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Growth and characterizations of tin-doped nickel-phthalocyanine thin film prepared by thermal co-evaporation as a novel nanomaterial
Growth and characterizations of tin-doped nickel-phthalocyanine thin film prepared by thermal co-evaporation as a novel nanomaterial N. Kayunkid, N. Tammarugwattana, K. Mano, A. Rangkasikorn, J. Nukeaw PII: DOI: Reference:
S0257-8972(16)30385-1 doi: 10.1016/j.surfcoat.2016.05.022 SCT 21177
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
2 February 2016 25 April 2016 8 May 2016
Please cite this article as: N. Kayunkid, N. Tammarugwattana, K. Mano, A. Rangkasikorn, J. Nukeaw, Growth and characterizations of tin-doped nickelphthalocyanine thin film prepared by thermal co-evaporation as a novel nanomaterial, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.05.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Growth and Characterizations of Tin-doped Nickel-phthalocyanine Thin Film Prepared by Thermal Co-evaporation as a Novel Nanomaterial
College of Nanotechnology King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok 10400,
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Thailand
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N. Kayunkida,b,c*, N. Tammarugwattanaa,d, K. Manoa,e, A. Rangkasikorna,b,c, and J. Nukeawa,b,c
Nanotec-KMITL Center of Excellence on Nanoelectronic Devices, Ladkrabang, Bangkok
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Faculty of Engineering King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520,
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10520, Thailand
Thailand
Faculty of Industry Education King Mongkut’s Institute of Technology Ladkrabang, Bangkok
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10520, Thailand
*Corresponding author: Email: [email protected]; Tel: +66-2-3298000 Ext. 2134; Fax: +66-2-3298106; Postal address: College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Chalongkrung road, Ladkrabang, Bangkok 10520, Thailand
Abstract The aim of this research is to control specific properties of nickel-phthalocyanine (NiPc) thin film by doping with tin (Sn). The hybrid thin films, Sn-doped NiPc, were fabricated by thermal co-evaporation as a function of Sn concentration. The quantity of Sn in NiPc matrix was controlled via the different deposition rate between Sn and NiPc. The specific properties of the hybrid films, e.g. morphology, optical absorption, chemical bonding as well as electrical
ACCEPTED MANUSCRIPT characteristics of the devices used such hybrid material as an active layer were characterized by combinations of microscopic and spectroscopic techniques. The experimental results evidently
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present the modification of thin film properties by adding Sn into NiPc matrix, i.e. the change of
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morphology from granules to fibers, the increase of beta-phase formation in the films as well as the enhancement of electrical properties resulting from the increase of both charge carrier
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mobility and carrier concentration in the hybrid material. Moreover, the internal formation of the
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Sn-doped NiPc reveals that Sn dopants are embedded in the NiPc matrix as Sn metal clusters coated with derivative metal oxide of Sn (SnOx). This research demonstrates that the doping
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metal-phthalocyanine with metal is an alternative approach to control the specific properties that
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possibly suit for organic electronic applications.
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Keywords: metal-phthalocyanine; metal-doped organic materials; thermal co-evaporation.
Abstract code: BO27
1. Introduction
During recent decades, organic semiconducting materials have become one of the key materials gaining tremendous interests from both industrial and academic viewpoints [1]. Due to highly versatile properties and simple preparation method of these materials, organic semiconducting materials have been used in several research fields such as organic electronics, chemical and biological sensors as well as renewable energy [2-5]. Contrast to the excellent properties, these materials encounter with some crucial drawbacks i.e. lower environmental stability and lower conductivity resulting from lacking of carrier concentration and carrier
ACCEPTED MANUSCRIPT mobility comparing to inorganic semiconducting materials [6-9]. Several approaches have been proposed to solve these limitations, for examples, modification of molecular structure via
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synthesis to achieve desirable properties or doping with donor or acceptor atom/molecules that
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shows significant enhancement in both electrical and optical properties [10-13]. It is well known that, by doping small amount of organic or metal dopants (~0.5-5%) into organic semiconducting
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material, it results in significant change in conductivity by many orders of magnitude [14]. In
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order to use the doping approach to enhance specific properties of organic semiconducting materials, three components need to be considered; (i) host material (ii) dopant and (iii) doping
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method.
Among the organic semiconducting materials, metal-phthalocyanine (M-Pcs) e.g. CuPc, ZnPc
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or NiPc, is one of the famous materials used as an injecting/transporting layer as well as an active layer especially in organic optoelectronic devices [15-16]. In these familial compounds,
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nickel-phthalocyanine (NiPc) presents outstanding specific properties such as high carrier conductivity, high chemical and thermal stability as well as suitable energy level and band gap for applying to organic optoelectronic applications [17-18]. Therefore, NiPc was chosen as the host material in this work. From the dopant viewpoints, alkali or halogen elements, transition metal as well as organic and inorganic compounds are famously used in order to enhance specific properties of organic semiconducting materials [14, 19-20]. Recently, Nukeaw et al. proposed an alternative approach to control the morphological and optical and properties of MPcs by doping with rich electron material such as metal in group IV. Moreover, by doping group IV metal into M-Pcs, the hybrid thin film presents significant enhancement in conductivity resulting from increment of both carrier mobility and carrier concentration [21]. So far, it has been only few publications reported the doping of organic semiconductor with metal in group IV
ACCEPTED MANUSCRIPT such as tin (Sn), germanium (Ge) or lead (Pb) [21]. Therefore, understanding of the effect of doping this type of dopant on the specific properties of doped materials is an interesting research
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topic that possibly provides an alternative novel nanomaterial for using in organic electronic
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applications. On the preparation point of views, there are a lot of approaches possibly employed for doping process i.e. vapor doping method that requires low manufacturing cost but allows
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limiting controllable parameters or ion-implantation method that provides precisely controllable
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doping parameters but exchange with not only expensive system but also complicating method [22-24]. Contrast to aforementioned methods, co-evaporation with thermal sources is one of the
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promising doping method due to less complicated process and possible to control doping parameters through controlling deposition rate between host and dopant. Moreover, co-
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evaporation allows the doping system to form thin film without the effect of host material structure found in abovementioned methods.
