arXiv subjects

Shao-Yu Chen

Publications and source records attributed to Shao-Yu Chen.

At least 19 recordsLinked to original sources

arXiv2025

Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides

In this work, we report a pronounced light upconversion in few-layer transition metal dichalcogenides. Our joint theory-experiment study attributes the upconversion photoluminescence to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons, which can have a high upconversion efficiency. Additionally, the upconversion photoluminescence is generic in MoS2, MoSe2, WS2, and WSe2, showing a high tuneability from green to ultraviolet light.

cond-mat.mtrl-sci↗

arXiv2023

Defects, band bending and ionization rings in MoS2

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS2 however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy (STM) and STS to study embedded sulphur vacancies in bulk MoS2 crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending (TIBB). The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.

cond-mat.mtrl-sci↗

arXiv2023

The key role of non-local screening in the environment-insensitive exciton fine structures of transition-metal dichalcogenide monolayers

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe$_2$-monolayers, one of the best known two-dimensional (2D) transition-metal dichalcogenides (TMD's), in various dielectric-layer environments by solving the first-principles-based Bethe-Salpeter equation. While the physical and electronic properties of atomically thin nano-materials are normally sensitive to the variation of surrounding environment, our studies reveal that the influence of dielectric environment on the exciton fine structures of TMD-ML's is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role to suppress the factor of dielectric environment and drastically shrink the fine structure splittings between bright exciton (BX) states and various dark exciton (DX) states of TMD-ML's. The intriguing non-locality of screening in 2D materials can be manifested by the measurable {\it non-linear} correlation between the BX-DX splittings and exciton binding energies with varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML's suggest the robustness of prospective dark-exciton-based opto-electronics against the inevitable variation of inhomogeneous dielectric environment.

cond-mat.mes-hall↗

arXiv2022

P-type Ohmic contact to monolayer WSe2 field-effect transistors using high electron affinity amorphous MoO3

1 School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia 2 Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia 3 Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States 5 School of Physics, the University of Melbourne, Melbourne, VIC 3010, Australia 6 Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 7 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 8 School of Physics, University of New South Wales, 2052 Sydney, Australia 9 Center of Condensed Matter Sciences and Center of Atomic Initiative for New Material, National Taiwan University, Taipei 106, Taiwan 10 Monash Centre for Atomically Thin Materials, Monash University, Clayton, 3800, VIC, Australia

cond-mat.mes-hall↗

arXiv2020

Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe$_2$

Monolayer transition metal dichalcogenides are a promising platform to investigate many-body interactions of excitonic complexes. In monolayer tungsten diselenide, the ground-state exciton is dark (spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons to form. Interactions between the dark exciton species and the optically accessible bright exciton (X) are likely to play significant roles in determining the optical properties of X at high power, as well as limiting the ultimate exciton densities that can be achieved, yet so far little is known about these interactions. Here, we demonstrate long-lived dense populations of momentum-indirect intervalley ($X_K$) and spin-indirect intravalley (D) dark excitons by time-resolved photoluminescence measurements (Tr-PL). Our results uncover an efficient inter-state conversion between X to D excitons through the spin-flip process and the one between D and $X_K$ excitons mediated by the exchange interaction (D + D to $X_K$ + $X_K$). Moreover, we observe a persistent redshift of the X exciton due to strong excitonic screening by $X_K$ exciton with a response time in the timescale of sub-ns, revealing a non-trivial inter-state exciton-exciton interaction. Our results provide a new insight into the interaction between bright and dark excitons, and point to a possibility to employ dark excitons for investigating exciton condensation and the valleytronics.

cond-mat.mes-hall↗

arXiv2020

Two-dimensional Ga$_2$O$_3$ glass: a large scale passivation and protection material for monolayer WS$_2$

Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all-dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, we show that the novel, ultrathin Ga$_2$O$_3$ glass is an ideal centimeter-scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga$_2$O$_3$ capping of commercial grade WS$_2$ monolayers outperforms hBN in both scalability and optical performance at room temperature. These properties make Ga$_2$O$_3$ highly suitable for large scale passivation and protection of monolayer TMDCs in functional heterostructures.

cond-mat.mtrl-sci↗

arXiv2020

Ground and Excited Exciton Polarons in Monolayer MoSe2

Monolayer transition metal dichalcogenide semiconductors, with versatile experimentally accessible exciton species, offer an interesting platform for investigating the interaction between excitons and a Fermi sea of charges. Using hexagonal boron nitride encapsulated monolayer MoSe2, we study the impact of charge density tuning on the ground and excited Rydberg states in the atomic layer. Consistent excitonpolaron behavior is revealed in both photoluminescence and reflection spectra of the A exciton 1s (A:1s) Rydberg state, in contrast to previous studies. The A:2s Rydberg state provides an opportunity to understand such interactions with greatly reduced exciton binding energy. We found that the impact of the Fermi sea becomes much more dramatic. With a photoluminescence upconversion technique, we further verify the 2s polaron-like behavior for the repulsive branch of B:2s exciton whose energy is well above the bare bandgap. Our studies show that the polaron-like interaction features are quite generic and highly robust, offering key insights into the dressed manybody state in a Fermi sea.

