Weitao Zhao

Françoise Maugé

Jianjun Chen

Laetitia Oliviero

Spectroscopic identification of active centers and reaction pathways on MoS 2 catalyst for H 2 production via water-gas shift reaction

Keywords: sulfur vacancy, MoS 2 edges, reaction mechanism, structure-activity relationships, deactivation

S/O exchange •

Introduction

Molybdenum-based sulfide catalysts find extensive applications in various catalytic processes such as hydrodesulfurization, CO 2 hydrogenation as well as hydrogen production by water electrolysis and the water gas shift (WGS) reaction [START_REF] Kibsgaard | Engineering the surface structure of MoS 2 to preferentially expose active edge sites for electrocatalysis[END_REF][START_REF] Liu | Phase-selective synthesis of 1T' MoS 2 monolayers and heterophase bilayers[END_REF][3][START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of Medge and S-edge[END_REF][START_REF] Sasaki | Reaction mechanism analysis for molybdenum-based water-gas shift catalysts[END_REF]. The catalytic activity for MoS 2 has been reported to mainly arises from the coordinatively unsaturated sites located along the edge of MoS 2 slabs rather than the saturatedcoordination sites in basal plane [START_REF] Hinnemann | Biomimetic hydrogen evolution: MoS 2 nanoparticles as catalyst for hydrogen evolution[END_REF][START_REF] Jaramillo | Identification of active edge sites for electrochemical H2 evolution from MoS 2 nanocatalysts[END_REF]. However, with the development of characterization technology, the understanding of active sites has also changed. Recently, in-plane sulfur atoms can be substituted by oxygen atoms doped into the MoS 2 [START_REF] Peto | Spontaneous doping of the basal plane of MoS 2 single layers through oxygen substitution under ambient conditions[END_REF][START_REF] Wang | Controllable etching of MoS 2 basal planes for enhanced hydrogen evolution through the formation of active edge sites[END_REF][START_REF] Grønborg | Basal plane oxygen exchange of epitaxial MoS 2 without edge oxidation[END_REF], and this formed MoS 2-x O x sites can also act as active sites for hydrogen evolution reaction (HER) due to its good electrical conductivity [START_REF] Xie | Controllable disorder engineering in oxygen-incorporated MoS 2 ultrathin nanosheets for efficient hydrogen evolution[END_REF]. Similarly, the importance of MoS 2-x O x site formed in situ as a result of hydrolysis has been recognized momentously for WGS reaction, and hitherto it has been proposed as an active center reacting with CO to release CO 2 [START_REF] Hou | Kinetic studies with a sulfur-tolerant water gas shift catalyst[END_REF]13]. However, due to the lack of the sophisticated in situ characterization techniques on supported MoS 2 catalysts, this hypothesis involving MoS 2-x O x site as WGS active center is not yet fully experimentally verified and thus understanding the actual surface reaction at surface of MoS 2 should be significant deepened.

Another ambiguous issue is the comparative study of edge sites for their catalytic reactivity.

Theoretical and experimental studies reveal the existence of two types of low-index edge terminations with different coordination environment over MoS 2 slab, i.e. the (10 0) M edge and the ( 010) S edge [START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of Medge and S-edge[END_REF][START_REF] Schweiger | Shape and Edge Sites Modifications of MoS 2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study[END_REF][START_REF] Lauritsen | Location and coordination of promoter atoms in Co-and Ni-promoted MoS2-based hydrotreating catalysts[END_REF][START_REF] Lauritsen | Atomic-scale insight into structure and morphology changes of MoS 2 nanoclusters in hydrotreating catalysts[END_REF][START_REF] Walton | MoS 2 nanoparticle morphologies in hydrodesulfurization catalysis studied by scanning tunneling microscopy[END_REF]. Such different coordination structures in M-and S-edge sites determine that they would have different reactivity. This point has been demonstrated in thiophene HDS reaction for which the intrinsic activity of S-edge sites is higher than that of M-edge sites [START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of Medge and S-edge[END_REF]. However, for HER and WGS reaction, the comparative study of the two edge sites activity is experimentally rare, and most of the studies mainly focus on theoretical calculation. For instance, comparing the hydrogen bond energy, hydrogen evolution on MoS 2 is expected to occur predominantly at the M-edge (∆G H = 0.08 eV) rather than S-edge (∆G H = 0.18 eV) [START_REF] Bonde | Hydrogen evolution on nanoparticulate transition metal sulfides[END_REF]. With respect to WGS reaction, there have been disagreements on mechanism over MoS 2 catalyst. Some researchers attribute formate species to spectator (formed by a side reaction between CO 2 and H) and claim that both M-and S-edge sites follow a redox mechanism of successive surface oxidation by H 2 O and reduction by CO [START_REF] Chen | On the Role of a Cobalt Promoter in a Water-Gas-Shift Reaction on Co-MoS 2[END_REF][START_REF] Chen | Mechanisms and Energies of Water Gas Shift Reaction on Fe-, Co-, and Ni-Promoted MoS 2 Catalysts[END_REF]; others correlate the formate to a favorable intermediate (formed via CO reaction with OH) on both edges of MoS 2 and thus propose a double mechanism through redox and associative paths [START_REF] Zhang | Insights into water-gas shift reaction mechanisms over MoS2 and Co-MoS 2 catalysts: a density functional study[END_REF]. However, even the widely accepted redox mechanism that supports the MoS 2-x O x site as active center does not allow to understand why sulfide catalysts would exhibit poor stability under H 2 S-free condition, which is often interpreted as poor antioxidation capability for the formation of oxy-sulfide phase during the WGS reaction. On the contrary, the catalytic performance and stability would be significantly enhanced in H 2 S-containing atmosphere [START_REF] Yun | Support Effects on Water Gas Shift Activity and Sulfur Dependence of Mo Sulfide Catalysts[END_REF][START_REF] Hulteberg | Sulphur-tolerant catalysts in small-scale hydrogen production, a review[END_REF]. These highly divergent results indicate that redox mechanism is not well understood, especially for the role of MoS 2-x O x sites. Furthermore, the role of the support also requires special consideration [START_REF] Singh | Gold Nanoparticles Supported on Carbon Nitride: Influence of Surface Hydroxyls on Low Temperature Carbon Monoxide Oxidation[END_REF]. On one hand, CO adsorbed on sulfide sites could spill over to the support [START_REF] Lorito | Determination of formate decomposition rates and relation to product formation during CO hydrogenation over supported cobalt[END_REF], implying that the support could be involved in catalytic reaction whereas only sulfide phase is considered in DFT calculation; on the other hand, the interaction between the sulfide phase and the support results in varying the proportion of M-edge to S-edge sites [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS 2 slab morphology[END_REF], also affecting the catalytic activity [START_REF] Yun | Support Effects on Water Gas Shift Activity and Sulfur Dependence of Mo Sulfide Catalysts[END_REF].

Therefore, a direct observation of the catalytic performance of specific sites is required to get insight into the reactivity of M-/S-edge site.

The crucial issue in the surface reaction investigation on MoS 2 is how to readily identify and quantify each type of edge sites. In general, visual observation of morphology can be directly achieved by electron microscopy to qualitatively distinguish the two exposed edges. However, active metal dispersion as measured by microscopy is limited by statistical information. Fourier transform infrared (FTIR) spectroscopy coupled with probe molecules appears to be a powerful method by monitoring with the sufficient sensitivity the probe-surface interaction. In particular, all aspects of the catalyst system in terms of the sulfide/support surface, titration of active edge sites, and electron transfer can be probed by CO adsorption at liquid nitrogen temperature followed by IR spectroscopy (CO/IR) [START_REF] Chen | How the CO molar extinction coefficient influences the quantification of active sites from CO adsorption followed by IR spectroscopy? A case study on MoS 2 /Al 2 O 3 catalysts prepared with citric acid[END_REF][START_REF] Travert | Parallel between infrared characterisation and ab initio calculations of CO adsorption on sulphided Mo catalysts[END_REF][START_REF] Mey | HDS of a model FCC gasoline over a sulfided CoMo/Al 2 O 3 catalyst: Effect of the addition of potassium[END_REF][START_REF] Chen | Effect of modification of the alumina acidity on the properties of supported Mo and CoMo sulfide catalysts[END_REF]. Meanwhile, CO can also represent the realistic active site information being a reactant in WGS reaction.

In this work, two kinds of MoS 2 /Al 2 O 3 catalysts have been prepared with/or without citric acid through wetness impregnation method in order to obtain two morphologies of MoS 2 slabs with different proportion of M-edge to S-edge sites [START_REF] Chen | IR spectroscopy evidence of MoS 2 morphology change by citric acid addition on MoS 2 /Al 2 O 3 catalysts -A step forward to differentiate the reactivity of Medge and S-edge[END_REF]. Next, the reactivity of each type of edge sites towards H 2 O and/or CO is studied using CO/IR spectroscopy technique. Then, the stepwise WGS surface reaction is carried out to elucidate the mechanism routes. Finally, operando study coupling IR analysis of the catalyst surface and IR/GC analysis of the gas phase confirms the routes through operando identification of relevant long-lived intermediates. This work is the first to explore the effect of H 2 O and CO on both M-edge and S-edge sites, allowing to identify the real active center of sulfide catalyst and to reveal the mechanism of WGS reaction. More importantly, understanding the reactivity of the M/S-edge sites with H 2 O and CO also contributes to the design of sulfide catalysts for other reactions involving H 2 O dissociation and CO conversion.

Experimental Section

Catalyst preparation

Mo/Al 2 O 3 catalyst was prepared by wetness impregnation method using ammonium heptamolybdate tetrahydrated salt as molybdenum precursor ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O, MERCK). The supports, γ-Al 2 O 3 (245 m 2 /g, Sasol), were crushed and sieved into 0.2-0.5 µm fraction and then pre-calcined at 773K during 4h. The loading of Mo was targeted at a sub monolayer with 3 atoms Mo/nm 2 . In addition, Mo(CA=2)/Al 2 O 3 catalyst was prepared by same method but with citric acid (CA, C 6 H 8 O 7 ·H 2 O, PROLABO) as a chelating agent and the molar ratio of citric acid to Mo was kept at 2. After impregnation, the catalysts were left for maturation at ambient for 2h and then dried at 383K (1K/min) during 16h. Note that the catalysts were not calcined in order to keep the chelating agent in its initial form. Mo/SiO 2 catalyst was prepared with same method than Mo/Al 2 O 3 on SiO 2 (506 m 2 /g, Merk).

Catalyst sulfidation

Catalyst sulfidation were performed in situ in a quartz glass IR cell. Sample was firstly grounded and pressed into a self-supporting pellet of ~20 mg and area of 2 cm 2 (precisely weighted). The pellet was then introduced into the glass cell and the cell was evacuated to ~1.0 ×10 -6 torr to remove the air and physisorbed water. Afterward, the pellet was activated at atmospheric pressure at 723K in a flow of 30 ml/min 10% H 2 S/H 2 . At the end of activation at 723K for 2h, the IR cell was flushed under Ar flow, then evacuated during 1h at 723K and finally cooled down to room temperature under vacuum.

