Ultrafast Excitonic Behavior in Two-Dimensional Metal

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Ultrafast Excitonic Behavior in Two-dimensional Metal-Semiconductor Heterostructure Deok Min Seo, Jeong-Hwan Lee, Suryeon Lee, Juyeon Seo, Changkyoo Park, Jaewook Nam, Yeonju Park, Sila Jin, Shubhda Srivastava, Mahesh Kumar, Young Mee Jung, Kyu-Hwan Lee, Yoon-Jun Kim, Sangwoon Yoon, Young Lae Kim, Pulickel M. Ajayan, Bipin Kumar Gupta, and Myung Gwan Hahm ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Ultrafast Excitonic Behavior in Two-dimensional Metal-Semiconductor Heterostructure Deok Min Seo,†,∇ Jeong-Hwan Lee,†,∇ Suryeon Lee,†,∇ Juyeon Seo,† Changkyoo Park,‡ Jaewook Nam,¶ Yeonju Park,§ Sila Jin,§ Shubhda Srivastava,k,†† Mahesh Kumar,k Young Mee Jung,§ Kyu-Hwan Lee,⊥ Yoon-Jun Kim,† Sangwoon Yoon,# Young Lae Kim,@ Pulickel M. Ajayan,4 Bipin Kumar Gupta,∗,k and Myung Gwan Hahm∗,† Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea, Laser and Electron Beam Application Department, Korea Institute of Machinery and Materials, Daejeon 34103, Republic of Korea, School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea, Department of Chemistry, Kangwon National University, Chunchon 24341, Republic of Korea, CSIR-National Physical Laboratory, New Delhi 110012, India, Department of Electrochemistry, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon-si, Gyeongsangnam-do 51508, Korea, Department of Chemistry, Chung-Ang University, Seoul 06974, Republic of Korea, Department of Electronic Engineering, Gangneung-Wonju National University, Gangneung 25457, Republic of Korea, and Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA E-mail: [email protected]; [email protected]

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Abstract The excitonic behavior in two-dimensional (2D) heterostructures of transition metal dichalcogenide (TMD) atomic layers have attracted much attention. Here, we very first report the ultrafast behavior of charge carriers in heterostructure of metal (NbSe2 ) and semiconductor (WSe2 ) atomic layers via ultrafast spectroscopy. We observe a blueshift of the excited-state absorption peak in time-resolved absorption spectra with time delays in both the as-grown semiconducting WSe2 and the metal-semiconductor heterostructure. However, the heterostructure shows a clear difference in the peak position and relaxation time of its electrons. This result indicates higher excited energy states in WSe2 in the presence of the NbSe2 metallic layer contact and implies the existence of interlayer electron quenching from WSe2 to NbSe2 layers. The heterostructure shows a shorter time scale in the peak rise time compared to bare WSe2 , due to interfacial defects between WSe2 and NbSe2 layers. The results offer a better understanding of the opto-electronic properties of 2D heterostructure interfaces.

Keywords: Transition metal dichalcogenides, two-dimensional materials, 2D van der waals heterostructure, ultrafast spectroscopy, exciton, metal-semiconductor heterostructure, excited-state absorption Low-dimensional heterostructures comprising two or more different component nanomaterials can have interesting opto-electronic properties. 1,2 In this regard, atomic-layered tran∗

To whom correspondence should be addressed Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea ‡ Laser and Electron Beam Application Department, Korea Institute of Machinery and Materials, Daejeon 34103, Republic of Korea ¶ School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea § Department of Chemistry, Kangwon National University, Chunchon 24341, Republic of Korea k CSIR-National Physical Laboratory, New Delhi 110012, India ⊥ Department of Electrochemistry, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon-si, Gyeongsangnam-do 51508, Korea # Department of Chemistry, Chung-Ang University, Seoul 06974, Republic of Korea @ Department of Electronic Engineering, Gangneung-Wonju National University, Gangneung 25457, Republic of Korea 4 Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005, USA ∇ These authors contributed equally to this work †† Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory Campus, New Delhi 110012, India †

