First Principle Investigation Of The Effect Of Organic Radical Termination On The Spin Property Of Graphene Nanoribbon

For a long time, the electrical and magnetic properties are investigated in the inorganic materials such as the conductivity of the gold, copper and other metals, the magnetism of iron, cobalt and nickel, and the semiconducting properties of silicon-based materials. Since the conducting behavior of the organic material TTF-TCNQ was discovered in the seventies of the last century, many (doped) organic polymers or small molecular systems have been found to be semiconductors or even conductors. Therefore, the nature of their conductive properties has also been extensively researched. Compared with traditional inorganic materials, organic polymers or small molecular materials have the advantages such as diverse structures, simple synthesis, low price and easy handling, and are used to design kinds of functional devices. These findings have brought the rise of organic electronics.With further research, it was found that the organic materials not only have excellent electronic properties, some organic materials are also magnetic. Provided by the different kinds of magnetic center, they can be distinguished into two categories. One is chemical doped organic materials by magnetic metals such as transition metal manganese, iron etc. These doped magnetic metals interact through organic materials to form a long-range magnetic order, namely micro-magnetism. The other is pure organic magnetic materials without magnetic metals. These materials are only composed from C, H, S, N, and/or O elements, such as some big-π planar conjugated molecules, organic radical groups including N-O or C-H radical, as well as some electron-transferred complexes. Different from traditional magnetic metals in which the magnetism is provided by d or f electrons, the magnetic properties of these pure organic materials come from the spin-related interaction between s and/or p electrons. Owing to the absence of heavy magnetic metals, spin polarized electrons in organic materials can travel quite a long distance without scattering. This feature is of broad usage in spintronic devices; however, the synthesis of these pure organic magnetic materials is a great challenge so far with only a few types prepared out.At the same time, the study of physics goes towards nanometer scale, in which many novel materials and physical phenomena have been discovered. For example, the new structures and physics are found in nano-scale carbon material, which is the main constituent element of organic materials. In the nanometer scale, zero-dimensional fullerenes and1-dimensional carbon nanotubes are fabricated. In2004, Andre Geim and Konstantin Novoselov succeeded in preparing graphene in the lab. This new material is consisted by a single layer of graphite surface, in which carbon atoms aligning in a hexagonal grid shape. The successful fabrication of graphene caused a huge stir in the scientific community, not only because it broke the theoretical prediction that real two-dimensional material could not exist, but also because it brought up a number of surprising new features. The two-dimensional crystal structure makes the mobility of carriers in graphene tens even hundreds times higher than that in traditional materials, making it suitable for ultra-fast transistors. Strong carbon-carbon bonds ensure its ultra-high hardness, which can be used for composite materials. The single atomic layer thickness brings about excellent light transmission, making it transparent conductor. The electron mobile behavior in graphene obeys relativistic quantum theory, making it a good platform for relativistic theory research. These features make graphene star material for next generation electronic devices.However, modern electronic devices rely highly on the semiconducting characteristics to achieve the transition between "ON" and "OFF" states. As the conduction and valence band of graphene meet at one point at the Fermi energy, graphene has zero band gap and nearly zero density of states. This is the typical characteristic of semi-metal, which limits the application of graphene in semiconducting devices. Therefore, it is an important prerequisite of graphene to open a band gap for its possible usage in modern electronic industry. At present the main methods for this perspective include preparation of double-layer or multi-layer graphene, stress constrained graphene and cutting large-area graphene into1-dimensional nanoribbons. Among these methods, fabrication of graphene nanoribbon is the hot topic of the current research. When large-area graphene is cut into nanoribbon, the quantum confinement effect turns it into semiconductor with an energy gap tunable with the variation of the ribbon width and/or external electric field. This feature quickly inspires the upsurge of graphene nanoribbon electronics. The prototype graphene nanoribbon transistor has been fabricated in the laboratory.Graphene nanoribbon can be divided into two types according to its edge shape, one is armchair edge and the other is zigzag edge. Due to the quantum confinement