Stacked printed MoS2 and Ag electrodes using electrohydrodynamic jet printing for thin-film transistors

Transition metal dichalcogenide-based thin-film transistors (TFTs) have drawn intense research attention, but they suffer from high cost of materials and complex methods. Directly printed transistors have been in the limelight due to low cost and an environmentally friendly technique. An electrohydrodynamic (EHD) jet printing technique was employed to pattern both MoS2 active layer and Ag source and drain (S/D) electrodes. Printed MoS2 lines were patterned on a silicon wafer using a precursor solution and simple annealing, and the patterns were transferred on other SiO2 substrates for TFT fabrication. On top of the patterned MoS2, Ag paste was also patterned for S/D electrodes using EHD jet printing. The printed TFTs had a high on–off current ratio exceeding 105, low subthreshold slope, and better hysteresis behavior after transferring MoS2 patterns. This result could be important for practical TFT applications and could be extended to other 2D materials.

www.nature.com/scientificreports/ simply without using any shadow masks compared to other methods such as E-beam or thermal evaporation. This merit facilitates the practical development of a variety of functional devices. Recently, direct-patterned source and drain (S/D) electrodes have been reported in oxide and organic thin film teansistor applications using this printing techinque 11,12 . In this work, MoS 2 thin-film transistors were fabricated with a unique combination of simple-strategy patterning MoS 2 from precursor solution and S/D electrodes using an EHD jet printing method, as shown in Fig. 1. For designing printed electronics, silver nanoparticle paste was selected for the S/D due to a host of advantages, such as printable material, affordable price, high conductivity, resistance to oxidization, good adhesion to oxide, and matching work function with the conduction band of MoS 2 . The TFT performance of multilayer MoS 2 transistors was evaluated before and after transfer on the same type of SiO 2 /Si substrate. Interestingly, transferred MoS 2 TFTs showed better electrical characteristics compared to as-grown MoS 2 TFTs. The hysteresis behavior was also characterized, and we obtained a remarkable decrease of the hysteresis phenomenon after transferring.

