Hybrid Characteristics of MoS2 Monolayer with Organic

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Functional Nanostructured Materials (including low-D carbon) 2

Hybrid Characteristics of MoS Monolayer with Organic Semiconducting Tetracene and Application to Anti-Ambipolar Field Effect Transistor Hyeon Jung Park, Cheol-Joon Park, Jun Young Kim, Min Su Kim, Jeongyong Kim, and Jinsoo Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10525 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Materials & Interfaces

Hybrid Characteristics of MoS2 Monolayer with Organic Semiconducting Tetracene and Application to Anti-Ambipolar Field Effect Transistor Hyeon Jung Park,‡ Cheol-Joon Park,‡ Jun Young Kim,‡ Min Su Kim,§ Jeongyong Kim*,⊥ and Jinsoo Joo*, ‡

AUTHOR ADDRESS ‡

Department of Physics, Korea University, Seoul 02842, Republic of Korea

§

Center for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Suwon

16419, Republic of Korea ⊥

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

KEYWORDS MoS2, tetracene, photoluminescence, anti-ambipolar, transistor, mobility

ACS Paragon Plus Environment

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ABSTRACT An n-type MoS2 monolayer grown by chemical vapor deposition method was partially hybridized with an organic semiconducting p-type tetracene thin film. The photoluminescence (PL) intensity in the hybrid region of the MoS2/tetracene is clearly lower than that of pristine tetracene because of the charge transfer effect, which was confirmed by the decrease in exciton lifetimes. By decreasing the temperature, the PL peak position of MoS2 layers was blue-shifted and consequently, the PL intensities of both tetracene and MoS2 considerably increased owing to the decrease in phonon interaction. The PL spectra of bound excitons in the hybrid region were clearly observed at low temperatures, indicating the formation of trap states. The lateral-type n–p heterojunction field-effect transistors (FETs) using the MoS2/tetracene hybrid as an active layer showed gate-tunable rectification I–V and anti-ambipolar field-effect characteristics with hysteresis effect. The charge transport characteristics across the n–p heterojunction of the hybrid region of the FET can be explained in terms of the Shockley–Read–Hall trap-intermediated tunneling and Langevin recombination mechanisms. To improve the performance of MoS2/tetracene-based FET, a dielectric hexagonal boron nitride (h-BN) thin layer was inserted between the SiO2 surface and active MoS2 layer. We observed the decrease in the hysteresis effect and threshold voltage of the h-BN/MoS2/tetracene-based FETs due to the decrease in the number of traps at the interface. The performance of h-BN/MoS2/tetracene FET device was also enhanced after the annealing process.


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1. INTRODUCTION Tetracene and pentacene are promising organic semiconducting p-type polyacenes, and their application in optoelectronic systems has been intensively studied.1–5 Polyacene systems can be easily condensed into a crystalline structure, and the energy band gap from anthracene to tetracene and pentacene can evolve because of the delocalization of π-electrons. Tetracene-based field-effect transistors (FETs) exhibit light emission and ambipolar transport characteristics.6,7 Electroluminescence from a tetracene-based light-emitting diode (LED) was observed near the drain electrode caused by the imbalance of electron and hole mobilities.6 For application in optoelectronic devices, uniform tetracene films have been grown using the organic vapor deposition method.8-11 With increasing deposition flux, a larger uniform surface can be obtained, and mobility () increases to 0.58 cm2/V∙s because the formation of well-interconnected grains leads to efficient charge transport and reduced scattering.8 Qin et al. reported mobility in FETs as a function of tetracene film thickness8. Charge transport and device performance of polyacenebased FET and LED are determined by the quality of the active film, such as crystallinity, thickness, roughness, uniformity, domain boundaries, etc.8-11 Transition metal dichalcogenides (TMDCs) such as MoS2, WSe2, and WS2 are outstanding two-dimensional (2D) nanosystems for optoelectronics, flexible electronic devices, and sensors because of their semiconducting characteristics that exhibit relatively high mobility and/or light emission.12-20 TMDCs have anisotropic electrical and optical properties with a direct or an indirect energy band gap depending on the number of layers. Therefore, the mobility (µ), photoluminescence (PL), and Raman properties of TMDCs vary with thickness.21-24 The µ of ntype semiconducting MoS2-based FETs has been demonstrated to be higher than 200 cm2/V∙s.25 Phototransistors using MoS2 flakes have also been fabricated and studied in terms of high


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detectivity and photo-responsivity.26 For the preparation of a monolayer (1L) and few-layer TMDCs, the chemical vapor deposition (CVD) and mechanical exfoliation methods have been used. The CVD method is more suitable to grow TMDCs because relatively large and homogeneous sample areas can be achieved.27 Furthermore, the CVD method has been adopted to grow TMDCs directly on dielectric substrates.28-31 Recently, nanostructures using van der Waals interaction between 2D layered systems including TMDCs enable new applications; for example, multifunctional optoelectronic devices with 2D heterojunction structures.32,33 Lee et al. reported the device characteristics of FETs consisting of an n–p heterojunction using a 1L n-type MoS2 and p-type WSe2, and the tunneling recombination of major carriers across the MoS2/WSe2 van der Waals interface controlled by the gate bias. Photovoltaic and gate-tunable diode characteristics were observed for MoS2/WSe2 FETs.32 A hybrid structure using an MoS2 with p-type organic semiconducting pentacene was reported by Hersam et al.33 Molecular and polymeric organic solids have saturated bonds in the surface atoms, resulting in a new type of van der Waals interaction in TMDCs. Jiang et al. reported on the transfer path of carriers at the vertical van der Waals interface of pentacene/MoS2 heterostructures.34

