A New Approach to 3D Plasmonic Characterization

Plasmonic nanostructures are known for their distinct optical characteristics that facilitate advanced applications in sensing, optoelectronics, and quantum technologies alike.¹ These structures concentrate light into sub-wavelength dimensions, improving electromagnetic fields at nanoscale "hot spots."

While planar plasmonics has been widely studied, extending it to 3D architectures unlocks new possibilities, such as vertical mode coupling and spatially tailored resonances for multifunctional systems. However, the shift to 3D plasmonics presents significant challenges.

At such small scales, even the smallest deviations in geometry could significantly change the targeted plasmonic behavior. To fully realize the potential of 3D plasmonics, precise characterization, and rigorous simulations are essential for verifying functionality and ensuring reliable performance as originally intended.

FEBID: Unlocking 3D Nanoprinting

Over the past decade, Focused Electron Beam Induced Deposition (FEBID) has emerged as a versatile and precise additive manufacturing technology for nanoscale 3D structures. By using a focused electron beam to locally dissociate precursor molecules (Figure 1A), FEBID enables the direct-write fabrication of complex architectures (Figure 1B–C) with spatial resolutions down to just a few nanometers.²

Its flexibility allows for the creation of 3D geometries that conventional lithographic techniques cannot achieve (Figure 1D). However, FEBID structures often contain a high proportion of carbon due to incomplete ligand dissociation.

As a result, purification processes are essential for removing this carbon while preserving the structures' geometry and enabling functional properties such as plasmonic activity.³ This highlights the crucial relationship between fabrication, purification, and characterization in ensuring successful outcomes.

Figure 1. 3D nanoprinting using focused electron beams (A) facilitates the flexible fabrication of plasmonic structures from simple (B) to intricate geometries (C), and even fully free-standing 3D architectures (D). Following purification, the material transforms into pure gold, enabling distinct plasmonic responses, as illustrated by the colored STEM-EELS maps. The objective of this study was to achieve full optimization to completely align with simulation predictions, as shown right in panel (D). Image Credit: Quantum Design UK and Ireland Ltd

The Challenge of Accurate Characterization

Purified FEBID structures require precise characterization to validate their design and optimize functionality, a task that becomes especially challenging at the nanoscale. One of the most effective techniques for visualizing plasmonic activity with spatial and energy resolution is Scanning Transmission Electron Microscopy (STEM) combined with Electron Energy Loss Spectroscopy (EELS).

While highly effective, experimental measurements can be further complemented by simulations, which are crucial for assessing whether the designs perform as intended (Figure 1D). However, accurate morphological data is essential at this scale, as even slight variations can significantly influence simulation outcomes.

While AFM characterization of such nanostructures on bulk substrates would be easy to carry out, it lacks comparability as growth and purification on ultra-thin sub-3 nm carbon grids, needed for STEM, is completely different. Thus, the only reliable approach for both characterization and simulation are primary STEM-EELS assessments, followed by AFM characterization of the same structures.

Although AFM characterization would be straightforward for nanostructures on bulk substrates, it lacks comparability. This is because growth and purification conditions on ultra-thin, sub-3 nm carbon grids—required for STEM—differ significantly from those on bulk substrates.

Missing this correlation, simulations founded on assumptions as opposed to real geometries are at risk of introducing considerable inaccuracies, which compromise the reliability of experimental outcomes and predictions.

While AFM assessments on such membranes are very challenging, the benefits of this technique are considerable and widely applicable. This approach can be used in any scenario demanding combined TEM and AFM.

Figure 2. Comprehensive workflow for 3D plasmonic nanostructure analysis. After 3D nanoprinting, STEM- EELS characterization is performed to evaluate the plasmonic functionality (top left box). The FusionScope then begins with SEM overviews to locate the region of interest and identify the specific nanowires of interest (red, top right). Next, the SEM guides the precise approach of the AFM cantilever (blue, bottom row) for high-resolution morphological characterization on ultra- thin membranes. The resulting cross- sectional profiles (bottom left) are used as input for simulations, enabling optimization of the fabrication process ^[4]. Image Credit: Quantum Design UK and Ireland Ltd

Strategy: Bridging the Morphology Gap with the FusionScope^®

To resolve the challenges of accurate characterization and simulation, Quantum Design UK and Ireland used a detailed approach that incorporates morphological characterization into the whole workflow.

