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Jed Kistner-Morris, Ao Shi, Erfu Liu, Trevor Arp, Farima Farahmand, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Vivek Aji, Chun Hung Lui, Nathaniel Gabor

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[Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayers](https://mdr.nims.go.jp/datasets/ee86352d-18bc-4fcc-9164-5e9abb058b64)

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Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayersArticle https://doi.org/10.1038/s41467-024-48321-1Electric-field tunable Type-I to Type-II bandalignment transition in MoSe2/WS2heterobilayersJed Kistner-Morris1,6, Ao Shi1,6, Erfu Liu 1,2, Trevor Arp1,3, Farima Farahmand 1,Takashi Taniguchi 4, Kenji Watanabe 5, Vivek Aji1, Chun Hung Lui 1 &Nathaniel Gabor 1Semiconductor heterojunctions are ubiquitous components of modern elec-tronics. Their properties depend crucially on the band alignment at theinterface, which may exhibit straddling gap (type-I), staggered gap (type-II) orbroken gap (type-III). The distinct characteristics and applications associatedwith each alignment make it highly desirable to switch between them within asingle material. Here we demonstrate an electrically tunable transitionbetween type-I and type-II band alignments in MoSe2/WS2 heterobilayers byinvestigating their luminescence and photocurrent characteristics. In theirintrinsic state, these heterobilayers exhibit a type-I band alignment, resultingin the dominant intralayer exciton luminescence from MoSe2. However, theapplication of a strong interlayer electric field induces a transition to a type-IIband alignment, leading to pronounced interlayer exciton luminescence.Furthermore, the formation of the interlayer exciton state traps free carriers atthe interface, leading to the suppression of interlayer photocurrent and highlynonlinear photocurrent-voltage characteristics. This breakthrough in elec-trical band alignment control, interlayer exciton manipulation, and carriertrapping heralds a new era of versatile optical and (opto)electronic devicescomposed of van der Waals heterostructures.Heterojunctions, where two different materials meet, form thefundamental building blocks formodern functional devices1, such aslight emitting diodes2, photodetectors3 and field-effect transistors4.The critical determinant of heterojunction device characteristics liesin the alignment of conduction and valence bands between the twomaterials5–7. Type-I band alignment occurs when the band gap of onematerial is fully encompassed within the band gap of the othermaterial, i.e., both the conduction band minimum (CBM) and thevalence band maximum (VBM) of the heterostructure reside in thesame material. This straddled band alignment causes photoexcitedelectrons and holes to relax into the samemedium8,9 (Fig. 1a, b). As aresult, the excitons have larger electron-hole wavefunction overlap,higher oscillator strength, and shorter radiation lifetime, whichfavor applications in light emitting devices10,11. In contrast, type-IIband alignment situates the CBM of the heterostructure in onematerial and the VBM in the other (Fig. 1c, d). This staggered align-ment causes the photoexcited electrons and holes to relax to dif-ferent materials. This facilitates exciton dissociation andReceived: 13 February 2023Accepted: 22 April 2024Check for updates1Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA. 2National Laboratory of Solid State Microstructures, School ofPhysics, and Collaborative Innovation Center of AdvancedMicrostructures, Nanjing University, Nanjing 210093, China. 3Department of Physics, University ofCalifornia, Santa Barbara, CA 93106, USA. 4International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba305-0044, Japan. 5Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 6These authorscontributed equally: Jed Kistner-Morris, Ao Shi. e-mail: joshua.lui@ucr.edu; nathaniel.gabor@ucr.eduNature Communications |         (2024) 15:4075 11234567890():,;1234567890():,;http://orcid.org/0000-0002-0231-798Xhttp://orcid.org/0000-0002-0231-798Xhttp://orcid.org/0000-0002-0231-798Xhttp://orcid.org/0000-0002-0231-798Xhttp://orcid.org/0000-0002-0231-798Xhttp://orcid.org/0009-0003-5189-0760http://orcid.org/0009-0003-5189-0760http://orcid.org/0009-0003-5189-0760http://orcid.org/0009-0003-5189-0760http://orcid.org/0009-0003-5189-0760http://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/0000-0003-3701-8119http://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/0000-0002-1470-328Xhttp://orcid.org/0000-0002-1470-328Xhttp://orcid.org/0000-0002-1470-328Xhttp://orcid.org/0000-0002-1470-328Xhttp://orcid.org/0000-0002-1470-328Xhttp://orcid.org/0000-0002-0351-2787http://orcid.org/0000-0002-0351-2787http://orcid.org/0000-0002-0351-2787http://orcid.org/0000-0002-0351-2787http://orcid.org/0000-0002-0351-2787http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-024-48321-1&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-024-48321-1&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-024-48321-1&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-024-48321-1&domain=pdfmailto:joshua.lui@ucr.edumailto:nathaniel.gabor@ucr.eduphotocarrier extraction and hence favors applications inphotodetection12,13 and photocatalysis14.In the conventional scenario, the band energy offset betweentwo materials is fixed; this imposes many constraints on devicefunctionality. To usher in the next generation of devices, it is highlydesirable to transition between different alignment types within asingle heterostructure. The realization of tunable band alignmentholds immense potential for unlocking unprecedented multi-functionality in device applications. However, achieving suchdemonstrations is challenging. While certain approaches involvematerial engineering through adjusting chemical compositions15–18 orlayer thickness19, these methods necessitate multiple synthesisiterations, precise dopant control, or the use of different samples. Amore efficient alternative envisions the capability to tune the bandalignment types through straightforward electrical means in a singledevice, though, to the best of our knowledge, such a demonstrationhas not been realized.A groundbreaking advancement in the fabrication of band-engineered heterostructures has emerged within the realm of two-dimensional (2D) van derWaals (vdW)materials20–24, notably includingmonolayer transition metal dichalcogenides (TMDs) like MoSe2 andWS2. These TMDs feature direct bandgaps, heavy carriers, robustexcitons25–32, and innovative spin-valley-coupled physics33–37. In con-trast to conventional grown heterostructures, 2D vdW hetero-structures exhibit reduced dimensions, atomically sharp interfaceswithout dangling bonds, and high tolerance for lattice mismatch.These attributes greatly enhance their stability, tunability and overallfunctionality. Moreover, the continually expanding library of diverse2D materials offers a rich palette of options for creating vdW hetero-structures. Indeed, recent theoretical propositions have delved intothe possibility of tuning band alignments in 2D vdW heterostructuresthrough various means, including electric field38,39, strain38, interlayerspacing38, and twist angle40. However, experimental demonstration ofsuch band alignment tuning is still lacking.In this Article, we demonstrate an electric-field-induced transi-tion between type-I and type-II band alignments in MoSe2/WS2 het-erobilayers, as evidenced through photoluminescence (PL) andphotocurrent (PC) spectroscopy. Figure 1 illustrates our keyfindings. Initially, in the absence of an external electric field, theheterojunction of monolayer MoSe2 and WS2 exhibits a type-I bandalignment. In this state, the WS2 CBM is marginally higher than theMoSe2 CBM, while the WS2 VBM is considerably lower than theMoSe2 VBM (Fig. 1a, b). This configuration, with its straddled bandgap, facilitates the relaxation of photo-excited electrons and holes tothe same (MoSe2) layer, leading to a pronounced PL peak of intra-layer excitons (AMo) at 1.6 eV (blue line in Fig. 