| One of the ultimate goal of the "bottom-up" nanotechnology is to build the so-called "molecular chip", that is, all the components in the chip are based on single or several molecules, which includes molecular wires, transistors, capacitors and et al. In order to allow a considerable carrier transportation within the chip, covalent connections should be established between these components. The organic chemistry has shed light on the realization of this goal due to diverse organic reactions that can be employed for covalent linking of versatile organic building blocks. Typically, organic materials are insulators, however, the conjugated polymers (one-dimensional chains) can behave as semiconductor. Since the electronic circuits are constructed with at least two dimensions to make closed loops for the carriers. Thus, extending the polymer conjugation into two dimensions maintains huge significance in the fabrication of the "molecular chip". One of the most promising strategy for the construction of 2D polymers is the on-surface polymerization in ultra-high vacuum (UHV). The employed atomically flat single crystal surfaces act as template to confine the polymerization in 2D and also enables the characterization with mature surface science techniques such as scanning tunnelling microscopy (STM). However, the general drawback of the on-surface polymerization approach is that the size and quality of the formed 2D polymers are still in a low level and far from their practical applications. This is mostly due to the irreversibility of the formed covalent C-C bonds between the precursor monomers. Once defects are formed in the structure, they cannot be healed by postreaction. In order to avoid the formation of structural defects in advance, the precursor monomers and substrates should be carefully chosen. Up to date, the rules for design of monomers and selection of substrates revealed by the reservoir of different monomer/substrate systems still remains incomplete. Only rough rules concerning the coupling probability p= Vcoupl/(vcoupl+Vdiff) factor of the precursor/substrate system or the control of growth dynamics via a hierarchical coupling strategy are promoted. Therefore, lots of further endeavors are required to be made to explore a systematic criterion for monomer design and substrate selection and additional pretreatment that leads to the formation 2D polymer nanostructures with enhanced quality and size.in this dissertation, I have investigated how to control the nanostructure formation with specially designed precursors (including 4,4"-dibromo-m-terphenyl (DMTP), 3,5,3",5"-tetrabromo-para-terphenyl (TBrTP), and 1,3,5-tribromo-benzene (TriBB)) and substrates (including Cu(111), Cu(110), Cu(110)-(2×1)O). Combining the revealed factors that influence the formation of surface nanostructures, I have also promoted alternative criterion for monomer design in chapter 8. The main achievements in this dissertation are presented as below:1. Using DMTP precursors in a UHV-compatible variant of the Ullmann reaction, we have successfully synthesized cyclo-octadecaphenylene (hyperbenzene), a novel hexagonal macrocycle consisting of 18 phenylene units, on a Cu(111) surface. The molecules form close-packed islands with a hexagonal unit cell. Formation and structure of hyperbenzene on Cu(111) were studied with STM, X-ray photoelectron spectroscopy (XPS), and first-principles theory. The large inner diameter of the hyperbenzene molecules of 2.13 nm makes them interesting candidates for nanotroughs that can enclose metal particles or organic molecules. Besides these large hydrocarbon molecules, we also obtained novel zigzag-shaped 1D organometallic polymers consisting of Cu-bridged terphenylene units. Furthermore, similar organometallic chains connected by C-Cu-C bonds were obtained after vapor deposition of submonolayer DMTP onto Cu(110) at 300 K. However, in comparison to six orientations of the organometallic chains formed on Cu(111) surface, the chains on Cu(110) surface oriented along only two directions. In contrast to Cu(111), annealing organometallic chains on Cu(110) to 458 K results in the formation of high yield of C-C covalent bonded zigzag oligophenylene chains rather than hyperbenzene macrocycles. The formation of different products from DMTP on Cu(111) and Cu(110) surface is mainly due to the surface template effects.2. The formation, structure, and dynamics of planar organometallic macrocycles (meta-terphenyl-Cu)n and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with STM and XPS. Vapor deposition of DMTP onto Cu(111) at 300 K leads to C-Br bond scission and formation of C-Cu-C bonds, which connect neighboring meta-terphenyl (MTP) fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (> 200×200 nm2). STM sequences show the formation of ordered structures through reversible scission and reformation of the C-Cu-C bonds. The cyclic organometallic species such as the hexamer (MTP-Cu)6 may represent intermediates in the surface-confined Ullmann synthesis of hyperbenzene.3. Using STM and low-energy electron diffraction (LEED), we have found that well-defined, ordered 1D zigzag organometallic oligomeric chains with uniform lengths can be fabricated from DMTP on the Cu stripes (width>5.6 nm) of the Cu(110)-(2 × 1)O surface. In addition, the lengths of the MTP-based chains can be adjusted by controlling the widths of the Cu stripes within a certain range. When reducing the widths of Cu stripes to a range of 2.6 to 5.6 nm, organometallic macrocycles including tetramer (MTP-Cu)4, hexamer (MTP-Cu)6, and octamer (MTP-Cu)s species are formed due to the spatial confinement effect and attraction to the Cu-O chains. An overview of all formed organometallic macrocycles on the Cu stripes with different widths reveals that the origin of the formation of these macrocycles is the c/s-configured organometallic dimer (MTP)2Cu3, which was observed on the extremely narrow Cu stripe with a width of 1.5 nm.4. The selective temperature-controlled surface-assisted synthesis of covalent, organometallic, and halogen-bonded nanomeshes based on TBrTP precursor was studied with STM and XPS in ultrahigh vacuum. Vapor deposition of TBrTP onto Cu(111) at 90 K leads to a highly ordered organic monolayer stabilized by Br…Br and Br H intermolecular bonds between the intact T-type assembled TBrTP molecules. Annealing the monolayer to 300 K results in C-Br bond scission and the formation of C-Cu-C bonds, which link adjacent para-terphenyl fragments such that stable organometallic frameworks are formed. Pore sizes in organometallic framework correlate with the number of enclosed Br adatoms, which presumably play a size-determining role during the process of the pore formation. Larger islands of the organometallic framework are obtained by deposition of TBrTP onto the copper surface held at 460 K. A further increase in sample temperature to 570 K during deposition gives rise to the formation of covalent organic frameworks with pores of tetragonal and trigonal symmetry. Comparison of the three different bonding regimes reveals that the degree of long-range order correlates inversely with the strength of the bonds between the building blocks.5. The temperature-controlled surface-assisted synthesis of halogen bonded, organometallic, and covalent nanostructures based on TriBB was studied with STM and XPS in ultrahigh vacuum. Vapor deposition of TriBB onto a Cu(111) surface held at 90 K leads to the formation of large domains of a honeycomb-like organic monolayer structure stabilized by triangular nodes with Br…Br intermolecular bonds. Upon annealing the organic monolayer to 140 K, a new hexagonal close-packed structure with intact TriBB molecules connected by Cu adatoms is formed. Further warming up the sample to 300 K gives rise to the scission of C-Br bonds and formation of C-Cu-C bonds between phenyl fragments such that stable dendritic organometallic networks are formed. Larger islands of organometallic networks are obtained by maintaining the temperature of Cu(111) at 420 K during deposition of TriBB. Simultaneously, large islands of Br atoms are formed around the organometallic networks. Annealing the more extended organometallic network (prepared at 420 K) to 520 K leads to the formation of a branched covalent organic framework (COF) which comprises structural elements of porous graphene and is surrounded by Br islands. These organometallic networks and COFs appear as small dendritic and branched domains, most likely due to the steric influence exerted by the Br islands... |
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