Rectifying effect in a MoS₂ monolayer crossed with an electro-spun PEDOT-PSS nano-ribbon

Abstract

A chemical vapor-deposited monolayer MoS₂ crystal was crossed with an electro-spun PEDOT-PSS nano-ribbon under ambient conditions. The current–voltage (I–V) curve measured across the hetero-junction was nonlinear and asymmetric, similar to a diode. Under thermal equilibrium, electrons flowed from the MoS₂ conduction band into the PEDOT-PSS LUMO level. This occurred via band bending that established a constant Fermi level across the interface. Consequently, a potential barrier was formed that restricted the current. Under normal operation in air, the diode turn-on voltage was 0.1 V and the rectification ratio at ± 1 V was 20. The thermionic emission Schottky junction model was employed for data analysis. The ideality factor was 1.9, and height of the barrier was 0.18 eV. The easy fabrication, low turn-on voltage and high rectification ratio could make this diode useful in cheap, low-power-consumption signal rectifiers.

1 Introduction

The layered transition metal dichalcogenide (TMD) MoS₂ belongs to a family of two-dimensional electronic materials similar to graphene [1]. In its monolayer form, a hexagonally arranged planar sheet of Mo atoms is sandwiched between two planar sheets of S atoms [2]. The thickness of a single S–Mo–S layer is ~ 0.7 nm [3], and van der Waals attractive forces between layers result in the formation of bulk MoS₂ crystals. Manual exfoliation of macroscopic crystals [4,5,6,7] and chemical vapor deposition (CVD) [8,9,10,11,12] are some of the techniques used to obtain MoS₂ monolayers. Like graphene, MoS₂ is stable in air; however, unlike graphene MoS₂ has a finite bandgap that varies from 1.2 eV (indirect) in the bulk [13] to 1.8 eV (direct) in a monolayer [1]. MoS₂ therefore exhibits useful semiconducting properties. The electronic configuration of d subshell electrons in Mo together with S vacancies result in MoS₂ being an n-type semiconductor [2]. Stable n-type materials are uncommon and hence desirable, especially in fabricating complementary electronic devices. Of the various TMDs currently known, MoS₂ is one of the most widely studied materials. While TMDs are popular industrial dry film lubricants, fabricating electronic devices from these materials extends their utility beyond tribological applications.

Diodes play an important role in converting AC to DC in power supplies and other electronic applications. A Schottky diode can be fabricated by forming a junction of an n-type semiconductor and a p-doped conducting polymer [14]. The effective turn-on voltages in Schottky diodes are lower than that of p–n junction diodes. Also, being a majority carrier device, Schottky diodes lack diffusion capacitance and hence can operate at higher frequencies [15]. Recently, we fabricated and analyzed a Schottky diode by crossing monolayer n-WS₂ with a poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid)—PEDOT-PSS nano-ribbon [16]. This paper extends the previous work to include a similar device, where monolayer MoS₂ is crossed with a PEDOT-PSS nano-ribbon. On comparing the WS₂/PEDOT-PSS and MoS₂/PEDOT-PSS diodes, the ideality parameters were similar for both diodes (1.9). However, the MoS₂/PEDOT-PSS diode had a lower turn-on voltage (0.1 V compared to 1.4 V). In addition, it possessed a higher rectification ratio (20 compared to 12) and a smaller barrier height (0.18 eV compared to 0.58 eV). The MoS₂/PEDOT-PSS diode was therefore superior to the WS₂/PEDOT-PSS diode, with greater potential for practical applications. The Ag contacts (with a lower Fermi energy) used in the MoS₂/PEDOT-PSS diode (compared to Au contacts used to for other) lower the contact resistance. Combining this with the smaller bandgap in MoS₂ (1.8 eV) compared to WS₂ (2.0 eV) would lead to better charge injection into MoS₂ due to the reduced barrier height. Finally, WS₂ is partially soluble in water, while MoS₂ is insoluble [17], and this would affect the surface states in WS₂ increasing the barrier height [18] for the WS₂/PEDOT-PSS diode.

