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.
Based on non-covalent functionalization of graphene, the thesis has prepared smart responsive as well as functional graphene/polymer composites and studied its properties wihtout destroying or changing the chemical structure of graphene. The main contents and conclusions were described as follows: The atoms transfer radical polymerization (ATRP) initiator containing pyrene group was designed and synthesized, which could anchor on surface of graphene oxide (GO) via π-π interaction between pyrene groups and sp2 -hybridized carbon structure. Then, GO/PDMAEMA composites were prepared by surface initiated-atom transfer radical polymerization (SI-ATRP) method. Studies indicate that the composite exihibits zwitterionic property and its charging state could be altered by changing solution with different pH values. By using zwitterionic property of the composite, through anion and cation ion exchanging respectively and following in-situ reduction method, the noble Pd-Au bimetal nanoparticle loaded graphene-based organic/ inorganic nanocomposites were prepared, which could effectively catalyze the reaction that reduction of p-nitrophenol by NaBH4. Using initiators which contain pyrene or catechol structure as anchoring ink, through non-covalent micro-contact printing and following SI-ATRP method, patterned polymer brushes with different functionalities (including pH, temperature responsive and positive/negative ionic polymer brushes) as well as binary polymer brush patterns on graphene substrate were successfully prepared. It was proved to be a general method for preparing polymer brush patterns on graphene surface. In this way, patterned POEGMA-OH brushes were grafted onto graphene surface, and were successfully used for anchoring proteins. The quasi-2D polymer brushes@graphene film with excellent properties such as different functionalities, freestanding, high transmistance, flexibility, was prepared, which could be used for a one-step, non-destructive modification of a variety of planar substrates. Using self-assembled reduced graphene oxide (rGO) film as substrate electrode, anodic aluminum oxide (AAO) as template, ordered PANI nanoarrays on rGO surface were prepared via potentiostatic method. The PANI nanoarray could firmly anchor on rGO surface via non-covalent interaction between conjugated polyaniline (PANI) and graphene. The results of the electrochemical test of PANI nanoarray indicate that PANI array has a higher specific capacitance than the PANI film; and its specific capacitance could reach up to 990 F/g. Keywords: Graphene, Atom transfer radical polymerization, Pyrene, Polymer brush, Noncovalent modification
Due to its extraordinary two-dimensional crystal structure and linear electronic structure, graphene has unique optical, electrical and other properties,thus making it a potential candidate for applications in nanoelectronics and photonics. Many remarkable physical properties can extend to bilayer and few-layers graphene. Disorder and edges in graphene materials has an enormous influence on their properties and applications. Raman spectroscopy plays a significant role in investigating the properties of graphene. In this thesis, based on the double resonant theory and Raman spectra, we study the ion-implanted graphene and the edges of multilayer graphene by Raman spectra, and so on. Raman modes of mono-layer and bi-layer graphene are investigated in detail in the frequency region between 1800 and 2150 cm?1. On the basis of the double resonance(DR) theory, we assign the respective 1808 cm?1 and 1909 cm?1 modes to the combination modes of the in-plane transverse acoustic(TA) and the longitudinal optical(LO) phonons near Γ, labeled as L1+D′, and of the in-plane longitudinal acoustic(LA) and LO phonons near Γ, labeled as L2+D′. Meanwhile, both the L1+D′ and L2+D′ modes are found to be strongly dispersive with the laser excitation energy, about 130 and 198 cm?1/eV. There are four dominant Raman modes in the region 1800-2150cm?1 of bilayer graphene. The frequencies of these modes dependent on excitation energy are revealed from the measurement with multi-wavelength lasers, which can be fully understood based on double resonance Raman scattering and the phonon dispersion relation of monolayer graphene. The results show that these Raman modes can be assigned to the combinational modes from the fundamental modes of iTA, LA and LO phonons, but not from iTO and oTO phonons as reported in the previous works. This study benefits us toward the full understanding of lattice dynamics of monolayer and multilayer graphenes. Raman spectroscopy has become a key way for characterizing and studying disorder in graphene, due to its nondestructive, rapid and sensitive technique. In this paper, ion implantation is used to produce the structural defects in singlelayer graphene (SLG) and bi-layer graphene (BLG). The first- and second-order modes of ion-implanted SLG and BLG and their physical origin were studied by Raman spectroscopy. The dispersive frequency of first- and second-order modes in SLG and BLG on the excitation energy was discussed in detail. The results show that the ～ 2450cm?1 peak is the combination mode of the D mode at ～ 1350cm?1 and the D” mode at ～ 1150cm?1. Edge naturally exists in a single-layer graphene sample. Similarly, each graphene layer in multilayer graphene (MLG) exists its own edge. We study the Raman spectrum at the edge of a graphene layer laid on an (n?1) layer graphene ((n?1)LG;n≥1). It found that the D mode at the edge of the top graphene layerexhibits an identical line shape to that of ion-implanted nLG (ion-nLG). Based on the spectral features of the D and 2D modes, we identified two types of alignment configurations at the edges of mechanically-exfoliated BLGs and TLGs, whose edges are likely well-aligned from their optical images. This study will benefit the experimental researches on the properties and applications of the edge states in MLGs.
