Debin Wang

Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics

Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude. Conducting regions can be drawn on graphene oxide sheets with a heated atomic force microscope tip. The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. We report on a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip. The rGO regions are up to four orders of magnitude more conductive than pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean, rapid, and reliable.

Reconciling disparate views of template-directed nucleation through measurement of calcite nucleation kinetics and binding energies

Significance Organisms use specialized macromolecules to direct the timing and placement of crystals during biomineral formation. This phenomenon has inspired synthetic approaches to templating but remains poorly understood. One view holds that the organic matrix promotes nucleation through stereochemical matching to guide the organization of solute ions, while another equates binding strength to promotion of nucleation. Our study reconciles these views with a mechanistic explanation for template-directed nucleation. Through measurements of calcite nucleation kinetics and substrate–crystal binding we show that nucleation barriers and binding free energies are linearly related for all functional group chemistries and conformations as predicted from classical nucleation theory. This model reconciles long-standing concepts of stereochemical matching with the conventional wisdom that good binders are good nucleators. The physical basis for how macromolecules regulate the onset of mineral formation in calcifying tissues is not well established. A popular conceptual model assumes the organic matrix provides a stereochemical match during cooperative organization of solute ions. In contrast, another uses simple binding assays to identify good promoters of nucleation. Here, we reconcile these two views and provide a mechanistic explanation for template-directed nucleation by correlating heterogeneous nucleation barriers with crystal–substrate-binding free energies. We first measure the kinetics of calcite nucleation onto model substrates that present different functional group chemistries (carboxyl, thiol, phosphate, and hydroxyl) and conformations (C11 and C16 chain lengths). We find rates are substrate-specific and obey predictions of classical nucleation theory at supersaturations that extend above the solubility of amorphous calcium carbonate. Analysis of the kinetic data shows the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energy of the system, γ. We then use dynamic force spectroscopy to independently measure calcite–substrate-binding free energies, ΔGb. Moreover, we show that within the classical theory of nucleation, γ and ΔGb should be linearly related. The results bear out this prediction and demonstrate that low-energy barriers to nucleation correlate with strong crystal–substrate binding. This relationship is general to all functional group chemistries and conformations. These findings provide a physical model that reconciles the long-standing concept of templated nucleation through stereochemical matching with the conventional wisdom that good binders are good nucleators. The alternative perspectives become internally consistent when viewed through the lens of crystal–substrate binding.

Local wettability modification by thermochemical nanolithography with write-read-overwrite capability

The wettability of a thin polymer film was modified twice by thermochemical nanolithography. By means of a first local chemical modification induced by an atomic force microscope tip heated to 110±20°C, hydrophilic patterns are written over an originally hydrophobic polymer surface. By further heating to 190±20°C, a second chemical modification reverses the local wettability change introduced by the first chemical modification. This write-read-overwrite capability can be particularly useful in the design of complex nanofluidic devices.

Fabricating Nanoscale Chemical Gradients with ThermoChemical NanoLithography

Production of chemical concentration gradients on the submicrometer scale remains a formidable challenge, despite the broad range of potential applications and their ubiquity throughout nature. We present a strategy to quantitatively prescribe spatial variations in functional group concentration using ThermoChemical NanoLithography (TCNL). The approach uses a heated cantilever to drive a localized nanoscale chemical reaction at an interface, where a reactant is transformed into a product. We show using friction force microscopy that localized gradients in the product concentration have a spatial resolution of ~20 nm where the entire concentration profile is confined to sub-180 nm. To gain quantitative control over the concentration, we introduce a chemical kinetics model of the thermally driven nanoreaction that shows excellent agreement with experiments. The comparison provides a calibration of the nonlinear dependence of product concentration versus temperature, which we use to design two-dimensional temperature maps encoding the prescription for linear and nonlinear gradients. The resultant chemical nanopatterns show high fidelity to the user-defined patterns, including the ability to realize complex chemical patterns with arbitrary variations in peak concentration with a spatial resolution of 180 nm or better. While this work focuses on producing chemical gradients of amine groups, other functionalities are a straightforward modification. We envision that using the basic scheme introduced here, quantitative TCNL will be capable of patterning gradients of other exploitable physical or chemical properties such as fluorescence in conjugated polymers and conductivity in graphene. The access to submicrometer chemical concentration and gradient patterning provides a new dimension of control for nanolithography.

