# Direct Reconstruction of the Band Diagram of Rhombohedral-Stacked Bilayer WSe 2 -Graphene Heterostructure via Photoemission Electron Microscopy

Mohamed Boutchich, Keiki Fukumoto, Aymen Mahmoudi, Alexandre Jaffré, José Alvarez, David Alamarguy, Chanan Euaruksakul, Fabrice Oehler, Abdelkarim Ouerghi

► To cite this version:

Mohamed Boutchich, Keiki Fukumoto, Aymen Mahmoudi, Alexandre Jaffré, José Alvarez, et al. Direct Reconstruction of the Band Diagram of Rhombohedral-Stacked Bilayer WSe 2 -Graphene Heterostructure via Photoemission Electron Microscopy. ACS Applied Electronic Materials, 2024, 6 (9), pp.6484-6492. 10.1021/acsaelm.4c00965. [PDF] [Full Article]

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Direct Reconstruction of the Band Diagram of
Rhombohedral-stacked Bilayer WSe₂-graphene
Heterostructure via Photoemission Electron
Microscopy

Mohamed Boutchich¹,⁴*, Keiki Fukumoto²*, Aymen Mahmoudi³, Alexandre Jaffré⁴, José Alvarez⁴,
David Alamarguy⁴, Chanan Euaruksakul⁵, Fabrice Oehler³, Abdelkarim Ouerghi³

¹Sorbonne Université, CNRS, Laboratoire de Génie Electrique et Electronique de Paris, 75252, Paris, France

²High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

³Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 91120, Palaiseau, France

⁴Université Paris-Saclay, CentraleSupélec, CNRS, Laboratoire de Génie Electrique et Electronique de Paris, 91192, Gif-sur-Yvette, France

⁵Synchrotron Light Research Institute, 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand
KEYWORDS 2D heterostructure, band diagram, Rhombohedral-stacked bilayer WSe$_{2}$, electronic properties, ER-PEEM, TR-PEEM

ABSTRACT The determination of energy levels at heterointerfaces is important for understanding charge transport mechanisms, enabling judicious assembly of various electronic and optoelectronic devices. Herein, we investigated the interface properties of a heterostructure consisting of two-dimensional (2D) transition-metal dichalcogenides rhombohedral 3R (AB stacking) bilayer WSe$_{2}$ (3R-2ML WSe$_{2}$) and epitaxial graphene using photoemission electron microscopy (PEEM) with a femtosecond laser excitation source. The 2D energy band diagram was imaged in an energy-resolved mode (ER-PEEM). For the 3R-2ML WSe$_{2}$, the conduction band minimum and the exciton were located at 2.0 and 2.6 eV, respectively, while the valence band maximum was at 4.18 eV. The Fermi level of graphene was located at 4.08 eV. These observations were supported by photoluminescence and Kelvin probe atomic force microscopy results. Furthermore, we investigated carrier dynamics using the system in the time-resolved mode (TR-PEEM). We evidenced that irradiation with 2.4-eV pulses induced a surface photovoltage that relaxed within ~25 ps. This methodology, coupling spectral and dynamic properties with space, time, and energy resolutions, allows the reconstruction of energy band diagrams and observation of the recombination mechanisms in nanoscale heterostructures. These parameters are instrumental for modeling and fabricating a wide range of heterojunction devices for photovoltaic and optoelectronic applications.

