Direct Selective Epitaxy of 2D Sb2Te3 onto Monolayer WS2 for Vertical p–n Heterojunction Photodetectors

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb2Te3, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p–n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb2Te3 onto inert mica substrates, providing valuable insights into the integration of Sb2Te3 with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb2Te3 on pre-existing WS2 surfaces on SiO2/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb2Te3/WS2 heterojunctions. Finally, the development of 2D Sb2Te3/WS2 optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively.


Introduction
Two-dimensional materials are pivotal in advancing the miniaturization of electronic devices to enhance Moore's Law, owing to their unique atomic-scale thickness [1,2].The van der Waals interlayer interaction in these materials overcomes lattice-matching challenges faced in conventional heterojunctions [3], enabling a "Lego-like" stacking approach in layered heterostructures, which serves as an excellent research platform in optics [4], electronics [5], magnetism [6,7], and other fields [8].
Two-dimensional transition metal dichalcogenides (2D-TMDs) are distinguished among the materials investigated due to their layered structure, which includes appropriate bandgaps and chemical stability [9,10].These characteristics render them highly suitable for optoelectronic device fabrication.In particular, the p-n heterojunction is a crucial structure in photodetectors, as it facilitates efficient separation of photogenerated electrons.Intrinsic two-dimensional transition metal chalcogenides are primarily n-type semiconductors, with p-type semiconductors being less common.For example, monolayer PtSe 2 is a p-type semiconductor with an indirect band gap of 1.16 eV [11][12][13].To achieve high-performance photodetectors, this work introduces Sb 2 Te 3 , a p-type semiconductor with a direct band gap of 0.33 eV [14,15], to construct p-n heterojunctions with n-type two-dimensional transition metal chalcogenides.Nevertheless, the current approach to Nanomaterials 2024, 14, 884 2 of 11 building novel two-dimensional layered material heterostructures primarily relies on the repetitive and inefficient mechanical exfoliation process [16], which is not conducive to mass industrial production.
Considered a method with potential for large-scale, controllable fabrication of 2D-TMD [2], chemical vapor deposition (CVD) is compatible with existing semiconductor processes and can produce high-quality crystals.The synthesis of two-dimensional transition metal dichalcogenide heterostructures via CVD involves both one-step and two-step processes.For instance, Jiao et al. demonstrated a one-step procedure wherein they sulfurized WO 3−x /MoO 3−x core-shell nanowires, resulting in vertically stacked MoS 2 /WS 2 heterojunctions [17].In contrast, a two-step CVD method, as employed by Duan et al., involves initial laser etching to create nucleation sites on large-area WSe 2 surfaces, followed by epitaxial growth of VSe 2 to produce arrayed VSe 2 /WSe 2 vertical heterojunctions [18].During the synthesis of two-dimensional transition metal chalcogenide heterostructures, the one-step method can easily lead to atomic intermixing within the heterostructures [19].In contrast, the two-step method requires careful avoidance of surface contamination on the two-dimensional materials produced in the first step to prevent interference with the epitaxial growth of the two-dimensional transition metal chalcogenides in the second step [20,21].
To date, there have been no reports on the preparation of Sb 2 Te 3 /WS 2 p-n heterojunctions using CVD.Therefore, this paper attempts to fabricate Sb 2 Te 3 /WS 2 p-n heterojunctions using a two-step chemical vapor deposition method.Initially, to explore the preparation conditions for two-dimensional Sb 2 Te 3 , we achieved large-scale fabrication on inert mica substrates as a reference for constructing related Sb 2 Te 3 heterostructures.Due to differences in surface migration barriers, van der Waals layered materials tend to selectively epitaxially grow on surfaces with lower migration barriers [22][23][24].Therefore, by exploiting the relatively low migration barrier of Sb 2 Te 3 on WS 2 surfaces, this work adopts a two-step chemical vapor deposition strategy to selectively epitaxially grow Sb 2 Te 3 on pre-prepared WS 2 on SiO 2 /Si substrates, achieving the assembly of Sb 2 Te 3 /WS 2 heterojunctions.
Lastly, the two-dimensional Sb 2 Te 3 /WS 2 heterostructures were utilized in the assembly of photodetectors, achieving in rapid response times of a 302-microsecond rise time and 503-microsecond decay time.This research provides valuable insights into constructing heterojunctions involving Sb 2 Te 3 and other two-dimensional layered materials, contributing to the advancement of high-performance photodetectors.Furthermore, given that two-dimensional materials with atomic-level thickness exhibit excellent mechanical flexibility [25] and high integration, they hold potential applications in industrial flexible robotic sensors.

