Surface modifications of nanocrystalline diamond films and their functionalization with phthalocyanines

Boron-doped diamond is a promising electrode material for a number of applications providing efficient carrier transport, a high stability of the electrolytic performance with time, a possibility for dye-sensitizing with photosensitive molecules, etc. It can be functionalized with electron donor molecules, like phthalocyanines or porphyrins, for the development of light energy conversion systems. For effective attachment of such molecules, the diamond surface has to be modified by plasma- or photo-chemical processes in order to achieve a desired surface termination. In the present work, the surface modifications of undoped and boron-doped nanocrystalline diamond (NCD) films and their functionalization with various phthalocyanines (Pcs) were investigated. The NCD films have been prepared by hot filament chemical vapor deposition (HFCVD) on silicon substrates and were thereafter subjected to modifications with O2 or NH3 plasmas or UV/O3 treatments for exchange of the H-termination of the as-grown surface. The effectiveness of the modifications and their stability with time during storage under different ambients were studied by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Furthermore, the surface roughness after the modifications was investigated with atomic force microscopy (AFM) and compared to that of as-grown samples in order to establish the appearance of etching of the surface during the treatment. The as-grown and the modified NCD surfaces were exposed to phthalocyanines with different metal centers (Ti, Cu, Mn) or with different side chains. The results of the Pc grafting were investigated by XPS and Raman spectroscopy. XPS revealed the presence of nitrogen stemming from the Pc molecules and traces of the respective metal atoms with ratios close to those in the applied Pc. In a next step Raman spectra of Ti-Pc, Cu-Pc and Mn-Pc were obtained with two different excitation wavelengths (488 and 785 nm) from droplet samples on Si after evaporation of the solvent in order to establish their Raman fingerprints. The major differences in the spectra were assigned to the effect of the size of the metal ion on the structure of the phthalocyanine ring. The spectra obtained were used as references for the Raman spectra of NCD surfaces grafted with Pc. Finally, selected boron doped NCD samples were used after their surface modification and functionalization with Pc for the preparation of electrodes which were tested in a photoelectrochemical cell with a Pt counter electrode and an Ag/AgCl reference electrode. The light sources and electrolytes were varied to establish their influence on the performance of the dye-sensitized diamond electrodes. Cyclic voltammetry measurements revealed broad electrochemical potential window and high stability of the electrodes after several cycles. The open circuit potential (OCP) measurements performed in dark and after illumination showed fast responses of the electrodes to the illumination resulting in photocurrent generation.

The Surface Plasmon Resonance of Supported Noble Metal Nanoparticles: Characterization, Laser Tailoring, and SERS Application

This work deals with the optical properties of supported noble metal nanoparticles, which are dominated by the so-called Mie resonance and are strongly dependent on the particles’ morphology. For this reason, characterization and control of the dimension of these systems are desired in order to optimize their applications. Gold and silver nanoparticles have been produced on dielectric supports like quartz glass, sapphire and rutile, by the technique of vapor deposition under ultra-high vacuum conditions. During the preparation, coalescence is observed as an important mechanism of cluster growth. The particles have been studied in situ by optical transmission spectroscopy and ex situ by atomic force microscopy. It is shown that the morphology of the aggregates can be regarded as oblate spheroids. A theoretical treatment of their optical properties, based on the quasistatic approximation, and its combination with results obtained by atomic force microscopy give a detailed characterization of the nanoparticles. This method has been compared with transmission electron microscopy and the results are in excellent agreement. Tailoring of the clusters’ dimensions by irradiation with nanosecond-pulsed laser light has been investigated. Selected particles are heated within the ensemble by excitation of the Mie resonance under irradiation with a tunable laser source. Laser-induced coalescence prevents strongly tailoring of the particle size. Nevertheless, control of the particle shape is possible. Laser-tailored ensembles have been tested as substrates for surface-enhanced Raman spectroscopy (SERS), leading to an improvement of the results. Moreover, they constitute reproducible, robust and tunable SERS-substrates with a high potential for specific applications, in the present case focused on environmental protection. Thereby, these SERS-substrates are ideally suited for routine measurements.

Calculation of the electronic properties of neutral and ionized divalent-metal clusters

The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized. Hg_n clusters, we determine the size dependence of the bond character and the ionization potential I_p(n). For neutral Hg_n clusters we obtain a transition from van del Waals to covalent behaviour at the critical size n_c ~ 10-20 atoms. Results for I_p(Hg_n) with n \le 20 are in good agreement with experiments, and suggest that small Hg_n^+ clusters can be viewed as consisting of a positive trimer core Hg_3^+ surrounded by n - 3 polarized neutral atoms.

