Color Centers in Hexagonal Boron Nitride.

Atomically thin two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as an essential material for the encapsulation layer in van der Waals heterostructures and efficient deep ultraviolet optoelectronics. This is primarily due to its remarkable physical properties and ultrawide bandgap (close to 6 eV, and even larger in some cases) properties. Color centers in hBN refer to intrinsic vacancies and extrinsic impurities within the 2D crystal lattice, which result in distinct optical properties in the ultraviolet (UV) to near-infrared (IR) range. Furthermore, each color center in hBN exhibits a unique emission spectrum and possesses various spin properties. These characteristics open up possibilities for the development of next-generation optoelectronics and quantum information applications, including room-temperature single-photon sources and quantum sensors. Here, we provide a comprehensive overview of the atomic configuration, optical and quantum properties, and different techniques employed for the formation of color centers in hBN. A deep understanding of color centers in hBN allows for advances in the development of next-generation UV optoelectronic applications, solid-state quantum technologies, and nanophotonics by harnessing the exceptional capabilities offered by hBN color centers.

Distribution of 239,240Pu in marine products from the seas around the Korean Peninsula after the Fukushima nuclear power plant accident.

In March 2011, an earthquake caused the shutdown of the active reactors at the Fukushima Daiichi Nuclear Power Plant (NPP), with the succeeding tsunami resulting in the release of radioactively contaminated water into the adjacent Japanese coastal waters. Marine biota selected from various trophic levels were collected in Korean coastal waters throughout 2014 and 2015 and their plutonium levels were measured to evaluate the radioactive contamination levels in the marine organisms that constitute the primary seafood diet in the Republic of Korea (ROK). The results showed that the activity concentrations of 239,240Pu in plankton, macroalgae, mollusks, crustaceans, and cephalopods ranged from 13 to 58, 0.64 to 0.80, 0.94 to 5.40, 0.06 to 0.50, and 0.26 mBq kg-1 of wet weight (w.w.), respectively. The activity concentrations of 239,240Pu measured in the muscles of fish varied from 0.09 to 0.30 mBq kg-1 (w.w.), relatively low values compared to those in other groups regardless of fish species, size, and sampling area. The concentration characteristics of 239,240Pu in the various organs in the respective marine products revealed that the internal organs showed higher concentrations than the muscle or skin (or exoskeleton). The highest concentration of 239,240Pu was measured in the viscera of an abalone, which had an activity concentration of 6.31 mBq kg-1 (w.w.). The concentration factors (CFs) in the >300-µm fraction of plankton and in anchovy, shrimp, and mackerel ranged 67-84% of the International Atomic Energy Agency (IAEA)-recommended values, although the CF in macroalgae was much lower at approximately 5% of IAEA values, suggesting a wide range of 239,240Pu CF in macroalgae. The mean transfer factor (TF) between macroalgae (0.6-0.8 mBq kg-1) and abalone (5.4 mBq kg-1) was estimated to be 7.5, implying that effective Pu transfer occurred between the two species. These figures equate to annual effective doses of 239,240Pu to Koreans through consumption of macroalgae, shellfish, and fish of 1.8 × 10-6, 1.4 × 10-6, and 7.1 × 10-7 mSv yr-1, respectively, and a total dose of 3.9 × 10-6 mSv yr-1, values that are negligible compared to the annual effective dose limit of 1 mSv yr-1.

Anthropogenic gadolinium in lakes and rivers near metrocities in Korea.

