Electronic Properties of Graphene–PtSe2 Contacts - ACS Applied

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Electronic Properties of Graphene−PtSe2 Contacts Shahid Sattar and Udo Schwingenschlögl* Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ABSTRACT: In this article, we study the electronic properties of graphene in contact with monolayer and bilayer PtSe2 using first-principles calculations. It turns out that there is no charge transfer between the components because of the weak van der Waals interaction. We calculate the work functions of monolayer and bilayer PtSe2 and analyze the band bending at the contact with graphene. The formation of an n-type Schottky contact with monolayer PtSe2 and a p-type Schottky contact with bilayer PtSe2 is demonstrated. The Schottky barrier height is very low in the bilayer case and can be reduced to zero by 0.8% biaxial tensile strain.

KEYWORDS: graphene, platinum diselenide, contact, band bending, heterostructure

I. INTRODUCTION Van der Waals heterostructures consisting of two-dimensional (2D) materials have opened up a new paradigm to design and manipulate nanoelectronic devices.1 The possibility of layer-bylayer assembly avoids the constraints of lattice matching and processing compatibility in standard growth processes, which can be exploited to fabricate field-effect tunneling transistors,2 photodetectors,3 and optoelectronic devices,4 for example. Because of the absence of a band gap,5 graphene has been used as metal contact that offers low contact resistance,6 superior mechanical properties,7 and the possibility to modulate the work function by impurity doping to achieve Ohmic contacts8 (which, e.g., are desirable for transistor operation9). Only recently, graphene electrodes have been employed successfully to inject electrons and holes into monolayers of transitionmetal dichalcogenides (TMDCs) to construct light-emitting diodes.10 TMDCs (MoS2 being investigated most frequently) are viable materials for low-power digital electronics because of their considerably large band gaps in the range of 1−2 eV.11−13 Although their room-temperature carrier mobilities are significantly lower than those in graphene,14,15 the electron mobility of monolayer PtSe2 has been predicted to be much higher than that in all other known TMDCs.16 It is, therefore, of great interest that recently high-quality monolayer PtSe2 samples have been fabricated by epitaxial growth on Pt substrate.17 The authors have shown that both monolayer and bilayer PtSe2 are semiconductors with indirect band gaps of 1.20 and 0.20 eV, respectively, whereas bulk PtSe2 is a semimetal.18 It has been demonstrated that the band gap can be tuned by uniaxial and biaxial tensile and compressive strains.19 The material is a promising photocatalyst for water splitting20 and a potential candidate for valleytronics.17 Furthermore, midinfrared photodetectors based on bilayer PtSe2 show short © XXXX American Chemical Society

response time and high responsivity under ambient conditions.21 To build electronic devices of monolayer or bilayer PtSe2, it is necessary to contact the material. The most natural 2D candidate for this purpose is graphene.22,23 As a consequence, it is important to establish insight into the electronic properties of graphene−PtSe2 contacts. Having in mind that the performance of monolayer MoS2 devices, for example, depends strongly on the properties of the metal−semiconductor contacts,24 both the band bending and Schottky barrier height (SBH) are critical quantities. We will examine these topics in the present study by addressing the following questions by means of first-principles calculations: (1) How does graphene interact with monolayer and bilayer PtSe2 compared with other TMDCs? (2) What are the contact characteristics of current-in-plane electronic devices?

II. COMPUTATIONAL METHOD We use density functional theory and the projector-augmented plane-wave method as implemented in the Vienna Ab-initio Simulation Package.25 For the exchange-correlation potential, the generalized gradient approximation is used in the Perdew− Burke−Ernzerhof parametrization and the van der Waals interaction is taken into account using the DFT-D3 method.26 The plane-wave cutoff energy is set to 500 eV, and a fine Monkhorst−Pack 15 × 15 × 1 k-mesh is used for the Brillouinzone integration (9 × 9 × 1 for the structural relaxation). We ensure an energy convergence of 10−6 eV in the iterative solution of the Kohn−Sham equations and a force convergence of 10−3 eV/Å for the structural relaxation. A 2 × 2 × 1 supercell Received: January 1, 2017 Accepted: April 18, 2017

A

DOI: 10.1021/acsami.7b00012 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of PtSe2 with 12 atoms and a 3 × 3 × 1 supercell of graphene with 18 atoms attached to each other result in a lattice mismatch of less than 1.3%, which is adequate for forming heterostructures. It turns out that relative shifts of the two supercells along the armchair and zigzag directions of graphene do not affect our conclusions. We use a 15 Å thick layer of vacuum on top of the heterostructures to exclude spurious interaction between periodic images in the out-of-plane direction, as three-dimensional periodic boundary conditions are applied.

