Superlattice Nanowire Pattern Transfer (SNAP)

During the past 15 years or so, nanowires (NWs) have emerged as a new and distinct class of materials. Their novel structural and physical properties separate them from wires that can be prepared using the standard methods for manufacturing electronics. NW-based applications that range from traditional electronic devices (logic and memory) to novel biomolecular and chemical sensors, thermoelectric materials, and optoelectronic devices, all have appeared during the past few years. From a fundamental perspective, NWs provide a route toward the investigation of new physics in confined dimensions. Perhaps the most familiar fabrication method is the vapor−liquid−solid (VLS) growth technique, which produces semiconductor nanowires as bulk materials. However, other fabrication methods exist and have their own advantages. In this Account, I review a particular class of NWs produced by an alternative method called superlattice nanowire pattern transfer (SNAP). The SNAP method is distinct from other nanowire preparation methods in several ways. It can produce large NW arrays from virtually any thin-film material, including metals, insulators, and semiconductors. The dimensions of the NWs can be controlled with near-atomic precision, and NW widths and spacings can be as small as a few nanometers. In addition, SNAP is almost fully compatible with more traditional methods for manufacturing electronics. The motivation behind the development of SNAP was to have a general nanofabrication method for preparing electronics-grade circuitry, but one that would operate at macromolecular dimensions and with access to a broad materials set. Thus, electronics applications, including novel demultiplexing architectures; large-scale, ultrahigh-density memory circuits; and complementary symmetry nanowire logic circuits, have served as drivers for developing various aspects of the SNAP method. Some of that work is reviewed here. As the SNAP method has evolved into a robust nanofabrication method, it has become an enabling tool for the investigation of new physics. In particular, the application of SNAP toward understanding heat transport in low-dimensional systems is discussed. This work has led to the surprising discovery that Si NWs can serve as highly efficient thermoelectric materials. Finally, we turn toward the application of SNAP to the investigation of quasi-one-dimensional (quasi-1D) superconducting physics in extremely high aspect ratio Nb NWs

Molecular Electronics

Molecular electronics describes the field in which molecules are utilized as the active (switching, sensing, etc.) or passive (current rectifiers, surface passivants) elements in electronic devices. This review focuses on experimental aspects of molecular electronics that researchers have elucidated over the past decade or so and that relate to the fabrication of molecular electronic devices in which the molecular components are readily distinguished within the electronic properties of the device. Materials, fabrication methods, and methods for characterizing electrode materials, molecular monolayers, and molecule/electrode interfaces are discussed. A particular focus is on devices in which the molecules or molecular monolayer are sandwiched between two immobile electrodes. Four specific examples of such devices, in which the electron transport characteristics reflect distinctly molecular properties, are discussed

Ultradense, Deep Subwavelength Nanowire Array Photovoltaics As Engineered Optical Thin Films

A photovoltaic device comprised of an array of 20 nm wide, 32 nm pitch array of silicon nanowires is modeled as an optical material. The nanowire array (NWA) has characteristic device features that are deep in the subwavelength regime for light, which permits a number of simplifying approximations. Using photocurrent measurements as a probe of the absorptance, we show that the NWA optical properties can be accurately modeled with rigorous coupled-wave analysis. The densely structured NWAs behave as homogeneous birefringent materials into the ultraviolet with effective optical properties that are accurately modeled using the dielectric functions of bulk Si and SiO_2, coupled with a physical model for the NWA derived from ellipsometry and transmission electron microscopy

Nanotechnology and cancer

The biological picture of cancer is rapidly advancing from models built from phenomenological descriptions to network models derived from systems biology, which can capture the evolving pathophysiology of the disease at the molecular level. The translation of this (still academic) picture into a clinically relevant framework can be enabling for the war on cancer, but it is a scientific and technological challenge. In this review, we discuss emerging in vitro diagnostic technologies and therapeutic approaches that are being developed to handle this challenge. Our discussion of in vitro diagnostics is guided by the theme of making large numbers of measurements accurately, sensitively, and at very low cost. We discuss diagnostic approaches based on microfluidics and nanotechnology. We then review the current state of the art of nanoparticle-based therapeutics that have reached the clinic. The goal of the presentation is to identify nanotherapeutic strategies that are designed to increase efficacy while simultaneously minimizing the toxic side effects commonly associated with cancer chemotherapies

Achieving the Theoretical Depairing Current Limit in Superconducting Nanomesh Films

