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Raman scattering study of InAs nanowires under high pressure
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 465704 (http://iopscience.iop.org/0957-4484/25/46/465704) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 61.129.42.30 This content was downloaded on 15/04/2017 at 19:37 Please note that terms and conditions apply.
You may also be interested in: Pressure dependence of Raman spectrum in InAs nanowires Sara Yazji, Ilaria Zardo, Simon Hertenberger et al. Vibrational properties of ZnTe at highpressures J Camacho, I Loa, A Cantarero et al. Effects of pressure and distortion on superconductivity in Tl2Ba2CaCu2O8+ Jian-Bo Zhang, Viktor V Struzhkin, Wenge Yang et al. Blue emitting organic semiconductors under high pressure: status and outlook Matti Knaapila and Suchismita Guha ScVO4 under non-hydrostatic compression: a new metastable polymorph Alka B Garg, D Errandonea, P Rodríguez-Hernández et al. Experimental and theoretical study of –Eu2(MoO4)3 under compression C Guzmán-Afonso, S F León-Luis, J A Sans et al. Pressure-induced phase transition in hydrothermally grown ZnO nanoflowers investigated by Raman and photoluminescence spectroscopy M Junaid Bushiri, R Vinod, Alfredo Segura et al. High-pressure phases of group IV and III-Vsemiconductors G J Ackland Pressure-induced phase transition inLaAlO3 P Bouvier and J Kreisel
Nanotechnology Nanotechnology 25 (2014) 465704 (8pp)
doi:10.1088/0957-4484/25/46/465704
Raman scattering study of InAs nanowires under high pressure Dipanwita Majumdar1, Abhisek Basu2, Goutam Dev Mukherjee2, Daniele Ercolani3, Lucia Sorba3 and Achintya Singha1 1
Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata 700009, India Department of Physical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur Campus, 741246 India 3 NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, I-56127 Pisa, Italy 2
E-mail: [email protected] Received 22 July 2014, revised 2 September 2014 Accepted for publication 3 September 2014 Published 31 October 2014 Abstract
The pressure-dependent phonon modes of InAs nanowires have been investigated by Raman spectroscopy under high pressure up to ∼58 GPa. X-ray diffraction measurements show that InAs nanowires at 21 GPa exhibit a phase transition from a wurtzite to an orthorhombic crystal structure, with a corresponding drastic change in the first-order Raman spectra. In the lowpressure regime, a linear increase in phonon frequencies is observed, whereas splitting between longitudinal and transversal optical phonon modes decreases as a function of applied pressure. The calculated mode Grüneisen parameters and Born’s transverse effective charge indicate that the wurtzite InAs nanowires exhibit a more covalent nature under compression. Keywords: high pressure Raman, high pressure XRD, wurtzite InAs nanowire, Grüneisen parameters, born’s transverse effective charge (Some figures may appear in colour only in the online journal) In recent years, one-dimensional semiconductor nanowires (NWs) with diameters in the range of several tens of nanometers have been identified as the next generation of building blocks for nanoscale electronics [1, 2], photonics [3], sensors [4], and lasers [5]. Among them, InAs NWs are of particular interest because they exhibit excellent electron transport properties due to a remarkably high bulk mobility that results from its small electron effective mass. Thermodynamic parameters play an important role in structural phase transitions, which have a large impact on the physical properties of a system. Application of external pressure can tune the electronic and structural properties of semiconductors [6, 7]. In a real system, a number of factors like defects, confinement, strain, and surface tension are important parameters for understanding pressure-induced electronic and structural transformations. Therefore, the investigation of low-dimensional materials under pressure can provide valuable information [8]. High-pressure Raman scattering is a powerful technique for extracting information regarding the vibrational properties and phase diagrams of both bulk and 0957-4484/14/465704+08$33.00
nanostructures [6, 9–14]. The use of a diamond anvil cell (DAC) and ruby fluorescence allows to reach high pressures, which enables the study of many new phenomena related to the behavior of solids [15–19]. In InAs, the stable structural phase in the bulk state is zincblende (ZB), but InAs NWs often stabilize in the wurtzite (WZ) structure [20, 21]. The phonon modes in the zone center of the two structures are different. For the WZ structure, the modes can be estimated by back-folding the phonon branches of ZB InAs along the [111] plane [22]. Raman spectroscopy can be used to probe the differences between the modes of WZ and ZB phases [22]. In the literature, several examples have shown that hydrostatic pressure can modify the structure of semiconductor bulk crystals. Based on resistivity measurements, Minomura and Drickamer reported a pressure-induced phase transition in bulk InAs at 8.46 GPa [23]. Using Raman spectroscopy, Jayaraman et al reported the occurrence of metallization in single-crystal InAs at 7.15 GPa [24]. In addition, Vohra et al studied bulk InAs crystals under pressure up to 27 GPa, and 1
© 2014 IOP Publishing Ltd Printed in the UK
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Nanotechnology 25 (2014) 465704
Figure 1. TEM dark-field images of InAs NWs: (a) low magnification, (b) high magnification. (c) Raman spectrum of bulk InAs (symbols: •); the red line is the result of a fit with three Lorentzian functions for TO (blue), LO (green), and 2LO phonons (orange). (d) WZ InAs NW Raman spectrum (symbols: +); the red line is the result of the fit with five Lorentzian components (see text). Inset of figure 1(d) shows the polarization-dependent Raman spectra collected in the x (z, z ) x¯ (pink) and x (y, y ) x¯ (violet) configurations.
