Strongly-bound excitons and trions in anisotropic 2D semiconductors
Sangho Yoon, Taeho Kim, Seung-Young Seo, Seung-Hyun Shin, Su-Beom Song, B. J. Kim, Kenji Watanabe, Takashi Taniguchi, Gil-Ho Lee, Moon-Ho Jo, Diana Y. Qiu, Jonghwan Kim
备注：
20 pages, 4 figures, Sangho Yoon, Taeho Kim contributed equally to this work
Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and novel photonics and optoelectronics based on strongly-bound excitons with two-fold anisotropy.
We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide MoS2. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18-0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime incre…
Monolayer 2D semiconductors provide an attractive option for valleytronics due to the valley-addressability by helicity-specific light beam. But the short valley lifetime for excitons have hindered potential valleytronic applications. In this paper, we demonstrate a strategy for prolonging the valley lifetime by converting excitons to trions through effective gate control and by taking advantage of much longer valley lifetime for trions than for excitons. In continuous-wave experiments, we found the valley polarization increases as gate voltage is tuned away from the charge neutrality, with th…
The possibility of electrical manipulation and detection of charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS$_{2}$/monolayer graphene vertical heterojunction, we report inter-layer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature, the resulting change in the WS$_{2}$ bandgap allows us to scan the excitation around the exciton-trion spectral overlap with …
Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe$_2$. We observe the intervalley dark excitons, which can recombine by the assistance of defect scatte…
We measure the optical absorption spectra and optical conductivities of excitons and trions in monolayers of metal dichalcogenide MoS$_{2}$ and compare the results with theoretical models. Our results show that the Wannier-Mott model for excitons, with modifications to account for small exciton radii and large exciton relative wavefunction spread in momentum space, phase space blocking due to Pauli exclusion in doped materials, and wavevector dependent dielectric constant, gives results that agree well with experiments. The measured exciton optical absorption spectra are used to obtain experim…
We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong chang…
We present a theoretical description of excitons and positively and negatively charged trions in "giant" CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a degenerate valence band. The calculations take into account the complex structure of the CdSe valence band and inter-part…
Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS$_2$, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion do…
We study theoretically fundamental Coulomb-correlated complexes: neutral and charged excitons, also known as trions, in transition metal dichalogenides monolayers. We focus on the situation where one of the electrons occupies excited, high-lying, conduction band characterized by a negative effective mass. We develop the theory of such high-lying excitons and trions with negative effective mass and demonstrate the key role of the non-parabolicity of the high-lying conduction band dispersion in formation of the bound exciton and trion states. We present simple, accurate and physically justified …
We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a …
The X$^-$ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X$^-$ trion creation operator in terms of exciton and electron. We can also derive the X$^-$ trion ladder diagram between an exciton and an electron. These are th…
We present experimental and theoretical results on the high-quality single-layer MoS$_{2}$ which reveal the fine structure of charged excitons, i.e., trions. In the emission spectra we resolve and identify two trion peaks, T$_{1}$ and T$_{2}$, resembling the pair of singlet and triplet trion peaks (T$_S$ and T$_{T}$) in tungsten-based materials. However, in polarization-dependent photoluminescence measurements we identify these peaks as novel intra- and inter-valley singlet trions, constituting the trion fine structure distinct from that already known in bright and dark 2D materials with large…
Excitonic quasi-particles, excitons/trions/bi-excitons, and their coupling with phonons and charge carriers play a crucial role in controlling the optical properties of atomically thin semiconducting 2D materials. In this work, we unravelled the dynamics of excitons/trions and their coupling with phonons and charge carriers in a few layers vertically and horizontally aligned MoS2. We observed trion signature up to the highest recorded temperature (330 K) in both systems and have shown that the dynamics of excitons/trions and their coupling with phonons and electrons are more affected in vertic…
The mechanisms for generation of long-lived spin coherence in a two-dimensional electron gas (2DEG) have been studied experimentally by means of a picosecond pump-probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum wells with a diluted 2DEG were investigated. The strong Coulomb interaction between electrons and holes, which results in large binding energies of neutral excitons and negatively charged excitons (trions), allows one to address selectively the exciton or trion states by resonant optical excitation. Different scenarios of spin coherence generation were analyzed theoretically, amon…
We present a collection of zero-, one- and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the exciton and trion nonlinear optical response. Exciton-exciton and exciton-trion coherent coupling is manifest as distinct peaks in the spectra, whereas signatures of trion-trion interactions are absent. Excellent agreement using density matrix calculations is obtained, which highlights the essential role of many-body effects on coherent interactions in the quantum well.
