Session T32: Focus Session: Self Assembly on Novel Templates

Directed assembly of diblock copolymers as a means of achieving functional structures

Block copolymers have tremendous potential for patterning and assembling functional material architectures at the nanoscale because their constituent, compositionally distinct blocks spontaneously form ordered structures (domains) with dimensions of 3 to 50 nm. The dimensions and shapes of the structures in the bulk represent a delicate balance of interfacial energy between blocks and the configurational entropy of the polymer chains. Unfortunately self-assembly results in the creation of features riddled with imperfection and non-uniformities. Intervention is required to overcome trapping of low energy defects. In our primary research approach, block copolymer films are equilibrated in the presence of chemical surface patterns that 1) have dimensions similar in size to individual domains, and 2) illicit differing interfacial interactions between the surface and the blocks of the copolymer film. By manipulating the geometry of the chemical patterns, and the magnitude of interfacial interactions, we can direct the block copolymer to assemble into two and three-dimensional architectures that have a high degree of perfection and uniformity of dimensions, and at resolution beyond that of traditional lithographic materials and processes. Specific applications of the assemblies will be discussed, including integrated circuits, patterned media, and electronic and photonic devices.   

Growth mechanism and control of pentacene thin film on chemically heterogeneous surface

Film growth on the substrate, which exhibits heterogeneity within the plane, is becoming increasingly important for the construction of future nanodevices. By means of photoemission electron microscopy (PEEM), we observed the evolution of pentacene ultrathin films grown on SiO$_{2}$ substrates with pre-deposited Au microelectrodes for organic thin film transistor fabrication. [1] The results clearly indicate that pentacene nucleation is suppressed around the periphery of bare Au electrodes, leaving grooves at the boundary. In the case of thiol-treated Au electrodes, however, the nucleation occurs even at the boundary, leaving no grooves there in successive growth. This improvement of connectivity between the pentacene film on a channel and that on a Au electrode can be ascribed to the reduction in mass flow of pentacene molecules from the channel to the electrode driven by heterogeneous surface energy. [1] Tsuruma \textit{et al}., Adv. Mater., published online.   

Self-assembly of Poly(3-hexylthiophene) with nanostructured inorganic semiconductors for applications in photovoltaics: a computational study

A number of functional materials based on blends of polythiophene derivatives and inorganic materials have been investigated in recent years for optoelectronic applications. For example, blends of Poly(3-hexylthiophene) (P3HT) and C60 are particularly efficient as donor-acceptor systems for bulk heterojunction organic photovoltaic solar cells due to high interfacial area and ultrafast photoinduced electron transfer. The increased availability of novel forms of nanostructured semiconductors with different surface functionalizations motivates the investigation of self-assembled P3HT with quantum dots, nanowires, thin films and other nanostructured semiconductors for applications in photovoltaics. Results based on classical molecular dynamics simulations of the self-assembly of P3HT with some of these materials will be presented. Density functional theory calculations are utilized to predict the dependence of the electronic structure of the self-assembled materials on interfacial surface chemistry and nanoscale curvature effects.   

Covalent assembly of molecular building blocks by ``on-surface-synthesis''

A key challenge in the field of molecular electronics is the bottom-up construction of stable molecular networks with pre-defined topology and shape, whereas covalent bonds are desired due to stability and charge transport requirements. We have developed the method of ``on-surface-synthesis,'' which allows the formation of covalent bonds by controlling the synthetic process directly on the surface. This technique has been used successfully for the controlled formation of covalently bound networks of porphyrin molecules on a gold surface, which were then characterized by low temperature scanning tunnelling microscopy (STM). The covalent character of the intermolecular bonds has been proven by manipulation and spectroscopy and is in agreement with calculations. We demonstrate that the dimensions and shape of these nanostructures can be precisely engineered, because the resulting nanostructures directly reflect the chemical structure of the individual building blocks, which makes this method highly interesting for functional molecules. Very recently, we have deposited ultrathin NaCl films on the metallic surface in order to achieve the interesting hybrid configuration of molecular wires on insulating films.   

Regular metal cluster arrays on graphene/Ir(111)

Clusters represent a distinct state of matter. Due to their size, they exhibit a broad range of new properties, such as magnetism for normally non magnetic materials, a highly increased catalytic activity, or size effects in the electronic spectrum. These properties are of relevance for information technology or energy efficiency. The controlled growth of clusters on a surface is still a challenge. We have grown regular arrays of various metal clusters (Ir, Pt, W, Re, Au, IrFe, IrAu) on a self organized graphene/Ir(111) moir\'{e} template by physical vapor deposition in ultra high vacuum and studied them with scanning tunneling microscopy. The template epitaxial graphene grown by the thermal decomposition of ethene (C$_2$H$_4$) on Ir(111). This yields a moir\'{e} superstructure with a periodicity of 25.3\AA. Upon physical vapor deposition of metals, clusters nucleate in the cells of moir\'{e} superstructure. The clusters are perfectly ordered in a hexagonal array with an nearest neighbour distance given by the 25.3{\AA} periodicity of the moir\'e. Clusters bind to the graphene layer via a local rehybridization of the carbon from sp$^2$ to sp$^3$ underneath the cluster. The average number of atoms in a cluster can be tuned by the amount of deposited metal, which is for Ir from 4.5 -- 130 atoms. Despite their small size, the clusters possess a high temperature stability. While small Au clusters are only stable at temperatures around 100\,K Ir, clusters are stable above 500\,K. The decay of the areal density of clusters can be described by an Boltzmann law with a very low effective attempt frequency below 1000\,Hz and energy barriers between 0.4 -- 0.8\,eV. The coalescence occurs through two different types of atomic process: complete coalescence for two small clusters results in one cluster occupying a single moir\'{e} cell, while for large clusters a sintering takes place. The resulting cluster spans multiple moir\'{e} cells. For some materials, which do not form ordered cluster arrays at room temperature, the formation of regular cluster superlattices can be enhanced by a lower substrate temperature. This method is demonstrated for Au and Re. The obtained Re clusters remain stable at room temperature. A second method, to form clusters containing materials, which do not form clusters at room temperature which is demonstrated for Fe and Au at room temperature. We seeded the superlattice with small Ir clusters and in a second step used these well ordered array of Ir clusters to anchor additional Fe or Au. The graphene/Ir(111) moir\'{e} is a versatile template for the growth of regular metal cluster arrays.   

