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MoS2 vertical heterostructures for fast visible-wavelength photodetectors
Journal Pre-proof Growth of CdSe/MoS2 vertical heterostructures for fast visible-wavelength photodetectors Yide Yuan, Xuehong Zhang, Huawei Liu, Tiefeng Yang, Weihao Zheng, Biyuan Zheng, Feng Jiang, Lihui Li, Dong Li, Xiaoli Zhu, Anlian Pan PII:
S0925-8388(19)33555-8
DOI:
https://doi.org/10.1016/j.jallcom.2019.152309
Reference:
JALCOM 152309
To appear in:
Journal of Alloys and Compounds
Received Date: 17 April 2019 Revised Date:
2 September 2019
Accepted Date: 15 September 2019
Please cite this article as: Y. Yuan, X. Zhang, H. Liu, T. Yang, W. Zheng, B. Zheng, F. Jiang, L. Li, D. Li, X. Zhu, A. Pan, Growth of CdSe/MoS2 vertical heterostructures for fast visiblewavelength photodetectors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152309. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Growth of CdSe/MoS2 vertical heterostructures for fast visible-wavelength photodetectors (Yide Yuan 1, Xuehong Zhang 1, Huawei Liu 1, Tiefeng Yang 1, Weihao Zheng 1, Biyuan Zheng 1, Feng Jiang 1, Lihui Li 1, Dong Li 2, Xiaoli Zhu 1*, Anlian Pan1,2*)
(1. School of Physics and Electronic Science, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha, 410082, China; 2. State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Materials Science and Engineering, Hunan University, Changsha, 410082, China) Correspondence: [email protected], [email protected].
Abstract: Heterostructures composed of different semiconducting materials have aroused wide attentions due to their fascinating properties originated from the interfaces. Particularly, the recent two dimensional layered materials (2DLM) have provided novel platforms to flexibly design the heterostructures for diverse electronic and optoelectronic applications. In this work, we have reported the growth of CdSe nanoplates/MoS2 monolayer vertical heterostructures with efficient and fast visible-wavelength photodetections. Highly dense CdSe nanoplates were vertically assembled on monolayer MoS2 through a two-step chemical vapor deposition (CVD) process. The interfacial photoinduced charge behaviors were investigated in detail via the time resolution photoluminescence (TRPL) measurements, revealing the efficient charge transfer across the heterointerface. Benefiting from the large CdSe coverage and efficient charge transfer, superior photodetection performances of the CdSe/MoS2 heterostructures can be obtained with an enhanced photoresponsivity of 1.63 A/W, which can be further improved to be 12 A/W via applying the gate voltage. Besides, the heterostructure detectors also exhibit a very fast photoresponse speed of 370 µs, much faster than previous photodetectors based on CVD-grown 2D heterostructures. The as-synthesized CdSe/MoS2 heterostructures may find important applications in integrated optoelectronic systems. Keywords: CdSe/MoS2; Vertical heterostructures; Photodetectors; Responsivity; Chemical vapor deposition.
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1 Introduction Since the groundbreaking discovery of graphene in 2004 [1], 2D materials, especially the layered transition-metal dichalcogenides (TMDs), have attracted more and more attention due to their extraordinary optoelectronic properties [2-4]. To further explore the potential of these materials, plenty of van der Waals (vdW) heterostructures have been successfully synthesized, such as SnS2/WSe2 [5], WS2/MoS2 [6], and so forth [7]. Unlike the traditional heterostructure that based mainly on group IV, II-VI, or III-V semiconductor materials with the connection depended on the covalent bond on the heterogeneous boundary surface, diverse vdW heterostructures can be factitiously combined without the lattice match restrictions. Meanwhile, these heterostructures have been proved to be promising candidates for next-generation optoelectronic integration systems [6,7]. Generally, 2DLMs with planar crystal structures are the preferred component for constructing 2D heterostructures to realize the enhanced optoelectronic performances. Nonetheless, large amounts of non-2D materials with excellent optical and electrical properties, such as CdS, CdSe, InGaAs, and so forth, can also demonstrate advantages in improving the 2DLM performances [8-11]. Indeed, the heterostructures composed of 2D and non-2D materials have been fabricated and investigated extensively. Indeed, the heterostructures composed of 2D and non-2D materials have been fabricated and investigated extensively [12,13]. For instance, rhodamine 6G/MoS2 heterostructures showed the enhanced responsivity of 1.17A/W [12]. The Se/ReS2 nanoplates/MoS2 heterostructures also realized the high photoresponsivity of 36 A/W [13]. However, these heterostructures generally demonstrate the slow response speed (milliseconds to seconds) when enhancing the responsivity, hindering their practical applications. In this work, we have reported the controlled growth of high-quality heterostructures with high-density CdSe nanoplates vertically assembled on monolayer MoS2. The photoinduced interfacial charge behavior was investigated in detail via TRPL measurements, revealing the efficient charge transfer from CdSe to MoS2. Photodetectors were then fabricated based on the as-synthesized CdSe/MoS2 heterostructures. Benefiting from the ultralarge coverage of CdSe nanoplates on MoS2 and charge transfer across the heterointerface, the heterostructure detectors exhibit an enhanced photoresponsivity of 1.63 A/W, which can be further improved to be 12 A/W via applying the gate voltage. Meanwhile, the heterostructure detectors also exhibit a very fast photoresponse speed of 370 µs.
