First-principles Study Of The Thermodynamic Properties Of One And Two Dimensional Nanomaterials

With decreasing device size, low dimensional materials are attracting extensive attention due to their novel properties and promising applications. Moreover, temperature is found to play significant roles in determining nanomaterial properties and applications, and the investigations on nanomaterial thermodynamic properties are insufficient. On the other hand, first-principles simulation is an efficient approach in scientific research accompanied by computer development. Thus, in this dissertation, we investigate the one and two dimensional (1D and2D) nanomaterial thermodynamic properties based on first-principles density functional theory. The thermodynamic properties investigated include specific heat, thermal conductivity and thermal expansion behaviour.In Chapter1,1D and2D material research status is discussed first. These low dimensional systems are found to possess novel properties different from their bulk allotropes, and many of them are fabricated in experiments using a variety of methods. The superior properties and mature experiment technologies enable low dimensional system potential applications. Secondly, the existing simulation methods for investigating thermodynamic properties are introduced. The widely used methods are the molecular dynamics (MD) simulations and various approaches based on phonon dispersion. The MD process and methods for obtaining phonon dispersion, including the frozen phonon approach and density functional perturbation theory are discussed. Finally, our formulas adopted in investigating specific heat, thermal conductivity and thermal expansion coefficient are presented.In Chapter2, the first-principles density functional theory (DFT) is introduced, including its original, fundamental theory and equations. Referring to the DFT calculations, the exchange and correlation functional significantly determines the result accuracy. Several existing approximations to this functional are introduced in sequence. Peosudopotentials are efficient in accelerating computation processes, and is widely used in simulations. The popular norm-conserving and ultrasoft pseudopotentials are also simply introduced in this chapter. The rest of this chapter deduces the specific heat, thermal conductivity and thermal expansion coefficient equations adopted based on phonon dispersion.In Chapter3, we focus on the1D material thermodynamic properties. Single-walled boron and zinc oxide (ZnO) nanotubes are investigated. For the single-walled boron nanotubes, their specific heats and thermal expansion behaviour are investigated based on a tube thermal isotropy assumption. The boron nanotube thermal expansion coefficient is determined to be dependent on tube chirality and diameter, and remarkable thermal contraction is found in small-sized tubes. For the single-walled ZnO nanotubes, we firstly propose an approximation for calculation of the ID system TEC, and the ZnO nanotube TECs are studied based on this assumption. All the single-walled ZnO nanotubes except for the (3,3) tubes are found to contract with increasing temperature. The thermal contraction amplitude presents an inverse ratio to tube mechanical strength. The (3,3) tube expands across the whole range of temperatures investigated, which is different from most atomic layer systems. A mechanism based on the structural properties for this abnormal thermal expansion behaviour is elucidated.In Chapter4, the2D system thermodynamic properties are discussed. In the first step, we investigate pristine monolayer graphene, silicene and germanene consisting of IV-A group elements, carbon, silicon and germanium, respectively. For the three monolayers graphene, silicene and germanene, the result implies that specific heat and thermal conductivity decrease, while the thermal contraction amplitude increases with increasing atomic number. These trends are determined to relate to atomic mass and radius. Secondly, we consider the thermal expansion behaviour of point defect graphenes. Four configurations, pristine graphene, graphene with one carbon atom replaced by boron and nitrogen, and graphene with two neighbouring carbon atoms replaced by boron nitrogen atoms, named Graphene, Graphene_B, Graphene_N and Graphene_BN respectively, are investigated. We find that all the point defects enhance graphene’s mechanical strength, and indeed weaken graphene’s thermal contraction. Moreover, p-doping is found to weaken graphene’s thermal contraction more evidently than n-doping. The point defect weakening of thermal contraction could be very useful in reducing the mismatch between graphene and its substrate under variable temperatures. Thirdly, the specific heat, thermal conductivity and thermal expansion behaviour of a monolayer ZnO sheet, a fascinating II-VI compound, are studied. Relatively low specific heat and thermal conductivity combined with remarkable thermal contraction are determined, which imply that the monolayer ZnO sheet is an ideal material to induce strain for other atomic films based on variable temperatures. Finally, the thermal expansion behaviour of ZnO sheets with up to five layers is researched. The thermal expansion increases with increasing layer number. The monolayer and bilayer ZnO sheets contract across the entire range of temperatures investigated, while the thermal contraction amplitude is much larger in the monolayer sheet. The trilayer sheet contracts under low temperature and expands under high temperature, so its thermal expansion coefficient absolute value is relatively small. The four-and five-layer sheets expand with increasing temperature. Combined with the decreasing energy bandgap of ZnO sheets with growing layer number, a temperature controlled tunable wavelength laser is proposed.A summary of the whole thesis is presented in Chapter5.