Highly nonlinear Moiré exciton and trion polaritons
Laboratory for Physical Sciences, University of Maryland（马里兰大学物理科学实验室）；University of Rochester（罗切斯特大学）；University of Notre Dame（圣母大学）；The Institute of Optics, University of Rochester（罗切斯特大学光学研究所）
The optical spectra of two dimensional (2D) materials exhibit sharp absorption peaks that are commonly identified with exciton and trions (or charged excitons). In this paper, we show that excitons and trions in doped 2D materials can be described by two coupled Schrodinger-like equations - one two-body equation for excitons and another four-body equation for trions. In electron doped 2D materials, a bound trion state is identified with a four-body bound state of an exciton and an excited conduction band electron-hole pair. In doped 2D materials, the exciton and the trions states are the not t…
We present a many-body formalism for the simulation of time-resolved nonlinear spectroscopy and apply it to study the coherent interaction between excitons and trions in doped transition-metal dichalcogenides. Although the formalism can be straightforwardly applied in a first-principles manner, for simplicity we use a parameterized band structure and a static model dielectric function, both of which can be obtained from a calculation using the $GW$ approximation. Our simulation results shed light on the interplay between singlet and triplet trions in molybdenum- and tungsten-based compounds. O…
We present a many-body theory for exciton-trion-polaritons in doped two-dimensional materials. Exciton-trion-polaritons are robust coherent hybrid excitations involving excitons, trions, and photons. Signatures of these polaritons have been recently seen in experiments. In these polaritons, the 2-body exciton states are coupled to the material ground state via exciton-photon interaction and the 4-body trion states are coupled to the exciton states via Coulomb interaction. The trion states are not directly optically coupled to the material ground state. The energy-momentum dispersion of these p…
We study optically the coherent evolution of trions and excitons in a $δ$-doped 3.5 nm-thick ZnO/Zn$_{0.91}$Mg$_{0.09}$O multiple quantum well by means of time-resolved four-wave mixing at temperature of 1.5~K. Employing spectrally narrow picosecond laser pulses in the $χ^{(3)}$ regime allows us to address differently localized trion and exciton states, thereby avoiding many-body interactions and excitation-induced dephasing. The signal in the form of photon echoes from the negatively charged A excitons (T$_\text{A}$, trions) decays with coherence times varying from 8 up to 60~ps, depending on…
ARPES signatures of trions in van der Waals materials
Philipps-Universität Marburg（马尔堡菲利普斯大学）；mar.quest | Marburg Center for Quantum Materials and Sustainable Technologies（mar.quest | 马尔堡量子材料与可持续技术中心）；Stockholm University（斯德哥尔摩大学）；Georg-August-Universität Göttingen（格奥尔格·奥古斯特哥廷根大学）
Monolayer transition metal dichalcogenide (TMD) semiconductors such as MoSe$_2$ host strongly bound excitons which are known to exhibit a strong resonant third-order nonlinear response. Although there have been numerous studies of the ultrafast nonlinear response of monolayer TMDs, a study of the steady-state nonlinear response is lacking. We report a comprehensive study of the steady-state two-color nonlinear response of excitons in hBN-encapsulated monolayer MoSe$_2$ at 7 K. We observe differential transmission (DT) signals associated with the neutral and charged exciton species, which is st…
We first derive a new ``commutation technique'' for an exciton interacting with electrons, inspired from the one we recently developed for excitons interacting with excitons. These techniques allow to take \emph{exactly} into account the possible exchanges between carriers. We use it to get the $\mathrm{X}^-$ trion creation operator in terms of exciton and free-electron creation operators. In a last part we generate the ladder diagrams associated to these trions. Although similar to the exciton ladder diagrams, with the hole replaced by an exciton, they are actually much more tricky : (i) Due …
Transition metal dichalcogenide (TMD) monolayers are two-dimensional semiconductors with two valleys in their band structure that can be selectively addressed using circularly polarized light. Their photoluminescence spectrum is characterized by neutral and charged excitons (trions) that form a chemical equilibrium governed by the net charge density. Here, we use chemical doping to drive the conversion of excitons into trions in $\text{WS}_{2}$ monolayers at room temperature, and study the resulting valley polarization via photoluminescence measurements under valley-selective optical excitatio…
Excitons, trions, biexcitons, and exciton-trion complexes in two-dimensional transition metal dichalcogenide sheets of MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$ and WSe$_2$ are studied by means of density functional theory and path integral Monte Carlo method in order to accurately account for the particle-particle correlations. In addition, the effect of dielectric environment on the properties of these exciton complexes is studied by modifying the effective interaction potential between particles. Calculated exciton and trion binding energies are consistent with previous experimental and computati…
