Epitaxial Growth of Crystalline CaF2 on Silicene.

Silicene is one of the most promising two-dimensional (2D) materials for the realization of next-generation electronic devices, owing to its high carrier mobility and band gap tunability. To fully control its electronic properties, an external electric field needs to be applied perpendicularly to the 2D lattice, thus requiring the deposition of an insulating layer that directly interfaces silicene, without perturbing its bidimensional nature. A promising material candidate is CaF2, which is known to form a quasi van der Waals interface with 2D materials as well as to maintain its insulating properties even at ultrathin scales. Here we investigate the epitaxial growth of thin CaF2 layers on different silicene phases by means of molecular beam epitaxy. Through electron diffraction images, we clearly show that CaF2 can be grown epitaxially on silicene even at low temperatures, with its domains fully aligned to the lattice of the underlying 2D structure. Moreover, in situ X-ray photoelectron spectroscopy data evidence that, upon CaF2 deposition, no changes in the chemical state of the silicon atoms can be detected, proving that no Si-Ca or Si-F bonds are formed. This clearly shows that the 2D layer is pristinely preserved underneath the insulating layer. Polarized Raman experiments show that silicene undergoes a structural change upon interaction with CaF2; however, it retains its two-dimensional character without transitioning to a sp3-hybridized silicon. For the first time, we have shown that CaF2 and silicene can be successfully interfaced, paving the way for the integration of silicon-based 2D materials in functional devices.

Optical Signatures of Dirac Electrodynamics for hBN-Passivated Silicene on Au(111).

The allotropic affinity for bulk silicon and unique electronic and optical properties make silicene a promising candidate for future high-performance devices compatible with mature complementary metal-oxide-semiconductor technology. However, silicene's outstanding properties are not preserved on its most prominent growth templates, due to strong substrate interactions and hybridization effects. In this letter, we report the optical properties of silicene epitaxially grown on Au(111). A novel in situ passivation methodology with few-layer hexagonal boron nitride enables detailed ex situ characterization at ambient conditions via µ-Raman spectroscopy and reflectance measurements. The optical properties of silicene on Au(111) appeared to be in accordance with the characteristics predicted theoretically for freestanding silicene, allowing the conclusion that its prominent electronic properties are preserved. The absorption features are, however, modified by many-body effects induced by the Au substrate due to an increased screening of electron-hole interactions.

Highly Biaxially Strained Silicene on Au(111).

Many of graphene's remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin-orbit coupling. Because of these outstanding electronic properties, silicene is considered as a promising building block for next-generation electronic devices. Recently, it has been shown that silicene grown on Au(111) still possesses a Dirac cone, despite the interaction with the substrate. Here, to fully characterize the structure of this 2D material, we investigate the vibrational spectrum of a monolayer silicene grown on Au(111) by polarized Raman spectroscopy. To enable a detailed ex situ investigation, we passivated the silicene on Au(111) by encapsulating it under few layers hBN or graphene flakes. The observed spectrum is characterized by vibrational modes that are strongly red-shifted with respect to the ones expected for freestanding silicene. By comparing low-energy electron diffraction (LEED) patterns and Raman results with first-principles calculations, we show that the vibrational modes indicate a highly (>7%) biaxially strained silicene phase.

Silicene Passivation by Few-Layer Graphene.

The stabilization of silicene at ambient conditions is essential for its characterization, future processing, and device integration. Here, we demonstrate in situ encapsulation of silicene on Ag(111) by exfoliated few-layer graphene (FLG) flakes, allowing subsequent Raman analysis under ambient conditions. Raman spectroscopy measurements proved that FLG capping serves as an effective passivation, preventing degradation of silicene for up to 48 h. The acquired data are consistent with former in situ Raman measurements, showing two characteristic peaks, located at 216 and 515 cm-1. Polarization-dependent measurements allowed to identify the two modes as A and E, demonstrating that the symmetry properties of silicene are unaltered by the capping process.

Ultrascaled Germanium Nanowires for Highly Sensitive Photodetection at the Quantum Ballistic Limit.

