Dacheng Wei

Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties

To realize graphene-based electronics, various types of graphene are required; thus, modulation of its electrical properties is of great importance. Theoretic studies show that intentional doping is a promising route for this goal, and the doped graphene might promise fascinating properties and widespread applications. However, there is no experimental example and electrical testing of the substitutionally doped graphene up to date. Here, we synthesize the N-doped graphene by a chemical vapor deposition (CVD) method. We find that most of them are few-layer graphene, although single-layer graphene can be occasionally detected. As doping accompanies with the recombination of carbon atoms into graphene in the CVD process, N atoms can be substitutionally doped into the graphene lattice, which is hard to realize by other synthetic methods. Electrical measurements show that the N-doped graphene exhibits an n-type behavior, indicating substitutional doping can effectively modulate the electrical properties of graphene. Our finding provides a new experimental instance of graphene and would promote the research and applications of graphene.

Controllable Synthesis of Graphene and Its Applications

Graphene, a two-dimensional material, is regarded as one of the most promising candidates for future nanoelectronics due to its atomic thickness, excellent properties and widespread applications. As the first step to investigate its properties and finally to realize the practical applications, graphene must be synthesized in a controllable manner. Thus, controllable synthesis is of great significance, and received more and more attention recently. This Progress Report highlights recent advances in controllable synthesis of graphene, clarifies the problems, and prospects the future development in this field. The applications of the controllable synthesis are also discussed.

Scalable Synthesis of Few-Layer Graphene Ribbons with Controlled Morphologies by a Template Method and Their Applications in Nanoelectromechanical Switches

Controllable and scalable production is of great importance for the application of graphene; however, to date, it is still a great challenge and a major obstacle which hampers its practical applications. Here, we develop a template chemical vapor deposition method for scalable synthesis of few-layer graphene ribbons (FLGRs) with controlled morphologies. The FLGRs have a good conductivity and are ideal for use in nanoelectromechanics (NEM). As an application, we fabricate a reversible NEM switch and a logic gate by using the FLGRs. This work realizes both controllable and scalable synthesis of graphene, provides an application of graphene in NEM switches, and would be valuable for both the scientific studies and the practical applications of graphene.

Spatially Resolved Electronic Structures of Atomically Precise Armchair Graphene Nanoribbons

Graphene has attracted much interest in both academia and industry. The challenge of making it semiconducting is crucial for applications in electronic devices. A promising approach is to reduce its physical size down to the nanometer scale. Here, we present the surface-assisted bottom-up fabrication of atomically precise armchair graphene nanoribbons (AGNRs) with predefined widths, namely 7-, 14- and 21-AGNRs, on Ag(111) as well as their spatially resolved width-dependent electronic structures. STM/STS measurements reveal their associated electron scattering patterns and the energy gaps over 1 eV. The mechanism to form such AGNRs is addressed based on the observed intermediate products. Our results provide new insights into the local properties of AGNRs, and have implications for the understanding of their electrical properties and potential applications.

Controllable unzipping for intramolecular junctions of graphene nanoribbons and single-walled carbon nanotubes

Graphene is often regarded as one of the most promising candidates for future nanoelectronics. As an indispensable component in graphene-based electronics, the formation of junctions with other materials not only provides utility functions and reliable connexions, but can also improve or alter the properties of pristine graphene, opening up possibilities for new applications. Here we demonstrate an intramolecular junction produced by the controllable unzipping of single-walled carbon nanotubes, which combines a graphene nanoribbon and single-walled carbon nanotube in a one-dimensional nanostructure. This junction shows a strong gate-dependent rectifying behaviour. As applications, we demonstrate the use of the junction in prototype directionally dependent field-effect transistors, logic gates and high-performance photodetectors, indicating its potential in future graphene-based electronics and optoelectronics.

