Mechanism of Visible Photoluminescence from μc-Ge Embedded in SiO2 Glassy Matrices

Abstract

The origin of visible photoluminescence of Ge microcrystals (pc-Ge) in SiOz glassy matrices was examined. The spectroscopic analyses and formation mechanism show that the origin of strong light emission with nanosecond decay time is attributed to the effective surface termination and the new nanostructure of pc-Ge with diameter less than 4 nm which forms from reduction reaction of germanium oxides at low annealing temperature.

Synthesis and Surface Functionalization of Hydride-Terminated Ge Nanocrystals Obtained from the Thermal Treatment of Ge(OH)2

“…As a result of the small band gap of Ge, electron–hole recombination in GeNCs is expected to exhibit PL in the NIR or IR spectral regions. However, reports are often contradictory regarding the source of PL, critical NC size, expected PL wavelength, quantum yield, effect of oxides and other impurities. ,,,, Adding further complexity, germanium oxides can often show visible PL. , There are also numerous examples showing UV–vis emitting GeNCs, ,, however it has been suggested that these emitters likely show PL resulting from surface species (e.g., oxides) rather than quantum confinement of GeNCs. ,− There are surprisingly few examples of colloidal GeNCs showing NIR or IR PL. ,, …”

“…Despite the average size of the functionalized GeNCs investigated here being smaller than the Bohr-exciton radius of Ge (i.e., 24.3), , they do not show detectable PL. We hypothesize that this may result from dark surface defects (e.g., impurities, vacancies, surface oxides, dangling bonds, and so forth) arising from the HF liberation protocol that remain after hydrogermylation that provide nonradiative pathways. ,, In an effort to passivate potential (as of yet unidentified) defects on the GeNC surfaces, we introduced a trace quantity of I 2 into the hydrogermylation reaction mixture.…”

“…In the early 1990s, the surprising discovery of visible photoluminescence (PL) from silicon and germanium nanostructures opened new possibilities. , Despite being in Group 14, germanium differs substantially from its more frequently studied periodic congener silicon. Compared to silicon, bulk Ge possesses a smaller band gap (0.67 vs 1.1 eV at 300 K), larger Bohr-exciton radius (24.3 vs 4.9 nm), , higher electron and hole mobility (≤3900 vs ≤1500 cm 2 /V·s), as well as greater capacity for and diffusivity of ions (e.g., Li + ) . In the context of these characteristics, comparatively large GeNCs and related structures (e.g., oxide-embedded GeNCs) are expected to possess favorable chemical, optical, and electronic properties useful in applications such as solar cells, biological imaging, , Bragg reflectors, light-emitting diodes, nonvolatile memory devices, − as well as battery electrode materials. , …”