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Electrical detection of atmospheric pollutants and detergents on silicon
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Date
2025
Authors
Vardhan, Vaishali
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Publisher
University College Cork
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Abstract
Air quality and climate change represent significant challenges today. Many air pollutants consistently exceed exposure levels the World Health Organization (WHO) recommends. Establishing an extensive network of high-quality sensors capable of accurately detecting primary pollutants is essential for conducting global air quality assessments and understanding pollution levels. This can be accomplished by developing sensors using the advanced silicon electronics platform.
This thesis presents silicon-based sensor technologies for detecting key atmospheric pollutants and radicals. Chapter 1 introduces and reviews current methods for sensing atmospheric pollutants, evaluating existing technologies. It highlights challenges such as the complexity of spectroscopic monitoring, high operating temperatures of solid-state sensors, sensitivity limitations of portable sensors, intricate multi-material fabrication for hybrid sensing, and scalability issues hindering the deployment of conventional sensors. Current silicon-based sensors are primarily constrained by the need to hybridise with other sensing materials. These limitations pave the way for developing innovative room-temperature sensor technology on a simplified silicon platform.
We have investigated interactions between gaseous oxidative species, specifically nitrogen dioxide (NO2), and silicon surfaces on ambipolar silicon junctionless nanowire transistors (Si-JNTs) (Chapter 2). Experimental work and density functional theory (DFT) modelling demonstrate that NO2 functions as a "pseudo" molecular dopant, influencing key device parameters across p-type and n-type conduction channels of the ambipolar device. Building on this understanding, we utilise the ambipolar Si-JNT platform to achieve dual-channel NO2 detection (Chapter 3). The simultaneous evolution of multiple electrical parameters in response to varying NO2 concentrations enhances the sensor's sensitivity and selectivity. The extensive parameter space in ambipolar JNTs is leveraged to develop a multivariate calibration model, improving their ability to differentiate between pollutant gases.
In Chapter 4, the focus shifts to reducing pollutant ammonia (NH3). Ambipolar Si-JNTs exposed to NH3 under ultraviolet light (254 nm) excitation exhibit a dual response to the gas. This method achieves high sensitivity, with detection limits as low as 200 ppb, and offers rapid response time, overcoming common issues of low sensitivity and poor stability in existing NH3 sensors. The dual-channel approach, showing markedly different responses in electron and hole conduction channels, enhances sensor performance by utilising optimal parameters from each channel.
A different silicon sensor technology was employed to detect atmospheric pollutant ozone (O3) and short-lived radical hydroxyl (•OH). A double Schottky junction device on a planar silicon-on-insulator (SOI) platform, with porous metal (Ni) contacts and organic surface modification of Si, exhibits remarkable sensitivity to O3 and •OH. Alkene self-assembled monolayers (SAMs) allow selective detection of O3 at room temperature and concentrations as low as 0.6 ppb, addressing challenges like high power consumption and interference from other gases (Chapter 5). The alkene and alkane functionalised SOI platforms show a consistent and linear electrical response to •OH across the atmospheric concentration range (Chapter 6), using chemical reactions between the organic monolayer and •OH radical. This advancement signals a shift in monitoring reactive atmospheric species through compact, cost-effective, on-chip sensors.
This thesis demonstrates that a single-material silicon sensor approach, capitalising on ambipolarity and dual-channel functionality of Si-JNTs and the adaptability of SOI-based devices, presents notable benefits over traditional methods for detecting atmospheric species. These benefits include easier fabrication, greater scalability, energy efficiency, and enhanced sensitivity and selectivity for various atmospheric pollutants. Findings from experimental studies and theoretical modelling form a foundation for future portable sensor networks, essential for advanced environmental monitoring and effective air quality management.
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Keywords
Silicon , Silicon on insulator , Junctionless nanowire transistor , Atmospheric radical , Hydroxyl radical , Atmospheric pollutants , Nitrogen dioxide , Ozone , Ammonia , Gas sensing , Ambipolar devices , Density functional theory , UV enhanced sensing
Citation
Vardhan, V. 2025. Electrical detection of atmospheric pollutants and detergents on silicon. PhD Thesis, University College Cork.