2D materials characterization using nanoscale IR spectroscopy and material property mapping

  • Complimentary Tapping AFM-IR and s-SNOM techniques provide 10nm spatial resolution mapping of chemical and complex optical properties for 2D materials
  • Correlative microscopy is provided with AFM based nanoscale thermal, electrical and mechanical property mapping
  • The nanoIR2-s platform is an easy to use, powerful characterization system for 2D materials enabling chemical and optical property mapping as well as AFM based nanoscale property mapping

Introduction

2D materials are an important emerging field of research due to their unique properties for important applications in photovoltaics, semiconductors, battery technology and many other areas. 2D materials have been characterized by multiple nanoscale and microscopy techniques in order to gain a better understanding of the nature of their properties. nanoIR techniques extend this characterization with critical nanoscale chemical and optical property mapping.

The nanoIR2-s system provides two complimentary nanoscale IR techniques, scattering-scanning near field optical microscopy (s-SNOM) and AFM-IR Photothermal based nanoscale IR imaging and spectroscopy, including Tapping AFM-IR. These techniques provide unique insights into the nanoscale chemical and complex optical properties of 2D materials. Complimentary atomic force microscopy techniques (AFM) such as mechanical and thermal property mapping also provide information on the electrical, thermal and mechanical properties of these materials. These techniques enable 10nm spatial resolution chemical and optical property mapping, well below the diffraction limit of conventional IR spectroscopy.

This note describes the application of the nanoIR2-s system to the characterization of a variety of 2D materials and structures including semiconductors, nanoantennae, graphene and more.

Nanoscale optical reflectivity of CVD grown graphene.
Sample courtesy of Oak Ridge National Laboratory

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Key words

s-SNOM | Nanoscale IR spectroscopy | 2D materials | Nanoantennas | Plasmonics | Phonon polaritons



Complimentary nanoscale IR techniques

The nanoIR2-s has the ability to acquire nanoscale images and IR spectra using two separate nearfield spectroscopy techniques; Photothermal AFM-IR and s-SNOM. These complementary techniques offer nanoscale chemical analysis, as well as optical, thermal, electrical and mechanical mapping with spatial resolutions down to a few nanometers for both soft and hard matter applications.

Nanoscale IR spectroscopy combines the precise chemical identification of infrared spectroscopy with the nanoscale capabilities of AFM to chemically identify sample components with a chemical spatial resolution down to 10 nm with monolayer sensitivity breaking the diffraction limit by >100x. AFM-IR absorption spectra are direct measurements of sample absorption, independent of other complex optical properties of the tip and sample. As such, the spectra correlate very well to that of conventional bulk transmission IR.



Imaging of plasmons and phonons

Surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials, with their high spatial confinement, can open up new opportunities for enhanced light-matter interaction, super lenses, subwavelength metamaterial, and other novel photonic devices. In situ characterization of these polaritonic excitations across different applications requires a versatile optical imaging and spectroscopy tool with nanometer spatial resolution. Through a non-invasive near-field light-matter interaction, s-SNOM provides a unique way to selectively excite and locally detect electronic and vibrational resonances in real space.

This technique is demonstrated by imaging the SPhPs of hexagonal boron nitride (hBN) as shown in Figure 1. Amplitude and phase near-field optical images provide complementary information for thorough characterization of the polaritonic resonances. >90° phase shift of SPhPs are observed on hBN, indicating strong light-matter coupling.

Similar to the visualization to the SPhPs in hBN, the SPPs of graphene can also be investigated using the nanoIR2-s; Figure 2 illustrates the standing wave of an SPP on a graphene wedge. Generally, the spatial resolution of s-SNOM is limited only by the end radius of the AFM probe, enabling the s-SNOM technique to measure cross sections of the SPP down to ~8 nm.

Figure 1. (a) AFM height image shows homogeneous hBN surface with different layers on 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.

Figure 2: (a) s-SNOM phase image of surfae plasmon polariton on graphene. (b) Cross section of SPP standing wave phase.



