Evolutionary selection growth of two-dimensional materials on polycrystalline substrates.

There is a demand for the manufacture of two-dimensional (2D) materials with high-quality single crystals of large size. Usually, epitaxial growth is considered the method of choice 1 in preparing single-crystalline thin films, but it requires single-crystal substrates for deposition. Here we present a different approach and report the synthesis of single-crystal-like monolayer graphene films on polycrystalline substrates. The technological realization of the proposed method resembles the Czochralski process and is based on the evolutionary selection 2 approach, which is now realized in 2D geometry. The method relies on 'self-selection' of the fastest-growing domain orientation, which eventually overwhelms the slower-growing domains and yields a single-crystal continuous 2D film. Here we have used it to synthesize foot-long graphene films at rates up to 2.5 cm h-1 that possess the quality of a single crystal. We anticipate that the proposed approach could be readily adopted for the synthesis of other 2D materials and heterostructures.

Improved Characterization of Aqueous Single-Walled Carbon Nanotube Dispersions Using Dynamic Light Scattering and Analytical Centrifuge Methods.

Aqueous dispersions of single-walled carbon nanotubes (SWCNTs) with a surfactant were studied by using a combination of differential sedimentation and dynamic light scattering methods. When applied to elongated particles like SWCNTs, the differential sedimentation method makes it possible to measure their diameters in dispersions, while the dynamic light scattering method allows to measure their lengths. Both methods have logarithmic dependence on the ratio between the length and diameter of the particles, and their simultaneous use improves the accuracy of measuring particles' dimensions. It was shown that sonication of dispersions leads not only to unbundling of agglomerates into individual nanotubes but also to a decrease in their lengths and the appearance of new defects detectable in increasing the D/G ratio in the Raman spectra. Unbundling into individual nanotubes occurs after exposure to 1 kWh/L energy density, and the single nanotube diameter with SDBS is ca. 3.3 nm larger than that of the naked nanotubes. Conductivity of thin SWCNT films made out of individual nanotubes demonstrates a power law dependence with the exponent close to the theoretical one for rigid rods.

Hydrogen bond geometries and proton tautomerism of homoconjugated anions of carboxylic acids studied via H/D isotope effects on 13C NMR chemical shifts.

Ten formally symmetric anionic OHO hydrogen bonded complexes, modeling Asp/Glu amino acid side chain interactions in nonaqueous environment (CDF(3)/CDF(2)Cl solution, 200-110 K) have been studied by (1)H, (2)H, and (13)C NMR spectroscopy, i.e. intermolecularly H-bonded homoconjugated anions of acetic, chloroacetic, dichloroacetic, trifluoroacetic, trimethylacetic, and isobutyric acids, and intramolecularly H-bonded hydrogen succinate, hydrogen rac-dimethylsuccinate, hydrogen maleate, and hydrogen phthalate. In particular, primary H/D isotope effects on the hydrogen bond proton signals as well as secondary H/D isotope effects on the (13)C signals of the carboxylic groups are reported and analyzed. We demonstrate that in most of the studied systems there is a degenerate proton tautomerism between O-H···O(-) and O(-)···H-O structures which is fast in the NMR time scale. The stronger is the proton donating ability of the acid, the shorter and more symmetric are the H-bonds in each tautomer of the homoconjugate. For the maleate and phthalate anions exhibiting intramolecular hydrogen bonds, evidence for symmetric single well potentials is obtained. We propose a correlation between H/D isotope effects on carboxylic carbon chemical shifts and the proton transfer coordinate, q(1) = ½(r(OH) - r(HO)), which allows us to estimate the desired OHO hydrogen bond geometries from the observed (13)C NMR parameters, taking into account the degenerate proton tautomerism.

Geometries and tautomerism of OHN hydrogen bonds in aprotic solution probed by H/D isotope effects on (13)C NMR chemical shifts.

The (1)H and (13)C NMR spectra of 17 OHN hydrogen-bonded complexes formed by CH(3)(13)COOH(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF(3)/CDF(2)Cl mixture as solvent. The slow proton and hydrogen bond exchange regime was reached, and the H/D isotope effects on the (13)C chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding (1)H chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the (13)C chemical shifts of the carboxyl group with the OHN hydrogen bond geometries.

