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Historically, there have been more examples of one-dimensional
inorganic nanostructures1,2 for use in nanoscale devices than
their organic semiconductor nanowire counterparts3,4. Organic
semiconductors are advantageous in general because of their facile
and large-scale synthesis, solution processability, and molecular
and electronic tunability by molecular design5,6. One-dimensional
organic nanostructures, such as nanowires, nanotubes,
nanoribbons, and nanofibers prepared via self-assembly4 from
conjugated small molecules or conjugated polymers constitute
next-generation materials for a vast array of electronic
applications7,8. They are promising materials for a multitude of
applications including vapor sensors9,10, phototransistors11,12,
solar cells13, nanoscale lasers14, memory elements15,
miniaturization of devices16, and as the active semiconductor
elements for organic field-effect transistors (OFETs)17,18.
It is well known that the performance of organic semiconductors
is governed by how molecules or polymer chains assemble in the solid
state19,20. Because organic molecules in nanowires self-assemble into
highly organized single-crystalline nanostructures4, they are ideal for
fundamental studies. Moreover, nanowires are model systems for
elucidating transport mechanisms8,21, addressing the role of nanoscale
domains in microstructures21,22, structure-property relationships23, and
for understanding intrinsic transport phenomena24. Enabling nanowire
synthesis from novel, never-before-used organic semiconductors may
very well open up new areas in science with a host of applications in
nanosensors, photovoltaics, wearable devices, and highly sophisticated
integrated logic.
This review highlights some current examples of organic nanowire
transistors as they make their debut in the rapidly evolving area of
organic electronics. Because of the enormous development in this
Organic nanowires self-assembled from small-molecule semiconductors and conducting polymers have attracted an enormous amount of interest for use in organic field-effect transistors. This new class of materials offers solution processability, the potential for elucidating transport mechanisms and structure-property relationships, and the realization of high-performance transistors that rival the performance of amorphous Si. We discuss the self-assembly of one-dimensional, single-crystalline organic nanowires, show the structures of commonly employed organic semiconductors, and review some of the advances in this field.
Alejandro L. Briseno1*, Stefan C. B. Mannsfeld2, Samson A. Jenekhe1,3, Zhenan Bao2, and Younan Xia1*
1Department of Chemistry, University of Washington, Seattle, WA 98195, USA
2Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA
3Department of Chemical Engineering, University of Washington, Seattle, WA 98195, USA
The molecules in one-dimensional nanostructures predominantly
self-assemble along the π−π stacking direction, which gives a high
charge-carrier mobility along the long axis of the nanostructure
as a result of the strong intermolecular coupling between the
packed molecules32,33. It is known that the tendency to form face-
to-face stacked structures can be enhanced by adding peripheral
substituents58–61,64,65, as well as with an increase of π-surface-to-
circumference ratios66. The face-to-face π-stacking motif is believed
to be more efficient for charge transport than most edge-to-face
‘herringbone-packing’ structures32,61. Fig. 2 shows examples of organic
semiconductors that form one-dimensional structures through strong
π-π interactions. These semiconductors have also been shown to
function successfully as one-dimensional organic crystalline transistors.
HTP is an example of a π-stacked small organic semiconductor
that adopts face-to-face packing in nanowires31. Fig. 3a shows HTP
molecules stacking face-to-face along the [100] crystallographic a-
axis with π-to-π distances of 3.54 Å64. In this example, π-stacking
is attributed to the chalcogen atoms located on the periphery of
pentacene’s backbone, which by steric hindrance prevent the edge-
to-face geometry that normally leads to the herringbone structure of
pentacene61. A distance of ~3.37 Å, which is smaller than twice the
van der Waals radius of the S atom (3.70 Å), is observed between the
central and outermost S atoms of a neighboring HTP molecule64. The
overlap of the molecular π-orbitals, crucial for the charge transport in
organic semiconductor materials, is significant only along the nanowire
direction (Fig. 3b). This type of stacking is also well known for PTCDI
Fig. 2 Known examples of (a) p-type, (b) n-type, and (c) polymer semiconductors that π-stack and function as one-dimensional organic single-crystalline FET s.
Solution deposition offers a cost-effective method for fabricating
large-area electronic components from organic materials73–75. While
solution-deposition techniques have been reported with some level
of success, there are still unresolved issues in controlling the crystal
packing and film-forming properties. Therefore, there is a growing
need to explore organic single-crystal nanostructures as solution-
processable materials. The idea is to employ high-performance organic
semiconductors that self-assemble into highly crystalline nanowires
either in solution or at the substrate-solution interface. So far, single-
crystal nanowire transistors have shown mobilities comparable to thin-
film transistors of the same material. In many cases, the single-crystal
nanowires have even shown mobilities larger than thin-films of the
same materials31,41,44,47,54,55.
