Abstract
Emerging photo-induced excitonic processes in transition metal dichalcogenide (TMD) heterobilayers, e.g., interplay of intra- and inter-layer excitons and conversion of excitons to trions, allow new opportunities for ultrathin hybrid photonic devices. However, with the associated large degree of spatial heterogeneity, understanding and controlling their complex competing interactions in TMD heterobilayers at the nanoscale remains a challenge. Here, we present an all-round dynamic control of interlayer-excitons and -trions in a WSe2/Mo0.5âW0.5âSe2 heterobilayer using multifunctional tip-enhanced photoluminescence (TEPL) spectroscopy with <20ânm spatial resolution. Specifically, we demonstrate the bandgap tunable interlayer excitons and the dynamic interconversion between interlayer-trions and -excitons, through the combinational tip-induced engineering of GPa-scale pressure and plasmonic hot electron injection, with simultaneous spectroscopic TEPL measurements. This unique nano-opto-electro-mechanical control approach provides new strategies for developing versatile nano-excitonic/trionic devices using TMD heterobilayers.
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Introduction
Stacking atomically thin layers of van der Waals (vdW) materials into bilayer heterostructures provides innovative strategies for the development of next-generation optoelectronic devices and substantially broadens the scope of material physics1,2,3,4. A plethora of intriguing phenomena has been already unveiled in vdW bilayers, but they are likely just the tip of the iceberg because many more structures remain unexplored with different chemical composition, stacking sequence and angle, interlayer distance, and other parameters. Hence, considerable efforts are currently focused on uncovering and controlling the inherent physical properties in vdW heterostructures.
In particular, interlayer excitons (IXs), formed by electrons and holes spatially separated in the top and bottom layers of transition metal dichalcogenide (TMD) heterobilayers5, show a range of distinct properties, which are promising for various optoelectronic applications. The reduced spatial overlap of the electron and hole wavefunctions in IXs brings about reduced radiative decay rates, with corresponding lifetimes up to μs6, while the interlayer distance and twist angle between the constituent monolayers provide knobs for tuning the IX quantum yield7. In addition, the out-of-plane component of the IX dipole moment enables straightforward electric field control. IXs in TMD heterobilayers also provide long-lived valley polarization and coherence8, circumventing the limits of TMD monolayers and enabling practical valleytronic applications. Additionally, the slight lattice mismatch and twist angles in heterobilayers give rise to moiré supperlattices and corresponding confinement potentials that can effectively trap IXs9. Therefore, IXs in TMD heterobilayers provide a promising element for realizing excitonic integrated circuits10,11 and possibly demonstrating high temperature many-body effects, such as Bose-Einstein condensates (BEC) and superfluidity12. In addition, since trions provide further opportunities for electrostatic control in excitonic circuits as well as longer radiative lifetimes compared to neutral excitons13, inducing and controlling the interlayer trions (IXâââor IXâ+â) in TMD heterobilayers are highly desirable.
However, in order to enable practical applications of TMD heterostructures using IX and IXâââ, several major challenges must be overcome, one of which is the large degree of spatial heterogeneity. The underlying processes, e.g., competing interactions of coupling, dephasing, and energy transfer of intra- and inter-layer excitons as well as IXâ interconversion, arise at the nanoscale and cannot be understood by diffraction-limited optical approaches, calling for the near-field optical probing14,15,16,17. Furthermore, beyond probing, it is highly important to achieve nanoscale control of local IX and IXâ properties in TMD heterostructures. However, dynamic control study of nanoscale properties of IXs with simultaneous nano-spectroscopic measurements has rarely been reported18,19 and the more desirable interlayer trion-conversion has not been investigated yet.
Here, we demonstrate an all-round dynamic control of intra- and inter-layer excitonic processes and interconversion between IX and IXâ in a WSe2/Mo0.5âW0.5âSe2 heterobilayer with <20ânm spatial resolution, achieved by multifunctional tip-enhanced photoluminescence (TEPL) spectroscopy and imaging. The use of the alloy-based heterobilayer allows us to tune the bandgap energy of IX and X depending on the molecular compositions. We optimize the W/Mo composition of 0.5 to induce strong electronic resonance between the tip-plasmon and the intra- and inter-layer exciton PLs while remaining well separated20. Through hyperspectral TEPL nano-imaging, we reveal nanoscale inhomogeneities of the IX emission and identify the regions with the different interlayer coupling strength. At the weak interlayer coupling region, we dynamically control the radiative recombination path and competing emission rates of intra- and inter-layer excitons through the engineering of Au tip-heterobilayer distance and interlayer distance, achieving increase of the IX quantum yield compared to that of intralayer excitons (Xs).
