Monolayer Semiconductors Research Articles

Room-Temperature Exciton Polaritons in a Monolayer Molecular Crystal.

Strong coupling between excitons and photons in optical microcavities leads to the formation of exciton polaritons, which maintain both the coherence of light and the interaction of matter. Recently, atomically thin monolayer semiconductors with a large exciton oscillator strength and high exciton binding energy have been widely used for realizing room-temperature exciton polaritons. Here, we demonstrated room-temperature exciton polaritons with a monolayer molecular crystal. The molecular monolayers behave as J-aggregates with comparable oscillator strength and narrow line width as inorganic monolayers, enabling exciton-photon strong coupling at the monolayer limit. Moreover, the coupling strength can be tuned systematically via engineering the in-plane polarization or by using a vertical stack of multiple molecular monolayers. Our research provides a new material platform for realizing strong light-matter interactions inside optical microcavities at room temperature and may motivate the development of molecular-crystal-based exciton-polaritonic devices with novel functions and new possibilities.

Increased Formation of Trions and Charged Biexcitons by Above-Gap Excitation in Single-layer WSe2.

Two-dimensional semiconductors exhibit pronounced many-body effects and intense optical responses due to strong Coulombic interactions. Consequently, subtle differences in photoexcitation conditions can strongly influence how the material dissipates energy during thermalization. Here, using multiple excitation spectroscopies, we show that a distinct thermalization pathway emerges at elevated excitation energies, enhancing the formation of trions and charged biexcitons in single-layer WSe2 by up to 2× and 5× , respectively. Power- and temperature-dependent measurements lend insights into the origin of the enhancement. These observations underscore the complexity of excited state relaxation in monolayer semiconductors, provide insights for the continued development of carrier thermalization models, and highlight the potential to precisely control excitonic yields and probe nonequilibrium dynamics in 2D semiconductors.

Photoinduced charge transfer and excitonic energy transfer in the CuInS2/ZnS/MoS2 heterotrilayer

Photo-induced charge transfer is the key process of many applications such as photovoltaics, photodetection, and light-emitting devices. With the outgrowth of a new class of low-dimensional semiconductors, i.e., monolayer transition metal dichalcogenides and semiconductor nanocrystals, charge transfer at the 2D/0D heterostructures has drawn many efforts because of the outstanding optical and electrical properties. This paper studies the dynamics of excitons of the CuInS2/ZnS/MoS2 heterotrilayer through femtosecond time-resolved transient absorption spectroscopy. The electron and hole transfer are observed by selectively exciting the electrons with tunable pump wavelengths. The exciton lifetimes are obtained on the picosecond scale. This work provides clues on exploring the non-toxic optoelectronic devices based on the CuInS2/ZnS/MoS2 heterotrilayer.

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

Solid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (&lt; 500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s‐1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

Enhanced spin Hall ratio in two-dimensional semiconductors

The conversion efficiency from charge current to spin current via the spin Hall effect is evaluated by the spin Hall ratio (SHR). Through state-of-the-art ab initio calculations involving both charge conductivity and spin Hall conductivity, we report the SHRs of the III-V monolayer family, revealing an ultrahigh ratio of 0.58 in the hole-doped GaAs monolayer. In order to find more promising 2D materials, a descriptor for high SHR is proposed and applied to a high-throughput database, which provides the fully relativistic band structures and Wannier Hamiltonians of 216 exfoliable monolayer semiconductors and has been released to the community. Among potential candidates for high SHR, the MXene monolayer Sc2CCl2 is identified with the proposed descriptor and confirmed by computation, demonstrating the descriptor validity for high SHR materials discovery.

CMOS-compatible strain engineering for monolayer semiconductor transistors

CMOS-compatible strain engineering for monolayer semiconductor transistors

Biodetector for chlordane using doped InP3 monolayers: a density functional theory study.

Chlordane is a serious pollutant in the environment, and it is necessary to monitor chlordane levels using biodetectors. We performed first-principles calculations to investigate the adsorption of chlordane on Ag, Pd, and Au doped InP3 semiconductor monolayers. The results indicated that the adsorption energies of chlordane adsorbed on Ag, Pd, and Au doped InP3 are -7.961 eV, -6.328 eV, and -7.889 eV respectively. The band gaps of the doped InP3 monolayers underwent drastic changes after coming in contact with chlordane, the largest change in band gap occurred for Pd-doped InP3, where the band gap changed from 0.024 eV to 0.335 eV. The large change in band gap shows that the monolayer is sensitive to the molecule, making it a good biodetector. Our results conclude that Pd-doped InP3 stands out as the most promising biodetector for chlordane. This result will benefit environmental experimentalists in their further research.

