From quantum to continuum mechanics in the delamination of atomically-thin layers from substrates

Anomalous proximity effects have been observed in adhesive systems ranging from proteins, bacteria, and gecko feet suspended over semiconductor surfaces to interfaces between graphene and different substrate materials. In the latter case, long-range forces are evidenced by measurements of non-vanishing stress that extends up to micrometer separations between graphene and the substrate. State-of-the-art models to describe adhesive properties are unable to explain these experimental observations, instead underestimating the measured stress distance range by 2–3 orders of magnitude. Here, we develop an analytical and numerical variational approach that combines continuum mechanics and elasticity with quantum many-body treatment of van der Waals dispersion interactions. A full relaxation of the coupled adsorbate/substrate geometry leads us to conclude that wavelike atomic deformation is largely responsible for the observed long-range proximity effect. The correct description of this seemingly general phenomenon for thin deformable membranes requires a direct coupling between quantum and continuum mechanics

Mica as an Ultra-Flat Substrate for Studying Mechanically Exfoliated Graphene

Silicon dioxide (SiO2) is a common support for studying two-dimensional materials and creating devices from them. However, graphene conformation to SiO2 roughness worsens the electronic properties, whereas graphene deposited on flat terraces of insulating mica is free of ripples. This thesis solves key challenges in the use of mica to support mechanically exfoliated graphene. Methods of mica cleavage and graphene exfoliation, and settings for electron microscopy, atomic force microscopy (AFM) and Raman spectroscopy were developed. Vacuum annealing was compared for graphene samples of different thicknesses, down to a single layer. Pre- and post-annealing, graphene on mica provided defect-free graphene and no observable strain or doping. In contrast, graphene on SiO2 showed disorder before annealing. Annealing up to 300°C reduced the Raman defect peak but did not remove it. Above 300°C, the defect peak increased. Graphene on SiO2 appeared to become ‘invisible’ with AFM after annealing at 500°C, in line with previous observations with scanning electron microscopy. Other studies attributed this to the graphene being removed, but, here, using substrate markers, Raman spectroscopy and line-averaged AFM showed that the graphene was still present but had conformed to the underlying roughness of the SiO2 so well as to appear nearly invisible. Mica annealed at 400°C showed the formation of potassium carbonate particles following dehydroxylation of the mica surface at a temperature lower than previously reported. In addition, the graphene appeared to act as a mask, protecting the mica underneath it while the surrounding surface was removed at 500°C. Patterning and etching mica are essential to create location grids and etch trenches to suspend deposited materials. The first patterning lithography recipe for mica was established herein using electron-beam lithography. Finally, mechanically exfoliated graphene was successfully transferred to the patterned mica and studied

Patterning methods for organic electronics

Organic electronics is an exciting new avenue for low cost electronics. The unique properties of organic semiconductors may enable a new generation of electronic devices to be fabricated into flexible, large area, and even transparent consumer products. However, for this to become a reality, many challenges must first be overcome. As the performance of these materials continues to improve, it is now necessary to look to new manufacturing methods and materials that can fully exploit the advantages of organic materials. The work presented in this thesis is focused on the development of new and high resolution fabrication methods which are compatible with organic electronic materials. The findings presented in the first half of this thesis are based on the idea that fundamentally new forms of manufacturing are required to match the unique properties of organic materials. Initially the adhesion properties of several materials are analysed with a focus on how they interact at the nano-scale. Further work then outlines how adhesion forces can be manipulated and used to produce highly aligned nano-scale electronic devices, something that until now has required high cost and specialist equipment. The second part of this thesis describes how existing fabrication methods can be modified to produce high performance organic devices. By creating self-aligned organic transistors, higher frequency device operation and enhanced performance may be possible. New materials such as graphene and low voltage nano-scale dielectrics are tested in this configuration and compared with similar devices reported in the literature.Open Acces

Graphene Photonics and Optoelectronics

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultra-wide-band tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres

Development of a Wireless MEMS Multifunction Sensor System and Field Demonstration of Embedded Sensors for Monitoring Concrete Pavements, Volume II

