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Unit for nanoscience and Theme Unit of Excellence in Nanodevices

Basics of Scanning probe microscopy A.K. Raychaudhuri. Unit for nanoscience and Theme Unit of Excellence in Nanodevices S.N. Bose National Centre for Basic Sciences Kolkata-700098. SNBNCBS and Bruker School December 14-15, 2011. www.bose.res.in. Basic concepts Simple components of SPM

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Unit for nanoscience and Theme Unit of Excellence in Nanodevices

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  1. Basics of Scanning probe microscopy A.K. Raychaudhuri Unit for nanoscience and Theme Unit of Excellence in Nanodevices S.N. Bose National Centre for Basic Sciences Kolkata-700098 SNBNCBS and Bruker School December 14-15, 2011 www.bose.res.in

  2. Basic concepts • Simple components of SPM • Cantilever Statics and Dynamics • The different modes of SPM

  3. I will assume: You have used SPM in some form before and have some acquaintance with it. However, the talk is not for experts.

  4. The Scanning Probe Microscope What are the basic components of a SPM A nano-positioning mechanism that can position the probe in “close proximity” of the surface Localized Probe that has an “interaction”with the substrate to be imaged SPM A mechanism to scan the probe relative to the substrate and measure the interaction as function of position A system to measure the interaction of the probe with the substrate

  5. Physical mechanism and contrast • Any microscopy will depend on some physical mechanism to create a contrast spatially. • It will also need a way to measure the “contrast” with spatial resolution. STM- Quantum mechanical tunneling between a tip and the substrate. The contrast comes from spatial variation of local electronic desnsity of states. AFM- Localized mechanical (attraction or repulsion) interaction between tip and surface.

  6. If the process of scanning does not measure the contrast that has a spatial dependence you will not get any image in any scanning microscope. • Being a computer operated system, any periodic noise in the system can create images because the scanning process can add it up to the main signal. These are plain artifacts. • How to detect artifacts ? – A quick thumb rule

  7. In contrast to TEM or Optical microscope there is no diffraction and reconstruction of diffracted wave front in SPM. Advantage: Resolution is not diffraction limited. Here the limitation comes from the “tip size” that interrogates and of course some fundamental limitations on detection process and electronics.

  8. Different SPM’s and different modes • The nature of the tip –surface interaction gives different types of microscopy. • The way we detect the “response” gives us the different modes of SPM.

  9. The Scanning Probe Microscope (SPM) family STM (Tunneling) SFM (Force) C-AFM Atomic Force Microscope (AFM) Lateral Force (LFM) Magnetic Force (MFM) Electrostatic Force (EFM) STS,STP,Scanning Electrochemical Microscope SPM Scanning Near Field Optical Microscope (Optical imaging) Scanning Thermal Microscope (Local Temperature)

  10. Scanning Force Microscope • It is nothing but a spring balance (the cantilever) that is scanned over a surface. • The cantilever is the precision force detection element- we can detect “atomic forces” • Type of force of interaction between the tip and substrate will determine what we are measuring and the mechanism that makes the contrast. How large are the atomic forces and can we really detect them by a cantilever that is much larger?

  11. How big is the “Atomic Force” The atomic spring constant What is the value of the spring constant of the bond connecting to atoms ? • 2  keff /M • - Is typically in IR range for atomic vibration • ~ 1013 - 1014cps, M ~ 5 x 10-26 Kg, keff= 2M ~5 x (1-102) N/m

  12. w L L One can make a cantilever as a force measuring element that can have the same order of k as that of a molecule. Si elastic modulus (E) [111] Young's modulus= 185GPa [110] Young's modulus=170 GPa [100] Young's modulus= 130 Gpa Si3N4~300 Gpa For a Si cantilever : t = 5m, w= 20 m, L= 200 m k=10N/m It can be softer than atomic spring constant

  13. L2 L1 b w Engineering cantilevers with different spring constant k- need for different applications t: thickness m*~0.24(mass of the cantilever) Advantages: 1.Less prone to vibrational noise. 2. Can go to lower k or resonance frequency.

