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Investigating 2D Materials with AFM: Five recent Nature Publications Bede Pittenger, Senior Applications Scientist
Enabling New Research on 2D materials Correlate chemistry and defects with topography, mechanical properties and electronic structure
7/30/2015 2 Bruker
Nanoscale deformation & other mechanical properties
-PeakForce QNM
SPIR illuminates defect rich area
-PeakForce IR at 1730cm-1
Maps of workfunction show layers and charges in substrate
-PeakForce KPFM
AFM Height:
Shows Graphene layer structure and points to
defect rich area
1 2
3
4
Graphene plasmons
-PeakForce IR at 870cm-1
Lattice orientation
-Contact Mode
Why AFM for 2d Materials?
• Atomic resolution:
• Z: easily identify monatomic steps and ripples
• XY: Lattice resolution for orientation, observation of defects
• Controlled force (some AFM modes)
• Preserves AFM tip and fragile samples
• Allows local stress to be applied to sample
• More than just topography
• surface potential, modulus, adhesion, conductivity, IR absorption & reflection, etc.
• Works under liquids as well as in ambient conditions
7/30/2015 3 Bruker
water
AFM Imaging Technology A Brief Review
• Mapping topography -> More information
• Contact mode & LFM (1986)
• Tapping Mode (1992)
• Force-Volume Mapping (~1992)
• PeakForce QNM (2010)
• PeakForce TUNA (2011)
• PeakForce KPFM (2012)
• PeakForce IR (2014)
• PeakForce XYZ (…)
7/30/2015 4 Bruker
Rich property mapping made possible by Bruker’s PeakForce Tapping
(ii)
The complete force curve from every interaction between tip and sample is analyzed in real-time, allowing:
• Feedback based on the peak force, protecting the tip and sample.
• Mechanical properties mapped simultaneously with topography.
• Individual curves can be examined and analyzed offline
• Off-resonance: allows combination with other techniques
7/30/2015 5 Bruker
AFM application of local stress Investigating a potential MoS2 strain sensor
• Piezoelectric effect observed in single layer MoS2 devices
• AFM and Raman used to confirm single layer
• Potential strain sensor application
• Found to be very high performance & expected to be suitable for multidirectional nanoscale sensors
7/30/2015 6 Bruker
Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.
AFM application of local stress Inducing piezoelectric effect in MoS2 monolayer
• PeakForce QNM measured deformation under varying load
• Saturation observed due to substrate
• AFM Force Spectroscopy used to apply load near center of device
• IV relations show decreasing current with increasing load
• Different S-D pairs provide very similar results
7/30/2015 7 Bruker
Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.
S S D
D
AFM application of local stress Piezoelectric effect explains device characteristics
• AFM tip loads device at different locations
• Results in either compressive or tensile strain
• Piezoelectric polarization changes Schottky barriers
• Causes different I-V curves
• Consistent with classic thermionic emission-diffusion theory
7/30/2015 8 Bruker
Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.
Compressive Tensile
+ + - -
Mechanical properties of 2d materials Suspended graphene
• Suspend graphene using SiN membrane = direct observation of deformation under load
• TappingMode AFM fails over the unsupported region.
• PeakForce Tapping allows imaging with controlled force
• Provides force curves at each point
• Fitting curves gives modulus ~350N/m
7/30/2015 9 Bruker
Clark, N. et al., 2013. Phys. Status Solidi B, doi:10.1002/pssb.201300137.
Mechanical properties of 2d materials Embedded Graphene
• Embedding the graphene in a polymer matrix allows use of a four point bending setup for more controlled strain
• Raman shift allows characterization of critical axial compressive strain ~-0.6% (or -6GPa)
• AFM enabled direct observation of wrinkling that occurs during compressive failure
7/30/2015 10 Bruker
PeakForce Tapping Height
Androulidakis, C. et al., 2014. Sci. Rep., doi:10.1038/srep05271.
Visualizing 2d structures in liquid Ordered gas molecules at solid-liquid interfaces
• Ordered gases at interfaces influence friction and biological processes involving collection of dissolved gases
• Interfacial Nanobubbles often form at solid-liquid interfaces, but sometimes other structures form as well…
• Why do they last so long? Young-Laplace equation predicts they should dissolve within a few milliseconds
7/30/2015 11 Bruker
Water supersaturated with O2 Water supersaturated with N2
Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.
Visualizing 2d structures in liquid Epitaxial O2 molecules at HOPG-water interface
• AFM lattice resolution allows comparison of epitaxial O2 layers to underlying HOPG lattice
• Two types of epitaxial O2
structures reflecting HOPG symmetry directions were observed
• Both had spacing ~4-5nm
7/30/2015 12 Bruker
Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.
