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I. 2D Materials for Advanced Devices: Direct Growth by Scalable Methods

A central problem in the development of devices based on 2 dimensional materials is the controlled direct growth of such films on suitable substrates by industrially practical and scalable methods. Our focus is on the direct growth of graphene, hexagonal BN (h-BN) and graphene/h-BN heterostructures on device-suitable substrates for spintronic and advanced electronic applications. Experiments with transition metal dichalcogenides are in progress. Current collaborations involve investigators at UNL, UMN, U Wisc., Caltech, SUNY-Buffalo and elsewhere. Recent highlights include:

Ø Fig. 1: Growth of graphene on Co3O4(111)/Co(0001): (a) LEED image and (b) line scan showing growth of few layer azimuthally oriented graphene on Co3O4(111) by MBE. The oxide/graphene interface is incommensurate. (c) MOKE data showing superparamagnetic behavior for the graphene/oxide/Co stack to at least 400 K. (MOKE data acquired at UNL). See Y. Wang, et al., J. Phys.: Cond. Matt. 25 (2013) 472203 The layer-by-layer growth of high quality (domain size > 1800 Å) graphene on Co₃O₄(111)/Co(0001) by molecular beam epitaxy (MBE) [1]. This results in significant grapheneàoxide charge transfer and synergistic magnetic properties for the graphene/oxide/Co stack: superparamagnetism to > 400 K [2]. Such phenomena (Fig. 1) are consistent with substrate spin-induced polarization of graphene charge carriers, with potential device implications.

Ø Fig. 2: Growth of h-BN on Co(0001) by atomic layer epitaxy. (a) optical micrograph of BN domain growth for sub-monolayer on Co(0001); (b) BN film thickness vs. number of ALE cycles showing precise control over BN thickness The layer-by-layer growth of h-BN(0001) on Co(0001) by atomic layer epitaxy [3]. Co/BN/Co heterojunctions with 5-8 BN layers are of great interest for spin filters, with predicted tunneling magnetoresistance values >> 1000 %. We are currently producing such structures (Fig. 2) for transport measurements.

Ø Figure 3. (Left) 60 eV LEED image of 3 ML graphene (MBE) on 3 ML h-BN(0001) (ALE) on Co(0001) deposited on a sapphire substrate. The direct growth of graphene by MBE on h-BN(0001). Fig 3. (left) shows a LEED image of 3 ML graphene on 3 ML h-BN(0001) on Co(0001). The 60 eV LEED data indicate that the Co, BN, and graphene lattices are in precise azimuthal registry—a critical factor for advanced spin and electronic tunneling devices. Fig. 3 (right) shows XPS data indicating that the hexagonal C layer in the LEED exhibits the expected asymmetric C 1s peak and associated πàπ* shakeup feature expected for graphene. The ability to directly grow graphene/BN heterostructures with varied graphene and BN layer thicknesses opens the way to a variety of spintronic and ultrahigh frequency electronic devices based on such heterostructures. Such heterostructures are now being produced for nanoscale-transport measurements.

Current work is focused on the fabrication of BN and graphene/BN heterostructures for device transport measurements, and the expansion of these growth efforts to other oxide substrates.

REFERENCES

[1] M. Zhou, F.L. Pasquale, P.A. Dowben, A. Boosalis, M. Schubert, V. Darakchieva, R. Yakimova, J.A. Kelber. Direct graphene growth on Co3O4(111) by molecular beam epitaxy. J Phys: Cond Matt. 24 (2012) 072201.

[2] Y. Wang, L. Kong, F.L. Pasquale, Y. Cao, B. Dong, I. Tanabe, C. Binek, P.A. Dowben, J.A. Kelber. Graphene mediated domain formation in exchange coupled graphene/Co₃O₄(111)/Co(0001) trilayers. J Phys : Cond Matt. 25 (2013) 472203.

[3] J. Beatty, Y. Cao, I. Tanabe, M. S. Driver, P. A. Dowben and J. A. Kelber, Atomic layer-by-layer depostion of h-BN(0001) on cobalt: a building block for spintronics and graphene electronics Mat. Res. Express 1 (2014) 046410