VEDA - Virtual Environment for Dynamic Atomic Force Microscopy

Transcription

1 VEDA - Virtual Environment for Dynamic Atomic Force Microscopy John Melcher, Daniel Kiracofe, doctoral students Steven Johnson, undergraduate Shuiqing Hu, Veeco Arvind Raman, Associate Professor Mechanical Engineering Birck Nanotechnology Center

2 Outline Introduction to dynamic AFM VEDA - dynamic approach curves tool VEDA - amplitude modulated d scanning tool VEDA- multi-mode (advanced) tool

3 A init Tapping mode AFM Z approach Scanning A A A ~h h Questions How to choose cantilever er stiffness? How to choose operating amplitude? What are the imaging forces? What does phase contrast mean? Influence of feedback on images? How to choose imaging controls? Do matl prop. influence images?

4 VEDA- Virtual Environment for Dynamic AFM Three tools released on 1 : Dynamic Approach Curves, Amplitude Modulated Scanning, Multi- mode Accurate numerical simulations Convenient input parameters Meaning of phase contrast Imaging forces Probe instabilities, stable scanning 1 J. Melcher, S. Hu and A. Raman,

5 Why VEDA? VEDA is free, sponsored by the NSF VEDA does not require you to download and maintain software (other simulators require simulink, matlab etc.) VEDA is cyber-enabled, run off the national teragrid, needs only a web-browser VEDA is extremely accurate, very few assumptions, fast FORTRAN recipes for stiff, nonlinear equations VEDA is supplemented by a detailed user manual, streaming video/ipod d learning modules Extremely versatile, many high end applications, simulations in liquids, bi-modal, higher harmonics all possible

6 Interaction forces in dynamic AFM Tip-sample interaction Force (F ts ) Nanosensors Gmbh Repulsive Tip-sample gap(d) Attractive Long-range electrostatic and magnetic forces (upto 100 nm) Capillary forces (few nm) Van der Waals forces (few nm) that are fundamentally quantum mechanical (electrodynamic) in nature Short-range chemical forces (fraction of nm) Elastic contact forces Electrostastic double-layer forces Solvation forces Nonconservative forces (Dürig (2003)) viscoelasticity and surface hysteresis included in VEDA Blue: included in VEDA

7 Microcantilever model in VEDA i th mode of microcantilever d(t) Z+x(t) k c, i Z ρ c, E c, I c, L c d(t) m i F ts (d(t)) ts( ()) x() t 2 Ω F(d) ts Fdrive cos(τ) Drive freq Ω x,ττ + x,τ+x= +, Ω= Q k k Re sonance freq c Nonlinear micromechanical oscillator model captures commonly observed dynamic AFM physics Accurate non-smooth, nonlinear simulations using DDASKR FORTRAN routine with root finding algorithm Probe driven near resonance and brought closer to/scanned over to sample Multi-mode tool allows simultaneous excitation of two modes c

8 How to access VEDA and supporting information Register on Look under simulate tab, and click c on Tools for NEMS and nanofluidics under which the VEDA tools appear. Click on any VEDA tool and click launch After launching the tool, click the tab About to access the comprehensive User manual Learning module (live streaming slide+audio) on VEDA also available. Under the Teach and learn tab click on more and select Resources:Learning modules scroll to find the Learning module for VEDA (I-Pod version also available)

9 Outline Introduction to dynamic AFM VEDA - dynamic approach curves tool VEDA - amplitude modulated d scanning tool VEDA- multi-mode (advanced) tool

10 Example 1 While imaging a polymer sample (E*=1GPa) with a tapping mode cantilever in air k=20 N/m, freq=250khz, Q=200), what initial amplitude and setpoint ratio to choose to keep the imagingi forces<5 nn? Answer Only meaningful way is to remain in the attractive regime of oscillation, so say at 15nm free amplitude, if the setpoint ratio is >90%

11 Outline Introduction to dynamic AFM VEDA - dynamic approach curves tool VEDA - amplitude modulated d scanning tool VEDA- multi-mode (advanced) tool

12 Example 2 While imaging a Silicon sample with trench with a tapping mode cantilever in air k=20 N/m, freq=250khz, Q=200, Ainit=30nm, 90% setpoint), how do the proportional and integral gains effect imagingi error? Answer Primary effect is from proportional gain, too small means insensitivity to topography, toolarge and feedback loop can become unstable

13 Outline Introduction to dynamic AFM VEDA - dynamic approach curves tool VEDA - amplitude modulated d scanning tool VEDA- multi-mode (advanced) tool

14 Example 3 How does a magnetically excited cantilever (slow stiffness) tap on a soft biological sample (0.1 GPa) in aqueous buffer? What are the interaction forces, contact times, and how do they dependd on amplitude stpoint? t? Answer In liquids the two-mode model must be used to understand the cantilever response, and the peak forces and contact times behave quite differently when compared to operation in air

15 Possible additions to VEDA Capability to enter user specific interaction laws Frequency modulated AFM Longer term- magnetic, electrostatic force microscopy Coupling with molecular dynamics simulations of tip-sample interactions Other ideas? Please

16 How to access VEDA and supporting information Register on Look under simulate tab, and click c on Tools for NEMS and nanofluidics under which the VEDA tools appear. Click on any VEDA tool and click launch After launching the tool, click the tab About to access the comprehensive User manual Learning module (live streaming slide+audio) on VEDA also available. Under the Teach and learn tab click on more and select Resources:Learning modules scroll to find the Learning module for VEDA (I-Pod version also available)

17 Questions? J. Melcher, S. Hu and A. Raman, Cover feature, Rev. Sci. Inst, 2008