7.2 Patterning and Assembly
223
acetone for 30 min. Thereafter, the sample was freed from residues via an IPA rinsing,
DI water rinsing and hotplate baking at 130
◦ C for 5 min.
In fact, by stacking van-der-Waals materials arbitrarily using different sequences
of material sheets, different twist angles and energy band gaps, a plethora of configurations become imaginable, and different effects on the electronic or optoelectronic
properties can be obtained, as aforementioned studies and reviews indicate.
7.2.4 Laser Processing and Ion Beam Milling
In addition to etching-based structuring techniques, laser processing can provide a
dry and similarly well-controllable method for microstructuring, as examples such as
beam-machined free-standing THz metamaterials indicate [62]. For metamaterials
preparation in metal foils by laser ablation, a shutter-controlled 10-Hz ns-pulsed
Nd:YAG laser and two motorised stages were employed in Marburg previously.
However, this comes at the cost of precision, as ablation of material by a laser beam
is not only limited by the beam’s focused spot size, but also by the heat transferred to
the material. Additional improvements can be obtained by ultrafast pulsed lasers (cf.
[24]), when samples are exposed to high peak powers within very short time scales
that prevent a large footprint due to suppressed heat transfer, which takes place on
longer time scales. Recently, pulsed CO 2 -laser pulses had been also used to postprocess selectively-etched fibre cores at their tips as a means of thermal treatment to
obtain smooth concave micromirror shapes [25] (Fig. 7.7).
In fact, laser cutting can be also applied for experimental studies on large-area 2D
materials. In one example, patterning of graphene and of aluminum shadow masks
were performed for channel definition and electrical-contact deposition, respectively,
in order to produce a field-effect-transistor device for photodetector studies involving
light-absorbing nanoparticles [23] (Fig. 7.3).
In contrast to laser cutting, milling on the nanoscale becomes feasible using
focused ion beams (FIBs). Typically, a FIB gun is found in scanning-electron microscopes used for sample preparation, e.g. prior to transmission-electron microscopy.
It has been recently shown that photonic structures of reasonable quality can
be achieved using FIB for experiments combining patterned landscapes and 2D
materials [50] (Fig. 7.8). In the work by Mey et al., FIB milling took place directly
on the GaP sample inside the evacuated chamber of a FIB device loaded with a
gallium gun, using a 30-KV ion beam with a beam current of 500 pA. Due to the
integrated scanning-electron microscope in such apparatus, in situ imaging of the
structure is usually possible.
Although cutting and milling techniques (both top–down techniques) can be very
useful for prototyping, a major drawback originates from the serial writing of patterns into the target material, which is time consuming. The same drawback holds
true for bottom-up laser writing (a form of 3D printing), which is an excellent tool
for prototyping and which even opens up the possibility of optically inscribing submicron-precise structures into photoresist material utilising two-photon polymerisa-
223
acetone for 30 min. Thereafter, the sample was freed from residues via an IPA rinsing,
DI water rinsing and hotplate baking at 130
◦ C for 5 min.
In fact, by stacking van-der-Waals materials arbitrarily using different sequences
of material sheets, different twist angles and energy band gaps, a plethora of configurations become imaginable, and different effects on the electronic or optoelectronic
properties can be obtained, as aforementioned studies and reviews indicate.
7.2.4 Laser Processing and Ion Beam Milling
In addition to etching-based structuring techniques, laser processing can provide a
dry and similarly well-controllable method for microstructuring, as examples such as
beam-machined free-standing THz metamaterials indicate [62]. For metamaterials
preparation in metal foils by laser ablation, a shutter-controlled 10-Hz ns-pulsed
Nd:YAG laser and two motorised stages were employed in Marburg previously.
However, this comes at the cost of precision, as ablation of material by a laser beam
is not only limited by the beam’s focused spot size, but also by the heat transferred to
the material. Additional improvements can be obtained by ultrafast pulsed lasers (cf.
[24]), when samples are exposed to high peak powers within very short time scales
that prevent a large footprint due to suppressed heat transfer, which takes place on
longer time scales. Recently, pulsed CO 2 -laser pulses had been also used to postprocess selectively-etched fibre cores at their tips as a means of thermal treatment to
obtain smooth concave micromirror shapes [25] (Fig. 7.7).
In fact, laser cutting can be also applied for experimental studies on large-area 2D
materials. In one example, patterning of graphene and of aluminum shadow masks
were performed for channel definition and electrical-contact deposition, respectively,
in order to produce a field-effect-transistor device for photodetector studies involving
light-absorbing nanoparticles [23] (Fig. 7.3).
In contrast to laser cutting, milling on the nanoscale becomes feasible using
focused ion beams (FIBs). Typically, a FIB gun is found in scanning-electron microscopes used for sample preparation, e.g. prior to transmission-electron microscopy.
It has been recently shown that photonic structures of reasonable quality can
be achieved using FIB for experiments combining patterned landscapes and 2D
materials [50] (Fig. 7.8). In the work by Mey et al., FIB milling took place directly
on the GaP sample inside the evacuated chamber of a FIB device loaded with a
gallium gun, using a 30-KV ion beam with a beam current of 500 pA. Due to the
integrated scanning-electron microscope in such apparatus, in situ imaging of the
structure is usually possible.
Although cutting and milling techniques (both top–down techniques) can be very
useful for prototyping, a major drawback originates from the serial writing of patterns into the target material, which is time consuming. The same drawback holds
true for bottom-up laser writing (a form of 3D printing), which is an excellent tool
for prototyping and which even opens up the possibility of optically inscribing submicron-precise structures into photoresist material utilising two-photon polymerisa-