Defect‐Mediated Efficient and Tunable Emission in van der Waals Integrated Light Sources at Room Temperature

AbstractDespite defect‐mediated states in monolayer semiconductors have been considered as efficient emitters in cryogenic conditions, they are severely quenched at room temperature (RT) because of multiple thermal‐induced non‐radiative channels, hindering their practical use. Here, robust and tunable defect‐mediated emissions at RT, designated as D1 (1.82 eV) and D2 (1.62 eV), are discovered in monolayer WS2 through argon plasma treatment, which, via multiple corroborative experiments, are attributed to distinct physical processes: bound excitons associated with sulfur vacancies (Vs) and the band‐to‐acceptor recombination, respectively. Remarkably, the defective sample exhibits over ten‐fold increase in both photoluminescence quantum yield and full‐width at half‐maximum (FWHM) compared to its intrinsic counterpart. Leveraging these pronounced edges, light‐emitting diodes (LEDs) functioning at RT achieve broadband (FWHM: 490 meV) and continuously tunable emission from 1.6 to 2.0 eV, as well as the highest electroluminescence (EL) external quantum efficiency (EQE) of ≈1.39% among transient LEDs based on monolayer semiconductors to date, thereby unveiling a new strategy for emission tailoring at the atomic‐scale limit.

Simultaneously Enhancing Brightness and Purity of WSe2 Single Photon Emitter Using High-Aspect-Ratio Nanopillar Array on Metal.

A monolayer semiconductor transferred on nanopillar arrays provides site-controlled, on-chip single photon emission, which is a scalable light source platform for quantum technologies. However, the brightness of these emitters reported to date often falls short of the perceived requirement for such applications. Also, the single photon purity usually degrades as the brightness increases. Hence, there is a need for a design methodology to achieve an enhanced emission rate while maintaining high single photon purity. By using WSe2 on high-aspect-ratio (∼3, at least 2-fold higher than previous reports) nanopillar arrays, here we demonstrate >10 MHz single photon emission rate in the 770-800 nm band that is compatible with quantum memory and repeater networks (Rb-87-D1/D2 lines) and satellite quantum communication. The emitters exhibit excellent purity (even at high emission rates) and improved out-coupling due to the use of a gold back reflector that quenches the emission away from the nanopillar.

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Ultrafast Polarization-Resolved Phonon Dynamics in Monolayer Semiconductors.

Monolayer transition metal dichalcogenide semiconductors exhibit unique valleytronic properties interacting strongly with chiral phonons that break time-reversal symmetry. Here, we observed the ultrafast dynamics of linearly and circularly polarized E'(Γ) phonons at the Brillouin zone center in single-crystalline monolayer WS2, excited by intense, resonant, and polarization-tunable terahertz pulses and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons producing directional sum-frequency generation from the incoherent phonon population emitting scattered photons. The longer incoherent population lifetime than what was expected from coherence lifetime indicates that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, the faster depolarization rate in circular bases than in linear bases suggests that the eigenstates are linearly polarized due to lattice anisotropy. Our results provide crucial information for improving the lifetime of chiral phonons in two-dimensional materials and potentially facilitate dynamic control of spin-orbital polarizations in quantum materials.

Photoluminescence of Chemically and Electrically Doped Two-Dimensional Monolayer Semiconductors.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers exhibit unique physical properties, such as self-terminating surfaces, a direct bandgap, and near-unity photoluminescence (PL) quantum yield (QY), which make them attractive for electronic and optoelectronic applications. Surface charge transfer has been widely used as a technique to control the concentration of free charge in 2D semiconductors, but its estimation and the impact on the optoelectronic properties of the material remain a challenge. In this work, we investigate the optical properties of a WS2 monolayer under three different doping approaches: benzyl viologen (BV), potassium (K), and electrostatic doping. Owing to the excitonic nature of 2D TMDC monolayers, the PL of the doped WS2 monolayer exhibits redshift and a decrease in intensity, which is evidenced by the increase in trion population. The electron concentrations of 3.79×1013 cm-2, 6.21×1013 cm-2, and 3.12×1012 cm-2 were measured for WS2 monolayers doped with BV, K, and electrostatic doping, respectively. PL offers a direct and versatile approach to probe the doping effect, allowing for the measurement of carrier concentration in 2D monolayer semiconductors.

Room-temperature field-free valley exciton splitting in monolayer semiconductors achieved by perpendicular magnetic proximity effect

Room-temperature field-free valley exciton splitting in monolayer semiconductors achieved by perpendicular magnetic proximity effect

Ultralow-Frequency Tip-Enhanced Raman Scattering Discovers Nanoscale Radial Breathing Mode on Strained 2D Semiconductors.

