The electron transport behavior through a series of molecular junctions composed of tetracene (TC... more The electron transport behavior through a series of molecular junctions composed of tetracene (TC) and S/O substituted-TC (S/O-TC) has been studied using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method. The unique transport behavior has been interpreted using correlated quantum interference and electron transport pathway models. In the TC system, two dominant electron transfer channels exist as demonstrated by detailed transmission pathway analysis. In the substituted S/O-TC systems, the electron transport behavior is regulated through either constructive or destructive quantum interference due to the existence of additional p-electrons, leading to a significant diversity of current-voltage curves. Compared to the TC molecule in the bias region from 0 to 1.0 V, an α-connected molecular junction exhibits greater current, whereas a β-connected molecular junction shows a smaller current. The substitution with O and S atoms shows a minor effect on the conductance of the molecular junctions. In order to clarify the role of heteroatoms, a series of artificial models designed by removing specific sulfur and carbon atoms in aS -TC have been investigated in detail. The results have demonstrated that only the S heteroatom on one side of the molecule contributes to the junction conductivity through constructive quantum interference. It has been also observed that current exchange occurs between the two electron transfer channels.
Nonetheless it has been reported that some PMMA residues still remain on the surface after therma... more Nonetheless it has been reported that some PMMA residues still remain on the surface after thermal annealing, resulting in scattering and defects. [3,9,12] In the absence of improved (i.e., low residue) graphene transfer techniques using PMMA or other polymers, [13-17] the other option for wafer-scale residue removal is wet-chemical cleaning typically using organic solvents such as acetone, chloroform, etc. [7] A major issue is that the presence and distribution of these polymer residues heavily depends on experimental conditions, the quality of graphene, etc., and therefore varies significantly from one process to another. An even bigger issue is that residues trapped at the metal-graphene interface generated during e-beam lithography cannot be removed by any of the techniques mentioned above as the only window for their removal is in between the steps of completion of resist development and metallization. This leads to large contact resistance and unwanted power losses at the contacts rather than in the device of interest. Few methods have been able to directly deal with PMMA residues in this window, necessitating complex bespoke equipment such as the use of CO 2 clusters from a jet nozzle, [18] plasma treatments, [19] and others have instead attempted post-fabrication treatments to reduce the contact resistance. [20,21] While the importance and challenges to remove transfer-induced PMMA residues on graphene surfaces are widely recognized, the majority of cleaning techniques are lacking in either reproducibility or efficiency, and meanwhile the issue of e-beam lithography-induced residues has been poorly addressed. In this work, we propose a simple and effective solution to removing PMMA residues on graphene surfaces as illustrated in Figure 1. By treating graphene surfaces with ionic solutions such as HCl and NaCl, we find that PMMA residues, especially the last thin layer (the layer in contact with graphene) are effectively removed. As these solutions have high chemical compatibility with the bulk PMMA layer, we also treated as-developed samples after e-beam exposure with our recipe and observed a significantly reduced contact resistance. CVD-grown graphene (on Cu foil) was transferred onto silicon substrates (300 nm SiO 2 / 500 µm Si) using PMMA 495 K A4 (495 K molecular weight, 4 wt% in anisole) as the supporting layer which was then dissolved in acetone for 3 h and then rinsed in isopropyl alcohol (IPA) to finish the transfer process. The residue cleaning effect of diluted HCl (1 m) and NaCl (1 m) was compared
We report the observation of long-range charge density oscillations within the vicinity of superl... more We report the observation of long-range charge density oscillations within the vicinity of superlattice boundaries on graphite at room temperature. These superlattices arise from rotational stacking faults between individual graphene layers on or just below the basal plane. Structural defects in the top graphene layer lead to elastic electron scattering, which is manifest as (i) a √ 3 superstructure at the atomic scale, and (ii) periodic modulations of the superlattice, at much lower spatial frequencies. The measured corrugation of these modulations is energy dependent and decays with increasing distance away from the defects, consistent with previously reported observations on Friedel oscillations in metals. This presents another charge-scattering mechanism limiting conductivity in honeycomb structures having rotational disorder. An understanding of such electronic modifications has important implications for tailoring the transport properties of future carbon electronics based on few-layer graphene.
