Browsing by Subject "Thermodynamic stability"

Now showing 1 - 13 of 13

Open Access
Band alignment issues in metal/dielectric stacks: a combined photoemission and inverse photoemission study of the HfO 2/Pt and HfO 2/Hf systems
(Electrochemical Society, 2004) Sayan, S.; Bartynski, R.A.; Robertson J.; Suehle, J. S.; Vogel, E.; Nguyen, N. V.; Ehrstein, J.; Kopanski, J. J.; Süzer, Şefik; Holl, M. B.; Garfunkel, E.
We have studied the HfO 2/Hf and HfO 2/Pt systems by photoemission and inverse photoemission spectroscopies. It is found that the "effective workfunction" of metals in multilayer structures are different than their vacuum workfunctions and are modified by their interface dipoles at the metal/high-k interface. The effective workfunction of Hf is 4.4 eV whereas that of Pt is 5.3 eV, within the range of acceptable values for PMOS and NMOS respectively.
Open Access
Conducting polymer composites: polypyrrole and poly (vinyl chloride-vinyl acetate) copolymer
(John Wiley & Sons, Inc., 1997) Balci, N.; Bayramli, E.; Toppare, L.
Composites of a polypyrrole (PPy) and poly (vinyl chloride-vinyl acetate) copolymer (PVC-PVA) were prepared both chemically and electrochemically. An insulating polymer was retained in the blend and the thermal stability of the polymer was enhanced by polymerizing pyrrole into the host matrix in both cases. The composites prepared electrochemically gave the best results in terms of conductivity and air stability. © 1997 John Wiley * Sons, Inc.
Open Access
Effects of temperature, pH and counterions on the stability of peptide amphiphile nanofiber structures
(Royal Society of Chemistry, 2016) Ozkan A.D.; Tekinay, A. B.; Güler, Mustafa O.; Tekin, E. D.
Peptide amphiphiles are a class of self-assembling molecules that are widely used to form bioactive nanostructures for various applications in bionanomedicine. However, peptide molecules can exhibit distinct behaviors under different conditions, suggesting that environmental variables such as temperature, pH, electrolytes and the presence of biological factors may greatly affect the self-assembly process. In this work, we used united-atom molecular dynamics simulations to understand the effects of three counterions (Na+, Ca2+ at pH 7 and Cl- at pH 2) and temperature change on the stability of the lauryl-VVAGERGD peptide amphiphile self-assembly. This molecule contains a bioactive RGD peptide sequence and has been shown to support cellular adhesion and proliferation in vitro. A 19-layered peptide nanostructure, containing 12 peptide amphiphile molecules per layer, was previously shown to exhibit optimal stability and it was used as the model nanofiber system. Peptide backbone stability was studied under increasing temperatures (300-358 K) using the number of hydrogen bonds and root-mean-square deviations of nanofiber size. At higher temperatures, fiber disintegration was observed to be dependent on the type of counter-ion used for nanofiber formation. Interestingly, rapid heating to higher temperatures could sometimes reestablish the integrity of the nanofiber backbone, possibly by allowing the system to bypass an energy barrier and assuming a more thermodynamically stable configuration. As counterion identity was observed to exhibit remarkable effects on the thermal stability of peptide nanofibers, we suggest that these behaviors should be considered while developing new materials for potential applications.
Open Access
Electrospinning of cyclodextrin/linalool-inclusion complex nanofibers: fast-dissolving nanofibrous web with prolonged release and antibacterial activity
(Elsevier, 2017-09) Aytac Z.; Yildiz, Z. I.; Kayaci-Senirmak, F.; Tekinay, T.; Uyar, Tamer
The volatility and limited water solubility of linalool is a critical issue to be solved. Here, we demonstrated the electrospinning of polymer-free nanofibrous webs of cyclodextrin/linalool-inclusion complex (CD/linalool-IC-NFs). Three types of modified cyclodextrin (HPβCD, MβCD, and HPγCD) were used to electrospin CD/linalool-IC-NFs. Free-standing CD/linalool-IC-NFs facilitate maximum loading of linalool up to 12% (w/w). A significant amount of linalool (45–89%) was preserved in CD/linalool-IC-NFs, due to enhancement in the thermal stability of linalool by cyclodextrin inclusion complexation. Remarkably, CD/linalool-IC-NFs have shown fast-dissolving characteristics in which these nanofibrous webs dissolved in water within two seconds. Furthermore, linalool release from CD/linalool-IC-NFs inhibited growth of model Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria to a great extent. Briefly, characteristics of liquid linalool have been preserved in a solid nanofiber form and designed CD/linalool-IC-NFs confer high loading capacity, enhanced shelf life and strong antibacterial activity of linalool.
Open Access
Electrospun nylon 6,6 nanofibers functionalized with cyclodextrins for removal of toluene vapor
(John Wiley and Sons Inc., 2015) Kayaci, F.; Sen, H. S.; Durgun, Engin; Uyar, Tamer
Functional nylon 6,6 nanofibers incorporating cyclodextrins (CD) were developed via electrospinning. Enhanced thermal stability of the nylon 6,6/CD nanofibers was observed due to interaction between CD and nylon 6,6. X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy studies indicated the existence of some CD molecules on the surface of the nanofibers. Electrospun nylon 6,6 nanofibers without having CD were ineffective for entrapment of toluene vapor from the environment, whereas nylon 6,6/CD nanofibrous membranes can effectively entrap toluene vapor from the surrounding by taking advantage of the high surface-volume ratio of nanofibers with the added advantage of inclusion complexation capability of CD presenting on the nanofiber surface. The modeling studies for formation of inclusion complex between CD and toluene were also performed by using ab initio techniques. Our results suggest that nylon 6,6/CD nanofibrous membranes may have potential to be used as air filters for the removal of organic vapor waste from surroundings.
Open Access
FTIR spectroscopic characterization of NOx species adsorbed on ZrO2 and ZrO2-SO42-
(American Chemical Society, 2002) Kantcheva, M.; Ciftlikli, E. Z.
The nature of the NOx species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on pure and sulfated zirconia has been investigated by means of in situ FTIR spectroscopy. The adsorption of NO on both samples occurs through disproportionation leading to the formation of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO-. A mechanism for the formation of these adsorption forms is proposed. The NO- species are stable on the surface of zirconia, whereas on the sulfated sample, they are readily oxidized by the SO42- groups. The process of NO disproportionation is favored by wet surfaces and occurs with participation of the tribridged (ZrO2) and terminal (ZrO2-SO42-) hydroxyl groups. Coadsorption of NO and O2 on pure zirconia leads to the formation of various kinds of nitrate species. The presence of sulfate ions reduces the amount of surface nitrates and decreases their thermal stability. An analysis of the combination bands of the nitrate species shows that this spectral region can be used for structural identification of bidentate and bridged nitrates.
Open Access
Functional electrospun polymeric nanofibers incorporating geraniol-cyclodextrin inclusion complexes: high thermal stability and enhanced durability of geraniol
(Elsevier Ltd, 2014) Kayaci, F.; Sen, H. S.; Durgun, Engin; Uyar, Tamer
In this study, solid geraniol/cyclodextrin inclusion complexes (geraniol/CD-IC) were successfully prepared by using three types of native CD (α-CD, β-CD and γ-CD). The modeling studies for inclusion complexation between CD and geraniol were performed by using ab initio techniques. Both experimentally and theoretically, the complexation efficiency between geraniol and γ-CD was higher; therefore, geraniol/γ-CD-IC was chosen and then incorporated into polyvinyl alcohol (PVA) nanofibers (NF) via electrospinning. The scanning electron microscopy imaging elucidated that the aggregates of geraniol/γ-CD-IC crystals were distributed in the PVA NF, whereas bead-free and uniform PVA and PVA/geraniol NF without CD-IC were obtained. Higher thermal stability of geraniol was observed in the electrospun PVA/geraniol/γ-CD-IC NF. However, geraniol molecules having volatile nature could not be preserved without CD-IC during electrospinning or during storage; therefore, the complete evaporation of geraniol in PVA/geraniol NF was unavoidable even after one day of its production. On the contrary, the loss of geraniol was minimal (~. 10%) for PVA/geraniol/γ-CD-IC NF even after storage of these NF for two years owing to inclusion complexation. Our study demonstrated that electrospun NF incorporating CD-IC may be quite applicable in food industry, e.g.: active food packaging or functional foods, due to very high surface area and nanoporous structure of NF; high thermal stability and enhanced durability of active agents and functional food ingredients.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).
Open Access
Non-linear thermoelectricity and cooling effects in metallic constrictions
(Institute of Physics Publishing Ltd., 1994) Kulik, I. O.
Linear as well as non-linear contributions to the Zeebeck and Peltier coefficients of a metallic film in contact with the equilibrium metal are calculated within a simple model. The non-linear part of the thermoelectric response survives down to a very low temperature which in principle permits thermoelectric cooling at these conditions. Thermal equilibrium in a metallic constriction between dissimilar metals is evaluated in the non-linear current-carrying regime.