Collective excitations including plasmons, magnons, and layer-breathing vibration modes emerge at an ultralow frequency (<1 THz) and are crucial for understanding van der Waals materials. Strain at the nanoscale can drastically change the property of van der Waals materials and create localized states like quantum emitters. However, it remains unclear how nanoscale strain changes collective excitations. Herein, ultralow-frequency tip-enhanced Raman spectroscopy (TERS) with sub-10nm resolution under ambient conditions is developed to explore the localized collective excitation on monolayer semiconductors with nanoscale strains. A new vibrational mode is discovered at around 12 cm-1 (0.36 THz) on monolayer MoSe2 nanobubbles and it is identified as the radial breathing mode (RBM) of the curved monolayer. The correlation is determined between the RBM frequency and the strain by simultaneously performing deterministic nanoindentation and TERS measurement on monolayer MoSe2. The generality of the RBM in nanoscale curved monolayer WSe2 and bilayer MoSe2 is demonstrated. Using the RBM frequency, the strain of the monolayer MoSe2 on the nanoscale can be mapped. Such an ultralow-frequency vibration from curved van der Waals materials provides a new approach to study nanoscale strains and points to more localized collective excitations to be discovered at the nanoscale.

Ultrafast Dynamics of Bloch Surface Wave Polaritons in Large-Area 2D Semiconductor Monolayers at Room Temperature.

The dynamics of strongly coupled polariton systems integrated with 2D transition metal dichalcogenides (TMDs) is key to enabling efficient coherent processes and achieving high-performance TMD-based polaritonic devices, such as ultralow-threshold polariton lasers and ultrafast optical switches. However, there has been a lack of a comprehensive understanding of the excited state dynamics in TMD-based polariton systems. In this work, ultrafast pump-probe optical spectroscopy is used to investigate the room temperature dynamics of the polariton systems consisting of TMD monolayer excitons strongly coupled with Bloch surface waves (BSWs) supported by all-dielectric photonic structures. The transient response is found for both above-exciton energy pumping and polariton-resonant pumping. The excited state population and ultrafast coherent coupling of the exciton reservoir and lower polariton (LP) branch are observed for resonant pumping. Moreover, it is found that the transient response of the LP first decays on a short-time scale of 0.15-0.25ps compared to the calculated intrinsic lifetime of 0.11-0.20ps, and is followed by a longer decay (>100ps) due to the dynamical evolution of the exciton reservoir. The results provide a fundamental understanding of the dynamics of TMD-based polariton systems while showing the potential for achieving efficient coherent optical processes for device applications.

Exchange Energy of the Ferromagnetic Electronic Ground State in a Monolayer Semiconductor.

Mobile electrons in the semiconductor monolayer MoS_{2} form a ferromagnetic state at low temperature. The Fermi sea consists of two circles: one at the K point, the other at the K[over ˜] point, both with the same spin. Here, we present an optical experiment on gated MoS_{2} at low electron density in which excitons are injected with known spin and valley quantum numbers. The resulting trions are identified using a model which accounts for the injection process, the formation of antisymmetrized trion states, electron-hole scattering from one valley to the other, and recombination. The results are consistent with a complete spin polarization. From the splittings between different trion states, we measure the exchange energy Σ, the energy required to flip a single spin within the ferromagnetic state, as well as the intervalley Coulomb exchange energy J. We determine Σ=11.2 meV and J=5 meV at n=1.5×10^{12} cm^{-2} and find that J depends strongly on the electron density n.

Electrical control of excitonic oscillator strength and spatial distribution in a monolayer semiconductor

Electrical control of excitonic oscillator strength and spatial distribution in a monolayer semiconductor

Monolayer Semiconductor Superlattices with High Optical Absorption.

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS2 monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.

Large-Scale Analysis of Defects in Atomically Thin Semiconductors using Hyperspectral Line Imaging.

Point defects play a crucial role in determining the properties of atomically thin semiconductors. This work demonstrates the controlled formation of different types of defects and their comprehensive optical characterization using hyperspectral line imaging (HSLI). Distinct optical responses are observed in monolayer semiconductors grown under different stoichiometries using metal-organic chemical vapor deposition. HSLI enables the simultaneous measurement of 400 spectra, allowing for statistical analysis of optical signatures at close to a centimeter scale. The study discovers that chalcogen-rich samples exhibit remarkable optical uniformity due to reduced precursor accumulation compared to the metal-rich case. The utilization of HSLI as a facile and reliable characterization tool pushes the boundaries of potential applications for atomically thin semiconductors in future devices.