Sp2 Hybridization State Research Articles

Новые соединения нитрида бора с атомами в sp+sp2-гибридизованном состоянии, сформированные на основе слоя BN-L4-8

The density functional theory (DFT) method was used to determine the crystal structures and electronic properties of new layered polymorphic varieties of boron nitride with a graphyne-like structure. The structures of new graphyne-like monolayers consist of boron and nitrogen atoms in the sp- and sp2-hybridized state and were model-built from a layer of graphene-like boron nitride BN-L4-8 by partial replacement of atoms in the three-coordinated (sp2-hybridized) state by atoms in the two-coordinated state (sp-hybridized). As a result of theoretical analysis, the possibility of the existence of seven sp + sp2 new structures of boron nitride was established: one α-type, three β-types and three γ-types, differing in the proportion of atoms in the sp-hybridized state and their spatial arrangement. However, two of the three structural varieties with a minimum content of atoms in the sp-hybridized state (γ-type) turned out to be unstable and, in the process of geometric optimization, were transformed into structures of graphene-like layers of boron nitride. The structure of the BN-L4-8-γ2 layer passed into the structure of the original graphene-like layer BN-L4-8, while the structure of the BN-L4-8-γ3 layer passed into the structure of the graphene-like layer BN-L4-6-8, where there are no atoms in the sp-hybridized state. The sublimation energies of the new polymorphic varieties range from 16.23 eV/(BN) to 16.70 eV/(BN). The band gap varies from 3.777 eV to 3.878 eV.

Characterization of BN interface and its effect on the mechanical behavior of SiCf/SiC composites

The boron nitride (BN) interface of SiC fiber-reinforced SiC matrix (SiCf/SiC) composites was fabricated by chemical vapor infiltration (CVI) using BCl3 and NH3 as precursor. The microstructure and chemical composition of the BN interface were characterized. The influence of the coating thickness on the tensile behavior of SiCf/SiC composites was then evaluated. The BN interface synthesized at 800 °C has a low degree of crystallization and displays a quasi-isotropic microstructure. The ratio of element B to element N is near 1 and the oxygen content is about 10%. Meanwhile, the electron structure of B, N atoms near the SiC fiber surface have more sp2 hybridization state. Increasing the BN interface thickness exerts a critical role in decreasing the interfacial sliding stress which improves the tensile strength and toughness of SiCf/SiC composites. The optimal thickness of the BN interface is about 670 nm and further increasing the thickness exhibits little improvement on the mechanical properties of SiCf/SiC composites.

Pulsed Vacuum Arc Deposition of Nitrogen-Doped Diamond-like Coatings for Long-Term Hydrophilicity of Electrospun Poly(ε-caprolactone) Scaffolds.

The surface hydrophobicity of poly(ε-caprolactone) electrospun scaffolds prevents their interactions with cells and tissue integration. Although plasma treatment of scaffolds enhances their hydrophilicity, this effect is temporary, and the hydrophobicity of the scaffolds is restored in about 30 days. In this communication, we report a method for hydrophilization of poly(ε-caprolactone) electrospun scaffolds for more than 6 months. To that end, diamond-like coating was deposited on the surface of the scaffolds in a nitrogen atmosphere using pulsed vacuum arc deposition with sputtering of graphite target. This approach allows for a single-side hydrophilization of the scaffold (water contact angle of 22 ± 3° vs. 126 ± 2° for pristine PCL scaffold) and preserves its structure. With increased nitrogen pressure in the chamber, sp3-hybridized carbon content decreased twice (sp2/sp3 ratio decreased from 1.06 to 0.52), which demonstrates the possibility of tailoring the content of carbon in sp2 and sp3 hybridization state. Nitrogen content in the deposited coatings was found at 16.1 ± 0.9 at.%. In vitro tests with fibroblast cell culture did not reveal any cytotoxic compounds in sample extracts.

