First-Principles Study on Layered C2NâMetal Interfaces - Langmuir

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A First-Principles Study on Layered C2N-Metal Interfaces Zhao Chen, Ruiqi Zhang, and Jinlong Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03801 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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A First-Principles Study on Layered C2N-Metal Interfaces Zhao Chen, Ruiqi Zhang and Jinlong Yang * Hefei National Laboratory for Physical Sciences at the Microscale, CAS Centre for Excellence Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China * Corresponding Author Email: [email protected]

ABSTRACT: Using first-principles calculations, we perform a comprehensive study of representative metal (Al, Sc, Pd, Ag, Pt and Au) contacts with monolayer (ML) and bilayer (BL) C2N, which is a low-cost and easily synthesized 2D metal-free semiconductor. Through analyzing the geometries, electronic structures, and Fermi level pinning effects of C2N-metal interfaces, we find metal Al and Sc top contact with ML C2N are Ohmic, which can be ascribed to the strong interactions and large orbital overlaps. Besides, owing to weak van der Waals (vdW) interactions at interfaces and low work functions of metallic materials, Ohmic contacts can also be realized in ML/BL C2N-Ag and BL C2N-Sc systems. Furthermore, it was also predicted that C2N-Sc and -Ag systems still maintain Ohmic features along edge contacts. Given the lower resistance of Ag electrode, the C2N-Ag electrode should be a more attractive electrode in practical applications. These predictions not only provide insights into the fundamental

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properties of layered C2N-metal interfaces but also pave the way to design high performance devices using low-cost layered C2N.

INTRODUCTION Since the successful isolation of graphene in 2004,1 novel two-dimensional (2D) materials have attracted tremendous interest due to their fascinating electronic, mechanical, optical or thermal properties.2–5 As a typical example, graphene is famous for its linear Dirac-like dispersion near the Fermi level, in which many unusual electronic and spintronic properties were discovered.3 However, the absence of a fundamental band gap severely limits its applications in field-effect transistors (FETs). Subsequently, possessing intrinsic direct band gaps, monolayer transition-metal dichalcogenides (TMDs)6–8 and phosphorene9–13 are considered to be superior in electronic and optoelectronic applications. Indeed, in a real device, 2D semiconductors need contact with metal electrodes. Thus, searching low resistance metal contacts with two-dimensional (2D) semiconductors has always been a focus of current research in 2D electronics.14–16 For examples, tremendous theoretical and experimental efforts have been devoted to understand the metal contacts with 2D MoS2,17–23 phosphorene,13,24–26 InSe,27 and other 2D semiconductor materials to design proper metal-2D semiconductor contacts.28,29 Most recently, a new and low-cost layered C2N with homogeneous holes and nitrogen atoms has been simply synthesized via a bottom-up wet-chemical reaction,30 which possesses a considerable band gap of ~1.96 eV. Furthermore, a FET device based on few-layers C2N shows a high on/off ratio of 107.30 Compared with the 2D materials mentioned above, layered C2N is low-cost and can be easily synthesized. These properties may make layered C2N becoming a very promising candidate material for applications in future low-cost nanoelectronic and


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nano-optoelectronic devices.30,31 However, studies about the C2N-metal contacts have not been reported so far. Hence, to fill this void, it is necessary to do a systematic study of C2N-metal contacts. In this paper, to understand the nature of monolayer (ML)/bilayer (BL) C2N-metal contacts, we have systematically investigated the interfacial properties of ML and BL C2N on several commonly used metals (Al, Sc, Pd, Ag, Pt and Au) based on first-principles calculations. These metals cover a wide range of work functions (Table 2) from Sc (3.42 eV) to Pt (5.75 eV) and have small lattice mismatches (less than 1%) with C2N, providing a suitable platform for studying the band structure realignments at the C2N-metal interfaces. It is shown that not only choosing suitable metals with low work functions but also the physical nature of interfaces play important roles for realizing low contact resistances in C2N-metal systems.