  14. Engineering cantilevers with different spring constant k-a real triangular cantilever Cantilever Tip What ever you do with SFM, the cantilever is the “key”. You need to know it. Estimated radius of curvature of the tip Rt ~ 30 nm Much softer than an atomic spring !!!! Kc=0.1 N/m

  15. Some feeling for numbers We have a cantilever as a force measuring element. F = k.δ If I want to measure F=1nN, k=1N/m. I should be able to measure a displacement δ=1 nm. Entering the world of nano

  16. At the heart of all scanning probe microscope is the cantilever with a tip. • How we position the tip? • How we scan the tip? • How we measure deflection of the cantilever?

  17. Laser QPD Scan Piezo Inertial drive piezo Electronics Demystifying AFM-A simple AFM(Home made) L. K. Brar, Mandar Pranjape, Ayan Guha and A.K.Raychaudhuri “Design and development of the scanning force microscope for imaging and force measurement with sub-nanonewton resolution” Current Science , 83, 1199 (2002) X-Y micrometer stage

  18. Schematic of SFM Keeps cantilever deflection or oscillation amplitude constant DEFLECTION SENSOR CANTILEVER FEEDBACK LOOP COMPUTER PROBE TIP XY-PIEZO SCANNER Z-PIEZO

  19. Practical Considerations for AFM/SFM • Cantilever deflection detection system. • Type of cantilevers that can be used. • Coarse and fine approach mechanism. • No net relative motion between sample, cantilever and detection system. • Scanner range and type of encoder for large size scanner. • Data acquisition system ,processing and display software. • Accessibility to all the parts of the SFM and capability of using image processing software on stored data. Where do the SPM sold by different vendors differ?

  20. Basic schematic for SPM Keeping “something” constant, need for feed back Pre-Amplifier Laser A B Tip & Cantilever A-B Quadrant Photo Detector Pixels PID Feedback DAC Scanner ADC X-Y scanner Z-scanner bits To Z-Piezo Need for calibration Coarse approach vs fine approach

  21. Calibration of scanning stage of SFM using commercial 2-D grating The grating has 2160 lines/mm 1000µm/2160=0.46µm The calibration: 40nm/V Brar et.al (2002)

  22. Arranging spheres of PS in an array by self-assembly Sub 500nm level calibration, works fine to 20nm Can find the size by Electron microscope or DLS Topography Can take care of image distortion Soma Das (2008)

  23. Calibration in atomic range- A freshly cleaved surface 7 nm x 7 nm Mica Freshly cleaved Can we assume a linear calibration ? The piezo -scanner is non-linear and has hysteresis

  24. Other calibrations: • Z-Calibration- large scale vs small scale • Force calibration-detection of exact k?

  25. Optical head and Detection electronics for scanning Pre-Amplifier Laser A B Tip & Cantilever A-B Quadrant Photo Detector Feedback DAC Scanner ADC To Z-Piezo

  26. Main components of the optical stage: • Laser diode • Cantilever • Quadrant photo-detector (QPD) • Collimating lenses • Mirror QPD is used as a position sensitive detector, its output signal is proportional to the position of the laser spot. Why we need smaller cantilever ? Optical lever = = 500 -100(for l=100mm)

  27. Calibration of the optical stage. Region of Gradient: 1000mm • Detects 4V for 1000μm movement • 1mV electrical noise , positional reolution~1/4μm • Using optical lever of 100, we can detect cantilever deflection of ~ 1/400 µm=2.5 nm. Source of noise in AFM

  28. Atomically resolved steps in Ti terminated SrTiO3 substrate-reaching the limits Size of step (1/2 unit cell) ~0.38nm Courtesy Dr. Barnali Ghosh. Taken in CP-II

  29. Resolution from optical detection Region of Gradient: 1000mm We have the “base” response of the QPD, need to enhance optical lever and reduce electrical noise to get better resolution Often it is good to have a cantilever –tip rest on a surface and record the output as a function of time • Detects 4V for 1000μm movement, 1mV electrical noise ~1/4μm. • Reduce noise to 0.1 mV, • Using optical lever of 100, we can detect cantilever deflection of ~ 1/4000 µm=0.25 nm.