The

device

configuration-dependent

transfer

characteristics

were

demonstrated, and the gate bias modulated the vertical space charge zone that existed in the n-p van der Waals interface.34 However, fundamental hybrid characteristics such as low-temperature PL spectra of organic semiconductor and TMDC systems have not yet to be studied. Moreover, in order to study new functions of devices, other hybrid FETs using different organic semiconducting nanostructures, such as anthracene and tetracene thin films, must be investigated.


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In this study, an organic tetracene thin film with a relatively wide energy band gap was partially deposited on the surface of a 1L MoS2 grown by CVD. We observed distinctive nanoscale optical and structural characteristics from the hybrid region of MoS2/tetracene. A shift in PL peak position and increase in PL intensities were observed with decreasing temperature due to the decrease in phonon interaction. PL characteristics from the bound excitons due to the trap states at the interface of the hybrid were clearly detected at low temperatures. From the 1L MoS2/tetracene-based FETs, we observed the gate-tunable rectification current-voltage (I-V) and anti-ambipolar characteristics. The trap states observed from low-temperature PL spectra of the hybrid play an important role for charge transport in MoS2/tetracene-based anti-ambipolar FET. The hysteresis and threshold voltage due to the traps at the interface of the FETs were reduced by the insertion of a nanoscale h-BN dielectric layer. The performance of the FETs was also improved by the annealing process.

2. MATERIALS AND METHODS 2.1. Synthesis of MoS2/tetracene hybrids For the growth of 1L and a few layer of MoS2 using the CVD method, reduced graphene oxide (rGO) and MoO3 chemicals were purchased from Sigma-Aldrich Co. and used without further purification. The solution of the rGO as seed particles and MoO3 powder dispersed in ethanol were spin coated on a SiO2/Si substrate, which was then placed on a quartz boat in a heating zone at 650 °C. Sulfur powder was placed on a quartz boat in another heating zone at 200 °C. To prevent oxidation and to flow chemical vapor, N2 gas was continuously flowed at a rate of 10 sccm. The MoO3–x suboxide compound was diffused to the substrate and further reacted with sulfur vapor to grow MoS2 layers.35


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To fabricate the n–p heterojunction consisting of MoS2 (n-type) and organic semiconducting tetracene (p-type) hybrid systems, tetracene molecules were partially deposited on the surface of the 1L MoS2 grown by CVD using an organic thermal evaporation system (DaeKi Hi-Tech Co. Ltd.).33,36 The deposition rate for organic tetracene molecules was 0.2 Å/s in ultra-high vacuum (3.1 × 10−7 Torr).

2.2. Measurements To measure the thickness and surface morphology of MoS2, tetracene, and their hybrid layers, an atomic force microscope (AFM) (Nano-Focus Ltd., Albatross), equipped with a laser confocal microscope (LCM) system, was used. For LCM PL and Raman mapping experiments, a He-Ar laser (λex = 514 nm) with a power of 2 mW was used. The spot diameter of the focused laser beam in the LCM system was approximately 500 nm. The acquisition time for the LCM PL and Raman mapping was 0.25 and 0.5 s, respectively. The LCM PL and Raman mapping spectra of MoS2, tetracene, and their hybrids were analyzed using the WITec software. Details of the nanoscale LCM PL and Raman mapping experiments have been previously reported.37,38 To investigate the temperature-dependent PL characteristics of MoS2, tetracene, and their hybrids from 300 to 77 K, a vacuum chamber (FTMICROHR2, Oxford Instruments) with liquid nitrogen was used. To determine the fluorescence lifetime decays, time-resolved PL (Tr-PL) spectra were measured using an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 40× objective. A 470-nm single-mode pulsed diode laser (~100 ps pulse width) was used as an excitation source. A dichroic mirror (490 DCXR, AHF), long-pass filter (HQ500lp, AHF), 50 µm pinhole, and single photon avalanche diode (PDM series, MPD) were used to collect light-emission data from the tetracene film through a 530-nm band-pass


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filter. The Tr-PL decay curves were fitted by iterative least-squares deconvolution using the SymPhoTime software (version 5.3). The electrical characteristics of the MoS2/tetracene-based FETs were measured using a Keithley 237 source measurement unit at room temperature in vacuum (25 mTorr) conditions.

