SWCNT Systems: A DFT Study

Apr 6, 2010 - For the Eu/SWCNT system, the hollow site on the outer wall of. SWCNT is the most favorable for the adsorption. The charge analysis resul...
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J. Phys. Chem. C 2010, 114, 7733–7737

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Hydrogen Adsorption on Eu/SWCNT Systems: A DFT Study Z. W. Zhang, J. C. Li, and Q. Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin UniVersity, Changchun 130022, China ReceiVed: January 2, 2010; ReVised Manuscript ReceiVed: March 12, 2010

Within the first-principles density functional theory, we investigate the interaction between H2 and Eu-doped single-walled carbon nanotubes (SWCNT). For the Eu/SWCNT system, the hollow site on the outer wall of SWCNT is the most favorable for the adsorption. The charge analysis results show that two 6s electrons in Eu transfer to SWCNT while 4f electrons remain in Eu, and the Eu atom is thus divalent. The results indicate that five H2 per Eu atom can be adsorbed in the Eu/SWCNT system while 4.44 wt % H2 can be stored in the Eu3/SWCNT system. The interaction between H2 and Eu/SWCNT is balanced by the electronic hybridization and electrostatic interactions. 1. Introduction Hydrogen as a promising alternative fuel is widely considered as an economical, nonpolluting, and renewable source of energy.1 The current state of the art is at an impasse in providing any material that meets the target of 6.0 wt % loaded hydrogen from the U.S. Department of Energy (DOE) or more required for practical applications.2,3 Nanostructured carbon materials with chemical stability and optimal density, such as carbon nanotubes (CNT), are one of the selections for hydrogen storage materials.4–6 Since CNT or single-walled CNT (SWCNT) adsorbs H2 molecules physically with the adsorption energy of Ead ≈ 0.03 eV,7 metal doping must be carried out to enhance the chemical reactivity of carbon surfaces where Ead increases to 0.2-0.4 eV.8–10 It is known that intercalated alkali atoms in CNT would significantly enhance the hydrogen storage.11 However, the corresponding Ead value is small due to the unstable physisorption state at the ambient condition. When the doping metals are transition metals (TM), such as the Ni/CNT system, Ead reaches -0.26 eV/H2 and the adsorption amount of H2 reaches 10 wt % at the ambient condition. The adsorbed Ni atoms on CNTs, however, prefer to assemble forming clusters with about 1 nm diameters.12 For the Ti/CNT case, although there is a storage ability of H2 of 7.7 wt %,13 Ti on CNT forms continuous chains with strong Ti-Ti interaction, which reduces hydrogen binding.14 RE doped C60 and CNT are other choices for hydrogen storage. Suzuk15 and Deng16 and their colleagues have studied the geometric and electronic structures of Eu/C60, Sm/C60, and Sm/C59, respectively. Wang et al.17 found that Eu prefers to adsorb at the hollow site of the hexagonal ring on the outer surface of the armchair CNT. An important advantage of RE (such as Eu) doping on CNT is that the electronic characteristics originating from the unpaired 4f electrons could serve as electron donors and thus increase Ead for H2 on CNT while this important H2/Eu/CNT system has not been systematically studied. Yoon et al.18 studied the interaction between H2 and La encapsulated carbon fullerenes. In despite of the high charge * To whom correspondence should be addressed. Phone: +86 431 85095371. Fax: +86 431 85095876, E-mail: [email protected].

