57, 1029-1080 (1994). Woodruff, D. P. ‘Adsorbate structure determination using photoelectron diffraction’. Surf. Sci. Rep




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Quantitative adsorbate structure determination using scanned-energy mode photoelectron diffraction


D.P. Woodruff


Physics Department, University of Warwick, Coventry CV4 7AL, UK


Through a Warwick-Berlin collaboration, initially with Alex Bradshaw’s group at the Fritz Haber Institute, and performing experiments at BESSY, we have developed and exploited the technique of scanned-energy mode photoelectron diffraction (PhD) to solve almost 100 distinct adsorbate structures on well-characterised single-crystal surfaces[1,2]. The new I09 beamline at Diamond and the associated infrastructure offers the opportunity to further develop and extend these measurements and to widen access to this method. In this brief review the basic technique and associated methodology will be explained and illustrated some of our most recent applications of the method to molecular adsorbates on metal and oxide surfaces.


References


  1. Woodruff, D.P., Bradshaw, A.M. 'Adsorbate structure determination on surfaces using photoelectron diffraction'. Rep.Prog.Phys. 57, 1029-1080 (1994).

  2. Woodruff, D.P. ‘Adsorbate structure determination using photoelectron diffraction’. Surf. Sci. Rep. 62, 1-38 (2007).



Photoelectron spectroscopy with X-ray standing waves for surface and interface analysis


Jörg Zegenhagen


European Synchrotron Radiation Facility, Grenoble, France


The X-ray standing wave (XSW) technique adds spatial resolution to the strong features of photoelectron spectroscopy (PES). When the standing wave is produced by Bragg reflection from a crystal, the accessible length-scale is limited to its unit cell dimensions. While a crystal is needed to produce the XSW, crystalline order or autocorrelation of the studied adsorbate system is not necessary. In principle the adsorption site of a single atom can be determined by the XSW technique. While this is just a Gedankenexperiment, it highlights a feature of the XSW technique, which is particularly important for the application in surface science. There are many cases where atomic arrangements on surfaces exhibit poor autocorrelation but pronounced correlation with the substrate bulk lattice because of a simple reason: Adsorbate surface distances, i.e. bond-lengths can only vary within a reasonable range. Setting up the XSW in a substrate crystal with an adsorbed layer, the (mean) surface-adsorbate distance can easily be determined by an XSW experiment employing diffraction planes parallel to the surface. However, one XSW/PES measurement using one diffraction plane determines one complex Fourier component (amplitude and phase) of the real space distribution of atoms element specific. Thus, the real space distribution can be reconstructed when using a larger set of diffraction planes,


I will briefly highlight the principle features of the XSW technique and present some examples for the analysis of surface adsorbate and ultra thin epitaxial systems using XSW combined with PES.


X-ray absorption spectroscopy of organic molecules


Georg Held


University of Reading


X-ray absorption spectroscopy (XES, NEXAFS, XANES) is a powerful method for determining the bond order and adsorption geometries of organic molecules. The method is based on the resonant excitation of core electrons into unoccupied states near the Fermi energy, which lead to maxima in the absorption cross section when the photon energy matches the energy difference between the core level and the unoccupied state. In addition, the cross section also depends on the symmetry of the electronic states and their orientation with respect to the polarization of the photon. Sharp NEXAFS resonances are usually associated with multiple chemical bonds involving the emitter atom. This allows targeting specific bonds or side groups of larger molecules by selecting appropriate core levels. Together with angle-resolved NEXAFS measurements symmetry selection rules can be used to determine the order and orientation of such bonds and/or the entire molecule, irrespective of their degree of lateral order. Examples of adsorbed water molecules and bio-molecular building blocks, such as amino acids and peptides, will be presented and discussed.


Shedding light on epitaxial graphene using x-rays


Alessandro Baraldi


Physics Department, University of Trieste, ITALY


While there are various methods for fabricating graphene films, such as through epitaxial growth on SiC or procedures where graphene oxide films are transferred to a substrate and reduced to graphene by chemical reaction or thermal annealing, the growth of graphene by means of hydrocarbon decomposition on transition metals such as Ni, Ir, Pt, Rh, Re and Ru represents a promising alternative for its production.


In this talk I will show few examples from our recent achievement in this field obtained by employing high-resolution core level photoelectron spectroscopy. This method, paralleled by density functional theory calculations, has proven to be a powerful approach to understand the interaction and the morphology of graphene with transition metal surfaces and its use as a template for the growth of nanoclusters.


