- Email: [email protected]
metal interface detected by electron paramagnetic resonance
Chemical Physics Letters 593 (2014) 45–47
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electron injection at the PTCDA/metal interface detected by electron paramagnetic resonance G.J. Gerardi a,⇑, J. Domenico a, A. Muraca a, H.K. Gerardi b a b
Department of Chemistry, William Paterson University, 300 Pompton Road, Wayne, NJ 07470, United States Ashland Incorporated, Molecular and Analytical Sciences Division, 1005 Highway 201/206, Bridgewater, NJ 08807, United States
a r t i c l e
i n f o
Article history: Received 6 November 2013 In final form 17 December 2013 Available online 22 December 2013
a b s t r a c t Electron paramagnetic resonance absorption was observed for samples prepared by vacuum vapor deposition of thin films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on metal films of magnesium (Mg), aluminum (Al) and silver (Ag) but not on gold (Au) or PTCDA on quartz. The resonance absorption is seen as a Lorentzian line with g-value of 2.0030 and a linewidth of 0.19 mT. The signal is attributed to PTCDA anions at the metal organic interface. The relative values of the metal work function and the PTCDA electron affinity appear to determine the occurrence of the EPR absorption. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Information concerning the junction formed between metal electrodes and organic semiconductor thin films is important for the development of flexible electronic devices. Formation of the metal organic (MO) interface under high vacuum conditions results in charge injection barriers, which play a major role in the performance of organic solar cells, light emitting diodes and field effect transistors. Interfaces formed from molecular solids have been investigated on a variety of metal substrates with energy level alignment and charge transfer largely inferred from electrical measurements, ultraviolet photoelectron spectroscopy (PES) and theoretical calculations [1–8]. In 1995, interface dipole barriers were observed from PES experiments of 5,10,15,20-zinctetraphenylporphyrin (ZnTPP) on Au, Ag, Al, and Mg and provided the first experimental evidence for the departure of MO interfaces from the vacuum level alignment or Schottky–Mott limit [8]. Similar experiments on 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films deposited on Mg, In, Sn, and Au, revealed Fermi level pinning at the metal organic semiconductor (MOS) interface to be essentially independent of the metal work function (UM) due to a linear dependence of the interface dipole (D) with UM [9]. This dependence leads to a fixed barrier for charge injection at PTCDA-MO interfaces. When the MO interface is formed, electron redistribution is largely determined by the metal work function (UM) and the semiconductor electron affinity (EA). The relative energies of these two quantities determine the magnitude and sign of the dipole that forms across the interface along with pinning the interface Fermi
⇑ Corresponding author. Fax: +1 9737202338. E-mail address: [email protected] (G.J. Gerardi). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.12.039
level, EF, with the density of states below the LUMO level of the OS [10]. This report presents results of electron paramagnetic resonance (EPR) studies of PTCDA evaporated on thin films of Mg, Al, Ag, and Au. EPR spectroscopy has been used extensively to detect and characterize defects at interfaces of semiconductor materials and more recently to characterize charged states of organic semiconductors [11–16]. The detection of an EPR absorption from an otherwise closed shell organic semiconductor deposited on a metal demonstrates charge transfer and the formation of a radical anion or cation at the interface. Our EPR measurements of PTCDA vacuum deposited on low work function metal films reveal a single absorption signal, which we attribute to PTCDA anions. Samples of PTCDA deposited on high work function metals or vapor depositions of PTCDA on inert substrates do not result in PTCDA radical ions. Our results show that EPR can distinguish the formation of MO interface dipoles as being due to charge transfer as opposed to only polarization, resulting from physical adsorption of the organic molecule on the metal, or to covalent bond formation [10]. 2. Experimental details 2.1. Materials PTCDA, magnesium ribbon, aluminum, silver and gold wire were purchased from Sigma–Aldrich Inc. and used as received. Quartz substrates were purchased from Ted Pella, Inc. 2.2. Sample preparation and characterization Samples were prepared by vacuum thermal deposition of Mg, Al, Ag and Au metals on quartz bars having dimensions 20 4 1 mm followed by deposition of thin films of PTCDA.
