Linear Carbon Chains under High-Pressure Conditions - The Journal

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Linear Carbon Chains under High-Pressure Conditions N. F. Andrade,† A. L. Aguiar,‡ Y. A. Kim,¶ M. Endo,§ P. T. C. Freire,† G. Brunetto,∥ D. S. Galvaõ ,∥ M. S. Dresselhaus,⊥ and A. G. Souza Filho*,† †

Departamento de Física, Universidade Federal do Ceará, 60455-900 Fortaleza, Ceará, Brazil Departamento de Física, Universidade Federal do Piauí, 64049-550 Teresina, Piauí, Brazil ¶ School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, Korea § Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan ∥ Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, 13083-859 Campinas, Sao Paulo, Brazil ⊥ Departments of Physics and Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡

ABSTRACT: A high-pressure resonance Raman spectroscopy study of linear carbon chains encapsulated inside multiwalled carbon nanotubes (MWCNTs) is reported. While the frequencies of the tangential modes of carbon nanotubes (G band) harden as the pressure increases, the vibrational frequencies of the chain modes (around 1850 cm−1) decrease, thus indicating a softening of the carbon−carbon bonds in this 1D solid. Pressure-induced irreversible structural changes in the linear carbon chains are unveiled by the red shift in the vibrational modes when pressure is released. These results have been interpreted as being due to a coalescence of carbon chains, and this hypothesis is supported by state-of-the-art atomistic reactive molecular dynamics simulations.

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INTRODUCTION The study of linear carbon chains originated from interstellar space research, and this early work is closely related to carbon nanoscience because these early investigations by Kroto historically led to the discovery of fullerenes.1 The discovery of these fullerene carbon cages inspired conjectures about the possible existence of tubular carbon structures,2 which were proven true by the report of multiwall carbon nanotubes (MWCNTs) by Iijima in 1991.3 Single-walled carbon nanotubes (SWCNTs) became known independently in 1993 after Iijima and Ichihashi4 and Bethune et al.5 attempted to produce MWCNTs filled with transition metals. Some years later, Smith et al. found that the core of SWCNTs could host fullerenes, thus leading to the discovery of the new hybrid carbon structure called peapods. 6 Due to their capability of encapsulating different species, the hollow cores of carbon nanotubes (CNTs) have been used as templates for confining different nanowires and molecule arrays within nanotube cores.7,8 In this scenario, the inner space of CNTs is found to be ideal for encapsulating and stabilizing 1D solids, such as © 2015 American Chemical Society

long linear carbon chains, which are not stable under ambient conditions.9−14 We should mention that small-length carbon chains have been investigated from both the experimental and the theoretical points of view, including looking at different phenomena and properties (electronic states, vibrational modes, transport properties, and mechanical properties) and proposing alternatives for designing and measuring a variety of chain-based systems such as carbon chains terminated with Pt atoms15 and with different aromatic end groups,16,17 bridging graphene domains,18 and N-doped carbon chains.19 All these studies pointed out relevant properties for these chains that are very promising for nanoelectronics and spintronic devices. MWCNTs are the category of carbon nanotubes with the greatest potential for technological applications, such as energy storage, electronic devices, and nanocomposites, among others, due to their remarkable mechanical, thermal, and electrical Received: January 28, 2015 Revised: April 10, 2015 Published: April 23, 2015 10669

