Steven Firth

Raman spectroscopy as a means for the identification of plattnerite (PbO2), of lead pigments and of their degradation products.

The Raman spectra of plattnerite [lead(IV) oxide, PbO2] and of the lead pigments red lead (Pb3O4), lead monoxide [PbO, litharge (tetragonal) and massicot (orthorhombic)], lead white [basic lead carbonate, 2PbCO3.Pb(OH)2] and of their laser-induced degradation products were recorded using a range of different excitation lines, spectrometer systems and experimental conditions. The degradation of PbO2 is more extensive along the pathway PbO2-->Pb3O4-->PbO (litharge)-->PbO (massicot) the shorter the wavelength of the excitation line and the higher its power. The Raman spectrum of PbO2, which is black and of the rutile structure, is particularly difficult to obtain but three bands, at 653, 515 and 424 cm-1, were identified as arising from the b2g, a1g and e(g) modes respectively, by analogy with the corresponding modes of isostructural SnO2 (776, 634 and 475 cm-1). A further oxide was identified, PbO1.55, the Raman spectrum of which does not correspond to that of any of the laser-induced degradation products of PbO2 at any of the wavelengths used. The Raman results are critical to the future use of Raman microscopy for the identification of lead pigments on artworks.

First- and second-order Raman spectra of galena (PbS)

Raman spectra of cleaved, millimeter-sized single crystals of mineral galena (PbS) have been measured between 100 and 1200 cm−1 over the temperature range 80 to 373 K. Three low wave number bands are observed which can be reconciled with the results of earlier reports. However, changes in the peak wave numbers and in the relative intensities of these bands in response to changes in temperature do not conform with previous assignments for the observed bands. Detailed assignments are proposed which are based on resonance Raman processes. The observed bands are assigned to forbidden longitudinal optical excitations, which are allowed under resonance conditions by a Frohlich interaction mechanism, and two-phonon excitations.

ZnO tetrapod Schottky photodiodes

The fabrication of an ultraviolet photodiode employing a single ZnO tetrapod nanocrystal is reported. This diode structure is prepared by depositing W and Pt electrodes to form Ohmic and Schottky contacts, respectively. Dark current-voltage measurements show rectifying behavior. The properties of the metal-semiconductor interface are studied with above and below band gap illumination. It is found that with increasing UV excitation the device converts from a rectifying to an Ohmic behavior. This effect is attributed to a flattening of the energy bands due to the migration of photogenerated carriers within the space charge region at the metal-semiconductor interface.

Boron-doping effects in carbon nanotubes

When nanotubes form in a carbon arc, the presence of boron results in long boron-doped carbon nanotubes which are generated as dominant zigzags. Metallic behaviour is observed, in contrast to carbon nanotubes, which are semi-conducting.

Metallic behaviour of boron-containing carbon nanotubes

Abstract Paramagnetism in boron-containing carbon nanotubes was detected by SQUID measurements, and a g -value of 2.0020, corresponding to the free carrier spin, was observed over a wide temperature range (108–455 K). This value implies intrinsic metallic behaviour and contrasts with thermally activated semiconduction which multi-walled carbon nanotubes usually exhibit.

Metallic molecular crystals containing chiral or racemic guest molecules

Metallic charge-transfer salts of BEDT-TTF containing both racemic (R/S)- and chiral (S)-sec-phenethyl alcohol as guest molecules have been crystallised and their structures and physical properties are compared.

Laser-induced degradation of lead pigments with reference to Botticelli’s Trionfo d’Amore

Abstract The recent analysis of the palette used in Botticelli’s Trionfo d’Amore by Raman microscopy has led to the erroneous identification of the spectrum of a degradation product, most likely that of massicot (orthorhombic PbO), as that of plattnerite (PbO 2 ) [Anal. Chim. Acta 429 (2001) 279]. The error is shown to arise from the laser-induced degradation of both the painting and a reference sample of pure PbO 2 . The spectrum of undegraded PbO 2 is presented, and the common problem of laser-induced degradation of lead pigments in art conservation studies utilising Raman microscopy is discussed.

Synthesis and characterisation of zinc oxide tetrapod nanocrystals

Zinc oxide is an important group II-VI semiconductor material with optical properties that permit stable emission at room temperature. We report on the synthesis of highly uniform nanocrystalline ZnO tetrapod (ZnO-T) nanostructures through a modified chemical vapour transport process. These self assembled nanocrystals are characterised by four cylindrical arms with a hexagonal facet all of which are joined at a tetrahedral core. Studies are carried out on ZnO tetrapods using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence spectroscopy (PLS) and Raman measurements. We find a simple technique to quench visible emission found in ZnO tetrapods as grown. We also observe Raman active modes suggesting that nitrogen is incorporated within our samples.

Nb-doped WS2 nanotubes

Abstract Nb-doped WS2 nanotubes, typically W 1−x Nb x S 2 (x⩽0.2) , generated by heating Nb2O5-coated W18O49 nanorods in H2S at 1100°C, contain more structural defects within the layers than do pure WS2 nanotubes. The tube tips are invariably closed. The presence of Nb appears to inhibit nanotube formation. A two-stage process is proposed to account for the observed doped nanotube growth.

Titanium‐Doped Molybdenum Disulfide Nanostructures

Ti-doped MoS2 nanotubes are produced by pyrolyzing a H2S/N2 mixture over an oxidized Ti–Mo alloy powder at elevated temperatures. Partial substitution of Mo by Ti does not significantly alter the 2H–MoS2 lattice.