effect, edge states emerge in zigzag graphene nanoribbon (zGNR). These states located at the edge atoms with opposite net spin moments, showing spatial magnetism. Owing to the emergence of magnetism in zGNR, it is natural to expect its application in spintronic devices such as spin filter and spin transistor. Unfortunately, the band structure of zGNR is spin degenerated which needs further modulation to get spin polarized. For example, under a transverse electric field, the energy bands near the Fermi energy for one spin component getting close to be metallic while the other spin component getting apart to be insulate, making the whole system half-metallic. Half-metallic materials allow only one spin component transport through, and thus can be used as spin valve or spin injection devices.The half-metallic property is an important research field. Several methods are proposed to realize the half-metallic property in zGNR. For example, due to the different electron affinities of boron and nitrogen atoms, an inner effective electric field is setup through delicate replacement of carbon atom chains by boron-nitrogen atom chains, turning the system half-metallic. To be more convenient, the sp2hybridization between graphene carbon atoms in the planar hexagonal lattice shares similar structure and chemical property with organic benzene-based molecules. As a result, graphene is easy to be chemically modified by kinds of organic groups. It is predicted that edge modification of zGNR by organic groups with different electron affinities at opposite side is also an efficient way to make zGNR half-metallic.At the same time, semiconducting zGNR based spintronic application is another crucial research field as it is often viewed as a significant option for the next generation spintronic devices. It requires that zGNR has both spin-polarized band structure and sizable energy gap, making it possible to be operated between "ON" and "OFF" states with spin-polarized electrons. At present, the design and fabrication of spin transistor based on zGNR is the bottleneck for real spintronic application of zGNR.Focused on this difficulty, we theoretically put forward a method of organic radical group termination of zGNR for realizing semiconducting zGNR with spin-split energy band using first principle calculation. After single edge termination, zGNR not only has sizable energy gap but also has100%spin-polarized energy band around the Fermi energy, ensuring its possible usage for spin transistor. Then we validate this method when the width of zGNR is ultra narrow (with width N=1～6). It should be noted that at this width the dimension effect is important for the ground state magnetism and spin related transport in zGNR. This dimension effect should be carefully considered in designing related devices.The main content of this thesis is summarized as follow:1. Ferromagnetic semiconducting zGNR obtained by organic radical termination.How to make the ground state of zGNR to be ferromagnetic semiconductor is the key issue of this thesis. In Chapter Ⅲ, semiconducting zGNR with spin split energy band is predicted by organic radical termination.1.1Single edge termination of zGNR with TMM groups (one kind of organic radical, see Chapter III for details) will induce a band gap of about0.5eV, with100%oppositely spin polarized energy band beside the Fermi energy. The net spin moment resides on TMM is opposite to which on the connected edge, and is the same with the farther edge. System with other spin configuration has higher total energy as well as structure distortion.1.2Replacing TMM by CH2group, the obtained band structure shares similar characteristic, showing that the emergence of ferromagnetic semiconducting ground state is robust for different radical groups, at least for the same kind of organic radical groups.1.3The inner mechanism is discovered to be the electron-spin-lattice interaction between radical and zGNR.1.4Manually rotating the terminating planar radical group turns the spin polarization near the Fermi energy from100%to nearly zero, indicating a possible new way of controlling the spintronic property of zGNR.2. Dimension effect in organic radical terminated ultranarrow zGNR.zGNR can be viewed as comprised by stacks of trans-polyacetylene-like carbon atom chains, with its number N representing the ribbon width. When N increases from1, the system turns from semiconducting polymer to magnetic zGNR The nature of the property of the system heavily changed with width N is called the dimension effect. In Chapter IV, we discuss whether the abovementioned method is still valid when N is extremely small. We also study whether dimension effect is important in ultranarrow zGNR after termination.2.1When N is small, such as N=1, organic radical termination can still stabilize the magnetic order in the system, leading to spin polarized semiconductor.2.2When N increased from1, for example to N=4, the energy gap of the system obviously changes, showing distinct dimension effect.2.3It indicate through spin-resolved transport calculation that dimension effect has crucial impact to the transmission when N is smaller than4.