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STM induced photon emission from adsorbed porphyrin molecules on a Cu(100) surface in ultrahigh vacuum
Surface Science 454–456 (2000) 1021–1025 www.elsevier.nl/locate/susc
STM induced photon emission from adsorbed porphyrin molecules on a Cu(100) surface in ultrahigh vacuum D. Fujita a, *, T. Ohgi a, W.-L. Deng a, H. Nejo a, T. Okamoto b, S. Yokoyama c, K. Kamikado c, S. Mashiko c a National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan b The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako 351-0198, Japan c Communication Research Laboratory, 588-2 Iwaoka, Kobe 651-2401, Japan
Abstract Photon emission induced by tunneling electrons has been observed from a submonolayer of Cu-tetra-[3,5-di-tbutylphenyl ]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface in ultrahigh vacuum. Near-field photons generated in a nanometer-scale region with Cu-TBPP molecules were collected effectively through the apex of a conductive optical fiber tip. The photon emission mechanism can be attributed to the inelastic tunneling events involving the tip, the Cu-TBPP molecules, and the Cu(100) substrate. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Copper; Electroluminescence; Low index single crystal surfaces; Photon emission; Scanning tunneling microscopy
1. Introduction Since the invention of the scanning tunneling microscope (STM ), this relatively simple tool has developed into a powerful instrument for the experimental researchers of nanometer-scale physics [1]. One exciting application of the STM technique is concerned with the detection of photons induced by tunneling electrons, where a part of the excess energy carried by the tunneling electron is transferred to a radiative inelastic process. These phenomena have been observed on metal surfaces [2–5], semiconductor surfaces [6,7], surfaces covered by chemisorbed molecules [8], conjugated polymer layers [9,10], and semiconductor multiquantum wells [11,12]. There are several proposed * Corresponding author. Fax: +81-298-59-2701. E-mail address: [email protected] (D. Fujita)
mechanisms for STM light emission. In the case of metal and indirect band-gap semiconductor surfaces, light emission can be explained by localized surface plasmons excited by tunneling electrons, whose radiative decay process causes photon emission [13–16 ]. For direct band-gap semiconductor surfaces and multiquantum wells, some of the electrons injected from the STM tip combine with holes to form excitons, which may subsequently decay radiatively. The electron–hole recombination process is also a reasonable explanation for the STM electroluminescence of organic semiconductors, which have recently attracted much interest for applications in light-emitting diodes [17,18]. In this paper we describe a new type of STM photon emission process observed on Cutetra-[3,5-di-t-butyl-phenyl ]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 7 4 -0
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D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
Fig. 1. Structure of Cu-tetra-[3,5-di-t-butylphenyl ]porphyrin (Cu-TBPP).
Cu-TBPP belongs to a category of metalloporphyrins, whose structure is shown in Fig. 1, and has absorption and fluorescence in visible wavelength. The driving force of this study is to demonstrate the possible existence of STM induced luminescence from a submonolayer of fluorescent molecules on a noble metal surface.
monitored by a quartz microbalance. Subsequent annealing at ~500 K equilibrated the randomly deposited Cu-TBPP molecules. STM induced photons were collected through a conductive transparent tip in outside detectors using optical fiber connections [19,20]. Optically transparent STM tips were made of multi-mode optical fibers of 100 mm diameter, whose apex was coated with conductive indium–tin–oxide (ITO) thin films of about 100 nm thickness. All the STM experiments were performed with the conductive optical-fiber tips at room temperature. Two types of STM induced photon measurement were performed, that is, photon mapping and optical spectroscopy. Photon mapping is spatial imaging of tunneling induced photons by simultaneous acquisition of photon intensity during STM topographic scans. In the mapping mode, the emitted photons were counted using a cooled photomultiplier tube (PMT; Hamamatsu, R2949). Because the PMT has sensitivity from ultraviolet (200 nm) to nearly infrared region (900 nm), the dark count of the photon counting system is about 100 cps. In order to perform STM induced luminescence spectroscopy, the emitted photons from a fixed position were introduced into a multichannel spectrometer with a liquid-nitrogen cooled CCD detector (Princeton Instruments).