Low-temperature CO adsorption followed by IR spectroscopy (CO/IR)

After sulfidation, the pressure in the cell is around ~1.0 ×10 -6 torr. Then the pellet was cooled down by liquid nitrogen to 100K for CO adsorption. Small calibrated doses of CO were introduced in the IR cell up to an equilibrium pressure of 133 Pa. IR spectra of adsorbed CO were recorded with a Nicolet iS50 FTIR spectrometer equipped with a MCT detector with 256 scans and a resolution of 4 cm -1 . For comparison, all the spectra presented were normalized to a sulfided catalyst pellet of 5 mg.cm -2 . The SI give the procedure for treatment after the sulfidation (SI1). The spectra treatment and subsequent calculation are also described in SI2.

Activity test

Before WGS reaction test, the as prepared catalyst was sulfided at 723K (3K/min) and 0.1MPa for 2h under a 30 ml/min flow of 10% H 2 S/H 2 . Then WGS reaction was carried out in a glass differential reactor under 0.5% CO and 1% H 2 O in Ar balance with GHSV~ 9000 h -1 at 573K and 0.1MPa. The vapor of water was introduced into the system using a saturator with Ar as carrier gas. The water vapor pressure was controlled through a temperature tank set at 17.6 o C (20 mbar). The pipelines were heated at 50 o C to avoid water condensation. The outlet gas was analyzed by Varian 450 gas chromatograph equipped with a PoraPlot Q column (Varian, 25 m, 0.53 mm, 20 µm) and thermal conductivity (TCD) detector. The catalytic activity was expressed by the conversion of CO. Stepwise test experiments were also performed in glass differential reactor under flow of pure CO or pure H 2 O flow (completed by Ar).

Operando IR/GC experiment

The operando experiments have been performed in a Nicolet 6700 FTIR spectrometer equipped with a specific IR-reactor cell, named IR "sandwich" cell. IR spectra of catalysts were directly recorded by FTIR spectrometer equipped with a MCT detector using 64 scans and a resolution of 4 cm -1 . Downstream the IR-reactor cell, the exhaust gases were analyzed simultaneously using an online IR-gas cell and a GC equipped with TCD detector. Catalyst sulfidation were performed in situ in the IR-reactor cell reactor. Sample was firstly grounded and pressed into a self-supporting pellet of ~15 mg and area of 2 cm 2 . The pellet was then installed into the IR-reactor cell. Catalyst sulfidation were performed in situ at atmospheric pressure under a flow of 30 ml/min 10% H 2 S/H 2 at 723K. Finally, the reactor was cooled down to 573K under pure Ar flow. Afterwards, WGS reaction was carried out under a 10 ml/min feed flow of 0.5% CO and 1% H 2 O in Ar balance.

Results and Discussion

S-edge sites of MoS 2 /Al 2 O 3 catalysts are not sensitive to water treatment

The reactivity with water of M-edge and S-edge sites of MoS 2 catalysts were evaluated on sulfided Mo/Al 2 O 3 and Mo(CA=2)/Al 2 O 3 catalysts. CO adsorption (SI1) and HAADF-STEM study [START_REF] Zavala-Sanchez | High-resolution STEM-HAADF microscopy on a gamma-Al 2 O 3 supported MoS 2 catalyst-proof of the changes in dispersion and morphology of the slabs with the addition of citric acid[END_REF] have previously demonstrated that the introduction of CA in the impregnation solution leads to higher S-/Medge ratio with a small decrease in total edge sites (reference J phys chem C jianjun to add). Then, the catalysts were characterized by CO/IR at around 100 K without any air contact after (i) in-situ sulfidation at 723 K, (ii) subsequent treatment under pure H 2 18 O vapor at 573 K and (iii) subsequent treatment under 10% H 2 S/H 2 at 723K i. e. resulfidation (Figure 1).

For both Mo/Al 2 O 3 and Mo(CA=2)/Al 2 O 3 catalysts, the CO uptake on M-edge sites (band at 2112 cm -1 ) decreases significantly after water treatment. This is in agreement with previous study and ascribed to an exchange reaction of sulfur-oxygen on the M-edge, making the oxygen-substituted MoS 2 sites unfavorable for CO adsorption [START_REF] Badawi | Effect of water on the stability of Mo and CoMo hydrodeoxygenation catalysts: A combined experimental and DFT study[END_REF]. Figure 1a ranging from 1700 to 1400 cm -1 (SI3 a). After H 2 18 O treatment at 573K, discernable changes in the complex signal of the carbonaceous residues are observed. Interestingly, carbon dioxide labeled by isotope 18 O is formed in the gas phase (SI3 b). Thus, H 2 18 O is reduced by the residual carbonaceous species and as a consequence, the formation of carbon dioxide should be accompanied by the formation of H 2 (undetectable on IR spectra). Specific formation of H 2 upon H 2 O feed on this catalyst will be further confirmed in the stepwise tests described later on. Finally, on Mo(CA=2)/Al 2 O 3 , the intensity of the v(CO/S-edge) band after the resulfidation treatment is lower than that after the initial sulfidation. In parallel, it has been verified that in absence of water treatment, resulfidation does not affect the amount of detected S-edge sites (SI4 a). Thus, water treatment also leads to some irreversible changes of S-edge sites that could be ascribed to some S/O exchanges during water treatment, in the same way as for the M-edge.

In conclusion, two features can occur during water treatment whose extent depend on the sulfide slab edges: (i) S/O exchange that occurs more easily on M-edge site than on S-edge. (ii) sulfur vacancies creation that occurs only on S-edge and only on catalyst presenting carbonaceous residues.

S-edge sites of MoS 2 /Al 2 O 3 catalysts form vacancy upon CO treatment

The effect of CO treatment (40% 13 CO/60% 12 CO at 573K) on edge sites of sulfided catalysts was probed by CO/IR ( 12 CO at 100K) with a similar strategy than for water treatment. After Mo/Al 2 O 3 and Mo(CA=2)/Al 2 O 3 catalysts were treated with 13 CO/ 12 CO at 573K, the CO uptake on the S-edge a b 10 increases significantly, while the CO adsorption at the M-edge is almost unchanged (Figure 2, Table 1).

Moreover, the ν(CO/S-edge) frequency undergoes a slight shift towards lower wavenumber after CO treatment in accordance with the formation of sulfur vacancy. In addition, resulfidation after CO treatment was carried out on Mo(CA=2)/Al 2 O 3 and the spectrum of CO adsorption (SI6 b) clearly show that the initial edge state is totally recovered.

a b c d

The IR spectra recorded during the CO treatment show the formation of 13 COS/ 12 COS in the gas phase (Figure 2c andd). These results evidence that CO treatment leads to sulfur vacancy creation through oxidation of CO into COS. Figure 2a and b points out that S-edge site is more reactive towards CO treatment than M-edge site. Interestingly, the signal of carbon dioxide was also clearly detected in gas phase during CO treatment. This could be ascribed to COS hydrolysis on hydroxyl group of support, or direct reaction of CO with OH group of the support. In order to check the direct reaction between CO and OH groups, CO treatment was conducted on pure alumina (Figure 3). The starting temperature of CO 2 formation on pure Al 2 O 3 is 623K, i.e. a higher temperature than on Mo/Al 2 O 3 . Thus, OH groups of alumina are not active on their own at 573K. However, we cannot exclude that the presence of sulfide phase enhances the reactivity of CO with OH groups through adsorbing and/or activating CO on sulfide site and subsequently spilling it over to the support.

Moreover, comparing the IR spectra of Al 2 O 3 before and after CO treatment at 623K, it can be found that a small amount of formate species (1589, 1394 and 1373cm 4a, the water treatment leads to S-O exchange reaction mainly at M-edge and formation of the Mo(S x O y ) zc sites. Following H 2 O treatment, we have seen that the Mo(S x O y ) zc on M-edge after resulfidation is not modified, but also subsequent CO treatment show no effect on the CO adsorption on M-edge site. In addition, the direct CO treatment after water treatment was also performed on sulfided Mo/Al 2 O 3 catalyst (SI7) and the CO uptake of M-edge after H 2 O + CO + resulfidation is the same as that obtained after water treatment, indicating that no Mo(S x O y ) zc site is reduced by CO. Thus, the formation of oxygen vacancy by CO on Mo(S x O y ) zc sites of M-edge can be discarded. Note that the CO uptake on the S-edge increases slightly after CO treatment, this could be attributed to the reaction between CO and MoS 2 site to create sulfur vacancy, as evidenced by the gas phase analysis which reveals the formation of COS (Figure 4b). As previously, CO 2 (C 16 O 2 + C 18 O 16 O) is also detected (Figure 4b) but the CO 2 /CO ratio is greater than for CO treatment performed directly after sulfidation (Figure 2). This could be ascribed to the enhanced COS hydrolysis and CO reactivity with hydroxyl group on the hydrated support. Indeed, formate species (Figure 4c) are observed on the catalyst surface.

The support also has a role in WGS pathway

In order to discriminate between the two possible pathways to form CO 2 from CO, i.e. COS hydrolysis pathway or formate pathway, similar study has been performed on Mo/SiO 2 . Indeed, on Mo/SiO 2, no formate is expected to be formed on the support. Meanwhile, its sulfide phase is known to have preferentially S-edge sites exposed as confirmed by Figure 5a [START_REF] Garcia | New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS 2 slab morphology[END_REF]. From the spectrum after sulfidation, the concentration of sites is calculated to be 17 µmol.g -1 for S-edge and 33 µmol.g -1 for Medge.

As shown in Figure 5a andc, CO (40% 13 CO/60% 12 CO) treatment of sulfided Mo/SiO 2 leads to the formation of vacancies mainly on S-edge along with the formation of carbonyl sulfide and carbon dioxide. This is consistent with results on Al 2 O 3 -supported Mo catalysts. However, on Mo/SiO 2 , the carbon dioxide should be formed through the carbonyl sulfide pathway since formate intermediates are, as expected, not detected (Figure 5b: no OH consumption, no formate formation). Water treatment of sulfided Mo/SiO 2 has similar effect as on Mo/Al 2 O 3 (Figure 5d), that is to say: almost no effect on Sedge site amount and strong decrease of M-edge sites. Similarly, subsequent CO treatment creates vacancy on both edges but mainly on S-edge. Meanwhile, COS and CO 2 are observed in gas phase in accordance with sulfur vacancy formation. Note that contrary to Mo/Al 2 O 3 , the proportion of CO 2 to CO formed on Mo/SiO 2 when the CO treatment is performed after the water one, is not greater than for direct CO treatment just after sulfidation. This is in accordance with the absence of direct reaction between SiOH and CO. Finally, resulfidation does not allow to reach back the initial sulfide site amount especially on M-edge and the corresponding CO uptake of M-edge is very similar to that measured after only water treatment. These results are totally in agreement with the ones obtained on Mo/Al 2 O 3 .

