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

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. 

Investigation of metal deposition in organs after joint replacement

Synchrotron analysis shows potentially harmful metals from implants can find their way into human organs.

The hip replacement is considered to be one of the most successful orthopaedic interventions, with 75,000 performed each year by the NHS alone. However, the implants used to replace hips contain metals, such as chromium and cobalt, which are potentially toxic and which can be deposited into tissues around the implant site due to wear and corrosion. A team of researchers used X-ray absorption spectroscopy (XAS) on the I18 beamline to show that these metals can also find their way into organ tissues. Their results suggest that chronic diseases, such as diabetes, may create conditions in which mildly toxic trivalent chromium (CrIII) particles from replacement joints are reoxidised within the body to form carcinogenic hexavalent chromium (CrVI). Their results have been published in the Journal of Trace Elements in Medicine and Biology.

>Read more on the Diamond Light Source website

Image: Overview of the study (entire figure to see here).

Cleaner diesel emissions

More effective control of diesel nitrogen oxides through dosed addition of ammonia

In diesel engines, the burning of the fuel releases nitrogen oxides (NOx), which are harmful to human health. The automobile industry therefore developed a technique that reduces these emissions: Gaseous ammonia is added to the exhaust and, prompted by a catalyst, reacts with the nitrogen oxides to produce harmless nitrogen and water. At low temperatures, however, this process does not yet work optimally. Now, for the first time, scientists at the Paul Scherrer Institute PSI have found a remedy which is based on observations at the molecular level: The precise amount of added ammonia needs to be varied depending on the temperature. With this knowledge, manufacturers can improve the effectiveness of their catalytic converters for diesel vehicles. The researchers have now published their findings in the journal Nature Catalysis.

>Read more on the Paul Scherrer Institute website

Image: At the X-ray beam line: Davide Ferri (left) and Maarten Nachtegaal at the SLS experimental station where they studied diesel catalysis.
Photo: Paul Scherrer Institute/Markus Fischer

Apply for the Kai Siegbahn prize 2018

The Prize was established in 2009 in honour of Kai Siegbahn, founder of Nuclear Instruments and Methods A (NIMA), who had a strong and lasting commitment to advancing synchrotron radiation science.

The Editorial Board of NIMA is currently accepting nominations for the 2018 award, and we are counting on you to help us identify potential honorees! We invite you to review the award criteria, and to nominate a worthy colleague.

All nominations should be submitted to the Committee Chair by March 31 2018:

Prof. Fulvio Parmigiani, Kai Siegbahn Chair
Department of Physics, University of Trieste
International Faculty, University of Cologne,
Elettra Sincrotrone Trieste S.C.p.A.
Email: fulvio.parmigiani@elettra.trieste.it

Nomination criteria:

The Prize aims to recognize and encourage outstanding experimental achievements in synchrotron radiation research with a significant component of instrument development. Particular preference will be given to the development of synchrotron radiation spectroscopies.

Rules and eligibility:

Nominations are open to scientists of all nationalities without regard to the geographical site at which the work was performed. Usually, the prize shall be awarded to one person but it may be shared if all recipients have contributed to the same accomplishment. The prize recipient should be 45 years old or younger at the time of selection.

Nominations are accepted from the NIMA advisory board, the NIMA board of editors, synchrotron radiation facility directors as well as from scientists engaged in synchrotron radiation science. Nomination packages should include a nominating letter, at least one supporting letter, a list of five papers on which the award is based as well as a proposed citation for the award.

High coherence and intensity at FERMI enables new X-ray interfacial probe

Interfaces are involved in a wide range of systems and have significant implications in many fields of scientific and technological advancement, often determining device performance or chemical reactivity. Vital examples include solar cells, protein folding, and computer chips. A class of commonly used surface science techniques are comprised of even-ordered nonlinear spectroscopies (i.e., second harmonic and sum frequency generation) which exhibit no response in centrosymmetric media due to symmetry constraints.As a result, they have been widely used at optical wavelengths to explore physical and chemical properties of interfaces, where centrosymmetry is broken. Extending this to x-ray wavelengths would effectively combine the element specificity and spectral sensitivity of x-ray spectroscopy with the rigorous interfacial/surface specificity of optical even-ordered nonlinear spectroscopies. Unfortunately, at hard x-ray energies (x-ray wavelength order of the spacing between atoms) these even-ordered nonlinear spectroscopies are effectively bulk probes, as each individual atom breaks inversion symmetry. As soft x-ray wavelengths fall in between the UV and hard x-ray regimes, there has been uncertainty regarding the interface specificity of soft x-ray second harmonic generation.