Controllable Chemical Vapor Deposition Growth of Few Layer Graphene for Electronic Devices

Because of its atomic thickness, excellent properties, and widespread applications, graphene is regarded as one of the most promising candidate materials for nanoelectronics. The wider use of graphene will require processes that produce this material in a controllable manner. In this Account, we focus on our recent studies of the controllable chemical vapor deposition (CVD) growth of graphene, especially few-layer graphene (FLG), and the applications of this material in electronic devices. CVD provides various means of control over the morphologies of the produced graph ene. We studied several variables that can affect the CVD growth of graphene, including the catalyst, gas flow rate, growth time, and growth temperature and successfully achieved the controlled growth of hexagonal graphene crystals. Moreover, we developed several modified CVD methods for the controlled growth of FLGs. Patterned CVD produced FLGs with desired shapes in required areas. By introducing dopant precursor in the CVD process, we produced substitutionally doped FLGs, avoiding the typically complicated post-treatment processes for graphene doping. We developed a template CVD method to produce FLG ribbons with controllable morphologies on a large scale. An oxidation-activated surface facilitated the CVD growth of polycrystalline graphene without the use of a metal catalyst or a complicated postgrowth transfer process. In devices, CVD offers a controllable means to modulate the electronic properties of the graphene samples and to improve device performance. Using CVD-grown hexagonal graphene crystals as the channel materials in field-effect transistors (FETs), we improved carrier mobility. Substitutional doping of graphene in CVD opened a band gap for efficient FET operation and modulated the Fermi energy level for n-type or p-type features. The similarity between the chemical structure of graphene and organic semiconductors suggests potential applications of graphene in organic devices. We used patterned CVD FLGs as the bottom electrodes in pentacene FETs. The strong π-π interactions between graphene and pentacene produced an excellent interface with low contact resistance and a reduced injection barrier, which dramatically enhances the device performance. We also fabricated reversible nanoelectromechanical (NEM) switches and a logic gate using the FLG ribbons produced using our template CVD method. In summary, CVD provides a controllable means to produce graphene samples with both large area and high quality. We developed several modified CVD methods to produce FLG samples with controlled shape, location, edge, layer, dopant, and growth substrate. As a result, we can modulate the properties of FLGs, which provides materials that could be used in FETs, OFETs, and NEM devices. Despite remarkable advances in this field, further exploration is required to produce consistent, homogeneous graphene samples with single layer, single crystal, and large area for graphene-based electronics.

Critical Crystal Growth of Graphene on Dielectric Substrates at Low Temperature for Electronic Devices

Graphene is regarded as one of the most promising candidate materials for future electronics. For usage in electrical devices, high-quality graphene needs to be placed on a dielectric surface. Currently, methods for graphene preparation require graphene to be transferred from scotch tape, metals, or solutions to the dielectric substrate for electrical applications. The transfer process normally induces contamination, wrinkles, or even breakage of the graphene samples and thus hampers the practical application of graphene in electronics. Moreover, chemical vapor deposition (CVD) on a metal catalyst normally requires a growth temperature as high as 800–1000 8C. For industrial-scale production, lowtemperature growth can lead to a decrease in energy consumption and cost, and an increase in compatibility. Therefore, a low-cost, controllable, and reliable method is required for the production of clean high-quality graphene directly on dielectric substrates at low temperature. One pioneering study towards the goal of the catalyst-free growth of graphene on dielectric materials examined the pyrolysis of CH4 on bare SiO2/Si; [5,6] however, this method requires a high growth temperature (1100–1650 8C). Another approach, plasma-enhanced CVD (PECVD), enables the low-temperature growth of graphene on metals, or even on dielectric surfaces (550–650 8C). However, without metal catalysts, structural defects readily form at the edges and terminate graphene growth. As a result, small graphene nanoclusters or noncrystalline samples are formed, the quality of which is lower than that required for electrical applications. Vertically oriented graphene nanosheets can be produced by PECVD; however, most of the products are multilayered. Herein, we describe the development of a critical PECVD (c-PECVD) growth method, in which a H2 plasma is introduced during graphene growth. H2 plasma is known to etch graphene from the edges. Moderate etching by a H2 plasma removes defects generated at the edges and thus keeps the edges atomically smooth and active during the whole process of graphene crystal growth. Therefore, in a critical equilibrium state between H2 plasma etching and CH4 or C2H4 plasma growth, we observed efficient catalyst-free crystal growth of graphene directly on crystalline sapphire, highly oriented pyrolytic graphite (HOPG), and the amorphous surface of Si substrates with a thermally grown 300 nm SiO2 overlayer (SiO2/Si), with crystal sizes up to the micrometer scale for single-layer hexagonal single crystals and up to the centimeter scale for continuous films. The growth temperature could be decreased to as low as 400 8C when C2H4 was used as the carbon source. To the best of our knowledge, this temperature is one of the lowest used for the catalyst-free growth of graphene. In the experiments, a homemade remote radiofrequency (13.56 mHz) PECVD system (80 W) was used (Figure 1a). Figure 1b illustrates the typical procedure for the c-PECVD growth of graphene on bare Si/SiO2 (see the Experimental Section for details). We first used peel-off graphene to clarify the etching, critical edge growth, and nucleation of graphene in PECVD. Figure 1c shows atomic force microscopy (AFM) images of a trilayer peel-off graphene flake before and after cPECVD growth (H2 plasma activation: 250 mTorr, 500 8C; growth: 30% H2, 90 mTorr, 600 8C, 60 min). The edges of the bottom, middle, and upper layer moved by 79, 117, and 158 nm, respectively, thus indicating that continuous growth of the flake took place on the edges, rather than in the plane. The different growth rate of each layer is attributed to the substrate (see Figure S1 in the Supporting Information). Control experiments show that the edge growth is highly dependent on the H2 content, the growth temperature, and the pressure (Figure 1d). Lower temperatures or higher H2 content tend to induce edge etching, whereas the opposite reaction conditions cause the nucleation of graphitic clusters (see Figure S2). For example, after CH4+H2 plasma CVD at a lower temperature (550 8C), instead of growth, the edges of the flakes were etched by about 168 nm (Figure 1e). Upon CH4+H2 plasma CVD at a lower H2 content (20%), besides the edge growth, small graphitic clusters were nucleated on whole surface of the graphene flakes and SiO2/Si surface with heights lower than 1 nm (Figure 1 f); the heights observed indicate the single-layered nature of the nucleated clusters. The edge growth (Figure 1c; see also Figure S3) only takes place at a well-controlled critical temperature between those needed for nucleation and edge etching, and the critical temperature decreases as the H2 content decreases (Figure 1d). When C2H4 was used as the carbon source in cPECVD (0%H2, 48 mTorr), the critical temperature for edge growth decreased to as low as 400 8C (Figure 1d). Moreover, low pressure can lead to a remarkable improvement in the growth rate. The growth rate (30% H2, 600 8C) was about [*] Dr. D. Wei, Dr. Y. Lu, Prof. W. Chen, Prof. A. T. S. Wee Department of Physics, National University of Singapore 2 Science Drive 3, Singapore 117542 (Singapore) E-mail: phyweid@nus.edu.sg phyweets@nus.edu.sg