Results and discussion
EHD jet patterning MoS 2 and Ag lines. Optimizing printing parameters is of importance to get a desirable jetting mode. In this work, Taylor cone-jet mode was used for both semiconductor and S/D electrodes in linear shape. Precursor fluids flowing through 100-µm inner-diameter capillary nozzles in the EHD jet printer resulted in 100-µm scale (Fig. 2a). A good combination of printing parameters is important to obtain Taylor cone jet mode for patterning linear MoS 2 and Ag lines as discussed previously the effect of printing parameters on pattern quality 13,14 .
For printing MoS 2 patterns, the parameters were a 1.8-mm tip height, 50 °C substrate temperature, 0.0032 µ L s −1 flow rate, and 1.8-kV applied voltage for the (NH 4 ) 2 MoS 4 precursor. The stage speed was then adapted to obtain uniformity and the required size of patterns. To investigate the substrate velocity's effect on pattern neatness, the movement speed was varied from 2000 to 7000 µ m s −1 while maintaining all other parameters during the printing process. A high-speed process with a lower contact time of the jetting solution leads to smaller line width, as shown in Fig. 2b. 0.025, 0.05, 0.075, and 0.1 M solutions of (NH 4 ) 2 MoS 4 were prepared at the same stage speed of 3000 µm s −1 to check a printing range of precursor concentrations. Optical microscope images of corresponding patterns were obtained after drying at 150 °C, as shown in Fig. 2c. Changing color of the pattern was observed from yellow (0.1 M) to green-like as the concentration was diluted from 0.1 M to lower concentrations. The shift in colors suggested that the (NH 4 ) 2 MoS 4 thickness increased with the precursor concentration. From the optical images of printed (NH 4 ) 2 MoS 4 lines, the printability of (NH 4 ) 2 MoS 4 solution by EHD jet printing was confirmed.  www.nature.com/scientificreports/ Figure 2d shows optical images of the same MoS 2 patterns printed on SiO 2 /Si from 0.05 M precursor concentration before annealing, after annealing, and after transferring them onto another SiO 2 substrate. No wrinkles or contaminants of MoS 2 thin films were observed after annealing and transferring thin films compared to preannealed MoS 2 . This demonstrated that undamaged MoS 2 thin films were successfully obtained, and the nature of the MoS 2 remained.
A drawback of this printing technique is that the significantly charged droplets give rise to the solution/ paste shooting out in all directions on the substrate. This could be reduced by modifying the original solution/ paste and relevant printing parameters. To create a clear Ag pattern on a MoS 2 pattern (Fig. 2e), tiny amounts of PGMEA and Silveray were added to the original silver nanoparticles. The PGMEA is used to improve droplet formation and enhances the printed feature resolution 13 . Due to different properties such as the low viscosity of (NH 4 ) 2 MoS 4 solution and high viscosity of Ag paste, the system of operating parameters was different. Those were well-discussed in our previous papers regarding patterning Ag and MoS 2 linear lines 13,14 . After optimization of the operating parameters, stacked Ag S/D on MoS 2 patterns were obtained using EHD jet printing.
Printed MoS 2 and Ag features. Controlling the smoothness and thickness of MoS 2 plays a crucial role because the number of layers considerably affects the optical and electrical properties of MoS 2 . Patterns printed from four concentrations were measured for their morphology and height profile using a contact-mode AFM. The root mean square (RMS) roughness values were 0.20-0.25 nm for the MoS 2 patterns (Fig. 3a-d). These values revealed reliable uniformity and smoothness of the MoS 2 surfaces, which is also important for upper layer deposition. The reliability of making uniform patterns from EHD jet printing could be understood. A computer-integrated EHD jet printer can make fine patterns with high resolution and uniformity. Therefore, with the same deposition parameters of both printing process and physics solution properties, the same MoS 2 layer can straightforwardly achieved after growth procedure. Then good uniformity can be obtained from this run-to-run process.
Raman spectroscopy was utilized to predict the thickness of the MoS 2 samples. Two typical concentrations of 0.025 and 0.05 M were selected.   The average values of electrical characteristics are summarized in Table 1. Among these concentrations, 0.05 M and 0.075 M exhibited the best electrical properties in printed TFT devices. Indeed, the average linear mobility of 0.05-0.08 cm 2 V -1 s -1 and on-to-off current ratio of 10 4 -10 5 are much higher than those of the 0.025 M ( ∼ 10 3 ) and 0.1 M ( ≈ 30) cases. The highest value of devices fabricated from 0.075 M was 0.18 cm 2 V −1 s −1 . It is noted that all the fabricated devices ( ≥ 20 devices per type of TFT) exhibit similar characteristics, which represents the uniformity and reproducibility of our method.
It is interesting that the non-monotonic relationship of the current ratio, subthreshold slope and mobility with the thickness of MoS 2 lines varied with the solution concentrations. This behavior can be explained by the resistance network 16 . While the Ag S/D contact the top layer of MoS 2 , access to lower layers involves additional inter layer resistors. The gate electrode mostly impacts the bottom layers of the active layer, and charge screening gives rise to degraded charge carriers for top MoS 2 layers.
In the thinnest MoS 2 active layer made with 0.025 M, a lack of sufficient screening of the substrate effect leads to low mobility. Much thicker MoS 2 layer is obtained from 0.1 M precursor, and the finite interlayer conductivity results in an effectively lower total mobility. The optimal thickness of MoS 2 was identified as 7-10 layers created from 0.05 to 0.075 M precursors. They possess proper thickness and good surface morphology, which create the highest mobility.
For the hysteresis behavior of TFTs, the relationship of the hysteresis width and the thickness of a MoS 2 layer was investigated before and after transferring. Figure 6 shows the hysteretic behavior from transfer curves measured at several V DS values for both as-grown and transferred MoS 2 TFTs. Various thicknesses of the MoS 2 layer manufactured from precursor concentrations of 0.05 and 0.075 M were chosen to represent all concentrations   17 . The reasons might come from the redistribution of charges mainly accumulating on the MoS 2 surface and/or the susceptibility of charge traps at the channel surface to the external electric field owing to the absence of electric screening. One can see the minimum obtained hysteretic gap of 4 V, which is similar to other reported MoS 2 TFTs with Au/Ti top contacts 18 .
Along with thickness dependence, the hysteresis is found to be related to the transfer of MoS 2 layer on another substrate in spite of using the same type of dielectric materials. After transferring, we observed 3-4 times smaller hysteresis, 50% decreased subthreshold swing, increased current ratio by 1-2 orders of magnitude, and better  www.nature.com/scientificreports/ mobility for both concentrations, as shown in Table 2. This is clear proof for very good MoS 2 patterns obtained after transferring by a conventional PMMA based method. The original SiO 2 /Si used as a substrate to build MoS 2 might meet undesired contaminants in the ambient environment during the printing process and be affected in annealing to some extent compared to new SiO 2 /Si target substrates. Therefore, owing to the cleaner and undamaged substrate, less scattering and charge carrier trapping might be obtained to promote the performance of fresh SiO 2 -based MoS 2 TFTs. The hysteresis and the large subthreshold swing are thus explained by interface trap charges present at the MoS 2 /SiO 2 interface.
The high on-off current ratio of 2.5 × 10 5 , good hysteresis behavior of 4 V, and acceptable subthreshold slope of 6 V dec −1 of EHD jet printed MoS 2 TFTs were comparable to TFTs from other works in terms of current ratio  Table 2. Analytical results for hysteresis and other performance of TFT devices with concentration of precursor solution and transferring step (AG; as-grown, TF; transferred MoS 2 films).

