Showing papers by "Jong Hyun Ahn published in 2021"

Impact of 2D-3D Heterointerface on Remote Epitaxial Interaction through Graphene.

Abstract: Remote epitaxy has drawn attention as it offers epitaxy of functional materials that can be released from the substrates with atomic precision, thus enabling production and heterointegration of flexible, transferrable, and stackable freestanding single-crystalline membranes. In addition, the remote interaction of atoms and adatoms through two-dimensional (2D) materials in remote epitaxy allows investigation and utilization of electrical/chemical/physical coupling of bulk (3D) materials via 2D materials (3D-2D-3D coupling). Here, we unveil the respective roles and impacts of the substrate material, graphene, substrate-graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film. We show that simply coating a graphene layer on wafers does not guarantee successful implementation of remote epitaxy, since atomically precise control of the graphene-coated interface is required, and provides key considerations for maximizing the remote electrostatic interaction between the substrate and adatoms. This was enabled by exploring various material systems and processing conditions, and we demonstrate that the rules of remote epitaxy vary significantly depending on the ionicity of material systems as well as the graphene-substrate interface and the epitaxy environment. The general rule of thumb discovered here enables expanding 3D material libraries that can be stacked in freestanding form.

2D Materials for Skin-Mountable Electronic Devices.

Abstract: Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Large-area synthesis of transition metal dichalcogenides via CVD and solution-based approaches and their device applications.

Abstract: For the last decade, two-dimensional transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their unique physical and chemical properties. Novel devices based on these materials are commonly fabricated using the exfoliated samples, which lacks control of the thickness and cannot be scaled. Therefore, the synthesis of large-area TMDC thin films with a high uniformity to advance the field is required. This article reviews the latest advances in the synthesis of wafer-scale thin films using chemical vapor deposition methods. The key factors that determine the electrical performance of TMDCs are introduced, including the interfacial properties and defects. The latest solution-based techniques which suggest the opportunity to obtain large-area TMDC thin films with a low-cost process and the potential applications in electronics and optoelectronics are also discussed. The outlook for future research directions, challenges, and possible development of 2D materials are further discussed.

MoS2/Graphene Photodetector Array with Strain-Modulated Photoresponse up to the Near-Infrared Regime.

Abstract: MoS2, an emerging material in the field of optoelectronics, has attracted the attention of researchers owing to its high light absorption efficiency, even as an atomically thin layer. However, the covered spectra of the reported MoS2-based photodetectors are restricted to the visible range owing to their electronic bandgap (∼1.9 eV). Strain engineering, which modulates the bandgap of a semiconductor, can extend the application coverage of MoS2 to the infrared spectral range. The shrinkage of the bandgap because of the tensile strain on MoS2 enhances the photoresponsivity in the visible range and extends its sensing capability beyond its fundamental absorption limit. Herein, we report a graphene/MoS2/graphene metal-semiconductor-metal photodetector (PD) array with a strain-modulated photoresponse up to the spectral range of the near-infrared (NIR). The MoS2 PD array on a flexible substrate was stretched in the biaxial direction to a tensile strain level of 1.19% using a pneumatic bulging process. The MoS2-based line-scanning system was implemented by digitizing the output photocurrent of the strained MoS2 linear array with a low-noise complementary metal-oxide-semiconductor (CMOS) readout integrated circuit (IC) and successfully captured vis-NIR images in foggy conditions. Therefore, we extended the application of the MoS2 PD array to the NIR regime and demonstrated its use in real-life imaging systems.

Biologically Plausible Artificial Synaptic Array: Replicating Ebbinghaus' Memory Curve with Selective Attention.

Abstract: The nature of repetitive learning and oblivion of memory enables humans to effectively manage vast amounts of memory by prioritizing information for long-term storage. Inspired by the memorization process of the human brain, an artificial synaptic array is presented, which mimics the biological memorization process by replicating Ebbinghaus' forgetting curve. To construct the artificial synaptic array, signal-transmitting access transistors and artificial synaptic memory transistors are designed using indium-gallium-zinc-oxide and poly(3-hexylthiophene), respectively. To secure the desired performance of the access transistor in regulating the input signal to the synaptic transistor, the content of gallium in the access transistor is optimized. In addition, the operation voltage of the synaptic transistor is carefully selected to achieve memory-state efficiency. Repetitive learning characterizing Ebbinghaus' oblivion curves is realized using an artificial synaptic array with optimized conditions for both transistor components. This successfully demonstrates a biologically plausible memorization process. Furthermore, selective attention for information prioritization in the human brain is mimicked by selectively applying repetitive learning to a synaptic transistor with a high memory state. The demonstrated biologically plausible artificial synaptic array provides great scope for advancement in bioinspired electronics.

Graphene-Based Nanomaterials for Flexible and Stretchable Batteries.

Abstract: Recently, as flexible and wearable electronic devices have become widely popular, research on light weight and large-capacity batteries suitable for powering such devices has been actively conducted. In particular, graphene has attracted considerable attention from researchers in the battery field owing to its good mechanical properties and its applicability in various processes to fabricate electrodes for batteries. Graphene is classified into two types: flake-type, fabricated from graphite, and film-type, synthesized using chemical vapor deposition. The unique processes involved in these two types enable the fabrication of flexible and stretchable batteries with various shapes and functions. In this article, the recent progress in the development of flexible and stretchable batteries based on graphene, as well as its important technical issues are reviewed.

