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Electronic polymer memory devices—Easy to fabricate, difficult to understand
Thin Solid Films 519 (2010) 587–590
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electronic polymer memory devices—Easy to fabricate, difficult to understand Shashi Paul ⁎, Iulia Salaoru Emerging Technologies Research Centre, De Montfort University, Leicester LE1 9BH, United Kingdom
a r t i c l e
i n f o
Available online 7 August 2010 Keywords: Polymer memory devices Charging mechanism Small molecules based memory devices Plastic electronic memory devices
a b s t r a c t There has been a number reports on polymer memory devices for the last one decade. Polymer memory devices are fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and nanoparticles) between two metal electrodes. These devices show two electrical conductance states (“1” and “0”) when voltage is applied, thus rendering the structures suitable for data retention. These two states can be viewed as the realisation of memory devices. However, polymer memory devices reported so far suffer from multiple drawbacks that render their industrial implementation premature. There is a large discrepancy in the results reported by different groups. This article attempts to answer some of the questions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently the usage of organic materials in electronic device fabrication has shown a rapid increase and there have been substantial advances in materials and devices such as organic light emitting diodes [1] organic field effect transistors [2], and solar cells [3], with some devices being developed to the point of commercialisation. The factors driving this growth can be attributed to the overall low cost of organic devices, material tailoring for specific properties, simple device structures, low temperature manufacturing processes and compatibility of organic materials with cheap flexible substrates. In the organic memory devices progress can be broadly split into three categories, namely molecular memory devices (MMDs), resistive switch and write-once, read-many times (WORM) devices and polymer memory devices (PMDs). In the case of molecular memory, a monolayer of molecules is deposited between metal electrodes [4]. In a polymer memory device an admixture (a blend) of small organic molecules and/or metal nanoparticles in a polymer matrix is deposited between metal electrodes to form an array of memory elements [5]. MMDs and PMDs classes of memory device can be switched between two conductivity states upon the application of write and erase voltages, with the state being sensed by an intermediate read voltage. This bistable behaviour of the devices renders them suitable for non-volatile organic memory. However, molecular memories have proved difficult to manufacture, with large variations between device characteristics. Polymer memory devices in contrast are typically fabricated by the spin coating and vacuum evaporation techniques and have shown
⁎ Corresponding author. E-mail address: [email protected] (S. Paul). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.07.004
potential for as a future memory technology. While significant advances have been made in the field of polymer memory devices, Scott et al. [6,7] highlighted several performance criteria that needed to be addressed before any new memory technologies would be capable of competing favourably with current silicon technologies. Polymer memory devices may be able to fulfil the criteria highlighted in the aforementioned papers, and so far there have been demonstrations of devices based on metal nanocluster layers by Ma et al. [8], Bozano et al. [9] and Tondelier et al. [10]. The use of gold nanoparticles (encapsulated in organic ligands) in hybrid organic/inorganic memory devices has been demonstrated by Paul et al. [11], Prime et al. [12– 14] and Kolliopoulou et al. [15]. Möller et al. [16] showed that the combination of organic materials and silicon diodes can be used to produce write-once read-many-times (WORM) memory device. Devices with gold nanoparticles embedded in a polymer matrix have also been demonstrated by Ouyang et al. [17] and Prakash et al. [18] and C60 based PMDs have successfully been implemented by Kanwal et al. [19] and Paul [20]. The recent development in the field organic memory devices has been discussed in the review article [21]. Recently, we have also demonstrated bistability in the admixute of polyvinyl acetate and ferroelectric nanoparticles [22]. This work investigates polymer memory devices made of an admixture of small organic molecules (electron donor and acceptor molecules) and a polymer, and the role electrode oxide, if any, in the creation of two electrical conductive states. The working principle of the devices is also discussed. 2. Experimental A polymer blend was prepared by dissolving 8-hydroxyquinoline (8HQ) tetracyanoethylene (TCNE) and polyvinyl-acetate in an organic solvent. The admixture was then spin coated onto glass substrate marked with thin Al tracks and a top contact was evaporated on to the
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S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
