We applied 2-photon laser ablation to write sub-diffraction nanoscale chemical patterns into ultrathin polymer films under ambient conditions. Poly(ethylene glycol) methacrylate brush layers were prepared on quartz substrates via surface initiated atom transfer radical polymerization (SI-ATRP) and ablated to expose the underlying substrate using the non-linear 2-photon absorbance of a frequency doubled Ti:Sapphire femtosecond laser. Single-shot ablation thresholds of polymer films were ~ 1.5 times smaller than that of a quartz substrate, which allowed patterning of nanoscale features without damage to the underlying substrate. At a 1/e2 laser spot diameter of 0.86 µm the features of exposed substrate approached ~ 80 nm, well below the diffraction limit for 400 nm light. Ablated features were chemically distinct and amenable to chemical modification.
Laser-based processing enables a wide variety of device configurations comprising thin films and nanostructures on sensitive, flexible substrates that are not possible with more traditional thermal annealing schemes. In near-field optical probing, only small regions of a sample are illuminated by the laser beam at any given time. Here we report a new technique that couples the optical near-field of the laser illumination into a transmission electron microscope (TEM) for real-time observations of the laser-materials interactions. We apply this technique to observe the transformation of an amorphous confined Si volume to a single crystal of Si using laser melting. By confinement of the material volume to nanometric dimensions, the entire amorphous precursor is within the laser spot size and transformed into a single crystal. This observation provides a path for laser processing of single-crystal seeds from amorphous precursors, a potentially transformative technique for the fabrication of solar cells and other nanoelectronic devices.
The application of nanoscale electrical and biological devices will benefit from the development of nanomanufacturing technologies that are high-throughput, low-cost, and flexible. Utilizing nanomaterials as building blocks and organizing them in a rational way constitutes an attractive approach towards this goal and has been pursued for the past few years. The optical near-field nanoprocessing of nanoparticles for high-throughput nanomanufacturing is reported. The method utilizes fluidically assembled microspheres as a near-field optical confinement structure array for laser-assisted nanosintering and nanoablation of nanoparticles. By taking advantage of the low processing temperature and reduced thermal diffusion in the nanoparticle film, a minimum feature size down to approximately 100 nm is realized. In addition, smaller features (50 nm) are obtained by furnace annealing of laser-sintered nanodots at 400 degrees C. The electrical conductivity of sintered nanolines is also studied. Using nanoline electrodes separated by a submicrometer gap, organic field-effect transistors are subsequently fabricated with oxygen-stable semiconducting polymer.
Among p–n
junction devices with multilayered heterostructures
with WSe2 and MoSe2, a device with the MoSe2–WSe2–MoSe2 (NPN) structure
showed a remarkably high photoresponse, which was 1000 times higher
than the MoSe2–WSe2 (NP) structure. The
ideality factor of the NPN structure was estimated to be ∼1,
lower than that of the NP structure. It is claimed that the NPN structure
formed a thinner depletion region than that of the NP structure because
of the difference of carrier concentrations of MoSe2 and
WSe2. Hence, the built-in electric field was weaker, and
the motion of the photocarriers was facilitated. These behaviors were
confirmed experimentally from a photocurrent mapping analysis and
Kelvin probe force microscopy. The work function depended on the wavelength
of the illuminator, and quasi-Fermi level was estimated. The surface
photovoltage on the MoSe2 region was higher than that on
WSe2 because the lower bandgap of MoSe2 induces
more electron–hole pair generation.
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