Graphene is an innovative 2-Dimensional material with exceptional properties and promising applications in the future. It has drawn a wide attention and got deeply researched. In this paper, the mechanical behaviors of graphene, as well as its derivatives, are to be studied utilizing theoretical analyses and molecular dynamics simulations. During the interaction of graphene nano-ribbon (GNR) on carbon nanotube (CNT), the GNR can be self-assembled onto the surface of CNT because of the van der Waals (vdW) interaction and two available configurations, helix or scroll, are to be presented. The final configuration is determined by the energy competition among the bending energy of GNR, vdW interaction between GNR and CNT and vdW between GNR itself. Phase diagram to predict the final configuration is obtained from theoretical modeling with those energies accounted. Molecular dynamics simulations are also performed to verify the results. Closely packed carbon nanoscrolls (CNSs) can form a macromolecular crystal. During uniaxial lateral compression and decompression, the mechanical behavior of the crystal is affected by interaction of adjacent CNSs . Theoretical modeling is conducted to describe the interaction and verified by molecular dynamics simulations. Hysteresis is observed during a compression and decompression loop and could be tuned by changing the interaction energy inside the CNSs. This discovery is applicable for energy-absorbing materials. Interactions of graphene sheet on different surfaces of crystal copper, including binding, folding and peeling behaviors, are also studied. The binding intensity follows the sequence of (111) > (100) > (110) > (112), with the Cu(111) being the strongest surface to graphene sheet. Folding of graphene is least possible on the Cu(111) as well, indicating the Cu(111) to be the most favorable surface for the synthesis of monolayer graphene by chemical vapor deposit method. Conventional peeling theory is proved to be suitable for peeling of graphene by analytical study and molecular dynamics simulations. Number of layers of peeled graphene can be determined by peeling tests.