COMPUTATIONAL METHODS The geometry optimizations are performed using the Vienna ab initio simulation package (VASP)32,33 with projector-augmented wave (PAW) pseudopotentials.34,35 The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)36 is used. The kinetic energy cutoff is 550 eV. Previous calculations suggest that six atomic layers of the slabs are enough to simulate metal surfaces.37 So we adopt six atomic layers to simulate surface and build a supercell with C2N absorbed on one side of the metal surface, as shown in Figure 1. A vacuum region at least 25 Å is set to remove the coupling between periodic slabs. The effect of van der Waals (vdW) interaction is accounted for by using empirical correction method proposed by Grimme


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(DFT-D3), which is a good description of long-range vdW interactions.38–40 As a benchmark, the DFT-D3 method gives an interlayer distance of 3.25 Å and a binding energy of -25 meV per carbon atom for bilayer graphene, consistent with previous experimental measurements and theoretical studies.41,42 Thus, the van der Waals (vdW) interaction can be considered by using the empirical correction method DFT-D3.43,44 Geometry structures are relaxed until the energy converged to 10-5 eV, and the force on each atom is less than 0.01 eV/Å. The dipole corrections are adopted in all calculations. The Monkhorst-Park k-point grids of 5×5×1 and 15×15×1 are chosen for ionic and electronic optimization for ML C2N, respectively. We fix the bottom three layers of metal atoms and other atoms, including C and N atoms, are fully relaxed. The √3×√3 ML C2N matches 5×5 (111) surfaces of Al, Ag, Au, the 1×1 ML C2N matches 3×3 (111) surfaces of Pd and Pt, the 2×2 ML C2N matches 5×5 (111) surfaces of Sc. The maximum strain in the five metals is 0.92% compression for Pd. We have calculated the band structures by fixing the lattice constant of C2N and Pd surface in Figure S1, respectively. As can be seen from the figure, band structures are almost unaffected using these two methods. Thus, in order to catch the detailed electronic behaviors at the studied interface in a comparative manner, the lattice constant of C2N is kept constant in all the models. The binding energies Eb between the six metals and C2N are calculated by the formula

Eb = (Emetal + EnC2N − Emetal−nC2N ) / N Where , and are the relaxed energies for clean metal surfaces, isolated C2N, and combined system, respectively. And the total number of C and N atoms in the interface of single-layer C2N is N.

RESULTS AND DISCUSSION


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Geometry and stability of ML/BL C2N-metal interfaces Firstly, the optimized lattice constant of ML C2N is 8.33 Å, in good agreement with previous value.45 Then, we fix the lattice constant of ML C2N, and extend or strain metal surfaces to match ML C2N in all calculations. The lattice mismatch for each metal is listed in Table 1, ranging from 0.06% to 0.92%. After these initial configurations of ML C2N-metal systems are optimized, the structural parameters of these metal contacts are summarized in Table 1. And optimized structures of them are depicted in Figure 1. In ML C2N-metal contacts, the equilibrium distances of the interface ( ) is defined as the averaged distance between ML C2N and the first metal atomic layer along the Z direction. The equilibrium distances range from 3.29 ~ 1.71 Å, decreasing in the order of Au > Pt > Ag > Pd > Al > Sc. Corresponding to the binding energies Eb have an increasing order, i.e., Au < Ag < Pt < Pd < Al < Sc. We note that C2N-Ag, -Pt and -Au interfaces have weak vdW interaction with Eb = 0.078 - 0.096 eV and the equilibrium distances range from 3.29 ~ 2.97 Å. It also can be confirmed by their optimized structures in Figure 1 (b), where ML C2N is slightly distorted. While for C2N-Pd contact, there is a medium binding energy with Eb = 0.136 eV. Additionally, ML C2N-Al and -Sc interfaces have strong interactions with binding energies of 0.220 and 0.778 eV, respectively, leading to strongly destroyed structure of ML C2N. (Figure 1 (c) and (d)). Such big difference of interactions in C2N-Ag and -Sc systems can be well explained by the d-band model.46 That is to say, Sc has open d-shells and the bonding between Sc and ML C2N is strong, while Ag has full d-shells and the bonding between Ag and ML C2N is weak. But for the C2N-Al system, there are large electrons transferring, leading to a strong interaction in this system. According to the Bader analyses,47,48 the number of electrons transfer from Sc slab to ML C2N is the largest (Table S1). And the values of Al and Sc contacts are almost one order of magnitude larger than other


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interfaces, agreeing well with the binding energies. Thus, strong orbital overlaps will exist at the ML C2N-Al and -Sc interfaces. Furtherly, we also study the configurations for BL C2N-metal contacts. The initial configuration of BL C2N (Figure S2) is adopted the most stable one in previous work.45 Our results show that ML and BL C2N-metal contacts nearly have the same

and (see Table 1 and Table S2).

