Silicon nanowire : fabrication, characterisation and application

PhD ThesisThis thesis focuses on the fabrication considerations and the characterisation of silicon nanowires and their integration into chemical sensors. One aim is to optimize a top-down fabrication process for silicon nanowires, in particular the methods that use optical lithography, wet etching and thermal oxidation. The main concerns here are to achieve a reproducible and high yield fabrication process and to obtain a controllable structure. Extensive work was carried out to study the parameters that affect the repeatability of the process. The properties of silicon nitride films, the oxidation method and the characteristics of the anisotropic etchant were found to be key parameters affecting the reproducibility of the process. Several silicon nitride films were deposited under various conditions and their optical properties were tested before and after thermal oxidation. It was found that the oxynitride thickness depends on the refractive index of the nitride film: the lower the refractive index, the thinner the oxynitride. Then an etching process was developed to selectively etch the oxidised silicon nitride over silicon dioxide. The etching process included two steps: firstly ion milling to remove the oxynitride film and secondly using boiling phosphoric acid to strip the silicon nitride film. Nitride-rich silicon nitride films exhibited higher etching selectivity over silicon dioxide compared with silicon-rich silicon nitride. Based on the etch selectivity, oxynitride thickness, and silicon dioxide thickness the maximum thickness of silicon nitride film that can be used to act as a mask during the fabrication of silicon nanowires was determined. The impact of oxidation method on the reliability of the process was studied, and SOI and bulk silicon samples were oxidised at the same temperature and time using lamp-based RTP radiation and also a furnace with resistive heating. The results showed that the SOI sample is colder than the bare silicon sample when both were heated using the lamp-based RTP. This effect was considered during the fabrication of silicon nanowires to obtain a reliable process. Comprehensive experimental measurements were carried out to compare the characteristics of Tetra-Methyl Ammonium Hydroxide (TMAH) and Potassium Hydroxide (KOH) etching to optimise the fabrication process. The use of TMAH was found to lead to a more reliable process. ii Another aim of the project was to characterise the fabricated devices, and for this the contact properties and the electrical properties of the silicon nanowires needed to be evaluated. Extensive electrical measurements were carried out to study the thermal stability and ohmic contact formation for the silicon nanowire. Three metallization schemes were studied: Al/Ti, Al/W/Ti and Al/Ti/AlOx. All these exhibited ohmic contact to the nanowires. However, Al/Ti/Si and Al/W/Ti/Si were found to be unstable after 425 °C RTP annealing. Al/Ti/AlOx/Si withstood this level of temperature but the contact resistance was about ten times higher than that of Al/W/Ti. The electrical resistivity of the silicon nanowires was then studied; it was found that the measured electrical resistivity decreases with the nanowire thickness. Several models were then developed to explain the apparent increase in resistivity. It was suggested it can be largely attributed to the reduction of the conductive area of the nanowire due to interface traps. Finally, a silicon nanowire sensor was designed and fabricated, and this sensor was used to detect the changes in pH. The preliminary results showed that the sensor detected the change of pH in the buffer solution. However, reliability and yield were low, which was assumed to be due to the large parasitic current between the source/drain and the buffer solution.For a scholarship to pursue my postgraduate studies, I am grateful to Damascus University in Syria

The effects of sulphur poisoning on the microstructure, composition and oxygen transport properties of perovskite membranes coated with nanoscale alumina layers

Perovskite oxides displaying mixed ionic and electronic conductivity have attracted a lot of interest for application in oxygen separation membranes. Such membranes could be used for a range of processes, including the conversion of natural gas to hydrogen or syngas. A major limitation of these materials is their tendency to segregate into simpler oxides under operating conditions, reacting with sulphur-based species often found in natural gas and leading to irreversible membrane degradation over time. Here we aim to delay or prevent this process by coating La0.6Sr0.4Co0.2Fe0.8O3-δ membranes with Alumina (Al2O3) layers of 1–100 nm thickness by using atomic layer deposition. We show that coatings of about 30 nm have negligible negative effect on O2 transport flux across the membrane and display good flux recovery when H2S is removed from the stream. Coatings thinner than this critical value provide little protection against irreversible poisoning while thicker coatings dramatically decrease overall O2 permeation fluxes. We also show that the irreversible sulphur poisoning under O2 permeation conditions is linked to microstructural and composition changes at the membrane surface caused predominantly by the formation of SrSO4 particles at the perovskite grain boundaries