Contrary to Ir, where graphene is only slightly corrugated and resist up to very high temperature, an extended single carbon layer prepared on Re(0001) and Ru(0001) represents the hallmark of a strongly interacting graphene-substrate system. The comparison of experimental results and DFT calculations shows that the graphene layer on Re presents a large corrugation which is closely knit with the thermal stability of the carbon network: C-C bond breaking is favored in the strongly buckled regions, though it requires the presence of diffusing graphene layer vacancies.


Graphene on TM surfaces offers also the great opportunity to finely control the morphology and the degree of structural order of metallic nanoclusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of Rh nanoclusters. The properties of these atoms will be discussed in view of their importance in heterogeneous catalysis and magnetism.


Graphene on Ir(111): growth and thermodynamics from combined experimental and theoretical methods


Dario Alfè1, Alessandro Baraldi2, Monica Pozzo1, Silvano Lizzit3, Paolo Lacovig3, Paolo Vilmercati2, Philip Hofmann4


1Department of Earth Sciences, UCL, Gower Street, WC1E6BT London, U.K.

2 Physics Department, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy

3Sincrotrone Trieste, Strada Statale 14 Km 163.5, 34012 Trieste, Italy

4Department of Physics and Astronomy and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark


The past few years have witnessed interest of the scientific community in graphene rising sharply. This is motivated by its unique physical properties which make it one of the most promising materials for nanoelectronics, electrochemistry and gas sensing, to mention a few. The growth of graphene by means of hydrocarbon dissociation on transition metal (TM) surfaces represents a challenging way to its synthesis, with peculiar growth mechanisms found depending on the metal substrate. A fundamental issue relates to the process that brings carbidic clusters to develop into a graphene island. It is evident that lattice mismatch and graphene island size play a crucial role, but little is known about the atomic mechanisms of the transition from strong- to weak-interacting C layers, an important target for tailoring the properties of graphene-based nanoscale devices.

In this talk I will present some recent results for the growth and the thermal properties of graphene on Ir(111), obtained from a combination of experimental techniques based on high resolution photo-electron spectroscopy, and theoretical methods based on density functional theory (DFT). The results highlight the mechanisms that bring dome-shaped nanoislands to the formation of an almost free standing graphene layer 1.

I will then describe the thermal expansion properties of graphene on Ir(111), showing how both experiment and theory predict a monotonic increase of the C-C distances with increasing temperature, despite an initial decrease of lattice parameter with increasing temperature. This is rationalised in terms of the excitation of out of plane ripples which are responsible for the negative thermal expansion at low temperature 2.


References


  1. Lacovig, P., Pozzo, M., Alfè, D., Vilmercati, P., Baraldi, A. and Lizzit, S. "Growth of dome-shaped carbon nanoislands on Ir(111): the intermediate between carbidic clusters and quasi free-standing graphene". Physical Review Letters. 103, 166101 1-4 (2009).

  2. Pozzo, M. Alfè, D. Lacovig, P. Hofmann, P. Lizzit, S. and Baraldi, A."Thermal expansion of supported and free-standing graphene". Physical Review Letters. 106, 135501 1-4 (2011).



Graphene and its intercalation compounds: an XSW study


Carsten Busse


II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany


Epitaxial growth of graphene on metals is an established way to obtain samples of high quality [1]. Depending on the substrate, the binding varies between pure covalent (e.g. on Ni(111)) to almost pure van-der-Waals (e.g. on Pt(111)). For graphene/Ir(111), the C layer is weakly bound to the substrate, which is indicated by the long distance between C and Ir 2. In this system also a large unit cell with varying local registries is found, leading to a local variation of the respective bond strength. Hence the graphene layer is not flat but significantly corrugated 2.

We performed X-ray Standing Wave (XSW) measurements for graphene/Ir(111) to determine the height distribution of the C atoms. The peak-to-peak corrugation for a fully closed film is 1 Å and about half this value for small graphene flakes. This can be rationalized by different strain states resulting from the cool-down from high growth temperatures under the influence of different thermal expansion coefficients.

A versatile tool to tailor the properties of graphene is intercalation of atoms between the carbon sheet and the substrate. This may cause doping which is a key for usability of graphene for next generation electronics, and can also induce spin-splitting of the Dirac cone 3. Furthermore it reduces the bonding to the substrate, thereby allowing mechanical exfoliation. We determined the binding type and strength using XSW in three model systems: 1) an electron acceptor O, 2) a weak electron donor Cs, and 3) a strong electron donor Eu, which is a good candidate to induce spin splitting due to its high magnetic moment makes it.