1e). However, theapplication of a strong vertical electric field, directed from the WS2to MoSe2 layer, triggers a critical shift—the WS2 CBM moves belowthe MoSe2 CBM (Fig. 1c, d). This alteration leads to a type-II bandalignment with staggered band gaps, where the photoexcited elec-trons and holes tend to relax to different layers—electrons to theWS2layer and holes to the MoSe2 layer. This separation leads to theformation of interlayer excitons (IX), which have a lower energy thantheir intralayer counterpart, resulting in a dominant PL peak at~1.57 eV (Fig. 1e). Beyond the striking shift of the PL spectrum, theemergence of interlayer excitons also effectively traps electron-holepairs at the heterojunction, thereby suppressing the interlayerphotocurrent (Ipc). This results in a highly nonlinear relationshipbetween Ipc and the applied interlayer voltage. Overall, our findingshighlight the profound impact of the type-I to type-II transition onthe optical and optoelectronic properties of devices. The ability toelectrically control this transition opens up exciting possibilities fordesigning innovative multifunctional devices using vdWheterostructures.Results and discussionPhotoluminescence measurementsOur experiment employs dual-gate MoSe2/WS2 heterobilayers encap-sulated in hexagonal boron nitride (BN)41 (Supplementary Fig. S1,Fig. 2a). We use thin graphite flakes to contact the TMDs and electro-des to enhance device performance. The heterobilayers for PL mea-surements have twist angles of either ~0� or ~ 60�. Deviation fromthese angles will suppress the interlayer emission due to electron-holemomentummismatch. Belowwewill present the PL results of Device 1while the reflectance contrast results are presented in Section 4 of theSupplementary Information.Type-I Band Alignment Type-II Band AlignmentIntralayer exciton (AMo)WS2MoSe2abeWS2MoSe2 HeterobilayerVBVBWS2MoSe2EEcdIntralayerExciton (AMo)InterlayerExciton (IX)hOnOffhWS2MoSe2 HeterobilayerVBCBhheeCBVBFig. 1 | Illustration of electric-field-induced type-I to type-II band alignmenttransition in MoSe2/WS2 heterobilayer. a, b Atomic layers and energy banddiagram of a MoSe2/WS2 heterobilayer with no electric field. The position-dependent conduction bands (CB) and valence bands (VB) of monolayer MoSe2(blue) and WS2 (red) exhibit the type-I alignment. The photoexcited electrons andholes relax to the MoSe2 layer to form intralayer excitons (AMo). c, d Atomic layersand energy band diagram under strong vertical electric field. The heterobilayerexhibits type-II band alignment. The photoexcited electrons (holes) relax to theWSe2 (MoSe2) layer to form interlayer excitons (IX). e Measured photo-luminescence (PL) spectra under zero (blue) and E =0.25 V/nm (red) electric field.The interlayer excitons are turned on and off by the electric field.Article https://doi.org/10.1038/s41467-024-48321-1Nature Communications |         (2024) 15:4075 2In Device 1, the top and bottom BN have similar thickness. Byapplying voltages with the same sign on the bottom gate (Vbg) and topgate (Vtg), we can inject carriers into the sample without inducing anyvertical electric field. The charge-density-dependent PL map of Device1 (Fig. 2b) exhibits a pronounced line (AMo) at 1.60 eV at the chargeneutrality region, whichmatches the reported A-exciton energy in BN-encapsulatedMoSe2monolayers42–45. Upon injecting electrons or holesinto the heterobilayer, the AMo peak subsides and two new PL peaks(A�Mo, A+Mo) emerge at ~30meV below the exciton on the electron andhole side, respectively. This energy separation is close to the knownexciton-polaron (or trion) binding energies in monolayer MoSe242–45.Therefore, we infer that AMo, A�Mo, A+Mo originate from the intralayerexciton and exciton polarons in the MoSe2 monolayer. We note thatthe weak lines at higher energies than AMo may arise frommoiré effector sample inhomogeneities, since they are not reproducible in otherdevices (Supplementary Figs. S8 and S9). Atomic reconstruction isunlikely to occur in this system due to the small moiré wave-length (~8 