46
G.J. Gerardi et al. / Chemical Physics Letters 593 (2014) 45–47
The metal films were deposited on quartz substrates at a rate of 3 Å/s for 15 min, resulting in an approximate film thickness of 270 nm followed by vacuum thermal deposition of PTCDA. The PTCDA layer thickness ranged from 24.0 to 25.5 lm as measured using an NT-MDT atomic force microscope. Vacuum depositions were performed at a pressure of 105 Torr. Reference samples of PTCDA deposited on identical quartz bars simultaneously with those on metals were prepared for comparison. Electron paramagnetic resonance spectra were recorded at room temperature using a CW Brüker EMX spectrometer and a HP5350B microwave frequency counter to measure resonance frequency. The free radical DPPH having g-value of 2.0036 served as the reference for g-values of the Mg, Al, and Ag EPR signals. Continuous wave power saturation method was used to obtain data for an estimation of the spin–lattice relaxation and decoherence times. This method requires a determination of H1, the magnetic field strength at the sample in the microwave cavity of the spectrometer. This value is related to the microwave power PW incident to the cavity by the relation H21 ¼ KP W . We obtained a value of K = 8.0 103 mT2/W, using a formula for K reported by Sands [17]. Spin density was determined by evaporation of micro liter quantities of a calibrated solution of DPPH onto gold coated quartz substrates from a calibrated micropipette. 3. Results and discussion 3.1. EPR characterization
Relative EPR Signal Intensity (A.U.)
The EPR spectra shown in Figure 1 are readily observed when PTCDA is vapor phase deposited on thin films of Mg and Al samples under high vacuum conditions. The signal observed on Ag was barely detectable but confirmed by signal averaging with intensity comparable to the noise level. However, samples of PTCDA on Au, or quartz alone, did not result in EPR absorption under the same conditions. The signals observed for PTCDA on Mg, Al, and Ag all had the same g-value of 2.0030 (±0.0001) and a low power limiting line width of 0.19 mT, but the relative intensity of the signals varied. The EPR signal intensity is proportional to the number of anion radicals resulting from the transfer of electrons from the metal to the PTCDA. The intensity of the Mg signal was approximately six times larger than the Al sample signal, which we attribute to the fact that Mg has a lower work function than Al. All of the samples
Figure 2. EPR absorption spectrum of the Mg/PTCDA sample (blue) and the data fitted to a pure Lorentzian function (red).
produced the same degree of cavity loading with quality factor Q of 2700. The resonance signal line shapes were all essentially Lorentzian. Figure 2 shows the absorption signal and the corresponding Lorentzian fit of the absorption, which had a coefficient of determination of 0.99879. The data could not be satisfactorily fit to a GAUSSIAN function. The resonances exhibited homogeneous saturation behavior. The saturation factor, s, based on the Bloch equations, is given by s1 ¼ 1 þ H21 c2 T 1 T 2 , where H1 is the microwave magnetic field and c the electron gyromagnetic ratio [18,19]. We obtained the spin decoherence time, T2, of 3.5 108 s from a linear extrapolation of the linewidths, DH0pp , with decreasing microwave power pffiffiffi using the relation T 2 ¼ 2= 3cDH0pp . The spin–lattice relaxation time T1 was 4.3 107 s from T2 and the slope of the power dependence of the inverse saturation factor relation (Figure 3). A Lorentzian line shape is expected for radical species in solid form due to electron-spin exchange between radicals or electron transfer between neutral molecules and radical ions [19]. The departure of the Lorentzian line shape in the wings of the spectrum, as shown in Figure 2, may be attributed to several factors, including spatial extension of radical anions on the sample substrate, hyperfine broadening, and the orientation distribution of the radical anions at the interface if they have some g-anisotropy and the PTCDA anions are randomly oriented. The possibility that the signal is due to the metal is not very likely since metal resonances have a Dysonian lineshape.