<footnote>*Email: mohamed.boutchich@sorbonne-universite.fr</footnote>

<footnote>*Email: keiki@post.kek.jp</footnote>
INTRODUCTION

Two-dimensional (2D) materials such as transition-metal dichalcogenides (TMDCs) and graphene are essential for various electronic and optoelectronic devices, such as light-emitting diodes and photodetectors<sup>1–3</sup>, field effect transistors<sup>4,5</sup>, and memory-storage devices<sup>6,7</sup>, owing to their unique band structures and low dimensionality. Combining these materials in van der Waals (vdW) heterostructures offers many advantages over bulk semiconductors, such as minimal or no lattice mismatch, bond-free interfaces, and layer-dependent properties. The development of large-scale chemical vapor deposition (CVD) methods to grow these materials on various substrates shows that their electronic properties may differ substantially compared to single crystals produced via mechanical exfoliation. The electronic and dynamic properties of materials may vary considerably depending on their synthesis method, defect density, or assembled heterostructure. The tunability of the electronic properties of vdW heterostructures is key to exploiting them for detection or emission purposes. To devise and model judicious heterojunctions, it is necessary to capture the precise energy offsets at the interfaces where charge transfer and separation occur. Probing these properties at the interface of device heterostructures is challenging, and various advanced techniques, such as angle-resolved photoemission spectroscopy (ARPES)<sup>8,9</sup>, time-resolved ARPES<sup>10</sup>, and X-ray and UV–photoemission spectroscopy<sup>11</sup>, are commonly utilized. While the latter is widely used for band alignment determination in surface science, it does not provide any information on carrier dynamics. In contrast, ARPES provides refined band structures in the reciprocal space and at low temperatures. The recent development of advanced photoemission electron microscopy (PEEM) systems typically accommodates two modes, energy-resolved PEEM (ER-PEEM) and time-
resolved PEEM (TR-PEEM), with superior spatial (50 nm) and temporal (100 fs) resolutions and imaging capabilities particularly suited for micron-size samples. The photon energy of the excitation source is tunable, and the energy resolution is not limited by the energy width of the femtosecond laser, i.e., 30 meV, rather than the detector. Such a technique has been utilized for various studies on the dynamics at the interface between TMDCs¹²,¹³. Of particular interest is the photoinduced charge transfer process at the interface between the 2D material and the graphene substrate¹⁴. This process is often ultrafast, in the picosecond range, and limited by exciton diffusion, nonradiative recombination, or trapping of carriers at defect centers, substantially reducing device efficiency¹⁵⁻¹⁷. These observations have triggered strong interest in various characterization communities in probing the optoelectronic properties of heterostructures and understanding the various mechanisms governing the modulation, transfer, and recombination of charge carriers using advanced techniques such as ultrafast spectroscopy with competitive energy, space, and time resolutions¹⁸,¹³. In this regard, ER/TR-PEEM is a versatile instrument that captures spectral and dynamic properties and allows for operando experiments. The WSe₂-graphene heterostructures are widely studied owing to their wide range of applications⁴,¹⁹. Zhang et al.²⁰ observed the modulation of electrical properties in a graphene-WSe₂ heterostructure using an optical-pump terahertz (THz)-probe, where WSe₂ transfers holes to graphene, lowering its Fermi level (E<sub>F</sub>) and thereby increasing its conductivity. Xing et al.¹⁶ conducted photon energy-dependent transient THz spectroscopy on WSe₂-graphene heterostructures with different stacking orders. They determined that for sub-A-exciton excitation, the charge transfer process occurs through thermionic emission, while for above-A-exciton, it is a direct electron (hole) transfer. Similarly, Wang et al.¹⁵ studied the dynamics of structural inhomogeneities in WSe₂ flakes via TR-PEEM and determined that the dynamics occur in two different regimes: a
subpicosecond one owing to electron cooling through interval scattering and an extended picosecond one owing to exciton-exciton annihilation. This procedure has been applied to various materials. Lyu et al.²¹ studied the layer dependence of ultrafast carrier dynamics in PdSe₂ using both the ER- and TR-PEEM modes and revealed time-constant variations between semimetal (bulk) and semiconducting (few layers) phases. Using a similar technique, we imaged defect distribution in 2D hexagonal boron nitride by tracing photogenerated electron dynamics¹² and observed operando carrier transport in organic transistors¹⁸. Recently, Fukumoto et al.²² studied photoexcited electron dynamics in three-dimensional topological insulators Bi₂Se₃ with time (100 fs), space (100 nm), and energy (30 meV) resolutions.

Herein, we employed energy and transient PEEM (ER- and TR-PEEM) to determine the energy band diagram of rhombohedral 3R (AB stacking) bilayer WSe₂ (3R-2ML WSe₂) and investigate its carrier dynamics transferred onto a graphene/4H-SiC (0001) substrate. The ER-PEEM probe was employed to determine the photoemission thresholds at the interface between the materials. We found that the heterostructure consists of n-type graphene assembled with p-type 3R-2ML WSe₂, and we precisely located the *E*<sub>F</sub> of graphene as well as the valence band maxima (VBM) of 3R-2ML WSe₂ (4.08 and 4.18 eV, respectively), its electron affinity (the conduction band minima, CBM), the exciton state, and its binding energy (~2.0 eV, 2.6 eV, and 610 meV, respectively). Furthermore, in the TR-PEEM mode, we detected a surface photovoltage of ~1 eV induced upon fast pumping of electrons at the 3R-2ML WSe₂-graphene interface that relaxes to equilibrium within ~25 ps.

