Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Shedding light on light absorption: titanium dioxide unveiled

13.04.2017

MPSD scientists have uncovered the hidden properties of titanium dioxide, one of the most promising materials for light-conversion technology.

The anatase crystal form of Titanium dioxide (TiO₂) is one of the most promising materials for photovoltaic and photocatalytic applications nowadays. Despite years of studies on the conversion of light absorbed by anatase TiO₂, into electrical charges, the very nature of its fundamental electronic and optical properties remained still unknown.


Lattice structure of anatase TiO2 with a graphical representation of the 2D exciton that is generated by the absorption of light. This 2D exciton is the lowest energy excitation of the material.

Scientists from the MPSD (Max Planck Institute for the Structure and Dynamics of Matter) at CFEL (Center for Free-Electron Laser Science) in Hamburg, together with their international partners at EPFL, Lausanne used a combination of cutting-edge steady-state and ultrafast spectroscopic techniques, as well as theoretical simulation tools to elucidate these fundamental properties of anatase TiO₂. Their work is published in Nature Communications.

Anatase TiO₂ is involved in a wide range of applications, ranging from photovoltaics and photocatalysis to self-cleaning glasses, and water and air purification, all of which are based on the absorption of light and its subsequent conversion into electrical charges. Given its widespread use in various applications, TiO₂ has been one of the most studied materials in the twentieth century, both experimentally and theoretically. Paradoxically, the very nature of what it is that actually absorbs light was unclear!

When light is shined on a semiconductor material, either free negative charges (electrons), positive charges (holes) or bound electron-hole pairs (excitons) are generated. Excitons can transport both energy and charge and are the basis of an entire field of next-generation electronics, called “excitonics”. So far we have lacked the ability to clearly identify the nature and properties of the physical object that absorbs light and characterizes the properties of TiO₂.

The group of Prof. Angel Rubio at the Theory Department of the MPSD along with its international collaborations have solved this problem using a combination of state-of-the-art first-principles theoretical tools along with cutting-edge experimental methods: steady-state angle-resolved photoemission spectroscopy (ARPES), which maps the energetics of the electrons (band-structure) along the different axis in the solid; and spectroscopic ellipsometry, which determines the macroscopic optical parameters (dielectric constant, etc.) of the solid with precision and ultrafast two-dimensional deep-ultraviolet spectroscopy, which is for the first time used in the study of materials. They discovered that the threshold of the absorption spectrum is due to a strongly bound exciton, which exhibits two remarkable novel properties:

a) it is confined on a 2-dimensional (2D) plane of the 3-dimensional lattice of the material. This is the first such case ever reported;

b) this 2D exciton is stable at room temperature and robust against defects, being present in any type of TiO₂-single crystals, thin films, and even nanoparticles used in devices.

This “immunity” of the exciton to long-range structural disorder and defects implies that it can store the incoming energy, in the form of light, and guide it at the nanoscale in a selective way. This promises a huge improvement compared to current technology, in which the conventional excitation schemes are extremely inefficient because the absorbed light energy is not stored but dissipated as heat to the crystal lattice. “The use of cutting-edge experimental techniques and theory allows us not only to understand but also design and create new, even more efficient materials for energy applications” says Adriel Domínguez.

Furthermore, the exciton parameters can be tuned by a variety of external and internal stimuli (temperature, pressure, excess electron density), promising a powerful, accurate and cheap detection scheme for sensors with an optical read-out. “Given the cheap and easy to fabricate anatase TiO₂ materials, these findings are crucial for such applications and beyond, to know how electrical charges are generated after light is absorbed”, says Prof. Majed Chergui from EPFL “These charges are the key players in solar energy conversion and photocatalysis.” Prof. Angel Rubio emphasizes that this sort of studies, resulting from the close collaboration between theoretical and experimental groups, are essential in order to unveil the microscopic origin of the light-energy conversion and energy transfer processes in materials of relevance for photovoltaic and photocatalytic applications and for the design of new artificial photosynthetic inorganic materials. “We’ll continue working with our international partners in EPFL in Lausanne to understand, even better, how this kind of bulk materials as well as many other low-dimensional oxide nanostructures behave when driven out of equilibrium by external stimuli such as light” he finalizes.

This work was carried out in a collaboration of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) with the EPFL’s Lausanne Centre for Ultrafast Science (LACS and the Institute of Physics (IPHYS), the University of Fribourg, the Università Campus Bio-Medico di Roma, the Center for Life Nano Science in Università di Roma “La Sapienza”, and the Universidad del Pais Vasco. It was funded by the Swiss National Science foundation (SNSF; NCCR:MUST), the European Research Council Advanced Grants “DYNAMOX” and “Qspec-Newmat”, Grupos Consolidados del Gobierno Vasco and the Austrian Science Fund.

Publication:
E. Baldini, L. Chiodo, A. Dominguez, M. Palummo, S. Moser, M. Yazdi-Rizi, G. Auböck, B.P.P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio, M. Chergui
Strongly bound excitons in anatase TiO2 single crystals and nanoparticles
Nature Communications Nature Communications 8, Article number: 13 (2017)

Weitere Informationen:

http://www.mpsd.mpg.de/399248/2017-04-Baldini-Rubio MPSD Research News including Contacts
http://www.mpsd.mpg.de/en/research/theo Theory Department of Prof. Angel Rubio
http://dx.doi.org/doi:10.1038/s41467-017-00016-6 Original Publication

Dr. Joerg Harms | Max-Planck-Institut für Struktur und Dynamik der Materie

More articles from Physics and Astronomy:

nachricht NASA's James Webb Space Telescope completes final cryogenic testing
21.11.2017 | NASA/Goddard Space Flight Center

nachricht Previous evidence of water on mars now identified as grainflows
21.11.2017 | US Geological Survey

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Previous evidence of water on mars now identified as grainflows

21.11.2017 | Physics and Astronomy

NASA's James Webb Space Telescope completes final cryogenic testing

21.11.2017 | Physics and Astronomy

New catalyst controls activation of a carbon-hydrogen bond

21.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>