2. Experimental methods
3. Results
The experiments were performed in a threechamber ultrahigh vacuum ( UHV ) system, which consists of STM (base pressure ~8×10−9 Pa), preparation, and deposition chambers. A Cu(100) single crystal (5 N purity) of rectangular shape (3×8×0.5 mm3) was used as a sample substrate, which was polished mechanically and electrochemically. The sample was then introduced into the preparation chamber, where the atomically clean surface was prepared by several cycles of Ar+ sputtering (1 keV ) and annealing at ~700 K in UHV. In the deposition chamber, Cu-TBPP molecules were sublimed and chemisorbed on the clean Cu(100) substrate at room temperature. The amount of chemisorbed Cu-TBPP molecules was
STM images of Cu-TBPP molecules adsorbed on Cu(100) after deposition in UHV and subsequent annealing are shown in Fig. 2a and b. Fig. 2a is a current image (V =−1.0 V, I =0.2 nA) tip tunnel of relatively large area (59×59 nm2), while Fig. 2b is a topographic image (V =−1.0 V, I = tip tunnel 0.2 nA) of smaller area (17.7×17.7 nm2). Each Cu-TBPP molecule was resolved as bright spots with square symmetry, which should correspond to the four di-t-butylphenyl (DBP) groups of the molecule. The overall structures of observed Cu-TBPP images are consistent with the previously reported STM images (E58×E58 superstructure) by Gimzewski’s group [21,22]. Molecular orbital calculation has suggested that the four DBP groups
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
(a)
(b) Fig. 2. STM images of Cu-TBPP molecules adsorbed on Cu(100) after deposition in UHV and subsequent annealing. (a) STM current image (V =−1.0 V, I =0.2 nA, tip tunnel area=59×59 nm2). (b) STM topographic image (V =−1.0 V, I =0.2 nA, area=17.7×17.7 nm2). tip tunnel
should be rotated ~90° from the central porphyrin ring. Therefore, it seems energetically favorable that the central porphyrin ring lies parallel to the surface supported by the four DBP groups as the
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legs. After annealing, Cu-TBPP molecules were found to form a simple square lattice on Cu(100), whose unit cell is shown in Fig. 2b, with lattice constant ~2.2 nm. Taking the size of a Cu-TBPP molecule into account, one unit cell can accommodate approximately two Cu-TBPP molecules. That is, the apparent coverage of the square lattice is about 0.5 ML. Because the image of Fig. 2a suggests that the Cu-TBPP molecules did not cover about 20% of the surface, especially near the step edges, the total coverage can be estimated as ~0.4 ML. In order to clarify the distribution of photon emission from the submonolayer coverage of Cu-TBPP molecules, simultaneous imaging of topography and photon emission was performed. Fig. 3a and b shows the typical topography/photon maps (17.7×17.7 nm2) obtained for submonolayer coverage (0.4 ML) of Cu-TBPP molecules on a Cu(100) surface. Relatively high tunneling voltage V =−6.0 V and tunneling current tip I =5.0 nA were employed to enhance photon tunnel intensity. In most of the region, the photon pattern reveals a loose correlation with the STM topography image. The number of photons induced by tunneling electrons was found to fluctuate between ~0.3 and ~4.0 kcps, with average number of photons ~1.0 kcps. Taking into account the total efficiency (~0.1%) of the photon detection system, the average number of emitted photons is estimated to be ~1×106 cps. Therefore, the quantum efficiency of the STM induced luminescence in the Cu-TBPP/Cu(100) system under the above conditions is roughly estimated to be ~2× 10−5 photons/electron. This value is of almost the same order as the quantum efficiencies (10−6 to 10−4) of tip induced plasmon modes on noble metal surfaces with metal tips [13,16 ]. Using conventional STM conditions (−3 to −10 V, 10 nA), the number of STM induced photons from a clean Cu(100) surface with ITO tip was found to be out of the detection limit, or less than 100 cps. This means that the quantum efficiency from a clean Cu(100) surface is approximately less than 10−6 photons/electron. Because the excitation of the tip induced plasmon mode is significantly dependent on the optical properties of the tip material, this relatively smaller value compared
