Nano-Porous Graphene as Free-Standing Membranes
69
Nanopore assessment is possible through several techniques. First, careful
imaging with high-resolution transmission electron microscopy (HRTEM) is the
most non-invasive method for this purpose. However, it requires a transfer of the
nano-porous graphene membrane over TEM grids which can be done following the
protocols in [158, 159]. In this method, not only the pore size and density can be
evaluated across the surface by multiple imaging, but the nanopore shape and form
can be assessed. Low temperature (LT) and ultra-high vacuum (UHV) scanning
tunneling microscopy (STM) allows for inspection of membrane flatness, defects,
nanopore size and terminations, topographically and electronically with atomic resolution. However, for resolving the nanopores, it is recommended that the membrane
be transferred onto an atomically flat and clean conductor like Au surface. A more
precise technique for nanopore examinations at the atomic scale is advanced noncontact (NC-) AFM operating in UHV and LT. This technique provides similar
topography information as STM but is superior to STM for two reasons: first, it
allows force spectroscopy on the surface providing mechanical information of the
membrane. Second, it does not require a conductive sample so the membrane can
be transferred onto any arbitrary atomically flat surface. If the nanopores are created
using oxygen plasma, investigating the evolution of Raman D peak and G peak of
graphene with plasma exposure time is very useful. Correlating the Raman results
with resolved topographic results on the nanopores obtained from HRTEM, STM
or NC-AFM gives insight into the distribution of the nanopores across large-area
graphene membranes [31, 158, 159].
If the nano-porous graphene membrane is functionalized with different functional
agents, a combination of X-ray photoelectron spectroscopy (XPS), Fourier transform
infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDX) gives
information on the percentage of these additives, defect interplay with the surface,
type of the agents and the bonds they have made with the carbonaceous membrane
in bulk. Again, investigating the evolution of Raman D peak and G peak of graphene
at different functionalization conditions gives insight into alterations induced on the
surface by functional groups [50, 51]. However, to demonstrate the termination of the
nanopores and what the electronic and mechanical impacts of the functional groups
are on the surface, atomic resolution imaging such as STM and NC-AFM in UHV
and LT are vital.
4.5 Realization of Nano-Porous Graphene Membranes
for Water Desalination
Early experimentally realized suspended atomically thin membranes were produced
by a combination of photolithography and mechanical exfoliation of graphene [27,
83]. The SiO 2 support layer consisted array of circles with diameters of 5 mm and
7 mm defined by photolithography. Reactive ion etching (RIE) was then used to etch
the circles into cylindrical cavities with a depth of 250–500 nm, leaving a series
69
Nanopore assessment is possible through several techniques. First, careful
imaging with high-resolution transmission electron microscopy (HRTEM) is the
most non-invasive method for this purpose. However, it requires a transfer of the
nano-porous graphene membrane over TEM grids which can be done following the
protocols in [158, 159]. In this method, not only the pore size and density can be
evaluated across the surface by multiple imaging, but the nanopore shape and form
can be assessed. Low temperature (LT) and ultra-high vacuum (UHV) scanning
tunneling microscopy (STM) allows for inspection of membrane flatness, defects,
nanopore size and terminations, topographically and electronically with atomic resolution. However, for resolving the nanopores, it is recommended that the membrane
be transferred onto an atomically flat and clean conductor like Au surface. A more
precise technique for nanopore examinations at the atomic scale is advanced noncontact (NC-) AFM operating in UHV and LT. This technique provides similar
topography information as STM but is superior to STM for two reasons: first, it
allows force spectroscopy on the surface providing mechanical information of the
membrane. Second, it does not require a conductive sample so the membrane can
be transferred onto any arbitrary atomically flat surface. If the nanopores are created
using oxygen plasma, investigating the evolution of Raman D peak and G peak of
graphene with plasma exposure time is very useful. Correlating the Raman results
with resolved topographic results on the nanopores obtained from HRTEM, STM
or NC-AFM gives insight into the distribution of the nanopores across large-area
graphene membranes [31, 158, 159].
If the nano-porous graphene membrane is functionalized with different functional
agents, a combination of X-ray photoelectron spectroscopy (XPS), Fourier transform
infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDX) gives
information on the percentage of these additives, defect interplay with the surface,
type of the agents and the bonds they have made with the carbonaceous membrane
in bulk. Again, investigating the evolution of Raman D peak and G peak of graphene
at different functionalization conditions gives insight into alterations induced on the
surface by functional groups [50, 51]. However, to demonstrate the termination of the
nanopores and what the electronic and mechanical impacts of the functional groups
are on the surface, atomic resolution imaging such as STM and NC-AFM in UHV
and LT are vital.
4.5 Realization of Nano-Porous Graphene Membranes
for Water Desalination
Early experimentally realized suspended atomically thin membranes were produced
by a combination of photolithography and mechanical exfoliation of graphene [27,
83]. The SiO 2 support layer consisted array of circles with diameters of 5 mm and
7 mm defined by photolithography. Reactive ion etching (RIE) was then used to etch
the circles into cylindrical cavities with a depth of 250–500 nm, leaving a series