This two-pronged study evaluated the performance of commercial off-the-shelf (COTS) micro-electromechanical sensors and systems (MEMS) embedded in concrete pavement (Final Report Volume I) and developed a wireless MEMS multifunctional sensor system for health monitoring of pavement systems (Final Report Volume II). The Volume I report focused on the evaluation of COTS MEMS sensors embedded in concrete pavement sections. The Volume II report covers the set of MEMS sensors that were developed as single-sensing units for measuring moisture, temperature, strain, and pressure. These included the following sensors: (1) nanofiber-based moisture sensors, (2) graphene oxide (GO)–based moisture sensors, (3) flexible graphene strain sensors with liquid metal, (4) graphene strain and pressure sensors, (5) three-dimensional (3D) planar and helical structured graphene strain sensors, (6) temperature sensors, and (7) water content sensors. In addition, the MEMS temperature sensors and the MEMS water content sensors were integrated into one sensing unit as a multifunctional sensor. A wireless signal transmission system was built for MEMS sensor signal readings. Characterization of the sensors was conducted and sensor responses were analyzed using different applications. The sensors developed were installed and tested inside concrete. The results demonstrated the capability to detect sensor response changes at the installed locations

NATURE-INSPIRED MATERIAL STRATEGIES TOWARDS FUNCTIONAL DEVICES

Naturally sourced, renewable biomaterials possess outstanding advantages for a multitude of biomedical applications owing to their biodegradability, biocompatibility, and excellent mechanical properties. Of interest in this dissertation are silk (protein) and chitin (polysaccharide) biopolymers for the fabrication of functional biodevices. One of the major challenges restricting these materials beyond their traditional usage as passive substrate materials is the ability to combine them with high-resolution fabrication techniques. Initial research work is directed towards the fabrication of micropatterned, flexible 2D substrates of silk fibroin and chitin using bench-top photolithographic techniques. Research is focused on imparting electrochemical properties to silk proteins using conducting polymers (PEDOT: PSS and PANI) and a naturally occurring semiconductor, eumelanin. The utility of conducting biomimetic composites in device applications was demonstrated by the fabrication of fully organic silk based flexible electrochemical biosensors. The biosensors display excellent detection of dopamine and ascorbic acid with high sensitivity. A flexible silk-PEDOT: PSS based temperature sensor is also demonstrated for the accurate monitoring of skin surface temperature. Finally, the challenge of conformability at the biological interface is addressed using structure-based design strategies. Inspiration from the Japanese art of paper cutting was taken for the formation of patterned cuts on silk fibroin films using photolithography. Micropatterned cuts can increase the conformability of films to soft biological interfaces by enhancing their strain tolerance. By doping with polyaniline (PANI), flexible, intrinsically conductive silk kirigami sheets could be fabricated. Such systems have potential in personalized healthcare monitoring devices, improving efficient disease detection and diagnosis

