Preparation and Charge Transfer at Sb₂Se₃/1L-MoS₂ Heterojunction

Abstract

Owing to the strong optical absorption of Sb2Se3, building heterojunctions (HJs) by using thin-layer Sb2Se3 and other two-dimensional (2D) materials is critical to the design and applications of ultrathin optoelectronic devices. However, the preparation of HJs using Sb2Se3 and other transition metal dichalcogenide (TMDC) thin layers is still challenging. Herein, a chemical vapor deposition (CVD) method was used to prepare monolayer MoS2(1L-MoS2) and Sb2Se3 thin layers. A dry transfer method was subsequently used to build their HJs. Individual PL spectra and PL mapping results obtained at the HJs indicate a charge injection from 1L-MoS2 into Sb2Se3 flake, which was further confirmed by contact potential difference (CPD) results obtained by using Kelvin probe force microscopy (KPFM). Further measurements indicate a type-Ⅰ band alignment with a band offset finally determined to be 157 meV. The obtained results of Sb2Se3/1L-MoS2 HJs will benefit the rational design of novel ultrathin optoelectronic devices based on novel 2D absorber layers working in visible light.

1. Introduction

In recent years, vertical van der Waals (vdW) HJs, stacked by two or more dissimilar 2D TMDC thin layers via van der Waal forces in the out-of-plane direction, inspired great enthusiasm in the application of novel 2D devices. The combination of different TMDC components offers extra manipulations of optical absorption, interlayer excitons [1], photovoltaic effect [2], tunneling-assisted carrier recombination [3], ultrafast and high-efficiency charge transfer [4], etc., which greatly enhances the external quantum efficiency and detectivity of optoelectronic devices from UV to near-infrared regions [5]. Since the emergence of the first vertically stacked graphene/h-BN HJ in 2010 [6], a variety of 2D bilayer TMDCs vertical HJs, denoted by MX₂/MX₂ (M = W, Mo, Nb, Pt, V, Re, Ta; X = S, Se, Te) [3,4,5,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] have been prepared. The manipulations through the composition, layer number, stacking sequence, and relative stacking angles are able to enhance light absorption ranging from the UV-light to near-infrared region and greatly elongate recombination lifetime of interlayer excitons up to five orders of magnitude (∼100 ns) [7,27], which greatly facilitates the design and preparation of novel optoelectronic devices with ultra-thin thickness.

Sb₂Se₃ have long been studied as a photovoltaic material. Its medium band gap of 1.04–1.18 eV [28,29] and strong absorption coefficient >10⁵ cm⁻¹ make it a potential absorber material in novel solar cells such as CdS/Sb₂Se₃ [30,31], Cd_xZn_1−xS/ Sb₂Se₃ [32], ZnO/Sb₂Se₃ [33], TiO₂/ Sb₂Se₃ [34]_, etc. In recent years, 2D Sb₂Se₃ nanoflakes with ultra-thin thickness as 1.28–1.54 nm have been prepared by the solvent-based method [35,36] and sodium-assisted CVD method [37]. Thicker layers with thickness of ~80 nm were also prepared by a hydrothermal method using SbCl₃ and Na₂SeO₃ as precursors [35]. The obtained Sb₂Se₃ nanoflakes exhibit a layer thickness dependent band gap, which varies from ~1.5 eV (flake thickness ~80 nm) [35] to ~1.8 eV(flake thickness (~1.3 nm) [37]. The ultra-thin Sb₂Se₃ flakes exhibit an interesting anisotropic response of Raman peak intensity and photocurrent as a function of the polarization angle of the incident light [36], which indicates Sb₂Se₃ can be used as an absorber material in novel atomic-scale anisotropic photodetectors. Building p-n junctions using Sb₂Se₃ nanoflakes with other 2D materials is crucial to designing novel Sb₂Se₃-based detectors.

One-dimensional (1D) Sb₂Se₃ nanowires have been used to build HJs with 1L WS₂ [38]. Regarding these processes, to the best of our knowledge, the preparation of HJs of Sb₂Se₃ with other 2D TMDCs is still challenging.