transfer from La to the carbon cage, Ead changes slightly since the transferred charge is highly localized near La ion and inside the cage, making the electric field of dipolar nature feel relatively weak outside the cage. Therefore, the metallofullerenes are not promising materials as high-capacity hydrogen storage medias. If the transferred charge is distributed over the outside of CNT and thus influences a relatively large region, there may be an electric field large enough near a H2 to increase Ead. This could be realized by locating the RE atoms on the outer surface of CNT. When H2 molecules are bound on RE atoms, electron transfers from H2 to RE, filling an acceptor-like state. As the electron configuration of Eu is [Xe]4f76s2, there are a large number of empty 4f orbital, and thus more H2 could be adsorbed. On the basis of the above discussion, one may hypothesize that the uptake capacity will increase if more Eu atoms are added. In addition, the binding would be strengthened if more charges are transferred between Eu and SWCNT. Obviously, the latter can also be addressed by adding more dopants with concomitant additional charges available for the electronic transfer. To examine the validity of this hypothesis, the Eu3/ SWCNT system, where the subscript indicates the number of the atom in a unit cell, is established in the present study. Our goal is to provide a new fundamental insight into the underlying mechanisms as well as the physical properties required for hydrogen storage. In this paper, we examine the hydrogen storage ability of the H2/Eu/SWCNT system through investigating the interaction, the bonding characteristics, and the adsorption ability with changes of Eu concentration. In particular, we focus on the role of transferred charges between Eu and SWCNT on the affinity of H2. Our results show that the hydrogen storage ability of SWCNT can be enhanced by doping more Eu atoms. Since metal atoms could merge into isolated particles once they meet each other, we also probe the case of Eu dimer. 2. Computational Framework Density functional theory (DFT) methods, implemented in the DMol3 package,19,20 are used to study the H2/Eu/SWCNT system, which has been widely utilized for SWCNT with functionalization of TM atoms.21,22 Spin unrestricted DFT in the generalized gradient approximation (GGA) with the

10.1021/jp100017y  2010 American Chemical Society Published on Web 04/06/2010

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Figure 1. Top and side views of the optimized structures of (a) the Eu/SWCNT system and (b) the Eu3/SWCNT system. The blue and gray spheres represent Eu and C atoms, respectively.

Perdew-Burke-Emzerhof (PBE) exchange-correlation functionals approach and Double Numerical plus polarization (DNP) atomic orbitals were taken as basis sets.23 Previous studies have shown that GGA prediction of the heavy element containing f electrons is in good agreement with experiments24,25 and is also better than the LDA+U calculation for the system doped with Eu. The reliability of GGA can be ascribed to that in most RE metals, DFT calculations in the GGA, including spin polarization, describe properly the strong Coulomb correction, because GGA treats the nonlocality of exchange-correlation better than LDA.26–28 For a cross check, we repeated part of calculations with the local density approximation (LDA) of Perdew-Wang (PWC) form.29 The DFT semicore pseudopotentials (DSPP) treatment was implemented for relativistic effects, which replaces core electrons by a single effective potential.30 It is known that relativistic effects play an important role in the chemical and physical properties of molecules containing heavier elements, such as Eu. We have applied the super tetragonal unit cell with the edge lengths of a and b of lattices of 20 Å, which is large enough to neglect the interaction between a nanotube and its periodic images. The c-lattice length aligned with the axis of the nanotube is tuned to match the periodic condition. To keep a reasonably large distance between the adsorbed atoms along the tube axis, two primitive cells are used for the zigzag (6,0) CNT. The Brillouin zone of the supercell was sampled by 1×1×6 k-points within the Monkhorst-Pack scheme.31 Full structural optimizations were obtained with symmetry constraints; by using a convergence tolerance of energy of 2.0 × 10-5 hartree (1 hartree ) 27.2114 eV), a maximum force is 0.004 hartree/Å and a maximum displacement of 0.005 Å. The orbital cutoff was set to be global with a value of 5.8 Å, and smearing was 0.002 Ha. For comparison, the identical simulation parameters were employed in all simulations. For H2 adsorption on the Eu/SWCNT system, we considered five possible H2 adsorption initial sites, i.e., the H site is above the hexagon, the B1 and B2 sites are above the midpoint of the zigzag and axial C-C bonds, and the T1 and T2 sites are on top of a C atom, as described in Figure 1a. The average adsorption energy Ead per H2 to the system is determined by,

