Ultrafast interfacial carrier dynamics and persistent topological surface states of Bi2Se3 in heterojunctions with VSe2

Vanadium diselenide (VSe2) has recently been highlighted as an efficient 2D electrode owing to its extra-high conductivity, thickness controllability, and van der Waals contact. However, as the electrode, applications of VSe2 to various materials are still lacking. Here, by employing ultrafast time-resolved spectroscopy, we study VSe2-thickness-dependent interfacial effects in heterostructures with topological insulator Bi2Se3 that is severely affected by contact with conventional 3D electrodes. Our results show unaltered Dirac surface state of Bi2Se3 against forming junctions with VSe2, efficient ultrafast hot electron transfer from VSe2 to Bi2Se3 across the interface, shortened metastable carrier lifetimes in Bi2Se3 due to dipole interactions enabling efficient current flow, and the electronic level shift (~tens meV) of bulk states of Bi2Se3 by interfacial interactions, which is ~10 times lower compared to conventional electrodes, implying weak Fermi level pinning. Our observations confirm VSe2 as an ideal electrode for efficient Bi2Se3-based-applications with full utilization of topological insulator characteristics. Topological insulators could be ideal materials for use in electronic devices but complications arising at the interface with metallic electrodes degrades performance values. Here, the authors propose VSe2 as an electrode material investigating the charge dynamics and interface quality using ultrafast transient reflectance measurements and demonstrating the preservation of Dirac surface states of the topological insulator.

T opological insulators (TIs) are a class of electronic materials that possess ordinary insulating states in the bulk and topologically protected metallic states at the surface 1,2 . The topological surface state with massless Dirac fermions exhibits unique properties of spin-momentum locking ensured by Z 2 invariant in three-dimensional (3D) TIs including Bi 2 Se 3 . A spinpolarized and defect-tolerant conducting channel of these TIs enables promising electronic, optoelectronic, and spintronic applications 2,3 , such as ultralow-power tunnel transistor 4,5 , optospintronics with polarized photocurrent 6,7 , high-performance broadband photodetector 8,9 , and nonvolatile memory by spintransfer torque 10 . For device applications in transistors or external circuits, contact with metal electrodes plays a crucial role. However, in the junction with metal electrodes for producing transistors or external circuits, unexpected contact issues occur for several reasons such as interfacial hybridization and defects during the manufacturing process, leading to relaxation of spinmomentum locking and large contact resistance by strong Fermi level pinning (FLP) 11,12 . This can severely degrade the device performance. Moreover, an inherent low dimensionality and the existence of Dirac surface state (DSS) make the material systems themselves very sensitive to the junction interfaces. Although several conventional 3D metals [13][14][15][16] , including Au, Cr, Ti, and Pd, have been adopted in Bi 2 Se 3 -based applications, the interfacial study of Bi 2 Se 3 -based junction has shown that there exist still several contact issues to be solved except for Au, which exhibits weak interfacial interaction with Bi 2 Se 3 12 .
A similar issue for the metal electrode contact has appeared at 2D semiconducting transition metal dichalcogenides (s-TMDs) [17][18][19] , which are critically affected by such interactions with metal electrodes owing to their inherent atomically thin thickness and low dimensionality. As a breakthrough strategy, van der Waals (vdW) contact has been illuminated to form atomically sharp interfaces and suppress chemical interactions at interfaces between two dissimilar materials [18][19][20][21][22] . The vdW metal-semiconductor junctions (MSJs) have been recently demonstrated as the ideal junction interfaces by transferring graphene 21,[23][24][25] or noble 3D metal films [18][19][20] . The desire for realizing an ideal junction interface and optimized metal electrode materials leads to considerable interest in metallic TMDs (m-TMDs) beyond graphene 21,[26][27][28][29] . The m-TMDs have been successfully adopted as metal electrodes for s-TMD because they provide weak FLP in vdW MSJs 21 , high field-effect mobility in VTe 2 / MoS 2 28 , low contact resistance in VS 2 /MoS 2 29 with high functionality, an integrated external circuit with advantages of high-conductivity, tunable thickness, and van der Waals epitaxial contacts 26 , in contrast to the use of conventional 3D metals. Among m-TMDs, VSe 2 , which exhibits thickness controllability and extra-high (10 2 -10 4 order) electrical conductivity compared to other 2D materials, turned out to be a promising electrode material 27 . Recently, VSe 2 at metallic 1T-phase has been applied as the electrode in diverse 2D material systems such as VSe 2 /WSe 2 , VSe 2 /MoSe 2 vdW MSJs with low contact resistance 30 , efficient electrocatalysts for Li-S batteries 31 , and superior K-ion storage 32 . Thus, it is considered important to expand and elucidate the role of VSe 2 as the metal electrode in diverse material systems where the contact issue still remains.
