705 research outputs found
Ab initio characterization of Hafnium oxides HfO2 and related derivatives as emerging nanomaterials for electronics applications
No full textQuesta tesi di dottorato mira a studiare, attraverso simulazioni atomistiche, materiali che possono fungere da efficienti raccoglitori di energia. In un'epoca in cui la domanda di energia e la necessità di fonti di energia sostenibili sono sempre più fondamentali, le tecniche di raccolta di energia possono offrire un modo efficiente per utilizzare forme di energia già disponibili nell'ambiente che altrimenti andrebbero perse. In letteratura sono descritte molte soluzioni per recuperare energia da radiazione solare ed elettromagnetica, vibrazioni meccaniche o differenziali termici. Tra i materiali che possono essere utilizzati a questo scopo, l'ossido di afnio (HfO2) ha ricevuto notevole attenzione per la sua combinazione unica di costante dielettrica elevata, stabilità termica e compatibilità con la tecnologia metallo-ossido-semiconduttore complementare (CMOS). In questo contesto, vengono esplorati alcuni aspetti relativi a questo materiale, avvalendosi di simulazioni teoriche ab initio; in particolare, HfO2 esiste in natura come diversi polimorfi associati a peculiari stabilità e proprietà. Inoltre, drogando l'HfO2 con diversi elementi, come Zr e Y, e con varie quantità, è possibile comprendere come le proprietà del materiale siano influenzate in termini di celle unitarie, energia di formazione, proprietà ottiche e funzioni dielettriche. Quindi, è stata condotta un'esplorazione teorica di diodi metallo-isolante-metallo (MIM) basati su HfO2 per studiare gli effetti dei polimorfi sulle proprietà del dispositivo e per stimare le curve IV (corrente-tensione). Analogamente, sono stati esplorati altri diodi MIM basati su HfO2 con un approccio di modellazione atomistica seguita da convalida sperimentale. Un dispositivo MIM è stato fabbricato interponendo uno strato dielettrico di HfO2, con uno spessore compreso tra 3 e 4 nm, tra due metalli diversi, Platino e Titanio. La curva IV ottenuta dalle simulazioni ab initio è stata sovrapposta alla curva ottenuta dalla misura sperimentale del diodo fabbricato e i risultati hanno mostrato l'eccellente capacità della metodologia proposta di descrivere accuratamente il comportamento del dispositivo MIM da un punto di vista atomistico. Inoltre, è stato valutato il comportamento piroelettrico in condensatori ferroelettrici basati su HfZrO2. Questa tesi descrive in dettaglio le simulazioni di dinamica molecolare ab initio utilizzate per analizzare le deformazioni della densità elettronica in funzione della temperatura, che determinano una modifica della permittività dielettrica dell'HfZrO2. Questa tesi esplora il potenziale delle simulazioni ab initio per prevedere, investigare ed esaminare le proprietà di una nuova classe di materiali, nonché dispositivi governati da fenomeni chimico-fisici in cui sono coinvolti i materiali stessi.This PhD thesis aims to investigate, through atomistic simulations, materials that can act as efficient energy
harvesters. In an era of increased energy demands and the need for sustainable power sources, energy
harvesting techniques can offer an efficient way to collect energy already available in the environment that
would otherwise be lost. In this context, many solutions have been presented in the literature to recover energy
from solar and electromagnetic radiation, mechanical vibrations, or thermal differentials. Among materials
that can be used for this purpose, hafnium oxide (HfO2) has received significant attention for its unique
combination of high dielectric constant, thermal stability, and compatibility with complementary metal-oxide-
semiconductor (CMOS) technology. In this context, a few aspects related to this material are explored, making
use of ab initio theoretical simulations; in particular, HfO2 exists in nature as different polymorphs associated
with peculiar stability and properties. Moreover, by doping the HfO2 with different elements, such as Zr and
Y, and with various amounts, it is possible to understand how the properties of the materials are affected in
terms of unit cells, formation energy, optical properties, and dielectric functions. Then, a first-principle
exploration of HfO2-based metal-insulator-metal (MIM) diodes was carried out to investigate the effects of
polymorphs on the device’s properties and to estimate the IV (Current-Voltage) curves. Similarly, other HfO2-
based MIM diodes have been explored with an atomistic-to-continuum modeling approach followed by
experimental validation. A MIM device has been fabricated by interposing a dielectric layer of HfO2, with a
thickness between 3 and 4 nm, between two different metals, platinum (Pt) and titanium (Ti), which are the
source and drain electrodes, respectively. The IV curve obtained by ab initio simulations has been traced back
to experimental measurements conducted on the fabricated diode, and the results showed the excellent
capability of the proposed methodology to accurately describe the behavior of a MIM device from an atomistic
point of view. Additionally, the pyroelectricity behavior in ferroelectric capacitors based on HfZrO2 has been
evaluated to test the harvesting capability of this material. This thesis details the ab initio molecular dynamics
simulations used to analyze the deformations of the electron density as a function of temperature, which results
in a change in the dielectric permittivity of the HfZrO2.
