Nanocrystalline silicon

A close examination of the voltage profile can yield much information about the physical and chemical process occurring during lithiation. In addition to the lithium concentration, phase transitions are also easily identified by plateaus in the voltage profile. This is a consequence of the Gibbs phase rule, which states that the number of degrees of freedom is a function of the number of components (c] and the number of phases (p] present  

Therefore, in a two-phase region, the chemical potential (or voltage] is fixed over changes in lithium concentration as one phase grows at the expense of the other. 

The voltage profile of Fig. 2.6a exhibits a reasonably smooth slope on both the charge and discharge cycles. The differential capacity, d x /d , where x is the lithium concentration and E is the cell potential, is used to accentuate changes in the slope of the potential curve (Fig. 2.6b]. Peaks in the differential capacity indicate regions of the potential where lithium ions are entering nearly equipotential sites. This plot suggests that the lithium insertion in nanocrystalline silicon occurs at 100 and 250 mV and lithium 

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A completely amorphous structure was found by X-ray diffraction on fibers which were pyrolysed at temperatures up to 1300 °C. A crystallization starts around 1400 °C and nanocrystalline silicon carbide is formed with a crystallite size of about 2 nm. Compared to an uncured sample the crystallization is retarded. A significant crystallite growth occurs around 1500 °C connected with a decreasing of the fiber properties. The oxygen content of these SiC fibers is less than 1 wt. % found by neutron activation... 

Getting Light from Silicon From Organosilanes to Light Emitting Nanocrystalline Silicon... 

Kerdihes S., Rizk R., Gourblleau F., Perez-Rodriguez A., Garrido B., Gonzalez-Varona O. and Morante J. R., Low temperature direct growth of nanocrystalline silicon carbide films, Mater. Sci. Eng. B 69 (2000) pp.530-535. 

Fig. 1. Photoluminescence (PL) spectra (solid line) and absorption respectively excitation spectra (PLE) (dotted line) of a) tetrakis(trimethylsilyl)silane (TTSS) (soluted in hexane) b) polydimethylsiloxane (PDS) c) oxidized nanocrystalline silicon d) bulk silicon [T transmission D crystallite size).

Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only metastable under ambient conditions and oxidation of freshly prepared porous silicon commences within minutes when exposed to air. While surface oxide can suitably passivate the nanocrystalline silicon and stabilize its photoluminescence, the electrically insulating and structurally defective character of this oxide layer... 

Beyond the motivation to fabricate working systems from functionalized porous silicon, there is also more fundamental interest in the reactions of its surfaces. Because the vast surface of nanocrystalline silicon contains a large fraction of the total atoms, the composition and environment of the silicon interface are believed to affect greatly the physical, chemical, and electronic properties of the material. In particular, functionalization of porous silicon with certain organic groups is known to diminish photoluminescence [27] however, the mechanism of quenching has... 

Marra, D. C., Edelberg, E. A., Nanone, R. L., and Aydil, E. S., Silicon hydride composition of plasma-deposited hydrogenated amorphous and nanocrystalline silicon films and surfaces. 7. Vac. Sci. Technol. A 16, 3199-3210 (1998a). 

The application to silicon crystalline nano structures in this article relates to the recently discovered visible luminescence of porous and nanocrystalline silicon. The discussion touches also on excited states. It is also an illustration of the application of symmetry and periodic boundary conditions to relatively large electron systems. 

XANES Si L2,3-spectra were obtained for the first time in Ge/Si quantum dots and in Si Er-based stmctures. Just nanocrystalline silicon structures demonstrate distinguished additional peaks in these spectra (about 102 - 103 eV). Corresponding energy levels in the conduction bands can participate in the visible luminescence. 

CHARACTERIZATION OF NANOCRYSTALLINE SILICON FILMS BY BEAM INDUCED CURRENT IN THE SCANNING TUNNELING... 

Electrically active regions of nanocrystalline silicon (nc-Si) films have been investigated by using a SEM/STM combined instrument. STM constant current images reveal a cell structure in the nc-Si which is also observed in the remote electron beam induced mode of the STM. The STM-REBIC contrast indicates the existence of space charge regions at the cell boundaries. 

HP Martin, R Ecke, and E Muller, Synthesis of nanocrystalline silicon carbide powder by carbothermal reduction, J. Ear Ceram. Soc.,18,1737-1742(1998). 

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. 

These facts make the possible application of nanocrystalline silicon in optoelectronic devices unlikely. Also, the majority of organopolysilane compounds do not represent suitable candidates because the transition involved in the PL (e.g. Figure 23.8a) involves the a o a (HOMO LUMO) transition which would also result in an irreversible photodegradation due to photolysis. 

Raman scattering measurements showed the presence of amorphous carbon clusters in porous layer after wet oxidation at 800 °C but no carbon-related RS was detected after oxidation at 950 °C. No carbon clusters was detected by RS after dry oxidation even at temperature as low as 650 °C. Only partial oxidation of nanocrystalline silicon was observed after dry oxidation at 800 °C. 

Encyclopedia of Nanoscience and Nanotechnology, Vol. 6, ed. H.S. Nalwa, American Scientific Publishers, Stevenson Ranch, Calif., 2004 R 181 T. Ehara, Nanocrystalline Silicon. Electron Spin Resonance , p. 495... 

As an example, bulk silicon has Eg = 1.1 eV. For nanocrystalline silicon Eg varies with the size of the crystals and for sizes less than 2nm > 2eV. 

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