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Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod Uttam Kumar Ghorai, Nilesh Mazumder, Hitesh Mamgain, Rajarshi Roy, Subhajit Saha, and Kalyan Kumar Chattopadhyay J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10620 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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The Journal of Physical Chemistry

Figure 1. (Color online) (a) Schematic of a CuPc molecule. (b) and (c) represent FESEM image and SAED of as prepared nanorods respectively. XRD profiles of α nanorods and bulk β-CuPc are shown in (d). (e) is a schematic of molecular stacking for α and β-CuPc 172x129mm (300 x 300 DPI)

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Figure 2. (Color online) Experimental Raman spectra of α and β with Lorentzian fitting of each modes are shown in (a) and (b) respectively.

206x179mm (300 x 300 DPI)


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Figure 3. (Color online) Raman shift and linewidth variation of B1g phonon mode as a function of temperature with inset showing the calculated in-plane Cartesian displacement vectors. 60x44mm (300 x 300 DPI)



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Figure 4. (Color online) Raman spectra of a α-CuPc nanorod at 316, 378 and 456 K. The spectral deconvolution (black square: experiment, red: B2g mode, blue: A1g mode, green: B1g mode, orange: sum) reflects gradual change in edge vibration symmetry and intermolecular interaction. Top and middle inset represent the calculated normal mode for Davydov doublet and B2g peak respectively. 155x303mm (300 x 300 DPI)


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Raman Spectroscopic Observation of Gradual Polymorphic Transition and Phonon Modes in CuPc Nanorod Uttam K. Ghorai,†§ Nilesh Mazumder,‡§ Hitesh Mamgain,ǁ Rajarshi Roy,ǂ Subhajit ⃰ Saha,ǂ and Kalyan K. Chattopadhyay ,ǂ,‡

ǂ

School of Materials Science and Nanotechnology and ‡Physics Department, Jadavpur

University, Kolkata 700032, India †

Department of Industrial Chemistry and Applied Chemistry, Swami Vivekananda

Research

Center, Ramakrishna

Mission

Vidyamandira,

Belurmath,

711202, India ǁ

WITec GmbH, Lise-Meitner-Strasse, 6 D-89081 Ulm, Germany

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Howrah-


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ABSTRACT A Raman scattering investigation of a single α-CuPc nanorod is demonstrated here within the temperature range 300 – 770 K. From the typical thermal dispersion of Raman shift and phonon linewidth of in-plane B1g mode at 1526 cm-1, the metastable α polymorph is observed to undergo unambiguous phase transition to thermally stable β phase. The phase transition temperature (456 K) is observed to be significantly lower than that reported for bulk samples. Extensive computation of normal modes associated with both α and β-CuPc reveal that the ‘fingerprint’ domain across 1300 – 1600 cm-1 is particularly sensitive to this polymorphic transition where a silent B2g mode appears at 1303 cm-1 in the spectral vicinity of molecular edge vibration sensitive Ag – Bg Davydov doublet.

KEYWORDS Raman spectroscopy, Copper phthalocyanine, Phase transition, Polymorphism, Phonon


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I.

INTRODUCTION As a promising model compound in organic electronics,[1] copper phthalocyanine

(CuPc) or Phthalo blue has wide ranging usage in organic light emitting diodes,[2,3] hybrid solar cells,[4] organic field-effect transistors,[5,6] gas sensors,[7] cold cathode emitter[8,9] and as artists’ pigment.[10] It has shown immense potential for spintronics applications[11] and quantum information processing[12] very recently as CuPc has surprisingly large population relaxation time and phase memory time at low temperature (5 – 80 K). Besides its applicability in organic-inorganic/all organic optoelectronics,[13,14] numerous polymorphs of CuPc[15] have poised them as good platforms to study fundamental spectroscopy.[10,16] So far, studies have been reported regarding temperature dependent electronic properties of CuPc across polymorphic transition temperature range,[7,17,18] but the practice of determining participating polymorph remain very limited which plays a major role on the exhibited properties with temperature. CuPc is the most widely used blue pigment in ink industry and extensively used in food packages, laser printers, paints, plastics and textiles.[19] Since pigments are highly insoluble in the delivery content, different crystal forms belonging to numerous polymorphs has a profound effect on end use. For example, solubility of β-CuPc is almost one third compared to its α phase.[20] On the other hand, β-CuPc exhibits superior photoresponse and transport properties.[21] To date, polymorphic transformation of CuPc had been characterized on the basis of X-ray diffraction,[22] pressure[23] and crystalline size.[24] But from an application point of view, it is nonetheless important to investigate thermal normalization of key phonon modes and related polymorphic phase transition of CuPc by Raman spectroscopy which is known to be a versatile tool for probing subtle structural distortion and related phase transitions in wide range of materials.[25-28] Surprisingly, a systematic demonstration of α to β-CuPc phase transition or any metal-phthalocyanine



