Influence of chemisorption on the thermal conductivity of graphene nanoribbons

The thermal conductivity of graphene nanoribbons (GNRs) functionalized by the chemical attachment of methyl and phenyl groups at random positions is calculated using reverse nonequilibrium molecular dynamics. The GNRs exhibit a rapid drop in thermal conductivity with increasing degree of functionalization; a functional group coverage regime of as little as 1.25% of GNR atoms reduces the thermal conductivity by about 50%. The thermal conductivity of nanoribbons with zigzag edges is more sensitive in the degree of functionalization than nanoribbons with armchair edges. The simulation results indicate that the rapid drop in thermal conductivity is a consequence of the higher angular momentum of functional groups, which rotate the unsupported sp³ bonds and thus reduce the phonon mean free paths.     

Thermal conductivity of graphene nanoribbons with defects and nitrogen doping

Thermal conductivity of defective graphene nanoribbons doped with nitrogen for different distributions around the defect edge at nanoscale is investigated using the reverse non-equilibrium molecular dynamics (RNEMD) method, which explores ways to improve thermal management. In addition, thermal conductivity of graphene nanoribbons with both defects and nearby nitrogen doping is investigated in comparison to that of nanoribbons with defects alone. The simulation results are analyzed from three perspectives: phonon match, concentration of N doping, and distribution of N doping. This approach reveals that a coupling effect is the cause of the observed results. Nitrogen doped graphene nanoribbons (both perfect and defective variants) perform better with thermal management than do graphene nanoribbons with defects alone, which is of considerable interest. Based on these investigations, a guide for graphene-interconnected circuits design is implied.     

Anomalous thermal transport along the grain boundaries of bicrystalline graphene nanoribbons from atomistic simulations

We investigate the thermal transport properties of bicrystalline graphene nanoribbons (bi-GNRs) with different symmetric tilt grain boundaries (GBs) by using the molecular dynamics (MD) simulations. It is found that the bi-GNR with the 10.98° GB (the highest dislocation density) has an anomalously enhanced thermal conductivity for the heat flux along the GB compared to other ribbons with lower dislocation densities. This is in strong contrast to the behavior of thermal conductivity across the GB, which decreases monotonically with increasing the dislocation density. We attribute this counterintuitive phenomenon to its non-folding structure and lower edge stress, which can reduce the phonon scatterings induced by GBs and rough edges. In addition, we also examine the effects of the characteristic length and temperature on the thermal conductivity of the bi-GNRs through the phonon Boltzmann transport equation. At any given temperature and characteristic length, the low-dislocation-density bi-GNR are shown to be much more efficient in suppressing the thermal conductivity, and has a higher tailoring rate. These facts reveal the bi-GNR with a lower dislocation density is more promising to be a high figure of merit thermoelectric material.     

甲烷水合物声子导热及量子修正

采用平衡分子动力学方法模拟了甲烷水合物的导热,给出了30～150 K甲烷水合物的热导率。采用量子修正对分子模拟结果进行处理,可以得到更接近实验值的结果。当模拟温度低于德拜温度时,量子效应对分子模拟结果的影响较大。通过对热流自相关函数拟合得到了声学声子和光学声子的弛豫时间。结果显示,声子弛豫时间随温度增加逐渐减小,声学声子导热在水合物的导热中比重最大。随着碳氧原子之间相互作用力的增加,碳氧原子之间振动的耦合程度增加,甲烷水合物的热导率增加。     

Structure-mediated thermal transport of monolayer graphene allotropes nanoribbons

We report the study of the thermal transport management of monolayer graphene allotrope nanoribbons (size ∼20 × 4 nm²) by the modulation of their structures via molecular dynamics simulations. The thermal conductivity of graphyne (GY)-like geometries is observed to decrease monotonously with increasing number of acetylenic linkages between adjacent hexagons. Strikingly, by incorporating those GY or GY-like structures, the thermal performance of graphene can be effectively engineered. The resulting hetero-junctions possess a sharp local temperature jump at the interface, and show a much lower effective thermal conductivity due to the enhanced phonon–phonon scattering. More importantly, by controlling the percentage, type and distribution pattern of the GY or GY-like structures, the hetero-junctions are found to exhibit tunable thermal transport properties (including the effective thermal conductivity, interfacial thermal resistance and rectification). This study provides a heuristic guideline to manipulate the thermal properties of 2D carbon networks, ideal for application in thermoelectric devices with strongly suppressed thermal conductivity.     

