26 research outputs found
Manipulating polarized light with a planar slab of Black Phosphorus
Wave polarization contains valuable information for electromagnetic signal
processing and the ability to manipulate it can be extremely useful in photonic
devices. In this work, we propose designs comprised of one of the emerging and
interesting two-dimensional media: Black Phosphorus. Due to substantial
in-plane anisotropy, a single slab of Black Phosphorus can be very efficient
for manipulating the polarization state of electromagnetic waves. We
investigate Black Phosphorus slabs that filter the fields along one direction,
or polarization axis rotation, or convert linear polarization to circular.
These slabs can be employed as components in numerous mid-IR integrated
devices
Quantum plasmons with optical-range frequencies in doped few-layer graphene
Although plasmon modes exist in doped graphene, the limited range of doping achieved by gating restricts the plasmon frequencies to a range that does not include the visible and infrared. Here we show, through the use of first-principles calculations, that the high levels of doping achieved by lithium intercalation in bilayer and trilayer graphene shift the plasmon frequencies into the visible range. To obtain physically meaningful results, we introduce a correction of the effect of plasmon interaction across the vacuum separating periodic images of the doped graphene layers, consisting of transparent boundary conditions in the direction perpendicular to the layers; this represents a significant improvement over the exact Coulomb cutoff technique employed in earlier works. The resulting plasmon modes are due to local field effects and the nonlocal response of the material to external electromagnetic fields, requiring a fully quantum mechanical treatment. We describe the features of these quantum plasmons, including the dispersion relation, losses, and field localization. Our findings point to a strategy for fine-tuning the plasmon frequencies in graphene and other two-dimensional materials.MIT/Army Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001
Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies
We determine the nature of coupled phonons and magnetic excitations in AlFeO3
using inelastic light scattering from 5 K to 315 K covering a spectral range
from 100-2200 cm-1 and complementary first-principles density functional
theory-based calculations. A strong spin-phonon coupling and magnetic ordering
induced phonon renormalization are evident in (a) anomalous temperature
dependence of many modes with frequencies below 850 cm-1, particularly near the
magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band
positions of high frequency Raman bands between 1100-1800 cm-1, in particular a
broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon
Raman scattering. We also observe weak anomalies in the mode frequencies at ~
100 K, due to a magnetically driven ferroelectric phase transition.
Understanding of these experimental observations has been possible on the basis
of first-principles calculations of phonons spectrum and their coupling with
spins
Engineering the electronic bandgaps and band edge positions in carbon-substituted 2D boron nitride: a first-principles investigation
Modification of graphene to open a robust gap in its electronic spectrum is essential for its use in field effect transistors and photochemistry applications. Inspired by recent experimental success in the preparation of homogeneous alloys of graphene and boron nitride (BN), we consider here engineering the electronic structure and bandgap of C2xB1−xN1−x alloys via both compositional and configurational modification. We start from the BN end-member, which already has a large bandgap, and then show that (a) the bandgap can in principle be reduced to about 2 eV with moderate substitution of C (x < 0.25); and (b) the electronic structure of C2xB1−xN1−x can be further tuned not only with composition x, but also with the configuration adopted by C substituents in the BN matrix. Our analysis, based on accurate screened hybrid functional calculations, provides a clear understanding of the correlation found between the bandgap and the level of aggregation of C atoms: the bandgap decreases most when the C atoms are maximally isolated, and increases with aggregation of C atoms due to the formation of bonding and anti-bonding bands associated with hybridization of occupied and empty defect states. We determine the location of valence and conduction band edges relative to vacuum and discuss the implications on the potential use of 2D C2xB1−xN1−x alloys in photocatalytic applications. Finally, we assess the thermodynamic limitations on the formation of these alloys using a cluster expansion model derived from first-principles
Non-linear hybrid surface-defect states in defective BiSe
Surface-states of topological insulators are assumed to be robust against
non-magnetic defects in the crystal. However, recent theoretical models and
experiments indicate that even non-magnetic defects can perturb these states.