We show the theoretical depairing current limit can be achieved in a robust fashion in highly ordered superconductor nanomesh films having spatial periodicities smaller than both the superconducting coherence length and the magnetic penetration depth. For a niobium nanomesh film with 34 nm spatial periodicity, the experimental critical current density is enhanced by more than 17 times over the continuous film and is in good agreement with the depairing limit over the entire measured temperature range. The nanomesh superconductors are also less susceptible to thermal fluctuations when compared to nanowire superconductors. T_c values similar to the bulk film are achieved, and the nanomeshes are capable of retaining superconductivity to higher fields relative to the bulk. In addition, periodic oscillations in T_c are observed as a function of field, reflecting the highly ordered nanomesh structure

Scanning Tunneling Microscopy Characterization of the Electrical Properties of Wrinkles in Exfoliated Graphene Monolayers

We report on the scanning tunneling microscopy study of a new class of corrugations in exfoliated monolayer graphene sheets, that is, wrinkles ~10 nm in width and ~3 nm in height. We found such corrugations to be ubiquitous in graphene and have distinctly different properties when compared to other regions of graphene. In particular, a “three-for-six” triangular pattern of atoms is exclusively and consistently observed on wrinkles, suggesting the local curvature of the wrinkle provides a sufficient perturbation to break the 6-fold symmetry of the graphene lattice. Through scanning tunneling spectroscopy, we further demonstrate that the wrinkles have lower electrical conductance and are characterized by the presence of midgap states, which is in agreement with recent theoretical predictions. The observed wrinkles are likely important for understanding the electrical properties of graphene

A Systems Approach to Molecular Electronics

Molecular electronics is an area of research in which molecules are employed to yield the active and passive device components (switches, diodes, resistors) of an electronic circuit or integrated circuit. Consider the crossbar circuits of nanowires in the electron micrograph at the left [1]. The smallest (100 element) crossbar in this image is patterned at a density approach 10^12/cm^2, and the wire diameter is approximately 8 nm. At a doping level (with species like Boron or Arsenic) of 10^18/cm^3, a similar 8 nm diameter, micrometer-long segment of silicon wires would have 20-30 dopant atoms, and a junction of two crossed wires would contain approximately 0.1-0.2 dopant atoms. Thus, field-effect transistors fabricated at these wiring densities might exhibit non-statistical, and perhaps non-predictable behavior. Related arguments, such as the gate oxide thickness, power consumption, (just from leakage currents through the gate oxide), fabrication costs, and others also highlight the difficulty of scaling standard electronics materials to molecular dimensions [2]. The point is that at device areas of a few tens of square nanometers, molecules have a certain fundamental attractiveness because of their size, because they represent the ultimate in terms of atomic control over physical properties, and because of the diversity of properties, such as switching, dynamic organization and recognition that can be achieved through such control.Molecular electronics circuits based on crossbar architectures can be utilized for both logic and memory applications [3], but in order to realize such applications, many things must be simultaneously considered. These include the design of the molecule, the molecule electrode interface, electronically configurable and defect tolerant circuit architectures, methods for bridging the nanometer-scale densities of these circuits to the sub-micrometer densities achievable with lithography, etc. [4] In this talk I will treat such circuits as a system, and discuss how all of these various properties are interrelated. I will also present experimental results of working random-access memory and configurable logic circuits, and FET-based multiplexers capable of bridging length scales.In these circuits the active device elements are molecular mechanical complexes organized at each of the junctions within the crossbar, as shown at left in the drawing. The molecules are switched via 1 or 2 electron process that results in a mechanical isomerization of the molecule, and thereby a change in the tunneling characteristics of the junction. Detailed electrical measurements on single molecule, three-terminal devices are revealing substantial information concerning how these types of devices can be better designed and optimized, and such measurements will also be presented and discussed

Molecular mechanics and molecular electronics

Electronic devices containing molecules as either passive or active (switching) components present the opportunity for scaling electronic circuitry down to near-molecular dimensions. In this paper the kinetic and thermodynamic properties of bistable molecular mechanical switches known as catenanes and rotaxanes are discussed. A defect-tolerant, binary tree demultiplexer architecture using Order log/sub 2/N submicron (lithographically patterned) wires to address TV nanowires are developed. Apart from traditional applications of memory, logic, and routing, new opportunities that include actuation, sensing, energy management, and possibly even peptide sequencing are enabled by these nanofabrication approaches

Nanotechnologies for biomedical science and translational medicine

In 2000 the United States launched the National Nanotechnology Initiative and, along with it, a well-defined set of goals for nanomedicine. This Perspective looks back at the progress made toward those goals, within the context of the changing landscape in biomedicine that has occurred over the past 15 years, and considers advances that are likely to occur during the next decade. In particular, nanotechnologies for health-related genomics and single-cell biology, inorganic and organic nanoparticles for biomedicine, and wearable nanotechnologies for wellness monitoring are briefly covered