Metal ad-cluster morphology vs. size on graphene/Ir(111)

The chemical activity of Au clusters adsorbed on oxides depends on how many Au atoms constitute a cluster, and on the clusters' morphology and charge [1]. This makes it interesting to study the systematics of the d-band metal clusters recently discovered to grow in regular arrays on a graphene/Ir(111) template [2]. Local density functional optimization of Ir clusters on this template accurately accounts for the observed transition from 2- to 3-dimensional at a cluster size of N=26 atoms [3]. New calculations for Au clusters imply that the transition to a 3-dimensional morphology occurs for smaller N. Wetting is the issue; it is governed by the relative strength of the ad-metal bonds with itself, as against its bonds with a buckled graphene sheet. [1] H. H\"{a}kkinen, S. Abbet, A. Sanchez, U. Heiz, and U. Landman, Angew. Chem. Int. Ed. 42, 1297(2003). [2] A. T. N'Diaye, T. Gerber, C. Busse, J. Mysilvecek, J. Coraux, T. Michely, New J. of Phys. 11, 103045(2009). [3] P. J. Feibelman, Phys. Rev. B80, 085412(2009).   

SAMs diversity via molecular conformation selection of functionalized C60

Structured organic thin films are relevant for many emerging technologies like optoelectronic molecular devices, energy conversion, and biosensors. The structural arrangement of molecules forming self-assembled monolayers (SAMs) is a consequence of molecular shape, and the competing molecule-substrate and intermolecular interactions. We will present a STM/DFT study of the self-assembly of C60 functionalized with alkyl chains of various lengths (F-C60) on compact metal surfaces [1] and graphene layers. We find that the molecule surface interaction drives the selection of particular molecular conformations resulting in diverse SAM structures as a function of the alkyl chain lengths. The SAM structures are ranging from zigzag to linear arrays of C60 cages. These results show that C60s can be assembled in 2D and non-compact molecular arrays with a unit cell symmetry and size controllable via appropriate chemical functionalization and surface selection. \\[4pt] [1] B. Diaconescu, et al., Phys. Rev. Lett., 102, 056102, 2009.   

C$_{60}$ on nanostructured Nb-doped SrTiO$_{3}$(001) surfaces

Nanostructured SrTiO$_{3}$(001) surfaces were carefully calibrated with respect to Si(111)-$(7\times 7)$ surface using STM. The surface patterns for observed nanostructures were assigned. Sequential C$_{60}$ deposition onto these nanostructured templates reveals distinct growth modes, including discrete small C$_{60}$ islands on $c(4\times 2)$ reconstruction surface, parallel one-dimensional C$_{60}$ chains on $(6\times 2)$dilines, C$_{60}$ double-chains on $(8\times 2)$trilines, epitaxial C$_{60}$ close packed adlayers over $(11\times 2)$tetralines, and two-dimensional ordered C$_{60}$ dimer arrays on $(7\times 6)$waffles. These structural diversities mainly stem from the relatively strong adsorbate-substrate interactions as well as the surface topography demands. The nanostructured oxide surfaces as templates are thus with great potentials in the molecular nanoarchitecture.   

Nano-ice on Boron Nitride Nanomesh: accessing proton disorder

The adsorption behavior of water and the concomitant proton arrangement have been investigated on a hexagonal boron nitride nanomesh ( $h$-BN/Rh(111) template using variable temperature scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations. Below 52 K, two distinct phases self-assemble within the 3.2 nm unit cell of the nanomesh that consists of ``holes'' and ``wires.'' In the 2 nm holes, an ordered and dense phase of nano-ice crystals with about 40 molecules is found. They form a hexagonal bilayer structure and obey the ice rules. On the 1-nm-wide wires, a low-density two-dimensional (2D) gas phase forms, which is characterized by contrast modulations and streaky noise in the STM images. Furthermore, by means of tunneling barrier height spectroscopy (dI/dz) measurements and the simulation of the electrostatic potential above the nano-ice clusters, the individual hydrogen positions in nano-ice clusters have been revealed. Based on the agreement between experiment and theory, the results give new insight into the self-assembly process of water and also indicate that the boron nitride nanomesh template is a good candidate to build artificial structures with new functionalities.\footnote{Boron Nitride Nanomesh: A template for Nano-ice, ArXiv: 090875.}   

Encapsulation of Graphene and Nanoparticles in Phospholipid Membranes

We demonstrate by coarse-grained molecular dynamics simulations that graphene sheets and surfactant coated nanoparticle clusters could be hosted in the hydrophobic interior of membranes formed by amphiphilic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phospholipids. These hybrid graphene-membrane superstructures might be prepared by forming hydrated micelles of individual graphene flakes or nanoparticles clusters covered by phospholipids, which are later fused with the membranes. We study the effect of interparticle coupling on the organization of the nanoparticles inside lipid bilayer membranes and investigate how small surfactant coated nanorods and nanodiscs are self-assembled over there. The composite systems might be used in the development of biosensors and bioelectronic materials.