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Anomalous c-axis shifts and symmetry enhancement in highly oriented pyrolytic graphite at the magic angle
Carbon 150 (2019) 27e31
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Letter to the editor
Anomalous c-axis shifts and symmetry enhancement in highly oriented pyrolytic graphite at the magic angle a b s t r a c t Observation of superconductivity in twisted-bilayer-graphene and highly-oriented-pyrolytic-graphite (HOPG) for certain magic-angles-of-rotation has recently attracted an important attention. Unusual temperature-(T)-induced-shifts in the graphitic c-axis have also been reported in HOPG in conditions of q misfit ~ 1 (first-magic-angle). We report a novel investigation of HOPGs with q misfit of 0.5 , 0.8 , 1.5 , [ 1.5 and of water-treatedturbostratic-graphite in the T-range from 298.15 to 673.15K. Presence of magic angles of rotations cor pattern analyses responding to the reported q misfit values is demonstrated by repeated HRTEM and Moire of HOPG lamellae extracted from individual samples with scotch tape methods. Interestingly in our measurements the c-axis-shift is found to depend strongly on the misfit-angle, with the highest-values of 0.00428 and 0.00426 nm in proximity of the first-magic-angle (q misfit ~ 0.8 and ~0.5 ). Two diffraction-peaks present at ~23.5 and ~48.5 2q (detector angle, in typical Bragg- Brentano configuration) for q misfit ~1 are also found to vanish for q misfit [ 1.5 . These findings imply existence of additional symmetry elements at the magic angle, which are not present in the standard space group notation used for structural characterization of graphite (P63/mmc). © 2019 Elsevier Ltd. All rights reserved.
The recent observation of superconductive effects in bilayer graphene and highly oriented pyrolytic graphite (HOPG) has attracted an important attention in the field of condensed matter physics and materials science [1e15]. Unusual superconductive effects in graphitic based materials were reported already in early 2000 in samples consisting of pyrolytic graphite with random atomiclayer orientation [1], for certain conditions of annealing. In these studies, observation of ferromagnetic and superconducting ordering was reported in un-annealed and annealed samples respectively [1]. In recent works, it has been proposed that quasitwo-dimensional (2D) interfaces in HOPG, described with the BurgerseBraggeReadeShockley (BBRS) dislocation model [5,6], can act as sources of granular superconductivity when certain conditions of rotational misfit-angles of the graphene layers are achieved [1e15]. In these materials, the misfit (rotational) angle between graphene layers has been reported to play a crucial role towards the appearance of such superconductive features [5,6,8,12]. Existence of logarithmic van Hove singularities has been reported for rotational misfit-angles of 1 qtwist 10 and give rise to the patterns with consequent appearance of flat bands in the enMoire ergy dispersion relation spectrum [5,6,8,12,13]. Also, the presence of rhombohedral stacking with relative abundances ranging from 10% to 20% and its interfacial contact with the Bernal A-B graphitic phase has been considered an additional parameter which could induce such superconductive effects [2,4,8]. Together with these interpretations, the presence of a temperature induced unit cell shift https://doi.org/10.1016/j.carbon.2019.05.003 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
in the c-axis of the graphitic unit cell has been reported in different type of carbon systems, namely multiwalled carbon nanotubes, carbon onions, commercial turbostratic graphite and HOPG [16e18]. In particular, appearance of anomalous c-axis shifts (i.e much different than those expected by considering the tabulated thermal expansion parameter reported at 300 K [18]) has been reported in HOPG under first-magic-angle conditions q misfit ~ 1 (0.8 average misfit angle) in the T-range from 298.15 to 673.15K [17]. In this letter we investigate further the T-induced reversible variation of the c-axis in HOPG samples with controlled misfit-angle. We report an in-depth study on the variation of the unit-cell c-axis shift for HOPG samples with average misfit-angles in proximity of first-magicangle, namely 0.5 , 0.8 , 1.5 ± 0:2 and [ 1.5 as confirmed also pattern analyses (ESI Fig.by HRTEM measurements and Moire Supp.26e28). XRD measurements were performed in the T-range from 298.15 to 673.15K. Comparative tests were also performed in water treated commercial turbostratic graphite samples in the same T-range. Our experimental measurements and Rietveld Refinement analyses show that the observed unit cell c-axis shift depends strongly on the misfit-angle, with the highest-values of 0.00428 and 0.00426 nm measured in proximity of the firstmagic-angle (see ESI for typical example of magic angle conditions demonstrated by repeated HRTEM measurements performed on the graphite layers of individual HOPG samples extracted by scotch tape method). Most importantly, note that two unlabeled peaks are found at approx. 23.5 and 48.5 2q (detector angle, in typical
