- Nano Express
- Open Access
A Facile Way to Fabricate High-Performance Solution-Processed n-MoS2/p-MoS2 Bilayer Photodetectors
© Ye et al. 2015
- Received: 18 October 2015
- Accepted: 17 November 2015
- Published: 25 November 2015
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.
- Liquid exfoliation method
- Partially oxided MoS2
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.
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.
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  (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.
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.
This study was financially supported by the Natural Science Research Projects Funded of Anhui Colleges and Universities (KJ2015A224) and Scientific Research Fund of Anhui Province Education Department (KJ2010B104).
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