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2. Experimental 2.1 Synthesis of CdSe/MoS2 heterostructures Single-layer MoS2 nanosheets and CdSe/MoS2 heterostructures were synthesized by CVD method on a SiO2 (300 nm)/Si substrate and the preparation process was presented (see Figure S1 in the supplementary material). Firstly, N2 gas (60 sccm) was introduced into the furnace (OTF-1200X) with the substrate to evacuate atmosphere at a rate of 60 sccm. Then, sulfur and MoO3 (Alfa Aesar, 99.99%) powders, separately loaded with a quartz boat in the furnace as the charge materials, were heated to 300
and 780 ,
respectively, and kept for 30-35 min. After the deposition, the achieved samples were transferred into another furnace with CdSe powders in a quartz boat (Alfa Aesar, 99.99%). Similar to the above deposition, N2 gas was injected into the second furnace at a rate of 50 sccm and the pressure inside the furnace was kept at 200−300 Torr. The substrate and CdSe powders were heated to 250-300
and 800 , respectively, and
kept for 60 min. Finally, the sample was cooled down to room temperature. 2.2 Characterization of materials The morphologies of the as-grown CdSe/MoS2 heterostructures were characterized by SEM (FE-SEM, Zeiss sigma-HD), atomic force microscope (AFM) (Bruker Multimode 8), X-ray Diffraction (XRD) (Rigaku D/Max 2500) and TEM (Tecai F20, voltage: 300 kV). Before TEM measurements, the heterostructures were exfoliated and transferred onto copper coated grids using PMMA solvent. MicroRaman and micro-photoluminescence (PL) measurements were performed using a confocal microscopy system (WITec, alpha-300) with a laser with excitation wavelength of 532 nm (excitation power: about 20 mW). 2.3 Fabrication and measurement of the devices Firstly, the SiO2 (300 nm)/Si substrate with CdSe/MoS2 heterostructures was coated with a layer of MMA copolymer (EL6, Microchem Company), followed by a 2 minutes bake at 150 . Secondly, another layer of PMMA (495 K, A4, Microchem Company) was spin-coated on the substrate followed by a 5 minutes bake at 160 . The patternings of drain and source electrodes were defined using Electron beam lithography (Raith 150 two). Ti/Au metal layer (Ti: 5 nm, Au: 50 nm) was deposited to form the sourcedrain electrodes by electron beam evaporation, and finally followed by lift-off process using acetone. The
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optoelectronic and electrical properties of the heterostructures were measured using the Lake Shore Probe Station and Keithley 4200 semiconductor analyzer in a vacuum (10-6 torr) desiccator at room temperature. The time response of the device was measured by switching the laser on and off with an internal squarewave trigger source and recorded by a digital oscilloscope. 3. Results and discussion The growth process of CdSe/MoS2 vertical heterostructures is schematically illustrated in Figure 1(a). Triangular shaped MoS2 monolayers were initially grown on the SiO2/Si substrate through a CVD route. The as-grown MoS2 nanosheets were subsequently employed as templates for the second-step CVD growth of CdSe nanoplates. The growth time of CdSe nanoplates is set to be a large value to obtain the high CdSe coverage. In order to confirm the crystallographic phase of the as-grown samples, X-Ray diffraction (XRD) measurements were conducted and the XRD information is collected from the whole area of SiO2/Si substrate (0.5 cm × 0.5 cm) fully covered with samples (MoS2 or CdSe/MoS2). As depicted in Figure 1(b), from the diffraction pattern of CdSe/MoS2, it is clear that the (002) and (110) peaks can be assigned to the characteristic peaks of MoS2 [14]. The other diffraction peaks corresponding to the CdSe are also observed [15]. This confirms the formation of the CdSe/MoS2 heterostructures. No other peaks are observed also indicates high phase purity of the as-grown heterostructures. Figure 1(c) gives the high-magnification scanning electron microscope (SEM) image of a selected CdSe/MoS2 heterostructure region, showing the well-defined morphology and smooth surface of these CdSe nanoplates with size of ~1 micron. The lowmagnification SEM image of the as-grown heterostructure sample indicates that large amounts of CdSe nanoplates uniformly distribute on the monolayer MoS2 (see Figure S2 in the supplementary material). The thickness of the achieved CdSe nanoplates is almost in the range of 10-50 nm (see Figure S3 in the supplementary material). The CdSe/MoS2 heterostructures was also investigated by transmission electron microscope (TEM) as shown in Figure 1(d). Low-resolution TEM image of a selected CdSe/MoS2 heterojunction area is shown in Figure 1(d) inset. From the selected area electron diffraction (SAED) pattern acquired at the junction area, two sets of six-fold symmetric diffraction spots can be clearly observed, where the brighter spots marked typically by red circles correspond to CdSe [14], and the weaker spots marked typically by the green circles belong to MoS2 [15]. Therefore, these TEM results also confirm the as-synthesized heterostructures.
