Machine learning electron-phonon interactions in 2D semiconducting materials: The case of zero-point renormalization

In the last decade, the rise of two-dimensional (2D) materials has attracted a tremendous amount of interest for the entire field of photonics and opto-electronics. The mechanism of light–matter interaction in 2D materials challenges the knowledge of materials physics, which drives the rapid development of materials synthesis and device applications. 2D materials coupled with plasmonic effects show impressive optical characteristics, involving efficient charge transfer, plasmonic hot electrons doping, enhanced light-emitting, and ultrasensitive photodetection. Here, we briefly review the recent remarkable progress of 2D materials, mainly on graphene and transition metal dichalcogenides, focusing on their tunable optical properties and improved opto-electronic devices with plasmonic effects. The mechanism of plasmon enhanced light–matter interaction in 2D materials is elaborated in detail, and the state-of-the-art of device applications is comprehensively described. In the future, the field of 2D materials holds great promise as an important platform for materials science and opto-electronic engineering, enabling an emerging interdisciplinary research field spanning from clean energy to information technology.

Atomically thin two-dimensional (2D) materials have been a famous and fascinating material in recent years due to the potential to replace conventional semiconducting bulk electronic materials. To control the performance of 2D materials, many methods have been proposed, including physical and chemical ways, to manipulate the electronic, atomic and microscopic properties. In this work, we would like to present a physical method based on the interactions of 2D materials with light to influence the 2D material properties and device performance. By reviewing some recent published work, we will show how effective the light can be to functionalize 2D materials. The fundamental fluorescence phenomenon and current applications using 2D materials in optoelectronics, such as photodetectors, solar cells and light emitting diodes, to obtain improved device properties will also be discussed.

A fascinating photonic platform with a small device scale, fast operating speed, as well as low energy consumption is two-dimensional (2D) materials, thanks to their in-plane crystalline structures and out-of-plane quantum confinement. The key to further advancement in this research field is the ability to modify the optical properties of the 2D materials. The modifications typically come from the materials themselves, for example, altering their chemical compositions. This article reviews a comparably less explored but promising means, through engineering the photonic surroundings. Rather than modifying materials themselves, this means manipulates the dielectric and metallic environments, both uniform and nanostructured, that directly interact with the materials. For 2D materials that are only one or a few atoms thick, the interaction with the environment can be remarkably efficient. This review summarizes the three degrees of freedom of this interaction: weak coupling, strong coupling, and multifunctionality. In addition, it reviews a relatively timing concept of engineering that directly applied to the 2D materials by patterning. Benefiting from the burgeoning development of nanophotonics, the engineering of photonic environments provides a versatile and creative methodology of reshaping light–matter interaction in 2D materials.

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides and phosphorene have emerged as brand new photonic materials with huge potential in a variety of optical applications in recent years, due to their unique properties and multiple functions. Owning to the significantly increased surface to volume ratio in the atomic thin layered structure, the interactions between 2D materials with light become remarkably strong in either elastic or inelastic interactions. Most of previous studies were focused on the inelastic interactions between 2D materials with light, namely Raman and photoluminescence properties. However, we found the elastic interactions between them and light are also very attractive. Based on the properties discovered, we demonstrated several optical applications, such as atomically thin optical lenses and gratings. On the other hand, we found the inelastic interactions with light in the 2D limitation affected a lot by the external environment, which could serve as a unique method to control the working status of the optical devices to meet multiple application needs. Several progresses were demonstrated such as control of excitons and trions in few-layer MoS2 via the co-modulations by both temperature and electric field and the defect engineering by oxygen plasma etching to stimulate new photon emission.

Enhanced optical absorption in two-dimensional (2D) materials has recently moved into the focus of nanophotonics research. In this work, we present a gain-assisted method to achieve critical coupling and demonstrate the maximum absorption in undoped monolayer graphene in the near-infrared. In a two-port system composed of photonic crystal slab loaded with graphene, the gain medium is introduced to adjust the dissipative rate to match the radiation rate for the critical coupling, which is accessible without changing the original structural geometry. The appropriate tuning of the gain coefficient also enables the critical coupling absorption within a wide wavelength regime for different coupling configurations. This work provides a powerful guide to manipulate light–matter interaction in 2D materials and opens up a new path to design ultra-compact and high-performance 2D material optical devices.