Our first-principles calculations demonstrate that the presence of Se vacancies
in BiSe, has a greater impact than a mere n-doping of the structure,
which would just shift the Fermi level relative to the Dirac point. We observe
the emergence of a non-linear band pinned near the Fermi level, while the Dirac
cone shifts deeper into the valence band. We attribute these features in the
bandstructure to the interaction between the surface and defect states, with
the resulting hybridization between these states itself depending on the
position and symmetry of the Se vacancy relative to the surfaces. Our results
bring us a step closer to understanding the exotic physics emerging from
defects in BiSe that remained unexplored in prior studies
First-principles study of coupled effect of ripplocations and S-vacancies in MoS2
Sensoy, Mehmet Gokhan/0000-0003-4815-8061; Shirodkar, Sharmila N/0000-0002-9040-5858; Tritsaris, Georgios/0000-0002-5738-4493WOS: 000483884600020Recent experiments have revealed ripplocations, atomic-scale ripplelike defects on samples of MoS2 flakes. We use quantum mechanical calculations based on density functional theory to study the effect of ripplocations on the structural and electronic properties of single-layer MoS2, and, in particular, the coupling between these extended defects and the most common defects in this material, S-vacancies. We find that the formation of neutral S-vacancies is energetically more favorable in the ripplocation. in addition, we demonstrate that ripplocations alone do not introduce electronic states into the intrinsic bandgap, in contrast to S-vacancies. We study the dependence of the induced gap states on the position of the defects in the ripplocation, which has implications for the experimental characterization of MoS2 flakes and the engineering of quantum emitters in this material. Our specific findings collectively aim to provide insights into the electronic structure of experimentally relevant defects in MoS2 and to establish structure-property relationships for the design of MoS2-based quantum devices. Published under license by AIP Publishing.ARO MURIMURI [W911NF14-0247]; DOE BES AwardUnited States Department of Energy (DOE) [DE-SC0019300]; National Science Foundation (NSF)National Science Foundation (NSF) [ACI-1053575]The authors would like to thank Venkataraman Swaminathan and Daniel Larson for helpful discussions. S.S. acknowledges support by the ARO MURI (Award No. W911NF14-0247). This work was supported by the DOE BES Award No. DE-SC0019300. For calculations, computational resources were used on the Odyssey cluster, which is maintained by the FAS Research Computing Group at Harvard University, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (NSF) under Grant No. ACI-1053575
First-principles study of coupled effect of ripplocations and S-vacancies in MoS 2
Sensoy, Mehmet Gokhan/0000-0003-4815-8061; Shirodkar, Sharmila N/0000-0002-9040-5858; Tritsaris, Georgios/0000-0002-5738-4493WOS: 000483884600020Recent experiments have revealed ripplocations, atomic-scale ripplelike defects on samples of MoS2 flakes. We use quantum mechanical calculations based on density functional theory to study the effect of ripplocations on the structural and electronic properties of single-layer MoS2, and, in particular, the coupling between these extended defects and the most common defects in this material, S-vacancies. We find that the formation of neutral S-vacancies is energetically more favorable in the ripplocation. in addition, we demonstrate that ripplocations alone do not introduce electronic states into the intrinsic bandgap, in contrast to S-vacancies. We study the dependence of the induced gap states on the position of the defects in the ripplocation, which has implications for the experimental characterization of MoS2 flakes and the engineering of quantum emitters in this material. Our specific findings collectively aim to provide insights into the electronic structure of experimentally relevant defects in MoS2 and to establish structure-property relationships for the design of MoS2-based quantum devices. Published under license by AIP Publishing.ARO MURIMURI [W911NF14-0247]; DOE BES AwardUnited States Department of Energy (DOE) [DE-SC0019300]; National Science Foundation (NSF)National Science Foundation (NSF) [ACI-1053575]The authors would like to thank Venkataraman Swaminathan and Daniel Larson for helpful discussions. S.S. acknowledges support by the ARO MURI (Award No. W911NF14-0247). This work was supported by the DOE BES Award No. DE-SC0019300. For calculations, computational resources were used on the Odyssey cluster, which is maintained by the FAS Research Computing Group at Harvard University, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (NSF) under Grant No. ACI-1053575