Bonding exercise: Quantifying biexciton binding energy
Applications in future devices based on biexcitons in TMDCs

Date:
March 2, 2022
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
A rare spectroscopy technique directly quantifies the energy
required to bind two excitons together. The experiment harnessed
interactions between real and virtual states to 'switch' the
electronic state of an atomically-thin (2D) material. As well as
improving fundamental understanding of biexciton dynamics and exotic
new quantum materials, the study aids work towards biexciton-based
devices such as compact lasers and chemical-sensors, and the search
for future low-energy electronics based on topological materials.



FULL STORY ==========================================================================
A rare spectroscopy technique performed at Swinburne University of
Technology directly quantifies the energy required to bind two excitons together, providing for the first time a direct measurement of the
biexciton binding energy in WS2.


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As well as improving our fundamental understanding of biexciton dynamics
and characteristic energy scales, these findings directly inform those
working to realise biexciton-based devices such as more compact lasers
and chemical- sensors.

The study also brings closer exotic new quantum materials, and quantum
phases, with novel properties.

The study is a collaboration between FLEET researchers at Swinburne and
the Australian National University.

Understanding Excitons Particles of opposite charge in close proximity
will feel the 'pull' of electrostatic forces, binding them together. The electrons of two hydrogen atoms are pulled in by opposing protons to
form H2, for example, while other compositions of such electrostatic (Coulomb-mediated) attraction can result in more exotic molecular states.



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The optical properties of semiconductors are frequently dominated by the behaviour of 'excitons'. These compound quasi-particles can be created via
the excitation of an electron from the valence to the conduction band,
with the negatively-charged conduction electron then electrostatically
binding to the positively-charged vacancy (known as a hole) its excitation
left in the valence band.

Understanding the interactions between excitons is crucial for realising
many of the proposed device applications, and in bulk materials they are
quite well understood. However, when things are reduced to two dimensions,
the ways they can interact change, and important quantum effect can
come into play. Monolayer semiconductors such as WS2 are introducing a materials revolution due to the novel properties uncovered by research
like this.

A Materials Revolution Due to the reduced dimensionality of
two-dimensional materials, the binding energy of excitons and exciton
complexes like biexcitons are greatly enhanced.

This increased binding energy makes the biexcitons more accessible, even
at room temperature, and introduces the possibility of using biexcitons
flowing in novel materials as the basis for a range of low-energy future technologies.

Atomically-thin transition metal dichalcogenides (TMDCs) like WS2 are
a family of semiconducting, insulating and semi-metallic materials that
have gained a significant amount of attention from researchers in recent
years for use in a future generation of 'beyond CMOS' electronics.



========================================================================== "Before we can apply these two-dimensional materials to the next
generation of low-energy electronic devices, we need to quantify the fundamental properties that drive their functionality," says lead author Mitchell Conway, a PhD student from Swinburne University of Technology (Australia).

A New Way to Quantify Biexciton Binding Energy The need to understand
the properties of biexcitons has driven significant conjecture and investigation in the semiconductor research community of their presence, binding energy, and nature. Attempts have been made to investigate how
much energy is required to separate the two excitons in a biexciton,
the obvious way being a comparison between the energy of the bound and
unbound excitons. Yet, this is not what is typically done.

The Swinburne-led study has identified the optically-accessible
biexciton in the atomically-thin TMDC tungsten disulphide (WS2). To unambiguously measure biexcitonic signatures, the team of researchers
employed a specific sequence of ultrashort optical pulses with a precisely controlled phase relation and well- defined wave-vectors.

"By using multiple pulses with a high degree of precision we can
selectively and directly probe the doubly excited biexciton state,
while eliminating any contributions from singly excited exciton states,"
says corresponding author Prof Jeff Davis (Swinburne).

"This ability to directly excite the biexciton is inaccessible to
more common techniques such as photoluminescence spectroscopy," says
Prof Davis.

The technique the team used is known as 'two-quantum multidimensional
coherent spectroscopy' (2Q-MDCS), which enables a direct experimental measurement of the biexciton binding energy. When the biexciton is
observed using 2Q-MDCS, a signal from an exciton pair that is interacting
but unbound is also generated, referred to as 'correlated excitons'.

"The energy difference between the biexciton peak and the correlated two- exciton peak is the best means to measure biexciton binding energy,"
Mitchell explains. "This was an exciting observation, since other
spectroscopic techniques don't observe these correlated excitons."
Techniques previously used to identify the biexciton are limited to
measuring photons from the biexciton to exciton transition. These
transitions may not reflect the precise energy of either relative to
the ground state.

In addition, the study identified the nature of the biexciton in
monolayer WS2.

The biexciton they observed was composed of two bright excitons
with opposite spin, which in WS2 is referred to as a 'bright-bright intervalley' biexciton.

In contrast, photoluminescence measurements reporting biexcitons in
monolayer WS2 are unable to identify the specific excitons involved, but
are typically assumed to involve bright exciton and one "dark" exciton,
due to the rapid relaxation into these lower energy exciton states that
don't absorb or emit light.

The ability to accurately identify biexciton signatures in monolayer semiconductors may also play a key role in the development of quantum
materials and quantum simulators. Higher-order electrostatic correlations provide a platform to construct coherent combinations of quantum states
and potentially tune the interactions in order to realise quantum phases
of matter that are still not well understood.

========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. M A Conway, J B Muir, S K Earl, M Wurdack, R Mishra, J O Tollerud,
J A
Davis. Direct measurement of biexcitons in monolayer WS2. 2D
Materials, 2022; 9 (2): 021001 DOI: 10.1088/2053-1583/ac4779 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220302092723.htm

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