Transferring self-assembled, nanoscale cables into electrical devices

Article abstract of DOI:10.1021/ja0642360

This study details a new derivative of the contorted HBCs that self-organizes into one-dimensional, single-crystalline fibers. X-ray diffraction, transmission electron microscopy, and electron diffraction studies show that they have an orthorhombic unit c

Full text of DOI:10.1021/ja0642360

Published on Web 07/28/2006
Transferring Self-Assembled, Nanoscale Cables into Electrical Devices
Shengxiong Xiao,† Jinyao Tang,† Tobias Beetz,‡ Xuefeng Guo,† Noah Tremblay,† Theo Siegrist,§
Yimei Zhu,‡ Michael Steigerwald,† and Colin Nuckolls*,†
Department of Chemistry and Center for Electronics of Molecular Nanostructures, Columbia UniVersity, New York,
New York 10027, Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York
11973-5000, and Bell Laboratories, Lucent Technologies, 600 Mountain AVenue, Murray Hill, New Jersey 07974
Received June 15, 2006; E-mail: cn37@columbia.edu
Here we describe the synthesis, self-assembly, and properties of
the contorted hexabenzocoronenes1 (HBC) 1. We show that this
material organizes into molecular stacks, and further that these
stacks organize into cables or fibers.2-5 These cables are ordered
both on the molecular scale and on the supramolecular scale having
an orthorhombic unit cell of 5.8 nm × 4.5 nm × 0.45 nm. We
exploit this self-assembly process and the integrity of the resulting
long-range order through the use of an elastomeric stamp to pick
individual cables out of tangled mats and place them into electrical
test structures.6 This pick-and-place procedure yields field effect
transistors7-9 constructed from isolated nanoscale fibers. This study
utilizes the power of self-assembly to create highly ordered
nanostructured materials and develops a general method to ma-
nipulate and position them in devices.
Figure 1. (A) Octa-substituted HBC 1. (B) SEM of a mat of fibers grown
from dodecane solution of 1 (inset: higher magnification micrograph).
Prior to this study, octa-substituted derivatives of the HBCs, such
as 1, had not been prepared, but their synthesis follows the
procedure developed for the tetrasubstituted versions.1 The strain
in the periphery of these compounds creates structures that are
bent2,10,11 significantly away from planarity. Experimental details
and spectroscopic characterization used for the synthesis of 1 are
contained in the Supporting Information.
Unlike the material with four alkoxyl side chains, which shows
a columnar liquid crystalline phase,1 differential scanning calorim-
etry (in the Supporting Information) for 1 shows no evidence of a
mesophase. This compound does organize supramolecularly; it
forms crystalline fibers when its solutions in dodecane are
concentrated by evaporation. Adjusting the solvent, temperature,
and concentration, we have coarse control over the dimensions of
the fibers. A movie showing the self-assembly of 1 into fibers is
contained in the Supporting Information. The fibers shown in Figure
1B were grown from dodecane (2 mg/mL) over the period of a
few days. Each of the fibers is ∼200 nm wide, implying that a few
thousand molecular strands of 5 nm in diameter would pack to form
the nanostructures.
We used transmission electron microscopy (TEM), electron
diffraction, and powder X-ray diffraction to elucidate the structure
of the fibers. Figure 2A and B shows TEM micrographs of fibers
grown on a lacey carbon grid. To circumvent the extreme sensitivity
of these fibers to the incident electrons, a CCD camera was used
to continuously record the diffraction patterns while the sample
was moved through the electron beam. Figure 2C shows an electron
diffraction pattern from a single HBC fiber similar to the one in
Figure 2B. The exposure time to record the pattern was 0.1 s. The
electron diffraction pattern features diffraction spots along three
lines, indicated by (hk0), (hk1) and (hk-1), which extend perpen-
dicular to the direction of the fiber. The distance between the lines
gives the unit cell spacing along the c-axis (0.45 nm). If this distance
Figure 2. (A) TEM images of fibers of 1 on a lacey carbon grid. (B) Higher
magnification image of an individual fiber. (C) Electron diffraction pattern
from a single HBC fiber. (D) Rectangular arrangement of 1. (E) Packing
of the columns into a fiber along the c-axis.
is the π-to-π, intermolecular spacing, it is atypically large (typically
∼3.5 Å). The intermolecular nestling that is required for the
contorted core of 1 to stack forces the arenes to offset from one
another in adjacent molecules, but the π-to-π distance is signifi-
cantly less than 0.45 nm.
An overall pseudohexagonal arrangement of the HBC core is
then assumed,1 further modified by the alkane chains, consistent
with the electron diffraction pattern, and an orthorhombic unit cell
with approximate dimensions of 6.0 nm × 4.5 nm × 0.45 nm was
inferred. A full pattern fit using the program Winprep12 of the
powder X-ray diffraction data gave a refined orthorhombic unit
cell with dimensions of a ) 5.8 nm, b ) 4.5 nm, and c ) 0.45 nm.
The most intense peak corresponds to a spacing (d ) 4.3 nm)
roughly the same as the long axis of the oval-shaped, HBC disk
