Impact of Organic Molecule-Induced Charge Transfer on Operating

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Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n-MoS and p-MoTe Transistors 2

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Yongjae Cho, Ji Hoon Park, Minju Kim, Yeonsu Jeong, Sanghyuck Yu, June Yeong Lim, Yeonjin Yi, and Seongil Im Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00019 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n-MoS2 and p-MoTe2 Transistors

Yongjae Cho‡, Ji Hoon Park‡, Minju Kim, Yeonsu Jeong, Sanghyuck Yu, June Yeong Lim, Yeonjin Yi and Seongil Im* Van der Waals Materials Research Center, Department of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea *Tel.: 82-2-2123-2842, fax: 82-2-392-1592, e-mail: [email protected]

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ABSTRACT

Since transition metal dichalcogenide (TMD) semiconductors are found as two dimensional Van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. But, overall reports indicate that threshold voltage (Vth) of those FETs are located much away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small-molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HATCN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposition, compared to its initial performance. When the same TMD FETs of the CMOS structure are integrated into an OLED pixel circuit for ambipolar switching, the circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which was not switched off without HAT-CN.

KEYWORDS: HAT-CN, TMD, threshold voltage, organic molecules, charge transfer


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TEXT Until recent years, many of two dimensional (2D) transition metal dichalcogenide (TMD) semiconductors and related electronic devices have extensively been studied.1–11 Among several types of devices, field effect transistors (FETs) with 2D channels must be the most important although their properties may not be compared to those of Si-based transistors yet.12 Contact resistance and chemical doping method for p-n type conversion still remain as issues to be resolved in spite of many efforts by researchers.13–15 Besides those issues, high operating voltage or threshold voltage (Vth) is another essential issue for p- or n-type 2D-like channels FETs. For instance, most of n-MoS2 FETs have shown quite negative Vth in general,16–21 and particularly the Vth of top-gated n-MoS2 FETs with atomic layer deposited (ALD) insulator appears much more negative due to ALD-induced electron doping.16,22 Vice versa, p-type 2D-like thin channel FETs generally display quite positive Vth.23–27 (Detailed information on the Vth of reported 2D material-based FETs is summarized as a plot in Figure S1). Expectedly, such depletion mode FETs cause high leakage current and power consumption at zero volt state, so they request high switching voltages to turn off the device. Hence, there have been a few reports to concern and control those Vth toward enhanced mode: using a TMD channel with high resistance or ungated region, fabricating dual gate FET to modulate Vth,25,28 depositing dipole-induced top passivation layer,29–32 and passivating the gate-dielectric surface.33 However, those techniques are not easy to control the Vth in device fabrication and only possible for single conduction polarity in FET (p- or n-channel), and moreover often cause parasitic channel resistance. Therefore, achieving Vth modulation toward enhancement mode operation of 2D channel FETs by means of a facile route in device fabrication is a highly desired objective in both of p- and n-channel TMD FET researches. 3 ACS Paragon Plus Environment

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In the present study, we introduce an easy way to simultaneously control the operating voltage of both p- and n-channel FETs with TMDs. We have deposited several types of organic molecules on top of p- and n-TMD channels (p-MoTe2 and n-MoS2) and found a specific organic film ideally working for both p- and n-TMDs. For instance, organic HAT-CN molecule (hexaazatriphenylenehexacarbonitrile) films make the Vth of n-MoS2 FET shift to more positive side as deposited on the TMD channel while shifting the Vth of pMoTe2 FET to more negative side, which means that the HAT-CN layer shifts p- and n-TMD FETs toward enhanced mode. It is because the organic HAT-CN molecule layer causes electron charge transfer mainly due to Fermi level difference between HAT-CN layer and TMD channels.34–38 As a result, a complementary metal oxide semiconductor (CMOS) inverter with HAT-CN deposited p-MoTe2 and n-MoS2 FETs becomes more desirable in such electrostatic behavior as voltage gain and power consumption than the other CMOS device without HAT-CN. The same CMOS device structure with HAT-CN film also demonstrates proper ON/OFF switching in an OLED diode pixel circuit, which could not be switched off without HAT-CN due to high OFF current and too high Vth. We regard that our finding on organic molecule-induced Vth modulation in 2D TMD FETs is novel and practical as well, resolving the high Vth issues of depletion mode TMD FETs. Figure 1a shows a cross-sectional schematic for an n-channel MoS2 FET with an organic molecule layer on top of the channel, while the absorption spectra of such organic layers

