Long-range transport of 2D excitons with acoustic waves

Interlayer and Moiré excitons in atomically thin double layers: From individual quantum emitters to degenerate ensembles

Abstract

Interlayer excitons (IXs), composed of electron and hole states localized in different layers, excel in bilayers composed of atomically thin van der Waals materials such as semiconducting transition-metal dichalcogenides (TMDs) due to drastically enlarged exciton binding energies, exciting spin-valley properties, elongated lifetimes, and large permanent dipoles. The latter allows modification by electric fields and the study of thermalized bosonic quasiparticles, from the single particle level to interacting degenerate dense ensembles. Additionally, the freedom to combine bilayers of different van der Waals materials without lattice or relative twist-angle constraints leads to layer-hybridized and Moiré excitons, which can be widely engineered. This article covers fundamental aspects of IXs, including correlation phenomena as well as the consequence of Moiré superlattices with a strong focus on TMD homo- and heterobilayers.

Graphical abstract

Acousto-Drag Photovoltaic Effect by Piezoelectric Integration of Two-Dimensional Semiconductors

Light-to-electricity conversion is crucial for energy harvesting and photodetection, requiring efficient electron-hole pair separation to prevent recombination. Traditional junction-based mechanisms using built-in electric fields fail in nonbarrier regions. Homogeneous material harvesting under a photovoltaic effect is appealing but is only realized in noncentrosymmetric systems via a bulk photovoltaic effect. Here we report the realization of a photovoltaic effect by employing surface acoustic waves (SAWs) to generate zero-bias photocurrent in the conventional layered semiconductor MoSe2. SAWs induce periodic modulation to electronic bands and drag the photoexcited pairs toward the traveling direction. The photocurrent is extracted from a local barrier. The separation of generation and extraction processes suppresses recombination and yields a large nonlocal photoresponse. We distinguish the acousto-electric drag and electron-hole pair separation effect by fabricating devices of different configurations. The acousto-drag photovoltaic effect, enabled by piezoelectric integration, offers an efficient light-to-electricity conversion method, independent of semiconductor crystal symmetry.

Room Temperature, Twist Angle Independent, Momentum Direct Interlayer Excitons in van der Waals Heterostructures with Wide Spectral Tunability

Interlayer excitons (IXs) in van der Waals heterostructures with static out of plane dipole moment and long lifetime show promise in the development of exciton based optoelectronic devices and the exploration of many body physics. However, these IXs are not always observed, as the emission is very sensitive to lattice mismatch and twist angle between the constituent materials. Moreover, their emission intensity is very weak compared to that of corresponding intralayer excitons at room temperature. Here we report the room-temperature realization of twist angle independent momentum direct IX in the heterostructures of bulk PbI2 and bilayer WS2. Momentum conserving transitions combined with the large band offsets between the constituent materials enable intense IX emission at room temperature. A long lifetime (∼100 ns), noticeable Stark shift, and tunability of IX emission from 1.70 to 1.45 eV by varying the number of WS2 layers make these heterostructures promising to develop room temperature exciton based optoelectronic devices.

Coherent optical coupling to surface acoustic wave devices

Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>105 measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems.

Gate-Tunable Quantum Acoustoelectric Transport in Graphene

Transport probes the motion of quasi-particles in response to external excitations. Apart from the well-known electric and thermoelectric transport, acoustoelectric transport induced by traveling acoustic waves has rarely been explored. Here, by adopting hybrid nanodevices integrated with piezoelectric substrates, we establish a simple design of acoustoelectric transport with gate tunability. We fabricate dual-gated acoustoelectric devices based on hBN-encapsulated graphene on LiNbO3. Longitudinal and transverse acoustoelectric voltages are generated by launching a pulsed surface acoustic wave. The gate dependence of zero-field longitudinal acoustoelectric signal presents strikingly similar profiles to that of Hall resistivity, providing a valid approach for extracting carrier density without magnetic field. In magnetic fields, acoustoelectric quantum oscillations appear due to Landau quantization, which are more robust and pronounced than Shubnikov-de Haas oscillations. Our work demonstrates a feasible acoustoelectric setup with gate tunability, which can be extended to the broad scope of various van der Waals materials.