This thesis explores the potential of ab initio simulations to predict, investigate, and examine the properties of
a novel class of materials, as well as devices governed by chemical-physical phenomena in which the same
materials are involved
Raman and photoemission spectroscopic analyses of explanted Biolox® delta femoral heads showing metal transfer
No full textBiolox® delta has been widely used in joint replacements thanks to its high strength and wear resistance. In this study, eleven Biolox® delta femoral head retrievals affected by metal transfer (MT) were analysed by Raman spectroscopy to estimate the tetragonal to monoclinic zirconia phase transformation, whose occurrence may compromise ceramic chemical stability and mechanical strength. The residual stress state was evaluated by both Raman and photoemission spectroscopy. Vmmonoclinic zirconia contents were higher near the centre of the articulating surface and in the MT area than in the border control area of the retrievals. In only one retrieval, stress related to MT appeared a more severe condition, able to induce zirconia phase transformation; for all the others, stresses related to loading in the central region and related to MT, were conducive to a zirconia phase transformation of nearly the same extent. Vmdepth profiling analyses showed that the transformation involved different thicknesses in different samples. Raman data allowed for the investigation of the mechanism of zirconia phase transformation and confirmed that the growth stage was absent and the nucleation stage was not occurring as freely as it would in unconstrained zirconia
Influence of grafting with acrylate compounds on the conformational rearrangements of silk fibroin upon electrospinning and treatment with aqueous methanol
No full textSilk fabrics from Bombyx mori silkworm were grafted with 2-hydroxyethyl methacrylate (HEMA) as well as a binary system of HEMA and 4-hydroxybutyl acrylate (HBA) and then analysed by Raman and infrared (IR) spectroscopy to elucidate the interactions between the components and their possible conformational changes. The samples were then dissolved in trifluoroacetic acid and electrospun; the influence of the grafted polymers on the silk fibroin rearrangements upon these treatments was investigated by vibrational spectroscopy. Upon grafting, the fabrics underwent conformational rearrangements towards a more unordered state, although they kept their prevailing β-sheet conformation; also the polymeric component underwent hydrogen bonding and backbone rearrangements upon interaction with silk fibroin and the occurrence of strong covalent bonds cannot be excluded. By immersing the as-electrospun grafted and pure fibroin nanofibres (prevalently unordered) in aqueous methanol, they partially recovered the β-sheet content observed in the corresponding starting fabrics; the percentage of recovery decreased along the series: pure silk > HEMA-grafted silk > HEMA and HBA-grafted silk. This trend suggests that the presence of the polyHEMA grafted component hinders the silk fibroin recrystallization into β-sheet upon aqueous methanol treatment; moreover, the addition of the more sterically hindered HBA monomer in the grafting system further prevented this process. Copyright © 2016 John Wiley & Sons, Ltd
Comparative micro-Raman study on standard, cross-linked and vitamin E-blended polyethylene acetabular cups after long-term in vitro testing and ageing
No full textUltra-high molecular-weight polyethylene has been used in total joint replacement for the last three decades. Many attempts have been made to improve the wear behaviour of this material. Highly cross-linked polyethylene (XLPE) has been developed to decrease the wear rate, while the addition of stabilizers such as vitamin E has been proposed to slow the oxidation processes. In this study, we compared the wear behaviour of conventional ultra-high molecular-weight polyethylene, XLPE and vitamin E-added XLPE acetabular cups for total hip arthroplasty. The samples were run for five million cycles on a hip joint simulator (first test), then underwent an accelerated ageing treatment, were tested again for two million cycles (second test) and finally were run for another two million cycles in presence of third-body particles (third test). Our study showed that vitamin E-added XLPE cups had a better wear behaviour only in the long-term and more severe testing conditions: in the third test, they underwent the lowest increase in wear rate. Micro-Raman spectroscopy allowed us to analyse the wear degradation of the cups on a molecular scale, and differences in the wear mechanisms were revealed. The presence of vitamin E did not prevent structural changes upon the first test, while it did in ageing and in the second test. In the third test, more significant structural changes were observed; the XLPE cups, which lost the lowest mass, had the most modified articular surface. These morphological aspects should be taken into account, together with gravimetric wear rates, in the characterization of wear degradation. Copyright © 2017 John Wiley & Sons, Ltd
Transfer of metallic debris after in vitro ceramic-on-metal simulation: wear and degradation in Biolox® Delta composite femoral heads
No full textThis study was aimed at investigating the effects of the transfer of metallic debris during an in vitro wear test on ceramic femoral heads articulating against metallic acetabular cups. In particular, Biolox® Delta ceramic femoral heads of two different diameters (32 and 36 mm, three samples of each set) were run for five million cycles onto a hip wear simulator using bovine calf serum as lubricant. Wear and degradation of Biolox® Delta Composite femoral heads were evaluated by a gravimetric method and by micro-Raman spectroscopy, which was used to investigate possible phase changes upon the particles deposit, by assessing the monoclinic zirconia content and quantifying the tetragonal → monoclinic zirconia transformation.
