STM-induced light emission from thin films of perylene derivatives on the HOPG and Au substrates
© Fujiki et al; licensee Springer. 2011
Received: 4 November 2010
Accepted: 19 April 2011
Published: 19 April 2011
We have investigated the emission properties of N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide thin films by the tunneling-electron-induced light emission technique. A fluorescence peak with vibronic progressions with large Stokes shifts was observed on both highly ordered pyrolytic graphite (HOPG) and Au substrates, indicating that the emission was derived from the isolated-molecule-like film condition with sufficient π-π interaction of the perylene rings of perylenetetracarboxylic diimide molecules. The upconversion emission mechanism of the tunneling-electron-induced emission was discussed in terms of inelastic tunneling including multiexcitation processes. The wavelength-selective enhanced emission due to a localized tip-induced surface plasmon on the Au substrate was also obtained.
Control of molecular emission from organic materials has attracted much attention owing to its potential applications not only in basic molecular science but also in research on soft material devices such as organic light-emitting diodes (OLEDs) and biosensors [1–4]. Scanning-tunneling-microscope-induced light emission (STM-LE) spectroscopy is highly effective for characterizing the optical and electronic properties of nanoscale materials such as organic single molecules or thin films at the atomic scale. However, it involves serious analytical difficulties in receiving extremely weak signals from the objective materials. To overcome such difficulties, it is promising to combine STM-LE spectroscopy with plasmon enhancement on surfaces. Surface plasmons at the interface between metallic and dielectric media generate an intense electromagnetic field on the surface, which provides an efficient enhancement field for some optical processes such as the fluorescence/phosphorescence emission and optical absorption of organic materials on a metal surface . We have first observed the fluorescence of Cu phthalocyanine under enhancement utilizing an STM-tip-induced plasmon (TIP) . For light emission from single molecules, Qiu et al.  reported light emission from individual Zn(II)-etioporphyrin I molecules adsorbed on Al2O3/NiAl(110), in which an oxide buffer layer is used to prevent fluorescence quenching and disturbance of pronounced plasmon emission [7–9]. They explained that the spectra were due to the de-excitation of excited anion states resulting from hot electron injection. The plasmon enhancement effect is also expected to be applied to the development of light-emitting diodes [2, 10]. Recently, we have developed a high-efficiency OLED including Au nanoparticles owing to the enhancement effect of localized surface plasmons on metal nanostructures .
Perylenetetracarboxylic diimide (PTCDI) and its derivatives are n-type semiconductors [11, 12], used in various optoelectronic devices such as thin-film transistors , photovoltaic , and light-emitting diodes . PTCDI molecules have been expected as a material of single-molecule devices  because of the high thermal and photostabilities of PTCDI. In this study, we have studied the STM-LE from N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C7) thin films on HOPG and Au substrates. We elucidated the intrinsic optical properties of PTCDI-C7 in terms of the STM-LE spectra on the HOPG substrate compared with the absorption and photoluminescence (PL) spectra, and demonstrated the wavelength control of enhanced molecular luminescence, i.e., the selective enhancement of the resonant wavelength of PTCDI-C7 through TIP enhancement effects on the Au substrate. We also discussed the emission mechanism of upconversion fluorescence.
PTCDI-C7 was synthesized by a modification of a previously reported method [17, 18]. A freshly cleaved HOPG and Au thin films evaporated on mica were used as the substrates. PTCDI-C7 thin films were prepared by spin-coating 0.4 mg/ml PTCDI-C7 solution in 1-tetradecene at a spin velocity of 1000 rpm, followed by rinsing with the solvent and drying in vacuum desiccators for 24 h. The film thickness was about 5-10 nm, which was determined by comparing the PL intensities of the PTCDI-C7 thin films fabricated by the spin coating method with those fabricated by evaporation in vacuum with thicknesses of 5, 10, 15, and 20 nm, which were estimated using a thickness monitor. STM (Digital Instruments Co. Ltd., USA, Nanoscope IIIa) measurement was carried out at room temperature under ambient conditions and a mechanically sharpened Pt/Ir tip was used. The collected photons were guided to a photomultiplier tube (Hamamatsu Photonics, Japan, R-649S) using an optical fiber to obtain a light intensity map (the dark count was less than 1 count per second (cps) at 253 K; the wavelength detection range was 300-850 nm). To acquire optical spectra, a grating spectrometer (Roper Scientific, USA, SpectraPro-300i) with a liquid-N2-cooled charge-coupled device camera (Roper Scientific, USA, Spec-10:100B/LN; the detection range was 200-1100 nm) was employed. The absorption and photoluminescence (PL) spectra of PTCDI-C7 were obtained using a UV-visible/NIR spectrophotometer (Hitachi High-Technologies Co., Japan, U-3010) and a custom-built system with an argon-ion laser (Edmond Optics, USA, Multi-Line 150 mW) at 514 nm, respectively.