In addition, by applying GPa scale tip-pressure to the heterobilayer, we directly modify its electronic bandstructure, which is demonstrated via blueshifted IX TEPL energy and supported by theoretical calculations. Furthermore, through the control of plasmonic hot electron injection from the Au tip, we convert neutral IXs into charged IXâ states. This approach presents the first-ever scanning-tip hot electron regulator, which can precisely control the hot electron transfer rate or trion-conversion rate in a fully reversible fashion. Our results demonstrate that IX and IXâââin TMD heterobilayers can be accurately controlled in nanoscopic volumes via a near-field approach, which opens up new avenues for the development of compact TMD based optoelectronic devices and provides insights for studying various many-body phenomena.
Results and discussion
Experimental configuration for TEPL spectroscopy
We fabricate a WSe2/Mo0.5âW0.5âSe2 heterobilayer on a Au film by stacking exfoliated ML flakes with their crystal axes aligned for optimized IX emission. The twist angle is measured to be ~1.1o via polarization-resolved second-harmonic generation (SHG) spectroscopy (see inset of Fig. 1a and Methods). In the assembled heterobilayer, IXs are formed by the spatially separated holes (h+) and electrons (eâ) in constituent layers in addition to intralayer excitons \({\rm{X}}_{{\rm{WSe}}_{2}}\) and \({\rm{X}}_{{\rm{Mo}}_{0.5}{\rm{W}}_{0.5}{\rm{Se}}_{2}}\). Hyperspectral far-field PL imaging shows that the spatial distributions of intra- and inter-layer excitons are considerably inhomogeneous at the microscale, as shown in Fig. 1a (see Fig. S1 for the optical microscopy image). Such spatial heterogeneity in vdW heterostructures is attributed to the non-uniform interlayer coupling strength, which depends sensitively on local strain-induced deformation and interfacial contamination21,22. Furthermore, on the smaller spatial scales below the diffraction-limit, nanoscale structural deformations, such as wrinkles, bubbles, and grain boundaries23,24,25, give rise to complex charge dynamics and interactions with competing recombination processes of intra- and inter-layer excitons.
To develop comprehensive understanding of the nanoscale heterogeneity in the WSe2/Mo0.5âW0.5âSe2 heterobilayer and demonstrate its precise control, we develop multifunctional TEPL spectroscopy. We use a radially polarized excitation beam in the bottom-illumination geometry to induce strong out-of-plane optical fields and plasmons at the Au tip-Au film junction (see Methods for more details). The tip plasmons induced by the dipole-dipole interaction between the Au tip and the Au substrate then couple with the Xs and IXs (Fig. 1b) in the heterobilayer and enhance their PL responses via the Purcell effect26. The tip-sample distance is regulated with a precision of ~0.2ânm using a shear-force feedback loop, with corresponding control on the plasmon enhancement and optical field strength. This allows us to dynamically manipulate the light-matter interactions at the nanoscale with the simultaneous spectroscopic TEPL measurements. Fig. 1c shows a schematic of the TEPL spectroscopy, including different multifunctional control modalities, i.e., GPa scale tip-pressure and plasmonic hot carrier injection, as well as tip-induced engineering of the interlayer distance (dI) in a TMD heterobilayer.
Near-field probing of the nanoscale heterogeneity in a TMD heterobilayer
To investigate the nanoscale heterogeneity of IXs originated from the non-uniform interlayer coupling strength, we perform hyperspectral TEPL imaging of the heterobilayer, with the experimentally observed spatial distribution of the tip-enhanced IX PL as shown in Fig. 2a. In our TEPL scanning, the tip-sample distance d is kept at ~5ânm to minimize tip-induced modification of the sample surface. To better visualize the spectroscopic information of the inhomogeneous IX distribution and corresponding topography, in Fig. 2b and c we present the TEPL intensity IIX, peak energy shift ÎE (ÎEâ=â0 at Eâ=â1.52âeV), linewidth Î, and height â along the line L1 (indicated in Fig. 2a). The variations in IIX indicate the non-uniform interlayer coupling strength and associated possible changes in the density, oscillator strength, and emission lifetimes of IXs.
The regions with higher IIX generally show narrower Î and blueshifted peak energy. The broader Î in the low-density IX regions is possibly due to the slight deviation of the IX dipole orientation, since it can cause PL energy variation due to the quantum confinement effect on the interlayer excitonic properties27. The observed blueshift of the high-density IXs is originated from the static electric dipole of IX because the repulsive interactions between the well-oriented IXs cause a mean-field shift, as revealed in previous far-field studies11. Note that the height â generally shows an uncorrelated behavior with the spectroscopic line profiles which means the interlayer coupling strength is not simply characterized by the surface profiling21,28. It should be noted that the whole region of Fig. 2a is measured with ~20ânm spatial resolution by TEPL imaging, which is much smaller than the diffraction-limited beam spot size. Hence, the observed spatio-spectral heterogeneity cannot be investigated using a conventional far-field imaging methods, such as confocal microscopy (see Fig. S2 for the confocal PL image of the same measured area).