Synthesis of 2D-Sb 2 Te 3
To synthesize 2D-Sb 2 Te 3 , a quartz boat containing 50 mg of Sb 2 Te 3 powder was placed at the center of a single-zone tube furnace (with a quartz tube diameter of 1.0 inch).A mica substrate was placed 5 cm away from the center of the boat.Prior to heating, the system underwent a 30-min purge with Ar gas; once heating commenced, the Ar flow rate was set to 50 standard cubic centimeters per minute (sccm).Subsequently, the temperature was increased at a rate of 25 • C/min up to 650 • C and maintained for 2 min.Ultimately, the quartz tube was swiftly cooled to room temperature, and the sample was extracted.

Synthesis of Monolayer WS 2
To synthesize monolayer WS 2 , the 280 nm SiO 2 /Si underwent a sequential treatment involving ultrasonic in acetone, isopropanol, and deionized water, each for 30 min.Subsequent to the ultrasonic treatment, the SiO 2 /Si was dried using a nitrogen gun, annealed in a muffle furnace at 600 • C for a duration of four hours, allowed to cool naturally to room temperature, and then set aside.Following this, 2.5 mg of WO 3 powder was precisely measured and uniformly dispersed on a custom-made graphite trough.A SiO 2 /Si substrate measuring 1.5 cm × 1.0 cm was delicately positioned 0.5 cm above the WO 3 powder in the trough.Additionally, 300 mg of sulfur powder was weighed and placed in a quartz trough fitted with a magnetic pull rod.The graphite trough containing SiO 2 /Si substrate and the quartz boat containing sulfur powder were positioned at the center and outside the heating region of a one-inch tube furnace.The quartz tube was purged with argon for 30 min to eliminate air and moisture.The argon flow rate was adjusted to 40 sccm, and the temperature was gradually increased at a rate of 25 • C/min up to 900 • C. Upon reaching the specified temperature, the sulfur powder was shifted to an area close to 380 • C, underwent a 5-min reaction, followed by the removal of the sulfur powder, rapid cooling to room temperature, and retrieval of the sample.

Synthesis of 2D-Sb 2 Te 3 /WS 2 Heterostructure
For the fabrication of the 2D-Sb 2 Te 3 /WS 2 heterostructure, a precise amount of 50 mg Sb 2 Te 3 powder was positioned within a quartz boat placed at the central temperature zone of a single-zone tube furnace (with a quartz tube diameter of 1.0 inch).Concurrently, a SiO 2 /Si substrate, coated with a monolayer of WS 2 flakes, was strategically placed 5 cm away from the center of the quartz tube.Before initiating the heating process, the system underwent a thorough 30-min Ar purging, followed by the adjustment of the Ar flow rate to 50 sccm.The temperature was then steadily increased at a rate of 25 • C/min until reaching 650 • C, where it was held for 2 min.Ultimately, the quartz tube was cooled rapidly to ambient temperature, and the sample was extracted.

Device Fabrication and Testing
For the fabrication of optoelectronic devices, standard e-beam lithography and metal thermal evaporation techniques were used to define electrodes Cr/Au (10/50 nm).Optoelectronic measurements were conducted with the probe station CRX-6.5Kfrom Lake Shore and the semiconductor parameter analyzer Keithley 4200 SCS (Cleveland, OH, USA) at room temperature under a 532 nm laser.Laser power density was collected with a powermeter (Thorlabs GmbH., PM 100D, Dachau, Germany).

Characterization
The morphology of the samples was examined using optical microscopy (OM, Carl Zeiss Microscopy GmbH, Jena, Germany) and scanning electron microscopy (SEM, Nova Nano SEM 200 FEI, Hillsboro, OR, USA).Their chemical compositions were assessed through Raman and photoluminescence (PL) spectra at room temperature.Raman spectra were obtained using a micro confocal Raman/PL spectrometer (Renishaw in Via, Gloucestershire, UK) with an excitation laser line operating at 532 nm.Flake thickness measurements were conducted using atomic force microscopy (AFM) with a Dimension Icon system from Bruker, San Diego, CA, USA.The crystallinity quality and chemical composition of the Sb 2 Te 3 and Sb 2 Te 3 /WS 2 flakes were investigated using transmission electron microscopy (TEM, JEOL 2100F, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) analysis performed on a PHI 5000 VP III instrument, Woodbury, MN, USA.
The band structure of monolayer WS 2 and the band structure of 2D Sb 2 Te 3 were calculated separately based on their respective structures.A kinetic cutoff energy of 450 eV was chosen for the planewave basis set.The valence electron configurations for W (5p 6 6s 2 5d 4 ), S (3s 2 3p 4 ), Sb (5s 2 5p 3 ), Te (5s 2 5p 4 ) were employed.The first Brillouin zone was characterized by a Γ-point-centered Monkhorst-Pack k-mesh with a grid configuration of 6 × 6 × 4. The energy convergence criterion was set at 1.0 × 10 −4 eV, for both structural optimizations and self-consistent-field (SCF) iteration.The force components convergence criterion was set at −0.02 eV/Å, with the charge density symmetrization employed.This paper describes the fabrication process of Sb 2 Te 3 /WS 2 heterostructures utilizing a two-step chemical vapor deposition method.Initially, we successfully prepared large-area monolayer WS 2 on SiO 2 /Si substrate.Figure S1a shows an optical image of monolayer WS 2 flakes uniformly distributed on SiO 2 /Si substrate.Additionally, a single WS 2 flake was subjected to AFM, revealing a measured thickness of approximately 0.83 nm, consistent with reported monolayer WS 2 thicknesses, as shown in Figure S1b [26].Moreover, Raman spectroscopy was performed on the obtained WS 2 .Figure S1c presents the Raman characteristic peaks of monolayer WS 2 , where 349.8 cm −1 represents the in-plane vibrational peak and 419.2 cm −1 represents the out-of-plane vibrational peak of WS 2 [27].Photoluminescence (PL) testing of the WS 2 showed a strong 632 nm PL peak, consistent with previous findings of monolayer WS 2 [28].Therefore, the uniformly distributed flakes on the SiO 2 /Si substrate were confirmed to be monolayer WS 2 .

