We present resugts of an extensive investigation with heat capacity measurements of the first lay... more We present resugts of an extensive investigation with heat capacity measurements of the first layer phases of 3He-4He mixtures adsorbed on graphite preplated with two layers of H2, for a wide range of coverages, and for temperatures between 0.2 K and 2.0 K. We observe the evolution of the twodimensional liquid(L)-vapor(V) coexistence region as a function of the 3He molar fraction(x). The L-V critical points, starting with 0.9 K at x=0, move to lower temperatures and the density of the self-bound liquid decreases as x increases, pointing towards a limiting value of x for L-V coexistence, in agreement with theoretical expectations. At densities higher than those producing L-V coexistence, we observe the onset of transitions to a phase present in the mixtures but apparently absent in the pure isotopic cases. Possible interpretations of the data will be discussed.
Aldo D. Migone, Achim Hofmann, JG Dash, and Oscar E. Vilches Department of Physics, University of... more Aldo D. Migone, Achim Hofmann, JG Dash, and Oscar E. Vilches Department of Physics, University of Washington, Seattle, Washington 98195. ... URL: http://link.aps.org/doi/10.1103/PhysRevB.37.5440 DOI: 10.1103/PhysRevB.37.5440 PACS: 68.10.Gw, 64.70.Dv, 68.45.Gd. ...
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
We report results from neutron diffraction measurements of five submonolayer coverages of Ne adso... more We report results from neutron diffraction measurements of five submonolayer coverages of Ne adsorbed on single-wall, closed-end carbon nanotube bundles (SWCNB). Our recent thermodynamic study of Ne adsorbed on SWCNB, Phys. Rev. B, 76, 075404 (2007), showed one-dimensional (1d) solid behavior below 4K at doses less than 0.08 monolayer, which crossed over to 3d-like behavior above 16K without signature of a phase transition. Above 0.18 monolayer there is a 2d-like solid behavior below 8K, with Debye temperatures in the 45 to 53K range, similar to those found for 2d Ne/graphite, but there is no melting transition at 13.5K as seen in 2d. Our structure measurements on Ne adsorbed on SWCNB similar to those used for the thermodynamic study were performed at 2K using beam line D20 at ILL. Results show the shift from a 1d to a 2d solid structure with increasing coverage. Lattice parameters, the relationship between the thermodynamic and structural measurements, and theoretical expectations will be shown and discussed.
In a previous experiment[1], we showed that one dimensional (1D) solid helium can be created on t... more In a previous experiment[1], we showed that one dimensional (1D) solid helium can be created on the surface of nanotube bundles. Specifically, when ^4He is first adsorbed on nanotubes, it forms a 1D linear solid along the grooves between two nanotubes on the bundle surface with lattice parameter, a1 = 3.40 ± 0.02 å. When more helium is added, 2D solid helium covers the whole bundle surface. We have now determined the vibrational dynamics of these 1D and 2D solids, the dynamic structure factor, S(Q,φ). From the inelastic intensity integrated over all φ we obtain the MS amplitude of vibration along 1D chain <u^2>= 0.28 å^2or Lindemann ratio γ= (<u^2>)^1/2/a1= 0.15 which is less than the bulk solid value near melting. The vibrational density states (DOS) of the 2D solid shows a gap at φ˜ 0.75 meV indicating a commensurate solid as found for ^3He and ^4He on graphite surfaces. In contrast the 1D DOS shows little or no gap and the DOS goes uniformly to zero as φ->0. [1] Pearce et al. Phys. Rev. Lett. 95, 185302 (2005).
We present resugts of an extensive investigation with heat capacity measurements of the first lay... more We present resugts of an extensive investigation with heat capacity measurements of the first layer phases of 3He-4He mixtures adsorbed on graphite preplated with two layers of H2, for a wide range of coverages, and for temperatures between 0.2 K and 2.0 K. We observe the evolution of the twodimensional liquid(L)-vapor(V) coexistence region as a function of the 3He molar fraction(x). The L-V critical points, starting with 0.9 K at x=0, move to lower temperatures and the density of the self-bound liquid decreases as x increases, pointing towards a limiting value of x for L-V coexistence, in agreement with theoretical expectations. At densities higher than those producing L-V coexistence, we observe the onset of transitions to a phase present in the mixtures but apparently absent in the pure isotopic cases. Possible interpretations of the data will be discussed.
Aldo D. Migone, Achim Hofmann, JG Dash, and Oscar E. Vilches Department of Physics, University of... more Aldo D. Migone, Achim Hofmann, JG Dash, and Oscar E. Vilches Department of Physics, University of Washington, Seattle, Washington 98195. ... URL: http://link.aps.org/doi/10.1103/PhysRevB.37.5440 DOI: 10.1103/PhysRevB.37.5440 PACS: 68.10.Gw, 64.70.Dv, 68.45.Gd. ...
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less of... more Adsorbates on a suspended single-walled carbon nanotube at a coverage of one monolayer or less offer the opportunity to study the various phases and phase transitions of a system where the dimensionality is below two. This is because such a monolayer resembles a well studied 2D monolayer on planar graphite, but with a tight cylindrical boundary condition imposed. The adsorbed density for any gas can be measured by using the nanotube itself as a vibrating microbalance, whose frequency varies with the adsorbed density and whose amplitude is detected by the way it modulates the conductance. We are focusing on two systems both thoroughly studied before on 2D graphite: the noble gases Xe and Kr; and oxygen. The noble gases are attractive for their simplicity, and because in 2D they exhibit discontinous phase transitions, which are not allowed in 1D according to an argument of Landau. They thus allow the possibility to confirm and explore this basic prediction of statistical mechanics for the first time. The magnetic and steric properties of phases of oxygen on 2D graphite, together with the question of its apparently unexplained large doping effect on nanotubes, make it particularly interesting and important. We have made suitable nanotube devices and will report on our progress in detecting monolayers on them.
