Conductive Paintable 2D Layered MoS2 Inks

Conductive and paintable inks of 2D layered MoS2 with aspect ratio-dependent conductivity are demonstrated. Using ultrasonically assisted solvent-exfoliation of MoS2, 2D and few-layer suspensions become inks that provide smooth films when painted. Conductivity of painted 2D MoS2 inks can be modulated by length and width, where the aspect ratio dependence of conductivity is linked to the painting direction. Inks of solvent-exfoliated MoS2 can be painted as conductive films without polymeric additives.

electrochemical methods. (17) Exfoliation of parent bulk crystals to give 2D and few-layer MoS2 dispersions in solvents may also be promising functional inks of various types. Whether thixotropic or dilatant, inks from particle dispersions are the basis of many printing and coating technologies.
Incorporating new functionality using 2D materials with standard processing has potential for interesting applications. (18) Inspired by MoS2 used in semi-fluid thixotropic lubricants, we developed a 2D material paint without using polymer additives. Using ultrasonically assisted solvent exfoliation, which is useful for high-through and high quality dispersions of graphene and a range of TMDs. (4,5,19,20), conductive MoS2 paints are reported here.
Modulating the electrical properties of MoS2 and many assemblies or networks of deposited TMD flakes is sensitive to the method of deposition and growth method of the 2D materials. Conductive polymer additives can hide underlying properties by adding sheet-to-sheet grain boundary and contact resistance, and this is important for paintable inks of 2D materials that may display conductivity that is dependent on a coating method for example (21).
Here, we developed inks using high concentrations of exfoliated MoS2 2D layer suspensions that can be painted as uniform films on substrates. Conductivity is controlled at material-level by high quality exfoliated 2D sheets. Painted MoS2 in-plane electronic conductivity is altered by the painting direction and aspect ratio of coated films. This 2D material paint and insights into the correlation between paint application has scope for diverse application requiring TMD and layers of 2D materials as a cohesive and conductive thin film that can be painted or sheer thinned into uniform layers or as more complex structure.

Experimental
MoS2 powder with as-received average particle size < 2µm (Aldrich) was used as received. MoS2 was subsequently exfoliated in solvent using ultrasonication into 2D and few-layer MoS2. To create inks, we followed a procedure like our work on 2D Bi2Te3.(21) A mass of 0.5 g of MoS2 was sonicated in 1 mL of 1-cyclohexenyl pyrrolidine (CHP) solvent for 800 mins. The 2D MoS2 suspension was then centrifuged at 4500 rpm for 45 min. For each dispersion, the supernatant was removed and the mixture (ink) was then painted onto substrates (glass, Si and SiO2) using an artist brush. The material was distributed evenly across the glass slides in several aspect ratios (width, length) to constant thickness of ~4 µm.
The substrates were then annealed at 100 °C for 3 h. Physical characterization was carried out using scanning electron microscopy (SEM) using a Hitachi S4800, transmission electron microscopy (TEM) at 200 kV using a JEOL 2100F, Raman scattering spectroscopy, and electrical transport measurements using a Keithley 2612B SourceMeter and a probe station.

Results and Discussion
Painted, conductive 2D layered MoS2 films with different aspect ratio (L × W) are shown in Fig. 1(a), shown schematically in Fig. 1(b). Using SEM, the morphology of the paint is shown in Fig. 1(c) and we note a relatively smooth, coherent deposition that does not contain voids or holes during painting or after drying. These films contain only exfoliated MoS2, no polymer or conductive additives to modify viscosity or conductivity. In Fig. 2, we show single 2D MoS2 (a,b) and few-layered MoS2 (c,d) morphology after solvent exfoliation.
Widening of the van der Waals spacing from solvent intercalation and buckling are consistently observed for few-layer MoS2. Raman scattering spectra confirm high quality MoS2 post exfoliation and (Fig. 2(e)), and MoS2 2D sheets exhibit an expected shift in optical phonon modes, and the higher frequency E 2 1u phonon compared to bulk MoS2. Normally, Coulombic interactions between adjacent layers cause Davydov splitting that defines the frequency difference between the (E 2 1u, E 1 2g) conjugate pair. The E 2 1u mode represents sulfur atoms on adjacent layers moving in phase and exfoliation appears to modify terminating sulfur bonds that alter this mode. The ratio of E 1 2g to A1g remains relatively unchanged, in agreement with the layered structure we see by TEM.
We investigated the painting of the MoS2 inks onto glass substrates that were cleaned by UV-ozone treatment. Two-probe transport using In-Ga eutectic ohmic contacts across various painted strips from the MoS2 ink with various aspect ratio are show in in Figure 3. Ohmic response is found in all cases, and roughly doubles in conductivity as the length is halved (Figure 4(a-c)). The seemingly simplistic ohmic behavior appears to follow a length-dependent resistivity interpretation, but tortuosity and complex minimum conductance paths are likely formed when the width is increased. The average in-plane conductivity is ~1.2 -1.6×10 -5 S cm -1 for these 2D MoS2 painted films, analysed along the direction of painting. As the aspect ratio is reduced to a square (equal length and width), the lateral 2D and few-layer sheet-to-sheet contacts provide alternative conduction paths between MoS2 or likely around scattering centres. The absolute conductance of wider films is evidently lower than for narrower ones (Fig. 4(a)), the conductivity slightly reduces to 1.5×10 -5 S cm -1 . A similar relative trend holds as a function of overall painted film area (Fig 4(b). When we analyse this using a linear trend, the conductivity varies as 2.8×10 -5 S cm -1 ·cm -1 (L×W). Thus, widening the 2D MoS2 paint provide conductance paths that improve overall conductivity measured along the length (L). This is confirmed in Fig. 4(c), where we find that lower aspect ratios (L/W) provide more conductive films using 2D MoS2 analyzed along the painted direction. As aspect ratio is dimensionless, the conductance is higher for any aspect ratio when the width (parallel to source-drain) is narrower. This behavior may be particular to layered materials whose shear-thinning during painting controls in-plane electronic conductivity. Other deposition methods might also impart a directional conductivity dependence (22). Wider films introduce tortuosity to the grain boundary resistivity for given thickness, and the nature of the grain-boundary contacts influence conductivity. Fundamental transport models for materials with either small or high bandgaps may explain conductivity with 3D vs 2D reduced dimensionality, and future studies underway will address some general queries in painted films or assemblies with random sheet-to-sheet contacts: does the density of contacts between 2D sheets (for a given thickness) control overall conductivity? Is the lowest resistance path a function of preferred directionality of 2D sheets? Conductivity measured along the width (W) is similarly modified by variation of the length (L), and so can anisotropy in 2D material deposition, variation in 2D sheet anisotropy or thickness (23), affect the global anisotropy in conductivity for different painted films of 2D materials we show here?
For any functional, complex shaped films, electrodes or related where the 2D material is printed, painted, sprayed or deposited in a manner that adds 'texture' to the material's nanostructure, or preferential orientation of 2D and few-layer sheets, the dependence of conductivity on film aspect ratio is intrinsically important.

Conclusions
Smooth, paintable films from 2D MoS2 ink are possible using solvent exfoliation without polymer additives. Electrical conductivity is enhanced by depositing narrower films at most aspect ratios, or shorter films at fixed width. Future work is necessary to understand the fundamental basis for reduced conductivity in wider painted films of equal length, and the prospects of paintable 2D materials in applications where electronic conductivity modification is important. The ability to paint arbitrary coatings provides a method for printable electronics, complex patterned catalytic, electrochemical or other coatings/electrodes on a wide range of substrates.