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

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The Journal of Physical Chemistry C properties.20 In this context, MWCNTs filled with carbon linear chains (Cn@MWCNTs) are very attractive for searching for new properties and for designing some novel applications. It is hoped that under appropriate conditions, such hybrid nanostructures as Cn@MWCNTs could have Young’s modulus, strength, and toughness properties superior to those of the carbon fibers, graphite whiskers, and bare MWCNTs, which would make this system very appealing to materials scientists.9 Many techniques have been used for characterizing carbon nanostructures, and among optical spectroscopies, Raman scattering has been very powerful for simultaneously studying both the vibrational and the electronic properties of these nanomaterials. In addition, the use of this technique can go beyond studying the normal conditions of temperature and pressure, being appropriate for studying nanocarbons under extreme conditions such as high pressure and high temperature. Pioneering work by Venkatesvaran21 in the late 90s showed that high-pressure Raman studies could provide insights into the properties of carbon nanotubes themselves, and since then, many works using this technique have been published for different nanotube samples (pristine and doped forms) such as SWCNTs, DWCNTs, and MWCNTs.22−34 The study of carbon chains by experimental means is difficult due to the fact that the chains are unstable under ambient conditions because the unsaturated bonds are highly reactive,35 and once they are placed and stabilized in the core of a MWCNTs, it is not easy to directly access them. In this scenario, the use of the high-pressure Raman technique appears as a suitable method for probing the carbon chains for three main reasons: (i) the Raman signal of carbon chains is very intense, (ii) high pressure is effective for tuning the degree of interaction between the carbon chains and its adjacent carbon nanotube wall, and (iii) the vibrational frequencies of the carbon chains are sensitive to any structural changes that the chain may experience. The aim of this study is to investigate the behavior of 1D linear carbon chains encapsulated into the innermost shells of MWCNTs by evaluating their structural stability at high pressure values using experimental and theoretical tools. The Raman band close to 1800 cm−1, which is characteristic of the 1D carbon chain, has been used in the present work as a probe for tracking the pressure-induced structural changes of this system. We report here a softening of the C−C stretch mode of the carbon chain as the pressure increases, and the results for the subsequent decompression show that the carbon chains experiences a pressure-induced irreversible process. This result has been interpreted as being a pressure-induced coalescence between the different carbon chains inside the carbon nanotubes, which is supported by fully atomistic reactive molecular dynamics simulations.

(∼1 mm), applying a constant current of 10 A (voltage of 20 V), and using argon gas at a flow rate of 1 L/min along the hollow tube of the graphite cathode, we obtained a gray tape spiraling on the outer surface of the cathode (width of 3−5 mm, thickness of ∼175 mm). Finally, cooling gas was used to detach the deposited thin tape from the cathode. Experimental Techniques. We measured the Raman spectra of the samples using different laser excitation energies (2.54, 2.33, and 1.57 eV) at ambient pressure, aiming to find the most sensitive resonant laser energy to probe pressureinduced changes in the linear chains. Several Raman spectra were collected at random points over the sample, and we attempted to verify the homogeneity of the sample regarding the observation of the vibrational modes associated with the carbon chains. A LabRam HR Jobin Yvon spectrometer was used for measuring the Raman spectra. This system contains a microscope with an objective lens containing a plan chromatic focal length of 20 mm and a numerical aperture (N.A.) of 0.35. In the high-pressure experiments, we used a diamond anvil cell (DAC) in the 0−10 GPa pressure range. The gasket used for mounting the sample chamber consists of a metal alloy foil with a thickness of 200 μm. The sample chamber is a drilled hole with a diameter of about 300 μm. The cell was loaded with a tiny piece of sample and a ruby pressure sensor and then filled with the pressure-transmitting fluid. We used Nujol mineral oil as the pressure transmitting medium because this oil does not react with the sample, and it maintains the hydrostatic behavior over the entire pressure range investigated in this experiment. To determine the pressure P within the cell, we used the ruby luminescence technique, which is based on the fact that the frequency of the two emission lines of ruby vary linearly as a function of the hydrostatic pressure P (given here in units of GPa), by using the expression P=