Abstract Carbonate reservoir rocks are normally mixed-wet or oil-wet, leading to low oil recovery... more Abstract Carbonate reservoir rocks are normally mixed-wet or oil-wet, leading to low oil recovery efficiency using water-based oil recovery methods. It is critical to understand the molecular composition of the organic material coating the surface of carbonate reservoir rock in order to design better enhanced oil recovery (EOR) methods. Herein, we extracted organic compounds from a carbonate reservoir rock and characterized their composition using high resolution mass spectrometry (HRMS). In contrast to conventional interpretation that the mixed-wet or oil-wet nature of carbonate reservoir rocks arises from the adsorption of carboxylic acids, our results demonstrated that the organic species strongly bound to carbonate reservoir rock surface are dominated by N-containing species, including a group of “sticky molecules”. Each of these molecules can form multiple hydrogen-bonds, therefore they might act as a “double-sided tape” which binds crude oil strongly to the carbonate rock surface. Furthermore, we applied atomic force microscopy (AFM) techniques to a model mineral surface with regions of positive change and negative charge which was contacted with the crude oil produced from the formation where the rock was sampled. It was found that only the organic molecules with positive charge in the oil were adsorbed onto the mineral. This supports HRMS results which suggest that the organic materials strongly bound to the carbonate reservoir rock surface are dominated by basic N-containing molecules. Overall, these findings suggest that, beside fatty acids, these “sticky molecules” might also play an important role in controlling the wetting state of carbonate reservoir rock.
Superlattice structures and rippling fringes were imaged on two separate pieces of graphite (HOPG... more Superlattice structures and rippling fringes were imaged on two separate pieces of graphite (HOPG) by scanning tunnelling microscopy (STM). We observed the corrugation conservation phenomenon on one of the superlattice structures where an overlayer does not attenuate the corrugation amplitude of the superlattice. Such a phenomenon may illustrate an implication that nanoscale defects a few layers underneath the surface may propagate through many layers without decay and form the superlattice structure on the topmost surface. Some rippling fringes with periodicities of 20 nm and 30 nm and corrugations of 0.1 nm and 0.15 nm were observed in the superlattice area and in nearby regions. Such fringes are believed to be due to physical buckling of the surface. The stress required to generate such structures is estimated, and a possible cause is discussed. An equation relating the attenuation factor to the number of overlayers is proposed.
TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference, 2007
... A DEPLETION LAYER ACTUATOR James HT Ransley1*, Colm Durkan1, and Ashwin A. Seshia1 1 Cambridg... more ... A DEPLETION LAYER ACTUATOR James HT Ransley1*, Colm Durkan1, and Ashwin A. Seshia1 1 Cambridge University Nanoscience Center, Cambridge, CB3 0FF, UNITED KINGDOM (*Corresponding author: Tel : +44-1223 ... 0 2 2 2 3 129.0 12 2 1 a L wEh dx dx ud wh E V L E ...
In this article, we describe nanometer scale characterization of piezoelectric thin films of Lead... more In this article, we describe nanometer scale characterization of piezoelectric thin films of Lead-Zirconate-Titanate (PZT). Using the electric field from a biased conducting atomic-force microscopy (AFM) tip, we show that it is possible to form and subsequently image ferroelectric domains. Using a sphere-plane model for the tip-sample system we calculate the distribution of electric potential, field and polarization charge, and find good agreement with the experimental values. We also discuss the effects of surface contaminants on domain formation.
The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance bet... more The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in 'plasmon rulers'. We explore this behaviour in the subnm regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-onmirror (NPoM) construct. These allow unprecedented sub-angstrom control over spacer thickness via choice of metal centre, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasising the need for new treatments of light at the sub-nanoscale.
