Imad Ibrahim

Graphene: Piecing it Together

Graphene has a multitude of striking properties that make it an exceedingly attractive material for various applications, many of which will emerge over the next decade. However, one of the most promising applications lie in exploiting its peculiar electronic properties which are governed by its electrons obeying a linear dispersion relation. This leads to the observation of half integer quantum hall effect and the absence of localization. The latter is attractive for graphene-based field effect transistors. However, if graphene is to be the material for future electronics, then significant hurdles need to be surmounted, namely, it needs to be mass produced in an economically viable manner and be of high crystalline quality with no or virtually no defects or grains boundaries. Moreover, it will need to be processable with atomic precision. Hence, the future of graphene as a material for electronic based devices will depend heavily on our ability to piece graphene together as a single crystal and define its edges with atomic precision. In this progress report, the properties of graphene that make it so attractive as a material for electronics is introduced to the reader. The focus then centers on current synthesis strategies for graphene and their weaknesses in terms of electronics applications are highlighted.

Synthesis of carbon nanotubes with and without catalyst particles

The initial development of carbon nanotube synthesis revolved heavily around the use of 3d valence transition metals such as Fe, Ni, and Co. More recently, noble metals (e.g. Au) and poor metals (e.g. In, Pb) have been shown to also yield carbon nanotubes. In addition, various ceramics and semiconductors can serve as catalytic particles suitable for tube formation and in some cases hybrid metal/metal oxide systems are possible. All-carbon systems for carbon nanotube growth without any catalytic particles have also been demonstrated. These different growth systems are briefly examined in this article and serve to highlight the breadth of avenues available for carbon nanotube synthesis.

Graphene oxide-based drug delivery vehicles: functionalization, characterization, and cytotoxicity evaluation

Abstract As a consequence of graphene oxides (GOs) high chemical versatility, there is great interest in functionalized as a nanocarrier for in vitro and in vivo drug delivery. Within this review, the structure and properties of GO that allow covalent and non-covalent functionalization are discussed. In short, toxicity investigations show functionalized GO is biocompatible. Various works demonstrate the potential of GO derivatives as exciting nanocarriers for the loading and delivery of therapeutic drugs.

CVD-Grown Horizontally Aligned Single-Walled Carbon Nanotubes: Synthesis Routes and Growth Mechanisms

Single-walled carbon nanotubes (SWCNTs) have attractive electrical and physical properties, which make them very promising for use in various applications. For some applications however, in particular those involving electronics, SWCNTs need to be synthesized with a high degree of control with respect to yield, length, alignment, diameter, and chirality. With this in mind, a great deal of effort is being directed to the precision control of vertically and horizontally aligned nanotubes. In this review the focus is on the latter, horizontally aligned tubes grown by chemical vapor deposition (CVD). The reader is provided with an in-depth review of the established vapor deposition orientation techniques. Detailed discussions on the characterization routes, growth parameters, and growth mechanisms are also provided.

Amorphous Carbon under 80 kV Electron Irradiation: A Means to Make or Break Graphene

Within graphene research, transmission electron microscopy (TEM) has proven to be an extremely useful and versatile characterization tool. [ 1 ] However, the electron beam can interact with the sample leading to its modifi cation during the process. This may be an undesirable effect and measures to avoid this do exist. In other cases, however, electron beam–sample interactions can be useful for nano-engineering or nano-manufacturing. [ 2 ] It is therefore crucially important to understand how a material responds to the electron beam and the environment inside a TEM. In general, carbon species are sensitive to a variety of irradiation effects including knock-on displacements, electronic excitations and radiolysis, and radiation-induced diffusion. [ 3 ] For example, the irradiation of amorphous carbon with a 100 or 200 keV electron beam has been shown to lead to the formation of sp 2 carbon onions. [ 4 ] The mechanism behind this catalyst-free electron beam induced graphitization is widely argued to arise from radiation-induced diffusion [ 5 ] which may qualitatively be thought of as similar to thermal diffusion. [ 3 ]

