Large Work Function Modulation of Monolayer MoS2 by Ambient

May 27, 2016 - §Device Laboratory and ∥AE group, Platform Technology Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Republic ...
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Large Work Function Modulation of Monolayer MoS2 by Ambient Gases Si Young Lee,†,⊥ Un Jeong Kim,*,§,⊥ JaeGwan Chung,∥ Honggi Nam,†,‡ Hye Yun Jeong,†,‡ Gang Hee Han,† Hyun Kim,†,‡ Hye Min Oh,†,‡ Hyangsook Lee,∥ Hyochul Kim,§ Young-Geun Roh,§ Jineun Kim,§ Sung Woo Hwang,§ Yeonsang Park,*,§ and Young Hee Lee*,†,‡ †

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), and ‡Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea § Device Laboratory and ∥AE group, Platform Technology Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Republic of Korea S Supporting Information *

ABSTRACT: Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors. KEYWORDS: monolayer MoS2, work function, Kelvin probe, diode, photocurrent spin-valley coupling,20 valley polarization,21 and valley quantum coherence,22 have been reported. Another interesting feature of the atomically thin layered materials is their sensitivity to adsorbents because all of the atoms are exposed to the surface.23 Therefore, the related intrinsic physical and chemical properties are no longer guaranteed, which often lead to a wrong conclusion. Work function, which is the energy required to remove an electron from the surface of a material, is a good example of easy modulation by ambient gases and is a key parameter to consider in designing numerous electronic and optoelectronic devices. Oxygen molecules, when adsorbed on a graphene surface, have been among the most representative gas species

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tomically thin layered materials have revealed numerous unique features as predicted by Feynman in his lecture note, “What could we do with layered structures of materials with just the right layers?”1 Graphene is a good example, which exhibits a massless Dirac particle feature with extremely high mobility. Recently, emerging transition-metal dichalocogenides (TMDs) have been discovered to be another class of layered materials that reveal a unique layer dependence, converting an indirect bandgap in a bulk to a direct bandgap semiconductor in a monolayer.2,3 The available metallic graphene, semiconducting TMD, and highly insulating hexagonal boron nitride as components for electronics, along with their flexibility, have opened new research areas for field effect transistors (FETs),4−8 photodetectors,9−13 light-emitting diodes,14−16 photovoltaic devices,17−19 and so on. Furthermore, owing to the inversion-asymmetric crystal structure in monolayer TMDs, new types of optical devices, such as giant © 2016 American Chemical Society

Received: March 11, 2016 Accepted: May 27, 2016 Published: May 27, 2016 6100

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Figure 1. (a) Schematic of the KP system. (b) Time-resolved work function changes measured in ambient, UHV (∼10−9 Torr), and O2 gas environment using the KP system. (c) Estimated band diagrams of the CVD-grown monolayer MoS2 in ambient, UHV, and O2 gas environment.

RESULTS AND DISCUSSION Centimeter-scale monolayer MoS2 was prepared by chemical vapor deposition (CVD), which is required for work function measurement using the electrical KP technique.28 The sample exhibited chemical and optical properties similar to exfoliated MoS2 (see the Supporting Information, Figure S1). The work function was measured in situ using the KP technique applied in an ultrahigh vacuum (UHV) chamber (Figure 1a). The measured work function of the monolayer MoS2 was 4.36 eV in air and was reduced to ∼4.04 eV under UHV pressure (∼10−9 Torr). The sample was then exposed to O2 gas, and its work function was further increased to a higher value of ∼4.47 eV. The time dependence of work function change under O2 gas flow (∼3 Torr) was also observed. After O2 exposure for 120 min, the work function was saturated (see the Supporting Information, Figure S2a). To confirm the origin of the work function change among the various adsorbents on the MoS2 surface, nitrogen gas was also introduced to the KP chamber under UHV pressure. The work function was slightly incremented by 0.02 eV with N2 exposure, which is a negligible change (see the Supporting Information, Figure S2b). The energy-level diagram of the MoS2 in ambient (left), in UHV (middle), and with O2 adsorption (right) is shown in Figure 1c using the information obtained from the KP technique (Figure 1b) and UPS studies (to be discussed later), and the band gap was taken from previous reports.29 The work function variation of the monolayer MoS2 was also carefully double-checked by UPS under UHV (∼10−9 Torr) using He II as a monochromatic excitation source (Figure 2a,b). The work function is defined as

