Design of Micro/Nanostructured Materials for Flexible Electronics and Sensing Applications

Department of Chemical EngineeringNowadays, the rapid development of modern society with the Fourth Industrial Revolution has initiated technological advances in electronic devices for improving the quality of human life. In addition, electronic devices have been evolved into human-interactive wearable electronics for smart healthcare systems, internet of things, augmented reality, and humanoid robots. Therefore, innovative and multirole-featured flexible sensing devices with advanced communication capability becomes considerably important for future electronics beyond the classical processing devices complying with Moore???s law. In particular, photodetectors are essential for detecting light by transducing an electrical signal, enabling recognition and imaging of some objects with contactless detection. In addition, mechanosensors offer useful physical information such as touch and gesture through direct contact with objects. The introduction of micro/nanostructures into electronic sensors can be helpful to maximize the performance of both photodetectors and mechanosensors with improved light absorption and effective transmission of external forces, respectively. Furthermore, the novel design of device architectures with appropriate use of unique properties of the base materials allows diverse functionalities to flexible sensors, which is advantageous in next-generation electronic devices. In this thesis, we demonstrate highly sensitive and multifunctional flexible sensing devices for future electronics via a novel design of micro/nanostructured materials. In Chapter 1, we briefly introduce the basic concepts and current technologies of flexible photodetectors and mechanosensors, respectively. In Chapter 2, we develop unique design of flexible photodetectors with both omnidirectional and broadband light-detection capabilities based on hierarchical ZnO nanowire arrays on flexible honeycomb-structured Si membranes. As another future proximity sensors, in Chapter 3, we suggest new fabrication method for hierarchical organic-inorganic hybrid perovskite nanoribbon arrays with controlled internal nanorod structures. The hierarchical perovskite nanoribbon based flexible photodetectors exhibit high photoresponsivity as well as self-powered operation and polarization-sensitive detection capabilities. Chapter 4 describes spacer-free, ultrathin, and highly sensitive triboelectric mechanosensors based on human skin-inspired hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness. Finally, we summarize this thesis with future prospects and challenges of this field in Chapter 5.clos

Fabrication of uniform layer-by-layer assemblies with complementary protein cage nanobuilding blocks via simple His-tag/metal recognition

A capsid-forming enzyme, lumazine synthase isolated from hyperthermophile Aquifex aeolicus (AaLS), is prepared and utilized as a template for constructing nanobuilding blocks to fabricate uniform layer-by-layer (LbL) assemblies. Two functionally complementary AaLS protein cage nanoparticles (PCNs) are generated either by genetically introducing His-tags on the surface of wild-type AaLS PCNs or by chemically attaching metal chelates (Ni-NTA moiety) to the surface of cysteine-bearing AaLS PCNs individually. The multivalent displays of His-tags (AaLS-His6 PCN) and Ni-NTA ligands (AaLS-NTA-Ni PCN) on the surface of each complementary AaLS PCN are successfully demonstrated by mass spectrometric and surface plasmon resonance analyses. By using these two complementary AaLS PCNs, uniform LbL assemblies are constructed via simple recognition between His-tags and metal chelates without the aid of additional binding mediators. This approach illustrates the potential of fabricating uniform nanostructures using protein-based hybrid functional nanobuilding blocks.close3

Large-Area, Solution-Processed, Hierarchical MAPbI3 Nanoribbon Arrays for Self-Powered Flexible Photodetectors

Organic-inorganic perovskites with micro-/nanostructures exhibit outstanding optical and electrical properties, thus attracting increased attention as components of high-performance optoelectronic devices, but the fabrication of complex micro-/nanostructured perovskites shows limited success. This study describes the fabrication of hierarchical methylammonium lead iodide (MAPbI3) nanoribbon (NR) arrays with controlled internal nanorod structures by simple spin-coating with solvent treatment process and investigates the suitability of these arrays for high-performance and multifunctional photodetectors. In the UV-to-800-nm range, photodetectors based on the hierarchical MAPbI3 NR arrays exhibit specific detectivities 18.1-23.7 times higher than those of photodetectors based on MAPbI3 films due to the effective photon management and reduced charge trap states in the hierarchical MAPbI3 NR arrays. The solution-processed hierarchical MAPbI3 NRs can be fabricated on various substrates including ultrathin polyimide, which facilitates the development of flexible photodetectors with highly stable photoresponse under bending strain. Furthermore, the ferroelectricity and the highly anisotropic alignment of MAPbI3 NR arrays allow multifunctional photodetectors capable of self-powered and polarization-sensitive light detection, respectively. The strategy used to fabricate hierarchical organic-inorganic perovskite NR arrays is believed to be applicable to other types of perovskites and can probably be used to construct various optoelectronic devices based on hierarchical nanostructures

