Gold nanorods have received much attention due to their unique optical and electronic properties which are dependent on their shape, size, and aspect ratio. This article covers in detail the synthesis, functionalization, self-assembly, and sensing applications of gold nanorods. The synthesis of three major types of rods is discussed: single-crystalline and pentahedrally-twinned rods, which are synthesized by wet chemistry methods, and polycrystalline rods, which are synthesized by templated deposition. Functionalization of these rods is usually necessary for their applications, but can often be problematic due to their surfactant coating. Thus, general strategies are provided for the covalent and noncovalent functionalization of gold nanorods. The review will then examine the significant progress that has been made in controllable assembly of nanorods into various arrangements. This assembly can have a large effect on measurable properties of rods, making it particularly applicable towards sensing of a variety of analytes. Other types of sensing not dependent on nanorod assembly, such as refractive-index based sensing, are also discussed.

Functional Gold Nanorods: Synthesis, Self-Assembly, and Sensing Applications

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

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Advanced scanning probe lithography

Self-assembling amyloid-like peptides and proteins give rise to promising biomaterials with potential applications in many fields. Amyloid structures are formed by the process of molecular recognition and self-assembly, wherein a peptide or protein monomer spontaneously self-associates into dimers and oligomers and subsequently into supramolecular aggregates, finally resulting in condensed fibrils. Mature amyloid fibrils possess a quasi-crystalline structure featuring a characteristic fiber diffraction pattern and have well-defined properties, in contrast to many amorphous protein aggregates that arise when proteins misfold. Core sequences of four to seven amino acids have been identified within natural amyloid proteins. They are capable to form amyloid fibers and fibrils and have been used as amyloid model structures, simplifying the investigations on amyloid structures due to their small size. Recent studies have highlighted the use of self-assembled amyloid-based fibers as nanomaterials. Here, we discuss the latest advances and the major challenges in developing amyloids for future applications in nanotechnology and nanomedicine, with the focus on development of sensors to study protein–ligand interactions.

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Amyloid-based nanosensors and nanodevices

Atomic force microscopy (AFM)-based methods have matured into a powerful nanoscopic platform, enabling the characterization of a wide range of biological and synthetic biointerfaces ranging from tissues, cells, membranes, proteins, nucleic acids and functional materials. Although the unprecedented signal-to-noise ratio of AFM enables the imaging of biological interfaces from the cellular to the molecular scale, AFM-based force spectroscopy allows their mechanical, chemical, conductive or electrostatic, and biological properties to be probed. The combination of AFM-based imaging and spectroscopy structurally maps these properties and allows their 3D manipulation with molecular precision. In this Review, we survey basic and advanced AFM-related approaches and evaluate their unique advantages and limitations in imaging, sensing, parameterizing and designing biointerfaces. It is anticipated that in the next decade these AFM-related techniques will have a profound influence on the way researchers view, characterize and construct biointerfaces, thereby helping to solve and address fundamental challenges that cannot be addressed with other techniques. Atomic force microscopy (AFM)-based approaches enable the characterization and manipulation of biological and synthetic biointerfaces, including tissues, cells, membranes, proteins, nucleic acid and functional materials. In this Review, the advantages and limitations of imaging, sensing, parameterizing and designing biointerfaces using AFM techniques are discussed.

Atomic force microscopy-based characterization and design of biointerfaces

We demonstrate plasmonic enhancement of upconversion luminescence in individual nanocrystal heterodimers formed by template-assisted self-assembly. Lithographically defined, shape-selective templates were used to deterministically coassemble single Au nanorods in proximity to single hexagonal (β-phase) NaYF4:Yb3+,Er3+ upconversion nanophosphors. By tailoring the dimensions of the rods to spectrally tune their longitudinal surface plasmon resonance to match the 977 nm excitation wavelength of the phosphors and by spatially localizing the phosphors in the intense near-fields surrounding the rod tips, several-fold luminescence enhancements were achieved. The enhancement effects exhibited a strong dependence on the excitation light’s polarization relative to the rod axis. In addition, greater enhancement was observed at lower excitation power densities due to the nonlinear behavior of the upconversion process. The template-based coassembly scheme utilized here for plasmonic coupling offers a versatile platfor...

