1

Charge Transport and Chemical Charge Transport and Chemical Sensing Properties of Organic Sensing Properties of Organic

Thin-filmsThin-films

Richard Yang Richard Yang Material Science & Engineering

University of California, San Diego06/12/2007

2

Project BackgroundProject Background

AFOSR MURI: Integrated nanosensors for bio/chemical warfare and explosive agents detection.

Organic thin-film chemical sensors• Chemiresistors• ChemFETs

Design objectives• Sensitivity, Stability, Selectivity• Integration in sensor platform

Conceptual design (2003)

3

Research ApproachResearch Approach

Charge transport

Device p

hysics

Chemical sensing

4

Results Before CandidacyResults Before Candidacy

• Analyte identification based on dispersive charge transport

• Electrode independent chemical responses in SCLC regime

10-1 100 101 102 103 104 105 106

-14

-12

-10

-8

-6

-4

-2

0

2 Methanol

G/G

(%

)

Frequency (Hz)

0 ppm 380 ppm 950 ppm 1900 ppm 9500 ppm 19000 ppm

Appl. Phys. Lett., 88 (2006) 074104 J. Phys. Chem. B, 110 (2006) 361

The above results were based on two-terminal chemiresistors.

5

Chemically Sensitive Field-Effect Chemically Sensitive Field-Effect TransistorsTransistors

Advantages of ChemFETs as compared to chemiresistors:• High chemical sensitivity and stability• High electrical conductivity, therefore, may utilize very thin films

Silicon Substrate (n+ )

S D

G

Gate dielectric

Vg

+ + + + + + + + +

Id

Vd

Ground

gasOrganic semiconductor thin-film

6

Device FabricationDevice Fabrication

25 m

Photolithography, e-beam evaporation, lift-off process

Organic thin-film deposited using molecular beam epitaxy

• Film thickness: 5 - 50 nm• Growth rate: 0.2 – 1 Å/sec • Growth temperature: 20 – 200 0C

Metal Phthalocyanine (MPc)

Metal center: Cu, Co, Fe etc

Silicon Substrate (n+ )

SiO2

S

G (Au)

D

7

Device CharacteristicsDevice Characteristics

0 -1 -2 -3 -4 -5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

Source-drain Voltage (V)

Dra

in C

urr

ent

(A

)Gate Voltage = -5 V

-4V

-3 V

-2 V

0 V

1, 2 V

30 nm CuPc/ 50 nm SiO2

• Low leakage current• Ideal FET behavior

• Small threshold voltage • Low operating voltage

Low voltage operating ChemFET has been fabricated ( since Feb 2005).

8

CuPc OTFT Characteristics in LiteratureCuPc OTFT Characteristics in Literature

The operation voltages are 10 times too high for ChemFET applications.

J. Appl. Phys. 92, 6028 (2002)Appl. Phys. Lett. 69, 3066 (1996)

SiO2 thickness = 300 nm

9

Fabrication Issues - Gate LeakageFabrication Issues - Gate Leakage

0 -2 -4 -6 -8 -10

0

-2

-4

-6

-8

-10

-12

-14

Ig (A

)

Vds (V)

Vg-14 V

+2 V

Silicon Substrate (n+ )

50 nm SiO2

Au

G (Au)

Au

Gate leakage problem persisted in first 3 months

Leakage sources and solutions:• Defective gate oxide: solved by careful growth and inspection• PR erosion by HF during backside SiO2 etching: solved by

developing BOE etching

Back gate process• Protect gate dielectric with PR• Dip into HF solution to remove

backside SiO2

• E-beam evaporation of Au

10

Fabrication Issue - Contact ResistanceFabrication Issue - Contact Resistance

0 -2 -4 -6 -8 -100.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

Ids

(A

)

Vds (V)

50 nm CuPcVg = -14 V

-10 V

-8 V

-6 V

-4 V

-2 V

0 V2 V

-1 V

Contact resistance limits current injection

Source and solution:• Residual PR forms hole injection blocking layer: solved by

developing cleaning procedure (three cycles of ultrasonication in trichloroethylene/ acetone/ isopropyl alcohol))

Residual PR

11

0 -2 -4 -6 -8 -10

0

-1

-2

-3

-4

-5

-67.5 micron channel

Source-Drain Voltage (V)

Dra

in C

urr

ent

(A

)