Ead ) [Et - EmEu+C - nEH]/n

(1)

where n indicates the number of H2 molecules. The subscripts t, mEu+C, and H denote the total amount of the considered system, the corresponding storage material with the number of Eu atoms m, and the free H2 molecules, respectively. The negative value of Ead indicates that the adsorption is exothermic and hence stable. The adsorption mechanisms are analyzed in

terms of the Mulliken population analysis and partial density of states (PDOS) plots, which can provide a clear and definitive description for charge redistribution. As the coverage becomes larger, the electrostatic repulsion increases with shortening of the Eu-Eu distance, which makes the system unstable. However, the surface curvature enlarges the Eu-Eu distance and thus enhances the stability of adsorbates with high coverage for narrow tubes. To achieve a stable structure, our doping follows the rule for the high metal covered nanotubes: If one hexagon has a dopant, all neighboring ones do not.32,33 Adsorption of a single layer of H2 molecules, where more than one H2 molecule per graphitic hexagon is covered, is unfeasible because the crowding of the nanotube surface leads to strong repulsive interactions between neighboring molecules.34 Twenty four H2 molecules are adsorbed around the above Eu3/ SWCNT model, where all initial molecules are parallel to SWCNT with the highest H2 uptake capacity. For simplicity, this system is called (H2)24/Eu3/SWCNT, which may render a gravimetric storage as high as 4.44 wt %. Moreover, the oneEu-doped (Eu/SWCNT) storage system is also simulated for comparison purposes. 3. Results and Discussion A single Eu atom prefers to adsorb strongly at the H site, as shown in Figure 1a. The Eu-C distance lEu-C is 2.48 Å, with the binding strength Eb ) 2.01 eV. The Mulliken charge analysis shows that Eu carries a 1.005 e positive charge, indicating the Eu atom is ionized and suggesting a possibility for H2 adsorption due to the polarization mechanism.35 The two nearest C atoms each get 0.264 e charge and four next-nearest C atoms are charged with 0.008 e. The charge around Eu clearly shows incomplete charge transfer from Eu to SWCNT, implying the presence of the hybridization between Eu and SWCNT. Mulliken population analysis shows changes of charge distribution on the atomic orbitals of Eu and C before and after doping. The electronic configurations of Eu and the two nearest C are [Xe]4f76s2 and [He]2s1.2722p2.660 separately before the doping. They become [Xe]4f6.9975d0.6936s0.2366p0.069 and [He]2s1.3062p2.890 after the doping. The interaction between Eu and SWCNT is primarily ionic due to the charge transfer from Eu-6s to C-2p orbital where the most 6s electrons transfer to π antibonding orbital of the C hexagonal ring. The hybridization essentially occurs among Eu-6s, Eu-5d, and C-2p orbitals. In fact, the 4f orbital of Eu also attends the electronic transitions although it is well shielded by the peripheral 6s and 5d ones. The occupation of 0.693 e and 0.069 e on Eu 5d and 6p orbitals is ascribed to a contribution of back-donation of the occupied orbitals of SWCNT. After this correction, the charge transfer from Eu to CNT is about 1.767 e, and the Eu atom is divalent

A DFT Study of Eu/SWCNT Systems

Figure 2. (a) Two Eu atoms aggregated on pristine SWCNT. (b) Two Eu atoms individually attached on pristine SWCNT. The total energy of the lower energy structure between aggregated and unaggregated structures is set to zero.

