Jaekyun Kim

Large-Scale Organic Single-Crystal Thin Films and Transistor Arrays via the Evaporation-Controlled Fluidic Channel Method

We report a facile and versatile approach for fabricating large-area organic thin film transistor (OTFTs) arrays via a fluidic channel method. Evaporation-controlled fluidic channel-containing organic semiconductors easily produce large-area organic single-crystalline thin films in a quite uniform manner. The unidirectional movement of the meniscus and the subsequent film growth via solvent evaporation inside the fluidic channel correspond to the simulation based on the finite element method. Utilizing this fluidic channel method, we fabricated high-performance 6,13-bis(triisopropylsilylethynyl)pentacene OTFT arrays with average and maximal mobilities of 0.71 and 2.18 cm(2) V(-1) s(-1), respectively, while exhibiting current on:off ratios of >1 × 10(6). We claim that this scalable fluidic channel method offers a competitive way to fabricate high-performance and large-area organic semiconductor devices for a variety of applications.

Ultrasensitive Room-Temperature Operable Gas Sensors Using p-Type Na:ZnO Nanoflowers for Diabetes Detection

Ultrasensitive room-temperature operable gas sensors utilizing the photocatalytic activity of Na-doped p-type ZnO (Na:ZnO) nanoflowers (NFs) are demonstrated as a promising candidate for diabetes detection. The flowerlike Na:ZnO nanoparticles possessing ultrathin hierarchical nanosheets were synthesized by a facile solution route at a low processing temperature of 40 °C. It was found that the Na element acting as a p-type dopant was successfully incorporated in the ZnO lattice. On the basis of the synthesized p-type Na:ZnO NFs, room-temperature operable chemiresistive-type gas sensors were realized, activated by ultraviolet (UV) illumination. The Na:ZnO NF gas sensors exhibited high gas response (S of 3.35) and fast response time (∼18 s) and recovery time (∼63 s) to acetone gas (100 ppm, UV intensity of 5 mW cm-2), and furthermore, subppm level (0.2 ppm) detection was achieved at room temperature, which enables the diagnosis of various diseases including diabetes from exhaled breath.

Water-Mediated Photochemical Treatments for Low-Temperature Passivation of Metal-Oxide Thin-Film Transistors

The low-temperature electrical passivation of an amorphous oxide semiconductor (AOS) thin-film transistor (TFT) is achieved by a deep ultraviolet (DUV) light irradiation-water treatment-DUV irradiation (DWD) method. The water treatment of the first DUV-annealed amorphous indium-gallium-zinc-oxide (a-IGZO) thin film is likely to induce the preferred adsorption of water molecules at the oxygen vacancies and leads to subsequent hydroxide formation in the bulk a-IGZO films. Although the water treatment initially degraded the electrical performance of the a-IGZO TFTs, the second DUV irradiation on the water-treated devices may enable a more complete metal-oxygen-metal lattice formation while maintaining low oxygen vacancies in the oxide films. Overall, the stable and dense metal-oxygen-metal (M-O-M) network formation could be easily achieved at low temperatures (below 150 °C). The successful passivation of structural imperfections in the a-IGZO TFTs, such as hydroxyl group (OH-) and oxygen vacancies, mainly results in the enhanced electrical performances of the DWD-processed a-IGZO TFTs (on/off current ratio of 8.65 × 10(9), subthreshold slope of 0.16 V/decade, an average mobility of >6.94 cm(2) V(-1) s(-1), and a bias stability of ΔVTH < 2.5 V), which show more than a 30% improvement over the simple DUV-treated a-IGZO TFTs.

Brain-Inspired Photonic Neuromorphic Devices using Photodynamic Amorphous Oxide Semiconductors and their Persistent Photoconductivity

The combination of a neuromorphic architecture and photonic computing may open up a new era for computational systems owing to the possibility of attaining high bandwidths and the low-computation-power requirements. Here, the demonstration of photonic neuromorphic devices based on amorphous oxide semiconductors (AOSs) that mimic major synaptic functions, such as short-term memory/long-term memory, spike-timing-dependent plasticity, and neural facilitation, is reported. The synaptic functions are successfully emulated using the inherent persistent photoconductivity (PPC) characteristic of AOSs. Systematic analysis of the dynamics of photogenerated carriers for various AOSs is carried out to understand the fundamental mechanisms underlying the photoinduced carrier-generation and relaxation behaviors, and to search for a proper channel material for photonic neuromorphic devices. It is found that the activation energy for the neutralization of ionized oxygen vacancies has a significant influence on the photocarrier-generation and time-variant recovery behaviors of AOSs, affecting the PPC behavior.

Scalable Sub-micron Patterning of Organic Materials Toward High Density Soft Electronics

The success of silicon based high density integrated circuits ignited explosive expansion of microelectronics. Although the inorganic semiconductors have shown superior carrier mobilities for conventional high speed switching devices, the emergence of unconventional applications, such as flexible electronics, highly sensitive photosensors, large area sensor array, and tailored optoelectronics, brought intensive research on next generation electronic materials. The rationally designed multifunctional soft electronic materials, organic and carbon-based semiconductors, are demonstrated with low-cost solution process, exceptional mechanical stability, and on-demand optoelectronic properties. Unfortunately, the industrial implementation of the soft electronic materials has been hindered due to lack of scalable fine-patterning methods. In this report, we demonstrated facile general route for high throughput sub-micron patterning of soft materials, using spatially selective deep-ultraviolet irradiation. For organic and carbon-based materials, the highly energetic photons (e.g. deep-ultraviolet rays) enable direct photo-conversion from conducting/semiconducting to insulating state through molecular dissociation and disordering with spatial resolution down to a sub-μm-scale. The successful demonstration of organic semiconductor circuitry promise our result proliferate industrial adoption of soft materials for next generation electronics.