References


  1. van Gastel, R., N’Diaye, A. T. et al. Selecting a single orientation for millimeter sized graphene sheets, Appl. Phys. Lett. 95, 121901 (2009).

  2. Busse, C., Lazic, P. et al., Graphene on Ir(111): Physisorption with chemical modulation, Phys. Rev. Lett. 107, 036101 (2011).

  3. Varykhalov, A., Sánchez-Barriga, J., et al., Electronic and Magnetic Properties of Quasifreestanding Graphene on Ni, Phys. Rev. Lett. 101, 157601 (2008).



High resolution X ray photoelectron and Near Edge X- ray Absorption Spectroscopy studies on graphene and carbon based nanomaterials


Pagona Papakonstantinou


Engineering Research Institute (ERI), School of Engineering, University of Ulster, Newtownabbey, BT37 0QB, UK


Despite the recent developments in Graphene Oxide, due to its importance as a host precursor of Graphene, the detailed electronic structure and its evolution during the thermal reduction remain largely unknown, hindering its potential applications. We show that a combination of high-resolution in situ X-ray photoelectron and X-ray absorption spectroscopies offer a powerful approach to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process(1). The studies help to establish that the edge plane carboxyl groups are highly unstable, whereas carbonyl groups are more difficult to remove. Out results consistently support the formation of phenol groups through reaction of basal plane epoxide groups with adjacent hydroxyl groups at moderate degrees of thermal activation (∼400 C). For the first time, a drastic increase in the density of states (DOS) near the Fermi level at 600 C is observed, suggesting a progressive restoration of aromatic structure in the thermally reduced graphene oxide.


In the talk I will also highlight some of our key contributions on the use of Near Edge X ray Absorption Fine Structure, NEXAFS spectroscopy as a powerful tool for simultaneously probing the electronic structure, chemical functionalities, and alignment in plasma functionalized carbon nanotubes and graphene nanoflakes(2-4). Moreover NEXAFS can be employed to determine the degree of sp2/sp3 bond hybridization as well the presence of ordered phases in self assembled diamond nanorods(5) and tetrahedral amorphous carbon. Although NEXAFS presents the constraint of limited accessibility to synchrotron facilities it is a non-destructive method that can reveal important information, which cannot be easily achieved by other spectroscopic techniques.


References


  1. Ganguly, A., Sharma, S., Papakonstantinou, P. and Hamilton, J. Probing the thermal deoxygenation of graphene oxide by high resolution in situ x-ray based spectroscopies. J. Phys. Chem. C. 115 17009 (2011).

  2. Pao, C.W., Ray, S.C., Tsai, H.M., Chen, H.C., Lin, I.N., Pong, W.F., Chiou J.W., Tsai M.H., Shang N.G., Papakonstantinou P., GuO G.H. Change of Structural Behaviors of Organo-Silane Exposed Graphene Nanoflakes, J. Phys. Chem. C, 114 8161 (2010).

  3. Abbas, G.A., Papakonstantinou, P., Iyer, G.R.S., Kirkman, I. W., Chen, L.C. Substitutional nitrogen incorporation through rf glow discharge treatment and subsequent oxygen uptake on vertically aligned carbon nanotubes. Phys. Rev. B. 75, 195429 (2007).

  4. Shang, N., Papakonstantinou, P., McMullan, M., Chu, M., Stamboulis, A., Potenza, A., Dhesi, S.S., Marchetto, H. Catalyst -free efficient growth, orientation, and biosening properties of multilayer graphene nanoflake films with sharp edge planes, Adv. Funct. Mater. 18, 3506 (2008) .

  5. Shang, N., Papakonstantinou, P., Wang, P., Zakharov, A., Palnitkar, U., Lin, I.N., Chu, M., Stamboulis, A. Self-Assembled Growth, Microstructure, and Field-Emission High-Performance of Ultrathin Diamond Nanorods. ACS Nano. 3, 1032 (2009).