Table 1. Calculated key parameters of ML C2N-metal contacts. The corresponding lattice mismatches are given

metal

mismatch (%)

dM-S (Å)

dM-N(Å)

dM-C(Å)

Eb(eV)

Al

0.78

1.80a

1.98

2.88

0.220

Sc

0.72

1.71a

2.16

2.28

0.778

Pd

0.92

2.55

2.32

2.85

0.136

Ag

0.11

2.97

2.90

3.07

0.088

Pt

0.06

3.15

3.12

3.42

0.096

Au

0.06

3.29

3.35

3.37

0.078

a

distances from the obvious ups of top layer metal atoms and downs of the C2N atoms in the z direction(C2N-Al/Sc contacts).The dM-N and dM-C is the minimum atomic distance from the topmost layer atom of metal to N and C atoms, respectively.


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Figure 1. Schematic diagram of a ML C2N FET in (a). The side and top views of the optimized structures for (b) ML C2N-Ag, (c) -Al, and (d) -Sc contacts, respectively. The blue and gray balls represent N atoms and C atoms, respectively.

Electronic structures and Schottky barrier heights (SBHs) of ML C2N-metal interfaces The schematic diagram of a ML C2N FET is shown in Figure 2(a), two kinds of contacts can form at two different interfaces: one is the top contact, consisting of metal surface and top contacted ML C2N in the vertical direction (the corresponding interface is labeled B and vertical


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SBH is labeled ); and the other is the edge contact, consisting of top contacted complex and the channel ML C2N in the lateral direction (the corresponding interface is labeled D and lateral SBH is labeled ). And the SBHs of top and edge contacts are listed in Table 2, respectively. To understand the coupling of metals with ML C2N, we plot the band structures of isolated ML C2N and top contacts in Figure 2 and Figure S3, 4. It is obviously that ML C2N is a direct-gap semiconductor with a band gap of 1.66 eV at the Γ point in the PBE calculation level, agreeing well with previous reports.45,49 This result will be compared later with combined systems. It is no doubt that DFT-GGA underestimates the band gap of C2N due to overlooking the many body effect among electrons. However, semiconductors are doped by metal electrodes or gates in practical FETs. Thus, the strong Coulombic screening by doped carriers minimizes the many body effect.50,51,28 Besides, many works have verified GGA calculations can get the consistent SBH with experimental one, such as MoS2-Ti52 contacts. Therefore, the GGA is appropriate for studying the metal-C2N contacts. /

The SBHs for top contacts ( ) can be evaluated by the energy difference between the Fermi level of the interfacial system and the band edge of underlying C2N. Actually, in heterogeneous interfaces with orbital hybridization, the CBM/VBM of the C2N can be identified by regarding the spilled hybridization states as gap states.51,28 The band gap of C2N is mainly determined by the projected orbitals states at the Γ-point rather than other K-points with small weights for orbitals hybridization (Figure S5), which are part of the interfacial gap states and don’t affect the SBH. Firstly, as shown in Figure 2(d) and Figure S4(b), (c), the conduction bands of ML C2N are slightly hybridized with bands of metals, while valence bands of ML C2N are preserved well in ML C2N-Ag, -Pt, and -Au system. And, as listed in Table 2, ML C2N-Pt and -Au interfaces are


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n-type Shottky contacts with larger = 0.74 and 0.32 eV ( is the SBH of electron in the top

contact), respectively. While, a negligible n-type vertical SB of 0.05 eV is found in ML C2N-Ag top contact. So an n-type Ohmic contact can form in ML C2N-Ag interfaces. Secondly, for ML C2N-Pd top contact with medium adhesion, conduction and valence bands of ML C2N are slightly hybridized with Pd (Figure S4(a)) and a vertical SB of 0.47 eV forms at this interface. Thirdly, as seen from Figure 2 (e) and (f), bands of ML C2N are strongly destroyed and across the Fermi level in ML C2N-Al and -Sc interfaces with strong interaction, inducing the metallization of ML C2N and strong orbital overlaps. Thus, ML C2N-Al and -Sc systems are Ohmic top contacts. Because we are only concerned about Ohmic contacts in these systems, only the detail discussions for C2N-Al and -Sc complexes with strong interaction and C2N-Ag with weak interaction are presented in next sections. /