In this work, we investigate the interfacial junction in VSe 2 / Bi 2 Se 3 heterostructures (VBHs) by employing ultrafast optical spectroscopy in reflection geometry. This allows us to examine b c a e f 10   b Raman spectra of VSe 2 and Bi 2 Se 3 . Inset, enlarged Raman spectra of VSe 2 . Each Raman active mode is indicated. c Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a VBH. The interface between VSe 2 and Bi 2 Se 3 shows the empty space of the carbon protective layer deposited before slice cutting by a focused ion beam. The clear vdW gap and atomic configuration are shown. d Corresponding energy-dispersive X-ray spectroscopy (EDS) elemental intensity maps for V, Se, and Bi along with a related schematic of the atomic structure. Note that the data of cross-sectional HAADF-STEM and EDS, which are required for deposition of carbon protecting layer, were collected after completing pump-probe experiments. e Atomic force microscopy (AFM) topography images of VBHs at positions 1-4. The dashed circles indicate the spots of interest in the pump-probe measurements. The scale bar is the same for all three images. f Height profiles along with the arrows at positions 1-4. Note that the measured height is the thickness of VSe 2 on Bi 2 Se 3 . the interfacial phononic/carrier dynamics in a highly sensitive manner [33][34][35] . Since the photoexcited carriers in Bi 2 Se 3 are quite sensitive to the topological state [36][37][38] and Fermi level 39,40 , crucial information on certain modulation of both DSS and bulk state of Bi 2 Se 3 by junctions with metallic VSe 2 can be obtained. An electron microscope and interfacial acoustic phonon dynamics show a good quality of interfacial contact in the VBHs. The hot electron transfer from VSe 2 to Bi 2 Se 3 across the interfaces in ultrashort timescale (~100 fs) leads to the improved efficiency of Bi 2 Se 3 -based optoelectronic devices. In addition, the photoexcited carrier dynamics in VBH provide direct evidence of unaltered DSS of Bi 2 Se 3 by the contact with VSe 2 . The VSe 2 -thicknessdependent spectral and dynamical response in the VBHs quantitatively gives electronic modulation of the bulk band in Bi 2 Se 3 , which originated by interfacial dipole interactions due to the charge transfer/redistribution for thermal equilibrium. The observed electronic level shifts of Bi 2 Se 3 by the junction with VSe 2 are~10 times lower compared to the junction with conventional 3D metals, indicating weak FLP. The lifetime of metastable carriers in Bi 2 Se 3 is shortened by 3 times due to dipole interactions in VBH, enabling highly efficient current flow. Overall results suggest that VSe 2 can be considered as a promising candidate for the vdW metal electrode in Bi 2 Se 3 -based diverse applications by fully utilizing the superior characteristics of topological insulators.

Results and discussion
Preparation and characterization of VBHs. For the investigation, single-crystalline metallic 1T-VSe 2 was grown by a chemical vapor transport method and transferred onto a 10 quintuple layer (QL) Bi 2 Se 3 thin film by mechanical exfoliation. The carrier concentration (N 0 ) of Bi 2 Se 3 was approximately N 0 = 3.13 × 10 20 cm −3 based on the Hall resistivity measurements (see Supplementary Fig. 1). The hall resistance of 10 QL Bi 2 Se 3 shows the characteristics of an n-type Bi 2 Se 3 . As marked positions by black arrows in Fig. 1a, we were able to achieve different thicknesses of VSe 2 on Bi 2 Se 3 in a single flake. The size of a VSe 2 flake on Bi 2 Se 3 was sufficiently large compared to the spatial resolution (<2 μm) of our spectroscopic system, and imperfectly exfoliated VSe 2 flakes showed regions with different thicknesses, enabling us to investigate the VSe 2 -thickness-dependent phononic and carrier dynamics of Bi 2 Se 3 . Different positions of VSe 2 with different thicknesses were marked as Pos. 1 to 4, as shown in Fig. 1a (additionally see Fig. 1e, f). The conspicuous and clear Raman modes (Fig. 1b) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image ( Fig. 1c) confirmed that both VSe 2 and Bi 2 Se 3 had a high-quality single-crystalline nature. Note that Fig. 1b shows the Raman spectra measured in individual VSe 2 and Bi 2 Se 3 , since only the predominant VSe 2 Raman peak at 206 cm −1 was obtained from the VBH owing to relatively thick VSe 2 , which obscures the weak Raman signal of Bi 2 Se 3 at appropriate laser powers below the VSe 2 damage threshold. Each Raman mode showed in good agreement with the previous studies of 1T-VSe 2 and Bi 2 Se 3 41,42 . Note that the absence of Raman mode of the semiconducting 2H-VSe 2 at 190 cm −1,43 which can be formed by structural phase transition during the preparation process, implies that the VSe 2 used in this study was a pure metallic 1T-phase. The thicknesses of VSe 2 at the marked positions (1, 2, 3, and 4) were measured to be 48, 74, 118, and 167 nm on 10 QL Bi 2 Se 3 by atomic force microscopy (AFM), respectively (Fig. 1f). The root means square roughness extracted from the AFM images ( Fig. 1e) was quite low (under~0.1 nm), indicating a clean and smooth surface.
