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Hugo Henck, Diego Mauro, Daniil Domaretskiy, Marc Philippi, Shahriar Memaran, Wenkai Zheng, Zhengguang Lu, Dmitry Shcherbakov, Chun Ning Lau, Dmitry Smirnov, Luis Balicas, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Vladimir I. Fal’ko, Ignacio Gutiérrez-Lezama, Nicolas Ubrig, Alberto F. Morpurgo

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[Light sources with bias tunable spectrum based on van der Waals interface transistors](https://mdr.nims.go.jp/datasets/01ceff04-9ea5-48cd-b813-57babfdf752f)

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Light sources with bias tunable spectrum based on van der Waals interface transistorsARTICLELight sources with bias tunable spectrum based onvan der Waals interface transistorsHugo Henck 1,2, Diego Mauro1,2, Daniil Domaretskiy 1,2, Marc Philippi1,2, Shahriar Memaran3,4,Wenkai Zheng3,4, Zhengguang Lu3,4, Dmitry Shcherbakov 5, Chun Ning Lau5, Dmitry Smirnov 3,4,Luis Balicas 3,4, Kenji Watanabe 6, Takashi Taniguchi 7, Vladimir I. Fal’ko 8,9,Ignacio Gutiérrez-Lezama1,2, Nicolas Ubrig 1,2✉ & Alberto F. Morpurgo 1,2✉Light-emitting electronic devices are ubiquitous in key areas of current technology, such asdata communications, solid-state lighting, displays, and optical interconnects. Controlling thespectrum of the emitted light electrically, by simply acting on the device bias conditions, is animportant goal with potential technological repercussions. However, identifying a materialplatform enabling broad electrical tuning of the spectrum of electroluminescent devicesremains challenging. Here, we propose light-emitting field-effect transistors based on van derWaals interfaces of atomically thin semiconductors as a promising class of devices to achievethis goal. We demonstrate that large spectral changes in room-temperature electro-luminescence can be controlled both at the device assembly stage –by suitably selecting thematerial forming the interfaces– and on-chip, by changing the bias to modify the deviceoperation point. Even though the precise relation between device bias and kinetics of theradiative transitions remains to be understood, our experiments show that the physicalmechanism responsible for light emission is robust, making these devices compatible withsimple large areas device production methods.https://doi.org/10.1038/s41467-022-31605-9 OPEN1 Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva, Switzerland. 2 Department of Applied Physics,University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva, Switzerland. 3 National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA.4Department of Physics, Florida State University, Tallahassee, FL 32306-4350, USA. 5Department of Physics, The Ohio State University, Columbus, OH43210, USA. 6 Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 7 InternationalCenter for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 8 National Graphene Institute,University of Manchester, Booth Street East, M13 9PL Manchester, UK. 9Henry Royce Institute for Advanced Materials, M13 9PL Manchester, UK.✉email: nicolas.ubrig@unige.ch; alberto.morpurgo@unige.chNATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunications 11234567890():,;http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-022-31605-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-022-31605-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-022-31605-9&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-022-31605-9&domain=pdfhttp://orcid.org/0000-0001-7441-7624http://orcid.org/0000-0001-7441-7624http://orcid.org/0000-0001-7441-7624http://orcid.org/0000-0001-7441-7624http://orcid.org/0000-0001-7441-7624http://orcid.org/0000-0002-6202-2550http://orcid.org/0000-0002-6202-2550http://orcid.org/0000-0002-6202-2550http://orcid.org/0000-0002-6202-2550http://orcid.org/0000-0002-6202-2550http://orcid.org/0000-0001-5125-8691http://orcid.org/0000-0001-5125-8691http://orcid.org/0000-0001-5125-8691http://orcid.org/0000-0001-5125-8691http://orcid.org/0000-0001-5125-8691http://orcid.org/0000-0001-6358-3221http://orcid.org/0000-0001-6358-3221http://orcid.org/0000-0001-6358-3221http://orcid.org/0000-0001-6358-3221http://orcid.org/0000-0001-6358-3221http://orcid.org/0000-0002-5209-0293http://orcid.org/0000-0002-5209-0293http://orcid.org/0000-0002-5209-0293http://orcid.org/0000-0002-5209-0293http://orcid.org/0000-0002-5209-0293http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0003-0828-0310http://orcid.org/0000-0003-0828-0310http://orcid.org/0000-0003-0828-0310http://orcid.org/0000-0003-0828-0310http://orcid.org/0000-0003-0828-0310http://orcid.org/0000-0002-1966-4435http://orcid.org/0000-0002-1966-4435http://orcid.org/0000-0002-1966-4435http://orcid.org/0000-0002-1966-4435http://orcid.org/0000-0002-1966-4435http://orcid.org/0000-0003-0974-3620http://orcid.org/0000-0003-0974-3620http://orcid.org/0000-0003-0974-3620http://orcid.org/0000-0003-0974-3620http://orcid.org/0000-0003-0974-3620mailto:nicolas.ubrig@unige.chmailto:alberto.morpurgo@unige.chwww.nature.com/naturecommunicationswww.nature.com/naturecommunicationsS ince the discovery that monolayer semiconducting transi-tion metal dichalcogenides (TMDs) are direct gap semi-conductors exhibiting strong luminescence1,2, two-dimensional (2D) materials have attracted interest for optoelec-tronic applications3–6. Their potential stems from the