MoS2 Contacts: Thickness Dependence of

Subscriber access provided by University of Sussex Library

Article 2

Band Alignment at Au/MoS Contacts: Thickness Dependence of Exfoliated Flakes Ahrum Sohn, Hankyoul Moon, Jayeong Kim, Miri Seo, Kyung-Ah Min, Sang Wook Lee, Seokhyun Yoon, Suklyun Hong, and Dong-Wook Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07511 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Band Alignment at Au/MoS2 Contacts: Thickness Dependence of Exfoliated Flakes Ahrum Sohn,†,§ Hankyoul Moon,† Jayeong Kim,† Miri Seo,† Kyung-Ah Min,‡ Sang Wook Lee,† Seokhyun Yoon,† Suklyun Hong,‡ and Dong-Wook Kim*,† †

Department of Physics, Ewha Womans University, Seoul 03760, Korea

§

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 16419,

Korea ‡

Department of Physics and Graphene Research Institute, Sejong University, Seoul 05006, Korea

ABSTRACT. We investigated the surface potential (Vsurf) of exfoliated MoS2 flakes on bare and Au-coated SiO2/Si substrates using Kelvin probe force microscopy. The Vsurf of MoS2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS2/Au interface leading to a modified band alignment. Vsurf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ~20 nm (~30 layers) on the bare and ~80 nm (~120 layers) on the Au-coated substrates, respectively. This thickness-dependence of Vsurf was attributed to electrostatic screening in the MoS2 layers. Thus, a difference in the thickness at which the bulk Vsurf appeared suggests that the underlying substrate has an effect on the electric-field

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

screening length of the MoS2 flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS2 contacts.

INTRODUCTION Atomic-layered 2D semiconductors (SCs) are expected to lead us into a new era of electronic and optoelectronic devices with superior and novel functionalities.1,2 2D-SC-based heterostructures fabricated using van der Waals epitaxy are tolerant with regard to the choice of materials that can be combined, which is a significant benefit they possess over their conventional 3D bulk SC counterparts. Metal/SC contacts are essential components for the electrical characterizations of SCs and for SC device fabrication. Theoretical models and experimental techniques to control the contact properties of 3D bulk SCs are well established. In particular, band diagrams are crucial for describing the charge transport across the metal/bulk SC interfaces (Figure 1a). Many researchers have attempted to understand the electrical properties of 2D SC contacts and have revealed that they have unique features distinct from those of 3D SC contacts.3-16 2D SCs do not form covalent bonds at the surface, resulting in the formation of a tunneling barrier at the metal/2D SC interface (a so-called van der Waals gap).7-9 Strong hybridization between the atoms of the metal electrodes and 2D SCs could distort the properties of the 2D SCs below the electrodes, modifying the sheet resistance of the 2D SC layer beneath the contacts.7,8,11 Thus, band diagrams should be provided along the in-plane direction (x-axis in Figure 1b) as well as the out-of-plane direction (z-axis in Figure 1b) to understand the transport characteristics of the metal/2D SC contacts. However, it has not been straightforward to separate the transport along the in-plane and out-of-plane directions in


Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

z WSCR

Metal

EVAC EC EF EV

z Metal

Bulk SC

Bulk SC

(b) Metal z

x

z ?

x ?

2D SC

Figure 1. Schematic illustrations and band diagrams for (a) metal/3D bulk semiconductor and (b) metal/2D SC contacts. EVAC, EC, EF, EV, and WSCR indicate the vacuum level, the conduction band minimum, the Fermi level, the valence band maximum, and the space charge region width, respectively.

experimental studies. The extraction of the key parameters that describe the metal/2D SC contacts requires careful measurements and elaborate data analyses.10-13 Of all the numerous known 2D SCs, MoS2 is one of the most intensively investigated because of its interesting physical properties, which include a high carrier mobility, sizable bandgap energy, and good mechanical felxibility.10-19 A clear understanding of the electrical transport properties at the metal/MoS2 contacts would help us in uncovering MoS2’s intrinsic material properties and improve the performance of MoS2-based devices. MoS2 multilayers have been widely used for device applications because they can be attained in high-yield using large-area synthesis processes.10-12,18 The charge distribution in each MoS2 layer and the layer-to-layer resistance (i.e., the resistance between the neighboring layers) have to be taken into account to explain the transport behaviors of metal/multi-layered 2D SC contacts.10,12 Therefore, it is important to explicitly study how the thickness of the 2D SC can affect the properties of a metal/2D SC contact. In this work, we investigated the surface potential of exfoliated MoS2 flakes on SiO2/Si and Aucoated SiO2/Si substrates. The thickness of the MoS2 flakes was examined by measuring their height profiles and Raman spectra. A comparison of the two substrates enabled us to deduce the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

