Direct measurement of key exciton properties: energy, dynamics and spatial distribution of the wave function
Shuo Dong, Michele Puppin, Tommaso Pincelli, Samuel Beaulieu, Dominik Christiansen, Hannes Hubener, Christopher W. Nicholson, R. Patrick Xian, Maciej Dendzik, Y… 展开作者
Shuo Dong, Michele Puppin, Tommaso Pincelli, Samuel Beaulieu, Dominik Christiansen, Hannes Hubener, Christopher W. Nicholson, R. Patrick Xian, Maciej Dendzik, Yunpei Deng, Yoav William Windsor, Malte Selig, Ermin Malic, Angel Rubio, Andreas Knorr, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer
Excitons, Coulomb-bound electron-hole pairs, are the fundamental excitations governing the optoelectronic properties of semiconductors. While optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy- and time-resolved perspective on excitons in the layered semiconductor WSe$_2$. By tuning the excitation wavelength, we determine the energy-momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allows to retrieve fundamental exciton properties like the binding energy and the exciton-lattice coupling and to reconstruct the real-space excitonic distribution function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton properties and is applicable to bright and dark excitons in semiconducting materials, heterostructures and devices.
Monolayer black and blue phosphorenes possess electronic and optical properties that result in unique features when the two materials are stacked. We devise a low-strain van-der-Waals double layer and investigate its properties with ab initio many-body perturbation theory techniques. A type-II band alignment and optical absorption in the visible range are found. The study demonstrates that spatially indirect excitons with full charge separation can be obtained between two layers with the same elemental composition but different crystalline structure, proving the system interesting for further …
Observing the spatial and temporal evolution of exciton wave functions
Philipps-Universität Marburg（马尔堡菲利普斯大学）；University of Regensburg（雷根斯堡大学）；University of Graz（格拉茨大学）；Forschungszentrum Jülich（于利希研究中心）；RWTH Aachen University（亚琛工业大学）
Probing exciton interaction with a spatially defined charge accumulation layer in the organic semiconductor Diindenoperylene
利用有机薄膜晶体管（OTFT）的光致发光（PL）和载流子输运测量，研究了激子和载流子之间的微观相互作用。为此目的，原型有机半导体二茚并萘嵌苯（DIP）被用作活性物质。OTFT累积层提供了空间限定的电荷和光生激子的相互作用区，导致PL强度降低高达4.5%。这种效应与累积的空穴载流子密度相关，并为非辐射激子-空穴过程的横截面提供了约1.3E-10 cm 3/s的较低估计。这是合理的，这些过程优先介导的陷阱空穴。
Emerging photo-induced excitonic processes in transition metal dichalcogenide (TMD) heterobilayers, e.g., coupling, dephasing, and energy transfer of intra- and inter-layer excitons, allow new opportunities for ultrathin photonic devices. Yet, with the associated large degree of spatial heterogeneity, understanding and controlling their complex competing interactions at the nanoscale remains a challenge. Here, we present an all-round dynamic control of intra- and inter-layer excitonic processes in a WSe$_2$/Mo$_{0.5}$W$_{0.5}$Se$_2$ heterobilayer using multifunctional tip-enhanced photolumines…
Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, due to their direct energy gaps in the optical range. Their electronic and optical properties, however, are complicated by exotic room-temperature excitons, whose fundamental structure and dynamics has been under intense investigation. While interband spectroscopy probes energies of excitons with vanishing centre-of-mass momenta, the majority of excitons has remained elusive, raising questions about their unusual in…
Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe$_2$ monolayers encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 monolayers at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening without …
The optical properties of particularly the tungsten-based transition-metal dichalcogenides are strongly influenced by the presence of dark excitons. Recently, theoretical predictions as well as indirect experimental insights have shown that two different dark excitons exist within the light cone. While one is completely dark, the other one is only dipole forbidden out-of-plane, hence referred to as grey exciton. Here, we present angle-resolved spectroscopic data of a high-quality hexagonal-BN-encapsulated WSe2 monolayer with which we directly obtain the radiation pattern of this grey exciton t…
Atomically thin MoS2 layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of MoS2 monolayer are extremely important in determining the strength of light matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in Mo…
We study conceptional problems of a photo-electron emission (PEE) process from a free exciton in insulating crystals. In this PEE process, only the electron constituting the exciton is suddenly emitted out of the crystal, while the hole constituting the exciton is still left inside and forced to be recoiled back to its original valence band. This recoil on the hole is surely reflected in the spectrum of the PEE with a statistical distribution along the momentum-energy curve of the valence band. This distribution is nothing but the square of the exciton wave function amplitude, since it shows h…
Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of…
Tightly bound excitons in monolayer semiconductors represent a versatile platform to study two-dimensional propagation of neutral quasiparticles. Their intrinsic properties, however, can be severely obscured by spatial energy fluctuations due to a high sensitivity to the immediate environment. Here, we take advantage of the encapsulation of individual layers in hexagonal boron nitride to strongly suppress environmental disorder. Diffusion of excitons is then directly monitored using time- and spatially-resolved emission microscopy at ambient conditions. We consistently find very efficient prop…
Coherent coupling between excitons is at the heart of many-body interactions with transition metal dichalcogenide (TMD) heterostructures as an emergent platform for the investigation of these interactions. We employ multi-dimensional coherent spectroscopy on monolayer MoSe\textsubscript{2}/WSe\textsubscript{2} heterostructures and observe coherent coupling between excitons spatially localized in monolayer MoSe$_2$ and WSe$_2$. Through many-body spectroscopy, we further observe the absorption state arising from free interlayer electron-hole pairs. This observation yields a spectroscopic measure…
Screening due to surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasi-particle bandgap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence (PL) peak shift at low temper…
Interlayer excitons in layered materials constitute a novel platform to study many-body phenomena arising from long-range interactions between quantum particles. The ability to localise individual interlayer excitons in potential energy traps is a key step towards simulating Hubbard physics in artificial lattices. Here, we demonstrate spatial localisation of long-lived interlayer excitons in a strongly confining trap array using a WS$_{2}$/WSe$_{2}$ heterostructure on a nanopatterned substrate. We detect long-lived interlayer excitons with lifetime approaching 0.2 ms and show that their confin…
Direct observation of ultrafast defect-bound and free exciton dynamics in defect-engineered WS$_2$ monolayers
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory（橡树岭国家实验室纳米材料科学中心）；Center for Integrated Nanotechnologies, Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室集成纳米技术中心）；Honda Research Institute USA Inc.（本田美国研究院）；Department of Materials Science and Engineering, University of Tennessee, Knoxville（田纳西大学诺克斯维尔分校材料科学与工程系）