observed a sequential structural transition from ZB to rock salt, and then to β-Sn [25]. Aoki et al studied volume-dependent Raman frequency, Born’s transverse effective charge (e*T), and resonance behavior of Raman intensity in bulk InAs under hydrostatic pressure [26]. Recently, studies on the influence of hydrostatic pressure on low-dimensional semiconductor systems have attracted extensive research interest due to their applications in optoelectronic devices. In ZB GaAs NWs, the pressure-dependent lattice constant, ionicity, and electron– phonon Fröhlich interactions have been investigated [6]. The pressure-dependent structural parameters and electronic band gap of WZ InAs NWs have recently been studied by Raman scattering technique up to 8.5 GPa [18, 19]. In the above work, no phase transition has been observed in WZ InAs NWs until 8.5 GPa, which is the highest pressure applied in that study. Yazji et al [19] also showed that the effective dynamical charge has a very weak dependence on lattice compression, which is different from the result for bulk InAs reported in reference [26]. In the present work, we report, for the first time, the pressure-dependent Raman response of InAs NWs up to 58 GPa. We determine the mode Grüneisen parameters (γ) for all observed first- and second-order phonon modes. Born’s transverse effective charge (e*T), obtained from the measured transverse optical (TO) and longitudinal optical (LO) frequencies as a function of applied pressure in the low pressure
regime up to about 11 GPa, agrees well with the reported results [23, 26]. At higher pressures, the Raman spectra show a drastic change, and also new Raman modes appear in the spectra. These changes are associated with a modification of the InAs NW crystal structure. This result was confirmed by x-ray diffraction (XRD) measurements, which showed that InAs nanowires at ∼21 GPa exhibit a phase transition from a WZ to an orthorhombic crystal structure. Aligned InAs NWs were grown on InAs(111)B substrates using the chemical beam epitaxy technique. The NWs were grown at 425 °C, with metal organic line pressure of 0.3 and 1.0 Torr for trimetyl indium and tertiarybutyl arsine, respectively. Detailed growth parameters are described elsewhere [27]. The shape, size, and crystal structure of the NWs were studied using transmission electron microscopy (TEM, Model: JEOL 2010). The high-pressure Raman measurements were performed using a DAC in a back-scattering geometry. A 4:1 mixture of methanol and ethanol was used as the pressure-transmitting medium. Pressure was monitored in situ by the shift in the ruby fluorescence peak. The excitation source was a 488 nm line of an Argon ion laser with a spot diameter of about 5 μm. All the Raman data were recorded with laser power density 0.5 mW μm−2. The spectra were collected with a LabRam HR monochromator and a charge coupled device (CCD) detector (Jobin Yvon) using an 1800 gr mm−1 grating. An edge filter has been used to cut the 2
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Nanotechnology 25 (2014) 465704
Figure 2. (a) Schematic representation of the high-pressure Raman measurement setup. The blue line represents both the incident beam and the backscattered signal. At the bottom, a magnified representation of the sample chamber with the NWs inside the DAC is shown. (b) Pressure dependence of the Raman spectra from 0 to 12.3 GPa. For clarity, the spectra are shifted vertically.
diameter, and the data were collected on a 2 M Pilatus (Dectris, Switzerland) silicon pixel detector. Figures 1(a) and (b) show low- and high-magnification TEM images of the InAs NWs. The stripes along the length of the NWs provide the signature of the presence of the ZB phase, which is present as stacking faults in the WZ NWs. The average diameter of the NWs is 40 nm. A recent report on the same NWs shows that the percentage of WZ and ZB structures on the whole NW length are 95% and 5%, respectively [28]. Figure 1(c) displays the Raman spectrum of bulk InAs with a ZB crystal structure. The peak positions are determined by deconvoluting the Raman spectrum using three Lorentzian lines. The positions of the zone center TO, LO, and second-order longitudinal optical (2LO) phonon modes are observed at 217.7 cm−1, 239.1 cm−1, and 478.9 cm−1, respectively. The Raman spectrum of the InAs NWs at ambient pressure is shown in figure 1(d). The Raman peaks of InAs NWs show a downshift and an increase in full width at half maximum (FWHM) compared to bulk InAs. A laserbeam-induced heating effect can lead to a shift of Raman modes towards lower wave numbers. To avoid this heating effect, we used laser power density 0.5 mW μm−2, similar to what was reported in Ref. [29]. In addition, the diamond absorbs a significant amount of laser power, and the absorption increases as the pressure increases [30]. Moreover, diamond is an excellent thermal conductor, and hence most of the heat is carried away. Therefore, the downshift and the increase in FWHM in the Raman peaks of InAs NWs are due to the relaxation of the q = 0 selection rule in the NWs with
Figure 3. Raman frequency as a function of applied pressure for the
various Raman modes. The lines are the result of a linear fit, according to equation (2).
Rayleigh line. High-pressure, angle-dispersive XRD measurements were carried out at the XRD1 beam line of the ELETTRA Synchrotron light source at a wavelength of 0.688 Å. For XRD measurements, InAs NWs were dispersed in Ag powder, which was used as a pressure calibrant, and loaded in a DAC along with a pressure-transmitting medium, as described above. The x-rays were collimated to 80 μm in 3
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Nanotechnology 25 (2014) 465704
Figure 4. (a) Splitting of the LO and TO Raman modes as a function of pressure. (b) Born’s transverse effective charge as a function of
pressure computed from the experimental data of figure 4(a) using equation (3).
⎛ ⎝ parameters (γi). Reported values of a i and γi for InAs are given in the last two columns.
