Optical luminescence from alkyl-passivated Si nanocrystals under vacuum ultraviolet excitation: Origin and temperature dependence of the blue and orange emissions

Abstract

The origin and stability of luminescence are critical issues for Si nanocrystals which are intended for use as biological probes. The optical luminescence of alkyl-monolayer-passivated silicon nanocrystals was studied under excitation with vacuum ultraviolet photons ͑5.1-23 eV͒. Blue and orange emission bands were observed simultaneously, but the blue band only appeared at low temperatures ͑Ͻ175 K͒ and with high excitation energies ͑Ͼ8.7 eV͒. At 8 K, the peak wavelengths of the emission bands were 430± 2 nm ͑blue͒ and 600± 2 nm ͑orange͒. The orange and blue emissions originate from unoxidized and oxidized Si atoms, respectively. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2216911͔Silicon nanostructures have attracted great interest since the observation of their efficient visible photoluminescence at room temperature in the early 1990s. 1-4 Silicon nanocrystals ͑SiNCs͒ are potential building blocks of future electronic and photonic devices. They also have promise as luminescent labels in biological applications, 5-9 because they show red-orange luminescence at small particle sizes ͑ca. 2 nm diam͒ and are not expected to show some aspects of the toxicity of cadmium-based semiconductors. However, the stability of SiNCs towards O 2 and H 2 O is a concern. Recently, several groups, including ourselves, have prepared alkyl-passivated silicon nanocrystals. [5][6][7][8][9][10][11][12][13][14][15][16] These are SiNCs whose surface is capped by a monolayer of saturated hydrocarbon molecules and anchored to the Si core via covalent Si-C bonds. Although the particles contain a little oxide from their preparation, the alkyl monolayer protects the Si core and they are not oxidized further under ambient conditions. 6 The reaction used to prepare the capping monolayer is sufficiently general to allow manipulation of the chemical functionality on the particle surface, e.g., for synthesis of DNA strands. 6 However, it is also important to characterize the physical properties, especially the luminescence, of these systems because the utility of the particles depends on their luminescence upon insertion into biological cells.Compared to the generally weak IR luminescence of bulk silicon ͑observed at low temperatures͒, the efficiency of photoluminescence ͑PL͒ from Si nanostructures is strongly increased due to the greater overlap of the electron and hole wave functions and the decrease in efficiency of nonradiative pathways. 17,18 A few electronic structure calculations on alkylated SiNCs have been made using density functional methods: these calculations include Si 29 clusters passivated with CH 3 or CH 2 ͑Ref. 19͒ and, more recently, alkyl monolayers up to C 4 H 9 on cluster sizes from Si 20 to Si 142 . 20 Theoretical work suggests that the band gap of SiNCs is little changed upon alkylation of the hydrogen-terminated particle surface, but the positions of the band edges are shifted significantly towards the vacuum level and the properties of the excited states are affected. 20 Experimental studies of the PL mechanism o...

Soft X‐ray induced oxidation on acrylic acid grafted luminescent silicon quantum dots in ultrahigh vacuum

“…Sham et al [38] measured X-ray excited optical luminescence (XEOL) and X-ray emission spectroscopy (XES) on silicon nanowires and showed that the blue emission is associated with the silicon oxide layer on the samples. This suggests that the blue PL emission is from oxidized Si species and the red PL emission originates from unoxidized Si core, which is in agreement with the previous results from alkyl Si-QDs [26]. It should be noted that the PL colour will be dominated by the main peak 604 nm here, which is red.…”

“…The motivation behind this work is twofold: over the course of exposure the QDs are observed to undergo a photon-induced oxidation, based on the evolution of the Si2p core level, and so is of interest from a purely investigative perspective in furthering our understanding of their luminescence behaviour. Similar photon-oxidation effects have been observed in light-emitting porous silicon [23], germanium nanocrystals [24], alkylated silicon nanocrystals [25,26], and crystalline silicon surfaces [27,28], where the photon source was either a laser, UV, EUV/VUV, or X-rays. This work is of particular value owing to the ability to disperse acrylic acid grafted Si-QDs in water which may offer new routes towards bio-imaging applications within living systems.…”

Optical Properties of Nanostructured Silicon

“…In order to investigate the detailed temperaturedependent behavior of Si NCs further, temperature quenching spectra were obtained by Chao et al [64] with 21 eV excitation energy as the temperature was increased at a rate of 15 K min À1 (Figure 10). The monochromator for the detector, a photomultiplier tube (PMT), was set at blue and orange wavelengths, 420 AE 10 nm (blue) and 610 AE 10 nm.…”

“…Higher-resolution images were obtained by STM on alkylated n-Si (111) substrates in which individual Si NCs could be resolved within the islands. If the source temperature was set to 200 C, the mean diameter of the Si cores was 1.8 AE 0.34 nm for the evaporated material compared to 2.2 AE 0.38 nm for as-prepared alkyl-Si NCs [12,64,71]. The ability to evaporate and deposit NCs in ultrahigh vacuum (UHV) should be useful for the size-controlled preparation of new nanoscale quantum-confined structures.…”

The effect of alkyl chain length on the level of capping of silicon nanoparticles produced by a one-pot synthesis route based on the chemical reduction of micelle

“…These are major factors which have helped drive interest in the synthesis and application of SiNPs over the past twenty years (Ahire et al, 2012, Wilson et al, 1993, Kang et al, 2011, Chao et al, 2007. Quantum confinement effects give SiNPs interesting optical, electronic and mechanical properties (Reboredo and Galli, 2005, Pavesi et al, 2000, Chao et al, 2006, O'Farrell et al, 2006, Rosso-Vasic et al, 2008.…”