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Pressure Tuning of Bromine Ionic States in Double-Walled Carbon Nanotubes
Abstract
The energetic, vibrational and electronic properties of bromine polyanions intercalated in Double-Walled Carbon Nanotubes (DWCNTs) were investigated by resonance Raman and X-Ray Absorption spectroscopies under high pressure conditions up to values close to 25 GPa. The mechanical resistance of the bromine intercalated DWCNTs is known to be affected by the presence of bromine polyanions, which induces uniaxial constrains in the interstitial regions of DWCNT bundles, thus leading to lower collapse pressure as compared with pristine DWCNTs. An upshift of bromine Raman frequencies concomitant to changes in the local structure of bromine atoms takes place at about 15 GPa, which is the pressure where the studied DWCNT intercalated bundles collapse. This suggests a differentiation of bromine polyanions interaction depending on the local curvature and the arrangement of the collapsed carbon structures. Supported by atomistic calculations, we suggest that those chains of Br$_2^-$ and Br$_5^-$ tend to dissociate to form Br-Br$_3$-Br complexes or elongated Br$_5^-$ polyanions in the interstitial regions of DWCNTs bundles after the nanotube collapse phase transition takes place. Chains of Br$_3^-$ could be found stable even after collapse. Those transitions appears to be reversible.
... The peak at ca. 235-239 cm −1 (denoted III) and its overtone at ca. 472-474 cm −1 (denoted VI) are usually assigned to the Br-Br stretching of intercalated Br 2 molecules. 4,14,26,[28][29][30]35,49 There are also peaks at ca. The latter can be assigned as a combination of I and II modes. ...
... 48 This frequency is downshifted to 233.9 cm −1 due to interaction of Br 2 with graphene that perfectly matches the position of Raman peak III. Previous theoretical investigations of brominated graphene and graphite showed significant overestimation of this frequency (up to 260-290 cm −1 ), 14,20,30,55 and to match experiment, an additional negative charge (ca. 0.5e) was assigned to Br 2 . ...
... Commonly bromine gets into inter-tubular spaces in the bundles. 4,14 The donor-acceptor interaction between CNTs and bromine and spatial confinement of intercalated molecules inside 1D-channels between tubes should drive the bromine polycondensation in CNTs and F-CNTs similarly to that in brominated graphite and lowfluorinated C 2 F x . To confirm this assumption, we synthesized brominated DWCNTs (denoted as DWCNT-Br) and fluorinated DWCNTs (denoted as F-DWCNT-Br) using a similar procedure to that described above for graphite. ...
Despite decades of study the precise behavior of bromine in graphitic carbons remains unclear. In this report, using Raman spectroscopy , we reveal two types of bromine structure in graphitic carbon materials. Between fluorinated graphene layers with a composition close to C2F, Br2 molecules are intercalated in a form similar to liquid bromine. Bromination of pristine and low-fluorinated graphitic carbons behaves very differently with distinct Br-related Raman spectra. With the guidance of density functional theory (DFT) calculations, all Raman features are assigned to normal vibration modes of specific bromine species over graphene and fluorinated graphene. When intercalated between extended non-fluorinated sp2-hybridized carbon regions, physisorbed Br2 molecules move freely across the non-functionalized region toward the CF border. Multiple Br2 molecules then combine spontaneously into Br3-based chains, whose coupling activates otherwise Raman inactive modes. Significant charge transfer to bromine species occurs in this case. DFT calculated frequencies match precisely the experimental Br-related Raman bands observed in the intercalation carbon compounds. The fluorine-catalyzed bromine chain-formation process shown here is general and should also operate with edges and other defect species.
... Is there a possibility for some inter-tube intercalation which we should have not detected by HRTEM? Halogens inter-tube intercalation has been in fact observed in other works but shown to reduce the value of the collapse pressure with respect to empty tubes [71]. We can then safely exclude inter-tube iodine intercalation in our study. ...
Carbon nanotubes have extraordinary mechanical properties, but modifications of their structure tend to weaken them. Here, we have studied by experiments and modelling the one-dimensional filling of single chirality (6,5) carbon nanotubes with iodine and water. We show that iodine-filling can enhance the pressure of radial collapse of these nanotubes by a factor 2 compared to the empty (6,5) tubes. For water filling, this enhancement factor reduces to 1.4. Our single-chirality study allows correlating the different Raman signatures of the radial collapsing process, which was not possible in samples with mixed chiralities. A clear spectroscopic signature of the collapse pressure can thus be given: it is the pressure at which the G-band frequency evolution with pressure softens while the radial breathing mode intensity vanishes. These new criteria for the detection of radial collapse allow correcting some existing discrepancies in the literature. Finally, we discuss the impact of molecular filling on the radial mechanical stability as a function of the tube diameter. It results that molecular filling allows for a superior stability effect than filling with tubes (i.e. multi-wall carbon nanotubes). The stability enhancement tends to grow with the tube diameter and depends strongly on the nature of the filling molecules.
... External parameters, as pressure, can be further used to tune the optical properties of nanomaterials [11,12]. Specifically, in hostguest systems, high pressure is an effective manner to further modulate molecule-molecule and host-molecule interactions [13,14], which offers opportunities for the fine-tuning of optoelectronic properties of quatertiophene (4T) filled SWCNTs (4T@SWCNTs) hybrid systems. ...
