Nanoporous metallic networks: growth process and optical properties

. Nanoporous metallic systems exhibit a new generation of advanced materials with potential in a wide variety of technological fields among them catalysis, photonics, optoelectronics and sensors. Their high surface-to-volume ratio, multimodal nanoscale moieties, ability to host guest materials, and inhomogeneous surface at the submicron scale distinct them from both bulk metals and conventional plasmonic materials as well as meta-surfaces. Those structures can be prepared through different fabrication and synthesis strategies including chemical dealloying, assembly of pre-synthesized metallic nanoparticles, and via templating. In a sharp contrast with these preparation strategies, we have demonstrated one can fabricate a macroscopic nanopourus metallic networks by using physical vapor deposition in a short single-step process. These materials are highly pure, and they show very unique linear and non-linear optical properties, among them high Second-Harmonic-generation response. Herein, we will discuss their growth process mechanism, and utilize it for more complex 3d structure which behave as SHG reflectors.

Disordered metallic networks have unique properties that derive from their increased disorder.Specific optical eigenmodes may appear over a broad optical range and their localization to a tiny sub-wavelength volume can occur.[1][2][3] In addition, when large enough, such nanoporous metallic networks could show resilience to damage, making them interesting for applications in extreme conditions, like aerospace and defense.Those structures can be prepared through different fabrication processes.Yet, practically, all the current available preparation techniques are multistep, and the resulting nanoporous metal contains foreign additives which dominate their optical properties and their performance.[1] We have demonstrated that one can fabricate a macroscopic nanopourus metallic networks by using physical vapor deposition (PVD) in a short single-step process.These materials are highly pure, and they show very unique linear and non-linear optical properties, among them a huge Second-Harmonic-generation (SHG) response.[1][2][3] Here in, we clearly show both experimentally and theoretically that the electrostatic nature of the substrate dictates the network growth, due to repulsion and attraction upon the PVD process.We show a network growth onto smooth surfaces such as a 2d material.We generalize the method for other hybrid materials and we show the unique linear and nonlinear optical properties emerging from such robust 3D metallic networks with nanosized building blocks.[4][5][6] Potentially, such 3D structures can be used as an efficient SHG reflectors for 2 photon and Raman imaging optical microscopies.

Formation and growth process of nanoporous metallic networks by PVD
We use a silica substrate, which due to the preparation process is electrostatic in nature.The electrostatic nature of the substate together with its morphology initiates the growth of the nanoporous networks.The primary parameter prompting the metal growth is the electrostatic nature of the substrate, whereas the surface morphology plays a secondary role.The substrate we use exhibits a non-homogeneous surface potential, and therefore direct the growth of the metallic ligaments.The descending metallic atoms interact with the non-homogenous field onto the surface, and atoms are repealed or attracted to specific nucleation sites.As the deposition process continues, a (semi)-conductive structure is formed, and the free electrons redistribute within it to equalize the Fermi level and to minimize the free energy of the system.Fig2.Shows a simulation of the growth process onto a smooth surface but with point charges which create surfaces with different polarizabilities.Fig. 2. Simulation of the PVD process onto substrates with different polarizability.The growth process is clearly non-homogenous.

Growth onto 2d material
In order to prove that indeed the electrostatic nature of the substrate plays a pivotal role, we deposited the following layers onto the silica aerogel substrate: a. a thin Cr layer.b. a 2d graphene layer.In case of the graphene, wrinkles have been presented at about every 1-2 microns.Afterwards, we run the PVD process Ag target onto those substrates.The results are shown in figs 2a& b.Although the substrates are smooth, one can see initiation porous metallic network growth.A 2d material is transparent to the electronic properties of the substrate below, and therefore does not screen the unique electronic properties of the original substrate.Of note are the areas (Fig2 b) in which a continues metallic films appears.In those areas, some folding in the graphene flack occurred, and thus screened the charges below.We further applied this technique, and used the PVD process onto a charged paper substrate (Fig2c).Herein, the building clocks are about 200 -500 nm, but the distances between the particles is ~2-5 nm, and thus provide enhanced electromagnetic field as is shown below.
Fig. 2d presents Transmission Electron Microscopy (TEM) image showing Au networks growth onto silica aerogel at early stage of the PVD process.Clearly, there is some penetration of the gold clusters into the aerogel, yet, holes and pillars are formed.

SHG responses
Second harmonic emission from the nanoporous metallic networks was characterized by 2 photon home build microscope.Fig. 4 a illustrates the major components of the setup.[7] The excitation source is a tunable Ti:sapphire laser (Mai-Tai HP, Spectra-Physics, 690−1080 nm) with 100 fs pulse duration and 80 MHz repetition rate.Alignment and collimation of the laser beam is carried out by a series of adjustable silver and dielectric mirrors, and appropriate lenses (Thorlabs).The laser beam is reflected into the sample through a dichroic mirror (Chroma) fixed at 45 0 , having a cutting wavelength at 700 nm.A long working distance ×50, 0.5 N/A objective lens (Olympus) is used so to have a loosely focused beam.

Fig. 1 .
Fig. 1. 3D nanoporous metallic networks made of silver produce by PVD on electrostatic silica aerogel.The connectivity between the building-blocks is observed, as well as the sub-wavelength pores size.

Fig. 3 .
Fig.3.Growth process of the metallic network.silver network grown onto (a) Cr and (b) graphene layer deposited on silica aerogel substrate.(c) Au network grown on paper.(d) TEM of the initial growth process of Au network onto silica aerogel.

Fig
Fig.4bshows a scanning of the SHG responses in reflection mode for such a nanoporous metallic network.Such a 3d structure has the potential to function as a disorder SHG reflector substrate.

Fig. 4 .
Fig. 4. SHG measurements of the 3d metallic network .(a) set-up (b) scanning of the sample with fundamental of 980 nm and power of 5mWatt.