Mg
Al Ag Au
333
334
335
336
337
338
Magnetic Field (mT) Figure 1. EPR spectra of PTCDA vapor phase deposited on thin films of Mg, Al, Ag and Au vapor deposited on quartz substrates at room temperature. The Mg and Ag spectra are single scans, while the Ag and Au spectra are the results of signal averaging 16 scans.
Figure 3. Inverse of the saturation factor, s1, is shown as a function of the microwave power for the Mg/PTCDA sample at room temperature.
G.J. Gerardi et al. / Chemical Physics Letters 593 (2014) 45–47
47
transfer. The Au work function is significantly below the LUMO level of PTCDA as indicated in Figure 4, and accounts for the absence of any PTCDA radical ions at the interface. These EPR results support recent theoretical calculations of Rusu et al. that for low work function metals electrons are transferred from the metal to the molecule [22]. Our results are also consistent with the work of Yoshida and Sato who found that, Al vapor deposited on PTCDA diffuses into the PTCDA layer and reacts with PTCDA molecules [23]. These observations further suggest that other organic semiconductors may produce radical ions when vapor phase deposited on active metal surfaces. In conclusion, electron transfer from a metal electrode to PTCDA occurs when the work function of the metal is less than the electron affinity of PTCDA and results in the formation of the PTCDA anion radical at the MOS interface. We expect this charge transfer to result in the formation of a dipole layer at the interface. References Figure 4. The relative energy alignments of the metal work function with the HOMO and LUMO levels of PTCDA before interface formation.
We expect the PTCDA anions to be concentrated at the MO interface. We obtained the spin density at the interface to be 1.5 1014 cm2 with a factor of two estimated uncertainty. Fenster et al. reported experimental evidence that PTCDA molecules, vapor phase deposited under high vacuum on a Au surface, are arranged with the plane of the molecule resting flat on the Au surface in a regular array consisting of 8 molecules within an area of dimensions 11.96 Å 19.91 Å [20]. That would correspond to a surface density of 3.35 1014 cm2. That estimate, taken in conjunction with our spin density result, would indicate that a significant fraction of the PTCDA molecules at the metal surface are PTCDA anions. Although it would be generally expected for the unpaired electron to be delocalized primarily over the ring portion of the PTCDA anion, it’s also possible that the spin density is localized at the acid anhydride end. This could result if the injected electron initiates a homolytic carbon–oxygen bond dissociation of the anhydride with the negative charge residing on the bridging oxygen and the unpaired electron on the carbonyl carbon in a r⁄ orbital. That would probably result in a more stable anion. Reactions of the type A–B + e ? A–B with spin on a r⁄ orbital have been investigated by Symons [21]. The relative EPR signal intensities of PTCDA on Mg, Al and Ag demonstrates that the lower work function metals have a greater tendency to transfer electrons to PTCDA and form a negative radical ion, while metals with work functions comparable to the LUMO level of PTCDA such as Ag have very little tendency for electron