We then position the tip in the weak interlayer coupling region and acquire different PL characteristics of IX and \({\rm{X}}_{{\rm{WSe}}_{2}}\) as a function of the tip-sample distance d for selected distances, as shown in Fig. 2d. At dâ=â20ânm, we observe far-field PL spectrum exhibiting IX and \({\rm{X}}_{{\rm{WSe}}_{2}}\) peaks at Eâ=â1.52âeV and 1.63âeV, respectively. The PL peak of \({\rm{X}}_{{\rm{Mo}}_{0.5}{\rm{W}}_{0.5}{\rm{Se}}_{2}}\) is not clearly observed due to its low quantum yield. At this relatively large tip-sample distance, the \({\rm{X}}_{{\rm{WSe}}_{2}}\) peak shows higher PL intensity than the IX peak due to the low interlayer coupling strength. In comparison, at dâ=â5ânm the intensities of the \({\rm{X}}_{{\rm{WSe}}_{2}}\) and IX PL become similar. Here, the plasmon-exciton coupling in the Au tip-Au film nanocavity is significantly stronger, and since the plasmonic resonance predominantly enhances out-of-plane optical fields, the PL of the vertically oriented IX dipoles is increased26. At the same time, the intralayer excitons, while efficiently excited in the far-field via in-plane polarized fields, show increasingly inhibited PL emission inside the plasmonic cavity at smaller distances d. When the tip approaches closely to the heterobilayer with dâ=â3ânm, the PL intensities of the \({\rm{X}}_{{\rm{WSe}}_{2}}\) and IX peaks are switched, and the IX emission dominates, while its spectral position and shape remain unchanged (see Fig. S3 for optical field distributions at the Au tip).
The TEPL enhancement of IXs is attributed to the increased excitation rate and Purcell effect (see the Supplementary information Section 4 for the calculated enhancement factor ~1.6âÃâ103)26. In addition, the tip-induced charge tunneling effect further influences the observed TEPL responses of IXs and Xs29,30. In the near-field regime approaching tip-sample contact, the effective overlap between electron wavefunctions of the Au tip and the heterobilayer can facilitate charge tunneling processes31 that cause the perturbation of the excitonic system. Fig. 2f illustrates the charge transport mechanism of the type-II band alignment when the tip approaches the 2D crystal surface. Since the Fermi level of Au lies lower than the conduction band minimum energy in WSe2, the electrons at the adjacent WSe2 tunnel into the Au tip. Additionally, the electrons and holes in the heterobilayer are redistributed via interlayer charge transfer. Consequently, the p-doped top layer and the n-doped bottom layer effectively facilitate the IX recombination at the local region with decreasing recombination rate of intralayer excitons, as experimentally confirmed in the result of Fig. 2d.
Tip-induced nano-engineering of TMD heterobilayer
In order to move towards practical opto-electronic device applications of vdW heterobilayers, the nanoscale heterogeneity of IX and X emission should be not only resolved, but also actively controlled. Recently, a few approaches for engineering local exciton properties in 2D heterostructures were demonstrated, for example, via electrostatic field10,32 or high magnetic field33. However, precise nanoscale control of emission beyond the tip-sample distance modulation is a significant challenge18,34. To further extend our tip-induced IX emission control, we present a nano-opto-mechanical tip-pressure engineering approach through the atomic force tip control combined with in-situ TEPL spectroscopy. As schematically illustrated in Fig. 3a, the tip exerts local pressure within a ~25ânm2 sample area, which is precisely regulated through changing the set-point in a shear-force feedback loop (see Methods). This pressure is expected to cause a local decrease in the interlayer distance and corresponding increase in the interlayer coupling strength. We experimentally verify this behavior by measuring TEPL spectra evolution in a reversible tip-press and -release process. As we demonstrate in Fig. S5 in the Supplementary information, tip pressure applied to a sample region with initially weak interlayer coupling results in stronger IX emission with simultaneously decreased X emission of WSe2 and Mo0.5âW0.5âSe2 which is attributed to the improved interlayer coupling strength35,36.
