| 1. |
- Xing, Xiuli, et al.
(författare)
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Modeling China's terrestrial ecosystem gross primary productivity with BEPS model : Parameter sensitivity analysis and model calibration
- 2023
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Ingår i: Agricultural and Forest Meteorology. - 0168-1923. ; 343
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Tidskriftsartikel (refereegranskat)abstract
- Terrestrial ecosystems are the largest sink for carbon, and their ecosystem gross primary productivity (GPP) regulates variations in atmospheric carbon dioxide (CO2) concentrations. Current process-based ecosystem models used for estimating GPP are subject to large uncertainties due to poorly constrained parameter values. In this study, we implemented a global sensitivity analysis (GSA) on parameters in the Boreal Ecosystem Productivity Simulator (BEPS) considering the parameters’ second-order impacts. We also applied the generalized likelihood estimation (GLUE) method, which is flexible for a multi-parameter calibration, to optimize the GPP simulation by BEPS for 10 sites covering 7 plant functional types (PFT) over China. Our optimized results significantly reduced the uncertainty of the simulated GPP over all the sites by 17 % to 82 % and showed that the GPP is sensitive to not only the photosynthesis-related parameters but also the parameters related to the soil water uptake as well as to the energy balance. The optimized GPP across South China showed that the mix forest, shrub, and grass have a higher GPP and are more controlled by the soil water availability. This study showed that the GLUE method together with the GSA scheme could constrain the ecosystem model well when simulating GPP across multiple ecosystems and provide a reasonable estimate of the spatial and temporal distribution of the ecosystem GPP over China. We call for more observations from more sites, as well as data on plant traits, to be collected in China in order to better constrain ecosystem carbon cycle modeling and understand its response to climate change.
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| 2. |
- He, Wei, et al.
(författare)
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China's Terrestrial Carbon Sink Over 2010–2015 Constrained by Satellite Observations of Atmospheric CO2 and Land Surface Variables
- 2022
-
Ingår i: Journal of Geophysical Research: Biogeosciences. - 2169-8953. ; 127:2
-
Tidskriftsartikel (refereegranskat)abstract
- The magnitude and distribution of China's terrestrial carbon sink remain uncertain due to insufficient constraints at large scales, whereby satellite data offer great potential for reducing the uncertainty. Here, we present two carbon sink estimates for China constrained either by satellite CO2 column concentrations (XCO2) within the Global Carbon Assimilation System or by remotely sensed soil moisture and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) in addition to in situ CO2 observations within the Carbon Cycle Data Assimilation System. They point to a moderate size of carbon sinks of 0.34 ± 0.14 (mean ± unc.) and 0.43 ± 0.09 PgC/yr during 2010–2015, which are supported by an inventory-based estimate for forest and soil carbon sink (0.26 PgC/yr) and fall in the range of contemporary ensemble atmospheric inversions (0.25–0.48 PgC/yr). They also agree reasonably well on interannual variations, which reflect the carbon sink anomalies induced by regional droughts in southwest China. Furthermore, their spatial distributions are broadly consistent that of the forest inventory-based estimate, indicating that the largest carbon sinks locate in central and eastern China. Their estimates for forest carbon sink coincide fairly well with the inventory-based estimate across different regions, especially when aggregated to the north and south of China. Although enhanced recently by afforestation, China's carbon sink was also significantly weakened by regional droughts, which were often not fully represented in previous in situ CO2-based inversions due to insufficient observations. Our results suggest that satellite-based atmospheric CO2 and land surface observations are vital in characterizing terrestrial net carbon fluxes at regional scales.
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| 3. |
- Jia, Tiekun, et al.
(författare)
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Facile Synthesis of Porous SnO(2) Spherical-Like Aggregates and Their Gas Sensing Property
- 2011
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Ingår i: Integrated Ferroelectrics. - : Informa UK Limited. - 1058-4587 .- 1607-8489. ; 128, s. 30-36
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Tidskriftsartikel (refereegranskat)abstract
- Porous spherical-like aggregates synthesis of SnO(2) crystals was achieved via a hydrothermal process in the mixed solvents of water and ethanol. The products were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman measurements. Morphological characterization showed that porous spherical-like aggregates were composed of short nanorods. Three main peaks broadening and shifting to high wavenumber in Raman spectrum implied that the components of the products had a smaller size. The result of the gas sensing measurement indicated that porous spherical-like aggregates had excellent ethanol gas sensing property compared to that of nanoparticles.
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| 4. |
- Wang, Meirong, et al.