Interfacial coherent acoustic phonon (CAP) dynamics in VBHs. To verify superior interfacial contact between VSe 2 and Bi 2 Se 3 in a large area, collinearly aligned two-color pump-probe spectroscopy was carried out on VBHs. The pump (1.5 eV) and probe (~0.93-0.95) pulses were normally incidents on the VBHs with a time delay. The pump fluence was~12 μJ/cm 2 and the probe fluence was kept sufficiently weak about two orders of magnitude compared to the pump beam. Time-resolved reflectance measurements were performed at each position of the VBHs. The inset of Fig. 2a illustrates a schematic of the process, where coherent acoustic phonons (CAPs) occurred at the interfaces of VBHs. The acoustic wave was generated at the VBH interfaces by the incident optical pulse and propagated into the VSe 2 flake. Figure 2a shows the background-free CAPs in 10 QL Bi 2 Se 3 and 118 nm VSe 2 /10 QL Bi 2 Se 3 (118 nm VBH) obtained by abstraction from the fitting with a biexponential representing the decay dynamics (more details in Supplementary Note 1). Earlier CAP before 40 ps and later echo signals around 80 ps time delay were observed. For the earlier CAP part, the oscillation frequencies of CAPs were obtained by fitting with damped oscillation functions as 125 and 43 GHz. The obtained frequencies well matched the interlayer vibrational modes in 10 QL Bi 2 Se 3 , in which the frequency depended on the number of layers 42,44 . Note that the CAP signal of Bi 2 Se 3 was observed in both Bi 2 Se 3 and the VBH at the same time delay. This result reflected that Bi 2 Se 3 was almost directly excited by the optical pulse.
After damping of the inherent earlier CAP signal in Bi 2 Se 3 , subsequent echo signals were observed. The acoustic wave generated by the optical pump reciprocated in VSe 2 by reflection at the air and Bi 2 Se 3 interface as shown in the inset of Fig. 2a. When the acoustic wave arrived at the VSe 2 -Bi 2 Se 3 interface, echo signals appeared with different intervals depending on the thickness of VSe 2 (Fig. 2b). The acoustic waves at interfaces can be described by the acoustic mismatch (AM) model for boundary conditions of continuous displacement and stress. This indicates perfect and atomically flat interfaces in our case 45 . Based on AM model, the amplitude reflection coefficient is given by Here, Z ¼ ρv s is the acoustic impedance of material, where ρ and v s are the density and the sound speed, respectively. The amplitude reflection coefficients of interfacial CAP were estimated to be r VSe 2 Àair ≈ -1 and r VSe 2 ÀBi 2 Se 3 = 0.29 with Z Bi 2 Se 3 = 21.78 46 , Z VSe 2 = 12, and Z air =0.0004 with a unit of 10 6 kg m −2 s −1 . The negative reflection coefficient indicates that the phase of the reflected acoustic wave is changed by π. Hence, the reflected echo exhibits a phase shift of π with each round trip, resulting from the reflection at the air interfaces. In the case of the heterostructure of 118 nm VSe 2 and 10 QL Bi 2 Se 3 (118 nm VBH in Fig. 2a), the first echo was observed at 87 ps, and subsequently, the second and third echoes emerged with alternating phases at the same interval of 86 ps. The similar times of the first echo (87 ps) and the interval (86 ps) imply that acoustic waves were launched from Bi 2 Se 3 through hot phonons and interlayer vibration after photoexcitation. Note that the reflected acoustic waves at the interface of Bi 2 Se 3 /Al 2 O 3 substrate were negligible in our analysis because the reflection coefficient of Bi 2 Se 3 /Al 2 O 3 is 0.18 with the acoustic impedance for Z Al 2 O 3 = 31.5 × 10 6 kg m −2 s −1 47 does not change the phase of acoustic waves. Moreover, the expected echo interval in Bi 2 Se 3 at 3.4 ps, where the thickness of Bi 2 Se 3 is 10 nm and the sound speed is 2.9 km s −1 , is substantially short compared to that observed in VSe 2 . The phase was also not changed by reflection at Bi 2 Se 3 /Al 2 O 3 interfaces, thus, we safely concluded that the echo signals were observed when the acoustic waves reached the interface at VSe 2 / Bi 2 Se 3 . The observed time intervals between echoes of 42, 54, 86, and 111 ps in the 48, 74, 118, and 167 nm VBHs, respectively (Fig. 2b), exhibited a linear dependence on the VSe 2 thickness, which means that so did the transit round-trip time of the acoustic wave in VSe 2 . The inset of Fig. 2b shows the VSe 2 -thickness-dependent time interval between echoes at each VBH, with a sound velocity of VSe 2 of v s = 2.71 km s −1 estimated from the linear dependence. This value is close to the previously reported value of 2.67 km s −1 48 . In general, defects and imperfect contact can reduce the coherence of acoustic waves and induce different frequencies, which are independent of the material thickness 45 . Moreover, as shown in the inset of Fig. 2b, the different amplitudes between echoes were consistent with the reflection coefficient of r VSe 2 ÀBi 2 Se 3 = 0.29, while r VSe 2 Àair was near unity, providing direct evidence of an atomically flat interface with negligible defects or poor contact of the VBHs 49 . Consequently, the interface of each VBH was assumed to be nearly perfect, and the effects of additional gap states due to defects that hinder the interpretation of the results could be excluded.
