One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{M}{\mathrm{o}}_{2}\mathrm{C}$

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One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting $α − M o_{2} C$

Jia-Bin Qiao, Yue Gong, Wei-Jie Zuo, Yi-Cong Wei, Dong-Lin Ma, Hong Yang, Ning Yang, Kai-Yao Qiao, Jin-An Shi, Lin Gu, and Lin He

Phys. Rev. B 95, 201403(R) – Published 8 May 2017

Abstract

Assembling different two-dimensional (2D) crystals, covering a very broad range of properties, into van der Waals (vdW) heterostructures enables unprecedented possibilities for combining the best of different ingredients in one objective material. So far, metallic, semiconducting, and insulating 2D crystals have been used successfully in making functional vdW heterostructures with properties by design. Here, we expand 2D superconducting crystals as a building block of vdW hererostructures. One-step growth of large-scale high-quality vdW heterostructures of graphene and 2D superconducting $α − M o_{2} C$ by using chemical vapor deposition is reported. The superconductivity and its 2D nature of the heterostructures are characterized by our scanning tunneling microscopy measurements. This adds 2D superconductivity, the most attractive property of condensed matter physics, to vdW heterostructures.

Received 21 March 2017

DOI:https://doi.org/10.1103/PhysRevB.95.201403

Physics Subject Headings (PhySH)

2-dimensional systems Graphene Van der Waals systems

Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jia-Bin Qiao¹, Yue Gong², Wei-Jie Zuo¹, Yi-Cong Wei¹, Dong-Lin Ma¹, Hong Yang¹, Ning Yang¹, Kai-Yao Qiao¹, Jin-An Shi², Lin Gu^2,3,4, and Lin He^1,*

¹Center for Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing 100875, China
²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
⁴School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China

^*Corresponding author: helin@bnu.edu.cn

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Vol. 95, Iss. 20 — 15 May 2017

