Strong Dependence of Structural and Electronic Properties of Rod-Shaped [RGaNH]_3n (R = H, CH₃) Oligomers on Terminal Groups

Abstract

On the basis of DFT calculations, we demonstrate that terminal effects have a crucial impact on electronic properties of rod-shaped [RGaNH] 3n (R = H, CH 3 ) oligomers. Capping of oligomers with GaR and NH groups results in an almost periodic structure in which terminal groups affect only the very edges of the oligomer. The [RGaNH] 3n+1 rod-shaped oligomers are considered as an intermediate between finite clusters and [RGaNH] 3∞ polymer. Electronic properties of the oligomers are extrapolated to the infinite limit and band structure of the polymer is extracted from oligomer calculations. The energy gap of the [RGaNH] 3n+1 is defined by states localized at the different ends of the oligomer. The value of the gap is converging fast with increase of n, and it is about 93% of the value of the band gap of the polymer at the infinite limit. In contrast, termination of the [RGaNH] 3n rod-shaped oligomer by saturation of dangling bonds with H or CH 3 groups destroys the periodic pattern and increases the number of ends localized sates. This way of termination is characterized by systematic change in structural parameters of the oligomer and near exponential reduction of the energy gap with the oligomer length. For the rod-shaped oligomer of 10 nm of length the gap is only 27% of the [RGaNH] 3∞ polymer band gap and the value is not yet converged. ■ INTRODUCTIONConfinement effect of nanoscaled semiconducting systems provides a link between properties of bulk semiconductors and those of discrete molecules. Size dependence and sensitivity of the properties of nanorods to subtle structural variations leads to immense design freedom for creating functional elements of nanoscaled devices. However, understanding of the electronic properties of nanoobjects to a great extent relies on a computer simulation. 1 Periodicity is an essential property of bulk semiconductors and band structure is an important characteristic of their electronic properties. Being intermediates between molecules and bulk crystals, semiconducting nanoparticles also exhibit periodic properties in structural as well as in electronic features. It is a common approach treating nanorods within periodic boundary conditions (PBC). 1−4 PBC approximation allows to study the effect of surface dangling bonds. 5,2 It was found that the band gaps of GaN 2 or GaAs 6 nanorods are ruled by surface states and do not change with the diameter of the nanorods according to the trend mandated by the quantum confinement effect. However, the PBC approach is not applicable for studying the effect of dangling bonds at the terminal edges of nanorods. Rather short nanorods could be considered in ab initio study of the edge effects. Investigation of finite size carbon nanotubes shows that terminal effects have strong impacts on the vibrational frequencies 7 and the study of finite graphene nanoribbons shows that the gap defined by the endlocalized states decreases exponentially as a function of the ribbon length. 8 It is expected that terminal effects should be profound in the case of I...

The effect of terminal substituents on the electronic properties of rod-shaped [HGaNH]n oligomers

“…[4] It was found that a choice of PBE0 instead of B3LYP causes increasing of HOMO-LUMO gaps values for about 0.55 eV, while the overall tendencies in the changes of the HOMO-LUMO gap upon the rod elongation are similar. [4] It was found that a choice of PBE0 instead of B3LYP causes increasing of HOMO-LUMO gaps values for about 0.55 eV, while the overall tendencies in the changes of the HOMO-LUMO gap upon the rod elongation are similar.…”

“…[1] A potentially interesting subclass of GaN based nanostructures is oligomeric clusters composed of [HGaNH] 3 rings with the wurtzitic arrangement of the GaN backbone. [2][3][4][5][6][7][8][9][10][11][12] Thermodynamic analysis [3,4] indicates that spontaneous generation of fairly long oligomers is thermodynamically feasible in a broad temperature range. [3] Stability, structural features and optical properties of the rod-like group 13-15 oligomeric clusters were the topics of many computational studies .…”

“…[2][3][4][5][6][7][8][9][10][11][12] Thermodynamic analysis [3,4] indicates that spontaneous generation of fairly long oligomers is thermodynamically feasible in a broad temperature range. [4,11] A variation of the HOMO-LUMO gap with the increase of the oligomerization degree n for the needle-shaped ("closed") oligomers RM[RMNH] n NH is significantly smaller than for the rod-like ("open") oligomers R 3 [RMNH] n H 3 . [5][6][7][8] 3 ; n = 4, 7, 10) was explored computationally.…”

“…The rod-shaped oligomers are energetically more favorable than cage-like isomers or 3-dimensional structures. [4] The strong influence of terminal groups at the oligomer ends on its electronic properties opens a way for fine tuning of the HOMO-LUMO gap by variation of only terminal substituents. [9][10][11][12] It was also shown that termination of the rod-shaped oligomers significantly influence their structural and electronic properties.…”

“…However, the substituents play a significant role for molecules with n=9 where the opposite ends of the molecule are close enough to interact with each other. Estimated band gap of [HGaNH] ∞ polymer, extrapolated from the oligomer computations, is circa 7.0 eV [4] . At such length (circa 10 nm) terminal groups do not affect the opposite ends of the rod.…”