In this study, we prepared a Sb₂Se₃/1L-MoS₂ HJ by a combined CVD and transfer method. Sb₂Se₃ flakes firstly deposited on mica substrate were transferred onto 1L-MoS₂ flakes using a dry-transfer method assisted by PDMS tape. Redshift of PL A excitons of MoS₂ at the junction area indicate a charge injection from 1L-MoS₂ into Sb₂Se₃. Further measurements using KPFM indicate this new HJ has a type-Ⅰ band alignment. This study provides a rational design of Sb₂Se₃-based ultrathin HJs, and the obtained results would be helpful for deeply understanding the photoresponse of this new HJ structure.

2. Experiment and Characterization

Sample preparation: The CVD method was employed for the 2D Sb₂Se₃ flakes grown using two different routines. In routine Ⅰ, CVD was performed in a two-heating-zone tube furnace (Figure 1). A quantity of 5 mg Sb₂O₃ (99.9%, Aladdin) was put in zone 1 at 700 °C. A quantity of 3 mg Se (99.999%, Macklin) was placed 8–10 cm away from Sb₂O₃ powder in the upper steam direction (roughly 300 °C). A freshly cleaved mica sheet (KMg₃AlSi₃O₁₀F₂) was placed in zone 2 (T2 = 400 °C). High purity Ar/H₂ (60 SCCM) was used as the carrier gas. In zone 2, the temperature was firstly ramped to 100 °C in 3 min and held for 10 min. Then the temperature was ramped to 400 °C and held for 33 min. In zone 1, the temperature was raised to 100 °C in 3 min and held for 26 min. Subsequently, the temperature was ramped to 700 °C and held for 5 min. During the whole process, the pressure was kept at 700–800 Torr. In routine Ⅱ, CVD was performed in a single heating zone tube furnace. A quantity of 10 mg of Sb₂Se₃ (99.9%, Aladdin) powder blended with a small amount of NaCl (10 wt.%) was used as precursor. A freshly cleaved mica sheet was directly placed on the top of the Sb₂Se₃ source. The temperature was ramped to 700 °C and held for 40 min. During the whole process, the pressure was kept at 700–800 Torr.

1L-MoS₂ samples were also prepared by the CVD method in another two-heating-zone tube furnace. A quartz boat containing sulfur powder was placed in heating zone 1. A Si/SiO₂ substrate (300 nm SiO₂) was placed upside-down on a Mo sheet in heating zone 2. High purity Ar gas was used as carrier gas at a flow rate of 60 SCCM. Firstly, the temperature of heating zone 2 was raised from ambient temperature to 550 °C in 25 min, then heating zone 1 was turned on. The temperatures of heating zones 1 and 2 were raised to 270 and 850 °C, respectively, and held for 15 min before cooling naturally to ambient temperature.

The Sb₂Se₃ nanoflakes growing on the mica were transferred onto 1L-MoS₂ flakes by the dry-transfer method. Firstly, PDMS was pasted onto the mica substrate and Sb₂Se₃ flakes were detached from mica and transferred to PDMS. Then, the Sb₂Se₃ flakes were released to Au thin film (60 nm) or 1L-MoS₂ after heating at 85 °C for 7 min. The obtained Sb₂Se₃/1L-MoS₂ HJs were annealed for 3 h in vacuum at 200 °C and a pressure of 3 × 10⁻² Torr.

Characterization: Optical images were captured by an optical microscope. A Raman spectrometer (Horiba Labram Nano) was used for the Raman measurement. This Raman spectrometer is equipped with 532, 632.8, and 785 nm lasers, 100x (NA = 0.9, Olympus) objective, two gratings (1800/150 mm), and a charge-coupled detector (CCD). Raman and PL measurements were conducted by switching the lasers and controlling laser power. Furthermore, an atomic force microscopy (AFM) (HORIBA, AIST-NT) was adopted to measure the morphology, CPD, and capacitance image. The Au–coated tips were used for the AFM and KPFM measurements. KPFM measurements were performed by lifting the tips 10 nm from the sample surface. A bias of 4 V was applied on the tips, while the substrates were grounded. The work function of the Au-coated tip was calibrated to be 4.9 eV by a highly oriented pyrolytic graphite (HOPG). I-V curves were measured in conductive mode using a Pt coated silicon tip working in contact mode. The element type and content of composition were analyzed by energy disperse spectroscopy (EDS) on a scanning electron microscope (SEM).

