Study On Electronic And Optical Properties Of Carbon-Based Nano-Devices And Behaviors Of Helium Atoms In Metal

In this work, we kept the focus on electronic and optical properties of carbon-based nano-materials and nano-devices and behaviors of He atoms in metal tritides and blanket materials for nuclear reactors. The main research contents and results are shown as follow:1. Carbon-based nano-materials (such as fullerene, monatomic carbon chains and graphene) by means of remarkable mechanical, electronic and optical properties have the potential to replace silicon-based materials as the electronic material of choice in the future. Currently, searching for novel carbon-based nano-materials to design nano-devices has been a highly concerned topic.(1) Recently, we found that SWCNCs can serve as electronic rectification devices. However, the rectification effects are strongly influenced by the electrode materials, e.g. the rectification effects with Au electrodes disappear if Pt electrodes is used. This phenomenon has not been investigated particularly and the mechanism is still unclear. In fact, similar effects exist commonly in molecular rectification devices. Here, we systematically investigate the dependence of electronic rectification effects on different electrode materials and different contact distances between SWCNCs and the electrodes via the non-equilibrium Green’s function method combined with the density functional theory (DFT). The results show that rectification mechanism rooted in the energy level of the SWCNCs, which are strongly influenced by the electrode materials. Materials with a lower bonding energy of them with SWCNCs (<2eV) should be selected as the electrode for SWCNC rectification devices. Moreover, the rectification effects can be significantly improved by increasing the contact distance between the SWCNC’s tip and the electrode. According to the present calculation, Cu is a better candidate for replacing Au electrodes in a SWCNC rectification device.(2) Since we theoretically proposed a scheme to mechanically pull monatomic carbon chains from single-layer graphene, some research groups have been achieved on the synthesis and characterization of MCCs. Up to now theoretical studies on basic properties of MCCs have been made by density functional theory for decades. But the calculation of bend gap (Eg) is far smaller than the experimental measurements, for example, the experiment value of Eg is 2.2 eV or 2.56 eV while the calculation is 0.3 eV. In this work, we employed the Heyd-Scuseria-Ernzerhof hybrid density functional (HSE06) to investigate the structural, mechanical, electronic and optical properties of MCCs. The results show that the calculations agree quite well with the experimentally extrapolated Eg for the infinite chain, suggesting that the HSE06 hybrid functional with optimized parameter xo is capable of describing the main structural and electronic properties of MCCs at equilibrium; MCCs have remarkable mechanical properties with the calculated 1D elastic modulus C1D of 90 eV/A, the ultimate strain yielded of 20% and breaking force of 12.2 nN (experimental result of 11.2 nN); and the band structure of MCCs exhibits a direct band gap with the corresponding Eg of 2.21 eV. It is worth noting that the strong anisotropic optical response and monochromaticity were observed from the absorption feature of MCCs, suggesting MCCs is a better candidate for the light-emitting materials.(3) During the last decade, the sizes of electro-optical devices have been greatly reduced to the scale of one hundred nanometers by applications of nano-size optical media such as quantum dots and wires. However, limited by the optical media, the wavelength of these devices can hardly be continuously tuned, which hinders relevant applications in on-chip communications and high sensitive detectors. Here, a scheme of mechanical stretching on MCCs is proposed to this issue. In the elastic area, the electronic and optical properties of the stretched MCCs were researched. The results show that the Eg of the MCC increase from 1.58 to 3.86 eV, and the direct gap feature holds within all elastic area (elastic strain from -5% to 10%). And we found that the absorption peak blue-shifts as the strain increases with the peak energy always equal to the band gap. Then we proposed a realistic stretching device by contacting a 10 atom-long chain with two graphene pads, which has already been realized experimentally, and calculated the mechanical property of this device. Considering the calculated the maximum strain of this device was 9%, the wavelength of this device can be continuously tuned from 345 to 561 nm.2. Tritium is an essential source for future thermonuclear energy production. However, due to its high mobility and radioactivity, tritium handling and storage raise safety issues. It is commonly stored in the form of a metal tritide for the safety, easy recovery and high stored density. The diffusion and accumulation of He atoms from tritium decay form high pressure He bubbles in metal tritide, which lead to the change of properties, such as swelling, surface roughening and blistering, and eventually fracture of the materials, reducing their lifetime. Thus, understanding the formation mechanism of He bubbles at atomic level is crucial for the material design and prolonging their lifetime by restraining bubble formation and growth. To address these issues, we made the following studies:(1) From the microstructure of practical materials, a kinetic model for describing the nucleation and growth of He bubbles was developed. The nucleation sites and growth rate were provided using this model combined with first principle calculation.(2) To apply the above model, we studied the mechanism of nucleation of He bubbles in titanium tritide (β-TiT2). The results show that He atoms were trapped in inherent big defects of grains, forming He bubbles and the calculated diameter of the bubbles increases linearly as t1/3 in titanium tritides at room temperature, which agrees quite well with the experimental measurements. Then, three critical factors (the temperature, energy barrier of He diffusion and defects density in grains) to restraining bubble growth were quantitatively discussed using our model. The way of reducing temperature down to 225 K or increasing diffusion barrier up to 1.1 eV can effectively restrain bubbles growth and prolong lifetime of titanium tritides more than 4 times.