In this study, the fabrication of highly efficient transparent conducting carbon nanotube (CNT) films is demonstrated by hybridization with metal nanoparticles. Pristine and acid-treated single-walled carbon nanotubes (SWNTs) are hybridized with gold nanoparticles to investigate charge transfer from SWNTs to gold ions. Double-walled carbon nanotubes (DWNTs) films are hybridized with gold nanoparticles to improve their electrical properties. Separation of SWNTs by their electronic types is achieved by combining hybridization with metal nanoaprticles and density gradient ultracentrifuge (DGU). Characterization such as optical absorption, electrical conductivity, work function and morphology is carried out for the basis of opto-electronic applications.
We examined the electrical conductivity and transmittance of six types of SWNT network films, de-veloped for potential use in transparent conductive films, to investigate the effect of Au doping on single-walled carbon nanotube (SWNT) defects: i) SWNT network films without Au doping (as-produced (AP-SWNTs), purified (P-SWNTs) and cut (C-SWNTs)), and ii) Au-doped films of AP-SWNTs (Au/AP SWNTs), P-SWNTs (Au/P-SWNTs) and C-SWNTs (Au/C-SWNTs). The density of SWNT network films was varied systematically by controlling the SWNT concentration and mediating defect formation. The electrical conductivity of each SWNT film was found to be significantly enhanced upon Au doping, with a negligible loss of optical transmittance. The effects of Au doping differed depending on the type of nanotube network present: the electrical conductivity of Au/C-SWNTs was found to be similar to that of Au/AP-SWNTs for a given transmittances, whereas the conductivity of C-SWNTs is much lower than that of the AP-SWNTs prior to Au doping. Therefore, the electrical conductivity was enhanced by the hybridization of gold nanoparticles to a larger degree in the defect-containing SWNT networks than in the AP-SWNT networks. This unusual elec-trical behavior of SWNT network films may be attributed to an increased number of gold nanoparticles depo-sited onto C-SWNT surfaces, which are formed from C-SWNTs containing an abundance of carboxylic groups.
The electrical conductivities and stability of double-walled carbon nanotube (DWNT) network films upon post-treatment via HNO3 or gold ions are investigated for their potential use as transparent conducting films. Post-treatment of DWNT films using HNO3 and deposition of gold nanoparticles, respectively, resulted in a significant decrease of the electrical resistance, while the initial value of the transmittance of pristine-DWNT films was maintained. The DWNT-Au nanoparticle hybrid films exhibited the best transmittance-conductivity performance among the evaluated network films, i.e., pristine-SWNT films, pristine-DWNT films, SWNT-Au nanoparticle hybrid films and HNO3 treated DWNT films. The electrical behaviors of the gold nanoparticle-coated and HNO3 treated DWNT films were examined by an analysis of the work-funtions of the DWNT films. The long-term stability of the films varied depending on nanotube type and post-treatment method, with gold ion-treated nanotubes exhibiting higher stability than HNO3-treated nanotubes. Moreover, DWNTs showed better stability than SWNTs, when treated with gold ions. Work function and optical absorption spectral measurements suggested that the de-doping process and the contact resistance of the nanotube networks may be important for stability under ambient conditions.
Next, a method for fabrication of highly efficient transparent conducting SWNT films is described. The process involves reduction of metal ions on SWNTs and DGU process. First, transparent conducting SWNT films using pre- and post-deposition of gold nanoparticles resulted in a significant decrease of the elec-trical resistance, while the initial value of the transmittance of pristine-SWNT films was maintained. The double Au nanoparticle hybrid films exhibited 5 times lower sheet resistance compared to pristine SWNT films. Second, metallic or semiconducting enriched SWNTs are obtained by hybridization with metal nanoa-particles and subsequent DGU. The metal nanoparticles (Pd or Pt) are spontaneously formed on the SWNTs via a mechanism involving reduction of metal ions due to differences in reduction potentials. An advantage of this method is that it facilitates a selective control of chiral index of SWNTs. Especially, metallic enriched SWNT films had a sheet resistance of 42% value compared to that of pristine SWNT films at similar transmittance of 95%. These results are important for the development of surface-modified SWNTs for potential applications in solar cells, light-emitting diodes, sensors, and field-effect transistors.
본 연구는 탄소나노튜브와 금속나노입자를 혼성화하여 ITO를 대체하기 위한 고효율 투명전도성 박막을 제작한 후, 광학적 및 전기적 특성을 고찰한 것이다.
첫번째 연구로, 산으로 기능화 (정제 및 커팅)된 단일벽 탄소나노튜브를 이용하여 박막을 제조하고 금 나노입자 도핑과 결함정도가 단일벽 탄소나노튜브 박막의 전기전도성에 미치는 영향에 대해 고찰하였다. 결함 정도가 큰 정제 및 커팅된 탄소나노튜브 박막은 상대적으로 낮은 전도도를 나타냈으나, 금 나노입자 도핑 이후에는 기능화 하지 않은 금 나노입자가 도핑된 탄소나노튜브 박막과 비슷한 전기전도도를 나타내었다. 이러한 특이한 전기적 거동은 산을 이용한 정제 및 커팅 과정에서 단일벽 탄소나노튜브 벽에 생성된 카르복실 그룹에 의한 탄소나노튜브와 금 이온 사이의 촉진된 전하 이동의 결과로 설명할 수 있다.
두번째와 세번째 연구에서는 단일벽 탄소나노튜브보다 좀 더 많은 전도성 파이 채널을 가지고 있는 이중벽 탄소나노튜브를 질산과 금 나노입자로 도핑한 후, 투명전도성 박막으로의 가능성을 고찰하였다. 단일벽 탄소나노튜브와 이중벽 탄소나노튜브를 질산과 금 나노입자로 각각 도핑한 결과, 이중벽 탄소나노튜브를 금 나노입자로 도핑한 박막이 가장 우수한 전기 전도도 및 안정성을 나타내었다. 또한, 탄소나노튜브의 종류에 관계없이 금 나노입자 도핑은 산처리보다 우수한 안정성을 나타내었다. 이러한 결과는 공기중에서 디-도핑과 접촉 저항의 영향으로 설명될 수 있다.
끝으로, 고효율의 투명전도성 탄소나노튜브 박막을 제조하기 위한 방법으로서 새로운 두 가지 방법을 제안하였다. 첫째로, 기존에 보고된 탄소나노튜브 박막을 금나노입자 용액에 담그는 방법에 앞서서 탄소나노튜브-계면활성제 분산액에 금나노입자를 혼합하여 반응시킨 후, 용액상에서 도핑시키는 방법을 개발하였다. 용액 상에서 탄소나노튜브를 금 나노입자로 도핑한 후, 박막을 제작하면 50% 감소된 면저항을 나타내었다. 추가적으로, 용액 상에서 금 나노입자로 도핑된 탄소나노튜브를 이용한 박막을 다시 금 나노입자로 도핑하면 이중처리 전보다 80% 감소된 면저항을 나타내었다 (투과도 83 %에서 최저 면저항90 Ω/sq.). 두 번째로, 탄소나노튜브를 계면활성제 용액상에서 백금이나 팔라듐 나노입자로 처리한 후 밀도차 원심분리 방법을 적용하여 금속성과 반도체성((6,5)와 (10,5))이 증대된 탄소나노튜브를 얻었다. 이러한 결과는 산화-환원 전위차로 인해 자발적으로 백금 이나 팔라듐 나노입자가 탄소나노튜브 벽면에 형성되고, 이로 인해 발생한 무게차이를 이용해 특정 키랄리티를 갖는 탄소나노튜브를 얻는 것이다. 또한, 금속성이 증대된 탄소나노튜브를 이용하여 박막을 제조한 후 금 나노입자를 이용하여 도핑한 결과 기존 탄소나노튜브 박막보다 동일한 투과도(95%)에서 면저항이 75% 감소된 박막을 확보하였다.