Energy issue is a hot topic in the current world. In the age of scarce energy resources, using new energy materials to develop solar energy, bio-energy and other new energies, exploring the new energy storage materials, and improving the energy efficiency are the main tasks for solving the energy problem. Graphene study has been one of the most important subjects in the current material science, physical and chemical fields since its discovery in 2004. Graphene is a wonder material with many superlatives features to its name. It is the thinnest known material in the universe and appeared to be one of the strongest materials known with a breaking strength. Its charge carriers exhibit giant intrinsic mobility, and have zero effective mass, which can travel for micrometers without scattering at room temperature. Due to these unique electronic properties, and the possibility of chemical doping, graphene will be the most potential material for electronic devices. The size bottleneck of using silicon as electronic materials fascinates the development of graphene. This thesis has studied the magnetic and electronic properties of doped graphene using first-principle electronic structure calculations based on spin-polarized density functional theory. Our results have given the dependence of magnetism(and band gap) on impurity types, as well as concentration and packing geometry of defects in graphene. This thesis consists of seven chapters. In Chapter 1, we analyze the current energy structures, and briefly explore the necessity to developing photovoltaic industry. According to the current status of solar cell industries, and high converting ratio of thin film cells, and after tracing the frontiers of graphene field in both theory and experiment, we propose our research task to do research on graphene, due to its special properties. In Chapter 2, we begin with introducing the basic concept of Density Functional Theory (DFT) and briefly review its process of improvement. Although the first-principle electronic structure calculations based on DFT reduce the original many particles problem into an equivalent single particle problem, there are still many difficulties to solve scientific questions. Due to uncertain functional forms to express electron exchange and correlation interactions, people use many kinds of approximations to describe these terms. In recent years, some theories have been developed, especially the hybirized density functional has made great progress in dealing with energy gaps. Now, the optimized first-principle electronic structure calculations are the most popular methods available in condensed-matter physics, computational physics and quantum chemistry. In Chapters 3 and 4, we have studied the electronic and magnetic properties of intrinsic graphene and graphene doped with nitrogen and fluorine atoms, respectively. The results show that introducing nitrogen doping would possibly perform the spin symmetry breaking, resulting energy degeneracy at some doping configurations. The spin symmetry breaking would cause spin-polarized effects, which induce magnetic response in graphene. Symmetric doping in the two sublattice makes the new symmetry, and the system has no spin-polarization. We can tailor the band gap according to the dependence of magnetic moments and band gaps in graphene on nitrogen-substitutional doping configurations. Fluorine adatoms change the bond types of carbon atoms in graphene. The average charges on carbons are reduced and the adsorption energies are increased along with increasing coverages. All of the stable configurations with different coverages have a band gap except for the situation with one atom adsorption. There is a magnetic moment when the number of F adatoms in a supercell on graphene sheet is odd. The magnetism comes from the distortion caused by the interaction between F atom and graphene. Band gap variations show th
Graphene is reported by the professor named Geim of the University of Manchester since 2004. Its unique performance has attracted wide attention from scientists. It is well known in physics, chemistry and materials science fields as a rising star. Graphene constitutes a nanocarbon comprising layers of carbon atoms arranged in six-membered rings (thickness calculated value, 0.334 nm). Graphene is the basic unit of all graphitic forms, while wrapped to form a zero-dimensional fullerene, rolled to form a one-dimensional carbon nanotube and stacked to form a three-dimensional graphite. Graphene is a zero gap semiconductor. The carrier mobility of graphene is 100 times higher than that of silicon 2×105 cm2/(V?s)]. Graphene has excellent thermal conductivity 3000 W/(m?K)], high fracture strength (110 GPa) and specific surface area (calculated value, 2630 m2/g). The electron transporting, optical coupling, electromagnetic, thermal and mechanical properties of graphene make it widely used in nano-electronic devices, the high-performance liquid crystal display materials, solar cells, gas sensors and energy storage. It shows that there are many methods for the synthesis of graphene, such as micromechanical exfoliation of graphite, solvent exfoliation of graphite, chemical vapor deposition, chemical exfoliation and reduction starting from the oxidation of graphite, cutting carbon nanotube, and synthesis of organic molecules. Whatever the method is used to prepare graphene, the obtained graphene unites are graphene nanosheets, and it is not so easy for the graphene to be directly applied in the devices. Therefore, it has become a key point for how to select and fabricate the appropriate graphene building blocks, and controlled assemble graphene-based nanostructures for its applications. The achievements have made in this thesis are summarized as follows: 1. Facile synthesis of