3. RESULTS AND DISCUSSION 3.1. LCM PL and Raman characteristics of MoS2 The 1L and few-layer MoS2 systems grown by CVD are triangle shaped, as shown in Fig. 1(a). The LCM Raman and PL mapping images of the samples are shown in Figs. 1(a) and (b), respectively. The inset of Fig. 1(a) shows the optical microscope image of the samples. Figure 1(c) shows the LCM Raman spectra (λex = 514 nm) of the 1L and few-layer MoS2. For region ① in Figs. 1(a) and (b), Raman characteristic modes were detected at approximately 381.19 and 401.83 cm−1 corresponding to the E12g and A1g modes, respectively. The difference in Raman shift was determined as approximately 20.64 cm−1, which corresponds to 1L MoS2. In region ② in Figs. 1(a) and (b), Raman characteristic modes corresponding to the E12g and A1g modes were detected at approximately 381.19 and 405.09 cm−1, respectively. The difference in Raman shift was determined to be approximately 23.89 cm−1, which corresponds to the tetra-layer (4L) MoS2. The difference in Raman characteristics of the 1L and 4L MoS2 systems originate from the interlayer interaction.39 The LCM PL spectrum of the 1L MoS2 shows the main emission peak at ~678 nm and shoulder peak at ~636 nm, as shown in Fig. 1(d), corresponding to the A and B excitonic transitions, respectively. The sharp PL peaks at ~540 nm are due to the Si background. The LCM PL intensity of the 1L MoS2 (red curve) is considerably higher than that of the 4L (blue curve) MoS2, as shown in Fig. 1(d), which is in agreement with the PL mapping image in


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Fig. 1(b). The relatively low PL intensity of the 4L MoS2 is due to the excitonic transition from the indirect band gap.39

3.2. LCM and time-resolved PL characteristics of MoS2/tetracene hybrids To investigate the nanoscale optical characteristics of MoS2/tetracene hybrids, the 1L MoS2, which was partially deposited with tetracene layer, was selected as shown in Fig. 2(a), where the white-dashed triangle and green region represent the 1L MoS2 and tetracene film, respectively. The overlapping regions marked by the red-dashed triangle indicate the hybrid. Figure 2(b) shows the AFM scanning image of the MoS2/tetracene hybrid. Figure 2(c) shows the AFM crosssectional surface profile of the corresponding sample. From the AFM experiment, the thickness of the deposited tetracene film was determined as approximately 80 nm. The successful hybridization of MoS2 and tetracene was also confirmed by the LCM Raman spectra, as shown in Fig. S1 (Supporting Information). Figure 3(a) shows the steady-state LCM PL mapping image of the MoS2/tetracene hybrid, which is the sample in Fig. 2. The inset of Fig. 3(a) shows the optical microscope image of the sample. Regions ⓐ, ⓑ, and ⓒ in Fig. 3(a) correspond to the tetracene, MoS2/tetracene hybrid, and MoS2 layers, respectively. Figure 3(b) shows the LCM PL spectra of tetracene (blue curve), 1L MoS2 (black curve), and MoS2/tetracene hybrid (red curve). The inset of Fig. 3(b) shows the LCM PL spectrum of the 1L MoS2. The 0–0 transition peak (corresponding to the singlet exciton) of tetracene film was observed at ~530 nm.40 The other PL characteristic peaks of the tetracene molecule were observed at 560 and 615 (670) nm, corresponding to the 0–1 transition peak and next-order transition peaks, respectively.40 As shown in the inset of Fig. 3(b), the PL characteristic peak of the 1L MoS2 observed at ~678 nm was caused by the direct band


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transition, with a shoulder peak at ~636 nm. The LCM PL intensity of the MoS2/tetracene (obtained from region ⓑ) is relatively lower than that of the pristine tetracene. The PL intensity of the 0–0 transition peak at 530 nm in the MoS2/tetracene hybrid region ⓑ decreased by approximately 63% compared to that of the pristine tetracene. This PL quenching effect in the MoS2/tetracene hybrid region suggests that the photo-generated excitons in the tetracene film were dissociated through a non-radiative decay process, and the charge transfer occurred at the interface of the hybrid. To study the charge transfer effect related to PL quenching, the Tr-PL decay spectra and Tr-PL mapping images of the pristine tetracene and MoS2/tetracene hybrid were measured and compared, as shown in Fig. 3(c) and its inset, respectively. To collect the signal of singlet excitons of the tetracene film, a 550 (±20) nm band-pass filter was used. The Tr-PL decay curves in Fig. 3(c) were analyzed using the multi-exponential fitting model I(t) = ∑Ai exp(-t/τi), where Ai and τi are the amplitude and fluorescence lifetimes of the ith component, respectively. The average lifetime (τavg) of the exciton was calculated using the equation τavg = ∑i(Aiτi2)/∑i(Aiτi).35 It should be noted that the decay time of a singlet state for the tetracene molecule is a few dozen ps.40,41 The τavg of a singlet exciton was measured as 0.89 and 0.75 ns for regions ⓐ (corresponding to tetracene) and ⓑ (corresponding to MoS2/tetracene hybrid), respectively. The decrease of exciton τavg in the MoS2/tetracene hybrid region implies the dissociation of the singlet exciton at the interface of the n–p heterojunction, supporting the results in Fig. 3(b). From the Tr-PL mapping image shown in the inset of Fig. 3(c), the hybrid region ⓑ is darker than the pristine region ⓐ, supporting the decrease of τavg.


