Background

Over the last decade, two-dimensional (2D) nanomaterials have drawn great attention because of their unique structures, large natural abundance, and distinctive properties compared to their bulk forms, and a broad range of applications in catalysis, electronics, energy-storage devices, optoelectronics, and so on [1–11]. In particular, the semiconducting layered transition metal dichalcogenides (LTMDs, e.g., WSe2, WS2, and MoS2) have gained significant interest on optoelectronics due to their direct bandgaps, possessing intriguing optical properties suitable for optoelectronic applications in light-emitting diodes and photovoltaics [12–14]. Usually, LTMDs have a unique 2D X–M–X structure in which the transition metal atom layer is sandwiched between two close-packed chalcogen atom layers [1, 2, 15–17].

As a prototypical compound of LTMDs, MoS2 has been extensively studied. Bulk MoS2 is a typical semiconductor with an indirect bandgap. Expectedly, monolayer MoS2 transistors have been demonstrated with on/off ratios of 108 and ultralow standby power dissipation [17–19]. However, to realize the highly efficient optoelectronic devices based on MoS2, it is also important to develop a strategy to prepare ultrathin MoS2 nanosheets and tune the bandgaps with facile process. Several methods, such as mechanical exfoliation (the so-called Scotch tape method), liquid exfoliation, colloidal synthesis, chemical vapor deposition, chemical exfoliation, and electrochemical exfoliation have been developed to prepare ultrathin MoS2 nanosheets [2, 20–30]. Among these methods, liquid exfoliation not only produces novel materials with the same composition yet dramatically changed electrical properties but also provides a facile way to prepare thin-layer nanosheets, which offers novel opportunities in the optoelectronics applications [17, 31–34].

In this work, we report that a novel liquid exfoliation method via ethyl cellulose-assisted doping can prepare an excellent thin MoS2 nanosheets and very effective method to generate the partially oxidized MoS2 (p-MoS2) nanosheets from the pristine n-type nanosheets. Moreover, an n-p junction type MoS2 photodetector device with the built-in potentials to separate the photogenerated charges can result in significantly improved visible light response. We have fabricated photodetector devices consisting of a vertically stacked indium tin oxide (ITO)/pristine n-type MoS2 nanosheets/p-MoS2/Ag structure, which exhibit reasonably good performance illumination, as well as high current values in the range of visible wavelength from 350 to 600 nm. This work provides important scientific insights for leveraging unique optoelectronic properties of 2D materials for photodetector applications.

Methods

Material Synthesis

Molybdenum disulfide (MoS2) nanosheets were synthesized by liquid ultrasound exfoliation as reported in the literature [35, 36]. Typically, MoS2 power (0.25 g, Aladdin) was dispersed in ethyl cellulose (EC) isopropanol solution (1 % w/v dispersion, 100 ml) in a SEBC bottle. The dispersion was sonicated for 24 h at 60 W in water bath. The resulting dispersion was centrifuged (Desktop High-speed Refrigerated Centrifuge Model TGL-16) at 5000 rpm for 15 min, and then the supernatant liquid was directly collected. Deionized water was mixed with the supernatant liquid (3:4 weight ratio) and subsequently centrifuged at 7500 rpm for 10 min. Whereafter, the lower precipitation was collected and dried. The resulting precipitation was redispersed in ethanol (10 mg/ml). NaCl aqueous solution (0.04 g/ml) was mixed with the redispersion (9:16 weight ratio) and centrifuged at 5000 rpm for 8 min, discarding the supernatant. To debride any residual salt, the resulting MoS2 precipitation was washed with deionized water and collected by vacuum filtration (0.45 μm filter paper). Finally, the MoS2 nanosheet product was dried as a fine black powder. The final MoS2 nanosheets were defined as n-MoS2. For the preparation of p-MoS2 nanosheets, the n-MoS2 powder was taken a UV-ozone plasma treatment for 40 min to completely change to p-MoS2 nanosheets.

Characterizations

TEM images were taken by a FEI TECNAI G2 F20-TWIN TEM. Raman spectra were recorded on inVia Raman microscope. XPS and UPS measurements were conducted using an ESCALAB 250Xi (Thermo) system. X-ray diffraction (XRD) patterns of the MoS2 was carried out on a Bruker D8 Focus X-ray diffractometer operating at 30 kV and 20 mA with a copper target (λ= 1.54 Å) and at a scanning rate of 1°/min.