Defects in Gr/h-BN nanoribbons heterostructure: Atomically precise cutting and defect amplification effect via hydrogenation pseudo-crack

Combining both defect and hydrogenation provides the possibility to create an unprecedented structure configuration using an optimally designed mechanical model. The effects of increasing the defect length, coverage of hydrogenation and also the effect of different locations of such defects (Stone-Wales (SW) and vacancy) in hybrid h-BN/Graphene/h-BN nanosheet on their mechanical properties have been studied. The result shows that hydrogenation can be regarded as a kind of pseudo-crack, and even the shape and structure of graphene heterojunction can be precisely controlled and cut. By creating the hydrogenation pseudo-crack in hybrid nanosheets, the carbon atoms in hydrogenated graphene are converted to sp3 hybridization, while the carbon atoms in un‑hydrogenated regions are still sp2 hybridization. The hydrogenation pseudo-crack can also improve the stability and interfacial adhesion of the important region (interface region) of hybrid nanosheets, and transform the sp2 hybridization state into sp3 hybridization state, which can effectively hinder the transformation of six-membered rings in the structure. Only high stress can destroy the structure, so it shows obvious brittle fracture characteristics in the process of tension. Temperature and interface connection also have important influence on the mechanical properties of graphene heterostructure. Defects and hydrogenation pseudo-crack in heterostructures will cause local stress, hybridization state transition and “defect amplification effect”, which will further affect physical properties of materials. In addition, the hydrogenation pseudo-crack is more suitable for atom cutting than vacancy and SW defects, and can precisely control the fracture path of graphene heterostructure.

USING QUANTUM-CHEMICAL PARAMETERS FOR PREDICTING ANTIRADICAL (HO•) ACTIVITY OF RELATED STRUCTURES CONTAINING A CINNAMOIL FRAGMENT. IV. STRUCTURE-ACTIVITY RELATIONSHIP BETWEEN UNSATURATION INDICES AND FLAVONE DERIVATIVES WITH FLOROGLUCIN RING “A”

The quantum-chemical parameters of 52 derivatives related to flavanones, flavanonoles, flavones and flavonoles with a phloroglucinic type of the A ring and containing electron-donating substituents in the B ring were studied.The aim is the analysis of the dynamics of changes in the electron density, bond numbers, free valence indices and unsaturation indices on carbon atoms C-7 → C-8 of the vinyl group of the main conjugation chain in relation to the position and number of substituents in the “B” ring and the type of the pharmacological activity.Materials and methods. The quantum-chemical parameters of the 4 analyzed groups of the compounds, have been calculated by the semi-empirical method PM7 (WinMopac 2016 program) on the workstation with an Intel Xeon E5-1620 3.5 GHz processor, 20 GB of RAM.Results and discussion. When comparing the quantum chemical parameters of the analyzed compounds, it was established that when the C-7 → C-8 multiple bond is formed, the free valency and unsaturation indices increase on both carbon atoms of the vinylene group in flavones and flavonols compared to the corresponding flavanones and flavanonols. This is explained by the fact that the value of the bond numbers Nµ on these atoms, on the contrary, decreases (Fµ = 4.732-Nµ). The transition from flavanone to flavone is accompanied by the formation of a vinyl group C-7 → C-8, and therefore both atoms from the sp3-hybridized state go into the sp2-state. The consequence of this transformation is a change in the electronegativity value and an increase in the unsaturation index of C-7 and C-8 atoms: C sp3 = 2.5; Csp2 = 2.8. At the same time, the transition from flavanone to flavone leads to the formation of a conjugated system with the participation of π-electrons of the aromatic system “B”, C-7, C-8 atoms and the carbonyl group, which is commonly called the “main conjugation chain”. These structural changes, namely, the transition from a less oxidized flavanone to a more oxidized flavone, contribute to a decrease in the electron density on C-7 and C-8 atoms, and an increase in the total unsaturation of the molecules in general. Mulliken charges on C-7 of all groups of compounds are characterized by a positive value. As for the carbon atoms of the B fragment, the following features are revealed here: in the presence of one substituent -OH or -OCH3 on the carbon atom to which the substituent is bounded, the Mulliken charge is positive; if there are two substituents in the B ring -OH or -OCH3, as well as two -OCH3 groups, then the carbon atoms bonded to the indicated substituents also have a positive Mulliken charge; in the case of trihydroxy substituted in the C-2, C-3 and C-4 B ring, all three carbon atoms are characterized by a positive Mulliken charge; if there are methoxy groups in positions C-2, C-3 and C-4, then the positive Mulliken charge is concentrated only on C-2 and C-4 atoms, and on C-3 atom this charge has a negative value.Conclusion. The above data on the quantum-chemical parameters of the main conjugation chain indicate that the transition of C-7 and C-8 atoms to the sp2-hybrid state, leads to a decrease in the electron density and a decrease in the bond numbers, with a simultaneous increase in the indices of unsaturation and free valence on these atoms. Thus, the trigger mechanism of the anti-radical activity, primarily with respect to the HO • radical, is determined by the fact that this particle, electrophilic in its properties, will attach in the C-8 atom during an initial attack.