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PL and Raman spectra measurements were performed for the samples to further confirm the as-grown heterostructures. Figure 2(a) showed typically the SEM image of a measured heterostructure region. The PL mapping at 675 nm and 710 nm are shown in Figures 2(b) and 2(c), respectively, where the shape of hexagon zone represents CdSe and the rest represents MoS2 [16,17]. This shows clearly the heterostructure. The corresponding PL spectra are given in Figure 2(d). For pure monolayer MoS2 region, a single PL peak at 675 nm is observed. For CdSe/MoS2 heterostructure region, the PL peak is only observed to be located at 710 nm, corresponding to the CdSe emission. However, the characteristic peaks of MoS2 disappeared in the CdSe/MoS2 heterostructure region, which may be attributed to that the emission of the MoS2 was quenched by the CdSe. Raman spectra of the pristine MoS2 and heterostructure are also measured and shown in Figure 2(e), where the peaks of E2g mode at 382 cm−1 and A1g mode at 402 cm−1 are belong to MoS2 [18], and the peaks at 207 cm−1 and 415 cm−1 can be identified as longitudinal optical (LO) and 2LO phonons for CdSe [19]. It can be seen that the Raman peaks of MoS2 and CdSe co-exist in the spectrum of the heterostructure region. Therefore, these spectra measurements further confirm the as-synthesized CdSe/MoS2 heterostructures. Apart from the heterostructure synthesis, we also investigated the photoinduced interfacial charge behavior of the heterostructure through TRPL measurements. Figures 3(a) and 3(b) give the streak camera images of the CdSe and CdSe/MoS2 heterostructures recorded at 710 nm. It can be clearly observed that the lifetime of CdSe nanoplate is shortened significantly after contacting with MoS2. Figure 3(c) showed the decay curves of pristine CdSe and MoS2/CdSe heterostructure. We fit the TRPL curves by a multiexponential function as follow: D（t）∝ I (t ) = A1 exp(-
t
τ1
) + A2 exp(-
t
τ2
) + A3 exp(-
t
τ3
)
(1)
where D(t) is the exciton concentration and is assumed proportional to I(t), the TRPL intensity. τ is the time constant and A is the amplitude of each component. The three components represent the change of exciton density due to charge transfer, carrier recombination, and surface state, respectively. The TRPL curve of CdSe/MoS2 heterostructure shows the peak decay significantly faster than that of pristine CdSe. We can explain it by following functions:
τ CdSe =
1 K1 + K 2
(2)
5
1 (3) K1 + K2 + K3 Here K1 and K2 are the surface states recombination, and the radiative recombination rate of CdSe, K3
τ CdSe/ MoS = 2
represents the rate of charge transfer from CdSe to the MoS2. That means CdSe/MoS2 heterostructure has an additional channel for charge decay compared to pristine CdSe. For the PL of CdSe, two components are fitted by equation (1) for CdSe decay curve (see Table S1 in the supplementary material). We attribute the shorter one (118.9 ps, 66.3%) and the longer one (1602.3 ps, 33.7 %) to the surface states recombination and the radiative recombination, respectively. Surprisingly, a striking shrink decay curve is obtained in CdSe/MoS2 heterostructure compared to the CdSe, and three components can be well fitted by equation (1). We also attribute the shorter one (2.6 ps, 52.8%) and the longer one (907.5 ps, 1.0 %) to the surface states recombination and the radiative recombination, respectively. The additional ultra-fast component (50.7 ps, 46.1%) is regarded as the charge transfer between the CdSe and the MoS2, which results in the decreased lifetime of the CdSe. The above charge transfer can be explained from the energy band diagram of CdSe/MoS2 heterojunction as shown in Figure 3(d). According to previously reported devices of CdSe and MoS2 [20,21], the heterojunction should be type
band alignment. Upon excitation, the charge separation
of photoinduced carriers can reduce the charge recombination, which also suggests that the CdSe/MoS2 heterostructure may show superior performances in optoelectronic devices. Photodetectors were then fabricated based on the as-grown CdSe/MoS2 heterostructures(see Figure S4 in the supplementary material). Figure 4(a) presents the schematic diagram of the device where two Ti/Au electrodes (5 nm Ti and 45 nm Au) were defined on the MoS2 as the drain and source electrodes. We first measured the wavelength dependent photocurrents of CdSe/MoS2 heterostructure and pristine MoS2 (see Figure S5 in the supplementary material). It is clear that the heterostructure exhibits a higher and broader photoresponse than pristine MoS2 due to the effective charge transfer at the heterointerface. Figure 4(b) further shows the power-dependent photoresponse of the heterostructure under 637 nm laser illumination and the extracted net photocurrent Iph (Iph= Ilight-Idark) was shown in Figure 4(c). It is clear that, with the increase of the drain voltage and power density, the Iph increases gradually. We then deduce the photoresponsivity (R) of the CdSe/MoS2 heterostructure device as shown in Figure 4(d). Here, the photoresponsivity is obtained using the formula R=Iph/P*A, where Iph is the net photocurrent, P is the light power density, and A is the effective area of the detector [22]. The maximum R of the device can reach up