Recent progress of exciton transport in two-dimensional semiconductors

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Mechanical, electronic, optical, piezoelectric and ferroic properties of strained graphene and other strained monolayers and multilayers: an update

This is an update of a previous review (Naumiset al2017Rep. Prog. Phys.80096501). Experimental and theoretical advances for straining graphene and other metallic, insulating, ferroelectric, ferroelastic, ferromagnetic and multiferroic 2D materials were considered. We surveyed (i) methods to induce valley and sublattice polarisation (P) in graphene, (ii) time-dependent strain and its impact on graphene's electronic properties, (iii) the role of local and global strain on superconductivity and other highly correlated and/or topological phases of graphene, (iv) inducing polarisationPon hexagonal boron nitride monolayers via strain, (v) modifying the optoelectronic properties of transition metal dichalcogenide monolayers through strain, (vi) ferroic 2D materials with intrinsic elastic (σ), electric (P) and magnetic (M) polarisation under strain, as well as incipient 2D multiferroics and (vii) moiré bilayers exhibiting flat electronic bands and exotic quantum phase diagrams, and other bilayer or few-layer systems exhibiting ferroic orders tunable by rotations and shear strain. The update features the experimental realisations of a tunable two-dimensional Quantum Spin Hall effect in germanene, of elemental 2D ferroelectric bismuth, and 2D multiferroic NiI2. The document was structured for a discussion of effects taking place in monolayers first, followed by discussions concerning bilayers and few-layers, and it represents an up-to-date overview of exciting and newest developments on the fast-paced field of 2D materials.

Enhanced Exciton Drift Transport through Suppressed Diffusion in One-Dimensional Guides

Drift-diffusion dynamics is investigated in a one-dimensional (1D) exciton guide at room temperature. Spatial engineering of the exciton energy in a WSe2 monolayer is achieved using local strain to confine and direct exciton transport. An unexpected and massive deviation from the Einstein relation is observed and correlated to exciton capture by defects. We find that the capture reduces exciton temperature and diffusion so much that drift transport visibility improves to 38% as excitons traverse asymmetrically over regions with occupied defect states. Based on measurements over multiple potential gradients, we estimate the exciton mobility to be 169 ± 39 cm2/(eV s) at room temperature. Experiments at elevated exciton densities reveal that the exciton drift velocity monotonically increases with exciton density, unlike exciton mobility, due to contributions from nonequilibrium many-body effects.

Acoustically Driven Magnon-Phonon Coupling in a Layered Antiferromagnet

Harnessing the causal relationships between mechanical and magnetic properties of Van der Waals materials presents a wealth of untapped opportunity for scientific and technological advancement, from precision sensing to novel memories. This can, however, only be exploited if the means exist to efficiently interface with the magnetoelastic interaction. Here, we demonstrate acoustically driven spin-wave resonance in a crystalline antiferromagnet, chromium trichloride, via surface acoustic wave irradiation. The resulting magnon-phonon coupling is found to depend strongly on sample temperature and external magnetic field orientation, and displays a high sensitivity to extremely weak magnetic anisotropy fields in the few mT range. Our work demonstrates a natural pairing between power-efficient strain-wave technology and the excellent mechanical properties of Van der Waals materials, representing a foothold toward widespread future adoption of dynamic magnetoacoustics.

Optical Imaging of Ultrafast Phonon-Polariton Propagation through an Excitonic Sensor

Hexagonal boron nitride (hBN) hosts phonon polaritons (PhP), hybrid light-matter states that facilitate electromagnetic field confinement and exhibit long-range ballistic transport. Extracting the spatiotemporal dynamics of PhPs usually requires "tour de force" experimental methods such as ultrafast near-field infrared microscopy. Here, we leverage the remarkable environmental sensitivity of excitons in two-dimensional transition metal dichalcogenides to image PhP propagation in adjacent hBN slabs. Using ultrafast optical microscopy on monolayer WSe2/hBN heterostructures, we image propagating PhPs from 3.5 K to room temperature with subpicosecond and few-nanometer precision. Excitons in WSe2 act as transducers between visible light pulses and infrared PhPs, enabling visible-light imaging of PhP transport with far-field microscopy. We also report evidence of excitons in WSe2 copropagating with hBN PhPs over several micrometers. Our results provide new avenues for imaging polar excitations over a large frequency range with extreme spatiotemporal precision and new mechanisms to realize ballistic exciton transport at room temperature.

The Acoustophotoelectric Effect: Efficient Phonon-Photon-Electron Coupling in Zero-Voltage-Biased 2D SnS2 for Broad-Band Photodetection

Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broad-band wavelength excitation─especially beyond the material's nominal band gap─while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling─which we term the acoustophotoelectric effect─in SnS2 that facilitates efficient photodetection through the application of 100 MHz order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS2 but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broad-band photodetection beyond the visible light range, while maintaining pA-order dark currents─ without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to 8 orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS2-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.

