Article

www.acsnano.org Atomic-Layer Controlled Interfacial Band Engineering at Two-Dimensional Layered ffi PtSe2/Si Heterojunctions for E cient Photoelectrochemical Hydrogen Production △ △ Cheng-Chu Chung, Han Yeh, Po-Hsien Wu, Cheng-Chieh Lin, Chia-Shuo Li, Tien-Tien Yeh, Yi Chou, Chuan-Yu Wei, Cheng-Yen Wen, Yi-Chia Chou, Chih-Wei Luo, Chih-I Wu, Ming-Yang Li, Lain-Jong Li, Wen-Hao Chang,* and Chun-Wei Chen*

Cite This: https://dx.doi.org/10.1021/acsnano.0c08970 Read Online

ACCESS Metrics & More Article Recommendations *sı Supporting Information

ABSTRACT: diselenide (PtSe2) is a group-10 two- dimensional (2D) transition metal dichalcogenide that exhibits the most prominent atomic-layer-dependent electronic behav- ior of “semiconductor-to-semimetal” transition when going from monolayer to bulk form. This work demonstrates an efficient photoelectrochemical (PEC) conversion for direct solar-to-hydrogen (H2) production based on 2D layered PtSe2/ Si heterojunction photocathodes. By systematically controlling fi the number of atomic layers of wafer-scale 2D PtSe2 lms through chemical vapor deposition (CVD), the interfacial band alignments at the 2D layered PtSe2/Si heterojunctions can be p- fi appropriately engineered. The 2D PtSe2/ Si heterojunction photocathode consisting of a PtSe2 thin lm with a thickness of 2.2 nm (or 3 atomic layers) exhibits the optimized band alignment and delivers the best PEC performance for hydrogen production with a photocurrent density of −32.4 mA cm−2 at 0 V and an onset potential of 1 mA cm−2 at 0.29 V versus a reversible hydrogen electrode (RHE) after post-treatment. The wafer-scale atomic-layer controlled band engineering of 2D fi ff PtSe2 thin- lm catalysts integrated with the Si light absorber provides an e ective way in the renewable energy application for

Downloaded via Chun-Wei Chen on March 3, 2021 at 01:49:08 (UTC). direct solar-to-hydrogen production.

KEYWORDS: atomic-layer-dependent electronic behavior, atomic layered catalyst, PtSe2/p-Si heterojunction, photoelectrochemical cell, solar-to-hydrogen conversion See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. hotoelectrochemical (PEC) conversion for direct solar- cathodes. Therefore, the effective integration of light absorbers to-hydrogen production has attracted great interest as a and catalysts at semiconductor/catalyst heterojunctions to renewable energy technology to store solar energy for achieve efficient carrier generation, charge transport, and P 1,2 hydrogen fuel. Silicon (Si) has been a promising photo- transfer is particularly important to fabricate an efficient PEC electrode candidate for photoelectrochemical (PEC) con- device for H2 generation using Si photocathode. version for direct solar-to-hydrogen because it consists of a Atomic-layer two-dimensional (2D) transition metal dichal- value of 1.1 eV to enable sufficient solar light cogenides (TMDs) have emerged as promsing catalysts for harvesting and exhibits a suitable band energy level for H − 2 generation.3 7 Nevertheless, because the bare Si surface has solar-driven HER on Si photocathodes. For example, the relatively low activity for hydrogen evolution reaction (HER), an effective catalyst which can enhance charge transfer Received: October 27, 2020 ffi Accepted: February 24, 2021 e ciencies and reduce the overpotential for H2 production is usually required.8 In addition to intrinsic catalytic activities, the surface coverage, morphology, transparency, and energy band structure of catalysts on the Si surface play crucial roles in ffi determining the PEC H2 conversion e ciency of Si photo-

© XXXX American Chemical Society https://dx.doi.org/10.1021/acsnano.0c08970 A ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article fi MoS2/Si heterojunction photocathodes present an outstanding has also been examined by annular-dark- eld (ADF) scanning PEC H production, where the overpotential at electrolyte/ transmission electron microscopy (STEM). Figure 1a shows 2 − solid interfaces can be largely reduced.9 11 The thin coating fi catalyst of MoS2 lms provides both catalytic activity and corrosion protection on the Si photocathode for HER. Recently, a discovered group-10 two-dimensional TMD − layered material, platinum selenide (PtSe ),12 19 also shows 2 − superior electrocatalytic activity for HER,16,19 21 compared to the most intensively studied group-6 TMD catalyst of 22,23 fi MoS2. 2D atomic layer PtSe2 thin lms exhibit the most prominent atomic-layer-dependent electronic behavior of “semiconductor-to-semimetal” transition as the thickness is increased from monolayer to bulk.12,14,16,24,25 The electro- catalytic activity of 2D layered PtSe2 was also found to be enhanced with an increased number of atomic layers, mainly attributed to increased conductivity.19 Nevertheless, most of the studies on PtSe -based catalysts for HER were only limited 2 − to the investigation as standalone electrocatalysts.16,19 21 Until now, solar-driven PEC H2 production based on the PtSe2 catalyst has not been reported yet. To fabricate an efficient Si photocathode for HER, another critical issue for pursuing an effective catalyst is to consider a suitable band alignment at the semiconductor/catalyst heterojunction to ensure efficient charge transfer and transport across the multiple interfaces.26 ffi In this work, we demonstrate an e cient and stable PEC H2 production based on 2D PtSe2/Si heterojunctions by system- atically controlling the number of atomic layers of 2D PtSe2 films from monolayer to bulk using the wafer-scale synthesis by chemical vapor deposition (CVD) growth technique. The 2D layered PtSe2/Si heterojunction photocathodes exhibit promis- ing PEC performance for H2 generation through atomic band fi engineering of PtSe2 thin lm catalysts integrated with the Si light absorber.