that exist in air and change the electrical properties of lowdimensional materials to p-type by trapping the electrons of the host materials.24−26 In the same manner, the Dirac point modulation of graphene has been systematically observed depending on the chemical compositions of ionic liquids as a gate dielectric.27 The intrinsic characteristics of these atomically thin layered materials, which are sensitive to the environment, have been recognized as a bottleneck in stable and reproducible device fabrication and performance. However, such susceptibility to the environment can be conversely considered as an advantage from an engineering perspective as a means to control the work function of a material, which is not available in bulk materials. In this study, we systematically measured the work function of monolayer MoS2 by an in situ Kelvin probe (KP) technique under various ambient conditions (air, oxygen, and nitrogen gases), which was further confirmed by ultraviolet photoemission spectroscopy (UPS) and density functional theory (DFT) calculations. We observed a large work function variation of ∼0.43 eV by varying the environment from vacuum to O2 exposure. The DFT calculations predicted that the work function can be modulated up to 1.2 eV by oxygen molecules. This large variation under gas exposure is comparable with that of graphene with gate modulation. A homojunction p−n diode, constructed by partially passivating the MoS2 channel with an Al2O3 thin film and exposing to oxygen molecules, clearly demonstrated a rectifying behavior with an ideality factor of almost one, which recovered to linear I−V characteristics upon removal of the gases. The reversibility of the I−V characteristics between the linear and rectifying characteristics is a unique property in the formation of oxygenadsorption-assisted ideal p−n junctions compared with conventional diodes. We also measured the photocurrent in the diode device by laser-beam scanning. A large photocurrent was observed at the junction interface, which proved the formation of an ideal depletion layer width of 200 nm with minimum recombination centers that are much narrower than those of conventional semiconductors.

ϕ = hv − W

(1)

where hν = 40.813 eV (He II) and W is the spectral width, which can be measured from the valence-band and secondary edges. The secondary-band edge, measured by UPS under UHV, and the valence-band spectra (see the Supporting Information, Figure S3) showed that the work function of the monolayer MoS2 was ∼4.023 eV, which was close to ∼4.05 eV measured by the KP technique. The work function of the bulk MoS2 under UHV by UPS was ∼4.542 eV, which was larger 6101

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than that of the MoS2 monolayer. The work functions for the monolayer and bulk MoS2 in the literature19,30,31 (measured by UPS) are listed in Table S1. The current work function value of the bulk MoS2 lies within a wide range of reported values (4.5− 4.9 eV) depending on the presence of sulfur vacancies or the amount of gas adsorbents. After O2 exposure for 2 h at 3 Torr, the sample was immediately transferred to another chamber for UPS measurement. The work function of the monolayer MoS2 measured after 2 h exposure was ∼4.28 eV (Figure 2b), which was lower than that measured by the KP technique. This lower work function was attributed to the partial desorption of oxygen molecules on the MoS2 surface by exposing the sample to UHV for UPS measurement. On the other hand, the valence band spectra (see the Supporting Information, Figure S3b) do not show any obvious shift in valence band maximum with O2 exposure. The p-doping effect by O2/H2O molecules has been extensively studied both theoretically and experimentally,24,32,33 which was also observed in our PL measurement change under ambient conditions (see the Supporting Information, Figure S4). In addition, the effect of adsorption of O2/H2O on the electronic band structure of MoS2 is not fully understood at this point, which may rely on O2/H2O coverage as well. This requires further studies. The work function change (ΔΦ) of the monolayer MoS2 as a function of oxygen-molecule coverage was theoretically investigated by DFT calculations using the generalized gradient approximation (GGA) and local density approximation (LDA), as shown in Figure 2c. The coverage (in percent) is defined as the number of oxygen molecules divided by the number of Mo atoms and multiplied by 100. The work function difference rapidly increased up to a 20% O2 coverage and then slowly saturated up to 1.2 eV at 100% coverage regardless of whether GGA or LDA was used. Meanwhile, the work function was not significantly modified by H2O molecules. The effect on the work function difference by H2O was much smaller than that by O2. The work function difference (0.32 eV) between ambient which includes moisture and UHV (as shown in Figure 1b) was smaller than that (0.43 eV) between the UHV and O2

Figure 2. Secondary-edge spectrum from (a) bulk and CVD-grown monolayer MoS 2 measured in UHV and (b) CVD-grown monolayer MoS2 measured in UHV and O2 gas environment. (c) Theoretical calculation of the work function difference of monolayer MoS2 as a function of O2/H2O adsorption rate by the GGA and LDA methods.