band alignment at the Au/MoS2 contacts, based on both the experimental and calculation results. In addition, the thickness dependence of the surface potential can reveal how the substrate affects the electric field screening behaviors of MoS2 flakes. EXPERIMENTAL SECTION Transfer of exfoliated MoS2 flakes MoS2 flakes were mechanically exfoliated from its bulk on SiO2/Si substrates. The thickness of the flakes were determined by atomic force microscopy (AFM) and Raman measurements. Poly(methyl methacrylate) (PMMA) layers were spin-coated onto the SiO2/Si source substrates and then baked at 180C for 2 min. to obtain PMMA layers containing the MoS2 flakes. The PMMA layers were peeled off from the SiO2/Si substrates in 4M potassium hydroxide water solution and transferred to Au-coated SiO2/Si target substrates. After the transfer, the PMMA was dissolved by acetone and the remaining MoS2 flakes on the target substrates were rinsed by isopropanol. The transferred MoS2 flakes with known thickness could be found by optical micrsocope, and the thickness was reconfirmed by AFM and Raman meausrements. Micro-Raman measurements Room-temperature micro-Raman spectra were measured using a spectrometer (Model 207, McPherson) equipped with a nitrogen-cooled charge coupled device array detector (SPEC-10, Roper Scientific). The samples were excited using a 488.0 nm (2.54 eV) diode laser (Sapphire CDRH HP, Coherent), focused to a ~1 m diameter spot using a microscope objective lens (100). The excitation power was kept less than 0.1 mW to minimize laser heating.


Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


KPFM

N2

AC

MoS2 z Au or SiO2

 DC 

Figure 2. Schematic of the experimental set-up used to measure the surface potential of MoS2 flakes on bare and Au-coated SiO2/Si substrates.

Kelvin probe force microscopy measurements Surface topography and surface potential (Vsurf) of the MoS2 flakes were measured using an AFM system (XE-100, Park Systems) in a glove box. We used Pt-coated Si cantilevers and the work function of the tip was calibrated using a highly ordered pyrolytic graphite reference sample before and after the measurements of each sample. We measured the Vsurf of a sample at several different locations in order to make sure that the measured data well represent the physical states of the sample. After loading a sample, the glove box was purged with N2 gas for at least 3 h and then the measurements were started. RESULTS AND DISCUSSION We measured the Vsurf of MoS2 flakes using Kelvin probe force microscopy (KPFM). Vsurf corresponds to the difference between the surface work function of the tip (Wtip) and that of MoS2 (WMoS2); thus eVsurf = Wtip – WMoS2 (where e is the magnitude of the electron charge). KPFM, a variant of AFM, enables the local Vsurf to be determined with a nanoscopic spatial resolution.20 For comparison, two kinds of substrates were used: SiO2(300 nm)/Si wafers (hereafter, referred to as ‘SiO2’) and SiO2/Si wafers coated with Au(100 nm)/Ti(20 nm) thin films (hereafter, referred to as



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

‘Au’). The KPFM measurements were conducted in a glove box filled with N2 gas, as schematically illustrated in Figure 2.

Figure 3. (a) Optical microscopy images of single-layer MoS2 flakes on bare and Au-coated SiO2/Si substrates. Red lines indicate the edges of single-layer MoS2 flakes. An identical flake was transferred from the bare to the Au-coated substrates. (b) AFM images and height profiles (blue lines) of the region in the black squares in (a). (c) Raman spectra of the MoS2 flakes on the bare (SiO2, blue) and Au-coated (Au, red) substrates.

Figure 3a, 3b, and 3c show optical microscopy images, AFM topographic data, and Raman spectra of exfoliated single-layer MoS2 (1-MoS2) flakes on SiO2 and Au, respectively. The MoS2 flakes on the substrates, which were obtained via mechanical exfoliation, were randomly sized with various thicknesses. The thickness of a MoS2 flake was identified via AFM height profiles (Figure 3b) and micro-Raman spectra (Figure 3c). A MoS2 flake with a known thickness could


Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thus be transferred from SiO2 to Au using a PMMA-mediated transfer technique.21 Figure 3a shows that a flake could be successfully transferred from one substrate to the other without significantly altering the flake, such that it can be considered to be ‘identical’ before and after the transfer. Comparing an identical sample on two different kinds of substrates allowed us to directly study the influence of the underlying substrate on the physical properties of MoS2. The Au thin films consisted of small grains and the root-mean-square (rms) roughness was as large as 3.5 nm. As a result, the 1-MoS2 flake on Au (rms roughness: 2.22 nm) had a much rougher surface compared with the one on SiO2 (rms roughness: 0.20 nm), as shown in Figure 3b. For the thick flakes, the surface roughness was not significantly affected by the underlying substrate: 50-nm-thick MoS2 flakes had a rms roughness of 1.4 nm on both SiO2 and Au. The Raman spectra of 1-MoS2 flakes on both SiO2 and Au show in-plane (E12g) and out-of-plane (A1g) vibration mode peaks, as shown in Figure 3c. The gap between the two peaks was 19.5 cm-1 and 21.0 cm-1 on SiO2 and Au, respectively. The peak separation is consistent with the literature results for 1-MoS2 flakes.22-24 The A1g peaks were blue shifted on Au (402.6 cm-1 on SiO2 and 404.1 cm-1 on Au), whereas the E12g peak positions were insensitive to the underlying substrates (383.1 cm-1 on both SiO2 and Au). The rough surface of the Au might cause elastic deformation of the extremely thin MoS2 flakes. Experimental studies and calculations have shown that MoS2 flakes subjected to uniaxial and biaxial tensile strain resulted in a more notable peak shift of the E12g mode compared with that of the A1g mode.22,23 Those reports demonstrate that the strain effects on their own cannot explain the Raman peak shift on Au that we observed. Chakraborty et al. reported that the A1g mode exhibited a peak shift that depended on the carrier concentration of MoS2, whereas the E12g mode did not change appreciably.25 They conducted density functional theory (DFT) calculations of the electron-phonon coupling as a function of elecrtron doping and



reported that the A1g mode coupled much more strongly with electrons than the E12g mode. Thus, the A1g mode peak shift in Figure 3c suggests that the substrate can modify the carrier concentration in the MoS2 flakes. In our experiments, the WMoS of the MoS2 flakes on SiO2 increased from 4.95 eV to 5.15 eV as 2

the number of layers, Nlayer (i.e., the thickness), of the flakes increased from 1 (thickness ~ 0.7 nm) to 30 (thickness ~ 20 nm) (see Figure S1 and S2 of Supporting Information). WMoS ceased to 2

increase when the thickness exceeded 20 nm. Kim et al. carried out photoemission spectroscopy measurements of MoS2, and their WMoS data, obtained after annealing the samples in ultra-high 2

vacuum, were comparable to our KPFM data.26 A similar thickness dependence of WMoS as in our 2

results has been reported by other research groups, who attributed the behavior to screening effects SiO2

0.8

Au

0.6 0.4

VsurfVsurf, max

(a)

Vsurf (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 21

0

10

-1

10

-2

10

-3

10

0

50

100

Nlayer

0.2 0.0 0

50

100

150

Nlayer (b)

eVsurf[1]

Wtip

eVsurf[Nlayer]

∆

EVAC

∆

...

WMoS2 EC

EF

EV Tip

1-MoS2

Au

Tip

m-MoS2

Au

eVsurf[1]  eVsurf[Nlayer] (Nlayer  2)

Figure 4. (a) Vsurf of exfoliated MoS2 flakes on SiO2 (blue) and Au (red) as a function of the layer number (Nlayer). Inset shows the surface potential normalized by the maximum of Vsurf (Vsurf,max). (b) Schematic band diagrams of single layer (1-MoS2) and multilayer (m-MoS2) flakes on Au. For the sake of the simplicity, the thickness-dependent bandgap energy of the MoS2 flake is not explicitly illustrated in the schematic band diagram.


Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the electric field present at the MoS2/SiO2 interface.26-28 This interfacial electric field is belived to be generated by charged impruties (e.g., surface adsorbates and trapped charges).27,28 To reveal the screening effects of the interface charges, the difference between the Vsurf of the MoS2 flakes and Vsurf of the bulk (Vsurf = Vsurf − Vsurf,bulk) was estimated as a function of the Nlayer.27 Vsurf,bulk was estimated using MoS2 flakes thicker than 60 layers (40 nm) and 120 layers (80 nm) for flakes on SiO2 and Au, respectively. As shown in Figure 4a, the Vsurf of the MoS2 flakes on SiO2 and Au gradually decreases and approaches zero, as the Nlayer increased. The electric field present at the interface causes a potential drop () and lowers WMoS2, as illustrated in Figure 4b. Further, electric-field screening occurs, and hence Vsurf will decrease for thicker MoS2 flakes, as shown in Figure 4a.27,28 In the MoS2 flakes, the resulting Vsurf distribution in each layer can be explained by the following equation, Vsurf[i] = KVsurf[i−1].10,27,29 Vsurf[i] is the Vsurf of the ith layer from the MoS2/substrate interface and K is the proportionality constant determined by the Thomas-Fermi screening length. The inset in Figure 4a shows the semi-log plot of Vsurf/(maximum of Vsurf) as a function of the Nlayer. This plot clearly shows the exponential relationship between Vsurf[i] and i, i.e., Vsurf[i] = K(i1)  Vsurf[1], where Vsurf[1] is the Vsurf of 1MoS2. The K values of the MoS2 flakes on SiO2 and Au were estimated to be 0.94 and 0.98, respectively. The electric field screening length of the MoS2 layers on SiO2 was shorter than that on Au. As shown in Figure 4a, the Vsurf of 1-MoS2 on Au (0.55 V) is much larger than that on SiO2 (0.30 V): the WMoS2 of 1-MoS2 on Au was 4.7 eV and the WMoS2 of 1-MoS2 on SiO2 was 4.95 eV (note: eVsurf = Wtip − WMoS2) (see Figure S1 and S3 of Supporting Information). Such difference cannot be explained by any contamination or oxidation during our transfer process, since the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

standard deviation of Vsurf was more or less the same before and after the transfer (see Figure S2 of Supporting Information). We carried out DFT calculations in order to understand the substrate effects on WMoS2. The DFT calculations were performed within generalized gradient approximation (GGA) for exchange-correlation (xc) functionals,30,31 implemented in the Vienna ab initio simulation package (VASP).32,33 The kinetic energy cut-off was set to 400 eV, and electron-ion interactions were represented by the projector augmented wave (PAW) potentials.34,35 Grimme’s DFT-D3 method36 based on a semi-empirical GGA-type theory was used for van der Waals corrections. In the calculations, (33) and (√ √ )R30° unit cells of MoS2 were adsorbed on α-quartz SiO2(0001) and Au(111), respectively. For the Brillouin-zone integration, we used (331) and (661) grids for atomic optimizations of MoS2 on SiO2 and Au, respectively, in the Gamma centered scheme. Atomic coordinates were fully optimized until the HellmannFeynman forces were less than 0.03 eV/Å. The calculations yield that the work function of 1-MoS2 on Au is lower than that on SiO2: WMoS2 = 4.91 eV on Au and WMoS2 = 5.05 eV on SiO2 (see Figure S4 of Supporting Information). This variation of WMoS2 for differing underlying substrates agrees well with the experimental results (The relative error between the experiments and calculations were 4.5 and 2.0% for Au and SiO2, respectively). Somewhat larger error for Au might be originated from the larger interface

Figure 5. Plane-averaged charge density difference (n) of 1-MoS2 on (a) SiO2 and (b) Au. n

> 0 indicates charge accumulation, while n < 0 indicates charge depletion. The regions of the MoS2-substrate interfaces are shaded in orange.


Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


roughness (see Figure 3b). Interestingly, the WMoS2 of 1-MoS2 on SiO2 is very close to the WMoS2 of isolated 1-MoS2 (5.07 eV). That is, the work function of 1-MoS2 is not significantly affected by the presence of SiO2, compared with for Au, highlighting the weak interaction between 1-MoS2 and SiO2. Figure 5 shows the plane-averaged charge density difference (n) of 1-MoS2 on SiO2 and Au: n = nMoS2/substrate – (nMoS2 + nsubstrate) (nMoS2/substrate, nMoS2, and nsubstrate are the planeaveraged charge densities of MoS2/substrate, isolated MoS2, and isolated substrate, respectively). The overlap of the wave functions of MoS2 and the substrate is dependent on the interaction between MoS2 and the underlying substrate. The resulting charge redistribution creates an interfacial electric dipole at the MoS2/substrate interface.3-5 Figure 5 suggests that larger electric dipoles are formed at the MoS2/Au interface than at the MoS2/SiO2 interface. The larger electric dipole results in a larger potential drop at the interface (represented by  in Figure 4b). This well explains the reason why the WMoS2 (Vsurf) of 1-MoS2 on Au is smaller (larger) than that on SiO2 (note: eVsurf = Wtip – WMoS2). Such a substrate-mediated interfacial electric field should affect the carrier concentration in the MoS2 flakes. The larger electric field at the MoS2/Au interface compared with at the MoS2/SiO2 interface causes a greater carrier concentration in the MoS2 flakes on Au. As a result, a more notable A1g mode peak shift occurs in the MoS2 flakes on Au, as observed in our Raman measurements (Figure 3c). In addition to the substrate-dependent Vsurf, the difference in the screening lengths of the MoS2 multilayers (Figure 4a) needs to be explained. The dielectric response of a material dominantly affects its electrostatic screening behaviors. Santos and Kaxiras claimed that the dielectric constant of multilayered MoS2 could increase under a large electric field.37 The larger Vsurf of 1-MoS2 on Au (Figure 4a) indicates a larger interfacial electric field at the MoS2/Au interface than that at the MoS2/SiO2 interface. Consequently, the screening length of MoS2 on Au is larger than that on SiO2.



