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Field emission from copper phthalocyanine and copper hexadecafluorophthalocyanine nanowires
Materials Letters 61 (2007) 3842 – 3846 www.elsevier.com/locate/matlet
Field emission from copper phthalocyanine and copper hexadecafluorophthalocyanine nanowires W.Y. Tong a , Z.X. Li b , A.B. Djurišić a,⁎, W.K. Chan c , S.F. Yu b a
b
Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore c Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Received 8 July 2006; accepted 21 December 2006 Available online 30 December 2006
Abstract Copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc) nanowires were fabricated by vapor deposition. The nanowires were studied by scanning electron microscopy and transmission electron microscopy. Field emission properties of these nanowires were studied. The field emission properties were strongly dependent on the substrate temperature and the material used, and the best results are obtained for β-phase CuPc nanoribbons. Different dependences of field emission properties on the substrate temperature were obtained for the two materials investigated. The obtained results are discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Vb; 81.07.Nb Keywords: Organic semiconductors; Nanostructures; Field emission
1. Introduction Among various applications of nanostructures, field emission is of considerable interest. Field emission properties of different materials, such as semiconductor nanowires [1–3], carbon nanotubes [4–6], and organic materials [7–9], have been studied. The properties are highly dependent on the type of material, as well as on the morphology and fabrication method [2]. For example, depending on the carbon nanotube electrode preparation method, turn-on fields in the range 1.33–6 V/μm can be obtained [4–6]. Compared to inorganic nanowires and carbon nanotubes, studies on the fabrication and field emission properties of organic materials have been scarce. Conducting poly(3,4-ethylenedioxythiophene) nanowires exhibited very good field emission properties, with turnon field of 3.5–4 V/μm (at 10 μA/cm2 ) [8]. Organic charge transfer complexes AgTCNQ and CuTCNQ (TCNQ = 7,7, 8,8-tetracyanoquinodimethane) also exhibit excellent turn-on fields of 2.58 and 3.13 V/μm [7]. On the other hand, tris ⁎ Corresponding author. E-mail address: [email protected] (A.B. Djurišić). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.044
(8-hydroxyquinoline) aluminum (Alq) nanowires exhibited a turn-on field of 10 V/μm (for 10 μA/cm2) and a field enhancement factor of 275 [9]. In this work, we report our study on the fabrication and field emission properties of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc) nanowires. CuPc is a p-type organic semiconductor, commonly used in organic light emitting diodes, organic solar cells, and organic field effect transistors. On the other hand, F16CuPc is an n-type organic semiconductor, whose main application is in organic field effect transistors. CuPc nanostructures (α-phase) have been previously reported for solar cell applications [10], while β-phase CuPc nanoribbons have been applied in field effect transistors [11]. While metal phthalocyanines can be used as precursors for the synthesis of carbon nanotubes [12,13], which can then be used as field emitters, no direct study of field emission from phthalocyanine nanostructures was reported. 2. Experimental details CuPc and F16CuPc nanowires were fabricated by thermal evaporation of CuPc (0.04 g, Fluka for CuPc, Aldrich for
W.Y. Tong et al. / Materials Letters 61 (2007) 3842–3846
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(ITO) glass, covered with a thin (10 nm) CuPc or F16CuPc film (to increase nanowire yield) for the corresponding nanowires evaporated in high vacuum. The films were evaporated in high vacuum (∼ 5 · 10− 4 Pa) using thermal evaporators PEVA 350T (Advanced Systems Technology, Hsinchu, Taiwan). The samples were examined by scanning electron microscopy (SEM) using Leo 1530 field emission SEM, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) using Philips Tecnai-20 TEM, and X-ray diffraction (XRD) using Bruker AXS SMART CCD diffractometer. For TEM, the samples were dispersed in ethanol in ultrasonic bath for 20 s and placed on a TEM grid (Cu with carbon film). The field emission measurements were carried out in a parallel plate configuration at a pressure of 1 × 10− 4 Pa. Indium tin oxide coated glass was used as the anode, which was separated 100 μm from the sample by Teflon spacers. The tested emission area was 0.35 cm2. 3. Results and discussion
Fig. 1. Representative SEM images for substrate temperatures of 273–293 °C: a) CuPc and b) F16CuPc; 195–216 °C: c) CuPc and d) F16CuPc; 142–170 °C: e) CuPc and f) F16CuPc.
