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Kim, Jayeong,Glier, Tomke E.,Grimm-Lebsanft, Benjamin,Buchenau, Sö,ren,Teubner, Melissa,Biebl, Florian,Kim, Nam-Jung,Kim, Heehun,Yi, Gyu-Chul,Rü,bhausen, Michael,Yoon, Seokhyun American Chemical Society 2019 The Journal of Physical Chemistry Part C Vol.123 No.40
<P>We studied surface-enhanced Raman spectroscopy (SERS) in 4-mercaptopyridine (4-Mpy) deposited on zinc oxide (ZnO) nanostructures, by using resonance Raman scattering covering a range of incident photon energies from 1.7 to 5.7 eV. We investigated all primary routes of the energy-specific resonances that are associated with the electronic transitions between the ZnO valence band (VB) to the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) to the ZnO conduction band (CB), respectively. Two resonances at 5.55 and 5.15 eV in the ultraviolet (UV) spectral range can be associated with transitions into the CB and most importantly into an excitonic-related state below the ZnO CB, respectively. The energy difference between the UV resonances is 0.4 eV corresponding to the excitonic binding energy as a result of excitonic quantum confinement in the 10-20 nm thick ZnO nanowalls. The observed excitonic SERS resonance enhancement of the ring-breathing mode of 4-Mpy is about 15 times stronger than for the VB resonance observed at 2.43 eV and free of luminescence background. Hence, we outline new pathways of improving the detectability of molecules by chemical SERS due to tuning of the quantum confinement in the excitonic resonance enhancement.</P> [FIG OMISSION]</BR>
Kim, Kihwan,Kim, Juran,Gang, Myeng Gil,Kim, Se-Ho,Song, Soomin,Cho, Yunae,Shin, Donghyeop,Eo, Young-Joo,Jeong, Inyoung,Ahn, Seung Kyu,Cho, Ara,Kim, Jayeong,Yoon, Seokhyun,Choi, Pyuck-Pa,Jo, William,Ki Elsevier 2019 Solar energy materials and solar cells Vol.195 No.-
<P><B>Abstract</B></P> <P>In this work, copper indium gallium selenide (Cu(In,Ga)Se<SUB>2</SUB>; CIGS) absorbers were grown on polyimide (PI)/molybdenum substrates by a three-stage co-evaporation process at various temperatures, film formation was systemically studied using various advanced characterization methods such as transmission electron microscopy, micro-Raman spectroscopy, Kelvin probe force microscopy, and atom probe tomography. The CIGS films on PI were found to exhibit considerable physical and electrical variations with respect to the process temperature of three-stage co-evaporation. In particular, when the process temperature reached 400 °C, the CIGS absorber on PI began to exhibit controlled microstructure and intergrain properties. By adjusting the microstructure and intergrain properties of the absorber films by means of the process temperature of three-stage co-evaporation, flexible CIGS solar cells on PI with an efficiency of 16.7% (with anti-refection coating) were achieved.</P> <P><B>Highlights</B></P> <P> <UL> <LI> CIGS absorber films were grown on flexible polyimide/molybdenum substrates. </LI> <LI> Low-temperature three-stage process (≤440 °C) with extrinsic Na addition was used to CIGS growth. </LI> <LI> CIGS film evolution was systemically observed using advanced material characterization techniques. </LI> <LI> Highly efficient CIGS cells on flexible polyimide substrates were realized while maintaining process manufacturability. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Kim, Juran,Kim, Jayeong,Yoon, Seokhyun,Kang, Jeong-yoon,Jeon, Chan-Wook,Jo, William American Chemical Society 2018 The Journal of Physical Chemistry Part C Vol.122 No.6
