Study of Reduced Graphene Oxide Property and Application : Reduced Graphene Oxide 속성과 응용에 관한 연구

우성용 DGIST 2016 국내석사

Graphene Oxide(GO)는 일반적인 실험실 혹은 실내 환경에서 작업이 가능한 물질이다. 사용방법도 간단하고 물질 접촉에 따른 독성도 없어서 작업에 아주 용이하다. 자체 특성상 비전도성 특성을 가지지만 열이나 레이저로 가공을 하면 에너지가 가해진 부분에서 환원과정이 발생하여 “Reduced Graphene Oxide(rGO)”가 되면서 전기 전도성을 가진다. 당연히 가공이 안된 면은 비전도성 성질을 그대로 가지면서 두 가지 특성을 동시에 가진 형태로 제작이 가능하게 된다. 입력된 에너지의 양에 따라서 전기적 특성이 달라지는데, 즉 전기적 저항에 차이가 발생한다. 레이저로 같은 위치를 반복하여 가공을 하면 저항 값이 감소하는 특성을 내장 하고 있다. 이러한 특성으로 인하여 반도체 및 메모리에 활용이 되고 있으며, 향후 자체 복합 회로도 제작이 가능할 것으로 보인다. 본 연구에서는 rGO 의 전기적 특성을 재 조명하면서 레이저 에너지의 양과 그에 따른 변화를 보고 활용분야의 확인과 적용을 하려고 한다. rGO 프로세서 과정에서 형태상 부풀어 오르는 성질을 이용하여 분자간의 접촉 면적이 커지는 점을 착안하여 다양한 이온을 혼합하여 특정 센서로 활용을 모색하고 있으며, 특히 의료 분야에서 혈액을 대상으로 하는 센서의 개발을 주로 이루어 지고 있다. 이러한 방법은 기존의 단백질과 각종 효소에 의지하여 제작되고 있는 고가의 반응 물질을 이용한 센서를 물리적인 성질을 이용한 센서로 대처하며, 효소나 단백질 센서의 고질적인 보관과 유통기한의 문제에서 해결의 돌파구를 제시할 것으로 보여진다. 의료분야에서 효소를 사용한 측정분야에서 가장 크게 사용되고 있는 분야는 혈당의 측정 즉 포도당을 측정하는 것으로 당뇨병 환자를 주 타깃으로 하는 분야이다. 본 분야의 센서부분은 위에서 언급한 효소를 사용한 센서로 인하여 생산단가가 높고 유통기한으로 이한 보관의 주의가 필요한 문제가 있다. 때문에 포도당 측정에 있어서 rGO 를 활용한 센서를 제작할 경우 낮은 가격에 대량의 생산의 길이 열릴 것으로 예상이 된다. 이러한 대량 생산을 위해서는 쉽고 빠르게 제작이 가능한 기기가 있어야 하며 본 연구에서는 직접 레이저 가공기를 활용한 방법을 제안한다. 레이저 가공기는 산업용 로봇이나 기타 범용 모터를 활용한 구성으로 제작과 구성이 용이하다. 또한 소형화를 통해 센서를 개인적으로 프린터에서 인쇄를 하듯 센서도 필요할 때 가정이나 장소에 구애가 없이 제작이 가능하며, 재료의 독성이 없으므로 폐기나 보관시 제약이 없어야 한다. 이러한 보관과 폐기의 용이성에 쉬운 제작과 대량 생산 및 자가 생산으로 접근성을 위한 여러 실험과 환경조건을 제시를 해야 한다. 본 연구는 독성이 없는 GO 를 사용하여 rGO 를 생성하는 방법을 쉽고 빠르게 제작이 가능한 레이저 스크라이버의 사용과 함께 제작된 rGO 의 성능 분석과 포도당의 직접적인 측정을 통하여 포도당측정의 가능여부 및 향후 개선안과 방향을 소개할 것이다. Graphene Oxide is a substance capable of working in a typical laboratory or indoor environment. Usage is also simple and also because of the toxic materials in contact is very easy to work. If the non-conductive nature of the process itself have only column characteristics or by laser energy reduction processes occurring in the applied part "Reduced Graphene Oxide" has electrical conductivity. While as the non-processed surface of non-conductive properties is manufactured in a form to enable this with the two characteristics simultaneously. The electrical resistance value varies according to the amount of energy input. That is the difference in electrical resistance occurs. When the process is repeated for the same position as the laser has a built-in property of the resistance value decreases. Because of this characteristic it has been utilized in the semiconductor and memory, which appears to be the next circuit making the composite itself. In this study, the trying to make the application of the application fields to see a change with the amount of laser energy thereto and refocus the electrical characteristics of rGO. rGO is utilized to expect sensor which using swell property in reduction processor and mixing nano particle. Particularly, recent rGO research is bio and blood base clinic (medical field) to target. This method is coping with the sensor using an expensive reactive substances that are produced by the will of the existing proteins and various enzymes to the sensor using the physical properties, the enzyme or the protein sensor a breakthrough resolved in a matter of persistent storage and expiration date. In fabricating the sensor, so that utilized rGO in glucose measurement is expected to be opened, the length of the large quantity of production at a lower price. These should be capable of quick and easy to manufacture device to mass production. In this study, we propose a method using direct laser scribe. Laser equipment is easy to manufacture and consists of a configuration using the industrial robot or other general-purpose motor. In addition, easily and expeditiously making processor can manufacture sensor with a laser scribe. There shall be no restrictions on the disposal or storage of materials. The storage and disposal easier and self-produced and mass production of the sensor should be presented several experimental and environmental conditions for access to the produce. This study presents a laser scriber capable of easily and quickly making how to create the rGO with the non-toxic GO. rGO sensor performance analysis that manufactured through laser fabrication process. These will be proposed the availability, future development direction and improvement of the glucose measurement.

