The controllable growth of graphene overlayers on metal substrates such as Ru(0001) and Pt(111) and intercalation reactions at graphene/metal interfaces have been studied in the home-made Deep Ultraviolet Photoemission Electron Microscopy (DUV-PEEM) system. The main results are as follows:1. A deep ultraviolet (DUV) laser (177.3 nm) was applied as the excitation source for Photoemission Electron Microscopy (PEEM). The spatial resolution of the DUV-PEEM facility reaches to 4.2 nm, which is in the top level in the world. The instrument has been also equipped with a field emission gun such that Low Energy Electron Microscopy (LEEM) and Mirror Electron Microscopy (MEM) can be made. The functions, including Micro probe Low Energy Electron Diffraction (μ-LEED), Micro probe Electron Energy Loss Spectroscopy (μ-EELS) and Intensity Voltage curves, can also be achieved.2. Layer number of graphene can be controlled on Ru(0001) and Pt(111) surface via chemical vapor deposition method or surface segregation method. The morphology of graphene on Ru(0001) can be controlled by engineering of substrate surface.3. A comparative study of Pb and Ni intercalation reactions at graphene/Ru(0001) interface was performed. The interaction between graphene and Ru(0001) substrate was decoupled after the intercalation of foreign atoms at the interface area. Two different intercalation mechanisms are suggested to explain the different intercalation process observed by PEEM/LEEM.4. O2 and CO can be intercalated to the interface area of graphene/Ru(0001). The diffusion of absorbed O on Ru(0001) surface is believed to be the rate limited step for O intercalation. CO cannot intercalate the graphene/Ru(0001) interface unless exposure to near ambient pressure of CO. The intercalation of CO and O can decouple the interaction between graphene and Ru substrate. The adsorption behavior of CO on Ru(0001) surface has been affected by the graphene cover indicating the confinement effect from the graphene overlayers.
Graphene is a single layer two-dimensional carbon sheet with the hexagonal lattice. Owing to its distinctive band structure, graphene has attracted an increasing considerable interest in the application in electronic and optoelectronic devices over recent years. Graphene-semiconductor contact is of critical importance to the fabrication of high performance graphene based devices. Understanding the contact properties of graphene on semiconductors is crucial to improving the performance of graphene devices.In this thesis, we, firstly, established a theoretical model for calculating the contact barrier height and the Fermi level shifts in graphene based on the traditional Schottky theory according to the special energy structure in graphene. The contact properties of graphene-semiconductor have been investigated using our model. And then, we systematically discussed and analyzed the contact properties of the pristine graphene (prepared by mechanical exfoliation of HOPG) or CVD grown graphene on wide bandgap semiconductors, such as n-type GaN, p-type GaN, n-type AlN and n-type SiC, respectively, using our home-build scanning near field multi-functional probe microscope system experimentally combined with the calculations of the theoretical model. The innovation of the work of this thesis can be listed as follows:
Stimuli-responsive patterned polymer brushes grafted micro/nano coatings with controlled chemical groups, grafting densities and grafting thickness respond well to environmental changes. Due to the expensive gold substrates and complex modification/treatment process of other substrates, further application is limited. Furthermore, the application of conventional controlled radical polymerization such as atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer polymerization (RAFT) is limited by the rigorous and complicated reaction conditions. supramolecular self-assembly is demonstrated as an aggregation of molecules driven by hydrogen bond, electrostatic interaction, etc. Recently, self-initiated photografting and photopolymerizaion (SIPGP) becomes a facile and high efficiency method even without surface-bonded initiator and any photo sensitizer. Thus the combination of supramolecular self-assembly and SIPGP may provide a simple and effective strategy to fabricate functional surface. In this thesis, we reported a series of photoactive molecules or graphene oxide (GO) assembled on the hydroxylated surface for constructing patterned stimuli-responsive polymer brushes. Further study is carried out to explore the supramolecular self-assembly mechanism and the flexibility of this method. The detailed contents are listed as follows: 1. Py-CH2NH2 modified hydroxylated silicon and graphene surface for fabricating patterned polymer brushes. Microcontact printing induced supramolecular self-assembly monolayers of photoactive Py-CH2NH2 on hydroxylated /graphene material surfaces through hydrogen-bond formation and π-π conjugations. The photoactive -NH2 groups point to the outer layer of monolayers, which offer the possible grafting points. Based on the chemical vapor deposition (CVD) graphene structure, we successfully transferred Py-CH2NH2 on graphene surface through π-π stacking interaction to fabricate patterned supramolecular self-assembled monolayers and subsequently amplified to stimuli-responsive polymer brushes. The facile strategy for functionalizing material surfaces has potential applications in developing microelectronic and sensors devices. 2. Graphene oxide modified hydroxylated silicon surface for fabricating patterned polymer brushes. The photoactive groups of hydroxyl and carboxyl in the basal plane of graphene oxide can form stably multi-hydrogen bond with hydroxyl groups on the silicon surface. While the photoactive groups can grafted stimuli-responsive polymer brushes through SIPGP. Due to the effective reduction of graphene oxide by UV light, SIPGP also plays an important role in GO reduction during the polymerization process, which extends the potential applications in filed effect transistors (FET), sensors, etc.