On the transition from localized to delocalized electronic states in divalent-metal clusters

The transition from van der Waals to covalent bonding, which is expected to occur in divalent-metal clusters with increasing cluster size, is discussed. We propose a model which takes into account, within the same electronic theory, the three main competing contributions, namely the kinetic energy of the electrons, the Coulomb interactions between electrons, and the s \gdw p intraatomic transitions responsible for van der Waals like bonding. The model is solved by taking into account electron correlations using a generalized Gutzwiller approximation (slave boson method). The occurrence of electron localization is studied as a function of the interaction parameters and cluster size.

Theory for the change of bond character in divalent-metal clusters

To determine the size dependence of the bonding in divalent-metal clusters we use a many-electron Hamiltonian describing the interplay between van der Waals (vdW) and covalent interactions. Using a saddle-point slave-boson method and taking into account the size-dependent screening of charge fluctuations, we obtain for Hg_n a sharp transition from vdW to covalent bonding for increasing n. We show also, by solving the model Hamiltonian exactly, that for divalent metals vdW and covalent bonding coexist already in the dimers.

Selbstassemblierte Monolagen und magnetisch strukturierte Submonolagen mit dia- und paramagnetischen Molekülen sowie Einzelmolekülmagneten auf Phthalocyanin- und Subphthalocyanin-Basis