In our previous study, we demonstrated that tip-induced local pressure can exceed 10âGPa owing to its nanoscale tip-sample contact area even though the tip-force is only on the order of pN21,37. Here, in the same fashion we induce ~GPa scale tip-pressure in a TMD heterobilayer (see the Supplementary information Section 6 and 7 for the estimation of pressure and compressive strain), which directly modifies its crystal structure and electronic bandstructure, resulting in the modified IX emission properties. Fig. 3b shows the modified TEPL spectra before (top panel) and after (bottom panel) inducing ~GPa scale tip-pressure in the WSe2/Mo0.5âW0.5âSe2 heterobilayer. The spectra are decomposed into 3 peaks corresponding to IX, \({\rm{X}}_{{\rm{WSe}}_{2}}\), and \({\rm{X}}_{{\rm{Mo}}_{0.5}{\rm{W}}_{0.5}{\rm{Se}}_{2}}\) via fitting by Lorentzian functions. In addition to the increase in the IX/X PL ratio discussed earlier, the IX TEPL peak exhibits a clearly discernible blueshift of ~7âmeV. In order to clarify the physical origin of the observed spectral changes, we simulate the associated electronic bandstructure modification with decreasing interlayer distance using density functional theory (DFT) calculations with 2âÃâ2 supercell as described in Methods.
The calculated electronic bandstructure for the experimentally studied WSe2/Mo0.5W0.5Se2 heterobilayer in the equilibrium configuration with AAâ stacking is shown in Fig. 3c. This configuration corresponds to the interlayer distance dIâ=â6.45âà , which is defined as the separation between the transition atom planes in the WSe2 and Mo0.5W0.5Se2 layers and determined numerically by relaxing the structure in DFT calculations using empirical van der Waals interactions38. The highest valence band and the lowest conduction band are highlighted with blue color and exhibit their maximum and minimum energy, respectively, at the K high-symmetry point (see schematic of the hexagonal Brillouin zone corresponding to the 2âÃâ2 supercell in the inset).
To study the effect of tip-induced strain, we calculate bandstructures for different values of interlayer distance dI and extract the corresponding energies of the K-K electronic transition. The results are plotted in Fig. 3d as shifts of the K-K transition energy from that at the equilibrium interlayer distance dIâ=â6.45âà (green circles). In the experimentally relevant range of energy shifts >10âmeV, the dependence on interlayer distance is well described by a linear fit (green line). Comparison with the experimentally observed IX energy blueshift of ~7âmeV (marked with black arrow in Fig. 3d) provides an estimate for the interlayer distance decrease in the maximally strained configuration of ÎdI~0.6 à relative to the equilibrium interlayer distance. Assuming a typical force constant per unit area for the vibrational breathing modes39 of a TMD heterobilayer of Kzâ=â8.6âÃâ1019âN/m3, we further estimate the tip-induced pressure as Pâ=âKzâ·âÎdIâââ5âGPa, which is of the same order of magnitude as values estimated from modelling in the Supplementary information Section 7 and reported previously37.
We note that the IX energy shift induced by compressive strain is sensitive to the chemical composition of the TMD bilayer. Our calculations for pristine bilayers (see the Supplementary information Section 8) show that similar tip-induced pressure would result in a higher ~20âmeV blueshift in a WSe2/MoSe2 heterobilayer, while for WSe2/WSe2 homobilayer it would result in ~20âmeV redshift. Therefore, our estimates of the applied pressure can be in general affected by fluctuations in local stoichiometry. Furthermore, our calculations do not account for local inhomogeneities of the initial strain or strain-dependent binding energies of IX states40, thus providing only an order-of-magnitude estimate of the induced strain.
Tip-induced hot electron injection control of charged IX
The experimental results reported so far have been measured at relatively low values of excitation intensity (â108âW/m2). By significantly increasing the excitation intensity, we can explore a different regime, characterized by electron transport from the Au tip to the heterobilayer, which is due to the hot electron generation at the plasmonic tip and subsequent injection into the Mo0.5âW0.5âSe2âML41,42. Our measurements of the excitation-intensity dependent IX PL confirm the increased charged interlayer exciton (IXâ) density in contrast to the saturating neutral IX density at the high-intensity regime attributed to the hot carrier injection43,44 (see the Supplementary information Section 9 for more details). By approaching the plasmonic âot tip with a strongly localized field close to the heterobilayer, we achieve the dynamic local control of the IXâ formation and recombination rate in the near-field regime. To demonstrate such control, we move to the sample region where the dominant IX emission is observed without the X emission and investigate the tip-induced hot eâ injection effect at the high excitation intensity (â 109âW/m2). Fig. 4a shows TEPL spectra of IX peak with respect to the tip-sample distance d, exhibiting apparent emergence of the IXâ peak when the Au tip closes to the crystal. To quantify the IX and IXâ peak energies for the TEPL spectrum obtained at dâ=â0.5ânm, we deconvolute the spectrum by the Lorentzian function. The fitted spectrum and its second derivative curve (d2 /dx2, gray) clearly show the pronounced IXâ peak at ~1.465âeV, in addition to the neutral IX peak at Eâ=â~1.509âeV, which is attributed to the hot eâ injection. By contrast, the IXâ peak is rarely seen in the TEPL spectrum at dâ=â6.5ânm due to the low hot eâ injection efficiency. Through the deconvolution of TEPL spectrum, we can derive the IX and IXâ peak energies of ~1.518âeV and ~1.474âeV. However, the energy assignment for the IXâ is not clearly seen in the second derivative curve due to the nonsignificant IXâ shoulder. Note that, from the comparison of IX peak energies at dâ=â6.5ânm and dâ=â0.5ânm, we find the TEPL redshift of ~8âmeV. This redshift is also supporting evidence of the hot eâ injection because photo-induced doping in TMD crystals gives rise to PL energy redshift.