Figure 3. (a) IDS−VDS transfer curves with VGS from +40 to −40 V measured in HV chamber. Inset: optical image of a monolayer MoS2 transistor on a 300 nm SiO2/Si substrate with Cr/Au electrodes. Scale bar, 10 μm. (b) IDS−VGS transfer curves measured in ambient, LV (∼10−3 Torr), and HV (∼10−6 Torr, after annealing) with VDS = 1 V. Inset: maximum transconductance changes. (c) IDS−VGS transfer curve measured in HV and after O2 gas exposure into the vacuum chamber. VDS = 1 V. (d) Band alignment of the MoS2−metal junction in ambient (black), LV (green), and HV (blue). 6102

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Figure 4. (a) Optical image of a half-covered MoS2 transistor. Scale bar, 5 μm. (b) Estimated band structure of a half-covered MoS2. Schematics of (c) as-prepared transistor under HV, (d) half-covered transistor under ambient, and (e) half-covered transistor under HV. IDS− VDS transfer curves with the transistor structures of (f) c, (g) d (inset: log-scaled transfer curve with the ideality factor), and (h) e.

ratio (Ion/Ioff ∼ 106) was observed under ambient conditions. As the vacuum level in the chamber changed from ambient to low vacuum (LV) and HV, the threshold voltage (Vth) shifted toward the negative direction, and the on-current level (Ion) increased by 2 orders of magnitude, as shown in Figure 3b. Thus, the transconductance (gm) increased from 10 to ∼50 nS (inset in Figure 3b). Under HV, the device displayed almost degenerate semiconductor behavior with the increased n-type doping density, which is congruent to the large work function difference observed in the KP technique. By introducing oxygen gas to the HV chamber, Vth and Ion partially recovered to their original values, as shown in Figure 3c. The typical field effect mobility (μ) values of the monolayer MoS2 transistor investigated in this work were ∼6, ∼11, and ∼20 cm2 V−1 s−1 under ambient, LV, and HV, respectively (see the Supporting Information, Figure S5b). Time dependent transfer curve (IDS-VGS) was measured at VDS = 1 V with 10 min span under O2 exposure (see Supporting Information Figure S5c). With 5 h O2 exposure, Vth was gradually upshifted to ∼10 V, and the on-current is also gradually decreased to ∼1.5 μA. With longer O2 exposure time, these values could be further changed until O2 coverage is saturated. The pristine monolayer MoS2 shows n-type behavior, which has been ascribed to S vacancies.36 These vacancies can be saturated by O2 gases or other environmental adsorbates, leading to n-type defect compensation. In HV, O2 molecules will be desorbed and n-type doping will recover to enlarge the n-type region, i.e., downshifting the threshold voltage and increasing the on-current. Thus, adsorption or desorption of O2 molecules modulates the Fermi level. The formation of a Schottky barrier is schematically shown in Figure 3d−f. The Fermi level increases under HV, creating a thinner Schottky barrier between the metal and MoS2 channel and allowing higher on-current by tunneling.

environment, which can be explained by the additional adsorption of water molecules in ambient. If the moisture effect is ignored, the expected coverage of oxygen molecules after UHV can be estimated to be less than ∼10% based on the DFT calculations. Pushing the theoretical limit of the work function modulation up to 1.2 eV by oxygen molecules can be a challenge for future experiment. Such a large work function modulation can also influence the photoluminescence (PL) spectra (see the Supporting Information, Figure S4). The PL of the monolayer MoS2 was measured in ambient and vacuum and after being vented to the atmosphere to understand the effect of O2 adsorption. Under vacuum (∼10−4 Torr), the intensity of A exciton decreased, and the relative intensity of A to B exciton became comparable. When the vacuum chamber was vented to the atmosphere, the PL spectrum recovered to the original spectrum in air. Oxygen molecules acted as a p-type dopant to reduce the n-doping effect.34 The PL intensity in vacuum was reduced by the increased free-carrier scattering. The effect of the work function modulation was also demonstrated by fabricating monolayer MoS2 FETs on a SiO2 (300 nm)/Si substrate using electron beam (e-beam) lithography (Figure 3). The I−V characteristics (IDS versus VDS), measured in air at room temperature (see the Supporting Information, Figure S5a), was nonlinear at the high-VDS regime, implying a Schottky contact formation originating from the work function difference between the MoS2 (∼4.36 eV in air) and Cr metal (4.5 eV).35 Interestingly, the IDS−VDS curves measured under HV of approximately ∼10−6 Torr became linear even at the high-VDS regime, as shown in Figure 3a, in spite of the reduced work function of MoS2 to ∼4.04 eV. The environmental effect on the transfer characteristics (IDS versus VGS) was studied at constant VDS = 1 V, as shown in Figure 3b,c. Typical n-type behavior with a large current on−off 6103