This suggests that the electric field screening length of the MoS2 flakes underneath electrodes may vary depending on the electrode materials. Kumar and Ahluwalia claimed that the out-of-plane static dielectric constant of bulk MoS2 (12.8) was larger than that of 1-MoS2 (4.8), based on their first principles calculations.38 Castellanos-Gomez et al. reported that interlayer hopping should be considered to explain the electrostatic screening processes in MoS2.27 The field/thicknessdependent dielectric constant and interlayer coupling will determine the electrostatic screening characteristics of MoS2 multilayers, as represented by the Vsurf data in Figure 4a. The surface photovoltage (SPV) of the MoS2 flakes on Au was measured, as shown in Figure 6a. SPV is the difference of Vsurf in the dark (Vsurf,dark) and under light (Vsurf,light): SPV = Vsurf,light − Vsurf,dark.20 The SPV measurements were done using the KPFM system and the light source was a green laser (wavelegnth of 525 nm) with an output power of 20 mW. SPV is almost zero for 1MoS2, and larger SPV values appear for thicker MoS2 layers. Since the photon energy (2.36 eV) is much larger than the bandgap of MoS2, the absorbed photons create electron and hole pairs in MoS2. The potential gradient in the multilayers, if there is any, can separate the electron and hole pairs, and hence the surface can be charged.20 This charge redistribution modifies Vsurf, resulting in a non-zero SPV. As illustrated in Figure 6b, the positive SPV values (Figure 6a) verify the

(a)

(b)

50

MoS2

Au

SPV

40 SPV (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

30 20 10 incident light

0 0

100 200 300

electron

hole

Nlayer

Figure 6. (a) SPV data of the MoS2 flakes on Au as a function of Nlayer under illumination of a green

laser light. (b) Schematic band diagrams in the dark (grey lines) and under light (red lines).


Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


potential gradient in the MoS2 layers that were inferred from the thickness-dependent Vsurf data. SPV increased along with the MoS2 thickness and saturated at Nlayer ~ 120 (80 nm). The optical penetration depth of bulk MoS2 is 30 nm for a wavelength of 525 nm.39 Such a short penetration depth limits any further increase of SPV in thick MoS2 flakes. Moreover, the hopping-mediated interlayer resistance should affect the SPV thickness depdence of the MoS2 flakes.10,27 Very recently, Li et al. reported their Vsurf studies of MoS2 layers grown via chemical vapor deposition method.28 They reported a non-zero SPV from their MoS2 single layers on Au, whereas the measured SPV of our MoS2 single layers on Au was zero (Figure 6a). They measured the Vsurf of their MoS2 layers grown via chemical vapor deposition using ultra-violet light (wavelength of 355 nm) in ambient air (containing water vapor and oxygen gases). In contrast, we measured the Vsurf of our exfoliated flakes using visible light (green) in dry N2 gas. In air, gas adsorption/desorption from the environment can change Vsurf under illumination by light, as discussed in Li et al.'s report.28 We compared the Vsurf of the MoS2 layers in N2, H2(5%)/N2(95%) and O2, to examine possible gas adsorption effects. To reveal the ambient effects on the sample, we calibrated the work function of the tip in each gas. In the case of single layers, Vsurf was 0.23 V smaller in N2 than in O2 (see Figure S5 of Supporting Information). It is well known that O2 molecules from air are readily adsorbed on MoS2 and greately increase Vsurf (e.g., up to 0.43 eV upon 10% surface coverage of O2).40 The Vsurf of our MoS2 sample was almost invariant in N2 and H2(5%)/N2(95%) (see Figure S5 of Supporting Information). Exposure of H2 will remove O2 adsorbates on the MoS2 surface. This demonstrates that the gas adsorption could be suppressed in our N2-ambient KPFM measurements, followed by N2 gas flushing for more than 3 h. In our SPV measurements, the potential gradient in the MoS2 multilayers, rather than gas adsorbates at the surface, predominantly determined the SPV characteristics (Figure 6a). The gas adsorption and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