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In this work, the novel nanomaterial “Sn-doped NiPc” thin films were prepared by thermal co-evaporation under high vacuum as a function of quantity of Sn component in the hybrid film and, afterward, characterized their morphological, optical, chemical bond and electrical properties to understand the fundamental knowledge and to investigate the possibility to use such novel nanomaterial as a candidate for organic electronic applications.
2. Experiments Nickel-phthalocyanine powder and tin pellets were purchased from Sigma-Aldrich Co. Ltd. and used without any purification. Sn-doped NiPc hybrid thin films were prepared on three different type of substrates: microscope slide, intrinsic silicon, and patterned indium tin oxide coated on a glass slide. All substrates were cleaned by deionized water, acetone, methanol and
ACCEPTED MANUSCRIPT isopropanol in sonicating bath for 15 minutes per step. After the cleaning process, the substrates were dried with nitrogen and were directly transferred to the evaporating chamber. The chamber
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was pumped down to base pressure of 5x10-6 mbar before starting the thermal co-evaporation
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process. NiPc powder was evaporated in the alumina crucible while Sn pellets were melted and vaporized in the tungsten boat. Each thermal source is equipped with individual quartz crystal
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microbalance (QCM) for monitoring the deposition rate and thin film thickness. Both thermal
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sources are covered by stainless plate to avoid thermal radiation taken place during evaporation process. Moreover, the temperature in the chamber during the deposition process was monitored
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by K-type thermocouple placed close to the substrate holder. The distance between deposition source and the rotating substrate holder was fixed at 30 centimeters. The amount of Sn
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component in the doped films was controlled by using different deposition rates between Sn and NiPc. The higher the evaporating rate of Sn used in the preparation, the higher the Sn
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concentration observed in hybrid film. The deposition rate of NiPc was fixed at 0.7 Å/s while the deposition rate of Sn was changed from 0, 0.1, 0.3 and 0.7 Å/s, respectively. The film’s total thickness of all preparation conditions was fixed to 200 nm. The topography of hybrid films was observed by field-emission scanning electron microscopy (FESEM; Hitachi S4700). The absorption spectrum of hybrid films in the range of 330-900 nm was collected by UV-visible spectroscopy (PG Instrument T90+). The bond vibrational strength in the range of 550-1650 cm-1 was collected by Raman spectroscopy (Thermo Scientific DXR Smart Raman Spectrometer with excitation wavelength of 532 nm). Moreover, the chemical bonds of Sn in the hybrid film were revealed by X-ray photoemission spectroscopy (XPS; Kratos Analytical AXIS Ultra DLD). Electrical properties, e.g., charge carrier mobility (μ) and carrier concentration (η), were extracted from current-voltage and capacitance-voltage (giving frequency of 900 Hz)
ACCEPTED MANUSCRIPT characteristics of Indium Tin Oxide (ITO)/Sn-doped NiPc (200 nm)/Aluminium (Al) (100 nm) devices (Agilent E4980A). The values of charge carrier mobility and carrier concentration of the
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devices were estimated from space-charge-limited current (SCLC) and capacitance-voltage
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characteristics as explained in reference 21.
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3. Results and discussion
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The surface morphology of Sn-doped NiPc thin films obtained from different concentrations of Sn are shown in Fig. 1. The undoped NiPc film clearly shows granular structure that is
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regularly obtained from NiPc film prepared by thermal evaporation [25]. By increasing Sn concentration into the doped film, the transition of morphology from granules to fibers is
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observed. Although the fibrillar structure of metal-phthalocyanine thin film is generally induced by substrate annealing process, all the samples in this work were prepared without giving
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temperature to the substrates [25]. Therefore, the morphological transition should be induced by the transferred energy from something to NiPc. Two possibilities are (i) the transferred energy from Sn dopants to NiPc due to the higher evaporating temperature of Sn than that of NiPc or (ii) the increasing temperature in the chamber caused by thermal radiation from evaporating sources during deposition process. Nevertheless, the temperature in the chamber during the deposition process measured by the thermocouple placed close to the substrate was around 60 oC. Such temperature does not provide enough energy to change NiPc’s morphology since NiPc requires higher temperature than 150 oC for morphological transition [25]. Therefore, the transition of the surface morphology of Sn-doped NiPc film can be concluded as a result of energy transferred from Sn dopants to NiPc domains.
