Understanding the Formation of Elemental Germanium by Thermolysis of Sol-Gel Derived Organogermanium Oxide Polymers

Thermolysis of organogermanium oxide sol-gel polymers yields germanium oxide-embedded germanium nanocrystals. In the present study, we investigate the influence of different organic substituents, R, on the sol-gel chemistry of organotrichloro- and organotrialkoxygermane precursors and the thermal behavior of the resulting organogermanium oxides (RGeO_1.5)_n. The organic substituent affects the structure of the sol-gel product, with bulky R groups hindering network polymer formation. Cage-like sol-gels formed in the presence of bulky substituents are volatile, while network polymers experience thermolytic cleavage of the Ge–C bond. This cleavage produces a Ge-rich oxide (GeO_1.5)_n, that undergoes thermally induced disproportionation into GeO₂ and elemental Ge. The onset temperature of the disproportionation reaction is profoundly influenced by the nature of the organic substituent. We propose the change in onset temperature arises from a shift in R-group cleavage pathways from radical to β-hydride elimination

Water-Assisted Transfer Patterning of Nanomaterials

We introduce a straightforward and cost-effective water-assisted approach to transfer patterns of nanomaterials onto diverse substrates. The transfer method relies on the hydrophobic effect and utilizes a water-soluble polymer film as a carrier to transfer hydrophobic nanomaterials from a patterned source substrate onto a target substrate. Using this approach, nanomaterials are transferred readily from solutions onto surfaces of various shapes and compositions with high fidelity for feature sizes approaching 10 microns

Thermally Induced Evolution of “Ge(OH)₂”: Controlling the Formation of Oxide-Embedded Ge Nanocrystals

Germanium nanocrystals (GeNCs) hold great promise as active materials in many applications including solar cells, biological imaging, Bragg reflectors, light-emitting diodes, and nonvolatile memory devices. However, because of the size-, shape-, and composition-dependent nature of their properties, it is essential that methods affording high-purity, well-defined GeNCs be developed. Herein, we report a systematic investigation of a series of “Ge­(OH)₂” precursors using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and solid-state ⁷³Ge NMR and correlate their properties with the GeNCs obtained from their thermal processing. This study provides important insights into the role of the Ge­(OH)₂ internal structure and will allow for future rational design and practical application of GeNCs

Water-Assisted Transfer Patterning of Nanomaterials

We introduce a straightforward and cost-effective water-assisted approach to transfer patterns of nanomaterials onto diverse substrates. The transfer method relies on the hydrophobic effect and utilizes a water-soluble polymer film as a carrier to transfer hydrophobic nanomaterials from a patterned source substrate onto a target substrate. Using this approach, nanomaterials are transferred readily from solutions onto surfaces of various shapes and compositions with high fidelity for feature sizes approaching 10 microns

Chloride Surface Terminated Silicon Nanocrystal Mediated Synthesis of Poly(3-hexylthiophene)

Abundant and environmentally benign metal-free silicon-based reagents, including chloride surface-terminated silicon nanocrystals (Cl-SiNCs) and silicon wafers as well as molecular chlorosilanes, were explored as catalysts for the synthesis of poly-3-hexylthiophene (P3HT) at room temperature. Cl-SiNC catalysts exhibit the highest activity of those investigated, and systems based upon single-crystal silicon wafers provide convenient, straightforward purification. The as-prepared P3HT exhibits moderate molecular weights and bears H/Br or Br/Br end groups; these properties will allow direct application and also facilitate their use as macroinitiators in the syntheses of block and/or telechelic polymers. The silicon-based systems are expected to provide an efficient metal-free catalytic preparation of functional polymers

A Convenient Method for Preparing Alkyl-Functionalized Silicon Nanocubes

The first solid-state synthesis of diamond structure silicon nanocube structures with edge lengths of 8–15 nm is reported. It is well-established that controlled high-temperature processing of hydrogen silsesquioxane produces exceptionally well-defined pseudospherical silicon nanocrystals. However, only a small number of accounts outlining shape-controlled synthesis have appeared. We report here that, upon prolonged annealing in an oxide matrix, nanocrystal surfaces thermodynamically self-optimize, yielding particles with cubic geometries. Surface functionalization of the resulting nanocubes is readily achieved via thermal hydrosilylation. Discussion will include description of the synthetic procedure, comprehensive material characterization, and the factors that lead to the formation of cubic structures

Surface-Induced Alkene Oligomerization: Does Thermal Hydrosilylation Really Lead to Monolayer Protected Silicon Nanocrystals?