# EXPERIMENTAL SECTIONS

## CVD synthesis of graphene and 3R-2ML WSe₂
Graphene on 4H-SiC (0001) is typically employed as a conductive host substrate for ARPES investigations. It is produced via a two-step process in a separate CVD system. Before graphitization, the substrate was hydrogen-etched (100% H₂) at 1550°C to produce well-ordered atomic terraces of SiC. Subsequently, the SiC sample was heated to 1000°C and further heated to 1550°C in an Ar atmosphere to obtain the desired single or few layers of graphene²³. Raman spectra measurements were conducted using a commercial confocal Horiba Labram micro-Raman microscope with a 532 nm laser in an ambient environment at room temperature. The excitation laser (wavelength 532 nm) was focused onto the samples with a spot diameter of ~1 µm²⁴,²⁵. Topographic imaging was realized using noncontact-atomic force microscopy (nc-AFM) experiments performed under the tapping mode with a Park System setup at atmospheric pressure at room temperature.

The 3R-2ML WSe₂ crystals were grown via CVD in a customized 4" Nabertherm (RST 120) horizontal oven on SiO₂/Si substrates (283 nm–thick SiO₂ on Si (100), Nova Wafers). Argon (Ar, 200 sccm) and hydrogen (H₂, 10 sccm) were used as carrier gases at a pressure of 2.5 mbar throughout the whole growth procedure. The effective growth duration at high temperatures was 30 min, excluding the temperature ramps from and to room temperature (60 min ramp up and 120 min ramp down). During growth, the selenium source was elemental Se (Neyco, 99.9%) in a quartz crucible heated to 250°C. The tungsten source was H₂WO₄ (Acros Organics, 99%) powder heated to 850°C and placed upstream of the SiO₂/Si substrates. The bilayers were found across the SiO₂/Si substrate (5 cm × 2 cm) with a preferential location downstream, toward the cold part of the substrate.

*Transfer of 3R-2ML WSe₂ onto graphene*
The wet transfer process was conducted using a KOH etching solution. First, the polymethyl methacrylate (PMMA) polymer was spin-coated onto the 3R-2ML WSe₂ flakes and subsequently peeled off from the SiO₂ substrate. The PMMA/3R-2ML WSe₂ layer was then transferred onto the graphene/SiC substrate, and the sacrificial polymer was finally removed using acetone. Following the transfer, the sample was annealed at 300°C for 30 min. The shape and size of the flake remained unchanged after the transfer. AFM and Raman characterization were subsequently performed.

*Micro-PL, Raman, and Kelvin probe force microscopy (KPFM) characterizations*

Herein, 2D Raman scattering images and Raman spectra were collected using a WiTec alpha 300R confocal Raman microscope with a UHTS300 spectrometer and a DV401A charge-coupled device (CCD) detector in an ambient environment at room temperature. The excitation laser, a frequency-doubled Nd:YAG laser at 532 nm, was focused onto the samples with a spot diameter of ~1 µm. The Raman signal was collected using a backscattering geometry. Similarly, photoluminescence (PL) spectra were collected using the same optical path but with a different filter and spectrograph than Raman. They were collected using an Acton SP2300 coupled with a DV420A OE Andor CCD camera. Temperature-controlled PL was acquired inside a Linkam scientific chamber positioned on the system stage. The Raman and PL mappings were 20 x 20 µm square with 500 nm pitch (i.e., 40 × 40 measurement points). Integration time was ~1 s per spectrum with 500 µW laser power. Average spectra were obtained by averaging all spectra within a specific region of interest (monolayer or multilayer regions) by convoluting the data object with a digital mask defined for these different regions.