ωRi − ωR0i 7.535

(1)

where ωRi is the wavenumber for any of the ruby’s two luminescent lines (units of cm−1 relative to the laser line) and ω0Ri is the relative wavenumber of the respective line at ambient pressure. Theoretical Methods. To investigate the structural behavior of linear carbon chains inside a carbon nanotube when the system is subjected to high external pressure values, we created a model system that mimics the main features of the experiments. The interactions in this model system are described using a reactive force field (ReaxFF) based on the bond energy−bond order (BEBO) method.37 The ReaxFF is designed to incorporate chemical reactions so that bond formation and bond breaking can be properly described. During chemical reactions, the bonded neighbors and the geometry both change and, consequently, the charge distribution also changes.38 ReaxFF is parametrized against DFT calculations and it is, in principle, able to describe the change in the coordination number in the carbon atoms.39 The methodology was employed successfully previously in different cases, and the results obtained are in good agreement with more accurate calculations and experimental evidence.40,41 ReaxFF uses the electronegativity equalization method (EEM)42 to assign partial point charges to every atom. Interatomic distances, computed as a sum of σ, π, and ππ terms (eq 2), are used to calculate the bond order contributions. The system energy is computed by a sum of

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METHODOLOGIES Sample Preparation. Linear carbon chains encapsulated within the hollow core of multiwalled carbon nanotubes (MWCNTs) were synthesized by the atmospheric arc discharge method, as described in ref 36. A hollow graphite anode (outer diameter of 10 mm, inner diameter of 4 mm) was moved with a speed of 170 mm/min, and a rod-type carbon cathode (diameter of 35 mm) with a high specific resistivity above 4000 mΩ·cm was rotated with a rate of 155 rpm. The cathode and anode were made of pure graphite with a purity of 99.99%. Then, by precisely controlling the gap between the electrodes 10670

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

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The Journal of Physical Chemistry C different contribution terms (eq 3) 37 in which Ebond is the bond energy, Eover and Eunder are, respectively, the over- and undercoordination energy, Eval is the angle-dependent strain energy, Epen is the energy penalty paid for introducing allenetype molecules into the chains, Etors is the torsion energy, Econj is the torsion conjugation energy, EvdW is the van der Waals interaction energy, and ECoul is the Coulomb interaction energy:

and red, and the water molecules are depicted as blue-colored balls surrounding the nanotube. To increase the external pressure experienced by the CNT, we decreased the box dimensions x and y with a constant rate of vwall = 2 × 10−5 Å/ps. The box dimension along the tube axial direction (periodic dimension) was kept constant to keep the tube length unchanged. The simulations were carried out with a Nosé−Hoover thermostat coupled to both the water and the CNT atoms with a target temperature of T = 300 K. The time constant considered in the temperature relaxation was τ = 10 fs. In all simulations, we used Δt = 0.025 fs for the time step size. To calculate the vibrational density of states (VDOS) Φ(ω) (eq 4) for the linear carbon chain, we use the relationship

BOij = BOijσ + BOijπ + BOijππ ⎡ ⎛ rij ⎞ pbo2 ⎤ ⎡ ⎛ rij ⎞ pbo4 ⎤ = exp⎢pbo1 ⎜ σ ⎟ ⎥ + exp⎢pbo3 ⎜ π ⎟ ⎥ ⎢⎣ ⎝ r0 ⎠ ⎥⎦ ⎢⎣ ⎝ r0 ⎠ ⎥⎦ ⎡ ⎛ rij ⎞ pbo6 ⎤ + exp⎢pbo5 ⎜ ππ ⎟ ⎥ ⎢⎣ ⎝ r0 ⎠ ⎥⎦

⎡ 1 Φ(ω) = ⎢ ⎣ 2

(2)

⎤2 (4)

The system was first thermalized during a time of 20 ps. After the system reached thermal equilibrium, the thermostat was removed and the system was placed in an adiabatic box, where it was allowed to evolve during a time of 500 ps with a constant energy (microcanonical or NVE ensemble where the system is isolated and preserves its number of moles (N), volume (V), and energy (E)) and where the atom velocities in the carbon chain were recorded every 10−3 ps. The function Z(t) in eq 4 is the autocorrelation of the velocities recorded while the system evolves within a constant energy (NVE) scheme and is described by