F16CuPc) in a three zone tube furnace in Ar gas flow (0.3 lpm for CuPc and 0.15 lpm for F16CuPc). The source temperature was 383 °C and the pressure during growth was ∼ 293.3 Pa. The growth time was 4 h. The substrate used was indium tin oxide
Fig. 1 illustrates the effects of the substrate temperature on the morphology of CuPc and F16CuPc. The morphologies of the CuPc and F16CuPc nanostructures for the same substrate temperatures are very similar. For both materials, at the highest temperature very long ribbons (tens of micrometers) are obtained, while the ribbon height and width were typically in the ranges ∼ 150–350 nm and ∼50–200 nm, respectively. With the reduction of the substrate temperature, the nanostructure size is also reduced. The length in the temperature range 195–216 °C is several micrometers, and the height and width are also reduced. At the lowest substrate temperature (142–170 °C), the majority of nanowires are parallel to the substrate (this is even more pronounced for F16CuPc), their diameter is typically below 100 nm, while the length is 1–2 μm.
Fig. 2. Representative TEM images of CuPc nanostructures for substrate temperatures: a) 170 °C and b) 293 °C; F16CuPc nanostructures for substrate temperatures: c) 195 °C and d) 273 °C.
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Fig. 3. XRD patterns of CuPc and F16CuPc nanowires deposited at different substrate temperatures.
To study the crystal structure of the obtained nanomaterials, TEM, SAED, and XRD were performed. TEM images, with corresponding SAED pattern shown in the inset, of two representative types of nanostructures for each material are shown in Fig. 2, while the representative XRD patterns for different substrate temperatures are shown in Fig. 3. Phthalocyanine materials can exist in several crystalline polymorphs [14]. The polymorphs have different tilt angles of molecules within the columns and different mutual arrangement of the columns [15]. The stable phase is β, while the films evaporated on substrates at low temperatures are usually polycrystalline with α-phase crystallites [14]. For deposition at temperatures above ∼ 210 °C, β-phase CuPc is obtained [14]. From the SAED and XRD patterns, we can observe that CuPc nanostructures grown at lower substrate temperatures (142–195 °C) consist predominantly of α-CuPc, while at 216 °C mixture of the two phases is obtained and finally at even higher temperatures β-CuPc is dominant, in agreement with previous studies of CuPc films [14]. The CuPc nanostructures grow along the [010] direction, similar to a previous report on CuPc nanoribbons [11]. F16CuPc nanoribbons grown at higher substrate temperature are also single crystalline. However, F16CuPc nanostructures grown at lower substrate temperature have inferior crystallinity. A weak peak at 6.15–6.2 °C can be observed in XRD patterns, in agreement with the previously reported one (at 6.1°) for α-F16CuPc [16], while SAED patterns typically show no clear indication of single crystalline structure. With further increase of the temperature, this peak disappears, and a new peak which likely corresponds to β (311) can be observed. In addition to differences in the crystal structure, CuPc polymorphs also have different conductivities [17]. Therefore, differences in field emission behavior can be expected. Field emission measurements were attempted for freshly prepared samples, but the observed field emission effects were negligible. The samples were then exposed to atmosphere for 24 h (relative humidity ∼ 50%), and the measurements were repeated. The obtained results are shown in Fig. 4. It is known that exposure of phthalocyanine films to air results in oxygen doping [18,19]. The charge density in a phthalocyanine film prior to oxygen
exposure is typically low [20]. The doping can affect the conductivity and the Fermi level, and consequently it can affect the field emission properties. The change of conductivity will reduce the voltage drop along the nanoribbon/nanowire and result in enhanced field at the tip [21]. In addition, the increased carrier concentration will result in the change of the Fermi level, and consequently the barrier height for the field emission [21]. The Fermi level shift due to oxygen exposure in CuPc is dependent on the film morphology, which was attributed to the increased diffusion of oxygen into CuPc with higher surface area [22]. After the measurement, the samples were stored in a vacuum dessicator for 1–2 weeks, and the reproducibility of the measured field emission curves was verified. The lowest turn-on field is obtained for β-CuPc nanoribbons. For obtaining the current density of 0.1 μA/cm2 (definition of turn-on field [1–3]), required fields are 5.1 V/μm for β-CuPc, and 6 V/μm for mixed and α-phase samples. For F16CuPc, the fields are 7.1 V/μm for α(142–170 °C)-F16CuPc, 6.3 V/μm for α(195–216 °C)-F16CuPc, and 8.9 V/μm for β-F16CuPc. For obtaining the current density of 10 μA/cm2 (alternative definition of turn-on field [7,9]), the field values are 8.1 V/μm for β-CuPc, 8.7 V/μm for α-CuPc, 9.7 for α + β-CuPc, 9.3 V/μm for α(195–216 °C)-F16CuPc, 9.8 V/μm for α(142–170 °C)-F16CuPc, and 12.7 V/μm for β-F16CuPc. From the measured field emission curves shown in Fig. 4, field enhancement factors can be determined. According to the Fowler–
Fig. 4. Field emission of a) CuPc and b) F16CuPc nanowires grown at different substrate temperatures; c) Fowler–Nordheim plot for CuPc and F16CuPc nanowires deposited at 170 °C.
