Using Nanoconfinment to Tailor Semiconducting Polymers: New Generation
Optoelectronics

B. G. Sumpter^1^, P. Kumar^2^, A. Mehta^3^, M. D. Barnes^3^, W. A.
Shelton^1^, R. J. Harrison^1^

^1^ Computer Science and Mathematics Division

^2^ Department of Chemistry, University of Tennessee

^3^ Chemical Sciences Division

Oak Ridge National Laboratory

Oak Ridge, TN 37831

**Summary and Conclusions**

Semiconductors and other inorganic crystals are the basis for
electronics and other technologies, but aside from small changes
available by doping with impurities, their chemical properties are
fairly inflexible. Soft materials such as polymers, on the other hand,
have almost unlimited possibilities since the chemical repeat groups can
be modified to suit a particular application. However, commonly used
techniques capable of producing the needed types soft-materials
structures such as thin-film or self-assembly processes suffer from
substrate and other molecular interactions which may dominate or obscure
the underlying polymer physics. In order to minimize these complexities
we have recently explored ink-jet printing methods for producing polymer
particles with arbitrary size and composition. This method is based on
using droplet-on-demand generation to create a small drop consisting of
a very dilute polymer mixture in a solvent.[^1]^,^[^2] As the solvent
evaporates, a polymer particle is produced whose size is defined by the
initial size of the droplet (typically between 5 and 30 µm), and the
weight fraction of polymer (or other non-volatile species) in solution.
Because the droplets are produced with small excess charge during
ejection from the nozzle, this approach lends itself naturally to
spatial manipulation of micro- and nanoparticles using electrodynamic
focusing techniques.[^3] The polymeric particles in the micro- and
nanometer size range provide many unique properties due to size
reduction to the point where critical length scales of physical
phenomena become comparable to or larger than the size of the structure.
Applications of these types of particles take advantage of high surface
area and confinement effects, leading to interesting nano-structures
with different properties that cannot be produced using

By combining experimental observations and developments with extensive
computational chemistry studies we have substantial evidence indicating
highly ordered rod-shaped structures for single molecule MEH-PPV and
CN-PPV systems. The chain organization is crucial to the photophysical
properties and can be controlled to a large extent by the solvent. A
relatively bad solvent leads to a structure that is tightly folded into
a rod-shaped morphology while a good solvent produces structures with
little or random folding and more of a defect, extended-chain
morphology. Secondary structural regularity is also induced by producing
single molecule nanoparticles from micro-droplets of the solution
without the presence of a substrate. For toluene and THF solvent
preparations of MEH-PPV and CN-PPV, the resulting solvent-free single
molecule nanoparticle structures show a very high level of organization
consisting of π-stacked folded chains. Due to this structural
organization which imposes a rod-shaped morphology, single molecule
nanoparticle orientation on an appropriate surface occurs such that the
particle stands on its end with near-perfect z-orientation. This
orientational alignment is caused by excess charges that are induced on
the particle during its production which tend to localize at the surface
causing the rod-shaped particle to minimize the Coulombic interactions
with the surface (Si-O)^-^ groups. The excess charge also appears to
increase the structural organization into π-stacks. The resulting
z-oriented single molecule rod-shaped nanoparticles are organized into
something like a core-shell system, where the inner core is a
self-solvated PPV system with inter-chain distances \~0.3 Å closer than
the surrounding chains. Since there is still orbital overlap throughout
the system, facile fluorescence resonant energy transfer without the
emission of a photon can occur to the core emission site. In addition
this type of inter-chain distance anisotropy can give rise to behavior
observed for a single quantum emitter in a dielectric medium at the
nanometer scale (Raleigh scattering regime) and thus show strong effects
due to classical electromagnetic interactions (vacuum fluctuations are
altered from boundary reflections) which would lead to the observed
altered fluorescence lifetimes.

**The overall ramifications of developing this fundamentally new
processing technique for generating optoelectronic materials, is
far-reaching. By achieving uniform orientation perpendicular to the
substrate with enhanced luminescence lifetimes and photostability under
ambient conditions, the door is now open for major developments in
molecular photonics, display technology and bio-imaging, as well as new
possibilities for optical coupling to molecular nanostructures and for
novel nanoscale optoelectronics devices.**

[^1]: C-Y. Kung, M. D. Barnes, N. Lermer, W. B. Whitten, and J. M.
    Ramsey, *Applied Optics* **38**, 1481 (1999).

[^2]: M. D. Barnes, K. C. Ng, K. Fukui, B. G. Sumpter, and D. W. Noid
    *Macromolecules* **32**, 7183 (1999).

[^3]: K. C. Ng, J. V. Ford, S. C. Jacobson, J. M. Ramsey, and M. D.
    Barnes, *Review of Scientific Instruments*, **71**, 2497 (2000).