ease withwhich the electronic properties of 2D semiconductors can betuned by different means7–11. In phosporene, for instance,mechanical strain can be used to tune the bandgap by a very largeamount, resulting in a controllable change of the wavelength ofemitted light, as observed in recent photoluminescencemeasurements12. In semiconducting TMDs, electrostatic gatinggives access to a variety of excitonic states with differentenergy13,14, an effect that has been used to realize bias-tunableelectroluminescent devices with emission energy that has beenvaried by several tens of meV9,11. Realizing electroluminescentdevices enabling much broader changes in the spectrum of theemitted light by simply acting on the device operation point (i.e.,on the device bias) has however not been possible so far.Light-emitting field-effect transistors (LEFETs) are three-terminal devices that allow switching of both the electrical con-ductance and light emission15–19. They rely on semiconductorsthat support ambipolar transport to inject simultaneously in thetransistor channel electrons and holes20, whose radiative recom-bination is the origin of the emitted light21,22. Past research onLEFETs has concentrated on organic semiconductors, which havesuitable properties for their realization18,19,23–25. Ionic-gatedLEFETs based on 2D semiconductors are a recently discoveredalternative that offer potential advantages, such as higher andwell-balanced electron and hole mobilities, as well as low-biasoperation26–30. Efficient LEFETs, however, require the use of 2Dsemiconductors with a direct bandgap, whose paucity limits thepossibility to tune the spectrum of the emitted light. Van derWaals (vdW) interfaces formed by atomically thin semi-conducting materials provide a strategy to address this issuebecause the wavelength of light emitted by interlayer transitions(electrons hosted in one layer recombining with holes hosted inthe other) can be engineered by selecting constituent materialswith an appropriate band alignment31–35.Here, we demonstrate experimentally LEFETs realized on vdWinterfaces, and show that they can be operated as electricallytunable light sources. As compared to LEFETs based on indivi-dual monolayers, devices fabricated on vdW interfaces potentiallyoffer more functionality. The electronic structure of the individuallayers, for instance, is often only minorly affected by the interfaceformation, so that a rich set of electronic levels—i.e., the bands ofthe two materials, including the sub-bands originating fromquantum confinement- is present36–40. If properly populated byacting on the device operation point (i.e., the applied source-drainand gate voltages, VSD and VG, respectively), these levels mayenable the energy of the emitted light to be tuned. Additionally,the electric field perpendicular to the transistor channel creates apotential difference between the layers forming the interface,which shifts the energy of the recombining electrons andholes41,42. The wavelength of light generated by interlayer tran-sitions is expected to shift accordingly, providing another route totune the emission spectrum by acting on the device operationpoint. These ideas disclose possible mechanisms to operateLEFETs based on vdW interfaces as electrically tunable lightsources. However, neither their validity nor the potential ofLEFETs based on vdW interfaces has been assessed so far.Here, we show that light-emitting transistors based on arecently discovered type of van der Waals (vdW) heterostructures—which we refer to as Γ–Γ interfaces (see discussion below)—exhibit room-temperature electroluminescence that can be tunedover a much broader spectral range (from below 1.2 to 1.7 eV, inthe devices reported here) by acting exclusively on the deviceoperation point. Our work relies on devices made of bilayers (2L)of semiconducting transition metal dichalcogenides (TMDs; weuse WS2 and MoS2) and InSe multilayers, to form vdW interfacesthat belong to a recently identified class exhibiting robust radia-tive interlayer transitions (e.g., transitions that are radiativeirrespective of the lattice structure of the constituent materials orof their relative orientation)38,43,44. The robustness originatesfrom having the conduction and valence band extrema in the twolayers at k= 0, i.e., at the Γ-point of the Brillouin zone (which iswhy we refer to these systems as to Γ–Γ interfaces. It is importantbecause it facilitates the device assembly, and makes it compatiblewith simple large-area production techniques9,45. Indeed, we findthat all our LEFETs exhibit electroluminescence, with a wave-length that can be engineered by selecting the constituent layers,and with a spectrum that can be tuned by acting on the deviceoperation point. Contrary to earlier studies of Γ–Γ interfaces44—in which photoluminescence (PL) was only observed at cryogenictemperatures– electroluminescence (EL) is already present atroom temperature, a key finding when assessing the technologicalpotential of these devices.ResultsDevice fabrication and characterization. Ionic-gated LEFETs(see Fig. 1a–c and Supplementary section 1) based on 2L-TMD/InSe vdW interfaces (see Fig. 1d) are realized using techniquescommonly employed for the assembly of structures based on 2Dmaterials46. TMD bilayers and InSe multilayers are exfoliatedfrom bulk crystals onto Si/SiO2 substrates. Heterostructures areformed by picking