Ling Zang, USTAR Prof. Department of Materials Science and Engineering Director, Utah Center of Trace Explosives Detection (UCTED) www.eng.utah.edu/~lzang Organic Semiconductor Nanowires: 1D Enhanced Optoelectronic Properties Applications in Vapor Sensing &
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Ling Zang, USTAR Prof.Department of Materials Science and EngineeringDirector, Utah Center of Trace Explosives Detection (UCTED) www.eng.utah.edu/~lzang
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
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)
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)