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To confirm the LCM PL characteristics in Fig. 3 and to eliminate the thickness effect of tetracene, we have prepared new MoS2/tetracene hybrid layer on transparent cover glass, and the focused-laser was incident from the glass side, as shown in Fig. S2(a). The LCM PL intensity of the MoS2/tetracene hybrid on the cover glass with the reverse direction of incident light also decreased in comparison with that of pristine tetracene [Figs. S2(c)-(f)], indicating the PL quenching on the interface region.

3.3. Temperature dependent LCM PL characteristics of MoS2/tetracene hybrids Figures 4(a), (b), and (c) show the LCM PL spectra of the 1L MoS2, tetracene, and MoS2/tetracene hybrid, respectively, at various low temperatures (Ts). By decreasing the temperature from 300 to 77 K, the intensities of PL characteristic peaks of MoS2, tetracene, and MoS2/tetracene hybrid increased, caused by the decrease in phonon interaction. In the normalized LCM PL spectrum of the 1L MoS2, the PL spectrum corresponding to exciton A was blue-shifted from ~681 nm (1.82 eV) at 300 K to ~661 nm (1.88 eV) at 100 K, as shown in Fig. 4(d). The variations in PL peak positions as a function of temperature [λpeak(T)] of excitons A (black markers) and B (red markers) of the 1L MoS2 are shown in Fig. 4(e). Both excitons A and B gradually blue-shifted with decreasing temperature, caused by the broadening of the energy band gap and/or exciton confinement by the weak phonon interaction.42 For semiconductors with light-emission characteristics, a temperature-dependent band gap extension can be described as Eg = E0 – αT2/(T+β), where Eg is the energy band gap, E0 is its value at 0 K, and α and β are proportionality constants.43 The relative position shift of the conduction band and valence band in the energy band gap and the confinement of excitons originated from the weak phonon interaction and the thermal expansion of the lattice with decreasing temperature.


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Figures 4(f) and (g) show the normalized PL spectra of the pristine tetracene and MoS2/tetracene hybrid, respectively, at various low temperatures. The narrowing of the full width at half maximum (FWHM) of the 0–0 transition peak of the pristine tetracene and hybrid systems with decreasing temperature was clearly observed. The narrowing of the FWHM of the tetracene and MoS2/tetracene hybrid systems was estimated as approximately 50 meV and 56 meV, respectively, with decreasing temperature, as shown in Figs. 4(h) and (i). At low temperatures, the decrease in phonon interaction results in the increase of PL intensity and narrowing of the FWHM. Figures 5(a) and (b) show the PL spectra of the pristine tetracene and MoS2/tetracene hybrid, respectively, at various low temperatures. For the MoS2/tetracene hybrid, new PL peaks in the range 700–780 nm with decreasing temperature [Fig. 5(b)] were clearly observed. For the pristine tetracene, a PL peak at ~730 nm was weakly observed at 77 and 100 K. Two PL peaks at ~722 nm and ~749 nm were observed below 125 K for the MoS2/tetracene hybrid, indicating the formation of trap states due to the hybridization of MoS2 and tetracene. To investigate the hybrid effect on PL characteristics, the PL spectra (measured at 100 K) of the pristine MoS2, tetracene, and MoS2/tetracene hybrid were decomposed using Gaussian fitting, as shown in Figs. 5(c), (d), and (e), respectively. The free and bound exciton characteristics can be obtained from the deconvolution of the PL spectra. Exciton A, exciton B, and negative trion (A−) for the 1L MoS2 were observed at ~678, ~636, and ~694 nm, respectively, as shown in Fig. 5(c). For the tetracene, the 0–0 transition peak at ~530 nm, the 0–1 transition peak at ~560 nm, and two shoulder peaks at ~615 and ~670 nm corresponding to free excitons were observed, as shown in Fig. 5(d). With decreasing temperature, the bound exciton peak can be observed because of the increasing PL intensity in our study. The bound exciton peaks at 100 K were clearly observed at


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~776 nm for the pristine MoS2 [Fig. 5(c)] and at ~730 nm for the pristine tetracene [Fig. 5(d)]. The bound exciton originates from a trap state caused by impurities and defects on the surface and/or interface, resulting in the broadening of the PL spectrum.42,44 The energy of the bound exciton is higher than the thermal activation energy; therefore, the bound exciton peak can clearly be observed in the low-temperature regime. This is the reason for investigating lowtemperature PL characteristics. Interestingly, for the MoS2/tetracene hybrid, the extra PL peaks corresponding to bound excitons (denoted by ① and ②) were observed at ~722 and ~749 nm, respectively, as shown in Figs. 5(b) and (e), which were newly formed through the trap states at the interfaces between MoS2 and tetracene. The two bound exciton ① and ② peaks were from the hybridization of MoS2 and tetracene, and not from each bound exciton peak of pristine MoS2 or pristine tetracene. By decreasing the temperature, these bound exciton PL peaks were gradually enhanced, as shown in Fig. 5(f), indicating the appearance of trap states owing to the hybridization of MoS2 with tetracene.

