Thermodynamics of resonating-valence-bond states toward the understanding of quantum spin liquid phenomena.

A quantum spin liquid (QSL) is a state of matter in which spins do not exhibit magnetic order. In contrast to paramagnets, the spins in a QSL interact strongly, similar to conventional ordered magnets; however the thermodynamic stability of QSLs is rarely studied. Here, the thermodynamic properties of centered hexagon nanoclusters were investigated using the Hubbard model, which was solved using exact numerical diagonalization. The total spin, spin-spin correlation functions, local magnetic moments, charge and spin gaps, and magnetocaloric effect were analyzed for a half-filled band as a function of the ratio between the on-site Coulomb repulsion and electronic hopping (U/t). The centered hexagon nanocluster exhibited an antiferromagnetic (AFM) behavior with exotic magnetic ordering. Resonating-valence-bond (RVB) states were observed for intermediate values of U/t, in which short-range spin-spin correlation functions were suppressed to minimize spin frustration. The AFM order was examined in terms of the Néel-like temperature derived from the temperature dependence of the magnetic susceptibility. An interesting result is that the systems under external magnetic fields exhibited an inverse magnetocaloric effect, which was remarkable for intermediate values of U/t, where the RVB state was observed. Owing to the novel discovery of exotic magnetic ordering in triangular moiré patterns in twisted bilayer graphene (TBLG) systems, these results provide insights into the onset of magnetism and the possible spin liquid states in these graphene moiré materials.

Electronic and magnetic properties of TATA-DNA sequence driven by chemical functionalization.

The TATA box is a promoter sequence able to interact directly with the components of the basal transcription initiation machinery. We investigate the changes in the electronic and magnetic properties of a TATA-DNA sequence when functionalized with different chemical groups; using the first-principles density functional theory specifically, the TATA-DNA sequences were functionalized with methyl groups (CH3 , methylation), amino groups (NH2 , amination), imine groups (NH, imination), chloroamine groups (NCl2 , chloramination), H-adatom (hydrogenation), and Cl-adatom (chlorination). The functional groups were anchored at nitrogen atoms from adenine and oxygen atoms from thymine at sites pointed as reactive regions. We demonstrated that chemical functionalization induces significant changes in charge transfer, hydrogen bond distance, and hydrogen bond energy. The hydrogenation and imination increased the hydrogen bond energy. Results also revealed that the chemical functionalization of DNA molecules exhibit a ferromagnetic ground state, reaching magnetization up to 4.665 µB and complex magnetic ordering. We further demonstrated that the functionalization could induce tautomerism (proton migration in the base pair systems). The present study provides a theoretical basis for understanding the functionalization further into DNA molecules and visualizing possible future applications.

Nitrogen-phosphorus codoped carbon nanotube sponges for detecting volatile organic compounds: experimental and DFT calculations.

The sensing of harmful gases and vapors is of fundamental interest to control the industrial emissions and environmental contamination. Nitrogen/phosphorus codoped carbon nanotube sponges (NP-CNTSs) were used to detect ethanol, acetone, cyclohexane, isopropanol, and methanol. The NP-CNTSs were produced through the aerosol-assisted chemical vapor deposition (AACVD) method using acetonitrile and triphenylphosphine as precursors at 1020 °C. The sensors based on NP-CNTSs were tested with varying operating temperatures (25-100 °C) and gas vapor concentrations (5-50 ppm). For instance, for a gas vapor concentration of 30 ppm and an operating temperature of 65 °C, the sensors showed changes in the electrical resistance of 1.12%, 1.21%, 1.09%, 2.4%, and 1.34% for ethanol, acetone, cyclohexane, isopropanol, and methanol, respectively. We found that the response and recovery times for isopropanol gas vapor are up to 43.7 s and 95 s, respectively. The current sensor outperformed the sensors reported in the literature by at least two times in the response measurement. Additionally, we performed van der Waals density functional theory calculations to elucidate the role of nitrogen and phosphorous codoped single-walled carbon nanotubes (SWCNTs) and their interaction with the considered gas molecule. We analyzed the molecular adsorption energy, optimized structures, and the density of states and calculated the electrostatic potential surface for N-doped, P-doped, NP-codoped, and OH-functionalized NP-codoped metallic SWCNTs-(6,6) and semiconducting SWCNTs-(10,0). Adsorption energy calculations revealed that in most cases the molecules are adsorbed to carbon nanotubes via physisorption. The codoping in SWCNTs-(6,6) promoted structural changes in the surface nanotube and marked chemisorption for acetone molecules.