nm).By applying voltages with opposite signs on the bottom and topgate (Vbg = −Vtg), we can apply a vertical electric field between theMoSe2 and WS2 layers while maintaining the heterobilayer in thecharge neutrality regime. Figure 2c displays the PL map of Device 1 atvarying Vbg = –Vtg (left axis), from which we extract the out-of-planeelectric field (right axis) with a BN dielectric constant of 3.4 (see thedetails in Supplementary Information, Section 2.1). At weak electricfield (E <0.16 V/nm pointing from WS2 to MoSe2), the AMo PL lineremains pronounced and exhibits no Stark shift; this observationconfirms its intralayer nature and supports the type-I alignment of theheterobilayer (Fig. 1b). When the electric field exceeds 0.16 V/nm, theAMo line subsides and below it emerges a newPLpeak (IX ). The IX peakredshifts linearly with a slope of 44 ± 11meV per 0.1 V/nm of field; the± 11meV error is mainly due to the uncertain BN dielectric constant(2.6–4.2) with a minor ±0.9meV linear-fit uncertainty. At high field(E >0.23 V/nm), IX becomes bright and dominates the PL.The Stark shift of IX indicates that it has an out-of-plane dipole, asignatureof interlayer excitons.By assuming that the electron andholeare localized in different layers, we deduce an electron-hole separationof 0.4 ± 0.1 nmbased on the Stark shift. This separation is comparableto the interlayer spacing (~0.6 nm) of the heterobilayer46, providingevidence for the origin of interlayer excitons.Whenwe extrapolate theStark shift of IX linearly to zero electric field, we arrive at an energy of~1.69 eV, which is ~90meV above the AMo line at 1.60 eV. Consideringthe different exciton binding energies between AMo and IX , we furtherestimate that the WS2 CBM resides ~40meV above the MoSe2 CBM,comparable to a predicted 0.03-eV band offset in ref. 47 (see Supple-mentary Information, Section 3).Our observation can be readily explained using the schematics inFig. 1. At lowfield, theheterobilayer exhibits a type-I alignmentwith theWS2 CBM lying slightly above the MoSe2 CBM. Consequently, photo-carriers relax to theMoSe2 layer to form intralayer excitons (Fig. 1a, b).As the electric field (directed from WS2 to MoSe2) increases, the WS2CBM is lowered, leading to a transition to a type-II alignment. In thetype-II configuration, photoexcited electrons and holes relax to dif-ferent layers to form interlayer excitons (Fig. 1c, d). We note that anopposite electric field (fromMoSe2 to WS2) elevates theWS2 CBM anddoes not induce the type-II alignment transition. This is consistentwithour observation that no IX peak appears at negative electricfield (Fig. 2c).Photocurrent measurementsIn addition to the striking PL shift, the emergenceof interlayer excitonscan also drastically affect the optoelectronic charge transport throughthe MoSe2/WS2 interface. We have measured photocurrent in anotherBN-encapsulated MoSe2/WS2 heterobilayer (Device 2), which hassource and drain contacts with a SiO2/Si back gate (Fig. 3a). We firstcharacterize the device by measuring the interlayer current with nooptical illumination as a function of source-drain voltage (Vsd) and gatevoltage (Vg) at room temperature. Vsd is applied to the WS2 flake andcurrent is measured from MoSe2 (Fig. 3a). The interlayer current issmall at negative Vg and becomes increasingly large at positive Vg(Fig. 3b). This indicates n-type transport mediated by electrons in theconduction bands. At constant positive Vg, the current is nearly zero atVsd < 0, but increases dramatically with an exponential turn-on atVsd > 0; such rectifying behavior is consistent with our band scheme inFig. 1b, where the MoSe2 CBM is lower than the WS2 CBM. This furthersupports the intrinsic type-I alignment (see more discussions in Sup-plementary Information, Section 5.1).Afterward wemeasure the interlayer photocurrent (Ipc) under theexcitation of an ultrafast laser. We tune the laser photon energy to be_ω = 1.49 eV (λ = 830nm),which is close to theMoSe2 exciton resonantenergy at room temperature (see Methods). Figure 3c displays aphotocurrent color map at varying Vsd and Vg. The photocurrentT ≈ 6 KT ≈ 6 KAMoIXAMoAMoAMo-+acbGraphiteBNBNSiO2Si GraphiteMoSe2WS2VtgVbgFig. 