Esystem = E bond + Eover + Eunder + Eval + Epen + Etors + Econj + EvdW + ECoul

∞

∫−∞ dt eiωt Z(t )⎥⎦

(3)

The structural changes in the carbon chains due to the external pressure increase were studied dynamically through molecular dynamics (MD) simulations. We used the ReaxFF model system as implemented in the Large-Scale Atomic/ Molecular Massively Parallel Simulator (LAMMPS) code.43,44 We used CNTs that are periodic along the tube axial direction and pairs of carbon chains of different sizes (two chains with 19 atoms (19−19), one chain with 19 and the other with 20 atoms (19−20), and two chains with 20 atoms each (20−20)). We used a (5,5) CNT with a diameter d = 6.8 Å and length l = 73.6 Å. The tube diameter was chosen to fit the carbon chains inside the tube and to reproduce a typical configuration obtained experimentally when the carbon chain is inside a MWCNT. The simulation box has dimensions of 32 × 32 × 73.6 Å3. In the nonperiodic directions, the box edges are constrained to deflect the atoms once the atoms are going to pass through the edges of the box. The atoms are elastically scattered and have no box wall in the periodic direction. Water molecules fill the space between the outside of the CNT and the inside of the box limits. A typical cross section view of the system is presented in Figure 1, in which the linear carbon chains (colored yellow) are in the middle, the CNT is colored gray

Z (t ) =

⟨v(0) ·v(t )⟩ ⟨v(0) ·v(0)⟩

(5)

The vibrational density of states was calculated for carbon chains with different lengths, ranging from 19 to 40 atoms.

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RESULTS AND DISCUSSION The Raman spectra of linear carbon chains encapsulated in the innermost shell of MWCNTs are shown in Figure 2 using the laser excitation energies 1.58, 1.96, 2.33, and 2.54 eV. All measured points (more than 20 different spots) over the sample using this laser energy showed the band at about 1850 cm−1 (labeled as Cn in Figure 2) associated with the carbon chain. This Cn band always appears with the same line shape

Figure 1. Carbon nanotube immersed in a water environment. A cross section of the simulation box showing the linear carbon chain (indicated in yellow) placed inside the (5,5) CNT (colored in gray) and immersed in water (colored in red and blue).

Figure 2. Average Raman spectra at ambient pressure obtained at different points of a sample showing linear carbon chains (Cn band) inside carbon nanotubes excited with different laser energies. All of the spectra are normalized to the peak intensity of the G band. 10671

