Study of the Optical UV-Visible Electron Induced Luminescence of Small Size Selected Metal Clusters in Rare Gas Solids

The optical properties of small size selected metal clusters have been studied by fluorescence spectroscopy. Cun (n = 1-5, 8), Agn (n = 1-5, 8, 9, 20), Aun (n = 1-5, 8) and Nin (n = 1-3) clusters embedded in condensed neon are photoionized by VUV photons in the solid, and the fluorescence originating from the reneutralization of the transient cluster ions is recorded. The VUV photons result from the radiative decay of excitons which are generated by the impact of electrons of variable energy on the solid. A new experimental setup has been developed, which permits the excitation of the rare gas solid by electrons with an energy between 0 and 200 eV. A field emission electron source based on carbon nanotubes provides an intense electron beam without any light generation. A custom built cryopumped gas purification system allows the production of rare gas solids with a purity in the order of up to 10-5. The characteristics and a physical descriptive model of electron induced luminescence is presented. The measurement of electron currents through the solid are thoroughly analyzed. Based on localized exciton production and radiative deexcitation of the host rare gas atoms near the surface of the solid, the excitation can be spread over distances as large as 20-30 µm by high energy photons which excite the species contained in the solid. Quenching and amplification effects on the luminescence intensities are observed depending on the material and the size of the dopants contained in the rare gas solids. These effects are believed to originate from the local enhanced electric field induced by the metal clusters which act as antennas. The analysis of the luminescence spectra reveals rich spectroscopic signatures from the UV to the near IR. A certain amount of fluorescent transitions could be identified from conventional excitation fluorescence spectroscopy. This proves that this new method is sensitive and reliable. The so far reported new transitions await interpretation on the basis of sophisticated quantum calculations.

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

Ultraviolet-visible absorption of small silver clusters in neon: Ag-n (n=1-9)

Wolfgang Harbich, Sylvain Lecoultre, Alexandre Rydlo

We present optical absorption and fluorescence spectra in the UV-visible range of size selected neutral Ag-n clusters (n = 1-9) in solid neon. Rich and detailed optical spectra are found with linewidths as small as 50 meV. These spectra are compared to time dependent density functional theory implemented in the TURBOMOLE package. Excellent agreement between theory and experiment is achieved in particular for the dominant spectroscopic features at photon energies below 4.5 eV. This allows a clear attribution of the observed electronic transitions to specific isomers. Optical transitions associated to the s-electrons are concentrated in the energy range between 3 and 4 eV and well separated from transitions of the d-electrons. This is in contrast to the other coinage metals (Au and Cu) which show a strong coupling of the d-electrons. (C) 2011 American Institute of Physics. [doi:10.1063/1.3589357]

American Institute of Physics2011

Optical properties of small metal clusters

Chongqi Yu

This thesis is devoted to the study of the optical properties of small metal clusters and metal cluster organic compounds. For this purpose a ultra-high vacuum (UHV) experimental apparatus has been built which allows for the deposition of mass selected neutral clusters in a size range up to 16,000 amu which corresponds to Ag150 or Au80. Cluster cations are produced in a gas aggregation cluster source, steered in an ion optics device and co-deposited with electrons and excess rare gas on a cryogenic deposition head. Neutral mono dispersed clusters are analyzed by optical absorption, fluorescence and excitation spectroscopy in the UV-visible energy range. A novel custom-made conical octupole combined to an organic molecule injector has been built, serving as a generator of metal cluster-organic compounds. These are atomically precise model systems which permit to study surface-enhanced Raman scattering (SERS) in order to test the theoretical models and better understand the mechanism of SERS. Optical absorption on size-selected silver clusters of Agn (n= 20 â 120) embedded in neon solid matrix was investigated. The spectra show plasmon-like absorption profiles around a photon energy of 4.0 eV which match in energy molecular like absorption spectra for the smallest Ag clusters but are sensitively higher than what would be expected for the dipolar absorption for bulk silver of about 3.3 eV. The central absorption peak, i.e. the plasmon energy as a function of cluster size is presented. The data fall in a transition between a blueshift of the plasmon resonance with decreasing size to a superimposed red shift for the very smallest sizes. Superimposed, we observe a distinct structure where clusters with atom number of 8,18,34,58,92 show a localized maximum value of the plasmon energy. These numbers correspond perfectly to the fully filled states of 1p, 1d, 1f, 1g and 1h according to the electronic shell model, manifesting a clear sub-shell effect. Optical absorption, fluorescence and excitation spectroscopy on size-selected small gold clusters of Aum (m= 19,20,21) embedded in solid neon were measured. These are the first optical data on ligand free neutral atomically precise Au clusters. Au20, the famous tetraedic structure, shows a rich structured absorption spectrum, superimposed on a continuous background increasing to the UV part of the spectrum. Au21 and Au19 show the same tendency, however without any superimposed structure. Very interestingly, all three Au clusters show strong fluorescence centered at around 1.85 eV, here again Au20 being the prominent species. Comparison to ligand protected Au clusters clearly proves that the fluorescence comes from the Au cluster itself and is only weakly influenced by the ligands. For the first time, size-selected small metal-organic compounds of silver-pyridine AgaPyb (a= 1,3; b= 1,2) which are prototypes for surface enhanced Raman scattering (SERS) have been produced and isolated in a solid Ne matrix. Optical absorption and fluorescence spectra have been measured. Raman scattering cross sections with an enhancement factor of 103 have been found in fair agreement with recent high level time-dependent density functional theory (TDDFT) calculations.