Ultrafast hot electron dynamics of Dirac surface states at VBHs. With the VBHs possessing a high-quality interface, we performed transient reflectance (TR) measurements to investigate the interfacial carrier dynamics in the DSS and the bulk electronic states of Bi 2 Se 3 in VBHs. In TR measurements with a pump fluence of 12 μJ cm −2 , the photoexcited carrier density estimated from the power density along with the complex refractive index of Bi 2 Se 3 50 was 1.6 × 10 19 cm −3 , which is only 5% of the background carrier concentration and implies a small perturbation to the charge carriers in Bi 2 Se 3 . Figure 3a shows TR spectra for 10 QL Bi 2 Se 3 , bare VSe 2 , and 118 nm VBH. For bare Bi 2 Se 3 , the TR spectral responses were significantly different before and after 2 ps. Just after photoexcitation, the negative differential reflectance (ΔR/R 0 ) at overall probe energies was observed before 2 ps, whereas, after 2 ps, a derivative-like TR response, i.e. changing the TR sign from negative to positive according to probe energy, was observed. Since the observed carrier dynamics are complex due to the coexistence of the metallic surface state and insulating bulk state of Bi 2 Se 3 thin film in optical spectroscopy 37,51 , a clear distinction is necessary for each contribution. The overall negative ΔR/R 0 before 2 ps was confirmed by the transient absorption (TA) measurement results (see Supplementary Note 2) to be related to the photobleaching through the injection of hot carriers 51,52 as indicated by stage (i) in Fig. 3b. After hot carrier injection, the rapid decay with 2.5 ps, as indicated τ 1 and stage (ii), was attributed to the intraband relaxation by electron-LO (longitudinal-optical) phonon scattering of excited carriers in bulk and surface states of Bi 2 Se 3 37,51,52 , which implies a dominant contribution of the DSSs on carrier dynamics in Bi 2 Se 3 thin film. After surface scattering relaxation, the derivative-like TR spectra (positive ΔR/R 0 to negative ΔR/R 0 according to probe energy) with a long lifetime appeared after~5 ps. The derivative TR with a long decay time was similar to the previously reported relaxation time of the bulk state toward their respective bottom bands, i.e. conduction band minimum/valence band maximum (CBM/ VBM) for the bulk state 51 . The derivative-like TR spectra and their decay dynamics, which are commonly observed in semiconductors by excitonic effects or state filling by photoexcitation [33][34][35] , can be regarded as a signature of the insulating bulk band of Bi 2 Se 3 as discussed in the following section.
For bare VSe 2 on SiO 2 substrate with a thickness of about 100 nm (see Supplementary Fig. 4a in Supplementary Note 3), the TR spectra show a negative ΔR/R 0 in overall delay time and probe energy. The probe energy used in our experiments was on the order of 1 eV, which corresponds to the photobleaching of intraband transition because the estimated energy difference between the Fermi energy and valence band maximum was about 1.7 eV reported in the previous theoretical calculations for the non-magnetic 1T-VSe 2 53,54 . The excited electron dynamics of VSe 2 were considerably slow compared to Bi 2 Se 3 as shown in Fig. 3b. The excited electrons in VSe 2 did not recover at all within a given time window (only 10-25% even within 300 ps time delay,   as shown in Supplementary Fig. 4b). This slow recovery characteristic can be influenced by very flat V-3d orbitals with energy of~1 eV where the excited electrons mainly located. Subsequently, the e-ph scattering rate is expected to be extremely low compared to Au 55 and graphene 56 in terms of energy/ momentum conservation. It seems to be a decay on the order of ns as shown in Supplementary Fig. 4b. Such a slow decay can be attributed to lattice contribution 57 . A similar level of decay property was found in the lattice temperature recovery time for Au thin film 58 . The TR traces in VSe 2 showed a fast-negative rise within~1.5 ps and subsequent slow decay. Although the thermalization time (τ therm ) of hot electrons in VSe 2 is about 1.5 ps, similar to the monolayer case 59 , the TR response is quite distinguishable, indicating that the excited electron dynamics have a thickness dependence as for other TMD systems 60,61 . Interestingly, for 118 nm VBH, the TR response and kinetics are almost the same as Bi 2 Se 3 . Furthermore, it is noteworthy that the positive ΔR/R 0 at VBH is improved by~2 times compared to Bi 2 Se 3 thin film, although the ΔR/R 0 for bare VSe 2 flakes is negative. The expected hot electron transfer time (τ trans ) from VSe 2 to Bi 2 Se 3 is comparable or faster than 100 fs. About 10 times faster electron transfer compared to the relaxation time of hot electrons in VSe 2 indicates that the photoexcited electrons in VSe 2 are directly transferring into Bi 2 Se 3 before thermalization owing to the fast decay channel of Bi 2 Se 3 (Fig. 3c). This is consistent with the previously reported VSe 2 /graphene heterostructures 59 . Fig. 3c shows the band diagram after junction in VBH. The work function (W) of VSe 2 is 5.8 eV 62 and the electron affinity (χ) of Bi 2 Se 3 is 5.3 eV 11 with the bulk bandgap (E g, bulk ) of 0.3 eV 14 , which creates an upward band bending of Bi 2 Se 3 63 and n-type junction with Schottky barrier height (SBH, ΦSB) of about 0.5 eV. Accordingly, the photoexcited electrons in VSe 2 with a 1.5-eV optical pump can transfer energetically into Bi 2 Se 3 , which has a fast decay channel through the DSS, across the vdW gap. The observed larger magnitude ΔR/R 0 and similar time constant of τ 1 in VBH imply that the excited electrons in VSe 2 can transfer to the bulk band in Bi 2 Se 3 within an ultrashort timescale (~100 fs), then efficiently relaxed by scattering with the DSS of Bi 2 Se 3 as illustrated in Fig. 3c.
