Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of ...NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ∼19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ∼250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.
Monolayer transition-metal dichalcogenides (TMDs) have the potential to become efficient optical-gain materials for low-energy-consumption nanolasers with the smallest gain media because of strong ...excitonic emission. However, until now TMD-based lasing has been realized only at low temperatures. Here we demonstrate for the first time a room-temperature laser operation in the infrared region from a monolayer of molybdenum ditelluride on a silicon photonic-crystal cavity. The observation is enabled by the unique combination of a TMD monolayer with an emission wavelength transparent to silicon, and a high-Q cavity of the silicon nanobeam. The laser is pumped by a continuous-wave excitation, with a threshold density of 6.6 W cm
. Its linewidth is as narrow as 0.202 nm with a corresponding Q of 5,603, the largest value reported for a TMD laser. This demonstration establishes TMDs as practical materials for integrated TMD-silicon nanolasers suitable for silicon-based nanophotonic applications in silicon-transparent wavelengths.
Context.
The carbon monoxide (CO) molecular line at around 46655 Å in solar infrared spectra is often used to investigate the dynamic behavior of the cold heart of the solar atmosphere, i.e., sunspot ...oscillation, especially at the sunspot umbra.
Aims.
We investigated sunspot oscillation at Doppler velocities of the CO 7-6 R67 and 3-2 R14 lines that were measured by the Cryogenic Infrared Spectrograph (CYRA), as well as the line profile of Mg
II
k line that was detected by the Interface Region Imaging Spectrograph (IRIS).
Methods.
A single Gaussian function is applied to each CO line profile to extract the line shift, while the moment analysis method is used for the Mg
II
k line. Then the sunspot oscillation can be found in the time–distance image of Doppler velocities, and the quasi-periodicity at the sunspot umbra are determined from the wavelet power spectrum. Finally, the cross-correlation method is used to analyze the phase relation between different atmospheric levels.
Results.
At the sunspot umbra, a periodicity of roughly 5 min is detected at the Doppler velocity range of the CO 7-6 R67 line that formed in the photosphere, while a periodicity of around 3 min is discovered at the Doppler velocities of CO 3-2 R14 and Mg
II
k lines that formed in the upper photosphere or the temperature minimum region and the chromosphere. A time delay of about 2 min is measured between the strong CO 3-2 R14 line and the Mg
II
k line.
Conclusions.
Based on the spectroscopic observations from the CYRA and IRIS, the 3 min sunspot oscillation can be spatially resolved in the Doppler shifts. It may come from the upper photosphere or the temperature minimum region and then propagate to the chromosphere, which might be regarded as a propagating slow magnetoacoustic wave.
In this study, we investigate the energy partition of four confined circular-ribbon flares (CRFs) that occurred close to solar disk center. The flares are observed simultaneously by the Solar ...Dynamics Observatory (SDO), the Geostationary Operational Environmental Satellites (GOES), and the Ramaty Hight Energy Solar Spectroscopic Imager (RHESSI). We calculate different energy components, including the radiative outputs in the ranges 1 – 8 Å, 1 – 70 Å, and 70 – 370 Å, total radiative loss, peak thermal energy derived from GOES and RHESSI, nonthermal energy in flare-accelerated electrons, and magnetic free energy before the flares. It is found that the values of energy components increase systematically with the flare class, indicating that more energy is involved in larger flares. The magnetic free energies (
E
m
a
g
) are larger than the nonthermal energies (
E
n
t
h
) and radiative outputs, which is consistent with the magnetic nature of flares. The ratio
E
n
t
h
E
m
a
g
of the four flares, being 0.70 – 0.76, is considerably higher than that of eruptive flares. Hence, this ratio may serve as an important factor for discriminating confined from eruptive flares. The nonthermal energies are sufficient to provide the heating requirements including the peak thermal energy and radiative loss. Our findings impose constraints on theoretical models of confined CRFs and have potential implications on space weather forecast.
ZrCu-based bulk metallic glass composites (BMGCs) are well known for their plastic deformability, superior to traditional metallic glasses (MGs), which is attributed to a unique dual-phases ...structure, namely, the glassy matrix and unstable B2 phase. In the present study, in-situ tensile testing is used to trace the deformation process of a ZrCu-based BMGC. Three deformation stages of the BMGC, i.e., the elastic-elastic stage, the elastic-plastic stage, and the plastic-plastic stage are identified. In the elastic-elastic and elastic-plastic stages, the yield strength and elastic limit are major influenced by the volume fraction of the B2 crystals. In the plastic-plastic stage, the B2 phase stimulates the formation of multiple shear bands and deflects the direction of shear bands by disturbing the stress field in front of the crack tip. The deformation-induced martensitic transformation of the metastable B2 phase contributes to the plasticity and work hardening of the composite. This study highlights the formation and propagation of multiple shear bands and reveals the interactions of shear bands with structural heterogeneities in situ. Especially, the blocking of shear bands by crystals and the martensitic transformation of the B2 phase are critical for the mechanistic deformation process and illustrate the function of the B2 phase in the present BMGCs.
