Constructing Graphene Grain Boundaries to Control Graphene Plasmons

Constructing graphene grain boundaries to control graphene plasmons

Although graphene offers enormous advantages for plasmon technologies, it is often difficult to excite multiple graphene plasmons together on a very dense single graphene plate. In a recently published article in the magazine ACS Nanothe researchers reported heteroepitaxially grown polycrystalline graphene monolayer with patterned gradient grain boundary (GB) density to aid the development of nanoscience with a single atomic layer, integrated photonics and optoelectronics.

Study: Engineering graphene grain boundaries for plasmonic multi-excitation and hot spots† Image credit: Marco de Benedictis/Shutterstock.com

Graphene GBs

The two-dimensional (2D) correlated graphene quasiparticles that can oscillate together form an interdisciplinary field in graphene plasmonics. The graphene plasmons have excellent electromagnetic controllability and confinement dominating far infrared and terahertz frequencies. In addition to microfabrication nanostructuring and chemical potential modulation, the GB of graphene serves as a promising candidate to obstruct or reflect the plasmons in real space.

Despite the efficiency of GBs in reconstructing the graphene structure, facilitating the tuning of graphene plasmonics, it suffers from extreme impurity doping and high density problems, preventing it from achieving a single graphene monolayer with diverse and dense GB. Thus, the application and influence of GBs on plasmonic modes remains elusive.

Several strategies have been developed to reduce and increase GB density; these strategies were limited to creating growth environments that are homogeneous throughout the substrate, limiting the diverse GB generation.

Graphene GBs for plasmonic multi-excitation and hot spots

The present study reported the heteroepitaxially grown polycrystalline graphene monolayer with a GB density with a patterned gradient. They used the chemical vapor deposition method to create several nano-sized local growth environments on a centimeter-scale substrate. These geometries allowed for plasmonic co-excitation with the diversification of wavelengths.

The team demonstrated the rich plasmonic standing waves and bright plasmonic hot spots using high-resolution scanning nearfield optical microscopy (SNOM). They noted that the local plasmonic wavelengths were tunable by glowing and varying the GB density. Based on theoretical modeling, they concluded that the reason for such plasmonic versatility was due to GB-induced phonon-plasmon interactions via the random phase approach. The reported seed-induced heteroepitaxial growth is a promising strategy for GB engineering of 2D materials. Moreover, the GB-based controllable plasmon co-generation and manipulation in a single graphene monolayer facilitates the use of graphene for nanophotonics and plasmonics.

Research results

The grain structure of the graphene film ring region was investigated using select-area electron diffraction (SAED) and transmission electron microscopy (TEM). The SAED pattern showed many families of spots indicating the presence of many differently oriented grains. The researchers obtained a real-space image of grain in a selected orientation through an objective aperture filter. The whole graphene grain structure maps were created and color coded by a lattice orientation.

The dark field TEM image showed that the ring region had a gradient grain structure and the GB density increased as it approached the ring center. The obtained grain sizes for the outer, ring and inner regions were 140 ± 56, 40 ± 21 and 30 ± 13 nanometers, respectively. The high-resolution TEM characterization with aberration correction showed that the polycrystalline graphene (PG) rings contained graphene grains without defects and attached to GBs with pentagons and heptagons.

The fast Fourier transform pattern revealed differently oriented grains in multiple numbers. The film showed highly crystalline regions of similar morphologies and variation in GB density. The plasmons in inhomogeneous PG (IPG) film were studied using SNOM, under an incident infrared (IR) wavelength of approximately 10 µm. Based on the darkfield TEM and growth mechanism observations, the team confirmed the formation of high-density GBs in the hole in the center of the individual graphene domain. They also observed a centrally located large plasmon hotspot in each domain, suggesting that the high-density GB regions are the origin of hotspots.

A nano-Fourier transform infrared (nano-FTIR) spectrum, collected from the SNOM tip around the hotspot, was used to demonstrate the local fingerprint spectroscopy. The spectrum showed an IR absorption stronger than pure silica (SiO2) substrate, indicating the plasmonic absorption of the graphene film hotspot.

Controlling the GB distribution or annealing the IPG helped reach the hot spots which expanded to an unpredictable size. The excited hot spots in this sample were about 1000 nanometers, which was twice the size of pristine samples. Here, the annealing of the IPG enhanced the carrier doping in it.

Conclusion

In conclusion, the researchers demonstrated the controlled growth of the IPG film with a pattern variation in GB density distribution, using PG ring seeds to create different nano-sized local growth environments over a centimeter-sized substrate.

The team also demonstrated the co-excitation of several plasmons in such IPGs with wavelength tunability measured using near-field optical imaging. These plasmons excited at the same time showed wavelength that increased exponentially and formed large plasmonic hot spots by increasing the density of the GB. Moreover, this plasmonic tunability was due to the plasmon-phonon interactions induced by GB and devoid of magnetic excitation or external gate bias.

Reference

Teng Ma, Baicheng Yao, Zebo Zheng, Zhibo Liu, Wei Ma, Maolin Chen, Huanjun Chen et al (2022). Engineering graphene grain boundaries for plasmonic multi-excitation and hot spots. ACS Nano. https://pubs.acs.org/doi/10.1021/acsnano.2c00396

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