W.Y. Tong et al. / Materials Letters 61 (2007) 3842–3846
Nordheim model, the emission current density J as a function of applied field E can be expressed as [1,2]: J ¼ Aðb2 E2 =/Þexpð−B/3=2 =bEÞ;
ð1Þ
where A = 1.54 × 1010 A eV V− 2, B = 6.83 × 109 V eV− 3/2 m− 1, ϕ is the work function, and β is the field enhancement factor. The field enhancement factor is related to the geometry, crystal structure, and nanostructure density [2,21]. It can be calculated from the slope of the Fowler–Nordheim plot (ln(J/E2) vs. 1/E) as [3,9]: Slope ¼
−B/3=2 : b
ð2Þ
In all cases, the Fowler–Nordheim plots exhibit two slopes, as shown in Fig. 4c for samples deposited at 170 °C (similar shape is observed in all samples). Deviation from linear relationship at low fields is commonly observed in carbon nanotubes [5] and organic nanostructures [7,8]. From the slopes at higher field, field enhancement factors can be calculated for known work function. For CuPc, the work function after exposure to oxygen is expected to be ∼ 4.62 eV [23]. In the case of F16CuPc, work function of 5.1 eV is assumed [24]. The obtained field enhancement factor values are β = 658 for αCuPc, 1383 for α + β-CuPc and 1347 for β-CuPc. For F16CuPc, β = 714 for β-F16CuPc, β = 904 for α(142–170 °C)-F16CuPc, and β = 742 for α (195–216 °C) -F 16 CuPc. Thus, the obtained field enhancement factors are higher than that reported for Alq nanowires, where β = 275 was obtained [9], as well as better than those of a variety of inorganic materials, such as ZnO nanotubes (turn-on field 7.0 V/μm) [2], and amorphous diamond (turn-on field 20 V/μm) [25]. Although the field emission performance of CuPc is not as good as that of highly conductive organic nanowires [7,8], the excellent thermal and environmental stability of phthalocyanine materials makes them excellent candidates for a variety of practical applications. The differences observed between different samples are due to different morphologies (size and density) of the nanostructures, different energy levels, and different charge transport properties. It has been shown that improved conductivity and resulting Fermi level shift of the samples result in improved field emission properties for ZnO nanorods [26] and sulphur-doped nanocrystalline diamond [27]. This is in agreement with the observation of the field emission from phthalocyanine nanowires after exposure to air, which results in oxygen doping of the material. However, we can also observe different behaviors between the α- and β-phases of CuPc and F16CuPc. Overall, CuPc exhibits better field emission properties compared to F16CuPc. This is likely due to the lowering of the HOMO and LUMO levels in F16CuPc compared to CuPc as an effect of the addition of electron-withdrawing substitutions, which can be as high as 1.6 eV [28]. This shift of the energy levels results in overall increased work function of F16CuPc compared to CuPc (5.1 eV [24] vs. 4.62 eV [23]), and consequently inferior field emission properties. Concerning the influence of the crystal structure on the field emission properties, it is difficult to conclusively establish which of the different factors (conductivity, work function, or morphology) plays the most significant role in the observed field emission properties. In the case of CuPc, conductivity at room temperature is higher for α-CuPc compared to β-CuPc [29]. Since CuPc is a p-type material, the Fermi level is located closer to the HOMO level for higher conductivity, so that worse field emission performance is expected for α-CuPc compared to β-CuPc. F16CuPc is an n-type material having different structures and properties, attributed to negative charging of the peripheral atoms [30], and it has been less thor-
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oughly studied than CuPc. Thus, further work on structural and electrical characterization of different F16CuPc polymorphs is needed before it can be conclusively established whether morphology or electronic structure plays a greater role in its field emission properties. The obtained results indicate that CuPc nanowires are promising materials for field emission applications. It is expected that their performance will be further improved by controlled doping. Successful doping of phthalocyanine films using acceptor molecules has been previously demonstrated [31]. Since the introduction of the dopant could change the nanostructure morphology, studies of the effects of co-deposition with acceptor materials on the morphology, structure, energy level positions, and conductivity of phthalocyanine nanowires need to be performed. Further work is in progress to optimize the substrate used, phthalocyanine nanostructure density, distribution and conductivity in order to obtain further improvement in the field emission properties.