0 -2 -4 -6 -8 -10

0

-2

-4

-6

-8

-10

-1210 micron channel

0 -2 -4 -6 -8 -10

0

-1

-2

-3

-4

-5

-615 micron channel

0 -2 -4 -6 -8 -10

0

-1

-2

-3

-4

-5

-620 micron channel

Vgs = -10 V

Vgs = +4 V

-2 V/step

nw/L=20,400 +/-1,200 nw/L=30,300

nw/L=14,666 nw/L=15,000

25ML CuPc Thin-films

Device ScalingDevice Scaling

12

5 10 15 20 25

-10

-20

-30

-40

-50

-60

-70

-80

Vg = -4 V

Vg = -6 V

I DS

/(nW

) (A

*m

)

Channel Length (m)

Vg = -8 VSaturated region: V

ds = -10 V

Linear fits with R > 0.9

2

d,sat 2 tgi

WI C V

LV

Linear Scaling of Current with Channel Linear Scaling of Current with Channel LengthLength

13

Charge Transport in Organic TransistorsCharge Transport in Organic Transistors

+ Localizedstates

EF

+ +

Delocalized valence band

Delocalized conduction band

p-type organic semiconductor

Trapping and release

Transport in delocalized band

G. Horowitz, M. E. Hajlaoui, and R. Hajlaoui, J. Appl. Phys. 87, 4456 (2000).

Trap energy distribution determines the device characteristics

Multiple trapping and release (MTR)

0 0 exp aeff

E

kT

eff= effective mobility0= free carrier mobility= free to total charge ratioEa = trap activation energy

14

Variable Temperature StudyVariable Temperature Study

8d

m Vds Vg

Ig

V

• Transconductance

0 exp( )am m

B

Eg g

k T

• Activation energy

• The charge transport is thermally activated. • The activation energy depends on the gate voltage.

15

Baseline Drift Reduction in OTFTsBaseline Drift Reduction in OTFTs

-2 0 2 4 6 8 10 12 14 16 18 20 22

0.4

0.6

0.8

1.0

No

rmal

ized

Id

Vg = -8 V, pulsing

Vg = -4 V, static

Time (hr)

Vg = -8 V, static

0 20 40 60 80 100 120 1400.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Duty Cycle

No

rma

lize

d I

d

Time (minute)

1% 2% 5% 10% 20% 100%

(a)

100 1000 10000

0

5

10

15

20

25

30

Dri

ft (

%)

Gate Bias Duration (ms)

threshold time

(b)

• Static gate operation reduce drain current 40% in 20 h• Pulsed gating (0.1 Hz, 1% duty cycle) reduce the drift to less than 1% in 20 h• There is threshold pulse duration in the baseline drift

16

Gate pulse train

t

Ev

Vg = 0 V

Ec

Ef

SiO2

OffState

Ef

Ev

SiO2Vg = -8 VEc

t

OnState

•A pulse train from “off” to “on” state is applied.

•Break lines represent trap states located near SiO2 interface and in the bulk.

•“Off State” – at flat band condition, no charge accumulation in the channel.

•“On State” – holes accumulate at the dielectric interface. There is finite amount of time (t) for the holes get trapped.

Pulsed Gating OperationPulsed Gating Operation

17

-2 0 2 4 6 8 10 12 14 16 18 20 22 24

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

(d)

No

rmal

ized

Dra

in C

urr

ent

Time (hr)

(a)

(b)(c)1900 ppm methanol pulses

(a) 1% 0.1 Hz gate with methanol

(b) static gate with methanol

(c) static bias without methanol

(d) 1900 ppm methanol pulses

20 ML CuPc

• Pulsed gating reduced the baseline drift to 0.09 + 0.016 %/h in exposure to 15 methanol pulses.

• Pulsed gating reduced the error in chemical response by 10%.