graphene-wrapped honeycomb manganese dioxide nanospheres and their application in supercapacitors In the redox reaction process, the negative graphene dispersion was prepared by controlling the proportion of graphene oxide, hydrazine, and ammonia. Monodisperse honeycomb manganese dioxide (MnO2) nanospheres were prepared via the micro-emulsion method and modified their surface with NH2 groups for the preparation of the positive honeycomb MnO2 nanospheres dispersion. Graphene-wrapped MnO2 (GW-MnO2) nanocomposites were fabricated by coassembly between honeycomb MnO2 nanospheres and graphene nanosheets via electrostatic interaction. The GW-MnO2 nanocomposites exhibited enhanced capacitive performance as high as 210 F/g at 0.5 A/g and retained about 82.4% of the original capacitance after 1000 cycles of charge?discharge. These results demonstrate exciting potentials of the GW-MnO2 nanocomposites for high performance supercapacitors. 2. Assembly and benign step-by-step post-treatment of oppositely charged reduced graphene oxides for transparent conductive thin films with multiple applications In the redox reaction process, positive and negative graphene dispersions were prepared by controlling the proportion of graphene oxide, hydrazine, ammonia, and poly(diallyldimethylammoniumchloride) (PDDA). Graphene-based transparent conductive thin films with different thicknesses were prepared by the layer-by-layer (LbL) assembly of oppositely charged reduced graphene oxides on the glass or poly(ethylene terephthalate) (PET) substrates. The conductivity of graphene-based transparent conductive thin films was enhanced by the nascent hydrogen reduction plus mild heat treatment in inert gas. The unique feature of the step-by-step mild post-treatments was ideal for flexible plastic substrates, such as PET. The graphene thin film on a flexible PET substrate maintained better flexibility and electrical conductivity as compared with an tin-doped indium oxide (ITO) film on the same PET substrate. The graphene thin film had a smooth surface with tunable wettability, while it had good
Nowadays, GaN-based Light-emitting Diodes (LEDs) have played an important role in the region of semiconductor solid-state lighting. Due to the limitation of the lattice mismatch, dislocation density and Quantum-confined Stark Effect (QCSE), the development of traditional LEDs with planar structures meet the bottleneck. But the one with low-dimensional structure (Nanorods and Core/Shell) which can release strain, reduce dislocation density and QCSE and enhance light extraction efficiency are able to solve the problem the conventional LEDs encountered. In this paper, we focused on studying the fabrication techniques of GaN-based LEDs with low-dimensional structure and characterizing the properties of devices. A method to obtain nanorod LEDs array using ICP etching with the self-assembly Ni nanoparticles as the marks was investigated. And the novel graphene transparent conductive electodes were adopted to interconnect the nanorods array, acheiving the electronic pump in devices successfully. The influence for graphne electrodes induced by different process techniques was also investigated. In addition, pyramid array InGaN/GaN core–shell light-emitting diodes (LEDs) were fabricated by using a highly homogeneous multilayer graphene with MOCVD. And we also studied the different way to get the GaN nanorods (nanowires) as the core of the Core/Shell LEDs. Primary research results are as follows. 1)Rapid thermal annealing was performed to Ni thin films to obtain self-assembly nanoparticles with the diameter of 300nm. We used these nanoparticles as the etching masks to fabricated the homogenuous nanorod LEDs array successfully with the diameter of 300nm and the height of 1.5μm. The multilayer graphene films obtained by a CVD method were transferred onto the top of nanorods array to interconnect the LEDs array , and hence achieving the current driven. And the process-induced damages to graphene electrodes were also investigated. And LEDs with graphene on the metal-pads exhibiting lower forward voltage (decreased by 20%) and higher electroluminescence intensity (improved by 55.3%) are obtained. Using scanning electron microscope and Raman spectroscopy, we have demonstrated that graphene transferred after the metal deposition remains intact and has much less damages than graphene under the metal during the fabrication of LEDs with nanorods; 2)Pyramid Core/Shell LEDs were obtained successfully with the method of Selective Area Epitaxial Growth using Metal-organic Chemical Vapor Deposition (MOCVD) on SiO2/sapphire templates. Pyramid array InGaN/GaN core–shell light-emitting diodes (LEDs) were fabricated by using a highly homogeneous multilayer graphene transparent conducting electrode. This novel electrode exhibited excellent optical, structural, and electrical properties. In this design, graphene connected each pyramid array as a top window electrode. The current-driven pyramid array InGaN/GaN core–shell LED was operated at a low current injection and exhibited bright electroluminescence. No apparent wavelength shift within EL spectra was observed, which demonstrated the degraded QCSE on semipolar planes; 3)Three fabrication methods to obtain GaN nanorods were investigated. (I) Using 30% KOH liquid in ethylene glycol, GaN nanorods with 45nm diameter and 1.6μm height were finally obtained with the radial etching rate of 80nm/min. (II) With the method of metal catalysis using HVPE, we investigated the influence of temperature to the morphology of GaN nanorods. (III) With selective-area growth in MOCVD, the hexagonal GaN columns with the diameter of 992nm and the height of 1000nm were obtained. We demonstrated that high temperature and low V/III ratio would faciliate the axial growth of nanorods.