for Eu/SWCNT system. In addition, minor electron accumulation forms a weak covalent bond between Eu and proximal C atoms. It is also found that 0.068 e occupies the C-3d orbital of SWCNTs, which is independent of the doping of Eu and is about 5% of that of the C-2s orbital. Thus, the effect of the C-3d orbital is considered to be nearly negligible. The generation of this phenomenon could be caused by the orbital projection. It is noteworthy that our calculated binding energy for Eu on SWCNT (2.01 eV) is larger than the calculated cohesive energy of bulk Eu, 1.69 eV (1.86 eV experimental value). Since the energy gained from cohesion is much lower than that from binding to SWCNT, clustering of Eu atoms would not occur on SWCNT. To provide a more explicit analysis, we further calculate the binding energy of the Eu dimer and two isolated Eu atoms on SWCNT, as shown in Figure 2. Parts a and b of Figure 2 show the optimized geometries for two aggregated Eu and for individually isolated Eu on SWCNT where the isolated case is energetically lower by 0.16 eV (per 2 Eu atoms) as compared with the aggregated case, providing ample evidence that Eu atoms do not cluster on SWCNT. This is ascribed to a significantly larger binding energy of Eu to the H site than the calculated Eu-dimer energy (0.30 eV). It is suggested that dispersed strong binding sites should exist in absorbents for better metal dispersions, particularly at low metal coverage. We further show the stable Eu adsorbates with a high coverage ratio of 1/8 on (6,0) SWCNT without the tendency for clustering in Figure 1b. Three Eu atoms are placed on the H site of SWCNT to obtain Eu3/SWCNT. After the full relaxation, all three Eu atoms still bind separately on the site without Eu aggregation due to a stronger metal-nanotube interaction with lEu-C ) 2.54 Å.36 The Mulliken charge analysis shows that Eu carries an average 0.756 e positive charge in the Eu3/SWCNT system. Eb ) 1.68 eV/Eu on average, which is lower than that in the Eu/SWCNT system. The minimum of lEu-Eu is 7.91 Å, which is almost two times that of the bulk lEu-Eu (3.97 Å). Thus, this adsorbate with a coverage ratio of 1/8 is stable. The relaxed geometry for the (H2)24/Eu3/SWCNT system is shown in Figure 3. Some regularity can be observed in the configurations while H2 is adsorbed not only on Eu atoms (region 1), but also on SWCNT (region 2). Three H2 molecules named “A”, “B”, and “C” are located at Atop, Bridge, and Hollow sites, respectively. H2A is still parallel to the dopant while H2B spontaneously tilts after relaxation, and is parallel to the zigzag C-C bond. The bond axis of H2C is oriented perpendicularly to the hexagonal ring of SWCNT since H2C is located at the middle point of the two dopants with symmetrical interactions. In addition, the distances between H2 and Eu have the sequence of C > A > B, which implies that H2C (or H2B) should have the weakest (or strongest) binding in the three sites.

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Figure 3. Top view (a) and side view (b) for the optimized atomic structure of maximal number of adsorbed H2 molecules for the Eu atoms doped SWCNT. Two insets show the fine views of regions 1 and 2, which indicate the areas close to and far from Eu atoms, respectively. The blue and gray spheres have the same meaning in Figure 1, and the white one shows H.

Figure 4. The relaxed hydrogen storage systems based on H2 adsorption on the Eu/SWCNT system with different numbers (n). Panels a-f show the optimized structures when n increases from 1 to 6.

In region 1, one Eu atom can adsorb up to five H2. In region 2, H2C moves to the center of the hexagonal ring and lies perpendicular to the tube. Ead ) -0.09 eV from GGA and -0.20 eV from LDA, exhibiting a physisorption. H2 adsorbs more strongly on the Eu/SWCNT system than on SWCNT due to the polarization of H2 and the charge presence in C induced by Eu. The Ead value is appropriate to be utilized for hydrogen storage. Ead ranges from -0.09 to -0.24 eV or from -0.20 to -0.42 eV depending on GGA or LDA, respectively. The transferred charges from Eu to CNT may produce a nonzero electric field near Eu, inducing a short-range effect. The high coverage influences a relatively large region where an electric field near a H2 molecule is large enough to increase the magnitude of Ead. H2 will then be polarized due to the doping-induced charge redistribution and Ead will be increased consequently. The enhanced Ead as well as the small amount of charge transfer indicates that the interaction between H2 and CNT is no longer pure van der Waals force after incorporating metal dopants. Similar phenomena have been observed in Lidoped CNT and C60.37,38 As Eu coverage reaches 1/8, see Figure 1b, 24 H2 with Ead ) -0.16 eV/H2 at GGA and -0.30 eV at LDA can be mostly adsorbed while DFT-GGA calculations usually underestimate while LDA calculations overestimate Ead.39,40 The true Ead could

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TABLE 1: The Adsorption Energy Ead Values in eV/H2 for H2 Adsorption on the Eu/SWCNT System with Different Number n Calculated by GGA-PBE and LDA-PWC, Respectively, and the Amount of Charge Transfer of Eu Atom in ea model H2/Eu/SWCNT (H2)2/Eu/SWCNT (H2)3/Eu/SWCNT (H2)4/Eu/SWCNT (H2)5/Eu/SWCNT a

functional

charge

first H2

GGA LDA GGA LDA GGA LDA GGA LDA GGA LDA

0.897 0.779 0.788 0.611 0.748 0.529 0.559 0.243 0.498 0.143

-0.26 -0.49 -0.26 -0.49 -0.25 -0.48 -0.24 -0.44 -0.24 -0.43

second H2

-0.26 -0.49 -0.25 -0.48 -0.24 -0.44 -0.24 -0.43

third H2

-0.16 -0.33 -0.24 -0.44 -0.24 -0.43

fourth H2

-0.24 -0.44 -0.24 -0.43

fifth H2

Ead

-0.14 -0.28

-0.26 -0.49 -0.26 -0.49 -0.22 -0.43 -0.24 -0.44 -0.22 -0.40

The last column denotes the average results of Ead.