up layers one after the other, and transferringthe resulting interface onto a fresh Si/SiO2 substrate, with theTMD layer covering the InSe one and effectively encapsulating it(which is important in view of the non-perfect stability of InSe inambient; the interface assembly process is carried out in thecontrolled atmosphere of a glove box). Source and drain contacts,as well as a large pad acting as gate electrode, are defined by meansof electron-beam lithography, electron-beam evaporation of a Pt-Au film (5/30 nm) and lift-off (see Fig. 1e). Subsequently, awindow in PMMA is patterned to define the region where theionic liquid ((N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammo-nium bis(trifluoromethylsulfonyl) imide) commonly referred to asDEME-TFSI) contacts the interface. The liquid is applied as a finalstep, prior to inserting the devices in a vacuum chamber withoptical access (see “Methods” section for more details).The WS2 and MoS2 crystals used for exfoliation are purchasedfrom HQ Graphene. InSe is a less commonly employedcompound and care is needed because the crystal quality variesstrongly depending on details of the growth process. Lowerquality can result in a large density of defects47–49 that create in-gap states acting as hole traps, which is why our earlier attemptsto use InSe crystals to realize vdW interface LEFETs failed (in-gapstates prevent the electrostatic accumulation of holes in thevalence band of the TMD, and impede ambipolar transport). TheInSe crystals that we employ here, grown at Florida StateUniversity (see “Methods” section for details of the growthprocess), appear to have high quality and a low density of defects,as we directly infer from the transistor electrical characteristicsand from the narrow lines observed in PL studies of h-BN-encapsulated multilayers (see Fig. 1f).Figure 2a shows the room-temperature transfer curve (source-drain current ISD vs. gate voltage VG at fixed source-drain voltageVSD= 50 mV) of a device realized on a 2L-WS2/4L-InSe interface.The behavior is typical of ambipolar transistors with currentmediated by holes and electrons flowing for sufficiently largenegative and positive VG, respectively24,26,50–55. Accumulation ofelectrons and holes leads to comparable current levels, confirmingARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-92 NATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsthat transport is well-balanced and that residual defects in InSe donot prevent high-quality device operation. When plotted inlogarithmic scale (Fig. 2b) the data allow determining thesubthreshold swing S ¼ ln 10 dVGdðln ISDÞ, equal to S= 115 and90 mV/decade near the threshold for hole and electron conduc-tion, respectively (other devices exhibit values even closer to theultimate room-temperature limit of 60 mV/decade)56. The outputcurves (ISD-vs.-VSD plotted for different, fixed VG) shown inFig. 2c, d also exhibit the expected behavior. Upon increasingVSD, ISD increases linearly at first then saturates, and eventuallyexhibits a very steep increase, when entering the ambipolarinjection regime. This regime—in which electrons and holes areinjected at opposite contacts—is the one of interest for LEFEToperation (see Supplementary section 1), and can be reachedirrespective of the polarity of the applied gate voltage.Electroluminescence from vdW interface LEFET. EL is expectedto occur concomitantly with ambipolar injection, with lightemission starting at one of the contacts and shifting into thechannel as VSD is further increased18. This is indeed what weobserve (see Fig. 3a as well as Supplementary Fig. 2 and accom-panying discussion in Supplementary section 1). The light emit-ted by the LEFET is collected by a microscope objective and fedinto a spectrometer. The spectral analysis performed on datameasured at fixed VG, by increasing VSD past the onset of theambipolar injection regime (VSD >+1.9 V in Fig. 3b for VG > 0and VSD < –2.2 V in Fig. 3e for VG < 0 V), is shown in Fig. 3c andd for VG > 0 V, and in Fig. 3f, g for VG < 0 V. As the currentincreases exponentially rapidly, we initially limit the maximumapplied VSD to avoid damaging the devices. EL is detected in allcases, with an intensity that increases rapidly with VSD. The lightexhibits a dominant spectral line just above 1.2 eV, irrespective ofthe precise value of VSD and of whether VG has positive ornegative polarity (an additional shoulder at 1.4 eV is visible forVG < 0, see Fig. 3g, is also present—albeit less pronounced—forVG > 0).In Fig. 3h, we plot the device EL spectrum (red line;VG=+0.5 V and VSD=+2.2 V) together with the PL spectrumof 4L-InSe (purple line; PL is measured at T= 5 K, since no signalis observed at room temperature) and 2L-WS2 (blue line). Theenergy of the EL peak is considerably lower than the recombina-tion energy in either 4L-InSe or 2L-WS2, as expected for aninterlayer transition44. The energy of the room-temperature ELsignal (Fig. 3j, thick line) matches that of low-temperatureFig. 