Results and Discussion
Subsequently, to explore the fabrication conditions for two-dimensional Sb 2 Te 3 , mica substrates, which lack dangling bonds similar to monolayer WS 2 surfaces, were utilized.Figure S2 shows optical images of two-dimensional Sb 2 Te 3 at different positions on a 1.5 cm × 1.5 cm mica substrate.Figure S3a presents an optical image of thin-layer Sb 2 Te 3 crystals, while Figure S3b shows an AFM image of Sb 2 Te 3 with a measured height of 3.8 nm corresponding to four layers of Sb 2 Te 3 [14].Figure S3c shows the Raman spectrum of Sb 2 Te 3 highlighting the characteristic Raman peaks A 1 1g (69.8 cm −1 ), E 2 g (112.1 cm −1 ), and A 2 1g (164.9 cm −1 ) [29].A low-resolution TEM image of two-dimensional Sb 2 Te 3 is depicted in Figure S4a. Figure S4b illustrates a high-resolution image, with a 0.22 nm interplanar spacing, consistent with the reported (100) plane spacing of Sb 2 Te 3 [14].Figure S4c displays a selected-area electron diffraction pattern of two-dimensional Sb 2 Te 3 , consistent with the reported hexagonal pattern.Finally, energy-dispersive spectroscopy testing of the crystals, as shown in Figure S4e,f, revealed the uniform distribution of Sb and Te elements within the flake.Thus, the flakes formed on the mica substrate were identified as large-area, uniformly distributed two-dimensional Sb 2 Te 3 .
The preparation process for the Sb 2 Te 3 /WS 2 vertical heterostructure involved substituting the mica substrate with a single layer of WS 2 flakes grown on a SiO 2 /Si substrate.Figure 1a illustrates the schematic of the synthesis of Sb 2 Te 3 /WS 2 vertical heterostructures utilizing a two-step chemical vapor deposition (CVD) method.During this process, Sb 2 Te 3 powders were placed at the central end of the quartz reactor, while a SiO 2 /Si substrate with pre-fabricated WS 2 flakes was positioned downstream.The thermal evaporation of Sb 2 Te 3 powders enabled the vertical epitaxial growth of Sb 2 Te 3 over the monolayer WS 2 (detailed growth process described in Section 2.4.Synthesis of 2D-Sb 2 Te 3 /WS 2 Heterostructure).The growth dynamics of Sb 2 Te 3 /WS 2 vertical heterostructures were meticulously examined.As depicted in Figure 1b, Sb 2 Te 3 selectively grew on the surface of WS 2 .The pronounced optical contrast vividly illustrates the morphological evolution of Sb 2 Te 3 /WS 2 grains, progressing from incomplete to complete coverage with increasing growth time from 0 to 90 s.Representative SEM micrographs of pure WS 2 samples and Sb 2 Te 3 /WS 2 heterostructures are shown in Figure 1c and Figure 1d, respectively.The SEM micrograph depicts a typical vertically arranged Sb 2 Te 3 /WS 2 heterojunction, with Sb 2 Te 3 partially cov-ering the underlying WS 2 layer.Evidently, Sb 2 Te 3 flakes tend to deposit on the WS 2 surface, with minimal residual sediment observed on the SiO 2 /Si substrate.AFM was utilized for sample characterization, as illustrated in Figure 1e,f.The thickness measurements of WS 2 and Sb 2 Te 3 at 0.857 nm and 1.598 nm, respectively, confirm a monolayer structure for WS 2 and a bilayer structure for Sb 2 Te 3 , termed as 2QL (with each quintuple layer (QL) in Sb 2 Te 3 arranged in a Te-Sb-Te-Sb-Te sequence) [30,31].Additionally, the analysis of the heterostructure morphology demonstrates the uniform surface quality of both Sb 2 Te 3 and WS 2 .
ture).The growth dynamics of Sb2Te3/WS2 vertical heterostructures were meticulously ex-amined.As depicted in Figure 1b, Sb2Te3 selectively grew on the surface of WS2.The pronounced optical contrast vividly illustrates the morphological evolution of Sb2Te3/WS2 grains, progressing from incomplete to complete coverage with increasing growth time from 0 to 90 s.Representative SEM micrographs of pure WS2 samples and Sb2Te3/WS2 heterostructures are shown in Figure 1c and Figure 1d, respectively.The SEM micrograph depicts a typical vertically arranged Sb2Te3/WS2 heterojunction, with Sb2Te3 partially covering the underlying WS2 layer.Evidently, Sb2Te3 flakes tend to deposit on the WS2 surface, with minimal residual sediment observed on the SiO2/Si substrate.AFM was utilized for sample characterization, as illustrated in Figure 1e,f.The thickness measurements of WS2 and Sb2Te3 at 0.857 nm and 1.598 nm, respectively, confirm a monolayer structure for WS2 and a bilayer structure for Sb2Te3, termed as 2QL (with each quintuple layer (QL) in Sb2Te3 arranged in a Te-Sb-Te-Sb-Te sequence) [30,31].Additionally, the analysis of the heterostructure morphology demonstrates the uniform surface quality of both Sb2Te3 and WS2.The atomic arrangement of the vertically stacked Sb2Te3/WS2 van der Waals heterojunction was further investigated using TEM coupled with energy-dispersive spectroscopy (EDS).Figure 2a displays a low-magnification TEM image of the as-transferred Sb2Te3/WS2 sample on a copper grid.Figure 2b presents an elemental distribution map of Te, Sb, S, and W originating from the marked sample region in Figure 2a.Notably, W and S elements exhibit uniform distribution throughout the entire area, whereas Sb and Te elements predominantly concentrate within the dark triangular region.Figure S5 presents the EDS spectra obtained from Figure 2a.The high-resolution TEM image in Figure 2c focuses on the region delineated by the red dashed line in Figure 2a.The contrasting regions reveal a lattice spacing of approximately 0.27 nm on the right, corresponding to the The atomic arrangement of the vertically stacked Sb 2 Te 3 /WS 2 van der Waals heterojunction was further investigated using TEM coupled with energy-dispersive spectroscopy (EDS).Figure 2a displays a low-magnification TEM image of the as-transferred Sb 2 Te 3 /WS 2 sample on a copper grid.Figure 2b presents an elemental distribution map of Te, Sb, S, and W originating from the marked sample region in Figure 2a.Notably, W and S elements exhibit uniform distribution throughout the entire area, whereas Sb and Te elements predominantly concentrate within the dark triangular region.Figure S5 presents the EDS spectra obtained from Figure 2a.The high-resolution TEM image in Figure 2c focuses on the region delineated by the red dashed line in Figure 2a.The contrasting regions reveal a lattice spacing of approximately 0.27 nm on the right, corresponding to the (100) plane of hexagonal WS 2 [32], and a lattice spacing of 0.21 nm on the left, corresponding to the (100) plane of hexagonal Sb 2 Te 3 [14].Furthermore, the clarity within the heterojunction of lattice fringe patterns demonstrates its superior crystal quality.The selected area electron diffraction (SAED) pattern featured in Figure 2d provides compelling evidence of the crystallographic structure of the Sb 2 Te 3 /WS 2 heterojunction.This pattern exhibits two distinct sets of diffraction patterns: one corresponding to the lattice of Sb 2 Te 3 , characterized by a spacing of 0.21 nm, and another corresponding to the lattice of WS 2 , characterized by a spacing of 0.27 nm.These distinct diffraction patterns unequivocally confirm the single-crystal nature of both Sb 2 Te 3 and WS 2 , thereby underscoring the high-quality crystalline properties exhibited by the heterojunction.
