196 research outputs found
Simple and efficient LCAO basis sets for the diffuse states in carbon nanostructures
We present a simple way to describe the lowest unoccupied diffuse states in
carbon nanostructures in density functional theory (DFT) calculations using a
minimal LCAO (linear combination of atomic orbitals) basis set. By comparing
plane wave basis calculations, we show how these states can be captured by
adding long-range orbitals to the standard LCAO basis sets for the extreme
cases of planar sp2 (graphene) and curved carbon (C60). In particular,
using Bessel functions with a long range as additional basis functions retain a
minimal basis size. This provides a smaller and simpler atom-centered basis set
compared to the standard pseudo-atomic orbitals (PAOs) with multiple
polarization orbitals or by adding non-atom-centered states to the basis.Comment: 3 pages, 3 figure
Removing all periodic boundary conditions: Efficient non-equilibrium Green function calculations
We describe a method and its implementation for calculating electronic
structure and electron transport without approximating the structure using
periodic super-cells. This effectively removes spurious periodic images and
interference effects. Our method is based on already established methods
readily available in the non-equilibrium Green function formalism and allows
for non-equilibrium transport. We present examples of a N defect in graphene,
finite voltage bias transport in a point-contact to graphene, and a
graphene-nanoribbon junction. This method is less costly, in terms of
CPU-hours, than the super-cell approximation.Comment: 8 pages, 7 figure
Physical insights on transistors based on lateral heterostructures of monolayer and multilayer PtSe2 via Ab initio modelling of interfaces
This work has been supported by the European Commission through the Horizon 2020 Framework Program, Future Emerging Technologies QUEFORMAL project (contract n. 829035). The authors thank Dr. Alessandro Fortunelli for useful discussions.Lateral heterostructures (LH) of monolayer-multilayer regions of the same noble transition metal
dichalcogenide, such as platinum diselenide (
PtSe2), are promising options for the fabrication of
efficient two-dimensional field-effect transistors (FETs), by exploiting the dependence of the energy
gap on the number of layers and the intrinsically high quality of the heterojunctions. Key for future
progress in this direction is understanding the effects of the physics of the lateral interfaces on farfrom-
equilibrium transport properties. In this work, a multi-scale approach to device simulation,
capable to include ab-initio modelling of the interfaces in a computationally efficient way, is
presented. As an application, p- and n-type monolayer-multilayer PtSe2
LH-FETs are investigated,
considering design parameters such as channel length, number of layers and junction quality. The
simulations suggest that such transistors can provide high performance in terms of subthreshold
characteristics and switching behavior, and that a single channel device is not capable, even in the
ballistic defectless limit, to satisfy the requirements of the semiconductor roadmap for the next
decade, and that stacked channel devices would be required. It is shown how ab-initio modelling of
interfaces provides a reliable physical description of charge displacements in their proximity, which
can be crucial to correctly predict device transport properties, especially in presence of strong dipoles,
mixed stoichiometries or imperfections.European Commission
European Commission Joint Research Centre 82903
Large-scale tight-binding simulations of quantum transport in ballistic graphene
Graphene has proven to host outstanding mesoscopic effects involving massless
Dirac quasiparticles travelling ballistically resulting in the current flow
exhibiting light-like behaviour. A new branch of 2D electronics inspired by the
standard principles of optics is rapidly evolving, calling for a deeper
understanding of transport in large-scale devices at a quantum level. Here we
perform large-scale quantum transport calculations based on a tight-binding
model of graphene and the non-equilibrium Green's function method and include
the effects of junctions of different shape, magnetic field, and
absorptive regions acting as drains for current. We stress the importance of
choosing absorbing boundary conditions in the calculations to correctly capture
how current flows in the limit of infinite devices. As a specific application
we present a fully quantum-mechanical framework for the "2D Dirac fermion
microscope" recently proposed by B{\o}ggild [Nat. Comm. 8, 10.1038
(2017)], tackling several key electron-optical effects therein predicted via
semiclassical trajectory simulations, such as electron beam collimation,
deflection and scattering off Veselago dots. Our results confirm that a
semiclassical approach to a large extend is sufficient to capture the main
transport features in the mesoscopic limit and the optical regime, but also
that a richer electron-optical landscape is to be expected when coherence or
other purely quantum effects are accounted for in the simulations.Comment: 12 pages, 10 figure
Local Coordination Modulates the Reflectivity of Liquefied Si-Ge Alloys
The properties of liquid Si-Ge binary systems at melting conditions deviate
from those expected by the ideal alloy approximation. Particularly, a
non-linear dependence of the dielectric functions occurs with the reflectivity
of liquid Si-Ge being 10\% higher at intermediate Ge content than in pure Si or
Ge. Using \textit{ab initio} methodologies, we revealed a direct correlation
between reflectivity and atomic coordination, discovering that Si-Ge's higher
local coordination drives the aforementioned optical behavior. These findings
extend the physical understanding of liquefied semiconductors and hold the
