1 Introduction

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 [26]. 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 [33]. The small lateral size and thermal instability of MXene also limits its application in planar optoelectronic devices [34]. 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 [35] 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 [43], 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 [44]. 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 [47]. 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. [48] (a = b ≈ 3.19 Å).

2.5 Characterization

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

A schematic illustration and the calculated electronic band structure of monolayer MoS2 are shown in Fig. 1a. The calculated electronic band structure shows that the band gap of monolayer MoS2 is 1.62 eV, which can efficiently absorb light with a wavelength below 765 nm. Figure 1b shows a schematic illustration and the calculated electronic band structure of monolayer Mo2C. Mo2C exhibits good metallic properties, similar to other TMCs without surface terminations. A schematic diagram of the Mo2C/MoS2/Au device and its energy band structure is shown in Fig. 1c; the device exhibits an asymmetric metal contact structure. The monolayer MoS2 absorbs light and internally generates electron–hole pairs. The photogenerated electrons and holes move to molybdenum carbide and gold, respectively, under the potential difference induced by the asymmetric metal contacts.

Fig. 1
figure 1

Schematic illustration (top) and calculated electronic band structure (bottom) of a monolayer MoS2 and b monolayer Mo2C. c Schematic diagram (top) and energy band diagram (bottom) of MoC2/MoS2/Au device

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A schematic diagram of the CVD growth process of Mo2C is shown in Fig. 2a. Methane is rapidly cracked at 1096 °C, providing the carbon source for the growth of Mo2C. There are two ways to grow molybdenum carbide on a copper/molybdenum film substrate using the principle of metal immiscibility. In the first method (denoted as “type I”), the copper foil is placed on molybdenum foil as a growth substrate for Mo2C crystals. Mo atoms diffuse toward the surface of the copper foil at high temperatures and eventually react with carbon atoms to form molybdenum carbide. This method can produce highly crystalline 2D Mo2C on the in situ grown graphene [36]. However, under the growth conditions of the type I method, the rapid diffusion of Mo atoms in the copper foil at high temperature results in the formation of dense Mo2C nucleation sites on the surface of the copper foil. Figures 2b and S1 show optical images of Mo2C prepared by the type I method with growth times of 20 and 180 min, respectively. The optical images reveal the presence of high-density, small-area molybdenum carbide on the surface of the copper foil. As shown in Fig. 2c, in the type I growth mode, the diffusion process of Mo atoms from the molybdenum foil to the liquid copper surface is unhindered. We developed a new Mo2C growth method, denoted as “type II.” In this method, copper foil is not placed on top of the molybdenum foil, but next to it. The distance between the Mo and the Cu foil is 2 mm. Copper vapor is deposited on the molybdenum foil under high-temperature and methane gas flow conditions. Graphene is continuously formed on the liquid copper surface during the copper vapor deposition process. Finally, Mo2C is formed on the surface of the copper film. Figure 2d shows an optical image of Mo2C prepared by the type II method with a growth time of 180 min. The comparison of Fig. 2b and S1 shows that the molybdenum carbide crystals prepared by the type II method exhibit significantly fewer nucleation sites as well as larger areas. Figure S2a, b shows the thickness of the Cu layer in the Cu/Mo substrate after 180 min of Mo2C growth by the type I and type II methods, respectively. The Cu layer thickness in Figure S2a is thicker than that in Figure S2b. Generally, the thinner the copper layer in the substrate, the higher the amount of molybdenum carbide grown on its surface [49]. However, under the same growth conditions, the copper layer obtained by the type II method is thinner and less molybdenum carbide is formed. Therefore, we can infer that the growth process of molybdenum carbide in the type II method is similar to that shown in Fig. 2e, which shows the diffusion process of molybdenum atoms in copper. Graphene has a very fast growth rate. During the deposition of copper vapor onto the molybdenum foil, a large amount of graphene crystals is formed on the copper surface and further covered by the newly deposited copper, eventually forming a graphene-intercalated copper structure. The intercalation of graphene in copper hinders the diffusion of molybdenum atoms, resulting in a small number of Mo atoms diffusing into the copper surface. Graphene on the copper surface also has a passivation effect on the nucleation of molybdenum carbide. Finally, only a small number of Mo2C nucleation sites are formed on the copper surface, which leaves room for the formation of larger Mo2C crystals. Figure S3a shows an optical image of Mo2C with graphene, which transferred on the SiO2 from the Cu/Mo substrate. Figure S3b shows the Raman spectra of graphene corresponding to the point marked “a” in Figure S3a.