Plasmon-Enhanced Upconversion Luminescence in Single Nanophosphor–Nanorod Heterodimers Formed through Template-Assisted Self-Assembly

We have used a temperature sensitive polymer film as a removable template to position, and align, gold nanorods onto an underlying target substrate. Shape-matching guiding structures for the assembly of nanorods of size 80 nm × 25 nm have been written by thermal scanning probe lithography. The nanorods were assembled into the guiding structures, which determine both the position and the orientation of single nanorods, by means of capillary interactions. Following particle assembly, the polymer was removed cleanly by thermal decomposition and the nanorods are transferred to the underlying substrate. We have thus demonstrated both the placement and orientation of nanorods with an overall positioning accuracy of ≈10 nm onto an unstructured target substrate.

Directed placement of gold nanorods using a removable template for guided assembly.

Scanning probe nanolithography (SPL) has demonstrated its potential in a variety of applications like 3D nanopatterning, 'direct development' lithography, dip-pen deposition or patterning of self-assembled monolayers. One of the main issues holding back SPL has been the limited throughput for patterning and imaging. Here we present a complete lithography and metrology system based on thermomechanical writing into organic resists. Metrology is carried out using a thermoelectric topography sensing method. More specifically, we demonstrate a system with a patterning pixel clock of 500 kHz, 20 mm s − 1 linear scan speed, a positioning accuracy of 10 nm, a read-back frequency bandwidth of 100 000 line-pairs s − 1 and a turnaround time from patterning to qualifying metrology of 1 min. Thus, we demonstrate a nanolithography system capable of implementing rapid turnaround.

https://iopscience.iop.org/article/10.1088/0957-4484/22/27/275306/pdf

Rapid turnaround scanning probe nanolithography.

Thermal scanning probe lithography is used for creating lithographic patterns with 27.5 nm half-pitch line density in a 50 nm thick high carbon content organic resist on a Si substrate. The as-written patterns in the poly phthaladehyde thermal resist layer have a depth of 8 nm, and they are transformed into high-aspect ratio binary patterns in the high carbon content resist using a SiO2 hard-mask layer with a thickness of merely 4 nm and a sequence of selective reactive ion etching steps. Using this process, a line-edge roughness after transfer of 2.7 nm (3σ) has been achieved. The patterns have also been transferred into 50 nm deep structures in the Si substrate with excellent conformal accuracy. The demonstrated process capabilities in terms of feature density and line-edge roughness are in accordance with today’s requirements for maskless lithography, for example for the fabrication of extreme ultraviolet (EUV) masks.

Thermal Probe Maskless Lithography for 27.5 nm Half-Pitch Si Technology

Heated tips offer the possibility to create arbitrary high-resolution nanostructures by local decomposition and
evaporation of resist materials. Turnaround times of minutes are achieved with this patterning method due to the high-speed
direct-write process and an in-situ imaging capability. Dense features with 10 nm half-pitch can be written into
thin films of organic resists such as self-amplified depolymerization (SAD) polymers or molecular glasses. The
patterning speed of tSPL has been increased far beyond usual scanning probe lithography (SPL) technologies and
approaches the speed of Gaussian shaped electron beam lithography (EBL) for <30 nm resolution. A single tip can write
complex patterns with a pixel rate of 500 kHz and a linear scan speed of 20 mm/s. Moreover, a novel scheme for
stitching was developed to extend the patterning area beyond the ≤100 μm range of the piezo stages. A stitching
accuracy of 10 nm is obtained without the use of markers. Furthermore, the patterning depth can be controlled
independently and accurately (~1 nm) at each position. Thereby, arbitrary 3D structures can be written in a single step.
Finally, we demonstrated an all-dry tri-layer pattern transfer concept to create high aspect ratio structures in silicon.
Dense fins and trenches with 27 nm half-pitch and a line edge roughness (LER) below 3nm (3σ) have been fabricated.

Thermal probe nanolithography: in-situ inspection, high-speed, high-resolution, 3D

The invention notably concerns a method for depositing nano-objects on a surface. The method includes: providing a substrate with surface patterns on one face thereof; providing a transfer layer on said face of the substrate; functionalizing areas on a surface of the transfer layer parallel to said face of the substrate, at locations defined with respect to said surface patterns, such as to exhibit enhanced binding interactions with nano-objects; depositing nano-objects and letting them get captured at the functionalized areas; and thinning down the transfer layer by energetic stimulation to decompose the polymer into evaporating units, until the nano-objects reach the surface of the substrate. The invention also provides a semiconductor device which includes a substrate and nano-objects accurately disposed on the substrate

Philip Paul

Papers

Directed placement of gold nanorods using a removable template for guided assembly.

Rapid turnaround scanning probe nanolithography.

Thermal Probe Maskless Lithography for 27.5 nm Half-Pitch Si Technology

Thermal probe nanolithography: in-situ inspection, high-speed, high-resolution, 3D

Accurate deposition of nano-objects on a surface