Featuring PiFM & PiF-IR chemical analysis Released: May 28, 2021

Whitepaper Article Nano-FTIR vs. PiF-IR: Comparing Nano‐IR Techniques

Background slightly better with a resolution 3 μm horizontally and a Ever since the invention of the atomic depth of 1.6 μm, but that is still too imprecise when the (AFM), researchers have sought to invent technologies goal is to look and nanoscale features. that would bring conventional chemical analysis tech‐ niqueslikeinfraredspectroscopytoamuchsmallerspatial Principle of Nano-FTIR volume. Currently, there are a few competing techniques Onemethodtoovercomethediffractionlimitandachieve which claim to offer these abilities. higher spatial resolution is to combine FTIR with tapping Given the popularity and utility of Fourier Transform mode (TM) atomic force (AFM) to realize (FTIR) spectroscopy, one natural option is to nano-FTIR. extend this technique to the nanoscale via nano-FTIR. Based on an apertureless near-field However, while FTIR is a robust and user-friendly tech‐ design (also known as scattering scanning near-field opti‐ nique at larger scales, the nanoscale variation has some cal microscopy, or s-SNOM), nano-FTIR utilizes a modern key limitations that other techniques like photo-induced broadband (white-) laser source instead of a fixed- force infrared (PiF-IR) spectroscopy have alleviated. wavelength laser as would normally be used in s-SNOM. The sample arm of the Michelson interferometer is FTIR replaced by the light scattering from the tip-sample inter‐ Conventional Fourier Transform Infrared (FTIR) spec‐ faceoftheTMAFM. troscopy is a well-established analytical technique that In nano-FTIR, the tip is typically metal coated, and the acquirestheinfrared(IR)spectrumofabsorption(ortrans‐ excitation light polarized along the tip direction to exploit mission) of a solid, liquid or gas sample. It utilizes a thehighintensityoftip-enhancednear-fieldillumination. broadband light source that contains the full spectrum of The near-field signal is measured optically by collecting wavenumbers to be measured (this is sometimes called a the light scattered off the tip and using lock-in amplifiers white-light source). to suppress the far-field signals. A sample and a reference mirror form two arms of a Michelsoninterferometer.Asthemirrorismoved,thetwo Principle of PiF-IR beamsfromthesampleandthereferencearminterfereat Another method to measure nanoscale chemical signa‐ a photodetector, forming a detector signal versus mirror tures is via photo-induced force infrared (PiF-IR) position graph called an interferogram. A complex Fourier spectroscopy.InPiF-IR,awidelytunablenarrowbandlaser transform is performed on the interferogram to acquire isusedtoexcitethesampleunderanAFMtip. thereal(reflection)andimaginary(absorption)IRspectra. Unlike sSNOM or nano-FTIR techniques thesignal is Given the limit of conventional optics, FTIR collectedusingmechanicalforcedetection ratherthanthe islimitedtoaspatialresolutionof~5μm.ATRFTIRcando optical detection of scattered light. This means that when

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comparing nano-FTIR to PiF-IR, there are a few inherent PiF-IR (in seconds) than with nano-FTIR (in tens of advantages in a PiF-IR system. Specific comparisons are minutes). discussed below. Selective Power Control Many nanoscale samples, especially organics and Comparison of nano-FTIR and PiF-IR biomolecules,canbeeasilydamagedmyhigh-poweredIR light. Therefore, careful power management of the excita‐ Spatial Resolution tion laser is crucial to any nanoscale analytical technique. The spatial resolution of nano-FTIR is reported to be WithPiF-IR,onsamplesthatcanbedamagedbyexcessive approximately equal to the tip radius, which typically is intensity and heating, an attenuator is used to reduce the around a few tens of nanometers for metal-coated tips. optical power to as little as 0.5% to 10% of the available This contrasts with PiF-IR where an even more confined QCLpower,whichisespeciallyimportantatwavenumbers near-field interaction provides a spatial resolution of ~ where the sample is highly absorptive. This power notch‐ 5nm for a similarly shaped tip. The relatively poor resolu‐ ing technology avoids sample damage while maintaining tion quoted for s-SNOM may not be a physical limitation high SNR and short acquisition time. ofthetechniquebutmaybeduetopoorerSNRbecauseof Unfortunately, because nano-FTIR uses a broadband thelowerpowerofthebroadbandlasersource(compared light source, this type of power control is impossible. to the sharply tuned laser source used for PiF-IR) and less Therefore,usablepowerlevelsareconstrainedbythepeak efficient near-field detection methodology. Nano-FTIR absorption of the sample to avoid damage. In many cases, suffers from the same low efficiency of light collection this means that to get sufficient SNR, multiple spectra inherenttos-SNOMduetothelimitednumericalaperture must be taken and averaged, further increasing the time of the collection optics and other factors. required to get meaningful data.

