**1. Introduction: the world of nanowires**

Semiconductor nanowires (NWs) are promising structures in the field of nanoscience. The name nanowire derives from the filamentary shape of these nanostructures. Indeed, they

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

have two dimensions in the range of few to tens of nanometers, while the third dimension is much longer, in the micrometer range. Typical NWs with a filamentary shape are shown in **Figure 1(a)**. The gold nanoparticles, which act as catalyzers during the growth, are clearly visible.

**Figure 1.** (a) Schematic and SEM image of gold‐catalyzed InAs NWs. Scale bar is 200 nm. Courtesy of Dr. L. Sorba, NEST (Pisa, Italy). (b) Schematic of a radial heterostructure, with top‐view SEM image of InAs‐GaSb core‐shell NWs (upper panel) and cross‐sectional TEM image of core‐multishell GaAs/AlGaAs NW heterostructures (lower panel). Courtesy of Dr. L. Sorba, NEST (Pisa, Italy) and Dr. G. Koblmuller, Walter Schottky Institut and Physik Department (Munchen, Germany), respectively. (c) Sketch of an axial NW heterostructure, together with a SEM image of an InP‐ InAs‐InSb NW. Scale bar is 200 nm. Reproduced with permission from Ref. [3]. © 2012, American Chemical Society.

In 1991, K. Hiruma et al. accidentally grew InAs nanowires on GaAs substrates [1]. Since the time, when the word "nanowire" first appeared in a paper, NWs have attracted interest from a large number of scientists around the world owing to the extraordinary opportunities that they enable. In fact, NWs are smaller than bulk crystals and larger than nanocrystals, thus providing a natural bridge between macroscopic and microscopic worlds in both research and technology fields. Moreover, due to the anisotropic shape of NWs and their high surface‐to‐ volume ratio, finite‐size and surface/interface effects are more important than the (more known) quantum confinement effects, a circumstance that renders NWs an ideal platform for the discovery of a variety of novel phenomena.

Semiconductor NWs can exist in many different chemical compositions, structures, and shapes. Regarding the chemical composition, NWs can be made by elemental semiconductors like Si and Ge, or by III–V compounds (e.g., GaAs), II–VI compounds (e.g., CdSe), III–V alloys (e.g., InGaAs), III‐nitrides (e.g., GaN), oxides (e.g., ZnO), etc. Not only homogeneously composed NWs exist, but also different materials can be mixed together in the same NW to form heterostructured NWs. Heterostructures are typically prepared in two ways [2]. In radial structures, one or more materials are grown around a NW, in a so‐called core‐shell or core‐ multishell arrangement. **Figure 1(b)** shows a schematic of this kind of structures along with a scanning electron microscope image (SEM) of an InAs‐GaSb core‐shell NW and a cross‐section transmission electron microscopy (TEM) image of a core‐multishell GaAs/AlGaAs NW. In axial structures, the NW composition is varied along the NW main axis, as depicted in **Figure 1(c)**, whereas an SEM image of an InP/InAs/InSb axial heterostructure is also displayed [3]. In the NW form, highly mismatched materials can grow on top of each other without misfit dislo‐ cations, due to the NW capability to accommodate strain by a coherent expansion of the lattice outward.

have two dimensions in the range of few to tens of nanometers, while the third dimension is much longer, in the micrometer range. Typical NWs with a filamentary shape are shown in **Figure 1(a)**. The gold nanoparticles, which act as catalyzers during the growth, are clearly

**Figure 1.** (a) Schematic and SEM image of gold‐catalyzed InAs NWs. Scale bar is 200 nm. Courtesy of Dr. L. Sorba, NEST (Pisa, Italy). (b) Schematic of a radial heterostructure, with top‐view SEM image of InAs‐GaSb core‐shell NWs (upper panel) and cross‐sectional TEM image of core‐multishell GaAs/AlGaAs NW heterostructures (lower panel). Courtesy of Dr. L. Sorba, NEST (Pisa, Italy) and Dr. G. Koblmuller, Walter Schottky Institut and Physik Department (Munchen, Germany), respectively. (c) Sketch of an axial NW heterostructure, together with a SEM image of an InP‐ InAs‐InSb NW. Scale bar is 200 nm. Reproduced with permission from Ref. [3]. © 2012, American Chemical Society.

In 1991, K. Hiruma et al. accidentally grew InAs nanowires on GaAs substrates [1]. Since the time, when the word "nanowire" first appeared in a paper, NWs have attracted interest from a large number of scientists around the world owing to the extraordinary opportunities that they enable. In fact, NWs are smaller than bulk crystals and larger than nanocrystals, thus providing a natural bridge between macroscopic and microscopic worlds in both research and technology fields. Moreover, due to the anisotropic shape of NWs and their high surface‐to‐ volume ratio, finite‐size and surface/interface effects are more important than the (more known) quantum confinement effects, a circumstance that renders NWs an ideal platform for

Semiconductor NWs can exist in many different chemical compositions, structures, and shapes. Regarding the chemical composition, NWs can be made by elemental semiconductors like Si and Ge, or by III–V compounds (e.g., GaAs), II–VI compounds (e.g., CdSe), III–V alloys (e.g., InGaAs), III‐nitrides (e.g., GaN), oxides (e.g., ZnO), etc. Not only homogeneously composed NWs exist, but also different materials can be mixed together in the same NW to form heterostructured NWs. Heterostructures are typically prepared in two ways [2]. In radial structures, one or more materials are grown around a NW, in a so‐called core‐shell or core‐ multishell arrangement. **Figure 1(b)** shows a schematic of this kind of structures along with a scanning electron microscope image (SEM) of an InAs‐GaSb core‐shell NW and a cross‐section transmission electron microscopy (TEM) image of a core‐multishell GaAs/AlGaAs NW. In axial structures, the NW composition is varied along the NW main axis, as depicted in **Figure 1(c)**,

the discovery of a variety of novel phenomena.

visible.

