Arrays of Colloidal Single Crystals Engineered with DNA in Lithographically Defined Microwells.

Lithographically defined microwell templates are used to study DNA-guided colloidal crystal assembly parameters, including superlattice position, habit orientation, and size, in an effort to increase our understanding of the crystallization process. In addition to enabling the synthesis of arrays of individual superlattices in arbitrary predefined patterns, the technique allows one to study the growth pathways of the crystals via ex situ scanning electron microscopy. Importantly, a Volmer-Weber (VM) (island formation)-like growth mode is identified, which has been reproduced via simulations. Notably, both experiment and simulation reveal that the crystallites merge and reorient within the microwells that defined the crystal growth to form single-crystalline structures, an observation not common for VM pathways. The control afforded by this platform will facilitate efforts in constructing metamaterials from colloidal crystals as well as their integration into optical devices and applications.

Combinatorial Synthesis and Screening of Mixed Halide Perovskite Megalibraries.

A significant bottleneck in the discovery of new mixed halide perovskite (MHP) compositions and structures is the time-consuming and low-throughput nature of current synthesis and screening methods. Here, a high-throughput strategy is presented that can be used to synthesize combinatorial libraries of MHPs with deliberate control over the halide mixing ratio and particle size (for example, CsPb(Br1-xClx)3 (0 < x < 1) with sizes between ∼100 and 400 nm). This strategy combines evaporation-crystallization polymer pen lithography (EC-PPL) and defect-engineered anion exchange to spatially encode particle size and composition, respectively. Laser exposure is used to selectively modify the defect concentration of individual particles, and thus the degree of subsequent anion exchange, allowing the preparation for ultra-high-density arrays of distinct compositions (>1 unique particle/µm2). This method was utilized to rapidly generate a library of ∼4000 CsPb(Br1-xClx)3 particles that was then screened for high-efficiency blue photoemission, which yielded CsPb(Br0.6Cl0.4)3 as the composition with the highest photoluminescence intensity. The combinatorial synthesis and screening strategy provided here, and the mechanistic understanding of the defect-engineering process gleaned from it, will enable the rapid discovery of exceptional MHP optoelectronic materials.

Regioselective Deposition of Metals on Seeds within a Polymer Matrix.

We use scanning probe block copolymer lithography in a two-step sequential manner to explore the deposition of secondary metals on nanoparticle seeds. When single element nanoparticles (Au, Ag, Cu, Co, or Ni) were used as seeds, both heterogeneous and homogeneous growth occurred, as rationalized using the thermodynamic concepts of bond strength and lattice mismatch. Specifically, heterogeneous growth occurs when the heterobond strength between the seed and growth atoms is stronger than the homobond strength between the growth atoms. Moreover, the resulting nanoparticle structure depends on the degree of lattice mismatch between the seed and growth metals. Specifically, a large lattice mismatch (e.g., 13.82% for Au and Ni) typically resulted in heterodimers, whereas a small lattice mismatch (e.g., 0.19% for Au and Ag) resulted in core-shell structures. Interestingly, when heterodimer nanoparticles were used as seeds, the secondary metals deposited asymmetrically on one side of the seed. By programming the deposition conditions of Ag and Cu on AuNi heterodimer seeds, two distinct nanostructures were synthesized with (1) Ag and Cu on the Au domain and (2) Ag on the Au domain and Cu on the Ni domain, illustrating how this technique can be used to predictively synthesize structurally complex, multimetallic nanostructures.

Single-Nanowire Strain Sensors Fabricated by Nanoskiving.

This article describes the fabrication of single-nanowire strain sensors by thin sectioning of gold films with an ultramicrotome-i.e., "nanoskiving." The nanowire sensors are transferred to various substrates from the water bath on which they float after sectioning. The electrical response of these single nanowires to mechanical strain is investigated, with the lowest detectable strain determined to be 1.6 × 10-5 with a repeatable response to strains as high as 7 × 10-4. The sensors are shown to have an enhanced sensitivity with a gauge factor of 3.1 on average, but as high as 9.5 in the low strain regime (ε ~ 1 × 10-5). Conventional thin films of gold of the same height as the nanowires are used as controls, and are unable to detect those same strains. The practicality of this sensor is investigated by transferring a single nanowire to polyimide tape, and placing the sensor on the wrist to monitor the pulse pressure waveform from the radial artery. The nanowires are fabricated with simple tools and require no lithography. Moreover, the sensors can be "manufactured" efficiently, as each consecutive section of the film is a quasi copy of the previous nanowire. The simple fabrication of these nanowires, along with the compatibility with flexible substrates, offers possibilities in developing new kinds of devices for biomedical applications and structural health monitoring.

