The Powerful Diversity of the AFM Probe Stefan B. Kaemmer, Bruker Nano Surfaces Division, Santa Barbara, CA 93117 stefan.kaemmer@bruker-nano.com March 21, 2011
Introduction The tip allows us to measure a quantity. This quantity is based on what interaction the selected tip is sensitive to. Surface topography Molecular forces (Pulling, molecular recognition) Nanomechanical information (PeakForce QNM) Electrical information (CAFM, SCM, SPoM, ) Optical information (Raman, IR, Fluorescence) Thermal information (SThM, nta) 2
What do we cover in this talk? In order to get quality information one needs the right tools: A high performance AFM, The correct tip 600 nm height image of Lambda DNA adsorbed onto a mica surface. (TappingMode TM in fluid.) 500 nm phase image of E. coli S-layer membranes exhibiting the characteristic 14nm lattice periodicity. (TappingMode TM in fluid.) 45 mm 180 mm image of Human endothelial cells captured at 2k x 2k pixel resolution. (Contact mode in fluid) 3
Cantilever and beam bounce setup Amplification: B = 3s/l but ultimate sensitivity is independent of l and proportional to 1/s l: length of cantilever S: distance between detector and cantilever 4
Where to find information about tips? www.brukerafmprobes.com 5
Probe selection Imaging Environment AFM Mode Sample Type Probe Family/Model Liquid Air Tapping Contact Force Curves Biomolecules (nucleic acids, proteins, lipids, carbohydrates, etc) Silicon OTESPA - X X - - RTESP - X X - - TESP - X X - - Biolever X - - - X Etched Silicon Cantilever 5-10 nm Biomolecules (nucleic acids, proteins, lipids, carbohydrates, etc) Silicon Nitride SNL X - X X X MSNL X - X X X NP-STT X - X X - Cells Silicon Biolever X - - X X Cells Silicon Nitride DNP X - X X X MLCT X - X X X Tissues Silicon TESP - X X - - Silicon Nitride Cantilever 10-40 nm Tissues Silicon Nitride DNP X - X X X MLCT X - X X X SNL X - X X X MSNL X - X X X 6
Example: Force measurements AFM is used for force measurements in pn (10-12 N) range Approach Retract z Hooke s law shows us that the force measured is directly proportional to the cantilever spring constant So the solution is easy: Just make a super soft cantilever and have a go. Or not? 7
Example: Force measurements 8
Example: Force measurements J Hutter, J Bechhoefer, Rev. Sci. Instrum. 64 (1993) 9
Example: Force measurements Example: OBL lever Biolever (from BrukerAFMprobes.com) 10
Tip functionalization Measure the specific interaction between a molecule attached to the tip apex (A) and another one attached to a support (from atomically flat support to living cells) in (most of the time) a liquid environment. A A A A A A A A A A A A A A A A A A A A B B B B B B B B B B B 11
Molecular Recognition Mapping Malaria-Infected Erythrocytes AFM Probe functionalized with endothelial surface receptor CD36. Malaria-infected RBCs (IE s) show different shape and appearance of knob-like surface structures. Knobs believed involved in adherence to endothelial cells Imaging of IE s with CD36 probe showed adhesion sites mapped to knob-like structures. Li et al. WCB 2010. 12
Mechanical Property Mapping Live and dead cells PeakForce deformation channel 13
Probe selection Imaging Environment AFM Mode Sample Type Probe Family/Model Liquid Air Tapping Contact Force Curves Biomolecules (nucleic acids, proteins, lipids, carbohydrates, etc) Silicon OTESPA - X X - - RTESP - X X - - TESP - X X - - Biolever X - - - X Etched Silicon Cantilever 5-10 nm Biomolecules (nucleic acids, proteins, lipids, carbohydrates, etc) Silicon Nitride SNL X - X X X MSNL X - X X X NP-STT X - X X - Cells Silicon Biolever X - - X X Cells Silicon Nitride DNP X - X X X MLCT X - X X X Tissues Silicon TESP - X X - - Silicon Nitride Cantilever 10-40 nm Tissues Silicon Nitride DNP X - X X X MLCT X - X X X SNL X - X X X MSNL X - X X X Resonance frequency of probes in fluid drops to 1/2-1/3 of the resonance frequency of the probe in air. Resonance frequency in fluid easily identified through use of thermal tune. 14
Tapping Mode Newtons 2 nd law of motion Effective resonance frequency 15
Tapping Mode Tapping Mode 16
Lateral resolution Smallest features that can be resolved In optics it is determined by the spot size of the focused beam, in SPM by tip size and tip-sample distance Tip size -> What does the sample actually see of the tip? E.g. STM resolution is restricted to area of tip at which current changes less than a magnitude (CJ Chen. Intro to STM 1993) In force microscopy we have to look at the change in tip-sample interaction forces to define the lateral resolution. 17
Why do we actually want to stay close to the surface? Lets take the example of electrostatic measurements. z r s x q Grounded tip away from a point charge q. The force on the tip will be F=f(x) with F max at x=0 The force is a function of F(x,s,r). One can show that: Force DLat DLat is: proportional to sqrt(r) directly proportional to the tip-sample distance s. q1 q2 This is why you want to be close to the surface and not far away like in noncontact for the highest lateral resolution. 18
What tip do we need? We want to stay close but at the same time want to avoid the jump to contact. F Spring s F Total = F Spring - F Ext F Total (s)=0 s F Ext s=0 time So we need a tip with a high enough spring constant to avoid the instability 19
Resonance versus Sub-Resonance Tapping Cantilever Response F ~ s k Solution: k 0.1~ 0.4 N/m F ~ A Q s ~ f Q Frequency k A0 ( A0 As ) k w ~ Q w<1x10-18 Joule/cycle A 0 ~ 20 nm, k=1n/m, Setpoint 0.9 Need: w>10x10-18 Joule/cycle, for a tip with R~10 nm 0 Bruker NanoSurfaces Division February 17, 2011 20
Peak Force Tapping Trajectory of the tip 1 nn approaching withdraw van der Waals Peak tapping force Time TESP (42 N/m) on Si, MM8 February 17, 2011 Bruker NanoSurfaces Division 21
Resolution & Force Control C 60 H 122 Height Stress = 1.27 GPa 1 nn C 36 H 74 Height Dia 1 nm C 18 H 38 Height 80x80 nm 500x500 nm 10pN 1uN 80x80 nm February 17, 2011 Bruker NanoSurfaces Division 22
True atomic resolution - Gibbsite in water 1 x 1 um 2 topography image of gibbsite platelets on mica substrate Tapping mode topography (left) and phase image (right) of a gibbsite surface in pure water. Data taken on regular MultiMode-AFM using Fastscan B cantilever. Courtesy of F. Mugele and D. Ebeling, Univ. of Twente/NL. 23
True atomic resolution - Mica in water Data taken on regular MultiMode-AFM. Courtesy of F. Mugele and D. Ebeling, Univ. of Twente/NL. 24
Conclusion Choosing the right cantilever for the job will unlock the full potential of your AFM It is beneficial to make some rough estimates on what can be achieved with a given cantilever Peak Force Tapping achieves extremely high resolution data in air due to superior force control and the ability to use soft cantilevers By working in liquids Tapping Mode using small amplitudes can produce true atomic resolution data High solution imaging: the sensitivity and even noise have been sufficient for a decade with Multimode IIIa I would like to thank my colleagues Andrea Slade, Steven Minne, James Shaw, and Chanmin Su for helpful discussions 25