DNA-MBF1 study using molecular dynamics simulations : On the road to understanding the heat stress response in DNA-protein interactions in plants.

Regulatory factor MBF1 is highly conserved between species and has been described as a cofactor and transcription factor. In plants, several reports associate MBF1 with heat stress response. Nevertheless, the specific physical processes involved in the MBF1-DNA interaction are still far from clearly understood. We thus performed extensive molecular dynamics simulations of DNA with a homology-based modethel of the MBF1 protein. Based on recent experimental data, we proposed two B-DNA sequences, analyzing their interaction with our model of the Arabidopsis MBF1c protein (AtMBF1c) at three different temperatures: 293, 300, and 320 K, maintaining a constant pressure of 1 bar. The simulations suggest that MBF1 binds directly to the DNA, supporting the idea of its role as a transcription factor. We identified two different conformations of the MBF1 protein when bound, and characterized the specific groups of amino acids involved in the formation of the DNA-MBF1 complex. These regions of amino acids are bound mostly to the minor groove of DNA by the attraction of positively charged residues and the negatively charged backbone, but subject to the compatibility of shapes, much in the sense of a lock-and-key mechanism. We found that only with a sequence rich in CTAGA motifs at 300 K does MBF1 bind to DNA in the DNA-binding domain Cro/C1-type HTH predicted. In the rest of the systems tested, we observed non-specific DNA-MBF1 interactions. This study complements findings previously reported by others on the role of CTAGA as a DNA-binding element for MBF1c at a heat stress temperature.

Tailoring the structure of MoS2 using ball-milled MoO3 powders: hexagonal, triangular, and fullerene-like shapes.

Single and few-layered MoS2 materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO3 films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS2 nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS2 materials using MoO3 powders previously exposed to a high-energy ball milling treatment by the salt-assisted CVD method. The MoO3 powders milled for 30, 120, and 300 min were used to synthesize sample MoS2-30, MoS2-120, and MoS2-300, respectively. We found morphologies mainly of hexagons (MoS2-30), triangles (MoS2-120), and fullerenes (MoS2-300). The MoS2 nanostructures and MoO3 powders were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray diffraction, and thermogravimetric analysis. It was found that MoO3 milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS2 crystals and a large number of milled MoO3-x  + NaCl promote greater nucleation sites for the formation of MoS2, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ∼30-600 nm with 1-12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS2 samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.

Nitrogen-phosphorus doped graphitic nano onion-like structures: experimental and theoretical studies.

Onion-like graphitic structures are of great importance in different fields. Pentagons, heptagons, and octagons are essential features of onion-like graphitic structures that could generate important properties for diverse applications such as anodes in Li metal batteries or the oxygen reduction reaction. These carbon nanomaterials are fullerenes organized in a nested fashion. In this work, we produced graphitic nano onion-like structures containing phosphorus and nitrogen (NP-GNOs), using the aerosol assisted chemical vapor deposition method. The NP-GNOs were grown at high temperature (1020 °C) using ferrocene, trioctylphosphine oxide, benzylamine, and tetrahydrofuran precursors. The morphology, structure, composition, and surface chemistry of NP-GNOs were characterized using different techniques. The NP-GNOs showed diameters of 110-780 nm with Fe-based nanoparticles inside. Thermogravimetric analysis showed that NP-GNOs are thermally stable with an oxidation temperature of 724 °C. The surface chemistry analysis by FTIR and XPS revealed phosphorus-nitrogen codoping, and several functionalities containing C-H, N-H, P-H, P-O, P[double bond, length as m-dash]O, C[double bond, length as m-dash]O, and C-O bonds. We show density functional theory calculations of phosphorus-nitrogen doping and functionalized C240 fullerenes. We present the optimized structures, electronic density of states, HOMO, and LUMO wave functions for P-doped and OH-functionalized fullerenes. The P[double bond, length as m-dash]O and P-O bonds attributed to phosphates or hydroxyl groups attached to phosphorus atoms doping the NP-GNOs could be useful in improving supercapacitor function.