Facing the increasingly serious environmental pollution problem and energy crisis, micro-nano sensors with miniaturization size, high sensitivity and fast response time are the main research and development directions. Carbon nano-materials have aroused much attention in sensor applications due to their unique physical and chemical properties. Novel high performance micro-nano sensors based on carbon nano-materials (single-wall carbon nanotube, graphene) were developed with micro-fabrication technology: 1. The photovoltaic self-powered p-n heterojunction micro-photodetectors based on p-type semiconducting reduced graphene oxide (rGO) and n-type silicon were successfully fabricated. The build-in electric field (BEF) of such structure confers the resulting PDs ultrafast response time (2 ms), zero energy consumption, and high sensitivity (1.64 A/W) towards UV-visible light (365 nm-600 nm). Meanwhile, the imported parameters such as the thickness and reducing degree of the rGO films and the architecture of the sensor device were systematically studied in this chapter; 2. Photo-assisted self-powered gas sensors based on p-type single-walled carbon nanotubes (SWNTs) and n-Si were successfully fabricated. Comparing with the conventional chemiresister type gas sensor based on SWNTs, the sensitivity and response time of the self-powered gas sensor were significantly enhanced. When exposed to 400 ppb H2S, the sensitivity increased from 0.49% to 2.23%. 3. With simple and cost-effective filteration method and Ni(OH)2 nano-sheets as the sacrificial layer, the controllable exfoliation of ultrathin graphene oxide (GO) films were realized. The obtained thinnest GO film was 5-10 nm. Flexible touchless capacitive sensors based on the exfoliated GO films as dielectric layers were constructed, and the sensors can rapidly identify the approaching of conductors and insulators and can also sensitively and rapidly monitor the approaching extend of the conductors. Key words：carbon nanotubes, graphene, self-powered photodetector, self-powered gas sensor, capacitive sensor?
In this thesis, we present our studies on electronic transport property of single-wall carbon nanotubes (SWNTs) and on that of few-layer graphite films (graphenes). For the study of SWNT, we have grown/fabricated SWNT devices and performed transport measurements at low temperatures. We focused on the Coulomb blockade and Kondo effect in nanotube quantum dot. In particular, spin related phenomena, such as multi-peaks Kondo effect and spin blockade, were observed in our SWNT devices coupled to a normal metal electrode and a ferromagnetic metal electrode. This thesis also present our experimental study on the magneto-conductance (MC) of disordered graphene and the transport property of graphene nanoribbon (GNR) quantum dot. In a disordered graphene sample, where the disorders were intentionally introducedby Ga+ ion irradiation, we show that the MC can be well described within the framework of two dimensional weak localization. We attribute the suppression of anti-weak localization effect in the disorded graphene sample to the disorder-induced inter-sublattice electron hopping and to the intervalley scattering mediated by the disorders. In a GNR quantum dot, we observed, for the first time, the two-fold shell filling structure and novel co-tunnling phenomenon, as shown in the Coulomb blockade diamond measurements. Our results indicate the lifting of both valley and spin degeneracy in the graphene nanoribbon, which demands further investigation on the underlying mechanisms.