Atomic and KPFM measurements were conducted using a scanning probe microscope system from AIST-NT (TRIOS platform) under ambient conditions. The measurements were conducted
in the frequency-modulated KPFM (FM-KPFM) mode with a two-pass scanning approach, where the second pass was performed at a constant distance (~10 nm) from the sample surface. Topographical data were collected on the first pass, while contact potential difference (CPD) was measured during the second pass. The compensation voltage $V_{DC}$ and the modulated voltage $V_{AC}$ (3 V peak-to-peak amplitude) were applied to the AFM tip (made of PtSi with a radius of <25 nm, a force constant of 2.8 N/m, and a resonance frequency close to 75 kHz), and the ground was connected to the graphene layer.

### ARPES and Kelvin probe measurements

The ARPES experiments were performed at the ANTARES beamline of the SOLEIL synchrotron light source (Saint-Aubin, France). ARPES data were acquired with a photon energy of 100 eV using linearly polarized light. Measurements were performed at a pressure of $10^{-11}$ mbar and 70 K.

### Energy and time-resolved photoemission electron microscopy

Time, space, and energy-resolved experiments were performed using a PEEM (FOCUS GmbH) with femtosecond laser pulses at the High Energy Accelerator Research Organization (KEK). A schematic of the system is shown in **Figure S1**. The femtosecond laser system is a Light Conversion PHAROS SP, with a fundamental wavelength of 1028 nm and an average power of 6 W. The repetition rate is tunable from single shots to 500 kHz. The laser pulses were split into two. The first pulse was injected into an Orpheus optical parametric amplifier (OPA), extending the output laser wavelength to the ultraviolet region (>220 nm) using nonlinear crystals. These laser pulses induced photoelectron emission (probe pulses). The second laser pulse was converted to the second harmonic (514 nm) for exciting electrons over the band gap of $WSe_2$ (pump pulses). The spatial resolution was ~100 nm in the present setup, and the repetition
rate was set to 20 kHz. These parameters were appropriate for obtaining sufficient photoelectron
intensity and clearly distinguishing between graphene and 3R-2ML WSe₂ flakes. Moreover, the
PEEM intensity was calculated by subtracting the intensity without pump excitation from that
with pump excitation, followed by normalization. Details of the system are described in
references²⁶,²⁷.

RESULTS AND DISCUSSION

Characterization of 3R-2ML WSe₂-graphene

Herein, we investigated 3R-2ML WSe₂²⁸ on an epitaxial graphene vdW heterostructure using PEEM. The heterostructure was fabricated using a wet transfer (see methods) of 3R-2ML WSe₂ flakes onto the epitaxial graphene substrate²⁸. The transferred bilayer flakes were then identified using their optical contrast with respect to the graphene substrate, as shown in Figure 1(a). The optical image shows a large flake of 3R-2ML WSe₂ on the graphene and confirms no damage from the transfer process. Figure 1(a–c) displays the AFM topography of the heterostructure and its thickness profile, indicating a bilayer WSe₂. The flakes measure ~20 µm side, with a large bilayer area and a central bulk nucleus, as confirmed through the AFM height profile of ~1.5 nm. A narrow monolayer area is visible at the peripheral edge of the flake. Figure S2 shows the room and low-temperature PL over the monolayer and bilayer regions of the 3R-2ML WSe₂ flake. The device was fabricated by transferring a 3R-2ML WSe₂ flake onto a SiO₂/Si substrate. Electrodes were patterned via laser lithography, and contacts (Ti (10 nm)/Au (80 nm)) were evaporated and lifted off in acetone. These ohmic contacts enabled the measurement of the I–V characteristics, revealing a p-type behavior, as shown in Figure S3.

The Raman spectrum at room temperature, shown in **Figure 1(d)**, exhibits the characteristic peaks $E_{1g}$ and $A_{2g}$ at 249 and 257 cm⁻¹, respectively, while the $B_{1g}$ peak at 308 cm⁻¹ indicates a
thickness larger than a monolayer. **Figure 1(e)** shows the PL spectrum as a function of temperature, with different excitons clearly resolved at low temperatures. The deconvolution, illustrated in **Figure S4**, indicates two exciton emissions at 1.52 and 1.61 eV²⁸, where the latter originates from the direct interband transition, whereas the former is from the indirect bandgap.