2 Experimental

Controlled growth of monolayer MoS₂ on Si⁺/SiO₂ substrates used in this work has been reported elsewhere [19]. After growth, the MoS₂ crystals were transferred to clean pre-patterned or un-patterned Si⁺/SiO₂ substrates (200 nm oxide thickness) via the use of a spin-coated PMMA film and a subsequent KOH oxide etch [20]. Some reasons for this transfer were: to avoid the possibility of using a substrate with a damaged oxide layer during CVD growth; to use new substrates that were pre-patterned and hence useful in a field-effect transistor configuration (FET); to increase the spacing between MoS₂ crystals on the new substrates, and hence fabricate an isolated MoS₂/PEDOT-PSS junction that eliminates cross-interference from other diodes. The adhesion of MoS₂ to the fresh substrates was sufficiently strong since solvent washing did not lead to delamination. Figure 1a shows a transmission electron microscope (TEM) image of isolated MoS₂ crystals, with a characteristic equilateral triangular shape. Figure 1b shows a representative Raman spectrum of the MoS₂ crystals used in this study. The peak at 383 cm⁻¹ corresponds to the in-plane $E_{2g}^{1}$ mode, while the peak at 404 cm⁻¹ corresponds to the out-of-plane $A_{1g}$ mode [21]. Figure 1c shows an atomic force microscope (AFM) image of an isolated as-grown single MoS₂ crystal together with a height profile along the black scan line seen in Fig. 1d. The height of the crystal from the AFM measurement is ~ 0.9 nm, which together with the Raman peak separation of 21 cm⁻¹ confirm that the MoS₂ crystals are one layer thick [21, 22].

Fig. 1

a TEM image of isolated CVD-grown MoS₂ crystals. b Raman spectrum of MoS₂ showing two peaks characteristic of MoS₂ single crystals. c AFM image of a section of a MoS₂ crystal. d Height profile along the black line shown in c

Commercial PEDOT-PSS was obtained from Agfa Gevaert N.V. as an aqueous dispersion. A 2 wt% of polyethylene oxide (PEO) was dissolved in it and used to fabricate ribbons. Using the electrospinning technique, nano-ribbons having widths of a few micrometers and height ~ 10 nm were prepared in air [23]. By quickly intercepting the electro-spun jet with the substrate containing MoS₂ crystals, isolated PEDOT-PSS nano-ribbons land on it at random. All these nano-ribbons stick to the substrate, and some also cross over the MoS₂ crystals as shown in Fig. 2a. Using a suitable TEM grid as a shadow mask, Ag contact electrodes were then evaporated after the grid was positioned in such a way that the grid bars covered the junction formed at the intersection of the MoS₂ and PEDOT-PSS. The advantage of using a TEM grid as a shadow mask for contact electrodes is that this strategy eliminates the need for harsh chemicals typically used in optical lithography and lift-off procedures. This technique preserves the conducting polymer component of the device from decomposition, and the device shape parameters were easily determined with a 100 × compound microscope. The silver contact pads are shown in Fig. 2b after evaporation, with the grid removed. Figure 2c shows a schematic image of the external electrical connections to the device.

Fig. 2

a Microscope image of an electro-spun PEDOT-PSS nano-ribbon crossing a MoS₂ monolayer crystal. b A lower magnification image of a after using a TEM grid as a shadow mask and evaporating Ag electrodes. The grid bars covered the PEDOT-PSS/MoS₂ intersection during evaporation. The dashed lines represent the edges of the MoS₂ crystals. Four Ag contacts can be seen, two of which contact the PEDOT-PSS nano-ribbon and one contacting the MoS₂ crystal. The fourth contact is open. c Simplified schematic diagram of the device shown in b together with the external electrical circuit

The device was electrically characterized in air using a Keithley electrometer model 6517A. For the three terminal field-effect transistor characterizations, an additional Keithley 2400 source meter supplied the gate bias. All the measurements were made at room temperature.

3 Results and discussion

Prior to investigating the diode, PEDOT-PSS and MoS₂ were electrically characterized separately. A PEDOT-PSS nano-ribbon was captured on un-patterned Si⁺/SiO₂ substrates and a TEM grid used to place contact pads on the ribbon as explained earlier. Figure 3 shows the current–voltage (I_DS–V_DS) curve for this nano-ribbon when used in a FET configuration. The inset to Fig. 3 shows an image of the nano-ribbon lying between two electrodes—the source (S) and drain (D). The ribbon (channel) length between the two contacts was 55 μm, and its thickness was 10 nm. The I_DS–V_DS curves in Fig. 3 corresponding to this device are symmetric and Ohmic, suggesting the absence of any Schottky barriers with the electrodes. In addition, there is no change in the channel resistance for different back-gate (G) voltages up to ± 30 V. The absence of a field effect could be due to screening of the electric field by highly conducting islands within the polymer [24] combined with the low dielectric permittivity of the gate insulator (SiO₂). The Fermi level in PEDOT-PSS (E_FP) is assumed to stay pinned midway between the LUMO and HUMO levels. The nano-ribbon conductivity was 0.23 S/cm, lower than reported earlier [16] due to a slightly larger fraction of PEO in the present composite nano-ribbon. The lower conductivity, however, does not affect diode operation.