ACCEPTED MANUSCRIPT The absorption spectra of Sn-doped NiPc films prepared with different deposition rates of Sn and NiPc are shown in Fig. 2(a). All the samples clearly exhibit two absorption peaks at 620 nm
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(~2 eV) and 675 nm (~1.84 eV) corresponding to the Q-band transition of NiPc formed as alpha-
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phase [25-26]. By increasing Sn concentration, the significant changes in both intensity and shape of the absorption spectrum are observed. The sequent decline in the absorption intensity
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with the increase of Sn concentration can be explained by the reduction in the NiPc content in
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the doped films. Moreover, the decreasing ratio between the peak intensity at 620 and 675 nm in the doped sample indicates the appearance of beta-phase NiPc as minority component in the
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doped film. It is generally known that the phase transition from alpha-phase to beta-phase can be achieved by substrate annealing (>150 oC) [25]. However, in this work, the transition is not
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caused by direct substrate annealing since all the samples were fabricated at room temperature
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and the chamber’s temperature during evaporation was approximately 60 oC. Moreover, the
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indirect annealing caused by thermal radiation from evaporation source can be neglected since there are thermal shields covering both thermal sources. Consequently, the key inducing phase transition of NiPc should be associated with the energy transferred from the Sn atom/cluster to the adjacent NiPc domain during the film formation process. Hence, the appearance of betaphase as the minority component in the doped film is an evidence to support the idea of energy transfer from Sn to NiPc that explains the morphological transition in the Sn-doped NiPc film. Additionally, the extension of the absorption in the range of 450 nm to approximately 520 nm that is observed in the higher Sn-doped samples is noteworthy to be considered. This expansion should be associated with the absorption of Sn compound formed into doped NiPc film. The origin of the expansion of the absorption tail will be discussed afterword in the section of chemical bonding analysis.
ACCEPTED MANUSCRIPT In order to understand the internal formation of Sn-NiPc matrix in the doped film, Raman spectroscopy and XPS were used to investigate the interaction between Sn and NiPc taken placed
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in the hybrid films. Raman absorption spectra in the range of 550-1650 cm-1 obtained from
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undoped NiPc and Sn-doped NiPc thin films with different Sn concentrations are shown in Fig. 2(b). The peaks appearing in this range of Raman shift are associated with the vibrational modes
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in the NiPc molecule, e.g. 592, 687, 1339, and 1552 cm-1 corresponding to benzene-ring
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deformation, macrocycle breathing, half-pyrrole stretching, and pyrrole stretching, respectively [27]. Hence, it is possible to identify the chemical bond between Sn and NiPc formed inside the
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Sn-NiPc matrix by considering the shift of the raman peak. Interestingly, no shift of raman peak is observed in the doped samples. This result indicates that Sn and NiPc are separately formed
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and no chemical bond is taken place between Sn atom and NiPc molecule in the Sn-doped NiPc thin film. Furthermore, the sequential declination of the raman peak intensity observed in the
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higher Sn concentration samples is simply described by the decrease of the NiPc component in the Sn-doped NiPc film. In addition, the phase formation of M-Pcs is able to distinguish by considering the peak intensity at 592 and 1552 cm-1. The dominant peak at 1552 cm-1 represents the alpha-phase formation while the dominant peak at 592 cm-1 indicates the beta-phase formation of M-Pcs thin film [28]. In this work, all the doped films show the dominant peak at 1552 cm-1 indicating the formation of alpha-phase as a majority in the doped films. Nevertheless, because beta-phase NiPc is formed in the hybrid film as minority component, the appearance of the beta-phase formation is not evidently observed in raman spectrum of the doped films. The XPS spectra represent the binding energy of the Sn3d core level obtained from Sn-doped NiPc films prepared under different evaporation conditions are exhibited in Fig. 3. All the samples exhibit two dominant binding energy in the range of 487.4-487.9 eV and 495.8-496.3
ACCEPTED MANUSCRIPT attributed to Sn3d3/2 and Sn3d5/2, respectively [29]. When the Sn content in the doped film is increased, the peaks tend to shift toward to lower binding energy, indicating the appearance of
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alternative species of Sn formed in the Sn-NiPc matrix. To further information, the fitting approach was applied to Sn3d spectra in order to identify Sn species formed in the hybrid films.
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Three fitting components, e.g., Sn4+ at 487.4 eV, Sn2+ at 496.0 eV and Sn-O at 487.9 and 496.0
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eV, are presented in Fig 3 [30]. The fitting results suggest two species of Sn formed in the hybrid
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film: (i) metal cluster of Sn and (ii) derivative oxide of Sn (SnOx). Furthermore, the increase of Sn4+ and Sn2+ components in the highly-doped samples may indicate that the Sn-O formed in
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hybrid film is native oxide covering the surface of Sn metal clusters embedded in Sn-doped NiPc thin film.
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According to the extension of the absorption tail observed in the samples with high Sn concentrations, two possible factors related to this expansion are the absorption from (i) tin metal
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cluster or (ii) tin oxide derivatives (SnOx). The absorption spectrum of tin metal clearly shows strong absorption over the range of 300-800 nm, but there is no dominant peak in this range [31]. Therefore, the effect of tin metal on the shape of the absorption spectrum should present as an upward shift of the baseline. Therefore, it can be concluded that the expansion of the absorption tail observed in the high-Sn-concentration samples results from the absorption of the derivative of tin oxide in the Sn-doped NiPc thin film. The current-voltage characteristics of ITO/Sn-doped NiPc/Al devices are presented in Fig. 4(a). The enhancement of the current flowed in the devices prepared with higher Sn concentration is observed. The electrical properties of the devices, e.g., charge carrier mobility and carrier concentration were extracted from SCLC approach and taking the capacitancevoltage characteristic curve into consideration [21]. Both the carrier mobility and carrier
ACCEPTED MANUSCRIPT concentration tend to increase with increasing quantity of Sn dopant in the hybrid film, as shown in Fig. 4(b). Since conductivity in material is directly related to these two parameters, it can be
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concluded that doping with Sn into NiPc can enhance the conductivity of the doped thin film.