3.4. Charge transport characteristics of MoS2/tetracene-based FETs Lateral-type n–p heterojunction FETs, using the hybrid of an n-type MoS2 and a p-type tetracene as an active layer, were prepared using e-beam lithography (EBL). Figures 6(a) and (b) show the schematic illustration and real optical microscope image of the MoS2/tetracene-based FET, respectively. Our FETs have a bottom gate coated with 300-nm SiO2 by heavily p-doped Si wafer. To fabricate the hybrid active layers, the SiO2 substrate was first covered with poly(methyl methacrylate), and the junction area was exposed using EBL. The tetracene layer (with thickness of approximately 35 nm) for the lateral-type n–p heterojunction was partially deposited on the MoS2 layer. For the electrodes of FETs, the Au 50 nm/Cr 10 nm/Al 50 nm


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bottom contacting the MoS2, and the Au 100 nm/Ti 10 nm top contacting the tetracene, were deposited using e-beam evaporation, as shown in Fig. 6(a). The black-dotted triangular region and the red-dotted square region in Fig. 6(a) represent the MoS2 and tetracene layers, respectively. The MoS2/tetracene hybrid region is located between the two inner electrodes [Fig. 6(b)]. This FET is denoted as device ①. Figure 6(c) shows the transfer characteristic curves at VD = 90 V of the MoS2/tetracene n–p heterojunction FET. The anti-ambipolar characteristics, i.e., the coexistence of n-type and p-type charge transport depending on VG, were observed. Furthermore, the off-states of n-type and ptype in the anti-ambipolar characteristics were obtained from a relatively high negative and positive gate bias, as shown in Fig. 6(c). In the forward bias direction (from −80 V to +80 V, black makers), with decreasing negative VG, ID rapidly increased (regime ①), exhibiting n-type characteristics. In regime ①, the value of ID increased owing to the relative increase of accumulated electrons (nelectron), while the concentration of accumulated holes (nhole) decreased at the MoS2/tetracene active layer. Then, ID reached its maximum of 1.6 × 10−9 A at VG = −50 V, caused by the balance of nelectron and nhole. By further decreasing negative VG values, ID rapidly decreased (regime ②), exhibiting p-type characteristics. In regime ②, ID decreased with decreasing negative VG because of the decrease in nhole while nelectron gradually increased. By significantly increasing the positive VG (≥ 20 V), ID fell below 1.9 × 10−10 A and saturated, exhibiting almost no current. Similarly, in the reverse bias direction (from +80 V to −80 V, red markers), ID increased as the positive VG decreased (regime ③). The maximum of ID was approximately 1.5 × 10−9 A at VG = −5 V, and then decreased again as VG increased in the


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negative region (regime ④), indicating n-type characteristics. With high negative VG (≤ −30 V), an n-type off-state was observed, as shown in Fig. 6(c). The forward and reverse transfer characteristic curves in Fig. 6(c) did not overlap, implying the hysteresis of the anti-ambipolar characteristics for our FETs, which is caused by the trap states at the interface. The difference of VG at the ID peaks between the forward and reverse bias, ∆ VG (ID,forward − ID,reverse), was determined as approximately 43 V. The charge mobility () can be calculated using the equation =(dID/dVG)/ (/) ⋅ ( ), where dID/dVG is the trans-conductance, W is the channel width, L is the channel length, and Cox is the capacitance of the oxide.37 The µ values in n-type regions ① and ④ were determined as 1.80 × 10−5 and 2.92 × 10−5 cm2/V·s, respectively. In p-type regions ② and ③, the µ values were determined as 1.40 × 10−5 and 2.48 × 10−5 cm2/V·s, respectively. The charge transport mechanism across the MoS2/tetracene n–p heterojunction can be described by the Shockley– Read–Hall (SRH) trap-intermediated tunneling and Langevin recombination processes,32 as shown in Fig. 6(d). The SRH recombination process was modified by the inelastic tunneling of major carriers across the trap states that formed in the energy gap [denoted by ⓐ process in Fig. 6(d)]. In our study, the formation of trap states was indirectly confirmed through two bound exciton peaks from low-temperature PL spectra for the hybrid region of MoS2 and tetracene, which plays an important role for charge transport at the interface of the n-p heterojunction. The electrons from MoS2 were transported to the trap states in tetracene, and the holes from tetracene were transported to the trap states in MoS2, resulting in a trap-intermediated tunneling current. The Langevin recombination process was modified by the charge transport through Coulomb


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interaction between electrons in the conduction band of MoS2 and holes in the HOMO level of tetracene [denoted by ⓑ process in Fig. 6(d)].32 Both SRH tunneling charges (ⓐ process) and

Langevin recombination charges by Coulomb interaction (ⓑ process) contributed to the total current in the MoS2/tetracene-based FET. Figure 6(e) shows the output characteristic curves (ID vs. VD) of the same MoS2/tetracenebased FET (device ①) with VG = −80, 0, and 80 V during reverse bias sweep. The rectifying behavior was observed on the ID–VD characteristic curves as shown in Fig. 6(e). This is caused by the energy band alignment at the n–p heterojunction and formation of potential barriers at the MoS2/tetracene interface. The conduction band (valence band) of pristine MoS2 and the lowest unoccupied molecular orbital (LUMO) [highest occupied molecular orbital (HOMO)] of tetracene are 4.5 eV (6.3 eV) and 2.4 eV (5.4 eV), respectively. It is noted that the results in Fig. 6(e) were not simultaneously measured during the experiment for Fig. 6(c). Considering the hysteresis of anti-ambipolar characteristics in our FET device ①, the values of ID in Figs. 6(c) and (e) do not exactly agree with each other. In the n-channel regime (i.e., VD > 0 V), ID–VD characteristic curves varied with VG, and the ID values decreased at VG = ±80 V. In the regime of VD ≥ 60 V at VG = 0 V, ID drastically increased, as shown in Fig. 6(e), which is in agreement with the observation of the maximum ID value at VG ≅ 0 V in the transfer characteristic curve [red markers in Fig. 6(c)]. This result is mainly attributed to the balance of electron and hole densities at VG = 0 V. Similar FET characteristics were observed in another batch of MoS2/tetracene-based FET device ③ (Fig. S3 in Supporting Information).