Light Sources Fixed-wavenumber imaging Nano-FTIRsufferssomeinherentpowercontrolandsignal Fixed-wavenumber imaging is incredibly useful for strength disadvantages due to the broadband white-light mapping chemical variations on the surface of a nano- source used. As an example, one company offers nano- scale sample. Such images are often especially useful for FTIR utilizing a state-of-the-art broadband mid-IR source understanding complex heterogeneous samples, where which spans 670 to ~2000 cm−1 with an emission band‐ multiple fixed-wavenumber images taken at different width of about 400 cm−1. This laser source generates an frequencies highlight material components separately in average power level of 1mW integrated over the band‐ a visually intuitive display. With photo-induced force width. 1 IfanIRspectrumwith10cm−1 spectralresolutionis microscopy (PiFM), the QCL can be tuned to a wavenum‐ desired, roughly 25 μW (or less due to loss from optics) of ber of interest (usually corresponding to a known power is available at each point of the resolved spectrum. molecular transition), and then a full image can be made This pales when compared to as much as 5 mW of laser inamatterofminutes. power that is available for each ~1 cm−1 bandwidth with Since s-SNOM uses a broadband light source, it cannot the quantum cascade laser (QCL) utilized for PiF-IR .2 Per acquire fixed wavenumber images directly. Instead, such wavenumber, this means a QCL can generate three orders imageshavetobeapproximatedbytakingahyperspectral of magnitude higher power than the broadband laser data-set and then extracting the intensities from a used with nano-FTIR. Naturally this allows high SNR spec‐ narrow-band of wavenumbers. Unfortunately, this tra with high resolution to be taken far more quickly with approach is extremely time consuming, generating a full

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spectrum at each image pixel when only single wavenum‐ fluctuates slightly. This necessitates that an interferogram berinformationisneeded.Whiletakingafullspectrumat on be acquired periodically to normalize the each pixel can be very useful on complex heterogeneous response from the sample.4 For the most accurate and samples with many unknown chemical species, the ideal reliable normalization, the reference interferogram needs case is for the user to be able to select between single to be acquired in identical experimental conditions (tip, wavenumber imaging and full spectrum imaging as harmonic detection, morphology, and other appropriate. PiFM provides both options – single factors).5 wavenumber imaging, and hyPIR spectra, which are WithPiF-IR,althoughthetunablelasermayalsohavea hyperspectral images that provide a full spectrum at each nonuniform power output as a function of wavenumber, a image pixel. reference spectrum is rarely needed. Instead, the power profile across the full spectral range can be made to be Far-field Background Suppression constantbyanactiveattenuator.Thismeansthatthereare Nano-FTIR collects scattered light from the tip to detect fewer opportunities for errors arising from improper thenear-fieldresponseofthesampleduetotheexcitation power normalization, and without the need for frequent light. While effective, this detection scheme comes with a reference spectra, data can be acquired much faster (in as few inherent problems that negatively impact signal little as 15 seconds for a fully normalized constant-power strength. Nano-FTIR relies on the fact that lock-in detec‐ spectrum or 100ms for a digitally normalized spectrum). tion of the interference signal at the tapping frequency Note: for some very thin samples (less than ~15 nm) will mostly reject the far-field background signal. differential measurements may be needed to remove However, the far-field light still contains some compo‐ substrate contributions. However, this is universally true nents modulated with the tapping frequency. For for all nanoscale molecular characterization. example,lightscatteringfromtheshankofthetip,orlight affected by the moving shadow of any part of the Thermal Stability or tip. Therefore, a higher harmonic of the Thermal drift can be a significant problem in s-SNOM- tapping frequency is usually utilized in an attempt to based nano-FTIR with the potential of introducing false maximize suppression of the unwanted signal. The cost of results or other normalization issues. With a Michelson using a higher harmonic, however, is reduced signal (each interferometer, differing thermal expansion in the refer‐ higher harmonic is about three- to fivefold loss in signal ence and sample arms causes unwanted phase drift. For a level3). representative commercial s-SNOM system, it is reported As explained inscientific principles, PiF-IR completely that a drift as small as 100 nm of path length difference and fundamentally rejects the far-field background signal (between sample and reference arms) shifts the nano- by its implementation of force measurement, achieving FTIR spectrum by about 6 degrees, which is in the same superior SNR. orderofmagnitudeasthephaseshiftproducedbyabsorp‐ tion in many samples.5 From literature, one can read that Power Normalization and Calibration enough phase drift takes place in about 120 seconds so All good analytic techniques need to have some reference that new interferograms of a reference area must be to make sure spectra accurately depict the sample acquired with each passage of about 120 seconds.5 Other response. Metals such as gold have a flat-IR response, so compensation methods can be applied, but phase drift they can be used as a reference material for calibration. remains a major nuisance for s-SNOM. With the broadband laser used for s-SNOM-based nano- On the other hand, because PiF-IR uses mechanical FITR, the power profile for a given center wavenumber force detection, drift is hardly an issue at all. For PiF-IR

