Transfer Printing of Nanomaterials as a Novel Device Fabrication Route

Department of Energy EngineeringTransfer printing is a promising fabrication process for the high-performance electronic devices, novel device structures, and flexible electronics. It enables the combination of the current low-cost and well-established standard silicon process with high performance of III-V compound semiconductor to overcome the limitations of silicon devices, resulting in low-cost fabrication of high-performance devices. This process can be applied to not only III-V compound semiconductor technology but also low-dimensional materials (i.e. graphene and nanowires). In addition, it guarantees the freedom of choice of materials and substrates. Thus, combination and stacking process of various nanomaterials can be potentially employed by transfer printing for the fabrication of diverse hetero-structured optoelectronic applications. Furthermore, transfer printing of inorganic semiconductor onto flexible substrate enables to achieve high-performance flexible electronic system. As a result, the transfer printing has been actively studied in various research fields. In this thesis, we introduce the various transfer printing methods and their applications in electronic and optoelectronic devices. Firstly, Chapter 1 introduces the historical background and the necessity of the transfer printing, the mechanism of the various transfer printing methods, and its applications. In Chapter 2, we introduce the dry transfer printing method of InAs high electron mobility transistor (HEMT) on silicon substrate for the silicon-based high-performance electronic device. In Chapter 3, we introduce the broadband and high-photoresponsivity photodiode through hetero-integration of III-V compound semiconductors on Si substrates with high-quality interface. In Chapter 4, we show the adhesive layer-assisted transfer printing method and the wrinkling process of the III-V compound semiconductor nanomembranes by using the vacuum-induced stress control of nanomembranes on polydimethylsiloxane (PDMS) microwell arrays. In this method, the size, direction, and location of wrinkle arrays can be easily controlled by changing the shape and location of the microwell arrays and the modulus of soft substrates. Finally, we introduce nanosheet-on-one-dimensional heterojunction photodiode by using wet transfer printing process in Chapter 5. The device shows high rectification ratio, very low dark current, and the high On/Off current ratio at room temperature under the light illumination. The transfer printing methods introduced in this thesis can be potentially employed in the fabrication of various heterostructure of semiconductors for diverse optoelectronic applications.ope

High-Performance Triboelectric Devices via Dielectric Polarization: A Review

Energy harvesting devices based on the triboelectric effect have attracted great attention because of their higher output performance compared to other nanogenerators, which have been utilized in various wearable applications. Based on the working mechanism, the triboelectric performance is mainly proportional to the surface charge density of the triboelectric materials. Various approaches, such as modification of the surface functional group and dielectric composition of the triboelectric materials, have been employed to enhance the surface charge density, leading to improvements in triboelectric performances. Notably, tuning the dielectric properties of triboelectric materials can significantly increase the surface charge density because the surface charge is proportional to the relative permittivity of the triboelectric material. The relative dielectric constant is modified by dielectric polarization, such as electronic, vibrational (or atomic), orientation (or dipolar), ionic, and interfacial polarization. Therefore, such polarization represents a critical factor toward improving the dielectric constant and consequent triboelectric performance. In this review, we summarize the recent insights on the improvement of triboelectric performance via enhanced dielectric polarization

Transparent and conductive nanomembranes with orthogonal silver nanowire arrays for skin-attachable loudspeakers and microphones

We demonstrate ultrathin, transparent, and conductive hybrid nanomembranes (NMs) with nanoscale thickness, consisting of an orthogonal silver nanowire array embedded in a polymer matrix. Hybrid NMs significantly enhance the electrical and mechanical properties of ultrathin polymer NMs, which can be intimately attached to human skin. As a proof of concept, we present a skin-attachable NM loudspeaker, which exhibits a significant enhancement in thermoacoustic capabilities without any significant heat loss from the substrate. We also present a wearable transparent NM microphone combined with a micropyramid-patterned polydimethylsiloxane film, which provides excellent acoustic sensing capabilities based on a triboelectric voltage signal. Furthermore, the NM microphone can be used to provide a user interface for a personal voice-based security system in that it can accurately recognize a user???s voice. This study addressed the NM-based conformal electronics required for acoustic device platforms, which could be further expanded for application to conformal wearable sensors and health care devices

Inkjet-printed CMOS-integrated graphene–metal oxide sensors for breath analysis

Abstract: Early diagnosis in exhaled breath is a key technology for next-generation personal healthcare monitoring. Current chemiresistive sensors, primarily based on metal oxide (MOx) thin films, have limited applicability in such portable systems due to their high power consumption, long recovery time, poor device-to-device consistency, and baseline drifts. To address these challenges for ammonia (NH3) detection in exhaled breath, a critical biomarker for a variety of kidney and liver problems, we present a formulation of a graphene–MOx functional ink-based sensing platform. We integrate our sensing layer directly onto miniaturized CMOS microhotplates (μHP) via inkjet printing, potentially enabling scalability and device-to-device performance repeatability. Using stage-by-stage temporal analysis, and a temperature-pulsed modulation (TM) strategy, we achieve ultrahigh responsivity (1500% at 10 ppm pure NH3), fast response and recovery time (28 and 43 s), ultralow power consumption (~6 mW), negligible baseline drift (<0.67%), excellent cross-device and cross-cycle consistency (<0.5% and <0.41% variation in responsivity) and long-term stability (<1% variation) in our graphene–zinc oxide (ZnO) formulation, outperforming conventional MOx chemiresistive sensors. We further mitigate the effect of humidity through our measurement protocols, while interference from acetone is compensated through the parallel deployment of an additional inkjet printed graphene–tungsten oxide (WO3) device as part of the sensor array. Our dual graphene–MOx formulations and their integration with ultralow power CMOS through inkjet printing represent a significant step towards reliable and portable multi-analyte breath diagnostics

