We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range ∼(4.84 ± 0.44) × 103 to (5.30 ± 0.48) × 103 W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.

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Superior Thermal Conductivity of Single-Layer Graphene

The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si∕SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼3080–5150W∕mK and phonon mean free path of ∼775nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.

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Extremely high thermal conductivity of graphene: Prospects for thermal management applications in nanoelectronic circuits

We investigated theoretically the phonon thermal conductivity of single-layer graphene. The phonon dispersion for all polarizations and crystallographic directions in graphene lattice was obtained using the valence-force field method. The three-phonon Umklapp processes were treated exactly using an accurate phonon dispersion and Brillouin zone, and accounting for all phonon relaxation channels allowed by the momentum and energy conservation laws. The uniqueness of graphene was reflected in the two-dimensional phonon density of states and restrictions on the phonon Umklapp scattering phase-space. The phonon scattering on defects and graphene edges has also been included in the model. The calculations were performed for the Gruneisen parameter, which was determined from the ab initio theory as a function of the phonon wave vector and polarization branch, and for a range of values from experiments. It was found that the near room-temperature thermal conductivity of single-layer graphene, calculated with a realistic Gruneisen parameter, is in the range $\ensuremath{\sim}2000--5000\text{ }\text{W}/\text{mK}$ depending on the flake width, defect concentration and roughness of the edges. Owing to the long phonon mean free path the graphene edges produce strong effect on thermal conductivity even at room temperature. The obtained results are in good agreement with the recent measurements of the thermal conductivity of suspended graphene.

Phonon thermal conduction in graphene: Role of Umklapp and edge roughness scattering

Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field.

Anti-reflecting and photonic nanostructures

In this work, a surface finish method for parts built-up by selective laser sintering (SLS) is presented One of the main drawbacks of the SLS technique is the high surface roughness of resulting parts Therefore, parts have to be polished to be valid for operation conditions Polishing processes are usually based on manual abrasive techniques However, in the present paper, a surface polishing method based on laser irradiation is presented The laser beam melts a microscopic layer on the surface, which re-solidifies under shielding gas protective conditions, resulting in a smoother surface Laser-polishing tests for lines, planar surfaces and inclined planes have been performed, with satisfactory results in all the cases The experimental tests were carried out on sintered test parts with an initial roughness of 75–78 μm Ra The tested material is a commercial alloy denominated LaserForm ST-100©, composed by sintered stainless steel and infiltrated bronze that it is used mainly for the constitution of injection moulds Experimental results present final surface roughness below 149 μm Ra, which represent an 801% reduction of the mean roughness Finally, a complete analysis of test probes and its metallurgical composition is presented Considering that the material presents a non-homogeneous structure, the polished surfaces present slightly higher hardness values and are more homogeneous than the initial ones Thus, polished surfaces do not present any heat affected zone or cracks, which could cause failure during the part operation

Laser polishing of parts built up by selective laser sintering

Thermal conductivity of CVD diamond at elevated temperatures

Large-area diamond deposition by microwave plasma

Experimental results on the formation of antire-flective periodical microrelief on the surface of CVD diamond films are presented. Diamond surface microstructuring was performed by a laser ablation technique by using either a tightly focused beam of a Nd:YAP laser (λ=1078nm) or a projection optical scheme with a KrF excimer laser (λ= 248 nm). The minimal period of the produced one- and two-dimensional surface gratings, consisting of parallel microgrooves or microcraters, was 3 μm. The optical transmission of the structured diamond surface was found to increase in the IR spectral region depending on the relief depth. The rise of the transmission at a wavelength of 10.6 μm from 70% to 80% was achieved for the diamond films, both sides of which were patterned.

Formation of antireflective surface structures on diamond films by laser patterning

UV laser polishing of thick diamond films for IR windows

Results are reported on laser polishing of 150–400-μm-thick free-standing diamond films with either a copper vapor laser (510 nm wavelength) or an ArF excimer laser (193 nm wavelength). Studies were focused on three particular goals. First, we aimed at a choice of optimum conditions for laser polishing of thick diamond films. It was shown that the laser polishing conditions and the resulting surface roughness were controlled by varying the angle of incidence of a scanning laser beam and by polishing time. Second, the laser ablation technique was applied to remove a defective layer from the “substrate” side of the diamond plates in order to reduce optical losses due to absorption in this layer. Third, the structure of the laser-graphitized diamond surface was studied using UV, visible, and IR optical spectroscopy techniques in the course of the “step-by-step” oxidative removal of the graphitic layer with increasing temperature of the oxidation in ambient air. Once the graphitic layer was removed, the optical transmission in the UV-visible-IR spectral range of the diamond films polished under optimum conditions was measured and compared with the optical transmission of the mechanically polished diamond films. It was shown that the optical quality (in the long-wave infrared region) of the laser-polished diamond plates was sufficient to reach the transmittance value very close to the theoretical limit.

A.V. Khomich

Papers

Thermal conductivity of CVD diamond at elevated temperatures

Large-area diamond deposition by microwave plasma

Formation of antireflective surface structures on diamond films by laser patterning

UV laser polishing of thick diamond films for IR windows

Laser polishing of diamond plates