Ultrafast carrier dynamics of insulating bulk states at VBHs. After the intraband relaxation process, a relatively long lifetime of carriers in the bulk band induced the derivative-like TR response, representing the optical transition in the insulating bulk band of Bi 2 Se 3 that was maintained for several hundred ps. The increased reflectance (red color) at lower energy and decreased reflectance (blue color) at higher energy were observed at the boundary of 0.96 eV in Bi 2 Se 3 and VBH (Figs. 3a, 4a). The shape of TR According to the fitting model, an assumption of photoexcited carrier concentration (n ex ) of 9.7 10 18 cm −3 , which is only~3% of the background carrier concentration (N 0 ), provided the best fit of the spectral response at 25 ps. We note that the photoexcited carrier concentration of 9.7 10 18 cm −3 well matched the estimated value of 1.6 10 19 cm −3 from the complex refractive index of Bi 2 Se 3 and optical power density, but was slightly small due to carrier recombination. In addition, the second surface states (SS 2 ) locate about 1.5 eV above the first conduction band (CB 1 ) edge 64 .
Therefore, the pump beam (ℏω pump = 1.5 eV) can directly excite 2D electrons in SS2 from the CB and 3D carriers from a deep level of VB to CB 1 band 64,65 . The latter density of optical transition was expected to be dominant 65 . Then, the probe energy (~0.8−1.1 eV) can be coupled with the optical transitions between SS 1 -SS 2 and discrete bulk bands. Subsequently, a non-resonant TR signal at SS 1 -SS 2 transitions and a resonant TR signal at discrete bulk bands can be expected. Therefore, the obtained transition energy of 0.96 eV with the derivative TR implied that the TR response originated from the pump-induced absorption between the first and second conduction bands (CB 1 -CB 2 ) in Bi 2 Se 3 66 . The obtained transition energy of 0.96 eV agreed well with the previous results of the bulk band in Bi 2 Se 3 66,67 .
In the VBHs with 48, 57, 118, and 167 nm thick VSe 2 , the changes of the spectral and dynamical response of Bi 2 Se 3 were observed according to VSe 2 thickness as shown in Fig. 4a, b. A notable change is that the derivative-like TR of Bi 2 Se 3 feature was redshifted with increasing thickness of VSe 2 , as indicated by the horizontal line in Fig. 4a and the asymmetry TR spectra (larger negative ΔR/R 0 than positive ΔR/R 0 ) became severe with increasing VSe 2 thickness (Fig. 4b). This asymmetric behavior of the TR spectra in the VBHs originated from the TR component of VSe 2 , which remained almost constant (Fig. 3a and Figs. S4c). We took the TR response of VSe 2 into account as a constant value during the model fitting process. Further details of TR spectrum modeling in VBHs can be found in the Experimental Methods section. The fitting result with our theoretical model provides the E opt and Γ of photoinduced absorption changes in the VBHs as shown in Fig. 4c. The magnitude of Δα is significantly increased in VBHs (~2.5 times in 167 nm VBH), which is consistent with the results shown in Fig. 3b as the larger excited carrier density (n ex ) by transferring from VSe 2 to Bi 2 Se 3 . E opt shows a linear dependence on the thickness of VSe 2 with the slope of 0.26 meV nm −1 (Fig. 4d). The factor of Γ also tends to increase with the thickness of VSe 2 (Fig. 4e). Figure 4f shows the decay characteristics of photoexcited carriers in the VBHs. In Bi 2 Se 3 , the pump-probe trace followed a biexponential decay. τ 1 and τ 2 indicate the decay times related to the electron-LO (longitudinaloptical) phonon relaxation and the interband relaxation involving unoccupied DSS, respectively 38,68 . The intraband relaxation by electron-LO phonon scattering is typically obtained with values of decay constants from 1 to 3 ps range, due to low LO-phonon energy and electron-phonon coupling in Bi 2 Se 3 . τ 1 decay time in Bi 2 Se 3 and VBHs were measured to be in the range of 1-3 ps, implying intrinsic elastic and electron-phonon coupling properties of Bi 2 Se 3 regardless of VSe 2 . However, τ 2 related to the interband relaxation including DSS was observed in a much longer decay time range. Compared to the longer decay time (τ 26 00 ps) in Bi 2 Se 3 thin film, τ 2 in VBHs was significantly shortened (see Supplementary Fig. 5). The fast relaxation in VBH indicates the efficient interband carrier recombination including coupling with DSS 68 , owing to the depletion of 3D carriers and stabilized 2D electrons by charge transfer after junction with VSe 2 .
Such spectral and dynamical changes of derivative TR spectra of the insulating bulk band suggest interactions between metallic VSe 2 and conduction/valence bands in Bi 2 Se 3 by forming junctions. Accordingly, this section mainly handles the electronic interaction between the insulating bulk band of Bi 2 Se 3 and metallic VSe 2 , which provides a good example of van der Waals MSJs. Previous ultrafast spectroscopic studies show that the interesting features in MSJ are determined by the interface 33,69,70 . Generally, certain changes in the electronic states of the semiconductor in MSJs can be explained by metal/defect-induced gap states (MIGS, DIGS), or interfacial dipoles that cause the FLP observed in different MSJs 20,21 . Since the MIGS store the electrons and holes and then pin the Fermi level, they can modulate the carrier dynamics of Bi 2 Se 3 . However, this can be sufficiently suppressed in the case of vdW MSJs owing to the vdW interaction [18][19][20][21] . Although DIGS can also affect the carrier responses, similar to MIGS, they are negligible in the present work due to the verified high-quality interface. Most noteworthy is that both MIGS and DIGS, due to the overlap of wave functions occurring at the MS interface, are independent of the thickness of the contact metal. Consequently, the observed VSe 2 -thicknessdependent redshift of transition energy suggests that the bulk band modulation can be much better interpreted by interfacial dipole interaction rather than MIGS and DIGS in MSJ.
