XPS – Lightsources.org

PHELIX beamline is ready to research

2020/10/072020/10/08

Synchrotron light has finally been observed for the first time on a sample at the end station of the experimental beamline PHELIX. This success is the crowning achievement of three years of hard work designing, constructing, fitting, and tuning its components to the synchrotron beam.

The installation of this new beamline began in mid-2018. In March of 2020, the final elements were delivered. Then on 18th September 2020, the scientific supervisors of beamline, Dr. Magdalena Szczepanik – Ciba and Tomasz Sobol, announced readiness for test experiments using the synchrotron beam.

The first results testing the capabilities with the active beam of the analyser at the PHELIX end station were performed using the sample of gold in the presence of a specialist from the SPECS company, Dr. Robert Reichelt. As a result of testing this calibration material, among others, the XPS Au4f spectrum was acquired (see pic.1). Additionally, an angle – resolved and spin – resolved measurements were performed .

During the latest open call for the beamtime the applications on the PHELIX beamline where included for the first time. This line will use soft X-ray radiation. The end-station will enable a wide range of spectroscopic and absorption researches, characterised by different surface sensitivity. Besides acquiring standard, high-resolution spectra, it will allow e.g. for the mapping of band structure in three dimensions and for the detection of spin in three dimensions.

Users will thus be able to conduct research on new materials, thin films, and multi-layer systems, catalysers and biomaterials, as well as research on solids, on spin-polarised surface states, and on chemical reactions taking place on the surface.

Read more on the SOLARIS website

Image: From left Tomasz Sobol, Dr. Robert Reichelt, Dr. Magdalena Szczepanik – Ciba. Credit – Solaris

A probe of light-harvesting efficiency at the nanoscale

2020/09/282020/10/01

SCIENTIFIC ACHIEVEMENT

Using time-resolved experiments at the Advanced Light Source (ALS), researchers found a way to count electrons moving back and forth across a model interface for photoelectrochemical cells.

SIGNIFICANCE AND IMPACT

The findings provide real-time, nanoscale insight into the efficiency of nanomaterial catalysts that help turn sunlight and water into fuel through artificial photosynthesis.

Solar-fuel tech goes for gold

In the search for clean-energy alternatives to fossil fuels, one promising solution relies on photoelectrochemical (PEC) cells: water-splitting, artificial-photosynthesis devices that turn sunlight and water into solar fuels such as hydrogen. In just a decade, researchers have achieved great progress in the development of PEC systems made of light-absorbing gold nanoparticles (NPs) attached to a semiconductor film of titanium dioxide (TiO₂).

Read more on the Advanced Light Source website

Image: Laser pulses were used to excite electrons in gold nanoparticles (AuNPs) on a titanium dioxide (TiO₂) substrate. X-ray pulses were used to count the electrons moving between the nanoparticles and the substrate. (Credit: Oliver Gessner/Berkeley Lab)

Transition-metal dichalcogenide NiTe2: an ambient-stable material for catalysis and nanoelectronics

2020/08/122020/08/13

Recently, transition-metal dichalcogenides hosting topological states have attracted considerable attention for their potential implications for catalysis and nanoelectronics. The investigation of their chemical reactivity and ambient stability of these materials is crucial in order to assess the suitability of technology transfer. With this aim, an international team of researchers from Italy, Russia, China, USA, India, and Taiwan has studied physicochemical properties of NiTe₂ by means of several experimental techniques and density functional theory. Surface chemical reactivity and ambient stability were followed by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) experiments at the BACH beamline, while the electronic band structure was probed by spin- and angle-resolved photoelectron spectroscopy (spin-ARPES) at the APE-LE beamline.

Read more on the Elettra website

Image: a) Ni-3p and b) Te-4d XPS core-level spectra collected from as-cleaved NiTe2 (black curves) and from the same surface exposed to 2·10⁴L of CO (red curves), H₂O (green curves) and O₂ (blue curves). Adapted from “S. Nappini et al., Adv. Funct. Mater. 30, 2000915 (2020); DOI: 10.1002/adfm.202000915” with permission from Wiley (Copyright 2020) with license 4873681106527

Surface instability and chemical reactivity of ZrSiS and ZrSiSe nodal-line semimetals