Soft and ion-conducting hydrogel artificial tongue for astringency perception

Artificial tongues have been receiving increasing attention for the perception of five basic tastes. However, it is still challenging to fully mimic human tongue-like performance for tastes such as astringency. Mimicking the mechanism of astringency perception on the human tongue, we use a saliva-like chemiresistive ionic hydrogel anchored to a flexible substrate as a soft artificial tongue. When exposed to astringent compounds, hydrophobic aggregates form inside the microporous network and transform it into a micro/nanoporous structure with enhanced ionic conductivity. This unique human tongue-like performance enables tannic acid to be detected over a wide range (0.0005 to 1 wt %) with high sensitivity (0.292 wt %(-1)) and fast response time (similar to 10 s). As a proof of concept, our sensor can detect the degree of astringency in beverages and fruits using a simple wipe-and-detection method, making a powerful platform for future applications involving humanoid robots and taste monitoring devices

Skin-Inspired Hierarchical Polymer Architectures with Gradient Stiffness for Spacer-Free, Ultrathin, and Highly Sensitive Triboelectric Sensors

The gradient stiffness between stiff epidermis and soft dermis with interlocked microridge structures in human skin induces effective stress transmission to underlying mechanoreceptors for enhanced tactile sensing. Inspired by skin structure and function, we fabricate hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness for spacer-free, ultrathin, and highly sensitive triboelectric sensors (TESs). The skin-inspired hierarchical polymers with gradient elastic modulus enhance the compressibility and contact areal differences due to effective transmission of the external stress from stiff to soft layers, resulting in highly sensitive TESs capable of detecting human vital signs and voice. In addition, the microridges in the interlocked polymers provide an effective variation of gap distance between interlocked layers without using the bulk spacer and thus facilitate the ultrathin and flexible design of TESs that could be worn on the body and detect a variety of pressing, bending, and twisting motions even in humid and underwater environments. Our TESs exhibit the highest power density (46.7 mu W/cm(2)), pressure (0.55 V/kPa), and bending (similar to 0.1 V/degrees) sensitivities ever reported on flexible TESs. The proposed design of hierarchical polymer architectures for the flexible and wearable TESs can find numerous applications in next-generation wearable electronics

High-Performance MoS2/CuO Nanosheet-on-One-Dimensional Heterojunction Photodetectors

van der Waals heterostructures based on stacked two-dimensional (2D) materials provide novel device structures enabling high-performance electronic and optoelectronic devices. While 2D-2D or 2D bulk heterostructures have been largely explored for fundamental understanding and novel device applications, 2D one-dimensional (1D) heterostructures have been rarely studied because of the difficulty in achieving high-quality heterojunctions between 2D and 1D structures. In this study, we introduce nanosheet-on-1D van der Wags heterostructure photodetectors based on a wet-transfer printing of a MoS2 nanosheet on top of a CuO nanowire (NW). MoS2/CuO nanosheet-on-1D photodetectors show an excellent photocurrent rectification ratio with an ideality factor of 1.37, which indicates the formation of an atomically sharp interface and a high-quality heterojunction in the MoS2/CuO heterostructure by wet-transfer-enhanced van der Waals bonding. Furthermore, nanosheet-on-1D heterojunction photodetectors exhibit excellent photodetection capabilities with an ultrahigh photoresponsivity (similar to 157.6 A/W), a high rectification ratio (similar to 6000 at +/- 2 V), a low dark current (similar to 38 fA at -2 V), and a fast photoresponse time (similar to 34.6 and 51.9 ms of rise and decay time), which cannot be achievable with 1D-on-nanosheet heterojunction photodetectors. The wet transfer printing of nanosheet-on-1D heterostructures introduced in this study provides a robust platform for the fundamental study of various combinations of 2D-on-1D heterostructures and their applications in novel heterojunction devices.clos

High-Performance Hybrid Photovoltaics with Efficient Interfacial Contacts between Vertically Aligned ZnO Nanowire Arrays and Organic Semiconductors

Hybrid photovoltaics (HPVs) incorporating both organic and inorganic semiconducting materials have attracted much attention as next-generation photovoltaics because of their advantage of combining both materials. The hybridization of ZnO nanowires (NWs) and organic semiconductors is expected to be a suitable approach to overcome the limited exciton diffusion length and low electron mobility associated with current organic photovoltaics. The use of ZnO NWs allows researchers to tune nanoscale dimensions more precisely and to achieve rod-to-rod spacing below 10 nm. However, the perfect incorporation of organic semiconductors into densely packed ZnO NW arrays has yet to be achieved. In this study, we report the fabrication of ZnO NW arrays and various organic heterojunction-based HPVs using the feasible and effective vacuum-assisted double coating (VADC) method, achieving full coverage of the organic semiconductors on the compact ZnO NW arrays. The newly proposed VADC method ensures perfect infiltration and full coverage of the organic semiconductors on the densely packed NW arrays. Compared with the conventional single spin-coating process, the use of the VADC method led to 11 and 14% increases in the power conversion efficiency of P3HT:PCBM- and PBDTTT-C-T:PC71BM-based HPVs, respectively. Our studies provide a feasible method for the fabrication of efficient HPVs

