Direct Contacting of 2D Nanosheets by Metallic Nanoprobes

: We present a simple and fast methodology to realize metal contacts on two-dimensional nanosheets. In particular, we perform a complete characterization of the transport properties of MoS 2 monolayer flakes on SiO 2 /Si substrates by using nano-manipulated metallic tips as metallic electrodes directly approached on the flake surface. We report detailed experimental investigation of transport properties and contact resistance in back-gated field effect transistor in which the Si substrate is used as the gate electrode. Moreover, profiting of the n-type conduction, as well as the high aspect ratio at the edge of the MoS 2 flakes, we also explored the possibility of exploiting the material as a field emitter. Indeed, by retracting one of the metallic probes (the anode) from the sample surface, it has been possible to switch on a field-emitted current by applying a relatively low external electric field of few-tens of Volts for a cathode-anode separation distance below 1 µm. Experimental data are then analyzed in the framework of Fowler-Nordheim theory and its extension to the two-dimensional limit.

MoS2 has a crystal structure characterized by a hexagonal layer of Mo atoms between two layers of S atoms. Layers are bonded together by van der Waals forces. MoS2 flakes can be fabricated either by mechanical exfoliation or chemical vapor deposition [14]. Bulk MoS2 has 1.2 eV indirect bandgap, while mono-layer (1 L) and bilayer (2 L) MoS2 have 1.8 eV and 1.6 eV indirect bandgap, respectively [15]. Consequently, both 1 L and 2 L MoS2 can be used to realize field-effect transistors with high On/Off ratio and photoresponse [16]. On the other hand, carrier mobility is typically limited to few-tens cm 2 V −1 s −1 . Moreover, ohmic contacts (with low resistance) are crucial to improving device performance [17].
In this paper, we demonstrate a simple method of realizing electrical contacts on MoS2 flakes by using nanomanipulated metallic probes inside a scanning electron microscope (SEM). We show that this technique allows complete characterization of the backgated field-effect transistor (FET), as well as checking the field emission properties of the MoS2 flake.

Materials and Methods
The MoS2 flakes studied in this work have been grown on Si/SiO2 substrates by means of a chemical vapour deposition technique, in which S powder and a saturated ammonium heptamolybdate solution have been used as precursors. Few-layer MoS2 flakes have been characterized by micro-Raman spectroscopy ( = 532 nm). The experimental setup for electrical characterization is realized inside a SEM chamber (see Figure 1a) provided with two piezo-driven nano-manipulators for precise positioning (step resolution ~5 nm) of metallic probes (tungsten tips). A semiconductor parameter analyzer (Keithley 4200-SCS) is then used as a source-measurement unit, to apply bias up to ±100 V and to measure current with resolution better than 0.1 pA. Electrical measurements are performed at room temperature and in high vacuum (10 −6 mbar) after gently approaching the tungsten tips on the MoS2 flake (a real image taken inside the SEM chamber is shown in Figure 1b) and using the Si substrate as a back gate. Micro-Raman analysis of the MoS2 flake has shown a spectrum (see Figure 1c) with two peaks corresponding to the and modes, separated by about 20 cm , indicating that the sample under investigation is a monolayer.

Results and Discussion
In Figure 2a, we report the output characteristics (  ) measured in the range of ±0.5 V for different values of the gate voltage ( ). We notice a slight rectification that can be explained as the result of asymmetric Schottky barriers forming at the tungsten/MoS2 interfaces [3]. By varying the distance between the two tungsten tips, we can modulate the channel length of the FET, thus realizing an experiment based on the Transfer Length Method (TLM) [18,19] to evaluate the contact resistance at the tungsten/MoS2 interface. In Figure 2b, we show the measured total resistance versus , with = 2 + , where is the contact resistance, is the MoS2 sheet resistance, W is the channel width (assumed to be equal to the tip diameter, 200 nm), and is the channel length, i.e., the separation between the two tips. Experimental data have linear behavior, from which specific area contact resistivity and sheet resistance can be evaluated as ≈ 4 × 10 Ωcm and ≈ 10 Ω/□ from the intercept and the slope of the linear fit, respectively.
The transfer characteristics (  ) reported in Figure 2c have been measured for different gate voltage ranges up to ±60 V, with = −5 V, and by positioning the tungsten tips at separation of 13 μm. The device has n-type behavior, with a threshold voltage of about −10 V, and it can be explained in terms of chemisorption of oxygen on MoS2 or sulphur vacancies [20][21][22]. From the transfer characteristic measured in the range ±50 V, we have estimated the on/off ratio as ~10 , a subthreshold swing of ≈ 4 , and a mobility of = 1 cm V s , a value within the typical range (0.02 − 100 cm V s ) reported for MoS2based FETs on SiO2 [23,24]. The low mobility can be attributed to the high contact resistance and to high defects or traps density [25].
In Figure 3c, we show the transfer characteristics measured by sweeping the gate voltage different ranges, from ±20 V up to ±60 V. The curves have a clear hysteresis that we explain as being caused by negative charge trapping [20]. We observe that the hysteresis width ( ), estimated at = 0.1 nA, has linear dependence on the sweeping range (See Figure 3d). This behavior can be ascribed to the trapping process driven by the gate voltage and the effects on the MoS2/Si-substrate capacitor.
Finally, we also investigated the field emission (FE) properties of the MoS2 flake, profiting of the n-type conduction and the high aspect ratio of the flake side. By retracting the tip-anode at a distance h = 900 nm from the MoS2 edge, we can measure the current emitted from the flake under the application of an external electric field (Figure 3a). More precisely, we applied a voltage bias of up to 120 V on the anode, and we measured the current emitted from the flake (cathode) with a resolution better that 0.1 pA. The current−voltage (  ) curves have been measured at fixed cathode-anode separation h and for two different values of gate voltage. The FE characteristics have been measured by applying a bias voltage on the anode up to +120 V, by keeping a fixed gate voltage of 10 V and 40 V, respectively. The measured curves are reported on a linear scale (Figure 3b) and on a logarithmic scale (Figure 3c). Interestingly, we observe that the FE current is larger for = 40 V, suggesting that the gate voltage increases the n-doping of the MoS2 flake [26]. We analyzed the FE curves in the framework of the Fowler-Nordheim (FN) theory [27], for which the FE current is expressed as where = 1.54 × 10 A V eV and = 6.83 × 10 V cm eV / , is the work function of the emitter, is the emitting surface area, and is the field enhancement factor. Accordingly, for FE curves, it is expected that ( / ) versus 1/ is linear (FN plot), and can be evaluated from its slope.
In Figure 3d, we report the FN plots that demonstrate the FE nature of the measured current. For = 40 V, we found a turn-on field = 40 V μm (defined as the field to obtain a FE current of 1 pA) and ≈ 200.

Conclusions
We demonstrate a simple and fast methodology to realize metal contacts on twodimensional nanosheets by gently approaching nanomanipulated tungsten tips inside a scanning electron microscope. We contacted a MoS2 monolayer to form a back-gated FET, and we performed complete electrical characterization, reporting specific area contact resistivity of 4 × 10 Ω cm , sheet resistance of 10 Ω/□, on/off ratio of 10 5 , subthreshold swing of 4 V/decade, and mobility of 1 cm V s . Finally, by retracting the tip-anode, we performed field emission characterization of the MoS2 flake, reporting that the FE current can be modulated by the gate bias. Data Availability Statement: Data available on request.

Conflicts of Interest:
The authors declare no conflict of interest.