One difference between Mo/SiO 2 and Mo/Al 2 O 3 catalysts concerns the effect of H 2 O treatment on Sedge sites. Indeed, even if S-edge sites are preferential sites of sulfide slabs on Mo/SiO 2 , no increase in S-edge sites is observed after water treatment in contrary to the case of Mo(CA=2)/Al 2 O 3 (Figure 1b).

This difference is related to the presence or absence of citric acid residues. As discussed in previous section, these species can react with water leading to some H 2 production and then formation of sulfur vacancies at S-edge. Hence, the involvement of carbonaceous residue proposed previously is confirmed by the absence of S-edge vacancy creation upon H 2 O treatment on Mo/SiO 2 .

WGS activity

0.5% CO and 1% H 2 O in Ar balance with GHSV~ 9000 h -1 at 573K (Figure 6). The two aluminasupported catalysts present a very strong deactivation up to 4 hours of time-on-stream (TOS). Then, the activity of Mo/Al 2 O 3 catalyst continues to slightly decrease while the activity of Mo(CA=2)/Al 2 O 3 catalyst remains stable. Finally, sulfided Mo(CA=2)/Al 2 O 3 catalyst exhibits a higher activity than Mo/Al 2 O 3 catalyst at high TOS. As for Mo/SiO 2 catalyst, it displays some deactivation up to 2 hours of TOS, and then presents a stable conversion. However, the activity of Mo/SiO 2 catalyst is lower than those of the alumina-supported catalysts whatever the TOS. Taking into account the complexity in terms of the continuous change in M-edge site environments and the role of the interface between support and sulfide sites, the intrinsic activity of the two edge sites is not possible to be determined from the initial site characterization. However, the different responses of the two edges to H 2 O and CO means that they follow different reaction pathways. Stepwise WGS reaction experiments were performed on the Al 2 O 3 supported catalysts to check if the oxidation/reduction pathway was occurring (Figure 7). After sulfidation of Mo/Al 2 O 3 catalyst, if only H 2 O is flowing at 573 K (without CO in the flow), no H 2 signal is detected (Figure 7a). Similar behavior is observed for the sulfided Mo(CA=2)/Al 2 O 3 catalyst, although a limited amount of H 2 is released at the beginning of the alone water feed (Figure 7c). As previously discussed, this small H 2 production comes from the reaction of water with residual carbonaceous species. Thus, the direct oxidation step by H 2 O giving H 2 can be discarded. For both catalysts, if CO is introduced alone at 573 K after the H 2 O flow, a high amount of H 2 is released and then H 2 formation reaches a much lower plateau. If now CO alone was introduced firstly after sulfidation (Figure 7b andd), a small production of H 2 occurs that is stable with TOS, and close to the plateau obtained in the previous experiment for long TOS (Figure 7a and c). Under CO alone flow, H can only come from the catalyst surface either as OH or SH species. In particular, knowing that OH species can be involved in both COS hydrolysis and formate formation, they can react directly (formate path) or indirectly with CO (COS path). Similar study was performed on Mo/SiO 2 (Figure 8). If CO is firstly introduced alone, H 2 production is almost undetectable (Figure 8b). Moreover, on this catalyst, the conversion of CO and formation of H 2 are not favored by the H 2 O pre-treatment. As shown in Figure 8a, water in the feed is necessary to produce H 2 on Mo/SiO 2 . These results confirm that SiOH are not involved in the reaction pathway.

The WGS reaction was further studied by operando IR/GC experiments that combined IR analysis of the surface species of the catalyst and on-line analysis by GC and gas phase IR of the reaction products (Figure 9). For the three catalysts, the conversion is in the same range than in the differential reactor although not totally the same. However, the same tendencies are observed as in terms of stability with TOS. Indeed, alumina-supported catalysts present a marked deactivation (up to ~0.5 h) that is not observed on silica-supported catalyst (Figure 9a). Moreover, alumina-supported catalyst is more stable when prepared with citric acid. After stabilization, the conversion is comprised between 15 to 10% for the three catalysts.

formate species are formed on the catalyst surface via reaction of adsorbed CO with surface OH groups, which then decompose to CO 2 and H 2 [START_REF] Zhang | Insights into water-gas shift reaction mechanisms over MoS2 and Co-MoS 2 catalysts: a density functional study[END_REF]. For the case of the redox mechanism, the surface of catalyst is alternatively oxidized by water and then reduced by CO [START_REF] Hou | Kinetic studies with a sulfur-tolerant water gas shift catalyst[END_REF]13].

Based (i) on the results of stepwise reactions and in particular on the production of H

This proposal is consistent with both γ-Al 2 O 3 and sulfide phase known as commercial COS hydrolysis catalysts [START_REF] Rhodes | The low-temperature hydrolysis of carbonyl sulfide and carbon disulfide: a review[END_REF][START_REF] George | Kinetics of cobalt-molybdate-catalyzed reactions of SO 2 with H 2 S and COS and the hydrolysis of COS*1[END_REF], and MoS 2 catalyst recognized as a good candidate for H 2 S decomposition to yield H 2 [START_REF] Sharma | Inhibitor, Co-Catalyst, or Co-Reactant? Probing the Different Roles of H 2 S during CO 2 Hydrogenation on the MoS 2 Catalyst[END_REF]. In addition, under the regime of the COS hydrolysis pathway during WGS reaction, the unconverted COS inevitably leads to sulfur loss, which also well explains why addition of H 2 S in the feed leads to a high and stable catalytic performance [START_REF] Sasaki | Reaction mechanism analysis for molybdenum-based water-gas shift catalysts[END_REF][START_REF] Yun | Support Effects on Water Gas Shift Activity and Sulfur Dependence of Mo Sulfide Catalysts[END_REF].

Interestingly, Table 1 shows that CO forms vacancies on S-edge, and not on M-edge. Hence, this novel redox mechanism involving COS can be exclusively ascribed to the S-edge sites. Note that one cannot exclude that the formate route may also happen on the S-edge of alumina-supported catalysts. On contrary, the activity of M-edge sites in WGS is only ascribed to the formate mechanism. Consequently, on Mo/SiO 2 , these M-edge sites should not be involved in reaction and CO conversion is entirely due to the contribution of the COS pathway at S-edge. Knowing that the M-edge sites are irreversibly sensitive to water, the deactivation with TOS observed on Al 2 O 3 supported catalysts and more strongly on Mo/Al 2 O 3 having lower S/M edge ratio (Figure 6) can then be ascribed to these M-edge sites.

Hence, on Al 2 O 3 -supported MoS 2 catalysts, both formate and COS redox reaction routes occur preferentially on M-and S-edge sites, respectively (Figure 10). As a general conclusion, the nature of the support and the S-/M-edge ratio of the sulfide slabs determine the mechanisms and the stability of the WGS reaction. Hydrogen can be produced by WGS reaction following formate mechanism i.e. through the reaction of CO with hydroxyl group of alumina support; (ii) Hydrogen can also be produced by a redox mechanism.

However, the conventional redox mechanism that occurs by direct oxidation by water and subsequent reduction by CO has to be excluded on the Mo-sulfide catalysts. Moreover, the stability of Mo(S x O y ) zc species towards CO exclude their role as WGS active centers. However, the detection of COS formation during WGS reaction leads to propose a novel redox mechanism associated with this intermediate.

Interestingly, the findings in different response of two edge site towards CO indicate that formate route occurs mainly on the M-edge, while the novel redox mechanism occurs exclusively on S-edge of the sulfide slabs. Finally, WGS tests show that the M-edge site is not a stable and durable active center compared with S-edge.

This work gives the direction for obtaining high stability sulfide catalyst for reactions involving water by improving stability of M-edge sites and also reveals that the active sites of CO activation and oxidation are located at the S-edge, which also contributes to the design of sulfide catalysts for other reaction, such as methanethiol synthesis.

Corresponding Author: * E-mail: laetitia.oliviero@ensicaen.fr ORCID: Laetitia Oliviero: https://orcid.org/0000-0002-7931-439X

Figure 1 .

 1 Figure 1. Investigation of the effect of water (H 2 18 O) treatment on M-edge and S-edge sites. IR

Figure 2 .

 2 Figure 2. Investigation of the effect of CO treatment on M-edge and S-edge sites. a, b, IR spectra of

Figure 3 .

 3 Figure 3. CO treatment on pure Al 2 O 3 . a, In-situ IR spectra of gas phase recorded during CO

Figure 4 .

 4 Figure 4. Subsequent CO treatment on water treated sulfided Mo/Al 2 O 3 catalyst. a, CO/IR spectra

Figure 5 .

 5 Figure 5. H 2 O or/and CO treatment on sulfided Mo/SiO 2 catalysts. a, CO/IR spectra of the Mo/SiO 2

Figure 6 .

 6 Figure 6. WGS reaction vs TOS. CO conversion over Mo/Al 2 O 3 , Mo(CA=2)/Al 2 O 3 and Mo/SiO 2

Figure 7 .

 7 Figure 7. Stepwise WGS reaction. a, c, Water feed followed by CO feed at 573K over sulfided

Figure 8 .

 8 Figure 8. Stepwise WGS reaction on sulfided Mo/SiO 2 . a, Water feed followed by CO feed at 573K

Figure 10 :

 10 Figure 10: Schematic of reaction routes for WGS reaction over M-edge and S-edge sites of MoS 2 slab.

  and b show that this S/O exchange on M-edge sites is not reversible under 10% H 2 S/H 2 treatment indicating the good stability of the formed Mo(S x O y ) zc sites. This notation is chosen to indicate some O in the coordination environment of Mo edge site without indication of the coordination number nor the proportion of S/O exchange.On Mo/Al 2 O 3 , the intensity of the v(CO/S-edge) band at 2064 cm -1 appears unaffected by water treatment whereas an increase of the CO/S-edge band area is observed with a downward shift on Mo(CA=2)/Al 2 O 3 . Moreover, the water effect on S-edge of Mo(CA=2)/Al 2 O 3 catalyst is reversible after the 10% H 2 S/H 2 treatment and thus seems to be similar to those observed when sulfur vacancies are

created during hydrogen post-treatment (SI2). The creation of vacancies could be related to H 2 production during the treatment with H 2 O of Mo(CA=2)/Al 2 O 3 . The surface IR spectrum reveals that carbonaceous residues are present after sulfidation of the catalyst prepared with CA, resulting in bands

Table 1 :

 1 -1 ) and carbonate species (1548 and 1494 cm -1 ) was accumulated on the surface of Al 2 O 3 . Formate are also observed after CO treatment at 573K on Mo/Al 2 O 3 (SI5). However, as described above, formate species could be involved in CO 2 production as active species or be only present as spectator ones. To check this point, the stability of formate species was studied. As shown in FigureS6, formate species on Mo/Al 2 O 3 decompose into CO 2 starting from 573 K. These results show that both the sulfide sites and OH groups are involved in the indirect or direct CO 2 formation on Mo/Al 2 O 3 . Concentration of edge sites on Mo/Al 2 O 3 and Mo(CA=2)/Al 2 O 3 catalysts after sulfidation and CO treatment detected by CO/IR spectra.