blend after drying—this resulted in a metal–admixture–metal (MAM) structure. Metal/Polyvinyl acetate/Polystyrene + 8HQ + TCNE/Silicon metal–insulator–semiconductor (MIS) structures were also prepared to understand the charging mechanism. The current–voltage and capacitance–voltage characteristics of the MAM and MIS structures respectively were measured in a screened sample chamber in the dark and at room temperatures, using a PC-driven pico-ammeter (HP4140B) and an LCR bridge (HP4192). Fourier Transformation infrared (FTIR) analysis was also carried out to understand the charge transfer and this study published somewhere else [23]. 3. Results and discussion Current–voltage behaviour of Metal-Polymer-Metal (MPM) and MAM structures is shown in Fig. 1. Typical I–V behaviour of Al/PVAc/Al MPM structure is shown in Fig. 1a. The devices exhibit symmetrical I–V characteristics for negative and positive applied voltages. The symmetrical I–V characteristics are typical of a bulk-limited mechanism. Current–voltage (I–V) scans for pristine devices were conducted to determine if the I–V behaviour showing any hysteresis. A small hysteresis was observed in pristine devices as compared to the devices made from admixture. The Al/PVAc (or Al/admixture) interface does not play any role in determining the electrical behaviour of these devices. Write and erase voltages were then selected to be significantly lower than the breakdown voltage, ensuring no physical damage took place during read, write and erase (RWE) cycles. The read voltage was then selected to be the point of greatest hysteresis in the I–V curves. To access the memory characteristics of the structures, RWE cycles were performed, with write and erase voltages of ±30 V respectively and a read voltage of +20 V, with characteristics shown in Fig. 2. In the ‘off’ state the typical measured current was on the order of 0.1 × 10− 6 A, while in the ‘on’ state a typical current of 0.25 × 10− 6 A
Fig. 1. I–V characteristics of (a) Al/PVAc/Al; (b) Al/PVAc + 8HQ + TCNE/Al.
Fig. 2. RWE characteristics of an Al/PVAc + 8HQ + TCNE/Al device.
was measured, giving a measurable current difference between the two states. Memory retention time was also investigated with the devices exhibiting a stable on/off current ratio for over 10,000 read cycles over a period of 3 h (data not shown). The working mechanism of our MAM devices promises to be complex. At the same time a broad qualitative understanding is possible. It appears likely that the small molecules play a significant part in the whole scheme. It is to be expected that the charge carriers (electrons) go from the 8HQ (which is an electron donor) into the TCNE molecules that are embedded in the PVAc insulator, at certain applied voltages and get delocalized there, thus making the TCNE negatively charged. This is schematically shown in Fig. 3. It is well know that a TCNE molecule can accept electrons [24] and the layer of PVAc around the TCNE helps to retain the charge in it. This gives rise to a form a charge separation. This creates surplus (or internal) electric field in the device. Using this concept, we tried to understand the change in the conductivity in our devices. Fig. 2 shows a qualitative current response of PMDs to the write–read–erase pulses. When a virgin devices which is then exposed to write voltage pulse. This results in the formation of dipoles in the polymer matrix and creates an internal electric field. The next pulse is read pulse. Now the effective voltage appears across the device is less than that of the applied voltage and results in the reduction in the conductivity. Thus,
Fig. 3. Schematic diagram explaining the memory effect in MAM devices.
S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
a lower current passes through the devices. A detailed working explanation has already been published elsewhere [20]. To further understand charging mechanism; we have extensively investigated the capacitance–voltage behaviour of MIS structure. Typical C–V curve obtained for Al/PVAc/PS+8HQ+TCNE/Si (p-type) is as shown in Fig. 4. Hysteresis observed in the C–V curves with small molecules was around 1.2 V and while it was a negligible small without the small molecules. Thus, further strengthening our hypothesis that dipole is created in the admixture (in PVAc matrix) due to exchange of electrons between electron donor and acceptor molecules thus causing the observed hysteresis in the electrical behaviour (both in I–V and C–V). So far, the worked carried out in our group; we have not found the difference between on and off states of several order of magnitude. The on/off states in this work are around 150 nA. There are number of factors suggested by others, such as, filamentary conduction, thin aluminium oxide layer on the aluminium electrode. In order to investigate the effect of the aluminium oxide layer two types, as shown in Fig. 5, of the structures were investigated. First one, Al bottom electrodes was evaporated onto a clean glass substrate, then spin coating the polyvinyl acetate layer and in finally the top Al electrodes was evaporated. The second structure was made of a deliberately formed aluminium oxide layer onto the bottom electrode. Al bottom electrodes were evaporated onto glass substrate and then were kept at 600 °C in air for an hour to form a oxide layer. The formation of oxide layer was checked with X-ray Diffraction (XRD) technique. The polymer (PVAc) was spin coated and top aluminium electrodes was evaporated through the shadow mask in the vacuum evaporator. The typical current–voltage characteristics of these structures are shown in Fig. 6. In a number of research papers it has been suggested
Fig. 4. C–V characteristics of (a) Al/PVAc/PS/Si (p-type); (b) Al/PVAc/PS + 8HQ + TCNE/ Si (p-type).