In this thesis, we investigate theoretically the spin-relatedoptical properties of low-dimensional semiconductor structureswithin the framework of effective-mass theory. The thesis consistsof five chapters, in which we study Faraday rotation of excitons ina GaAs quantum well or GaMnAs bulk materials embedded in amicrocavity, the oscillation behavior of magnetic circular dichroism(MCD), the direct detection of pure spin current, the visibility ofthe graphene layers and the magneto-optical far-infrared absorptionbetween Landau levels in graphene, respectively.In Chapters 1 and 5 we introduce the emerging field of Spintronicsand give the conclusions the thesis, respectively.In Chapter 2, the Faraday rotation of an exciton in a GaAs quantumwell (QW) embedded in a microcavity is investigated theoretically.We find that the Faraday rotation is enhanced remarkably by themicrocavity, about two orders of magnitude larger than that of asingle QW without microcavity. The Faraday rotation can be tuned bychanging the incident angle of the pump and probe lights, or byvarying the temperature or an external electric field. With anappropriate detuning between the cavity mode of the pump and probelights, the Faraday rotation spectrum displays a strongly asymmetricline-shape, which can easily be detected experimentally. The giantFaraday rotation enhanced by the microcavity makes it possible todetect a weak electron spin signal, and could play an important rolein investigating exciton spin dynamics. The band structure and MCDof GaMnAs are also investigated theoretically. We find thenon-monotonic magnetic field dependence of MCD signal, the resultsare in good agreement with the experiment. We demonstratetheoretically that the Faraday rotation of GaMnAs materials embeddedin a microcavity can reach 45 degree.In Chapter 3, We suggest a new practical scheme for the directdetection of pure spin current by using the two-color Faradayrotation of optical quantum interference process (QUIP) in asemiconductor system. We prove theoretically that the Faradayrotation of QUIP depends sensitively on the spin orientation andwave vector of the carriers, and can be tuned by the relative phaseand the polarization direction of the ω and 2ω laser beams. Byadjusting these parameters, the magnitude and direction of the spincurrent can be detected. This scheme may also be important fordistinguishing the ESHE and ISHE from the different dependence ofthe pure spin current on the crystal orientation. Our scheme canalso be used to detect the spin polarization of the charge current.In Chapter 4, We investigate theoretically the light reflectance ofa graphene layer prepared on the top of one-dimensional Si/SiO2photonic crystal (1DPC). It is shown that the visibility of thegraphene layers is enhanced greatly when 1DPC is added, and thevisibility can be tuned by changing the incident angle and lightwavelengths. This phenomenon is caused by the absorption of thegraphene layer and the enhanced reflectance of the 1DPC. The largedifferences in the reflectance make it possible for the graphenelayers of different thicknesses to be more easily observed anddistinguished experimentally. Recently we investigate theoreticallythe optical absorption of graphene in a perpendicular magnetic fieldwith an integral number of filled Landau levels and study thedispersion of single particle-hole excitations. Theinter-Landau-level absorption spectrum and the magneto-opticaleffect in single graphene layers is calculated. We find that themagneto-optical effect in single graphene layers depends on thenumber of filled sublevels in Landau level.
