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At least 199 records · Page 11

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DOE OSTI2024

Evidence for widespread thermal acclimation of canopy photosynthesis

Plants acclimate to temperature by adjusting their photosynthetic capacity over weeks to months. However, most evidence for photosynthetic acclimation derives from leaf-scale experiments. Here we address the scarcity of evidence for canopy-scale photosynthetic acclimation by examining the correlation between maximum photosynthetic rates (A max,2,000 ) and growth temperature $(\overline{T_{air}})$ across a range of concurrent temperatures and canopy foliage quantity, using data from >200 eddy covariance sites. We detect widespread thermal acclimation of canopy-scale photosynthesis, demonstrated by enhanced A max,2,000 under higher $\overline{T_{air}}$, across flux sites with adequate water availability. A 14-day period is identified as the most relevant timescale for acclimation across all sites, with a range of 12-25 days for different plant functional types. The mean apparent thermal acclimation rate across all ecosystems is 0.41 (-0.38-1.04 for 5th-95th percentile range) µmol m -2 s -1 °C -1 , with croplands showing the largest acclimation rates and grasslands the lowest. Incorporating an optimality-based prediction of leaf photosynthetic capacities into a biochemical photosynthesis model is shown to improve the representation of thermal acclimation. Our results underscore the critical need for enhanced understanding and modelling of canopy-scale photosynthetic capacity to accurately predict plant responses to warmer growing seasons.

59 BASIC BIOLOGICAL SCIENCES↗

NASA NTRS1994

Automated protein crystal growth facility

A customer for the protein crystal growth facility fills the specially designed chamber with the correct solutions, fills the syringes with their quenching solutions, and submits the data needed for the proper growth of their crystal. To make sure that the chambers and syringes are filled correctly, a NASA representative may assist the customer. The data needed is the approximate growth time, the growth temperature, and the desired crystal size, but this data can be changed anytime from the ground, if needed. The chambers are gathered and placed into numbered slots in special drawers. Then, data is entered into a computer for each of the chambers. Technicians map out when each chamber's growth should be activated so that all of the chambers have enough time to grow. All of this data is up-linked to the space station when the previous growth session is over. Anti-vibrational containers need to be constructed for the high forces encountered during the lift off and the landing of the space shuttle, and though our team has not designed these containers, we do not feel that there is any reason why a suitable one could not be made. When the shuttle reaches the space station, an astronaut removes a drawer of quenched chambers from the growth facility and inserts a drawer of new chambers. All twelve of the drawers can be replaced in this fashion. The optical disks can also be removed this way. The old drawers are stored for the trip back to earth. Once inside the growth facility, a chamber is removed by the robot and placed in one of 144 active sites at a time previously picked by a technician. Growth begins when the chamber is inserted into an active site. Then, the sensing system starts to determine the size of the protein crystal. All during the crystal's growth, the customer can view the crystal and read all of the crystal's data, such as growth rate and crystal size. When the sensing system determines that the crystal has reached the predetermined size, the robot is told to pick up a syringe filled with the correct quenchant solution and inject it into the chamber to stop the crystal growth. The chamber is then removed from the active site and placed into its original storage slot. Another chamber is then placed into the active site and the process is repeated in all of the active sites until all of the chambers have complted their growth. After ninety days (the scheduled time between shuttle visits), the crystal growth is completed, and the old drawers are replaced with new ones. Once the customer extracts the crystals, the chambers are retrieved for future customers.

Donald, Stacey↗

DOE OSTI2022

High Carrier Lifetimes in Epitaxial Germanium-Tin/Al(In)As Heterostructures with Variable Tin Compositions