2 | Photoluminescence signature of type-I to type-II band alignmenttransition in MoSe2/WS2 heterobilayers. a The schematic of a dual-gate MoSe2/WS2 heterobilayer device encapsulated by boron nitride. b The charge-density-dependent PL map of Device 1. Equal voltages Vbg =Vtg are applied to the bottomand top gates. The charge density is proportional to the gate voltages. The AMo,A�Mo, A+Mo features arise from the intralayer excitons, electron-side and hole-sideexciton polarons (or trions) in the MoSe2 layer, respectively. c The electric-field-dependent PLmapofDevice 1. Opposite voltagesVbg = −Vtg (left axis) are applied tothe bottom and top gates to induce an interlayer electric field (right axis). Aninterlayer exciton (IX) feature appears at high electric field. The dashed line is alinear extrapolation of its Stark shift to zero field. All measurements were per-formed with 532-nm laser excitation (incident power ≈ 3μW) at sample tempera-ture T ≈ 6 K.Article https://doi.org/10.1038/s41467-024-48321-1Nature Communications |         (2024) 15:4075 3occurs predominantly in forward bias (Vsd > 0 V applied to WS2), con-sistent with the type I band alignment shown in Fig. 1b. Notably, thephotocurrent map exhibits a narrow, curved region where Ipc dropswith increasing Vsd. Such an anomalous suppression of interlayerphotocurrent canbe seen clearly fromthe Ipc-Vsd line traces at differentVg (Fig. 3d). As Vsd increases, the interlayer photocurrent first increa-ses, then drops in the range of Vsd = 1–2 V, and afterward increasesagain. Similar non-monotonic Ipc-Vsd characteristics are found for awide range of Vg from −3.0 to −7.5 V, where the suppression regionshifts from lower to higherVsd values (Fig. 3c). This shift of suppressionregion with Vg is likely due to the change of contact resistance whenthe carrier density is modulated by the global silicon gate. A highercontact resistancemeans a higherVsd is required to drive an equivalentinterlayer voltage drop across the heterobilayer. By imaging the spatialphotocurrent response, we confirm that this photocurrent suppres-sion only occurs in the heterobilayer region (Supplementary Fig. S7).We have extracted the interlayer electric field (E) from Vsd bymodeling the device as a p-n junction and fitting its charge transportdata (see Supplementary Information, Section 2.2). The anomaloussuppression of photocurrent starts near E ~ 0.23 V/nm, comparable tothe critical field (E ~ 0.16 V/nm) of interlayer exciton formationobserved in the PL map (right axis in Fig. 2c). This suggests that thephotocurrent suppression is induced by the interlayer excitonformation.To clarify the origin of the photocurrent suppression, we haveinvestigated its dependence on the excitation laser power P. Fig-ure 4a shows the photocurrent Ipc at varying Vsd (top axis) andelectric field E (bottom axis) under increasing laser power atVg = −6.5 V. At low laser power, Ipc increases monotonically withincreasing Vsd. At high laser power, however, Ipc drops in the range ofVsd = 1.0–1.5 V. As the laser power increases, the photocurrentsuppression becomes more severe. By examining the powerdependence of Ipc at varying Vsd and Vg, we find that the photo-current generally increases sublinearly with the laser power (e.g., seethe inset of Fig. 4a) and, remarkably, the degree of sublinearity isclosely related to the photocurrent suppression.The photocurrent dynamics under pulsed excitation can be cap-tured by using a simple model. Each laser pulse instantaneously gen-erates N0 electron-hole pairs, which drops to zero population beforeFig. 