Applications of AFM in the Characterization of 2D Materials

Since graphene was first isolated in 2004, the world of two-dimensional (2D) materials has expanded massively.1 In little more than a decade, the library of single- and few-layer materials has grown to more than a dozen compound and elemental crystals.

Synthesis methods have also grown beyond the original “sticky tape” mechanical exfoliation to encompass chemical exfoliation and chemical vapor deposition (CVD, Figures 1 and 2), to name a few. This growth shows no sign of slowing down.

CVD growth of molybdenum disulfide (MoS2 ) on graphene

Figure 1: CVD growth of molybdenum disulfide (MoS2 ) on graphene – Three-dimensional topography image of MoS2 (triangles) grown by CVD on epitaxial graphene. The terraces are attributed to miscut of the silicon carbide wafer substrate. Subsequent growth precipitates new nuclei on previously grown triangles, forming multilayered MoS2 pyramids. Scan size 2 μm. Imaged on the Cypher AFM in tapping mode. Image courtesy of I. Balla, S. Kim and M. Hersam, Northwestern University.

Controlling MoS2 crystalline orientation in CVD processes

Figure 2: Controlling MoS2 crystalline orientation in CVD processes – “Bottom-up” strategies such as CVD have potential for commercial-scale synthesis of MoS2 but need further refinement to enable growth with controlled crystalline orientation. (right) Topography images of a MoS2 domain grown by CVD on annealed sapphire. (left) The height profile (blue line) reveals a domain thickness of <1 nm. Individual terraces in the atomically flat sapphire can also be seen with step height ~0.22 nm, which is sufficiently low to allow continuous, single-crystal MoS2 growth. Imaged on the Cypher AFM in tapping mode. Adapted from Ref. 4. Image Credit: Asylum Research

With a shift in focus towards synthesis in commercial-scale quantities and predictions of up to 500 more 2D systems2, new opportunities and challenges continue to flourish.3 This boom is not only encouraged by fundamental interest in a completely new class of materials, but also by the huge potential of 2D materials for next-generation technology.

Decreased dimensionality creates fascinating mechanical, optical, electrical, and related properties that could be used in a myriad of ways.3 Although optoelectronic and electronic applications are key targets, other possibilities appear everywhere from water purification to chemical sensing and tissue engineering.

The atomic force microscope (AFM) has played an essential role in 2D materials research, ever since it was employed to confirm the first isolation of graphene.1 Just as the world of 2D materials has grown, so has the power of AFMs.

Today, AFMs can image crystal lattice structure in addition to nanoscale morphology and sensing local mechanical, electrical, and functional response in more ways than ever before. These capabilities and how AFMs contribute to continued progress in 2D materials research are outlined here.

Evaluating Nanoscale Morphology

Understanding basic morphology, shape and size, is crucial for both brand-new 2D materials and existing ones made in new ways. Simply detecting the material’s presence may be enough in some instances, although not trivial.

In other instances, measuring flake or device dimensions, the dispersion of flakes, or sample roughness and uniformity may be vital. Data on layer thickness and the amount of layers are also frequently required, partly because these quantities can affect a number of sample properties.

AFMs excel at tasks such as these and have distinct advantages over optical tools, with spatial resolution well below a nanometer. Unlike white light interferometry, any substrate may be utilized. A bigger range of sample thickness and quality can be accommodated than with Raman methods.

Instead of giving a single, spatially-averaged value, AFMs also visualize the distribution in features on sub-µm length scales. Information on morphology is gathered by imaging topography (height), usually in contact or tapping mode (Figures 1 and 2).

When performed carefully, both techniques generate accurate results.5 For example, in tapping mode it is crucial to make sure that the cantilever does not switch between the repulsive and attractive regimes.

Environmental control can be desirable or even vital during topographic imaging or other AFM experiments. For example, humidity control may enhance day-to-day reproducibility of high-resolution thickness measurements.

Stopping irreversible changes from oxidation reactions with the use of an inert gas environment is a more dramatic example. To meet these and related requirements, specialized measurement cells are available. The whole AFM can be enclosed in a glovebox if even more stringent environmental control is needed.

