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.

Colloidal crystals can be assembled using a variety of entropic, [1][2][3] depletion, [4,5] electrostatic, [6][7][8] or biorecognition forces [9][10][11][12] 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, [1,3,[13][14][15][16][17] electronic, [13,18] and magnetic properties. [19,20] These properties can be sensitive to the composition, symmetry, and distance between nanoparticles, in addition to the number of layers and orientation. [9,15,16] DNA-mediated nanoparticle crystallization is particularly attractive for preparing these materials because the nanoparticle building blocks can be considered a type of "pro-

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.T he integration of nanoparticles into devices has enabled applications spanning sensing (1, 2), catalysis (3), electronics (2), photonics (4), and plasmonics (5, 6), but synthesizing individual nanoparticles with control over size, composition, and placement on substrates is challenging (1-3, 6, 7). With conventional approaches, nanoparticles are synthesized and subsequently positioned on a surface using techniques such as parallel printing (8), surface dewetting (9, 10), microdroplet molding (7), nanoparticle sliding (11), direct writing (4, 12, 13), and self-assembly (2, 14, 15). However, it is difficult and in most cases, impossible to use these methods to reliably make and position a single particle on a surface with nanometer-scale control.In contrast with the conventional synthesis-then-positioning paradigm, which is the basis for most single-particle device incorporation schemes, scanning probe block copolymer lithography (SPBCL) is an example of precursor positioning-then-synthesis. The technique uses concepts from the block copolymer community (16) and the positional control offered by dip-pen nanolithography (DPN) (17) to deliver attoliter volumes of a metalcoordinated block copolymer onto a surface, which then can be used to synthesize individual nanoparticles (18,19). Importantly, SPBCL allows one to directly synthesize arbitrary patterns of single nanoparticles over large areas on a surface, which has been usef...

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.

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this sense, the particles act as "programmable atom equivalents" (PAEs), where each particle behaves as an "atom" with bonding behavior that can be tuned via the DNA interconnects. The synthetic tailorability afforded by DNA allows independent control over the superlattice connectivity and nanoparticle core, thereby enabling the design and synthesis of colloidal crystals with widely varying symmetry, scale, and composition. [14][15][16][17][18][19] While DNAmediated assembly is a useful approach for spatially arranging noble metal nanoparticles, this method has not succeeded in forming crystals with short rigid DNA strands, especially for larger diameter particles. [ 14 ] This inability to crystallize large particles with small separations is likely a result of kinetic jamming behavior, which is often seen in colloidal systems [ 20 ] and may be related to the rigidity of the double-stranded DNA ligands.Initial evidence for the importance of fl exibility came from the observation that certain PAE systems which form amorphous aggregates when linked with fully duplexed DNA will crystallize if a single unpaired base is added to the DNA strands that link the particles. [ 12 ] Given that single-stranded DNA has a shorter persistence length than double-stranded DNA, this can be rationalized by considering that adding some degree of fl exibility may be required to allow the packing of the inherently polydisperse particles into a regular lattice. Further, it is well known that the phase behavior of colloids depends on their range of interaction; long-range interactions result in a "softer" pair-potential that may allow for crystallization, while short-range interactions often result in jamming and gel formation. [ 21 ] Based on these observations, we hypothesize that adding fl exibility to the DNA ligand could allow each strand to explore a wider space, thereby increasing the interaction range and decreasing jamming during PAE crystallization.Herein, we explore the relationship between crystallization and linker fl exibility by systematically adding various lengths of a fl exible oligomer to the DNA ligand ( Figure 1 ). Through annealing at different temperatures, we fi nd that the highest quality crystals (defi ned here as crystals with the largest grain sizes and lowest microstrain as measured by X-ray diffraction) are formed at temperatures close to the melting temperature. Signifi cantly, increasing ligand fl exibility increases the temperature range under which crystallization occurs while simultaneously minimizing microstrain and maximizing domain size.In a typical assembly experiment, PAE superlattices were observed by correlated synchrotron small-angle X-ray scattering (SAXS) and UV-vis spectroscopy ( Figure 2 ), a set of measurements that allowed for the simultaneous determination of structural and thermodynamic information. Specifi cally, gold nanoparticles (AuNPs) were coated with a dense, oriented Superlattices composed of noble metal nanoparticles display considerable promise for the development of optical m...

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.

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.