(författare)
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Recent recovery of the boreal spring sensible heating over the Tibetan Plateau will continue in CMIP6 future projections
- 2019
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 14:12, s. 1-9
-
Tidskriftsartikel (refereegranskat)abstract
- The spring sensible heating (SH) over the Tibetan Plateau (TP) serves as a huge 'air pump', significantly influencing the Asian summer monsoon, has experienced a decreasing trend. However, it remains unclear whether this decline will continue. Therefore, we here examine the long-term trends of spring SH over the central and eastern TP (CETP) based on a meteorological station-based calculated SH dataset, and CMIP6 multi-model simulations. These two sources confirmed the previous finding that the SH peaks in May. Further, we find that the declining SH was replaced by a fast recovery after approximate 2000 in the station-based SH. This is to some extent verified by the historical simulations of CMIP6 models. Importantly, CMIP6 future projections suggest that this increasing trend will continue, and get stronger with higher radiative forcing from SSP126 to SSP585. Mechanism analysis indicates that the previous decreasing trend in SH was mainly caused by the decline of 10 m wind speed, while the recent and future increasing trend results from the rising ground-air temperature difference. We suggest that this increasing trend of spring SH over the CETP may serve as an alternative driver for the enhancement of the East Asian summer monsoon in the future.
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| 5. |
- Zhang, Tiankai, et al.
(författare)
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Ion-modulated radical doping of spiro-OMeTAD for more efficient and stable perovskite solar cells
- 2022
-
Ingår i: Science. - : AMER ASSOC ADVANCEMENT SCIENCE. - 0036-8075 .- 1095-9203. ; 377:6605, s. 495-501
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Tidskriftsartikel (refereegranskat)abstract
- Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2,7,7-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4-tert-butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.
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| 6. |
- Buyanova, Irina A, et al.
(författare)
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Effects of Ga doping on optical and structural properties of ZnO epilayers
- 2009
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 45:4-5, s. 413-420
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Tidskriftsartikel (refereegranskat)abstract
- Effects of Ga incorporation on electrical, structural and optical properties of ZnO epilayers are systematically studied by employing structural and optical characterization techniques combined with electrical and secondary ion mass spectrometry measurements. A non-monotonous dependence of free electron concentrations on Ga content is observed and is attributed to defect formation and phase separation. The former process is found to dominate for Ga concentrations of around 2-3x1020 cm-3. corresponding defects are suggested to be responsible for a broad red emission, which peaks at around 1.8 eV at K. Characteristic properties of this emission are well accounted for by assuming intracenter transitions at a deep center, of which the associated Huang-Rhys factor and mean phonon energy are determined. For higher Ga doping levels, the phase separation is found to be significant. It is that under these conditions only a minor fraction of incorporated Ga atoms form shallow donors, which leads to the observed dramatic decrease of carrier concentration.
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| 7. |
- Ji, Fuxiang, 1991-, et al.
(författare)
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Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2024
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Ingår i: Advanced Optical Materials. - : John Wiley & Sons. - 2162-7568 .- 2195-1071. ; 12:8
-
Tidskriftsartikel (refereegranskat)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron-phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK(-1) based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III-V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 8. |
- Lim, W., et al.
(författare)
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Migration and Luminescence Enhancement Effects of Deuterium in ZnO/ZnCdO Quantum Wells
- 2008
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92, s. 032103-
-
Tidskriftsartikel (refereegranskat)abstract
- ZnO/ZnCdO/ZnO multiple quantum well samples grown on sapphire substrates by molecular beam epitaxy and annealed in situ were exposed to D2 plasmas at 150 °C. The deuterium showed migration depths of ~0.8 µm for 30 min plasma exposures, with accumulation of 2H in the ZnCdO wells. The photoluminescence (PL) intensity from the samples was increased by factors of 5 at 5 K and ~20 at 300 K as a result of the deuteration, most likely due to passivation of competing nonradiative centers. Annealing up to 300 °C led to increased migration of 2H toward the substrate but no loss of deuterium from the sample and little change in the PL intensity. The initial PL intensities were restored by annealing at >=400 °C as 2H was evolved from the sample (~90% loss by 500 °C). By contrast, samples without in situ annealing showed a decrease in PL intensity with deuteration. This suggests that even moderate annealing temperatures lead to degradation of ZnCdO quantum wells.
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| 9. |
- Lu, Haibo, et al.
(författare)
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Comparing machine learning-derived global estimates of soil respiration and its components with those from terrestrial ecosystem models
- 2021
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9318 .- 1748-9326. ; 16:5
-
Tidskriftsartikel (refereegranskat)abstract
- The CO2 efflux from soil (soil respiration (SR)) is one of the largest fluxes in the global carbon (C) cycle and its response to climate change could strongly influence future atmospheric CO2 concentrations. Still, a large divergence of global SR estimates and its autotrophic (AR) and heterotrophic (HR) components exists among process based terrestrial ecosystem models. Therefore, alternatively derived global benchmark values are warranted for constraining the various ecosystem model output. In this study, we developed models based on the global soil respiration database (version 5.0), using the random forest (RF) method to generate the global benchmark distribution of total SR and its components. Benchmark values were then compared with the output of ten different global terrestrial ecosystem models. Our observationally derived global mean annual benchmark rates were 85.5 ± 40.4 (SD) Pg C yr-1 for SR, 50.3 ± 25.0 (SD) Pg C yr-1 for HR and 35.2 Pg C yr-1 for AR during 1982-2012, respectively. Evaluating against the observations, the RF models showed better performance in both of SR and HR simulations than all investigated terrestrial ecosystem models. Large divergences in simulating SR and its components were observed among the terrestrial ecosystem models. The estimated global SR and HR by the ecosystem models ranged from 61.4 to 91.7 Pg C yr-1 and 39.8 to 61.7 Pg C yr-1, respectively. The most discrepancy lays in the estimation of AR, the difference (12.0-42.3 Pg C yr-1) of estimates among the ecosystem models was up to 3.5 times. The contribution of AR to SR highly varied among the ecosystem models ranging from 18% to 48%, which differed with the estimate by RF (41%). This study generated global SR and its components (HR and AR) fluxes, which are useful benchmarks to constrain the performance of terrestrial ecosystem models.
