Pressure-enabled phonon engineering in metals.

We present a combined first-principles and experimental study of the electrical resistivity in aluminum and copper samples under pressures up to 2 GPa. The calculations are based on first-principles density functional perturbation theory, whereas the experimental setup uses a solid media piston-cylinder apparatus at room temperature. We find that upon pressurizing each metal, the phonon spectra are blue-shifted and the net electron-phonon interaction is suppressed relative to the unstrained crystal. This reduction in electron-phonon scattering results in a decrease in the electrical resistivity under pressure, which is more pronounced for aluminum than for copper. We show that density functional perturbation theory can be used to accurately predict the pressure response of the electrical resistivity in these metals. This work demonstrates how the phonon spectra in metals can be engineered through pressure to achieve more attractive electrical properties.

Understanding the charge transfer mechanism in CsPbBr3 nanocrystals and nitrogen-doped carbon quantum dot heterostructures: effect of nanocrystal encapsulation.

Recently, lead halide perovskite nanocrystal (NC)-based heterostructures have demonstrated significant promise in various research areas, including solar cells, CO2 reduction, and photocatalysis. These hybrid structures have also played a crucial role in advancing our fundamental conception of charge transfer mechanisms occurring at the interface. A thin shell around the NCs is not suitable for the formation of stable and luminescent materials. However, such NCs are suitable for solar cells, LEDs, CO2 reduction, and photocatalytic applications due to higher carrier mobility. Thick-shelled NCs are highly stable but hinder charge transport among the NCs which is beneficial for bio-imaging and color-converted LED fabrication. So, understanding the mechanism of charge transport among the NCs dependent on the shell materials is important. Here, we synthesized CsPbBr3 NCs with various coating materials to vary the effective distance between the perovskite and nitrogen-doped carbon quantum dots (NCQDs) to understand the charge transfer process among them. We encapsulated the NCs with different coating materials (i.e., oleic acid, oleylamine, polyvinylpyrrolidone, and silica) such that the thickness of the NCs' shell can differ. We observed that the charge transfer rate between thick-shelled NCs and NCQDs is slow. The faster charge transfer among the thinner-shelled NCs and NCQDs is feasible due to the bonding of the N-state of NCQDs with Pb-atoms of the CsPbBr3 structure. The density functional theory (DFT) calculations of the heterostructure indicate that the electron acceptor state of the N-atom in NCQDs lies below the conduction band of perovskite NCs, which is accountable for such charge transfer. This study has immense significance as it provides crucial insights into the design and application of heterostructures, which can be extended to various novel opportunities for progress and innovation.

Quasiparticle band gap engineering of graphene and graphone on hexagonal boron nitride substrate.

Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. In principle, graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. Recent theoretical and experimental studies have provided conflicting results: while theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. Using the first-principles density functional method and the many-body perturbation theory, we have studied graphene on hBN substrate. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. While the band gap opening in graphene/hBN is due to symmetry breaking and is vulnerable to slight perturbation such as misalignment, the graphone band gap is due to chemical functionalization and is robust in the presence of misalignment. The band gap of graphone reduces by about 1 eV when it is supported on hBN due to the polarization effects at the graphone/hBN interface. The band offsets at graphone/hBN interface indicate that hBN can be used not only as a substrate but also as a dielectric in the field effect devices employing graphone as a channel material. Our study could open up new way of band gap engineering in graphene based nanostructures.

Electronic structure of neighboring extein residue modulates intein C-terminal cleavage activity.

Protein splicing is an autocatalytic reaction where an intervening element (intein) is excised and the remaining two flanking sequences (exteins) are joined. The reaction requires specific conserved residues, and activity may be affected by both the intein and the extein sequence. Predicting how sequence will affect activity is a challenging task. Based on first-principles density functional theory and multiscale quantum mechanics/molecular mechanics, we report C-terminal cleavage reaction rates for five mutations at the first residue of the C-extein (+1), and describe molecular properties that may be used as predictors for future mutations. Independently, we report on experimental characterization of the same set of mutations at the +1 residue resulting in a wide range of C-terminal cleavage activities. With some exceptions, there is general agreement between computational rates and experimental cleavage, giving molecular insight into previous claims that the +1 extein residue affects intein catalysis. These data suggest utilization of attenuating +1 mutants for intein-mediated protein manipulations because they facilitate precursor accumulation in vivo for standard purification schemes. A more detailed analysis of the "+1 effect" will also help to predict sequence-defined effects on insertion points of the intein into proteins of interest.