1024
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
(a)
(b) Fig. 3. STM topography image (a) and its simultaneous photon map (b) obtained for submonolayer (0.4 ML) coverage of Cu-TBPP molecules on Cu(100) (V =−6.0 V, tip I =5.0 nA, area=17.7×17.7 nm2). tunnel
with the metal tip case is reasonable. Compared with the quantum efficiency of the bare surface, the submonolayer of Cu-TBPP admolecules was found to enhance the STM induced photon emission significantly. In order to clarify the mechanism of the enhanced STM induced photon emission, optical spectra in the visible and near infrared regions were measured from the same surface as above. The typical spectra are shown in Fig. 4, which
Fig. 4. Optical spectra of STM induced photon emission obtained from 0.4 ML coverage of Cu-TBPP molecules on Cu(100) with different conditions (V =−3.5 to −6.0 V, tip I =10.0 nA). tunnel
indicate that there exist at least two peaks. The observed features in the optical spectra were mostly well reproduced. The larger peak (Peak 1) is broad and located around 1.6 eV (~770 nm). Previously reported data suggest that the STM induced plasmons on Cu(111) have a broad maximum around 1.8 eV [2]. Compared with this, the Peak 1 around 1.6 eV can be attributed to the radiative decay of tip induced local surface plasmons excited by inelastic tunneling events. The smaller peak (Peak 2) has a sharper peak shape than Peak 1 and is located around 2.2 eV (~560 nm). It is also possible to assign Peak 2 to a part of the broad plasmon peak. Another possible assignment is the Cu-TBPP molecule itself. It is well known that the irradiation of ultraviolet light causes porphyrins or metalloporphyrins to emit strong fluorescent light in the red wavelength region from a conjugated porphyrin ring [23], which is caused by the radiative decay of Q-band excitons located around 2–2.2 eV. Therefore, it is also possible to assign Peak 2 to the STM induced fluorescence of Cu-TBPP molecules excited by inelastic tunneling events. Decoupling of the conjugated porphyrin ring from the surface by the four DBP legs also plays an important role in the molecular fluorescence, otherwise the non-radiative decay process would dominate.
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
4. Conclusions Photon emission induced by tunneling electrons has been observed from a submonolayer of Cu-TBPP molecules chemisorbed on a Cu(100) surface in UHV. Near-field photons generated in a nanometer-scale region were collected through the apex of a conductive optical fiber tip coated with ITO film. Simultaneous mapping of the piezo deviation and the emitted photon showed some loose correlation between surface topography and photon intensity. Photon emission spectra suggest the possible existence of two radiative processes, decay of local surface plasmon and molecular fluorescence, both of which can be attributed to the inelastic tunneling events involving the tip, molecules and substrate.
References [1] R. Wiesendanger, In: Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge, 1994, p. 4. [2] R. Berndt, J.K. Gimzewski, P. Johansson, Phys. Rev. Lett. 67 (1991) 3796. [3] R. Berndt, J.K. Gimzewski, P. Johansson, Phys. Rev. Lett. 71 (1993) 3493. [4] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Phys. Rev. Lett. 74 (1995) 102.