Mit dieser Arbeit wurde die Selbstassemblierung von dia- und paramagnetischen Molekülen sowie Einzelmolekülmagneten auf Goldsubstraten und magnetisch strukturierten Substraten untersucht. Dazu wurden drei verschiedene Klassen an Phthalocyaninderivaten verwendet: Diamagnetische Subphthalocyanine, paramagnetische Phthalocyaninatometalle und Diphthalocyaninatolanthanidkomplexe. Alle synthetisierten Verbindungen sind peripher thioethersubstituiert. Die Alkylketten (a: n-C8H17, b: n-C12H25) vermitteln die Löslichkeit in vielen organischen Solventien und sorgen für eine geordnete Assemblierung auf einer Oberfläche, wobei die Bindung auf Gold hauptsächlich über die Schwefelatome stattfindet. Die aus Lösung abgeschiedenen selbstassemblierten Monolagen wurden mit XPS, NEXAFS-Spektroskopie und ToF-SIMS untersucht. Bei der Selbstassemblierung auf magnetisch strukturierten Substraten stehen die Moleküle unter dem Einfluss magnetischer Streufelder und binden bevorzugt nur in bestimmten Bereichen. Die gebildeten Submonolagen wurden zusätzlich mit X-PEEM untersucht. Die erstmals dargestellten Manganphthalocyanine [MnClPc(SR)8] 1 wurden ausgehend von MnCl2 erhalten. Hier fand bei der Aufarbeitung an Luft eine Oxidation zu Mangan(III) statt; +III ist die stabilste Oxidationsstufe von Mangan in Phthalocyaninen. Der Nachweis des axialen Chloridoliganden erfolgte mit Massenspektrometrie und FIR- sowie Raman-Spektroskopie. SQUID-Messungen haben gezeigt, dass die Komplexe 1 vier ungepaarte Elektronen haben. Bei den Subphthalocyaninen [BClSubpc(SR)6] 2 wurde der axiale Chloridoligand mit dem stäbchenförmigen Phenolderivat 29-H substituiert und die erfolgreiche Ligandensubstitution durch NMR- und IR-Spektroskopie sowie Massenspektrometrie an den Produkten [BSubpc(SR)6(29)] 30 belegt. Der Radikalcharakter der synthetisierten Terbiumkomplexe [Tb{Pc(SR)8}2] 3 wurde spektroskopisch nachgewiesen; SQUID-Messungen ergaben, dass es sich um Einzelmolekülmagnete mit einer Energiebarriere U des Doppelpotentialtopfs von 880 K oder 610 cm-1 bei 3a handelt. Zunächst wurden die SAMs der Komplexverbindungen 1, 2, 30 und 3 auf nicht magnetisch strukturierten Goldsubstraten untersucht. Die Manganphthalocyanine 1 bilden geordnete SAMs mit größtenteils flach liegenden Molekülen, wie die XPS-, NEXAFS- und ToF-SIMS-Analyse zeigte. Die Mehrzahl der Thioether-Einheiten ist auf Gold koordiniert und die Alkylketten zeigen ungeordnet von der Oberfläche weg. Bei der Adsorption findet eine Reduktion zu Mangan(II) statt und der axiale Chloridoligand wird abgespalten. Das beruht auf dem sog. Oberflächen-trans-Effekt. Im vorliegenden Fall übt die Metalloberfläche einen stärkeren trans-Effekt als der axiale Ligand aus, was bisher experimentell noch nicht beobachtet wurde. Die thioethersubstituierten Subphthalocyanine 2 und 30 sowie die Diphthalocyaninatoterbium-Komplexe 3 sind ebenfalls für SAMs geeignet. Ihre Monolagen wurden mit XPS und NEXAFS-Spektroskopie untersucht, und trotz einer gewissen Unordnung in den Filmen liegen die Moleküle jeweils im Wesentlichen flach auf der Goldoberfläche. Vermutlich sind bei diesen Systemen auch die Alkylketten größtenteils parallel zur Oberfläche orientiert. Im Gegensatz zu den Manganphthalocyaninen 1 tritt bei 2b, 30a, 30b und 3b neben der koordinativen Bindung der Schwefelatome auf Gold auch eine für Thioether nicht erwartete kovalente Au–S-Bindung auf, die durch C–S-Bindungsbruch unter Abspaltung der Alkylketten ermöglicht wird. Der Anteil, zu dem dieser Prozess stattfindet, scheint nicht mit der Molekülstruktur zu korrelieren. Selbstassemblierte Submonolagen auf magnetisch strukturierten Substraten wurden mit dem diamagnetischen Subphthalocyanin 2b hergestellt. Der Nachweis der Submonolagen war schwierig und gelang schließlich durch eine Kombination von ToF-SIMS, NEXAFS Imaging und X-PEEM. Die Analyse der ToF-SIMS-Daten zeigte, dass tatsächlich eine Modulation der Verteilung der Moleküle auf einem unterwärts magnetisch strukturierten Substrat eintritt. Mit X-PEEM konnte die magnetische Struktur der ferromagnetischen Schicht des Substrats direkt der Verteilung der adsorbierten Moleküle zugeordnet werden. Die Subphthalocyanine 2b adsorbieren nicht an den Domänengrenzen, sondern vermehrt dazwischen. Auf Substraten mit abwechselnd 6.5 und 3.5 µm breiten magnetischen Domänen binden die Moleküle bevorzugt in den Bereichen geringster magnetischer Streufeldgradienten, also den größeren Domänen. Solche Substrate wurden für die ToF-SIMS- und X-PEEM-Messungen verwendet. Bei größeren magnetischen Strukturen mit ca. 400 µm breiten Domänen, wie sie aufgrund der geringeren Ortsauflösung dieser Methode für NEXAFS Imaging eingesetzt wurden, binden die Moleküle dann in allen Domänen. Die diamagnetischen Moleküle werden nach dieser Interpretation aus dem inhomogenen Magnetfeld über der Probenoberfläche heraus gedrängt und verhalten sich analog makroskopischer Diamagnete. Die eindeutige Detektion der Moleküle auf den magnetisch strukturierten Substraten konnte bisher nur für die diamagnetischen Subphthalocyanine 2b erfolgen. Um die Interpretation ihres Verhaltens bei der Selbstassemblierung in einem inhomogenen Magnetfeld weiter voranzutreiben, wurde das Subphthalocyanin 37b dargestellt, welches ein stabiles organisches TEMPO-Radikal in seinem axialen Liganden enthält. Das paramagnetische Subphthalocyanin 37b sollte auf den magnetisch strukturierten Substraten in Regionen starker magnetischer Streufelder binden und damit das entgegengesetzte Verhalten zu den diamagnetischen Subphthalocyaninen 2b zeigen. Aus Zeitgründen konnte dieser Nachweis im Rahmen dieser Arbeit noch nicht erbracht werden.

