Nanowire‐Based Sensors

Nanowires are important potential candidates for the realization of the next generation of sensors. They offer many advantages such as high surface‐to‐volume ratios, Debye lengths comparable to the target molecule, minimum power consumption, and they can be relatively easily incorporated into microelectronic devices. Accordingly, there has been an intensified search for novel nanowire materials and corresponding platforms for realizing single‐molecule detection with superior sensing performance. In this work, progress made towards the use of nanowires for achieving better sensing performance is critically reviewed. In particular, various nanowires types (metallic, semiconducting, and insulating) and their employment either as a sensor material or as a template material are discussed. Major obstacles and future steps towards the ultimate nanosensors based on nanowires are addressed.     

Tuning the Electrical Properties of Si Nanowire Field‐Effect Transistors by Molecular Engineering

Exposed facets of n‐type silicon nanowires (Si NWs) fabricated by a top‐down approach are successfully terminated with different organic functionalities, including 1,3‐dioxan‐2‐ethyl, butyl, allyl, and propyl‐alcohol, using a two‐step chlorination/alkylation method. X‐ray photoemission spectroscopy and spectroscopic ellipsometry establish the bonding and the coverage of these molecular layers. Field‐effect transistors fabricated from these Si NWs displayed characteristics that depended critically on the type of molecular termination. Without molecules the source–drain conduction is unable to be turned off by negative gate voltages as large as ?20 V. Upon adsorption of organic molecules there is an observed increase in the “on” current at large positive gate voltages and also a reduction, by several orders of magnitude, of the “off” current at large negative gate voltages. The zero‐gate voltage transconductance of molecule‐terminated Si NW correlates with the type of organic molecule. Adsorption of butyl and 1,3‐dioxan‐2‐ethyl molecules improves the channel conductance over that of the original SiO₂? Si NW, while adsorption of molecules with propyl‐alcohol leads to a reduction. It is shown that a simple assumption based on the possible creation of surface states alongside the attachment of molecules may lead to a qualitative explanation of these electrical characteristics. The possibility and potential implications of modifying semiconductor devices by tuning the distribution of surface states via the functionality of attached molecules are discussed.     

CMOS‐Compatible Silicon Nanowire Field‐Effect Transistors for Ultrasensitive and Label‐Free MicroRNAs Sensing

MicroRNAs (miRNAs) have been regarded as promising biomarkers for the diagnosis and prognosis of early‐stage cancer as their expression levels are associated with different types of human cancers. However, it is a challenge to produce low‐cost miRNA sensors, as well as retain a high sensitivity, both of which are essential factors that must be considered in fabricating nanoscale biosensors and in future biomedical applications. To address such challenges, we develop a complementary metal oxide semiconductor (CMOS)‐compatible SiNW‐FET biosensor fabricated by an anisotropic wet etching technology with self‐limitation which provides a much lower manufacturing cost and an ultrahigh sensitivity. This nanosensor shows a rapid (< 1 minute) detection of miR‐21 and miR‐205, with a low limit of detection (LOD) of 1 zeptomole (ca. 600 copies), as well as an excellent discrimination for single‐nucleotide mismatched sequences of tumor‐associated miRNAs. To investigate its applicability in real settings, we have detected miRNAs in total RNA extracted from lung cancer cells as well as human serum samples using the nanosensors, which demonstrates their potential use in identifying clinical samples for early diagnosis of cancer.     

Organic Field‐Effect Transistor Memory Devices Using Discrete Ferritin Nanoparticle‐Based Gate Dielectrics

Organic field‐effect transistor (OFET) memory devices made using highly stable iron‐storage protein nanoparticle (NP) multilayers and pentacene semiconductor materials are introduced. These transistor memory devices have nonvolatile memory properties that cause reversible shifts in the threshold voltage (V_th) as a result of charge trapping and detrapping in the protein NP (i.e., the ferritin NP with a ferrihydrite phosphate core) gate dielectric layers rather than the metallic NP layers employed in conventional OFET memory devices. The protein NP‐based OFET memory devices exhibit good programmable memory properties, namely, large memory window ΔV_th (greater than 20 V), a fast switching speed (10 μs), high ON/OFF current ratio (above 10⁴), and good electrical reliability. The memory performance of the devices is significantly enhanced by molecular‐level manipulation of the protein NP layers, and various biomaterials with heme Fe^III/Fe^II redox couples similar to a ferrihydrite phosphate core are also employed as charge storage dielectrics. Furthermore, when these protein NP multilayers are deposited onto poly(ethylene naphthalate) substrates coated with an indium tin oxide gate electrode and a 50‐nm‐thick high‐k Al₂O₃ gate dielectric layer, the approach is effectively extended to flexible protein transistor memory devices that have good electrical performance within a range of low operating voltages (<10 V) and reliable mechanical bending stability.