Session V36: General Chemical Physics

Measurement of kinetic and potential energy deposition in highly charged ion collisions with surfaces

We measure craters in thin dielectric films formed by highly charged Xe$^{Q+}$ (26 $\leq Q \leq$ 44) projectiles [1]. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater $\sigma_{c}(Q)$ increased by 4 orders of magnitude ($7.9 \times 10^{-4}\mbox{ }\mu$S to \\6.1 $\mu$S) as $Q$ increased, corresponding to crater depths ranging from 2 to 11 \mbox{\AA}. By employing a heated spike model, we determine that the energy required to produce the craters spans from 8 to 25 keV over the investigated charge states where kinetic energies were $(8 \times Q)$ keV. We partition crater formation energy into potential and kinetic contributions to find that at least (27 $\pm$ 2) $\%$ of the available ion potential energy is required. Decreasing projectile kinetic energy at constant $Q$, provides a new test for charge-dependent kinetic energy loss theory.\\[4pt] [1] R.E. Lake, J.M. Pomeroy, H. Grube, C.E. Sosolik, Phys. Rev. Lett. \textbf{107}, 063202 (2011)   

Charge transport in single photochromic molecular junctions

Recently, photoswitchable molecules, i.e. diarylethene, gained significant interest due to their applicability in data storage media, as optical switches, and in novel logic circuits [1]. Diarylethene-derivative molecules are the most promising candidates to design electronic functional elements, because of their excellent thermal stability, high fatigue resistance, and negligible change upon switching [1]. Here, we present the preferential conductance of specifically designed sulfur-free diarylethene molecules [2] bridging the mechanically controlled break-junctions at low temperatures [3]. The molecular energy levels and electrode couplings are obtained by evaluating the current-voltage characteristics using the single-level model [4]. The charge transport mechanism of different types of diarylethene molecules is investigated, and the results are discussed within the framework of novel theoretical predictions. \\[4pt] [1] M. Del Valle etal., \textit{Nat Nanotechnol} \textbf{2}, 176 (2007) S. J. van der Molen etal., \textit{Nano. Lett.} \textbf{9}, 76 (2009).\\[0pt] [2] D. Sysoiev etal., \textit{Chem. Eur. J.} \textbf{17}, 6663 (2011).\\[0pt] [3] Y. Kim etal., \textit{Phys. Rev. Lett. }\textbf{106}, 196804 (2011).\\[0pt] [4] Y. Kim etal., \textit{Nano Lett.} \textbf{11}, 3734 (2011). L. Zotti etal., \textit{Small} \textbf{6}, 1529 (2010).   

Length and temperature dependent crossover of charge transport across molecular junctions

We study the electronic transport in a molecular junction, in which each unit is coupled to a local phonon bath, using the non-equilibrium Green's function method. We observe the conductance oscillates with the molecular chain length and the oscillation period in odd-numbered chains depends strongly on the applied bias. This oscillatory behavior is smeared out at the bias voltage near the phonon energy. For the phonon-free case, we find a crossover from tunneling to thermally activated transport as the length of the molecule increases. In the presence of electron-phonon interaction, the transport is thermally driven and a crossover from the thermally suppressed to assisted conduction is observed.   

Electrical properties of a layered manganese vanadate

We present the electrical characteristics of a layered manganese vanadate. Octahedral Mn and tetrahedral V units form layers that in turn are connected to each other via weakly bonded strontium ions. The Mn sites are also connected to each other through bridging oxygen atoms that are partially protonated, allowing for possible proton conduction in the material. The conductivity is dependent on crystal direction. Variable temperature conductivity measurements, from 160 to 830 K, show semiconducting behavior with average activation energy of 0.35 eV. Around 670 K a dip in the conductivity is observed, correlated with loss of water from the structure inferred from thermogravimetric analysis. Above 760K, an increase in conductivity is observed. Single crystal x-ray analysis is performed on samples heated above 670 K, to probe temperature induced structural changes. Preliminary results show a contraction of one of the unit cell axes corresponding to the loss of the bridging oxygen and the ensuing movement of the two Mn sites closer to each other. Single crystal x-ray investigations of the material under hydrostatic pressure in a DAC are also performed, to probe the influence of structural changes on electronic transport properties.   

Capturing Ion-Solid Interactions with MOS structures

We have fabricated metal-oxide-semiconductor (MOS) devices for a study of implantation rates and damage resulting from low energy ion-solid impacts. Specifically, we seek to capture ion irradiation effects on oxides by exposing as-grown SiO$_{2}$ layers (50 nm to 200 nm) to incident beams of alkali ions with energies in the range of 100 eV to 10 keV. The oxide is analyzed post exposure by encapsulating the irradiated region under a top metallic contact or within a finished MOS device. Characterization of the resulting ion-modified MOS device involves the standard techniques of room temperature and bias-dependent capacitance-voltage (C-V) measurements. The C-V results reveal alkali ion-induced changes in the flatband voltage of irradiated devices which can be used to extract both the range and implantation probabilities of the ions. Biased C-V measurements are utilized to confirm the concentration or dosage of ions in the oxide. A triangular voltage sweep (TVS) measurement at elevated temperatures also reveals the total ionic space charge in the oxide and can be used to extract a mobility for the ions as they pass through the damaged oxide. Comparisons of these measurements to standard device models as well as to ion range calculations in the oxide are presented.   