Heavy metal and microbial loads in sewage irrigated vegetables of Kabul, Afghanistan

Little is known about the heavy metal and microbial contamination of vegetables produced in Central Asian cities. We therefore measured the concentration of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) and of faecal pathogens (Coliform bacteria, Salmonella sp., Shigella sp., Ascaris lubricoides, Entamoeba sp. and pinworms [Oxyuris vermicularis syn. Enterobius vermicularis]) in soil, irrigation water, and marketed vegetables of Kabul City, Afghanistan. Leaf Pb and Zn concentrations of leafy vegetables were with 1–5 and 33–160 mg kg^{-1} dry weight (DW) several-fold above respective international thresholds of 0.3 mg Pb kg^{-1} and 50 mg Zn kg^{-1}. The tissue concentration of Cu was below threshold limits in all samples except for spinach in one farm. Above-threshold loads of microbes and parasites on vegetables were found in five out of six gardens with coliforms ranging from 0.5–2 × 10^7 cells 100g^{-1} fresh weight (FW), but no Salmonella and Shigella were found. Contamination with 0.2 × 10^7 eggs 100g^{-1} FW of Ascaris was detected on produce of three farms and critical concentrations of Entamoeba in a single case, while Oxyuris vermicularis, and Enterobius vermicularis were found on produce of three and four farms, respectively. Irrigation water had Ascaris, Coliforms, Salmonella, Shigella, Entamoeba, and Oxyuris vermicularis syn. Enterobius vermicularis ranging from 0.35 × 10^7 to 2 × 10^7 cells l^{-1}. The heavy metal and microbial loads on fresh UPA vegetables are likely the result of contamination from rising traffic, residues of the past decades of war and lacking treatment of sewage which needs urgent attention.

Theoretical study of magnetism, structure and chemical order in transition-metal alloy clusters

Research on transition-metal nanoalloy clusters composed of a few atoms is fascinating by their unusual properties due to the interplay among the structure, chemical order and magnetism. Such nanoalloy clusters, can be used to construct nanometer devices for technological applications by manipulating their remarkable magnetic, chemical and optical properties. Determining the nanoscopic features exhibited by the magnetic alloy clusters signifies the need for a systematic global and local exploration of their potential-energy surface in order to identify all the relevant energetically low-lying magnetic isomers. In this thesis the sampling of the potential-energy surface has been performed by employing the state-of-the-art spin-polarized density-functional theory in combination with graph theory and the basin-hopping global optimization techniques. This combination is vital for a quantitative analysis of the quantum mechanical energetics. The first approach, i.e., spin-polarized density-functional theory together with the graph theory method, is applied to study the Fe$_m$Rh$_n$ and Co$_m$Pd$_n$ clusters having $N = m+n \leq 8$ atoms. We carried out a thorough and systematic sampling of the potential-energy surface by taking into account all possible initial cluster topologies, all different distributions of the two kinds of atoms within the cluster, the entire concentration range between the pure limits, and different initial magnetic configurations such as ferro- and anti-ferromagnetic coupling. The remarkable magnetic properties shown by FeRh and CoPd nanoclusters are attributed to the extremely reduced coordination number together with the charge transfer from 3$d$ to 4$d$ elements. The second approach, i.e., spin-polarized density-functional theory together with the basin-hopping method is applied to study the small Fe$_6$, Fe$_3$Rh$_3$ and Rh$_6$ and the larger Fe$_{13}$, Fe$_6$Rh$_7$ and Rh$_{13}$ clusters as illustrative benchmark systems. This method is able to identify the true ground-state structures of Fe$_6$ and Fe$_3$Rh$_3$ which were not obtained by using the first approach. However, both approaches predict a similar cluster for the ground-state of Rh$_6$. Moreover, the computational time taken by this approach is found to be significantly lower than the first approach. The ground-state structure of Fe$_{13}$ cluster is found to be an icosahedral structure, whereas Rh$_{13}$ and Fe$_6$Rh$_7$ isomers relax into cage-like and layered-like structures, respectively. All the clusters display a remarkable variety of structural and magnetic behaviors. It is observed that the isomers having similar shape with small distortion with respect to each other can exhibit quite different magnetic moments. This has been interpreted as a probable artifact of spin-rotational symmetry breaking introduced by the spin-polarized GGA. The possibility of combining the spin-polarized density-functional theory with some other global optimization techniques such as minima-hopping method could be the next step in this direction. This combination is expected to be an ideal sampling approach having the advantage of avoiding efficiently the search over irrelevant regions of the potential energy surface.