Our results showed that after five million cycles, the 32 mm-acetabular cups lost a higher mass than the 36 mm-ones. Metal transfer was observed on all the Biolox® Delta ceramic femoral heads and determined a worsening of all roughness parameters. The micro-Raman analyses of the in vitro tested femoral heads confirmed the results previously obtained on retrievals, i.e. the stress related to metal transfer appeared a particularly severe condition able to induce the tetragonal→monoclinic zirconia phase transformation. The extent of the transformation appeared higher than that observed in vivo for Biolox® Delta-on-Biolox® Delta couplings. On average, the 36 mm-femoral heads seemed more detrimentally affected by metal transfer than the 32 mm-ones; one femoral head belonging to the first set of samples underwent the highest extent of phase transformation and the highest depth involved, which in the worn centre area appeared meanly higher for the 36 mm-components than for the 32 mm-ones
Stability toward alkaline hydrolysis of B. mori silk fibroin grafted with methacrylamide
No full textBombyx mori silk fibroin fibers were grafted with methacrylamide (MAA) and characterized by Raman and infrared (IR) vibrational spectroscopy before and after hydrolysis in NaOH 5% to elucidate the possible interactions between the two components and the stability of the fibers toward alkaline hydrolysis. Upon grafting, the fibers underwent conformational rearrangements toward a more unordered state and lost orientation at weight gains higher than 60%. Vibrational spectroscopy disclosed the occurrence of intermolecular interactions (mainly hydrogen bonds) between B. mori silk fibroin and polyMAA in the grafted fibers, and the formation of covalent bonds has been explored. These strong interactions made the grafted fibers as a whole more stable toward alkaline hydrolysis because they prevented the solubilization of the polymer upon hydrolysis and made slower the transformation of its CONH2 groups into COOH and COO− groups. Upon hydrolysis, silk fibroin underwent an enrichment in the β-sheet crystalline domains, because of the preferential removal of the unordered domains, which were more prone to the OH− attack. IR and Raman spectroscopy proved valid techniques to investigate the degradation mechanism and kinetics of grafted silk fibroin fibers and so for designing high-performing silk-based materials. The A731/A1004 Raman intensity ratio was proposed to spectroscopically evaluate the composition of the grafted samples; its value was found to linearly increase with weight gain (R2 = 0.998), envisaging the possibility of using Raman spectroscopy as a routine analytical technique for qualitative and quantitative characterization of grafted industrial samples. Copyright © 2016 John Wiley & Sons, Ltd
Structural study on methacrylamide-grafted Tussah silk fibroin fibres
No full textTussah silk fibroin fibres were modified by grafting with methacrylamide (MAA), with weight gains ranging between 2.6% and 71.4%. Raman and IR spectroscopic analyses showed that upon grafting the fibres underwent slight conformational changes towards a more unordered state, due to the covalent and hydrogen bonds interactions occurring between the polymer (polyMAA) and the amorphous domains of silk fibres. To test the stability towards alkaline hydrolysis, the untreated and MAA-grafted silk fibres (weight gain of 71.4%) were immersed in NaOH 5% at 50 °C for different times; the IR and Raman spectroscopic techniques were utilized to elucidate the degradation mechanism as well as the rearrangements of the fibres induced by the treatment. Upon hydrolysis, both the untreated and grafted fibres underwent an enrichment in β-sheet conformation, due to the preferential removal of the unordered domains. As a result of the covalent interactions with silk fibroin, the polymer increased its stability towards alkaline hydrolysis, since its complete solubilization was avoided and the transformation of its CONH2 groups into COO- and COOH was delayed. Vibrational spectroscopy proved to be a valid technique to investigate the mechanism and the effects of the hydrolytic attack, which are both fundamental to design new-generation silk-based materials
Inverted Scanning Microwave Microscopy
No full textScanning Microwave Microscopy (SMM) is prominent for providing imaging of sub-surface structures and allowing local quantitative characterization of the sample. A novel technique known as Inverted Scanning Microwave Microscopy (iSMM) is the improvement developed recently to