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sition metal dichalcogenide (TMD) heterostructures, in particular, have attracted strong interest for diverse applications. 3–8 Several stoichiometries enable 2D TMDs to be conductors, semiconductors, or superconductors. 9 Variable electronic band structures and carrier mobility of component atomic layers 10–12 should facilitate various devices based on such heterostructures. 13–17 To utilize single 2D TMDs per se in many nanoscale devices, contact with metallic materials as electrodes is indispensable. Therefore, the interfacial characteristics of 2D metal-semiconductor TMD heterostructures are important for tailoring electronic and optical properties. A recent flurry of activity in van der Waals (vdW) heterostructure research on 2D nanomaterials such as graphene and TMDs has shown interfacial characteristics such as Fermi level pinning and spin-orbit interaction. 18–21 Moreover, experiments recently demonstrated that electron-hole exchanges are efficiently constrained by the Schottky barrier lowering and tunneling phenomena between atomic-layered semiconducting WSe2 and metallic NbSe2 . Atomic-layer NbSe2 is a promising candidate as an electrode for 2D TMD-based devices owing to its metallic characteristics, lattice structure, low degree of surface defects, and difference in electron affinity from other TMDs. 19,22–24 However, many underlying physics of vdW interfaces between semiconducting and metallic TMDs are still under scrutiny. Herein, we report the excitonic behavior at the vdW interface between atomic-layered metallic NbSe2 and semiconducting WSe2 via ultrafast time-resolved absorption spectroscopy and study the femtosecond charge transfer processes in the WSe2 /NbSe2 heterostructure. To create an atomic-layered metal-semiconductor heterostructure, first, metallic NbSe2 and semiconducting WSe2 were synthesized via a selenization process of niobium pentoxide (Nb2 O5 ) and tungsten trioxide (WO3 ) thin film, respectively (see detailed information in Methods). Figure 1b (left) and S1 show the cross-sectional transmission electron microscopy (TEM) images of as-synthesized semiconducting WSe2 and metallic NbSe2 . Clearly, as-grown WSe2 and NbSe2 are multilayered atomic structures. The metal-semiconductor heterostructure was fabricated by the poly(methyl methacrylate) (PMMA)-assisted wet-transfer process. 4


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Figure 1: Investigation of bare WSe2 and WSe2 /NbSe2 van der Waals heterostructure. (a) Schematics of bare WSe2 and WSe2 /NbSe2 heterostructure samples. The bare WSe2 has approximately 3-4 layers, and the WSe2 /NbSe2 heterostructure sample has 10 layers. (b) High-resolution cross-sectional TEM images of bare WSe2 and WSe2 /NbSe2 heterostructure on a SiO2 wafer. The film thickness is approximately 20 Å for bare WSe2 and 120 Å for the WSe2 /NbSe2 heterostructure sample. (c) Raman spectra of WSe2 and WSe2 /NbSe2 heterostructure samples. The E12g peak comes from the in-plane displacement of W and Se atoms, and the A1g peak is associated with the interlayer interaction of Se atoms in WSe2 . (d) Absorption spectra and (e) band alignment scheme of WSe2 . Each characteristic exciton energy of WSe2 is displayed for both the absorption spectra and band alignment scheme of A (1.65 eV), B (2.10 eV), A’ (2.25 eV), and B’ (2.70 eV). The A exciton comes from the electron transition between the VBM and conduction band minimum of WSe2 . The A’ exciton is associated with VBM and the second-lowest excited state (S2 ) of WSe2 . The B and B’ exciton energy originates from valence band splitting, and the resulting energy state is switched by -0.45 eV from VBM (Ev0 ). Hence B and B’ excitons have higher energy compared to A and A’ excitons, respectively.