Surface functionalization of hydride-terminated silicon nanocrystals (SiNCs) with dodecene via thermal hydrosilylation has been reexamined. We observed the formation of dodecyl oligomers (<i>n</i> ≤ 4) during the reaction under an argon atmosphere at various predesigned temperatures (100–190 °C). In a comparative study, surface hydrosilylation and ligand oligomerization were found to be more pronounced under air (<i>n</i> ≤ 7) at the same temperatures. These observations strongly suggest that hydrogen abstraction by oxygen accelerates hydrosilylation and generates sufficient silyl radical as initiator to interact with unsaturated bonds, promote chain propagation, and generate ligand oligomers. We further propose that, to inhibit ligand oligomerization and obtain monolayer coverage on SiNC surfaces, it is feasible to apply comparatively low temperatures, inert atmosphere, and dilute ligand concentration during thermal hydrosilylation

Photothermal Response of Photoluminescent Silicon Nanocrystals

We demonstrate that silicon nanocrystals (Si-NCs) exhibiting relatively high near-IR photoluminescent quantum yields also exhibit a notable photothermal (PT) response. The PT effect has been quantified as a function of NC size, defect concentration, and irradiating energy, suggesting that the origin of the PT response is a combination of carrier thermalization and defect-mediated heating. The PT effect observed under NIR irradiation suggests that Si-NCs could find use in combined in vivo PL imaging and PT therapy

Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation

Spectrally resolved photoluminescence (PL) decays were measured for samples of colloidal, ligand-passivated silicon nanocrystals. These samples have PL emission energies with peak positions in the range ∼1.4–1.8 eV and quantum yields of ∼30–70%. Their ensemble PL decays are characterized by a stretched-exponential decay with a dispersion factor of ∼0.8, which changes to an almost monoexponential character at fixed detection energies. The dispersion factors and decay rates for various detection energies were extracted from spectrally resolved curves using a mathematical approach that excluded the effect of homogeneous line width broadening. Since nonradiative recombination would introduce a random lifetime variation, leading to a stretched-exponential decay for an ensemble, we conclude that the observed monoexponential decay in size-selected ensembles signifies negligible nonradiative transitions of a similar strength to the radiative one. This conjecture is further supported as extracted decay rates agree with radiative rates reported in the literature, suggesting 100% internal quantum efficiency over a broad range of emission wavelengths. The apparent differences in the quantum yields can then be explained by a varying fraction of “dark” or blinking nanocrystals

Revisiting an Ongoing Debate: What Role Do Surface Groups Play in Silicon Nanocrystal Photoluminescence?

The origin of photoluminescence (PL) in silicon nanocrystals (SiNCs) remains a subject of considerable debate. Size-dependent PL that supports the quantum confinement model has been proposed by several researchers. On the other hand, SiNC PL arising from surface states that are independent of nanocrystal size has also been shown. This work addresses the origin of surface-functionalized SiNC PL as relating to surface states and the NC size. SiNCs of different sizes (3 and 5 nm diameters) were prepared with three distinct surface chemistries. Steady-state and time-resolved PL measurements were performed at temperatures ranging from 37 to 377 K. Temperature-dependent luminescence consistent with core emission was observed for alkyl-terminated SiNCs, while alkylamine-functionalized SiNCs displayed minimal temperature-dependent luminescence, consistent with a charge-transfer mechanism. Lightly oxidized alkyl SiNCs had similar emission profiles to alkyl SiNCs; however, they showed longer luminescence lifetimes and their luminescence spectrum was shifted to shorter wavelengths than their nonoxidized counterparts. A general mechanism is proposed to explain all three phenomena, suggesting that surface groups play a crucial role in SiNC optical response