An analysis of polycrystalline Au thin-film interconnects of widths ranging from 850 to 25 nm, an... more An analysis of polycrystalline Au thin-film interconnects of widths ranging from 850 to 25 nm, and lengths ranging from 1 microm to 20 nm which have been electrically stressed to the point of failure is presented. A new method for testing failure of interconnects is proposed, based on a quantity we call the failure current density. The mean time to failure for fixed current density and also the failure current density are seen to decrease with decreasing wire width contrary to expectations. The failure current density for a given wire width increases as the length decreases. An analysis of the temperature and stress profiles based on calculations of a simple model is presented which shows that the length dependence is due to thermal stresses rather than electromigration, and the width dependence is due to enhanced electromigration due to surface diffusion.
The authors report the electrical characterization of gold and gold-palladium nanowires failed by... more The authors report the electrical characterization of gold and gold-palladium nanowires failed by electromigration. Nanogaps 1–2nm in size are reliably made from metal nanowires by controlling the electromigration failure process, opening up the possibility of using these metal nanowires with nanogaps for molecular conduction studies and large-scale molecular junction device fabrication. Nanogaps are formed by applying a voltage sweep to the wires at a ramp rate of 4mV∕s. The interplay between Joule heating and electromigration means that reliable nanogaps can be formed without the need of a feedback circuit, rendering the technique relatively simple to implement.
Grain-boundary and surface scattering are known to increase the electrical resistivity of thin me... more Grain-boundary and surface scattering are known to increase the electrical resistivity of thin metallic films and wires. The length scale at which these produce appreciable effects is of the order of the electronic mean free path. For the well-studied case of thin films, both mechanisms can, in principle, be used to explain the observed thickness dependence on resistivity. In order to evaluate which of these mechanisms is more relevant, we have carried out an experimental study of the width dependence of the resistivity of narrow thin-film polycrystalline gold wires ͑nanowires͒, and computed the expected behavior on the basis of both surface and grain-boundary scattering mechanisms independently. We find that the resistivity increases as wire width decreases in a manner which is dependent on the mean grain size and cannot be explained adequately by either model alone. We propose a modification to the well-known model of Mayadas and Shatzkes, incorporating the variation of mean grain size on wire dimensions.
The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally f... more The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally for ribbon widths from 16 nm-320 nm and is shown to validate the expected quantum scattering model for conduction through confined graphene structures. The experimental findings are that the resistivity follows a more dramatic trend than that seen for metallic nanowires of similar dimensions, due to a combination of the nature of the charge carriers in this 2D material, surface scattering from the edges, band-gap related effects and shifts in the Fermi level due to edge effects. We show that the Charge Neutrality point switches polarity below a ribbon width of around 50 nm, and that at this point, the thermal coefficient of resistance is a maximum. The majority doping type therefore can be controlled by altering ribbon width below 100 nm. We also demonstrate that an alumina passivation layer has a significant effect on the mean free path of the charge carriers within the graphene, which can be probed directly via measurements of the widthdependent resistivity.
A technique that is easy to implement and sensitive for measuring lateral oscillation amplitudes ... more A technique that is easy to implement and sensitive for measuring lateral oscillation amplitudes of optical fibers on the nanometer scale for shear-force microscopy is described. The measurement system analyzed here is based on using the optical fiber tip as a cylindrical lens to focus and deflect a detection beam. It is shown that for our experimental arrangement, this technique is at least 2.5 times as sensitive as merely shadowing the edge of such a beam, as in most commonly used configurations. As a result, oscillation amplitudes of the order of 2-3 nm may easily be measured. An advantage of this system is that absolute vibration amplitudes are easily measured. A simple geometric model is used to describe the operation of the system.
Page 1. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics Colm Durkan Page 2. ... Page ... more Page 1. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics Colm Durkan Page 2. ... Page 4. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics ColmDurkan University of Cambridge, UK ICP Imperial College Press Page 5. ...