2D materials (2DMs), due to spin-valley locking degree of freedom, exhibit strongly bound exciton and chiral optical selection rules and become promising material candidates for optoelectronic and spin/valleytronic devices. Over the last decade, the manifesting of 2D materials by circularly polarized lights expedites tremendous fascinating phenomena, such as valley/exciton Hall effect, Moiré exciton, optical Stark effect, circular dichroism, circularly polarized photoluminescence, and spintronic property. In this review, recent advance in the interaction of circularly polarized light with 2D materials covering from graphene, black phosphorous, transition metal dichalcogenides, van der Waals heterostructures as well as small proportion of quasi-2D perovskites and topological materials, is overviewed. The confronted challenges and theoretical and experimental opportunities are also discussed, attempting to accelerate the prosperity of chiral light-2DMs interactions.

Symbolic regression for defect interactions in 2D materials

Emerging 2D van der Waals compounds have gained increasing attention due to their unique electronic and optical properties, and have shown promise in sensing applications. The realization of sensing devices using these materials still faces several challenges. For example, it is critical to gain clear understandings of (1) the fundamental light-matter interactions and their relations to the atomic structures, which govern many key material properties and device performances; and (2) the coupling with other nanostructures and molecules, which is a required structure for sensing devices and systems. This talk introduces new discoveries and pioneering works on these critical challenges, and novel applications of these materials in biochemical sensing. The first part of this talk presents light-matter interactions and techniques to augment material performance, such as 2D Janus transition metal dichalcogenides. The second part of this talk focuses on the interaction of 2D materials with organic molecules and related sensing applications. In particular, a novel enhancement effect of molecular Raman signals on 2D surface was discovered, which offers a new paradigm of biochemical sensing with high specificity, high multiplexity, and low noise. The selection rule for the 2D material substrates has been revealed, which is critical for device design. Two sensing applications for Alzheimer’s disease and respiratory viruses will also be discussed. Overall, the works presented in this talk are significant in fundamental quantum science, and offer important guidelines for practical applications in sensing and quantum technologies. The methodologies used here also provide a framework for the future study of many emerging materials and sensing scenarios.

The 2D materials based on the layered structures, from graphene, to transition metal dichalcogenides (TMDs), to black phosphorous and few-layer topological insulators have emerged as novel platforms for both fundamental studies and device applications. Here, we review the progress in microcavity-enhanced light-matter interaction in 2D materials. In particular, we focus our attention on the devices based on microcavities and photonic crystals embedded with graphene and TMDs. Effects observed under the weak and strong coupling regimes are discussed followed by brief overview of future directions.

Strong Coulomb interactions in 2D materials results in a wide variety of correlated many body states. In this talk we present optical spectroscopy results and theoretical results on strongly correlated electron states in Landau levels in bi-layer graphene and on trions and trion-polaritons in transition metal dichalcogenides. Our results show a rich variety of features exhibited by these correlated states in 2D materials. In the case of Landau levels, we observe valley-dependent optical transitions that violate the conventional optical selection rules. At low magnetic fields, the oscillator strengths of forbidden transitions are as large as those of the allowed transitions. Moreover, we can tune the relative oscillator strength by tuning the bandgap of bilayer graphene. Our findings provide new insights into the interplay between magnetic field, band structure and many-body interactions in tunable semiconductor systems, and the experimental technique paves the way to studying symmetry-broken states and low energy magneto-optical properties of novel materials. In the case of trions and trion-polaritons, despite the nomenclature, our results show that the actual quantum states are many body states involving at least four or five particles. A trion state in n-doped 2D materials consists of two electrons, one valence band hole, and one conduction band hole. The conduction band hole is weakly bound to the other three particles. However, inside an optical microcavity the hole becomes strongly bound to the other three particles because of strong light-matter interaction. Our results shed new light on the nature of trions and trion-polaritons in 2D materials. We will discuss the connections of our models with the recently proposed exciton-polaron picture of trions.

