posted on 2020-02-05, 15:04authored byXiaotian Zhu, Gert H. ten Brink, Sytze de Graaf, Bart J. Kooi, George Palasantzas
Size-dependent
optical properties of germanium (Ge) nanocrystals
(NCs) make them a desirable material for optoelectronic applications.
So far, the synthesis of ligand-free and tunable-size Ge NCs by inert
gas condensation has been scarcely reported. In this work, we introduce
a gas-phase approach to synthesize quantum-confined Ge NCs by inert
gas condensation, where the size of the Ge NCs can be readily tuned
by controlling the thickness of a Cu plate supporting the Ge target.
As explained by simulations using the finite element method, the magnetic
field configuration above the target can be manipulated by varying
the thickness of the Cu backing plate. In-depth analysis based on
transmission electron microscopy (TEM) results reveals the morphology
and crystalline structure of Ge NCs. X-ray photoelectron spectroscopy
has proven the formation of a substoichiometric Ge oxide shell for
the as-deposited Ge NCs. In addition, Raman spectroscopy indicated
peak shifts according to the phonon confinement model that yielded
nanoparticle sizes in a good agreement with the TEM results. Furthermore,
the quantum confinement effect for Ge NCs was demonstrated by analysis
of the absorption (UV–vis–NIR) spectrum, which indicated
that the band gap of the Ge NCs was increased from ∼0.8 to
1.1 eV with decreasing size of Ge NCs. Comparison with theory shows
that the quantum confinement effect on the band gap energy for different-sized
Ge NCs follows the tight-binding model rather well.