Silicon nanowire piezoresistor and its applications: a review.

Monocrystalline bulk silicon with doped impurities has been the widely preferred piezoresistive material for the last few decades to realize micro-electromechanical system (MEMS) sensors. However, there has been a growing interest among researchers in the recent past to explore other piezoresistive materials with varied advantages in order to realize ultra-miniature high-sensitivity sensors for area-constrained applications. Of the various alternative piezoresistive materials, silicon nanowires (SiNWs) are an attractive choice due to their benefits of nanometre range dimensions, giant piezoresistive coefficients, and compatibility with the integrated circuit fabrication processes. This review article elucidates the fundamentals of piezoresistance and its existence in various materials, including silicon. It comprehends the piezoresistance effect in SiNWs based on two different biasing techniques, viz., (i) ungated and (ii) gated SiNWs. In addition, it presents the application of piezoresistive SiNWs in MEMS-based pressure sensors, acceleration sensors, flow sensors, resonators, and strain gauges.

Piezoresistive Composite Silicon Dioxide Nanocantilever Surface Stress Sensor: Design and Optimization.

In this paper, we present the design and optimization of a rectangular piezoresistive composite silicon dioxide nanocantilever sensor. Unlike the conventional design approach, we perform the sensor optimization by not only considering its electro-mechanical response but also incorporating the impact of self-heating induced thermal drift in its terminal characteristics. Through extensive simulations first we comprehend and quantify the inaccuracies due to self-heating effect induced by the geometrical and intrinsic parameters of the piezoresistor. Then, by optimizing the ratio of electrical sensitivity to thermal sensitivity defined as the sensitivity ratio (υ) we improve the sensor performance and measurement reliability. Results show that to ensure υ ≥ 1, shorter and wider piezoresistors are better. In addition, it is observed that unlike the general belief that high doping concentration of piezoresistor reduces thermal sensitivity in piezoresistive sensors, to ensure υ ≥ 1 doping concentration (p) should be in the range: 1E18 cm-3 ≤ p ≤ 1E19 cm-3. Finally, we provide a set of design guidelines that will help NEMS engineers to optimize the performance of such sensors for chemical and biological sensing applications.

Impact of Isolation and Immobilization Layers on the Electro-Mechanical Response of Piezoresistive Nano Cantilever Sensors.

In the last decade, piezoresistive nano cantilever sensors have been extensively explored, especially for chemical and biological sensing applications. Piezoresistive cantilever sensors are multi-layer structures with different constituent materials. Performance of such sensors is a function of their geometry and constituent materials. For a fixed material set, the pre-requisite for optimizing the performance of a composite piezoresistive cantilever sensor is careful geometrical design of its constituent layers. Even though, treatise encompasses various designs of such sensors, typically for computational simplicity the functional layers i.e., the isolation and immobilization layers are neglected in the modeling stages. In this paper, we elucidate the impact of the functional layers on the electro-mechanical response of composite piezoresistive nano cantilever sensors. Systematic and detailed computations are performed using theoretical models and numerical simulations. Results show that both the isolation and immobilization layers play a critical role in governing the sensor performance. Simulation results depict that compared to a sensor with an isolation layer of thickness 100 nm, a sensor without isolation layer has 36.29% and 42.51% better deflection sensitivity and electrical sensitivity respectively. Furthermore, it is found that when an immobilization layer of thickness 40 nm is added atop the isolation layer, the deflection sensitivity and electrical sensitivity reduces by 12.98% and 15.83% respectively. Through our investigation it is shown that the isolation and immobilization layers not only play a vital role in determining the stability and electro-mechanical response of the sensor but their negligence in the design stages can be detrimental. Apart from investigating the impact of the immobilization layer thickness, to model the sensor closer to real time operational conditions, we have performed analysis to understand the impact of non-uniformity in the immobilization layer thickness and non-uniform surface stress loading on the electro-mechanical response of the sensor. Results and inferences obtained from this study will help NEMS engineers to optimize the performance of piezoresistive nano cantilever sensors and to design multi-layer cantilever platform structures for other transducers.

Robust mixed 10/12 helices promoted by "alternating chirality" in a new family of C-linked carbo-beta-peptides.

The C-linked carbo-beta-peptides, oligomers of a new class of C-linked carbo-beta3-amino acids, have been shown to generate mixed 12/10 and 10/12 helices. The design involves use of "alternating chirality" of the epimeric (at the amine center) monomers to control the stability of these helices. The observation of stable 12/10 helix in a tripeptide and 10/12 helix in a tetrapeptide is unprecedented.

Unusual cyclo-tetra and hexa peptidation of bis-boc-cystine with cystine-di-OMe: one step preparation of the novel 32- and 48-membered cyclotetracystine and cyclohexacystine.

The unprecedented formation of 32- and 48-membered macrocycles that inscribe 4 and 6 cystine units, in the peptidation of bis-Boc-cystine with cystine di-OMe is reported.