			Sulfidation	573K CO
	Catalyst	edge	v(CO)	n	v(CO)	n
			(cm -1 )	(µmol g -1 )	(cm -1 )	(µmol g -1 )
		M	2112	59	2112	60
	Mo/Al 2 O 3
		S	2064	19	2062	25
		M	2110	38	2110	39
	Mo(CA=2)/Al 2 O 3
		S	2063	31	2062	37
	3.3. S/O

exchanged sites are not involved in the redox pathway of WGS on MoS 2 /Al 2 O 3 catalysts

	As mentioned above, because the M-edge site is prone to the S/O exchange reaction, it is most likely
	to follow the conventional redox route, with the Mo(S x O y ) zc site as the active center. In order to examine
	the reactivity of Mo(S x O y ) zc sites toward CO, CO/IR was performed stepwise after successive H 2 O

treatment, resulfidation and CO treatment at 573 K on sulfided Mo/Al 2 O 3 catalyst. As shown in Figure

  2 under the CO alone feed, (ii) from the direct observation of formate intermediates in operando studies and (iii) the observed decomposition of formate at 573K on Mo/Al 2 O 3 , one can conclude that the formate mechanism occurs on Al 2 O 3 -supported MoS 2 catalysts at 573K. It must be underlined that at this temperature, WGS reaction does not occur on pure alumina, indicating that the vicinity between the Al-OH groups and the sulfide phase facilitates the formate species formation and thus play a role in the catalyst reactivity.However, various observations point out that another reaction route exists. Indeed, silica-supported MoS 2 catalyst present a stable CO conversion whereas no formate species are formed during the WGS reaction. The conventional redox mechanism was firstly considered, in which the water-induced Mo(S x O y ) zc site is proposed as the active center and then reduced by CO to produce CO 2 and give rise to a vacancy site[START_REF] Hou | Kinetic studies with a sulfur-tolerant water gas shift catalyst[END_REF] 13]. However, the CO/IR studies (Figure4) vividly demonstrate that this oxygen substituted Mo(S x O y ) zc site formed in situ by water treatment is stable toward CO treatment, thus preventing a direct redox process. In addition, this Mo(S x O y ) zc site also shows excellent stability even in 10% H 2 S/H 2 treatment and thus cannot be re-sulfided back to the initial state. These results suggest that the proposition of the conventional redox mechanism associated with Mo(S x O y ) zc site might not be correct, and this water-induced S/O exchange reaction is an irreversible poisoning process for MoS 2 ,

	CO + MoS 2 → MoS∆ + COS	(1)
	COS + H 2 O → CO 2 +H 2 S	(2)
	H 2 S + MoS∆ → MoS 2 + H 2
	especially for M-edge site.
	The IR gas phase spectra collected during the WGS reaction substantiate the formation of COS
	production. Thus, a novel redox mechanism associated with COS route can be proposed (equation 1-3).
	In this three-stage novel mechanism, the catalyst surface is firstly reduced by CO to form COS, then

Acknowledgments

Alexandre Vimont, Philippe Bazin and Yoann Levaque from LCS are greatly acknowledged for the IR experiments and activity measurements for WGS reaction. The authors thank Jinxing Mi and Xiaoping Chen for kindly helping with Graphical abstract and Scheme design.

Funding Sources

W. Zhao is grateful for the MOPGA grant (Campus France MOPGA-927541K)

Meanwhile, the IR spectra of the Mo/Al 2 O 3 show after a few minutes of reaction the appearance of a band at 2910 cm -1 as well as bands at 1590 and the doublet at 1392 and 1377 cm -1 which show the formation of formate species (Figure 9b). After pure Ar flow purge (red spectrum), the signal of water in gas phase is eliminated which makes clearer the observation of the bands at 2910, 1590, 1392 and 1377 cm -1 . These are respectively assigned to (CH), (OCO) asym , δ(CH) and (OCO) sym of formate species on the Mo/Al 2 O 3 catalyst surface [START_REF] Nelson | Carboxyl intermediate formation via an in situ-generated metastable active site during water-gas shift catalysis[END_REF]. In parallel, the gas phase IR spectra reveals CO consumption and CO 2 formation as well as the formation of a limited amount of COS (Figure 9c).

On the IR spectra of Mo(CA=2)/Al 2 O 3 catalyst, despite the presence of the complex massif due to carbonaceous residues, bands characterizing formate intermediate can be observed. As with Mo/Al 2 O 3 , CO consumption and CO 2 formation are also observed in the gas phase spectra. In addition, at the beginning of the reaction, larger COS amounts are released on this catalyst than on the catalyst without citric acid. However, the COS production decreases strongly and rapidly after the first TOS, and then COS signal stays relatively stable and at value close to Mo/Al 2 O 3 .

On Mo/SiO 2 catalyst, no formate species are formed in agreement with the previous IR observations. Meanwhile, gas phase IR spectra show CO consumption and CO 2 production as well as COS formation.

In this case, the COS production decreases at beginning but more slightly than for the aluminasupported catalysts and then presents a stable value.

So in conclusion, WGS study performed in differential reactor and in the operando set up points out that (i) activity of WGS reaction on alumina-supported catalysts is higher than that on silica-supported spectra of the gas phase during the WGS reaction (blue spectra correspond to that taken at the beginning and at the end of the reaction. All spectra correspond to gas phase spectra recorded at given TOS minus spectrum under the initial WGS feed flow)

New insights on WGS mechanisms on sulfide catalysts

Two mechanisms are usually considered for WGS reaction over sulfide catalysts, such as the associative mechanism and the redox mechanism [START_REF] Ojifinni | Water-enhanced low-temperature CO oxidation and isotope effects on atomic oxygencovered Au(111)[END_REF]. In the associative mechanism, intermediate

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Supporting


Ankita Ram

Krishna Maity

Cédric Marchand

Aymen Mahmoudi

Aseem Rajan Kshirsagar

Mohamed Soliman

Takashi Taniguchi

Kenji Watanabe

Bernard Doudin

Abdelkarim Ouerghi

Sven Reichardt

Ian O'connor
email: ian.oconnor@ec-lyon.fr

Jean-Francois Dayen
email: dayen@unistra.fr

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

Keywords: reconfigurable electronics, 2D materials, ferroelectricity, electronic band structure, optoelectronics, logic gate, circuit design

Emerging reconfigurable devices are fast gaining in popularity in the search for next-generation computing hardware, while ferroelectric engineering of the doping state in semiconductor materials has the potential to offer alternatives to the traditional von-Neumann architecture. In this work, we combine these concepts and demonstrate the suitability of Reconfigurable Ferroelectric Field-Effect-Transistors (Re-FeFET) for designing non-volatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe 2 ) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP 2 S 6 ) layer. Controlling the state encoded in the Program Gate enables switching between p, n and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 10 6 and hysteresis windows of up to 10 V width. The homojunction can change from ohmic-like to diode behavior, with a large rectification ratio of 10 4 . When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with long retention time exceeding 10^4 seconds. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key non-volatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates remarkable compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit groundbreaking potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.

INTRODUCTION

Modern integrated circuits face significant challenges in the quest for continuous device miniaturization, pushing the boundaries of Moore's Law. [START_REF] Moore | Cramming More Components onto Integrated Circuits[END_REF][START_REF] Skotnicki | The End of CMOS Scaling: Toward the Introduction of New Materials and Structural Changes to Improve MOSFET Performance[END_REF] Key concerns include device downscaling, circuit design and energy consumption. Conventional circuit design is based on elementary devices with fixed functionality such as Field Effect Transistors (FET), diodes, and memories. The prevailing technology in this domain is Complementary Metal Oxide Semiconductor (CMOS), which classically employs unipolar FETs that are fixed to either n-type or p-type operation based on their underlying fabrication process and chemical doping. However, as device downscaling progresses, this approach poses increasing complexity-related challenges. [START_REF] Irds Tm | More Moore -IEEE IRDS TM[END_REF][START_REF] Ahopelto | NanoElectronics Roadmap for Europe: From Nanodevices and Innovative Materials to System Integration[END_REF] To overcome some of the limitations of CMOS, an elegant emerging approach leverages the concept of Reconfigurable FETs (ReFET). [START_REF] Irds Tm | Beyond CMOS and Emerging Research Materials -IEEE IRDS TM[END_REF][START_REF] Mikolajick | 20 Years of Reconfigurable Field-Effect Transistors: From Concepts to Future Applications[END_REF] Unlike conventional FETs, these devices offer a promising solution by providing the ability to adjust their functionality. A ReFET typically consists of two gate electrodes, either lateral or vertical, that independently control the energy band profile within the semiconductor channel. Specifically, the program gate (PG) determines the type of carrier, while the control gate (CG) toggles the transistor on and off. This disruptive FET concept allows for a reversible reconfiguration between n-type and p-type operating modes. [START_REF] De Marchi | Polarity Control in Double-Gate, Gate-All-around Vertically Stacked Silicon Nanowire FETs[END_REF] The inclusion of these additional functionalities in a single device enhances logic capabilities and device versatility and potentially mitigates the challenges posed by miniaturization. For instance, a logic gate based on ReFETs can dynamically adjust its configuration during operation to implement a different truth table. [START_REF] Raitza | Exploiting Transistor-Level Reconfiguration to Optimize Combinational Circuits[END_REF] Furthermore, this technology holds promise for next-generation hardware security applications by safeguarding against the direct reading of the layout of an integrated circuit and dramatically complicating reverse engineering through key protection strategies. [START_REF] Wu | Two-Dimensional Transistors with Reconfigurable Polarities for Secure Circuits[END_REF][START_REF]Securing Hardware through Reconfigurable Nano-structures[END_REF] Lastly, as inherently multifunctional systems, reconfigurable electronic devices find applications in various fields such as neuromorphics [START_REF] Pan | Reconfigurable Logic and Neuromorphic Circuits Based on Electrically Tunable Two-Dimensional Homojunctions[END_REF][START_REF] Tian | A Dynamically Reconfigurable Ambipolar Black Phosphorus Memory Device[END_REF] , in-memory computing [START_REF] Sun | Reconfigurable Logic-in-Memory Architectures Based on a Two-Dimensional van Der Waals Heterostructure Device[END_REF][START_REF] Huang | An Ultrafast Bipolar Flash Memory for Self-Activated in-Memory Computing[END_REF][START_REF] Marchand | FeFET Based Logic-in-Memory: An Overview[END_REF] , energy harvesting, [START_REF] Gréboval | Split-Gate Photodiode Based on Graphene/HgTe Heterostructures with a Few Nanosecond Photoresponse[END_REF][START_REF] Pospischil | Solar-Energy Conversion and Light Emission in an Atomic Monolayer p-n Diode[END_REF][START_REF] Buscema | Photovoltaic Effect in Few-Layer Black Phosphorus PN Junctions Defined by Local Electrostatic Gating[END_REF] and nanoelectronics in general. [START_REF] Fei | Emerging Reconfigurable Electronic Devices Based on Two-Dimensional Materials: A Review[END_REF][START_REF] Noumbé | Reconfigurable 2D/0D p-n Graphene/HgTe Nanocrystal Heterostructure for Infrared Detection[END_REF][START_REF] Baugher | Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide[END_REF][START_REF] Zhao | Nonvolatile Reconfigurable 2D Schottky Barrier Transistors[END_REF] Integrating nanomaterials into the technological roadmap is a promising approach to surpass the limitations of Moore's Law. [START_REF] Skotnicki | The End of CMOS Scaling: Toward the Introduction of New Materials and Structural Changes to Improve MOSFET Performance[END_REF][START_REF] Irds Tm | Beyond CMOS and Emerging Research Materials -IEEE IRDS TM[END_REF][START_REF] Franklin | Nanomaterials in Transistors: From High-Performance to Thin-Film Applications[END_REF] Among these nanomaterials, two-dimensional (2D) materials have emerged as viable candidates due to their favorable electrical performance, scalability, and compatibility with silicon platforms. [START_REF] Ahopelto | NanoElectronics Roadmap for Europe: From Nanodevices and Innovative Materials to System Integration[END_REF][START_REF] Liu | Two-Dimensional Materials for next-Generation Computing Technologies[END_REF][START_REF] Fiori | Electronics Based on Two-Dimensional Materials[END_REF][START_REF] Akinwande | Graphene and Two-Dimensional Materials for Silicon Technology[END_REF] The unique characteristic of 2D materials lies in their dangling bond-free interfaces, which offer unprecedented flexibility in combining different materials, thereby opening up possibilities for uncharted device concepts. [START_REF] Jariwala | Mixed-Dimensional van Der Waals Heterostructures[END_REF][START_REF] Novoselov | 2D Materials and van Der Waals Heterostructures[END_REF] Furthermore, their ultimate atomic thickness and high sensitivity to external electric fields enable efficient engineering of the energy band profiles. [START_REF] Briggs | A Roadmap for Electronic Grade 2D Materials[END_REF][START_REF] Yang | Two-Dimensional Materials Prospects for Non-Volatile Spintronic Memories[END_REF] Consequently, certain 2D semiconductors, such as WSe 2 , MoTe 2 or black phosphorus, can demonstrate ambipolar characteristics when used in conjunction with suitable metal contacts. [START_REF] Hu | Ambipolar 2D Semiconductors and Emerging Device Applications[END_REF] These combined properties position van der Waals materials as an ideal platform for reconfigurable electronics.