Low temperature critical growth of high quality nitrogen doped graphene on dielectrics by plasma-enhanced chemical vapor deposition.

Nitrogen doping is one of the most promising routes to modulate the electronic characteristic of graphene. Plasma-enhanced chemical vapor deposition (PECVD) enables low-temperature graphene growth. However, PECVD growth of nitrogen doped graphene (NG) usually requires metal-catalysts, and to the best of our knowledge, only amorphous carbon-nitrogen films have been produced on dielectric surfaces by metal-free PECVD. Here, a critical factor for metal-free PECVD growth of NG is reported, which allows high quality NG crystals to be grown directly on dielectrics like SiO2/Si, Al2O3, h-BN, mica at 435 °C without a catalyst. Thus, the processes needed for loading the samples on dielectrics and n-type doping are realized in a simple PECVD, which would be of significance for future graphene electronics due to its compatibility with the current microelectronic processes.

Tunable optical absorption and interactions in graphene via oxygen plasma

We report significant changes of optical conductivity in single layer graphene induced by mild oxygen plasma exposure, and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, the real part of the frequency-dependent conductivity is nearly completely suppressed below an exposure-dependent threshold in the near infrared range. The clear step-like suppression follows the Pauli blocking behaviour expected for doped monolayer graphene. The nearly zero residual conductivity at frequencies below 2Ef can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controlably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

Controllable Synthesis of Graphene by Plasma‐Enhanced Chemical Vapor Deposition and Its Related Applications

Graphene and its derivatives hold a great promise for widespread applications such as field‐effect transistors, photovoltaic devices, supercapacitors, and sensors due to excellent properties as well as its atomically thin, transparent, and flexible structure. In order to realize the practical applications, graphene needs to be synthesized in a low‐cost, scalable, and controllable manner. Plasma‐enhanced chemical vapor deposition (PECVD) is a low‐temperature, controllable, and catalyst‐free synthesis method suitable for graphene growth and has recently received more attentions. This review summarizes recent advances in the PECVD growth of graphene on different substrates, discusses the growth mechanism and its related applications. Furthermore, the challenges and future development in this field are also discussed.