4. Conclusions
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Sn-doped NiPc thin films were fabricated and characterized to investigate the influences of
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the Sn concentration on specific properties of hybrid films. The results show clear evidence that doping Sn into NiPc significantly modifies the surface morphology and absorption behavior as
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well as the crystalline packing of the hybrid films. The key parameter controlling morphological, optical, and crystalline transitions is associated with the transferred energy between metal
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dopants to organic host material. Moreover, spectroscopic results revealed that no interaction between Sn and NiPc taken place in the hybrid film and Sn dopants in the doped films are
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formed as metal clusters covered by derivative of tin oxide (SnOx). Furthermore, the increment of Sn concentration in the doped films significantly enhances both the carrier mobility and carrier concentration of Sn-doped NiPc films. Interestingly, the metal-doping approach is an alternative way to modify the specific properties of NiPc that should provides greatly benefits for applying this novel nanomaterial to organic electronic applications.
Acknowledgements This work has been supported by National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of the Center of Excellence Network.
ACCEPTED MANUSCRIPT References [1] S. R. Forrest, Nature 428 (2004) 911.
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[2] B. Geffroy, P. L. Roy, C. Prat, Polym. Int. 55 (2006) 572.
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[3] C. D. Dimitrakopoulos, D. J. Mascaro, IBM J. Res. Dev. 45 (2001) 11. [4] S. Gunes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 107 (2007) 1324.
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[5] F. I. Bohrer, A. Sharoni, C. Colesniuc, J. Park, I. K. Schuller, A. C. Kummel, W. C. Trogler,
[6] S. Riad, Thin Solid Films 370 (2000) 253.
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[10] M. Pfeiffer, K. Leo, X. Zhou, J. S. Huang, M. Hofmann, A. Wemer, J. Blochwitz-Nimoth, Org. Electron. 4 (2003) 89.
[11] M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 3202. [12] J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D. M. Alloway, P. A. Lee, N. R. Armstrong, Org. Electron. 12 (2001) 97. [13] W. Gao, A. Kahn, Appl. Phys. Lett. 79 (2001) 4040. [14] B. Lussem, M. Riede, K. Leo, Phys. Status Solidi A. 210 (2013) 9. [15] C. C. Leznoff and A. B. P. Lever, Phthalocyanines: Properties and Applications (WileyVCH, Weinheim, 1992). [16] B. Nell Mckeown, Phthalocyanine Materials: Synthesis, Structure and Function (Cambridge University Press, Cambridge, U.K., 1998).
ACCEPTED MANUSCRIPT [17] M. M. El-Nahass, K. F. Abd-El-Rahman, A. A. M. Farag, A. A. A. Darwish, Org. Electron. 6 (2005) 129.
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[18] M. Neghabi, M. Zadzar, S. M. B. Ghorashi, Mater. Sci. Semicond. Process. 17 (2014) 13.
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[19] K. Hayashi, T. Shinano, Y. Miyazaki, T. Kajitani, J. Appl. Phys. 109 (2011) 023712. [20] B. Fang, Z. Haoshen, I. Honma, Appl. Phys. Lett. 89 (2006) 023102.
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[21] N. Kayunkid, A. Rangkasikorn, C. Saributr, J. Nukeaw, Jpn. J. Appl. Phys. 55 (2016)
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[25] M. T. Hussein, E. M. Nasir, A. H. Al-Aarajiy, Adv. Mater. Phys. Chem. 3 (2013) 113. [26] I. Bruder, J. Schoneboom, R. Dinnebier, A. Ojala, S. Schafer, R. Sens, P. Erk, J. Weisi, Org. Electron. 11 (2010) 377. [27] M. Szybowicz, W. Bata, K. Fabisiak, K. Paprocki, M. Drozdowski, J. Mater. Sci. 46 (2011) 6589.
[28] L. Gaffo, M. R. Cordeiro, A. R. Freitas, W. C. Moreira, E. M. Girotto, V. Zucolotto, J. Mater. Sci. 45 (2010) 1366. [29] Y. Her, J. Wu, Y. Lin, S. Tsai, Appl. Phys. Lett. 89 (2006) 043115. [30] B. V. Crist, Handbook of Monochromatic XPS Spectra (Wiley-VCH, Weinheim, 2000). [31] M. F. Melendreza, C. Vargas-Hernandez, Rev. Mex. Fis. 59 (2013) 39.
ACCEPTED MANUSCRIPT Figure captions Fig. 1. FE-SEM images represent surface morphology of undoped NiPc film and Sn-doped NiPc
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films with different concentrations of Sn. The label in each figure exhibits the ratios of the
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evaporation rates between Sn and NiPc in the unit of Å/s. The bar in each image represents the
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scale of 500 nm.
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Fig. 2. (a) Absorption spectra in the range of 330-900 nm and (b) Raman spectra in the range of 550-1650 cm-1 of undoped NiPc and Sn-doped NiPc films prepared with different concentrations
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of Sn. The labels in each figure represent the ratios of the evaporation rates of Sn and NiPc in the
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unit of Å/s.
Fig. 3. XPS spectra of Sn3d core level obtained from Sn-doped NiPc films prepared with
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different concentration of Sn. The numbers in the parentheses in each figure indicate the ratios of the evaporation rates of Sn and NiPc in the unit of Å/s.
Fig. 4. (a) Current-voltage characteristics and (b) Charge carrier concentration (η) (circles) and carrier mobility (μ) (squares) obtained from ITO/NiPc/Al and ITO/Sn-doped NiPc/Al devices prepared with different Sn concentrations.