Electron transfer in platinum(II) diimine-centered triads: mechanistic insights from photoinduced transient displacement current measurements.

The consequences of photoexcitation of a platinum diimine bisacetylide complex and its triads with phenothiazine species (McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355-4365) were investigated by the photoinduced transient displacement current (PTDC) method, with the aim of understanding the role of solvent in defining the nature and extent of intratriad electron transfer. PTDC enables reports on the distance of charge separation in photoexcited states. Photoexcition of the triad, Pt(dbbpy)(CCC(6)H(4)CH(2)(PTZ))(2) (where dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), leads to formation of the (3)MLCT excited state, which in CH(2)Cl(2) is intramolecularly quenched by PTZ to form a charge-separated (CS) state, Pt(dbbpy(*-))(CCC(6)H(4)CH(2)(PTZ) CCC(6)H(4)CH(2)(PTZ(*+)); the CS state features a dipole moment oriented in essentially the opposite direction to that of the ground state. In toluene the last step of charge separation is shut down. In THF these two transient states are equilibrated, approximately as a 1:1 mixture of (3)MLCT and CS states, causing a complex, but instructive, PTDC response. The PTDC response for Pt(dbbpy)(CCC(6)H(5))(2), on the other hand, is similar in all solvents and shows a negative signal corresponding to a long-lived, and comparatively nonpolar, (3)MLCT state. The ground-state dipole moment, mu(g), weakly increases with solvent polarity, from approximately 11.5 D in toluene to approximately 15.5 D in THF and CH(2)Cl(2).

Exclusively Proton Conductive Membranes Based on Reduced Graphene Oxide Polymer Composites.

Proton exchange membranes are at the heart of various technologies utilizing electrochemical storage of intermittent energy sources and powering electrical devices. Current state of the art membranes are based on perfluorosulfonic acid, introduced more than a half century ago. Low specificity to protons accompanied by permeance by other species is one of the main impediments for various promising applications in green technologies in an energy sustainable economy. Here we present composite membranes that are exclusively proton selective and do not allow crossover of any ionic or molecular species other than protons. Membranes have high proton conductivity and exceptional mechanical and chemical stability and thus may significantly improve performance of hydrogen-based technologies such as electrolyzers, various kinds of fuel cells, and flow batteries in the future.

Ionic Conductance through Graphene: Assessing Its Applicability as a Proton Selective Membrane.

Inspired by recent reports on possible proton conductance through graphene, we have investigated the behavior of pristine graphene and defect engineered graphene membranes for ionic conductance and selectivity with the goal of evaluating a possibility of its application as a proton selective membrane. The averaged conductance for pristine chemical vapor deposited (CVD) graphene at pH1 is ∼4 mS/cm2 but varies strongly due to contributions from the unavoidable defects in our CVD graphene. From the variations in the conductance with electrolyte strength and pH, we can conclude that pristine graphene is fairly selective and the conductance is mainly due to protons. Engineering of the defects with ion beam (He+, Ga+) irradiation and plasma (N2 and H2) treatment showed improved areal conductance with high proton selectivity mostly for He-ion beam and H2 plasma treatments, which agrees with primarily vacancy-free type of defects produced in these cases confirmed by Raman analysis.

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers.

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.

Surface-assisted transient displacement charge technique. II. Effect of gases on photoinduced charge transfer in self-assembled monolayers.

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was used to study charge transfer in self-assembled monolayers of 7-diethylaminocoumarin covalently linked to an oxide surface in the atmosphere of different gases. The dipole signal was found to be opposite to that in solution and dependent on the nature of the gas and its pressure. The results were explained by collision-induced relaxation that impedes uninhibited tilting of molecules onto the surface. Collisions with paramagnetic oxygen induce intersystem crossing to long-lived triplet dipolar states of coumarin with the rate close to half of that for the collision rate.

Simple and Versatile Detection of Viruses Using Anodized Alumina Membranes.

A simple sensor for viral particles based on ionic conductivity through anodized alumina membranes was demonstrated using MS2 bacteriophage as an example. A facile two-point measuring scheme is geared toward realization using a computer's sound card input/output capabilities suitable for a fast and inexpensive point of care testing. The lowest detection concentration down to ~7 pfu/mL and a large dynamic range up to ~2000 pfu/mL were obtained due to physical optimization that included proper length and diameter for the pores, removing the oxide layer at the electrode, as well as the chemical optimization of covalent binding of antibodies to the pore's walls.