Note that lateral SBs ( ) may form in these metallic systems along edge contacts, as illustrated in Figure 2(a). For ML C2N-Pd, -Ag, -Pt and Au systems with vdW interactions, ϕL are calculated by the CBM difference between interfacial and free standing C2N. But, ML C2N is metallization due to strong hybridization with Al and Sc electrodes. Thus, the ϕL in C2N-Al and -Sc systems are calculated by the energy difference between the Fermi level of the interfacial system and the CBM of the free standing C2N.28,37,53 These method also can be get qualitatively consistent results for ML MoS2-Sc system (0.11 eV)37 with ATK method (0.15 eV).52 Based on our calculations (Table 2), for ML C2N-Al and -Sc systems are 0.20 eV and -0.41 eV, respectively. Consequently, Ohmic contacts exist in ML C2N-Sc top and edge contacts and the band diagram for this are shown in Figure 2(b).


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Figure 2. (a) Schematic diagram of interface. A, C and E are three regions, while there are two interfaces B and D to separate them. Red arrows indicate the pathway (A→B→C→D→E) of electron injection from contact metal (A) to ML C2N channel (E). (b) Band diagram of ML C2N-Sc contacts. The EFM is the Fermi level of the ML C2N-Sc top contact. The EC and ECh, Ev are the CBM and Fermi level, VBM of channel C2N, respectively. (c) Band structures for ML C2N. (d), (e), (f) are band structures for ML C2N-Ag, -Al, -Sc complexes, respectively. Gray lines represent ML C2N-metal surface bands; red dots represent bands of C2N.


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Table 2. The work function (W) for clean metal surface and WML for metal surface adsorbed by ML C2N, respectively. and are the vertical and lateral SBHs of electron, respectively. metal

W (eV)

WML (eV)

(eV)

(eV)

Al

4.05

4.62

0

0.20

Sc

3.42

4.01

0

-0.41

Pd

5.27

4.74

0.47

0

Ag

4.45

4.58

0.05

0

Pt

5.73

5.16

0.74

0

Au

5.20

4.73

0.32

0

The metallization of C2N-Al and -Sc top contacts with strong interaction can be further verified by their Partial density of states (PDOS). As illustrated in Figure 3, we plot density of states (DOS) of free standing ML C2N and PDOS of ML C2N in these complexes. For ML C2N-Al and -Sc systems, large PDOS spread all over the primary band gap of ML C2N, leading to large orbital overlaps in these complexes (see Figure 3 (b) and (c)). Especially, we also note that overlapping states are delocalized (Figure S6), indicating a high injection efficiency and a low contact resistance in ML C2N-Al/Sc interface. Thus, Ohmic contacts forming in ML C2N-Al, and -Sc systems can be ascribed to the large orbital overlaps and metallization of ML C2N at interfaces. Charge transfer in ML C2N-metals can be used to understand above results. The plane-averaged charge differences are shown in the Figure 3 (d) and (e). As seen from the figure, electrons accumulate at different degree for different interfaces. When ML C2N contact with


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Al/Sc, much larger charge transfer per unit area occurs at interfaces from top Al/Sc atoms to ML C2N and corresponding value is 0.026 e Å-2/0.052 e Å-2. This large charge transfer implies that strong orbital overlaps at ML C2N-Al/Sc interface. In particular, as seen from in Figure 3 (d) and (e), accumulated electrons are not only from metal atoms, but also from ML C2N. This obvious charge transfer contributes to form substantial and strong covalent bonding in ML C2N-Al/Sc interface. Based on our above analyses, the nature of metallization in the C2N-Al and -Sc complexes can be well explained by the strong orbital overlaps at these interfaces, leading to efficient electron transfer.

Figure 3. (a) DOS of ML C2N, and PDOS of ML C2N for the (b) ML C2N-Al and (c) ML C2N-Sc complexes, respectively. The Fermi level is set to zero. (d) and (e) are plane-averaged charge density difference along the Z-direction parallel to the interfaces of ML C2N-Al and -Sc, respectively. In the Figure 3(d) and (e), the vertical dashed lines indicate the interfaces of ML C2N-Al and Sc contacts, respectively. The average ∆ρ (e Å-2) is the value of the electron transfer per unit area from metal to ML C2N by integrating the plane-averaged electron density difference on the side of C2N along Z-direction.