ing to the (100) plane of hexagonal Sb2Te3 [14].Furthermore, the clarity within the heterojunction of lattice fringe patterns demonstrates its superior crystal quality.The selected area electron diffraction (SAED) pattern featured in Figure 2d provides compelling evidence of the crystallographic structure of the Sb2Te3/WS2 heterojunction.This pattern exhibits two distinct sets of diffraction patterns: one corresponding to the lattice of Sb2Te3, characterized by a spacing of 0.21 nm, and another corresponding to the lattice of WS2, characterized by a spacing of 0.27 nm.These distinct diffraction patterns unequivocally confirm the single-crystal nature of both Sb2Te3 and WS2, thereby underscoring the highquality crystalline properties exhibited by the heterojunction.X-ray photoelectron spectroscopy (XPS) analysis (shown in Figure 2e-h) was utilized as a tool for elucidating the chemical composition of the Sb2Te3/WS2 heterostructure.In Figure 2e, distinct peaks observed at binding energies of 33.7 and 35.8 eV correspond to the chemical states of W 4f7/2 and W 4f5/2, respectively.Additionally, peaks observed at binding energies of 163.4 and 164.5 eV are indicative of the chemical states of S 2p3/2 and S 2p1/2 in Figure 2f, respectively [33].The XPS data for Sb 3d and Te 3d are presented in Figure 2g and Figure 2h, respectively.The fitted curves for Sb 3d at 528.8 and 538.2 eV correspond to Sb 3d3/2 and Sb 3d5/2 [34].Meanwhile, Te 3d displays a peak at 569.7 eV for Te 3d5/2, and 580.1 eV for Te 3d3/2 [15,35].Notably, higher binding energy peaks at 573.5 eV and 582.9 eV are attributed to tellurium oxide formation resulting from surface oxidation [36].The values measured for S 2p, W 4f, Sb 3d, and Te 3d are consistent with reported values for WS2 and Sb2Te3.These analyses suggest that, during the top Sb2Te3 vapor growth process, no additional impurities infiltrate the underlying WS2 flake.Overall, the TEM and XPS investigations affirm the high crystal quality exhibited by the fabricated Sb2Te3/WS2 van der Waals heterostructure.
Raman and photoluminescence (PL) measurements were conducted utilizing a 532 nm laser excitation to comprehensively characterize the Sb2Te3/WS2 vertical heterojunction.Figure 3a exhibits the optical image of the partially covered Sb2Te3/WS2 X-ray photoelectron spectroscopy (XPS) analysis (shown in Figure 2e-h) was utilized as a tool for elucidating the chemical composition of the Sb 2 Te 3 /WS 2 heterostructure.In Figure 2e, distinct peaks observed at binding energies of 33.7 and 35.8 eV correspond to the chemical states of W 4f 7/2 and W 4f 5/2 , respectively.Additionally, peaks observed at binding energies of 163.4 and 164.5 eV are indicative of the chemical states of S 2p 3/2 and S 2p 1/2 in Figure 2f, respectively [33].The XPS data for Sb 3d and Te 3d are presented in Figure 2g and Figure 2h, respectively.The fitted curves for Sb 3d at 528.8 and 538.2 eV correspond to Sb 3d 3/2 and Sb 3d 5/2 [34].Meanwhile, Te 3d displays a peak at 569.7 eV for Te 3d 5/2 , and 580.1 eV for Te 3d 3/2 [15,35].Notably, higher binding energy peaks at 573.5 eV and 582.9 eV are attributed to tellurium oxide formation resulting from surface oxidation [36].The values measured for S 2p, W 4f, Sb 3d, and Te 3d are consistent with reported values for WS 2 and Sb 2 Te 3 .These analyses suggest that, during the top Sb 2 Te 3 vapor growth process, no additional impurities infiltrate the underlying WS 2 flake.Overall, the TEM and XPS investigations affirm the high crystal quality exhibited by the fabricated Sb 2 Te 3 /WS 2 van der Waals heterostructure.
Raman and photoluminescence (PL) measurements were conducted utilizing a 532 nm laser excitation to comprehensively characterize the Sb 2 Te 3 /WS 2 vertical heterojunction.Figure 3a exhibits the optical image of the partially covered Sb 2 Te 3 /WS 2 heterojunction sample.Moving to Figure 3b, the Raman spectrum of the Sb 2 Te 3 /WS 2 heterostructure reveals distinct Raman peaks corresponding to WS 2 (E 2g 1 at 350 cm −1 and mode A 1g at 412 cm −1 ) [37] and Sb 2 Te 3 (A 2u 3 at 135 cm −1 ) [34] within the junction region.This observation serves to validate the vertical configuration of the Sb 2 Te 3 /WS 2 junction.Further examination through Raman mapping, as shown in Figure 3c,d, provides additional insights.The Raman mapping at 135 cm −1 , shows uniform signal intensity distribution at the center of the crystal, significantly stronger than at the periphery, indicating uniform Sb 2 Te 3 in the crystal center.Conversely, the Raman mapping at 350 cm −1 displays a uni-form signal intensity distribution at the crystal's periphery, stronger than at the center, signifying uniform WS 2 at the crystal's outer region.Notably, the stronger signal of WS 2 at 350 cm −1 , combined with the central Raman spectrum analysis, confirms a vertically stacked Sb 2 Te 3 /WS 2 structure within the central circle region.Room temperature PL spectra and mapping are presented in Figure 3e,f.The PL mapping depicts strong PL emission at 631 nm from the single WS 2 region (blue line in Figure 3e), while the PL of WS 2 is significantly quenched in the vertically stacked Sb 2 Te 3 /WS 2 heterostructure region (red line in Figure 3e).Normalization of the peak intensity of the PL spectra, as illustrated in Figure S6, unveils a redshift in the PL peak of the Sb 2 Te 3 /WS 2 heterostructure.This redshift is attributed to strong charge transfer between WS 2 and Sb 2 Te 3 , serving as the primary cause of the significant PL quenching and redshift in the PL peak position [38].In summary, the successful fabrication of the Sb 2 Te 3 /WS 2 vertical heterostructure is confirmed through comprehensive Raman and PL characterizations.
erostructure reveals distinct Raman peaks corresponding to WS2 (E2g 1 at 350 cm −1 and mode A1g at 412 cm −1 ) [37] and Sb2Te3 (A2u 3 at 135 cm −1 ) [34] within the junction region.This observation serves to validate the vertical configuration of the Sb2Te3/WS2 junction.Further examination through Raman mapping, as shown in Figure 3c,d, provides additional insights.The Raman mapping at 135 cm⁻¹, shows uniform signal intensity distribution at the center of the crystal, significantly stronger than at the periphery, indicating uniform Sb2Te3 in the crystal center.Conversely, the Raman mapping at 350 cm⁻¹ displays a uniform signal intensity distribution at the crystal's periphery, stronger than at the center, signifying uniform WS2 at the crystal's outer region.Notably, the stronger signal of WS2 at 350 cm⁻¹, combined with the central Raman spectrum analysis, confirms a vertically stacked Sb2Te3/WS2 structure within the central circle region.Room temperature PL spectra and mapping are presented in Figure 3e,f.The PL mapping depicts strong PL emission at 631 nm from the single WS2 region (blue line in Figure 3e), while the PL of WS2 is significantly quenched in the vertically stacked Sb2Te3/WS2 heterostructure region (red line in Figure 3e).Normalization of the peak intensity of the PL spectra, as illustrated in Figure S6, unveils a redshift in the PL peak of the Sb2Te3/WS2 heterostructure.This redshift is attributed to strong charge transfer between WS2 and Sb2Te3, serving as the primary cause of the significant PL quenching and redshift in the PL peak position [38].In summary, the successful fabrication of the Sb2Te3/WS2 vertical heterostructure is confirmed through comprehensive Raman and PL characterizations.