The phonon transport properties in cubic graphene with entirely sp2 hybridization state

Cubic graphene is an intrinsic semiconductor with hollow geometric structure and entirely sp2 hybridization state. In this paper, using first principles calculations, we investigate the phonon transport properties of this novel carbon allotrope. The calculations show that at room temperature the thermal conductivity of cubic graphene is approximately 266.17 W/mK, which is obviously lower than that of diamond with fully sp3 hybridization state (2033 W/mK). Such low thermal conductivity mainly originated from the distorted sp2 bond and complex structure which give rise to the flatted and mixed phonon branches as well as strong phonon anharmonicity. Meanwhile, to provide detectable structure fingerprints for experiment, the Raman spectrum and vibrational features of Raman-active phonon modes are also calculated in this work. These results elucidate the intrinsic phonon transport properties of cubic graphene as compared with the case of diamond, and could underpin its potential applications in the field of thermal management.

Impacts of Multilayer Hybrid Coating on PSF Hollow Fiber Membrane for Enhanced Gas Separation.

One of the most critical issues encountered by polymeric membranes for the gas separation process is the trade-off effect between gas permeability and selectivity. The aim of this work is to develop a simple yet effective coating technique to modify the surface properties of commonly used polysulfone (PSF) hollow fiber membranes to address the trade-off effect for CO2/CH4 and O2/N2 separation. In this study, multilayer coated PSF hollow fibers were fabricated by incorporating a graphene oxide (GO) nanosheet into the selective coating layer made of polyether block amide (Pebax). In order to prevent the penetration of Pebax coating solution into the membrane substrate, a gutter layer of polydimethylsiloxane (PDMS) was formed between the substrate and Pebax layer. The impacts of GO loadings (0.0–1.0 wt%) on the Pebax layer properties and the membrane performances were then investigated. XPS data clearly showed the existence of GO in the membrane selective layer, and the higher the amount of GO incorporated the greater the sp2 hybridization state of carbon detected. In terms of coating layer morphology, increasing the GO amount only affected the membrane surface roughness without altering the entire coating layer thickness. Our findings indicated that the addition of 0.8 wt% GO into the Pebax coating layer could produce the best performing multilayer coated membrane, showing 56.1% and 20.9% enhancements in the CO2/CH4 and O2/N2 gas pair selectivities, respectively, in comparison to the membrane without GO incorporation. The improvement is due to the increased tortuous path in the selective layer, which created a higher resistance to the larger gas molecules (CH4 and N2) compared to the smaller gas molecules (CO2 and O2). The best performing membrane also demonstrated a lower degree of plasticization and a very stable performance over the entire 50-h operation, recording CO2/CH4 and O2/N2 gas pair selectivities of 52.57 (CO2 permeance: 28.08 GPU) and 8.05 (O2 permeance: 5.32 GPU), respectively.

