First Principles Study Of The Electronic And Optical Properties Of Cu Doped ZnO Single-walled Nanotubes

In recent years, zinc oxide (ZnO) has been intensively investigated due to theextraordinary optical and electrical properties. Since the1960s, with the rapid developmentof the nano-technology, the investigation of one dimensional nanomaterial has become thefrontier of nanoscience and nanotechnology. As the size decreases, the surface/volume ratioincreases, the system exhibts a uniform single crystal structure and the growth has adirectional characteristics. These make one-dimensional nanomaterials showing differentproperties from the bulk ones, such as electronic, mechanical, chemical and opticalproperties. Therefore, one-dimensional nanomaterials have more extensive applicationprospect than the bulk ones, and attract great research interest.One-dimensional ZnO nano-devices have great prospects in the visible and ultravioletphotoelectron applations. Due to the excellent physical properties and various potentialapplications, a lot of one-dimensional ZnO nanomaterials with different morphology havebeen successfully synthesized, such as nanowires, nanorods, nanotube, nanoneedle,nanotower, nanobelts, nanometer nail, nano helix, nano branch structure, comb-likenanostructure, nano limbs, and dumbbell structure. The syntheses mainly include thevapour-liquid-solid growth, laser ablation, layered curling, law enforcement, template,sol-gel and hydrothermal methods.Compared with other one-dimensional structures, tubular nanostructures have theporous characteristic and the large surface/volume ratio, which makes them a preferential choice for the applations in dye-sensitized photovoltaic cells, low dimensional stability ofelectrode, metal ion battery, electrochemical super capacitor, hydrogen storage devices,biosensors, gas sensors and the optimal photocatalytic decomposition of water. Up to now,there are no experimental reports that one-dimensional ZnO single-wall nanotubes weresuccessfully synthesized. However, a lot of theories have predicted the feasibility ofsynthesis of single nanotubes. ZnO single-wall nanotubes can be regarded as a cylindricalstructure rolled by the ZnO monolayer with a hexagonal lattice. To describe the ZnOsingle-wall nanotube, the feature vector similar to single-walled carbon nanotube has beenintroduced. The chiral vector Ch is defined with a pair of parameters (n, m), which arenamed the index of the single-wall nanotube. The binding energies of different ZnOsingle-wall nanotubes can be negative, which indicates that ZnO single-wall nanotubes canexist stably.The main studies of the paper are on the above basis, and can be divided into three partsas following:Firstly, we investigated the electronic and optical properties of ZnO single-wallnanotube doped by Cu atoms in the segregation manner. We found that the electronic andoptical properties depend on the doping concentration. As the doping concentration of Cuatoms increases, the conduction band minimum drops and the band gap decreases too.While the impurity levels become more and deeper, the electron transition becomes easier.The dielectric constant and refractive index obviously change with the increase of theimpurity concentration, from ultraviolet region red-shift to the visible light region andfinally to the infrared range. Namely, we can tune the adsorption/emission wavelength by controlling the impurity concentration of the nanotubes.Secondly, the modulation of the electronic and optical properties of ZnO single-wallnanotube by the dispersed doping of Cu atoms has been considered. We calculated asingle-wall (Zn4/6Cu2/6O)3/(Zn5/6Cu1/6O)3nanotube superlattice using the GGA+Utechnique. The results show that the superlattice could improve the solar energy conversionefficiency and be a potential material for visible light water photoelectrolysis. The designof band gap is related to the superlattice structure where the variation of Cu concentrationand the coherent interface exist. The both affect electronic structures of the superlattice andlead to the possible use in redox reaction of water. This new concept will be important inmodulating band gaps of semiconductors with special electronic and optical properties forvarious functional applications.Finally, the modulation of electronic and optical properties and the physical mechanismfor two possible methods to dispersedly dope Cu atoms in ZnO single-wall nanotube arestudied. First, the effects of the interface and middle layers on the band gap are consideredin the variation of periodic (Zn4/6Cu2/6O)L/(Zn5/6Cu1/6O)Lsuperlattices by changing the sizeof L. The band gap exhibits a minimum in S4(L=4) which is nearly a conductor. The bandgaps of S3(L=3) and S5(L=5) also decrease obviously compared to the other threeconfigurations. Especially, the band gap of S3has decreased to2.16eV which is in therange of narrow bandgap semiconductor. The middle layer can increase the VBM ordecrease CBM rapidly. The band gap can be modulated by alloying through constructingappropriate periodic length. And these properties make the superlattices as a potentialmaterial in photocatalysis and the visible light emitter. Then, the other case has been considered. We found that the band gap of ZnO SWNTs can also be modulated by adjustingtotal doping concentration and the concentration gradient. The band gap can decrease from4.5eV of the perfect ZnO single-wall nanotube to1.95eV of ZnO single-wall nanotubedoped by Cu. In same concentration without gradient, the band gap is largest, and then itdecreases as concentration gradient decreases. Under the same concentration gradient, bandgap decreases with the decrease of the concentration.Although it is difficult to synthetize ZnO single-wall nanotubes in the manufacturingfield of nanoscale devices, they will play an important role in the next generation ofelectronic devices due to their excellent optical and electrical properties. Many propertiesof these single-wall nanotubes lead to some technological challenges, however they alsobring the special application performance and prospects. Once the synthesis factors can beprecisely controlled, the practical application of ZnO single-walled nanotubes will beachieved.