posted on 2021-06-23, 17:33authored byRichard Murray, Micheal Burke, Daniela Iacopino, Aidan J. Quinn
Realization of graphene-based
sensors and electronic devices remains
challenging, in part due to integration challenges with current fabrication
and manufacturing processes. Thus, scalable methods for in situ fabrication
of high-quality graphene-like materials are essential. Low-cost CO2 laser engravers can be used for site-selective conversion
of polyimide under ambient conditions to create 3-D, rotationally
disordered, few-layer, porous, graphene-like electrodes. However,
the influences of non-linear parameter terms and interactions between
key parameters on the graphitization process present challenges for
rapid, resource-efficient optimization. An iterative optimization
strategy was developed to identify promising regions in parameter
space for two key parameters, laser power and scan speed, with the
goal of optimizing electrode performance while maximizing scan speed
and hence fabrication throughput. The strategy employed iterations
of Design of Experiments Response Surface (DoE-RS) methods combined
with choices of readily measurable parameters to minimize measurement
resources and time. The initial DoE-RS experiment set employed visual
response parameters, while subsequent iterations used sheet resistance
as the optimization parameter. The final model clearly demonstrates
that laser graphitization through raster scanning is a highly non-linear
process requiring polynomial terms in scan speed and laser power up
to fifth order. Two regions of interest in parameter space were identified
using this strategy: Region 1 represents the global minimum for sheet
resistance for this laser (∼16 Ω/sq), found at a low
scan speed (70 mm/s) and a low average power (2.1 W) . Region 2 is
a local minimum for sheet resistance (36 Ω/sq), found at higher
values for scan speed (340 mm/s) and average power (3.4 W), allowing
∼5-fold reduction in write time. Importantly, these minima
do not correspond to constant ratios of average laser power to scan
speed. This highlights the benefits of DoE-RS methods in rapid identification
of optimum parameter combinations that would be difficult to discover
using traditional one-factor-at-a-time optimization. Verification
data from Raman spectroscopy showed sharp 2D peaks with mean full-width-at-half-maximum
intensity values <80 cm–1 for both regions, consistent
with high-quality 3D graphene-like carbon. Graphene-based electrodes
fabricated using the parameters from the respective regions yielded
similar performance when employed as capacitive humidity sensors with
hygroscopic dielectric layers. Devices fabricated using Region 1 parameters
(16 Ω/sq) yielded capacitance responses of 0.78 ± 0.04
pF at 0% relative humidity (RH), increasing to 31 ± 7 pF at 85.1%
RH. Region 2 devices (36 Ω/sq) showed comparable responses (0.88
± 0.04 pF at 0% RH, 28 ± 5 pF at 85.1% RH).