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

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The Journal of Physical Chemistry C for Elaser = 1.96 eV. The Cn mode from the chain is more intense than the G band from the MWCNTs, thus indicating a strong resonance process when the chain and MWCNTs are excited with Elaser = 1.96 eV. The peaks characteristic of carbon nanotubes (D and G bands) are also shown in Figure 2. In fact, the band associated with the carbon chain is actually formed by four peaks according to the line shape analysis (discussed in the high-pressure experiments in the next paragraphs). By using only Raman spectroscopy, it is not possible to address the geometry of the carbon chain confined in the central hollow part of the nanotubes and the determination of what length that particular carbon chain actually has. However, some studies reported in the literature have attempted to characterize similar systems by means of Raman spectroscopy. Kurti et al.45 performed theoretical calculations of the Raman spectra of molecules containing linear carbon chain segments to model cumulene (···CCC···) and polyyne (···CC C···) structures. As a result, they predicted the presence of an intense peak at about 2000 cm−1, which corresponds to the stretching mode of the linear carbon chain. This result was confirmed by the experimental data recorded in nanostructured carbon films,46 which showed the presence of modes in this frequency range by using the Elaser = 2.33 eV excitation and observing contributions from different polyyne and cumulene species. Our experimental data described here do not show Raman modes around 2000 cm−1 for the 2.33 eV laser energy. It is known that the polyyne configuration is less stable than the cumulene, because the latter has a degenerate pair of halffilled energy bands that experience a Peirels distortion.47,48 Also, the polyyne structure is unstable under ambient conditions.49 However, when these structures are encapsulated into a carbon nanotube core, these carbon chains become considerably more stable, even when subjected to high temperatures.13 This is due to the more favorable nanoscale confinement provided by the carbon nanotube cage.12 The observation of intense Raman modes at around 1850 cm−1 has been reported in the literature for linear chains (cumulene or polyyne structures) encapsulated into carbon nanotubes.9,13,50 These modes were attributed to the vibrations of onedimensional carbon chains. It is difficult to distinguish between cumulene and polyyne structures on the basis of Raman spectra because the frequencies also depend on the chain lengths.45 We can observe in Figure 2 that the mean frequency of the vibrational bands from the carbon chains increases as the laser excitation energy increases. This upshift can be associated with shorter chains whose carbon−carbon bond strength (assuming the same bonding type) is expected to be weaker than for longer chain lengths. To understand the behavior of the chains inside MWCNTs when they are subjected to pressure variations, we show in Figure 3 the Raman spectrum obtained for different pressure values. Because this mode was observed along the entire sample, and its intensity was the highest at 1.96 eV for all tested lasers, we used this laser excitation energy for studying the carbon chains under high-pressure conditions in which the limited sample volume leads to a low-intensity Raman signal. We notice here that as the pressure increases, the G band frequency value increases (hardens) whereas the band at 1850 cm−1 downshifts (softens). The pressure dependence observed for the G band frequency is well documented in the literature for different sp2 carbon structures.31,51,52 The pressure coefficients (∂ωG)/(∂P) reported for the multiwall carbon nanotubes measured here are almost the same as those obtained for graphite (see Figure 4).53

Figure 3. Observed Raman spectra measured with Elaser = 1.96 eV as a function of the pressure applied to linear carbon chains encapsulated in MWCNTs.

Figure 4. Comparison between the behavior of the G band of the sample under study and of the G band of a graphite sample when both are subjected to high pressure. The data for graphite were taken from ref 53.

In the case of the band around 1850 cm−1 after the line shape analysis, it is possible to construct the frequency versus pressure plots for the three peaks assigned to the chains during the compression (Figure 5). In Figure 6 we show the behavior of the sample during the decompression processes at pressure values of 9, 6, 3, and 0 GPa and outside the cell after decompression. We can clearly see from Figure 6 that the sample exhibits a tendency to return to its original conformation, thus increasing the intensity of the G band and recovering the line shape of the band at about 1850 cm−1. Just one pressure cycle was run due to difficulties in the experiments. First, the metal piece used for placing the sample lost its elasticity and became plastic, which did not allow us to run the hydrostatic pressure experiment again. Second, to run the experiment on the same sample, it is necessary to recover the sample from the pressure cell after one pressure cycle and to prepare another sample for loading. This is difficult to do because the sample is tiny and we have no guarantee that we are going to measure in the new cycle the same part of the sample that was previously measured. This point is important because the sample is not homogeneous (as we discussed above). However, by comparing the Raman spectra of the sample before and after the high-pressure experiments, we can observe that the overall spectrum is not completely recovered, thus 10672

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

The Journal of Physical Chemistry C

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ATOMISTIC MODELING To understand the pressure-induced red shifts in the vibrational bands of the chain after the pressure cycle, we ran atomistic simulations. Because longer chains have lower frequencies, we propose that the observed red shifts come from the coalescence between two or more chains that are confined within the nanotube core. This hypothesis was tested and validated by the atomistic MD simulations. However, the observed changes were not phase-transition-like because in the experiment we have a distribution of chain lengths, and the coalescence can occur over a range of pressure values. These aspects were taken into account in the atomistic simulations considering two chains of different lengths inside a single-walled nanotube. To study the chain coalescence, we considered two carbon chains within a given tube, each having even (20−20 atoms), odd (19−19 atoms), and mixed (19−20 atoms) numbers of carbon atoms confined inside an infinite tube with periodic boundary conditions along the main tube axis. Figure 7a shows the two

Figure 5. Frequency dependence of the Raman peaks around 1850 cm−1 with the pressure applied to the linear carbon chains inside the MWCNTs. Solid (compression) and open (decompression) symbols stand for data taken during the compression and decompression processes, respectively. The dotted lines are intended to guide the eye.

