SUPPORTING INFORMATION

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1 SUPPORTING INFORMATION Non-Plasmonic SERS with Silicon: Is It Really Safe? New Insights into the Opto-Thermal Properties of Core/Shell Microbeads Nicolò Bontempi, a,d Irene Vassalini, a,b Stefano Danesi, a,b Matteo Ferroni, c,d Maurizio Donarelli, c Paolo Colombi, e Ivano Alessandri a,c,d* a INSTM-UdR Brescia, via Branze 38, 2513 Brescia, Italy b Department of Mechanical and Industrial Engineering, via Branze 38, 2513 Brescia, Italy c Department of Information Engineering, University of Brescia, via Branze 38, 2513 Brescia, Italy d INO-CNR, via Branze 38, 2513 Brescia, Italy e CSMT, via Branze 45, Brescia, Italy SI 1. Fabrication of SiO 2 /Si beads. SI 2. SEM images: 2_Si and 6_Si series. SI 3. Simulations of the optical absorption of 2_Si100 beads. SI 4. Details on SERS experiments. SI 5. Light-induced modifications in 3D core/shell Si-rex samples (2_Si series) irradiated at λ=633 nm: power threshold. SI 6. SEM images of 2_Si samples irradiated at λ=532 nm (laser power: 50mW. Irradiation time: 1-20 s). SI 7. Examples of laser writing by irradiation at λ=532 nm. 1

2 SI 1. Fabrication of SiO 2 /Si core/shell beads. The SiO 2 /Si core/shell beads were obtained by coating commercial silica microspheres (Microparticles GmbH) with amorphous Si thin shells deposited by rf-magnetron sputtering. Silica spheres with two different sizes (2.06±0.05 and 6.47±0.32 µm, respectively) were selected. The deposition of the amorphous Si shell layer was carried out with a MHz radio-frequency power (AE CESAR RF Power Supply) at 400 W while keeping the reflected power below 1 W by means of a AE VarioMatch platform. The chamber was pumped down to a base pressure of Pa. Ar % pure was introduced to a working pressure of Pa during sputtering. Substrates were maintained at a distance of 100 mm from the target. The substrate temperature was measured by thermocouple at the bottom of the aluminum sample holder and was in the range 20 C - 80 C for the whole deposition time. The thickness of the amorphous Si shell layers was 25, 50, 75 and 100 nm, as assessed by stylus profilometry on step structures obtained by transparency markers prior to coating. 2

3 SI 2. SEM images: 2_Si and 6_Si series Figure S2.1. Representative SEM images of SiO2/a-Si core/shell spheres (2_Si series): a) 2_Si25; b) 2_Si50; c) 2_Si75 and d) 2_Si100. Scale bars (red): 1 µm. Inset Figure d): magnification of a crushed sphere region, showing the a-si shell layer. Scale bar (green): 100 nm. 3

4 Figure S2.2. Comparison between 2_Si100 (a) and 6_Si100 (b) core/shell samples (scale bar: 2 µm). 4

5 Absorptio n SI 3. FDTD simulations of the optical absorption of 2_Si100 and 6_Si100 beads. Electromagnetic Finite-Difference Time-Domain (FDTD) calculations (Lumerical FDTD solutions) were performed on a schematized free standing core/shell system. 1 The diameters of the two silica cores are 2.06 µm and 6.47 µm, respectively and the shell has been modeled as a 100 nm a-si conformal coating. The source is a focalized Gaussian beam, with focal plane located on the top of the shell, a central wavelength of 650 nm and a pulse-length of 2.65 fs. The problem was solved on a two-dimensional plane crossing the geometry in the middle, in order to get rid of the high computational cost of solving the full 3D problem. Both TE and TM polarization fields were analyzed and the result is reported in Figure S μm diameter 6.47 μm diameter TE TE TM Wavelength (nm) TM Wavelength (nm) Figure S3. TE and TM absorption spectra of 2_Si100 and 6_Si100 beads. The dashed lines show the three laser wavelengths used in the experiments (λ=532, 633 and 785 nm) and the corresponding dots refer to the absorption value at any given laser wavelength. 5

6 SI 4. Details on SERS experiments. The Raman experiments were carried out by means of a Labram HR-800 (Horiba) spectrophotometer coupled to an optical microscope (objective 100X, NA: 0.9). A CW He-Ne laser (λ=633 nm) was utilized as an excitation source. Methylene Blue (MB) solutions with concentration of 10-5 and 10-9 M were utilized as analytical targets for SERS experiments. In a typical detection test a 5 µl droplet of the MB solution at a given concentration was infiltrated into the 3D 6Si_100 substrates. At least ten different regions of the substrate were sampled per each detection experiment. The same experiments were carried out on planar silicon samples that represent a reference for normal Raman scattering (NRS). A rough estimate of the analytical enhancement was calculated from the formula 2 : = The analytical enhancement factor extracted from experimental data is about 2x

7 SI 5. Light-induced modifications in 3D core/shell Si-rex samples (2_Si series) irradiated at λ=633 nm: power threshold. Figure S5.1. Representative optical images of 3D SiO 2 /a-si core/shell beads (2_Si series) exposed at λ=633nm laser exposure for different powers (P) and times (t), in particular: a) 2_Si100 P = 7 mw t = 1s, b) 2_Si75 P = 7 mw t = 1s, c) 2_Si50 P = 7 mw t = 1s, d) 2_Si25 P = 7 mw t = 1s, Scale bar: 5µm. Figure S5.2. Power threshold as a function of irradiation time at 633 nm for 3D colloidal crystals (2_Si series). 7

8 SI 6. SEM images of 2_Si samples irradiated at λ=532 nm (laser power: 50mW. Irradiation time: 1-50 s). Figure S6. Representative SEM images of 3D SiO2/ a-si core/shell spheres (2_Si series) irradiated at λ=532 nm (laser power: 50mW) at different exposure times: a) 1s; b) 2s; c) 5s; d) 10s; e) 20s.f) 50 s Scale bars 200nm. 8

9 SI 7. Examples of laser writing by irradiation at λ=532 nm Figure S7. Example of laser writing on 3D 2Si_100 samples achieved by irradiation with a green laser (λ=532 nm, power: 50 mw, exposure time: 20 s): a) optical image (scale bar: 5 µm) and b) SEM image (scale bar: 2 µm). c) SEM image of the detail highlighted in b (scale bar: 200nm) 9

10 References 1) see, for example, Yan, J.; Ma, C.; Liu, P.; Yang, G. Plasmon-Induced Energy Transfer and Photoluminescence Manipulation in MoS 2 with a Different Number of Layers. ACS Photonics 2017, 4, ) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. SERS Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C. 2007, 111,