- Email: [email protected]
The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies
Applied Surface Science 255 (2008) 764–766
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies N. Peltekis a, B.N. Holland a, L.F.J. Piper b, A. DeMasi b, K.E. Smith b, J.E. Downes c, I.T. McGovern a, C. McGuinness a,* a b c
School of Physics, Trinity College Dublin, Dublin 2, Ireland Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, MA 02215, USA Physics Department, Macquarie University, NSW 2109, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
The electronic structure of thin films of the organic semiconductor tin phthalocyanine (SnPc) has been investigated by resonant and non-resonant soft X-ray emission (RXES and XES) at the carbon and nitrogen K-edges with excitation energies determined from near edge X-ray absorption fine structure (NEXAFS) spectra. The resultant NEXAFS and RXES spectra measure the unoccupied and occupied C and N 2p projected density of states, respectively. In particular, RXES results in site-specific C 2p and N 2p local partial density of states (LPDOS) being measured. An angular dependence of C 2p and N 2p RXES spectra of SnPc was observed. The observed angular dependence, the measured LPDOS and their correspondence to the results of density functional theory calculations are discussed. Observed differences on the same site-specific features between resonant (non-ionising) and non-resonant (ionising) NXES spectra are attributed to symmetry selection and screening. ß 2008 Elsevier B.V. All rights reserved.
Available online 11 July 2008 PACS: 78.70En 78.70Dm 71.20.Rv Keywords: Tin phthalocyanine SnPc X-ray fluorescence Resonant X-ray emission spectroscopy X-ray absorption spectroscopy
1. Introduction Phthalocyanine (Pc) organic semiconductors materials are used in a wide variety of technological applications from optical recording to their use as transport layers in organic light emitting diodes. Recent current–voltage (I–V) experiments on thin organic interlayers of tin phthalocyanine (SnPc) in metal-inorganic semiconductor Schottky diode unexpectedly resulted in a large reduction in the Ag/n-GaAs barrier height for certain thicknesses [1] where this was ascribed as being due to an organic-induced change in the n-GaAs space charge region. Clearly, it is desirable to further investigate the electronic structure of both SnPc-inorganic semiconductor (or metal) interfaces as well as the electronic structure of bulk SnPc. The electronic structure of thin layers of SnPc deposited on various GaAs and Ge substrates has previously been studied by both valence band photoemission spectroscopy (VBXPS) and near edge X-ray absorption fine structure (NEXAFS) [2,3]. NEXAFS, in addition to providing information on the orientation of the
* Corresponding author. E-mail address: [email protected] (C. McGuinness). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.058
molecule on the substrate, also probes the partial density of states (PDOS) of the lowest unoccupied molecular orbitals (or LUMO band) while VBXPS measures the total density of states of the highest occupied molecular orbitals (HOMO band). A complementary technique allowing the measurement of the occupied partial density of states on a site- and symmetry-specific basis is that of resonant soft X-ray emission spectroscopy (RXES) which is applied to tin phthalocyanine here. 2. Experimental details The RXES measurements were performed at the X1B soft X-ray undulator beamline of the National Synchrotron Light Source (NSLS). All RXES spectra were obtained using a Nordgren-type grazing incidence X-ray emission spectrometer positioned at 908 to the beam [4]. NEXAFS spectra were obtained at C and N K-edges with monochromator resolutions of 0.11 eV and were obtained in total electron yield (TEY) by the current drain method. RXES spectra of the excited C and N K-edges were obtained with nominal instrumental resolutions of 0.5–0.8 eV and 0.45 eV, respectively. A SnPc film of 1000 A´˚ thickness was deposited by sublimating from a Knudsen cell at <1 10 7 Torr onto the native oxide of a Si(1 0 0) substrate. The substrate had been degassed for 6 h at 200 8C in the
