Nanoscale engineered materials with tailored thermal properties are desirable for applications such as highly efficient thermoelectric, microelectronic and optoelectronic devices. It has been shown earlier that by judiciously varying the interface thermal boundary resistance (TBR), thermal conductivity in nanostructures can be controlled. In the presented investigation, the role of TBR in controlling thermal conductivity at the nanoscale is analyzed by performing non-equilibrium molecular dynamics (NEMD) simulations to calculate thermal conductivity of a range of Si-Ge multilayered structures with 1-3 periods, and with four different layer thicknesses. The analyses are performed at three different temperatures (400, 600 and 800 K). As expected, the thermal conductivity of all layered structures increases with the increase in the number of periods as well as with the increase in the monolayer thickness. Invariably, we find that the TBR offered by the interface nearest to the hot reservoir is the highest. This effect is in contrast to the usual notion that each interface contributes equally to the heat transfer resistance in a layered structure. Findings also suggest that for high period structures the average TBR offered by the interfaces is not equal. Findings are used to derive an analytical expression that describes period-length-dependent thermal conductivity of multilayered structures.

Superlattices are considered one of the most promising material systems for nanotechnological applications in fields such as high figure of merit (ZT) thermoelectrics, microelectronics, and optoelectronics owing to the possibility that these materials could be tailored to obtain desired thermal properties. Factors that could be adjusted for tailoring the thermal conductivity of the superlattices include the monolayer film thickness, periodicity, heat flow direction, straining, and temperature of operation. In the presented study, nonequilibrium molecular dynamics (NEMD) simulations are performed to obtain an understanding of the effect of such factors on the thermal conductivity of Si–Ge superlattices at three different temperatures (400, 600, and 800 K). The NEMD simulations are performed using Tersoff bond-order potential. The thermal conductivity is found to increase with an increase in the number of periods as well as with the increase in the period thickness. The dependence of thermal conductivity on the direction of heat flow is found to be sensitive to the extent of acoustic mismatch at the interface (i.e., heat flowing from Si to Ge versus heat flowing from Ge to Si in a single period). Superlattices with Ge–Si interfaces (heat flows from Ge monolayer to Si monolayer in a period) are found to have lower thermal conductivity than superlattices with Si–Ge interfaces (heat flows from Si monolayer to Ge monolayer in a period). The superlattices thermal conduction, therefore, can be considered to have a characteristic somewhat similar to a thermal diode. Both compressive and tensile strains are observed to be an important factor in tailoring the thermal conductivity of the analyzed superlattices. Particularly, straining can help in reducing the thermal conductivity. The influence of straining is found to increase with increasing period thickness and periodicity.

The ability to alter the thermal and mechanical properties of nanostructures by tailoring nanoscale morphology has led to vast activity in applications such as high figure of merit (ZT) thermoelectric, microelectronic and optoelectronic devices. Two types of nanostructures that have gained significant attention are Si–Ge superlattices and Si–Ge biomimetic nanocomposites, in which one phase is distributed in the other phase in a staggered biomimetic manner similar to biological materials. A systematic comparison of the atomistic factors that affect their thermal behaviour under different extents of straining at a range of temperatures remains to be performed. In this investigation, such analyses are performed for a set of Si–Ge superlattices and Si–Ge biomimetic nanocomposites using non-equilibrium molecular dynamics (NEMD) simulations at three different temperatures (400, 600 and 800 K) and at strain levels varying between −10% and 10%. Analyses indicate that the nanoscale morphology differences between the superlattices and the nanocomposites lead to a striking contrast in the phonon spectral density, interfacial thermal boundary resistance and thermal conductivity. In the case of the nanocomposites, morphology variation at the nanoscale and the tensile or compressive straining at temperatures from 400 to 800 K do not have a significant effect on the changes in thermal conductivity values. Such factors, however, strongly influence the thermal conductivity of superlattices. The thickness of the nanocomposites, however, is found to influence the thermal conductivity values significantly under straining, with the effect of straining increasing with increasing nanocomposite thickness. A relation based on the effective medium approach is shown to fit the NEMD calculated nanocomposite thermal conductivity values.

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