Filling carbon nanotubes with molecules is a route for the development of electronically modified one-dimensional hybrid structures for which the interplay between the electronic structure of molecules and nanotubes is a key factor. Tuning these energy levels with external parameters is an interesting strategy for the engineering of new devices and materials. Here we show that the hybrid system composed by quaterthiophene (4T) molecules confined in single-walled carbon nanotubes, presents a piezo-Raman-resonance of the molecule vibrational pattern. This behavior manifests as a rapid pressure induced enhancement of the 4T Raman mode intensities compared to the tubes G-band Raman modes. Density functional theory calculations allow to explain the spectral behaviour through the pressure-enhanced quaterthiophene resonance evolution. By increasing pressure, the tube cross-section deformation leads to a reduction of the intermolecular distance, to the splitting of the molecular levels and then to an increase of resonance channels. Calculations and experiments converge to the 4T piezo-resonance scenario associated with the pressure-induced nanotube radial collapse observed at about 0.8 GPa. Our findings offer possibilities for the development of pressure transducers based on molecule-filled carbon nanotubes.
... At this excitation energy, Br-Br stretching vibrations are in resonance giving an intense peak at 232 cm -1 . Additionally, the Raman spectrum of brominated samples exhibited a peak at 153 cm -1 , attributed to polybromine Br n chains [45,48]. Our recent calculations have shown the chains are mainly composed of the Br 3 -based species [49]. ...
Bromination of double-walled carbon nanotubes (DWCNTs) was carried out using a saturated vapor of Br2 at room temperature with or without a pretreatment in bromine water. X-ray photoelectron spectroscopy revealed that ultrasound pretreatment modified the chemical state of bromine in the product. The binding energies of the Br 3d electrons in the pre-sonicated DWCNT sample were between those characteristic of the covalent C–Br bonds and the negatively charged Br2 molecules, observed when the pretreatment was not performed. Raman spectroscopy, however, clearly evidenced Br–Br vibrations in both brominated samples. Calculations of CNT–Br2 models within density functional theory were used to propose that the electronic state of a Br2 molecule depends on the adsorption site. The bromine molecules prefer to be located near edge hydroxyl groups, which acept the electron density from Br2. This increases the binding energy of Br 3d levels as compared to that for Br2 molecules in other adsorption sites.
This work presents macroscopic fibers of aligned double-walled carbon nanotubes (DWCNTs) intercalated with long-range ordered bromine, with a stoichiometry close to C17Br. Tribromide ions lie inside interstitial sites between hexagonally-packed DWCNTs and extend parallel to their axis as ordered supramolecular “wires”. First-principles simulations confirm this structure and a transfer of 0.13 electrons per Br atom. The structure of nested bundles of CNTs with a homogeneous distribution of bromine species in the interstitial sites of the superlattice is directly imaged by cross-sectional HRTEM, showing full intercalation of highly dense and aligned fibers. The presence of Br2 and is confirmed by Raman spectroscopy, and their supramolecular organization is resolved by 2D wide-angle X-ray scattering. Intercalation increases room-temperature longitudinal electrical conductivity by a factor of 8.4. Through low-temperature transport measurements in the longitudinal and transverse directions, we show that the intercalate reduces the tunneling-dominated resistance associated with transport between adjacent CNTs, rather than exclusively acting as a dopant that increases conductance of individual CNTs. By preserving the separation between CNTs, the exceptional mechanical properties of the CNT fiber host are retained. The combined tensile strength above 2.46 GPa and conductivity of 10.68 MSm−1 makes intercalated CNT fibers attractive lightweight conductors with combined properties superior to metals and graphite intercalation compounds.
Highly volatile and toxic bromine (Br2) molecules can be utilized safely in various chemical processes when coupled with efficient separation systems. Herein, we present two different N-containing porous organic cages...
Pressure and temperature phase transitions of nanomaterials often differ significantly from those of their bulk parents, offering novel approaches for the engineering of original materials. The importance or even the dominance of surface atoms in the nanoworld enhances the effects of environment, geometry, and intercalation. In the present article, we explore the current knowledge of these effects, as evidenced in the high pressure phase diagrams of nanomaterials such as nanocrystals, carbon nanotubes, fullerites, graphene, and other 2D systems, as well as nanoporous structures like clathrates or zeolites. Recent advances and future challenges in the use of extreme thermodynamic conditions to develop new functional nanomaterials, composites, or devices will be reviewed, along with the specificities of the experimental environment required for these investigations.
We present a joint experimental and theoretical study on the high-pressure behavior of bromine confined in the one-dimensional (1D) nanochannels of zeolite AlPO4-5 (AFI) single crystals. Raman scattering experiments indicate that loading bromine into AFI single crystals can lead to the formation of bromine molecular chains inside the nanochannels of the crystals. High-pressure Raman and X-ray diffraction studies demonstrate that high pressure can increase the length of the confined bromine molecular chains and modify the inter- and intramolecular interactions of the molecules. The confined bromine shows a considerably different high-pressure behavior to that of bulk bromine. The pressure-elongated bromine molecular chains can be preserved when the pressure is reduced to ambient pressure. Theoretical simulations explain the experimental results obtained from the Raman spectroscopy and X-ray diffraction studies. Furthermore, we find that the intermolecular distance between confined bromine molecules gradually becomes comparable to the intramolecular bond length in bromine molecules upon compression. This may result in the dissociation of the bromine molecules and the formation of 1D bromine atomic chains at pressures above 24 GPa. Our study suggests that the unique nanoconfinement has a considerable effect on the high-pressure behavior of bromine, and the confined bromine species concomitantly enhance the structural stability of the host AFI single crystals.