(4

nm-

and

40

nm-thick

HAT-CN

and

~75

nm-thick

Alq3:

tris-(8-

hydroxyquinoline)aluminum) are displayed in Supporting Information Figure S2a and b along with their respective molecular structures (inset).39–43 For the global gate dielectric of our 2D FETs, we have used a few nm-thin polystyrene brush (PS-brush) on 285 nm-thick SiO2 as a dielectric,16,44–47 of which the structure details and capacitance-voltage 4 ACS Paragon Plus Environment

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characteristics are shown in Supporting Information Figure S3a. According to the absorption spectra, an approximate highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of HAT-CN is expected to be at ~380 nm (3.26 eV),48–50 and that of Alq3 is 2.60 eV.51,52 When we compare two MoS2 FETs with and without HAT-CN top layer (35 nm) each other, it is found from the transfer characteristics (drain current-gate voltage; ID-VGS) of Figure 1c that the threshold voltage of MoS2 FET is considerably shifted to the positive direction by ~25 V due to the 35 nm-thick HAT-CN layer but without much difference in mobility (~30 cm2/V s, see the inset plots). For the extraction of Vth, we applied constant current method, designating 1 pA as the reference current.53,54 Figure 1d displays output characteristics (drain current-drain voltage; ID-VDS) of corresponding devices, of which results appear consistent with their transfer characteristics and also show good ohmic behavior in source and drain contact. The Vth shift in MoS2 FET is attributed to the electron charge transfer from MoS2 to HAT-CN, originating from their Fermi level (EF or electron affinity; EA) difference as depicted in the energy band diagrams of Figure 1b.36,55 This HATCN-induced Vth shift is certainly a desirable result since most of reported n-MoS2 FETs have shown much negative Vth values. On the one hand, such charge transfer direction could be opposite when 35 nm-thick HAT-CN layer is replaced by only 2 nm-thin Alq3 because it has higher EA than that of MoS2 (see the energy band diagram of Figure 1b, where Alq3 would donate electrons to MoS2).56,57 In this opposite case, Vth of MoS2 FET becomes shifted to more negative direction as seen in Figure 1e although mobility and ON-state ID appear almost the same (see the inset of Figure 1e and output curves of Figure 1f). We applied the same molecule layers on p-MoTe2 channel FETs, of which device schematic is shown in Figure 2a. According to the transfer characteristics of Figure 2b as obtained with HAT-CN molecule application, the Vth of p-MoTe2 FET is systematically 5 ACS Paragon Plus Environment

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shifted to the negative direction with the thickness of HAT-CN (thickness range between 0.5 and 32 nm). Resultant ON-state ID somewhat decreases as seen in the transfer and output curves as well (Figure 2c), and the mobility appears reduced from 23 to 17 cm2/V s (Figure 2d). Detail information on the change of Vth and mobility is displayed in the plots of Figure 2e, where threshold voltage change is seen very considerable by more than 20 V in the HATCN thickness range. These results indicate that HAT-CN layer somehow provides electrons to p-channel MoTe2 reducing hole concentrations in the channel. So, if we apply Alq3 layer on another p-MoTe2 FET, a similar result should be observed in the direction of Vth shift because Alq3 has even higher EA level than that of HAT-CN layer. Figure 2f and its inset plot show such expected results in transfer curves and mobility plots of p-MoTe2 FETs, respectively. In fact, however, the negative Vth shift of p-MoTe2 FET by HAT-CN molecule needs more detail explanation, since the EF level of HAT-CN is reported lower than that of pMoTe2 in general. Explanation comes from another fact that our p-MoTe2 channel goes through air ambient annealing at 200

o

C, which introduces ultrathin MoO3 (or MoOx) on

MoTe2 surface to improve its p-type conduction for an optimum FET channel.58,59 Transfer curve characteristics of Figure 3a shows such results from pristine (before anneal), N2 annealed, and air ambient annealed MoTe2 FETs while corresponding output curves are presented in Figure 3b. (We used one single MoTe2 channel for these sequential annealing experiments). The most desirable output/transfer I-V performances come from air ambient annealing due to MoO3-induced p-type doping, and the chemical existence of surface MoO3, which should have much lower EF than that of p-MoTe2,60–62 is confirmed by x-ray photoelectron spectroscopy (XPS) as shown in Figure 3c.62,63 Because our p-MoTe2 channel has ultrathin MoO3 on its surface, realistic energy band diagrams of Figure 3d for MoTe2, 6 ACS Paragon Plus Environment