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Fig. 1. FE-SEM images represent surface morphology of undoped NiPc film and Sn-doped NiPc
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films with different concentrations of Sn. The label in each figure exhibits the ratios of the evaporation rates between Sn and NiPc in the unit of Å/s. The bar in each image represents the scale of 500 nm.
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Fig. 2. (a) Absorption spectra in the range of 330-900 nm and (b) Raman spectra in the range of 550-1650 cm-1 of undoped NiPc and Sn-doped NiPc films prepared with different concentrations of Sn. The labels in each figure represent the ratios of the evaporation rates of Sn and NiPc in the unit of Å/s.
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Fig. 3. XPS spectra of Sn3d core level obtained from Sn-doped NiPc films prepared with
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different concentration of Sn. The numbers in the parentheses in each figure indicate the ratios of
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the evaporation rates of Sn and NiPc in the unit of Å/s.
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Fig. 4. (a) Current-voltage characteristics and (b) Charge carrier concentration (η) (circles) and carrier mobility (μ) (squares) obtained from ITO/NiPc/Al and ITO/Sn-doped NiPc/Al devices prepared with different Sn concentrations.
ACCEPTED MANUSCRIPT Highlights: Sn-doped NiPc is a novel hybrid nanomaterial
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Morphological and Phase transition induced by energy transfer from Sn to NiPc
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Enhancement of electrical properties by means of doping Sn into NiPc film
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• Combining spectroscopy techniques to determine formation of Sn in the hybrid film
S0257-8972(16)30385-1 doi: 10.1016/j.surfcoat.2016.05.022 SCT 21177
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
2 February 2016 25 April 2016 8 May 2016
Please cite this article as: N. Kayunkid, N. Tammarugwattana, K. Mano, A. Rangkasikorn, J. Nukeaw, Growth and characterizations of tin-doped nickelphthalocyanine thin film prepared by thermal co-evaporation as a novel nanomaterial, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.05.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Growth and Characterizations of Tin-doped Nickel-phthalocyanine Thin Film Prepared by Thermal Co-evaporation as a Novel Nanomaterial
College of Nanotechnology King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok 10400,
c
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Thailand
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b
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a
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N. Kayunkida,b,c*, N. Tammarugwattanaa,d, K. Manoa,e, A. Rangkasikorna,b,c, and J. Nukeawa,b,c
Nanotec-KMITL Center of Excellence on Nanoelectronic Devices, Ladkrabang, Bangkok
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Faculty of Engineering King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520,
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10520, Thailand
Thailand
Faculty of Industry Education King Mongkut’s Institute of Technology Ladkrabang, Bangkok
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10520, Thailand
*Corresponding author: Email: [email protected]; Tel: +66-2-3298000 Ext. 2134; Fax: +66-2-3298106; Postal address: College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Chalongkrung road, Ladkrabang, Bangkok 10520, Thailand
Abstract The aim of this research is to control specific properties of nickel-phthalocyanine (NiPc) thin film by doping with tin (Sn). The hybrid thin films, Sn-doped NiPc, were fabricated by thermal co-evaporation as a function of Sn concentration. The quantity of Sn in NiPc matrix was controlled via the different deposition rate between Sn and NiPc. The specific properties of the hybrid films, e.g. morphology, optical absorption, chemical bonding as well as electrical
ACCEPTED MANUSCRIPT characteristics of the devices used such hybrid material as an active layer were characterized by combinations of microscopic and spectroscopic techniques. The experimental results evidently
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present the modification of thin film properties by adding Sn into NiPc matrix, i.e. the change of
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morphology from granules to fibers, the increase of beta-phase formation in the films as well as the enhancement of electrical properties resulting from the increase of both charge carrier
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mobility and carrier concentration in the hybrid material. Moreover, the internal formation of the
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Sn-doped NiPc reveals that Sn dopants are embedded in the NiPc matrix as Sn metal clusters coated with derivative metal oxide of Sn (SnOx). This research demonstrates that the doping
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metal-phthalocyanine with metal is an alternative approach to control the specific properties that
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possibly suit for organic electronic applications.
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Keywords: metal-phthalocyanine; metal-doped organic materials; thermal co-evaporation.
Abstract code: BO27
1. Introduction
During recent decades, organic semiconducting materials have become one of the key materials gaining tremendous interests from both industrial and academic viewpoints [1]. Due to highly versatile properties and simple preparation method of these materials, organic semiconducting materials have been used in several research fields such as organic electronics, chemical and biological sensors as well as renewable energy [2-5]. Contrast to the excellent properties, these materials encounter with some crucial drawbacks i.e. lower environmental stability and lower conductivity resulting from lacking of carrier concentration and carrier
ACCEPTED MANUSCRIPT mobility comparing to inorganic semiconducting materials [6-9]. Several approaches have been proposed to solve these limitations, for examples, modification of molecular structure via
PT
synthesis to achieve desirable properties or doping with donor or acceptor atom/molecules that
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shows significant enhancement in both electrical and optical properties [10-13]. It is well known that, by doping small amount of organic or metal dopants (~0.5-5%) into organic semiconducting
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material, it results in significant change in conductivity by many orders of magnitude [14]. In
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order to use the doping approach to enhance specific properties of organic semiconducting materials, three components need to be considered; (i) host material (ii) dopant and (iii) doping
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method.
