Virgil B. Shields

Oriented 2D Covalent Organic Framework Thin Films on Single-Layer Graphene

Microporous covalent organic frameworks, which usually form as insoluble powders, grow as crystalline films on graphene. Covalent organic frameworks (COFs), in which molecular building blocks form robust microporous networks, are usually synthesized as insoluble and unprocessable powders. We have grown two-dimensional (2D) COF films on single-layer graphene (SLG) under operationally simple solvothermal conditions. The layered films stack normal to the SLG surface and show improved crystallinity compared with COF powders. We used SLG surfaces supported on copper, silicon carbide, and transparent fused silica (SiO2) substrates, enabling optical spectroscopy of COFs in transmission mode. Three chemically distinct COF films grown on SLG exhibit similar vertical alignment and long-range order, and two of these are of interest for organic electronic devices for which thin-film formation is a prerequisite for characterizing their optoelectronic properties.

Ultrafast relaxation dynamics of hot optical phonons in graphene

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.

Very Slow Cooling Dynamics of Photoexcited Carriers in Graphene Observed by Optical-Pump Terahertz-Probe Spectroscopy

Using optical-pump terahertz-probe spectroscopy, we study the relaxation dynamics of photoexcited carriers in graphene at different substrate temperatures. We find that at lower temperatures the tail of the relaxation transients measured by the differential probe transmission become slower, extending beyond several hundred picoseconds below 50 K. We interpret the observed relaxation transients as resulting from the cooling of the photoexcited carriers via phonon emission. The slow cooling of the photoexcited carriers at low temperatures is attributed to the bulk of the electron and hole energy distributions moving close enough to the Dirac point that both intraband and interband scattering of carriers via optical phonon emission become inefficient for removing heat from the carriers. Our model, which includes intraband carrier scattering and interband carrier recombination and generation, agrees very well with the experimental observations.

Correlated conductivity and work function changes in epitaxial graphene

Correlation between conductance and surface work function (SWF) changes caused by molecular adsorption on epitaxial graphene on both faces of 6 H-SiC has been investigated. The SWF and conductance changes, explained on the basis of graphene band diagram, indicate C-face multilayer and Si-face few layer graphene behave as p and n-type sensing layers, respectively. A quantitative model correlating conductance and SWF changes has been proposed within the framework of Boltzmann transport theory. Our results further indicate that for epitaxial graphene, the charge interaction by the adsorbed molecules and related work function changes can be strongly influenced by the SiC substrate.

Molecular Adsorption Behavior of Epitaxial Graphene Grown on 6H-SiC Faces

Epitaxial graphene layers grown on 6H-SiC faces were investigated for molecular adsorption by electron withdrawing NO2 and electron donating NH3. From amperometric measurements performed on these samples, we observed that epitaxial graphene grown on C-face SiC mostly behaved as a p-type sensing layer in contrast to the Si-face graphene, which behaved as n-type. Potentiometric sensing experiments performed reveal that epitaxial graphene on both C- and Si-faces have similar charge transfer mechanism with respect to a specific adsorbent gas.

Graphene on different substrates for sensing applications

Comparison between the sensing behavior of graphene grown on different substrates has been carried out in the light of charge carrier transport properties and molecular doping. Experimentally obtained sensing results indicate that, in epitaxial multilayer graphene (MLG) grown on C-face SiC, p-type charge carriers are dominant and in Si-face few layer graphene (FLG), charge carriers are mostly n-type. Graphene grown by chemical vapor deposition (CVD) on thin copper foils has strong p-type carrier density while graphene on sapphire appears to be almost neutral in nature. Molecular doping induced changes in carrier densities (both n- and p-type) in C and Si-faces epitaxial graphene, CVD graphene on copper and sapphire has been correlated with the shifts in Fermi level.

Work function and conductivity changes due to molecular adsorption in epitaxial graphene on 6H-SiC

Charge carrier transport properties and molecular doping in graphene grown epitaxially on both Si and C-faces of 6H-SiC have been investigated. Our results indicate that in C-face multiplayer graphene (MLG) p-type charge carriers are dominant and in Si-face few layer graphene (FLG), charge carriers are mostly n-type. A strong correlation between conductance change and SWF change due to molecular doping in C and Si-faces epitaxial graphene has been found based on type of carrier, Fermi level shifts, and carrier density of the graphene film. An analytical model for the dependence of conductivity on the adsorbate-induced doping and impurities is being developed based on work function change and Boltzmann transport theory.

The effects of dielectric layers on SiC based epitaxial graphene in transistor applications

By characterizing the mobility of Si-face/C-face of SiC graphene before and after stacking the layer of HfO2 with a polyvinyl alcohol (PVA) treatment on the device active layer, we have explored the properties of SiC based epitaxial graphene and the effects of the dielectric film with PVA treatment. Epitaxial graphene grown on the carbon face produces a higher mobility than film grown on the silicon face. Also, the mobility, in the presence of the PVA treatment, improves after the deposition of gate dielectrics: ∼20% in C-face graphene and ∼100% in Si-face graphene. This is unlike the degradation normally observed with dielectric/graphene systems.