J.W. Niemantsverdriet

Characterization of polymer solar cells by TOF-SIMS depth profiling

Abstract Solar cells consisting of polymer layers sandwiched between a transparent electrode on glass and a metal top electrode are studied using dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS) in dual-beam mode. Because depth profiling of polymers and polymer–metal stacks is a relatively new field the craters were thoroughly investigated by environmental SEM (ESEM), interferometry, surface profilometry and tapping mode AFM. A huge increase in crater bottom roughness was observed when starting from the aluminum top layer going in depth, resulting in a loss of depth resolution. It is shown that layer-to-layer diffusion and contaminants at buried interfaces can be extracted from the depth profiles when taking into account the loss of depth resolution.

Deposition of inorganic salts from solution on flat substrates by spin-coating: theory, quantification and application to model catalysts

The theory of spin-coating is applied to predict the amount of inorganic material that is deposited from a solution on a flat substrate on the basis of concentration, density, viscosity and evaporation rate of the solution and the spin speed applied during spin-coating. Measurements by Rutherford backscattering spectrometry (RBS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) confirm the validity of the theory. Applications of the method in the preparation of model catalysts are discussed.

Surface roughness effects in quantitative XPS: magic angle for determining overlayer thickness

Abstract The use of X-ray photoelectron spectroscopy (XPS) as a technique for non-destructive depth profiling of technical samples is often hindered by their roughness. In this paper, we elaborate on earlier work where we reported on the apparent existence of a ‘magic’ angle for the determination of the thickness of uniform overlayers on rough substrates. Simple calculations for fully three-dimensional model rough surfaces strongly suggest that the thickness of an overlayer on a rough substrate can be determined accurately without taking the roughness into account in the analysis of the XPS intensities, the neglect of roughness effects leading to an average error less than 10% in the determined thickness.

Thermal desorption of strained monoatomic Ag and Au layers from Ru(001)

Model‐independent or complete analysis of thermal desorption spectra suggests, in agreement with low‐energy electron diffraction, that attractive lateral interactions between Ag atoms on a Ru(001) substrate lead to island formation for Ag coverages above 0.15 ML, with zeroth‐order desorption kinetics and coverage‐independent activation energy of desorption E, and preexponential factor ν. For the system Au/Ru(001), on the other hand, E and ν decrease monotonically with coverage, indicating that Au–Au interactions on Ru(001) are repulsive. For ΘAg<0.15 and ΘAu<1 the systems exhibit a clear compensation effect between E and ν.

On the formation of cobalt-molybdenum sulfides in silica-supported hydrotreating model catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO2 layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a Co–Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower activity in thiophene HDS.

The compensation effect and the manifestation of lateral interactions in thermal desorption spectroscopy

Abstract If thermal desorption spectra are analysed in terms of the Polanyi-Wigner equation lateral interactions between the adsorbates may lead to coverage (θ) dependent pre-exponential factors, v, and activation energies of desorption, E. Evidence from the literature shows that E and v often satisfy the well-known compensation effect 1n v(θ) = bE(θ) + c, with constants b and c. Here we insert this compensation effect into the rate equation of desorption and simulate spectra which illustrate the influence of the compensation effect in thermal desorption spectra of adsorbate systems where pairwise lateral interactions prevail.

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly[2-methoxy-5-(3′,7′dimethyl-octyloxyl)-1,4-phenylenevinylene] (MDMO-PPV) or 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C61 (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Al+3LiF→AlF3+3Li. If this reaction takes place, AlF3 should be detectable at the surface. However, SIMS measurements showed that AlF3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS m...

The adsorption of NH3 on Rh(111)

The adsorption of NH a on Rh(lll) has been investigated by temperature programmed desorption, work function measurements, low energy electron diffraction and secondary ion mass spectrometry. TPD indicates the existence of three distinct desorption states; el-, ~2- and 13-NHa with peak maxima at 320, 155 and 130 K, respectively. For the most strongly chemisorbed state, ~I-NH3, the desorption energy is 81.5 kJ mol-k A (2 x 2) LEED pattern is observed after complete filling of the ~1- and ~2-NH3 states, which is attributed to saturation of the first adsorption layer, corresponding to an NH3 coverage of 0.25 ML. The sticking coefficient for NH 3 adsorption at 120 K, is on the order of unity and independent of coverage during filling of the first adsorption layer. NH 3 adsorption causes a significant work function decrease of 2.4 eV below the value of clean Rh(lll). The initial work function decrease corresponds to an average dipole per NH 3 molecule of 1.9 D, i.e. higher than that of the NH3 molecule in the gas phase. SIMS spectra of NH a on Rh(lll) contain Rh(NH3) + as the predominant ammonia-derived duster ion. The intensity of the Rh(NH3) + duster ion behaves strongly non-linear with the NH3 coverage and indicates some changes in the NH3 adlayer at a coverage of ~0.12 ML.

Direct versus hydrogen-assisted CO dissociation on the Fe (100) surface: a DFT study

CO dissociation: Three most probable pathways to CO dissociation on the Fe (100) surface exist: a) direct, CO→C+O (-) and H-assisted b) H+CO↔HCO→CH+O (-) or c) CO+H↔COH→C+OH (-). Under high hydrogen pressure conditions and highly occupied surfaces the formation of HCO and subsequent dissociation to CH+O may at best compete with direct dissociation.

Quantification of lateral repulsion between coadsorbed CO and N on Rh(100) using temperature-programmed desorption, low-energy electron diffraction, and Monte Carlo simulations

This has important implications for the lateral distribution of coadsorbed CO and N at different adsorbate coverages. Regarding the different lateral interactions and mobility of adsorbates, we propose a structural model which satisfactorily explains the observed effects of atomic N on the desorption of CO. Dynamic Monte Carlo simulations were used to verify the experimentally obtained value for the CO‐N interaction, by using the kinetic parameters and interaction energy derived from the temperature-programmed desorption experiments.