Real-time TEM imaging of the formation of crystalline nanoscale gaps

Disordered Ballistic Bismuth Nano-waveguides for Highly Efficient Thermoelectric Energy Conversion

Junctions based on electronic ballistic waveguides, such as semiconductor nanowires or nanoribbons with transverse structural variations in the order of a large fraction of their Fermi wavelength, are suggested as highly efficient thermoelectric (TE) devices. Full harnessing of their potential requires a capability to either deterministically induce structural variations that tailor their transmission properties at the Fermi level or alternatively to form waveguides that are disordered (chaotic) but can be structurally modified continuously until favorable TE properties are achieved. Well-established methods to realize either of these routes do not exist. Here, disordered bismuth (Bi) waveguides are reported, which are both formed and structurally tuned by electromigration until their efficiency as TE devices is maximized. In accordance with theory, the conductance of the most efficient TE waveguides is in the sub quantum of conductance regime. The stability of these structures is found to be substantially higher than other actively studied devices such as single molecule junctions.

Tracking the Growth of Superparamagnetic Nanoparticles with an In-Situ Magnetic Particle Spectrometer (INSPECT)

Magnetic Particle Spectroscopy (MPS) is a measurement technique to determine the magnetic properties of superparamagnetic iron oxide nanoparticles (SPIONs) in an oscillating magnetic field as applied in Magnetic Particle Imaging (MPI). State of the art MPS devices are solely capable of measuring the magnetization response of the SPIONs to an oscillatory magnetic excitation retrospectively, i.e. after the synthesis process. In this contribution, a novel in-situ magnetic particle spectrometer (INSPECT) is presented, which can be used to monitor the entire synthesis process from particle genesis via growth to the stable colloidal suspension of the nanoparticles in real time. The device is suitable for the use in a biochemistry environment. It has a chamber size of 72 mm such that a 100 ml reaction flask can be used for synthesis. For an alkaline-based precipitation, the change of magnetic properties of SPIONs during the nucleation and growth phase of the synthesis is demonstrated. The device is able to record the changes in the amplitude and phase spectra, and, in turn, the hysteresis. Hence, it is a powerful tool for an in-depth understanding of the nanoparticle formation dynamics during the synthesis process.

Evaluating nanoscale ultra-thin metal films by means of lateral photovoltaic effect in metal-semiconductor structure

Nanoscale metal-semiconductor (MS) structure materials occupy an important position in semiconductor and microelectronic field due to their abundant physical phenomena and effects. The thickness of metal films is a critical factor in determining characteristics of MS devices. How to detect or evaluate the metal thickness is always a key issue for realizing high performance MS devices. In this work, we propose a direct surface detection by use of the lateral photovoltaic effect (LPE) in MS structure, which can not only measure nanoscale thickness, but also detect the fluctuation of metal films. This method is based on the fact that the output of lateral photovoltaic voltage (LPV) is closely linked with the metal thickness at the laser spot. We believe this laser-based contact-free detection is a useful supplement to the traditional methods, such as AFM, SEM, TEM or step profiler. This is because these traditional methods are always incapable of directly detecting ultra-thin metal films in MS structure materials.

In situ observation of the formation process for free-standing Au nanowires with a scanning electron microscope

We have developed a simultaneous electronic and structural characterization method for studying the formation process for Au nanowires. The method is based on two-probe electronic transport measurement of free-standing Au nanowires and simultaneous structural characterization using scanning electron microscopy (SEM). We measured the electronic currents during the electromigration (EM)-induced narrowing process for the free-standing Au nanowires. A free-standing Au nanowire with a desired conductance value was fabricated by EM. Simultaneous SEM and conductance measurements revealed the EM-induced narrowing process for the Au wires, in which material transfer in the nanowires caused growth towards the positively biased electrode and contact failure at the negatively biased electrode. The narrowed free-standing Au nanowires were stable and could be maintained for more than 10 h without their conductance changing. These results indicate the high stability of the EM-processed Au nanowires compared to Au nanowires fabricated by mechanical elongation or the breaking of Au nanocontacts.

