Ordering of Ge quantum dots on silicon surfaces via bottom-up and top-down approaches

The nanoscale ordering of inorganic semiconductor quantum dots (QDs) is crucial to obtain reliable structures for novel nanotechnological applications such as nanomemories, nanolasers and nanoelectronic devices. We have directly grown Ge QDs by physical vapour deposition (PVD) on Si(111), Si(100) and some of its vicinal surfaces and studied innovative bottom up techniques to order such nanostructures. Specifically, we harnessed naturally occurring instabilities due to reconstruction and intrinsic anisotropic diffusion in Si bare surfaces, such as step bunching and natural steps occurring in silicon vicinal surfaces, to order the QDs both in one dimension and in the plane. We have also shown the use of controlled quantities of surfactants, like Sb, dramatically improves the desired ordering. Moreover, we have assisted these self-assembling processes using top-down approaches like Focused Ion Beam (FIB) milling and STM nanoindentation to control the nucleation sites and the density of the Ge QDs. Real-time study of growth and self-assembly has been accomplished using Scanning Tunneling Microscopy imaging in UHV. An explanation of the occurring processes is given, and a software routine is used to quantify the ordering of the QDs both in pre-patterned and bare surfaces. Applications, mainly in the field of Nanocrystal Nonvolatile Memories, are discussed

A study of the pair distribution function of self-organized Ge quantum dots

We explore the use of the pair distribution function to study the self-organization process of Gequantum dots on both nanopatterned and nonpatterned oxidized Si(001) surfaces.Dots formation and ordering upon annealing of a Ge thin film are analyzed. The method we use is not limited to this case study. We show how it can be applied to determine short and long range self-ordering of nanostructures. We support our results by applying a software routine to simulate patterns of dots to finally spot the relevant physical aspects of Ge islands self-assembly

Photobiomodulation at Multiple Wavelengths Differentially Modulates Oxidative Stress In Vitro and In Vivo

Photobiomodulation (PBM) is emerging as an effective strategy for the management of multiple inflammatory conditions, including oral mucositis (OM) in cancer patients who receive chemotherapy or radiotherapy. Still, the poor understanding of the mechanisms by which the light interacts with biological tissues and the heterogeneity of light sources and protocols employed worldwide significantly limits its applicability. Reactive oxygen species (ROS) are massively generated during the early phases of OM and play a major role in the pathogenesis of inflammation in general. Here, we report the results of a clinical and experimental study, aimed at evaluating the effect of laser light at different wavelengths on oxidative stress in vivo in oncologic patients suffering from OM and in vitro in two cell types abundantly present within the inflamed oral mucosa, neutrophil polymorphonuclear (PMN) granulocytes, and keratinocytes. In addition to standard ROS detection methods, we exploited a roGFP2-Orp1 genetically encoded sensor, allowing specific, quantitative, and dynamic imaging of redox events in living cells in response to oxidative stress and PBM. We found that the various wavelengths differentially modulate ROS production. In particular, the 660\u2009nm laser light increases ROS production when applied either before or after an oxidative stimulus. In contrast, the 970\u2009nm laser light exerted a moderate antioxidant activity both in the saliva of OM patients and in both cell types. The most marked reduction in the levels of ROS was detected in cells exposed either to the 800\u2009nm laser light or to the combination of the three wavelengths. Overall, our study demonstrates that PBM exerts different effects on the redox state of both PMNs and keratinocytes depending on the used wavelength and prompts the validation of a multiwavelength protocol in the clinical settings

Modified strain and elastic energy behavior of Ge islands formed on high-miscut Si(0 0 1) substrates

Abstract We investigate here the influence of Si substrate miscut on the strain and elastic energy of Ge islands. We show how the morphology, composition and the elastic energy for 4 and 13 monolayers (ML) Ge islands grown at 600 °C and 730 °C on vicinal Si(0 0 1) surfaces change with miscut angles ranging between 0° and 10°. Scanning Tunneling Microscopy is used to determine the island morphology. Resonant x-ray diffraction near the Ge-K absorption edge allows the determination of the Ge concentration as well as the elastic energy stored on such structures from their dependency on the lattice parameter. Simulations using the Finite Elements Method indicate that the enlargement of the SiGe broad peak retrieved from the x-ray diffraction measurements is actually caused by the asymmetrical faceting induced by large miscut angles. Such faceting has a strong effect on island density and elastic energy, producing differences that are proportional to those observed in conditions with distinct SiGe content

Interplay between Kolmogorov-Johnson-Mehl-Avrami kinetics and Poisson-Voronoi tessellation

In this paper we investigate the connection between Voronoi tessellation and the KJMA approach of space filling. In particular, we study how nuclei, in their growth, cover a given Voronoi cell. This approach leads to an integral equation for the cell-size distribution function. Starting from the 1D case, that is solved exactly, we extend the results to the dDcase. The analysis allows to find a rationale to the phenomenological parameter entering the Gamma distribution function and to improve the description of the transformation through the knowledge of the kinetics of grain formation. Moreover, the nucleus size distribution function has been calculated as a function of the transformed fraction

I fenomeni fisici e chimici irreversibili: la teoria termodinamica dell'osmosi e la sua verifica sperimentale

In questo articolo vengono forniti ai docenti delle scuole di I e II grado alcuni spunti didattici per l’elaborazione di una lezione sull’ irreversibilit`a dei fenomeni naturali. Il processo dell’ osmosi, di straordinaria importanza nella chimica, nella biologia e nella fisiologia, `e analizzato quale caso paradigmatico di un fenomeno irreversibile. La prima parte dell’articolo ne esamina alcuni aspetti termodinamici sulla base del I e del II principio della termodinamica. La seconda parte `e interamente dedicata alla discussione di semplici esperimenti e alla possibilit`a di ottenere informazioni quantitative sul fenomeno in esame

I fenomeni fisici e chimici irreversibili: la teoria termodinamica dell'osmosi e la sua verifica sperimentale

In questo articolo vengono forniti ai docenti delle scuole di I e II grado alcuni spunti didattici per l’elaborazione di una lezione sull’ irreversibilit`a dei fenomeni naturali. Il processo dell’ osmosi, di straordinaria importanza nella chimica, nella biologia e nella fisiologia, `e analizzato quale caso paradigmatico di un fenomeno irreversibile. La prima parte dell’articolo ne esamina alcuni aspetti termodinamici sulla base del I e del II principio della termodinamica. La seconda parte `e interamente dedicata alla discussione di semplici esperimenti e alla possibilit`a di ottenere informazioni quantitative sul fenomeno in esame

Idrogeno ed elio : atomi di storia

Viene ripercorsa, molto brevemente, la strana storia della scoperta di un atomo di idrogeno "esotico" nel periodo pioneristico della spettroscopia ottica. Un periodo pieno di fervore e scoperte , che, al di là di qualche incidente di percorso, fornì dati che contribuirono non poco alla definitiva determinazione della struttura dell'atomo di idrogeno, più in generale della tabella periodica degli elementi, e che spianò la strada alla seconda rivoluzione scientifica: la nascita della meccanica quantistica