RESULTS AND DISCUSSION Figure 1. Wafer-scale PtSe2 synthesis via CVD process. (a) Preparations and Characterizations of Wafer-Scale Annular-dark-field (ADF) scanning transmission electron micros- ff fi 2D PtSe2 Thin Films with Di erent Atomic Layers. For copy (STEM) image of a PtSe2 thin lm. The Pt and Se sites were practical applications of PtSe /Si heterojunctions for PEC H marked by orange and red dots, respectively. (b) High-resolution 2 2 fi ∼ − fi TEM (HRTEM) image of PtSe2 thin lm with 10 20 nm domain generation, growing large-area and high-quality PtSe2 thin lms fi size. (c) Photographic image of the as-grown PtSe2 thin lms with with well-controlled layer thickness and uniformity is required. − fi various thicknesses ranging 0.8 3.0 nm. (d,e) Corresponding XPS To date, most reported PtSe2 lms were synthesized by direct spectra of the Pt 4f and the Se 3d core level peaks. selenization of predeposited Pt thin films via thermally assisted 27 fi conversion (TAC). Although large-area PtSe2 thin lms can be synthesized by direct selenization through the TAC the atomically resolved Z-contrast image of the PtSe2, where fi approach, it is still challenging to obtain PtSe2 lms thinner each Pt atom is surrounded by six Se atoms, indicating a 1T- than 4 nm, below which the “semiconductor-to-semimetal” phase octahedral structure with AA stacking. This is also the 24,25 24 transition is expected to occur. Accordingly, most PtSe2 most stable stacking structure as predicted theoretically and fi lms obtained from the direct selenization through the TAC consistent with the structure of PtSe2 bulk crystals synthesized approach usually exhibit semimetallic electronic behavior with by the CVT method.19 High-resolution TEM (HRTEM) 27 fl less optical transparency. Recently, PtSe2 akes with imaging shown in Figure 1b suggests that the CVD-grown ∼ fi ∼ − controlled thicknesses ranging from monolayer to 20 layers PtSe2 lms are polycrystalline, with a typical grain size of 10 can be directly synthesized by the modified chemical vapor 20 nm. The film thicknesses can be distinguished by their transport (CVT) method through adjusting the growth optical contrast and have been further identified by AFM 19 fi temperature and the amounts of reactants. However, the measurements (Figure S1). Large-area PtSe2 lms down to fl sizes of the CVT-grown PtSe2 atomic akes are still too small monolayer thickness with a root-mean-square (RMS) surface (∼10−20 μm) to form a continuous film, which may limit the roughness of only ∼0.2 nm can be grown by our CVD fabrications of large-area PtSe2/Si heterojunction photo- technique, which is not easy to achieve by using the fi cathodes for PEC H2 production. Here, wafer-scale PtSe2 conventional selenization of deposited Pt lms (Figure S2). fi fi lms were synthesized on c-plane sapphire substrates by Figure 1c shows the optical images of the PtSe2 lms on CVD using PtCl2 and Se powders as source precursors. By sapphire substrates with controlled thickness of 0.8, 1.4, 2.2, fi controlling the growth time and temperature, PtSe2 lms with and 3.0 nm, corresponding to monolayer (1L), two-layer (2L), ∼ fi thickness ranging from monolayer ( 0.8 nm) to bulk (21 nm) three-layer (3L), and four-layer (4L) PtSe2 thin lms, can be grown. The of the CVD-grown PtSe2 respectively. The chemical compositions of the CVD-grown

B https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article fi PtSe2 lms have been analyzed by X-ray photoelectron wavelength compared to those samples of 0.8 and 1.4 nm. The fi spectroscopy (XPS) shown in Figure 1d and e, where the Pt band gap values of these 2D PtSe2 lms were further evaluated α 1/2 4f and Se 3d core levels with the binding energies located at by using the Tauc plot method of ( hv) vs (hv-Eg) as shown α ffi 73.03 and 76.36 eV for the Pt 4f7/2 and 4f5/2, and 54.7 and 55.5 in Figure 2d and e, where is the absorption coe cient, hv is 28 eV for the Se 3d5/2 and 3d3/2, respectively, are consistent with the photon energy, and Eg is the optical band gap. The band 14−16,19 fi the reported spectral signatures of the PtSe2. The gap of PtSe2 lms decreases monotonically from 1.24 to 0.63 corresponding atomic ratio of Pt and Se was evaluated to be eV as the thickness is increased from 0.8 to 2.2 nm, while the ∼ fi 1:2, indicating the high quality of our CVD-grown PtSe2 thin 3.0 nm lm exhibits a semimetallic electronic behavior with a films (Figure S3). zero band gap according to the intersection of the Tauc plot. Figure 2a shows the layer-dependent Raman spectra of the For comparison, we further measured the optical transmittance fi ∼ fi PtSe2 lms. Two prominent vibrational modes at 180 and and the band gap value of 2D PtSe2 lm with a thickness of 4.0 nm as shown in the Supporting Information. The 4.0 nm film also exhibits a semimetallic electronic behavior with a zero band gap similar to that of the film thickness of 3.0 nm with a reduced transmittance due to an increased thickness. The decreasing band gap with the increased layer number clearly demonstrates the distinct signature of “semiconductor-to- ” semimetal transition of PtSe2, consistent with the theoretical band structure calculation results.12,19,24,25 p- Photoelectrochemical H2 Generation for 2D PtSe2/ fi Si Heterojunctions. The as-grown PtSe2 lms on sapphire substrates were transferred to p-Si substrates for the fabrication of PtSe2/p-Si heterojunction photocathodes for PEC H2 generation. Figure 3a briefly illustrates the transfer process;

Figure 2. Atomic-layer dependent optical properties of PtSe2 thin fi fi lms. (a) Raman spectra of as-grown PtSe2 thin lms on sapphire substrates. (b) The A1g/Eg intensity ratios and the red-shifted Eg peaks (inset) with increased PtSe2 thickness. (c) The FTIR and UV−vis-NIR (inset) spectra. (d) Tauc plot fittings and (e) the fi corresponding optical band gap values of the PtSe2 thin lms with various thickness.