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| 10. |
- Luo, Yi, et al.
(författare)
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A Facile and Green Method for the Synthesis of SFE Borosilicate Zeolite and Its Heteroatom-Substituted Analogues with Promising Catalytic Performances
- 2018
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 24:2, s. 306-311
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Tidskriftsartikel (refereegranskat)abstract
- Synthesis of SFE-type borosilcate zeolite was successfully carried out using a commercially available low-cost organic structure directing agent (OSDA) with ultra-low OSDA and water contents within a short crystallization time. Heteroatom (Al, Ti, V, or Fe)-substituted SFE-type zeolite analogues were also directly synthesized for the first time. The obtained Al containing zeolites exhibited promising catalytic performances in the disproportion-ation of isopropylnaphthalene.
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| 11. |
- Luo, Ziyu, et al.
(författare)
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An Efficient Deep-Subwavelength Second Harmonic Nanoantenna Based on Surface Plasmon-Coupled Dilute Nitride GaNP Nanowires
- 2021
-
Ingår i: Nano Letters. - : AMER CHEMICAL SOC. - 1530-6984 .- 1530-6992. ; 21:8, s. 3426-3434
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Tidskriftsartikel (refereegranskat)abstract
- High-index semiconductor nanoantennae represent a powerful platform for nonlinear photon generation. Devices with reduced footprints are pivotal for higher integration capacity and energy efficiency in photonic integrated circuitry (PIC). Here, we report on a deep subwavelength nonlinear antenna based on dilute nitride GaNP nanowires (NWs), whose second harmonic generation (SHG) shows a 5-fold increase by incorporating similar to 0.45% of nitrogen (N), in comparison with GaP counterpart. Further integrating with a gold (Au) thin film-based hybrid cavity achieves a significantly boosted SHG output by a factor of similar to 380, with a nonlinear conversion efficiency up to 9.4 x 10(-6) W-1. In addition, high-density zinc blende (ZB) twin phases were found to tailor the nonlinear radiation profile via dipolar interference, resulting in a highly symmetric polarimetric pattern well-suited for coupling with polarization nano-optics. Our results manifest dilute nitride nanoantenna as promising building blocks for future chip-based nonlinear photonic technology.
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| 12. |
- Riera-Galindo, Sergi, et al.
(författare)
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Impact of Singly Occupied Molecular Orbital Energy on the n-Doping Efficiency of Benzimidazole Derivatives
- 2019
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Ingår i: ACS Applied Materials and Interfaces. - : AMER CHEMICAL SOC. - 1944-8244 .- 1944-8252. ; 11:41, s. 37981-37990
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the impact of singly occupied molecular orbital (SOMO) energy on the n-doping efficiency of benzimidazole derivatives. By designing and synthesizing a series of new air-stable benzimidazole-based dopants with different SOMO energy levels, we demonstrated that an increase of the dopant SOMO energy by only similar to 0.3 eV enhances the electrical conductivity of a benchmark electron-transporting naphthalenediimide-bithiophene polymer by more than 1 order of magnitude. By combining electrical, X-ray diffraction, and electron paramagnetic resonance measurements with density functional theory calculations and analytical transport simulations, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and crystallinity of the doped polymer as a function of the dopant SOMO energy. Our findings strongly indicate that charge and energy transport are dominated by the (relative) position of the SOMO level, whereas morphological differences appear to play a lesser role. These results set molecular-design guidelines for next-generation n-type dopants.
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| 13. |
- Shen, Qian, et al.
(författare)
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The Genome of Artemisia annua Provides Insight into the Evolution of Asteraceae Family and Artemisinin Biosynthesis
- 2018
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Ingår i: Molecular Plant. - : Cell Press. - 1674-2052 .- 1752-9867. ; 11:6, s. 776-788
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Tidskriftsartikel (refereegranskat)abstract
- Artemisia annua, commonly known as sweet wormwood or Qinghao, is a shrub native to China and has long been used for medicinal purposes. A. annua is now cultivated globally as the only natural source of a potent anti-malarial compound, artemisinin. Here, we report a high-quality draft assembly of the 1.74-gigabase genome of A. annua, which is highly heterozygous, rich in repetitive sequences, and contains 63 226 protein-coding genes, one of the largest numbers among the sequenced plant species. We found that, as one of a few sequenced genomes in the Asteraceae, the A. annua genome contains a large number of genes specific to this large angiosperm clade. Notably, the expansion and functional diversification of genes encoding enzymes involved in terpene biosynthesis are consistent with the evolution of the artemisinin biosynthetic pathway. We further revealed by transcriptome profiling that A. annua has evolved the sophisticated transcriptional regulatory networks underlying artemisinin biosynthesis. Based on comprehensive genomic and transcriptomic analyses we generated transgenic A. annua lines producing high levels of artemisinin, which are now ready for large-scale production and thereby will help meet the challenge of increasing global demand of artemisinin.