4. Conclusion We have fabricated and characterized CuPc and F16CuPc nanowires. Their morphology and crystal structure were strongly dependent on the substrate temperature, and different morphologies and crystal structures affected their field emission properties. The obtained results indicate that CuPc nanowires have the potential for field emission applications. Acknowledgements This work is supported by the University of Hong Kong seed funding grant, Strategic Research Theme, Science Faculty Development grant, and the University of Hong Kong University Development Fund Grant. References [1] S. Kar, S. Chaudhuri, Chem. Phys. Lett. 422 (2006) 424. [2] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, M.B. Yu, W. Huang, Appl. Phys. Lett. 88 (2006) 213102. [3] S.K. Marathe, P.M. Koinkar, S.S. Ashtaputre, M.A. More, S.W. Gosawi, D.S. Joag, S.K. Kulkarni, Nanotechnology 17 (2006) 1932. [4] K.-C. Chen, C.-F. Chen, J.-S. Chiang, C.-L. Hwang, Y.-Y. Chang, C.-C. Lee, Thin Solid Films 498 (2006) 198. [5] Y.D. Lee, H.J. Lee, J.H. Han, J.E. Yoo, Y.H. Lee, J.K. Kim, S. Nahm, B.K. Ju, J. Phys. Chem., B 110 (2006) 5310. [6] E. Itoh, I. Suzuki, K. Miyari, Jpn. J. Appl. Phys. 44 (636) (2005). [7] H.B. Liu, Q. Zhao, Y. Li, F. Lu, J. Zhuang, S. Wang, L. Jiang, D. Zhu, D. Yu, L. Chi, J. Am. Chem. Soc. 127 (2005) 1120. [8] B.H. Kim, M.S. Kim, K.T. Park, J.K. Lee, D.H. Park, J. Joo, S.G. Yu, S.H. Lee, Appl. Phys. Lett. 83 (539) (2003). [9] J.J. Chiu, C.C. Kei, W.S. Wang, T.P. Perng, Adv. Mater. 15 (2003) 1361. [10] F. Yang, M. Shtein, S.R. Forrest, Nat. Mater. 4 (2005) 37. [11] Q. Tang, H. Li, M. He, W. Hu, C. Liu, K. Chen, C. Wang, Y. Liu, D. Zhu, Adv. Mater. 18 (2006) 65. [12] J. Song, M. Sun, Q. Chen, J. Wang, G. Zhang, Z. Xue, J. Phys. D: Appl. Phys. 37 (2004) 5. [13] N.S. Kim, Y.T. Lee, J. Park, J.B. Han, Y.S. Choi, S.Y. Choi, J. Choo, G.H. Lee, Phys. Chem. B 107 (2003) 9249. [14] O. Berger, W.J. Fischer, B. Adolphi, S. Tierbach, V. Melev, J. Schreiber, J. Mater. Sci., Mater. Electron. 11 (2000) 331. [15] R. Resel, M. Ottmar, M. Hanack, J. Keckes, G. Leising, J. Mater. Res. 15 (2000) 934. [16] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207.
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[17] S.I. Shihub, R.D. Gould, Thin Solid Films 290–291 (1996) 390. [18] T. Nishi, K. Kanai, Y. Ouchi, M.R. Willis, K. Seki, Chem. Phys. Lett. 414 (2005) 479. [19] T. Anthopoulos, T.S. Shafai, J. Appl. Phys. 94 (2003) 2426. [20] T. Nishi, K. Kanai, Y. Ouchi, M.R. Willis, K. Seki, Chem. Phys. 325 (2006) 121. [21] C.X. Xu, X.W. Sun, B.J. Chen, Appl. Phys. Lett. 84 (2004) 1540. [22] L. Grządziel, J. Żak, J. Szuber, Thin Solid Films 436 (2003) 70. [23] J. Szuber, L. Grządziel, Thin Solid Films 376 (2000) 214. [24] K.M. Lau, J.X. Tang, H.Y. Sun, C.S. Lee, S.T. Lee, D. Yan, Appl. Phys. Lett. 88 (2006) 173513.