III. RESULTS AND DISCUSSION Monolayer PtSe2 consists of Pt atoms sandwiched between Se atoms with Pt−Se bond lengths of 2.54 Å. The 1T structure has been predicted to be thermodynamically stable27 and is therefore used in the present study. We obtain optimized lattice constants of 3.72 and 3.75 Å for monolayer and bilayer PtSe2, respectively (in agreement with refs 28 and 29), and the electronic band structures in Figure 1 show band gaps of 1.20 Figure 2. (a) Top and (b) side views of graphene on monolayer PtSe2.

agreement with weak van der Waals interaction. The interaction can be quantified by calculating the binding energy per C atom E b = (E[graphene‐PtSe2] − E[graphene] − E[PtSe2])/18 (1)

where E[graphene−PtSe2] is the total energy of the heterostructure, E[graphene] is the total energy of isolated graphene, and E[PtSe2] is the total energy of isolated monolayer PtSe2. We obtain Eb = −48 meV, which is less than that reported for the graphene−phosphorene heterostructure (Eb = −60 meV31) but almost doubled that of the heterostructures of graphene and other TMDCs (Eb = −21 meV in the case of the 2H phase of MoSe2, for example, ref 32), indicating that graphene binds to monolayer PtSe2 unexpectedly strong. However, the interaction is still of van der Waals type, and no charge transfer (∼10−4 electrons per C atom, according to Bader charge analysis) is observed between the components. Strong van der Waals interaction turns out to be characteristic of PtSe2 and can be explained by its 1T structure. For graphene on ZrS2, which also realizes a 1T structure, Eb = −92 meV has been reported in ref 33. The band structure of graphene on monolayer PtSe2 in Figure 3a,b consists of the largely preserved band structures of the components. We observe a band gap of 4 meV at the Γpoint (which corresponds to the K-point of graphene due to Brillouin-zone folding). This value is larger than that reported for graphene on monolayer MoS2 and WS2 (1−2 meV34) due to the higher binding energy discussed above. Graphene on bilayer PtSe2 (Figure 3c,d) behaves similarly (the band structure being essentially a merger of the individual band structures); however, the band gap is enhanced to 10 meV because Eb = −52 meV and the distance between the components is reduced to 3.40 Å. Because the obtained band gaps are still small compared with the thermal fluctuations at room temperature, it is anticipated that graphene will behave as metal when placed on monolayer and bilayer PtSe2. For a current-in-plane electronic device with monolayer or bilayer PtSe2 as semiconducting channel material and graphene as metal contact, the band offset, band bending, and SBH are key quantities. We first calculate the work functions of the components (Figure 4) as the difference between the vacuum

Figure 1. Band structures of (a, b) monolayer and (c, d) bilayer PtSe2. The spin−orbit coupling is neglected on the left-hand side and taken into account on the right-hand side.

and 0.21 eV, incorporating spin−orbit coupling (in agreement with ref 17). We note that the correct band gap of bilayer PtSe2 can be obtained only by taking into account the van der Waals interaction.30 The sharp decrease in the band gap from the monolayer to the bilayer reflects the interlayer interaction and continues to show a semimetallic behavior for the trilayer and beyond.17 Figure 2 shows the top and side views of the minimumenergy structure of graphene on monolayer PtSe2. Besides relaxing the atomic positions, we have varied the lateral position between the components along the armchair and zigzag directions of graphene, which results in an energy variation of only 5 meV and therefore indicates homogeneous interaction. We obtain a distance of 3.44 Å between the components, in B

DOI: 10.1021/acsami.7b00012 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

