Wang Haomin

Chemical vapor deposition of graphene on insulating substrates and its potential applications

As the first two-dimensional crystal isolated in 2004, graphene has triggered numerous fundamental and technological studies because of its unique properties and a wide range of potential applications. To take these applications to an industrial level requires successful large scale growth of high quality graphene. Among popular methods for graphene synthesis, chemical vapor deposition (CVD) has received a lot of attention because of its relatively high yields, high quality and low cost in preparation of graphene. In addition, CVD is compatible with the existing silicon semiconductor processes, and exhibits potential for synthesis and applications of graphene at industrial scale. CVD graphene has been synthesized on various metal substrates such as ruthenium, iridium, platinum, nickel and copper. Though suitable for mass production, the need to transfer the graphene to different substrates has so far constrained its up-scaling to roll-to-roll production methods. However, CVD graphene on metal need transfer to insulating substrates for further device fabrication and characterization, and the extra transfer step inevitably causes the degradation of the graphene quality because of crack formation or resists residues. Researchers have developed some novel technologies aiming at reducing the influence of transfer process on the quality of graphene, but transfer process is still time-consuming and relatively expensive. An alternative to overcome the transfer difficulty is to synthesize graphene directly on insulating substrates. Epitaxial growth of graphene on single crystal SiC is one route towards mass production of graphene, however, single crystal SiC wafers are still very expensive. The search for better production techniques of graphene, in particular a transfer-free production method of high quality graphene, has intensified over the last few years. An amount of effective strategies have been utilized to realize the direct synthesis of high quality graphene on insulating substrates such as h-BN, silicon oxide, quartz, sapphire, SrTiO3 and even normal glass. Due to the lack of catalytic capability and carbon-dissolving ability in insulating substrates, the direct growth of graphene often associates with the problems of high nucleation density, small domain size, poor layer control and slow growth rate. As such, more research works should be conducted to explore the growth mechanism on the insulating substrates. In this paper, we reviewed recent progresses about direct growth of graphene on insulating substrates by chemical vapor deposition and its electronic applications. Firstly, we briefly discussed the newly progresses of graphene preparation and application. Great progress have been achieved in the field of direct synthesis of graphene on insulating substrates, however, there is still much room to improve further in many aspects, such as quality, domain size, layer and substrate suitability of graphene. We classified the growth strategies on insulating substrates into four groups: non-catalyst assisted method, plasma enhanced method, interface catalyzed growth and metal-vapor assisted growth. In each strategy, details of technology and merits were carefully discussed. In addition, we also introduced the typical applications of each strategy, which exhibits extraordinary prospect for improving our daily life. Finally, we shared our perspective views on future trend of this research field. We believe that with more and more efforts from the graphene-research community, technologies about direct synthesis of graphene on insulating substrates will be developed further. The future of directly grown graphene will become clearer in industrial applications.

Synthesis of multilayer hexagonal boron nitride on Cu-Ni alloy by chemical vapor deposition

Hexagonal boron nitride (h-BN) is isoelectronic to graphite with an equal number of boron (B) and nitrogen (N) atoms. For bulk h-BN, B and N atoms are bonded together by strong covalent bonds in plane while weak van der Waals force dominates the interaction in-between the layers. h-BN has attractive properties including high mechanical strength, high thermal conductivity, chemical inertness, and excellent electrical insulation. The unique properties of h-BN provide a potential for a wide range of applications as both a structural and electronic material. As h-BN is of a wide band gap, a ultra-smooth surface and a low density of surface charge states, it is always regarded as an ideal substrate for other two dimensional crystals, such as graphene and transition metal dichalcogenides, in electronic applications. Although monolayer single crystal of h-BN has been synthesized on Cu and Cu-Ni alloy foil by chemical vapor deposition (CVD), multilayer h-BN single-crystal has yet not been synthesized successfully. Normally the growth of h-BN by CVD method on metal surface shows an obvious self-limited behavior, and thus h-BN monolayer is always obtained. As a dielectric material for two dimensional semiconductors, the screening effect of monolayer h-BN is limited for electronics. As such, multilayer crystals of h-BN are required to overcome the inextricable difficulty. In this work, we demonstrate an approach to synthesize multilayer single crystal of h-BN on Cu-Ni alloy by using growth-etching-regrowth strategy. In the strategy, growth of the first layer of h-BN was suppressed when H2 flow is increased and the supply of B-N precursors is reduced. The exposed Cu-Ni alloy significantly improves the growth rate of multilayer h-BN in the next growth process. The morphology of multilayer h-BN samples was characterized by field-emission scanning electron microscope. It is found that multilayer domains of h-BN finally obtained are in triangle with each side up to ~20 μ m. X-ray photoemission spectroscopy (XPS) measurement was also done on the h-BN/Cu-Ni sample, the results show that the spectra of B 1s and N 1s were at 190.4 and 397.8 eV, respectively. Raman spectroscopy is used to understand the lattice structure of the h-BN. In the Raman spectrum, only the E 2g Raman peak at 1365 cm - 1 was observed, it indicates that the configuration for B and N atoms is B-N bonding, implying that the hexagonal phase exists in our BN films. The full width at half maximum (FWHM) of Raman peak is 18 cm - 1, which is less than the values in earlier literatures and comparable to the single-crystalline bulk h-BN fabricated by high temperature high pressure method. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) were further conducted to characterize the microstructure of the multilayer h-BN domains. Cross-sectional TEM image in a high magnification shows that the interlayer distance of multilayer h-BN is 0.33 nm, and the SAED results indicates that h-BN domains is well-stacked with and AA′ stacking order and each layer of h-BN has the same lattice orientation. All the results show that the multilayer h-BN domains we synthesized are in single-crystalline with high quality.