Superior Dielectric Screening in Two-Dimensional MoS2 Spirals

Subscriber access provided by LONDON METROPOLITAN UNIV

Article

Superior Dielectric Screening in Two-dimensional MoS2 Spirals Thuc Hue Ly, Hyun Kim, Quoc Huy Thi, Shu Ping Lau, and Jiong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11468 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 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.

ACS Applied Materials & Interfaces 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 19

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

ACS Applied Materials & Interfaces

Superior Dielectric Screening in Two-dimensional MoS2 Spirals ,

Thuc Hue Ly, ‡,†,* Hyun Kim, # Quoc Huy Thi, # Shu Ping Lau, † and Jiong Zhao† *

‡

Department of Chemistry and Center of Super-Diamond & Advanced Films (COSDAF), City

University of Hong Kong, Hong Kong, China. †

Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong,

China. #

IBS Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan

University, Suwon 440-746, Korea.

Keywords: Molybdenum disulfide, spiral, Kelvin force microscopy, screening, thickness dependence

*

Corresponding author E-mail: (T. H. Ly) [email protected], (J. Zhao) [email protected]

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 19

Abstract

Metals have the best dielectric screening capability among all the materials, however it is usually difficult to fabricate continuous and uniform ultra-thin (few-atomic-layer thickness) metal films. Conversely, high quality atomic-thick semiconductor or semimetal materials (so called twodimensional materials) such as graphene or MoS2 can be readily obtained and robust in ambient conditions, however their dielectric screening capabilities are greatly reduced by the reduced dimensionality. Particularly, in the vertical direction the dielectric screening of two-dimensional materials is insufficient thus the performances of devices by two-dimensional materials were easily affected by the coulomb scattering or other kind of sources. Herein we propose with a screw dislocation connecting the van der Waals layers in two-dimensional MoS2 spiral structures, excellent dielectric screening in the vertical direction can be achieved. Our Kelvin force microscopy directly demonstrates that the external impurity charges can be perfectly screened by a theoretically minimum number of layers (two layers) in the MoS2 spirals. This spiral structure assisted screening approach paves new way to the design of high performance ultrathin electrical and optical devices.

Introduction Two-dimensional (2D) materials are currently attracting tremendous interests due to novel emergent properties accompanying the reduced dimensionality. Graphene,1 h-BN2 and transition metal dichalcogenides (TMD) such as MoS23,4 have demonstrated extraordinary electronic or optical performances as well as the chemical stability up to single atomic layer, intriguing for the next-generation device applications. Meanwhile, surfaces, boundaries or other defects which might be unimportant in bulk materials could be essential for 2D materials, and they can critically influence their electronic structures as well as the dielectric properties.5,6 In the scheme of 2D atomic layered materials, dielectric screening is significantly reduced owing to the prevalent surfaces and reduced dimensionality,7,8 hence the exciton binding energy 2


Page 3 of 19

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


can be increased until up to ten times larger than the bulk.

9

In previous works it has been

denoted that the dielectric screening in vertical direction (along c axis or perpendicular to the layers) is also greatly suppressed in ultrathin 2D graphene5 and MoS210-13 flakes as compared to bulk materials and have clear thickness dependence, whereas the thickness dependence is relatively weak for the insulator h-BN.14 This reduction in dielectric screening may introduce more coulomb scattering for electron transport in electrical or optical applications. Currently the reported experimental results by electrostatic force microscopy (EFM) or Kelvin force microscopy (KFM) methods10-16 as well as theoretical investigations by first principle calculations17 on the dielectric properties of 2D systems have lots of discrepancies, in part due to the effects from different substrates or surface/interface conditions,18,19 and the difficulties in abinitio descriptions of long-range van der Waals (vdW) interactions and sampling of supercells or k points.17,20 In particular, in few-layer MoS2 the importance of coupling (electron hopping) between vdW layers has been addressed,11 in correspondence to a nonlinear Thomas-Fermi model.21 As the key parameters of 2D materials, exciton binding energy,22 surface potential11 and work function23 are all closely related with the dielectric properties, thus the knowledge about the dielectric properties of these new materials is indispensable. Here we focused on the electrostatic screening ability of a new kind of structure: the spiral structure, which has been found in MoS2,24,25 WS2,26 graphene,27,28 etc. They can be synthesized by chemical vapor deposition (CVD) method via a screw dislocation in the center.25 The results of our KFM experiments clearly reveal the enhancement of dielectric screening by this special structure, in comparison to the mechanically exfoliated MoS2 flakes. Results Similar to our previous study,25 the 2D MoS2 spiral samples were prepared by CVD and then immediately transferred onto Si/SiO2 wafer by polymethylmethacrylate (PMMA) (see methods). KFM technique has been widely applied to study the electrostatic potential, surface potential, contact potential difference (CPD) or work functions.10-13 Figure 1a shows the schematic of our KFM measurement on the MoS2 spiral sample. The capacitive electrostatic force is modulated by 3



