Understanding Cantilever Transduction Efficiency and Spatial Resolution in Nanoscale Infrared Microscopy

Abstract

Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale infrared (IR) imaging and spectroscopy by using the tip of an atomic force microscope to transduce the local photothermal expansion and contraction of a sample. The signal transduction efficiency and spatial resolution of PTIR depend on a multitude of sample, cantilever, and illumination source parameters in ways that are not yet well understood. Here, we elucidate and separate the effects of laser pulse length, pulse shape, sample thermalization time (τ), interfacial thermal conductance, and cantilever detection frequency by devising analytical and numerical models that link a sample’s photothermal excitations to the cantilever dynamics over a broad bandwidth (10 MHz). The models indicate that shorter laser pulses excite probe oscillations over broader bandwidths and should be preferred for measuring samples with shorter thermalization times. Furthermore, we show that the spatial resolution critically depends on the interfacial thermal conductance between dissimilar materials and improves monotonically, but not linearly, with increasing cantilever detection frequencies. The resolution can be enhanced for samples that do not fully thermalize between pulses (i.e., laser repetition rates ≳ 1/3τ) as the probed depth becomes smaller than the film thickness. We believe that the insights presented here will accelerate the adoption and impact of PTIR analyses across a wide range of applications by informing experimental designs and measurement strategies as well as by guiding future technical advances.

Beating thermal noise in a dynamic signal measurement by a nanofabricated cavity optomechanical sensor

“…The tip (≈30-nm radius) is operated in contact with the sample (contact mode; Fig. 1A) and is characterized by a mechanical in-contact lowest eigenfrequency of ≈10 MHz, which is advantageous for imaging with high spatial resolution ( 39 ). The rapid thermal expansion of the sample excites efficiently only the fundamental in-plane mode (Fig.…”

Mid-Infrared, Near-Infrared, and Visible Nanospectroscopy of Hydrogen-Intercalated MoO3

“…By contrast, in the mid-IR spectral range, we use a resonant-enhanced method that amplifies the signal by the cantilever’s mechanical quality factor by matching the laser pulse frequency to one of the contact resonance modes and maintained using a phase-locked loop . Details of the PTIR signal transduction can be found in recent publications. , Importantly, only the fast sample expansion from directly beneath the probe tip is transduced, leading to spatial resolutions of ≈10 nm in contact mode and ≈5 nm in tapping mode, far smaller than the optical diffraction limit (e.g., ≈200 nm in the blue and >5 μm in the mid-IR) and the thermal diffusion limit (typically >1 μm) . Since, in our measurements, the penetration depth of light (typically >1 μm) exceeds the thicknesses of the MoO 3 flakes (≲ 500 nm), PTIR probes optical absorption throughout the entire thickness of such thin flakes.…”

“…Here, we use the photothermal induced resonance (PTIR) technique − to investigate nanoscale compositional and optical heterogeneities occurring in α-MoO 3 single crystals annealed in hydrogen gas. The PTIR technique combines capabilities of atomic force microscopy (AFM) with absorption spectroscopy yielding absorption spectra and maps with ≈10 nm spatial resolution from the infrared (IR) to the visible spectral ranges. ,, We detect clear changes in IR and near-IR absorption strongly correlated with nanoscale topographic features and optically distinct regions of hydrogen-annealed MoO 3 crystals. A trend of greater oxygen deficiencies in samples annealed for longer times and at higher temperatures is suggested based on semiquantitative energy-dispersive X-ray spectroscopy (EDS) compositional analyses of hydrogen-annealed flakes.…”

Visible to Mid-IR Spectromicroscopy with Top-Down Illumination and Nanoscale (≈10 nm) Resolution

“…In PTIR experiments, either case leads to the rapid heating and thermal expansion of the sample which, in turn, excites the oscillation of the AFM cantilever . PTIR signal transduction details are discussed in full in the recent literature. , As a proof-of-concept, we prepared a sample consisting of a commercially available polycyanine dye (IR Dye 9658, Adam Gates & Company) dispersed in PMMA, as described previously . The sample has a thickness up to 425 ±25 nm, as determined by AFM.…”

“…21 PTIR signal transduction details are discussed in full in the recent literature. 14,44 As a proof-of-concept, we prepared a sample consisting of a commercially available polycyanine dye (IR Dye 9658, Adam Gates & Company) dispersed in PMMA, as described previously. 21 The sample has a thickness up to 425 ±25 nm, as determined by AFM.…”