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polymorphic transition in terms of temperature dependent Raman spectroscopy is scarce in literature. In this work, we demonstrate a Raman spectroscopic investigation of a single α-CuPc nanorod as a function of temperature to explore (i) polymorphic phase transition temperature of the nanorod with respect to bulk samples, and (ii) to analyze the effect of phase sequence on the specific phonon modes. For this study, two vibration modes are identified: B1g mode at 1526 cm-1 which is sensitive to in-plane macroring peripheral C1-N2 vibration (Fig. 1a) and Ag – Bg symmetric Davydov doublet situated at 1341 cm-1. B1g mode is a well-known ‘crystal marker’ for phthalocyanine with a large isotopic shift[29] and sensitive to structure modification. On the other hand, thermal dispersion of Ag+Bg doublet is a distinct portrayal of competition between intramolecular and intermolecular interaction during the phase transition window. For our study, abrupt change in the thermal dispersion of Raman shift and phonon linewidth along with the appearance of a new intermolecular vibration mode is considered to be the confirmatory identification of phase transition with associated critical temperature.

II.

EXPERIMENTAL SECTION α-CuPc nanorods are prepared by self-assembly as described in our earlier works.[8,9]

Micro-Raman (beam diameter 1 µm) spectra are collected in backscattering configuration at temperatures ranging 303–817 K and analyzed by a WITec alpha 300RS spectrometer with confocal optics. The spectrograph is connected with a Peltier-cooled back-illuminated charge-coupled-device. We have used 532 nm laser (solid-state frequency doubled Nd:YAG) as the excitation line with 100X (NA = 0.9) achromatic objective (Zeiss). All spectra are acquired using an 1800 lines/mm grating (spectral resolution of 1.1 cm-1) with 10 seconds of total integration time for ten spectral acquisitions. The wavenumber error is considered ±0.5 4 ACS Paragon Plus Environment

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cm-1 for our calculations. For Raman analysis, synthesized nanorods are dispersed in ethanol by sonication. Then the solution was drop casted onto aluminum substrate followed by vacuum drying for 8 hr at 40°C. To distinguish between α and β-CuPc, we have acquired room temperature Raman spectra for both α-CuPc nanorod and bulk β-CuPc (Merck) separately using 50 µW laser power across 700 – 1700 cm-1. To avoid laser induced melting (873 K for CuPc) of nanorod surface, substrate temperature is maintained well under 800 K. For Raman analysis, Breit-Wigner-Fano (BWF) fitting[30] of B1g mode is carried out for the observed thermal range. According to this model, the Raman intensity can be described by =

, where = − ⁄ , ω0 being the renormalized phonon frequency in the

presence of the Fano coupling, q is the asymmetry and Γ is the phonon linewidth.

III.

RESULTS AND DISCUSSION

CuPc is a planar aromatic molecule of D4h point group symmetry. Pc molecules with a central metal ion usually crystallize with various polymorphic herringbone type arrangements between the molecular columns. Amongst several different reported polymorphic forms,[29] βCuPc is the most stable bulk phase.[32] A schematic of an isolated CuPc molecule is shown in Fig. 1a, where carbon atoms are characterized by 1 – 4 according to their distance from the central macroring. As synthesized α-CuPc nanorods have varying diameter of 150 – 200 nm and length of few µm as seen from the FESEM (Hitachi S4800) image (Fig. 1b). The nanorods are not entirely straight but their diameters are nearly constant across the length. Also, good crystalline quality of the nanorods having dominant growth direction of [001] is reflected by the SAED (JEOL JEM-2100 HRTEM) shown in Fig. 1c. The X-ray diffraction profiles (Rigaku-Ultima-ІІІ) of α and β-CuPc are shown in Fig. 1d. Two strong peaks at 2θ = 7.08 and 9.25° are the typical signature of bulk β-CuPc (JCPDS card 39-1881). Peak positions for α-CuPc at 6.76 and 7.30° are in accordance with previous reports by Jung et 5 ACS Paragon Plus Environment