∼207 cm−1 can be clearly observed accompanying one less intense peak at ∼230 cm−1 for these samples. The peaks at ∼ ∼ −1 180 and 207 cm are the in-plane Eg mode and the out-of- 14,17 plane A1g mode, respectively. The Eg peak exhibits a slight fi red shift as the PtSe2 lm thickness is increased (inset of Figure 2b). The relative peak intensity ratio of the A1g mode to the Eg mode increases with the increasing film thickness as shown in Figure 2b, due to an increased out-of-plane contribution caused by the increased van der Waals interaction with film Figure 3. (a) Schematic illustration of the fabrication process of p- 14,16,17 ffi PtSe2/ Si heterojunction photocathode. (b) Cross-sectional TEM thickness. To fabricate an e cient device for PEC H2 p- generation using the Si photocathode, an optimum trans- image of the PtSe2/ Si heterojunction photocathode. mittance at the catalyst layer is another important factor to ensure sufficient light harvesting at the underlying Si absorber the details are further described in the Experimental Section. layer, except its intrinsic HER catalytic activity. The trans- The supporting polymer of poly[methyl methacrylate] fi ff fi mittances of the PtSe2 thin lms with di erent thicknesses of (PMMA) was rst spin-coated on the as-grown PtSe2 thin fi fi 0.8, 1.4, 2.2, and 3.0 nm were measured using UV-vis-NIR lm. Afterward, the PMMA/PtSe2 thin lm was slowly spectroscopy and Fourier-transform infrared (FTIR) spectros- detached from the sapphire substrate by immersion in a copy over a broad wavelength range 400−5000 nm (0.25−3.1 NaOH solution and followed by rinsing in DI water (Step A). fl fi eV). Figure 2c shows the transmittance spectra of these PtSe2 Next, the oating PMMA/PtSe2 thin lm was transferred onto films in the NIR-IR (800−5000 nm) and the UV-vis (400− buffered oxide etchant (BOE) treated p-Si substrate and blown 800 nm) spectral ranges (see the inset of Figure 2c). It clearly by a nitrogen gun until the film was well-stuck on the targeted fi ° shows that the transmittance decreases with the lm thickness substrate. After baking at 90 C for 5 min, the PMMA/PtSe2 over the entire wavelength range. In particular, the 3.0 nm film thin film was rinsed in acetone solution to remove all the exhibits a much lower transmittance in the IR region as PMMA layer (Step B), giving a well-attached PtSe2/p-Si compared to the other three thinner samples. The 2.2 nm heterojunction photocathode structure. The backside contact sample exhibits the optical absorption edge at a longer was contacted to a copper tape and a defined area of the

C https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article

p- − Figure 4. Photoelectrochemical (PEC) measurements of the PtSe2/ Si heterojunction photocathodes. (a) Photocurrent density potential J−V ff fi ( ) curves and (b) summarized PEC performances for the photocathodes consisting of di erent PtSe2 thin lm thickness. (c) Tafel slopes of photocathodes shown as log(J) against overpotential vs RHE. (d) Electrochemical impedance spectroscopic (EIS) analyses of the p- corresponding photocathodes. (e) Stability test of the normalized limiting current density of the 2.2 nm PtSe2/ Si heterojunction photocathodes at −0.1 V and PEC HER performance before/after 24 h. photocathode was exposed for subsequent PEC measurements. of 0.22 V (vs RHE) and a current density of −24.8 mA/cm2 at We further performed cross-sectional transmission electron 0 V (vs RHE). Figure 4b summarizes the corresponding microscopy (TEM) analyses to examine the interface quality at overpotentials (vs RHE) at a photocurrent density of 1 mA/ 2 the PtSe2/Si heterojunctions. Figure 3b shows the cross- cm and the photocurrent densities at 0 V of the PtSe2/p-Si fi fi sectional bright- eld TEM image of the PtSe2/Si hetero- heterojunction photocathodes with various PtSe2 lm thick- fi junction device. The PtSe2 lm is well aligned parallel to the nesses. The PtSe2/Si heterojunction photocathode consisting fi substrate, indicating the formation of a PtSe2/Si hetero- of a 2.2 nm PtSe2 thin lm exhibits the best performance with junction. an onset potential of 0.27 V, a photocurrent of −28.1 mA/cm2 The photocathodic activities of the PtSe2/p-Si hetero- at 0 V, demonstrating its outstanding PEC performance for junction photocathodes for H2 generation were carried out HER. in a standard three-electrode system with a stirred solution We further conducted electrochemical impedance spectros- containing 1 M HClO4 electrolyte (pH = 0) under simulated copy (EIS) measurements to elucidate the charge carrier A.M 1.5 irradiation (100 mW/cm2). Figure 4a shows the resistance during the solar-to-hydrogen conversion among current density (J) as a function of potential (V) vs reversible various PtSe2/Si heterojunction photocathodes as shown in hydrogen electrode (RHE) for the PtSe2/p-Si heterojunction Figure 4d. The corresponding Nyquist impedance plots were ff fi photocathodes with di erent PtSe2 lm thicknesses: 0.8, 1.4, recorded under 1 sun irradiation at a bias near the onset 2.2, and 3.0 nm. For comparison, the PEC performance of a potentials of these photocathodes, reflecting the catalytic bare pristine Si is also shown. The referenced bare Si activities of the PEC devices during HER. The semicircle-like photocathode exhibits a saturated photocurrent of −28.7 Nyquist plot can be fitted by an equivalent circuit consisting of mA/cm2 and an onset potential of −0.24 V, which corresponds constant phase elements (CPEs) and charge-transfer resistan- 2 to the potential at a photocurrent density of 1 mA/cm .Itis ces (Rct) according to the reported model of catalyst- 9 found that a large potential still needs to be applied to trigger semiconductor system. Accordingly, Rct1 which corresponds the solar-to-hydrogen conversion for base p-Si due to its to the charge-transfer resistance from Si to PtSe2 and Rct2 inherently higher kinetic barrier for proton reduction. As the which corresponds to that from PtSe2 to the redox couple in fi PtSe2 thin lms with various thicknesses were transferred onto electrolyte can be deduced resepctively. Both charge-transfer p-Si substrates, all the onset potentials were shifted gradually resistances (Rct1, Rct2) of the photocathodes with 0.8, 1.4, 2.2, fi toward positive potential due to the lowering kinetic barrier of and 3.0 nm PtSe2 thin lms were evaluated to be (94.0, 13.1), photoexcited electron transfer from Si to the electrolyte, (60.3, 9.8), (50.8, 3.7), and (71.8, 7.6) Ω/cm2, respectively. ff fi indicating the catalytic e ect of PtSe2 thin lms. The PtSe2/p- The variations of both Rct1 and Rct2 of the photocathodes with ff Si heterojunction photocathodes exhibit a monotonic shift in di erent PtSe2 thicknesses agree well with the trend of their the onset potential toward more positive values of 0.19, 0.24, corresponding PEC performances for H2 generation. The fi and 0.27 V (vs RHE) and increased photocurrent densities of photocathode with a 2.2 nm PtSe2 thin lm exhibits the −20.9, −26.7, and −28.1 mA/cm2 at 0 V (vs RHE), as the smallest semicircle and the lowest charge-transfer resistances, fi thickness of PtSe2 thin lms is increased from 0.8, 1.4, to 2.2 representing the fastest electron shuttling from the PtSe2/Si nm, respectively. As the thickness of PtSe2 is further increased heterojunction photocathode to electrolyte during HER under to 3.0 nm, the performance of PtSe2/p-Si heterojunction solar illumination. The result is consistent with its best PEC photocathode begins to decline, exhibiting an onset potential performances for H2 production among all these PtSe2/Si

