41 research outputs found
Exploring a novel class of Janus MXenes by first principles calculations: structural, electronic and magnetic properties of Sc2CXT, X = O, F, OH; T = C, S, N
The already intriguing electronic and optical properties of the MXene Sc2C family can be further
tuned through a wide range of possible functionalizations. Here, by means of Density Functional
Theory, we show that the 36 possible elements of the Janus MXT (M:Sc2C, X:O, F, OH, T:C, N, S)
family, built by considering the four possible structural models (i) FCC, (ii) HCP , (iii) FCC + HCP,
and (iv) HCP + FCC, are all potentially stable. The analysis of their mechanical properties shows
the excellent mechanical flexibility of functionalized MXenes (f-MXenes) under large strain, making
them more suitable for applications where stress could be an issue. Interestingly, while Sc2C presents
a metallic character, Sc2COS, Sc2CFN and Sc2COHN are found to be semiconductors with bandgaps
of 2.5 eV (indirect), 1.67 eV (indirect) and 1.1 eV (direct), respectively, which presents promising
applications for nano- and optoelectronics. Moreover, Sc2CFC presents a ferromagnetic ground state
with the 2x2x1 supercell magnetic moment of 3.99 mB, while the ground state of Sc2COHC might be
antiferromagnetic with a magnetic moment of 3.98 mB, depending on the environment. Remarkably,
the band structures of Sc2CFC and Sc2COHC present a half-metallic character with an HSE06
fundamental band gap of 0.60 eV and 0.48 eV, respectively. Our results confirm the extraordinary
potential of the Janus MXT (M:Sc2C, X:O, F, OH, T:C, N, S) family for novel applications in 2D
nano-,opto- and spintronics.Junta de Andalucia P18-FR-4834AEI PID2021-125604NB-I0
Transport Length Scales in Disordered Graphene-based Materials: Strong Localization Regimes and Dimensionality Effects
We report on a numerical study of quantum transport in disordered two
dimensional graphene and graphene nanoribbons. By using the Kubo and the
Landauer approaches, transport length scales in the diffusive (mean free path,
charge mobilities) and localized regimes (localization lengths) are computed,
assuming a short range disorder (Anderson-type). In agreement with localization
scaling theory, the electronic systems are found to undergo a conventional
Anderson localization in the zero temperature limit. Localization lengths in
weakly disordered ribbons are found to differ by two orders of magnitude
depending on their edge symmetry, but always remain several orders of magnitude
smaller than those computed for 2D graphene for the same disorder strength.
This pinpoints the role of transport dimensionality and edge effects.Comment: 4 pages, Phys. rev. Lett. (in press
DNA/RNA sequencing using germanene nanoribbons via two dimensional molecular electronic spectroscopy: an ab initio study
Developing fast, reliable, and cost effective, yet practical DNA/RNA sequencing methods and devices is a
must. In this regard, motivated by the recently introduced two-dimensional electronic molecular spectroscopy
(2DMES) technique for molecular recognition, and the compatibility of 2D layers of group IV
elements with the current technology of manufacturing electronic devices, we investigate the capability
of germanene nanoribbons (GeNRs) as a feasible, accurate, and ultra-fast sequencing device under the
application of 2DMES. We show that by employing 2DMES, not only can GeNRs unambiguously distinguish
different nucleobases to sequence DNA/RNA, they are also capable of recognizing methylated
nucleobases that could be related to cancerous cell growth. Our calculations indicate that, compared to
frequently used graphene layers, germanene provides more distinct adsorption energies for different
nucleobases which implies its better ability to recognize various molecules unambiguously. By calculating
the conductance sensitivity of the system for experimental purposes, we also show that the introduced
sequencing device possesses a high sensitivity and selectivity characteristic. Thus, our proposed system
would be a promising device for next-generation DNA sequencing technologies and would be realizable
using the current protocols of fabricating electronic devices.H2020 Marie Sklodowska-Curie Actions 841673European Commission MAT2017-88258-RPrograma Operativo FEDER of Andalucia 2014-2020 B-FQM-272-UGR20
AEI MAT2017-88258-
Simulaciones numéricas en sistemas de baja dimensionalidad: superficies semiconductoras y nanotubos de carbono
Tesis doctoral inédita leida en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 20-12-200
Anderson localization in carbon nanotubes: defect density and temperature effects
The role of irradiation induced defects and temperature in the conducting
properties of single-walled (10,10) carbon nanotubes has been analyzed by means
of a first-principles approach. We find that di-vacancies modify strongly the
energy dependence of the differential conductance, reducing also the number of
contributing channels from two (ideal) to one. A small number of di-vacancies
(5-9) brings up strong Anderson localization effects and a seemly universal
curve for the resistance as a function of the number of defects. It is also
shown that low temperatures, around 15-65 K, are enough to smooth out the
fluctuations of the conductance without destroying the exponential dependence
of the resistivity as a function of the tube length.Comment: 4 pages, 4 figure
Atomistic Boron-Doped Graphene Field Effect Transistors: A Route towards Unipolar Characteristics
We report fully quantum simulations of realistic models of boron-doped
graphene-based field effect transistors, including atomistic details based on
DFT calculations. We show that the self-consistent solution of the
three-dimensional (3D) Poisson and Schr\"odinger equations with a
representation in terms of a tight-binding Hamiltonian manages to accurately
reproduce the DFT results for an isolated boron-doped graphene nanoribbon.