2019/06/072019/06/13

Among topological semimetals, in nodal-line semimetals (NLSM) conduction and valence bands cross each other. In particular, in NLSM, topological constraints protect band crossings and, moreover, band touching forms nodal lines or rings. Recently, topological nodal lines have been observed in bulk ZrSiX compounds (X = S, Se, Te). In ZrSiX, a tetragonal structure is formed by the stacking of X-ZrSi-Zr-X slabs covalently bonded between each other, whose strength decreases by replacing S with Se or Te ions. This class of materials exhibits large and non-saturating magnetoresistance and ultrahigh mobility of charge carriers.
The control over surface phenomena, including oxidation, degradation, and surface reconstruction is a crucial step in order to evaluate the feasibility of the exploitation in technology of ZrSiX.
By means of X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline, high-resolution electron energy loss (HREELS) and density functional theory, an international team of researchers from Italy, China, Russia, Taiwan, and USA (coordinated by University of L’Aquila) has studied the evolution of ZrSiS and ZrSiSe surfaces in oxygen and ambient atmosphere.
The chemical activity of ZrSiX compounds is mainly determined by the interactions of Si layer with ZrX sublayer. Any adsorption provides distortion of the Si layer (flat in bulk). In the case of ZrSiS, the ZrS sublayer is almost the same as in bulk and therefore adsorption is unfavorable because it provides distortions of Si layer. In the case of ZrSiSe, the ZrSe sublayer is already strongly distorted (structure different from bulk), and, therefore, further distortion of Si layer by adsorption is favorable (see figure).

>Read more on the Elettra website

Image: Atomic structure of different steps of the process of the oxidation of ZrSiSe from (a-d) Zr-sites and (e-h) Si-sites. Red, light blue, black and yellow balls represent O, Zr, Se, and Si atoms, respectively. On panels (a) and (e) physical adsorption of single oxygen molecule is depicted. Panels (b) and (f) represent the situation of uniform coverage of the surfaces by molecular oxygen. In panels (c) and (g), decomposition of single oxygen molecule on the surfaces is represented. Panels (d) and (h) show total oxidation of the surfaces.

Why having your head in the clouds could be a really good thing

2019/04/152019/04/17

The ATMOS research group in the NANOMO unit, led by Nønne Prisle, Associate Professor at the University of Oulu, are trying to find out what kind of chemistry is happening in cloud droplets and tiny nanometer-sized aerosol particles in the atmosphere. This knowledge could eventually, hopefully, give us more accurate theoretical models to understand the ongoing climate change.
– The only thing that can halter climate change is to stop emitting CO₂. Nønne Prisle is very, very clear on that. Even so, she says, if we want to take any other step to try to counter climate change, we really need to know what’s going on in the clouds since these processes could be quite critical.
The ATMOS team are using the beamline HIPPIE at MAX IV being so-called commissioning experts, which means that the experiment is done both to provide useful data but also to verify the capacity and capability of the beamline experimental station.

Image: From left to right: Robert Seidel, Helmholtz Zentrum Berlin; Nønne Prisle, Kamal Raj and Jack Lin, University of Oulu at the HIPPIE beamline.

Progress on low energy electronics

2018/12/112018/12/12

Soft X-ray experiments used to characterise new thin film topological Dirac Semimetal

A large international collaboration including scientists from Monash University, the ARC Centre for Future Low Energy Electronics (FLEET), the Monash Centre for Anatomically Thin Materials and the Australian Synchrotron reported today in Nature on the development of an advanced material that is able to switch between an electrically conductive state to an insulating state, simply by applying an electric field.
The work represents a step towards the development of a new generation of ultra-low energy electronics at room temperature.
Co-author Dr Anton Tadich, a beamline scientist at the Soft X-ray beamline and Partner Investigator with FLEET, collaborated with investigators from Monash University, Singapore and Lawrence Berkeley National Lab on the use of photoemission techniques at the Australian Synchrotron X-ray Photoelectron Spectroscopy (XPS) and the Advanced Light Source in the US Angle Resolved Photoelectron Spectroscopy, (ARPES).
The chemical composition and growth mechanisms of thin films of the topological Dirac semi-metal sodium bismuthide Na₃Bi on a silicon substrate was investigated using XPS at the Australian Synchrotron’s Soft X-ray beamline.