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Figure 1. (Color online) (a) Schematic of a CuPc molecule. (b) and (c) represent FESEM image and SAED of as prepared nanorods respectively. XRD profiles of α nanorods and bulk β-CuPc are shown in (d). (e) is a schematic of molecular stacking for α and β-CuPc.

al.[21] The columnar orientations of CuPc molecules for α and β phase are shown in Fig. 1e. For the analysis of vibrational modes, we have considered α-CuPc with triclinic (space-group P-1, point group Ci) and β-CuPc with monoclinic (space-group P21/a, point group C2h) unit cell.[33] The corresponding irreducible representation for α is Γα = 60Ag +63Au and for β is Γβ = 60Ag +60Bg +63Au +63Bu of which only Ag and Bg are Raman active. However, the isolated molecule, in general has 165 normal modes of vibration belonging to D4h point group given by = 14 13 14 14 13 6 8 7 7 28 , of which the in-plane A1g, B1g, B2g and out-of-plane Eg modes are Raman active. Initially, we have collected Raman spectra of both α and β-CuPc individually at 50 µW across 700 – 1700 cm-1 which are shown in Fig. 2a and b respectively. Spectra are fitted with a sum of Lorentzian type phonon functions in accordance with damped harmonic oscillator formalism. The individual modes are represented by solid lines (red for α and orange for β-CuPc) and the resultant sum is represented by the dotted lines (green for α and purple for β-CuPc). The mode assignment is in accordance with the extensive study of CuPc Raman modes by Basova 6 ACS Paragon Plus Environment

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Figure 2. (Color online) Experimental Raman spectra of α and β with Lorentzian fitting of each modes are shown in (a) and (b) respectively.

et al.[28] Clearly, the observed Raman modes can be categorized into two types: (a) exclusive Raman modes present in β-CuPc only and (b) modes present in both the phases. In β-CuPc, three distinct additional peaks are observed at 771, 1195 and 1303 cm-1 (green columns, Fig. 1e-f) and are attributed to Eg, B1g and B2g modes respectively. Importantly, all three additional Raman modes originate from terminal carbon atoms of molecular arm; Eg from C3-C4-H and C2-C3-H out-of-plane, B1g from C4-C4-H and C4-C3-H in-plane and B2g from C3-C4-H and C2-C3-H in-plane vibrations. Within the domain of 1300 – 1600 cm-1



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(indicated by grey column in Fig. 2a-b), B1g Raman mode at 1526 cm-1 in α-CuPc is due to central in-plane vibration of C1-N2. Notably, the higher energy phonons outside 1300 -1600 cm-1 do not undergo similar extent of Stokes shift upon polymorphic transition (Figure S1, Supporting Information). For example, change in the Raman peak position (∆ω) for the phonon modes within this region is at least 6 cm-1 upon phase transition. On the other hand, for other modes the ∆ω is only about 1 – 2 cm-1. This is evidencing the ‘fingerprint’ nature of associated spectral domain in distinguishing polymorphic transition. Fig. 3 shows the evolution of Raman shift and phonon linewidth of the B1g phonon mode as a function of temperature for CuPc nanorod. Here, B1g spectral shape for each temperature is fit with a Fano line shape and the resulting parameters obtained from each fit are used to plot the thermal dispersion of the Raman shift and width of the phonon mode. Overall, the in-plane macroring breathing mode (inset) exhibits a shift in Raman peak toward lower wavenumbers along with the broadening of phonon linewidth with increasing temperature due to anharmonic effects of lattice like dilatation of lattice and thermal disorders. The linewidth increases from 3 to 5.7 cm-1 while the Raman peak shifts from 1526 to 1516 cm-1 with the increase in temperature from 300 to 770 K. An abrupt variation in Raman shift at the temperature region between 425 and 456 K can easily be identified. Another important finding is that the Raman linewidth becomes nearly independent of temperature 456 K onwards. Careful observation reveals that the evolution of both Raman shift and linewidth between 300 and 425 K consist of regions with varying slopes indicating either intermediate polymorphic transitions or gradual conversion from α to β-CuPc. Annealing time dependent monitoring of electronic absorption spectra for H2Pc thin film have also recorded similar intermediate steps between sharp α to β phase transition.[32] However, nearly temperature independent behavior of linewidth or phonon lifetime of C1-N2 vibration 456 K onwards is a spectral signature of phase transformation of α-CuPc into a


