriers. Interestingly, the CT mechanism changes abruptly, but reversibly, from incoherent tunnelling (which is thermally activated) to coherent tunnelling (which is activationless) across the E2-LFtn-based tunnel junctions with the highest Fe ion loading at a temperature of 220-240 K. During this transition the current density across the junctions increases by a factor of 13 at an applied voltage of V = -0.8 V. X-ray absorption spectroscopy indicates that the iron oxide NPs inside the E2-LFtn cages undergo a reversible phase transition; this phase transition opens up new a tunnelling pathway changing the mechanism of CT from thermally activated to activationless tunnelling despite the large size of the E2-LFtn and associated distance for tunnelling.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{ISI:000658406400001,
title = {The energy level alignment of the ferrocene-EGaIn interface studied with photoelectron spectroscopy},
author = {Nipun Kumar Gupta and Thorsten Schultz and Senthil Kumar Karuppannan and Ayelet Vilan and Norbert Koch and Christian A Nijhuis},
doi = {10.1039/d1cp01690c},
times_cited = {0},
issn = {1463-9076},
year = {2021},
date = {2021-05-26},
journal = {PHYSICAL CHEMISTRY CHEMICAL PHYSICS},
volume = {23},
number = {24},
pages = {13458-13467},
publisher = {ROYAL SOC CHEMISTRY},
address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND},
abstract = {The energy level alignment after the formation of a molecular tunnel junction is often poorly understood because spectroscopy inside junctions is not possible, which hampers the rational design of functional molecular junctions and complicates the interpretation of the data generated by molecular junctions. In molecular junction platforms where the top electrode-molecule interaction is weak; one may argue that the energy level alignment can be deduced from measurements with the molecules supported by the bottom electrode (sometimes referred to as "half junctions"). This approach, however, still relies on a series of assumptions, which are challenging to address experimentally due to difficulties in studying the molecule-top electrode interaction. Herein, we describe top electrode-molecule junctions with a liquid metal alloy top electrode of EGaIn (which stands for eutectic alloy of Ga and In) interacting with well-characterised ferrocene (Fc) moieties. We deposited a ferrocene derivative on films of EGaIn, coated with its native GaOx layer, and studied the energy level alignment with photoelectron spectroscopy. Our results reveal that the electronic interaction between the Fc and GaOx/EGaIn is very weak, resembling physisorption. Therefore, investigations of "half junctions" for this system can provide valuable information regarding the energy level alignment of complete EGaIn junctions. Our results help to improve our understanding of the energy landscape in weakly coupled molecular junctions and aid to the rational design of molecular electronic devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}