Browsing by Subject "nitrogen"

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    Differential effects of nitrogen and sulfur deprivation on growth and biodiesel feedstock production of Chlamydomonas reinhardtii
    (2012) Cakmak, T.; Angun P.; Demiray, Y.E.; Ozkan, A.D.; Elibol, Z.; Tekinay, T.
    Biodiesel production from microalgae is a promising approach for energy production; however, high cost of its process limits the use of microalgal biodiesel. Increasing the levels of triacylglycerol (TAG) levels, which is used as a biodiesel feedstock, in microalgae has been achieved mainly by nitrogen starvation. In this study, we compared effects of sulfur (S) and nitrogen (N) starvation on TAG accumulation and related parameters in wild-type Chlamydomonas reinhardtii CC-124 mt(-) and CC-125 mt(+) strains. Cell division was interrupted, protein and chlorophyll levels rapidly declined while cell volume, total neutral lipid, carotenoid, and carbohydrate content increased in response to nutrient starvation. Cytosolic lipid droplets in microalgae under nutrient starvation were monitored by three-dimensional confocal laser imaging of live cells. Infrared spectroscopy results showed that relative TAG, oligosaccharide and polysaccharide levels increased rapidly in response to nutrient starvation, especially S starvation. Both strains exhibited similar levels of regulation responses under mineral deficiency, however, the degree of their responses were significantly different, which emphasizes the importance of mating type on the physiological response of algae. Neutral lipid, TAG, and carbohydrate levels reached their peak values following 4 days of N or S starvation. Therefore, 4 days of N or S starvation provides an excellent way of increasing TAG content. Although increase in these parameters was followed by a subsequent decline in N-starved strains after 4 days, this decline was not observed in S-starved ones, which shows that S starvation is a better way of increasing TAG production of C. reinhardtii than N starvation. © 2012 Wiley Periodicals, Inc.
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    Nitrogen and sulfur deprivation differentiate lipid accumulation targets of chlamydomonas reinhardtii
    (2012) Cakmak, T.; Angun P.; Ozkan, A.D.; Cakmak, Z.; Olmez, T.T.; Tekinay, T.
    Nitrogen (N) and sulfur (S) have inter-related and distinct impacts on microalgal metabolism; with N starvation having previously been reported to induce elevated levels of the biodiesel feedstock material triacylglycerol (TAG), while S deprivation is extensively studied for its effects on biohydrogen production in microalgae.1,2 We have previously demonstrated that N- and S-starved cells of Chlamydomonas reinhardtii display different metabolic trends, suggesting that different response mechanisms exist to compensate for the absence of those two elements.3 We used C. reinhardtii CC-124 mt(-) and CC-125 mt(+) strains to test possible metabolic changes related to TAG accumulation in response to N and S deprivation, considering that gamete differentiation in this organism is mainly regulated by N.4 Our findings contribute to the understanding of microalgal response to element deprivation and potential use of element deprivation for biodiesel feedstock production using microalgae, but much remains to be elucidated on the precise contribution of both N and S starvation on microalgal metabolism. © 2012 Landes Bioscience.
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    Plasma-assisted atomic layer deposition of III-nitride thin films
    (2014) Özgit-Akgün, Çağla
    III-nitride compound semiconductors and their alloys have emerged as versatile and high-performance materials for a wide range of electronic and optoelectronic device applications. Besides possessing very unique material properties individually, members of the III-nitride family with wurtzite (hexagonal) crystal structure also exhibit direct band gaps, which cover a wide range with values of 6.2, 3.4 and 0.64 eV for AlN, GaN and InN, respectively. In this respect, ternary and quaternary alloys of this family are particularly important since their bandgaps can easily be tuned by adjusting the alloy composition. Although high quality IIInitride thin lms can be grown at high temperatures (>1000 XC) with signi cant rates, deposition of these lms on temperature-sensitive device layers and substrates necessitates the adaptation of low-temperature methods such as atomic layer deposition (ALD). ALD is a special type of chemical vapor deposition, in which the substrate surface is exposed to sequential pulses of two or more precursors separated by purging periods. When compared to other low-temperature thin lm deposition techniques, ALD stands out with its self-limiting growth mechanism, which enables the deposition of highly uniform and conformal thin lms with sub-angstrom thickness control. Moreover, alloy thin lms can be easily deposited by ALD, where lm composition is digitally controlled by the relative number of subcycles. In this thesis, we report on the development of plasma-assisted ALD (PAALD) processes for III-nitrides, and present detailed characterization results for the deposited thin lms and fabricated nanostructures. PA-ALD of polycrystalline wurtzite AlN thin lms was realized at temperatures ranging from 100- 500 XC using trimethylaluminum (AlMe3) as the Al precursor. Films deposited at temperatures within the ALD window (100-200 XC for both ammonia (NH3) and N2/H2 plasma processes) were C-free and had relatively low O concentrations (<3 at.%). We also demonstrated the conformality of AlMe3-NH3 plasma process by fabricating high surface area AlN hollow nano bers using electrospun nylon nano ber mats as sacri cial templates. Our initial e orts for depositing GaN and InN resulted in thin lms with high O concentrations. Although - at rst - the most probable source of this contamination was presumed as the O-containing impurities in the unpuri ed 5N-grade NH3 gas, subsequent experiments revealed the true source as the quartz tube of inductively coupled RF-plasma (ICP) source itself. In view of these circumstances, the choice of N-containing plasma gas (NH3, N2/H2 or N2) determined the severity of O incorporation into AlN and GaN lms deposited by PA-ALD. As an e ort to completely avoid this plasma-related oxygen contamination problem, we replaced the original quartz-based ICP source of the ALD system with a stainless steel hollow cathode plasma (HCP) source. Thereby we demonstrated the low-temperature hollow cathode PA-ALD (HCPAALD) of crystalline AlN, GaN and AlxGa1−xN thin lms with low impurity concentrations (O, C <1 at.%) using AlMe3 and trimethylgallium (GaMe3) as the Al and Ga precursors, respectively. Optical band edge values of the AlxGa1−xN lms shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with alloy composition. HCPA-ALD of InN was also investigated within the scope of this study. Initial results revealed the possibility to obtain single-phase wurtzite InN thin lms using cyclopentadienyl indium (CpIn) as the In precursor.