We noticed that some flakes do not exhibit PL quenching on graphene compared to similar samples on SiO₂. This suggests that the work functions (WF) of both materials are aligned or that the interface separation is large enough to prevent electron-hole pairs in 3R-2ML WSe₂ from quickly recombining through the nonradiative channel of metallic graphene²⁹. A strong substrate-layer interaction might pull the graphene away from the 3R-2ML WSe₂, thereby increasing the interlayer distance. Consequently, we only observe the PL of the topmost layer, as if graphene and 3R-2ML WSe₂ are independent of each other. Yang et al.²⁹ calculated that the separation required to prevent quenching could be as small as 0.01–0.03 Å. We then conducted ARPES measurements to visualize the band structure of the 3R-2ML WSe₂ on epitaxial graphene heterostructure. **Figure 1(f)** represents the electronic dispersion along the Γ–K direction. Two branches are clearly split at the top of the valence band (VB) at the Γ point, indicating a bilayer film. As 3R-2ML WSe₂ is essentially a bilayer, the graphene bands⁹ do not appear and do not hybridize with WSe₂. Moreover, the VB at the K point is slightly lower than at the Γ point, suggesting that 3R-2ML WSe₂ has an indirect bandgap between the VBM at Γ and the CBM at K. Notably, our 3R-2ML WSe₂ exhibits a strong spin–orbit coupling of ~520 ± 20 meV, as reported in the literature³⁰. The energy difference between the Fermi level ($E_F$) and the VB for this sample is ~0.7 eV at 70 K.
<figure><img src="image_1.png" /><figcaption>**Figure 1.** (a) Optical image of a 3R-2ML WSe₂ flake transferred onto a graphene/SiC (0001) substrate. (b and c) AFM image and profile of the bilayer WSe₂. (d) Raman spectrum of the 3R-2ML WSe₂ flake. (e) Photoluminescence spectra of 3R-2ML WSe₂ on graphene at different temperatures. (f) ARPES measurements of the 3R-2ML WSe₂/graphene heterostructure along the high-symmetry direction Γ-K.</figcaption></figure>

*Surface potential mapping*

To probe the surface and interface electrostatic properties of the heterostructure, we conducted KPFM experiments. KPFM quantifies the vacuum level shift at the surface of a 3R-2ML WSe₂ flake induced by charge transfer or dipole presence, providing valuable insights into the energy band offsets at the interface. Interestingly, not all the flakes transferred exhibited a CPD, defined as $V_{CPD} = \frac{\phi_{sample} - \phi_{tip}}{e}$, supporting the PL measurements that either aligned Fermi levels or
notable separation between 3R-2ML WSe₂ and graphene²⁹ (Figure S5). In contrast, the large 3R-2ML WSe₂ flake shown in Figure 2(a) is in intimate contact with the graphene. We measured the surface potential map of the WSe₂ flake simultaneously with the topographic image, revealing distinct features, including the bulk center, flake edges, terraces, and step edges of the epitaxial graphene. Figure 2(b) shows the topography profile and $V_{CPD}$ variation from a graphene terrace to the flake along the cut marked by the black arrow (1). The bilayer nature of the flakes (~1.5 nm) and varying surface potential levels are clearly resolved.

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) KPFM image of the rhombohedral-stacked bilayer WSe₂ on epitaxial graphene.</figcaption></figure>

(b) Topographic and potentiometric cross-section profiles along the black arrow (scan 1) in
(a). The AFM profile indicates the bilayer nature of the 3R-2ML WSe₂. (c) Zoom-in image of (a) showing the center nucleus. (d) Height and CPD profile over the center (scan 2). The thickness is ~20 nm, representing more than 20 layers.