The electron transport behavior through a series of molecular junctions composed of tetracene (TC... more The electron transport behavior through a series of molecular junctions composed of tetracene (TC) and S/O substituted-TC (S/O-TC) has been studied using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method. The unique transport behavior has been interpreted using correlated quantum interference and electron transport pathway models. In the TC system, two dominant electron transfer channels exist as demonstrated by detailed transmission pathway analysis. In the substituted S/O-TC systems, the electron transport behavior is regulated through either constructive or destructive quantum interference due to the existence of additional p-electrons, leading to a significant diversity of current-voltage curves. Compared to the TC molecule in the bias region from 0 to 1.0 V, an α-connected molecular junction exhibits greater current, whereas a β-connected molecular junction shows a smaller current. The substitution with O and S atoms shows a minor effect on the conductance of the molecular junctions. In order to clarify the role of heteroatoms, a series of artificial models designed by removing specific sulfur and carbon atoms in aS -TC have been investigated in detail. The results have demonstrated that only the S heteroatom on one side of the molecule contributes to the junction conductivity through constructive quantum interference. It has been also observed that current exchange occurs between the two electron transfer channels.
Nonetheless it has been reported that some PMMA residues still remain on the surface after therma... more Nonetheless it has been reported that some PMMA residues still remain on the surface after thermal annealing, resulting in scattering and defects. [3,9,12] In the absence of improved (i.e., low residue) graphene transfer techniques using PMMA or other polymers, [13-17] the other option for wafer-scale residue removal is wet-chemical cleaning typically using organic solvents such as acetone, chloroform, etc. [7] A major issue is that the presence and distribution of these polymer residues heavily depends on experimental conditions, the quality of graphene, etc., and therefore varies significantly from one process to another. An even bigger issue is that residues trapped at the metal-graphene interface generated during e-beam lithography cannot be removed by any of the techniques mentioned above as the only window for their removal is in between the steps of completion of resist development and metallization. This leads to large contact resistance and unwanted power losses at the contacts rather than in the device of interest. Few methods have been able to directly deal with PMMA residues in this window, necessitating complex bespoke equipment such as the use of CO 2 clusters from a jet nozzle, [18] plasma treatments, [19] and others have instead attempted post-fabrication treatments to reduce the contact resistance. [20,21] While the importance and challenges to remove transfer-induced PMMA residues on graphene surfaces are widely recognized, the majority of cleaning techniques are lacking in either reproducibility or efficiency, and meanwhile the issue of e-beam lithography-induced residues has been poorly addressed. In this work, we propose a simple and effective solution to removing PMMA residues on graphene surfaces as illustrated in Figure 1. By treating graphene surfaces with ionic solutions such as HCl and NaCl, we find that PMMA residues, especially the last thin layer (the layer in contact with graphene) are effectively removed. As these solutions have high chemical compatibility with the bulk PMMA layer, we also treated as-developed samples after e-beam exposure with our recipe and observed a significantly reduced contact resistance. CVD-grown graphene (on Cu foil) was transferred onto silicon substrates (300 nm SiO 2 / 500 µm Si) using PMMA 495 K A4 (495 K molecular weight, 4 wt% in anisole) as the supporting layer which was then dissolved in acetone for 3 h and then rinsed in isopropyl alcohol (IPA) to finish the transfer process. The residue cleaning effect of diluted HCl (1 m) and NaCl (1 m) was compared
We report the observation of long-range charge density oscillations within the vicinity of superl... more We report the observation of long-range charge density oscillations within the vicinity of superlattice boundaries on graphite at room temperature. These superlattices arise from rotational stacking faults between individual graphene layers on or just below the basal plane. Structural defects in the top graphene layer lead to elastic electron scattering, which is manifest as (i) a √ 3 superstructure at the atomic scale, and (ii) periodic modulations of the superlattice, at much lower spatial frequencies. The measured corrugation of these modulations is energy dependent and decays with increasing distance away from the defects, consistent with previously reported observations on Friedel oscillations in metals. This presents another charge-scattering mechanism limiting conductivity in honeycomb structures having rotational disorder. An understanding of such electronic modifications has important implications for tailoring the transport properties of future carbon electronics based on few-layer graphene.