Baseline Drift to Volatile VaporsBaseline Drift to Volatile Vapors

18

0 4 8 12 16 20 240.75

0.80

0.85

0.90

0.95

1.00

No

rmal

ized

Dra

in C

urr

ent

Constant flow gas pulses

Time (h)

(a)

(b)

(c)

(a) 1% 0.1 Hz gate with 32 ppm DMMP

(b) 1% 0.1 Hz gate with 19 ppm DIMP

(c) Analyte pulse sequence

• Chemical source of baseline drift has been tested with low vapor pressure analytes

•There is 10% baseline drift due the tight binding of analytes

Baseline Drift to Low Vapor Pressure Baseline Drift to Low Vapor Pressure AnalytesAnalytes

19

0 4 8 12 16 20 24 28

1.00

0.95

0.90

0.85 (b)

No

rmal

ized

Id

Time (h)

0 4 8 12 16 20 24 28

0

10

20

30

(iii)

(ii)

(i)

DIM

P (

pp

m)

(a)

(iii)(ii)(i)

(a) 20% DIMP duty cycle

(b) 15% DIMP duty cycle

(c) 8% DIMP duty cycle

• Even in the presence of very low volatility analytes, the drift can be reduced to zero by lowering the duty cycle of the analyte pulse.

Chemical Drift ReductionChemical Drift Reduction

20

Physical Structure Based Sensing ModelPhysical Structure Based Sensing Model

T. Someya, et. al. APL, 81, 3079 (2004)L. Torsi, et. al., Ana. Chem. 77, 308 A (2005)

Assumptions

• Film mobility is determined by traps located at grain boundary (GB)

• Analytes adsorbed at grain surface change the GB barrier height EB and therefore change device mobility and threshold voltage

Grain boundary model

Limitations

• No definite proof of trap state locating at GBs in organic films by SKPM

• Weak correlation of chemical response with grain size

• Electronic effect of oxygen doping ignored

EB: Charge trapping barrier

Polycrystalline pentacene film

21

Scanning Kelvin Probe MicroscopeScanning Kelvin Probe Microscope

200 nm200 nm

Topography color scale: 20nm Potential color scale: 50mV

The potential drop between GBs is less than thermal energy.

Data acquired by Xiaotian Zhou

22

Evidences of Oxygen DopingEvidences of Oxygen Doping

• CuPc and F16CuPc sensing films out of vacuum are doped by oxygen • Oxygen is an acceptor-like dopant as it withdraws electron from phthalocyanines• Displacing oxygen reduces p-channel device current, while increases n-channel device current

0 10 20 30 40 50

1.6

1.8

2.0

2.2

-2.6

-2.8

-3.0

-3.2

-3.4

-3.6p-type

p-c

han

nel

I d (A

)

n-c

han

nel

I d(

A)

Time (h)

n-type

23

Electronic Model of Chemical Sensing Electronic Model of Chemical Sensing

• Organic sensing films are doped by chemisorbed oxygen once outside of vacuum.

• The surface layer has higher dopant concentration.

• Chemical analytes adsorption on film surface has 2 effects:– Surface doping level change due to oxygen displacement– Trapping energy change due to new energy states formed by analyte

adsorption charge transfer Ionization

Si CuPc Air

x0

s

Ec

Ev

Ef -2 2O /O

AirSiO2O2

“delta-doping”

31 2 kk k + - - +2 2 2 2MPc + O MPc-O MPc -O MPc-O +h

24

Chemical Sensing MechanismsChemical Sensing Mechanisms

-2 0 2 4 6 8 10121416182022242628301.20

1.25

1.30

1.35

1.40

1.45

1.50

0 60 120 180

1.34

1.35

1.36

1.37

1.38

1.39

I(A

)

MeOH (1520 ppm)

p-channel

DMMP (68 ppm)

Ab

s (

I d)

(A

)

Time (h)

n-channelI

25

The Exponential DecaysThe Exponential Decays

0 2000 4000 6000 8000 10000 120001.34

1.36

1.38

1.40

1.42

1.44

1.46

Equation: y = A1*exp(-x/t1) + y0Weighting: y No weighting Chi^2/DoF = 2.1604E-6R^2 = 0.9964 y0 1.33026 ±0.0006A1 0.11914 ±0.00048t1 7084.0095 ±73.686

(a). n-channel: DMMP

0 2000 4000 6000 8000 10000 12000-1.045

-1.040

-1.035

-1.030

-1.025

-1.020

-1.015

(b). p-channel: DMMP

Equation: y = A1*exp(-x/t1) + y0Weighting: y No weighting Chi^2/DoF = 1.3636E-7R^2 = 0.99732 y0 -1.04583 ±0.0001A1 0.03262 ±0.00008t1 5736.22502 ±42.75252

