Electrochemical nanoimprinting of silicon.

Scalable nanomanufacturing enables the commercialization of nanotechnology, particularly in applications such as nanophotonics, silicon photonics, photovoltaics, and biosensing. Nanoimprinting lithography (NIL) was the first scalable process to introduce 3D nanopatterning of polymeric films. Despite efforts to extend NIL's library of patternable media, imprinting of inorganic semiconductors has been plagued by concomitant generation of crystallography defects during imprinting. Here, we use an electrochemical nanoimprinting process-called Mac-Imprint-for directly patterning electronic-grade silicon with 3D microscale features. It is shown that stamps made of mesoporous metal catalysts allow for imprinting electronic-grade silicon without the concomitant generation of porous silicon damage while introducing mesoscale roughness. Unlike most NIL processes, Mac-Imprint does not rely on plastic deformation, and thus, it allows for replicating hard and brittle materials, such as silicon, from a reusable polymeric mold, which can be manufactured by almost any existing microfabrication technique.

3D Printing-Enabled Nanoparticle Alignment: A Review of Mechanisms and Applications.

3D printing (additive manufacturing (AM)) has enormous potential for rapid tooling and mass production due to its design flexibility and significant reduction of the timeline from design to manufacturing. The current state-of-the-art in 3D printing focuses on material manufacturability and engineering applications. However, there still exists the bottleneck of low printing resolution and processing rates, especially when nanomaterials need tailorable orders at different scales. An interesting phenomenon is the preferential alignment of nanoparticles that enhance material properties. Therefore, this review emphasizes the landscape of nanoparticle alignment in the context of 3D printing. Herein, a brief overview of 3D printing is provided, followed by a comprehensive summary of the 3D printing-enabled nanoparticle alignment in well-established and in-house customized 3D printing mechanisms that can lead to selective deposition and preferential orientation of nanoparticles. Subsequently, it is listed that typical applications that utilized the properties of ordered nanoparticles (e.g., structural composites, heat conductors, chemo-resistive sensors, engineered surfaces, tissue scaffolds, and actuators based on structural and functional property improvement). This review's emphasis is on the particle alignment methodology and the performance of composites incorporating aligned nanoparticles. In the end, significant limitations of current 3D printing techniques are identified together with future perspectives.

Optical characterization and modeling of nanoporous gold absorbers fabricated by thin-film dealloying.

This work studies the optical reflectance of nanoporous gold (NPG) thin films of varying pore volume fraction (PVF) synthesized by chemical dealloying of Ag-Au alloy precursors. The fabricated samples are characterized by scanning electron microscopy, and spectral hemispherical reflectance is measured with an integrating sphere. The effective isotropic optical constants of NPG with varying PVF are modeled for the wavelength range from 0.4 to 1.6 µm using the Bruggeman effective medium theory. As the thickness of the NPG thin films is more than ten times larger than the effective penetration depth, the spectral reflectance is simply modeled with the Fresnel coefficients at the interface of air and semi-infinite NPG with different incident angles and polarizations. Consistent with the modeling results, the optical measurement data shows that the spectral normal reflectance of NPG significantly decreases with larger PVF values in the near-infrared regime. On the other hand, the reflectance increases greatly only within visible range at larger oblique angles for transverse-electric polarized waves compared to transverse-magnetic waves. Moreover, the NPG samples demonstrate good thermal stability from room temperature up to 100 °C with little changes in the temperature-dependent spectral hemispherical reflectance.

Quenched phonon drag in silicon nanowires reveals significant effect in the bulk at room temperature.

Existing theory and data cannot quantify the contribution of phonon drag to the Seebeck coefficient (S) in semiconductors at room temperature. We show that this is possible through comparative measurements between nanowires and the bulk. Phonon boundary scattering completely quenches phonon drag in silicon nanowires enabling quantification of its contribution to S in bulk silicon in the range 25-500 K. The contribution is surprisingly large (∼34%) at 300 K even at doping of ∼3 × 10(19) cm(-3). Our results contradict the notion that phonon drag is negligible in degenerate semiconductors at temperatures relevant for thermoelectric energy conversion. A revised theory of electron-phonon momentum exchange that accounts for a phonon mean free path spectrum agrees well with the data.

Controllable doping and wrap-around contacts to electrolessly etched silicon nanowire arrays.