Fig. 2
figure 2

a Schematic diagram of the CVD method to grown Mo2C. In the type I method, copper foil is placed on molybdenum foil to directly grow molybdenum carbide. In the type II method, the molybdenum foil is placed near the copper foil. Copper evaporates at high temperatures and adsorbs on the molybdenum foil for the growth of Mo2C. b Optical image of Mo2C prepared by the type I method with a growth time of 20 min. c Schematic diagram of the type I Mo2C growth process. d Optical image of Mo2C prepared by the type II method. e Schematic diagram of the type II Mo2C growth process. The black line represents graphene

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Figure 3a, b shows a high-resolution TEM image of Mo2C and the selected area electron diffraction (SAED) pattern along the [\( \overline{1} \) 00] zone axis, respectively. The interplanar distances of the (002) and (\( 02\overline{1} \)) planes are 2.60 and 2.61 Å, respectively. Figure 3c shows the Raman spectrum of Mo2C; the two characteristic Raman peaks of Mo2C crystals are located near 140 and 650 cm−1, respectively, in good agreement with previous reports [43, 49]. Figure 3d, e shows the high-resolution TEM image of MoS2 and the corresponding SAED pattern along the [0001] zone axis, respectively. The interplanar distances of the (\( 10\overline{1} 0 \)) and (\( 11\overline{2} 0 \)) planes are 2.79 and 1.60 Å, respectively. Figure S4 displays optical images of Mo2C and MoS2 on SiO2. The Raman spectrum of MoS2 is shown in Fig. 3f, where the \( E_{{2{\text{g}}}}^{1} \) and \( A_{{1{\text{g}}}}^{{}} \) peaks are found at 385.5 and 404.9 cm−1, respectively. The difference between the positions of the two peaks (19.4 cm−1) is characteristic of monolayer MoS2 [50]. To confirm the stoichiometry of Mo2C, the elemental distribution and electron energy-dispersive spectroscopy (EDS) data of Mo2C are shown in Figures S5 and S6, respectively.

Fig. 3
figure 3

a High-resolution TEM image of Mo2C, with space group Pbcn. b SAED pattern along the [\( \bar{1}00 \)] zone axis. c Raman spectrum of MoS2. d High-resolution TEM image of MoS2, with space group P63/mmc. e SAED pattern along the [0001] zone axis. f Raman spectrum of MoS2

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In order to investigate the characteristics of Mo2C as an electrode material, we prepared a Mo2C/MoS2/Mo2C device and tested its I–V characteristics in the dark and under illumination. Figure 4a shows the optical image of the Mo2C/MoS2/Mo2C device. At zero bias, the device does not have photodetection capabilities. At small bias voltages, the dark current of the device does not increase significantly, while the light current shows a marked increase, as shown in Fig. 4b. This result indicates that Mo2C can form a good metal–semiconductor (MS) contact with MoS2, although the Mo2C/MoS2/Mo2C device with symmetrical electrode structure cannot provide self-powered detection of light.

Fig. 4
figure 4

a Optical image of the Mo2C/MoS2/Mo2C device. b Dark (black) and light (red) I–V curves of the Mo2C/MoS2/Mo2C photodetector. The inset image shows the magnified dark I–V curve of the photodetector

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Figure 5a shows the morphology and a schematic illustration of the Mo2C/MoS2/Au photodetector. The size of the channel between the gold electrode and the Mo2C material is approximately 5 μm. Au and Mo2C are connected to the source and drain electrodes, respectively. Figure 5b shows the |I|–V curves of Mo2C/MoS2/Au photodetectors. No significant increase in dark current is observed at small bias voltages. At bias voltages of 1 and − 1 V, different light currents are generated in the device as a result of the asymmetry of the work functions of the two electrodes. Figure 5c shows the magnified I–V curves of Mo2C/MoS2/Au photodetectors in the dark and under irradiation. Under 80 mW cm−2 illumination, the photodetector exhibits an open circuit voltage of 0.16 mV and a short-circuit current of approximately 15 pA. Figure 5d shows that the Mo2C/MoS2/Au device exhibits a clear photocurrent response under irradiation with 80 mW cm−2 white light. Since the electrodes at both ends have identical work functions, no significant photocurrent response is detected for the Au/MoS2/Au device. Figure 5e shows the transfer characteristics of the Mo2C/MoS2/Au devices on 300 nm SiO2 and 300 μm n-type Si substrates. The inset shows an enlarged view of the transfer curve in the dark. The transfer curves show that the devices exhibit n-type field effect transistor (FET) characteristics, due to the sulfur vacancies and the surface states of MoS2. Since electrons in n-type MoS2 are more likely to move from the valence to the conduction band under forward bias conditions, a larger source–drain current is generated when the device is illuminated under forward bias.