Graphene oxide를 기반으로 한 PEBAX 혼합막의 CO2/N2 기체 투과 특성

이은선 상명대학교 일반대학원 2023 국내석사

In this study, graphene oxide (GO), polyethylenimine-graphene oxide (PEI-GO), and polyethylenimine-graphene oxide@zeolite imidazolate framework-8 (PEI-GO@ZIF-8) were added to poly (ether-block-imide) 2533 (PEBAX 2533) by content and mixed to be used as mixed matrix membranes (MMMs). The MMMs penetrated single CO2 and N2 gas, to investigate the performance of each filler and content. Through Fourier-transform infrared spectrometer (FT-IR) and powder X-ray diffractometer (XRD), it confirmed GO, PEI-GO, and PEI-GO@ZIF-8 were synthesized well, and changes in crystallinity of the materials were confirmed due to modification. When MMMs, FT-IR peak change was not observed, because the small amount of the filler. As a result of analysis through Thermogravimetric Analyzer (TGA), it was confirmed that the thermal stability of the filler increased as GO was modified to PEI and ZIF-8. The thermal stability of the MMMs was also increased with the content of the filler, and it was confirmed that the PEBAX/PEI-GO@ZIF-8 MMMs had the best thermal stability. In Gas permeation, case of the PEBAX/GO MMMs, the permeability of both N2 and CO2 gas tended to decrease as the GO content increased. On the other hand, the CO2/N2 selectivity steadily increased to 58.89 which was the highest at 0.3 wt%, decreased at 0.5 wt% to 55.34 which was similar to 0.1 wt%, and the overall selectivity was improved overall compared to the pure PEBAX 2533 membrane. In the PEBAX/PEI-GO MMMs, the N2 permeability decreased steadily. The CO2 permeability decreases from 0.1wt% and then increases from 0.3wt%, and decreases slightly from 0.5wt%, but shows a value similar to 0.3wt%. The CO2/N2 selectivity steadily increased to 73.5, the highest at 0.3wt%, and slightly decreased at 0.5wt%. The N2 permeability of the PEBAX/PEI-GO@ZIF-8 MMMs shows a decreasing trend. The CO2 permeability was from 0.1wt% to 221.88 Barrer, showing the highest permeability in all MMMs. It was confirmed that the CO2 permeability gradually decreased at the above content. Unlike the PEBAX/GO and PEBAX/PEI-GO MMMs, the CO2/N2 selectivity recorded the highest 60 at a content of 0.1wt%, and then decreased. When the MMMs subjected to gas permeation were illustrated in upper bound (2008), it was confirmed that all membranes were close to the upper bound compared to the pure PEBAX 2533 membrane. Among them, PEBAX/PEI-GO of 0.3wt% and 0.5wt% showed excellent performance beyond the upper bound. 본 연구는 합성한 graphene oxide (GO), polyethylenimine-graphene oxide (PEI-GO), polyethylenimine-graphene oxide@zeolite imidazolate framework-8 (PEI-GO@ZIF-8)을 poly(ether-b-amide) 2533 (PEBAX 2533)에 함량별로 첨가, 혼합하여 mixed matrix membranes (MMMs)로 사용하였다. 제조된 혼합막은 단일 기체인 CO2, N2 기체에 투과하여 충진재별, 함량별 성능을 알아보았다. PEBAX 2533에 혼입되는 GO는 기존 발표된 논문을 기초로 합성하였고, PEI-GO는 GO 표면에 PEI 사슬을 접합하는 형태로 합성되었다. PEI-GO@ZIF-8의 경우 PEI-GO 내 GO 표면에 ZIF-8 입자가 형성된 구조를 이루도록 합성하였다. Fourier-transform infrared spectrometer (FT-IR)를 통해 GO, PEI-GO, PEI-GO@ZIF-8의 합성이 잘 되었음을 판단하였다. 각 충진재의 powder X-ray diffractometer (XRD)를 통해 PEI와 ZIF-8을 통한 개질이 GO sheets 간 거리에 영향을 미친 것으로 확인하였다. Thermogravimetric analyzer (TGA)를 통한 분석 결과 GO를 PEI와 ZIF-8으로 개질할수록 충진재의 열적 안정성이 증가하는 것을 확인할 수 있었다. 또한 GO, PEI-GO, PEI-GO@ZIF-8을 첨가한 혼합막의 열적 안정성은 충진재의 함량에 따라 증가하였으며, PEBAX/PEI-GO@ZIF-8 혼합막이 가장 우수한 열적 안정성을 가진다는 것을 확인하였다. 기체 투과는 3atm, 25˚C 조건에서 이루어졌다. PEBAX/GO 혼합막의 경우 GO의 함량 증가에 따라 N2 기체와 CO2 기체 모두에서 투과도가 감소하는 경향을 보였다. 반면 CO2/N2 선택도는 꾸준히 증가하여 0.3wt%에서 가장 높은 58.89를 보였으며, 0.5wt%에서는 감소하여 0.1wt%와 유사한 55.34를 기록하였고 전체적인 선택도는 순수한 PEBAX 2533 막에 비해 전반적으로 개선된 값을 보였다. PEBAX/PEI-GO 혼합막에서는 N2 투과도가 꾸준히 감소하는데, 그 감소 폭은 PEBAX/GO 막에서보다 큰 것으로 나타났다. CO2 투과도는 0.1 wt%에서 일시적으로 감소하였다가 0.3wt%에서 증가하고, 0.5wt%에서 약간 감소하나 0.3wt%와 유사한 값을 보인다. CO2/N2 선택도는 PEBAX/GO 혼합막과 유사하게 꾸준히 증가하여 0.3wt%에서 가장 높은 73.5를 기록하였으며, 0.5wt%에서 약간 감소한다. PEBAX/PEI-GO@ZIF-8 혼합막의 N2 투과도는 감소하는 추세를 보이나 PEBAX/PEI-GO 혼합막보다 완만하게 감소한다. CO2 투과도는 0.1wt%에서 221.88 Barrer로 모든 혼합막에서 가장 높은 투과도를 보였으며 그 이상의 함량에서는 서서히 투과도가 감소하는 것을 확인하였다. CO2/N2 선택도는 PEBAX/GO, PEBAX/PEI-GO 혼합막과 달리 0.1wt% 함량에서 가장 높은 60을 기록하였으며 그 후 감소하였다. 혼합막의 기체 투과 결과를 upper bound (2008)에 도시하였을 때 순수한 PEBAX 2533 막에 비해 모든 혼합막이 upper bound에 근접한 것을 확인할 수 있었다. 그 중 PEBAX/PEI-GO 0.3wt%, 0.5wt% 혼합막은 upper bound를 넘는 우수한 성능을 보였다.

