Kamal B. Dhungana

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).

Giant amplification of tunnel magnetoresistance in a molecular junction: Molecular spin-valve transistor

Amplification of tunnel magnetoresistance by gate field in a molecular junction is the most important requirement for the development of a molecular spin valve transistor. Herein, we predict a giant amplification of tunnel magnetoresistance in a single molecular spin valve junction, which consists of Ru-bis-terpyridine molecule as a spacer between two ferromagnetic nickel contacts. Based on the first-principles quantum transport approach, we show that a modest change in the gate field that is experimentally accessible can lead to a substantial amplification (320%) of tunnel magnetoresistance. The origin of such large amplification is attributed to the spin dependent modification of orbitals at the molecule-lead interface and the resultant Stark effect induced shift in channel position with respect to the Fermi energy.

Unlocking the Origin of Superior Performance of a Si–Ge Core–Shell Nanowire Quantum Dot Field Effect Transistor

The sustained advancement in semiconducting core-shell nanowire technology has unlocked a tantalizing route for making next generation field effect transistor (FET). Understanding how to control carrier mobility of these nanowire channels by applying a gate field is the key to developing a high performance FET. Herein, we have identified the switching mechanism responsible for the superior performance of a Si-Ge core-shell nanowire quantum dot FET over its homogeneous Si counterpart. A quantum transport approach is used to investigate the gate-field modulated switching behavior in electronic current for ultranarrow Si and Si-Ge core-shell nanowire quantum dot FETs. Our calculations reveal that for the ON state, the gate-field induced transverse localization of the wave function restricts the carrier transport to the outer (shell) layer with the pz orbitals providing the pathway for tunneling of electrons in the channels. The higher ON state current in the Si-Ge core-shell nanowire FET is attributed to the pz orbitals that are distributed over the entire channel; in the case of Si nanowire, the participating pz orbital is restricted to a few Si atoms in the channel resulting in a smaller tunneling current. Within the gate bias range considered here, the transconductance is found to be substantially higher in the case of a Si-Ge core-shell nanowire FET than in a Si nanowire FET, which suggests a much higher mobility in the Si-Ge nanowire device.

Electrical tuning of spin current in a boron nitride nanotube quantum dot

Controlling spin current and magnetic exchange coupling by applying an electric field and achieving high spin injection efficiency at the same time in a nanostructure coupled to ferromagnetic electrodes have been the outstanding challenges in nanoscale spintronics. A relentless quest is going on to find new low-dimensional materials with tunable spin dependent properties to address these challenges. Herein, we predict, from first-principles, the transverse-electric-field induced switching in the sign of exchange coupling and tunnel magneto-resistance in a boron nitride nanotube quantum dot attached to ferromagnetic nickel contacts. An orbital dependent density functional theory in conjunction with a single particle Greens function approach is used to study the spin dependent current. The origin of switching is attributed to the electric field induced modification of magnetic exchange interaction at the interface caused by the Stark effect. In addition, spin injection efficiency is found to vary from 61% to 89% depending upon the magnetic configurations at the electrodes. These novel findings are expected to open up a new pathway for the application of boron nitride nanotube quantum dots in next generation nanoscale spintronics.