(NH 4 ) 2 MoS 4 concentration [M] MoS 2 I on /I off S-S [V dec −1 ] µ lin [cm 2 V −1 s -1 ] Hysteretic width [V]
0.05 AG (3.0 ± 0.5) × 10 4  www.nature.com/scientificreports/ (~ 10 2 -10 4 ), such as paper-based MoS 2 TFTs with printed Cu electrodes 19 , solution-based MoS 2 FETs with Au S/ D 20 , and CVD MoS 2 TFTs with ink-jet printed Ag or graphene contacts 21,22 . However, the carrier mobility of TFTs was quite modest compared to other MoS 2 TFTs with different source/drain materials, such as expensive Au/Ti 23 and so forth. The reason may be poor charge injection from Ag electrodes to the MoS 2 layer and diffusion of Ag into the underlying layer and channel area. The high contact resistance at the interface between the MoS 2 active layer and Ag source/drain could be assigned to several factors, such as the high work function of Ag, residual impurities in the original Ag paste or in the printing process, the drying step, and the spatial potential barrier at the MoS 2 -Ag interface. To improve device characteristics, other materials with lower work function could be employed. However, for printed electronics, EHD jet printing technique was used as much as possible to produce our current devices for the purpose of low-cost and environmentally friendly. Consequently, improvement of the interface of the semiconductor and printed Ag electrodes should be focused on the future to optimize device fabrication and circuit integration. Printed 0.05 M MoS 2 TFTs had the highest on-off current ratio of 2.3 ± 0.8 × 10 5 and lowest subthreshold slope of 8.47 ± 1.59 V/dec. TFTs made with 0.075 M showed the highest mobility (0.070 ± 0.010 cm 2 /V s) and the best hysteresis behavior (~ 4 V). Thinner (0.025 M) or thicker MoS 2 (0.1 M) layers gave rise to degraded TFT properties. The ideal thickness of MoS 2 for the best performance of TFTs with Ag S/D was 7-10 layers. Although TFTs with an EHD jet-printed active layer and Ag S/D have limited performance, the stacked printed MoS 2 semiconductor and Ag S/D electrodes using EHD jet printing proved that this printing is a very attractive and effective method for multi-printing technology for 2D materials.

Conclusions
In summary, MoS 2 TFTs have been successfully fabricated by EHD jet printing MoS 2 as a semiconductor and Ag as S/D electrodes for the first time. The MoS 2 -based device exhibits a current ratio of over 10 5 and acceptable SS slope of 6.0 V dec −1 , which is important for application in high-speed circuits in the case of SiO 2 gate insulators. The electrical characteristics of TFTs were observed with different MoS 2 thicknesses. Higher mobility was found for the most optimal thickness of five to seven layers.
The transferred MoS 2 TFTs showed better electrical performance compared to as-grown MoS 2 TFT devices. In addition, after transferring printed MoS 2 , better hysteresis behavior and higher mobility were obtained due to less scattering and charge trapping in the cleaner substrate. The doubled EHD jet printing of the MoS 2 active layer and Ag S/D electrodes proved the possibility of a low-cost stacked printing method for 2D materials and next-generation optoelectronic applications.

