Plasmonic enhancements of photoluminescence in hybrid Si nanostructures with Au fabricated by fully top-down lithography
© Nakaji et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 31 October 2012
Published: 16 November 2012
The authors study plasmonic enhancements of photoluminescence (PL) in Si nanodisk (ND) arrays hybridized with nanostructures such as nanoplates of Au, where these hybrid nanostructures are fabricated by fully top-down lithography: neutral-beam etching using bio-nano-templates and high-resolution electron-beam lithography. The separation distance between the Si ND and Au nanostructure surfaces is precisely controlled by inserting a thin SiO2 layer with a thickness of 3 nm. We observe that PL intensities in the Si NDs are enhanced by factors up to 5 depending on the wavelength by integrating with the Au nanoplates. These enhancements also depend on the size and shape of the Au nanoplates.
KeywordsAu Bio-nano-templates Electron-beam lithography Hybrid nanostructures Neutral-beam etching Plasmonic enhancements Photoluminescence Si nanodisk
Plasmonic effects induced by metallic nanostructures, such as remarkable enhancements of the intensity of photoluminescence (PL) in semiconductor nanostructures, are very attractive because optical responses of the semiconductor nanostructures can be controlled in the relatively wide spectral range[1–5]. However, fully top-down fabrication of semiconductor nanostructures exhibiting the plasmonic effects has not been established yet because the plasmonic effects appear only when optically active nanomaterials are close to the metallic nanostructures, typically at a distance of 10 nm. Therefore, conventional lithography techniques are not applicable to the fabrication of the plasmon-coupled hybrid nanostructures of semiconductors with metals.
To solve this problem, we have employed Si nanodisks (NDs) fabricated by damage-free neutral-beam etching using bio-nano-templates[6–8]. A closely packed two-dimensional alignment of the Si ND with a diameter of 10 nm and interspacing of 2 nm was formed, where the sheet density was 5 × 1011 cm−2. This alignment of the Si ND provides a good opportunity to prepare the abovementioned hybrid nanostructures, where metallic nanostructures can be closely arranged on a surface of the Si ND array. Moreover, we can place some NDs at the most appropriate position for realizing the plasmonic enhancement; this position must be very close to the edge of metallic nanostructures in the in-plane lateral direction as well as the perpendicular one. It is rather difficult to realize a similar situation when we use self-assembled quantum dots with sheet densities of 109 to 1010 cm2. We demonstrate plasmonic enhancements of PL in a visible light region in the hybrid nanostructures of Si ND with Au. The separation distance between the Si ND layer and the Au nanostructure was precisely controlled by inserting a 3-nm-thick SiO2 layer. Improved optical functionalities such as efficient absorption and bright PL are highly expected for applications of the Si nanostructures to optical devices such as efficient solar cells and optical interconnections in Si-based integrated circuits.
PL spectra were observed at a temperature of 150 K using micro-PL equipment with a spatial resolution of 1 μm, corresponding to a diameter of an excitation light spot. An InGaN laser with a wavelength of 408 nm and an excitation power of 0.3 mW was used as an excitation light source.
Results and discussion
As can be seen, this spectral dip of the reflectivity change coincides well with the spectral peak at 640 nm of the PL enhancement. Therefore, the PL enhancements observed in these hybrid Si/Au nanostructures can be attributed to the excitations of localized surface plasmons around the Au nanoplates, particularly around sharp edges of the corner of the Au square pattern. The PL enhancement factors for the limited number of Si ND arranged at the appropriate places for strong plasmonic effects should be significantly higher than the averaged values observed by the present micro-PL, if we accept the above scenario.
We have observed plasmonic enhancements of PL in hybrid nanostructures of Si NDs with Au nanoplates, where the Au nanoplate is separated from the surface of the Si ND array by inserting a SiO2 layer with a thickness of 3 nm. We demonstrate that PL intensities in the Si NDs are enhanced by integrating the Au nanoplates, depending on the wavelength. This enhancement also depends on the size and shape of the Au nanoplates. We find that the PL spectral region indicating the enhancement correlates well with the spectral dip of the reflectivity change originating from the Au nanoplates. Therefore, we attribute the PL enhancements observed in these hybrid Si/Au nanostructures to plasmonic effects induced by the square-shaped Au nanostructures.
KN fabricated the Au nanostructures. MI fabricated the Si NDs. KN, HL, TK, and AM performed the spectroscopic study and analyzed the structural and optical data. KN, TK, and AM drafted the manuscript. KN, TK, MI, SS, and AM conceived the fabrication process and participated in its design and coordination. All authors read and approved the final manuscript.
scanning electron microscopy.
This work is supported in part by the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Scientific Research (S) No. 22221007 and for Challenging Exploratory Research.
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