Ranjit Pati

Effect of H2O adsorption on electron transport in a carbon nanotube

We have studied the adsorption of H2O molecules on a single-wall carbon nanotube (NT) using first-principles gradient-corrected density-functional theory. Subsequently, Green’s function-based Landauer–Buttiker multichannel formalism within a tight-binding model is used to calculate the electron transport, and our results suggest that H2O molecules adsorbed on the NT surface reduce the electronic conduction in the tube in agreement with recent experimental measurements. The decrease of conductance with water adsorption is explained on the basis of charge transfer between the adsorbate and the NT.

Massively parallel computing on an organic molecular layer

Modern computers operate at enormous speeds—capable of executing in excess of 1013 instructions per second—but their sequential approach to processing, by which logical operations are performed one after another, has remained unchanged since the 1950s. In contrast, although individual neurons of the human brain fire at around just 103 times per second, the simultaneous collective action of millions of neurons enables them to complete certain tasks more efficiently than even the fastest supercomputer. Here we demonstrate an assembly of molecular switches that simultaneously interact to perform a variety of computational tasks including conventional digital logic, calculating Voronoi diagrams, and simulating natural phenomena such as heat diffusion and cancer growth. As well as representing a conceptual shift from serial-processing with static architectures, our parallel, dynamically reconfigurable approach could provide a means to solve otherwise intractable computational problems. The processors of most computers work in series, performing one instruction at a time. This limits their ability to perform certain types of tasks in a reasonable period. An approach based on arrays of simultaneously interacting molecular switches could enable previously intractable computational problems to be solved.

Origin of Negative Differential Resistance in a Strongly Coupled Single Molecule-Metal Junction Device

A new mechanism is proposed to explain the origin of negative differential resistance (NDR) in a strongly coupled single molecule-metal junction. A first-principles quantum transport calculation in a Fe-terpyridine linker molecule sandwiched between a pair of gold electrodes is presented. Upon increasing the applied bias, it is found that a new phase in the broken symmetry wave function of the molecule emerges from the mixing of occupied and unoccupied molecular orbitals. As a consequence, a nonlinear change in the coupling between the molecule and the lead is evolved resulting in NDR. This model can be used to explain NDR in other classes of metal-molecule junction devices.

What Determines the Sign Reversal of Magnetoresistance in a Molecular Tunnel Junction

The observations of both positive and negative signs in tunneling magnetoresistance (TMR) for the same organic spin-valve structure have baffled researchers working in organic spintronics. In this article, we provide an answer to this puzzle by exploring the role of metal-molecule interface on TMR in a single molecular spin-valve junction. A planar organic molecule sandwiched between two nickel electrodes is used to build a prototypical spin-valve junction. A parameter-free, single-particle Greens function approach in conjunction with a posteriori, spin-unrestricted density functional theory involving a hybrid orbital-dependent functional is used to calculate the spin-polarized current. The effect of external bias is explicitly included to investigate the spin-valve behavior. Our calculations show that only a small change in the interfacial distance at the metal-molecule junction can alter the sign of the TMR from a positive to a negative value. By changing the interfacial distance by 3%, the number of participating eigenchannels as well as their orbital characteristics changes for the antiparallel configuration, leading to the sign reversal in TMR.

Theoretical study of electron transport in boron nanotubes

The electron transport in single-walled boron nanotube (BNT) is studied using the Landauer-Buttiker [R. Landauer, J. Phys.: Condens: Matter 1, 8099 (1989); M. Buttiker, Phys. Rev. Lett. 57, 1761 (1986)] multichannel approach in conjunction with the tight-binding method. In the range of the calculated length (1–5.0nm) of the tubes, the calculations predict a ballistic transport in BNT and find a relatively low resistance for BNTs as compared to that of the single-walled carbon nanotubes (CNTs) of comparable length. A lower resistance in the case of BNT than the CNT may be attributed to electron-deficient nature of boron characterized by the presence of two-center, and multicenter bonds in the former.

