Michael A. Chesters

FT-rairs, eels and leed studies of the adsorption of carbon monoxide on Cu(111)

Abstract The adsorption of CO on Cu(111) has been re-investigated using Fourier transform reflection absorption infrared spectroscopy (FT-RAIRS), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). The combination of these techniques has revealed important new information about this system. At 95 K three adsorption stages (1 to 3) can be distinguished, leading to successive ordered overlayer structures (I to III). Stage 1 exists for coverages up to 0.33 and involves linearly bound species only. The sharpest band observed during the course of these experiments, with an intrinsic FWHM of 4.5 cm −1 , is associated with a coverage of 0.33 and an ordered (√3 × √3) R 30 ° structure (I). Stage 2 is associated with the coverage regime 0.33−0.44. In this stage EELS data show tilted, linear CO species are present. No bridged species exist at this point. On the basis of this, we propose a new overlayer structure (II), for θ = 0.44. Increasing the coverage gives stage 3. During this process, the high resolution FT-RAIRS spectra resolved two bands associated with linear CO species. The behaviour of these bands suggests that the final structure (III) is formed in a step-wise manner, with islands of (II) and (III) coexisting in the adlayer. Both linear and bridged species exist in stage 3. The bandshapes suggest significant inhomogeneities in the adlayer are present at the saturation coverage. Finally, the concentration of bridged species is found to be very sensitive to the adsorption temperature.

Organic electroluminescent devices : enhanced carrier injection using an organosilane self assembled monolayer (SAM) derivatized ITO electrode

A protocol for the reproducible silylation of indium–tin oxide coated glass (ITO) using small molecule chlorosilanes is reported, and shown to be a convenient means of dramatically improving the performance of the ITO anode invariable used in organic electroluminescent devices. Using the model system: ITO|TPD|Alq3|Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′diphenyl-1,1′biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato)aluminium) luminance-voltage, current–voltage, quantum efficiency and luminance efficiency data illustrate the superior performance of the silane modified device over a reference. Static contact angle measurements confirming the successful silylation of the ITO electrode surface, are supported by direct measurement of the effect the dipolar monolayer has on the work function of the underlying ITO, using a scanning Kelvin probe. This research builds on our earlier work with dipolar phosphonic acids. However, unlike phosphonic acids, chlorosilanes are known to adhere to oxide surfaces via a covalent bond, and so the silylated ITO electrodes are expected to exhibit improved durability. Such electrode modification provides a method of tuning the work function of the ITO electrode to the HOMO of the hole-transporting layer and thus, improving device performance.

The adsorption and reaction of methanol on oxidized copper(111) studied by Fourier transform reflection-absorption infrared spectroscopy

Abstract At 91 K, methanol adsorbs on clean and oxidised copper (111) to give a hydrogen-bonded multilayer. On heating to 150 K, the methanol is completely desorbed from the clean surface, while on the oxidized surface, a deprotonation reaction takes place resulting in the production of a methoxy species, CH 3 0. The Fourier transform reflection-absorption i.r. spectrum (FT-RAIRS) measured between 4000 and 800 cm -1 shows only four peaks, three of which appear in the region 3000ȓ2800 cm -1 . The spectrum is assigned to an upright species which has C 3 v symmetry. The assignment is assisted by comparison to the i.r. spectrum of a model cluster compound containing a μ 2 methoxy ligand. The methoxy species decomposes above 300 K and is removed completely from the surface at 400 K.

Infrared spectroscopic comparison of the chemisorbed species from ethene, propene, but-1-ene and cis- and trans-but-2-ene on Pt(111) and on a platinum/silica catalyst

The chemisorption of ethene, propene, but-1-ene, cis- and trans-but-2-ene and but-2-yne has been studied on a Pt(111) surface using reflection absorption infrared spectroscopy (RAIRS). The results have been compared with transmission infrared spectra of the alkenes adsorbed on a finely divided impregnated Pt/SiO2 catalyst.On the Pt(111) surface ethene, propene and but-1-ene are adsorbed in the corresponding n-alkylidyne form CH3(CH2)nCPt3(n= 0, 1, 2) at room temperature. The cis- and trans-but-2-enes are adsorbed in the form of but-2-yne bonded to the surface in a di-σ/π fashion, as was confirmed by adsorption of but-2-yne itself.The room-temperature spectra of ethene, propene and but-1-ene on Pt/SiO2 all exhibited prominent absorptions from the n-alkylidyne, but other adsorbed species also contributed to the spectra, considered to be predominantly the appropriate di-σ species PtCH2CH(R)Pt (R = H, CH3, C2H5) together with smaller amounts of the relevant π-complexes. In the case of the cis- and trans-but-2-enes, virtually identical spectra were obtained on Pt/SiO2 but with many of the same prominent features as from but-1-ene. Clearly the finely divided catalyst had sites which led to double-bond isomerization. A smaller proportion of di-σ/π bonded but-2-yne was also present.The infrared spectra proved to be very effective at delineating the similarities and differences between the species chemisorbed on Pt(111) and on the Pt/SiO2 catalyst.