Crypto primitive of MOCVD MoS2 transistors for highly secured physical unclonable functions

Abstract: Physically unclonable crypto primitives have potential applications for anti-counterfeiting, identification, and authentication, which are clone proof and resistant to variously physical attack. Conventional physical unclonable function (PUF) based on Si complementary metal-oxide-semiconductor (CMOS) technologies greatly suffers from entropy loss and bit instability due to noise sensitivity. Here we grow atomically thick MoS2 thin film and fabricate field-effect transistors (FETs). The inherently physical randomness of MoS2 transistors from materials growth and device fabrication process makes it appropriate for the application of PUF device. We perform electrical characterizations of MoS2 FETs, collect the data from 448 devices, and generate PUF keys by splitting drain current at specific levels to evaluate the response performance. Proper selection of splitting threshold enables to generate binary, ternary, and double binary keys. The generated PUF keys exhibit good randomness and uniqueness, providing a possibility for harvesting highly secured PUF devices with two-dimensional materials.

Damage-free transfer mechanics of 2-dimensional materials: competition between adhesion instability and tensile strain

Abstract: The transfer of two-dimensional (2D) materials is crucial to the realization of 2D material-based devices for practical applications. The thinness of 2D materials renders them prone to mechanical damage during the transfer process and to degradation of their superior electrical and mechanical properties. Herein, the mechanisms involved in the damage of chemical vapor deposition-grown graphene (Gr) and MoS2 are investigated during a roll-based transfer process. We identify two different damage mechanisms, i.e., instability-induced damage and tensile strain-induced damage. The two mechanisms compete, depending on the thickness of the transfer medium, and induce dissimilar damage. By minimizing these two mechanisms, we realize and demonstrate the damage-free transfer of 2D materials. The sheet resistance and mobility of transferred Gr are 235 ± 29 Ω sq–1 and 2250 cm2 V–1 s–1, respectively, with no microscopic cracks or tear-out damage. We observe instability-induced damage to be ubiquitous in monolayer MoS2, thin metals, and thin oxide films. By understanding the instability-induced damage mechanism, a broad range of 2D materials and thin films can be transferred without mechanical damage. Damage-free transfer will contribute to the high-yield fabrication of 2D material-based electronic devices. The mechanisms involved in the damage of CVD-grown graphene (Gr) and MoS2 are investigated during a roll-based transfer process. We identify two different damage mechanisms, i.e., instability-induced damage and tensile strain-induced damage. The two mechanisms compete, depending on the thickness of the transfer medium, and induce dissimilar damage. By optimizing the thickness, we realize and demonstrate the damage-free transfer of 2D materials. The sheet resistance and mobility of transferred Gr are 235 ± 29 Ω sq–1 and 2250 cm2 V–1 s–1, respectively, with no microscopic cracks or tear-out damage.

Graphene-electrode array for brain map remodeling of the cortical surface

Abstract: Cortical maps, which are indicative of cognitive status, are shaped by the organism’s experience. Previous mapping tools, such as penetrating electrodes and imaging techniques, are limited in their ability to be used to assess high-resolution brain maps largely owing to their invasiveness and poor spatiotemporal resolution, respectively. In this study, we developed a flexible graphene-based multichannel electrode array for electrocorticography (ECoG) recording, which enabled us to assess cortical maps in a time- and labor-efficient manner. The flexible electrode array, formed by chemical vapor deposition (CVD)-grown graphene, provided low impedance and electrical noise because a good interface between the graphene and brain tissue was created, which improved the detectability of neural signals. Furthermore, cortical map remodeling was induced upon electrical stimulation at the cortical surface through a subset of graphene spots. This result demonstrated the macroscale plasticity of cortical maps, suggesting perceptual enhancement via electrical rehabilitation at the cortical surface. The spatial organization of neural networks representing sensorimotor behavior and cognition has been mapped by flexible devices placed on the cortex, the outer layer of the brain. Sensory messages from each part of the body are processed in a specific area in the brain and maps of the cortex can help understand these areas. A person’s cortical map is shaped by their early development and experiences. Study of such maps can help identify and treat sensory disorders. Existing techniques for brain mapping require placement of penetrating electrodes which is time-consuming and risks brain damage. Minseok Lee from City University of Hong Kong and colleagues made arrays of electrodes using graphene membranes and constructed sensory maps of rats and mice by placing these arrays directly on the cortical surface. The timely and continuous measurement of cortical maps is required for studying the nature and plasticity of brain maps. In this work, we developed the multichannel graphene array that enables high-resolution brain mapping, facilitating rapid and repetitive assessments of brain maps. The advanced graphene array with intervening thru-hole enables large-scale mapping simultaneously in the surface and deep of cortical areas, also improving conformality for better detection of electrocorticography signals. In a subset of the graphene array, cortical surface stimulation can remodel cortical maps, therein enhancing cortical plasticity. This technology provides potential therapeutic applications for various brain disorders by correcting brain maps.

Si nanomebranes: Material properties and applications

Abstract: Silicon (Si) has widely been used as an essential material in the modern semiconductor industry. Recently, new attempts have been actively made to apply Si to a variety of fields such as flexible electronic devices and biosensors by manufacturing Si nanomembranes (NMs) having nanometer thickness. In particular, as the thickness of Si is reduced to a nanometer scale, its mechanical, electrical, and optical properties differ from that of its bulk form, which provides opportunities for the development of new conceptual devices. In this review, we present recent advances in Si NM technology that exhibit functional features different from the bulk materials. In addition, we discuss the opportunities and current challenges related to this field.