Plasmon-enhanced S2 electroluminescence from the high-lying excited state of a single porphyrin molecule

Abstract

We demonstrate the B-band electroluminescence from the high-lying S2 excited state of a single zinc porphyrin molecule with the scanning tunneling microscope-induced luminescence technique by using an aluminum tip. The nanocavity plasmon mode is found to be critical for the occurrence of S2 electroluminescence. When using a silver tip to excite the molecule electronically decoupled from the Ag(100) substrate by an ultrathin sodium chloride spacer, we only observe the Q-band electroluminescence originating from the radiative decay of the S1 first excited state, without any B-band emission due to the lack of effective plasmonic enhancement for the B-band. However, when the nanocavity plasmon resonance is tuned to a bluer range by using an aluminum tip, the S2 electroluminescence from a single zinc porphyrin shows up because the nanocavity plasmon mode can now spectrally overlap with the B-band emission to generate efficient plasmonic enhancement for the radiative decay directly from the S2 state. Interestingly, the excitation mechanisms for these two types of emission are found to be different. While the Q-band emission is attributed mainly to a carrier-injection mechanism, the B-band electroluminescence is found to be excited via an inelastic electron scattering process. Our results open a route to investigate the photophysical property and dynamic behavior of isolated molecules in their excited states.

Optical Images of Molecular Vibronic Couplings from Tip-Enhanced Fluorescence Excitation Spectroscopy

“… 37 As a result, the fluorescence emission will be dominated by emissions from the ground vibrational level of the first excited state, i.e., a feature known as Kasha’s rule, 38 as shown by the downward red-solid arrows in Figure 1 b. It should be noted that the breakdown of Kasha’s rule has been reported in several STM electroluminescence (STM-EL) measurements 10 , 14 , 15 , 39 − 41 because of the strong enhancement of the radiative channels in a plasmonic nanocavity. 42 − 45 We have thus checked the possible influences of the breakdown of Kasha’s rule on the TEFE images of the H 2 P molecule.…”

“…As a result, the fluorescence emission will be dominated by emissions from the ground vibrational level of the first excited state, i.e., a feature known as Kasha’s rule, as shown by the downward red-solid arrows in Figure b. It should be noted that the breakdown of Kasha’s rule has been reported in several STM electroluminescence (STM-EL) measurements ,,,− because of the strong enhancement of the radiative channels in a plasmonic nanocavity. − We have thus checked the possible influences of the breakdown of Kasha’s rule on the TEFE images of the H 2 P molecule. Our numerical results via rate equation-based simulations reveal that the main features of the vibronic couplings for the H 2 P molecule remain largely unchanged even when the non-Kasha’s emissions are artificially increased by six orders because the fluorescence of the molecule is dominated by HT terms (see section S3 in Supporting Information for details).…”

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

“…We next review studies of hot electroluminescence using STM. ,− Dong et al demonstrated hot electroluminescence, of which the de-excitation process is identical to ultrafast SEF, directly from the S 1 vibrational excited states of porphyrin molecules located inside a HS between an STM tip and metal substrate, along with the upconversion electroluminescence, by spectrally tuning the frequency of plasmons . Chen et al developed the model to evaluate the hot electroluminescence by combining the classical generalized Mie theory with the quantum master equation .…”

“Awakening” of the S2 Emission of Tricarbocyanine Dyes Stimulated by Interaction with Carbon Quantum Dots

“…The Purcell effect is due to the resonance of the nanocavity electric field and a quantum emitter (i.e., the dye molecule in our case), when the cavity size is much smaller compared with the emission wavelength. 28−30 Recently, an enhancement of the S 2 emission of a single zinc porphyrin molecule due to the plasmon resonance nanocavity was reported by Tian et al 31 A plasmonic nanocavity can even induce molecular emission from the vibronic (i.e., hot excited levels) because of strongly enhanced spontaneous emission rates which compete with IC rates. 32 In our case, the CQD can play the role of a nanocavity, whose electric field oscillation is in resonance with the S 2 emission field of the dye.…”