589
Fig. 5. Schematic structure of (a) Al/PVAc/Al; (b) Al/Al2O3/PVAc/Al.
that the native aluminium oxide layer is created on the aluminium bottom electrode [25–27]. It is very clear from Fig. 6 that I–V characteristics of these two structures are very different and the structure with a thin oxide layer shows large hysteresis in the I–V behaviour. This may suggest that devices reported by other in the polymer memory devices, which are showing very large hysteresis, caused by the fact that a thin oxide layer is presented if they are using aluminium as an electrode material. In conclusion, we can say that the observed memory effect in our devices is due to the creation of internal electrical field in the polymer matrix. No oxide is presented on our electrode material and the memory behaviour reported in this work is solely due the admixture.
Fig. 6. Current–voltage characteristics of (a) Al/PVAc/Al; (b) Al/Al2O3/PVAc/Al devices.
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S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
Acknowledgement The authors would like to thank EPSRC (Grant #EP/E047785/1) for supporting this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (12) (1987) 913. F. Garnier, et al., Science 265 (5179) (1994) 1684. G. Yu, et al., Science 270 (5243) (1995) 1789. M.A. Reed, et al., Appl. Phys. Lett. 78 (23) (2001) 3735. S. Paul, A. Kanwal, M. Chhowalla, Nanotechnology 17 (1) (2006) 145. J.C. Scott, Science 304 (5667) (2004) 62. J.C. Scott, L.D. Bozano, Adv. Mater. 19 (2007) 1452. L. Ma, et al., Appl. Phys. Lett. 82 (9) (2003) 1419. L.D. Bozano, et al., Appl. Phys. Lett. 84 (4) (2004) 607. D. Tondelier, et al., Appl. Phys. Lett. 85 (23) (2004) 5763. S. Paul, C. Pearson, A. Molloy, M.A. Cousins, M. Green, S. Kolliopoulou, P. Dimitrakis, P. Normad, D. Tsoukalas, M.C. Petty, Nano Lett. 3 (2003) 533. [12] D. Prime, S. Paul, P.W. Josephs–Franks, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4215. [13] D. Prime, S. Paul, Appl. Phys. Lett. 96 (4) (2010) art. no. 043120.
[14] D. Prime, S. Paul, CIMTEC 2008—Proceedings of the 3rd International Conference on Smart Materials, Structures and Systems—Smart Materials and Micro/ Nanosystems, vol. 54, 2008, p. 480. [15] S. Kolliopoulou, P. Dimitrakis, P. Normad, H.L. Zhang, N. Cant, S.D. Evans, S. Paul, C. Pearson, A. Molloy, M.C. Petty, D. Tsoukalas, J. Appl. Phys. 94 (2003) 5234. [16] S. Möller, C. Perlov, W. Jackson, C. Taussig, S.R. Forrest, Nature 426 (2003) 166. [17] J.Y. Ouyang, et al., Appl. Phys. Lett. 86 (12) (2005) 123507. [18] A. Prakash, et al., J. Appl. Phys. 100 (2006) 054309. [19] A. Kanwal, S. Paul, M. Chhowalla, Organic memory devices using C60 and insulating polymer, Materials Research Society Symposium Proceedings, vol. 830, 2005, p. 349. [20] S. Paul, IEEE Trans. Nanotechnol. 6 (2) (2007) 191. [21] D. Prime, S. Paul, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4141. [22] I. Salaoru, S. Paul, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4227. [23] I. Salaoru, S. Paul, Material Research Society Symposium Proceedings, 2009, 1114G12-09. [24] D.A. Clemente, A. Marzotto, J. Mater. Chem. 6 (6) (1996) 941. [25] Sungho Kim, Yang-Kyu Choi, Appl. Phys. Lett. 92 (22) (2008) 223. [26] M. Vilkman, K. Solehmainen, A. Laiho, H.G.O. Sandberg, O. Ikkala, Org. Electron. 10 (8) (2009) 1478. [27] Artur Hefczyc, Lars Beckmann, Eike Becker, Hans-Hermann Johannes, Wolfgang Kowalsky, Phys. Status Solidi A 205 (3) (2008) 647.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electronic polymer memory devices—Easy to fabricate, difficult to understand Shashi Paul ⁎, Iulia Salaoru Emerging Technologies Research Centre, De Montfort University, Leicester LE1 9BH, United Kingdom
a r t i c l e
i n f o
Available online 7 August 2010 Keywords: Polymer memory devices Charging mechanism Small molecules based memory devices Plastic electronic memory devices
a b s t r a c t There has been a number reports on polymer memory devices for the last one decade. Polymer memory devices are fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and nanoparticles) between two metal electrodes. These devices show two electrical conductance states (“1” and “0”) when voltage is applied, thus rendering the structures suitable for data retention. These two states can be viewed as the realisation of memory devices. However, polymer memory devices reported so far suffer from multiple drawbacks that render their industrial implementation premature. There is a large discrepancy in the results reported by different groups. This article attempts to answer some of the questions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently the usage of organic materials in electronic device fabrication has shown a rapid increase and there have