Field Emission from Carbon Nanotubes on Titanium Nitride-Coated Planar and 3D-Printed Substrates.

Carbon nanotubes (CNTs) are well known for their outstanding field emission (FE) performance, facilitated by their unique combination of electrical, mechanical, and thermal properties. However, if the substrate of choice is a poor conductor, the electron supply towards the CNTs can be limited, restricting the FE current. Furthermore, ineffective heat dissipation can lead to emitter-substrate bond degradation, shortening the field emitters' lifetime. Herein, temperature-stable titanium nitride (TiN) was deposited by plasma-enhanced atomic layer deposition (PEALD) on different substrate types prior to the CNT growth. A turn-on field reduction of up to 59% was found for the emitters that were generated on TiN-coated bulk substrates instead of on pristine ones. This observation was attributed exclusively to the TiN layer as no significant change in the emitter morphology could be identified. The fabrication route and, consequently, improved FE properties were transferred from bulk substrates to free-standing, electrically insulating nanomembranes. Moreover, 3D-printed, polymeric microstructures were overcoated by atomic layer deposition (ALD) employing its high conformality. The results of our approach by combining ALD with CNT growth could assist the future fabrication of highly efficient field emitters on 3D scaffold structures regardless of the substrate material.

Subtractive Low-Temperature Preparation Route for Porous SiO2 Used for the Catalyst-Assisted Growth of ZnO Field Emitters.

The possibility to gradually increase the porosity of thin films facilitates a variety of applications, such as anti-reflective coatings, diffusion membranes, and the herein investigated tailored nanostructuring of a substrate for subsequent self-assembly processes. A low-temperature (<160 °C) preparation route for porous silicon oxide (porSiO2) thin films with porosities of about 60% and effective refractive indices down to 1.20 is tailored for bulk as well as free-standing membranes. Subsequently, both substrate types are successfully employed for the catalyst-assisted growth of nanowire-like zinc oxide (ZnO) field emitters by metal organic chemical vapor deposition. ZnO nanowires can be grown with a large aspect ratio and exhibit a good thermal and chemical stability, which makes them excellent candidates for field emitter arrays. We present a method that allows for the direct synthesis of nanowire-like ZnO field emitters on free-standing membranes using a porSiO2 template. Besides the application of porSiO2 for the catalyst-assisted growth of nanostructures and their use as field emission devices, the herein presented general synthesis route for the preparation of low refractive index films on other than bulk substrates-such as on free-standing, ultra-thin membranes-may pave the way for the employment of porSiO2 in micro-electro-mechanical systems.

Culturing human iPSC-derived neural progenitor cells on nanowire arrays: mapping the impact of nanowire length and array pitch on proliferation, viability, and membrane deformation.

Nanowire arrays used as cell culture substrates build a potent tool for advanced biological applications such as cargo delivery and biosensing. The unique topography of nanowire arrays, however, renders them a challenging growth environment for cells and explains why only basic cell lines have been employed in existing studies. Here, we present the culturing of human induced pluripotent stem cell-derived neural progenitor cells on rectangularly arranged nanowire arrays: In detail, we mapped the impact on proliferation, viability, and topography-induced membrane deformation across a multitude of array pitches (1, 3, 5, 10 µm) and nanowire lengths (1.5, 3, 5 µm). Against the intuitive expectation, a reduced proliferation was found on the arrays with the smallest array pitch of 1 µm and long NWs. Typically, cells settle in a fakir-like state on such densely-spaced nanowires and thus experience no substantial stress caused by nanowires indenting the cell membrane. However, imaging of F-actin showed a distinct reorganization of the cytoskeleton along the nanowire tips in the case of small array pitches interfering with regular proliferation. For larger pitches, the cell numbers depend on the NW lengths but proliferation generally continued although heavy deformations of the cell membrane were observed caused by the encapsulation of the nanowires. Moreover, we noticed a strong interaction of the nanowires with the nucleus in terms of squeezing and indenting. Remarkably, the cell viability is maintained at about 85% despite the massive deformation of the cells. Considering the enormous potential of human induced stem cells to study neurodegenerative diseases and the high cellular viability combined with a strong interaction with nanowire arrays, we believe that our results pave the way to apply nanowire arrays to human stem cells for future applications in stem cell research and regenerative medicine.