0 2000 4000 6000

1.35

1.35

1.36

1.36

1.37

1.37

1.38

1.38

1.39Equation: y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0Weighting: y No weighting Chi^2/DoF = 8.5004E-7R^2 = 0.99005 y0 1.35052 ±0.00026A1 -0.01143 ±0.00066t1 57.13782 ±5.90263A2 0.03778 ±0.0002t2 2331.16822 ±44.41452

(c). n-channel: MeOH

I d (

t) (A

)

Time (second)

raw data Exponetial fit

0 2000 4000 6000-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

(d). p-channel: MeOH

Equation: y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0Weighting: y No weighting Chi^2/DoF = 1.5898E-7R^2 = 0.99774 y0 -1.04404 ±0.00007A1 0.04283 ±0.00025t1 189.38934 ±2.13073A2 0.01364 ±0.00019t2 1622.09746 ±43.98648

26

Concentration DepedendentConcentration Depedendent

0 2 4 6 8 10 12 14 16 18

1.36

1.38

1.40

1.42

1.44

1.46

0

10

20

30

40

50

Dra

in C

urr

ent

(A

)

Time (h)

Co

nc.

(p

pm

)

(a). n-channel: DMMP

-2 0 2 4 6 8 10 12 14 16 18

-1.02

-1.03

-1.03

-1.04

-1.04

-1.05

-1.05

-1.06

50

40

30

20

10

0

Dra

in C

urr

ent

(A

)

Time (h)

Co

nc.

(p

pm

)(b). p-channel: DMMP

0 10 20 30 40 50 600

15

30

45

60

75

90

105

p-channel

I (

nA

)

Concentration (ppm)

n-channel

1exp bEd I

SI c kT

27

Binding to a Weak BinderBinding to a Weak Binder

0 500 1000 15000

15

30

45

60

75

n-channel

I

(nA

)

Concentration (ppm)

p-channel

0

500

1000

1500

2000

2000

1500

1000

500

0

0 2 4 6 8 10 12

-1.36

-1.38

-1.40

-1.42

-1.44

-1.46(b). n-channel

0 2 4 6 8 10 121.34

1.36

1.38

1.40

Co

nc

en

tra

tio

n (

pp

m)

(a). n-channel

Time (h)

Dra

in C

urr

ent

(A

)

28

Ultrasensitive Sensor DesignUltrasensitive Sensor Design

In conventional OTFT sensors (> 10 nm), the chemical sensing and charge transport interfaces are separated.

Merge the 2 interfaces: ultrasensitive ChemFET design

29

Chemical Response ComparisonChemical Response Comparison

Appl. Phys. Lett., In Press

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0

4

8

12TE/44

EA/150

MeOHC

onc

(ppm

)

Time (h)

DIMP/1.9

NB/0.35

MeOH/190

Air

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0-2-4-6-8

-10

EA TE

AirDIMP NBMeOH50 ML CoPc

Res

pons

e (%

)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0-2-4-6-8

-10-12-14-16

EA TE AirNBDIMP

4 ML CoPc

30

Chemical Sensitivity EnhancementChemical Sensitivity Enhancement

0 exp aE

kT

0 is related to carrier density

Ea is the trap energy at CoPc/SiO2 interface

0 1 2 3 40

2

4

6

8

10

12

14

16

Ethyl acetate2.2

Sen

siti

vity

En

han

cem

ent

Dipole Moment (Debye)

Toluene1.7

MeOH4.0

Nitrobenzene15.8

DIMP3.62

Effective field-effect mobility

In the ultrathin device, the air/CoPc and CoPc/SiO2 interfaces are so close that analytes affect both carrier density and trap energy.

Sensitivity enhancement has been observed on all 5 analytes.

31

Detection of Nitrobenzene VaporsDetection of Nitrobenzene Vapors

0 1 2 3 4 5 6 7 8 9 10 11 12

0.0

0.5

1.0

Flo

w R

ate

(sc

cm)

Time (hr)

0.1 0.2

0.60.8

1.00 1 2 3 4 5 6 7 8 9 10 11 12

-2.79

-2.70

-2.61

I d (A

)

Time (hr)

Vg = - 8 VVds = -4 V

35 ppb 70 ppb

210 ppb

350 ppb280 ppb

70 ppb nitrobenzene has been detected without a precentrator

Simulant for TNT

Nitrobenzene

32

2004

2005

2006 – 6 Pack

2004: Three parallel electrodes

2005: Interdigitated electrodes

2006: 6 pack ChemFET for an e-nose.