Among the organic semiconducting materials, metal-phthalocyanine (M-Pcs) e.g. CuPc, ZnPc
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or NiPc, is one of the famous materials used as an injecting/transporting layer as well as an active layer especially in organic optoelectronic devices [15-16]. In these familial compounds,
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nickel-phthalocyanine (NiPc) presents outstanding specific properties such as high carrier conductivity, high chemical and thermal stability as well as suitable energy level and band gap for applying to organic optoelectronic applications [17-18]. Therefore, NiPc was chosen as the host material in this work. From the dopant viewpoints, alkali or halogen elements, transition metal as well as organic and inorganic compounds are famously used in order to enhance specific properties of organic semiconducting materials [14, 19-20]. Recently, Nukeaw et al. proposed an alternative approach to control the morphological and optical and properties of MPcs by doping with rich electron material such as metal in group IV. Moreover, by doping group IV metal into M-Pcs, the hybrid thin film presents significant enhancement in conductivity resulting from increment of both carrier mobility and carrier concentration [21]. So far, it has been only few publications reported the doping of organic semiconductor with metal in group IV
ACCEPTED MANUSCRIPT such as tin (Sn), germanium (Ge) or lead (Pb) [21]. Therefore, understanding of the effect of doping this type of dopant on the specific properties of doped materials is an interesting research
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topic that possibly provides an alternative novel nanomaterial for using in organic electronic
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applications. On the preparation point of views, there are a lot of approaches possibly employed for doping process i.e. vapor doping method that requires low manufacturing cost but allows
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limiting controllable parameters or ion-implantation method that provides precisely controllable
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doping parameters but exchange with not only expensive system but also complicating method [22-24]. Contrast to aforementioned methods, co-evaporation with thermal sources is one of the
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promising doping method due to less complicated process and possible to control doping parameters through controlling deposition rate between host and dopant. Moreover, co-
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evaporation allows the doping system to form thin film without the effect of host material structure found in abovementioned methods.
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In this work, the novel nanomaterial “Sn-doped NiPc” thin films were prepared by thermal co-evaporation under high vacuum as a function of quantity of Sn component in the hybrid film and, afterward, characterized their morphological, optical, chemical bond and electrical properties to understand the fundamental knowledge and to investigate the possibility to use such novel nanomaterial as a candidate for organic electronic applications.
2. Experiments Nickel-phthalocyanine powder and tin pellets were purchased from Sigma-Aldrich Co. Ltd. and used without any purification. Sn-doped NiPc hybrid thin films were prepared on three different type of substrates: microscope slide, intrinsic silicon, and patterned indium tin oxide coated on a glass slide. All substrates were cleaned by deionized water, acetone, methanol and
ACCEPTED MANUSCRIPT isopropanol in sonicating bath for 15 minutes per step. After the cleaning process, the substrates were dried with nitrogen and were directly transferred to the evaporating chamber. The chamber
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was pumped down to base pressure of 5x10-6 mbar before starting the thermal co-evaporation
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process. NiPc powder was evaporated in the alumina crucible while Sn pellets were melted and vaporized in the tungsten boat. Each thermal source is equipped with individual quartz crystal
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microbalance (QCM) for monitoring the deposition rate and thin film thickness. Both thermal
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sources are covered by stainless plate to avoid thermal radiation taken place during evaporation process. Moreover, the temperature in the chamber during the deposition process was monitored
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by K-type thermocouple placed close to the substrate holder. The distance between deposition source and the rotating substrate holder was fixed at 30 centimeters. The amount of Sn
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component in the doped films was controlled by using different deposition rates between Sn and NiPc. The higher the evaporating rate of Sn used in the preparation, the higher the Sn
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concentration observed in hybrid film. The deposition rate of NiPc was fixed at 0.7 Å/s while the deposition rate of Sn was changed from 0, 0.1, 0.3 and 0.7 Å/s, respectively. The film’s total thickness of all preparation conditions was fixed to 200 nm. The topography of hybrid films was observed by field-emission scanning electron microscopy (FESEM; Hitachi S4700). The absorption spectrum of hybrid films in the range of 330-900 nm was collected by UV-visible spectroscopy (PG Instrument T90+). The bond vibrational strength in the range of 550-1650 cm-1 was collected by Raman spectroscopy (Thermo Scientific DXR Smart Raman Spectrometer with excitation wavelength of 532 nm). Moreover, the chemical bonds of Sn in the hybrid film were revealed by X-ray photoemission spectroscopy (XPS; Kratos Analytical AXIS Ultra DLD). Electrical properties, e.g., charge carrier mobility (μ) and carrier concentration (η), were extracted from current-voltage and capacitance-voltage (giving frequency of 900 Hz)
ACCEPTED MANUSCRIPT characteristics of Indium Tin Oxide (ITO)/Sn-doped NiPc (200 nm)/Aluminium (Al) (100 nm) devices (Agilent E4980A). The values of charge carrier mobility and carrier concentration of the
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devices were estimated from space-charge-limited current (SCLC) and capacitance-voltage
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characteristics as explained in reference 21.
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3. Results and discussion
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The surface morphology of Sn-doped NiPc thin films obtained from different concentrations of Sn are shown in Fig. 1. The undoped NiPc film clearly shows granular structure that is
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regularly obtained from NiPc film prepared by thermal evaporation [25]. By increasing Sn concentration into the doped film, the transition of morphology from granules to fibers is
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observed. Although the fibrillar structure of metal-phthalocyanine thin film is generally induced by substrate annealing process, all the samples in this work were prepared without giving
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temperature to the substrates [25]. Therefore, the morphological transition should be induced by the transferred energy from something to NiPc. Two possibilities are (i) the transferred energy from Sn dopants to NiPc due to the higher evaporating temperature of Sn than that of NiPc or (ii) the increasing temperature in the chamber caused by thermal radiation from evaporating sources during deposition process. Nevertheless, the temperature in the chamber during the deposition process measured by the thermocouple placed close to the substrate was around 60 oC. Such temperature does not provide enough energy to change NiPc’s morphology since NiPc requires higher temperature than 150 oC for morphological transition [25]. Therefore, the transition of the surface morphology of Sn-doped NiPc film can be concluded as a result of energy transferred from Sn dopants to NiPc domains.