Design of narrowband perfect absorber for enhancing photoluminescence in atomically thin WSe2

Electron–phonon interaction (EPI) plays an important role in the transport property of metals. Nevertheless, EPI in polymer nanocomposites containing metal fillers has not been fully investigated due to the complicated physical mechanisms. In this work, the copper–polyethylene (Cu–PE) nanocomposite is used as a model system, and nonequilibrium molecular dynamics (NEMD) simulation combined with the two-temperature model (TTM) is conducted to reveal the role of EPI in Cu–PE systems. The effects of filler length, filler property, and EPI strength on thermal conductivity are revealed. A dimensionless number (Θ) is defined to characterize the electron–phonon nonequilibrium (EPN) degree. The results show that the EPN degree has a significant impact on thermal conductivity. When the system is in a strong EPN state, the effects of EPI on thermal conductivity can be negligible. However, when the EPN degree gradually decreases, ignoring EPI can severely underestimate the thermal conductivity. Furthermore, the temperature profile of electrons and phonons is extracted and the equivalent thermal circuit is presented. The additional thermal resistances induced by the nonlocal and nonequilibrium effects are revealed. Phonon spectra analysis and heat current decomposition are performed to uncover the underlying mechanisms. In addition, the effects of atomic mass density and filler types on thermal conductivity are revealed. Stress–strain simulation is conducted to unravel the effects of strain rates on the mechanical property. The results are useful for designing polymer nanocomposites with controlled thermal conductivity.

We have studied four 2D layered perovskites, including OA2PbI4 (RP phase), ODAPbI4 and BDAPbI4 (DJ phase), (GA)MAPbI4 (ACI phase), where OA is [(C m H2m+1)NH3](m = 8), ODA is [NH3(CH2) m NH3](m = 8), BDA is [NH3(CH2) m NH3](m = 4), and GA is [C(NH2)3]; RP, DJ, and ACI means Ruddlesden-Popper, Dion-Jacobson and alternating cations in the interlayer, respectively. The temperature dependence of absorption and photoluminescence (PL) spectra have been measured. From which the average phonon energy (electron-phonon interaction strength) is analyzed as around 34 (80), 47 (184), 50 (402), and 63 (758) with the unit of meV for OA2PbI4, ODAPbI4, BDAPbI4, and (GA)MAPbI4, respectively. Larger phonon energy indicates the involvement of more phonons in organic spacer layer, with the corresponding stronger electron-phonon interaction. Furthermore, ultrafast transient absorption spectroscopy proves that, when the excitation photon energy is serval hundred meV higher than bandgap, the excitons still are the major photoexcitations in OA2PbI4, but polarons are major one in ODAPbI4, BDAPbI4, and (GA)MAPbI4 films, no matter the excitonic transitions dominate the absorption at their band edges. This work proves the organic spacers can regulate electron-phonon interaction then optoelectronic properties in 2D perovskites profoundly, which have implications toward future rational design for relevant devices.

Through in situ control of the growth condition in the vapor phase transport and condensation, we intentionally prepared ZnO nanorods with different concentrations of oxygen vacancy (VO), as confirmed by the X-ray photoelectron spectra. A spectral shift in the ultraviolet (UV) emission between these nanorods, as large as ∼80 meV, has been observed in the room temperature photoluminescence (PL) spectra, showing strong correlation to the VO concentration. With the help of the variable-temperature PL, this spectral shift is clearly attributed to the different spectral contributions of the free exciton emission and its phonon replicas. Furthermore, a remarkable variation in the electron–phonon interaction strength among these samples is unambiguously revealed by the Raman spectra, which is in good consistence with the Huang–Rhys parameters obtained from the PL. This study implies that VO in ZnO nanostructures not only modulates the visible emission as intensively previously investigated but also significantly...

Strong enhancement of Raman signal upon integration of trilayer MoS 2 with Fabry-Perot (FP) optical cavities is reported. We also discuss prospective application of enhanced light matter interaction in 2D materials for energy harvesting purposes.