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[5] R. Berndt, J.K. Gimzewski, Phys. Rev. B 48 (1993) 4746. [6 ] A. Downes, M.E. Welland, Phys. Rev. Lett. 81 (1998) 1857. [7] C. Thirstrup, M. Sakurai, K. Stokbro, M. Aono, Phys. Rev. Lett. 82 (1999) 1241. [8] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Appl. Phys. A 57 (1993) 513. [9] S.F. Alvarado, W. Rieß, P.F. Seidler, P. Strohriegl, Phys. Rev. B 56 (1997) 1269. [10] D.G. Lidzey, S.F. Alvarado, P.F. Seidler, A. Bleyer, D.D.C. Bradley, Appl. Phys. Lett. 71 (1997) 2008. [11] T. Tsuruoka, Y. Ohizumi, S. Ushioda, U. Ohno, H. Ohno, Appl. Phys. Lett. 73 (1998) 1544. [12] S. Evoy, C.K. Harnett, H.G. Craighead, S. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 74 (1999) 1457. [13] P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210. [14] Y. Uehara, Y. Kimura, S. Ushioda, K. Takeuchi, Jpn. J. Appl. Phys. 31 (1992) 2465. [15] A. Downes, M.E. Taylor, M. Welland, Phys. Rev. B 57 (1998) 6706. [16 ] P. Johansson, Phys. Rev. B 58 (1998) 10 823. [17] D.G. Lidzey, D.D.C. Bradley, M.S. Skolnick, T. Virgili, S. Walker, D.M. Whittaker, Nature 395 (1998) 53. [18] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Sandos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [19] T. Murashita, Jpn. J. Appl. Phys. 34 (1995) 4398. [20] T. Murashita, J. Vac. Sci. Technol. B 15 (1997) 32. [21] T.A. Jung, R.R. Schlittler, J.K. Gimzewski, Nature 386 (1997) 696. [22] J.K. Gimzewski, T.A. Jung, M.T. Cuberes, R.R. Schlittler, Surf. Sci. 386 (1997) 101. [23] D. Dolphin ( Ed.), The Porphyrins Vol. III, Academic Press, New York, 1978, p. 1.
STM induced photon emission from adsorbed porphyrin molecules on a Cu(100) surface in ultrahigh vacuum D. Fujita a, *, T. Ohgi a, W.-L. Deng a, H. Nejo a, T. Okamoto b, S. Yokoyama c, K. Kamikado c, S. Mashiko c a National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan b The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako 351-0198, Japan c Communication Research Laboratory, 588-2 Iwaoka, Kobe 651-2401, Japan
Abstract Photon emission induced by tunneling electrons has been observed from a submonolayer of Cu-tetra-[3,5-di-tbutylphenyl ]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface in ultrahigh vacuum. Near-field photons generated in a nanometer-scale region with Cu-TBPP molecules were collected effectively through the apex of a conductive optical fiber tip. The photon emission mechanism can be attributed to the inelastic tunneling events involving the tip, the Cu-TBPP molecules, and the Cu(100) substrate. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Copper; Electroluminescence; Low index single crystal surfaces; Photon emission; Scanning tunneling microscopy
1. Introduction Since the invention of the scanning tunneling microscope (STM ), this relatively simple tool has developed into a powerful instrument for the experimental researchers of nanometer-scale physics [1]. One exciting application of the STM technique is concerned with the detection of photons induced by tunneling electrons, where a part of the excess energy carried by the tunneling electron is transferred to a radiative inelastic process. These phenomena have been observed on metal surfaces [2–5], semiconductor surfaces [6,7], surfaces covered by chemisorbed molecules [8], conjugated polymer layers [9,10], and semiconductor multiquantum wells [11,12]. There are several proposed * Corresponding author. Fax: +81-298-59-2701. E-mail address: [email protected] (D. Fujita)
mechanisms for STM light emission. In the case of metal and indirect band-gap semiconductor surfaces, light emission can be explained by localized surface plasmons excited by tunneling electrons, whose radiative decay process causes photon emission [13–16 ]. For direct band-gap semiconductor surfaces and multiquantum wells, some of the electrons injected from the STM tip combine with holes to form excitons, which may subsequently decay radiatively. The electron–hole recombination process is also a reasonable explanation for the STM electroluminescence of organic semiconductors, which have recently attracted much interest for applications in light-emitting diodes [17,18]. In this paper we describe a new type of STM photon emission process observed on Cutetra-[3,5-di-t-butyl-phenyl ]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(100) surface.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 7 4 -0
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D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
Fig. 1. Structure of Cu-tetra-[3,5-di-t-butylphenyl ]porphyrin (Cu-TBPP).