Table 1. Results of linear regression of the pressure dependence of phonon modes ⎜ ωi 0 and a i =
Present work Phonon modes
and calculated mode Grüneisen
Reported values
ωi0 (cm−1)
a i (cm−1 GPa−1)
γi
TO SO LO
214.6 ± 1 215.5 ± 0.7 231.6 ± 0.7 239.3 ± 0.7
3.9 ± 0.2 4.6 ± 0.1 3.8 ± 0.1 3.3 ± 0.1
1.08 ± 0.0.06 1.27 ± 0.03 0.98 ± 0.03 0.82 ± 0.02
2LO
476.9 ± 1.6
7.5 ± 0.9
0.94 ± 0.11
EH 2
⎞ ∂ω i ⎟ ∂P P = 0 ⎠
( )
a i (cm−1 GPa−1)
γi
5.25 ± 0.35 (NW) [19] 4.39 (bulk) [26] — 4.57 (bulk) [26] 4.16 ± 0.32 (NW) [19] —
1.42 ± 0.08 (NW) [19] 1.21 (bulk) [26] — 1.06 (bulk) [26] 1.00 ± 0.08 (NW) [19] —
vanish above 10.9 GPa. The frequencies of the EH 2 , TO, SO, LO, and 2LO modes as a function of applied hydrostatic pressure are shown in figure 3. It is important to stress that the shift of lattice vibrational modes under applied pressure allows us to evaluate two important parameters: the behavior of the material under volume variations, indicated by the modes’ Grüneisen parameter, γi, where the index i is relative to the individual phonon modes; and the character of chemical bonds in the crystal, parameterized by the transverse (or Born) effective charge, e*T, which can be related to the bond-length dependence of the ionicity and polarity. The mode Grüneisen parameter can be calculated from Raman modes using following equation [32, 33]:
lattice defects [31]. The peak at the lower wave number is a convolution of EH 2 (arising due to the WZ structure) and the TO modes. This has been confirmed from polarizationdependent Raman measurements where the TO mode is dominant in the x (z, z ) x¯ configuration and EH 2 is dominant in the x (y, y ) x¯ configuration, as shown in the inset of figure 1(d). Thus, we have fitted the Raman spectrum of InAs NWs by five Lorentzians. The peak at about 216.2 cm−1 is the TO mode (blue line), while the LO mode (green line) appears at 238.4 cm−1. The lower lying mode at 213.6 cm−1 (purple line) is the EH 2 WZ mode. The surface phonon (SO) mode (magenta line) and 2LO mode (orange line) are observed at 230.1 and 477.4 cm−1, respectively. Thus, the Raman analysis enables us to confirm the WZ structure in the InAs NWs [22, 28]. A schematic of the high-pressure Raman setup using a DAC in a back-scattering geoemetry is shown in figure 2(a). The Raman spectra of InAs NWs as a function of hydrostatic pressure up to 12.3 GPa are plotted in figure 2(b). With the increase in pressure, all phonon modes shift towards higher frequencies with gradually decreasing intensity, and finally
γi =
∂ ln ωi K0 = ai, ∂ ln V ωi 0
(1)
where i is an index for the specific Raman modes, V is the crystal volume, K0 = − V ((∂P ) (∂V )) is the isothermal bulk modulus (P: pressure), ω is the frequency of the mode, and ω0 and a are the coefficients of first-order expansion of the 4
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Nanotechnology 25 (2014) 465704
dimensionality, random orientation of the bundles of NWs measured, defects in the NWs crystal structure, etc) is presented. The other derived parameter is the transverse effective charge, e*T, which is associated with the absorption induced by TO phonons and can be derived from the longitudinal and transverse optical phonon frequencies as [40]
(
( e*T )2 ε0 ε∞ μV
(3)
where μ is the reduced mass of the anion-cation pair, ε0 is the vacuum permittivity, and ε∞ is the infrared dielectric constant. Figure 4(a) shows LO-TO splitting (ωLO-ωTO) as a function of applied pressure. The splitting diminishes with increasing pressure, which is consistent with most of the tetrahedrally bonded GaN, AIN, GaAs, and GaP semiconductors [6, 10, 19, 26, 39, 41]. Using equation (3), the calculated e*T in the units of elementary charge decreases with increasing pressure, as shown in figure 4(b). Similar pressure dependence for e*T has been observed in ZB InAs [26], WZ ZnO bulk crystals [37], and GaAs NWs [6]. The decrease of e*T with pressure indicates a decrease in the ionicity of the system [6], which could imply that at high pressures, the NWs become more covalent in nature compared to NWs at ambient pressure. Raman spectra of InAs NWs for different applied pressures from 0 to ∼58 GPa are shown in figure 5. For clarity, only selected spectra are plotted, and they have been shifted vertically. As we apply pressure, higher-order Raman modes (at about the fourth order), marked as X1 and X2, start appearing. The intensity variations of the first- and higherorder (X1 and X2) Raman modes as a function of pressure are shown in figures 6(a), (b), and (c), respectively. The intensity profile of the first-order modes shows resonant behavior around 1.6 GPa (see figure 6(a)), which is consistent with an earlier report [18]; the intensity of these modes reduces significantly above 6 GPa. On the other hand, the X1 and X2 mode intensities have a maximum at about 8 GPa. In fact, the Raman spectra do not show any clear signals of the EH 2 , TO, LO, SO, and 2LO phonon modes above 10.9 GPa, whereas higher-order modes are visible up to much higher pressure levels, and their frequencies increase monotonically with the increase in pressure, as shown in the insets of figures 6(b) and (c). The reason behind the anomalous behavior of the Raman mode intensity will be discussed later. In addition to the above modes, some new modes labeled A, B, and C appear around 16 GPa. In the literature, new modes have been observed in bulk WZ InN semiconductors as a characteristics of the WZ-torock-salt phase transition above 13.5 GPa [35]. Therefore, in order to learn more about the Raman results observed at high pressure, we carried out high-pressure XRD measurements. Figure 7 shows the pressure evolution of XRD patterns of InAs NWs at 5 GPa, 21.7 GPa, and 28 GPa. The XRD pattern at 5 GPa (see figure 7(a)) could be indexed well with the WZ phase in space group P63mc with lattice parameters a = 4.194 (2), c = 6.857(4) Ǻ. However, as shown in figure 7(b), at
Figure 5. Pressure dependence of the Raman spectra from 0 to
58 GPa. For clarity, the spectra are shifted vertically. Peaks X1 and X2 are higher-order Raman modes. A, B, and C are new modes that appear at high pressure.
pressure dependent phonon frequencies, ωi = ωi 0 ± a i P
)
ω2 LO − ω2TO =
(2)
It is evident from the data in figure 3 that a first-order fit is adequate to describe our data, although in most ZB semiconductors, the pressure dependence of phonon frequencies is modeled by a quadratic equation [6, 26]. However, a linear pressure dependence of the phonon modes has been observed for WZ InN [34–36] and WZ ZnO bulk semiconductors [37]. We have used the reported values of K0 (59.60 GPa) [38] for WZ InAs to calculate mode Grüneisen parameters using equation (1); other reported values (i.e., for bulk ZB InAs [26] and for InAs NWs [19]) differ only slightly from the ones used here. Table 1 summarizes the dependence of the EH 2 , TO, SO, LO, and 2LO phonon modes of WZ InAs NWs as a function of applied pressure obtained from the deconvoluted experimental spectra. The obtained values of ω0, a, and γ from the present work, the corresponding values determined by Yazji et al in their recent report [19], and those reported for bulk ZB InAs [26] are given in the table. For most of the tetrahedrally bound semiconductors, the measured γi values are between 1 and 1.6 [39]. The estimated mode Grüneisen parameters are close to the reported values for bulk InAs [26] (for zone center TO and LO modes) and InAs NWs in [19], where a detailed discussion of the possible sources of deviation (one5
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Nanotechnology 25 (2014) 465704
Figure 6. (a) Intensity variation of EH 2 , TO, SO, and LO modes as a function of the applied pressure. (b) and (c) are the intensity variation of
higher-order Raman modes X1 and X2, respectively. The pressure-dependent Raman shifts of X1 and X2 are shown in the insets of figures 6(a) and (b), respectively. (d) Schematic diagram of the formation of the bundle structure of InAs NWs with increasing pressure.