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 19

the ac bias voltage applied on the sample stage, while the feedback voltage Voff is measured as the surface potential Vs according to the lock-in detected cantilever amplitude. Here we used standard Au tip and ambient conditions for KFM measurement, see methods for the specification of the KFM experiments. The potential resolution of our KFM measurement can reach 1~3 mV. Figure 1b presents the TEM image of the MoS2 spiral, the screw dislocation contrast in the center of the flake can be clearly identified. Figure 2a,b demonstrates the atomic force microscopy (AFM) topography image and the KFM potential image of the MoS2 spiral. The magnified topography image for the same sampleis shown in Figure 2cwhich clearly reveals the atomic steps near the edge of the MoS2 spiral. The height profile in Fig. 2c inset shows each step height roughly equal to 0.8 nm, close to the theoretical thickness of one layer of MoS2. A bump in the center highlighted in Fig. 2a is a protruded stage (~25nm in height) containing the screw dislocation.25 The height and potential profiles for the A,B dashed lines in Fig. 2a,b are presented in Fig. 2d,e. Fig. 2f is the larger scale KFM result for the same MoS2 spiral. The uniform surface potential images by KFM (Fig. 2b) shows there is no apparent thickness dependence of the surface potential, down to the minimum thickness for the bilayer part in MoS2 spiral. However, the surface potential for the monolayer part in MoS2 spiral is apparently higher than the thicker parts. This trend in the surface potential are present in many other spirals (Supporting Information Figure S1). In addition, the edge effect on surface potential for MoS2 spiral is negligible, in contrast to previous studies,6 suggesting the clean chemical states of these free edges. To compare with the MoS2 spirals, we also prepared the conventional MoS2 flakes from bulk hexagonal (2H) MoS2 by mechanical exfoliation method.29 Figure 3a,b shows the AFM topography image and the corresponding KFM potential image of one conventional MoS2 flake. The thickness of the exfoliated MoS2 sample varies from 1 nm to 14 nm, while the surface potential gradually decreases by around 200 mV from the thickness 1 nm to 6 nm (1~10 atomic layers). In the surface potential image (Fig.3b) the steps corresponding to the atomic steps on the MoS2 samples (Fig.3a) can be clearly identified, in sharp contrast to the varnished steps in the potential maps of the MoS2 spiral (Fig. 2b,f).

4


Page 5 of 19

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 surface potential Vs (the offset potential between the Au tip and the sample surface) we have measured by KFM can be easily correlated with the work functions (Φ) of the samples by the following relationship, Vs = ΦAu - ΦMoS2.

