Impact of Polymer Residue Level on the In-Plane Thermal Conductivity of Suspended Large-Area Graphene Sheets.

The presence of polymer transfer residues on graphene surfaces is a major bottleneck to overcome for the commercial and industrial viability of devices incorporating graphene layers. In particular, how clean the surface must be to recover high (>2500 W/mK) thermal conductivity and maximize the heat spreading capability of graphene for thermal management applications remains unclear. Here, we present the first systematic study of the impact of different levels of polymer residues on the in-plane thermal conductivity (κr) of single-layer graphene (SLG) fabricated by chemical vapor deposition (CVD). Control over the quantity of surface residue was achieved by varying the length of time each sample was rinsed in toluene to remove the poly(methyl methacrylate) (PMMA) support layer. The level of residue contamination was assessed using atomic force microscopy (AFM) and optical characterization. The thermal conductivity of the suspended SLG was measured using an optothermal Raman technique. We observed that the presence of polymer surface residue has a significant impact on the thermal properties of SLG, with the most heavily contaminated sample exhibiting a κr as low as (905 +155/-100) W/mK. Even without complete eradication of surface residues, a thermal conductivity as high as (3100 +1400/-900) W/mK was recovered, where the separation between adjacent clusters was sufficiently large (>700 nm). The proportion of the SLG surface covered by residues and the mean separation distance between clusters were found to be key factors in determining the level of κr suppression. This work has important implications for future large-scale graphene fabrication and transfer, particularly where graphene is to be used as a heat spreading layer in devices. The possibility of new opportunities for manipulation of the thermal properties of SLG via PMMA nanopatterning is also raised.

Correlating Thermionic Emission with Specific Surface Reconstructions in a <100> Hydrogenated Single-Crystal Diamond.

Thermionic emission relies on the low work function and negative electron affinity of the, often functionalized, surface of the emitting material. However, there is little understanding of the interplay between thermionic emission and temperature-driven dynamic surface transformation processes as these are not represented on the traditional Richardson-Dushman equation for thermionic emission. Here, we show a new model for thermionic emission that can reproduce the effect of dynamic surface changes on the electron emission and correlate the components of the thermionic emission with specific surface reconstruction phases on the surface of the emitter. We use hydrogenated <100> single-crystal and polycrystalline diamonds as thermionic emitters to validate our model, which shows excellent agreement with the experimental data and could be applicable to other emitting materials. Furthermore, we find that tailoring the coverage of specific structures of the C(100)-(2 × 1):H surface reconstruction could increase the thermionic emission of diamond by several orders of magnitude.

Barrier-Layer Optimization for Enhanced GaN-on-Diamond Device Cooling.

GaN-on-diamond device cooling can be enhanced by reducing the effective thermal boundary resistance (TBReff) of the GaN/diamond interface. The thermal properties of this interface and of the polycrystalline diamond grown onto GaN using SiN and AlN barrier layers as well as without any barrier layer under different growth conditions are investigated and systematically compared for the first time. TBReff values are correlated with transmission electron microscopy analysis, showing that the lowest reported TBReff (∼6.5 m2 K/GW) is obtained by using ultrathin SiN barrier layers with a smooth interface formed, whereas the direct growth of diamond onto GaN results in one to two orders of magnitude higher TBReff due to the formation of a rough interface. AlN barrier layers can produce a TBReff as low as SiN barrier layers in some cases; however, their TBReff are rather dependent on growth conditions. We also observe a decreasing diamond thermal resistance with increasing growth temperature.

Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination.

We present a phenomenon concerning electromagnetic enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The local electric field is sensed by micro Raman spectroscopy, which allows the enhancement of the Raman signal arising from the heterojunction region to be revealed; the Raman signal per unit volume increases at least ten times with respect to the homogeneous Si and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a three-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by finite element methods. A local enhancement of the electric field at the heterojunction was deduced. However, the magnitude of the electromagnetic field enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this effect promises a way of improving photon harvesting using axially heterostructured semiconductor nanowires.