The $V_{\text{CPD}}$ directly measures the interface dipole induced by $E_F$ alignment when both materials are in contact. In the central bulk nucleus region of the flake (Figure 2(c–d)), the $V_{\text{CPD}}$ increases from ~100 mV at the center (cut along arrow (2)), indicating a WF closer to the KPFM tip (PtIr: 4.89 eV) up to a $V_{\text{CPD}}$ on the 3R-2ML WSe₂ flake, which has a larger WF than the tip. The 3R-2ML WSe₂ flake and graphene step edges exhibit nearly homogenous $V_{\text{CPD}}$ of ~270 mV, while the graphene terraces present a $V_{\text{CPD}}$ of ~90 mV, as determined from the representative yellow and gray homogeneous areas with minimal defect contributions (wrinkles and boundaries), as shown in Figure 3(a). Figure 3(b) illustrates the n type-doped Dirac structure of epitaxial graphene along with the energy band diagram of the p type-doped 3R-2ML WSe₂ flake before and after their contact. As the WF of 3R-2ML WSe₂ is larger than graphene, the vacuum level shifts up by 180 mV ($\Delta\Phi$), consistent with Richheimer et al.³¹. X-ray photoelectron spectroscopy (XPS) survey (Figure S6(a)) shows that 3R-2ML WSe₂ is not oxidized and retains its integrity. Figure S6(b) displays a PEEM image acquired under a synchrotron beamline on a different 3R-2ML WSe₂/graphene sample and its corresponding cut (Figure S6(c)). The WF difference of ~125 meV aligns with the KPFM trend. This small WF difference may not induce substantial charge transfer between graphene and 3R-2ML WSe₂.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

**Figure 3.** (a) Histogram of the surface potential integrated over the regions indicated by the gray and yellow rectangles depicted in Figure 2(a). (b) Schematic illustration of the band alignment of the heterostructure before and after contact. At thermal equilibrium after contact, the Fermi levels align, causing the vacuum level (E<sub>vac</sub>) of 3R-2ML WSe<sub>2</sub> to shift up by the V<sub>CPD</sub> measured (ΔΦ ≈ 180 mV) owing to its larger WF compared to the KPFM
tip and graphene.

### Energy level mapping

To determine the band alignment of the heterostructure in real space (energy level mapping), we conducted an ER-PEEM acquisition using a charge density-contrast PEEM (CDC-PEEM) with an energy filter (EF) to monitor photoelectron kinetic energy (see method). First, we determined the Fermi level of graphene and the VBM of 3R-2ML $WSe_2$ by tuning the photon energy to 5.6 eV while sweeping the EF voltage from -2 to 2 eV at 0.1 V steps. For each step, a PEEM image was acquired (Figure 4(a)). The averaged photoelectron emission intensity from the red and green regions depicted in Figure 4(a) is plotted versus energy from the vacuum level in Figure 4(b). These are thresholds of photoelectron emission of ~4.1 and ~4.2 eV for the Fermi level of graphene and the VBM of 3R-2ML $WSe_2$, respectively. To probe the upper levels of the CBM and the exciton state of $WSe_2$, the photon energy was set to 3.0 eV to avoid the photoemission of the deep background levels. The onset of photoemission corresponds to the CBM located at 2.0 eV (inset of Figure 4(c)), with a second threshold locating the exciton at ~2.6 eV (Figure 4(c)). Notably, almost no photoemission is observed from the graphene covered by 3R-2ML $WSe_2$.

The exciton energy measured via PL, shown in **Figure 1(e)**, is located at 1.57 eV above the VBM, indicating an electronic bandgap of ~2.18 eV (the difference between CBM and VBM). Hence, the exciton binding energy is ~610 meV. This value is higher than those reported for sapphire and Au substrates³² and larger than that of conventional semiconductors such as GaAs³³,³⁴ but is within the range predicted for TMDCs³⁴. Such high binding energy explains the stability of excitons in $WSe_2$ even at room temperature. The reconstructed band diagram is depicted in **Figure 4(d)**. At room temperature, the VBM of 3R-2ML $WSe_2$ is located at -0.1 eV
below the $E_F$ of the heterostructure, which is lower than observed using ARPES at 70 K. This discrepancy is ascribed to sample variations and contamination during preparation, rendering the 3R-2ML WSe₂ flake p-type.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Direct reconstruction of the energy band diagram using PEEM. (a) PEEM image at $EF = 0$ V. The red and green regions of interest (ROIs) correspond to the area on 3R-2ML WSe₂ and graphene, respectively, from which photoemission spectra are extracted. (b) The CBM threshold, corresponding to the electron affinity of 3R-2ML WSe₂, is ~2.0 eV, while the exciton threshold is located at ~2.6 eV below the vacuum level. (c) The onset of</figcaption></figure>
photoemission for each ROI corresponds to the $E_F$ of graphene at 4.08 eV and the VBM of 3R-2ML $WSe_2$ at 4.18 eV. (d) Illustration of the reconstructed band diagram. Movies in the supplementary information show the PEEM acquisition process during the energy scan.