D https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article heterojunction photocathodes. Figure 4e shows the stability test of the normalized saturation current density at −0.1 V for fi the PtSe2/Si photocathode device with a 2.2 nm PtSe2 thin lm for 24 h. The inset of Figure 4e shows the linear sweep voltammogram (LSV) curves of the corresponding PEC device before and after 24 h of PEC H2 production. The saturated current is approximately 90% of the initial current after the first 10 h and remains ∼80% after the 24 h testing period of PEC H2 production. By contrast, the referenced pristine Si photocathode exhibits a decreased saturation current to only ∼20% of the initial current after 12 h under the same operation conditions (Figure S4). The result suggests that the formation of the PtSe2/Si heterojunction in PEC conversion not only exhibits enhanced photoelectrochemical activity but also significantly improves the operational stability of devices. Atomic-Layer Dependent Band Alignments at 2D p- PtSe2/ Si Heterojunctions. The above result clearly indicates that the PEC performance of the PtSe2/Si heterojunction photocathodes strongly depends on the atomic fi layer number or thickness of the 2D PtSe2 thin lms. According to previous reports, it is known that standalone electrocatalytic activity of 2D layered PtSe2 can be enhanced by the increased number of atomic layers as a result of Figure 5. Ultraviolet photoelectron spectroscopy (UPS) analyses. 16,19,20 ff ΔE E − E increased conductivity. Nevertheless, in addition to the (a) Work function values and (b) energy di erences = f v fi ff fi ff values of PtSe2 thin lms with di erent thicknesses. (c) The energy intrinsic catalytic activity of the PtSe2 thin lms, the e ective p- fi band diagram of Si, PtSe2 thin lms with a thickness of 0.8, 1.4, integration between the p-Si light absorber and the 2D + ∼− 2.2, and 3.0 nm and redox couple (2H /H2)( 4.5 eV). catalysts at PtSe2/Si heterojunctions is crucial to obtain high- ffi performance PEC devices for H2 production with e cient light harvesting, carrier generation, and charge transfer. Next, we the underlying p-Si substrate, the photogenerated electrons in have performed ultraviolet photoemission spectroscopy (UPS) the Si conduction band are expected to transport to the PtSe2/ to investigate the corresponding band alignments of p-Si with electrolyte interface for HER. For the heterostructured fi ff fi the PtSe2 thin lms of di erent thicknesses at the PtSe2/p-Si photocathode consisting of a monolayer PtSe2 thin lm (0.8 heterojunction. The work function can be determined from the nm), there exists a large energy barrier of ∼0.60 eV in the ff ff di erence between the cuto kinetic energy and the excitation conduction band at the PtSe2/p-Si interface. Although it is photon energy (hν = 21.2 eV) as shown in Figure 5a. The possible for electrons to tunnel through the monolayer PtSe fi 2 work functions of the PtSe2 thin lms with thicknesses of 0.8, thin film to the electrolyte, the charge transfer efficiency and 1.4, 2.2, and 3.0 nm are −4.32, −4.35, −4.43, and −4.31 eV, the corresponding photocurrent could be limited by the large respectively. All these samples exhibit similar work function energy barrier at the PtSe2/p-Si heterojunction. As the fi values. The valence band maximum (Ev) can be determined thickness of PtSe2 thin lm is further increased to 1.4 and from the cutoff of the lowest kinetic energy9,16,29 as shown in 2.2 nm, the energy barriers in the conduction band at the ff Δ Figure 5b, which corresponds to the energy di erence E = Ef PtSe2/p-Si interface are further reduced by a downward shift of − 9,16,29 ff Ev, where Ef is the Fermi level). The energy di erence the E of the PtSe thin films. This explains the monotonically Δ c 2 E for these samples are 0.21, 0.16, 0.12, and 0 eV. The values decreased charge-transfer resistances in the PtSe2/p-Si of conduction band minimum (Ec) can then be determined by heterojunction photocathodes as the PtSe2 thickness is adding the optical band gap values obtained in the above increased from 0.8 to 2.2 nm, as shown in Figure 4c. It is section. The corresponding band diagrams for the PtSe2 thin noted that although the optical band gap values deduced from films with different thicknesses are constructed, as shown in Tauc plots may be different from the quasiparticle band gap of Figure 5c. For comparison, the work function and the energy 2D atomic layer materials by further considering the exciton ff 30 di erence between the Ef and Ev of p-Si were also measured binding energy, the trend of reduced energy barriers in the fi (see the Supporting Information). The PtSe2 thin lms with conduction band at the PtSe2/p-Si heterojunctions with thicknesses of 0.8, 1.4, and 2.2 nm have their work function increased PtSe2 thicknesses is clearly observed. The con- fi positions located close to Ev and exhibit a p-type semi- duction band edge of the 2.2 nm PtSe2 thin lm exhibits the fi conductor behavior, while the 3.0 nm PtSe2 thin lm exhibits a optimum band alignment with that of Si for charge transfer, Δ fi semimetallic behavior with the E value equal to zero. It is compared to those of the 0.8 and 1.4 nm PtSe2 thin lms. For fi fi known that both n-type and p-type PtSe2 thin lms can be the 3.0 nm PtSe2 thin lm, although the band alignment at the obtained through controlling the growth condition of the Se interface of PtSe2/p-Si heterojunction may allow the photo- 13,15,21 fi precursor supply. The Ev values for the PtSe2 thin lms generated electrons at Si to be transferred to PtSe2, the lower with thicknesses of 0.8, 1.4, and 2.2 nm are quite close. By PEC performance of the device as compared to the 2.2 nm contrast, the corresponding Ec positions exhibit a large device is mainly caused by its intrinsic semimetallic nature with fi ffi variation with increased thickness of PtSe2 thin lms, as a zero band gap, leading to a reduced light harvesting e ciency ff result of their di erent band gap values. In the PtSe2/p-Si in the underlying Si substrate. This can be further evident from heterojunction photocathodes, as incident photons are trans- the PEC performance of the PtSe2/p-Si heterojunction fi mitted through catalytic ultrathin PtSe2 lms and absorbed by photocathode as the thickness of PtSe2 is further increased

