Heating-up Synthesis of MoS2 Nanosheets and Their Electrical Bistability Performance
© Li et al. 2016
Received: 12 February 2016
Accepted: 21 March 2016
Published: 31 March 2016
Molybdenum disulfide (MoS2) nanosheets were synthesized by using a simple heating-up approach, in which 1-dodecanethiol (DDT) was used not only as a sulfur source but also as the surface ligand. The sheet-like morphology was confirmed by the transmission electron microscopy (TEM) and atomic force microscopy (AFM) results, and the X-ray diffraction (XRD) patterns and Raman spectrum were employed to characterize the structure of the as-synthesized MoS2 nanosheets. The as-obtained MoS2 nanosheets blending with a polymer could be used to fabricate an electrically bistable device through a simple spin-coating method, and the device exhibited an obvious electrical bistability in the I-V curve. The charge transport of the device was discussed based on the organic electronic models.
In the past few decades, much attention has been paid to the two-dimensional (2D) nanomaterials due to their surprising unique physical, electrical, and chemical properties arising from high surface area and quantum confinement effects in two dimensions [1–3]. Therefore, the 2D nanomaterials exhibit wide potential applications in various applications, such as optoelectronic devices, energy conversion, and bio-sensing [4–9]. To date, different types of 2D compounds have been developed, and layered transition metal dichalcogenides (TMDCs) have become one of the most popular objectives due to their indirect-to-direct bandgap transition when exfoliated to monolayer and possessing intriguing optical properties [4, 9]. Therefore, it is necessary to exploit synthetic strategies to prepare TMDC nanocrystals. Till now, different synthetic approaches including chemical vapor deposition [10, 11], ion-intercalation and exfoliation [12, 13], and colloidal chemical synthesis [14, 15] have been developed for synthesis of TMDCs. Among these different synthetic methods, colloidal chemical synthesis is widely used in the preparation of ultrathin 2D nanomaterials. Although some progress has been made in the colloidal synthesis of 2D nanomaterials, some challenges still remain in this field. Therefore, it is interesting to develop a simple heating-up (non-injection) method to prepare molybdenum disulfide (MoS2) nanosheets, which is reliable and does not need any injection and pre-synthesis of precursors.
In this paper, a simple heating-up colloidal chemical approach has been developed to prepare MoS2 nanosheets, which involves the direct heating of the mixture of molybdenum precursors and 1-dodecanethiol (DDT) in the non-coordinating solvent. The crystal structure, morphology, and chemical composition of the as-obtained products were characterized by X-ray diffraction (XRD) patterns, Raman spectra, transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopic (XPS) techniques. The MoS2 nanosheets could disperse well in the semiconducting polymer, which was used to fabricate sandwiched structured electrically bistable devices. An obvious electrical bistability was observed in the current-voltage (I-V) results, and the charge transport mechanism was discussed based on the organic electronic models.
A typical synthesis MoS2 nanosheets is described as follows. Stoichiometric of Mo(acac)2, DDT, and 1-octadecene (ODE) were mixed in a 50-ml three-necked flask, and then the reaction solution was purged through nitrogen for 20 min under magnetic stirring. Afterwards, the mixture was heated to 280 °C slowly under the protection of nitrogen and kept at the temperature for 4 h. After the completion of the reaction, the reaction mixture was cooled down to room temperature naturally after removal of the heating source. The MoS2 nanosheets were obtained by addition of excess ethanol and centrifuged at 6000 rpm for 10 min, and then the precipitates were redispersed in chloroform. The precipitation and washing process was repeated twice, and the as-obtained products were dispersed in chloroform or dried in vacuum for next characterization.
The as-obtained MoS2 nanosheets were mixed with poly (N-vinylcarbazole) (PVK) in chlorobenzene, in which the weight ratio of MoS2 to PVK was 1:1 and the total concentration was 20 mg/mL. The electrically bistable device was fabricated as follows: the glass substrate coated with an indium-tin-oxide (ITO) anode was pre-cleaned and then the poly (3, 4-ethylenedioxy- thiophene):poly-(styrene-sulfonate) (PEDOT:PSS) was spin-coated onto the substrate as a buffer layer and then annealed at 150 °C for 15 min. Afterwards, the MoS2:PVK thin film was formed by using a spin-coating technique. Finally, the Al top electrodes were thermally evaporated through a shadow mask at a pressure of approximately 10−6 Torr.
The XRD patterns were measured by a D8 ADVANCE X-ray diffractometer. The thermogravimetric analysis (TGA) and differential thermal gravity (DTG) were taken on a PYRISI thermal gravimetric analyzer. The Raman spectrum was obtained using HR Evolution Raman spectrometer. TEM images were collected by Tecnai G2 F20 transmission electron microscope. The AFM patterns of MoS2 nanosheets were measured in Bruker Multimode 8 Scanning probe microscopes (SPMs). The XPS measurements were performed on an ESCALAB 250 spectrometer with a 300-W Al Ka radiation source. The I-V characteristics of the devices were measured by using a Keithley Source Meter 2612 controlled by a computer. All the measurements were carried out at room temperature.
Results and Discussion
In summary, a simple heating-up colloidal approach was developed to prepare MoS2 nanosheets, which was synthesized in DDT and non-coordinating solvent. The as-obtained products had a sheet-like morphology, whose crystal structure was characterized by the XRD patterns and Raman spectrum. An electrically bistable device was fabricated based on the blends of MoS2 nanosheets and PVK, and an obvious electrical bistability and NDR behavior was observed in the I-V curves. The charge transport could be described in terms of the organic electronic models, and the charge transport mechanism changed from the thermionic emission to the ohmic model during the transition of the conducting state from an OFF state to ON state. The result indicates that the MoS2 nanosheets may have potential application in the organic/inorganic hybrid electrically bistable devices.
This work is partly supported by the Fundamental Research Funds for the Central Universities (2014JBZ010) and and the National Nature Science Foundation of China (No. 61125505, No.61205180). The authors (XL, LG) also appreciate the financial support from the Midwest Universities Comprehensive Strength Promotion Project.
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