열처리 방법으로 제조된 환원된 그래핀 산화물의 물리화학적 특성

이두원 인하대학교 대학원 2015 국내석사

Graphene, a monolayer form of graphite with a two-dimensional honeycomb lattice, has shown many interesting properties: electrical, thermal, mechanical properties. These properties make graphne for many applications. For example, electronic devices, composites, sensors, energy storage and so on. These applications of graphene, the development of suitable synthesis processes is necessary. This research studied the synthesis method of graphene oxide via a low-cost and efficient manufacturing method and reduction method of graphene oxide. The synthesis started with the chemical oxidation of graphite powder to graphene oxide by several modified Hummers' method. And using thermal reduction method form a tube funace, exfoliation of graphene oxide was processing. Finally, exfoliated graphene oxide was analyzed various method, found the best method of graphene synthesis. The samples of reduced graphene were characterized by XRD, FT-IR, optical analysis, BET, Raman spectroscopy and TEM methods. Also, through electrochemical analysis, capacitance was calculated.

Functionalization and Manipulation of Graphene oxide and Reduced Graphene Oxides for Supercapacitor Application : 슈퍼커패시터 응용을 위한 산화흑연 및 환원형 산화흑연의 기능화 및 개질

전하이 대구대학교 2017 국내박사

긴 수명 시간, 높은 출력 밀도, 그리고 친환경적 특성을 가진 그래핀 기반 슈퍼커패시터가 최근 뜨거운 주제이다. 그러나 이중층 커패시터인 슈퍼커패시터는 실제 사용 및 응용을 위하여 에너지 밀도를 향상시켜야 하는 단점이 있다. 이를 보완하기 위하여 그래핀 기반 슈퍼커패시터의 개질 및 기능화를 해야한다. 특히 그래핀은 자연적으로 층층이 다시 모이는 적층 성질이 있어 그래핀 고유의 좋은 특성들을 방해한다. 이는 동시에 슈퍼커패시터 응용을 위한 전기화학적 특성을 저해한다. 본 논문은 슈퍼커패시터 응용을 위한 산화흑연 및 환원형 산화그래핀의 개질 및 기능화에 대한 제안 및 연구결과이다. 산화흑연은 브로디 방법에 의해 합성되었으며 여러가지 방법으로 환원형 산화흑연을 합성하였다. 열 및 화학적 방법에 의한 환원형 산화흑연을 제조하여 이들의 전기화학적 특성을 비교하였으며, 플라즈마 처리 전∙후의 산화흑연 (그래핀옥사이드)의 특성을 조사하였다. 또한 폴리머, 글루코스, 니켈, 은 나노입자, 활 물질, 나노튜브 등을 이용한 산화흑연 (그래핀옥사이드) 복합체를 합성하였으며 이들의 물리화학적 특성 및 전기화학적 특성들을 다양한 각도로 조사하였다. 결과적으로 산화흑연 (그래핀 옥사이드)은 슈퍼커패시터 응용에 매우 적합한 물질이며, 에너지 저장 소자인 배터리의 대체물질이다는 것을 증명하였고, 앞으로 이에 관련된 연구들을 지속해야 할 것이다. Graphene based supercapacitors have become hot topics recently for alternative conventional battery with outstanding properties such as longer cycle life time, higher power density, and safer environments. However, supercapacitors, based on pure double-layer capacitance, need to improve energy density for practical applications. Modification and functionalization of the graphene based supercapacitors are necessary for better performance of the supercapacitor. Besides, graphene layers tend to reagglomerate naturally, obstructing its unique excellent properties which limit electrochemical performance simultaneously. This dissertation will propose the modification and functionalization of graphene oxide and reduced graphene oxide, for supercapacitor applications. Graphene oxide (GO) was synthesized by Brodie method and then reduced by different ways in order to obtain reduced graphene oxide (RGO). This study represents comparison of electrochemical performance of reduced graphene oxide by thermal and chemical methods. A new approach for reduction of graphene oxide by plasma was also investigated. Besides, there are different composites of RGO (or GO) and diversity agents were explored, including of poly (sodium 4-styrensulfonate) (PSS), glucose/ nickel, silver nanoparticles and activated carbon/carbon nanotube. Each composite represent electrochemical properties superior to pure precursors. It proved that graphene is suitable material for supercapacitor application, a potential battery alternative device for energy storage and should be studied more interesting researches in the future.

