Daniel J. Eichelsdoerfer

Delineating the pathways for the site-directed synthesis of individual nanoparticles on surfaces.

Although nanoparticles with exquisite properties have been synthesized for a variety of applications, their incorporation into functional devices is challenging owing to the difficulty in positioning them at specified sites on surfaces. In contrast with the conventional synthesis-then-assembly paradigm, scanning probe block copolymer lithography can pattern precursor materials embedded in a polymer matrix and synthesize desired nanoparticles on site, offering great promise for incorporating nanoparticles into devices. This technique, however, is extremely limited from a materials standpoint. To develop a materials-general method for synthesizing nanoparticles on surfaces for broader applications, a mechanistic understanding of polymer-mediated nanoparticle formation is crucial. Here, we design a four-step synthetic process that enables independent study of the two most critical steps for synthesizing single nanoparticles on surfaces: phase separation of precursors and particle formation. Using this process, we elucidate the importance of the polymer matrix in the diffusion of metal precursors to form a single nanoparticle and the three pathways that the precursors undergo to form nanoparticles. Based on this mechanistic understanding, the synthetic process is generalized to create metal (Au, Ag, Pt, and Pd), metal oxide (Fe2O3, Co2O3, NiO, and CuO), and alloy (AuAg) nanoparticles. This mechanistic understanding and resulting process represent a major advance in scanning probe lithography as a tool to generate patterns of tailored nanoparticles for integration with solid-state devices.

Stepwise Evolution of DNA‐Programmable Nanoparticle Superlattices

Colloidal crystals can be assembled using a variety of entropic, depletion, electrostatic, or biorecognition forces and provide a convenient model system for studying crystal growth. Although superlattices with diverse geometries can be assembled in solution and on surfaces, the incorporation of specific bonding interactions between particle building blocks and a substrate would significantly enhance control over the growth process. Herein, we use a stepwise growth process to systematically study and control the evolution of a body-centered cubic (bcc) crystalline thinfilm comprised of nanoparticle building blocks functionalized with DNA on a complementary DNA substrate. We examine crystal growth as a function of temperature, number of layers, and substrate–particle bonding interactions. Importantly, the judicious choice of DNA interconnects allows one to tune the interfacial energy between various crystal planes and the substrate, and thereby control crystal orientation and size in a stepwise fashion using chemically programmable attractive forces. This is a unique approach since prior studies involving superlattice assembly typically rely on repulsive interactions between particles to dictate structure, and those that rely on attractive forces (e.g. ionic systems) still maintain repulsive particle–substrate interactions. In addition to providing a model for crystallization, the field of particle assembly has garnered considerable interest because materials generated from ordered particle arrays can have novel optical, 13–17] electronic, and magnetic properties. These properties can be sensitive to the composition, symmetry, and distance between nanoparticles, in addition to the number of layers and orientation. DNA-mediated nanoparticle crystallization is particularly attractive for preparing these materials because the nanoparticle building blocks can be considered a type of “programmable atom equivalent” with tailorable size, composition, shape, and bonding interactions. This tunability allows one to access a diverse class of crystal symmetries, tailor lattice parameters with sub-nanometer resolution, and create structures that have no known mineral equivalent. Indeed, to date, 17 unique symmetries have been realized and over 100 unique crystal structures have been synthesized, all of which conform to a key hypothesis: these atom equivalents assemble into structures that maximize the total number of hybridized DNA interconnects between particles. While these structures have enormous potential, their use is limited because they are typically formed in solution as polycrystalline aggregates with little control over crystal size or orientation. Consequently, it is difficult to measure their properties or integrate them with other device elements using existing microfabrication techniques. The development of thin-film superlattices is therefore necessary to fully realize the potential of these structures as metamaterials, photonic crystals, and data storage elements. The growth of DNA-mediated monoand multi-layered nanoparticle structures was first examined by our group and later by Niemeyer and co-workers. However, the use of strong DNA interactions prevented nanoparticle crystallization. Herein, we exploit multiple weak DNA interactions for superlattice growth to examine the development of crystal orientation (texture) and control film thickness. Body-centered cubic colloidal crystals composed of spherical nucleic acid gold nanoparticle conjugates (SNA-AuNPs) were used as a model system since these structures require two complementary particle types and therefore allow the stepwise introduction of each layer. Alternatively, other crystal symmetries such as face-centered cubic (fcc) require

Large-area molecular patterning with polymer pen lithography

The challenge of constructing surfaces with nanostructured chemical functionality is central to many areas of biology and biotechnology. This protocol describes the steps required for performing molecular printing using polymer pen lithography (PPL), a cantilever-free scanning probe-based technique that can generate sub-100-nm molecular features in a massively parallel fashion. To illustrate how such molecular printing can be used for a variety of biologically relevant applications, we detail the fabrication of the lithographic apparatus and the deposition of two materials, an alkanethiol and a polymer onto a gold and silicon surface, respectively, and show how the present approach can be used to generate nanostructures composed of proteins and metals. Finally, we describe how PPL enables researchers to easily create combinatorial arrays of nanostructures, a powerful approach for high-throughput screening. A typical protocol for fabricating PPL arrays and printing with the arrays takes 48–72 h to complete, including two overnight waiting steps.