ACCEPTED MANUSCRIPT The absorption spectra of Sn-doped NiPc films prepared with different deposition rates of Sn and NiPc are shown in Fig. 2(a). All the samples clearly exhibit two absorption peaks at 620 nm
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(~2 eV) and 675 nm (~1.84 eV) corresponding to the Q-band transition of NiPc formed as alpha-
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phase [25-26]. By increasing Sn concentration, the significant changes in both intensity and shape of the absorption spectrum are observed. The sequent decline in the absorption intensity
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with the increase of Sn concentration can be explained by the reduction in the NiPc content in
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the doped films. Moreover, the decreasing ratio between the peak intensity at 620 and 675 nm in the doped sample indicates the appearance of beta-phase NiPc as minority component in the
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doped film. It is generally known that the phase transition from alpha-phase to beta-phase can be achieved by substrate annealing (>150 oC) [25]. However, in this work, the transition is not
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caused by direct substrate annealing since all the samples were fabricated at room temperature
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and the chamber’s temperature during evaporation was approximately 60 oC. Moreover, the
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indirect annealing caused by thermal radiation from evaporation source can be neglected since there are thermal shields covering both thermal sources. Consequently, the key inducing phase transition of NiPc should be associated with the energy transferred from the Sn atom/cluster to the adjacent NiPc domain during the film formation process. Hence, the appearance of betaphase as the minority component in the doped film is an evidence to support the idea of energy transfer from Sn to NiPc that explains the morphological transition in the Sn-doped NiPc film. Additionally, the extension of the absorption in the range of 450 nm to approximately 520 nm that is observed in the higher Sn-doped samples is noteworthy to be considered. This expansion should be associated with the absorption of Sn compound formed into doped NiPc film. The origin of the expansion of the absorption tail will be discussed afterword in the section of chemical bonding analysis.
ACCEPTED MANUSCRIPT In order to understand the internal formation of Sn-NiPc matrix in the doped film, Raman spectroscopy and XPS were used to investigate the interaction between Sn and NiPc taken placed
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in the hybrid films. Raman absorption spectra in the range of 550-1650 cm-1 obtained from
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undoped NiPc and Sn-doped NiPc thin films with different Sn concentrations are shown in Fig. 2(b). The peaks appearing in this range of Raman shift are associated with the vibrational modes
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in the NiPc molecule, e.g. 592, 687, 1339, and 1552 cm-1 corresponding to benzene-ring
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deformation, macrocycle breathing, half-pyrrole stretching, and pyrrole stretching, respectively [27]. Hence, it is possible to identify the chemical bond between Sn and NiPc formed inside the
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Sn-NiPc matrix by considering the shift of the raman peak. Interestingly, no shift of raman peak is observed in the doped samples. This result indicates that Sn and NiPc are separately formed
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and no chemical bond is taken place between Sn atom and NiPc molecule in the Sn-doped NiPc thin film. Furthermore, the sequential declination of the raman peak intensity observed in the
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higher Sn concentration samples is simply described by the decrease of the NiPc component in the Sn-doped NiPc film. In addition, the phase formation of M-Pcs is able to distinguish by considering the peak intensity at 592 and 1552 cm-1. The dominant peak at 1552 cm-1 represents the alpha-phase formation while the dominant peak at 592 cm-1 indicates the beta-phase formation of M-Pcs thin film [28]. In this work, all the doped films show the dominant peak at 1552 cm-1 indicating the formation of alpha-phase as a majority in the doped films. Nevertheless, because beta-phase NiPc is formed in the hybrid film as minority component, the appearance of the beta-phase formation is not evidently observed in raman spectrum of the doped films. The XPS spectra represent the binding energy of the Sn3d core level obtained from Sn-doped NiPc films prepared under different evaporation conditions are exhibited in Fig. 3. All the samples exhibit two dominant binding energy in the range of 487.4-487.9 eV and 495.8-496.3
ACCEPTED MANUSCRIPT attributed to Sn3d3/2 and Sn3d5/2, respectively [29]. When the Sn content in the doped film is increased, the peaks tend to shift toward to lower binding energy, indicating the appearance of
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alternative species of Sn formed in the Sn-NiPc matrix. To further information, the fitting approach was applied to Sn3d spectra in order to identify Sn species formed in the hybrid films.
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Three fitting components, e.g., Sn4+ at 487.4 eV, Sn2+ at 496.0 eV and Sn-O at 487.9 and 496.0
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eV, are presented in Fig 3 [30]. The fitting results suggest two species of Sn formed in the hybrid
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film: (i) metal cluster of Sn and (ii) derivative oxide of Sn (SnOx). Furthermore, the increase of Sn4+ and Sn2+ components in the highly-doped samples may indicate that the Sn-O formed in
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hybrid film is native oxide covering the surface of Sn metal clusters embedded in Sn-doped NiPc thin film.