E https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article to 4.0 nm as shown in Figure S6 (Supporting Information). CONCLUSION The observation of a drastically decreased PEC performance is In summary, we have demonstrated an efficient and stable PEC mainly attributed to the large attenuation of light for the 4.0 H production based on the 2D layered PtSe /Si hetero- nm PtSe thin film although the 4.0-nm-thick PtSe thin film 2 2 2 2 junction photocathodes. Interfacial band engineering at the exhibits a higher electrochemical (EC) HER activity PtSe /Si heterojunctions can be achieved by controlled (Supporting Information). Accordingly, the PEC performances 2 synthesis of PtSe thin films from monolayer to bulk using based on the PtSe /p-Si heterojunction photocathodes strongly 2 2 the wafer-scale CVD growth technique. Integrating the atomic- depend on the atomic layer numbers of PtSe thin films. By 2 layer dependent electronic band structures with the excellent controlling the atomic layer number, the PtSe /p-Si hetero- 2 catalytic activities of 2D PtSe thin films, the optimum 2D junction photocathode consisting of a 2.2 nm PtSe thin film 2 2 layered PtSe /Si heterojunction photocathode exhibits out- exhibits the outperformed PEC H production among all these 2 2 standing PEC performance for HER. The success of PtSe /Si heterojunction photocathodes, which has shown the 2 controlling the atomic-layer dependent electronic behaviors most effective integration with the Si light absorber by from semiconductor to semimetal of 2D PtSe thin films considering all the aspects of optical transparency, band 2 provides a great variety to adjust their band alignments with alignment, and catalytic activity. various semiconductors to form heterojunctions on the Enhanced Performance by Oxygen Plasma Treat- emerging energy conversion or optoelectronic applications. ments. Previously, it has been demonstrated that the HER catalytic activities of the monolayer pristine MoS2 can be effectively improved by introducing more active sites via the EXPERIMENTAL SECTION 31 fi formation of defects through plasma exposure or annealing. Growth of PtSe2. Large-area PtSe2 lms were grown on c-plane Here, we further employed the oxygen (O2) plasma post- sapphire substrates by CVD in a 1 in. tube furnace with one heating treatments to perform defect engineering on the 2.2 nm PtSe2 zone. High-purity Se (Aldrich, 99.99%) and PtCl2 (Alfa Aesar, thin film. Oxygen plasma treatments with various exposure 99.99%) powders were used as the source precursors. The Se powder was placed at the upstream, while the PtCl2 powder was placed in the periods were performed on these devices as shown in the fi Supporting Information (Figure S8). Figure 6 shows the middle of the heating zone. The sapphire substrates were rst cleaned by piranha solution and then loaded into the downstream of the − ° furnace. The PtSe2 growth was carried out at 350 450 C using argon as the carrier gas (60 sccm) under a base pressure of 20 Torr. The film thickness was controlled by the growth time (10−30 min) and the growth temperature. After growth, the furnace was cooled naturally to room temperature. Characterization of PtSe2. Raman spectra were measured using a 532 nm solid-state laser in the backscattering configuration under a home-built optical microscope equipped with a 100× objective lens (N.A. = 0.9). The Raman signals were analyzed by a grating monochromator (750 cm) and detected by a cooled CCD camera. fi Optical transmittance measurements of the PtSe2 thin lms with different thicknesses were measured over a broad wavelength range 400−5000 nm (0.25−3.1 eV). The spectra in visible range (400−800 fi nm) were measured on PtSe2 lms transferred onto quartz substrates p- using an UV-vis-NIR spectrometer (PerkinElmer LAMBDA 365). Figure 6. PEC performance and Tafel slope of 2.2 nm PtSe2/ Si − before and after O plasma post-treatment. The spectra in the IR range (800 5000 nm) were measured on PtSe2 2 films grown on double-side polished sapphire substrates using a FTIR spectrometer (Bruker VERTEX 70v). We used the measured transmittance spectra to deduce the absorption spectra α(hν). For current density (J) as a function of potential (V) vs RHE for semiconductors with an indirect gap, the optical absorption near the α ν ∝ ν 2 α the 2.2 nm PtSe2/p-Si heterojunction photocathodes without band gap exhibits the form: h (h -Eg) , where is the absorption ffi ν and with O2 plasma treatments. The device with exposure time coe cient, h is the photon energy, and Eg is the optical bandgap. We 28 of 60 s shows an improved PEC performance for H2 determined the optical bandgap by the Tauc plot method, i.e., production compared to the device without plasma treatments, extrapolating the linear region in the plot of (αhν)1/2 vs hν. For XPS fi − and UPS measurements, the PtSe2 lms were transferred onto ITO/ yielding an onset potential of 0.29 V, a photocurrent of 32.4 α mA/cm2 at 0 V, and the lowest Tafel slope of 49 mV/dec, quartz substrates. XPS spectra were measured using an Mg K X-ray source and calibrated using the C 1s peak. The UPS spectra were demonstrating its excellent PEC performance for HER. It is excited by the He I (21.2 eV) and He II (40.8 eV) photon lines and known that enhanced catalytic activity of 2D-TMD monolayer analyzed by a hemispherical analyzer with an energy resolution of 50 MoS2 achieved by defect engineering is mainly attributed to meV. The CVD-grown PtSe2 were transferred onto a Cu grid for the creation of S-vacancies, where the increased number of S- TEM characterizations. Scanning transmission electron microscopy vacancysitesmaystrengthenhydrogenadsorptionand (STEM) and HRTEM imaging were performed in a spherical increase the density of active sites.23,31 The presence of Se aberration-corrected transmission electron microscope (JEOL-ARM 200F) operating at 80 kV. vacancies in 2D PtSe2 after plasma post-treatments has also been reported recently because the anions (chalcogen) are Fabrication of PtSe2/Si Heterojunction Photocathode. The supporting polymer of poly[methyl methacrylate] (PMMA) was first easier to dissociate from the crystal than cations in typical fi 32 spin-coated on the as-grown PtSe2 thin lm at the speed of 4500 rpm TMD materials. The enhanced PEC H2 production of the for 40 s and then placed in the ambient environment to dry for 15 PtSe2/Si heterojunction photocathode through the post- min. Second, the tip of the plastic tweezer was utilized to scrape the treatment strategy using O2 plasma exposure is mainly four sides of the PMMA-coated sample for the following infiltration of attributed to the increased number of active sites in the 2D NaOH solution. Afterward, the PMMA/PtSe thin film was slowly fi 2 PtSe2 thin lm. detached from the sapphire substrate by immersion in a NaOH

