Note: Optical optimization for ultrasensitive photon mapping with submolecular resolution by scanning tunneling microscope induced luminescence

Abstract

We report the development of a custom scanning tunneling microscope equipped with photon collection and detection systems. The optical optimization includes the comprehensive design of aspherical lens for light collimation and condensing, the sophisticated piezo stages for in situ lens adjustment inside ultrahigh vacuum, and the fiber-free coupling of collected photons directly onto the ultrasensitive single-photon detectors. We also demonstrate submolecular photon mapping for the molecular islands of porphyrin on Ag(111) under small tunneling currents down to 10 pA and short exposure time down to 1.2 ms/pixel. A high quantum efficiency up to 10(-2) was also observed.

Self-Decoupled Porphyrin with a Tripodal Anchor for Molecular-Scale Electroluminescence

“…Otherwise, because plasmonic excitation is bias-polarity irrelevant, molecular fluorescence should be observed for both polarities. The exact reason for the inefficient excitation of molecules by plasmons in the present system is still unclear but may pertain to the very low efficiency of electron energy being inelastically converted to the NCP emission in the tunneling regime (less than 10 –3 photons per electron). ,− As a result, the excitation probability of molecules by plasmons is much smaller compared to the direct excitation via elastic electron injection. However, it should be pointed out that resonant NCP modes are believed to play an important role in the emission enhancement through improving the radiative decay rates. ,,, Figure illustrates such an excitation and emission process schematically: when both the HOMO and LUMO levels fall inside the electrochemical potential window defined by the bias voltage, the molecule is excited predominantly by hot electron injection; , the excited molecule then decays back to the ground state via Franck–Condon π*−π transitions with the radiative rate enhanced by the nanocavity plasmons resonant with the selected channels, producing enhanced molecular electroluminescence.…”

“…Photons emitted from the tunnel junction were collected with a lens inside the UHV chamber and then refocused into an optical fiber by another lens placed outside the UHV chamber. The collected light was guided into a grating spectrometer and analyzed by a cooled CCD detector (Princeton Instruments) for spectral information. ,, Photon intensities, measured by the SPAD (PerkinElmer, ∼72% detection efficiency of around 660 nm), were corrected for the detection efficiency of the SPAD and the collection efficiency of both the lens system (∼10%) and the beamsplitter (50%) . Nevertheless, luminescence spectra presented in the article were not corrected for the wavelength-dependent sensitivity of photon collection and detection systems.…”

Probing Adsorption Configurations of Small Molecules on Surfaces by Single‐Molecule Tip‐Enhanced Raman Spectroscopy

“…A continuous‐wave laser at 532 nm is used as a Raman excitation source. The photon collection and detection systems were described elsewhere ,…”

Subnanometer-resolved chemical imaging via multivariate analysis of tip-enhanced Raman maps

“…Our experiments were performed using a custom low-temperature ultrahigh-vacuum STM (Unisoku) at ~80 K under a base pressure of ~1 × 10 –10 Torr, equipped with a side-illumination confocal optical system 47 , as shown in Figure 1a and the Supplementary Information (see Supplementary Section S1 for more details). Two different porphyrin molecules, that is, metal-centered zinc-5,10,15,20-tetraphenyl-porphyrin (ZnTPP) and free-base meso-tetrakis (3,5-di-tertiarybutyl-phenyl)-porphyrin (H 2 TBPP) (upper-right corner of Figure 1a ), were used to build different molecular nanostructures on metal surfaces via thermal evaporation.…”