Integration of Topological Insulator Josephson Junctions in Superconducting Qubit Circuits.

The integration of semiconductor Josephson junctions (JJs) in superconducting quantum circuits provides a versatile platform for hybrid qubits and offers a powerful way to probe exotic quasiparticle excitations. Recent proposals for using circuit quantum electrodynamics (cQED) to detect topological superconductivity motivate the integration of novel topological materials in such circuits. Here, we report on the realization of superconducting transmon qubits implemented with (Bi0.06Sb0.94)2Te3 topological insulator (TI) JJs using ultrahigh vacuum fabrication techniques. Microwave losses on our substrates, which host monolithically integrated hardmasks used for the selective area growth of TI nanostructures, imply microsecond limits to relaxation times and, thus, their compatibility with strong-coupling cQED. We use the cavity-qubit interaction to show that the Josephson energy of TI-based transmons scales with their JJ dimensions and demonstrate qubit control as well as temporal quantum coherence. Our results pave the way for advanced investigations of topological materials in both novel Josephson and topological qubits.

Evaporation-driven colloidal cluster assembly using droplets on superhydrophobic fractal-like structures.

Microparticles can be considered building units for functional systems, but their assembly into larger structures typically involves complex methods. In this work, we show that a large variety of macro-agglomerate clusters ("supra-particles") can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades. The key is the use of robust superhydrophobic substrates: in this study we make use of a recently discovered kind of patterned surface with fractal-like microstructures which dramatically reduce the contact of the droplet with the solid substrate. Our results show a clear transition from quasi-2D to 3D clusters as a function of the initial particle concentration, and a clear transition from unstable to stable 3D spheroids as a function of the evaporation rate. The origin of such shape transitions can respectively be found in the dynamic wetting of the fractal-like structure, but also in the enhanced mechanical stability of the particle agglomerate as its particle packing fraction increases.

Attomolar SERS detection of organophosphorous pesticides using silver mirror-like micro-pyramids as active substrate.

Surface-enhanced Raman spectroscopy (SERS) is gaining importance as an ultrasensitive analytical tool for routine high-throughput analysis of a variety of molecular compounds. One of the main challenges is the development of robust, reproducible and cost-effective SERS substrates. In this work, we study the SERS activity of 3D silver mirror-like micro-pyramid structures extended in the z-direction up to 3.7 µm (G0 type substrate) or 7.7 µm (G1 type substrate), prepared by Si-based microfabrication technologies, for trace detection of organophosphorous pesticides, using paraoxon-methyl as probe molecule. The average relative standard deviation (RSD) for the SERS intensity of the peak displayed at 1338 cm-1 recorded over a centimetre scale area of the substrate is below 13% for pesticide concentrations in the range 10-6 to 10-15 mol L-1. This data underlies the spatial uniformity of the SERS response provided by the microfabrication approach. According to finite-difference time-domain (FDTD) simulations, such remarkable feature is mainly due to the contribution on electromagnetic field enhancement of edge plasmon polaritons (EPPs), propagating along the pyramid edges where the pesticide molecules are preferentially adsorbed. Graphical abstract.

Scalable 3D Nanoparticle Trap for Electron Microscopy Analysis.

Arrays of nanoscale pyramidal cages embedded in a silicon nitride membrane are fabricated with an order of magnitude miniaturization in the size of the cages compared to previous work. This becomes possible by combining the previously published wafer-scale corner lithography process with displacement Talbot lithography, including an additional resist etching step that allows the creation of masking dots with a size down to 50 nm, using a conventional 365 nm UV source. The resulting pyramidal cages have different entrance and exit openings, which allows trapping of nanoparticles within a predefined size range. The cages are arranged in a well-defined array, which guarantees traceability of individual particles during post-trapping analysis. Gold nanoparticles with a size of 25, 150, and 200 nm are used to demonstrate the trapping capability of the fabricated devices. The traceability of individual particles is demonstrated by transferring the transmission electron microscopy (TEM) transparent devices between scanning electron microscopy and TEM instruments and relocating a desired collection of particles.

Let's twist again: elasto-capillary assembly of parallel ribbons.

We show the self-assembly through twisting and bending of side by side ribbons under the action of capillary forces. Micro-ribbons made of silicon nitride are batch assembled at the wafer scale. We study their assembly as a function of their dimensions and separating distance. Model experiments are carried out at the macroscopic scale where the tension in ribbons can easily be tuned. The process is modeled considering the competition between capillary, elastic and tension forces. Theory shows a good agreement for macroscale assemblies, while the accuracy is within 30% at the micrometer scale. This simple self-assembly technique yields highly symmetric and controllable structures which could be used for batch fabrication of functional 3D micro-structures.

AFM cantilever with in situ renewable mercury microelectrode.

