A Facile Way to Fabricate High-Performance Solution-Processed n-MoS2/p-MoS2 Bilayer Photodetectors

Two-dimensional (2D) material has many advantages including high carrier mobilities and conductivity, high optical transparency, excellent mechanical flexibility, and chemical stability, which made 2D material an ideal material for various optoelectronic devices. Here, we developed a facile method of preparing MoS2 nanosheets followed by a facile liquid exfoliation method via ethyl cellulose-assisted doping and utilizing a plasma-induced p-doping approach to generate t effectively the partially oxided 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 is able to 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. We believe that this work provides important scientific insights for photoelectric response properties of emerging atomically layered 2D materials for photovoltaic and other optoelectronic applications.

In this work, we report that a novel liquid exfoliation method via ethyl cellulose-assisted doping can prepare an excellent thin MoS 2 nanosheets and very effective method to generate the partially oxidized MoS 2 (p-MoS 2 ) nanosheets from the pristine n-type nanosheets. Moreover, an n-p junction type MoS 2 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 MoS 2 nanosheets/p-MoS 2 /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.

Material Synthesis
Molybdenum disulfide (MoS 2 ) nanosheets were synthesized by liquid ultrasound exfoliation as reported in the literature [35,36]. Typically, MoS 2 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 MoS 2 precipitation was washed with deionized water and collected by vacuum filtration (0.45 μm filter paper). Finally, the MoS 2 nanosheet product was dried as a fine black powder. The final MoS 2 nanosheets were defined as n-MoS 2 . For the preparation of p-MoS 2 nanosheets, the n-MoS 2 powder was taken a UV-ozone plasma treatment for 40 min to completely change to p-MoS 2 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 MoS 2 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-MoS 2 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-MoS 2 nanosheets (15 mg/ml, in isopropanol) was spin coated on n-MoS 2 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-MoS 2 nanosheets layer by thermal

Results and Discussion
The equal concentration of pristine MoS 2 and MoS 2 nanosheets after the liquid ultrasound exfoliation solution   (Fig. 1a) and evident MoS 2 particles adhere to the sidewall. In contrast, the MoS 2 nanosheets after the liquid ultrasound exfoliation solution show a highly uniform and homogeneous suspension solution (Fig. 1b), indicating the successful preparation of MoS 2 nanosheets with the good dispensability.
In order to verify the degree of dispersion of exfoliated MoS 2 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 MoS 2 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 MoS 2 , and clearly stacked MoS 2 can be seen. However, Fig. 2b displays an individual MoS 2 sheet with six spot pattern in the selected-area electron diffraction (SAED) of MoS 2 , suggesting that MoS 2 is scattered as individual MoS 2 nanosheet [38,39]. Also, the severe aggregation of the pristine MoS 2 can be observed in SEM images (Fig. 2c), intriguingly, after being treated by ethyl cellulose ethanol solution via liquid ultrasound exfoliation, MoS 2 nanosheets can fully cover and tightly attach on the ITO substrate with a quite smooth surface morphology (Fig. 2d).
To further verify morphology results, the XRD patterns of pristine and exfoliated MoS 2 nanosheets (Fig. 3a)  only the peaks of (103) and (002) plane remain after liquid exfoliation which confirms that the MoS 2 nanosheets were successfully striped [40,41]. Moreover, the disappearance of other peaks could prove that ultrathin MoS 2 nanosheets are tightly deposited on the ITO glass with preferred ductility. The Raman spectrum can once again prove the exfoliation of MoS 2 nanosheets. The two peaks (1 and 2 g) between 360 and 430 cm −1 are the main peak of MoS 2 [42][43][44]. After liquid exfoliation, the obvious decrease of the intensity of the two peaks was observed.
It is well known that the MoS 2 nanosheets are n-type semiconductor materials and several researches have been reported that MoS 2 could be changed as a p-type semiconductor material with a relative high work function after UV-ozone plasma treatment. Thus, the properties of MoS 2 nanosheets with or without the UV-ozone plasma treatment were also investigated. Figure 4a is the X-ray photoelectron spectroscopy (XPS) profile of n-MoS 2 nanosheets (without plasma treatment) and p-MoS 2 nanosheets (with plasma treatment). The Mo 3D spectra of pristine MoS 2 nanosheets demonstrate outstanding Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 bands at 228.7 and 231.5 eV, in agreement with the other works for n-MoS 2 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 MoO 3 [45,46]. Therefore, it proved that n-MoS 2 nanosheets can be successfully oxidized to p-type materials after UV-ozone plasma treatment. Since the MoS 2 layer is very thin via the spin-coating method, it is important to analyze the bilayer junction existing at the interface of n-MoS 2 /p-MoS 2 . To gain insight into the electronic structures of the n-MoS 2 /p-MoS 2 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 MoS 2 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 MoS 2 nanosheets change to p-type MoS 2 nanosheets [47].
On the basis of the above results, we have constructed an energy diagram showing the band bending behavior at the n-MoS 2 /p-MoS 2 bilayer junction interface, as shown in Fig. 5a. The n-MoS 2 /p-MoS 2 bilayer junction with a built-in potential promises an excellent photodetector performance with a ITO/n-MoS 2 /p-MoS 2 /Ag device structure (Fig. 5b) which will be discussed later. The photocurrent-voltage curves and the photocurrentvoltage 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-MoS 2 /p-MoS 2 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-MoS 2/ p-MoS 2 bilayer junction can harvest nearly the whole energy range of visible light.

Conclusions
We have demonstrated a high-quality n-MoS 2 /p-MoS 2 bilayer junction-based device to achieve the high performance photoresponse which can harvest nearly the whole energy range of visible light. Excellent, thin exfoliated MoS 2 nanosheets are realized by a facile liquid exfoliation, changing the n-type MoS 2 nanosheets to p-type MoS 2 nanosheets via a simple plasma treatment. This work shows that thin MoS 2 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.