Other applications benefit from AFM experiments carried out in a liquid environment. For example, many “top-down” routes to facile synthesis of 2D materials involve liquid-phase reactions.

Here, assessing morphology in situ generates much richer information than simple post-process imaging in air. Experiments were carried out to monitor the spontaneous exfoliation of graphene in an ionic liquid in the example shown in Figure 3.

Monitoring spontaneous graphene exfoliation in ionic liquids (ILs)

Figure 3: Monitoring spontaneous graphene exfoliation in ionic liquids (ILs) – Exfoliation methods show promise for large-scale production but are prone to aggregation. The tapping mode phase images were acquired on freshly-cleaved highly oriented pyrolytic graphite (HOPG) immersed in the IL 1-ethyl-3-methylimidazolium acetate (EMIm Ac) at 25 °C. Distinct surface changes are observed as immersion time increases: HOPG’s characteristic step edges gradually erode and are replaced by amorphous domains (see arrows). Analogous experiments were performed at other temperatures and in a second IL, 1-ethyl-3-methylimidazolium trifluoromethansulfonate (EMIm TFMS). The graph shows that the time to exfoliation decreases sharply with increasing temperature. This Arrhenius-like behavior supports the hypothesis of spontaneous exfoliation via ionic intercalation between sheets. Imaged with the Cypher ES AFM. Adapted from Ref. 6. Image Credit: Asylum Research

Sophisticated fully-sealed fluid cells are available to surround the entire cantilever and sample surface in a liquid solvent. These cells are extremely versatile because of their capabilities for operating under either static or perfusion conditions make.

Evaluating Morphology with Asylum AFMs

  • Setup for tapping mode is both faster and simpler on MFP-3D Infinity™ and Cypher AFMs with GetStarted™. Before the tip ever touches the sample, the predictive algorithm automatically optimizes imaging parameters. Neither the sample nor tip is damaged by non-optimal settings, and high-quality data is gathered from the first scan line.
  • SpotOn™ automated laser alignment decreases setup time on Cypher™ AFMs. Just click the mouse on the desired position, and the fully-motorized stages align the laser spot on the cantilever and center the photodetector.
  • Easily locate single-layer flakes and other areas of interest with the high-resolution optics of Cypher family AFMs. Diffraction-limited optics with Köhler illumination provide sub-micrometer resolution for digital zooms and pans.
  • GetReal™ software simplifies accurate measurements of absolute force and stiffness on all Asylum AFMs. It calibrates the cantilever spring constant and deflection sensitivity automatically with a single click and without touching the sample.

Environmental Control with Asylum AFMs

  • Turnkey glovebox solutions supply full environmental isolation for MFP-3D and Cypher AFMs, stopping irreversible surface chemical reactions with water vapor or ambient oxygen for more reliable measurements.
  • Environmental control options for MFP-3D family AFMs include the Closed Fluid Cell for static or perfusion operation in gases and liquids, the BioHeater for heating (ambient to 80 °C), the MicroFlow Cell, and the Humidity Sensing Cell. For all MFP-3D AFMs except Origin, the PolyHeater (ambient to 300 °C), PolyHeater+ (ambient to 400 °C), and CoolerHeater (-30 °C to +120 °C) can be utilized for temperature control.
  • Tapping mode in liquid is more stable, simpler, and more quantitative with blueDrive™ Photothermal Excitation on Cypher family AFMs. Cantilever actuation with a blue laser generates exceptionally clean and stable signals.
  • The Cypher ES AFM contains a sealed cell with broad chemical compatibility for static or perfusion operation and relative humidity control for precise environmental control. The Heater (ambient to 250 °C) and CoolerHeater (0 °C -120 °C) stages provide precise temperature control without extra electronics or cooling pumps.

Yet traditionally, tapping mode imaging in liquid has been challenged by the “forest of peaks,” that is, spurious signals produced by piezo acoustic excitation. This phenomenon, which can make tapping in liquid less stable and more challenging, has been eliminated by new photothermal excitation methods.

Driving the cantilever oscillation via laser light leads to a near-ideal response on any cantilever, in any environment. Temperature is another experimental variable which is vital. For example, it allows fine-tuning of liquid-phase processes and permits studies of long-term reliability under realistic conditions.