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The cross-sectional TEM image in Figure 1b (right) shows the layer-by-layer heterostructure consisting of atomic-layered WSe2 and NbSe2 . The Raman spectra recorded from the surfaces of WSe2 and NbSe2 obviously exhibit an out-of-plane mode (A1g ) and an in-plane-mode (E2g ) on each spectrum (Figure 1c). Two distinguishable phonon modes indicate that the synthesis and transfer process successfully created the atomic-layered WSe2 and the heterostructure. The strong out-of-plane mode of the spectrum recorded from the surface of WSe2 indicates that the multilayer corresponds to the cross-sectional TEM image. Interestingly, redshifts of Raman spectra recorded from the heterostructure are observed owing to the interaction of out-of-plane (228 cm−1 ) and in-plane (246 cm−1 ) modes from NbSe2 . 19,24 The redshifts of Raman spectrum in the heterostructure originated from a combination of strain of TMDs, charge transfer on the interface, and overlap with spectrum of NbSe2 . It is well-known that the strain of lattice of 2D materials leads shift of peak position of Raman spectrum. The wet transfer process for fabrication of heterostructure can cause partial strain of lattice structures of TMDs that contributes to redshift in Raman spectra. 25 The interlayer couplings in TMD heteroctructure contribute to the charge transfer between layers, which usually can be seen in PL quenching and time-resolved absorption spectra. 26,27 Owing to phonon renormalization in 2D materials, the change also can be seen in the difference in FWHM and redshift in Raman spectra. 28 WSe2 and NbSe2 have close peak positions(A1g mode and E12g mode of NbSe2 and E12g mode and A1g mode of WSe2 ) in Raman spectra, shown in Figure 1c and Figure S3. Hence, Taman spectrum of NbSe2 can act as background of the spectrum recorded from heterostructure and it contributes to redshift of the Raman spectrum of heterostructure. Excitonic absorption spectra A and B of WSe2 exhibit direct gap transitions at the K point of the Brillouin zone (Figure 1d). The energy states of the metallic NbSe2 and semiconducting WSe2 heterostructure also have similar absorption spectra to bare semiconducting WSe2 as shown in Figure 1d. They have two ground (Ev and Ev0 ) and excited (S1 and S2 ) states at the resonance wavelengths (A, A’, B, and B’). 29,30 As shown in Figure 1e, the A exciton is formed from ground state Ev to the lowest excited state S1 , and the B exciton has 6


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higher exciton energy resonance from Ev0 to S1 . 10,29–31 The A’ and B’ excitons originate from energy band splitting of the valence band maximum (VBM) by spin-orbit coupling (SOC) of WSe2 . The differences of exciton energy between the A, B excitons and A’, B’ excitons are 0.45 eV, and the deviation matches well with the SOC-based valence band splitting. 30,32 To elucidate the excitonic behavior of atomic-layered WSe2 and the metal-semiconductor heterostructure, femtosecond time-resolved transient absorption difference spectroscopy was conducted with different delay times. A pump pulse with a 420-nm wavelength (2.95 eV) produces photo-excitation from the valence band to the S2 excitation state at the K (K’) point of WSe2 as depicted in Figure 1e. A probe pulse with a certain delay after the pump pulse produces an absorption difference (∆A) at the WSe2 film as shown in Figure 2a. As a result, spectroscopic data comprise a series of positive and negative spectra according to the absorption difference caused by the transition of electrons. At first, the pump pulse induces ground state bleaching (GSB) to WSe2 layers. After GSB, excited electrons are relaxed with internal conversion (IC) and stimulated emission (SE) through the optically allowed energy level difference of WSe2 . The relaxations explain the negative absorption difference displayed as negative spectra in Figure 2a. Negative spectra with signals from 530 nm to 570 nm and from 700 nm to 760 nm are resonant with relaxations that have the same energy as the A’ and A excitons, respectively. The transition of electrons from the S2 to the S1 state by IC is not shown in the spectroscopic data because it has a relatively smaller relaxation time and resonant energy than the measurement range. In contrast, positive spectra result from excited-state absorption (ESA) in the WSe2 film. Especially, the positive spectrum from 575 nm to 705 nm shows a significant peak shift depending on the delay time. This can be interpreted as the contribution of two different spectra as indicated in the inset of Figure 2a. The broad ESA peak has two components-one at 627 nm and the other at 650 nm-and those two peaks are attributed to the electron transition from the S1 level to the S4 level and from the S1 level to the S3 level, respectively. The Franck-Condon principle clarifies that excitation and relaxation energies are not perfectly equal to the calculated energy difference 7