(1)

It has been further validated by our reference KFM experiment with one Au tip (Au work function 5.4 eV) 30 on standard clean SiO2 surface (work function 5.05 eV).31 By statistics on all the pixels of AFM and KFM images in Fig. 2a,b and Fig. 3a,b, we can plot the work functions versus thickness for both the spiral and conventional MoS2 samples (Figure 4). The work functions for the conventional exfoliated MoS2 samples increase from 5.05 eV to 5.25 eV, for thickness ranging from 1 nm to 6 nm. And it approaches the bulk MoS2 value (5.25 eV) for conventional MoS2 sample with thickness larger than 6 nm as expected. However, for MoS2 spiral samples, the work function keeps stable around the bulk MoS2 work function value (5.25 eV), for thickness ranging from 1 nm to 6 nm, except for the monolayer part (< 1 nm) the work function falls down into the range of MoS2 monolayer. The spiral work function has slightly increase when the thickness increases further, reaching around 5.4 eV until 25nm thickness. Discussions The thickness dependence of the surface potential (or work function) for the conventional MoS2 samples can be attributed to the finite dielectric screening ability. Usually some positive charged impurities can locate at the interface between the substrate and MoS2 after we transfer the samples,19 thus the area covered by thinner conventional MoS2 flakes which has been less screened have higher surface potentials (or lower work function), evidenced by the KFM results in Fig. 3. The screening length for the MoS2 is reduced with the increasing thickness,8 as a result the surface potential approaches the bulk MoS2 as the thickness is over 6 nm. On the other hand, due to the same sample transfer procedures were applied, positive charges should remain between the MoS2 spirals and SiO2 substrates. Revealed by our published transmission electron microscopy (TEM) and AFM results on the MoS2 spirals,24,25 the MoS2 atomic layers in the spirals have quasi-rhombohedral stacking order, and a screw dislocation in 5



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 19

the center continuously connected all the layers. Therefore the MoS2 spiral is vertically conductive,25 consequently the atomic layers in MoS2 spiral can no longer be treated as vdW layers which has been used to explain the conventional MoS2 screening properties .11 Instead, as electrons in the conduction band of MoS2 spirals can freely transport across the layers, we expect the external net charges can be screened in the vertical direction by a bilayer MoS2 spiral in the minimum requirement. The carrier density in the conduction band of our MoS2 spiral was measured by conductive AFM,25 estimated to be around 4×1018 cm-3, which is adequate to supply the free electrons for screening. Monolayer MoS2 spiral sample is identical to the conventional MoS2 whereas significantly reduced screening effect can be expected. Our KFM results on the MoS2 spirals are in coincidence with the above analysis. For all the thicknesses except for the monolayer part in the MoS2 spiral (ranging from ~2 nm to 6 nm), the surface potential (or work function) is fluctuating around 5.25 eV. In other words, the electrostatic screening ability for the MoS2 spiral is significantly enhanced as compared to the conventional MoS2. It is noted that the work functions of rhombohedral stacking MoS2 and hexagonal stacking MoS2 are very close due to their similar electronic structure,32 although the vdW layers sometimes can have pronounced electronic couplings.33,34 The remained residual charges at the interface have almost no effect on the MoS2 spiral with thickness larger than 2 nm while in conventional MoS2 approximately 6 nm thickness as minimum is required to achieve similar screening effect. The slightly dark contrast in the triangle region in the KFM image (Figure 2b) means lower surface potential, with higher work function than bulk hexagonal MoS2, which might be affected by the electronic structure changes by rhombohedral stacking. The above results can give many insights to the understanding of the spiral structures. First, the work function or surface potential does not depend solely on the local atomic arrangement. If we examine a local position in any places excluding the central screw dislocation in the MoS2 spiral, the atomic structure is no different from the conventional rhombohedral stacked 2D MoS2, which will lead to the misunderstanding that MoS2 spiral has the same dielectric screening as conventional 2D MoS2. However, as we have seen, one screw dislocation can overwhelmingly introduce the dielectric screening for the whole flake, and the influential area can be up to 20~30 µm in lateral size. This is also determined by the enormous anisotropy (in-basal plane and out-of6