Density Functional Theory Calculations of Band Structures for Monolayer (1L) WS2 and Two-Dimensional (2D) Sb2Te3
Using density functional theory calculations, the band structure of monolayer WS2 and the band structure of 2D Sb2Te3 were individually calculated based on their respective structures, as illustrated in Figure 4a,b.According to existing reports, the conduction band minimum (CBM) and valence band maximum (VBM) of WS2 are, respectively, at −4.39 eV and −6.46 eV [39]; whereas for Sb, the CBM and VBM are located at −4.15 eV and −4.45 eV [40], as demonstrated in Figure 4c.Upon the formation of heterostructures, a type-II band  Using density functional theory calculations, the band structure of monolayer WS 2 and the band structure of 2D Sb 2 Te 3 were individually calculated based on their respective structures, as illustrated in Figure 4a,b.According to existing reports, the conduction band minimum (CBM) and valence band maximum (VBM) of WS 2 are, respectively, at −4.39 eV and −6.46 eV [39]; whereas for Sb, the CBM and VBM are located at −4.15 eV and −4.45 eV [40], as demonstrated in Figure 4c.Upon the formation of heterostructures, a type-II band alignment is observed at the junction interface.Upon laser excitation at 532 nm, photoexcited electrons in Sb 2 Te 3 tend to transfer to WS 2 , while holes in WS 2 preferentially migrate to Sb 2 Te 3 .This charge separation mechanism hinders recombination within the heterostructure, leading to notable photoluminescence (PL) quenching (Figure 4d) and redshift in the PL peak position (Figure S6), consistent with the observed PL test results [38][39][40][41].
alignment is observed at the junction interface.Upon laser excitation at 532 nm, photoexcited electrons in Sb2Te3 tend to transfer to WS2, while holes in WS2 preferentially migrate to Sb2Te3.This charge separation mechanism hinders recombination within the heterostructure, leading to notable photoluminescence (PL) quenching (Figure 4d) and redshift in the PL peak position (Figure S6), consistent with the observed PL test results [38][39][40][41].

