Arm Chair Research Articles

Polarization independent and non-reciprocal absorption in multi-layer anisotropic black phosphorus metamaterials.

The polarization independent and non-reciprocal absorption is particularly crucial for the realization of non-reciprocal absorption devices. Herein, we proposed and studied the absorption response of two- and three-layer anisotropic black phosphorus (BP) metamaterials by using the finite-difference time-domain (FDTD) simulation and radiation oscillator theory (ROT) analysis. It is shown that, due to unequal surface plasmon resonant modes excited in zigzag (ZZ) and armchair (AC) directions of the anisotropic BP layer, tunable polarization independent and dependent absorption can be achieved for the proposed multi-layer anisotropic BP metamaterials with AC-AC, AC-ZZ, ZZ-AC, AC-AC-φ, AC-ZZ-φ, and ZZ-AC-φ configurations. Especially, the polarization independent absorption also can be realized for odd-layer BP nanostructures. Unlike previous reports, polarization independence only can be achieved in the even-layer BP nanostructure. Moreover, tunable non-reciprocal absorption with the extremely large non-reciprocal degree (NRD) is also found in the case of AC-ZZ and ZZ-AC configurations and AC-ZZ-φ and ZZ-AC-φ configurations. These results may open up the possibility of realizing tunable polarization independent and non-reciprocal plasmonic devices based on 2D materials.

Density functional theory study of the mechanical behavior of silicene and development of a Tersoff interatomic potential model tailored for elastic behavior

Silicene, a graphene-like 2D material made from Si atoms, has been fabricated and studied for its promising applications in micro/nanoelectronics. For the reliable function of silicene devices, it is important to investigate silicene’s mechanical properties. In this study, the authors conducted density functional theory (DFT) simulations of mechanical tests of silicene and investigated the elastic modulus and mechanical response such as structural transformation. In addition, the authors optimized the Tersoff potential parameters using a gradient-based minimization with a grid search method in hyperdimensional parameter space, to match the DFT calculation results in the elastic regime. With the new parameter set, the elastic moduli of silicene in the zigzag (ZZ) and armchair (AC) directions were computed with molecular statics (MS) simulations and compared with those of other Si interatomic potential models and DFT results. In addition, uniaxial tensile tests along the ZZ and AC directions were performed to examine how far the Tersoff model is transferable with our new parameter set to describe the nonlinear mechanical behavior of silicene. The results of uniaxial tensile tests suggest that the angle penalty function in the Tersoff model needs to be modified and that the stress–strain curve predicted with this modification shows improvement compared to the original function.

Reconstructed edges of T phase transition metal dichalcogenides

As surfaces are to bulk materials, edge configurations greatly influence the properties and ensuing applications of two-dimensional (2D) materials. Being a large family of “beyond graphene”, 2D transition metal dichalcogenides (TMDCs) have many potential applications due to diverse phases and tunable properties. Unlike the well-studied H phase TMDCs initiated by MoS2, the edge structures of T phase TMDCs remain poorly studied. Herein, taking freestanding T phase PtSe2 as a prototype, we rationally construct 43 edge structures on the basis of conventional zigzag (ZZ) and armchair (AC) edges, and systematically evaluate their thermodynamic stabilities and relevant properties using density functional theory. Twelve most stable reconstructed edges (five ZZ-oriented and seven AC-oriented) are found to be highly stable at different experimental conditions, which can be achieved by precise control of synthesis conditions. Further Wulff constructions suggest hexagonal shapes with ZZ edges would be the equilibrium structures of the freestanding T phase PtSe2 clusters or quantum dots. Electronic structure calculations show tunable band gap via edge reconstruction. Some reconstructed edges also exhibit excellent catalytic activity for hydrogen evolution reaction. Our work is expected to advance the knowledge of edge structures of T-phase TMDCs, and motivates materials design via TMDC edge engineering.

The Mechanism of Graphene Vapor-Solid Growth on Insulating Substrates.