Fig. 3

Drain–source current (I_DS) versus drain–source voltage (V_DS) curves measured in vacuum of the PEDOT-PSS nano-ribbon for different back-gate voltages; red 0 V—(open circle); green − 30 V—(open square); blue + 30 V—(open diamond). Inset: PEDOT-PSS nano-ribbon lying between two Ag contact electrodes (S/D). Ag epoxy (seen at opposite corners) was used to hard-wire the device

Figure 4a shows an image of a MoS₂ monolayer transferred to a pre-patterned Si⁺/SiO₂ substrate. The MoS₂ sample is seen bridging two electrodes (S/D) 4 μm apart. Figure 4b shows the I_DS–V_DS curves of the device when connected in a FET configuration. For each back-gate voltage, the current tends to saturate for V_DS > 0.5 V after an initial linear increase. Such measurements were made on numerous devices. All devices showed an increase in I_DS for increasing V_GS. This implies that MoS₂ is an n-type semiconductor [2]. Without a gate voltage, there is no induced charge in the channel and the conductivity was calculated to be 1.1 S/cm. This value is consistent with un-doped semiconductors (amorphous Si). The inset to Fig. 4b shows the device trans-conductance (I_DS–V_GS) curve that is consistent with the I_DS–V_DS measurements. From the linear portion of this curve, the trans-conductance was calculated to be 3.3 nS and the mobility was 0.5 cm²/V s.

Fig. 4

a Optical microscope image of a MoS₂ crystal bridging two electrodes. b Drain–source current (I_DS) versus drain–source voltage (V_DS) curves measured in vacuum for various back-gate bias voltages (V_GS). The gate bias V_GS is shown to the right above each curve. Inset: drain–source current (I_DS) versus gate–source voltage (V_GS) for fixed V_DS = 0.1 V

Figure 5 shows the I–V characteristics of the MoS₂/PEDOT-PSS diode shown in Fig. 2b. The electrode contacting PEDOT-PSS was first connected to the source voltage positive terminal and the negative terminal to the MoS₂ arm. There is a linear current increase for increasing voltage as seen in the figure. The device is then turned on in the first quadrant for V > 0.1 V. This is the forward bias condition of a diode [14]. The current was negligible under reverse bias. Reversing the voltage supply connections and repeating the experiment enable diode turn on in the third quadrant as seen in Fig. 5. The nonlinear asymmetric curve is primarily due to the Schottky junction formed where PEDOT-PSS crosses MoS₂, and is not due to the metal/MoS₂ or metal/PEDOT-PSS contacts. Comparable values of the conductivity in PEDOT-PSS and mobility in MoS₂ suggest that the contact resistance is insignificant and does not affect diode performance. The on/off ratio calculated by dividing the on current at + 1 V to the off current at − 1 V was 20. This value depends on the off state current, the device series resistance and the barrier height. The diode turn-on voltage from Fig. 5 was seen to be 0.1 V.

Fig. 5

I–V curve measured in air for the device seen in Fig. 2b with the voltage source positive terminal connected to PEDOT-PSS and the negative to MoS₂ (blue). The red curve is the same I–V with the external connections reversed. Inset: semi-logarithmic plot of the current as a function voltage in the first quadrant. I_S is the reverse bias saturation current. I_s and n were calculated from the intercept and the slope of the line shown

Figure 6 shows the band diagram for PEDOT-PSS and MoS₂ before and after physical contact. Studies have shown the bandgap in PEDOT-PSS ~ 1.6 eV [25]. The absence of a field effect as seen in Fig. 3 implies that the Fermi level stays fixed midway between the HOMO and LUMO levels. MoS₂ was also seen to be an n-type material as verified in Fig. 4 with a bandgap of ~ 1.8 eV. The Fermi energy in MoS₂ is located closer to the conduction band edge in this semiconductor. The band diagram of PEDOT-PSS and MoS₂ together with key energy values for the components prior to the formation of the hetero-junction is shown in Fig. 6a. Upon physical contact, in thermal equilibrium, the Fermi energies of PEDOT-PSS and MoS₂ must coincide at the hetero-junction. When this happens, electrons from MoS₂ flow into the LUMO level of the polymer lowering its Fermi energy via band bending. A barrier is then formed that prevents further flow of electrons as seen in Fig. 6b. From the key energy values shown in Fig. 6a, the expected barrier height is 0.4 eV. The MoS₂ surface states and the PEDOT-PSS work function are some of the parameters that determine this barrier height [18, 26]. These remain unaffected by moisture even though the measurements were made in air.