A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extendin... more A monolayer of adsorbates on a single-walled carbon nanotube presents the possibility of extending earlier studies of two-dimensional monolayer systems on graphite to the quasi-one-dimensional regime, by effectively imposing cylindrical boundary conditions. The monolayer can be detected either via its effect on the nanotube's conductance or by using the nanotube itself as a vibrating microbalance. Many adsorbates are known to affect the conductance, through a variety of mechanisms. Amongst these are O2 and the noble gases Xe and Kr, whose phases and ordering on 2D graphite are well known. Our experiments so far have indicated that the presence of an O2 layer on a nanotube close to liquid nitrogen temperatures can be detected using a threshold shift. We are now fabricating individual suspended nanotube devices with the initial aim of studying cylindrical commensurability effects on the phases of noble gases using the microbalance technique.
We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspend... more We investigate the effects of adsorbed monolayers of Ar, Kr and other gases on individual suspended single-walled carbon nanotubes. The down-shifts of the vibrational resonances of a nanotube can be used to determine the monolayer densityootnotetextZ. Wang et al, Science 327, 552 (2010) while the electrical conductance is measured simultaneously, at temperatures as low as 4.3 K. In the case of Ar, by studying density isotherms in the range 38 to 65 K, we see behavior resembling that of the well known two-dimensional vapor, liquid and solid phases on exfoliated graphite, although the correspondence is not exact and is device dependent. In addition, we find that the conductance changes significantly and non-monotonically with the density, and there are indications that it is sensitive to ordering in the monolayer.
We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on ... more We report a study of the adsorption of ^3He, ^4He, H2, HD, D2, Ne, Ar, N2, Kr and Xe adsorbed on samples of BuckyPearls^TM, a form of HiPCo-type^TM carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of ^4He and Ne at low temperatures. For each gas, except ^3He and ^4He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N2 monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.
We have fabricated mass balances each consisting of an individual single-walled carbon nanotube s... more We have fabricated mass balances each consisting of an individual single-walled carbon nanotube suspended across a micron-sized trench in an oxidized Si wafer. The vibrational resonance frequency of a nanotube, which is in the range 50-500 MHz, is determined by monitoring the current through it while applying an electrostatic driving signal. By tracking changes in the resonance frequency we have measured isotherms of adsorbed mass vs vapor pressure for Ar ot Kr at liquid nitrogen temperatures. The sensitivity of the balances corresponds to just a few atoms. We have compared the monolayer mass shifts due to Ar and Kr, and measured a family of isotherms of Ar below 77 K. From the latter we calculated the isosteric heat of adsorption on the nanotube surface, which is found to be lower than that of Ar on basal plane graphite and only slightly larger than the latent heat of sublimation of bulk Ar at these temperatures. In one device we observed a phase transition in the adsorbed Ar near monolayer completion. In another device, which probably consists of two nanotubes joined in parallel, we observed enhanced adsorption at lower coverages which may be in the groove between the two nanotubes. This work is supported by the NSF, grant number 0606078.
We report results from neutron diffraction measurements of five submonolayer coverages of Ne adso... more We report results from neutron diffraction measurements of five submonolayer coverages of Ne adsorbed on single-wall, closed-end carbon nanotube bundles (SWCNB). Our recent thermodynamic study of Ne adsorbed on SWCNB, Phys. Rev. B, 76, 075404 (2007), showed one-dimensional (1d) solid behavior below 4K at doses less than 0.08 monolayer, which crossed over to 3d-like behavior above 16K without signature of a phase transition. Above 0.18 monolayer there is a 2d-like solid behavior below 8K, with Debye temperatures in the 45 to 53K range, similar to those found for 2d Ne/graphite, but there is no melting transition at 13.5K as seen in 2d. Our structure measurements on Ne adsorbed on SWCNB similar to those used for the thermodynamic study were performed at 2K using beam line D20 at ILL. Results show the shift from a 1d to a 2d solid structure with increasing coverage. Lattice parameters, the relationship between the thermodynamic and structural measurements, and theoretical expectations will be shown and discussed.
In a previous experiment[1], we showed that one dimensional (1D) solid helium can be created on t... more In a previous experiment[1], we showed that one dimensional (1D) solid helium can be created on the surface of nanotube bundles. Specifically, when ^4He is first adsorbed on nanotubes, it forms a 1D linear solid along the grooves between two nanotubes on the bundle surface with lattice parameter, a1 = 3.40 ± 0.02 å. When more helium is added, 2D solid helium covers the whole bundle surface. We have now determined the vibrational dynamics of these 1D and 2D solids, the dynamic structure factor, S(Q,φ). From the inelastic intensity integrated over all φ we obtain the MS amplitude of vibration along 1D chain <u^2>= 0.28 å^2or Lindemann ratio γ= (<u^2>)^1/2/a1= 0.15 which is less than the bulk solid value near melting. The vibrational density states (DOS) of the 2D solid shows a gap at φ˜ 0.75 meV indicating a commensurate solid as found for ^3He and ^4He on graphite surfaces. In contrast the 1D DOS shows little or no gap and the DOS goes uniformly to zero as φ->0. [1] Pearce et al. Phys. Rev. Lett. 95, 185302 (2005).
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Papers by oscar vilches