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| 14. |
- Wang, Hao, et al.
(författare)
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In-situ growth of low-dimensional perovskite-based insular nanocrystals for highly efficient light emitting diodes
- 2023
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Ingår i: Light. - : SPRINGERNATURE. - 2095-5545 .- 2047-7538. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Regulation of perovskite growth plays a critical role in the development of high-performance optoelectronic devices. However, judicious control of the grain growth for perovskite light emitting diodes is elusive due to its multiple requirements in terms of morphology, composition, and defect. Herein, we demonstrate a supramolecular dynamic coordination strategy to regulate perovskite crystallization. The combined use of crown ether and sodium trifluoroacetate can coordinate with A site and B site cations in ABX(3) perovskite, respectively. The formation of supramolecular structure retard perovskite nucleation, while the transformation of supramolecular intermediate structure enables the release of components for slow perovskite growth. This judicious control enables a segmented growth, inducing the growth of insular nanocrystal consist of low-dimensional structure. Light emitting diode based on this perovskite film eventually brings a peak external quantum efficiency up to 23.9%, ranking among the highest efficiency achieved. The homogeneous nano-island structure also enables high-efficiency large area (1 cm(2)) device up to 21.6%, and a record high value of 13.6% for highly semi-transparent ones.
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| 15. |
- Wang, Shu Min, 1963, et al.
(författare)
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Metamorphic InGaAs quantum wells for light emission at 1.3-1.6 μm
- 2007
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 515:10, s. 4348-4351
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Tidskriftsartikel (refereegranskat)abstract
- Metamorphic InGaAs quantum well structures grown on GaAs reveal strong light emission at 1.3-1.6 μm, smooth surface with an average roughness below 2 nm and good rectifying I-V characteristics. Dark line defects are found in the QW. Post growth thermal annealing further improves the luminescence efficiency but does not remove those dark line defects. Some challenges of epitaxial growth using this method for laser applications are discussed. © 2006 Elsevier B.V. All rights reserved.
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| 16. |
- Wang, Suhao, et al.
(författare)
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Sequential Doping of Ladder-Type Conjugated Polymers for Thermally Stable n-Type Organic Conductors
- 2020
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 12:47, s. 53003-53011
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Tidskriftsartikel (refereegranskat)abstract
- Doping of organic semiconductors is a powerful tool to optimize the performance of various organic (opto)electronic and bioelectronic devices. Despite recent advances, the low thermal stability of the electronic properties of doped polymers still represents a significant obstacle to implementing these materials into practical applications. Hence, the development of conducting doped polymers with excellent long-term stability at elevated temperatures is highly desirable. Here, we report on the sequential doping of the ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL) with a benzimidazole-based dopant (i.e., N-DMBI). By combining electrical, UV-vis/infrared, X-ray diffraction, and electron paramagnetic resonance measurements, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and microstructure of the sequentially doped polymer films as a function of the thermal annealing temperature. Importantly, we observed that the electrical conductivity of N-DMBI-doped BBL remains unchanged even after 20 h of heating at 190 °C. This finding is remarkable and of particular interest for organic thermoelectrics.
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| 17. |
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| 18. |
- Battiato, M., et al.
(författare)
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Distinctive Picosecond Spin Polarization Dynamics in Bulk Half Metals
- 2018
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 121:7
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Tidskriftsartikel (refereegranskat)abstract
- Femtosecond laser excitations in half-metal (HM) compounds are theoretically predicted to induce an exotic picosecond spin dynamics. In particular, conversely to what is observed in conventional metals and semiconductors, the thermalization process in HMs leads to a long living partially thermalized configuration characterized by three Fermi-Dirac distributions for the minority, majority conduction, and majority valence electrons, respectively. Remarkably, these distributions have the same temperature but different chemical potentials. This unusual thermodynamic state is causing a persistent nonequilibrium spin polarization only well above the Fermi energy. Femtosecond spin dynamics experiments performed on Fe3O4 by time- and spin-resolved photoelectron spectroscopy support our model. Furthermore, the spin polarization response proves to be very robust and it can be adopted to selectively test the bulk HM character in a wide range of compounds.
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| 19. |
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| 20. |
- Beyer, Jan, et al.
(författare)
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Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures
- 2012
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:14, s. 145304-
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Tidskriftsartikel (refereegranskat)abstract
- Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in optical and spin polarization of the QDs is observed with increasing magnetic field in the range of 0-2 T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields when these spin depolarization processes are quenched, electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits rather strong field dependence under non-resonant excitation. In contrast, such field dependence is practically absent in more "isotropic" QD structures (e.g. single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e. wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.
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| 21. |
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| 22. |
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| 23. |
- Bubnova, Olga, et al.
(författare)
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Corrigendum: Semi-metallic polymers
- 2014
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 13, s. 662-662
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Tidskriftsartikel (refereegranskat)
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| 24. |
- Bubnova, Olga, et al.
