Session V10: Focus Session: Surfaces and Interfaces in Electronic Materials III

Formation Processes and (Photo-)electrochemical Properties of Organic Monolayers on Hydrogen Terminated Si(111).

Construction of molecular layers on solid surfaces is one of the most important subjects not only for fundamental science but also for a wide range of applications. The most studied mono-molecular layer system is self-assembled monolayers (SAMs) of alkanethiols on various metals especially on gold and SAMs with a wide variety of functionalities have been constructed. It may be more important, however, to construct ordered molecular layers with various functionalities on a semiconductor surface, Si in particular, as far as technological applications are concerned because of possible applications for molecular and biomolecular devices in conjunction with the advanced silicon technology. Here we followed the thermal and photochemical organic monolayer formation process of by ATR-IR, ellipsometry, and sum frequency generation spectroscopy, and constructed and investigated electrochemical and photoelectrochemical properties of the organic monolayers with electron transfer function, i.e., viologen moiety, on hydrogen-terminated Si(111) surface. Furthermore, effect of platinum nanoparticles deposited on the surface on hydrogen evolution reaction was also investigated (1). Photo-switching property was also introduced by incorporating a diarylethene moiety. (1) Masuda, T.; Uosaki, K. \textit{Chem. Lett.} \textbf{2004, }$33, $788.   

New SiOH complexes and proton release mechanism in silica as a source of Si/SiO$_{2}$ interface-trap build up

Water molecules in SiO$_{2}$ have been studied extensively in the context of reliability of electronic devices. Here we report results of new density-functional first-principles calculations and experimental data that demonstrate a key role of H$_{2}$O molecules in the long-term degradation of MOSFETs by increases in interface-trap densities. A new low energy complex formed by H$_{2}$O has been identified. The complex consists of two SiOH groups located on neighboring rings. The energy of the complex is 0.3 eV lower than that for the free interstitial water molecule in the ring of $\sim $ 0.7 nm. The two silanol groups have different local topology, which results in different ability of the SiOH elements to capture holes and release protons under X-irradiation. The release of H$^{+}$ has a barrier of the reaction $\sim $ 0.45 eV and is accompanied by creation of a peroxy-type structure inside network ring. The released protons can diffuse or drift (driven by electric fields) to the interface, where they depassivate dangling bonds by forming H$_{2}$ molecules. We will present experimental data on the radiation response of devices that have been in storage for 20 years. The results are consistent with the theoretical picture, when water molecules are responsible for a substantial increase of interface-trap densities over time.   

Scanning tunnelling spectroscopy of single molecule on a semiconductor surface

Scanning tunnelling spectroscopy was performed on 1,3-cyclohexadiene molecules on Si(100) surface at 5K. Degenerated N-type semiconductor and platinum covered tungsten tips were used. For the first time a vibrational spectrum of chemisorbed molecule on semiconductor surface was obtained. The probe induced perturbations of the molecule electronic density of states and its vibrational properties were also investigated. Transition from tunnelling to contact regime between the probe and the molecule was successfully monitored.   

X-ray Photoelectron Spectroscopy of Buried Electronic Layers and Interfaces

Miniaturization of integrated circuits requires ever more detailed nanoscale physical and chemical characterization to engineer successful devices, as critical device layers are now only nanometers thick and frequently buried within complex material stacks. Nonetheless, correlating electronic device behavior with internal chemical structure remains essential for producing reliable devices. We present a new method for accessing the internal chemical structure of critical nanoscale layers in electronic device stacks via x-ray photoelectron spectroscopy (XPS). The method is based on engineering a weakened interface between two critical layers, then cleaving the stacks at this interface in a UHV environment and using XPS to characterize the layers and interfaces adjacent the cleave-plane. We present data from Pt/Pt-oxide/organic-monolayer/metal device stacks which show useful electrical switching behavior. This method reveals unexpected changes to the metastable Pt-oxide occur during stack fabrication. These changes to the buried nanoscale Pt-oxide layer are also shown to be inaccessible with conventional ion-milling or sputtering techniques that destroy the evidence of these subtle changes.   

