Luminescence experiments on supported molecules with the scanning tunneling microscope

The present review on light emission stimulated by electrons tunneling inelastically through a junction formed by a sample and a tip of a scanning tunneling microscope (STM) focusses on the most relevant results obtained for a variety of systems, including metal surfaces, single molecules on ultrathin dielectric films, and molecules assembled in thin films or in nanostructures on metal surfaces or on dielectric films. The decisive role of the localized surface plasmon for the enhanced luminescence from supported molecules is highlighted. The progress so far achieved for both experimental techniques and theoretical analysis is addressed. Current trends are discussed and possible future developments are indicated. STM-induced photon emission has come a long way. More than a decade ago, simple photon maps reflected only the emitted light intensity on a local scale. Today this technique has advanced to spectroscopically resolve optical emission from electronic and vibrational molecular modes in single molecules, demonstrating the capability of STM-induced light emission for chemical recognition on the single-molecule scale. © 2010 Elsevier B.V. All rights reserved.

Scanning Tunneling Luminescence of Pentacene Nanocrystals

Alexander Kabakchiev

Organic semiconductors are promising materials for future electronic and electroluminescence applications. A detailed understanding of organic layers and nano-sized crystals down to single molecules can address fundamental questions of contacting organic semiconductors at the nanometer limit and obtaining luminescence from them. In this thesis, electroluminescence spectra from pentacene, a policyclic hydrocarbon (acene), are discussed. The luminescence is induced by the current from the tip of a scanning tunneling microscope (STM). Pentacene is an organic semiconductor which gained a lot of attention in technology because of its electronic properties suitable for applications in thin film devices. Moreover, recent fundamental studies employed pentacene as a standard to demonstrate sub-molecular resolution by scanning probe techniques. This work reports the first observation of light emission from nanometer-sized pentacene crystals grown on an ultrathin insulating layer on noble metal surfaces. Different STM-techniques are combined to characterize the individual systems studied with respect to topography, crystal structure, electronic properties and work function changes. The initial step of the project was the implementation of an optical system for light collection from the tunnel-junction of a low-temperature STM. In the set-up, a novel approach based on the use of in situ adjustable lenses has been realized. Three lenses placed in the vicinity of the tip-apex allow light collection into three independent channels which can be used for versatile optical analysis. An important part in the characterization of the organic system was to clarify the interaction between adsorbed molecules and substrates. We investigated individual pentacene molecules on different ultrathin insulator-metal systems. With the STM tip in tunnel contact, the molecules are situated in a double barrier junction formed by the insulating layer on one side and the vacuum gap on the other side. The metal surface and the STM-tip form the electrodes. The electronic properties of pentacene in this configuration have been characterized. Insulator-metal-systems, which provide a good electronic decoupling for pentacene from the metal, have been chosen as substrates for the growth of pentacene nanocrystals. Using the sub-molecular resolution of the STM we resolved the structure of the top-layer of the pentacene nanocrystals and found that the crystal phase agrees with the pentacene bulk structure. Moreover, the comparison of charge injection barriers between individual molecules and nanocrystals of pentacene indicates a significant change of electronic properties after the formation of ordered structure from the individual building blocks. Optical spectroscopy of the emitted light reveals an excitonic emission from the nanocrystals, which is in very good agreement with photoemission spectra of macroscopic crystals. Although a highly localized current-injection by the STM tip is used for excitation, it can be concluded that the source of light emission is delocalized. In contrast to luminescence measurements reported for other organic materials in STM, the excitation is not localized on the individual molecule, into which a charge is injected by the STM-tip. Our study indicates the importance of inter-molecular coupling leading to a high mobility of the excited state which has to be considered in the framework of STM-induced luminescence. A further result is the observation of the Stark shift of pentacene luminescence, that has so far not been measured at large electric fields of the order of 1V/nm. The evaluation of the Stark shift provides information on dipole and polarizability change during the decay of an exciton. The measured large dipole change of the lowest singlet exciton in pentacene indicates a mixing of the Frenkel exciton (FE) with a small contribution of charge-transfer (CT). Finally, we developed a set-up for photon correlation measurements in the STM. The properties of photon statistics is a means which can unambiguously prove that the luminescence is due to a single-photon source, i.e. a light source which emits no more than one photon at a time. A typical example of such a source is the emission from a single molecule. We performed photon correlations measurements of the luminescence from pentacene and C60 nanocrystals. While the excitonic bulk emission of pentacene is not expected to represent a single photon emitter, we also did not observe anticorrelations in the case of C60 indicating the importance of the local environment in the STM-junction, e.g. the short distance to the metal electrodes.