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The anti-ambipolar charge transport, hysteresis, and shape of I–V characteristic curves of our FETs were determined by the balance of electron and hole mobilities, relative charge densities, channel lengths of n-type and p-type layers, hybrid area, etc. It was observed that the symmetry of anti-ambipolar characteristic curves and maximum ID were related to the balance of the nelectron and nhole. The negative shift in the position of the VG peak in the transfer characteristic curve indicates that the major contribution of p-type materials (i.e., holes) to charge transport is dominant because of the relatively lower and n of electrons. Therefore, the charge transport and FET performance are dependent on physical size (including thickness), overlapping area, and relative channel length of n-type and p-type materials. We observed different FET performance with different tetracene thickness of active layers (FET devices ①–⑤).

3.5. Device characteristics of MoS2/tetracene-based FET with h-BN dielectric layer To improve the performance of our FET, we fabricated another MoS2/tetracene-based FET hybridized with dielectric hexagonal boron nitride (h-BN) thin layer (thickness of approximately 20 nm), which was inserted between the MoS2 and SiO2 layers, as shown in Fig. 7(a). This FET is denoted as device ②. Figures 7(b) and (c) show the optical microscope image and transfer characteristic curves (at VD = 85 V) of the h-BN/MoS2/tetracene-based FET, respectively. Antiambipolar characteristic curves similar to those in Fig. 6 were also observed for this hBN/MoS2/tetracene-based FET in the range −35 V ≤ VG ≤ 10 V. By decreasing negative VG, ID increased (regimes ① and ④), indicating n-type characteristics. The µ values in regimes ① and ④ were determined as 2.30 ×10−5 and 2.59 ×10−5 cm2/V·s, respectively. In regimes ② and ③, ID


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decreased with decreasing negative VG, indicating a p-type nature. The µ values in regimes ② and ③ were determined as 1.12 ×10−5 and 2.16 ×10−5 cm2/V·s, respectively. The µ values of the FET with h-BN (device ②) are comparable with those of the FET (device ①) without h-BN. However, the hysteresis effect was reduced. The difference ∆VG of the ID peaks between the forward and reverse bias was approximately 5 V, which is much smaller than that (≅ 43 V) of FET device ① without h-BN. The h-BN dielectric layer reduced the hysteresis effect in the MoS2/tetracene-based FET because of the decrease in trapped charges,45 and it can protect from Coulomb scattering caused by charged impurities on the SiO2 surface.17 Figure 7(d) shows the output characteristic curves of the h-BN/MoS2/tetracene-based FET in the range −30 V ≤ VG ≤ +10 V. For negative VG values, the turn-on VD was observed at 60, 68, and 74 V by varying VG = −30, −20, and −10 V, respectively. The turn-on VD was controlled through the applied VG. The shift of the turn-on VD can be explained in terms of the decrease in VT due to the insertion of the h-BN layer, which resulted in the effective removal of traps between the MoS2 and SiO2 layers. The VT value of FET device ① (MoS2/tetracene-based FET without h-BN) was −74 V (regime ①) and −26 V (regime ②) in the forward and reverse bias directions, respectively. However, for FET device ② (MoS2/tetracene-based FET with h-BN), VT was −32 V (regime ①) and −26 V (regime ②) in the forward bias and reverse bias directions, respectively. These results suggest that device characteristics, such as the VT and hysteresis of the FET device using the MoS2/tetracene hybrid, can be improved by the insertion of a nanoscale


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thin dielectric h-BN layer which decreases the number of traps at the interfaces. The similar results were observed from the different batch of h-BN/MoS2/tetracene-based FET (device ④), as shown in Fig. S4 in Supporting Information. It is noted that for device ②, the h-BN was partially loaded in the hybrid region. When it was loaded in the entire hybrid region including tetracene, the FET performance can be improved because the h-BN dielectric layer reduces more traps. To further improve the performance of FET device, we investigated the effects of annealing48,49 on the h-BN/MoS2/tetracene FET (device ⑤). Figure 8(a) shows an optical microscope image of our h-BN/MoS2/tetracene-based FET after the annealing process. To increase hole mobility, the tetracene thickness was greater (approximately 500 nm) than in previous devices. Device ⑤ was annealed by baking at 45 ℃ for 5 h. Figure 8(b) shows the transfer characteristic curves at 40 V. We observed similar anti-ambipolar characteristic curves, but they were turned-on at lower VD values such as +30 V (not shown here) and +40 V [Fig. 8(b)]. In the forward bias direction (from −80 V to 60 V, blue makers), ID rapidly increased with decreasing negative VG (from −80 V to −60 V), and ID reached its maximum value at VG= −60 V. By further decreasing negative VG, ID rapidly decreased. By significantly the positive VG (> 0 V), exhibiting almost no current. In reverse bias direction (from 60 V to −80 V, red makers), the anti-ambipolar characteristic curve was observed similarly. The µ values in n-type regimes ① and ④ were determined as 2.36 × 10−5 and 2.68 × 10−5 cm2/V·s, respectively. The p-type regions ② and ③, the µ values were determined as 4.33 × 10−5 and 2.44 × 10−5 cm2/V·s, respectively.