  30. Quadrant photo-detectors Why use 4 quadrant detector ? Vertical deflection of cantilever-Topography Lateral deflection of cantilever-Lateral Force Microscopy (LFM)

  31. Thermal Noise limited resolution If k is reduced the force sensitivity is increased Cantilever displacement = Force/k K ~ 0.1N/m , displacement of 1nm will come from a force of 100pN Does any thing limit us ? Yes it is the thermal noise. It can be very high for “soft” cantilevers (those with very small k)

  32. Thermal Noise limited resolution For any oscillatory system we can apply Equi-partition theorem For a 0.1N/m cantilever the thermal noise induced root mean-square amplitude 0.14 nm. For a deflection of 1nm of the cantilever it is a substantial amount. Force uncertainty~(100±14)pN

  33. I have discussed some of the basic concepts of the SFM and the main components that go with it and their functions as well as limitations. Cantilevers and force detection, Scanner calibrations, Optical detections and sources of noise It will be best if your reflect upon your experience of using SFM and connect to this presentation

  34. Cantilever Statics and Dynamics The different modes of SPM

  35. Source: PhD thesis Soma Das , SNBNCBS

  36. Tip sample interaction model Statics and Dynamics of cantilever • Interaction between the tip and the substrate will decide the nature of force and hence the statics and dynamics of the cantilever

  37. Driving term for dynamic mode Any force  velocity will add to damping and reduce amplitude of vibration-dissipation Any force  displacement will change the frequency of vibration Dynamics of cantilever Simple ball and spring model Different types of force microscopy depends on the dynamics of cantilever and the mode of detection

  38. Static mode (contact mode) AFM ω=0

  39. Static mode: • Mostly for contact-mode – the cantilever deflection is such that the bending force is balanced by the force of interaction: • F(z) =-U/z=-k.z • U = Total energy that includes the surface as well as elastic deformation energy. a0~Atomic dimension (hard sphere) E*~ Effective elastic constant Rt- Tip radius of curvature. H=Hamakar cosntant

  40. Elastic force wins over. The deformation of the surface should be larger than the features you would like to see

  41. Si tip pressing on Si substrate One can evaluate the contact radius Herzian contact The contact area depends on Elastic modulus

  42. A thumb rule to select cantilever in contact mode imaging Cantilever touching a surface is like two springs connected back to back, The force applied is balanced by displacement The softer spring wins

  43. A thumb rule to select cantilever in contact mode imaging The softer spring wins Correct condition for topography in contact mode Will image the elastically deformed surface A surface with mixed k (elastic constants) like a composite of soft and hard matter will not image the topography. What you image is actually a “mixture” of both

  44. Some tips for good contact mode imaging • Get a soft cantilever that is realistically needed. • Do a force spectr0scopy (F-d) curve • Have some idea about the elastic modulus of the surface you image. • For soft materials when you cannot have very soft cantilever use LFM

  45. ODT self-assembled monolayer on Ag Sai and AKR, J.Phys.D Appl. Phys. 40, 3182 (2007)

  46. Some useful applications of contact mode AFM Force spectroscopy Piezo-force spectroscopy Conducting –AFM Local charge measurements

  47. Dynamic mode Driving force Controlled by experimenter Force of interaction of tip with substrate and surrounding

  48. Dynamic mode (all non-contact modes): Cantilever is modulated at resonance frequency and the shift in resonance frequency , phase or amplitude measures the force gradient -F/z=-k+(2U/z2)

  49. Dynamic mode -what do we do ? • Oscillate the cantilever at close to resonance frequency • Interaction with the substrate will change the resonance frequency and /or amplitude of oscillation (through the viscous force on the surface) • Detect the departure from resonance or damping detected by amplitude, phase or frequency shift as the cantilever scans the surface • This leads to contrast and the imaging

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