4 | Photocurrent signature of type-I to type-II band alignment transition inMoSe2/WS2 heterobilayer. a Photocurrent (Ipc) as a function of source-drain vol-tage (Vsd, top axis) and interlayer electric field (bottom axis) under various exci-tation laser powers. The color of each curve represents the excitation power, asindicated by the color scale bar. The inset shows the photocurrent with increasinglaser power at Vsd = 1.15 V (E =0.24V/nm). The red line is a fit based on the modeldescribed in the text. The gate voltage is Vg = −6.5 V in all measurements. b Thebest-fit γ=ðα +βÞ value as a function of electric field. The vertical dashed linesapproximately define three regions that correspond to the three scenarios depic-ted by the lower insets, which, from left to right, illustrate the absence, formationand dissociation of interlayer excitons at increasing field. The measurements wereconducted at 830-nm excitation wavelength at room temperature.Fig. 3 | Photocurrent of MoSe2/WS2 heterobilayer. a The schematic of a MoSe2/WS2 heterobilayer device with silicon back gate (Device 2). b Grayscale map ofinterlayer current I as a function of the source-drain voltage (Vsd) and gate voltage(Vg)with nooptical excitation. cColormapof interlayer photocurrent Ipc vs.Vsd andVg and (d) Cross-cut Ipc-Vsd profiles extracted from panel c at specific Vg values,denoted by dashed lines with corresponding colors. e Color map of the log of therelative rates of multi-particle decay (Auger processes) to single electron-hole pairdecay γ=ðα +βÞ as a function of Vsd and Vg over the same range as in (c).Article https://doi.org/10.1038/s41467-024-48321-1Nature Communications |         (2024) 15:4075 4the next pulse arrives. The photocurrent is obtained asIPC = efαZ 10N tð Þdt ð1ÞHere e is the elementary charge; f is the pulse repetition rate; α is thecarrier extraction rate; N tð Þ is the time-dependent population ofelectron-hole pairs after the pulsed excitation. NðtÞ decays accordingto the equationdN=dt = � ðα + βÞN � γN2: ð2ÞHere β is the recombination rate of the electron-hole pairs. The�ðα + βÞN termdescribes the decrease of carrier population due to theextraction and first-order recombination of photocarriers. γ is thedecay rate due to exciton-exciton annihilation or Auger process.Whenexciton density N increases at increasing laser power, the nonlineardecay term (�γN2) becomesmore important, leading to faster excitondecay and photocurrent suppression.Solving Eq. (2) givesN tð Þ= N0e� α +βð Þt1 + γN0α +β 1� e� α +βð Þt� � ð3Þwhich, when combined with Eq. (1), yields an analytical expression forthe photocurrent under pulsed excitation:IPC =ef α +βð ÞγlnγN0α +β+ 1� �ð4ÞBy assuming that N0 scales linearly with the laser power (P), wecan use Εq. (4) to fit the Ipc—P data. The fitting is excellent for a broadrange ofVsd andVg values (e.g., see the line in the inset of Fig. 4a). Fromthe fitting, we extract the relative rate γ= α +βð Þ (up to a proportionalityconstant) between multi-particle decay (Auger-like processes) andsingle electron-hole pair decay at different Vsd and Vg (Fig. 3e).Figure 4b displays the best-fit γ= α + βð Þ value as a function ofelectric field at Vg = −6.5 V. We observe a striking peak of γ= α +βð Þ inthe rangeVsd = 1–1.5 V,which coincideswith the E-field rangewhere thephotocurrent is suppressed in Fig. 4a. To consolidate this observation,we extract the γ= α +βð Þ value at varying Vsd and Vg (Fig. 3e) and com-pare itwith the photocurrentmap (Fig. 3c). The γ= α +βð Þ enhancementis found to coincide well with the photocurrent suppression. As thecarrier extraction rateα and theAuger-like decay rate γ typically evolvesmoothly with electric field, the sharp peak of γ= α + βð Þ implies asudden decrease of the electron-hole recombination rate β. This isconsistent with the formation of interlayer excitons in a type-I to type-II transition because the interlayer excitons have much smallerrecombination rate (longer lifetime) than the intralayer excitonsdue tothe spatial separation of electrons and holes.The insets of Fig. 4b illustrate how the interlayer exciton forma-tion may account for the observed photocurrent behavior. At weakinterlayer electric field (left inset), the heterobilayer has the type-Iband alignment and hence exhibits rectifying behavior, in which thephotocurrent increases monotonically with increasing interlayer