STANFORD, Calif., Nov. 1, 2023 — When combined with electronics, infrared (IR) light can enable small, fast devices for sensing, imaging, and signaling at the molecular level. To fully harness the advantages of IR light, the materials used for IR optical and optoelectronic applications require defect-free crystallinity.

To make high-quality crystals that resonate strongly with IR light, researchers at Stanford University and Lawrence Berkeley National Laboratory (LBNL) developed a bottom-up, self-assembly approach to synthesize nanostructures with crystal qualities consistent with bulk single crystals. The ultrathin nanostructures act as ultrahigh-quality, nanoscale resonators of lattice vibrations at IR frequencies, to provide a high-performance, low-loss platform for IR applications.

To build the nanoresonators, the researchers used van der Waals (vdW) nanomaterials. VdW materials support strong resonances between IR photons and crystal-lattice vibrations (i.e., phonons) and form hybrid photon-phonon quasiparticles known as phonon polaritons.

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The tip of an AFM focuses IR light from an x-ray beamline onto a tiny spot, enabling researchers to detect the lattice vibrations of an ultrathin, ribbon-like nanocrystal (yellow). Courtesy of Lawrence Berkeley National Laboratory and Stanford University.

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The researchers synthesized phonon polaritonic vdW materials using a fast, inexpensive, scalable process called flame vapor deposition (FVD). According to the team, FVD is a significant advancement over current mechanical exfoliation methods, which are labor-intensive and unsystematic, and slower vapor deposition techniques, which are costly and require lithography treatments that can damage the crystals.

The team used FVD to grow nanoribbons of molybdenum oxide (MoO₃), a vdW phonon polaritonic material that has the potential to tune resonances to IR frequencies. The researchers controlled the sizes and shapes of the synthesized MoO₃ nanostructures by varying temperature, molybdenum concentration, and time.

The MoO₃ nanoribbons produced via FVD were found to have smooth, parallel edges that could function as reflecting surfaces and, thus, as resonating cavities for IR phonon polariton standing waves.

To measure the quality of the IR nanoresonators, the researchers probed the resonators with Synchrotron Infrared Nanospectroscopy (SINS) at the Advanced Light Source (ALS), a Department of Energy Office of Science user facility at LBNL.

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Top: Synthesis of MoO₃ micro- and nanostructures using FVD. The structures were dry-transferred to a target substrate and characterized with IR light focused by an atomic force microscope (AFM) tip. Bottom: Scanning electron microscope images of MoO₃ samples (microplates, nanoribbons, and nanowires) prepared under different FVD conditions. Courtesy of Lawrence Berkeley National Laboratory and Stanford University.

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The broadband IR light provided by ALS Beamline 2.4 enabled the researchers to map phonon polariton resonances spanning mid- to far-IR wavelengths, covering four distinct frequency bands where resonances occurred. In addition, the researchers used Beamline 5.4, which covers the mid-IR range with much higher spectral resolution than is found in typical commercial systems.

SINS uses the tip of an atomic force microscope (AFM) to focus IR light beams from the synchrotron radiation down to a spot size that is smaller than the wavelength of the IR light. The researchers dry-transferred the MoO₃ structures to a target substrate and characterized the structures with IR light focused by the AFM tip.

The resulting resonance maps fully characterized, for the first time, the broadband IR response of FVD-synthesized, MoO₃ nanoribbons with high spatial and spectral resolution, detecting resonance modes beyond the tenth order. The resonances were stronger and more discernible than those from nanostructures prepared using alternative methods. The quality factors (Q-factors) obtained via FVD are the highest reported for a phonon polariton resonator to date, demonstrating the high crystal quality of the synthesized nanostructures.

The bottom-up-synthesized, polaritonic vdW nanostructures can act as ultrahigh-quality nanoscale resonators of lattice vibrations at IR frequencies and serve as high-performance, low-loss platforms for IR optical and optoelectronic applications. Applications that use light in the IR regime include systems for subwavelength imaging, thermal emission, and molecular sensing.

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.1c10489).

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