82 Raman Spectroscopy and Applications

Regarding the shape, NWs have been grown in many different, sometimes funny, shapes that could add functionalities to nanoscale devices [4]. For instance, NWs were grown in branched or flower‐like morphologies [5], where an increased surface area ensures higher power‐ conversion efficiency compared to straight and vertical NWs.

NWs are the nanomaterial system in which pivotal key parameters, such as composition, structure, morphology, and doping, have been best controlled to date. At the heart of this control is the development of successful methods for NW growth. In the top‐down technology, lithographic techniques allow to carve the nanowire structure out of a bulk material. This kind of approach ensures fine control over the position of the NWs, but the crystal quality of the NWs is not excellent (due to the surface damage produced by etching processes), and large‐ area production is problematic and expensive. Therefore, bottom‐up NW fabrication is the most diffused. Since one‐dimensional growth on a substrate is not energetically favored with respect to a two‐dimensional growth, a change of the initial surface/interface is required to activate the NW growth. This change can be done by creating holes in the substrate (selective area growth) or by using metal particles, such as gold, to induce the crystal growth (particle‐ assisted growth) via the so‐called "vapor‐liquid‐solid" (VLS) mechanism that was first invoked in the 1960s to explain the growth of Si whiskers [6]. In both cases, epitaxial techniques are employed to fabricate NWs. In **Figure 2**, the main steps of a typical VLS growth process leading

**Figure 2.** Main steps of the VLS growth process of III–V nanowire grown by metal‐organic vapor phase epitaxy (MOVPE). First, a gold particle (with diameter <100 nm) is deposited/formed on a substrate made by III–V crystal. Then, the particle is heated until it melts. The liquid droplet often consists of a molten eutectic alloy between Au and the substrate. Afterwards, group‐III and group‐V precursors in the vapor phase are introduced. They do not decom‐ pose on the solid substrate, as typically the temperature used during growth is too low. Decomposition only occurs at the droplet interface due to the catalytic effect of the metal. The red path represents absorption, diffusion in the liquid droplet, and precipitation of adatoms at the base of the droplet. The precipitation is due to the continued incorporation into the liquid droplet leading to a supersaturation. Finally, axial and radial growth of the NW occurs as soon as addi‐ tional material precipitates on the growth interface. The three possible diffusion paths of adatoms before adsorption are sketched by black arrows.

to free‐standing III–V NWs are explained, since this is the most widely used technique to grow high‐quality nanowires. Each gold droplet represents the nucleation site of a NW, so there are approximately as many NWs as the droplets are. The gold droplets can be directly deposited on the substrate or result from the annealing of an Au thin film. To achieve a perfect control on the NW position, an array of gold particles can be even prepared by using lithography techniques.

Thanks to the high degree of control reached on the NW growth process, nowadays most of NW properties can be finely tuned, to such an extent that the creation of NWs tailor fit to specific applications is close to be achieved. Due to the several technological applications enabled by NWs, the interest of the scientific community on them is rapidly growing, as testified by the exponentially growing number of papers published on NWs in the last two decades. The field where the peculiar shape and dimensions of NWs have revealed to have great potential in enabling new functions and/or simply enhancing performances of existing devices is very broad, as it includes electronics, photonics, biosensing, energy conversion, and storage [7]. Therefore, we will pick few examples taken from such a huge variety. For instance, the capability to controllably dope NWs is routinely exploited in field effect transistors [8] while the low mass peculiar to NWs renders them ideal to be used in cantilever force sensors [9] and the NW flexibility makes them easily integrable in devices like flexible displays and artificial skin [10]. Moreover, the NW diameter, smaller than the light wavelength in the visible range, allows NWs to confine electromagnetic waves in the radial direction while guiding light in the axial direction, a property that has been exploited to build NW‐based antennas, lasers, and light‐emitting diodes (LEDs). An example of a multicolor NW‐based LED is shown in **Figure 3(a)** [11]. In the field of energy conversion, we mention that the typically low thermal

**Figure 3.** Two very recent examples of the technological power of NWs. (a) Schematic of monolithically integrated multicolor single InGaN/GaN dot‐in‐nanowire LED pixels on a single chip. Light emission wavelength is tuned across the visible spectrum by varying the nanowire diameter. Reproduced with permission from [11]. Copyright 2016, Amer‐ ican Chemical Society. (b) SEM image showing the deformation of an array of InP NWs in direct contact with the body of a phytopathogen Xylella fastidiosa cell (colorized in green). The ordered NW array allows to evaluate single cell adhesion forces and to explore their dependence on organochemical surface compositions. Reproduced with permis‐ sion from Ref. [14]. © 2016, American Chemical Society.

conductivity of NWs makes them ideal building blocks of thermoelectric devices able to convert wasted heat into electricity [12], while the high surface‐to‐volume ratio of NWs ensures great light absorption capability, which is currently exploited in the fabrication of high‐ efficiency solar cells [13]. Finally, interfacing NWs with living cells for delivering drugs or doing sensing activity is a very fascinating field. As an example, in **Figure 3(b)** we show an array of NWs acting as force sensors for bacterial cell adhesion [14].

After this brief overview on the NW world, including growth processes and technological applications, we will describe the properties of NWs that can be addressed by Raman spec‐ troscopy. Indeed, a deep understanding of the NW properties is the starting point to design powerful devices as well as to perform important fundamental physics studies.
