| 1. |
- Zhao, Jie
(författare)
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Solution-Processable Conductive Graphene-Based Materials for Flexible Electronics
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work explores electrical conductors based on few-layer graphene flakes as an enabler for low-cost, mechanically flexible, and high-conductivity conductors in large area flexible and printed electronic devices. The flakes are deposited from aqueous solutions and processed at low temperature.Graphene is selected for its excellent properties in mechanical, optical, electronic, and electrical aspects. However, thin films of pristine few-layer graphene flakes deposited from dispersions normally exhibit inferior electrical conductivity. One cause responsible for this problem is the loose stacking and random orientation of graphene flakes in a graphene deposition. We have solved this problem by implementing a simple post-deposition treatment leading to dramatically densified and planarized thin films. Significantly increased electrical conductivity by ~20 times is obtained. The 1-pyrenebutyric acid tetrabutylammonium salt as an exfoliation enhancer and dispersant in water yields ~110 S/m in conductivity when the graphene based thin films are processed at 90 °C. In order to achieve higher conductivity, a room-temperature method for site-selective copper electroless deposition has been developed. This method is of particular interest for the self-aligned copper deposition to the predefined graphene films. The resultant two-layer graphene/copper structure is characterized by an overall conductivity of ~7.9 × 105 S/m, an increase by ~7000 times from the template graphene films. Several electronic circuits based on the graphene/copper bilayer interconnect have been subsequently fabricated on plastic foils as proof-of-concept demonstrators. Alternatively, highly conductive composites featuring graphene flakes coated with silver nanoparticles with electrical conductivity beyond 106 S/m can be readily obtained at 100 oC. Moreover, a highly conductive reduced-graphene-oxide/copper hybrid hydrogel has been achieved by mixing aqueous graphene oxide solution and copper-containing Fehling's solution. The corresponding aerogel of high porosity exhibits an apparent electrical conductivity of ~430 S/m and delivers a specific capacity of ~453 mAh g−1 at current density of 1 A/g. The experimental results presented in this thesis show that the solution-phase, low-temperature fabrication of highly conductive graphene-based materials holds promises for flexible electronics and energy storage applications.
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| 2. |
- Chen, Xi
(författare)
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Silicon Nanowire Field-Effect Devices as Low-Noise Sensors
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the past decades, silicon nanowire field-effect transistors (SiNWFETs) have been explored for label-free, highly sensitive, and real-time detections of chemical and biological species. The SiNWFETs are anticipated for sensing analyte at ultralow concentrations, even at single-molecule level, owing to their significantly improved charge sensitivity over large-area FETs. In a SiNWFET sensor, a change in electrical potential associated with biomolecular interactions in close proximity to the SiNW gate terminal can effectively control the underlying channel and modulate the drain-to-source current (IDS) of the SiNWFET. A readout signal is therefore generated. This signal is primarily determined by the surface properties of the sensing layer on the gate terminal, with sensitivity close up to the Nernstian limit widely demonstrated. To achieve a high signal-to-noise ratio (SNR), it is essential for the SiNWFETs to possess low noise of which intrinsic device noise is one of the major components. In metal-oxide-semiconductor (MOS)-type FETs, the intrinsic noise mainly results from carrier trapping/detrapping at the gate oxide/semiconductor interface and it is inversely proportional to the device area.This thesis presents a comprehensive study on design, fabrication, and noise reduction of SiNWFET-based sensors on silicon-on-oxide (SOI) substrate. A novel Schottky junction gated SiNWFET (SJGFET) is designed and experimentally demonstrated for low noise applications. Firstly, a robust process employing photo- and electron-beam mixed-lithography was developed to reliably produce sub-10 nm SiNW structures for SiNWFET fabrication. For a proof-of-concept demonstration, MOS-type SiNWFET sensors were fabricated and applied for multiplexed ion detection using ionophore-doped mixed-matrix membranes as sensing layers. To address the fundamental noise issue of the MOS-type SiNWFETs, SJGFETs were fabricated with a Schottky (PtSi/silicon) junction gate on the top surface of the SiNW channel, replacing the noisy gate oxide/silicon interface in the MOS-type SiNWFETs. The resultant SJGFETs exhibited a close-to-ideal gate coupling efficiency (60 mV/dec) and significantly reduced device noise compared to reference MOS-type SiNWFETs. Further optimization was performed by implementing a three-dimensional Schottky junction gate wrapping both top surface and two sidewalls of the SiNW channel. The tri-gate SJGFETs with optimized geometry exhibited significantly enhanced electrostatic control over the channel, thereby confined IDS in the SiNW bulk, which greatly improved the device noise immunity to the traps at bottom buried oxide/silicon interface. Finally, a lateral bipolar junction transistor (LBJT) was also designed and fabricated on a SOI substrate aiming for immediate sensor current amplification. Integrating SJGFETs with LBJTs is expected to significantly suppress environmental interference and improve the overall SNR especially under low sensor current situations.