We report an experimental study on quasi-one-dimensional Al-Ge-Al nanowire (NW) heterostructures featuring unmatched photoconductive gains exceeding 107 and responsivities as high as 10 A/µW in the visible wavelength regime. Our observations are attributed to the presence of GeO x related hole-trapping states at the NW surface and can be described by a photogating effect in accordance with previous studies on low-dimensional nanostructures. Utilizing an ultrascaled photodetector device operating in the quantum ballistic transport regime at room temperature we demonstrate for the first time that individual current channels can be addressed directly by laser irradiation. The resulting quantization of the photocurrent represents the ultimate limit of photodetectors, allowing for advanced concepts including highly resolved imaging, light effect transistors and single photon detectors with practically zero off-state current.

Direct writing of gold nanostructures with an electron beam: On the way to pure nanostructures by combining optimized deposition with oxygen-plasma treatment.

This work presents a highly effective approach for the chemical purification of directly written 2D and 3D gold nanostructures suitable for plasmonics, biomolecule immobilisation, and nanoelectronics. Gold nano- and microstructures can be fabricated by one-step direct-write lithography process using focused electron beam induced deposition (FEBID). Typically, as-deposited gold nanostructures suffer from a low Au content and unacceptably high carbon contamination. We show that the undesirable carbon contamination can be diminished using a two-step process - a combination of optimized deposition followed by appropriate postdeposition cleaning. Starting from the common metal-organic precursor Me2-Au-tfac, it is demonstrated that the Au content in pristine FEBID nanostructures can be increased from 30 atom % to as much as 72 atom %, depending on the sustained electron beam dose. As a second step, oxygen-plasma treatment is established to further enhance the Au content in the structures, while preserving their morphology to a high degree. This two-step process represents a simple, feasible and high-throughput method for direct writing of purer gold nanostructures that can enable their future use for demanding applications.

Room-Temperature Quantum Ballistic Transport in Monolithic Ultrascaled Al-Ge-Al Nanowire Heterostructures.

Conductance quantization at room temperature is a key requirement for the utilizing of ballistic transport for, e.g., high-performance, low-power dissipating transistors operating at the upper limit of "on"-state conductance or multivalued logic gates. So far, studying conductance quantization has been restricted to high-mobility materials at ultralow temperatures and requires sophisticated nanostructure formation techniques and precise lithography for contact formation. Utilizing a thermally induced exchange reaction between single-crystalline Ge nanowires and Al pads, we achieved monolithic Al-Ge-Al NW heterostructures with ultrasmall Ge segments contacted by self-aligned quasi one-dimensional crystalline Al leads. By integration in electrostatically modulated back-gated field-effect transistors, we demonstrate the first experimental observation of room temperature quantum ballistic transport in Ge, favorable for integration in complementary metal-oxide-semiconductor platform technology.

Highly conductive and pure gold nanostructures grown by electron beam induced deposition.

This work introduces an additive direct-write nanofabrication technique for producing extremely conductive gold nanostructures from a commercial metalorganic precursor. Gold content of 91 atomic % (at. %) was achieved by using water as an oxidative enhancer during direct-write deposition. A model was developed based on the deposition rate and the chemical composition, and it explains the surface processes that lead to the increases in gold purity and deposition yield. Co-injection of an oxidative enhancer enabled Focused Electron Beam Induced Deposition (FEBID)-a maskless, resistless deposition method for three dimensional (3D) nanostructures-to directly yield pure gold in a single process step, without post-deposition purification. Gold nanowires displayed resistivity down to 8.8 µΩ cm. This is the highest conductivity achieved so far from FEBID and it opens the possibility of applications in nanoelectronics, such as direct-write contacts to nanomaterials. The increased gold deposition yield and the ultralow carbon level will facilitate future applications such as the fabrication of 3D nanostructures in nanoplasmonics and biomolecule immobilization.

Electrical transport properties of single-crystal Al nanowires.

