SILICON-INTEGRATED SENSING DEVICES BASED ON PLASMONIC NANOANTENNAS AND TWO-DIMENSIONAL FIELD-EFFECT TRANSISTORS

This thesis investigates silicon-integrated, label-free sensing concepts along two technologically related research lines. The first line concerns plasmonic nanoantennas operating in the terahertz (THz, 1–10 THz) and mid-infrared (mid-IR, 25–100 THz) ranges, where an analyte is expected to modify the resonant electromagnetic response. The second line concerns two-dimensional (2D) hafnium disulfide (HfS2) flakes integrated in atransistor-like electrode configuration, intended as a first step towards electrically read-out sensors. Biosensing is the motivating application of both lines and is treated in this thesis as a prospective application, to be addressed by future experiments with biological analytes.The main original contribution is the design and full-wave numerical study (CST Studio Suite, time-domain solver) of a new multiresonant THz geometry, the Hole V-Fork nanoantenna, which supports resonances at about 1.5, 2.6, 3.8 and 5.1 THz, together with a graphene-loaded variant whose resonances can be shifted by changing the graphene chemical potential. For an analyte layer 13 μm thick, the simulated refractive-index sensitivity reported in the associated publication is 253 GHz/RIU for the bare antenna and about 300 GHz/RIU with graphene; Chapter 6 discusses these values together with the sensitivity of the fundamental mode alone (130 ± 5 GHz/RIU). The Hole V-Fork array was fabricated by electron-beam lithography (EBL) and lift-off by the poliFAB facility (Milan), on the layout designed in this work, and inspected by scanning electron microscopy (SEM); its optical characterization, which requires a far-infrared/THz set-up covering the 1–10 THzrange, is left to future work.The butterfly and shamrock geometries developed earlier in the research group, together with a conventional bowtie used as a reference, are presented as the starting point of the design evolution, and their THz versions were studied numerically. Scaled mid-IR arrays of butterfly and shamrock antennas, fabricated and measured by FTIR at CNR-ISASI (Naples), are reported as bare-array (“naked sensor”) resonances: they show that the resonance can be tuned by the geometry, and they are the starting point for surface-enhanced infrared absorption (SEIRA) experiments.In the 2D-material line, carried out during a research stay at University College London, HfS2 flakes were mechanically exfoliated, transferred onto Cr/Au electrodes and characterized electrically. Two- and four-terminal current–voltage measurements are reported; under the tested conditions the drain current was not modulated by either a back gate or a lateral (scratch-defined) gate, a behavior attributed mainly to leakage through the very thin plasma-grown oxide. Thermal and oxygen-plasma treatments reduced the optical contrast of all the treated HfS2 flakes, a change consistent with the formation of a hafnium oxide layer.