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MoO3, and HAT-CN (or Alq3) could be expected to explain electron charge transfer when they are in contact.36,62–67 It is known that EF of MoO3 is even lower than that of HAT-CN.48– 50

Although MoTe2 can initially give away electrons to ultrathin MoO3,60,68 it is regarded that

those charges come back to MoTe2 if organic HAT-CN or Alq3 causes strong coulombic repulsion by donating electron charges to MoO3. For more details and possible explanations, other band diagrams might be helpful as provided in Figure S4, considering the sequence of materials formation on MoTe2 channel: thin MoO3 and then thick HAT-CN. Figure S5 also shows that HAT-CN deposition following N2 ambient-anneal in MoTe2 FET only causes a positive shift of Vth. These results indicate that there would be no negative shift of Vth in MoTe2 FET without the thin MoO3 formation. At this point, it would be worth to recognize that organic HAT-CN layer can be very useful as Vth modulator for both p- and n-channel TMD FETs, since the layer shifts the Vth of n-MoS2 and p-MoTe2 FETs to the positive and negative side, respectively. Based on above findings and understandings, we have fabricated a low voltage operating CMOS inverter with n-MoS2 and p-MoTe2 FETs, considering that HAT-CN deposition would give any benefits to the 2D TMD CMOS in performance.4,25 Both FETs have 50 nm-thick atomic layer deposited (ALD) Al2O3 and patterned back gate metal (Au/Ti, 10/5 nm) in common. Optical microscope (OM) image of Figure 4a displays our CMOS inverter while the CMOS circuit and the schematic device cross section of low voltage operating TMD FETs are shown in Figure 4b. According to the respective transfer characteristics of Figure 4c and d for p-MoTe2 and n-MoS2 FETs, Vth shifts toward enhanced mode direction for both FETs are clearly observed with 35 nm-thick HAT-CN application although slight decrease of their mobilities seems unavoidable (their respective output characteristics are seen in Figure S6 and mobilities were calculated by using the dielectric 7 ACS Paragon Plus Environment

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capacitance (98 nF/cm2) of PS-brush/50 nm-thick Al2O3 as obtained from C-V measurements in Figure S3b). Voltage transfer characteristics (VTCs) of CMOS inverters without and with 35 nm-thick HAT-CN layer are presented in Figure 4e and f, respectively. Higher voltage gain (-dVOUT/dVIN) is observed from HAT-CN-applied CMOS inverter, to be ~17 at a supplied voltage (VDD) of 3 V, which is about two times higher than that of pristine CMOS at the same voltage. In addition, the static power consumption of HAT-CN applied CMOS device appears remarkably smaller than that of our pristine CMOS device before HAT-CN deposition (Figure 4g and h). According to the inset plots of Figure 4g and h, the peak power consumption of HAT-CN-involved CMOS device is 4 nW only at 1 V of VIN while that of the other device shows ~100 nW. It is remarkable because the two CMOS inverters came from the same MoTe2 and MoS2 flakes FETs, and such results are attributed to lower OFFstate ID of FETs with HAT-CN at the transition voltage of VIN in CMOS inverter (between 0 and 1 V). Figure S7a and b display the crossing voltage points in transfer curves without and with HAT-CN application, respectively. When the transfer curves of p- and n-FETs cross each other, lower OFF-state ID current is achieved with HAT-CN at the crossing point. Consequently, much lower supplied current (IDD) in CMOS inverter appears at the transition voltage of VIN as shown in Figure S7c. As a final application to stress the importance of low OFF-state ID and operational switching voltage, we prepared another circuit using p-MoTe2 and n-MoS2 FETs on a common gate as shown in the insets of Figure 5a and Figure 5b, which would cause ambipolar transistor switching and ambipolar-induced organic light emitting diode (OLED) switching, respectively. These basically exploit the same structure as that of CMOS FETs in Figure 4a but only changing the circuit setup. According to the transfer curves of Figure 5a, the ambipolar behavior of the circuit with HAT-CN shows at least an order of magnitude 8 ACS Paragon Plus Environment