Photodetector Device Fabrication

All devices were fabricated on pre-treatment ITO glass substrates [37] (sheet resistance <10 Ωsq−1, ShenZhen NanBo Display Technology Co., Ltd.); cleaned sequentially using sonication in acetone, detergent, deionized water, and isopropanol; and then dried under a nitrogen stream, followed by ultraviolet light irradiation. Then, the n-MoS2 nanosheets (10 mg/ml, in isopropanol) spin coated with 2000 rpm and thermally annealed at 150 °C for 15 min receive a thickness of 80 nm. Thereafter, the p-MoS2 nanosheets (15 mg/ml, in isopropanol) was spin coated on n-MoS2 nanosheets layer, followed by thermal annealing at 150 °C for 10 min in atmospheric environment. Eventually, Argentum Ag (150 nm) was deposited over the p-MoS2 nanosheets layer by thermal evaporation under a vacuum of 6 × 10−6 Torr to accomplish the device fabrication. The effective area of one cell was ~1 cm2. The photocurrent-voltage curves and I-T curves were measured with a Keithley 2400 source meter and a 150-W Xe lamp light source. The dark current-voltage curves were measured by Keithley 2400 source meter under dark. All the measurements were performed under ambient atmosphere at room temperature. The incident photo-to-electron conversion efficiency spectrum (IPCE) were detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the incident light was performed with a monocrystalline silicon diode.

Results and Discussion

The equal concentration of pristine MoS2 and MoS2 nanosheets after the liquid ultrasound exfoliation solution (10 mg/ml) was treated with ultrasound in ethanol for 30 min, respectively. The detailed process is demonstrated in experimental section. The photographs of pristine MoS2 and MoS2 nanosheets isopropanol dispersion solutions after ultrasound treatment are shown in Fig. 1. After storing for 48 h, humorous aggregation can be observed in pristine MoS2 solution (Fig. 1a) and evident MoS2 particles adhere to the sidewall. In contrast, the MoS2 nanosheets after the liquid ultrasound exfoliation solution show a highly uniform and homogeneous suspension solution (Fig. 1b), indicating the successful preparation of MoS2 nanosheets with the good dispensability.

Fig. 1
figure 1

The images are camera pictures of a pristine MoS2 and b MoS2 nanosheets dispersion

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In order to verify the degree of dispersion of exfoliated MoS2 nanosheets by ethyl cellulose ethanol solution via liquid ultrasound exfoliation, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed (Fig. 2). For comparison, the morphologies of the pristine MoS2 nanosheets prepared by 150 °C thermal annealing for 10 min were also determined. All of samples were spin-coated on ITO and tested in the same testing conditions. Figure 2a shows a rough morphology of the pristine MoS2, and clearly stacked MoS2 can be seen. However, Fig. 2b displays an individual MoS2 sheet with six spot pattern in the selected-area electron diffraction (SAED) of MoS2, suggesting that MoS2 is scattered as individual MoS2 nanosheet [38, 39]. Also, the severe aggregation of the pristine MoS2 can be observed in SEM images (Fig. 2c), intriguingly, after being treated by ethyl cellulose ethanol solution via liquid ultrasound exfoliation, MoS2 nanosheets can fully cover and tightly attach on the ITO substrate with a quite smooth surface morphology (Fig. 2d).

Fig. 2
figure 2

The transmission electron microscopy (TEM) images of a pristine MoS2 and b MoS2 nanosheets films on glass substrate, and the inset is selected area electron diffraction (SAED) pattern of the MoS2 nanosheets. The scanning electron microscopy (SEM) images of c pristine MoS2 and d MoS2 nanosheets films on glass substrate

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To further verify morphology results, the XRD patterns of pristine and exfoliated MoS2 nanosheets (Fig. 3a) only the peaks of (103) and (002) plane remain after liquid exfoliation which confirms that the MoS2 nanosheets were successfully striped [40, 41]. Moreover, the disappearance of other peaks could prove that ultrathin MoS2 nanosheets are tightly deposited on the ITO glass with preferred ductility. The Raman spectrum can once again prove the exfoliation of MoS2 nanosheets. The two peaks (1 and 2 g) between 360 and 430 cm−1 are the main peak of MoS2 [42–44]. After liquid exfoliation, the obvious decrease of the intensity of the two peaks was observed.