Ab initio Hartree–Fock study of electron transfer in organic molecules

Electron transfer (ET) in σ-bonded organic cage structures (bicyclo[1.1.1]pentane, cubane, and bicyclo[2.2.2]octane) has been studied with the help of ab initio Hartree–Fock calculations in the framework of a two-state model. The calculated values of the ET coupling matrix element VAB exhibit strong dependence on the basis set employed. A minimal basis set underestimates the value of VAB with respect to an extended (double-zeta and polarization) basis set. The ET shows correlation with the electronic and geometrical structure of the molecules studied. It is found that the more strained the chemical bonds in the cage structure are, the stronger is the coupling between the two states participating in ET. Furthermore, the ET matrix element VAB is calculated to have its maximum value when the two end groups attached to the cage structures are coplanar, and its minimum value when two end π groups are perpendicular to each other. However, for coplanar end-groups, minimal changes are noted in the value of VAB wi...

Boron Nitride Nanotubes for Spintronics

With the end of Moores law in sight, researchers are in search of an alternative approach to manipulate information. Spintronics or spin-based electronics, which uses the spin state of electrons to store, process and communicate information, offers exciting opportunities to sustain the current growth in the information industry. For example, the discovery of the giant magneto resistance (GMR) effect, which provides the foundation behind modern high density data storage devices, is an important success story of spintronics; GMR-based sensors have wide applications, ranging from automotive industry to biology. In recent years, with the tremendous progress in nanotechnology, spintronics has crossed the boundary of conventional, all metallic, solid state multi-layered structures to reach a new frontier, where nanostructures provide a pathway for the spin-carriers. Different materials such as organic and inorganic nanostructures are explored for possible applications in spintronics. In this short review, we focus on the boron nitride nanotube (BNNT), which has recently been explored for possible applications in spintronics. Unlike many organic materials, BNNTs offer higher thermal stability and higher resistance to oxidation. It has been reported that the metal-free fluorinated BNNT exhibits long range ferromagnetic spin ordering, which is stable at a temperature much higher than room temperature. Due to their large band gap, BNNTs are also explored as a tunnel magneto resistance device. In addition, the F-BNNT has recently been predicted as an ideal spin-filter. The purpose of this review is to highlight these recent progresses so that a concerted effort by both experimentalists and theorists can be carried out in the future to realize the true potential of BNNT-based spintronics.

BODIPY-Based Fluorescent Probes for Sensing Protein Surface-Hydrophobicity

Mapping surface hydrophobic interactions in proteins is key to understanding molecular recognition, biological functions, and is central to many protein misfolding diseases. Herein, we report synthesis and application of new BODIPY-based hydrophobic sensors (HPsensors) that are stable and highly fluorescent for pH values ranging from 7.0 to 9.0. Surface hydrophobic measurements of proteins (BSA, apomyoglobin, and myoglobin) by these HPsensors display much stronger signal compared to 8-anilino-1-naphthalene sulfonic acid (ANS), a commonly used hydrophobic probe; HPsensors show a 10- to 60-fold increase in signal strength for the BSA protein with affinity in the nanomolar range. This suggests that these HPsensors can be used as a sensitive indicator of protein surface hydrophobicity. A first principle approach is used to identify the molecular level mechanism for the substantial increase in the fluorescence signal strength. Our results show that conformational change and increased molecular rigidity of the dye due to its hydrophobic interaction with protein lead to fluorescence enhancement.

Fluorinated Boron Nitride Nanotube Quantum Dots: A Spin Filter

Spin filtering requires a selective transmission of spin-polarized carriers. A perfect spin filter allows all majority (or minority) spin carriers to pass through a channel while blocking the minority (or majority) carriers. The quest for a novel low-dimensional metal-free magnetic material that would exhibit magnetism at a higher temperature with an excellent spin filtering property has been intensively pursued. Herein, using a first-principles approach, we demonstrate that the fluorinated boron nitride nanotube (F-BNNT) quantum dot, which is ferromagnetic in nature, can be used as a perfect spin filter with efficiency as high as 99.8%. Our calculation shows that the ferromagnetic spin ordering in F-BNNT is stable at a higher temperature. Comparison of the conductance value of the F-BNNT quantum dot with that of the pristine BNNT quantum dot reveals a significantly higher conductance in F-BNNT, which is in very good agreement with the experimental report (Tang, C., et al. J. Am. Chem. Soc. 2005, 127, 6552).

First-principles investigation of spin-polarized conductance in atomic carbon wires

We analyze spin-dependent energetics and conductance for one-dimensional (1D) atomic carbon wires consisting of terminal magnetic (Co) and interior nonmagnetic (C) atoms sandwiched between gold electrodes, obtained by employing first-principles gradient-corrected density functional theory and Landauers formalism for conductance. Wires containing an even number of carbon atoms are found to be acetylenic with