3. Results and Discussions

Optical images of obtained Sb₂Se₃ nanoflakes prepared on a mica substrate by routines Ⅰ and Ⅱ are presented in Figure 2a and Figure S1a, respectively. The obtained nanoflakes exhibit a long rhombic shape, which is analogous to that of Sb₂Se₃ nanoflakes prepared by the CVD method reported by Valdman et al. [39] and Wang et al. [35]. The atomic structure of Sb₂Se₃ is composed of 1D chain-like [Sb₄Se₆]_n ribbons along the [1] direction. Within each [Sb₄Se₆]_n ribbon, the atoms are bonded by strong Sb–Se covalent bonds. Trigonal SbSe₃ units and tetragonal SbSe₅ pyramids are coordinated alternately. Along the [100] and [10] directions, the ribbons are stacked by relatively weak van der Waals forces [36,37]. An AFM topographic image of a Sb₂Se₃ nanoflake prepared by routine Ⅰ is presented in Figure 2b. This flake has a rhombic flake shape and a larger thickness on one side. This feature is also quite similar to that of epitaxial Sb₂Se₃ on mica [39]. The thickness of this flake can be determined to be 245 nm from the height profile along the dashed line (inset in Figure 2b). An SEM image of a Sb₂Se₃ nanoflake prepared on Au-coated substrate is shown in Figure 2c. The Sb and Se atomic percentages were measured to be 22.5% and 77.5%, respectively, by EDS collected at the marked point in Figure 2c. The large deviation in stoichiometry may be induced by large uncertainties in EDS measurement.

Raman spectra of the Sb₂Se₃ nanoflakes prepared by routine Ⅰ were measured by 532, 632.8, and 785 nm lasers and are presented in Figure 2d. The Raman spectrum of Sb₂Se₃ nanoflakes prepared by routine Ⅱ was measured by 532 nm laser and is plotted in Figure S1b. It can be seen these spectra exhibit similar features, i.e., a broad Raman band centered at ~190 cm^−1, which can be assigned to A_g mode, in which the Sb-Se bond-stretching vibration is orthogonal to the ribbon direction [37]. It should be noted that these spectra were measured at extremely low laser power. For an excitation laser wavelength of 532 nm and larger laser power of ~34 μW at the focus, the features of Raman spectra strongly depend on the scanning time (Figure 2e). At the beginning of the scan (<20 s), the obtained Raman spectra are similar to those in Figure 2d, whereas for scanning time >40 s, three Raman peaks at 193, 215, and 251 cm⁻¹ were predominant (Figure S2a). These features are similar to those of Sb₂O₃ oxidized from Sb₂Se₃ [16,40]. The peak at ~193 cm⁻¹ can still be assigned to A_g of Sb₂Se₃, whereas the peak at 251 cm⁻¹ can be assigned to A_g mode of Sb₂O₃ [41]. Similar oxidation was not observed by using excitation laser wavelength of 632.8 and 785 nm (Figure S2b,c), which might be induced by low optical absorption at these two wavelengths [37]. The scanning time-dependent Raman spectra of Sb₂Se₃ flakes prepared by routine Ⅱ are presented in Figure S2d. The intensity of the A_g peak also decreases with long exposure time, indicating the oxidation is still seen at long exposure time, which is similar to the flakes prepared by routine Ⅰ. These results indicate oxidation of Sb₂Se₃ nanoflakes prepared by routine Ⅰ and Ⅱ occurs when strong laser power and long exposure time are used. This also indicates the similar crystal structure and quality of Sb₂Se₃ prepared by the two routines. Therefore, the Sb₂Se₃ flakes prepared by the two routines are intentionally not discussed in the following section.