However, the current implementation of ReFETs heavily relies on volatile external voltages to program their operating modes. The p-and n-conduction modes are configured by connecting the program gates to fixed voltage levels. [START_REF] Mikolajick | 20 Years of Reconfigurable Field-Effect Transistors: From Concepts to Future Applications[END_REF][START_REF] Wu | Two-Dimensional Transistors with Reconfigurable Polarities for Secure Circuits[END_REF] This approach raises concerns regarding energy consumption and reliability, and limits the benefits of reconfigurability. To overcome these challenges, it is crucial to develop non-volatile ReFETs that can maintain their doping profile without the need to continuously maintain the gate voltages. One promising approach to introduce non-volatility in ReFETs is through the integration of ferroelectric materials as the gate medium. The ferroelectric gate can retain non-volatile polarization states, which are then used to electrostatically dope the adjacent semiconductor layer. This working principle is a cornerstone of the field of ferroelectronics, which encompasses various novel device concepts such as ferroelectric memories, ferroelectric tunnel junctions, photodetectors, neuromorphic circuits and ferroelectric FETs (FeFET). [START_REF] Martin | Thin-Film Ferroelectric Materials and Their Applications[END_REF][START_REF] Chanthbouala | A Ferroelectric Memristor[END_REF][START_REF] Khan | The Future of Ferroelectric Field-Effect Transistor Technology[END_REF][START_REF] Garcia | Giant Tunnel Electroresistance for Non-Destructive Readout of Ferroelectric States[END_REF][START_REF] Gruverman | Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale[END_REF][START_REF] Soliman | Photoferroelectric All-van-Der-Waals Heterostructure for Multimode Neuromorphic Ferroelectric Transistors[END_REF][START_REF] Luo | Artificial Optoelectronic Synapses Based on Ferroelectric Field-Effect Enabled 2D Transition Metal Dichalcogenide Memristive Transistors[END_REF][START_REF] Cavallo | Coupling Ferroelectric to Colloidal Nanocrystals as a Generic Strategy to Engineer the Carrier Density Landscape[END_REF] In addition, novel circuit design concepts are being explored, where the internal polarization state of the FeFET serves as an input parameter for a new class of ferroelectric logic gates. [START_REF] Marchand | FeFET Based Logic-in-Memory: An Overview[END_REF][START_REF] Chen | The Impact of Ferroelectric FETs on Digital and Analog Circuits and Architectures[END_REF][START_REF] Breyer | Reconfigurable NAND/NOR Logic Gates in 28 Nm HKMG and 22 Nm FD-SOI FeFET Technology[END_REF][START_REF] Mikolajick | The Past, the Present, and the Future of Ferroelectric Memories[END_REF][START_REF] Yin | Ferroelectric FETs-Based Nonvolatile Logic-in-Memory Circuits[END_REF] Recent advancements in the field have introduced ferroelectric van der Waals systems, [START_REF] Ryu | Empowering 2D Nanoelectronics via Ferroelectricity[END_REF][START_REF] Si | A Ferroelectric Semiconductor Field-Effect Transistor[END_REF][START_REF] Liu | Room-Temperature Ferroelectricity in CuInP2S6 Ultrathin Flakes[END_REF][START_REF] Qi | Review on Recent Developments in 2D Ferroelectrics: Theories and Applications[END_REF][START_REF] Sun | Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection[END_REF] which include van der Waals ferroelectric heterostructures with tunable interfacial physics. [START_REF] Zhao | Nonvolatile Reconfigurable 2D Schottky Barrier Transistors[END_REF][START_REF] Soliman | Photoferroelectric All-van-Der-Waals Heterostructure for Multimode Neuromorphic Ferroelectric Transistors[END_REF][START_REF] Luo | Artificial Optoelectronic Synapses Based on Ferroelectric Field-Effect Enabled 2D Transition Metal Dichalcogenide Memristive Transistors[END_REF][START_REF] Jin | Ferroelectrics-Integrated Two-Dimensional Devices toward Next-Generation Electronics[END_REF][START_REF] Wang | 2D Layered Materials for Memristive and Neuromorphic Applications[END_REF][START_REF] Cao | 2D Material Based Synaptic Devices for Neuromorphic Computing[END_REF][START_REF] Wang | Van Der Waals Negative Capacitance Transistors[END_REF][START_REF] Wu | High Tunnelling Electroresistance in a Ferroelectric van Der Waals Heterojunction via Giant Barrier Height Modulation[END_REF] These discoveries have brought novel approaches into the field and opened up new possibilities. However, the exploration of reconfigurable ferroelectric electronics and their potential for Logic-in-Memory remains mostly uncharted territory.

In this work, we demonstrate reconfigurable ferroelectric devices based on van der Waals ferroelectric heterostructures. Our device architecture is based on a split-gates geometry and employs a layer of the CuInP 2 S 6 (CIPS) van der Waals material [START_REF] Belianinov | CuInP₂S₆ Room Temperature Layered Ferroelectric[END_REF][START_REF] Brehm | Tunable Quadruple-Well Ferroelectric van Der Waals Crystals[END_REF] as the ferroelectric gate medium. This configuration enables precise, efficient, and remanent control of the energy band profile within the WSe 2 homojunction. Our reconfigurable Ferroelectric FET (Re-FeFET) exhibits three distinct operating electronic modes: ambipolar FeFET, unipolar n-FeFET, and unipolar p-FeFET. Furthermore, we evaluate the optoelectronic properties of the Re-FeFET. It demonstrates the capability for photodetection in both phototransistor and photodiode modes. When programmed in the photodiode mode, open-circuit voltages and short-circuit photocurrents are generated, highlighting its potential for photovoltaic applications.

Finally, we explore the versatility of these tunable electronic devices by designing reconfigurable ferroelectric logic gates, which can be programmed to implement several Boolean functions (such as NAND, NOR or XOR). We conduct a comprehensive logic gate design exploration based on structure consisting of two series-connected devices. Our results demonstrate the superiority of ferroelectric reconfigurable circuits over classical CMOS circuits. In addition, we introduce a simple model for the Re-FeFET, which facilitates further circuit design exploration and behavioral simulations, while also allowing for a deeper understanding and analysis of the device's behavior in various circuit configurations.

RESULTS AND DISCUSSION

Figure 1a depicts a schematic of the van der Waals Re-FeFET studied in this work, which is constructed from the CIPS/hBN/WSe 2 heterostructure. The fabrication process involves mechanically exfoliating and sequentially transferring flakes of CIPS, h-BN, and WSe 2 onto pre-patterned metallic back-gate electrodes. The two back-gate electrodes were patterned beforehand with a split-gate geometry and an inter-electrode spacing of 3 µm. Source and drain electrodes are then fabricated by electron-beam lithography on top of the WSe 2 channel. Figure 1b provides an optical image of a typical Re-FeFET device. To optimize the ferroelectric polarization while minimizing leakage currents, few-layered h-BN is used as a dielectric spacer. It also improves the transport characteristics of WSe 2 with respect to defects and trap states. [START_REF] Rhodes | Disorder in van Der Waals Heterostructures of 2D Materials[END_REF][START_REF] Illarionov | Insulators for 2D Nanoelectronics: The Gap to Bridge[END_REF] More detailed information on the fabrication process can be found in the Supplementary Information Section 1. In Figure 1c, the micro-Raman spectrum of the van der Waals heterostructure is shown, which was obtained from a typical Re-FeFET as described in the Methods section. The spectrum displays the first-order in-plane and out-of-plane Raman modes of the WSe 2 bilayer sample around 249 cm -1 and 251 cm -1 , respectively. The recorded spectra reveal a complete suppression of degeneracy, and the two modes exhibit a frequency splitting of ∼2 cm -1 . The ratio of A 1g to E' 2g peak intensities is about 3. The peak observed at 308 cm -1 (labeled as B' 2g ) is indicative of regions with a thickness of 2 monolayers. Based on the described Raman data, the intense B' 2g peak confirms the bilayer behavior of the deposed WSe 2 , [START_REF] Mahmoudi | Electronic Properties of Rhombohedrally Stacked Bilayer WSe2 Obtained by Chemical Vapor Deposition[END_REF][START_REF] Li | Stacking-Dependent Exciton Multiplicity in WSe2 Bilayers[END_REF] and the splitting of the two main out-of-and in-plane modes indicates a type of stacking that can be attributed to the 3R phase. Additional micro-Raman spectra, obtained from different regions of the heterostructures with varying distinct numbers of stacked van der Waals materials, are provided in Supplementary Information Section 2.

