483 research outputs found
Effects of low energy electron irradiation on formation of nitrogen-vacancy centers in single-crystal diamond
Exposure to beams of low energy electrons (2 to 30 keV) in a scanning
electron microscope locally induces formation of NV-centers without thermal
annealing in diamonds that have been implanted with nitrogen ions. We find that
non-thermal, electron beam induced NV-formation is about four times less
efficient than thermal annealing. But NV-center formation in a consecutive
thermal annealing step (800C) following exposure to low energy electrons
increases by a factor of up to 1.8 compared to thermal annealing alone. These
observations point to reconstruction of nitrogen-vacancy complexes induced by
electronic excitations from low energy electrons as an NV-center formation
mechanism and identify local electronic excitations as a means for spatially
controlled room-temperature NV-center formation
Local formation of nitrogen-vacancy centers in diamond by swift heavy ions
We exposed nitrogen-implanted diamonds to beams of swift uranium and gold
ions (~1 GeV) and find that these irradiations lead directly to the formation
of nitrogen vacancy (NV) centers, without thermal annealing. We compare the
photoluminescence intensities of swift heavy ion activated NV- centers to those
formed by irradiation with low-energy electrons and by thermal annealing. NV-
yields from irradiations with swift heavy ions are 0.1 of yields from low
energy electrons and 0.02 of yields from thermal annealing. We discuss possible
mechanisms of NV-center formation by swift heavy ions such as electronic
excitations and thermal spikes. While forming NV centers with low efficiency,
swift heavy ions enable the formation of three dimensional NV- assemblies over
relatively large distances of tens of micrometers. Further, our results show
that NV-center formation is a local probe of (partial) lattice damage
relaxation induced by electronic excitations from swift heavy ions in diamond.Comment: to be published in Journal of Applied Physic
Blue-Light-Emitting Color Centers in High-Quality Hexagonal Boron Nitride
Light emitters in wide band gap semiconductors are of great fundamental
interest and have potential as optically addressable qubits. Here we describe
the discovery of a new color center in high-quality hexagonal boron nitride
(h-BN) with a sharp emission line at 435 nm. The emitters are activated and
deactivated by electron beam irradiation and have spectral and temporal
characteristics consistent with atomic color centers weakly coupled to lattice
vibrations. The emitters are conspicuously absent from commercially available
h-BN and are only present in ultra-high-quality h-BN grown using a
high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these
emitters originate from impurities or related defects specific to this unique
synthetic route. Our results imply that the light emission is activated and
deactivated by electron beam manipulation of the charge state of an
impurity-defect complex
A framework for mapping cultural resources in landscape conservation planning
Abstract Cultural resources can be an important means of connecting people to conservation efforts. Currently, it is difficult to identify and map these tangible and intangible aspects of the landscape. We propose a framework to include cultural resources in spatial conservation planning that acknowledges the different scales of importance and management of sites. Through categorizing and mapping sites of national, state, local, interest group, and community management, cultural resources can be measured and included in existing planning tools. Data on cultural resources are a primary limitation due to availability and fragmentation. Until data sources improve, our framework provides a stopgap that allows for cultural resources to be included in conservation‐planning efforts
Electrically driven photon emission from individual atomic defects in monolayer WS2.
Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources
Effects of Crime Type and Location on Park Use Behavior
Crime and the fear of crime can be a barrier to park use, and locations of crimes can have varied effects. Unsafe areas in or around the park, around the residence, or along the route to the park can alter park use behavior. Our study aimed to examine associations between objective measures of types and location of crimes and park use behaviors. In 2013 we surveyed a sample (N = 230) of residents in Greensboro, North Carolina, about park use, with responses matched to objective crime and spatial measures. We measured all crimes and violent crimes near home, near the closest park, and along the shortest route between home and park. By using ordered and binary logistic modeling, we examined the relationships between the locations of crime and park use and duration of park visit, park rating, and never visiting parks. Additional models included distance to the closest park. Increased crime in parks and near home was associated with fewer park visits. Greater violent crime in all locations was related to fewer park visits. Park ratings were lower for parks with high violent crime rates. Given the importance of parks as settings for outdoor recreation and physical activity, crime may have a detrimental effect on physical activity and, therefore, public health
Associations of parks, greenness, and blue space with cardiovascular and respiratory disease hospitalization in the US Medicare cohort
Natural environments have been linked to decreased risk of cardiovascular disease (CVD) and respiratory disease (RSD) mortality. However, few cohort studies have looked at associations of natural environments with CVD or RSD hospitalization. The aim of this study was to evaluate these associations in a cohort of U.S. Medicare beneficiaries (∼63 million individuals). Our open cohort included all fee-for-service Medicare beneficiaries (2000-2016), aged ≥65, living in the contiguous U.S. We assessed zip code-level park cover based on the United States Geological Survey Protected Areas Database, average greenness (Normalized Difference Vegetation Index, NDVI), and percent blue space cover based on Landsat satellite images. Cox-equivalent Poisson models were used to estimate associations of the exposures with first CVD and RSD hospitalization in the full cohort and among those living in urban zip codes (≥1000 persons/mile2). NDVI was weakly negatively correlated with percent park cover (Spearman ρ = -0.23) and not correlated with percent blue space (Spearman ρ = 0.00). After adjustment for potential confounders, percent park cover was not associated with CVD or RSD hospitalization in the full or urban population. An IQR (0.27) increase in NDVI was negatively associated with CVD (HR: 0.97, 95%CI: 0.96, 0.97), but not with RSD hospitalization (HR: 0.99, 95%CI: 0.98, 1.00). In urban zip codes, an IQR increase in NDVI was positively associated with RSD hospitalization (HR: 1.02, 95%CI: 1.00, 1.03). In stratified analyses, percent park cover was negatively associated with CVD and RSD hospitalization for Medicaid eligible individuals and individuals living in low socioeconomic status neighborhoods in the urban population. We observed no associations of percent blue space cover with CVD or RSD hospitalization. This study suggests that natural environments may benefit cardiorespiratory health; however, benefits may be limited to certain contexts and certain health outcomes
Associations of greenness, parks, and blue space with neurodegenerative disease hospitalizations among older US adults
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