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V ac sin  0 t

MC based Potentiometric detection. NO CANTILEVER COATING. Displacement, a  = (F  /k) Q. The total capacitive force,. V ac sin  0 t. V DC. Tip sample capacitive force, Fcap=F0+F +F2. Only  component is extracted.

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V ac sin  0 t

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  1. MC based Potentiometric detection NO CANTILEVER COATING Displacement, a= (F/k) Q • The total capacitive force, Vacsin0t VDC Tip sample capacitive force, Fcap=F0+F+F2 Only  component is extracted G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105(2007).

  2. (....continued)  The displacement Amplitude, VACsin0t When molecules adsorb on the surface VDC  G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105(2007).

  3. Example problem Assume a noise limited deflection sensitivity of 0.1 nm at resonance, calculate the work function resolution that can be detected. What is the fixed charge density that can be sensed? Assume Q = 50, Vac = 5 V rms, and k = 0.1 N/m. G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105(2007).

  4. MC based Potentiometric detection: Principle Molecular Adsorption Surface Work Function Change Capacitive Force (F) Change Oscillation amplitude Change Transduction

  5. Advantages of potentiometric detection Technique: • Sample Functionalization: • Less complicated • No need to replace the cantilever • Single cantilever can oscillate over different sensing layers and detect • Sensitivity can be tuned: • By spring constant, k; external voltages Vac and Vdc; the cantilever sample separation, d • Only surface property change matters: • SWF is a surface electrical property. Hence, bulk conductivity is does not affect the response

  6. Potentiometric detection set up AFM Based Experimental Setup Gas Flow • Surface Work Function measurements are performed • Optical transduction is applied through laser and PSPD • Sensing was performed in ambient conditions Gas Flow Fixture

  7. Gas sensing using metallic layer H2 sensing using platinum thin film H2 : Electron Donating Molecule Gas ON Maximum possible change with H2 : 0.9 eV G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys. Lett. 90, 173105(2007).

  8. (…continued) H2 sensing using platinum thin film H2 : Electron Donating Molecule • 8 ppm H2 detected • Hydrogen concentration in Air : 0.55 ppm • SWF based sensing is possible on metallic layers where conductance change measurement is not possible G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys. Lett. 90, 173105(2007).

  9. Si piezoresistive microcantilever Tip bias->oscillate microcantilever R3 R4 R2 R1 R1 R4 n – type (001) Si R2 R3 Piezoresistors in [110] or [110] Piezoresistors  longitudinalπl , transverse πt Readout of Wheatstone bridge VG y: deflection; ξ: average piezoresistive coefficient ; F: force ; V: applied voltage on Wheatstone bridge; E: Young’s modulus, L: length, H: thickness 9

  10. Finite Element Simulations Stress and strain analysis L = 300 µm W = 140 µm H = 2 µm E = 179 GPa F = 100 nN Ρ = 2330 kg/m3 Force applied at the center x Stress Strain Deviation from analytical results due to finite dimensions of the cantilever Analytical COMSOL Analytical COMSOL 10

  11. 3D Stress and deflection distribution analysis Stress-enhanced cantilever Regular cantilever Deflection distribution Deflection distribution L = 300 µm W = 140 µm H = 2 µm E = 179 GPa F = 10 μN Ρ = 2330 kg/m3 Max : 5.475 µm Base Base Min : 0 µm Stress distribution Stress distribution Max : 3.75e+7 Pa R3 R3 R2 Base R4 R2 R4 Base R1 R1 Min: 6.895e+4 Pa

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