N. Peltekis et al. / Applied Surface Science 255 (2008) 764–766
765
chamber which had a base pressure of 5 10 9 Torr. The thickness was estimated by a quartz crystal monitor. As RXES measurements require long accumulation times, sample exposure was limited to avoid beamdamage [2,4] by vertically translating the sample every 300 ms to obtain a fresh unexposed surface. 3. Results and discussion The non-planar SnPc has a shuttlecock shape and forms a triclinic crystal structure [5]. Individual molecules stack along the a-axis, alternating convex and concave sides with all stacks identical. NEXAFS of thin films of SnPc on Ge and GaAs have shown SnPc molecular planes lying close to parallel to the substrate [2]. In Fig. 1 NEXAFS spectra of SnPc films on Si, measured at 208 and 708 angles of incidence at both the C and N K-edges, show a similar angular dependence; in contrast the molecular plane of FePc on Si(1 0 0) is near vertical [6]. XES spectra for these angular extremes (limited due to geometry) at either the C or N K-edge can be recorded with different excitation energies from threshold upwards. These XES spectra represent a measure of the 2p PDOS of the selected element, where in the case of the C K-edge, a difference in 1s binding energy between the pyrrole C site and the ring or aromatic C site can be exploited at or near threshold, energies A (285.6 eV) and B (286.3 eV) in Fig. 1, respectively, to almost separately measure their respective PDOS [4]. Instead we discuss here the angular dependence of the observed SXE spectra measured on resonance
Fig. 2. Diagram illustrating angular dependence of RXES arising from the various C sites for a single SnPc molecule.
and above threshold at both C and N K-edges as shown in the lower panels of Fig. 1. The middle panel of Fig. 1 shows the C K-edge XES spectra taken at both the pyrrole 1s-p* resonance (B) and above threshold (E). It is seen clearly that the near-normal incidence (708) spectrum shows significantly greater spectral weight at 282 eV as well as increased weight at 284 eV. This corresponds to the HOMO-1 and HOMO levels as determined from density functional theory calculations (not shown) [7] where the HOMO-1 shows a strong angular dependence. The C PDOS of LUMO and HOMO is found to be dominated by pyrrole carbons with mainly out of plane spatial orbital distribution. Similarly, the N K-edge RXES spectra (F) show increased spectral weight for the 708 angles at 396.5 eV corresponding to the HOMO levels. The RXES angular dependence arises as the SnPc molecular planes lie at slight angles to the substrate (see Fig. 2) and even if randomly oriented, low angles of incidence onto the surface will have E predominantly parallel to p* orbitals. Due to the 908 scattering geometry, emission then occurs only from occupied inplane (s-like) orbitals [8]. Angles of incidence near the normal to the molecular plane result in emission from both occupied inplane (s) and out-of-plane (p) orbitals giving spatial information of the C and N LPDOS components of the HOMO and HOMO-1 levels as indicated above. RXES spectra and above threshold NXES spectra can be considered as one-step or two-step processes, respectively with a relaxation of selection rules for the latter [8,9]. Consider the N Kedge non-resonant NXES spectra (G): in these, at both angles, a much greater spectral weight from 396 eV and higher is observed which arises almost exclusively from the out of plane aza (or mesobridging) outer N sites where this is suppressed in the RXES spectrum (F). In both C and N RXES spectra it seems that symmetry selection suppresses the HOMO which is distinctive only in the nonresonant emission. Finally there also appears a shift in the peak of 0.3 eV between RXES (F) and non-resonant XES spectra (G) at N K-edge which is not completely explained by symmetry site selection and may be due to screening of the core hole in the resonant emission process. 4. Summary
Fig. 1. Top panels: SnPc NEXAFS at C (left) and N (right) K-edges, respectively; at 208 and 708 incidence of beam to substrate. Bottom panels: C K and N K XES spectra at the same angles in resonant and non-resonant mode.