We next calculate the band bending (difference of Fermi energies) from the work functions: W[graphene−PtSe2] − W[PtSe2]. By definition, electrons will flow from PtSe2 to graphene when the band bending is positive (p-doping of the channel material) and in the other direction when it is negative (n-doping of the channel material). The band bending is found to be negative for monolayer PtSe2 (−0.38) and positive for bilayer PtSe2 (0.06), which implies that the conduction at the contact occurs via electrons and holes, respectively. The absence of chemical bonds and charge transfer between the components of the heterostructures under investigation allows us to apply the Schottky−Mott model to determine the SBH.36 For the n-type semiconductor, the SBH is given by ϕn = Ec[PtSe2] − W [graphene‐PtSe2]

(2)

and for the p-type semiconductor by ϕp = W [graphene‐PtSe2] − Ev [PtSe2]

(3)

where Ev and Ec refer to the PtSe2-dominated bands of the heterostructure. According to Figure 4, the Dirac point of graphene is close to Ec of the heterostructure with monolayer PtSe2, representing an n-type Schottky contact with ϕn = 0.35 eV. This SBH is about half the value recently reported for the PtSe2/n-Si contact (0.71 eV) and therefore a key improvement from a technological perspective.37 On the other hand, for the heterostructure with bilayer PtSe2, the Dirac point of graphene is close to Ev so that the SBH is very low, ϕp = 0.04 eV. We have also studied whether the SBH can be modified by strain. Figure 5 shows the

Figure 3. (Weighted) electronic band structures of graphene on (a, b) monolayer and (c, d) bilayer PtSe2.

Figure 5. SBH at the contact between graphene and bilayer PtSe2 under (biaxial) tensile strain.

results and demonstrates a strong dependence of ϕp on applied (biaxial) tensile strain. We find that the contact becomes Ohmic (as desired for current-in-plane devices) already for a small strain of around 0.8%, which can be easily sustained by both graphene and bilayer PtSe2. Figure 4. Energy-band diagrams for monolayer and bilayer PtSe2 as well as for graphene on monolayer or bilayer PtSe2.

IV. CONCLUSIONS In conclusion, we have investigated the possibility of contacting monolayer and bilayer PtSe2 with graphene. To this aim, we have calculated the band structures and work functions of monolayer and bilayer PtSe2 isolated and in a heterostructure with graphene. Structure optimizations result in large interlayer distances due to weak van der Waals interaction between the components (essentially no chemical bonding). The band structures of the components are found to be largely preserved in the heterostructures, and no charge transfer is observed. Analysis of the work functions shows that the band bending is negative (positive) for graphene in contact with monolayer (bilayer) PtSe2 so that the conduction occurs via electrons (holes). Graphene is found to form an n-type Schottky contact

and Fermi energies, Evac − EF (the Fermi energy being defined as the average of the valence-band and conduction-band edges). For monolayer (bilayer) PtSe2, we obtain a work function of 4.89 eV (4.62 eV). In addition, the valence-band maximum, Ev, is found at an energy of −5.49 eV (−4.72 eV), with respect to the vacuum level, and the conduction-band minimum, Ec, is found at −4.29 eV (−4.51 eV). The work function of graphene turns out to be 4.25 eV, consistent with an earlier result.35 For the graphene−PtSe2 heterostructure, we obtain work functions of 4.51 and 4.68 eV in the monolayer and bilayer cases, respectively. C

DOI: 10.1021/acsami.7b00012 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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with monolayer PtSe2 (SBH, 0.35 eV) and a p-type Schottky contact with bilayer PtSe2 (SBH, 0.04 eV). In the bilayer case, the SBH can be easily tuned by applying biaxial tensile strain, the contact becoming Ohmic at a critical value of 0.8%. These findings suggest that graphene is ideal for contacting PtSe2 in electronic applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +966(0) 544700080. ORCID

Udo Schwingenschlögl: 0000-0003-4179-7231 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). Fruitful discussions with M. Sajjad and N. Singh are gratefully acknowledged. This publication was made possible by a National Priorities Research Program grant (NPRP 7-665-1-125) from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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DOI: 10.1021/acsami.7b00012 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX