Three different Terahertz quantum-cascade-laser designs supporting clean n-level systems were ana... more Three different Terahertz quantum-cascade-laser designs supporting clean n-level systems were analyzed using nonequilibrium Green’s functions. In clean n-level systems, most of the electrons occupy the active laser levels, with thermally activated leakage channels being suppressed almost entirely up to room temperature. Simulations of the three designs, namely a resonant phonon design, a two-well design, and a split-well direct-phonon design were investigated. The results from the simulations indicated that the two-well design would perform best overall, in terms of variations in current density, interface roughness, and ionized impurity scattering. We conclude that future research aiming to improve the temperature performance of such laser designs should be based on a two-well design.
Various conditions cause dispersions of particulate matter to circulate inside the anterior chamb... more Various conditions cause dispersions of particulate matter to circulate inside the anterior chamber of a human eye. These dispersed particles might reduce visual acuity or promote elevation of intraocular pressure (IOP), causing secondary complications such as particle related glaucoma, which is a major cause of blindness. Medical and surgical treatment options are available to manage these complications, yet preventive measures are not currently available. Conceptually, manipulating these dispersed particles in a way that reduces their negative impact could prevent these complications. However, as the eye is a closed system, manipulating dispersed particles in it is challenging. Standing acoustic waves have been previously shown to be a versatile tool for manipulation of bioparticles from nano-sized extracellular vesicles up to millimeter-sized organisms. Here we introduce for the first time a novel method utilizing standing acoustic waves to noninvasively manipulate intraocular pa...
Micro-patterning of a metal organic framework (MOF) from a solution of precursors is achieved by ... more Micro-patterning of a metal organic framework (MOF) from a solution of precursors is achieved by local laser heating, alleviating the need for pre-preparation and stabilization.
It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacryla... more It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacrylate (PMMA) or Nylon as well as non-rubbed PMMA and Nylon induce "redox" reactions, including those of the reduction of Pd(+2) and Cu(+2) ions. On this basis, it was deduced that these dielectric materials may hold ≅10(13)-10(14) of "hidden" electrons cm(-2), a value at least three orders of magnitude higher than the charge that a dielectric surface can accumulate without being discharged in air. The "hidden" electrons were termed "cryptoelectrons". In variance to these reports, we offer here an alternative interpretation. Our model is supported by X-ray photoelectron spectroscopy, contact angle and vibrating electrode (modified Kelvin probe) measurements performed on representative examples. Rubbing of the polymers was found to transfer polymer fragments between the rubbed surfaces altering their physical properties. The transferred polymer fragments promote adsorption of Cu(2+) and Pd(2+) ions. It was found that Teflon and PE rubbed with PMMA and Nylon, and non-rubbed PMMA and non-rubbed Nylon do not induce "redox" reactions of Cu(2+) and Pd(2+) ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd(2+) to Pd(0) by electrons, as detected by catalytic activity of Pd(0) in a Cu-plating bath, can be alternatively explained by reduction of adsorbed Pd(2+) by the reducing agents of the bath itself. Based on these findings, we support the hypothesis that charging of dielectric polymers is due to ions or free radicals rather than electrons and there is no evidence to invoke a hypothesis of "cryptoelectrons".
... A small (2 mm) vibrating electrode (Kelvin probe) was placed at a distance (5 cm) that was gr... more ... A small (2 mm) vibrating electrode (Kelvin probe) was placed at a distance (5 cm) that was greater than the dimensions of a charged plate of Teflon (0.2×0.2×0.1 cm) placed on a large (8×15 cm) metallic ... MM Apodaca, PJ Wesson, KJM Bishop, MA Ratner, BA Grzybowski, Angew ...
Recent reports in the literature claimed that Teflon and Polyethylene charged by rubbing with eit... more Recent reports in the literature claimed that Teflon and Polyethylene charged by rubbing with either Polymethylmethacrylate(PMMA) or Nylon and, similarly, spontaneously charged pure PMMA and pure Nylon induce a variety of "redox" reactions, including those of Pd +2 and Cu +2 ions. On the basis of these findings it was concluded that these polymers are charged spontaneously by electrons, defined as "crypto-electrons" .The concentration of the latter was estimated as high as >10 13 -10 14 electrons/cm 2 . X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), contact angle and vibrating electrometer (modified Kelvin probe) measurements show, however, that the surface charge created on the above dielectric polymers does not reduce Cu 2+ and Pd 2+ ions. Instead, rubbing of polymers causes material exchange, accompanied by transfer of polar active species (containing >C=O groups), which promotes adsorption of Cu 2+ and Pd 2+ ions. Surface coverage o...
We have studied the effect of doping on the temperature performance of a split-well (SW) direct-p... more We have studied the effect of doping on the temperature performance of a split-well (SW) direct-phonon (DP) terahertz (THz) quantum-cascade laser (QCL) scheme supporting a clean three-level system. Achieving a system that is as close as possible to a clean n-level system proved to be the strategy that led to the best temperature performance in THz-QCLs. We expected to obtain a similar improvement to that observed in resonant-phonon (RP) schemes after increasing the carrier concentration from 3 × 1010 cm−2 to 6 × 1010 cm−2. Our goal was to improve the temperature performance by increasing the doping, ideally the results should have improved. To our surprise, in the devices we checked, the results show the contrary. Although an increase in doping had previously shown a positive effect in RP schemes, our results indicated that this does not happen with SW–DP devices. However, we observed a significant increase in gain broadening and a reduction in the dephasing time as the doping and t...
We studied the temperature performance of split-well direct phonon terahertz quantum cascade lase... more We studied the temperature performance of split-well direct phonon terahertz quantum cascade lasers and found that it is limited by a lasing instability that becomes significant as the temperature increases. When the hot electrons of the upper laser level cannot scatter effectively to excited states due to the high radiative barriers of the structures, a lasing instability occurs, which limits the temperature performance.
Acoustic manipulation is an emerging non-invasive method enabling precise spatial control of cell... more Acoustic manipulation is an emerging non-invasive method enabling precise spatial control of cells in their native environment. Applying this method for organizing neurons is invaluable for neural tissue engineering applications. Here, we used surface and bulk standing acoustic waves for large-scale patterning of Dorsal Root Ganglia neurons and PC12 cells forming neuronal cluster networks, organized biomimetically. We showed that by changing parameters such as voltage intensity or cell concentration we were able to affect cluster properties. We examined the effects of acoustic arrangement on cells atop 3D hydrogels for up to 6 days and showed that assembled cells spontaneously grew branches in a directed manner towards adjacent clusters, infiltrating the matrix. These findings have great relevance for tissue engineering applications as well as for mimicking architectures and properties of native tissues.
We describe the fabrication of polymer nanofibers with entrapped molecularly imprinted polymer (M... more We describe the fabrication of polymer nanofibers with entrapped molecularly imprinted polymer (MIP) nanoparticles and study their possible use in a fluorescence-based biosensor application. The MIP was imprinted with the fluorescent amino acid derivative dansyl-L-phenylalanine. Poly(vinyl alcohol) was used as a support for MIP nanoparticles because it is water-soluble and can be spun into very thin fibers. The fibers were characterized by atomic force microscopy and optical microscopy, and fluorescence microscopy was used for the characterization of target binding to the MIP. The fibers show close to 100% recovery upon extraction and rebinding of the target molecule. The selectivity of the system has been demonstrated through competitive binding experiments with nonfluorescent analogues boc-L-phenylalanine and boc-D-phenylalanine.
