Introducing carbon diffusion barriers for uniform, high-quality graphene growth from solid sources

Abstract

Carbon diffusion barriers are introduced as a general and simple method to prevent premature carbon dissolution and thereby to significantly improve graphene formation from the catalytic transformation of solid carbon sources. A thin Al2O3 barrier inserted into an amorphous-C/Ni bilayer stack is demonstrated to enable growth of uniform monolayer graphene at 600 °C with domain sizes exceeding 50 μm, and an average Raman D/G ratio of <0.07. A detailed growth rationale is established via in situ measurements, relevant to solid-state growth of a wide range of layered materials, as well as layer-by-layer control in these systems.

Figure 1

SEM micrographs of Ni(550 nm)/ta-C(10 nm) (A–C) and Ni(550 nm)/Al₂O₃(2 nm)/ta-C(10 nm) (D–F) annealed at 200 °C (A,D), 300 °C (B), 500 °C (E), 600 °C (C,F) for 5 min (heated and cooled at a fixed rate of 100 °C min^–1). The inset of D shows a higher-magnification micrograph of the sample showing the Ni grain structure (scale bar is 200 nm). The insets of E,F show lower-magnification micrographs of the same samples (scale bars are 100 μm). Sketches indicating the effect of annealing for each of the samples are also shown.

Figure 2

(A) Raman spectra of the M-/FLG grown from Ni(550 nm)/ta-C(10 nm) (corresponding to Figure 1C), Ni(550 nm)/Al₂O₃(2 nm)/ta-C(10 nm) (corresponding to Figure 1F), and Au(5 nm)/Ni(550 nm)/Al₂O₃(2 nm)/ta-C(10 nm) samples annealed for 5 min at ∼600 °C and subsequently transferred to Si/SiO₂(300 nm) using the bubbling transfer method. (B) Optical micrograph of the as-transferred MLG grown from a Au(5 nm)/Ni(550 nm)/Al₂O₃(2 nm)/ta-C(10 nm) sample under the annealing conditions used in (A). The sheet resistance (R_S) of the as-transferred graphene is ∼1 kΩ/□, measured using six contact Hall-geometry devices (see Methods). (C,D) Raman maps of 2D/G peak intensity (average 2D/G ratio of ∼3.5 with 100% of the area >2 ) (C) and D/G peak intensity (average D/G ratio of <0.07) (D) for the region of graphene corresponding to the optical micrograph in (B).

Figure 3

(A,B) In situ XRR curves of a Ni(70 nm)/ta-C(10 nm)/SiO₂(300 nm)/Si sample taken during vacuum heating (base pressure ∼10^–6 mbar) for reflecting angles (2θ) of 0.5–0.9° (A) and 2–4.0° (B). The dashed horizontal arrow in A indicates the shift in total reflective angle on heating, while the vertical dashed lines in A and B indicate the oscillations associated with the Ni and ta-C layers respectively. (C) In situ grazing incidence XRD of a Ni(70 nm)/ta-C(10 nm)/SiO₂(300 nm)/Si sample taken during the same stepwise annealing process with a fixed incident angle of α_i = 0.75° (information depth of ∼80 nm). Note that the temperature-dependent shift in the reflection angles is due to thermal expansion. A monochromatic X-ray beam of 11.5 keV and a wavelength of 1.07812 Å (selected by a Si(111) double crystal monochromator) is used, and the reflected/diffracted X-rays are measured using a Mythen detector system. (D) Sketch showing the Ni/ta-C stacks that were probed and indicating the diffusion of carbon to the exposed catalyst surface which leads to M-/FLG formation.

Figure 4

(A) Time-resolved in situ XPS C1s core level lines for Ni(550 nm)/ta-C(10 nm) stacks during vacuum heating to ∼600 °C at ∼100 °C/min. Acquisition times are relative to the start of the heating ramp from room temperature. Spectra are collected in normal emission geometry at photon energies of 435 eV (surface sensitive; λ_escape ≈ 7 Å) with a spectral resolution of ∼0.3 eV. (B) Depth-resolved in situ XPS Ni2p_3/2 core level lines for Ni(550 nm)/ta-C(10 nm) stacks at the end of vacuum annealing at ∼600 °C. Spectra are background corrected (Shirley) and collected in normal emission geometry at photon energies of 1010 eV (surface sensitive; λ_escape ≈ 7 Å) and 1300 eV (bulk sensitive; λ_escape ≈ 10 Å) with a spectral resolution of ∼0.3 eV. Increased information depth is achieved using higher-incident X-ray energies and hence increased electron mean free path lengths. The spectra are fitted using Doniach-Šùnjić functions convoluted with Gaussian profiles with an accuracy of ∼0.05 eV. All binding energies are referenced to the Fermi edge.

Figure 5

Scanning electron micrographs of Ni(550 nm)/diamond(∼100 nm) (A) and Ni(550 nm)/HOPG (B) annealed at ∼600 °C for 5 min in vacuum (heated at a fixed rate of 100 °C min^–1, cooled at ∼300 °C min^–1). All scalebars are 2 μm. Insets show the corresponding Raman spectra measured on the as-grown samples. Sketches indicating the effect of annealing on each of the samples are also shown.