field.When the electric field reaches a critical value Ec ~ 0.15 V/nm (com-parable to Ec ~ 0.16 V/nm in the PL results in Fig. 2c), the heterobilayertransitions from type-I to type-II band alignment, enabling the forma-tion of interlayer excitons (middle inset). The interlayer exciton for-mation traps the carriers at the interface, simultaneously suppressingthe photocurrent and reducing the photocarrier decay rate β (i.e.boosting γ= α +βð Þ). When the increasing electric field becomes strongenough to dissociate the interlayer excitons (right inset), the excitoneffect subsides and the photocurrent resumes its normal increasingtrend with increasing Vsd.Besides Devices 1 and 2 presented above, we have also measuredDevices 3 to reproduce themajor PL results andDevice 4 to reproduceboth the major PL and photocurrent results (see SupplementaryInformation, Sections 1, 6 and 7).In summary, we demonstrate controlled on/off switching ofinterlayer excitons in MoSe2/WS2 heterobilayers through a type-I totype-II transition, which substantially influences the optical propertiesand photocurrent behavior. This phenomenon stems from the closelyaligned conduction band minima with field-tunable offset betweenmonolayer MoSe2 and WS2, and it is not expected to occur in otherTMD heterobilayers with large band offsets47. Our findings establishMoSe2/WS2 heterobilayers as a highly adaptable platform for excitonicresearch and applications. For instance, onemay harness this effect forswitching a hypothetical interlayer excitonic Bose-Einstein con-densate, tuning exciton potential depth, and realizing depth-adjustable exciton traps within van der Waals heterostructure mate-rials. The tuning mechanism complements other ‘live’ tunable para-meters, such as strain, stress, and twist angles, and achieves ‘in-situ’control of band-engineered exciton behaviors. This integration pro-mises a new level of precision and adaptability in manipulating exci-tonic properties in these advanced materials.MethodsDevice fabricationAll MoSe2/WS2 heterobilayer devices are fabricated by applying apolycarbonate-based dry-transfer technique to stack different 2Dcrystals together. The substrates are silicon wafers with 300-nm-thick oxide layer. For the dual-gate Devices 1, 3, 4, we use a poly-carbonate stamp to sequentially pick up a thin graphite flake (ser-ving as the top-gate electrode), a thin BN flake (as the top-gatedielectric), monolayer MoSe2, monolayer WS2, a second thin gra-phite flake (as the contact electrode), another thin BN flake (as thebottom gate dielectric), and a third thin graphite flake (as thebottom-gate electrode). During the stacking process, we align thesharp edges of the MoSe2 and WS2 crystals so that the twist anglesbetween them are expected to be close to 0° or 60°. Afterward, wedeposit the stack of materials onto the Si/SiO2 substrate. Finally, weuse the standard electron-beam lithography to deposit the goldcontacts (70-nm thickness) onto the devices.For single-gate Device 2 used in the photocurrent experiment, wefirst use electron-beam lithography to deposit the two gold contacts(as source and drain electrodes) on a Si/SiO2 substrate. Afterward, weuse a polycarbonate stamp to transfer a BN flake to cover the areabetween the two electrodes. Upon this surface with pre-patternedelectrodes, we transfer a MoSe2/WS2 heterobilayer stack by using alarge thin BN flake to sequentially pick up monolayer MoSe2 andmonolayer WS2. We align the sample position so that the WS2 layercontacts one electrode and the MoSe2 layer contacts the other elec-trode. This allows us to apply a bias voltage between the two layers.Photoluminescence experimentsThe photoluminescence (PL) experiments are performed in a closed-cycle cryostat (Montana), where the sample temperature is estimatedto be T ~ 6 K. The excitation light source is a 532-nm continuous-wavelaser (Torus 532, Laser Quantum). The laser is focused onto the samplewith a spotdiameter of 1~2μmbyanobjective lens (numerical aperture0.6). The incident laser power is P ~ 3μW. The PL is collected