Suppressed phase separation of mixed-halide perovskites confined in endotaxial matrices.

The functionality and performance of a semiconductor is determined by its bandgap. Alloying, as for instance in InxGa1-xN, has been a mainstream strategy for tuning the bandgap. Keeping the semiconductor alloys in the miscibility gap (being homogeneous), however, is non-trivial. This challenge is now being extended to halide perovskites - an emerging class of photovoltaic materials. While the bandgap can be conveniently tuned by mixing different halogen ions, as in CsPb(BrxI1-x)3, the so-called mixed-halide perovskites suffer from severe phase separation under illumination. Here, we discover that such phase separation can be highly suppressed by embedding nanocrystals of mixed-halide perovskites in an endotaxial matrix. The tuned bandgap remains remarkably stable under extremely intensive illumination. The agreement between the experiments and a nucleation model suggests that the size of the nanocrystals and the host-guest interfaces are critical for the photo-stability. The stabilized bandgap will be essential for the development of perovskite-based optoelectronics, such as tandem solar cells and full-color LEDs.

Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids.

The study of ionic liquids that may be compatible with the type of radiation chemistry events occurring in nuclear separation processes is a topic of high current interest. In this article, we focus on two ionic liquids based on the benzylpyridinium cation. This cation has been proposed to be able to capture either an excess electron or hole without undergoing fast dissociation. Shkrob, Wishart, and collaborators ( J. Phys. Chem. B 2013 , 117 ( 46 ), 14385 - -14399 ) have indicated that the stabilization is likely in the form of dimers in solution with the excess electron localized on adjacent pyridinium rings and the excess hole localized on phenyl rings. Our first-principles dynamical studies support these ideas but present a more nuanced view of the time-dependent behavior that is likely to occur at short time for systems at room temperature.

Comparison of the Structural Response to Pressure of Ionic Liquids with Ether and Alkyl Functionalities.

The response of ionic liquids to external perturbations including elevated pressure is a topic of current interest for applications such as tribology. Ionic liquids come in many classes, including those that are amphiphilic and some that are mostly polar having either cationic or anionic tails that are functionalized. The current study compares the effect of elevated pressure on two ionic liquids with different types of cationic tail. 1-Decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C10mim+/NTf2-) is amphiphilic whereas isoelectronic ether-functionalized 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C7O3mim+/NTf2-) has cationic tails that are more polar and conformationally different. We find that the response to elevated pressure for these two systems is quite unsimilar. Intramolecular conformational changes as well as changes in the structure of liquid nanodomains appear to be significantly more prominent in the case of C10mim+/NTf2-. Whereas both the density of C10mim+/NTf2- and C7O3mim+/NTf2- change at elevated pressure, the change is more dramatic for C10mim+/NTf2-. The very different response for each of these two types of ionic liquids can be clearly gleaned from distribution functions in real space and the partial subcomponents of the X-ray structure function, S(q), in reciprocal space. Liquid structure in the case of C7O3mim+/NTf2-, and the intramolecular conformational structure of C7O3mim+ in particular, appear to be more resilient to pressure changes than those in the isoelectronic amphiphilic analogue.

Structure of cyano-anion ionic liquids: X-ray scattering and simulations.

Ionic liquids with cyano anions have long been used because of their unique combination of low-melting temperatures, reduced viscosities, and increased conductivities. Recently we have shown that cyano anions in ionic liquids are particularly interesting for their potential use as electron donors to excited state photo-acceptors [B. Wu et al., J. Phys. Chem. B 119, 14790-14799 (2015)]. Here we report on bulk structural and quantum mechanical results for a series of ionic liquids based on the 1-ethyl-3-methylimidazolium cation, paired with the following five cyano anions: SeCN(-), SCN(-), N(CN)2 (-), C(CN)3 (-), and B(CN)4 (-). By combining molecular dynamics simulations, high-energy X-ray scattering measurements, and periodic boundary condition DFT calculations, we are able to obtain a comprehensive description of the liquid landscape as well as the nature of the HOMO-LUMO states for these ionic liquids in the condensed phase. Features in the structure functions for these ionic liquids are somewhat different than the commonly observed adjacency, charge-charge, and polarity peaks, especially for the bulkiest B(CN)4 (-) anion. While the other four cyano-anion ionic liquids present an anionic HOMO, the one for Im2,1 (+)/B(CN)4 (-) is cationic.

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.

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.

Boron nitride nanotubes for spintronics.

With the end of Moore's 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.

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).

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 Green's 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.