Using a 3D Poisson/Schr\"odinger solver within the Non-Equilibrium Green's
Functions (NEGF) formalism, self-consistent calculations of the gate-screened
scattering potentials induced by the boron impurities have been performed,
allowing the theoretical exploration of the tunability of transistor
characteristics. The boron-doped graphene transistors are found to approach
unipolar behavior as the boron concentration is increased, and by tuning the
density of chemical dopants the electron-hole transport asymmetry can be finely
adjusted. Correspondingly, the onset of a mobility gap in the device is
observed. Although the computed asymmetries are not sufficient to warrant
proper device operation, our results represent an initial step in the direction
of improved transfer characteristics and, in particular, the developed
simulation strategy is a powerful new tool for modeling doped graphene
nanostructures.Comment: 7 pages, 5 figures, published in ACS Nan
Anderson localization in carbon nanotubes: Defect density and temperature effects
The role of irradiation induced defects and temperature in the conducting properties of single-walled (10,10) carbon nanotubes has been analyzed by means of a first-principles approach. We find that divacancies modify strongly the energy dependence of the differential conductance, reducing also the number of contributing channels from two (ideal) to one. A small number of divacancies (5-9) brings up strong Anderson localization effects and a seemly universal curve for the resistance as a function of the number of defects. It is also shown that low temperatures, about 15-65 K, are enough to smooth out the fluctuations of the conductance without destroying the exponential dependence of the resistivity as a function of the tube length. © 2005 The American Physical Society.The authors were partially supported by the Spanish MCyT under Contract No. MAT2002-01534 and the EC 6th Framework Network of Excellence NANOQUANTA (NMP4-CT-2004-500198). B. B. is indebted to MEC (Spain) for financial support.Peer Reviewe
Nonperturbative indirect exchange in spin valley coupled two-dimensional crystals
We study indirect exchange interactions between localized spins of magnetic impurities in spin valley coupled systems described with the Kane-Mele model. Our model captures the main ingredients of the energy bands of the 1H transition metal dichalcogenide (TMD) monolayers, such as 1H-MoS2 and 1H-NbSe2. To obtain the effective interactions, we use the exact diagonalization of the Hamiltonian, avoiding momentum cutoffs. We start by comparing the standard perturbation expansion in terms of the Kondo exchange with the exact calculation of the interaction, treating the local spins classically. We find that perturbation theory works well even beyond the regime where the relevant figure of merit, the ratio between the exchange J and the hopping t, is small. We verify that the effective indirect exchange Hamiltonian derived from perturbation theory also works in the nonperturbative regime. Additionally, we analyze the interplay between the symmetry of the different terms of the interaction (Heisenberg, Ising, and Dzyaloshinskii-Moriya), the Fermi-surface topology, and the crystallographic direction in which the impurities are placed. We show that the indirect exchange along the armchair direction is actually Heisenberg-like, due to the reflection symmetry of the crystal structure around this direction. Finally, we explore the exploitation of indirect exchange, combined with atomic manipulation, to engineer the Majumdar-Ghosh model. Our results show that TMDs provide an extremely versatile platform to engineer indirect exchange interactions.This study forms part of the Advanced Materials programme and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana (MFA/2022/045). We acknowledge Programa Operativo FEDER/Junta de Andalucía—Consejería de Transformación Económica, Industria, Conocimiento, y Universidades (Grant No. P18-FR-4834). The Albaicín supercomputer of the University of Granada is also acknowledged for providing computational time and facilities. B.B. acknowledges financial support from AEI under Project No. PID2021-125604NB-I00. J.F.R. acknowledges financial support from FCT (Grant No. PTDC/FIS-MAC/2045/2021), SNF Sinergia (Grant Pimag), Generalitat Valenciana funding Prometeo 2021/017 and MFA/2022/045, and funding from MICIIN-Spain (Grant No. PID2019-109539GB-C41)
Anomalous Doping Effects on Charge Transport in Graphene Nanoribbons
We present first-principles calculations of quantum transport in chemically
doped graphene nanoribbons with a width of up to 4 nm. The presence of boron
and nitrogen impurities is shown to yield resonant backscattering, whose
features are strongly dependent on the symmetry and the width of the ribbon, as
well as the position of the dopants. Full suppression of backscattering is
obtained on the pi-pi* plateau when the impurity preserves the mirror symmetry
of armchair ribbons. Further, an unusual acceptor-donor transition is observed
in zig-zag ribbons. These unconventional doping effects could be used to design
novel types of switching devices.Comment: Accepted in Physical Review Letter
Ab initio study of transport properties in defected carbon nanotubes: an O(N) approach
A combination of ab initio simulations and linear-scaling Green's functions
techniques is used to analyze the transport properties of long (up to one
micron) carbon nanotubes with realistic disorder. The energetics and the
influence of single defects (mono- and di-vacancies) on the electronic and
transport properties of single-walled armchair carbon nanotubes are analyzed as
a function of the tube diameter by means of the local orbital first-principles
Fireball code. Efficient O(N) Green's functions techniques framed within the
Landauer-Buttiker formalism allow a statistical study of the nanotube
conductance averaged over a large sample of defected tubes and thus extraction
of the nanotubes localization length. Both the cases of zero and room
temperature are addressed.Comment: 15 pages, 12 figures (submitted to J. Phys: Condens. Matter