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to 1.63 A/W (Vds=10 V, device area of 204 µm2), which can be further improved to be 12 A/W through applying the gate voltage (see Figure S6 in the supplementary material). This value is much higher than that of previously as-grown MoS2 [23,24], and even comparable with that of most 2D/2D vertically stacked heterostructures [25,26]. The high photoresponse is mainly attributed to the efficient photogenerated carrier transfer at the heterostructure interface. Particularly, the highly dense CdSe coverage can make this process more prominent. Reliable and fast photoresponse speed is critical for a photodetector. time-resolved photoresponse measurements were performed by turning on and off the laser light (637 nm) with a chopper worked at 1 Hz. A high speed oscilloscope was used to monitor the fast-varying signal. As shown in Figure 4(e), the CdSe/MoS2 heterostructure device exhibit excellent stability and reliability with the on/off photoswitching behavior at different bias voltages. The rise time and the decay time are 370 and 380 µs, respectively as shown in Figures 4f and 4(g). Importantly, as far as we known, the photoresponse speed of the present nonlayered CdSe/layered MoS2 heterostructure photodetector is much faster than previous photodetectors.based on CVD-grown 2D heterostructures [12,13,27,28]. Particularly, as compared to previous non-2D Se/ReS2 and non-2D CdS/MoS2 heterostructures photodetectors [12,27], the present detectors demonstrate much faster response speed, which is mainly attributed to the larger CdSe coverage on the MoS2, enabling the efficient passivation of the MoS2 surface and thus the fast speed [29]. 4. Conclusions In conclusion, through a two-step CVD method, we have synthesized the CdSe/MoS2 vertical heterostructures with very large CdSe nanoplate coverage. Importantly, the efficient charge transfer can be observed via detailed TRPL measurements. Benefiting from the large CdSe coverage and charge transfer at the heterointerface, photodetectors based on the CdSe/MoS2 heterostructures show high performance with the high photoresponsivity of 12 A/W and the ultra-fast photoresponse speed of ~370 µs, which is much faster than previous photodetectors based on CVD-grown 2D heterostructures. These as-grown heterostructures may provide a stage to investigate the interaction between layered and non-layered materials and show potential applications in future optoelectronic applications. Acknowledgement
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The authors are grateful to the National Natural Science Foundation of China (Nos. 51525202, 61505051, 61574054, 51772084, 61635001, 51902098, and 51972105), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and The Foundation for Innovative Research Groups of NSFC (Grant 21521063). Authors’ contributions Y. D. Yuan and X. H. Zhang contributed equally to this work. Competing interests The authors declare that they have no competing interests.
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Figure captions Fig. 1 (a) Schematic illustration of the growth of CdSe/MoS2 heterostructures. (b) XRD patterns of the CdSe/MoS2 heterostructures and pristine MoS2. (c) The high-magnification SEM image of a selected CdSe/MoS2 heterostructure region. (d) SAED pattern of the heterostructure. The inset is the TEM image of the CdSe/MoS2 heterostructure. Fig. 2 (a) SEM image of hexagonal CdSe on MoS2. (b) PL mapping at 675 nm. (c) PL mapping at 710 nm. (d) PL spectra of the MoS2 and CdSe/MoS2 heterostructures. (e) Raman spectra of the MoS2 and CdSe/MoS2 heterostructures. Fig. 3. Streak camera images of (a) CdSe and (b) CdSe/MoS2. (c) Decay curves of pristine CdSe and CdSe/MoS2 heterostructure. (d) Band alignment and charge transfer of the heterostructure. Fig. 4 (a) Schematic of CdSe/MoS2 heterostructure photodetector. (b) I-V curves of the CdSe/MoS2 heterostructure photodetector under 637 nm laser illuminations at different incident power density. (c) Extracted power density dependent net photocurrent curves. (d) Calculated power dependent photoresponsivity at different bias voltages. Power density: 0.105 mW/cm2. (e) Photoswitching characteristics of the photodetectors based on CdSe/MoS2 heterostructures under 637 nm laser illuminations at various bias voltages. (f) Decay and (g) rise times of the CdSe/MoS2 heterostructure photodetector at bias voltage of 1 V.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Highlights
• Highly dense CdSe nanoplates were vertically assembled on monolayer MoS2
• The interfacial photoinduced charge behaviors were investigated in detail via the TRPL
• The CdSe/MoS2 heterostructures detectors demonstrate high photoresponsivity of 12 A/W
• The detectors show the very fast photoresponse speed of 370 µs
S0925-8388(19)33555-8
DOI:
https://doi.org/10.1016/j.jallcom.2019.152309
Reference:
JALCOM 152309
To appear in:
Journal of Alloys and Compounds
Received Date: 17 April 2019 Revised Date:
2 September 2019
Accepted Date: 15 September 2019
Please cite this article as: Y. Yuan, X. Zhang, H. Liu, T. Yang, W. Zheng, B. Zheng, F. Jiang, L. Li, D. Li, X. Zhu, A. Pan, Growth of CdSe/MoS2 vertical heterostructures for fast visiblewavelength photodetectors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.152309. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Growth of CdSe/MoS2 vertical heterostructures for fast visible-wavelength photodetectors (Yide Yuan 1, Xuehong Zhang 1, Huawei Liu 1, Tiefeng Yang 1, Weihao Zheng 1, Biyuan Zheng 1, Feng Jiang 1, Lihui Li 1, Dong Li 2, Xiaoli Zhu 1*, Anlian Pan1,2*)
(1. School of Physics and Electronic Science, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha, 410082, China; 2. State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Materials Science and Engineering, Hunan University, Changsha, 410082, China) Correspondence: [email protected], [email protected].
