Electronic properties of hybrid WS2/MoS2 multilayer on flexible PET

Two-dimensional (2D) layered materials transition metal dichalcogenide compound (TMDC), which stack-together and form van der Waals heterostructures, have created interesting phenomena due to their interlayer interactions and their great potential for atomic-scale devices. Various electrical properties have been investigated. The presence of vacancies and their related charge trappings have been reported to affect the electrical properties. In this study, we investigate the electrical properties of hybrid WS2/MoS2 multilayer film deposited on polyethylene terephthalate (PET). The hybrid morphology and signatures are confirmed by the scanning electron microscope image and Raman shift spectra, respectively. We observed a semiconductor like behaviour as well as the large hysteresis which indicates the vacancies inducing charge trappings. This characteristics is different with the electronic characteristics of WS2 and MoS2 multilayer which tend to exhibit insulating behaviours and small hysteresis. This study shows how hybrid dichalcogenide WS2/MoS2 multilayer might create new features for future electronic devices.


Introduction
The van der Waals interaction along c axis of TMDC provides the possibilities to create thin transparent flexible films which have different electrical and optical properties from the thick ones. The number of layers affect the electronic band structure [1][2][3][4]. Various studies reported the intriguing properties such as tunability of photoluminescence by strain [5,6] and circularly polarized light [7,8]. Those phenomena are suggested to have application in strain engineering devices [9,10], optoelectronics [11,12], and pseudo spin valleytronics [13,14]. The possibility to create two dimensional materials with strong mechanical properties also trigger suggestions the exploration of TMDC deposited on flexible substrate to create flexible electronics [15,16].
In addition to the study of the excellent pure TMDC properties, tremendous efforts have been also dedicated to investigate the properties of TMDC hybrids [17,18] and heterostructure [19,20]. The TMDC combination affects the electronic bandgap. For example, the WS 2 /MoS 2 heterostructure has a bandgap of 1.42 eV [21], whereas the WS 2 and MoS 2 monolayer have bandgaps of 2.1 eV and 1.8 eV, respectively [1]. Lateral WS 2 /MoS 2 heterostructures exhibits an intrinsic p-n junctions characteristics and a photovoltaic effect with an open-circuit voltage (Voc) of 0.15 V and a short-circuit current (Isc) of 5.2 pA [22].
The presence of hysterisis in I-V curve characteristics of hybrid TMDC has revealed various scenarios related to the presence of sulphur atom vacancies or defects in WS 2 and MoS 2 [23,24]. The adsorbed oxygen and moisture or fabrication residues on the sample surface have been also discussed [25,26]. The ohmic characteristics, the space charge limited current, and the trap filled limited current are suggested to be dominant factors influencing the hysteresis [27] . Despite its potential for memory devices, hysteresis influences the stability of the devices and required further investigations.
This study is aimed to explore the electronic characteristics of hybrid WS 2 /MoS 2 multilayer on flexible substrate. We modified the thickness of WS 2 and MoS 2 layer by using liquid exfoliation method. A thin multilayer hybrid film is fabricated by dropping the WS 2 and MoS 2 supernatant on flexible Polyethylene terephthalate (PET) substrate. A semiconducting characteristics is shown by the I-V curve characterization. Furthermore, we elucidate three different hysteresis characteristics and discuss the possible reason behind them. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Experimental methods
The 1 mg ml −1 WS 2 and 1 mg ml −1 MoS 2 solutions were modified by liquid-phase exfoliation (LPE) [28,29]. Detail of exfoliation process is explained in supplementary information (available online at stacks.iop.org/ MRX/8/016409/mmedia). The supernatant was subsequently deposited by drop-casting on the Polyethylene terephthalate (PET) substrate. The sample was dried using a commercial oven (oxone OX 858) for 1-3 h at a temperature of 70°C to remove the remaining organic solvents. The surface morphology of the hybrid WS 2 /MoS 2 thin film was characterized using scanning electron microscopy (SEM SU3500). To confirm the presence of WS 2 and MoS 2 , we conducted the energy dispersive x-ray spectroscopy (EDS) measurement. Furthermore, the phonon vibrations were also investigated using Raman spectroscopy measurement (XploRA ONE Horiba) with a laser spot size of ∼1 μm and a laser excitation of 532 nm. Measurement of the electrical properties was conducted using 2400 Keithley in two probes configuration. Silver paste were used as electrodes on both sides of the sample.  2(a)). The modes have 65.6 cm −1 difference indicating the formation of multilayer [30].   vice versa, as indicating by red and black arrows, respectively. The WS 2 and MoS 2 exhibit insulating behavior whereas the hybrid WS 2 /MoS 2 tends to have semiconductor characteristics. Our result is different with the one reported by Choudhary et al which shows rectifying p-n juction behavior in heterostructure MoS 2 /WS 2 [20]. The mixed WS 2 /MoS 2 flakes could be the reason behind the non rectifiying behavior of our sample.

Results and discussion
We also observed a clear hysteresis when the voltage polarity was varried. The suspected intrinsic factors behind the hysteresis were the presence of sulphur atom vacancies or defects in WS 2 and MoS 2 [23,24]. Besides, there might be also extrinsic factors such as adsorbed oxygen and moisture or fabrication residues on the sample surface [25,26]. Both intrinsic and extrinsic factors can cause charge trappings. Following the discussion at [27], the current characteristics at our hybrid sample can be divided into three regions which are (1) the linear ohmic, (2) the large slope nonlinear , and (3) the small slope nonlinear characteristic. The linear ohmic behaviour indicates that the thermally generated carriers are dominating the system below 2 V. For applied voltage larger than 2 V, the injected carriers are more dominant. The large and small slopes at nonlinear regimes might due to abrupt and gradual distribution of injected carriers in the sample, respectively. The wider hysteresis at negative voltage polarity indicates the positive entrapped charges are more dominant influencing the electronic characteristics. We should note that smaller sample is more sensitive to any fluctuation or disturbance in electronic system such as trap states from edge defect or mismatched layer stacking which affects charge transport. This sensitivity might generate more visible hysteresis in I-V curve which was not observed in previous study [20].

Conclusions
We conclude that the hybrid WS 2 /MoS 2 multilayer thin film deposited on PET flexible substrate exhibits semiconducting electronic properties. It is different from the MoS 2 and WS 2 ones which tend to have insulating properties. The hysteresis are also observed more clearly in the hybrid sample. Three different characteristics of I-V curve suggest various charge carrier distributions mechanism. The wider hysteresis curve at negative voltage polarity indicates that positive trapped charges are more dominant in the sample. Further investigation to control the hysteresis will be useful for memory application.