Temperature-dependence of ink transport during thermal dip-pen nanolithography

We investigate the control of tip temperature on feature size during dip-pen nanolithography (DPN) of mercaptohexadecanoic acid (MHA) on Au. Heated atomic force microscopy (AFM) probes operated between 25 °C and 50 °C wrote nanostructures of MHA for various dwell times and tip speeds. The feature size exhibited an exponential dependence on tip temperature with an apparent activation barrier of 165 kJ/mol. Analysis of the ink transfer process shows that, while ∼1/3 of the barrier is from ink dissolution into the meniscus, the rest reflects the barrier to adsorption onto the growing feature, a process that has been ignored in previous DPN models.

Direct writing and characterization of poly(p-phenylene vinylene) nanostructures

We report the use of thermochemical nanolithography to convert a precursor polymer film to poly(p-phenylene vinylene) with sub-100 nm spatial resolution, in ambient conditions. The local thermochemical conversion is verified by Raman spectroscopy, fluorescence imaging, and atomic force microscopy. This convenient direct writing of conjugated polymer nanostructures could be desirable for the design and fabrication of future nanoelectronic, nanophotonic, and biosensing devices.

Hierarchical assembly of plasmonic nanostructures using virus capsid scaffolds on DNA origami templates

Building plasmonic nanostructures using biomolecules as scaffolds has shown great potential for attaining tunable light absorption and emission via precise spatial organization of optical species and antennae. Here we report bottom-up assembly of hierarchical plasmonic nanostructures using DNA origami templates and MS2 virus capsids. These serve as programmable scaffolds that provide molecular level control over the distribution of fluorophores and nanometer-scale control over their distance from a gold nanoparticle antenna. While previous research using DNA origami to assemble plasmonic nanostructures focused on determining the distance-dependent response of single fluorophores, here we address the challenge of constructing hybrid nanostructures that present an organized ensemble of fluorophores and then investigate the plasmonic response. By combining finite-difference time-domain numerical simulations with atomic force microscopy and correlated scanning confocal fluorescence microscopy, we find that the use of the scaffold keeps the majority of the fluorophores out of the quenching zone, leading to increased fluorescence intensity and mild levels of enhancement. The results show that the degree of enhancement can be controlled by exploiting capsid scaffolds of different sizes and tuning capsid-AuNP distances. These bioinspired plasmonic nanostructures provide a flexible design for manipulating photonic excitation and photoemission.

A New AFM-Based Lithography Method: Thermochemical Nanolithography

In the last decade, there has been a tremendous increase in the number of techniques for patterning materials on the nanoscale (10-100nm), driven by numerous potential applications, for example, in sensing[1], data storage [2], optoelectronic [3], display [4], nanofluidic [5], and biomimetic [6] devices. An ideal nanolithography technique would be able to: (1) write with nm resolution; (2) write with speeds of multiple centimeters per second (while preserving nanometer scale registry) for wafer-scale lithography; (2) impart different chemical functionality and/or physical properties (with or without topographical changes) as desired; (4) function in different laboratory environments (for example, under ambient pressure or in solution); (5) be capable of massive parallelization for both writing and metrology; and (6) write on a variety of materials deposited on a variety of substrates. Specific applications will require one or more of the attributes described earlier, but the most versatile technique would encompass as many as possible. To our knowledge, no technique currently in practice can simultaneously attain all of these features.

Nanofabrication of Functional Nanostructures by Thermochemical Nanolithography

Nanofabrication is the process of building functional structures with nanoscale dimensions, which can be used as components, devices, or systems with high density, in large quantities, and at low cost. Since the invention of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in 1980s, the application of scanning probe based lithography (SPL) techniques for modification of substrates and creation of functional nanoscale structures and nanostructured materials has been widespread, resulting in the emergence of a large variety of methodologies. In this chapter, we review the recent development of a thermal probe based nanofabrication technique called thermochemical nanolithography (TCNL). We start with a brief review of the evolution of the thermal AFM probes integrated with resistive heaters. We then provide an overview of some established nanofabrication techniques in which thermal probes are used, namely thermomechanical nanolithography, the Millipede project, and thermal dip-pen nanolithography. We discuss the heat transfer mechanisms of the thermal probes in the thermal writing process of TCNL. The remainder of the chapter focuses on the use of TCNL on a variety of systems and thermochemical reactions. TCNL has been successfully used for fabrication of functional nanostructures that are appealing for various applications in nanofluidics, nanoelectronics, nanophotonics, and biosensing devices. Finally, we close this chapter by discussing some future research directions where the capabilities and robustness of TCNL can be further extended.