Characterizing 2D Materials with Nanoscale IR Spectroscopy and Property Mapping

The exceptional properties of 2D materials for important applications in semiconductors, battery technology, photovoltaics, and several other fields render them a key developing area of research. Various nanoscale and microscopy methods have been used to characterize 2D materials to gain better insights into the nature of their properties. NanoIR methods extend this characterization with crucial nanoscale chemical and optical property mapping.

The nanoIR3-s system offers two complementary nanoscale IR methods, AFM-IR photothermal-based nanoscale IR imaging and spectroscopy (including Tapping AFM-IR) and scattering-scanning near-field optical microscopy (s-SNOM). These methods offer an exceptional understanding of the nanoscale chemical and complex optical properties of 2D materials. Complementary atomic force microscopy (AFM) methods such as mechanical and thermal property mapping also offer information on the thermal, mechanical, and electrical properties of these materials. These methods allow chemical and optical property mapping with 10 nm spatial resolution, which is much less than the diffraction limit of traditional IR spectroscopy.

In this article, the application of the nanoIR3-s system for characterizing a range of 2D structures and materials, including nanoantennae, graphene, semiconductors, and more is explained.

Complementary Nanoscale IR Techniques

The nanoIR3-s is capable of acquiring nanoscale images and IR spectra using two different near-field spectroscopy methods: s-SNOM and photothermal AFM-IR. These complementary methods enable nanoscale chemical analysis, as well as electrical, thermal, mechanical, and optical mapping with spatial resolution down to a few nanometers for hard as well as soft matter applications.

Nanoscale IR spectroscopy integrates the exact chemical identification of infrared spectroscopy with the nanoscale capabilities of AFM to enable chemical detection of sample components at a chemical spatial resolution down to 10 nm with monolayer sensitivity, breaking the diffraction limit by greater than 100x. AFM-IR absorption spectra are direct measurements of sample absorption and are not dependent on other complicated optical properties of the sample and the tip. Therefore, the spectra compare extremely well to that of traditional bulk transmission IR.

Imaging of Plasmons and Phonons

Due to their high spatial confinement, surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials can create new possibilities for improved light-matter interaction, subwavelength metamaterial, super lenses, and other innovative photonic devices. An adaptable optical imaging and spectroscopy tool with nanometer spatial resolution is required for the in situ characterization of these polaritonic excitations across various applications. s-SNOM offers an exceptional means to selectively excite and locally detect vibrational and electronic resonances in real space through a non-invasive near-field light-matter interaction.

This method is demonstrated by imaging the SPhPs of hexagonal boron nitride (hBN) as illustrated in Figure 1. Complementary information for a detailed characterization of the polaritonic resonances is offered by amplitude and phase near-field optical images. The observation of greater than 90° phase shift of SPhPs on hBN signifies strong light-matter coupling.

Figure 1. (a) AFM height image shows homogeneous hBN surface with different layers on a Si substrate; (b) s-SNOM amplitude shows strong interference fringes due to propagating SPhP along the surface on hBN; (c) s-SNOM phase shows a different phase signal with layer thickness.

The SPPs of graphene can also be examined using the nanoIR3-s analogous to the visualization of the SPhPs in hBN. The standing wave of an SPP on a graphene wedge is shown in Figure 2. Usually, only the end radius of the AFM probe limits the spatial resolution of s-SNOM, thereby allowing the s-SNOM method to measure the cross sections of SPP down to nearly 8 nm.

Figure 2. (a) s-SNOM phase image of surface plasmon polariton on graphene; (b) cross section of SPP standing wave phase.

Nanocontamination of Graphene

The unique electrical and mechanical properties of graphene are reliant on maintaining the overall conjugated structure of the sheet. The nanoIR3-s can simply evaluate the quality of exfoliated graphene obtained by different techniques, as illustrated in Figure 3. Contamination that cannot be easily identified in the AFM height image is visible in the s-SNOM reflection image. In addition, difference in the s-SNOM reflection image changes with the number of existing graphene layers, showing nanocontamination on the sample.

Figure 3. (a) s-SNOM reflection image, showing nanocontamination (dirt), and (b) AFM height image of exfoliated graphene.

Characterizing Nanoantenna Resonance

Nanoantennae have a wide range of applications, varying from sensing to energy conversion. For the construction of exact and reliable devices, the capability of measuring and tuning the resonance structures of these antennas is of crucial importance. Arrays of nanoantennae are common as they enable a large number of individual antennas to be packed within a compact area. An AFM topography image of an antenna array including coupled antennas and single bar antennas is illustrated in Figure 4a.

Figure 4. (a) AFM height image of assembled antenna array, (b) s-SNOM phase, and (c) s-SNOM amplitude images of antenna dipole.

The contact point to the antennas is an important factor in achieving optimal energy transfer efficiency during the fabrication of antenna arrays. s-SNOM imaging enables simple detection of the antenna resonance hot spots and the perfect contact point. The s-SNOM amplitude and phase image of a single bar antenna included inside the array is shown in Figure 4b. The dipole antenna resonance can be observed with 11 µm excitation. A phase change of ~180° can be observed at dipole resonance.