cond-mat.mes-hall↗

arXiv2019

Oxidized-monolayer Tunneling Barrier for Strong Fermi-level Depinning in Layered InSe Transistors

In 2D-semiconductor-based field-effect transistors and optoelectronic devices, metal-semiconductor junctions are one of the crucial factors determining device performance. The Fermi-level (FL) pinning effect, which commonly caused by interfacial gap states, severely limits the tunability of junction characteristics, including barrier height and contact resistance. A tunneling contact scheme has been suggested to address the FL pinning issue in metal-2D-semiconductor junctions, whereas the experimental realization is still elusive. Here, we show that an oxidized-monolayer-enabled tunneling barrier can realize a pronounced FL depinning in indium selenide (InSe) transistors, exhibiting a large pinning factor of 0.5 and a highly modulated Schottky barrier height. The FL depinning can be attributed to the suppression of metal- and disorder-induced gap states as a result of the high-quality tunneling contacts. Structural characterizations indicate uniform and atomically thin surface oxidation layer inherent from nature of van der Waals materials and atomically sharp oxide-2D-semiconductor interfaces. Moreover, by effectively lowering the Schottky barrier height, we achieve an electron mobility of 2160 cm$^2$/Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The realization of strong FL depinning in high-mobility InSe transistors with the oxidized monolayer presents a viable strategy to exploit layered semiconductors in contact engineering for advanced electronics and optoelectronics.

cond-mat.mes-hall↗

arXiv2019

Luminescent emission of excited Rydberg excitons from monolayer WSe2

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of g-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

cond-mat.mes-hall↗

arXiv2019

Friend, Collaborator, Student, Manager: How Design of an AI-Driven Game Level Editor Affects Creators

Machine learning advances have afforded an increase in algorithms capable of creating art, music, stories, games, and more. However, it is not yet well-understood how machine learning algorithms might best collaborate with people to support creative expression. To investigate how practicing designers perceive the role of AI in the creative process, we developed a game level design tool for Super Mario Bros.-style games with a built-in AI level designer. In this paper we discuss our design of the Morai Maker intelligent tool through two mixed-methods studies with a total of over one-hundred participants. Our findings are as follows: (1) level designers vary in their desired interactions with, and role of, the AI, (2) the AI prompted the level designers to alter their design practices, and (3) the level designers perceived the AI as having potential value in their design practice, varying based on their desired role for the AI.

cs.HC↗

arXiv2018

Coulomb-bound four- and five-particle valleytronic states in an atomically-thin semiconductor

As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are so far elusive and controversial. Here we present experimental evidence for valleytronic four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tungsten diselenide samples. Through charge doping, thermal activation, and magnetic-field tuning measurements, we determined that the biexciton and exciton-trion optical emissions are bound with respect to the bright exciton and the trion respectively. Further, both the biexciton and the exciton-trion are intervalley bound states involving dark excitons, giving rise to emissions with large, negative valley polarizations in contrast to that of the well-known two-particle excitons. Our studies provide new opportunities for building valleytronic quantum devices harnessing high-order excitations.

cond-mat.mes-hall↗

arXiv2017

Superior valley polarization and coherence of 2s excitons in monolayer WSe2

We report experimental observation of 2s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2s luminescence is highly robust and persists up to 150K, offering a new quantum entity for manipulating the valley degree of freedom. Remarkably, the 2s exciton displays superior valley polarization and coherence than 1s under similar experimental conditions. This observation provides evidence that the Coulomb-exchange-interaction-driven valley-depolarization process, the Maialle-Silva-Sham mechanism, plays an important role in valley excitons of monolayer transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2016

Intrinsic Phonon Bands in High-Quality Monolayer T' Molybdenum Ditelluride

The topologically nontrivial and chemically functional distorted octahedral (T') transition metal dichalcogenides (TMDCs) are a type of layered semimetal that has attracted significant recent attention. However, the properties of monolayer (1L) T'-TMDC, a fundamental unit of the system, is still largely unknown due to rapid sample degradation in air. Here we report that well-protected 1L CVD T'-MoTe2 exhibits sharp and robust intrinsic Raman bands, with intensities about one order of magnitude stronger than those from bulk T'-MoTe2. The high-quality samples enabled us to reveal the set of all nine even-parity zone-center optical phonons, providing reliable fingerprints for the previously elusive crystal. By performing light polarization and crystal orientation resolved scattering analysis, we can effectively distinguish the intrinsic modes from Te-metalloid-like modes A (~122 cm-1) and B (~141 cm-1) which are related to the sample degradation. Our studies offer a powerful non-destructive method for assessing sample quality and for monitoring sample degradation in situ, representing a solid advance in understanding the fundamental properties of 1L-T'-TMDCs.

