Increasing Growth Temperature Research Articles

Tunable Grain Boundary of Lead‐Free All‐Inorganic Perovskite Films for Smart Photodetectors

AbstractThe nonradiative recombination caused by the trap states at the grain boundaries (GBs) and surfaces directly limits the photoelectronic performance of lead‐free all‐inorganic perovskites films. In this work, the typical CsSnBr3 films with controlled GBs and surfaces are prepared successfully on glass by a solid‐source chemical vapor deposition method. The grain size of CsSnBr3 films increases with the increase of growth temperature, indicating the reduction of GBs. Time‐resolved photoluminescence spectra show that the lifetime of photogenerated carrier is longer in the large‐grain‐size CsSnBr3 film. Furthermore, the photodetector based on CsSnBr3 film with decreased GBs shows excellent performance with photocurrent up to 760 nA, responsivity up to 9200 mA W−1, and fast response times of 5/12 ms under the illumination of 405 nm laser. Moreover, the photodetector is also fabricated on a flexible substrate, demonstrating a good bending stability. This work provides a guideline in the growth of high‐quality perovskites films with controlled GBs and surfaces for next‐generation high‐performance photoelectronic devices.

Temperature acclimation of leaf respiration differs between marsh and mangrove vegetation in a coastal wetland ecotone.

Temperature acclimation of leaf respiration (R) is an important determinant of ecosystem responses to temperature and the magnitude of temperature-CO2 feedbacks as climate warms. Yet, the extent to which temperature acclimation of R exhibits a common pattern across different growth conditions, ecosystems, and plant functional types remains unclear. Here, we measured the short-term temperature response of R at six time points over a 10-month period in two coastal wetland species (Avicennia germinans [C3 mangrove] and Spartina alterniflora [C4 marsh grass]) growing under ambient and experimentally warmed temperatures at two sites in a marsh-mangrove ecotone. Leaf nitrogen (N) was determined on a subsample of leaves to explore potential coupling of R and N. We hypothesized that both species would reduce R at 25°C (R25 ) and the short-term temperature sensitivity of R (Q10 ) as air temperature (Tair ) increased across seasons, but the decline would be stronger in Avicennia than in Spartina. For each species, we hypothesized that seasonal temperature acclimation of R would be equivalent in plants grown under ambient and warmed temperatures, demonstrating convergent acclimation. Surprisingly, Avicennia generally increased R25 with increasing growth temperature, although the Q10 declined as seasonal temperatures increased and did so consistently across sites and treatments. Weak temperature acclimation resulted in reduced homeostasis of R in Avicennia. Spartina reduced R25 and the Q10 as seasonal temperatures increased. In Spartina, seasonal temperature acclimation was largely consistent across sites and treatments resulting in greater respiratory homeostasis. We conclude that co-occurring coastal wetland species may show contrasting patterns of respiratory temperature acclimation. Nonetheless, leaf N scaled positively with R25 in both species, highlighting the importance of leaf N in predicting respiratory capacity across a range of growth temperatures. The patterns of respiratory temperature acclimation shown here may improve the predictions of temperature controls of CO2 fluxes in coastal wetlands.

Fatty Acid Profiles of Selected Microalgae Used as Live Feeds for Shrimp Postlarvae in Vietnam

The importance of microalgal lipids for the survival and growth of shrimp postlarvae has been recognized in a range of studies. Microalgae with fast growth rates and high levels of polyunsaturated fatty acids (PUFA) are considered vital to maximise production and minimise cost in shrimp larviculture. The lipid content and fatty acid composition of microalgae used in shrimp production varies substantially between the algal classes and species being used in Vietnam. This study aims to characterise microalgal lipid and fatty acid (FA) profiles and evaluate the most promising species under growth conditions that are most suitable for shrimp aquaculture. Here, we report that the highest lipid contents were obtained in the Haptophyta microalgae, Tisochrysis lutea and Isochrysis galbana, at 90.3 and 61.1 mg/g, respectively. In contrast, two of the most popular diatom species being used for shrimp larval cultivation in Vietnam, Thalassiosira pseudonana and T. weissflogii, displayed the lowest lipid contents at 16.1 mg/g. Other microalgal species examined showed lipid contents ranging from 28.6 to 55 mg/g. Eicosapentaenoic acid (EPA, 20:5ω3) ranged from 0.6 to 29.9% across the species, with docosahexaenoic acid (DHA, 22:6ω3) present at 0.01 to 11.1%; the two omega (ω)–3 long-chain (LC, ≥C20) LC-PUFA varied between the microalgae groups. Polar lipids were the main lipid class, ranging from 87.2 to 97.3% of total lipids, and triacylglycerol was detected in the range of 0.01 to 2.5%. Saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) increased and PUFA decreased with increasing growth temperatures. This study demonstrated the differences in the lipid contents and FA profiles across 10 microalgal species and the effect of the higher temperature growing conditions encountered in Vietnam.