The I-V curve of a Sb₂Se₃ nanoflake prepared on gold thin film (topography image is shown in Figure S3) was measured in a dark state and is plotted in Figure 2f. The semi-linear curve indicates a typical response of a resistor, whereas, with illumination of a 532 nm laser, the enhanced current indicates the enlarged conductivity induced by generation of photocarriers. This indicates the semiconductor properties of the Sb₂Se₃ nanoflake, which agree well with those reported by Vidal-Fuentes et al. [16] and Chen et al. [29].

The CPD of a Sb₂Se₃ nanoflake prepared on gold thin film was measured and is shown in Figure 2g. CPD profiles along two dashed lines across the flake are plotted as an inset. The CPD on Au thin film is 0.02 ± 0.01 V, which is close to zero, the expected value. The average CPD on the Sb₂Se₃ nanoflake is measured to be −125 mV, corresponding to a Fermi energy of 5.025 eV by using Equation (1) [42]:

$$E_f={\Phi }_{Au}-e\times CPD$$

(1)

where ${\Phi }_{Au}$ is the work function of the Au tip (4.9 eV). The obtained Fermi level of the nanoflake is slightly larger than the 4.64 eV of Sb₂Se₃ thin films reported by Mamta et al. [43]. The capacitance image of this Sb₂Se₃ nanoflake is shown in Figure 2h. The capacitance fluctuation may be induced by the thickness fluctuation, as shown by a topography image in Figure S3.

An optical image of a Sb₂Se₃/MoS₂ HJ sample is presented in Figure 3a. The MoS₂ triangular flake presents a light purple contrast, which is similar to those in a previous report [44]. A rhombic multilayer Sb₂Se₃ with bright contrast is placed in the center of this MoS₂ flake. AFM topography image of this HJ is presented in Figure 3b. The thickness of the multilayer Sb₂Se₃ can be determined to be 535 nm by a height line profile across the flake (inset in Figure 3b). The Raman spectrum of a clean MoS₂ area is plotted in Figure S4a. Prominent Raman peaks can be seen at 383.1, 403.3, and 449.7 cm⁻¹, which can be assigned to be two first-order modes of E′ and A′, and a second-order mode 2LA(M), respectively [45]. The separation between the E′ and A′ modes, 20.2 cm⁻¹, agrees with that of the CVD-prepared 1L-MoS₂ [46], and indicate the 1L nature of the MoS₂ flake in Figure 3a. Compared to the 1L-MoS₂ area, the intensities of A′ and E′ peaks of MoS₂ are too weak to be observed in the HJ area, indicating extremely strong optical absorption of the incident laser photons. The Raman spectra of the Sb₂Se₃ on the HJ are quite similar to those in Figure S1b and indicate the good quality of the transferred heterojunctions. On the PL spectra collected from 1L-MoS₂ and the HJ area (Figure 3c), prominent PL peaks at ~1.97 and 1.80 eV can be assigned to B and A excitons, respectively, induced by spin-orbit splitting of the valence band [47]. Compared to that of 1L-MoS₂, the A exciton redshifts by 3 meV. Further deconvolution using Gaussian functions indicates a negligible exciton energy shift of B excitons in the HJ area. These exciton features of 1L-MoS₂ in the HJ area indicate charge depletion of 1L-MoS₂.

The PL intensity and energy images of the HJ sample are shown in Figure 3d,e, respectively. The A exciton PL intensity and energy images of this 1L-MoS₂ flake before transfer are presented in Figure S4b,c for comparison. At the junction area, the PL intensity of MoS₂ is lowered by 90%, which can be attributed to the strong absorption of Sb₂Se₃. The shape of the PL lowered area (dashed ellipse) agrees well with the shape of the Sb₂Se₃ nanoflake. Meanwhile, the lowered PL area gives a brighter contrast area (dashed ellipse) with redshifted PL peak energy. This HJ area also gives lowered Raman intensity of MoS₂, which is further shown by a Raman image presented in Figure 3f and agrees with the Raman spectra in Figure S4a.