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The µ values of the FET after annealing (device ⑤) increased by approximately 2–3 times compared with the FETs (devices ① and ②) without annealing. The results indicate that the performance of the h-BN/MoS2/tetracene FET was improved by annealing because of reduced traps and defects. Figures 8(c) and (d) show the ID vs. VD curves of the FET device ⑤ in the ranges VG ≤ 0 V, and VG > 0 V, respectively. In the n-channel regime (i.e., VD > 0 V), the rectifying ID–VD characteristic behavior was observed due to the energy band alignment at the n–p heterojunction. The turn-on VD values were in the range 20 V ≤ VD,turn-on ≤ 30 V, which are much lower than those of previous devices ① and ②. For VG ≤ 0 V, the turn-on VD was observed at 21, 23, 26 and 29 V by varying VG = −60, −40, −20, and 0 V, respectively, as shown in Fig. 8(c). We clearly observed that the performance of h-BN/MoS2/tetracene-based FET was improved by the annealing process and use of thicker tetracene layer. For discussion, the difference of our work compared with the MoS2/pentacene FETs reported by Hersam et al.33 is the nanoscale optical characteristics of the MoS2/tetracene hybrid, including the experimental observation of trap states from low-temperature PL spectra. The trap states are essential for the charge transport and hysteresis characteristics in TMDC/organic van der Waals heterojunction. The advance of our study is that the hysteresis of anti-ambipolar characteristics was controlled using the h-BN dielectric layer. The insertion of the h-BN layer reduced the SiOH silanol groups owing to the binding of SiO2 and H2O at the interface between SiO2 and MoS2. Therefore, the FET performance using MoS2/tetracene was improved by the insertion of the hBN dielectric layer. In terms of the fabrication of FET, our anti-ambipolar FETs were large-scale


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devices using monolayer MoS2 grown by CVD rather than exfoliation,32-34 and OMBD-deposited organic tetracene film. For the FETs using the MoS2/tetracene hybrids (devices ① and ②), the ID were relatively low34,46,47 as the order of 10-9 A with applying relatively high VD (such as 80–90 V). The other FETs using the TMDC hybrids also showed the similar ID values (0.6~7 nA).32,46-47 Notably, the FETs using p-type tetracene films showed lower mobility (~10-5 cm2/V∙s), resulting in the poor performance of FETs using the MoS2/tetracene hybrids. We improved the performance, such as lowering the turn-on VD of our h-BN/MoS2/tetracene-based FET, via the annealing process and use of a thicker p-type tetracene layer (device ⑤).

4. CONCLUSIONS We have synthesized 2D nanostructures using 1L MoS2 grown by CVD method, which was partially hybridized with organic semiconducting tetracene. PL quenching was observed from the MoS2/tetracene hybrid region because of the charge transfer effect, which was confirmed by the decrease in exciton lifetime. By decreasing the temperature, the PL peak position of the MoS2 was blue-shifted; the PL intensities of the pristine and hybrid systems increased; and the FWHM of the hybrid was narrowed owing to the decrease in phonon interaction. For the MoS2/tetracene hybrid region, the PL peaks corresponding to two new bound excitons due to the trap states at the interface were clearly observed at 722 and 749 nm under 125 K. We fabricated n–p heterojunction FET devices using the MoS2/tetracene hybrids as an active layer. The gatetunable rectification I–V and anti-ambipolar characteristics were simultaneously observed from the MoS2/tetracene-based FETs. The charge transport of the FET was explained in terms of SRH


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trap-intermediated tunneling and Langevin recombination processes at the interface of the hybrid. By inserting a nanoscale thin h-BN dielectric layer, the hysteresis and large VT value due to the trap states at the interface were considerably reduced. Reduced turn-on VD and enhanced mobilities were observed for the h-BN/MoS2/tetracene-based FET as a result of the annealing process and use of thicker tetracene layer.


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Figure 1. (a) LCM Raman mapping image of the 1L MoS2 (region ①) and 4L MoS2 (region ②) grown by CVD. Inset: Optical microscope image of the corresponding samples. (b) LCM PL mapping image of the corresponding samples. (c) LCM Raman spectra (λex = 514 nm) of the 1L and 4L MoS2 samples. (d) LCM PL spectra (λex = 514 nm) of the 1L and 4L MoS2 samples.


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Figure 2. (a) Optical microscope image of the MoS2/tetracene hybrid. (b) AFM image of the corresponding samples. (c) Cross-sectional profile of the AFM image along the yellow line in Fig. 2 (b). The thickness of the deposited tetracene is approximately 80 nm.