Single-crystal Al nanowires (NWs) were fabricated by thermally induced substitution of vapor-liquid-solid grown Ge NWs by Al. The resistivity of the crystalline Al (c-Al) NWs was determined to be ρ = (131 ± 27) × 10(-9) Ω m, i.e. approximately five times higher than for bulk Al, but they withstand remarkably high current densities of up to 1.78 × 10(12) A m(-2) before they ultimately melt due to Joule heating. The maximum current density before failure correlates with the NW diameter, with thinner NWs tolerating significantly higher current densities due to efficient heat dissipation and the reduced lattice heating in structures smaller than the electron-phonon scattering length. The outstanding current-carrying capacity of the c-Al NWs clearly exceeds those of common conductors and surpasses requirements for metallization of future high-performance devices. The linear temperature coefficient of the resistance of c-Al NWs appeared to be lower than for bulk Al and a transition to a superconducting state in c-Al NWs was observed at a temperature of 1.46 K.

Synthesis, Morphological, and Electro-optical Characterizations of Metal/Semiconductor Nanowire Heterostructures.

In this letter, we demonstrate the formation of unique Ga/GaAs/Si nanowire heterostructures, which were successfully implemented in nanoscale light-emitting devices with visible room temperature electroluminescence. Based on our recent approach for the integration of InAs/Si heterostructures into Si nanowires by ion implantation and flash lamp annealing, we developed a routine that has proven to be suitable for the monolithic integration of GaAs nanocrystallite segments into the core of silicon nanowires. The formation of a Ga segment adjacent to longer GaAs nanocrystallites resulted in Schottky-diode-like I/V characteristics with distinct electroluminescence originating from the GaAs nanocrystallite for the nanowire device operated in the reverse breakdown regime. The observed electroluminescence was ascribed to radiative band-to-band recombinations resulting in distinct emission peaks and a low contribution due to intraband transition, which were also observed under forward bias. Simulations of the obtained nanowire heterostructure confirmed the proposed impact ionization process responsible for hot carrier luminescence. This approach may enable a new route for on-chip photonic devices used for light emission or detection purposes.

Gate-Tunable Electron Transport Phenomena in Al-Ge⟨111⟩-Al Nanowire Heterostructures.

Electrostatically tunable negative differential resistance (NDR) is demonstrated in monolithic metal-semiconductor-metal (Al-Ge-Al) nanowire (NW) heterostructures integrated in back-gated field-effect transistors (FETs). Unambiguous signatures of NDR even at room temperature are attributed to intervalley electron transfer. At yet higher electric fields, impact ionization leads to an exponential increase of the current in the ⟨111⟩ oriented Ge NW segments. Modulation of the transfer rates, manifested as a large tunability of the peak-to-valley ratio (PVR) and the onset of impact ionization is achieved by the combined influences of electrostatic gating, geometric confinement, and heterojunction shape on hot electron transfer and by electron-electron scattering rates that can be altered by varying the charge carrier concentration in the NW FETs.

Investigation of neurotrophic factor concentrations with a novel in vitro concept for peripheral nerve regeneration.

The regeneration of nerves of the peripheral nervous system after injuries is a complex process. This study presents a novel in vitro neurite regeneration concept to investigate the regeneration of neurons and their processes with different concentrations of neurotrophic factors. The core part of the concept is a transparent microfluidic neurite isolation (NI) device affixed on top of a microelectrode array (MEA), providing a fast and easy way to assess both the growth and the electrical activity of neurites. The NI-MEA isolates neurites from the culture with microchannels that serve as guidance tubes, equipped with microelectrodes. Thus, the NI-MEA allows neurite growth, as observed by microscopy, to be correlated with neurite electrical activity, as measured by electrophysiological recordings. To demonstrate proof of concept of neurite regeneration, we cultured cells from the superior cervical ganglion of postnatal mice under different concentrations of nerve growth factor (NGF). During the regeneration process, we observed an increase in the number of neurites entering the microchannels along with an increase in spike activity recorded by the microelectrodes in the microchannels. We also observed a concentration-dependent effect of neurotrophic factor on the excitability of the growing neurites, with neurites bathed in 20 ng/ml NGF exhibiting enhanced early growth. Thus, our neurite regeneration concept with the NI-MEA device allows further study of neurotrophic factors and reduces the requirement for in vivo experiments on the regeneration of peripheral nerves after injury.