Cu-TBPP belongs to a category of metalloporphyrins, whose structure is shown in Fig. 1, and has absorption and fluorescence in visible wavelength. The driving force of this study is to demonstrate the possible existence of STM induced luminescence from a submonolayer of fluorescent molecules on a noble metal surface.
monitored by a quartz microbalance. Subsequent annealing at ~500 K equilibrated the randomly deposited Cu-TBPP molecules. STM induced photons were collected through a conductive transparent tip in outside detectors using optical fiber connections [19,20]. Optically transparent STM tips were made of multi-mode optical fibers of 100 mm diameter, whose apex was coated with conductive indium–tin–oxide (ITO) thin films of about 100 nm thickness. All the STM experiments were performed with the conductive optical-fiber tips at room temperature. Two types of STM induced photon measurement were performed, that is, photon mapping and optical spectroscopy. Photon mapping is spatial imaging of tunneling induced photons by simultaneous acquisition of photon intensity during STM topographic scans. In the mapping mode, the emitted photons were counted using a cooled photomultiplier tube (PMT; Hamamatsu, R2949). Because the PMT has sensitivity from ultraviolet (200 nm) to nearly infrared region (900 nm), the dark count of the photon counting system is about 100 cps. In order to perform STM induced luminescence spectroscopy, the emitted photons from a fixed position were introduced into a multichannel spectrometer with a liquid-nitrogen cooled CCD detector (Princeton Instruments).
2. Experimental methods
3. Results
The experiments were performed in a threechamber ultrahigh vacuum ( UHV ) system, which consists of STM (base pressure ~8×10−9 Pa), preparation, and deposition chambers. A Cu(100) single crystal (5 N purity) of rectangular shape (3×8×0.5 mm3) was used as a sample substrate, which was polished mechanically and electrochemically. The sample was then introduced into the preparation chamber, where the atomically clean surface was prepared by several cycles of Ar+ sputtering (1 keV ) and annealing at ~700 K in UHV. In the deposition chamber, Cu-TBPP molecules were sublimed and chemisorbed on the clean Cu(100) substrate at room temperature. The amount of chemisorbed Cu-TBPP molecules was
STM images of Cu-TBPP molecules adsorbed on Cu(100) after deposition in UHV and subsequent annealing are shown in Fig. 2a and b. Fig. 2a is a current image (V =−1.0 V, I =0.2 nA) tip tunnel of relatively large area (59×59 nm2), while Fig. 2b is a topographic image (V =−1.0 V, I = tip tunnel 0.2 nA) of smaller area (17.7×17.7 nm2). Each Cu-TBPP molecule was resolved as bright spots with square symmetry, which should correspond to the four di-t-butylphenyl (DBP) groups of the molecule. The overall structures of observed Cu-TBPP images are consistent with the previously reported STM images (E58×E58 superstructure) by Gimzewski’s group [21,22]. Molecular orbital calculation has suggested that the four DBP groups
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
(a)
(b) Fig. 2. STM images of Cu-TBPP molecules adsorbed on Cu(100) after deposition in UHV and subsequent annealing. (a) STM current image (V =−1.0 V, I =0.2 nA, tip tunnel area=59×59 nm2). (b) STM topographic image (V =−1.0 V, I =0.2 nA, area=17.7×17.7 nm2). tip tunnel
should be rotated ~90° from the central porphyrin ring. Therefore, it seems energetically favorable that the central porphyrin ring lies parallel to the surface supported by the four DBP groups as the
