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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.