21.7 GPa, new XRD lines belonging to a new phase appear together with the diffraction lines corresponding to the WZ phase. The transition to a new phase was completed at about 28 GPa, as shown in figure 7(c). This new phase could not be indexed to any known polymorphs of InAs, including rock salt and the Cmcm phase or β-Sn type phase [25, 42]. A careful indexing of the XRD pattern at 28 GPa, using both the program DICVOL [43] and further refinement of the lattice parameters using the program CHECKCELL, confirmed that the new structure is orthorhombic with space group Pnnm and lattice parameters a = 5.051(2), b = 3.258(3), and c = 2.595 Ǻ. Since the XRD measurements at 21.7 GPa showed the presence of the WZ phase in the InAs NWs, the absence of low-frequency Raman lines above 11 GPa cannot be related to any structural change. Application of high pressure induces large strain on individual NWs, which effectively damps the low-frequency atom vibrations, and hence the LO and TO modes disappear. Similar anomalous Raman mode features have been observed in Si nanowires, where the second-order mode intensities are found to increase with pressure [44]. This behavior was attributed to the formation of bundles of aligned Si NWs that can sustain the pressure-induced deformation. In
the present case, the appearance of higher-order Raman modes (X1 and X2) under high pressures can similarly be attributed to the formation of closed packed bundles of InAs NWs, as shown in figure 6(d), in which the individual NWs may be aligned in a particular direction. The alignment, and hence the bundle size, increases as pressure increases, which is reflected in the increase of higher-order Raman intensities. Above 8 GPa, pressure-induced strain effects dominate, and that results in a broadening of all modes. The new Raman modes (marked as A, B, and C) corresponding to the new phase start to appear around 16 GPa, which is consistent with the fact that they are seen at about 21 GPa in the XRD measurements. In conclusion, application of pressure is found to strongly modify the vibrational properties of InAs NWs. At high pressures above 16 GPa, new phonon modes appear in the Raman spectra. This change is associated with the occurrence of a phase transition from a WZ to an orthorhombic structure in InAs NWs. In addition, the decrease in effective dynamical charge with pressure indicates an increase of covalent character in the NWs. These results emphasize the prominent role of pressure in modifying the vibrational and electronic properties of one-dimensional systems. 6
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[6] Zardo I, Yazji S, Marini C, Uccelli E, Morral A F, Abstreiter G and Postorino P 2012 ACS Nano 6 3284 [7] Yao L D et al 2010 J. Mater. Res. 25 2330 [8] Khachadorian S, Papagelis K, Scheel H, Colli A, Ferrari A C and Thomsen C 2011 Nanotechnology 22 195707 [9] Trommer R, Anastassakis E and Cardona M 1976 Light Scattering in Solids ed M Balkanski, R C C Leite and S P S Porto (Paris: Flammarion) p 396 [10] Trommer R, Miiller H, Cardona M and Vogl P 1980 Phys. Rev. B 21 4869 [11] Weinstein B A, Renucci J B and Cardona M 1973 Solid State Commun. 12 473 [12] Weinstein B A and Piermarini G 1975 Phys. Rev. B 12 1172 [13] Olego D and Cardona M 1982 Phys. Rev. B 25 1151 [14] Olego D, Cardona M and Vogl P 1982 Phys. Rev. B 25 3878 [15] Mizuta M, Tachikawa M, Kukimoto H and Minomura S 1985 Jpn. J. Appl. Phys. 24 L143 [16] Stradling R A 1985 Festkörperprobleme (Advances in Solid State Physics) vol 25 ed P Grosse (Braunschweig: Pergamon/Vieweg) p 591 [17] Porowski S 1980 Proc. 4th Int. Conf. on the Physics of Narrow Gap Semiconductors Linz 1980 ed E Gornick, H Henrich and L Palmetshofer (Berlin: Springer) p 420 Konczewicz L, Staszewska E L and Porowski S 1977 Proc. 3rd Int. Conf. on Narrow Gap Semiconductors (Warsaw: Elsevier) p 211 [18] Zardo I, Yazji S, Hörmann N, Hertenberger S, Funk S, Mangialardo S, Morkötter S, Koblmüller G, Postorino P and Abstreiter G 2013 Nano Lett. 13 3011 [19] Yazji S, Zardo I, Hertenberger S, Morkötter S, Koblmüller G, Abstreiter G and Postorino P 2014 J. Phys.: Condens. Matter 26 235301 [20] Froberg L E, Seifert W and Johansson J 2007 Phys. Rev. B 76 153401 [21] Persson A I, Froberg L E, Jeppesen S, Bjork M T and Samuelson L 2007 J. Appl. Phys. 101 034313 [22] Hörmann N G, Zardo I, Hertenberger S, Funk S, Bolte S, Döblinger M, Koblmüller G and Abstreiter G 2011 Phys. Rev. B 84 155301 [23] Minomura S and Drickamer H G 1962 J. Phys. Chem. Solids 23 451 [24] Jayaraman A, Swaminathan V and Batlogg B 1984 Pramana 23 405 [25] Vohra Y K, Weir S T and Ruoff A L 1985 Phys. Rev. B 31 7344 [26] Aoki K, Anastassakis E and Cardona M 1984 Phys. Rev. B 30 681 [27] Viti L, Vitiello M S, Ercolani D, Sorba L and Tredicucci A 2012 Nanoscale Res. Lett. 7 159 [28] Panda J K, Roy A, Singha A, Gemmi M, Ercolani D, Pellegrini V and Sorba L 2012 Appl. Phys. Lett. 100 143101 [29] Begum N, Bhatti A S, Jabeen F, Rubini S and Martelli F 2009 J. Appl. Phys. 106 114317 [30] Yagi T and Susaki J 1992 High-pressure Research: Applications to Earth and Planetory Sciences ed Y Syono and H P Manghnani (American Geophysical Union) (Tokyo: Terra Scientific) 51 [31] Begum N, Piccin M, Jabeen F, Bais G, Rubini S, Martelli F and Bhatti A S 2008 J. Appl. Phys. 104 104311 [32] Blackman M 1957 Proc. Phys. Soc. London, Sect. B 70 827 [33] Daniels W B 1965 Lattice Dynamics ed R F Wallis (Oxford: Pergamon Press) p 273 [34] Pinquier C et al 2004 Phys. Rev. B 70 113202 [35] Pinquier C et al 2006 Phys. Rev. B 73 115211 [36] Ibáñez J, Manjón F J, Segura A, Oliva R, Cuscó R, Vilaplana R, Yamaguchi T, Nanishi Y and Artús L 2011 Appl. Phys. Lett. 99 011908
Figure 7. XRD patterns of InAs NWs dispersed in an Ag powder,
which was used as a pressure calibrant, at pressures of: (a) 5 GPa, (b) 21.7 GPa, and (c) 28 GPa.
