2,043 research outputs found
The Influence of Minimum Tillage Systems on Soil Organic Matter and Water Conservation in some Soils of Romania
First-principles study of the interaction and charge transfer between graphene and metals
Measuring the transport of electrons through a graphene sheet necessarily
involves contacting it with metal electrodes. We study the adsorption of
graphene on metal substrates using first-principles calculations at the level
of density functional theory. The bonding of graphene to Al, Ag, Cu, Au and
Pt(111) surfaces is so weak that its unique "ultrarelativistic" electronic
structure is preserved. The interaction does, however, lead to a charge
transfer that shifts the Fermi level by up to 0.5 eV with respect to the
conical points. The crossover from p-type to n-type doping occurs for a metal
with a work function ~5.4 eV, a value much larger than the work function of
free-standing graphene, 4.5 eV. We develop a simple analytical model that
describes the Fermi level shift in graphene in terms of the metal substrate
work function. Graphene interacts with and binds more strongly to Co, Ni, Pd
and Ti. This chemisorption involves hybridization between graphene -states
and metal d-states that opens a band gap in graphene. The graphene work
function is as a result reduced considerably. In a current-in-plane device
geometry this should lead to n-type doping of graphene.Comment: 12 pages, 9 figure
Surface Dipoles and Work Functions of Alkylthiolates and Fluorinated Alkylthiolates on Au(111)
We study the dipole formation at the surface formed by -CH3 and -CF3
terminated shortchain alkyl-thiolate monolayers on Au(111). In particular, we
monitor the change in work function upon chemisorption using density functional
theory calculations. We separate the surface dipole into two contributions,
resulting from the gold-adsorbate interaction and the intrinsic dipole of the
adsorbate layer, respectively. The two contributions turn out to be
approximately additive. Adsorbate dipoles are defined by calculating dipole
densities of free-standing molecular monolayers. The gold-adsorbate interaction
is to a good degree determined by the Au-S bond only. This bond is nearly
apolar and its contribution to the surface dipole is relatively small. The
surface dipole of the self-assembled monolayer is then dominated by the
intrinsic dipole of the thiolate molecules. Alkyl-thiolates increase the work
function of Au(111), whereas fluorinated alkyl-thiolates decrease it.Comment: 24 pages, 5 figures, 4 table
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