Visualizing 2d structures in liquid Dissecting O2 structures with PeakForce Tapping
• TappingMode (FM) shows nanobubble on disordered micropancake
• PeakForce Tapping at different forces allows the penetration of the tip into the surface to be controlled
• Allows observation of epitaxial layers beneath micropancake
• Micropancakes only form on epitaxial layers – need crystalline, hydrophobic substrate
7/30/2015 13 Bruker
Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.
Visualizing 2d structures in liquid Dissecting N2 structures with PeakForce Tapping
• N2 also forms 2d structures
• Epitaxial rows only along the HOPG zig-zag direction
• Micropancakes not observed
• PeakForce Tapping at different forces shows
• Nanobubbles are disordered (Adhesion map)
• Nanobubbles form directly on the HOPG, not on an epitaxial overlayer
• Epitaxial domains form the perimeter of the bubble
• Interfacial water may be stabilizing the structures
7/30/2015 14 Bruker
150pN: Height
1500pN 2250pN
150pN: Adhesion
Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.
A nanoscale pressure reactor Graphene on diamond (100)
• When graphene is placed on diamond in the presence of water, interfacial energy drives water from some areas while accumulating it in others
• Annealing to ~1275K causes graphene to bond to diamond, trapping accumulated water into nanobubbles
• A mat of nanoscale hydrothermal anvils will be formed containing any molecules present
• Raman and FTIR can study the contents of the nanobubbles
7/30/2015 15 Bruker
Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579.
A nanoscale pressure reactor Observation of the nanobubbles (GNB)
• AFM imaging allows direct observation and measurement of the GNB
• Dimensions can be used to obtain a measure of the strain in the graphene (~6%)
• Also allow estimation of the pressure within a GNB
• Combined AFM-Raman allows selection of areas without GNB for control measurements
• PeakForce QNM modulus mapping shows the GNB to be ~10x softer than the area where the graphene is bonded to the diamond
7/30/2015 16 Bruker
Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579.
A nanoscale pressure reactor High-pressure chemistry
• If temperature increased >1500K, GNBs rupture
• AFM and SEM images show square shaped voids (100) diamond planes
• At 1375K and 1GPa, the water in the GNB is superheated
• Capable of etching the diamond
• Concave graphene is less reactive than diamond
• More recently, similar methods were used to study polymerization of C60
7/30/2015 17 Bruker
Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579. Lim, C. H. Y. X. et al., 2014. Angew. Chemie, doi: 10.1002/anie.201308682.
Strain in 2d heterostructures Moire patterns and commensurate domains
• Lattice mismatch between graphene and hBN causes a moiré pattern that affects electrical, optical properties
• Periodicity depends on rotation of lattices
• When lattices are nearly aligned, there is a transition to a state with commensurate domains where graphene deforms to match the hBN lattice
• Domains are separated by a domain wall where strain is accumulated
7/30/2015 18 Bruker
Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.
Aligned Misaligned ~3deg
Strain in 2d heterostructures PeakForce QNM ‘Modulus’ shows clear transition
• Different periodicities of moiré visible in CAFM
• ‘Modulus’ cross section changes when lattices are aligned
• Misaligned lattices: ~sinusoidal modulus
• Aligned lattices: narrow domain walls separating commensurate regions
• PeakForce QNM shows clear transition when lattices are aligned (period>~10nm)
• Variation in strain at domain walls causes contrast in modulus
7/30/2015 19 Bruker
Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.
CA
FM
8nm 14nm
‘Modulus’
Strain in 2d heterostructures STM confirms source of ‘modulus’ contrast
• STM confirms that the source of the modulus contrast is strain in the graphene
• Lattice constant in domain wall ~2% smaller than within domain
• Transition appears to be responsible for discrepancies in earlier transport measurements of graphene/hBN devices
7/30/2015 20 Bruker
Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.
• AFM with PeakForce Tapping enabled…
• Stimulation of device at precise location with controlled force
• Observation of deformation of graphene under tension and compression
• Dissection of 2d structures at solid-liquid interfaces with controlled force
• Quantification of pressure in nanoanvils by measurement of nanobubbles
• Observation of strain in graphene on BN through high resolution mapping of ‘modulus’
Showcasing the range of AFM enabled research on 2d materials
7/30/2015 21 Bruker
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contact me here:
Bede Pittenger, PhD
bede.pittenger@bruker.com
www.bruker.com/service/education-training/webinars/afm.html
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