We measured dissolved rare earth elements (REEs) in the water samples from Shihwa Lake (SL), which was assumed to be highly polluted, and in the downstream portion of the Han River (HR), which runs through Seoul, Korea. Among the investigated REEs, only gadolinium (Gd) was found to be significantly enhanced after REE concentrations were shale-normalized (SN). The calculated Gd anomaly (Gd/Gd* = 3 × GdSN/(SmSN + 2 × TbSN)) was about 1.5 ± 0.1 (n = 16) in SL and 1.6 ± 0.3 in the HR (n = 26) water relative to other types of natural water such as groundwater, seawater, and river water in uncontaminated areas (Gd/Gd* ∼ 1.2, n > 400). These significant Gd anomalies seem to be due to the inputs of anthropogenic Gd (Gdanth), especially from the use of Gd-based contrast agents for magnetic resonance imaging (MRI) tests from a number of hospitals and medical institutes surrounding our study areas. The amount of Gdanth was estimated to be 190 ± 80 g and 680 ± 360 kg Gd in SL and the HR (watersheds in our study area), respectively. The Gdanth flux to the Yellow Sea from the HR is estimated to be 530 ± 330 g Gd d-1. These results suggest that quantitative evaluation of anthropogenic REEs in natural waters near big cities is needed, because considerable amounts of REEs are now used by modern high-tech industries.

[Distal Ileal Lymphoma Presenting Ileocecal Intussusception with Spontaneous Reduction].

Intussusception is a rare disease in adults. A demonstrable etiology is found in approximately 85% of all cases, and approximately 40% of them are caused by malignant tumors. A 65-year-old patient visited the outpatient department with mild abdominal pain without other symptoms. The initial laboratory test and simple X-ray showed normal findings. CT revealed intussusception in the ileocecal area. The initial colonoscopic biopsy revealed atypical cells. Follow up colonoscopy showed spontaneous reduction of the intussusception. Diffuse large B-cell lymphoma was suspected in the second colonoscopic biopsy. An elective operation was performed. This case reports a case of a spontaneous reduction of adult intussusception with a brief review of literature.

On the role of 210Bi in the apparent disequilibrium of 210Pb-210Po at sea.

The disequilibrium of the grandparent-daughter pair 210Pb (t1/2=22.3 years)-210Po (t1/2=138 days) has been used to estimate the export fluxes of particulate organic carbon in the ocean using particulate-matter-associated 210Po. 210Po is produced from 210Bi, not from 210Pb. The half-life of 210Bi (t1/2=5.01 days) is sufficiently long compared to the rates of biological particle formation and decomposition or dissolution occurring at sea. The role of 210Bi has not yet been assessed quantitatively in the apparent disequilibrium between 210Pb and 210Po, partly due to the non-existence of 210Bi depth profile measurements at sea up to now. However, greater affinity of 210Bi over 210Po and 210Pb was found recently in coastal waters and phytoplankton 207Bi uptake experiments. Build upon these findings, we developed a primitive and simple analytical approach to elucidate the role of 210Bi in the 210Po-210Pb pair in the ocean using a simplified two-box irreversible steady-state ocean model. We assumed that the activity concentrations in the dissolved and particulate phases of 210Pb, 210Bi, and 210Po in a given water column are solely determined by the concentration of the particles, their input and output, the distribution coefficients between dissolved and particulate phases, and decay constants of these radionuclides in the steady-state ocean. The 210Bi contribution to the 210Pb-210Po activity difference in seawater is found to be significant, therefore, it needs to be considered in estimating particle fluxes using 210Pb-210Po secular equilibrium at sea.

Distribution and accumulation of artificial radionuclides in marine products around Korean Peninsula.

We investigated the distributions of the artificial radionuclides 137Cs, 239+240Pu, and 90Sr in the various marine products around Korean Peninsula collected in 2015-2017. The activities of 137Cs, 239+240Pu, and 90Sr were in the ranges of 12.2-258.4 mBq kg-1, 0.014-1.879 mBq kg-1, and 11.3-18.3 mBq kg-1, respectively. The higher 239+240Pu (a factor of 10-50) relative to fishes were observed in mollusks. 137Cs in fishes were significantly enriched with growths. The calculated concentration factors (CFs) of 137Cs, 239+240Pu, and 90Sr in marine products were 7-143, 3-407, and 15-25, respectively. Overall, no noticeable artificial radioactivity was detected in marine organisms when comparing these results with previous data reported before the Fukushima nuclear accident and recommended CF values. The annual effective doses of 137Cs, 239+240Pu, and 90Sr by seafood consumption in South Korea were estimated to be 1.1 × 10-4, 1.6 × 10-5 and, 2.9 × 10-5 mSv yr-1, respectively, which is insignificant relative to that of natural radionuclides.