Wafer-scale single-crystal graphene film directly grown on insulating substrates via the chemical vapor deposition (CVD) method is desired for building high-performance graphene-based devices. In comparison with the well-studied mechanism of graphene growth on transition metal substrates, the lack of understanding on the mechanism of graphene growth on insulating surfaces greatly hinders the progress. Here, by using first-principles calculation, we systematically explored the absorption of various carbon species CHx (x = 0, 1, 2, 3, 4) on three typical insulating substrates [h-BN, sapphire, and quartz] and reveal that graphene growth on an insulating surface is dominated by the reaction of active carbon species with the hydrogen-passivated graphene edges and thus is less sensitive to the type of the substrate. The dominating gas phase precursor, CH3, plays two key roles in graphene CVD growth on an insulating substrate: (i) to feed the graphene growth and (ii) to remove excessive hydrogen atoms from the edge of graphene. The threshold reaction barriers for the growth of graphene armchair (AC) and zigzag (ZZ) edges were calculated as 3.00 and 1.94 eV, respectively; thus the ZZ edge grows faster than the AC one. Our theory successfully explained why the circumference of a graphene island grown on insulating substrates is generally dominated by AC edges, which is a long-standing puzzle of graphene growth. In addition, the very slow graphene growth rate on an insulating substrate is calculated and agrees well with existing experimental observations. The comprehensive insights on the graphene growth on insulating surfaces at the atomic scale provide guidance on the experimental design for high-quality graphene growth on insulating substrates.

Dynamically tunable narrowband anisotropic total absorption in monolayer black phosphorus based on critical coupling

A dynamically tunable anisotropic narrowband absorber based on monolayer black phosphorous (BP) is proposed in the terahertz (THz) band. The proposed absorber consists of a monolayer BP and a silicon (Si) grating, which is placed on a silica (SiO2) isolation layer and a gold (Au) substrate. The benefit from the critical coupling mechanism with guided resonance is the efficiency of the absorption can reach 99.9% in the armchair (AC) direction and the natural anisotropy of BP makes it only 87.2% in the zigzag (ZZ) direction. Numerical and theoretical studies show that the absorption efficiency of the structure is operatively controlled by critical coupling conditions, including the geometric parameters of the Si grating, the electron doping of BP and the angle of incident light, etc. More importantly, in the absence of plasmon response, this structure greatly enhances the interaction between light and matter in monolayer BP. In particular, there are several advantages in this structure, such as extremely high-efficiency absorption, excellent tunability, outstanding intrinsic anisotropy and easy manufacturing, which will show unusual and promising potential applications in the design of BP-based tunable high-performance devices.

Fracture behavior dependent on crack-tip shapes in nanoscale crack-defect monolayer boron nitride sheets

ABSTRACT Nanoscale defects, including cracks, circular holes, and the triangular-shaped defects, often occur in the growth of boron nitride nanosheets (BNNS). In this study, the fracture behavior of chiral BNNS with different crack-tip shapes and the interactions of nanoscale crack-defects are studied using molecular dynamics (MD) simulations and finite element (FE) analysis. Both MD and FE results indicate that the fracture strength of BNNS with two crack tips (t = 2) is significantly higher than that with one crack tip (t = 1), in which the difference in zigzag (ZZ) direction is more obvious than that in armchair (AC) direction, mainly due to the fact that the change of bond angles near the crack tips is more substantial in the ZZ direction than those in the AC direction. Our results show that the fracture strength of BNNS strongly depends on crack-tip shapes, chiral angles, the defect-to-crack tip spacing and deflection angles. Checking against the current MD simulations and FE analysis shows the present results are reasonable. This study should be of great importance for enhancing the fracture performance of BNNS by modulating their crack-tip shapes and the interactions of nanoscale crack-defects.

