Patrick Galliker

Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets

Nanotechnology, with its broad impact on societally relevant applications, relies heavily on the availability of accessible nanofabrication methods. Even though a host of such techniques exists, the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters equal to that of a single ejected nanodroplet. The virtually material-independent growth of nanostructures into the third dimension is then governed by an autofocussing phenomenon caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate the capabilities of our electrohydrodynamic printing technique with several examples, including the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.

Observation of all the intermediate steps of a chemical reaction on an oxide surface by scanning tunneling microscopy.

By means of high-resolution scanning tunneling microscopy (STM), we have revealed unprecedented details about the intermediate steps for a surface-catalyzed reaction. Specifically, we studied the oxidation of H adatoms by O(2) molecules on the rutile TiO(2)(110) surface. O(2) adsorbs and successively reacts with the H adatoms, resulting in the formation of water species. Using time-lapsed STM imaging, we have unraveled the individual reaction intermediates of HO(2), H(2)O(2), and H(3)O(2) stoichiometry and the final reaction product-pairs of water molecules, [H(2)O](2). Because of their different appearance and mobility, these four species are discernible in the time-lapsed STM images. The interpretation of the STM results is corroborated by density functional theory calculations. The presented experimental and theoretical results are discussed with respect to previous reports where other reaction mechanisms have been put forward.

Near-Field Light Design with Colloidal Quantum Dots for Photonics and Plasmonics

Colloidal quantum-dots are bright, tunable emitters that are ideal for studying near-field quantum-optical interactions. However, their colloidal nature has hindered their facile and precise placement at desired near-field positions, particularly on the structured substrates prevalent in plasmonics. Here, we use high-resolution electro-hydrodynamic printing (<100 nm feature size) to deposit countable numbers of quantum dots on both flat and structured substrates with a few nanometer precision. We also demonstrate that the autofocusing capability of the printing method enables placement of quantum dots preferentially at plasmonic hot spots. We exploit this control and design diffraction-limited photonic and plasmonic sources with arbitrary wavelength, shape, and intensity. We show that simple far-field illumination can excite these near-field sources and generate fundamental plasmonic wave-patterns (plane and spherical waves). The ability to tailor subdiffraction sources of plasmons with quantum dots provides a complementary technique to traditional scattering approaches, offering new capabilities for nanophotonics.

A Novel 3D Integrated Platform for the High-Resolution Study of Cell Migration Plasticity

Understanding the mechanisms of interstitial cancer migration is of great scientific and medical interest. Creating 3D platforms, conducive to optical microscopy and mimicking the physical parameters (in plane and out of plane) involved in interstitial migration, is a major step forward in this direction. Here, a novel approach is used to directly print free-form, 3D micropores on basal scaffolds containing microgratings optimized for contact guidance. The platforms so formed are validated by monitoring cancer cell migration and micropore penetration with high-resolution optical microscopy. The shapes, sizes and deformability of the micropores are controllable, paving the way to decipher their role in interstitial migration.

Dielectrophoretic bending of directly printed free-standing ultra-soft nanowires

Electrohydrodynamic printing has shown superior resolution compared to conventional ink-jet printing, but the use of electrically charged liquid commonly leads to unwanted repulsion effects posing a threshold to resolution capabilities. However, a recently demonstrated controlled dripping process of nanoscale, particle-laden droplets, could circumvent such resolution obstacles even on insulating substrates. Here, we show that so-printed free-standing nanostructures can be autonomously deformed, and mechanically characterized due to the presence of the electrified nozzle, or, after voltage termination, due to transient charge residuals on the structures themselves. Dielectrophoretic forces, arising between two subsequently printed nanopillars lead to their contactless bending and to the formation of out-of-plane arc structures arising from the connection of the pillar apexes. Once connected, the ultra-soft nanopillars are found to be tightly merged and could, for example, serve in electronics as out of plane nanobonds.

Full-Spectrum Flexible Color Printing at the Diffraction Limit

Color printing at the diffraction limit has been recently explored by fabricating nanoscale plasmonic structures with electron beam lithography. However, only a limited color range and constant intensity throughout the structure have been demonstrated. Here we show an alternative, facile approach relying on the direct, open-atmosphere electrohydrodynamic rapid NanoDrip printing of controlled amounts of red, green, and blue (RGB) quantum dots at a resolution of 250 nm. The narrow emission spectrum of the dots allows the coverage of a very broad color space, exceeding standard RGB (sRGB) of modern display devices. We print color gradients of variable intensity, which to date could not be achieved with diffraction-limited resolution. Showcasing the capabilities of the technology, we present a photorealistic printed image of a colorful parrot with a pixel size of 250 nm.

Open-atmosphere sustenance of highly volatile attoliter-size droplets on surfaces

The controlled formation and handling of minute liquid volumes on surfaces is essential to the success of microfluidics in biology, chemistry, and materials applications. Even though current methods have demonstrated their potential in a variety of experimental assays, there remain significant difficulties concerning breadth of applicability, standardization, throughput, and economics. Here we introduce a unique microfluidic paradigm in which microscopic volatile droplets are formed, sustained, and manipulated in size and content at any desired spot on unpatterned substrates. Their sustainability is warranted by continuous replacement of the rapidly vaporizing sessile fluid through controlled equivalent volume deposition of smaller discrete liquid entities by an electrohydrodynamic nanodripping process. Using nanoparticle inks we show that the concentration of solutes in so-stabilized droplets can be linearly increased at isochoric conditions and user-defined rates. An intriguing insensitivity of the droplet shape toward surface heterogeneities ensures robustness and experimental reproducibility, even when handling attoliter quantities. The unique capabilities and technical simplicity of the presented method introduce a high degree of flexibility and make it pertinent to a diverse range of applications.