Fig. 3
figure 3

a XRD patterns of the MoS2 films on glass substrate. b Raman spectrum of MoS2 films on glass substrate

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It is well known that the MoS2 nanosheets are n-type semiconductor materials and several researches have been reported that MoS2 could be changed as a p-type semiconductor material with a relative high work function after UV-ozone plasma treatment. Thus, the properties of MoS2 nanosheets with or without the UV-ozone plasma treatment were also investigated. Figure 4a is the X-ray photoelectron spectroscopy (XPS) profile of n-MoS2 nanosheets (without plasma treatment) and p-MoS2 nanosheets (with plasma treatment). The Mo 3D spectra of pristine MoS2 nanosheets demonstrate outstanding Mo4+3d5/2 and Mo4+3d3/2 bands at 228.7 and 231.5 eV, in agreement with the other works for n-MoS2 nanosheets. However, the two strong peaks have a notable shift to 235.3 and 232.5 eV, respectively, which is similar with the spectra of MoO3 [45, 46]. Therefore, it proved that n-MoS2 nanosheets can be successfully oxidized to p-type materials after UV-ozone plasma treatment. Since the MoS2 layer is very thin via the spin-coating method, it is important to analyze the bilayer junction existing at the interface of n-MoS2/p-MoS2. To gain insight into the electronic structures of the n-MoS2/p-MoS2 bilayer junction, we have performed the UPS analysis. The work function was calculated through the difference between the cutoff of the highest binding energy and the photon energy of the exciting radiation. The valence band (VB) can be calculated from the cutoff from the lowest binding energy. As shown in Fig. 4b, after UV-ozone plasma treatment, the work function of the MoS2 nanosheets has increased from 4.3 to 5.2 eV. The energy difference between the Fermi level and valence band maximum is decreased from 1.4 to 0.4 eV, demonstrating the n-type MoS2 nanosheets change to p-type MoS2 nanosheets [47].

Fig. 4
figure 4

a Mo 3D region and of X-ray photoelectron spectroscopy (XPS) profiles of MoS2 nanosheets with or without plasma treatment. b The ultraviolet photoelectron spectroscopy (UPS) spectra of MoS2 nanosheets with or without plasma treatment

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On the basis of the above results, we have constructed an energy diagram showing the band bending behavior at the n-MoS2/p-MoS2 bilayer junction interface, as shown in Fig. 5a. The n-MoS2/p-MoS2 bilayer junction with a built-in potential promises an excellent photodetector performance with a ITO/n-MoS2/p-MoS2/Ag device structure (Fig. 5b) which will be discussed later. The photocurrent-voltage curves and the photocurrent-voltage were measured with the Keithley 2400 source meter. As shown in Fig. 6a, b, the device shows the photovoltaic response under a 150-W Xe lamp light source illumination. The result shows the device have a p-n junction inside. In order to understand the photoelectric response properties in more detail and detect potential application in photoelectronic fields, we have performed further experiments of photodetector at a 1-V DC bias as shown in Fig. 7a, b. As seen from Fig. 7a, b, the photocurrent increases at an applied dc bias voltage of 0 and 1 V. Moreover, the photoresponse is steady, prompt, and reproducible during repeated on/off cycles of visible light illumination. More importantly, the n-MoS2/p-MoS2 bilayer junction-based device shows a very broad photoelectric response range from 350 to 600 nm, as shown in Fig. 7c, and therefore, the n-MoS2/p-MoS2 bilayer junction can harvest nearly the whole energy range of visible light.

Fig. 5
figure 5

a The schematic energy diagram of the MoS2 photodetector b the structure of MoS2 photodetector device

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Fig. 6
figure 6

Current-voltage curves of the device a under a 150-W Xe lamp light source illumination and b in dark

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Fig. 7
figure 7

a The output signal of photocurrent under alternating light on and light off, where the entire device was illuminated by a 150-W Xe lamp irradiation. Photoresponse of MoS2-based photodetector at a 0-V DC bias voltage. b Photoresponse of MoS2-based photodetector at 1-V DC bias voltage. c The spectral photoresponse vs. wavelength, showing a broad photoresponse range from 350 to 650 nm, which is, the absorption spectrum of the nanohybrid covers the whole energy range of visible light

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Conclusions

We have demonstrated a high-quality n-MoS2/p-MoS2 bilayer junction-based device to achieve the high performance photoresponse which can harvest nearly the whole energy range of visible light. Excellent, thin exfoliated MoS2 nanosheets are realized by a facile liquid exfoliation, changing the n-type MoS2 nanosheets to p-type MoS2 nanosheets via a simple plasma treatment. This work shows that thin MoS2 nanosheets can be fully integrated into the photodetector manufacturing process, which holds promise for realizing 2D materials in a variety of optical electronic and optical devices.