Formation of Size and Density Controlled Nanostructures by Galvanic Displacement

Abstract

Gold (Au) and copper (Cu)-based nanostructures are of great interest due to their applicability in various areas including catalysis, sensing and optoelectronics. Nanostructures synthesized by the galvanic displacement method often lead to non-uniform density and poor size distribution. Here, density and size-controlled synthesis of Au and Cu-based nanostructures was made possible by galvanic displacement with limited exposure to hydrofluoric (HF) acid and the use of surfactants like L-cysteine (L-Cys) and cetyltrimethylammonium bromide (CTAB). An approach involving cyclic exposure to HF acid regulated the nanostructure density. Further, the use of surfactants generated monodisperse nanoparticles in the initial stages of the deposition with increased density. The characterization of Au and Cu-based nanostructures was performed by scanning electron microscopy, atomic force microscopy, UV-Visible spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and X-ray diffraction. The surface enhanced Raman spectroscopic measurements demonstrated an increase in the Raman intensity by two to three orders of magnitude for analyte molecules like Rhodamine 6G dye and paraoxon.

Keywords: gold; monodisperse; nanostructures; sensing; surfactant.

Figure 1

Experimental steps in the synthesis process: (a) cleaved and cleaned Si substrate; (b) immersion in 10% HF for 2 min, to dissolve the surface oxide; (c) immersion in 0.3 mM metal precursor solution for 5 min; (d) rinse sample with deionized water.

Figure 2

SEM images of Au nanostructures deposited on Si substrate after (a) first, (b) second, (c) third, (d) fourth, (e) fifth, (f) sixth, (g) eighth, and (h) tenth deposition cycle. The scale bar is 500 nm.

Figure 3

SEM images of Cu-based nanostructures deposited on Si substrate after (a) first, (b) second, (c) third, (d) fourth and (e) fifth deposition cycle. The scale bar is 500 nm.

Figure 4

SEM images of Au nanostructures deposited on the Si substrate after (a) first, (b) second, (c) third, (d) fourth, (e) fifth, (f) sixth, (g) eighth, and (h) tenth deposition cycle with L-Cys added as surfactant. The scale bar is 500 nm.

Figure 5

SEM images of Au nanostructures deposited on Si substrate after (a) first, (b) second, (c) third, (d) fourth, (e) fifth, (f) sixth, (g) eighth, and (h) tenth deposition cycle with CTAB added as surfactant. The scale bar is 500 nm.

Figure 6

SEM images of Cu-based nanostructures deposited on Si substrate after (a) first, (b) second, (c) third, (d) fourth, (e) fifth deposition cycle with L-Cys added as surfactant. The scale bar is 500 nm.

Figure 7

SEM images of Cu-based nanostructures deposited on Si substrate after (a) first, (b) second, (c) third, (d) fourth, (e) fifth deposition cycle with CTAB added as surfactant. The scale bar is 500 nm.

Figure 8

Representative AFM topography maps of the Au samples after one (1X), three (3X), and five (5X) deposition cycles. Scan area of 2 μm × 2 μm. Height scale is in nm.

Figure 9

RMS roughness of Au samples after one (1X), three (3X), and five (5X) deposition cycles. Mean values from five different locations on each sample are shown. Error bars represent 95% confidence intervals.

Figure 10

Absorption spectra of Au nanoparticles after the first deposition cycle in the absence and presence of surfactants.

Figure 11

XPS spectra showing (a) Cu 2p peaks corresponding to different oxidation states of Cu, (b) O 1s peaks corresponding to different oxide species, and (c) C 1s peaks corresponding to various carbon bonds (d) AES spectrum showing Cu LMM peak that resemble Cu(OH)₂ and Cu₂O. The dashed curves in (a) and (b) represent Gaussian curve fits.

Figure 12

(a) Raman spectra of R6G on Si, and Au nanostructures samples after five, eight, and ten deposition cycles. (b) Raman spectra showing R6G modes for Au and Cu-based nanostructures. R6G concentration was 10⁻⁵ M.

Figure 13

Raman spectra of paraoxon adsorbed on Si substrate and Au nanostructures deposited on Si after ten deposition cycles.