Fractionation and risk assessment of metals in sediments of an ocean dumping site.

Sediments of Yellow Sea dumping sites (YSDS) collected in 2015 were analyzed using the European Community Bureau of Reference (BCR) sequential extraction method to assess the contamination and potential risk to the environment. Cr, Ni, Cu, and Zn exhibited the most dominant residual fraction, whereas Cd was mostly in the exchangeable fraction and Pb in the reducible fraction. Cr contains a significant amount of oxidizable fraction in the dumping area due to the dumping of tannery sludge with high concentrations of Cr, mainly in the organic matter-bound form. The global contamination factor (GCF) showed that high non-residual fractions of Cd and Pb contributed considerably to contamination. Nevertheless, modified potential ecological risk index (MRI) suggested low ecological risk for metals in YSDS because of the low total content of Cd and the small mobile fraction of other metals.

Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe2 Transistors.

We investigate the valley Hall effect (VHE) in monolayer WSe2 field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n -doped MoS2, we observe the VHE in WSe2 in both the n - and p -doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/µm, integrated over the edge region in the p -doped regime. Assuming a spin diffusion length less than 0.1 µm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

Ultrafast Graphene Light Emitters.

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

Native aortic valve thrombosis resembling papillary fibroelastoma.

The differential diagnosis of cardiac mass is important in determining the therapeutic plan and avoiding unnecessary surgical intervention. Non-invasive imaging methods would be useful in the diagnosis of suspected cardiac mass, because they may provide earlier diagnosis and more accurate assessment of cardiac mass. Native aortic valve thrombosis is a rare disorder and difficult to differentiate from a tumor, and in particular, a papillary fibroelastoma. Thus, the clinical decision making with imaging modalities should be performed cautiously. We recently met a female patient who had a aortic valve mass resembling papillary fibroelastoma in normal native valve. The patient underwent a surgical resection and the pathologic finding showed an organized thrombus with no evidence of papillary fibroelastoma.

Evaluation of plating conditions for the recovery of ²¹°Po on a Ag planchet.

The polonium-210 in the sea and its radiological consequences have been widely studied. Current processes for (210)Po recovery from seawater vary significantly. We compared selected processes to determine optimal conditions for recovery in modestly equipped laboratories. Plating (210)Po onto a Ag planchet with constant stirring for 15 h at room temperature after preconcentration from seawater samples with Mn was preferred, achieving more than 96% recovery with 3% or less precision. Possible contaminants were masked only by ascorbic acid treatment.

Valley splitting and polarization by the Zeeman effect in monolayer MoSe2.

We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of ∓0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to ∓0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.

Relationship between total concentration and dilute HCl extraction of heavy metals in sediments of harbors and coastal areas in Korea.

Total concentrations of Cr, Ni, Cu, Zn, Cd and Pb in surface sediments were determined to investigate the regional trends of heavy metal contamination in 11 coastal areas in Korea. Enrichment factor (EF) of heavy metals was calculated by comparing the level of their regional background. The averages of EF values in study areas were 0.99 for Cr, 1.05 for Ni, 4.23 for Cu, 1.80 for Zn, 3.92 for Cd and 1.54 for Pb, respectively. Dilute HCl extractions were useful to deduce the anthropogenic sources of heavy metals and the 1 M HCl extractable fractions of each metal varied from 0.3 to 37.3% for Cr, 1.9 to 66.3% for Ni, 4.2 to 92.9% for Cu, 7.1 to 99.7% for Zn, 10.9 to 98.9% for Cd and 15.0 to 99.1% for Pb. Comparing 1 M HCl extractable fraction to total concentration, large portions of Cu, Zn, Cd and Pb were present as potentially bioavailable fractions from anthropogenic input and were significantly correlated with their EF values showing r > 0.68.