F https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article solution and followed by 3 times wash in DI water. A p-type Si wafer Authors with a resistivity of 1−10 ohm−cm was cleaned in an ultrasonic bath Cheng-Chu Chung − Department of Materials Science and ff of acetone and isopropanol (IPA) followed by the bu ered oxide etch Engineering, National Taiwan University, Taipei 10617, (BOE) treatment for 2 min to remove the native oxide. The BOE Taiwan solution has a 6:1 volume ratio of 40% NH4F and 49% HF in water. fl fi Han Yeh − Department of Electrophysics, National Chiao The oating PMMA/PtSe2 thin lm was transferred onto BOE- treated p-Si substrate and blown by nitrogen gun until the film was Tung University, Hsinchu 30010, Taiwan well-attached on the targeted substrate. After baking on a hot plate at Po-Hsien Wu − Department of Materials Science and ° fi 90 C for 5 min, the PMMA/PtSe2 thin lm was rinsed in the acetone Engineering, National Taiwan University, Taipei 10617, solution to remove the PMMA layer, giving a well-attached PtSe2/p-Si Taiwan heterojunction structure. A backside electrode consisting of 7-nm- Cheng-Chieh Lin − International Graduate Program of thick Cr and 60-nm-thick Au was deposited through thermal vapor Molecular Science and Technology, National Taiwan evaporation. The backside contact was placed in contact with the fi University (NTU-MST), Taipei 10617, Taiwan; Molecular copper tape and a de ned area of the photocathode was exposed to Science and Technology Program, Taiwan International allow the subsequent PEC measurement. Oxygen Plasma Post-Treatment to PtSe Thin Film. The Graduate Program (TIGP), Academia Sinica, Taipei 11529, 2 Taiwan; orcid.org/0000-0003-1895-493X PtSe2 sample was placed in a glass dish and exposed to a pressure of 70 Torr pure oxygen (99.99%) atmosphere in a plasma cleaner Chia-Shuo Li − Graduate Institute of Photonics and chamber. The surface modification was then carried out under 30 W Optoelectronics, National Taiwan University, Taipei 10617, oxygen plasma with various treatment period. Taiwan; orcid.org/0000-0002-6735-3228 Photo-Electrochemical Measurements. The photoelectro- Tien-Tien Yeh − Department of Electrophysics, National chemical performance of bare Si and PtSe2/p-Si were analyzed in 1 Chiao Tung University, Hsinchu 30010, Taiwan M HClO4 (Sigma-Aldrich, 70% solution in water) electrolyte in a Yi Chou − Department of Electrophysics, National Chiao three-electrode system, where silicon, Pt wire, and Ag/AgCl with Tung University, Hsinchu 30010, Taiwan saturated KCl solution were served as working, counter, and reference Chuan-Yu Wei − Department of Materials Science and electrode, respectively. The electrolyte was a 1 M HClO4 solution with pH of 0 (Sigma-Aldrich, 70% solution in water). Linear sweep Engineering, National Taiwan University, Taipei 10617, voltammetry (LSV) and chronoamperometry measurements were Taiwan performed under 1 sun irradiation (100 mW/cm2, AM 1.5 G) and the Cheng-Yen Wen − Department of Materials Science and data were recorded by Autolab PGSTAT302N station. J−V curves Engineering, National Taiwan University, Taipei 10617, ranged from +0.3 V to −1.0 V versus Ag/AgCl (saturated KCl) at a Taiwan; International Graduate Program of Molecular scan rate of 0.1 V/s were measured during the LSV measurement. For Science and Technology, National Taiwan University (NTU- the electrochemical impedance spectroscopy (EIS) measurement, the MST), Taipei 10617, Taiwan; orcid.org/0000-0002- working electrode was applied for a constant potential of +0.17 V vs 9788-4329 RHE in a sweeping frequency ranging from 100 kHz to 0.1 Hz with an − AC amplitude of 10 mV. Yi-Chia Chou Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan; orcid.org/ ASSOCIATED CONTENT 0000-0002-7775-2927 Chih-Wei Luo − Department of Electrophysics, National *sı Supporting Information Chiao Tung University, Hsinchu 30010, Taiwan; The Supporting Information is available free of charge at orcid.org/0000-0002-6453-7435 https://pubs.acs.org/doi/10.1021/acsnano.0c08970. Chih-I Wu − Graduate Institute of Photonics and fi fi OM images of PtSe2 thin lms, AFM identi cation of Optoelectronics, National Taiwan University, Taipei 10617, fi surface roughness of PtSe2 lms, XPS measurements of Taiwan; orcid.org/0000-0003-3613-7511 fi PtSe2 lms, charge-transfer resistance of PtSe2/p-Si Ming-Yang Li − Corporate Research, Taiwan Semiconductor ff photocathode, HER activities of PtSe2 with di erent Manufacturing Company (TSMC), Hsinchu 30075, Taiwan thicknesses, UPS of p-Si, stability measurement of bare Lain-Jong Li − Corporate Research, Taiwan Semiconductor p-Si photocathode under a voltage of −0.1 V, PEC Manufacturing Company (TSMC), Hsinchu 30075, fi performance of 2.2 nm PtSe2/p-Si modi ed by O2 Taiwan; orcid.org/0000-0002-4059-7783 plasma post-treatments (PDF) Complete contact information is available at: https://pubs.acs.org/10.1021/acsnano.0c08970 AUTHOR INFORMATION Corresponding Authors Author Contributions △ Wen-Hao Chang − Department of Electrophysics and Center C.C.C and H.Y. contributed equally to this work. for Emergent Functional Matter Science (CEFMS), National Notes Chiao Tung University, Hsinchu 30010, Taiwan; Research fi Center for Applied Sciences, Academia Sinica, Taipei 11529, The authors declare no competing nancial interest. Taiwan; orcid.org/0000-0003-4880-6006; Email: [email protected] ACKNOWLEDGMENTS Chun-Wei Chen − Department of Materials Science and The authors would like to thank the financial support from the Engineering, National Taiwan University, Taipei 10617, Minister of Science and Technology (MOST), Taiwan and Taiwan; International Graduate Program of Molecular Taiwan Consortium of Emergent Crystalline Materials Science and Technology, National Taiwan University (NTU- (TCECM), (Project No. 107-2119-M-002-028-MY2, 107- MST), Taipei 10617, Taiwan; Center of Atomic Initiative for 2112-M-002-024 -MY3, 107-2112-M-009-024-MY3 and 108- New Materials (AI-MAT), National Taiwan University 2119-M-009-011-MY3). In addition, financial support by the (NTU), Taipei 10617, Taiwan; orcid.org/0000-0003- Center of Atomic Initiative for New Materials (AI-MAT), 3096-249X; Email: [email protected] National Taiwan University, from the Featured Areas Research