The behavior of diatomic molecular solids under pressure have attracted great interest and been extensively studied. Under ambient pressure, the structure of bromine is known to be a molecular phase (phase I). With increasing pressure, it transforms into an incommensurate phase (phase V) before eventually to a monoatomic phase (phase II). However, between phases I and V, the interatomic distance was found to first increase with pressure and then decreased abruptly. This anomalous bond length behavior is accompanied by the splitting of the Raman bands. These phenomena have not been resolved. Here we suggest a new solid phase that explains the Raman spectra. Furthermore, the anomalous bond length behavior is found to be the result of subtle second neighbor intermolecular interactions and is an intrinsic property of bromine in molecular phases.
The vibrational and electronic properties of Br2-adsorbed double-wall carbon nanotubes (DWNTs) were investigated by resonance Raman scattering. Special attention was given to distinguish the behavior between S/M and M/S outer/inner semiconducting (S) and metallic (M) tubes. By using three laser excitation energies 2.33, 1.96, and 1.58 eV, resonance Raman spectra were obtained for the DWNTs before and after bromine adsorption, and also for the Br-Br molecular resonance, thereby facilitating the study of charge transfer between the DWNTs and the bromine. It was found that Br2 molecules act as acceptors and that metallic nanotubes are specially sensitive to the presence of Br2 molecules even when they constitute the inner tubes of DWNTs.
We report intercalation of charged polyiodide chains into the interstitial channels in a single-wall carbon nanotube (SWNT) rope lattice, suggesting a new carbon chemistry for nanotubes, distinctly different from that of graphite and C60. This structural model is supported by results from Raman spectroscopy, x-ray diffraction, Z-contrast electron microscopy, and electrical transport data. Iodine-doped SWNTs are found to be air stable, permitting the use of a variety of techniques to explore the effect of charge transfer on the physical properties of these novel quantum wires.
New models of fluid transport are expected to emerge from the confinement of liquids at the nanoscale, with potential applications in ultrafiltration, desalination and energy conversion. Nevertheless, advancing our fundamental understanding of fluid transport on the smallest scales requires mass and ion dynamics to be ultimately characterized across an individual channel to avoid averaging over many pores. A major challenge for nanofluidics thus lies in building distinct and well-controlled nanochannels, amenable to the systematic exploration of their properties. Here we describe the fabrication and use of a hierarchical nanofluidic device made of a boron nitride nanotube that pierces an ultrathin membrane and connects two fluid reservoirs. Such a transmembrane geometry allows the detailed study of fluidic transport through a single nanotube under diverse forces, including electric fields, pressure drops and chemical gradients. Using this device, we discover very large, osmotically induced electric currents generated by salinity gradients, exceeding by two orders of magnitude their pressure-driven counterpart. We show that this result originates in the anomalously high surface charge carried by the nanotube's internal surface in water at large pH, which we independently quantify in conductance measurements. The nano-assembly route using nanostructures as building blocks opens the way to studying fluid, ionic and molecule transport on the nanoscale, and may lead to biomimetic functionalities. Our results furthermore suggest that boron nitride nanotubes could be used as membranes for osmotic power harvesting under salinity gradients.
The vibrational and structural properties of modified double-wall carbon nanotubes (DWNTs) were investigated by high-pressure resonance Raman scattering. We studied bromine-intercalated DWNTs grown by chemical vapor deposition (CVD) and 13 C 60 peapod-derived DWNTs in comparison with pristine CVD-grown DWNTs. The effects of chemical modification, carbon interwall geometry, and inhomogeneous filling on the high-pressure evolution of the DWNTs have been investigated. We find that the mechanical resistance of the DWNT system is affected both in the case of bromine-intercalated CVD-DWNTs and also for the 13 C 60 peapod-derived DWNTs, thus lowering the onset of collapse pressure P (onset) c compared with pristine CVD-DWNTs. For bromine CVD-DWNTs, P (onset) c was observed to be 13 GPa, well below the 21 GPa found for pristine CVD-DWNTs. Uniaxial constrains in the interstitial regions of the DWNT bundle due to the presence of bromine arrangements explains this mechanical instability rather than a charge transfer process. Isotopic 13 C enrichment of the inner tube reduces the frequency of its tangential contribution to the G-band Raman spectrum, which appears to be an effective method to separate the contribution of inner-and outer-tube G + components during pressure evolution. P (onset) c was found to be 12 GPa for the 13 C 60 -derived DWNT system. In this case, the instability of the DWNT is mainly due to the high inhomogeneous filling of the outer tube, as a consequence of the conversion method used to produce the inner-wall nanotube from the peapods, which produces inner tubes which are usually shorter than the outer tubes, leading to the outer tube not being completely filled.
