Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity.

Penta-graphene (PG) has been identified as a novel two-dimensional (2D) material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative, hydrogenated penta-graphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity, HPG shows a notable increase in thermal conductivity, which is much higher than that of PG. Considering the necessity of using the same thickness when comparing thermal conductivity values of different 2D materials, hydrogenation leads to a 63% reduction in thermal conductivity for graphene, while it results in a 76% increase for PG. The high thermal conductivity of HPG makes it more thermally conductive than most other semiconducting 2D materials, such as the transition metal chalcogenides. Our detailed analyses show that the primary reason for the counterintuitive hydrogenation-induced thermal conductivity enhancement is the weaker bond anharmonicity in HPG than PG. This leads to weaker phonon scattering after hydrogenation, despite the increase in the phonon scattering phase space. The high thermal conductivity of HPG may inspire intensive research around HPG and other derivatives of PG as potential materials for future nanoelectronic devices. The fundamental physics understood from this study may open up a new strategy to engineer thermal transport properties of other 2D materials by controlling bond anharmonicity via functionalization.

Hookah use among college students from a Midwest University.

National data indicate nearly a quarter of college students smoked from a hookah at some point in their lifetime regardless of gender. To address this issue, researchers assessed the perceptions, knowledge, beliefs of hookah users at a large Midwestern University and also determined what other drug related high-risk behaviors were associated with this behavior. An anonymous, online survey was sent to 2,000 randomly selected undergraduate students from a large Midwestern University. Researchers used a cross sectional research design to determine the prevalence and motivating factors associated with hookah use. Respondents included 438 individuals (60% female) with an average age of 23.1 (SD = 12.32), yielding a response rate of 22%. Approximately 15.4% of the sample had previously smoked hookah, while 6% used hookah within the past 30 days. Common motivating factors associated with smoking hookah included socializing/partying (29%), peer influence (27%), and for relaxation (25%). Correlations were calculated comparing hookah use to other high risk behaviors with the two highest correlations consisted of 30-day tobacco use (r = 0.67) and marijuana (r = 0.39). The results from this study suggest hookah use is limited to a small percentage of students. Students appear to smoke hookah for social reasons and underestimate the addictive properties associated with the product. Researchers and practitioners need to develop and evaluate specific interventions to educate college students about the health hazards associated with hookah use.

Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics--a Comparative Study with Gallium Nitride.

Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN)--another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

Molecular dynamics simulations of SrTiO3 thin-film growth from cluster deposition.

We use classical molecular dynamics simulations to examine the deposition of SrTiO(3) stoichiometric clusters on (001) SrTiO(3). The simulations consider the deposition of clusters that consist of one, two, three or four stoichiometric units that have incident energies of 1.0 eV/atom. Two types of beam compositions are considered: those that are comprised of mono-sized clusters and those that are comprised of mixed-sized clusters along with individual SrO and TiO(2) particles. The results are analyzed to determine the effect of surface termination layer (SrO versus TiO(2)), cluster size and beam composition on the resulting thin-film structure. The simulations indicate that termination layer and beam composition have an impact on the resulting film structure with mixed-beam composition and TiO(2) termination yielding films with a structure similar to that of bulk STO.