G https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article

Center Program within the framework of the Higher Education (15) Xu, H.; Zhang, H. M.; Liu, Y. W.; Zhang, S. M.; Sun, Y. Y.; Sprout Project by the Ministry of Education in Taiwan Guo, Z. X.; Sheng, Y. C.; Wang, X. D.; Luo, C.; Wu, X.; Wang, J. L.; (108L9008) is acknowledged. Financial support from the Hu, W. D.; Xu, Z. H.; Sun, Q. Q.; Zhou, P.; Shi, J.; Sun, Z. Z.; Zhang, Center for Emergent Functional Matter Science (CEFMS) of D. W.; Bao, W. Z. Controlled Doping of Wafer-Scale PtSe2 Films for Device Application. Adv. Funct. Mater. 2019, 29, 1805614. National Chiao Tung University supported by the Ministry of (16) Shi, J. P.; Huan, Y. H.; Hong, M.; Xu, R. Z.; Yang, P. F.; Zhang, Education of Taiwan is also acknowledged. Z. P.; Zou, X. L.; Zhang, Y. F. Chemical Vapor Deposition Grown Large-Scale Atomically Thin with Semimetal- REFERENCES Semiconductor Transition. ACS Nano 2019, 13, 8442−8451. (17) Zhao, Y. D.; Qiao, J. S.; Yu, Z. H.; Yu, P.; Xu, K.; Lau, S. P.; (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Zhou, W.; Liu, Z.; Wang, X. R.; Ji, W.; Chai, Y. High-Electron- Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. − Mobility and Air-Stable 2D Layered PtSe2 FETs. Adv. Mater. 2017, Chem. Rev. 2010, 110, 6446 6473. 29, 1604230. (2) Bolton, J. R.; Strickler, S. J.; Connolly, J. S. Limiting and (18) Yim, C.; McEvoy, N.; Riazimehr, S.; Schneider, D. S.; Gity, F.; Realizable Efficiencies of Solar Photolysis of Water. Nature 1985, 316, − Monaghan, S.; Hurley, P. K.; Lemme, M. C.; Duesberg, G. S. Wide 495 500. Spectral Photoresponse of Layered Platinum Diselenide-Based (3) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Photodiodes. Nano Lett. 2018, 18, 1794−1800. Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using (19) Hu, D. K.; Zhao, T. Q.; Ping, X. F.; Zheng, H. S.; Xing, L.; Liu, Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science X. Z.; Zheng, J. Y.; Sun, L. F.; Gu, L.; Tao, C. G.; Wang, D.; Jiao, L. Y. 2011, 334, 645−648. Unveiling the Layer-Dependent Catalytic Activity of PtSe2 Atomic (4) Boettcher, S. W.; Spurgeon, J. M.; Putnam, M. C.; Warren, E. L.; Crystals for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. Turner-Evans, D. B.; Kelzenberg, M. D.; Maiolo, J. R.; Atwater, H. A.; 2019, 58, 6977−6981. Lewis, N. S. Energy-Conversion Properties of Vapor-Liquid-Solid- − (20) Lin, S. H.; Liu, Y.; Hu, Z. X.; Lu, W.; Mak, C. H.; Zeng, L. H.; Grown Silicon Wire-Array Photocathodes. Science 2010, 327, 185 Zhao, J.; Li, Y. Y.; Yan, F.; Tsang, Y. H.; Zhang, X. M.; Lau, S. P. 187. Tunable Active Edge Sites in PtSe2 Films towards Hydrogen (5) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Evolution Reaction. Nano Energy 2017, 42,26−33. Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 Coatings Stabilize (21) Chia, X. Y.; Adriano, A.; Lazar, P.; Sofer, Z.; Luxa, J.; Pumera, Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science M. Layered Platinum Dichalcogenides (PtS , PtSe , and PtTe ) − 2 2 2 2014, 344, 1005 1009. Electrocatalysis: Monotonic Dependence on the Chalcogen Size. Adv. (6) Chen, Y. W.; Prange, J. D.; Duhnen, S.; Park, Y.; Gunji, M.; Funct. Mater. 2016, 26, 4306−4318. Chidsey, C. E. D.; McIntyre, P. C. Atomic Layer-Deposited Tunnel (22) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Oxide Stabilizes Silicon Photoanodes for Water Oxidation. Nat. Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Mater. 2011, 10, 539−544. Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science (7) Ji, L.; McDaniel, M. D.; Wang, S. J.; Posadas, A. B.; Li, X. H.; 2007, 317, 100−102. Huang, H. Y.; Lee, J. C.; Demkov, A. A.; Bard, A. J.; Ekerdt, J. G.; Yu, (23) Li, H.; Tsai, C.; Koh, A. L.; Cai, L. L.; Contryman, A. W.; E. T. A Silicon-Based Photocathode for Water Reduction with an Fragapane, A. H.; Zhao, J. H.; Han, H. S.; Manoharan, H. C.; Abild- Epitaxial SrTiO3 Protection Layer and a Nanostructured Catalyst. Pedersen, F.; Norskov, J. K.; Zheng, X. L. Activating and Optimizing Nat. Nanotechnol. 