At the center of this technique is the FusionScope by Quantum Design, which merges the imaging capabilities of Scanning Electron Microscopy (SEM) for nanoscale feature identification with the accurate surface analysis abilities of Atomic Force Microscopy (AFM).⁵ The workflow (see Figure 2) follows these key steps:

1. Fabrication and Initial Characterization: Following 3D nanoprinting on sub-3 nm membranes, STEM-EELS was used for comprehensive functional characterization of the plasmonic nanowires.
2. Correlative Morphology: The FusionScope facilitated the localization and analysis of the same nanowires. SEM-guided positioning enabled precise nanowire identification, ensuring accurate AFM measurements.
3. Simulation Validation: Quantitative AFM cross-sectional data provided input parameters for plasmonic simulations, ensuring that the modeled geometries closely matched the actual structures. This significantly improved the predictive accuracy of the simulations.

This approach demonstrated that direct morphological correlation is not just beneficial but essential for achieving reliable, reproducible results—serving as a crucial step toward full process optimization.

Experimental Success: FusionScope in Action

The FusionScope was found to be the optimal tool for this high-precision workflow (Figure 2). Its integrated SEM-AFM abilities enabled the location and characterization of nanowires on fragile, ultra-thin sub-3 nm films—a considerable achievement demonstrating the tool's sensitivity and stability.

SEM-guided navigation was essential, enabling the AFM probe to precisely approach structures without causing damage. As shown in Figure 3, AFM measurements closely matched TEM data, confirming the FusionScope’s reliability and accuracy for nanoscale imaging.

The direct correlation between TEM and AFM data allowed for the refinement of simulation inputs and the optimization of both fabrication and purification processes, paving the way for predictable 3D plasmonic structures. This high level of predictability transformed the entire workflow into a deductive design process—starting with simulations and progressing to fabrication.

Ultimately, this approach fulfilled the long-standing potential of FEBID as a dependable method for creating 3D plasmonic architectures.

Figure 3. TEM-validated AFM data, demonstrating very good agreement in morphological characterization. This comparison highlights the exceptional sensitivity, stability, and reliability of the FusionScope, even when analyzing structures on ultra-thin sub-3 nm membranes, reinforcing its credibility for high-precision nanoscale analysis. Image Credit: Quantum Design UK and Ireland Ltd

Conclusion: A New Standard in Correlative Microscopy

The FusionScope has transformed applicable workflows for activities in 3D plasmonic nanostructures by bridging the important gap between fabrication and simulation for the first time in both a consistent and reliable way.^6,7,8 In a field where nanoscale precision is critical, this instrument establishes a new benchmark for next-generation correlative microscopy, delivering capabilities that surpass those of alternative systems.

Beyond this specific use case, the FusionScope has reimagined the potential of correlated microscopy. Its seamless integration of SEM-guided AFM with sophisticated operational modes makes it invaluable for pre- and post-characterization workflows even on highly exposed surface areas.⁹

The tool is ideally suited for assessing difficult surfaces, as shown on sub-3 nm membranes, and consistently delivers outstanding sensitivity, precision, and stability with an easy-to-use operation philosophy.

For researchers standing at the forefront of nanotechnology, the FusionScope provides unequaled flexibility and reliability, representing a key technological advancement in nanoscale characterization.

Acknowledgments

This article is based on materials originally authored by Harald Plank, Department Head for Functional Nanofabrication / FIB / AFM at the Graz Centre of Electron Microscopy.

References and Further Reading

1. Gu, P., Zhang, W. and Zhang, G. (2018). Plasmonic Nanogaps: From Fabrications to Optical Applications. Advanced Materials Interfaces, 5(19), p.1800648. https://doi.org/10.1002/admi.201800648.
2. Winkler, R., et al. (2019). 3D nanoprinting via focused electron beams. Journal of Applied Physics, [online] 125(21), p.210901. https://doi.org/10.1063/1.5092372.
3. V. Reisecker, Winkler, R. and Plank, H. (2024). A Review on Direct‐Write Nanoprinting of Functional 3D Structures with Focused Electron Beams. Advanced Functional Materials. https://doi.org/10.1002/adfm.202407567.
4. Reisecker, V., et al. (2023). Spectral Tuning of Plasmonic Activity in 3D Nanostructures via High‐Precision Nano‐Printing. Advanced Functional Materials. https://doi.org/10.1002/adfm.202310110.
5. FusionScope by Quantum Design – a new AFM SEM Correlative Microscopy Platform. https://www.fusionscope.com/.
6. Kuhness, D., et al. (2020). High-Fidelity 3D Nanoprinting of Plasmonic Gold Nanoantennas. ACS Applied Materials & Interfaces, 13(1), pp.1178–1191. https://doi.org/10.1021/acsami.0c17030.
7. Winkler, R., et al. (2016). Direct-Write 3D Nanoprinting of Plasmonic Structures. ACS Applied Materials & Interfaces, 9(9), pp.8233–8240. https://doi.org/10.1021/acsami.6b13062.

This information has been sourced, reviewed and adapted from materials provided by Quantum Design

For more information on this source, please visit Quantum Design.