Correlation of Defects and Lasing Threshold for AlGaN Deep Ultraviolet Lasers Grown by Molecular Beam Epitaxy

Herein, aluminum gallium nitride (AlGaN)/AlN double‐heterostructure (DH) laser at 287 nm with a lasing threshold of 530 kW cm−2 is shown. The laser structure is grown on the AlN‐on‐sapphire template by molecular beam epitaxy. This lasing threshold is drastically reduced compared with the previously reported AlGaN DH lasers at 297 nm with a threshold of around 1 MW cm−2. The detailed studies indicate that this improvement is mainly attributed to the control of defect formation largely due to the increased growth temperature of the AlGaN active layer. This is consistent with the improved internal quantum efficiency at low excitations. Moreover, the control of Al vacancy related point defects is clearly demonstrated by the absence of defect emission in the visible band.

The insertion of the ALD diffusion barriers: An approach to improve the quality of the GaN deposited on Kapton by PEALD

The influence of growth temperature (200–300 ℃) on the properties of gallium nitride (GaN) directly deposited on Kapton by plasma-enhanced atomic layer deposition (PEALD) has been systematically investigated. With increasing growth temperatures, the O content and surface roughness of the GaN thin films increase. Besides, the diffusion behavior of GaN in Kapton was found when the growth temperature exceeds 250 ℃. In this study, ultrathin ALD aluminum oxide (Al2O3) and aluminum nitride (AlN) films were proposed to see if they could alleviate and/or even suppress the negative effects of interface diffusion during high-temperature (≥250 ℃) GaN growth. It was found that oxygen impurities were significantly decreased within the as-deposited GaN. Furthermore, we observed a complete suppression of GaN diffusion into the Kapton throughout the entire growth process. Additionally, an introduction of either the Al2O3 or AlN before the GaN deposition could improve the surface morphology of the GaN deposited on Kapton.

Growth temperature effect on physical and mechanical properties of nitrogen rich InN epilayers

A set of N-polar InN epilayers has been grown at different temperatures by plasma–assisted molecular beam epitaxy (PA-MBE) on GaN/AlN/Al2O3(0001) templates. The purpose is to understand how the variation of crucial factor, such as the temperature, impacts the growth process and the resulting samples’ properties. The characterization of these InN samples using atomic force microscopy and scanning electron microscopy showed different island distributions and shapes by varying the growth temperature. High resolution-X-ray diffraction (HRXRD) enabled to identify a single crystalline phase (hexagonal wurtzite), whatever the growth temperature. Actually, the increase of growth temperature up to 560 °C has improved the crystalline quality; whereas for high temperature, the crystalline quality degrades. The dislocation density of the epilayer grown at this optimum temperature (around 560 °C) is about 1.9 × 1010 cm−2, which is determined using HRXRD spectra analysis. High compressive residual stress value of 0.54 GPa was derived using Raman spectroscopy. Room temperature photoluminescence (PL) displays a band gap energy around 0.69 eV. Besides, Burstein-Moss effect, the PL band gap energy measured at 10 K is dictated by the biaxial compressive residual stresses. Nanoindentation tests were carried out on InN epilayers. Only, the sample grown at 560 °C exhibited a pop-in event, for which the measured hardness and Young’s modulus are of 4.5 ± 0.5 GPa and of 171 ± 8 GPa, respectively. Accordingly, the growth temperature of InN epilayers influences the resulting physical and mechanical performances, thus a good compromise between physical and mechanical features permits to manufacture efficient devices.