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Figure 3. (Color online) Raman shift and linewidth variation of B1g phonon mode as a function of temperature with inset showing the calculated in-plane Cartesian displacement vectors.

thermally stable polymorph. In 1965, J. M. Assour have determined the α to β-CuPc transition temperature to be 300 °C (573 K) for bulk CuPc crystals.[35] Later, Hassan et al. have identified the transition temperature to be 240 °C (513 K) for CuPc thin films.[36] Besides, report of polymorphic (α→β) phase transition at 210 °C (483 K) can also be found in literature.[37] In comparison, our study performed on a single nanorod yields the transition temperature to be 456 K (183 °C), which is much lower than the previous reports. Enhanced surface energy of a nanocrystal system can be attributed for this downshift of transition temperature. The extended Gibbs-Thomson expression involving greater surface energy leads to displacement of the phase transition line in the P-T diagram. For a nanocrystal system, higher surface energy results in higher effective temperature with a consequent drop in transition temperature. Our observation is in accordance with the lower polymorphic transition temperature observed for WO3 nanowires.[25] Moreover, (A1g+B1g) peak at 1341 cm-1 exhibits similar temperature dependence like B1g at 1526 cm-1 indicating identical transition temperature (Figure S2, Supporting Information). So, two in-plane vibrations



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Figure 4. (Color online) Raman spectra of a α-CuPc nanorod at 316, 378 and 456 K. The spectral deconvolution (black square: experiment, red: B2g mode, blue: A1g mode, green: B1g mode, orange: sum) reflects gradual change in edge vibration symmetry and intermolecular interaction. Top and middle inset represent the calculated normal mode for Davydov doublet and B2g peak respectively.


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undergoing similar thermal dispersions are strongly implying the diagnostic possibility of sub-micron scale polymorphic transition for CuPc using the ‘fingerprint’ spectral domain across 1300 – 1600 cm-1. To probe the modification in the symmetry of CuPc terminal vibrations during α→β transformation, we have analyzed the thermal dispersion of a Davydov doublet situated at 1341 cm-1. This doublet consists of A1g and B1g modes situated at 1337 and 1343 cm-1 for α phase, as seen from Lorentzian deconvolution resembling a single Raman mode (Fig. 4). In case of phthalocyanine Raman spectrum, single or isolated phonon modes are usually associated with the vibrations of internal macroring.[38] On the contrary; a Davydov doublet predominantly contains the vibration of peripheral atoms. Besides, Davydov doublet with a Bg component is particularly important for studying intermolecular interactions. Moreover, from the intensity ratio of intramolecular A1g mode to the intermolecular B1g mode, the molecular environment is reflected qualitatively during structural transition (Figure S3, Supporting Information). As seen from Fig. 4, a new vibration mode at 1303 cm-1 appears upon spectral fitting at 378 K, much lower than the phase transition temperature. The existence of this mode is in accordance with the study performed on single crystal of βCuPc[29] and is attributed to in-plane C2-C3-H and C3-C4-H vibration sensitive B2g mode. Our calculation reveals the in-plane displacement vectors associated with collective doublet and the new peak separately (top and middle inset) with the finding that B2g is primarily governed by hydrogen related edge vibrations. Evidently, this is an important feature of structurally planar α to herringbone type β phase transition depicting the gradual change in the edge symmetry. Besides, contribution of A1g vibration in the Davydov doublet grows significantly in comparison to its B1g counterpart, implying a stronger C1-N1-C1 and associated weakening of C3-C4 contribution (top inset) for β-CuPc. This is suggesting that α→β transition is not discrete as it seems from thermal dispersion of B1g Raman shift (Fig. 3);



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rather gradual temperature dependent modification in edge symmetry is initiated from a temperature lower than 456 K. Finally, we have observed that α→β polymorphic transition is not reversible. B2g mode at 1303 cm-1 remains as a permanent feature even if temperature is lowered afterwards. This is indicating B2g to be a β-exclusive Raman mode using which ‘polymorphic imaging’ can be accomplished for mixed phase CuPc devices.