been substantial advances in materials and devices such as organic light emitting diodes [1] organic field effect transistors [2], and solar cells [3], with some devices being developed to the point of commercialisation. The factors driving this growth can be attributed to the overall low cost of organic devices, material tailoring for specific properties, simple device structures, low temperature manufacturing processes and compatibility of organic materials with cheap flexible substrates. In the organic memory devices progress can be broadly split into three categories, namely molecular memory devices (MMDs), resistive switch and write-once, read-many times (WORM) devices and polymer memory devices (PMDs). In the case of molecular memory, a monolayer of molecules is deposited between metal electrodes [4]. In a polymer memory device an admixture (a blend) of small organic molecules and/or metal nanoparticles in a polymer matrix is deposited between metal electrodes to form an array of memory elements [5]. MMDs and PMDs classes of memory device can be switched between two conductivity states upon the application of write and erase voltages, with the state being sensed by an intermediate read voltage. This bistable behaviour of the devices renders them suitable for non-volatile organic memory. However, molecular memories have proved difficult to manufacture, with large variations between device characteristics. Polymer memory devices in contrast are typically fabricated by the spin coating and vacuum evaporation techniques and have shown
⁎ Corresponding author. E-mail address: [email protected] (S. Paul). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.07.004
potential for as a future memory technology. While significant advances have been made in the field of polymer memory devices, Scott et al. [6,7] highlighted several performance criteria that needed to be addressed before any new memory technologies would be capable of competing favourably with current silicon technologies. Polymer memory devices may be able to fulfil the criteria highlighted in the aforementioned papers, and so far there have been demonstrations of devices based on metal nanocluster layers by Ma et al. [8], Bozano et al. [9] and Tondelier et al. [10]. The use of gold nanoparticles (encapsulated in organic ligands) in hybrid organic/inorganic memory devices has been demonstrated by Paul et al. [11], Prime et al. [12– 14] and Kolliopoulou et al. [15]. Möller et al. [16] showed that the combination of organic materials and silicon diodes can be used to produce write-once read-many-times (WORM) memory device. Devices with gold nanoparticles embedded in a polymer matrix have also been demonstrated by Ouyang et al. [17] and Prakash et al. [18] and C60 based PMDs have successfully been implemented by Kanwal et al. [19] and Paul [20]. The recent development in the field organic memory devices has been discussed in the review article [21]. Recently, we have also demonstrated bistability in the admixute of polyvinyl acetate and ferroelectric nanoparticles [22]. This work investigates polymer memory devices made of an admixture of small organic molecules (electron donor and acceptor molecules) and a polymer, and the role electrode oxide, if any, in the creation of two electrical conductive states. The working principle of the devices is also discussed. 2. Experimental A polymer blend was prepared by dissolving 8-hydroxyquinoline (8HQ) tetracyanoethylene (TCNE) and polyvinyl-acetate in an organic solvent. The admixture was then spin coated onto glass substrate marked with thin Al tracks and a top contact was evaporated on to the
588
S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
blend after drying—this resulted in a metal–admixture–metal (MAM) structure. Metal/Polyvinyl acetate/Polystyrene + 8HQ + TCNE/Silicon metal–insulator–semiconductor (MIS) structures were also prepared to understand the charging mechanism. The current–voltage and capacitance–voltage characteristics of the MAM and MIS structures respectively were measured in a screened sample chamber in the dark and at room temperatures, using a PC-driven pico-ammeter (HP4140B) and an LCR bridge (HP4192). Fourier Transformation infrared (FTIR) analysis was also carried out to understand the charge transfer and this study published somewhere else [23]. 