28
Letter to the editor / Carbon 150 (2019) 27e31
Bragg- Brentano configuration) in proximity of first magic angle conditions with an intensity that decreases with the increase of the misfit-angle and almost vanishes for q misfit [ 1.5 . These peaks can no be explained on the basis of the standard database cards with space group P63/mmc typically used for structural analyses of commercial graphite (CIF 1200017 from Crystal Open Database). These observations imply that misfit angle of graphene layers may play a crucial role in controlling structural, symmetry and thermal characteristics of unit-cell c-axis in HOPG. These observations were further confirmed by additional measurements on commercial graphite powder and water treated graphite powders where no such additional symmetry characteristics were detected. HOPG samples with dimensions of 5 5 1 mm and average pattern analyses) of misfit angles (as extracted by repeated Moire 0.5 , 0.8 , 1.5 ± 0:2 and [ 1.5 were purchased from XFNANO, INC. Commercial graphite powder was purchased from Xiya Reagents China Cas number 7782-42-5 and treated with water following the method reported in Ref. [7]. Variable temperature dependent XRD measurements were performed on a Rigaku Smart-lab powder X-ray diffractometer (Cu K-a average, l ¼ 0.15418 nm) with Bragg- Brentano configuration under vacuum values below 7 Pa in the temperature range from 298.15 to 673.15 K, with an Anton Paar TTK450. The HOPG samples were positioned with the c-axis perpendicular to the substrate holder, in analogy to conditions used in Ref. [17]. The measurements were performed in a Ni substrate. Comparative measurements were also performed in HOPG samples with q misfit [ 1.5 positioned with the c-axis perpendicular to a glass substrate. These measurements revealed values of c-axis shifts similar to those extracted in measurements performed with Ni substrate. This is shown in the table of Fig. Supp. 34 (see ESI Figs.Supp. 31e39 for analyses of the comparative measurements performed in HOPG samples with q misfit [ 1.5 positioned in a glass substrate). Additional measurements with c-axis orientation parallel to the substrate were performed at room temperature, see Supp.Fig.42e44. TEM measurements were performed with a 200 kV American FEI Tecnai G2F20 on lamellae extracted with scotch-tape method from the main HOPG sample (these lamellae were extracted after preliminary removal of surface layers). Magnetometry measurements were performed on a VSM Quantum Design system at 300K, 200K and 100K for HOPG samples with average qmisfit of 1.5 and at 350K, 300K and 150K for HOPG samples with average qmisfit [ 1.5 . These measurements revealed presence of superconducting-like hysteresis in samples with average qmisfit of 1.5 but not in samples with average qmisfit [ 1.5 , see ESI Fig.Supp.45,46 (in agreement with structural characterization in Figs. 1e5). Note also that EDX analyses did not reveal presence of significant impurities in the samples (Fig.Supp.47e49). The layered structure of HOPG samples with misfit-angle q misfit of 0.5 was firstly revealed by room temperature XRD measurements, as indicated in Fig. 1, with the observation of preferred 002 and 004 reflections of graphitic carbon with space group P63/mmc. By analyzing the temperature dependent X-ray diffractograms in Fig. 1 for HOPG with q misfit of 0.5 , in the range from 298.15 to 673.15K, it is possible to notice that a significant temperature dependent shift in the position of the 002 and 004 diffraction peaks is present towards lower values of degrees 2q. This observation has similarities with that reported in the case of HOPG samples with q misfit of 0.8 [17]. This significant transition can be observed in the complete plot of Fig. 1 and in the detailed diffractograms of Fig. 2, where the 002 (Fig.2A) and 004 (Fig. 2B) peak-shifts are shown as a function of the temperature. Comparisons with other HOPG samples with different misfit angle are shown in Fig. 3 and in Fig. 4 (see ESI for detailed Rietveld refinement analyses of the graphitic c-axis in these samples and table showing results of
Fig. 1. XRD diffractograms showing the structural shifts of the 002 and 004 reflections of HOPG (0.5 q misfit misfit-angle) as a function of the temperature from 298.15 to 673.15K. (A colour version of this figure can be viewed online.)