1 LEFETs based on van der Waals interfaces. a Schematics of an ionic-gated field-effect transistor, with the source-drain (S-D) contactsconnected to a semiconducting layer and the gate electrode, all in contactwith an ionic liquid. The zoom in on the channel regions shows that uponthe application of a gate voltage charge is accumulated on thesemiconductor (see Supplementary Section 1 for details). b, c Electron-holerecombination in the channel of an ionic liquid (IL)-based LEFET operated inthe ambipolar injection regime (with electrons and holes injected atopposite contacts) for a device based on an individual 2D material (b) andon a vdW interface (c). In the latter case, light is emitted by therecombination of electrons and holes hosted in different layers, whichoffers new opportunity to control its spectrum (see main text; here we useinterfaces of bilayer TMDs, hosting holes, and InSe multilayers, hostingelectrons). d The type-II band alignment between InSe and TMDs aroundthe Γ-point enables k-direct interlayer radiative transitions between the twoband edges (as indicated by the orange arrow and labeled IX). Higherenergy bands originating from the quantum confinement of charge carriersin the two layers (as indicated by the thin lines) can lead to radiativetransitions with different energy. e Optical micrograph of a device used inthis work, based on 2L-WS2 and 4L InSe (the contours of the layers aremarked by the blue and white lines; the stacking sequence is indicated ontop). The vdW interface is in contact with the liquid through an opening inthe PMMA layer covering the entire sample. The scale bar is 2 μm. f Peak inthe PL spectrum of a 5L InSe crystal encapsulated between two thicker hBNlayers: the narrow width indicates the high quality of the material.I SD(nA)050100150200VG (V)−2 −1.5 −1 −0.5 090 mV/dec115 mV/decI SD( μA)10−1010−910−810−7VG (V)−2 −1 0-1.8V-2VI SD(μA)−2−1.5−1−0.50VSD (V)−2−1.5−1−0 5.00.1V0.5VI SD(μA)−10123VSD (V)0 0.5 1 1.5 2 2.5a bc dVSD = 50 mVFig. 2 Transistor characteristics of a 2L-WS2/4L InSe device. a Room-temperature transfer curve (ISD-vs.-VG at fixed VSD= 50mV) of a 2L-WS2/4L-Inse. b The high device quality is evidenced by the low subthresholdswings for electron and hole transport, whose values (115 mV/decade and90mV/decade, respectively, for this specific transistor) are close to theultimate limit of 60mV/decade. c and d Output characteristics (ISD asfunction of VSD) of the same device for different negative and positive gatebiases, respectively. Linear, saturation, and ambipolar injection regimes canbe clearly identified upon increasing the magnitude of VSD, as described inthe main text (see also Supplementary section 1).NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-9 ARTICLENATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunications 3www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsinterlayer transitions seen in PL (Fig. 3j, thin line) due toelectrons in InSe recombining with holes in WS2, if we take intoaccount that the TMD and the InSe gap typically increases byapproximatively 50-100 meV upon cooling from 300 to5 K14,57–59. The measurements, therefore, confirm that ourLEFET operates as anticipated, with electrons injected in theInSe layer and holes in the WS2 one recombining via an interlayertransition. Finding that this transition results in EL even at roomtemperature is a positive, unexpected surprise.Devices based on other Γ–Γ vdW interfaces should exhibit allkey properties of 2L-WS2/4L-InSe LEFETs. We verify that this isindeed the case using transistors realized on interfaces of 2L-MoS2 (instead of 2L-WS2), and 3L-, 4L-, and 5L-InSe. Withoutgoing through all details (see Supplementary section 2), the datashow the occurrence of ambipolar transport (Fig. 4a) and of theambipolar injection regime past saturation (Fig. 4b). Uponentering the ambipolar injection regime, EL is observed resultingin a line at 1.3 eV (for 2L-MoS2/5L-InSe) independently of VSD(see Fig. 4c, d), i.e., an energy lower than that of the transitions inthe constituent materials (see Fig. 4e). Figure 4f overviews theresults obtained, by plotting together the room-temperature ELspectrum of LEFETs fabricated on all different vdW interfaces,and shows that combining different 2D materials indeed allows adense coverage of part of the near-infrared and visible spectralrange. Selecting multilayers of different thicknesses or havingdifferent compositions (e.g., MoSe2 or MoTe2) would furtherbroaden the accessible spectrum, both on the higher and lowerend38,43,44.Electrically tunable EL spectrum in vdW interface LEFET.Having established that LEFETs based on Γ–Γ interfaces provide arobust platform to generate room-temperature EL, we test whe-ther the light spectrum can be controlled by varying the deviceoperation point. Figure 5 illustrates the evolution of the spectrumof the light emitted by a 2L-WS2/4L-InSe (Fig. 5a–c) and by a 2L-MoS2/5L-InSe (Fig. 5d–f) LEFETs, upon pushing the source-drainbias VSD to reach deeper in the ambipolar transport regime. Thegreen rectangle in Fig. 5a delimits the VSD interval discussedearlier, and the corresponding part of the spectrum in Fig. 5c(also delimited by a green rectangle) shows emission from theinterlayer transition just above 1.2 eV, in agreement with the dataFig. 