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Posted: Dec 21, 2016

Black gold maximizes the light absorption of nanomaterials

(Nanowerk Spotlight) Maximizing light absorption of nanomaterials has been an emerging research field in the recent years due to its attractiveness in a wide range of applications that involves 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 requires complicated non-scalable fabrication process such as electron beam lithography.
Efforts on maximizing 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.
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").
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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 of an appropriate fabrication strategy that is simple, cheap and robust to create plasmonic nanostructures that exhibits 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 human eyes due to its broadband high absorption of the visible light," adds Gomez. "It can be fabricated over large area surfaces in a robust and cost-efficient manner. Furthermore, it also exhibits the flexibility to adhere to arbitrary surfaces that warrants its attractiveness 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 exhibiting as a broadband super absorber that is capable of absorbing >92% of the incident light energy up to 600 nm.
The intriguing light absorption capability is related to its tapered wall structure and gap plasmon modes between the tubes.
The majority of nanomaterials that exhibit high light absorption involves the complexity of employing multiple materials, high costs 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.
Furthermore, the black gold film consists of high aspect ratio and closely packed gold nanotubes with a tapered wall thickness and high surface area exposed.
"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 photodetector.
Other applications could be plasmon-derived photocatalytic systems, Surface-enhanced Raman spectroscopy, or solar water purification.
"Future directions in this research field will be moving towards more incorporation of cost efficient fabrication strategies to create structures that can maximize its light absorption such as bottom-up strategies and large scale deposition of thin films, in order to integrate these nanomaterials into practical devices for applications," concludes Ng. "However, the challenge still lies in the availability of such fabrication strategies that can provide devices that are of macroscopic areas."
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