Cost-Effective Ultrabright Silicon Quantum Dots and Highly Efficient LEDs from Low-Carbon Hydrogen Silsesquioxane Polymers

Abstract

Cost-effective methods of synthesizing bright colloidal silicon quantum dots (SiQDs) for use as heavy-metal-free QDs, which have applications as light sources in biomedicine and displays, are required. We report simple protocols for synthesizing ultrabright colloidal SiQDs and fabricating SiQD LEDs based on hydrogen silsesquioxane (HSQ) polymer synthesis. Red photoluminescence with a quantum yield (PLQY) of 60−80% and LEDs with an external quantum efficiency (EQE) of >10% were obtained at 1/3600th of the cost of existing methods. This was achieved by using HSiCl 3 and a low-polarity solvent to prepare the HSQ polymer and by optimizing the LED hole-injection layer thickness. A stochastic analysis of 31 SiQD syntheses revealed that SiQDs with the highest PLQYs were obtained from a hard, low-carbon HSQ polymer precursor containing many Si−H groups and cage structures. Notably, simple FTIR measurements predicted whether a HSQ polymer would yield high-PLQY SiQDs and high-EQE LEDs. These straightforward, cost-effective protocols should lead to advances in SiQD synthesis and LED fabrication methods.

Ligand Stabilities and Reactivities of Green Photoluminescent Silicon Quantum Dots: Positive Aging in Solution

“…All of the spectra featured bands that were assignable to the transverse optical modes of crystalline (521 cm –1 ) and amorphous (480 cm –1 ) Si, and hence the synthesized SiQDs included crystalline and amorphous content. The same features were observed in Raman spectra of colloidal SiQDs prepared using other synthesis methods. ,,, In particular, the integrated Raman intensities were used to compute the crystalline fraction f c ; f c = 1/(1 + γ I a r ), where I a r is the relative Raman intensity for the bands, obtained by dividing the integrated intensity of the amorphous band ( I a ) by that of the nanocrystalline band ( I c ), and γ is the relative Raman cross section (γ = 0.1025 for a 5.9 nm Si nanocrystal) . The same analyses were previously conducted in SiQDs prepared by the HSQ method. , Accordingly, the calculated crystalline fractions of the samples for which the Raman spectra are shown in Figure g,h were estimated to be 69 and 52%, respectively.…”

“…Although Si is widely used in electronic devices, it is known to be a poor optical material. This is because the luminescence wavelength of bulk Si is in the invisible NIR region (λ = 1100 nm), and its photoluminescence quantum yield (PLQY) is typically ∼0.01% owing to the indirect nature of its interband transition. − However, Si quantum dots (SiQDs) exhibit luminescence across the entire visible spectral region. − In addition, for colloidal SiQDs, PLQYs of up to 80% have been reported, ,, and colloidal SiQD dispersions can be used to manufacture devices via printing and roll-to-roll processing. Thus, colloidal SiQDs have enormous potential as environmentally benign, heavy-metal-free materials for displays, light-emitting diodes (LEDs), lighting, and biomedical imaging .…”

“…Moreover, SiQDs have been used as nanomedical agents, in applications including disease diagnosis/therapeutics, bioanalyte sensing, and tissue engineering . Thus, many methods for synthesizing colloidal SiQDs, such as plasma synthesis, pulsed laser ablation, − halosilane reduction, − and thermal pyrolysis of hydrogen silsesquioxane (HSQ), ,− ,− have been demonstrated, and the optical properties of colloidal SiQDs have been extensively investigated over the past decade.…”

“…The properties of the photoluminescence (PL) of colloidal SiQDs have been characterized based on the quantum confinement effect (S-band) and surface ligand effect (F-band). , S-band PL is orange-red and has a slow (μs) decay time, and the PLQY can be as much as 70 , or 80%. , In contrast, F-band PL is associated with surface ligand states and is blue-yellow with a fast (ns) decay time. PLQYs of up to 90% have been observed for the yellow PL of amine-terminated Si nanoparticles, , and replacing the hydrogens in hydride SiQDs with Cl, I, and Br ligands and then performing an alkyl termination reaction have been shown to produce blue, yellow, and red PL, respectively .…”

1D, 2D, and 3D Mapping of Plasmon and Mie Resonances: A Review of Field Enhancement Imaging Based on Electron or Photon Spectromicroscopy

“…2D and/or 3D maps of PL, fluorescence, and Raman spectra can be obtained, and this data can be compared with AFM and/or SEM measurements and FDTD simulations corresponding to the same positions within the sample. This method can be applied to samples of noble metals with nano or submicrometer structural features, ,, semiconductors with submicrometer structural features, − ,, semiconducting organic materials, − and colloidal quantum dot materials. − ,− , To the best of our knowledge, only a few reports of the 2D and/or 3D mapping of electromagnetic field enhancement owing to the Mie resonances of semiconductors have been published. − …”