The angular dependence of C and N K-edge RXES from thick films of SnPc has been observed, and is directly related to the spatial distribution of the occupied p and s molecular orbitals and these correspond well with DFT calculations. We have indicated the complexity of this spectroscopy in the accurate interpretation of electronic structure of these organic molecular semiconductors through highlighting differences between resonant and nonresonant SXE spectra, such as suppression of the HOMO on the
766
N. Peltekis et al. / Applied Surface Science 255 (2008) 764–766
former due to symmetry selection, and shift due to screening effect in the latter. Application to planar-Pc molecules where herringbone stacking occurs may be problematic even though desirable. Acknowledgments This work was supported in part by Science Foundation Ireland, Enterprise Ireland, the U.S. AFOSR under FA9550-06-1-0157 and the US ARO under DAAD19-01-1-0364 and DAAH04-95-0014. The National Synchrotron Light Source is supported by the U.S. DOE under DE-AC02-98CH10886.
References [1] A.R. Vearey-Roberts, D.A. Evans, Appl. Phys. Lett. 86 (2005). [2] G. Cabailh, J.W. Wells, I.T. McGovern, A.R. Vearey-Roberts, A. Bushell, D.A. Evans, Appl. Surf. Sci. 234 (2004) 144–148. [3] G. Cabailh, B.N. Holland, C. Stephens, I.T. McGovern, C. McGuinness, A. Cafolla, A.R. Vearey-Roberts, D.A. Evans, J. Phys. IV 132 (2006) 11–15. [4] J.E. Downes, C. McGuinness, P.A. Glans, T. Learmonth, D.F. Fu, P. Sheridan, K.E. Smith, Chem. Phys. Lett. 390 (2004) 203–207. [5] R. Kubiak, J. Janczak, J. Alloys Compd. 189 (1992) 107–111. [6] J. Ahlund, K. Nilson, J. Schiessling, L. Kjeldgaard, S. Berner, N. Martensson, C. Puglia, B. Brena, M. Nyberg, Y. Luo, J. Chem. Phys. 125 (2006) 034709. [7] C. McGuinness, J.E. Downes, N. Peltekis, Unpublished. [8] F. Gel’mukhanov, H. Agren, Phys. Rep. Rev. Sec. Phys. Lett. 312 (1999) 87–330. [9] Y. Luo, H. Agren, F. Gelmukhanov, Phys. Rev. A 53 (1996) 1340.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies N. Peltekis a, B.N. Holland a, L.F.J. Piper b, A. DeMasi b, K.E. Smith b, J.E. Downes c, I.T. McGovern a, C. McGuinness a,* a b c
School of Physics, Trinity College Dublin, Dublin 2, Ireland Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, MA 02215, USA Physics Department, Macquarie University, NSW 2109, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
The electronic structure of thin films of the organic semiconductor tin phthalocyanine (SnPc) has been investigated by resonant and non-resonant soft X-ray emission (RXES and XES) at the carbon and nitrogen K-edges with excitation energies determined from near edge X-ray absorption fine structure (NEXAFS) spectra. The resultant NEXAFS and RXES spectra measure the unoccupied and occupied C and N 2p projected density of states, respectively. In particular, RXES results in site-specific C 2p and N 2p local partial density of states (LPDOS) being measured. An angular dependence of C 2p and N 2p RXES spectra of SnPc was observed. The observed angular dependence, the measured LPDOS and their correspondence to the results of density functional theory calculations are discussed. Observed differences on the same site-specific features between resonant (non-ionising) and non-resonant (ionising) NXES spectra are attributed to symmetry selection and screening. ß 2008 Elsevier B.V. All rights reserved.
