Measuring the Exciton Valleys in WSe2

By Liam CritchleyApr 13 2017

Image Credit: Shutterstock.com/ogwen

Excitons, also referred to as Coulomb bound electron-hole pairs, are composite bosons which lead to condensation and light amplification processes in semiconductors.

It is the optical properties in a semiconductor that are governed by the excitons, with one of the key effects being simulated scattering, where the bosons scatter into an already occupied quantum state.

An exciton is the bound state of an electron and an electron hole, which become attracted to each other through Coulombic interactions. Normal excitons are electrically neutral and can transport energy without transporting any net electric charge, but charge excitons can also be found.

In semiconductors, excitons are formed when a photon is absorbed into the semiconductor. The absorbance excites the electrons and promotes them into the conductance band, leaving behind a hole.

The electrons then return to the holes through repulsive Coulombic forces that arise from the large number of electrons surrounding the hole and the excited electron.

The exciton effects are much stronger in 2D materials, including in transition metal dichalcogenide (TMDC) monolayers. Monolayer tungsten diselenide (WSe₂) is one such TMDC and presents a much stronger Coulombic attraction than many other materials.

WSe₂ is an inorganic-based semiconductor with interesting properties in its monolayer form. The structure of the sheet is similar to other 2D materials in that it adopts a hexagonal array of atoms, where every tungsten atom is covalently bound to 3 selenium atoms and vice versa.

The extremes of the valence and conductance bands in WSe₂ are located at the corners of Brillouin Zone (BZ), where the time-reversal degenerate (K and -K valley) points give rise to a direct band gap, which leads to a degree of freedom in the excitons at the band-edge of the electrons and holes.

When an exciton, as a bound electron-hole pair, gains a degree of freedom in one (or more) of the degenerate K valleys, it is known as an exciton valley. Due to optical selection rules, exciton valleys can only be excited through circularly polarized light. The classification of exciton valleys is not only dependent upon the excitation, but also the spin. Excitons are considered to possess a pseudospin.

There have been many different methods tried to measure various aspects, parameters and the properties that exciton valleys possess. As they are an integral part of semiconductors, the need in recent years to further the understanding of these valleys has been strong.

Ways in Which Researchers Have Measured Excitons and Exciton Valleys in WSe₂

One team of researchers have probed the exciton valley dynamics through pump-probe Kerr rotation dynamics using a mode-locked Ti:Sapphire laser. The process yielded a direct measure for the exciton valley depolarisation time, which was found to be 6 ps under low temperatures (4 K), with a fast relaxation time.

This is due to the strong electron-hole Coulomb exchange interactions in the bright excitons. The researchers also increased the lattice temperature to 125 K, which led to a significant decrease in the depolarisation time and timeframes of 1.5 ps being recorded. The researchers defined the dependence on temperature to be due to exchange interactions and fast exciton scattering times on a short-range potential.

Another way that valley dynamics have been studied is through charged and neutral exciton emissions using a frequency-doubled optical parametric oscillator, a mode-locked Ti:Sa laser and a synchro-scan Hamamatsu Streak Camera.

The researchers monitored the emission and polarisation dynamics of both the neutral (bound electron hole pairs) and charged (trions) excitons through photoluminescence. The intensity of the photoluminescence decay in the neutral excitons was found to be 4 ps, whereas the trions decayed occur over seveal tens of ps.

The trion polarisation was found to a partial, fast initial decay within tens of ps before reaching a stable polarisation of 20%. The observed signatures were found to showcase a stable, optically initialized valley polarization.

The most recent method is through enabled selective excition scattering from WSe₂ upconversion. The research was undertaken due to a lack of understanding towards the bosonic character of exciton scattering in WSe₂ monolayers.

The researchers found that two different excitons, named A-excitons and B-excitons, occur in the same valley in momentum space. The researchers generated an upconversion in the 2D sheet using a low power excitation laser (Ti-Sa Laser SolsTis, M SQUARED), alongside a low vibration attoDry cryostat for micro-spectroscopy measurements, to generate B-excitons in the presence of resonantly excited A-excitons.

This process was found to lead to a power-dependent, negative polarisation of the B-exciton emissions; and by detuning the wavelength to outside of the A-exciton resonance field, the researchers found that the upconversion signal vanished. The upconverted B-excitons were also found to emit cross-polarised light, where an increase in the polarisation was observed when the laser intensity was increased, and gave the researchers the first fingerprint of boson scattering for 2D excitons.

Sources:

Manca M., Glazov M. M., Robert C., Cadiz F., Taniguchi T., Watanabe K., Courtade E., Amand T., Renucci P., Marie X., Wang G., Enabling valley selective exciton scattering in monolayer WSe2 through upconversion, Nature Communications, 2017, 8, 14927

Yu H., Cui X., Xu X., Yao W., Valley excitons in two-dimensional semiconductors, National Science Review, 2015, 2(1), 57-70

Zhu C. R., Zhang K., Glazov M., Urbaszek B., Amand T., Ji W. Z., Liu B. L., Marie X., Exciton Valley Dynamics probed by Kerr Rotation in WSe2 Monolayers, Physical Review B, 2014, 90, 161302

Wang G., Bouet D., Lagarde M., Vidal A., Balocchi A., Amand T., Marie X., Urbaszek B., Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2, Physical Review B, 2014, 90, 075413

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