scholarly journals Size, Shape, and Crystallinity of Luminescent Structures in Oxidized Si Nanoclusters and H-Passivated Porous Si

1994 ◽  
Vol 358 ◽  
Author(s):  
S. Schuppler ◽  
S. L. Friedman ◽  
M. A. Marcus ◽  
D.L. Adler ◽  
Y.-H. Xie ◽  
...  
Keyword(s):  
Visible Region ◽  
Weighted Average ◽  
Ir Absorption ◽  
Porous Si ◽  
Emission Data ◽  
X Ray ◽  
Visible Luminescence ◽  
Si Nanocrystals ◽  
X Ray Absorption ◽  
Average Structures

ABSTRACTNear-edge and extended x-ray absorption fine structure measurements from a wide variety of H-passivated porous Si samples and oxidized Si nanocrystals, combined with electron microscopy, ir-absorption, α-recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For luminescent porous Si samples peaking in the visible region, i. e., ≤700nm, their mass-weighted-average structures are determined here to be particles–not wires, whose short-range character is crystalline – not amorphous, and whose dimensions – typically <15 Å – are significantly smaller than previously reported or proposed. These results depend only on sample luminescence behavior, not on sample preparation details, and thus have general implications in describing the mechanism responsible for visible luminescence in porous silicon. New results are also presented which demonstrate that the observed luminescence is unrelated to either the photo-oxidized Si species in porous Si or the interfacial suboxide species in the Si nanocrystals.

scholarly journals X-Ray Absorption Spectroscopy from H-Passivated Porous Si and Oxidized Si Nanocrystals

1994 ◽  
Vol 375 ◽  
Author(s):  
S. Schuppler ◽  
S. L. Friedman ◽  
M. A. Marcus ◽  
D. L. Adler ◽  
Y.-H. Xie ◽  
...  
Keyword(s):  
Fine Structure ◽  
Quantum Confinement ◽  
Ir Absorption ◽  
X Ray ◽  
Coordination Numbers ◽  
Absorption Fine ◽  
Visible Luminescence ◽  
Si Nanocrystals ◽  
Wide Range ◽  
X Ray Absorption

AbstractQuantum confinement in nanoscale Si structures is widely believed to be responsible for the visible luminescence observed from anodically etched porous silicon (por-Si), but little is known about the actual size or shape of these structures. Extended x-ray absorption fine structure data from a wide variety of por-Si samples show significantly reduced average Si coordination numbers due to the sizable contribution of surface-coordinated H. (The H/Si ratios, as large as 1.2, were independently confirmed by ir-absorption and α-recoil measurements.) The Si coordinations imply very large surface/volume ratios, enabling the average Si structures to be identified as crystalline particles (not wires) whose dimensions are typically <15 Å. Comparison of the size-dependent peak luminescence energies with those of oxidized Si nanocrystals, whose shapes are known, shows remarkable agreement. Furthermore, near-edge x-ray absorption fine structure measurements of the nanocrystals shows the outer oxide and interfacial suboxide layers to be constant over a wide range of nanocrystal sizes. The combination of these results effectively rules out surface species as being responsible for the observed visible luminescence in por-Si, and strongly supports quantum confinement as the dominant mechanism occurring in Si particles which are substantially smaller than previously reported or proposed.

Structure of Two Disordered Molybdates, Li2MoIVO3 and Li4Mo3 IVO8, from Total Neutron Scattering

1997 ◽  
Vol 53 (4) ◽  
pp. 604-612 ◽  
Author(s):  
S. J. Hibble ◽  
I. D. Fawcett ◽  
A. C. Hannon
Keyword(s):  
Neutron Scattering ◽  
Rietveld Analysis ◽  
Total Neutron ◽  
Bragg Scattering ◽  
Pair Correlations ◽  
X Ray ◽  
Metal Metal ◽  
X Ray Absorption ◽  
Average Structures ◽  
Insight Into

The structures of the disordered lithium molybdates Li2MoO3 and Li4Mo3O8 have been investigated using total neutron scattering from polycrystalline powders. Rietveld analysis of the Bragg scattering is used to determine the average structures. Shortcomings in this method of analysis are demonstrated by comparing the total correlation function, T(r), determined from total neutron scattering, with those calculated from the structures determined from Rietveld analysis. Much more satisfactory models for these materials are derived from the structurally related ordered material LiZn2Mo3O8, using information from Mo K-edge extended X-ray absorption fine-structure spectroscopy (EXAFS). These models include metal–metal-bonded Mo3O13 clusters [d(Mo—Mo) = 2.58 Å in Li2MoO3 and 2.56 Å in Li4Mo3O8] not present in the average structure determined from Rietveld analysis [d(Mo—Mo) = 2.88 Å in Li2MoO3]. In contrast to EXAFS studies neutron diffraction yields information on all the pair correlations in the material, not merely those involving molybdenum, and allows, for example, the location of lithium. Remaining discrepancies between our models and the experimental T(r)'s give an insight into the disorder in the two materials.