Apart from the potential to collect high-resolution images of optical phenomenon, the nanoIR3-s offers the ability to spectrally probe nanoscale surface features.

Figure 5 illustrates the AFM-IR spectra collected on single rod and coupled antenna, where the antenna resonance can be distinctly resolved at 910 cm^-1, in accordance with theoretical predictions.

Figure 5. AFM-IR spectrum collected on single rod and coupled antenna; the peak at 910 cm^-1 corresponds to the antenna resonance of the single rod antenna, while the peak at 1100 cm^-1 shows the Si-O mode shared by both antennas.

Effects of Polarized Light on Metasurface Chirality

This is the first time the combination of the complementary nanoscale imaging techniques, s-SNOM, and AFM-IR have been applied to study the role of chirality in the origins of circular dichroism in 2D nanoscale materials. Chiral molecules are a specific kind of molecule that have a non-superimposable mirror image. The mirror images of chiral molecules are usually known as left handed and right handed, and because of the vector nature of light, it can also exist with two forms of handedness, left and right circularly polarized.

Fully two-dimensional (2D) metamaterials, also referred to as metasurfaces, made up of planar-chiral plasmonic metamolecules with a thickness of just a few nanometers, have been shown to reveal chiral dichroism in transmission (CDT). Theoretical calculations signify that this unexpected effect is dependent on finite non-radiative (ohmic) losses of the metasurface. To date, this astonishing theoretical prediction has never been experimentally proven due to the difficulty in measuring the non-radiative loss on the nanoscale.

s-SNOM is used to map the optical energy distribution when the structures are subjected to RCP and LCP IR radiation; on the other hand, AFM-IR was subsequently used to detect the radically different ohmic heating seen under RCP and LCP radiation.¹

For the first time, it has been unquestionably proven that the circular dichroism observed in 2D metasurfaces can be attributed to handedness-dependent ohmic heating, as shown in Figure 6.

Figure 6. Experimentally measured AFM cantilever deflection amplitudes. The cantilever deflection is directly proportional to temperature increase in the sample during the laser pulse; this confirms that the magnitude and spatial distribution the Ohmic heating of a chiral 2D metasurface markedly depends on the handedness of light.¹

Analysis of Carbon Nanotubes with nanoIR

The AFM-IR method operates by detecting the thermal expansion of a material induced by the absorption of infrared illumination. A material’s thermal expansion depends on a number of factors, such as the thickness of the material and the coefficient of thermal expansion. 1D and 2D materials, such as single-walled carbon nanotubes (CNT) and single-layer graphene, possess a low coefficient of thermal expansion as well as a thickness of around 1–2 nm. Characterization with AFM-IR is rendered difficult due to the nature of these 1D and 2D samples.

A two orders of magnitude increase in AFM-IR signal intensity is observed by positioning a thin layer of polymeric material below the graphene and CNT samples.^2,3 Since the incident IR radiation is absorbed by the thin sample absorbs, the generated heat is shifted to the thin polymer that has a considerably higher coefficient of thermal expansion, making it expand. Shown in Figure 7 is the finite element analysis model used to simulate the effects that polymer thickness has on the temperature changes and thermal expansion.

Figure 7. (a) Temperature rise (ΔT) and expansion (ΔZ) as a function of polymer thickness beneath the sample; Temperature rise (b) with no polymer and (c) with polymer beneath the sample; Vertical thermomechanical expansion (d) with no polymer and (e) with polymer beneath the sample.

The model was verified by inspecting a series of CNTs placed on top of a 150 nm thick polystyrene layer on a ZnSe prism. Before CNT deposition, an area of the polymer substrate was removed in order to ensure there was a region of CNT without polymer underneath. In Figure 8, it can be observed that the IR chemical image obtained at 4000 cm^-1 shows clear signal from the CNT in the region which has polystyrene underneath, while no signal is seen where the polymer substrate has been removed. It has been suggested that the changing AFM-IR signal from different CNTs is because of the difference between metallic and semiconducting tubes.

The AFM-IR imaging of graphene on top of a 106 nm thick layer of PMMA is illustrated in Figure 8c. This image reveals the extension of this technique to monolayer 2D materials. Due to the amplification of the AFM-IR signal by a thin layer of polymer, the signal intensity is increased by two orders of magnitude. This new technique enables the AFM-IR characterization of 1 nm thick 1D and 2D materials, which was not possible earlier. Moving ahead, this dramatic signal improvement may be applied to a range of applications, including ultrathin biologicals and a variety of 1D and 2D materials.

Figure 8. (a) AFM topography imaging of CNTs deposited on polystyrene substrate; (b) IR chemical mapping image at 4000 cm^-1 showing absorption by CNTs; (c) IR chemical mapping image of monolayer graphene captured at 4000 cm^-1.

