It is understood that defects of the atomic arrangement of the lattice in 2D molybdenum disulfide (MoS 2 ) grown by chemical vapor deposition (CVD) can have a profound effect on the electronic and optical properties. Beyond these it is a major prerequisite to also understand the fundamental effect of such defects on phonon transport, to guarantee the successful integration of MoS 2 into the solid-state devices. A comprehensive joint experiment-theory investigation to explore the effect of lattice defects on the thermal transport of the suspended MoS 2 monolayer grown by CVD is presented. The measured room temperature thermal conductivity values are 30 ± 3.3 and 35.5 ± 3 W m −1 K −1 for two samples, which are more than two times smaller than that of their exfoliated counterpart. High-resolution transmission electron microscopy shows that these CVD-grown samples are polycrystalline in nature with low angle grain boundaries, which is primarily responsible for their reduced thermal conductivity. Higher degree of polycrystallinity and aging effects also result in smoother temperature dependency of thermal conductivity (κ) at temperatures below 100 K. First-principles lattice dynamics simulations are carried out to understand the role of defects such as isotopes, vacancies, and grain boundaries on the phonon scattering rates of our CVD-grown samples.

The thermal properties of nano-scale materials are largely influenced by their geometry. The zero, one and quasi one dimensional forms of the same material could exhibit unique thermal transport properties depending upon the shape and nano-scale feature size. In order to gain a clear understanding of the contributions from geometrical scattering effects on thermal transport, it is required to study these nano-materials in a single isolated form rather than in clusters or films. In the past decade, titanium dioxide nanotube arrays fabricated by anodic oxidation of titanium emerged as a useful semiconductor architecture for a variety of applications, particularly for solar energy conversion. Nonetheless, the thermal properties of individual nanotubes that are important for their use in high temperature applications have not been clearly understood. Here we report the thermal transport properties of individual titania nanotubes as revealed by our preliminary study using a suspended microdevice that facilitates the thermal conductivity measurements and crystal structure investigation on the same nanotube. The nanotubes were prepared by anodic oxidation of a titanium foil in HF-DMSO electrolyte at 60 V, having outer diameters in the range of 200 to 300 nm and wall thicknesses of ∼30 to 70 nm in either amorphous or polycrystalline anatase phase. The thermal conductivity of single nanotubes was found to be very close to that of the amorphous phase (1.5 W mK(-1) and 0.85 W mK(-1) respectively) and it was only half of the thermal conductivity of the nanotube arrays in the film form. The thermal conductivity of bulk TiO2 is known to be almost six times higher. The observed thermal conductivity suppression in single nanotubes was explained using a transport model developed by considering diffuse phonon-surface scattering and scattering of phonons by ionized impurities of concentrations in the order of 10(18)-10(19) cm(-3).

It is well understood that defect engineering can give rise to exotic electronic properties in transition-metal dichalcogenides, but to this date, there is no detailed study to illustrate how defects can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches based on the first-principles density functional theory and Boltzmann transport equations, we have explored the effect of lattice vacancies and substitutional tungsten (W) doping on the thermal transport of the suspended molybdenum diselenide (MoSe) monolayers grown by chemical vapor deposition (CVD). The results show that even though the isoelectronic substitution of the W atoms for Mo atoms in CVD-grown MoWSe monolayers reduces the Se vacancy concentration by 50% compared to that found in the MoSe monolayers, the thermal conductivity remains intact in a wide temperature range. On the other hand, Se vacancies have a detrimental effect for both samples and more so in the MoWSe monolayers, which results in thermal conductivity reduction up to 72% for a vacancy concentration of 4%. This is because the mass of the W atom is larger than that of the Mo atom, and missing a Se atom at a vacancy site results in a larger mass difference and therefore kinetic energy and potential energy difference. Furthermore, the monotonically increasing thermal conductivity with temperature for both systems at low temperatures indicates the importance of boundary scattering over defects and phonon-phonon scattering at these temperatures.

The effect of physisorbedvs.chemisorbed oxygen on highly organized single walled carbon nanotube (SWCNT) ultrathin films is investigated by correlating the thermoelectric properties measured by a suspended micro-device to the SWCNT structure.

Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 10 cm, which counteracted the low mobility values ∼0.006 cm V s. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (E) from the conduction band to the valence band.