Theoretical and Empirical Approaches to Easy Band Gap Engineering of Graphene for a Variety of Applications

박지수 서울대학교 대학원 2020 국내박사

그래핀은 그 자체가 가지고 있는 기존 재료 대비 뛰어난 특성 때문에 그 동안 큰 주목을 받아왔다. 이에 따라, 다양한 응용 분야에서 그래핀을 사용하여 고성능 장치를 구현하고자 수 많은 연구들이 진행되어 왔다. 그러나, 그래핀 제조 공정에 의해 변경 될 수 있는 그래핀의 그 자체의 재료 특성에 대한 인지 부족으로 인해 실제 디바이스에 대한 적용 시 요구되는 다양한 범위의 그래핀을 만족시키지 못하여 그래핀이 가지고 있는 뛰어난 물성을 완전하게 구현하는데 실패하였다. 특히, 전자소자로의 응용의 경우, 전기적 특성을 결정하는 중요한 특성인 밴드갭은 그래핀의 경우 존재하지 않으며, 물리적 혹은 화학적 구조 변화에 의해서만 확보 될 수 있으므로, 그래핀 기반 장치의 성능의 비약적인 향상을 위해 그래핀의 밴드갭 확보 과정에 대한 분석이 필수적이라고 할 수 있다. 이를 위해 일부 연구자들은 밴드갭을 결정하는 그래핀 자체의 물질적 파라미터를 찾아내고자 하였으나, 그 밴드갭과 물질적 파라미터 간의 관계를 실질적으로 명확하게 밝혀내는 데에는 실패하였다. 이러한 그래핀 연구의 장애물을 극복하기 위해서는 기존 연구진이 시도해왔던 것과는 차별화하여, 그래핀이 가지고 있는 탄소원자를 기반으로 그래핀의 밴드갭을 제어하는 숨겨진 파라미터를 밝혀내는 것이 중요하다. 따라서, 이 연구는 기존의 관점에서 탈피하여 공액구조를 가지고 있는 탄소원자의 비율과 경계탄소의 비율, 그리고 그래핀의 밴드갭 사이의 관계 확립에 중점을 두어 명확한 선형관계가 성립함을 입증하였다. 또한, 대표적인 응용 분야들을 위해 실제 미세 조정된 밴드갭을 갖는 그래핀을 실험적으로 적용하여 그 효과를 실질적으로 증명하고자 하였다. Part I 에서는 그래핀의 밴드갭 미세 조정에 대한 배경과 필요성을 정리하였다. 그래핀의 밴드갭 엔지니어링에 대한 현재 연구 동향을 면밀하게 검토하였으며, 이를 기반으로 그래핀의 이론적으로 예측된 성능과 실제 그래핀 기반 장치 사이의 격차를 극복하기 위한 이 연구의 목적을 확립하고자 하였다. Part II에서는 이론적으로 그래핀의 밴드갭과 표면 파라미터 간의 관계를 예측하고, 이를 검증하기 위해 기본적인 이론을 확립하는데 중점을 두었으며, 공액구조를 가지는 탄소원자의 비율과 그 중 경계탄소의 비율을 기반으로 표면 특성에 따라 실제 밴드갭을 결정하는 주요 파라미터를 도출하고자 하였다. 주요 재료적 파라미터들은 기본 전자 공학 이론을 기반으로 검토 후 도출하였다. 이를 바탕으로 화학적인 방법으로 합성된 그래핀 옥사이드 기반 그래핀의 밴드갭을 예측하기 위해 공액구조를 가지는 탄소원자 및 공액구조 경계 탄소원자의 비율을 도입하였으며, 이들 파라미터들과 실제 그래핀 옥사이드 기반 그래핀의 관계에 대한 실험적인 검증을 통해 확립된 이론의 신뢰성을 확보하고자 하였다. Part III은 Part II에서 제시된 그래핀 밴드갭과 그 자체의 물질적 파라미터 간의 관계를 따라 맞춤형 밴드갭을 갖는 그래핀의 설계 및 합성에 중점을 둔다. Chapter 3에서는 그래핀의 가장자리 선택적 산화에 의해 환원 후 그래핀 옥사이드 기반 그래핀의 밴드갭을 낮추는 것을 시도한다. 그 결과 그래핀 산화물의 환원공정 동안 발생하는 결함을이 기존 그래핀 옥사이드에 비해 크게 감소하었으며 그 결과 환원된 그래핀의 전기전도도를 크게 향상시키는데 성공하였다. Chapter 4 에서는 그래핀 옥사이드의 표면의 에폭사이드 작용기를 선택적으로 감소시킴으로써, 표적 가스 분자의 흡착 및 전하 운반체의 이동을 향상 시켜 타겟 물질에 대한 감도를 증가시키는데 성공하였다. Chapter 5에서는 인듐-갈륨-아연-산화물 (IGZO) 박막 트랜지스터의 채널 물질의 전기적 수송을 개선하기 위해 그래핀의 양에 따른 전체 계의 밴드갭을 미세하게 조정하였고, 채널 물질의 열처리 동안 기공 발생을 최소화하기 위해 에폭사이드 작용기가 감소된 그래핀 옥사이드를 사용하여 밴드갭이 조정된 그래핀 / IGZO 복합체의 전하 수송을 효과적으로 증가시켰다. 이 연구는 그래핀 기반 장치의 성능과 관련된 변수에 대한 이론적 고려를 통해 그래핀의 밴드갭에 영향을 미치는 주요 재료 매개 변수를 도출하고 이를 실질적으로 검증한다는 점에서 그 의의를 찾을 수 있으며, 실제적으로 대표적인 적용 분야에서 성능 향상을 보인 만큼, 이 연구에서 제시된 그래핀 설계 지침은 이론적으로 예측된 그래핀의 성능과 실제 장치 성능 사이의 격차를 극복하고 차세대 그래핀 기반 재료로 연구를 발전시키는 것에 대한 통찰력을 제공 할 수 있을 것이라 사려된다. Graphene has been taken attention because of its astounding properties than other transitional materials. Therefore, there have been tremendous studies on developing high performance devices using graphene in a variety of applications. However, many researchers failed to achieve devices with high performance because of negligence about the material properties of graphene that can be altered by the preparation process. Especially in the case of electronic applications, band gap, which considered as important property that decides electrical properties, can be altered by the physical and chemical characteristics of the graphene itself. Therefore, for the improved performance of graphene based devices, fine tuning of band gap is essential. For the fine tuning of graphene’s band gap, some of the researchers tried to find the materials parameters of graphene which decide the band gap itself, but failed to find the clear relationship. In order to overcoming this huddle, unveiling of hidden parameters which control the band gap of graphene based on carbon atom is important. Therefore this research is focused on the establishment of the relationship between sp2 carbon and graphene’s band gap. In addition, through the empirical application of graphene which has fine-tuned band gap for representative applications. Part I summarizes the background and necessity for fine tuning of graphene’s band gap. The current research trends in the band gap engineering of graphene are reviewed. In order to overcome the gap between the theoretically predicted performance of graphene and the practical graphene based devices, the objectives and scopes of this study were established. In Part II, basic fundamentals are studied to examine the theoretically predicted band gap of graphene. Based on the sp2 carbon, key parameters that determine practical band gap according to the surface characteristics are derived. Key material parameters are derived by examining the theory of basic electronics, while the area of sp2 carbon and boundary sp2 carbon is introduced to predict the band gap of graphene from chemical route. And this relationship between sp2 carbon and band gap is experimentally reviewed and extended to graphene embedded materials. Part III focuses on the design and synthesis of the graphene with tailor fitted band gap according to the design guidelines presented in Part II. Chapter 4 attempt to overcome the lower the band gap of graphene by induced defect. In Chapter 4, edge selectively oxidation of graphene are used to overcome the induced defect on basal plane during reduction process of graphene oxide, resulting in improved electrical conductivity. In Chapter 5, the surface of the graphene oxide is modified to improve the absorption of target gas molecule and transfer of charge carrier by selectively reduction of epoxide group, thereby increasing the sensitivity of the sensing material. In Chapter 6, graphene is embedded in indium-gallium-zinc-oxide (IGZO) for the improvement of electrical transport of channel materials of thin film transistor. By using lower functionalized graphene oxide to minimize the pore generation during heat treatment of channel materials to effectively increase the charge transport of band gap tuned graphene / IGZO composite. As a result, the mobility of graphene embedded thin film transistor is maximized resulting in much lowered roughness. This study derives key material parameters that affect the band gap of graphene through theoretical considerations for variables related to the performance of graphene based devices. The graphene design guidelines presented in this study can provide insight to overcoming the gap between the theoretically predicted performance of graphene and practical device performance in addition to advancing the research into next generation graphene based materials.

그래핀 산화물을 다양한 역할로 적용한 광전자소자의 특성 향상 연구

류버들 전북대학교 일반대학원 2016 국내박사

Graphene is a one atom-thick and closely packed two-dimensional (2D) sp2-bonded carbon honeycomb lattice. Among the obtained methods of graphene sheets, graphene oxide (GO) is generated through exfoliation of graphite via oxidation, which is referred to as the modified Hummer’s method. GO has received much attention because not only it is more available and easily scalable than other 2D carbon materials, but also it has tunable electrical and optical properties that are tuned through thermal annealing process. In chapter 3, the performance of Si-based solar cells based on GO sheet’s size was investigated. Large-sized reduced GO (rGO) with an in-plane crystalline diameter of 3.42 nm has smaller defect sites and thus the Si/rGO Schottky junction solar cell shows a lower leakage current than the solar cell with small-sized rGO (i.e. an in-plane crystalline diameter of 3.03 nm). Enhanced open-circuit voltage (Voc) and improved short-circuit current (Jsc) are observed for the solar cell with large-sized rGO due to the increased work function and Schottky barrier height at the Si and rGO junction. In other words, an increased built-in potential and a wider depletion region of the solar cell with large-sized rGO contribute to the increased carrier absorption and generation. These findings indicate that (i) rGO acts as a good transparent conducting layer and hole-transporting layer, and (ii) the control of rGO size in Si/rGO Schottky junction solar cell is important to improve the performance. Next chapter, a reduced graphene oxide (rGO) layer inserted between a poly(3,4-etylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Si interface improves the stability of Si-organic hybrid solar cells. The thermal reduction of graphene oxide layer on a Si substrate formed an rGO layer that acted as an effective passivation layer to suppress the surface oxidation, results to small variation in water contact angle over one-month. It indicates that the high-temperature thermal reduction generates smaller aromatic sp2 domain size that changes the GO surface from hydrophilic to hydrophobic. Additionally, the thin thickness between reduced GO layers can serve as a barrier for both liquid and vapor permeation. Consequently, the PEDOT:PSS/Si device with rGO passivation layer shows a significantly enhanced air-stability from the J-V characteristic under illumination during a one-month storage period. The application of rGO is a promising technique to extend the stability of Si-organic solar cells without the encapsulation process. In chapter 5, a rGO layer was produced on undoped and n-type GaN, and its effect on the current- and heat spreading properties of GaN-based light-emitting diodes (LEDs) was studied. The rGO inserted between metal electrode and GaN semiconductor acted as a conducting layer and enhanced lateral current flow in the device. Especially, introduction of the rGO layer on the n-type GaN improved the electrical performance of the device, relative to that of conventional LEDs, due to a decrease in the series resistance of the device. The enhanced current spreading was further of benefit, giving the device a higher light output power and a lower junction temperature at high injection currents. These results therefore indicate that rGO can be a suitable current and heat-spreading layer for GaN-based LEDs. Additionally, the temperature-dependence optical and electrical performances of rGO electrode embedded LED with increasing temperature figure out. The optical properties of rGO electrode embedded LED show a slight decrease with increasing temperature, as compared to the conventional LED. The inserted rGO electrode is beneficial for long current spreading length, wide effective active area, and better heat dissipation, resulting to the enhanced optical property. Moreover, the crystal quality enhances during regrown GaN on the rGO layer, whereupon the non-radiative recombination lifetime is longer in MQWs by TRPL data at room temperature. The defect-assist tunneling current of rGO electrode embedded LED impedes by the low defect state density in MQWs. Consequently, the rGO electrode embedded LED enhances the optical and electrical performance. Our study shows that the rGO layer is suitable as an electrode and as heat dissipation layer in high-efficiency LEDs. In the final chapter, the three-dimensional hybrid and layer-by-layer structures coated on the Al heat sink by use of the GO and Copper nanoparticles (Cu NPs) for heat distribution. The combination of Cu NPs and GO enabled improvement of the heat dissipation property. The thermal resistance of our proposed structure is lower 46 % than that of Al heat sink. It means that the horizontal heat transfer is better through GO surface, and Cu NPs help to better dissipation on vertical direction between GO and GO layer. Therefore, the proposed structures are possible effective thermal management for long operating lifetime in the high performance of LEDs.

Graphene-based electrode materials for supercapacitor applications

Khoh, Wai Hwa Incheon National University 2016 국내박사

Extensive research efforts have been made on development of high performance Li-ion batteries and fuel cells in the past. However, low power density, low charge/discharge rate and environmental unfriendliness have kept them away from many applications. Recently, supercapacitors have drawn great attention because of their high charge/discharge rate, long life cycle, high power density and environmental friendliness. However, supercapacitors generally have low energy density. Electrode material with desirable properties is the key for improving performance supercapacitors. Graphene has been considered a promising electrode material for supercapacitor applications due to its unique lattice structure, excellent electrical conductivity, chemical stability and high surface area (2600 m2/g). However, graphene nanosheets tend to aggregate and restack to multilayer structure, thus, leading to a dramatic decrease in the surface area and hinder ion-diffusion from the electrolyte to the electrode, resulting in low electrochemical performance. Therefore, our research have mainly focused on development graphene-based supercapacitor by introducing either metal oxide nanoparticles or conducting polymer into the interlayer spacing of graphene nanosheets to prevent them from aggregating and also to enhance the energy density of the supercapacitor. These materials were designed to enlarge the interplanar spacing with aiming at maintaining the high surface area of graphene nanosheet, and making both sides of the nanosheets accessible. Moreover, these metal oxides and conducting polymer were able to contribute an additional electrochemical performance in the supercapacitor. This dissertation is divided into several chapters, each discussing a specific topic. Chapter 2 describes the application of layer-by-layer (LBL) self-assembled multilayer film composed of Fe3O4/reduced graphene oxide in supercapacitor applications. Chapter 3 and 4 described the assembly of asymmetric supercapacitor using ITO substrate and flexible PET substrate, respectively. Chapter 2 illustrated the electrochemical properties of LBL self-assembled multilayer films composed of magnetite (Fe3O4) nanoparticle and chemically reduced graphene oxide (RGO) for supercapacitor application. The thickness of the Fe3O4/GO bilayer was determined by using optical ellipsometry to be 6.53±0.17 nm, which agreed well with the sum of the independently measured thickness; 5.61±0.14 nm for a Fe3O4 layer and 0.91±0.1 nm for a GO layer component, respectively. Each layer was found to be deposited uniformly and regularly. Fe3O4/RGO multilayer film was obtained by reducing a Fe3O4/GO film using hydrazine. The multilayer film yielded a minimum resistance of 1.0 × 104 Ω/sq (2.8 S/cm) for a film composed of 50 Fe3O4/GO bilayers. The electrodes fabricated from 30 Fe3O4/RGO bilayers exhibited excellent capacitive performances with a maximum specific gravimetric capacitance (151 F/g) at a current density of 0.9 A/g. In Chapter 3, manganese dioxide/reduced graphene oxide/indium tin oxide (MRI) and polypyrrole/reduced graphene oxide/indium tin oxide (PRI) electrodes were prepared via the chronopotentiometric deposition of either manganese oxide or polypyrrole, respectively, onto a RGO/ITO film at a constant current density. Solid-state asymmetric supercapacitors (ASC) were assembled with MRI as the positive electrode and PRI as the negative electrode in a PVA/LiCl gel electrolyte. These devices displayed a power density of 7.4 kW/kg (for an energy density of 13 Wh/kg), an energy density of 16 Wh/Kg (for a power density of 0.3 kW/kg), and a capacitance retention of 75% over 2000 cycles. The MRI//PRI ASC exhibited a much improved capacitive performance compared to the symmetric PRI//PRI (3 Wh/kg at 0.47 kW/kg) and MRI//MRI (9 Wh/kg at 0.12 kW/kg) supercapacitors. The superior capacitive performance of the MRI//PRI ASC was ascribed to the improved conductivities and mechanical stabilities of MRI and PRI electrodes, which were obtained by fabricating either the polypyrrole or manganese oxide films on a graphene-coated electrode. In Chapter 4, flexible solid-state asymmetric supercapacitor based on multilayer of [Polyaniline(PANI)/PEDOT/PANI/reduced ultra-large GO]n (PPPrG) and PEDOT/MoS2 (PMo) multilayer film were successfully fabricated on flexible PET substrate using a facile coating method like either LBL self-assembly or drop-coating. The PPPrG and PMo electrode were integrated into a flexible supercapacitor cell with including PVA/H2SO4 gel electrolyte. The structure of flexible supercapcitor was simplified with no need of expensive current collector and binders, because the multilayer film work well as the active electrode and current collector owing to the high conductivity. This ASC showed the maximum energy density of 5.4 mWh/cm3 at a power density of 110 mW/cm3, and still maintained 4.0 mWh/cm3 at the power density of 265 mW/cm3 in optimized cell voltage of 0.8 V. Moreover, the ASC exhibited excellent mechanical flexibility without sacrificing electrochemical performances. The superior capacitive performance of the PPPrG//PMo ASC was ascribed to the synergistic combination of good electrical conductivity in the rULGO and a high pseudocapacitance of the eletroactive materials including PANI and PEDOT. Despite the intensive research activities for the enhancement electrochemical performance of supercapacitor, scientists are still looking for the advanced novel materials, which would satisfy the complex requirement for high performance of the supercapacitor including energy density, power density, long-term cycle stability, and low cost production.

Band Gap Engineering of Graphene Oxide by Controlling Amount of Oxygen-containing Functionality

김유리 서울대학교 대학원 2019 국내석사

Graphene is a two-dimensional nanomaterial with carbon atoms arranged in a hexagonal lattice. Due to its excellent physical, optical and electrical properties, various applications have been proposed. Particularly, when graphene is applied to a transistor device, it becomes possible to integrate more transistors on a circuit than a silicon semiconductor. However, graphene is semimetal with band gap close to zero, which make it not suitable for realizing a logic circuit. Therefore, studies for increasing the band gap of graphene have been actively conducted in various fields. Among them, the method of utilizing the oxygen functional group of graphene oxide, which is an intermediate stage of graphene synthesis, is excellent in commercial value in that the process is simple and mass production is possible. In addition, analogous effects can be obtained without introducing hetero elements, and the hydrophilicity of the graphene is enhanced due to the oxygen functional group, which enables various applications. However, experimental data on how various oxygen functional groups affect the band structure of each graphene are insufficient, and the application of graphene oxide to the transistor material is very limited. Therefore, in order to overcome these limitations, this study investigate the specific relationship between the oxygen functional group of graphene and the electrical properties including band gap. For the purpose, different reduction reactions were applied to graphene oxide, and the composition of the functional groups were adjusted by controlling degree of reduction. The changed band gaps were measured to analyze the quantitative relationship with the functional groups. Analysis of the relationship between the material properties and electrical properties of graphene showed that single and double bonds of carbon and oxygen have significantly different effects on the band gap. Also, the band gap tuning effect of the hydroxyl group was figured out to be tenuous. Based on the analytical results, we suggested the synthesis direction for synthesizing graphene with specific performance. The results of this experiment can be expected to improve the usability of graphene oxide in electronic devices. 그래핀은 탄소 원자가 육방 격자로 배열된 2차원 나노물질이며, 뛰어난 물리적, 광학적, 전기적 성질로 인해 다양한 응용이 제시되어 왔다. 특히 그래핀을 트랜지스터 소자에 적용하면 실리콘 반도체 이상의 집적화가 가능하게 된다. 그러나 그래핀은 밴드 갭이 0에 가까운 반금속으로, 논리 회로를 구현하는 데에는 적절하지 않다. 따라서 그래핀의 밴드 갭을 증가시키기 위한 연구가 다방면에서 활발하게 진행되어 왔다. 그 중에서도 흑연을 산화시켜 박리한 형태인 그래핀 옥사이드의 산소 작용기를 활용하는 방법은, 공정이 경제적이고 대량 생산이 가능하다는 점에서 상업적 가치가 뛰어나다. 더불어 이종 원소를 도입하지 않고도 유사한 효과를 이끌어낼 수 있고, 산소 작용기로 인해 그래핀의 친수성이 향상되어 다양한 응용이 가능하다. 그런데 다양한 종류의 산소 작용기들이 각각 그래핀의 밴드구조에 어떻게 영향을 미치는지에 대한 실험적인 자료가 미비하고, 그로 인해 그래핀 옥사이드의 트랜지스터 재료에의 적용이 매우 제한적이다. 따라서 본 연구에서는 이러한 한계점을 극복하기 위해서, 그래핀의 산소 작용기와 밴드 갭을 비롯한 전기적 성질과의 실험적 관계를 연구하였다. 이를 위해 그래핀 옥사이드에 서로 다른 종류의 환원 반응을 적용하였고, 환원 정도를 조절하여 작용기의 조성을 제어하였다. 그리고 그로써 변화한 밴드 갭을 측정하여 작용기와의 정량적 관계를 분석하였다. 이와 같이 그래핀의 재료적 성질과 전기적 성질의 관계를 분석한 결과, 그래핀 원자와 산소 원자의 서로 다른 결합이 밴드 갭에 미치는 영향이 상당히 다름을 보였다. 또한 수산기의 밴드 갭 조율 효과는 매우 미비한 것으로 나타났다. 이러한 분석 결과를 바탕으로 특정 성능을 갖는 그래핀을 합성하기 위한 합성 방향성을 제시할 수 있었다. 본 연구를 통해 전자 소자에서 그래핀 옥사이드의 활용성이 향상될 것을 기대할 수 있을 것이다.