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Figure 2: Ultrafast spectroscopy on the bare WSe2 sample. (a) Characteristic positive and negative peaks from the WSe2 sample. The positive peaks at 500 nm, 627 nm, and 650 nm represent electron excitation by the probe pulse via excited state absorption (ESA) after initial excitation by the pump pulse. The negative peaks at 556 nm and 735 nm show relaxation (recombination) of excited electrons to the ground state by stimulated emission (SE). The inset shows Gaussian-Lorentz fitting of the ESA band of 575-705 nm. There are two distinguishable positive peaks, 627 nm and 650 nm, as depicted. (b) Absorption difference versus delay time and exponential decay fitting results (red solid line) of absorption difference peaks at 627 nm and 650 nm in the bare WSe2 sample as a function of the delay time between the pump and probe pulses. The peaks at 627 nm and 650 nm show 424-ps and 352-ps relaxation times, respectively. (c) Generation of two positive (upper panels) and two negative (lower panels) peaks on a few-picosecond scale. The difference between the peak rise time of the negative peaks (∼1 ps) and the positive peaks (∼5 ps) can be seen. The excitation of electrons occurs immediately after the start of electron relaxation.

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of states. 33 This is derived from the vibrational wave function difference between the original state and final state. The recorded positive and negative spectra approximately are in tune with the resonant transition based on the calculated energy states. The Stokes shift in the WSe2 film is not strong, so the energy difference between the A exciton and SE is negligible. 34,35 As a result, positive spectra induced by ESA and negative spectra induced by SE are originally shown together; however, they are merged into a single positive signal in the recorded spectra. Next, we focused on the positive ESA spectra divided into two different peaks to derive a detailed interpretation of the higher excited states (S3 and S4 ) and ESA of the carriers. The peak relaxation time of the positive spectra at 627 nm and 650 nm were investigated as depicted in Figure 2b. The 627-nm peak has a relaxation time of 424 ps, whereas the 650-nm peak relaxes rapidly in 352 ps. This difference causes the peaks of the ESA spectra to shift over time. Electrons excited by ESA take up the lower excited state (S3 state; 1.9 eV from the S1 state) and higher excited state (S4 state; 1.98 eV from the S1 state) simultaneously. However, because the peak is shifted from a longer wavelength to a shorter wavelength with time, the dominant state where electrons arrive is changed from the S3 state to the S4 state with time. In addition, as shown in Figure 2c, the positive peaks (627 nm and 650 nm) concomitantly rise after the negative peaks (556 nm and 735 nm). This may imply that electron excitation by the ESA occurs in these positive peaks, which have already experienced relaxation from the S2 state to the S1 state by IC. During the relaxation and excitation process, an electron requires less than ∼4 ps, which can be deduced from the difference in rise time between negative (∼1 ps) and positive (∼5 ps) peaks. Moreover, the electron transition at the K (K’) point in WSe2 from the S1 state to the S3 and S4 states is originated by ESA. 36 This transition is highly resonant with the ESA positive peak (∼1.95 eV) from our spectroscopic data. Exponential decay fittings of additional positive and negative spectra are displayed in Figure S2 for additional information. We now turn our attention to the excitonic effect on the metal-semiconductor heterostructure. The recorded spectroscopic data for the WSe2 /NbSe2 heterostructure are depicted in 9