2006: Handheld package. On-board integration of temperature, humidity sensors and current amplifier.

2006 – 8 Packwith a blower in handheld package

Project EvolutionProject Evolution

33

Wireless Handheld PackageWireless Handheld Package

Labview interfaceLabview interface

Vapor sampling with a vacuumVapor sampling with a vacuum

Integrated PCBIntegrated PCB

Sensor enclosureSensor enclosureBlue tooth transmitterBlue tooth transmitter

34

SummarySummary

• Process of low voltage operating and repeatable ChemFETs have been developed.

• Trap states are found to dominate charge transport in organic transistors.

• Pulsed gating technique has been developed to reduce drift to less than 0.1%/h in ChemFETs.

• A ChemFET sensing model has been developed: gas adsorption on organic semiconductor surface changes both doping concentration and trap energy.

• Ultrasensitive ChemFETs have been developed to detect explosive simulant at ppb level.

• The project has evolved from discrete device to integrated circuits to handheld packages.

35

AcknowledgementsAcknowledgements

Committee Members:• Prof. Andrew Kummel (Chair)• Prof. Sungho Jin (Co-chair)• Prof. Yu-Hwa Lo• Prof. William Trogler• Prof. Edward Yu

Collaborators (MSE, Chemistry, Physics)Jeongwon Park, Xiaotian Zhou, Corneliu Colesniuc, Dr. Karla Miller, Dr. Amos Sharoni and Dr. Thomas Gredig

Funding from AFOSR MURI

Undergrads (ECE, CSE and MAE)Ti, Tammy, Kate, Casey, Jordan, Sureel, Byron and Vince

36

Education/Research BackgroundEducation/Research Background

B.S. B.S. Chemical EngineeringChemical Engineering

M.S. M.S. Surface ChemistrySurface Chemistry

Ch

emistry

Ch

emistry M.S. M.S.

Advanced MaterialsAdvanced Materials

Research Offer Research Offer Inst. of Mater. Res & EngInst. of Mater. Res & Eng

Materials

Materials

Ph.D. Ph.D. Materials Science & EngineeringMaterials Science & Engineering

37

Chemical Response of p and n ChannelsChemical Response of p and n Channels

-20 -15 -10 -5 0 52

0

-2

-4

-6

-8

-10

-12

-14

-16

-18 CuPc 50nm, L = 5 micron

Dra

in C

urr

ent

(A

)

Gate Voltage (V)

air DIMP

Vds = -6 V25 20 15 10 5 0 -5

0

1

2

3

4

5

6

7 F16CuPc 50nm, L = 5 micron

Dra

in C

urr

ent

(A

)

Gate Voltage (V)

air DIMP

Vds = +6 V

p-channel n-channel

Analyte adsorption changes free carrier concentration and trap energy.

190 ppm DIMP

EEoxygenoxygen

EEcc

EEvv

EEffEEDIMPDIMP

EEcc

EEvv

EEff EEDIMPDIMP

EEoxygenoxygen

CuPc F16CuPc

38

Role of Oxygen in SensingRole of Oxygen in Sensing

-2 0 2 4 6 8 10 12 14 16 18 20-0.6-0.7-0.8-0.9-1.0-1.1-1.2

air(b). p-channel

Dra

in C

urr

ent

(A

)

Time (h)

N2

-2 0 2 4 6 8 10 12 14 16 18 201.31.41.51.61.71.81.92.0

air

N2(a). n-channel

0 2 4 6 8 10

-0.7

-0.8

-0.9

-1.0

-1.1

N2

(b). p-channelair

Dra

in C

urr

en

t (

A)

Time (h)

0 2 4 6 8 10

1.4

1.5

1.6

1.7N

2

air

(a). n-channel

Not direct displacement.Not direct displacement.A mixture of the co-A mixture of the co-adsorption and remote adsorption and remote adsorptionadsorption

39

Macroscopic View of Charge TransportMacroscopic View of Charge Transport

•Low voltage region, Ohmic conduction

0J N ed

V

• High voltage region, space-charge limited conduction

2

3

9

8 dJ

V

J = current densityN0= thermal carrier concentration = permittivity of materialV = voltage biasd = film thickness

40

Scanning Kelvin Probe MicroscopeScanning Kelvin Probe Microscope

An oscillating voltage is applied on the cantilever tip, Vac sint, which creates an oscillating electrostatic force at the frequency

( sin ( ))2 dc ac

dCF V V t x

dz

When Vdc = (x) , the cantilever feels no electrostatic force, the surface potential x is recorded as the tip voltage.

