Metal nanofibrils embedded in long free-standing carbon nanotube fibers with a high critical current density

Rho, Hokyun,Park, Min,Park, Mina,Park, Junbeom,Han, Jiyoon,Lee, Aram,Bae, Sukang,Kim, Tae-Wook,Ha, Jun-Seok,Kim, Seung Min,Lee, Dong Su,Lee, Sang Hyun Springer Science and Business Media LLC 2018 NPG Asia Materials Vol.10 No.-

Porous copper-graphene heterostructures for cooling of electronic devices

Rho, Hokyun,Jang, Yea Sol,Kim, Sungmin,Bae, Sukang,Kim, Tae-Wook,Lee, Dong Su,Ha, Jun-Seok,Lee, Sang Hyun The Royal Society of Chemistry 2017 Nanoscale Vol.9 No.22

<P>Recently, research on micro-electronic and optoelectronic devices has been rapidly increasing. Parts and products related to these devices are becoming smaller and more integrated within circuits. As a result, the heat generated in devices has increased greatly. When excess heat is generated, important properties are affected such as efficiency and lifetime and, in severe cases, this can result in the failure of devices. Therefore, efficient cooling is required and it becomes necessary to study the heat dissipation properties of device materials. Research on heat-dissipating materials with high thermal conductivities and large surface areas, and which can transfer heat rapidly to facilitate progressive heat-release, is being actively pursued. In this study, a porous copper with reduced graphene oxide (pCu-rGO) heterostructure was fabricated by thermal annealing using Cu powder and GO. The thermal properties were then investigated and the results indicated that the pCu-rGO heterostructure exhibits a higher thermal conductivity than porous Cu. In addition, the thermal resistance of the sample was measured by applying it as a heat sink of a light emitting diode (LED). The result was 18.33% lower than that of bulk Cu. Also, when an overcurrent of 750 mA was applied for 144 hours, the luminance of bulk Cu decreased from 100% to 86.07%. On the other hand, the pCu-rGO showed that the luminance was maintained at 95.64%. Therefore, it is expected to resolve the existing problem of heat generation in electronic and optical devices.</P>

N-GaN 접촉 전극의 크기 및 배열 변화에 따른 패드리스 수직형 발광다이오드의 구동전압의 변화에 관한 연구

노호균(Hokyun Rho),하준석(Jun-Seok Ha) 한국마이크로전자및패키징학회 2015 마이크로전자 및 패키징학회지 Vol.21 No.1

LED (Light Emitting Diode) 시장의 발전이 빠르게 이루어지고 있음에 따라 점차 고효율 LED의 필요성이 증가하고 있다. 이에 우리는 Hole Type의 Padless 신 구조 수직형 LED에서, 접촉 전극의 크기와 그 배치가 Chip의 가동 전압에 어떠한 영향을 미치는지 알아보았다. 이를 위하여 LED simulation을 통한 계산과 실제 Chip 제작을 통한 전기적 특성 평가를 하였다. 그 결과, Simulation 을 통하여 n전극의 크기가 커질수록 구동전압이 낮아짐을 확인하였고, N 전극의 형태가 확산됨에 따라서도 구동전압이 낮아짐을 확인하였다. 이러한 추세는 실제 제작한 LED Chip의 측정 결과와 비슷한 경향을 나타내었다. For the application of light-emitting diodes (LEDs) for general illumination, the development of high power LEDs chips became more essential. For these reasons, recently, modified vertical LEDs have been developed to meet various requirements such as better heat dissipation, higher light extraction and less cost of production. In this research, we investigate the effect of Size and Array of N-GaN contact on operation voltage with new structured padless vertical LED. We changed the size and array of N-electrodes and investigated how they affect the operation voltage of LEDs. We simulated the current crowding and expected operation voltage for different N-contact structures with commercial LED simulator. Also, we fabricated the padless vertical LED chips and measured the electrical property. From the simulation, we could know that the larger size and denser array of n-electrodes could make operation voltage decrease. These results are well in accordance with those measured values of real padless vertical LED chips.

Rapid Fabrication of Cu/Cu2O/CuO Photoelectrodes by Rapid Thermal Annealing Technique for Efficient Water Splitting Application

Minjeong Lee(Minjeong Lee),Hyojung Bae(Hyojung Bae),Hokyun Rho(Hokyun Rho),Vishal Burungale(Vishal Burungale ),Pratik Mane(Pratik Mane ),Chaewon Seong(Chaewon Seong),Jun-Seok Ha(Jun-Seok Ha) 한국마이크로전자및패키징학회 2020 마이크로전자 및 패키징학회지 Vol.27 No.4

The Cu/Cu2O/CuO photoelectrode has been successfully fabricated by Rapid Thermal Annealing technique. The structural characterization of fabricated photoelectrode was performed using X-Ray diffraction, while elemental composition of the prepared material has been checked with X-Ray Photoelectron Spectroscopy. The synthesis parameters are optimized on the basis of photoelectrochemical performance. The best photoelectrochemical performance has been observed for the Cu/Cu2O/CuO photoelectrode fabricated at 550 ℃ oxidation temperature and oxidation time of 50 seconds with highest photocurrent density of -3 mA/㎠ at -0.13 V vs. RHE.