Potential of pH-Sensitive Polymer-Anchored Cationic Liposomes for Combinatorial Anticancer Therapy with Doxorubicin and siRNA

To explore a potential of pH-sensitive polymer-liposome complexes for the tumor-specific combinatorial delivery of anticancer agents and siRNA, conventional liposomes (ConL), polymer-liposome complexes (PLC) and polymer-cationic liposome complexes (PCLC) were prepared. Pluronic P104-based multiblock copolymer (MBCP-2) was included as pH-sensitive polymer. Physicochemical properties, release under different pH, cytotoxicity and in vitro cellular uptake of DOX-loaded liposomes were investigated. From the release test, an acidic pH was determined to be an important factor for release from the PLC vehicles. The novel PLC vehicle itself showed low cytotoxicity demonstrating suitable viability. Observing cellular uptake of DOX by confocal microscopy imaging, a greater amount of DOX was delivered to cells with the pH-sensitive polymer- anchored vehicles than that with free DOX and ConL. It was verified that the novel vehicles could effectively deliver both DOX and GFP-siRNA. Novel pH-sensitive PCLC have a potential for targeted therapy of anticancer agents and gene therapy under acidic tumor microenvironment.

High electron mobility of β-HgS colloidal quantum dots with doubly occupied quantum states

Electron occupation of the lowest electronic state of the conduction band (1Se) of a semiconducting nanocrystal offers numerous opportunities to efficiently utilize the quantization of the colloidal quantum dot. The steady-state electron occupation of the 1Se gives rise to unprecedented electrical, optical, and magnetic properties. We report an electron mobility of ∼1.29 cm2 V−1 s−1 measured in a mercury sulfide (β-HgS) quantum dot field effect transistor (FET), demonstrating the best carrier mobility for the HgS colloidal nanocrystal solid. The high electron mobility of the HgS nanocrystals with the doubly occupied quantum state originates from the efficient ligand exchange from oleylamine to thiocyanate, better carrier hopping via shortened inter-dot-distance, and the packing of nanocrystals by optimized thermal annealing conditions.

High-performance and scalable metal-chalcogenide semiconductors and devices via chalco-gel routes

Chalco-gel–based metal-chalcogenide semiconductors and scalable high-performance electronic devices were demonstrated. We report a general strategy for obtaining high-quality, large-area metal-chalcogenide semiconductor films from precursors combining chelated metal salts with chalcoureas or chalcoamides. Using conventional organic solvents, such precursors enable the expeditious formation of chalco-gels, which are easily transformed into the corresponding high-performance metal-chalcogenide thin films with large, uniform areas. Diverse metal chalcogenides and their alloys (MQx: M = Zn, Cd, In, Sb, Pb; Q = S, Se, Te) are successfully synthesized at relatively low processing temperatures (<400°C). The versatility of this scalable route is demonstrated by the fabrication of large-area thin-film transistors (TFTs), optoelectronic devices, and integrated circuits on a 4-inch Si wafer and 2.5-inch borosilicate glass substrates in ambient air using CdS, CdSe, and In2Se3 active layers. The CdSe TFTs exhibit a maximum field-effect mobility greater than 300 cm2 V−1 s−1 with an on/off current ratio of >107 and good operational stability (threshold voltage shift < 0.5 V at a positive gate bias stress of 10 ks). In addition, metal chalcogenide–based phototransistors with a photodetectivity of >1013 Jones and seven-stage ring oscillators operating at a speed of ~2.6 MHz (propagation delay of < 27 ns per stage) are demonstrated.

High-performance organic circuits based on precisely aligned single-crystal arrays

In this paper, we demonstrate high-performance organic logic circuits based on precisely controlled organic single-crystal arrays. Well-aligned microrod shaped 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single-crystal organic thin-film-transistors (OTFTs) were fabricated via solvent mediated molecular tailoring with a polymeric sacrificial layer, exhibiting saturation mobility of >2 cm2 V−1 s−1. Using this approach, precise placement of organic crystal arrays in a controlled orientation was successfully achieved, enabling the fabrication of OTFT-based inverter circuits with a gain of 1.37 (V V−1). Furthermore, it was demonstrated that, by varying the number of single-crystal microrods, the device dimension and corresponding circuit performance can be modulated. A high-performance inverter operation with various interdigitating single-crystal microrod arrays can thus be achieved.

Static and Dynamic Water Motion-Induced Instability in Oxide Thin-Film Transistors and Its Suppression by Using Low-k Fluoropolymer Passivation

Here, we report static and dynamic water motion-induced instability in indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) and its effective suppression with the use of a simple, solution-processed low-k (ε ∼ 1.9) fluoroplastic resin (FPR) passivation layer. The liquid-contact electrification effect, in which an undesirable drain current modulation is induced by a dynamic motion of a charged liquid such as water, can cause a significant instability in IGZO TFTs. It was found that by adopting a thin (∼44 nm) FPR passivation layer for IGZO TFTs, the current modulation induced by the water-contact electrification was greatly reduced in both off- and on-states of the device. In addition, the FPR-passivated IGZO TFTs exhibited an excellent stability to static water exposure (a threshold voltage shift of +0.8 V upon 3600 s of water soaking), which is attributed to the hydrophobicity of the FPR passivation layer. Here, we discuss the origin of the current instability caused by the liquid-contact electrification as well as various static and dynamic stability tests for IGZO TFTs. On the basis of our findings, we believe that the use of a thin, solution-processed FPR passivation layer is effective in suppressing the static and dynamic water motion-induced instabilities, which may enable the realization of high-performance and environment-stable oxide TFTs for emerging wearable and skin-like electronics.