Fig. 6

The hetero-junction band diagram when MoS₂ is crossed with PEDOT-PSS: a before crossing b after crossing. E_F, E_C and E_V are the Fermi, conduction and valence energy levels of MoS₂ and E_FP is the Fermi energy of PEDOT-PSS. ΔE₁ (1.6 eV) and ΔE₂ (1.8 eV) are the energy bandgaps for PEDOT-PSS and MoS₂, respectively, φm (5.1 eV) and φs (4.7 eV) are the work functions of PEDOT-PSS and MoS₂, and V_b (0.4 eV) is the expected built-in potential barrier. A common Fermi level exists at thermal equilibrium via band bending. This gives rise to a Schottky barrier (indicated by the arrow) that blocks the flow of electrons from MoS₂ into the polymer

Using the standard thermionic emission model of a Schottky junction, we have analyzed the diode performance. According to this model [27]:

$$J = J_{\text{s}} \left[ {{ \exp }\frac{qV}{nkT} - 1} \right]$$

(1)

$$J_{\text{s}} = A^{*} T^{2} \exp \left[ {\frac{ - q\varphi }{kT}} \right]$$

(2)

where J_s is the saturation current density, J is the current density, q is the electronic charge, k is the Boltzmann constant, T is the temperature (300 K), φ is the barrier height and n is the ideality parameter that takes into account corrections to the original simple model, e.g., lowering of the image-force barrier. The Richardson constant $A*\left( { = \frac{{4\pi qm^{*} k^{2} }}{{h^{3} }}} \right)$ is 120 A/K² cm² where m* is the electron mass. At low voltage, a linear current increase in the diode is predicted by Eq. 1. Once the Schottky barrier is overcome, the current should increase exponentially. However, this increase is gradual (above 0.1 V) due to heating losses in the semiconductor. Figure 5 (inset) also shows the semi-logarithmic plot of the diode current versus applied voltage in the first quadrant. For low voltages (V < 0.1 V) and extrapolating the linear section to 0 V, we extract a saturation current (I_S) of 0.18 pA. The diode ideality parameter was calculated as follows:

$$n = \frac{q}{kT}\left[ {\frac{\partial V}{\partial lnJ}} \right]$$

(3)

and was 1.9. Using these equations, the barrier height was 0.18 eV. It is known that some polymers tend to lower the work function of PEDOT-PSS [28]. The lower-than-expected value for the barrier height could be due to the presence of PEO that lowers the PEDOT-PSS work function below 5.1 eV that we assumed. Values of n > 1 have been attributed to several factors that include the recombination of holes and electrons in the depletion layer [29], the presence of an interfacial layer and interface states at the polymer–semiconductor interface [30] or even a tunneling process [31]. PEDOT-PSS nano-ribbons can detect alcohol vapors [32], and therefore, it makes this diode attractive for use as a gas sensor, thereby rendering it multifunctional. Using Ag contact electrodes (instead of Au) imply efficient charge injection into MoS₂ due to its lower Fermi energy and low contact resistance. Also, MoS₂ has a lower bandgap compared to WS₂; hence, the barrier height for electrons to flow into the LUMO level of PEDOT-PSS is lowered. This manifests itself as a smaller turn-on voltage as seen in Fig. 5 and better performance compared to the WS₂/PEDOT-PSS diode.

4 Conclusions

PEDOT-PSS was conducting with no field-dependent charge transport. MoS₂, however, was shown to be n-type and had a charge mobility of 0.5 cm²/V s. Forming a junction of these two materials lead to a Schottky junction having an on/off ratio of 20 and a turn-on voltage of 0.1 V. Using the thermionic emission Schottky junction model, we calculated that the diode had n = 1.9 and a barrier height of 0.18 eV. A low turn-on voltage and small barrier height make this diode superior to that fabricated from WS₂/PEDOT-PSS. The PEDOT-PSS nano-ribbon and MoS₂ have large surface areas; this also makes the diode potentially useful as a gas sensor. The diode could be multifunctional in nature making it suitable for use in complex electronic circuits. Efforts are underway to fabricate an array of such diodes for signal processing.