Using density-functional theory calculations, we investigate the atomic and electronic structure of the bromine species encapsulated in carbon nanotubes. We find that the odd-membered molecular structures (Br3 and Br5) are energetically favored than the common Br2 molecule. The transformation from bromine molecules (Br2) into Br3 or Br5 is found to be almost barrierless. A strong electron transfer from the nanotube to the adsorbates, which has been doubtful in previous studies, is accompanied by the formation of such odd-membered polybromine anions. We suggest that the tip-opened carbon nanotube samples can be heavily hole-doped after exposure to Br2 gas.
One of the most pervasive problems afflicting people throughout the world is inadequate access to clean water and sanitation. Problems with water are expected to grow worse in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Addressing these problems calls out for a tremendous amount of research to be conducted to identify robust new methods of purifying water at lower cost and with less energy, while at the same time minimizing the use of chemicals and impact on the environment. Here we highlight some of the science and technology being developed to improve the disinfection and decontamination of water, as well as efforts to increase water supplies through the safe re-use of wastewater and efficient desalination of sea and brackish water.
IT has been suggested1 that the tensile strength of carbon nanotubes2 might exceed that of other known fibres because of the inherent strength of the carbon-carbon bond. Calculations of the elastic properties of nanotubes confirm that they are extremely rigid in the axial direction and are most likely to distort perpendicular to the axis3'4. Carbon nanotubes with localized kinks and bends5'6, as well as minor radial deformations7'8, have been observed. Here we report the existence of multi-shelled carbon nanotubes whose overall geometry differs radically from that of a straight, hollow cylinder. Our observations reveal nanotubes that have suffered complete collapse along their length. Theoretical modelling demonstrates that, for a given range of tube parameters, a completely collapsed nanotube is favoured energetically over the more familiar 'inflated' form with a circular cross-section.
Combining a classical force field, a tight-binding model, and first-principle calculations, we have studied structural, electronic, and optical properties of double-walled carbon nanotube (DWNT) bundles under hydrostatic pressure. We find that the outer tube acts as a protection shield for the inner tube and the inner tube increases the structure stability and the ability to resist the pressure of the outer tube. Moreover, the collapsed structures of the double-walled carbon nanotube bundle called "parallel" and "in-between" are more stable than the one called "herringbone". The structural phase transition induces a pseudogap along the symmetry line ΓX. Furthermore, the optical properties change greatly after the collapse and a strong anisotropy appears in the collapsed structure. This provides an efficient experimental way to detect structural phase transitions in DWNT bundles.
The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in CNTs research. In this work, the capillary effect was used to fill double wall carbon nanotubes with iron. The samples are characterized by Mossbauer and Raman spectroscopy, transmission electron microscopy, scanning area electron diffraction, and magnetization. In order to investigate their structural stability and compare it with that of single wall carbon nanotubes (SWNTs), elucidating the differences induced by the inner-outer tube interaction, unpolarized Raman spectra of tangential modes of double wall carbon nanotubes (DWNTs) filled with 1D nanocrystallin -Fe excited with 514 nm were studied at room temperature and elevated pressure. Up to 16GPa we find a pressure coefficient for the internal tube of 4.3cm-1GPa-1 and for the external tube of 5.5cm-1GPa-1. In addition, the tangential band of the external and internal tubes broadens and decreases in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tubes (at least up 16GPa). Structural phase transitions were not observed in this range of pressure.
We use sulfuric acid as pressure medium to extrapolate the G -band position of the inner and outer tubes of double-wall carbon nanotubes. Keeping the G -band position of the inner and outer tubes constant, we can determine the fraction of double-wall and single-wall tubes in samples containing a mixture of the two. A -band-related electronic interwall interaction at 1560 cm-1 is observed, which is associated with the outer tube walls. This band is observed to shift with pressure at the same rate as the G band of outer tubes and is not suppressed with chemical doping. Differences in the interwall interaction is discussed for double-wall carbon nanotubes grown by the catalytic chemical-vapor method and double-wall carbon nanotubes obtained through transformation of peapods. © 2008 The American Physical Society.
We use the signal from the internal tubes of double-wall carbon nanotubes as an ideal pressure ref. The intensity assocd. with the G band of the external tubes is shown to be related to the interaction of the pressure medium and the carbon nanotube. We observe clear pressure medium dependent pressure coeffs. of the Raman G band using at. argon, oxygen, and alc. The G band of the internal tubes shifts between 5.1 and 3.3 cm-1/GPa and for the external tubes between 5.8 and 8.6 cm-1/GPa for the different pressure media used. We find that the spectral shape of the optical phonon band depends clearly on the pressure medium. Ab initio calcns. support local partial ordering and shell formation of the pressure medium around the nanotube. The shell formation around the tube has a strong impact on the local pressure transmission. [on SciFinder(R)]
We use sulfuric acid as pressure medium to extrapolate the G-band position of the inner and outer tubes of double-wall C nanotubes. Keeping the G-band position of the inner and outer tubes const., we can det. the fraction of double-wall and single-wall tubes in samples contg. a mixt. of the 2. A-band-related electronic interwall interaction at 1560 cm-1 is obsd., which is assocd. with the outer tube walls. This band is obsd. to shift with pressure at the same rate as the G band of outer tubes and is not suppressed with chem. doping. Differences in the interwall interaction is discussed for double-wall C nanotubes grown by the catalytic chem.-vapor method and double-wall C nanotubes obtained through transformation of peapods. [on SciFinder(R)]
This report focuses on the effects of different Br2 doping levels on the radial breathing modes of "double-wall carbon nanotube (DWNT) buckypaper". The resonance Raman profile of the Br2 bands are shown for different DWNT configurations with different Br2 doping levels. Near the maximum intensity of the resonance Raman profile, mainly the Br2 molecules adsorbed on the DWNT surface contribute strongly to the observed omega(Br-Br) Raman signal.