[25] Y.H. Yang, C.X. Wang, B. Wang, N.S. Xu, G.W. Yang, Chem. Phys. Lett. 403 (2005) 248. [26] Q. Zhao, X.Y. Xu, X.F. Song, X.Z. Zhang, D.P. Yu, C.P. Li, L. Guo, Appl. Phys. Lett. 88 (2006) 033102. [27] G. Morell, A. González-Berríos, B.R. Weiner, S. Gupta, J. Mater. Sci., Mater. Electron. 17 (2006) 443. [28] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207. [29] B.N. Achar, K.S. Lokesh, J. Solid State Chem. 177 (2004) 1987. [30] Q. Tang, H. Li, Y. Liu, W. Hu, J. Am. Chem. Soc. 128 (2006) 14634. [31] M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 3202.
Field emission from copper phthalocyanine and copper hexadecafluorophthalocyanine nanowires W.Y. Tong a , Z.X. Li b , A.B. Djurišić a,⁎, W.K. Chan c , S.F. Yu b a
b
Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore c Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Received 8 July 2006; accepted 21 December 2006 Available online 30 December 2006
Abstract Copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc) nanowires were fabricated by vapor deposition. The nanowires were studied by scanning electron microscopy and transmission electron microscopy. Field emission properties of these nanowires were studied. The field emission properties were strongly dependent on the substrate temperature and the material used, and the best results are obtained for β-phase CuPc nanoribbons. Different dependences of field emission properties on the substrate temperature were obtained for the two materials investigated. The obtained results are discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Vb; 81.07.Nb Keywords: Organic semiconductors; Nanostructures; Field emission
1. Introduction Among various applications of nanostructures, field emission is of considerable interest. Field emission properties of different materials, such as semiconductor nanowires [1–3], carbon nanotubes [4–6], and organic materials [7–9], have been studied. The properties are highly dependent on the type of material, as well as on the morphology and fabrication method [2]. For example, depending on the carbon nanotube electrode preparation method, turn-on fields in the range 1.33–6 V/μm can be obtained [4–6]. Compared to inorganic nanowires and carbon nanotubes, studies on the fabrication and field emission properties of organic materials have been scarce. Conducting poly(3,4-ethylenedioxythiophene) nanowires exhibited very good field emission properties, with turnon field of 3.5–4 V/μm (at 10 μA/cm2 ) [8]. Organic charge transfer complexes AgTCNQ and CuTCNQ (TCNQ = 7,7, 8,8-tetracyanoquinodimethane) also exhibit excellent turn-on fields of 2.58 and 3.13 V/μm [7]. On the other hand, tris ⁎ Corresponding author. E-mail address: [email protected] (A.B. Djurišić). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.044
(8-hydroxyquinoline) aluminum (Alq) nanowires exhibited a turn-on field of 10 V/μm (for 10 μA/cm2) and a field enhancement factor of 275 [9]. In this work, we report our study on the fabrication and field emission properties of copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F16CuPc) nanowires. CuPc is a p-type organic semiconductor, commonly used in organic light emitting diodes, organic solar cells, and organic field effect transistors. On the other hand, F16CuPc is an n-type organic semiconductor, whose main application is in organic field effect transistors. CuPc nanostructures (α-phase) have been previously reported for solar cell applications [10], while β-phase CuPc nanoribbons have been applied in field effect transistors [11]. While metal phthalocyanines can be used as precursors for the synthesis of carbon nanotubes [12,13], which can then be used as field emitters, no direct study of field emission from phthalocyanine nanostructures was reported. 2. Experimental details CuPc and F16CuPc nanowires were fabricated by thermal evaporation of CuPc (0.04 g, Fluka for CuPc, Aldrich for
W.Y. Tong et al. / Materials Letters 61 (2007) 3842–3846
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(ITO) glass, covered with a thin (10 nm) CuPc or F16CuPc film (to increase nanowire yield) for the corresponding nanowires evaporated in high vacuum. The films were evaporated in high vacuum (∼ 5 · 10− 4 Pa) using thermal evaporators PEVA 350T (Advanced Systems Technology, Hsinchu, Taiwan). The samples were examined by scanning electron microscopy (SEM) using Leo 1530 field emission SEM, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) using Philips Tecnai-20 TEM, and X-ray diffraction (XRD) using Bruker AXS SMART CCD diffractometer. For TEM, the samples were dispersed in ethanol in ultrasonic bath for 20 s and placed on a TEM grid (Cu with carbon film). The field emission measurements were carried out in a parallel plate configuration at a pressure of 1 × 10− 4 Pa. Indium tin oxide coated glass was used as the anode, which was separated 100 μm from the sample by Teflon spacers. The tested emission area was 0.35 cm2. 3. Results and discussion
Fig. 1. Representative SEM images for substrate temperatures of 273–293 °C: a) CuPc and b) F16CuPc; 195–216 °C: c) CuPc and d) F16CuPc; 142–170 °C: e) CuPc and f) F16CuPc.