Water desalination using nanoporous single-layer graphene.

By creating nanoscale pores in a layer of graphene, it could be used as an effective separation membrane due to its chemical and mechanical stability, its flexibility and, most importantly, its one-atom thickness. Theoretical studies have indicated that the performance of such membranes should be superior to state-of-the-art polymer-based filtration membranes, and experimental studies have recently begun to explore their potential. Here, we show that single-layer porous graphene can be used as a desalination membrane. Nanometre-sized pores are created in a graphene monolayer using an oxygen plasma etching process, which allows the size of the pores to be tuned. The resulting membranes exhibit a salt rejection rate of nearly 100% and rapid water transport. In particular, water fluxes of up to 10(6) g m(-2) s(-1) at 40 °C were measured using pressure difference as a driving force, while water fluxes measured using osmotic pressure as a driving force did not exceed 70 g m(-2) s(-1) atm(-1).

Voltage-gated hydrophobic nanopores.

Hydrophobicity is a fundamental property that is responsible for numerous physical and biophysical aspects of molecular interactions in water. Peculiar behavior is expected for water in the vicinity of hydrophobic structures, such as nanopores. Indeed, hydrophobic nanopores can be found in two distinct states, dry and wet, even though the latter is thermodynamically unstable. Transitions between these two states are kinetically hindered in long pores but can be much faster in shorter pores. As it is demonstrated for the first time in this paper, these transitions can be induced by applying a voltage across a membrane with a single hydrophobic nanopore. Such voltage-induced gating in single nanopores can be realized in a reversible manner through electrowetting of inner walls of the nanopores. The resulting I-V curves of such artificial hydrophobic nanopores mimic biological voltage-gated channels.

H/D isotope effects on NMR chemical shifts of nuclei involved in a hydrogen bridge of hydrogen isocyanide complexes with fluoride anion.

(1)H, (2)H, (19)F and (15)N NMR spectra of a strongly hydrogen-bonded anionic cluster, CNHF(-), as an ion pair with a tetrabutylammonium cation dissolved in CDF(3)-CDF(2)Cl mixture were recorded in the slow exchange regime at temperatures down to 110 K. The fine structure due to spin-spin coupling of all nuclei involved in the hydrogen bridge was resolved. H/D isotope effects on the chemical shifts were measured. The results were compared with those obtained earlier for a similar anion, FHF(-), and interpreted via ab initio calculations of magnetic shielding as functions of internal vibrational coordinates, namely an anti-symmetric proton stretching and a doubly-degenerate bending. The values of primary and secondary isotope effects on NMR chemical shifts were estimated using a power expansion of the shielding surface as a function of vibrational coordinates. A positive primary isotope effect was explained as a result of the decrease of the hydron stretching amplitude upon deuteration. We show that the proton shielding surface has a minimum close to the equilibrium geometry of the CNHF(-) anion, leading to the positive primary H/D isotope effect in a rather asymmetric hydrogen bond. We conclude that caution should be used when making geometric estimations on the basis of NMR data, since the shapes of the shielding functions of the internal vibrational coordinates can be rather exclusive for each complex.

Low-temperature NMR studies of the structure and dynamics of a novel series of acid-base complexes of HF with collidine exhibiting scalar couplings across hydrogen bonds.

The low-temperature (1)H, (19)F, and (15)N NMR spectra of mixtures of collidine-(15)N (2,4,6-trimethylpyridine-(15)N, Col) with HF have been measured using CDF(3)/CDF(2)Cl as a solvent in the temperature range 94-170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the (19)F(1)H(19)F and (19)F(1)H(15)N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex [ColHFHCol](+)[FHF](-) is a contact ion pair. Collidinium hydrogen difluoride [ColH](+)[FHF](-) is an ionic salt exhibiting a strong hydrogen bond between collidinium and the [FHF](-) anion. In this complex, the anion [FHF](-) is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol(-)(1) for the reorientation. Finally, collidinium dihydrogen trifluoride [ColH](+)[F(HF)(2)](-) is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the [F(HF)(n)()](-) clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant (1)J(FH) when the F.H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, (1)J(FH) = 0, the heavy atom coupling constant (2)J(FN) remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.

