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High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c -axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm −1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.

We report the preparation and solid state characterization of a silicon (IV) salt based on a new redox active tripodal hexadentate ligand [H3L = l,l,l-Tris(l′-oxophenalenyl-9′-N-methyl) ethane]. X-ray analysis of the complex reveals a short Si-N bond distance (av 1.878(16)). The compound is shown to undergo three reversible one-electron reduction steps (without decomposition) due to the formation of radical, anion and dianion of the phenalenyl moiety.

An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of ∼0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

We report a systematic study of the purification of nitric acid treated single-walled carbon nanotubes (SWNTs) by centrifugation at constant pH. The progress of the purification procedure was followed by estimating the purity by solution-phase near-infrared spectroscopy, and it was found that the sediment obtained by centrifugation at pH 2 had the highest purity. The stability of the aqueous dispersions of SWNTs and carbonaceous impurities (CIMP) after nitric acid treatment was investigated by measuring their zeta potential and the concentration of acidic sites. We found that, at lower pH, the CIMP has a higher zeta potential than nitric acid treated SWNTs, and it is this factor that allowed the separation of the carbonaceous impurities via centrifugation and the concomitant purification of the SWNTs.

We report the preparation, crystallization, and solid-state characterization of the first members of a new family of spiro-bis-(1,9-diamino-substituted-phenalenyl)boron neutral radicals. The crystal structures show that the three radicals are monomeric and without close contacts in the crystal lattice. In all cases magnetic susceptibility measurements confirm the presence of free radicals with one unpaired spin per molecule. Two of the new radical compounds are among the most highly conducting neutral organic solids, with room-temperature conductivities reaching sigma(RT) = 4 x 10(-2) S/cm. The measured conductivities correlate with the closest intermolecular contacts in the solid state and with the calculated band dispersions, even though the bandwidths are much smaller than those found in other organic conductors.

We present a study on the electronic behavior of films of as-prepared and purified single-walled carbon nanotubes (SWNTs) and demonstrate the important role that chemical functionalization plays in modifying their electronic properties, which in turn throws further light on the mechanism of action of SWNT-based sensors. Films of electric arc SWNTs were prepared by spraying, and optical spectroscopy was used to measure the effective film thickness. The room-temperature conductivities (sigma(RT)) of thin films deposited from as-prepared and purified SWNTs are in the range sigma(RT) = 250-400 S/cm, and the nonmetallic temperature dependence of the conductivity indicates the presence of tunneling barriers, which dominate the film conductivity. Chemical functionalization of SWNTs with octadecylamine (ODA) and poly(m-aminobenzenesulfonic acid) (PABS) significantly decreases the conductivity; sigma(RT) = 3 and 0.3 S/cm for SWNT-ODA and SWNT-PABS, respectively.

We report a chemical processing technology that allows the continuous spinning of single-walled carbon nanotubes (SWNTs)-nylon 6 (PA6) fibers by the in-situ polymerization of caprolactam in the presence of SWNTs, which simultaneously optimizes the morphology of the composite. We show that caprolactam is an excellent solvent for carboxylic-acid-functionalized SWNTs (SWNT-COOH) and that this allows the efficient dispersal of the SWNTs and subsequent grafting of PA6 chains to the SWNTs through condensation reactions between the carboxylic-acid group on SWNT-COOH and the terminal amine group of PA6. The existence of a graft copolymer between the PA6 chains and the SWNTs is demonstrated by IR, TGA, and AFM studies, and we show that the solubility of the polymerized material in formic acid is controlled by the degree of graft copolymerization. The amount of grafted PA6 chains that are attached to the SWNTs can be adjusted by controlling the concentration of the initiator (6-aminocaproic acid). The process leads to a uniform dispersion of the SWNTs, and the presence of the graft copolymer increases the polymer/SWNT compatibility while strengthening the interfacial interaction between the nanotube and matrix. The Young's modulus, tensile strength, and thermal stability of the SWNT-reinforced composite fibers produced by this process are significantly improved.