Origin of anomalous piezoresistive effects in VLS grown Si nanowires.

Although the various effects of strain on silicon are subject of intensive research since the 1950s the physical background of anomalous piezoresistive effects in Si nanowires (NWs) is still under debate. Recent investigations concur in that due to the high surface-to-volume ratio extrinsic surface related effects superimpose the intrinsic piezoresistive properties of nanostructures. To clarify this interplay of piezoresistive effects and stress related surface potential modifications, we explored a particular tensile straining device (TSD) with a monolithic embedded vapor-liquid-solid (VLS) grown Si NW. Integrating the suspended NW in a gate all around (GAA) field effect transistor (FET) configuration with a transparent gate stack enables optical and field modulated electrical characterization under high uniaxial tensile strain applied along the ⟨111⟩ Si NW growth direction. A model based on stress-induced carrier mobility change and surface charge modulation is proposed to interpret the actual piezoresistive behavior of Si NWs. By controlling the nature and density of surface states via passivation the "true" piezoresistance of the NWs is found to be comparable with that of bulk Si. This demonstrates the indispensability of application-specific NW surface conditioning and the modulation capability of Si NWs properties for sensor applications.

Direct-write deposition and focused-electron-beam-induced purification of gold nanostructures.

Three-dimensional gold (Au) nanostructures offer promise in nanoplasmonics, biomedical applications, electrochemical sensing and as contacts for carbon-based electronics. Direct-write techniques such as focused-electron-beam-induced deposition (FEBID) can provide such precisely patterned nanostructures. Unfortunately, FEBID Au traditionally suffers from a high nonmetallic content and cannot meet the purity requirements for these applications. Here we report exceptionally pure pristine FEBID Au nanostructures comprising submicrometer-large monocrystalline Au sections. On the basis of high-resolution transmission electron microscopy results and Monte Carlo simulations of electron trajectories in the deposited nanostructures, we propose a curing mechanism that elucidates the observed phenomena. The in situ focused-electron-beam-induced curing mechanism was supported by postdeposition ex situ curing and, in combination with oxygen plasma cleaning, is utilized as a straightforward purification method for planar FEBID structures. This work paves the way for the application of FEBID Au nanostructures in a new generation of biosensors and plasmonic nanodevices.

Free-standing magnetic nanopillars for 3D nanomagnet logic.

Nanomagnet logic (NML) is a relatively new computation technology that uses arrays of shape-controlled nanomagnets to enable digital processing. Currently, conventional resist-based lithographic processes limit the design of NML circuitry to planar nanostructures with homogeneous thicknesses. Here, we demonstrate the focused electron beam induced deposition of Fe-based nanomaterial for magnetic in-plane nanowires and out-of-plane nanopillars. Three-dimensional (3D) NML was achieved based on the magnetic coupling between nanowires and nanopillars in a 3D array. Additionally, the same Fe-based nanomaterial was used to produce tilt-corrected high-aspect-ratio probes for the accurate magnetic force microscopy (MFM) analysis of the fabricated 3D NML gate arrays. The interpretation of the MFM measurements was supported by magnetic simulations using the Object Oriented MicroMagnetic Framework. Introducing vertical out-of-plane nanopillars not only increases the packing density of 3D NML but also introduces an extra magnetic degree of freedom, offering a new approach to input/output and processing functionalities in nanomagnetic computing.

Multimode silicon nanowire transistors.

The combined capabilities of both a nonplanar design and nonconventional carrier injection mechanisms are subject to recent scientific investigations to overcome the limitations of silicon metal oxide semiconductor field effect transistors. In this Letter, we present a multimode field effect transistors device using silicon nanowires that feature an axial n-type/intrinsic doping junction. A heterostructural device design is achieved by employing a self-aligned nickel-silicide source contact. The polymorph operation of the dual-gate device enabling the configuration of one p- and two n-type transistor modes is demonstrated. Not only the type but also the carrier injection mode can be altered by appropriate biasing of the two gate terminals or by inverting the drain bias. With a combined band-to-band and Schottky tunneling mechanism, in p-type mode a subthreshold swing as low as 143 mV/dec and an ON/OFF ratio of up to 10(4) is found. As the device operates in forward bias, a nonconventional tunneling transistor is realized, enabling an effective suppression of ambipolarity. Depending on the drain bias, two different n-type modes are distinguishable. The carrier injection is dominated by thermionic emission in forward bias with a maximum ON/OFF ratio of up to 10(7) whereas in reverse bias a Schottky tunneling mechanism dominates the carrier transport.