Abstract Carbonate reservoir rocks are normally mixed-wet or oil-wet, leading to low oil recovery... more Abstract Carbonate reservoir rocks are normally mixed-wet or oil-wet, leading to low oil recovery efficiency using water-based oil recovery methods. It is critical to understand the molecular composition of the organic material coating the surface of carbonate reservoir rock in order to design better enhanced oil recovery (EOR) methods. Herein, we extracted organic compounds from a carbonate reservoir rock and characterized their composition using high resolution mass spectrometry (HRMS). In contrast to conventional interpretation that the mixed-wet or oil-wet nature of carbonate reservoir rocks arises from the adsorption of carboxylic acids, our results demonstrated that the organic species strongly bound to carbonate reservoir rock surface are dominated by N-containing species, including a group of “sticky molecules”. Each of these molecules can form multiple hydrogen-bonds, therefore they might act as a “double-sided tape” which binds crude oil strongly to the carbonate rock surface. Furthermore, we applied atomic force microscopy (AFM) techniques to a model mineral surface with regions of positive change and negative charge which was contacted with the crude oil produced from the formation where the rock was sampled. It was found that only the organic molecules with positive charge in the oil were adsorbed onto the mineral. This supports HRMS results which suggest that the organic materials strongly bound to the carbonate reservoir rock surface are dominated by basic N-containing molecules. Overall, these findings suggest that, beside fatty acids, these “sticky molecules” might also play an important role in controlling the wetting state of carbonate reservoir rock.
Superlattice structures and rippling fringes were imaged on two separate pieces of graphite (HOPG... more Superlattice structures and rippling fringes were imaged on two separate pieces of graphite (HOPG) by scanning tunnelling microscopy (STM). We observed the corrugation conservation phenomenon on one of the superlattice structures where an overlayer does not attenuate the corrugation amplitude of the superlattice. Such a phenomenon may illustrate an implication that nanoscale defects a few layers underneath the surface may propagate through many layers without decay and form the superlattice structure on the topmost surface. Some rippling fringes with periodicities of 20 nm and 30 nm and corrugations of 0.1 nm and 0.15 nm were observed in the superlattice area and in nearby regions. Such fringes are believed to be due to physical buckling of the surface. The stress required to generate such structures is estimated, and a possible cause is discussed. An equation relating the attenuation factor to the number of overlayers is proposed.
TRANSDUCERS 2007 - 2007 International Solid-State Sensors, Actuators and Microsystems Conference, 2007
... A DEPLETION LAYER ACTUATOR James HT Ransley1*, Colm Durkan1, and Ashwin A. Seshia1 1 Cambridg... more ... A DEPLETION LAYER ACTUATOR James HT Ransley1*, Colm Durkan1, and Ashwin A. Seshia1 1 Cambridge University Nanoscience Center, Cambridge, CB3 0FF, UNITED KINGDOM (*Corresponding author: Tel : +44-1223 ... 0 2 2 2 3 129.0 12 2 1 a L wEh dx dx ud wh E V L E ...
In this article, we describe nanometer scale characterization of piezoelectric thin films of Lead... more In this article, we describe nanometer scale characterization of piezoelectric thin films of Lead-Zirconate-Titanate (PZT). Using the electric field from a biased conducting atomic-force microscopy (AFM) tip, we show that it is possible to form and subsequently image ferroelectric domains. Using a sphere-plane model for the tip-sample system we calculate the distribution of electric potential, field and polarization charge, and find good agreement with the experimental values. We also discuss the effects of surface contaminants on domain formation.
The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance bet... more The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in 'plasmon rulers'. We explore this behaviour in the subnm regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-onmirror (NPoM) construct. These allow unprecedented sub-angstrom control over spacer thickness via choice of metal centre, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasising the need for new treatments of light at the sub-nanoscale.