Fig. 2. XRD diffractograms showing with a higher detail the structural shifts of the 002 (A) and 004 (B) reflections of HOPG (0.5 q misfit misfit-angle) as a function of the temperature from 298.15 to 673.15K. (A colour version of this figure can be viewed online.)
Letter to the editor / Carbon 150 (2019) 27e31
29
Fig. 3. Plots showing the variation of the unit cell c-axis of HOPG samples with misfit-angle q misfit of 0.5 , 1.5 , [ 1.5 and turbostratic graphite extracted with the Rietveld refinement method (see ESI for detailed analyses) as a function of the temperature from 298.15 to 673.15K. See ESI Fig.Supp.26e28 for typical HRTEM analyses of lamellae extracted periodicities corresponding to q misfit of ~1 and ~1.5 respectively. from HOPG samples with misfit-angle q misfit of 0.8 and 1.5 revealing Moire
Fig. 4. XRD diffractograms showing the variation in the intensity of the 002,004 and unlabeled peaks reflections (see black stars) of HOPG samples with misfit-angleq misfit of 0.5 , 0.8 , 1.5 and [ 1.5 . (A colour version of this figure can be viewed online.)
30
Letter to the editor / Carbon 150 (2019) 27e31
Fig. 5. XRD diffractograms showing the variation in the intensity of the unlabeled peak observed in the region of approximately 23.5 2q as function of the level of misfit-angle in the sample (misfit angle of the graphene layers, as confirmed by HRTEM measurements in ESI). See also ESI Fig.Supp.25 for analyses performed on turbostratic (doped and undoped) graphite powder revealing disappearance of the observed peak. Note the increase of the relative intensity of this reflection in proximity of first magic angle conditions. An additional unlabeled peak is found at approx. 48.5 2q in the analysed HOPG samples with an intensity that decreases with the increase of the misfit-angle, as shown in ESI Fig.Supp. 29, 30. Also this peak is found to vanish in turbostratic graphite powder (see ESI Fig.Supp. 30). (A colour version of this figure can be viewed online.)
Rietveld refinement calculation of c-axis shift, Fig.Supp.24). A significant variation in the c-axis shift was found. Large-shift values of approximately 0.00428 and 0.00426 nm were extracted from HOPGs with average q misfit of 0.8 and 0.5 (in proximity to first magic angle ± 0:2 ). Also, slightly lower shifts of 0.00398 nm were found for q misfit of 1.5 . Instead a much lower shift value of 0.00311 nm could be extracted for q misfit [1.5 . Note that the observed c-axis shift values are much different with respect to those expected if calculated with the previously reported thermal expansion factor of 26.7 106 K1 reported at 300K for graphite [18]. This important difference may be related to variation in the degree of misfit angle of graphite layers or to possible presence of multiple graphite interfaces with different ordering which may then play also a role on the room temperature granular superconductivity characteristics as recently suggested by Scheike T. [7,8]. Further comparisons were then sought with turbostratic graphite, produced with water treated method produced following the method reported by Scheike T. in Ref. [7]. As shown in Fig. 3D, Rietveld refinement analyses of the diffractograms revealed a much weaker variation of the unit-cell c-axis in the same temperature range (0.00183 nm) (see ESI for examples of Rietveld refinement analyses of the graphitic c-axis for all the analysed samples) and absence of additional symmetry characteristics. These observations appear to imply that the higher the degree of turbostratic carbon, the weaker the variation in the unit cell c-axis values with the temperature. In our measurements the highest value of c-axis shift is found for conditions in proximity to first magic angle. Most importantly, two unlabeled peaks are found at approx. 23.5 and 48.5 2q
only in the analysed HOPG samples (and not in water treated and undoped graphite) with an intensity that decreases with the increase of the misfit-angle. Almost complete disappearance of these peaks is interestingly found for q misfit [1.5 together with a dramatic decrease in the intensity of 004 peaks, as shown in Figs. 4 and 5 and ESI Fig.Supp.25, 29, 30. These observations imply the possible presence of magic-angle-induced superlattices symmetries which are observable with XRD when conditions of q misfit ~1 are reached. Note that no such peak features could be detected in turbostratic graphite powder (see ESI Fig.Supp.25, 30). See also Fig. 4 for variation of relative intensities of 002 and 004 reflections with the misfit angle in the analysed HOPGs. In conclusion, in this letter we have investigated the variation of the temperatureinduced unit-cell c-axis shift in HOPG samples with misfit-angles q misfit of 0.5 , 0.8 1.5 and [1.5 in the temperature range from 298.15 to 673.15 K. Additional comparisons were also performed with water treated commercial turbostratic graphite, in the same temperature range. Our results show that the unit cell c-axis shift has maximum values in proximity of first- magic-angle of rotation. Two unlabeled peaks (possibly indicating additional superlattice symmetry characteristics) are also observed at approx. 23.5 and 48.5 2q under first magic angle conditions (q misfit ~1 ) and almost vanish for q misfit [ 1.5 .