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Electronic structures and SBHs of BL C2N-metal interfaces To study influences of the layer numbers of C2N, we also calculate band structures of BL C2N-metal contacts, which are shown in Figure 4 and Figure S4. And the SBHs are listed in the Table S4. It can be seen from Figure 4 (a), a negligible in BL C2N-Ag contact is 0.04 eV.

Combined with above results, it is clearly that n-type Ohmic contacts can form in both ML and BL C2N-Ag interfaces. As the number of layer increases, SBHs have a slightly decrease from 0.05 eV to 0.04 eV for top the contact of Ag with C2N (C2N-Pd/Pt contact has the same tendency in Figure S4), which attributes to the increase of electron affinity of C2N from 4.42 to 4.57 eV. Because of the strong band hybridization, ML C2N-Al and -Sc systems can be regarded as new metallic materials, marked as C2N/Al and C2N/Sc. Thus, the definition of SBH for BL C2N-Sc (Al) interface is the same with ML C2N-metal system, which can be extracted from the energy difference between Fermi level of ML C2N-C2N/Sc (Al) system and the CBM of underlying ML C2N. Obviously, an n-type SB with a value of 0.17 eV form in BL C2N-Al complex. While for BL C2N-Sc complex, the interface still keep Ohmic contact with of -0.01 eV. At the same

time, from band alignments, evaluated lateral SBHs ( ) for BL C2N-Al, and -Sc, -Ag systems are 0.11, and -0.09, 0 eV, respectively. Hence, due to free-SB in the contacts, Sc and Ag electrodes can spontaneously inject electrons into channel BL C2N.


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Figure 4. (a) Band structure of BL C2N-Ag, gray dots represent bands the of BL C2N-Ag complex; red dots represent the projected band structure of BL C2N. Band structures for (b) BL C2N-Al, (c) BL C2N-Sc complexes, respectively. Gray lines represent bands of the BL C2N-metal complexes; red dots represent the projected band structures on the underlying ML C2N singlet layer.

Fermi level pinning According to Liu’s work,15,54 the reason for Ohmic contacts forming in ML C2N-Ag and ML C2N-C2N/Sc top contacts can be understood by low work functions of metallic materials, and weak vdW interactions at these interfaces, which has been considered to reduce the Fermi level pinning (FLP) effect. To reflect these, we should firstly note that C2N/Al and C2N/Sc systems can be considered as new metallic materials based on above analyses. Then, according to our calculations, the work functions of Ag and C2N/Sc systems are relatively low (Table S3). To address weak FLP effect at interfaces with weak vdW interactions, we carried out a quantitative analysis of FLP by the linearly fitting slope of the -versus-W (work function)15 ,! / "


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Where the is extracted from electronic structures (Figure 2, 4 and Figure S4). EF is the Fermi level of contacts and ECBM is the CBM of the C2N adsorbed on metal substrates. The slope being close to 1 indicates no Fermi level pinning, and 0 suggest a strong Fermi level pinning.55 Notice that there are weak vdW interactions in ML (BL) C2N-Ag, which has been verified by large dM-S and small binding energies Eb (Table 1 and Table S2). And for ML C2N-C2N/Sc system, there is also a weak interaction between the underlying ML C2N and C2N/Sc electrode with a small binding energy (Eb2 = 0.027 eV) and the large distance (d2 = 3.145 Å). Thus, we plotted the values of vs. W for weak vdW interaction of ML C2N systems in Figure 5. As seen from this figure, the slope is only 0.43, which is much larger than corresponding values in MoS255,56 (0.1) and silicon57 (0.27). Thus, compared with MoS2 and silicon, there is a weaker FLP effect in the weak vdW interaction ML C2N contacts. Therefore, low work functions of metals and weak vdW interactions at interfaces lead to Ohmic contacts occurring in ML C2N-Ag and ML C2N-C2N/Sc systems. And the Ohmic character for BL C2N-Ag is the same with ML C2N-Ag.

Figure 5. Calculated as a function of work function of metals for ML C2N contacts, respectively. The black diamonds in figure are calculation results and a linear fit by red line.


