# Structure and Morphology-Controlled Synthesis of Colloidal Ge1–x–y Si y Sn x Quantum Dots with Composition-Tunable Energy Gaps and Visible to Near-IR Optical Properties

## TL;DR

Researchers developed a method to synthesize quantum dots with tunable optical properties by adjusting their composition and size.

## Contribution

A new colloidal synthesis method for Ge1–x–ySi y Sn x quantum dots with controllable composition and optical properties is introduced.

## Key findings

- Alloying Si and Sn into Ge expands the optical tunability of quantum dots from visible to near-IR.
- Surface and core elemental composition significantly influence absorption and photoluminescence properties.
- The synthesized QDs show blue-shifted energy gaps compared to bulk materials and previously reported QDs.

## Abstract

Ge1–x–y
Si
y
Sn
x
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 Ge1–x–y
Si
y
Sn
x
QDs with narrow size dispersity (4.0 ± 0.4 – 5.2 ±
0.6 nm) and variable Si (y = 0.030 – 0.252)
and Sn (x = 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
n+/Si
n+/Sn
n+ (1 ≤ n ≥ 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 x = 0.044 – 0.059 and y = 0.030 – 0.252,
which are prominently blue-shifted from bulk counterparts and previously
reported Ge1–x
Sn
x
QDs. PL spectra of Ge1–x–y
Si
y
Sn
x
QDs exhibit nanosecond-scale emission from 1.84 – 1.88
eV for y ≤ 0.134 and 2.32 – 2.43 eV
for y ≥ 0.177 compositions, displaying similarly
pronounced blueshifts from comparable Ge1–x
Sn
x
QDs. This correlated absorption/PL
tunability expands upon that demonstrated by Ge and Ge1–x
Sn
x
counterparts widens
the optical window of Group IV semiconductor nanostructures, making
them attractive for visible-to-near-IR optoelectronic studies.