Open Access
Polymer-free nanofibers from vanillin/cyclodextrin inclusion complexes: high thermal stability, enhanced solubility and antioxidant property
(Royal Society of Chemistry, 2016) Celebioglu A.; Kayaci-Senirmak, F.; Ipek, S.; Durgun, Engin; Uyar, Tamer
Vanillin/cyclodextrin inclusion complex nanofibers (vanillin/CD-IC NFs) were successfully obtained from three modified CD types (HPβCD, HPγCD and MβCD) in three different solvent systems (water, DMF and DMAc) via an electrospinning technique without using a carrier polymeric matrix. Vanillin/CD-IC NFs with uniform and bead-free fiber morphology were successfully produced and their free-standing nanofibrous webs were obtained. The polymer-free CD/vanillin-IC-NFs allow us to accomplish a much higher vanillin loading (∼12%, w/w) when compared to electrospun polymeric nanofibers containing CD/vanillin-IC (∼5%, w/w). Vanillin has a volatile nature yet, after electrospinning, a significant amount of vanillin was preserved due to complex formation depending on the CD types. Maximum preservation of vanillin was observed for vanillin/MβCD-IC NFs which is up to ∼85% w/w, besides, a considerable amount of vanillin (∼75% w/w) was also preserved for vanillin/HPβCD-IC NFs and vanillin/HPγCD-IC NFs. Phase solubility studies suggested a 1:1 molar complexation tendency between guest vanillin and host CD molecules. Molecular modelling studies and experimental findings revealed that vanillin:CD complexation was strongest for MβCD when compared to HPβCD and HPγCD in vanillin/CD-IC NFs. For vanillin/CD-IC NFs, water solubility and the antioxidant property of vanillin was improved significantly owing to inclusion complexation. In brief, polymer-free vanillin/CD-IC NFs are capable of incorporating a much higher loading of vanillin and effectively preserve volatile vanillin. Hence, encapsulation of volatile active agents such as flavor, fragrance and essential oils in electrospun polymer-free CD-IC NFs may have potential for food related applications by integrating the particularly large surface area of NFs with the non-toxic nature of CD and inclusion complexation benefits, such as high temperature stability, improved water solubility and an enhanced antioxidant property, etc.
Open Access
Solid or liquid? Solidification of a nanoconfined liquid under nonequilibrium conditions
(American Chemical Society, 2006) Patil, S.; Matei, G.; Oral, A.; Hoffmann, P. M.
There has been a long-standing debate about the physical state and possible phase transformations of confined liquids. In this report, we show that a model-confined liquid can behave both as a Newtonian liquid with very little change in its dynamics and as a pseudosolid, depending solely on the rate of approach of the confining surfaces. Thus, the confined liquid does not exhibit any confinement-induced solidification in thermodynamic equilibrium. Instead, solidification is induced kinetically when the two confining surfaces are approached with a minimum critical rate. This critical rate is surprisingly slow (on the order of 6 Å/s), explaining the frequent observation of confinement-induced solidification.
Open Access
Thermal degradation of polythiophene-natural rubber and polythiophene-synthetic rubber conducting polymer composites
(1997) Hacaloǧlu J.; Yiǧit, S.; Akbulut, U.; Toppare L.
Thermal degradation of conducting polymer composites of polythiophene and rubbers was studied by direct and indirect pyrolysis mass spectrometry techniques. The samples were prepared by electrooxidation of polythiophene using natural rubber or synthetic rubber as the insulating matrix. Presence of decomposition products which were not observed during pyrolysis of pure polythiophene and rubbers, and disappearance of rubber-based pyrolysis mass peaks, together with changes in thermal stability and behaviour, may directly be related to a chemical interaction between the components of the composites. The pyrolysis data were used to propose possible polymerization and degradation mechanisms. © 1997 Elsevier Science Ltd.
Open Access
Trends in molecular design strategies for ambient stable n-channel organic field effect transistors
(Royal Society of Chemistry, 2017) Dhar, J.; Salzner, U.; Patil, S.
In recent years, organic semiconducting materials have enabled technological innovation in the field of flexible electronics. Substantial optimization and development of new π-conjugated materials has resulted in the demonstration of several practical devices, particularly in displays and photoreceptors. However, applications of organic semiconductors in bipolar junction devices, e.g. rectifiers and inverters, are limited due to an imbalance in charge transport. The performance of p-channel organic semiconducting materials exceeds that of electron transport. In addition, electron transport in π-conjugated materials exhibits poorer atmospheric stability and dispersive transient photocurrents due to extrinsic carrier trapping. Thus development of air stable n-channel conjugated materials is required. New classes of materials with delocalized n-doped states are under development, aiming at improvement of the electron transport properties of organic semiconductors. In this review, we highlight the basic tenets related to the stability of n-channel organic semiconductors, primarily focusing on the thermodynamic stability of anions and summarizing the recent progress in the development of air stable electron transporting organic semiconductors. Molecular design strategies are analysed with theoretical investigations.