This study demonstrates a novel approach toward development of advanced chemical sensors based on chemically functionalized single-walled carbon nanotubes (SWNTs). SWNTs with covalently attached poly(m-aminobenzene sulfonic acid), SWNT-PABS, have shown improved sensor performance for detection of NH3. Compared to purified SWNTs, devices fabricated with SWNT-PABS have shown more than 2 times higher change of resistance upon exposure to NH3. Importantly, the SWNT-PABS sensors rapidly recover their resistance when NH3 is replaced with nitrogen. Exposure to NH3 induces significant changes in the electronic structure of SWNT-PABS, which allow detection of NH3 at concentrations as low as 5 ppm. Thin film deposited between interdigitated electrodes was explored as a device configuration for development of gas sensors.

The influence of Ni−Y catalyst composition on the bulk scale synthesis of single-walled carbon nanotubes (SWNTs) is studied using solution phase near-infrared spectroscopy. We found that a wide ran...

UV illumination of semiconducting single wall carbon nanotubes (SWNTs) results in a substantial increase of the intensity of the first interband transition (S11) followed by dark or IR light induced restoration of the initial intensity. The intensity of the S11 transition could also be modulated by light-induced reversible conversion of spiropyran molecules attached to SWNTs from the spiro conformation to its merocyanine form. Both surface-located monomeric and end-located aggregated merocyanines were found in UV illuminated spiropyran-SWNT supramolecular conjugates.

The measurement of the bulk purity of single-walled carbon nanotubes (SWNTs) is an important outstanding problem. We report a solution-phase near-IR (NIR) spectroscopic study of a range of carbon materials with an emphasis on SWNTs, and we show that NIR spectroscopy is an extremely powerful tool in the assessment of the carbonaceous purity of SWNTs. We demonstrate the applicability of Beer's law for all of the carbon materials included in this study within a range of concentrations in dimethylformamide (DMF), and we are able to solve for the effective extinction coefficients. By analyzing the areal absorptivities of the second interband transition of semiconducting SWNTs for a number of samples of differing purities, we are able to derive an absolute molar extinction coefficient for the carbonaceous impurities in SWNT samples. We demonstrate significant progress toward the establishment of an absolute scale for the bulk carbonaceous purity of SWNTs.

A general synthetic route to the pyridine-bridged dithiazolodithiazolyl framework R2BPR1, involving N-alkylation of a 4-substituted 2,6-dichloropyridine, followed by amination and double Herz cyclization with S2Cl2, has been developed. The radicals R2BPMe (R2 = Me, Ph) have been prepared and characterized by EPR spectroscopy and cyclic voltammetry. Their crystal structures have been determined by X-ray crystallography. Both structures consist of undimerized slipped radical π-stacks. Lateral interactions in MeBPMe (space group P21/c) generate chainlike arrays with radicals linked by inversion centers; there are no close interchain S−S contacts. By contrast, in PhBPMe (space group P3121) the radical π-stacks are nested about 31 axes, so as to produce an extensive 2-dimensional network of intermolecular S−S interactions. Variable-temperature magnetic susceptibility measurements reveal that MeBPMe is essentially paramagnetic, whereas PhBPMe displays strong antiferromagnetic coupling. A value of J = −149 cm-1 ...

Single-walled carbon nanotube (SWNT) reinforced polymer composite membranes have been fabricated using the electrospinning technique. Nanofibers with a diameter in the range 50−100 nm were obtained by electrospinning SWNT-filled polystyrene composites. TEM observations revealed incorporation of small SWNT bundles oriented parallel to the nanofiber axis. As-prepared (AP) and ester (EST) functionalized SWNTs have been electrospun with polyurethane (PU) to demonstrate the effect of the chemical functionalization of SWNTs on the mechanical properties of SWNT-reinforced composites. The tensile strength of EST-SWNT-PU membranes is enhanced by 104% as compared to electrospun pure polyurethane membranes, while an increase of only 46% was achieved incorporating AP−SWNT in the polyurethane matrix. The tangent moduli of AP- and EST-SWNT-PU membranes were found to be respectively 215% and 250% higher than the control polyurethane membranes.

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-step stack. This particular mode of 1-D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the pi-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm(-1). Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10(-3) S/cm and is best described as a Mott insulator.