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ACS Nano The amount of Vth shift of MoS2 transistor after exposing O2 under HV condition is quite low compared to the expected value from the shift of its work function by KP measurement, and Vth is not even restored to the original value, but the work function is increased to a higher value than that under ambient conditions. This can be explained by several points. First, the vacuum conditions of IV and KP measurements are quite different, which is HV (∼10−6 Torr) and UHV (∼10−9 Torr), respectively. Thus, O2 and other adsorbents are not sufficiently removed from MoS2 surface under HV condition for IV measurement. Second, the amount of charge transfer by O2 molecule to ideal MoS2 and sulfur vacancy in defective MoS2 has been theoretically reported by 0.021 and 0.997 eV per O2, respectively.33 Centimeter-scale MoS2 used for KP measurements can contain higher defect densities, including sulfur vacancies and grain boundaries, than single-crystal triangularshaped MoS2 used for IV measurements.37 Thus, large area MoS2 including several types of defects can be more sensitive in doping by O2 molecules. The difference in the amount of charge transfer due to defect density can result in less Vth shift than work function after introduction of O2 in the vacuum chamber. Finally, residues on the MoS2 surface occurring by device fabrication processes can inhibit the efficient removal of O2 molecules and other adsorbents, which will result in relatively less dramatic change in Vth compared to work function shift by KP measurement. The device performance and environmental effect on the CVD-grown monolayer MoS2 FETs are also compared with those on the mechanically exfoliated MoS2 from bulk (see the Supporting Information, Figures S6 and S7). The IDS−VGS curves exhibit increased on-current and Ion/Ioff ratio (∼107) with higher Vth, which might be related to the sulfur deficiency in the MoS2 prepared by the CVD method, as discussed earlier. Similar environmental effects were investigated using the exfoliated samples. Under HV, the sample became more ntype, revealing higher on-current and downshifted threshold voltage. The general feature of the environmental effects on the optical and electrical properties in both samples was the same. In addition, control of H2O vapor also induces p-type doping effect but a weaker effect than O2 gas (see the Supporting Information, Figure S5d), which is consistent with our theoretical calculation and previous reports.32 N2 gas did not appreciably change the on-current (see the Supporting Information, Figure S7b). Meanwhile, the H2 gas exposure created slight n-doping by donating electrons to the MoS2, consequently increasing the on-current and downshifting the threshold voltage, although much less severely compared with those of the O2 gas (see the Supporting Information, Figure S7c). We next demonstrate a controllable and reproducible work function of 2D materials via environmental change to facilitate viable device fabrications without intentional doping processes, which have been a bottleneck in actual device applications. A lateral homojunction p−n diode was formed by partially depositing an Al2O3 thin film via e-beam evaporator as a passivation layer on a monolayer MoS2 channel, as shown in Figure 4. Prior to the passivation, two-terminal devices were annealed at 200 °C for several hours to remove residual molecules before Al2O3 deposition. After the half-passivated device was exposed to air, ambient gases were adsorbed again on the open MoS2 channel. The passivated MoS2 region acted as an n++ type, and the air-exposed region acted as an n-type. Thus, an n++−n homojunction was formed at the junction, and

the related energy band diagram is shown in Figure 4b. The asymmetric metal junction was necessary to improve the oncurrent characteristics in the forward bias. When the device is operated under HV, the IDS−VDS curves were linear and slightly asymmetric with a lower current level at the Pd electrode side (Figure 4c,f). When exposed to air (Figure 4d,g), the device revealed rectifying behavior by forming an n++−n junction at the junction. When exposed again to HV, the device recovered to its original state (Figure 4e,h). The reversibility of the I−V characteristics between the linear and rectifying behavior is simply engineered by the environmental gases, which is clearly distinct from the devices that use traditional doping processes,38 physical contact of pand n- material,15 or an asymmetric electrode.39 The diode performance shown in Figure 4g was evaluated using the Shockley diode equation expressed as follows I = Is(e VD/ nVT − 1)

