Single-Molecule Imaging of Activated Nitrogen Adsorption on

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Single-Molecule Imaging of Activated Nitrogen Adsorption on Individual Manganese Phthalocyanine Jia Lin Zhang, Zhunzhun Wang, Jian-Qiang Zhong, Kaidi Yuan, Qian Shen, Leilie Xu, Tian Chao Niu, Chengding Gu, Christopher Wright, Anton Tadich, Dongchen Qi, Hexing Li, Kai Wu, Guo Qin Xu, Zhenyu Li, and Wei Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00290 • Publication Date (Web): 23 Apr 2015 Downloaded from http://pubs.acs.org on April 28, 2015

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Single-Molecule Imaging of Activated Nitrogen Adsorption on Individual Manganese Phthalocyanine Jia Lin Zhang,1,2# Zhunzhun Wang,3,4# Jian Qiang Zhong, 1,2# Kai Di Yuan,2 Qian Shen,1 Lei Lei Xu,1 Tian Chao Niu,1 Cheng Ding Gu,1 Christopher A. Wright,5 Anton Tadich,5,6 Dongchen Qi,5 He Xing Li,7 Kai Wu,8,9 Guo Qin Xu,1,8 Zhenyu Li,3* Wei Chen1,2,8,10* 1

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore 2

Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore

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Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China 4 Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Normal College, Guiyang 550018, China 5 Department of Chemistry and Physics, La Trobe University, Victoria, 3086, Australia 6 Australian Synchrotron, 800 Blackburn Road Clayton, Victoria 3168, Australia 7 Chinese Education Ministry Key Laboratory of Resource Chemistry, Shanghai Normal University, Shanghai 200234, China 8 Singapore-Peking University Research Center for a Sustainable Low-Carbon Future, 1 CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore 9

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Jiang Su 215123, China

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ACS Paragon Plus Environment

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Abstract An atomic-scale understanding of gas adsorption mechanisms on metal-porphyrins or metal-phthalocyanines is essential for their practical application in biological processes, gas sensing and catalysis. Intensive research efforts have been devoted to the study of coordinative bonding with relatively active small molecules, such as CO, NO, NH3, O2 and H2. However, the binding of single nitrogen atoms has never been addressed, which is both of fundamental interest and indeed essential for revealing the elementary chemical binding mechanism in nitrogen reduction processes. Here, we present a simple model system to investigate, at the single-molecule level, the binding of activated nitrogen species on the single Mn atom contained within the manganese phthalocyanine (MnPc) molecule supported on an inert graphite surface. Through the combination of in situ low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy and density functional theory calculations, the active site and the binding configuration between the activated nitrogen species (neutral nitrogen atom) and the Mn center of MnPc is investigated at the atomic scale. Keywords: single molecule, activated nitrogen, manganese phthalocyanine, axial coordination