Investigating Exothermic Peaks of Polyethylene Using nanoTA and LCR

One of the most extensively utilized polymers is polyethylene (PE), with applications in several industries, including 2D materials applications. Inorganic fillers such as graphite and metallic particles have been included to vary the thermal, electrical, and mechanical properties of PE. Recently, hexagonal boron nitride (hBN) has exhibited potential as a filler owing to its high mechanical strength, insulating properties, and thermal conductivity. Researchers at Sichuan University characterized this effect of hBN particles on the melting behavior of polyethylene using Lorentz Contact Resonance (LCR) and nano-thermal analysis (nanoTA).³

As shown in Figure 9a and b, LCR imaging can vividly show regions of high hBN concentration on the surface. Subsequently, nanoTA was used to measure the softening temperature of different regions of the material; as shown in Figure 9, a rise in the transition temperature of 4–8 °C was observed for areas of the PE sample near hBN aggregates when compared to areas without hBN. Using the bulk transition temperature within the standard deviation of the nanoTA values, the accuracy of this technique was verified in comparison with conventional DSC analysis.

These results, combined with DSC analysis, demonstrate that the meso-phase of the PE develops near h-BN particles during crystallization, which brings about a weak exothermic peak that was unsolved earlier. Shown in Figure 9 is nanoTA measurement also carried out directly on the hBN particles, for which no thermal transition was measured at temperatures up to 400 °C.

Figure 9. (a) LCR-AFM height image; (b) AFM mechanical image (using LCR) of the PE/BN composites, showing boron nitride clusters in the areas A, D, and E; (c) Local thermal analysis data of the assigned positions were obtained by nano-TA, comparing the melting temperatures of PE and BN; (d) DSC from the PE/BN composites (heating rate of 2 °C min^-1).

Analyzing Thermal Conductivity of Graphene Sheets with SThM

Recent research has been focused on graphene because of its high thermal conductivity and potential in optoelectronics. Scanning Thermal Microscopy (SThM) characterizes thermal conductivity of 2D materials because it yields high sensitivity in resistance detection between the probe and the sample. These high spatial resolutions eliminate uncertainty in the detection of the source of a sample’s electrical capabilities, which makes SThM a reliable technique for monitoring sample temperature and also thermal conductivity in a qualitative manner.

Researchers at Durham University and Lancaster University employed SThM to study thermal conductivity of single and multilayer graphene sheets.⁴ Graphene was deposited on Si/SiO₂ substrate with pre-patterned trenches, with both graphene suspended above the trench and supported by the substrate imaged. It was discovered that increasing the number of supported graphene layers resulted in an apparent decrease in thermal resistance. A main observation was that the thermal conductance of both bilayer and multilayer graphene suspended over the trench was higher than that of the supported layer, in contrast to expectations that conduction from the graphene to the substrate would yield greater heat dissipation.

Since the mean free path of thermal phonons in graphene is much higher when compared to the height of the trench, it is hypothesized that ballistic acoustic phonons from the SThM tip are the major source of heat transfer, with 90% approaching the trench in the ballistic regime. A graphene bulge that was still suspended over the trench showed similar properties, excluding experimental differences like SThM contact area as the cause for such behavior.

These measurements concluded that three-layer graphene had nearly 68% of the thermal conductance than the single layer. Finally, thermal mapping of border regions between supported graphene layers demonstrates that the thermal transition region has a width of 50–100 nm, verifying theoretical estimates for the mean free path.

Figure 10. (a) SThM image of supported graphene, showing varied thicknesses throughout the sample; (b) measured contact thermal resistance as a function of the number of graphene layers, showing reduction in thermal resistance as the number of layers increases.

Conclusion

The nanoIR3-s offers exceptional characterization of 2D material properties with complimentary photothermal-based Tapping AFM-IR and near-field s-SNOM methods. AFM-based nanoscale property mapping offers correlative microscopy capability for electrical, thermal, and mechanical property mapping.

References

1. Khanikaev AB, Arju N, Fan Z, Purtseladze D, Lu F, Lee J, Sarriugarte P, Schnell M, Hillenbrand R, Belkin MA, Shvets G. Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials. Nature Communications. 2017.
2. Rosenberger MR, Wang MC, Xie X, Rogers JA, N S, K WP. Measuring individual carbon nanotubes and single graphene sheets using atomic force microscope infrared spectroscopy. Nanotechnology. 2017.
3. Zhang X, Wu H, Guo S, Wang Y. 2015. Understanding in crystallization of polyethylene: the role of boron nitride (BN) particles. Royal Social of Chemistry Advances. 2015(121):99585-100407.
4. Pumarol ME, Rosamond MC, Tovee P, Petty MC, Zeze DA, Falko V, Kolosov OV. Direct Nanoscale Imaging of Ballistic and Diffusive Thermal Transport in Graphene Nanostructures. Nano Letters. 2012(12)2906-2911.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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