Robert S. Weatherup

In Situ Characterization of Alloy Catalysts for Low-Temperature Graphene Growth

Low-temperature (∼450 °C), scalable chemical vapor deposition of predominantly monolayer (74%) graphene films with an average D/G peak ratio of 0.24 and domain sizes in excess of 220 μm(2) is demonstrated via the design of alloy catalysts. The admixture of Au to polycrystalline Ni allows a controlled decrease in graphene nucleation density, highlighting the role of step edges. In situ, time-, and depth-resolved X-ray photoelectron spectroscopy and X-ray diffraction reveal the role of subsurface C species and allow a coherent model for graphene formation to be devised.

Observing Graphene Grow: Catalyst-Graphene Interactions during Scalable Graphene Growth on Polycrystalline Copper

Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration.

Kinetic Control of Catalytic CVD for High-Quality Graphene at Low Temperatures

Low-temperature (∼600 °C), scalable chemical vapor deposition of high-quality, uniform monolayer graphene is demonstrated with a mapped Raman 2D/G ratio of >3.2, D/G ratio ≤0.08, and carrier mobilities of ≥3000 cm(2) V(-1) s(-1) on SiO(2) support. A kinetic growth model for graphene CVD based on flux balances is established, which is well supported by a systematic study of Ni-based polycrystalline catalysts. A finite carbon solubility of the catalyst is thereby a key advantage, as it allows the catalyst bulk to act as a mediating carbon sink while optimized graphene growth occurs by only locally saturating the catalyst surface with carbon. This also enables a route to the controlled formation of Bernal stacked bi- and few-layered graphene. The model is relevant to all catalyst materials and can readily serve as a general process rationale for optimized graphene CVD.

In Situ Observations of the Atomistic Mechanisms of Ni Catalyzed Low Temperature Graphene Growth

The key atomistic mechanisms of graphene formation on Ni for technologically relevant hydrocarbon exposures below 600 °C are directly revealed via complementary in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. For clean Ni(111) below 500 °C, two different surface carbide (Ni2C) conversion mechanisms are dominant which both yield epitaxial graphene, whereas above 500 °C, graphene predominantly grows directly on Ni(111) via replacement mechanisms leading to embedded epitaxial and/or rotated graphene domains. Upon cooling, additional carbon structures form exclusively underneath rotated graphene domains. The dominant graphene growth mechanism also critically depends on the near-surface carbon concentration and hence is intimately linked to the full history of the catalyst and all possible sources of contamination. The detailed XPS fingerprinting of these processes allows a direct link to high pressure XPS measurements of a wide range of growth conditions, including polycrystalline Ni catalysts and recipes commonly used in industrial reactors for graphene and carbon nanotube CVD. This enables an unambiguous and consistent interpretation of prior literature and an assessment of how the quality/structure of as-grown carbon nanostructures relates to the growth modes.

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

Graphene-Passivated Nickel as an Oxidation-Resistant Electrode for Spintronics

We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapor deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrate that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magneto-resistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.

Introducing Carbon Diffusion Barriers for Uniform, High-Quality Graphene Growth from Solid Sources

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.