Quantum Size Effects in Nanostructures

Quantum size effects in metal thin films and metallic clusters are studied using first-principles density functional theory. For metal thin films, Pb(111), Pb(100), Al(110), and Al(111) films up to 30 monolayers are calculated. Significant oscillatory quantum size effects are found on surface energy, work function, and surface relaxations. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modelled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. It is found that a quantitative description of these quantum size effects requires full consideration of the crystal band structure. For metallic clusters, the highly symmetric particles of sizes up 4 nm (Al$_{923}$, Pb$_{923}$, and Au$_{309})$ in the icosahedral (ico), decahedral, and cubotohedral (fcc) structures are calculated. We propose a simple scheme to compare their relative stability and to identify the quantum size effect. In addition, the famous Mackay (fcc-to-ico) transition for metallic clusters is investigated by \textit{ab-initio} elastic-band method. The transition path can in general be described by an angular variable \textbf{\textit{s}}. The barriers of the Mackay transition for large Al, Pb, and Au clusters are found to be smaller than the thermal energy at room temperature. Finally CO oxidation on metallic clusters will be presented. A catalytic reaction path for CO oxidation on Au$_{55}$, Ag$_{55}$, and Au$_{25}$Ag$_{30}$ ico clusters is found with activation energies of less than 0.5 eV. The reaction consists of a peroxolike transition intermediate involving the OOCO configuration. A crucial factor to determine the reaction rate on these clusters is identified as the co-adsorption energy of CO and O$_{2}$ on these clusters.   

Direct visualization of metal ions in supramolecules.

We performed high resolution scanning-tunneling microscopy with simultaneous current-voltage characteristics (STS) measurements on single molecules deposited on graphite surfaces. We present our recent results on Co [2$\times $2], Mn [3$\times $3] grid-type molecules, Cu$_{20}$ wheel-shaped polyoxoanions, as well as on Cu coordination polymers. In our STS measurements we found a rather large signal at the positions of the metal centers in the molecules i.e. the location of the individual metal ions in their organic matrix is directly addressable by STS even if these ions are covered by the organic ligands.   

Surface-enhanced Raman scattering of wurtzite-type GaN(0001) and ZnO(0001):

We first-time report surface-enhanced Raman scattering (SERS) of Ag-deposited wurtzite-type GaN(0001) epitaxial film and ZnO(0001) substrate. On non-deposited region, two Raman-active modes, A$_{1}$(LO) and E$_{2}$(high), were observed in backscattering geometry, which is consistent with the Raman selection rule on wurtzite structure. In contrast, on the Ag-deposited region of both samples, only A$_{1}$(LO) mode (734 cm$^{-1}$ for GaN and 572 cm$^{-1}$ for ZnO) exhibits clear Raman enhancement. We propose that the macroscopic polarization field accompanied by LO-phonons is responsible for this anomalous Raman enhancement. The study of SERS effect on ionic crystals thus provides a simple test to investigate the mechanism beside electromagnetic effect in enhanced Raman scattering.   

Organic Field Effect Transistor Interfaces Probed In-situ during operation by Sum Frequency Generation

In order to gain a molecular level understanding of the electrical conduction in thin film organic semiconductors, we have integrated an electrical probe station with an SFG spectroscopy system. Thin film transistors consisting of organic semiconductors 5,5'-bis(4-hexylphenyl)-2,2'-bithiophene (6PTTP6) and pentacene, were fabricated on silicon substrates with top contacts. SFG spectra show strong correlations with the electrical characteristics measured. In particular, the SFG spectra of the interfaces changed with increasing gate voltage (Vg). For both 6pttp6 and pentacene the non-resonant SFG background correlated with the increase in conductivity (slope of the IV curve after turn on) measured of the semiconductor layer with increasing Vg. For 6pttp6 we observed that the dependency of the methylene peak intensity on gate voltage correlated well with the dependence of the saturation current on gate voltage. These results point to the fact that charging of molecules and the field effects in OFETs can be probed in-situ using SFG and electrical testing, and we can gain a molecular level understanding of OFET interfaces from the results.