| 1. |
- Feng, Yi
(author)
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Printed RFID Humidity Sensor Tags for Flexible Smart Systems
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Radio frequency identification (RFID) and sensing are two key technologies enabling the Internet of Things (IoT). Development of RFID tags augmented with sensing capabilities (RFID sensor tags) would allow a variety of new applications, leading to a new paradigm of the IoT. Chipless RFID sensor technology offers a low-cost solution by eliminating the need of an integrated circuit (IC) chip, and is hence highly desired for many applications. On the other hand, printing technologies have revolutionized the world of electronics, enabling cost-effective manufacturing of large-area and flexible electronics. By means of printing technologies, chipless RFID sensor tags could be made flexible and lightweight at a very low cost, lending themselves to the realization of ubiquitous intelligence in the IoT era.This thesis investigated three construction methods of printable chipless RFID humidity sensor tags, with focus on the incorporation of the sensing function. In the first method, wireless sensing based on backscatter modulation was separately realized by loading an antenna with a humidity-sensing resistor. An RFID sensor tag could then be constructed by combining the wireless sensor with a chipless RFID tag. In the second method, a chipless RFID sensor tag was built up by introducing a delay line between the antenna and the resistor. Based on time-domain reflectometry (TDR), the tag encoded ID in the delay time between its structural-mode and antenna-mode scattering pulse, and performed the sensing function by modulating the amplitude of the antenna-mode pulse.In both of the above methods, a resistive-type humidity-sensing material was required. Multi-walled carbon nanotubes (MWCNTs) presented themselves as promising candidate due to their outstanding electrical, structural and mechanical properties. MWCNTs functionalized (f-MWCNTs) by acid treatment demonstrated high sensitivity and fast response to relative humidity (RH), owing to the presence of carboxylic acid groups. The f-MWCNTs also exhibited superior mechanical flexibility, as their resistance and sensitivity remained almost stable under either tensile or compressive stress. Moreover, an inkjet printing process was developed for the f-MWCNTs starting from ink formulation to device fabrication. By applying the f-MWCNTs, a flexible humidity sensor based on backscatter modulation was thereby presented. The operating frequency range of the sensor was significantly enhanced by adjusting the parasitic capacitance in the f-MWCNTs resistor. A fully-printed time-coded chipless RFID humidity sensor tag was also demonstrated. In addition, a multi-parameter sensor based on TDR was proposed.The sensor concept was verified by theoretical analysis and circuit simulation.In the third method, frequency-spectrum signature was utilized considering its advantages such as coding capacity, miniaturization, and immunity to noise. As signal collision problem is inherently challenging in chipless RFID sensor systems, short-range identification and sensing applications are believed to embody the core values of the chipless RFID sensor technology. Therefore a chipless RFID humidity sensor tag based on near-field inductive coupling was proposed. The tag was composed of two planar inductor-capacitor (LC) resonators, one for identification, and the other one for sensing. Moreover, paper was proposed to serve as humidity-sensing substrate for the sensor resonator on accounts of its porous and absorptive features.Both inkjet paper and ordinary packaging paper were studied. A commercial UV-coated packaging paper was proven to be a viable and more robust alternative to expensive inkjet paper as substrate for inkjet-printed metal conductors. The LC resonators printed on paper substrates showed excellent sensitivity and reasonable response time to humidity in terms of resonant frequency. Particularly, the resonator printed on the UV-coated packaging paper exhibited the largest sensitivity from 20% to 70% RH, demonstrating the possibilities of directly printing the sensor tag on traditional packages to realize intelligent packaging at an ultra-low cost.
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| 2. |
- Hakkarainen, Teemu, et al.
(author)
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Te incorporation and activation as n-type dopant in self-catalyzed GaAs nanowires
- 2019
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In: Physical Review Materials. - 2475-9953. ; 3:8
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Journal article (peer-reviewed)abstract
- Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for n-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is 100% at a doping level of 4×1018cm-3, which is a significant result in terms of future device applications.
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| 3. |
- Li, Miao, et al.
(author)
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0.7-GHz Solution-Processed Indium Oxide Rectifying Diodes
- 2020
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In: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9383 .- 1557-9646. ; 67:1, s. 360-364
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Journal article (peer-reviewed)abstract
- Solution-based deposition, with its simplicity and possibility for upscaling through printing, is a promising process for low-cost electronics. Metal oxide semiconductor devices, especially indium oxide with its excellent electrical properties, offer high performance compared to amorphous Si-based rivals, and with a form factor conducive to flexible and wearable electronics. Here, rectifying diodes based on an amorphous spin-coated indium oxide are fabricated for high-speed applications. We report a solution-processed diode approaching the UHF range, based on indium oxide, with aluminum and gold as the electrodes. The device was spin-coated from a precursor material and configured into a half-wave rectifier. The J-V and frequency behavior of the diodes were studied, and the material composition of the diode was investigated by X-ray photoemission spectroscopy (XPS). The 3-dB point was found to be over 700 MHz. The results are promising for the development of autonomously powered wireless Internet-of-Things systems based on scalable, low-cost processes.