EPFL2010

Single molecule fluorescence and phosphorescence studies using a scanning tunneling microscope

Abhishek Grewal

This thesis investigates novel single-molecule luminescence phenomena at their inherent, sub-molecular length scale. The microscopic understanding of luminescence processes will be crucial for the continued improvement of organic optoelectronic and semiconductor devices.In this work, a scanning tunneling microscope (STM) is combined with optical spectroscopy and Hanbury Brown and Twiss interferometry to image topography, electronically characterize, and analyze the vibrational mode excitations and emission statistics, all at the atomic scale. Moreover, the precise STM tip positioning allows tuning the coupling between the molecular exciton and the tip-induced plasmon with sub-molecular spatial control, which is key to the decay rate enhancement of molecular phosphors.A prototypical platinum phthalocyanine (PtPc) molecule, decoupled from a metal substrate by a thin film of NaCl, provides a model platform for examining STM-induced bipolar fluorescence, phosphorescence, and energy upconversion electroluminescence (UCEL). First, an STM investigation of only the ionic insulator NaCl demonstrates that both valence and conduction bands are of anionic Cl- character. This is relevant for the coupling between the molecule and the metal substrate. Theoretical considerations reveal a dominant contribution from low-lying molecular orbitals under certain conditions.Voltage and polarity of fluorescence onsets can be controlled using different metal substrates because their different work functions tune the energy alignment of the molecular orbitals with respect to the substrate Fermi level. For PtPc atop an Au(111) substrate, fluorescence is observed at both voltage polarities, even under tunneling conditions where UCEL occurs. This remarkable behavior allows the testing of different emission mechanisms and reveals new general guidelines for designing future optoelectronic devices. An analysis of photon emission statistics from a single molecule under UCEL conditions refutes previously proposed excitation mechanisms. It guides theory towards a novel mechanism, briefly introduced in the thesis.PtPc atop NaCl on Ag(111) exhibits fluorescence, and in this thesis, its phosphorescence is also observed by local excitation in the STM. Electronic characterization reveals a simultaneous voltage onset of both types of emission. Phosphorescence is also detected using tip-enhanced resonant photoexcitation of the singlet state. Since direct optical excitation of the triplet state is forbidden, this observation is the first demonstration of intersystem crossing in a tip-controlled single-molecule experiment. Exciton-plasmon coupling can be varied by fine positioning the tip in proximity to the molecule. This allows tuning the triplet emission intensity through control of the singlet lifetime. These findings are directly transferrable to highly topical phosphorescent OLEDs that employ plasmonic enhancement.The phenomenon of plasmonic antibunching is discovered for electron tunneling from a few monolayer thick C60 films to the STM tip. The layer thickness and the applied voltage control the sequential tunneling of charges by a Coulomb blockade mechanism. This mechanism eventually causes the emission of one photon at a time. Such a plasmonic source allows for a higher single-photon emission rate than an individual molecule. The design principles for other semiconducting films are discussed, suggesting a quest for similar nanostructured single-photon emitters.

EPFL2022

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

Frédéric Rossel

This thesis reports on the optical properties of fullerene molecules forming nanoscale crystals grown on ultrathin NaCl films on Au(111) investigated by Scanning Tunneling Microscope (STM)-induced light emission spectroscopy. In this regard, an existing optical setup has been improved. The new instrument facilitates the alignment of the detection lens with the tunnel junction. STM images of the investigated samples show that, upon adsorption on a NaCl covered Au(111) surface, C60 and C70 crystallize into multilayer islands with a hexagonally arranged close-packed top layer in which electron-poor regions on one molecule face electron-rich regions on the adjacent molecule. Luminescence spectra from these nanocrystals induced by electrons tunneling through a double barrier STM junction are presented. Their bias dependence as well as their similarity with laser-induced photoluminescence spectra indicate a hot electron injection mechanism followed by a radiative decay associated with the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of the molecules. Local fluorescence and phosphorescence of C70 molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover, the molecular luminescence is demonstrated to be selectively enhanced by localized surface plasmons in the STM tip-sample gap, the tunnel junction acting as an optical antenna. Finally, the results of a scanning tunneling microscopy and scanning tunneling spectroscopy investigations of self-assembled islands of Azure A molecules on a Au(111) substrate are presented. Although no final conclusion can be drawn on the molecular arrangement owing to the impossibility to recognize specific parts of the molecule, the presence of these structures is found to modify the Au(111) herringbone reconstruction.