Tuning the bandgap of exfoliated InSe nanosheets by quantum confinement

Strong quantization effects and tuneable near-infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ-rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

Topical review: recent progress of charge density waves in 2D transition metal dichalcogenide-based heterojunctions and their applications

Charge density wave (CDW) is an intriguing physical phenomenon especially found in two-dimensional (2D) layered systems such as transition-metal dichalcogenides (TMDs). The study of CDW is vital for understanding lattice modification, strongly correlated electronic behaviors, and other related physical properties. This paper gives a review of the recent studies on CDW emerging in 2D TMDs. First, a brief introduction and the main mechanisms of CDW are given. Second, the interplay between CDW patterns and the related unique electronic phenomena (superconductivity, spin, and Mottness) is elucidated. Then various manipulation methods such as doping, applying strain, local voltage pulse to induce the CDW change are discussed. Finally, examples of the potential application of devices based on CDW materials are given. We also discuss the current challenge and opportunities at the frontier in this research field.

Ultrafast dynamics of hot carriers in a quasi-two-dimensional electron gas on InSe

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas-Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high-κ dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

Real-Time GW-Ehrenfest-Fan-Migdal Method for Nonequilibrium 2D Materials

Quantum simulations of photoexcited low-dimensional systems are pivotal for understanding how to functionalize and integrate novel two-dimensional (2D) materials in next-generation optoelectronic devices. First-principles predictions are extremely challenging due to the simultaneous interplay of light-matter, electron-electron, and electron-nuclear interactions. We here present an advanced ab initio many-body method that accounts for quantum coherence and non-Markovian effects while treating electrons and nuclei on equal footing, thereby preserving fundamental conservation laws like the total energy. The impact of this advancement is demonstrated through real-time simulations of the complex multivalley dynamics in a molybdenum disulfide (MoS2) monolayer pumped above gap. Within a single framework, we provide a parameter-free description of the coherent-to-incoherent crossover, elucidating the role of microscopic and collective excitations in the dephasing and thermalization processes.

Grazing-angle reflectivity setup for the low-temperature infrared spectroscopy of two-dimensional systems

A new optical setup is described that allows the reflectivity at grazing incidence to be measured, including ultrathin films and two-dimensional electron systems (2DES) down to liquid-helium temperatures, by exploiting the Berreman effect and the high brilliance of infrared synchrotron radiation. This apparatus is well adapted to detect the absorption of a 2DES of nanometric thickness, namely that which forms spontaneously at the interface between a thin film of LaAlO₃ and its SrTiO₃ substrate, and to determine its Drude parameters.

Quantitative regulation of electron-phonon coupling

Electron-phonon (e-p) coupling plays a crucial role in various physical phenomena, and regulation of e-p coupling is vital for the exploration and design of high-performance materials. However, the current research on this topic lacks accurate quantification, hindering further understanding of the underlying physical processes and its applications. In this work, we demonstrate quantitative regulation of e-p coupling, by pressure engineering andin-situspectroscopy. We successfully observe both a distinct vibrational mode and a strong Stokes shift in layered CrBr3, which are clear signatures of e-p coupling. This allows us to achieve precise quantification of the Huang-Rhys factorSat the actual sample temperature, thus accurately determining the e-p coupling strength. We further reveal that pressure efficiently regulates the e-p coupling in CrBr3, evidenced by a remarkable 40% increase inSvalue. Our results offer an approach for quantifying and modulating e-p coupling, which can be leveraged for exploring and designing functional materials with targeted e-p coupling strengths.