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| 3. |
- Hinnemo, Malkolm, 1986-
(författare)
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On the Road to Graphene Biosensors
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biosensors are devices that detect biological elements and then transmit a readable signal. Biosensors can automatize diagnostics that would otherwise have to be performed by a physician or perhaps not be possible to perform at all. Current biosensors are however either limited to particular diseases or prohibitively expensive. In order to further the field, sensors capable of many parallel measurements at a lower cost need to be developed. Field effect transistor (FET) based sensors are possible candidates for delivering this, mainly by allowing miniaturization. Smaller sensors could be cheaper, and enable parallel measurements.Graphene is an interesting material to use as the channel of FET-sensors. The low electrochemical reactivity of its plane makes it possible to have graphene in direct contact with the sample liquid, which enhances the signal from impedance changes. Graphene-FET based impedance sensors should be able to sense almost all possible analytes and allow for scaling without losing sensitivity.In this work the steps needed to make graphene based biosensors are presented. An improved graphene transfer is described which by using low pressure to dry the graphene removes most contamination. A method to measure the contamination of graphene by surface enhanced Raman scattering is presented. Methods to produce double gated and electrolyte gated graphene transistors on a large scale in an entirely photolithographic process are detailed. The deposition of 1-pyrenebutyric acid (PBA) on graphene is studied. It is shown that at high surface concentrations the PBA stands up on graphene and forms a dense self-assembled monolayer. A new process of using Raman spectroscopy data to quantify adsorbents was developed in order to quantify the molecule adsorption. Biosensing has been performed in two different ways. Graphene FETs have been used to read the signal generated by a streaming potential setup. Using FETs in this context enables a more sensitive readout than what would be possible without them. Graphene FETs have been used to directly sense antibodies in high ionic strength. This sensing was done by measuring the impedance of the interface between the FET and the electrolyte.
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| 4. |
- Wen, Chenyu
(författare)
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Solid-State Nanopores for Sensing : From Theory to Applications
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanopore based sensing technology has been widely studied for a broad range of applications including DNA sequencing, protein profiling, metabolite molecules, and ions detection. The nanopore technology offers an unprecedented technological solution to meeting the demands of precision medicine on rapid, in-field, and low-cost biomolecule analysis. In general, nanopores are categorized in two families: solid-state nanopore (SSNP) and biological nanopore. The former is formed in a solid-state membrane made of SiNx, SiO2, silicon, graphene, MoS2, etc., while the latter represents natural protein ion-channels in cell membranes. Compared to biological pores, SSNPs are mechanically robust and their fabrication is compatible with traditional semiconductor processes, which may pave the way to their large-scale fabrication and high-density integration with standard control electronics. However, challenges remain for SSNPs, including poor stability, low repeatability, and relatively high background noise level. This thesis explores SSNPs from basic physical mechanisms to versatile applications, by entailing a balance between theory and experiment.The thesis starts with theoretical models of nanopores. First, resistance of the open pore state is studied based on the distribution of electric field. An important concept, effective transport length, is introduced to quantify the extent of the high field region. Based on this conductance model, the nanopores size of various geometrical shapes can be extracted from a simple resistance measurement. Second, the physical causality of ionic current rectification of geometrically asymmetrical nanopores is unveiled. Third, the origin of low-frequency noise is identified. The contribution of each noise component at different conditions is compared. Forth, a simple nano-disk model is used to describe the blockage of ionic current caused by DNA translocation. The signal and noise properties are analyzed at system level.Then, nanopore sensing experiments are implemented on cylinder SiNx nanopores and truncated-pyramid silicon nanopores (TPP). Prior to a systematic study, a low noise electrical characterization platform for nanopore devices is established. Signal acquisition guidelines and data processing flow are standardized. The effects of electroosmotic vortex in TPP on protein translocation dynamics are excavated. The autogenic translocation of DNA and proteins driven by the pW-level power generated by an electrolyte concentration gradient is demonstrated. Furthermore, by extending to a multiple pore system, the group translocation behavior of nanoparticles is studied. Various application scenarios, different analyte categories and divergent device structures accompanying with flexible configurations clearly point to the tremendous potential of SSNPs as a versatile sensor.
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