IV.

CONCLUSION

In summary, we have demonstrated the temperature dependent local Raman spectroscopic detection of polymorphic transition (α→β) for a CuPc nanorod. Phase transition temperature is identified on the basis of thermal dispersion of macroring peripheral B1g vibration at 1526 cm-1. Change in the edge vibration symmetry is observed even below the transition temperature in terms of thermal evolution of Davydov doublet at 1341 cm-1, and the appearance of terminal hydrogen related B2g vibration at 1303 cm-1. Group theoretical calculations are carried out to identify the exact vibrational configurations of the analyzed Raman modes and it is found that the change in the relative molecular orientation from α to β phase is affecting the hydrogen related edge vibrations giving rise to the β exclusive B2g vibration. The α→β transition temperature of nanorod is found to be significantly lower than bulk CuPc due to high surface energy. In line of our investigation, we hope that our results elucidate the scope of micron level local spectroscopic distinction between different polymorphs for mixed phase CuPc devices and is applicable in phase sensitive electrical and spintronics applications.


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AUTHOR INFORMATION *Corresponding author email: [email protected] FAX: 91 33 2114 6584; Phone: 91 33 24138917 §U. K. G and N. M have contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS N.M and R.R wish to acknowledge the financial support by the University Grants Commission (UGC), Govt. of India under UPE-II scheme. N.M gratefully acknowledges Prof. Mike Glazer of Oxford University for the copy of ‘VIBRATE!’. The authors also acknowledge Nirmalya Sankar Das and Prasanta Mandal for technical assistance.

ASSOCIATED CONTENT Supporting Information The supporting information for this work contains details of Raman spectra along with BWF fitting, and extensive phonon mode analysis chart with VIBRATE!. This information is available free of charge via the Internet at http://pubs.acs.org.



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REFERENCES [1] Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W. L.; Sai, N.; Gearba, R.; Kaake, L. G.; Willaims, K. J.; Leung, K.; Rossky, P. J. et al. Hot Charge-Transfer Excitons Set the Time Limit for Charge Separation at Donor/Acceptor Interfaces in Organic Photovoltaics. Nat. Mater. 2013, 12, 66 – 73. [2] Cheng, C. H.; Fan, Z. Q.; Yu, S. K.; Jiang, W. H.; Wang, X.; Du, G. T.; Chang, Y. C.; Ma, C. Y. 1.1 µm Near-Infrared Electrophosphorescence from Organic Light-Emitting Diodes Based on Copper Phthalocyanine. Appl. Phys. Lett. 2006, 88, 213505 (1 – 3). [3] Yan, F.; Li, W. L.; Chu, B.; Li, T. L.; Su, W. M.; Su, Z. S.; Zhu, J. Z.; Yang, D. F.; Zhang, G.; Bi, D. F. et al. Sensitized Electrophosphorescence of Infrared Emission Diode Based on Copper Phthalocyanine by an Ytterbium Complex. Appl. Phys. Lett. 2007, 91, 203512 (1 – 3). [4] Levitsky, I. A.; Euler, W. B.; Tokranova, N.; Xu, B.; Castracane, J. Hybrid Solar Cells Based on Porous Si and Copper Phthalocyanine Derivatives. Appl. Phys. Lett. 2004, 85, 6245 – 6247. [5] Zeis, R.; Siegrist, T.; Kloc, Ch. Single-Crystal Field-Effect Transistors Based on Copper Phthalocyanine. Appl. Phys. Lett. 2005, 86, 022103 (1 – 3). [6] Wei, Z.; Cao, Y.; Ma, W.; Wang, C.; Xu, W.; Guo, X.; Hu, W.; Zhu, D. Langmuir– Blogett Monolayer Transistors of Copper Phthalocyanine. Appl. Phys. Lett. 2009, 95, 033304 (1 – 3). [7] Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Langmuir−Blodgett Multilayers Based on Copper Phthalocyanine as Gas Sensor Materials: Active Layer−Gas Interaction Model and Conductivity Modulation. Langmuir 1997, 13, 6562 – 6567.