[1] Ruchi Agrawal, Subhasis Gosh, Appl. Phys. Lett. 89 (2006) 222114. [2] S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R.L. Johnson, F. Schreiber, N. Koch, Org. Electron. 9 (2008) 111. [3] Y. Zou, L. Kilian, A. Schöll, T. Schmidt, R. Fink, E. Umbach, Surf. Sci. 600 (2006) 1240. [4] Xian Ning Xie, Xingyu Gao, Yuzhan Wang, Junzhong Wang, Kian Ping Loh, Andrew Thye Shen Wee, Appl. Phys. Lett. 95 (2009) 063301. [5] Hiroyuki Yoshida, Naoki Sato, Appl. Phys. Lett. 91 (2007) 141915. [6] H. Vasquez, R. Oszwaldowski, P. Pou, J. Ortgea, R. Perez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 107. [7] J. Hwang, A. Wan, A. Kahn, Mater. Sci. Eng. R 64 (2008) 1. [8] S. Narioka et al., Appl. Phys. Lett. 67 (1995) 1899. [9] I.G. Hill, A. Rajagopal, A. Kahn, Appl. Phys. Lett. 73 (1998) 662. [10] A. Kahn, N. Koch, W. Gao, J. Polym. Sci. Polym. Phys. 41 (2003) 2529. [11] G.J. Gerardi, E.H. Poindexter, P. Caplan, N.M. Johnson, Appl. Phys. Lett. 49 (1986) 348. [12] G.J. Gerardi, E.H. Poindexter, P. Caplan, M. Harmatz, W.R. Buchwald, N.M. Johnson, J. Electrochem. Soc. 136 (1989) 2609. [13] Si-Guang Chen, Howard M. Branz, Sandra S. Eaton, J. Phys. Chem. B 108 (2004) 17327. [14] Kazuhiro Marumoto, Shin-ichi Kuroda, Taishi Takenobu, Yoshihiro Iwasa, Phys. Rev. Lett. 97 (2006) 256603. [15] Hideo Sato, Lauraine A. Dalton, Duc Ha, Richard W. Quine, J. Phys. Chem. B 111 (2007) 7972. [16] Hiroyuki Matsui, Tatsuo Hasegawa, Yoshinori Tokura, Phys. Rev. Lett. 100 (2008) 126601. [17] R.H. Sands, NMR and EPR Spectroscopy, Pergamon Press Inc., 1960, pp. 44–80. [18] Charles Poole, Electron Spin Resonance, second ed., John Wiley, New York, 1983, p. 589. [19] C.P. Poole, H.A. Farach, Theory of Magnetic Resonance, Wiley, New York, 1972, p. 201. [20] P. Fenter et al., Phys. Rev. B 56 (1997) 3046. [21] M.C.R. Symons, Foundations of Modern EPR, G.R. Eaton, S.S. Eaton, K.M. Salikov, World Scientific Pub. Co., Ltd., New Jersey, 1998, p. 301. [22] P.C. Rusu, G. Giovannetti, C. Weijtens, R. Coehoorn, G. Brocks, J. Phys. Chem. C 113 (2009) 9974. [23] H. Yoshida, N. Sato, J. Appl. Phys. 91 (2002) 4312.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electron injection at the PTCDA/metal interface detected by electron paramagnetic resonance G.J. Gerardi a,⇑, J. Domenico a, A. Muraca a, H.K. Gerardi b a b
Department of Chemistry, William Paterson University, 300 Pompton Road, Wayne, NJ 07470, United States Ashland Incorporated, Molecular and Analytical Sciences Division, 1005 Highway 201/206, Bridgewater, NJ 08807, United States
a r t i c l e
i n f o
Article history: Received 6 November 2013 In final form 17 December 2013 Available online 22 December 2013
a b s t r a c t Electron paramagnetic resonance absorption was observed for samples prepared by vacuum vapor deposition of thin films of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on metal films of magnesium (Mg), aluminum (Al) and silver (Ag) but not on gold (Au) or PTCDA on quartz. The resonance absorption is seen as a Lorentzian line with g-value of 2.0030 and a linewidth of 0.19 mT. The signal is attributed to PTCDA anions at the metal organic interface. The relative values of the metal work function and the PTCDA electron affinity appear to determine the occurrence of the EPR absorption. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Information concerning the junction formed between metal electrodes and organic semiconductor thin films is important for the development of flexible electronic devices. Formation of the metal organic (MO) interface under high vacuum conditions results in charge injection barriers, which play a major role in the performance of organic solar cells, light emitting diodes and field effect transistors. Interfaces formed from molecular solids have been investigated on a variety of metal substrates with energy level alignment and charge transfer largely inferred from electrical measurements, ultraviolet photoelectron spectroscopy (PES) and theoretical calculations [1–8]. In 1995, interface dipole barriers were observed from PES experiments of 5,10,15,20-zinctetraphenylporphyrin (ZnTPP) on Au, Ag, Al, and Mg and provided the first experimental evidence for the departure of MO interfaces from the vacuum level alignment or Schottky–Mott limit [8]. Similar experiments on 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films deposited on Mg, In, Sn, and Au, revealed Fermi level pinning at the metal organic semiconductor (MOS) interface to be essentially independent of the metal work function (UM) due to a linear dependence of the interface dipole (D) with UM [9]. This dependence leads to a fixed barrier for charge injection at PTCDA-MO interfaces. When the MO interface is formed, electron redistribution is largely determined by the metal work function (UM) and the semiconductor electron affinity (EA). The relative energies of these two quantities determine the magnitude and sign of the dipole that forms across the interface along with pinning the interface Fermi
