Interconnect Fabrication by Electroless Plating on 3D-Printed Electroplated Patterns

The metallic interconnects are essential components of energy devices such as fuel cells and electrolysis cells, batteries, as well as electronics and optoelectronic devices. In recent years, 3D printing processes have offered complementary routes to the conventional photolithography- and vacuum-based processes for interconnect fabrication. Among these methods, the confined electrodeposition (CED) process has enabled a great control over the microstructure of the printed metal, direct printing of high electrical conductivity (close to the bulk values) metals on flexible substrates without a need to sintering, printing alloys with controlled composition, printing functional metals for various applications including magnetic applications, and for in situ scanning electron microscope (SEM) nanomechanical experiments. However, the metal deposition rate (or the overall printing speed) of this process is reasonably slow because of the chemical nature of the process. Here, we propose using the CED process to print a single layer of a metallic trace as the seed layer for the subsequent selected-area electroless plating. By controlling the activation sites through printing by the CED process, we control, where the metal grows by electroless plating, and demonstrate the fabrication of complex thin-film patterns. Our results show that this combined process improves the processing time by more than 2 orders of magnitude compared to the layer-by-layer printing process by CED. Additionally, we obtained Cu and Ni films with an electrical resistivity as low as ∼1.3 and ∼2 times of the bulk Cu and Ni, respectively, without any thermal annealing. Furthermore, our quantitative experiments show that the obtained films exhibit mechanical properties close to the bulk metals with an excellent adhesion to the substrate. We demonstrate potential applications for radio frequency identification (RFID) tags, for complex printed circuit board patterns, and resistive sensors in a Petri dish for potential biological applications.

d-sp Interband Transition Excited Carriers Promoting the Photochemical Growth of Plasmonic Gold Nanoparticles

As an important damping way of the light absorption of plasmonic nanoparticles, the d-sp interband transition within the short wavelength regime has been recently drawing attentions in photochemistry. Compared with the intraband Landau damping, the d-sp interband transition excited carriers have larger populations and longer relaxation times, which is promising to match the photochemical reactions in time scale. Here, we propose a novel approach to the growth of plasmonic gold nanoparticles more efficiently promoted by d-sp interband transition excited charge carriers than by plasmon excited carriers. It is founded that photochemical growth of plasmonic gold nanoparticles can be modulated by engineering the electrochemical potential of precursor using different surfactants. From the distribution of surfactant on a single gold nanoparticle revealed by nanoscale resolved IR mapping, a reasonable photochemical reaction mechanism mediated by d-sp interband excitation is suggested. The results highlight the importance of d-sp interband transition excited carriers in photochemical reactions especially for materials science.

Direct-Write Printing Copper-Nickel (Cu/Ni) Alloy with Controlled Composition from a Single Electrolyte Using Co-Electrodeposition

Although various processes for metal printing at the micro- and mesoscale have been demonstrated, printing functional devices such as thermocouples, thermopiles, and heat flux sensors that function based on interfaces between an alloy and another alloy/metal demands processes for printing alloys. Furthermore, a high-quality and crystalline alloy is required for acceptable function of these devices. This article reports for the first time co-electrodeposition-based printing of single-phase solid solution nanocrystalline copper/nickel (Cu/Ni) alloy with various controllable compositions (Cu100Ni0 to Cu19Ni81) from a single electrolyte. The printed alloy is nanocrystalline (<35 nm), continuous, and dense with no apparent porosity, with remarkable mechanical and magnetic properties, without any postprocessing annealing such as heat treatment. In addition, a functional thermocouple fabricated using this process is demonstrated. Such a process can not only be used for fabrication of functional devices, it may also facilitate fundamental studies on alloys by printing a continuous library of alloy composition for material characterization.