Grains of Porous Silicon Embedded in SiO2:Studies of Optical Gain and Electroluminescence

2004 ◽  
Vol 99-100 ◽  
pp. 31-36 ◽  
Author(s):  
K. Dohnalová ◽  
K. Luterová ◽  
J. Valenta ◽  
Jiří Buršík ◽  
M. Procházka ◽  
...  
Keyword(s):  
Porous Silicon ◽  
Silicon Nanocrystals ◽  
Visible Region ◽  
Optical Gain ◽  
Intrinsic Property ◽  
Stimulated Emission ◽  
Silicon Nanostructures ◽  
Porous Si ◽  
Si Nanocrystals ◽  
Excitation Spot

Recent reports on experimental observation of optical gain in silicon nanostructures in the visible region, performed at several laboratories all over the world, have triggered an extraordinary surge of interest in silicon lasing. However, attempts aimed at reproducing the red stimulated emission from „standard“silicon nanocrystals (sized 3-5 nm) at some other laboratories either failed, or. did not come to definite conclusions. Therefore, more detailed measurements of optical gain in a wider variety of samples containing Si nanocrystals are required in order to unravel whether or not the observation of optical gain is an intrinsic property of Si nanocrystals. We have performed a detailed study of optical gain in layers of densely packed Si nanocrystals in SiO2, prepared on the basis of porous Si, using the variable-stripe-length (VSL) method in combination with the shifted-excitation-spot (SES) method. In selected samples we have observed a distinct difference in behaviour between VSL and SES curves, indicating the occurrence of positive optical gain of ~ 24 cm-1. Preliminary reports on transport and electroluminescence measurements in thin films of SiO2 doped with porous silicon grains, prepared by spin-coating technique, are also discussed.

CONTINUOUS-WAVE PHOTOLUMINESCENCE AND NANOSTRUCTURAL PROPERTIES OF POROUS SILICON

2009 ◽  
Vol 08 (03) ◽  
pp. 311-318
Author(s):  
ADWAN Al-AJILI
Keyword(s):  
Electron Microscopy ◽  
Porous Silicon ◽  
Continuous Wave ◽  
Structural Information ◽  
Photoluminescence Excitation ◽  
Porous Si ◽  
X Ray ◽  
Diffraction Patterns ◽  
Silicon Based ◽  
X Ray Absorption

The photoluminescence (PL) emitted by porous silicon has been investigated by using the continuous tuneable UV Synchrotron Radiation Source. One sample was investigated for orange PL emission wavelength at temperatures 77–295 K. The PL peak is found to shift to higher frequency with decreasing temperature. Information about the nanostructure of porous silicon has been determined from PL and Extended X-ray Absorption Fine Structure (EXAFS), as well as from electron microscopy. In particular, the optical properties of silicon-based nanostructured materials, obtained from PL and photoluminescence excitation measurements, have been correlated with structural information from Si –K-edge EXAFS. Electron microscopy was used to study the relation between the nanostructure and PL of porous Si , and to investigate porous Si structure. Platelet Si and Si crystallites in porous Si layers were observed. The size of crystallites ranged from 4 to 6.5 nm. Diffraction patterns show these porous Si samples have a crystalline structure.

Structural Characterization of Porous Silicon Fabricated by Electrochemical and Chemical Dissolution of Si Wafers

1991 ◽  
Vol 256 ◽  
Author(s):  
S. Miyazaki ◽  
T. Yasaka ◽  
K. Okamoto ◽  
K. Shiba ◽  
K. Sakamoto ◽  
...  
Keyword(s):  
Porous Silicon ◽  
Electron Beam Irradiation ◽  
Complete Recovery ◽  
Porous Si ◽  
Beam Irradiation ◽  
X Ray Diffraction ◽  
X Ray ◽  
Disordered Structure ◽  
Visible Luminescence

ABSTRACTThe structure of porous silicon exhibiting efficient visible photoluminescence has been characterized by using Fourier transformed infrared absorption, Raman scattering and x-ray diffraction. It is shown that the lattice spacing in the porous Si layer expands by about 0.3% in the direction perpendicular to the surface and also a partially disordered structure is existing. Electron beam irradiation causes desorption of hydrogen and fluorine bonds which terminate the surface, resulting in the quenching of the visible luminescence. The chemical etching of such layer has led to complete recovery of the luminescence intensity as well as the hydrogen and fluorine bonds termination.