Methods
Printing setup. All printing processes for MoS 2 and Ag were conducted using an EHD jet printer. The system setup included an XY moving stage, a Z motor for three-dimensional movement, a DC power supply, a pneumatic regulator for pressure requirements, a syringe pump, and a nozzle tip with a 100-µm inner diameter for injecting and ejecting (NH 4 ) 2 MoS 4 solution and Ag paste. A voltage was applied from the DC power supply between the nozzle and a conducting stage, which induces an electrostatic field that drives the flow of solution ejected from the nozzle tip, and the flow meets a target. To observe the whole dynamics of the electric fielddriven jetting behavior, a high-speed camera was set up to capture or film the process of meniscus deformation and droplet ejection at the tip head.
Material preparation. The precursor solutions for producing MoS 2 were synthesized using our recently developed approach 10 . A 1 M sulfur solution was prepared by dissolving S (Alfa Aesar, Fisher Scientific) in carbon disulfide (CS 2 , Yakuri Pure Chemicals Co., Ltd). The precursor solution was obtained by dissolving ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 , 99.97%, Sigma Aldrich) in 4 parts of ethanolamine (Sigma Aldrich) and 4 parts of butylamine (Sigma Aldrich) with the S solution. Then, 2 parts of n,n dimethylformamide (DMF, Sigma Aldrich) were added to the solution to form the precursor solution. In our CVD-free method, S was added during solution preparation to yield an S-rich precursor instead of adding S powder separately, as proposed in other CVD methods.
The silver paste was formulated by mixing 100 parts of original Ag paste (4000 cps, AD-V7-108) with 1 part of Silveray (solvent) and 3 parts of propylene glycol methyl ether acetate (PGMEA, Sigma Aldrich). This was done to make an even paste and prevent clogging at the tip capillarity during the printing process according to our recent publication 13 . We modified it in order to be relevant to this research.

MoS 2 growth and TFT fabrication.
Prior to printing, heavily p-doped Si substrates covered by 300-nm thermally grown SiO 2 were simply cleaned, and 40 min of UV/O 3 exposure was used to enhance the hydrophilicity. The precursor solution was EHD jet printed on the SiO 2 with a combination of optimized parameters. The viscosity and surface tension of the precursor solution are around 14 cP and 38 mN m −1 , respectively. After the printing process, precursor patterns were dried on a hot plate at 150 °C for 30 min and annealed in a furnace tube with an N 2 environment at 1000 °C for 1 h to form MoS 2 patterns as a semiconductor layer. MoS 2 was crystallized through thermolysis of (NH 4 ) 2 MoS 4 24 . MoS 2 patterns were then transferred onto another 300-nm SiO 2 /Si substrate for transferred-MoS 2 TFT fabrication using the conventional wetting method with support of PMMA. Silver paste was subsequently printed transversely on both as-grown and transferred MoS 2 patterns using the EHD jet printer and dried at 150 °C for 30 min on a hot plate to form S/D electrodes. To obtain a clear channel, printing neat Ag pattern lines was necessary with optimized parameters such as a standoff height of 1. 5  www.nature.com/scientificreports/ speed of 2000 m µ s −1 , pressure of 80 kPa, and voltage electricity of 1.5 kV. The whole TFT fabrication process is shown in Fig. 1.
Characterizations. The morphology and thickness of each layer and whole devices were observed by an optical microscope (Olympus-BX51M), atomic force microscope (AFM, Nano expert II EM4SYS), scanning electron microscope (SEM, Tescan, Lyra 3 XMHS) and Raman spectroscopy at a 532-nm excitation wavelength. 2H crystal MoS 2 structure was measured by transmission electron microscope (TEM, Zeiss Libra 200FE). The electrical characteristics of MoS 2 TFTs were studied a using semiconductor parameter analyzer in ambient air (Keithley 4200). In order to assess them objectively, a series of at least 20 TFTs for each type was fabricated and characterized.

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.