Nanomechanical 2D-scanning photothermal microscopy for analysis and imaging of single sub-10 nm nanoparticles

Label-free optical detection and imaging of single nanoparticles and molecules are of fundamental interests in many research fields due to its simpler detection scheme, reduced limitations on samples and higher stability. For... [ view full abstract ]

Label-free optical detection and imaging of single nanoparticles and molecules are of fundamental interests in many research fields due to its simpler detection scheme, reduced limitations on samples and higher stability. For sub-100 nm nanoparticles, absorption cross section exceeds scattering by several orders of magnitude, and the difference increases as the diameter of particles reduce, which makes photothermal spectroscopy a powerful technique for the imaging of single Au nanoparticles even under sub-10 nm regime. Instead of using inhomogeneous refractive index [1,2] to create contrast, photothermal analysis via the thermal frequency detuning of nanomechanical 1D string resonators of single metal [3] and polymer [4] nanoparticles has been demonstrated alternatively. We hereby introduce highly sensitive nanomechanical membrane resonator as a novel methodology and optical platform for single nanoparticle detection and 2D imaging, as shown in figure 1. The experiments were done with a rectangular silicon-rich silicon nitride membrane with 500 µm in width, 50 nm in thickness and pre-stress of 250 MPa and 30 MPa, and the results are shown in figure 2 and 3, respectively, along with their corresponding SEM images. When scanning a heating laser over the membrane, the local photothermal heating of the nanoparticles causes a heat influx into substrate and thus a measurable detuning of the membrane resonance frequency, which was monitored with a phase-locked loop. With pumping power of only 300 µW and spot size of 1 µm, we achieve the detection and imaging of 10 nm Au nanoparticles, as shown in figure 3, with a dissipated power of 300 fW and integration time of 100 ms, which demonstrates even higher sensitivity than state-of-the-art photothermal microscopy [1,2], making this novel spectroscopic technique a potential candidate toward single molecule detection and imaging.

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[2] A. Gaiduk et al., Chem. Sci. 1, 343–350, 2010.

[3] S. Schmid et al., Nano Lett. 14,2318–2321, 2014.

[4] T. Larsen et al., ACS Nano 7, 6188–6193, 2013.