3 Electroluminescence from a Γ–Γ interface. a Images of the channel of a transistor based on a 2L-WS2/4L-InSe heterostructure taken with an opticalmicroscope (top) and with the camera of our spectrometer (bottom). The bottom image, taken with the device biased at the onset of the ambipolarinjection regime, shows a bright spot due to EL. The scale bars are 1 μm. b Device output curve measured at, VG=+0.5 V (electron accumulation). c False-color plot of the EL intensity measured by the spectrometer, as a function of photon energy and applied VSD, showing a peak centered around 1.25 eV thatemerges at VSD=+1.9 V, corresponding to the onset of ambipolar injection (see b). d Individual EL spectra at selected VSD values extracted from c: in thisbias range the spectrum remains unchanged, and the intensity increases following the increase in source-drain current. e–g Data analogous to those ofpanels b–d are shown for VG= –2.2 V (hole accumulation), demonstrating that the presence of EL is robust and that at sufficiently low bias the spectrum isvirtually identical for electron and hole accumulation. h The peak in the EL spectrum (acquired with VSD=+2.2 V and VG=+0.5 V) is red-shifted relativeto the PL emission energies of the layers forming the interface (purple: 4L Inse; blue: 2L-WS2; for InSe, PL data are taken at 5 K, because no PL is observedat room temperature), as expected from an interlayer Γ–Γ transition (see i). j Comparison between the normalized EL (thick red line) and PL (thin red line)emission spectra of the interface. The energy difference originates from having to measure PL at cryogenic temperatures (T= 5 K), since no PL is observedat room temperature.ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-94 NATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsshown in Fig. 3b, c. When larger VSD is applied, corresponding tothe interval in the rectangle delimited by the red line in Fig. 5a,the spectrum evolves. Additional transitions appear, visible inFig. 5b in the region delimited by the red rectangle, as well as inFig. 5c, which shows the spectrum of the emitted light at specificvalues of VSD. A qualitatively identical behavior is observed inLEFETs based on 2L-MoS2/5L-InSe, with the regime of lower andhigher VSD highlighted by red and green rectangles in Fig. 5d andthe corresponding spectra shown in Fig. 5e, f.We have also measured the spectrum of the emitted light at afixed VSD value, as a function of VG, and found that in that case aswell, the spectrum depends strongly on the device operationpoint. Figure 6a–d show the spectrum of the light emitted by adevice realized on a 2L-WS2/4L-InSe interface, as a function ofgate voltage, for two different values of source-drain bias(VSD=+2.4 V in Fig. 6a, b and VSD=+3.12 V in Fig. 6c, d).Changing the gate voltage at fixed VSD allows switching thespectrum of the light between two transitions visible in Fig. 6b, c.In particular, at large positive gate voltage (VG >+0.4 V in Fig. 6aand VG >+0.8 V in Fig. 6c), the spectrum of the emitted light isdominated by the interlayer transition at 1.2 eV between thebottom of the InSe conduction band and the top of the TMDvalence band. At low gate voltage (VG <+0.2 V in Fig. 6a andVG <+0.6 V in Fig. 6c), instead, light is emitted by anothertransition (possibly by multiple transitions, as suggested by thebroad linewidth) at higher energy (approximately 1.4 eV), whichappears to blue shift upon increasing VG. Unexpectedly, the tworegimes are separated by an interval of gate voltages in which thepower of emitted light vanishes (or is below the sensitivity of ourdetector). Finding that the gate allows switching the spectrumbetween two different emission lines is interesting, as it mayprovide new functionality to these LEFETs devices.As part of our experiments, we have also determined theexternal quantum efficiency of our devices—i.e., the ratio betweenthe number of emitted photons detected in our set-up and thenumber of injected electrons—by comparing the measured ELsignal to the signal measured (with the same set-up) when using acommercial light-emitting diode as source. We found that, in theexperimental configuration employed to detect EL, the externalquantum efficiency of our devices is approximately 0.005%, threeorders of magnitude smaller than that of commercial devices. Itshould be realized, however, that in our set-up the externalquantum efficiency is much lower than the actual quantumefficiency, because the optical selection rules for interlayertransitions37,60 dictate that light is mostly emitted in the planeof the interface. This implies that the majority of the photonsemitted by our light-emitting transistors are not collected by themicroscope objective, and that adopting strategies to improve thelight outcoupling should lead to a drastic enhancement of themeasured external quantum efficiency.Irrespective of these considerations about the external quantumefficiency, our observation that the EL spectrum does depend on thedevice operation point proves that LEFETs based on vdW interfacesare indeed electrically tunable light sources. Understanding in detailhow the EL spectrum depends on the LEFET operation point ishowever complex, both because different processes likely play a role,and because screening due to charges accumulated in the transistorchannel can strongly (and non-linearly) affect the potentialdifference between the two layers forming the interface, especiallyin the region of the transistor channel where electron-holerecombination occurs. At sufficiently large VSD, we expect thatelectrons are injected not only in the conduction band of InSe butalso in that of the TMD, so that light can be emitted also fromintralayer transitions within the TMD. This may account for thepeak centered around 1.6 eV in 2L-WS2/4L-InSe, which correspondswell to one of the 2L-WS2 PL peaks. An intralayer transition in thesemiconducting TMD is likely also responsible for part of the broadpeak around 1.5 eV in the MoS2-based interfaces (the energymatches one of the peaks observed in PL of 2L-MoS2). The lesspronounced peaks near 1.4 eV (at comparable but different energiesin the 2L-WS2 and the 2L-MoS2 devices; see Fig. 5b, e) occur at anenergy that changes slightly upon changing VSD. As the energy ofthese peaks is lower than all known intralayer transitions in therespective systems, we attribute their origin to an interlayer transitionbetween an electron in a higher energy InSe sub-band recombiningwith a hole in the TMD (whose precise energy is affected by theelectrostatic potential difference between the layers). This attributionis also consistent with data taken at fixed VSD upon varying VG (seeFig. 