Acknowledgments The authors acknowledge useful discussions with Dr Biswajit Karmakar, Saha Institute of Nuclear Physics, Kolkata, India and thank Dr P V Satyam, IOP, Bhubaneshwar, India, for his help in the TEM work. Authors also gratefully acknowledge all the help and support from Dr Maurizio Polentarutti, Dr Giorgio Bais and Dr Nicola Demitri for successfully carrying out the high pressure XRD experiments. This work was partly supported by MIUR under PRIN 2009 prot. 2009HS2F7N_003. A.B. and G.D.M. gratefully acknowledge funding from the Indo-Italian Program of Co-operation for performing the high-pressure XRD measurements at the ELETTRA synchrotron light source. G.D.M. also gratefully acknowledges funding from the Ministry of Earth Sciences for developing the DACs for high-pressure experiments.
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Raman scattering study of InAs nanowires under high pressure
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 465704 (http://iopscience.iop.org/0957-4484/25/46/465704) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 61.129.42.30 This content was downloaded on 15/04/2017 at 19:37 Please note that terms and conditions apply.
You may also be interested in: Pressure dependence of Raman spectrum in InAs nanowires Sara Yazji, Ilaria Zardo, Simon Hertenberger et al. Vibrational properties of ZnTe at highpressures J Camacho, I Loa, A Cantarero et al. Effects of pressure and distortion on superconductivity in Tl2Ba2CaCu2O8+ Jian-Bo Zhang, Viktor V Struzhkin, Wenge Yang et al. Blue emitting organic semiconductors under high pressure: status and outlook Matti Knaapila and Suchismita Guha ScVO4 under non-hydrostatic compression: a new metastable polymorph Alka B Garg, D Errandonea, P Rodríguez-Hernández et al. Experimental and theoretical study of –Eu2(MoO4)3 under compression C Guzmán-Afonso, S F León-Luis, J A Sans et al. Pressure-induced phase transition in hydrothermally grown ZnO nanoflowers investigated by Raman and photoluminescence spectroscopy M Junaid Bushiri, R Vinod, Alfredo Segura et al. High-pressure phases of group IV and III-Vsemiconductors G J Ackland Pressure-induced phase transition inLaAlO3 P Bouvier and J Kreisel
Nanotechnology Nanotechnology 25 (2014) 465704 (8pp)
doi:10.1088/0957-4484/25/46/465704
Raman scattering study of InAs nanowires under high pressure Dipanwita Majumdar1, Abhisek Basu2, Goutam Dev Mukherjee2, Daniele Ercolani3, Lucia Sorba3 and Achintya Singha1 1
Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata 700009, India Department of Physical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur Campus, 741246 India 3 NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, I-56127 Pisa, Italy 2
E-mail: [email protected] Received 22 July 2014, revised 2 September 2014 Accepted for publication 3 September 2014 Published 31 October 2014 Abstract
The pressure-dependent phonon modes of InAs nanowires have been investigated by Raman spectroscopy under high pressure up to ∼58 GPa. X-ray diffraction measurements show that InAs nanowires at 21 GPa exhibit a phase transition from a wurtzite to an orthorhombic crystal structure, with a corresponding drastic change in the first-order Raman spectra. In the lowpressure regime, a linear increase in phonon frequencies is observed, whereas splitting between longitudinal and transversal optical phonon modes decreases as a function of applied pressure. The calculated mode Grüneisen parameters and Born’s transverse effective charge indicate that the wurtzite InAs nanowires exhibit a more covalent nature under compression. Keywords: high pressure Raman, high pressure XRD, wurtzite InAs nanowire, Grüneisen parameters, born’s transverse effective charge (Some figures may appear in colour only in the online journal) In recent years, one-dimensional semiconductor nanowires (NWs) with diameters in the range of several tens of nanometers have been identified as the next generation of building blocks for nanoscale electronics [1, 2], photonics [3], sensors [4], and lasers [5]. Among them, InAs NWs are of particular interest because they exhibit excellent electron transport properties due to a remarkably high bulk mobility that results from its small electron effective mass. Thermodynamic parameters play an important role in structural phase transitions, which have a large impact on the physical properties of a system. Application of external pressure can tune the electronic and structural properties of semiconductors [6, 7]. In a real system, a number of factors like defects, confinement, strain, and surface tension are important parameters for understanding pressure-induced electronic and structural transformations. Therefore, the investigation of low-dimensional materials under pressure can provide valuable information [8]. High-pressure Raman scattering is a powerful technique for extracting information regarding the vibrational properties and phase diagrams of both bulk and 0957-4484/14/465704+08$33.00
nanostructures [6, 9–14]. The use of a diamond anvil cell (DAC) and ruby fluorescence allows to reach high pressures, which enables the study of many new phenomena related to the behavior of solids [15–19]. In InAs, the stable structural phase in the bulk state is zincblende (ZB), but InAs NWs often stabilize in the wurtzite (WZ) structure [20, 21]. The phonon modes in the zone center of the two structures are different. For the WZ structure, the modes can be estimated by back-folding the phonon branches of ZB InAs along the [111] plane [22]. Raman spectroscopy can be used to probe the differences between the modes of WZ and ZB phases [22]. In the literature, several examples have shown that hydrostatic pressure can modify the structure of semiconductor bulk crystals. Based on resistivity measurements, Minomura and Drickamer reported a pressure-induced phase transition in bulk InAs at 8.46 GPa [23]. Using Raman spectroscopy, Jayaraman et al reported the occurrence of metallization in single-crystal InAs at 7.15 GPa [24]. In addition, Vohra et al studied bulk InAs crystals under pressure up to 27 GPa, and 1
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Figure 1. TEM dark-field images of InAs NWs: (a) low magnification, (b) high magnification. (c) Raman spectrum of bulk InAs (symbols: •); the red line is the result of a fit with three Lorentzian functions for TO (blue), LO (green), and 2LO phonons (orange). (d) WZ InAs NW Raman spectrum (symbols: +); the red line is the result of the fit with five Lorentzian components (see text). Inset of figure 1(d) shows the polarization-dependent Raman spectra collected in the x (z, z ) x¯ (pink) and x (y, y ) x¯ (violet) configurations.