Temperature control in AFM experiments is typically attained by utilizing specialized sample stages. Current AFMs feature sample stages that supply highly stable, precise temperature control up to 400 °C.

Imaging Lattice Structure

In a number of instances, characterizing the crystalline nature of 2D materials is vital. Knowing lattice structure verifies that the desired crystal was created and helps to assess the synthesis process. Sometimes the relative crystalline orientation of different layers may influence the overall device performance.

Traditionally, measurements of lattice structure have been thought of as the domain of  low-temperature, ultra-high-vacuum scanning tunneling microscopy (LT-UHV STM) or high-resolution transmission electron microscopy (HRTEM).

Both methods supply enough resolution to visualize the crystal structure and evaluate characteristics like quality, symmetry, and orientation. Measurements of lattice constants can be gathered either directly from an image or its spatial Fourier transform.

Yet, imaging the atomic lattice of 2D crystals is also viable on some of today’s commercial AFMs, like the Cypher. Instrumentation improvements mean spatial resolution comparable to, or even better than, HRTEM’s current limit7 of ~50 pm can now be routinely achieved.

Regions of 2D materials with local atomic flatness supply optimal conditions for high resolution AFM imaging, as they permit fewer atoms (or even a single atom) in the tip to interact with the surface. AFM methods also have multiple practical benefits over HRTEM and LT-UHV STM for lattice imaging.

AFMs usually work under ambient conditions, without vacuum or cryogenic equipment. Substrates and samples may be insulating, conducting, or semiconducting. Sample preparation is generally minimal, significantly shortening the time before the first image is gathered.

AFM images represent the surface topography and so are more directly interpreted than images of tunneling current (LT-UHV STM) or phase interference (HRTEM). These concepts are shown in Figures 4 and 5, which illustrate high-resolution AFM images of 2D materials gathered in ambient conditions.

Measuring lattice constants of dioctylbenzothieno-benzothiophene (C8-BTBT) molecular crystals

Figure 4: Measuring lattice constants of dioctylbenzothieno-benzothiophene (C8-BTBT) molecular crystals – Solution processing of 2D single-crystal organic semiconductors could enable high-throughput manufacturing of novel optoelectronic devices. In this work, single- and few-layer sheets of C8-BTBT were formed by floating-coffee-ring-driven assembly on a silicon/silicon oxide substrate. The image of a C8-BTBT bilayer (left) was acquired with the amplitude signal in tapping mode. The 2D Fourier transform (right) reveals peaks corresponding to the crystal lattice. Vectors indicate the oblique unit cell with lattice constants a and b with angle θ. The histograms of a, b, and θ were obtained from analysis of 10 images acquired at different places on a bilayer region with total area >5×104 μm2. The narrow distributions indicate the bilayer is composed of a single-crystal phase. Imaged with the Cypher AFM in air. Adapted from Ref. 8. Image Credit: Asylum Research

Engineering strain in epitaxial graphene

Figure 5: Engineering strain in epitaxial graphene – Understanding the strain created during graphene deposition will facilitate control of its electronic properties. (a) Topography image of graphene grown on hexagonal boron nitride (hBN) by high-temperature molecular beam epitaxy (MBE). Two regions of graphene, separated by a central crack, display hexagonal moiré patterns that arise from a lattice mismatch between the graphene and the hBN substrate. Defects and variable periodicity in the moiré patterns represent strain-induced spatial variations in lattice constant. (b)-(d) Topography images acquired within the square regions indicated in (a). The scan size of each image is 5 nm [see scale bar in (b)]. The vectors indicate that the graphene regions in (b) and (d) have the same lattice orientation and are aligned with the hBN substrate in (c), confirming epitaxial growth. Imaged in air with contact mode on the Cypher AFM. Adapted from Ref. 9. Image Credit: Asylum Research

The figures also demonstrate that imaging may be carried out in either tapping mode (Figure 4) or contact mode (Figure 5). The height (Z-sensor) signal can be employed for imaging in both tapping mode and contact mode; in tapping mode, the amplitude signal can also be utilized.

