Chemistry of Silicon Nanocrystal Surfaces Exposed to Ammonia

Abstract

Silicon nanocrystals (NCs) 8-10 nm in diameter are grown on SiO 2 surfaces in an ultrahigh-vacuum chamber using hot wire chemical vapor deposition. These NCs are subjected to varying exposures of deuterated ammonia (ND 3 ). The surface chemistry of Si NCs is studied using X-ray photoelectron (XP) spectroscopy and temperature-programmed desorption (TPD). The dissociative adsorption of ND 3 on Si NCs results in the formation of ND 2 species prior to TPD, and Si 3 N (nitride) formation is observed after TPD. D 2 desorption is observed only from the monodeuteride species at 780 K. In separate experiments, a hot tungsten filament is used to predissociate ND 3 before adsorption on the NC surface. XP spectra reveal that ND 2 species form initially, and as the dose is increased, ND species dominate. After TPD, a Si x N y species is observed. D 2 desorption is observed from the mono-, di-, and trideuteride species when ND 3 is predissociated. Irrespective of the technique used for dosing ammonia, TPD spectra do not contain any ND 3 fragments, indicating that the ND 2 species are not thermally stable on the NC surface. The photoluminescence (PL) emitted from Si NCs (diameter ∼ 4.1 nm) is reported for an excitation wavelength of 405 nm. PL is observed only when the hot filament is used to predissociate ND 3 , and this can be attributed to the presence of di-and trideuteride species on the nanocrystal surface, which results in better passivation.

Optical absorption and emission of nitrogen-doped silicon nanocrystals

“…Without being embedded in a matrix, freestanding Si NCs have gained popularity owing to their easily accessible surface. 2,[11][12][13][14][15][16][17] Both ammonia exposure of as-synthesized Si NCs 16 and intentional nitrogen doping during Si NC synthesis 17 have attempted to improve the stability of freestanding Si NCs in air.…”

Aerosol Process for theIn SituCoating of Nanoparticles with a Polymer Shell

“…As shown in Figure 8, these particles acquire a polymer coating of only about 4 nm which is less than the one of silica agglomerates. The dissociative adsorption of ammonia on silica surfaces has been studied using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) (Taylor et al 1989;Salivati et al 2010). It was found that the dissociative adsorption of ammonia on Si (100) at 300 K leads to the formation of NH 2 (a) and H(a) species.…”

Effect of Surface Chemistry on Quantum Confinement and Photoluminescence of Ammonia-Passivated Silicon Nanocrystals

“…The mono- and dideuteride are denoted by β 1 and β 2 , which desorb around 780 and 680 K, respectively; the trideuteride denoted by β 3 appears as a broad low temperature feature before the dideuteride feature. In the N 1s XP spectra, the Si−ND 2 species appear at 398.5 eV and the Si 2 ND species appear at 398 eV . ND 3 exposures are reported in Langmuirs (1 L = 10 −6 Torr s).…”

“…Umemoto et al have shown that ammonia decomposition on a hot tungsten filament results in the formation of predominantly H atoms and NH 2 radicals, with NH radicals formed as a minor product . In separate experiments, a hot tungsten filament was used to predissociate ND 3 before adsorption on the NC surface . XP spectra revealed that Si−ND 2 species are present initially, but as the ND 3 dose is increased, Si 2 ND species dominate.…”

“…Si NCs grown using 15 min of HWCVD (∼4 nm diameter) were subjected to various hot wire doses of ND 3 at 375 K and were moved to the transfer chamber where PL was measured (Figure ) at 310 K. No PL was observed when the NCs were subjected to dissociative (thermal) adsorption of ND 3 at 375 K . Hot wire predissociation of ND 3 was necessary to observe PL.…”