In situ gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals

Mechanical behavior of SiNC layers on PDMS: effects of layer thickness, PDMS modulus, and SiNC surface functionality

Thin layers of nanomaterials on stretchable substrates have the potential to enable stretchable, bendable optoelectronic devices, wearable diagnostics, and more. Recently, our group reported on a novel method for finding the neo-Hookean coefficient of thin layers of silicon nanocrystals (SiNCs) on polydimethylsiloxane (PDMS). Here we elaborate on that initial study by examining the effects of the SiNC layer thickness, PDMS neo-Hookean coefficient, and SiNC surface functionality on the neo-Hookean coefficient of the SiNC layers. We found that, while the layer thickness and PDMS neo-Hookean coefficient influence the behavior of the SiNC layers, layers of surface-functionalized SiNCs do not exhibit disparate behavior from layers of bare SiNCs.

Experimental/theoretical estimations of the neo-Hookean coefficients of SiNC layers on PDMS show a dependence on layer thickness as well as on the modulus of the PDMS, but not on the surface functionality of the SiNCs.

Aerosol-Phase Synthesis and Processing of Luminescent Silicon Nanocrystals

Silicon quantum dots are attractive materials for luminescent devices and bioimaging applications. For these light-emitting applications, higher photoluminescence efficiency is desired in order to achieve better device performance. Nonthermal plasma synthesis successfully allows for the continuous production of silicon nanocrystals, but postprocessing is necessary to improve photoluminescence quantum yields so that nanocrystals can be used for luminescence applications. In this work, we demonstrate an all-aerosol-phase synthesis and processing route that integrates nonthermal plasma synthesis, plasma-assisted surface functionalization with alkene ligands, and in-flight annealing within one flow stream. Here, luminescent silicon nanocrystals are synthesized and postprocessed on a time scale of only 100 ms, which is orders of magnitude faster than previous synthesis and functionalization schemes. The as-produced silicon nanocrystals have photoluminescence quantum yields exceeding 20%, which is a 5-fold increase compared to previous silicon nanocrystals synthesized with all-aerosol-phase approaches. We attribute the enhanced photoluminescence to the reduced "dark" nanocrystal fraction due to reduction of dangling bond density and desorption of surface silyl species induced by the in-flight annealing. We also demonstrate that the ligand coverage plays a minor role for the photoluminescence properties, but that the nature of the silicon hydride surface groups is a major factor.

A novel approach to finding mechanical properties of nanocrystal layers

Flexible, bendable, stretchable devices represent the future of electronics for a wide range of real-world applications. Due to the fact that these technologies deviate significantly from traditional wafer technologies there is a need to understand and engineer material systems that allow large elastic deformations present in such devices, which requires knowledge about the mechanical properties of these material systems. Here we evaluate the mechanical properties of a bilayer polydimethylsiloxane (PDMS)/silicon nanocrystal system. By observing the formation of instabilities due to finite bending deformation and applying theoretical modeling, we estimated the neo-Hookean coefficient (analogous to shear modulus at low stress/strain) of the silicon nanocrystal film to be 345 ± 23 kPa. The method used here represents a novel approach to evaluating these properties and is widely applicable to many different combinations of systems of nanocrystals and elastomers.

Gas Phase Organic Functionalization of SiO2 with Propanoyl Chloride

The reaction mechanism of propanoyl chloride (C₂H₅COCl) with -SiOH-terminated SiO₂ films was studied using in situ surface infrared spectroscopy. We show that this surface functionalization reaction is temperature dependent. At 230 °C, C₂H₅COCl reacts with isolated surface -SiOH groups to form the expected ester linkage. Surprisingly, as the temperature is lowered to 70 °C, the ketone groups are transformed into the enol tautomer, but if the temperature is increased back to the starting exposure temperature of 230 °C, the ketone tautomer is not recovered, indicating that the enol form is thermally stable over a wide range of temperatures. Further, the enol form is directly formed after exposure of a SiO₂ surface to C₂H₅COCl at 70 °C. We speculate that the enol form, which is energetically unfavorable, is stabilized because of hydrogen bonding with adjacent enol groups or through hydrogen bonding with unreacted surface -SiOH groups. The surface coverage of hydrocarbon molecules is calculated as ∼6 × 10¹² cm^-2, assuming each reacted -SiOH group contributes to one hydrocarbon linkage on the surface. At a substrate temperature of 70 °C, the enol form is unreactive with H₂O, and H₂O molecules simply physisorb on the surface. At higher temperatures, H₂O converts the ketone to the enol tautomer and reacts with Si-O-Si bridges, forming more -SiOH reactive sites. The overall hydrocarbon coverage on the surface can then be further increased through cycling H₂O and C₂H₅COCl doses.