Graphene Oxide-Coupled Surface Plasmon Resonance Detection of Immunoassays

Ryu, Yeonsoo 연세대학교 대학원 2014 국내박사

As our society ages, the number of patients suffering from chronic diseases, including dementia, cardiovascular disease, hypertension, and cancer, is rapidly increasing. New epidemics such as severe acute respiratory syndrome, foot-and-mouth disease, and avian influenza, which are caused by various types of molecular-sized viruses, have been frequent recently. To diagnosis and cure these diseases in the early stage, detection and monitoring of low-molecular analytes have become more important than ever before. To satisfy these demands, it is necessary to develop high-performance biosensors. Although surface plasmon resonance (SPR) biosensors have received much attention for their label-free detection and real-time sensing, it is still difficult to detect small molecules (< 2 kDa) and monitor inter-molecular reactions using them. This thesis briefly reviews the important issues with regard to the development of SPR biosensors, such as the types of biosensors and detection, surface plasmon phenomenon, graphene oxide application, surface functionalization, and sensitivity. This thesis also presents two studies: (1) a study of the correlation between the field-target overlap and sensitivity of SPR biosensors and (2) a comparison of graphene oxide-coupled SPR detection and metal-based SPR detection. Many experimental results have demonstrated improved sensitivity, but the mechanism behind the enhancement is not yet clear. Therefore, I investigated the correlation of the detection sensitivity of SPR biosensors with the optical signatures related to the near-field overlap of biomolecules. The sandwich antibody-antigen interaction and reverse sandwich interaction were used to measure the resonant angle shift. Human and antihuman immunoglobulin G molecules were used as the antigen and antibody, respectively. The detection sensitivity of the biosensor was examined using the correlation with the field-target overlap in the near-field. The results show that the overlap and detection sensitivity are strongly correlated, both theoretically and experimentally, with correlation coefficients exceeding 95% in all cases. A gold surface has been used as a platform for biosensors, but it is limited by relatively poor adsorption with biomolecules. Since Prof. Geim received the Nobel Prize in 2010, there have been many studies of graphene and graphene oxide because of its superior electrical and optical properties. Graphene oxide is an intermediatry obtained during fabrication of graphite into graphene. It has great potential for application in SPR biosensors because of its superior characteristics such as good water dispersibility, high mechanical strength, facile surface modification and sp2/sp3 existing structure. Therefore, I tried to measure the detection sensitivity of graphene oxide-coupled SPR after depositing a SiO2 spacer and graphene oxide on the gold surface. The resonance angle shift of the biosensors was studied by experimentally comparing graphene oxide-coupled SPR detection and conventional SPR detection. Graphene oxide-coupled SPR detection enhances the resonant shift to at least 113% of that of conventional SPR detection. This is the first attempt to calculate and measure the detection sensitivity of an SPR biosensor based on graphene oxide deposited by Langmuir-Blodgett assembly. This thesis confirms the feasibility of applying graphene oxide to SPR biosensors. The results of this research will contribute to the development of a high-performance SPR biosensor that can detect small molecules, in particular, at low concentrations.

Reduced graphene oxide sponge and its applications as oil-water separator and stress sensor

Khan, Fakhre Alam Sungkyunkwan university 2019 국내박사

Graphene as nanomaterial has contributed significantly towards the recent advancements in engineering applications. Graphene oxide (GO) functional groups provide advantage to use it for synthesis of free standing three dimensional sponge structure. Three dimensional graphene oxide sponge can be synthesized using two main techniques. Chemical reduction (self-assembly) and freeze drying (direct drying) methods. Due to inherent porous structure, graphene oxide sponge can be used in various engineering applications including, energy storage devices, sensors, composites materials, absorbents and separation membrane. This thesis focuses on graphene oxide based sponge synthesis and their application as oil-water separation membrane and stress sensor. Membranes with fixed wettabilities have an issue of separating only one type of emulsion, i.e. either oil-in-water or water-in-oil emulsion. Graphene oxide sponge is also fixed wettability membrane because chemically reduced graphene oxide sponge is hydrophobic and super-oleophilic in nature. It can only be used for separating of only one type of emulsion i.e. water-in-oil. It will allow oil to pass through its pores while rejecting water. This issue can be resolved by inducing Janus (materials with dual behavior on its opposite ends) characteristic to graphene oxide sponge. One side of Janus graphene oxide (J-GO) will be super-hydrophilic due to inducing oxygen functionalization by O2 plasma and the opposite end of graphene oxide sponge will chemically fluorinated to induce super-hydrophobicity. The oxygen functionalized side of Janus graphene oxide sponge wettability was changed from (oleophilic  oleophobic) when the sponge was immersed in water from air. This change in wettability behavior was supported by Young-Laplace theory of wettability. Finally J-GO sponge was used to separate both oil-in-water and water-in-oil emulsions by switching the flow directions at low pressure compared with other reported Janus membranes. The separation permeability of Janus graphene oxide sponge was an order of magnitude higher than reported Janus membranes in literature. Next, graphene oxide sponge can also be used as stress sensor due to its three dimensional porous structure. Pure graphene oxide sponge has low structural strength, due to which it cannot be used for higher stress applications. So in order to use graphene oxide sponge for higher stress applications, we introduced polyimide (PI) polymer to enhance its mechanical strength. The polymer addition causes lowering in electrical conductivity of graphene oxide-polymer composite sponge. Soft conductive materials should have higher conductivity even at large deformation. In order to compensate for lower electrical conductivity we introduced highly conductive and high surface area, flower shaped metal nano-particles to the composite sponge structure. Sensitivity (ratio of normalized resistance to applied stress) of the composite graphene oxide sponge was 2 times higher than that of reported literature and with wide operating stress range. Conductivity of nanoparticle based sponge was an order of magnitude higher than graphene oxide and polymer based sponge. This sensor can be used to detect low stress (holding flower) as well as high stress (holding vase) applications. In order to increase structural integrity of GO sponge, addition of polymer using conventional techniques results in drastic electrical conductivity degradation. In order to increase mechanical strength without compromising on electrical conductivity, a novel Joule heat polymer curing technique at GO junction was introduced. Due to low mass of polymer attachment to sponge structure, the electrical conductivity of Joule heat cured GO sponge was more than 2 orders of magnitude higher than that of conventional oven cured GO sponge.