Electrospun a-Si using Liquid Silane/Polymer Inks
Douglas L. Schulz, Justin M. Hoey, Jeremiah Smith, John Lovaasen, Chris Braun,
Xuliang Dai, Kenneth Anderson, Arumugasamy Elangovan, Xiangfa Wu, Scott Payne,
Konstantin Pokhodnya, Iskander Akhatov, Larry Pederson, Philip Boudjouk
North Dakota State University, 1805 NDSU Research Park Drive, Fargo, ND USA 58102
ABSTRACT
Amorphous silicon nanowires (a-SiNWs) were prepared
by electrospinning cyclohexasilane (Si6H12) admixed with
polymethylmethacrylate (PMMA) in toluene. Raman
spectroscopy characterization of these wires (d~50-2000
nm) shows 350 °C treatment yields a-SiNWs. Porous
a-SiNWs are obtained using a volatile polymer.
1. INTRODUCTION
In this paper, we describe an application of
cyclohexasilane (Si6H12) [1]. Si6H12 is a high melting point
liquid (18 °C) that is stable toward reduced-pressure
distillation as well as ambient light. Liquid silanes have
been considered as precursors in direct-write fabrication
of printed electronics [2]. Si6H12 can be transformed into
solid polydihydrosilane -(SiH2)n- by thermal treatment or
light activation via radical polymerization. Additional
thermolysis causes evolution of H2 (g) giving a-Si:H at
~350 °C and crystalline silicon at ~750 °C (Fig. 1) [3].
Δ, hν
-(SiH2)nSi 6H12
diradical
polysilane
Δ
-H2 (g)
a-Si:H
Δ
-H2 (g)
amorphous
silicon
c-Si:H
crystalline
silicon
Fig. 1 Thermolytic pathway for Si6H12.
Marked microstructural changes are associated with
this thermolytic transformation. Figure 2 shows a scanning
electron microscopy image of an amorphous silicon film
produced by heating a polysilane film to 350 °C using a hot
plate located inside an inert gas glovebox. Polysilane films
~3 μm in thickness were prepared by spin-coating a
Si6H12-based “liquid silane material” (Si6LSM) that was
prepared in analogy to Shimoda et al. [2] who used Si5H10
as the ring-opening monomer and solubilizing reagent.
Shrinkage is observed to occur around 290 °C in these
Si6H12-derived films and it appears related to the evolution
of SiH2 and SiH3 fragments as previously reported [2].
This shrinkage does not lead to cracking when films are
less than a thickness of ~200 nm. In this paper we report
the translation of solution-processed amorphous silicon
from 2-dimesional thin films to 1-dimensional wires. An
electrospinning polymer is utilized as a carrier for Si6H12
with a-Si wires formed after appropriate treatment. We
anticipate that this technology could displace other
methods of manufacturing Si NWs (e.g., etching, VLS).
Fig. 2 SEM image of an a-Si film ~3 μm thick on Al2O3
substrate prepared from a Si6H12-based “liquid
silane material” after thermal treatment at 350 °C.
2. RESULTS AND DISCUSSION
2.1 Electrospinning PMMA-Si6H12 Inks
Initial attempts at directly spinning the Si6LSM were
unsuccessful as the fluid ejected from the nozzle tip
breaking into droplets. For this reason, polymer solutions
that possess viscosity and chain length characteristics
amenable to electrospinning were used as carriers
(Scheme 1). All procedures were performed under inert
N2 conditions as Si6H12 sparks and pops in the air.
2.1.1 PMMA and Si6H12 in Toluene
A polymer solution was prepared by adding 0.52 g
PMMA (Aldrich, MW= 996,000) and 4.60 g of dry toluene
to a dried glass vial. The stirring mixture was heated to
75 °C to expedite dissolution and then cooled to roomtemp. Next, 500 μL of the PMMA/toluene was placed into
another dried glass vial and 100 μL of Si6H12 was added
dropwise giving colorless immiscible phases with one
being rather viscous. After stirring 15 min, the mixture
appeared to be homogeneous with an apparent viscosity
that was higher than either of the immiscible phases.
