Optical Transitions and Excitonic Properties of Ge_1–xSn_x Alloy Quantum Dots

Abstract

Using hybrid functional calculations and experimental characterization, we analyze optical properties of 2–3 nm Ge1–x Sn x alloy quantum dots, synthesized by colloidal chemistry methods. Hybrid functional theory, tuned to yield experimental bulk band structure of germanium, reproduces directly measured properties of Ge1–x Sn x quantum dots, such as lattice constants, energy gaps, and absorption spectra. Time-dependent hybrid functional calculations yield optical absorption in good agreement with experiments, and allow probing the nature of the dark excitons in quantum dots. Calculations suggest a spin-forbidden dark exciton ground state, which is supported by the changes in the photoluminescence lifetimes with temperature and tin concentrations. The synthesis and theoretical understanding of Ge1–x Sn x alloy quantum dots will add to the overall toolbox of low to nontoxic, silicon-compatible group IV semiconductors with potential application in visible to near-infrared optoelectronics.

Structural Characterization of Oleylamine- and Dodecanethiol-Capped Ge1–xSnx Alloy Nanocrystals

“…To date, literature on the synthesis and characterization of Ge 1– x Sn x NCs has focused on convection heating methods to achieve a wide range of Sn compositions while controlling the particle diameter. Esteves et al, Hafiz et al, and Demchenko et al have synthesized Ge 1– x Sn x NCs with diameters less than 4 nm, demonstrating size control across a range of low compositions, 0.04 ≤ x ≤ 0.24. Ramasamy et al have demonstrated similar control for larger diameter Ge 1– x Sn x NCs with larger Sn compositions. , In this work, facile microwave heating methods are used to achieve a wide range of particle sizes, 3.8–9.3 nm in diameter, across a narrow range of Sn compositions, 0.12 ≤ x ≤ 0.18, by varying the synthesis temperature (Figure ).…”

“…Experimental band gap energies, determined using a polynomial fitting, agree well with literature values for Ge 1– x Sn x NCs in the 1–5 nm range (Figure ). − ,, However, few values have been reported for Ge 1– x Sn x NCs in the 5–9 nm range. ,, …”

“…It has been predicted that incorporation of Sn in a cubic Ge 1– x Sn x alloy structure would result in an indirect-to-direct band gap crossover. − Incorporation of the zero-gap semiconductor Sn lowers the overall energy of the conduction band of Ge, and the energy of the Γ point ( k = 0) is predicted to decrease more rapidly than that of the L point ( k ≠ 0), making the direct band gap transition lower in energy for the alloy . Using Vegard’s law and assuming properties are linearly dependent on composition, indirect-to-direct crossover has been interpolated to occur at x = 0.22 for Ge 1– x Sn x . , Other theoretical predictions place the indirect-to-direct crossover for unstrained Ge 1– x Sn x at Sn compositions ranging from x = 0.016 to 0.17, ,,,,,− with the range of x = 0.06–0.11 being generally accepted. − Computational studies have been employed to better understand the band structure and emission properties of Ge 1– x Sn x NCs. ,,, Emission similar in energy to the absorption onset is attributed to a fundamental energy gap transition, and nonradiative decay can occur from surface trap states caused by Sn incorporation and alloy disorder. , …”

“…Microwave-assisted heating has been applied to the synthesis of doped and alloyed Ge nanomaterials. Bismuth-doped Ge NCs were synthesized by microwave-assisted reduction of GeI 2 and BiCl 3 . , Similar to the incorporation of Bi in Ge structures, the synthesis of Ge 1– x Sn x nanomaterials is challenging due to the low solubility of Sn in Ge (<1% Sn) and a large lattice mismatch that makes the alloy thermodynamically unstable. − Ge 1– x Sn x nanowires have been synthesized by the microwave-assisted reduction of bis­[bis­(trimethylsilyl)­amino]­tin­(II) and bis­[bis­(trimethylsilyl)­amino]­germanium­(II). , Ge 1– x Sn x NCs have also been achieved by the microwave-assisted reduction of GeI 2 and bis­[bis­(trimethylsilyl)­amino]­tin­(II) in oleylamine (OAm); however, synthesis using convection heating has been studied more extensively. − ,,,− …”

Recent Progress in the Synthesis and Applications of Composite Photocatalysts: A Critical Review

“…In case of binary alloy QDs, the band gaps could be altered by controlling the QDs particles size and this feature of QDs is highly significant due to the quantum confinement effect, as compared to the bulk nanomaterials. [107,108] Thus, the extremely small particles size of alloy QDs make them highly effective visible light catalysts for solar photocatalysis, compared to the bulk semiconductors. Some recent reports on alloy QDs based composite photocatalysts are provided in Table 1.…”