observed a sequential structural transition from ZB to rock salt, and then to β-Sn [25]. Aoki et al studied volume-dependent Raman frequency, Born’s transverse effective charge (e*T), and resonance behavior of Raman intensity in bulk InAs under hydrostatic pressure [26]. Recently, studies on the influence of hydrostatic pressure on low-dimensional semiconductor systems have attracted extensive research interest due to their applications in optoelectronic devices. In ZB GaAs NWs, the pressure-dependent lattice constant, ionicity, and electron– phonon Fröhlich interactions have been investigated [6]. The pressure-dependent structural parameters and electronic band gap of WZ InAs NWs have recently been studied by Raman scattering technique up to 8.5 GPa [18, 19]. In the above work, no phase transition has been observed in WZ InAs NWs until 8.5 GPa, which is the highest pressure applied in that study. Yazji et al [19] also showed that the effective dynamical charge has a very weak dependence on lattice compression, which is different from the result for bulk InAs reported in reference [26]. In the present work, we report, for the first time, the pressure-dependent Raman response of InAs NWs up to 58 GPa. We determine the mode Grüneisen parameters (γ) for all observed first- and second-order phonon modes. Born’s transverse effective charge (e*T), obtained from the measured transverse optical (TO) and longitudinal optical (LO) frequencies as a function of applied pressure in the low pressure
regime up to about 11 GPa, agrees well with the reported results [23, 26]. At higher pressures, the Raman spectra show a drastic change, and also new Raman modes appear in the spectra. These changes are associated with a modification of the InAs NW crystal structure. This result was confirmed by x-ray diffraction (XRD) measurements, which showed that InAs nanowires at ∼21 GPa exhibit a phase transition from a WZ to an orthorhombic crystal structure. Aligned InAs NWs were grown on InAs(111)B substrates using the chemical beam epitaxy technique. The NWs were grown at 425 °C, with metal organic line pressure of 0.3 and 1.0 Torr for trimetyl indium and tertiarybutyl arsine, respectively. Detailed growth parameters are described elsewhere [27]. The shape, size, and crystal structure of the NWs were studied using transmission electron microscopy (TEM, Model: JEOL 2010). The high-pressure Raman measurements were performed using a DAC in a back-scattering geometry. A 4:1 mixture of methanol and ethanol was used as the pressure-transmitting medium. Pressure was monitored in situ by the shift in the ruby fluorescence peak. The excitation source was a 488 nm line of an Argon ion laser with a spot diameter of about 5 μm. All the Raman data were recorded with laser power density 0.5 mW μm−2. The spectra were collected with a LabRam HR monochromator and a charge coupled device (CCD) detector (Jobin Yvon) using an 1800 gr mm−1 grating. An edge filter has been used to cut the 2
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Figure 2. (a) Schematic representation of the high-pressure Raman measurement setup. The blue line represents both the incident beam and the backscattered signal. At the bottom, a magnified representation of the sample chamber with the NWs inside the DAC is shown. (b) Pressure dependence of the Raman spectra from 0 to 12.3 GPa. For clarity, the spectra are shifted vertically.
diameter, and the data were collected on a 2 M Pilatus (Dectris, Switzerland) silicon pixel detector. Figures 1(a) and (b) show low- and high-magnification TEM images of the InAs NWs. The stripes along the length of the NWs provide the signature of the presence of the ZB phase, which is present as stacking faults in the WZ NWs. The average diameter of the NWs is 40 nm. A recent report on the same NWs shows that the percentage of WZ and ZB structures on the whole NW length are 95% and 5%, respectively [28]. Figure 1(c) displays the Raman spectrum of bulk InAs with a ZB crystal structure. The peak positions are determined by deconvoluting the Raman spectrum using three Lorentzian lines. The positions of the zone center TO, LO, and second-order longitudinal optical (2LO) phonon modes are observed at 217.7 cm−1, 239.1 cm−1, and 478.9 cm−1, respectively. The Raman spectrum of the InAs NWs at ambient pressure is shown in figure 1(d). The Raman peaks of InAs NWs show a downshift and an increase in full width at half maximum (FWHM) compared to bulk InAs. A laserbeam-induced heating effect can lead to a shift of Raman modes towards lower wave numbers. To avoid this heating effect, we used laser power density 0.5 mW μm−2, similar to what was reported in Ref. [29]. In addition, the diamond absorbs a significant amount of laser power, and the absorption increases as the pressure increases [30]. Moreover, diamond is an excellent thermal conductor, and hence most of the heat is carried away. Therefore, the downshift and the increase in FWHM in the Raman peaks of InAs NWs are due to the relaxation of the q = 0 selection rule in the NWs with
Figure 3. Raman frequency as a function of applied pressure for the
various Raman modes. The lines are the result of a linear fit, according to equation (2).