InGaAs Nanomembrane/Si van der Waals Heterojunction Photodiodes with Broadband and High Photoresponsivity

Development of broadband photodetectors is of great importance for applications in high-capacity optical communication, night vision, and biomedical imaging systems. While heterostructured photodetectors can expand light detection range, fabrication of heterostructures via epitaxial growth or wafer bonding still faces significant challenges because of problems such as lattice and thermal mismatches. Here, a transfer printing technique is used for the heterogeneous integration of InGaAs nanomembranes on silicon semiconductors and thus the formation of van der Waals heterojunction photodiodes, which can enhance the spectral response and photoresponsivity of Si photodiodes. Transfer-printed InGaAs nanomembrane/Si heterojunction photodiode exhibits a high rectification ratio (7.73 ?? 104 at ??3 V) and low leakage current (7.44 ?? 10-5 A/cm2 at -3 V) in a dark state. In particular, the photodiode shows high photoresponsivities (7.52 and 2.2 A W-1 at a reverse bias of -3 V and zero bias, respectively) in the broadband spectral range (400-1250 nm) and fast rise-fall response times (13-16 ms), demonstrating broadband and fast photodetection capabilities. The suggested III-V/Si van der Waals heterostructures can be a robust platform for the fabrication of high-performance on-chip photodetectors compatible with Si integrated optical chips.clos

Gate-Controlled Spin-Orbit Interaction in InAs High-Electron Mobility Transistor Layers Epitaxially Transferred onto Si Substrates

We demonstrate gate-controlled spin-orbit interaction (SOI) in InAs high-electron mobility transistor (HEMT) structures transferred epitaxially onto Si substrates. Successful epitaxial transfer of the multilayered structure after separation from an original substrate ensures that the InAs HEMT maintains a robust bonding interface and crystalline quality with a high electron mobility of 46200 cm2/(V s) at 77 K. Furthermore, Shubnikov-de Haas (SdH) oscillation analysis reveals that a Rashba SOI parameter (??) can be manipulated using a gate electric field for the purpose of spin field-effect transistor operation. An important finding is that the ?? value increases by about 30% in the InAs HEMT structure that has been transferred when compared to the as-grown structure. First-principles calculations indicate that the main causes of the large improvement in ?? are the bonding of the InAs HEMT active layers to a SiO2 insulating layer with a large band gap and the strain relaxation of the InAs channel layer during epitaxial transfer. The experimental results presented in this study offer a technological platform for the integration of III-V heterostructures onto Si substrates, permitting the spintronic devices to merge with standard Si circuitry and technology.close0

High-Performance MoS₂/CuO Nanosheet-on-One-Dimensional Heterojunction Photodetectors

van der Waals heterostructures based on stacked two-dimensional (2D) materials provide novel device structures enabling high-performance electronic and optoelectronic devices. While 2D–2D or 2D–bulk heterostructures have been largely explored for fundamental understanding and novel device applications, 2D–one-dimensional (1D) heterostructures have been rarely studied because of the difficulty in achieving high-quality heterojunctions between 2D and 1D structures. In this study, we introduce nanosheet-on-1D van der Waals heterostructure photodetectors based on a wet-transfer printing of a MoS₂ nanosheet on top of a CuO nanowire (NW). MoS₂/CuO nanosheet-on-1D photodetectors show an excellent photocurrent rectification ratio with an ideality factor of 1.37, which indicates the formation of an atomically sharp interface and a high-quality heterojunction in the MoS₂/CuO heterostructure by wet-transfer-enhanced van der Waals bonding. Furthermore, nanosheet-on-1D heterojunction photodetectors exhibit excellent photodetection capabilities with an ultrahigh photoresponsivity (∼157.6 A/W), a high rectification ratio (∼6000 at ±2 V), a low dark current (∼38 fA at −2 V), and a fast photoresponse time (∼34.6 and 51.9 ms of rise and decay time), which cannot be achievable with 1D-on-nanosheet heterojunction photodetectors. The wet-transfer printing of nanosheet-on-1D heterostructures introduced in this study provides a robust platform for the fundamental study of various combinations of 2D-on-1D heterostructures and their applications in novel heterojunction devices