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lower ID current (~10 nA) at its OFF-state than that of our reference ambipolar circuit without HAT-CN. Such behavior is clearly displayed with a green OLED device integrated to the ambipolar circuits. Figure 5c demonstrates the OLED display switching behavior which is driven by our ambipolar circuit without HAT-CN layer at VDD (=2.7 V). VIN sweep for ambipolar switching was periodically conducted from -5 to +5 V in many periods of triangular waveform, which alternately causes p- and n-FET to turn on. Dotted curve comes out as VOUT in the circuit with OLED. Since the OLED diode is always under forward bias state, OLED is brightened the most as driven by n-FET/p-FET at maximum VIN =+5/-5 V where minimum VOUT and peak IDD are obtained. One problem in this case is that minimum IDD for OFF-state is as large as ~200 nA which is still enough to shine the green light with some intensity as seen in the photo of Figure 5c. In contrast, such no-OFF situation completely changes with the HAT-CN top layer under the same dynamic and periodic voltage conditions. Figure 5d shows the similar VOUT and IDD behavior under the same periodic VIN condition on our ambipolar circuit with HAT-CN layer. Minimum IDD appears to be as small as ~10 nA which is 20 times lower than that of Figure 5c, and it can work as complete OFF-state which is shown as a dark OLED pixel in Figure 5d. These dynamic OLED switchings are nicely demonstrated in Supporting Information OLED switching.avi, comparing the dynamic behavior of two ambipolar-OLED circuits with and without HAT-CN. All of the thickness information on exfoliated MoTe2 and MoS2 for Figure 1~5 is prepared by atomic force microscopy (AFM) scan and summarized in Figure S8. In summary, we have introduced a facile and practical way to simultaneously control the operating voltages of both p- and n-channel FETs with TMDs. For the purpose of operating voltage control, we have deposited two types of organic molecules (Alq3 and HATCN) on top of p-MoTe2 and n-MoS2 channels, finding that organic HAT-CN molecule makes 9 ACS Paragon Plus Environment

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the Vth of n-MoS2 FET shift to more positive side while shifting the Vth of p-MoTe2 FET to more negative side. Consequently, HAT-CN layer can concurrently shift both p- and n-TMD channel toward more enhanced mode. It is because the organic HAT-CN molecule layer causes electron charge transfer due to Fermi level difference between HAT-CN layer and nMoS2 channel or between HAT-CN and ultrathin MoO3 (on p-MoTe2 channel). As an application, a complementary metal oxide semiconductor (CMOS) inverter with HAT-CNdeposited p-MoTe2 and n-MoS2 channels has been fabricated to show more desirable electrostatic behavior in voltage gain and power consumption. When the same p- and n-TMD FETs of CMOS structure were modified to be an ambipolar device for ON/OFF switching of an OLED diode, the ambipolar device circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which could not be switched off without HAT-CN due to high OFF current. We thus conclude that HAT-CN molecule-induced charge transfer in 2D TMD FETs is essentially practical in regards of future 2D transistor electronics, providing such a great benefit as operating/switching voltage and OFF-state ID control for both p- and n-channel FETs. It is highly regarded that the organic layer approach may play an important role of resolving the high Vth issues of TMD FETs.

Experimental Section Device fabrication: A SiO2 (285 nm)/p+-Si substrate was chosen as a substrate for a global bottom gate structure. The substrate was thoroughly cleaned by acetone and ethanol for 15 minutes using an ultrasonicator, in that order. Dimethyl chlorosilane terminated polystyrene (Polymer Source, Product No. P3881-SSiCl) was dissolved in toluene (Aldrich) solvent to prepare 1 wt% PS-brush solution. Prior to spin-coating the PS-brush solution, the SiO2 surface of the substrate was exposed to oxygen plasma (150 W, 50 sccm, 20 sec) to allow 10 ACS Paragon Plus Environment

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hydroxylgroup-rich SiO2 surface. Detailed annealing procedure followed as reported in the literature.16,44–47 As next step, ultrathin MoS2 and MoTe2 nanosheets (HQ Graphene) were mechanically exfoliated by polydimethylsiloxane (PDMS) and transferred onto the PS-brush treated SiO2 substrate. After transfer, Pt/Ti/Pt (30/20/50 nm) or Au (50 nm) contact electrodes were patterned by conventional photolithography and DC sputter-deposition/liftoff processes. Finally, HAT-CN or Alq3 molecule was thermally-deposited (and selectively patterned through shadow mask) using organic-molecule beam (OMBD) deposition system onto the 2D channel. In order to check the thickness effect of HAT-CN on the 2D MoTe2 channel in Figure 2, an identical MoTe2 FET was used for gradual deposition of HAT-CN (thickness from 0.5 to 32 nm). For fabrication of low voltage driving MoS2 and MoTe2 FETs, Au/Ti (10/5 nm) thin gate electrode on the glass substrate (Eagle 2000) was patterned by conventional photolithography and DC sputter-deposition/lift-off processes. A 50 nm-thick Al2O3, as processed by atomic-layer deposition (ALD) at 100 ºC, was served as a bottom-gate dielectric layer. Following procedures such as dry transfer of 2D nanosheets, contact patterning, and deposition of organic doping molecules were conducted in the identical manner as done for a global bottom gate device of Figure 1. In the case of CMOS structure in Figure 4 and 5, identical contact metal of Au (50 nm) was employed for both n- and pchannel FET. For the individual 2D channel transistors (Figure 1~3), proper post-annealing was conducted: MoS2 FET at 250 °C using nitrogen gas by rapid thermal annealing process for 5 min and MoTe2 at 200 °C using hot-plate at ambient condition for 3 min. However, it should be noted that for CMOS device, post-annealing was done under the conditions for MoTe2 FETs (ambient annealing on a hot plate at 200 °C for 3 min). Materials characterization: For XPS measurements, bulk MoTe2 flakes (a few mm size) with and without ambient annealing process were prepared on a 100 nm-thick Au-deposited glass 11 ACS Paragon Plus Environment