Light-Emitting Diodes Based on Colloidal Silicon Quantum Dots with Octyl and Phenylpropyl Ligands

Colloidal silicon quantum dots (Si QDs) hold ever-growing promise for the development of novel optoelectronic devices such as light-emitting diodes (LEDs). Although it has been proposed that ligands at the surface of colloidal Si QDs may significantly impact the performance of LEDs based on colloidal Si QDs, little systematic work has been carried out to compare the performance of LEDs that are fabricated using colloidal Si QDs with different ligands. Here, colloidal Si QDs with rather short octyl ligands (Octyl-Si QDs) and phenylpropyl ligands (PhPr-Si QDs) are employed for the fabrication of LEDs. It is found that the optical power density of PhPr-Si QD LEDs is larger than that of Octyl-Si QD LEDs. This is due to the fact that the surface of PhPr-Si QDs is more oxidized and less defective than that of Octyl-Si QDs. Moreover, the benzene rings of phenylpropyl ligands significantly enhance the electron transport of QD LEDs. It is interesting that the external quantum efficiency (EQE) of PhPr-Si QD LEDs is lower than that of Octyl-Si QD LEDs because the benzene rings of phenylpropyl ligands suppress the hole transport of QD LEDs. The unbalance between the electron and hole injection in PhPr-Si QD LEDs is more serious than that in Octyl-Si QD LEDs. The currently obtained highest optical power density of ∼0.64 mW/cm² from PhPr-Si QD LEDs and highest EQE of ∼6.2% from Octyl-Si QD LEDs should encourage efforts to further advance the development of high-performance optoelectronic devices based on colloidal Si QDs.

Surface Phenomena During Plasma-Assisted Atomic Layer Etching of SiO2

Surface phenomena during atomic layer etching (ALE) of SiO₂ were studied during sequential half-cycles of plasma-assisted fluorocarbon (CF_x) film deposition and Ar plasma activation of the CF_x film using in situ surface infrared spectroscopy and ellipsometry. Infrared spectra of the surface after the CF_x deposition half-cycle from a C₄F₈/Ar plasma show that an atomically thin mixing layer is formed between the deposited CF_x layer and the underlying SiO₂ film. Etching during the Ar plasma cycle is activated by Ar⁺ bombardment of the CF_x layer, which results in the simultaneous removal of surface CF_x and the underlying SiO₂ film. The interfacial mixing layer in ALE is atomically thin due to the low ion energy during CF_x deposition, which combined with an ultrathin CF_x layer ensures an etch rate of a few monolayers per cycle. In situ ellipsometry shows that for a ∼4 Å thick CF_x film, ∼3-4 Å of SiO₂ was etched per cycle. However, during the Ar plasma half-cycle, etching proceeds beyond complete removal of the surface CF_x layer as F-containing radicals are slowly released into the plasma from the reactor walls. Buildup of CF_x on reactor walls leads to a gradual increase in the etch per cycle.

Exclusively Gas-Phase Passivation of Native Oxide-Free Silicon(100) and Silicon(111) Surfaces

Reactions in the gas phase are of primary technological importance for applications in nano- and microfabrication technology and in the semiconductor industry. We present exclusively gas-phase protocols to chemically passivate oxide-free Si(111) and Si(100) surfaces with short-chain alkynes. The resulting surfaces showed equal or better oxidation resistance than most existing liquid-phase-derived surfaces and rivaled the outstanding stability of a full-coverage Si(111)-propenyl surface.1,2 The most stable surface (Si(111)-ethenyl) grew one-fifth of a monolayer of oxide (0.04 nm) after 1 month of air exposure. We monitored the regrowth of oxides on passivated Si(111) and Si(100) surfaces by X-ray photoelectron spectroscopy (XPS) and observed a significant crystal-orientation dependence of initial rates when total oxide thickness was below approximately one monolayer (0.2 nm). This difference was correlated with the desorption kinetics of residual surface Si-F bonds formed during HF treatment. We discuss applications of the technology and suggest future directions for process optimization.

Radical Initiated Hydrosilylation on Silicon Nanocrystal Surfaces: An Evaluation of Functional Group Tolerance and Mechanistic Study

Hydrosilylation is among the most common methods used for modifying silicon surface chemistry. It provides a wide range of surface functionalities and effective passivation of surface sites. Herein, we report a systematic study of radical initiated hydrosilylation of silicon nanocrystal (SiNC) surfaces using two common radical initiators (i.e., 2,2'-azobis(2-methylpropionitrile) and benzoyl peroxide). Compared to other widely applied hydrosilylation methods (e.g., thermal, photochemical, and catalytic), the radical initiator based approach is particle size independent, requires comparatively low reaction temperatures, and yields monolayer surface passivation after short reaction times. The effects of differing functional groups (i.e., alkene, alkyne, carboxylic acid, and ester) on the radical initiated hydrosilylation are also explored. The results indicate functionalization occurs and results in the formation of monolayer passivated surfaces.