We report atomic resolution Z-contrast scanning transmission electron microscopy images that reveal the incorporation of I atoms in the form of helical chains inside single-walled carbon nanotubes. Density functional calculations and topological considerations provide a consistent interpretation of the experimental data. Charge transfer between the nanotube walls and the I chains is associated with the intercalation.
We present simulations of the collapse under hydrostatic pressure of carbon nanotubes containing either water or carbon dioxide. We show that the molecules inside the tube alter the dynamics of the collapse process, providing either mechanical support and increasing the collapse pressure, or reducing mechanical stability. At the same time the nanotube acts as a nanoanvil, and the confinement leads to the nanostructuring of the molecules inside the collapsed tube. In this way, depending on the pressure and on the concentration of water or carbon dioxide inside the nanotube, we observe the formation of 1D molecular chains, 2D nanoribbons, and even molecular single and multi-wall nanotubes. The structure of the encapsulated molecules correlates with the mechanical response of the nanotube, opening opportunities for the development of new devices or composite materials. Our analysis is quite general and it can be extended to other molecules in carbon nanotube nanoanvils, providing a strategy to obtain a variety of nano-objects with controlled features.
The mechanical stability of single-wall carbon nanotubes (SWCNTs) at high pressure was studied by high-resolution resonant Raman and wavelength-dependent fluorescence-excitation (PLE) spectroscopy resolving the vibrational and electronic resonances of 18 individual chiralities and furthermore even resolving the different behaviour of empty (closed, pristine) and water-filled (opened) SWCNTs (diameter range = 0.6-1.42 nm). We find that water-filling exerts a stabilizing counter-pressure on the SWCNT walls, leading to an increasing difference between the radial breathing mode frequencies of water-filled and empty SWCNTs at elevated pressures. For small diameter SWCNTs (d < 1 nm) with a chiral angle of ∼12°, in particular for the (7,2) chirality, an anomalous behaviour is observed, revealing an increased mechanical instability for these SWCNTs. We furthermore ascribe the longstanding contradiction between experiments and theory on the collapse pressure of SWCNTs to the presence of filling in most experiments to date, while empty SWCNTs follow the theoretically predicted collapse behaviour.
Raman spectroscopy of the one-dimensional atomic or molecular chains, which are the attractive building blocks of advanced nanoscale materials, is crucial in understanding the physical properties of the one-dimensional atomic or molecular chains. Here, we introduce the bromine into the one-dimensional channels of AlPO4-5 single crystals through a physical vapor diffusion method. Raman spectroscopy indicates that the confined bromine structures mainly exist as (Br2)n chains, individual Br2 molecules, and a small amount of Br3− chains inside the channels of AlPO4-5 single crystals. Polarized Raman spectra demonstrate that the bromine molecular chains are approximately parallel to the channel direction of AlPO4-5 single crystals. Copyright © 2015 John Wiley & Sons, Ltd.
The behavior of molecules and molecular chains confined in 1D nanochannels imposed by external interactions is a problem of fundamental interest. Here, we report structural manipulation of iodine confined inside zeolite (AFI) nanochannels by the application of high pressure. Structural transformations of the confined iodine under pressure have been unambiguously identified by polarized Raman spectroscopy combined with theoretical simulation. The length of the iodine chains and the orientation and intermolecular interaction of the confined iodine have been tuned at the molecular level by applied pressure. Almost all the confined iodine can be tuned into an axially oriented state upon compression, favoring the formation of long chains. The long iodine chains can be preserved to ambient pressure when released from intermediate pressures.
The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of triple-wall carbon nanotubes (TWCNTs) under hydrostatic pressure is reported. Pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high-pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters because their pressure response is different. Furthermore, it was possible to observe similar RBM profiles for the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure.
Being confined within nanoscale space, substances may exhibit unique physicochemical properties. The effect of nanoconfinement on molecular interactions is of significance, but a sound understanding has not been established yet. Here we present a quantitative study on boronate affinity (covalent) and electrostatic (non-covalent) interactions confined within mesoporous silica. We show that both interactions were enhanced by the confinement and that the enhancement depended on the closeness of the interacting location, as well as on the difference between the pore size and the molecular size. The overall enhancement could reach 3 orders of magnitude.
The vibrational and electronic properties of Br2-adsorbed double-wall carbon nanotubes (DWNTs) were investigated by resonance Raman scattering. We have found that Br2 molecules interact with the DWNTs and their intercalation characteristics are completely reversible upon thermal annealing. Upshifts in the Raman frequencies for the tangential modes and depression of their Raman intensities indicate that electrons are transferred from the nanotubes to the Br2 molecules. Metallic nanotubes are specially sensitive to the adsorption of Br2 molecules, even when they are the inner tubes of DWNTs. The vibrational spectra of the bromine dopant also provide information about the intercalation process.