Group IV-based germanium-tin (Ge1-ySny) compositional materials have recently shown great promise for infrared detection, light emission and ultra-low power transistors. High carrier lifetimes are desirable for enhancing the detection limit and efficiency of photodetectors, low threshold current density in lasers, and low tunneling barrier height by lowering defects and dislocations at the heterointerface of a source and a channel. Here, carrier lifetimes in epitaxial germanium (Ge) and variable tin (Sn) compositional Ge1-ySny materials were experimentally determined on GaAs substrates using the contactless microwave photoconductive decay (u-PCD) technique at an excitation wavelength of 1500 nm. Sharp (2 x 2) reflection high energy electron diffraction patterns and low surface roughness were observed from the surface of the Ge0.97Sn0.03 epilayer. X-ray rocking curves from Ge0.97Sn0.03 and Ge0.94Sn0.06 layers demonstrated the pseudomorphic and lattice-matched growth on AlAs and In0.12Al0.88As buffers, respectively, further substantiated by reciprocal space maps and abrupt heterointerfaces evident from the presence of Pendellosung oscillations. High effective carrier lifetimes of 150 ns to 450 ns were measured for Ge1-ySny epilayers as a function of Sn composition, surface roughness, growth temperature, and layer thickness. The observed increase in the carrier lifetime with an increasing Ge layer thickness and a reducing surface roughness, by incorporating Sn, were explained. The enhancement of the carrier lifetime with an increasing Sn concentration was achieved by controlling the defects with lattice-matched Ge0.94Sn0.06/In0.12Al0.88As heterointerfaces or the pseudomorphic growth of Ge0.94Sn0.06 on GaAs. Therefore, our monolithic integration of variable Sn alloy compositional Ge1-ySny materials with high carrier lifetimes opens avenues to realize electronic and optoelectronic devices.

carrier lifetimes↗

DOE OSTI2022

The Role of Oxygen Transfer in Oxide Heterostructures on Functional Properties

A variety of mechanisms are reported to play critical roles in contributing to the high carrier/electron mobility in oxide/SrTiO 3 (STO) heterostructures. By using La 0.95 Sr 0.05 TiO 3 (LSTO) epitaxially grown on different single crystal substrates (such as STO, GdScO 3 , LaAlO 3 , (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 , and CeO 2 buffered STO) as the model systems, the formation of a conducting substrate surface layer (CSSL) on STO substrate is shown at relatively low growth temperature and high oxygen pressure (725 °C, 5 × 10 –4 Torr), which contributes to the enhanced conductivity of the LSTO/STO heterostructures. Different from the conventional oxygen vacancy model, this work reveals that the formation of the CSSL occurs when growing an oxide layer (LSTO in this case) on STO, while neither annealing nor the growth of an Au layer alone at the exact same growth condition generates the CSSL in STO. It demonstrates that the oxide layer actively pulls oxygen from STO substrate at given growth conditions, leading to the formation of the CSSL. Here, the observations emphasize the oxygen transfer across film/substrate interface during the synthesis of oxide heterostructures playing a critical role in functional properties.

36 MATERIALS SCIENCE↗

NASA NTRS1999

Vapor Growth and Characterization of Cr-Doped ZnSe Crystals

Cr-doped ZnSe single crystals were grown by a self-seeded physical vapor transport technique in both vertical (stabilized) and horizontal configurations. The source materials were mixtures of ZnSe and CrSe. Growth temperatures were in the range of 1140-1150 C and the furnace translation rates were 1.9-2.2 mm/day. The surface morphology of the as-grown crystals was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Different features of the as-grown surface of the vertically and horizontally grown crystals suggest that different growth mechanisms were involved in the two growth configurations. The [Cr] doping levels were determined to be in the range of 1.8-8.3 x 10 (exp 19) cm (exp -3) from optical absorption measurements. The crystalline quality of the grown crystals were examined by high-resolution triple-crystal X-ray diffraction (HRTXD) analysis.

Su, Ching-Hua↗

NASA NTRS1995

The growth of low band-gap InAs on (111)B GaAs substrates

The use of low band-gap materials is of interest for a number of photovoltaic and optoelectronic applications, such as bottom cells of optimized multijunction solar cell designs, long wavelength light sources, detectors, and thermophotovoltaics. However, low band-gap materials are generally mismatched with respect to lattice constant, thermal expansion coefficient, and chemical bonding to the most appropriate commercially available substrates (Si, Ge, and GaAs). For the specific case of III-V semiconductor heteroepitaxy, one must contend with the strain induced by both lattice constant mismatch at the growth temperature and differences in the rates of mechanical deformation during the cool down cycle. Several experimental techniques have been developed to minimize the impact of these phenomena (i.e., compositional grading, strained layer superlattices, and high-temperature annealing). However, in highly strained systems such as InAs-on-GaAs, three-dimensional island formation and large defect densities (greater than or equal to 10(exp 8)/ cm(exp -2)) tend to limit their applicability. In these particular cases, the surface morphology and defect density must be controlled during the initial stages of nucleation and growth. At the last SPRAT conference, we reported on a study of the evolution of InAs islands on (100) and (111)B GaAs substrates. Growth on the (111)B orientation exhibits a number of advantageous properties as compared to the (100) during these early stages of strained-layer epitaxy. In accordance with a developing model of nucleation and growth, we have deposited thin (60 A - 2500 A), fully relaxed InAs films on (111)B GaAs substrates. Although thicker InAs films are subject to the formation of twin defects common to epitaxy on the (111)B orientation, appropriate control of the growth parameters can greatly minimize their density. Using this knowledge base, InAs films up to 2 microns in thickness with improved morphology and structural quality have been grown on (111)B GaAs substrates, thereby enabling the measurement of electronic and optical properties.