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

Electric-arc produced as-prepared single-walled carbon nanotube (AP-SWNT) films have been purified by a selective oxidation process. The purification is monitored by near-infrared (NIR) spectroscopy, which gives a quantitative estimate of relative purities. This technique also gives an estimate of the change in relative amounts of SWNTs and amorphous carbon impurities after each purification step, thereby providing accurate and selective control over the oxidation process. Oxidation was carried out by reaction in flowing oxygen at different temperatures and for different durations of time; the purity was evaluated after each oxidation step and compared to the starting as-prepared film. The purities and amounts of SWNTs and carbonaceous impurities were monitored as a function of oxidation temperature and time. Optimization of the process allowed a 3-fold improvement in the AP-SWNT purity with almost quantitative retention of the SWNT portion of the sample. A purification efficiency is defined, which is exp...

The nature of single-walled carbon nanotube (SWNT)−solvent interaction that is responsible for the stabilization of ultrasonicated dispersions was investigated using various techniques. It was found that the sonochemical decomposition and polymerization of o-dichlorobenzene (ODCB) is necessary for the stabilization of SWNTs in ODCB. The rate of decomposition of ODCB was shown to be dependent on the shape of the sonication vessel. The decomposition products of sonicated ODCB interact irreversibly with SWNTs. Under similar sonication conditions, N,N-dimethylformamide (DMF) did not induce discernible modification of the near-infrared spectrum of the dispersed SWNTs.

We report the crystal structure of a spiro-bis(1,9-disubstituted-phenalenyl)boron cation ( 2 + ) and by comparison with the structures of a series of radicals we conclude that the changes in bond lengths in the radicals compared to those of the corresponding cation can be qualitatively predicted by the sign of the partial bond order of the singly occupied molecular orbital (SOMO), which contains the additional electron. By use of this information, we can analyse the structures of the π-dimer radicals 1 and 2 and ascertain the location of the unpaired electron in these compounds. Where these radicals exist as paramagnetic π-dimers (high-temperature regime), the unpaired electrons reside mainly in the phenalenyl units not directly involved in the π-dimer interaction. At low temperatures, where the diamagnetic π-dimers exist, the electrons localise in the phenalenyl units directly involved in the π-dimer, but there still exists some degree of delocalisation of the electrons to the other halves of the molecules. We postulate that the increase in delocalisation of the unpaired electrons is related to the increase of the conductivity by about two orders of magnitude as the electrons spin pair.

We report a rapid, quantitative procedure for the evaluation of the carbonaceous purity of bulk quantities of as-prepared single-walled carbon nanotube (SWNT) soot by the utilization of solution-phase near-IR spectroscopy. The procedure starts with two steps of homogenization followed by solution/dispersion spectroscopy of a representative part of the bulk sample. The purity is evaluated against a reference sample by utilizing the region of the second interband transition (S22) for semiconducting SWNTs. The procedure is found to be capable of reliably analyzing the carbonaceous purity of a 10-g batch of SWNTs produced by the electric arc discharge method to within 3%.

A new class of phenalenyl-based neutral radical molecular conductors has recently been reported; a distinctive feature of these molecular solids is the absence of the well-defined conducting pathway(s) that are characteristic of the organic charge- transfer salts. These radicals do not stack in the solid state, and the requisite carbon- carbon contacts are all larger than the sum of the van der Waals distances. Magnetic susceptibility measurements show that these compounds usually exist as isolated free radicals with one spin per molecule (at least over some temperature range), apparently supporting the idea that there is little interaction between the molecules in the solid state. Nevertheless, these radicals show the highest conductivity (RT 0.05 S/cm) of any neutral organic solid and the conduction mechanism is at present unresolved. In this article we report a structurally related radical with a resistivity five orders of magnitude higher than that of the previously reported radicals. We analyze the crystallographic and electronic structure of this solid in detail and point out some of the challenges that remain in relating structure, magnetism, and conductivity in this new class of organic solids. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 853- 865, 2003

The passage of a current through a solid requires a pathway for the movement of electrons or holes. A new class of phenalenyl-based neutral radical molecular conductors has recently been reported; a distinctive feature of these molecular solids is the absence of any obvious conducting pathway(s). These radicals do not stack in the solid state, and the requisite carbon−carbon contacts are all larger than the sum of the van der Waals distances. Magnetic susceptibility measurements show that the first compound to be isolated behaves as a free radical with one spin per molecule, apparently supporting the idea that there is little interaction between the molecules in the solid state. Nevertheless, this radical shows the highest conductivity (σRT = 0.05 S/cm), of any neutral organic solid, and the conduction mechanism is presently unresolved. In the present study, we report a structurally related radical with a resistivity 4 orders of magnitude higher than that of the previously reported radicals. We analyze th...

