Doctor of Philosophy
Nanoscience and Nanotechnology
Dr. Indika U. Arachchige
Semiconductors are a necessary component for many important electrical and optical devices such as lithium-ion batteries1, photodetectors2, transistors3, solar cells4, and bio-imaging5. As a result, many different semiconducting materials have been investigated, the most popular being direct-gap materials such as CdSe, GaAs, or PbTe due to their excellent light-matter interactions and high molar absorptivity.6 Unfortunately, the majority of these semiconductors are extremely toxic and rare, and therefore, unable to integrate into devices for biomedical or environmental applications. In contrast, Group IV semiconducting materials such as Ge and Si are much more bio-friendly but have an indirect bandgap hindering their absorption and emission properties. Sn alloying into Si and Ge modifies the valence and conduction band minima, causing an indirect-to-direct gap transition and enhancing light-matter interactions and consequently optical properties.7,8 With the addition of Sn, the bandgap is significantly reduced in the alloy, shifting the optical properties towards the IR region, but with the effects of quantum confinement, the energy gap can be increased back towards the visible or near IR spectrum.8–10 There has always been some difficulty alloying Sn with Ge and Si due to large discrepancies in lattice constants as well as high temperatures and long growth times used in traditional vapor phase syntheses, which promote Sn segregation. However, moderately low-temperature solution synthesis routes have been demonstrated to alloy Ge and Sn and produce homogeneous alloy nanocrystals (NCs) with varying sizes and compositions. Synthesis of both strongly and weakly confined Ge1-xSnx NCs were investigated both within this work and beyond.8–12 These reports established the size, composition-dependent energy gaps, and enhanced optical properties for particles ranging from 2 to 15 nm and with Sn incorporation up to 95%.8–12 To utilize these advanced properties, Ge1−xSnx NCs must be assembled into devices for optoelectronic, charge storage, and electronic applications. The first step towards this is the fabrication of these NCs into low-cost, solution-processed thin films.
Herein, we report the synthesis of Ge1−xSnx NCs with varying sizes (ranging from 4.7 ± 0.6 to 8.6 ± 1.9 nm) and Sn compositions (x = 0.01−0.08), followed by the successful exchange of insulating surfactant ligands with molecular metal chalcogenides (MCCs), to produce solution-processed conductive NC thin films.12 The ligand exchange of NCs was confirmed qualitatively as well as through FTIR spectroscopy. Structural and surface analysis of pre- and post-exchanged NCs indicates a diamond cubic structure and replacement of amine surface ligands with the MCC. Electron micrographs of alloy NCs show a notable decrease in size upon ligand exchange, which is consistent with the etching induced by chalcogenide ligands. The size confinement effects have resulted in energy gaps that are significantly blue-shifted from bulk Ge for the Ge1−xSnx alloy quantum dots with composition-tunable solution-state (1.68−1.26 eV for x = 0.01−0.08) energy gaps and solid-state (1.54−1.20 eV for x = 0.01−0.08) absorption onsets. Subsequently, the colloidal solution was spin-coated onto a glass substrate producing continuous, homogenous thin films with a thickness of 197 ± 5nm.12 Electrical characterization of the uniform NC films using the 2-point probe technique reveals that the films are insulating prior to ligand exchange and show >3 orders of magnitude increase in conductivity (3.5 × 10−6 S/cm for Ge0.92Sn0.08 NCs) upon functionalization with MCC. The electrical conductivity of the films increases with the increasing Sn composition (1.2 × 10−6 −3.5 × 10−6 S/cm for x = 0.01− 0.08), which is consistent with the increased spin-orbital coupling and reduction in energy gaps realized through homogeneous alloying of cubic Ge and α-Sn.12 The fabrication of solution-processed Ge1-xSnx NC thin films with high electronic conductivity provides an efficient methodology for cost-efficient processing of optoelectronic and charge storage devices.
With the successful fabrication of Ge1-xSnx alloyed NC thin films, our focus shifted to the development of low-temperature chemical routes for Si and Si1-xSnx alloy NCs. Ge and Ge1-xSnx NCs show direct energy gaps and promising optical properties in the near IR spectrum whereas Si and Si1-xSnx NCs are expected to show enhanced optical properties in the visible to near IR region.15 This not only allows for the opportunity for light-harvesting in the visible spectrum but also if coupled with Ge, a Si1-x-yGe1-xSny alloy could be produced that shows high molar absorptivity (and consequently, emissivity) throughout the visible to near IR spectral region, ultimately offering fabrication of efficient optoelectronics that could operate over a wider spectral region. To investigate this potential, we have recently studied the incorporation of elemental Sn into Si nanostructures via a room-temperature colloidal synthetic route as well as a moderately high-temperature solid state synthetic route. The room-temperature synthesis involved the co-reduction of halides whereas the high-temperature synthesis employed thermal decomposition of a Si precursor to produce Si NCs. Through the room-temperature synthetic route, Si0.66Sn0.33 nanoparticles with 5.88 ± 0.78 nm average size were produced and imaged with TEM. EDS was used to confirm the elemental composition of the alloy NPs. PXRD data suggest that these NPs are amorphous and do not have any elemental Sn impurities. However, the amorphous particles did not show any visible luminescence. This could be due to the fact these particles may have dangling bonds seen in amorphous Si13 or they may be poorly passivated by surfactant ligands. Both create a large number of surface traps/defects, which are detrimental for efficient emission.
The influence of Sn on a high-temperature, solid-state synthetic route was investigated to allow for the study of highly crystalline Si NCs. The Si NCs produced via the high-temperature route (at 1100°C) without Sn show sizes from 4.77 ± 0.88 nm to 7.77 ± 1.31 nm and tunable energy gaps from 2.08 eV to 2.21 eV. These particles were indeed highly crystalline having a diamond cubic structure. The Si NCs were found to have luminescence maxima from 650 – 750 nm for sizes between 4.77 ± 0.88 nm and 5.33 ± 1.03 nm. The larger particles (7.77 ± 1.31 nm) did not show luminescence in the visible region due to their size. Attempts to alloy Sn into Si NCs proved unsuccessful due to lattice mismatch, but with the Sn addition into the synthesis, it was discovered that larger Si NCs could be produced at moderately low temperatures. The resulting Si NCs were highly crystalline and showed average sizes of 7.83 ± 1.61 – 13.29 ± 2.10 nm for 0.2-3.0% Sn compositions. In this way, Sn acts as a catalyst for the synthesis of Si NCs producing particles at significantly low temperatures (450°C). With more Sn present during the synthesis, larger NCs can be produced. From this study, it can be concluded that the Sn-catalyzed, solid-state Si NC synthesis produces NCs through the vapor-liquid-solid mechanism. This discovery establishes a more cost-effective and efficient methodology for the synthesis of larger and smaller Si NCs.
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Available for download on Tuesday, May 11, 2027