Performance analysis of SnS thin films fabricated using thermal evaporation technique for photodetector applications

In the present work, SnS thin films were prepared using the thermal evaporation technique at room temperature and varying annealing temperatures from 200 to 350 °C for photodetector applications. The prepared samples were characterized using different techniques for analyzing the structural, optical, morphological, and photosensing properties of the samples. From XRD analysis, the diffraction pattern of all the prepared thin films shows the pristine SnS phase of the samples possessing an orthorhombic phase without the presence of any impurity phases. Among the fabricated thin films, the SnS thin film annealed at a temperature of 300 °C reveals the highest crystallite size. The Raman results showed the vibrational modes of SnS films and with the increase in growth temperature, the peaks are slightly shifted towards the lower wavelength region. Morphological results show that the SnS thin films exhibit a uniform morphology of 2-D petal-like morphology with different sizes. The UV–vis spectroscopic study shows the decrease in the bandgap value of the samples with the increase in annealing temperature. The photosensing properties of the fabricated samples show the SnS sample annealed at 300 °C has a higher responsivity value of 6.40 × 10-2 AW-1, external quantum efficiency (EQE) value of 14.9%, and the detectivity value of 6.05 × 109 Jones. Finally, the transient photoresponse results suggest that the SnS annealed at 300 °C shows a rise and fall time of 1.5 and 2.5 s compared to the other samples which would be better suited for photodetector applications.

Temperature dependence of β-Ga2O3 heteroepitaxy on c-plane sapphire using low pressure chemical vapor deposition

β-Ga2O3 has drawn significant attention for power electronics and deep ultraviolet (UV) photodetector applications owing to its wide bandgap of ~4.4–4.9 eV and high electric breakdown strength of ~7–8 MV/cm. Growth of β-Ga2O3 epitaxial thin films with high growth rate has been recently reported using low pressure chemical vapor deposition (LPCVD) technique. In this work, we have investigated the effect of growth temperature on β-Ga2O3 films grown on c-plane sapphire substrates using LPCVD. We performed growths by varying temperatures from 800 °C to 950 °C while keeping all other growth parameters (Ar/O2 gas flow rates, growth pressure, and Gallium precursor to substrate distance) constant. Optical, structural, and surface characterizations are performed to determine the thickness, bandgap, phase purity, crystal orientation, and crystalline quality of the grown thin films. Amorphous islands of Ga2O3 are observed at growth temperature of 800 °C while continuous and crystalline (−201) oriented β-Ga2O3 thin films are achieved for growth temperatures of 850–950 °C. Crystallinity of the films is found to improve with increase in growth temperature with a minimum rocking full width at half maximum of 1.52º in sample grown at 925 °C. For all the samples grown at and above 875 °C, transmittance measurements revealed an optical bandgap of ~4.77–4.80 eV with high growth rate of ~6 µm/h.

Controlled lateral epitaxial growth in vertical β-Ga2O3 nanowires on sapphire by MOCVD

Lateral epitaxial growth in vertical β-Ga2O3 nanowires was studied on sapphire substrates via metal organic chemical vapor deposition by controlling growth temperature and O2 flow rate. The lateral epitaxial growth rate of β-Ga2O3 rise up with increasing growth temperature and decreasing O2 flow rate, which are related to reaction kinetics and parasitic reaction, respectively. In addition, a growth model is proposed to explain the morphology of nanowires when growth condition changes. These results can provide important guidance on addressing slow coalescence rate of β-Ga2O3 nuclear islands on heterogeneous substrates and open new doors to β-Ga2O3 optoelectronic and electronic devices on cost-effective foreign substrates.

X-Ray CT Phenotyping Reveals Bi-Phasic Growth Phases of Potato Tubers Exposed to Combined Abiotic Stress.

As a consequence of climate change, heat waves in combination with extended drought periods will be an increasing threat to crop yield. Therefore, breeding stress tolerant crop plants is an urgent need. Breeding for stress tolerance has benefited from large scale phenotyping, enabling non-invasive, continuous monitoring of plant growth. In case of potato, this is compromised by the fact that tubers grow belowground, making phenotyping of tuber development a challenging task. To determine the growth dynamics of tubers before, during and after stress treatment is nearly impossible with traditional destructive harvesting approaches. In contrast, X-ray Computed Tomography (CT) offers the opportunity to access belowground growth processes. In this study, potato tuber development from initiation until harvest was monitored by CT analysis for five different genotypes under stress conditions. Tuber growth was monitored three times per week via CT analysis. Stress treatment was started when all plants exhibited detectable tubers. Combined heat and drought stress was applied by increasing growth temperature for 2 weeks and simultaneously decreasing daily water supply. CT analysis revealed that tuber growth is inhibited under stress within a week and can resume after the stress has been terminated. After cessation of stress, tubers started growing again and were only slightly and insignificantly smaller than control tubers at the end of the experimental period. These growth characteristics were accompanied by corresponding changes in gene expression and activity of enzymes relevant for starch metabolism which is the driving force for tuber growth. Gene expression and activity of Sucrose Synthase (SuSy) reaffirmed the detrimental impact of the stress on starch biosynthesis. Perception of the stress treatment by the tubers was confirmed by gene expression analysis of potential stress marker genes whose applicability for potato tubers is further discussed. We established a semi-automatic imaging pipeline to analyze potato tuber delevopment in a medium thoughput (5 min per pot). The imaging pipeline presented here can be scaled up to be used in high-throughput phenotyping systems. However, the combination with automated data processing is the key to generate objective data accelerating breeding efforts to improve abiotic stress tolerance of potato genotypes.

