Fabrication of Chemical Sensors, Optical Detectors, and Optical Sources From Metal Nanogap Structures

With the continuing miniaturization of functional devices, nano-structured materials have become of significant scientific and practical interest. In this dissertation, we created a nanogap in silver (Ag) nanowires or electrodeposited nanocrystalline cadmium selenide (nc-CdSe) into FIB cut gold nanogaps and explored their applications for chemical sensors, optical detectors and optical sources.;In the first part, the formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A single Ag nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition (LPNE) method. Then a 1-5 nm nanogap was created by electromigration and reconnected by applying a voltage ramp across the nanowire resulting in the formation of a resistive (MegaOhms), ohmic CRJ. The resistance changes upon exposure to ammonia (NH3), nitrogen dioxide (NO2) and water vapor have been studied. The proposed mechanism of the enhanced resistance response for a CRJ, supported by Density Function Theory (DFT) calculations is that semiconducting p-type AgxO is formed within the CRJ and the binding of molecules to this AgxO modulates its electrical resistance.;In the second part, nc-CdSe was electrodeposited within a sub-50 nm electromigrated gold nanogap, to form a photoconductive metal-semiconductor-metal (M-S-M) nanojunction. The photoconductivity of these nc-CdSe-lled gold nanogaps was characterized by a detectivity of 6.9 × 10 10 Jones and a photosensitivity of 500. These devices also demonstrated a maximum photoconductive gain of ∼45 and response and recovery times below 2 μs, corresponding to a 3 dB bandwidth of at least 175 kHz.;In the third part, similar nc-CdSe based M-S-M nanojunctions as in the above photodetector case were demonstrated to be able to emit near-infrared light with a threshold voltage right above the bandgap of CdSe ∼1.7 V. Two different deposition temperatures (20 °C versus 75 °C) were used and the resulting device performance indicated that the bigger the grain size (75 °C), the higher the electroluminescence (EL) efficiency. Voltage dependent EL spectra revealed that light emission occurred through inelastic scattering of tunneling electrons from band-edge and defect states transitions.