Slide #

Vacsin0t

VDC

Tip sample capacitive force,

Fcap=F0+F+F2

Displacement,

a= (F/k) Q

• The total capacitive force,

G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105 (2007).

NO CANTILEVER COATING

1

Only component is extracted

Slide #

VACsin0t

VDC

The displacement Amplitude,

k

QVφV

z

Cacdc

G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105 (2007).

2

When molecules adsorb on the surface

Slide #G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys.Lett. 90, 173105 (2007).

3

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.

Slide #4

Molecular Adsorption

Surface Work Function Change

Capacitive Force (F) Change

Oscillation amplitude Change

Transduction

Slide #

Sample Functionalization:

Less complicated No need to replace the cantilever Single cantilever can oscillate over

different sensing layers and detect

Only surface property change matters:

SWF is a surface electrical property. Hence, bulk conductivity is does not affect the response

Sensitivity can be tuned:

By spring constant, k; external voltages Vac and Vdc; the cantilever sample separation, d

5

k

QVφV

z

Ca acdc

Slide #

AFM BASED EXPERIMENTAL SETUP

GAS FLOW FIXTURE

Surface Work Function measurements are performed Optical transduction is applied through laser and PSPD Sensing was performed in ambient conditions

6

GAS FLOW

Slide #

G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys. Lett. 90, 173105 (2007).

Maximum possible change with H2 : 0.9 eV

H2 SENSING USING PLATINUM THIN FILM

7

H2 : Electron Donating Molecule

Gas ON

Slide #

G. Koley, M. Qazi, L. Laxminayaranan, and T. G. Thundat, "Gas Sensing using Electrostatic Force Potentiometry", Appl. Phys. Lett. 90, 173105 (2007).

8 ppm H2 detected

Hydrogen concentration in Air : 0.55 ppm

H2 SENSING USING PLATINUM THIN FILM

8

SWF based sensing is possible on metallic layers where conductance change measurement is not possible

H2 : Electron Donating Molecule

Slide #9

Si piezoresistive microcantilever

9

n – type (001) Si

Piezoresistors longitudinal πl , transverse πt

yL

HEVVG 2

3

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

1111070)(*2/1 PaTL

R3R2

R1 R4

R1R2R4

R3

ttlloR

R

Piezoresistors in [110] or [110]

Tip bias->oscillate microcantilever

Slide #

0

5 10-7

1 10-6

1.5 10-6

2 10-6

0 50 100 150 200 250 300

Str

ain

Position along length (m)

0

5 104

1 105

1.5 105

2 105

2.5 105

3 105

3.5 105

0 50 100 150 200 250 300

Str

ess

(P

a)

Position along length (m)

10

Finite Element Simulations

Stress and strain analysis

10

Force applied at the center

x

Analytical

COMSOL

Strain

Deviation from analytical

results due to finite

dimensions of the

cantilever

Stress

L = 300 µm

W = 140 µm

H = 2 µm

E = 179 GPa

F = 100 nN

Ρ = 2330 kg/m3

Ex F

WH

xLx 2

)(6

Analytical

COMSOL

Slide #11

Min : 0 µm

Min: 6.895e+4 Pa

Max : 3.75e+7 Pa

Max : 5.475 µm

Stress distribution

Deflection distribution

Deflection distribution

Stress distribution

3D Stress and deflection distribution analysis

Base

Base

BaseBase

R3

R1

R2 R4

R3

R2

R1

R4

L = 300 µmW = 140 µmH = 2 µmE = 179 GPaF = 10 μN Ρ = 2330 kg/m3

Regular cantilever Stress-enhanced cantilever