High-Performance Carbon-Based Optoelectronic Nanodevices

Unique electron configuration and multiple chemical bonding possibility make carbon form various carbon allotropes with completely different properties, which can exhibit semiconducting, metallic or insulating, highly transparent or extremely dark, and variable thermal conductivity. Nowadays, a large number of carbon nanomaterials with zero-, one- and two-dimensional structures have been extensively explored and utilized in the different fields. Especially, single-walled carbon nanotubes (SWCNTs) and graphene are considered as great promising candidates for next-generation nanoelectronics and nano-optoelectronics. Therefore, low-dimensional carbon nanomaterials have attracted more and more attentions from both academia and industry. With the development of preparation and process technologies, they are expected to be more widely practically applied in various fields. Herein, we will comprehensively introduce the carbon and its low-dimensional allotropes. And the current applications of carbon nanotubes and graphene in various fields will be simply introduced. Finally, the current challenges and further perspectives will also be discussed at the end of this chapter.

Carbon nanotubes (CNTs) and graphene are typical low-dimensional nanomaterials with large specific surface area and exceptional band structures. For example, single-walled CNTs (SWCNTs) can be considered as an ideal 1D quantum wires and possess novel electronics and optical properties which are strongly depended on the chiral indices and surface states. While monolayer graphene is a true 2D system in which also exhibits many exceptional electronics and optical properties closely related to the band structures. Therefore, understanding the basic physics of CNTs and graphene is very critical to exploring high performance carbon-based electronic and optical devices. In this chapter, the basic atomic structures of SWCNTs and graphene are first introduced, and then we will mainly introduce their electronics and optical properties since this book mainly focuses on carbon-based photoelectric devices.

The large-scale controlled synthesis of high-quality single-walled carbon nanotubes (SWCNTs) and graphene are the basis of high-performance carbon-based electronic and photoelectric devices. With the rapid development of synthesis and characterization techniques, great achievements have been made in understanding the nucleation and growth mechanisms of SWCNTs and graphene, thereby promoting the development of the selective synthesis of specific SWCNTs and large-area graphene. Since carbon-based photoelectric devices in this book are mainly fabricated using SWCNTs and graphene, we mainly introduce the synthesis of SWCNTs and chemical vapor deposition (CVD) graphene in this chapter. The recent advances in the controlled synthesis of SWCNTs will be highlighted. And then, the main achievements in the CVD growth of graphene will be summarized. Finally, the current challenges will also be summarized and further perspectives are given.

The detailed structural characterizations are one of the important foundations for the understanding of basic properties of carbon nanotubes (CNTs) and graphene, which plays an important role in the controlled growth, physical property research and applications. In this chapter, we mainly introduce several typical optical and electron microscopy techniques to characterize the structural information of single-walled CNTs (SWCNTs) and graphene. Especailly, we have highlighted commonly-used assignment methods of chiral structures of SWCNTs, including Raman, UV-vis-NIR absorption spectra, photoluminescence, and electron diffraction patterns, etc. For graphene, the determination of layer number and structural defects has been focused on using Raman sepectroscopy and atomic-resolution electron microscopy. With the further development of characterization techniques, the detailed structure and physical properties of SWCNTs and graphene will be ananlyzed more accurately and quantitatively. However, the collaborative utilization of several technologies is still recommended when both the characterization accuracy and efficiency are considered.

Ultrahigh carrier mobility, unique electronic bandgaps, excellent optical and chemical properties allow carbon nanomaterials (carbon nanotubes and graphene) to show great potential for next-generation photoelectronic devices. Especially, high-performance photodetectors based on single-walled carbon nanotubes, graphene and their related heterostructures have attracted increasing attentions in the past ten years. In this chapter, we firstly introduce the basic physical mechanism and figures-of-merit of photodetectors, and then the state-of-art research progress about carbon-based broadband photodetectors has been summarized systematically. Finally, the current challenges and future perspectives of carbon-based photodetectors are discussed and given.

Carbon nanomaterials can be excellent conductors or the semiconductors with variable band gap according to their different atom hybridizations. For example, two-thirds of single-walled carbon nanotubes (SWCNTs) and fullerenes exhibit semiconducting properties, while one-thirds of SWCNTs and graphene are metallic with ultrahigh mobility. Except for SWCNT- and graphene-based heterostructures with other materials, all-carbon van der Waals (vdW) heterojunctions composed of two or three type carbon nanostructures have also great potential for next-generation ultrahigh performance photodetectors. In this chapter, the formation methods of all-carbon vdW heterojunctions will be firstly introduced, and then we will summarize systematically the state-of-art research progress on all-carbon vdW heterojunction photodetectors, including zero dimensional (0D)/one dimensional (1D), 0D/2D, and 1D/2D vdW heterojunctions. Finally, we also highlight the current challenges and the future perspectives of all-carbon vdW heterojunction photodetectors.

Unique one-dimensional structure and excellent diameter-depended optoelectrical properties make CNTs have great potential for novel high-performance photovoltaic applications. Among them, van der Waals heterojunction (vdW) solar cells have been demonstrated by combining the excellent optical and electrical properties of CNTs with the photoelectric properties of bulk semiconductors since 2007. Owing to simple device structure and easy fabrication process, these vdW heterojunction solar cells have received more and more attentions in the past ten years with the development of high-quality CNTs (conducting type, chirality and purity, etc.). Especially, the sharply atomic interface can remarkably reduce the recombination probability of charge carriers, and the carrier separation efficiency can be improved by changing the diameter and doping states of CNTs. Meanwhile, High transparency and high carrier mobility of thin CNT film also make it to be beneficial to collect the photo-generated carriers. In this chapter, the work mechanism of CNT/semiconductor vdW heterojunctions are firstly introduced when semiconducting or metallic CNTs are used, and some key parameters of solar cells are also introduced. Then, we mainly introduce the state-of-the-art research progress about the vdW heterojunction solar cells, which consist of bulk semiconductor (Si and GaAs) and CNTs with different wall number. Several key technologies have been optimized to improve the photovoltaic performance of CNT/Si vdW heterojunction solar cells, such as high-quality CNT film, Fermi level controlling, and interface engineering, and so on. Finally, the current challenges and future perspectives of highly efficient vdW heterojunction solar cells have also been discussed.

As one of p-type semiconductors with direct bandgap, semiconducting single-walled carbon nanotubes (sc-SWCNTs) possess ultrahigh carrier mobility, unique electron structures, high light absorption coefficient and other physical properties, which have been widely used in novel photodetectors and photovoltaic devices. While fullerenes are typical n-type semiconductors and metallic SWCNTs (or graphene) usually behave as carrier transport channel due to the high carrier mobility. Taking advantage of the excellent physical properties of different carbon nanomaterials, a new concept photovoltaic device, all-carbon solar cell, is expected to be developed by combing semiconducting and metallic carbon allotropes on the basis of the optimal design of band arrangements. In this chapter, we firstly introduce the photoexcitation transfer dynamics and bandgap structure limits of sc-SWCNTs in the sc-SWCNT/fullerene heterojunctions, respectively. Then, the solar cells based on all-carbon bulk and planar heterojunctions as active layers have been discussed. Finally, the recent progress about all-carbon solar cells have been highlighted, and several feasible approaches to improve the performance have also been proposed.