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Figure 1
Schematic of synthesis and optical characterizations of graphene/ $α − M o_{2} C$ heterostructures. (a) Schematic illustration of the growth process for the heterostructures. A Cu foil is supported by a Mo substrate, and then Mo atoms start to diffuse across the liquid Cu at high temperatures with the flow of $H_{2}$ . With a low ratio of $C H_{4}$ to $H_{2}$ , C atoms are predominantly captured by Mo atoms (marked by luminous Mo and C atoms) and react into $α − M o_{2} C$ crystals. With a high ratio of $C H_{4}$ to $H_{2}$ , we directly obtained graphene/ $α − M o_{2} C$ heterostructures. (b) Optical image of graphene/ $α − M o_{2} C$ heterostructures on a Cu (Cu-Mo alloy)/Mo substrate. Scale bar: $50 μ m$ . Optical images exhibit as-grown [(c) and (d)] and transferred [(e) and (f)] regular polygon-shaped heterostructures on Cu and $Si O_{2} / Si$ targets, respectively. All the scale bars are 5 μm. (g) Raman spectra for the transferred graphene sheets and graphene/ $α − M o_{2} C$ heterostructures on $Si O_{2} / Si$ targets. The green squares denote the Raman signals of $α − M o_{2} C$ , while the peak at low wave number $(\sim 970 c m^{- 1})$ in the graphene spectrum arises from the $Si O_{2} / Si$ substrate.
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Figure 2
Structural characterization of graphene/ $α − M o_{2} C$ heterostructures. (a) Low-magnification HAADF-STEM image of the hexagonal graphene/ $α − M o_{2} C$ heterostructures. Scale bar: $1 μ m$ . (b) SAED pattern along the [100] zone axis, where some blue (green) spots belonging to graphene ( $α − M o_{2} C$ ) are circled. Scale bar: $5 n m^{- 1}$ . The estimated lattice parameters of graphene and $α − M o_{2} C$ are $a_{0} \sim 0.24 nm$ and $b \sim 0.603 nm$ , $c \sim 0.523 nm$ , respectively. The twist angle between graphene and $α − M o_{2} C$ is extracted as $\sim 11^{\circ}$ . Atomic-resolved HAADF-STEM images of the heterostructure surface parallel to the (100) facet (c) and cross section parallel to the (010) facet (d). This confirms the nature of the underlying $α − M o_{2} C$ crystals, where the center C atom (gray spot, marked by the green ball) is surrounded by six Mo atoms (white spots, marked by the purple balls) in a hcp arrangement in the (100) face and the Mo layers are arranged in ABAB-stacking form in the (010) face [insets in (c) and (d)], respectively. Both the scale bars are 1 nm.
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Figure 3
Surface topography and superconducting gap of graphene/ $α − M o_{2} C$ heterostructures. (a) Large-scale STM image of the heterostructures $(V_{b} = 1 V, I_{S} = 0.1 nA)$ . Scale bar: 5 nm. (b) Topographic closeup of the patterned surface ( $V_{b} = 500 mV$ , $I_{S} = 0.1 nA$ ). Scale bar: 1 nm. (c) Fast Fourier transform (FFT) image of the topography in (b). The outer six spots (marked by blue circles) and an inner pair of spots (marked by green circles) are the reciprocal lattice of graphene and $α − M o_{2} C$ , respectively. The derived lattice parameter of graphene is $a_{0} \sim 0.24 nm$ . The distance between the inner pair of spots is consistent with the crystalline interplanar spacing between the (002) and $(00 \bar{2})$ faces of $α − M o_{2} C$ crystal $(\sim 7.68 n m^{- 1})$ , and one of the lattice constants is estimated as $c \sim 0.52 nm$ . The twist angle between graphene and $α − M o_{2} C$ is measured as $\sim 14^{\circ}$ . Scale bar: $3 n m^{- 1}$ . (d) STS spectra recorded in the patterned regions of the heterostructures at different temperatures. (e) A zooming-in $d I / d V$ spectrum taken in the same region at 400 mK showing the superconducting gap (indicated by red dots). The black solid line is the theoretical BCS fitting with $Δ = 1.58 meV$ and $Γ = 0.4 meV$ .
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Figure 4
Superconducting properties of the heterostructures. (a) Temperature evolution of superconducting tunneling spectra. The curves are offset on the $y$ axis for clarity. a.u.: arbitrary units. (b) Normalized tunneling spectra as a function of applied magnetic fields. The curves are also offset on the $y$ axis for clarity. (c) Temperature dependence of the superconducting gap. The measured $Δ (T)$ is determined from a BCS fit to the tunneling spectra, and the light blue line is a theoretical result using the BCS gap function. (d) Magnetic field dependence of the superconducting gap. The measured $Δ (T)$ is deduced from fitting the spectra in (b) to the BCS density of states, consistent with the relationship $Δ (H) / Δ (0) = \sqrt{1 - H / H_{c}}$ . (e) Different superconducting transition temperatures and magnetic fields determined from the STS measurements. Red dots denote the variable $T_{c}$ in the range of 2–4 K, while green and blue dots indicate the critical fields $μ_{0} H_{c 2} (T)$ , where $μ_{0} H_{c 2}$ (0.4 K) varies from around 80 to 200 mT.
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Physical Review B

covering condensed matter and materials physics

One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting $α − M o_{2} C$

Jia-Bin Qiao, Yue Gong, Wei-Jie Zuo, Yi-Cong Wei, Dong-Lin Ma, Hong Yang, Ning Yang, Kai-Yao Qiao, Jin-An Shi, Lin Gu, and Lin He

Phys. Rev. B 95, 201403(R) – Published 8 May 2017

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Physical Review B

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One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting α−Mo2C

Jia-Bin Qiao, Yue Gong, Wei-Jie Zuo, Yi-Cong Wei, Dong-Lin Ma, Hong Yang, Ning Yang, Kai-Yao Qiao, Jin-An Shi, Lin Gu, and Lin He

Phys. Rev. B 95, 201403(R) – Published 8 May 2017

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One-step synthesis of van der Waals heterostructures of graphene and two-dimensional superconducting $α − M o_{2} C$