Controlled assembly of Ag nanoparticles and carbon nanotube hybrid structures for biosensing.

Here we report a chemical-free, simple, and novel method in which a part from a silver-based anode is controllably used in a straightforward manner to produce silver nanoparticles (Ag NPs) in order to fabricate a controlled assembly of Ag NPs and single walled carbon nanotube (SWCNT) hybrid structures. The attachment and distribution of Ag NPs along SWCNTs have been investigated and characterized by field emission scanning electron microscopy (FESEM). We have achieved the decoration of SWCNTs with different densities of Ag NPs by changing the deposition time, the applied voltage, and the location of carbon nanotubes with respect to the anode. At low voltage, single silver nanoparticle is successfully attached at the open ends of SWCNTs whereas at high voltage, intermediate and full coverage densities of Ag NPs are observed. As voltage is further increased, fractals of Ag NPs along SWCNTs are observed. In addition, a device based on a Ag NPs-SWNT hybrid structure is used for the label-free detection of ssDNA molecules immobilized on it. We believe that the proposed method can be used to decorate and/or assemble metal nanoparticles or fractal patterns along SWCNTs with different novel metals such as gold, silver, and copper and can be exploited in various sensitive applications for fundamental research and nanotechnology.

Stable aqueous dispersions of noncovalently functionalized graphene from graphite and their multifunctional high-performance applications.

We present a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions. The exfoliation mechanism is established using stringent control experiments and detailed characterization steps. Using the exfoliated graphene, we demonstrate highly sensitive and selective conductometric sensors (whose resistance rapidly changes >10,000% in saturated ethanol vapor), and ultracapacitors with extremely high specific capacitance (∼ 120 F/g), power density (∼ 105 kW/kg), and energy density (∼ 9.2 Wh/kg).

Gold Nanoparticles Augment N-Terminal Cleavage and Splicing Reactions in Mycobacterium tuberculosis SufB.

Protein splicing is a self-catalyzed event where the intervening sequence intein cleaves off, joining the flanking exteins together to generate a functional protein. Attempts have been made to regulate the splicing rate through variations in temperature, pH, and metals. Although metal-regulated protein splicing has been more captivating to researchers, metals were shown to only inhibit splicing reactions that confine their application. This is the first study to show the effect of nanoparticles (NPs) on protein splicing. We found that gold nanoparticles (AuNPs) of various sizes can increase the splicing efficiency by more than 50% and the N-terminal cleavage efficiency by more than 45% in Mycobacterium tuberculosis SufB precursor protein. This study provides an effective strategy for engineering splicing-enhanced intein platforms. UV-vis absorption spectroscopy, isothermal titration calorimetry (ITC), and transmission electron microscopy (TEM) confirmed AuNP interaction with the native protein. Quantum mechanics/molecular mechanics (QM/MM) analysis suggested a significant reduction in the energy barrier at the N-terminal cleavage site in the presence of gold atom, strengthening our experimental evidence on heightened the N-terminal cleavage reaction. The encouraging observation of enhanced N-terminal cleavage and splicing reaction can have potential implementations from developing a rapid drug delivery system to designing a contemporary protein purification system.

Insight into anomalous hydrogen adsorption on rare earth metal decorated on 2-dimensional hexagonal boron nitride: a density functional theory study.

Hydrogen interaction with metal atoms is of prime focus for many energy related applications like hydrogen storage, hydrogen evolution using catalysis, etc. Although hydrogen binding with many main group alkaline and transition metals is quite well understood, its binding properties with lanthanides are not well reported. In this article, by density functional theory studies, we show how a rare earth metal, cerium, binds with hydrogen when decorated over a heteropolar 2D material, hexagonal boron nitride. Each cerium adatom is found to bind eight hydrogen molecules which is a much higher number than has been reported for transition metal atoms. However, the highest binding energy occurs at four hydrogen molecules. This anomaly, therefore, is investigated in the present article using first-principles calculations. The number density of hydrogen molecules adsorbed over the cerium adatom is explained by investigating the electronic charge volume interactions owing to a unique geometrical arrangement of the guest hydrogen molecules. The importance of geometrical encapsulation in enhancing electronic interactions is explained.