Abstract: Heterostructures composed of different semiconducting materials have aroused wide attentions due to their fascinating properties originated from the interfaces. Particularly, the recent two dimensional layered materials (2DLM) have provided novel platforms to flexibly design the heterostructures for diverse electronic and optoelectronic applications. In this work, we have reported the growth of CdSe nanoplates/MoS2 monolayer vertical heterostructures with efficient and fast visible-wavelength photodetections. Highly dense CdSe nanoplates were vertically assembled on monolayer MoS2 through a two-step chemical vapor deposition (CVD) process. The interfacial photoinduced charge behaviors were investigated in detail via the time resolution photoluminescence (TRPL) measurements, revealing the efficient charge transfer across the heterointerface. Benefiting from the large CdSe coverage and efficient charge transfer, superior photodetection performances of the CdSe/MoS2 heterostructures can be obtained with an enhanced photoresponsivity of 1.63 A/W, which can be further improved to be 12 A/W via applying the gate voltage. Besides, the heterostructure detectors also exhibit a very fast photoresponse speed of 370 µs, much faster than previous photodetectors based on CVD-grown 2D heterostructures. The as-synthesized CdSe/MoS2 heterostructures may find important applications in integrated optoelectronic systems. Keywords: CdSe/MoS2; Vertical heterostructures; Photodetectors; Responsivity; Chemical vapor deposition.
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1 Introduction Since the groundbreaking discovery of graphene in 2004 [1], 2D materials, especially the layered transition-metal dichalcogenides (TMDs), have attracted more and more attention due to their extraordinary optoelectronic properties [2-4]. To further explore the potential of these materials, plenty of van der Waals (vdW) heterostructures have been successfully synthesized, such as SnS2/WSe2 [5], WS2/MoS2 [6], and so forth [7]. Unlike the traditional heterostructure that based mainly on group IV, II-VI, or III-V semiconductor materials with the connection depended on the covalent bond on the heterogeneous boundary surface, diverse vdW heterostructures can be factitiously combined without the lattice match restrictions. Meanwhile, these heterostructures have been proved to be promising candidates for next-generation optoelectronic integration systems [6,7]. Generally, 2DLMs with planar crystal structures are the preferred component for constructing 2D heterostructures to realize the enhanced optoelectronic performances. Nonetheless, large amounts of non-2D materials with excellent optical and electrical properties, such as CdS, CdSe, InGaAs, and so forth, can also demonstrate advantages in improving the 2DLM performances [8-11]. Indeed, the heterostructures composed of 2D and non-2D materials have been fabricated and investigated extensively. Indeed, the heterostructures composed of 2D and non-2D materials have been fabricated and investigated extensively [12,13]. For instance, rhodamine 6G/MoS2 heterostructures showed the enhanced responsivity of 1.17A/W [12]. The Se/ReS2 nanoplates/MoS2 heterostructures also realized the high photoresponsivity of 36 A/W [13]. However, these heterostructures generally demonstrate the slow response speed (milliseconds to seconds) when enhancing the responsivity, hindering their practical applications. In this work, we have reported the controlled growth of high-quality heterostructures with high-density CdSe nanoplates vertically assembled on monolayer MoS2. The photoinduced interfacial charge behavior was investigated in detail via TRPL measurements, revealing the efficient charge transfer from CdSe to MoS2. Photodetectors were then fabricated based on the as-synthesized CdSe/MoS2 heterostructures. Benefiting from the ultralarge coverage of CdSe nanoplates on MoS2 and charge transfer across the heterointerface, the heterostructure detectors exhibit an enhanced photoresponsivity of 1.63 A/W, which can be further improved to be 12 A/W via applying the gate voltage. Meanwhile, the heterostructure detectors also exhibit a very fast photoresponse speed of 370 µs.