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Figure 3: Ultrafast spectroscopy of the WSe2 /NbSe2 heterostructure sample. (a) Characteristic positive peaks from the WSe2 /NbSe2 heterostructure sample. The positive peaks are at 508 nm, 639 nm, and 690 nm. The inset shows Gaussian-Lorentz fitting of the broad positive peak near 650 nm. There are two distinguishable positive peaks, 639 nm and 690 nm, as depicted. (b) Absorption difference versus delay time and exponential decay fitting results (red solid line) of the absorption difference peaks at 639 nm and 690 nm as a function of the delay time between the pump and probe pulses. The peak relaxation time is 132 ps for the 639-nm peak and 90 ps for the 690-nm peak. (c) Generation of two positive peaks in the WSe2 /NbSe2 heterostructure sample on a few-picosecond scale. The peaks rise in approximately 3 ps, shorter than the bare WSe2 positive peak rise time.

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Figure 3a. The same pump pulse as used with WSe2 is used to excite electrons close to the S2 state. Most spectra recorded on the surface of the metallic NbSe2 -semiconducting WSe2 heterojunction are near the position of a bare WSe2 sample. The ESA spectrum near 650 nm consists of two different positive peaks (639 nm and 690 nm) (Figure 2a). This peak is significantly redshifted from 630 nm to 650 nm compared to bare WSe2 , as shown in Figure 2a and 3a. This result shows the energy level reduction at higher excited states (S3 and S4 ) at WSe2 . Moreover, the negative spectrum with a signal from 530 nm to 570 nm and 700 nm to 760 nm of bare semiconducting WSe2 disappears at the metallic NbSe2 -semiconducting WSe2 heterojunction. This result implies that the population of electrons that relaxed from the S2 and S1 states to the ground state is considerably reduced (almost disappears) owing to the metallic NbSe2 contact. Figure 3b shows the result of exponential decay fitting of two positive peaks of the ESA spectrum (639 nm and 690 nm). The two peaks have different relaxation times of 132 ps and 90 ps, respectively. This shows a significant reduction of the peak relaxation time compared with bare semiconducting WSe2 (Figure 2b). The rise time of positive peaks in the heterostructure is displayed in Figure 3c. Compared with bare WSe2 , positive peaks at the heterojunction show shorter rise times. The positive peaks recorded from bare semiconducting WSe2 (Figure 2c upper panels) show a rise time of approximately 5 ps; in contrast, peaks recorded from the surface of the metallic NbSe2 -semiconducting WSe2 heterojunction show a rise time of approximately 3 ps. The reduced time scale of the positive peak rise and relaxation indicates that initially excited electrons tend to be relaxed via IC and excited via ESA more rapidly by the effect of the metallic NbSe2 contact. According to the spectroscopic data, Figure 4 describes how the electron transition mechanism is altered by comparing the energy level diagrams of bare WSe2 (Figure 4a) and the WSe2 /NbSe2 heterostructure (Figure 4b). Electrons at the valence band were initially excited to the S2vib state and then vibrationally relaxed to the S2 state with phonon scattering. After these electrons in the S2 state relaxed to the S1 state via IC, the population of the S1 state decayed to Ev (via SE) or upper electronic states (via ESA) by probe pulses. From 11