Fig. 5
figure 5

a Optical image and schematic illustration of the Mo2C/MoS2/Au device. b |I|–V curves of Mo2C/MoS2/Au devices with (red) and without (black) 80 mW cm−2 white light irradiation. The inset image shows the magnified dark current. c Magnified I-V curves of Mo2C/MoS2/Au devices with a bias voltage range of − 0.1 to 0.3 mV. d Photocurrent response of self-powered Mo2C/MoS2/Au (red) and Au/MoS2/Au (black) devices under 80 mW cm−2 white light irradiation. The image in the inset shows the magnified dark current. e Transfer characteristic curves of the photodetector with and without white light irradiation

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Figure 6 shows the band energy diagram of Mo2C/MoS2/Au, without considering the surface states of MoS2. As MoS2 is connected to Au and Mo2C, a potential difference ∆Eeff is generated between Au and Mo2C. The value of ∆Eeff corresponds to the difference between the work functions of Au (ΦAu = 5.1 eV) and Mo2C (ΦMo2C = 3.8 eV): ∆Eeff = (5.1–3.8) eV = 1.3 eV. The contact between n-type MoS2 and Au forms an electron blocking layer at the interface. Because MoS2 induces the accumulation of a large amount of holes in the electron blocking layer and of electrons on the Au surface, its energy band is bent upward at the interface and generates a built-in electric field (E1) directed from MoS2 to Au. The work function of MoS2 on SiO2 is 4.49 eV [51]. In the contact between MoS2 and Au, the barrier height on the molybdenum sulfide side (∆Φ1) caused by the band bending is 0.6 eV (corresponding to the difference between the work functions of MoS2 and Au). The contact of MoS2 with Mo2C leads to the formation of an electron anti-blocking layer at the interface. Electrons and holes accumulate on the MoS2 and Mo2C side, respectively, and generate a built-in electric field (E2) from Mo2C to MoS2. The barrier height (∆Φ2) produced by the MoS2–Mo2C contact of is − 0.7 eV. Due to this band structure, electrons photogenerated in MoS2 can smoothly move from MoS2 to Mo2C instead of Au. The transfer direction of photogenerated holes in molybdenum sulfide is opposite to that of photogenerated electrons.

Fig. 6
figure 6

Schematic band energy diagram of the Mo2C/MoS2/Au device

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Figure 7a shows that the response of the device to 0.5 mW cm−2 light at 600 nm is slightly higher than that at 400, 500, and 650 nm. The Mo2C/MoS2/Au photodetector exhibits almost no response to 700 nm light. Figure S7 shows the current responses of the Mo2C/MoS2/Au photodetector to light of various wavelengths. When the wavelength of the incident light is increased to 700 nm, the photocurrent response of the device drops rapidly; therefore, the band gap of MoS2 is about 1.77 eV. Figure 7b shows the photodetector response to 600 nm light of different intensities. The photocurrent increases with increasing light intensity. At an energy density of 1.78 mW cm−2 and a wavelength of 600 nm, the device has a switching ratio of approximately 10 and a responsivity of approximately 10−1 mA W−1. As shown in Fig. 7c, the response and recovery times of the photodetectors are 23 and 28 s, respectively. Moreover, Fig. 7d shows that the responsivity of the photodetector remains approximately constant over 110 days, indicating the high reliability and stability of self-powered Mo2C/MoS2/Au photodetectors.

Fig. 7
figure 7

a Photodetector responses to light with various wavelengths and energy density of 0.5 mW cm−2. b Photodetector responses to 600 nm light of different intensities. c Response and recovery times of the photodetectors. The intensity of the incident light (600 nm) is 0.56 mW cm−2. d Long-term performance of the Mo2C/MoS2/Au photodetector under illumination with 600 nm light

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The characteristics of the Mo2C/MoS2/Au device are compared to those of photodetectors with different structures in Table 1. Au/MoS2/Mo2C shows a larger difference between the work functions of the electrodes than that of other asymmetric structures. The self-powered Au/CVD-MoS2/Mo2C device has a slightly faster response speed than Au/CVD-MoS2/Au which should work at bias voltage. Although the responsiveness of the Au/MoS2/Mo2C devices measured in this study is not outstanding, the responsivity of photodetectors driven by an asymmetric contact can be improved by reducing the spacing between the electrodes or decorating light-absorbing materials with quantum dots [27, 52]. We thus believe that the Mo2C/semiconductor/Au configuration is an effective asymmetrical structure for self-powered photodetectors.

Table 1 Comparison of the characteristics and performance of Mo2C/MoS2/Au photodetectors with photodetectors with other structures
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4 Conclusions

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.