Electroluminescence properties of organic nanostructures studied by scanning tunnelling microscopy

Abstract

The control of light emission on the scale of individual quantum systems, like molecules or quantum dots, is a field of intense current research. One way to induce light emission from these systems is the local charge injection through the tip of a scanning tunnelling microscope (STM). Studies which employ this method have to address one basic question: Does the detected luminescence provide information precisely from the molecule into which charge is injected by the STM tip apex or are the luminescence properties determined by a larger volume?In this article, we focus on the investigation of organic nanocrystals and discuss the relation between the local excitation, the intermolecular coupling and the influence of the STM as a measuring instrument. Choosing pentacene as an organic emitter, we present results, which suggest that the STMinduced luminescence cannot be attributed simply to the emission by a single molecule. We discuss how information about locality can be obtained and comment on the present experimental limitations and possible future improvements.

Atomic-Scale Imaging and Spectroscopy of Electroluminescence at Molecular Interfaces

“…The main line has been ascribed to a self-trapped exciton. Interestingly, the free singlet exciton with an energy of 1.78 eV is also observed with very low intensity . The free exciton has been interpreted in terms of an excimer state in the literature and has a short lifetime, which may explain the weak intensity in the spectrum.…”

“…This cross-talk can be avoided by spectral filtering in front of the detectors 97 or by the use of an HBT geometry where the optical paths are long enough to time-separate the flash from the studied correlation. 98 In a start−stop measurement where the start pulse is given by an external trigger, the timer will be stopped by one of the time-resolving detectors. This setup is unaffected by the discussed artifacts and has been employed for the voltage pulse measurements described in section 7.2.…”

“…Interestingly, the free singlet exciton with an energy of 1.78 eV is also observed with very low intensity. 98 The free exciton has been interpreted in terms of an excimer state in the literature 250 and has a short lifetime, which may explain the weak intensity in the spectrum. Free pentacene excitons can stabilize in the self-trapped state (gaining an energy of ΔE = 0.18 eV) or undergo fission into two triplet excitons in a process liberating ΔE = 0.25 eV.…”

“…The emitted light travels back through the optical system and can lead to detection of spurious photons in the respective other detector. This cross-talk can be avoided by spectral filtering in front of the detectors or by the use of an HBT geometry where the optical paths are long enough to time-separate the flash from the studied correlation . In a start–stop measurement where the start pulse is given by an external trigger, the timer will be stopped by one of the time-resolving detectors.…”

Quantitative mapping of fast voltage pulses in tunnel junctions by plasmonic luminescence

“…The achievable experimental time resolution is limited by the time jitter of the photon detector and estimated to be less than 1.1 ns for the APD used here. 22 The only requirement to use the photon intensity as a voltage probe is its strictly monotonic dependence on the applied voltage, since the photon emission is instantaneous on our measurement time scale (see above). Once known, this dependence can be used to convert the photon intensity back to an instantaneous transient voltage in the junction.…”