scholarly journals Characterization, X-ray Absorption Spectroscopic Analysis and Photocatalytic Activity of Co/Zn Co-Doped TiO2 Nanoparticles Synthesized by One-Step Sonochemical Process

Crystals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1254
Author(s):  
Wanichaya Mekprasart ◽  
Sorapong Pavasupree ◽  
C. K. Jayasankar ◽  
Balaji Rao Ravuri ◽  
Chakkaphan Wattanawikkam ◽  
...  
Keyword(s):  
Photocatalytic Activity ◽  
Tio2 Nanoparticles ◽  
Visible Region ◽  
Sonochemical Method ◽  
Edge Structure ◽  
Doped Tio2 ◽  
X Ray ◽  
One Step ◽  
X Ray Absorption ◽  
Co Doped

A novel one-step preparation of sonochemical method was applied to synthesize Co/Zn co-doped TiO2 nanoparticles using a sonicator of 750 W, 20 kHz for 30 min at room temperature. The formation of the anatase TiO2 phase for all as-prepared samples was observed from XRD results with a crystalline size in nanoscale. The use of ultrasound allowed for the successful doping of both Co and Zn into the TiO2 lattice, which was confirmed by Synchrotron light including X-ray near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy. Ti K-edge, Co K-edge, and Zn K-edge XANES spectra exhibited the dominating +4, +2, and +2 valence state of Ti, Co, and Zn in as-prepared samples, respectively. A detailed XANES and EXAFS data analysis give strong evidence that the Co/Zn dopants partially replace the Ti atom of the TiO2 host. The Co/Zn co-doping extends the light absorption of the host to the visible region and restricts the e+/h+ recombination. The photocatalytic activity of samples was tested for degradation of Rhodamine B dye solution under visible light irradiation. The as-synthesized of the co-doped catalyst was presented as highly efficient, with 2.5 and 5 times dye degradation compared with single-doped and bare TiO2.

Exafs and Exelfs Study of the Structure of Pd-Ni-P Bulk Metallic Glasses

2000 ◽  
Vol 644 ◽  
Author(s):  
Faisal M. Alamgir ◽  
Himanshu Jain ◽  
David B. Williams ◽  
Gilles Hug ◽  
Ricardo B. Schwarz ◽  
...  
Keyword(s):  
Fine Structure ◽  
Metallic Glasses ◽  
Nearest Neighbor ◽  
Bulk Metallic Glasses ◽  
Weighted Average ◽  
Ternary Alloys ◽  
X Ray ◽  
Efficient Packing ◽  
X Ray Absorption ◽  
Nearest Neighbor Distances

AbstractWe have explored the short-range order around all three constituent atoms in (Pd-Ni)80P20 bulk metallic glasses (BMGs), a system that is a prototype for a whole class of BMG formers containing 80% transition metal and 20% metalloid. We have examined the changes in the nearest neighbor environments around the transition metals in (Pd-Ni)80P20 glasses using extended X-ray absorption fine structure (EXAFS) in comparison to their binary counterparts. We have done similar studies around the coordination of P using extended energy loss fine structure (EXELFS). The environment around the all the atoms in Pd60Ni20P20 and Pd30Ni50P20are very similar to those of the binary phosphides at the ends of the composition range. However, the (Pd-Ni)80P20 glasses are not simply solid solutions of the phases. The nearest neighbor distances of the metals are reduced in the ternary alloys with respect to those of the binary phosphides. The best glass former in this series, Pd40Ni40P20 is nearly isostructural to Pd30Ni50P20 but shows shorter distances at the 2nd and 3rd coordination shells, which we believe is due to more efficient packing in this glass. The metal environments in Pd40Ni40P20, on the other hand, are better described by a weighted average of those of Pd30Ni50P20 and Pd60Ni20P20.

Unimportance of Siloxene in Luminescent Porous Silicon as Determined by Nexafs & Exafs

1993 ◽  
Vol 298 ◽  
Author(s):  
S.L. Friedman ◽  
M.A. Marcus ◽  
D.L. Adler ◽  
Y.-H. Xie ◽  
T.D. Harris ◽  
...  
Keyword(s):  
Fine Structure ◽  
Porous Silicon ◽  
Room Temperature ◽  
Emission Spectra ◽  
Porous Si ◽  
X Ray ◽  
Absorption Fine ◽  
X Ray Absorption ◽  
Room Temperature Luminescence ◽  
Combined Data

AbstractNear-edge-- and extended--x-ray absorption fine structure measurements, as well as luminescence excitation and emission spectra, were obtained from samples of porous Si and siloxene. Contrary to a recently proposed explanation for the room temperature luminescence in porous Si, the combined data indicate that siloxene is not principally responsible for the observed effect.

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