Self-consistent full counting statistics of inelastic transport

The full counting statistics (FCS) of inelastic transport in molecular junctions is considered for the case of weak electron-vibration coupling. We introduce a self-consistent procedure for FCS within the non-equilibrium Green function (NEGF) method, and discuss its importance in two aspects. First, we show that in the case of FCS the self-consistent treatment provides a conserving approximation. Second, we discuss the importance of molecular vibration renormalization for the counting statistics of electron transport. We consider two-level bridge with diagonal and off-diagonal electron-vibration couplings. The latter model is shown to be especially sensitive to renormalization of the vibration. We show that heating the molecular vibration may lead to either an increase or a decrease in current through the junction depending on the strength of electron-vibration coupling in the bridge compared to molecule-contact coupling. We report an appearance of super-Poissonian noise induced by the non-equilibrium vibration at resonance, which is similar to the effect of the avalanche transport previously reported in the literature for a system with a strong electron-vibration interaction.   

Measurement of the Spectral Distribution of Low Energy Electrons Emitted as a Result of M$_{3}$VV Auger Transitions in Cu(100)

Auger Photoelectron Coincidence Spectroscopy (APECS) was used to investigate the physics of the Low Energy Tail (LET) region of the Auger spectrum of a Cu(100) sample. A beam of 200eV photons was used to probe the sample. Two Cylindrical Mirror Analyzers (CMAs) were used to select the energy of electrons emitted from the sample. An APECS technique was used to obtain an Auger spectrum with one of the CMAs fixed at the core photoemission peak. The spectrum contains the extrinsic contributions from electrons excited by the M$_{3}$VV Auger transition plus a background due to true coincidence between photo-emitted valence band electrons that undergo inelastic~scattering and other valence electrons. To remove the extrinsic contribution to the LET of the Auger Spectrum, Coincidence measurements were made with the fixed analyzer set at various energies (150eV, 165eV, 180eV, 190eV and 197eV) between the core and the valence band and obtain an estimate of the background due to the inelastic scattering of the valence band electrons. The extrinsic contribution to the LET was then subtracted to get the final spectrum consisting of the secondary electrons that are intrinsic to the M$_{3}$VV Auger transition only.   

Interaction of Water Layers on Calcite Surfaces

Calcite is a mineral of great interest because its abundance in both geological and biological systems. While the $\{10\hat{1}4\}$ surface largely dominates the calcite morphology, other surfaces consisting of $\{10\hat{1}4\}$ terraces and steps are important for the crystal dissolution or growth in aquas environment. We use ab-initio calculations to study the interaction of single water molecule and one and two water layers with the flat $\{10\hat{1}4\}$ calcite surface and two step surfaces: $\{10\hat{1}3\}$ and $\{10\hat{1}5\}$ made of $\{10\hat{1}4\}$ terraces offset by one atomic layer along the $\{10\hat{1}1\}$ and the $\{0001\}$ surface respectively. Preliminary results show that the first layer of water bond strongly to the calcite surface. However, dissociation of the water molecules is not favored on the surface.   

Tracking Amino Acids in Chiral Quantum Corrals

Engineering molecular superstructures on metals opens great possibilities for the control and exploration of complex nanosystems for technological applications. Of particular interest is the use of chiral molecules, such as alanine, to build self-assembled nanoscale structures for the trapping of the two-dimensional free electron gas of a metal. In the present work, molecules of D- or L-alanine were deposited on Cu(111). Scanning tunneling microscopy, spectroscopy, and density functional theory (DFT) revealed the formation of a uniform network of hexagonal chiral pores of average diameter $\sim $ 1.2 nm. Each pore acts as a quantum corral by confining the two-dimensional electron gas of the Cu(111) surface state. Furthermore, each hexagonal pore acts as nanoscale tracks when excess alanine molecules were trapped at the inner perimeter of the pore, and were observed as rotating spatial states periodically moving between the six vertices of the hexagon. This study demonstrates the engineering of one of the smallest quantum confined structure, and the dynamics of molecular motion within these chiral potentials wells.   

