Abstract
Atomic vibrations due to stretching or bending modes cause optical phonon modes in the solid phase. These optical phonon modes typically lie in the frequency range of 102 to 104 cm−1. How much can the frequency of optical phonon modes be lowered? Herein we show an extremely low-frequency optical phonon mode of 19 cm−1 (0.58 THz) in a Rb-intercalated two-dimensional cyanide-bridged Co–W bimetal assembly. This ultralow frequency is attributed to a millefeuille-like structure where Rb ions are very softly sandwiched between the two-dimensional metal–organic framework, and the Rb ions slowly vibrate between the layers. Furthermore, we demonstrate temperature-induced and photo-induced switching of this low-frequency phonon mode. Such an external-stimulation-controllable sub-terahertz (sub-THz) phonon crystal, which has not been reported before, should be useful in devices and absorbers for high-speed wireless communications such as beyond 5G or THz communication systems.
Highlights
A monoatomic molecule in the gas or liquid phase moves with free motion
If a heavy monoatomic molecule is so ly caught by an organic cage or a metal–organic framework cage (Fig. 1(ii)), the translational mode switches to a vibrational mode
From the viewpoint of phonon design, we demonstrate extremely low-frequency optical phonon modes of 0.58 THz (580 GHz, 2.4 meV) and 0.78 THz (780 GHz, 3.2 meV)
Summary
A monoatomic molecule in the gas or liquid phase moves with free motion. Such a movement of a monomolecule is called a translational mode (Fig. 1(i)). The vibrational motion is observed as the optical phonon mode.[1,2,3,4,5,6,7] Typical stretching vibrational modes are observed at several tens of terahertz (THz), e.g., CO stretching mode (2143 cmÀ1, 64 THz) and CN stretching mode (2200 cmÀ1, 66 THz). To design a low-frequency optical phonon mode, the size and shape of the cage and the chemical affinity of the surrounding framework are both important.
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