Acknowledgments We acknowledge NSFC Grant No 11750110413.
Letter to the editor / Carbon 150 (2019) 27e31
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.05.003. References [1] Y. Kopelevich, P. Esquinazi, J.H.S. Torres, S. Moehlecke, Ferromagnetic- and superconducting-like behavior of graphite, J. Low Temp. Phys. 119 (2000) 691e702. [2] M. Zoraghi, J. Barzola-Quiquia, M. Stiller, P.D. Esquinazi, I. Estrela-Lopis, Influence of interfaces on the transport properties of graphite revealed by nanometer thickness reduction, Carbon 139 (2018) 1074e1084. [3] D. Marchenko, D.V. Evtushinsky, E. Golias, A. Varykhalov, Th Seyller, O. Rader, Extremely flat band in bilayer graphene, Sci. Adv. 4 (2018) 11. [4] C.E. Precker, P.D. Esquinazi, A. Champi, J. Barzola-Quiquia, M. Zoraghi, ~ os-Landin, A. Setzer, W. Bo €hlmann, D. Spemann, J. Meijer, S. Muin T. Muenster, O. Baehre, G. Kloess, H. Beth, Identification of a possible superconducting transition above room temperature in natural graphite crystals, New J. Phys. 18 (2016) 113041. €, Y.V. Lysogorskiy, D.A. Tayurskii, G.E. Volovik, On the [5] P. Esquinazi, T.T. Heikkila superconductivity of graphite interfaces, JETP Lett. (Engl. Transl.) 100 (2014) 336e339. [6] G.E. Volovik, Graphite, graphene, and the flat band superconductivity, JETP Lett. (Engl. Transl.) 107 (2018) 516e517. €hlmann, P. Esquinazi, J. Barzola-Quiquia, A. Ballestar, [7] T. Scheike, W. Bo A. Setzer, Can doping graphite trigger room temperature superconductivity? Evidence for granular high-temperature superconductivity in water-treated graphite powder, Adv. Mater. 24 (2012) 5826. €hlmann, Granular superconductivity at [8] T. Scheike, P. Esquinazi, A. Setzer, W. Bo room temperature in bulk highly oriented pyrolytic graphite samples, Carbon 59 (2013) 140. [9] M. Saad, I.F. Gilmutdinov, A.G. Kiiamov, D.A. Tayurskii, S.I. Nikitin, R.V. Yusupov, Observation of persistent currents in finely dispersed pyrolytic graphite, JETP Lett. (Engl. Transl.) 107 (2018) 37e41. [10] A. Ballestar, J. Barzola-Quiquia, T. Scheike, P. Esquinazi, Josephson-coupled superconducting regions embedded at the interfaces of highly oriented pyrolytic graphite, New J. Phys. 15 (2013), 023024. lez-Herrero, G.T. de Laissardie re, M.M. Ugeda, [11] I. Brihuega, P. Mallet, H. Gonza mez-Rodríguez, F. Yndur L. Magaud, J.M. Go ain, J.-Y. Veuillen, Unraveling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis, Phys. Rev. Lett. 109 (2012) 196802. [12] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, P. Jarillo-Herrero, Unconventional superconductivity in magic-angle graphene superlattices, Nature 556 (2018) 43e50. patterns on STM images of [13] M. Flores, E. Cisternas, J. Correa, P. Vargas, Moire graphite induced by rotations of surface and subsurface layers, Chem. Phys. 423 (2013) 49.