3. Results and discussion Current–voltage behaviour of Metal-Polymer-Metal (MPM) and MAM structures is shown in Fig. 1. Typical I–V behaviour of Al/PVAc/Al MPM structure is shown in Fig. 1a. The devices exhibit symmetrical I–V characteristics for negative and positive applied voltages. The symmetrical I–V characteristics are typical of a bulk-limited mechanism. Current–voltage (I–V) scans for pristine devices were conducted to determine if the I–V behaviour showing any hysteresis. A small hysteresis was observed in pristine devices as compared to the devices made from admixture. The Al/PVAc (or Al/admixture) interface does not play any role in determining the electrical behaviour of these devices. Write and erase voltages were then selected to be significantly lower than the breakdown voltage, ensuring no physical damage took place during read, write and erase (RWE) cycles. The read voltage was then selected to be the point of greatest hysteresis in the I–V curves. To access the memory characteristics of the structures, RWE cycles were performed, with write and erase voltages of ±30 V respectively and a read voltage of +20 V, with characteristics shown in Fig. 2. In the ‘off’ state the typical measured current was on the order of 0.1 × 10− 6 A, while in the ‘on’ state a typical current of 0.25 × 10− 6 A
Fig. 1. I–V characteristics of (a) Al/PVAc/Al; (b) Al/PVAc + 8HQ + TCNE/Al.
Fig. 2. RWE characteristics of an Al/PVAc + 8HQ + TCNE/Al device.
was measured, giving a measurable current difference between the two states. Memory retention time was also investigated with the devices exhibiting a stable on/off current ratio for over 10,000 read cycles over a period of 3 h (data not shown). The working mechanism of our MAM devices promises to be complex. At the same time a broad qualitative understanding is possible. It appears likely that the small molecules play a significant part in the whole scheme. It is to be expected that the charge carriers (electrons) go from the 8HQ (which is an electron donor) into the TCNE molecules that are embedded in the PVAc insulator, at certain applied voltages and get delocalized there, thus making the TCNE negatively charged. This is schematically shown in Fig. 3. It is well know that a TCNE molecule can accept electrons [24] and the layer of PVAc around the TCNE helps to retain the charge in it. This gives rise to a form a charge separation. This creates surplus (or internal) electric field in the device. Using this concept, we tried to understand the change in the conductivity in our devices. Fig. 2 shows a qualitative current response of PMDs to the write–read–erase pulses. When a virgin devices which is then exposed to write voltage pulse. This results in the formation of dipoles in the polymer matrix and creates an internal electric field. The next pulse is read pulse. Now the effective voltage appears across the device is less than that of the applied voltage and results in the reduction in the conductivity. Thus,
Fig. 3. Schematic diagram explaining the memory effect in MAM devices.
S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
a lower current passes through the devices. A detailed working explanation has already been published elsewhere [20]. To further understand charging mechanism; we have extensively investigated the capacitance–voltage behaviour of MIS structure. Typical C–V curve obtained for Al/PVAc/PS+8HQ+TCNE/Si (p-type) is as shown in Fig. 4. Hysteresis observed in the C–V curves with small molecules was around 1.2 V and while it was a negligible small without the small molecules. Thus, further strengthening our hypothesis that dipole is created in the admixture (in PVAc matrix) due to exchange of electrons between electron donor and acceptor molecules thus causing the observed hysteresis in the electrical behaviour (both in I–V and C–V). So far, the worked carried out in our group; we have not found the difference between on and off states of several order of magnitude. The on/off states in this work are around 150 nA. There are number of factors suggested by others, such as, filamentary conduction, thin aluminium oxide layer on the aluminium electrode. In order to investigate the effect of the aluminium oxide layer two types, as shown in Fig. 5, of the structures were investigated. First one, Al bottom electrodes was evaporated onto a clean glass substrate, then spin coating the polyvinyl acetate layer and in finally the top Al electrodes was evaporated. The second structure was made of a deliberately formed aluminium oxide layer onto the bottom electrode. Al bottom electrodes were evaporated onto glass substrate and then were kept at 600 °C in air for an hour to form a oxide layer. The formation of oxide layer was checked with X-ray Diffraction (XRD) technique. The polymer (PVAc) was spin coated and top aluminium electrodes was evaporated through the shadow mask in the vacuum evaporator. The typical current–voltage characteristics of these structures are shown in Fig. 6. In a number of research papers it has been suggested
Fig. 4. C–V characteristics of (a) Al/PVAc/PS/Si (p-type); (b) Al/PVAc/PS + 8HQ + TCNE/ Si (p-type).