Figure 6. Raman spectra measured with Elaser = 1.96 eV, recorded at 9, 6, 3, and 0 GPa and outside the cell before compression. Figure 7. Representation of the linear carbon chain (in yellow) confined inside the carbon nanotubes. Some atoms belonging to the tubes were made transparent to facilitate the visualization. (a) The red double arrow indicates the chain separation before the increase of the external pressure value. (b) Pressure induces the coalescence of the two chains.

suggesting that the pressure cycle has induced some irreversible changes, and also that the linear carbon chains have possibly undergone some modification in their structure at high pressure values. This assumption is based on the downshift (about 13 cm−1 for the highest intense peak of the chain) associated with the chain band after the pressure loading in comparison to the spectrum obtained before the pressure loading. This shift in peak position may be associated with some structural changes experienced by the linear chain resulting from the pressure loading experiment. It is expected that longer carbon chains will show a lower frequency in the vibrational peaks as discussed above, and our hypothesis is that after the pressure loading, the average length of the carbon chains has increased. This is possibly due to a pressure-induced coalescence of the individual chains forming longer individual chains. In addition to this shift, we also observed that the profile of the band is slightly different for samples exposed to high pressure cycles, which confirms the structural changes in the carbon chain. To validate our hypothesis and get further insights into the pressure effects in the carbon chain structure, we performed atomistic modeling using reactive molecular dynamics simulations, which is discussed in the next section.

chains (colored in yellow) confined inside a (5,5) carbon nanotube; some atoms were made transparent to facilitate the visualization. The red arrow indicates that initially there was a distance between the two chains (the distance between the C ending atoms of adjacent chains shown by a red double arrow), which are kept separated before the compression process starts. Although the external pressure is increased, the reaction between the two chains can be identified by an abrupt decrease in the chain potential energy, as shown in Figure 8 at 60 ps. The atomic structure of the new chain is shown in Figure 7b. We observed that the coalescence of the two chain pairs composed of 19−20 (red) atoms occurs at a much lower pressure (0.6 GPa) in comparison to that of the 19−19 (green) and 20−20 (blue) atom chain pairs (10.2 and 11.4 GPa, respectively). Also, the energy variation for the chain pair composed of 19−20 atoms is about 1.4 times larger than that for the other chain pairs (19−19 and 20−20), thus leading to a 10673

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

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The Journal of Physical Chemistry C

Figure 9. Distance values between the two carbon atoms that participate directly in the coalescence process (19−19 atom carbon chain). During the pressure increase stages (t = 0 until 1200 ps), the two chains show a bouncing movement until the pressure reaches a value that is high enough to induce the coalescence between the chains (t > 1200 ps).

Figure 8. Evolution of the chain potential energy during the hydrostatic compression. Red, green, and blue curves show the chain potential energy during the coalescence process for pair chain lengths of 19−20, 19−19, and 20−20 atoms, respectively. The yellow filled curve represents the system pressure evolution with time.