First scan: topography

Second scan: potential

SKPM:SKPM:Surface potential, electrical field and charge distributionSurface potential, electrical field and charge distribution

41

Microscopic View of Charge TransportMicroscopic View of Charge Transport

( )d xE x

dx

• Ohmic conduction (low voltage): linear V(x) and uniform E(x) in the channel. No net charge in the film.

• SCLC (high voltage): parabolic V(x) and non-uniform E(x) as a consequence of space charge buildup.

42

Electrode Independent Chemical Electrode Independent Chemical Response in SCLC RegionResponse in SCLC Region

J. Phys. Chem. B, 110 (2006) 361

• At high voltage, chemical response is independent of contact and historyAt high voltage, chemical response is independent of contact and history• The interface traps are filled up that do not affect chemical sensingThe interface traps are filled up that do not affect chemical sensing

43

Impedance SpectroscopyImpedance Spectroscopy

Input Output

J. Phys. Chem. B, 110 (2006) 361

(( )) Rv

iXZ i

1Resistance: R

G( )

1

Reactance: XC

• Low and high frequency semicircles co-exists

• The low frequency semicircle deceases with increasing field

• The 2 semicircles relate to interface and bulk traps

44

Analyte Identification Using ImpedanceAnalyte Identification Using Impedance

10-1 100 101 102 103 104 105 106

-14

-12

-10

-8

-6

-4

-2

0

2 Methanol

G/G

(%

)

Frequency (Hz)

0 ppm 380 ppm 950 ppm 1900 ppm 9500 ppm 19000 ppm

• AC conductance change (> 10kHz) is independent of methanol concentration above 950 ppm.

• DC conductance changes linearly with concentration.

(( )) ac

ac

iY

vCG i

AC conductivity

Differential AC conductance on 50 nm CoPc thin film w/o analyte

Input Output

G () AC conductance.

C () capacitance.

45

AC Conductance vs. ConcentrationAC Conductance vs. Concentration

10-1 100 101 102 103 104 105 106-12

-10

-8

-6

-4

-2

0

2

4Isopropanol

G/G

(%

)

Frequency (Hz)

525 ppm 1050 ppm 4200 ppm 5250 ppm 21000 ppm

10-1 100 101 102 103 104 105 106-10-8-6-4-202468

1012

Ethanol

G/G

(%

)

Frequency (Hz)

275 ppm 850 ppm 4250 ppm 8500 ppm 17000 ppm

• AC conductance change is concentration independent for ethanol and isopropanol above critical levels.

• There are distinct binding sites with different analyte absorption energies, which can be used for analyte identification.

Appl. Phys. Lett., 88 (2006) 074104

46

Resonance Frequency Detection Resonance Frequency Detection

1( )( )Z i

YXR

1

Reactance: - LXC

Dissipation factor

DFX

R

Impedance Spectroscopy

11.5 11.6 11.7 11.8-500

-400

-300

-200

-100

0

100

200

300

400

500

600

700

Dis

sip

ati

on

(a

.u.)

Frequency (kHz)

Air

Methanol

NitrobenzeneDIMP

(1900 ppm)

(19 ppm)(2 ppm)

103 104 105103

104

Frequency (Hz)

X (

w)

103 104 105

-100000

-50000

0

50000

Frequency (Hz)

X

()

Frequency (Hz)

0

0X

Resonance

Frequency (Hz)

Dis

sip

atio

n

(a.u

.)

Low High

HighLow

Appl. Phys. Lett., 88 (2006) 074104

47

Summary – 2 Terminal DeviceSummary – 2 Terminal Device

• Charge transport in organic thin-film is Ohmic at low field and SCLC at high field.

• Operating Chemiresistors in SCLC region gives contact independent chemical responses.

• There are co-existence of low frequency and high frequency transport states in organic thin-film.

• An impedance spectroscopy technique has been developed to identify chemical analytes based on dispersive charge transport.