with its side chains fully extended. If we assume this reflection to
be from the b-axis of the orthorhombic lattice and further assign
the second most intense reflection (d ) 1.8 nm) the index (220),
the remaining reflections in the diffraction pattern refine well.
Fibers have been formed from flat HBCs13,14 prepared by Aida
and co-workers.3 These materials organize into two-dimensional
† Columbia University.
‡ Brookhaven National Laboratory.
§ Lucent Technologies.
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J. AM. CHEM. SOC. 2006, 128, 10700-10701
10.1021/ja0642360 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
is methodological, where we use elastomer stamps to pick-and-
place self-organized organic cables into devices. It would have been
difficult to study the former without the latter, as it would be
difficult to exploit the latter without a material like 1. This method
is broadly applicable in materials chemistry for testing the properties
of individual, self-assembled organic nanostructures and the
elucidation of intrinsic properties.
Acknowledgment. We acknowledge support from the Nano-
scale Science and Engineering Initiative of the NSF under Award
Number CHE-0117752 and by the New York State Office of
Science, Technology, and Academic Research (NYSTAR); the
DOE, Nanoscience Initiative (NSET#04ER46118); the NSF CA-
REER award (#DMR-02-37860). We thank the MRSEC Program
of the NSF under Award Number DMR-0213574 and by the New
York State Office of Science, Technology and Academic Research
(NYSTAR). The work at Brookhaven National Laboratory was
supported by the Office of Basic Energy Sciences, U.S. DOE. This
manuscript has been authored by Brookhaven Science Associates,
LLC under Contract No. DE-AC02-98CH10886 with the U.S. DOE.
Figure 3. (A) Schematic of a FET from an isolated fiber. (B) Micrograph
of a device formed from an isolated fiber that spans Au (200 nm thick) on
Cr (5 nm thick) electrodes. The silicon wafer forms a back gate for the
device. (C) Transfer characteristics (VDS ) -50 V) and (D) transistor output.
VG ) 0 and -24 V in 4 V steps. The electrical characteristics are for the
device pictured in (B).
Supporting Information Available: Experimental details for
synthesis, structure determination, and electrical measurements, and a
movie showing the self-assembled fiber growth. This material is
available free of charge via the Internet at http://pubs.acs.org.
sheets because the molecules are substituted to be amphiphilic.
These sheets then roll themselves into a hollow helix. In our present
case, the structure is much different: the columns are solid rods
rather than hollow tubes. The oval cross section of the columns
causes them to pack into a rectangular lattice, as shown in Figure
2D,E. By aligning the fibers in devices, we can probe the electrical
conductivity along the one-dimensional stacks where they should
be most highly conductive.15,16
We developed a method to manipulate, align, and transfer isolated
fibers into transistor devices using elastomer stamps. This technique
follows those developed for carbon nanotubes6,17 and nanowires.18,19
First, we grow a mat of nanofibers from dodecane solution (shown
in Figure 1B) by slow evaporation of the solvent. An elastomer
stamp made from polydimethoxysilane (PDMS) is gently pressed
into the mat of fibers. Remarkably only a few fibers transfer to the
stamp. Pressing the “loaded” stamp onto a substrate or device test
structure then transfers this fiber.
We tested two methods for making devices out of these fibers.
One was to transfer the fibers onto prefabricated electrodes, and
another was to transfer the fiber to a clean wafer and then evaporate
electrodes onto them through a shadow mask (100 µm × 100 µm
squares separated by 5 µm). We found better electrical character-
istics when the fibers were first transferred and the electrodes
subsequently deposited.
A device constructed on an individual fiber and its electrical
characteristics are shown in Figure 3. The material behaves as a
p-type, hole-transporting semiconductor. The length of the fiber
spanning the Au/Cr electrodes is 6.1 µm, and its width is ∼250
nm. The carrier mobility in these fibers is estimated to be ∼0.02
cm2/V‚s.20 These values are quite good compared to other columnar
materials1,21,22 but are still a lower limit on the values that are
possible with these materials. The nonlinear current voltage curves
at low VD in Figure 3D indicate they are limited by contact
resistance.23,24 Moreover, the drain current in the gate sweep (Figure
3C) is greater than that in the corresponding source-drain sweep
(Figure 3D), indicating there is a measurable amount of carrier
trapping.25,26 We tested around a 100 devices made from fibers and
found that the mobility varied sample to sample, ranging between
10-4 and g10-2 cm2/V‚s. This variation is likely due to the
differences in morphology of individual fibers (as seen in Figure
1B) and changes in the physical contact to each of the fibers.
This study describes two new items: one is material, a new set
of molecules that self-organize into molecular cables, and the other
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