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.

We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Å d-spacing polymorph are noticeably larger than for the 14.5 Å d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.

Covalent chemistry exerts characteristic changes to the geometric and electronic structures of the carbon allotropes associated with modification of the conjugation. There are two limiting cases: (a) conventional addition chemistry leading to the formation of σ-bonds to the graphitic surface in which there is full rehybridization of the derivatized carbon atoms from sp2 to sp3; in this case the carbon atoms are removed from conjugation (destructive rehybridization) and (b) covalent chemisorption with formation of an organometallic hexahapto-metal bond that largely preserves the graphitic band structure (constructive rehybridization). 1, 2 We focus our discussion on the organometallic chemistry of the carbon allotropes and the possibility to electronically interconnect graphitic surfaces by means of covalent bonding. This mode of functionalization allows the formation of atomic scale interconnects that consist of bis-hexahapto-metal-bonds between benzenoid ring systems and it may be used to increase the dimensionality of the electronic structure of the materials leading to enhanced conductivity. We will discuss the effect of bis-hexahapto bond formation on the electronic properties of percolating networks of single walled carbon nanotubes, small scale graphite nanoplatelets and graphene. 1. Bekyarova, E.; Sarkar, S.; Wang, F.; Itkis, M. E.; Kalinina, I.; Tian, X.; Haddon, R. C., Effect of Covalent Chemistry on the Electronic Structure and Properties of Carbon Nanotubes and Graphene. Acc. Chem. Res. 2013, 46, 65-76. 2. Bekyarova, E.; Niyogi, S.; Sarkar, S.; Tian, X.; Chen, M.; Moser, M. L.; Ayub, K.; Mitchell, R. H.; Haddon, R. C., Stereochemical Effect of Covalent Chemistry on the Electronic Structure and Properties of the Carbon Allotropes and Graphene Surfaces. Synth. Met. 2015, 210, 80-84.

We report the crystallization of a subsitutionally doped organic conductor based on a host lattice composed of spiro-bis(phenalenyl)boron radicals. Co-crystallization of solutions of spiro-bis(9-oxidophenalenone)boron radical [PLY(O,O)]2B mixed with selected amounts of spiro-bis(9-oxidophenalenone)beryllium [PLY(O,O)]2Be leads to the formation of a series of solid-state solutions of composition [PLY(O,O)]2B(1-x)Be(x). The dopant molecules [PLY(O,O)]2Be serve to introduce holes into the lattice of spins provided by the [PLY(O,O)]2B radicals and lead to a systematic increase in the conductivity while decreasing the activation energy of the conduction process and leaving the solid-state structure relatively unperturbed. While the energies of the hole sites are expected to be high, the results are consistent with the interpretation of the electronic structure of [PLY(O,O)]2B in terms of the resonating valence bond model.

The formation of covalent bonds to single-walled carbon nanotube (SWNT) or graphene surfaces usually leads to a decrease in the electrical conductivity and mobility as a result of the structural rehybridization of the functionalized carbon atoms from sp(2) to sp(3). In the present study, we explore the effect of metal deposition on semiconducting (SC-) and metallic (MT-) SWNT thin films in the vicinity of the percolation threshold and we are able to clearly delineate the effects of weak physisorption, ionic chemisorption with charge transfer, and covalent hexahapto (η(6)) chemisorption on these percolating networks. The results support the idea that for those metals capable of forming bis-hexahapto-bonds, the generation of covalent (η(6)-SWNT)M(η(6)-SWNT) interconnects provides a conducting pathway in the SWNT films and establishes the transition metal bis-hexahapto organometallic bond as an electronically conjugating linkage between graphene surfaces.

We report the response of the electrical conductivity of semiconducting single-walled carbon nanotube (SWNT) thin films on exposure to metal vapors of the early lanthanides under high vacuum conditions. We attribute the strongly enhanced conductivities observed on deposition of samarium and europium to charge transfer from the metals to the SWNT backbone, thereby leading to the first examples of mixed covalent-ionic bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = Sm, Eu].