FERROELECTRIC FIELD EFFECT TRANSISTOR WITH RECONFIGURABLE POLARITIES

We first study how the split-gate architecture makes it possible to emulate different FeFET operating behavior depending on whether the device is operated in ambipolar or unipolar mode (cf. Figure 2a). Transconductance measurements under different Program Gate configurations are performed while sweeping the Control Gate bias with 10 ms long voltage pulses. The Figure 2b shows the transconductance of the Re-FeFET operating in a symmetric configuration while biasing both gates with the same voltage (cf. "Ambipolar FeFET mode" depicted in Figure 2a). This operating mode is mostly equivalent to the single-gate FeFET and is well-established. [START_REF] Miller | Physics of the Ferroelectric Nonvolatile Memory Field Effect Transistor[END_REF][START_REF] Kim | Ferroelectric Field Effect Transistors: Progress and Perspective[END_REF] The remanent polarization state of the CIPS can be inverted by applying a gate voltage of opposite polarity, exceeding the ferroelectric coercive fields ( or respectively for the positive or negative sides). The polarization up (P up ) or down (P down ) leads to the accumulation of negative or positive mobile charges within the WSe 2 channel. The Re-FeFET exhibits an ambipolar I ds -V gs transfer characteristic, enabling conduction in both the electron and hole accumulated regimes. Moreover, it exhibits a well-defined hysteresis loop that can be decomposed into an anti-clockwise and a clockwise hysteresis loop respectively for the n-and p-branches. Such characteristics show that the transconductance properties of the WSe 2 bilayer are driven by the CIPS ferroelectric polarization switching. The analysis of the transconductance demonstrates excellent performance, with the on/off current ratio exceeding 10 5 (resp. 10 4 ) in the p-(resp. n-) branch, and the ferroelectric window approaching a width of 10 V. These features attest to the efficient electrostatic coupling between the CIPS and the WSe 2 , and the good interface quality of the van der Waals stack without dominating Fermi level pinning or trap states. [START_REF] Liu | Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects[END_REF] Next, we investigate the reconfigurable properties of the Re-FeFET while operating it in asymmetric configurations (cf. the two "Unipolar FeFET modes" depicted in Figure 2a). Here, we firstly polarize the Program Gate either to the upward polarized state PG  (V PG = +12 V >

) or to the downward polarized state PG ↓ (V PG = -12 V < -). This enables the WSe 2 channel part positioned over the PG Gate to be set selectively to a predetermined doping state. Transconductance measurements are then performed while modulating only the Control Gate voltage V CG . The Re-FeFET demonstrates now very different characteristics than those observed in the ambipolar mode: the device behaves in both asymmetric modes as unipolar-like FeFETs of opposite polarities. In the PG ↓ state (Figure 2b), the Re-FeFET mimics the behavior of a p-FeFET and shows a clockwise transconductance hysteresis with an on/off current ratio of six orders of magnitude. When programmed in the PG  state, the device behaves as an n-FeFET, with a clockwise transconductance hysteresis with an on/off current ratio of four orders of magnitude (Figure 2d). For both unipolar modes, the high and low conductance states are obtained, respectively, when the homojunction is set to unipolar (p-p, or n-n) or ambipolar (p-n, or n-p) configurations.

To further illustrate the ambipolarity of the Re-FeFET, we perform density functional theory (DFT) calculations to find the relative alignment of the electronic energy levels of WSe 2 and the Ti metal contacts (see Methods for the computational details of the calculation). Figure 2e (left graph) depicts the WSe 2 and Ti slab components of the density of states (DOS) of a combined WSe 2 -Ti system corresponding to the different states of the Re-FeFET. In Figure 2e (middle and right graphs), the effect of the finite surface charge of the CIPS is qualitatively illustrated by a sketch of the effective Fermi level. The WSe 2 and Ti slab components of the DOS are obtained by projecting the DFT orbitals onto atomic orbitals. In the absence of any surface charge in the CIPS, the Fermi level of the combined system lies in the middle of the WSe 2 electronic band gap (Left graph of Figure 2e), due to the presence of the partially filled metallic band of Ti within the gap. The choice of Ti as the contact material with its convenient level alignment to the WSe 2 bands thus allows for the efficient electrostatic doping of the WSe 2 layer with both p-and n-type carriers, leading to the achieved ambipolarity of the Re-FeFET.

(LEFT) The electrochemical potential lies within the Ti metallic band within the band gap of WSe 2 . (MIDDLE) Same as (LEFT) but for the p-doped state, illustrating the electrostatic doping of the WSe 2 layer, with the hole charge carriers supplied by the Ti reservoir. (RIGHT) same as (MIDDLE) but for

the n-doped state. Inset: Sketch of the vertical stacking of the Ti contact layer (red), the WSe 2 layer, and the polarized CIPS layer (teal), which is connected to the Ti through a metal contact.

RECONFIGURABLE HOMOJUNCTION AND PHOTOVOLTAIC DIODE

We now fully explore the electrical and optoelectronic properties of the reconfigurable homojunction. We first characterize the tunability of the homojunction while investigating systematically the transport properties of the WSe 2 homojunction as a function of the drain-source (V DS ) and gate-source (V PG , V CG ) bias voltages. Figure 3a shows the transconductance map I DS (V PG, V CG ) measured at fixed drain-source voltage of value 0.5V under a systematic sweep on the gate bias voltage parameters (V PG, V CG ). Four distinct homojunction states can be identified, corresponding to the four quadrants labeled on the transconductance map: the p-p, n-n, p-n and n-p states (see corresponding energy band diagrams in Supplementary Information Section 3). We further characterize the rectifying capability of the homojunction with these four states and report their output characteristics in Figure 3b. In the unipolar states (n-n or p-p), the I DS (V DS ) traces show quasilinear behavior, while in the ambipolar states (n-p or p-n) the I DS (V DS ) traces demonstrate clear nonlinear rectifying characteristics. The rectification ratio (defined as the ratio of the forward current to the reverse current obtained for opposite polarities of V DS ) exceeds 10 4 at V DS = 1 V in the n-p and p-n states, while it stays close to 1 in the unipolar cases (approaching respectively 1.5 and 2 for the n-n and p-p homojunctions). Such a high rectification ratio indicates the excellent quality of the p-n junction formed in WSe 2 and its efficient coupling with CIPS. We then investigate the potential of the Re-FeFET to convert light energy into electricity by the photovoltaic effect, taking advantage of the large built-in electric field of the p-n junction to dissociate photoexcited carriers. Figure 3c illustrates the separation of photogenerated electron-hole pairs due to the built-in electric field formed within the WSe 2 bilayer. Upon illumination, the internal built-in electric field dissociates the photogenerated carriers and gives rise to a photocurrent at zero applied voltage (short-circuit photocurrent, I SC ) and a photovoltaic voltage with zero current flow (open circuit voltage, V OC ). Figure 3d presents I-V curves under optical illumination ( = 522 nm, P = 0.001-0.1 mW.mm -2 , see Methods for experimental details). When the Re-FeFET is set to a diode configuration p-n (resp. n-p), the I-V characteristics are shifted upward (resp. downward) compared to the dark characteristics. This demonstrates that the Re-FeFET operates as a photodiode. Figure 3e and Figure 3f report the color maps of the I SC and the V OC respectively, as a function of the split-gate parameters (V PG , V CG ) measured under illumination conditions ( = 522 nm, P = 0.1 mW.mm -2 ). The color maps precisely identify the existence of short-circuit photocurrents and open-circuit voltages with the diode states previously pinpointed in dark conditions (Figure 3a). When the Re-FeFET is set to a unipolar state (n-n or p-p-), no I SC or V OC is detected (see Supplementary Information Section 4). Consistently, we observe in the p-n (n-p) configuration the apparition of a large negative (positive) V OC (Figure 3d and Figure 3e) and a positive (negative) I SC (Figure 3d and Figure 3f). The fact that I SC and V OC change sign while flipping the diode polarities is a clear indication that the photoresponse does not originate from the Schottky contacts. [START_REF] Pospischil | Solar-Energy Conversion and Light Emission in an Atomic Monolayer p-n Diode[END_REF][START_REF] Baugher | Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide[END_REF][START_REF] Fontana | Electron-Hole Transport and Photovoltaic Effect in Gated MoS2 Schottky Junctions[END_REF] For an optimally engineered junction profile, the maximum short-circuit photocurrent detected reaches 1.2 nA (resp. -1.3 nA) in p-n (resp. n-p) configuration, while the open circuit voltage created reaches +0.7 V (resp. -0.7 V). The photovoltaic voltages produced are among the best reported so far for WSe 2 , [START_REF] Pospischil | Solar-Energy Conversion and Light Emission in an Atomic Monolayer p-n Diode[END_REF][START_REF] Baugher | Optoelectronic Devices Based on Electrically Tunable p-n Diodes in a Monolayer Dichalcogenide[END_REF][START_REF] Long | Progress, Challenges, and Opportunities for 2D Material Based Photodetectors[END_REF][START_REF] Groenendijk | Photovoltaic and Photothermoelectric Effect in a Double-Gated WSe2 Device[END_REF] and outperform alternative van der Waals doping strategies such as superionic gating [START_REF] Lee | Programmable Devices Based on Reversible Solid-State Doping of Two-Dimensional Semiconductors with Superionic Silver Iodide[END_REF] and contact engineering. [START_REF] Du | Gate-Controlled Polarity-Reversible Photodiodes with Ambipolar 2D Semiconductors[END_REF][START_REF] Kim | High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts[END_REF] This demonstrates that the Re-FeFET can also be used for energy harvesting. Moreover, the non-volatile ferroelectric control of the photodiode properties opens new prospects for in-memory sensing and computing. [START_REF] Wu | Ferroelectric-Defined Reconfigurable Homojunctions for in-Memory Sensing and Computing[END_REF]

POLYMORPHIC FERROELECTRIC AND ELECTRONIC LOGIC GATES

As mentioned before, the Re-FeFET architecture enables the programming of distinct homojunction states with different conduction levels. This naturally hints at reconfigurable Boolean logic operations. The schematic of Figure 4a shows a simple logic circuit built from a Re-FeFET in series with a passive load. We now investigate the reconfigurability of this logic circuit while operating it either as an electronic or a ferroelectric logic gate. In the electronic mode, the parameters (V CG , V PG ) define the two input variables (schematic of Figure 4a). Here, V CG and V PG are set independently to the "1" state (resp. the "0" state) while polarizing the CIPS to the upward (resp. downward) polarization with V CG or V PG > (resp. V CG or V PG < -). As shown in the measured output voltage V out in Figure 4a, the Re-FeFET circuit operated in the electronic logic mode allows emulation of the XNOR logic function. In the case of the ferroelectric mode, the logic state is now set by the input pair (FE Branch , V CG ). The program gate (V PG ) is pre-set to a fixed polarized state (either in the PG  state or the PG ↓ state) as detailed in the schematics of Figure 4b andFigure 4c. The new variable FE Branch is specific to ferroelectric logic. [START_REF] Marchand | FeFET Based Logic-in-Memory: An Overview[END_REF][START_REF] Yin | Ferroelectric FETs-Based Nonvolatile Logic-in-Memory Circuits[END_REF] It is set to the "0" state (resp. the "1" state) when the FeFET is operating in the low (resp. high) threshold voltage branch of the transconductance hysteresis that transits at the negative (resp. positive) coercive field (resp.