Determination of sp2 and sp3 phase fractions on the surface of diamond films from C1s, valence band X-ray photoelectron spectra and CKVV X-ray-excited Auger spectra

The practice of applying the methods for determining the relative contents of carbon atoms in sp2 and sp3 hybridization states from the profiles of C1s X-ray photoelectron and carbon KVV (CKVV) Auger spectra is analyzed. Difficulties in using C1s and CKVV spectra for determining the sp2/sp3 ratios are discussed. A method is proposed for evaluating the fractions of sp2 and sp3 phases based on the fitting of the valence band X-ray photoelectron spectra by superposition of valence band spectra of samples with carbon atoms in pure sp2 and sp3 states. Relative fractions of sp2 and sp3 phases on the virgin surfaces of two ultrananocrystalline diamond films, and on their surfaces subjected to friction, were determined using C1s spectra, CKVV Auger spectra, and valence band spectra. All three methods gave the results that are consistent for virgin film surfaces with accuracy of 2%, and for the surfaces of films subjected to friction, 15%. In contrast to the ultrananocrystalline films, three XPS-based methods applied to a microcrystalline diamond film yielded significantly different sp2/sp3 ratios. The reasons for this discrepancy are shown to lie in different analysis depths of these methods, and in different morphology of the film surfaces.

SIMS Analysis of Carbon-Containing Materials: Content of Carbon Atoms in sp2 and sp3 Hybridization States

SIMS Analysis of Carbon-Containing Materials: Content of Carbon Atoms in sp2 and sp3 Hybridization States

An environmental evaluation of carbonaceous aerosols in PM10 at micro- and nano-scale levels reveals the formation of carbon nanodots.

Carbonaceous aerosols play significant roles in air quality and the climate; their oxidation at the nano-scale level may possibly increase the reactivity and toxicity of atmospheric particulates. In the present study, a laboratory experiment on the atmospheric carbonaceous aerosol was done by using H2O2 as an oxidizing agent. An extensive study made with advanced analytical tools revealed the formation of photoluminescent carbon nanoparticles (carbon nanodots) in the carbonaceous aerosol. The carbon nanoparticles are mostly at the sp2 hybridization state and contain various surface functional groups such as carboxyl and carbonyl groups. The properties of these carbon nanoparticles resemble the engineered carbon nanoparticles such as carbon dots (CDs). The carbon nanoparticles, mainly less than 10nm, are composed of carbon nanocrystals containing a few other elements such as Ca and Fe. Fluorescence spectroscopy revealed the characteristic excitation-dependent emission spectra of blue fluorescent carbon nanoparticles. The results indicate the presence of characteristic carbon nanoparticles in the carbonaceous aerosol in PM10, opening a new road for predicting environmental processes occurring in the atmospheric environment.

Improving the thermoelectric properties of carbon nanotubes through introducing graphene nanosprings

Carbon nanotube (CNT) is a typical one-dimensional nanomaterial containing sp2 hybridization states. In this paper, we investigate the ballistic thermoelectric performance of CNTs incorporating graphene nanosprings by using non-equilibrium Green's function. The calculations reveal that the thermoelectric figure of merit could be obviously improved by introducing graphene nanosprings, which is about ten times of that of pristine CNTs at 700 K. Such enhancement is mainly attributed to the remarkable suppression of phononic and electronic thermal conductance and improvement of Seebeck coefficient. In addition, compared to the zigzag graphene nanospring, introducing of the armchair case possesses better thermoelectric performance. The results presented in this paper indicate that embedding graphene nanospring is a viable method to optimize the thermoelectric performance of CNTs and could provide useful theoretical guidance for design and fabrication of CNTs-based thermoelectric devices.