promise of further generalization
A two-dimensional Dirac fermion microscope
The electron microscope has been a powerful, highly versatile workhorse in
the fields of material and surface science, micro and nanotechnology, biology
and geology, for nearly 80 years. The advent of two-dimensional materials opens
new possibilities for realising an analogy to electron microscopy in the solid
state. Here we provide a perspective view on how a two-dimensional (2D) Dirac
fermion-based microscope can be realistically implemented and operated, using
graphene as a vacuum chamber for ballistic electrons. We use semiclassical
simulations to propose concrete architectures and design rules of 2D electron
guns, deflectors, tunable lenses and various detectors. The simulations show
how simple objects can be imaged with well-controlled and collimated in-plane
beams consisting of relativistic charge carriers. Finally, we discuss the
potential of such microscopes for investigating edges, terminations and
defects, as well as interfaces, including external nanoscale structures such as
adsorbed molecules, nanoparticles or quantum dots.Comment: 34 pages; 14 pages; 6 figures; Supplementary informatio
A self-sterilizing fluorescent Nanocomposite as versatile material with broad-spectrum Antibiofilm features
Hematogenous spread of infections from colonized central intravenous catheters or central lines is a long-recognized problem with infection rates of 2 and 6.8 per 1000âŻdays, respectively. Besides, removal of severe microbial colonization of implanted biomaterials is still a challenge and usually requires invasive operations. Hence, on demand self-sterilizing materials are required to avoid explant of colonized biomaterials and improve patient compliance. Moreover, photoluminescence is needed to make trackable biomaterials, which can be easily monitored upon implanting them in the body. Here, we propose the incorporation of near infrared (NIR) sensitive red-emitting carbon nanodot (CDs) into a polymeric matrix to give rise to innovative biomaterials with self-tracking and photothermal antimicrobial abilities. We obtain a material which can be processed to obtain medical devices using different techniques, among which, for instance, electrospinning. Herein, a proof-of-concept preparation of electrospun scaffolds is reported as it is highly desired in biomedical applications. Beside to confer imaging properties to the scaffold, that would allow an easy control over the in vivo positioning of implanted biomaterials as well as its degradation state and grade of integration with the surrounding native tissues, thanks to the capability to convert NIR light into local heat CDs can be exploited to exert broad-spectrum antimicrobial effect toward several pathogens. The rise in temperature can be easily modulated by controlling the irradiation time to achieve both an in vitro self-sterilization of the device and eventually in vivo destabilization of the microbial colonization. This innovative biomaterial could successfully inhibits biofilm formation and might be used as a powerful tool to treat antibiotic-resistant nature of biofilm-related infections in implanted medical devices
Physicochemical and rheological characterization of different low molecular weight gellan gum products and derived ionotropic crosslinked hydrogels
A series of four different low molecular weight gellan gum products was obtained by alkaline hydrolysis with the aim to investigate the impact of the molecular weight on the rheological properties of the polysaccharide aqueous dispersions and on the physicochemical characteristics of derived ionotropic crosslinked hydrogels. In particular, thermo-rheological analysis was conducted on aqueous dispersions to study the influence of molecular weight on the thermogelation properties typical of the native polysaccharide while strain sweep experiments were conducted to establish if aqueous dispersion shows a viscoelastic behavior. The effect of different Ca2+ on the rheological properties of hydrogels were studied. Furthermore, ionotropic crosslinked hydrogels were analyzed in terms of morphology on the dried state and swelling behavior, while their viscoelastic properties were studied by means of rheological analysis conducted in frequency sweep regime after different time points of incubation in phosphate buffer at pH 7.4. Release experiments conducted using fluorescein isothiocyanate labelled dextran as a model diffusion agent and was performed to investigate the possibility of using the low molecular weight GG-derived hydrogels as an active molecule-releasing device. Finally, the cytocompatibility of hydrolysis products was investigated, as well as the capacity of hydrogels to encapsulate viable MC3T3-E1 preosteoblastic cells
Multi-scale approach to first-principles electron transport beyond 100 nm
Multi-scale computational approaches are important for studies of novel,
low-dimensional electronic devices since they are able to capture the different
length-scales involved in the device operation, and at the same time describe
critical parts such as surfaces, defects, interfaces, gates, and applied bias,
on a atomistic, quantum-chemical level. Here we present a multi-scale method
which enables calculations of electronic currents in two-dimensional devices
larger than 100 nm, where multiple perturbed regions described by density
functional theory (DFT) are embedded into an extended unperturbed region
described by a DFT-parametrized tight-binding model. We explain the details of
the method, provide examples, and point out the main challenges regarding its
practical implementation. Finally we apply it to study current propagation in
pristine, defected and nanoporous graphene devices, injected by chemically
accurate contacts simulating scanning tunneling microscopy probes
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