Catalyst particle size dependent carbon nanotube cloning

The single walled carbon nanotube (SWCNT) cloning or its seeded growth with a catalyst particle docked on one of its ends was proposed as the ultimate solution for chirality-specific SWCNT synthesis and, therefore, revealing the effect of catalyst particle size on the robustness of the SWCNT cloning is crucial for experimental designs. Through systematic atomic simulations, we have clearly demonstrated that the robustness of SWCNT cloning depends on both the chiral angle of the SWCNT and the ratio of the diameters between the SWCNT and the docked catalyst particle. The zigzag (ZZ) or near ZZ SWCNTs are highly sensitive to the size of the docked catalyst particle. A small change of the catalyst particle size could lead to a variation of the SWCNT’s chirality. In contrast, armchair (AC) or near-AC SWCNTs are less sensitive to the size of the docked catalyst particle and able to maintain their original chiralities within ∼30% variation of the catalyst particle size. This study greatly deepens our understanding of SWCNT’s growth mechanism and provides a quantitative guidance for SWCNT cloning.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Prolonged unsupervised Nordic walking and walking exercise following six months of supervision in adults with overweight and obesity: A randomised clinical trial

Background and aimsThis study is a randomized trial that examined the effects of 6 months of unsupervised Nordic walking (NW) and walking (W) exercise following 6 months of supervised training in overweight/obese adults. Methods and resultsAfter a 6-month program of diet and supervised training participants (n = 27) of NW (66 ± 7 yrs, body mass index (BMI) 34 ± 5) and W (66 ± 8 yrs, BMI 32 ± 5) group continue the training without supervision for other 6 months. Steps count and mean heart rate (HRmean) were performed in each session; anthropometric and body composition, aerobic capacity and strength of the upper and lower limbs were evaluated at baseline, after 6 months of supervised and 6 months of unsupervised training. In the unsupervised training, monthly sessions and steps count decreased over time in both groups (p < 0.05), with no significant changes in HRmean. Compared to the supervised phase, adherence decreased significantly only in the W group in the last 3 months of unsupervised training. Compared to baseline in both groups BMI did not change, but W group lost total fat; only the NW group maintained (p < 0.05) the gains in arm curl (33%) and chair stand (31%); both groups improved in six-minute walking test (p < 0.05). ConclusionDespite unsupervised training was not effective for a further increase in performance, participants, especially in NW, maintained some of the improvements achieved during the supervision. However, the presence of instructor that guides training, may enhance adherence and health benefits of NW and W exercise. Clinical trial registrationclinicaltrials.gov Identifier: NCT03212391 (July 11, 2017).

COVID-19 Transition: Does virtual exercise maintain physical function?

Background: In March 2020, COVID-19 mandates to restrict face to face exercise and group based gatherings were enacted. These mandates were enforced within most states in the US. Gerofit, a facility-based exercise program for older Veterans in Durham, NC, transitioned to remote virtual exercise instruction to accommodate continuity of care. Objectives: To explore whether remote virtual exercise (RVE) can sustain physical function within individuals previously participating in onsite face to face exercise (OFF). Methods: Physical function assessments performed during OFF were compared with assessments conducted remotely over virtual platform. Assessments included the 30-second arm curl, the 30-second chair stand, time to complete five chair stands, and either 6-minute walk or 2-minute step test. All assessments for RVE were completed via a remote virtual platform. Only participants enrolled in both OFF and home based RVE with functional assessments within 6-months of pre and post COVID-19 transition were compared. Descriptive comparisons, opposed to statistical, were reported due to the limited sample size. Results: Fourteen OFF Gerofit participants were reassessed remotely within the first 6-months of transitioning to RVE (12 male, 2 female, mean age 73.1, mean body mass index 31.5). Functional assessments between OFF versus RVE were arm curls (21.0 vs 20.4 repetitions), chair stands (15.0 vs 17.5 repetitions), and time to 5 chair stands (9.0 vs 8.4 seconds). Cardiovascular function, reported in normalized percentiles (46.4%tile vs 58.9%tile) Conclusion: Among older Veterans engaged in regular structured exercise, physical function was preserved with transition to virtual exercise.