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Introduction In metal-porphyrins and metal-phthalocyanines, the metal center is usually coordinatively unsaturated and provides a local reactive site for axial coordination.1-3 In addition to the interesting physical phenomena exhibited by these molecules, such as the Kondo effect4 and negative differential resistance5, these systems are also able to interact with small molecules like CO, NO, NH3, O2 and H2,1, 3, 6-20 showing promising applications in biological processes,21 gas sensing18, 19 and catalysis10, 15. Elegant examples include dioxygen storage and transport by metallotetrapyrrole units;21 nitrogen monoxide detection using iron porphyrins7 and oxygen reduction reaction by iron phthalocyanines10. In turn, the adsorption of small molecules onto active sites of metal complexes can also be used to engineer the properties of the metal complexes themselves. This opens a unique possibility for charge manipulation,14 chirality manipulation,12 spin switching1, 11 and modification of the molecule-substrate interaction by external chemical stimuli3, 22. Most of these studies investigate the coordination of relatively active small molecules or their corresponding monatomic atoms. An atomic scale investigation of the binding between a single nitrogen atom and metal complexes has, however, never been addressed. In addition to being of fundamental interest, atomic scale characterization of the binding configuration between the nitrogen atoms and the transition metal centers are essential for elucidating the elementary chemical binding mechanism in nitrogen reduction processes.23-41 Moreover, previous studies mainly focus on the molecules on metal substrates, where strong coupling between the molecule and the substrate can greatly modify the intrinsic properties of the molecular system.2, 42-46 When an external stimulus is applied to the molecule, both the molecule-substrate interaction and the response of the molecule to the external stimuli will contribute to the final state of the molecule, which complicate the coordination process.3 To reduce the interaction between the molecule and the substrate, it is therefore desirable to study the coordination process on an inert substrate; to date, this has been rarely addressed. Manganese phthalocyanine (MnPc), a 3d transition-metal phthalocyanine with an intermediate spin state of S = 3/2, has been attracting considerable interest.11, 47-49 In this work, by using a surface confined MnPc monolayer as a model system, we demonstrate single-molecule imaging of the bonding between the activated nitrogen and the coordinatively unsaturated single manganese (Mn) metal atom center at room temperature (RT) and low pressure. Through surface anchoring of one monolayer of MnPc onto highly oriented pyrolytic graphite (HOPG), arrays of coordinatively unsaturated Mn centers can be generated, allowing their response upon exposure to nitrogen to be investigated by using in situ low-temperature scanning tunneling microscopy (LT-STM) at the atomic scale. Dinitrogen molecules were first activated by a hot filament and then selectively coordinatively bonded with the center Mn atom of the surface confined MnPc monolayer. Such coordinative bonding of the activated nitrogen (i.e., the neutral nitrogen atom of N) can greatly modify the electronic structure of the center Mn atom as well as the MnPc monolayer, as corroborated by the in situ scanning tunneling spectroscopy (STS), ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS) investigations and density functional theory (DFT) calculations. XPS core level spectroscopy, temperature dependent desorption and STM tip manipulated desorption experiments revealed chemical bonding between the N and the Mn center atom of the MnPc molecule. The combination of heat-induced or tip-induced nitrogen-desorption with the exposure-driven nitrogen-adsorption allows a completely reversible chemical cycle to be established in this system.


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Figure 1. STM images of MnPc molecules on HOPG after nitrogen exposure. (a) Large scale STM image of MnPc on HOPG after room temperature deposition at the pressure of 3 × 10-9 mbar (Vtip = 1.7 V, 28.8 × 40 nm2). (b) Large scale STM image after exposing the sample in panel (a) to 1000 L N2 (Vtip = 1.6 V, 28.8 × 40 nm2). (c) STM image of MnPc on HOPG by further exposing