F16CuPc) in a three zone tube furnace in Ar gas flow (0.3 lpm for CuPc and 0.15 lpm for F16CuPc). The source temperature was 383 °C and the pressure during growth was ∼ 293.3 Pa. The growth time was 4 h. The substrate used was indium tin oxide
Fig. 1 illustrates the effects of the substrate temperature on the morphology of CuPc and F16CuPc. The morphologies of the CuPc and F16CuPc nanostructures for the same substrate temperatures are very similar. For both materials, at the highest temperature very long ribbons (tens of micrometers) are obtained, while the ribbon height and width were typically in the ranges ∼ 150–350 nm and ∼50–200 nm, respectively. With the reduction of the substrate temperature, the nanostructure size is also reduced. The length in the temperature range 195–216 °C is several micrometers, and the height and width are also reduced. At the lowest substrate temperature (142–170 °C), the majority of nanowires are parallel to the substrate (this is even more pronounced for F16CuPc), their diameter is typically below 100 nm, while the length is 1–2 μm.
Fig. 2. Representative TEM images of CuPc nanostructures for substrate temperatures: a) 170 °C and b) 293 °C; F16CuPc nanostructures for substrate temperatures: c) 195 °C and d) 273 °C.
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Fig. 3. XRD patterns of CuPc and F16CuPc nanowires deposited at different substrate temperatures.
To study the crystal structure of the obtained nanomaterials, TEM, SAED, and XRD were performed. TEM images, with corresponding SAED pattern shown in the inset, of two representative types of nanostructures for each material are shown in Fig. 2, while the representative XRD patterns for different substrate temperatures are shown in Fig. 3. Phthalocyanine materials can exist in several crystalline polymorphs [14]. The polymorphs have different tilt angles of molecules within the columns and different mutual arrangement of the columns [15]. The stable phase is β, while the films evaporated on substrates at low temperatures are usually polycrystalline with α-phase crystallites [14]. For deposition at temperatures above ∼ 210 °C, β-phase CuPc is obtained [14]. From the SAED and XRD patterns, we can observe that CuPc nanostructures grown at lower substrate temperatures (142–195 °C) consist predominantly of α-CuPc, while at 216 °C mixture of the two phases is obtained and finally at even higher temperatures β-CuPc is dominant, in agreement with previous studies of CuPc films [14]. The CuPc nanostructures grow along the [010] direction, similar to a previous report on CuPc nanoribbons [11]. F16CuPc nanoribbons grown at higher substrate temperature are also single crystalline. However, F16CuPc nanostructures grown at lower substrate temperature have inferior crystallinity. A weak peak at 6.15–6.2 °C can be observed in XRD patterns, in agreement with the previously reported one (at 6.1°) for α-F16CuPc [16], while SAED patterns typically show no clear indication of single crystalline structure. With further increase of the temperature, this peak disappears, and a new peak which likely corresponds to β (311) can be observed. In addition to differences in the crystal structure, CuPc polymorphs also have different conductivities [17]. Therefore, differences in field emission behavior can be expected. Field emission measurements were attempted for freshly prepared samples, but the observed field emission effects were negligible. The samples were then exposed to atmosphere for 24 h (relative humidity ∼ 50%), and the measurements were repeated. The obtained results are shown in Fig. 4. It is known that exposure of phthalocyanine films to air results in oxygen doping [18,19]. The charge density in a phthalocyanine film prior to oxygen