more stable configuration when the two chains are joined. The coalescence procedure causes the chain energy to drop by around 3.5 kcal/mol/atom (around 136 kcal/mol for the considered chain lengths). The obtained value is in good agreement with a previous value reported by Kertesz et al.35 and points out that the aggregation reaction is not reversible under the investigated conditions. In the atomistic simulations we did not observe any reaction between the chain and the tube walls for pressure values up to 50 GPa. Besides the energy drop (shown in Figure 8), the coalescence between two adjacent chains can be identified by measuring the distance between the two chains (the red double arrow in Figure 7a). By measuring the distance between the terminating atoms of the chains while the external hydrostatic pressure is increased, we observed that the two chains executed a large back and forth movement, as indicated in Figure 9. This implies that the natural chain movement inside the CNT due to the thermal energy is not enough to overcome the reaction barrier to the coalescence between two chains. The back and forth movements continuously exist until the external pressure value is high enough and the conditions become favorable for the reaction that joins the two chains to occur. The energy barrier for merging the two linear chains is estimated to be around 40 kcal/mol.35 This energy is provided to the chains by the external pressure applied to the system. Once the reaction occurs, the distance between the two former outermost carbon atoms reaches a constant value of 1.2 Å, which is characteristic of a CC bond. Larger chains have lower vibrational frequency energies, which is consistent with the experimental observations (Figure 6). Before submitting the system to an external hydrostatic pressure, the CNT is filled by chains of small length (around 10 atoms), on average. Calculating the vibrational spectra for a carbon chain with a length of 9 atoms, we observed an intense peak around 1870 cm−1 (Figure 10). The applied external pressure induces the coalescence between neighbor chains, as shown before, and increases the chain length in general. Yang et al.54 showed that longer chains have a characteristic frequency

Figure 10. Vibrational density of the states for linear carbon chains (LCC) of different lengths. Increasing the carbon chain length from 9 (black trace) to 40 (blue trace) atoms red-shifts the strongest peak from 1870 to around 1850 cm−1.

around 1850 cm‑1 (red shifted in comparison with the frequency of the 9 atoms length chain). The peak around 1660 cm−1 could be an artifact of the force field used and should not be considered in the analysis. The red shift in the most intense peak in the initial vibrational spectrum, when the carbon chain length is increased, indicates that the C−C bonds become softer when the chain size increases.

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CONCLUSIONS In this work, we studied the behavior of linear carbon chains encapsulated inside of carbon nanotubes under high-pressure conditions (up to 10 GPa). We investigated the vibrational properties of this hybrid Cn@MWCNT material as well as the structural changes occurring under externally applied hydrostatic pressure. The enclosed chains are protected by the surrounding carbon nanotube layers in the MWCNT, which makes it difficult to directly access the chains by several experimental techniques. We showed that high-pressure Raman spectroscopy can provide a sensitive experimental probe for studying these systems because the Raman signal is very strong due to resonance conditions, and the vibrational frequencies ωCn are very sensitive to any structural and electronic changes induced in the chains. Upon applying external pressure, we 10674

DOI: 10.1021/acs.jpcc.5b00902 J. Phys. Chem. C 2015, 119, 10669−10676

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The Journal of Physical Chemistry C observed that the carbon−carbon bond in the sp2 lattice (carbon nanotube) becomes stronger, while the carbon−carbon bond in the sp lattice (1D chain) softens. Furthermore, pressure can induce irreversible structural changes in the linear carbon chains, as revealed by the red shift in the vibrational modes when the pressure is released. These results have been interpreted as being due to the coalescence of different carbon chains, and this hypothesis is supported by the results obtained by fully atomistic reactive molecular dynamics simulations.

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AUTHOR INFORMATION

Corresponding Author

*A.G.S.F. E-mail: agsf@fisica.ufc.br. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.F.A., P.T.C.F., A.G.S.F., A.L.A., G.B., and D.S.G. acknowledge funding from the Brazilian agencies CNPq, CAPES, and FAPESP. A.G.S.F. acknowledges CNPq (grant no. 307317/ 2010-2 and INCT NanoBioSimes) and Fundaçaõ Cearense de ́ Apoio ao Desenvolvimento Cientifico e Tecnológico (FUNCAP) through PRONEX (grant no. PR2-0054-00022.01.00/ 11). G.B. and D.S.G. thank the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID, grant no. 2013/08293-7. M.S.D. acknowledges financial support from NSF grant DMR1004147. Y.A.K. acknowledges NRF-2014R1A2A1A10050585.

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