Proteomic profile in congenital microcephaly

Autosomal recessive primary microcephaly (MCPH) consists of a group of disorders characterized by microcephaly and intellectual disability. This study is essential to complement previous findings of MCPH as it helps clarify the role of different genes and proteins involved in the underlying pathophysiology of MCPH. To date, 27 different mutations have been identified. This study defines a number of changes in gene expression occurring in MCPH. This helps deepen our understanding of the effect of MCPH mutations on gene expression. This study also shows the functions of proteins that increase, are unaffected or become dysfunctional due to MCPH. We identified a marked reduction of about 30 proteins with vital roles in several processes including cell cytoskeleton dynamics, cell cycle progression, ciliary functions, and apoptosis. We used Cdk5rap2 (Hartwig's anemia mice (an/an)), which is a model that closely represents MCPH3. Gel electrophoresis was utilized in order to separate brain proteins. Fixation and protein identification was then done in order to detect changes in the level of the tested protein

Electroless Nickel Deposition:An Alternative for Graphene Contacting

We report the first investigation into the potential of electroless nickel deposition to form ohmic contacts on single layer graphene. To minimize the contact resistance on graphene, a statistical model was used to improve metal purity, surface roughness, and coverage of the deposited film by controlling the nickel bath parameters (pH and temperature). The metalized graphene layers were patterned using photolithography and contacts deposited at temperatures as low as 60 °C. The contact resistance was 215 ± 23 ω over a contact area of 200 μm × 200 μm, which improved upon rapid annealing to 107 ± 9 ω. This method shows promise toward low-cost and large-scale graphene integration into functional devices such as flexible sensors and printed electronics

Large-Area Electrodeposition of Few-Layer MoS2 on Graphene for 2D Material Heterostructures

Heterostructures involving two-dimensional (2D) transition metal dichalcogenides and other materials such as graphene have a strong potential to be the fundamental building block of many electronic and opto-electronic applications. The integration and scalable fabrication of such heterostructures is of essence in unleashing the potential of these materials in new technologies. For the first time, we demonstrate the growth of few-layer MoS2 films on graphene via non-aqueous electrodeposition. Through methods such as scanning and transmission electron microscopy, atomic force microscopy, Raman spectroscopy, energy and wavelength dispersive X-ray spectroscopies and X-ray photoelectron spectroscopy, we show that this deposition method can produce large-area MoS2 films with high quality and uniformity over graphene. We reveal the potential of these heterostructures by measuring the photo-induced current through the film. These results pave the way towards developing the electrodeposition method for the large-scale growth of heterostructures consisting of varying 2D materials for many applications.Comment: 11 pages and 6 figure

On-chip integrated graphene aptasensor with portable readout for fast and label-free COVID-19 detection in virus transport medium

Graphene field-effect transistor (GFET) biosensors exhibit high sensitivity due to a large surface-to-volume ratio and the high sensitivity of the Fermi level to the presence of charged biomolecules near the surface. For most reported GFET biosensors, bulky external reference electrodes are used which prevent their full-scale chip integration and contribute to higher costs per test. In this study, GFET arrays with on-chip integrated liquid electrodes were employed for COVID-19 detection and functionalized with either antibody or aptamer to selectively bind the spike proteins of SARS-CoV-2. In the case of the aptamer-functionalized GFET (aptasensor, Apt-GFET), the limit-of-detection (LOD) achieved was about 103 particles per mL for virus-like particles (VLPs) in clinical transport medium, outperforming the Ab-GFET biosensor counterpart. In addition, the aptasensor achieved a LOD of 160 aM for COVID-19 neutralizing antibodies in serum. The sensors were found to be highly selective, fast (sample-to-result within minutes), and stable (low device-to-device signal variation; relative standard deviations below 0.5%). A home-built portable readout electronic unit was employed for simultaneous real-time measurements of 12 GFETs per chip. Our successful demonstration of a portable GFET biosensing platform has high potential for infectious disease detection and other health-care applications

Effect of Bistorta affinis (D. Don) and Malcolmia cabulica (Boiss) Essential Oils on Human and Foodborne Pathogens, Insects and Termites

This current study investigates the biological efficiency of essential oils extracted from Bistorta affinis and Malcolmia cabulica on human and foodborne pathogens as well as against insects and termites. The oils were obtained by steam distillation using a Clevenger-type system and analyzed for their constituents using GC-MS. Twenty compounds were identified, wherein carvacrol and thymol were the main constituents in both plants. Essential oils exhibited significant activity against all tested foodborne bacteria, fungi, and other pathogens. In addition, essential oils from both plants have shown promising activity against all tested insects, which is a positive sign of being used as an insect repellent. In contrast, no activity was observed against termites. The bioactivities are mainly due to carvacrol and thymol’s levels in the essential oils that known for their effectiveness against foodborne pathogens and pests. The present study constitutes a basis for further investigation and development of bioactive compounds in Bistorta affinis and Malcolmia cabulica

