Effect of interfacial oxide layers on the current-voltage characteristics of Al-Si contacts

Aluminum-silicon contacts with very thin interfacial oxide layers and various surface impurity concentrations are studied for both n and p-type silicon. To determine the surface impurity concentrations on p(+)-p and n(+)-n structures, a modified C-V technique was utilized. Effects of interfacial oxide layers and surface impurity concentrations on current-voltage characteristics are discussed based on the energy band diagrams from the conductance-voltage plots. The interfacial oxide and aluminum layer causes image contrasts on X-ray topographs

Development of Capacitive Imaging Technology for Measuring Skin Hydration and Other Skin Properties

In this thesis, capacitive imaging systems are assessed for their suitability in skin research studies, as multi-purpose and portable laboratory equipment. The water content of the human skin, the status of the skin barrier, its permeability by solvents, and the skin texture are crucial pieces of information in pharmaceutical and cosmetic industries for the development of skin treatment products. Normally, multiple high-end scientific instruments with expensive dedicated analysis software are employed to measure the above skin properties. The aim of this work is to demonstrate how fingerprint sensors, originally designed for biometric security, can be exploited to achieve reliable skin hydration readings and analyse multiple other skin properties while maintaining low cost and portability. To begin with, the anatomy of human skin is summarised alongside the functional properties of each skin layer. The skin hydration instruments study the outermost layer of skin and its appendages, so their thickness, biology, functions, hydration levels and water holding capabilities are presented in the literature review in order to understand the target measurands. Since capacitive imaging, rather than single sensor, probes are employed in this work, the skin texture and its importance in cosmetic science are also studied as a part of the target measurand. In order to understand how this technology fits in the current skin research instrument market, well established measurement apparatuses are presented. These include opto-thermal transient emission radiometry and confocal Raman microspectroscopy for skin hydration and solvent permeation measurements as well as depth profiling. Then, electrical hygrometry and the dynamic vapour sortpion measurement principles are outlined, which focus on water diffusion and sorption measurements correspondingly. Since the skin texture will also be studied in this work, dermatoscopy is also summarised. A literature review on the non-invasive electrical-based measurement method is achieved, alongside the stratum corneum and viable skin capacitance and conductance as functions of sampling frequency. The latter allows to establish the criteria for the suitability of electrical based apparatuses in skin hydration measurements. More specifically, it is concluded that the measurement depth of the instrument should not be reaching viable skin and that the sampling frequency should be constant and below 100kHz for capacitive measurements. The presentation of existing electrical based skin hydration probes in the market demonstrates the current development stage of this technology, and it enables the expression of the research aim and its objectives for this work. In order to improve trust in the use of capacitive imaging technology for measuring skin hydration, apart from visualisation, established electrical based skin hydration probes are examined and compared with a capacitive imaging sensor. The criteria for this comparison derive from the literature review, i.e. the sampling frequency and the penetration depth of the electric field. The sampling frequency is measured directly on the hardware using an oscilloscope, while the measurement depth is estimated using an electrostatic model. The development of this model for different sensor geometries is presented and it is evaluated against different models as well as experimental results in the literature. It is concluded that low cost instruments tend to have high measurement depth that makes them unsuitable for stratum corneum hydration measurements. Higher end instruments, although they are using high sampling frequency, have safe penetration depth but low measurement sensitivity. The capacitive imaging sensor shown acceptable penetration depth, on the high end of the expected range, and good measurement sensitivity due to the miniaturisation of the technology. A common disadvantage of most of these instruments is that the readouts are provided in arbitrary units, so experimental results cannot be compared directly with the literature when different scientific equipment has been used. To overcome this disadvantage, and based on the previous analysis of capacitive measurement principle, a system calibration is proposed to convert system capacitance or arbitrary units to dielectric permittivity units, a property of the sample measurand. This allows the calculation of hydration and solvent percentage concentration within the sample and so direct comparison with a wider range of reported results in the literature. Furthermore, image analysis techniques are applied on the dielectric permittivity images to allow targeting and relocating skin regions of interest, as well as excluding pixels with bad sample contact that distort the results. Next, the measurement reliability of the capacitive imaging arrays is examined through in-vivo and in-vitro experiments as well as side-by-side comparative measurements with single sensor skin hydration probes. The advantages of the developed calibration method and image analysis tools are demonstrated via the introduction of new system applications in the skin research, including skin damage characterisation via occlusion, skin solvent penetration and water desorption in hair samples experiments. It has to be mentioned that a small number of subjects is used in these experiments and the results are compared with the literature, so the statistical significance is not clearly examined. Next, advanced image processing techniques are adapted and applied on the capacitive skin images to expand further the application of this technology. More specifically, the skin micro-relief aspects of interest in cosmetic industry are summarised, and algorithmic approaches for measuring the micro-relief orientation and intensity as well as the automatic skin grids account are reviewed and experimentally evaluated. The main research aim and its objective have been achieved, with their methodologies clearly presented the their implementations evaluated with experimental results. However, vulnerabilities of this technology have also been exposed and suggestions for further improvement are provided in the conclusions