We report here first results obtained on a novel, in situ renewable mercury microelectrode integrated into an atomic force microscopy (AFM) cantilever. Our approach is based on a fountain pen probe with appropriate dimensions enabling reversible filling with (nonwetting) mercury under changing the applied pressure at a connected mercury supply in a dedicated experimental setup. The fountain pen probe utilizes a special design with vertical pillars inside the channel to minimize mechanical perturbation. In proof of principle experiments, dropping and hanging mercury drop were observed as a function of the applied pressure at the external mercury supply. Electrical conductivity occurred only through the mercury after filling, and the empty fountain pen probe showed excellent electrical insulation. This was demonstrated by chronoamperometric measurements in the electrolyte and by mechanical and electrical contacting of an ITO substrate with a mercury-filled and empty probe in air. Finally, cyclic voltammetry and square wave voltammetry were done in a static mercury electrode fountain pen configuration, demonstrating the principle usability of the mercury probe for electrochemical studies. Our findings are of fundamental importance as they enable further integration of a renewable mercury electrode probe into an AFM setup, which is the subject of ongoing work.

Wafer-scale fabrication of nanoapertures using corner lithography.

Several submicron probe technologies require the use of apertures to serve as electrical, optical or fluidic probes; for example, writing precisely using an atomic force microscope or near-field sensing of light reflecting from a biological surface. Controlling the size of such apertures below 100 nm is a challenge in fabrication. One way to accomplish this scale is to use high resolution tools such as deep UV or e-beam. However, these tools are wafer-scale and expensive, or only provide series fabrication. For this reason, in this study a versatile method adapted from conventional micromachining is investigated to fabricate protruding apertures on wafer-scale. This approach is called corner lithography and offers control of the size of the aperture with diameter less than 50 nm using a low-budget lithography tool. For example, by tuning the process parameters, an estimated mean size of 44.5 nm and an estimated standard deviation of 2.3 nm are found. The technique is demonstrated--based on a theoretical foundation including a statistical analysis--with the nanofabrication of apertures at the apexes of micromachined pyramids. Besides apertures, the technique enables the construction of wires, slits and dots into versatile three-dimensional structures.

3D nanofabrication of fluidic components by corner lithography.

A reproducible wafer-scale method to obtain 3D nanostructures is investigated. This method, called corner lithography, explores the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as structural material or as an inversion mask in subsequent steps. The potential of corner lithography is studied by fabrication of functional 3D microfluidic components, in particular i) novel tips containing nano-apertures at or near the apex for AFM-based liquid deposition devices, and ii) a novel particle or cell trapping device using an array of nanowire frames. The use of these arrays of nanowire cages for capturing single primary bovine chondrocytes by a droplet seeding method is successfully demonstrated, and changes in phenotype are observed over time, while retaining them in a well-defined pattern and 3D microenvironment in a flat array.

Large Dense Periodic Arrays of Vertically Aligned Sharp Silicon Nanocones.

Convex cylindrical silicon nanostructures, also referred to as silicon nanocones, find their value in many applications ranging from photovoltaics to nanofluidics, nanophotonics, and nanoelectronic applications. To fabricate silicon nanocones, both bottom-up and top-down methods can be used. The top-down method presented in this work relies on pre-shaping of silicon nanowires by ion beam etching followed by self-limited thermal oxidation. The combination of pre-shaping and oxidation obtains high-density, high aspect ratio, periodic, and vertically aligned sharp single-crystalline silicon nanocones at the wafer-scale. The homogeneity of the presented nanocones is unprecedented and may give rise to applications where numerical modeling and experiments are combined without assumptions about morphology of the nanocone. The silicon nanocones are organized in a square periodic lattice, with 250 nm pitch giving arrays containing 1.6 billion structures per square centimeter. The nanocone arrays were several mm2 in size and located centimeters apart across a 100-mm-diameter single-crystalline silicon (100) substrate. For single nanocones, tip radii of curvature < 3 nm were measured. The silicon nanocones were vertically aligned, baring a height variation of < 5 nm (< 1%) for seven adjacent nanocones, whereas the height inhomogeneity is < 80 nm (< 16%) across the full wafer scale. The height inhomogeneity can be explained by inhomogeneity present in the radii of the initial columnar polymer mask. The presented method might also be applicable to silicon micro- and nanowires derived through other top-down or bottom-up methods because of the combination of ion beam etching pre-shaping and thermal oxidation sharpening. A novel method is presented where argon ion beam etching and thermal oxidation sharpening are combined to tailor a high-density single-crystalline silicon nanowire array into a vertically aligned single-crystalline silicon nanocones array with < 3 nm apex radius of curvature tips, at the wafer scale.

Method for Keyhole-Free High-Aspect-Ratio Trench Refill by LPCVD.

