Low temperature synthesis of silicon quantum dots with plasma chemistry control in dual frequency non-thermal plasmas

Abstract

The advanced materials process by non-thermal plasmas with a high plasma density allows the synthesis of small-to-big sized Si quantum dots by combining low-temperature deposition with superior crystalline quality in the background of an amorphous hydrogenated silicon nitride matrix. Here, we make quantum dot thin films in a reactive mixture of ammonia/silane/hydrogen utilizing dual-frequency capacitively coupled plasmas with high atomic hydrogen and nitrogen radical densities. Systematic data analysis using different film and plasma characterization tools reveals that the quantum dots with different sizes exhibit size dependent film properties, which are sensitively dependent on plasma characteristics. These films exhibit intense photoluminescence in the visible range with violet to orange colors and with narrow to broad widths (∼0.3-0.9 eV). The observed luminescence behavior can come from the quantum confinement effect, quasi-direct band-to-band recombination, and variation of atomic hydrogen and nitrogen radicals in the film growth network. The high luminescence yields in the visible range of the spectrum and size-tunable low-temperature synthesis with plasma and radical control make these quantum dot films good candidates for light emitting applications.

Controlled Growth, Microstructure, and Properties of Functional Si Quantum Dot Films via Plasma Chemistry and Activated Radicals

“…Additionally, we correlate the HRTEM (Figure ) and XRD (Figure ) data with the Raman analysis (Figure a,c) to obtain the information about the QD structures. From the individual Raman spectrum, the average QD size is determined as d Si ≈ (1/3)·exp­(−π 2 )/[(ω d – ω 0 ) 2 + (δ 0 /2) 2 ]. , In this estimation, ω d represents the frequency of the nc-Si like mode for an NC of size d Si . The other parameters ω 0 and δ 0 are, respectively, 520 and 3.5 cm –1 for nc-Si.…”

“…Silicon (Si) nanocrystals (NCs) and Si quantum dots (QDs) have recently shown the immense attention because of their compatibility with the Si technology that makes Si NCs and QDs very attractive for the fabrication of microelectronics, optoelectronics, and energy producing devices like solar cells. − Particularly, Si QDs embedded in a dielectric matrix have emerged as the material for devices such as next-generation photovoltaic cells, thin-film transistors (TFTs), light emitting diodes (LEDs), and other applications. − When the size of the Si QDs in the confined system approaches the Si Bohr diameter (∼10 nm), the overlapping of the electron–hole wave function is significantly enhanced, which, in turn, enhances the likelihood of radiative electron–hole recombination. ,− Due to this reason, nanocrystalline Si-based nanostructured materials have a distinct advantage over bulk Si that exhibits an indirect band-gap semiconductor, which, in turn, necessitates phonon interactions while absorbing or emitting photons. , Further, note that the band gap ( E g ) of bulk Si is ∼1.12 eV, which can be much higher for Si QDs depending on the size of the QD. Because of QD’s enlarged E g compared to the bulk Si, an intense visible photoluminescence (PL) can be possible at room temperature . Thus, by using properly sized Si QDs, it is possible to cover the maximum visible length of the electromagnetic spectrum .…”

“…5,7−9 Due to this reason, nanocrystalline Si-based nanostructured materials have a distinct advantage over bulk Si that exhibits an indirect band-gap semiconductor, which, in turn, necessitates phonon interactions while absorbing or emitting photons. 8,9 Further, note that the band gap (E g ) of bulk Si is ∼1.12 eV, which can be much higher for Si QDs depending on the size of the QD. Because of QD's enlarged E g compared to the bulk Si, an intense visible photoluminescence (PL) can be possible at room temperature.…”

Energy/Electron Transfer Switch for Controlling Optical Properties of Silicon Quantum Dots

“…Several methods have been reported in literature for preparing SQDs including laser pyrolysis 17 , etching of bulk silicon 18 , nonthermal plasma 19 , and preparation in supercritical fluids 20 . In this work, a facile solution-based reduction method has been adopted with minor modifications to prepare SQDs 21 .…”

Freeze-Resistant Cadmium-Free Quantum Dots for Live-Cell Imaging

“…Quantum dots (QDs) have gained enormous popularity among the scientific community as a fluorescent marker for its unique intrinsic photophysical properties such as high brightness and quantum yield (QY), excellent photostability, broad absorption and narrow emission, size, and composition-based tuning of emission properties ranging from visible to near-IR spectrum. − QDs have been extensively utilized in various research fields such as biological imaging, determining the interaction between biomolecules, production of efficient solar cells, sensing, etc. ,, A general route for QD synthesis from organometallic precursors involves the use of high-boiling long-chain hydrophobic molecules, for example, trioctylphosphine, trioctyl phosphine oxide, oleyl amine, oleic acid, dodecanethiol, etc. − These molecules, also acting as surface-passivating ligand, are attached to the outer surface of the QDs and are essential for their colloidal stability in the solution. Because of the presence of hydrophobic surface ligands, they are only soluble in nonpolar solvents like hexane, toluene, chloroform, etc.…”