MoS2-Based Photodetectors Powered by Asymmetric Contact Structure with Large Work Function Difference
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2D Mo2C was produced by a modified chemical vapor deposition method, and 2D Mo2C-Au was formed as an asymmetric contact structure with a large work function difference.
Mo2C/MoS2/Au photodetectors powered by asymmetric contact structure can work under self-powered condition with a responsivity of 10−1 mA W−1. The detection performance of the photodetectors can be stable for at least 110 days.
KeywordsMo2C MoS2 Chemical vapor deposition Asymmetric metal contacts Photodetector
Self-powered and high-sensitivity miniature photodetectors are very attractive for a broad range of applications, spanning the imaging, environmental monitoring, and biomedical diagnostics fields [1, 2, 3, 4, 5, 6, 7]. Two-dimensional (2D) materials offer the opportunity to produce the performance of self-powered and high-sensitivity miniature photodetectors [8, 9, 10, 11, 12, 13, 14]. 2D material-based photodetectors with various structures have been widely reported [15, 16, 17, 18, 19, 20, 21, 22, 23]. The fabrication of asymmetric metal contact structures based on 2D materials is an effective method to obtain self-powered photodetectors [24, 25]. One advantage of self-powered devices driven by an asymmetric contact is that they are suitable for long operation times . The large difference in work function between asymmetric source and drain contact metal is beneficial for the fabrication of efficient detection devices [27, 28]. However, common metal electrodes have a high work function, which makes it difficult to create an asymmetric structure with a large work function difference between the contacts. Obtaining ultrathin and stable electrode materials with a low work function is still a major challenge. Some ultrathin transition metal carbides (TMCs) with low work function may be potential candidates for the fabrication of asymmetric contact structures [29, 30, 31, 32]. Nevertheless, the materials prepared by chemically etching methods (known as MXenes) always exhibit surface terminations, such as hydroxyl, oxygen, or fluorine, which affect their properties and structures . The small lateral size and thermal instability of MXene also limits its application in planar optoelectronic devices . The chemical vapor deposition (CVD) method offers a new way to synthesize large-area 2D TMCs (such as Mo2C, TaC, and WC) and their derivatives, without surface terminations [35, 36, 37, 38, 39, 40]. Ultrathin Mo2C crystals with a work function of 3.8 eV, prepared by the CVD method, are both thermally (they remain stable in air at 200 °C) and environmentally stable under H2 plasma treatment [41, 42, 43]. It has been reported that CVD-grown ultrathin Mo2C (CVD-Mo2C) has significant superconducting properties, with a high critical temperature  and is suitable for the preparation of asymmetric metal structures. However, dense nucleation sites are often formed during the Mo2C preparation by the CVD method, which limits the lateral growth of Mo2C.
In this work, we first optimized the CVD process for preparing Mo2C. The use of graphene-intercalated copper can reduce the number of Mo2C nucleation sites in the initial growth stage, thereby facilitating the synthesis of large-area Mo2C. Then, a self-powered Mo2C/MoS2/Au device with an asymmetric metal contact structure was fabricated. The large work function difference (1.3 eV) between Mo2C and Au favors the fabrication of an efficient MoS2-based photodetector. The Mo2C/MoS2/Au-structured device could enable the detection of light without external electric power. We believe that this novel device provides a new direction for the design of self-powered miniature detectors. The present work also highlights the great potential of ultrathin CVD-Mo2C in electrode applications.
2 Experimental and Computational Methods
2.1 Preparation of MoS2
Monolayer triangular MoS2 was synthesized through an atmospheric pressure CVD system with a two-temperature-zone tube furnace. Thoroughly cleaned n-type SiO2 (300 nm) on Si was used as the substrate. Molybdenum trioxide powder (Mo source) and sulfur powder (S source) were vaporized at temperatures of 850 and 180 °C, respectively, for the synthesis of MoS2 on the SiO2 substrate. Finally, the furnace was cooled down to room temperature naturally.
2.2 Preparation of Mo2C
Three-layer, 25-μm-thick Cu foil (Alfa Aesar, 99.95% purity) was cut into pieces and placed on a 50-μm-thick Mo foil (Alfa Aesar, 99.95% purity). The Cu/Mo substrate was placed in a single-temperature-zone CVD system and heated to above 1096 °C under 200 sccm Ar (type I method, see below). The other Mo foil was placed near the Cu/Mo substrate (type II method). CH4 (5 sccm) and H2 (300 sccm) were introduced into the chamber to grow the Mo2C crystals. Finally, the tube furnace was naturally cooled to room temperature before collecting the Mo2C sample.