(2)

where I is the diode current, Is is the reverse-bias saturation current, VD is the voltage across the diode, n is the ideality factor, and VT is the thermal voltage (∼26 mV at 300 K). The ideality factor n is considered as the junction quality factor whose value typically varies from one to two depending on the fabrication process and semiconductor materials. Surprisingly, by fitting eq 2 to the diode I−V characteristics, the ideality factor is found to be n = 1, which represents an ideal junction. The ideality factor is larger than 1 in reality and can be between 1 and 2 in the Si diode.40 For the chemically doped p−n junction for random network carbon nanotubes, this is 4.8.41 The ideality factor of a 7 nm thick MoS2 diode formed by AuCl3 doping was 1.8,38 whereas the stacked multilayer MoS2/ monolayer WSe2 showed an ideality factor of n = 1.2.15 In our case, the n++−n homojunction formed by half-passivation with environmental gases created a trap-free junction and an ideal depletion region. To obtain the Schottky barrier height at the n++−n junction, thermionic emission law for reverse saturation current (Is) in the p−n junction diode equation was applied, which is expressed as Is = AA * T 2 exp(−qφB / kbT )

(3)

where A, A*, T, q, kb, and φB are the contact area between the electrode and channel, Richardson’s constant, temperature, magnitude of the electronic charge, Boltzmann’s constant, and potential difference across the n++−n junction, respectively. A* = 58.8 A cm−2 K−2 was obtained using the electron effective mass of MoS2, i.e., m* = 0.6m0.42 Is was obtained from the diode I−V curve. Here, a symmetric electrode was assumed. The energy-barrier height at room temperature, namely, qφB, was found to be approximately 0.373 eV, which is reasonable compared with the result from the KP measurement of ∼0.4 eV in Figure 1. This value was not altered much when an asymmetric electrode was used (see the Supporting Information, Figure S8). Photocurrent mapping of the n++−n junction MoS2 transistor was done using a focused laser beam with a wavelength of 488 nm at 100 μW. The source and drain electrode was maintained at zero gate (VGS = 0 V) and zero drain bias (VDS = 0 V). Figure 5a shows the real image of the whole device taken by optical microscope; the dotted square indicates the position of the patterned MoS2 channel. Figure 5b shows the photocurrentmapping image of the dotted square area scanned with a 200 6104

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When each parameter was calculated for the photocurrent generated at the n++−n junction at VDS = 0 V and VGS = 0 V, R was ∼0.1 A/W, EQE = ∼ 25%, and D* = ∼ 109 (Jones). These values are not the best recorded values but are reasonably comparable with those of reported MoS2 optical devices at VDS = 0 V and VGS = 0 V. The best records in the literature were obtained under biased conditions.12 The above factors that characterized the optical devices are critically dependent on the device configurations and dimensions. The simple method of n++−n junction formation on 2D materials can be beneficial in effectively optimizing the optical devices. Vertically stacked structure can be adapted to use the entire active-layer area. The nature of the metal−MoS 2 contact has been controversial. Recently, a partial Fermi-level pinning behavior has been suggested by observing the Schottky contacts among various metals with a work function difference larger than 2 eV. According to the DFT calculations, the Fermi levels in all studied MoS2−metal complexes with work functions ranging over 4.2−6.1 eV are generally pinned above the mid gap of MoS2 (except for platinum) but are not strictly pinned at a specific energy and distributed in a 0.5−0.64 eV window within the gap, still forming an n-type contact.44 This behavior is explained as follows: (i) by the metal work function modification using the interface dipole formation originating from charge redistribution and (ii) by the generated gap states mainly from the Mo d-orbitals by the weakened intralayer S− Mo bonding resulting from the strong contact metal−S interaction. Linear behavior of the I−V characteristics of a two-terminal device with asymmetric metal electrodes in the absence of a junction was exhibited, as shown in Figure 4f in our case. This can be explained by the negligible Schottky barrier height difference between two metal junctions due to partial Fermi-level pinning. Nevertheless, the barrier thinning at the Pd−MoS2 junction under HV plays a role in allowing the tunneling current, as shown in Figure 3f. Recently, a MoS2 Schottky p−n diode has been demonstrated using asymmetric metal contact (Pt and Ti). Pt is known as a p-type Schottky contact material in the literature.39,44 Thus, the rectification behavior of our device appears to mainly originate from the n++−n-junction formation instead of the asymmetric electrodes (Cr/Au−Pd). Understanding work function dependence on environmental gases opens the possibility of 2D materials for large-scale integration of electronics and optoelectronic devices. Unlike quantum dots and nanowires/nanotubes which require challenges of bottom-up assembly of such nanocomponents for device fabrication, 2D materials can be prepared on waferscale by, for example, a chemical vapor deposition method. Therefore, the device integration can be simply done by adopting the current state-of-the-art top-down approaches. This is a great advantage from integration point of view.