We demonstrate for the first time herein that electrospinning of soluble crosslinked polymer part... more We demonstrate for the first time herein that electrospinning of soluble crosslinked polymer particles (microgels) is feasible and that it can be used to obtain micron-sized fibers from these macromolecules. Most notably, the electrospinning approach is found to be successful also in the case of microgels bearing metal nanoclusters, and allows to prepare composite fibers containing a homogeneous dispersion of
Three different Terahertz quantum-cascade-laser designs supporting clean n-level systems were ana... more Three different Terahertz quantum-cascade-laser designs supporting clean n-level systems were analyzed using nonequilibrium Green’s functions. In clean n-level systems, most of the electrons occupy the active laser levels, with thermally activated leakage channels being suppressed almost entirely up to room temperature. Simulations of the three designs, namely a resonant phonon design, a two-well design, and a split-well direct-phonon design were investigated. The results from the simulations indicated that the two-well design would perform best overall, in terms of variations in current density, interface roughness, and ionized impurity scattering. We conclude that future research aiming to improve the temperature performance of such laser designs should be based on a two-well design.
Various conditions cause dispersions of particulate matter to circulate inside the anterior chamb... more Various conditions cause dispersions of particulate matter to circulate inside the anterior chamber of a human eye. These dispersed particles might reduce visual acuity or promote elevation of intraocular pressure (IOP), causing secondary complications such as particle related glaucoma, which is a major cause of blindness. Medical and surgical treatment options are available to manage these complications, yet preventive measures are not currently available. Conceptually, manipulating these dispersed particles in a way that reduces their negative impact could prevent these complications. However, as the eye is a closed system, manipulating dispersed particles in it is challenging. Standing acoustic waves have been previously shown to be a versatile tool for manipulation of bioparticles from nano-sized extracellular vesicles up to millimeter-sized organisms. Here we introduce for the first time a novel method utilizing standing acoustic waves to noninvasively manipulate intraocular pa...
Micro-patterning of a metal organic framework (MOF) from a solution of precursors is achieved by ... more Micro-patterning of a metal organic framework (MOF) from a solution of precursors is achieved by local laser heating, alleviating the need for pre-preparation and stabilization.
It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacryla... more It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacrylate (PMMA) or Nylon as well as non-rubbed PMMA and Nylon induce "redox" reactions, including those of the reduction of Pd(+2) and Cu(+2) ions. On this basis, it was deduced that these dielectric materials may hold ≅10(13)-10(14) of "hidden" electrons cm(-2), a value at least three orders of magnitude higher than the charge that a dielectric surface can accumulate without being discharged in air. The "hidden" electrons were termed "cryptoelectrons". In variance to these reports, we offer here an alternative interpretation. Our model is supported by X-ray photoelectron spectroscopy, contact angle and vibrating electrode (modified Kelvin probe) measurements performed on representative examples. Rubbing of the polymers was found to transfer polymer fragments between the rubbed surfaces altering their physical properties. The transferred polymer fragments promote adsorption of Cu(2+) and Pd(2+) ions. It was found that Teflon and PE rubbed with PMMA and Nylon, and non-rubbed PMMA and non-rubbed Nylon do not induce "redox" reactions of Cu(2+) and Pd(2+) ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd(2+) to Pd(0) by electrons, as detected by catalytic activity of Pd(0) in a Cu-plating bath, can be alternatively explained by reduction of adsorbed Pd(2+) by the reducing agents of the bath itself. Based on these findings, we support the hypothesis that charging of dielectric polymers is due to ions or free radicals rather than electrons and there is no evidence to invoke a hypothesis of "cryptoelectrons".
... A small (2 mm) vibrating electrode (Kelvin probe) was placed at a distance (5 cm) that was gr... more ... A small (2 mm) vibrating electrode (Kelvin probe) was placed at a distance (5 cm) that was greater than the dimensions of a charged plate of Teflon (0.2×0.2×0.1 cm) placed on a large (8×15 cm) metallic ... MM Apodaca, PJ Wesson, KJM Bishop, MA Ratner, BA Grzybowski, Angew ...
