The lubricating properties of water have been discussed extensively for millennia. Water films can exhibit wearless high friction in the form of cold ice, or act as lubricants in skating and skiing when a liquid. At the fundamental level, friction is the result of a balance between the rate of energy generation by phonon excitation during sliding and drainage of the energy from the interface by coupling with bulk atoms. Using atomic force microscopy, we found that when H2O intercalates between graphene and mica, it increases the friction between the tip and the substrate, dependent on the thickness of the water and graphene layers, while the magnitude of the increase in friction was reduced by D2O intercalation. With the help of first-principles density functional theory calculations, we explain this unexpected behavior by the increased spectral range of the vibration modes of graphene caused by water, and by better overlap of the graphene vibration modes with mica phonons, which favors more efficient energy dissipation. The larger increase in friction with H2O versus D2O shows that the high-frequency vibration modes of the water molecules play a very important role in the transfer of the vibrational energy of the graphene to the phonon bath of the substrate.

Carrier multiplication (i.e. generation of multiple electron-hole pairs from a single high-energy electron, CM) in graphene has been extensively studied both theoretically and experimentally, but direct application of hot carrier multiplication in graphene has not been reported. Here, taking advantage of efficient CM in graphene, we fabricated graphene/TiO2 Schottky nanodiodes and found CM-driven enhancement of quantum efficiency. The unusual photocurrent behavior was observed and directly compared with Fowler's law for photoemission on metals. The Fowler's law exponent for the graphene-based nanodiode is almost twice that of a thin gold film based diode; the graphene-based nanodiode also has a weak dependence on light intensity-both are significant evidence for CM in graphene. Furthermore, doping in graphene significantly modifies the quantum efficiency by changing the Schottky barrier. The CM phenomenon observed on the graphene/TiO2 nanodiodes can lead to intriguing applications of viable graphene-based light harvesting.

Objective

In this prospective cohort study, we investigated cerebral glucose metabolism reductions on [(18)F]-fluorodeoxyglucose (FDG)-PET in progranulin (GRN) mutation carriers prior to frontotemporal dementia (FTD) onset.

Methods

Nine mutation carriers (age 51.5 ± 13.5 years) and 11 noncarriers (age 52.7 ± 9.5 years) from 5 families with FTD due to GRN mutations underwent brain scanning with FDG-PET and MRI and clinical evaluation. Normalized FDG uptake values were calculated with reference to the pons. PET images were analyzed with regions of interest (ROI) and statistical parametric mapping (SPM) approaches.

Results

Compared with noncarriers, GRN mutation carriers had a lowered anterior-to-posterior (AP) ratio of FDG uptake (0.86 ± 0.09 vs 0.92 ± 0.05) and less left-right asymmetry, consistent with an overall pattern of right anterior cerebral hypometabolism. This pattern was observed regardless of whether they were deemed clinically symptomatic no dementia or asymptomatic. Individual ROIs with lowered FDG uptake included right anterior cingulate, insula, and gyrus rectus. SPM analysis supported and extended these findings, demonstrating abnormalities in the right and left medial frontal regions, right insular cortex, right precentral and middle frontal gyri, and right cerebellum. Right AP ratio was correlated with cognitive and clinical scores (modified Mini-Mental State Examination r = 0.74; Functional Rating Scale r = -0.73) but not age and years to estimated onset in mutation carriers.

Conclusion

The frontotemporal lobar degenerative process associated with GRN mutations appears to begin many years prior to the average age at FTD onset (late 50s-early 60s). Right medial and ventral frontal cortex and insula may be affected in this process but the specific regional patterns associated with specific clinical variants remain to be elucidated.