In micro-machined micro-electromechanical systems (MEMS), refilled high-aspect-ratio trench structures are used for different applications. However, these trenches often show keyholes, which have an impact on the performance of the devices. In this paper, explanations are given on keyhole formation, and a method is presented for etching positively-tapered high-aspect ratio trenches with an optimised trench entrance to prevent keyhole formation. The trench etch is performed by a two-step Bosch-based process, in which the cycle time, platen power, and process pressure during the etch step of the Bosch cycle are studied to adjust the dimensions of the scallops and their location in the trench sidewall, which control the taper of the trench sidewall. It is demonstrated that the amount of chemical flux, being adjusted by the cycle time of the etch step in the Bosch cycle, relates the scallop height to the sidewall profile angle. The required positive tapering of 88° to 89° for a keyhole-free structure after a trench refill by low-pressure chemical vapour deposition is achieved by lowering the time of the etch step.

Self-Aligned Crystallographic Multiplication of Nanoscale Silicon Wedges for High-Density Fabrication of 3D Nanodevices.

High-density arrays of silicon wedges bound by {111} planes on silicon (100) wafers have been created by combining convex corner lithography on a silicon dioxide hard mask with anisotropic, crystallographic etching in a repetitive, self-aligned multiplication procedure. A mean pitch of around 30 nm has been achieved, based on an initial pitch of ∼120 nm obtained through displacement Talbot lithography. The typical resolution of the convex corner lithography was reduced to the sub-10 nm range by employing an 8 nm silicon dioxide mask layer (measured on the {111} planes). Nanogaps of 6 nm and freestanding silicon dioxide flaps as thin as 1-2 nm can be obtained when etching the silicon at the exposed apices of the wedges. To enable the repetitive procedure, it was necessary to protect the concave corners between the wedges through "concave" corner lithography. The produced high-density arrays of wedges offer a promising template for the fabrication of large arrays of nanodevices in various domains with relevant details in the sub-10 nm range.

Capillarity at the nanoscale.

In this critical review we treat the phenomenon of capillarity in nanoscopic confinement, based on application of the Young-Laplace equation. In classical capillarity the curvature of the meniscus is determined by the confining geometry and the macroscopic contact angle. We show that in narrow confinement the influence of the disjoining pressure and the related wetting films have to be considered as they may significantly change the meniscus curvature. Nanochannel based static and dynamic capillarity experiments are reviewed. A typical effect of nanoscale confinement is the appearance of capillarity induced negative pressure. Special attention is paid to elasto-capillarity and electro-capillarity. The presence of electric fields leads to an extra stress term to be added in the Young-Laplace equation. A typical example is the formation of the Taylor cone, essential in the theory of electrospray. Measurements of the filling kinetics of nanochannels with water and aqueous salt solutions are discussed. These experiments can be used to characterize viscosity and apparent viscosity effects of water in nanoscopic confinement. In the final section we show four examples of appearances of capillarity in engineering and in nature (112 references).

Long-range energy propagation in nanometer arrays of light harvesting antenna complexes.

Here we report the first observation of long-range transport of excitation energy within a biomimetic molecular nanoarray constructed from LH2 antenna complexes from Rhodobacter sphaeroides. Fluorescence microscopy of the emission of light after local excitation with a diffraction-limited light beam reveals long-range transport of excitation energy over micrometer distances, which is much larger than required in the parent bacterial system. The transport was established from the influence of active energy-guiding layers on the observed fluorescence emission. We speculate that such an extent of energy migration occurs as a result of efficient coupling between many hundreds of LH2 molecules. These results demonstrate the potential for long-range energy propagation in hybrid systems composed of natural light harvesting antenna molecules from photosynthetic organisms.

Complex Tumor Spheroid Formation and One-Step Cancer-Associated Fibroblasts Purification from Hepatocellular Carcinoma Tissue Promoted by Inorganic Surface Topography.

In vitro cell models play important roles as testbeds for toxicity studies, drug development, or as replacements in animal experiments. In particular, complex tumor models such as hepatocellular carcinoma (HCC) are needed to predict drug efficacy and facilitate translation into clinical practice. In this work, topographical features of amorphous silicon dioxide (SiO2) are fabricated and tested for cell culture of primary HCC cells and cell lines. The topographies vary from pyramids to octahedrons to structures named fractals, with increased hierarchy and organized in periodic arrays (square or Hexagonal). The pyramids were found to promote complex 2D/3D tissue formation from primary HCC cells. It was found that the 2D layer was mainly composed of cancer-associated fibroblasts (CAFs), while the 3D spheroids were composed of tumor cells enwrapped by a CAF layer. Compared with conventional protocols for 3D cultures, this novel approach mimics the 2D/3D complexity of the original tumor by invading CAFs and a microtumor. Topographies such as octahedrons and fractals exclude tumor cells and allow one-step isolation of CAFs even directly from tumor tissue of patients as the CAFs migrate into the structured substrate. Cell lines form spheroids within a short time. The presented inorganic topographical surfaces stimulate complex spheroid formation while avoiding additional biological scaffolds and allowing direct visualization on the substrate.