Sculpting Artificial Edges in Monolayer MoS2 for Controlled Formation of Surface-Enhanced Raman Hotspots.

Hotspot engineering has the potential to transform the field of surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive and reproducible detection of analytes. However, the ability to controllably generate SERS hotspots, with desired location and geometry, over large-area substrates, has remained elusive. In this study, we sculpt artificial edges in monolayer molybdenum disulfide (MoS2) by low-power focused laser-cutting. We find that when gold nanoparticles (AuNPs) are deposited on MoS2 by drop-casting, the AuNPs tend to accumulate predominantly along the artificial edges. First-principles density functional theory (DFT) calculations indicate strong binding of AuNPs with the artificial edges due to dangling bonds that are ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation of AuNPs along the artificial edges intensifies plasmonic effects in these regions, creating hotspots exclusively along the artificial edges. DFT further indicates that adsorption of AuNPs along the artificial edges prompts a transition from semiconducting to metallic behavior, which can further intensify the plasmonic effect along the artificial edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated) edges of the MoS2 sheet. To demonstrate the practical utility of this concept, we use our substrate to detect Rhodamine B (RhB) with a large SERS enhancement (∼104) at the hotspots for RhB concentrations as low as ∼10-10 M. The single-step laser-etching process reported here can be used to controllably generate arrays of SERS hotspots. As such, this concept offers several advantages over previously reported SERS substrates that rely on electrochemical deposition, e-beam lithography, nanoimprinting, or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety of 2D material platforms.

Highly conserved histidine plays a dual catalytic role in protein splicing: a pKa shift mechanism.

Protein splicing is a precise autocatalytic process in which an intein excises itself from a precursor with the concomitant ligation of the flanking sequences. Protein splicing occurs through acid-base catalysis in which the ionization states of active site residues are crucial to the reaction mechanism. In inteins, several conserved histidines have been shown to play important roles in protein splicing, including the most conserved "B-block" histidine. In this study, we have combined NMR pK(a) determination with quantum mechanics/molecular mechanics (QM/MM) modeling to study engineered inteins from Mycobacterium tuberculosis (Mtu) RecA intein. We demonstrate a dramatic pK(a) shift for the invariant B-block histidine, the most conserved residue among inteins. The B-block histidine has a pK(a) of 7.3 +/- 0.6 in a precursor and a pK(a) of <3.5 in a spliced intein. The pK(a) values and QM/MM data suggest that the B-block histidine has a dual role in the acid-base catalysis of protein splicing. This histidine likely acts as a general base to initiate splicing with an acyl shift and then as a general acid to cause the breakdown of the scissile bond at the N-terminal splicing junction. The proposed pK(a) shift mechanism accounts for the biochemical data supporting the essential role for the B-block histidine and for the near absolute sequence conservation of this residue.

Synthesis of a 3D free standing crystalline NiSex matrix for electrochemical energy storage applications.

The electrochemical performance for energy storage of three-dimensional (3D) self-supported heterogeneous NiSex cubic-orthorhombic nanocrystals grown by a facile one-step chemical vapour deposition (CVD) approach on Ni foam substrates has been explored. NiSex shows a high specific capacitance of 1333 F g-1 with ultra-high energy (105 W h kg-1) and power (54 kW kg-1) densities. Furthermore, by integrating the as-grown NiSex as the anode and reduced graphene oxide as the cathode, a hybrid supercapacitor (HSC) prototype with a coin cell configuration has been fabricated. The device shows better capacitance (40 F g-1) with high energy (22 W h kg-1) and power (5.8 kW kg-1) densities and robust cycling durability (∼88% capacitance retention after 10 000 repeated cycles). For practical reliability of the as-fabricated HSC, a red LED has been illuminated by connecting it with two charged coin cells. These outstanding performances of the HSC prove to be promising for applications in high energy storage systems.