Page 7 of 19

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


basal plane) for this kind of 2D vdW layered materials. The influential area of the screw dislocation here is different from the exciton radius or the screening length. The influential area describes the area where the screening enhancement effect takes place, while the screening length or exciton radius are indicators of how strong the screening effect can be. Although the screening is enhanced in the spiral MoS2 compared to the conventional MoS2, the exciton radius of the MoS2 spiral is still on the order of nanometer. Second, in agreement with our previous transport measurement by conductive (C)-AFM,25 the MoS2 spiral has an ultrahigh vertical conductivity as compared to conventional MoS2. In the KFM case, we are talking about the static electric field, the real part of the relative permittivity (ε’) is the main concern; while in the (C)-AFM case, we are talking about the static current, the static conductivity (σs) is the main concern. The screening length in c-axis can be considered as one interlayer distance (~0.8 nm) in the spiral, reaching the possible minimum limit as the vertical screening process requires at least two atomic layers (Figure 5). According to theoretical calculations9 as well as experiments,5,7 the vertical screening has been shown to be much weaker in single layer 2D materials like monolayer graphene and MoS2, in spite of the MoS2 layer is actually formed three layers of atoms (one molybdenum atomic layer sandwiched by two sulphur atomic layers). The contribution from intralayer for vertical screening in MoS2 spiral is negligible compared with the interlayer screening. In contrast to the conventional MoS2 which usually has environmental sensitive work function,23 we found the work function and surface potentials of ultrathin semiconductor spirals is robust, unsusceptible to the external fields and impurity charges. This is a great advantage of the spirals for future electronic device applications. The screw dislocations do have some negative effects on the planar FET devices (performances of vertical FET devices using the spirals will be greatly damaged due to the leakage current in vertical direction brought by the screw dislocation). However, the density of the screw dislocations in devices are extremely low, only one dislocation per device, so the overall effect on planar devices will not be significant. The superior screening effect brought by the dislocation will dominates and the in-plane mobility can be enhanced due to the suppression of interfacial excess charge scattering. In addition, we propose the 2D spirals can be applied to the induction devices,25 the charge screening layers for other devices, etc. 7



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 19

Conclusion In summary, we explored the dielectric screening effects on the novel MoS2 spiral structures via KFM approach. The surface potential or work function displays no apparent thickness dependence for the MoS2 spirals, in sharp contrast to the mechanical exfoliated MoS2. This is mainly attributed to the enhanced screening caused by the electronically connected vdW layers. The work function of MoS2 spiral is robust and kept at the bulk MoS2 value because of this excellent screening capability. The achieved perfect bilayer screening scheme in spirals has reached the theoretical lower bound for vertical dielectric screening. It is comparable with metals but can be more easily handled than the metals to make atomically thin films. This peculiar ultrathin spiral structure is common for most of the vdW layered semiconductor materials, and can find wide applications in future miniaturized devices or other related fields. Experimental Section: Synthesis of molybdenum disulphide spiral on SiO2/Si substrate. The synthesis method was mentioned in our previous work25 and can be summarized as following. The promoter was firstly prepared by dissolving sodium cholate (SC) hydrate (Sigma-Aldrich, C6445) into DI water (0.1 g of SC in 10 ml of DI water). We also separately prepared water-soluble molybdenum precursor (11.5 mM of ammonium molybdate tetrahydrate, Sigma-Aldrich, 431346) and mixed with promoter. The mixture solution were then dropped onto SiO2/Si wafer and spincasted at 3000 rpm for 1 min. 200 mg of S source and the prepared substrate were placed in zone 1 and zone 2, respectively. Zone 1 was heated up to 210 oC at a rate of 42 °C /min, whereas zone 2 was heated up to 780 oC. This whole process was carried out under 500 sccm N2 for 17 minutes. It is worth noting that the spiral structure of MoS2 is chosen randomly among flakes. Preparation of MoS2 spiral on pre-etched Si substrate for Kelvin Force microscopy measurement. The as-grown MoS2 spiral was transferred via PMMA method onto pre-etched Si substrates for further Kelvin force microscopy (KFM) experiments. The purpose of the preetching is to remove the surface oxide layer in order to improve the electrical conductance between substrate and MoS2 spiral.

