Versailles Project on Advanced Materials and Standards interlaboratory study on intensity calibration for x-ray photoelectron spectroscopy instruments using low-density polyethylene.

We report the results of a Versailles Project on Advanced Materials and Standards interlaboratory study on the intensity scale calibration of x-ray photoelectron spectrometers using low-density polyethylene (LDPE) as an alternative material to gold, silver, and copper. An improved set of LDPE reference spectra, corrected for different instrument geometries using a quartz-monochromated Al Kα x-ray source, was developed using data provided by participants in this study. Using these new reference spectra, a transmission function was calculated for each dataset that participants provided. When compared to a similar calibration procedure using the NPL reference spectra for gold, the LDPE intensity calibration method achieves an absolute offset of ∼3.0% and a systematic deviation of ±6.5% on average across all participants. For spectra recorded at high pass energies (≥90 eV), values of absolute offset and systematic deviation are ∼5.8% and ±5.7%, respectively, whereas for spectra collected at lower pass energies (<90 eV), values of absolute offset and systematic deviation are ∼4.9% and ±8.8%, respectively; low pass energy spectra perform worse than the global average, in terms of systematic deviations, due to diminished count rates and signal-to-noise ratio. Differences in absolute offset are attributed to the surface roughness of the LDPE induced by sample preparation. We further assess the usability of LDPE as a secondary reference material and comment on its performance in the presence of issues such as variable dark noise, x-ray warm up times, inaccuracy at low count rates, and underlying spectrometer problems. In response to participant feedback and the results of the study, we provide an updated LDPE intensity calibration protocol to address the issues highlighted in the interlaboratory study. We also comment on the lack of implementation of a consistent and traceable intensity calibration method across the community of x-ray photoelectron spectroscopy (XPS) users and, therefore, propose a route to achieving this with the assistance of instrument manufacturers, metrology laboratories, and experts leading to an international standard for XPS intensity scale calibration.

Photoinduced processes in a dyad made of a linear and an angular perylene bisimide.

A dyad (PI0-PIa) made of a linear (PI0) and an angular (PIa) perylene biscarboximide is synthesized and its spectroscopic, electrochemical and photophysical properties investigated in solvents of various polarity. PIa is characterized by a high intersystem crossing. The spectroscopy and electrochemistry data point to a modest electronic coupling. LUMO-LUMO electron transfer from the singlet excited state PI0-¹PIa is thermodynamically feasible in polar solvents but its occurrence is precluded by a very fast energy transfer to yield ¹PI0-PIa, k(en) ≥ 10(11) s(-1). A HOMO-HOMO electron transfer in the latter state in polar solvents is precluded by the poor driving force, the reaction being unable to compete with the radiative deactivation of the excited state. The efficient energy transfer process is quantitatively examined in the frame of current theories and ascribed to a dipole-dipole (Förster) mechanism.

Angular benzoperylenetetracarboxylic bisimides.

Benzoperylene derivatives with two angularly attached dicarboxylic imide rings, which were prepared by the Diels-Alder-reaction, exhibit strong fluorescence and their free peri positions allow either control of the UV/Vis spectra through their substituents or form anchor positions for the attachment of functional units. The angular chromophore 3 may be used both for fluorescent labeling such as for primary amines or enzymes or as building blocks for more complex assemblies where they may act as energy donors for FRET or electron acceptors in PET such as for photovoltaic solar cells.

Photophysical and redox properties of perylene bis- and tris-dicarboximide fluorophores with triplet state formation: transient absorption and singlet oxygen sensitization.

A detailed photophysical characterization of a couple of new perylene imide derivatives, a carboxylic trisimide PIx, and an asymmetrically substituted carboxylic bisimide PIa is presented. PIx and PIa have the lowest singlet excited state just below 2.6 eV. The dyes are remarkably fluorescent (ϕ(f) = 0.37 ± 0.03 for PIa and ϕ(f) = 0.58 ± 0.04 for PIx in toluene), but they also display an efficient intersystem crossing. This leads to typical excited triplet photophysics/photochemistry, with intense triplet state absorption spectra and efficient singlet oxygen ((1)Δ(g)) photosensitization (ϕ(Δ) = 0.68 ± 0.02 for PIa and 0.44 ± 0.02 for PIx in toluene). On the basis of the measured ϕ(Δ), a ϕ(isc) of 0.65 ± 0.02 for PIa and 0.43 ± 0.02 for PIx in toluene is derived. PIx reduces at -0.58 eV vs SCE, almost similarly to the corresponding symmetrically substituted perylene bisimide PI0, but unlike the latter, it has the first oxidation potential above +1.9 V. PIa is more electron rich and displays a more difficult first reduction at -0.95 V with a more facile oxidation at +1.75 V, similar to that of the parent PI0. The absorption spectra of the excited singlet and triplet states and that of electrochemically generated monoanions are reported.

Versailles Project on Advanced Materials and Standards Interlaboratory Study on Measuring the Thickness and Chemistry of Nanoparticle Coatings Using XPS and LEIS.

We report the results of a VAMAS (Versailles Project on Advanced Materials and Standards) inter-laboratory study on the measurement of the shell thickness and chemistry of nanoparticle coatings. Peptide-coated gold particles were supplied to laboratories in two forms: a colloidal suspension in pure water and; particles dried onto a silicon wafer. Participants prepared and analyzed these samples using either X-ray photoelectron spectroscopy (XPS) or low energy ion scattering (LEIS). Careful data analysis revealed some significant sources of discrepancy, particularly for XPS. Degradation during transportation, storage or sample preparation resulted in a variability in thickness of 53 %. The calculation method chosen by XPS participants contributed a variability of 67 %. However, variability of 12 % was achieved for the samples deposited using a single method and by choosing photoelectron peaks that were not adversely affected by instrumental transmission effects. The study identified a need for more consistency in instrumental transmission functions and relative sensitivity factors, since this contributed a variability of 33 %. The results from the LEIS participants were more consistent, with variability of less than 10 % in thickness and this is mostly due to a common method of data analysis. The calculation was performed using a model developed for uniform, flat films and some participants employed a correction factor to account for the sample geometry, which appears warranted based upon a simulation of LEIS data from one of the participants and comparison to the XPS results.

Phosphorescent perylene imides.

Asymmetrically substituted perylene imide derivatives PIa and PIx display phosphorescence in glassy matrices at 77 K. The lifetime is 49.0 ms for PIa and 13.5 ms for PIx. The triplet energy is 1.79 eV for PIa and 1.68 eV for PIx as confirmed by sensitization experiments of the C(60) triplet.

Improving the photoinduced charge separation parameters in corrole-perylene carboximide dyads by tuning the redox and spectroscopic properties of the components.

A couple of corrole-perylene carboximide dyads (C2-PIa and C2-PIx) have been synthesized and their photoreactivity has been evaluated. We aimed at obtaining better performances for photoinduced charge separation, both in terms of efficiency and in terms of lifetime, with respect to formerly studied systems. The energy level of the charge-separated state was tuned by selecting perylene and corrole components with diverse redox and spectroscopic properties. High spectroscopic energy levels of the perylene carboximide derivatives (PIs) allow a fast charge separation to be maintained in competition with an energy-transfer process from the PI to the corrole unit. Yields and lifetimes of charge separation in toluene are, respectively, 75% and 2.5 µs for C2-PIa and 65% and 24 ns for C2-PIx. The results and the effect of solvent polarity are discussed in the framework of current energy- and electron-transfer theories.