Modelling functional interfaces for energy harvesting


Feliciano Giustino


Department of Materials, University of Oxford


Understanding and designing functional interfaces has become a primary challenge in many areas of science and technology, ranging from photovoltaics and photocatalysis to electronics and biosensing. A fundamental property of functional interfaces is the alignment of the quasiparticle energy levels between the two materials. Such alignment underpins a variety of complex phenomena such as charge-transfer doping, carrier injection, and exciton dynamics. In the context of solar energy technology the level alignment determines the ability of the interface to transfer energy from a donor to an acceptor by exchanging photoexcited charges. While the physics of the energy-level alignment at conventional semiconductor heterojunctions is currently well established, little is known about functional interfaces involving metal oxides and soft materials such as polymers and light-harvesting complexes. Here I will review our recent activity (1-3) in the computational modelling of TiO2 and ZnO interfaces of current interest for nanostructured solar cells, and of experimental probes such as X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. In this area the first challenge that we have to face is to determine the structure of the interface at the atomic scale. Here standard optimization techniques are bound to fail due to the large number of possible interface morphologies. We will argue that a possible way forward is to build interface models by reverse-engineering experimental data using first-principles computational spectroscopy. This notion will be illustrated by discussing core-level photoemission at interfaces between TiO2 and metal-organic complexes. The second challenge is the development of robust computational methods for studying the electronic structure of functional interfaces. In fact standard density-functional techniques are rarely in quantitative agreement with photoemission data, and in some cases they fail to correctly describe the charge transfer at the interface. This will be illustrated by discussing semiconductor-polymer and metal-oxide/molecule interfaces (1). Our findings point to the necessity of developing accurate and reliable methods for investigating electronic and optical excitations at complex interfaces. Here I will briefly review our activity in the area of many-body perturbation theory techniques (GW method), and in particular the use of quasiparticle methods in the case of transition metal oxides with localized d electrons (3).


References


  1. Patrick, C. E., Giustino F., O-1s core-level shifts at the anatase TiO2(101)/N3 photovoltaic interface: signature of H-bonded supramolecular assembly. Phys. Rev. B. 84, 085330 (2010).

  2. Patrick, C. E., Giustino F., Structural and electronic properties of semiconductor-sensitized solar cells interfaces. Adv. Funct. Mater. 21, 4663 (2011).

  3. Patrick, C. E., Giustino F., GW quasiparticle band gaps of anatase TiO2 starting from DFT+U. J. Phys.: Condens. Matter. 24, 202201 (2012).



In situ studies of chemical reactions on surfaces and in ionic liquids by X-ray photoelectron spectroscopy


Hans-Peter Steinrück


Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Germany


Chemical reactions can be followed in detail using X-ray photoelectron spectroscopy (XPS or ESCA). From the binding energies of adsorbate and substrate core levels, detailed information on the chemical composition, chemical state (e.g. oxidation state), adsorption sites, but also on the photoemission process itself can be derived. Based on the understanding obtained for simple adsorbate systems, now complex molecular systems can be studied in great detail. In this presentation two research fields will be addressed. The first deals with the interaction of small molecules with metal surfaces. The second concerns ionic liquids. Due to their low vapor pressure the full arsenal of UHV-based surface science methods can be applied to investigate this material class and detailed information can be derived. Particular emphasis will be given on surface and interface properties of imidazoliumbased ionic liquids and in-situ monitoring of organic liquid phase reactions in ionic liquids.


References


  1. Papp, C., Fuhrmann, T., Tränkenschuh, B. Denecke R. and Steinrück, H.-P. Kinetic isotope effects and reaction intermediates in the decomposition of methyl on flat and stepped platinum (111) surfaces. Chem. Phys. Letters. 442, 176-181 (2007).

  2. Streber, R., Papp, C., Lorenz, M.P.A., Bayer, A., Denecke, R. and Steinrück, H.-P. Sulfur Oxidation on Pt(355) – It’s the Steps!. Angew. Chem. Int. 48, 9743-9746 (2009).

  3. Lovelock, K. R. J., Villar-Garcia, I. J., Maier, F., Steinrück, H.-P. & Licence, P. Photoelectron Spectroscopy of Ionic Liquid-Based Interfaces. Chem. Rev. 110, 5158-5190 (2010).

  4. Kolbeck, C., Niedermaier, I., Taccardi, N., Schulz, P. S., Maier, F., Wasserscheid, P. & Steinrück, H.-P Monitoring of liquid-phase organic reactions by photoelectron spectroscopy. Angew. Chem. Int. 51, 2610-2613 (2012).

  5. Steinrück, H.-P. Recent developments in the study of ionic liquid interfaces using X-ray photoelectron spectroscopy and potential future directions. Phys. Chem. Chem. Phys. 14 (2012) 2510-2529, Invited Perspective.



