Lattice-Planarity and Carrier Density in Organometallic, Ring-Functionalized Graphene and Boron Nitride
thesisposted on 06.08.2019, 00:00 authored by Songwei Che
Binding molecular groups and nanoparticles on graphene and hexagonal boron nitride (h-BN) will expand their applications scope by offering sites to interface with other material systems. However, such functionalization makes it challenging to retain the trigonal sp2 hybridized lattice. The conventional modification routes for graphene are “carbon-centered” that change its local hybridization of carbon atoms from sp2 to tetrahedral sp3. This disrupts the lattice planarity of graphene, thus dramatically deteriorating its innate superior properties. The low intensity of defect-induced peak (D-band) in Raman spectroscopy and slight change in carrier-mobility from electronic measurements after η6-functionalization strongly proved that graphene’s planar lattice and high carrier-mobility were retained. The grafted chromium carbonyl (Cr(CO)3) groups further acted as reactive sites for silver nanoparticles (AgNPs)’ nucleation and growth. The AgNPs attached on η6-graphene were applied as plasmonic centers in Schottky junction solar cells, improving power conversion efficiency by 11-times. Based on the η6-chemistry, transition metal oxide nanoparticles (TMONs, CrxO3, MoO3 and WO3) were successfully incorporated on graphene via a one-step, photo-organometallic route. Specially, a bracelet-shape CrxO3 nanoparticles were deposited on graphene only at room temperature via dipole-directed self-assembly. The slight change of D-band intensity showed that graphene’s sp2 structure has been retained after the attachment of TMONs. The Fermi level of graphene was reduced by 250 mV with enhanced conductivity and only one order of magnitude reduced in carrier mobility. As an analogy of graphene, h-BN also has π-electrons at the interplane sheets, which are essential for organometallic, ring-centered modification. The Cr(CO)3 moieties are bonded to hexagon rings of h-BN’s basal plane, overcoming the challenge that chemistries only occur at edges and defect points in transitional functionalization. A downshift of E2g-phonon mode by about ~4 cm-1 with a reduction of optical band gap was achieved in η6-h-BN. Graphene grain boundaries (GB) are formed while two grains coalesce together, which are more active for chemical reactions. By bonding with pentagon-rings, a technique to selectively modify graphene at GB regions is proposed. The doping effect η5-chemistry enhanced the D-band signal at GB regions with reduced carrier-mobility.