Optoelectronic Testing of Sb2Te3/WS2
The optoelectronic performance of the Sb2Te3/WS2 nanoflakes was investigated by fabricating Sb2Te3/WS2 based photodetectors on a SiO2/Si substrate, as illustrated in Figure 5a.Employing a Cr/Au electrode, one end was connected to the upper Sb2Te3 layer, while the other was linked to the lower WS2 layer.Figure 5b demonstrates the transfer characteristic curves of the Sb2Te3/WS2 p-n heterojunction.The prevailing n-type transfer curve suggests that electron transport dominates the charge transport within WS2. Figure 5c presents the Ids-Vds curves of the Sb2Te3/WS2 p-n heterojunction under dark conditions and various power levels of 532 nm laser irradiation.With increasing optical power, the photogenerated current increases.Moreover, the power-dependent photoresponse was modeled using the power law (  ) to examine the trap states within the Sb2Te3/WS2 nanoflakes [42,43], as depicted in Figure 5d.The fitting coefficient of 0.62 implies that as laser intensity rises, light absorption gradually saturates.To assess the photoresponse speed of the Sb2Te3/WS2 heterojunction photodetector, time-resolved photoresponse measurements were conducted, with results depicted in Figure 5e.The photocurrent demonstrates efficient switching between on and off states by periodic activation and deactivation of the laser, showcasing exceptional stability.Figure 5f shows the rise/decay time of the photocurrent in the photodetector, where the rise/decay time is defined as the time required for the photocurrent to increase from 10% to 90% of the peak value and decrease from 90% to 10% of the peak value, respectively.Further analysis reveals a rise  The optoelectronic performance of the Sb 2 Te 3 /WS 2 nanoflakes was investigated by fabricating Sb 2 Te 3 /WS 2 based photodetectors on a SiO 2 /Si substrate, as illustrated in Figure 5a.Employing a Cr/Au electrode, one end was connected to the upper Sb 2 Te 3 layer, while the other was linked to the lower WS 2 layer.Figure 5b demonstrates the transfer characteristic curves of the Sb 2 Te 3 /WS 2 p-n heterojunction.The prevailing n-type transfer curve suggests that electron transport dominates the charge transport within WS 2 .Figure 5c presents the I ds -V ds curves of the Sb 2 Te 3 /WS 2 p-n heterojunction under dark conditions and various power levels of 532 nm laser irradiation.With increasing optical power, the photogenerated current increases.Moreover, the power-dependent photoresponse was modeled using the power law (I ph = αP θ ) to examine the trap states within the Sb 2 Te 3 /WS 2 nanoflakes [42,43], as depicted in Figure 5d.The fitting coefficient of 0.62 implies that as laser intensity rises, light absorption gradually saturates.To assess the photoresponse speed of the Sb 2 Te 3 /WS 2 heterojunction photodetector, time-resolved photoresponse measurements were conducted, with results depicted in Figure 5e.The photocurrent demonstrates efficient switching between on and off states by periodic activation and deactivation of the laser, showcasing exceptional stability.Figure 5f shows the rise/decay time of the photocurrent in the photodetector, where the rise/decay time is defined as the time required for the photocurrent to increase from 10% to 90% of the peak value and decrease from 90% to 10% of the peak value, respectively.Further analysis reveals a rise time (τ rise ) of 305 µs and a decay time (τ decay ) of 503 µs for the device, surpassing those of the WSe 2 /WS 2 vertical heterostructure [44].Given the excellent mechanical flexibility [25] and high integration of two-dimensional layered materials, their potential applications in future industrial flexible robotic sensors warrant additional research into two-dimensional optoelectronic devices.

