While graphene offers enormous advantages for plasmon technologies, it is often difficult to co-excite multiple graphene plasmons into a single, highly dense graphene sheet. In a recent article in the journal ACS Nano, researchers reported a heterolithically cultured polycrystalline graphene monolayer with a model gradient (GB) grain boundary density to aid the development of single-atom atomic nanoscience. , integrated photonics and optoelectronics.
Study: Graphene grain boundary engineering for multi-plasmonic excitation and hot spots. Image credit: Marco de Benedictis / Shutterstock.com
GB of graphene
Two-dimensionally correlated (2D) correlated particles of graphene that can collectively oscillate make an interdisciplinary field in graphene plasmonics. Graphene plasmons have excellent electromagnetic control and confinement that dominate far infrared and terahertz frequencies. In addition to microstructuring nanostructuring and chemical potential modulation, graphene GB serves as a promising candidate for obstructing or reflecting plasmons in real space.
Despite the efficiency of the GB in reconstructing the structure of graphene, which facilitates the adjustment of graphene plasmonics, it suffers from doping of extreme impurities and high density problems, which prevents it from achieving a single monolayer. of graphene with diverse and dense GB. Thus, the application and influence of GB in plasmonic modes remains elusive.
Various strategies have been developed to reduce and increase GB density; these strategies were limited to creating homogeneous growth environments across the substrate, which restricts the diverse generation of GB.
GB of graphene for plasmonic multiexcitation and hot spots
The present study reported the polycrystalline graphene monolayer grown heteroepitaxially with a GB density of modeled gradient. They used the chemical vapor deposition method to create several nanometer-sized local growth environments on a one-centimeter-scale substrate. These geometries allowed the plasmonic coexcitation with the diversification of the wavelengths.
The team demonstrated rich plasmonic standing waves and bright plasmonic hot spots using high-resolution scanning near-field optical microscopy (SNOM). They observed that local plasmonic wavelengths were adjustable by annealing and GB density variation. From the theoretical modeling, they inferred that the reason for this plasmonic versatility was due to GB-induced phonon-plasmon interactions using the random phase approximation method. Seed-induced heteroepitaxial growth reported is a promising strategy for GB 2D materials engineering. In addition, the cogeneration and handling of GB-based controllable plasmons in a single graphene monolayer facilitates the use of graphene for nanophotonics and plasmonics.
Research results
The grain structure of the ring area of graphene film was investigated using selected area electron diffraction (SAED) and transmission electron microscopy (TEM). The SAED pattern showed many spot families indicating the presence of many differently oriented grains. The researchers obtained an image of the actual grain space in a selected orientation using a target aperture filter. The entire graphene grain structure maps were created and color-coded using a lattice orientation.
The dark field TEM image showed that the ring area had a gradient grain size structure and as it approached the center of the ring, the density of GB increased. The grain sizes obtained for the outer, ring, and inner area were 140 ± 56, 40 ± 21, and 30 ± 13 nanometers, respectively. Aberration-corrected high-resolution TEM characterization showed that polycrystalline graphene (PG) rings contained flawless graphene grains bound to GB with pentagons and heptagons.
The Fourier transform pattern revealed grains oriented differently in multiple numbers. The film showed highly crystalline areas with similar morphologies and variation in GB density. Plasmons from the non-homogeneous PG (IPG) film were studied by SNOM, under an incident infrared (IR) wavelength of approximately 10 micrometers. Based on dark field TEM observations and growth mechanism, the team confirmed the formation of high-density GB within the hole in the center of the individual graphene domain. They also observed a large plasmon hot spot centrally located in each domain, suggesting that the high-density GB regions are the source of the hot spots.
A nano-Fourier transform (nano-FTIR) infrared spectrum, which was collected from the SNOM tip around the hot spot, was used to demonstrate local fingerprint spectroscopy. The spectrum showed stronger IR absorption than the pristine silica (SiO2) substrate, indicating the hot spot plasmonic absorption of graphene film.
Controlling the GB distribution or IPG annealing helped get the hotspots expanded to an unpredictable size. The hot spots in this sample were about 1,000 nanometers, which was twice as large as the pristine samples. Here IPG annealing improved carrier doping.
Conclusion
In conclusion, the researchers demonstrated the controlled growth of IPG film with a modeled variation in GB density distribution, where PG ring seeds were used to create different nanoscale local growth environments on a centimeter-sized substrate.
The team also demonstrated the coexcitation of several plasmons in these IPGs with a wavelength tuning that was measured using near-field optical imaging. These simultaneously excited plasmons showed an exponentially increased wavelength and formed large plasmonic hotspots increasing GB density. In addition, this plasmon tuning was due to GB-induced plasmon-phonon interactions and devoid of magnetic excitation or bias of the outer gate.
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 Hotspots. ACS Nano. https://pubs.acs.org /doi/10.1021/acsnano.2c00396
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