Sung-Yong Min

Large-scale organic nanowire lithography and electronics

Controlled alignment and patterning of individual semiconducting nanowires at a desired position in a large area is a key requirement for electronic device applications. High-speed, large-area printing of highly aligned individual nanowires that allows control of the exact numbers of wires, and their orientations and dimensions is a significant challenge for practical electronics applications. Here we use a high-speed electrohydrodynamic organic nanowire printer to print large-area organic semiconducting nanowire arrays directly on device substrates in a precisely, individually controlled manner; this method also enables sophisticated large-area nanowire lithography for nano-electronics. We achieve a maximum field-effect mobility up to 9.7 cm(2) V(-1) s(-1) with extremely low contact resistance (<5.53 Ω cm), even in nano-channel transistors based on single-stranded semiconducting nanowires. We also demonstrate complementary inverter circuit arrays comprising well-aligned p-type and n-type organic semiconducting nanowires. Extremely fast nanolithography using printed semiconducting nanowire arrays provide a simple, reliable method of fabricating large-area and flexible nano-electronics.

Molecularly Controlled Interfacial Layer Strategy Toward Highly Efficient Simple-Structured Organic Light-Emitting Diodes

A highly efficient simplified organic light-emitting diode (OLED) with a molecularly controlled strategy to form near-perfect interfacial layer on top of the anode is demonstrated. A self-organized polymeric hole injection layer (HIL) is exploited increasing hole injection, electron blocking, and reducing exciton quenching near the electrode or conducting polymers; this HIL allows simplified OLED comprised a single small-molecule fluorescent layer to exhibits a high current efficiency (∼20 cd/A).

Organic core-sheath nanowire artificial synapses with femtojoule energy consumption

Researchers report an organic nanowire artificial synapse that emulates working principles and energy consumption of biological synapses. Emulation of biological synapses is an important step toward construction of large-scale brain-inspired electronics. Despite remarkable progress in emulating synaptic functions, current synaptic devices still consume energy that is orders of magnitude greater than do biological synapses (~10 fJ per synaptic event). Reduction of energy consumption of artificial synapses remains a difficult challenge. We report organic nanowire (ONW) synaptic transistors (STs) that emulate the important working principles of a biological synapse. The ONWs emulate the morphology of nerve fibers. With a core-sheath–structured ONW active channel and a well-confined 300-nm channel length obtained using ONW lithography, ~1.23 fJ per synaptic event for individual ONW was attained, which rivals that of biological synapses. The ONW STs provide a significant step toward realizing low-energy–consuming artificial intelligent electronics and open new approaches to assembling soft neuromorphic systems with nanometer feature size.

N-Doped Graphene Field-Effect Transistors with Enhanced Electron Mobility and Air-stability

Although graphene can be easily p-doped by various adsorbates, developing stable n-doped graphene that is very useful for practical device applications is a difficult challenge. We investigated the doping effect of solution-processed (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) on chemical-vapor-deposited (CVD) graphene. Strong n-type doping is confirmed by Raman spectroscopy and the electrical transport characteristics of graphene field-effect transistors. The strong n-type doping effect shifts the Dirac point to around -140 V. Appropriate annealing at a low temperature of 80 ºC enables an enhanced electron mobility of 1150 cm(2) V(-1) s(-1). The work function and its uniformity on a large scale (1.2 mm × 1.2 mm) of the doped surface are evaluated using ultraviolet photoelectron spectroscopy and Kelvin probe mapping. Stable electrical properties are observed in a device aged in air for more than one month.

Non-Volatile Ferroelectric Memory with Position-Addressable Polymer Semiconducting Nanowire

One-dimensional nanowires (NWs) have been extensively examined for numerous potential nano-electronic device applications such as transistors, sensors, memories, and photodetectors. The ferroelectric-gate field effect transistors (Fe-FETs) with semiconducting NWs in particular in combination with ferroelectric polymers as gate insulating layers have attracted great attention because of their potential in high density memory integration. However, most of the devices still suffer from low yield of devices mainly due to the ill-control of the location of NWs on a substrate. NWs randomly deposited on a substrate from solution-dispersed droplet made it extremely difficult to fabricate arrays of NW Fe-FETs. Moreover, rigid inorganic NWs were rarely applicable for flexible non-volatile memories. Here, we present the NW Fe-FETs with position-addressable polymer semiconducting NWs. Polymer NWs precisely controlled in both location and number between source and drain electrode were achieved by direct electrohydrodynamic NW printing. The polymer NW Fe-FETs with a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) exhibited non-volatile ON/OFF current margin at zero gate voltage of approximately 10(2) with time-dependent data retention and read/write endurance of more than 10(4) seconds and 10(2) cycles, respectively. Furthermore, our device showed characteristic bistable current hysteresis curves when being deformed with various bending radii and multiple bending cycles over 1000 times.

Rapid Fabrication of Designable Large-Scale Aligned Graphene Nanoribbons by Electro-hydrodynamic Nanowire Lithography

A new technique, electro-hydrodynamic nanowire (e-NW) lithography , is demonstrated for the rapid, inexpensive, and efficient fabrication of graphene nanorib bons (GNRs) on a large scale while simultaneously controlling the location and alignment of the GNRs. A series of interesting GNR architectures, including parallel lines, grids, ladders, and stars are produced. A sub-10-nm-wide GNR is obtained to fabricate field-effect transistors that show a room-temperature on/off current ratio of ca. 70.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

Individually Position‐Addressable Metal‐Nanofiber Electrodes for Large‐Area Electronics

A individually position-addressable large-scale-aligned Cu nanofiber (NF) array is fabricated using electro-hydrodynamic nanowire printing. The printed single-stranded Cu NF has a diameter of about 710 nm and resistivity of 14.1 μΩ cm and is effectively used as source/drain nanoelectrode in pentacene transistors, which show a 25-fold increased hole mobility than that of a device with Cu thin-film electrodes.

Versatile Metal Nanowiring Platform for Large-Scale Nano- and Opto-Electronic Devices.

A versatile metal nanowiring platform enables the fabrication of Ag nanowires (AgNW) at a desired position and orientation in an individually controlled manner. A printed, flexible AgNW has a diameter of 695 nm, a resistivity of 5.7 μΩ cm, and good thermal stability in air. Based on an Ag nanowiring platform, an all-NW transistors array, as well as various optoelectronic applications, are successfully demonstrated.

Electrospun polymer/quantum dot composite fibers as down conversion phosphor layers for white light-emitting diodes

Color control without severe photoluminescence (PL) quenching is one of main issues in white light emission technology. White light emission was successfully achieved using phosphor layers made of electrospun quantum dot (QD) embedded polymer fibers as color down-conversion layers of blue light-emitting diodes (LEDs). Down-conversion from blue to longer wavelength was characterized by fluorescence microscopy and photoluminescence (PL) spectroscopy. Using orange QD-embedded fiber-based phosphor layers, a broad spectrum of white-light was demonstrated with the CIE coordination of (0.367, 0.367). The QDs in the polymer fiber matrix did not show the aggregation of QDs unlike the QDs in a thin film matrix. Furthermore, from Time-Correlated Single Photon Counting (TCSPC) analysis, the QDs in fiber mats have longer PL lifetime (∼3.95 ns) than that in a thin film matrix (∼3.20 ns) due to the lower aggregation-induced luminescence concentration quenching. Our results suggest that the simple electrospinning method may be a very good method to obtain uniform and bright QD phosphors for white LEDs which can be used for solid-state illumination sources and lighting devices.