Results and discussion
Our interest in both PL and STM-LE spectra was aroused by our observation of distinct vibronic progressions, similar to the case of the isolated molecular condition, even in the thin-film configuration of PTCDI-C7 where a moderate intermolecular interaction appeared on the fluorescence spectra in the form of a large (approximately 100 nm) Stokes shift. We assumed that PTCDI-C7 molecules had a poorly crystalline orientation/distribution in the thin film fabricated by the spin coating method due to the steric effect of long alkane substituents, which led them to have a quasi-isolated molecular condition in the thin film structure in terms of the perylene-ring-stretching vibration, although proper π-π stacking exhibiting a large Stokes shift and peak broadening in the spectra of the thin film structures remained. How to extend the fact that the electronic configurations of the PTCDI-C7 molecules are modified by the distribution in the thin film, such as induction, conjugation, and electrostatic, remains controversial. To evaluate such electronic effects, other experiments, such as photoemission spectroscopy and scanning tunneling spectroscopy, should be required.
The most surprising result in terms of the excitation mechanism in this study was that the sample bias voltage (the energy of tunneling electrons) of all the observed STM-LE emissions shown in Figure 6 did not satisfy the excitation energy of the S1(0-0) transition of 2.36 eV. Currently, it is difficult to precisely clarify the excitation mechanism. To realize the obtained phenomena, a total emission process must contain (i) an upconversion process, (ii) a novel excited state (S'1) energetically lower than the S1 state, and (iii) an initial S0 state of molecular excitation consisting of higher vibrational states of PTCDI-C7 (following the electronic excitation of S0(n) → S1(0)). (i) In the first scheme, multielectron/multistep excitation processes should be introduced; however, these multiexcitation processes must be excluded because of the low quantum efficiency of inelastic tunneling , which is also supported by the sample bias dependence of the STM-LE results (Figure 6) in which all of the emissions satisfied the cutoff condition (hν ≤ eVs). The triplet-triplet annihilation (TTA) mechanism enhanced by TIP (we observed the TTA fluorescence in Cu phthalocyanine thin films on the Au substrate ) could not be accepted since we observed sufficient intensity of the emission on the HOPG substrate and the free-base PTCDI has a low intersystem crossing probability from the singlet state to the triplet state. (ii) In the second scheme, the molecules are excited to the S'1 state derived from an intermolecular interaction due to molecular aggregation in the film. We observed a new peak (565 nm) below the S1 state in the absorption spectrum, which was also reported in previous works [22, 33]. Note that the energy difference between the S1 and S'1 states was estimated to be 0.34 eV, which is about twice the energy intervals of vibronic levels, suggesting that a reassignment of the vibronic transitions of the observed peaks is required. (iii) The third scheme of the emission mechanism should include, e.g., thermally assisted excitation to the S0(n) states and the direct excitation of vibrational levels by inelastic tunneling. Thermal excitation is easily excluded because the excitation of vibrational levels by heat requires a high temperature of >1800 K in the nanocavity of the STM system (k T = approximately 0.17 eV), which is refuted by the result of first-principles calculations  and the observed molecular stability. Recently, Dong et al.  have observed unexpected upconversion electroluminescence such as S1(0) → S0(n) for porphyrine molecules adsorbed on a Au(111) surface and proposed that the considerable population rate of electrons moving into higher vibrational states in S0 state is induced by plasmon-assisted multistep excitation via virtual electronic all excited states in analogy to surface-enhanced Raman scattering. In their case, TIP, excited by both tunneling electrons and plasmon-exciton coupling and acting as a near-field light source, was pumping molecules into higher vibrational excited states of S0. In this study, their proposed mechanism could be applied to the emission of the PTCDI-C7 thin film on the Au substrate. We observed a strong sample bias dependence of the peak intensity of the PTCDI-C7 thin films on the Au substrate, i.e., the emission peaks considerably decreased in intensity upon decreasing sample bias voltage in the TIP resonance energy region. However, the above mechanism was hardly accepted in the case of the HOPG substrate because of the lack of assistance from TIP in the observed energy range. This suggests that the plasmon-assisted direct vibrational excitation of the ground state S0 occurs in the case of the HOPG substrate, since the surface plasmon energy of the HOPG surface is approximately 60 meV  and the energy of TIP generated between the HOPG surface and the Pt/Ir tip covers the excitation energy of vibronic levels of approximately 0.17 eV. In either case, the overall excitation and radiation perspectives remain controversial and theoretical support for the STM-LE mechanism is highly required.
We have investigated the STM-LE from a PTCDI-C7 thin film on HOPG and Au substrates fabricated by spin coating. On the HOPG substrate, we obtained significantly high-emission intensity from the PTCDI-C7 thin films in spite of the lack of the TIP enhancement effect. In the comparison with those of the absorption and PL spectra, the peaks of the STM-LE spectra were attributed to vibronic progressions of the S1(0-0) transition. Using the Au substrate, the emission intensities of the higher index of vibronic peaks, whose energy matched the energy of TIP, were selectively enhanced compared with those in the case of the HOPG substrate. The emission mechanism of the upconversion STM-LE for the PTCDI-C7 thin films could be interpreted by the inelastic tunneling including the multiexcitation of the S0 states on both HOPG and Au substrates. Such a selective enhancement of molecular emission is quite useful for various applications of OLEDs, plasmonic devices, ultrasensitive sensors, and other devices, through the control of radiative transitions via an intense plasmon enhancement effect.
count per second
highly ordered pyrolytic graphite
organic light-emitting diodes
scanning-tunneling-microscope-induced light emission
This research was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas "Emergence in Chemistry" from the Ministry of Education, Culture, Sports, Science and Technology in Japan. The first author would like to express her gratitude to "The Center of Excellence Program for Atomically Controlled Fabrication Technology" for educational and financial support.
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