Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal properties of graphene and nanostructured carbon materials

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range – of over five orders of magnitude – from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

A review of electrolyte materials and compositions for electrochemical supercapacitors

Thermal diffusivity (α) has been correlated with the electrical and structural properties of hydrogenated microcrystalline silicon (µc-Si:H) films. In the heterogeneous microcrystalline network, α and electrical conductivity (σD) maintain a one-to-one correspondence, and both these parameters are directly related to the crystalline volume fraction (Fc) of the network. For the amorphous silicon network, α is ~ 0.2 cm2s1 and has a very low σD of ~ 10-8 Scm-1. When the crystalline volume fraction (Fc) exceeds a certain percolation threshold, both α and σD increase abruptly. Undoped µc-Si:H films having Fc~94% exhibit a high magnitude of α~0.80 cm2s-1 and a very high σD of ~ 10-3 Scm-1.

Correlation of Electrical, Thermal and Structural Properties of Microcrystalline Silicon Thin Films : Semiconductors

Integration of Thin Film Transistor Controlled Carbon Nanotubes for Field Emission Devices

This paper introduces a photo-assisted atomic force microscope (AFM) local oxidation technique which is capable of producing highly smooth oxide patterns with heights reaching several tens of nanometres on both n- and p-types of GaN (and in principle on most semiconductors) without the use of chemicals. The novel methodology relies on UV illumination of the surface of the substrate during conventional AFM local oxidation. A low 1.2 V threshold voltage for n-type GaN was obtained, which can be explained by UV photo-generation of excess electron–hole pairs in the substrate near the junction, thereby reducing the electric field required to drive carrier flow through the tip–sample Schottky barrier. It was demonstrated that the presence or absence of light alone was sufficient to switch the growth of the oxide on or off. The photo-assisted AFM oxidation technique is of immediate interest to the semiconductor industry for the fabrication of GaN-based complementary metal–oxide–semiconductor devices and nanodevices, improves chances for AFM-type data storage, and presents new degrees of freedom for process control technique.

http://iopscience.iop.org/article/10.1088/0957-4484/17/13/036/pdf

Photo-assisted local oxidation of GaN using an atomic force microscope

The objective of this chapter is to provide a concise review of the growth, characterization and properties of the nanorods and nanowires in two nitrides systems: the group III-nitrides (specifically, AlN, GaN, InN and ternary Ga-In-N) and group IV-nitrides (Si3N4 and ternary Si-C-N). During the last three decades, tremendous efforts have been devoted to the physics, chemistry and synthesis of GaN and related materials. The wurtzite polytypes of AlN, GaN and InN form a nearly continuous alloy system with direct band gaps between 1.9 and 6.2 eV, therefore, suitable for optoelectronic devices that are active at wavelengths ranging from the green well into the ultraviolet. In fact, all primary and mixed colors can be obtained by using nitride emitters as pumps. Progress in III-nitride based devices has been made at an astonishing rate in the last decade [1]. Besides the applications of III-nitrides in light emitting diodes and laser diodes, high-speed field-effect transistors, ultraviolet photodetectors and high-temperature electronic devices have been demonstrated [2–4]. Furthermore, GaN is also chemically inert and radiation resistant and has been considered as a stable photocatalyst in photo-electrochemical fuel cells [5], while InN/Si tandem cells have been proposed for high efficiency solar cells [6, 7]. Of equal importance to just being an active component of the nitride optoelectronic devices, AlN has also attracted extensive interest for applications as electrical packaging material due to its high thermal conductivity and low coefficient of thermal expansion that closely matches that of silicon, and as components in structural composites owing to its excellent mechanical strength and chemical stability [8].

Group III- and Group IV-Nitride Nanorods and Nanowires

We report the structure and emission properties of ternary (In,Ga)N nanowires (NWs) embedded with self-assembled quantum dots (SAQDs). InGaN NWs are fabricated by the reaction of In, Ga and NH 3 via a vapor-liquid-solid (VLS) mechanism, using Au as the catalyst. By simply varying the growth temperature, In-rich or Ga-rich ternary NWs have been produced. X-ray diffraction, Raman studies and transmission electron microscopy reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming SAQDs embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich NWs. Temperature-dependent photoluminescence (PL) measurements indicate relaxation of excited electrons from the matrix of the Ga-rich NWs to their embedded SAQDs. A multi-level band schema is proposed for the case of In-rich NWs, which showed an anomalous enhancement in the PL peak intensity with increasing temperature accompanies with red shift in its peak position.

Kuei-Hsien Chen

Papers

Correlation of Electrical, Thermal and Structural Properties of Microcrystalline Silicon Thin Films : Semiconductors

Integration of Thin Film Transistor Controlled Carbon Nanotubes for Field Emission Devices

Photo-assisted local oxidation of GaN using an atomic force microscope

Group III- and Group IV-Nitride Nanorods and Nanowires

Enhanced Emission of (In, Ga) Nitride Nanowires Embedded with Self-Assembled Quantum Dots†