Nanowires

The design, development and understanding of synthetic materials, with at least one dimension below 100 nm, have been driving a broad range of research in the scientific community for a number of years given the potential of such materials to substantially impact many areas of science and technology. In particular, one-dimensional nanowires, with diameters reaching to the molecular or quantum regime, have been a focus of research over the past two decades. The underlying principles for synthesis of one-dimensional materials have been investigated in different contexts for almost half a century ago, although significant challenges existed in developing the critical understanding to control (i) diameters to the deep nanoscale dimensions as well as (ii) structure and composition in the axial and radial coordinates as necessary for the synthesis of materials with designed and tunable functionality. In this chapter, the emergence of the nanowire research platform is introduced, including the concept and importance, synthetic challenges and initial design, and the development of vapor-liquid-solid crystal growth mechanism. In addition, other nanofabrication based approaches explored in the early years of this field will be briefly introduced.

Over the past two decades, remarkable progress has been made in research focused on the synthesis of 1D NWs leading to the rational design and synthetic control of key properties, where the capability of design and control of NWs has opened up the potential for revolutionary advances in diverse areas ranging from electronics and photonics to energy and healthcare. Scaling NW diameters to the deep nanometer and even molecular regime, as well as controlling their morphology, composition and structure represents fundamental challenges critical to exploiting NWs for applications in science and technology. In this chapter, we overview major bottom-up strategies for the synthesis of NWs, including vapor phase, templated, and solution-based methods. The advantages and challenges of different methods will be discussed, with representative examples illustrated.

Advances in nanoscience and nanotechnology critically depend on the development of nanostructures whose properties are controlled during synthesis. The ability to control and modulate the composition, doping, crystal structure and morphology of semiconductor NWs allows researchers to explore applications of NWs for investigating fundamental scientific questions through developing new technologies. The chapter expands significantly upon the basic methods introduced in the previous chapter for NW synthesis by focusing on controlled growth of a host of NWs with modulated morphologies and structures, including axial and radial heterostructures, kinked, branched, and/or modulated doped structures, where the increased complexity in the NWs can enable unique functional properties.

The rationally designed and synthesized semiconductor NWs offer as a platform material with the potential to realize unprecedented structural and functional complexity as building blocks. To utilize these building blocks for nanoscale devices through integrated systems, for example in electronics and photonics, requires controlled and scalable assembly of NWs on either rigid or flexible substrates. In this chapter, we will summarize recent advances in large-scale NW assembly and hierarchical organization with two general approaches. First, organization of pre-grown NWs onto target substrates in one or more independent steps, where distinct NW building blocks can be used in each assembly step, and second, the direct growth of aligned NWs on substrates will be discussed.

As electronic device features have been pushed into the deep sub-100-nm regime, conventional scaling strategies in the semiconductor industry have faced technological and economic challenges. Electronics obtained through the bottom-up approach of molecular-level control of material composition and structure may lead to devices and fabrication strategies as well as new architectures not readily accessible or even possible within the context of the top-down driven industry and manufacturing infrastructure. This chapter presents a summary of recent advances in basic nanoelectronics devices, simple circuits and nanoprocessors assembled by semiconductor NWs.

Single crystalline semiconductor NWs have been extensively investigated as building blocks for ultra-small and entirely new electronic and photonic devices, due to their unique electronic and optical properties. The sub-wavelength diameters of NW structures and tunable energy band gaps provide a host of advantages for investigating generation, detection, amplification and modulation of light. Photonic platforms using NW building blocks also offers the promise of integrated functionalities at dimensions compatible with top-down fabricated electronics. With rational design and synthesis of the NW structures, the capability of controlling and manipulating these structures on surface to form single devices and networks is a crucial step for realizing these chemically synthesized NWs into photonic circuitry. In this chapter we will review progress made in the area of NW photonic devices, including waveguides, light-emitting diodes, lasers, and photodetectors.

The NW structure is highly anisotropic with radial dimensions confined by the diameters to <100 nm and axial lengths >1000 nm, although the axial device dimension can be confined during synthesis, for example by modulation doping, or by device fabrication. When the confined dimensions of the NWs become comparable to the electron wavelength, the fundamental quantum properties of charge carriers dominate the charge transport and new device properties become possible. In this chapter, we introduce studies over the past decade where quantum properties are critical to the observed behavior, including quantum dot systems in semiconductor NWs, hybrid superconductor-semiconductor NW devices, and NW topological insulators.

A variety of energy storage systems are currently being explored and in some cases commercialized to meet the needs for both small and large-scale energy storage/usage. Among these systems, rechargeable batteries have been extensively investigated in the research community in efforts to make breakthroughs beyond existing commercial lithium ion systems and thereby provide enhancements to capacity, power density and other metrics that would be beneficial to ubiquitous consumer electronic devices through electric automobiles. In this chapter, the advantages of NW structures for efficient energy storage will be illustrated and discussed, including their high surface area, efficient charge transport and capability to sustain large volume expansion/contraction during charge/discharge cycles. In particular, we will introduce and discuss representative works focused on lithium ion batteries, electrochemical capacitors, and sodium ion batteries. Finally, prospects and challenges for implementing NWs for practical energy storage solutions will be briefly discussed.

Substantial recent scientific effort has been focused on the development of renewable energy sources, such as solar energy, in order to lower the carbon footprint for energy usage. Semiconductor NWs are attractive candidates for energy conversion materials since their composition, size and other factors that determine basic electronic and optical properties can be synthetically manipulated in complex ways. In this chapter, we discuss representative NW-based structures and devices for energy conversion, particularly focusing on photovoltaic, thermoelectric, and piezoelectric systems that have been used produce energy by converting light, heat, and mechanical sources.

Sensitive and quantitative analysis of proteins and other biochemical species are central to disease diagnosis, drug screening and proteomic studies. Research advances exploiting SiNWs configured as FETs for biomolecule analysis have emerged as one of the most promising and powerful platforms for label-free, real-time, and sensitive electrical detection of proteins as well as many other biological species. In this chapter, we first briefly introduce the fundamental principle for semiconductor NW-FET sensors. Representative examples of semiconductor NW sensors are then summarized for sensitive chemical and biomolecule detection, including proteins, nucleic acids, viruses and small molecules. In addition, this chapter discusses several electrical and surface functionalization methods for enhancing the sensitivity of semiconductor NW sensors.

The interface between nanosystems and biosystems is emerging as one of the broadest and most dynamic areas of science and technology, bringing together biology, chemistry, physics and many areas of engineering, biotechnology and medicine. The combination of these diverse areas of research promises to yield revolutionary advances in healthcare, medicine and the life science through, for example, the creation of new and powerful tools that enable direct, sensitive and rapid analysis of biological species and cellular activities. Research at the interface between nanomaterials and biology could yield breakthroughs in fundamental science and lead to revolutionary technologies. In this chapter, we will introduce studies focused on building the interface of NWs to cells and tissues, including extracellular and intracellular signal recording, synthetic cyborg tissues and in vivo recording.

The development of NW-based materials has led to breakthrough achievements with rapid expanding impact in all areas of nanotechnology, including but not limited to, electronics, optoelectronics, energy science, sensors and the life sciences. In spite of the progresses discussed in this book, substantial room still exists for NW research and development, and the opportunities may be realized by exquisite control of NW synthesis and assembly, as well as large scale production. In this chapter, we will summarize the basic NW research and applications introduced in this book, and challenges and exciting future opportunities will be discussed.