Long- versus Short-Range Scattering in Doped Epitaxial Graphene

Carrier Relaxation and Multiplication in Bi Doped Graphene

By introducing different contents of Bi adatoms to the surface of monolayer graphene, the carrier concentration and their dynamics have been effectively modulated as probed directly by the time- and angle-resolved photoemission spectroscopy technique. The Bi adatoms are found to assist acoustic phonon scattering events mediated by supercollisions as the disorder effectively relaxes the momentum conservation constraint. A reduced carrier multiplication has been observed, which is related to the shrinking Fermi sea for scattering, as confirmed by time-dependent density functional theory simulation. This work gives insight into hot carrier dynamics in graphene, which is crucial for promoting the application of photoelectric devices.

Regulating the Electronic Structure of Freestanding Graphene on SiC by Ge/Sn Intercalation: A Theoretical Study

The intrinsic n-type of epitaxial graphene on SiC substrate limits its applications in microelectronic devices, and it is thus vital to modulate and achieve p-type and charge-neutral graphene. The main groups of metal intercalations, such as Ge and Sn, are found to be excellent candidates to achieve this goal based on the first-principle calculation results. They can modulate the conduction type of graphene via intercalation coverages and bring out interesting magnetic properties to the entire intercalation structures without inducing magnetism to graphene, which is superior to the transition metal intercalations, such as Fe and Mn. It is found that the Ge intercalation leads to ambipolar doping of graphene, and the p-type graphene can only be obtained when forming the Ge adatom between Ge layer and graphene. Charge-neutral graphene can be achieved under high Sn intercalation coverage (7/8 bilayer) owing to the significantly increased distance between graphene and deformed Sn intercalation. These findings would open up an avenue for developing novel graphene-based spintronic and electric devices on SiC substrate.

Suppression of Intervalley Coupling in Graphene via Potassium Doping

The quantum interference patterns induced by impurities in graphene can form the (√3 × √3)R30° superlattice with intervalley scattering. This superlattice can lead to the folded Dirac cone at the center of Brillouin zone by coupling two non-equivalent valleys. Using angle-resolved photoemission spectroscopy (ARPES), we report the observation of suppression of the folded Dirac cone in mono- and bilayer graphene upon potassium doping. The intervalley coupling with chiral symmetry broken can persist upon a light potassium-doped level but be ruined at the heavily doped level. Meanwhile, the folded Dirac cone can be suppressed by the renormalization of the Dirac band with potassium doping. Our results demonstrate that the suppression of the intervalley scattering pattern by potassium doping could pave the way toward the realization of novel chiraltronic devices in superlattice graphene.

Enhanced Environmental Stability Coupled with a 12.5% Power Conversion Efficiency in an Aluminum Oxide-Encapsulated n-Graphene/p-Silicon Solar Cell

A significant improvement in the power conversion efficiency (PCE) and the environmental stability of n-Graphene/p-Si solar cells is indicated through effective n-doping of graphene, using low work function oxide capping layers. AlO _x, deposited through atomic layer deposition, is particularly effective for such doping and in addition serves as an antireflection coating and a cell encapsulating layer. It is shown that the related charge transfer doping and interfacial engineering was crucial to achieve a record PCE of 12.5%. The work indicates a path forward, through work function engineering, for further efficiency gains in Gr-based solar cells.

Tunable terahertz photoconductivity of hydrogen functionalized graphene using optical pump-terahertz probe spectroscopy

We show that the terahertz photoconductivity of monolayer graphene following 800 nm femtosecond optical pump excitation can be tuned by different levels of hydrogenation (graphane) and provide a quantitative understanding of the unique spectral dependence of photoconductivity. The real part of terahertz photoconductivity (Δσ_Re(ω)), which is negative in doped pristine graphene, becomes positive after hydrogenation. Frequency and electronic temperature T_e dependent conductivity σ(ω, T_e) is calculated using the Boltzmann transport equation taking into account the energy dependence of different scattering rates of the hot carriers. It is shown that the carrier scattering rate dominated by disorder-induced short-range scattering, though sufficient for pristine graphene, is not able to explain the observed complex Δσ(ω) for graphane. Our results are explained by considering the system to be heterogeneous after hydrogenation where conductivity is a weighted sum of conductivities of two parts: one dominated by Coulomb scattering coming from trapped charge impurities in the underlying substrate and the other dominated by short-range scattering coming from disorder, surface defects, dislocations and ripples in graphene flakes. A finite band gap opening due to hydrogenation is shown to be important in determining Δσ(ω).

Electronic transport properties of graphene doped by gallium

In this work we present the effect of low dose gallium (Ga) deposition (<4 ML) performed in UHV (10^-7 Pa) on the electronic doping and charge carrier scattering in graphene grown by chemical vapor deposition. In situ graphene transport measurements performed with a graphene field-effect transistor structure show that at low Ga coverages a graphene layer tends to be strongly n-doped with an efficiency of 0.64 electrons per one Ga atom, while the further deposition and Ga cluster formation results in removing electrons from graphene (less n-doping). The experimental results are supported by the density functional theory calculations and explained as a consequence of distinct interaction between graphene and Ga atoms in case of individual atoms, layers, or clusters.

Electronic transport properties of Ir-decorated graphene

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature, and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.