Structural Studies at Liquid Interfaces: Liquid/Gas and Liquid/Solid


Robert G Jones


School of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK


Until now studying the surfaces of liquids at the molecular level has proved difficult, tending to impossible, due to their high vapour pressures which preclude their study using ultra-high vacuum surface science techniques. With the advent of ionic liquids, we now have a class of chemically-tunable solvents, exhibiting a range of uses from catalysis and gas capture to electrochemistry, which are electrically conductive and ultra-high vacuum compatible, allowing them to be studied by surface science techniques. For ionic liquids we expect structuring along the surface normal at both the liquid/solid and liquid/gas (1) interfaces (Fig.1, A & B). At the liquid/solid interface, we also expect ordering parallel to the surface, as shown by 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [C2C1Im][Tf2N], (Fig. 1B) on Au(110) (Fig.1 C & D) (2). The SISA beamline is designed for surface structural studies using synchrotron radiation. Here we describe the progress that has been made in structural studies of liquid interfaces, and how the techniques on the SISA beamline can be applied to liquid samples.



Figure : A: potential energy diagram for the [C2C1Im][Tf2N]/H2O vapour interface(1). B: [C2C1Im][Tf2N]. C: Au(111)-(1x3)-reconstruction induced by [C2C1Im][Tf2N] adsorption (2). D: STM of submonolayer [C2C1Im][Tf2N] on Au(111) showing ion pairs (2).





In particular we describe how normal incidence X-ray standing wave (NIXSW) can be applied to liquid/crystalline-solid interfaces; long period XSW could be applied to the study of liquid/gas interfaces; the related techniques of X-ray reflectivity and variable period XSW could be applied to both liquid/gas and liquid/solid surfaces and how all of these plus photoelectron diffraction can be applied to the study of ad/absorbates at liquid interfaces.


References


1. Deyko A, Jones R G, Faraday Discussions 154, 265-288 (2012)

2. Foulston R, Gangopadhyay S, Chiutu C, Moriarty P, Jones R G, Phys. Chem. Chem. Phys. 14, 6054-6066 (2012).


Chirality at Surfaces: Handedness and Footedness


Matthew Forster, Matthew S. Dyer, Mats Persson and Rasmita Raval


Surface Science Research Centre and Department of Chemistry, University of Liverpool, Liverpool, L69 3BX, UK


Chirality can be bestowed upon a surface by the adsorption of molecules and is usually discussed in terms of the molecular handedness. However, the adsorption process often leads to a new manifestation of chirality in the form of the adsorption footprint which can also be chiral, generating mirror-images in 2-D. Therefore, it has recently been proposed that in describing the chirality of the interface, one must consider both the handedness and the ‘footedness’ of the system (1). We have previously demonstrated the ability to map both molecular handedness and footedness with organised amino-acid assemblies on surfaces (2,3). Here, we show that a surprising level of complexity in chiral behaviour at interfaces be realized, encompassing chiral arrangements ranging from enantiopure assemblies, conglomerates, ordered racemic compounds to random solid solutions, which can be expressed at both the handed and footed levels. Specifically, we have demonstrated that the (4 × 2) assembly of the enantiopure amino-acid (S)-proline on a Cu(110) surface organises with a strict heterochiral footprint arrangement, with insights provided by scanning tunnelling microscopy (STM), reflection absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT) calculations (2,3). Interestingly, the racemic (RS)-proline (4 × 2) assembly is also governed by a strict footprint template but the molecular chirality is randomly dispersed, creating a 2-D random solid solution. In addition, we show the ability to engineer surface footedness by carrying out targeted structural modifications to drive the system to complete homochirality (4). The modification of proline by the addition of a double bond within the pyrrolidine ring, yielding 3-pyrroline-2-carboxylic acid (PCA), directly affects the footprints adopted within the analogous (4 × 2) organisation on the Cu(110) surface and forces enantiopure (S)-PCA to project a homochiral footprint assembly, in contrast to the heterochiral footprint arrangement favoured by the (S)-proline (4 × 2) assembly. The corresponding racemic (RS)-PCA assembly is found to possess a random arrangement at both the handed and footed level, representing a truly random solid solution in 2-D (5). The ability to both identify and control adsorption footprints is not only pivotal to tailoring chirality at surfaces but also plays a key role in dictating the organisation, the outward facing functionalities and the response of the interface with potential applications in sensors, enantioselective catalysis and non-linear optical materials.