Page 18 of 23

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[8] Ghorai, U. K.; Das, S.; Saha, S.; Mazumder, N.; Sen, D.; Chattopadhyay, K. K. Efficient and Persistent Cold Cathode Emission from CuPc Nanotubes: A Joint Experimental and Simulation Investigation. Dalton Trans., 2014, 43, 9260 – 9266. [9] Ghorai, U. K.; Saha, S.; Mazumder, N.; Das, N. S.; Banerjee, D.; Sen, D.; Chattopadhyay, K. K. Experimental and Theoretical Investigation of Enhanced Cold Cathode Emission by Plasma-Etched 3d Array of Nanotips Derived from CuPc Nanotube. RSC Adv., 2015, 5, 23847 – 23854. [10] Defeyt, C.; Vandenabeele, P.; Gilbert, B.; Pevenage, J. V.; Cloots R.; Strivay, D. Contribution to the Identification of α-, β- and ε-Copper Phthalocyanine Blue Pigments in Modern Artists' Paints by X-ray Powder Diffraction, Attenuated Total Reflectance MicroFourier Transform Infrared Spectroscopy and Micro-Raman Spectroscopy. J. Raman. Spectrosc. 2012, 43, 1772 (1 – 9). [11] Cinchetti, M.; Heimer, K.; Wüstenberg, J. P.; Andreyev, O.; Bauer, M.; Lach, S.; Ziegler, C.; Gao Y.; Aeschlimann, M. Determination of Spin Injection and Transport in a Ferromagnet/Organic Semiconductor Heterojunction by Two-Photon Photoemission. Nat. Mater. 2009, 8, 115 – 119. [12] Warner, M.; Din, S.; Tupitsyn, I. S.; Morley, G. W.; Stoneham, A. M.; Gardener, J. A.; Wu, Z.; Fisher, A. J.; Heutz, S.; Kay C. W. M. et al. Potential for Spin-Based Information Processing in a Thin-Film Molecular Semiconductor. Nature 2013, 503, 504 – 508. [13] Takada, J.; Awaji, H.; Koshioka, M.; Nevin, W. A.; Imanishi M.; Fukada, N. Copper Phthalocyanine‐Titanium Oxide Multilayers. J. Appl. Phys. 1994, 75, 4055 – 4059. [14] Chunder, A.; Pal, T.; Khondaker, S. I.; Zhai, L. Reduced Graphene Oxide/Copper Phthalocyanine Composite and Its Optoelectrical Properties. J. Phys. Chem C 2010, 114, 15129 – 15135.



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[15] Hoshino, A.; Takenaka, Y.; Miyaji, H. Redetermination of the Crystal Structure of Copper Phthalocyanine Grown on KCl. Acta Cryst. B 2003, 59, 393 – 403. [16] Shaibat, M. A.; Casabianca, L. B.; Siberio-Pérez, D. Y.; Matzger, A. J.; Ishii, Y. Distinguishing Polymorphs of the Semiconducting Pigment Copper Phthalocyanine by SolidState NMR and Raman Spectroscopy. J. Phys. Chem. B 2010, 114, 4400 – 4406. [17] Ambily, S.; Menon, C. S. The Effect of Growth Parameters on the Electrical, Optical and Structural Properties of Copper Phthalocyanine Thin Films. Thin Solid Films 1999, 347, 284 – 288. [18] Schuster, C.; Kraus, M.; Opitz, A.; Brütting, W.; Eckern, U. Transport Properties of Copper Phthalocyanine Based Organic Electronic Devices. Eur. Phys. J. 2010, 180, 117 – 134. [19] Gregory, P. Industrial Applications of Phthalocyanines. J. Porphyrins Phthalocyanines 2000, 4, 432 – 437. [20] Mutaftschiev, R. Interfacial Aspects of Phase Transformations (Reidel, Boston, 1982) p. 537. [21] Jung, J. S.; Lee, J. W.; Kim, K.; Cho, M. Y.; Jo, S. G.; Joo, Rectangular Nanotubes of Copper Phthalocyanine: Application to a Single Nanotube Transistor. J. Chem. Mat. 2010, 22, 2219 – 2225. [22] Berger, O.; Fischer, W. J.; Adolphi, B.; Tierbach, S.; Melev, V.; Schreiber, J. Studies on Phase Transformations of Cu-Phthalocyanine Thin Films. J. Mat. Sc. 2000, 11, 331 – 346. [23] Houle, G. B.; Gilson, D. F.; Butler, I. S. Pressure-Tuning Micro-Raman Spectra of Artists’ Pigments: α- and β-Copper Phthalocyanine Polymorphs. Spec. Acta A 2014, 117, 61 – 64. [24] Iwatsu, F. Size Effects on the α-β Transformation of Phthalocyanine Crystals. J. Phys. Chem. 1988, 92, 1678 – 1681.


