Nanocombinatorics with Cantilever-Free Scanning Probe Arrays.

The effectiveness of combinatorial experiments is determined by the rate at which distinct experimental conditions can be prepared and interrogated. This has been particularly limiting at the intersection of nanotechnology and soft materials research, where structures are difficult to reliably prepare and materials are incompatible with conventional lithographic techniques. For example, studying nanoparticle-based heterogeneous catalysis or the interaction between biological cells and abiotic surfaces requires precise tuning of materials composition on the nanometer scale. Scanning probe techniques are poised to be major players in the combinatorial nanoscience arena because they allow one to directly deposit materials at high resolution without any harsh processing steps that limit material compatibility. The chief limitation of scanning probe techniques is throughput, as patterning with single probes is prohibitively slow in the context of large-scale combinatorial experiments. A recent paradigm shift circumvents this problem by fundamentally altering the architecture of scanning probes by replacing the conventionally used cantilever with a soft compliant film on a rigid substrate, a substitution that allows a densely packed array of probes to function in parallel in an inexpensive format. This is a major lithographic advance in terms of scalability, throughput, and versatility that, when combined with the development of approaches to actuate individual probes in cantilever-free arrays, sets the stage for scanning-probe-based tools to address scientific questions through nanocombinatorial studies in biology and materials science. In this review, we outline the development of cantilever-free scanning probe lithography and prospects for nanocombinatorial studies enabled by these tools.

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.

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).

Capillary bridge rupture in dip-pen nanolithography.

Here, we explore fluid transfer from a nanoscale tip to a surface and elucidate the role of fluid flows in dip-pen nanolithography (DPN) of liquid inks. We find that while fluid transfer in this context is affected by dwell time and tip retraction speed from the substrate, their specific roles are dictated by the contact angle of the ink on the surface. This is shown by two observations: (1) the power law scaling of transferred fluid with dwell time depends on contact angle, and (2) slower retraction speeds result in more transfer on hydrophilic surfaces, but less transfer on hydrophobic surfaces. These trends, coupled with the observation of a transition from quasi-static to dynamic capillary rupture at a capillary number of 6 × 10(-6), show that the transfer process is a competition between surface energy and viscosity. Based on this, we introduce retraction speed as an important parameter in DPN and show that it is possible to print polymer features as small as 14 nm. Further explorations of this kind may provide a useful platform for studying capillary phenomena at the nanoscale.

Combinatorial screening of mesenchymal stem cell adhesion and differentiation using polymer pen lithography.

The extracellular matrix (ECM) is a complex, spatially inhomogeneous environment that is host to myriad cell-receptor interactions that promote changes in cell behavior. These biological systems can be probed and simulated with engineered surfaces, but doing so demands careful control over the arrangement of ligands. Here, we describe how such surfaces can be fabricated by utilizing polymer pen lithography (PPL), which is a cantilever-free scanning probe lithographic method that utilizes polymeric pen arrays to generate patterns over large areas. With the advent of PPL, fundamental questions in cell biology can be answered by recapitulating cell-ECM interactions to explore how these interactions lead to changes in cell behavior. Here, we describe an approach for the combinatorial screening of cell adhesion behavior to gain understanding of how ECM protein feature size dictates osteogenic differentiation of mesenchymal stem cells. The technique outlined here is generalizable to other biological systems and can be paired with quantitative analytical methods to probe important processes such as cell polarization, proliferation, signaling, and differentiation.

Role of absorbed solvent in polymer pen lithography.

We report on the dynamic role of solvents in molecular printing and show that material transport can be mediated by both environmental solvent (i.e., humidity) and solvent absorbed in the pen. To explore the transport of materials in the absence of environmental solvent, a hydrophobic polymer was patterned using a polydimethylsiloxane (PDMS) pen array that had been soaked in undecane, a nonpolar solvent that readily absorbs into PDMS. We also explored the patterning of the hydrophilic polymer polyethylene glycol (PEG) and found that, even though PDMS only absorbs trace amounts of water, soaking a PDMS pen array in water enables PEG deposition in completely dry environments for over 2 h. We find that the length of time one can pattern in a dry environment is determined by the availability of absorbed solvent, a relationship that we elucidate by comparing the performance of pens with varying ability to absorb water. Furthermore, a calculation accounting for the dynamics of retained water captures these effects completely, allowing for generalization of this result to other solvents and providing a way to tune the desired solvent retention profile. Taken together, this work explores the subtle and dynamic role of solvent on molecular printing and provides an alternative to strict environmental humidity control for reliable molecular printing.

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.

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.

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.

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 (Fe(2)O(3), Co(2)O(3), 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.

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.

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.