6), in which the transition energy is seen to blue shift uponchanging the gate voltage.Fig. 4 LEFET based on a MoS2/InSe interface. a, b Transfer and output characteristics of a 2L-MoS2/5L-InSe transistor. c EL spectra of the device biasednear the onset of ambipolar injection, for electron and hole accumulation (the gray and blue curves are measured respectively at VG= –0.7 V andVSD=+2.1 V, and at VG= –2.7 V and VSD= –2.3 V), showing a peak around 1.3 eV independently of the applied gate voltage. d Dependence of EL intensityon photon energy and VSD, measured at VG= –0.7 V, showing that near the onset of ambipolar transport the spectrum is independent of VSD. e Also in thiscase, the EL spectrum (thick orange line) is red-shifted as compared to the PL emission energy of the layers forming the interface (purple: 4L InSe; green:2L-MoS2), as expected for an interlayer transition. f EL spectrum of LEFETs realized using four different interfaces, based on two different 2L TMDs (blue:MoS2; red: WS2) and three different thicknesses of the InSe layer (3L, 4L, and 5L, as indicated in the figure), showing a dense coverage of part of the visiblespectrum (a broader range of photon energy can be spanned using other semiconducting TMD compounds).NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-9 ARTICLENATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunications 5www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsMore work is clearly needed to understand in detail theelectroluminescence spectrum of Γ–Γ interfaces at large biases, aswell as its evolution with both source-drain and gate biases (asmentioned earlier, screening plays a major and complex role inthe way spectral features shift as function of biases, and aquantitative description will require a separate, dedicatedmodeling effort). We note, however, that the spectrum of lightemitted by LEFETs based on monolayer TMDs remainsunchanged even under driving the device with very largesource-drain biases, as discussed in Supplementary section 3and shown in Supplementary Fig. 4 in there. The data, therefore,appear to substantiate our initial idea—namely that LEFETsbased on vdW interfaces offer more functionalities than similardevices realized from individual monolayers—irrespective of theprecise microscopic origin of the emitted light (i.e., of the specifictransitions involved in the light emission process).DiscussionThe results presented above demonstrate the operation of vdWinterface LEFETs and show that these transistors do allow therealization of light sources with a bias-tunable spectrum. Findingthat devices realized with multiple semiconducting TMDs andwith InSe layers of different thickness lead to a qualitativelysimilar evolution of the spectrum of the emitted light with biasindicates that the operation mechanism is robust and of generalvalidity. This robustness is important because—whereas only afew examples of electroluminescent devices with an electricallycontrollable spectrum have been reported in the past61–64—alarge variety of 2D semiconductors exists that can be employed torealize light-emitting Γ–Γ interfaces.It is clear that at this stage light-emitting transistors based onΓ–Γ vdW interfaces remain proof-of-principle devices and thatconsiderable research is needed to characterize them and opti-mize their operation. We anticipate, for instance, that ionic liquidgating can be replaced by conventional solid-state gates usingnm-thick h-BN dielectrics to separate the gate electrode from thevdW heterostructure. Indeed, h-BN layers that are a few nan-ometers thick exhibit breakdown field values approaching 1 V/nm65, which are likely sufficient to achieve ambipolar transportand to operate the devices with VSD larger than VG. Similarly—asalready mentioned—the external quantum efficiency of ourdevices will need to be improved by adopting suitable strategies tooptimize light outcoupling. While it is clear that at this stageFig. 5 Bias-tunable light emission from vdW interface LEFETs. a Outputcharacteristics of the 2L-WS2/4L-InSe device, whose data are shown inFig. 