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CONCLUSIONS In summary, we have investigated interfacial properties of ML/BL C2N-metal (Al, Sc, Pd, Ag, Pt, and Au) contacts based on the first-principles calculations. We find when ML C2N contacts with metal Al and Sc, strongly interface interactions and the consequent metallization induce SB-free contacts forming in these interfaces. Besides, ML/BL C2N-Ag and BL C2N-Sc complexes also show Ohmic features. The reason for these can be understood by weak vdW interactions between layered C2N and metals with low work functions. Moreover, SB-free contact can form in Sc/Ag edge contact with C2N based on band alignments. Furtherly, considering the lower resistance of Ag electrode, ML/BL C2N-Ag contact should have a better electron injection efficiency. It is also worth to note that compared with other star 2D materials, such as MoS2 and phosphorene, layered C2N is simply synthesized and low-cost. Thus, these results presented here not only elucidate the nature of C2N-metal contacts but also provide a theoretical reference to design high performance 2D nanodevices based on low-cost 2D crystals.

ASSOCIATED CONTENT Supporting Information. The bader electron transfer of ML C2N-metal systems; the calculated key parameters of BL C2N-metal contacts; the work function W for clean metal surface and WML, WBL for metal surface adsorbed by ML and BL C2N, respectively; the vertical and lateral SBHs of BL C2N-metal contacts, respectively; band structure of ML C2N-Pd are calculated by fixing C2N and Pd surface, respectively; side and top view of the optimized


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structures of BL C2N-Sc; the band structures for 1×1 and 2×1 ML C2N, respectively; band structures of ML/BL C2N-Pd, -Pt, -Au complexes, respectively; projected orbitals states of C2N at the C2N-Ag interface; PDOS of ML C2N-Sc complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jinlong Yang: 0000-0002-5651-5340

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is partially supported by the National Natural Science Foundation of China (Grants No. 21503204, No. 21688102 and No. 21421063), by the National Key Research & Development Program of China (Grant No. 2016YFA0200604). REFERENCES (1)

Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V., Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science. 2004, 306, 666–669.

(2)

Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191.


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(3)

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Neto, A. H. C.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109.

(4)

Mas-Ballesté, R.; Gómez-Navarro, C.; Gómez-Herrero, J.; Zamora, F. 2D Materials: To Graphene and beyond. Nanoscale 2011, 3, 20–30.

(5)

Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R. Recent Advances in Two-Dimensional Materials beyond Graphene. ACS Nano 2015, 9, 11509–11539.

(6)

Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263–275.

(7)

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699–712.

(8)

Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. Progress, Challenges, and Opportunities in Two-Dimensional Materials beyond Graphene. ACS Nano 2013, 7, 2898–2926.

(9)

Xia, F.; Wang, H.; Jia, Y. Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 4458.

(10)

Fei, R.; Yang, L. Strain-Engineering the Anisotropic Electrical Conductance of Few-Layer Black Phosphorus. Nano Lett. 2014, 14, 2884–2889.


Page 19 of 24 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

Langmuir

(11)

Qiao, J.; Kong, X.; Hu, Z.-X.; Yang, F.; Ji, W. High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Commun. 2014, 5, 4475.

(12)

Ye, P. D.; Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Engineering, C.; Lafayette, W.; States, U.; Lansing, E.; States, U.; Engineering, M.; Lafayette, W.; States, U. Phosphorene : An Unexplored 2D Semiconductor with a High Hole. ACS Nano 2014, 8, 4033–4041.

(13)

Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372–377.

(14)

Allain, A.; Kang, J.; Banerjee, K.; Kis, A. Electrical Contacts to Two Dimensional Semiconductors. Nat. Mater. 2015, 14, 1195–1205.

(15)

Liu, Y.; Stradins, P.; Wei, S.-H. Van Der Waals Metal-Semiconductor Junction: Weak Fermi Level Pinning Enables Effective Tuning of Schottky Barrier. Sci. Adv. 2016, 2, 1600069.

(16)

Xu, Y.; Cheng, C.; Du, S.; Yang, J.; Yu, B.; Luo, J.; Yin, W.; Li, E.; Dong, S.; Ye, P.; Duan, X. Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and P-N Heterojunctions. ACS Nano 2016, 10, 4895–4919.