). As shown in Figure 4b and Figure 4c (see also Supplementary Information Section 5), the Re-FeFET logic can switch between a ferroelectric NAND (FE-NAND) and a ferroelectric AND (FE-AND) function respectively, depending on the pre-programmed FeFET state (either p-or n-type, respectively). Hence, various ferroelectric logic functions can be obtained, providing polymorphic logic functionality, which is of interest for hardware security, custom operations, and circuit design. It is important to underline that the logic gate inputs can be provided by voltage pulses. In this case, the I DS current is measured after applying the various gate pulses, in open-circuit gate conditions, and the ferroelectric logic states correspond to remanent states. The retention properties of the four homojunction states are reported in Figure 4d and Figure 4e. The Program and Control Gates have been programmed independently either in the up or down state, and subsequently grounded. The output voltage is presented as a function of time, and demonstrates successful retention after more than 10^4 seconds for each state, comparable to the best values reported so far on van der Waals ferroelectric heterostructures. [START_REF] Wang | Van Der Waals Engineering of Ferroelectric Heterostructures for Long-Retention Memory[END_REF] Such remanence is a major added value when compared with traditional CMOS-based circuits that rely on a constant supply of external voltages to maintain the desired doping levels. This represents a paradigm shift with respect to conventional von Neumann architectures, as the information processing and storing are combined in a single device. This demonstrates the promise of van der Waals ferroelectric circuits for Logic-In-Memory hardware.

c, Same circuit with the Re-FeFET operating in the p-FeFET mode and showing AND logic. When used as a polymorphic ferroelectric logic gate (a,b), both inputs represent the inmemory state corresponding to the FE Branch (cf. top schemes V out (V CG ) hysteresis traces in a, and b, and the V CG voltage (state '1' for > V CG > 0 V, state '0' for V CG > When operating as an electric logic gate, both inputs are provided by V CG and V PG terminals (cf. color plot scheme in c,). Waveforms V out vs. logic state are traced based on experimental data, with V DD = 1 V. d, and e, show the retention properties of the four ferroelectric logic states of the Fe-XNOR gate. The output voltage (normalized respect to V DD set at 0.5 Volt) was measured after the Program and Control Gates have been programmed independently either in the up or down state, and subsequently grounded.

CIRCUIT DESIGN EXPLORATION WITH RE-FEFETS

In this section, we assess the potential of ferroelectric reconfigurable logic circuits based on Re-FeFETs through a circuit design exploration and modeling study. In order to set a framework and terminology for the analysis, we abstract the single device to a set of parameters where:

1-represents the non-volatile configured type of the transistor, where indicates p-type operation and indicates n-type operation.

2-

represents the volatile voltage applied to the program gate (also referred as polarity gate) after type configuration, causing a shift of the threshold voltage.

3-

represents the non-volatile polarization of the second part of the ferroelectric layer within the Re-FeFET. Here, (resp. ) corresponds to a negative (resp. positive) polarization.

4-

represents the volatile voltage applied to the control gate after polarization configuration.

This parameter set for the Re-FeFET allows us to establish conditions that determine when the device is in the state or the state. These conditions are summarized in the conduction table, as presented in Figure 5a. For more detailed explanations regarding the axioms and underlying principles used to construct this conduction table, the reader is referred to the Supplementary Information Section 6.

With this formalization of the Re-FeFET functionality, it becomes possible to explore the diverse logical functionalities that can be achieved with simple circuit structures. Let us consider a specific case where the circuit consists of two series-connected Re-FeFETs (shown in Figure 5c). By leveraging the conduction table, non-volatile logic operations can be implemented using this circuit. The process involves searching for solutions within the table that satisfy the constraints imposed by the truth table of the desired logical operation. Each solution corresponds to a unique parameter set for each Re-FeFET in the circuit (and -).

To create any logical or computing function, six elementary logic gates are needed: and The conduction table provides direct implementations for and functions. However, the function requires a more complex implementation. Detailed information on the implementation of functions are given in the Supplementary Information Section 6. From the truth table, it is possible to find solutions within the conduction table by setting and , given by:

 - ,  - .
It is worth noting that these solutions only utilize as the volatile operand. This implies that there is no need for an additional inverter to physically implement this circuit, unlike conventional implementations of 2-input XOR gates. Additionally, the non-volatile operand is linked to the type of the Re-FeFET. Consequently, and are set within each Re-FeFET using an external shared programming circuit and it is not necessary to generate with a local inverter. In some cases, when an n-type device is used to pass to the output, there can be a logic '1' degradation. To mitigate logic degradation, it is common practice to regenerate the output voltage level (usually via an inverter and/or a level restorer), either for each logic gate or for a set of cascaded logic gates.

Finally, it is demonstrated that all six elementary logic gates can be accessed using the single seriesconnected Re-FeFET (Figure 5c) circuit configuration, while keeping and . An example configuration for each logic gate implementation is depicted in Figure 5b. As shown in Figure 5, Re-FeFETs offer the advantage of very compact logic gates compared to CMOS circuits. In the case of the inverter, the same transistor-count is required for both Re-FeFET and CMOS implementations. However, Figure 5d and Figure 5e show that for gates, the Re-FeFET based circuit requires only two devices, while the equivalent CMOS circuits necessitate four transistors. In the case of the operation presented in Figure 5f, the CMOS implementation typically requires 12 transistors, considering the two additional inverters required to convert and to their complemented forms ( and ). On the other hand, the Re-FeFET based can be implemented with just two devices. This results in a significant reduction in transistor count by a factor of 6. Consequently, Re-FeFET devices offer the potential for compact circuits, which can be beneficial in a wide range of applications such as reconfigurable circuits with systematic and regular structures (such as FPGAs), or obfuscated circuits aimed at preventing reverse engineering, among others. The use of an identical basic circuit element for all logic gates provides additional advantages. Fabrication constraints are alleviated, leading to more compact and reliable chip designs. Furthermore, hardware security can be enhanced as the time-dependent power consumption of each logic gate remains identical, making it more challenging to derive information from power measurements. This inherent resilience to side channel attacks adds to the appeal of Re-FeFET-based circuits in terms of hardware security.

To facilitate further design considerations and enable device simulation, an equivalent circuit is created in the Cadence software. The details of this circuit can be found in the Supplementary Information Figure S5. Using this equivalent circuit, behavioral simulations of the three main logical operations ( , , and ) are conducted. The simulation results are presented in Figure 5g.

Figure 5. Implementation of NAND, NOR and XOR functions using Re-FeFETs as compared to conventional CMOS circuits. a, Re-FeFET conduction table specifying when the Re-FeFET is conducting ("On") or not ("Off") depending on the set of parameters. b, Table of series-connected Re-FeFET circuit (c,) configurations for six different logic functions (NOT, NAND, OR, NOR, AND and XOR). c, Ferroelectric circuit consisting of two seriesconnected Re-FeFETs and detailed configurations to the logic functions NAND (d,), NOR (e,), and XOR (f,).

For each logic function, the ferroelectric logic demonstrate superiority in terms of circuit compactness with a factor between 2 and 6. f, Behavioral simulations (created with the Cadence software) of the three main logic operations (NAND, NOR, XOR) performed by the in-series Re-FeFET circuit.

CONCLUSION

In conclusion, our exploration of reconfigurable ferroelectric devices based on a WSe 2 semiconducting channel controlled by split-gates and the CuInP 2 S 6 van der Waals material has revealed significant properties and functionalities. We demonstrate full electrostatic control of the device by means of the split-gate electrodes. This architecture enables the precise tuning of the energy band profile along the WSe 2 channel from unipolar (n-n or p-p) to ambipolar (n-p or p-n) homojunction configurations. The Re-FeFET can be switched between p-FeFET, n-FeFET, and ambipolar-FeFET modes. Using DFT-level electronic structure calculations, we showed that the favorable alignment of the Fermi level of the Ti contacts within the band gap of the WSe 2 layer allows its electrostatic doping with both p-type and n-type carriers. With careful device engineering, the Re-FeFET demonstrates an on-off ratio of 10 6 (10 4 ) for the p-branch (n-branch), clear carrier-polarity clockwise/anticlockwise reversal of ferroelectric hysteresis and a wide memory window of up to 10 V. Additionally, the Re-FeFET can be configured as a diode (pn or np) to provide photovoltaic functionalities. The diode homojunction configurations demonstrate an excellent rectification ratio exceeding 4 order of magnitude. The observed photocurrent and large open-circuit photovoltage are attributed to the efficient electron-hole separation facilitated by the built-in electric field at the PN junction. Furthermore, we have demonstrated a proof-of-concept reconfigurable logic unit that can function either as a polymorphic ferroelectric logic or as an electronic logic unit. In the ferroelectric mode, the reconfigurable logic unit can be switched from a Fe-NAND gate to a Fe-AND gate based on the state stored in the Program Gate, and demonstrate long retention capability exceeding 10^4 s. Such promissing characteristics call for future works to explore the endurance of Re-FeFET for dataintensive applications and the effect of device downsizing. When operated in the electronic mode, the independent control of the Program and Control Gates allows for encoding the XNOR logic. We have also explored the further potential of the Re-FeFET for reconfigurable logic circuit design purposes by first formalizing the device as a set of parameters, then proposing a structure composed of two series-connected Re-FeFETs. By applying the formalized parameter set, we have analyzed various configurations of the structure to implement 2-input NOT, NAND, OR, NOR, AND, and XOR logic functions.