(författare)
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Semi-metallic polymers
- 2014
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Ingår i: Nature Materials. - : Nature Publishing Group. - 1476-1122 .- 1476-4660. ; 13:2, s. 190-194
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Tidskriftsartikel (refereegranskat)abstract
- Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly( 3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.
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| 25. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Effects of hydrogen on the optical properties of ZnCdO/ZnO quantum wells grown by molecular beam epitaxy
- 2008
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92, s. 261912-
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Tidskriftsartikel (refereegranskat)abstract
- Temperature-dependent cw- and time-resolved photoluminescence (PL), as well as optically detected magnetic resonance (ODMR) measurements are employed to evaluate effects of deuterium (2H) doping on optical properties of ZnCdO/ZnO quantum well structures grown by molecular beam epitaxy. It is shown that incorporation of 2H from a remote plasma causes a substantial improvement in radiative efficiency of the investigated structures. Based on transient PL measurements, the observed improvements are attributed to efficient passivation by hydrogen of competing nonradiative recombination centers via defects. This conclusion is confirmed from the ODMR studies.
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| 26. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Optical characterization of Zn(1-x)Cd(x)O alloys grown by molecular-beam epitaxy
- 2006
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Ingår i: 210th ECS Meeting Volume 3, Issue 5. - : The Electrochemical Society. ; , s. 391-398
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We have carried out comprehensive optical studies to evaluate structural and bandgap properties of Zn1-xCdxO alloys with x{less than or equalto}0.17 grown by molecular beam epitaxy. High crystalline quality ofthe alloys was concluded from cathodoluminescence measurements. Based on absorptionand reflectance measurements, the compositional dependence of the bandgap energyof ZnCdO, estimated without taking into account excitonic effects, wasfound to follow the trend Eg(x)=3.28-2.23x+0.45x2. Degradation in the alloyquality due to possible phase separation was found to causedeviations from this trend, evident from a more rapid redshift of the absorption edge. Effects of Cd incorporation onthe variation of the bandgap energies with temperature are alsodiscussed.
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| 27. |
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| 28. |
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| 29. |
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| 30. |
- Chen, Weimin, et al.
(författare)
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How to Deactivate Harmful Defects and Active them for New Spin Functionalities in a Semiconductor?
- 2015
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Ingår i: Abstract Book. ; , s. FF3.02-
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Konferensbidrag (refereegranskat)abstract
- We demonstrate a general approach via spin engineering that is capable of not only deactivating defect-mediated efficient non-radiative carrier recombination channels in a semiconductor that are harmful to photonic and photovoltaic device performance, but also adding new room-temperature (RT) spin functionalities that are desirable for future spintronics and spin-photonics but so far unachievable otherwise. This approach exploits the Pauli Exclusion Principle that prohibits occupation of a non-degenerate defect level by two spin-parallel electrons, thereby providing spin blockade of carrier recombination via the defect level. The success of the approach is demonstrated in the dilute nitride of Ga(In)NAs, which holds promises for low-cost, highly efficient lasers for fiber-optic communications as well as for multi-band and multi-junction solar cell applications. First we identify that Gai self-interstitials and their complexes are the most common grown-in defects found in Ga(In)NAs grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). They provide a dominant non-radiative shunt path for non-equilibrium carriers, leading to low efficiencies of light-emitting and photon-charge carrier conversion. Spin blockade is shown to lead to a giant enhancement by up to 800% in light emission intensity at RT.Furthermore we show that via spin engineering these seemingly harmful defects can be turned into advantages by adding unconventional defect-enabled spin functionalities that are highly effective at RT, including some of the fundamental building blocks essential for future spintronics. We demonstrate efficient defect-engineered spin filtering in Ga(In)NAs, which is capable of generating a record-high degree (> 40%) of electron spin polarization at RT [Nature Materials 8, 198 (2009), Phys. Rev. B 89, 195412 (2014)]. We also provide the first experimental demonstration of an efficient RT spin amplifier based on defect engineered Ga(In)NAs with a spin gain up to 2700% [Adv. Materials 25, 738 (2013)]. Such a spin amplifier is shown to be capable of amplifying a fast-modulating input spin signal while truthfully maintaining its time variation of the spin-encoded information [7]. By taking advantage of the spin amplification effect, we show that Ga(In)NAs can be employed as efficient RT spin detectors, with spin detection efficiency well exceeding 100% [8,9]. By combining the spin-filtering effect and hyperfine coupling, we further achieve the first realization of RT nuclear spin hyperpolarization in semiconductors via conduction electrons [Nature Communications. 4, 1751 (2013)], relevant to nuclear spin qubits. We believe that such defect-enabled spin functionalities could potentially provide an attractive, alternative solution to the current and important issues on RT spin injection, spin amplification and spin detection in semiconductors for future spintronics.
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| 31. |
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| 32. |
- Chernenk, Kirill, et al.
