Since it was first discovered in 2004, graphene has been a rapidly rising star on the horizon of materials science and condensed-matter physics. Unique two-dimensional structure and excellent electrical, optical, mechanical, thermal characteristics make graphene a broad prospect of application in theoretical researches, nanoelectronic devices, energy storages, transparent conductive films, sensors and composite materials. Since the preparation of material is the premise and basis for the systematic study of its properties and applications, we study on the synthesis and transfer of graphene grown by chemical vapor deposition in this paper. The main results are summarized as follows: 1. Few-layer graphene films on Ni films and Cu foils have been successfully synthesised by the self-built CVD equipment. 2. The correlation between the annealing time, surface treatment, gas flow and the quality of graphene has been studied. The results indicate that the size of Ni crystalline grain and the area of fewer layers graphene increase with the annealing time, but the amount of hydrogen blisters also increases. On the other hand, surface treatment of Cu foil can improve the crystallization of graphene, and gas flow rate optimizing makes the graphene film more homogeneous, more orderly and fewer layers. Finally, we obtained homogeneous graphene films under two layers on the condition of CH4 : H2 = 200 : 0 sccm on surface treated Cu foil. 3. The graphene films synthesised on Ni films and Cu foils have been successfully transferred to SiO2, glasses, surface treated-glasses and PET films by self-designed method. 4. The correlation between the pressure duration, heating temperature, target substrates’ surface treatment and the transfer percentage has been reserched, finding that longer pressure duration, higher heating temperature and surface treatment can improve the transfer percentage of graphene. 5. The graphene films synthesised on Ni films exhibit a high optical transmittance of more than 90%, which is superior to commercial transparent electrodes such as indium tin oxides.
In this thesis, we have prepared n-layer graphenes through several approaches and studied Raman features of graphenes with strain, deposited with gold particles or covered by crystal violet molecules. The thesis includes the following parts:
er of carbon atoms arranged in honeycomb lattice, is a newly discovered allotrope form of carbon after fullerene and carbon nanotubes. As a strictly 2D atomic crystal, graphene possess peculiar electronic structures and on this basis holds fundamental importance for condensed-matter physics research. In the meanwhile, the remarkable material properties of graphene, ranging from ultra-high carrier nobilities to extreme mechanical stiffness, also lend itself to a plethora of potential applications such as post-silicon nanoelectronics, optoelectronics, reinforced nanocomposite materials, energy storage materials and so on.
Graphene, a one-atom-layer of sp2 carbon atoms densely packed into a honeycomb lattice, is the first discovered, truly two-dimensional material. Owing to its unique physical and chemical properties, graphene has become a “star” material and triggered tremendous research interests in such fields as condensed matter physics, material sciences and chemistry, etc, since it was discovered in 2004. In the early stage, most research attention was focused on its basic physical properties; thereafter, with accumulated and deeper research, more interests were cast on its material properties, such as its huge application prospects in nanoelectronics, transparent conducting film, physical, chemical and biological sensors and energy storage material.
This dissertation dealt with in-situ synthesis and characterization of PET/graphene nanocomposites using a TiO2/SiO2 sol catalyst supported on the surface of graphite oxide (GO) nanosheets. A preliminary study on melt spinning of the PET/graphene nanocomposites was also performed. The main contents of the research are listed as follows:1. GO was synthesized through the modified Hummers method. GO has quantities reactive oxygen-containing functional groups, which can react with various compounds to form covalent or non-covalent attachment resulting in chemically modified GO, that will enable tailoring or imparting desired surface properties. Two approaches were used to decorate the surface of GO. The first one is using organic modifiers with short alkyl chains, and the second one is grafting polymer chains to the GO surface. It has been found that GO can be directly used as a polymerization initiator for vinyl monomers. Poly(vinyl pyrrolidone) (PVP) can be readily grafted to the surface of GO sheets by using GO as the initiator, which was confirmed by FTIR, Raman, 13C-NMR, Elementary Analysis (EA), XPS and so on. Besides, GO was found to be partly reduced during the polymerization of NVP. As for different monomers, GO showed different initiating efficiency. Poly(methyl methacrylate)/GO nanocomposite was form meanwhile surface functional and polymer with high molecular weight can be obtained by GO initating the polymerization of methyl methacrylate.
