Min Suk Oh

Low-temperature annealed PbS quantum dot films for scalable and flexible ambipolar thin-film-transistors and circuits

Thiocyanate (SCN)-treated lead sulfide (PbS) quantum dot thin-film-transistors (QD TFTs) and CMOS-compatible circuits were fabricated on a flexible substrate via a scalable photolithography process. Spectroscopic and electrical investigations demonstrated that the thermal treatments induce ligand decomposition and densification of the QD arrays at around 170 °C. High temperature annealing above 200 °C induces an aggregation of the QD particles, resulting in a degradation of device performance, such as the field-effect mobility and the on-/off-current ratio. It is also noted that the surface defects which act as charge carrier traps are increased with the annealing temperature, possibly due to the decomposition of the SCN leading to an aggregation of the QD particles. On the basis of the experimental results, bottom-gate and bottom-contact ambipolar PbS QD TFTs with an electron/hole mobility of 0.47/0.43 cm2 V−1 s−1 and CMOS inverter circuits with gains of >14 V at a supply bias of 10 V were successfully fabricated on spin-on thin plastic substrates.

Crack-induced Ag nanowire networks for transparent, stretchable, and highly sensitive strain sensors

Crack-based strain sensor systems have been known for its high sensitivity, but suffer from the small fracture strain of the thin metal films employed in the sensor which results in its negligible stretchability. Herein, we fabricated a transparent (>90% at 550u2009nm wavelength), stretchable (up to 100%), and sensitive (gauge factor (GF) of 30 at 100% strain) strain gauge by depositing an encapsulated crack-induced Ag nanowire (AgNW) network on a hydroxylated poly(dimethylsiloxane) (PDMS) film. Stretching the encapsulated AgNWs/PDMS resulted in the formation of a percolation network of nanowire ligaments with abundant percolation paths. The encapsulating polymer was designed to adhere strongly to both the AgNW and PDMS. The improved adhesion ensured the resistance of the crack-induced network of AgNWs varied reversibly, stably, and sensitively when stretched and released, at strains of up to 100%. The developed sensor successfully detected human motions when applied to the skin.

Increased shell thickness in indium phosphide multishell quantum dots leading to efficiency and stability enhancement in light-emitting diodes

We report efficient indium phosphide (InP) quantum dot-based light-emitting diodes (QD-LEDs). The current efficiency and the device stability of QD-LEDs were enhanced by increasing the thickness of ZnS outer shell of InP/ZnSe/ZnS multishell QDs. Reversible luminance degradation was observed in operation of QD-LEDs, which was hypothesized to result from QD charging. QDs having thicker ZnS shell with strong confinement suppressed the luminescence quenching as well as QD charging. Our findings about the reversible QD charging and the developed performance by the thick ZnS outer shell would help to rationalize the luminance quenching issue in QD-LED operation.

Electrical Contact Analysis of Multilayer MoS 2 Transistor With Molybdenum Source/Drain Electrodes

We demonstrate a two-dimensional (2D) multilayered molybdenum disulfide (MoS2) transistor with molybdenum (Mo) side and edge contacts, which is deposited using a dc-sputtering method. It exhibits field-effect mobility of 23.9 cm2/Vs and ON/OFF ratio of 106 in a linear region. A current-voltage study under different temperatures (300-393 K) reveals that the Mo-MoS2 transistor shows a band transport characteristics, and a Schottky barrier height of 0.14 eV is estimated using a thermionic emission theory. Finally, the side and edge contacts of Mo-MoS2 are confirmed through the transmission electron microscope analysis. Our results not only show that Mo can be an alternative contact metal to other low work-function metals but also that the edge contact may play an important role in resolving the performance degradation over thickness increase of the MoS2 channel layer.

Flexible and Wavelength-Selective MoS2 Phototransistors with Monolithically Integrated Transmission Color Filters.

Color-selective or wavelength-tunable capability is a crucial feature for two-dimensional (2-D) semiconducting material-based image sensor applications. Here, we report on flexible and wavelength-selective molybdenum disulfide (MoS2) phototransistors using monolithically integrated transmission Fabry-Perot (F-P) cavity filters. The fabricated multilayer MoS2 phototransistors on a polyarylate substrate exhibit decent electrical characteristics (μFEu2009>u200964.4u2009cm2/Vs, on/off ratiou2009>u2009106), and the integrated F-P filters, being able to cover whole visible spectrum, successfully modulate the spectral response characteristics of MoS2 phototransistors from ~495u2009nm (blue) to ~590u2009nm (amber). Furthermore, power dependence of both responsivity and specific detectivity shows similar trend with other reports, dominated by the photogating effect. When combined with large-area monolayer MoS2 for optical property enhancement and array processing, our results can be further developed into ultra-thin flexible photodetectors for wearables, conformable image sensor, and other optoelectronic applications.