Imaging Atomic Lattices with Asylum AFMs

  • Small cantilevers (<10 µm long) and small laser spot sizes (3×9 μm2) make line scan rates up to 40 Hz routinely achievable on Cypher AFMs. This means it only takes a few seconds to gather complete 256×256 pixel images.
  • Cypher AFMs do not need add-on vibration isolation in most labs, even when imaging atomically flat samples. An integrated enclosure and super-stable mechanical design makes them inherently immune to normal environmental vibration. Thermal drift is also lower by 10× than in less advanced AFMs.
  • All Asylum AFMs feature closed-loop scanners with position sensors to eliminate image distortions for high-precision offsets and zooms. Cypher and MFP-3D Infinity AFMs feature the latest generation of position sensors with exceptionally low noise, as low as 35 pm in Z (Infinity) and 60 pm in X and Y (Cypher).

The excellent spatial resolution in these figures has been acquired by careful instrument design. In the lateral direction, resolution is dependent on XY scanner performance and tip sharpness.

Closed-loop control supplies more accurate XY scanning than the open-loop operation of older AFMs. Utilization of more thermally stable materials, symmetric design, and smaller enclosures decrease additional scanning distortion due to thermal drift.

Transient noise that might break sharp tips is also minimized as newer AFMs are more stable. Vertical resolution depends on the noise floor, below which real features cannot be determined from random mechanical and electrical fluctuations.

By making the mechanical loop between the tip and sample as short and stiff as possible the noise floor can be lowered. This minimizes disruptions from environmental vibrations when combined with improved acoustic isolation. Another factor in attaining higher spatial resolution is the utilization of smaller cantilevers.

With lengths that are ten times smaller than their conventional counterparts, smaller cantilevers have intrinsically lower thermal noise for the same spring constant and resonant frequencies that are much higher.

AFMs must possess faster photodiodes and control electronics, smaller laser spot sizes, and higher instrument resonances than older AFMs in order to accommodate small cantilevers and allow quicker scanning. Images resolving the atomic lattice can also be gathered by measuring the cantilever’s lateral deflection in contact mode.

This method is known as lateral force microscopy (LFM), or friction force microscopy if the difference between trace and retrace signals is employed. In both instances, image contrast arises from periodic slip-stick of atomic-scale frictional forces. (The actual tip sample contact area is much bigger, usually ~100 nm2.)

As the contrast is largely independent of topography, these methods are especially useful for samples fabricated on rougher substrates. Figure 6 shows an example of LFM imaging of the graphene lattice.

Mapping graphene grain orientation with fast, high-resolution scanning

Figure 6: Mapping graphene grain orientation with fast, high-resolution scanning – Low-pressure CVD growth of graphene on copper foil is an attractive option for large-area synthesis. This image of CVD graphene on copper was acquired with the lateral deflection signal in contact mode (LFM) and is part of a larger image (scan size 20 nm). The 2D Fourier transform of the larger image displays the hexagonal pattern of peaks indicative of crystalline graphene (dashed lines) and enables the local lattice orientation to be determined. By collecting more than 1000 such images unattended over 7 h (<30 s per 512×512 pixel image), the grain orientation over a 25×25 μm2 region was mapped. Imaged on the Cypher AFM in air. Adapted from Ref. 10. Image Credit: Asylum Research

Probing Electrical and Functional Response

Ranging from insulating, to highly conductive, to semiconducting, the electrical properties of 2D materials could allow multiple types of disruptive technology.11 They may hold the key to extremely small, fast transistors with superior performance at minimal power; other possibilities include new flexible displays, photovoltaic devices, and light-emitting diodes (LEDs).

Yet, realization of such next-generation devices needs knowledge of 2D materials beyond simply topography. Information on relevant physical behavior, particularly electrical and related functional response, is crucial.

A number of AFM modes interrogate optoelectronic and electrical behavior with nanoscale spatial resolution.12 For example, nanoscale current mapping is carried out with conductive AFM (CAFM), supplying complementary information to macroscale techniques like four-point probes which test a whole device.

