Ling Zang, USTAR Prof.Department of Materials Science and EngineeringDirector, Utah Center of Trace Explosives Detection (UCTED) www.eng.utah.edu/~lzang

Organic Semiconductor Nanowires:

1D Enhanced Optoelectronic Properties

Applications in Vapor Sensing&

1D self-assembly through solution or surface processing

Zang et al. Accounts of Chemical Research,  2008, Special Issue on Nanoscience, vol. 41, pp1596-1608.

Advantages of Organic Materials:

• Unlimited choices of molecules: electronic

structure (color), configuration, size, shape …

• Easy to modify: chemical interactions.

• Flexible for processing: vapor, liquid/solution,

solid.

• Adaptable to various substrate.

• Cheap for manufacturing, processing,

packaging.

• …

Organic Semiconductors: perspectives and challengesnanowires, nanodevices, optoelectronic sensors, lasers, and

more…the Zang Research Group, Dept of MSE, University of Utah

Integrated for multi-target detection

Linearly polarized emission : single-nanobelt study by NSOM

J. Phys. Chem. B, 110 (2006), 12327-12332

Waveguide: just another 1D confinement

Chem. Mater. 21(2009) 2930-34.

Waveguide: just another 1D confinement

Chem. Mater. 21(2009) 2930-34.

Self-waveguide emission: dominated by exciton migration at elevated temperature

Lupton, Zang, et al. Nano. Lett. 11 (2011) 488-492.

300 K 4 K

Thermo-enhanced exciton diffusion

Waveguidingdominated

Lupton, Zang, et al. Nano. Lett. 11 (2011) 488-492.

Nanofiber: enhanced fluorescence sensing

illuminationFluorescenceemission

*

illuminationFluorescenceemission

* *

illuminationFluorescenceemission

* *TNT X

Charge transfer occurs betweenthe Excited state (exciton) and TNT

Long-range exciton migration enables amplification of fluorescence quenching: locally formed excited state can be quenched by an explosive molecule randomly adsorbed on surface.

Nanofibril film: for improved sensitivity

• Amplified emission quenching;

piling

Enhanced sensitivity

Zang et al. Accounts of Chemical Research,  2008, Special Issue on Nanoscience, invited.

• Continuous porosity expedient diffusion of gaseous molecules;

• Large surface area increased adsorption.

Efficient fluorescence quenching upon exposure to TNT vapor

CH3

NO2

NO2

O2N

TNT

5 ppb

J. Am. Chem. Soc. 129 (2007) 6978-6979

detection limit,< 10 ppt

Quenching efficiency independent on film thickness--- easy for manufacturing

Long-range exciton migration

+Cross-film diffusion

of explosives

Thickness independence

J. Am. Chem. Soc. 129 (2007) 6978-6979

15 30 45 60 75 900

20

40

60

80

100

Que

nchi

ng (

%)

Film Thickness / nm

CH3

NO2

NO2

O2N

TNT

CH3

NO2

NO2

DNT

Efficient fluorescence sensing of amines vapor

ON

O

O

O

O

1

amine

Nan

o L

ett

., 8

(2

00

8)

22

19

-22

23

Maximal adsorption produces maximal sensing sensitivity

continuous porosity

expedient diffusion of guest molecules

fast sensing response:

milliseconds

Nano Lett., 8 (2008) 2219-2223

Tubular fibrils for enhanced vapor sampling and trapping

0 200 400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0TNT vapor: 20-50 ppt

TNT vapor: 40-100 ppt

TNT vapor: 0.2-1.5 ppb

No

rm.

Time (s)

TNT vapor: 2-8 ppb

vapor input

Emission intensity of tubular fibrils in response to TNT

Emission quenching data from NRL vapor generator

0 400 800 1200 1600 2000

0.6

0.7

0.8

0.9

1.0

I

nte

nsi

ty (

No

rm.)

Time (s)

RDX vapor

(50 ppt - 3.2 ppb)

open to air

Emission intensity of tubular fibrils in response to RDX

Emission quenching data from NRL vapor generator

1D enhancement of electrical conductivity via cofacial p-electronic delocalization of doped charges

J. Am. Chem. Soc. 129 (2007) 6354-6355 and 129 (2007) 7234-7235.

Leading to a sensor for reducing reagents.