A reactive molecular dynamics study of hyperthermal atomic oxygen erosion mechanisms for graphene sheets

Carbon-based composite materials are widely used in the aerospace field due to their light weight and excellent physical/chemical properties. The mechanisms of the erosion process, e.g., surface catalysis and ablation, during the impact of oxygen atoms, however, remain unclear. In this study, the surface catalysis and ablation behavior during the erosion process of hyperthermal atomic oxygens were achieved through the molecular dynamics method with the reactive force field potential. The concomitant impacts of energy flux density of energetic oxygen atoms, the presence of multiple layers beneath the graphene sheet, and the morphology of graphite surfaces, i.e., graphite basal plane, armchair (AC) edge surface, and zigzag edge surface, respectively, were discussed. The results show that the adsorption of oxygen atoms dominates at the beginning by generating O2 molecules, suggesting the importance of surface catalytic for any ablation study. A unique “layer-by-layer” ablation phenomenon by hyperthermal atomic oxygen is observed for multi-layered graphite slab, and the ablation rate reduces as the number of graphene layers increases. The morphology/structure of the surface shows significant effects on the ablation rate, with AC surfaces showing the largest etching rate and the basal one showing the lowest. The low binding energies of the AC edge are responsible for the difficulty in the formation of stable functional group structures to resist the etching of high-enthalpy oxygen atoms. Such revelation of the detailed surface catalysis and ablation mechanism at the atomistic scale provides insight into design of future materials for the augmentation of the thermal protection effect.

Tuning the electronic properties of hydrogen passivated C3N nanoribbons through van der Waals stacking

The two-dimensional (2D) C3N has emerged as a material with promising applications in high performance device owing to its intrinsic bandgap and tunable electronic properties. Although there are several reports about the bandgap tuning of C3N via stacking or forming nanoribbon, bandgap modulation of bilayer C3N nanoribbons (C3NNRs) with various edge structures is still far from well understood. Here, based on extensive first-principles calculations, we demonstrated the effective bandgap engineering of C3N by cutting it into hydrogen passivated C3NNRs and stacking them into bilayer het-erostructures. It was found that armchair (AC) C3NNRs with three types of edge structures are all semiconductors, while only zigzag (ZZ) C3NNRs with edges composed of both C and N atoms (ZZ-CN/CN) are semiconductors. The bandgaps of all semiconducting C3NNRs are larger than that of C3N nanosheet. More interestingly, AC-C3NNRs with CN/CN edges (AC-CN/CN) possess direct bandgap while ZZ-CN/CN have indirect bandgap. Compared with the monolayer C3NNR, the bandgaps of bilayer C3NNRs can be greatly modulated via different stacking orders and edge structures, varying from 0.43 eV for ZZ-CN/CN with AB’-stacking to 0.04 eV for AC-CN/CN with AA-stacking. Particularly, transition from direct to indirect bandgap was observed in the bilayer AC-CN/CN heterostructure with AA’-stacking, and the indirect-to-direct transition was found in the bilayer ZZ-CN/CN with AB-stacking. This work provides insights into the effective bandgap engineering of C3N and offers a new opportunity for its applications in nano-electronics and optoelectronic devices.

Indium doped phosphorene as a potential gas sensor: a study using density functional theory

The adsorption of molecules NH3, CH4, and H2O are studied on pristine and indium doped phosphorene (In@P). For adsorption, armchair (AC), zigzag, and isotropic/square supercells are used. We have calculated structural, adsorption energies and electronic properties using the first-principles method. The most stable configurations for adsorption on doped phosphorene are determined, and the adsorption sites, energies, binding distances, electronic structure, and charge transfers are calculated. The adsorption energies of adsorbate molecules on In@P are significantly improved compared to undoped phosphorene. The electrical conductivity is calculated using the Boltzmann transport equation. It is found that all three molecules are physisorbed on In@P, and NH3 and H2O can be detected with high sensitivity. The outstanding recovery time (few μs) is found for H2O and NH3 on AC supercells. These findings indicate that In@P could be a promising material for gas sensing applications.