References


  1. Mark, A.G., Forster, M. and Raval, R. Chemphyschem, 12, 1474 (2011)

  2. Forster, M., Dyer, M.S., Persson, M. and Raval, R. J. Am. Chem. Soc. 131, 10173 (2009)

  3. Forster, M., Dyer, M.S., Persson, M. and Raval, R. Angew. Chem. Int. Ed. 49, 2344 (2010)

  4. Forster, M., Dyer, M.S., Persson, M. and Raval, R. J. Am. Chem. Soc. 133, 15992 (2011)

  5. Forster, M., Dyer, M.S., Persson, M. and Raval, R. J. Phys. Chem. C. 115, 1180 (2011)



Cormac McGuiness


Characterization of monomolecular films and related systems by advanced synchrotron soft X-ray spectroscopy


M. Zharnikov


Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253,

69120 Heidelberg, Germany


Frontier areas of modern science and technology rely on the possibility to tailor interfacial properties such as wetting, adhesion, lubrication, corrosion, and biocompatibility on both microscopic and macroscopic scale. To large extent, these objectives can be achieved with the help of self-assembled monolayers (SAMs), which are 2D polycrystalline films of semi-rigid molecules that are chemically anchored to a substrate by a suitable headgroup and carrying, at the other end of the molecular chain, a specific tail group, which redefines the physical and chemical properties of the substrate. In addition, such layers represent a general platform for sensor fabrication, possible arrangement of future molecular electronics devices, framework for Chemical Lithography, and a model system for macromolecular and biological assemblies. Advanced synchrotron-based soft X-ray spectroscopic techniques represent a powerful tool box for the characterization of SAMs, which is a prerequisite for their design and applications (1,2). Basic information on the chemical identity, integrity, molecular conformation and orientation in these films can be obtained by a combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy, which can be, if necessary, complemented by laboratory-based methods. More specific questions, such as, e.g., bonding configuration of the anchor group (3) or dynamics of the charge transfer through the molecular backbone (4,5) require special experimental tools such as high-resolution XPS and resonant Auger electron spectroscopy. Using several standard and specially designed systems, I will illustrate the application of the above techniques for the characterization and design of SAMs. In addition, I will show that SAMs can be used as prototypes of highly organized biological systems and provide important implications for the cryogenic approach in advanced electron and x-ray spectroscopy and microscopy of biological macromolecules and cells (6). Finally, I will demonstrate that SAM concept can be extended to more complex systems of biological significance such as single strand DNA films (7).


References


  1. Zharnikov, M., Grunze, M. Spectroscopic characterization of thiol-derived self-assembled monolayers, J. Phys. Condens. Matter. 13, 11333-11365 (2001)

  2. Zharnikov, M. High-resolution X-ray photoelectron spectroscopy in studies of self-assembled organic monolayers, J. Electr. Spectr. Rel. Phenom. 178-179, 380-393 (2010)

  3. Chesneau, F., Zhao, J. et al. Adsorption of long-chain alkanethiols on Au(111) - a look from the substrate by high resolution X-ray photoelectron spectroscopy, J. Phys. Chem. C 114, 7112–7119 (2010)

  4. Neppl, S. Bauer, U. et al. Dynamics of the charge transfer in self-assembled monolayers, Chem. Phys. Lett. 447, 227-231 (2007)

  5. Hamoudi, H., Neppl, S. et al. Orbital-dependent charge transfer dynamics in conjugated self-assembled monolayers, Phys. Rev. Lett. 107, 027801 (2011)

  6. Feulner, P., Niedermayer, T. et al. Strong temperature dependence of irradiation effects in organic layers, Phys. Rev. Lett. 93, 178302 (2004)

  7. Schreiner, S., Hatch, A. et al. Impact of DNA-surface interactions on the stability of DNA hybrids, Anal. Chem. 83, 4288-4295 (2011)



Charge transfer dynamics of complex molecules on surfaces


James N. O’Shea, Andrew Britton, Matthew Weston, Louise C. Mayor, J. Ben Taylor


School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK





Charge-transfer between molecules and the surfaces to which they are bound lies at the heart of a number of really important and – from a molecular nanoscience point of view – incredibly interesting devices. The most obvious of these are dye-sensitised solar cells. Techniques that can probe charge-transfer dynamics in molecular systems are therefore excellent tools for understanding these devices at the most fundamental level. These include the core-hole clock implementation of resonant photoemission (RPES) which we have used to study electron injection from adsorbed molecules into the conduction band of a solid substrate on the low femtosecond timescale.1,2 Amongst the electronic debris that accompanies core-hole decay, participator electrons that indicate that a core-excited electron remains on the molecule during the core-hole lifetime are easily distinguished from Auger electrons that indicate that it has tunnelled into the surface conduction band. But charge injection is only half the story. What about getting electrons back into molecules through charge transfer from the surface? Information concerning this process can be obtained by monitoring the spectator electrons emitted during core-hole decay with extraordinarily high kinetic energy superspectator electrons revealing charge transfer from the surface into the lowest unoccupied molecular orbital of the core-excited molecules.3,4