Photo-organometallic, Nanoparticle Nucleation on Graphene for Cascaded Doping

Controlling the doping levels in graphene by modifying the electric potential of interfaced nanostructures is important to understand "cascaded-doping"-based applications of graphene. However, graphene does not have active sites for nanoparticle attachment, and covalently adding functional groups on graphene disrupts its planar sp2-hybridization, affecting its cascaded doping. Here we show a hexahepto (η6) photo-organometallic chemistry to interface nanoparticles on graphene while retaining the sp2-hybridized state of carbon atoms. For testing cascaded doping with ethanol interaction, transition metal oxide nanoparticles (TMONs) (Cr2O3/CrO3, MoO3, and WO3) are attached on graphene. Here, the transition metal forms six σ-bonds and π-back-bonds with the benzenoid rings of graphene, while its opposite face binds to three carbonyl groups, which enable nucleation and growth of TMONs. With a radius size ranging from 50 to 100 nm, the TMONs downshift the Fermi level of graphene (-250 mV; p-doping) via interfacial charge transfer. This is consistent with the blue shift of graphene's G and 2D Raman modes with a hole density of 3.78 × 1012 cm-2. With susceptibility to ethanol, CrxO3 nanoparticles on graphene enable cascaded doping from ethanol that adsorbs on CrxO3, leading to doping of graphene to increase the electrical resistance of the TMONs-graphene hybrid. This nanoparticle-on-graphene construct can have several applications in gas/vapor sensing, electrochemical catalysis, and high-energy-density supercapacitors.

Pulsed laser deposition of monolayer and bilayer graphene

The properties of graphene vary with the number of layers, thus determining its usefulness in different applications. Consequently, it is important to develop a method for precisely controlling the number of layers for various application demands. In this study, high quality monolayer graphene was successfully prepared using pulsed laser deposition. Meanwhile, the growth of monolayer and bilayer graphene can be controlled by adjusting the laser energy density. Raman spectra show that high quality monolayer films grew at a laser energy density of 5.66 J/cm2 and bilayer graphene films grew at a laser energy density of 8.49 J/cm2. X-ray photoelectron spectroscopy indicate that the main chemical state of carbon is the sp2 hybrid state, and the maximum atomic ratio of sp2 and sp3 results from the sample with laser energy density of 5.66 J/cm2. Scanning electron microscope images show that monolayer graphene is connected by small sheets with different sizes, and the largest graphene sheet reaches 20 μm. High resolution transmission electron microscope images and selected area electron diffraction patterns show that graphene samples are highly crystalline and yield significant layer information. UV–Vis spectra indicate that the transmittance through monolayer and bilayer graphene at 550 nm are 96.93% and 95.05%, respectively. Ultraviolet photoelectron spectroscopic measurements show that the work function of graphene ranges from 3.72 eV to 4.76 eV; the variation in the work function is related to growth defects. Finally, the relationship between the structure of the graphene films and laser energy density are investigated in detail, and these results indicate that the controllable growth of graphene can be achieved by adjusting the laser energy density. This means that a suitable laser energy density is conducive to defining an equilibrium between diffusion and condensation of carbon atoms, which is crucial to the growth of high-quality monolayer and bilayer graphene. It is expected that this study might be facilitate the preparation of graphene-based composite films and devices.

Tailoring the Grain Size of Bi-Layer Graphene by Pulsed Laser Deposition.

Improving the thermoelectric efficiency of a material requires a suitable ratio between electrical and thermal conductivity. Nanostructured graphene provides a possible route to improving thermoelectric efficiency. Bi-layer graphene was successfully prepared using pulsed laser deposition in this study. The size of graphene grains was controlled by adjusting the number of pulses. Raman spectra indicated that the graphene was bi-layer. Scanning electron microscopy (SEM) images clearly show that graphene changes from nanostructured to continuous films when more pulses are used during fabrication. Those results indicate that the size of the grains can be controlled between 39 and 182 nm. A detailed analysis of X-ray photoelectron spectra reveals that the sp2 hybrid state is the main chemical state in carbon. The mobility is significantly affected by the grain size in graphene, and there exists a relatively stable region between 500 and 800 pulses. The observed phenomena originate from competition between decreasing resistance and increasing carrier concentration. These studies should be valuable for regulating grains sizes for thermoelectric applications of graphene.