Graphene (G) is an ideal two-dimensional structural carbonaceous material with thickness of only one atomic layer. Due to their large theoretical specific surface area and high ability of modification, potential environmental applications of graphene-based nanomaterials (GBNs) as superior adsorbents have been recognized for removal of organic and inorganic contaminants from environment. Dissolved organic materials (DOM) are ubiquitous in natural waters and have been found to interact with nanomaterials in the environment. The interactions between DOM and nanomaterials not only alter the surface properties and environmental behavior of these nanomaterials, but also affect the adsorption of coexisting contaminants. Therefore, it is necessary to study the interactions between DOM and GBNs with different structures and properties, and to examine their effects on the adsorption of coexisting contaminants. In this dissertation, we choose G and graphene oxide (GO) with different π-electron density and O-containing functional groups as adsorbents to examine the interactions of DOM (humic acid (HA) and fulvic acid (FA)) with G and GO and their effects on the adsorption of metal ions (Cr3+ and Cu2+) using a batch equilibration technique. Combined with X-ray photoelectron spectroscopy, Raman spectroscopy, micro-Fourier transform infrared spectroscopy and extended edge X-ray adsorption fine structure spectroscopy, we reveal the mechanisms of DOM adsorption and sequent effects on heavy metals adsorption on G and GO. The main conclusions of this dissertation included: 1. It is found that DOM is adsorbed on G and GO through different adsorption mechanisms due to the property difference between G and GO. Except for hydrophobic partition interaction, DOM is adsorbed on G mainly through π-π interaction. In contrast, DOM is adsorbed on GO surface mainly through polar interactions due to its rich O-containing functional groups. 2. The adsorption of Cr3+ on GO is a rapid process. The adsorption kinetic data were well described with pseudo-second-order model and the equilibrium data were well fitted by Langmuir model. The calculated thermodynamic parameters indicate that the adsorption of Cr3+ on GO is spontaneous and endothermic. It is found that Cr3+ adsorption on GO is strongly dependent on solution pH, but weakly dependent on ionic strength. Fourier transform infrared spectra suggest that Cr3+ is adsorbed on GO mainly through the formation of inner-sphere complexes with the O-containing functional groups on GO surface. 3. The presence of HA increases the adsorption of Cu2+ on G significantly. It is because the adsorbed HA introduced O-containing functional groups and negative charges to G surfaces, increasing Cu2+ adsorption through chemical complexation and electrostatic attraction. Compared to G, the adsorbed HA had little effect on Cu2+ adsorption onto GO. HA was adsorbed on GO through polar interactions. On the one hand, these adsorbed HA shielded the surface of GO and occupied parts of surface sites on GO, which would decrease the adsorption of Cu2+. On the other hand, the adsorbed HA can introduce new adsorption sites for Cu2+ adsorption onto the GO surface. The combined result is that the adsorption of Cu2+ is independent on the presence of HA. 4. EXAFS results suggest that O-containing functional groups of G and GO are the main adsorption sites for Cu2+. Compared to GO, G has fewer O-containing functional groups on its surfaces, which led to a portion of Cu2+ precipitate on G surfaces at pH 8.0. The addition of HA increased the amount of adsorption sites on G, causing most of Cu2+ to be adsorbed on G surfaces as G-HA-Cu ternary surface complexes at the examined pH range.