References


  1. Charge transfer dynamics of model charge transfer centres of a multi-centre water splitting dye complex on rutile TiO2(110), M. Weston et al. J. Chem. Phys. 134, 054705 (2011)

  2. Photoemission, resonant photoemission and X-ray absorption of a Ru(II) complex adsorbed on rutile TiO2(110) prepared by in-situ electrospray deposition, L. C. Mayor et al. J. Chem. Phys. 129, 114701 (2008)

  3. Charge transfer interactions of a Ru(II) dye complex and related ligand molecules adsorbed on Au(111), A. J. Britton et al. J. Chem. Phys. 135, 164702(2011)

  4. Charge transfer between the Au(111) surface and adsorbed C60: Resonant photoemission and new core-hole decay channels, A. Britton et al. J. Chem. Phys. 133, 094705 (2010)


Co-Crystals and Salts, Coatings and Clusters: Core Level Spectroscopies as a Probe for Local Bonding in Pharmaceutical Systems

Joanna S. Stevens1, Lauren K. Newton1, Matthew Thomason2, Guy A. Hembury1, Angela M. Beesley1, Luanga N. Nchari1, Hamizah M. Zaki2, A. Murray Booth1, Emad F. Aziz3, Sven L.M. Schroeder1,2


1School of Chemical Engineering and Analytical Science, The University of Manchester, UK

2School of Chemistry, The University of Manchester, UK

3BESSY II, Helmholtz-Zentrum Berlin, Germany


Core level spectroscopies with soft X-ray excitation, especially X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS; particularly NEXAFS), have for decades been part of a standard suite of instruments used for the characterisation of surfaces and interfaces of inorganic condensed matter. Originally ultra-high vacuum (UHV) techniques, instrument development since the 1990s has facilitated their application under non-UHV conditions, for in situ and/or operando studies under control of environmental conditions. A consequence is the increasing use of soft X-ray spectroscopies for pharmaceutical applications, including soft matter, molecular crystals and molecular solutes in solutions. My presentation will focus on nitrogen 1s core level measurements for (i) the examination of H-bonding and proton transfer in organic systems, (ii) the detection of coatings on pharmaceutical actives, (iii) the determination of the causes underlying facet-specific interactions on paracetamol crystals, and (iv) for following the local structure in the imidazole/water system during the transition from dilute to concentrated solutions.


Elucidating the active site in heterogeneous catalysis by in-situ X-ray spectroscopies


Adam F. Lee1


1Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK


Catalysis is a rich, multidisciplinary field with a global socio-economic impact ranging from improved air quality to the design of new HIV therapies, and creation of bulletproof fabrics. Heterogeneous (solid) catalysis is currently experiencing a global renaissance, with interest soaring in new predictive quantum chemical models and experimental synthetic and analytical methodologies able to deliver tailored catalyst formulations offering precise activities, and selectivities, for diverse chemistries such as biofuels production and artificial photosynthesis. The rational design of new heterogeneous catalysts for sustainable chemical technologies can be accelerated by molecular level insight into surface chemistry,1 and nanoengineering approaches to achieve precise control over the structure and reactivity of novel functional materials.2 Here we highlight how in-situ and time-resolved X-ray spectroscopies can be used to visualise chemistry in action (Figure 1) and identify the active catalytic site responsible,3 and thereby help direct the design of tunable nanocrystalline and nanoporous catalysts for clean technologies such as the aerobic selective oxidation (selox) of allylic alcohols.4





Figure 1. (a) Cartoon of stainless-steel in-situ reactor; (b) DRIFTS/MS/XANES intensities as a function of alternating CrOH/O2 cycles over 2.37 wt% Pd/meso-Al2O3 catalyst at 80 C:1712 cm-1;O2 (m/z = 32),crotonaldehyde (m/z = 70), XANES Pd2+ concentration.


References

  1. Lee, A.F., Vinod, C.P., Wilson, K. Surface X-ray studies of clean catalytic technologies. Chem. Comm. 46, 3827 (2010).