Understanding the electrochemistry of armchair graphene nanoribbons containing nitrogen and oxygen functional groups: DFT calculations.

The surface and edge chemistry are vital points to assess a specific application of graphene and other carbon nanomaterials. Based on first-principles density functional theory, we investigate twenty-four chemical functional groups containing oxygen and nitrogen atoms anchored to the edges of armchair graphene nanoribbons (AGNRs). Results for the band structures, formation energy, band gaps, electronic charge deficit, oxidation energy, reduction energy, and global hydrophilicity index are analyzed. Among the oxygen functional groups, carbonyl, anhydride, quinone, lactone, phenol, ethyl-ester, carboxyl, α-ester-methyl, and methoxy act as electron-withdrawing groups and, conversely, pyrane, pyrone, and ethoxy act as electron-donating groups. In the case of nitrogen-functional groups, amine, N-p-toluidine, ethylamine, pyridine-N-oxide, pyridone, lactam, and pyridinium transfer electrons to the AGNRs. Nitro, amide, and N-ethylamine act as electron-withdrawing groups. The carbonyl and pyridinium group-AGNRs show metallic behavior. The formation energy calculations revealed that AGNRs with pyridinium, amine, pyrane, carbonyl, and phenol are the most stable structures. In terms of the global hydrophilicity index, the quinone and N-ethylamine groups showed the most significant values, suggesting that they are highly efficient in accepting electrons from other chemical species. The oxidation and reduction energies as a function of the ribbon's width are discussed for AGNRs with quinone, hydroquinone, nitro, and nitro + 2H. Besides, we discuss the effect of nitrogen-doping in AGNRs on the oxidation and reduction energies for the quinone and hydroquinone functional groups.

Oxygenated Surface of Carbon Nanotube Sponges: Electroactivity and Magnetic Studies.

We report the synthesis of nitrogen-doped carbon nanotube sponges (N-CNSs) by pyrolysis of solutions of benzylamine, ferrocene, thiophene, and isopropanol-based mixture at 1020 °C for 4 h using an aerosol-assisted chemical vapor deposition system. The precursors were transported through a quartz tube using a dynamic flow of H2/Ar. We characterized the N-CNSs by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetric analysis. We found that isopropanol, isopropanol-ethanol, and isopropanol-acetone as precursors promote the formation of complex-entangled carbon fibers making knots and junctions. The N-CNSs displayed an outstanding oxygen concentration reaching a value of 9.2% for those synthesized with only isopropanol. We identified oxygen and nitrogen functional groups; in particular, the carbon fibers produced using only isopropanol exhibited a high concentration of ether groups (C-O bonds). This fact suggests the presence of phenols, carboxyl, methoxy, ethoxy, epoxy, and more complex functional groups. Usually, the functionalization of graphitic materials is carried out through aggressive acid treatments; here, we offer an alternative route to produce a superoxygenated surface. The understanding of the chemical surface of these novel materials represents a huge challenge and offers an opportunity to study complex oxygen functional groups different from the conventional quinone, carboxyl, phenols, carbonyl, methoxy, ethoxy, among others. The cyclic voltammetry measurements confirmed the importance of oxygen in N-CNSs, showing that with high oxygen concentration, the highest anodic and cathodic currents are displayed. N-CNSs displayed ferromagnetic behavior with an outstanding saturation magnetization. We envisage that our sponges are promising for anodes in lithium-ion batteries and magnetic sensor devices.

Synthesis of ZnMn2O4 Nanoparticles by a Microwave-Assisted Colloidal Method and their Evaluation as a Gas Sensor of Propane and Carbon Monoxide.