In 2014, three Japanese, who invention of efficient blue-light LED, won The Nobel Prize in Physics.This marks that commercial gallium nitride (GaN) based Light-emitting Diode (LED) technology has matured. LED has been widely used because of its high efficiency, small size, environmental protection, and other excellent performance.LED is gradually becoming the mainstream of lighting sources, will lighting the future of mankind.In 2010, Two scientists who discover the graphene won The Nobel Prize in Physics. It ignited the enthusiasm of the scientific and technological research on graphene. Graphene is a two-dimensional crystal structure consisted of the hexagonal honeycomb lattice. It has a unique and excellent properties, especially excellent in the optical and electrical properties, so scientists research focuses on the optical and electrical properties of graphene, graphene want to play an active role in the photovoltaic sector and will lead mankind to era of graphene.This thesis, investigations on the application of graphene in GaN LED, aim to find some graphene’s applications in GaN LED. Our exploratory research work consist of direct growth of graphene on GaN, growth of free-standing GaN by using stacked graphene, the wettability of graphene, the preparation for GaN LED with graphene wetting layer and its performance analysis. The main work is as follows: (1) Direct growth of graphene on GaN LED without extra catalyst has been achieved. When methane as a carbon source in the process of direct growth of graphene transparent electrodes on GaN LED, GaN LED is almost no thermal degradation; graphene growth condition is optimized; possible mechanism is proposed: the Ga atoms on GaN surface is catalyst. Considering the acetylene as a carbon source can reduce the growth temperature, thereby, it can reduce the influence to the performance of GaN LED in the graphene growth process. the effect of carbon source, growth temperature and pressure are also studied.(2) Self lift-off GaN epitaxial film has been fabricated by using one graphene stacking on another graphene as sacrificial layer. We note the force between two stacking graphene is physical adsorption in the process of graphene transfer. When the stress in heteroepitaxy over come it, the purpose of lift-off epitaxy can be achieved. The process of self lift-off is analysed and the principle of self lift-off is verifed. The influences of stacking graphene with different fluorination power on the quality of free-standing GaN are compared. The thinner free-standing GaN substrate and the reusable sapphire substrate reduced the cost of GaN homogeneous substrate. It is a very promising technique for preparing GaN homogeneous substrate.(3) Investigations on the wettability of graphene and the graphene nanodrums power.When the morphology of graphene fit to the morphology of substrate, the graphene wettability is partially transparent, no matter plane substrate or patterned substrate. The performances of graphene nondrums were tested. The experiment shows that the direction of induced current is determined by the direction of graphene nanodrums movement, the size of the induced current is closely related to amplitude of graphene. We propose a possible mechanism: electromagnetic induction from the magnetic graphene edges.(4) A transfer-free graphene was grown on c-plane sapphire, this provide a new method for fabricating the graphene covered c-plane sapphire. The advantages of graphene as wetting layer to epitaxial growth III-nitride (III-N): The increase in the migration of metal atoms on the graphene, release stress in epitaxial layer which is next to graphene. GaN LED with graphene wetting layer was prepared and its performance is analysed. The graphene wetting layer increased the surface roughness of the substrate, which is similar to the role of nano-patterned substrate, it can increase the light extraction efficiency. The LED with graphene wetting layer improves the electrical perf
Terahertz (THz) waves are genarally in the frequency range of 0.1 to 10 THz. Due to the characteristics of spectral resolution, good perspective and security, THz technology has important application prospects in the field of materials detection, quality and quantity analyse of materials, and biomedical imaging. However, most of natural materials have no electromagnetic response in THz region, resulting in a lack of THz materials and devices. The development of THz functional devices will greatly promote the development and practical application of THz technology. Metamaterials (MMs), as a kind of electromagnetic material with controllable properties, can be designed artificially, providing an effective method for developing THz functional devices. In this thesis, we carry out research work concerning the mechanism and characteristics of THz MM functional devices; and various THz MM functional devices were proposed. The main work are summarized and listed as follows:1. A multiband THz MM absorber with three absorption peaks was proposed, with all absorption intensity more than 85%. The proposed structure has the feature of polarization independent, and has potential application in THz spectroscopy, sensing and other fields. Furthermore, a multiband THz MM filter was proposed, and all pass bands have the fearture of low average insertion loss, steep skirts, high out-of-band rejection and polarization independent. This filter has broad application prospects in THz photodetectors, imaging, and communications. 2. A planar THz MM structure with electromagnetically induced transparency (EIT) effect was proposed, and the unit cell of the proposed structure consists of two different split ring resonantors (SRRs). Due to the destructive interference between these two SRRs, the proposed structure generates a very narrow transparent window in the transmission spectrum, which is a typical EIT-like effect. In addititon, a graphene-based THz MM structure composed of multilayer gr