Deformation Behavior of Foam Laser Targets Fabricated by Two-Photon Polymerization.

Two-photon polymerization (2PP), which is a three-dimensional micro/nano-scale additive manufacturing process, is used to fabricate component for small custom experimental packages (“targets”) to support laser-driven, high-energy-density physics research. Of particular interest is the use of 2PP to deterministically print millimeter-scale, low-density, and low atomic number (CHO) polymer matrices (“foams”). Deformation during development and drying of the foam structures remains a challenge when using certain commercial acrylic photo-resins. Acrylic resins were chosen in order to meet the low atomic number requirement for the foam; that requirement precludes the use of low-shrinkage organic/inorganic hybrid resins. Here, we compare the use of acrylic resins IP-S and IP-Dip. Infrared and Raman spectroscopy are used to quantify the extent of the polymerization during 2PP vs. UV curing. The mechanical strength of beam and foam structures is examined, particularly the degree of deformation that occurs during the development and drying processes. The magnitude of the shrinkage is quantified, and finite element analysis is used in order to simulate the resulting deformation. Capillary drying forces during development are shown to be small and are likely below the elastic limit of the foam log-pile structures. In contrast, the substantial shrinkage in IP-Dip (~5⁻10%) causes large shear stresses and associated plastic deformation, particularly near constrained boundaries and locations with sharp density transitions. Use of IP-S with an improved writing procedure results in a marked reduction in deformation with a minor loss of resolution.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons.

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.

Surface diffusion of gold nanoclusters on Ru(0001): effects of cluster size, surface defects and adsorbed oxygen atoms.

Understanding thermal behavior of metallic clusters on their solid supports is important for avoiding sintering and aggregation of the active supported metallic particles in heterogeneous catalysis. As a model system we have studied the diffusion of gold nano-clusters on modified Ru(0001) single crystal surfaces, employing surface density grating formation via a laser induced ablation technique. Surface modifications included damage induced by varying periods of Ne(+) ion sputtering at a collision energy of 2.8 keV and the effect of pre-adsorbed oxygen on the clean, defect free ruthenium surface. High density of surface damage, obtained at long sputter times, has led to enhanced diffusivity with lower onset temperature for diffusion. It is attributed to reduced cluster-surface commensurability which gives rise to smaller effective activation energy for diffusion. The diffusion of gold nano-clusters, 2 nm in size, was found to be insensitive to the oxygen surface concentration. The adsorbed oxygen acted as an "atomic layer lubricant", reducing friction between the cluster and the underlying surface. This has led to lower diffusivity onset temperatures (150 K) of the nano-clusters, with a stronger effect on smaller clusters.

Selective ablation of Xe from silicon surfaces: molecular dynamics simulations and experimental laser patterning.

The mechanism of laser-induced removal of Xe overlayers from a Si substrate has been investigated employing MD simulations and evaluated by buffer layer assisted laser patterning experiments. Two distinct regimes of overlayer removal are identified in the simulations of a uniform heating of the Si substrate by a 5 ns laser pulse: The intensive evaporation from the surface of the Xe overlayer and the detachment of the entire Xe overlayer driven by explosive boiling in the vicinity of the hot substrate. Simulations of selective heating of only a fraction of the silicon substrate suggest that the lateral heat transfer and bonding to the unheated, colder regions of the Xe overlayer is very efficient and suppresses the separation of a fraction of the overlayer from the substrate. Interaction with surrounding cold Xe is responsible for significant increase in the substrate temperature required for achieving the spatially selective ablation of the overlayer. The predictions of the MD simulations are found to be in a qualitative agreement with the results of experimental measurements of the threshold laser power required for the removal of Xe overlayers of different thickness and the shapes of metallic stripes generated by buffer-assisted laser patterning.

Adsorption of H2O, CO2 and Xe on Soft Surfaces.

The interactions of water, carbon dioxide, and Xe with octadecanethiol (C(18)H(37)SH, ODT) self-assembled monolayers (SAMs) were studied under ultrahigh vacuum conditions employing temperature-programmed desorption and optical diffraction measurements. The ODT layer was grown on a 1 nm thick gold film deposited over a Ru(001) single-crystal substrate. The gases used in this report differ in their lateral interactions while adsorbed on ODT-SAM being either repulsive (Xe) or attractive (H(2)O, CO(2)). The activation energies for desorption of the first layer from ODT are E(a) = 3.6 +/- 0.9, 4.1 +/- 0.5, and 8.5 +/- 0.9 kcal/mol for Xe, CO(2), and H(2)O, respectively. Sticking probabilities of the three gases on the soft ODT surface are S(0) = 0.7 +/- 0.1, 0.8 +/- 0.1, and 0.95 +/- 0.05 for xenon, CO(2), and water, respectively, derived from the respective adsorption curves. Optical diffraction studies from multilayer coverage grating of Xe on ODT-SAM have demonstrated that sublimation is a thermodynamically more favorable process over diffusion and wetting. The significantly lower binding energy of the first layers of H(2)O and CO(2) adsorbed on the soft surface of ODT compared to that on clean metals and oxides, reflects generally weak (CO(2)) and hydrophobic (H(2)O) interactions that are important for understanding the behavior of these molecules on interfaces that are found in biological systems.