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[14] L.-J. Yin, J.-B. Qiao, W.-X. Wang, Z.-D. Chu, K.F. Zhang, R.-F. Dou, C.L. Gao, J.F. Jia, J.-C. Nie, L. He, Tuning structures and electronic spectra of graphene layers with tilt grain boundaries, Phys. Rev. B 89 (2014) 205410. €mmeli, T. Gemming, B. Büchner, G.A.D. Briggs, Direct im[15] J.H. Warner, M.H. Ro aging of rotational stacking faults in few layer graphene, Nano Lett. 9 (2009) 102. [16] F.S. Boi, X. Zhang, A. Corrias, Temperature driven structural-memory-effects in carbon nanotubes filled with Fe3C nano crystals, Mater. Res. Express 5 (2018), 025010. [17] F.S. Boi, M. Liu, J. Xia, O. Odunmbaku, A. Taallah, J. Wen, Temperature driven anomalous unit-cell c-axis shifts in magic-angle highly oriented pyrolytic graphite, Carbon (2019). Accepted/In press, https://doi.org/10.1016/j.carbon. 2019.01.066. [18] M. Harb, C. von Korff Schmising, H. Enquist, A. Jurgilaitis, I. Maximov, P.V. Shvets, A.N. Obraztsov, D. Khakhulin, M. Wulff, J. Larsson, The c-axis thermal conductivity of graphite film of nanometer thickness measured by time resolved X-ray diffraction, Appl. Phys. Lett. 101 (2012) 233108.
Filippo S. Boi* College of Physical Science and Technology, Sichuan University, Chengdu, China Sino-British Joint Materials Research Institute, Sichuan University, Chengdu, China Mengjiao Liu College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China JiaChen Xia, Omololu Odunmbaku, Ayoub Taallah College of Physical Science and Technology, Sichuan University, Chengdu, China Sino-British Joint Materials Research Institute, Sichuan University, Chengdu, China Jiqiu Wen Analytical & Testing Center, Sichuan University, Chengdu, China *
Corresponding author. College of Physical Science and Technology, Sichuan University, Chengdu, China. E-mail address: [email protected] (F.S. Boi). 11 March 2019 Available online 3 May 2019
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Letter to the editor
Anomalous c-axis shifts and symmetry enhancement in highly oriented pyrolytic graphite at the magic angle a b s t r a c t Observation of superconductivity in twisted-bilayer-graphene and highly-oriented-pyrolytic-graphite (HOPG) for certain magic-angles-of-rotation has recently attracted an important attention. Unusual temperature-(T)-induced-shifts in the graphitic c-axis have also been reported in HOPG in conditions of q misfit ~ 1 (first-magic-angle). We report a novel investigation of HOPGs with q misfit of 0.5 , 0.8 , 1.5 , [ 1.5 and of water-treatedturbostratic-graphite in the T-range from 298.15 to 673.15K. Presence of magic angles of rotations cor pattern analyses responding to the reported q misfit values is demonstrated by repeated HRTEM and Moire of HOPG lamellae extracted from individual samples with scotch tape methods. Interestingly in our measurements the c-axis-shift is found to depend strongly on the misfit-angle, with the highest-values of 0.00428 and 0.00426 nm in proximity of the first-magic-angle (q misfit ~ 0.8 and ~0.5 ). Two diffraction-peaks present at ~23.5 and ~48.5 2q (detector angle, in typical Bragg- Brentano configuration) for q misfit ~1 are also found to vanish for q misfit [ 1.5 . These findings imply existence of additional symmetry elements at the magic angle, which are not present in the standard space group notation used for structural characterization of graphite (P63/mmc). © 2019 Elsevier Ltd. All rights reserved.