Furthermore, CAFM current images are acquired simultaneously with topography images. This facilitates correlations between local structure and characteristics such as charge distribution and transport, as seen in Figure 7. For deeper analysis, CAFM can also be employed to gather I-V curves at user-defined device locations.

Creating a photoswitchable diode with MoS2

Figure 7: Creating a photoswitchable diode with MoS2 – Optically controlling the electronic response of 2D materials will facilitate development of next-generation photodetectors and LEDs. Here, MoS2 flakes were exfoliated on mixed-self-assembled monolayers (mSAMs) of azobenzene chemisorbed on gold (Au). As a photochromic material, azobenzene molecules switch from a stable trans-configuration to a metastable cis-configuration when exposed to UV light. The CAFM current-voltage curves were acquired before illumination (trans 1, blue, after exposure to UV light (cis, red), and after exposure to white light (trans 2, green). Current rectification occurs for the trans- but not the cis-mSAM. Rectification is further demonstrated in the image of CAFM current for one, two, and three layers of MoS2 (1L, 2L, and 3L, respectively) on a trans-mSAM. With a bias voltage (0.1 V) below the heterostructure turn-on voltage (0.5 V), the image and its histogram indicate lower conductance in the MoS2 flakes than the bare mSAM. Scan size 1 μm; acquired on the MFP-3D AFM with the ORCA module. Adapted from Ref. 13. Image Credit: Asylum Research

CAFM is called photoconductive AFM when carried out on optically responsive systems with an illumination source. As CAFM scans in contact mode, lateral forces emerge which could potentially damage the sample or tip. The resulting wear may influence the measured current and complicate image interpretation.

Fast current mapping methods have been recently developed in order to circumvent such issues. Here, instead of scanning in contact, current is measured whilst gathering a high-speed array of force curves.

Fast current mapping is based on a fast-force-curve method, in which the cantilever is moved vertically in a continuous sinusoidal motion while it is also scanned laterally. Fast current mapping techniques yield parallel topography and current data to elucidate local structure-property relations, as in CAFM.

As long as complete curves of current and deflection versus time are stored, they also provide a wealth of data analysis choices. Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM) are other modes to assess local electrical behavior on both operating devices and sheet materials, either with or without illumination.

EFM senses electric field variations because of long-range electrostatic force gradients and supplies useful images of qualitative contrast with minimal setup. In comparison, KPFM quantitatively measures the contact potential difference between the sample and tip.

This gives valuable contrast even when topography cannot: for example, to distinguish crystalline grain boundaries and orientation, detect single layers on rough substrates, or differentiate single- and multiple-layer regions.

Electrical and Functional Response with Asylum AFMs

  • Enhance photocurrent measurements on all Asylum AFMs using the dual-pass approach of EclipseTM mode. Topography is measured in contact mode in the first scan pass. The laser is turned off in the second pass, and CAFM measurements are performed.
  • Asylum Research provides the only commercial high-voltage PFM mode (up to ±220 V for MFP-3D Origin AFMs; up to ±150 V for Cypher and MFP-3D Infinity AFMs). High sensitivity measurements with resonance-enhanced PFM are integrated into software on using Dual AC Resonance Tracking (DART) mode or the Band Excitation option.
  • Utilize the ORCA module for CAFM experiments on Cypher S and MFP-3D AFMs. Its cantilever holder features a sensitive, low-noise transimpedance amplifier operable over a large range of currents (~1 pA to 20 nA) and is available in a variety of gain choices. For an even wider current range, the Dual Gain ORCA option contains two separate amplifiers. The outputs from both amplifiers may be measured at the same time for high sensitivity and high resolution from ~1 pA to 10 μA.
  • Gather force and current curve arrays at the same time with Fast Current Mapping Mode on Cypher and MFP-3D Infinity AFMs. With pixel rates up to 300 Hz on Infinity AFMs and 1 kHz on Cypher AFMs, a 256×256 pixel array can take under 10 minutes to obtain. Use of ORCA and Dual Gain ORCA amplifiers ensure high measurement sensitivity over a wide current range.

As outlined in Figure 8, a further use of KPFM is to examine band bending in semiconductors. In addition, proper calibration of KPFM experiments permits quantitative measurements of the work function in conductors and semiconductors.

