Growth of uniform Mg-doped p-AlGaN nanowires using plasma-assisted molecular beam epitaxy technique for UV-A emitters.

In this manuscript, we have shown the growth and extensive structural and optical characteristic of the uniformly Mg-doped Al0.23Ga0.77N (UV-A region,λ∼ 323 nm) nanowire. The Kelvin probe force microscopy was employed to determine the profile of holes in p-type AlGaN nanowires by measuring the work function changes induced by Mg incorporation. The influence of surface band bending on doping concentration has thoroughly been discussed. Our experiment confirms the homogeneous incorporation of Mg throughout the nanowire without any top surface Mg segregation. In this work, we have also demonstrated a comprehensive analysis of acceptor states induced thermal quenching behaviour in the optical transition of Mg-doped AlGaN nanowire. We propose a phenomenological model, based on the rate equation which confirms that achieving higher 'hole' (p-doping) concentration in AlGaN nanowire (>1018cm-3) is more conducive than the planar counterpart if the growth of NWs is carried out at optimized process conditions. This rate equation-based model has also demonstrated the influence of sidewall surface passivation in those AlGaN nanowires.

Role of defect saturation in improving optical response from InGaN nanowires in higher wavelength regime.

Growth of InGaN, having high Indium composition without compromising crystal quality has always been a great challenge to obtain efficient optical devices. In this work, we extensively study the impact of non-radiative defects on optical response of the plasma assisted molecular beam epitaxy (PA-MBE) grown InGaN nanowires, emitting in the higher wavelength regime ([Formula: see text] nm). Our analysis focuses into the effect of defect saturation on the optical output, manifested by photoluminescence (PL) spectroscopy. Defect saturation has not so far been thoroughly investigated in InGaN based systems at such a high wavelength, where defects play a key role in restraining efficient optical performance. We argue that with saturation of defect states by photo-generated carriers, the advantages of carrier localization can be employed to enhance the optical output. Carrier localization arises because of Indium phase segregation, which is confirmed from wide PL spectrum and analysis from transmission electron microscopy (TEM). A theoretical model has been proposed and solved using coupled differential rate equations in steady state to undertake different phenomena, occurred during PL measurements. Analysis of the model helps us understand the impact of non-radiative defects on PL response and identifying the origin of enhanced radiative recombination.

Triaxially uniform high-quality Al x Ga(1-x)N (x ∼ 50%) nanowires on template free sapphire substrate.

We have demonstrated the growth of high-quality Al x Ga(1-x)N (x âˆ¼ 50%) nanowires (NWs) for the first time on the sapphire substrate without using GaN NWs as the template, by plasma-assisted molecular beam epitaxy. Our newly developed process elucidates that depending on the substrate temperature and V/III ratio an AlGaN network is formed on sapphire substrate prior to the NWs growth. We find that the ledges of this kinked shaped network act as nucleation sites for the NW growth. The present observations suggest that availability of nucleation sites and higher substrate temperature during growth are the key parameters for the growth of homogeneous AlGaN NWs on the sapphire substrates. Energy dispersive x-ray spectroscopy, high-resolution transmission electron microscopy, Raman spectroscopy, x-ray diffraction, photoluminescence spectroscopy, and scanning electron microscopy analysis show that AlGaN NWs exhibit near-atomic scale compositional uniformity along the length as well as across the diameter.

Co-production of 3-hydroxypropionic acid and 1,3-propanediol from glycerol using resting cells of recombinant Klebsiella pneumoniae J2B strain overexpressing aldehyde dehydrogenase.

The co-production of 3-hydroxypropionic acid (3HP) and 1,3-propanediol (PDO) from glycerol was studied using the resting cells of a recombinant Klebsiella pneumoniae J2B strain that overexpresses an aldehyde dehydrogenase (KGSADH). Active biomass was produced in a mineral salt medium containing yeast extract and glycerol under a range of aeration conditions, and shifted to potassium phosphate buffer containing glycerol for bioconversion. The microaerobic or anaerobic conditions were favorable for both the production of active biomass and subsequent bioconversion. At the flask level, the recombinant strain (2.0 g CDW/L) grown under microaerobic conditions produced 43.2 mM 3HP and 59.0 mM PDO from glycerol (117 mM) in 30 min with a cumulative yield of 0.87 (mol/mol). The fed-batch bioconversion, which was performed in a 1.5-L bioreactor with 1.0 g CDW/L at a constant pH 7.0 under anaerobic conditions, resulted in 125.6 mM 3HP and 209.5 mM PDO in 12 h with a cumulative overall productivity, yield, and maximum specific production rate of 27.9 mmol/L/h, 0.71 (mol/mol), and 128.5 mmol/g CDW/h, respectively. Lactate, succinate and 2,3-butanediol were the major by-products, whereas the production of acetate and ethanol was marginal. This is the first report of the simultaneous production of 3HP and PDO from glycerol using a resting cell system.

Simultaneous production of 3-hydroxypropionic acid and 1,3-propanediol from glycerol using resting cells of the lactate dehydrogenase-deficient recombinant Klebsiella pneumoniae overexpressing an aldehyde dehydrogenase.

In the present study, the lactate dehydrogenase-deficient (ldhA(-)) recombinant Klebsiella pneumoniae overexpressing an ALDH (KGSADH) was developed and the co-production of 3-HP and PDO from glycerol by this recombinant under resting cell conditions was examined. The new recombinant did not produce any appreciable lactate, which seriously inhibits the production of 3-HP and PDO. The final titers of 3-HP and PDO by the ldhA(-) recombinant strain at 60 h were 252.2 mM and 308.7 mM, respectively, which were improved by approximately 30% and 50%, respectively, compared to those by the counterpart recombinant strain, which was the wild type for ldhA. In addition, after deleting ldhA, the cumulative yield on glycerol and specific production rate of these two metabolites (3-HP and PDO) were enhanced by 41.4% and 52%, respectively.