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legs. After annealing, Cu-TBPP molecules were found to form a simple square lattice on Cu(100), whose unit cell is shown in Fig. 2b, with lattice constant ~2.2 nm. Taking the size of a Cu-TBPP molecule into account, one unit cell can accommodate approximately two Cu-TBPP molecules. That is, the apparent coverage of the square lattice is about 0.5 ML. Because the image of Fig. 2a suggests that the Cu-TBPP molecules did not cover about 20% of the surface, especially near the step edges, the total coverage can be estimated as ~0.4 ML. In order to clarify the distribution of photon emission from the submonolayer coverage of Cu-TBPP molecules, simultaneous imaging of topography and photon emission was performed. Fig. 3a and b shows the typical topography/photon maps (17.7×17.7 nm2) obtained for submonolayer coverage (0.4 ML) of Cu-TBPP molecules on a Cu(100) surface. Relatively high tunneling voltage V =−6.0 V and tunneling current tip I =5.0 nA were employed to enhance photon tunnel intensity. In most of the region, the photon pattern reveals a loose correlation with the STM topography image. The number of photons induced by tunneling electrons was found to fluctuate between ~0.3 and ~4.0 kcps, with average number of photons ~1.0 kcps. Taking into account the total efficiency (~0.1%) of the photon detection system, the average number of emitted photons is estimated to be ~1×106 cps. Therefore, the quantum efficiency of the STM induced luminescence in the Cu-TBPP/Cu(100) system under the above conditions is roughly estimated to be ~2× 10−5 photons/electron. This value is of almost the same order as the quantum efficiencies (10−6 to 10−4) of tip induced plasmon modes on noble metal surfaces with metal tips [13,16 ]. Using conventional STM conditions (−3 to −10 V, 10 nA), the number of STM induced photons from a clean Cu(100) surface with ITO tip was found to be out of the detection limit, or less than 100 cps. This means that the quantum efficiency from a clean Cu(100) surface is approximately less than 10−6 photons/electron. Because the excitation of the tip induced plasmon mode is significantly dependent on the optical properties of the tip material, this relatively smaller value compared
1024
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
(a)
(b) Fig. 3. STM topography image (a) and its simultaneous photon map (b) obtained for submonolayer (0.4 ML) coverage of Cu-TBPP molecules on Cu(100) (V =−6.0 V, tip I =5.0 nA, area=17.7×17.7 nm2). tunnel
with the metal tip case is reasonable. Compared with the quantum efficiency of the bare surface, the submonolayer of Cu-TBPP admolecules was found to enhance the STM induced photon emission significantly. In order to clarify the mechanism of the enhanced STM induced photon emission, optical spectra in the visible and near infrared regions were measured from the same surface as above. The typical spectra are shown in Fig. 4, which
Fig. 4. Optical spectra of STM induced photon emission obtained from 0.4 ML coverage of Cu-TBPP molecules on Cu(100) with different conditions (V =−3.5 to −6.0 V, tip I =10.0 nA). tunnel
indicate that there exist at least two peaks. The observed features in the optical spectra were mostly well reproduced. The larger peak (Peak 1) is broad and located around 1.6 eV (~770 nm). Previously reported data suggest that the STM induced plasmons on Cu(111) have a broad maximum around 1.8 eV [2]. Compared with this, the Peak 1 around 1.6 eV can be attributed to the radiative decay of tip induced local surface plasmons excited by inelastic tunneling events. The smaller peak (Peak 2) has a sharper peak shape than Peak 1 and is located around 2.2 eV (~560 nm). It is also possible to assign Peak 2 to a part of the broad plasmon peak. Another possible assignment is the Cu-TBPP molecule itself. It is well known that the irradiation of ultraviolet light causes porphyrins or metalloporphyrins to emit strong fluorescent light in the red wavelength region from a conjugated porphyrin ring [23], which is caused by the radiative decay of Q-band excitons located around 2–2.2 eV. Therefore, it is also possible to assign Peak 2 to the STM induced fluorescence of Cu-TBPP molecules excited by inelastic tunneling events. Decoupling of the conjugated porphyrin ring from the surface by the four DBP legs also plays an important role in the molecular fluorescence, otherwise the non-radiative decay process would dominate.