⇑ Corresponding author. Fax: +1 9737202338. E-mail address: [email protected] (G.J. Gerardi). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.12.039
level, EF, with the density of states below the LUMO level of the OS [10]. This report presents results of electron paramagnetic resonance (EPR) studies of PTCDA evaporated on thin films of Mg, Al, Ag, and Au. EPR spectroscopy has been used extensively to detect and characterize defects at interfaces of semiconductor materials and more recently to characterize charged states of organic semiconductors [11–16]. The detection of an EPR absorption from an otherwise closed shell organic semiconductor deposited on a metal demonstrates charge transfer and the formation of a radical anion or cation at the interface. Our EPR measurements of PTCDA vacuum deposited on low work function metal films reveal a single absorption signal, which we attribute to PTCDA anions. Samples of PTCDA deposited on high work function metals or vapor depositions of PTCDA on inert substrates do not result in PTCDA radical ions. Our results show that EPR can distinguish the formation of MO interface dipoles as being due to charge transfer as opposed to only polarization, resulting from physical adsorption of the organic molecule on the metal, or to covalent bond formation [10]. 2. Experimental details 2.1. Materials PTCDA, magnesium ribbon, aluminum, silver and gold wire were purchased from Sigma–Aldrich Inc. and used as received. Quartz substrates were purchased from Ted Pella, Inc. 2.2. Sample preparation and characterization Samples were prepared by vacuum thermal deposition of Mg, Al, Ag and Au metals on quartz bars having dimensions 20 4 1 mm followed by deposition of thin films of PTCDA.
46
G.J. Gerardi et al. / Chemical Physics Letters 593 (2014) 45–47
The metal films were deposited on quartz substrates at a rate of 3 Å/s for 15 min, resulting in an approximate film thickness of 270 nm followed by vacuum thermal deposition of PTCDA. The PTCDA layer thickness ranged from 24.0 to 25.5 lm as measured using an NT-MDT atomic force microscope. Vacuum depositions were performed at a pressure of 105 Torr. Reference samples of PTCDA deposited on identical quartz bars simultaneously with those on metals were prepared for comparison. Electron paramagnetic resonance spectra were recorded at room temperature using a CW Brüker EMX spectrometer and a HP5350B microwave frequency counter to measure resonance frequency. The free radical DPPH having g-value of 2.0036 served as the reference for g-values of the Mg, Al, and Ag EPR signals. Continuous wave power saturation method was used to obtain data for an estimation of the spin–lattice relaxation and decoherence times. This method requires a determination of H1, the magnetic field strength at the sample in the microwave cavity of the spectrometer. This value is related to the microwave power PW incident to the cavity by the relation H21 ¼ KP W . We obtained a value of K = 8.0 103 mT2/W, using a formula for K reported by Sands [17]. Spin density was determined by evaporation of micro liter quantities of a calibrated solution of DPPH onto gold coated quartz substrates from a calibrated micropipette. 3. Results and discussion 3.1. EPR characterization
Relative EPR Signal Intensity (A.U.)