Capillary negative pressure measured by nanochannel collapse.

A new method is presented to measure capillarity-induced negative pressure. Negative pressures of several bars have been measured for five different liquids (ethanol, acetone, cyclohexane, aniline, and water) over a range of surface tension. Capillary negative pressure was measured in 79 +/- 3 nm silica nanochannels on the basis of the determination of the critical channel width for elastocapillary collapse of the flexible plate covering the channels. The results are consistent with the Young-Laplace equation.

Wafer-scale 3D shaping of high aspect ratio structures by multistep plasma etching and corner lithography.

The current progress of system miniaturization relies extensively on the development of 3D machining techniques to increase the areal structure density. In this work, a wafer-scale out-of-plane 3D silicon (Si) shaping technology is reported, which combines a multistep plasma etching process with corner lithography. The multistep plasma etching procedure results in high aspect ratio structures with stacked semicircles etched deep into the sidewall and thereby introduces corners with a proper geometry for the subsequent corner lithography. Due to the geometrical contrast between the gaps and sidewall, residues are left only inside the gaps and form an inversion mask inside the semicircles. Using this mask, octahedra and donuts can be etched in a repeated manner into Si over the full wafer area, which demonstrates the potential of this technology for constructing high-density 3D structures with good dimensional control in the bulk of Si wafers.

Multi-silicon ridge nanofabrication by repeated edge lithography.

We present a multi-Si nanoridge fabrication scheme and its application in nanoimprint lithography (NIL). Triple Si nanoridges approximately 120 nm high and 40 nm wide separated by 40 nm spacing are fabricated and successfully applied as a stamp in nanoimprint lithography. The fabrication scheme, using a full-wet etching procedure in combination with repeated edge lithography, consists of hot H(3)PO(4) acid SiN(x) retraction etching, 20% KOH Si etching, 50% HF SiN(x) retraction etching and LOCal Oxidation of Silicon (LOCOS). Si nanoridges with smooth vertical sidewalls are fabricated by using Si 110 substrates and KOH etching. The presented technology utilizes a conventional photolithography technique, and the fabrication of multi-Si nanoridges on a full wafer scale has been demonstrated.

Postdeposition UV-Ozone Treatment: An Enabling Technique to Enhance the Direct Adhesion of Gold Thin Films to Oxidized Silicon.

We found that continuous films of gold (Au) on oxidized silicon (SiO2) substrates, upon treatment with ultraviolet (UV)-ozone, exhibit strong adhesion to the SiO2 support. Importantly, the enhancement is independent of micro- or nanostructuring of such nanometer-thick films. Deposition of a second Au layer on top of the pretreated Au layer makes the adhesion stable for at least 5 months in environmental air. Using this treatment method enables us to large-scale fabricate various SiO2-supported Au structures at various thicknesses with dimensions spanning from a few hundreds of nanometers to a few micrometers, without the use of additional adhesion layers. We explain the observed adhesion improvement as polarization-induced increased strength of Auδ-Siδ+ bonds at the Au-SiO2 interface due to the formation of a gold oxide monolayer on the Au surface by the UV-ozone treatment. Our simple and enabling method thus provides opportunities for patterning Au micro/nanostructures on SiO2 substrates without an intermediate metallic adhesion layer, which is critical for biosensing and nanophotonic applications.

Wafer-scale fabrication of high-quality tunable gold nanogap arrays for surface-enhanced Raman scattering.

We report a robust and high-yield fabrication method for wafer-scale patterning of high-quality arrays of dense gold nanogaps, combining displacement Talbot lithography based shrink-etching with dry etching, wet etching, and thin film deposition techniques. By using the self-sharpening of <111>-oriented silicon crystal planes during the wet etching process, silicon structures with extremely smooth nanogaps are obtained. Subsequent conformal deposition of a silicon nitride layer and a gold layer results in dense arrays of narrow gold nanogaps. Using this method, we successfully fabricate high-quality Au nanogaps down to 10 nm over full wafer areas. Moreover, the gap spacing can be tuned by changing the thickness of deposited Au layers. Since the roughness of the template is minimized by the crystallographic etching of silicon, the roughness of the gold nanogaps depends almost exclusively on the roughness of the sputtered gold layers. Additionally, our fabricated Au nanogaps show a significant enhancement of surface-enhanced Raman scattering (SERS) signals of benzenethiol molecules chemisorbed on the structure surface, at an average enhancement factor up to 1.5 × 106.