The recent observation of superconductive effects in bilayer graphene and highly oriented pyrolytic graphite (HOPG) has attracted an important attention in the field of condensed matter physics and materials science [1e15]. Unusual superconductive effects in graphitic based materials were reported already in early 2000 in samples consisting of pyrolytic graphite with random atomiclayer orientation [1], for certain conditions of annealing. In these studies, observation of ferromagnetic and superconducting ordering was reported in un-annealed and annealed samples respectively [1]. In recent works, it has been proposed that quasitwo-dimensional (2D) interfaces in HOPG, described with the BurgerseBraggeReadeShockley (BBRS) dislocation model [5,6], can act as sources of granular superconductivity when certain conditions of rotational misfit-angles of the graphene layers are achieved [1e15]. In these materials, the misfit (rotational) angle between graphene layers has been reported to play a crucial role towards the appearance of such superconductive features [5,6,8,12]. Existence of logarithmic van Hove singularities has been reported for rotational misfit-angles of 1 qtwist 10 and give rise to the patterns with consequent appearance of flat bands in the enMoire ergy dispersion relation spectrum [5,6,8,12,13]. Also, the presence of rhombohedral stacking with relative abundances ranging from 10% to 20% and its interfacial contact with the Bernal A-B graphitic phase has been considered an additional parameter which could induce such superconductive effects [2,4,8]. Together with these interpretations, the presence of a temperature induced unit cell shift https://doi.org/10.1016/j.carbon.2019.05.003 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
in the c-axis of the graphitic unit cell has been reported in different type of carbon systems, namely multiwalled carbon nanotubes, carbon onions, commercial turbostratic graphite and HOPG [16e18]. In particular, appearance of anomalous c-axis shifts (i.e much different than those expected by considering the tabulated thermal expansion parameter reported at 300 K [18]) has been reported in HOPG under first-magic-angle conditions q misfit ~ 1 (0.8 average misfit angle) in the T-range from 298.15 to 673.15K [17]. In this letter we investigate further the T-induced reversible variation of the c-axis in HOPG samples with controlled misfit-angle. We report an in-depth study on the variation of the unit-cell c-axis shift for HOPG samples with average misfit-angles in proximity of first-magicangle, namely 0.5 , 0.8 , 1.5 ± 0:2 and [ 1.5 as confirmed also pattern analyses (ESI Fig.by HRTEM measurements and Moire Supp.26e28). XRD measurements were performed in the T-range from 298.15 to 673.15K. Comparative tests were also performed in water treated commercial turbostratic graphite samples in the same T-range. Our experimental measurements and Rietveld Refinement analyses show that the observed unit cell c-axis shift depends strongly on the misfit-angle, with the highest-values of 0.00428 and 0.00426 nm measured in proximity of the firstmagic-angle (see ESI for typical example of magic angle conditions demonstrated by repeated HRTEM measurements performed on the graphite layers of individual HOPG samples extracted by scotch tape method). Most importantly, note that two unlabeled peaks are found at approx. 23.5 and 48.5 2q (detector angle, in typical
28
Letter to the editor / Carbon 150 (2019) 27e31
Bragg- Brentano configuration) in proximity of first magic angle conditions with an intensity that decreases with the increase of the misfit-angle and almost vanishes for q misfit [ 1.5 . These peaks can no be explained on the basis of the standard database cards with space group P63/mmc typically used for structural analyses of commercial graphite (CIF 1200017 from Crystal Open Database). These observations imply that misfit angle of graphene layers may play a crucial role in controlling structural, symmetry and thermal characteristics of unit-cell c-axis in HOPG. These observations were further confirmed by additional measurements on commercial graphite powder and water treated graphite powders where no such additional symmetry characteristics were detected. HOPG samples with dimensions of 5 5 1 mm and average pattern analyses) of misfit angles (as extracted by repeated Moire 0.5 , 0.8 , 1.5 ± 0:2 and [ 1.5 were purchased from