<P>Tin monosulfide (SnS) is one of the most promising binary compounds for thin-film solar cells owing to its suitable optical properties and abundance in nature. However, in solar cells it displays a low open circuit voltage and power conversion efficiency owing to multiphases in the absorber layers. In this study, we investigated approximately 1.2-μm-thick SnS thin films prepared via a two-step process involving (1) the deposition of metal precursor layers and (2) sulfurization at 400 °C. To investigate the phase variations inside the thin films we employed a dimpling method to get a vicinal cross-section of the sample. Kelvin probe force microscopy, conductive atomic force microscopy, and micro-Raman scattering spectroscopy were used to characterize the local electrical and optical properties of the sample. We studied the distribution of the Sn–S polytypes in the film and analyzed their electrical performances for solar cell applications. The work functions of SnS and SnS<SUB>2</SUB> were determined to be 4.3–4.9 and ∼5.3 eV, respectively. The local current transport properties were also measured; they displayed an interesting transition in the conduction mechanism, namely from Ohmic shunt current at low voltages to space-charge-limited current at high voltages.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2018/jpccck.2018.122.issue-6/acs.jpcc.8b00179/production/images/medium/jp-2018-001792_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp8b00179'>ACS Electronic Supporting Info</A></P>
Kim, Nam-Jung,Kim, Jayeong,Park, Jun-Beom,Kim, Hyemin,Yi, Gyu-Chul,Yoon, Seokhyun The Royal Society of Chemistry 2019 Nanoscale Vol.11 No.1
<P>We used high-quality ZnO nanostructures/graphene substrates for understanding the mechanisms of charge transfer (CT) that take place under nonplasmonic conditions. As the optimal conditions for CT processes are found, we studied the range of CT normal to the ZnO surface that is coated with nanoscale HfO2 layers with different thicknesses. We could observe that CT decays over a few nanometers. In addition, we also observed a unique oscillation of the SERS intensity in the atomically thin oxide layers, which reflects the quantum tunneling effects of CT electrons across the oxide layers. To the best of our knowledge, this is the first direct observation of SERS-active charge transport and measurement of a CT span with atomic-scale accuracy.</P>
Backlog Estimation for Uplink/Downlink Fairness in Unsaturated Bidirectional CSMA Networks
Kim, JaYeong,Jin, Hu,Kim, Seong Hwan,Sung, Dan Keun IEEE 2017 IEEE communications letters Vol.21 No.12
<P>Bidirectional transmission (BT)-based protocols have been proposed to achieve uplink/downlink fairness in IEEE 802.11 systems. To successfully guarantee the fairness in unsaturated carrier sensing multiple access (CSMA) systems, it is important to accurately estimate the number of backlogged nodes considering both of the uplink (UL) and downlink (DL) traffic. In this letter, we propose a backlog estimation mechanism for the bidirectional media access control to guarantee UL/DL fairness in an unsaturated CSMA environment. We derive the optimal transmit probability and the optimal DL transmit opportunity for BT as a function of the estimated backlog size. The proposed algorithm ensures the near-perfect UL/DL fairness while improving the overall system performance.</P>
Hybrid ARQ-Based Fairness Enhancement in Uplink WLAN
Kim, Jayeong,Kim, Seong Hwan,Sung, Dan Keun IEEE 2018 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.17 No.7
<P>Per-station fairness is one of the important and challenging issues in wireless local area network based on a contention-based medium access control protocol. The unfairness problem among stations having different channel qualities is caused by the imbalance in access probability and outage probability among them. In this paper, we investigate the per-station fairness by resolving the imbalance in the above two factors through the enhanced distributed coordination function (DCF) adopting hybrid automatic repeat request (HARQ), named the DCF-H. We suggest adopting the HARQ with Chase combining as a solution for the imbalance in the outage probability on top of the loss differentiation mechanism which resolves the imbalance in the access probability. We also analyze the performance of the DCF-H by proposing a new Markov chain model and derive a closed-form expression of system throughput, delay, and outage probability. We compare the performance of the DCF-H with that of the conventional DCF protocols and find out that the DCF-H not only guarantees the near-perfect per-station fairness but also improves the whole system performance, such as throughput, delay, and outage probability.</P>