Single-Atom Switches and Single-Atom Gaps Using Stretched Metal Nanowires

Utilizing individual atoms or molecules as functional units in electronic circuits meets the increasing technical demands for the miniaturization of traditional semiconductor devices. To be of technological interest, these functional devices should be high-yield, consume low amounts of energy, and operate at room temperature. In this study, we developed nanodevices called quantized conductance atomic switches (QCAS) that satisfy these requirements. The QCAS operates by applying a feedback-controlled voltage to a nanoconstriction within a stretched nanowire. We demonstrated that individual metal atoms could be removed from the nanoconstriction and that the removed metal atoms could be refilled into the nanoconstriction, thus yielding a reversible quantized conductance switch. We determined the key parameters for the QCAS between the "on" and "off" states at room temperature under a small operating voltage. By controlling the applied bias voltage, the atoms can be further completely removed from the constriction to break the nanowire, generating single-atom nanogaps. These atomic nanogaps are quite stable under a sweeping voltage and can be readjusted with subangstrom accuracy, thus fulfilling the requirement of both reliability and flexibility for the high-yield fabrication of molecular devices.

Highly stable, extremely high-temperature, nonvolatile memory based on resistance switching in polycrystalline Pt nanogaps

Highly stable, nonvolatile, high-temperature memory based on resistance switching was realized using a polycrystalline platinum (Pt) nanogap. The operating temperature of the memory can be drastically increased by the presence of a sharp-edged Pt crystal facet in the nanogap. A short distance between the facet edges maintains the nanogap shape at high temperature, and the sharp shape of the nanogap densifies the electric field to maintain a stable current flow due to field migration. Even at 873 K, which is a significantly higher temperature than feasible for conventional semiconductor memory, the nonvolatility of the proposed memory allows stable ON and OFF currents, with fluctuations of less than or equal to 10%, to be maintained for longer than eight hours. An advantage of this nanogap scheme for high-temperature memory is its secure operation achieved through the assembly and disassembly of a Pt needle in a high electric field.

Depth profiles of the interfacial strains of Si0.7Ge0.3/Si using three-beam Bragg-surface diffraction

Interfacial strains are important factors affecting the structural and physical properties of crystalline multilayers and heterojunctions, and the performance of the devices made of multilayers used, for example, in nanowires, optoelectronic components, and many other applications. Currently existing strain measurement methods, such as grazing incidence X-ray diffraction (GIXD), cross-section transmission electron microscope, TEM, and coherent diffractive imaging, CDI, are limited by either the nanometer spatial resolution, penetration depth, or a destructive nature. Here we report a new non-destructive method of direct mapping the interfacial strain of [001] Si_0.7Ge_0.3/Si along the depth up to ~287 nm below the interface using three-beam Bragg-surface X-ray diffraction (BSD), where one wide-angle symmetric Bragg reflection and a surface reflection are simultaneously involved. Our method combining with the dynamical diffraction theory simulation can uniquely provide unit cell dimensions layer by layer, and is applicable to thicker samples.

Nonlinear Ballistic Transport in an Atomically Thin Material

Ultrashort devices that incorporate atomically thin components have the potential to be the smallest electronics. Such extremely scaled atomically thin devices are expected to show ballistic nonlinear behavior that could make them tremendously useful for ultrafast applications. While nonlinear diffusive electron transport has been widely reported, clear evidence for intrinsic nonlinear ballistic transport in the growing array of atomically thin conductors has so far been elusive. Here we report nonlinear electron transport of an ultrashort single-layer graphene channel that shows quantitative agreement with intrinsic ballistic transport. This behavior is shown to be distinctly different than that observed in similarly prepared ultrashort devices consisting, instead, of bilayer graphene channels. These results suggest that the addition of only one extra layer of an atomically thin material can make a significant impact on the nonlinear ballistic behavior of ultrashort devices, which is possibly due to the very different chiral tunneling of their charge carriers. The fact that we observe the nonlinear ballistic response at room temperature, with zero applied magnetic field, in non-ultrahigh vacuum conditions and directly on a readily accessible oxide substrate makes the nanogap technology we utilize of great potential for achieving extremely scaled high-speed atomically thin devices.

Visualizing size-dependent deformation mechanism transition in Sn

Displacive deformation via dislocation slip and deformation twinning usually plays a dominant role in the plasticity of crystalline solids at room temperature. Here we report in situ quantitative transmission electron microscope deformation tests of single crystal Sn samples. We found that when the sample size was reduced from 450 nm down to 130 nm, diffusional deformation replaces displacive plasticity as the dominant deformation mechanism at room temperature. At the same time, the strength-size relationship changed from "smaller is stronger" to "smaller is much weaker". The effective surface diffusivity calculated based on our experimental data matches well with that reported in literature for boundary diffusion. The observed change in the deformation mode arises from the sample size-dependent competition between the Hall-Petch-like strengthening of displacive processes and Coble diffusion softening processes. Our findings have important implications for the stability and reliability of nanoscale devices such as metallic nanogaps.