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the sample in panel (b) to 20000 L N2 (Vtip = 1.89 V, 28.8 × 40 nm2). (d-f) The corresponding high resolution STM images of panel (a), (b) and (c) (Vtip = 1.7 V, 10 × 10 nm2; Vtip = 1.6 V, 10 × 10 nm2; Vtip = 1.89 V, 10 × 10 nm2). (g) Schematic of the MnPc molecule before and after N* species adsorption. (h) dI/dV spectra taken at the center and lobe positions of MnPc and (i) N*-MnPc. Results and discussions A MnPc monolayer on HOPG was prepared via a conventional organic molecular beam deposition method under ultrahigh vacuum (UHV). As shown by the large scale (Fig. 1a) and the corresponding high-resolution (Fig. 1d) STM images in Fig. 1a and Fig. 1d, MnPc molecules self-assemble into a densely packed monolayer on HOPG with their molecular π-plane lying flat on surface. A closer inspection reveals that the pristine MnPc molecules appear as a four-lobe feature without a central bright protrusion, as highlighted by the red dashed circles and referred to as configuration 1.1, 11, 49 Dinitrogen molecules were activated by a hot filament in the presence of nitrogen introduced into the chamber at a controlled pressure using a leak valve. The electron emission current emitted from the heated filament results in electrons colliding with the gas molecules in the enclosed volume and causing the ionization and/or dissociation of the molecules. The activated nitrogen may be in the form of N, N+, N2+, N2+ or N2*. N2* here represents the excited neutral dinitrogen molecule. The details of the products also depend on the activation method.50-52 Figure 1b and c show large scale STM images of a MnPc/HOPG monolayer island after exposure to nitrogen with a dose of 1000 Langmuir (L) and 20000L at RT, respectively (Detailed STM images at different dosages of nitrogen can be found in the Supplementary Information). The pristine MnPc (i.e. configuration 1) molecules presented in Fig. 1a progressively disappear upon nitrogen exposure and switch to configuration 2, which features a bright protrusion in the center, as shown in Fig 1b, 1c with their corresponding high resolution STM images shown in Fig 1e and 1f. We propose that the bright protrusion is caused by the selective bonding of one specific activated nitrogen species (N*) to the Mn atom in MnPc, as illustrated by the schematic in Fig 1g. Such selective bonding can substantially alter the electronic properties of MnPc molecules, as revealed by the site-specific Scanning Tunneling Spectroscopy STS (i.e., dI/dV spectra) measurements shown in Fig. 1h and 1i. Here, only the dI/dV spectra measured at positive tip bias (occupied states) are presented. The upper panel of Fig. 1h displays the density-of-state (DOS) measured on the center Mn atom of the pristine MnPc. The first peak located at 0.75 V is due to Mn 3d dominant HOMO of MnPc, which is mainly localized on the central Mn atom. The second peak at around 1.45 V present in both the center and lobe of the pristine MnPc is due to the π electron dominant orbital.53 The selective bonding between the activated nitrogen species (N*) and the center Mn atom in MnPc significantly modifies the local DOS on the Mn center of the N*-MnPc (upper panel of Fig. 1i) with the Mn 3d dominant HOMO now completely vanished. For the DOS measured on the lobe of N*-MnPc (lower panel of Fig. 1i), the intensity of the first peak is slightly reduced compared with the pristine MnPc, whereas the second peak is much less affected. The significant difference between the DOS measured on the center of the pristine MnPc and N*-MnPc indicates that the coordination with N* modifies the electronic structure of the central Mn atom, originating via electron donation from the Mn 3d orbital into the bonded N* species.


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Figure 2. UPS and XPS spectra for monolayer MnPc on HOPG before and after N2 exposure at 1.0 × 10-5 mbar for 10 hours at RT. (a) UPS spectra in the HOMO band region. (b) XPS core level spectra of Mn 2p3/2. (c) XPS core level spectra of N 1s. Fig. 2a shows the UPS spectra near the HOMO region for the as-deposited MnPc/HOPG and after exposure to nitrogen at 1.0 × 10-5 mbar for 10 hours at RT. For the pristine MnPc, the first peak at a binding energy of 0.63 eV is assigned to the Mn 3d dominant HOMO as mentioned previously, and the second peak at 1.23 eV is assigned to the ligand π electron dominant orbital.53 Upon exposure to nitrogen, the first peak decreases in intensity, while the second peak remains intact. This is in accordance with our STS observations, and can be attributed to N* coordination to the center Mn atom. A striking difference between the pristine and the nitrogen-dosed MnPc is also seen in the Mn 2p3/2 XPS spectrum. As shown in Fig. 2b, for the pristine MnPc, the Mn 2p3/2 shows a broad structure with main peak located at 640.5 eV, followed by a broad satellite extending to higher binding energies (642.5-645.5 eV) caused by the open shell structure of the Mn atom.13, 54 Upon nitrogen exposure, the peak intensity at 640.5 eV strongly decreases, accompanied by the emergence of a sharp peak at 642.8 eV. This new intense peak can be attributed to the newly N* coordinated Mn atom. The narrowing of the line-shape9, 55 and a shift toward higher binding energy compared with the pristine MnPc has also been observed for small molecules (CO and NO) coordinated on other transition metals.2, 3, 13 It is a sign of strong interaction between the N* and the Mn atom, and signals a variation of the Mn charge state (now oxidized) in MnPc. As shown in Fig. 2c, exposure to nitrogen leads to a slight increase of the N 1s peak intensity, again confirming the adsorption of nitrogen onto the sample.