Current enhancement upon exposure to hydrazine vapor

+ -P T C D I na now ire

glass

Bare nanowire

The conductivity estimated: 1.310-3 S m-1, about 1 order of magnitude higher than that measured from polymer nanowires, e.g., polythiophene, F8T2.

hydrazine(140 ppm) ~ 103

The conductivity estimated: ca. 1.0 S m-1, about 3 order ofmagnitude higher than that ofundoped silicon, 1.610-3 S m-1.

J. Am. Chem. Soc. 129 (2007) 6354-6355

aminee-

Photo-doping via D-A charge separation to enhance the conductivity

J. Am. Chem. Soc. 132 (2010) 5743-5750.

Low conductivity for pristine organic semiconductor: neutral molecules, zero doping

zero charge carriers

Long axis of nanowire

Photo-doping of n-type nanowires via D-A charge separation

Photo

induce

dET

electrons

OO

OO

N

O N

NN

OO

OO

NN

OO

OO

NN

No E

T

Too f

ast

Just

rig

ht

Ph

oto

ind

uce

dE

THigh conductivity: balance between intra- and inter-molecular ET.

J. Am. Chem. Soc. 132 (2010) 5743-5750.

High 1D photo-conductivity

O

O

O

O

NN

N

On/off ratio > 1,000@low irradiation0.4 mW/mm2

J. Am. Chem. Soc. 132 (2010) 5743-5750.

0.3 mW/mm2

0.03

Vapor sensing through charge-carrier depletion

Photo

induce

dET

OO

OO

NN

N

electrons

explosives

Suited for sensing weak-oxidizing reagents that are difficult to detect by fluorescent sensors. J. Am. Chem. Soc. 132 (2010) 5743-5750.

Enhanced electrical vapor sensing via photo-doping

Fast blowing of nitro-methane vapor

volatile, weak-oxidizing, difficult to detect …

CH3O2N

Ideal sensor for vapor detection

High sensitivity or low detection limit: stand-off detection (> 50 m, ideally 100 m), trace TNT (40 ppt) over buried landmines.

Fast response: seconds, porous structure and continuous channel both enhancing the penetration of gaseous molecules into the film, strong chemical interaction (sticking) at interface improving the accumulation of target molecules within the film.

Stability: thermal damage, photobleaching, thick film desired for improved stability, sustainability, reliability and reproducibility.

Selectivity: against environment interferences. Cost effective: cheap for materials and processing, flexible for

materials modification and improvement, adaptable to various substrates for device fabrication --- all can be satisfied with organic materials.

Easy to use, minimal maintenance, …

ON

O

O

O

O

1

amine

Thinner Fibers for Enhanced Vapor Sensing

ChemComm. 2009, p5106.

diameter350 nm 40 nm

1E-3 0.01 0.1 1 10 100 10001E-4

1E-3

0.01

0.1

1

Vapor concentration (ppb)

5 ppt

0.1 ppb 1 ppb5 ppb

Qu

ench

ing

eff

icie

ncy

(1-I

/Io)

Enhanced Vapor Sensing of Aniline by shrinking down the size of fibers

350 nm nanofiber

Detection limit down to

a few ppt

40 nm nanofiber

TNT EtOH MeOH Acetone H2O2 NH3 Hexane0

20

40

60

80

100

I / I 0

% (

corr

. fo

r 2

% p

ho

tob

lea

chin

g)

Before exposure After exposure

Potential Interference from Common Liquids (10 s exposed to sat. vapor)

1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

1. TNT (5 ppb); 2. Pantene Pro-V Mousse; 3. Loreal Studioline Hair Spray; 4. Head&Shoulders 2 in1 shampoo; 5. Pond's dry skin cream; 6. Olay UV moisturizing lotion; 7. Neutrogena men face lotion SPF 20; 8. Colgate Total toothpaste; 9. Chanel Allure perfume; 10. Fendi Theorema perfume.

I / I 0

% (

corr

. fo

r 2

% p

ho

tob

lea

chin

g)

Before exposure After exposure

Potential Interference from Cosmetics (10 s exposed to sat. vapor)

TNT

1 2 3 4 5 60

20

40

60

80

100

1. TNT (5 ppb); 2. concentrated cigarette smoke; 3. car exhaust, 1 foot, Toyota Camry 1991, 7:00am; 4. car exhaust, 1 foot, Toyota Camry 2003, 5:00pm, sunny; 5. opening gas tank of Camry; 6. boiled water vapor.

I / I 0

% (

corr

. fo

r 2

% p

ho

tob

lea

chin

g)

Before exposure After exposure

Potential environmental interference (10 s exposure)

TNT