Molecular beam epitaxy (MBE) plays an important role in the preparation of nanomaterials, such as ultrathin films, nanowires and superlattices; scanning tunneling microscope/spectroscopy (STM/STS) is among the most important experimental tools in the study of condensed matter physics and nanoscience. In this thesis, by using MBE and LT-STM/STS, we have studied several frontier research projects in condensed matter physics, such as graphene, low dimensional superconductivity and topological insulators. The thesis consists of three parts: (1) Graphene, a recently discovered two dimensional material, has attracted great attention since its successful preparation in 2004. Graphene can be applied in nanoelectronic devices due to its high carrier mobility. The carrier concentration and type of graphene can be tuned by adsorption of metal atoms and molecules. In this part, the mono-, bi- and multi-layer graphene were prepared by thermal decomposition of 6H-SiC(0001), then F16CuPc molecules were deposited onto them. The interaction between F16CuPc and graphene is van der Waals type combined with π-orbital hybridization and charge-transfer. The electronic states of graphene are modified via introducing extra unoccupied states, showing a new state slightly above the Fermi energy. Our study provides very useful information on developing graphene-based electronic devices.(2) Low dimensional superconductivity has been the subject of intense interest for decades. In this part, we prepared ultrathin Pb islands/films on Graphene/6H-SiC(0001) and Pb-SIC/Si(111) surfaces by MBE and studied their superconducting properties by STM/STS. The superconducting transition temperatures (Tc) of 3-ML Pb films grown on Graphene/6H-SiC(0001) and Pb-SIC/Si(111) are revealed to be 5.8 K and 6.9 K, respectively, indicating that the Cooper pairing is strongly affected by the interface. Although the superconducting gap and vortex were observed on both 2-ML and 3-ML Pb films grown on Pb-SIC/Si(111) by STM, a superconductor (3-ML)–insulator (2-ML)transition was observed by transport measurement, which possibly indicates a BKT transition of superconductivity in two dimensional (2D) system. (3)Topological insulator, a new state of quantum matter, is currently one of the hot topics in condensed matter physics. Topological insulators (Bi2Te3, Bi2Se3 and Sb2Te3) doped with Cu, Fe, Mn, and Cr exhibit some unusual properties, for example, Bi2Se3 can be made into a superconductor by Cu intercalation. We prepared high quality Bi2Se3 films and studied the structural and electronic properties of Bi2Se3 doped with Cu and Cr atoms. The Cu atoms can form interstitial and intercalated defects, while all Cu atoms form CuBiSe2 compound by annealing at 200℃. The Cr atoms substitute Bi atoms and form substituted defects. The Cu and Cr atoms behave as donors, leading to n-type doping. The surface states of Bi2Se3 are remained in Cu doped films while are destroyed in Cr doped films, a gap is opened at the Dirac point. The Landau levels of surface states of Bi2Se3 are also destroyed in samples with high Cu- and Cr-induced defect density. In order to search for Majorana fermions, we studied the interface between Bi2Se3 and FeSe, finding the atomically sharp interface can be formed between topological insulators and layered superconductors with inert surface.
