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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