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substrate. XPS characterization was carried out with K-Alpha (Thermo fisher scientific) using a monochromatic Al Kα source of 1486.6 eV, and beam width of 400 µm. The thickness of MoTe2 nanoflake channels was characterized by AFM (Park System) directly on FET samples. For UV-visible absorption spectra, Alq3 and HAT-CN thin films with certain thickness were deposited by OMBD on the glass (Eagle 2000) substrate. Then, the UVvisible spectra were characterized by V-650 (JASCO Corporation). Device and Electrical characterization: All static and dynamic electrical measurements of our devices were performed with a semiconductor parameter analyzer (Agilent 4155C) and function generator (AFG310, Tektronix) in the dark at room temperature. OLED pixel samples were obtained from a company.


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FIGURES

Figure 1. (a) Cross section schematics of a device. Two kinds of organic molecules (HATCN and Alq3) are deposited on two independent n-FETs with a MoS2 channel FETs. (b) Energy band diagram of MoS2 / HAT-CN and MoS2 / Alq3 indicating electron transfer. At 13 ACS Paragon Plus Environment

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MoS2 / HAT-CN interface, electrons are transferred from MoS2 to HAT-CN, whereas such electron transfer occurs from Alq3 to MoS2 at the MoS2 / Alq3 interface. (c) Transfer characteristics of the n-FET with (w/) and without (w/o) 35 nm-thick HAT-CN layer. Corresponding linear mobility plots are shown in the inset where slight mobility degradation is shown with a 35 nm-thick HAT-CN layer. (d) Output characteristics of the n-FET with and without HAT-CN deposition (e) Transfer characteristics of the n-FET with and without at 2 nm-thick Alq3 layer. Corresponding linear mobility plots are shown in the inset. (f) Output characteristics of the n-FET with and without Alq3 deposition


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Figure 2. (a) Cross section schematics of the device. Two types of organic molecules (HATCN and Alq3) are deposited on two independent p-FETs with a MoTe2 channel. (b) Transfer characteristics of the p-FET observed with increasing thickness of HAT-CN layer. HAT-CN was additionally deposited after ex-situ electrical measurements. (c) Output characteristics of the p-FET with (w/) and without (w/o) 35 nm-thick HAT-CN layer. (d) Linear mobility plots of the p-FET with increasing thickness of HAT-CN. (e) Plot of the threshold voltage shifts (black) and linear mobility (red) as modulated by the thickness increase of the HAT-CN layer. The modulated ΔVth appeared as large as ~20 V along with the final Vth of 2 V while the degradation amount of linear mobility looks relatively as small as 6.7 cm2/V s (initial mobility: 23.4 cm2/V s). (f) Transfer characteristics of the p-FET with and without 2 nm-thick Alq3 deposition. Corresponding linear mobility plots are shown in the inset. Alq3 shows very strong Vth modulation effects with only a 2 nm-thin layer, which also gives strong suppression of mobility.


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Figure 3. (a) Transfer characteristics of the p-FET with MoTe2 before and after sequential annealing at 200 o C (N2-ambient: red line, and then air ambient: orange). (b) Output characteristics of the p-FET for corresponding annealing conditions. (c) XPS spectra of 2H MoTe2 before and after air ambient annealing. The bottom spectrum evidences the formation of ultrathin MoO3 on MoTe2 layer after air ambient annealing. (d) Energy band diagram of MoTe2, thin MoO3, and HAT-CN (or Alq3) in contact. Dotted arrow indicates the electron charge transfer from MoTe2 to MoO3 before deposition of organic molecule layer. Solid arrows indicate electron transfers caused by the organic layer deposition. The electron transfer from organic layer to MoO3 may induce backward electron charge transfer at the MoTe2/MoO3 interface.