A Hybrid Process for Printing Pure and High Conductivity Nanocrystalline Copper and Nickel on Flexible Polymeric Substrates

Printing functional devices on flexible substrates requires printing of high conductivity metallic patterns. To prevent deformation and damage of the polymeric substrate, the processing (printing) and post-processing (annealing) temperature of the metal patterns must be lower than the glass transition temperature of the substrate. Here, a hybrid process including deposition of a sacrificial blanket thin film, followed by room environment nozzle-based electrodeposition, and subsequent etching of the blanket film is demonstrated to print pure and nanocrystalline metallic (Ni and Cu) patterns on flexible substrates (PI and PET). Microscopy and spectroscopy showed that the printed metal is nanocrystalline, solid with no porosity and with low impurities. Electrical resistivity close to the bulk (~2-time) was obtained without any thermal annealing. Mechanical characterization confirmed excellent cyclic strength of the deposited metal, with limited degradation under high cyclic flexure. Several devices including radio frequency identification (RFID) tag, heater, strain gauge, and temperature sensor are demonstrated.

Ultrafast nonlinear optical properties and carrier dynamics of silver nanoparticle-decorated ZnO nanowires

Silver (Ag) nanoparticle-decorated zinc oxide (ZnO) nanowires (Ag–ZnO) have been successfully synthesized by chemical vapour deposition and the magnetron sputtering method. Scanning electron microscopy images indicate that Ag nanoparticles are distributed uniformly on the surface of the ZnO nanowires. The results of room temperature photoluminescence (RTPL) reveal two major emission peaks for the Ag–ZnO nanowires, and the emission peaks in the visible region are stronger than those of the unmodified ZnO nanowires. The mechanism of RTPL and low temperature photoluminescence (LTPL) emission is discussed in detail. Nonlinear optical properties and ultrafast dynamics have been investigated using the Z-scan and two color pump–probe (TCPP) techniques, respectively. The nonlinear absorption properties in the nano-, pico- and femto-second regime have been analyzed using the singlet state three-level and four-level models, respectively. The samples show self-focusing nonlinearity and good two-photon absorption (TPA)-induced ground state saturation absorption as well as excited state reverse saturable absorption behavior. For the nanosecond and picosecond pulses, the reverse saturated absorption in the excited state mainly originates from the absorption at low excited states or deep levels; however, for the femtosecond pulse, it is caused by the absorption at high excited states. The TCPP results show that the ground state or deep level light bleaching (for nano- and pico-second regime) and TPA-induced excited-state absorption (for femtosecond regime) behaviors range from 470 nm to 620 nm. The remarkable nonlinear optical properties reveal that Ag–ZnO nanowires are potential nanocomposite materials for the development of nonlinear optical devices.

Silver (Ag) nanoparticle-decorated zinc oxide (ZnO) nanowires (Ag–ZnO) have been successfully synthesized by chemical vapour deposition and the magnetron sputtering method.

Additive manufacture of complex 3D Au-containing nanocomposites by simultaneous two-photon polymerisation and photoreduction

The fabrication of complex three-dimensional gold-containing nanocomposite structures by simultaneous two-photon polymerisation and photoreduction is demonstrated. Increased salt delivers reduced feature sizes down to line widths as small as 78 nm, a level of structural intricacy that represents a significant advance in fabrication complexity. The development of a general methodology to efficiently mix pentaerythritol triacrylate (PETA) with gold chloride hydrate (HAuCl4∙3H2O) is reported, where the gold salt concentration is adjustable on demand from zero to 20 wt%. For the first-time 7-Diethylamino-3-thenoylcoumarin (DETC) is used as the photoinitiator. Only 0.5 wt% of DETC was required to promote both polymerisation and photoreduction of up to 20 wt% of gold salt. This efficiency is the highest reported for Au-containing composite fabrication by two-photon lithography. Transmission Electron Microscopy (TEM) analysis confirmed the presence of small metallic nanoparticles (5.4 ± 1.4 nm for long axis / 3.7 ± 0.9 nm for short axis) embedded within the polymer matrix, whilst X-ray Photoelectron Spectroscopy (XPS) confirmed that they exist in the zero valent oxidation state. UV-vis spectroscopy defined that they exhibit the property of localised surface plasmon resonance (LSPR). The capability demonstrated in this study opens up new avenues for a range of applications, including plasmonics, metamaterials, flexible electronics and biosensors.

Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes

We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm², corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >10⁵ and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices.

Additive Manufacturing of Metal Structures at the Micrometer Scale

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro-stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser-assisted electrophoretic deposition, laser-induced forward transfer, local electroplating methods, laser-induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1-10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.