Rayleigh line. High-pressure, angle-dispersive XRD measurements were carried out at the XRD1 beam line of the ELETTRA Synchrotron light source at a wavelength of 0.688 Å. For XRD measurements, InAs NWs were dispersed in Ag powder, which was used as a pressure calibrant, and loaded in a DAC along with a pressure-transmitting medium, as described above. The x-rays were collimated to 80 μm in 3
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Figure 4. (a) Splitting of the LO and TO Raman modes as a function of pressure. (b) Born’s transverse effective charge as a function of
pressure computed from the experimental data of figure 4(a) using equation (3).
⎛ ⎝ parameters (γi). Reported values of a i and γi for InAs are given in the last two columns.
Table 1. Results of linear regression of the pressure dependence of phonon modes ⎜ ωi 0 and a i =
Present work Phonon modes
and calculated mode Grüneisen
Reported values
ωi0 (cm−1)
a i (cm−1 GPa−1)
γi
TO SO LO
214.6 ± 1 215.5 ± 0.7 231.6 ± 0.7 239.3 ± 0.7
3.9 ± 0.2 4.6 ± 0.1 3.8 ± 0.1 3.3 ± 0.1
1.08 ± 0.0.06 1.27 ± 0.03 0.98 ± 0.03 0.82 ± 0.02
2LO
476.9 ± 1.6
7.5 ± 0.9
0.94 ± 0.11
EH 2
⎞ ∂ω i ⎟ ∂P P = 0 ⎠
( )
a i (cm−1 GPa−1)
γi
5.25 ± 0.35 (NW) [19] 4.39 (bulk) [26] — 4.57 (bulk) [26] 4.16 ± 0.32 (NW) [19] —
1.42 ± 0.08 (NW) [19] 1.21 (bulk) [26] — 1.06 (bulk) [26] 1.00 ± 0.08 (NW) [19] —
vanish above 10.9 GPa. The frequencies of the EH 2 , TO, SO, LO, and 2LO modes as a function of applied hydrostatic pressure are shown in figure 3. It is important to stress that the shift of lattice vibrational modes under applied pressure allows us to evaluate two important parameters: the behavior of the material under volume variations, indicated by the modes’ Grüneisen parameter, γi, where the index i is relative to the individual phonon modes; and the character of chemical bonds in the crystal, parameterized by the transverse (or Born) effective charge, e*T, which can be related to the bond-length dependence of the ionicity and polarity. The mode Grüneisen parameter can be calculated from Raman modes using following equation [32, 33]:
lattice defects [31]. The peak at the lower wave number is a convolution of EH 2 (arising due to the WZ structure) and the TO modes. This has been confirmed from polarizationdependent Raman measurements where the TO mode is dominant in the x (z, z ) x¯ configuration and EH 2 is dominant in the x (y, y ) x¯ configuration, as shown in the inset of figure 1(d). Thus, we have fitted the Raman spectrum of InAs NWs by five Lorentzians. The peak at about 216.2 cm−1 is the TO mode (blue line), while the LO mode (green line) appears at 238.4 cm−1. The lower lying mode at 213.6 cm−1 (purple line) is the EH 2 WZ mode. The surface phonon (SO) mode (magenta line) and 2LO mode (orange line) are observed at 230.1 and 477.4 cm−1, respectively. Thus, the Raman analysis enables us to confirm the WZ structure in the InAs NWs [22, 28]. A schematic of the high-pressure Raman setup using a DAC in a back-scattering geoemetry is shown in figure 2(a). The Raman spectra of InAs NWs as a function of hydrostatic pressure up to 12.3 GPa are plotted in figure 2(b). With the increase in pressure, all phonon modes shift towards higher frequencies with gradually decreasing intensity, and finally
γi =
∂ ln ωi K0 = ai, ∂ ln V ωi 0
(1)
where i is an index for the specific Raman modes, V is the crystal volume, K0 = − V ((∂P ) (∂V )) is the isothermal bulk modulus (P: pressure), ω is the frequency of the mode, and ω0 and a are the coefficients of first-order expansion of the 4
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dimensionality, random orientation of the bundles of NWs measured, defects in the NWs crystal structure, etc) is presented. The other derived parameter is the transverse effective charge, e*T, which is associated with the absorption induced by TO phonons and can be derived from the longitudinal and transverse optical phonon frequencies as [40]
(
( e*T )2 ε0 ε∞ μV
(3)
where μ is the reduced mass of the anion-cation pair, ε0 is the vacuum permittivity, and ε∞ is the infrared dielectric constant. Figure 4(a) shows LO-TO splitting (ωLO-ωTO) as a function of applied pressure. The splitting diminishes with increasing pressure, which is consistent with most of the tetrahedrally bonded GaN, AIN, GaAs, and GaP semiconductors [6, 10, 19, 26, 39, 41]. Using equation (3), the calculated e*T in the units of elementary charge decreases with increasing pressure, as shown in figure 4(b). Similar pressure dependence for e*T has been observed in ZB InAs [26], WZ ZnO bulk crystals [37], and GaAs NWs [6]. The decrease of e*T with pressure indicates a decrease in the ionicity of the system [6], which could imply that at high pressures, the NWs become more covalent in nature compared to NWs at ambient pressure. Raman spectra of InAs NWs for different applied pressures from 0 to ∼58 GPa are shown in figure 5. For clarity, only selected spectra are plotted, and they have been shifted vertically. As we apply pressure, higher-order Raman modes (at about the fourth order), marked as X1 and X2, start appearing. The intensity variations of the first- and higherorder (X1 and X2) Raman modes as a function of pressure are shown in figures 6(a), (b), and (c), respectively. The intensity profile of the first-order modes shows resonant behavior around 1.6 GPa (see figure 6(a)), which is consistent with an earlier report [18]; the intensity of these modes reduces significantly above 6 GPa. On the other hand, the X1 and X2 mode intensities have a maximum at about 8 GPa. In fact, the Raman spectra do not show any clear signals of the EH 2 , TO, LO, SO, and 2LO phonon modes above 10.9 GPa, whereas higher-order modes are visible up to much higher pressure levels, and their frequencies increase monotonically with the increase in pressure, as shown in the insets of figures 6(b) and (c). The reason behind the anomalous behavior of the Raman mode intensity will be discussed later. In addition to the above modes, some new modes labeled A, B, and C appear around 16 GPa. In the literature, new modes have been observed in bulk WZ InN semiconductors as a characteristics of the WZ-torock-salt phase transition above 13.5 GPa [35]. Therefore, in order to learn more about the Raman results observed at high pressure, we carried out high-pressure XRD measurements. Figure 7 shows the pressure evolution of XRD patterns of InAs NWs at 5 GPa, 21.7 GPa, and 28 GPa. The XRD pattern at 5 GPa (see figure 7(a)) could be indexed well with the WZ phase in space group P63mc with lattice parameters a = 4.194 (2), c = 6.857(4) Ǻ. However, as shown in figure 7(b), at
Figure 5. Pressure dependence of the Raman spectra from 0 to
58 GPa. For clarity, the spectra are shifted vertically. Peaks X1 and X2 are higher-order Raman modes. A, B, and C are new modes that appear at high pressure.
