Black gold maximizes the light absorption of nanomaterials

(Nanowerk Spotlight) Maximizing light absorption of nanomaterials has been an emerging research field in recent years due to its attractiveness in a wide range of applications that involve conversion or utilization of solar energy.
However, most of the concepts reported are based on multi-layered architecture inspired by optical impedance matching concepts that require complicated, non-scalable fabrication processes such as electron beam lithography.
Efforts to maximize light absorption via nanostructuring remain scarce. A group of researchers in Australia is now one of the first to report such a material – a nanolayer of black gold nanotubes.
The team, led by associate professor Daniel E. Gomez from RMIT University, has published their findings in Advanced Functional Materials ("Black Gold: Broadband, High Absorption of Visible Light for Photochemical Systems").
A) The plan-view SEM image of the black Au film illustrating the presence of Au nanotubes and a measurement of the average wall thickness at the top surface of the nanotubes. B) The cross-sectional SEM image reflecting the nanotube structure and thickness of the film. C) XRD spectra of the black Au film depicting the presence of Au and Ti on the fabricated surface. D) EDAX analysis on the surface of the black Au to confirm the homogeneity of Au on the surface of nanotubes. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"Maximizing light absorption of plasmonic nanostructures for photochemistry systems has always been the core of our research interest," says Dr. Charlene Ng, a Postdoctoral Fellow at CSIRO and the paper's first author. "The motivation to conduct this work comes from the search for an appropriate fabrication strategy that is simple, cheap and robust to create plasmonic nanostructures that exhibit strong light absorption in the visible range."
"The most exciting result in our paper is the ability to create gold surfaces that are nanostructured and appears black to the human eye due to its broadband high absorption of the visible light," adds Gomez. "It can be fabricated over large surface areas in a robust and cost-efficient manner. Furthermore, it also exhibits the flexibility to adhere to arbitrary surfaces that make it attractive to a wide range of photo-related applications."
"Most importantly" he adds, the fabrication process is not limited to only gold; we have also demonstrated the fabrication of black nickel using a similar method."
The researchers strongly believe that this current study could provide a new paradigm for the use of highly absorbing metal nanostructures to effectively harvest the entire visible spectrum for photo-related applications such as solar fuel production, photo-detection and photovoltaics.
In this present work, the team demonstrates a black gold film of merely 400 nm in thickness acting as a broadband super absorber that is capable of absorbing >92% of the incident light energy up to a wavelength of 600 nm.
The intriguing light absorption capability of this black gold film is related to the high aspect ratio and closely packed gold nanotubes with a tapered wall thickness and high exposed surface area.
The majority of nanomaterials that exhibit high light absorption involves the complexity of employing multiple materials, high cost of fabrication process (e.g. lithography methods) and planar geometry that limits its exposed surface area for photochemical reactions to occur.
On the other hand, the black gold film is fabricated by a simple, cheap and scalable template-assisted physical vapor technique, which makes it highly attractive and versatile to advance the field.
"We also present experimental evidence that shows how this material can drive photochemical transformations under visible light, demonstrating its attractiveness as a novel material for a wide range of photochemical applications," Ng notes.
The next stage in the team's investigations is to integrate this novel black gold material into plasmon-based devices such as plasmonic solar cells or photodetectors.
Other applications could be plasmon-derived photocatalytic systems, surface-enhanced Raman spectroscopy, or solar water purification.
"Future directions in this research field will move towards more cost-efficient fabrication strategies such as bottom-up processes and large scale deposition of thin-films, in order to integrate these nanomaterials into practical devices," concludes Ng. "However, the challenge still lies in the availability of fabrication strategies that can provide devices at macroscopic scales."
By Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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