十八烷酸热传导机制的尺度效应研究

十八烷酸的热导率具有显著的尺寸依赖性,然而只有宏观热输运性质得到充分研究,限制了微尺度特征结构的复合材料热性能的进一步提高。因此,为满足新型复合相变材料的研究需求,十八烷酸的纳米尺度热输运特性亟待解决。基于分子动力学模拟,系统性地研究了块体、纳米线和纳米链三种形态的十八烷酸热导率的演变规律,通过平衡法模拟的热导率分别为0.4546、0.2213和0.0085 W·m^-1·K^-1;结合声子态密度和重叠能分析,发现在较低频率声子振动的衰减导致纳米线热导率低于块体,显著降低的声子重叠能严重阻碍纳米链的声子输运导致极低热导率。     

二维氮化铝材料传热性能的模拟研究

二维氮化铝材料是一种新型Ⅲ-Ⅴ族二维材料,具有与石墨烯相似的分子结构和材料性能,受到了广泛的关注,然而其导热性能尚未被充分探讨。应用分子动力学模拟的方法研究了单层二维氮化铝在不同温度的热稳定性和导热性能,并分析了其声子频谱。结果表明,单层二维氮化铝材料可以在极高温度（3500 K）下保持结构稳定性,同时在常温情况热导率可达264.2 W·m^-1·K^-1;在500 K以上温度时,声子色散现象使得该材料热导率明显降低。为二维氮化铝材料导热过程的调控和高温导热材料的应用提供了理论指导。     

A molecular dynamics study of the thermal conductivity of graphene nanoribbons containing dispersed Stone–Thrower–Wales defects

Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone–Thrower–Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100–600 K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.     

First principle calculations of the electronic properties of nitrogen-doped carbon nanoribbons with zigzag edges

Calculations have been performed for carbon nanoribbons (CNRs) with zigzag edges containing one substitutional nitrogen atom per 154 carbon atoms, using ab initio density functional theory. It is found that the formation energies of these nanoribbons depend on the nitrogen doping site, as do the electrical properties. The doping nitrogen atom energetically prefers to distribute near the nanoribbon edges, and there is an impurity state below or above the Fermi level for the nitrogen-doped CNR, which depends on the nitrogen doping site. Also, the distribution of non-bonding electrons of nitrogen atom depends on the nitrogen doping site.     

Thermal conductivity in porous silicon nanowire arrays

The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.     

缺陷对氮化硅导热性能影响的模拟研究

通过反应或热压烧结制备氮化硅器件过程中,产生的晶格空位和杂质氧等缺陷会严重影响氮化硅材料的导热性能。为了探究空位和氧杂质对氮化硅材料导热性能的影响规律,利用分子动力学模拟方法设计了多种不同缺陷状态的氮化硅模型,分析了空位/氧杂质的比例、分布状态、晶格位置以及温度对氮化硅材料导热性能的影响。研究结果表明:随着空位/氧杂质比例的增加以及温度的升高,氮化硅体系的热导率都呈明显的下降趋势;当空位/氧杂质由原本随机分布逐渐向导热通路中间聚集时,氮化硅的热导率急剧降低;空位/氧杂质所处不同晶格位置,体系热导率有明显差异。另外,通过计算氮化硅模型的声子态密度,进一步验证了空位/氧杂质比例以及温度对体系导热性能的影响规律。研究结果为制备具有高导热性的氮化硅陶瓷提供了重要的指导。     

Amplified spontaneous emission from single CdS nanoribbon with low symmetric cross sections

CdS nanoribbons with various cross sections offer the opportunity to deeply understand the interaction between optical cavity and spontaneous emission. Herein, long tapered nanoribbons with the cross sections gradually changing were synthesized by a simple physical vapour deposition method. Morphology dependent micro-region photoluminescence (PL) spectroscopy is employed to show Purcell effect along different low symmetry cross sections. Spikes on the PL spectra reveal that local density of optical modes increases when the mode match happens between optical cavity and spontaneous emission. Bound exciton complex related amplified spontaneous emission is observed in a single CdS nanoribbon with well-defined elliptical cross sections and optimized width/thickness ratio ～1.45. Polarized Raman and TEM confirmed that the nanoribbon with the elliptical cross section adopts the [0002] growth direction with good quality. The results suggest that the cross section resonant cavity would be of importance for both fundamental and practical application of cavity quantum electrodynamics in CdS nanoribbon.     