N-GaN 접촉 전극의 크기 및 배열 변화에 따른 패드리스 수직형 발광다이오드의 구동전압의 변화에 관한 연구

노호균,하준석,Rho, Hokyun,Ha, Jun-Seok 한국마이크로전자및패키징학회 2014 마이크로전자 및 패키징학회지 Vol.21 No.1

LED (Light Emitting Diode) 시장의 발전이 빠르게 이루어지고 있음에 따라 점차 고효율 LED의 필요성이 증가하고 있다. 이에 우리는 Hole Type의 Padless 신 구조 수직형 LED에서, 접촉 전극의 크기와 그 배치가 Chip의 가동 전압에 어떠한 영향을 미치는지 알아보았다. 이를 위하여 LED simulation을 통한 계산과 실제 Chip 제작을 통한 전기적 특성 평가를 하였다. 그 결과, Simulation 을 통하여 n전극의 크기가 커질수록 구동전압이 낮아짐을 확인하였고, N 전극의 형태가 확산됨에 따라서도 구동전압이 낮아짐을 확인하였다. 이러한 추세는 실제 제작한 LED Chip의 측정 결과와 비슷한 경향을 나타내었다. For the application of light-emitting diodes (LEDs) for general illumination, the development of high power LEDs chips became more essential. For these reasons, recently, modified vertical LEDs have been developed to meet various requirements such as better heat dissipation, higher light extraction and less cost of production. In this research, we investigate the effect of Size and Array of N-GaN contact on operation voltage with new structured padless vertical LED. We changed the size and array of N-electrodes and investigated how they affect the operation voltage of LEDs. We simulated the current crowding and expected operation voltage for different N-contact structures with commercial LED simulator. Also, we fabricated the padless vertical LED chips and measured the electrical property. From the simulation, we could know that the larger size and denser array of n-electrodes could make operation voltage decrease. These results are well in accordance with those measured values of real padless vertical LED chips.

Graphene–Carbon–Metal Composite Film for a Flexible Heat Sink

Cho, Hyunjin,Rho, Hokyun,Kim, Jun Hee,Chae, Su-Hyeong,Pham, Thang Viet,Seo, Tae Hoon,Kim, Hak Yong,Ha, Jun-Seok,Kim, Hwan Chul,Lee, Sang Hyun,Kim, Myung Jong American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.46

<P>The heat generated from electronic devices such as light emitting diodes (LEDs), batteries, and highly integrated transistors is one of the major causes obstructing the improvement of their performance and reliability. Herein, we report a comprehensive method to dissipate the generated heat to a vast area by using the new type of graphene carbon metal composite film as a heat sink. The unique porous graphene-carbon-metal composite film that consists of an electrospun carbon nanofiber with arc-graphene (Arc-G) fillers and an electrochemically deposited copper (Cu) layer showed not only high electrical and thermal conductivity but also high mechanical stability. Accordingly, superior thermal management of LED devices to that of conventional Cu plates and excellent resistance stability during the repeated 10 000 bending cycles has been achieved. The heat dissipation of LEDs has been enhanced by the high heat conduction in the composite film, heat convection in the air flow, and thermal radiation at low temperature in the porous carbon structure. This result reveals that the graphene carbon metal composite film is one of the most promising materials for modern electronics.</P>

Improvement of efficiency in graphene/gallium nitride nanowire on Silicon photoelectrode for overall water splitting

Bae, Hyojung,Rho, Hokyun,Min, Jung-Wook,Lee, Yong-Tak,Lee, Sang Hyun,Fujii, Katsushi,Lee, Hyo-Jong,Ha, Jun-Seok Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.422 No.-

<P><B>Abstract</B></P> <P>Gallium nitride (GaN) nanowires are one of the most promising photoelectrode materials due to their high stability in acidic and basic electrolytes, and tunable band edge potentials. In this study, GaN nanowire arrays (GaN NWs) were prepared by molecular beam epitaxy (MBE); their large surface area enhanced the solar to hydrogen conversion efficiency. More significantly, graphene was grown by chemical vapor deposition (CVD), which enhanced the electron transfer between NWs for water splitting and protected the GaN NW surface. Structural characterizations of the prepared composite were performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocurrent density of Gr/GaN NWs exhibited a two-fold increase over pristine GaN NWs and sustained water splitting up to 70min. These improvements may accelerate possible applications for hydrogen generation with high solar to hydrogen conversion efficiency.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Pristine GaN NWs were grown on n-type Si (111) substrates using radio-frequency plasma-assisted MBE. </LI> <LI> The graphene grown by CVD was coated on surface of GaN nanowires (Gr/GaN NWs). </LI> <LI> The photoelectrochemical properties of these structures were investigated. </LI> <LI> The saturated photocurrent density increased in Gr/GaN NWs than ref GaN NWs. </LI> <LI> We reported the reason for this difference. </LI> </UL> </P>