## Carrier dynamics

Finally, we studied the photoinduced charge transfer occurring at the 3R-2ML $WSe_2$ interface on epitaxial graphene. Fast pumping affords photoinduced carriers generated in the CBM of 3R-2ML $WSe_2$, which partially transfer to graphene, altering its conductivity. This change in conductivity results in varying photoemission intensity, which can be readily monitored using PEEM. This ultrafast process occurs on a picosecond timescale and is not observable in the transient photoresponses of a device owing to high cut-off frequencies. We utilized our PEEM system in the TR-PEEM mode to observe carrier dynamics. The experimental setup is described in references and the releated experimental section$^{12,27,35}$. Herein, we precisely locate the heterostructure ROIs previously observed using Raman spectroscopy and AFM. The sample was illuminated with a constant pump energy ($h\nu \ge 2.4$ eV A-exciton energy) while sweeping the probe energy from 2.99 to 4.13 eV and varying the pump-probe delay. A PEEM image was acquired for each delay, and data analysis provided the carrier dynamics averaged over each ROI, fitted with a double exponential function.

Photogenerated electron dynamics in graphene show no probe energy dependence when the probe energy is larger than the Fermi level of graphene (~4.08 eV). **Figure 5** displays the time profiles for graphene and 3R-2ML $WSe_2$. The vertical axes of panels (a) and (b) are the normalized photoelectron emission intensity, calculated by subtracting photoelectron emission intensity without pump pulse irradiation from the photoelectron emission intensity with the pump pulse and normalizing by the intensity at negative delay and at 0 ps. Owing to laser instability,
the intensities at 0 ps for various photon energies vary by within 10%. The first decay time constant, approximately 0.2 ± 0.1 ps for all measurements, reflects hot electron thermalization via electron-electron scattering, consistent with similar observations²⁰,³⁵. The second decay for graphene, in the picosecond range (5 ± 0.5 ps), corresponds to the recombination of photoinduced hot carriers in the VB³⁵, as shown in **Figure 5(a)**.

In contrast, the carrier dynamics in 3R-2ML WSe₂ differ substantially from those in graphene alone. The time profiles depend strongly on the probe energy, as shown in **Figure 5(b)**. For probe energies above 3.63 eV, electron thermalization and recombination processes are observed with time constants of 0.2 ± 0.1 and 7 ± 0.1 ps, respectively. However, for probe energies below 3.63 eV, the photoemission intensity initially increases sharply before dropping to negative intensity. This mechanism can be understood by considering the origin of the photoelectron emission intensity. In the TR-PEEM mode, photoelectron emission is induced by tuning the photon energy and depends on two factors: electron density at a given energy level and the depth of that energy level from the vacuum level. Therefore, the reduction in photoemission intensity is ascribed to shifts in the energy level of the band diagram relative to the vacuum level, indicating a change in surface potential upon illumination owing to surface photovoltage (SPV). As the probe energy approaches 2.99 eV, photoemission efficiency drops and the time profiles of 3R-2ML WSe₂ exhibit a low signal-to-noise ratio. At a threshold photon energy of ~3.63 eV, the photoelectron emission intensity drops to zero, indicating that the initial photoemission threshold, i.e., 2.6 eV from the exciton to CBM levels, has shifted downward owing to SPV. This suggests that the exciton state has crossed the photoemission threshold at ~3.63 eV, revealing an SPV of ~1 eV (3.63 eV minus the initial 2.6 eV exciton energy). The SPV occurs within ~0.25 ±
0.1 ps and relaxes within 24.7 ± 0.1 ps, as shown in the **inset** of Figure 5(b), beyond which the band diagram recovers its equilibrium position.