desorption can affect the Vsurf of MoS2 layers in dark and under illumination, since they have extremely large surface-to-volume ratio.41 More details regarding the ambient gas effects on the photocurrent and SPV behaviors of MoS2 layers will be published in a near future. CONCLUSION We investigated the Vsurf of exfoliated MoS2 flakes on SiO2 and Au. Increasing the MoS2 layer thickness decreased the Vsurf of the flakes. Vsurf reached the bulk value at a number of layers of ~30 and ~120 on SiO2 and Au, respectively. Additionally, we noted that the Vsurf of MoS2 single layers was much larger on Au than on SiO2. DFT calculations showed that the electric dipoles, formed by the overlap of the wave functions of the MoS2 layers and the underlying substrate, generated an electric field at the MoS2/substrate interface. Consequently, electrostatic screening in the MoS2 layers resulted in a smaller Vsurf for thicker flakes. The DFT calculations also showed that larger electric dipoles were formed at the MoS2/Au interface than at the MoS2/SiO2 interface. The calculation results thus support the experimental results, which showed a larger Vsurf for MoS2 single layers on Au. The thickness-dependence of Vsurf showed that the electrostatic screening length in the MoS2 flakes was shorter on SiO2 than Au. This suggests that the difference in the interfacial electric field, which depends on the underlying layer, can affect the dielectric response and result in a screening behavior of the MoS2 flakes. Combined, all the experimental and calculation results yielded the band alignment at the Au/MoS2 interface, which provides us with valuable insights to help us understand the electrical properties of metal/MoS2 contacts.


Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Thickness-dependent work function data of MoS2 flakes on SiO2 and Au; Work function maps and data of MoS2 flakes on SiO2 and Au with various thickness; Electrostatic potential of MoS2 single layers on SiO2 and Au; Work function of MoS2 flakes in N2, H2(5%)/N2(95%), and O2

AUTHOR INFORMATION Corresponding Author * Dong-Wook Kim. E-mail: [email protected]

Author Contributions A.S. and D.-W.K. conceived and designed the research study. A.S., H.M., J.K., M.S., S.W.L., S.Y., and D.-W.K. performed the experiments and analyzed the data. K.-A.M. and S.H. carried out the theoretical calculations. A.S., M.S., K.-A.M., S.W.L., S.Y., S.H., and D.-W.K. contributed to preparing the manuscript and all the authors have given approval to the submitted manuscript.

Acknowledgements This work was supported by Priority Research Center Program (2010-0020207) and Basic Science Research Program (NRF-2015R1A2A2A05050829, NRF-2016R1D1A1B01009032, and NRF-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

2016R1D1A1A09917491, and NRF-2017R1A2B2010123) through the National Research Foundation of Korea (NRF).

Notes The authors declare no competing financial interest.

REFERENCES 1. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals, PNAS 2005, 102, 10451–10453. 2. Ganatra, R.; Zhang, Q. Few-Layer MoS2: A Promising Layered Semiconductor. ACS Nano 2014, 8, 4074–4099. 3. Gong, C.; Colombo, L.; Wallace, R. M.; Cho, K. The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces. Nano Lett. 2014, 14, 1714−1720. 4. Min, K.-A.; Park, J.; Wallace, R. M.; Cho, K.; Hong, S. Reduction of Fermi Level Pinning at Au–MoS2 Interfaces by Atomic Passivation on Au Surface. 2D Mater. 2016, 4, 01509. 5. Min, K.-A.; Cha, J.; Cho, K.; Hong, S. Ferromagnetic Contact between Ni and MoX2 (X = S, Se, or Te) with Fermi-Level Pinning. 2D Mater. 2017, 4, 024006. 6. Guo, Y.; Liu, D.; Robertson, J. 3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides. ACS Appl. Mater. Interfaces 2015, 7, 25709–25715. 7. Kang, J.; Liu, W.; Sarkar, D.; Jena, D.; Banerjee, K. Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors. Phys. Rev. X 2014, 4, 031005. 8. Allain, A.; Kang, J.; Banerjee, K.; Kis, A. Electrical Contacts to Two-Dimensional Semiconductors. Nat. Mater. 2015, 14, 1195–1205.


Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Zhong, H.; Quhe, R.; Wang, Y.; Ni, Z.; Ye, M.; Song, Z.; Pan, Y.; Yang, J.; Yang, L.; Lei, M.; et al. Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations. Sci. Rep. 2016, 6, 21786. 10. Das, S.; Appenzerller, J. Screening and Interlayer Coupling in Multilayer MoS2. Phys. Status Solidi RRL 2013, 7, 268. 11. Guo, Y.; Han, Y.; Li, J.; Xiang, A.; Wei, X..; Gao, S.; Chen, Q. Study on the Resistance Distribution at the Contact between Molybdenum Disulfide and Metals. ACS Nano 2014, 8, 7771-7779. 12. Son, Y.; Wang, Q. H.; Paulson, J. A.; Shih, C.-J.; Rajan, A. G.; Tvrdy, K.; Kim, S.; Alfeeli, B.; Braatz, R. D.; Strano, M. S. Layer Number Dependence of MoS2 Photoconductivity Using Photocurrent Spectral Atomic Force Microscopic Imaging. ACS Nano 2015, 9, 2843-2855. 13. Kim, C.; Moon, I.; Lee, D.; Choi, M. S.; Ahmed, F.; Nam, S.; Cho, Y.; Shin, H.-J.; Park, S.; Yoo, W. J. Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides. ACS Nano 2017, 11, 1588-1596. 14. Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M. Phase-Engineered Low-Resistance Contacts for Ultrathin MoS2 Transistors. Nat. Mater. 2014, 13, 1128–1134. 15. Kim, A. R.; Kim, Y.; Nam, J.; Chung, H.-S.; Kim, D. J.; Kwon, J.-D.; Park, S. W.; Park, J.; Choi, S. Y.; Lee, B. H.; et al. Alloyed 2D Metal–Semiconductor Atomic Layer Junctions. Nano. Lett. 2016, 16, 1890–1895. 16. Jeong, H. Y.; Kim, U. J.; Kim, H.; Han, G. H.; Lee, H.; Kim, M. S.; Jin, Y.; Ly, T. H.; Lee, S. Y.; Roh, Y. G.; et al. Optical Gain in MoS2 via Coupling with Nanostructured Substrate: Fabry– Perot Interference and Plasmonic Excitation. ACS Nano 2016, 10, 8192–8198. 17. Bertolazzi, S.; Brivio, J.; Kis, A. Stretching and Breaking of Ultrathin MoS2. ACS Nano 2011, 5, 9703–9709. 18. Cho, Y.; Cho, B.; Kim, Y.; Lee, J.; Kim, E.; Nguyen, T. T. T.; Lee, J. H.; Yoon, S.; Kim, D.-H.; Choi, J.; et al. Broad-Band Photocurrent Enhancement in MoS2 Layers Directly Grown on



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

Light-Trapping Si Nanocone Arrays. ACS Appl. Mater. Interfaces 2017, 9, 6314–6319. 19. Le, C. T.; Clark, D. J.; Ullah, F.; Jang, J. I.; Senthikumar, V.; Sim, Y.; Seong, M-J.; Chung, K.H.; Kim, J. W.; Park, S.; et al. Tuning Resonant Second Harmonic Generation of Monolayer MoS2 by Selenium Doping. ACS Photon. 2017, 4, 38–44. 20. Kim, E. ; Cho, Y.; Sohn, A.; Hwang, H.; Lee, Y. U.; Kim, K.; Park, H.-H.; Kim, J.; Wu, J. W.; Kim, D.-W. Mie Resonance-Modulated Spatial Distributions of Photogenerated Carriers in Poly (3-hexylthiophene-2, 5-diyl)/Silicon Nanopillars. Sci. Rep. 2017, 6, 29472. 21. Seo, M.; Kim, H.; Kim, Y. H.; Na, J.; Lee, B. J.; Kim, J. J.; Lee, I. ; Yun, H.; McAllister, K.; Kim, K. S.; et al. Fabrication and Electrical Properties of Single Wall Carbon Nanotube Channel and Graphene Electrode based Transistors Arrays. Appl. Phys. Lett. 2015, 107, 033103. 22. Rice, C.; Young, R. J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K. S. Raman-Scattering Measurements and First-Principles Calculations of Strain-Induced Phonon Shifts in Monolayer MoS2. Phys. Rev. B 2013, 87, 081307(R). 23. Li, H.; Contryman, A. W.; Qian, X.; Ardakani, S. M.; Gong, Y.; Wang, X.; Weisse, J. M.; Lee, C. H.; Zhao, J.; Ajayan, P. M.; et al. Optoelectronic Crystal of Artificial Atoms in StrainTextured Molybdenum Disulphide. Nat. Commun. 2015, 6, 7381. 24. Lee, K. C. J.; Chen, Y.-H.; Lin, H.-Y.; Cheng, C.-C.; Chen, P.-Y.; Wu, T.-Y.; Shih, M.-H.; Wei, K.-H.; Li, L.-J.; Chang, C.-W. Plasmonic Gold Nanorods Coverage Influence on Enhancement of the Photoluminescence of Two-Dimensional MoS2 Monolayer. Sci. Rep. 2015, 5, 16374. 25. Chakraborty, B.; Bera, A.; Muthu, D. V. S.; Bhowmick, S.; Waghmare, U. V.; Sood, A. K. Symmetry-Dependent Phonon Renormalization in Monolayer MoS2 Transistor. Phys. Rev. B 2012, 85, 161403(R). 26. Kim, J. H.; Lee, J.; Kim, J. H.; Hwang, C. C.; Lee, C.; Park, J. Y. Work Function Variation of MoS2 Atomic Layers Grown with Chemical Vapor Deposition: The Effects of Thickness and the Adsorption of Water/Oxygen Molecules. Appl. Phys. Lett. 2015, 106, 251606. 27. Castellanos-Gemez, A.; Cappelluti, E.; Roldán, R.; Agraït, N.; Guinea, F.; Rubio-Bolliner, G.


Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electric-Field Screening in Atomically Thin Layers of MoS2: the Role of Interlayer Coupling. Adv. Mater. 2013, 25, 899-903. 28. Li, F.; Qi, J.; Xu, M.; Xiao, J.; Xu, Y.; Zhang, X.; Liu, S.; Zhang, Y. Layer Dependence and Light Tuning Surface Potential of 2D MoS2 on Various Substrates. Small. 2017, 13, 1603103. 29. Lee, N. J.; Yoo, J. W.; Choi, Y. J.; Kang, C. J.; Jeon, D. Y.; Kim, D. C.; Seo, S.; Chung, H. J. The Interlayer Screening Effect of Graphene Sheets Investigated by Kelvin Probe Force Microscopy. Appl. Phys. Lett. 2009, 95, 222107. 30. Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133. 31. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. 32. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. 33. Kresse, G.; Furthmüller, J. Efficiency of ab initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 34. Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. 35. Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758. 36. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate ab initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements HPu. J. Chem. Phys. 2010, 132, 154104. 37. Santos, E. J. G.; Kaxiras, E. Electrically Driven Tuning of the Dielectric Constant in MoS2 Layers. ACS Nano 2013, 7, 10741-10746. 38. Kumar, A.; Ahluwalia. P. K. Tunable Dielectric Response of Transition Metals Dichalcogenides MX2 (M=Mo, W; X=S, Se, Te): Effect of Quantum Confinement. Physica B 2012, 407, 4627-4634.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

39. Yim, C.; O’Brien, M.; McEvoy, N.; Winters, S.; Mirza, I.; Lunney, J. G.; Duesberg, G. S. Investigation of the Optical Properties of MoS2 Thin Films using Spectroscopic Ellipsometry. Appl. Phys. Lett. 2014, 104, 103114. 40. Lee, S. Y.; Kim, U. J.; Chung, J.; Nam, H.; Jeong, H. Y.; Han, G. H.; Kim, H.; Oh, H. M.; Lee, H.; Kim, H.; et al. Large Work Function Modulation of Monolayer MoS2 by Ambient Gases. ACS Nano 2016, 10, 6100-6107. 41. Kim, H.; Chan, N. Y.; Dai, J.-y.; Kim, D.-W. Enhanced Surface-and-Interface Coupling in Pd-

Nanoparticle-coated LaAlO3/SrTiO3 Heterostructures: Strong Gas- and Photo-Induced Conductance Modulation. Sci. Rep. 2015, 5, 8531.


Page 21 of 21

SiO2

0.8

Au

0.6 0.4

VsurfVsurf, max

TOC FIGURE;

Vsurf (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

eVsurf

0

-1

10

-2

10

Dipole-induced potential drop EVAC

-3

10

0

50

100

...

Nlayer

0.2

EC

EF

EV

0.0 0

50

100

Nlayer

150

Tip

MoS2

Au


MoS2 Contacts: Thickness Dependence of

Recommend Documents