Graphene Biosensors for Diabetic Foot Ulcer Monitoring

The prevalence of Diabetes Mellitus (DM) in the twenty-first century has increased drastically, consequently, the incidence of DM-related complications has increased as well. According to the International Diabetes Federation (IDF) in 2021, globally one in every ten adults aged from 20 to 79 years had DM. Approximately 15-34% of individuals with DM are likely to develop a Diabetic Foot Ulcer (DFU) throughout their lifetime. Unmonitored and in- fected DFU can lead to non-traumatic lower extremity amputation and worst-case cause morbidity. Therefore, it is of great importance to develop effective, rapid production, bio- compatible, low-cost, flexible, wearable, sustainable sensors to monitor objectively the ulcer healing state. This dissertation aims to meet this need through the development of tempera- ture and pH laser-induced graphene (LIG) sensors on paper, that could be included in smart bandages and medical wound dressings. During this dissertation, LIG on paper fabrication parameters were studied to obtain the most reproducible, durable, and good electrical per- formance. The production condition of the LIG used for the development of the sensors had an average sheet resistance value of 24.9Ω/ with 1.2 Ω/ of standard deviation. The ther- moresistive sensor developed is characterized by a negative temperature coefficient with a highly linear response, and a sensitivity of 0.71 %℃−1 from 26℃ to 40℃, a suitable interval for its application. The electrochemical cell produced works as a potentiometric pH sensor. Its working electrode (WE) was electropolymerized with polyaniline (PANI) a pH-sensitive bio- compatible electrolyte. The sensor demonstrated a Nernstian behavior with a sensitivity of 53.0 / and 2.3 / of standard deviation on the interval from 2 pH to 9 pH.A prevalência da Diabetes Mellitus (DM) no século XXI aumentou drasticamente, con- sequentemente, a incidência de complicações relacionadas com a DM também aumentou. Segundo a Federação Internacional de Diabetes em 2021, globalmente um em cada dez adultos com idades compreendidas entre os 20 e os 79 anos tem DM. Aproximadamente 15- 34% dos indivíduos com DM são suscetíveis de desenvolver uma úlcera do pé diabético (DFU) durante toda a sua vida. A DFU não monitorizada e infetada pode levar a uma amputa- ção não traumática das extremidades inferiores e causar morbilidade no pior dos casos. Por conseguinte, é de grande importância desenvolver sensores eficazes, de produção rápida, biocompatíveis, de baixo custo, flexíveis, viáveis e sustentáveis para monitorizar objetivamen- te o estado de cicatrização da úlcera. Esta tese visa responder a esta necessidade através do desenvolvimento de sensores de temperatura e pH induzidos por laser (LIG) em papel, que poderiam ser incluídos em ligaduras inteligentes e curativos médicos de feridas. Durante esta dissertação, foram estudados parâmetros de fabrico de LIG em papel para obter o mais re- produtível, durável, e bom desempenho elétrico. O valor da resistência da folha média da condição de produção utilizada para o desenvolvimento foi de 24.9 Ω/ com um desvio padrão de 1.2 Ω/. O sensor termoresistivo desenvolvido é caracterizado por um coeficiente de temperatura negativa com uma resposta altamente linear, e uma sensibilidade de 0.71 %℃−1 entre os 26℃ e 40℃, um intervalo adequado para a sua aplicação. A célula ele- troquímica produzida funciona como um sensor de pH potenciométrico. O seu elétrodo de trabalho (WE) foi electropolimerizado com polianilina (PANI) um eletrólito biocompatível sensível ao pH. O sensor demonstrou um comportamento Nernstiano com uma sensibilidade de 53.0 / e desvio padrão de 2.3/ no intervalo de 2 a 9 pH

Single-Walled Carbon Nanotube electrodes for all-plastic, electronic device applications

In this thesis, new mechanically robust, high performance transparent conducting films of commercially sourced arc-made Single-Walled Carbon Nanotubes (SWCNTs) on both glass and flexible substrates were produced using spin-coating or spray deposition, interlayer or stencil patterning methods and used for fabricating efficient, flexible polymer-fullerene bulk hetero-junction solar cells. After carefully optimizing the dispersion process of SWCNTs with H2O:SDS (up to 0.03 wt.%) and developing and efficient surfactant removal/p-doping procedure with nitric acid, highly conductive and smooth SWCNT thin films (ca. 30 nm) were obtained with more than 6,500 Scm-1 at > 69 % transmittance and 7 nm (r.m.s.) roughness. In particular, SWCNT films spray coated from H2O:SDS exhibited electrical conductivities of up to 7694 ± 800 Scm-1. To our knowledge, these values are the highest so far reported for SWCNT electrodes. Peak values for the ratio of the dc conductivity to the optical conductivity (σdc/σop) were obtained as up to 24, which is quite similar to state of the art SWCNT films so far reported. In addition, two patterning methods were developed to define electrode patterns of SWCNT thin films for electronic device applications. Interlayer lithography provided a fast and high resolution patterning procedure for SWCNT thin films at micron and sub-micron length scales, which is important for the fabrication of high-speed transistors requiring short channel lengths, and offers an attractive route to fabricating high-density integrated circuits. In addition, stencil patterning provides a simple and fast method, which is well suited for low resolution electronic device applications such as organic solar cells. The patterned highly conductive SWCNT electrodes were incorporated into P3HT:PCBM bulk heterojunction solar cell applications, obtaining the best device performance of 3.6 %, which is the best result so far reported in the literature. Finally, to break through the limited performance (σdc/σop < 25) of SWCNT thin films, layered hybrid thin films of SWCNTs on reduced Graphene-Oxide were fabricated by a simple spray coating method and the optimised hybrid films were incorporated into relatively efficient organic solar cells (2 % efficiency)