The electronic structure of single-walled carbon nanotubes (SWNTs) is investigated by transmission spectroscopy of thin films. Hole-doping of SWNTs, as a result of purification, leads to free carrier absorption in the far-IR spectral range. Thermal cycling de-dopes SWNTs, transferring spectral weight from the far-IR to the interband transitions in the near-IR spectral range. After the nanotubes are de-doped, a diameter dependent peak in the far-IR is apparent at ∼100 cm-1, which we ascribe to a curvature induced energy gap in the DOS of nonarmchair metallic SWNTs.

By use of solution phase mid-IR spectroscopy we are able to obtain an estimate for the ratio of the carbon atoms in the single-walled carbon nanotube (SWNT) backbone to the carbon atoms in the end-groups and at defect sites of the octadecylamido (ODA) functionalized soluble SWNTs (s-SWNT-CONH(CH2)17CH3). This analysis shows that the weight percentage of the octadecylamido functionality in the s-SWNTs is about 50%.

We report the measurement of the acidic sites in three different samples of commercially available full-length purified single-walled carbon nanotubes (SWNTs) – as obtained from CarboLex (CLI), Carbon Solutions (CSI) and Tubes@Rice (TAR) – by simple acid–base titration methods. Titration of the purified SWNTs with NaOH and NaHCO3 solutions was used to determine the total percentage of acidic sites and carboxylic acid groups, respectively. The total percentage of acidic sites in full length purified SWNTs from TAR, CLI and CSI are about 1–3%.

We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.

Full-length single-walled carbon nanotubes (SWNTs) were rendered soluble in common organic solvents by noncovalent (ionic) functionalization of the carboxylic acid groups present in the purified SWNTs. Atomic force microscopy (AFM) showed that the majority of the SWNTs ropes were exfoliated into small ropes (2−5 nm in diameter) and individual nanotubes with lengths of several micrometers during the dissolution process. The combination of multiwavelength laser excitation Raman scattering spectroscopy and solution-phase visible and near-infrared spectroscopies was used to characterize the library of SWNTs that is produced in current preparations. The average diameter of metallic nanotubes was found by Raman spectroscopy to be smaller than that of semiconducting nanotubes in the various types of full-length SWNT preparations. This observation sheds new light on the mechanism of SWNT formation.

The infrared transmission of the quasi-one dimensional charge-density-wave (CDW) conductor blue bronze (K0.3MoO3) is affected by polarization of the CDW, and therefore by application of a voltage near or above the threshold for CDW depinning. In this paper, we compare the spectra associated with the relative change in transmission taken for different temperatures and oscillating voltages. We find that the phonon spectrum is affected by CDW polarization; the linewidths or frequencies of most phonons change by cm-1. However, no new intragap states that can be associated with current injection are observed; i.e. the spectra associated with polarization of the CDW in the crystal bulk is identical to that associated with CDW current injection near the contacts. Our results indicate that, for light polarized perpendicular to the conducting chains, the density (n), cross-section , and bandwidth of intragap states are related by: n (Acm-1)-1. For expected values of the cross-section and bandwidth, this implies that the intragap states can be optically excited for a time less than s.

We have used electromodulated infrared transmission spectroscopy to search for states associated with soliton excitations of the CDW phase in blue bronze with tunable diode lasers. In particular, we have searched the spectral region near the edge of the CDW gap and compared the spectra associated with contact polarization (i.e. current injection) with that associated with bulk CDW polarization. No new states are observed, implying that the lifetime of injected solitons is < 0.5 μs. Both bulk and contact polarization are observed to shift the frequency of high energy phonons, suggesting that the density of electronic states in the gap, present because of CDW fluctuations, depends on CDW polarization.

We report the preparation, crystallization, and solid-state characterization of a spiro-biphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state and withou...