XFNANO, INC. Commercial graphite powder was purchased from Xiya Reagents China Cas number 7782-42-5 and treated with water following the method reported in Ref. [7]. Variable temperature dependent XRD measurements were performed on a Rigaku Smart-lab powder X-ray diffractometer (Cu K-a average, l ¼ 0.15418 nm) with Bragg- Brentano configuration under vacuum values below 7 Pa in the temperature range from 298.15 to 673.15 K, with an Anton Paar TTK450. The HOPG samples were positioned with the c-axis perpendicular to the substrate holder, in analogy to conditions used in Ref. [17]. The measurements were performed in a Ni substrate. Comparative measurements were also performed in HOPG samples with q misfit [ 1.5 positioned with the c-axis perpendicular to a glass substrate. These measurements revealed values of c-axis shifts similar to those extracted in measurements performed with Ni substrate. This is shown in the table of Fig. Supp. 34 (see ESI Figs.Supp. 31e39 for analyses of the comparative measurements performed in HOPG samples with q misfit [ 1.5 positioned in a glass substrate). Additional measurements with c-axis orientation parallel to the substrate were performed at room temperature, see Supp.Fig.42e44. TEM measurements were performed with a 200 kV American FEI Tecnai G2F20 on lamellae extracted with scotch-tape method from the main HOPG sample (these lamellae were extracted after preliminary removal of surface layers). Magnetometry measurements were performed on a VSM Quantum Design system at 300K, 200K and 100K for HOPG samples with average qmisfit of 1.5 and at 350K, 300K and 150K for HOPG samples with average qmisfit [ 1.5 . These measurements revealed presence of superconducting-like hysteresis in samples with average qmisfit of 1.5 but not in samples with average qmisfit [ 1.5 , see ESI Fig.Supp.45,46 (in agreement with structural characterization in Figs. 1e5). Note also that EDX analyses did not reveal presence of significant impurities in the samples (Fig.Supp.47e49). The layered structure of HOPG samples with misfit-angle q misfit of 0.5 was firstly revealed by room temperature XRD measurements, as indicated in Fig. 1, with the observation of preferred 002 and 004 reflections of graphitic carbon with space group P63/mmc. By analyzing the temperature dependent X-ray diffractograms in Fig. 1 for HOPG with q misfit of 0.5 , in the range from 298.15 to 673.15K, it is possible to notice that a significant temperature dependent shift in the position of the 002 and 004 diffraction peaks is present towards lower values of degrees 2q. This observation has similarities with that reported in the case of HOPG samples with q misfit of 0.8 [17]. This significant transition can be observed in the complete plot of Fig. 1 and in the detailed diffractograms of Fig. 2, where the 002 (Fig.2A) and 004 (Fig. 2B) peak-shifts are shown as a function of the temperature. Comparisons with other HOPG samples with different misfit angle are shown in Fig. 3 and in Fig. 4 (see ESI for detailed Rietveld refinement analyses of the graphitic c-axis in these samples and table showing results of
Fig. 1. XRD diffractograms showing the structural shifts of the 002 and 004 reflections of HOPG (0.5 q misfit misfit-angle) as a function of the temperature from 298.15 to 673.15K. (A colour version of this figure can be viewed online.)
Fig. 2. XRD diffractograms showing with a higher detail the structural shifts of the 002 (A) and 004 (B) reflections of HOPG (0.5 q misfit misfit-angle) as a function of the temperature from 298.15 to 673.15K. (A colour version of this figure can be viewed online.)
Letter to the editor / Carbon 150 (2019) 27e31
29
Fig. 3. Plots showing the variation of the unit cell c-axis of HOPG samples with misfit-angle q misfit of 0.5 , 1.5 , [ 1.5 and turbostratic graphite extracted with the Rietveld refinement method (see ESI for detailed analyses) as a function of the temperature from 298.15 to 673.15K. See ESI Fig.Supp.26e28 for typical HRTEM analyses of lamellae extracted periodicities corresponding to q misfit of ~1 and ~1.5 respectively. from HOPG samples with misfit-angle q misfit of 0.8 and 1.5 revealing Moire
Fig. 4. XRD diffractograms showing the variation in the intensity of the 002,004 and unlabeled peaks reflections (see black stars) of HOPG samples with misfit-angleq misfit of 0.5 , 0.8 , 1.5 and [ 1.5 . (A colour version of this figure can be viewed online.)
