Simultaneous ambient pressure X-ray photoelectron spectroscopy and grazing incidence X-ray scattering in gas environments

We have developed an experimental system to simultaneously observe surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) in gas pressures as high as the multi-Torr regime, while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano- to the meso-scale. The grazing incidence geometry provides tunable depth sensitivity while scattered X-rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and in ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O$_2$ atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent X-ray scattering experiments can greatly benefit from the concepts we present here.Comment: 21 pages, 4 figure

In-situ study of the carbon gasification reaction of highly oriented pyrolytic graphite promoted by cobalt oxides and the novel nanostructures appeared after reaction

Cobalt interaction and its effects on carbon-based systems at the nanoscale have recently attracted much attention in different fields, such as catalysis of carbon nanotubes or graphene and graphite nano-patterning taking advantage of its ferromagnetic behavior. Experiments performed in our laboratories show how the re-oxidation process of two equivalent monolayers of CoO deposited on highly oriented pyrolytic graphite at 400 °C leads to the formation of nanochannels at lower temperature than using other methods. Here we present the in-situ characterization of the carbon gasification reaction that drives this process by means of near ambient pressure X-ray photoelectron spectroscopy performed at the ALBA synchrotron facility. The reason why this reaction takes place at such low temperature compared to other methods is due to the weakening of the carbon σ bonds by the initial CoO wetting layer formed at the early stages of growth on the graphite surface. Besides nanochannels, ex-situ atomic force microscopy measurements also show the appearance of two more kinds of nanostructures: nano-strips and nano-rings. The appearance of these nanostructures reveals the impressive modification of the surface after the re-oxidation process mediated by the cobalt oxideThis investigation has been funded by the MINECO of Spain through the FIS2015-67367-C2-1-P P and MAT2017-85089-C2-1-R projects and by the Comunidad de Madrid through the NANOMAGCOST-CM Ref: P2018/NMT4321 project. The experiments were performed at CIRCE/NAPP beamline at ALBA Synchrotron with the collaboration of ALBA staff. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH1123

Structure of the Clean and Oxygen-Covered Cu(100) Surface at Room Temperature in the Presence of Methanol Vapor in the 10-200 mTorr Pressure Range.

Using ambient pressure X-ray photoelectron spectroscopy (APXPS) and high pressure scanning tunneling microscopy (HPSTM), we show that in equilibrium with 0.01-0.2 Torr of methanol vapor, at room temperature, the Cu(100) surface is covered with methoxy species forming a c(2 × 2) overlayer structure. In contrast, no methoxy is formed if the surface is saturated with an ordered oxygen layer, even when the methanol pressure is 0.2 Torr. At oxygen coverages below saturation, methanol dissociates and reacts with the atomic oxygen, producing methoxy and formate on the surface, and formaldehyde that desorbs to the gas phase. Unlike the case of pure carbon monoxide and carbon dioxide, methanol does not induce the restructuring of the Cu(100) surface. These results provide insight into catalytic anhydrous production of aldehydes.Marie-Sklodowska Curie Individual Fellowship from European Union Horizon 2020 programme. St John's College Cambridge. US Department of Energy

Islands and despots

This paper challenges a conventional wisdom: that when discussing political systems, small is democratic. And yet, can there be paradises without serpents? The presumed manageability of small island spaces promotes and nurtures dispositions for domination and control over nature and society. In such dark circumstances, authoritarian rule is a more natural fit than democracy. By adopting an inter-disciplinary perspective, this paper argues that small island societies may be wonderful places to live in, as long as one conforms to a dominant cultural code. Should one deviate from expected and established practices, the threat of ostracism is immense. Formal democratic institutions may and often do exist, and a semblance of pluralism may be manifest, but these are likely to be overshadowed by a set of unitarist and homogenous values and practices to which many significant social players, in politics and civil society, subscribe (at least in public).peer-reviewe

