Nano-Architecture of nitrogen-doped graphene films synthesized from a solid CN source

New synthesis routes to tailor graphene properties by controlling the concentration and chemical configuration of dopants show great promise. Herein we report the direct reproducible synthesis of 2-3% nitrogen-doped ‘few-layer’ graphene from a solid state nitrogen carbide a-C:N source synthesized by femtosecond pulsed laser ablation. Analytical investigations, including synchrotron facilities, made it possible to identify the configuration and chemistry of the nitrogen-doped graphene films. Auger mapping successfully quantified the 2D distribution of the number of graphene layers over the surface, and hence offers a new original way to probe the architecture of graphene sheets. The films mainly consist in a Bernal ABA stacking three-layer architecture, with a layer number distribution ranging from 2 to 6. Nitrogen doping affects the charge carrier distribution but has no significant effects on the number of lattice defects or disorders, compared to undoped graphene synthetized in similar conditions. Pyridinic, quaternary and pyrrolic nitrogen are the dominant chemical configurations, pyridinic N being preponderant at the scale of the film architecture. This work opens highly promising perspectives for the development of self-organized nitrogen-doped graphene materials, as synthetized from solid carbon nitride, with various functionalities, and for the characterization of 2D materials using a significant new methodology

UV photodissociation dynamics of iodobenzene: effects of fluorination.

The UV photochemistry of various fluorinated iodobenzenes (4-fluoro-, 2,4-difluoro-, 3,5-difluoro-, and perfluoro-iodobenzene) has been investigated at many wavelengths by velocity map imaging, time-resolved near infrared absorption spectroscopy and (spin-orbit resolved) ab initio calculations of the ground and excited state potentials along the C-I stretch coordinate, R(C-I). The textbook description of the near UV photochemistry of CH(3)I, i.e., σ∗←n excitation to the (3)Q(0+) state, followed by direct dissociation (to yield spin-orbit excited iodine atom (I∗) products) or by non-adiabatic coupling via a conical intersection (CI) with the (1)Q(1) potential (to yield ground state iodine (I) atoms) is shown to provide a good zero-order model for aryl iodide photochemistry also. However, the aryl halides also possess occupied π and low-lying π∗ orbitals, and have lower (C(2v) or C(s)) symmetry than CH(3)I. Both of these factors introduce additional subtleties. For example, excitations to and predissociation of ππ∗ excited states provide additional routes to I products, most obviously at long UV wavelengths. nσ∗∕πσ∗ configuration mixing stabilizes the (analogue of the) (3)Q(0+) potential energy surface (PES), to an extent that scales with the degree of fluorination; the corresponding 4A(1) PES in C(6)F(5)I is actually predicted to exhibit a minimum at extended R(C-I). This has the effect of extending the long wavelength threshold for forming I∗ products. The lowered symmetry enables an additional (sloped) CI with the 5A(2) (9A(") in 2,4-difluorobenzene) PES, which provides an extra non-adiabatic route to (fast) ground state I atoms when populating the 4A(1) PES at shorter UV excitation wavelengths

Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with 'universal' detection.

We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels

nσ* and πσ* excited states in aryl halide photochemistry: a comprehensive study of the UV photodissociation dynamics of iodobenzene.

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (ϕ(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (ϕ(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised

Conformer specific dissociation dynamics of iodocyclohexane studied by velocity map imaging.

The photodissociation dynamics of iodocyclohexane has been studied using velocity map imaging following excitation at many wavelengths within its A-band (230 ≤ λ ≤ 305 nm). This molecule exists in two conformations (axial and equatorial), and one aim of the present experiment was to explore the extent to which conformer-specific fragmentation dynamics could be distinguished. Ground (I) and spin-orbit excited (I∗) state iodine atom products were monitored by 2 + 1 resonance enhanced multiphoton ionization, and total kinetic energy release (TKER) spectra and angular distributions derived from analysis of images recorded at all wavelengths studied. TKER spectra obtained at the longer excitation wavelengths show two distinct components, which can be attributed to the two conformers and the different ways in which these partition the excess energy upon C-I bond fission. Companion calculations based on a simple impulsive model suggest that dissociation of the equatorial (axial) conformer preferentially yields vibrationally (rotationally) excited cyclohexyl co-fragments. Both I and I∗ products are detected at the longest parent absorption wavelength (λ ∼ 305 nm), and both sets of products show recoil anisotropy parameters, β > 1, implying prompt dissociation following excitation via a transition whose dipole moment is aligned parallel to the C-I bond. The quantum yield for forming I∗ products, Φ(I∗), has been determined by time resolved infrared diode laser absorption methods to be 0.14 ± 0.02 (at λ = 248 nm) and 0.22 ± 0.05 (at λ = 266 nm). Electronic structure calculations indicate that the bulk of the A-band absorption is associated with transition to the 4A(') state, and that the (majority) I atom products arise via non-adiabatic transfer from the 4A(') potential energy surface (PES) via conical intersection(s) with one or more PESs correlating with ground state products