8


Page 9 of 19

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


Preparation of exfoliated MoS2 on pre-etched Si substrate for Kelvin Force microscopy measurement. MoS2 flake was purchased from 2D-Materials Company, MoS2 was prepared on pre-etched Si substrate using conventional mechanical exfoliation methods, where the description can be found anywhere. Kelvin force microscopy (KFM) measurements. The KFM study was performed with Esweep system (Seiko, Japan). An Au tip (SI-DF3-A) with an approximately 10 nm tip radius was used. The force constant and resonant frequencies of the tips were approximately 1.6 N/m and 27 kHz, respectively KFM study was done monolayer MoS2 transferred on pre-etched Si substrate. Topographic and potential images were acquired simultaneously in active trace KFM mode. In this mode, the line scan for topography measurement and potential measurement can be completely separated. The vibration for topography measurement (ωr) was cut during the potential measurement, and the potential measurement was performed only by using the vibration of ac frequency (ω). For the topography imaging, the tip was vibrating at its resonance frequency ωr (~27 kHz) with the tip bias of zero. For the potential distribution measurement, the cantilever was moved at trace distance z parallel to the substrate plane. An AC voltage of 0.4 V at frequency ω (~28 kHz), which was slightly higher than ωr, was applied between the tip and the substrate. The feedback voltage Voff is measured as the surface potential Vs so the lock-in detected amplitude Aω of the cantilever excited with static electric force from the modulation voltage applied to the sample. The area selected to extract data of Figure 4 is the whole flake area shown in Figure 2b and Figure 3b. For extraction of the work function versus height, the pixels in the AFM/KFM images are correlated and categorized, firstly meshed by 0.1nm interval according to AFM data, followed by averaging the work functions (KFM) for all the pixels belong to this height (AFM) interval.

9



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 19

AUTHOR INFORMATION Corresponding Author *E-mail address: (T. H. Ly) [email protected], (J. Zhao) [email protected] ACKNOWLEDGMENT This work was supported by a grant from City University of Hong Kong (Project No. 7200551) and the Hong Kong Polytechnic University Grant (No. G-YW1U, No. 1-ZE8C), Institute for Basic Science (IBS-R011-D1) and by the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

ASSOCIATED CONTENT Supporting Information. Supporting Figure S1. Supporting information is available free of charge at http://pubs.acs.org.

REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6 (3), 183-191. (2) Song, L.; Ci, L.; Lu, H.; Sorokin, P. B.; Jin, C.; Ni, J.; Kvashnin, A. G.; Kvashnin, D. G.; Lou, J.; Yakobson. B. I.; Ajayan, P. M. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Lett. 2010, 10 (8), 3209-3215. (3) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, I. V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6 (3), 147-150.

10


Page 11 of 19

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


(4) Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7 (8), 494-498. (5) Datta, S. S.; Strachan, D. R.; Mele, E. J.; Johnson, A. C. Surface Potentials and Layer Charge Distributions in Few-Layer Graphene Films. Nano Lett. 2008, 9 (1), 7-11. (6) Hao, G.; Huang, Z.; Liu, Y.; Qi, X.; Ren, L.; Peng, X.; Yang, L.; Wei, X.; Zhong, J. Electrostatic Properties of Few-Layer MoS2 Films. AIP Adv. 2013, 3 (4), 042125. (7) Prins, F.; Goodman, A. J.; Tisdale, W. A. Reduced Dielectric Screening and Enhanced Energy Transfer in Single-and Few-Layer MoS2. Nano Lett. 2014, 14 (11), 6087-6091. (8) Santos, E. J.; Kaxiras, E. Electrically Driven Tuning of the Dielectric Constant in MoS2 Layers. ACS Nano 2013, 7 (12), 10741-10746. (9) Latini, S.; Olsen, T.; Thygesen, K. S. Excitons in van der Waals Heterostructures: The Important Role of Dielectric Screening. Phys. Rev. B 2015, 92 (24), 245123. (10) Kaushik, V.; Varandani, D.; Mehta, B. R. Nanoscale Mapping of Layer-Dependent Surface Potential and Junction Properties of CVD-Grown MoS2 Domains. J. Phys. Chem. C 2015, 119 (34), 20136-20142. (11) Castellanos‐Gomez, A.; Cappelluti, E.; Roldán, R.; Agraït, N.; Guinea, F.; Rubio‐Bollinger, G. Electric-Field Screening in Atomically Thin Layers of MoS2: the Role of Interlayer Coupling. Adv. Mater. 2013, 25 (6), 899-903. (12) Li, Y.; Xu, C. Y.; Zhen, L. Surface Potential and Interlayer Screening Effects of Few-Layer MoS2 Nanoflakes. Appl. Phys. Lett. 2013, 102 (14), 143110. (13) Choi, S.; Shaolin, Z.; Yang, W. Layer-Number-Dependent Work Function of MoS2 Nanoflakes. J. Korean Phys. Soc. 2014, 64 (10), 1550-1555.