The newly discovered graphene shows great potential for applications because of its exciting and unique properties, therefore, it is becoming demanding to fabricate large scale graphene layers and to study the problems appeared in the devices. This study explores the physical properties of the epitaxial graphene grown on metal surfaces. The chemical vapor deposition (CVD) method to get large scale and high quality graphene is introduced first, and the effects of the substrate to the epitaxial graphene are particularly explored. Meanwhile, the construction work on scanning tunneling microscope is also briefly introduced. First, the formation of graphene based quantum dots on Ru(0001) made from about 90 carbon atoms is evidenced by means of scanning tunneling spectroscopy. The local tunneling conductance peaks at distinct tunneling voltages are explained with a quantum well resonance and field emission resonances. The QWR on the hills of the corrugated graphene is very strong and has a distinctly lower energy (≈500meV) compared to that in the valleys. Our observations demonstrate that graphene on ruthenium constitutes an ordered array of quantum dots, with both lateral and vertical confinement. The structures are small enough that they are candidates for single electron physics at room temperature and they bridge zero-dimensional molecule like and two-dimensional graphene. Second, the thesis demonstrates the selective adsorption and the formation of ordered molecular arrays of iron phthalocyanine and pentacene molecules of different structural symmetries on the graphene/Ru(0001) templates. With in-depth investigations of the molecular adsorption and assembly processes we reveal the existence of site-specific, lateral electric dipoles (or lateral electric fields) in the epitaxial graphene monolayers and the capability of the dipoles in directing and driving the molecular adsorption and assembly. We are also able to show that the lateral dipoles originate from the inhomogeneous distribution of charge due to the epitaxial constraint of graphene on Ru (0001) surface and that the dipole-driven assembly mechanism is rather general and applicable to similar molecular systems on triangular graphene monolayers formed on other transition metal surfaces, for example, Ir(111).The third part reports on measurements of phonon excitations on monolayer graphene grown on Ru(0001) with high spatial resolution. The observed d2I/dV2 spectra and maps features are in good agreement with the vibrational density of states calculated from first principle caculations. We observe a significant spatial variation of the intensity of the out-of-plane acoustic mode at 16 meV in the d2I/dV2map over a unit cell of the moiré pattern, in contrast to a rather uniform feature of the transverse acoustic mode at 81 meV. This mode-dependent spatial localization of the surface phonon of graphene is due to the different influence of the graphene-substrate interaction upon the out-of-plane and in-plane acoustic phonon modes. Our results point to the importance of interfacial bonding on phonon properties and, consequently, electronic and thermal transport properties of graphene based devices.The forth part briefly introduces the work on manipulation of single molecular rotator on Au(111) surface. The manipulations include STM tip, tunneling current, molecular structure, etc.The last part introduces the construction of a scanning tunneling microscopy. Detailed description will cover the electronic controller, scanner head, and other vacuum parts. The whole project has been finished and test successfully.
An efficient order N real space Kubo approach is developed for the calculation of the thermal conductivity of complex disordered materials. The method, which is based on the Chebyshev polynomials expansion of an evolution operator and the Lanczos tri-diagonalisation scheme, efficiently treats the propagation of phonon wave packets in real space and calculate the phonon diffusion coefficients. The phonon transport mean free path and the thermal conductance can be determined from the diffusion coefficients. These quantities can be extracted simultaneously for all different frequency modes, which is another advantage in comparison to the Green’s function-based Landauer approach. Additionally, multiple scattering phenomena, which is related to weak or strong phonon localization, can be followed through the time dependence of the diffusion coefficient deep into the diffusive regime. The accuracy of our computational scheme is demonstrated by comparing the calculated phonon mean free paths in isotope-disordered carbon nanotubes with Landauer simulations and analytical results. One then illustrate its efficient upscalability by exploring the phonon mean free paths and the thermal conductance features in edge disordered graphene nanoribbons with lateral sizes of several tens of nanometers, definitely beyond the reach of other numerical techniques. The phonon mean free paths of armchair nanoribbons are found to be smaller than those of zigzag nanoribbons for intermediate frequency modes dominating the thermal conductance at low temperatures. The thermal conductance can be reduced by a factor of 10. We then study two-dimensional graphene-like structures. The first considered structure is the 13C isotope-disordered graphene. We have shown that the transport mean free paths are completely different from the elastic mean free paths in the case that the 13C impurities aggregate as clusters, which means it makes nonsense to approximate transport mean free paths by elastic mean free paths to calculate thermal conductivity. The second structure consist of hybrid BN-C domains. Our calculations treat domain concentrations in the whole range from 0 to 100% and domain size up to 8nm. The mean free paths minimize at 50% domain concentration, which agrees with the effective medium picture. The room temperature thermal conductivity is found to increase with domain size. This computational strategy is applicable to higher dimensional systems, as well as non carbon-based structures.