Influence of ligand and environment substitution on photo-triggered linkage isomerization of photochromic ruthenium sulfoxide complexes

The group of ruthenium polypyridine sulfoxides features a pronounced photochromism in the UV/VIS spectral range based on an ultrafast photo-triggered linkage isomerization located at the SO-ligand. This isomerization exhibits a tremendous photosensitivity and a high thermal stability of the two metastable structural isomers. Here, we discuss the characteristic photochromic properties of the compounds in the frame of ligand substitution and the replacement of the dielectric environment. The complex [Ru(bpy)$_2$(ROSO)]$\cdot$PF$_6$ [1] (with OSO: 2-methylsulfinylbenzoate) has been modified with the groups R = H, Bn, BnCl and BnMe [2] and studied in different solvents as well as in polydimethylsiloxane. The analysis is performed by cw-pump-probe technique as a function of temperature and exposure. Our results reveal a selective adjustability of the thermal stability in the compounds, while the photosensitivity and the characteristic absorption spectra remain unchanged. We discuss the impact of sulfoxide compounds with the desired features in view of application in molecular photonic devices.\\[4pt] [1] V. Dieckmann et al., Opt. Express 17, 15052 (2009)\newline [2] V. Dieckmann et al., Opt. Express 18, 23495 (2010)   

Classical models for electron capture by highly charged ions in the thin film regime

We present an extension of the classical over the barrier (COB) model[1] for highly charged ions (HCIs) that describes thin dielectric films on metal surfaces, bridging the bulk metal and bulk insulator regimes. Motivated by recent experiments [2,3], we detail the onset of charge transfer between a HCI and metal covered with a dielectric thin film. In this talk, capture distances as a function of C$_{60}$ film thickness on Au(111) will be presented. For ultrathin films, electron capture begins from filled levels in the metal and the C$_{60}$ film decreases the potential barrier for charge transfer and increases the critical distance compared to clean Au(111), increasing the time available for above-surface relaxation. ~This is consistent with the new observation of increasing HCI-induced electron emission yield as a function of film thickness [3]. As film thickness grows and reaches a critical value, the first captured electrons originate from the film at the distance expected for an insulator target.\\[4pt] [1] J. Burgd\"{o}rfer et al. Phys. Rev. A 44, 5674--5685 (1991) \\[0pt] [2] R.E. Lake et al. Phys. Rev. Lett. 107, 063202 (2011) \\[0pt] [3] E. Bodewits et al. Phys. Rev. A 84, 042901 (2011)   

Coupling of Cobalt-Tetraphenylporphyrin Molecules in Copper Nitride Molecular Network

We have used low temperature scanning tunneling microscopy to study the interaction between individual cobalt-tetraphenylporphyrin molecules and a molecular copper nitride network. We demonstrated that the molecular Cu$_{3}$N-Cu(110) network promotes the decoupling of the porphiryn, allowing to visualize the molecular orbitals and vibronic states of the molecule, while keeping a strong coupling of the Co atom in the center of macrocycle with the substrate. The reverse behavior was observed when the molecule was sitting on the Cu(110) metallic surface. First principle calculations confirm the assembled position of the molecule on top of N atoms and the decoupling from the surface.   

A New Series of Donor-Acceptor Substituted Small Organic Molecules With Large Third Order Molecular Polarizability

We report on the third-order nonlinear optical properties of a series of new small organic molecules with a non-planar structure. A large third-order molecular polarizability is achieved thanks to a lower excited state energy obtained due to donor-acceptor substitution. We determined the influence of variations in the donor-acceptor substitution pattern and relate them to the nonlinear response by modeling the molecular properties computationally, and by experimentally determining the rotational average of their third-order polarizability by degenerate four-wave mixing. We found that the best molecules are extremely efficient both in relation to their size and to the fundamental quantum limit. These molecules show great potential for applications where the molecules are combined into dense supramolecular solid state assemblies in the form of high optical quality thin films obtained by molecular beam deposition.   

Organic molecular crystals: materials with competing orders

The search for multiferroics has become an important research topic in the last few years. Almost all newly discovered multiferroics are transition metal compounds where spin, lattice and charge degrees of freedom are strongly entangled. The possibility of finding organic multiferroics can open up a new area of research where new mechanisms, different from those active in standard transition metal oxide multiferroics, may have a role for the simultaneous occurrence of magnetic and ferroelectric order. By means of ab-initio and model calculations, we show an instability towards multiferroicity in the organic molecular crystal TMTTF$_2$-PF$_6$. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase with the tendency to the dimerization magnetically driven. The Multiferroicity is induced by competing orders: charge distribution and lattice distortions coupled with the magnetic state.   

Energy Transport in Quantum Systems with Discrete Spectrum

Energy transport in quantum system driven by stochastic perturbations is examined. One of the goals of this study is to determine how the Landauer channels can be defined in a system with discrete energy spectrum. A model describes a particle trapped in a confining potential and subjected to a stochastic perturbation localized off-center of the potential well. The perturbation pumps energy into the system which results in non-zero average energy flux between different regions of the confining potential. The energy flux can be defined in terms of quantum advection modes, where each mode is associated with an off-diagonal element of the density matrix and carries a finite, quantized amount of energy per unit of the probability flux. Statistical correlations between different modes and the net energy flux will be discussed.