The mechanism of the tip-induced IXâ generation is schematically illustrated in Fig. 4b. The hot electrons injected into the heterobilayer within the nanoscale region under the plasmonic tip couple with the neutral IX and form the IXâ. Since this process becomes increasingly efficient as the tip approaches the heterobilayer, the local density of the neutral IX shows saturating feature (see the Supplementary information Section 9 for more details). To demonstrate this behavior from the result of Fig. 4a, we plot the changes of the linewidth (Î, yellow), IXâ intensity (IIXâ, red), and intensity ratio of IX/IXâ (IIX/IIXâ, blue) as a function of d, as shown in Fig. 4c. Note that, Î, IIXâ, and IIX/IIXâ are directly derived from the experimentally observed TEPL spectra. As seen in the figure, Î and IIXâ start to increase at dâââ10ânm, which means the threshold distance for the hot eâ -induced IXâ generation. This distance is in good agreement with the results of previous surface-enhanced Raman spectroscopy studies for molecular samples45. In addition, the abruptly decreasing IIX/IIXâ from dâ=â10ânm, also indicates a highly efficient conversion from the neutral IX to IXâ under the plasmonic tip as expected.
We further enhance and control the charged IX emission by applying GPa scale pressure with the tip under the high-power excitation. As shown in Fig. 4d, under the tip-induced pressure the contributions to the total TEPL intensity from both IXâ and IX peaks are increased, which we attribute to the higher interlayer coupling strength and correspondingly increased recombination rate for both neutral and charged IX species. This is accompanied by redshift of the IX TEPL spectrum and increased linewidth, which contrasts with the observed blueshift of the TEPL spectrum at low excitation powers presented in Fig. 3b. Additionally, we observe that the TEPL intensity of intralayer excitons (IX for a WSe2 ML) is decreased, which is naturally understood from the competing recombination process between the intra- and inter-layer excitons, as discussed earlier regarding the data presented in Fig. 3. Furthermore, the precise modification of IX, IXâ and X emissions is clearly demonstrated in Fig. 4e. When we press the sample with GPa scale tip pressure, the TEPL intensity ratios for IXâ/IX (red) and \({\rm{X}}_{{\rm{WSe}}_{2}}\) /IX (blue) show opposite behaviors with the pressure. This result shows a distinct advantage of our work compared to the previous hot eâ injection studies, i.e., the ability to dynamically control the hot eâ density and the corresponding IXâ conversion rate. By regulating the tip-sample distance precisely (~0.2ânm26) using the scanning probe tip, we can control the hot eâ injection at the nanoscale in a fully reversible manner, which was not possible in the previous studies46,47 (see Fig. S10 for demonstration of reversible control).
Conclusion
In summary, we have investigated the nanoscale heterogeneity of the interlayer coupling strength in an aligned WSe2/Mo0.5âW0.5âSe2 heterobilayer and demonstrated active control of its emission via multifunctional TEPL spectroscopy inside a plasmonic tip-substrate cavity in two distinct power regimes. At low excitation powers, we control the interplay between the intralayer and neutral interlayer exciton PL via distance-tunable Purcell enhancement, where IX emission becomes dominant at small tip-sample distances. At high excitation powers, the plasmonic tip acts as a source of hot electrons, which are injected into the heterobilayer and facilitate formation of IXâ with distance-tunable efficiency. Beyond the simple control of interlayer excitons via tip-sample distance modulation, we reversibly modify their spectral response via applying nano-localized tip-induced GPa scale pressure. We support the observed local pressure-dependent IX spectral evolution with DFT simulations, which provide insights into interlayer distance dependent band structure in aligned TMD bilayers. The presented results demonstrate new approaches to study the nanoscale heterogeneity of the IX response in TMD heterobilayers and suggest ways to control that response within nanoscopic sample areas. This manifests an important step towards the development of next-generation optoelectronic devices, such as nano-integrated excitonic/trionic circuits, and investigation of novel many-body effects with TMD-based heterobilayers.