Characterization of CNT and It’s carrier mobility dependency on diameter in Back-Gated CNT-FET

Based on the chiral indices, different structures like an arm chair, zigzag and chiral are formed and carbon nanotube (CNT) shows semiconducting and metallic properties. This is a very interesting phenomenon in the field of nanoelectronics research. Some fundamental parameters like diameter, bandgap, number of hexagons in a unit cell, and temperature affect the carbon nanotube properties. These parameters may also change the carrier mobility in the carbon nanotube based back-gated field effect transistor (FET). Diameter (directly proportional with mobility) and temperature (inversely proportional to the mobility) dependency is very much important on charge density.

Anisotropic basic electronic properties of strained black phosphorene

The gap modulation is an intriguing feature of the single-layer black phosphorene (BP) in optoelectronics. In this paper, the uniaxial and biaxial strain effects are systematically investigated to tailor the band gap, Fermi velocity, and effective mass along both armchair (AC) and zigzag (ZZ) directions in BP. The tight-binding model and the Harrison rule propose the electronic phase transitions. In the presence of uniaxial strains, in-plane strains keep the semiconducting phase, while the semiconductor-to-insulator and semiconductor-to-semimetal phase transitions emerge for biaxial strains. Although the anisotropic Fermi velocity introduces critical strains for which the concavity inversion occurs, Fermi velocity increases (decreases) with in-plane (out-of-plane) strain along the AC direction, while it remains constant along the ZZ direction. Further, we found that the heavy (light)-effective mass is related to the ZZ (AC) direction. Moreover, both in-plane and out-of-plane strains lead to effective mass sign-changing due to the concavity inversion of bands. Additionally, the heaviest effective masses are dedicated to critical strains corresponding to the phase transitions after which the concavity inversion of bands emerges. Our results propose interesting information for the strain tuning of electronic properties in BP.

A comparison study between the Lennard-Jones and DRIP potentials for friction of graphene layers

Graphene is a promising solid lubricant, in particular for small-length scale devices such as nano/micro-electro-mechanical systems. Atomistic simulations such as molecular dynamics is a popular tool to study the frictional behaviors of graphene layers and it is of critical importance to accurately describe the interlayer interactions in order to give a reliable prediction on the friction of graphene. In this study, the interlayer interactions between graphene layers are examined by using two interatomic potentials, Lennard-Jones (LJ) potential and dihedral-angle-corrected registry-dependent interlayer (DRIP) potential, in the molecular dynamics simulations of friction sliding of multilayer graphene structures. While both potentials have the identical attractive interactions, DRIP models the repulsive interaction by registry-dependent modifiers considering transverse distance and dihedral angle. The sliding simulations mimic the atomic force microscope experiment and are carried out in the zigzag (ZZ) and armchair (AC) directions. The simulation results reveal that the friction forces of the DRIP models are about one order of magnitude larger than those of the LJ models in both sliding directions. It turns out that the modification of the repulsive term in DRIP introduces additional energy corrugations which increase the friction force. Moreover, it is found that the sliding direction is another important factor on friction force of graphene layers so that in both LJ and DRIP models the friction force in the AC sliding direction is about two to three times larger than that of the ZZ sliding.

Muscle Mass and Muscle Strength Relationships to Balance: The Role of Age and Physical Activity.

To investigate the relations of balance to muscle mass (MM) and muscle strength (MS), depending on age and physical activity, which is of particular importance to functional independence in older people. This cross-sectional study included 802 volunteers (69.82 ± 5.60 years). The Fullerton Advanced Balance scale was used to assess balance and a composite score, including arm curl and chair stand tests for assessing MS. MM was estimated by dual-energy X-ray absorptiometry and physical activity by questionnaire. Greater MM (r = .26, p < .001) and MS (r = .53, p < .001) were positively correlated to balance. The old-old adults and less active individuals attained lower balance. Notably, moderation and subsequent simple slope analyses revealed that the relations of balance, MM, and MS were larger in less active and the old-old adults. The old-old and less active adults were more prone to muscle weakness and balance impairments. Tailored interventions should particularly consider these vulnerable groups.