by thesame objective and analyzed by a spectrometer (HRS-500-MS, Prin-ceton Instruments) equipped with a charge-coupled-device (CCD)camera. Two Keithley K2400 sourcemeters are used to independentlycontrol the top and bottom gate voltages.Photocurrent experimentsPhotocurrent experiments are performed in vacuum in a customizedJanis Research ST-3T-2 optical cryostat. Device 2 ismeasured at roomArticle https://doi.org/10.1038/s41467-024-48321-1Nature Communications |         (2024) 15:4075 5temperature; Device 4 is measured at T = 50 K. The light source is anultrafast Coherent Mira laser that generates pulses with 150-fsduration and 75-MHz repetition rate. The laser wavelength is tunedto either 790 nm or 830 nm, close to the optical band gap ofmonolayer MoSe2 as well as the energy of the interlayer exciton. Thelaser is focusedonto the samplewith a spot diameter of ~2 µmusing aThorlabs gradient-index (GRIN) lens. The laser position on thesample is controlled by a Thorlabs galvanometer. The galvo position,Vsd, and Vg are controlled by two data acquisition cards (DAQs) fromNational Instruments. We measure the interlayer current with a pre-amplifier (DL Instruments 1211). The optically induced current isextracted from the total current by using a lock-in amplifier (Stan-ford Research) and optical chopper.Data availabilityThe data generated in this study have been deposited into https://github.com/qmolabucr/EField-Tunable-MoSe2WS2. This repositoryincludes all the relevant data and the python scripts that are used togenerate the figures such that the results can be fully replicated.References1. Sze, S. M., Li, Y. & Ng, K. K. Physics of Semiconductor Devices (JohnWiley & Sons, 2021).2. Yao, Y., Hoffman, A. J. & Gmachl, C. F. Mid-infrared quantum cas-cade lasers. Nat. Photonics 6, 432–439 (2012).3. Michel, J., Liu, J. & Kimerling, L. C. High-performance Ge-on-Siphotodetectors. Nat. 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N00014-19-1-2574 (N.M.G. and T.B.A.).C.H.L. acknowledges support from the National Science Foundation(NSF) Division of Materials Research CAREER Award No. 1945660 andfrom the American Chemical Society Petroleum Research Fund No.61640-ND6. K.W. and T.T. acknowledge support from the JSPS KAKENHI(Grant Numbers 19H05790, 20H00354 and 21H05233).Author contributionsA.S., E.L. and J.K.-M. fabricated the devices. A.S. and E.L. performed thephotoluminescence and reflectance contrast experiments. J.K.-M. andF.F. performed thephotocurrent experiments. T. A. andN.M.G. designedthe photocurrent experiments, T.A. and J.K.-M. obtained initial photo-current data that stimulated further study. V.A. provided theoreticalsupport to the interpretation of the experimental data. T.T. and K.W.provided boron nitride crystals. C.H.L. and N.M.G. supervised the pro-ject, while C.H.L., A.S., J.K.-M. wrote the manuscript with input from allother authors.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s41467-024-48321-1.Correspondence and requests for materials should be addressed toChun Hung Lui or Nathaniel Gabor.Peer review information Nature Communications thanks the anon-ymous reviewer(s) for their contribution to thepeer reviewof thiswork. 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If material is notincluded in the article’s Creative Commons licence and your intendeduse is not permitted by statutory regulation or exceeds the permitteduse, you will need to obtain permission directly from the copyrightholder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2024Article https://doi.org/10.1038/s41467-024-48321-1Nature Communications |         (2024) 15:4075 7https://doi.org/10.1038/s41467-024-48321-1http://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/ Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayers Results and discussion Photoluminescence measurements Photocurrent measurements Methods Device fabrication Photoluminescence experiments Photocurrent experiments Data availability References Acknowledgements Author contributions Competing interests Additional information