In situ transmission electron microscopy imaging of electromigration in platinum nanowires

In situ transmission electron microscopy was performed on the electromigration in platinum (Pt) nanowires (14 nm thick, 200 nm wide, and 300 nm long) with and without feedback control. Using the feedback control mode, symmetric electrodes are obtained and the gap usually forms at the center of the Pt nanowire. Without feedback control, asymmetric electrodes are formed, and the gap can occur at any position along the wire. The three-dimensional gap geometries of the electrodes in the Pt nanowire were determined using high-angle annular dark-field scanning transmission electron microscopy; the thickness of the nanowire is reduced from 14 nm to only a few atoms at the edge with a gap of about 5-10 nm.

A training effect on electrical properties in nanoscale BiFeO3

We report our observation of the training effect on dc electrical properties in a nanochain of BiFeO3 as a result of large scale migration of defects under the combined influence of electric field and Joule heating. We show that an optimum number of cycles of electric field within the range zero to ~1.0 MV cm(-1) across a temperature range 80-300 K helps in reaching the stable state via a glass-transition-like process in the defect structure. Further treatment does not give rise to any substantial modification. We conclude that such a training effect is ubiquitous in pristine nanowires or chains of oxides and needs to be addressed for applications in nanoelectronic devices.

Single molecule electronics and devices

The manufacture of integrated circuits with single-molecule building blocks is a goal of molecular electronics. While research in the past has been limited to bulk experiments on self-assembled monolayers, advances in technology have now enabled us to fabricate single-molecule junctions. This has led to significant progress in understanding electron transport in molecular systems at the single-molecule level and the concomitant emergence of new device concepts. Here, we review recent developments in this field. We summarize the methods currently used to form metal-molecule-metal structures and some single-molecule techniques essential for characterizing molecular junctions such as inelastic electron tunnelling spectroscopy. We then highlight several important achievements, including demonstration of single-molecule diodes, transistors, and switches that make use of electrical, photo, and mechanical stimulation to control the electron transport. We also discuss intriguing issues to be addressed further in the future such as heat and thermoelectric transport in an individual molecule.

Scanning the potential energy surface for synthesis of dendrimer-wrapped gold clusters: design rules for true single-molecule nanostructures

The formation of true single-molecule complexes between organic ligands and nanoparticles is challenging and requires careful design of molecules with size, shape, and chemical properties tailored for the specific nanoparticle. Here we use computer simulations to describe the atomic-scale structure, dynamics, and energetics of ligand-mediated synthesis and interlinking of 1 nm gold clusters. The models help explain recent experimental results and provide insight into how multidentate thioether dendrimers can be employed for synthesis of true single-ligand-nanoparticle complexes and also nanoparticle-molecule-nanoparticle "dumbbell" nanostructures. Electronic structure calculations reveal the individually weak thioether-gold bonds (325 ± 36 meV), which act collectively through the multivalent (multisite) anchoring to stabilize the ligand-nanoparticle complex (∼7 eV total binding energy) and offset the conformational and solvation penalties involved in this "wrapping" process. Molecular dynamics simulations show that the dendrimer is sufficiently flexible to tolerate the strained conformations and desolvation penalties involved in fully wrapping the particle, quantifying the subtle balance between covalent anchoring and noncovalent wrapping in the assembly of ligand-nanoparticle complexes. The computed preference for binding of a single dendrimer to the cluster reveals the prohibitively high dendrimer desolvation barrier (1.5 ± 0.5 eV) to form the alternative double-dendrimer structure. Finally, the models show formation of an additional electron transfer channel between nitrogen and gold for ligands with a central pyridine unit, which gives a stiff binding orientation and explains the recently measured larger interparticle distances for particles synthesized and interlinked using linear ligands with a central pyridine rather than a benzene moiety. The findings stress the importance of organic-inorganic interactions, the control of which is central to the rational engineering and eventual large-scale production of functional building blocks for nano(bio)electronics.

Graphene: nanoscale processing and recent applications

One of the most interesting features of graphene is the rich physics set up by the various nanostructures it may adopt. The planar structure of graphene makes this material ideal for patterning at the nanoscale. The breathtakingly fast evolution of research on graphene growth and preparation methods has made possible the preparation of samples with arbitrary sizes. Available sample production techniques, combined with the right patterning tools, can be used to tailor the graphene sheet into functional nanostructures, even whole electronic circuits. This paper is a review of the existing graphene patterning techniques and potential applications of related lithographic methods.

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-of-states engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of metallic optical antennas based on the background of both well-developed radiowave antenna engineering and plasmonics. In particular, we discuss the role of plasmonic resonances on the performance of nanoantennas and address the influence of geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.

In situ electronic characterization of graphene nanoconstrictions fabricated in a transmission electron microscope

We report electronic measurements on high-quality graphene nanoconstrictions (GNCs) fabricated in a transmission electron microscope (TEM), and the first measurements on GNC conductance with an accurate measurement of constriction width down to 1 nm. To create the GNCs, freely suspended graphene ribbons were fabricated using few-layer graphene grown by chemical vapor deposition. The ribbons were loaded into the TEM, and a current-annealing procedure was used to clean the material and improve its electronic characteristics. The TEM beam was then used to sculpt GNCs to a series of desired widths in the range 1-700 nm; after each sculpting step, the sample was imaged by TEM and its electronic properties were measured in situ. GNC conductance was found to be remarkably high, comparable to that of exfoliated graphene samples of similar size. The GNC conductance varied with width approximately as G(w)=(e2/h)w0.75, where w is the constriction width in nanometers. GNCs support current densities greater than 120 μA/nm2, 2 orders of magnitude higher than that which has been previously reported for graphene nanoribbons and 2000 times higher than that reported for copper.

Field deposition from metallic tips onto insulating substrates

The deposition of gold ions from atomic force microscope cantilever tips onto bulk insulating substrates with nearby surface electrodes is discussed. Numerical models of the potential distribution are used to estimate potential barriers for the desorption process. These models indicate deposition height thresholds of 7-10 nm with the tip 20-25 nm from the metallic electrode edge over a KBr surface but greater than 20 nm high for InP/GaAs/InP substrates with a two-dimensional electron gas (2DEG) as the back electrode. Experimental results for the deposition of gold clusters over KBr surfaces near metal electrodes in ultra-high vacuum (UHV) are presented and show promising agreement with calculations of the deposition threshold heights. Deposition of clusters over InP is discussed for comparison and indicates similar trends.

Electrical breakdown of nanowires

Instantaneous electrical breakdown measurements of GaN and Ag nanowires are performed by an in situ transmission electron microscopy method. Our results directly reveal the mechanism that typical thermally heated semiconductor nanowires break at the midpoint, while metallic nanowires breakdown near the two ends due to the stress induced by electromigration. The different breakdown mechanisms for the nanowires are caused by the different thermal and electrical properties of the materials.

Precise in situ tuning of the critical current of a superconducting nanowire using high bias voltage pulses

We present a method for in situ tuning of the critical current (or switching current) and critical temperature of a superconducting MoGe nanowire using high bias voltage pulses. Our main finding is that as the pulse voltage is increased, the nanowire demonstrates a reduction, a minimum and then an enhancement of the switching current and critical temperature. Using controlled pulsing, the switching current of a superconducting nanowire can be set exactly to a desired value. These results correlate with in situ transmission electron microscope imaging where an initially amorphous nanowire transforms into a single crystal nanowire by high bias voltage pulses. We compare our transport measurements to a thermally activated model of Little's phase slips in nanowires.

Shock-induced breaking in the gold nanowire with the influence of defects and strain rates

Defects in metallic nanowires have raised concerns about the applied reliability of the nanowires in nanoelectromechanical systems. In this paper, molecular dynamics simulations are used to study the deformation and breaking failure of the [100] single-crystal gold nanowires containing defects at different strain rates. The statistical breaking position distributions of the nanowires show mechanical shocks play a critical role in the deformation of nanowires at different strain rates, and deformation mechanism of the nanowire containing defects is based on a competition between shocks and defects in the deformation process of the nanowire. At low strain rate of 1.0% ps(-1), defect ratio of 2% has changed the deformation mechanism because micro-atomic fluctuation is in an equilibrium state. However, owing to strong symmetric shocks, the sensitivity of defects is not obvious before a defect ratio of 25% at high strain rate of 5.0% ps(-1).

Verification of unzipping models of electromigration in gold nanocontacts by in situ high-resolution transmission electron microscopy

We observed in situ the electromigration process of gold (Au) nanocontacts (NCs) by high-resolution transmission electron microscopy. The structural dynamics of the interior and surfaces of the NCs were investigated at the atomic level. In particular, we directly verified the evidence of the unzipping model of electromigration with the in situ observation of surface-edge movement. The fundamental parameters of NCs, i.e., conductance and tensile force, were also measured during in situ lattice imaging of electromigration. Atoms migrating from the negative electrode accumulated at the most constricted regions of the NCs, leading to expansion. As a result, the NCs were compressed by the two electrodes. We demonstrated the magnitude of the force acting on the NCs during electromigration. The critical voltage of electromigration was approximately 80 mV, and the current density at the critical voltage was 60 TA m(-2). We found that Au nanogaps could be fabricated by applying this bias voltage to Au NCs.

High-on/off-ratio graphene nanoconstriction field-effect transistor

A method is reported to pattern monolayer graphene nanoconstriction field-effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback-controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. The opening of a confinement-induced energy gap is observed as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios obtained with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room-temperature on/off ratios for patterned graphene FETs.

Parallel fabrication of polymer-protected nanogaps

A method to create an array of sub-5 nm nanogaps with self-aligned holes in a protective polymer overlayer is presented. The parallel formation of the nanogaps, intended for electrical sensing of biomolecules in an aqueous environment, is achieved by electromigration using a simple voltage ramp across parallel-connected electrode patterns with individual constrictions. It was observed that the nanogap always formed on the cathode side of a bowtie electrode, with corresponding hillocks on the anode side, with the distance of the gap/hillock formation from the constriction depending on the ambient temperature. This technique provides a practical means to fabricate a series of polymer-protected nanogaps with considerably higher efficiency than afforded by the normally slow serial process of electromigration.

Memristive switching of single-component metallic nanowires

Memristors have recently generated significant interest due to their potential use in nanoscale logic and memory devices. Of the four passive circuit elements, the memristor (a two-terminal hysteretic switch) has so far proved hard to fabricate out of a single material. Here we employ electromigration to create a reversible passive electrical switch, a memristive device, from a single-component metallic nanowire. To achieve resistive switching in a single-component structure we introduce a new class of memristors, devices in which the state variable of resistance is the system's physical geometry. By exploiting electromigration to reversibly alter the geometry, we repeatedly switch the resistance of single-component metallic nanowires between low and high states over many cycles. The reversible electromigration causes the nanowire to be cyclically narrowed to approximately 10 nm in width, resulting in a change in resistance by a factor of two. As a result, this work represents a potential route to the creation of nanoscale circuits from a single metallic element.

Reconnectable sub-5 nm nanogaps in ultralong gold nanowires

A protocol is described for forming reconnectable sub-5 nm nanogaps in single ultralong (>100 microm) gold nanowires fabricated by lithographically patterned nanowire electrodeposition (LPNE). During an initial computer-controlled electromigration process, gold nanowires with a rectangular cross-section were transformed by the formation of a constriction at a single point along the 250 microm length of the nanowire, and within this constriction a nanogap of width <5 nm. After this initial nanogap formation, 42% (19 of 45) of the gaps could be reconnected by applying a voltage ramp, restoring the electrical resistance of the original nanowire to within 10%. The voltage threshold for nanogap reconnection was narrowly distributed across multiple wires and nanogaps and in the range from 2 to 3 V. Using voltage programming, it was possible to cycle between the open and closed states for some nanogaps more than 100 times. We propose that the mechanism for reconnection involves the field evaporation of gold, qualitatively as observed previously for metal transfer from the tip of a scanning tunneling microscope.

Exploring the conduction in atomic-sized metallic constrictions created by controlled ion etching

A novel technique to establish atomic-sized contacts in metallic materials is shown. It is based on etching a (sub)micrometric electrode via a low-energy focused ion beam. The in situ measurements of the nanoconstriction resistance during the etching process permit control of the formation of atomic-sized constrictions with milling time, observing steps in the conductance in the range of the conductance quantum (G(0) = 2e(2)/h), just before entering the tunnelling regime. These constrictions are highly stable with time due to the adherence to a substrate, which allows further studies such as the detailed current-voltage transport investigation reported here. Scanning electron microscopy images are used to correlate the etching process and the constriction microstructure. The high control achieved in the process makes us suggest this technique as a promising route to study physical phenomena in the verge of the metal-tunnel conduction crossover.