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Posted: Sep 01, 2017
Nanophotonic AFM probe provides ultrafast and ultralow noise detection
(Nanowerk Spotlight) Photothermal induced resonance (PTIR) has found application in the characterization of materials in fields spanning from photovoltaics, plasmonic, polymer science, biology and geology to name a few. PTIR combines the spatial resolution of atomic force microscopy (AFM) with the specificity of absorption spectroscopy, enabling mapping of composition and electronic bandgap, material identification and biomolecule conformational analysis with nanoscale spatial resolution.
PTIR signal transduction relies on the thermal expansion of the sample, which is in general a small quantity, especially for very thin samples. Furthermore, PTIR is a relatively slow technique.
Scientists at the Center for Nanoscale Science and Technology at National Institute of Standards and Technology (NIST) have now implemented, for the first time, an integrated near-field cavity-optomechanics readout concept to realize fully functional nanoscale AFM probes capable of ultralow detection noise (∼3 fm•Hz-0.5) within an extremely wide measurement bandwidth (>25 MHz) in ambient conditions, surpassing all previous AFM probes.
"The sensitive transducers used in our work allow measuring samples as thin as a molecular monolayer with high signal to noise ratio and improve the measurement throughput," Dr. Andrea Centrone, who together with Dr. Vladimir A. Aksyuk led this work, tells Nanowerk.
Nanophotonic optomechanical transducers enabling large-bandwidth, low-noise PTIR measurements. (A) Conventional PTIR: a wavelength-tunable pulsed laser (red) excites the sample in total internal reflection. Following the absorption of a laser pulse, the sample rapidly expands and excites the oscillation modes of an AFM cantilever. The oscillation amplitude (peak to peak) is proportional to the absorbed energy, enabling nanoscale IR spectroscopy; however, the SNR and bandwidth of conventional AFM cantilevers are insufficient for capturing the fast sample thermal expansion and thermalization dynamics. (B) Conventional cantilever PTIR signal for a thin, 50 nm PMMA film. (C) Photonic transducer PTIR: a fiber-pigtailed, integrated transducer leverages cavity optomechanics to measure motion of a nanoscale probe, radically reducing the noise and increasing the measurement bandwidth for capturing the sample’s fast thermalization dynamics induced by laser pulses. (D) The optomechanical transducer improves the PTIR SNR by ∼50 for a 50 nm PMMA film under the same conditions of panel B (1160 cm-1, 256 pulses averaged). (E) Transducer colorized scanning electron micrograph: a nanoscale Si probe is near-field coupled across a nanoscale gap to high-Q whispering-gallery optical modes of a Si microdisk optical cavity. The cavity is evanescently coupled to an integrated waveguide. (F) During the measurement, the fiber-coupled laser wavelength (λ0) is fixed; the sample expansion reduces the cantilever-disk gap, shifting the resonance to longer wavelengths and increases the transmitted intensity (I) proportionally to the displacement. The measured spectrum before the laser pulse (black) is shifted 15 pm (blue), illustrating a sample thermal expansion of ∼3.75 nm. (G) The transducer provides an ultralow input-referred measurement noise spectral density (blue) across a wide (25 MHz) bandwidth; integrated over the full bandwidth, the probe thermodynamic noise (black line) slightly exceeds the detection noise (red). (Reprinted with permission by American Chemical Society) (click on image to enlarge)
"A couple of years ago, we developed a technique named scanning thermal infrared microscopy (STIRM) (Nanoscale, "Mid-infrared spectroscopy beyond the diffraction limit via direct measurement of the photothermal effect") that relies on temperature sensitive AFM probes to provide both chemical composition and thermal conductivity information at the nanoscale," notes Centrone. "However, STIRM probes trade measurement sensitivity for time resolution and were of limited use. For such measurements to be successful, it was clear that a new approach capable of overcoming the trade-off between measurement sensitivity and time resolution was necessary."
The transducers resulting from this new work break the trade-off between AFM measurement precision and ability to capture transient events. For PTIR, this capability improves the time resolution, signal-to-noise ratio and throughput by a few orders of magnitude each.
As first practical application the scientists leveraged these characteristics to measure the intrinsic thermal conductivity of metal-organic framework (MOF) individual microcrystals, a property not measurable by conventional techniques. MOFs are a class of nanoporous materials promising for catalysis, gas storage, sensing and thermoelectric applications where accurate knowledge of thermal conductivity is critically important.
"Capturing the sample thermalization dynamics with high precision enable measuring the thermal conductivity with low uncertainty (<10%) of many nanomaterials that consist of nanoparticles or microcrystals that are too small or heterogeneous for macroscale thermal conductivity measurements," says Aksyuk. "We believe that the novel measurement method we demonstrated will foster the development of nanomaterials in thermoelectric, electronic and other applications."
"Furthermore" he adds, "because these measurements provide information on both the chemical composition and thermal conductivity of the sample, the measurement could provide the thermal conductivity of samples whose composition is initially unknown."
According to the team, there are several aspects that they believe are important, innovative and exciting. First, the use of AFM is already pervasive in nanoscience and biology because it can provide maps of many materials properties with high spatial resolution.
In their work, the researchers revolutionize AFM signal transduction by integrating and using cavity-optomechanics for high-precision motion detection of nanosized/picogram scale AFM probes, which are much smaller and faster than conventional cantilevers.
The resulting opto-mechanical AFM probes have unprecedented precision and bandwidth, thereby breaking the trade-off between AFM measurement precision and ability to capture transient events. Applied in PTIR, the probe near-field ultralow detection noise and wide bandwidth improves the time resolution (10 ns, 1500 x), signal-to-noise ratio (50x) and throughput (2500x) simultaneously.
"Remarkably, this synergy enables a new PTIR measurement modality: capturing the previously inaccessible fast thermal-expansion response of the sample to nanosecond laser pulses, thus allowing concurrent measurement of the chemical composition and thermal conductivity, at the nanoscale, for the first time," notes Centrone.
According to the team, the nanophotonic transducer probes hold promise for further giant leaps in PTIR and AFM measurements. For example, they are working to leverage the transducers in combination with recently developed high-repetition rate tunable lasers, to further increase the PTIR throughput 200-fold (i.e. 500000-fold with respect to conventional measurements) and enable IR hyperspectral imaging at the nanoscale.
Furthermore, they believe that the nanoscale transducer's very low drag cross-section could open new opportunities for nanoscale chemical imaging in liquids, which is of paramount importance in biological applications.
Furthermore, the high signal to noise ratio could be leveraged to study defects in semiconductors.
Going forward, the team is interested in improving the measurement time-resolution further, possibly a few orders of magnitude; they caution, though, that such challenge will require additional breakthroughs in fabrication and measurements.