Boya Radha

Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

We synthesized fluorescent, porphyrin-anchored, Au(22) clusters in a single step, starting from well-characterized Au(25) clusters protected with glutathione (-SG) by a combined core reduction/ligand exchange protocol, at a liquid-liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au(25) and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au(22) core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H(2)TPPOAS-), on the Au(22) core was confirmed by fluorimetric titrations with Cu(2+) and Zn(2+) ions. From all of these, the composition of the cluster was determined to be Au(22)[(-SG)(15)(-SAOPPTH(2))(2)], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging.

Metal hierarchical patterning by direct nanoimprint lithography

Three-dimensional hierarchical patterning of metals is of paramount importance in diverse fields involving photonics, controlling surface wettability and wearable electronics. Conventionally, this type of structuring is tedious and usually involves layer-by-layer lithographic patterning. Here, we describe a simple process of direct nanoimprint lithography using palladium benzylthiolate, a versatile metal-organic ink, which not only leads to the formation of hierarchical patterns but also is amenable to layer-by-layer stacking of the metal over large areas. The key to achieving such multi-faceted patterning is hysteretic melting of ink, enabling its shaping. It undergoes transformation to metallic palladium under gentle thermal conditions without affecting the integrity of the hierarchical patterns on micro- as well as nanoscale. A metallic rice leaf structure showing anisotropic wetting behavior and woodpile-like structures were thus fabricated. Furthermore, this method is extendable for transferring imprinted structures to a flexible substrate to make them robust enough to sustain numerous bending cycles.

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 Modified Micromolding Method for Sub‐100‐nm Direct Patterning of Pd Nanowires

An unconventional modification of a poly(dimethylsiloxane) stamp hosting microchannels leads to the formation of nanochannels entrapping a precursor, which metallizes into sub-100-nm Pd nanowires under the molding conditions (see image). This is a single-step direct-patterning technique termed nanoentrapment molding.

Flexible Palladium-Based H2 Sensor with Fast Response and Low Leakage Detection by Nanoimprint Lithography

Flexible palladium-based H2 sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H2 sensor that is capable of detecting H2 gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H2 sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H2 sensors with improved durability, sensitivity, and response time in comparison to palladium thin films. At ambient pressure and temperature, the device showed a fast response time of 18 s at a H2 concentration of 3500 ppm. At 50 ppm concentration, the response time was found to be 57 s. The flexibility of the sensor does not appear to compromise its performance.

Flexible and Semitransparent Strain Sensors Based on Micromolded Pd Nanoparticle–Carbon μ-Stripes

Flexible resistive strain sensors have been fabricated by micromolding Pd alkanethiolate on polyimide substrates and subjecting to thermolysis in air. Thus produced stripes were ∼1 μm wide with spacing of ∼0.5 μm and contained Pd nanoparticles in carbon matrix. The nanoparticle size and the nature of carbon are much dependent on the thermolysis temperature as is also the resistance of the microstripes. Generally, lower thermolysis temperatures (<230 °C) produced stripes containing small Pd nanoparticles with significant fraction of carbon from the precursor decomposition. The stripes were poorly conducting yet interestingly, exhibited change of resistance under tensile and compressive strain. Particularly noteworthy are the stripes produced from 195 °C thermolysis, which showed a high gauge factor of ∼390 with strain sensitivity, 0.09%. With molding at 230 °C, the stripes obtained were highly conducting, and amazingly did not change the resistance with strain even after several bending cycles. The latter are ideal as flexible conduits and interconnects. Thus, the article reports a method of producing flexible sensitive strain sensors on one hand and on the other, flexible conduits with unchanging resistance, merely by fine-tuning the precursor decomposition under the molding conditions.

Metal Anion–Alkyl Ammonium Complexes as Direct Write Precursors to Produce Nanopatterns of Metals, Nitrides, Oxides, Sulfides, And Alloys

The study explores the possibility of using metal anions complexed with tetraoctylammonium bromide (ToABr) as single-source direct write precursors in e-beam and soft lithography processes to obtain micro- and nanoscale patterns of various metals, i.e., Au, Pd, Pt, Ag, Pb and Cu, as well as of their alloys (AuCu), oxides (Co(3)O(4), ZnO), nitrides (CoN, InN, GaN), and sulfides (Ag(2)S). The extraction efficiency of ToABr for different metal anions is found to be varied (40-90%), but the obtained precursors are easily processable as they have reasonable solubility in common solvents and are obtainable as smooth films, both being important for high-resolution patterning. The e-resist action of the precursors originates from the extreme e-beam sensitivity of the hydrocarbon chain present in ToABr, while direct micromolding has been possible due to easy flow of the precursor solutions in capillaries. The interaction of the anion and ToABr being mainly electrostatic enables easy removal of the hydrocarbon from patterned regions by thermolysis on a hot plate in the ambient or in controlled atmosphere to form the desired product. This method can be easily generalized.

Reconstitutable nanoparticle superlattices.

Colloidal self-assembly predominantly results in lattices that are either: (1) fixed in the solid state and not amenable to additional modification, or (2) in solution, capable of dynamic adjustment, but difficult to transition to other environments. Accordingly, approaches to both dynamically adjust the interparticle spacing of nanoparticle superlattices and reversibly transfer superlattices between solution-phase and solid state environments are limited. In this manuscript, we report the reversible contraction and expansion of nanoparticles within immobilized monolayers, surface-assembled superlattices, and free-standing single crystal superlattices through dehydration and subsequent rehydration. Interestingly, DNA contraction upon dehydration occurs in a highly uniform manner, which allows access to spacings as small as 4.6 nm and as much as a 63% contraction in the volume of the lattice. This enables one to deliberately control interparticle spacings over a 4-46 nm range and to preserve solution-phase lattice symmetry in the solid state. This approach could be of use in the study of distance-dependent properties of nanoparticle superlattices and for long-term superlattice preservation.

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.

Coexistence of Vapor–Liquid–Solid and Vapor–Solid–Solid Growth Modes in Pd-Assisted InAs Nanowires

During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire-tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor-liquid-solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor-solid-solid (VSS) mode.