exposure is typically low [20]. The doping can affect the conductivity and the Fermi level, and consequently it can affect the field emission properties. The change of conductivity will reduce the voltage drop along the nanoribbon/nanowire and result in enhanced field at the tip [21]. In addition, the increased carrier concentration will result in the change of the Fermi level, and consequently the barrier height for the field emission [21]. The Fermi level shift due to oxygen exposure in CuPc is dependent on the film morphology, which was attributed to the increased diffusion of oxygen into CuPc with higher surface area [22]. After the measurement, the samples were stored in a vacuum dessicator for 1–2 weeks, and the reproducibility of the measured field emission curves was verified. The lowest turn-on field is obtained for β-CuPc nanoribbons. For obtaining the current density of 0.1 μA/cm2 (definition of turn-on field [1–3]), required fields are 5.1 V/μm for β-CuPc, and 6 V/μm for mixed and α-phase samples. For F16CuPc, the fields are 7.1 V/μm for α(142–170 °C)-F16CuPc, 6.3 V/μm for α(195–216 °C)-F16CuPc, and 8.9 V/μm for β-F16CuPc. For obtaining the current density of 10 μA/cm2 (alternative definition of turn-on field [7,9]), the field values are 8.1 V/μm for β-CuPc, 8.7 V/μm for α-CuPc, 9.7 for α + β-CuPc, 9.3 V/μm for α(195–216 °C)-F16CuPc, 9.8 V/μm for α(142–170 °C)-F16CuPc, and 12.7 V/μm for β-F16CuPc. From the measured field emission curves shown in Fig. 4, field enhancement factors can be determined. According to the Fowler–
Fig. 4. Field emission of a) CuPc and b) F16CuPc nanowires grown at different substrate temperatures; c) Fowler–Nordheim plot for CuPc and F16CuPc nanowires deposited at 170 °C.
W.Y. Tong et al. / Materials Letters 61 (2007) 3842–3846
Nordheim model, the emission current density J as a function of applied field E can be expressed as [1,2]: J ¼ Aðb2 E2 =/Þexpð−B/3=2 =bEÞ;
ð1Þ
where A = 1.54 × 1010 A eV V− 2, B = 6.83 × 109 V eV− 3/2 m− 1, ϕ is the work function, and β is the field enhancement factor. The field enhancement factor is related to the geometry, crystal structure, and nanostructure density [2,21]. It can be calculated from the slope of the Fowler–Nordheim plot (ln(J/E2) vs. 1/E) as [3,9]: Slope ¼
−B/3=2 : b
ð2Þ
In all cases, the Fowler–Nordheim plots exhibit two slopes, as shown in Fig. 4c for samples deposited at 170 °C (similar shape is observed in all samples). Deviation from linear relationship at low fields is commonly observed in carbon nanotubes [5] and organic nanostructures [7,8]. From the slopes at higher field, field enhancement factors can be calculated for known work function. For CuPc, the work function after exposure to oxygen is expected to be ∼ 4.62 eV [23]. In the case of F16CuPc, work function of 5.1 eV is assumed [24]. The obtained field enhancement factor values are β = 658 for αCuPc, 1383 for α + β-CuPc and 1347 for β-CuPc. For F16CuPc, β = 714 for β-F16CuPc, β = 904 for α(142–170 °C)-F16CuPc, and β = 742 for α (195–216 °C) -F 16 CuPc. Thus, the obtained field enhancement factors are higher than that reported for Alq nanowires, where β = 275 was obtained [9], as well as better than those of a variety of inorganic materials, such as ZnO nanotubes (turn-on field 7.0 V/μm) [2], and amorphous diamond (turn-on field 20 V/μm) [25]. Although the field emission performance of CuPc is not as good as that of highly conductive organic nanowires [7,8], the excellent thermal and environmental stability of phthalocyanine materials makes them excellent candidates for a variety of practical applications. The differences observed between different samples are due to different morphologies (size and density) of the nanostructures, different energy levels, and different charge transport properties. It has been shown that improved conductivity and resulting Fermi level shift of the samples result in improved field emission properties for ZnO nanorods [26] and sulphur-doped nanocrystalline diamond [27]. This is in agreement with the observation of the field emission from phthalocyanine nanowires after exposure to air, which results in oxygen doping of the material. However, we can also observe different behaviors between the α- and β-phases of CuPc and F16CuPc. Overall, CuPc exhibits better field emission properties compared to F16CuPc. This is likely due to the lowering of the HOMO and LUMO levels in F16CuPc compared to CuPc as an effect of the addition of electron-withdrawing substitutions, which can be as high as 1.6 eV [28]. This shift of the energy levels results in overall