Towards High Quality and Large Area Two Dimensional Layered Materials: Synthesis, Transfer and Electronic Properties.

摘要

In 2004,a new era of two-dimensional (2D) layered materials in the material science and nanoelectronics was heralded. These new materials such as graphene and h-BN, consisting of planar sheets of sp2 bonded atoms arranged in a honeycomb crystal lattice, have drawn intense attention due to their exceptional properties. Despite intense interest and remarkably rapid progress in the field of 2D material-related research, there still exists a long way ahead for the widespread use of these 2D materials in the electronics/optoelectronics community. It primarily results from the difficulty in reliably producing high quality samples, especially in a scalable fashion. In this thesis, high-quality, large-area 2D materials have been successfully synthesized by utilizing our home-built cold/hot wall reactors, and transferred onto both conventional and modified SiO2/Si substrates for comprehensive characterization.;Firstly, in graphene growth, recent studies show that at the initial stage of chemical vapor deposition (CVD), the isolated carbon monomers form defective carbon clusters with pentagons that inevitably degrade the quality of synthesized graphene. To circumvent this hurdle, we demonstrate that high-quality centimeter-sized graphene sheets can be synthesized on Cu foils by a self-assembled approach with defect-free polycyclic aromatic hydrocarbons (PAHs) in a high vacuum (HV) chamber (cold wall reactor) without hydrogen. Different molecular motifs, namely coronene, pentacene, and rubrene, can lead to significant difference in the quality of synthesized graphene. When coronene used as precursor, monolayer graphene flakes with an adequate quality can be achieved at a growth temperature as low as 550°C. For the graphene sheets obtained at 1000°C, a carrier mobility up to ∼5300 cm2/V˙s on bare SiO2/Si at room temperature has been attained through transport measurements performed on back-gated field effect transistors (FETs) composed of large channel lengths (∼30 microm). The underlying growth mechanism mainly involves surface-mediated nucleation process of dehydrogenated PAHs rather than segregation or precipitation process of small carbon species decomposed from the precursors.;Secondly, based on our proposed growth mechanism, high quality graphene sheets have been obtained from another PAH, namely triphenylene. Due to the low melting point of triphenylene, nickel foils fastened by a magnet outside of the growth chamber were used as thermal shield to avoid any unwanted sublimation of triphenylene. Moreover, during the transfer process, Poly(Bisphenol A carbonate) was selected as the supporting layer instead of commonly used poly (methyl methacrylate, PMMA) to attain the clean graphene surface on a large scale.;Thirdly, for the best-quality graphene sheets derived from coronene precursor, it is found that an OTMS-SAM modified SiO2/Si substrate can consistently enhance the performance of large-area graphene FETs. The improved transport properties in terms of boosted carrier mobility (up to 10,700 +/- 300 cm2/V˙s), long mean free path of carriers, nearly vanished hysteretic behavior, and remarkably low intrinsic doping level are mainly attributed to the strong suppression of interfacial charge impurity scattering and remote interfacial phonon (RIP) scattering, less adsorption of dipolar adsorbates, and the attenuated charger transfer at the interface of graphene and dielectric. The intrinsic doping levels (the Fermi energy) of graphene on OTMS-modified and bare SiO2/Si have been quantitatively estimated and corroborated by the Dirac point location of GFETs, the Raman spectroscopic mapping of G-peak position, and the surface potential distribution by Kelvin probe force microscopy (KPFM).;Finally, we have presented the LPCVD synthesis (hot wall reactor) to obtain both monolayer and multilayer h-BN ultrathin films by using copper-foil enclosure and Cu tube, respectively. Unlike the hexagonal shape of graphene nucleation islands, h-BN starts to grow with a triangular shape/asymmetric diamond, possibly due to the more energetically favored nitrogen-atom-terminated edges. With prolonged growth time, the h-BN nuclei/islands extend in plane and merge with each other, resulting in a complete layer covering the Cu surface. Multilayer h-BN ultrathin films in large area (∼cm2) have been successfully transferred on SiO2/Si substrate and characterized by XPS, Raman spectroscopy, and AFM. With the synthesized h-BN ultrathin films, one can envisage many interesting research aspects, which will definitely bring about important applications, e.g., UV light emitting thin films for optoelectronics applications, and gate dielectric layers/substrates for graphene electronics applications.