Spinel-type ZnMn2O4 nanoparticles were synthesized via a simple and inexpensive microwave-assisted colloidal route. Structural studies by X-ray diffraction showed that a spinel crystal phase of ZnMn2O4 was obtained at a calcination temperature of 500 °C, which was confirmed by Raman and UV-vis characterizations. Spinel-type ZnMn2O4 nanoparticles with a size of 41 nm were identified by transmission electron microscopy. Pellet-type sensors were fabricated using ZnMn2O4 nanoparticles as sensing material. Sensing measurements were performed by exposing the sensor to different concentrations of propane or carbon monoxide at temperatures in the range from 100 to 300 °C. Measurements performed at an operating temperature of 300 °C revealed a good response to 500 ppm of propane and 300 ppm of carbon monoxide. Hence, ZnMn2O4 nanoparticles possess a promising potential in the gas sensors field.

Two Sprayer CVD Synthesis of Nitrogen-doped Carbon Sponge-type Nanomaterials.

Nitrogen-doped carbon sponge-type nanostructures (N-CSTNs) containing coaxial multiwalled carbon nanotubes are synthesized at 1020 °C by using a modified chemical vapor deposition (CVD) arrangement. Here, the CVD reactor is supplied by two flows coming from two independent sprayers (called sprayer A and sprayer B). The nebulized material in each sprayer is transported by two different gases with different flow velocities. The synthesis of carbon N-CSTNs is performed using different precursors: sprayer A contains a solution composed of ethanol, thiophene and ferrocene, whereas sprayer B contains a solution of benzylamine, thiophene and ferrocene. Samples are classified according to the position inside the reactor and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and thermogravimetric analysis (TGA). Samples collected at the beginning of the reactor contain curly structures with diameters of 10-100 nm. At the end of the reactor, the sample is mainly formed by one type of structure. A spongy-type material is mainly formed in the hottest zone of the tubular furnace. The N-CSTNs are highly hydrophobic with oil sorption properties, which could be used for adsorption of oil spills.

Wrinkled Nitrogen-doped Carbon Belts.

Graphene, carbon nanotubes, and fullerenes are nanomaterials with outstanding properties such as electrical, thermal, mechanical strength, flexibility, and high surface area. These nanomaterials are used as building blocks for the construction of novel and astonishing 3D-dimensional networks. In the present work, nitrogen-doped carbon belt (N-CB) structures containing wrinkled carbon fibres as building blocks were synthesized under unstable conditions in a chemical vapour deposition experiment. N-CB structures with 0.2-3.0 microns of wide and 350 nm thick were assembled from complex individual wrinkled carbon fibres grown on Co/Cu films. These complex structures have a tubular appearance, showing holed and wrinkled graphite layers. Sulphur and copper atoms drastically affect the catalytic role of cobalt, changing the conventional growth of carbon nanotubes. Chemical functional groups, N-doping, and carbons hybridizations involved in the winkled carbon fibres are investigated. These findings provides a novel material that can be used as an excellent oxygen-reduction reaction catalyst or nano-electronics component.

Synthesis, Characterization, and Sensor Applications of Spinel ZnCo2O4 Nanoparticles.

Spinel ZnCo2O4 nanoparticles were synthesized by means of the microwave-assisted colloidal method. A solution containing ethanol, Co-nitrate, Zn-nitrate, and dodecylamine was stirred for 24 h and evaporated by a microwave oven. The resulting solid material was dried at 200 °C and subsequently calcined at 500 °C for 5 h. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy, confirming the formation of spinel ZnCo2O4 nanoparticles with average sizes between 49 and 75 nm. It was found that the average particle size decreased when the dodecylamine concentration increased. Pellets containing ZnCo2O4 nanoparticles were fabricated and tested as sensors in carbon monoxide (CO) and propane (C3H8) gases at different concentrations and temperatures. Sensor performance tests revealed an extremely high response to 300 ppm of CO at an operating temperature of 200 °C.

GaN Haeckelite Single-Layered Nanostructures: Monolayer and Nanotubes.

Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube's chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures.

Large-area Si-doped graphene: controllable synthesis and enhanced molecular sensing.

Large-area Si-doped graphene (SiG) is controllably synthesized for the first time. A much-enhanced molecular-sensing performance is achieved when SiG is used as a probing surface. This will open up opportunities for developing high-performance sensors that are able to detect trace amounts of organic and fluorescent molecules. Furthermore, many fascinating properties predicted by theoretical calculations can be tested based on the as-synthesized SiG.

