Category Materials Challenges

Summary of Metal Nanoparticle Fabrication

Despite the large variety of fabrication methods, metal island fllms are currently the most common type of nanoparticle used in laboratory solar cell experiments. This is because the technique requires no additional equip­ment or expertise than is already available in a typical solar cell fabrication laboratory, needing only a single metal evaporation and a low temperature anneal. Most other metal nanoparticle fabrication techniques require addi­tional equipment or extensive process development. The MIF technique is very simple, but the resulting arrays are highly complex, often being comprised of closely spaced, irregularly shaped nanoparticles with a wide distribution of sizes.

Low cost, ease of process integration and large area coverage make metal island fllms attractive for industri...

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Thermal Recrystallization

Thermal recrystallization is the process of forming or modifying metal nanoparticles from larger or smaller starting materials using heat.176 Temperatures well below the melting point can still result in dramatic reshaping of metal films and nanoparticles due to the diffusion of metal atoms along grain boundaries and the substrate. Annealing semi-continuous or continuous metal thin films at temperatures on the order of 100 °C or higher can result in the formation of nanoparticle arrays, known as metal island films (MIFs). These are dense arrays of nanoparticles with a wide distribution of nanoparticle size and shape, which is a complex function of both the properties of the starting layer and of the annealing conditions...

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Chemical Synthesis

Chemical synthesis can be used to produce metal nanoparticle colloids, which can subsequently be precipitated onto a substrate in 2D or 3D arrays. Synthesis is typically achieved by the reduction of metal salts, and involves separate stages to nucleate, grow and sort the nanoparticles within solution. Simple one-pot recipes can be used to produce spherical nanoparticles in gram scale quantities.169-171 In contrast to other fabrication methods, chemical synthesis also allows a large degree of control over the three­dimensional (3D) shape of the nanoparticles, as the growth of specific crystal facets can be enhanced or suppressed by changing the reaction conditions...

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Lithography

Lithographic techniques utilize a resist layer to mask the deposition or etching of a planar metal film, resulting in an array of planar metal nano­particles. The primary difference between lithographic techniques is the method used to pattern the resist layer. The most flexible option is electron beam lithography (EBL), where an electron beam is used to sequentially write the pattern of each nanoparticle into the resist. This allows complete freedom of the lateral design of the array: each nanoparticle can have an arbitrarily chosen size, shape and position, with feature sizes down to tens of nanometres.161’162 The drawback is that EBL equipment is expensive and complex, and requires many hours to write a 1 mm2 array. Interference lithography, as introduced in Section 8.7.2...

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Fabrication of Metal Nanoparticles

Metal nanoparticles can be fabricated at low cost over large areas and so are a viable option for the PV industry. For PV applications there is a consider­able difference in the industrial and laboratorial requirements of a metal nanoparticle fabrication technique. Industrial applications require a tech­nique that is fast, low-cost and large area, and does not require complex equipment. These criteria are less important for laboratory use, and instead the level of control over particle size, shape and arrangement are prioritized such that the parameter space available for experiments is maximized.

Metal nanoparticles can be fabricated using a wide variety of techniques, which can broadly be split into three categories: lithography; chemical synthesis; and thermal recrystallization...

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Constituent Metal

So far in this section we have only considered Ag nanoparticles, but other metals also support LSPs in the visible and NIR range.157-159 LSPs can only be excited efficiently at wavelengths where the optical properties of the metal are predominantly due to the behaviour of electrons in the conduction band (i. e. free electrons). Interband transitions act as additional non-radiative decay channels and so either damp or prohibit the excitation of LSPs. The metals with predominantly free-electron behaviour in the visible and NIR are the noble metals (Ag, Au, Cu), the alkali metals (K, Na, Li) and Al. The alkali metals are too chemically unstable to use in practical applications...

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Shape: Spheroids and Prisms

The optical properties of metal nanoparticles are highly sensitive to the particle shape because this influences the collective electron oscillation dynamics and hence the resonance frequency. Therefore it is possible to tune the optical properties of metal nanoparticles while keeping the particle volume constant. For example, stretching one axis of a sphere results in a prolate spheroid, which supports distinct resonances along the longitu­dinal and transverse axes.154 The result is a polarization-sensitive extinction spectrum, with a weak short wavelength peak for polarization aligned along the transverse axes, and a strong long wavelength peak for polarization aligned along the longitudinal axis...

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Size and Surrounding Medium

Small metal nanoparticles exhibit a single narrow extinction peak, which corresponds to the dipolar LSP resonance.146 As the diameter is increased the dipolar peak is broadened, attenuated and shifted to longer wavelengths (Figure 8.16). Additional peaks occur in the spectra of large nanoparticles due to the excitation of higher order modes, with the overall spectra being superpositions of each individual mode.146’151

Spherical nanoparticles in air (n = 1) require large diameters to achieve extinction peaks in the NIR. However, metal nanoparticles embedded within the solar cell structure will have a higher surrounding refractive index (n $ 1.5)...

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