Figure 5. (a) Optical image of a half-covered MoS2 transistor. Scale bar, 1 μm. (b) Photocurrent mapping (top) and its profile along the dotted line (bottom) of a half-covered MoS2 transistor (VDS = 0 V, VGS = 0 V). (c) Band diagrams with photoexcited carriers on different positions of a half-covered MoS2 diode.

nm step (upper image) and a color scale bar on the right. The photocurrent profile along the dotted line in the upper panel of Figure 5b is provided at the bottom panel. A peculiar peak was observed at the junction region, which corresponds to the depletion layer formed at the junction where the excited electrons and holes were extracted from the adjacent n- and pregions. Interestingly, the depletion layer width was estimated to be ∼200 nm. Although this value could be obscured by the large beam size (∼500 nm), Gaussian distribution of the focused beam still provided a discernible width. This value is much smaller than the few micrometers typically observed in bulk p−n junction semiconductors or electrostatic p−n junctions.9,43 Whether the small depletion layer width originates from the monolayer nature of 2D materials or junction character controlled by environmental gases is not clear now and requires further study. The photocurrent near the metal contact is higher than that in the middle region, which is attributed to the reduced recombination rate of photogenerated carriers by the shortened diffusion length. In particular, higher photocurrent was obtained near the Pd contact where the photogenerated carriers are accelerated by the steep potential due to large Schottky barrier height formed at the contact, as schematically shown in Figure 5c. The performance of the optical device was evaluated using responsivity (R(A/W) = Iph/P), external quantum efficiency (EQE (%) = (hc/e)(Iph/P) × 100), and specific detectivity (D*(Jones) = RA1/2/(2eIdark)1/2). Iph is the photocurrent, P is the laser power, h is the Planck’s constant, c is the speed of light, Idark is the dark current, and A is the device dimension.

CONCLUSIONS In this study, the work function of MoS2 could be remarkably modulated by environmental gas species up to 1.2 eV, as estimated from the DFT calculations. Under the experimental conditions, ∼ 0.43 eV of the work function modulation was observed using oxygen gases, which corresponds to less than 10% adsorption per Mo atom. A high-quality homojunction n++−n diode (ideality factor n = 1) was demonstrated by controlling the oxygen molecule densities in two different areas through partial passivation of the monolayer MoS2. n = 1 for the monolayer MoS2 was realized by sharp work function 6105

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oxygen gas canister were connected to the KP chamber for ambient gas study. Large-scale monolayer MoS2 (1 cm × 1 cm) was placed on the highly p-doped Si substrate by “wet” transfer method. DFT Calculations. DFT calculations were performed using the ab initio Quantum ESPRESSO code (1). The exchange correlation energy was described by both relativistic LDA and GGA using the Perdew− Burke−Ernzerhof exchange model (2) with 0.01-Ry Gaussian broadening and DFT-D2 energy correction. A lattice parameter of 3.12 Å for the LDA and 3.18 Å for the GGA was obtained using geometry optimization of the primitive unit cell of the MoS2. A 25-Å vacuum slab was used to simulate the monolayer MoS2 along the caxis. The oxygen and water molecules were physically attached onto the S atom with a distance of 3.061 Å for the LDA and 3.389 Å for the GGA.