2.1.2 Electrospinning PMMA-Si6H12
The sample was taken up into a 1 mL syringe fitted
with a blunt-nosed 21 gauge stainless steel needle 1.5
inches in length. A six-inch square sheet of aluminum foil
(2 mil Reynolds WrapTM) was placed on a grounding pad
with a needle standoff distance of ~25 cm. A high voltage
source (Gamma High Voltage Research Inc. model
ES40P-12W/DDPM)
was
connected
with
the
Scheme 1. Schematic diagram illustrating the transformation of PMMA-Si6H12 into a-Si:H materials.
CH3
Electrospin
+
O
-toluene (g)
O
CH 3
Si6H12
n
Heat
PMMA in toluene
MW= 996,000
Polydihydrosilane / PMMA
Heat (350 °C)
TEM micrograph
SEM image showing beads
positive terminal on the needle and the negative (ground)
on the aluminum foil. After the flow of the ink was
established, 15 kV was applied and fibers were seen
spinning horizontally from the nozzle to the foil. After
electrospinning, the sample was heated to ~350 °C with a
hot plate at which time a slight yellow tint was observed.
Raman Intensity (a.u.)
2.2 Characterization of the 1D Si from PMMA/Si6H12
A Raman microscope system (Horiba Jobin Yvon,
LabRAM ARAMIS, 532 nm illumination) was employed to
prove that the Si6H12 was transformed into a-Si during the
processing detailed above. Indeed, the Raman spectra for
the heat-treated sample exhibited a broad band at 485
cm-1 that corresponds to a-Si (Fig. 3 red). Interestingly,
350
Fig. 4 Optical image of wires formed
electrospinning PMMA/Si6H12-based ink.
Before Laser
513 cm‐1
c‐Si
After Laser
485 cm‐1
a‐Si:H
400
450
500
cm-1
550
600
650
Fig. 3 Raman spectra for silicon wires formed by
electrospinning a PMMA/Si6H12-derived ink (red)
before and (blue) after melting with the Raman laser.
the Raman laser can transform the a-Si wires into
-1
crystalline Si as evidenced by a band at 513 cm (Fig. 3
blue). In addition, the Si wires are melted when the laser
2
beam is focused to ~100 kW/cm (Fig. 4).
by
2.3 Electrospinning QPAC® 100-Si6H12 Inks
As a step toward to production of higher purity Si NWs,
a
thermalizable
polypropylene
carbonate/
polycyclohexene carbonate polymer system (referred to
as QPAC® 100) was employed as the carrier.
2.3.1 QPAC® 100 and Si6H12 in Toluene
The ink was prepared by dispersing the polymer into
the solvent with subsequent addition of the liquid silane.
Toward this end, 0.12 g of QPAC® 100 (Empower
Materials, Inc.) and 1.06 g of dried, degassed toluene
were added to a flame dried vial and the mixture was
stirred with a magnetic stir bar at 500 rpm for 2.5 hr.
Upon introduction of 50 µL Si6H12 by pipette, a slight
immiscibility was noted with a homogeneous mixture
observed after an additional ~40 hr of stirring.
2.3.2 Electrospinning QPAC®100-Si6H12
The QPAC-Si6H12 ink was electrospun using the
identical setup as above. Copper foil (500 µm thickness)
was used as the substrate for this part of the study and
subjected to the following protocol prior to electrospinning:
isopropanol rinse to remove residual organics; cleaning
with 1% HCl in water solution; rinsing with copious
amounts of deionized water; and, transferring into an inert
atmosphere glovebox and heating to 350 °C for one min to
desorb trace solvent. Electrospinning was performed with
a 30 cm stand-off distance, 0.5 mL/hr ink feed rate and a
10 kV excitation. After spinning for one hour, the sample
was cut into several pieces for further processing.
2.3.3 Post-Treatment of QPAC®100-Si6H12
The electrospun samples derived from the
QPAC-Si6H12 ink were heated to ~350 °C using a hot plate
in the glovebox. In contrast to the previous observations
for PMMA-based inks (see above, Section 2.1.2),
thermolysis did not result in the formation of a-Si NWs.
SEM analysis showed a 2D outline of the original
nanowires where SEM-EDS confirmed the dark lines in
Figure 5 as silicon-rich. It was speculated that the QPAC
polymer vaporized prior to the formation of -(SiH2)n- and
partially-polymerized liquid Si6H12 flowed down to give the
thin Si films. To address this shortcoming in the process,
the electrospun QPAC-Si6H12 deposits were subjected to
laser treatment giving -(SiH2)n- prior to QPAC thermolysis.