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Figure 4: Summary of different exciton behavior between the bare WSe2 and WSe2 /NbSe2 heterostructure samples. (a) Electrons are initially excited by the pump pulse to S2vib states and then relaxed to the S1 state via internal conversion (IC) and to the valence band via stimulated emission (SE). Electrons in the S1 state are excited to the S3 or S4 state by excited state absorption (ESA). The S3 state is filled with electrons first because of the higher electron transition rate than the S4 state. (b) Schematic of exciton behavior in the WSe2 /NbSe2 heterostructure sample. The exciton behavior, including initial excitation and internal conversion (IC) to the S1 state, is the same as the bare WSe2 sample. However, the energy levels of the S3 and S4 states are redshifted by the effect of NbSe2 contact. The lowered S3 and S4 excited states result in the redshift of the positive absorption difference peak of the WSe2 /NbSe2 heterostructure sample. The excited electrons at the WSe2 layers in the WSe2 /NbSe2 heterostructure sample can be quenched to NbSe2 layers at the interface because of the higher work function of NbSe2 than the electron affinity of WSe2 (blue arrow). This transition reduces the electron population reaching the S1 state by relaxation from the S2 state, resulting in a shorter peak relaxation time of excitation from the S1 state to higher excited states. In addition, quenching reduces the relaxation of electrons from the S1 and S2 states to the ground state, resulting in the disappearance of the negative SE peak near 550 nm and 730 nm. the perspective of the broad positive ESA spectrum (located near 630 nm in the bare WSe2 and 650 nm in the WSe2 /NbSe2 heterostructure), there is a significant peak shift from lower energy to higher energy. From the result of Figure 2b and 3b, positive peaks at longer wavelengths (with lower energy; 650 nm in Figure 2b and 690 nm in Figure 3b) relax more rapidly than peaks with shorter wavelengths (627 nm in Figure 2b and 639 nm in Figure 3b). This also can be confirmed by the blueshift of the ESA spectrum peak in Figure 2a and 3a. Because the S3 and S4 states have very similar energies, excitations by ESA of electrons begin to fill the S3 and S4 states simultaneously. However, the S3 state is filled with electrons earlier than the S4 state. This can be interpreted by the difference of the transition rate 12


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between the two states. The electron transition to the S3 state is terminated in advance of the S4 state. This proves that the transition rate of electrons is faster in the case of the S3 state than that of the S4 state and indicates that the density of states (DOS) of the S3 state is higher than that of the S4 state according to the Fermi golden rule. This phenomenon occurs in both the bare WSe2 and WSe2 /NbSe2 heterostructure samples. However, although the two individual samples share a certain regime in the existence of peaks and the overall mechanism, these samples have clear differences in details, such as the accurate peak position and relaxation time. The broad positive ESA spectrum near 630 nm in the bare WSe2 sample has been redshifted to near 650 nm in the WSe2 /NbSe2 heterostructure. This results from the shift of higher excited energy states in WSe2 (S3 and S4 ) when in contact with metallic NbSe2 . The electron band structure can be changed with various effects derived from semiconductor contact with metallic materials. When forming metal-semiconductor junctions, even with 3D metallic materials, metal-induced gap states are formed in semiconductors. Such a state can store electrons or holes, so Fermi level pinning may occur at the junction and band alignment of semiconducting material deviates. 37–39 Additionally, at the interface between two different materials, an interface dipole can be formed by charge redistribution, which can shift the energy state levels of semiconductors. 23,37,40 In addition, the band alignment of semiconducting TMDs can be changed by the types of contact metal and contact conditions. 41,42 The entire band alignment and carrier transition mechanisms are not fully understood owing to a lack of theoretical studies. In this case, the higher excited state energy level of bare WSe2 has been redshifted to a lower energy level after contact with NbSe2 . As a result, the broad positive ESA spectrum shifted from approximately 630 nm to 650 nm with NbSe2 . In other words, electron transitions from the S1 state to the S3 and S4 states have different energies between the bare WSe2 and the WSe2 /NbSe2 heterostructure. The results we have discussed above will be used to understand the fundamental property of this unique material system in an alloyed 2D heterostructure. The investigation of further details such as the band alignment of higher excited states will expand the research from experimental 13