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Letter to the editor / Carbon 150 (2019) 27e31
Fig. 5. XRD diffractograms showing the variation in the intensity of the unlabeled peak observed in the region of approximately 23.5 2q as function of the level of misfit-angle in the sample (misfit angle of the graphene layers, as confirmed by HRTEM measurements in ESI). See also ESI Fig.Supp.25 for analyses performed on turbostratic (doped and undoped) graphite powder revealing disappearance of the observed peak. Note the increase of the relative intensity of this reflection in proximity of first magic angle conditions. An additional unlabeled peak is found at approx. 48.5 2q in the analysed HOPG samples with an intensity that decreases with the increase of the misfit-angle, as shown in ESI Fig.Supp. 29, 30. Also this peak is found to vanish in turbostratic graphite powder (see ESI Fig.Supp. 30). (A colour version of this figure can be viewed online.)
Rietveld refinement calculation of c-axis shift, Fig.Supp.24). A significant variation in the c-axis shift was found. Large-shift values of approximately 0.00428 and 0.00426 nm were extracted from HOPGs with average q misfit of 0.8 and 0.5 (in proximity to first magic angle ± 0:2 ). Also, slightly lower shifts of 0.00398 nm were found for q misfit of 1.5 . Instead a much lower shift value of 0.00311 nm could be extracted for q misfit [1.5 . Note that the observed c-axis shift values are much different with respect to those expected if calculated with the previously reported thermal expansion factor of 26.7 106 K1 reported at 300K for graphite [18]. This important difference may be related to variation in the degree of misfit angle of graphite layers or to possible presence of multiple graphite interfaces with different ordering which may then play also a role on the room temperature granular superconductivity characteristics as recently suggested by Scheike T. [7,8]. Further comparisons were then sought with turbostratic graphite, produced with water treated method produced following the method reported by Scheike T. in Ref. [7]. As shown in Fig. 3D, Rietveld refinement analyses of the diffractograms revealed a much weaker variation of the unit-cell c-axis in the same temperature range (0.00183 nm) (see ESI for examples of Rietveld refinement analyses of the graphitic c-axis for all the analysed samples) and absence of additional symmetry characteristics. These observations appear to imply that the higher the degree of turbostratic carbon, the weaker the variation in the unit cell c-axis values with the temperature. In our measurements the highest value of c-axis shift is found for conditions in proximity to first magic angle. Most importantly, two unlabeled peaks are found at approx. 23.5 and 48.5 2q
only in the analysed HOPG samples (and not in water treated and undoped graphite) with an intensity that decreases with the increase of the misfit-angle. Almost complete disappearance of these peaks is interestingly found for q misfit [1.5 together with a dramatic decrease in the intensity of 004 peaks, as shown in Figs. 4 and 5 and ESI Fig.Supp.25, 29, 30. These observations imply the possible presence of magic-angle-induced superlattices symmetries which are observable with XRD when conditions of q misfit ~1 are reached. Note that no such peak features could be detected in turbostratic graphite powder (see ESI Fig.Supp.25, 30). See also Fig. 4 for variation of relative intensities of 002 and 004 reflections with the misfit angle in the analysed HOPGs. In conclusion, in this letter we have investigated the variation of the temperatureinduced unit-cell c-axis shift in HOPG samples with misfit-angles q misfit of 0.5 , 0.8 1.5 and [1.5 in the temperature range from 298.15 to 673.15 K. Additional comparisons were also performed with water treated commercial turbostratic graphite, in the same temperature range. Our results show that the unit cell c-axis shift has maximum values in proximity of first- magic-angle of rotation. Two unlabeled peaks (possibly indicating additional superlattice symmetry characteristics) are also observed at approx. 23.5 and 48.5 2q under first magic angle conditions (q misfit ~1 ) and almost vanish for q misfit [ 1.5 .
Acknowledgments We acknowledge NSFC Grant No 11750110413.
Letter to the editor / Carbon 150 (2019) 27e31
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Filippo S. Boi* College of Physical Science and Technology, Sichuan University, Chengdu, China Sino-British Joint Materials Research Institute, Sichuan University, Chengdu, China Mengjiao Liu College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, China JiaChen Xia, Omololu Odunmbaku, Ayoub Taallah College of Physical Science and Technology, Sichuan University, Chengdu, China Sino-British Joint Materials Research Institute, Sichuan University, Chengdu, China Jiqiu Wen Analytical & Testing Center, Sichuan University, Chengdu, China *
Corresponding author. College of Physical Science and Technology, Sichuan University, Chengdu, China. E-mail address: [email protected] (F.S. Boi). 11 March 2019 Available online 3 May 2019