AFM topographic and CPD images of a Sb₂Se₃/1L-MoS₂ HJ are presented in Figure 4a,b, respectively. This Sb₂Se₃ nanoflake exhibits a nonuniform thickness, which can be determined from the height profile along the dashed line. The flake thickness at the edge and the center is 362 and 679 nm, respectively. On the CPD image, lower contrast of Sb₂Se₃ flake indicates its lower CPD than that of the 1L-MoS₂ area. In order to obtain the accurate CPD of 1L-MoS₂, a CPD profile was taken along the dashed line in Figure S5b and is plotted in Figure 4c. The CPD of the 1L-MoS₂ was measured to be 389 mV, corresponding to a Fermi level of 4.511 eV calculated by using Equation (1) and agreeing well with the value (4.47 eV) in the literature [48]. The CPD profile across the Sb₂Se₃/1L-MoS₂ HJ is plotted in Figure 4d. At the junction edge, the CPD difference of 1L-MoS₂ and Sb₂Se₃ is measured to be 157 mV.

Since the thickness of our Sb₂Se₃ flake is several hundreds of nanometers, the band gap is expected to be close to that of the bulk layer, which can be set to 1.18 eV [28]. The band gap of 1L-MoS₂ can be set to be 2.42 eV using the B exciton energy of 1.98 eV and exciton binding energy of 0.44 eV [49]. The electron affinity of MoS₂ and Sb₂Se₃ can be assumed to be 4 [50] and 4.2 eV [51], respectively. Therefore, the band alignment of the Sb₂Se₃/1L-MoS₂ HJ can be plotted as a diagram in Figure 4e. The valence band difference ΔE_V (ΔE_V = E_v-MoS2 − E_v-Sb2Se3) and conduction band difference ΔE_c (ΔE_c = E_c-MoS2 − E_c-Sb2Se3) are calculated to be −0.043 eV and 1.197 eV, respectively. It can be immediately seen that Sb₂Se₃/1L-MoS₂ HJ is a type-Ⅰ junction. Such band alignment results in the electron transport from 1L-MoS₂ to Sb₂Se₃ nanoflake. The charge depletion of the 1L-MoS₂ area is in good agreement with charge transfer from 1L-MoS₂ to Sb₂Se₃ indicated by PL spectral measurements in Figure 3.

The capacitance image of the Sb₂Se₃/1L-MoS₂ HJ is presented in Figure 4f. The capacitance is qualitatively proportional to the measured value. Brighter contrast in Figure 4f indicates larger capacitance. According to the definition of a planner capacitor, C = εS/d, where ε, S, and d are the dielectric constant, area of electrode, and the separation between two electrodes, respectively [52]. Given a constant electrode area and an ideal dielectric with no charging effect, the capacitance is in a reciprocal relationship with the thickness of the dielectric layer. The low thickness of 1L-MoS₂ indicates the largest capacitance. It is expected that the Sb₂Se₃ nanoflake, in which the thickness is hundreds of nanometers, gives low contrast and small capacitance. However, the center area of the Sb₂Se₃ layer gives a much higher contrast than the edge area. This could be induced by the charge transfer effect, in which the injected electron accumulation changes the dielectric properties of Sb₂Se₃ flakes and changes the capacitance.

In summary, Sb₂Se₃/1L-MoS₂ HJs were prepared by transferring Sb₂Se₃ nanoflakes to 1L-MoS₂ deposited by the CVD method. PL spectra measured at the junction area indicate a charge transfer from 1L-MoS₂ to Sb₂Se₃ flakes, which was further confirmed by an A exciton redshift indicated by PL imaging using A exciton intensity. KPFM measurement further indicates charge depletion and a band offset of 157 meV at the junction interface. Our study helps to understand the physical properties of 2D HJs using Sb₂Se₃ as an absorber layer. The obtained results would be instructive for the design of ultrathin novel optoelectronic devices such as solar cells and novel detectors.