cond-mat.mtrl-sci↗

arXiv2016

Activation of new Raman modes by inversion symmetry breaking in type II Weyl semimetal candidate T'-MoTe2

We synthesized distorted octahedral(T') molybdenum ditelluride (MoTe2) and investigated its vibrational properties with Raman spectroscopy, density functional theory and symmetry analysis. Compared to the results from high temperature centrosymmetric monoclinic (T'mo) phase, four new Raman bands emerge in the low temperature orthorhombic (T'or) phase, which was recently predicted to be a type II Weyl semimetal. Crystal-angle-dependent, light-polarization-resolved measurements indicate that all the observed Raman peaks belong to two categories: those vibrating along the zigzag Mo atomic chain (z-mode) and those vibrating in the mirror plane (m-mode) perpendicular to the zigzag chain. Interestingly the low energy shear z-mode and shear m-mode, absent from the T'mo spectra, become activated when sample cooling induces a phase transition to the T'or crystal structure. We interpret this observation as a consequence of inversion-symmetry breaking, which is crucial for the existence of Weyl fermions in the layered crystal. Our temperature dependent Raman measurements further show that both the high energy m-mode at 130 cm-1 and the low energy shear m-mode at 12 cm-1 provide useful gauges for monitoring the broken inversion symmetry in the crystal.

cond-mat.mes-hall↗

arXiv2016

Raman scattering and anomalous Stokes anti-Stokes ratio in MoTe2 atomic layers

Stokes and anti-Stokes Raman scattering are performed on atomic layers of hexagonal molybdenum ditelluride (MoTe2), a prototypical transition metal dichalcogenide (TMDC) semiconductor. The data reveal all six types of zone center optical phonons, along with their corresponding Davydov splittings, which have been challenging to see in other TMDCs. We discover that the anti-Stokes Raman intensity of the low energy layer-breathing mode becomes more intense than the Stokes peak under certain experimental conditions, and find the effect to be tunable by excitation frequency and number of atomic layers. These observations are interpreted as a result of resonance effects arising from the C excitons in the vicinity of the Brillouin zone center in the photon-electron-phonon interaction process.

cond-mat.mtrl-sci↗

arXiv2016

Inversion-symmetry-breaking-activated shear Raman bands in $T'$-MoTe$_2$

Type-II Weyl fermion nodes, located at the touching points between electron and hole pockets, have been recently predicted to occur in distorted octahedral ($T'$) transition metal dichalcogenide semimetals, contingent upon the condition that the layered crystal has the noncentrosymmetric orthorhombic ($T'_{or}$) stacking. Here, we report on the emergence of two shear Raman bands activated by inversion symmetry breaking in $T'$-MoTe$_2$ due to sample cooling. Polarization and crystal orientation resolved measurements further point to a phase transition from the monoclinic ($T'_{mo}$) structure to the desired $T'_{or}$ lattice. These results provide spectroscopic evidence that low-temperature $T'$-MoTe$_2$ is suitable for probing type-II Weyl physics.

cond-mat.mes-hall↗

arXiv2015

Extrinsic Origin of Persistent Photoconductivity in Monolayer MoS2 Field Effect Transistors

Recent discoveries of the photoresponse of molybdenum disulfide (MoS2) have shown the considerable potential of these two-dimensional transition metal dichalcogenides for optoelectronic applications. Among the various types of photoresponses of MoS2, persistent photoconductivity (PPC) at different levels has been reported. However, a detailed study of the PPC effect and its mechanism in MoS2 is still not available, despite the importance of this effect on the photoresponse of the material. Here, we present a systematic study of the PPC effect in monolayer MoS2 and conclude that the effect can be attributed to random localized potential fluctuations in the devices. Notably, the potential fluctuations originate from extrinsic sources based on the substrate effect of the PPC. Moreover, we point out a correlation between the PPC effect in MoS2 and the percolation transport behavior of MoS2. We demonstrate a unique and efficient means of controlling the PPC effect in monolayer MoS2, which may offer novel functionalities for MoS2-based optoelectronic applications in the future.

cond-mat.mes-hall↗

arXiv2015

Demonstration of distinct semiconducting transport characteristics of monolayer graphene functionalized via plasma activation of substrate surfaces

We report semiconducting behavior of monolayer graphene enabled through plasma activation of substrate surfaces. The graphene devices are fabricated by mechanical exfoliation onto pre-processed SiO2/Si substrates. Contrary to pristine graphene, these graphene samples exhibit a transport gap as well as nonlinear transfer characteristics, a large on/off ratio of 600 at cryogenic temperatures, and an insulating-like temperature dependence. Raman spectroscopic characterization shows evidence of sp3 hybridization of C atoms in the samples of graphene on activated SiO2/Si substrates. We analyze the hopping transport at low temperatures, and weak localization observed from magnetotransport measurements, suggesting a correlation between carrier localization and the sp3-type defects in the functionalized graphene. The present study demonstrates the functionalization of graphene using a novel substrate surface-activation method for future graphene-based applications.

cond-mat.mes-hall↗