3 (VG=+0.2 V). The colored dashed lines delimit the low-bias (greenline) and high-bias (red line) regime, which exhibit different EL spectralproperties. b Color plot of the EL spectrum as a function of photon energyand VSD. In the low-bias regime (region inside the green rectangle) ELexhibit a single peak, due to a Γ–Γ interlayer transition from the bottom ofthe InSe conduction band to the top of the WS2 valence band (see Fig. 3). Inthis regime the spectrum is independent of bias. In the high-bias regime(region inside the red rectangle), the EL spectrum evolves upon increasingVSD, showing that the LEFET acts a light source with bias-tunable spectrum.c Individual EL emission spectra measured in the high-bias regime, uponvarying VSD from 2.3 to 2.5 V. d–f Same measurements as those shown inpanels a–c, but performed on a 2L-MoS2/5L-InSe LEFET (data taken atVG= –0.7 V). The data illustrate that the evolution of the EL spectrumobserved in this MoS2/InSe LEFET is fully analogous that observed in theWS2/InSe LEFET, showing the robustness of the device operation.Fig. 6 Gate-tuning of the spectrum of the light emitted from a 2L-WS2/4L-InSe LEFET device. a Color plot of the intensity of the light emitted by a 2L-WS2/4L-InSe LEFET device as function of photon energy and gate voltage (VSD is fixed to +2.4 V). The data show that varying VG allows the frequency ofthe emitted light to be tuned: for VG > 0.4 V, the spectrum is peaked at 1.25 eV, i.e., the characteristic energy of the interlayer transition between thebottom of the InSe conduction band and the top of the WS2 valence band; for VG < 0.2 V a broader peak centered around 1.4 eV and shifting with varyingVG is observed. b Individual horizontal cuts of the color plot shown in a, for VG varying from –0.1 to +0.7 V in 0.1 V steps. c and d Same measurements as ina and b with VSD fixed at +3.12 V. The individual spectral cuts range from VG=+0.2 V to +1.1 V in 0.1 V steps.ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-96 NATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunicationswww.nature.com/naturecommunicationsdifferent aspects of the device require improvements, it is worthre-iterating that Γ–Γ interfaces used within a light-emittingtransistor configuration represent a platform that offers a veryhigh potential for the realization of electroluminescent deviceswith bias-tunable spectrum, and that satisfies many key require-ments essential to the development of a successful technology.These include room-temperature operation and insensitivity ofthe devices to details of their assembly process, which ensurestheir robust operation. It is for these reasons that exploring thedevelopment of light-emitting transistors based on Γ–Γ interfacesappears to be promising for future device applications.MethodsCrystal growth. InSe single crystals were grown through the Bridgman method:6N-pure indium and 5N-pure selenium pellets in an atomic ratio of 52:48 weresealed in an evacuated quartz ampule and subsequently placed into a radio fre-quency (RF) furnace where the RF power was gradually increased to raise thetemperature up to 800 ∘C. The ampule was then pulled through the hottest zone ata rate of 2 mm/h. Single crystals were characterized via electron dispersive spec-troscopy and aberration-corrected transmission electron microscopy. X-ray dif-fraction measurements confirmed that γ-InSe adopts the R3m space group (160)with unit cell dimensions a= 4.08(2)Å and c= 24.938(24)Å. The quality of thecrystals grown in this way is illustrated by the different measurements presented inthe main text, including the narrow photoluminescence spectrum measured onexfoliated multilayers encapsulated in between hBN crystals, and the ambipolartransport properties of InSe/TMD heterostructures.Sample fabrication. The fabrication of the heterostructures used to perform themeasurements discussed in the main text relies on conventional techniques that arecommonly employed to manipulate atomically thin crystals46 and is briefly out-lined here for completeness. Atomically thin layers of TMDs and InSe are obtainedby mechanical exfoliation of bulk crystals in a nitrogen gas-filled glove box with a<0.5 ppm concentration of oxygen and water. The exfoliated crystals are trans-ferred onto Si/SiO2 substrates and suitable layers are identified by looking at theiroptical contrast under an optical microscope. The heterostructures are thenassembled in the same glove box with by-now conventional pick-up and releasetechniques based on either PPC/PDMS (poly(propylene carbonate)/poly-dimethylsiloxane) or PC/PDMS (polycarbonate) polymer stacks placed on glassslides46. To avoid degradation of air-sensitive InSe crystals, the structures areassembled so that the InSe layer lays on a thick hBN layer ( ≈ 20 nm) and is coveredby the TMD layer (thereby ensuring that the InSe layer is properly encapsulated inbetween air-stable materials). Metallic electrodes (Pt/Au) are attached to the TMDlayer by conventional nanofabrication techniques using electron-beam lithography,electron-beam evaporation, and lift-off. The sample is wire-bonded with indium orgold wires to a chip carrier and a small amount of ionic liquid ((N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl) imide) com-monly referred to as DEME-TFSI) is dropcasted onto the surface of the substrate tocover the metallic gate electrode and the transistor channel. The device is thenrapidly transferred into the vacuum chamber with optical access and pumpedovernight to remove moisture from the ionic liquid prior to the optical and elec-trical investigations.Optical measurements. The sample is mounted in a vacuum chamber positionedunder an optical microscope, providing optical and electrical access to the sample.The photoluminescence and electroluminescence of our light-emitting field-effecttransistors are collected with help of a microscope objective and sent to a Czerny-Turner