Structure and stability of germanium nanoparticles

Abstract

In order to control and tailor the properties of nanodots, it is essential to separate the effects of quantum confinement from those due to the surface, and to gain insight into the influence of preparation conditions on the dot physical properties. We address these issues for the case of small Ge clusters (1-3 nm), using a combination of empirical and first-principles molecular dynamics techniques. Our results show that over a wide temperature range the diamond structure is more stable than tetragonal, ST12-like structures for clusters containing more than 50 atoms; however, the magnitude of the energy difference between the two geometries is strongly dependent on the surface properties. Based on our structural data, we propose a mechanism which may be responsible for the formation of metastable ST12 clusters in vapor deposition experiments, by cold quenching of amorphous nanoparticles with unsaturated, reconstructed surfaces.In semiconductor nanoparticles quantum confinement leads to an increase of the optical gap compared to the bulk value and thus opens new possibilities for controlling photoluminescence effects, with narrow emission spectra tunable over a wide range of wavelengths [1,2]. These properties make semiconductor dots attractive for many applications including photovoltaics, lasers and infrared dyes. Furthermore, their brightness, low toxicity and the ability to use a single excitation wavelength make them good alternatives to organic dyes for biological labelling, but their low water solubility has limited biological applications. However, recent experiments have shown that using specific coatings, the surface of selected semiconductor nanodots can be tailored to enhance the chemical interaction with a biological sample and the water solubility [3,4].Understanding the influence of surface reconstruction and passivation on the ground state properties of semiconductor nanodots is a key prerequisite not only in designing biological applications, but also for controlling deposition of nanoparticles on surfaces and aggregation of multiple dots into new structures. In order to tailor the properties of nanodots, it is important to separate the effects of quantum confinement from those due to the surface, and to gain insight into the mechanism by which preparation conditions can influence the dot atomic structure and thus its optical properties.Here we address these issues for the case of small Ge dots (1-3 nm), whose atomic structure is the most controversial amongst those of group IV and II-VI semiconductors. While some preparation techniques, including chemical methods [5][6][7][8][9], yield diamond-like Ge dots irrespective of size, several experiments [10-12] suggest a structural transition, as the dot diameter becomes smaller than 4-5 nm. In particular, some experiments [13,14] using vapor deposition techniques indicate a change from a cubic diamond (DIA) to a tetragonal structure, possibly ST12, in contrast to the behavior found for Si [15][16][17] and other II-VI dots [18,19]. In the bulk, the ST...

Optical properties and structure of nanodiamonds

“…All clusters exhibit the Td geometry except for the smallest (CH3 and C2H6). Upon relaxation, we observe strong angular distortions at the surface and we note that, conversely to the other group IV nanoclusters [7], size reduction to a few nanometers causes a tensile stress to act on the structure. This results in a significant increase, up to 6% in comparison to bulk, of the C-C bond length.…”

“…In this paper, we present a theoretical study showing that diamond has unique properties not only as a bulk material but also on the nanoscale, where size reduction and surface reconstruction effects are fundamentally different from those found, e.g. in Si and Ge [7]. We performed ab-initio calculations using Density Functional Theory (DFT), and compared the results with previous X-ray absorption and emission measurements on nanodiamonds synthesized in detonation waves from high explosives [8].…”

Quantum chemistry of the minimal CdSe clusters

“…20 The possibility of surface reorganization by partially saturating Se dangling bonds thus has been neglected in previous theoretical investigations. More realistic simulations of the surface reconstruction using first principle methods had been published recently, 16,19,21 and include self-healing and ligand effect of CdSe NCs.…”

Enhanced radiative transition inSinGemnanoclusters

“…Theoretical studies on the structure and stability of Ge clusters [18], the polarizabilities of small Ge clusters [19], and the quantum confinement effect on excitons in Ge quantum dots [20] have also been reported. Since Ge has smaller electron and hole effective masses and a larger dielectric constant than the corresponding quantities for Si, the effective Bohr radius of the exciton in Ge is larger than that in Si, and the quantum confinement effect appears more pronounced in Ge than in Si [16,17,20,21].…”