Electrostatic actuated strain engineering in monolithically integrated VLS grown silicon nanowires.

In this paper we demonstrate the fabrication and application of an electrostatic actuated tensile straining test (EATEST) device enabling strain engineering in individual suspended nanowires (NWs). Contrary to previously reported approaches, this special setup guarantees the application of pure uniaxial tensile strain with no shear component of the stress while e.g. simultaneously measuring the resistance change of the NW. To demonstrate the potential of this approach we investigated the piezoresistivity of about 3 µm long and 100 nm thick SiNWs but in the same way one can think about the application of such a device on other geometries, other materials beyond Si as well as the use of other characterization techniques beyond electrical measurements. Therefore single-crystal SiNWs were monolithically integrated in a comb drive actuated MEMS device based on a silicon-on-insulator (SOI) wafer using the vapor-liquid-solid (VLS) growth technique. Strain values were verified by a precise measurement of the NW elongation with scanning electron microscopy (SEM). Further we employed confocal µ-Raman microscopy for in situ, high spatial resolution measurements of the strain in individual SiNWs during electrical characterization. A giant piezoresistive effect was observed, resulting in a fivefold increase in conductivity for 3% uniaxially strained SiNWs. As the EATEST approach can be easily integrated into an existing Si technology platform this architecture may pave the way toward a new generation of nonconventional devices by leveraging the strain degree of freedom.

Generation of 3D nanopatterns with smooth surfaces.

Ga implantation into Si and reactive ion etching has been previously identified as candidate techniques for the generation of 3D nanopatterns. However, the structures manufactured using these techniques exhibited impedingly high surface roughness. In this work, we investigate the source of roughness and introduce a new patterning process to solve this issue. The novel patterning process introduces an additional layer absorbing the implanted Ga, thus preventing the clustering of the implanted Ga observed with uncoated Si substrates. This process enables 3D nanopatterning with sub-100 nm lateral resolution in conjunction with smooth height transitions and surface roughness down to 4 nm root mean square. Such patterns are ideally suited for optical applications and enable the manufacturing of nanoimprint lithography templates for low-profile Fresnel lenses.

Peculiarities of temperature dependent ion beam sputtering and channeling of crystalline bismuth.

In this paper, we report on the surface evolution of focused ion beam treated single crystalline Bi(001) with respect to different beam incidence angles and channeling effects. 'Erosive' sputtering appears to be the dominant mechanism at room temperature (RT) and diffusion processes during sputtering seem to play only a minor role for the surface evolution of Bi. The sputtering yield of Bi(001) shows anomalous behavior when increasing the beam incidence angle along particular azimuthal angles of the specimen. The behavior of the sputtering yield could be related to channeling effects and the relevant channeling directions are identified. Dynamic annealing processes during ion irradiation retain the crystalline quality of the Bi specimen allowing ion channeling at RT. Lowering the specimen temperature to T = -188 °C reduces dynamic annealing processes and thereby disables channeling effects. Furthermore unexpected features are observed at normal beam incidence angle. Spike-like features appear during the ion beam induced erosion, whose growth directions are not determined by the ion beam but by the channeling directions of the Bi specimen.

Streptavidin binding as a model to characterize thiol-ene chemistry-based polyamine surfaces for reversible photonic protein biosensing.

Biotin- and iminobiotin-bonded surfaces obtained by thiol-ene chemistry and subsequent modification with polyamines were characterized with respect to streptavidin-binding capacity and reversibility for photonic biosensing using X-ray photoelectron spectroscopy and Mach-Zehnder-interferometric sensors. The streptavidin-iminobiotin system was exploited for reversible multilayer deposition and determination of affinity constants on each layer.