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Figure 3. Temperature dependent desorption of N* from MnPc. (a) XPS core level spectra of Mn 2p3/2 after annealing the N*-MnPc on HOPG at different temperatures. Large scale STM images of MnPc on HOPG (b) after exposure to 20000 L N2 (Vtip = 2.1 V, 30 × 40 nm2), and (c) after annealing the sample in panel (b) at 280 °C for 1 hour (Vtip = 1.654 V, 30 × 40 nm2). After annealing, most of the N*-MnPc on HOPG converted to pristine MnPc, indicating the desorption of N* from the MnPc center. We have also carried out temperature programmed desorption experiments to examine the thermal stability of N* coordinated MnPc. As shown in Fig. 3a, nitrogen exposure at 6.0 × 10-5 mbar for 2 hours on MnPc/HOPG at RT produces a XPS spectrum with two features: a peak at 642.8 eV originating from the N* coordinated MnPc, and a peak at 640.5 eV which can be attributed to residual pristine MnPc on the surface. After annealing the nitrogen-exposed sample at 120 °C, 165 °C, and 215 °C for 1 hour, respectively, we did not observe any significant change of the peak shape, position and intensity. Further increasing the annealing temperature to 260 °C resulted in a slight intensity decrease for the peak at 642.8 eV (N* coordinated MnPc), and a simultaneous increase in intensity for the peak at 640.5 eV (pristine MnPc). This indicates that the desorption starts to occur at 260 °C. Such a high desorption temperature suggests good thermal stability of the N* coordinated MnPc, and points toward a chemical bond between the Mn atom and the N* species. After heating the sample at 305 °C for 1 hour, the Mn 2p3/2 spectrum completely restores to that of the pristine MnPc/HOPG, revealing a complete desorption of the N* species from the MnPc monolayer. This can be further corroborated by STM experiments. As shown in Fig. 3b, the molecules with a small bright dot in the center are identified as N* coordinated MnPc whilst the others are pristine MnPc. After annealing the sample at 280 °C for 1 hour, all of the N* coordinated MnPc converts to the pristine MnPc, without any disruption of the molecule (Fig. 3c). Moreover, such adsorption and desorption of the activated nitrogen species (N*) on MnPc can be repeated many times. This suggests that the MnPc molecule can recover its


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activity after annealing without any loss of catalytic activity.

Figure 4. Sequential STM images showing the STM tip bias induced desorption of N* species from the MnPc center (a-c) (Vtip = 2.9 V, 7 × 10 nm2). The molecule indicated by the yellow circle is switched from N*-MnPc (a) to MnPc (b) by scanning at high tip bias voltage of 2.9 V, and then the molecule indicated by the red circle is switched from N*-MnPc (b) to MnPc (c) in the following scan. The desorbed N* species attaches to the neighboring MnPc, and hence the molecule indicated by the black circle simultaneously switches from MnPc (b) to N*-MnPc (c). (d-f) Bias dependent scanning of a same area of the MnPc:PFP (2:1) binary molecular networks (Vtip = 1.8 V, Vtip = 2.0 V, Vtip = 2.5 V, 7 × 8 nm2). The molecules indicated by the red circles are N*-MnPc molecules, scanning at low tip bias cannot trigger the N* desorption, at higher voltages (> 2.0 V), the N* have a larger probability of being desorbed from the MnPc center. (g, h) STM images showing desorption and re-adsorption of N* (Vtip = 2.0 V, Vtip = 2.3 V, 10 × 10 nm2). The molecule indicated by the red circle switches from N*-MnPc (g) to MnPc (h), at the same time, the desorbed N* species re-adsorbs on the neighboring MnPc and hence, the molecule indicated by the yellow circle switches from MnPc to N*-MnPc. (i) Schematic packing structure of the MnPc:PFP 2:1 binary molecular network. STM tip manipulated desorption can provide detailed insights into the adsorption/desorption behavior of the activated nitrogen N* species on MnPc at the atomic scale; here the tunneling electrons can induce the desorption of N* from the N* coordinated MnPc. From our experiments, a threshold voltage of 1.7 V is required to detach the N* from the Mn center, suggesting a strong bonding between the N* and the Mn atom. Sequential STM images obtained at a bias voltage of


