Fabrication And Physical Properties Of Novel Graphene And FeTPP Molecular Nanostructures

Accelerating miniaturization of electronic devices is crucial for information technology.Both graphene nanostructures and organic molecules emerge as ideal candidates for design and construction of feature practical nano-devices.Graphene nanostructures,that is,low-dimensional sp²carbon-based structures,are predicted to exhibit unique properties beyond pristine graphene.It is well-known that the properties of graphene nanostructures rely vastly on their shape,size,edge config-urations,defects and adatoms et.al.Therefore,synthesis and characterization of high-quality graphene nanostructures with well-defined atomic structures including uniform shape,edge configurations,structural defects and adsorb structures,is one of crucial issues in the field of graphene and two-dimensional materials.Meanwhile,organic molecules such as porphyrin molecules have an indispensable role in control-ling a wide range of functionalities due to their great chemical stability,distinctive optical and electrical properties,making them equally appreciated for technological applications as well as for fundamental studies.The thesis discussed the synthesis of several high-quality graphene nanostruc-tures with well-defined atomic structures on Ir?111?,HOPG?0001?,Ru?0001?via using both bottom-up and top-down methods.Utilizing low-temperature scan-ning tunneling microscopy/spectroscopy?LT-STM/STS?,which allow to resolve the atomic structures and to measure corresponding electronic properties at atomic scales,the atomic configurations and unique electronic properties of as-fabricated nanostructures are studied.In addition,self-assemble structures,electronic states and ultra-low energy vibrational modes of iron?II?[tetra-?pentafluorophenyl?]porphyrin(FeF₂₀TPP)on Au?111?were also studied.Finally,bulk Ni tips suitable for spin- polarized STM which are regarded as powerful tools for further study of spin-polarizated states that exist on magnetic molecules and graphene defects,have been successfully fabricated.The thesis has five parts as following:1.Si intercalation of graphene nanoislands?GNIs?grown on Ir?111?and edge states at well-defined zigzag edges after intercalation.Spin-polarized edge states have been predicted to exist on graphene nanostructures with well-defined zigzag edges,making them promising candidates for practical applications toward graphene-based spintronics.Studying the edge states on GNIs with various shapes and sizes can lead to understanding of the interactions among edge states at var-ious finite edges,which can be helpful in the design and construction of practical graphene-based spintronics.However,the strong coupling with these metal sub-strates results in the absence of edge states.Here,by inserting a single layer of Si atoms between the GNIs with zigzag edges and the supporting Ir substrate,the edge states of GNIs with zigzag edges on Ir?111?can be successfully observed and studied.Extensive measurements and calculations show that the GNIs are effec-tively decoupled from the Ir substrate by the intercalated Si layer,leading to the appearance of the edge states that were originally suppressed by the graphene-metal interaction.The edge states gradually shift to the Fermi level with increasing lateral sizes of the GNIs.We also demonstrate that the electronic and magnetic properties of graphene nanostructures can be tuned by their geometries,which might be useful for the fabrication of graphene-based electronic and spintronic devices.2.Atomic-precise manipulations of disk-shaped GNIs.Manipulations of GNIs emerge as crucial important ways to construct complex graphene nanostructures with novel properties.Here,two efficient ways are presented.One is coalescence of GNIs at high temperature:at first,graphene nanopores were fabricated through cycles of hydrogen atoms etching and thermal post-annealing at about 850^?C.Then,the as-fabricated pores were transformed into graphene nanoribbons after thermal post-annealing at about 1000^?C.Finally,ribbons were torn into small pieces and merged into disk-shaped GNIs after thermal post-annealing at about 1200^?C.LT-STM/STS combined with calculations based on DFT show that 5-7-5-7 GBs with a configuration of a periodic alternation of pentagons and heptagons are widely formed on the as-grown GNIs.Furthermore,the two domains abutting GBs on GNIs are found to be stacked differently with graphite substrate,that is,AB and twist stacking.The carrier scattering behavior on the two domains abutting 5-7-5-7GBs are found to depend on the stacking mode.Another way of manipulation is using STM tip to manipulate the as-fabricated GNIs on HOPG at 4.2 K:at first,we demonstrate that GNIs can be easily displaced and rotated on HOPG.Then,by putting a STM tip closer to edge of GNIs and moving forward and backward,the GNF can be folded and unfolded.The atomic structures of folded edges can be precisely controlled by altering the folding axes of a single island.STS measurements found that the electronic properties of folded edges can be tuned by their detailed atomic structures.The coalescence and STM tip manipulations of GNIs provide new ways to design and construct complex graphene nanostructures for practical applications.3.Synthesis of crystalline partially hydrogenated graphene with anisotropic properties on Ru?0001?.It is generally thought that hydrogen atoms can chemisorb on graphene in a perfect periodic manner,forming new crystalline two-dimensional?2D?materials that exhibit unique electronic properties beyond pristine graphene,such as graphane,graphone and 2D C_xH_y.So far,only samples with small patches of order-arranged hydrogen adatoms can only be produced.Here,atomically re-solved scanning tunneling microscope?STM?images demonstrate the fabrication of one-third hydrogenated graphene with a prefect ordered structure over hundreds of nanometers.Moreover,we show that,this crystalline one-third-hydrogenated graphene is decoupled from the substrate and structurally anisotropic.Calcula-tions show that the structural anisotropy of one-third hydrogenation results in an anisotropic electronic structure.This work directly provides atomic-scale evidences that new two-dimensional crystals with designed electronic properties can be real-ized by attaching other atoms and molecules onto graphene.4.Ultra-low-energy?<20 meV?vibraitons of FeF₂₀TPP on Au?111?.The quantum efficiency or the rate of conversion of incident photon to free electron in photosynthesis is known to be extremely high.It has long been thought that the origin of this efficiency are molecular vibrations leading to a very fast sepa-ration of electrons and holes within the involved molecules.However,molecular vibrations are commonly in the range above 100 meV,which is too high for exci-tations in an ambient environment.Here,we analyze experimental spectra of single organic molecules on metal surfaces at 4.2 K,which often exhibit a pronounced dip.We show that measurements on iron?II?[tetra-?pentafluorophenyl?]porphyrin resolve this single dip at 4.2 K into a series of step-shaped inelastic excitations at 0.4 K.Via extensive spectral maps under applied magnetic fields and corresponding theoreti-cal analysis,the dip is due to ultra-low-energy vibrations of the molecular frame,typically in the range below 20 meV.The result indicates that ultra-low energy vi-brations in organic molecules are much more common than currently thought,and may be all-pervasive for molecules above a certain size.5.Fabrication of Ni tips for spin-polarized scanning tunneling microscopy?SP-STM?.SP-STM is a state-of-the-art technique that allows us to study the spin texture of low-dimensional nanostructures at an atomic level.An ideal SP-STM tip should offer simultaneously a high spatial resolution down to the atomic level and a high spin polarization with tunable spin orientation and negligible magnetic stray field.Here,an efficient method to fabricate Ni tips for SP-STM via electrochemical etching of Ni wires in a constant-current mode.Instead of applying constant voltages to trigger the electrochemical etching of Ni wires in previous reports,here a constant current is applied,which ensures a stable etching process and favors a high yield of Ni tips with good quality.The prepared Ni tips have been applied to obtain atomic resolution images on various surfaces in conventional STM measurements and to resolve magnetic-state-dependent contrast of Co islands grown on a Cu?111?surface in SP-STM experiments.This work provides a powerful tool for studying spin-polarized properties of graphene nanostructures and organic molecules in the feature.