Single-step plasma synthesis of carbon-coated silicon nanoparticles

We have developed a novel single-step technique based on nonthermal, radio frequency (rf) plasmas to synthesize sub-10 nm, core-shell, carbon-coated crystalline Si (c-Si) nanoparticles (NPs) for potential application in Li(+) batteries and as fluorescent markers. Hydrogen-terminated c-Si NPs nucleate and grow in a SiH4-containing, low-temperature plasma in the upstream section of a tubular quartz reactor. The c-Si NPs are then transported downstream by gas flow, and are coated with amorphous carbon (a-C) in a second C2H2-containing plasma. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and in situ attenuated total reflection Fourier transform infrared spectroscopy show that a thin, < 1 nm, 3C-SiC layer forms at the c-Si/a-C interface. By varying the downstream C2H2 plasma rf power, we can alter the nature of the a-C coating as well as the thickness of the interfacial 3C-SiC layer. The transmission electron microscopy (TEM) analysis is in agreement with the Si NP core size determined by Raman spectroscopy, photoluminescence spectroscopy, and XRD analysis. The size of the c-Si NP core, and the corresponding light emission from these NPs, was directly controlled by varying the thickness of the interfacial 3C-SiC layer. This size tunable emission thus also demonstrates the versatility of this technique for synthesizing c-Si NPs for potential applications in light emitting diodes, biological markers, and nanocrystal inks.

Newly synthesized silicon quantum dot-polystyrene nanocomposite having thermally robust positive charge trapping

Striving to replace the well known silicon nanocrystals embedded in oxides with solution-processable charge-trapping materials has been debated because of large scale and cost effective demands. Herein, a silicon quantum dot-polystyrene (SiQD-PS) nanocomposite (NC) was synthesized by post-functionalization of hydrogen-terminated silicon quantum dots (H-SiQDs) with styrene using a thermally induced surface-initiated polymerization approach. The NC contains two miscible components: PS and SiQD@PS which, respectively, are polystyrene and polystyrene chains-capped SiQDs. Spin-coated films of the nanocomposite on various substrate were thermally annealed at different temperatures and subsequently used to construct metal-insulator-semiconductor (MIS) devices and thin film field-effect transistors (TFTs) having a structure of p-Si++/SiO2/NC/pentacene/Au source-drain. Capacitance-voltage (C-V) curves obtained from the MIS devices exhibit a well-defined counterclockwise hysteresis with negative fat band shifts, which was stable over a wide range of curing temperatures (50-250 °C). The positive charge trapping capability of the NC originates from the spherical potential well structure of the SiQD@PS component while the strong chemical bonding between SiQDs and polystyrene chains accounts for the thermal stability of the charge trapping property. The transfer curve of the transistor was controllably shifted to the negative direction by varying applied gate voltage. Thereby, this newly synthesized and solution processable SiQD-PS nanocomposite is applicable as charge trapping materials for TFT based memory devices.

Gas-phase hydrosilylation of plasma-synthesized silicon nanocrystals with short- and long-chain alkynes

Surface passivation of Si nanocrystals (NCs) is necessary to enable their utilization in novel photovoltaic and optoelectronic devices. Herein, we report the surface passivation of plasma-synthesized, H-terminated Si NCs via gas-phase hydrosilylation using a combination of short- and long-chain alkynes. Specifically, using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that a sequential exposure of the Si NC surface to acetylene and phenylacetylene results in a surface alkenyl coverage of ∼58%, which is close to the theoretical maximum of ∼55% and ∼60% predicted for alkyl- and alkenyl-terminated Si(111) surfaces, respectively. We attribute this unprecedented high surface hydrocarbon coverage to the combination of short- and long-chain alkynes that reduce the steric hindrance on the surface, higher reactivity of 1-alkynes versus 1-alkenes of the same chain length, and the smaller van der Waals radius of the alkenyl groups compared to the alkyl groups. Unlike 1-alkenes, 1-alkynes also react with the surface to form the 1,1- and 1,2-bridge structures via the bis-hydrosilylation reaction. However, our data clearly show that this reaction pathway cannot account for the enhanced surface coverage in the sequential exposure experiments, since exposure of the surface to just acetylene or phenylacetylene results in an almost identical surface coverage due to the 1,1- and 1,2-bridge sites.

Silicon nanocrystal OLEDs: effect of organic capping group on performance

The synthesis of highly luminescent, colloidally-stable and organically-capped silicon nanocrystals (ncSi) and their incorporation into a visible wavelength organic light-emitting diode (OLED) is reported. By substituting decyl chains with aromatic allylbenzene capping ligands and size-selecting visible emitting ncSi, superior packing density, enhanced charge transport, and an improved photoluminescence absolute quantum yield of the ncSi is obtained in the active layer of an OLED.