An analysis of polycrystalline Au thin-film interconnects of widths ranging from 850 to 25 nm, an... more An analysis of polycrystalline Au thin-film interconnects of widths ranging from 850 to 25 nm, and lengths ranging from 1 microm to 20 nm which have been electrically stressed to the point of failure is presented. A new method for testing failure of interconnects is proposed, based on a quantity we call the failure current density. The mean time to failure for fixed current density and also the failure current density are seen to decrease with decreasing wire width contrary to expectations. The failure current density for a given wire width increases as the length decreases. An analysis of the temperature and stress profiles based on calculations of a simple model is presented which shows that the length dependence is due to thermal stresses rather than electromigration, and the width dependence is due to enhanced electromigration due to surface diffusion.
The authors report the electrical characterization of gold and gold-palladium nanowires failed by... more The authors report the electrical characterization of gold and gold-palladium nanowires failed by electromigration. Nanogaps 1–2nm in size are reliably made from metal nanowires by controlling the electromigration failure process, opening up the possibility of using these metal nanowires with nanogaps for molecular conduction studies and large-scale molecular junction device fabrication. Nanogaps are formed by applying a voltage sweep to the wires at a ramp rate of 4mV∕s. The interplay between Joule heating and electromigration means that reliable nanogaps can be formed without the need of a feedback circuit, rendering the technique relatively simple to implement.
Grain-boundary and surface scattering are known to increase the electrical resistivity of thin me... more Grain-boundary and surface scattering are known to increase the electrical resistivity of thin metallic films and wires. The length scale at which these produce appreciable effects is of the order of the electronic mean free path. For the well-studied case of thin films, both mechanisms can, in principle, be used to explain the observed thickness dependence on resistivity. In order to evaluate which of these mechanisms is more relevant, we have carried out an experimental study of the width dependence of the resistivity of narrow thin-film polycrystalline gold wires ͑nanowires͒, and computed the expected behavior on the basis of both surface and grain-boundary scattering mechanisms independently. We find that the resistivity increases as wire width decreases in a manner which is dependent on the mean grain size and cannot be explained adequately by either model alone. We propose a modification to the well-known model of Mayadas and Shatzkes, incorporating the variation of mean grain size on wire dimensions.
The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally f... more The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally for ribbon widths from 16 nm-320 nm and is shown to validate the expected quantum scattering model for conduction through confined graphene structures. The experimental findings are that the resistivity follows a more dramatic trend than that seen for metallic nanowires of similar dimensions, due to a combination of the nature of the charge carriers in this 2D material, surface scattering from the edges, band-gap related effects and shifts in the Fermi level due to edge effects. We show that the Charge Neutrality point switches polarity below a ribbon width of around 50 nm, and that at this point, the thermal coefficient of resistance is a maximum. The majority doping type therefore can be controlled by altering ribbon width below 100 nm. We also demonstrate that an alumina passivation layer has a significant effect on the mean free path of the charge carriers within the graphene, which can be probed directly via measurements of the widthdependent resistivity.
A technique that is easy to implement and sensitive for measuring lateral oscillation amplitudes ... more A technique that is easy to implement and sensitive for measuring lateral oscillation amplitudes of optical fibers on the nanometer scale for shear-force microscopy is described. The measurement system analyzed here is based on using the optical fiber tip as a cylindrical lens to focus and deflect a detection beam. It is shown that for our experimental arrangement, this technique is at least 2.5 times as sensitive as merely shadowing the edge of such a beam, as in most commonly used configurations. As a result, oscillation amplitudes of the order of 2-3 nm may easily be measured. An advantage of this system is that absolute vibration amplitudes are easily measured. A simple geometric model is used to describe the operation of the system.
Page 1. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics Colm Durkan Page 2. ... Page ... more Page 1. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics Colm Durkan Page 2. ... Page 4. CURRENT AT THE NANOSCALE An Introduction to Nanoelectronics ColmDurkan University of Cambridge, UK ICP Imperial College Press Page 5. ...
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