Silicon-Doped Graphene Oxide Quantum Dots as Efficient Nanoconjugates for Multifunctional Nanocomposites

Graphene oxide quantum dots (GOQDs) hold great promise as a new class of high-performance carbonaceous nanomaterials due to their numerous functional properties, such as tunable photoluminescence (PL), excellent thermal and chemical stability, and superior biocompatibility. In this study, we developed a facile, one-pot, and effective strategy to engineer the interface of GOQDs through covalent doping with silicon. The successful covalent attachment of the silane dopant with pendant vinyl groups to the edges of the GOQDs was confirmed by an in-depth investigation of the structural and morphological characteristics. The Si-GOQD nanoconjugates had an average dimension of ∼8 nm, with a graphite-structured core and amorphous carbon on their shell. We further used the infrared nanoimaging based on scattering-type scanning near-field optical microscopy to unveil the spectral near-field response of GOQD samples and to measure the nanoscale IR response of its network; we then demonstrated their distinct domains with strongly enhanced near fields. The doping of Si atoms into the sp²-hybridized graphitic framework of GOQDs also led to tailored PL emissions. We then sought to explore the potential applications of Si-GOQDs on the surface of plastic films where poly(dimethylsiloxane) (PDMS) served as a bridge to tightly anchor the Si-GOQDs to the surface. The bi-layered coated films which were built with co-assembly of Si-GOQDs and PDMS contributed to suppressing the transmission of water molecules due to the generation of compact and less accessible passing sites, achieving a nearly twofold reduction in water permeability compared to the single-layered coated films. The nanoindentation and PeakForce quantitative nanomechanical mapping showed that Si-GOQD-coated substrates were softer and more deformable than those coated only with PDMS. The co-assembly of PDMS and Si-GOQDs yielded films that were less stiff than those made from PDMS alone. Our findings provided conceptual insights into the importance of nanoscale surface engineering of GOQDs in conferring excellent dispersibility and enhancing the performance of nanocomposite films.

Quantification of Trace-Level Silicon Doping in Al x Ga1-xN Films Using Wavelength-Dispersive X-Ray Microanalysis

Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35-100 ppm [corresponding to (3-9) × 1018 cm-3] in doped AlxGa1-xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1-xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1-xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

Superior and Reversible Lithium Storage of SnO2/Graphene Composites by Silicon Doping and Carbon Sealing

The poor cycle stability and reversibility seriously hinder the widespread application of SnO₂ materials as anodes for lithium-ion batteries (LIBs). A novel sandwich-architecture composite of Si-doped SnO₂ nanorods and reduced graphene oxide with carbon sealing (Si-SnO₂@G@C) is engineered and fabricated by a facile two-step hydrothermal process and subsequent annealing treatment, which exhibit not only extraordinary rate performance and ultrahigh reversible capacity but also excellent cycle stability and high electrical conductivity as the anode of LIBs. The Si-doped SnO₂ nanoparticles on the surface of graphene were firmly wrapped in the C-coating and formed a porous sandwich structure, which can efficiently prevent the Sn nanoparticles from aggregation and provide more extra space for accommodating the volume variations and more active sites for reactions. The carbon layer also blocks the direct contact of the SnO₂ nanorods with electrolyte and prevents the graphene nanosheets from the restacking. More importantly, the reversibility of lithiation/delithiation reactions can be remarkably improved by the doping silicon. The doped Si not only accelerates the diffusion of Li⁺ but also brings a significant increase in the specific capacity. As a consequence, the Si-SnO₂@G@C nanocomposite can maintain a high capacity of 654 mAh/g at 2 A/g even after 1200 cycles with negligible capacity loss and excellent reversibility with a Coulombic efficiency retention over 99%, which can be capable of the alternative to commercial graphite anodes. This work provides a new strategy for the reasonable design of advanced anode materials with superior and reversible lithium storage capacity.

Surface Characteristics and Biological Evaluation of Si-DLC Coatings Fabricated Using Magnetron Sputtering Method on Ti6Al7Nb Substrate

Diamond-like carbon (DLC) coatings are well known as protective coatings for biomedical applications. Furthermore, the incorporation of different elements, such as silicon (Si), in the carbon matrix changes the bio-functionality of the DLC coatings. This has also been proven by the results obtained in this work. The Si-DLC coatings were deposited on the Ti6Al7Nb alloy, which is commonly used in clinical practice, using the magnetron sputtering method. According to the X-ray photoelectron spectroscopy (XPS) analysis, the content of silicon in the examined coatings varied from ~2 at.% up to ~22 at.%. Since the surface characteristics are key factors influencing the cell response, the results of the cells’ proliferation and viability assays (live/dead and XTT (colorimetric assays using tetrazolium salt)) were correlated with the surface properties. The surface free energy (SFE) measurements, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the polarity and wettability of the surfaces examined increase with increasing Si concentration, and therefore the adhesion and proliferation of cells was enhanced. The results obtained revealed that the biocompatibility of Si-doped DLC coatings, regardless of the Si content, remains at a very high level (the observed viability of endothelial cells is above 70%).

Silicon-Doped Titanium Dioxide Nanotubes Promoted Bone Formation on Titanium Implants

While titanium (Ti) implants have been extensively used in orthopaedic and dental applications, the intrinsic bioinertness of untreated Ti surface usually results in insufficient osseointegration irrespective of the excellent biocompatibility and mechanical properties of it. In this study, we prepared surface modified Ti substrates in which silicon (Si) was doped into the titanium dioxide (TiO₂) nanotubes on Ti surface using plasma immersion ion implantation (PIII) technology. Compared to TiO₂ nanotubes and Ti alone, Si-doped TiO₂ nanotubes significantly enhanced the expression of genes related to osteogenic differentiation, including Col-I, ALP, Runx2, OCN, and OPN, in mouse pre-osteoblastic MC3T3-E1 cells and deposition of mineral matrix. In vivo, the pull-out mechanical tests after two weeks of implantation in rat femur showed that Si-doped TiO₂ nanotubes improved implant fixation strength by 18% and 54% compared to TiO₂-NT and Ti implants, respectively. Together, findings from this study indicate that Si-doped TiO₂ nanotubes promoted the osteogenic differentiation of osteoblastic cells and improved bone-Ti integration. Therefore, they may have considerable potential for the bioactive surface modification of Ti implants.

Boosting the Boron Dopant Level in Monolayer Doping by Carboranes

Monolayer doping (MLD) presents an alternative method to achieve silicon doping without causing crystal damage, and it has the capability of ultrashallow doping and the doping of nonplanar surfaces. MLD utilizes dopant-containing alkene molecules that form a monolayer on the silicon surface using the well-established hydrosilylation process. Here, we demonstrate that MLD can be extended to high doping levels by designing alkenes with a high content of dopant atoms. Concretely, carborane derivatives, which have 10 B atoms per molecule, were functionalized with an alkene group. MLD using a monolayer of such a derivative yielded up to ten times higher doping levels, as measured by X-ray photoelectron spectroscopy and dynamic secondary mass spectroscopy, compared to an alkene with a single B atom. Sheet resistance measurements showed comparably increased conductivities of the Si substrates. Thermal budget analyses indicate that the doping level can be further optimized by changing the annealing conditions.

Nanocathodoluminescence Reveals Mitigation of the Stark Shift in InGaN Quantum Wells by Si Doping

Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.