broaden the application beyond the current focus on surface physics and semiconductor technology. With a simple metal probe, the iSMM can be converted from existing atomic force microscopes (AFM) or scanning tunneling microscopes (STM) outperforming the conventional SMM in terms of bandwidth, sensitivity, and dynamic range. The iSMM was primarily used to analyze biological samples as it can operate in liqui
Insights into first-principles characterization of the monoclinic VO2(B) polymorph via DFT + U calculation: electronic, magnetic and optical properties
No full textWe have studied the structural, electronic, magnetic, and optical properties of the VO2(B) polymorph using first-principles calculations based on density functional theory (DFT). This polymorph was found to display four optimized structures namely VO2(B)PP, VO2(B)LP, VO2(B)PPD, and VO2(B)LPD using the generalized gradient approximation (GGA) PBE exchange-correlation functional by including/excluding van der Waals interaction. Our derivation provides a theoretical justification for adding an on-site Coulomb U value in the conventional DFT calculations to allow a direct comparison of the two methods. We predicted a zero bandgap of the VO2(B) structure based on GGA/PBE. However, by GGA/PBE + U, we found accurate bandgap values of 0.76, 0.66, and 0.70 eV for VO2(B)PP, VO2(B)LP, and VO2(B)PPD, respectively. The results obtained from DFT + U were accompanied by a structural transition from the metallic to semiconductor property. Here, we verified the non-magnetic characteristic of the monoclinic VO2(B) phase with some available experimental and theoretical data. However, the debate on the magnetic property of this polymorph remains unresolved. Imaginary and real parts of the dielectric function, as computed with the GGA/PBE functional and the GGA/PBE + U functional, were also reported. The first absorption peaks of all considered geometries in the imaginary part of the dielectric constants indicated that the VO2(B) structure could perfectly absorb infrared light. The computed static dielectric constants with positive values, as derived from the optical properties, confirmed the conductivity of this material. Among the four proposed geometries of VO2(B) in this study, the outcomes obtained by VO2(B)PPD reveal good results owing to the excellent consistency of its bandgap, magnetic and optical properties with other experimental and theoretical observations. The theoretical framework in our study will provide useful insight for future practical applications of the VO2(B) polymorph in electronics and optoelectronics
PBEsol/HSE functional: a promising candidate for vanadium dioxide (B) characterization
No full textA VO(2)(B) polymorph has been thoroughly investigated by means of density functional theory (DFT) calculations to evaluate the structure, Raman spectrum, cohesive energy, phonon band structure, an delectronic and optical properties. Among the computed Raman modes, eight of them have been assigned to the VO(2)(B) structure in full agreement with the corresponding experimental spectra. The minimized structure of the VO(2)(B) polymorph indicated the presence of negative frequencies in its phonon dispersion curves, confirming the dynamic instability of this material. Herein, the combination of generalized gradient approximation (GGA)/PBEsol with a hybrid HSE functional has been employed to perform ab initio calculations on VO(2)(B), since the conventional semi-local DFT calculations are believed to underestimate the band gap of materials. By considering the electronic structure calculations, for the first time, we found that the calibration of the PBEsol functional can efficiently model the metallic-like properties of VO(2)(B) with a band gap of 0.26 eV, while the corresponding electronic bandgap of VO(2)(B) based on the HSE functional possesses a larger band gap of 0.67 eV. The prediction of optical characteristics of VO(2)(B) indicated that the optical conductivity of VO(2)(B) lies in the infrared region of light. This work strongly suggests the combination of GGA/PBEsol with HSE hybrid functionals to carefully describe the physical properties of smart materials exploitable in electronics and optoelectronics applications. The nanostructure of VO(2)(B) looks promising for IR photodetectors and smart windows applications as a semiconductor material with excellent optical features. It is predicted that in the future VO(2)(B) will continue to expand the envelope of its capabilities because of its remarkable properties
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