A new route to produce efficient surface. gold decorated CdSe nanowires

Transcription

1 A new route to produce efficient surface enhanced Raman spectroscopy substrates: gold decorated CdSe nanowires Gobind Das 1, Ritun Chakraborty 1,2, Anisha Gopalakrishnan 1,2, Dmitry Baranov 3, Enzo Di Fabrizio 1 and Roman Krahne 1 1 Nanostructure Div., Italian Institute of Technology, Via Morego 30, Genoa, Italy 2 University of Genoa, Genoa, Italy 3 Dept. of Chem. and Biochem., University of Colorado at Boulder, UCB 215, Boulder, CO, USA 1. Flat Au-coated Si substrate 2. Particle analysis 3. SERS enhancement calculation 4. Image analysis at different location of SERS substrate Supplementary Information

2 Section 1. Analysis of Au-NP distribution obtained by drop-cast deposition of Au-decorated CdSe NWs. The particle analysis was performed based on the transmission-electron-microscopy (TEM) image shown in Fig. S2a, using ImageJ software, from the area depicted in (b). The number of Au NPs in this area was 65. In Fig. S2c, the Au particles are indicated by encircled red dots. The blended image of Fig. (S2b,c) is shown in Fig. S2d. Surface area covered by the Au-NPs is also estimated using the ImageJ software, and is found close to 1%. Figure S1. (a) TEM image of Au CdSe NWs. The selected area for the analysis is highlighted by the red square in (a) and displayed in (b)., The analyzed particles is shown in (c), and the blended image of (b) and (c) is illustrated in (d). Section 2. SERS enhancement factor calculation We can estimate the SERS enhancement factor G by the following expression: I N *t *P G = SERS * Raman Raman Raman I Raman N SERS * t SERS * PSERS Here ISERS and IRaman are the intensities of the peak at 590 cm 1 in the spectra in Fig. 6, respectively, and N, t, and P are the number of molecules under laser spot, time of

3 acquisition and laser power. The subscript indicates the measurement type, i.e. SERS for the Au-CdSe NWs substrate and Raman for the reference Au substrate. The mean Au NPs diameter is found to be around 15 nm. The area covered by Au-NPs is estimated to be 1 % of the total area. In this way the total area available for the molecules to be adsorbed is just 1% and other 99% can be assumed free from molecules. For the reference Au-film substrate we assumed full area coverage of molecules. Thus, for t SERS =10s, P SERS =27µW, I Raman = 780, t Raman =50s, P Raman =55µW and N Raman /N SERS =99 we obtain the enhancement factors reported in table 1 for 10 different spectra recorded from CV on Au-CdSe NWs. Spectrum ISERS (counts) G Table 1: SERS enhancement factor G calculated for different spectra recorded from CV on Au- CdSe NWs. The parameters for the calculation are reported in the text above. Section 3. Morphology of the Au film used as reference sample to obtain the SERS enhancement factor. Figure S2. (a) Atomic-force-microscopy image of a Au marker fabricated by the same process of metal evaporation as the reference Au film that was taken for the Raman experiments. (b) Height profile along the line indicated in (a). The peak-topeak roughness of the Au film is around 1.5 nm.

4 Section 4. SERS data recorded from a different area covered by Au- decorated CdSe NWs. Figure S4. (a) Optical microscope image of Au CdSe NWs, where the region of the Raman and fluoresecence mapping is indicated by the rectangular box. (b) Spatial Raman mapping of Au- CdSe NWs (b) on the CaF 2 substrate using the CdSe LO phonon mode as the signal. The amplitude of the CdSe LO phonon is displayed in color code and clearly allows imaging of the NWs. (c) Spatial PL mapping of the NWs plotting the onset of fluorescence at 654 nm that demonstrates the low PL signal of the Au- decorated NWs.

5 Section 5. Raman and SERS spectra recorded from CV molecules deposited on flat Au substrates and Au- decorated CdSe NWs Figure S5. SERS spectra recorded from CV deposited onto a Au- coated substrate (a), and from the Au decorated CdSe NWs (b). The arrows indicate the bands that are additionally resolved due to the better enhancement of the Au- CdSe NW substrates.