Signatures of Valley Drift in the Diversified Band Dispersions of Bright, Gray, and Dark Excitons in Molybdenum Disulfide (MoS2) Monolayers Under Uni-axial Strains: A First-Principles Study

Основні поняття

Uni-axial strain applied to molybdenum disulfide (MoS2) monolayers leads to diversified band dispersions of bright, gray, and dark excitons, impacting their optical properties and diffusion behavior, as revealed by first-principles calculations.

Анотація

Bibliographic Information: Shih, C.-H.; Peng, G.-H.; Lo, P.-Y.; Li, W.-H.; Xu, M.-L.; Chien, C.-H.; Cheng, S.-J. Signatures of valley drift in the diversified band dispersions of bright, gray, and dark excitons in MoS2 monolayers under uni-axial strains. arXiv, 2024, 2410.03209.
Research Objective: This study investigates the impact of uni-axial strain on the excitonic properties of molybdenum disulfide (MoS2) monolayers using first-principles calculations based on the Bethe-Salpeter equation (BSE).
Methodology: The researchers employed density functional theory (DFT) to calculate the electronic band structure of strained MoS2 monolayers. They then used these results to solve the BSE and obtain the excitonic fine structures and band dispersions for different types of excitons (bright, gray, and dark) under varying uni-axial strain conditions.
Key Findings: The study reveals that uni-axial strain significantly affects the band dispersions of excitons in MoS2 monolayers. While bright exciton dispersions remain relatively insensitive to strain, gray exciton masses decrease, and dark exciton dispersions transform into a Mexican-hat-like profile, exhibiting negative effective mass and strain-activated brightness. These changes are attributed to the interplay between strain-induced valley drift and electron-hole exchange interactions.
Main Conclusions: The authors conclude that uni-axial strain can be used to manipulate the optical and transport properties of different exciton types in MoS2 monolayers. The diversified band dispersions result in distinct exciton diffusivities and angle-resolved optical patterns, suggesting potential applications in spatially resolving different exciton species.
Significance: This research provides valuable insights into the strain-engineering of excitonic properties in 2D materials, paving the way for novel optoelectronic and valleytronic devices.
Limitations and Future Research: The study focuses on MoS2 monolayers, and further research is needed to explore the strain-dependent excitonic properties in other TMD materials. Experimental verification of the predicted optical and transport characteristics is crucial for future device applications.

Статистика

Applying a tensile uni-axial strain to a MoS2 monolayer increases its energy gap, transitioning from an intra-valley to an inter-valley indirect gap.
The calculated drift of the conduction valley under 5% tensile strain is 0.091˚A−1, while the valence valley drifts by 0.074˚A−1.
The unstrained MoS2 monolayer exhibits a bright exciton-dark exciton splitting of 14 meV.
The effective mass of the gray exciton along the strain axis drops from 1.455m0 to 0.125m0 under 5% tensile strain.
The dark exciton band exhibits a sign-reversed effective mass of -0.150m0 at the gamma point under 5% tensile strain.
The critical strain for the emergence of the Mexican-hat-like dispersion in the dark exciton band is approximately 1.7%.

Цитати

"The unequal momentum VDs of conduction and valence bands implies the possible momentum shift of exciton ground states and the strain-reshaped exciton band structures."
"In contrast to spin-allowed BXs, our first principles studies reveal that an imposed uni-axial stress directly impacts the spin-forbidden exciton states in both the band dispersions as well as the optical selection rules."
"This suggests the emergent importance of DX in nano-optics based on strained 2D materials."
"The predicted diversification in the effective masses of exciton reveals the possibility to spatially resolve the BX, GX, and DX in exciton transport experiments by means of imposing an uni-axial strain to a TMD-ML."

Ключові висновки, отримані з

Signatures of valley drift in the diversified band dispersions of bright, gray, and dark excitons in MoS2 monolayers under uni-axial strains

by Ching-Hung S... о arxiv.org 10-07-2024

https://arxiv.org/pdf/2410.03209.pdf

Глибші Запити

How would the findings of this study be affected by considering different substrates or encapsulating materials beyond hBN?

This is a very astute question, as the choice of substrate and encapsulating material can significantly influence the excitonic properties of 2D materials like MoS2. Here's a breakdown of how different substrates/encapsulants could affect the findings:
1. Dielectric Screening:

Impact: The surrounding dielectric environment directly affects the strength of Coulomb interactions within the MoS2 monolayer. hBN is known for its relatively large band gap and low dielectric constant, leading to weak dielectric screening and thus pronounced excitonic effects.
Using different materials:

High-κ dielectrics: Materials like HfO2 or high-κ polymers would increase dielectric screening. This would weaken the exciton binding energy, potentially reducing the bright-dark exciton splitting and diminishing the strain-induced diversification of exciton masses.
Other layered materials: Using different van der Waals materials, each with unique dielectric properties, could lead to a range of tunable excitonic behaviors.
2. Strain Transfer:

Impact: The efficiency of strain transfer from the substrate or encapsulant to the MoS2 layer is crucial. hBN, with its similar lattice structure to MoS2, allows for efficient strain transfer.
Using different materials:

Lattice mismatch: Substrates with significant lattice mismatch could lead to interfacial strain, defects, or even suppression of strain transfer, altering the predicted strain-dependent exciton behavior.
Substrate rigidity: Flexible substrates might not transfer strain as effectively as rigid ones, again affecting the observed strain-induced modifications in exciton properties.
3. Interfacial Effects:

Impact: The interface between MoS2 and the substrate/encapsulant can introduce new effects.
Using different materials:

Charge transfer: Some materials might induce charge transfer with MoS2, leading to doping and modifying the exciton dynamics.
Interfacial phonons: New phonon modes arising from the interface could provide additional scattering channels for excitons, influencing their diffusion lengths.
In summary: Moving beyond hBN to different substrates or encapsulating materials opens up exciting possibilities for tailoring the excitonic properties of MoS2. However, careful consideration of dielectric screening, strain transfer efficiency, and potential interfacial effects is crucial for accurate prediction and interpretation of experimental results.

Could the strain-induced manipulation of exciton properties in MoS2 monolayers be leveraged for developing novel quantum information processing platforms?

The strain-induced control over exciton properties in MoS2 monolayers, as highlighted in the study, holds significant promise for quantum information processing (QIP). Here's how:
1. Strain-Tunable Qubits:

Exciton-based qubits: Excitons in TMDs, with their valley degree of freedom, are candidates for qubits. Strain could offer a way to:

Control qubit energy levels: Strain-induced shifts in exciton energy levels (e.g., the splitting of the bright exciton doublet) could be used to define and manipulate qubit states.
Couple qubits: Controlled strain gradients or patterns could mediate interactions between neighboring exciton qubits, enabling two-qubit gates essential for QIP.
2. Valleytronics for QIP:

Valley coherence: Strain can influence the valley degree of freedom of excitons. This opens possibilities for:

Encoding information: Using the valley index (K or K' valley) as a quantum bit.
Valley-based logic gates: Strain-controlled valley splitting and selective manipulation of excitons in different valleys could enable the realization of valleytronic logic operations.
3. Single-Photon Sources:

Strain-controlled emission: The study shows that strain can modify the brightness and even the directionality of light emission from different exciton species. This could be used to:

Create deterministic single-photon sources: Precise strain engineering could lead to on-demand generation of single photons from localized excitons, a key resource for quantum communication.
Control photon polarization: Strain-induced anisotropy in optical selection rules could enable the generation of single photons with tailored polarization states.
Challenges and Outlook:

Coherence times: Long exciton coherence times are crucial for QIP. Investigating how strain affects decoherence mechanisms in MoS2 is essential.
Scalability: Developing scalable techniques for applying and controlling strain at the nanoscale is vital for realizing practical QIP devices.
In conclusion: The ability to manipulate exciton properties through strain in MoS2 monolayers offers a compelling pathway towards novel QIP platforms. Further research on coherence properties, strain control techniques, and integration with other QIP components will be key to unlocking the full potential of this approach.

How does the understanding of strain-dependent exciton behavior in 2D materials contribute to the broader field of condensed matter physics and materials science?

The study of strain-dependent exciton behavior in 2D materials like MoS2 has profound implications, pushing the boundaries of our understanding in condensed matter physics and opening doors for new possibilities in materials science:
Condensed Matter Physics:

Exciton Physics in Reduced Dimensions:

Stronger interactions: 2D materials exhibit enhanced Coulomb interactions due to confinement, leading to robust excitons even at room temperature. Strain provides a unique tool to probe and manipulate these interactions, deepening our understanding of excitonic phenomena in low-dimensional systems.
Valleytronics: The study highlights how strain can be used to control the valley degree of freedom, a key aspect of valleytronics. This field explores the use of valleys in momentum space for information processing, and strain-controlled 2D materials offer a fertile ground for its development.

Symmetry and Topology:

Symmetry breaking: Strain breaks the crystal symmetry, leading to novel effects like the emergence of out-of-plane dipoles in dark excitons. This provides insights into the interplay between symmetry, electronic structure, and optical properties in 2D materials.
Topological phases: Strain engineering in certain 2D materials can induce topological phase transitions, potentially leading to exotic states of matter with unique properties. The understanding of strain-dependent exciton behavior can be a stepping stone towards controlling and harnessing these phases.

Materials Science:

Strain Engineering of 2D Materials:

Property tuning: The study demonstrates how strain can be used to tailor the optical and electronic properties of 2D materials. This opens avenues for designing materials with desired characteristics for applications in optoelectronics, sensing, and energy harvesting.
Device design: Strain-engineered 2D materials can be integrated into flexible and stretchable devices, enabling novel functionalities and improved performance in areas like flexible displays, wearable electronics, and bio-integrated sensors.

Beyond Electronics:

Exciton-based devices: The findings pave the way for developing exciton-based devices, such as exciton transistors, light-emitting diodes, and lasers, with enhanced performance and novel functionalities through strain control.
Quantum technologies: As discussed earlier, the ability to manipulate exciton properties via strain holds promise for quantum information processing, quantum communication, and other emerging quantum technologies.

In essence: The study of strain-dependent exciton behavior in 2D materials is not merely an academic pursuit. It provides fundamental insights into condensed matter physics, while simultaneously driving the development of advanced materials and devices with transformative potential across various technological domains.