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

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

5. 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. 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.

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

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  k eff /M  - Is typically in IR range for atomic vibration  ~ 10 13 - 10 14 cps, M ~ 5 x 10 -26 Kg, k eff =  2 M ~5 x (1-10 2 ) N/m

12. One can make a cantilever as a force measuring element that can have the same order of k as that of a molecule. w L L Si elastic modulus (E) [111] Young's modulus= 185GPa [110] Young's modulus=170 GPa [100] Young's modulus= 130 Gpa Si 3 N 4 ~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. L2L2 b w L1L1 t: thickness m*~0.24(mass of the cantilever) Engineering cantilevers with different spring constant k- need for different applications Advantages: 1.Less prone to vibrational noise. 2. Can go to lower k or resonance frequency.

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

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. Demystifying AFM-A simple AFM (Home made) Laser QPD Inertial drive piezo Scan Piezo Electronics 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 DEFLECTION SENSOR FEEDBACK LOOP CANTILEVER Z-PIEZO PROBE TIP COMPUTER XY-PIEZO SCANNER Keeps cantilever deflection or oscillation amplitude constant

19. Practical Considerations for AFM/SFM 1.Cantilever deflection detection system. 2.Type of cantilevers that can be used. 3.Coarse and fine approach mechanism. 4.No net relative motion between sample, cantilever and detection system. 5.Scanner range and type of encoder for large size scanner. 6.Data acquisition system,processing and display software. 7.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. Scanner Feedback A-B Pre-Amplifier AB Quadrant Photo Detector Tip & Cantilever Basic schematic for SPM To Z-Piezo Laser ADC DAC Need for calibration Keeping “something” constant, need for feed back X-Y scanner Z-scanner Coarse approach vs fine approach Pixels bits PID

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

22. Topography Can take care of image distortion 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 Soma Das (2008)

23. Mica Freshly cleaved 7 nm x 7 nm Calibration in atomic range- A freshly cleaved surface 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 Scanner Feedback A-B Pre-Amplifier AB Quadrant Photo Detector Tip & Cantilever To Z-Piezo Laser ADC DAC

26. Optical lever = = 500 -100(for l=100mm) Main components of the optical stage: 1.Laser diode 2.Cantilever 3.Quadrant photo-detector (QPD) 4.Collimating lenses 5.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 ?

27. Calibration of the optical stage. Region of Gradient: 1000  m 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 SrTiO 3 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: 1000  m 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. Often it is good to have a cantilever –tip rest on a surface and record the output as a function of time We have the “base” response of the QPD, need to enhance optical lever and reduce electrical noise to get better resolution

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

37. 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 Tip sample interaction model

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

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

40. 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. a 0 ~Atomic dimension (hard sphere) E * ~ Effective elastic constant R t - Tip radius of curvature. H=Hamakar cosntant

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

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

43. 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

44. A thumb rule to select cantilever in contact mode imaging 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 Correct condition for topography in contact mode The softer spring wins Will image the elastically deformed surface

45. 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

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

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

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

49. 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+(  2 U/  z 2 )

50. 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

51. Dynamics of cantilever

52. In dynamic mode spectroscopy the resonance curve and its modifications during imaging provides the image

53. what happens to resonance frequency in dynamic mode when there is additional force Start with a cantilever that is free Shift in resonance frequency when the interaction is turned on Force derivative is the important parameter in dynamic mode

54. NC Tapping

55. Force Derivative 55 NC Tapping

56. Two paradigms of dynamic mode Detection by amplitude modulation If the resonant frequency of a cantilever shifts, then the amplitude of cantilever vibration at a given frequency changes. Near a cantilever’s resonant frequency, this change is large.  Non-contact (tip does not touch the substrate,) - This also encompasses the EFM and MFM.  Tapping or IC mode (the tip touches the surface at some part of the swing)

57. The set frequency is somewhat larger than the free resonance frequency. Non-contact

58. IC/tapping-mode The set frequency is somewhat smaller than the free resonance frequency.

59. From simulation of data-what happens to the resonance curve in Tapping mode Das, Sreeram,AKR, Nanotechnology 18, 035501 (2007),Nanotechnology 21, 045706 (2010),Journal of Nanoscience and Nanotechnology 7, 2167 (2007)

60. Amplitude vs. distance curves for mica for three different free vibration amplitude of the cantilever. Sample: Mica K= 0.68N/m Resonance Frequency = 86KHz

61. Amplitude vs Height (in absence of feedback) 61

62. Application of Non-contact mode Magnetic Force Microscopy MFM Measuring long-range force Any force that decays slower than inverse square

64. This mode is realized by employing suitable probes (magnetic tip) and utilizing their specific dynamic properties. MFM is an important analytical tool whenever the near-surface stray-field variation of a magnetic sample is of interest. MFM can be used to image flux lines in low- and high-Tc superconductors. MFM have even extended local detection of magnetic interactions to eddy currents and magnetic dissipation phenomena.

65. The interpretation of images acquired by magnetic force microscopy requires some basic knowledge about the specific near-field magnetostatic interaction between probe and sample. How to take care of the topography ???

66. The magnetic stray field produced by a magnetized medium and the “contrast” mechanism The shift in frequency the MFM detects is the gradient of the magnetic force

67. Magnetic Force Microscopy of hard disk (No applied field) MFM maps the magnetic domains on the sample surface Stored data in a hard disk The stray field is maximum when the anisotropy is perpendicular

68. Magnetic Force Microscopy (with applied field)

69. Requirements for MFM tips These tips can be coated with a thin layer of magnetic material for the purpose of MFM observations. A lot of effort has been spent on the optimization of magnetic tips in order to get quantitative information from MFM data. The problem is that in the coating of conventional tips, a pattern of magnetic domains will arrange, which reduces the effective magnetic moment of the tip. The exact domain structure is unknown and can even change during MFM operation. Best tip is the one that has a single “mono-domain” magnetic particle !!!!!

70. Lorentz Microscopy of field around a tip

71. Effect of tip sharpness Stray field line scan Observed Simulated Ordinary tip Mono-domain tip In SFM, what ever you do the most significant role is played by the tip and the cantilever

72. I have tried to give a basic introduction to SFM and some of its different modes and shared my experience with you. SFM images are not just picture gallery The more knowledge you acquire and more quantitative you become you can get more value from your SFM. Thank you