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According to the extension of the absorption tail observed in the samples with high Sn concentrations, two possible factors related to this expansion are the absorption from (i) tin metal
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cluster or (ii) tin oxide derivatives (SnOx). The absorption spectrum of tin metal clearly shows strong absorption over the range of 300-800 nm, but there is no dominant peak in this range [31]. Therefore, the effect of tin metal on the shape of the absorption spectrum should present as an upward shift of the baseline. Therefore, it can be concluded that the expansion of the absorption tail observed in the high-Sn-concentration samples results from the absorption of the derivative of tin oxide in the Sn-doped NiPc thin film. The current-voltage characteristics of ITO/Sn-doped NiPc/Al devices are presented in Fig. 4(a). The enhancement of the current flowed in the devices prepared with higher Sn concentration is observed. The electrical properties of the devices, e.g., charge carrier mobility and carrier concentration were extracted from SCLC approach and taking the capacitancevoltage characteristic curve into consideration [21]. Both the carrier mobility and carrier
ACCEPTED MANUSCRIPT concentration tend to increase with increasing quantity of Sn dopant in the hybrid film, as shown in Fig. 4(b). Since conductivity in material is directly related to these two parameters, it can be
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concluded that doping with Sn into NiPc can enhance the conductivity of the doped thin film.
4. Conclusions
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Sn-doped NiPc thin films were fabricated and characterized to investigate the influences of
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the Sn concentration on specific properties of hybrid films. The results show clear evidence that doping Sn into NiPc significantly modifies the surface morphology and absorption behavior as
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well as the crystalline packing of the hybrid films. The key parameter controlling morphological, optical, and crystalline transitions is associated with the transferred energy between metal
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dopants to organic host material. Moreover, spectroscopic results revealed that no interaction between Sn and NiPc taken place in the hybrid film and Sn dopants in the doped films are
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formed as metal clusters covered by derivative of tin oxide (SnOx). Furthermore, the increment of Sn concentration in the doped films significantly enhances both the carrier mobility and carrier concentration of Sn-doped NiPc films. Interestingly, the metal-doping approach is an alternative way to modify the specific properties of NiPc that should provides greatly benefits for applying this novel nanomaterial to organic electronic applications.
Acknowledgements This work has been supported by National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of the Center of Excellence Network.
ACCEPTED MANUSCRIPT References [1] S. R. Forrest, Nature 428 (2004) 911.
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[28] L. Gaffo, M. R. Cordeiro, A. R. Freitas, W. C. Moreira, E. M. Girotto, V. Zucolotto, J. Mater. Sci. 45 (2010) 1366. [29] Y. Her, J. Wu, Y. Lin, S. Tsai, Appl. Phys. Lett. 89 (2006) 043115. [30] B. V. Crist, Handbook of Monochromatic XPS Spectra (Wiley-VCH, Weinheim, 2000). [31] M. F. Melendreza, C. Vargas-Hernandez, Rev. Mex. Fis. 59 (2013) 39.
ACCEPTED MANUSCRIPT Figure captions Fig. 1. FE-SEM images represent surface morphology of undoped NiPc film and Sn-doped NiPc
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films with different concentrations of Sn. The label in each figure exhibits the ratios of the
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evaporation rates between Sn and NiPc in the unit of Å/s. The bar in each image represents the
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scale of 500 nm.
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Fig. 2. (a) Absorption spectra in the range of 330-900 nm and (b) Raman spectra in the range of 550-1650 cm-1 of undoped NiPc and Sn-doped NiPc films prepared with different concentrations
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of Sn. The labels in each figure represent the ratios of the evaporation rates of Sn and NiPc in the
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unit of Å/s.
Fig. 3. XPS spectra of Sn3d core level obtained from Sn-doped NiPc films prepared with
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different concentration of Sn. The numbers in the parentheses in each figure indicate the ratios of the evaporation rates of Sn and NiPc in the unit of Å/s.
Fig. 4. (a) Current-voltage characteristics and (b) Charge carrier concentration (η) (circles) and carrier mobility (μ) (squares) obtained from ITO/NiPc/Al and ITO/Sn-doped NiPc/Al devices prepared with different Sn concentrations.
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Fig. 1. FE-SEM images represent surface morphology of undoped NiPc film and Sn-doped NiPc
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films with different concentrations of Sn. The label in each figure exhibits the ratios of the evaporation rates between Sn and NiPc in the unit of Å/s. The bar in each image represents the scale of 500 nm.
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Fig. 2. (a) Absorption spectra in the range of 330-900 nm and (b) Raman spectra in the range of 550-1650 cm-1 of undoped NiPc and Sn-doped NiPc films prepared with different concentrations of Sn. The labels in each figure represent the ratios of the evaporation rates of Sn and NiPc in the unit of Å/s.
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Fig. 3. XPS spectra of Sn3d core level obtained from Sn-doped NiPc films prepared with
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different concentration of Sn. The numbers in the parentheses in each figure indicate the ratios of
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the evaporation rates of Sn and NiPc in the unit of Å/s.
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Fig. 4. (a) Current-voltage characteristics and (b) Charge carrier concentration (η) (circles) and carrier mobility (μ) (squares) obtained from ITO/NiPc/Al and ITO/Sn-doped NiPc/Al devices prepared with different Sn concentrations.
ACCEPTED MANUSCRIPT Highlights: Sn-doped NiPc is a novel hybrid nanomaterial
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Morphological and Phase transition induced by energy transfer from Sn to NiPc
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Enhancement of electrical properties by means of doping Sn into NiPc film
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• Combining spectroscopy techniques to determine formation of Sn in the hybrid film