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2. Experimental 2.1 Synthesis of CdSe/MoS2 heterostructures Single-layer MoS2 nanosheets and CdSe/MoS2 heterostructures were synthesized by CVD method on a SiO2 (300 nm)/Si substrate and the preparation process was presented (see Figure S1 in the supplementary material). Firstly, N2 gas (60 sccm) was introduced into the furnace (OTF-1200X) with the substrate to evacuate atmosphere at a rate of 60 sccm. Then, sulfur and MoO3 (Alfa Aesar, 99.99%) powders, separately loaded with a quartz boat in the furnace as the charge materials, were heated to 300
and 780 ,
respectively, and kept for 30-35 min. After the deposition, the achieved samples were transferred into another furnace with CdSe powders in a quartz boat (Alfa Aesar, 99.99%). Similar to the above deposition, N2 gas was injected into the second furnace at a rate of 50 sccm and the pressure inside the furnace was kept at 200−300 Torr. The substrate and CdSe powders were heated to 250-300
and 800 , respectively, and
kept for 60 min. Finally, the sample was cooled down to room temperature. 2.2 Characterization of materials The morphologies of the as-grown CdSe/MoS2 heterostructures were characterized by SEM (FE-SEM, Zeiss sigma-HD), atomic force microscope (AFM) (Bruker Multimode 8), X-ray Diffraction (XRD) (Rigaku D/Max 2500) and TEM (Tecai F20, voltage: 300 kV). Before TEM measurements, the heterostructures were exfoliated and transferred onto copper coated grids using PMMA solvent. MicroRaman and micro-photoluminescence (PL) measurements were performed using a confocal microscopy system (WITec, alpha-300) with a laser with excitation wavelength of 532 nm (excitation power: about 20 mW). 2.3 Fabrication and measurement of the devices Firstly, the SiO2 (300 nm)/Si substrate with CdSe/MoS2 heterostructures was coated with a layer of MMA copolymer (EL6, Microchem Company), followed by a 2 minutes bake at 150 . Secondly, another layer of PMMA (495 K, A4, Microchem Company) was spin-coated on the substrate followed by a 5 minutes bake at 160 . The patternings of drain and source electrodes were defined using Electron beam lithography (Raith 150 two). Ti/Au metal layer (Ti: 5 nm, Au: 50 nm) was deposited to form the sourcedrain electrodes by electron beam evaporation, and finally followed by lift-off process using acetone. The
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optoelectronic and electrical properties of the heterostructures were measured using the Lake Shore Probe Station and Keithley 4200 semiconductor analyzer in a vacuum (10-6 torr) desiccator at room temperature. The time response of the device was measured by switching the laser on and off with an internal squarewave trigger source and recorded by a digital oscilloscope. 3. Results and discussion The growth process of CdSe/MoS2 vertical heterostructures is schematically illustrated in Figure 1(a). Triangular shaped MoS2 monolayers were initially grown on the SiO2/Si substrate through a CVD route. The as-grown MoS2 nanosheets were subsequently employed as templates for the second-step CVD growth of CdSe nanoplates. The growth time of CdSe nanoplates is set to be a large value to obtain the high CdSe coverage. In order to confirm the crystallographic phase of the as-grown samples, X-Ray diffraction (XRD) measurements were conducted and the XRD information is collected from the whole area of SiO2/Si substrate (0.5 cm × 0.5 cm) fully covered with samples (MoS2 or CdSe/MoS2). As depicted in Figure 1(b), from the diffraction pattern of CdSe/MoS2, it is clear that the (002) and (110) peaks can be assigned to the characteristic peaks of MoS2 [14]. The other diffraction peaks corresponding to the CdSe are also observed [15]. This confirms the formation of the CdSe/MoS2 heterostructures. No other peaks are observed also indicates high phase purity of the as-grown heterostructures. Figure 1(c) gives the high-magnification scanning electron microscope (SEM) image of a selected CdSe/MoS2 heterostructure region, showing the well-defined morphology and smooth surface of these CdSe nanoplates with size of ~1 micron. The lowmagnification SEM image of the as-grown heterostructure sample indicates that large amounts of CdSe nanoplates uniformly distribute on the monolayer MoS2 (see Figure S2 in the supplementary material). The thickness of the achieved CdSe nanoplates is almost in the range of 10-50 nm (see Figure S3 in the supplementary material). The CdSe/MoS2 heterostructures was also investigated by transmission electron microscope (TEM) as shown in Figure 1(d). Low-resolution TEM image of a selected CdSe/MoS2 heterojunction area is shown in Figure 1(d) inset. From the selected area electron diffraction (SAED) pattern acquired at the junction area, two sets of six-fold symmetric diffraction spots can be clearly observed, where the brighter spots marked typically by red circles correspond to CdSe [14], and the weaker spots marked typically by the green circles belong to MoS2 [15]. Therefore, these TEM results also confirm the as-synthesized heterostructures.