Toward this end, electrospun deposits were transferred
to a glovebox that contained a 355 nm beam from a
HIPPO laser (Spectra Physics, Inc.). The Si6H12 was
transformed into polysilane using a laser power that
ranged from 0.5 to 4.0 W and a beam diameter of 1.0 cm.
A yellow/brown discoloration was visible for all laser-
Fig. 5 SEM micrograph of the residue after an
electrospun QPAC-Si6H12 wire was subjected to a
thermal treatment at 350 °C for 20 min under an N2
atmosphere (from Ref [3], Supplementary Materials).
treated areas. Subsequent thermal treatment for 20 min
at ~350 °C gave porous a-Si NWs. This processing route
is illustrated in Scheme 2.
2.4 Thermogravimetric Analysis of Ink Components
Thermogravimetric analysis (TGA) of the various ink
components was performed to provide detailed
information regarding thermal processing of these
Si6H12-derived a-Si wires. Figure 6 shows TGA data for
four samples collected using a TA Instruments Q600
SDT with N2 carrier gas (100 sccm) and a temperature
ramp rate of 10 °C/min. PMMA and QPAC samples were
used as-received. Si6H12 was prepared according to [1]
and purified by short path distillation. The
Si6H12/-(SiH2)n- sample was prepared by irradiating
Scheme 2. Schematic diagram illustrating the transformation of QPAC-Si6H12 into a-Si:H materials.
Electrospin
O
O
O
O
O
O
n
QPAC100®
polypropylene carbonate/
polycyclohexene carbonate
in toluene MW = 660,000
m
-toluene (g)
+
Laser (355 nm @ ~250mW/cm2)
1) Si6H12 transformed to -(SiH2)n-
Porous a-Si NWs
HRSEM micrographs
Heat (350 °C/20 min)
2) QPAC 100® volatizes
3) -(SiH2)n- transformed to a-Si
Fig. 6 TGA curves for the materials employed in this study.
200 µL Si6H12 with 250 mW of 355 nm laser light for 23 min
with stirring to give a viscous, honey-like fluid. The
-(SiH2)n- sample was the solid deposit formed upon storing
Si6H12 in a clear glass vial inside a glovebox over a period
of about two months. This TGA data can be used to help
conceptualize the dynamics of the electrospinning
processes that result in the formation of a-Si NWs from
Si6H12. For example, when nanowires prepared from
PMMA-Si6H12 inks are subjected to heat treatment, the
polymer remains as a stable support until Si6H12 
-(SiH2)n-  a-Si. By way of comparison, thermoloysis of
QPAC-Si6H12 wires leads to volatilization of the QPAC
before the Si6H12 is cured into a dimensionally-stable solid.
3. CONCLUSION
The thermal conversion of Si6H12 and/or -(SiH2)n- into
a-Si occurs with marked shrinkage around 290 °C. This
phenomenon may limit electrical transport owing to
microcracking within the thin films. The electrospinning
method described in this paper appears to manage the
stress by reducing the dimensionality from 2D films to 1D
wires. Some of the results in this paper show non-ideal
morphologies (e.g., wires with beads on Scheme 1) and a
greater understanding of the physics of the
electrospinning process will likely be required prior to
technology deployment. It is clear that Si6H12 associates
with oxygen-containing polymers and further details of
Lewis acid-base interactions are forthcoming. It is obvious
that the inclusion of secondary semiconductor
nanomaterials (e.g., C nanotubes, II-VI metal
chalcogenide nanoparticles, or Si-Ge nanoparticles) into
these Si6H12 inks offers a manifold for the exploration of
inorganic-inorganic hybrid materials and such studies
are ongoing toward reducing these concepts to practice.
ACKNOWLEDGEMENTS
Technical discussions and collegial discourse with
Kimihiro Matsukawa are kindly acknowledged. This
presentation is based on research sponsored by
Defense Microelectronics Activity (DMEA) under
agreement number H94003-08-2-0805, the National
Science Foundation under grant EPS-0447679 and the
Department of Energy under DE-FC36-08GO88160.
The United States Government is authorized to
reproduce and distribute reprints for Government
purposes, notwithstanding any copyright notation
thereon.
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