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Figure 3. (a) LCM PL mapping image of the MoS2/tetracene hybrid on the SiO2/Si substrate. Inset: Optical microscope image of the corresponding samples. Region ⓐ: pristine tetracene, region ⓑ: MoS2/tetracene hybrid, and region ⓒ: pristine MoS2. (b) LCM PL spectra of tetracene film (blue curve), MoS2/tetracene hybrid (red curve), and 1L MoS2 (black curve). Inset: LCM PL spectrum of 1L MoS2. (c) Time-resolved (Tr)-PL decay curves (λex = 470 nm) of tetracene (blue curve) of MoS2/tetracene hybrid (red curve). Inset: Tr-PL mapping image of the corresponding samples.


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Figure 4. LCM PL spectra of the (a) MoS2, (b) tetracene, and (c) MoS2/tetracene hybrid at various low temperatures. (d) Normalized LCM PL spectra for MoS2 at various low temperatures. (e) Temperature dependence of PL peak positions corresponding to excitons A (black markers) and B (red markers) of pristine MoS2. Variation in the normalized LCM PL spectra for (f) tetracene and (g) MoS2/tetracene hybrid at various low temperatures. Temperature dependence of the FWHM for (h) tetracene and (i) MoS2/tetracene hybrid.


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Figure 5. LCM PL spectra of the (a) tetracene and (b) MoS2/tetracene hybrid at various low temperatures to identify new bound excitons corresponding to trap states. LCM PL spectra with deconvoluted curves using Gaussian fitting for (c) pristine MoS2 (region ⓒ), (d) tetracene (region ⓐ ), and (e) MoS2/tetracene hybrid (region ⓑ ) at 100 K. The dotted color curves represent the deconvoluted curves for PL spectra. (f) Variation in LCM PL intensities corresponding to the bound excitons (denoted as ① and ②) for the hybrid sample as a function of temperature.


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Figure 6. (a) Schematic illustration of the MoS2/tetracene n–p heterojunction FET. Inset: Schematic chemical structures of MoS2 and tetracene molecules. (b) Optical microscope image of the MoS2/tetracene n–p heterojunction FET. Black-dashed triangular region: 1L MoS2, red dotted square area: tetracene (thickness ~35 nm). (c) Transfer characteristic curves (ID vs.VG) at VD = +90 V for FET. Black line: forward bias. Red line: backward bias. (d) Schematic energy band diagram of the MoS2/tetracene n–p heterojunction with explanation of the Langevin (blue arrow) and Shockley–Read–Hall (red arrow) recombination processes. The numerical values indicate the valence band energy (HOMO) and conduction band energy (LUMO) levels. (e) FET output characteristic curves (ID vs. VD) for different VG = −80, 0, and +80 V.


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Figure 7. (a) Schematic illustration of the MoS2/tetracene n–p heterojunction FET with h-BN dielectric layer. (b) Optical microscope image of the FET. Region of white dotted line: 1L MoS2, red dotted square area: tetracene (thickness of ~35 nm), blue dotted line region: h-BN (thickness of ~20 nm). (c) Transfer characteristic curves (ID vs. VG) at VD= +85 V for h-BN/MoS2/tetracene n–p heterojunction FET. Black curve: forward bias. Red curve: reverse bias. (d) Output characteristic curves (ID vs. VD) for different VG= −30, −20, −10, 0, and +10 V.


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Figure 8. (a) Optical microscope image of the MoS2/tetracene n–p heterojunction FET with hBN dielectric layer (device ⑤). The device ⑤ was annealed by baking at 45℃ for 5h. Red dotted line region: h-BN (thickness of ~20 nm), white dotted triangular area: 1L MoS2, blue dotted square area: tetracene (thickness of ~500 nm). (b) Transfer characteristic curves (ID vs.VG) at VD = +40 V for the annealed h-BN/MoS2/tetracene-based FET. Output characteristic curve of the same FET (ID vs. VD) for different (c) VG = −60, −40, −20, and 0 V and (d) VG = 20, 40, and 60 V.


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ASSOCIATED CONTENT Supporting Information LCM Raman spectra of tetracene film and MoS2/tetracene hybrid, optical microscope image of the MoS2/tetracene-based FET, LCM PL spectra and Tr-PL decay curves using the MoS2/tetracene hybrid layer on the surface of glass, typical transfer characteristic curves of the pristine 1L MoS2 and pristine tetracene, transfer characteristic curves for different batch of MoS2/tetracene-based FET, optical microscope image and transfer characteristics curves of the MoS2/tetracene-based FET with h-BN dielectric layer (PDF).

AUTHOR INFORMATION Corresponding Authors *J. Joo E-mail: [email protected] *J. Kim E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (No. 2015R1A2A2A01003805 and 2018R1A2B2006369); Center for Advanced Meta-Materials (CAMM, as Global Frontier Project) (CAMM-2014M3A6B3063710). The authors thank Dr. Weon-Sik Chae, KBSI Daegu Center, for the fluorescence lifetime measurements.


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39

Hybrid Characteristics of MoS2 Monolayer with Organic

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