thin films – Lightsources.org

Sharper insights into thin-film systems

2021/09/162021/10/21

Interfaces in semiconductor components or solar cells play a crucial role for functionality. Nevertheless, until now it has often been difficult to investigate adjacent thin films separately using spectroscopic methods. An HZB team at BESSY II has combined two different spectroscopic methods and used a model system to demonstrate how well they can be distinguished.

Photoelectron spectroscopy (PES) enables the chemical analysis of surfaces and semiconductor layers. In this process, an X-ray pulse (photons) hits the sample and excites electrons to leave the sample. With special detectors, it is then possible to measure the direction and binding energy of these electrons and thus obtain information about electronic structures and the chemical environment of the atoms in the material. However, if the binding energies are close to each other in adjacent layers, then it is hardly possible to distinguish these layers from each other with PES.

A team at HZB has now shown how precise assignments can nevertheless be achieved: they combined photoelectron spectroscopy with a second spectroscopic method: Auger electron spectroscopy. Here, photoelectrons and Auger electrons are measured simultaneously, which gives the resulting method its name: APECS for Auger electron photoelectron coincidence spectroscopy (APECS).

Read more on the HZB website

Image: The illustration shows how the APECS measurement works on a nickel single crystal with an oxidised surface. An X-ray beam ionises atoms, either in the nickel crystal or on the surface. The excited photoelectrons from the surface and from the crystal have slightly different binding energies. The Auger electrons make it possible to determine the origin of the photoelectrons.

Credit: © Martin Künsting /HZB

Hybrid photoactive perovskites imaged with atomic resolution for the first-time

2020/10/302020/11/05

A huge step towards better performing solar cells – a collaboration identified information previously invisible using Diamond’s ePSIC microscopes of Oxford University’s Departments of Materials and Physics

A new technique has been developed allowing reliable atomic-resolution images to be taken, for the first time, of hybrid photoactive perovskite thin films.- highly favourable materials for efficient photovoltaic and optoelectronic applications. These images have significant implications for improving the performance of solar cell materials and have unlocked the next level of ability to understand these technologically important materials. The breakthrough was achieved by a joint team from the University of Oxford and Diamond who have just released a new paper published in Science.

Using the ePSIC (the Electron Physical Science Imaging Centre) E02 microscope and the ARM200 microscope in at the Department of Materials, University of Oxford, the team developed a new technique which allowed them to image the hybrid photoactive perovskites thin films with atomic resolution. This gave them unprecedented insights into their atomic makeup and provided them with information that is invisible to every other technique.

Read more on the Diamond website

Image: An example of one of the images obtained using the new protocol, which illustrates several of the phenomena that the team has been able to describe for the first time, including a range of grain boundaries, extended planar defects, stacking faults, and local inclusions of non-perovskite material.

Magnetic patterning by electron beam assisted carbon lithography

2019/02/112019/03/11

The exploitation of the unique physical properties of thin films and heterostructures are opening intriguing opportunities for magnetic storage technology. These artificial materials will in fact enable novel architectures for a multitude of magnetic devices and sensors, promoting a significant improvement in storage density, functionality and efficiency. Their usage will also contribute to diminish the consumption of materials that are rare and difficult to extract, being often detrimental to the environment. With these objectives in mind, researchers are now looking with great attention at the combination of thin ferromagnetic layers with 2-dimensional crystals like graphene and transition metal dichalcogenides. Due to their layered structure, these systems exhibit very favorable magnetic properties, which can be tuned through thickness and interfacial interactions. For instance, graphene-cobalt stacks display an enhanced perpendicular magnetic anisotropy, a feature that is especially important for non-volatile memories.
The fabrication of layered materials, however, is still a very challenging process. Not only it requires atomic precision in the deposition of the various layers but also the ability to create nano or microstructures of arbitrary shape. Conventional lithography in conjunction with chemical etching permits nowadays to sculpture the matter with great accuracy, at lateral resolution close to the nanometer. Yet, this approach poses an important limitation, that is, the material can only be shaped by erosion. The ability to vary the chemical composition, by adding atoms for example, is instead very desirable for many applications. To date, this can be done by stimulating the fragmentation of suitable carrier molecules using photons or electrons. So far, various methods based on focused beam induced processing methods have been devised, which can be readily employed to deposit carbonaceous layers and metallic nanostructures. These methods, however, cannot be applied when ultra-clean, ultra-high vacuum (UHV) conditions are needed, as happens for the case of semiconductor industry.