The Reflection Absorption Infrared-Spectra of N-Alkanes Adsorbed on Pt(111)

Abstract Reflection-absorption infrared spectra of the series of C 2 to C 6 normal alkanes in the form of both monolayers and multilayers on Pt(111) are presented. The spectra are interpreted to reveal the orientation of the plane containing the carbon chains to be almost exclusively parallel to the metal surface in the case of monolayers. The multilayers appeared to be crystalline again with molecules aligned so that the plane containing the carbon atoms was parallel to the substrate in the case of butane and hexane but with this plane at an angle to the surface in the case of pentane. The spectra of ethane and propane multilayers were consistent with liquid-like films. The monolayer spectra show evidence for infrared activity induced in some modes as a result of the metal-alkane interaction but there is no evidence for C-H … metal hydrogen bonding.

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C2D2 spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.

Vibrational spectroscopic characterisation of hydrogen bridged between metal atoms: A model for the adsorption of hydrogen on low-index faces of tungsten

Abstract Vibrational spectra of hydrogen chemisorbed on metal surfaces are discussed in the context of data from model cluster compounds and adsorption sites for hydrogen on low index planes of tungsten are suggested on the basis of reported electron energy loss spectra. A hydrogen adsorption site bridging two metal atoms is proposed for both the low coverage, β 2 , and the high coverage, β 1 , states of hydrogen on W(100) and the change in the symmetric metal-hydrogen stretching frequency is correlated with the reconstruction of the tungsten surface in the β 2 state.

The chemisorption of CO on Pd(110) at 110 and 300 K studied by electron energy loss spectroscopy

The vibrational electron energy loss spectrum of CO adsorbed on Pd(110) at both 110 and 300 K has been recorded as a function of CO exposure. At both temperatures the spectra indicate that CO adsorption occurs predominantly in two-fold bridged sites along the rows of metal atoms. At 110 K the surface may be saturated to give a two-fold bridged (2 × 1) CO overlayer of p2mg unit mesh symmetry, yielding a LEED pattern with absences at the (±12hodd, 0) positions along the 〈110〉 azimuth. Near saturation, (θ ≥ 1.0), a weak band due to CO adsrobed in on-top sites is observed. At 300 K, LEED studies show that a c(2 × 2) pattern forms at low exposures ( <1 L), followed by a p(4 × 2) pattern at higher exposures. The latter structure persists up to a dynamic CO pressure of 1 × 10−7 mbar where an incomplete p2mg pattern forms, with residual four-fold periodicity along the rows of metal atoms. EEL spectra corresponding to the p(4 × 2) overlayer indicate that at least two distinct two-fold bridge CO species are present giving rise to νCO loss features separated by only 60 cm−1.

Infrared spectroscopy of molecules on metal single-crystal surfaces

Abstract The current status of reflection-absorption infrared spectroscopy (RAIRS) will be reviewed with examples illustrating its application to studies of structural chemical transformations at surfaces. The sensitivity of the technique will be compared with that of electron energy loss spectroscopy in particular where infrared spectra have been measured over a wide spectral range using Fourier transform spectrometers. Further examples illustrating the sensitivity of absorption band shapes to changes in the structure of the adsorbed layer will be, given. Finally the prospects for the future will be briefly discussed including the use of synchrotron and laser i.r. sources and the achievement of short recording times for dynamic measurements.

Infrared microscopy of epithelial cancer cells in whole tissues and in tissue culture, using synchrotron radiation

Oral epithelial tumour tissue, and cultured cervical epithelial carcinoma cells have been studied using synchrotron infrared microspectroscopy. Mid infrared absorption spectra collected at cellular spatial resolution from within oral tumours were found to be sufficiently distinct, when analysed by principal component analysis, to distinguish between three different cell types within the tumour. The resulting data were sufficiently robust to allow correct classification of spectra from cells within subsequent tissue samples. These results go some way to demonstrate the potential of infrared spectroscopy as a tool in the post-operative screening of oral cancer patients by the examination of exfoliated epithelial cells. To gain a better understanding of the inherent variability in the infrared spectra of such epithelial cells, we have studied A431 carcinoma cells under the stimulus of the growth-stimulating hormone EGF. We have detected key changes in the infrared spectrum that relate to the activation of the growth factor signalling mechanism.