Nanocontamination of graphene

The exceptional mechanical and electrical properties of graphene are dependent on maintaining the overall conjugated structure of the sheet. The nanoIR2-s can easily assess the quality of exfoliated graphene obtained by various methods as shown in Figure 3. Contamination that is not easily recognizable in the AFM height image is visible in the s-SNOM reflection image; furthermore, contrast in the s-SNOM reflection image varies with the number of graphene layers present, showing nanocontamination on the sample.

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



Characterizing nanoantenna resonance

The applications of nanoantennae are very diverse, ranging from sensing to energy conversion. The ability to measure and tune the resonance structures of these antennas is of vital importance to the construction of accurate and reliable devices. Arrays of nanoantennae are common as they allow for the packing of a large number of individual antennas in a compact area. Figure 4a shows an AFM topography image of an antenna array consisting of single bar antennas as well as coupled antennas.

When fabricating antenna arrays, the contact point to the antennas is an important consideration to achieve optimum energy transfer efficiency. s-SNOM imaging allows of the easy detection of the antenna resonance hot spots, the ideal contact point. Figure 4b demonstrates the s-SNOM amplitude and phase image of a single bar antenna contained within the array. The dipole antenna resonance is observed with 11 µm excitation; note the ~180° phase change observed at dipole resonance.

In addition to the ability to collect high resolution images of optical phenomenon, the nanoIR2-s provides the capability to spectrally probe nanoscale surface features. Figure 5 shows the AFM-IR spectra collected on single rod and coupled antenna and the antenna resonance can be clearly resolved at 910 cm-1, in agreement with theoretical predictions.

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

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 1100cm-1 shows the Si-O mode shared by both antennas


Effects of polarized light on metasurface chirality1

For the first time, the combination of the complimentary nanoscale imaging techniques, AFM-IR and s-SNOM have been used to investigate the role of chirality in the origins of circular dichroism in 2D nanoscale materials. Chiral molecules are a certain type of molecule that have a non super imposable mirror image. These mirror images of chiral molecules are often called left handed and right handed, and due to 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 known as metasurfaces, comprised of planar-chiral plasmonic metamolecules that are just nanometres thick, have been shown to exhibit chiral dichroism in transmission (CDT). Theoretical calculations indicate that this surprising effect relies on finite non-radiative (Ohmic) losses of the metasurface. Until now this surprising theoretical prediction has never been experimentally verified because of the challenge of measuring non-radiative loss on the nanoscale.

s-SNOM is used to map the optical energy distribution when the structures are exposed to RCP and LCP IR radiation while AFM-IR was then used to detect the drastically different Ohmic heating observed under RCP and LCP radiation.1

For the first time it has been conclusively established the circular dichroism observed in 2D metasurfaces is attributed to handedness dependent Ohmic heating as seen 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 distributino o the Ohmic heating of a chiral 2D metasurface markedly depends on the handedness of light1.



Analysis of carbon nanotubes with nanoIR

The AFM-IR technique functions by detecting the thermal expansion of a material, induced by the absorption of infrared illumination. The thermal expansion of a material is dependent on several factors, including, the coefficient of thermal expansion and the thickness of the material. 1D and 2D materials such as single walled carbon nanotubes (CNT) and single layer graphene have both a low coefficient of thermal expansion and also have a thickness of roughly 1-2 nm. The nature of these 1D and 2D samples make characterization with AFM-IR challenging.

By placing a thin layer of polymeric material underneath graphene and CNT samples a two orders of magnitude increase in AFM-IR signal intensity is observed2, 3. As the thin sample absorbs the incident IR radiation, the heat generated is transferred to the thin polymer, which has a significantly higher coefficient of thermal expansion, and it expands. Figure 7 illustrates the finite element analysis model used to simulate the effects polymer thickness on the thermal expansion and temperature changes.

The model was verified by examining an array of CNTs deposited on top of a layer of 150 nm thick polystyrene on a ZnSe prism. A region of the polymer substrate was removed prior to CNT deposition to ensure there was a region of CNT without polymer underneath. As can be seen in Figure 8, the IR chemical image collected at 4000 cm-1 shows clear signal from the CNT in the region that is supported by polystyrene, while no signal is observed where the polymer substrate has been removed. It has been suggested that the varying AFM-IR signal from different CNTs is caused by the difference between metallic and semiconducting tubes.

