L.C.P.M. de Smet

Covalent Attachment of Bent-Core Mesogens to Silicon Surfaces

Two vinyl-terminated bent core-shaped liquid crystalline molecules that exhibit thermotropic antiferroelectric SmCPA phases have been covalently attached onto a hydrogen-terminated silicon(111) surface. The surface attachment was achieved via a mild procedure from a mesitylene solution, using visible light at room temperature. AFM measurements indicate that a smooth monolayer has been formed. The thickness of the monolayer was evaluated with ellipsometry and X-ray reflectivity. Although the molecules differ in length by four carbon atoms, the thickness of the resulting monolayers was the same. The measured thicknesses correspond quite well with the smectic layer thickness in the bulk liquid crystalline material, suggesting a similar self-organization within the monolayer. From attenuated total reflectance infrared (ATR-IR), which clearly shows the C-H and C-O vibrations, a tilt angle of the mesogens is deduced that also corresponds well with the tilt angle in the liquid crystalline state. X-ray photoelectron spectroscopy (XPS) measurements confirm the high quality of the monolayers, with only marginal silicon oxide formation. The elemental composition and amounts of different O and C atoms deduced from the high-resolution XPS correspond very well with the calculated compositions.

Hydrogen termination of CVD diamond films by high-temperature annealing at atmospheric pressure

A high-temperature procedure to hydrogenate diamond films using molecular hydrogen at atmospheric pressure was explored. Undoped and doped chemical vapour deposited (CVD) polycrystalline diamond films were treated according to our annealing method using a H2 gas flow down to ~50 ml∕min (STP) at ~850 °C. The films were extensively evaluated by surface wettability, electron affinity, elemental composition, photoconductivity, and redox studies. In addition, electrografting experiments were performed. The surface characteristics as well as the optoelectronic and redox properties of the annealed films were found to be very similar to hydrogen plasma-treated films. Moreover, the presented method is compatible with atmospheric pressure and provides a low-cost solution to hydrogenate CVD diamond, which makes it interesting for industrial applications. The plausible mechanism for the hydrogen termination of CVD diamond films is based on the formation of surface carbon dangling bonds and carbon-carbon unsaturated bonds at the applied tempera-ture, which react with molecular hydrogen to produce a hydrogen-terminated surface.

IC compatible top down process for Silicon Nanowire fet arrays with three {100} surfaces for (BIO) chemical sensing

We report on a relatively simple and cost-effective method for fabrication of Silicon Nanowire Field-Effect Transistor (SiNW FET) arrays with three {100} surfaces via IC-compatible, top-down processes. Devices fabricated with this process offers an extra advantage compared to other top down fabrication techniques by not just only offering one or two, but instead three well-defined surfaces. The method offers a precise control of the NWs width (down to sub-100nm) without the need for nanolithography. In addition, very smooth and straight sidewalls are obtained. The process flow requires just one mask step and room-temperature, wet etching for the nanowire patterning and is thus suitable for wafer-scale fabrication of nanowires arrays. Device characterization includes electrical characterization and pH measurements were performed using the fabricated SiNW FET arrays.

Controlled amino-functionalization by electrochemical reduction of bromo and nitro azobenzene layers bound to Si(111) surfaces

4-Nitrobenzenediazonium (4-NBD) and 4-bromobenzenediazonium (4-BBD) salts were grafted electrochemically onto H-terminated, p-doped silicon (Si) surfaces. Atomic force microscopy (AFM) and ellipsometry experiments clearly showed layer thicknesses of 2-7 nm, which indicate multilayer formation. Decreasing the diazonium salt concentration and the reaction time resulted in a smaller layer thickness, but did not prevent the formation of multilayers. It was demonstrated, mainly by X-ray photoelectron spectroscopy (XPS), that the diazonium salts not only react with the H-terminated Si surface, but also with electrografted phenyl groups via azo-bond formation. These azo bonds can be electrochemically reduced at Ered = -1.5 V, leading to the corresponding amino groups. This reduction resulted in a modest decrease in layer thickness, and did not yield monolayers. This indicates that other coupling reactions, notably a biphenyl coupling, induced by electrochemically produced phenyl radicals, take place as well. In addition to the azo functionalities, the nitro functionalities in electrografted layers of 4-NBD were independently reduced to amino functionalities at a lower potential (Ered = -2.1 V). The presence of amino functionalities on fully reduced layers, both from 4-NBD- and 4-BBD-modified Si, was shown by the presence of fluorine after reaction with trifluoroacetic anhydride (TFAA). This study shows that the electrochemical reduction of azo bonds generates amino functionalities on layers produced by electrografting of aryldiazonium derivatives. In this way multifunctional layers can be formed by employing functional aryldiazonium salts, which is believed to be very practical in the fabrication of sensor platforms, including those made of multi-array silicon nanowires.

Protein detection on biotin-derivatized polyallylamine by optical microring resonators.

Silicon optical microring resonators (MRRs) are sensitive devices that can be used for biosensing. We present a novel biosensing platform based on the application of polyelectrolyte (PE) layers on such MRRs. The top PE layer was covalently labeled with biotin to ensure binding sites for antibodies via a streptavidin-biotin binding scheme. Monitoring the shift in the microring resonance wavelength allows real-time, highly sensitive detection of the biomolecular interaction.

Silicon nanowire FET arrays for real time detection of chemical activation of cells

Silicon nanowire FET arrays have been designed and fabricated with a relatively low-cost, but wafer-scale process without the use of e-beam lithography. An optimized fabrication process that improves the etching uniformity of NWs and also provides a better control of the NW dimensions is presented. The fabricated devices are electrically characterized and their performance is discussed. A dedicated measurement set-up and a preparation procedure to employ these SiNW FET arrays for real time monitoring of the substance P activation of neurokinin (NK1) receptors in HEK 293 cells are presented.

Pick-up and drop transfer of diamond nanosheets

Nanocrystalline diamond (NCD) is a promising material for electronic and mechanical micro- and nanodevices. Here we introduce a versatile pick-up and drop technique that makes it possible to investigate the electrical, optical and mechanical properties of as-grown NCD films. Using this technique, NCD nanosheets, as thin as 55 nm, can be picked-up from a growth substrate and positioned on another substrate. As a proof of concept, electronic devices and mechanical resonators are fabricated and their properties are characterized. In addition, the versatility of the method is further explored by transferring NCD nanosheets onto an optical fiber, which allows measuring its optical absorption. Finally, we show that NCD nanosheets can also be transferred onto two-dimensional crystals, such as MoS2, to fabricate heterostructures. Pick-up and drop transfer enables the fabrication of a variety of NCD-based devices without requiring lithography or wet processing.

Current-induced nanogap formation and graphitization in boron-doped diamond films

A high-current annealing technique is used to fabricate nanogaps and hybrid diamond/graphite structures in boron-doped nanocrystalline diamond films. Nanometer-sized gaps down to ∼1 nm are produced using a feedback-controlled current annealing procedure. The nanogaps are characterized using scanning electron microscopy and electronic transport measurements. The structural changes produced by the elevated temperature, achieved by Joule heating during current annealing, are characterized using Raman spectroscopy. The formation of hybridized diamond/graphite structure is observed at the point of maximum heat accumulation.