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| 4. |
- Lopez Cabezas, Ana, 1980-
(author)
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Nanofibrillar Materials for Organic and Printable Electronics
- 2013
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Doctoral thesis (other academic/artistic)abstract
- In recent years, organic electronics have attracted great attention due to their multiple advantages such as light weight, flexibility, large area fabrication and cost-effective production processes. The recent progress in fabricating organic electronic devices has been achieved with the development of new materials which provide competing functionalities to the electronics devices. However, as it happens with all type of technologies, organic electronics is not free from challenges. In the latest OE-A Roadmap for organic and printed electronics (2011), the “red brick walls” were identified, and the following three main challenges were pointed out as the potential roadblocks from the material point of view: electrical performance, solution processability (especially formulations in non-toxic solvents) and environmental stability. Currently there is a significant increasing interest in optimizing or developing novel materials to meet those requirements. This thesis presents processing development and study of nanofibrillar materials and deals with the optimization for its applicability for organic electronics. The overall work presented in the thesis is based on three nanofibrillar materials: Polyaniline (PANI), carbon nanotubes (CNTs) and the CNT/PANI composite. First, the solution processability of carbon nanotubes and polyaniline is studied respectively, and through covalent and non-covalent methods, stable aqueous dispersions of these materials are successfully achieved. Second, a composite consisting of multi-walled carbon nanotubes (MWCNTs) and PANI with a core-shell structure is developed and characterized. The investigation of the effects of the loading and type of nanotubes incorporated in the composite material, led to understanding on the fundamental theory underlying the composite morphology. Based on those findings and by carefully optimizing the synthesis procedure, water dispersible MWCNT/PANI nanofibrillar composite is successfully synthesized becoming compatible with solution processable techniques, such as spray coating and potentially with printing technology. With the incorporation of carbon nanotubes, the nanofibrillar composite reaches conductivities 20 times higher than that of the pure polymer. Moreover, the presence of the nanotubes in the composite material decelerates up to 60 times the thermal ageing of its conductivity, making the polymer more robust and suitable for possible manufacturing processes. Furthermore, the composite material still retains the advantageous properties of PANI: electrochromism, tunable conductivities, and sensing capabilities. Third, the stable dispersions of PANI, CNTs and MWCNT/PANI composite were effectively deposited by spray coating technique on several low-cost substrates (PET, PEN, polyimide and papers), and homogeneous, flexible, large-area films were fabricated. Additionally, by spraying the materials on pre-fabricated inkjet printed electrodes, a pH sensor based on the MWCNT/PANI composite and a humidity sensor based on functionalized MWCNTs capable of working at GHz range were demonstrated, which shows that the nanofibrillar materials studied in this thesis work are promising sensor materials for wireless application at ultra-high frequency (UHF) band. Finally, the humidity sensor was integrated into a sensor-box demonstrating a hybrid interconnection platform where printed electronics can be seamlessly integrated with silicon-based electronics. The integration closes the gap between the two technologies, anticipating the adaption of organic electronic technologies.
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| 5. |
- Sollami Delekta, Szymon, 1990-
(author)
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Inkjet Printing of Graphene-based Microsupercapacitors for Miniaturized Energy Storage Applications
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Printing technologies are becoming increasingly popular because they enable the large-scale and low-cost production of functional devices with various designs, functions, mechanical properties and materials. Among these technologies, inkjet printing is promising thanks to its direct (mask-free) patterning, non-contact nature, low material waste, resolution down to 10 µm, and compatibility with a broad range of materials and substrates. As a result, inkjet printing has applications in several fields like wearables, opto-electronics, thin-film transistors, displays, photovoltaic devices, and in energy storage. It's in energy storage that the technique shows its full potential by allowing the production of miniaturized devices with a compact form factor, high power density and long cycle life, called microsupercapacitors (MSCs). To this end, graphene has a number of remarkable properties like high electrical conductivity, large surface area, elasticity and transparency, making it a top candidate as an electrode material for MSCs.Some key drawbacks limit the use of inkjet printing for the production of graphene-based MSCs. This thesis aims at improving its scalability by producing fully inkjet printed devices, and extending its applications through the integration of inkjet printing with other fabrication techniques.MSCs typically rely on the deposition by hand of gel electrolyte that is not printable or by submerging the whole structure into liquid electrolyte. Because of this, so far large-scale production of more than 10 interconnected devices has not been attempted. In this thesis, a printable gel electrolyte ink based on poly(4-styrene sulfonic acid) was developed, allowing the production of large arrays of more than 100 fully inkjet printed devices connected in series and parallel that can be reliably charged up to 12 V. Also, a second electrolyte ink based on nano-graphene oxide, a solid-state material with high ionic conductivity, was formulated to optimize the volumetric performance of these devices. The resulting MSCs were also fully inkjet printed and exhibited an overall device thickness of around 1 µm, yielding a power density of 80 mW cm-3.Next, the use of inkjet printing of graphene was explored for the fabrication of transparent MSCs. This application is typically hindered by the so-called coffee-ring effect, which creates dark deposits on the edges of the drying patterns and depletes material from the inside area. In light of this issue, inkjet printing was combined with etching to remove the dark deposits thus leaving uniform and thin films of graphene with vertical sidewalls. The resulting devices showed a transmittance of up to 90%.Finally, the issue of the substrate compatibility of inkjet printed graphene was addressed. Although inkjet printing is considered to have broad substrate versatility, it is unreliable on hydrophilic or porous substrates and most inks (including graphene inks) require thermal annealing that damages substrates that are not resistant to heat. Accordingly, a technique based on inkjet printing and wet transfer was developed to reliably deposit graphene-based MSCs on a number of substrates, including flat, 3D, porous, plastics and biological (plants and fruits) with adverse surfaces.The contributions of this thesis have the potential to boost the use of inkjet printed MSCs in applications requiring scalability and resolution (e.g. on-chip integration) as well as applications requiring conformability and versatility (e.g. wearable electronics).