Kapitza-resistance-like exciton dynamics in atomically flat MoSe2-WSe2 lateral heterojunction

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe2-WSe2) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

Extended Spatial Coherence of Interlayer Excitons in MoSe_{2}/WSe_{2} Heterobilayers

We report on the spatial coherence of interlayer exciton ensembles as formed in MoSe_{2}/WSe_{2} heterostructures and characterized by point-inversion Michelson-Morley interferometry. Below 10 K, the measured spatial coherence length of the interlayer excitons reaches values equivalent to the lateral expansion of the exciton ensembles. In this regime, the light emission of the excitons turns out to be homogeneously broadened in energy with a high temporal coherence. At higher temperatures, both the spatial coherence length and the temporal coherence time decrease, most likely because of thermal processes. The presented findings point towards a spatially extended, coherent many-body state of interlayer excitons at low temperature.

Observation of optical anisotropy and a linear dichroism transition in layered silicon phosphide

The investigation of in-plane two-dimensional (2D) anisotropic materials has garnered significant attention due to their exceptional electronic, optical, and mechanical characteristics. The anisotropic optical properties and angle-dependent photodetectors based on 2D anisotropic materials have been extensively studied. However, novel in-plane anisotropic materials still need to be explored to satisfy for distinct environments and devices. Here, we report the remarkable anisotropic behavior of excitons and demonstrate a unique linear-dichroism transition of absorption between ultraviolet and visible light in layered silicon phosphide (SiP) through the analysis of polarization photoluminescence (PL) and absorbance spectra. Its high absorption linear dichroism ratio of 1.16 at 388 nm, 1.15 at 532 nm, and 1.19 at 733 nm is revealed, suggesting the brilliant non-isotropic responses. The robust periodic variation of the A₁ and A₂ Raman modes in 2D SiP materials allows for the determination of their crystal orientation. Furthermore, the presence of indirect excitons with phonon sidebands in the temperature-dependent PL spectra exhibits non-monotonic energy shifts with increasing temperature, which is attributed to an enhanced electron-phonon interaction and thermal expansion. Our findings provide valuable insights into the fundamental physical properties of layered SiP and offer guidelines for designing polarization-sensitive photodetectors and angle-dependent devices based on 2D anisotropic materials.

Surface acoustic wave induced phenomena in two-dimensional materials

Surface acoustic wave (SAW)-matter interaction provides a fascinating key for inducing and manipulating novel phenomena and functionalities in two-dimensional (2D) materials. The dynamic strain field and piezo-electric field associated with propagating SAWs determine the coherent manipulation and transduction between 2D excitons and phonons. Over the past decade, many intriguing acoustic-induced effects, including the acousto-electric effect, acousto-galvanic effect, acoustic Stark effect, acoustic Hall effect and acoustic exciton transport, have been reported experimentally. However, many more phenomena, such as the valley acousto-electric effect, valley acousto-electric Hall effect and acoustic spin Hall effect, were only theoretically proposed, the experimental verification of which are yet to be achieved. In this minireview, we attempt to overview the recent breakthrough of SAW-induced phenomena covering acoustic charge transport, acoustic exciton transport and modulation, and coherent acoustic phonons. Perspectives on the opportunities of the proposed SAW-induced phenomena, as well as open experimental challenges, are also discussed, attempting to offer some guidelines for experimentalists and theorists to explore the desired exotic properties and boost practical applications of 2D materials.

Interlayer Exciton Diode and Transistor

Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles can potentially lead to low loss excitonic circuits. Here, we report unidirectional transport of IXs along nanoscale electrostatically defined channels in an MoSe₂-WSe₂ heterostructure. These results are enabled by a lithographically defined triangular etch in a graphene gate to create a potential energy "slide". By performing spatially and temporally resolved photoluminescence measurements, we measure smoothly varying IX energy along the structure and high speed exciton flow with a drift velocity up to 2 × 10⁶ cm/s, an order of magnitude larger than previous experiments. Furthermore, exciton flow can be controlled by saturating exciton population in the channel using a second laser pulse, demonstrating an optically gated excitonic transistor. Our work paves the way toward low loss excitonic circuits, the study of bosonic transport in one-dimensional channels, and custom potential energy landscapes for excitons in van der Waals heterostructures.