In the case of interfacial dipoles due to charge transfer and redistribution, they can shift the electronic level, deviating from the Schottky-Mott limit 71,72 . Fig. 5a illustrates the band alignment of VSe 2 and Bi 2 Se 3 after contact. Due to the low χ (~5.3 eV) and small bulk bandgap (E g~0 .3 eV) in Bi 2 Se 3 compared to the W of VSe 2 (~5.8 eV), electrons are transferred from Bi 2 Se 3 to VSe 2 due to the Fermi level alignment after contact (Fig. 5a). Subsequently, the VSe 2 layer close to the interface becomes negatively charged, while positive charges are created in the Bi 2 Se 3 layer close to the interface, leading to interfacial dipoles (Fig. 5a). These interfacial dipoles due to charge transfer lift the electronic level of Bi 2 Se 3 layers near the interface, whereas the Bi 2 Se 3 layers far from the interface remain unchanged due to the rapid decay with distance from the interface 71,72 . With this modulation, a band offset (Δ) is created as displayed in Fig. 5a. We note that the photoinduced built-in electric field is negligible at the interface of the MSJ in our experiment due to a small perturbation of~3% of background carrier density. Hence, dipole interactions are dominant, which enables us to estimate the Δ (= E opt,VBH -E opt,Bi2Se3 ) created by the junction with metallic VSe 2 . The redshift of the optical transition energy in the VBHs makes it possible to move the transitions of CB 1 -CB 2 to promoted by Δ. Thus, the observed transition energy directly reflects the Δ induced by the interfacial dipole interaction with the electronic level of Bi 2 Se 3 . Moreover, the upward bending in the electronic level of Bi 2 Se 3 layers with increasing VSe 2 thickness in VBH also resulted in the increasing broadening factor of the projected Bi 2 Se 3 band structure as shown in Fig. 4e. The effect through the interfacial dipole interaction depends on the thickness of the metallic contact layer because more electrons require severer Fermi level alignment with increasing VSe 2 thickness, and thus, the dipole strength becomes stronger (Fig. 5a). Particularly, in the 167 nm thick VBH (dashed box in Fig. 4a), the observed additional optical transition over 1 eV reflects the highly distorted electronic level due to strong dipole interactions. Note that the electronic level shift of Bi 2 Se 3 by the junction with VSe 2 was~10 times lower compared to conventional 3D metals 11 , including Au, Pd, and Pt. This implies that extremely weak interactions can be achieved with VSe 2 as the 2D metal electrode.
The estimated Δ showed a linear dependence on the thickness of VSe 2 with a slope of 0.26 meV nm −1 (Fig. 5b). The Δ on the order of several tenths of meV is comparable to the order of the first-principles calculation result for metal-2D semiconductor junctions 72 . Moreover, observed VSe 2 -thickness-dependent electronic level shift (Δ) in Bi 2 Se 3 suggests that Δ becomes almost zero with monolayer VSe 2 , expecting extrapolation in Fig. 5b. This interpretation is consistent with the recent angle-resolved photoemission spectroscopy (ARPES) study of monolayer VSe 2 / Bi 2 Se 3 heterostructures 73 , which directly observe the unchanged band structure and no band bending of Bi 2 Se 3 under VSe 2 monolayer (0.3-2 ML). Since Ф SB is determined by the energy difference between the conduction band edges and the Fermi level in Bi 2 Se 3 (Ф SB = E CBM -E F ) for electrons, the 4 observed from their inherent levels leads to a deviation from the Schottky-Mott limit (S = 1) 21,71,72 . The pinning factor S, which is defined as S = 1− Δ/Ф SB,0 , where Ф SB,0 is the SBH in non-interacting subsystem and denotes the strength of FLP, which is deduced from the estimated Δ (Fig. 5c). Note that S became closer to the Schottky-Mott limit as the thickness of VSe 2 decreased. Consequently, the changes in the SBH due to charge transfer and interfacial dipoles are expected to be weak in vdW MSJs with an ultrathin layer (monolayer to few-layer), and further approaching the ideal Schottky-Mott limit (S = 1) is possible. The broadening factor of the optical transition in Bi 2 Se 3 also increased with the increasing thickness of the VSe 2 layer (Fig. 4e). The gradual shift of the electronic bands in the Bi 2 Se 3 layers reflects that the width of the optical transition in Bi 2 Se 3 is broadened in the projected band structure, as illustrated in Fig. 5a. This trend was also observed in previous studies on electronic band modulation under an external electric field [74][75][76] . Moreover, since the lifetime of metastable carriers (τ 2 ) strongly depends on the Fermi level of Bi 2 Se 3 39,40 , strong FLP can pin the τ 2 regardless of VSe 2 thickness. Accordingly, significant VSe 2thickness-dependence of τ 2 shows additional evidence of weak FLP, originated by interfacial hybridization or defect states. On the other side, the interband recombination time of metastable carriers decreased with increasing Δ, owing to the stronger interfacial dipole interaction. The electrons and holes contributed to the surface recombination in band edges and DSS enables the current to flow efficiently (Fig. 5a). Then, it becomes possible for the metastable carriers of Bi 2 Se 3 to flow efficiently to each surface, giving rise to the shortening of the metastable carrier lifetime (Fig. 4f). This result is consistent with the observed redshift of the transition energy and broadening of the optical transition and shows good agreement with previous reports on exciton dissociation mechanisms, which were previously studied in 2D semiconductors with in-plane dissociation of strongly bound exciton under an external electric field 74 and out-of-plane dissociation due to the built-in-field of the p-n doping 77 .