The effect of strain on the zigzag and armchair phosphorene nanoribbon

Abstract In this work, the band structure and electronic properties of different nanoribbons of phosphorene along two typical crystal directions of armchair (AM) and zigzag (ZZ) were investigated under compression and tensile strain by density functional theory. The results were compared to the band structure and electronic properties of phosphorene monolayer at similar conditions. The electronic properties of armchair nanoribbons revealed to be resistant to tensile strain in the direction of the armchair, consistent to phosphorene monolayer, but they undergo band structure change due to the applied compression strain at armchair direction. By comparing the effects of strain on the zigzag nanoribbon and phosphorene monolayer, it was found that the zigzag direction neither tolerates the tensile strain nor resists compression strain.

Defect Interaction and Deformation in Graphene

Interactions between defects in graphene and the lattice distortion and electronic charge localization induced by the defect interactions are studied by tight-binding (TB) calculations using the recently developed three-center TB potential model. The interaction between two 5–7 Stone–Wales defects gliding along the zig-zag (ZZ) direction of graphene, which has been observed by experiment, is studied at first to validate the TB calculations. Reconstructed divacancy defect pairs and di-adatom defect pairs separated along the glide ZZ and armchair (AC) directions in graphene, respectively, are then studied. We show that the characteristics (i.e., attractive or repulsive) and the strength of interactions between these defects are dependent on the type of defects and on the direction and distance of the defect separation on graphene. Although elastic interaction due to graphene lattice distortion induced by the defect has significant contribution to the total interaction energy, redistribution of electron char...

Mystery Music

Mystery Music Eric Burger (bio) I liked the joyful sound of the piano coming from the open door where a few departing partygoers giggled, arms linked. I went in. But despite the pianist's spirit and those lively partygoers, this party was on its last legs. A brown-haired woman with a brass hoop around her neck and an empty martini glass in her hand was sleeping, curled in an armchair. A pudgy man in a silver shirt, looking both pleased with and done with his party effort, tucked in his shirt and said to his friend, "I just can't help but leave a bit of myself everywhere I go." Then they left. None of him lingered in the air. There was just a winding down down down feeling and the now-quieter piano music filling the big, uncarpeted space. I walked over to the hoop woman to check out her grip on the martini glass. The piano playing suddenly got icy and jagged, like some brutal directive. It was telling me something. But what was I supposed to do, whip out a kitchen knife and stab her for crying out loud? Have a psychotic break? The piano must have been in the back room, but it felt like it was on top of me, inside me. Then the playing switched to wooly and sad, like the feeling that might accompany remembrance of a keen-eyed tenth-grade girlfriend who first drifted away—you saw her sometimes in the hallways—and then moved to Argentina. But what to do with this? This was a bossy, yet vague piano. I sat down in the empty arm chair next to the hoop woman, who still slept with the martini glass clutched in her fist like a rattle. The piano playing got quieter, sweeter. I closed my eyes. A sunlit commuter train clattered into my head bearing characters from my childhood that I hadn't thought about in decades: an elderly customer on my paper route who had a parakeet named Clyde, a massively mustachioed hotdog vendor whom I found curious once at a basketball game, a quiet girl from art class who threw crayons at the window. The train slowed down down down while I remembered, while people lost from my life came back to me. A strange mercy from the universe, which tries to help us in ways clumsy and divine. [End Page 49] Eric Burger Eric Burger has received fellowships/awards from the Wisconsin Institute for Creative Writing, the Arizona Commission on the Arts, the Sewanee Writers' Conference, the Wesleyan Writers Conference, and Writers at Work. His poems have appeared in Denver Quarterly, Black Warrior Review, The Missouri Review Online, Harvard Review Online, Indiana Review, Rattle, Quarterly West, The Southeast Review, and many others. He is a senior instructor in the University of Colorado-Boulder's Program for Writing and Rhetoric, and lives in Longmont, CO. Copyright © 2019 Emerson College