화학기상증착법을 이용한 그래핀 메쉬구조의 단일공정 합성과 가장자리 기능화

Abstract

Despite the theoretical prediction of the unique and fascinating properties of graphene, single layered graphene could not be successfully achieved over several decades. Recently, however, single layered graphene was successfully exfoliated from the graphite via mechanical peeling-off method (so-called ‘scotch tape method’), which significantly stimulated the researches on atomically thin two-dimensional (2D) materials. Following the discovery of graphene, new properties and the related applications of graphene have been explosively revealed within a decade. However, most of researches were focused on the advantageous aspects of graphene, while only a few attempts have been paid to make up for disadvantages of graphene. Graphene is the name given to a 2D sheet of carbon atoms densely packed into a honeycomb lattice. In particular, graphene has very strong carbon-carbon boding and long-range π-conjugation in graphene structure. This unique structure allows not only extraordinary properties (i.e., high thermal/electrical conductivity and mechanical strength) but also negative effect on chemical reactivity with foreign molecules. Therefore, these structural properties could become both pros and cons depending on its applications. For example, graphene can be useful as a passivation layer for protecting the penetration of gas molecules from the external environment or as a filter that can allow only specific molecules to permeate selectively. On the other hand, its strong stability that is associated with strong carbon-carbon boding has restricted modification of electrical properties of graphene. To solve this problem, some techniques based on the surface adsorption and fabrication of dangling bond have been adopted. In this dissertation, we have focused on the graphene mesh structures, in which the edges along the circular holes provide the stable and efficient sites for electrical property tuning. First, to synthesize the graphene meshes with clean and empty holes with very abrupt edge, direct chemical vapor deposition method have been developed. Before graphene growth, hexagonally-closed packed silica sphere array was deposited on metal foil as a growth mask. In this synthesize steps, silica bead prevents graphene growth under the silica bead and minimizes contamination which generally occurred during conventional lithography and etching processes. The edge sites of as-synthesized graphene meshes could help a covalent functionalization of graphene meshes, and as results, more reliable nitrogen doping was achieved after thermal treatment with ammonia gas. The existence of nitrogen (N)-dopants in graphene was investigated by Auger electron spectroscopy. In addition, the field effect transistors (FET) based on graphene mesh exhibited the remarkable shift of Dirac point to negative direction after N-doping, and moreover, this carbon-nitrogen bonding remained even after vacuum annealing. This result illustrates that N doping occurs mostly through covalent functionalization at the edges of graphene meshes and supports the possibility of semi-permanent N doping in graphene. For further investigation, graphene and graphene mesh were fabricated as a sensor by decoration with Pd nanoparticles. Compared with pristine graphene, the graphene meshes showed the sensitivity enhancement in low concentration of hydrogen gas, and also, it exhibited faster response to hydrogen gas. This result indicated the outstanding charge transport property of functionalized edges in the graphene meshes. This dissertation demonstrated that graphene meshes with functionalized edges can provide additional opportunities that could be limited in pristine graphene counterparts. By taking advantages of graphene meshes, the applications of graphene as future electronics could be greatly extended. |비록 그래핀의 존재 및 가능성은 오래 전부터 예견 되어왔지만, 2004년에 있었던 그래핀의 실제 구현은 특별한 이슈가 없이 정체 되어 있던 이차원 재료의 연구 분야에 있어 단비와 같은 사건이었다. 때문에, 그래핀과 그 응용소자에 대한 연구는 문자 그대로 폭발적으로 이루어져 왔다. 이로 인하여 매우 짧은 시간 만에 그래핀의 특성 및 응용분야에 대한 연구는 양적으로도, 질적으로도 크게 성장하였다. 하지만, 이런 빠른 흐름 속에 이루어진 대부분의 연구는 그래핀의 우수한 특성과 이를 이용한 응용에 집중되어 왔으며, 상대적으로 그래핀의 단점이나 이를 보완할 방법에 대한 연구는 미진한 형편이다. 그래핀은 그래핀을 이루는 탄소원자간의 매우 강한 공유 결합으로 이루어진 육각형 그물 모양 구조이며, 여분의 최외각 전자가 평면에 수직한 상태로 π-오비탈 선형결합을 이루고 있다. 이러한 구조적 특성으로 인하여, 그래핀은 매우 우수한 열적, 기계적, 전기적 특성을 가지고 있다. 한편으로는, 그래핀을 이루는 모든 탄소 원자가 서로간의 강한 결합에 속박되어 있기 때문에, 외부의 다른 분자들과 직접적인 화학적 결합이 불가능한 특성을 가지고 있다. 이는 마치 양날의 검과 같아, 경우에 따라서 장점도 단점도 모두 될 수 있다. 외부로부터 무언가를 보호하는 패시베이션(passivation)이나 물질간의 이동을 통제하는 필터(filter) 등으로써 역할을 할 때는 매우 큰 이점이 될 수 있는 반면, 도핑 등의 방법을 통해서 전기적인 특성을 바꾸는 데는 큰 제약이 되기 때문이다. 특히, 전자의 결정운동량과 에너지가 선형관계를 가지기 때문에, 일반적인 반도체 물질과는 달리 밴드갭이 없는 그래핀이기 때문에 이는 더욱 큰 약점으로 다가올 수 있다. 이러한 점을 극복 하고자, 그래핀 표면에 다른 분자들을 흡착 시키거나, 결함을 만들어주는 등의 시도들이 있어 왔다. 본 연구에서는 이들 여러 방법들 중에서, 가장 안정적이며, 동시에 효과적인 방법인 그래핀 나노메쉬 구조를 만들고, 이 구조의 가장자리를 기능화시키는 연구를 진행하였다. 특히, 본 연구는 정렬된 실리카 비드를 금속 포일 위에 올린 후, 화학기상증착법으로 그래핀을 성장시키기 때문에 단 한번의 공정만으로 그래핀 메쉬 구조를 성장시킬 수 있는 장점을 가지고 있다. 뿐만 아니라, 이렇게 만들어진 그래핀 메쉬는 일반적인 그래핀 패터닝 방법인 포토리소그래피나 플라즈마식각을 배재하였기 때문에 이로 인하여 발생될 수 있는 오염을 방지할 수 있는 큰 장점 역시 가지고 있다. 만들어진 메쉬는 암모니아 가스 분위기에 열처리를 해주어서 가장자리부분을 질소와 결합시키는 기능화에 성공하였으며, 실제로 오제 전자 분석과 전기적인 특성을 확인 하였을 때, 확실한 특성변화가 이루어졌음을 확인하였다. 또한 열증착기를 이용하여 무작위로 Pd particle을 올려 센서로서의 성능을 테스트 하였는데, 작은 농도의 수소 분위기에서 그래핀 메쉬의 경우가 일반 그래핀 보다 더 높은 민감도와 빠른 반응속도를 보임을 확인하였다. 무작위로 올라 갔음에도 이러한 차이가 나는 것은 가장자리가 일반 표면보다 더 우수한 전하전이특성(charge transport)을 가지고 있음을 암시한다. 이들 실험을 통해서, 메시구조가 그래핀의 단점들을 보완할 수 있음을 확인하였으며, 둘 간의 조화를 통해서, 그래핀 기반 소자의 응용분야를 확대시키는데 크게 기여할 수 있을 것이라 기대 된다.; Despite the theoretical prediction of the unique and fascinating properties of graphene, single layered graphene could not be successfully achieved over several decades. Recently, however, single layered graphene was successfully exfoliated from the graphite via mechanical peeling-off method (so-called ‘scotch tape method’), which significantly stimulated the researches on atomically thin two-dimensional (2D) materials. Following the discovery of graphene, new properties and the related applications of graphene have been explosively revealed within a decade. However, most of researches were focused on the advantageous aspects of graphene, while only a few attempts have been paid to make up for disadvantages of graphene. Graphene is the name given to a 2D sheet of carbon atoms densely packed into a honeycomb lattice. In particular, graphene has very strong carbon-carbon boding and long-range π-conjugation in graphene structure. This unique structure allows not only extraordinary properties (i.e., high thermal/electrical conductivity and mechanical strength) but also negative effect on chemical reactivity with foreign molecules. Therefore, these structural properties could become both pros and cons depending on its applications. For example, graphene can be useful as a passivation layer for protecting the penetration of gas molecules from the external environment or as a filter that can allow only specific molecules to permeate selectively. On the other hand, its strong stability that is associated with strong carbon-carbon boding has restricted modification of electrical properties of graphene. To solve this problem, some techniques based on the surface adsorption and fabrication of dangling bond have been adopted. In this dissertation, we have focused on the graphene mesh structures, in which the edges along the circular holes provide the stable and efficient sites for electrical property tuning. First, to synthesize the graphene meshes with clean and empty holes with very abrupt edge, direct chemical vapor deposition method have been developed. Before graphene growth, hexagonally-closed packed silica sphere array was deposited on metal foil as a growth mask. In this synthesize steps, silica bead prevents graphene growth under the silica bead and minimizes contamination which generally occurred during conventional lithography and etching processes. The edge sites of as-synthesized graphene meshes could help a covalent functionalization of graphene meshes, and as results, more reliable nitrogen doping was achieved after thermal treatment with ammonia gas. The existence of nitrogen (N)-dopants in graphene was investigated by Auger electron spectroscopy. In addition, the field effect transistors (FET) based on graphene mesh exhibited the remarkable shift of Dirac point to negative direction after N-doping, and moreover, this carbon-nitrogen bonding remained even after vacuum annealing. This result illustrates that N doping occurs mostly through covalent functionalization at the edges of graphene meshes and supports the possibility of semi-permanent N doping in graphene. For further investigation, graphene and graphene mesh were fabricated as a sensor by decoration with Pd nanoparticles. Compared with pristine graphene, the graphene meshes showed the sensitivity enhancement in low concentration of hydrogen gas, and also, it exhibited faster response to hydrogen gas. This result indicated the outstanding charge transport property of functionalized edges in the graphene meshes. This dissertation demonstrated that graphene meshes with functionalized edges can provide additional opportunities that could be limited in pristine graphene counterparts. By taking advantages of graphene meshes, the applications of graphene as future electronics could be greatly extended.