The optical spectra of transition metal dichalcogenide (TMDC) monolayers are dominated by excitons and trions. Here we establish the dependences of these optical transitions on disorder from hyperspectral imaging of h-BN encapsulated monolayer MoSe2. While both exciton and trion energies vary spatially, these two quantities are almost perfectly correlated, with spatial variation in the trion binding energy of only ~0.18 meV. In contrast, variation in the energy splitting between the two lowest energy exciton states is one order of magnitude larger at ~1.7 meV. Statistical analysis and theoreti…
So far in the literature the terms "charged exciton" and "trion" are often confused with each other and mostly considered as the same. In this work we show this is not the case in 2D quantum dots with a parabolic confinement. By using the unrestricted Hartree-Fock method the energy and binding energy of both charged excitons and trions in 2D parabolic quantum dots are calculated in dependence on the confinements of charge carriers in quantum dot. It is shown that the binding energies of the charged exciton and the trion behave differently in regard to the ratios of the confinements between the…
The anisotropic nature of the new two-dimensional (2D) material phosphorene, in contrast to other 2D materials such as graphene and transition metal dichalcogenide (TMD) semiconductors, allows excitons to be confined in a quasi-one-dimensional (1D) space predicted in theory, leading to remarkable phenomena arising from the reduced dimensionality and screening. Here, we report a trion (charged exciton) binding energy of 190 meV in few-layer phosphorene at room temperature, which is nearly one to two orders of magnitude larger than those in 2D TMD semiconductors (20-30 meV) and quasi-2D quantum …
Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we resolve the conflict by employing low-temperature photoluminescence spectroscopy to id…
The influence of a perpendicular magnetic field on the binding energy and structural properties of excitons, trions, and biexcitons in monolayers of semiconducting transition metal dichalcogenides (TMDs) is investigated. The stochastic variational method (SVM) with a correlated Gaussian basis is used to calculate the different properties of these few-particle systems. In addition, we present a simplified variational approach which supports the SVM results for excitons as a function of magnetic field. The exciton diamagnetic shift is compared with recent experimental results and we extend this …
The optical properties of weakly-doped two-dimensional materials are dominated by strong exciton and trion absorption and luminescence features. In this article we examine the influence of moiré patterns in semiconductor heterobilayers on exciton and trion states in the limit of strong moiré modulation potentials, commenting on similarities and differences compared to the case of excitons and trions in semiconductor quantum dots. We discuss strategies for using optical properties as quantitative probes of moiré materials, and the prospects for exploiting moiré materials to design unique light …
The optoelectronic properties of metal chalcogenide colloidal nanoplatelets are often interpreted in terms of excitonic states. However, recent spectroscopic experiments evidence the presence of trion states, enabled by the slow Auger recombination in these structures. We analyze how the presence of an additional charge in trions modifies the emission energy and oscillator strength as compared to neutral excitons. These properties are very sensitive to dielectric confinement and electronic correlations, which we describe accurately using image-charge and variational Quantum Monte Carlo methods…
We study the impact of a free carrier reservoir on the optical properties of excitonic and trionic complexes in a MoSe$_2$ monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser the electron density can be controlled in our sample and in turn the exciton and trion densities can be tuned. We find a significant increase of the trion binding energy in the presence of an induced electron gas both in power- and in time-resolved photoluminescence spectra. This behaviour is reproduced within the original variational approach that takes into account both screening a…
Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS$_2$ encapsulated in hexagonal BN (hBN). We find that this system is an $n$-type doped semiconductor and that dark trions dominate the emission spectrum. In line with previous studies on WSe$_2$, we identify the Coulomb exchange interaction coupled neutral dark and grey excitons through their polarization properties, while an analogous effect is not observed for dark trions. Applying the magnetic field in both perpendicular …
As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are so far elusive and controversial. Here we present experimental evidence for valleytronic four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tu…