The synthesis and solid-state characterization of the heterocyclic π-radical 1,2,5-thiadiazolo[3,4-b]-1,2,3-dithiazolo[3,4-b]pyrazin-2-yl, 1,2,3-TDTA, is described. The ESR spectrum of 1,2,3-TDTA (in CH2Cl2, 293 K, g = 2.009) confirms a highly delocalized spin distribution, with observable hyperfine coupling to all five nitrogen atoms of the tricyclic molecule (aN = 0.514, 0.343, 0.109, 0.051, and 0.045 mT). While chemical and electrochemical oxidation (E1/2(ox) = 1.14 V vs SCE) of 1,2,3-TDTA requires relatively harsh conditions, reduction is extremely facile (E1/2(red) = 0.15 V vs SCE). More importantly both the observed cell potential Ecell and computed (MNDO) gas-phase enthalpy ΔHdisp for the disproportionation of this and other 1,2,3-dithiazolyls are significantly lower than those observed for their 1,3,2-isomers. Crystals of 1,2,3-TDTA are monoclinic P21/n, with a = 6.6749(16) A, b = 11.7178(14) A, c = 8.6148(14) A, β = 103.297(16)°, and Z = 4. The crystal structure consists of slipped stacks of head...

We report EPR studies on pristine, purified, shortened and soluble SWNTs in various solution phases. Some of these samples give rise to strong, sharp EPR signals, and this technique is useful for monitoring the presence of SWNTs in aqueous and organic solvents. The soluble SWNTs carry about 1 unpaired electron per 10000 carbon atoms and give a free electron g-value.

We had previously shown that the infrared transmission in blue bronze $({\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3})$ is modulated by an applied electric field, and showed that this change was related to deformations of the charge-density wave (CDW) [M. E. Itkis and J. W. Brill, Phys. Rev. Lett. 72, 2049 (1994)]. Since the CDW is also deformed by temperature changes, we expected the infrared (IR) transmission to also exhibit thermal hysteresis, and have observed this. Comparison of the magnitude and temperature dependences of the thermal hysteresis in the dc conductance and IR transmission indicate that the changes in the latter are not primarily due to changes in quasiparticle absorption. Comparisons of the electromodulated and temperature difference IR and Raman spectra suggest that the electromodulated changes result from quasiparticle screening of phonons, and we estimate that application of a field near the threshold for CDW depinning changes the quasiparticle density by a few percent near the current contacts.

Resonance stabilized bis-1,2,3-thiaselenazolyl radicals associate in the solid state to afford Se-Se sigma-bonded dimers.

Ruthenium oxide (RuO2) nanoparticles were supported on the outer surface of single walled carbon nanotubes (SWNTs) using chemical reduction technique. The nanocomposite was employed as an electrode material for charge storage capacitor using ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte solution. The electrochemical charge storage properties of the composite were investigated using two electrode cyclic voltammetry and galvanostatic charge-discharge techniques. The specific capacitance and energy density of the RuO2-SWNT nanocomposite electrode material based on the fabricated coin cell supercapacitor were measured to be as high as 174 F/g and 74 Wh/kg at current density of 1 A/g.

Thermal interface materials (TIMs) are crucial components of high density electronics and the high thermal conductivity of graphite makes this material an attractive candidate for such applications. We report an investigation of the in-plane and through-plane electrical and thermal conductivities of thin thermal interface layers of graphite nanoplatelet (GNP) based composites. The in-plane electrical conductivity exceeds its through-plane counterpart by three orders of magnitude, whereas the ratio of the thermal conductivities is about 5. Scanning electron microscopy reveals that the anisotropy in the transport properties is due to the in-plane alignment of the GNPs which occurs during the formation of the thermal interface layer. Because the alignment in the thermal interface layer suppresses the through-plane component of the thermal conductivity, the anisotropy strongly degrades the performance of GNP-based composites in the geometry required for typical thermal management applications and must be taken into account in the development of GNP-based TIMs.

Chemically modified single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization were utilized for the fabrication of SWNT thin film catalyst support layers (CSLs) in polymer electrolyte membrane fuel cells (PEMFCs), which were suitable for benchmarking against the US DOE 2017 targets. Use of the optimum level of SWNT -COOH functionality allowed the construction of a prototype SWNT-based PEMFC with total Pt loading of 0.06 mg(Pt)/cm²--well below the value of 0.125 mg(Pt)/cm² set as the US DOE 2017 technical target for total Pt group metals (PGM) loading. This prototype PEMFC also approaches the technical target for the total Pt content per kW of power (0.125 g(PGM)/kW) at cell potential 0.65 V: a value of 0.15 g(Pt)/kW was achieved at 80°C/22 psig testing conditions, which was further reduced to 0.12 g(Pt)/kW at 35 psig back pressure.