2.3 Device Fabrication
After cleaning the Mo2C on Cu/Mo with H2 plasma to remove graphene from the surface of the sample , the Mo2C on the Cu/Mo substrate was placed face down on the silicon oxide wafer on which MoS2 was grown. A 1 M (NH4)2S2O8 aqueous solution was then used for etching the Cu layer, following which the Mo2C on the substrate surface dropped onto the silicon oxide wafer. After the sample was dried, a photoresist was spin-coated on the Mo2C–MoS2 sample at 4000 rpm for 60 s. After drying at 90 °C for 1.5 min, the target site for the vapor deposition of a gold electrode on the Mo2C-MoS2 was exposed with a 200 mW cm−2 laser. Then, the sample was placed in the developing solution and allowed to stand for 1 min. After rinsing the developing solution with deionized water, gold was evaporated onto the sample. Finally, we obtained the multifunctional and horizontally structured Mo2C/MoS2/Au sample by washing away the remaining photoresist with acetone.
2.4 Theoretical Calculations
Structural relaxations and electronic calculations were performed by first-principles simulations based on density functional theory, as implemented in the CASTEP package . The exchange–correlation interaction was treated within the generalized gradient approximation (GGA), using the Perdew–Burke–Ernzerhof (PBE) functional and a plane-wave basis with a kinetic energy cutoff of 500 eV [45, 46]. The long-range van der Waals interactions were considered using the DFT-D2 dispersion correction proposed by Grimme . The atomic positions and cell vectors were relaxed until the maximum force and maximum stress tolerance were less than 0.01 eV Å−1 and 0.02 GPa, respectively. Vacuum gaps of at least 15 Å were used to minimize the interactions between adjacent images of the single-layer structure. The reciprocal space was sampled with dense grids of 16 × 16 × 1 (for structural optimizations) or 20 × 20 × 1 (for accurate band structure calculations) k-points in the Brillouin zone. The lattice constant, Mo–S bond length, and S–Mo–S bond angle of MoS2 after the structural optimization were 3.218 Å, 2.437 Å, and 80.63°, respectively. The calculated bond length of 3.218 Å is slightly different from that of the optimized lattice structure reported by Saha et al.  (a = b ≈ 3.19 Å).
Raman spectroscopy (LabRAM HR800, He–Ne laser excitation at 532 nm) was used for structural characterization. Optical images were acquired by a Leica DM4000 M microscope. Field-emission scanning electron microscopy (FE-SEM, FEI Nova Nano-SEM 450) and transmission electron microscopy (TEM, Tecnai G220 U-TWIN) were used for investigating the morphology and structure of the samples. The current–time characteristics of the photodetector were measured by a low-temperature cryogenic probe station (Lake Shore CRX-6.5 K), a semiconductor parameter analyzer (Keithley 4200-SCS), and a light source (Energetiq EQ-1500).
3 Results and Discussion
Comparison of the characteristics and performance of Mo2C/MoS2/Au photodetectors with photodetectors with other structures
Work function difference
4.9 mA W−1
< 7 ns/< 7 ns
52 mA W−1
5 V (bias voltage)
2.97 × 104 A W−1
30 s/32 s
1 V (bias voltage)
0.42 mA W−1
50 ms/50 ms
0.1 mA W−1
23 s/28 s
In conclusion, our results show that the use of graphene-intercalated copper to prepare Mo2C can affect the diffusion of Mo atoms in copper and reduce the number of Mo2C nucleation sites in the initial growth stage, thereby facilitating the synthesis of large-area Mo2C. The physical and chemically stable Mo2C has a low work function and is well suited for the preparation of asymmetric metal contact structures. The MoS2-based photodetectors powered by asymmetric contact structure with large work function difference can detect light of wavelength below 700 nm without external power. The responsivity of Mo2C/MoS2/Au photodetectors is approximately 10−1 mA W−1 under light irradiation at 600 nm and 1.78 mW cm−2. The response and recover times are 23 and 28 s, respectively. This novel device may open new avenues for the design of self-powered multifunctional miniature devices. The present study also reveals the great potential of ultrathin CVD-Mo2C in electrode applications.
This work was supported by the National Natural Science Foundation of China (11674113, U1765105). Y.H.G. thanks Prof. Zhong Lin Wang for the support of experimental facilities in WNLO of HUST. The authors thank Analysis and Testing Center of HUST for support.
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