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2.9 V are shown in Fig 4a-c. Initially, the N*-MnPc complex, as indicated by the yellow circle (Fig. 4a), is switched to MnPc (Fig. 4b) by scanning this area at a bias voltage of 2.9 V. Subsequently, another N*-MnPc complex, now denoted by a red circle (Fig. 4b), is switched to MnPc (Fig. 4c) by scanning at the same bias voltage. It is worth noting that the neighboring molecule indicated by the black circle (Fig. 4b) switches from MnPc to N*-MnPc simultaneously. We propose that the desorbed N* species re-adsorbs onto the neighboring MnPc molecule. Similar switching can also be realized in a stable intermolecular hydrogen bonded binary molecular network comprising MnPc and perfluoropentacene (PFP) with a 2:1 molecular ratio. The schematic packing structure of this binary molecular network is displayed in Fig. 4i (see detailed description in Supplementary Information). A time series of STM images of this binary molecular network scanned at different bias voltages is shown in Fig. 4d-f. At the beginning, four N*-MnPc complexes can be observed within the frame, as indicated by the red circles (Fig. 4d). No desorption of the N* species occurs (Fig. 4e) after scanning the same area at 2.0 V. Upon increasing the bias voltage to 2.5 V, two N*-MnPc complexes are switched to pristine MnPc (Fig. 4f). Translation of the N* species from one N*-MnPc to the neighboring pristine MnPc can also be observed in this binary molecular network. As shown in Fig. 4g-h, a N*-MnPc complex indicated by the red circle (Fig. 4g) is switched to MnPc after scanning this area at 2.3 V, and subsequently, the desorbed N* species is transferred to the neighboring MnPc denoted by the yellow circle (Fig. 4h). When the MnPc monolayer was exposed to dinitrogen with the hot filament off (without any activation of dinitrogen), all the pristine MnPc remained intact, as confirmed by the UPS and XPS measurements (Fig. S2). This suggests that only activated N* species can strongly interact with MnPc. As mentioned previously, electrons emitted from the hot filament can collide with the dinitrogen molecules to produce the activated positive, negative nitrogen ions or neutral nitrogen atoms. To elucidate the nature of the activated N* species that can selectively coordinates to the center Mn atom in MnPc, we performed the XPS measurements with the application of voltages of different polarities to the sample during the exposure to nitrogen and monitored the evolution of the Mn 2p3/2 spectra. If the activated N* species are positive or negative nitrogen ions, they would be easily repelled by the positively or negatively biased sample, respectively, thereby leading to a largely modified intensity of the N* coordinated MnPc. As clearly demonstrated in Fig. S5 (Supplementary Information), the exact same N* adsorption always occurs with either positive or negative or zero sample bias. This suggests that the activated N* species are most likely the neutral, active, nitrogen atom. Density functional theory (DFT) calculation was employed to further understand the nature of the activated nitrogen adsorbed on MnPc. As shown in Fig. 5, monolayer MnPc adsorbed on graphene was used as a model system for the DFT calculation. N and N2 adsorption possess adsorption energies of -6.63 and -0.12 eV, respectively. Therefore, if neutral nitrogen atoms are formed in the gas phase, they will adsorb much more easily onto MnPc. To compare with the experimental STS data, we calculated the projected density of states (PDOS) of center atoms (including Mn and possible adsorbate atoms) and lobe atoms (including one benzene ring) for three systems (MnPc, N2-MnPc, N-MnPc). Energy alignment for these three systems was calibrated via core levels as shown in Fig. S7 (Supplementary Information). There is almost no


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difference between N2-MnPc and MnPc when comparing their PDOS near the Fermi level, thus excluding the possibility of N2 adsorption in the experiment. This is in good agreement with our XPS/UPS results (Fig. S2). In the PDOS of N-MnPc, the -0.16 eV HOMO peak disappears, which is consistent with the experimental STS observation of the vanishing of the Mn 3d character HOMO peak. This is accompanied with a disappearance of spin polarization of the N-MnPc system. It further confirms that the adsorbed N* species are neutral nitrogen atoms.

Figure 5. Optimized structures and PDOS of MnPc on graphene before and after nitrogen adsorption. (a) Top and side views and (b) PDOS of MnPc. (c) Top and side views and (d) PDOS of MnPc with an N2 molecule. (e) Top and side views and (f) PDOS of MnPc with an N adatom adsorbed. The Fermi levels are set to zero. DFT calculations were also carried out to simulate the STM images of MnPc and N-MnPc molecules. The HOPG substrate is approximated with a monolayer graphene. Fig. 6a and 6b show the high resolution STM images for a single MnPc and N-MnPc molecule; while Fig. 6c and 6d are the corresponding simulated STM images. Both of them show good agreement between experiment and simulation.


