106 research outputs found
Quasiparticle Levels at Large Interface Systems from Many-body Perturbation Theory: the XAF-GW method
We present a fully ab initio approach based on many-body perturbation theory
in the GW approximation, to compute the quasiparticle levels of large interface
systems without significant covalent interactions between the different
components of the interface. The only assumption in our approach is that the
polarizability matrix (chi) of the interface can be given by the sum of the
polarizability matrices of individual components of the interface. We show
analytically, using a two-state hybridized model, that this assumption is valid
even in the presence of interface hybridization to form bonding and
anti-bonding states, up to first order in the overlap matrix elements involved
in the hybridization. We validate our approach by showing that the band
structure obtained in our method is almost identical to that obtained using a
regular GW calculation for bilayer black phosphorus, where interlayer
hybridization is significant. Significant savings in computational time and
memory are obtained by computing chi only for the smallest sub-unit cell of
each component, and expanding (unfolding) the chi matrix to that in the unit
cell of the interface. To treat interface hybridization, the full wavefunctions
of the interface are used in computing the self-energy. We thus call the method
XAF-GW (X: eXpand-chi, A: Add-chi, F: Full wavefunctions). Compared to
GW-embedding type approaches in the literature, the XAF-GW approach is not
limited to specific screening environments or to non-hybridized interface
systems. XAF-GW can also be applied to systems with different dimensionalities,
as well as to Moire superlattices such as in twisted bilayers. We illustrate
the generality and usefulness of our approach by applying it to self-assembled
PTCDA monolayers on Au(111) and Ag(111), and PTCDA monolayers on
graphite-supported monolayer WSe2, where good agreement with experiment is
obtained.Comment: More detailed proof of Add-Chi for hybridized states added in this
versio
Dielectric Screening by 2D Substrates
Two-dimensional (2D) materials are increasingly being used as active
components in nanoscale devices. Many interesting properties of 2D materials
stem from the reduced and highly non-local electronic screening in two
dimensions. While electronic screening within 2D materials has been studied
extensively, the question still remains of how 2D substrates screen charge
perturbations or electronic excitations adjacent to them. Thickness-dependent
dielectric screening properties have recently been studied using electrostatic
force microscopy (EFM) experiments. However, it was suggested that some of the
thickness-dependent trends were due to extrinsic effects. Similarly, Kelvin
probe measurements (KPM) indicate that charge fluctuations are reduced when BN
slabs are placed on SiO, but it is unclear if this effect is due to
intrinsic screening from BN. In this work, we use first principles calculations
to study the fully non-local dielectric screening properties of 2D material
substrates. Our simulations give results in good qualitative agreement with
those from EFM experiments, for hexagonal boron nitride (BN), graphene and
MoS, indicating that the experimentally observed thickness-dependent
screening effects are intrinsic to the 2D materials. We further investigate
explicitly the role of BN in lowering charge potential fluctuations arising
from charge impurities on an underlying SiO substrate, as observed in the
KPM experiments. 2D material substrates can also dramatically change the
HOMO-LUMO gaps of adsorbates, especially for small molecules, such as benzene.
We propose a reliable and very quick method to predict the HOMO-LUMO gap of
small physisorbed molecules on 2D and 3D substrates, using only the band gap of
the substrate and the gas phase gap of the molecule.Comment: 24 pages, 5 figures, Supplementary Informatio
Energy Level Alignment at Hybridized Organic-metal Interfaces: the Role of Many-electron Effects
Hybridized molecule/metal interfaces are ubiquitous in molecular and organic
devices. The energy level alignment (ELA) of frontier molecular levels relative
to the metal Fermi level (EF) is critical to the conductance and functionality
of these devices. However, a clear understanding of the ELA that includes
many-electron self-energy effects is lacking. Here, we investigate the
many-electron effects on the ELA using state-of-the-art, benchmark GW
calculations on prototypical chemisorbed molecules on Au(111), in eleven
different geometries. The GW ELA is in good agreement with photoemission for
monolayers of benzene-diamine on Au(111). We find that in addition to static
image charge screening, the frontier levels in most of these geometries are
renormalized by additional screening from substrate-mediated intermolecular
Coulomb interactions. For weakly chemisorbed systems, such as amines and
pyridines on Au, this additional level renormalization (~1.5 eV) comes solely
from static screened exchange energy, allowing us to suggest computationally
more tractable schemes to predict the ELA at such interfaces. However, for more
strongly chemisorbed thiolate layers, dynamical effects are present. Our ab
initio results constitute an important step towards the understanding and
manipulation of functional molecular/organic systems for both fundamental
studies and applications.Comment: main text - first 22 page
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