Metallic Nanoislands on Graphene as Highly Sensitive Transducers of Mechanical, Biological, and Optical Signals.

This article describes an effect based on the wetting transparency of graphene; the morphology of a metallic film (≤20 nm) when deposited on graphene by evaporation depends strongly on the identity of the substrate supporting the graphene. This control permits the formation of a range of geometries, such as tightly packed nanospheres, nanocrystals, and island-like formations with controllable gaps down to 3 nm. These graphene-supported structures can be transferred to any surface and function as ultrasensitive mechanical signal transducers with high sensitivity and range (at least 4 orders of magnitude of strain) for applications in structural health monitoring, electronic skin, measurement of the contractions of cardiomyocytes, and substrates for surface-enhanced Raman scattering (SERS, including on the tips of optical fibers). These composite films can thus be treated as a platform technology for multimodal sensing. Moreover, they are low profile, mechanically robust, semitransparent and have the potential for reproducible manufacturing over large areas.

Using the thickness of graphene to template lateral subnanometer gaps between gold nanostructures.

This work demonstrates the use of single-layer graphene as a template for the formation of subnanometer plasmonic gaps using a scalable fabrication process called "nanoskiving." These gaps are formed between parallel gold nanowires in a process that first produces three-layer thin films with the architecture gold/single-layer graphene/gold, and then sections the composite films with an ultramicrotome. The structures produced can be treated as two gold nanowires separated along their entire lengths by an atomically thin graphene nanoribbon. Oxygen plasma etches the sandwiched graphene to a finite depth; this action produces a subnanometer gap near the top surface of the junction between the wires that is capable of supporting highly confined optical fields. The confinement of light is confirmed by surface-enhanced Raman spectroscopy measurements, which indicate that the enhancement of the electric field arises from the junction between the gold nanowires. These experiments demonstrate nanoskiving as a unique and easy-to-implement fabrication technique that is capable of forming subnanometer plasmonic gaps between parallel metallic nanostructures over long, macroscopic distances. These structures could be valuable for fundamental investigations as well as applications in plasmonics and molecular electronics.

From Heterostructures to Solid-Solutions: Structural Tunability in Mixed Halide Perovskites.

The stability, reliability, and performance of halide-perovskite-based devices depend upon the structure, composition, and particle size of the device-enabling materials. Indeed, the degree of ion mixing in multicomponent perovskite crystals, although challenging to control, is a key factor in determining properties. Herein, an emerging method termed evaporation-crystallization polymer pen lithography is used to synthesize and systematically study the degree of ionic mixing of Cs0.5 FA0.5 PbX3 (FA = formamidinium; X = halide anion, ABX3 ) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A-site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs-FA ion segregation. This preference for segregation can be overcome to form a solid-solution by decreasing crystal size (<60 nm) or increasing temperature. Finally, these tools are utilized to identify and synthesize solid-solution nanocrystals of Cs0.5 FA0.5 Pb(Br/I)3 that significantly suppress photoinduced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.

Massively Parallel Nanoparticle Synthesis in Anisotropic Nanoreactors.

This work reports a massively parallel approach for synthesizing inorganic nanoparticles (Au, Ag, Se, and mixed oxides of Cu, Co, Ni, Ge, and Ta) based upon lithographically generated arrays of square pyramidal nanoholes, which serve as nanoreactors. Particle precursor-containing polymers are spin-coated onto the nanoreactors, which upon dewetting generate a morphology of isolated polymer droplets in each nanoreactor. This dewetting process yields a well-defined and precisely controlled volume of polymer and therefore particle precursor in each nanoreactor. Subsequent stepwise annealing (first at 150 °C and then at 500 °C) yields arrays of monodisperse, site-isolated particles with sub-5 nm position control. By varying the precursor loading of the polymer, particle size can be systematically controlled in the 7-30 nm range. This work not only introduces the concept of merging block copolymer inks with nanohole arrays in the synthesis of nanoparticles but also underscores the value of the nanoreactor shape in controlling resulting particle position.