We present a detailed investigation of the Doppler shift oscillations in a hot loop during an M7.1 flare on 2014 October 27 observed by the Interface Region Imaging Spectrograph. The periodic ...oscillations are observed in the Doppler shift of Fe xxi 1354.09 (log T ∼ 7.05 ), and the dominant period is about 3.1 minutes. However, such 3.1 minute oscillations are not found in the line-integrated intensity of Fe xxi 1354.09 , AIA EUV fluxes, or microwave emissions. Solar Dynamics Observatory/AIA and Hinode/XRT imaging observations indicate that the Doppler shift oscillations locate at the hot loop-top region (≥11 MK). Moreover, the differential emission measure results show that the temperature is increasing rapidly when the Doppler shift oscillates, but the number density does not exhibit the corresponding increases nor oscillations, implying that the flare loop is likely to oscillate in an incompressible mode. All of these facts suggest that the Doppler shift oscillations at the shorter period are most likely the standing kink oscillations in a flare loop. Meanwhile, a longer period of about 10 minutes is identified in the time series of Doppler shift and line-integrated intensity, GOES SXR fluxes, and AIA EUV light curves, indicating the periodic energy release in this flare, which may be caused by a slow mode wave.
•Primary particle exhibits four types of nanostructure with and without H2 addition.•Effect of H2 on size of primary/aggregate particle is majorly engine load dependent.•Oxidation reactivity of ...primary particle is evaluated with and without H2 addition.•Oxidation reactivity of primary particle is morphology-controlled.
Experimental investigations were conducted on a 4-cylinder natural-aspirated direct-injection diesel engine with naturally aspirated hydrogen, focusing on the effects of hydrogen addition on the physico-chemical properties of the diesel particulate. Diesel particulates were sampled for off-line analysis, with the aid of TEM and TGA facilities. Hydrogen addition promotes particle oxidation at low engine load and speed due to the increase of exhaust temperature, resulting in smaller particles, but it inhibits particle oxidation at high engine load due to the competition of oxygen between hydrogen and diesel fuel which results in larger primary particles. The replacement of injected diesel fuel by hydrogen inhibits the formation of soot nuclei and decreases its volume density, hence reduces the size of aggregate particles which are more spherical as indicated by an increase of fractal dimension and a decrease of radius of gyration. With increase of engine load, primary particles exhibit more graphitic structure, changing from “onion like” to “shell–core” structure. Hydrogen addition promotes and inhibits primary particle oxidation at low and high engine loads, respectively, and the corresponding primary particles are “turbostratic interlayer” and “shell-amorphous” in structure, respectively. The results of recognized fringe length, tortuosity and fringe separation distance are consistent with the observed morphology. The oxidation reactivity is related to equivalence ratio, being higher at low engine load and speed, which is indicated by the variation of activation energy and ignition temperature. The oxidation reactivity is validated to be related to the nanostructure of primary particles.
This review discusses the complementary relationship between radio and hard X-ray observations of the Sun using primarily results from the era of the
Reuven Ramaty High Energy Solar Spectroscopic ...Imager
satellite. A primary focus of joint radio and hard X-ray studies of solar flares uses observations of nonthermal gyrosynchrotron emission at radio wavelengths and bremsstrahlung hard X-rays to study the properties of electrons accelerated in the main flare site, since it is well established that these two emissions show very similar temporal behavior. A quantitative prescription is given for comparing the electron energy distributions derived separately from the two wavelength ranges: this is an important application with the potential for measuring the magnetic field strength in the flaring region, and reveals significant differences between the electrons in different energy ranges. Examples of the use of simultaneous data from the two wavelength ranges to derive physical conditions are then discussed, including the case of microflares, and the comparison of images at radio and hard X-ray wavelengths is presented. There have been puzzling results obtained from observations of solar flares at millimeter and submillimeter wavelengths, and the comparison of these results with corresponding hard X-ray data is presented. Finally, the review discusses the association of hard X-ray releases with radio emission at decimeter and meter wavelengths, which is dominated by plasma emission (at lower frequencies) and electron cyclotron maser emission (at higher frequencies), both coherent emission mechanisms that require small numbers of energetic electrons. These comparisons show broad general associations but detailed correspondence remains more elusive.
Blinking and time correlation between fluorescences of neighboring negatively charged CdSe/CdS and CdSe/ZnS colloidal quantum dots have been studied experimentally. A tendency of synchronous ...blinking, that is, a bunching effect, is clearly observed from two neighboring QDs with a spatial separation up to 1.1 μm. We believe that our observations will help to better understand the mechanisms for the blinking.
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