Methods
Sample preparation
Cover glass (170âμm thickness) was ultrasonicated in acetone and isopropanol for 10âmin each and cleaned again by O2 plasma treatment for 10âmin. Then, a Cr adhesion layer (2ânm thickness) and an Au film (9ânm thickness) were deposited subsequently on the glass with a rate of 0.1âà /s each at the base pressure of ~10â6âtorr using a conventional thermal evaporator. The prepared substrate was covered with a 0.5ânm thick layer of Al2O3 via atomic layer deposition to reduce the PL quenching with maintaining the large TEPL enhancement. TMD monolayers (Mo0.5âW0.5âSe2,âWSe2) were mechanically exfoliated from corresponding bulk crystals (HQ Graphene) onto polydimethylsiloxane (PDMS) stamps. For better homogeneity of the target heterobilayer, the monolayers were exposed to UV light48 for 10âmin. To achieve accurate layer alignment in the heterobilayer, the directions of crystallographic axes for the monolayers were determined from polarization-resolved second-harmonic generation (SHG) measurements under excitation with laser pulses of 1200ânm center wavelength and 100âfs duration. The WSe2 and Mo0.5âW0.5âSe2 monolayers were then stacked together on a PDMS substrate with their crystallographic axes aligned via dry transfer at a temperature of 60â°C. The twist angle between the monolayers in the resulting heterobilayer was measured again with polarization-resolved SHG. Finally, the heterobilayer was placed onto the Au-covered substrate for near-field measurements via dry transfer.
Multifunctional TEPL spectroscopy and imaging setup
Multifunctional TEPL spectroscopy is based on the bottom-illumination mode confocal optics setup combined with shear-force AFM using the Au tip. For the excitation beam, He-Ne laser (λâ=â632.8ânm, optical power P of â¤0.5âmW) was passed through a radial polarizer and then focused at the Au tip-Au film junction by an oil immersion objective lens (PLN100x, 1.25âNA, Olympus). The radial polarizer was used to make vertically polarized beam component as large as possible at the tip apex which leads to effective coupling of exciton and cavity plasmon inducing highly enhanced TEPL signals. The backscattered TEPL signals from a sample were collected by the same objective lens. Note that we use high NA objective lens for efficient collection of the interlayer exciton emissions which has out-of-plane dipole moment. In addition, undesirable far-field background noise was reduced by using a pinhole in the detection scheme. TEPL signals (633ânm cut-off) were then sent to a spectrometer (fâ=â320âmm, 150âg/mm, ~1.6ânm spectral resolution, Monora320i, Dongwoo Optron) and finally imaged onto a thermoelectrically cooled charge-coupled device (CCD, DU971-BV, Andor) to obtain TEPL spectra. For hyperspectral nano-imaging, TEPL spectra at each pixel were recorded during an AFM scanning by a digital controller (Solver next SPM controller, NT-MDT) based on the Au tip attached on a quartz tuning fork. The Au tip (apex radius of ~10ânm) was prepared by the refined electrochemical etching protocol49 and attached to a tuning fork with a super glue. The tip-sample distance was regulated by the shear-force feedback through monitoring the changing dithering amplitude of the tuning fork/tip assembly.
Tip-induced pressure-engineering of heterobilayer
To perform the nanoscale pressure-engineering of the heterobilayer using the Au tip, we gradually changed the setpoint of the shear-force feedback. To modify the electronic bandstructure (Fig. 3), we gradually lowered the setpoint to ~75% of the initial oscillating amplitude to induce ~GPa pressure to the crystal structures.
Simulations of electronic bandstructures in TMD hetero- and homo-bilayers
For this study, we performed first-principle calculations for the geometry optimization and electronic band structures using density functional theory (DFT) implemented in Quantum ESPRESSO50. The electronic exchange-correlation interactions were treated using generalized gradient approximation (GGA) with the method of Perdew-Burke-Ernzerhof (PBE)51. The optimized norm-conserving Vanderbilt fully relativistic pseudopotentials52 from the PseudoDojo library [http://www.pseudo-dojo.org/] were used. The Brillouin zone was sampled by a 18âÃâ18âÃâ1 Monkhorst-Pack k-point mesh for all calculations in a 2âÃâ2 supercell. The kinetic energy cut-off was 70âRy. The full geometry optimizations were performed with energy and force tolerances of 10â6âeV and 10â6âeVâà â1, respectively. To simulate the hydrostatic pressure, we fixed metal atoms in two planes separated by a fixed distance dI and relaxed vertical positions of Se atoms. A vacuum space of 20âà was considered between the bilayers to avoid any interaction between them. To account for the interlayer vdW interactions, we used vdW density functionals53 for all the simulations.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
Yu, W. J. et al. Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials. Nat. Nanotechnol. 8, 952â958 (2013).