The EPR spectra shown in Figure 1 are readily observed when PTCDA is vapor phase deposited on thin films of Mg and Al samples under high vacuum conditions. The signal observed on Ag was barely detectable but confirmed by signal averaging with intensity comparable to the noise level. However, samples of PTCDA on Au, or quartz alone, did not result in EPR absorption under the same conditions. The signals observed for PTCDA on Mg, Al, and Ag all had the same g-value of 2.0030 (±0.0001) and a low power limiting line width of 0.19 mT, but the relative intensity of the signals varied. The EPR signal intensity is proportional to the number of anion radicals resulting from the transfer of electrons from the metal to the PTCDA. The intensity of the Mg signal was approximately six times larger than the Al sample signal, which we attribute to the fact that Mg has a lower work function than Al. All of the samples
Figure 2. EPR absorption spectrum of the Mg/PTCDA sample (blue) and the data fitted to a pure Lorentzian function (red).
produced the same degree of cavity loading with quality factor Q of 2700. The resonance signal line shapes were all essentially Lorentzian. Figure 2 shows the absorption signal and the corresponding Lorentzian fit of the absorption, which had a coefficient of determination of 0.99879. The data could not be satisfactorily fit to a GAUSSIAN function. The resonances exhibited homogeneous saturation behavior. The saturation factor, s, based on the Bloch equations, is given by s1 ¼ 1 þ H21 c2 T 1 T 2 , where H1 is the microwave magnetic field and c the electron gyromagnetic ratio [18,19]. We obtained the spin decoherence time, T2, of 3.5 108 s from a linear extrapolation of the linewidths, DH0pp , with decreasing microwave power pffiffiffi using the relation T 2 ¼ 2= 3cDH0pp . The spin–lattice relaxation time T1 was 4.3 107 s from T2 and the slope of the power dependence of the inverse saturation factor relation (Figure 3). A Lorentzian line shape is expected for radical species in solid form due to electron-spin exchange between radicals or electron transfer between neutral molecules and radical ions [19]. The departure of the Lorentzian line shape in the wings of the spectrum, as shown in Figure 2, may be attributed to several factors, including spatial extension of radical anions on the sample substrate, hyperfine broadening, and the orientation distribution of the radical anions at the interface if they have some g-anisotropy and the PTCDA anions are randomly oriented. The possibility that the signal is due to the metal is not very likely since metal resonances have a Dysonian lineshape.
Mg
Al Ag Au
333
334
335
336
337
338
Magnetic Field (mT) Figure 1. EPR spectra of PTCDA vapor phase deposited on thin films of Mg, Al, Ag and Au vapor deposited on quartz substrates at room temperature. The Mg and Ag spectra are single scans, while the Ag and Au spectra are the results of signal averaging 16 scans.
Figure 3. Inverse of the saturation factor, s1, is shown as a function of the microwave power for the Mg/PTCDA sample at room temperature.
G.J. Gerardi et al. / Chemical Physics Letters 593 (2014) 45–47
47
transfer. The Au work function is significantly below the LUMO level of PTCDA as indicated in Figure 4, and accounts for the absence of any PTCDA radical ions at the interface. These EPR results support recent theoretical calculations of Rusu et al. that for low work function metals electrons are transferred from the metal to the molecule [22]. Our results are also consistent with the work of Yoshida and Sato who found that, Al vapor deposited on PTCDA diffuses into the PTCDA layer and reacts with PTCDA molecules [23]. These observations further suggest that other organic semiconductors may produce radical ions when vapor phase deposited on active metal surfaces. In conclusion, electron transfer from a metal electrode to PTCDA occurs when the work function of the metal is less than the electron affinity of PTCDA and results in the formation of the PTCDA anion radical at the MOS interface. We expect this charge transfer to result in the formation of a dipole layer at the interface. References Figure 4. The relative energy alignments of the metal work function with the HOMO and LUMO levels of PTCDA before interface formation.