Effects of oxide electron transport layer on quantum dots light emitting diode with an organic/inorganic hybrid structure

We report on the effects of an oxide semiconductor as an electron transport layer (ETL) on a quantum dots light emitting diode (QD-LED). To improve the properties of QD-LED, we optimized the process parameters for the deposition and post-annealing steps of an oxide ETL. When zinc tin oxide (ZTO) was deposited by radio-frequency magnetron sputtering in a gas mixture of argon and oxygen (Ar : O2 = 2 : 1) and then annealed under 760 Torr O2 for 10 min, our QD-LED showed improved luminescence characteristics. Additionally, to overcome the problem of non-uniform luminescence, we optimized the concentration and process conditions of colloidal quantum dot materials. Finally, we fabricated QD-LED devices with luminescence of 4,874 cd/m2 and luminous efficiency of 2.68 cd/A.

Photo-induced fabrication of Ag nanowire circuitry for invisible, ultrathin, conformable pressure sensors

We demonstrate ultrathin (thickness 88% at 550 nm), and conformable pressure-induced bending sensors with unprecedented flexibility and stretchability, produced by developing a photo-induced pattern of silver nanowires (AgNWs) on a 1.4 μm-thick polyethylene terephthalate (PET) sheet. This patterning approach does not require any additional materials to form a patterned barrier for AgNW etching or to enhance the adhesion between the AgNWs and PET. Simple irradiation using pulsed light on a masked AgNWs/1.4 μm-thick PET assembly followed by sonication formed a finely patterned AgNW network well-adhered to the underlying PET without impacting the optical transparency of the ultrathin PET. A pressure-induced bending-sensitive capacitive sensor fabricated by this approach was extremely flexible and stretch-compatible up to 100% uniaxial strain. This sensor, based on a simple tandem compound pattern, was reproducible, durable, and 90% more sensitive than an elastomeric pressure sensor made using the same sensor design. The functionality of the developed sensor system was successfully demonstrated in a sensitive acupressure sensor mounted on a gloved fingertip, in which the capacitance coincidently varied with the force applied to the fingertip.

Inverted InP quantum dot light-emitting diodes using low-temperature solution-processed metal–oxide as an electron transport layer

The present work shows the inverted InP quantum dot light-emitting diodes (QD-LEDs) with inorganic metal oxide layers. In the inverted structure of ITO/ZnO/InP QDs/CBP/MoO3/Al, a sol?gel derived ZnO film was used as an electron transport layer (ETL) and MoO3 was used as a hole injection layer (HIL). In contrary to high annealing temperature (>200 ?C) for conventional ZnO films, low temperature annealing (?150 ?C) was performed for sol?gel derived ZnO film. The performance of the inverted QD-LEDs was efficiently improved by optimization of the annealing time and temperature of ZnO ETL. The current efficiency was significantly improved about 215% by lowering annealing temperature of ZnO ETL.

Study of ethanolamine surface treatment on the metal-oxide electron transport layer in inverted InP quantum dot light-emitting diodes

The present work shows the effect of ethanolamine surface treatment on inverted InP quantum dot light-emitting diodes (QD-LEDs) with inorganic metal oxide layers. In the inverted structure of ITO/ZnO/InP QDs/CBP/MoO3/Al, a sol-gel derived ZnO film was used as an electron transport layer (ETL) and MoO3 was used as a hole injection layer (HIL). First, ethanolamine was treated as a surface modifier on top of the ZnO electron transport layer. The optical performance of the QD-LED device was improved by the ethanolamine surface treatment. Second, low temperature annealing (<200°C) was performed on the ZnO sol-gel electron transport layer, followed by an investigation of the effect of the ZnO annealing temperature. The efficiency of the inverted QD-LEDs was significantly enhanced (more than 3-fold) by optimization of the ZnO annealing temperature.

Electrical aging effect of ZnS based quantum dots for white light-emitting diodes

The present work reports cadmium-free colloidal ZnS:Al quantum dot (QD) based white quantum dot light-emitting diodes (QD-LEDs). The device was fabricated with a structure of ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al using synthesized ZnS:Al QDs which has 393 nm of peak wavelength and sub peaks in visible wavelength. White emission with high color rending index (CRI) was achieved by the combination of blue emission from PVK and ZnS:Al QDs, electroplex emission at the interface between PVK and ZnS:Al QDs, and Al traps/defects emission, which are controlled by electrical aging effect. The characteristic of our device shows the potential for spectrum tunable and Cd-free white QD-LEDs in the near future.