Figure 1 .
Figure 1.Synthesis of vertically stacked Sb2Te3/WS2 van der Waals heterojunctions: (a) schematic overview of the growth process for the synthesis of Sb2Te3/WS2 heterojunctions using a dual-stage chemical vapor deposition process; (b) schematic diagrams illustrating the growth process of the Sb2Te3/WS2 heterostructures over time, in which the deep purple in the same triangular flake represents the Sb2Te3/WS2 heterostructures, while the light purple represents monolayer WS2 (scale bar: 10 µm); (c) scanning electron microscope (SEM) image exhibiting a typical monolayer WS2 triangular flakes, while (d) exhibits the partially covered vertically stacked Sb2Te3/WS2 heterostructures; (e) Atomic Force Microscope (AFM) images of the partially covered Sb2Te3/WS2 heterojunction; (f) the magnified view of the red square area depicted in (e).

Figure 1 .
Figure 1.Synthesis of vertically stacked Sb 2 Te 3 /WS 2 van der Waals heterojunctions: (a) schematic overview of the growth process for the synthesis of Sb 2 Te 3 /WS 2 heterojunctions using a dual-stage chemical vapor deposition process; (b) schematic diagrams illustrating the growth process of the Sb 2 Te 3 /WS 2 heterostructures over time, in which the deep purple in the same triangular flake represents the Sb 2 Te 3 /WS 2 heterostructures, while the light purple represents monolayer WS 2 (scale bar: 10 µm); (c) scanning electron microscope (SEM) image exhibiting a typical monolayer WS 2 triangular flakes, while (d) exhibits the partially covered vertically stacked Sb 2 Te 3 /WS 2 heterostructures; (e) Atomic Force Microscope (AFM) images of the partially covered Sb 2 Te 3 /WS 2 heterojunction; (f) the magnified view of the red square area depicted in (e).