pressure dependent phonon frequencies, ωi = ωi 0 ± a i P
)
ω2 LO − ω2TO =
(2)
It is evident from the data in figure 3 that a first-order fit is adequate to describe our data, although in most ZB semiconductors, the pressure dependence of phonon frequencies is modeled by a quadratic equation [6, 26]. However, a linear pressure dependence of the phonon modes has been observed for WZ InN [34–36] and WZ ZnO bulk semiconductors [37]. We have used the reported values of K0 (59.60 GPa) [38] for WZ InAs to calculate mode Grüneisen parameters using equation (1); other reported values (i.e., for bulk ZB InAs [26] and for InAs NWs [19]) differ only slightly from the ones used here. Table 1 summarizes the dependence of the EH 2 , TO, SO, LO, and 2LO phonon modes of WZ InAs NWs as a function of applied pressure obtained from the deconvoluted experimental spectra. The obtained values of ω0, a, and γ from the present work, the corresponding values determined by Yazji et al in their recent report [19], and those reported for bulk ZB InAs [26] are given in the table. For most of the tetrahedrally bound semiconductors, the measured γi values are between 1 and 1.6 [39]. The estimated mode Grüneisen parameters are close to the reported values for bulk InAs [26] (for zone center TO and LO modes) and InAs NWs in [19], where a detailed discussion of the possible sources of deviation (one5
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Figure 6. (a) Intensity variation of EH 2 , TO, SO, and LO modes as a function of the applied pressure. (b) and (c) are the intensity variation of
higher-order Raman modes X1 and X2, respectively. The pressure-dependent Raman shifts of X1 and X2 are shown in the insets of figures 6(a) and (b), respectively. (d) Schematic diagram of the formation of the bundle structure of InAs NWs with increasing pressure.
21.7 GPa, new XRD lines belonging to a new phase appear together with the diffraction lines corresponding to the WZ phase. The transition to a new phase was completed at about 28 GPa, as shown in figure 7(c). This new phase could not be indexed to any known polymorphs of InAs, including rock salt and the Cmcm phase or β-Sn type phase [25, 42]. A careful indexing of the XRD pattern at 28 GPa, using both the program DICVOL [43] and further refinement of the lattice parameters using the program CHECKCELL, confirmed that the new structure is orthorhombic with space group Pnnm and lattice parameters a = 5.051(2), b = 3.258(3), and c = 2.595 Ǻ. Since the XRD measurements at 21.7 GPa showed the presence of the WZ phase in the InAs NWs, the absence of low-frequency Raman lines above 11 GPa cannot be related to any structural change. Application of high pressure induces large strain on individual NWs, which effectively damps the low-frequency atom vibrations, and hence the LO and TO modes disappear. Similar anomalous Raman mode features have been observed in Si nanowires, where the second-order mode intensities are found to increase with pressure [44]. This behavior was attributed to the formation of bundles of aligned Si NWs that can sustain the pressure-induced deformation. In
the present case, the appearance of higher-order Raman modes (X1 and X2) under high pressures can similarly be attributed to the formation of closed packed bundles of InAs NWs, as shown in figure 6(d), in which the individual NWs may be aligned in a particular direction. The alignment, and hence the bundle size, increases as pressure increases, which is reflected in the increase of higher-order Raman intensities. Above 8 GPa, pressure-induced strain effects dominate, and that results in a broadening of all modes. The new Raman modes (marked as A, B, and C) corresponding to the new phase start to appear around 16 GPa, which is consistent with the fact that they are seen at about 21 GPa in the XRD measurements. In conclusion, application of pressure is found to strongly modify the vibrational properties of InAs NWs. At high pressures above 16 GPa, new phonon modes appear in the Raman spectra. This change is associated with the occurrence of a phase transition from a WZ to an orthorhombic structure in InAs NWs. In addition, the decrease in effective dynamical charge with pressure indicates an increase of covalent character in the NWs. These results emphasize the prominent role of pressure in modifying the vibrational and electronic properties of one-dimensional systems. 6
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Figure 7. XRD patterns of InAs NWs dispersed in an Ag powder,
which was used as a pressure calibrant, at pressures of: (a) 5 GPa, (b) 21.7 GPa, and (c) 28 GPa.
Acknowledgments The authors acknowledge useful discussions with Dr Biswajit Karmakar, Saha Institute of Nuclear Physics, Kolkata, India and thank Dr P V Satyam, IOP, Bhubaneshwar, India, for his help in the TEM work. Authors also gratefully acknowledge all the help and support from Dr Maurizio Polentarutti, Dr Giorgio Bais and Dr Nicola Demitri for successfully carrying out the high pressure XRD experiments. This work was partly supported by MIUR under PRIN 2009 prot. 2009HS2F7N_003. A.B. and G.D.M. gratefully acknowledge funding from the Indo-Italian Program of Co-operation for performing the high-pressure XRD measurements at the ELETTRA synchrotron light source. G.D.M. also gratefully acknowledges funding from the Ministry of Earth Sciences for developing the DACs for high-pressure experiments.
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