The Formation Mechanism And Mechanical Properties Of Nanocrystal Superlattices

Nanocrystal superlattices?NCSLs?are a class of two-dimensional or three-dimensional highly ordered superstructures self-assembled from inorganic nanocrystals?NCs?capped with organic ligands.Due to the unique optical,electronic and magnetic properties as well as good tunability,NCSLs are very promising for a wide of applications in electronic,photoelectronic,thermal conductivity and solar cell devices,which currently become a study hotspot in the areas of chemistry and material science.Uuderstanding the structure formation and mechanical properties of NCSLs is of great importance for their structural design and device applications.In this thesis,we theoretically investigate the formation mechanism and mechanical properties of NCSLs.The main content is as follows:Brownian dynamics?BD?simulation based on a coarse-grained model is performed to study the assembly of dodecanethiol-ligated Au NCs at a toluene-air interface.With increasing diameter from 3 nm to 9 nm,the NCs are found to form three different phases-a dispersed phase without aggregation,a mixture of dispersed NCs and rounded clusters,and a compactly packed solid phase of the fractal structure.Such size dependence of the assembled phase is attributed to the variation in the well depth ?c of the interaction potential between NCs,and the value of ?_c=-3.5KBT turns out to be most suitable to form monolayers with hexagonal packing.The result is of universal importance for assembling complete monolayers,because the valley of the interaction potential can be well tailored by properly choosing the NC size,ligand length and solvent.Interaction between NCs is important to understand the formation mechanism and mechanical properties of NCSLs.We report on steered molecular dynamics?SMD?study of two identical interacting alkylthiol-coated tetrahedral gold NCs in vacuum.It is shown that the interaction between NCs greatly depends on their effective softness?the ratio between ligand length and core size?.In the case of small softness,the tetrahedron-tetrahedron interaction is orientation dependent,exhibiting the strongest face-face interaction and weakest vertex-vertex interaction among all the other orientations.However,with the increasing NC softness,a clear anisotropy-isotropy transformation of the tetrahedron-tetrahedron interaction is observed.In addition,it is found that the ligand length and core size can synergistically influence the strength and interparticle spacing of the tetrahedron-tetrahedron interaction.Interparticle spacing in NCSLs is a crucial parameter for their photoelectric properties.Based on large-scale atomic MD simulations,we systematically calculate interparticle spacing in alkylthiol-stabilized gold NCSLs as a function of the NC size,ligand length and external pressure.Our computations indicate that the interparticle spacing increases linearly with increasing ligand length and decreases monotonically with increasing hydrostatic pressure,while no dependence of the interparticle spacing on NC size is found.These are consistent with available experiments.In addition,through a comparison with the case of a single pair of ligated NPs,we show the influence of repulsive many-body interactions on the interparticle spacing in bulk NCSLs.Furthermore,the longer the ligand chains and the smaller the NP size,the more significant the many-body effects.Mechanical property of superlattices is important for their practical applications.Atomistic MD simulations are performed to study the elastic properties of alkylthiol-functionalized gold NCSLs.The predicted Young's and shear moduli are around 1 GPa and 100 MPa,respectively.We show that,with increasing NC size,the Young's modulus decreases while the shear modulus essentially remains invariant;with increasing ligand length,the Young's modulus increases but the shear modulus decreases.Moreover,significant increase in the Young's modulus is seen when the polycrystalline NCs are replaced by single-crystal ones of the same size.We attribute the mechanisms to the interaction between capping ligands as well as its variations caused by the change in ligand length and NC geometry.Using atomistic MD simulations,we report that alkylthiol-capped gold NCSLs have obvious asymmetric elastic constants C₁₁ and C₁₂ in compression and tension and a symmetric C₄₄ in shear deformation.Our results show that the elastic asymmetry of C₁₁ and C₁₂ is attributed to flexible ligands,which can sterically interact more to stiffen the superlattice in compression,while in tension they interact less and cause a less stiffness.Moreover,the degree of elastic asymmetry is found to be independent of ligand length and core size.We perform a coarse-grained MD simulation to study the unidirectional tension of 2D alkythiol-ligated gold NCSLs.Consistent with available experiments,the predicted Young's modulus is in the range of 6-15 GPa,exhibiting a trend of decreasing with the increasing NCs' size and decreasing ligand length.Our simulation shows that the deformation of the superlattice experiences elastic and nonelastic stages before defect nucleation at the NC level.The larger tensile strain gives rise to slips along the most densely packed lines,making them equal to ±60°with the tensile direction before deformation,which further triggers the occurrence of cavities and cracks and finally leads to the fracture of the specimen.This thesis from the perspective of the structure formation and mechanical properties of NCSLs,further study the interparticle spacing,many-body effect,elastic properties and elastic asymmetry of bulk gold NCSLs,and the formation mechanism and tensile behavior of 2D gold NCSLs,and the interaction between tetrahedral gold NCs.These studies provide a theoretical basis for the structural design and device applications of NCSLs.