Structure and Morphology-Controlled Synthesis of Colloidal Ge _{1– <i>x</i> – <i>y</i>} Si _<i>y</i> Sn _<i>x</i> Quantum Dots with Composition-Tunable Energy Gaps and Visible to Near-IR Optical Properties

Ge_{1-<i>x</i>-<i>y</i>} Si _<i>y</i> Sn _<i>x</i> quantum dots (QDs) are an attractive class of low-to-nontoxic and earth-abundant semiconductors exhibiting size and composition-tunable optical properties. Their electronic structure can be modified by varying elemental composition and quantum confinement to achieve tunable absorption and photoluminescence (PL) across the visible to near-IR spectrum. Alloying with Sn enhances oscillator strengths, whereas decreasing size and incorporating Si increase energy gaps. Herein, we report a facile colloidal route to produce Ge_{1-<i>x</i>-<i>y</i>} Si _<i>y</i> Sn _<i>x</i> QDs with narrow size dispersity (4.0 ± 0.4 - 5.2 ± 0.6 nm) and variable Si (<i>y</i> = 0.030 - 0.252) and Sn (<i>x</i> = 0.044 - 0.059) compositions and investigate the influence of core/surface species on optical properties. Structural analysis reveals an expanded diamond cubic Ge lattice, a red-shifted Ge-Ge Raman peak, and the emergence of a Ge-Si peak with increasing Si composition. Successful alloying of Si and Sn into Ge host lattice is confirmed by electron microscopy, suggesting homogeneous solid solution behavior of ternary QDs. Surface analysis further indicates the presence of Ge⁰/Si⁰/Sn⁰ core species alongside charged Ge ^<i>n</i>+/Si ^<i>n</i>+/Sn ^<i>n</i>+ (1 ≤ <i>n</i> ≥ 4) surface species coordinated to passivating organic ligands. The effects of confinement and surface/core elemental composition on optical properties were revealed through composition-tunable absorption onsets (1.15 - 2.33 eV) and associated Tauc direct (1.86 - 3.03 eV) and indirect (1.01 - 1.81 eV) energy gaps achieved for QDs with <i>x</i> = 0.044 - 0.059 and <i>y</i> = 0.030 - 0.252, which are prominently blue-shifted from bulk counterparts and previously reported Ge_1-<i>x</i> Sn _<i>x</i> QDs. PL spectra of Ge_{1-<i>x</i>-<i>y</i>} Si _<i>y</i> Sn _<i>x</i> QDs exhibit nanosecond-scale emission from 1.84 - 1.88 eV for <i>y</i> ≤ 0.134 and 2.32 - 2.43 eV for <i>y</i> ≥ 0.177 compositions, displaying similarly pronounced blueshifts from comparable Ge_1-<i>x</i> Sn _<i>x</i> QDs. This correlated absorption/PL tunability expands upon that demonstrated by Ge and Ge_1-<i>x</i> Sn _<i>x</i> counterparts widens the optical window of Group IV semiconductor nanostructures, making them attractive for visible-to-near-IR optoelectronic studies.