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 Smallest transistor worldwide switches current with a single atom in solid electrolyte
17.08.2018 | Karlsruher Institut für Technologie (KIT)

nachricht Protecting the power grid: Advanced plasma switch for more efficient transmission
17.08.2018 | DOE/Princeton Plasma Physics Laboratory

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: It’s All in the Mix: Jülich Researchers are Developing Fast-Charging Solid-State Batteries

There are currently great hopes for solid-state batteries. They contain no liquid parts that could leak or catch fire. For this reason, they do not require cooling and are considered to be much safer, more reliable, and longer lasting than traditional lithium-ion batteries. Jülich scientists have now introduced a new concept that allows currents up to ten times greater during charging and discharging than previously described in the literature. The improvement was achieved by a “clever” choice of materials with a focus on consistently good compatibility. All components were made from phosphate compounds, which are well matched both chemically and mechanically.

The low current is considered one of the biggest hurdles in the development of solid-state batteries. It is the reason why the batteries take a relatively long...

Im Focus: Color effects from transparent 3D-printed nanostructures

New design tool automatically creates nanostructure 3D-print templates for user-given colors
Scientists present work at prestigious SIGGRAPH conference

Most of the objects we see are colored by pigments, but using pigments has disadvantages: such colors can fade, industrial pigments are often toxic, and...

Im Focus: Unraveling the nature of 'whistlers' from space in the lab

A new study sheds light on how ultralow frequency radio waves and plasmas interact

Scientists at the University of California, Los Angeles present new research on a curious cosmic phenomenon known as "whistlers" -- very low frequency packets...

Im Focus: New interactive machine learning tool makes car designs more aerodynamic

Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.

When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...

Im Focus: Robots as 'pump attendants': TU Graz develops robot-controlled rapid charging system for e-vehicles

Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.

Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

LaserForum 2018 deals with 3D production of components

17.08.2018 | Event News

Within reach of the Universe

08.08.2018 | Event News

A journey through the history of microscopy – new exhibition opens at the MDC

27.07.2018 | Event News

 
Latest News

Quantum bugs, meet your new swatter

20.08.2018 | Information Technology

A novel synthetic antibody enables conditional “protein knockdown” in vertebrates

20.08.2018 | Life Sciences

Metamolds: Molding a mold

20.08.2018 | Information Technology

VideoLinks
Science & Research
Overview of more VideoLinks >>>