approach the lowest requirement of -0.20 eV/H2 as proposed by the DOE. The corresponding hydrogen-storage capacity is 4.44 wt %. Figure 4 shows the interaction behaviors of H2 and Eu/ SWCNT. The Ead value and the amount of electron transfer calculated are summarized in Table 1. Both GGA and LDA results are given for comparison. When one H2 is around the adsorbed Eu atom, H2 is molecularly adsorbed with Ead ) -0.26 eV for GGA and -0.49 eV for LDA, as shown in Figure 4a. lH-H, lEu-H, and the shortest lEu-C are respectively 0.77, 2.54, and 2.50 Å. This lH-H is 0.02 Å longer than that of a free H2 molecule (0.75 Å) and lEu-C is 0.02 Å longer than that in Eu/ SWCNT (2.48 Å), implying the weakness of the H-H and Eu-C bonds. Panels b-e in Figure 4 illustrate the top views of the equilibrium configurations for the adsorption of two to five H2 on the Eu/SWCNT system, respectively. When the sixth H2 is added onto the (H2)5/Eu/SWCNT system, it moves away (see Figure 4f). Thus, at most five H2 can be adsorbed around one Eu. lEu-C ) 2.52 Å while lH-H varies from 0.75 to 0.77 Å and lEu-H for the sixth H2 is 3.14 Å and Ead ) -0.07 eV for GGA, exhibiting a weak physisorption. As listed in Table 1, Ead slightly reduces from -0.26 to -0.22 eV/H2 or from -0.49 to -0.40 eV/H2 depending on GGA or LDA, respectively, which may be due to the steric repulsion as n changes. Meanwhile the charge on Eu decreases with increased n, and the electric field around Eu also decreases. This, in turn, results in the distance drop between adsorbed Eu and polarized H2. The charge on CNT decreases as well. A single Eu on CNT can adsorb up to five H2 molecules with Ead ≈ -0.22 eV/H2 at GGA and -0.40 eV/H2 at LDA, similar to the case of the Ca/ CNT system.41 GGA calculations give similar trends as those obtained with LDA although Ead values of the former are only a half of the later. They both give respectively an upper (lower) bound of van der Waals type binding of H2 while the real Ead value may lie in between.39,40 These optimal Ead values make hydrogen adsorption and desorption feasible at the ambient condition, which is critical for practical applications. The charge transfer from Eu to SWCNT gives rise to electric fields surrounding SWCNT and enhances the H2 adsorption. As a H2 approaches Eu, Eu accepts electrons from H2 while subsequently Eu transfers them to C atoms. This is indicated by the electronic configuration of Eu of [Xe]4f6.9915d0.7326s0.2506p0.130 in the H2/Eu/SWCNT system. For the (H2)5/Eu/SWCNT system, the electronic configuration of Eu is [Xe]4f6.9695d0.9286s0.3096p0.296, four other H2 molecules thus add partial charge to Eu and occupy more orbitals of 5d and 6p subshells of Eu. Eu has difficulty donating electrons to H2, but polarizes H2 and binds them by electrostatic interaction. The adsorbed H2 molecules in all systems have positive charges.

Figure 5. The partial density of states (PDOS) plots for C atoms, Eu atom, and H2 molecules of the (H2)5/Eu/SWCNT system in Figure 4e. The Fermi level is set to zero and indicated by a dotted line.