We have used electromodulated infrared transmission to image charge-density-wave strains in blue bronze. Two components of the strain are identified: a bulk strain, which appears throughout the crystal above an applied voltage ${\mathit{V}}_{\mathit{P}}$, and a contact strain, which appears within \ensuremath{\approxeq}100 \ensuremath{\mu}m of the contacts with the onset of nonlinear current at ${\mathit{V}}_{\mathit{T}}$\ensuremath{\gtrsim}${\mathit{V}}_{\mathit{P}}$. The two components have very different voltage, frequency, and temperature dependencies.

Taking advantage of the weak attraction forces between gold surface and silica zeolite colloids, selective growth of zeolite on silicon wafer over gold was exploited to fabricate continuous micropatterned b-oriented zeolite MFI monolayer film with excellent adhesion.

Spin transport in carbon-based materials has stimulated much interest due to their ballistic conductance and a long phase coherence length. While much research has been conducted on individual carbon nanotubes, current growth and placement techniques are incompatible with large-scale fabrication. Here, we report on nonlocal spin injection and detection in single-walled carbon nanotube networks. We observe spin transport over a distance of 1 \ensuremath{\mu}m and extract a spin diffusion length of 1.6--2.4 \ensuremath{\mu}m with an injected spin polarization from CoFe into nanotube network of 18$%$--41$%$. Our observations demonstrate that spin transport is possible in carbon nanotube networks due to the formation of natural tunnel barriers between nanotubes and metallic contacts.

Chemistry will play an increasingly important role in the realization of graphene applications. The chemical formation of covalent carbon–carbon bonds involving the basal plane carbon atoms offers an alternative approach to the control of the electronic properties of graphene, and potentially allows the generation of insulating and semiconducting regions in graphene wafers. This review summarizes recent progress in the covalent modification of epitaxial graphene and the effect that chemistry has on the electronic and magnetic properties of the material.

We report the preparation, crystallization and solid state characterization of a cyclohexanemethylamine substituted spirobiphenalenyl radical; in the solid state the compound is iso-structural with its dehydro-analog (benzylamine-substitued compound), and the molecules packed in a one-dimensional fashion that we refer to as a π-step stack. Neighboring molecules in the stack interact via the overlap of one pair of active (spin bearing) carbon atoms per phenalenyl unit. The magnetic susceptibility measurement indicates that in the solid state the radical remains paramagnetic and the fraction of Curie spins is 0.75 per molecule. We use the analytical form of the Bonner-Fisher model for the S = 1/2 antiferromagnetic Heisenberg chain of isotropically interacting spins with intrachain spin coupling constant J = 6.3 cm−1, to fit the experimentally observed paramagnetism [χp (T)] in the temperature range 4–330 K. The measured room temperature conductivity (σRT = 2.4 × 10–3 S/cm) is comparable with that of the iso-structural benzyl radical, even though the calculated band dispersions are smaller than that of the unsaturated analog.

International audience; The authors report the effects of radical functionalization on the electrostatic force microscopy (EFM), the scanning tunneling spectra (STS), and Raman spectroscopy of epitaxial graphene. The EFM studies show the existence of layer dependent trapped charges in the pristine graphene. The uniform enhancement of energy gap is observed through STS. Raman spectra show nonuniformly distributed D-band intensities throughout the functionalized sample as a result of the inhomogeneous distribution of covalent bonds to the graphene sheets. The functionalization chemistry has a marked effect on the homogeneity of the electrostatic charge and leads to an increase of the energy of the band gap.

A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic. In the paramagnetic state, the unpaired electrons are located in the exterior phenalenyl units of the dimer, whereas in the diamagnetic state the electrons migrate to the interior phenalenyl units and spin pair as a π-dimer. Against all expectations, the conductivity increases by two orders of magnitude in the diamagnetic state, and the band gap decreases. This type of multifunctional material has the potential to be used as the basis for new types of electronic devices, where multiple physical channels are used for writing, reading, and transferring information.