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[25] Lu, D. Y.; Chen, J.; Chen, H. J.; Gong, L.; Deng, S. Z.; Xu, N. S.; Liu, Y. L. Raman Study of Thermochromic Phase Transition in Tungsten Trioxide Nanowires. Appl. Phys. Lett. 2007, 90, 041919 (1 – 3). [26] Mazumder, N.; Sen, D.; Ghorai, U. K.; Roy, R.; Saha, S.; Das, N. S.; Chattopadhyay, K. K. Realizing Direct Gap, Polytype, Group IIIA Delafossite: Ab Initio Forecast and Experimental Validation Considering Prototype CuAlO2. J. Phys. Chem. Lett. 2013, 4, 3539 – 3543. [27] Li, S.; Li, Q.; Wang, K.; Zhou, M.; Huang, X.; Liu, J.; Yang, K.; Liu, B.; Cui, T.; Zou, G. et al. Pressure-Induced Irreversible Phase Transition in the Energetic Material Urea Nitrate: Combined Raman Scattering and X-ray Diffraction Study. J. Phys. Chem. C 2013, 117, 152 – 159. [28] Yan, T.; Wang, K.; Tan, X.; Yang, K.; Liu, B.; Zou, B. Pressure-Induced Phase Transition in N–H···O Hydrogen-Bonded Molecular Crystal Biurea: Combined Raman Scattering and X-ray Diffraction Study. J. Phys. Chem. C 2014, 118, 15162 – 15168. [29] Basova, T. V.; Kiselev, V. G.; Schuster, B. E.; Peisert, H.; Chassé, T. Experimental and Theoretical Investigation of Vibrational Spectra of Copper Phthalocyanine: Polarized SingleCrystal Raman Spectra, Isotope Effect and DFT Calculations. J. Raman Spectrosc. 2009, 40, 2080 (1 – 8). [30] Gupta, R.; Xiong, Q.; Adu, C. K.; Kim, U. J.; Eklund, P. C. Laser-Induced Fano Resonance Scattering in Silicon Nanowires. Nano Lett. 2003, 3, 627 – 631. [31] Erk, P.; Hengelsberg, H. The Porphyrin Handbook (Academic, Boston, 2002) [32] Brown, C. J. Crystal Structure of β-Copper Phthalocyanine. J. Chem. Soc. A 1968, 1968, 2488 – 2493.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[33] Giovannetti, G.; Brocks, G.; van den Brink, J. Ab Initio Electronic Structure and Correlations in Pristine and Potassium-Doped Molecular Crystals of Copper Phthalocyanine. Phys. Rev B 2008, 77, 035133 (1 – 8). [34] Heutz, S.; Bayliss, S. M.; Middleton, R. L.; Rumbles, G.; Jones, T. S. Polymorphism in Phthalocyanine Thin Films: Mechanism of the α → β Transition. J. Phys. Chem. B 2000, 104, 7124 – 7129. [35] Assour, J. M. On the Polymorphic Modifications of Phthalocyanines. J. Phys. Chem. 1965, 69, 2295 – 2299. [36] Hassan, A. K.; Gould, R. D. Structural Studies of Thermally Evaporated Thin Films of Copper Phthalocyanine. Phys. Stat. Sol. (a) 1992, 132, 91 – 101. [37] Snow, A. W.; Barger, W. R. Phthalocyanine. Properties and Applications (VCH, New York, 1989) p. 362. [38] Cerdeira, F.; Garriga, M.; Alonso, M. I.; Ossó, J. O.; Schreiber, F.; Dosch, H.; Cardona, M. Raman Spectroscopy as a Probe of Molecular Order, Orientation, and Stacking of Fluorinated Copper-Phthalocyanine (F16CuPc) Thin Films. J. Raman Spectrosc. 2013, 44, 597 (1 – 11).


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