Acid-base equilibria: not exactly like you remember in chemistry class

2018/10/18

Work published in the Royal Society of Chemistry with the support of the Helmholtz Association through the Center for Free-Electron Laser Science at DESY, MAX IV Laboratory, Lund University, Sweden, European Research Council (ERC) under the European Union’s Horizon 2020 and the Academy of Finland.

Remember doing titrations in chemistry class? Adding acid drop-by-drop to the beaker and the moment you took your eye off it the solution completely changed colour.
We learned in chemistry that by doing this titration, we were actually affecting an important equilibrium in the beaker between acids and bases. This equilibrium was first described at the turn of the 20th century by American biochemist Lawrence Henderson and modified by Karl Hasselbalch giving us the Henderson-Hasselbalch equation. The discovery and subsequent study of acids and bases using this equation has led to the discovery of many important phenomena in the natural world from as how cells function to how materials are formed.

However, after years of study, an idea arose that questioned the validity of the Henderson-Hasselbalch equation, what happens at the surface? If you have a beaker filled with a dilute acid, what happens at the very top atomic layer? The top layer of a liquid in a beaker is special for many reasons, but if you’re a dissolved molecule, it means that you’re no longer surrounded by water on all sides. For hydrophobic molecules, this means that it is favourable to be at the surface. With this in mind, the scientists took another look at the Henderson-Hasselbalch equilibrium equation and thought that it couldn’t work at the surface. Many studies have measured indicator chemical species, and determined that the Henderson-Hasselbalch equation does not seem to apply at the surface, and concluded that the concentration of hydronium or hydroxide ions, which determines the acidity/basicity, is different at the air-liquid interface than in the bulk.

Tailoring the surface chemical reactivity of transition‐metal dichalcogenide PtTe2 crystals

2018/03/262018/03/28

Recently, the PtX2 (X=S, Se, Te) class of transition-metal dichalcogenides has emerged as one of the most promising among layered materials “beyond graphene” for the presence of high room-temperature electron mobility and, moreover, bulk type-II Dirac fermions, arising from a tilted Dirac cone.
Information on the ambient stability of PtTe2 is a crucial step in order to evaluate the feasibility of its exploitation in technology. Moreover, the possibility to tune surface chemical reactivity by appropriate surface modification is an essential step for its employment for diverse applications, especially in catalysis.
By means of experiments with several surface-science spectroscopies and density functional theory, an international team of researchers from Italy, Republic of Korea, and Taiwan (coordinated by Graphene Labs of Istituto Italiano di Tecnologia) has investigated the reactivity of the PtTe2 surface toward most common ambient gases (oxygen and water), under the framework of the European Graphene Flagship-Core1 project.
To assess the surface chemical reactivity of PtTe2, X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline has been combined with high-resolution electron energy loss (HREELS) experiments and with density functional theory.
From the analysis of Te 3d core-level spectra in XPS and from the featureless vibrational spectrum in HREELS, it has been demonstrated that as-cleaved defect-free PtTe2 surface is inert toward most common ambient gases (oxygen and water).
In the evaluation of the ambient stability of PtTe2, the possible influence of Te vacancies on surface chemical reactivity deserves particular attention. As a matter of fact, Te vacancies may appear on non-stoichiometric samples during the growth process. To check the influence of Te vacancies on ambient stability of PtTe2, Te vacancies have been intentionally introduced in stoichiometric PtTe2 samples by Ar-ion sputtering. After exposing to O2 the PtTe2 surface defected by ion sputtering, with a Pt:Te ratio of 39:61, spectral features related to Te(IV) species appear, arising from the formation of Te=O bonds in a tellurium-oxide phase. The Te(IV) components are the most intense lines in the Te 3d XPS spectra for the case of air-exposed defected samples (see Figure 1). Concerning reactivity to water, it adsorbs molecularly even at room temperature on defected PtTe2. These findings also imply that the presence of Te vacancies is able to jeopardize the ambient stability of uncapped PtTe2-based devices, with a subsequent necessity to reduce the amount of Te vacancies for a successful technological exploitation of PtTe2.

>Read more on the Elettra website

Figure: XPS spectra of Te-3d core levels acquired for: defected PtTe₂ (green curve), the same surface exposed to 10⁶ L of O₂(black curve) and air-exposed defected PtTe₂ (yellow curve). The photon energy is 745 eV.