Monolayer molybdenum disulfide (MoS$_2$) has emerged as an excellent 2D model system because of its two inequivalent, direct-gap valleys that lead to exotic bound and excited states. Here we focus on one such bound state, the negatively charged trion. Unlike excitons, trions can radiatively decay with non-zero momentum by kicking out an electron, resulting in an asymmetric trion photoluminescence (PL) peak with a long low-energy tail. As a consequence, the peak position does not correspond to the zero momentum trion energy. By including the trion's long tail in our analysis we are able to accu…
Charge doping in transition metal dichalcogenide is currently a subject of high importance for future electronic and optoelectronic applications. Here we demonstrate chemical doping in CVD grown monolayer (1L) of WS2 by a few commonly used laboratory solvents by investigating the room temperature photoluminescence (PL). The appearance of distinct trionic emission in the PL spectra and quenched PL intensities suggest n-type doping in WS2. The temperature-dependent PL spectra of the doped 1L-WS2 reveal significant enhancement of trion emission intensity over the excitonic emission at low tempera…
We first show that, for problems dealing with trions, it is totally hopeless to use the standard many-body description in terms of electrons and holes and its associated Feynman diagrams. We then show how, by using the description of a trion as an electron interacting with an exciton, we can obtain the trion absorption through far simpler diagrams, written with electrons and \emph{excitons}. These diagrams are quite novel because, for excitons being not exact bosons, we cannot use standard procedures designed to deal with interacting true fermions or true bosons. A new many-body formalism is n…
We present results of atomistic empirical pseudopotential calculations and configuration interaction for excitons, positive and negative trions (X\pm), positive and negative quartons (X2\pm) and biexcitons. The structure investigated are laterally aligned InGaAs quantum dot molecules embedded in GaAs under a lateral electric field. The rather simple energetic of excitons becomes more complex in the case of charged quasiparticles but remains tractable. The negative trion spectrum shows four anticrossings in the presently available range of fields while the positive trion shows two. The magnitud…
We present a theoretical study of photo-absorption in n-doped two-dimensional (2D) and quasi-2D semiconductors that takes into account the interaction of the photocreated exciton with Fermi-sea (FS) electrons through (i) Pauli blocking, (ii) Coulomb screening, and (iii) excitation of FS electron-hole pairs---that we here restrict to one. The system we tackle is thus made of one exciton plus zero or one FS electron-hole pair. At low doping, the system ground state is predominantly made of a "trion-hole"---a trion (two opposite-spin electrons plus a valence hole) weakly bound to a FS hole---with…
Strong Coulomb interaction in atomically-thin transition metal dichalcogenides makes these systems particularly promising for studies of excitonic physics. Of special interest are the manifestations of the charged excitons, also known as trions, in the optical properties of two-dimensional semiconductors. In order to describe the optical response of such a system, the exciton interaction with resident electrons should be explicitly taken into account. In this paper we demonstrate that this can be done both in the trion (essentially, few-particle) and Fermi-polaron (many-body) approaches, which…
The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO$_2$/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the $"parent"$ WS$_2$ and WSe$_2$ MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in …
We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{a…
We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreover, the effective radiative lifetime of a trion was evaluated to be approximately 20 ns.
The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional (2D) semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding…
The nature of trions and their interaction with light has remained a puzzle. The composition and dispersion of polaritons involving trions provide insights into this puzzle. Trions and excitons in doped two-dimensional (2D) materials are not independent excitations but are strongly coupled as a result of Coulomb interactions. When excitons in doped 2D materials are also strongly coupled with light inside an optical waveguide, the resulting polariton states are coherent superpositions of exciton, trion, and photon states. We realize these exciton-trion-polaritons by coupling an electron-doped m…
We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schrödinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induc…
Strong Coulomb interaction in monolayer transition metal dichalcogenides can facilitate nontrivial many-body effects among excitonic complexes. Many-body effects like exciton-exciton annihilation (EEA) have been widely explored in this material system. However, a similar effect for charged excitons (or trions), that is, trion-trion annihilation (TTA), is expected to be relatively suppressed due to repulsive like-charges, and has not been hitherto observed in such layered semiconductors. By a gate-dependent tuning of the spectral overlap between the trion and the charged biexciton through an "a…
