X.-L. Guo

Tunneling-electron-induced molecular luminescence from a nanoscale layer of organic molecules on metal substrates

Molecular luminescence from an ultrathin layer of free-base porphyrin molecules has been generated by a scanning tunneling microscope on top of a monolayer spacer of perinone derivatives on Cu(100). Tunneling-electron-induced fluorescence spectra are in good agreement with the conventional photoluminescence data of the molecule. The dominant molecular luminescence peak becomes clear and sharp for bias voltages above ∼2.1 V. The perinone monolayer does not emit light because of quenching effects; it acts as a buffer layer to enhance the decoupling of the electronic state of the porphyrin molecules from the Cu substrate. The molecular luminescence from porphyrin is attributed to the hot electron injection excitation. These results demonstrate the feasibility of electrically driven molecular luminescence on metal substrates by a nanoscale probe.

Tunneling electron induced photon emission from monolayered H2TBP porphyrin molecules on Cu(1 0 0)

Positioning of a clean scanning tunneling microscope tip above a monolayer of free-base porphyrin (H 2 TBPP) molecules on Cu(100) is found to induce merely plasmon-mediated emission with molecular fluorescence completely quenched. The molecule acts as a spacer to increase the tip-metal substrate distance to make spectra blue-shifted. Additional broad emissions at low energies may be associated with the molecules either adsorbed onto the tip apex or on the second monolayer and might suggest the involvement of modified molecular fluorescence. In both cases, the energy transfer from molecular excited states to the metal substrate is overwhelmingly dominant, leading to enhanced plasmon-mediated emission.

Molecular fluorescence from ZnTBP porphyrin molecular layers on Cu(100) induced by tunnelling currents

Molecular fluorescence from the surface of ZnTBP porphyrin (ZnTBPP) molecular layers on Cu(100) is induced with nanoprobe excitation in the tunnelling regime. The observed well-defined molecular fluorescence is a perfect match with the standard photoluminescence data of ZnTBPP molecules. The decoupling of the electronic state of the top layer ZnTBPP is controlled by the thickness of the molecular layers. The excitation mechanism of molecular luminescence may be attributed to the hot electron injection into the molecules in proximity to the tip apex of a scanning tunnelling microscope. This approach features simplicity, bipolar operation and good reproducibility. The research provides a new way for the integration of molecular fluorescence with a nanoprobe and the development of a nanoscale molecular light source.

Nanoscale electroluminescence from n-type GaAs(110) in tunnel junctions

Nanoscale electroluminescence (EL) was induced from n-type GaAs(110) in tunnel junctions using an indium tin oxide (ITO)-coated optical fibre probe at both polarities, room temperature (RT), and 80xa0K. The quantum efficiency of photon emission at negative bias is much higher than at positive bias at both RT and 80xa0K. A high quantum efficiency of about ~10−4(photons/electron) was achieved at 80xa0K. The well-defined optical spectra exhibit two-peak features at 1.49 and 1.39xa0eV which are generated by the radiative recombination of hole–electron pairs over the direct band gap and surface states, respectively.

Molecular-Scale Organic Electroluminescence From Tunnel Junctions

We report the generation and detection of bipolar organic electroluminescence of porphyrin molecules from a nanoscale junction in an ultrahigh vacuum scanning tunneling microscope (STM). Clear molecular fluorescence from porphyrin molecules near metal substrates has been realized through highly localized electrical excitation of molecules in proximity to a sharp tip apex. The molecular origin of the luminescence, arising from the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) radiative transitions of neutral molecules, is clearly established by the observed well-defined vibrationally resolved fluorescence spectra that match perfectly with conventional photoluminescence data from molecular thin films. The molecules fluoresce at low onset voltages for both bias polarities, presenting an example of bipolar organic electroluminescence at the nanoscale. Such bipolar operation suggests a double-barrier model for electron transport, with hot electron injection into unoccupied states of molecules in both polarities. The optical behavior of molecules in the tunnel junction is also found sensitive to the electronic properties of molecules and energy level alignment at the interface. These results offer new information to the optoelectronic behavior of molecules in a nanoscopic environment and may open up new routes to the development of single-molecule science and molecular scale electronics.