Camilla Coletti: CVD synthesis of high-mobility seeded graphene and toilered van der Waals heterostacks for optoelectronic and spintronic applications

Although obtained with a cost effective approach, CVD graphene on Cu was up to recently not preferred for electronic and photonic applications due to its polycrystalline nature and consequently impoverished electronic performances. In the last couple of years significant steps forwards have been made and a number of groups have reported on the synthesis of mm-sized single-crystal graphene on Cu. However, long growth processes are typically needed for the synthesis of grains with lateral size of 1 mm. Recently, we showed that single crystals of graphene with dimensions up to 3.5 mm can be obtained in just 3 hours using a commercially available cold-wall CVD reactor [1]. For the synthesized graphene single-crystals, we have measured remarkable transport properties, even on non-ideal Si/SiO2 substrates, such as mobilities of 12000 cm2/Vs and up to 12 well-defined Landau levels [2]. Furthermore, during this talk a novel CVD approach – namely seeded growth - with promising prospects for the wafer scale integration of high-mobility graphene will be discussed. By using pre-patterned Cu substrates with chromium nucleation sites, arrays of graphene single-crystals as large as several hundred microns with a periodicity of up to 1 mm can be obtained. The graphene is transferred to target substrates using aligned and contamination-free semi-dry transfer. The high quality of the synthesized graphene is confirmed by Raman spectroscopy and transport measurements, demonstrating room-temperature carrier mobility of 21 000 cm2 / V s when transferred on top of hexagonal boron nitride [3]. By tailoring the nucleation of large single-crystals according to the desired device geometry, it will be possible to produce complex device architectures based on single-crystal graphene, thus paving the way to the adoption of CVD graphene in wafer-scale fabrication. CVD can be also successfully used as a scalable technique to synthetize highly-crystalline van der Waals heterostructures. By exhibiting a measurable bandgap and exotic valley physics, atomically-thick tungsten disulfide (WS2) offers exciting prospects for optoelectronic and spintronic applications. The synthesis of continuous WS2 films on other two-dimensional (2D) materials would greatly facilitate the implementation of novel all-2D photoactive devices. During this talk the scalable growth of WS2 on graphene and h-BN via a CVD approach will be demonstrated [4]. Detailed spectroscopic and microscopic analysis will be presented revealing that the synthesized films have an epitaxial relation to the substrate. Also, angle resolved photoemission spectroscopy (ARPES) data for the heterostack WS2/graphene will be discussed. Remarkable room temperature conservation of polarization – peaking at 74% for bilayer WS2 films – will be reported. The scalable synthesis and design on 2D substrates of WS2 films with outstanding optical properties is instrumental in the development of novel all-2D quantum optoelectronic and valleytronic devices.