increased work function of F16CuPc compared to CuPc (5.1 eV [24] vs. 4.62 eV [23]), and consequently inferior field emission properties. Concerning the influence of the crystal structure on the field emission properties, it is difficult to conclusively establish which of the different factors (conductivity, work function, or morphology) plays the most significant role in the observed field emission properties. In the case of CuPc, conductivity at room temperature is higher for α-CuPc compared to β-CuPc [29]. Since CuPc is a p-type material, the Fermi level is located closer to the HOMO level for higher conductivity, so that worse field emission performance is expected for α-CuPc compared to β-CuPc. F16CuPc is an n-type material having different structures and properties, attributed to negative charging of the peripheral atoms [30], and it has been less thor-
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oughly studied than CuPc. Thus, further work on structural and electrical characterization of different F16CuPc polymorphs is needed before it can be conclusively established whether morphology or electronic structure plays a greater role in its field emission properties. The obtained results indicate that CuPc nanowires are promising materials for field emission applications. It is expected that their performance will be further improved by controlled doping. Successful doping of phthalocyanine films using acceptor molecules has been previously demonstrated [31]. Since the introduction of the dopant could change the nanostructure morphology, studies of the effects of co-deposition with acceptor materials on the morphology, structure, energy level positions, and conductivity of phthalocyanine nanowires need to be performed. Further work is in progress to optimize the substrate used, phthalocyanine nanostructure density, distribution and conductivity in order to obtain further improvement in the field emission properties.
4. Conclusion We have fabricated and characterized CuPc and F16CuPc nanowires. Their morphology and crystal structure were strongly dependent on the substrate temperature, and different morphologies and crystal structures affected their field emission properties. The obtained results indicate that CuPc nanowires have the potential for field emission applications. Acknowledgements This work is supported by the University of Hong Kong seed funding grant, Strategic Research Theme, Science Faculty Development grant, and the University of Hong Kong University Development Fund Grant. References [1] S. Kar, S. Chaudhuri, Chem. Phys. Lett. 422 (2006) 424. [2] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, M.B. Yu, W. Huang, Appl. Phys. Lett. 88 (2006) 213102. [3] S.K. Marathe, P.M. Koinkar, S.S. Ashtaputre, M.A. More, S.W. Gosawi, D.S. Joag, S.K. Kulkarni, Nanotechnology 17 (2006) 1932. [4] K.-C. Chen, C.-F. Chen, J.-S. Chiang, C.-L. Hwang, Y.-Y. Chang, C.-C. Lee, Thin Solid Films 498 (2006) 198. [5] Y.D. Lee, H.J. Lee, J.H. Han, J.E. Yoo, Y.H. Lee, J.K. Kim, S. Nahm, B.K. Ju, J. Phys. Chem., B 110 (2006) 5310. [6] E. Itoh, I. Suzuki, K. Miyari, Jpn. J. Appl. Phys. 44 (636) (2005). [7] H.B. Liu, Q. Zhao, Y. Li, F. Lu, J. Zhuang, S. Wang, L. Jiang, D. Zhu, D. Yu, L. Chi, J. Am. Chem. Soc. 127 (2005) 1120. [8] B.H. Kim, M.S. Kim, K.T. Park, J.K. Lee, D.H. Park, J. Joo, S.G. Yu, S.H. Lee, Appl. Phys. Lett. 83 (539) (2003). [9] J.J. Chiu, C.C. Kei, W.S. Wang, T.P. Perng, Adv. Mater. 15 (2003) 1361. [10] F. Yang, M. Shtein, S.R. Forrest, Nat. Mater. 4 (2005) 37. [11] Q. Tang, H. Li, M. He, W. Hu, C. Liu, K. Chen, C. Wang, Y. Liu, D. Zhu, Adv. Mater. 18 (2006) 65. [12] J. Song, M. Sun, Q. Chen, J. Wang, G. Zhang, Z. Xue, J. Phys. D: Appl. Phys. 37 (2004) 5. [13] N.S. Kim, Y.T. Lee, J. Park, J.B. Han, Y.S. Choi, S.Y. Choi, J. Choo, G.H. Lee, Phys. Chem. B 107 (2003) 9249. [14] O. Berger, W.J. Fischer, B. Adolphi, S. Tierbach, V. Melev, J. Schreiber, J. Mater. Sci., Mater. Electron. 11 (2000) 331. [15] R. Resel, M. Ottmar, M. Hanack, J. Keckes, G. Leising, J. Mater. Res. 15 (2000) 934. [16] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207.
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