Strain engineering for thermal conductivity of single-walled carbon nanotube forests

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling.     

Knitted graphene-nanoribbon sheet: a mechanically robust structure

In this paper, a new nanostructure is proposed, namely, the knitted graphene-nanoribbon sheet (KGS), which consists of zigzag and/or armchair graphene nanoribbons. The knitting technology is introduced to graphene nanotechnology to produce large area graphene sheets. Compared with pristine graphene, the chirality of a knitted graphene-nanoribbon sheet is much more flexible and can be designed on demand. The mechanical properties of KGSs are investigated by molecular dynamics simulations, including the effect of vacancies. With hydrogen atoms saturating the ribbon edges, the structure (KGS + H) is found to be of significant mechanical robustness, whose fracture does not rely on the critical bonds. The fracture strain of KGS + H remains nearly unchanged as long as there remains a single defect-free graphene nanoribbon in the tensile direction. This graphene nano knitting technique is experimentally feasible, inspired by a recent demonstration by Fournier et al. [Phys. Rev. B, 2011, 84, 035435] of lifting a single molecular wire using a combined frequency-modulated atomic force and tunnelling microscope.     

三维石墨烯-碳纳米管复合结构热导率的分子动力学模拟

采用非平衡分子动力学方法模拟了三维石墨烯-碳纳米管复合结构的法向热导率。结果表明相比于多层石墨烯,其法向热导率提高了一个量级,其界面热阻相比碳纳米管的接触热阻降低了一个量级,但是石墨烯和碳纳米管的界面形变又阻碍了三维石墨烯-碳纳米管复合热导率的进一步提高。通过其振动态密度和重叠能进一步探究了三维石墨烯-碳纳米管复合结构结构能量的传递及声子的局域化情况。结果表明,碳管的添加激发了更多中高频声子振动参与传热,但是依然是低频声子占据主导;验证了界面处的形变是阻止法向热导率进一步提升的主要因素。     

Effect of yttria on thermal transport and vibrational modes in yttria-stabilized hafnia

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y₂O₃ and HfO₂. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y₂O₃, and then reaches a limiting value (～2.1 W m^?1K^?1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y₂O₃ exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y₂O₃. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R_p to the phonon mean free path for zero-porosity, l₀. The thermal conductivity for l₀/R_p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l₀/R_p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R _p to the phonon mean free path for zero-porosity, l ₀. The thermal conductivity for l ₀/R _p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l ₀/R _p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

In-plane lattice thermal conductivities of multilayer graphene films

The in-plane lattice thermal conductivities of a single layer and multilayer graphene films are investigated using nonequilibrium molecular dynamics simulations. It is found the thermal conductivity of a single layer graphene is higher than that of multilayer graphene. Increasing the bonding strength between neighboring layers will reduce the in-plane thermal conductivity for multilayer graphene films. The constraints from the neighboring layer play the role of impeding phonon transport along the in-plane direction in multilayer graphene films. This observation implies the thermal conductivity of a single layer graphene will be reduced in practical applications once it is bonded on a substrate.     

气体-溶液-固体法制备铜基Cu（OH）2纳米带阵列

文章报道一种新的Cu（OH）2纳米带阵列合成方法,即气体-溶液-固体法。这种方法主要特点是把铜片置于氨水的液面上,使铜片基板表面有氧气、氨水和铜参与反应,最终在铜片基板上生长出Cu（OH）2纳米带阵列。Cu（OH）2纳米带的尺寸可以通过控制氨水的浓度进行调节,即氨水浓度降低,Cu（OH）2纳米带的尺寸也变小了;而且Cu（OH）2在一定温度下可以脱水转变成为CuO纳米带;最后研究了Cu（OH）2和CuO纳米带阵列的生长机理。