Available online 11 July 2008 PACS: 78.70En 78.70Dm 71.20.Rv Keywords: Tin phthalocyanine SnPc X-ray fluorescence Resonant X-ray emission spectroscopy X-ray absorption spectroscopy
1. Introduction Phthalocyanine (Pc) organic semiconductors materials are used in a wide variety of technological applications from optical recording to their use as transport layers in organic light emitting diodes. Recent current–voltage (I–V) experiments on thin organic interlayers of tin phthalocyanine (SnPc) in metal-inorganic semiconductor Schottky diode unexpectedly resulted in a large reduction in the Ag/n-GaAs barrier height for certain thicknesses [1] where this was ascribed as being due to an organic-induced change in the n-GaAs space charge region. Clearly, it is desirable to further investigate the electronic structure of both SnPc-inorganic semiconductor (or metal) interfaces as well as the electronic structure of bulk SnPc. The electronic structure of thin layers of SnPc deposited on various GaAs and Ge substrates has previously been studied by both valence band photoemission spectroscopy (VBXPS) and near edge X-ray absorption fine structure (NEXAFS) [2,3]. NEXAFS, in addition to providing information on the orientation of the
* Corresponding author. E-mail address: [email protected] (C. McGuinness). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.058
molecule on the substrate, also probes the partial density of states (PDOS) of the lowest unoccupied molecular orbitals (or LUMO band) while VBXPS measures the total density of states of the highest occupied molecular orbitals (HOMO band). A complementary technique allowing the measurement of the occupied partial density of states on a site- and symmetry-specific basis is that of resonant soft X-ray emission spectroscopy (RXES) which is applied to tin phthalocyanine here. 2. Experimental details The RXES measurements were performed at the X1B soft X-ray undulator beamline of the National Synchrotron Light Source (NSLS). All RXES spectra were obtained using a Nordgren-type grazing incidence X-ray emission spectrometer positioned at 908 to the beam [4]. NEXAFS spectra were obtained at C and N K-edges with monochromator resolutions of 0.11 eV and were obtained in total electron yield (TEY) by the current drain method. RXES spectra of the excited C and N K-edges were obtained with nominal instrumental resolutions of 0.5–0.8 eV and 0.45 eV, respectively. A SnPc film of 1000 A´˚ thickness was deposited by sublimating from a Knudsen cell at <1 10 7 Torr onto the native oxide of a Si(1 0 0) substrate. The substrate had been degassed for 6 h at 200 8C in the
N. Peltekis et al. / Applied Surface Science 255 (2008) 764–766
765
chamber which had a base pressure of 5 10 9 Torr. The thickness was estimated by a quartz crystal monitor. As RXES measurements require long accumulation times, sample exposure was limited to avoid beamdamage [2,4] by vertically translating the sample every 300 ms to obtain a fresh unexposed surface. 3. Results and discussion The non-planar SnPc has a shuttlecock shape and forms a triclinic crystal structure [5]. Individual molecules stack along the a-axis, alternating convex and concave sides with all stacks identical. NEXAFS of thin films of SnPc on Ge and GaAs have shown SnPc molecular planes lying close to parallel to the substrate [2]. In Fig. 1 NEXAFS spectra of SnPc films on Si, measured at 208 and 708 angles of incidence at both the C and N K-edges, show a similar angular dependence; in contrast the molecular plane of FePc on Si(1 0 0) is near vertical [6]. XES spectra for these angular extremes (limited due to geometry) at either the C or N K-edge can be recorded with different excitation energies from threshold upwards. These XES spectra represent a measure of the 2p PDOS of the selected element, where in the case of the C K-edge, a difference in 1s binding energy between the pyrrole C site and the ring or aromatic C site can be exploited at or near threshold, energies A (285.6 eV) and B (286.3 eV) in Fig. 1, respectively, to almost separately measure their respective PDOS [4]. Instead we discuss here the angular dependence of the observed SXE spectra measured on resonance
Fig. 2. Diagram illustrating angular dependence of RXES arising from the various C sites for a single SnPc molecule.
