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All-Optical Ultrasound Transducer Using CNT-PDMS and Etalon Thin-Film Structure
Geonwook Yoo,Hyunmin Yoon,Jeongmin Heo,Thakur, Ujwal Kumar,Hui Joon Park,Hyoung Won Baac,Junseok Heo IEEE 2015 IEEE photonics journal Vol.7 No.6
<P>Compared with conventional piezoelectric transductions, an all-optical high-frequency ultrasound (HFUS) transducer is a promising modality for high-resolution ultrasound imaging. We demonstrate an all-optical HFUS transducer by integrating a carbon nanotube-polydimethylsiloxane composite film with an etalon thin-film structure incorporating SiO<SUB>2</SUB>/TiO<SUB>2</SUB> distributed Bragg reflectors and an SU-8 resonator. The optical and acoustic characteristics are investigated for two different configurations (forward and backward operation modes). The maximum amplitude of the pulse echo in backward mode is approximately twofold higher than that of the forward mode. This difference is contributed by the increased reflectance and the absorptive loss of the incident pulsed laser in the forward mode. The pulse echo from the transducer exhibits a broad frequency bandwidth of 27 MHz. Furthermore, the scalability of the 2-D all-optical transducer array is also evaluated by characterizing the optical properties of the etalon across an area of 0.1 × 0.2 mm<SUP>2</SUP>. Our experimental results show that the proposed transducer is a promising candidate for high-resolution ultrasound imaging systems.</P>
Yoo, Geonwook,Lee, Sungho,Yoo, Byoungwook,Han, Chuljong,Kim, Sunkook,Oh, Min Suk IEEE 2015 IEEE electron device letters Vol.36 No.11
<P>We demonstrate a two-dimensional (2D) multilayered molybdenum disulfide (MoS<SUB>2</SUB>) transistor with molybdenum (Mo) side and edge contacts, which is deposited using a dc-sputtering method. It exhibits field-effect mobility of 23.9 cm <TEX>$^{\mathrm {2}}$</TEX>/Vs and ON/OFF ratio of <TEX>$10^{\mathrm { {6}}}$</TEX> in a linear region. A current–voltage study under different temperatures (300–393 K) reveals that the Mo–MoS<SUB>2</SUB> 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–MoS<SUB>2</SUB> 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 MoS<SUB>2</SUB> channel layer.</P>
Akter, Afroja,Yoo, Geonwook,Kim, Sangin,Baac, Hyoung Won,Heo, Junseok American Scientific Publishers 2017 Journal of Nanoscience and Nanotechnology Vol.17 No.5
<P>The electronic intraband absorption in InGaN nanodisks embedded in GaN nanowires with several kinds of cladding materials and without cladding was theoretically investigated. The cladding layer was 5 nm thick, and AlN, GaN, and Al0.4Ga0.6N were considered. The strain distribution, internal electric field, and intraband absorption in the nanodisks were calculated using the elastic energy minimization method and the single-band Schrodinger equation implemented in Nextnano3. For a plain nanowire without cladding, an inhomogeneous strain in the disk caused a piezoelectric field and deformation potential, yielding band-bending and a higher electron probability density in the periphery of the disk. An InGaN nanodisk embedded in a cladding GaN nanowire exhibited a higher intraband absorption. The case of the GaN cladding was optimal owing to the homogeneous surroundings of the disk.</P>
Hong, Seongin,Yoo, Geonwook,Kim, Dong Hak,Song, Won Geun,Le, Ong Kim,Hong, Young Ki,Takahashi, Kaito,Omkaram, Inturu,Son, Do Ngoc,Kim, Sunkook WILEY‐VCH Verlag Berlin GmbH 2017 Physica status solidi. PSS. C, Current topics in s Vol.14 No.3
<P>Seongin Hong et al. (article no. <url href='http://doi.wiley.com/10.1002/pssc.201600262'>1600262</url>) have investigated the polyethylenimine (PEI) doping mechanism and its effect on the electrical and optical properties of multilayer MoS<SUB>2</SUB> field effect transistors (FETs). Density functional theory (DFT) calculation and X‐ray photoelectron spectroscopy (XPS) measurement confirm that the PEI molecules were successfully doped and formed Mo–N bonds on the MoS<SUB>2</SUB> channel, generating new energy states near the valence band. The strong n‐doping changed the threshold voltage as well as the Schottky barrier width attributed to the induced interfacial dipoles. Therefore, the ON‐current of the doped MoS<SUB>2</SUB> FETs was improved in comparison with the pristine FETs. Furthermore, the PEI doping also enhanced the photoresponsivity of the MoS<SUB>2</SUB> FETs from 0.14 A/W to 4.41 A/W. This study suggests that PEI molecular doping could be widely applicable to two‐dimensional materials in order to improve the electrical and optical properties of respective devices.</P>
Bias-dependent photoresponsivity of multi-layer MoS <sub>2</sub> phototransistors
Park, Jinwu,Park, Youngseo,Yoo, Geonwook,Heo, Junseok Springer US 2017 NANOSCALE RESEARCH LETTERS Vol.12 No.1
<P>We studied the variation of photoresponsivity in multi-layer MoS<SUB>2</SUB> phototransistors as the applied bias changes. The photoresponse gain is attained when the photogenerated holes trapped in the MoS<SUB>2</SUB> attract electrons from the source. Thus, the photoresponsivity can be controlled by the gate or drain bias. When the gate bias is below the threshold voltage, a small amount of electrons are diffused into the channel, due to large barrier between MoS<SUB>2</SUB> and source electrode. In this regime, as the gate or drain bias increases, the barrier between the MoS<SUB>2</SUB> channel and the source becomes lower and the number of electrons injected into the channel exponentially increases, resulting in an exponential increase in photoresponsivity. On the other hand, if the gate bias is above the threshold voltage, the photoresponsivity is affected by the carrier velocity rather than the barrier height because the drain current is limited by the carrier drift velocity. Hence, with an increase in drain bias, the carrier velocity increases linearly and becomes saturated due to carrier velocity saturation, and therefore, the photoresponsivity also increases linearly and becomes saturated.</P>
Lee, Sanghyun,Park, Youngseo,Yoo, Geonwook,Heo, Junseok American Institute of Physics 2017 APPLIED PHYSICS LETTERS Vol.111 No.22
<P> The responsivities of MoS<SUB>2</SUB> based photodetectors or phototransistors have been improved by integrating additional layers or implementing a hetero-structure. Despite complicated processes, these methods only improve the overall responsivity and are not capable of wavelength selective enhancement. This study reports the wavelength-selective enhancement of photo-responsivity in a multi-layer MoS<SUB>2</SUB> phototransistor by employing a reflective Al metal gate. We have observed that the photo-responsivities are enhanced at a wavelength of ~500 nm (FWHM=140 nm) in the Al metal-gated multi-layer MoS<SUB>2</SUB> phototransistors. The finite difference time domain method also confirmed that the wavelength for enhanced responsivity can be adjusted according to the thicknesses of the MoS<SUB>2</SUB> and gate insulating layer. Using this effect, the Al metal-gated multi-layer MoS<SUB>2</SUB> phototransistors can be tuned for specific applications. Published by AIP Publishing. </P>