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| 6. |
- Xie, Li
(author)
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Heterogeneous Integration of Silicon and Printed Electronics for Intelligente Sensing Devices
- 2014
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Doctoral thesis (other academic/artistic)abstract
- Driven by the exploding popularity of the Internet-of-Things (IoT), the demand for thin, flexible, lightweight intelligent sensingdevices is growing rapidly. Two attractive examples are intelligent packaging and wearable healthcare monitoring devices, which help to connect and track / monitor everything / everybody at any time and in any place. The remarkably swift development of flexible and printed electronics is promoting new possibilities for cost-effective manufacturing of such devices. However, compared with silicon-based circuits, state-of-the-art all-printed circuits are encountering low integration density, long switching time and corresponding high cost per function. Therefore, a heterogeneous platform is in great demand, which employs a cost-effective, large-area manufacturing technique while keeping the same complex functionality and processing capability as silicon-based systems. Due to temperature and mechanical reasons, traditional silicon integration methods, such as solder bonding and wire bonding, are not suitable for flexible printed electronics. This thesis aims to develop a generally applicable hetero-geneous integration platform for the realization of intelligent sensing devices on flexible substrates.First, inkjet printing technique is introduced and studied. As the basic and key element, inkjet printing technology is employed to fabricate interconnections as well as electrodes of the printed sensors. Novel flexible media, plastic and paper, are evaluated as the substrates of printed electronic systems from two aspects: the electrical characteristics and performance reliability. In addition to widely used inkjet/photo paper, packaging paper is presented as a promising candidate for intelligent packaging applications due to the advantages in terms of lower price, higher temperature endurance and better reliability against 85◦/85% RH aging.Second, the heterogeneous integration platform enabled byinkjet printing is presented. Benefiting from the non-contact, accurate alignment and fine resolution features, this integration technique has the advantages of simplified fabrication process and multi-substrate compatibility. The design rules have been studied and the integration process is optimized for silicon chips with/without packaging.Finally, to verify the suitability, the heterogeneous integration platform is applied to two representative applications, each with unique emphasis and requirements.For intelligent packaging, low-cost is one crucial requirement. Paper substrate is selected because it is cost-effective, recyclableand a commonly-used packaging material in industry. In order to fit into non-regular shape pack-ages, the intelligent packaging needs to be bent or folded, which brings about reliability concern for paper electronics. Therefore, bending and folding tests are applied to reveal the capability and the limitation of paper electronics in terms of flexibility. For applications such as fresh food tracking, humidity is an important physical quantity to monitor during transportation and storage. Therefore, a resistive humidity sensor based on multi-walled carbon nanotubes is fabricated and integrated. A commercial packaged microcontroller is used to sense and store the resistance of the sensor and control the LEDs to indicate the ambient humidity level. By integratingthe microcon-troller, LEDs and a switch with the printed sensor and battery, a prototype of a paper-based humidity sensor card is implemented.For the healthcare application, user comfort is an essential element. Future long-term healthcare devices require a bio-sensing system which is small, thin, lightweight and wearable, has a long-battery life, and is easy to customize. The heterogeneous platform offers a promising solution for such systems from three aspects. 1) A fully integrated system-on-chip (SoC) is embedded to detect and process the bio-signal. The SoC solution features tiny size and low-power consumption, which contribute to system miniaturization and long battery lifetime. 2) Inkjet printing offers a cost-effective approach to fabricate personalized electrodes. 3) Inkjet printed interconnections enable the direct integration of the bare die instead of the packaged chip. This significantly reduces the physical size of the system, simplifies the manufacture process and lowers the cost. The concept is demonstrated by aminiaturized wearable Bio-Patch with the size of 4.5 cm×2.5 cm.
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