X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under pressure are probes of local order and microscopic magnetic properties. XMCD is a selective probe that has access to a large variety of elements. The dispersive extended X-ray absorption fine structure (EXAFS) station at SOLEIL (ODE beam line) provides the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic. The main advantages of dispersive XAFS are the focusing optics, the short acquisition time (few μs) and great stability during the measurements due to the absence of any mechanical movement. These advantages allow the study of small samples, 70 μm at SOLEIL, which is mandatory in the case of high-pressure studies. We present the new ODE beam line at SOLEIL and its first high-pressure XMCD results.
Extended X-ray absorption fine structure (EXAFS) measurements are presented for dilute graphite-Br2 intercalation compounds containing 0.27 mole % and 0.75 mole % Br2. Intercalated Br2 molecules are found to have a BrBr distance of and predominate in the 0.75% sample. A unique feature of the present work is the discovery of another type of bromine molecule with a BrBr distance of . This molecule seems to be associated with defect or edge sites and predominates in the 0.27% sample. It is reasonable to speculate that this molecule acts like a “can opener”, preparing the graphite planes for intercalation. The implication of these results with respect to charge transfer is discussed, and it is estimated that roughly 0.16 electrons are transferred to each intercalated molecule, while ∼0.6 electrons are transferred to each “can opener” molecule.
We use ab initio density-functional calculations to investigate the electronic structure of the bromine-adsorbed carbon nanotubes. When a Br2 molecule is inside the (10,0) carbon nanotube, a trace of electron charge transfers from the nanotube to the Br2 adsorbate, resulting in an increased Br–Br bond length. When the supercell contains two Br2 molecules, total energy calculations reveal the formation of a linear chain of bromine atoms inside the carbon nanotube. Electron transfer from the nanotube to the atomic chains of the bromine adsorbates is much enhanced even in large-diameter nanotubes. We suggest that an exposure of the tip-opened carbon nanotube samples to a modest Br2 partial pressure could result in a strong hole-doping of the nanotube, which makes the semiconducting nanotubes nearly metallic.
The structural and electronic properties of homoatomic chains consisting of up to five Br-atoms are studied by means of the linear combination of Gaussian-type orbitals-local spin density method including nonlocal corrections to the exchange and correlation energy. A highly flexible basis set is used and the effects of introducing additional diffuse basis functions are examined. By comparison of the results for atomic Br and Br2 with those from very accurate correlated ab initio calculations the quality of the present method is established. Based on these results neutral and singly charged Br3, Br4 and Br5 are investigated, for which very few accurate data exist in literature. Geometries, harmonic vibrational frequencies, ionization potentials, electron affinities and charge distributions are reported and found in satisfactory agreement with available experimental data.
Raman results for different single-walled carbon nanotube bundles doped with Br2
were studied both at ambient pressure and under high pressure up to 6 GPa. Our
study indicates that bromine resides in the interstitial channel of nanotube
bundles as a form of polymer.
The high-pressure Raman response of double-wall carbon nanotubes (DWCNTs) in the radial breathing mode (RBM) frequency region can be used for the identification of both the inner and their respective outer tubes. A simple anharmonic model of this system demonstrates that the pressure response of the inner tube in a DWCNT is uniquely defined by the inner-outer tube spacing. Consequently, a plot of the normalized pressure coefficients of the inner tube RBM frequencies as a function of their ambient pressure frequency serves as a map which allows the identification of unknown DWCNTs. The present experimental results form the basis of such a plot.
The behavior of single-walled carbon nanotubes has been investigated under high pressures with the help of classical molecular dynamics simulations in two configurations: when bundles are empty and when argon is present as a pressure transmitting medium. Our calculations show that for the empty tubes, depending on the pressure step, relaxation times, and temperature, several different organizations of collapsed tubes exist for the high-pressure phase above 2.4 GPa. When the nanotubes are filled with argon (as well as surrounded by it), the high-pressure behavior is found to be substantially different. The phase transition shifts to higher pressures as the number of argon atoms inside the nanotubes is increased beyond a critical value and becomes close to 7 GPa for the calculated optimum Ar density. Computed x-ray diffraction patterns of argon-filled nanotubes show that the intensity of the first diffraction peak, which experimentally has been taken as indicative of two-dimensional order in bundles, persists up to higher pressures. We propose that seemingly varied experimental observations in the high-pressure phase transitions of carbon nanotubes are due to the pressure transmitting medium at different densities.
Results of the temperature-dependent Raman spectra of graphite-bromine intercalation compounds are reported in the temperature range 290<T<380 K. Of particular importance is the first observation of phase transitions in intercalated graphite using the Raman scattering technique, including (1) a commensurate-incommensurate phase transition and (2) a melting transition in the intercalate bromine layer. For a stage-3 compound, the commensurate-incommensurate phase transition occurs at T0=335±10 K, while the melting transition occurs continuously from 345 to 380 K. The temperature dependences of the Raman line shape, linewidth, frequency, and the intensity of the intercalate bromine molecular stretch mode have been utilized to investigate the phase transitions. The amount of charge transfer per intercalate bromine molecule has been estimated to be 0.34 by using the frequency shift of the intercalate bromine molecular stretch mode.