Welser, R. E.↗

NASA NTRS1998

Vapor Growth and Characterization of Cr-Doped ZnSe Crystals

Cr-doped ZnSe single crystals were grown by self-seeded physical vapor transport technique under both vertical (stabilized) and horizontal configurations. The source materials were mixtures of ZnSe and CrSe. The growth temperatures were in the range of 1140 to 1150C and the furnace translation rates were 1.9 to 2.2 mm/day. The surface morphology of the as-grown crystals was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The different features on the as-grown surface of the vertically and horizontally grown crystals suggests that different growth mechanisms were involved for the two growth configurations. The [Cr] doping levels were determined to be in the range of 1.8 to 8.3 x 10(exp 19)/cubic centimeter from optical absorption measurements. The crystalline quality of the grown crystals were examined by high resolution triple crystal X-ray diffraction (HRTXD) analysis.

Su, Ching-Hua↗

NASA NTRS2001

Characterization and In-Situ Monitoring of ZnSe Crystal Growth by Seeded PVT for Microgravity Applications

Crystal growth from the vapor phase continues to play a significant role in the production of II-VI semiconductor compounds (ZnO, ZnTe, CdTe, etc.) and SiC. As compared to melt growth methods (where available) the advantages are: (1) lower growth temperature(s); (2) reduction in defect concentration; (3) additional purification; and (4) enhanced crystal perfection. A powerful tool in determining the mechanism of PVT is microgravity. Under normal gravity conditions the transport mechanism is a superposition of diffusive and convective fluxes. Microgravity offers the possibility of studying the transport properties without the influence of convective effects. Research on the crystal growth of ZnSe by PVT (P.I.: Su of NASA/MSFC) will help to clarify the effects of convection on crystal growth. A crystal growth furnace with in-situ and real time optical monitoring capabilities was constructed and used to monitor the vapor composition and growing crystal surface morphology during the PVT growth of ZnSe. Using photoluminescence and SIMS, ex-situ, the incorporation of point defects (Zn vacancy) and impurities was found to be correlated to the gravity vector due to the influence of the convective flow. A summary of the results to date will be presented.

Feth, Shari T.↗

DOE OSTI2023

Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures

Warming shifts the thermal optimum of net photosynthesis ( T optA ) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO 2 ) conditions. It is unclear whether shifts in T optA of field-grown trees will keep pace with the temperatures predicted for the 21 st century under elevated atmospheric CO 2 concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO 2 levels, we show that T optA of mature boreal conifers increased with warming. However, shifts in T optA did not keep pace with warming as T optA only increased by 0.26–0.35°C per 1°C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO 2 spruce, while remaining constant in ambient CO 2 spruce and in both ambient CO 2 and elevated CO 2 tamarack with warming. Although shifts in T optA of these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Thermal, water, and land cover factors led to contrasting urban and rural vegetation resilience to extreme hot months

Abstract With continuing global warming and urbanization, it is increasingly important to understand the resilience of urban vegetation to extreme high temperatures, but few studies have examined urban vegetation at large scale or both concurrent and delayed responses. In this study, we performed an urban–rural comparison using the Enhanced Vegetation Index and months that exceed the historical 90th percentile in mean temperature (referred to as “hot months”) across 85 major cities in the contiguous United States. We found that hot months initially enhanced vegetation greenness but could cause a decline afterwards, especially for persistent (≥4 months) and intense (≥+2 °C) episodes in summer. The urban responses were more positive than rural in the western United States or in winter, but more negative during spring–autumn in the eastern United States. The east–west difference can be attributed to the higher optimal growth temperatures and lower water stress levels of the western urban vegetation than the rural. The urban responses also had smaller magnitudes than the rural responses, especially in deciduous forest biomes, and least in evergreen forest biomes. Within each biome, analysis at 1 km pixel level showed that impervious fraction and vegetation cover, local urban heat island intensity, and water stress were the key drivers of urban–rural differences. These findings advance our understanding of how prolonged exposure to warm extremes, particularly within urban environments, affects vegetation greenness and vitality. Urban planners and ecosystem managers should prioritize the long and intense events and the key drivers in fostering urban vegetation resilience to heat waves.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2024