Characterization of fluxional hydrogen-bonded complexes of acetic acid and acetate by NMR: geometries and isotope and solvent effects.

1H, (2)H, and (13)C NMR spectra of enriched CH(3)(13)COOH acid without and in the presence of tetra-n-butylammonium acetate have been measured around 110 K using a liquefied Freon mixture CDF(3)/CDF(2)Cl as a solvent, as a function of the deuterium fraction in the mobile proton sites. For comparison, spectra were also taken of the adduct CH(3)(13)COOH.SbCl(5) 1 and of CH(2)Cl(13)COOH under similar conditions, as well as of CH(3)(13)COOH and CH(3)(13)COO(-) dissolved in H(2)O and D(2)O at low and high pH at 298 K. The low temperatures employed allowed us to detect several well-known and novel hydrogen-bonded complexes in the slow hydrogen bond exchange regime and to determine chemical shifts and coupling constants as well as H/D isotope effects on chemical shifts from the fine structure of the corresponding signals. The measurements show that self-association of both carboxylic acids in Freon solution gives rise exclusively to the formation of cyclic dimers 2 and 3 exhibiting a rapid degenerate double proton transfer. For the first time, a two-bond coupling of the type (2)J(CH(3)COOH) between a hydrogen-bonded proton and the carboxylic carbon has been observed, which is slightly smaller than half of the value observed for 1. In addition, the (1)H and (2)H chemical shifts of the HH, HD, and the DD isotopologues of 2 and 3 have been determined as well as the corresponding HH/HD/DD isotope effects on the (13)C chemical shifts. Similar "primary", "vicinal", and "secondary" isotope effects were observed for the novel 2:1 complex "dihydrogen triacetate" 5 between acetic acid and acetate. Another novel species is the 3:1 complex "trihydrogen tetraacetate" 6, which was also characterized by a complex degenerate combined hydrogen bond- and proton-transfer process. For comparison, the results obtained previously for hydrogen diacetate 4 and hydrogen maleate 7 are discussed. Using an improved (1)H chemical shift-hydrogen bond geometry correlation, the chemical shift data are converted into hydrogen bond geometries. They indicate cooperative hydrogen bonds in the cyclic dimers; i.e., widening of a given hydrogen bond by H/D substitution also widens the other coupled hydrogen bond. By contrast, the hydrogen bonds in 5 are anticooperative. The measurements show that ionization shifts the (13)C signal of the carboxyl group to low field when the group is immersed in water, but to high field when it is embedded in a polar aprotic environment. This finding allows us to understand the unusual ionization shift of aspartate groups in the HIV-pepstatin complex observed by Smith, R.; Brereton, I. M.; Chai, R. Y.; Kent, S. B. H. Nature Struct. Biol. 1996, 3, 946. It is demonstrated that the Freon solvents used in this study are better environments for model studies of amino acid interactions than aqueous or protic environments. Finally, a novel correlation of the hydrogen bond geometries with the H/D isotope effects on the (13)C chemical shifts of carboxylic acid groups is proposed, which allows one to estimate the hydrogen bond geometries and protonation states of these groups. It is shown that absence of such an isotope effect is not only compatible with an isolated carboxylate group but also with the presence of a short and strong hydrogen bond.

NMR parameters and geometries of OHN and ODN hydrogen bonds of pyridine-acid complexes.

In this paper, equations are proposed which relate various NMR parameters of OHN hydrogen-bonded pyridine-acid complexes to their bond valences which are in turn correlated with their hydrogen-bond geometries. As the valence bond model is strictly valid only for weak hydrogen bonds appropriate empirical correction factors are proposed which take into account anharmonic zero-point energy vibrations. The correction factors are different for OHN and ODN hydrogen bonds and depend on whether a double or a single well potential is realized in the strong hydrogen-bond regime. One correction factor was determined from the known experimental structure of a very strong OHN hydrogen bond between pentachlorophenol and 4-methylpyridine, determined by the neutron diffraction method. The remaining correction factors which allow one also to describe H/D isotope effects on the NMR parameters and geometries of OHN hydrogen bond were determined by analysing the NMR parameters of the series of protonated and deuterated pyridine- and collidine-acid complexes. The method may be used in the future to establish hydrogen-bond geometries in biologically relevant functional OHN hydrogen bonds.