Shape and Size Dependence of Electronic Properties of InSb Diamondoids and Nanocrystals: A Density Functional Theory Study

Shape and size dependence of electronic properties of InSb diamondoids and nanocrystals is investigated using density functional theory. Cluster and large unit cell methods are combined with molecular orbital methods to obtain electronic structure of InSb diamondoids and nanocrystals. Starting from the simple molecules of hydrogenated InSb clusters such as InSbH6, In3Sb3H12, InSb-diamantane, InSb-tetramantane, and InSb-hexamantane and ending with InSb large unit cell method we were able to obtain the electronic structure of a wide range of InSb nanostructures. Results showed that energy gap and In–Sb bond lengths generally decrease as the number of atoms increases with remarkable dependence on the shape of the molecule or nanocrystal. Atomic charges, tetrahedral angles, and bond lengths are used to compare different sizes, locations, and shapes of InSb diamondoids and nanocrystals. 1. Introduction InSb is a unique semiconductor. It has one of the largest lattice constants and one of the smallest energy gaps. These properties give InSb the opportunity to cover some applications that no other semiconductor can be used for. As an example, the small energy gap (0.17？eV [1]) nominated InSb to be used in infrared devices and other applications. These applications span photodiodes [2], thermal imaging [3], terahertz radiation [4], and so forth. Transforming materials to their nanoscale size change, many of their properties. These changes include their electronic, mechanical, and optical properties. InSb nanoparticles undergo these changes that might include new applications as is the case for other materials. In the present work we introduce InSb diamondoids as a molecular limit and building blocks for larger nanocrystals. These molecules (diamondoids) are well known and found in nature as cage like molecules that resemble the tetrahedral bonding of carbon [5]. Cage like molecules other than carbon are found in nature or synthesized with a variety of atoms such as Si, P, and N compounds. We shall show that even numbered diamondoids (diamantane, tetramantane, and hexamantane) can also form stable molecules for the III–V InSb semiconductor compound. Odd numbered diamondoids (adamantane, triamantane, etc.) produce molecules with unequal number of In and Sb atoms. 2. Theory Geometrical optimization method is used in the present work to obtain the electronic structure of InSb molecules and nanocrystals (Figure 1.) These include the following: InSbH6, In3Sb3H12, InSb-diamantane, InSb-tetramantane, and InSb-hexamantane. Figure 1: Shape of geometrically optimized (a)