Top-down electroless chemical etching enables non-lithographic patterning of wafer-scale nanostructured arrays, but the etching on highly doped wafers produces porous structures. The lack of defect-free nanostructures at desired doping and the difficulties in forming reliable electrical side-contacts to the nanostructure arrays limits their integration into high performance nanoelectronics. We developed a barrier layer diffusion technique to controllably dope wafer-scale silicon nanowire arrays (10(17)-10(20) cm(-3)) produced by chemically etching lightly doped silicon wafers. In order to achieve low resistance top-side electrical contacts to the arrays, we developed a two step tip-doping procedure to locally dope the tips (∼10(20) cm(-3)) to metallic levels. The dopant concentration is characterized by depth profiling using secondary ion mass spectroscopy and four-point probe electrical measurements. Further, array scale electrical measurements show that the tip-doping lowers the specific contact resistivity (∼10(-5) Ω cm(2)) since the metallic tips enable direct tunneling of electrons across the nickel silicide contacts to the nanowire arrays. This work provides a scalable and cost-effective doping approach to control charge injection and charge conduction in nanowire arrays, thus advancing their integration into various device applications.

Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires.

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.

Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers.

Metal-assisted electrochemical imprinting (Mac-Imprint) is a combination of metal-assisted chemical etching (MACE) and nanoimprint lithography that is capable of direct patterning 3D micro- and nanoscale features in monocrystalline group IV (e.g., Si) and III-V (e.g., GaAs) semiconductors without the need of sacrificial templates and lithographical steps. During this process, a reusable stamp coated with a noble metal catalyst is brought in contact with a Si wafer in the presence of a hydrofluoric acid (HF) and hydrogen peroxide (H2O2) mixture, which leads to the selective etching of Si at the metal-semiconductor contact interface. In this protocol, we discuss the stamp and substrate preparation methods applied in two Mac-Imprint configurations: (1) Porous Si Mac-Imprint with a solid catalyst; and (2) Solid Si Mac-Imprint with a porous catalyst. This process is high throughput and is capable of centimeter-scale parallel patterning with sub-20 nm resolution. It also provides low defect density and large area patterning in a single operation and bypasses the need for dry etching such as deep reactive ion etching (DRIE).

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics.

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size ≈45 nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter ≈42 nm) onto silicon. In this work, roughness is reduced to sub-10 nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7 nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10 nm while providing depth uniformity (i.e., 42.9 ± 5.5 nm), and limited levels of silicon defect formation (e.g., Raman peak shift < 0.1 cm-1 ) and sidewall scattering.

Electroless Dealloying of Thin-Film Nanocrystalline Au-Ag Alloys: Mechanisms of Ligament Nucleation and Sources of Its Synthesis Variability.

Control of ligament size in nanoporous gold through process inputs in chemical dealloying holds the potential to exploit its size dependent properties in applications in energy and biomedicine. While its morphology evolution is regulated by the kinetics of coarsening, recent studies are focused on the early stage of dealloying (e.g., ∼ 5-42 at. % in residual alloy content) to understand mechanisms of ligament nucleation and its role in altering process-structure relationships. This paper examines this stage in chemical dealloying of nanocrystalline Au49Ag51 thin films and finds that ligaments are nucleated uniformly through its thickness due to the dealloying front rapidly propagating through the thickness of the film. Further, through the establishment of process-structure relationships with large data sets (i.e., 80 samples), this paper quantifies sources of variability that alter the kinetics of ligament growth such as aging of the precursor (e.g., grain growth) and solution evaporation. It is found that ligament diameter is better predicted by the residual silver content rather than by the dealloying time even amidst both effects and independent control of ligament diameter and solid area fraction is demonstrated within a limited window.

Nonlithographic patterning and metal-assisted chemical etching for manufacturing of tunable light-emitting silicon nanowire arrays.

Semiconductor nanowires have potential applications in photovoltaics, batteries, and thermoelectrics. We report a top-down fabrication method that involves the combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical-etching (MacEtch), to produce silicon nanowire arrays with defined geometry and optical properties in a manufacturable fashion. Strong light emission in the entire visible and near infrared wavelength range at room temperature, tunable by etching condition, attributed to surface features, and enhanced by silver surface plasmon, is demonstrated.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode.

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO3 and (Er,W):BiVO4 is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er3+ is substituted at the Bi3+ sites in the BiVO4 lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO3/(Er,W):BiVO4 electrode shows photocurrent densities of 4.1 and 7.2 mA cm-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K2HPO4 electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO3/(Er,W):BiVO4/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi3+ and V5+ sites to enhance absorption, this study demonstrates for the first time a planar WO3/BiVO4 heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.