Atomic-layer deposition of TiO2 thin films with a thermally stable (CpMe5)Ti(OMe)3 precursor

Atomic layer deposition (ALD) of TiO2 films from (CpMe5)Ti(OMe)3 as precursor and O3 as co-reactant was examined. The high thermal stability of (CpMe5)Ti(OMe)3 enabled ALD reaction up to a high temperature of 345 °C. A wide temperature window from 182 to 345 °C was achieved in the ALD process, and the growth per cycle increased with increasing the temperature from 0.025 to 0.06 nm/cycle in the ALD window. The impurity content of the films decreased with increasing growth temperature. Above 291 °C, the carbon content in the films decreased to the level in a single crystalline Si substrate. The morphology with patterns spreading radially from the multiple points developed above 236 °C, and the size of the grains decreased as the growth temperature increased. Eventually, a uniform morphology with fine grains was obtained at temperatures > 300 °C. The films grown at the high temperatures exhibited superior dielectric properties. Other common metalorganic precursors of Ti usually restrict the use of high-temperature ALD because they are thermally unstable and decompose below 300 °C. Therefore, (CpMe5)Ti(OMe)3 is favorable for forming dense and high-purity TiO2 films by ALD.

Influence of the PALE growth temperature on quality of MOVPE grown AlN/Si (111)

In the present study, the PALE-AlN (pulsed atomic layer epitaxy) epilayers were grown on the Si (111) substrates at different growth temperatures by metal organic vapor phase epitaxy (MOVPE) technique. The oxygen (O) and silicon (Si) concentrations of grown PALE-AlN epilayers and interface between epilayer and substrate were investigated by secondary ion mass spectroscopy (SIMS). It was observed that O and Si concentration change with growth temperature of epilayers as well as the interface significantly. HRXRD (high-resolution x-ray diffraction) analyses showed that the highest growth temperature results with the lowest full width at half maximum (FWHM) value for both ɷ scans. Scanning electron microscope (SEM) and atomic force microscopy (AFM) analyses indicated that relatively low growth temperature grown samples gave rise to 2D-like growth mode with openings while increased growth temperature resulted in change the growth mode to a columnar mode with increasing V-shape pits because of the increasing diffusion coefficient of O impurities and Si atoms in AlN epilayers.

Kinetic model for molecular beam epitaxy growth of InAsSbBi alloys

The growth of Bi-containing III-V alloys requires careful control over temperature and group-V fluxes due to the low equilibrium solubility of Bi and its tendency to surface segregate into Bi-rich droplet features. A model for molecular beam epitaxy growth based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation is applied to the bismide alloy InAsSbBi grown on GaSb substrates. A steady-state solution is derived for the Bi, Sb, and As mole fractions and surface layer coverages based on the Bi, Sb, and As fluxes. A nonlinear least-squares algorithm is used to fit the growth model parameters to experimentally measured Bi mole fractions in bulk and quantum well InAsSbBi samples grown at 400 °C and 420 °C. The Bi mole fraction ranges from 0.12% to 1.86% among 17 samples examined. The results indicate that as the growth temperature increases, the rate of Bi incorporation decreases and the rate of Bi self-desorption increases. A strong interaction is observed between Bi and As that plays a role in the desorption of excess Bi from the growth surface, thus reducing the likelihood of Bi-rich droplet formation when an excess As flux is present. Significantly, the model predicts that the incorporation of Bi is limited to mole fractions of 1.43% at 400 °C and 0.30% at 420 °C in lattice-matched bulk InAsSbBi grown on GaSb substrates.