Direct Formation of C-C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocoupling of gem-Dibromomethyl Molecules.

Conductive polymers are of great importance in a variety of chemistry-related disciplines and applications. The recently developed bottom-up on-surface synthesis strategy provides us with opportunities for the fabrication of various nanostructures in a flexible and facile manner, which could be investigated by high-resolution microscopic techniques in real space. Herein, we designed and synthesized molecular precursors functionalized with benzal gem-dibromomethyl groups. A combination of scanning tunneling microscopy, noncontact atomic force microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and density functional theory calculations demonstrated that it is feasible to achieve the direct formation of C-C double-bonded structural motifs via on-surface dehalogenative homocoupling reactions on the Au(111) surface. Correspondingly, we convert the sp3-hybridized state to an sp2-hybridized state of carbon atoms, i. e., from an alkyl group to an alkenyl one. Moreover, by such a bottom-up strategy, we have successfully fabricated poly(phenylenevinylene) chains on the surface, which is anticipated to inspire further studies toward understanding the nature of conductive polymers at the atomic scale.

(Invited) Understanding ORR Reaction on Nitrogen Doped Carbon Materials: Insight from Experiments and Calculations

In this communication, we will present recent results on the oxygen reduction reaction (OOR) on nitrogen doped carbons. DFT calculations on nitrogen doped graphene systems will be used to understand the key elements that activate the reaction on these materials. The study is centered on the monodentate chemisorption of molecular oxygen as the first step of the process. It will be shown that carbon atoms are the active sites provided that several conditions are fulfilled. First, water should be included in the models, since it stabilizes the adsorption of the oxygen molecule with the characteristics of a superoxide species on the carbon atoms, which are the active sites. This carbon atom should be able to transition from a sp2 hybridization state to sp3 upon adsorption of the oxygen molecule to have a favorable interaction energy with the oxygen molecule. This transition is favored destabilized carbon atoms, that is, those neighboring a nitrogen dopant. Second, a significant charge transfer to this site should occur, so that charge can be concentrated on the adsorbed oxygen molecule. This charge transfer requires the presence of additional nitrogen dopants, which are charge donors, even when they are in distant positions. When the nitrogen dopant is located in an edge of the graphene structure, a clearly favorable chemisorbed state for molecular oxygen was found when nitrogen-dopant is hydrogenated and located at an armchair edge. The chemisorbed state is further favored by additional available charge from other nitrogen. By contrast, the chemisorbed state of oxygen is much less favorable when the hydrogenated pyridinic nitrogen-dopants are located at zigzag edges. To demonstrate the applicability of this approach, the ORR was experimentally studied on an aza-fused π-conjugated microporous polymer, which has a very well characterized structure. The experimental results using a rotating disk configuration indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. The DFT calculations indicate half of the nitrogen atoms of the material are hydrogenated at the potentials at which the reaction takes place. This hydrogenation process destabilizes some carbon atoms in the lattice and also provide a source of charge. On these carbon atoms, molecular oxygen could be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects, as in the previous cases. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state is only slightly stable, promoting the formation of hydrogen peroxide.