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insight to established fact. In addition, the rise and relaxation times of the positive ESA spectrum are different between the bare WSe2 and the WSe2 /NbSe2 heterostructure. In the case of relaxation time (Figure 2b and 3b), the reduced time scale of the WSe2 /NbSe2 heterostructure suggests the existence of interlayer electron quenching from WSe2 layers to NbSe2 layers. The effect of quenching results from the band alignment of excited states of WSe2 and the Fermi level of NbSe2 . Because the work function of NbSe2 is higher than the electron affinity of WSe2 excited states (S1 and S2 states), 43,44 excited electrons in the S1 and S2 states of WSe2 can interlayer transit to NbSe2 layers (Figure 4b blue dotted arrows). Thus, an electron initially excited to the S2 state or relaxed to the S1 state may quench toward the NbSe2 layers. This transition reduces the population of electrons excited from the S1 state to the S3 and S4 states. As a result, the peak relaxation time for the positive ESA spectrum in the WSe2 /NbSe2 heterostructure is reduced as shown in Figure 2b and 3b. The peak relaxation time is reduced from 352 ps to 90 ps for excitation to the S3 state and from 424 ps to 132 ps for excitation to the S4 state; i.e., the peak relaxation time is reduced by approximately 70-75% when WSe2 is in contact with NbSe2 . The negative peak at 735 nm in the bare WSe2 sample, indicating electrons in the S1 state that have relaxed to the ground state (Ev ), has disappeared in the WSe2 /NbSe2 heterostructure. In the WSe2 /NbSe2 heterostructure, electrons in the S1 state tend to excite to higher states and simultaneously quench toward NbSe2 layers by interlayer transition rather than transit to the ground state (Ev ) by radiative recombination (SE). In the case of the peak rise time reduction (Figure 2c and 3c), the time scale is shorter in the WSe2 /NbSe2 heterostructure than the bare WSe2 . This result suggests that the relaxation of electrons from the S2 state to the S1 state after the pump-induced GSB is faster in the heterostructure. We must focus on the interfacial defect introduced by the NbSe2 contact. TMDs are basically two-dimensional materials, and the carriers have a large effective mass and small exciton radii. This increases the correlation and scattering between carriers as a result of enhanced Coulomb interaction. From this feature, excitons in TMDs tend to be captured rapidly by midgap defects via the evident 14


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Auger scattering effect. 45,46 The recombination lifetime is inversely proportional to the radiative recombination coefficient and trap-assisted Auger coefficient. Thus, the existence of defect or trap states can reduce the recombination lifetime of semiconducting materials. 47 In the WSe2 /NbSe2 heterostructure sample, WSe2 layers make contact with metallic NbSe2 layers by the transfer technique. During this process, interfacial defects can be formed at the interface of two different materials. In a few-layer TMD system, this interfacial defect state can contribute to the recombination mechanism by acting as a center state for trapassisted nonradiative recombination. 45,48,49 This recombination center state formed between the VBM and the conduction band minimum causes excited electrons to rapidly relax to the ground state. Only a picosecond to a few picoseconds of time is needed for electrons to be captured by the recombination center state. 46 In summary, we investigated the carrier transition behavior in bare WSe2 and a WSe2 /NbSe2 heterostructure with femtosecond ultrafast transient absorption difference spectroscopy. Most peaks are at similar wavelengths in both samples. However, in the case of the positive ESA spectrum near 630 nm in the bare WSe2 , a significant redshift of the wavelength to near 650 nm is observed, and this broad positive ESA spectrum consists of two different peaks according to Gaussian-Lorentz peak fitting. As a result, we suppose that electrons excited by the probe pulse from the S1 state to higher excited states arrive at two different states (S3 and S4 ) at the K (K’) point via ESA, and the energy level of the S3 and S4 states is reduced by NbSe2 contact. Additionally, the exciton lifetime is reduced, and electrons in the S1 and S2 states can be quenched toward NbSe2 layers. Thus, the relaxation time of the positive ESA spectrum near 650 nm for the WSe2 /NbSe2 heterostructure is reduced by approximately 70-75%, and the negative peak with a signal at 550 nm and 730 nm disappears, proving the significant reduction in the population of electrons radiatively relaxing from the S2 and S1 states to the ground state by SE. Utilizing ultrafast spectroscopy for 2D semiconducting TMDs and their semiconducting-metallic TMD heterostructure, we unveiled the effect of NbSe2 contact on the carrier transition behavior and band alignment of WSe2 at 15


