Tag: x-ray photoelectron spectroscopy

Unlocking the secrets of hafnia: a new era in ferroelectric materials

Unlocking the secrets of hafnia: a new era in ferroelectric materials

Depth-resolved X-ray Photoelectron Spectroscopy probes the link between polarisation and electrochemistry

Ferroelectric materials exhibit a unique property called spontaneous polarisation. Their built-in electric dipole moment can be switched between different directions by applying an external electric field. This makes them incredibly useful for a wide range of applications, including memory storage devices, sensors, and energy harvesters. The discovery of ferroelectricity in nanoscale hafnia-based films has spurred extensive research to understand its origin and unlock its full potential. Hafnia displays unusual behaviour in that its ferroelectricity becomes stronger as the material gets thinner, and one theory suggests that the electrochemical state within the hafnia film is directly linked to its polarisation and responsible for the unique size-dependent properties.

In work recently published in Advanced Materials, researchers from the University of Cambridge used depth-resolved X-ray Photoelectron Spectroscopy (XPS) at Diamond’s I09 beamline to investigate the intricate relationship between polarisation and electrochemical changes in hafnia-based ferroelectrics. The results suggest that the electrochemical state is not directly linked to polarisation, and that certain dopants can suppress the electrochemical changes that cause degradation without sacrificing polarisation, opening up exciting possibilities for engineering more robust and reliable ferroelectric devices.

Surprising findings in nanoscale hafnia films

The research team focused on two specific compositions, Hf0.5Zr0.5O2 (HZO) and Hf0.88La0.04Ta0.08O2(HLTO), both in the form of single-phase epitaxial films. These films were chosen to minimise the influence of grain boundaries and other structural complexities that could complicate the analysis. The first step was to meticulously characterise the structure and ferroelectric properties of the HLTO and HZO films using a combination of techniques. They used X-ray Diffraction (XRD) to determine the crystallographic phase and orientation of the films, Piezoresponse Force Spectroscopy (PFS) and Microscopy (PFM) to confirm the presence of ferroelectricity and visualise the domain structure and Positive-Up Negative-Down (PUND) measurements to measure the remnant polarisation and coercive field, key parameters describing the ferroelectric behaviour.

These initial characterisations confirmed the presence of the desired ferroelectric phases in both HLTO and HZO and identified 24 areas on the samples, two sets of each specific polarisation state (P-up, P-down, or as-grown), to analyse using depth-resolved XPS.

Dr Nives Strkalj explained:

Our hafnia samples were intended to be very similar in terms of polarisation, but we were expecting to see changes in their electrochemistry when we used an electric field to change the polarisation. We opted for the I09 beamline because it’s a unique setting where you can change between X-rays that probe deep and shallow with just the click of a button. Usually, if you want to probe depth, you have to realign the incidence angle, then you have to realign the detector, and it’s very time consuming. We had to check many areas of our samples, areas which were P-up, or which were P-down, and on I09 we can get depth probing very quickly.

During the XPS experiments, the researchers discovered a surprising difference in the electrochemical behaviour between HLTO and HZO. In the P-up state, HLTO showed an increase in non-lattice oxygen (NL-O) primarily at the surface, suggesting that the electric field was driving oxygen species from the atmosphere onto the film. In contrast, HZO displayed an increase in NL-O distributed throughout the bulk of the film, accompanied by reduction of the Hf and Zr cations. These findings suggest that the polarisation state is not solely responsible for the changes in oxygen electrochemistry in these materials. Instead, the electric field used to switch the polarisation plays a crucial role.

Read more on Diamond website

Real-time observation of hydrocarbon polymerization

Real-time observation of hydrocarbon polymerization

The polymerization of hydrocarbons into linear chains lies at the heart of many industrially relevant chemical reactions. One prominent example is the alkene polymerization with the Ziegler Natta catalysts, which is responsible for 2/3 of the global production of polyolefins. Long-chain hydrocarbons can also be produced from syngas (a mixture of carbon monoxide and hydrogen) through the so-called Fischer-Tropsch synthesis (FTS), occurring typically on cobalt-based catalysts. This process is experiencing a renewed interest, especially in the context of modern power-to-gas and power-to-liquid plants.

From a microscopic point of view, the identification of the complex series of reaction steps involved in the polymerization of small molecules into long hydrocarbon chains is still under debate. Surface-science techniques have proved to be extremely powerful to explore the mechanisms of heterogeneous catalysis. However, due to the harsh reaction conditions of FTS, analysis using such techniques poses a real experimental challenge.

Though the step sites of the catalytic surface are also commonly assumed to promote the C-C coupling of CHx monomers, the typical strong adsorption of small molecules at the step edges could trap the CHx species, hindering the polymerization. This behavior can be understood in the framework of the Sabatier’s principle, stating that if the adsorption energy of the substrate is too low, then the catalytic activity is suppressed; if it is too large, then the product will not desorb and blocks the surface, leading to catalyst poisoning. Therefore, elucidating the actual role of the step sites is crucial for an in-depth atomistic understanding of the hydrocarbon chain growth process.

Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X-ray photoelectron spectroscopy (XPS) performed at the SuperESCA beamline of Elettra, the intermediate species and reaction products have been directly identified. This has been enabled by the high energy resolution (about 100 meV) of the instrument, allowing resolving vibrational fine structures.

Read more on the Elettra website

Connecting the dots between material properties and qubit performance

Connecting the dots between material properties and qubit performance

Engineers and materials scientists studying superconducting quantum information bits (qubits)—a leading quantum computing material platform based on the frictionless flow of paired electrons—have collected clues hinting at the microscopic sources of qubit information loss. This loss is one of the major obstacles in realizing quantum computers capable of stringing together millions of qubits to run demanding computations. Such large-scale, fault-tolerant systems could simulate complicated molecules for drug development, accelerate the discovery of new materials for clean energy, and perform other tasks that would be impossible or take an impractical amount of time (millions of years) for today’s most powerful supercomputers.

An understanding of the nature of atomic-scale defects that contribute to qubit information loss is still largely lacking. The team helped bridge this gap between material properties and qubit performance by using state-of-the-art characterization capabilities at the Center for Functional Nanomaterials (CFN) and National Synchrotron Light Source II (NSLS-II), both U.S. Department of Energy (DOE) Office of Science User Facilities at Brookhaven National Laboratory. Their results pinpointed structural and surface chemistry defects in superconducting niobium qubits that may be causing loss. 

Read more on the BNL website

Image: Scientists performed transmission electron microscopy and x-ray photoelectron spectroscopy (XPS) at Brookhaven Lab’s Center for Functional Nanomaterials and National Synchrotron Light Source II to characterize the properties of niobium thin films made into superconducting qubit devices at Princeton University. A transmission electron microscope image of one of these films is shown in the background; overlaid on this image are XPS spectra (colored lines representing the relative concentrations of niobium metal and various niobium oxides as a function of film depth) and an illustration of a qubit device. Through these and other microscopy and spectroscopy studies, the team identified atomic-scale structural and surface chemistry defects that may be causing loss of quantum information—a hurdle to enabling practical quantum computers.