Many-body
interactions in photoexcited semiconductors can bring
about strongly interacting electronic states, culminating in the fully
ionized matter of electron–hole plasma (EHP) and electron–hole
liquid (EHL). These exotic phases exhibit unique electronic properties,
such as metallic conductivity and metastable high photoexcitation
density, which can be the basis for future transformative applications.
However, the cryogenic condition required for its formation has limited
the study of dense plasma phases to a purely academic pursuit in a
restricted parameter space. This paradigm can potentially change with
the recent experimental observation of these phases in atomically
thin MoS2 and MoTe2 at room temperature. A fundamental
understanding of EHP and EHL dynamics is critical for developing novel
applications on this versatile layered platform. In this work, we
studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results
reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP
formation. Within the free-standing geometry, photoexcitation is accompanied
by an unconstrained thermal expansion, resulting in a direct-to-indirect
gap electronic transition at a critical lattice spacing and fluence.
This dramatic altering of the material’s energetic landscape
extends carrier lifetimes by 2 orders of magnitude and allows the
density required for EHP formation. The result is a stable dense plasma
state that is sustained with modest optical photoexcitation. Our findings
pave the way for novel applications based on dense plasma states in
two-dimensional semiconductors.
Atomically thin (1L)-MoS 2 emerged as a direct band gap semiconductor with potential optical applications. The photoluminescence (PL) of 1L-MoS 2 degrades due to aging-related defect formation. The passivation of these defects leads to substantial improvement in optical properties. Here, we report the enhancement of PL on aged 1L-MoS 2 by laser treatment. Using photoluminescence and Raman spectroscopy in a gas-controlled environment, we show that the enhancement is associated with efficient adsorption of oxygen on existing sulfur vacancies preceded by removal of adsorbates from the sample's surface. Oxygen adsorption depletes negative charges, resulting in suppression of trions and improved neutral exciton recombination. The result is a 6-to 8-fold increase in PL emission. The laser treatment in this work does not cause any measurable damage to the sample as verified by Raman spectroscopy, which is important for practical applications. Surprisingly, the observed PL enhancement is reversible by both vacuum and ultrafast femtosecond excitation. While the former approach allows switching a designed micropattern on the sample ON and OFF, the latter provides a controllable mean for accurate PL tuning, which is highly desirable for optoelectronic and gas sensing applications.
Light-matter interactions can create and manipulate collective many-body phases in solids 1-3 , which are promising for the realization of emerging quantum applications. However, in most cases these collective quantum states are fragile, with a short decoherence and dephasing time, limiting their existence to precision tailored structures under delicate conditions such as cryogenic temperatures and/or high magnetic fields. In this work, we discovered that the archetypal hybrid perovskite, MAPbI3 thin films, exhibit such a collective coherent quantum many-body phase, namely superfluorescence, at 78 K and above. Pulsed laser excitation first creates a population of high energy electron-hole pairs, which quickly
Nonuniform strain on multilayer transition-metal
dichalcogenide
(TMDC) nanosheets is an exciting path toward practical optoelectronic
devices, as it combines the advantages of localized control of optical
and electronic properties with ease of fabrication. However, the weaker
photoluminescence (PL) due to their indirect nature poses a challenge
to their application. Here, we demonstrate extraordinary enhancement
of PL from multilayer MoS2 nanosheets under nonuniform
strain generated by nanopillars. We observe charge and exciton funneling
to the pillar strain apex. The screening from the increased exciton
and charge density lowers the exciton binding energy and renormalizes
the band gap. Hence, we attribute the dramatic increase in PL to dissociation
of bound excitons to free electron–hole pairs, showing that
nonuniform strain on TMDC nanosheets can effectively manipulate the
nature of light–matter interaction in these atomically thin
materials for application in novel strain-engineered optoelectronics.
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