In a foundational study, Pavel V. Kolesnichenko, Lukas Wittenbecher, Qianhui Zhang, Run Yan Teh, Chandni Babu, Michael S. Fuhrer, Anders Mikkelsen, and Donatas Zigmantas present the first direct measurement of the ultrafast dynamics of such dark excitons in semiconducting monolayers of WS₂, enabled via femtosecond photoemission spectro-microscopy allowing tracing their ultrafast behavior in real-time.
“This study is about the first direct measurement of the formation of elusive dark excitons and their subsequent dynamics in semiconducting monolayers of tungsten disulphide (WS2),” says Pavel Kolesnichenko.
Imaging method of Einstein’s photoelectric effect
“The measurement was enabled by a unique combination of two advanced experimental methods. The first is a pump-and-probe spectroscopy utilizing broadband ultrashort optical pulses in the deep ultraviolet region, which we also previously reported; generating such pulses is a challenge on its own. The second is the imaging method that leverages the photoelectric effect, which was described by Albert Einstein who was later awarded the 1921 Nobel Prize in physics. The high-resolution images are ultimately formed by electrons which, according to quantum physics, can be also described by waves (or, de Broglie waves, or matter waves). It is the very short wavelengths (compared to visible light) of electron waves that ultimately allow imaging with very high spatial resolution beating that of modern optical microscopes. This imaging method nowadays is referred to as photoemission electron microscopy (or, PEEM).
Combining the two methodologies allowed us to trace the formation of dark excitons – for the first time – in real time (!) and conclude that they form on ultrafast time scales, as fast as a few tens of femtoseconds! To imagine how fast it is, the following statement is commonly used: One femtosecond is to one second as one second is to approximately 32 million years!
High imaging resolution of PEEM allowed us to additionally examine the ultrafast behavior of dark excitons occurring in different locations on the semiconducting monolayer, in turn allowing us to conclude that material defects mediated the evolution of dark excitons,” says Pavel Kolesnichenko.
The results are interesting along several dimensions, he states. First, monolayer matter is on its own of high interest in science because of the possibility of combining different monolayer materials to achieve material properties otherwise not achievable in conventional bulk matter. Monolayers of transition metal dichalcogenides (TMD) in particular (such as WS2) are promising for various applications including in electronics, spintronics, optoelectronics, and valleytronics, because of the peculiar combination of their physical properties. Their light absorption properties are predominantly governed by strongly bound excitons. Excitons are the entities, which consist of a negatively charged electron and a positively charged hole (absence of electron) bound to each other via Coulomb forces. This binding is much stronger than for excitons in conventional semiconductors, making excitons in TMD monolayers stable at room temperatures. This, in turn, indicates that these excitons can be exploited in the applications mentioned above as they live long enough to utilize their properties.
Bright and dark excitons
“Now, amongst different kinds of excitons, one can separate all of them into two main types: bright and dark excitons. Bright excitons are called this way because they are directly visible under light illumination, which is not the case for dark excitons that cannot be directly observed with light. Some of the dark excitons can acquire intrinsic directionality to their motion making them potential suitable candidates for charge (electron or hole) transport from point A to point B. This is one of the first requirements in solar cell technology where sunlight generates charges, which have to be transported from the point (point A) of their generation towards the solar cell’s electrical contact (point B) to ultimately be converted to electricity running along the wires attached to this contact. Dark excitons could play a role of charge carriers in this case. Moreover, these dark excitons can be coupled to external fields, which could also be leveraged to enhance charge transport,” says Pavel Kolesnichenko.
The fact that this is the first real-time tracking of dark exciton dynamics is fascinating on its own.
“The study is also highly interesting from a fundamental science perspective. As mentioned above, excitons in TMD monolayers are very stable, even at room temperature and high exciton concentrations. This makes 2D materials ideal platforms for investigating experimentally many-body interactions arising in theoretical quantum physics. The ability to access experimentally dark excitons adds another facet to experimental investigations of a multitude of exciton complexes. The fact that it is the first real-time tracking of dark exciton dynamics is fascinating on its own.
The developed methodology opens a new avenue for studying ultrafast dynamics of dark carriers in other 2D materials,” says Pavel Kolesnichenko.
When asked what the most important thing learned from this study is, he mentions the fact that the so-called optical bandgap renormalization plays a crucial role in defining the course of evolution of dark excitons in WS2 monolayers.
Exploiting the motion of dark excitons
“Optical bandgap renormalization is the effect of exciton becoming less energetic in the presence of other excitons around it, so that as a result all excitons are ultimately found at lower energies. This phenomenon is one of the multitude of complex phenomena arising in many-body physics, and has been observed previously in few studies of 2D materials. It is very interesting that, in our study, it is only by involving the bandgap renormalization that made it possible to reconcile the complexity of all observations made in our experiments, which means that this effect is significant and has to be taken into account in future technologies based on 2D materials that exploit the motion of dark excitons.”
The results, he states, are useful for future research aiming at studying dark excitons in 2D materials as it lays the foundation for studying their properties by more advanced methodologies, which can be built on top of the method described in their work.
“Additional experimental degrees of freedom that can be used to expand the information content about these excitons are, for example, the addition of momentum resolution to our approach, as well as excitation-energy resolution and electron kinetic energy resolution, to mention a few.
In addition, other materials can be studied with the described methodology providing more knowledge on dark carriers in other TMD materials as well as other 2D materials,” he says.
Science combating global environment issues
“As an outreach, it is very important to keep public aware of the current progress in science, for the public to have a better feel where science is at combating global environmental issues. Although the intrinsic mobility of dark excitons may be suitable for charge transport in future solar cell technologies, it is still far from commercialization, and more studies are needed to gain even more insights into exciton behavior in these materials.”
“Sub-100 fs formation of dark excitons in monolayer WS₂” published in Nano Letters
