A new method utilizing subsequent depositions of thin crack-free nanoparticle layers is demonstrated to avoid the formation of cracks within silica nanoparticle films. Using this method, films can be assembled with thicknesses exceeding the critical cracking values. Explanation of this observed phenomenon is hypothesized to mainly arise from chemical bond formation between neighboring silica nanoparticles. Application of this method for fabricating crack-free functional structures is demonstrated by producing crack-free Bragg reflectors that exhibit structural color.

Active materials that respond to physical and chemical stimuli can be used to build dynamic micromachines that lie at the interface between biological systems and engineered devices. In principle, the specific hybridization of DNA can be used to form a library of independent, chemically driven actuators for use in such microrobotic applications and could lead to device capabilities that are not possible with polymer- or metal-layer-based approaches. Here, we report shape changing films that are powered by DNA strand exchange reactions with two different domains that can respond to distinct chemical signals. The films are formed from DNA-grafted gold nanoparticles using a layer-by-layer deposition process. Films consisting of an active and a passive layer show rapid, reversible curling in response to stimulus DNA strands added to solution. Films consisting of two independently addressable active layers display a complex suite of repeatable transformations, involving eight mechanochemical states and incorporating self-righting behaviour.

We report a bioinspired hygromorphic double-layered actuator (HDA), of which the movement is controlled by cyclical changes in relative humidity (RH). The basic principle of the HDA lies in the rapid swelling and deswelling of highly hygroscopic layer-by-layer (LbL) assembled films deposited on a moisture-resistant and flexible polytetrafluoroethylene (PTFE) ribbon. We engineer the geometry of the HDA to induce locomotion on a ratchet track. By controlling the exposure time and RH, the HDA is remotely controlled to move a precise number of steps on the ratchet track during one cycle of RH changes. We demonstrate that the step length of the HDA depends on the relative thickness change of the LbL film. We also provide theoretical considerations based on a plate theory and the Flory–Huggins theory to describe the actuation of the HDA. Our work provides fundamental insights into the fabrication and design of hygromorphic actuators driven by RH changes.

High effi ciency dye-sensitized solar cells (DSSCs) are fabricated with a heterostructured photoanode that consists of a 500-nm-thick organized mesoporous TiO 2 (om-TiO 2 ) interfacial layer (IF layer), a 7 or 10-μ m thick nanocrystalline TiO 2 layer (NC layer), and a 2-μ m-thick mesoporous Bragg stack (meso-BS layer) as the bottom, middle and top layers, respectively. An om-TiO 2 layer with a high porosity, transmittance, and interconnectivity is prepared via a sol-gel process, in which a poly(vinyl chloride)-gpoly(oxyethylene methacrylate) (PVC-g -POEM) graft copolymer is used as a structure-directing agent. The meso-BS layer with large pores is prepared via alternating deposition of om-TiO 2 and colloidal SiO 2 (col-SiO 2 ) layers. Structure and optical properties (refractive index) of the om-TiO 2 and meso-BS layers are studied and the morphology of the heterostructured photoanode is characterized. DSSCs fabricated with the heterostructured IF/NC/BS photoanode and combined with a polymerized ionic liquid (PIL) exhibit an energy conversion effi ciencies of 6.6% at 100 mW/cm 2 , one of the highest reported for solid-state DSSCs and much larger than cells prepared with only a IF/NC layer (6.0%) or a NC layer (4.5%). Improvements in energy conversion effi ciency are attributed to the combination of improved light harvesting, decreased resistance at the electrode/electrolyte interface, and excellent electrolyte infi ltration.

Nanoparticle thin films (NTFs) exhibit multifunctionality, making them useful for numerous advanced applications including energy storage and conversion, biosensing and photonics. Poor mechanical reliability and durability of NTFs, however, limit their industrial and commercial applications. Atomic layer deposition (ALD) represents a unique opportunity to enhance the mechanical properties of NTFs at a relatively low temperature without drastically changing their original structure and functionality. In this work, we study how ALD of different materials, Al(2)O(3), TiO(2), and SiO(2), affects the mechanical properties of TiO(2) and SiO(2) NTFs. Our results demonstrate that the mechanical properties of ALD-reinforced NTFs are dominantly influenced by the mechanical properties of the ALD materials rather than by the compositional matching between ALD and nanoparticle materials. Among the three ALD materials, Al(2)O(3) ALD provides the best enhancement in the modulus and hardness of the NTFs. Interestingly, Al(2)O(3) ALD is able to enhance not only the modulus and hardness but also the toughness of NTFs. Our study presents an additional benefit of depositing nanometer scale ALD layers in NTFs; that is, we find that the hardness and modulus of ultrathin ALD layers (<5 nm) can be estimated from the mechanical properties of ALD-reinforced NTFs using a simple mixing rule. This investigation also provides insight into the use of nanoindentation for testing the mechanical properties of ultrathin ALD-reinforced NTFs.