(17)

Kang, J.; Liu, W.; Banerjee, K. High-Performance MoS2 Transistors with Low-Resistance Molybdenum Contacts. Appl. Phys. Lett. 2014, 104, 2–7.

(18)

Xu, S.; Wu, Z.; Lu, H.; Han, Y.; Long, G.; Chen, X.; Han, T.; Ye, W.; Wu, Y.; Lin, J.; Shen, J.; Cai, Y.; He, Y.; Zhang, F.; Lortz, R.; Cheng, C.; Wang, N. Universal Low-Temperature Ohmic Contacts for Quantum Transport in Transition Metal Dichalcogenides. 2D Mater. 2016, 3, 21007.


Langmuir 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

(19)

Page 20 of 24

Liu, H.; Neal, A. T.; Ye, P. D. Channel Length Scaling of MoS2 MOSFETs. ACS Nano 2012, 6, 8563–8569.

(20)

Popov, I.; Seifert, G.; Tománek, D. Designing Electrical Contacts to MoS2 Monolayers: A Computational Study. Phys. Rev. Lett. 2012, 108, 156802.

(21)

RadisavljevicB; RadenovicA; BrivioJ; GiacomettiV; KisA; Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat Nano 2011, 6, 147–150.

(22)

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, 031005/1-14.

(23)

Liu, H.; Si, M.; Deng, Y.; Neal, A. T.; Du, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Ye, P. D. Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: An Insight into Current Flow across Schottky Barriers. ACS Nano 2014, 8, 1031–1038.

(24)

Das, S.; Zhang, W.; Demarteau, M.; Hoffmann, A.; Dubey, M.; Roelofs, A. Tunable Transport Gap in Phosphorene. Nano Lett. 2014, 14, 5733–5739.

(25)

Chanana, A.; Mahapatra, S. First Principles Study of Metal Contacts to Monolayer Black Phosphorous. J. Appl. Phys. 2014, 116, 204302.

(26)

Gong, K.; Zhang, L.; Ji, W.; Guo, H. Electrical Contacts to Monolayer Black Phosphorus: A First Principles Investigation. Phys. Rev. B 2014, 90, 125441/1-6.

(27)

Jin, H.; Li, J.; Wan, L.; Dai, Y.; Wei, Y.; Guo, H. Ohmic Contact in Monolayer InSe-Metal Interface. 2D Mater. 2017, 4, 25116.


Page 21 of 24 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

Langmuir

(28)

Wang, Y.; Yang, R. X.; Quhe, R.; Zhong, H.; Cong, L.; Ye, M.; Ni, Z.; Song, Z.; Yang, J.; Shi, J.; Li, J.; Lu, J. Does P-Type Ohmic Contact Exist in WSe2-Metal Interfaces? Nanoscale 2016, 1179, 1179–1191.

(29)

Liu, W.; Kang, J.; Sarkar, D.; Khatami, Y.; Jena, D.; Banerjee, K. Role of Metal Contacts in Designing High-Performance Monolayer N-Type WSe2 Field Effect Transistors. Nano Lett. 2013, 13, 1983–1990.

(30)

Mahmood, J.; Lee, E. K.; Jung, M.; Shin, D.; Jeon, I.-Y.; Jung, S.-M.; Choi, H.-J.; Seo, J.-M.; Bae, S.-Y.; Sohn, S.-D.; Park, N.; Oh, J. H.; Shin, H.-J.; Baek, J.-B. Nitrogenated Holey Two-Dimensional Structures. Nat. Commun. 2015, 6, 6486.

(31)

Guan, Z.; Lian, C.-S.; Hu, S.; Ni, S.; Li, J.; Duan, W. Tunable Structural, Electronic, and Optical Properties of Layered Two-Dimensional C2N and MoS2 van Der Waals Heterostructure as Photovoltaic Material. J. Phys. Chem. C 2017, 121, 3654–3660.

(32)

Kresse, G.; Hafner, J. Ab Initio. Phys. Rev. B 1993, 47, 558–561.

(33)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186.

(34)

Kresse, G. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775.

(35)

Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979.

(36)

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys.Rev.Lett. 1996, 77, 3865–3868.

(37)

Li, Z. J.; Li, X. X.; Yang, J. L. Comparative Study on Electronic Structures of Sc and Ti Contacts with Mono Layer and Multilayer MoS2. ACS Appl. Mater. Interfaces 2015, 7, 12981–12987.


