Quantum Confined Intense Red Luminescence from Large Area Monolithic Arrays of Mesoporous and Nanocrystal-Decorated Silicon Nanowires for Luminescent Devices

Abstract

We report intense red luminescence from mesoporous n + -Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching. n + -Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure. p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surfacebound nanocrystals. Both mesoporous n + -Si NWs and rough, solid p-Si NWs exhibit red luminescence at ∼1.7 and ∼1.8 eV, respectively. Time-resolved photoluminescence (PL) measurements indicated long (tens of μs) radiative recombination lifetimes. The red luminescence is visible with the naked eye and the red light is most intense from mesoporous n + -Si NWs, which exhibit a red-shift in the emission maximum to 1.76 eV at 100 K. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. X-ray photoelectron spectroscopy identifies a stoichiometric SiO 2 on the rough p-Si NWs with a SiO x species at the NW surface. No distinct oxide is found on the mesoporous NWs. The analysis confirms that long life-time PL emission arises from quantum confinement from internal nanoscale crystallites, and oxidized surface-bound crystallites, on n + -and p-Si NWs respectively. Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled 1-3 to modify thermal and optical properties. 4,5 In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity 6-9 and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources.10 Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics.11 In photovoltaics, polycrystalline silicon comprises more than 90% of the PV cell production. The efficiency is limited to ∼33-35%, with most of the power converted to heat via phonon emission.12 Recent developments in nanoscale silicon crystals that provide multiple exciton generation events (at energies greater than twice the bandgap) per incident photon may improve efficiencies. 13Porous semiconductors and clusters of quantum dots, particularly of silicon, have also been the subject of intense research recently for Li-ion batteries [14][15][16] water oxidation 17 and electroluminescent devices 18 and those exhibiting optical gain, 19-21 and a range of (bio)sensors. 22,23 Since the discovery of visible light emission in porous silicon [24][25][26] that in the main, arose from quantum confinement effects, porous silicon has had some usefulness for nanoelectronic devices, 27 and field ef...

The Nature of Silicon Nanowire Roughness and Thermal Conductivity Suppression by Phonon Scattering Mechanisms

“…Characteristically, as identified in previous work, 40 rough Si NWs formed by MAC etching retain a stoichiometric SiO 2 oxide coating due to the process of Si oxidation that facilitates etching in HF acid. 25 Interestingly, computational analysis of NW with amorphous silica coatings shows a 30-fold decrease in thermal conductivity compared to bulk silicon for 15 nm diameter NWs.…”

“…Separate evidence of these crystallites within the roughness was obtained by photoluminescence, whereby light emission signatures matched those of Si nanocrystals. 40 STEM images use secondary electrons and the information avoids contributions from electron transparent sections of NW roughness features in projection, thus the nature of the roughness crystallinity is accurately representative. NWs.-Figures 3a, 3b show the temperature-dependent Raman scattering spectra for the electrochemically roughened p-type Si NWs, acquired at low laser power density to avoid localized heating during equilibrated sample heating.…”

(Invited) Material Porosity

“…The rough surface comprises various orientations of Si nanocrystals that also show red luminescence ~1.8 eV, respectively. 79 Finally, the roughness and degree of porosity affect temperature-dependent propagation of phonons. In order to realize increasingly efficient thermoelectric materials and devices, various methods have been used to suppress with phonon propagation and heat conduction.…”