We expect the PTCDA anions to be concentrated at the MO interface. We obtained the spin density at the interface to be 1.5 1014 cm2 with a factor of two estimated uncertainty. Fenster et al. reported experimental evidence that PTCDA molecules, vapor phase deposited under high vacuum on a Au surface, are arranged with the plane of the molecule resting flat on the Au surface in a regular array consisting of 8 molecules within an area of dimensions 11.96 Å 19.91 Å [20]. That would correspond to a surface density of 3.35 1014 cm2. That estimate, taken in conjunction with our spin density result, would indicate that a significant fraction of the PTCDA molecules at the metal surface are PTCDA anions. Although it would be generally expected for the unpaired electron to be delocalized primarily over the ring portion of the PTCDA anion, it’s also possible that the spin density is localized at the acid anhydride end. This could result if the injected electron initiates a homolytic carbon–oxygen bond dissociation of the anhydride with the negative charge residing on the bridging oxygen and the unpaired electron on the carbonyl carbon in a r⁄ orbital. That would probably result in a more stable anion. Reactions of the type A–B + e ? A–B with spin on a r⁄ orbital have been investigated by Symons [21]. The relative EPR signal intensities of PTCDA on Mg, Al and Ag demonstrates that the lower work function metals have a greater tendency to transfer electrons to PTCDA and form a negative radical ion, while metals with work functions comparable to the LUMO level of PTCDA such as Ag have very little tendency for electron
[1] Ruchi Agrawal, Subhasis Gosh, Appl. Phys. Lett. 89 (2006) 222114. [2] S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R.L. Johnson, F. Schreiber, N. Koch, Org. Electron. 9 (2008) 111. [3] Y. Zou, L. Kilian, A. Schöll, T. Schmidt, R. Fink, E. Umbach, Surf. Sci. 600 (2006) 1240. [4] Xian Ning Xie, Xingyu Gao, Yuzhan Wang, Junzhong Wang, Kian Ping Loh, Andrew Thye Shen Wee, Appl. Phys. Lett. 95 (2009) 063301. [5] Hiroyuki Yoshida, Naoki Sato, Appl. Phys. Lett. 91 (2007) 141915. [6] H. Vasquez, R. Oszwaldowski, P. Pou, J. Ortgea, R. Perez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 107. [7] J. Hwang, A. Wan, A. Kahn, Mater. Sci. Eng. R 64 (2008) 1. [8] S. Narioka et al., Appl. Phys. Lett. 67 (1995) 1899. [9] I.G. Hill, A. Rajagopal, A. Kahn, Appl. Phys. Lett. 73 (1998) 662. [10] A. Kahn, N. Koch, W. Gao, J. Polym. Sci. Polym. Phys. 41 (2003) 2529. [11] G.J. Gerardi, E.H. Poindexter, P. Caplan, N.M. Johnson, Appl. Phys. Lett. 49 (1986) 348. [12] G.J. Gerardi, E.H. Poindexter, P. Caplan, M. Harmatz, W.R. Buchwald, N.M. Johnson, J. Electrochem. Soc. 136 (1989) 2609. [13] Si-Guang Chen, Howard M. Branz, Sandra S. Eaton, J. Phys. Chem. B 108 (2004) 17327. [14] Kazuhiro Marumoto, Shin-ichi Kuroda, Taishi Takenobu, Yoshihiro Iwasa, Phys. Rev. Lett. 97 (2006) 256603. [15] Hideo Sato, Lauraine A. Dalton, Duc Ha, Richard W. Quine, J. Phys. Chem. B 111 (2007) 7972. [16] Hiroyuki Matsui, Tatsuo Hasegawa, Yoshinori Tokura, Phys. Rev. Lett. 100 (2008) 126601. [17] R.H. Sands, NMR and EPR Spectroscopy, Pergamon Press Inc., 1960, pp. 44–80. [18] Charles Poole, Electron Spin Resonance, second ed., John Wiley, New York, 1983, p. 589. [19] C.P. Poole, H.A. Farach, Theory of Magnetic Resonance, Wiley, New York, 1972, p. 201. [20] P. Fenter et al., Phys. Rev. B 56 (1997) 3046. [21] M.C.R. Symons, Foundations of Modern EPR, G.R. Eaton, S.S. Eaton, K.M. Salikov, World Scientific Pub. Co., Ltd., New Jersey, 1998, p. 301. [22] P.C. Rusu, G. Giovannetti, C. Weijtens, R. Coehoorn, G. Brocks, J. Phys. Chem. C 113 (2009) 9974. [23] H. Yoshida, N. Sato, J. Appl. Phys. 91 (2002) 4312.