Photoluminescence silicon clusters

Clusters of small and intermediate size play an important role in the explanation of the chemical and physical properties of matter on the way from molecules to solids. The great interest in silicon clusters stems from the importance of silicon in solid-state physics and from the photoluminescence properties of porous silicon. In Chapter 20, A. F. Sax presents theoretical models to describe bare and hydrogenated small silicon clusters and discusses... 

One of the great issues in the field of silicon clusters is to understand their photoluminescence (PL) and finally to tune the PL emission by controlling the synthetic parameters. The last two chapters deal with this problem. In experiments described by F. Huisken et al. in Chapter 22, thin films of size-separated Si nanoparticles were produced by SiLL pyrolysis in a gas-flow reactor and molecular beam apparatus. The PL varies with the size of the crystalline core, in perfect agreement with the quantum confinement model. In order to observe an intense PL, the nanocrystals must be perfectly passivated. In experiments described by S. Veprek and D. Azinovic in Chapter 23, nanocrystalline silicon was prepared by CVD of SiH4 diluted by H2 and post-oxidized for surface passivation. The mechanism of the PL of such samples includes energy transfer to hole centers within the passivated surface. Impurities within the nanocrystalline material are often responsible for erroneous interpretation of PL phenomena. 

The study of small and intermediate-sized clusters has become an important research field because of the role clusters play in the explanation of the chemical and physical properties of matter on the way from molecules to solids/ Depending on their size, clusters can show reactivity and optical properties very different from those of molecules or solids. The great interest in silicon clusters stems mainly from the importance of silicon in microelectronics, but is also due in part to the photoluminescence properties of silicon clusters, which show some resemblance to the bright photoluminescence of porous silicon. Silicon clusters are mainly produced in silicon-containing plasma as used in chemical vapor deposition processes. In these processes, gas-phase nucleation can lead to amorphous silicon films of poor quality and should be avoided.On the other hand, controlled production of silicon clusters seems very suitable for the fabrication of nanostructured materials with a fine control on their structure, morphological, and functional properties. ... 

The optical band gap of silicon clusters increases with decreasing cluster size and all surface atoms with dangling bonds contribute to mid-gap electronic states (defects) which are assumed to quench photoluminescence. Saturation of the surface dangling bonds with hydrogen is the simplest way to eliminate defects, and therefore theoretical studies of the electronic structure of silicon clusters without defects are most easily performed on hydrogenated silicon clusters. The thermodynamic stability of these species is also of interest, because hydrogenated silicon clusters are formed in thermal and plasma CVD of silane. ... 

Cubic Si-skeleton clusters were used to model the optical properties of porous silicon (40). In the octasilacubane 15, the absorption edge is observed at approxiamtely 3.2 eV, and a broad photoluminescence spectrum is also observed with a peak of 2.50 eV. Both porous silicon and octasilacubane show broad photoluminescence spectra and large Stokes shifts. 

Photoluminescence excitation (PLE) spectroscopy was carried out at 77K on oxidized porous silicon containing iron/erbium oxide clusters. The novel PLE spectrum of the 1535 nm Er PL band comprises a broad band extending from 350 to 570 nm and very week bands located at 640, 840, and 895 nm. The excitation at wavelengths of 400 - 560 nm was shown to be the most effective. No resonant PLE peaks related to the direct optical excitation of Er by absorption of pump photons were observed. The lack of the direct optical excitation indicates conclusively that Er is in the bound state and may be excited by the energy transfer within the clusters. 

Optical properties of dielectrics can be modified by incorporating nanosize clusters of foreign materials. Recently Si nanoclusters were shown to excite rare-earth element Er in the silica glass host [1,2]. The favorable effect of Si nanoclusters on the photoluminescence of Er in oxidized porous silicon (OPS) was also demonstrated [3], In silica hosts doped with Si nanoclusters it was shown that the excitation energy can be transferred from nanoclusters to Er ions located in a silicalike environment near the clusters. Nowadays, there is a principal interest to incorporate Er ions inside clusters due to influence on the excitation process. 

CO2 laser pyrolysis of silane in a gas flow reactor and the extraction of the resulting silicon nanoparticles into a cluster beam apparatus has been shown to offer an excellent means for the production of homogeneous films of size-separated quantum dots. Their photoluminescence varies with the size of the crystalline core. All observations are in perfect agreement with the quantum confinement model, that is, the photoluminescence is the result of the recombination of the electron-hole pair created by the absorption of a UV photon. Other mechanisms involving defects or surface states are not operative in our samples. 

Hydrogenated amorphous silicon was formed by plasma decomposition of monosilane gas. The network has the dimension of close to 3. Polysilane alloy was formed by plasma decomposition of disilane gas.33 The network consists of a mixture of 1-dimensional polysilane and 3-dimensional silicon micro clusters.34 The effective network dimension is lower than that of amorphous silicon. Photoluminescence observations for various silicon-based materials are shown in Figure 14. The peak energy values of the photoluminescence spectra for amorphous silicon, polysilane alloy, hexyl-silicon network polymer and dihexylpolysilane are 0.8, 1.2, 2.8 and 3.3 eV, respectively. This result confrrms that a wide continuous spectra range from ultraviolet to infrared can be covered by the luminescence spectra of silicon based polymers. 

Using Dye/amine/alkyl bromide, iridium-containing photoluminescent polyacrylate films have also been manufactured. Polyoxometallate (POM)/polymer hybrid materials exhibiting modified mechanical properties (polymerized EPOX/TMPTA matrix with inserted POM clusters [XIA 15]) were synthesized in a one-step process using a silicon or phosphor polyoxomolybdate/iodonium salt/silane system. 

Indirect band-gap materials exhibit no significant photoluminescence (PL) at room temperature. As a specific nano-effect the quantum confinement in semiconductor clusters induces visible room-temperature PL. Thus, nano-colloidal silicon, molybdenum sulfide, and pyrite shows intense luminescence. 

Abstract This chapter reports numerical models devoted to predict the optical and vibrational properties of nanoparticles treated as isolated objects or confined in host matrixes. The theoretical data obtained by the numerical simulations were confronted with the experimental investigations carried out by several spectroscopic methods such as Raman, IR, and UV-Vis absorption as well as photoluminescence. As model cluster systems, the physical properties of nanosized silicon carbide (SiC) particles in vacuum were numerically modeled. The computer simulations were also performed for SiC confined in polymeric matrix, namely, poly(methyl methacrylate), poly-N-vinylcarbazole, and polycarbonate. The obtained host-guest nanocomposites exhibit original optical and electro-optical features. 

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