Zhaofeng Zhai

Photochemical Modification of Single Crystalline GaN Film Using n-Alkene with Different Carbon Chain Lengths as Biolinker

As a potential material for biosensing applications, gallium nitride (GaN) films have attracted remarkable attention. In order to construct GaN biosensors, a corresponding immobilization of biolinkers is of great importance in order to render a surface bioactive. In this work, two kinds of n-alkenes with different carbon chain lengths, namely allylamine protected with trifluoroacetamide (TFAAA) and 10-aminodec-1-ene protected with trifluoroacetamide (TFAAD), were used to photochemically functionalize single crystalline GaN films. The successful linkage of both TFAAA and TFAAD to the GaN films is confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurement. With increased UV illumination time, the intensity of the secondary ions corresponding to the linker molecules initially increases and subsequently decreases in both cases. Based on the SIMS measurements, the maximum coverage of TFAAA is achieved after 14 h of UV illumination, while only 2 h is required in the case of TFAAD to reach the situation of a fully covered GaN surface. This finding leads to the conclusion that the reaction rate of TFAAD is significantly higher compared to TFAAA. Measurements by atomic force microscopy (AFM) indicate that the coverage of GaN films by a TFAAA layer leads to an increased surface roughness. The atomic terraces, which are clearly observable for the pristine GaN films, disappear once the surface is fully covered by a TFAAA layer. Such TFAAA layers will feature a homogeneous surface topography even for reaction times of 24 h. In contrast to this, TFAAD shows strong cross-polymerization on the surface, this is confirmed by optical microscopy. These results demonstrate that TFAAA is a more suitable candidate as biolinker in context of the GaN surfaces due to its improved controllability.

Fabrication of silicon-vacancy color centers in diamond films: tetramethylsilane as a new dopant source

Color centers in diamonds hold great promise for applications in optical sensors, bio-imaging, and quantum communication. Here, we synthesize Si-doped diamond films with Si-vacancy (SiV) centers by the flow of tetramethylsilane (TMS, Si(CH3)4) gas using the microwave plasma chemical vapor deposition technique. In order to achieve high emission efficiency of the SiV centers, the effect of the TMS content on the microstructural evolution and photoluminescence (PL) of this type of color center is investigated using various spectroscopic techniques and high resolution transmission electron microscopy (HRTEM). The introduction of TMS gas in the diamond films leads to grain refinement of the diamond crystals and a weak SiV PL intensity located at 738 nm, at a growth temperature of 650 °C. For diamond films grown at 870 °C, the addition of Si atoms results in grain refinement and the transition of the diamond grains from micro-size without doping (no TMS) to nano-level at a Si/C ratio of 1/100. The SiV PL intensity exhibits a non-monotonic behavior with increasing Si/C ratios. At a Si/C ratio of 1/3100, the diamond film features a structure of nano-grains separated with (100) micro-grains, and displays a maximum in the PL intensity of the SiV centers: a very strong narrow peak at 738 nm with a FWHM of about 5.1 nm. Increasing the Si/C ratio promotes the formation of a nanocrystalline structure and the decrease of the SiV PL intensity. The combination of Raman spectral and HRTEM analysis implies that the PL quenching of the SiV center with the increasing Si/C ratios is attributed to the formation of amorphous carbon. Our results not only demonstrate that the diamond film, featuring a structure of nano-crystals with (100) micro-crystals, could be a promising material with high-efficiency SiV centers, but also highlight that this approach to balancing the concentration of Si impurities and the crystalline quality of the diamond films could advance the fabrication of high-emission SiV centers for optical applications.

Hydration-induced nano- to micro-scale self-recovery of the tooth enamel of the giant panda

The tooth enamel of vertebrates comprises a hyper-mineralized bioceramic, but is distinguished by an exceptional durability to resist impact and wear throughout the lifetime of organisms; however, enamels exhibit a low resistance to the initiation of large-scale cracks comparable to that of geological minerals based on fracture mechanics. Here we reveal that the tooth enamel, specifically from the giant panda, is capable of developing durability through counteracting the early stage of damage by partially recovering its innate geometry and structure at nano- to micro- length-scales autonomously. Such an attribute results essentially from the unique architecture of tooth enamel, specifically the vertical alignment of nano-scale mineral fibers and micro-scale prisms within a water-responsive organic-rich matrix, and can lead to a decrease in the dimension of indent damage in enamel introduced by indentation. Hydration plays an effective role in promoting the recovery process and improving the indentation fracture toughness of enamel (by ∼73%), at a minor cost of micro-hardness (by ∼5%), as compared to the dehydrated state. The nano-scale mechanisms that are responsible for the recovery deformation, specifically the reorientation and rearrangement of mineral fragments and the inter- and intra-prismatic sliding between constituents that are closely related to the viscoelasticity of organic matrix, are examined and analyzed with respect to the structure of tooth enamel. Our study sheds new light on the strategies underlying Natures design of durable ceramics which could be translated into man-made systems in developing high-performance ceramic materials. STATEMENT OF SIGNIFICANCE: Tooth enamel plays a critical role in the function of teeth by providing a hard surface layer to resist wear/impact throughout the lifetime of organisms; however, such enamel exhibits a remarkably low resistance to the initiation of large-scale cracks, of hundreds of micrometers or more, comparable to that of geological minerals. Here we reveal that tooth enamel, specifically that of the giant panda, is capable of partially recovering its geometry and structure to counteract the early stages of damage at nano- to micro-scale dimensions autonomously. Such an attribute results essentially from the architecture of enamel but is markedly enhanced by hydration. Our work discerns a series of mechanisms that lead to the deformation and recovery of enamel and identifies a unique source of durability in the enamel to accomplish this function. The ingenious design of tooth enamel may inspire the development of new durable ceramic materials in man-made systems.