Facile synthesis of single crystal of two-dimensional mixed-halide copper-based perovskites with tunable band gaps and their capability of exfoliation and reversible thermochromism.
Conventionally,
graphene is a poor thermoelectric material with
a low figure of merit (ZT) of 10–4–10–3. Although nanostructuring was proposed
to improve the thermoelectric performance of graphene, little experimental
progress has been accomplished. Here, we carefully fabricated as-grown
suspended graphene nanoribbons with quarter-micron length and ∼40
nm width. The ratio of electrical to thermal conductivity was enhanced
by 1–2 orders of magnitude, and the Seebeck coefficient was
several times larger than bulk graphene, which yielded record-high ZT values up to ∼0.1. Moreover, we observed a record-high
electronic contribution of ∼20% to the total thermal conductivity
in the nanoribbon. Concurrent phonon Boltzmann transport simulations
reveal that the reduction of lattice thermal conductivity is mainly
attributed to quasi-ballistic phonon transport. The record-high ratio
of electrical to thermal conductivity was enabled by the disparate
electron and phonon mean free paths as well as the clean samples,
and the enhanced Seebeck coefficient was attributed to the band gap
opening. Our work not only demonstrates that electron and phonon transport
can be fundamentally tuned and decoupled in graphene but also indicates
that graphene with appropriate nanostructures can be very promising
thermoelectric materials.
Adding a mechanical degree of freedom to the electrical and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelectrical physics and devices with mechanical motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom–up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined experiments, molecular dynamics simulations and theoretical calculations with a phase-diagram analysis. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelectrical physics and industrial applications.
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