Conclusion
In summary, by employing ultrafast spectroscopy, we investigated the interfacial carrier dynamics in both of metallic surface state and insulating bulk state in VSe 2 /Bi 2 Se 3 , for which a nearly perfect interface was realized and the dependence of the dipole effect hidden by MIGS and DIGS on the metal thickness was observed. The excellent interfacial contact in the MSJs was verified by precise STEM images and interfacial CAP dynamics. The hot electron transfer from VSe 2 to Bi 2 Se 3 across vdW gap in ultrashort timescale (~100 fs) before thermalization in VSe 2 enables to improve the efficiency of Bi 2 Se 3 -based optoelectronic devices. Furthermore, it was found that the metallic surface state of Bi 2 Se 3 was well preserved even after contact with VSe 2 . The VSe 2 thickness-dependent TR measurements quantitatively provided electronic level shifts of the bulk band in Bi 2 Se 3 due to dipole interactions, leading to deviation from the Schottky-Mott limit. In addition, the observed carrier long-decay characteristics in VBHs show a strong dependence on the VSe 2 thickness and there were no significant FLP effects. The metastable carrier lifetime was noticeably shorter due to the built-in field, enabling a highly efficient current flow of Bi 2 Se 3 -based devices.
Our results show that VSe 2 is a promising candidate as the 2D metal electrode for diverse applications based on Bi 2 Se 3 and other topological insulators. Moreover, the interfacial dipole interaction created by charge transfer and redistribution allows us to understand the carrier dynamics and modulated electronic states in MSJs, approaching the ideal Schottky-Mott limit. Although thickness control of the semiconductor part in MSJs has been suggested to cover the effects of interfacial dipoles due to the depinning effect between the semiconductor layers, i.e., a multilayer (thick) semiconductor is an advantageous option for suppressing FLP 71,72 , the practical development remains elusive due to the significant changes in the electrical and optical properties from the monolayer counterpart. Thus, the combination of defining an approach to perceive the modulated band structures in MSJs that may encompass delicate materials and finding suitable vdW materials close to the ideal junctions shown in the present work provides a strategy for the contact fabrication process for optimizing device performance by fully utilizing the intrinsic properties of Bi 2 Se 3 and other topological insulators.

Methods
Fabrication of VSe 2 /Bi 2 Se 3 heterostructures. A Bi 2 Se 3 thin film was grown by a molecular beam epitaxy system with an ultrahigh vacuum chamber equipped with VEECO (Knudsen) cells of high purity bismuth (99.999+%) and selenium (99.999%) sources 78 . Prior to growth, the Al 2 O 3 (001) substrate was cleaned with acetone and isopropyl alcohol and baked at 700°C for 30 min to remove any residual contaminants. A Bi 2 Se 3 /Al 2 O 3 (001) film was grown at 250°C for 90 min, followed by a post-annealing process of 450°C for 30 min for self-crystallization. The ratio of Bi and Se fluxes was kept at~1:15 to minimize Se vacancies. The formation of crystalline Bi 2 Se 3 on the substrate was monitored by an in situ reflections high-energy electron diffraction system. The carrier concentration was determined by Hall resistivity measurement at 300 K. The carrier concentration (N 0 ) of Bi 2 Se 3 is given by N 0 = (B·I)/(e·d·V H ), where I is current, d is the thickness of the Bi 2 Se 3 thin film, e is the electron charge, and V H is the Hall voltage; thus, we can estimate N 0 = 3.13 × 10 20 cm −3 .
Single-crystalline 1T-VSe 2 was grown by the chemical vapor transport method 41 . V (99.5% purity) and Se (99.999% purity) powders were used as the precursors with a stoichiometric amount of 2:1 wt%. As a transport agent (concentration of 1.5 mg/cm 3 ), VCl 3 powder (99% purity) was used to provide sufficient vapor pressure. The mixture of precursors and transport agents was   Fig. 5 Schematic of changes of the VSe2-thickness-dependent optical transition behavior in VBHs after contact with VSe2. a Schematic of charge transfer and redistribution due to the thermal equilibrium by the junction at VSe 2 /Bi 2 Se 3 and corresponding charge density near the junction interface. The increase of VSe 2 thickness requires a larger amount of electron from Bi 2 Se 3 for Fermi level alignment, leading to an increase of the interfacial dipole strength. Corresponding band diagram for VSe 2 /Bi 2 Se 3 junctions with thin and thick VSe 2 are illustrated with the electron Schottky barrier height (SBH, Ф SB ) and band offset (4) as indicated by black and red vertical arrows, respectively. The CB1/CB2, VB, and SS imply the first/second conduction band, valence band, and surface state of Bi 2 Se 3 , respectively. The green vertical arrow indicates optical transitions corresponding to probe energies. b VSe 2thickness-dependent band offset (4) by dipole interactions and c estimated electron Ф SB . The expected pinning factor S is listed. The error bars in (b) and (c) come from the standard error for fitted values of E opt in Fig. 4d. placed in a quartz ampule. The ampule was evacuated below 10 -6 Torr, followed by melting and sealing. The sealed quartz ampule was placed at the center between two heating zones, which were heated slowly in five steps to approach their target temperatures (750°C for vaporization and 850°C for growth). After being held for 72 h at the maximum temperatures, the system was cooled down naturally, and shiny silver-gray VSe 2 crystals were collected after breaking the ampule. We adopted mechanical exfoliation to transfer VSe 2 onto Bi 2 Se 3 for the fabrication of VSe 2 /Bi 2 Se 3 heterostructures. VSe 2 flakes were transferred onto the Bi 2 Se 3 thin film by using a simple blue tape method. We selected one of them, and different thicknesses were exposed on one VSe 2 flake for a consistent experimental environment.
