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Nano-Chemical Imaging and -Spectroscopy (IR, nIR, VIS)

Summary

Conventional techniques for analyzing chemical composition, such as infrared (IR) microscopy often miss critically important nanoscale details.  To overcome this limitation, we rely on the photothermal Induced Resonance (PTIR) technique where the tip of an atomic force microscope (AFM) transduces the light induced photothermal expansion of a sample with few (≈ 10) nm spatial resolution and with amplitude proportional to the absorbed energy in the sample. Importantly, PTIR spectra enable chemical identification by comparison with far-field FTIR spectral databases. Our work aims to drastically improve and to widely apply PTIR to characterize nanomaterials in many important applications.

Description

PTIR combines the spatial resolution of AFM with the specificity of absorption spectroscopy

Fig. 1. PTIR combines the spatial resolution of AFM with the specificity of absorption spectroscopy. a) PTIR Schematic with (total reflection) illumination. b) AFM topography and c) PTIR absorption image (1720 cm-1) of a sample made of polymethylmethacrylate (PMMA, large particle) and polystyrene (PS, small particles) within an epoxy matrix. At 1720 cm-1 light is absorbed by PMMA but not by PS and epoxy.

The aim of this project is twofold: 1) improve the PTIR measurement (spectral range, signal to noise ratio, throughput, time resolution, information content, range of environmental conditions) and 2) apply PTIR to characterize nanomaterials in important applications. Examples or recent PTIR applications from NIST include: plasmonic nanomaterials, organic inorganic perovskites (solar cells and detectors), hexagonal boron nitride nanostructures, 2D materials and their heterostructures, CdTe solar cells, metal organic frameworks, oil paints, nanomaterials for drug-delivery, and peptides nanostructures involved in neurodegenerative diseases. A recent PTIR review is available elsewhere.1

Advanced PTIR Measurement Schemes

By carefully matching the laser repetition rate to the AFM cantilever resonances it is possible to improve (≈ 50x) the PTIR signal to noise ratio. For example, with this method we measured ≈ 1000 cytarabine molecules inside a single liposome and determined the conformation of single peptide nanostructures relevant to understanding Alzheimer’s disease in water.2 

In tapping-mode PTIR, we resonantly excite the AFM cantilever by non-linear mixing two cantilever modes (f1, f2) with the laser induced sample photothermal expansion (fL = f2-f1). For example, with this method we determined the nanoscale composition of rough paint samples.

Recent PTIR innovations at NIST

Nanophotonic optomechanical probes enable fast, low noise PTIR measurements
Fig. 2. Nanophotonic optomechanical probes enable fast, low noise PTIR measurements. a) PTIR schematic with NIST photonic probes. A CW tunable laser and high bandwidth detector are fiber-coupled to the probe replace conventional AFM detection. b) False color SEM image of the probe. c) IR spectrum of an Octadecylchlorosilane (OTS) monolayer (red) and its 95 % confidence uncertainty (blue).

We recently extended PTIR to the visible, near-IR and far-IR ranges, covering the spectral range from 405 nm to 16,000 nm, continuously. These improvements, for example, enable measuring chemical composition, bandgap and defects with wavelength independent spatial resolution.

Using new nanoscale optomechanical transducer probes3 (Fig. 3) developed and fabricated at NIST , we improved the PTIR signal to noise ratio (50x), throughput (2500x) and time resolution (10 ns, 1500x). These novel AFM probes enable capturing the sample thermal expansion dynamic directly yielding chemical composition and the thermal conductivity at the nanoscale. For example, with these probes we measured the thermal conductivity of metal-organic framework single microcrystals,3 and monolayers with high signal to noise ratio.3 Currently we are integrating these optomechanical probes with high repetition rate tunable lasers to further increase the chemical and thermal imaging throughput 200-fold i.e. up to ≈ 500,000x better than conventional ringdown measurements.

We believe that our instrument will impact many nanotechnology applications in fields such as material science, energy, photonics, quantum physics, biology, and medicine. 
 

Created December 10, 2010, Updated December 12, 2019