Fig. 4 shows (a) the synthesis of HTP and (b) a dispersion of
nanowires in chloroform, as well as a high-performance nanowire
transistor exhibiting a mobility of 0.27 cm2/Vs31. The single-crystal
HTP nanowire transistor exhibits a mobility over six times greater than
that of vapor-deposited HTP thin-film transistors64. The higher mobility
of the nanowire transistors compared with vapor-deposited thin-film
transistors can be attributed to the high level of structural perfection
of the single-crystal HTP nanowires in addition to the fact that they
are essentially free of grain boundaries68–72. Furthermore, it is not
uncommon for mobility to be higher in single-crystals than in the thin-
film form of a semiconductor material31,41,44,47,54,55,58,76.
Nuckolls and coworkers recently described the synthesis, solution
self-assembly, and electrical properties of a hexabenzocoronene (HBC)
derivative (Fig. 5)34. A pick-and-place procedure using an elastomeric
stamp was used to pick individual ‘cables’ from tangled mats to
fabricate single nanowire transistors with mobilities as large as
0.02 cm2/Vs. The structure and packing of HBC molecules within the
nanowires were studied by transmission electron microscopy (TEM) and
it was determined that the molecules self-assemble in one-dimensional
stacks along the nanowire axis where charge transport should be
optimal.
Solution processability of bulk quantities of one-dimensional
wires has also been demonstrated with highly soluble, air-stable
oligoarenes56. Single oligoarene microwire transistors were tested with
hole mobilities on the order of ~10–2 cm2/Vs. Cho and coworkers
recently reported a record mobility of 1.42 cm2/Vs for a single-
crystalline triisopropylsilylethynyl pentacene microribbon41. To date,
this is the largest mobility of an individual one-dimensional organic
single crystal via solution processing (Fig. 6).
Nanowires via physical vapor transportGrowth of single-crystal organic nanowires via physical vapor transport
is an alternative way of producing high-performance materials for
use in FETs77. The main advantage is that one does not need to
handle the organic nanowires and this eliminates the possibility of
contamination (single-crystal nanowires can be directly grown onto
prefabricated source-drain electrodes)68. If care is not taken, solution
deposition of nanowires may incorporate debris and increase the
chance of contamination at the substrate-nanowire interface. However,
by growing nanowires directly onto substrates, contamination can
be greatly minimized. Some drawbacks to growing organic nanowires
via physical vapor transport, however, are low throughput of device
fabrication and, depending on the organic material being thermally
evaporated, it may not be possible to grow nanowires directly onto
plastic substrates unless low melting point (high vapor pressure)
materials are employed (e.g. anthracene, tetracene, etc.). Nevertheless,
some of the highest-mobility nanowire transistors have been
Fig. 5 (a) Schematic of an individual HBC nanowire transistor; (b) scanning electron micrograph of an HBC nanowire bridging the source-drain electrodes;
substrates with mobilities as large as 4.6 cm2/Vs have recently
been reported72. The flexible devices bend to a radius of less than
1 cm without any significant loss in performance. Large-area arrays
of patterned rubrene microcrystal transistors on flexible polyimide
substrates also show similar results68. CuPc single-crystal nanowires
also exhibit a high degree of mechanical flexibility43. This study was
carried out by mechanically bending a nanowire with a microprobe
to well over 180° without any fracturing of the nanowire. It did not,
however, report the effects of mechanical deformation on device
performance.
We have recently fabricated HTP nanowire transistors on
mechanically flexible substrates to evaluate their performance for
applications in flexible electronics31. Our approach to measuring the
Fig. 6 (a) Schematic of the molecular packing of TIPS-PENT molecules self-assembled into a single-crystalline ribbon. (b) Micrograph of a single TIPS-PENT
Fig. 8 (a) A series of optical photographs showing the self-assembly of PTCDI nanowires over time. (b) Schematic of an inverter with p- and n-type nanowire
networks. (c) Digital photograph of a substrate containing 13 discrete nanowire inverters. (d) Static inverter transfer characteristics. (Reprinted with permission
Fig. 11 (a) Optical micrograph of P3HT single-crystal microwires, (b) scanning electron micrograph with a close-up showing the well-defined facets, and (c) a
TEM micrograph with the corresponding electron diffraction pattern (d). (e) Molecular packing of P3HT chains in an orthorhombic unit cell, and (f) a schematic