Controlled synthesis and transfer of large-area WS2 sheets: from single layer to few layers.

The isolation of few-layered transition metal dichalcogenides has mainly been performed by mechanical and chemical exfoliation with very low yields. In this account, a controlled thermal reduction-sulfurization method is used to synthesize large-area (~1 cm(2)) WS2 sheets with thicknesses ranging from monolayers to a few layers. During synthesis, WOx thin films are first deposited on Si/SiO2 substrates, which are then sulfurized (under vacuum) at high temperatures (750-950 °C). An efficient route to transfer the synthesized WS2 films onto different substrates such as quartz and transmission electron microscopy (TEM) grids has been satisfactorily developed using concentrated HF. Samples with different thicknesses have been analyzed by Raman spectroscopy and TEM, and their photoluminescence properties have been evaluated. We demonstrated the presence of single-, bi-, and few-layered WS2 on as-grown samples. It is well known that the electronic structure of these materials is very sensitive to the number of layers, ranging from indirect band gap semiconductor in the bulk phase to direct band gap semiconductor in monolayers. This method has also proved successful in the synthesis of heterogeneous systems of MoS2 and WS2 layers, thus shedding light on the controlled production of heterolayered devices from transition metal chalcogenides.

Novel hetero-layered materials with tunable direct band gaps by sandwiching different metal disulfides and diselenides.

Although bulk hexagonal phases of layered semiconducting transition metal dichalcogenides (STMD) such as MoS2, WS2, WSe2 and MoSe2 exhibit indirect band gaps, a mono-layer of STMD possesses a direct band gap which could be used in the construction of novel optoelectronic devices, catalysts, sensors and valleytronic components. Unfortunately, the direct band gap only occurs for mono-layered STMD. We have found, using first principles calculations, that by alternating individual layers of different STMD (MoS2, WS2, WSe2 and MoSe2) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We foresee that the alternation of different STMD would result in the fabrication of materials with unprecedented optical and physico-chemical properties that would need further experimental and theoretical investigations.

Extraordinary room-temperature photoluminescence in triangular WS2 monolayers.

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from those of their bulk counterparts. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL weakens with increasing number of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet's center. The structure and chemical composition of the platelet edges appear to be critical for PL enhancement.

Quantum transport in graphene nanonetworks.

The quantum transport properties of graphene nanoribbon networks are investigated using first-principles calculations based on density functional theory. Focusing on systems that can be experimentally realized with existing techniques, both in-plane conductance in interconnected graphene nanoribbons and tunneling conductance in out-of-plane nanoribbon intersections were studied. The characteristics of the ab initio electronic transport through in-plane nanoribbon cross-points is found to be in agreement with results obtained with semiempirical approaches. Both simulations confirm the possibility of designing graphene nanoribbon-based networks capable of guiding electrons along desired and predetermined paths. In addition, some of these intersections exhibit different transmission probability for spin up and spin down electrons, suggesting the possible applications of such networks as spin filters. Furthermore, the electron transport properties of out-of-plane nanoribbon cross-points of realistic sizes are described using a combination of first-principles and tight-binding approaches. The stacking angle between individual sheets is found to play a central role in dictating the electronic transmission probability within the networks.

Millimeter-long carbon nanotubes: outstanding electron-emitting sources.

We are reporting the fabrication of a very efficient electron source using millimeter-long and highly crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.17 V/µm and reach threshold emission at 0.24 V/µm. In addition, these electron sources are very stable and can achieve a peak current density of 750 mA cm(-2) at only 0.45 V/µm. In order to demonstrate intense electron beam generation, these devices were used to produce visible light by cathodoluminescence. Finally, density functional theory calculations were used to rationalize the measured electronic field emission properties in open carbon nanotubes of different lengths. The modeling establishes a clear correlation between length and field enhancement factor.

Phosphorus and phosphorus-nitrogen doped carbon nanotubes for ultrasensitive and selective molecular detection.

A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus-nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P-N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH3 adsorbed on PN-doped nanotubes, O2 on P-doped nanotubes, and NO2 and SO2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO2 and CO adsorptions cause minor shifts in electronic conductance, NH3, NO2, and O2 adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells.