(B, N) 도펀트와 산소기능기가 그래핀과 그 응용의 구조적, 전기적, 광학적 성질에 미치는 영향

Abstract

Graphene, graphene oxide (GO) 그리고 reduced graphene oxide (RGO) 와 같은 graphene-based sheets (GBS) 는 그 특별한 구조 및 우수한 물리적, 화학적 특성으로 인해 기초 과학 및 잠재적인 응용 프로그램 모두에서 엄청난 주목을 받고 있다. 보통 재료의 특성은 구조와 관련되므로 전기적 특성의 변화 및 graphene 의 밴드 갭 조정과 같은 많은 효과를 나타낼 수 있다. 다른 원자의 화학적 도핑은 본질적으로 원재료의 특성을 변화시킬 수 있는 효과적인 방법이다. 수많은 도펀트 중 (Li, P, B 및 N), 질소 (N)와 붕소는 (B) 탄소 재료의 우수한 도핑 물질 중 하나이다. 이는 탄소 원자와 크기가 유사하고 탄소 원자와 5 개, 3 개의 전자를 통하여 강력한 본드를 형성할 수 있기 때문에 각각 p 와 n 형 반도체의 거동을 보인다. 더 중요한 건, 탄소 네트워크에 B 타입과 N 타입의 다양한 결합과 기능성 그룹을 통해 B-및 N-도핑된 graphene 의 특성을 변화 시킬 수 있다는 것이다. B-및 N-도핑된 graphene 의 구조, 전기적 촉매 및 전계 방출 특성의 연구는 많이 이루어진 반면, 전기적 광학적 특성에 대한 연구는 거의 없다. 본 논문에서는 도핑 되지 않은 GBS 와 B/N-도핑된 GBS 의 구조적, 전기적, 광학적 특성에 초점을 맞추어 연구했다. 첫 번째 실험에서는 GO 필름의 기능성 그룹이 구조적 전기적 성질에 미치는 영향을 연구했다. 단일층 또는 이중층의 GO 는 수정된 Hummers 방법으로 합성했으며, 이는 전기 절연체이다. 고전도도의 GO 필름은 화학적으로 환원된 GO 분산액을 스프레이 증착법으로 석영 기판에 필름을 형성한 뒤 H2 분위기에서 열처리를 통하여 합성한다. GO 필름의 기능성 그룹이 구조적 전기적 성질에 미치는 영향은 SEM, TEM, AFM, XRD, FTIR, UV-vis, XPS 그리고 전기전도도 측정기를 통하여 분석했다. 화학적으로 감소된 GO (RGO)는 graphene 시트의 산소 기능성 그룹과 SP2 탄소 네트워크의 환원으로 인해 전기 전도성이 크게 향상 되는 것으로 나타났다. 또한, H2 분위기에서 열처리 후 RGO 필름의 전도성도 RGO 의 잔류 산소 기능성 그룹과 SP2 탄소 네트워크의 환원으로 인해 향상 되는 것으로 나타났다. 열처리된 RGO 필름은 1.25×103 Ohm/ 의 낮은 저항을 나타내며 550 nm 의 파장에서 80%의 투과율을 갖는다. 이러한 특성은 고감도 가스 센서, 투명 전극, 태양 전지 및 전계 방사 트랜지스터와 같은 광범위한 기술 분야의 어플리케이션으로 사용될 것으로 예상된다. 두 번째 실험에서는 N-도핑된 GO 필름의 구조적 광학적 성질에 대하여 연구했다. N 은 600-900oC 의 온도와 NH3 가스 분위기에서 도핑 되었다(3.63-7.45%). XPS 와 FTIR 분석을 통하여 GO sheet 의 주된 결합은 C-N 과 C=N 본드임을 밝혀냈다. 라만스펙트라 분석을 통해 G 밴드는 도핑 온도의 증가에 따라 흑연의 G 밴드와 가까워짐을 알 수 있었고, N-도핑은 G 밴드의 blue-shift 를 야기함을 알 수 있었다. 상온 PL 분석을 통해서 N-도핑은 최대 파장뿐만 아니라 전체적인 PL intensity 의 증가를 야기함을 알 수 있었다. N-도핑된 graphene 의 PL 이동은 N 도핑 농도 증가에 기인함을 알 수 있었다. 세 번째 실험에서는 RGO 를 1100oC 와 NH3 분위기에서 열처리를 통하여 합성된 N-도핑 graphene 의 기능, 전기전도도 그리고 광학적 성질에 대하여 연구했다. 다른 농도로 도핑된 (2.3-4.7%) N-도핑 graphene 은 시간을 변수로 하여 합성했다. 이는 다양한 분석 기술을 통하여 체계적으로 분석했다. XPS, FTIR, Raman, XRD 분석을 통하여 N-도핑된 graphene 은 열처리 시간의 변화에 따라 구조적으로 변화가 생김을 알 수 있었으며, 이는 상대적으로 C 의 양의 증가와 O 과 N 의 감소로 나타났다. 고해상도 N1s XPS 에서는 열처리 시간의 증가에 따라 pyridine-N 과 pyrrolic-N 은 감소하는 것으로 나타났고, quaternary-N 은 크게 증가함을 알 수 있었다. 중요한 것은, 어닐링 시간의 변화가 N-도핑된 graphene 의 전기적 광학적 특성 모두에서 큰 변화를 일으킨 것으로 확인되었다. N-도핑된 graphene 의 전기적 저항은 GO 와 RGO 에 비해 크게 감소하였고, 열처리 시간의 증가에 따라 감소하였다. 이는 sp2 탄소 네트워크의 증가와 산소의 감소 그리고 N-도핑과 관련된 결함에 기인하는 것으로 예상된다. GO, RGO 그리고 N-도핑된 graphene 의 상온 PL 특성은 열처리 시간에 따라 분석하였다. 결과에 의하면 GO 의 최대 PL 픽은 700 nm 근처이며, 동일조건에서 RGO 는 두 개의 뚜렷한 픽: green emission (485-500 nm), blue emission (420-428 nm)과 함께 강하게 blue-shift 됨을 알 수 있었다. N-도핑된 graphene 의 경우, blue emission 의 intensity 는 열처리 시간의 증가에 따라 강화 되었으며 이는 blue- 와 green light-emission 의 광학적 성질을 컨트롤 할 수 있음을 보인다. 마지막 실험에서는 B-도핑된 GO 필름의 구조적 광학적 성질에 대하여 연구했다. B-도핑된 graphene 은 GO 와 H3BO3를 N, N-Dimethylformamide 용매에 녹인 뒤 열처리를 통하여 합성하였다. XRD 에서 B-도핑된 GO 와 도핑되지 않은 GO 의 거리는 열처리를 통해 감소되었다. 라만스펙트라 분석에서 B-도핑된 GO 의 D 와 G 밴드의 intensity 비율은 도핑없이 열처리된 GO 보다 낮았다. 이는 도핑으로 인하여 좀 더 탄화가 이루어졌을 것이라 예상된다. B-도핑된 GO필름의 C1s XPS 에서는 많은 양의 기능성 그룹뿐만 아니라 283.7 eV 근처의 C-B밴드도 제거되었다. 또한 B-도핑된 GO 필름의 B1s XPS 에서는 187.2, 188.9, 190.3, 192.0 그리고 193.7 eV 으로 deconvolution 할 수 있었으며, 이는 B4C, Bsub-C, BC2O, BCO2 그리고 B2O3 에 기인한다. B-도핑된 GO 를 1100oC 에서 열처리된 GO 와 PL intensity 를 비교한 결과 전체적으로 감소함을 알 수 있었으며, 이는 B-도핑으로 인한 탄화에 기인함이라 예상된다. B-도핑된 GO 의 600-700 nm 근처의 다른 밴드는 BC 상의 여기에 기인한다.|Due to their special structure and outstanding physical and chemical properties, graphene-based sheets (GBS) such as graphene, graphene oxide (GO) and reduced graphene oxide (RGO) have attracted tremendous attention in both fundamental science and potential applications. Since material properties are usually related to the structure, many effects have been made to modify the electrical properties and tune the band gap of graphene. Chemical doping with foreign atoms is an effective method to intrinsically modify the properties of the host materials. Among the numerous potential dopants (Li, P, B and N), nitrogen (N) and boron (B) are considered to be excellent candidates for chemical doping of carbon materials. This is because of their comparable atomic size and the presence of five and three valence electrons available to form strong valence bonds with carbon atoms, which would provide p- and n-type semiconductor behavior, respectively. More importantly, incorporating different of types B and N into the carbon network would provide the B- and N-doped graphene with more functional groups for property design. While many attempts have been made to explore the structural, electrocatalytic, and field emission properties of B and N-doped graphene, very little work has been done on the investigation of electrical and optical properties. The work presented in this thesis focuses on the structure, electrical, optical properties of non- and B/N-doped GBS. In the first set of experiments we study the influence of oxygen functional groups on the structural and electrical properties of graphene films. Single or bilayer GO sheets were prepared by a modified Hummers method, which is an electrical insulator. Highly conducting graphene films were synthesized on quartz substrates by spray deposition of a chemically reduced-GO/GO dispersions, the followed by a thermal treatment under H2. The influence of oxygen functional groups on the structural and electrical properties of graphene films were investigated by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Atomic force microscopy (AFM), Raman, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Ultraviolet-visible (UV-vis), X-ray photoelectron spectroscopy (XPS) and by electrical conductivity measurements. The results showed that chemically reduced GO (RGO) resulted in significant increase in electrical conductivity because of the removal of oxygen functional groups and restoration of sp2 carbon network in the graphene sheets. Furthermore, after thermal annealing in H2, the conductivity of RGO film is further improved, indicating that the residual oxygen functional groups in the RGO are removed and the better restoration of sp2 carbon networks by thermal annealing. Upon thermal annealing of the RGO films, the obtained films possess: a low sheet resistance of 1.25×103 Ohm/ with 80% transparency at a typical wavelength of 550 nm. These properties promise a wide range of applications in technology fields, such as ultra-sensitive gas sensor, transparent electrodes, solar cell, and field effect transistors. The second part of the thesis concentrates on the structural and optical properties of N-doped graphene oxide films. N was doped into GO films (with an atomic concentration of 3.63-7.45%) at temperatures of 600-900oC under NH3 gas. XPS and FTIR spectra show that there are mainly single C-N and double C=N bonds in the GO sheet. Raman spectra indicate that the G band becomes closer to the position of the G band of graphite with increasing doping temperature, and thus reveal that N doping produces a blue-shift of the G-band. In room-temperature photoluminescence (PL) spectra, N-doping produces an increase not only in the overall PL intensity, but also in the wavelength of the peak maxima. The shift of the induced PL of N-doped graphene is attributed mainly to the increased number of graphitic (or quaternary) N. The third part the thesis, we present a simple approach to tune the contents of N functionalities, electrical conductivity and optical properties of graphene, through the thermal annealing of RGO with ammonia atmosphere at 1100oC. N-doped graphene with different atomic concentration (2.3-4.7 at. %) of N has been synthesized by thermal annealing of RGO in NH3 gas during different times. The effects of annealing time on the structure, electrical and optical properties of N-doped graphene have been systematically investigated by using various analytical techniques. XPS, FTIR, Raman, XRD studies show that there is a gradual structural change in N-doped graphene sheets with increasing annealing time, resulting from the increase of carbon and simultaneous decrease of oxygen and N contents. High resolution N1s XPS spectra reveal that the pyridine-N and pyrrolic-N content decreases with increasing the annealing time, whereas the amount of quaternary-N increases. Importantly, it has been found that the annealing time caused significant changes in both the electrical and the optical properties of N-doped graphene. The electrical resistance of N-doped graphene is greatly reduced compared to that of GO and RGO, and found to further decrease with increasing annealing time, possibly due to the increase of sp2 carbon networks and decrease of oxygen contents as well as defects associated with the incorporation of N. The room-temperature PL properties of GO, RGO and N-doped graphene were systematically studied with regard to annealing time. The results showed that the PL spectrum of GO exhibits a peak emission maximum at around 700 nm, while RGO is found to be strongly blue-shifted with two distinct emission peaks: green emission at 485-500 nm and blue emission at 420-428 nm. For N-doped graphene samples, the blue emission intensity could be enhanced by increasing the annealing time, which shows a promising blue- and green light-emitting material with controllable optical properties. The final part of the thesis focuses on the study of B-doping effect on the structural and optical properties of GO films. We prepared boron-doped GOs by means of annealing the films, which were obtained from the suspensions of GO and H3BO3 in N, N-Dimethylformamide solvent. The interplanar spacing of both B-doped and undoped GOs in XRD patterns have been reduced by the thermal annealing at 1100oC. First-order Raman spectra revealed that the intensity ratio of the D and G bands of Bdoped GO was significantly lower than those of as-synthesized and annealed GOs, suggesting more graphitization of the B-doped GO due to doping effect. The C1s XPS spectrum of B-doped GO films not only indicated that considerable amount of functional groups has been removed but also exhibited the peak of C-B band at around 283.7 eV. Additionally, the B1s XPS spectrum of B-doped GOs could be deconvoluted into several peaks centered at 187.2, 188.9, 190.3, 192.0 and 193.7 eV, being attributed to the presence of boron atom in B4C, B-sub-C, BC2O, BCO2 and B2O3, respectively. Comparison of the photoluminescence (PL) spectra of B-doped GO with that of 1100oC-annealed GO indicated that the overall intensity was decreased, presumably due to the B-induced graphitization. An additional band at around 600-700 nm from B-doped GO is attributed to the generated boron carbide phases.; Due to their special structure and outstanding physical and chemical properties, graphene-based sheets (GBS) such as graphene, graphene oxide (GO) and reduced graphene oxide (RGO) have attracted tremendous attention in both fundamental science and potential applications. Since material properties are usually related to the structure, many effects have been made to modify the electrical properties and tune the band gap of graphene. Chemical doping with foreign atoms is an effective method to intrinsically modify the properties of the host materials. Among the numerous potential dopants (Li, P, B and N), nitrogen (N) and boron (B) are considered to be excellent candidates for chemical doping of carbon materials. This is because of their comparable atomic size and the presence of five and three valence electrons available to form strong valence bonds with carbon atoms, which would provide p- and n-type semiconductor behavior, respectively. More importantly, incorporating different of types B and N into the carbon network would provide the B- and N-doped graphene with more functional groups for property design. While many attempts have been made to explore the structural, electrocatalytic, and field emission properties of B and N-doped graphene, very little work has been done on the investigation of electrical and optical properties. The work presented in this thesis focuses on the structure, electrical, optical properties of non- and B/N-doped GBS. In the first set of experiments we study the influence of oxygen functional groups on the structural and electrical properties of graphene films. Single or bilayer GO sheets were prepared by a modified Hummers method, which is an electrical insulator. Highly conducting graphene films were synthesized on quartz substrates by spray deposition of a chemically reduced-GO/GO dispersions, the followed by a thermal treatment under H2. The influence of oxygen functional groups on the structural and electrical properties of graphene films were investigated by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Atomic force microscopy (AFM), Raman, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Ultraviolet-visible (UV-vis), X-ray photoelectron spectroscopy (XPS) and by electrical conductivity measurements. The results showed that chemically reduced GO (RGO) resulted in significant increase in electrical conductivity because of the removal of oxygen functional groups and restoration of sp2 carbon network in the graphene sheets. Furthermore, after thermal annealing in H2, the conductivity of RGO film is further improved, indicating that the residual oxygen functional groups in the RGO are removed and the better restoration of sp2 carbon networks by thermal annealing. Upon thermal annealing of the RGO films, the obtained films possess: a low sheet resistance of 1.25×103 Ohm/&#61450