Real-time multiscale monitoring and tailoring of graphene growth on liquid copper

15-07-2021

Complementary in-situ methods of synchrotron X-ray diffraction at beamline ID10, Raman spectroscopy, and optical microscopy have been used to monitor graphene chemical vapour deposition on liquid copper (at 1370 K at atmospheric pressure conditions), enabling the control of graphene growth at multiscale, including atomic structure, flake shape, and flake supra-organisation.

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Reproducible mass production of large, defect-free two-dimensional materials (2DMs) such as graphene is a major challenge for their industrial applications. Currently the state-of-the-art method for graphene production is chemical vapour deposition (CVD) on solid copper at elevated temperatures (~1270 K) [1]. As the nucleation of graphene happens at random places on the copper surface (often on defects such as grain boundaries, dislocations, roughness, etc.), this method results in an imperfect multi-domain layer, with domain sizes in the order of micrometres.

Recently, liquid metal catalysts (LMCats) such as molten copper have been employed for the fast growth of uniform hexagonal graphene and other 2DMs of significantly higher quality [2]. The quality of the 2DM is less affected by the catalyst surface structure as it grows on an atomically flat isotropic melt. Intense efforts have been made to optimise the CVD process on the liquid phase. However, the lack of precise, multiscale in-situ monitoring techniques enabling direct feedback on the growth parameters (including temperature, gas composition, and pressures), has led primarily to empirical recipes. Such recipes intrinsically suffer from a limited understanding of the graphene formation process and low reproducibility of the product due to the complex and stochastic nature of the growth phenomena.

 

Figure 1

Fig. 1: a) Configuration of in-situ monitoring methods applied to a graphene layer grown on liquid copper. b) An example of in-situ radiation-mode optical microscopy of self-organised hexagonal graphene flakes on liquid copper. c) One single-crystal flake growing to millimetre size [4].

 

Recent publications report the development [3] and successful implementation [4] of four in-situ techniques for multiscale monitoring of graphene growth on liquid copper at 1370 K and under atmospheric-pressure CVD conditions. The LMCat reactor, located at ID10, enables the tailoring of the crystal size, shape, and quality of graphene while optimising the growth speeds (Figure 1a). Radiation-mode optical microscopy provides essential information on growth morphology and dynamics in real time at macroscopic scales. In-situ Raman spectroscopy confirms the presence of monolayer graphene and yields information about its crystallinity and defects from mesoscopic to nano scales. At the atomic scale, the lattice constant and corrugation of graphene sheets floating on liquid copper are derived from the positions and angular spread of the Bragg rods measured by grazing-incidence X-ray diffraction. The number of graphene layers, its roughness, and the separation between graphene and liquid copper are provided by synchrotron X-ray reflectivity. Multi-scale simulations have been used to analyse and understand the obtained experimental results [4,5].

To demonstrate the wealth of information and control capability that can be achieved by multiscale in-situ monitoring, CVD growth processes for which the nucleation of graphene seeds is induced by an injection of a short pulse of methane at high partial pressure were investigated. This procedure produces many flakes that grow and form a super-ordered assembly due to short-range electrostatic and long-range capillary interactions (Figure 1b). Simulations have reproduced the observed assembly of flakes into a 2D hexagonal network on liquid copper. Such spontaneous ordering is not possible on a solid catalyst due to immobility of the flakes on a solid surface. Ultimately, flakes merge into a continuous film; however, slight misorientations of neighbouring flakes remain upon their coalescence, ultimately leaving domain boundaries where they have merged. Next, monitoring and feedback-control was used to improve the ordering of the flakes and reduce the remnant defects upon merging. Finally, the growth parameters were tailored to nucleate only a single flake and grow it to millimetre size (Figure 1c). The spectra obtained using X-ray scattering and Raman spectroscopy compare well to those of single-layer exfoliated graphene [4].

With a growth speed of about ~1 µm/s, a sheet resistance of 280 Ohms/sq, and superior crystalline quality, this process is practically viable and paves the way for single-crystal graphene production suitable for different electronic applications. The devised real-time monitoring/tailoring methodology can also be implemented for the scientific study or industrial production of other nanomaterials such as h-BN, GaN, or ultrathin oxide layers. Finally, 2DM growth on LMCats has the potential to open up a radically new 2DM production method: that of continuous production via direct separation of 2DMs from the liquid catalyst [6].

 

Principal publication and authors
Real-time multiscale monitoring and tailoring of graphene growth on liquid copper, M. Jankowski (a,b), M. Saedi (c), F. La Porta (b), A.C. Manikas (d), C. Tsakonas (d), J.S. Cingolani, M. Andersen (e), M. de Voogd (f), G.J.C. van Baarle (f), K. Reuter (e), C. Galiotis (d), G. Renaud (a), O.V. Konovalov (b), I.M.N. Groot (c), ACS Nano 15(6), 9638-9648 (2021); https://doi.org/10.1021/acsnano.0c10377
(a) Université Grenoble Alpes, CEA, IRIG/MEM/NRS, Grenoble (France)
(b) ESRF
(c) Leiden Institute of Chemistry, Leiden University, Leiden (The Netherlands)
(d) FORTH/ICE-HT and Department of Chemical Engineering, University of Patras (Greece)
(e) Chair for Theoretical Chemistry and Catalysis Research Center, Technische Universität München, Garching (Germany)
(f) Leiden Probe Microscopy (LPM), Leiden (The Netherlands)

References
[1] C. Mattevi et al., J. Mater. Chem. 21, 3324 (2011).
[2] C. Tsakonas et al., Nanoscale 13, 3346 (2021).
[3] M. Saedi et al., Rev. Sci. Instrum. 91, 013907 (2020).
[4] M. Jankowski et al., ACS Nano 15, 9638 (2021).
[5] J.S. Cingolani et al., J. Chem. Phys. 153, 074702 (2020).
[6] More information can be found on the project website: www.lmcat.eu