CVD growth of 1D and 2D sp 2 carbon nanomaterials

The discovery of graphene and carbon nanotubes (rolled-up graphene) has excited the world because their extraordinary properties promise tremendous developments in many areas. Like any materials with application potential, it needs to be fabricated in an economically viable manner and at the same time provides the necessary quality for relevant applications. Graphene and carbon nanotubes are no exception to this. In both cases, chemical vapor deposition (CVD) has emerged as the dominant synthesis route since it is already a well-established process both in industry and laboratories. In this work, we review the CVD fabrication of graphene and carbon nanotubes. Initially, we briefly introduce the materials and the CVD process. We then discuss pretreatment steps prior to the CVD reaction. The discussion then switches to the CVD process, provides comparative data for thermal CVD and plasma-enhanced CVD, and includes coverage of kinetics, thermodynamics, catalyst choice, and other aspects of growth as well as post production treatments. Finally, conclusions are drawn and presented.

Understanding High-Yield Catalyst-Free Growth of Horizontally Aligned Single-Walled Carbon Nanotubes Nucleated by Activated C60 Species

Our understanding of the catalyst-free growth of single-walled carbon nanotubes by chemical vapor deposition is limited. Toward improving our knowledge base, we conducted systematic investigations into the initial preparation of C(60) fullerenes as nucleation precursors for single-wall and even double-wall carbon nanotube fabrication. The role of the dispersing media is shown to be crucial and is related to the initial fullerene cluster size. Oxygen-based groups, in particular, epoxy groups, are shown to be vital prior to actual growth. Moreover, the presence of oxygen groups during the growth phase is necessary for tube development. We also demonstrate the possibility of fabricating the tubes in crossbar configurations with bespoke crossing angles in a single synthesis step, unlike other routes which require at least two synthesis steps. The systematic studies significantly advance our understanding of the growth mechanisms involved in all-carbon catalyst-free growth of single- and double-walled carbon nanotubes.

Current Progress in the Chemical Vapor Deposition of Type-Selected Horizontally Aligned Single-Walled Carbon Nanotubes

Exciting electrical properties of single-walled carbon nanotubes show promise as a future class of electronic materials, yet the manufacturing challenges remain significant. The key challenges are to determine fabrication approaches for complex and flexible arrangements of nanotube devices that are reliable, rapid, and reproducible. Realizing regular array structures is an important step toward this goal. Considerable efforts have and are being made in this vein, although the progress to date is somewhat modest. However, there are reasons to be optimistic. Positive steps of being able to control not only the spatial location and diameter of the tubes but also their electronic type (chiral control) are being made. Two primary approaches are being exploited to address the challenges. Tube deposition techniques, on the one hand, and direct growth of the desired tube at the target location are being explored. While this review covers both approaches, the emphasis is on recent developments in the direct fabrication of type-selected horizontally aligned single-walled carbon nanotubes by chemical vapor deposition.

Growth of all-carbon horizontally aligned single-walled carbon nanotubes nucleated from fullerene-based structures

All-carbon single-walled carbon nanotubes (SWCNTs) were successfully synthesized, nucleated using a fullerene derivative. A systematic investigation into the initial preparation of C60 fullerenes as growth nucleators for the SWCNTs was conducted. Enhancement in the yield of the produced SWCNT has been achieved with exploring different dispersing media for the fullerenes, the period, and environment of the initial thermal treatment of the fullerenes in addition to the use of different fullerene-based structures. The systematic studies significantly advance our understanding of the growth of the all-carbon catalyst-free single-walled carbon nanotubes. Field-effect transistors were fabricated using the catalyst-free SWCNT and then electrically characterized, showing current capacity as high as the well-studied catalyst-assisted nanotubes.

Spatial recognition of defects and tube type in carbon nanotube field effect transistors using electrostatic force microscopy

The exceedingly high current capacity of single wall carbon nanotubes (SWCNTs) and the high currents that SWCNT thin films can sustain are driving significant efforts to fabricate high quality horizontally aligned SWCNTs. Dielectrophoresis is being increasingly used to prepare aligned nanotubes. However, the aligned nanotubes are generally of low quality since the processing involved can introduce defects. In addition, in arrays of tubes, tube-tube contact resistance can be high. Electrostatic force microscopy is a technique similar to atomic force microscopy, but in this case a bias voltage between the conductive tip and sample enables one to probe the long range electrostatic force. In this study we confirm that the technique can be successfully used to spatially determine defects, crossover points between tubes and nanotube electronic type.