Britnell, L. et al. Strong light-matter interactions in heterostructures of atomically thin films. Science 340, 1311â1314 (2013).
Ciarrocchi, A. et al. Excitonic devices with van der Waals heterostructures: valleytronics meets twistronics. Nat. Rev. Mater. 7, 449â464 (2022).
Kennes, D. M. et al. Moire heterostructures as a condensed-matter quantum simulator. Nat. Phys. 17, 155â163 (2021).
Policht, V. R. et al. Dissecting interlayer hole and electron transfer in transition metal dichalcogenide heterostructures via two-dimensional electronic spectroscopy. Nano Lett. 21, 4738â4743 (2021).
Rivera, P. et al. Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures. Nat. Commun. 6, 6242 (2015).
Nayak, P. K. et al. Probing evolution of twist-angle-dependent interlayer excitons in MoSe2/WSe2 van der Waals heterostructures. ACS Nano 11, 4041â4050 (2017).
Rivera, P. et al. Interlayer valley excitons in heterobilayers of transition metal dichalcogenides. Nat. Nanotechnol. 13, 1004â1015 (2018).
Yu, H. Y. et al. Moire excitons: from programmable quantum emitter arrays to spin-orbit-coupled artificial lattices. Sci. Adv. 3, e1701696 (2017).
Unuchek, D. et al. Room-temperature electrical control of exciton flux in a van der Waals heterostructure. Nature 560, 340â344 (2018).
Jauregui, L. A. et al. Electrical control of interlayer exciton dynamics in atomically thin heterostructures. Science 366, 870â875 (2019).
Wang, Z. F. et al. Evidence of high-temperature exciton condensation in two-dimensional atomic double layers. Nature 574, 76â80 (2019).
Harats, M. G. et al. Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2. Nat. Photonics 14, 324â329 (2020).
Zhang, S. et al. Nano-spectroscopy of excitons in atomically thin transition metal dichalcogenides. Nat. Commun. 13, 542 (2022).
Yao, K. Y. et al. Nanoscale optical imaging of 2D semiconductor stacking orders by exciton-enhanced second harmonic generation. Adv. Optical Mater. 10, 2200085 (2022).
Siday, T. et al. Ultrafast nanoscopy of high-density exciton phases in WSe2. Nano Lett. 22, 2561â2568 (2022).
Tran, T. N. et al. Enhanced emission from interlayer excitons coupled to plasmonic gap cavities. Small 17, 2103994 (2021).
May, M. A. et al. Nanocavity clock spectroscopy: resolving competing exciton dynamics in WSe2/MoSe2 heterobilayers. Nano Lett. 21, 522â528 (2021).
Park, S., Kim, D. & Seo, M. K. Plasmonic photonic crystal mirror for long-lived interlayer exciton generation. ACS Photonics 8, 3619â3626 (2021).
Kravtsov, V. et al. Spin-valley dynamics in alloy-based transition metal dichalcogenide heterobilayers. 2D Mater. 8, 025011 (2021).
Cho, C. et al. Highly strain-tunable interlayer excitons in MoS2/WSe2 heterobilayers. Nano Lett. 21, 3956â3964 (2021).
Schwartz, J. J. et al. Chemical identification of interlayer contaminants within van der Waals heterostructures. ACS Appl. Mater. Interfaces 11, 25578â25585 (2019).
Koo, Y. et al. Tip-induced nano-engineering of strain, bandgap, and exciton funneling in 2D semiconductors. Adv. Mater. 33, 2008234 (2021).
Darlington, T. P. et al. Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature. Nat. Nanotechnol. 15, 854â860 (2020).
Park, K. D. et al. Probing bilayer grain boundaries in large-area graphene with tip-enhanced Raman spectroscopy. Adv. Mater. 29, 1603601 (2017).
Park, K. D. et al. Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect. Nat. Nanotechnol. 13, 59â64 (2018).
Sichert, J. A. et al. Quantum size effect in organometal halide perovskite nanoplatelets. Nano Lett. 15, 6521â6527 (2015).
Rodriguez, A. et al. Nano-optical visualization of interlayer interactions in WSe2/WS2 heterostructures. J. Phys. Chem. Lett. 13, 5854â5859 (2022).
Kravtsov, V. et al. Control of plasmon emission and dynamics at the transition from classical to quantum coupling. Nano Lett. 14, 5270â5275 (2014).
Ji, J. et al. Selective chemical modulation of interlayer excitons in atomically thin heterostructures. Nano Lett. 20, 2500â2506 (2020).
Zhang, Y. C. et al. Improving resolution in quantum subnanometre-gap tip-enhanced Raman nanoimaging. Sci. Rep. 6, 25788 (2016).