(författare)
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Performance and characterization of the FinEstBeAMS beamline at the MAX IV Laboratory
- 2021
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Ingår i: Journal of Synchrotron Radiation. - 0909-0495. ; 28, s. 1620-1630
-
Tidskriftsartikel (refereegranskat)abstract
- FinEstBeAMS (Finnish-Estonian Beamline for Atmospheric and Materials Sciences) is a multidisciplinary beamline constructed at the 1.5 GeV storage ring of the MAX IV synchrotron facility in Lund, Sweden. The beamline covers an extremely wide photon energy range, 4.5-1300 eV, by utilizing a single elliptically polarizing undulator as a radiation source and a single grazing-incidence plane grating monochromator to disperse the radiation. At photon energies below 70 eV the beamline operation relies on the use of optical and thin-film filters to remove higher-order components from the monochromated radiation. This paper discusses the performance of the beamline, examining such characteristics as the quality of the gratings, photon energy calibration, photon energy resolution, available photon flux, polarization quality and focal spot size.
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| 33. |
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| 34. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Activation of defects in GaNP by post-growth hydrogen treatment
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Effect of post-growth hydrogen treatment on defects and their role in carrier recombination in molecular beam epitaxial GaNP alloys is examined by means of photoluminescence and optically detected magnetic resonance. We present direct experimental evidence for effective activation of several different defects in carrier recombination by the hydrogen treatment. Among them, two defect complexes are identified to contain a Ga interstitial (Gai). None of the activated Gai complexes was previously observed in GaNP. Possible mechanisms for the hydrogen-induced defect activation are discussed.
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| 35. |
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| 36. |
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| 37. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Effect of growth conditions on grown-in defect formation and luminescence efficiency in GaInNP epilayers grown by molecular-beam epitaxy.
- 2008
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Ingår i: Physica status solidi (c)Special Issue: E-MRS 2007 Spring Meeting – Symposium F and Conference on Photonic Materials. - Weinheim : Wiley-VCH Verlagsgesellschaft. ; , s. 460-463
-
Konferensbidrag (refereegranskat)abstract
- A detailed study of the impact of different growth conditions (i.e. ion bombardment, nitrogen flow and In content) on the defect formation in Ga(In)NP epilayers grown on GaP substrates by solid-source molecular beam epitaxy is performed. Reduced nitrogen ion bombardment during the growth is shown to significantly reduce formation of defects acting as competing recombination centers, such as a Ga interstitial defect and other unidentified defects revealed by optically detected magnetic resonance. Further, high nitrogen flow is found to be even more effective than the ion bombardment in introducing the defects. The incorporation of In by 5.1% is, on the other hand, found not to affect the introduction of defects. The results provide a useful insight into the formation mechanism of the defects that will hopefully shed light on a control of the defect introduction in the alloys by optimizing growth conditions.
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| 38. |
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| 39. |
- Dagnelund, Daniel, et al.
(författare)
-
Effect of postgrowth hydrogen treatment on defects in GaNP
- 2011
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Ingår i: APPLIED PHYSICS LETTERS. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 98:14, s. 141920-
-
Tidskriftsartikel (refereegranskat)abstract
- Effect of postgrowth hydrogen treatment on defects and their role in carrier recombination in GaNP alloys is examined by photoluminescence (PL) and optically detected magnetic resonance. We present direct experimental evidence for effective activation of several defects by low-energy subthreshold hydrogen treatment (andlt;= 100 eV H ions). Among them, two defect complexes are identified to contain a Ga interstitial. Possible mechanisms for the H-induced defect activation and creation are discussed. Carrier recombination via these defects is shown to efficiently compete with the near band-edge PL, explaining the observed degraded optical quality of the alloys after the H treatment.
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| 40. |
- Dagnelund, Daniel, et al.
(författare)
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Effects of hydrogenation on non-radiative defects in GaNP and GaNAs alloys: An optically detected magnetic resonance study
- 2012
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 111:023501
-
Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence and optically detected magnetic resonance techniques are utilized to study defect properties of GaNP and GaNAs alloys subjected to post-growth hydrogenation by low-energy sub-threshold ion beam irradiation. It is found that in GaNP H incorporation leads to activation of new defects, which has a Ga interstitial (Ga-i) atom at its core and may also involve a H atom as a partner. The observed activation critically depends on the presence of N in the alloy, as it does not occur in GaP with a low level of N doping. In sharp contrast, in GaNAs hydrogen is found to efficiently passivate Ga-i-related defects present in the as-grown material. A possible mechanism responsible for the observed difference in the H behavior in GaNP and GaNAs is discussed.