>Read more on the FERMI webpage

Figure: (extract)  Experimental Design. X-ray pulses are passed through a 2 mm iris and focused onto the graphite sample at normal incidence. The transmitted beam is then passed through a 600 nm aluminum filter and onto a spectrometer grating, spatially resolving the second harmonic signal from the fundamental. Inset: A schematic energy level diagram of the second harmonic generation process. (entire figure here)


Questioning the universality of the charge density wave nature…

… in electron-doped cuprates

The first superconductor materials discovered offer no electrical resistance to a current only at extremely low temperatures (less than 30 K or −243.2°C). The discovery of materials that show superconductivity at much higher temperatures (up to 138 K or −135°C) are called high-temperature superconductors (HTSC). For the last 30 years, scientists have researched cuprate materials, which contain copper-oxide planes in their structures, for their high-temperature superconducting abilities. To understand the superconducting behavior in the cuprates, researchers have looked to correlations with the charge density wave (CDW), caused by the ordered quantum field of electrons in the material. It has been assumed that the CDW in a normal (non-superconducting) state is indicative of the electron behavior at the lower temperature superconducting state. A team of scientists from SLAC, Japan, and Michigan compared the traits of superconducting and non-superconducting cuprate materials in the normal state to test if the CDW is correlated to superconductivity.

>Read more on the SSRL website

Picture: explanation in detail to read in the full scientific highlight (SSRL website)




Complex tessellations, extraordinary materials

Simple organic molecules form complex materials through self-organisation

An international team of researchers lead by the Technical University of Munich (TUM) has discovered a reaction path that produces exotic layers with semiregular structures. These kinds of materials are interesting because they frequently possess extraordinary properties. In the process, simple organic molecules are converted to larger units which form the complex, semiregular patterns. With experiments at BESSY II at Helmholtz-Zentrum Berlin this could be observed in detail.

Only a few basic geometric shapes lend themselves to covering a surface without overlaps or gaps using uniformly shaped tiles: triangles, rectangles and hexagons. Considerably more and significantly more complex regular patterns are possible with two or more tile shapes. These are so-called Archimedean tessellations or tilings.

Materials can also exhibit tiling characteristics. These structures are often associated with very special properties, for example unusual electrical conductivity, special light reflectivity or extreme mechanical strength. But, producing such materials is difficult. It requires large molecular building blocks that are not compatible with traditional manufacturing processes.


>Read more on the Bessy II website

Image: The new building block (left, red outline) comprises two modified starting molecules connected to each other by a silver atom (blue). This leads to complex, semiregular tessellations (right, microscope image).
Credit: Klappenberger and Zhang / TUM

Control of magnetoresistance in spin valves

Molecules, due to their wide-ranging chemical functionalities that can be tailored on demand, are becoming increasingly attractive components for applications in materials science and solid-state physics. Remarkable progress has been made in the fields of molecular-based electronics and optoelectronics, with devices such as organic field-effect transistors and light emitting diodes. As for spintronics, a nascent field which aims to use the spin of the electron for information processing, molecules are proposed to be an efficient medium to host spin-polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin.

Spin-polarized hybrid states induced by the interaction of the first molecular monolayers on ferromagnetic substrates are expected to govern the spin polarization at the molecule–metal interface, leading to changes in the sign and magnitude of the magnetoresistance in spin-valve devices. The formation of spin-polarized hybrid states has been determined by spin-polarized spectroscopy methods and principle-proven in nanosized molecular junctions, but not yet verified and implemented in large area functional device architectures.

>Read more on the ALBA website

Image: Magnetoresistance (top) and X-ray spectroscopy (bottom) measurements, evidencing the control of the magnetoresistance sign and amplitude by engineering spin valves with NaDyClq/NiFe and NaDyClq/Co interfaces, and their corresponding interfacial molecule-metal hybridization states.

First Pilot Experiment at SwissFEL-Alvra

UV photo-induced charge transfer in OLED system

On the 17th of December 2017 SwissFEL saw its first pilot experiment in the Alvra experimental station of the SwissFEL ARAMIS beamline. A team of scientists from the University of Bremen, Krakow and PSI, led by Matthias Vogt (Univ. Bremen) and Chris Milne (PSI)in collaboration with J. Szlachetko, J. Czapla-Masztafiak, W. M. Kwiatek (Inst. of Nucl.Phys. PAN (Krakow), successfully did the first pilot experiment at SwissFEL-Alvra on UV photo-induced charge transfer in OLED system.

With ever-increasing demands on low-cost, low-power display technology, significant resources have been invested in identifying OLED materials that are based on Earth-abundant materials while maintaining high internal quantum efficiencies. The recent pilot experiment performed at SwissFEL’s Alvra experimental station aimed to use X-ray spectroscopy to investigate a promising OLED candidate based on copper and phosphorus. This molecule, synthesized by Dr. Matthias Vogt from the University of Bremen, is based on a physical phenomenon called thermally activated delayed fluorescence, which allows for extremely high energy efficiencies to be achieved. The experiment probed how the phosphorus atoms are involved in the fluorescence process as a complement to longer-timescale measurements on the copper atoms performed at the Swiss Light Source’s SuperXAS beamline by Dr. Grigory Smolentsev and collaborators. The SwissFEL measurements confirm that the phosphorus atoms are directly involved in the charge transfer process in the molecule, and lay the foundation for future investigations of the mechanisms behind the efficiency of the delayed fluorescence process.

>Read more on the SwissFEL website

Figure: please find here the full figure