Tuning electrical properties of tungsten diselenide (WSe2)

Figure 8: Tuning electrical properties of tungsten diselenide (WSe2) – The ability to tailor band gaps in transition metal dichalcogenides (TMDs) would enhance the performance of 2D semiconductor devices. In this work, a transistor was created with WSe2 as the channel, and defects were selectively induced by irradiation with a focused helium-ion (He+) beam. The images show topography (gray) and KPFM surface potential (color) after half the channel was exposed to a dose of 5 × 1014 He+/cm2. The surface potential profile across the dotted line in the KPFM image shows a sharp interface. The results indicate band bending due to Se vacancies, which act as electron donors in the area exposed to He+. Imaged on the Cypher AFM. Adapted from Ref. 14. Image Credit: Asylum Research

Using a near-field method known as scanning microwave impedance microscopy (sMIM), characterization of electrical response at microwave frequencies is possible. Utilizing a microwave source coupled to a shielded probe, sMIM detects local changes in sample conductivity and permittivity.

Applications include analyzing electric defects, sensing buried conducting layers, and measuring type and concentration of semiconductor dopants with high sensitivity. Other types of functional response in 2D materials could also result in technology breakthroughs.

For example, piezoelectric and ferroelectric behavior could bring innovations in logic and memory devices, actuators and sensors, and other products. Piezoresponse force microscopy (PFM) is a powerful tool for characterizing ferroelectric, piezoelectric, and multiferroic materials on the nanoscale.

It supplies information on both dynamic and static electromechanical properties including domain structure, growth, and polarization reversal. An example of PFM imaging on ultrathin CuInP2S6 (CIPS) is shown in Figure 9.

Evaluating room-temperature ferroelectricity in CuInP2S6 (CIPS)

Figure 9: Evaluating room-temperature ferroelectricity in CuInP2S6 (CIPS) Ferroelectricity in 2D materials could be exploited for non-volatile memory and other devices but remains relatively unexplored. Here, CIPS flakes were mechanically exfoliated onto heavily-doped silicon containing an oxide layer and a gold topcoat. The images of topography (gray) and PFM amplitude (color) contain flakes with two, three, and four layers (2L, 3L, and 4L, respectively). The corresponding sections across the dashed lines reveal ferroelectric behavior in flakes only a few nanometers thick, although the magnitude of response decreases with decreasing thickness. Imaged on the MFP-3D AFM. Adapted from Ref. 15. Image Credit: Asylum Research

Ferromagnetic response in 2D materials could also be exploited for technology advances such as quicker data transfer, higher storage capacity, and new spintronic devices.

Magnetic force microscopy (MFM) utilizes the interaction forces between a magnetized tip and a magnetic sample to probe ferromagnetic and multiferroic materials on the nanoscale sensitively. Magnetic features such as vortices, and domain patterns and walls can be imaged using MFM.

Measuring Mechanical Properties

Although most proposed uses of 2D materials exploit their functional and electrical response, their mechanical properties can also have a huge impact. For instance, the high tensile strength of graphene affects its performance in flexible displays, while the high modulus of MoS2 facilitates strain engineering of nanoelectronic heterostructures.

To both confirm theoretical predictions and hasten development of commercial products, more experimental data on mechanical properties are currently required. To assist with this, a range of AFM methods have been developed to supply unique information on nanoscale mechanical properties.

A familiar example is the force curve technique. This well-established method for measuring elastic modulus was utilized in order to gather the first result on graphene of 1 TPa.16

Yet, these measurements needed a specialized cantilever with a very high spring constant and diamond tip to ensure sufficient sensitivity on such a stiff material. Conventional force curve mapping is also extremely slow, though recently introduced fast force curve techniques permit much quicker operation.

Other AFM nanomechanical techniques have been developed to increase imaging speed and expedite measurements on higher-modulus materials. Two of these are contact resonance AFM (CR-AFM) and AM-FM mode.

Both modes permit qualitative, fast, contrast imaging in addition to quantitative modulus mapping, even on very stiff materials (to 100+ GPa). Additional information on damping and viscoelastic response can be gathered by AFM loss tangent imaging, which is performed either as part of AM-FM mode or in standard tapping mode.