Characterization of thin carbon films capable of low-field electron emission

Previous experiments have demonstrated that carbon nanoisland films (or disordered quantum-dot arrays) deposited on silicon wafers may possess the property of low-field electron emission. This paper presents our new work on comparative characterization of emitting and nonemitting thin carbon films. The experimental results acquired by Auger spectroscopy, electron energy loss spectroscopy, Anderson's technique for workfunction measurement, and secondary-emission techniques confirmed that the emitting films are discontinuous and consist of carbon in sp2-hybridization state, while their workfunction is relatively high (>4 eV). These experimental data clearly contradict the commonly accepted Fowler–Nordheim theory of field emission and suggest that the observed emission phenomenon has a different nature. A novel model is proposed as a development of the well-known hot-electron emission mechanism supplemented with nanoscale-related features of thermoelectric phenomena.

Recent progress in molecular simulation of nanoporous graphene membranes for gas separation

If an ideal membrane for gas separation is to be obtained, the following three characteristics should be considered: the membrane should be as thin as possible, be mechanically robust, and have welldefined pore sizes. These features will maximize its solvent flux, preserve it from fracture, and guarantee its selectivity. Graphene is made up of a hexagonal honeycomb lattice of carbon atoms with sp 2 hybridization state forming a one-atom-thick sheet of graphite. Following conversion of the honeycomb lattices into nanopores with a specific geometry and size, a nanoporous graphene membrane that offers high efficiency as a separation membrane because of the ultrafast molecular permeation rate as a result of its one-atom thickness is obtained. Applications of nanoporous graphene membranes for gas separation have been receiving remarkably increasing attention because nanoporous graphene membranes show promising results in this area. This review focuses on the recent advances in nanoporous graphene membranes for applications in gas separation, with a major emphasis on theoretical works. The attractive properties of nanoporous graphene membranes introduce make them appropriate candidates for gas separation and gas molecular-sieving processes in nanoscale dimensions.

Optimizing the thermoelectric performance of graphyne nanotube via applying radial strain

Graphyne nanotubes (GNTs) are a new kind of one-dimensional carbon nanomaterials containing both sp and sp2 hybridization states. In this paper, taking gamma-GNTs as an example, we investigate the effect of radial strain on the thermoelectric conversion efficiency by means of nonequilibrium Green's function method as implemented in the density functional based tight-binding framework. The results reveal that the thermoelectric figure of merit could be obviously improved by applying appropriate radial strain. Such enhancement mainly originates from the reserved pristine electronic transport properties and significant suppressed phononic thermal conductance. However, as radial strain increases further, such enhanced thermoelectric performance will be reduced rapidly, which is attributed to the serious electron scattering caused by the change of bonding configuration of GNTs. These phenomena are quite different from previous studies that axial strain has a weak influence on the thermoelectric performance of carbon nanotubes and graphene nanoribbons. The findings presented in this paper indicate that radial strain is a viable way to optimize the thermoelectric performance of GNTs and could provide helpful theoretical guideline for designing and fabricating GNT-based thermoelectrics.

Structural basis for guanidine sensing by the ykkC family of riboswitches.

Regulation of gene expression by cis-encoded riboswitches is a prevalent theme in bacteria. Of the hundreds of riboswitch families identified, the majority of them remain as orphans, without a clear ligand assignment. The ykkC orphan family was recently characterized as guanidine-sensing riboswitches. Herein we present a 2.3 Å crystal structure of the guanidine-bound ykkC riboswitch from Dickeya dadantii. The riboswitch folds into a boot-shaped structure, with a coaxially stacked P1/P2 stem forming the boot, and a 3′-P3 stem–loop forming the heel. Sophisticated base-pairing and cross-helix tertiary contacts give rise to the ligand-binding pocket between the boot and the heel. The guanidine is recognized in its positively charged guanidinium form, in its sp2 hybridization state, through a network of coplanar hydrogen bonds and by a cation–π stacking contact on top of a conserved guanosine residue. Disruption of these contacts resulted in severe guanidinium-binding defects. These results provide the structural basis for specific guanidine sensing by ykkC riboswitches and pave the way for a deeper understanding of guanidine detoxification—a previously unappreciated aspect of bacterial physiology.