Crossover from weak anti-localization to weak localization in inkjet-printed Ti3C2Tx MXene thin-film

Two-dimensional (2D) transition metal carbides/nitrides or &quot;MXenes&quot; belong to a diverse-class of layered compounds, which offer composition- and electric-field-tunable electrical and physical properties. Although the majority of the MXenes, including Ti3C2Tx, are metallic, they typically show semiconductor-like behaviour in their percolated thin-film structure; this is also the most common structure used for fundamental studies and prototype device development of MXene. Magnetoconductance studies of thin-film MXenes are central to understanding their electronic transport properties and charge carrier dynamics, and also to evaluate their potential for spin-tronics and magnetoelectronics. Since MXenes are produced through solution processing, it is desirable to develop deposition strategies such as inkjet-printing to enable scale-up production with intricate structures/networks. Here, we systematically investigate the extrinsic negative magnetoconductance of inkjet-printed Ti3C2Tx, MXene thin-films and report a crossover from weak anti-localization (WAL) to weak localization (WL) near 2.5 K. The crossover from WAL to WL is consistent with strong, extrinsic, spin-orbit coupling, a key property for active control of spin currents in spin-orbitronic devices. From WAL/WL magnetoconductance analysis, we estimate that the printed MXene thin-film has a spin orbit coupling field of up to 0.84 T at 1.9 K. Our results and analyses offer a deeper understanding into microscopic charge carrier transport in Ti3C2Tx, revealing promising properties for printed, flexible, electronic and spinorbitronic device applications

Plasma etching and surface characteristics depending on the crystallinity of the BaTiO3 thin film

Due to its high dielectric constant (κ), the BaTiO3 (BTO) thin film has significant potential as a next-generation dielectric material for metal oxide semiconductor field-effect transistors (MOSFETs). Hence, the evaluation of the BTO thin film etching process is required for such nanoscale device applications. Herein, the etching characteristics and surface properties are examined according to the crystallinity of the BTO thin film. The results demonstrate that the etching rate is low in the high-crystallinity thin film, and the surface residues are much lower than in the low-crystallinity thin film. In particular, the accelerated Cl radicals in the plasma are shown to penetrate more easily into the low-crystallinity thin film than the high-crystallinity thin film. After the etching process, the surface roughness is significantly lower in the high-crystallinity thin film than in the low-crystallinity thin film. This result is expected to provide useful information for the process design of high-performance electronic devices

The Reflectance Characteristics of an Inverse Moth-Eye Structure in a Silicon Substrate Depending on SF6/O2 Plasma Etching Conditions

The global RE100 campaign is attracting attention worldwide due to climate change caused by global warming, increasingly highlighting the efficiency of renewable energy. Texturing of photovoltaic devices increases the devices’ efficiency by reducing light reflectance at their surfaces. This study introduces the change in light reflectance following the process conditions of plasma etching as a texturing process to increase the efficiency of photovoltaic cells. Isotropic etching was induced through plasma using SF6 gas, and the etch profile was modulated by adding O2 gas to reduce light reflectance. A high etch rate produces high surface roughness, which results in low surface reflectance properties. The inverse moth-eye structure was implemented using a square PR pattern arranged diagonally and showed the minimum reflectance in visible light at a tip spacing of 1 μm. This study can be applied to the development of higher-efficiency optical devices

Tunable physical properties of Al-doped ZnO thin films by O2 and Ar plasma treatments

Al-doped ZnO (AZO) is a promising transparent conducting oxide that can replace indium tin oxide (ITO) owing to its excellent flexibility and eco-friendly characteristics. However, it is difficult to immediately replace ITO with AZO because of the difference in their physical properties. Here, we study the changes in the physical properties of AZO thin films using Ar and O _2 plasma treatments. Ar plasma treatment causes the changes in the surface and physical properties of the AZO thin film. The surface roughness of the AZO thin film decreases, the work function and bandgap slightly increase, and the sheet resistance significantly decreases. In contrast, a large work function change is observed in the AZO thin film treated with O _2 plasma; however, the change in other characteristics is not significant. Therefore, the results indicate that post-treatment using plasma can accelerate the development of high-performance transparent devices

High-density plasma etching characteristics of aluminum-doped zinc oxide thin films in Cl2/Ar plasma

We investigated the etching characteristics of aluminum-doped zinc oxide (AZO) thin films in an adaptively coupled plasma (ACP) system. The dry etching characteristics of AZO films were studied by changing the Cl _2 /Ar gas mixing ratio, RF power, DC bias voltage. We determined the following optimized process conditions: RF power of 500 W, DC bias voltage of −100 V, process pressure of 15 mTorr. In Cl _2 /Ar plasma (=50:50%), the maximum etching rate of AZO films is 70.45 nm min ^−1 . The ion composition of Cl _2 /Ar plasma was determined by optical emission spectrometry (OES). The chemical reactions on the surface of AZO films were analyzed by x-ray photoelectron spectroscopy (XPS)

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