2015, 10,84−90. MoS2 Basal Planes for Hydrogen Evolution through the Formation of (8) Khaselev, O.; Turner, J. A. A Monolithic Photovoltaic- Strained Sulphur Vacancies. Nat. Mater. 2016, 15,48−53. Photoelectrochemical Device for Hydrogen Production via Water (24) Kandemir, A.; Akbali, B.; Kahraman, Z.; Badalov, S. V.; Ozcan, − Splitting. Science 1998, 280, 425 427. M.; Iyikanat, F.; Sahin, H. Structural, Electronic and Phononic (9) Kwon, K. C.; Choi, S.; Hong, K.; Moon, C. W.; Shim, Y. S.; Kim, Properties of PtSe2: From Monolayer to Bulk. Semicond. Sci. Technol. D. H.; Kim, T.; Sohn, W.; Jeon, J. M.; Lee, C. H.; Nam, K. T.; Han, 2018, 33, 085002. S.; Kim, S. Y.; Jang, H. W. Wafer-Scale Transferable Molybdenum (25) Villaos, R. A. B.; Crisostomo, C. P.; Huang, Z. Q.; Huang, S. Sisulfide Thin-Film Catalysts for Photoelectrochemical Hydrogen M.; Padama, A. A. B.; Albao, M. A.; Lin, H.; Chuang, F. C. Thickness Production. Energy Environ. Sci. 2016, 9, 2240−2248. Dependent Electronic Properties of Pt Dichalcogenides. Npj 2D (10) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Mater. Appl. 2019, 3,2. Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for (26) McKone, J. R.; Lewis, N. S.; Gray, H. B. Will Solar-Driven Solar Hydrogen Production Using Molybdenum Sulfide Nanomateri- Water-Splitting Devices See the Light of Day? Chem. Mater. 2014, 26, als. Adv. Energy Mater. 2014, 4, 1400739. 407−414. (11) Ding, Q.; Meng, F.; English, C. R.; Caban-Acevedo, M.; (27) Yim, C.; Lee, K.; McEvoy, N.; O’Brien, M.; Riazimehr, S.; Shearer, M. J.; Liang, D.; Daniel, A. S.; Hamers, R. J.; Jin, S. Efficient Berner, N. C.; Cullen, C. P.; Kotakoski, J.; Meyer, J. C.; Lemme, M. Photoelectrochemical Hydrogen Generation Using Heterostructures C.; Duesberg, G. S. High-Performance Hybrid Electronic Devices of Si and Chemically Exfoliated Metallic MoS2. J. Am. Chem. Soc. from Layered PtSe2 Films Grown at Low Temperature. ACS Nano 2014, 136, 8504−8507. 2016, 10, 9550−9558. (12) Wang, Y. L.; Li, L. F.; Yao, W.; Song, S. R.; Sun, J. T.; Pan, J. B.; (28) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Ren, X.; Li, C.; Okunishi, E.; Wang, Y. Q.; Wang, E. Y.; Shao, Y.; Electronic Structure of Amorphous Germanium. Phys. Status Solidi B Zhang, Y. Y.; Yang, H. T.; Schwier, E. F.; Iwasawa, H.; Shimada, K.; 1966, 15, 627−637. Taniguchi, M.; Cheng, Z. H.; Zhou, S. Y.; et al. Monolayer PtSe2,a (29) Kwon, K. C.; Choi, S.; Lee, J.; Hong, K.; Sohn, W.; Andoshe, D. New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially M.; Choi, K. S.; Kim, Y.; Han, S.; Kim, S. Y.; Jang, H. W. Drastically Grown by Direct Selenization of Pt. Nano Lett. 2015, 15, 4013−4018. Enhanced Hydrogen Evolution Activity by 2D to 3D Structural (13) Wang, Z. G.; Li, Q.; Besenbacher, F.; Dong, M. D. Facile Transition in Anion-Engineered Molybdenum Disulfide Thin Films SynthesisofSingleCrystalPtSe2 Nanosheets for Nanoscale for Efficient Si-Based Water Splitting Photocathodes. J. Mater. Chem. Electronics. Adv. Mater. 2016, 28, 10224−10229. A 2017, 5, 15534−15542. (14) Yan, M. Z.; Wang, E. Y.; Zhou, X.; Zhang, G. Q.; Zhang, H. Y.; (30) Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J. Q.; Zhang, K. N.; Yao, W.; Lu, N. P.; Yang, S. Z.; Wu, S. L.; Yoshikawa, Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; T.; Miyamoto, K.; Okuda, T.; Wu, Y.; Yu, P.; Duan, W. H.; Zhou, S. Cobden, D. H.; Xu, X. D. Electrically Tunable Excitonic Light- Y. High Quality Atomically Thin PtSe2 Films Grown by Molecular Emitting Diodes Based on Monolayer WSe2 p-n Junctions. Nat. Beam Epitaxy. 2D Mater. 2017, 4, 045015. Nanotechnol. 2014, 9, 268−272.

H https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article

(31) Ye, G. L.; Gong, Y. J.; Lin, J. H.; Li, B.; He, Y. M.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097−1103. (32) Yu, X. C.; Yu, P.; Wu, D.; Singh, B.; Zeng, Q. S.; Lin, H.; Zhou, W.; Lin, J. H.; Suenaga, K.; Liu, Z.; Wang, Q. J. Atomically Thin Noble Metal Dichalcogenide: A Broadband Mid-Infrared Semi- conductor. Nat. Commun. 2018, 9, 1545.

I https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX