Massively parallel fabrication of crack-defined gold break junctions featuring sub-3 nm gaps for molecular devices

Flexible Crossbar Molecular Devices with Patterned EGaIn Top Electrodes for Integrated All-Molecule-Circuit Implementation

Here, a unique crossbar architecture is designed and fabricated, incorporating vertically integrated self-assembled monolayers in electronic devices. This architecture is used to showcase 100 individual vertical molecular junctions on a single chip with a high yield of working junctions and high device uniformity. The study introduces a transfer approach for patterned liquid-metal eutectic alloy of gallium and indium top electrodes, enabling the creation of fully flexible molecular devices with electrical functionalities. The devices exhibit excellent charge transport performance, sustain a high rectification ratio (>103), and stable endurance and retention properties, even when the devices are significantly bent. Furthermore, Boolean logic gates, including OR and AND gates, as well as half-wave and full-wave rectifying circuits, are successfully implemented. The unique design of the flexible molecular device represents a significant step in harnessing the potential of molecular devices for high-density integration and possible molecule-based computing.

Enhancing Field Emission in Air via Ultrascaled Nanorod Electrodes

Enhancing field emission in ultrascaled electronics improves the device performance and energy efficiency. Conventional lithography defines electrodes with a few-nanometer spacing on the cost of strengthened electron scattering and the reduced field enhancement factor, thus presenting challenges to enhance field emission at a small bias. Here, we used self-assembled nanorods with sub-5 nm spacing as electrodes to overcome these challenges. Intrinsic ballistic transport through high-crystallinity solution-synthesized nanorods minimized charge scattering; meanwhile ultrascaled anisotropic morphologies concentrated local electric fields and thereby lowered the barrier height. Enabled by these structural features, we demonstrated field emission density up to 4.1 × 104 A cm-2 at 1 V in air, more than 10-fold higher than typical molecular and vacuum electronics at similar conditions, and constructed an air-operating electron source with an on/off ratio of 105 at the collector electrode. Energy-efficient high-conductance electron emission suggested the potential of using solution-synthesized nanomaterials in ultrascaled electronics.

Electromigrated Gold Nanogap Tunnel Junction Arrays: Fabrication and Electrical Behavior in Liquid and Gaseous Media

Tunnel junctions have been suggested as high-throughput electronic single molecule sensors in liquids with several seminal experiments conducted using break junctions with reconfigurable gaps. For practical single molecule sensing applications, arrays of on-chip integrated fixed-gap tunnel junctions that can be built into compact systems are preferable. Fabricating nanogaps by electromigration is one of the most promising approaches to realize on-chip integrated tunnel junction sensors. However, the electrical behavior of fixed-gap tunnel junctions immersed in liquid media has not been systematically studied to date, and the formation of electromigrated nanogap tunnel junctions in liquid media has not yet been demonstrated. In this work, we perform a comparative study of the formation and electrical behavior of arrays of gold nanogap tunnel junctions made by feedback-controlled electromigration immersed in various liquid and gaseous media (deionized water, mesitylene, ethanol, nitrogen, and air). We demonstrate that tunnel junctions can be obtained from microfabricated gold nanoconstrictions inside liquid media. Electromigration of junctions in air produces the highest yield (61-67%), electromigration in deionized water and mesitylene results in a lower yield than in air (44-48%), whereas electromigration in ethanol fails to produce viable tunnel junctions due to interfering electrochemical processes. We map out the stability of the conductance characteristics of the resulting tunnel junctions and identify medium-specific operational conditions that have an impact on the yield of forming stable junctions. Furthermore, we highlight the unique challenges associated with working with arrays of large numbers of tunnel junctions in batches. Our findings will inform future efforts to build single molecule sensors using on-chip integrated tunnel junctions.

High-bandwidth low-current measurement system for automated and scalable probing of tunnel junctions in liquids

Tunnel junctions have long been used to immobilize and study the electronic transport properties of single molecules. The sensitivity of tunneling currents to entities in the tunneling gap has generated interest in developing electronic biosensors with single molecule resolution. Tunnel junctions can, for example, be used for sensing bound or unbound DNA, RNA, amino acids, and proteins in liquids. However, manufacturing technologies for on-chip integrated arrays of tunnel junction sensors are still in their infancy, and scalable measurement strategies that allow the measurement of large numbers of tunneling junctions are required to facilitate progress. Here, we describe an experimental setup to perform scalable, high-bandwidth (>10 kHz) measurements of low currents (pA-nA) in arrays of on-chip integrated tunnel junctions immersed in various liquid media. Leveraging a commercially available compact 100 kHz bandwidth low-current measurement instrument, we developed a custom two-terminal probe on which the amplifier is directly mounted to decrease parasitic probe capacitances to sub-pF levels. We also integrated a motorized three-axis stage, which could be powered down using software control, inside the Faraday cage of the setup. This enabled automated data acquisition on arrays of tunnel junctions without worsening the noise floor despite being inside the Faraday cage. A deliberately positioned air gap in the fluidic path ensured liquid perfusion to the chip from outside the Faraday cage without coupling in additional noise. We demonstrate the performance of our setup using rapid current switching observed in electromigrated gold tunnel junctions immersed in deionized water.

Determination of the Thermomigration Force on Adatoms

Thermal gradients in nanomaterials can cause surface mass transport phenomena. However, the atomic fluxes are challenging to quantify and the underlying atomic mechanisms are complex. Using low energy electron microscopy we have examined in operando, under a thermal gradient of 10^{4}  K/m, the thermomigration of supercooled Si(111)-1×1 advacancy islands. The islands move in the direction of the thermal gradient at 0.26±0.06  nm/s. This reveals that the adatoms move toward the cold region and the effective force exerted on Si adatoms is 1.4±0.4×10^{-8}  eV/nm. We quantify the heat of transport of Si atoms Q^{*}=1.2±0.4  eV and show that it corresponds to the combined effects of adatom creation at step edges and adatom diffusion on atomically flat terraces.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Fabrication of electron tunneling probes for measuring single-protein conductance

Studying the electrical properties of individual proteins is a prominent research area in the field of bioelectronics. Electron tunnelling or quantum mechanical tunnelling (QMT) probes can act as powerful tools for investigating the electrical properties of proteins. However, current fabrication methods for these probes often have limited reproducibility, unreliable contact or inadequate binding of proteins onto the electrodes, so better solutions are required. Here, we detail a generalizable and straightforward set of instructions for fabricating simple, nanopipette-based, tunnelling probes, suitable for measuring conductance in single proteins. Our QMT probe is based on a high-aspect-ratio dual-channel nanopipette that integrates a pair of gold tunneling electrodes with a gap of less than 5 nm, fabricated via the pyrolytic deposition of carbon followed by the electrochemical deposition of gold. The gold tunneling electrodes can be functionalized using an extensive library of available surface modifications to achieve single-protein-electrode contact. We use a biotinylated thiol modification, in which a biotin-streptavidin-biotin bridge is used to form the single-protein junction. The resulting protein-coupled QMT probes enable the stable electrical measurement of the same single protein in solution for up to several hours. We also describe the analysis method used to interpret time-dependent single-protein conductance measurements, which can provide essential information for understanding electron transport and exploring protein dynamics. The total time required to complete the protocol is ~33 h and it can be carried out by users trained in less than 24 h.

Periodic fracture behaviour of nanomembranes

Fracture control in membranes is highly desirable in nano-technology, but is a great challenge because of the 'multi-scale' complexity of fracture initiation and propagation. Here, we devise a method that can controllably direct fractures in stiff nanomembranes, realized by 90° peeling of the nanomembrane overlaid on a soft film, i.e., a stiff/soft bilayer, from a substrate underneath. The peeling allows the stiff membrane to be creased into a soft film periodically in the bending region and fractured along the unique bottom line of the crease, i.e, the fracture route is strictly straight and periodic. The facture period is tunable, because the surface perimeter of the creases is determined by the thickness and modulus of the stiff membranes. This is a new kind of fracture behavior of stiff membranes, which is unique to stiff/soft bilayers but universally exists in such systems, promising a new generation of technology for cutting nanomembranes.

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

Current strategies of plasmonic nanoparticles assisted surface-enhanced Raman scattering toward biosensor studies

With the progressive nanofabrication technology, plasmonic nanoparticles (PNPs) have been increasingly deployed in the field of biosensing. PNPs have favorable biocompatibility, conductivity, and tunable optical properties. In addition, the localized surface plasmon resonance (LSPR) of PNPs plays a vital role in surface-enhanced Raman scattering (SERS). PNPs-based SERS biosensing enables wide-ranging applications for sensitive detection and high spatial and temporal resolution imaging. Numerous reviews of PNPs in the field of SERS biosensing highlight the fabrication or applications in one or more fields. However, the specific strategies for the SERS biosensor construction had not been summarized systematically. Thus, this work offers a comprehensive overview of SERS enhancement strategies based on PNPs, with a focus on SERS label-free detection along with label detection sensing construction, as well as its challenges and future trends.

Design of Pyrrole-Based Gate-Controlled Molecular Junctions Optimized for Single-Molecule Aflatoxin B1 Detection

Food contamination by aflatoxins is an urgent global issue due to its high level of toxicity and the difficulties in limiting the diffusion. Unfortunately, current detection techniques, which mainly use biosensing, prevent the pervasive monitoring of aflatoxins throughout the agri-food chain. In this work, we investigate, through ab initio atomistic calculations, a pyrrole-based Molecular Field Effect Transistor (MolFET) as a single-molecule sensor for the amperometric detection of aflatoxins. In particular, we theoretically explain the gate-tuned current modulation from a chemical-physical perspective, and we support our insights through simulations. In addition, this work demonstrates that, for the case under consideration, the use of a suitable gate voltage permits a considerable enhancement in the sensor performance. The gating effect raises the current modulation due to aflatoxin from 100% to more than 103÷104%. In particular, the current is diminished by two orders of magnitude from the μA range to the nA range due to the presence of aflatoxin B1. Our work motivates future research efforts in miniaturized FET electrical detection for future pervasive electrical measurement of aflatoxins.

Ferroelectric Nanogap-Based Steep-Slope Ambipolar Transistor

The subthreshold swing (SS) of metal-oxide-semiconductor field-effect transistors is limited to 60 mV dec^-1 at room temperature by the Boltzmann tyranny, which restricts the scaling of the supply voltage. A nanogap-based transistor employs a switchable nanoscale air gap as the channel, offering a steep-slope switching process. Meanwhile, nanogaps featuring even sub-3 nm can efficiently block the current flow, exhibiting the potential for tackling the short-channel effect. Here, an electrically switchable ferroelectric nanogap to construct steep-slope transistors, is exploited. An average SS of 15.9 mV dec^-1 across 5 orders and a minimum SS of 13.23 mV dec^-1 are obtained in the high current density range. The transistor exhibits excellent performance with near-zero off-state leakage current and a maximum on-state current of 202 µA µm^-1 at V_DS  = 0.5 V. In addition, the transistor can turn off with either a positive or negative increase in the gate voltage, exhibiting ambipolar characteristics.

Electrical devices designed based on inorganic clusters

The idea of exploring the bottom brink of material science has been carried out for more than two decades. Clusters science is the frontmost study of all nanoscale structures. Being an example of 0-dimensional quantum dot, nanocluster serves as the bridge between atomic and conventionally understood solid-state physics. The forming mechanism of clusters is found to be the mutual effects of electronic and geometric configuration. It is found that electronic shell structure influences the properties and geometric structure of the cluster until its size becomes larger, where electronic effects submerge in geometric structure. The discrete electronic structures depend on the size and conformation of clusters, which can be controlled artificially for potential device applications. Especially, small clusters with a size of 1-2 nm, whose electronic states are possibly discrete enough to overcome thermal fluctuations, are expected to build a single-electron transistor with room temperature operation. However, exciting as the progress may be seen, cluster science still falls within the territory of merely the extension of atomic and molecular science. Its production rate limits the scientific and potential application research of nanoclusters. It is suggested in this review that the mass-produce ability without losing the atomic precision selectivity would be the milestone for nanoclusters to advance to material science.

In Situ Force Microscopy to Investigate Fracture in Stretchable Electronics: Insights on Local Surface Mechanics and Conductivity

Stretchable conductors are of crucial relevance for emerging technologies such as wearable electronics, low-invasive bioelectronic implants, or soft actuators for robotics. A critical issue for their development regards the understanding of defect formation and fracture of conducting pathways during stress-strain cycles. Here we present a combination of atomic force microscopy (AFM) methods that provides multichannel images of surface morphology, conductivity, and elastic modulus during sample deformation. To develop the method, we investigate in detail the mechanical interactions between the AFM tip and a stretched, free-standing thin film sample. Our findings reveal the conditions to avoid artifacts related to sample bending modes or resonant excitations. As an example, we analyze strain effects in thin gold films deposited on a soft silicone substrate. Our technique allows one to observe the details of microcrack opening during tensile strain and their impact on local current transport and surface mechanics. We find that although the film fractures into separate fragments, at higher strain a current transport is sustained by a tunneling mechanism. The microscopic observation of local defect formation and their correlation to local conductivity will provide insight into the design of more robust and fatigue resistant stretchable conductors.

Sub-5 nm nanogap electrodes towards single-molecular biosensing

Nanogap electrodes (NGEs) with sub-5 nm gap has been widely used in single-molecule sensing and sequencing, with the characteristics of label-free, high sensitivity, rapid detection and low-cost. However, the fabrication of sub-5 nm gap electrodes with high controllability and reproducibility still remains a great challenge that impedes the experimental research and the commercialization of the nanogap device. Here, we review the common currently used fabrication methods of nanogap electrodes, such as gap narrowing deposition, mechanical controllable break junctions and the fabrication methods combined with nanopore or nanochannel. We then highlight the typical applications of nanogap electrodes in biological/chemical sensing fields, including single molecule recognition, single molecule sequencing and chemical kinetics analysis. Finally, the challenges of nanogap electrodes in single molecule sensing/sequencing are outlined and the future directions for sensing perspectives are suggested.

High-Throughput Fabrication of Triangular Nanogap Arrays for Surface-Enhanced Raman Spectroscopy

Squeezing light into nanometer-sized metallic nanogaps can generate extremely high near-field intensities, resulting in dramatically enhanced absorption, emission, and Raman scattering of target molecules embedded within the gaps. However, the scarcity of low-cost, high-throughput, and reproducible nanogap fabrication methods offering precise control over the gap size is a continuing obstacle to practical applications. Using a combination of molecular self-assembly, colloidal nanosphere lithography, and physical peeling, we report here a high-throughput method for fabricating large-area arrays of triangular nanogaps that allow the gap width to be tuned from ∼10 to ∼3 nm. The nanogap arrays function as high-performance substrates for surface-enhanced Raman spectroscopy (SERS), with measured enhancement factors as high as 10⁸ relative to a thin gold film. Using the nanogap arrays, methylene blue dye molecules can be detected at concentrations as low as 1 pM, while adenine biomolecules can be detected down to 100 pM. We further show that it is possible to achieve sensitive SERS detection on binary-metal nanogap arrays containing gold and platinum, potentially extending SERS detection to the investigation of reactive species at platinum-based catalytic and electrochemical surfaces.

Nonequilibrium Green's function method for phonon heat transport in quantum system

Phonon heat transport property in quantum devices is of great interesting since it presents significant quantum behaviors. In the past few decades, great efforts have been devoted to establish the theoretical method for phonon heat transport simulation in nanostructures. However, modeling phonon heat transport from wavelike coherent regime to particlelike incoherent regime remains a challenging task. The widely adopted theoretical approach, such as molecular dynamics, semiclassical Boltzmann transport equation, captures quantum mechanical effects within different degrees of approximation. Among them, Non-equilibrium Green's function (NEGF) method has attracted wide attention, as its ability to perform full quantum simulation including many-body interactions. In this review, we summarized recent theoretical advances of phonon NEGF method and the applications on the numerical simulation for phonon heat transport in nanostructures. At last, the challenges of numerical simulation are discussed.

Wafer-Scale and Cost-Effective Manufacturing of Controllable Nanogap Arrays for Highly Sensitive SERS Sensing

The metallic nanogap has been proved as an efficient architecture for surface-enhanced Raman scattering (SERS) applications. Although a lot of nanogap fabrication methods have been proposed in the last few decades, the economical and high-yield manufacturing of sub-10 nm gaps remains a challenge. Here, we present a convenient and cost-effective fabrication method for wafer-scale patterning of metallic nanogaps, which simply combines photolithographic metal patterning, swelling-induced nanocracking, and superimposition metal sputtering without requiring expensive nanofabrication equipment. By controlling the swelling time and metal deposition thickness, the gap size can be precisely defined, down to the sub-10 nm scale. Furthermore, we demonstrate that the fabricated nanogap array can be used as an excellent SERS substrate for molecule measurements and shows a high Raman enhancement factor of ∼10⁸ and a high sensitivity for the detection of rhodamine 6G (R6G) molecules, even down to 10^-14 M, indicating an extraordinary capability for single-molecule detection. Due to its high controllability and wafer-scale fabrication capability, this nanogap fabrication method offers a promising route for highly sensitive and economical SERS detections.

Scalable Fabrication of Metallic Nanogaps at the Sub-10 nm Level

Metallic nanogaps with metal-metal separations of less than 10 nm have many applications in nanoscale photonics and electronics. However, their fabrication remains a considerable challenge, especially for applications that require patterning of nanoscale features over macroscopic length-scales. Here, some of the most promising techniques for nanogap fabrication are evaluated, covering established technologies such as photolithography, electron-beam lithography (EBL), and focused ion beam (FIB) milling, plus a number of newer methods that use novel electrochemical and mechanical means to effect the patterning. The physical principles behind each method are reviewed and their strengths and limitations for nanogap patterning in terms of resolution, fidelity, speed, ease of implementation, versatility, and scalability to large substrate sizes are discussed.

Sub-Picosecond Nanodiodes for Low-Power Ultrafast Electronics

The tradeoff between ultrahigh speed and low power is a dominant challenge in continuously improving modern electronics. Fundamental electronic devices with ultrafast response are highly desired in low-power electronics. However, conventional semiconductor electronic devices now near the speed limit from the physical roadblocks including short-channel effect, restricted carrier velocity, and heat death. Currently emerging electronic devices also face formidable difficulties to achieve high-speed performance at low operating voltage without heat disturbance. Here, a novel fabricated coplanar tip-to-edge semiconductor-free nanostructure with asymmetric sub-10 nm air channel is reported, stimulating electric-field accelerated scattering-free transport of electrons and resulting in ultrafast response of record sub-picoseconds at a low turn-on voltage around 0.7 V. Simulation results show a typical electrical response down to 64 fs, which is ≈10³ times faster than that of conventional semiconductor electronic devices. The coplanar asymmetric nanostructure allows a high rectifying ratio up to 10⁶ which is superior to that of the most promising 2D semiconducting nanodiodes. In addition, heat death is overcome due to the inherent advantages from the novel nanostructure and underlying working mechanism. The intriguing nanodiodes will attract broadly interests in electronics due to their potential as rudimentary building blocks in ultrafast electronic integrated circuits.

Edge-Trimmed Nanogaps in 2D Materials for Robust, Scalable, and Tunable Lateral Tunnel Junctions

Lateral tunnel junctions are fundamental building blocks for molecular electronics and novel sensors, but current fabrication approaches achieve device yields below 10%, which limits their appeal for circuit integration and large-scale application. We here demonstrate a new approach to reliably form nanometer-sized gaps between electrodes with high precision and unprecedented control. This advance in nanogap production is enabled by the unique properties of 2D materials-based contacts. The large difference in reactivity of 2D materials' edges compared to their basal plane results in a sequential removal of atoms from the contact perimeter. The resulting trimming of exposed graphene edges in a remote hydrogen plasma proceeds at speeds of less than 1 nm per minute, permitting accurate control of the nanogap dimension through the etching process. Carrier transport measurements reveal the high quality of the nanogap, thus-produced tunnel junctions with a 97% yield rate, which represents a tenfold increase in productivity compared to previous reports. Moreover, 70% of tunnel junctions fall within a nanogap range of only 0.5 nm, representing an unprecedented uniformity in dimension. The presented edge-trimming approach enables the conformal narrowing of gaps and produces novel one-dimensional nano-trench geometries that can sustain larger tunneling currents than conventional 0D nano-junctions. Finally, the potential of our approach for future electronics was demonstrated by the realization of an atom-based memory.

Application of Micro/Nanofabrication Techniques to On-Chip Molecular Electronics

Molecular electronics is a promising subject to overcome the size limitation of silicon-based electronic devices. In the past decades, various micro/nanofabrication techniques have been developed for constructing molecular junctions, and a number of breakthroughs are made in the characterizations and applications of the single-molecule device. The history and progress are reviewed in this article, laying emphasis on the recent works on the combination of micro/nanofabrication techniques with other techniques such as electrochemical deposition and surface-enhanced Raman spectroscopy (SERS). Some prototypical single-molecule devices such as molecular transistors are presented. Finally, the challenges and prospects in the fabrication of single-molecule devices are discussed.

Hybrid Approach to Fabricate Uniform and Active Molecular Junctions

Molecules can serve as ultimate building blocks for extreme nanoscale devices. This requires their precise integration into functional heterojunctions, most commonly in the form of metal-molecule-metal architectures. Structural damage and nonuniformities caused by current fabrication techniques, however, limit their effective incorporation. Here, we present a hybrid fabrication approach enabling uniform and active molecular junctions. A template-stripping technique is developed to form electrodes with sub-nanometer smooth surfaces. Combined with dielectrophoretic trapping of colloidal nanorods, uniform sub-5 nm junctions are achieved. Uniquely, in our design, the top contact is mechanically free to move under an applied stimulus. Using this, we investigate the electromechanical tuning of the junction and its tunneling conduction. Here, the molecules help control sub-nanometer mechanical modulation, which is conventionally challenging due to instabilities caused by surface adhesive forces. Our versatile approach provides a platform to develop and study active molecular junctions for emerging applications in electronics, plasmonics, and electromechanical devices.

Photoswitching Molecular Junctions: Platforms and Electrical Properties

Remarkable advances in technology have enabled the manipulation of individual molecules and the creation of molecular electronic devices utilizing single and ensemble molecules. Maturing the field of molecular electronics has led to the development of functional molecular devices, especially photoswitching or photochromic molecular junctions, which switch electronic properties under external light irradiation. This review introduces and summarizes the platforms for investigating the charge transport in single and ensemble photoswitching molecular junctions as well as the electronic properties of diverse photoswitching molecules such as diarylethene, azobenzene, dihydropyrene, and spiropyran. Furthermore, the article discusses the remaining challenges and the direction for moving forward in this area for future photoswitching molecular devices.

Recent progress in controlled nano/micro cracking as an alternative nano-patterning method for functional applications

Generally, cracking occurs for many reasons connected to uncertainties and to the non-uniformity resulting from intrinsic deficiencies in materials or the non-linearity of applied external (thermal, mechanical, etc.) stresses. However, recently, an increased level of effort has gone into analyzing the phenomenon of cracking and also into methods for controlling it. Sophisticated manipulation of cracking has yielded various cutting-edge technologies such as transparent conductors, mechanical sensors, microfluidics, and energy devices. In this paper, we present some of the recent progress that has been made in controlling cracking by giving an overview of the fabrication methods and working mechanisms used for various mediums. In addition, we discuss recent progress in the various applications of methods that use controlled cracking as an alternative to patterning tools.

Top-down GaN nanowire transistors with nearly zero gate hysteresis for parallel vertical electronics

This paper reports on the direct qualitative and quantitative performance comparisons of the field-effect transistors (FETs) based on vertical gallium nitride nanowires (GaN NWs) with different NW numbers (i.e., 1–100) and diameters (i.e., 220–640 nm) fabricated on the same wafer substrate to prove the feasibility of employing the vertical 3D architecture concept towards massively parallel electronic integration, particularly for logic circuitry and metrological applications. A top-down approach combining both inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching is applied in the realization of vertically aligned GaN NWs on metalorganic vapor-phase epitaxy (MOVPE)-based GaN thin films with specific doping profiles. The FETs are fabricated involving a stack of n-p-n GaN layers with embedded inverted p-channel, top drain bridging contact, and wrap-around gating technology. From the electrical characterization of the integrated NWs, a threshold voltage (V_th) of (6.6 ± 0.3) V is obtained, which is sufficient for safely operating these devices in an enhancement mode (E-mode). Aluminium oxide (Al₂O₃) grown by atomic layer deposition (ALD) is used as the gate dielectric material resulting in nearly-zero gate hysteresis (i.e., forward and backward sweep V_th shift (ΔV_th) of ~0.2 V). Regardless of the required device processing optimization for having better linearity profile, the upscaling capability of the devices from single NW to NW array in terms of the produced currents could already be demonstrated. Thus, the presented concept is expected to bridge the nanoworld into the macroscopic world, and subsequently paves the way to the realization of innovative large-scale vertical GaN nanoelectronics.

From Single Molecules to Thin Film Electronics, Nanofibers, e-Textiles and Power Cables: Bridging Length Scales with Organic Semiconductors

Organic semiconductors are the centerpiece of several vibrant research fields from single-molecule to organic electronics, and they are finding increasing use in bioelectronics and even classical polymer technology. The versatile chemistry and broad range of electronic functionalities of conjugated materials enable the bridging of length scales 15 orders of magnitude apart, ranging from a single nanometer (10^-9 m) to the size of continents (10⁶ m). This work provides a taste of the diverse applications that can be realized with organic semiconductors. The reader will embark on a journey from single molecular junctions to thin film organic electronics, supramolecular assemblies, biomaterials such as amyloid fibrils and nanofibrillated cellulose, conducting fibers and yarns for e-textiles, and finally to power cables that shuffle power across thousands of kilometers.

Scalable Manufacturing of Single Nanowire Devices Using Crack-Defined Shadow Mask Lithography

Single nanowires (NWs) have a broad range of applications in nanoelectronics, nanomechanics, and nanophotonics, but, to date, no technique can produce single sub-20 nm wide NWs with electrical connections in a scalable fashion. In this work, we combine conventional optical and crack lithographies to generate single NW devices with controllable and predictable dimensions and placement and with individual electrical contacts to the NWs. We demonstrate NWs made of gold, platinum, palladium, tungsten, tin, and metal oxides. We have used conventional i-line stepper lithography with a nominal resolution of 365 nm to define crack lithography structures in a shadow mask for large-scale manufacturing of sub-20 nm wide NWs, which is a 20-fold improvement over the resolution that is possible with the utilized stepper lithography. Overall, the proposed method represents an effective approach to generate single NW devices with useful applications in electrochemistry, photonics, and gas- and biosensing.

Sub-5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

Sub-5 nm metal nanogaps have attracted widespread attention in physics, chemistry, material sciences, and biology due to their physical properties, including great plasmon-enhanced effects in light-matter interactions and charge tunneling, Coulomb blockade, and the Kondo effect under an electrical stimulus. These properties especially meet the needs of many cutting-edge devices, such as sensing, optical, molecular, and electronic devices. However, fabricating sub-5 nm nanogaps is still challenging at the present, and scaled and reliable fabrication, improved addressability, and multifunction integration are desired for further applications in commercial devices. The aim of this work is to provide a comprehensive overview of sub-5 nm nanogaps and to present recent advancements in metal nanogaps, including their physical properties, fabrication methods, and device applications, with the ultimate aim to further inspire scientists and engineers in their research.

Scalable Manufacturing of Nanogaps

The ability to manufacture a nanogap in between two electrodes has proven a powerful catalyst for scientific discoveries in nanoscience and molecular electronics. A wide range of bottom-up and top-down methodologies are now available to fabricate nanogaps that are less than 10 nm wide. However, most available techniques involve time-consuming serial processes that are not compatible with large-scale manufacturing of nanogap devices. The scalable manufacturing of sub-10 nm gaps remains a great technological challenge that currently hinders both experimental nanoscience and the prospects for commercial exploitation of nanogap devices. Here, available nanogap fabrication methodologies are reviewed and a detailed comparison of their merits is provided, with special focus on large-scale and reproducible manufacturing of nanogaps. The most promising approaches that could achieve a breakthrough in research and commercial applications are identified. Emerging scalable nanogap manufacturing methodologies will ultimately enable applications with high scientific and societal impact, including high-speed whole genome sequencing, electromechanical computing, and molecular electronics using nanogap electrodes.