On the Mechanisms of Ni-Catalysed Graphene Chemical Vapour Deposition

The development of a scalable, economical production technique for monoand few-layer graphene (M-/FLG) is a key requirement to exploit its unique properties for applications. Catalytic chemical vapour deposition (CVD) has emerged as one of the most promising and versatile methods for M-/FLG growth. The generic principle of catalytic, rather than pyrolytic, CVD is to expose a catalyst template to a gaseous precursor at temperatures/conditions for which the precursor preferentially dissociates on the catalyst. Hence, the catalyst is key to M-/FLG formation, in particular its role in precursor dissociation, C dissolution, M-/FLG nucleation and domain growth/merging. Although the structure of as-formed graphitic layers on crystalline transition metal surfaces under ultra-high vacuum conditions has been extensively studied in surface science, a central question remains: what M-/FLG quality can be achieved with CVD, in particular, if for cost effectiveness sacrificial polycrystalline metal films/foils and less stringent vacuum/CVD process conditions are used. There have been numerous recent reports of large area M-/FLG CVD on for instance poly-crystalline Ni and Cu, including integrated roll-to-roll processing. However, there is currently very limited understanding of the detailed growth mechanisms, and the mostly empirical process calibrations provide little fundamental insight in to how the process and M-/FLG quality/domain size can be optimised. Herein, we study M-/FLG CVD by complementary in situ probing under realistic process conditions with the aim of revealing the key growth mechanisms. We focus on poly-crystalline Ni films and simple one-step hydrocarbon exposure conditions. However, as highlighted by Figure 1, even for such seemingly simple CVD conditions, the parameter space is manifold which leads to ambiguity in the interpretation of post-growth process characterisation and motivates our in situ approach. For catalyst metals with a high C solubility, such as Ni, current literature typically assumes C precipitation upon cooling as the main growth process. 8] M-/FLG precipitation has been studied in detail for slow, near thermodynamic equilibrium thermal cycling of C doped crystals. 9] For CVD, however, the conditions are distinctly different (Figure 1): an isothermal C precursor exposure phase, which represents a variation in composition rather than temperature, is followed by a typically fast cooling or thermal quenching. Hence kinetic aspects are important. Additionally, competing processes might influence the growth outcome such as etching of M-/FLG in a reactive atmosphere, for example, hydrogen or water, during the CVD process. By combining in situ, timeand depth-resolved X-ray photoelectron spectroscopy (XPS) and in situ X-ray diffraction (XRD), we can clearly show that M-/FLG growth occurs during isothermal hydrocarbon exposure and is not limited to a precipitation process upon cooling. While the fraction of M-/FLG due to isothermal growth and precipitation upon cooling strongly depends on process conditions, we show that the former is dominant for the low-temperature CVD conditions used. We find that M-/FLG nucleation is preceded by an increase in (subsurface) dissolved C with the formation of a solid solution of C in the Ni film, which indicates that graphene CVD is not a purely surface process. We discuss our data here in the context of simple considerations of C solubility and diffusivity as well as rate equations of the basic contributing processes, in order to establish a framework to guide future improvements in graphene CVD by a more fundamental understanding. We perform in situ XPS during low-pressure CVD of M-/FLG from hydrocarbon precursors on Ni(550 nm) films. Figure 2A Figure 1. Illustrative processing profile for a simple one-step hydrocarbon exposure consisting of four major phases: catalyst pretreatment, C dissolution into the catalyst during initial precursor exposure, isothermal M-/FLG growth with continued precursor exposure, M-/FLG growth by precipitation upon cooling. The key catalyst and M-/FLG properties that may be defined at each phase of growth are also listed.

Substrate-assisted nucleation of ultra-thin dielectric layers on graphene by atomic layer deposition

We report on a large improvement in the wetting of Al2O3 thin films grown by un-seeded atomic layer deposition on monolayer graphene, without creating point defects. This enhanced wetting is achieved by greatly increasing the nucleation density through the use of polar traps induced on the graphene surface by an underlying metallic substrate. The resulting Al2O3/graphene stack is then transferred to SiO2 by standard methods.

Sub-nanometer Atomic Layer Deposition for Spintronics in Magnetic Tunnel Junctions Based on Graphene Spin-Filtering Membranes

We report on the successful integration of low-cost, conformal, and versatile atomic layer deposited (ALD) dielectric in Ni–Al2O3–Co magnetic tunnel junctions (MTJs) where the Ni is coated with a spin-filtering graphene membrane. The ALD tunnel barriers, as thin as 0.6 nm, are grown layer-by-layer in a simple, low-vacuum, ozone-based process, which yields high-quality electron-transport barriers as revealed by tunneling characterization. Even under these relaxed conditions, including air exposure of the interfaces, a significant tunnel magnetoresistance is measured highlighting the robustness of the process. The spin-filtering effect of graphene is enhanced, leading to an almost fully inversed spin polarization for the Ni electrode of −42%. This unlocks the potential of ALD for spintronics with conformal, layer-by-layer control of tunnel barriers in magnetic tunnel junctions toward low-cost fabrication and down-scaling of tunnel resistances.