572 research outputs found
Effects of extrinsic point defects in phosphorene: B, C, N, O and F Adatoms
Phosphorene is emerging as a promising 2D semiconducting material with a
direct band gap and high carrier mobility. In this paper, we examine the role
of the extrinsic point defects including surface adatoms in modifying the
electronic properties of phosphorene using density functional theory. The
surface adatoms considered are B, C, N, O and F with a [He] core electronic
configuration. Our calculations show that B and C, with electronegativity close
to P, prefer to break the sp3 bonds of phosphorene, and reside at the
interstitial sites in the 2D lattice by forming sp2 bonds with the native
atoms. On the other hand, N, O and F, which are more electronegative than P,
prefer the surface sites by attracting the lone pairs of phosphorene. B, N and
F adsorption will also introduce local magnetic moment to the lattice.
Moreover, B, C, N and F adatoms will modify the band gap of phosphorene
yielding metallic transverse tunneling characters. Oxygen does not modify the
band gap of phosphorene, and a diode like tunneling behavior is observed. Our
results therefore offer a possible route to tailor the electronic and magnetic
properties of phosphorene by the adatom functionalization, and provide the
physical insights of the environmental sensitivity of phosphorene, which will
be helpful to experimentalists in evaluating the performance and aging effects
of phosphorene-based electronic devices
Atomically thin group-V elemental films: theoretical investigations of antimonene allotropes
Group-V elemental monolayers including phosphorene are emerging as promising
2D materials with semiconducting electronic properties. Here, we present the
results of first principles calculations on stability, mechanical and
electronic properties of 2D antimony (Sb), antimonene. Our calculations show
that free-standing {\alpha} and \b{eta} allotropes of antimonene are stable and
semiconducting. The {\alpha}-Sb has a puckered structure with two atomic
sub-layers and \b{eta}-Sb has a buckled hexagonal lattice. The calculated Raman
spectra and STM images have distinct features thus facilitating
characterization of both allotropes. The \b{eta}-Sb has nearly isotropic
mechanical properties while {\alpha}-Sb shows strongly anisotropic
characteristics. An indirect-direct band gap transition is expected with
moderate tensile strains applied to the monolayers, which opens up the
possibility of their applications in optoelectronics
Modeling Of Legged Locomotion With A Suspended Load In The Sagittal Plane
Walking or running while carrying loads has always been a tedious task, more so when the loads are heavy. Such a task of carrying loads not only requires extra effort but also leads to physical pain and in some cases injury. Prior studies on human locomotion with a suspended load have used models that are restricted in their DOFs and so are not able to take into account the fore aft movement in human beings. The objective of this thesis is to model the dynamics of sagittal plane center-of-mass locomotion with a suspended load and apply findings to carrying loads with an elastic pole.
The approach taken was to develop and analyze a variant of the Hip Actuated Spring-Loaded Inverted Pendulum (SLIP) model of locomotion that has a second sprung mass added to represent a suspended load. This model showed a large increment in human running speed and stride frequency as the suspension stiffness was increased. A stability analysis on the model showed branching among fixed points with one branch nearly stable while the other branch has greater stability. This particular model was able to show a reduction in peak forces and amplitude of the load for very compliant suspensions.
In order to limit velocity change that occurs with changing suspension stiffness, a variable torque model was developed. This model was able to limit the velocity magnitude and stride frequency near target values. It also showed reduction in peak shoulder forces and has better stability.
One direct application of this work is to inform and potentially influence better practices involving the ancient human behavior of carrying heavy loads with bamboo poles, which remains common in some regions of Asia. The dynamic aspects of the hip actuated SLIP were synthesized with those of the beam bending model to design a compliant pole. Optimizing the design parameters of bamboo helped us to obtain a region which provided suitable reduction in peak shoulder forces within the safety limits of avoiding fracture
Degradation of Phosphorene in Air: Understanding at Atomic Level
Phosphorene is a promising two dimensional (2D) material with a direct band
gap, high carrier mobility, and anisotropic electronic properties.
Phosphorene-based electronic devices, however, are found to degrade upon
exposure to air. In this paper, we provide an atomic level understanding of
stability of phosphorene in terms of its interaction with O2 and H2O. The
results based on density functional theory together with first principles
molecular dynamics calculations show that O2 could spontaneously dissociate on
phosphorene at room temperature. H2O will not strongly interact with pristine
phosphorene, however, an exothermic reaction could occur if phosphorene is
first oxidized. The pathway of oxidation first followed by exothermic reaction
with water is the most likely route for the chemical degradation of the
phosphorene-based devices in air
Electronic conduction in a three-terminal molecular transistor
The electronic conduction of a novel, three-terminal molecular architecture,
analogous to a heterojunction bipolar transistor is studied. In this
architecture, two diode arms consisting of donor-acceptor molecular wires fuse
through a ring, while a gate modulating wire is a \pi-conjugated wire. The
calculated results show the enhancement or depletion mode of a transistor by
applying a gate field along the positive or negative direction. A small gate
field is required to switch on the current in the proposed architecture. The
changes in the electronic conduction can be attributed to the intrinsic dipolar
molecular architecture in terms of the evolution of molecular wavefunctions,
specifically the one associated with the terphenyl group of the modulating wire
in the presence of the gate field.Comment: 13 pages, 5 figure
Phosphorene Oxide: Stability and electronic properties of a novel 2D material
Phosphorene, the monolayer form of the (black) phosphorus, was recently
exfoliated from its bulk counterpart. Phosphorene oxide, by analogy to graphene
oxide, is expected to have novel chemical and electronic properties, and may
provide an alternative route to synthesis of phosphorene. In this letter, we
investigate physical and chemical properties of the phosphorene oxide including
its formation by the oxygen adsorption on the bare phosphorene. Analysis of the
phonon dispersion curves finds stoichiometric and non-stoichiometric oxide
configurations to be stable at ambient conditions, thus suggesting that the
oxygen absorption may not degrade the phosphorene. The nature of the band gap
of the oxides depends on the degree of the functionalization of phosphorene;
indirect gap is predicted for the non-stoichiometric configurations whereas a
direct gap is predicted for the stoichiometric oxide. Application of the
mechanical strain and external electric field leads to tunability of the band
gap of the phosphorene oxide. In contrast to the case of the bare phosphorene,
dependence of the diode-like asymmetric current-voltage response on the degree
of stoichiometry is predicted for the phosphorene oxide
Electronic Structure Calculations of Static Hyper(Polarizabilities) of Substrate-Supported Group-IV and -V Elemental Monolayers
The substrate-induced effects on the polarizability (α) and first dipole hyperpolarizability (β) of group-IV (i.e., graphene, silicene, germanene, stanene) and group-V (i.e., phosphorene, arsenene, antimonene, and bismuthene) elemental monolayer nanoflakes are investigated. Density functional theory calculations show that these monolayers are bound with varying degrees of interaction strength with the Ag(111) substrate surface. Calculated dipole moment and β values are zero for the centrosymmetric configurations of the pristine elemental monolayers. On the other hand, substrate-induced changes in the electronic densities at the interface lead to substantially enhanced values of β, making these materials attractive for applications in the next-generation photonic technologies at the nanoscale
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