Scanning Tunneling Luminescence of Pentacene Nanocrystals

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.

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

Christoph Grosse

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of

EPFL2015

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

Dynamics of nanoscale systems probed by time-resolved STM-induced luminescence

Anna Maria Roslawska

The efficiency and peculiarities of processes such as surface adsorption or electron-to-photon energy conversion in organic and inorganic structures are determined by the dynamics at the scale of individual molecules, atoms and charges. The timescales of such effects reach down to the ps regime, which can be routinely probed using ultrafast optics methods. However, these far-field techniques suffer from ensemble-averaging that obscures the information encoded at the atomic level. These limits can be overcome by combining scanning tunnelling microscopy (STM) with time-resolved light detection so that pm and ps scales can be reached simultaneously. STM-induced luminescence (STML) methodologies are employed in this thesis to explore the nanoscale dynamics of different systems: light emitting defects in C60 thin films, H2 molecules weakly physisorbed on Au(111), metal-metal tunnel junctions and single atom contacts. First, we study the dynamics associated with the radiative decay of single excitons at defect-related emission centres localized in the C60 molecular film. Such emission centres serve as the smallest possible model of an organic light emitting diode where a single electron and hole recombine producing a photon. Using time-resolved STML (TR-STML) excited by voltage pulses, we follow the formation of an individual exciton and map this process in real space with ns resolution. Besides the excitonic emission, on C60 thin films we observe luminescence due to the radiative decay of plasmons. We study the interplay of these two processes and create a nanoscale bimodal source whose emission can be dynamically tuned by controlling the charge injection rate. A kinetic model is employed to thoroughly describe the charge and exciton dynamics in this system. The developed kinetic model can be also used to analyse the motion of hydrogen molecules adsorbed on an Au(111) crystal. The weak interaction with the metal and the continuous charge injection by the tip leads to recurrent excursions of the molecule inside and outside from the tunnelling junction, which chop the plasmonic luminescence intensity. These dynamical changes in the light intensity can be followed by correlation spectroscopy and allow extraction of the molecule's residence and excursion times. In the next step, we probe the dynamics of a generic metal-metal tunnel junction. Interestingly, when a high bias is applied, the emission from the junction exhibits photon bunching on a time scale below 50 ps as evidenced by Hanbury Brown-Twiss interferometry. Such a process is a result of a distribution of the energy of one electron between two photons, which may possibly be entangled. The opposite process may occur as well, under conditions where two electrons interact supplying energy to one photon. This overbias emission is a feature of single atom contacts and is used here to probe the fluctuations of plasmonic enhancement driven by the motion of single atoms in the vicinity of such a contact.

EPFL2019