monochromator with a grating of 150 grooves/mm (Andor Shamrock500i). The signal is detected with a Silicon Charge Coupled Device (CCD) array(Andor Newton 970 EMCCD). For photoluminescence measurements, the sampleis illuminated with a laser beam generated by a supercontinuum white light lasersource combined with a contrast filter, allowing to set the illumination wavelengthto 610 nm. The laser power is kept at 50 μW, to avoid damaging the structures.Transport measurements. The electrical characterization of our FET is performedin the same chamber used for optical measurements. The gate bias voltage isapplied using either a Keithley 2400 source unit or a homemade low-noise voltagesource. The current and voltage signals are amplified with homemade low-noiseamplifiers, and the amplified signals are recorded with an Agilent 34410A digitalmultimeter unit.Data availabilityThe data supporting the findings of this study are available free of charges from theYareta repository of the University of Geneva. https://doi.org/10.26037/yareta:mze42hgmc5cqtdsqmeszkprfxm.Received: 15 February 2022; Accepted: 13 June 2022;References1. Splendiani, A. et al. Emerging photoluminescence in monolayer MoS2. NanoLett. 10, 1271–1275 (2010).2. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS_{2}:a new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).3. Mak, K. F. & Shan, J. Photonics and optoelectronics of 2D semiconductortransition metal dichalcogenides. Nat. Photon. 10, 216–226 (2016).4. Avouris, P., Heinz, T. F. & Low, T. 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Room temperature upconversion electroluminescencefrom a mid-infrared In(AsN) tunneling diode. Appl. Phys. Lett. 116, 142108(2020).65. Hattori, Y., Taniguchi, T., Watanabe, K. & Nagashio, K. Layer-by-layerdielectric breakdown of hexagonal boron nitride. ACS Nano 9, 916–921(2015).AcknowledgementsWe gratefully acknowledge Alexandre Ferreira for continuous and precious technicalsupport. A.F.M. gratefully acknowledges financial support from the Swiss National Sci-ence Foundation (Division II) and from the EU Graphene Flagship project. V.F.acknowledges support from EC-FET Quantum Flagship Project 2D-SIPC and EPSRCgrant EP/S030719/1. L.B. acknowledges support from US NSF-DMR 1807969 (synthesis,physical characterization, and heterostructure fabrication) and the Office Naval ResearchDURIP Grant 11997003 (stacking under inert conditions). L.B. acknowledges the use ofthe facilities at the Platform for the Accelerated Realization, Analysis, and Discovery ofInterface Materials (PARADIM), which is supported by the US-NSF under the Coop-erative Agreement No. DMR-2039380. D.S. acknowledges support from the U.S.Department of Energy (DE-FG02-07ER46451) for photoluminescence measurements ofInSe. D.S. and C.N.L. acknowledge the support by NSF/ECCS award 2128945. TheNational High Magnetic Field Laboratory acknowledges support from the US-NSFCooperative agreement Grant number DMR-1644779 and the state of Florida. K.W. andT.T. acknowledge support from the Elemental Strategy Initiative conducted by theMEXT, Japan (Grant Number JPMXP0112101001) and JSPS KAKENHI (Grant Num-bers 19H05790, 20H00354, and 21H05233).Author contributionsH.H., D.M., D.D., and M.P. fabricated the devices with the help of I.G.L.; H.H., and N.U.measured the devices and analyzed the data. S.M., W.Z., and L.B. grew the InSe crystals.Z.L., D.S., C.N.L., and D.S. performed photoluminescence measurements on hBNencapsulated InSe multilayer, to characterize their quality. V.F. identified the high qualityof the InSe crystals and suggested their use the realization of different devices. K.W. andT.T. provided the hBN crystals. H.H., I.G.L., N.U., and A.F.M. discussed the data. A.F.M.wrote the manuscript with input from N.U., H.H., and I.G.L. All the authors read andcommented on the manuscript. N.U. and A.F.M. supervised the research.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version contains supplementary materialavailable at https://doi.org/10.1038/s41467-022-31605-9.Correspondence and requests for materials should be addressed to Nicolas Ubrig orAlberto F. Morpurgo.Reprints and permission information is available at http://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, as long as you giveappropriate credit to the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. If material is not included in thearticle’s Creative Commons license and your intended use is not permitted by statutoryregulation or exceeds the permitted use, you will need to obtain permission directly fromthe copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2022ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31605-98 NATURE COMMUNICATIONS |         (2022) 13:3917 | https://doi.org/10.1038/s41467-022-31605-9 | www.nature.com/naturecommunicationshttps://doi.org/10.1038/s41467-022-31605-9http://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/www.nature.com/naturecommunications Light sources with bias tunable spectrum based on van der Waals interface transistors Results Device fabrication and characterization Electroluminescence from vdW interface LEFET Electrically tunable EL spectrum in vdW interface LEFET Discussion Methods Crystal growth Sample fabrication Optical measurements Transport measurements Data availability References References Acknowledgements Author contributions Competing interests Additional information