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| 41. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Formation of grown-in defects in molecular beam epitaxial Ga(In)NP : effects of growth conditions and post-growth treatments
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103, s. 063519-
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Tidskriftsartikel (refereegranskat)abstract
- Effects of growth conditions and post-growth treatments, such as presence of N ions, N2 flow, growth temperature, In alloying, and postgrowth rapid thermal annealing (RTA), on formation of grown-in defects in Ga(In)NP prepared by molecular beam epitaxy are studied in detail by the optically detected magnetic resonance (ODMR) technique. Several common residual defects, such as two Ga-interstitial defects (i.e., Gai-A and Gai-B) and two unidentified defects with a g factor around 2 (denoted by S1 and S2), are closely monitored. Bombardment of impinging N ions on grown sample surface is found to facilitate formation of these defects. Higher N2 flow is shown to have an even more profound effect than a higher number of ions in introducing these defects. Incorporation of a small amount of In (e.g., 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In; however, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai-related defects formed during the growth. In the alloys where the Gai-A and Gai-B defects are absent in the as-grown samples (i.e., GaNP grown at a low temperature of 460 °C), the concentrations of the two Gai defects are found to increase after postgrowth RTA. This indicates that the defects originally introduced in the as-grown alloys have been transformed into the more thermally stable Gai-A and Gai-B during RTA. On the other hand, when the Gai-A and Gai-B are readily abundant (e.g., at higher growth temperatures (>=500 °C), RTA leads to a slight reduction of the Gai-A and Gai-B ODMR signals. The S2 defect is also shown to be thermally stable upon the RTA treatment.
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| 42. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Optically detected cyclotron resonance studies of InGaNAs/GaAs structures
- 2007
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Ingår i: Physics of semiconductors : 28th International Conference on the Physics of Semiconductors, ICPS 2006, Vienna, Austria, 24-28 July 2006. - : American Institute of Physics (AIP). - 9780735403970 ; , s. 383-384
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report on our recent results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well (QW), by the optically detected cyclotron resonance (ODCR) technique. By monitoring photoluminescence (PL) emissions from various layers the predominant ODCR peak is shown to be related to carriers with a 2D character and an effective mass of 0.51 m∗e. The responsible carriers are ascribed to be electrons in GaAs/AlAs superlattices (SL) that are employed in the laser structures to prevent carrier leak by sandwiching the InGaNAs/GaAs QW. This conclusion is based on the following observations: (a) the ODCR peak is only observed in the structures containing the SL; (b) the effective mass value determined by the ODCR peak is independent of In and N compositions; (c) the same ODCR peak is also observed by monitoring PL from the SL. Unfortunately no ODCR signal related to InGaNAs was observed
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| 43. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Optically detected cyclotron resonance studies of InxGa1-xNyAs1-y/GaAs quantum wells sandwiched between type-II AlAs/GaAs superlattices.
- 2007
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101, s. 073705-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on our results from a systematic study of layered structures containing an InGaNAs/GaAs single quantum well (SQW) enclosed between staggered type II AlAs/GaAs superlattices (SL), by the photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. Besides the ODCR signal known to originate from electrons in GaAs, the predominant ODCR peak is shown to be related to carriers with a two-dimensional character and a cyclotron resonance effective mass of m*[approximate](0.51-0.56)m0. The responsible carriers are ascribed to electrons on the ellipsoidal equienergy surface at the AlAs X point of the Brillouin zone within the SL, based on results from angular and spectral dependences of the ODCR signal. No ODCR signal related to the InGaNAs SQW was detected, presumably due to low carrier mobility despite the high optical quality. Multiple absorption of photons with energy below the band gap energy of the SL and the GaAs barriers was observed, which bears implication on the efficiency of light-emitting devices based on these structures.
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| 44. |
- Dagnelund, Daniel, et al.
(författare)
-
Phosphorus-related interfacial defect complex at a GaP/GaNP heterojunction
- 2010
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- For full exploration of dilute nitrides in device applications, a better understanding and control of defects located at interfaces involving e.g. Ga(In)NP are required. In this work we report on the first identification of a point defect situated at an interface between two semiconductors: GaNP and GaP. The defect is concluded to be a complex involving a P antisite or a P interstitial in its core, partnered with a neighboring impurity/defect aligned along a <111> direction, from detailed angular dependence studies of the optically detected magnetic resonance (ODMR) spectra at both X- and Q-band microwave frequencies. The principal g and A values, g┴=2.013, g║=2.002, A┴=130x10-4 cm-1 and A║=330x10-4 cm-1, are obtained from a spin Hamiltonian analysis. The interface nature of the defect is clearly evident from the absence of the ODMR lines originating from two out of four equivalent <111> orientations. Defect formation is shown to be facilitated by severe nitrogen ion bombardment under non-equilibrium growth conditions during solid-source molecular beam epitaxy and the defect is thermally stable upon post-growth thermal annealing.
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| 45. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
-
Spin resonance spectroscopy of grown-in defects in Ga(In)NP alloys
- 2008
-
Ingår i: 7th International Conference on Physics of Light-Matter Coupling in Nanostructures, 2007. - : Elsevier Ltd.. ; , s. 620-625
-
Konferensbidrag (refereegranskat)abstract
- We employ the optically detected magnetic resonance (ODMR) technique to study and identify important grown-in defects in Ga(In)NP grown by molecular-beam epitaxy (MBE). Several types of defects were revealed from ODMR studies. The dominant defects were found to be related to Ga interstitials, evident form their characteristic hyperfine interaction arising from the spin interaction between the electron and the Ga nucleus. Some other as yet unidentified intrinsic defects were also found to be commonly present in the alloys. The effects of growth conditions (ion bombardment, N2 gas flow, etc.) and post-growth rapid thermal annealing on the formation of these defects were studied in detail, shedding light on the formation mechanism of defects.