>Read more on the Elettra website

Figure 1. (left) Scheme of the protocol for printing chemo-magnetic patterns in ultrathin Co on Re(0001). (a) The film is exposed to CO at room temperature. The irradiation with a focused electron beam (yellow) stimulates the dissociation of the molecule, which results in the accumulation of atomic carbon on the surface. (b) Subsequently, the sample is annealed above 170 °C to desorb molecularly adsorbed CO from the non-irradiated surface regions. (c) LEEM image of an e-beam irradiated disk. Disk diameter: 1 μm; Co thickness: 4 atomic layers; irradiation energy: 50 eV; CO dose: 9.75 L; (d) Intensity profile across the orange line in the LEEM image in (c) and fit using a step function convoluted with a Gaussian of full width at half-maximum of 30 nm. The dashed blue lines indicate the 15–85% distance between minimum and maximum intensity. (e) XMCD-PEEM image of the same region at the Co L3 edge. (f) Intensity profiles across the blue and orange dashed lines in the XMCD-PEEM image in (e). The magnetic stripes indicate out-of-plane magnetic anisotropy. The stripe period is 120 nm. Adapted with permission from [1].
Copyright (2018) American Chemical Society.

Modifications to novel non-fullerene small molecule acceptor in organic thin film

2018/03/112018/03/16

… for solar cells demonstrates improved power conversion efficiency.

Scientists from the Imperial College London, Monash University, CSIRO, and King Abdullah University of Science and Technology have reported an organic thin film for solar cells with a non-fullerene small molecule acceptor that achieved a power conversion efficiency of just over 13 per cent.

By replacing phenylalkyl side chains in indacenodithieno[3,2-b]thiophene-based non-fullerene acceptor (ITIC) with simple linear chains to form C8-ITIC, they improved the photovoltaic performance of the material.

C8-ITIC was blended with a fluorinated analog of the donor polymer PBDB-T to form bulk-heterojunction thin films.

The research was recently published in Advanced Materials.

Dr Xuechen Jiao of McNeill Research Group at Monash University carried out grazing incidence wide angle X-ray scattering (GIWAXS) measurements at the Australian Synchrotron to gain morphological information on pure and blended thin films.

“By changing the chemical structure of the organic compound, a promising boost in efficiency was successfully achieved in an already high-performing organic solar cells” said Jiao.

Ubiquitous formation of type-I and type-II bulk Dirac cones

2017/11/182018/03/12

… and topological surface states from a single orbital manifold in transition-metal dichalcogenides

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied properties. They range from metals and superconductorsto strongly spin-orbit-coupled semiconductors and charge-density-wave systems with their single-layer variants one of the most prominent current examples of two-dimensional materials beyond graphene.Their varied ground states largely depend on the transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date.

>Read more on the Elettra website.

Image: Chalcogen-derived topological ladder in PdTe2.(a) Orbitally-resolved bulk electronic structure of PdTe2, indicating dominantly chalcogen-derived orbital character for the states in the vicinity of the Fermi level. (b) The measured out-of-plane dispersion together with the calculated band structure. Measured (c) and calculated (d) in-plane dispersion. (e,f) Spin-resolved energy distribution curves along the lines shown in (c).

Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking

2017/10/202018/01/16

Research collaboration led by Professor Philip King from University of St. Andrews, and comprising the researchers from Max Planck Institute for Chemical Physics of Solids in Dresden, Institute for Theoretical Physics of the University of Heidelberg and researchers from I05 beamline at Diamond Light Source and APE beamline at Elettra, described a new route to maximise the spin-splitting of surface states.
The electronic properties of surfaces are often different from those of the bulk. In particular, the intrinsically broken symmetries of the surface compared with the bulk of the material allow for appearance of the new electronic surface states. For the systems in which spin-orbit interaction is strong, a non-negligible separation of these states according to their spin takes place. The spin splitting of surface- or interface-localized two-dimensional electron gases is characterized by a locking of the electron spin perpendicular to its momentum.

Highly Crystalline C8-BTBT Thin-Film Transistors by Lateral Homo-Epitaxial Growth on Printed Templates

2017/10/112018/01/17

The latest generation of organic semiconductors display excellent characteristics, with charge mobilities surpassing those of amorphous silicon thin film transistors (TFTs) that are commonly used in today’s flat panel displays. The integration of organic TFTs (OTFTs) into real applications requires high performance and low spread of the electrical characteristics. As transport properties are greatly influenced by the microstructure of the organic layer, single crystalline films offer the greatest potential for high-performance OTFTs.