Magnetic interactions between transition metal impurities and clusters mediated by low-dimensional metallic hosts: A first principles theoretical investigation

The magnetic properties and interactions between transition metal (TM) impurities and clusters in low-dimensional metallic hosts are studied using a first principles theoretical method. In the first part of this work, the effect of magnetic order in 3d-5d systems is addressed from the perspective of its influence on the enhancement of the magnetic anisotropy energy (MAE). In the second part, the possibility of using external electric fields (EFs) to control the magnetic properties and interactions between nanoparticles deposited at noble metal surfaces is investigated. The influence of 3d composition and magnetic order on the spin polarization of the substrate and its consequences on the MAE are analyzed for the case of 3d impurities in one- and two-dimensional polarizable hosts. It is shown that the MAE and easy- axis of monoatomic free standing 3d-Pt wires is mainly determined by the atomic spin-orbit (SO) coupling contributions. The competition between ferromagnetic (FM) and antiferromagnetic (AF) order in FePtn wires is studied in detail for n=1-4 as a function of the relative position between Fe atoms. Our results show an oscillatory behavior of the magnetic polarization of Pt atoms as a function of their distance from the magnetic impurities, which can be correlated to a long-ranged magnetic coupling of the Fe atoms. Exceptionally large variations of the induced spin and orbital moments at the Pt atoms are found as a function of concentration and magnetic order. Along with a violation of the third Hund’s rule at the Fe sites, these variations result in a non trivial behavior of the MAE. In the case of TM impurities and dimers at the Cu(111), the effects of surface charging and applied EFs on the magnetic properties and substrate-mediated magnetic interactions have been investigated. The modifications of the surface electronic structure, impurity local moments and magnetic exchange coupling as a result of the EF-induced metallic screening and charge rearrangements are analysed. In a first study, the properties of surface substitutional Co and Fe impurities are investigated as a function of the external charge per surface atom q. At large inter-impurity distances the effective magnetic exchange coupling ∆E between impurities shows RKKY-like oscillations as a function of the distance which are not significantly affected by the considered values of q. For distances r < 10 Å, important modifications in the magnitude of ∆E, involving changes from FM to AF coupling, are found depending non-monotonously on the value and polarity of q. The interaction energies are analysed from a local perspective. In a second study, the interplay between external EF effects, internal magnetic order and substrate-mediated magnetic coupling has been investigated for Mn dimers on Cu(111). Our calculations show that EF (∼ 1eV/Å) can induce a switching from AF to FM ground-state magnetic order within single Mn dimers. The relative coupling between a pair of dimers also shows RKKY-like oscillations as a function of the inter-dimer distance. Their effective magnetic exchange interaction is found to depend significantly on the magnetic order within the Mn dimers and on their relative orientation on the surface. The dependence of the substrate-mediated interaction on the magnetic state of the dimers is qualitatively explained in terms of the differences in the scattering of surface electrons. At short inter-dimer distances, the ground-state configuration is determined by an interplay between exchange interactions and EF effects. These results demonstrate that external surface charging and applied EFs offer remarkable possibilities of manipulating the sign and strength of the magnetic coupling of surface supported nanoparticles.

First-principles electronic theory of non-collinear magnetic order in transition-metal nanowires

The structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM) monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of first-principles density-functional theory. The stability of long-range magnetic order along the nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a function of the 'spin-wave' vector q. First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium wave vectors q that depend on the composition and interatomic distance. The electronic and magnetic properties of these novel spin-spiral structures are discussed from a local perspective by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the spin-density distributions for representative values q. Second, we investigate the stability of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0) dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the corresponding effective exchange interactions J(ij) between the local magnetic moments μ(i) and μ(j) at atoms i and j. The effective couplings are derived by fitting a classical Heisenberg model to the ab initio magnon dispersion relations. In addition they are analyzed in the framework of general magnetic phase diagrams having arbitrary first, second, and third nearest-neighbor (NN) interactions J(ij). The effect of external electric fields (EFs) on the stability of NC magnetic order has been quantified for representative monoatomic free-standing and deposited chains. We find that an external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains, whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs. In this case the NC spiral order, which was unstable in the absence of EF, becomes the most favorable one when perpendicular fields of the order of 0.1 V/Å are applied. As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited on graphene sheets. One observes that even weak chain substrate hybridizations can modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains on graphene are correlated to charge transfers from the TMs to NN carbon atoms. Goals and limitations of this study and the resulting perspectives of future investigations are discussed.