Moreover, the compression of interlayer separation between the two WSe₂ layers in 3R-2ML WSe₂ and between 3R-2ML WSe₂ and graphene during pumping affects the carrier dynamics. Mannebach et al.³⁶ observed that ultrafast pumping above the optical bandgap of the material increases carrier density, which enhances vdW interlayer attractive forces and reduces interlayer spacing between the materials. Consequently, charge transfer and recombination processes occur rapidly with shortened time constants. This is consistent with our observation, where the recombination lifetime for 3R-2ML WSe₂ is in the picosecond range, compared to the nanosecond range observed for bulk WSe₂ by Liu et al.¹⁹. These time scales are consistent with those observed in TR-PEEM studies of WSe₂, highlighting the impact of strain and defects on exciton dynamics³⁶. However, the SPV relaxation time of ~24.7 ps is not necessarily equivalent to the recombination lifetime of carriers, as trap-assisted mechanisms could often affect the recombination process.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

**Figure 5.** Electron dynamics spectra in the TR-PEEM mode. (a) Time profile of the epitaxial graphene substrate showing a decay time response of 5 ± 0.5 ps. (b) Time profiles of 3R-2ML WSe₂ acquired for varying probe energies. Carriers relax within 7 ps for probe energies from 4.13 to 3.63 eV, while below 3.63 eV, the SPV mechanism dominates, relaxing within ~24.7 ps (inset at 2.99 eV). (c) Illustration of carrier dynamics and SPV relaxation processes within the heterostructure band diagram. **Figure S7(a)** displays all the time profiles acquired for probe energies ranging from 4.13 to 2.99 eV in 0.1 eV steps. **Figure S7(b)** illustrates the changes in the band diagram at equilibrium and during pumping, where SPV appears.
## CONCLUSION

Herein, we measured the electronic properties of 3R-2ML WSe₂ grown via CVD on epitaxial graphene. KPFM measurements show that the dipole strength of the 3R-2ML WSe₂ relative to the graphene increases up to ~180 mV. Using ER-PEEM, we precisely located the exciton state relative to the vacuum level and correlated it with PL measurements. Moreover, we successfully measured the CBM and VBM thresholds, allowing us to directly reconstruct the energy band diagram of the 3R-2ML WSe₂-graphene heterostructure. In TR-PEEM, we observed the carrier dynamics and observed a rigid shift of the 3R-2ML WSe₂ energy levels upon illumination owing to an induced SPV of at least ~1 eV. This band bending relaxes within ~25 ps. ER and TR-PEEM instrumentation, with resolved time, energy, and space, enables the determination of band diagrams in complex heterostructures and provides key parameters for modeling photovoltaic and optoelectronic devices.

## ASSOCIATED CONTENT

Additional datasets including temperature controlled and deconvoluted PL, XPS, and illustration of the PEEM system are available in the supporting information.

## AUTHOR INFORMATION

### Corresponding Authors

Mohamed Boutchich-Sorbonne Université, CNRS, Laboratoire de Génie Electrique et Electronique de Paris, 75252, Paris, France; https://orcid.org/0000-0002-8460-448X

Email: mohamed.boutchich@sorbonne-universite.fr
Keiki Fukumoto-High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba,
Ibaraki 305-0801, Japan; https://orcid.org/0000-0002-7742-5862

Email: keiki@post.kek.jp

## Author Contributions

M.B and K.F. conceived and designed the project. F.O, A.M and A.O performed material synthesis transfer and AFM characterization. A.J and J.A carried out PL, Raman and KPFM, D.A and C.E the XPS characterization. M.B carried out the device fabrication, the electrical measurement. K.F and M.B carried out ER and TR-PEEM characterization and data analysis. All authors discussed and commented on the manuscript.

## Funding Sources

Any funds used to support the research of the manuscript should be placed here (per journal style).

## Data Availability Statement

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

## Declaration of Competing Interest

The authors declare no competing interests.

## ACKNOWLEDGMENTS

This work was financially supported by the "PHC Sakura" program (project number: 46982WB), implemented by the French Ministry for Europe and Foreign Affairs, the French Ministry of
Higher Education and Research and the Japan Society for Promotion of Science. K. Fukumoto wishes to thank MEXT for Q-LEAP Attosecond lasers for next frontiers in science and technology (ATTO) grant, JSPS KAKENHI (15K17677 and 21H01752), as well as JST FOREST Program, Grant Number JPMJFR203P.

ABBREVIATIONS

ARPES, Angle-resolved photoemission spectroscopy; CCD, Charge-coupled device; CPD, Contact potential difference; CVD, Chemical vapor deposition; EF, Energy filter; ER, Energy-resolved; KPFM, Kelvin probe force microscopy; OPA, Optical parametric amplifier; ROI, Regions of interest; VB, Valence band; VBM, Valence band maxima; WF, Work functions; XPS, X-ray photoelectron spectroscopy

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