modulation across the homojunction MoS2, which led to a relatively low trap density. The reversibility of the I−V characteristics between the linear and rectifying behavior is a unique property of the oxygen-adsorption-assisted pseudo p−n junction, in sharp contrast with those of the conventional diodes. The responsivity, EQE, and specific detectivity of the photocurrent generated at the n++−n junction at VDS = 0 V and VGS = 0 V were 0.1 A/W, 25%, and 109 (Jones), respectively. These values are reasonably comparable with those of the reported MoS2 optical devices. Further studies on the diodelike rectifying behavior of our MoS2 transistor with partial passivation would be necessary for gas sensors, in addition to optoelectronic devices.

ASSOCIATED CONTENT

EXPERIMENTAL SECTION

S Supporting Information *

Synthesis of Monolayer MoS2. Ammonium heptamolybdate (AHM, Sigma-Aldrich, 431346) powder, as a Mo precursor, was dissolved in deionized water. The solution (6 μL) was dropped onto a quartz wafer (2 mm × 20 mm). The quartz wafer was placed into a dry oven (∼80 °C) next to the target wafer in a reactor that was coated by sodium cholate solution, as previously described. The S source (200 mg) was placed inside and sublimated prior to the growth process. The Mo heating zone was heated to 780 °C at a ramping rate of 78 °C/min, and the S-zone temperature was ramped up to 210 °C (42 °C/min). During the entire process, N2 (500 sccm) was injected as a carrier gas. Device Fabrication. The CVD-grown monolayer MoS2 was coated by poly(methyl methacrylate) (PMMA) as a supporting layer, and the MoS2/PMMA was separated from the SiO2/Si wafer by KOH solution. The MoS2/PMMA was transferred onto the SiO2/Si wafer after rinsing using deionized water. The PMMA was then removed using acetone. After the transfer, the MoS2 was annealed in LV (∼10−2 Torr) with N2 gas at 150 °C for 1 h. E-beam lithography was used to fabricate the electrodes on the flakes. Cr/Au (10 nm/50 nm) was evaporated using an e-beam/thermal evaporator for the source and drain electrodes. Pd (50 nm) was used in another electrode to form an asymmetric electrode. The MoS2 was patterned by e-beam lithography and etched by reactive-ion etching using SF6 gas. Al2O3 (20 nm) was evaporated on the half of the channel defined by e-beam lithography after keeping the device in HV (∼10−7 Torr) overnight. Measurements. The photocurrent mapping was implemented using a modified Raman instrument (XperRam 200, Nanobase) by coupling various lasers using optical fibers at a 200 nm per step with laser wavelength of 488 nm (laser power of 100 μW). Raman and PL mapping was performed using a confocal Raman/PL microscopy (NTEGRA Spectra, NT-MDT) system with an exciting laser wavelength of 532 nm. A vacuum probe station system (M5VC, MS Tech) and a semiconductor characterization system (4200-SCS, Keithley) were used for the I−V measurements. In situ UPS and Xray photoelectron spectroscopy (XPS) were performed under UHV (∼10−9 Torr) using He II (40.813 eV) and Al Kα (hν = 1486.6 eV) as monochromatic excitation sources, respectively. KP (McAllister, INC. KP4500) is a nondestructive device to measure work functions (Φ), which consist of a vibrating reference electrode (3 mm in diameter and a stainless steel disk) in a plane parallel to the sample, creating a capacitor. The sample and probe are connected via a voltage source called the “backing potential” (Vb). When Vb is set to zero, a contact potential difference (Vcpd), which is equal to the difference in the probe and sample work functions, appears between the probe and sample faces. The change in the work function is detected via Δwf = eΔVcpd, where e is the electron charge. At the unique point where Vb = −Vcpd, the circuit is balanced, and the electric field between the plates vanishes, resulting in a null output signal. This null condition and its deviation can be detected with high precision, thereby directly measuring the sample work function. By measuring Vcpd between the probe tip and the sample, the work function of the MoS2 can be obtained because the work function of the tip (4.4 eV) is already known. The UHV system and the nitrogen/

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01742. Figures S1−S8 and Table S1 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

S.Y.L. and U.J.K. contributed equally.

Notes

The authors declare no competing financial interest.

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