11



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 19

(14) Li, L. H.; Santos, E. J.; Xing, T.; Cappelluti, E.; Roldán, R.; Chen, Y.; Wanatabe, K.; Taniguchi, T. Dielectric Screening in Atomically Thin Boron Nitride Nanosheets. Nano Lett. 2014, 15 (1), 218-223. (15) Li, S.; Zhou, Y.; Zi, Y.; Zhang, G.; Wang, Z. L. Excluding Contact Electrification in Surface Potential Measurement Using Kelvin Probe Force Microscopy. ACS Nano 2016, 10 (2), 2528-2535. (16) Bertolazzi, S.; Brivio, J.; Radenovic, A.; Kis, A.; Wilson, H.; Prisbrey, L.; Minot, E.; Tselev, A.; Philips, M.; Viani, M.; Walters, D.; Proksch, R. Exploring Flatland: AFM of Mechanical and Electrical Properties of Graphene, MoS2 and Other Low-Dimensional Materials. Microsc. Anal. 2013, 27, 21-24. (17) Qiu, D. Y.; da Jornada, F. H.; Louie, S. G. Screening and Many-Body Effects in TwoDimensional Crystals: Monolayer MoS2. Phys. Rev. B 2016, 93, 235435. (18) Robinson, B. J.; Giusca, C. E.; Gonzalez, Y. T.; Kay, N. D.; Kazakova, O.; Kolosov, O. V. Structural, Optical and Electrostatic Properties of Single and Few-Layers MoS2: Effect of Substrate. 2D Mater. 2015, 2 (1), 015005. (19) Varghese, J. O.; Agbo, P.; Sutherland, A. M.; Brar, V. W.; Rossman, G. R.; Gray, H. B.; Heath, J. R. The Influence of Water on the Optical Properties of Single-Layer Molybdenum Disulfide. Adv. Mater. 2015, 27 (17), 2734-2740. (20) Hüser, F.; Olsen, T.; Thygesen, K. S. How Dielectric Screening in Two-Dimensional Crystals Affects the Convergence of Excited-State Calculations: Monolayer MoS2. Phys. Rev. B 2013, 88 (24), 245309. (21) Pietronero, L.; Strässler, S.; Zeller, H. R.; Rice, M. J. Charge Distribution in c Direction in Lamellar Graphite Acceptor Intercalation Compounds Phys. Rev. Lett. 1978, 41 (11), 763.