589
Fig. 5. Schematic structure of (a) Al/PVAc/Al; (b) Al/Al2O3/PVAc/Al.
that the native aluminium oxide layer is created on the aluminium bottom electrode [25–27]. It is very clear from Fig. 6 that I–V characteristics of these two structures are very different and the structure with a thin oxide layer shows large hysteresis in the I–V behaviour. This may suggest that devices reported by other in the polymer memory devices, which are showing very large hysteresis, caused by the fact that a thin oxide layer is presented if they are using aluminium as an electrode material. In conclusion, we can say that the observed memory effect in our devices is due to the creation of internal electrical field in the polymer matrix. No oxide is presented on our electrode material and the memory behaviour reported in this work is solely due the admixture.
Fig. 6. Current–voltage characteristics of (a) Al/PVAc/Al; (b) Al/Al2O3/PVAc/Al devices.
590
S. Paul, I. Salaoru / Thin Solid Films 519 (2010) 587–590
Acknowledgement The authors would like to thank EPSRC (Grant #EP/E047785/1) for supporting this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (12) (1987) 913. F. Garnier, et al., Science 265 (5179) (1994) 1684. G. Yu, et al., Science 270 (5243) (1995) 1789. M.A. Reed, et al., Appl. Phys. Lett. 78 (23) (2001) 3735. S. Paul, A. Kanwal, M. Chhowalla, Nanotechnology 17 (1) (2006) 145. J.C. Scott, Science 304 (5667) (2004) 62. J.C. Scott, L.D. Bozano, Adv. Mater. 19 (2007) 1452. L. Ma, et al., Appl. Phys. Lett. 82 (9) (2003) 1419. L.D. Bozano, et al., Appl. Phys. Lett. 84 (4) (2004) 607. D. Tondelier, et al., Appl. Phys. Lett. 85 (23) (2004) 5763. S. Paul, C. Pearson, A. Molloy, M.A. Cousins, M. Green, S. Kolliopoulou, P. Dimitrakis, P. Normad, D. Tsoukalas, M.C. Petty, Nano Lett. 3 (2003) 533. [12] D. Prime, S. Paul, P.W. Josephs–Franks, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4215. [13] D. Prime, S. Paul, Appl. Phys. Lett. 96 (4) (2010) art. no. 043120.
[14] D. Prime, S. Paul, CIMTEC 2008—Proceedings of the 3rd International Conference on Smart Materials, Structures and Systems—Smart Materials and Micro/ Nanosystems, vol. 54, 2008, p. 480. [15] S. Kolliopoulou, P. Dimitrakis, P. Normad, H.L. Zhang, N. Cant, S.D. Evans, S. Paul, C. Pearson, A. Molloy, M.C. Petty, D. Tsoukalas, J. Appl. Phys. 94 (2003) 5234. [16] S. Möller, C. Perlov, W. Jackson, C. Taussig, S.R. Forrest, Nature 426 (2003) 166. [17] J.Y. Ouyang, et al., Appl. Phys. Lett. 86 (12) (2005) 123507. [18] A. Prakash, et al., J. Appl. Phys. 100 (2006) 054309. [19] A. Kanwal, S. Paul, M. Chhowalla, Organic memory devices using C60 and insulating polymer, Materials Research Society Symposium Proceedings, vol. 830, 2005, p. 349. [20] S. Paul, IEEE Trans. Nanotechnol. 6 (2) (2007) 191. [21] D. Prime, S. Paul, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4141. [22] I. Salaoru, S. Paul, Philos. Trans. R. Soc. Lond. A: Math. Phys. Eng. Sci. 367 (1905) (2009) 4227. [23] I. Salaoru, S. Paul, Material Research Society Symposium Proceedings, 2009, 1114G12-09. [24] D.A. Clemente, A. Marzotto, J. Mater. Chem. 6 (6) (1996) 941. [25] Sungho Kim, Yang-Kyu Choi, Appl. Phys. Lett. 92 (22) (2008) 223. [26] M. Vilkman, K. Solehmainen, A. Laiho, H.G.O. Sandberg, O. Ikkala, Org. Electron. 10 (8) (2009) 1478. [27] Artur Hefczyc, Lars Beckmann, Eike Becker, Hans-Hermann Johannes, Wolfgang Kowalsky, Phys. Status Solidi A 205 (3) (2008) 647.