Unconventional and Exotic Magnetism in Carbon-Based Structures and Related Materials

The detailed analysis of the problem of possible magnetic behavior of the carbon-based structures was fulfilled to elucidate and resolve (at least partially) some unclear issues. It was the purpose of the present paper to look somewhat more critically into some conjectures which have been made and to the peculiar and contradictory experimental results in this rather indistinct and disputable field. Firstly the basic physics of magnetism was briefly addressed. Then a few basic questions were thoroughly analyzed and critically reconsidered to elucidate the possible relevant mechanism (if any) which may be responsible for observed peculiarities of the "magnetic" behavior in these systems. The arguments supporting the existence of the intrinsic magnetism in carbon-based materials, including pure graphene were analyzed critically. It was concluded that recently published works have shown clearly that the results of the previous studies, where the "ferromagnetism" was detected in pure graphene, were incorrect. Rather, graphene is strongly diamagnetic, similar to graphite. Thus the possible traces of a quasi-magnetic behavior which some authors observed in their samples may be attributed rather to induced magnetism due to the impurities, defects, etc. On the basis of the present analysis the conclusion was made that the thorough and detailed experimental studies of these problems only may shed light on the very complicated problem of the magnetism of carbon-based materials. Lastly the peculiarities of the magnetic behavior of some related materials and the trends for future developments were mentioned.Comment: 40 pages, 5 tables, 221 Reference

Metallic magnetic nanoparticles

In this paper, we reviewed some relevant aspects of the magnetic properties of metallic nanoparticles with small size ( below 4 nm), covering the size effects in nanoparticles of magnetic materials, as well as the appearance of magnetism at the nanoscale in materials that are nonferromagnetic in bulk. These results are distributed along the text that has been organized around three important items: fundamental magnetic properties, different fabrication procedures, and characterization techniques. A general introduction and some experimental results recently obtained in Pd and Au nanoparticles have also been included. Finally, the more promising applications of magnetic nanoparticles in biomedicine are indicated. Special care was taken to complete the literature available on the subject

Synthesis of a mesoscale ordered 2D-conjugated polymer with semiconducting properties

2D materials with high charge carrier mobility and tunable electronic band gaps have attracted intense research effort for their potential use as active components in nanoelectronics. 2D-conjugated polymers (2DCP) constitute a promising sub-class due to the fact that the electronic band structure can be manipulated by varying the molecular building blocks, while at the same time preserving the key features of 2D materials such as Dirac cones and high charge mobility. The major challenge for their use in technological applications is to fabricate mesoscale ordered 2DCP networks since current synthetic routes yield only small domains with a high density of defects. Here we demonstrate the synthesis of a mesoscale ordered 2DCP with semiconducting properties and Dirac cone structures via Ullmann coupling on Au(111). This material has been obtained by combining rigid azatriangulene precursors and a hot dosing approach which favours molecular diffusion and reduces the formation of voids in the network. These results open opportunities for the synthesis of 2DCP Dirac cone materials and their integration into devices.Comment: 21 pages, 3 figure

Molecular Networks Through Surface-Mediated Reactions - From Hydrogen Bonds to Covalent Links

This thesis deals with adsorption, self-assembly, and surface reactions of organic molecules on solid substrates, with the aim to fabricate higher hierarchical twodimensional (2D) structures. It is of genuine interest in materials science to develop strategies and methods for reproducible growth of extended molecular assemblies with specific and desired chemical, physical and functional properties. The experimental technique used was Scanning Tunneling Microscopy (STM) - an outstanding method to gain real space information of the atomic-scale realm of adsorbates on crystalline surfaces. The investigated systems are characterized by a complex interplay between adsorbateadsorbate interactions and adsorbate-substrate interactions. In one series of experiments this could be illustrated through self-assembly of hydrogen bonded heteromeric molecular networks on a chemically relatively inert graphite substrate. In this case, van-der-Waals forces between adsorbate and substrate have to be balanced with intermolecular hydrogen bonds in concert with weaker van-der-Waals forces. Since the magnitude of van-der-Waals forces between adsorbates and substrates correlates with the contact area, this type of interaction becomes more dominant for larger molecules. By stronger interactions which do not depend on molecule size, it was also possible to grow isotopological molecular networks, i.e. networks following a similar building plan. By varying for instance the length of aliphatic spacers, supramolecular structures with tuneable lattice parameter could be formed. Studies of organic molecules on chemically more active metal substrates show that more complex processes can be involved. In particular the concept of reactivity and surface-catalyzed reactions are discussed and illustrated by an intuitive example. It is demonstrated that strong molecule-substrate interaction can induce unimolecular reactions such as deprotonation of molecules or more generally dissociation of intramolecular bonds. This interaction strength, thus substrate reactivity is highly influenced by a variety of factors which include material, crystallographic surface orientation, and temperature. Further more the importance of so-called active sites on crystal surfaces, i.e. special sites with significantly increased interaction strength, is taken into account and exemplified with experimental results. Exploiting these fundamental principles, C-Br bond scission of brominated aromatic compounds was demonstrated upon adsorption on reactive substrates and followed by successful incorporation in covalently bonded networks. However, irreversibility of covalent bonds prevents similar control and error correction mechanisms over the system as compared to hydrogen bonded networks. A high defect density and a low degree of ordering is the consequence for the resulting 2D structures. In a final set of experiments aromatic thiol molecules could be assembled into highly or dered structures via metal-coordination bonds. The 2D gas of freely diffusing adatoms of a copper surface was thermally excited to finally transform a trithiolate precursor structure into metal-coordination networks via Cu-S metal coordination bonds. Two different coordination geometries were observed giving rise to the formation of two morphologically distinct phases. These studies revealed the impact of the adatom gas for surface reactivity and chemistry of metals

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

Polymerization of functional organics into covalently cross-linked nanostructures via bottom-up approach on solid surfaces has attracted tremendous interest recently, due to its appealing potentials in fabricating novel and artificial low dimensional nanomaterials. While there are various synthetic approaches being proposed and explored, this paper reviews the recent progress of on-surface coupling strategies towards the synthesis of low dimensional nanostructures ranging from 1D nanowire to 2D network and describes their advantages and drawbacks during on-surface process and phase transformations, for example, from molecular self-assembly to on-surface polymerization. Specifically, Ullmann reaction is discussed in detail and the mechanism governing nanostructures’ transforming effect by surface treatment is exploited. In the end, it is summarized that the hierarchical polymerization combined with Ullmann coupling makes it possible to realize the selection of different synthetic pathways and phase transformations and obtain novel organometallic nanowire with metalorganic bonding

Metal–organic framework growth at functional interfaces: thin films and composites for diverse applications

Porous metal–organic frameworks (MOFs) are highly ordered crystalline materials prepared by the self-assembly of metal ions with organic linkers to yield low density network structures of diverse topology. MOFs have attracted considerable attention over the last decade due to their facile preparation, tunable pore metrics and the ease of functionalisation of their internal surfaces, such that designer frameworks with exceptional properties for application in gas-storage, separation of small molecules, heterogeneous catalysis and drug delivery are becoming commonplace. For any material to find practical utility however, there is a need for processing and formulation into application-specific configurations. One way to do this is to prepare composite materials where the MOF is supported on a planar substrate or some other shaped body through interaction with functional groups at the support interface. This is a rapidly developing research area, and this review provides an overview of the diverse MOF composite materials prepared up to now, organised by interface type. The importance of the interface is explored within each section and while the overall emphasis is on applications of the composites, coatings and MOF-based devices, the most widely-used and successful synthetic strategies for composite formation are also presented