Responses of Lotus corniculatus to environmental change 3: The sensitivity of phenolic accumulation to growth temperature and light intensity and effects on tissue digestibility

Growth temperature and light intensity are major drivers of phenolic accumulation in Lotus corniculatus resulting in major changes in carbon partitioning which significantly affects tissue digestibility and forage quality. The response of plant growth, phenolic accumulation and tissue digestibility to light and temperature was determined in clonal plants of three genotypes of Lotus corniculatus (birdsfoot trefoil) cv Leo, with low, intermediate or high levels of proanthocyanidins (condensed tannins). Plants were grown from 10 °C to 30 °C, or at light intensities from 20 to 500 µm m-2 s-1. Plants grown at 25 °C had the highest growth rate and highest digestibility, whereas the maximum tannin concentration was found in plants grown at 15 °C. Approximately linear increases in leaf flavonol glycoside levels were found with increasing growth temperature in the low tannin genotype. Tannin hydroxylation increased with increasing growth temperature but decreased with increasing light intensity. The major leaf flavonols were kaempferol glycosides of which kaempferol-3-glucoside and kaempferol-3,7-dirhamnoside were the major components. Increases in both tannin and total flavonol concentrations in leaves were linearly related to light intensity and were preceded by a specific increase in the transcript level of a non-legume type chalcone isomerase. Changes in growth temperature and light intensity, therefore, result in major changes in the partitioning of carbon into phenolics, which significantly affects tissue digestibility and nutritional quality with a high correlation between tannin concentration and leaf digestibility.

Oxygen defect dominated photoluminescence emission of ScxAl1−xN grown by molecular beam epitaxy

A fundamental understanding and control of impurity incorporation and charge carrier recombination are critical for emerging ScxAl1−xN electronics, optoelectronics, and photonics. We report on the photoluminescence properties of ScxAl1−xN grown by plasma-assisted molecular beam epitaxy with varying growth temperatures and Sc contents. Bright and broad emission comprising a dominant peak at ∼3.52 eV and a weak peak at ∼2.90 eV was observed in Sc0.05Al0.95N. The origin of the ∼3.52 eV emission line is attributed to charge carrier recombination from the localized excited state of (Vcation-ON)2−/− to its ground state, whereas the second peak at ∼2.90 eV results from charge carrier recombination of isolated Vcation3−/2− to the valence band. We further show that oxygen defect-related emission can be significantly suppressed by increasing growth temperature. This work sheds light on the recombination dynamics of photoexcited carriers in ScxAl1−xN and further offers insight into how to improve the optical and electrical properties of ScxAl1−xN that are relevant for a broad range of applications.

Evolution of amorphous carbon films into nano-crystalline graphite with increasing growth temperature in plasma-enhanced chemical vapor deposition

Amorphous carbon films (ACFs) have recently emerged as one of the best candidates for etching-resistant hardmask materials in advanced semiconductor manufacturing processes. Etching resistivity of ACFs is known to be improved by controlling the relative abundance between sp2 and sp3 bonds. We have investigated the relative abundance between sp2 and sp3 bonds in several ACFs, fabricated by plasma-enhanced chemical vapor deposition at different temperatures, which were analyzed by using X-ray photoemission spectroscopy, Raman spectroscopy, and transmission electron microscopy. We found that the relative abundance of sp2 bond increased as the growth temperature was raised. Furthermore, the ACFs eventually evolved into nano-crystalline graphite with increasing growth temperature.

Optical Properties of ZnO Deposited by Atomic Layer Deposition on Sapphire: A Comparison of Thin and Thick Films

Optical properties of thin (≈100 nm) ZnO films in comparison with thick (≈1 μm) layers deposited on sapphire substrates by atomic layer deposition (ALD) at temperatures from 100 to 300 °C are investigated by optical transmittance and luminescence. It is found that growth temperature, the thickness of the layers, and post‐grown annealing influence the measured optical bandgap values and low‐temperature photoluminescence. However, the values of the bandgaps corrected according to the Burstein–Moss effect do not depend on layer thickness, whereas an increase in growth temperature leads to a decrease in the energy gap. On the other hand, post‐growth annealing of the layers results in increasing the value of the energy gap and decreasing Urbach energy. It is observed that thicker and annealed samples show more intensive and narrower PL bands, which are assigned to donor and acceptor bound excitons, free electrons to acceptors, and donor–acceptor pair transitions. Their energy position and relative intensity depend on the growth temperature applied.