Post-vasectomy semen analysis: Optimizing laboratory procedures and test interpretation through a clinical audit and global survey of practices

Purpose: The success of vasectomy is determined by the outcome of a post-vasectomy semen analysis (PVSA). This article describes a step-by-step procedure to perform PVSA accurately, report data from patients who underwent post vasectomy semen analysis between 2015 and 2021 experience, along with results from an international online survey on clinical practice. Materials and Methods: We present a detailed step-by-step protocol for performing and interpretating PVSA testing, along with recommendations for proficiency testing, competency assessment for performing PVSA, and clinical and laboratory scenarios. Moreover, we conducted an analysis of 1,114 PVSA performed at the Cleveland Clinic’s Andrology Laboratory and an online survey to understand clinician responses to the PVSA results in various countries. Results: Results from our clinical experience showed that 92.1% of patients passed PVSA, with 7.9% being further tested. A total of 78 experts from 19 countries participated in the survey, and the majority reported to use time from vasectomy rather than the number of ejaculations as criterion to request PVSA. A high percentage of responders reported permitting unprotected intercourse only if PVSA samples show azoospermia while, in the presence of few non-motile sperm, the majority of responders suggested using alternative contraception, followed by another PVSA. In the presence of motile sperm, the majority of participants asked for further PVSA testing. Repeat vasectomy was mainly recommended if motile sperm were observed after multiple PVSA’s. A large percentage reported to recommend a second PVSA due to the possibility of legal actions. Conclusions: Our results highlighted varying clinical practices around the globe, with controversy over the significance of non-motile sperm in the PVSA sample. Our data suggest that less stringent AUA guidelines would help improve test compliance. A large longitudinal multi-center study would clarify various doubts related to timing and interpretation of PVSA and would also help us to understand, and perhaps predict, recanalization and the potential for future failure of a vasectomy.American Center for Reproductive Medicin

Effects of fluoxetine on functional outcomes after acute stroke (FOCUS): a pragmatic, double-blind, randomised, controlled trial

Background Results of small trials indicate that fluoxetine might improve functional outcomes after stroke. The FOCUS trial aimed to provide a precise estimate of these effects. Methods FOCUS was a pragmatic, multicentre, parallel group, double-blind, randomised, placebo-controlled trial done at 103 hospitals in the UK. Patients were eligible if they were aged 18 years or older, had a clinical stroke diagnosis, were enrolled and randomly assigned between 2 days and 15 days after onset, and had focal neurological deficits. Patients were randomly allocated fluoxetine 20 mg or matching placebo orally once daily for 6 months via a web-based system by use of a minimisation algorithm. The primary outcome was functional status, measured with the modified Rankin Scale (mRS), at 6 months. Patients, carers, health-care staff, and the trial team were masked to treatment allocation. Functional status was assessed at 6 months and 12 months after randomisation. Patients were analysed according to their treatment allocation. This trial is registered with the ISRCTN registry, number ISRCTN83290762. Findings Between Sept 10, 2012, and March 31, 2017, 3127 patients were recruited. 1564 patients were allocated fluoxetine and 1563 allocated placebo. mRS data at 6 months were available for 1553 (99·3%) patients in each treatment group. The distribution across mRS categories at 6 months was similar in the fluoxetine and placebo groups (common odds ratio adjusted for minimisation variables 0·951 [95% CI 0·839–1·079]; p=0·439). Patients allocated fluoxetine were less likely than those allocated placebo to develop new depression by 6 months (210 [13·43%] patients vs 269 [17·21%]; difference 3·78% [95% CI 1·26–6·30]; p=0·0033), but they had more bone fractures (45 [2·88%] vs 23 [1·47%]; difference 1·41% [95% CI 0·38–2·43]; p=0·0070). There were no significant differences in any other event at 6 or 12 months. Interpretation Fluoxetine 20 mg given daily for 6 months after acute stroke does not seem to improve functional outcomes. Although the treatment reduced the occurrence of depression, it increased the frequency of bone fractures. These results do not support the routine use of fluoxetine either for the prevention of post-stroke depression or to promote recovery of function. Funding UK Stroke Association and NIHR Health Technology Assessment Programme