Recent reports in the literature claimed that Teflon and Polyethylene charged by rubbing with eit... more Recent reports in the literature claimed that Teflon and Polyethylene charged by rubbing with either Polymethylmethacrylate(PMMA) or Nylon and, similarly, spontaneously charged pure PMMA and pure Nylon induce a variety of "redox" reactions, including those of Pd +2 and Cu +2 ions. On the basis of these findings it was concluded that these polymers are charged spontaneously by electrons, defined as "crypto-electrons" .The concentration of the latter was estimated as high as >10 13 -10 14 electrons/cm 2 . X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), contact angle and vibrating electrometer (modified Kelvin probe) measurements show, however, that the surface charge created on the above dielectric polymers does not reduce Cu 2+ and Pd 2+ ions. Instead, rubbing of polymers causes material exchange, accompanied by transfer of polar active species (containing >C=O groups), which promotes adsorption of Cu 2+ and Pd 2+ ions. Surface coverage o...
We have studied the effect of doping on the temperature performance of a split-well (SW) direct-p... more We have studied the effect of doping on the temperature performance of a split-well (SW) direct-phonon (DP) terahertz (THz) quantum-cascade laser (QCL) scheme supporting a clean three-level system. Achieving a system that is as close as possible to a clean n-level system proved to be the strategy that led to the best temperature performance in THz-QCLs. We expected to obtain a similar improvement to that observed in resonant-phonon (RP) schemes after increasing the carrier concentration from 3 × 1010 cm−2 to 6 × 1010 cm−2. Our goal was to improve the temperature performance by increasing the doping, ideally the results should have improved. To our surprise, in the devices we checked, the results show the contrary. Although an increase in doping had previously shown a positive effect in RP schemes, our results indicated that this does not happen with SW–DP devices. However, we observed a significant increase in gain broadening and a reduction in the dephasing time as the doping and t...
We studied the temperature performance of split-well direct phonon terahertz quantum cascade lase... more We studied the temperature performance of split-well direct phonon terahertz quantum cascade lasers and found that it is limited by a lasing instability that becomes significant as the temperature increases. When the hot electrons of the upper laser level cannot scatter effectively to excited states due to the high radiative barriers of the structures, a lasing instability occurs, which limits the temperature performance.
Acoustic manipulation is an emerging non-invasive method enabling precise spatial control of cell... more Acoustic manipulation is an emerging non-invasive method enabling precise spatial control of cells in their native environment. Applying this method for organizing neurons is invaluable for neural tissue engineering applications. Here, we used surface and bulk standing acoustic waves for large-scale patterning of Dorsal Root Ganglia neurons and PC12 cells forming neuronal cluster networks, organized biomimetically. We showed that by changing parameters such as voltage intensity or cell concentration we were able to affect cluster properties. We examined the effects of acoustic arrangement on cells atop 3D hydrogels for up to 6 days and showed that assembled cells spontaneously grew branches in a directed manner towards adjacent clusters, infiltrating the matrix. These findings have great relevance for tissue engineering applications as well as for mimicking architectures and properties of native tissues.
We describe the fabrication of polymer nanofibers with entrapped molecularly imprinted polymer (M... more We describe the fabrication of polymer nanofibers with entrapped molecularly imprinted polymer (MIP) nanoparticles and study their possible use in a fluorescence-based biosensor application. The MIP was imprinted with the fluorescent amino acid derivative dansyl-L-phenylalanine. Poly(vinyl alcohol) was used as a support for MIP nanoparticles because it is water-soluble and can be spun into very thin fibers. The fibers were characterized by atomic force microscopy and optical microscopy, and fluorescence microscopy was used for the characterization of target binding to the MIP. The fibers show close to 100% recovery upon extraction and rebinding of the target molecule. The selectivity of the system has been demonstrated through competitive binding experiments with nonfluorescent analogues boc-L-phenylalanine and boc-D-phenylalanine.
We demonstrate for the first time herein that electrospinning of soluble crosslinked polymer part... more We demonstrate for the first time herein that electrospinning of soluble crosslinked polymer particles (microgels) is feasible and that it can be used to obtain micron-sized fibers from these macromolecules. Most notably, the electrospinning approach is found to be successful also in the case of microgels bearing metal nanoclusters, and allows to prepare composite fibers containing a homogeneous dispersion of
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Papers by Silvia Piperno