Preparation and orientation mechanism analysis of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> magneto-optical single crystal film with out-of-plane orientation

Liquid-phase epitaxy (LPE) is one of the best techniques for the preparation of single crystal garnet films. However, the specific Faraday rotation angle of Yttrium iron garnet (YIG) is small, and its easy magnetization axis is parallel to the film surface. The YIG requires a large external saturation field, which cannot meet the development needs of magneto-optical devices. It is found that Bi-substituted YIG(Bi:YIG) film has a larger specific Faraday angle. By adjusting the easy magnetization axis of Bi: YIG perpendicular to the film surface, the saturation magnetization of Bi: YIG can be reduced, so that it can work under a small external magnetic field. This meets the development needs of miniaturization and energy saving of magneto-optical device. The saturation magnetization of garnet film can be effectively reduced by substituting Ga<sup>3+</sup> for YIG crystal, mainly for Fe<sup>3+</sup> at the 24d position of its tetrahedron. And the lattice constants of Gd<sub>3</sub>Ga<sub>5</sub>O<sub>12</sub> (GGG) and YIG are 1.2383 nm and 1.2376 nm, respectively. However, the radius of Bi<sup>3+</sup> (10.8 nm) is larger than that of Y<sup>3+</sup> (9.0 nm), the lattice mismatch of garnet film increases with the incorporation of Bi<sup>3+</sup>. In order to neutralize the lattice expansion caused by Bi<sup>3+</sup>, Tm<sup>3+</sup> (8.69 nm) with a radius smaller than that of Y<sup>3+</sup> (9.0 nm) is selected. Based on the theoretical analysis of the magnetocrystalline anisotropy of garnet film, (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films with different growth temperatures and different thickness values are grown by LPE on GGG (111) substrates. The experimental results show that when the thickness of epitaxial film is greater than 1 μm, the influence of shape anisotropy on magnetocrystalline anisotropy can be ignored. With the increase of growth temperature, the substitution number of Bi<sup>3+</sup> ions decreases gradually, the lattice constant of epitaxial film decreases gradually, and the lattice mismatch first decreases and then increases. Then, the state of compressive stress gradually changes into that of tensile stress. Compared with growth-induced anisotropy, the stress-induced anisotropy is dominant in the change of magnetocrystalline anisotropy. The Verdet constant of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> film is 11.8 × 10<sup>4</sup> rad/Tm@1064 nm. The results show that the prepared (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> mono-crystalline films have great development potential in magneto-optical devices.

Theoretical Analysis of XRD Data by X-Ray Peak Profile Analysis for Estimation of lattice Strain and crystallite Size and study of the Effect of Growth Temperature in CdS Nanoparticles

CdS is an important material of low cost photovoltaic devices. Use of capping agents during synthesis of nanomaterials has drawn a lot of attention due to their size dependent properties. Here we reported the synthesis of CdS nanoparticles using a simple chemical precipitation method at 40°C to 70°C. The reaction was carried out at constant temperature in aqueous medium with stirring. The hydrazine hydrate was used as capping agent. The as prepared samples were characterized using X-ray powder diffraction (XRD) technique for structural analysis. The effects of growth temperature on structural properties are evaluated. The lattice strain, crystallite size and dislocation densities were calculated using the Williamson-Hall (W-H) method. The results show that the crystallite size decreases with increase in growth temperature.

Thermotolerance, Survival, and Stability of Lactic Acid Bacteria After Spray Drying as Affected by the Increase of Growth Temperature

Microencapsulation of lactic acid bacteria (LAB) via spray drying differs from that of common bioactive substances in that the intrinsic stress tolerance of cells can be modulated to improve cell survival. In this study, elevated growth temperatures that were 3–5 °C above the standard conditions were used to culture Lactococcus lactis subsp. cremoris, Lactobacillus rhamnosus GG (LGG), and Lactobacillus acidophilus for spray drying. The heat-adapted cultures showed lower bacterial population than the controls by 0.45 log at stationary growth phase and produced lactobacilli cells with elongated shape, while their metabolic activities were maintained similar to the controls. Heat-adapted L. cremoris and LGG demonstrated increases in survival by 0.7–1.5 log and 0.3 log, respectively, after heat treatment at 60 °C. The thermotolerance of L. acidophilus grown at 42 °C was dependent on growth phase, and the culture entered death phase within 24 h of incubation. The survival of heat-adapted L. cremoris and L. acidophilus after spray drying was increased by 21.0% and 13.7%, respectively, whereas the increase shown by LGG was relatively insignificant (9.9%). Spray-dried powders containing heat-adapted cells showed substantial reduction of viability at the first week of storage, reaching 1.03–1.23 log, compared to 0.87–0.90 log of reduction shown by the controls. The findings demonstrated that strain-specific cellular response toward variations in growth conditions is crucial to the intrinsic properties of LAB and to cell survival during spray drying and storage. Controlling cellular response is one of the key factors in developing a viable spray drying scheme for active LAB.