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Figure 4. (a) Optical microscope image of hetero 2D CMOS inverter with n-MoS2 and pMoTe2 channel (dotted lines indicate each TMD flake). Single Au/Ti electrode has been used for the gate of both channels in common while Au electrodes were used as the source/drain contacts for both channels. (b) CMOS circuit diagram and the schematic device cross section of our low voltage operating FETs. (c) Transfer characteristics of the low voltage operating p-FET before and after HAT-CN deposition. Corresponding linear mobilities appear to be 10 cm2/V s and 4 cm2/V s. (d) Transfer characteristics of the low voltage operating n-FET before and after HAT-CN deposition. Corresponding highest linear mobilities are 15 cm2/V s and 9 cm2/V s. Voltage transfer characteristics (VTC) of the CMOS inverter (e) without and (f) with HAT-CN deposition at various VDD. The corresponding voltage gain (= -dVOUT/dVIN) values are indicated at the right axis. Power consumption characteristics of our CMOS inverter (g) without and (h) with 35 nm-thick HAT-CN deposition. The insets show power consumption characteristics under low supplied voltage, VDD. It is regarded that HAT-CN clearly provides benefits such as at least 10 fold power reduction and 2 fold voltage gain, all of which are attributed to the low OFF ID current.


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Figure 5. (a) Transfer characteristics of the ambipolar FET setup combined both MoS2 nchannel and MoTe2 p-channel when applied with and without 35 nm-thick HAT-CN. The inset shows corresponding electrical circuit diagram, where the source/drain/gate electrodes are shared for n-FET and p-FET. With HAT-CN applied, the OFF-state of the ambipolar FET appears more than an order of magnitude lower. (b) Optical microscope image (back illumination) of the ambipolar switching device with p-MoTe2 and n-MoS2 FETs (dotted lines indicate each TMD). The inset shows the electrical circuit diagram for the OLED display switching using the ambipolar FET setup. (c) Time-domain VIN plot, and VOUT and IDD plots as synchronously obtained under constant VDD of 2.7 V before HAT-CN deposition. Input voltage is triangular waveform with a peak voltage of ± 5 V. Output voltage plot is shown as the dotted line. Top and bottom images of the left inset show the ON-states of green OLED pixel at peak input voltages of -5 V (from p-FET) and 5 V (from n-FET), respectively under VDD of 2.7 V. The middle image is supposed to show the OFF-state of green OLED, but shows some brightness because IDD level is still high as 216 nA. (d) Time-domain VIN, VOUT and IDD plots under the same VDD of 2.7 V after 35 nm-thick HAT-CN deposition. In this case, the OFF-state pixel (IDD= ~10 nA) appears absolutely dark unlike the case without HAT-CN.

ASSOCIATED CONTENT Supporting Information . Plot of threshold voltages of reported FETs with a variety of 2D semiconductor channels. Capacitance of gate insulator layers of FETs including a PSbrush/SiO2 (285 nm) layer and a PS-brush/Al2O3 layer. Energy band diagram of MoTe2, MoO3 and HAT-CN in more details. Effect of HAT-CN deposition on N2 ambient annealed 20 ACS Paragon Plus Environment

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MoTe2 p-FET. ID-VDS output characteristics of the p-MoTe2 FET and n-MoS2 FET in the CMOS inverter before and after HAT-CN deposition. ID-VGS transfer characteristics of pMoTe2 FET and n-MoS2 FET before and after HAT-CN deposition. Thickness profiles of our MoS2 and MoTe2 channels shown in Figure 1, 2, 3, 4 and 5, as measured with AFM scan.

Movie S1: Short movie showing the effects of HAT-CN molecules on the threshold voltage shift, which results in proper OLED switching

AUTHOR INFORMATION Corresponding Author *Seongil Im, E-mail: [email protected]. Phone: 82-2-2123-2842. Fax: 82-2-392-1592. Address: Van der Waals Materials Research Center, Science Building, Room 240, Yonsei University, Seoul, 03722, Korea. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGEMENT


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The authors acknowledge the financial support from NRF (NRL program: Grant No. 2017R1A2A1A05001278, SRC program: Grant No.2017R1A5A1014862, vdWMRC center). J.H.P acknowledges this research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1A6A3A11035872) REFERENCES (1)

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