1차원 및 2차원 나노소재 하이브리드 기반 3차원 나노구조 설계 및 응용

Abstract

일차원-이차원 나노구조의 특성을 모두 가지는 하이브리드 구조는 다양한 분야에서 적용이 가능하고, 최근 들어 다양한 형태의 나노소재를 하이브리드 시킨 삼차원 개념의 나노구조가 필요로 되고 있다. 본 연구에서는 대표적인 일차원 나노구조인 나노선/막대와 이차원 나노구조인 그래핀을 이용한 삼차원 나노구조를 형성하여 일-이차원의 장점을 모두 가지는 하이브리드구조 제작의 관한 연구를 진행했다. 첫째로, 열저항가열(Joul heating)방식을 이용한 화학기상증착법(CVD)을 이용하여 이차원 나노구조인 대면적의 그래핀을 실리콘옥사이드( SiO2/Si) 기판 위에 성장시켰다. 이렇게 성장된 그래핀의 구조적, 광학적, 전기적 특성을 투과전자현미경 (TEM), 라만스팩트럼, 광학적 투과도, 전류-전압 (I-V) 특성 분석을 통해 조사한 결과, 기존의 열기상증착법(thermal CVD)을 이용해서 성장된 그래핀과 동일한 특성을 보임을 확인하였다. 둘째로, 이렇게 형성된 그래핀을 플렉시블한 기판위에 옮긴 후 수열합성법을 통해 일차원 나노구조체인 ZnO 나노막대를 이차원 나노구조체인 그래핀 위에 선택적으로 성장시켜 ZnO 나노막대-그래핀의 하이브리드 구조 (ZnO nanorod-graphene hybrid architectures)를 제작하였다. ZnO 나노막대-그래핀의 하이브리드 구조는 우수한 전기전도도를 보이며, 70-80%정도의 투과도를 나타냄을 확인하였다. 음극선발광 사진 (cathodoluminescence images)과 광발광 (photoluminescence) 스펙트럼 분석 결과 ZnO 나노막대에서 자외선 빛이 발광함을 보였다.. 또한, ZnO 나노막대-그래핀의 하이브리드 구조는 뛰어난 기계적 특성 및 구조적 안전성을 가지고 있음을 bending test를 통하여 알 수 있었다. . 셋째로, 일차원 구조인 필라 위에 이차원 구조인 그래핀을 상부전극으로 형성하여 LED를 제작하였다. 자가 정렬된 실리카 볼을 에칭마스크로 이용하여 GaAs 기반 다층우물(MQW) 나노필라 어레이(nanopillar array)를 제조하고 여기에 그래핀을 top window electrode로 활용해서 발광소자(LED)를 제작하였다. 열처리 과정을 통해 그래핀과 필라의 접촉저항을 줄이고 뛰어난 전기적 특성을 확인 할 수 있었다. 나노필라-그래핀 하이브리드 구조는 필라 어레이를 사용함으로써 빛을 방출하는 활성층의 면적을 증가시키고 광추출 효율 향상시킬 수 있으며, 그래핀의 우수한 전기전도도, 광학적 투명성 및 기계적 특성을 모두 가지는 발광소자를 제작하였다. 이렇듯, 일-이차원 나노구조를 하이브리드 시킨 삼차원 나노 구조체는 에너지소자, 발광소자, 전기소자 등 다양한 분야에 적용이 가능할 것입니다. |The hybridization of different types of materials is crucial as it can enable versatile and tailor-made properties with performances far beyond those of the individual materials. Especially for two-dimensional (2D) graphene, hybridization with one-dimensional (1D) semiconductor nanostructures enables the construction of three-dimensional architectures and the imposition of multi-functionalities. Here we have studied on 3D hybrid nanoarchitectures composed of regular arrays of 1D nanorods and 2D graphene sheets. First, we studied on the large-area synthesis of few-layer graphene films by chemical vapor deposition (CVD) in a cold-wall reactor. The key feature of this method is that the catalytic metal layers on the SiO2/Si substrates are self-heated to high growth temperature (900?1,000 ?C) by high current Joule heating. The synthesis of high-quality graphene films, whose structral and electrical charactercteristics are comparable to those grown by hot wall CVD systems, was confirmed by transmission electron microscopy images, Raman spectra, optical transmittance spectra, and current-voltage analysis. Second, we fabricated ZnO nanorod-graphene hybrid architectures (ZnO-Gr HAs) composed of regular arrays of ZnO nanorods formed on a few-layer graphene films transferred to the transparent and/or flexible substrates. The ZnO-Gr HAs exhibited a high current flow reaching ~1.1 mA at an applied bias of 1 V and good optical transmittance in the range of 70-80%, comparable to those of a graphene layer. In addition, cathodoluminescence images and photoluminescence spectra of the ZnO-Gr HAs showed the distinct light emission involving optical transitions in the ZnO nanorod array. Moreover, a bending test demonstrated that the ZnO-Gr HAs exhibits the excellent mechanical flexibility and structural stability for the bending radius down to ~4 mm. Our results suggest that the 1D-2D HAs provide the unique and multiple functions as can be applicable for next-generation electronic and optoelectronic systems. Last, we studied on a type of device that combines vertical arrays of 1D pillar-superlattice (PSL) structures with 2D graphene sheets to yield a class of light emitting diode (LED) with interesting mechanical, optical and electrical characteristics. In this application, graphene sheets coated with very thin metal layers exhibit good mechanical and electrical properties, and an ability to mount, in a freely suspended configuration, on the PSL arrays as a top window electrode. Optical characterization demonstrates that graphene exhibits excellent optical transparency even after deposition of the thin metal films. Thermal annealing of the graphene/metal (Gr/M) contact to the GaAs decreases the contact resistance, to provide enhanced carrier injection. The resulting PSL-Gr/M LEDs exhibit bright light emission over large areas. The result suggests the utility of graphene-based materials as electrodes in devices with unusual, non-planar 3D architectures.; The hybridization of different types of materials is crucial as it can enable versatile and tailor-made properties with performances far beyond those of the individual materials. Especially for two-dimensional (2D) graphene, hybridization with one-dimensional (1D) semiconductor nanostructures enables the construction of three-dimensional architectures and the imposition of multi-functionalities. Here we have studied on 3D hybrid nanoarchitectures composed of regular arrays of 1D nanorods and 2D graphene sheets. First, we studied on the large-area synthesis of few-layer graphene films by chemical vapor deposition (CVD) in a cold-wall reactor. The key feature of this method is that the catalytic metal layers on the SiO2/Si substrates are self-heated to high growth temperature (900?1,000 ?C) by high current Joule heating. The synthesis of high-quality graphene films, whose structral and electrical charactercteristics are comparable to those grown by hot wall CVD systems, was confirmed by transmission electron microscopy images, Raman spectra, optical transmittance spectra, and current-voltage analysis. Second, we fabricated ZnO nanorod-graphene hybrid architectures (ZnO-Gr HAs) composed of regular arrays of ZnO nanorods formed on a few-layer graphene films transferred to the transparent and/or flexible substrates. The ZnO-Gr HAs exhibited a high current flow reaching ~1.1 mA at an applied bias of 1 V and good optical transmittance in the range of 70-80%, comparable to those of a graphene layer. In addition, cathodoluminescence images and photoluminescence spectra of the ZnO-Gr HAs showed the distinct light emission involving optical transitions in the ZnO nanorod array. Moreover, a bending test demonstrated that the ZnO-Gr HAs exhibits the excellent mechanical flexibility and structural stability for the bending radius down to ~4 mm. Our results suggest that the 1D-2D HAs provide the unique and multiple functions as can be applicable for next-generation electronic and optoelectronic systems. Last, we studied on a type of device that combines vertical arrays of 1D pillar-superlattice (PSL) structures with 2D graphene sheets to yield a class of light emitting diode (LED) with interesting mechanical, optical and electrical characteristics. In this application, graphene sheets coated with very thin metal layers exhibit good mechanical and electrical properties, and an ability to mount, in a freely suspended configuration, on the PSL arrays as a top window electrode. Optical characterization demonstrates that graphene exhibits excellent optical transparency even after deposition of the thin metal films. Thermal annealing of the graphene/metal (Gr/M) contact to the GaAs decreases the contact resistance, to provide enhanced carrier injection. The resulting PSL-Gr/M LEDs exhibit bright light emission over large areas. The result suggests the utility of graphene-based materials as electrodes in devices with unusual, non-planar 3D architectures.