Extended-x-ray-absorption-fine-structure (EXAFS) measurements have been made on 0.6- and 0.9- monolayer samples of Br2 adsorbed on Grafoil, a form of graphite, and on an intercalated sample between 100 and 293 K. For both of the adsorbed samples the Br2 molecule is found to lie parallel to the basal-plane surface with each atom aligned as well as it can be above adjacent hexagonal sites. The Br-Br distance increases about 0.03 Å to accommodate part of the lattice mismatch. The average Br-C distance is 2.9 Å. Both of these coverages seem to be in the two-dimensional liquid phase seen in low-energy-electron-diffraction measurements, although analysis of the Br-Br internuclear vibrational amplitude suggests increased ordering for the 0.9-monolayer sample as the temperature is lowered. These two coverages are distinctly different from results previously reported for 0.2 monolayer, demonstrating the importance of Br2-Br2 interactions. In the intercalated sample the average Br-C distance decreases to 2.5 Å, and the bromine seems to be mainly molecular with the Br-Br distance increasing to match the periodicity of the graphite lattice. There is also evidence that the intercalated sample is a mixture of two phases. Finally, the amplitude of the Br-Br EXAFS is found to exhibit puzzling deviations from that of the vapor.
Results of Raman scattering experiments associated with in-plane intercalate modes in graphite-bromine are reported. The spectra are interpreted in terms of models in which the molecular character of the bromine is retained and the molecular ordering of the intercalate layer is closely related to that in the layer structure of solid molecular bromine. The symmetry and group-theoretical analysis of several possible molecular arrangements are given. A large resonant enhancement of the Br2 stretch mode is reported, which suggests molecular excitations for intercalated Br2 at energies ≳2.8 eV.
Librational lattice modes have been observed at 15°K in solid Cl2 at 83, 100, and 118 cm−1, and at 55, 74, 86, and 101 cm−1 in solid Br2. Their intensities relative to one another can be qualitatively accounted for by the oriented gas model. The stretching frequencies [ν(1–0)] of Cl2 and Br2 are shifted −13 and −20 cm−1, respectively, from their gas phase values and show structure due to isotope splittings and inter‐molecular coupling. The spectra of the lattice modes, the Br2 stretching motion, and the Cl2 stretching motion indicate, respectively, strong, intermediate, and weak intermolecular coupling relative to the isotope splittings. Both lattice and internal frequencies indicate stronger intermolecular forces in solid Br2 than in solid Cl2.
The infrared and Raman spectra for the polyhalide ions (ICl2−, ICl4−, BrCl2−, and Br3−) are presented. From these data, the X—Y stretching force constant fr, the interaction force constants frr between bond stretching coordinates at 180° to each other, and the interaction force constant frr′ for bond stretching coordinates at an angle of 90° (for ICl4−) have been calculated. The values of fr for the trihalide ions are roughly one‐half the values for the free halogens, and the values of the interaction force constants frr are very large (approximately 35% the value of the stretching constant fr). These rather unusual force constants have been interpreted in terms of the description of the bonding in these ions using p orbitals, which was first suggested by Pimentel. In fact, these results offer rather strong support for the recent evidence from nuclear quadrupole coupling constant measurements of Cornwell and Yamasaki favoring this structure. Attention is drawn to the qualitative similarity between these force constants and those for the HF2− ion.
High-quality x-ray absorption fine structure (XAFS) spectra of the Br2, GeCl4, and BBr3 molecules, collected at a third generation synchrotron radiation source above the Br or Ge K-edges, are presented. Excellent fits are obtained using model spectra calculated in the muffin-tin approximation assuming Gaussian atomic distributions. The extended energy ranges of the spectra (up to 24 Å−1 for Br2) contribute to the reduction of the statistical errors in the structural parameters. We show that the potential accuracy of present XAFS determinations is 0.001 Å in bond lengths and 0.0001 Å2 in vibrational amplitudes. These results demonstrate that XAFS is nowadays competitive with electron diffraction in the determination of simple molecular structures in the presence of heavy atomic species. © 1998 American Institute of Physics.
Single-walled carbon nanotubes (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements, suggests that doping does increase the carrier concentration of the SWNT bundles.
The vibrational properties of double-walled carbon nanotubes (DWNTs) is investigated by high-pressure resonance Raman scattering up to 30 GPa in two different pressure-transmitting media (PTM): paraffin oil and NaCl. The protection effect on the outer tube during compression is verified .The collapse of DWNTs is experimentally observed for the first time, showing to be two-step: the onset of the outer 1.56 nm diameter tube collapse at 21 GPa is followed by the collapse of the inner 0.86 nm diameter tube at a higher pressure of 25 GPa. This observation is supported by calculations. We show that filling a tube with another tube leads to a pressure stabilization against collapse, in strong opposition to what is observed when filling a tube with fullerenes or iodine. The collapse pressure in DWNTs appears to follow a 1/dtav3 law, where dtav is the average diameter from the inner and outer tubes, in agreement with predictions [Yang, X.; Appl. Phys. Lett. 2006, 89, 113101]. Contrary to SWNTs and peapods, for DWNTs, the observed collapse pressure is independent of the PTM nature. Those differences are discussed in terms of tube filling homogeneity and of the separate roles of inner and outer tubes: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. This leads to high collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs ideal fillers for composite nanomaterials for high load mechanical support.