Transparent Electrodes Fabricated from Single Walled Carbon Nanotubes and Ni Nanoparticles with Low Contact Resistance to p-Al_0.3Ga_0.7N

Park, Jun-Beom,Rho, Hokyun,Bae, Hyojung,Chung, Tae Hun,Jeong, Tak,Ha, Jun-Seok The Electrochemical Society 2018 ECS journal of solid state science and technology Vol.7 No.12

<P>The transparent conductive electrodes (TCEs) that have high-transmittance in the ultra-violet region are reported. The TCEs in this paper consist of combination of single-walled carbon nanotubes (SWCNTs) and nanoscale Ni particles (Ni NPs). TCEs can form low-resistance by directly contacting to p-AlGaN without ohimic contact layer of p-GaN that absorbed light energy in the UV region. Ni NPs are combined on the surface of the SWCNTs to enhance the work function, resulting in the low contact resistance between TCEs and p-Al<SUB>0.3</SUB>Ga<SUB>0.7</SUB>N. And, the specific contact resistance of TCEs and p-Al<SUB>0.3</SUB>Ga<SUB>0.7</SUB>N is 8.97 × 10<SUP>−2</SUP> Ωcm,<SUP>2</SUP> which means that it is possible to improve the output power of light emitting diodes by directly forming TCEs on p-AlGaN without inserting p-GaN layer.</P>

금속/그래핀 이중 구조 와이어의 합성 및 전기적 특성 연구

정민희,김동영,노호균,신한균,이효종,이상현,Jeong, Minhee,Kim, Dongyeong,Rho, Hokyun,Shin, Han-Kyun,Lee, Hyo-Jong,Lee, Sang Hyun 한국마이크로전자및패키징학회 2021 마이크로전자 및 패키징학회지 Vol.28 No.1

본 연구에서는 금속 와이어를 촉매로 화학기상증착법을 이용하여 그래핀을 합성하고 구조 및 전기적 특성 변화를 분석하였다. 구리와 니켈의 탄소에 대한 용해도 차이로 인해 구리와이어에서는 단층 그래핀이 성장하였고, 니켈와이어의 표면에는 다층 그래핀이 성장되었다. 또한. 고온의 그래핀 성장 조건에서 구리와 니켈의 재결정화를 통해 결정립의 크기가 증가한 것을 확인하였다. 표면에 그래핀이 합성된 구리와이어의 경우, 최대전류허용치는 1.91×105 A/㎠으로 합성 전 구리와이어에 비해 약 27% 향상되었다. 이와 유사하게, 다층 그래핀이 합성된 니켈와이어의 경우에도 최대전류 허용치는 순수한 니켈와이어 대비 약 36% 향상된 4.41×104 A/㎠으로 측정되었다. 이러한 그래핀/금속 복합소재의 우수한 전기적 특성은 고전류를 요구하는 소자 및 부품에서 안정적인 전기적 흐름을 공급하는데 기여할 수 있을 것이다. In this study, graphene layer was grown on metal microwire using chemical vapor deposition. The difference of carbon solubility between copper and nickel resulted in the formation of mono-layer and multi-layer graphene were formed on the surfaces of copper and nickel microwires, respectively. During the growth of graphene at high temperature, copper and nickel were recrytallized and the grain size increased. The ampacity of graphene/copper microwire was improved by approximately 27%, 1.91×105 A/㎠, compared to pristine copper microwire. Similar to this behavior, the ampacity of multilayer graphene/nickel microwire was 4.41×104 A/㎠ which is about about 36% improved compared to the pure nickel microwire. The excellent electrical properties of graphene/metal composites are beneficial for supplying the electrical energy to the high-power electronic devices and equipment.

Hydrothermal Synthesis of CaMn2O4·xH2O Nanorods as Co-Catalysts on GaN Nanowire Photoanode

Bae Hyojung,Kim Hyunggu,Burungale Vishal,Min Jung‐Wook,Cha An‐na,Rho Hokyun,Ryu Sang‐Wan,Kang Soon Hyung,Ha Jun‐Seok 대한화학회 2021 Bulletin of the Korean Chemical Society Vol.42 No.6

Co-catalysts play a crucial role in photoelectrochemical (PEC) water splitting reactions by improving slow kinetics and reducing surface recombination, thereby enhancing PEC performance. However, achieving a well-defined interface between low-dimensional semiconductors and co-catalysts is challenging due to difficulties in depositing a uniform co-catalyst layer and attaining mixed and disordered states. CaMn2O4 xH2O has been extensively studied for the purpose of decreasing the overpotential in the oxygen evolution reaction. In this study, CaMn2O4xH2O was synthesized via a hydrothermal method, and loaded onto a GaN nanowire (NW) photoanode. Optimized CaMn2O4xH2O layers were decorated on GaN NWs to create CaMn2O4@GaN, which delivered a maximum photocurrent of 0.4 mA/cm2 at 0 V vs. the Ag/AgCl reference electrode under an AM 1.5 G solar simulator, 1.6-fold higher than that (0.25 mA/cm2) of bare GaN NWs. Thus, highly efficient and stable PEC water splitting was achieved on the GaN NW photoanode.