Development of a method to study retention of hydrophobic actives from cosmetic emulsions on optimized skin biomimics

All-in-one products are a popular trend in cosmetics, personal and home care. In particularly, personal care products that serve multiple purposes are especially popular due to their multifunctional action which simultaneously provides cleaning, conditioning and protection of the treated area such as, skin or hair, in a fraction of the time in comparison to when using traditional products. In this research programme, a dedicated method was developed towards the evaluation of the conditioning performance of ‘all-in-one’ cleansing products. Hence, all steps of product usage were replicated in a controlled environment and suitable characterization methods were employed. Optimized skin mimics were fabricated to be used as test substrates and model systems of cosmetic formulations were produced, which demonstrated both cleaning and moisturizing capabilities as well as a repetitive and highly-controlled deposition set-up and a cleaning set-up. The conditioning performance of the emulsions was studied and post-wash retention levels of the hydrophobic active on skin bio-mimics were systematically characterised. Techniques including fluorescent microscopy, gravimetric analysis and tribometry were employed to provide further, into-depth quantitative data of the retention. Properties of the formulation including oil droplet-size, viscosity and volume-fraction and cleaning parameters including rinsing-duration and flow-angle were tested for their impact on retention

Real-Time Impedance Monitoring of Epithelial Cultures with Inkjet-Printed Interdigitated-Electrode Sensors

From electronic devices to large-area electronics, from individual cells to skin substitutes, printing techniques are providing compelling applications in wide-ranging fields. Research has thus fueled the vision of a hybrid, printing platform to fabricate sensors/electronics and living engineered tissues simultaneously. Following this interest, we have fabricated interdigitated-electrode sensors (IDEs) by inkjet printing to monitor epithelial cell cultures. We have fabricated IDEs using flexible substrates with silver nanoparticles as a conductive element and SU-8 as the passivation layer. Our sensors are cytocompatible, have a topography that simulates microgrooves of 300 µm width and ~4 µm depth, and can be reused for cellular studies without detrimental in the electrical performance. To test the inkjet-printed sensors and demonstrate their potential use for monitoring laboratory-growth skin tissues, we have developed a real-time system and monitored label-free proliferation, migration, and detachment of keratinocytes by impedance spectroscopy. We have found that variations in the impedance correlate linearly to cell densities initially seeded and that the main component influencing the total impedance is the isolated effect of the cell membranes. Results obtained show that impedance can track cellular migration over the surface of the sensors, exhibiting a linear relationship with the standard method of image processing. Our results provide a useful approach for non-destructive in-situ monitoring of processes related to both in vitro epidermal models and wound healing with low-cost ink-jetted sensors. This type of flexible sensor as well as the impedance method are promising for the envisioned hybrid technology of 3D-bioprinted smart skin substitutes with built-in electronics.The work by D.M.-M. has been performed in the frame of an FPU Program, FPU015/06208, and a Mobility Fellows Program, both granted by the Spanish Ministry of Education, Culture and Sports. This work has been funded by the Comunidad de Madrid under the grant BIOPIELTEC-CM (P2018/BAA-4480) and the Ministerio de Ciencia e Innovación under the grant PARAQUA (TEC2017-86271-R)