Figure 2 .
Figure 2. Atomic configuration of the vertically aligned Sb2Te3/WS2 heterojunction: (a) low-magnification TEM image of a vertically aligned Sb2Te3/WS2 vdW heterojunction, the red dashed rectangular region corresponds to the location in the high-resolution TEM image in (c); (b) a 2D elemental mapping visualizing the distribution of W, S, Sb, and Te within the Sb2Te3/WS2 heterostructure; (c) high-resolution TEM image capturing the interface region of the heterojunction; (d) electron diffraction pattern captured from the aligned region of the Sb2Te3/WS2 vdW heterojunction; (e-h) XPS spectra of W 4f, S 2p, Sb 3d, and Te 3d levels in the Sb2Te3/WS2 heterostructure, with black lines denoting measured data and dots representing fitting curves.

Figure 2 .
Figure 2. Atomic configuration of the vertically aligned Sb 2 Te 3 /WS 2 heterojunction: (a) lowmagnification TEM image of a vertically aligned Sb 2 Te 3 /WS 2 vdW heterojunction, the red dashed rectangular region corresponds to the location in the high-resolution TEM image in (c); (b) a 2D elemental mapping visualizing the distribution of W, S, Sb, and Te within the Sb 2 Te 3 /WS 2 heterostructure; (c) high-resolution TEM image capturing the interface region of the heterojunction; (d) electron diffraction pattern captured from the aligned region of the Sb 2 Te 3 /WS 2 vdW heterojunction; (e-h) XPS spectra of W 4f, S 2p, Sb 3d, and Te 3d levels in the Sb 2 Te 3 /WS 2 heterostructure, with black lines denoting measured data and dots representing fitting curves.

Figure 3 .
Figure 3. Utilization of 532 nm laser excitation for Raman and photoluminescence (PL) characterization of the Sb2Te3/WS2 heterojunction: (a) optical photograph of the Sb2Te3/WS2 heterojunction; (b) Raman spectra collected from the red dots (dark covered region at the crystal center) and blue dots (light region at the outer edge of the crystal) in (a); (c,d) frequency-specific Raman mapping of the Sb2Te3/WS2 heterojunction at 135 cm −1 and 350 cm −1 , correspondingly; (e) photoluminescence spectra obtained from the isolated WS2 area (blue curve) and the intersected region (red curve); (f) PL cartography of the vertical stacked Sb2Te3/WS2 heterojunction, at 631 nm.

Figure 3 .
Figure 3. Utilization of 532 nm laser excitation for Raman and photoluminescence (PL) characterization of the Sb 2 Te 3 /WS 2 heterojunction: (a) optical photograph of the Sb 2 Te 3 /WS 2 heterojunction; (b) Raman spectra collected from the red dots (dark covered region at the crystal center) and blue dots (light region at the outer edge of the crystal) in (a); (c,d) frequency-specific Raman mapping of the Sb 2 Te 3 /WS 2 heterojunction at 135 cm −1 and 350 cm −1 , correspondingly; (e) photoluminescence spectra obtained from the isolated WS 2 area (blue curve) and the intersected region (red curve); (f) PL cartography of the vertical stacked Sb 2 Te 3 /WS 2 heterojunction, at 631 nm.

Figure 4 .
Figure 4. Schematic diagram of theoretical calculations: (a,b) calculated band structures of 1L WS2 and two-dimensional (2D) Sb2Te3; (c) energy band profiles of the 1L WS2 and Sb2Te3 before contract; (d) band alignment illustration displaying the process of charge transfer at the interface of the junction under 532 nm laser exposure.

Figure 4 .
Figure 4. Schematic diagram of theoretical calculations: (a,b) calculated band structures of 1L WS 2 and two-dimensional (2D) Sb 2 Te 3 ; (c) energy band profiles of the 1L WS 2 and Sb 2 Te 3 before contract; (d) band alignment illustration displaying the process of charge transfer at the interface of the junction under 532 nm laser exposure.