Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6

In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18

1.1. What is Plasmonics?
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal.

While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light.



Figure 1

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ...



In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

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Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

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Metal-Assisted Chemical Etching of Silicon: A Review

Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from preceramic polymers.
(5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

Silicon nanowire (SiNW)-based solar cells on glass substrates have been fabricated by wet electroless chemical etching (using silver nitrate and hydrofluoric acid) of 2.7 μm multicrystalline p+nn+ doped silicon layers thereby creating the nanowire structure. Low reflectance ( 90% at 500 nm) have been measured. The highest open-circuit voltage (Voc) and short-circuit current density (Jsc) for AM1.5 illumination were 450 mV and 40 mA/cm2, respectively at a maximum power conversion efficiency of 4.4%.

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Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters.

The rapidly growing fi eld of nano-optics requires the development of fl exible and reliable fabrication methods at the nanometer scale. For that purpose direct writing of nanostructures is ideal. [ 1 ] One direct writing method at the nanometer scale is electron-beam-induced deposition (EBID). [ 2 ] This method makes use of a precursor gas, inserted into the vacuum chamber of an electron microscope, that is cracked under the focused electron beam. For precursor gases of metal organic (MO) nature (e.g., dimethyl-gold(III)-acetylacetonate as used in the work reported here), the deposits contain the metal component in the form of metal nanoparticles embedded in an amorphous carbonaceous matrix. [ 3 ]

The Direct Writing of Plasmonic Gold Nanostructures by Electron‐Beam‐Induced Deposition

Silicon nanowire (SiNW) ensembles with vertical and zigzag architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(111) and p-Si(100)) with a mask of silver nanoparticles that are deposited by wet electroless deposition. The etching of SiNWs is based on subsequent treatments in chemical solutions such as 0.02 M aqueous solutions of silver nitrate (AgNO3) followed by 5 M hydrofluoric acid and 30% hydrogen peroxide (H2O2). The etching of the Si wafers is mediated by the reduction of silver on the silicon surface and in parallel by the oxidation of Si thereby forming SiO2 which is dissolved in the HF surroundings. The morphology of the starting silver (Ag) layer/Ag nanoparticles that form during processing on the Si wafer surfaces strongly influences the morphology of the SiNW ensembles and homogeneity of the etch profile. Our observations suggest that the Ag layer/Ag nanoparticles not only catalyze the wet chemical etching of silicon but also strongly catalyze the decomposit...

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Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire Architectures

[*] Dr. Y. Wang, Prof. M. Steinhart, Dr. Y. Qin, Dr. A. Berger, E. Yau, L. Zhang, Prof. U. Gosele, Dr. M. Knez Max Planck Institute of Microstructure Physics Weinberg 2, 06120 Halle/Saale (Germany) E-mail: steinhart@mpi-halle.de Dr. Y. Wang State Key Laboratory of Materials-Oriented Chemical Engineering Nanjing University of Technology Nanjing 210009, Jiangsu (P. R. China) E-mail: Yongwang@njut.edu.cn Prof. M. Steinhart Intitute for Chemistry University of Osnabruck Barbarastrase 7 D-49069 Osnabruck

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Nanoscopic Morphologies in Block Copolymer Nanorods as Templates for Atomic-Layer Deposition of Semiconductors

Strong room temperature visible (red-orange) photoluminescence (PL) has been observed in silicon nanowires (SiNWs) that were realized by wet chemical etching of heavily (arsenic, As: ${10}^{20}\text{ }{\text{cm}}^{\ensuremath{-}3}$) and lowly doped (boron, B: ${10}^{15}\text{ }{\text{cm}}^{\ensuremath{-}3}$) single crystalline silicon (Si) wafers. Optical characterization of these SiNWs by PL combined with structural characterization by transmission and scanning electron microscopy strongly suggest that the visible PL at room temperature results from the rough SiNW sidewall structure that is composed of nanoscale features in which quantum confinement effects may occur.

/pdf/roughness-of-silicon-nanowire-sidewalls-and-room-temperature-2akqnaju11.pdf

Andreas Berger

Papers

Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters.

The Direct Writing of Plasmonic Gold Nanostructures by Electron‐Beam‐Induced Deposition

Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire Architectures

Nanoscopic Morphologies in Block Copolymer Nanorods as Templates for Atomic-Layer Deposition of Semiconductors

Roughness of silicon nanowire sidewalls and room temperature photoluminescence