Langmuir 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

(38)

Page 22 of 24

Grimme, S.; Mück-Lichtenfeld, C.; Antony, J. Noncovalent Interactions between Graphene Sheets and in Multishell (Hyper) Fullerenes. J. Phys. Chem. C 2007, 111, 11199–11207.

(39)

Hu, W.; Wu, X.; Li, Z.; Yang, J. Helium Separation via Porous Silicene Based Ultimate Membrane. Nanoscale 2013, 5, 9062–9066.

(40)

Zhang, R.; Li, B.; Yang, J. A First-Principles Study on Electron Donor and Acceptor Molecules Adsorbed on Phosphorene. J. Phys. Chem. C 2015, 119, 2871–2878.

(41)

Zacharia, R.; Ulbricht, H.; Hertel, T. Interlayer Cohesive Energy of Graphite from Thermal Desorption of Polyaromatic Hydrocarbons. Phys. Rev. B. 2004, 69, 155406.

(42)

Baskin, Y.; Meyer, L. Lattice Constants of Graphite at Low Temperatures. Phys. Rev. 1955, 100, 544.

(43)

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 H-Pu. J. Chem. Phys. 2010, 132, 154104/1-19.

(44)

Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Correct Density Functional Theory. J. Comput. Chem. 2011, 32, 1456–1465.

(45)

Zhang, R.; Li, B.; Yang, J. Effects of Stacking Order, Layer Number and External Electric Field on Electronic Structures of Few-Layer C2N-h2D. Nanoscale 2015, 7, 14062–14070.

(46)

Nørskov, J. K.; Abild-Pedersen, F.; Studt, F.; Bligaard, T. Density Functional Theory in Surface Chemistry and Catalysis. Proc. Natl. Acad. Sci. 2011, 108, 937–943.

(47)

Yu, M.; Trinkle, D. R. Accurate and Efficient Algorithm for Bader Charge Integration. J. Chem. Phys. 2011, 134, 064111/1-8.


Page 23 of 24 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

Langmuir

(48)

Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354–360.

(49)

Sun, J.; Zhang, R.; Li, X.; Yang, J. A Many-Body GW+ BSE Investigation of Electronic and Optical Properties of C2N. Appl. Phys. Lett. 2016, 109, 133108/1-4.

(50)

Gong, C.; Huang, C.; Miller, J.; Cheng, L.; Hao, Y.; Cobden, D.; Kim, J.; Ruoff, R. S.; Wallace, R. M.; Cho, K. Metal Contacts on Physical Vapor Deposited Monolayer MoS2. ACS Nano 2013, 7, 11350–11357.

(51)

Gong, C.; Colombo, L.; Wallace, R. M.; Cho, K. The Unusual Mechanism of Partial Fermi Level Pinning at Metal-MoS2 Interfaces The Unusual Mechanism of Partial Fermi Level Pinning at Metal-MoS2 Interfaces. Nano Lett. 2014, 14, 1714–1720.

(52)

Zhong, H.; Quhe, R.; Wang, Y.; Ni, Z.; Ye, M.; Song, Z.; Pan, Y.; Yang, J.; Yang, L.; Lei, M. Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations. Sci. Rep. 2016, 6, 21786.

(53)

Pan, Y.; Wang, Y.; Ye, M.; Quhe, R.; Zhong, H.; Song, Z.; Peng, X.; Yu, D.; Yang, J.; Shi, J. Monolayer Phosphorene–Metal Contacts. Chem. Mater. 2016, 28, 2100–2109.

(54)

Liu, Y.; Xiao, H.; Goddard III, W. A. Schottky-Barrier-Free Contacts with Two-Dimensional Semiconductors by Surface-Engineered MXenes. J. Am. Chem. Soc. 2016, 138, 15853–15856.

(55)

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.

(56)

Das, S.; Chen, H. Y.; Penumatcha, A. V.; Appenzeller, J. High Performance Multilayer MoS2 Transistors with Scandium Contacts. Nano Lett. 2013, 13, 100–105.


Langmuir 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

(57)

Page 24 of 24

Sze, S. M. Semiconductor Devices: Physics and Technology; John Wiley & Sons: New York, 2008.

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