R.I.R. Blyth

XPS studies of graphite electrode materials for lithium ion batteries

Abstract Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation of the so-called solid electrolyte interphase (SEI) during battery charging. In this paper, we compare graphite powders and carbon fibres as model materials for X-ray photoemission spectroscopy (XPS) studies of the effects of surface pre-treatments. For carbon fibres, the surface carbon percentage was found to vary from 70–95% depending on the surface treatment, with corresponding changes in the relative proportion of graphitic compared to CO bonds, as determined from C 1s curve fits. In contrast, results from the graphite powders show very little change in surface chemical composition and an essentially constant C 1s lineshape dominated by graphitic carbon. SEM data show the carbon fibre cross-section to be composed of a radial array of layered graphite, leaving a surface consisting largely of prismatic planes, while the graphite powder consists of graphite platelets with the surface area predominantly of basal planes. We conclude that the chemical modification occurs at the prismatic planes, and that the powders are unsuitable as models for XPS studies of electrode surface modification, while the fibres are very well suited.

The adsorption of aromatics on sp-metals: benzene on Al 111

Abstract We have studied the adsorption of benzene on Al(111) using angle-resolved ultraviolet photoelectron, high-resolution electron energy loss, and thermal desorption spectroscopies (ARUPS, HREELS, and TDS, respectively), work function measurements, and by density functional theory (DFT) calculations using the ab-initio vasp code. The analysis of ARUPS and HREELS spectra of a benzene monolayer unambiguously indicate C 6v symmetry and a weak benzene–Al interaction in an adsorption geometry with the ring plane parallel to the surface. The weak interaction is confirmed by TDS. The DFT calculations indicate an electrostatic bond and yield an average benzene–Al(111) distance of 3.7 A. A weak minimum of the potential energy is observed at the hollow adsorption position.

Band alignment at the organic-inorganic interface

The band alignment of the bithiophene interface with a diverse range of substrates has been determined by a combination of ultraviolet photoemission and work function measurements. Not only is vacuum level alignment clearly shown to be invalid but also any sort of linear relationship between band alignment and substrate work function is shown not to be the case. Rather, the alignment is determined by the interface dipole, which is specific to the interaction at the inorganic-organic interface. The interface dipoles, which always appear, while dominated by the first monolayer interaction, are completed after two to three monolayers. As the ionization potentials of the films are shown to be constant, it is argued that a simple work function measurement, for an organic film on a particular substrate, quantifies the band alignment.

Low-onset organic blue light emitting devices obtained by better interface control

We demonstrate that highly efficient single-layer light emitting devices (LED) can be realized by better control in the device production process, especially regarding the interface between the active material and the metal cathode. The device cross-section was investigated using Auger depth profiling and spectroscopy. Following this approach, LEDs with para-sexiphenyl (optical gap: 3.1 eV) as active medium and aluminum as cathode were realized, which emit blue light at a bias of only 3.5 V, when the aluminum deposition rate is drastically reduced, namely from 10 to below 1 A/s. We find that the lower deposition rate of Al results in a decreased width of the interfacial region, where carbon, aluminum, and oxygen are intermixed. At the same time the relative oxygen concentration at the cathode interface is increased. However, the presence of oxygen does not lead to the formation of oxidized aluminum species, as verified by the Al local mixing model (LMM) Auger signal.

Influence of oxygen on band alignment at the organic/aluminum interface

The presence of adventitious oxygen is inevitable when organic/metal interfaces are formed by evaporation in high vacuum (10−6 mbar.). In this letter, we highlight the importance of this oxygen for band alignment, and hence, performance, in organic-based devices. The influence of controlled amounts of oxygen on band alignment in benzene/aluminum model cathode interfaces has been studied using ultraviolet photoemission in ultrahigh vacuum. We show that even small amounts of oxygen significantly lower the aluminum work function with concomitant improvement in band alignment.