Figure 8c shows the AFM-IR imaging of graphene on top of a 106 nm thick layer of PMMA. This image shows the extension of this technique to monolayer 2D materials.The amplification of the AFM-IR signal by a thin layer of polymer increases the signal intensity by two orders of magnitude. This new technique allows for the AFM-IR characterization of 1 nm thick 1D and 2D materials that was previously impossible. Going forward, this dramatic signal enhancement may be applied to a variety of applications including ultrathin biologicals and a variety of 1D and 2D materials.

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

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

Polyethylene (PE) is one of the most widely utilized polymers, with uses in numerous industries including 2D materials applications. In order to change the mechanical, thermal, and electrical properties of PE, inorganic fillers such as graphite and metallic particles have been added. In recent years, hexagonal boron nitride (hBN) has shown promise as a filler due to its high mechanical strength, thermal conductivity, and insulating properties. Researchers at Sichuan University used nano thermal analysis (nanoTA) and Lorentz Contact Resonance (LCR) to characterize this effect of hBN particles on the melting behavior of polyethylene3.

LCR imaging is able to clearly show regions of high hBN concentration on the surface as shown in Figure 9a and b. nanoTA was then used to measure the softening temperature of various regions of the material; there was an increase shown in the transition temperature of 4-8 °C for areas of the PE sample near hBN aggregates when compared to areas without hBN, as shown in Figure 9. The accuracy of this technique was verified when compared to traditional DSC analysis, shown in Figure 9d, with the bulk transition temperature within the standard deviation of the nanoTA values. These results, combined with DSC analysis, show that the meso-phase of the PE forms near h-BN particles during crystallization, which induces a weak exothermic peak that was previously unexplained. Figure 9 illustrates nanoTA measurement also performed directly on the hBN particles, for which no thermal transition was measured at temperatures up to 400 °C.

Figure 9: (a) AFM mechanical image (using LCR) of the PE/BN composites, showing boron nitride clusters in the areas A,D and E; (b) LCR-AFM height image; (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 (heaing rate of 2 °C min-1)



Analyzing thermal conductivity of graphene sheets with SThM

Graphene has been a focus of recent research due to its high thermal conductivity and potential in optoelectronics. Scanning Thermal Microscopy (SThM) characterizes thermal conductivity of 2D materials, as it yields high sensitivity in resistance detection between the probe and the sample. These high spatial resolutions remove ambiguity in the detection of the source of a sample’s electrical capabilities, making SThM a reliable method of monitoring a sample temperature, as well as thermal conductivity in a qualitative manner.

Researchers at Lancaster University and Durham University used SThM to investigate thermal conductivity on single and multilayer graphene sheets4. Graphene was deposited on Si/SiO2 substrate with prepatterned trenches, with both graphene suspended over the trench and supported by the substrate imaged. It was found that increasing the number of supported graphene layers led to a clear decrease in thermal resistance. A key observation was that the thermal conductance of both bilayer and multilayer graphene suspended over the trench was greater than that of the supported layer, contrary to expectations that conduction from the graphene to the substrate would produce greater heat dissipation. As the mean free path of thermal phonons in graphene is much greater than the height of the trench, it is hypothesized that ballistic acoustic phonons from the SThM tip are the main source of heat transfer, with 90% reaching the trench in the ballistic regime. A graphene bulge that was still suspended on over the trench exhibited similar properties, ruling out experimental differences such as SThM contact area as the reason for such behavior.

These measurements concluded that three layer graphene had approximately 68% of the thermal conductance when compared to single layer. Finally, thermal mapping of border regions between supported graphene layers show 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 nanoIR2-s provides unique characterization of 2D material properties with complimentary photothermal based Tapping AFM-IR and near field s-SNOM techniques. AFM based nanoscale property mapping provides correlative microscopy capability for mechanical, electrical and thermal 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.