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the very first. A similar crystal structure and lowered Schottky barrier height can improve the conductive efficiency of semiconducting TMD-based electronic devices. However, the reduced exciton lifetime and radiative recombination behavior by NbSe2 contact adversely affect the luminescent properties of the semiconducting TMDs. Further theoretical and experimental study of electron interband and interlayer transitions will open the pathway to the broad application of TMDs to optoelectronic devices.

1 1.1

Methods Preparation of atomic-layered metal-semiconductor heterostrucrture

WSe2 and NbSe2 atomic layers were prepared by the direct selenization method using the chemical vapor deposition process. Transition metal oxide thin films (WO3 (3 nm) and Nb2 O5 (1 nm) film) were thermally evaporated on SiO2 /Si substrates and then thermally selenized at 900◦ C and 1000◦ C, respectively. After synthesis, the as-grown WSe2 layer was transferred onto a quartz substrate. As-synthesized NbSe2 was then transferred onto a WSe2 /quartz substrate for femtosecond ultrafast transient absorption difference spectroscopy.

1.2

Femtosecond ultrafast transient absorption difference spectroscopy

Femtosecond ultrafast transient absorption difference spectroscopy was performed in the pump-probe configuration using femtosecond pulses. The setup comprised a Ti:Sapphire oscillator (Mira by Coherent), Ti:Sapphire amplifier (Legend by Coherent), optical parametric amplifier (OPA, TOPAS by Light Conversion), and spectroscopy system (Helios by Ultrafast Systems). The 3-mJ (1 kHz) output of the amplifier with a pulse width of ∼35 fs centered at 800 nm was split into two parts. One of the parts was fed to the OPA with 1.8 mJ of energy, and the other part with 0.5 mJ was fed to the spectrometer via a delay stage of 0-8 ns. The

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pulse width after passing through TOPAS was approximately 70 fs. The broadband white light was generated inside the spectrometer by passing the split beam through sapphire and CaF2 crystals, whose combination can generate probe pulses in the range of 350-1600 nm. A highly stable 420 nm of the OPA output was selected as a pump beam, and the fluence was fixed at 40 mJ/cm2 . The fitting of the data was performed with Surface Xplorer analysis software. The Helios spectrometer was calibrated using the ZnTPP dye.

1.3

Raman spectroscopy

The Raman spectra were obtained at room temperature using a Jobin Yvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with an integral BX 41 confocal microscope. Radiation from a HeNe laser (633 nm) was used as the excitation source. Raman scattering was detected at a 180◦ geometry using a multichannel air-cooled (-60◦ C) chargecoupled device camera (1024 × 256 pixels). We performed Raman shift calibration with a Si wafer, which has a characteristic Raman shift at 520 cm−1 . The Raman spectra at 50-500 cm−1 were collected with 1 s of exposure time and two accumulations using the 50 × (NA 0.8) objective.

Acknowledgement We thank M. Choi for helpful discussions, and authors B.K.G., M.K., and S.S. thank Dr. D.K. Aswal (Director, National Physical Laboratory, New Delhi) for his deep interest in this work. Funding: M.G.H. is grateful for the support from the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF-2017R1D1A1B03030225). S.S. would like to thank the Department of Science and Technology (Ministry of Science Technology) for the fellowship under the Women Scientist (SR/WOS-A/ET-49/2013) Scheme. This study was supported by the Research Program (POC2930) of the Korean Institute of Materials Science (KIMS). 17


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Supporting Information Available The following files are available free of charge. • TEM image of NbSe2 and peaks relaxation fitting of bare WSe2 . This material is available free of charge via the Internet at http://pubs.acs.org/.

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Ultrafast Excitonic Behavior in Two-Dimensional Metal

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