An experimental and theoretical investigation of the thiophene/aluminum interface

The adsorption of thiophene and 2, 2′-bithiophene on Al(111) has been studied using thermal desorption spectroscopy (TDS), angle-resolved UV photoemission (ARUPS), and work function measurements. Ab initio density functional theory calculations have been performed for thiophene on Al(111). Both thiophene and bithiophene bond only very weakly to Al(111), as indicated by TDS and calculations of the thiophene absorption energy, which is found to be only 0.54 eV. There is no indication of π-bonding in either the ARUPS data or the calculations. The calculated S–Al distance, 3.7 A, is much greater than either measured or calculated S–metal distances for covalent bonding. The bonding is shown to be almost entirely electrostatic, with a small contribution from the sulfur lone pair. This is in direct contrast to calculations for Al–thiophene complexes which show covalent bonds between the Al atoms and the thiophene α carbons. The calculations show the molecule to be essentially flat, with a tilt angle of only 2° o...

X-ray photoemission studies of surface pre-treated graphite electrodes

Abstract Surface pre-treatment of graphitic electrode materials for lithium ion cells has recently been shown to significantly reduce the irreversible consumption of material and charge due to the formation of the so-called solid electrolyte interphase, during battery charging. In this paper we compare graphite powders and carbon fibres as model materials for studies of the effects of CO 2 pre-treatments on solid electrolyte interphase (SEI) formation using X-ray photoemission spectroscopy (XPS). We observe significant modification in the surface elemental composition as a function of CO 2 treatment temperature for carbon fibres, but no strong changes for graphite powders. We conclude that the predominance of the relatively unreactive basal plane in the surface area of the powders makes them far less suitable for detailed XPS studies than the fibres, where the more reactive prismatic planes predominate.

Band alignment in organic devices: Photoemission studies of model oligomers on In2O3

The interfaces of In2O3, a model for indium–tin–oxide (ITO), with benzene, thiophene, and benzaldehyde, models for technologically important organic molecules, are studied using angle resolved ultraviolet photoemission and work function measurements. Band alignment diagrams for hypothetical Al/organic/ITO devices have been drawn, using values determined from this work and previously published studies of these molecules on Al(111). The similarity between the bonding of benzene and thiophene on Al(111) and In2O3, i.e., largely electrostatic, leads to near identical alignment at both metal and oxide interfaces. This indicates that clean Al and ITO will make a very poor electron/hole injecting pair. We suggest that the apparent efficiency of Al as an electron injecting contact in real devices is due to the presence of oxygen at the Al/organic interface. For benzaldehyde the interaction with In2O3 is largely electrostatic, in contrast to the covalent bonds formed on Al(111). This leads to very different alignm...

Photoemission study of In2O3 thin films on NiIn(0001): valence bands and indium 4d levels of a model for ITO

Ultraviolet photoemission has been used to study the growth of ultrathin films of In2O3 on NiIn(0001), a model material for technologically important indium–tin–oxide (ITO). He II excited In 4d spectra are shown to be an excellent monitor of the indium oxidation, which allows an interface layer to be discerned between the alloy and In2O3 film. The growth of the second layer begins before the completion of this layer. Valence band spectra show that the electronic structure of the films is consistent with photoemission data from bulk In2O3 and In2O3 grown on Si(111), and with published density of states calculations. From a comparison of He I and He II excited valence band spectra we suggest that the valence band contains a finite contribution from In 4d antibonding states, as seen in the published calculations. With the exception of oxygen plasma treated ITO, which also shows significant differences to the calculations, the In2O3/NiIn(0001) valence band spectra are in good agreement with spectra from differently treated ITO samples.

Preparation and characterisation of the (0001) surface of single crystal NiIn

NiIn is one of the few intermetallics to adopt the highly unusual CoSn (B35) structure. In this paper we describe the preparation of the clean, ordered (0001) surface of stoichiometric single crystal NiIn in ultra-high vacuum, using Ar ion bombardment and annealing, with characterisation by X-ray photoelectron spectroscopy (XPS), Auger spectroscopy (AES), and low energy electron diffraction (LEED). Preferential sputtering occurs, but a stoichiometric, ordered surface can be obtained by annealing to 700±25 K. This surface appears to be a simple bulk termination, with the same lateral periodicity and composition, indicating the stability of the non-close packed indium layers in the CoSn structure.