Vibrational spectra (laser-excited Raman and infrared) for the bromite and hypobromite ions in aqueous solution were obtained. Bond-stretching force constants were calculated and these, taken with published data for BrO3- and the ClOn- (n = 1, 2, or 3) series, indicate that the chemical-bonding changes in these two series are similar. If the observed increase in bond strength with increase in n is due to increasing p-d π bonding, it appears that 2p-4d π bonding in BrOn- is as effective as 2p-3d π bonding is in ClOn-. A proposed explanation for the instability of BrO4- assumes that this is not the case. Spectral features additional to those assigned to the oxyanions were observed under certain conditions; these were assigned to Br3- and Br5-.
In situ Raman scattering measurements of pristine and fluorinated double-walled carbon nanotube samples under high pressure have been performed. Due to the mechanical shielding effect of the outer tubes of the double-walls to the applied pressure, the Raman scattering lines from the inner tubes of both samples remained to be observed even at high pressure.
We have implemented a linear scaling, fully self-consistent density-functional method for performing first-principles calculations on systems with a large number of atoms, using standard norm-conserving pseudopotentials and flexible linear combinations of atomic orbitals (LCAO) basis sets. Exchange and correlation are treated within the local-spin-density or gradient-corrected approximations. The basis functions and the electron density are projected on a real-space grid in order to calculate the Hartree and exchange–correlation potentials and matrix elements. We substitute the customary diagonalization procedure by the minimization of a modified energy functional, which gives orthogonal wave functions and the same energy and density as the Kohn–Sham energy functional, without the need of an explicit orthogonalization. The additional restriction to a finite range for the electron wave functions allows the computational effort (time and memory) to increase only linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, allowing structural relaxation and molecular dynamics simulations. We present test calculations beginning with small molecules and ending with a piece of DNA. Using double-z, polarized bases, geometries within 1% of experiments are obtained. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 453–461, 1997
In this work, we report a theoretical coupled study of the structural and
phonons properties of bundled single- and double-walled carbon nanotubes
(DWNTs), under hydrostatic compression. Our results confirm drastic changes in
volume of SWNTs in high-pressure regime as assigned by a phase transition from
circular to collapsed phase which are strictly dependent on the tube diameter.
For the DWNTs, those results show first a transformation to a polygonized shape
of the outer tube and subsequently the simultaneous collapse of the outter and
inner tube, at the onset of the inner tube ovalization. Before the DWNT
collapse, phonon calculations reproduce the experimentally observed screening
effect on the inner tube pressure induced blue shift both for RBM and
tangential G$_z$ modes . Furthermore, the collapse of CNTs bundles induces a
sudden redshift of tangential component in agreement with experimental studies.
The G$_z$ band analysis of the SWNT collapsed tubes shows that the flattened
regions of the tubes are at the origin of their G-band signal. This explains
the observed graphite type pressure evolution of the G band in the collapsed
phase and provides in addition a mean for the identification of collapsed tubes
Bromine has been studied up to a pressure of 110 GPa by X-ray absorption spectroscopy (XAS) at the bromine K-edge, that allows
to measure the pressure evolution of the width of the unoccupied conduction band. At GPa we observe a slope change in the evolution of this width. Comparison with published calculations of the electronic density
of states indicates that the physical origin of the slope change is compatible with the metallisation process. This is also
confirmed by a simple tight binding calculation. In addition, the metallisation pressure value is in agreement with calculated
ones. At GPa a discontinuity in the evolution of the width of the sigma antibonding band points out the onset of a phase transformation.
This result is compatible with the observed phase transformation near GPa by X-ray diffraction that is associated with the molecular dissociation.
A brief review of the SIESTA project is presented in the context of
linear-scaling density-functional methods for electronic-structure calculations
and molecular-dynamics simulations of systems with a large number of atoms.
Applications of the method to different systems are reviewed, including carbon
nanotubes, gold nanostructures, adsorbates on silicon surfaces, and nucleic
acids. Also, progress in atomic-orbital bases adapted to linear-scaling
methodology is presented.
We present a simple procedure to generate first-principles norm-conserving pseudopotentials, which are designed to be smooth and therefore save computational resources when used with a plane-wave basis. We found that these pseudopotentials are extremely efficient for the cases where the plane-wave expansion has a slow convergence, in particular, for systems containing first-row elements, transition metals, and rare-earth elements. The wide applicability of the pseudopotentials are exemplified with plane-wave calculations for copper, zinc blende, diamond, alpha-quartz, rutile, and cerium.
The high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
The possibility that steadily compressed hydrogen might undergo a transition from a proton-paired insulator to a monatomic metal was first suggested in 1935. But experimental realization of metallic hydrogen in solid form has remained elusive, despite studies at pressures as high as 342 GPa. The pairing structure is known to be robust (from the persistence of its associated vibron mode), leading to the suggestion of an alternative route to the metallic state, involving a band-overlap transition in which the pairing is preserved. Here we report density functional calculations within the local density approximation that predict a range of densities for hydrogen where a paired or molecular metallic state may be energetically preferred. The transition to this metallic state is naturally associated with the closing of an overall bandgap; but the pressures required to effect the transition are shown to change significantly when the gaps are corrected by approximate inclusion of many-electron effects. The implication is that a complete resolution of the structural and phase problem in dense hydrogen may require methods beyond the local density approximation.