Enhanced Nonenzymatic Glucose-Sensing Properties of Electrodeposited NiCo2O4-Pd Nanosheets: Experimental and DFT Investigations.

Here, we report the facile synthesis of NiCo2O4 (NCO) and NiCo2O4-Pd (NCO-Pd) nanosheets by the electrodeposition method. We observed enhanced glucose-sensing performance of NCO-Pd nanosheets as compared to bare NCO nanosheets. The sensitivity of the pure NCO nanosheets is 27.5 µA µM-1 cm-2, whereas NCO-Pd nanosheets exhibit sensitivity of 40.03 µA µM-1 cm-2. Density functional theory simulations have been performed to qualitatively support our experimental observations by investigating the interactions and charge-transfer mechanism of glucose on NiCo2O4 and Pd-doped NiCo2O4 through demonstration of partial density of states and charge density distributions. The presence of occupied and unoccupied density of states near the Fermi level implies that both Ni and Co ions in NiCo2O4 can act as communicating media to transfer the charge from glucose by participating in the redox reactions. The higher binding energy of glucose and more charge transfer from glucose to Pd-doped NiCo2O4 compared with bare NiCo2O4 infer that Pd-doped NiCo2O4 possesses superior charge-transfer kinetics, which supports the higher glucose-sensing performance.

Room-temperature ferromagnetism in doped face-centered cubic fe nanoparticles.

The magnetism of Fe and its alloys has been at the center of scientific and technological interest for decades. Along with the ferromagnetic nature of body-centered cubic Fe, the magnetic properties of face-centered cubic (fcc) Fe have attracted much attention. It is well known that fcc Fe is thermodynamically unstable at ambient conditions and not ferromagnetic. Contrary to what is known, we report that elongated nanoparticles of fcc Fe, grown within graphitic nanotubes, remain structurally stable and appear ferromagnetic at room temperature. The magnetic moment (2+/-0.5 microB) in these nanoparticles and the hyperfine fields for two different components of 57Fe (33 and 21 T), measured by Mössbauer spectroscopy, are explained by carbon interstitials in the expanded fcc Fe lattice, that is, FeC(x) where x approximately 0.10, which result in the formation of a dominant Fe4C stoichiometry. First-principles calculations suggest that the ferromagnetism observed in the fcc Fe is related to both lattice expansion and charge transfer between iron and carbon. The understanding of strain- and dopant-induced ferromagnetism in the fcc Fe could lead to the development of new fcc Fe-based alloys for magnetic applications.

Strain engineering the work function in monolayer metal dichalcogenides.

We use first-principles density functional theory to investigate the effect of both tensile and compressive strain on the work functions of various metal dichalcogenide monolayers. We find that for all six species considered, including MoS2, WS2, SnS2, VS2, MoSe2 and MoTe2, that compressive strain of up to 10% decreases the work function continuously by as much as 1.0 eV. Large enough tensile strain is also found to decrease the work function, although in some cases we observe an increase in the work function for intermediate values of tensile strain. This work function modulation is attributed to a weakening of the chalcogenide-metal bonds and an increase in total energy of each system as a function of strain. Values of strain which bring the metal atoms closer together lead to an increase in electrostatic potential energy, which in turn results in an increase in the vacuum potential level. The net effect on the work function can be explained in terms of the balance between the increases in the vacuum potential levels and Fermi energy.

Atomically thin layers of B-N-C-O with tunable composition.

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Substrate-induced band gap renormalization in semiconducting carbon nanotubes.

The quasiparticle band gaps of semiconducting carbon nanotubes (CNTs) supported on a weakly-interacting hexagonal boron nitride (h-BN) substrate are computed using density functional theory and the GW Approximation. We find that the direct band gaps of the (7,0), (8,0) and (10,0) carbon nanotubes are renormalized to smaller values in the presence of the dielectric h-BN substrate. The decrease in the band gap is the result of a polarization-induced screening effect, which alters the correlation energy of the frontier CNT orbitals and stabilizes valence band maximum and conduction band minimum. The value of the band gap renormalization is on the order of 0.25 to 0.5 eV in each case. Accounting for polarization-induced band gap changes is crucial in comparing computed values with experiment, since nanotubes are almost always grown on substrates.