  2. Parlett, C.M.A., Bruce, D.W., Hondow, N.S., Lee, A.F., Wilson, K. Support-enhanced selective aerobic oxidation of allylic alcohols over Pd/Silicas. ACS Catalysis 1, 636 (2011).

  3. Lee, A.F., Ellis, C.V., Naughton, J., Newton, M.A., Parlett, C.M.A., Wilson, K. Reaction-driven surface restructuring and selectivity control in allylic alcohol catalytic aerobic oxidation over Pd. J. Am. Chem. Soc. 133, 5724 (2011).

  4. Vinod, C.P., Wilson, K., Lee, A.F. Recent advances in the heterogeneously catalysed aerobic selective oxidation of alcohols. J. Chem. Tech. Biotech 86, 161 (2011).



Simulating Near-Edge X-ray Absorption Fine Structure Spectra from First Principles


David Prendergast


The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA


Near-edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy reveals details of the local electronic structure of materials accessible via resonant excitation of core electrons of a given element to available unoccupied states with the correct symmetry. However, interpretation of such measurements presumes an a priori understanding of a given material’s excited-state electronic structure. In practice, we rely heavily on comparison with previous experiments on standard materials and theoretical estimates of the x-ray absorption cross-section. Recently, we have developed a first-principles approach to accurately predict x-ray absorption spectra (XAS) of a wide range of materials,1 with specific application to isolated molecules,2 molecular liquids,1 aqueous solvation of organic3-5 and ionic species,6-9 molecular,10 ionic and covalent crystals,11 transition metals12 and their oxides, nanostructures,13,14 lithium battery materials, and actinide complexes. The details of this approach will be presented, with particular emphasis on the role of dynamics in modifying measured spectral features and the estimation of chemical shifts in NEXAFS based on theoretical atomic references.

References


  1. Prendergast, D. and Galli G., “X-ray absorption spectra of water from first-principles calculations”, Physical Review Letters 96, 215502 (2006).

  2. Uejio, J. S., Schwartz, C. P., et al., “Effects of vibrational motion on core-level spectra of prototype organic molecules”, Chemical Physics Letters 467, 195 (2008).

  3. Schwartz, C. P., Uejio, J. S., et al., “Auto-oligomerization and hydration of pyrrole revealed by x-ray absorption spectroscopy”, Journal of Chemical Physics 131, 114509 (2009).

  4. Uejio, J. S., Schwartz, C. P., et al., “Monopeptide vs. Monopeptoid: Insights on Structure and Hydration of Aqueous Alanine and Sarcosine via X-Ray Absorption Spectra”, Journal of Physical Chemistry B 114, 4702 (2010).

  5. Schwartz, C. P., Uejio, J. S., et al., “Investigation of Protein Conformation and Interactions with Salts via X-ray Absorption Spectroscopy”, Proceedings of the National Academy of Sciences 107, 14008 (2010).

  6. Kulik, H. J., Marzari, N., et al., “Local Effects in the X-ray Absorption Spectrum of Salt Water”, Journal of Physical Chemistry B 114, 9594 (2010).

  7. Duffin, A. M., Schwartz, C. P., et al., “pH-Dependent X-ray Absorption Spectra of Aqueous Boron Oxides”, Journal of Chemical Physics 134, 154503 (2011).

  8. Duffin, A. M., England, A. H., et al., “Electronic structure of aqueous borohydride: a potential hydrogen storage medium”, Physical Chemistry Chemical Physics 13, 17077 (2011).

  9. England, A. H., Duffin, A. M., et al., “On the Hydration and Hydrolysis of Carbon Dioxide”, Chemical Physics Letters 514, 187 (2011).

  10. Schwartz, C. P., Saykally, R. J., and Prendergast, D., “An analysis of the NEXAFS Spectra of a molecular crystal: α-Glycine”, Journal of Chemical Physics 133, 044507 (2010).

  11. Lin, J. F., Fukui, H., et al., “Electronic Bonding Transition in Compressed SiO2 Glass”, Physical Review B 75, 012201 (2007).

  12. Cho, B. I., Engelhorn, K., et al., “Electronic structure of warm dense copper studied by ultrafast x-ray absorption spectroscopy”, Physical Review Letters 106, 167601 (2011).

  13. Lee, J. R. I., Whitley, H. D., et al., “Ligand-mediated modification of the electronic structure of CdSe quantum dots”, Nano Letters ASAP (May 2012).

  14. Schultz, B. J., Patridge, C. J., et al., “Imaging local electronic corrugations and doped regions in graphene”, Nature Communications 2, 372 (2011).



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