and above threshold at both C and N K-edges as shown in the lower panels of Fig. 1. The middle panel of Fig. 1 shows the C K-edge XES spectra taken at both the pyrrole 1s-p* resonance (B) and above threshold (E). It is seen clearly that the near-normal incidence (708) spectrum shows significantly greater spectral weight at 282 eV as well as increased weight at 284 eV. This corresponds to the HOMO-1 and HOMO levels as determined from density functional theory calculations (not shown) [7] where the HOMO-1 shows a strong angular dependence. The C PDOS of LUMO and HOMO is found to be dominated by pyrrole carbons with mainly out of plane spatial orbital distribution. Similarly, the N K-edge RXES spectra (F) show increased spectral weight for the 708 angles at 396.5 eV corresponding to the HOMO levels. The RXES angular dependence arises as the SnPc molecular planes lie at slight angles to the substrate (see Fig. 2) and even if randomly oriented, low angles of incidence onto the surface will have E predominantly parallel to p* orbitals. Due to the 908 scattering geometry, emission then occurs only from occupied inplane (s-like) orbitals [8]. Angles of incidence near the normal to the molecular plane result in emission from both occupied inplane (s) and out-of-plane (p) orbitals giving spatial information of the C and N LPDOS components of the HOMO and HOMO-1 levels as indicated above. RXES spectra and above threshold NXES spectra can be considered as one-step or two-step processes, respectively with a relaxation of selection rules for the latter [8,9]. Consider the N Kedge non-resonant NXES spectra (G): in these, at both angles, a much greater spectral weight from 396 eV and higher is observed which arises almost exclusively from the out of plane aza (or mesobridging) outer N sites where this is suppressed in the RXES spectrum (F). In both C and N RXES spectra it seems that symmetry selection suppresses the HOMO which is distinctive only in the nonresonant emission. Finally there also appears a shift in the peak of 0.3 eV between RXES (F) and non-resonant XES spectra (G) at N K-edge which is not completely explained by symmetry site selection and may be due to screening of the core hole in the resonant emission process. 4. Summary
Fig. 1. Top panels: SnPc NEXAFS at C (left) and N (right) K-edges, respectively; at 208 and 708 incidence of beam to substrate. Bottom panels: C K and N K XES spectra at the same angles in resonant and non-resonant mode.
The angular dependence of C and N K-edge RXES from thick films of SnPc has been observed, and is directly related to the spatial distribution of the occupied p and s molecular orbitals and these correspond well with DFT calculations. We have indicated the complexity of this spectroscopy in the accurate interpretation of electronic structure of these organic molecular semiconductors through highlighting differences between resonant and nonresonant SXE spectra, such as suppression of the HOMO on the
766
N. Peltekis et al. / Applied Surface Science 255 (2008) 764–766
former due to symmetry selection, and shift due to screening effect in the latter. Application to planar-Pc molecules where herringbone stacking occurs may be problematic even though desirable. Acknowledgments This work was supported in part by Science Foundation Ireland, Enterprise Ireland, the U.S. AFOSR under FA9550-06-1-0157 and the US ARO under DAAD19-01-1-0364 and DAAH04-95-0014. The National Synchrotron Light Source is supported by the U.S. DOE under DE-AC02-98CH10886.
References [1] A.R. Vearey-Roberts, D.A. Evans, Appl. Phys. Lett. 86 (2005). [2] G. Cabailh, J.W. Wells, I.T. McGovern, A.R. Vearey-Roberts, A. Bushell, D.A. Evans, Appl. Surf. Sci. 234 (2004) 144–148. [3] G. Cabailh, B.N. Holland, C. Stephens, I.T. McGovern, C. McGuinness, A. Cafolla, A.R. Vearey-Roberts, D.A. Evans, J. Phys. IV 132 (2006) 11–15. [4] J.E. Downes, C. McGuinness, P.A. Glans, T. Learmonth, D.F. Fu, P. Sheridan, K.E. Smith, Chem. Phys. Lett. 390 (2004) 203–207. [5] R. Kubiak, J. Janczak, J. Alloys Compd. 189 (1992) 107–111. [6] J. Ahlund, K. Nilson, J. Schiessling, L. Kjeldgaard, S. Berner, N. Martensson, C. Puglia, B. Brena, M. Nyberg, Y. Luo, J. Chem. Phys. 125 (2006) 034709. [7] C. McGuinness, J.E. Downes, N. Peltekis, Unpublished. [8] F. Gel’mukhanov, H. Agren, Phys. Rep. Rev. Sec. Phys. Lett. 312 (1999) 87–330. [9] Y. Luo, H. Agren, F. Gelmukhanov, Phys. Rev. A 53 (1996) 1340.