Development of a Miniaturized Electro-Fluidic Detector for Medical Diagnostics

In the last decades the interest towards personalized therapies has fostered a big number of studies dedicated to the realization and the optimization of bio-detectors to be used as fast diagnostic tools during medical treatment [1, 2, 3, 4]. Among the proposed devices the best performances, both in terms of multiplexing and cost reduction, are expected by the detectors based on electrical readout. These sensors can be integrated with microfluidic networks in the so called Lab-on-a-Chip systems and o ffer the possibility to develop complete diagnostic kits for the use as a medical practitioner\u2019s bench tool and, ultimately, for rapid and reliable analysis in low-resource areas and in the developing world [5, 6]. In this framework we focused on the development of an electrochemical biosensor based on capacitance readout, for the detection of biomolecules in small sample volumes. We performed electrochemical impedance spectroscopy (EIS) measurements of DNA-hybridization and protein-protein interaction in electrochemical cells with microfabricated gold electrodes. The time stability of the device was tested in two di erent configurations: two microelectrodes in a microfluidic channel; two microelectrodes plus a reference electrode in an electrochemical cell. Our results demonstrate that the three-electrode setup is more stable, more reproducible, and suitable for real-time measurements. A thorough study of the immobilization strategy of the DNA-molecules on the gold electrodes was carried out. In the last part of the work we performed a test study of DNA-hybridization in real time and we showed that the three-electrode configuration can measure the process in-situ. [1] Vladimir Gubala, Leanne F Harris, Antonio J Ricco, Ming X Tan, and David E Williams. Point of care diagnostics: status and future. Analytical chemistry, 84(2):487\u2013515, 2011. [2] V Tsouti, C Boutopoulos, I Zergioti, and S Chatzandroulis. Capacitive microsystems for biological sensing. Biosensors and Bioelectronics, 27(1):1\u201311, 2011. [3] Sandro Carrara. Nano-Bio-Technology and Sensing Chips: New Systems for Detection in Personalized Therapies and Cell Biology. Sensors, 10(1):526\u2013543, January 2010. [4] Shaurya Prakash, M.B. Karacor, and S. Banerjee. Surface modification in microsystems and nanosystems. Surface Science Reports, 64(7):233\u2013254, July 2009. [5] Paul Yager, Gonzalo J Domingo, and John Gerdes. Point-of-care diagnostics for global health. Annual review of biomedical engineering, 10:107\u201344, January 2008. [6] Xiaole Mao and Tony Jun Huang. Microfluidic diagnostics for the developing world. Lab on a chip, 12(8):1412\u20136, April 2012

A scanning tunneling microscope control system with potentiometric capability

Includes bibliographical references.This report starts by describing the background research and work that had already been done on the UCT scanning tunneling microscope (STM). This system is being developed in the Department of Electrical Engineering at UCT. It goes on to describe the continuation of the research work that was done for this dissertation on the STM at UCT. The work was originally started by Dr. Tapson for his PhD (1994). and continued by the author for his MTech degree in ) 997 and 1998. The work was temporary discontinued from May 2000 till August 2002 to enable the author to work as a contract engineer at the Institute of Physics in Basel, Switzerland to learn more about the construction of probe microscopes. The new work evolved around the need to implement scanning tunneling potentiometry (STP) capability in the new STM. This capability should give the end-user the capability of looking at the sub-surface structure of any material on a sub-micron scale. The basic STP function must be implemented in two dimensions in the plane of the specimen. The STM tip is then used as a highly localized voltmeter to sense what the potential distribution is at that point on the surface. The potential information that is obtained is then used to plot two images of the potential distribution over the surface in the X and Y directions. The topographic information is obtained in the usual way from the STM scan. This method gives three collocated imagesas the result and a better understanding of the surface structure is obtained in this way. The penetration depth of the potential scan can be varied by adjusting the frequency of the applied AC signal in the X and Y directions. This use of the skin effect should allow the end user to obtain slices of the surface at various penetration levels of the specimen. These slices will give a picture of what happens from the surface up to a certain penetration depth. The interpretation of these images could be very difficult because the skin effect does not stop at a defined penetration depth. Only the 3 dB point is defined, which means that sub surface structures below the 3 dB point will also have an influence on the obtained image. During the course of the research new hardware and scanning software was implemented to enable the error-free acquisition of new data. This entailed splitting the existing XY controller into three separate parts namely a Communications interface, and two STP measurement boards. This was suggested as one of the conclusions of the MTech thesis results. The PC software stayed the same but for a change in the array size, that holds theacquired data. This was again changed after the work experience in Basel and is explained in chapter 6