Dynamics of Surface Alloys: Rearrangement of Pd/Ag(111) Induced by CO and O2

Alloys of Ag and small amounts of Pd are promising as bifunctional catalysts, potentially combining the inherent selectivity of the noble Ag with that of the more reactive Pd. Stable PdAg surface alloys are prepared via evaporation of Pd onto Ag(111) at room temperature followed by annealing at 400 K to create a model system. Using this procedure, the most stable form of the surface alloy under vacuum was determined to be a Ag-capped PdAg surface alloy, on the basis of a combination of X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). Extensive roughening of the surface was apparent in STM images, characterized by islands of the Ag/PdAg/Ag(111) alloy of several layers thickness. The roughening is attributed to transport of Ag from the Ag(111) surface into the alloy islands. Within these islands, there is a driving force for Pd to be dispersed, surrounded by Ag, on the basis of DFT modeling. Exposure of these Ag/PdAg/Ag(111) islands to CO (0.5 Torr) at 300 K induces migration of Pd to the surface, driven by the energetic stabilization of the Pd-CO bond based on ambient-pressure XPS. Once the Pd is drawn to the surface by higher pressures of CO at room temperature, it remains stable even under very low CO partial pressures at temperatures of 300 K and below, on the basis of DFT-modeled phase behavior. Exposure to 1 Torr of O2 at 400 K also causes Pd to resurface, and the resulting structure persists even at low pressures and temperatures below 300 K. These results establish that the state of the PdAg catalyst surface depends strongly on pretreatment and operational conditions. Hence, exposure of an alloy catalyst to CO or O2 at moderate temperatures and pressures can lead to catalyst activation by bringing Pd to the surface. Furthermore, these results demonstrate that exposure to CO at room temperature, which is often used as a proxy for evaluating the Pd coordination sites available in a catalyst, changes the surface structure. Therefore, the CO vibrational frequencies measured with diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) on PdAg catalyst materials do not necessarily provide information about their working state, and fundamental understanding of the CO-PdAg alloy is crucial

Monolayer Phases of a Dipolar Perylene Derivative on Au 111 and Surface Potential Build Up in Multilayers

9-(Bis-<i>p</i>-<i>tert</i>-octylphenyl)-amino-perylene-3,4-dicarboxy anhydride (BOPA-PDCA) is a strongly dipolar molecule representing a group of asymmetrically substituted perylenes that are employed in dye-sensitized solar cells and hold great promise for discotic liquid crystal applications. Thin BOPA-PDCA films with orientated dipole moments can potentially be used to tune the energy-level alignment in electronic devices and store information. To help assessing these prospects, we here elucidate the molecular self-assembly and electronic structure of BOPA-PCDA employing room temperature scanning tunneling microscopy and spectroscopy in combination with ultraviolet and X-ray photoelectron spectroscopies. BOPA-PCDA monolayers on Au(111) exclusively form in-plane antiferroelectric phases. The molecular arrangements, the increase of the average number of molecules per unit cell via ripening, and the rearrangement upon manipulation with the STM tip indicate an influence of the dipole moment on the molecular assembly and the rearrangement. A slightly preferred out-of-plane orientation of the molecules in the multilayer induces a surface potential of 1.2 eV. This resembles the giant surface potential effect that was reported for vacuum-deposited tris­(8-hydroxyquinoline)aluminum and deemed applicable for data storage. Notably, the surface potential in the case of BOPA-PDCA can in part be <i>reversibly</i> removed by visible light irradiation

Near total reflection x-ray photoelectron spectroscopy: Quantifying chemistry at solid/liquid and solid/solid interfaces

Near total reflection regime has been widely used in x-ray science, specifically in grazing incidence small angle x-ray scattering and in hard x-ray photoelectron spectroscopy (XPS). In this work, we introduce some practical aspects of using near total reflection (NTR) in ambient pressure XPS and apply this technique to study chemical concentration gradients in a substrate/photoresist system. Experimental data are accompanied by x-ray optical and photoemission simulations to quantitatively probe the photoresist and the interface with the depth accuracy of ∼1 nm. Together, our calculations and experiments confirm that NTR XPS is a suitable method to extract information from buried interfaces with highest depth-resolution, which can help address open research questions regarding our understanding of concentration profiles, electrical gradients, and charge transfer phenomena at such interfaces. The presented methodology is especially attractive for solid/liquid interface studies, since it provides all the strengths of a Bragg-reflection standing-wave spectroscopy without the need of an artificial multilayer mirror serving as a standing wave generator, thus dramatically simplifying the sample synthesis

Near total reflection x-ray photoelectron spectroscopy: Quantifying chemistry at solid/liquid and solid/solid interfaces

Near total reflection regime has been widely used in x-ray science, specifically in grazing incidence small angle x-ray scattering and in hard x-ray photoelectron spectroscopy (XPS). In this work, we introduce some practical aspects of using near total reflection (NTR) in ambient pressure XPS and apply this technique to study chemical concentration gradients in a substrate/photoresist system. Experimental data are accompanied by x-ray optical and photoemission simulations to quantitatively probe the photoresist and the interface with the depth accuracy of ∼1 nm. Together, our calculations and experiments confirm that NTR XPS is a suitable method to extract information from buried interfaces with highest depth-resolution, which can help address open research questions regarding our understanding of concentration profiles, electrical gradients, and charge transfer phenomena at such interfaces. The presented methodology is especially attractive for solid/liquid interface studies, since it provides all the strengths of a Bragg-reflection standing-wave spectroscopy without the need of an artificial multilayer mirror serving as a standing wave generator, thus dramatically simplifying the sample synthesis