Characterization of VSe 2 /Bi 2 Se 3 heterostructures. The thickness of the VSe 2 flake was analyzed by AFM (n-Tracer, Nano Focus Inc.) operated in a non-contact mode. Raman measurements were performed with a Raman spectrometer (FEX, NOST) by focusing the laser radiation with a 0.3 mW power centered at 531 nm. A microstructural study was carried out using a STEM system (JEM-ARM200F, JEOL) equipped with a probe spherical aberration corrector. HAADF-STEM images were obtained to confirm the atomic structure in real space. For HAADF-STEM images, the convergent semiangle was fixed at 22 mrad, and the collection semiangle was 67-170 mrad. TEM specimens were thinned for electron transparency by a focused ion beam system. Transient reflectance (TR) measurements. CAP dynamics and TR measurements were performed by employing a Ti:sapphire laser oscillator (MAITAI, Spectra-Physics), which produced 100-fs pump pulses at an 80-MHz repetition rate. The main portion of the output from the oscillator at a central wavelength of 830 nm was further used to pump a synchronously pumped optical parametric oscillator (SPOPO) for the generation of tuneable 150-fs probe pulses (1150-1550 nm) at the same repetition rate. With a dichroic mirror, the pump and probe pulses were collinearly combined and focused on the sample to a beam waist of~2 μm by a single objective lens (M Plan Apo 100×, Mitutoyo). The pump fluence adjusted with two polarizers was~12 μJ/cm 2 . The time delay between the pump and probe pulses was controlled by using a motorized linear stage. The reflected probe beam was recorded with a Ge photodetector (DET50B, Thorlabs) connected to a lock-in amplifier (SR830, Stanford Research Systems) after blocking the reflected pump beams. Based on this setup, we measured the time-resolved differential reflectance defined by ΔR/R 0 ≡ (R 0 −R(t))/R 0 where R 0 and R(t) are the reflectance of the sample without and with pump excitation, respectively. All measurements were carried out under ambient conditions at room temperature.
Modeling of TR spectra. The theoretical model for analyzing the TR feature is based on pump-induced refractive index changes for semiconducting materials 34 . The pump pulses excite electrons in that ground state, then perturb the dielectric response and fractionally change the complex refractive index of e n ¼ n þ ik. First, we calculated the imaginary part of k corresponding to absorption coefficient as k ¼ ðλ=4πÞα, where λ is the probe wavelength and α is the absorption coefficient. Once the change of absorption coefficient ðΔαÞ is obtained, the change of real part of the refractive index (4n) is derived by using the Kramers-Kronig transformation Δαð_ωÞ ω 02 À ω 2 dω0 ð 2Þ where c is the speed of light, and P is the Cauchy principal value for the integral. We calculated the absorption coefficient change with optical absorption of δð_ω opt Þ with perturbed factor as photoexcited carrier concentration divided by doping concentration (ΔN/N 0 ) and with homogeneous broadening by deconvolution with Gaussian function. The broadening factor Γ is included in the calculation. In the measurement, since the observed fractional reflectance change (ΔR/R 0 ) was too small ($ 10 À4 ) by normal incident pump pulses, the observed differential reflectance was dominated by refractive index changes. From this, ΔR/R 0 can be approximated as where n 0 ¼ 5:5 is the averaged refractive index of Bi 2 Se 3 in the interesting spectral region 50 . To obtain the optical transition energy and broadening factor in VBHs, we performed the model fit in TR spectra of VBH at the same time delay of 25 ps. In Bi 2 Se 3 , asymmetrical positive and negative peaks in TR spectra were observed as displayed in Supplementary Notes 2, 3. However, in VBHs, the asymmetrical TR spectra were observed with small positive peaks at lower energy and large negative peaks at higher energy as can be seen in Fig. 4b. Subsequently, we modified the Eq.
(3) for TR spectra in VBHs as where C VSe 2 is the constant TR response of the bare VSe 2 as shown in Supplementary Fig. 4c. The value of C VSe 2 became larger with the increase of VSe 2 thickness and is consistent with the modified model of TR response in VBHs.

Data availability
The data of this work are available from the corresponding authors upon request.

Code availability
The code that supports the findings of this study are available from the corresponding author upon reasonable request.