Low‐Temperature Synthesis of Stable CaZn 2 P 2 Zintl Phosphide Thin Films as Candidate Top Absorbers

Abstract The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps in the range of ≈1.5–2.3 eV, for use in the top cell paired with a narrower‐gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn 2 P 2 as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn 2 P 2 thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible spectroscopy shows that CaZn 2 P 2 films exhibit an optical absorptivity of ≈10 4 cm −1 at ≈1.95 eV direct bandgap. Room‐temperature photoluminescence (PL) measurements show near‐band‐edge optical emission, and time‐resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ≈30 ns. CaZn 2 P 2 is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first‐principles calculations, which indicate the absence of low‐formation‐energy, deep intrinsic defects. Overall, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells.

14 SOLAR ENERGY↗

NASA NTRS1999

Chemical Vapor Deposition at High Pressure in a Microgravity Environment

In this paper we present an evaluation of critical requirements of organometallic chemical vapor deposition (OMCVD) at elevated pressure for a channel flow reactor in a microgravity environment. The objective of using high pressure is to maintain single-phase surface composition for materials that have high thermal decomposition pressure at their optimum growth temperature. Access to microgravity is needed to maintain conditions of laminar flow, which is essential for process analysis. Based on ground based observations we present an optimized reactor design for OMCVD at high pressure and reduced gravity. Also, we discuss non-intrusive real-time optical monitoring of flow dynamics coupled to homogeneous gas phase reactions, transport and surface processes. While suborbital flights may suffice for studies of initial stages of heteroepitaxy experiments in space are essential for a complete evaluation of steady-state growth.

McCall, Sonya↗

NASA NTRS2010

Single Spatial-Mode Room-Temperature-Operated 3.0 to 3.4 micrometer Diode Lasers

Compact, highly efficient, 3.0 to 3.4 m light emitters are in demand for spectroscopic analysis and identification of chemical substances (including methane and formaldehyde), infrared countermeasures technologies, and development of advanced infrared scene projectors. The need for these light emitters can be currently addressed either by bulky solid-state light emitters with limited power conversion efficiency, or cooled Interband Cascade (IC) semiconductor lasers. Researchers here have developed a breakthrough approach to fabrication of diode mid-IR lasers that have several advantages over IC lasers used for the Mars 2009 mission. This breakthrough is due to a novel design utilizing the strain-engineered quantum-well (QW) active region and quinternary barriers, and due to optimization of device material composition and growth conditions (growth temperatures and rates). However, in their present form, these GaSb-based laser diodes cannot be directly used as a part of sensor systems. The device spectrum is too broad to perform spectroscopic analysis of gas species, and operating currents and voltages are too high. In the current work, the emitters were fabricated as narrow-ridge waveguide index-guided lasers rather than broad stripe-gain guided multimode Fabry-Perot (FP) lasers as was done previously. These narrow-ridge waveguide mid-IR lasers exhibit much lower power consumptions, and can operate in a single spatial mode that is necessary for demonstration of single-mode distributed feedback (DBF) devices for spectroscopic applications. These lasers will enable a new generation of compact, tunable diode laser spectrometers with lower power consumption, reduced complexity, and significantly reduced development costs. These lasers can be used for the detection of HCN, C2H2, methane, and ethane.