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PL and Raman spectra measurements were performed for the samples to further confirm the as-grown heterostructures. Figure 2(a) showed typically the SEM image of a measured heterostructure region. The PL mapping at 675 nm and 710 nm are shown in Figures 2(b) and 2(c), respectively, where the shape of hexagon zone represents CdSe and the rest represents MoS2 [16,17]. This shows clearly the heterostructure. The corresponding PL spectra are given in Figure 2(d). For pure monolayer MoS2 region, a single PL peak at 675 nm is observed. For CdSe/MoS2 heterostructure region, the PL peak is only observed to be located at 710 nm, corresponding to the CdSe emission. However, the characteristic peaks of MoS2 disappeared in the CdSe/MoS2 heterostructure region, which may be attributed to that the emission of the MoS2 was quenched by the CdSe. Raman spectra of the pristine MoS2 and heterostructure are also measured and shown in Figure 2(e), where the peaks of E2g mode at 382 cm−1 and A1g mode at 402 cm−1 are belong to MoS2 [18], and the peaks at 207 cm−1 and 415 cm−1 can be identified as longitudinal optical (LO) and 2LO phonons for CdSe [19]. It can be seen that the Raman peaks of MoS2 and CdSe co-exist in the spectrum of the heterostructure region. Therefore, these spectra measurements further confirm the as-synthesized CdSe/MoS2 heterostructures. Apart from the heterostructure synthesis, we also investigated the photoinduced interfacial charge behavior of the heterostructure through TRPL measurements. Figures 3(a) and 3(b) give the streak camera images of the CdSe and CdSe/MoS2 heterostructures recorded at 710 nm. It can be clearly observed that the lifetime of CdSe nanoplate is shortened significantly after contacting with MoS2. Figure 3(c) showed the decay curves of pristine CdSe and MoS2/CdSe heterostructure. We fit the TRPL curves by a multiexponential function as follow: D（t）∝ I (t ) = A1 exp(-
t
τ1
) + A2 exp(-
t
τ2
) + A3 exp(-
t
τ3
)
(1)
where D(t) is the exciton concentration and is assumed proportional to I(t), the TRPL intensity. τ is the time constant and A is the amplitude of each component. The three components represent the change of exciton density due to charge transfer, carrier recombination, and surface state, respectively. The TRPL curve of CdSe/MoS2 heterostructure shows the peak decay significantly faster than that of pristine CdSe. We can explain it by following functions:
τ CdSe =
1 K1 + K 2
(2)
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1 (3) K1 + K2 + K3 Here K1 and K2 are the surface states recombination, and the radiative recombination rate of CdSe, K3
τ CdSe/ MoS = 2
represents the rate of charge transfer from CdSe to the MoS2. That means CdSe/MoS2 heterostructure has an additional channel for charge decay compared to pristine CdSe. For the PL of CdSe, two components are fitted by equation (1) for CdSe decay curve (see Table S1 in the supplementary material). We attribute the shorter one (118.9 ps, 66.3%) and the longer one (1602.3 ps, 33.7 %) to the surface states recombination and the radiative recombination, respectively. Surprisingly, a striking shrink decay curve is obtained in CdSe/MoS2 heterostructure compared to the CdSe, and three components can be well fitted by equation (1). We also attribute the shorter one (2.6 ps, 52.8%) and the longer one (907.5 ps, 1.0 %) to the surface states recombination and the radiative recombination, respectively. The additional ultra-fast component (50.7 ps, 46.1%) is regarded as the charge transfer between the CdSe and the MoS2, which results in the decreased lifetime of the CdSe. The above charge transfer can be explained from the energy band diagram of CdSe/MoS2 heterojunction as shown in Figure 3(d). According to previously reported devices of CdSe and MoS2 [20,21], the heterojunction should be type
band alignment. Upon excitation, the charge separation
of photoinduced carriers can reduce the charge recombination, which also suggests that the CdSe/MoS2 heterostructure may show superior performances in optoelectronic devices. Photodetectors were then fabricated based on the as-grown CdSe/MoS2 heterostructures(see Figure S4 in the supplementary material). Figure 4(a) presents the schematic diagram of the device where two Ti/Au electrodes (5 nm Ti and 45 nm Au) were defined on the MoS2 as the drain and source electrodes. We first measured the wavelength dependent photocurrents of CdSe/MoS2 heterostructure and pristine MoS2 (see Figure S5 in the supplementary material). It is clear that the heterostructure exhibits a higher and broader photoresponse than pristine MoS2 due to the effective charge transfer at the heterointerface. Figure 4(b) further shows the power-dependent photoresponse of the heterostructure under 637 nm laser illumination and the extracted net photocurrent Iph (Iph= Ilight-Idark) was shown in Figure 4(c). It is clear that, with the increase of the drain voltage and power density, the Iph increases gradually. We then deduce the photoresponsivity (R) of the CdSe/MoS2 heterostructure device as shown in Figure 4(d). Here, the photoresponsivity is obtained using the formula R=Iph/P*A, where Iph is the net photocurrent, P is the light power density, and A is the effective area of the detector [22]. The maximum R of the device can reach up