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| 46. |
- Filippov, Stanislav, et al.
(författare)
-
Effects of Ni-coating on ZnO nanowires : A Raman scattering study
- 2013
-
Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 113:21, s. 214302-1-214302-6
-
Tidskriftsartikel (refereegranskat)abstract
- Structural properties of ZnO/Ni core/shell nanowires (NWs) are studied in detail by means of Raman spectroscopy. It is shown that formation of the Ni shell leads to passivation of surface states responsible for the observed enhanced intensity of the A1(LO) Raman mode of the bare ZnO NWs. It also causes appearance of 490 cm−1 and 710 cm−1 modes that are attributed to local vibrational modes of a defect/impurity (or defects/impurities). This defect is concluded to be preferably formed in annealed ZnO/Ni NWs and is unlikely to contain a Ni atom, as the same Raman modes were also reported for the Ni-free ZnO nanostructures. From our resonant Raman studies, we also show that the ZnO/Ni core/shell NWs exhibit an enhanced Raman signal with a multiline structure involving A1(LO). This observation is attributed to combined effects of an enhanced Fröhlich interaction at the ZnO/Ni heterointerface and coupling of the scattered light with local surface plasmons excited in the Ni shell. The plasmonic effect is also suggested to allow detection of carbon-related species absorbed at the surface of a single ZnO/Ni NW, promising for applications of such structures as efficient nano-sized gas sensors.
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| 47. |
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| 48. |
- Gong, S., et al.
(författare)
-
An investigation on dynamic thickness of a boiling liquid film
- 2015
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 90, s. 636-644
-
Tidskriftsartikel (refereegranskat)abstract
- Motivated by understanding the micro-hydrodynamics of boiling heat transfer and the mechanism of critical heat flux (CHF) occurrence, the present study is to investigate the boiling phenomenon in a liquid film whose dynamic thickness is recorded by a confocal optical sensor with the measurement accuracy of micrometres, while the bubble dynamics of the boiling in the film is visualized by a high-speed photography. This paper is focused on a statistical analysis of the measured thickness signals for the boiling condition ranging from low heat flux to high heat flux (near or at CHF). The dynamic thickness of liquid film appears oscillating with peak values, resulting from the liquid film movements due to nucleation of bubble(s) and its growth and rupture. The statistical analysis in a certain period indicates there emerge three distinct liquid film thickness ranges: 0-50 μm, 50-500 μm and 500-2500 μm, seemingly corresponding to the microlayer, macrolayer and bulk layer. With increasing heat flux to a specific extent, the bulk layer disappears, and then the macrolayer gradually decreases to ∼105 μm, beyond which the liquid film may lose its integrity and CHF occurs at 1.563 MW/m2.
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| 49. |
- Gong, Yaopeng, et al.
(författare)
-
Thermophysical properties of UO2-ZrO2 melt measured by aerodynamic levitation
- 2024
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 602
-
Tidskriftsartikel (refereegranskat)abstract
- This study addresses the measurement of thermophysical properties in molten core materials (corium), which is crucial for modeling severe accidents in light water reactors. Due to the measurement challenges at temperatures above 3000 K, there is a significant lack of experimental data for corium. Focusing on a UO2-ZrO2 melt (atomic ratio U/Zr=0.456), density and viscosity measurements were conducted using aerodynamic levitation, a non-contact method chosen to avoid interactions between the samples and container walls. Samples were prepared using the pressing sintering method and levitated by argon gas above a conical converging-diverging nozzle. Density was measured using the cooling traces method under the axisymmetric ellipsoid hypothesis, while viscosity was measured by inducing damped oscillations which occurred when the levitated droplet was forced to oscillate around its resonant frequency through acoustic excitation, followed by the cessation of the acoustic excitation. The provided data and derived equations for the density and viscosity of the UO2-ZrO2 melt contribute to enriching the thermophysical property database for materials in nuclear reactors.
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| 50. |
- Gu, H., et al.
(författare)
-
Investigation on contact angle measurement methods and wettability transition of porous surfaces
- 2016
-
Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 292, s. 72-77
-
Tidskriftsartikel (refereegranskat)abstract
- Various solid surfaces (e.g., smooth titanium surface, smooth aluminum surface, polished copper surfaces, polished silver surfaces and porous copper surfaces) were prepared to quantify the reliability of half-angle algorithm and axisymmetric drop shape analysis (ADSA) algorithm for calculating contact angles. Besides, the effects of surface conditions on contact angle values were also investigated. The experimental results of 10 repeated tests for each surface show that both algorithms have good accuracy for an acute contact angle, while the ADSA algorithm is better than the half-angle algorithm for an obtuse contact angle. Furthermore, with the decrease of surface roughness, the contact angle increases but the standard deviation of contact angles by 10 repeated tests decreases. In addition, the porous layer on copper surface by electrochemical deposition shows a super hydrophilic property, but it could change to be super hydrophobic after exposed in ambient air for 24 h. Interestingly, the surface wettability reverses to be super hydrophilic again after it is immersed in water, and the inorganic contamination is the reason of formal change from the super hydrophilic status to the super hydrophobic status.
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