Giant conductivity switching of LaAlO3/SrTiO3 heterointerfaces governed by surface protonation.

Complex-oxide interfaces host a diversity of phenomena not present in traditional semiconductor heterostructures. Despite intense interest, many basic questions remain about the mechanisms that give rise to interfacial conductivity and the role of surface chemistry in dictating these properties. Here we demonstrate a fully reversible >4 order of magnitude conductance change at LaAlO3/SrTiO3 (LAO/STO) interfaces, regulated by LAO surface protonation. Nominally conductive interfaces are rendered insulating by solvent immersion, which deprotonates the hydroxylated LAO surface; interface conductivity is restored by exposure to light, which induces reprotonation via photocatalytic oxidation of adsorbed water. The proposed mechanisms are supported by a coordinated series of electrical measurements, optical/solvent exposures, and X-ray photoelectron spectroscopy. This intimate connection between LAO surface chemistry and LAO/STO interface physics bears far-reaching implications for reconfigurable oxide nanoelectronics and raises the possibility of novel applications in which electronic properties of these materials can be locally tuned using synthetic chemistry.

Positionally Defined, Binary Semiconductor Nanoparticles Synthesized by Scanning Probe Block Copolymer Lithography

We report the first method for synthesizing binary semiconductor materials by scanning probe block copolymer lithography (SPBCL) in desired locations on a surface. In this work, we utilize SPBCL to create polymer features containing a desired amount of Cd(2+), which is defined by the feature volume. When they are subsequently reacted in H(2)S in the vapor phase, a single CdS nanoparticle is formed in each block copolymer (BCP) feature. The CdS nanoparticles were shown to be both crystalline and luminescent. Importantly, the CdS nanoparticle sizes can be tuned since their diameters depend on the volume of the originally deposited BCP feature.

Oligonucleotide flexibility dictates crystal quality in DNA-programmable nanoparticle superlattices.

The evolution of crystallite size and microstrain in DNA-mediated nanoparticle superlattices is dictated by annealing temperature and the flexibility of the interparticle bonds. This work addresses a major challenge in synthesizing optical metamaterials based upon noble metal nanoparticles by enabling the crystallization of large nanoparticles (100 nm diameter) at high volume fractions (34% metal).

A cantilever-free approach to dot-matrix nanoprinting

Scanning probe lithography (SPL) is a promising candidate approach for desktop nanofabrication, but trade-offs in throughput, cost, and resolution have limited its application. The recent development of cantilever-free scanning probe arrays has allowed researchers to define nanoscale patterns in a low-cost and high-resolution format, but with the limitation that these are duplication tools where each probe in the array creates a copy of a single pattern. Here, we report a cantilever-free SPL architecture that can generate 100 nanometer-scale molecular features using a 2D array of independently actuated probes. To physically actuate a probe, local heating is used to thermally expand the elastomeric film beneath a single probe, bringing it into contact with the patterning surface. Not only is this architecture simple and scalable, but it addresses fundamental limitations of 2D SPL by allowing one to compensate for unavoidable imperfections in the system. This cantilever-free dot-matrix nanoprinting will enable the construction of surfaces with chemical functionality that is tuned across the nano- and macroscales.

A methodology for preparing nanostructured biomolecular interfaces with high enzymatic activity

The development of a novel method for functionalizing nanopatterned surfaces with catalytically active proteins is reported. This method involves using dip-pen nanolithography (DPN) and polymer pen lithography (PPL) to generate nanoscale patterns of coenzyme A, followed by a phosphopantetheinyl transferase-mediated coupling between coenzyme A and proteins fused to the ybbR-tag. By exploiting the ability to generate protein features over large areas afforded by DPN and PPL, it was now possible to measure protein activity directly on these surfaces. It was found that proteins immobilized on the nanoscale features not only display higher activity per area with decreasing feature size, but are also robust and can be used for repeated catalytic cycles. The immobilization method is applicable to a variety of proteins and gives rise to superior activity compared to proteins attached in random orientations on the surface.

Layer-by-layer assembly of a metallomesogen by dip-pen nanolithography.

Palladium alkanethiolates are introduced here as a novel liquid ink for dip-pen nanolithography (DPN). These structures exhibit the unusual characteristic of layer-by-layer assembly, allowing one to deposit a desired number of metal ions on a surface, which can subsequently be reduced via thermolysis to form active catalytic structures. Such structures have been used to generate contiguous metallic or conducting polymer nanoscale architectures by electroless deposition.

Tuning the Spring Constant of Cantilever-Free Tip Arrays

A method to measure and tune the spring constant of tips in a cantilever-free array by adjusting the mechanical properties of the elastomeric layer on which it is based is reported. Using this technique, large-area silicon tip arrays are fabricated with spring constants tuned ranging from 7 to 150 N/m. To illustrate the benefit of utilizing a lower spring constant array, the ability to pattern on a delicate 50 nm silicon nitride substrate is explored.