Shanks, D. N. et al. Nanoscale trapping of interlayer excitons in a 2D semiconductor heterostructure. Nano Lett. 21, 5641â5647 (2021).
Ciarrocchi, A. et al. Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures. Nat. Photonics 13, 131â136 (2019).
Zhao, W. Y. et al. Dynamic tuning of moire excitons in a WSe2/WS2 heterostructure via mechanical deformation. Nano Lett. 21, 8910â8916 (2021).
Ma, X. L. et al. Robust interlayer exciton in WS2/MoSe2 van der Waals heterostructure under high pressure. Nano Lett. 21, 8035â8042 (2021).
Xia, J. et al. Strong coupling and pressure engineering in WSe2-MoSe2 heterobilayers. Nat. Phys. 17, 92â98 (2021).
Lee, H. et al. Tip-induced strain engineering of a single metal halide perovskite quantum dot. ACS Nano 15, 9057â9064 (2021).
Grimme, S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 27, 1787â1799 (2006).
Zhao, Y. Y. et al. Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2. Nano Lett. 13, 1007â1015 (2013).
Benimetskiy, F. A. et al. Measurement of local optomechanical properties of a direct bandgap 2D semiconductor. APL Mater. 7, 101126 (2019).
Currie, M. et al. Optical control of charged exciton states in tungsten disulfide. Appl. Phys. Lett. 106, 201907 (2015).
Ghods, S. & Esfandiar, A. Plasmonic enhancement of photocurrent generation in two-dimensional heterostructure of WSe2/MoS2. Nanotechnology 32, 325203 (2021).
Deilmann, T. & Thygesen, K. S. Interlayer trions in the MoS2/WS2 van der Waals heterostructure. Nano Lett. 18, 1460â1465 (2018).
Choi, C. et al. Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures. npj 2D Mater. Appl. 2, 30 (2018).
Zhang, H. et al. Plasmon-induced interfacial hot-electron transfer directly probed by Raman spectroscopy. Chem 6, 689â702 (2020).
Reddy, H. et al. Determining plasmonic hot-carrier energy distributions via single-molecule transport measurements. Science 369, 423â426 (2020).
Tagliabue, G. et al. Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures. Nat. Mater. 19, 1312â1318 (2020).
Jain, A. et al. Minimizing residues and strain in 2D materials transferred from PDMS. Nanotechnology 29, 265203 (2018).
Neacsu, C. C., Steudle, G. A. & Raschke, M. B. Plasmonic light scattering from nanoscopic metal tips. Appl. Phys. B 80, 295â300 (2005).
Giannozzi, P. et al. Advanced capabilities for materials modelling with quantum espresso. J. Phys.: Condens. Matter 29, 465901 (2017).
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865â3868 (1996).
Hamann, D. R. Optimized norm-conserving vanderbilt pseudopotentials. Phys. Rev. B 88, 085117 (2013).
Thonhauser, T. et al. Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond. Phys. Rev. B 76, 125112 (2007).
Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2020R1C1C1011301, 2021R1A6A1A10042944, and NRF-2021R1A2C1008452) and supported by the MSIT, Korea, under the ITRC support program (IITP-2022-RS-2022-00164799) supervised by the IITP and supported by internal fund/grant of Electronics and Telecommunications Research Institute (ETRI) [21YB2100, Research on Device Technology of SiC-based Single Photon Sources for Quantum Cryptography] and supported by the Samsung Science and Technology Foundation (SSTF-BA2102-05) and supported by the COMPA funded by the MSIT (2023-22030003-30). This work was supported by Priority 2030 Federal Academic Leadership Program and the Ministry of Science and Higher Education of Russian Federation, goszadanie no. 2019-1246. 2D materials fabrication was funded by Russian Science Foundation, project 21-72-10100. V.P. acknowledges the support by the National Science Foundation under Grant No. 2230727.
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Y.K., H.L., V.K., and K.-D.P. conceived the experiments. Y.K. and H.L. performed the TEPL spectroscopy and control experiments. T.I., E.K., and V.K. prepared and transferred TMD MLs on the Au substrate. V.P., A.K. performed theoretical calculation and modeling of the electronic bandstructure. Y.K., H.L., V.K., and K.-D.P. analyzed the data, and all authors discussed the results. Y.K., H.L., V.K., and K.-D.P. wrote the manuscript with contributions from all authors. K.-D.P. supervised the project.
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Koo, Y., Lee, H., Ivanova, T. et al. Tunable interlayer excitons and switchable interlayer trions via dynamic near-field cavity. Light Sci Appl 12, 59 (2023). https://doi.org/10.1038/s41377-023-01087-5
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DOI: https://doi.org/10.1038/s41377-023-01087-5