AFM loss tangent results to evaluate surface water layers on graphene are shown in Figure 10. In Figure 11, CR-AFM was employed to detect subsurface structural variations in oxygen intercalated graphene.

Investigating interfacial phenomena in graphene

Figure 10: Investigating interfacial phenomena in graphene – elucidating the effects of water vapor layers could improve the accuracy of thickness measurements on 2D materials. These tapping mode images of AFM loss tangent tan δ acquired at different relative humidity (RH) values indicate a water adlayer (arrows) between the few-layer graphene flake and a silicon dioxide (SiO2) film on silicon. The graph reveals that tan δ is higher on graphene than SiO2 at low RH but decreases for both materials with increasing RH, until it reaches similar values for both materials at ~70% RH. The results support a model of RH-dependent water adsorption developed from thickness measurements (not shown). Imaged with the MFP-3D AFM and the Humidity Sensing Cell. Adapted from Ref. 17. Image Credit: Asylum Research

Mapping subsurface variations in graphene

Figure 11: Mapping subsurface variations in graphene – Understanding and controlling the interface between a 2D material and substrate is imperative for successful device engineering. In this work, the power of contact resonance AFM (CR-AFM) to sense subsurface structural and compositional variations was demonstrated. Epitaxial graphene on silicon was annealed in air to form deliberately modified subsurface regions via oxygen intercalation. (a) The topography image shows only graphene terrace steps, but (b) the CR-AFM frequency image contains numerous other features. (c) Topography and (d) CR-AFM frequency images corresponding to the boxes in (a) and (b). Using a combined density functional theory and continuum modeling approach (not shown), the nanomechanical response in regions 1, 2, and 3 could be attributed to three different interfacial atomic structures. Imaged on the MFP-3D in DART mode. Adapted from Ref. 18. Image Credit: Asylum Research

In many cases, tribological properties of 2D materials also influence performance. Nanoscale surface adhesion, which gives insight into reactivity and wetting effects in chemical sensors and other devices, is measured with force curves.

Vital in applications like solid lubricant layers for nano- and micromechanical systems (NEMS and MEMS), frictional behavior, is often characterized using LFM. These methods can image spatial variations in lateral forces or gather friction loops of lateral force versus sliding distance. Calibrated measurements at different applied loads allow the coefficient of friction to be established.

Nanomechanical Measurements with Asylum AFMs

  • The NanoRack Stretch Stage on MFP-3D AFMs permits measurements on samples under tensile or compressive strain.
  • Asylum Research provides a large range of nanomechanical methods, so that the most suitable one for a given application can be chosen or results from different techniques compared.
  • Contact Resonance Viscoelastic Mapping Mode is integrated into software on all Asylum AFMs. It employs either Dual AC Resonance Tracking (DART) or the Band Excitation option.
  • AM-FM Viscoelastic Mapping Mode is exclusive on all Asylum AFMs. It performs quantitative nanomechanical mapping quicker than any other method, when small cantilevers are employed.

Explore Flatlands with Asylum AFMs

The world of 2D materials has continued to grow at an exponential rate. Once limited to small flakes of graphene painstakingly prepared, it is now poised to encompass a plethora of materials, facile synthesis processes, and high-impact applications.

The AFM is already a vital tool for characterizing 2D materials, but the enhanced power of today’s models ensure it will maintain this role into the future. Enhancements like faster imaging rates, higher spatial resolution, and better environmental control, in addition to advanced capabilities for measuring functional, electrical, and mechanical properties, make AFMs more valuable than ever for 2D materials research.

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Acknowledgments

Asylum Research acknowledges the assistance of R. Atkin, I. Balla, J. Becker, A. Belianinov, P. Beton, A. Elbourne, J. Finley, M. Hersam, S. Kim, A. Kis, V. Korolkov, Y. Li, F. Liu, Z. Liu, E. Margapoti, P. Rack, Y. Shi, A. Summerfield, M. Stanford, Q. Tu, N. Wilson, J. Wang, Q. Wang, X. Wang, Y. Wu, L. You, and S. Zauscher with figure preparation.

This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.

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