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to 1.63 A/W (Vds=10 V, device area of 204 µm2), which can be further improved to be 12 A/W through applying the gate voltage (see Figure S6 in the supplementary material). This value is much higher than that of previously as-grown MoS2 [23,24], and even comparable with that of most 2D/2D vertically stacked heterostructures [25,26]. The high photoresponse is mainly attributed to the efficient photogenerated carrier transfer at the heterostructure interface. Particularly, the highly dense CdSe coverage can make this process more prominent. Reliable and fast photoresponse speed is critical for a photodetector. time-resolved photoresponse measurements were performed by turning on and off the laser light (637 nm) with a chopper worked at 1 Hz. A high speed oscilloscope was used to monitor the fast-varying signal. As shown in Figure 4(e), the CdSe/MoS2 heterostructure device exhibit excellent stability and reliability with the on/off photoswitching behavior at different bias voltages. The rise time and the decay time are 370 and 380 µs, respectively as shown in Figures 4f and 4(g). Importantly, as far as we known, the photoresponse speed of the present nonlayered CdSe/layered MoS2 heterostructure photodetector is much faster than previous photodetectors.based on CVD-grown 2D heterostructures [12,13,27,28]. Particularly, as compared to previous non-2D Se/ReS2 and non-2D CdS/MoS2 heterostructures photodetectors [12,27], the present detectors demonstrate much faster response speed, which is mainly attributed to the larger CdSe coverage on the MoS2, enabling the efficient passivation of the MoS2 surface and thus the fast speed [29]. 4. Conclusions In conclusion, through a two-step CVD method, we have synthesized the CdSe/MoS2 vertical heterostructures with very large CdSe nanoplate coverage. Importantly, the efficient charge transfer can be observed via detailed TRPL measurements. Benefiting from the large CdSe coverage and charge transfer at the heterointerface, photodetectors based on the CdSe/MoS2 heterostructures show high performance with the high photoresponsivity of 12 A/W and the ultra-fast photoresponse speed of ~370 µs, which is much faster than previous photodetectors based on CVD-grown 2D heterostructures. These as-grown heterostructures may provide a stage to investigate the interaction between layered and non-layered materials and show potential applications in future optoelectronic applications. Acknowledgement
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The authors are grateful to the National Natural Science Foundation of China (Nos. 51525202, 61505051, 61574054, 51772084, 61635001, 51902098, and 51972105), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and The Foundation for Innovative Research Groups of NSFC (Grant 21521063). Authors’ contributions Y. D. Yuan and X. H. Zhang contributed equally to this work. Competing interests The authors declare that they have no competing interests.
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Figure captions Fig. 1 (a) Schematic illustration of the growth of CdSe/MoS2 heterostructures. (b) XRD patterns of the CdSe/MoS2 heterostructures and pristine MoS2. (c) The high-magnification SEM image of a selected CdSe/MoS2 heterostructure region. (d) SAED pattern of the heterostructure. The inset is the TEM image of the CdSe/MoS2 heterostructure. Fig. 2 (a) SEM image of hexagonal CdSe on MoS2. (b) PL mapping at 675 nm. (c) PL mapping at 710 nm. (d) PL spectra of the MoS2 and CdSe/MoS2 heterostructures. (e) Raman spectra of the MoS2 and CdSe/MoS2 heterostructures. Fig. 3. Streak camera images of (a) CdSe and (b) CdSe/MoS2. (c) Decay curves of pristine CdSe and CdSe/MoS2 heterostructure. (d) Band alignment and charge transfer of the heterostructure. Fig. 4 (a) Schematic of CdSe/MoS2 heterostructure photodetector. (b) I-V curves of the CdSe/MoS2 heterostructure photodetector under 637 nm laser illuminations at different incident power density. (c) Extracted power density dependent net photocurrent curves. (d) Calculated power dependent photoresponsivity at different bias voltages. Power density: 0.105 mW/cm2. (e) Photoswitching characteristics of the photodetectors based on CdSe/MoS2 heterostructures under 637 nm laser illuminations at various bias voltages. (f) Decay and (g) rise times of the CdSe/MoS2 heterostructure photodetector at bias voltage of 1 V.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Highlights
• Highly dense CdSe nanoplates were vertically assembled on monolayer MoS2
• The interfacial photoinduced charge behaviors were investigated in detail via the TRPL
• The CdSe/MoS2 heterostructures detectors demonstrate high photoresponsivity of 12 A/W
• The detectors show the very fast photoresponse speed of 370 µs