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Figure 6. High resolution STM images and simulated STM images of MnPc and N-MnPc on HOPG. (a) and (b) High resolution STM images for a single MnPc and a single N-MnPc molecule on HOPG. (c) and (d) The corresponding simulated STM images for MnPc and N-MnPc. Conclusions We have demonstrated single-molecule imaging of the activated nitrogen (neutral nitrogen atom) adsorbed on the coordinatively unsaturated single Mn metal atom in a surface confined MnPc monolayer at room temperature and low pressure through the combination of LT-STM/STS and XPS/UPS investigations and DFT calculations. The electronic structures of the center Mn atom are greatly modified upon nitrogen atom coordination. This is further verified by temperature programmed desorption and STM tip manipulated desorption experiments. This heterogeneous process on surface can also be extended to other well characterized transition metal complexes, such as Mo and Fe, which are responsible for the biological nitrogen fixation and Haber-Bosch process. Methods In situ LT-STM were carried out in a custom-built multichamber UHV system with base pressure better than 1.0 × 10-10 mbar, housing an Omicron LT-STM interfaced to a Nanonis controller. All STM imaging were performed at 77K using constant current mode with an electrochemically etched tungsten tip. All the bias voltages were applied to the tip.56 The differential conductance dI/dV spectra were obtained with a lock-in technique using a modulation voltage of 50 mV and a frequency of 500 Hz. When ramping the voltage, the feedback loop was opened.57 In situ photoelectron spectroscopy (PES) experiments were performed in a custom-designed UHV system with a base pressure better than 2.0 × 10-10 mbar. He Iα (hυ = 21.2 eV) and Mg Ka (hυ = 1253.6 eV) were used as the excitation sources for UPS and XPS, respectively. All UPS and XPS measurements were performed at room temperature, and the binding energies of all PES spectra were calibrated and referenced to the Fermi level of a


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sputter-cleaned Ag(110) surface. The total instrumental energy resolution was estimated to be about 100 meV for UPS and 850 meV for XPS58. For both LT-STM and PES experiments, a fresh cleaved HOPG substrate was thoroughly degassed in the UHV chamber at around 500 °C overnight before molecule deposition. Vacuum-sublimated purified MnPc molecules were thermally evaporated onto the HOPG substrate from Knudsen cells in the growth chamber (base pressure better than 2.0 × 10-9 mbar). For N2 exposure, the gas was introduced into the growth chamber through a leak valve and the pressure was monitored by an ion gauge. The MnPc/HOPG system was exposed to N2 at a pressure around 1.5 × 10-6 mbar for STM and 10-5 mbar for UPS/XPS. The HOPG substrate was kept at room temperature for both MnPc deposition and N2 exposure processes. All the calculations were performed using DFT implemented in the Vienna Ab Initio Simulation Package (VASP)59, 60 within the projector-augmented wave (PAW) framework.61, 62 Exchange correlation interactions were considered with the generalized gradient approximation (GGA-PBE)63 plus a DFT-D2 van der Waals (vdW) correction.64 And spin polarized calculation is performed. In order to describe the d bands of manganese better, we used the GGA+U method65, 66 with an on-site Coulomb (U) and exchange (J) parameters of 2 and 0 eV, respectively. Test calculations with these parameter values agree with previous results well.48, 67 The plane-wave basis cutoff energy was set to 550 eV. The criteria of convergence for energy and force were set to 10-6 eV and 0.01 eV/Å. For the (25 Å×25 Å×25 Å) supercell, a (1×1×1) k-point grid was used. ASSOCIATED CONTENT Supporting Information Additional experimental details, STM images, UPS/XPS spectra and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] Author Contributions # These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKONWLEDGEMENTS The authors acknowledge the financial support from Singapore MOE grant R143-000-559-112, Singapore National Research Foundation CREATE-SPURc program R-143-001-205-592, and NFSC program (21222304). Synchrotron-based photoelectron measurements were undertaken on the soft x-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia. References 1.

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