12


Page 13 of 19

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


(22) Hanbicki, A. T.; Currie, M.; Kioseoglou, G.; Friedman, A. L.; Jonker, B. T. Measurement of High Exciton Binding Energy in the Monolayer Transition-Metal Dichalcogenides WS2 and WSe2 Solid State Commun. 2015, 203, 16-20. (23) Lee, S. Y.; Kim, U. J.; Chung, J. G.; Nam, H.; Jeong, H. Y.; Han, G. H.; Kim, H.; Oh, H. M.; Lee, H.; Kim, H.; Roh, Y.-G.; Kim, J.; Hwang, S. W.; Park, Y.; Roh, Y. G. Large Work Function Modulation of Monolayer MoS2 by Ambient Gases. ACS Nano 2016, 10 (6), 6100-6107. (24) Zhang, L.; Liu, K.; Wong, A. B.; Kim, J.; Hong, X.; Liu, C.; Cao, T.; Louie, S. G.; Wang, F.; Yang, P. Three-Dimensional Spirals of Atomic Layered MoS2. Nano Lett. 2014, 14 (11), 64186423. (25) Ly, T. H.; Zhao, J.; Kim, H.; Han, G. H.; Nam, H.; Lee, Y. H. Vertically Conductive MoS2 Spiral Pyramid. Adv. Mater. 2016, 28 (35), 7723-7728. (26) Sarma, P. V.; Patil, P. D.; Barman, P. K.; Kini, R. N.; Shaijumon, M. M. Controllable Growth of Few-Layer Spiral WS2. RSC Adv. 2016, 6 (1), 376-382. (27) Avdoshenko, S. M.; Koskinen, P.; Sevinçli, H.; Popov, A. A.; Rocha, C. G. Topological Signatures in the Electronic Structure of Graphene Spirals. Sci. Rep. 2013, 3, 1632. (28) Xu, F.; Yu, H.; Sadrzadeh, A.; Yakobson, B. I. Riemann Surfaces of Carbon as Graphene Nanosolenoids. Nano Lett. 2015, 16 (1), 34-39. (29) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324 (5934), 1530-1534. (30) Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48 (11), 4729-4733. (31) 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 (22), 222107.

13



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 19

(32) He, J.; Hummer, K.; Franchini, C. Stacking Effects on the Electronic and Optical Properties of Bilayer Transition Metal Dichalcogenides MoS2, MoSe2, WS2, and WSe2. Phys. Rev. B 2014, 89 (7), 075409. (33) Liu, K.; Zhang, L.; Cao, T.; Jin, C.; Qiu, D.; Zhou, Q.; Zettl, A.; Yang, P.; Louie, S.G.; Wang, F. Evolution of interlayer coupling in twisted molybdenum disulfide bilayers. Nat. Commun. 2014, 5, 4966. (34) Zhang, J.; Wang, J.; Chen, P.; Sun, Y.; Wu, S.; Jia, Z.; Lu, X.; Yu, H.; Chen, W.; Zhu, J.; Xie,G.; Yang, R.; Shi, D.; Xu, X.; Xiang, J.; Liu, K.; Zhang, G. Observation of strong interlayer coupling in MoS2/WS2 heterostructures. Adv. Mater. 2016, 28, 1950-1956.

FIGURE LEGENDS:

Figure 1. (a) Schematic of KFM setup for surface potential measurement on MoS2 spiral. (b) Dark field TEM image for one MoS2 spiral.

Figure 2. (a) The AFM topography image for MoS2 spiral, inset shows the conductive AFM image for the whole spiral sample while the white dashed rectangle in the inset marked the corresponding area for the topography AFM image. The area is 10 µm×10 µm. (b) The KFM surface potential image for the same area MoS2 spiral sample as in (a). (c) The magnified AFM topography image for the white dashed rectangle part in MoS2 spiral. Inset shows the height profile corresponding to the white dashed line in (c). The area is 5 µm×5 µm. (d) (e) The height and surface potential profiles derived along the dashed lines A and B in (a) and (b) (f) The larger scale KFM image for the MoS2 spiral flake. The area is 20 µm×20 µm.

Figure 3. (a) The AFM topography image for the mechanical exfoliated MoS2 sample. The area is 10 µm×10 µm. (b) The KFM surface potential image for the same area in exfoliated MoS2 as in (a). 14


Page 15 of 19

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


Figure 4. Statistical results for the relationship of surface work function and sample thickness of spiral MoS2 and mechanical exfoliated MoS2, experimentally measured by KFM and AFM.

Figure 5. The atomic schematic for the perfect dielectric screening by a bilayer MoS2 spiral structure.

15



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 19

Figures:

Figure 1

16


Page 17 of 19

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


Figure 2

Figure 3

17



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 19

Figure 4

Figure 5

18


Page 19 of 19

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


TOC GRAPHIC

19


Superior Dielectric Screening in Two-Dimensional MoS2 Spirals

Recommend Documents