Frez, Clifford F.↗

NASA NTRS2017

Relationship Between Unusual High-Temperature Fatigue Crack Growth Threshold Behavior in Superalloys and Sudden Failure Mode Transitions

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Fatigue Crack Growth Thresholds; Grain boundary ox↗

NASA NTRS2017

Relationship Between Unusual High-Temperature Fatigue Crack Growth Threshold Behavior in Superalloys and Sudden Failure Mode Transitions

Fatigue Crack Growth Thresholds; Grain boundary ox↗

NASA NTRS2002

Reduction of Convection in Closed Tube Vapor Transport Experiments

The primary objective of this effort was to develop a method for suppressing convective flows during the growth of mercurous chloride crystals by vapor transport in closed tubes to levels approaching those obtained in the microgravity environment. Mercurous chloride was chosen because it is a technologically interesting acoustical optical material whose optical properties are believed to be affected by convective flows. Since the Grashof number scales as the cube of the smallest dimension in the flow system, reduction of the size scale can be extremely effective in reducing unwanted convective flows. However, since materials of practical interest must be grown at least on the cm scale, reduction of the overall growth system is not feasible. But if the region just above the growing crystal could be restricted to a few mm, considerable reduction in flow velocity would result. By suspending an effusive barrier in the growth ampoule just above the growth interface, it should be possible to reduce the convective velocity in this vicinity to levels approaching flows in microgravity. If successful, this growth technique will offer a screening test for proposed space experiments that involve vapor transport to see if reduction of convection will result in improved material and will set a new standard against which the improvements obtained in microgravity may be judged. In addition, it may provide an improved method for preparing materials on Earth whose growth is affected adversely by convection. If the properties of this material can be improved there is a potential commercial interest from Brimrose Inc., who has agreed to fabricate and test devices from the crystals we have grown. This report describes the development of the growth facility, the purification processes developed for preparing the starting material, and the results from growth experiments with and without the effusive baffle. Mercurous chloride turned out to be a more difficult material to deal with than originally anticipated. At growth temperatures, it is extremely sensitive to practically any impurity which causes it to form oxychlorides and/or to decompose into elemental mercury and bichloride of mercury. We were unable to find a suitable method for protecting the magnetic material used to suspend the effusion barrier from the attack of mercurous chloride vapor. Although we were successful in growing single crystals of mercurous chloride without the effusion baffle, they exhibited severe microcracking which we attribute to wall-induced thermal stresses. This leads us to believe that uncontrolled convection may not be the most important problem in the development of this material and a new growth process was attempted that eliminates the wall-induced stress. Unfortunately, the grant ran out before this new method could be adequately tested.

Naumann, R. J.↗

DOE OSTI2024

Temperature-dependent iron motion in extremophile rubredoxins – no need for ‘corresponding states’

Extremophile organisms are known that can metabolize at temperatures down to – 25 °C (psychrophiles) and up to 122 °C (hyperthermophiles). Understanding viability under extreme conditions is relevant for human health, biotechnological applications, and our search for life elsewhere in the universe. Information about the stability and dynamics of proteins under environmental extremes is an important factor in this regard. Here we compare the dynamics of small Fe-S proteins – rubredoxins – from psychrophilic and hyperthermophilic microorganisms, using three different nuclear techniques as well as molecular dynamics calculations to quantify motion at the Fe site. The theory of ‘corresponding states’ posits that homologous proteins from different extremophiles have comparable flexibilities at the optimum growth temperatures of their respective organisms. Although ‘corresponding states’ would predict greater flexibility for rubredoxins that operate at low temperatures, we find that from 4 to 300 K, the dynamics of the Fe sites in these homologous proteins are essentially equivalent.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2024

Buffer-layer-controlled nickeline vs zinc-blende/wurtzite-type MnTe growths on c -plane Al 2 O 3 substrates

In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family [MnTe] m [Bi2Te 3 ] n , which hosts a wide range of magneto-topological properties depending on the choice of m and n. However, bulk crystal growth allows only a few combinations of m and n for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of [MnTe]m and [Bi 2 Te 3 ] n to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe - nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of 4.20 ± 0.04 Å and 4.39 ± 0.04 Å, respectively - can be selectively grown on c-plane Al 2 O 3 substrates using different buffer layers and growth temperatures. Moreover, we provide the first comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy and show that ZB and WZ-like stacking sequences can easily alternate between the two. Surprisingly, In 2 Se 3 buffer layer, despite its lattice constant (4.02 Å) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas Bi 2 Te 3 , sharing the same lattice constant (4.39 Å) with the ZB-WZ phase, fosters the NC phase. Furthermore, these discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magneto-topological applications.

36 MATERIALS SCIENCE↗