Owing to a unique structure, a single or few-layered two-dimensional (2D) sp2-bonded carbon sheet, graphene has attracted considerable interest over the past years, offering an increasing application in electronic and optoelectronic devices, catalysis and sensors. Recently, graphene was attempted to be utilized in a variety of biomedical application, including cellular imaging and drug delivery, bio-analysis, stem cell proliferation & differentiation and even photothermal therapy for tumor.Neural cells involve electro-active functions of nerve system with electrical activities. Neuronal stimulation and monitor are needed for a variety of clinical diagnostics and treatments. Besides extraordinary elasticity and stiffness and good chemical stability, graphene can be tailored to match the charge transport for electrical-cellular interfacing applications, which makes it a promising candidate as a neural interfacial material.
This thesis is about the study on preparing high quality epitaxial films and investigating their properties by in-situ ultra high vacuum scanning tunneling microscopy (STM). The development of technology promotes the need for new materials to obtain much smaller size, higher efficiency, lower power consumption, more stable and enviroment friendly devices. The reseach on new materials is based on this requirement. In this Ph.D thesis, there are two kinds of epitaxial films: graphene and manganites. Two dimensional graphene has attracted much attention, and the CMR effect in manganites is also one of hot topics. The first part is tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the grapheneme＜x＞tal systems. These findings may give new insights to the interfacial interactions in the grapheneme＜x＞tal systems and promote the use of graphene-based hetero-structures in devices.The second part is about epitaxial growth of graphene on Pt(111) surface. It was found out that the proportion of different rotational domains varies with growth temperature and the graphene quality can be improved by adjusting both the growth temperature and ethylene exposure. Rippled and unrippled domains of high quality graphene are observed. The adhesive energy and electronic structure of two models, representing rippled and unrippled graphene, are obtained with density functional theory calculation, which shows that the interaction between graphene and Pt(111) surface is very weak and the electronic structure is nearly the same as that of a free standing graphene.In the third part, the intercalation of silicon can tune the intercalation between epitaxial single layer graphene and substrate. Silicon can be intercalated underneath graphene after annealing, then the graphene/Si/Ru(0001) system was studied by scanning tunneling microscopy (STM) and Raman spectra. STM images show that different amount of Si can induce different structures in the system, which can modulate the interaction between graphene and substrate. When small amount of Si is penetrated, graphene on Si islands still ripples largely showing strong interaction between graphene and substrate. A large amount of Si can result in a （√7×√7） R 19.1°structure respecting to Ru(0001), on which graphene shows a honeycomb structure in high resolution STM image. Furthermore, Raman spectra show the intercalation of Si can weaken the interaction between graphene and substrate.The last part is about STM study on the phase separation and electronic properties of manganite. A bulk-like metal-insulator transition and phase separation has been observed on 100 nm thick single crystalline epitaxial La5/8-xPrxCa3/8MnO3 (LPCMO) films using variable temperature scanning tunneling spectroscopy. In contrast, tunneling spectra from a 25 nm thick LPCMO film reveal a significantly lower transition temperature as compared to the thick film and doesn’t show any phase separation, which is consistent with the transport and magnetic properties. The observation of contrasting properties in films having the same surface termination settles the debate: tunneling spectroscopy at the surface of manganite films is able to probe bulk properties and is not limited to surface (state) effects.