Our findings demonstrate that the Re-FeFET based structure enables significantly more compact circuit implementations compared to conventional CMOS circuit implementations, resulting in up to 80% reduction in transistor count. However, it will be necessary to reduce the voltages used to program and operate the Re-FeFET in future work in order to compare fairly with conventional CMOS platforms. Identified pathways include reducing the thickness of the ferroelectric layer [START_REF] Liu | Room-Temperature Ferroelectricity in CuInP2S6 Ultrathin Flakes[END_REF][START_REF] Wang | Evolution of Pronounced Ferroelectricity in Hf0.5Zr0.5O2thin Films Scaled down to 3 Nm[END_REF] and contact engineering. [START_REF] Du | Gate-Controlled Polarity-Reversible Photodiodes with Ambipolar 2D Semiconductors[END_REF][START_REF] Kim | High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts[END_REF] Nevertheless, the high logic expressivity enabled by the simple Re-FeFETbased circuit under study will help to reduce complexity and increase logic circuit compactness through reconfigurability. We have also validated the behavior of our logic gate using equivalent circuit-based simulations, further confirming the feasibility and performance of the Re-FeFET in logic circuit applications. This work thus establishes the foundations for future research on reconfigurable ferroelectric devices and circuits, showcasing their potential in logic functions, reconfigurable circuits, in-memory computing, and photodetection. Our proof-of-concept device is thus a step toward the fabrication of non-volatile reconfigurable electronic circuits. We envision that ferroelectric circuits represent a large playground for heterostructure engineering with 2D materials and for nanoelectronics systems design. Hence, while the gate overlap scheme was engineered in our device, the Re-FeFET operating mechanism is also fully compatible with the gate underlap geometry. This comes with anticipated added benefices in terms of device performance and upscaling. [START_REF] Trivedi | Nanoscale FinFETs with Gate-Source/Drain Underlap[END_REF] Also, alternative ambipolar van der Waals semiconductors with smaller bandgap energy, such as black phosphorus or molybdenum ditelluride, should be investigated with expected smaller ferroelectric operating window. [START_REF] Hu | Ambipolar 2D Semiconductors and Emerging Device Applications[END_REF] Furthermore, the fascinating van der Waals sliding ferroelectrics are promissing candidates to enable the ferroelectric gate control down to thin layer of just few atoms. [START_REF] Wu | Sliding Ferroelectricity in 2D van Der Waals Materials: Related Physics and Future Opportunities[END_REF][START_REF] Bian | High-Performance Sliding Ferroelectric Transistor Based on Schottky Barrier Tuning[END_REF] Finally, our results highlights the promising nature of van-der-Waals materials as a platform for engineering high-performance beyond-Moore devices. The ability of the Re-FeFET to offer multiple functionalities in a single unit (electronic, memory, optoelectronic, and photovoltaic), with reconfigurability at runtime, holds promise for both More-than-Moore and beyond-Moore strategies in the field of advanced electronics.

METHODS/EXPERIMENTAL

Optical spectroscopy measurements

Micro-Raman spectroscopy measurements were conducted at room temperature using a commercial confocal Horiba Labram Raman spectrometer operating with a laser of wavelength λ = 532 nm. The laser beam was focused onto a small spot with diameter of about 1 μm on the sample and incident power of about 250 μW. All measurements are acquired at room temperature.

Opto-electrical measurements

The high precision source meters K2634B were used for electrical characterization and gate pulse control. For the ferroelectric logic circuit, the K2182A nanovoltmeter was used to extract the voltage output parameter, while measuring the voltage drop across a resistance connected in series with the device. Voltage pulses, or combination of voltage pulses and voltage sweeps, were applied to the PG (Program Gate) and CG (Control Gate) inputs to produce different states to characterize the different gate logics (XNOR, NAND, AND). In-situ optical fiber was used for photoexcitation together with COHERENT OBIS LS/LX 522 nm source in CW-power mode.

Circuit modeling and simulation

Circuit modeling and simulation are performed using an equivalent circuit implemented in Cadence software for behavioral simulations.

DFT modeling

The DFT calculations for the level alignment of WSe 2 and the Ti contact are performed with the PWscf program of the QuantumESPRESSO suite [START_REF] Giannozzi | QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials[END_REF][START_REF] Giannozzi | Advanced Capabilities for Materials Modelling with Quantum ESPRESSO[END_REF] . A plane-wave basis for the single-electron wave functions is used, truncated at a kinetic energy cutoff of 60 Ry. The interaction of the ionic core with the valence electrons is modeled using ultrasoft pseudopotentials from the PS library [START_REF] Dal Corso | Pseudopotentials Periodic Table: From H to Pu[END_REF] . The planewave basis for the electronic density is truncated at 540 Ry. The exchange and correlation interactions are described using the Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation. [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] The spin-orbit interaction is neglected in the interest of computational efficiency, as the density of states of WSe 2 is only marginally effected. The structures of the individual materials were fully optimized, yielding in-plane lattice constants of 3.32 Å and 2.93 Å for WSe 2 and Ti, respectively.

To get the absolute level alignment with respect to a common vacuum level, a commensurate hexagonal supercell of a monolayer WSe 2 (

), rotated by 18°, and a 10-atom thick Ti (0001) slab (3 3) is constructed using the CellMatch package [START_REF] Lazić | CellMatch: Combining Two Unit Cells into a Common Supercell with Minimal Strain[END_REF] to achieve a minimal strain of 0.01%. As we are interested in finding the relative electronic level alignment of the metal and freestanding 2D semiconductor, rather than the complex potential landscape at their interface, [START_REF] Kang | Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors[END_REF][START_REF] Chen | Metal Contacts with Moire Interfaces on WSe2for Ambipolar Applications[END_REF] the WSe 2 layer and Ti slab are separated in our simulation by a vacuum distance of 12.5 Å. For integrations over the first Brillouin zone, we use a non-shifted Monkhorst-Pack grid of 6 6 1 points. We use a Gaussian smearing for the metallic Ti states with a smearing energy of 50 meV. The Ti slab is structurally relaxed to obtain the ground state geometry of the surface. The band energies obtained using DFT are aligned by setting the electrostatic potential (ionic+Hartree) to zero at a point in the vacuum region of the supercell, equidistant from the WSe 2 layer and the Ti slab.

Figure 1 .

 1 Figure 1. Van de Waals Reconfigurable Ferroelectric Field Effect Transistor. a, Schematic of the Re-FeFET, with split-gate architecture combined as two independent bottom gate control terminals (Program Gate and Control Gate). The van der Waals heterostructure is composed of a WSe 2 /h-BN/CuInP 2 S 6 stack. b, Optical microscope image of a typical FeFET device (scale bar=10µm). c, Micro-Raman spectra of the CuInP 2 S 6 taken from a Re-FeFET device. d, Micro-Raman subtracted spectra of the WSe 2 channel taken from the WSe 2 /hBN/CIPS heterostructure region.

Figure 2 .

 2 Figure 2. Ferroelectric Field Effect Transistor with reconfigurable polarities. a, Schematic of the various operating modes of the Re-FeFET. In the ambipolar mode, both gates are biased at the same sweeping voltage. In the unipolar n-FeFET (resp. p-FeFET) mode, the program gate V PG is set above (resp. below) the positive (resp. negative) ferroelectric coercive voltage V FE , while the control gate voltage V CG is swept. b, Transconductance loop of the Re-FeFET operating in the ambipolar mode and demonstrating ambipolar hysteresis with anticlockwise hysteresis in the n-branch and clockwise hysteresis in the p-branch. c, and d, show the transconductance loops of the Re-FeFET operating respectively in the p-FeFET and n-FeFET modes. The ferroelectric polarization switching of the CIPS is responsible for the anticlockwise (resp. clockwise) hysteresis of the n-FeFET (resp. p-FeFET) transconductance. e, Atom-projected density of states (DOS) of the combined

Figure 3 .

 3 Figure 3. Electrical and optoelectronics tunable properties of the Re-FeFET. a, Contour map of the drain-source current I DS as a function of the voltages applied to the program gate (V PG ) and the control gate (V CG ). b, I DS -V DS characteristics of the Re-FeFET programmed in the n-n (grey), p-p (black), p-n (pink) and n-p (purple) states. c, Energy band diagram illustrating the photovoltaic effect in (i) p-n and (ii) n-p homojunction configuration of the WSe 2 channel. d, Drain-source current vs voltage characteristics of the Re-FeFET programmed in p-n (red) and n-p (blue) configurations, under various light intensity "P" (P = 0.1 mW.mm-2 at 100%, λ = 522 nm). e, Color map of the open circuit voltage V OC (I DS = 0 A) as the program (V PG ) and the control (V CG ) gates voltages are tuned independently. f, Color map of the short circuit photocurrent I SC (V DS = 0 V) as the two side-gates are swept independently.

15

 15

Figure 4 .

 4 Figure 4. Polymorphic ferroelectric logic gate. a, Ferroelectric circuit (cf. circuit schematic) based on Re-FeFET operating in ambipolar mode, and demonstrating XNOR logic. b, Same circuit with the Re-FeFET programmed to the n-FeFET mode, demonstrating NAND logic. c, Same circuit with the Re-FeFET operating in the p-FeFET mode and showing AND logic. When used as a polymorphic ferroelectric logic gate (a,b), both inputs represent the inmemory state corresponding to the FE Branch (cf. top schemes V out (V CG ) hysteresis traces in a, and b, and the V CG voltage (state '1' for > V CG > 0 V, state '0' for V CG > When operating as an electric logic gate, both inputs are provided by V CG and V PG terminals (cf. color plot scheme in c,). Waveforms V out vs. logic state are traced based on experimental data, with V DD = 1 V. d, and e, show the retention properties of the four ferroelectric logic states of the Fe-XNOR gate. The output voltage (normalized respect to V DD set at 0.5 Volt) was measured after the Program and Control Gates have been programmed independently either in the up or down state, and subsequently grounded.

ACKNOWLEDGMENTS

We also acknowledge the Agence Nationale de la Recherche for financial support through the grant MixDFerro (ANR-21-CE09-0029) and MIXES (ANR-19-CE09-028), the Region Grand-Est (Project Phoenix), IdEx Unistra (ANR 10 IDEX 0002), SFRI STRAT'US project (ANR 20 SFRI 0012) and EUR (QMat-ANR-18-EUR-0016) under the framework of the French Investments for the Future Program. We acknowledge support from Eucor -The European Campus for financial support through the QUSTEC funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 847471, and support of the Institut Universitaire de France (IUF). A.R.K. and S.R. acknowledge funding from the National Research Fund (FNR) Luxembourg, project « RESRAMAN » (grant no. C20/MS/14802965). We thank Bohdan Kundys for providing diodes and his techinal advise on optical measurements, Fabien Chevrier for cryogenic technical assistance, and the STnano nanofabrication platform.

SUPPORTING INFORMATION

Device Fabrication and AFM characterization; Raman characterization; Reconfigurable Energy band profiles; Reconfigurable FeFET operating in Phototransistor mode; Ferroelectric logic gate measurements; Circuit design exploration with Re-FeFETs.

AUTHOR CONTRIBUTIONS