Distributions of heavy metals in the sediments of South Korean harbors.

Bottom sediments of harbors in the South Korea have been long suspected for metal contamination due to ship-based and urban-based activities for the past several decades. A number of areas have been suspected to impair ecosystem services to the local residents and drawn complaints from main stakeholders. Twelve contamination suspected harbors were subject to evaluate the level of contamination of Cr, Ni, Cu, Zn, As, Cd, Pb and Hg. The level of sediment contamination for each metal was evaluated comparing the relative enrichment of a given metal to pollution-insensitive aluminum. Regional background concentration of a given metal was also determined based on its down core measurement and sediment texture. Ecological risk posed by the presence of heavy metals was evaluated using the sediment quality guidelines (SQGs) developed by United States National Ocean and Atmosphere Administration (US NOAA) as benchmarks for evaluating sediment chemistry to aquatic organisms. Cu, Zn, Cd, Pb and Hg in the surface sediment were found to be higher than a factor of 1.5 than background sediments, and the overall metal contaminations of surface sediment can be regarded as medium-high- to high-priority sites in the sense of SQGs.

Multicriteria decision analysis to assess options for managing contaminated sediments: Application to Southern Busan Harbor, South Korea.

Many years of untreated effluent discharge from residential areas, a shipyard, a marina, and a large fish market resulted in substantial contamination of bottom sediment in Southern Busan Harbor, South Korea. Contaminants in these sediments include heavy metals and organic compounds. Newly introduced regulations for ocean disposal of dredged material in South Korea pose significant challenges, because the previous practice of offshore disposal of contaminated dredged material was no longer possible after August 2008. The South Korean government has mandated that such sediments be assessed in a way that identifies the most appropriate dredged material management alternative, addressing environmental, social, and cost objectives. An approach using multicriteria decision analysis (MCDA) in combination with comparative risk assessment was used as a systematic and transparent framework for prioritizing several dredged sediment management alternatives. We illustrate how MCDA can recognize the multiple goals of contaminated sediment management. Values used in weighting decision criteria were derived from surveys of stakeholders who were sediment management professionals, business owners, or government decision makers. The results of the analysis showed that land reclamation was the preferred alternative among cement-lock, sediment washing, 3 contained aquatic disposal alternatives (one in combination with a hopper dredge), geotextile tubes, solidification, and land reclamation after solidification treatment. Land reclamation was the preferred alternative, which performed well across all MCDA objectives, because of the availability of a near-shore confined disposal facility within a reasonable distance from the dredging area.

A decision-analysis approach for contaminated dredged material management in South Korea.

To meet London Protocol requirements, South Korea is preparing to reduce the need for disposal of dredged material at sea. The new requirements controlling ocean disposal of dredged material pose significant challenges to the South Korean government, because the previous practice of offshore disposal of contaminated dredged material is no longer permitted. Hence, other alternatives for treating and disposing of contaminated dredged material are being evaluated and selected for implementation. A new management and decision approach is therefore needed for regulators and implementers to show what information and what decision-making processes were used to make the decision, to increase administrative transparency for such projects in the public domain. To address this need, an iterative approach was developed for dredged material management that includes the essential elements of process, people, and tools needed for successful environmental decision making. The approach has 6 steps: problem definition, developing objectives and criteria, identifying alternatives, performing the evaluation, comparing alternatives, and selecting the preferred alternative. The primary objective of the approach is to provide a systematic means of exploring contaminated dredged material management alternatives in South Korea using criteria that integrate risk with economic and stakeholder value information. The approach incorporates the desired decision-making attributes of transparency, comparative analysis, and inclusion of public input. Although it was developed for South Korea, the approach can be applied in any situation in which dredged material management alternatives are being considered to manage contaminated sediment risks.