A sub-150-nanometre-thick and ultraconformable solution-processed all-organic transistor

Recent advancements in the field of electronics have paved the way to the development of new applications, such as tattoo electronics, where the employment of ultraconformable devices is required, typically achievable with a significant reduction in their total thickness. Organic materials can be considered enablers, owing to the possibility of depositing films with thicknesses at the nanometric scale, even from solution. However, available processes do not allow obtaining devices with thicknesses below hundreds of nanometres, thus setting a limit. Here, we show an all-organic field effect transistor that is less than 150 nm thick and that is fabricated through a fully solution-based approach. Such unprecedented thickness permits the device to conformally adhere onto nonplanar surfaces, such as human skin, and to be bent to a radius lower than 1 μm, thereby overcoming another limitation for field-effect transistors and representing a fundamental advancement in the field of ultrathin and tattoo electronics

Multi-dimensional Resistivity Models of the Shallow Coal Seams at the Opencast Mine 'Garzweiler I' (Northwest of Cologne) inferred from Radiomagnetotelluric, Transient Electromagnetic and Laboratory Data

The entire Cenozoic unconsolidated fill of the Lower Rhine Embayment in Germany hosts the largest single lignite, or brown coal, deposit in Europe which covers an area of some 2,500 km2 to the northwest of Cologne. Rhineland brown coal is mined in large-scale opencast mining and accounts for around one-quarter of the public electricity supply in Germany. The present study was devoted to carrying out radiomagnetotelluric (RMT) and transient electromagnetic (TEM) investigations over the shallow coal seams at the opencast mine 'Garzweiler I.' The main objectives of the survey were to highlight the applicability and efficiency of RMT and TEM methods in an area like brown coal exploration, and to image the vertical electrical resistivity structure of these coal seams. Therefore, the vertical and lateral resolution capabilities of such methods were as necessary as the ability to cover large areas. Consequently, a total of 86 azimuthal RMT and 33 in-loop TEM soundings were carried out along six separate profiles over two opencast benches at the 'Garzweiler I' mine. The local stratigraphy at the survey areas comprises a layer-cake sequence, from top to bottom, of Garzweiler, Frimmersdorf and Morken coal seams embedded in a sand background, consisting of Surface, Neurath, Frimmersdorf and Morken Sands. A considerable amount of clay and silt intervenes the whole succession. The data were interpreted extensively and consistently in terms of one-dimensional (1D) RMT and TEM resistivity models, without using any complex multi-dimensional interpretation. However, the presence of thin, surficial clay masses (or lenses) broke down such interpretation scheme. In this case, to greatly improve the resistivity resolution for these surficial masses and the underlying coal seams, two-dimensional (2D) RMT and three-dimensional (3D) TEM interpretations have been carried out. They could be used effectively to study the local EM distortion on the measured data, where these surficial masses were found, as well as to cross-check the nearby-topography effect. Because the RMT data are usually skin-depth limited, they only provided a resolution depth between 25 and 30 m for the shallow resistivity structures. Whereas, the TEM data still have sufficiently early- to late-time information, and therefore resulted in a better resolution depth of about 100 m for the shallow to sufficiently-deep resistivity structures. The final 1D/2D RMT and 1D/3D TEM resistivity models displayed a satisfied correlation with both thicknesses derived from the stratigraphic-control boreholes and resistivities measured from direct-current (DC) and spectral induced polarization (SIP) laboratory techniques on 16 rock samples. As demonstrated, the integrated use of azimuthal RMT and in-loop TEM soundings was highly successful and effective at mapping the major stratigraphic units at the survey areas, i.e. the shallowest conductive Garzweiler and Frimmersdorf Coals within their fairly resistive sand background. They could not distinguish between Neurath Sand and the underlying sand/silt or between Frimmersdorf Coal and the underlying organic clay. The deepest Morken Coal was beyond the depth-of-investigation of the present measurements. Finally, the resistivity models revealed that both coal seams gently dip in the southwesterly direction. This should be in fairly good agreement with the regional structural makeup of the Rhineland brown coal. However, they showed that Garzweiler Coal is gradually thinned northeastwards, while Frimmersdorf Coal still has almost a regular thickness