D. Fujita et al. / Surface Science 454–456 (2000) 1021–1025
4. Conclusions Photon emission induced by tunneling electrons has been observed from a submonolayer of Cu-TBPP molecules chemisorbed on a Cu(100) surface in UHV. Near-field photons generated in a nanometer-scale region were collected through the apex of a conductive optical fiber tip coated with ITO film. Simultaneous mapping of the piezo deviation and the emitted photon showed some loose correlation between surface topography and photon intensity. Photon emission spectra suggest the possible existence of two radiative processes, decay of local surface plasmon and molecular fluorescence, both of which can be attributed to the inelastic tunneling events involving the tip, molecules and substrate.
References [1] R. Wiesendanger, In: Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge, 1994, p. 4. [2] R. Berndt, J.K. Gimzewski, P. Johansson, Phys. Rev. Lett. 67 (1991) 3796. [3] R. Berndt, J.K. Gimzewski, P. Johansson, Phys. Rev. Lett. 71 (1993) 3493. [4] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Phys. Rev. Lett. 74 (1995) 102.
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[5] R. Berndt, J.K. Gimzewski, Phys. Rev. B 48 (1993) 4746. [6 ] A. Downes, M.E. Welland, Phys. Rev. Lett. 81 (1998) 1857. [7] C. Thirstrup, M. Sakurai, K. Stokbro, M. Aono, Phys. Rev. Lett. 82 (1999) 1241. [8] R. Berndt, R. Gaisch, W.D. Schneider, J.K. Gimzewski, B. Reihl, R.R. Schlittler, M. Tschudy, Appl. Phys. A 57 (1993) 513. [9] S.F. Alvarado, W. Rieß, P.F. Seidler, P. Strohriegl, Phys. Rev. B 56 (1997) 1269. [10] D.G. Lidzey, S.F. Alvarado, P.F. Seidler, A. Bleyer, D.D.C. Bradley, Appl. Phys. Lett. 71 (1997) 2008. [11] T. Tsuruoka, Y. Ohizumi, S. Ushioda, U. Ohno, H. Ohno, Appl. Phys. Lett. 73 (1998) 1544. [12] S. Evoy, C.K. Harnett, H.G. Craighead, S. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 74 (1999) 1457. [13] P. Johansson, R. Monreal, P. Apell, Phys. Rev. B 42 (1990) 9210. [14] Y. Uehara, Y. Kimura, S. Ushioda, K. Takeuchi, Jpn. J. Appl. Phys. 31 (1992) 2465. [15] A. Downes, M.E. Taylor, M. Welland, Phys. Rev. B 57 (1998) 6706. [16 ] P. Johansson, Phys. Rev. B 58 (1998) 10 823. [17] D.G. Lidzey, D.D.C. Bradley, M.S. Skolnick, T. Virgili, S. Walker, D.M. Whittaker, Nature 395 (1998) 53. [18] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Sandos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [19] T. Murashita, Jpn. J. Appl. Phys. 34 (1995) 4398. [20] T. Murashita, J. Vac. Sci. Technol. B 15 (1997) 32. [21] T.A. Jung, R.R. Schlittler, J.K. Gimzewski, Nature 386 (1997) 696. [22] J.K. Gimzewski, T.A. Jung, M.T. Cuberes, R.R. Schlittler, Surf. Sci. 386 (1997) 101. [23] D. Dolphin ( Ed.), The Porphyrins Vol. III, Academic Press, New York, 1978, p. 1.