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spectra, there is no thermal drift. When taking a PiFM wide variety of energy states and spatial orientations, imagethermaldriftcanslowlyshifttheimagingregionas resulting in very wide spectral peaks. However, as the itdoesinanyAFM–aneffectwhichisminimizedbygood volume of probed becomes smaller, there will instrument design. As long as precautions are taken to be be fewer molecular states represented. This results in aware of any residual drift, and to make sure the correct sharper peak shapes, and is one of the reasons why spec‐ location is being imaged, thermal drift has no impact on tral resolution matters for nano-scale IR analysis. PiFM. PiF-IR uses a QCL with an incredibly narrow spectral resolution of approximately one wavenumber. This means Spectral Resolution, Acquisition Time, and SNR that PiF-IR spectra are extremely detailed to the point For bulk analysis techniques like FTIR that take spectra where more information can be collected than is possible from much larger volumes, spectral resolution beyond a with conventional FTIR. This opens up many possibilities certain point doesn’t matter very much. This is because in for groundbreaking research. For example, changes in the a larger volume of material there may be molecules in a secondary structures of some proteins can shift a molecu‐

nano-FTIR PiF-IR

Laser source Broadband (~350 cm−1) Narrowband (~1 cm−1)

Scattered photons mixed with far-field Near-field signal background photons Photo-induced force only under the tip apex

Background suppression Lock-in detection of higher harmonics No background signal

Spectrum technique Fourier transform of interferograms Wavenumber sweep

Spectral resolution 6.4cm−1 (optional 3 cm−1) ~1cm−1

Signal to noise Poor Excellent

Speed Slow, especially for high-resolution Very fast

Need for reference Yes No

Selective power control No Yes (notching)

Spatial resolution ~20nm lessthan10nm

Agreement with FTIR Good Excellent

Fixed-wavenumber imaging No, only via hyperspectral imaging Yes

Table 1: Summarizes the key differences between nano-FTIR and PiF-IR spectra.

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lar transition by approximately ten wavenumbers, which signal strength. Additionally, PiF-IR uses a high-powered would be easily detectable in PiF-IR spectra. narrow-band tunable laser which offers much greater While nano-FTIR often has better spectral resolution control and signal strength than the slower broadband than many bulk FTIR machines, it can’t match the resolu‐ white-light sources used in nano-FTIR experiments. These tionofferedbyPiF-IR,afactorwhichlimitsitscapabilityin key differences mean that PiF-IR based measurements some application spaces. Due to the interferometer have tremendous advantages in terms of better signal design, higher spectral resolution requires more time for strength, faster data acquisition, higher thermal stability spectral acquisition. For nano-FTIR, the length that the and precise laser power management. Photo-induced reference mirror travels to generate the interferogram force microscopy and spectroscopy techniques are there‐ determines the spectral resolution of the nano-FTIR; the fore the most valuable nanoscale characterization longer the travel, the higher the spectral resolution. The techniques for all researchers and technicians. speed of the travel governs the SNR. Examples from published literature suggest a single pass of the mirror References withatravelrangesufficientfor8.3cm− spectralresolution [1] TopticaFemtoFiberdichroMidIRBrochure,https:// takesaround40secondstocoverabandwidth200to400 www.toptica.com/fileadmin/Editors_English/ cm−1 wide. Unfortunately, due to poor signal strength, 11_brochures_datasheets/01_brochures/topti‐ multiple passes may be needed for sufficient SNR. ca_BR_Ultrafast_Fiber_Lasers.pdf Furthermore, at this resolution, it is likely necessary to [2] QCL Brochure,https://www.blockeng.com/products/ interrupt the measurement multiple times for reference miniqcl.pdf spectra (because of thermal drift), meaning the whole [3] S. Amarie and F.Keilmann, “Broadband-infrared process for a high-resolution spectrum can take as much assessment of phonon in scattering-type asanhourormoreforgoodSNRifthewavelengthrangeis near-field microscopy,” Phys. Rev.83 B ,045404 large. Therefore, the types of high-resolution spectra that (2011) can be recorded in seconds using PiF-IR become impracti‐ [4] M.Autore,L.Mester,M.Goikoetxea,andR.Hillen‐ cal to replicate using nano-FTIR. In general, calculations brand, “Substrate Matters: Surface-Polariton suggest that PiF-IR measurements are between 100 to Enhanced Infrared Nanospectroscopy of Molecular 1000 times faster than comparable nano-FTIR measure‐ Vibrations,” Nano Lett. 19, 8066 (2019) ments. [5] I.Amenabar,S.Poly,M.Goikoetxea,W.Nuansing,P. Lasch & R. Hillenbrand, “Hyperspectral infrared Summary nanoimaging of organic samples based on Fourier Table 1 summarizes the comparisons for nano-FTIR and transform infrared nanospectroscopy,” PiF-IR Spectra. Communications8 ,Article#:14402(2017) PiF-IR and nano-FTIR offer two different methods of probing surface chemistry at the nanoscale. While each claimstooffersimilarfeatures,thedetectionschemesand lightsourcesmakeahugedifferenceinhoweffectiveeach technology actually is. PiF-IR relies on a mechanical detec‐ tion scheme that fundamentally eliminates background signals from contaminating the near-field response while nano-FTIR collects scattered light and uses lock-in detec‐ tion to filter competing background signals at the cost of

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