Low-Cost Synthesis of Silicon Quantum Dots with Near-Unity Internal Quantum Efficiency

Abstract

As a cost-effective batch synthesis method, Si quantum dots (QDs) with near-infrared photoluminescence, high quantum yield (>50% in polymer nanocomposite), and near-unity internal quantum efficiency were fabricated from an inexpensive commercial precursor (triethoxysilane, TES), using optimized annealing and etching processes. The optical properties of such QDs are similar to those prepared from state-of-the-art precursors (hydrogen silsesquioxane, HSQ) yet featuring an order of magnitude lower cost. To understand the effect of synthesis parameters on QD optical properties, we conducted a thorough comparison study between common solid precursors: TES, HSQ, and silicon monoxide (SiO), including chemical, structural, and optical characterizations. We found that the structural nonuniformity and abundance of oxide inherent to SiO limited the resultant QD performance, while for TES-derived QDs this drawback can be avoided. The presented low-cost synthetic approach would significantly favor applications requiring high loading of good-quality Si QDs, such as light conversion for photovoltaics.

Figure 1

Three types of silicious precursors for the synthesis of Si QDs, their chemical formulas, and illustrations of their molecular structures. For TES, there are a series of pretreatments to form xerogels for the subsequent annealing.

Figure 2

Chemical, optical, and structural characterizations of Si QDs synthesized from three precursors. (a) X-ray powder diffraction (XRD) spectra of the annealed HSQ (1200 °C), annealed TES-derived xerogels (950 °C), annealed SiO (920 °C). (b) Emission and absorption spectra of Si QDs in toluene synthesized from three precursors. (c–e) Transmission electron microscope (TEM) images of these Si QDs. The inset of each TEM image is the size distribution of QDs.

Figure 3

Effect of high annealing temperature on the peak position of the PL emission of Si QDs synthesized from TES (green squares). After HF etching was extended, the peak position can be tuned to the range of ∼850 nm (red triangles). The inset illustrates the two-stage annealing process of TES-derived xerogels. Note that for Si QDs with a peak wavelength exceeding 900 nm, the PLQY values were not shown due to the limitation of detector sensitivity.

Figure 4

A spectrally resolved PL decay of QDs-TES detected at 878 nm and stretched exponential fit. The decays of QDs-SiO and QDs-HSQ detected at the same wavelength are shown in Figure S7.

Figure 5

Total recombination rates of Si QDs synthesized from HSQ, SiO, and TES in toluene solution (a) and OSTE matrix (b). The error bar is inside the dot. Note that the reference sample was claimed to have a near-unity internal quantum efficiency in a previous paper. The gray bands at the background indicate that curves included inside have similar values and trends within experimental and analysis errors, revealing their near-unity IQE.

Figure 6

(a) Attenuated total reflectance (ATR) spectra of thin dried films of Si QDs synthesized from SiO (red), HSQ (blue), and TES (green). (b) The ratio of areas under the Si–O–Si and C=O stretching modes obtained from (a).