Dedicated to Professor Jozef T. Devreese on the occasion of his 65th birthday PACS 78.55.Cr, 78.60.Hk, 78.67.De We report on a detailed study of low temperature photoluminescence (PL) in Al 0.07 Ga 0.93 N/GaN multiple quantum wells (MQWs). The structures were grown on sapphire with the conventional low temperature AlN nucleation layer and thick GaN buffer layer. Several sets of 5 QW MQW samples were studied, one set with Si doping in the barriers up to or above the metallic limit. Nominally undoped MQW samples were also studied. The spectral behaviour of the doped samples was strongly affected by the near surface depletion field, causing overlap of different spectra from non-equivalent QWs. The QWs closest to the surface are presumably inactive in some samples, due to a very high depletion field. For the case of undoped samples, on the other hand, the near surface QWs are active and most prominent in the PL spectra. The structure from discrete well width variations is here resolved in the PL spectra. The results demonstrate that for structures with no additional capping layer both the depletion field and the polarisation fields need to be considered in the interpretation of experimental data. The theoretically estimated fields in this work are consistent with the experimental spectra. The presence of localisation even in the case of metallic samples, as observed by a constant PL decay time independent of doping, is discussed in terms of penetration of the hole wave functions into the AlGaN barriers. This localisation is also manifested in a sizeable LO phonon coupling strength in all samples studied.
A nanocomposite packaging film (NPF) prepared by blending polyethylene (PE) with nanoclay (closite 20A) was used. The effects of NPF on the quality parameters of mature peach fruit were investigated during 42 d of storage at 2 °C. The results showed that barrier properties of active film properly suited fruit respiration and significantly inhibited bacterial growth. The weight loss, softening and internal browning of peach fruit were also significantly inhibited by 42%, 28%, and 15%, respectively, which indicated that NPF could delay peach ripening. In addition, pH, total soluble solid, and titratable acidity of fruit in the neat PE film and the unpacked fruit changed more significantly compared with the NPF‐packed fruit. Additionally, peach in NPF exhibited lower polyphenol oxidase and higher peroxidase and catalase activities than the others. These results shown that NPF may be an applicable method to control fruit loss and prolong quality of peach during storage.
Practical applications
In this study a clay‐nanocomposite film with modified barrier and unti‐microbial properties on storage of peach fruit was investigated. The barrier properties of the nanocomposite film improved compared to the neat LDPE film so that causes to decrease the respiration rate of the product. Also it is capable to increase the shelf life and storage life of the product. In addition this nanocomposite film can be used as an active packaging with anti‐microbial property. Furthermore, the melt blending extrusion for developing of the nanocomposite film can be used as an industrial and cost effective method for reduction of the product losses in postharvest period.
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