The Eu center in Eu/SWCNT bears a positive charge. As more H2 molecules are adsorbed, the Eu becomes less positively charged. The charge change on the Eu is due to the electron donation from the dihydrogen bond to Eu. Consequently, the H-H bonds are elongated, and H2 is mainly bound by the charge polarization mechanism with a small amount of electron transfer to Eu. This “insensitivity” in Ead as a function of n is also a consequence of the mechanism that binds H2, namely the positive charge of the cation polarizes H2, and the bonding results from an electrostatic interaction. This mechanism has also been demonstrated for TM ions where the binding of H2 to the metal ion is even larger, namely about -0.5 eV/H2.42,43 Thus, the key to H2 adsorption with binding energies at proper physisorbed states is to find a system where the positively charged state of metal atoms can remain. As n increases, Eu is close to a limit state, which in turn reduces polarization. The PDOS for (H2)5/Eu/SWCNT is shown in Figure 5. The overlap of density state between H-1s orbital and Eu-4f, -5d, -6s, -6p orbitals appears at about 8 eV below the Fermi level EF, where the hybridization takes place. This is especially evident between Eu-5d, Eu-6p, and H-1s orbitals. There are three peaks between -9 and 0 eV in PDOS of the Eu-5d orbital. The first two peaks around -0.5 and -2 eV correspond to the hybridization of Eu-5d and C-2p orbitals, which is responsible for the bonding of Eu and SWCNT. The third peak is related to the hybridization among Eu-5d, C-2p, and H-1s orbitals. This suggests that Eu acts as a “bridge” in this reaction, which interacts with H2 and SWCNT simultaneously. This result rationalizes the effect of Eu dopants on hydrogen storage. The

A DFT Study of Eu/SWCNT Systems PDOS of the Eu-4f orbital is much localized with a sharp peak. The PDOS of Eu-5d, -6s, and -6p orbitals are continuous at EF, and the PDOS of Eu-5d, -6s, and -6p and C-2p have similar peaks and characteristics, which implies hybridization among them, being similar to the electronic structures of Eu/C60.44 Thus, the electronic hybridization of Eu and H2 orbitals leads to partial H2 adsorption to Eu/SWCNT apart from the electrostatic interaction. The former is always favorable to the binding, whereas the latter may change their roles, depending on n. Although there are abundant half-filled Eu-4f orbitals, this Eu4f band exhibits a high stable state, which brings out little electron transfer. As a result, Eu-4f electrons have little impact on the hybridization. It is expected that if the 4f orbital of doped RE elements is not empty, half-filled, or full-filled, the 4f hybridization could begin. 4. Conclusions In summary, the Eu atom prefers to adsorb at the hollow site of the hexagonal ring on the outer surface of SWCNT, with electronic charge transfer to the C atoms and strong bonding. It is found that Eu atoms without tend to form clusters on the surface of SWCNT. Five H2 molecules can bind with the Eu/ SWCNT system where Ead ) -0.22 eV/H2. The (H2)24/Eu3/ SWCNT system is a potential media for hydrogen storage (4.44 wt %) with an Ead of -0.16 eV/H2 from GGA and -0.30 eV/ H2 from LDA. The nature of bonding between Eu and H2 is due to the hybridization of Eu-5d, -6p orbitals with the H-1s orbital. Eu-4f electrons, however, have no contribution on the hybridization due to the stability of the 4f7 state as that of 4f 0 and 4f14 states. Acknowledgment. We acknowledge support by the National Key Basic Research and Development Program (Grant No. 2010CB631001) and by the Program for Changjiang Scholars and Innovative Research Team in University. References and Notes (1) Dresselhaus, M. S.; Williams, K. A.; Eklund, P. C. MRS Bull. 1999, 24, 4550. (2) Coontz, R.; Hanson, B. Science 2004, 305, 957. (3) Chan, S. P.; Chen, G.; Gong, X. G.; Liu, Z. F. Phys. ReV. Lett. 2001, 87, 205502. (4) van Setten, M. J.; de Wijs, G. A.; Brocks, G. Phys. ReV. B 2007, 76, 075125. (5) Cho, J. H.; Park, C. R. Catal. Today 2007, 120, 407. (6) Durgun, E.; Ciraci, S.; Yildirim, T. Phys. ReV. B 2008, 77, 085405. (7) Vidalig, G.; Ihm, G.; Kim, H. Y.; Cole, M. W. Surf. Sci. Rep. 1991, 12, 133. (8) Calbi, M. M.; Cole, M. W.; Gatica, S. M.; Bojan, M. J.; Stan, G. ReV. Mod. Phys. 2001, 73, 857.

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