In recent years, graphene nanoribbons (GNR), which are precisely controlled at the atomic level, have attracted much attention due to their special electronic structure, magnetic edge state and carrier transport characteristics. Whether it is to explore synthesis methods or physical properties, related research results have been repeatedly published in "Science" and "Nature", and they have become the current hot nanomaterials.
On June 25, 2020, the team of Professor A.-P. Li from the Oak Ridge National Laboratory in the United States and Professor K. Amsharov from the University of Erlangen-Nuremberg in Germany proposed a method that can directly synthesize atomically accurate metal oxides on the surface of semiconductor metal oxides. New method of GNR. The method realizes the pre-designed continuous activation of C-Br bond, C-F bond, dehydrogenation and ring formation on the rutile TiO2 (011)-(2×1) surface through annealing treatment.
The results of scanning tunneling microscope and spectroscopic analysis confirmed that the method successfully achieved the synthesis of a plane armchair-type GNR with a zigzag boundary, and the author revealed the weak interaction between GNR and rutile-type TiO2 substrate. The research was published in "Science" as a paper entitled "Rational synthesis of atomically precise graphene nanoribbons directly on metal oxide surfaces".
Recently, Professors Michael F. Crommie and Steven G. Louie of the Department of Physics at the University of California, Berkeley, and Felix R. Fischer, professors of the Department of Chemistry, and others have demonstrated the use of atomic level by embedding a symmetrical superlattice in a zero-energy mode into a semiconductor GNR. A general strategy for the design and preparation of metal GNR using precise bottom-up synthesis methods. Scanning tunneling microscope and first-principles density functional theory calculation results confirm that the obtained zigzag metal sGNR is metallic.
The research results show that by deliberately breaking the sublattice symmetry to control the overlap of the zero-mode wave function, the metal bandwidth in the GNR can be adjusted in a large range, thereby obtaining stable metallicity. The result was published online on "Science" on September 25, 2020 with the title "Inducing metallicity in graphene nanoribbons via zero-mode superlattices".
What are graphene nanoribbons? What are metal graphene nanoribbons?
Generally, in order to impart certain electrical properties to the single-layer graphene, the graphene is cut according to a specific pattern to form graphene strips with a approximate width of less than 50 nm, which is called Graphene nanoribbon (GNR). According to the cut edge shape, it can be divided into zigzag type and armchair type GNR. Earlier calculations made by scientists based on the tight-binding approximation model predicted that the zigzag type has metallic properties, while the armchair type has metallic or semiconducting properties; which property depends on the nanobelt bandwidth.
In recent years, density functional theory calculations combined with experimental results show that the armchair GNR has semiconductor properties, and its energy gap is inversely proportional to the bandwidth of the nanobelt. And when the size of the bandwidth is close to the atomic level, GNR will show metallic behavior, also known as the metalization of GNR. Therefore, researchers believe that GNR is expected to replace copper as an ideal connection material for future electronics.
In 2016, researchers at Aalto University have successfully prepared metallized graphene nanoribbons, which are only 5 carbon atoms wide (Reference Nature Communications, 2015; 6: 10177). In the article, the research members used scanning tunneling microscope (STM) to probe the electronic structure and performance of GNR at atomic resolution. The study found that when the GNR bandwidth exceeds 5 nm, it exhibits metallic behavior.
Although there have been previous reports of metallized graphene nanoribbons, the design and fabrication of stable metal states in GNRs are still very tricky and challenging because of the lateral quantum confinement and multi-electron interaction when graphene is patterned on the nanoscale. The effect causes an electronic band gap.
How to realize the design of stable metal state in GNR?
The researchers' strategy for designing metal GNRs is based on the basic tight-binding electronic structure model, that is, electrons jumping between adjacent local zero modes will cause metal bands. By providing a unit cell for the metal GNR, the unit cell contains two extra carbon atoms on the sublattice A, which can be used to design the metal GNR. Under this structure, there are two related hopping amplitudes: intra-cell hopping amplitude (t1) and inter-cell hopping amplitude (t2). When the jumping amplitude is the same, the energy gap is reduced to zero, and the resulting one-dimensional electronic structure is infinitely close to metal.
Based on the above strategy, the researchers designed the GNR precursor molecule 1 and deposited it on a clean Au surface. By heating the surface to 200°C for 20 minutes, and then performing a second annealing step at 300°C for 20 minutes to complete the ring dehydrogenation reaction, a gradual growth polymerization of 1 is induced. Under ring dehydrogenation, the methyl carbon atom attached to the central naphthacene will fuse and provide an extra carbon atom on each sublattice.
The previous gradual growth polymerization of structurally related molecules indicated that the surface polymerization of molecule 1 would place the central naphthacene unit on both sides of the GNR growth axis in an alternating pattern. If the polymerization reaction proceeds from start to finish, the resulting GNR has two additional carbon atoms on the sublattice A of each unit cell A.
After ring hydrogenation, the expected GNR structure consists of short sawtooth edges and protruding concave regions, called sawtooth GNR (sGNR). Judging from the symmetry of the sGNR unit cell, the researchers predict that the jump amplitudes t1 and t2 will be equal, resulting in the metal band structure of sGNR. In addition, long-term annealing of sGNRs at temperatures> 300°C will cause the dehydrogenation of the secondary ring in the red line region, resulting in the formation of five-membered rings along the edges of the sGNRs. The resulting zigzag GNR is called 5-sGNR.
Subsequently, the researchers used STM spectroscopy to characterize the electronic structure of sGNR to verify the metallicity of GNR. The result is as expected in theory, both sGNR and 5-sGNR have topological homojunction and metal-semiconductor heterojunction, showing metallic behavior. However, the energy of the metal DOS characteristic of 5-sGNR is much wider than that of the EF narrow peak of sGNR.
Combined with DFT calculations, it is shown that both experimentally and theoretically, 5-sGNR has a more stable metallicity, and its bandwidth is much wider than that of sGNR, and no magnetism occurs when transferring from Au (111) to an insulator. Phase change. This is because the formation of the five-membered ring destroys the binary lattice symmetry of graphene, resulting in the loss of sublattice polarization.
That is to say, the intentional fusion of the five-membered ring along the edge of the concave corner greatly increases the effective overlap of adjacent local zero modes and greatly enhances the metal bandwidth, even if the gap between the zero modes along the GNR backbone in space The interval is fixed.
Application prospects
This research is a general strategy for the design and preparation of metal GNRs using atomic-level precise bottom-up synthesis methods, creating opportunities for the development of nano-scale electrical devices and the exploration of electronic and magnetic phenomena in such one-dimensional metal systems . At the same time, metal graphene nanoribbons can be used to explore single-dimensional strange quantum phases, and can be used as metal connection lines in future microprocessors, which have great prospects in the fields of microelectronics, graphene circuits, and nanoribbon connection lines. .