A conserved threonine spring-loads precursor for intein splicing.

Protein splicing is an autocatalytic process where an "intein" self-cleaves from a precursor and ligates the flanking N- and C-"extein" polypeptides. Inteins occur in all domains of life and have myriad uses in biotechnology. Although the reaction steps of protein splicing are known, mechanistic details remain incomplete, particularly the initial peptide rearrangement at the N-terminal extein/intein junction. Recently, we proposed that this transformation, an N-S acyl shift, is accelerated by a localized conformational strain, between the intein's catalytic cysteine (Cys1) and the neighboring glycine (Gly-1) in the N-extein. That proposal was based on the crystal structure of a catalytically competent trapped precursor. Here, we define the structural origins and mechanistic relevance of the conformational strain using a combination of quantum mechanical simulations, mutational analysis, and X-ray crystallography. Our results implicate a conserved, but largely unstudied, threonine residue of the Ssp DnaE intein (Thr69) as the mediator of conformational strain through hydrogen bonding. Further, the strain imposed by this residue is shown to position the splice junction in a manner that enhances the rate of the N-S acyl shift substantially. Taken together, our results not only provide fundamental understanding of the control of the first step of protein splicing but also have important implications in various biotechnological applications that require precursor manipulation.

Superior field emission properties of layered WS2-RGO nanocomposites.

We report here the field emission studies of a layered WS2-RGO composite at the base pressure of ~1 × 10(-8) mbar. The turn on field required to draw a field emission current density of 1 µA/cm(2) is found to be 3.5, 2.3 and 2 V/µm for WS2, RGO and the WS2-RGO composite respectively. The enhanced field emission behavior observed for the WS2-RGO nanocomposite is attributed to a high field enhancement factor of 2978, which is associated with the surface protrusions of the single-to-few layer thick sheets of the nanocomposite. The highest current density of ~800 µA/cm(2) is drawn at an applied field of 4.1 V/µm from a few layers of the WS2-RGO nanocomposite. Furthermore, first-principles density functional calculations suggest that the enhanced field emission may also be due to an overalp of the electronic structures of WS2 and RGO, where graphene-like states are dumped in the region of the WS2 fundamental gap.

Aluminum conducts better than copper at the atomic scale: a first-principles study of metallic atomic wires.

Using a first-principles density functional method, we have studied the electronic structure, electron-phonon coupling, and quantum transport properties of atomic wires of Ag, Al, Au, and Cu. Non-equilibrium Green's function-based transport studies of finite atomic wires suggest that the conductivity of Al atomic wires is higher than that of Ag, Au, and Cu in contrast to the bulk where Al has the lowest conductivity among these systems. This is attributed to the higher number of eigenchannels in Al wires, which becomes the determining factor in the ballistic limit. On the basis of density functional perturbation theory, we find that the electron-phonon coupling constant of the Al atomic wire is lowest among the four metals studied, and more importantly, that the value is reduced by a factor of 50 compared to the bulk.

Optical and sensing properties of 1-pyrenecarboxylic Acid-functionalized graphene films laminated on polydimethylsiloxane membranes.

We present fabrication and characterization of macroscopic thin films of graphene flakes, which are functionalized with 1-pyrenecarboxylic acid (PCA) and are laminated onto flexible and transparent polydimethylsiloxane (PDMS) membranes. The noncovalently (π-stacked) functionalization of PCA allows us to obtain a number of unique optical and molecular sensing properties that are absent in pristine graphene films, without sacrificing the conducting nature of graphene. The flexible PCA-graphene-PDMS hybrid structure can block 70-95% of ultraviolet (UV) light, while allowing 65% or higher transmittance in the visible region, rendering them potentially useful for a number of flexible UV absorbing/filtering applications. In addition, the electrical resistance of these structures is found to be sensitive to the illumination of visible light, atmospheric pressure change, and the presence of different types of molecular analytes. Owing to their multifunctionality, these hybrid structures have immense potential for the development of versatile, low-cost, flexible, and portable electronic and optoelectronic devices for diverse applications.