Topology assisted self-organization of colloidal nanoparticles: application to 2D large-scale nanomastering

Deterministic positioning of few aqueous colloidal quantum dots

Emerging quantum technologies that critically require the integration of quantum emitters on photonic platforms are hindered by the control over their position, quantity, and scalability. Herein, we describe a facile strategy to deposit aqueous silica-coated quantum dots (QDs) in a template of polymethyl methacrylate (PMMA) nanoholes that leverages saturated ethanol vapor drop-casting and subsequent lift-off of the template. Ethanol vapor incorporation into water droplets during the drying process reduces the meniscus contact angle, which increases capillary forces and enhances particle confinement within the pinning contact region. Furthermore, induced Marangoni flow controls the particle transport dynamics inside the droplets, making large-scale deposition possible. Controlling the hole diameter of the template demonstrates changes in the number of QDs per hole, which is consistent with the Poissonian distribution with the best results of ∼40% single-particle yield from an ∼80% total site occupancy. This method employs a simple setup, eliminating the need for intricate optimization, yet offers the potential for deterministic patterning within complex photonic platforms.

A review of cost-effective black silicon fabrication techniques and applications

Ever since the discovery of black silicon, scientists around the world have been trying to come up with novel, cost-effective methods of utilizing this super material in a variety of different industries due to its remarkably low reflectivity and excellent electronic and optoelectronic properties. In this review, many of the most common methods of black silicon fabrication are exhibited, including metal-assisted chemical etching, reactive ion etching, and femto-second laser irradiation. Different nanostructured silicon surfaces are assessed based on their reflectivity and applicable properties in both the visible wavelength range and the infrared range. The most cost efficient technique for the mass production of black silicon is discussed, as well as some promising contender materials ready to replace silicon. Also, solar cell, IR photo-detector, and antibacterial applications are looked into, along with their respective challenges to date.

Controlling disorder in self-assembled colloidal monolayers via evaporative processes

Monolayers of assembled nano-objects with a controlled degree of disorder hold interest in many optical applications, including photovoltaics, light emission, sensing, and structural coloration. Controlled disorder can be achieved through either top-down or bottom-up approaches, but the latter is more suited to large-scale, low-cost fabrication. Disordered colloidal monolayers can be assembled through evaporatively driven convective assembly, a bottom-up process with a wide range of parameters impacting particle placement. Motivated by the photonic applications of such monolayers, in this review we discuss the quantification of monolayer disorder, and the assembly methods that have been used to produce them. We review the impact of particle and solvent properties, as well as the use of substrate patterning, to create the desired spatial distributions of particles.

Templated Assembly of Nanoparticles into Continuous Arrays

The templated assembly of nanoparticles has been limited so far to yield only discontinuous nanoparticle clusters confined within lithographically patterned cavities. Here, we explored the templated assembly of nanoparticles into continuous 2D structures, using lithographically patterned templates with topographical features sized as the assembled nanoparticles. We found that these features act as nucleation centers, whose exact arrangement determines four possible assembly regimes (i) rotated, (ii) disordered, (iii) closely packed, and (iv) unpacked. These regimes produce structures strikingly different from their geometry, orientation, long-range and short-range orders, and packing density. Interestingly, for templates with relatively distant nucleation centers, these four regimes are replaced with three new ones, which produce large monocrystalline domains that are either (i) uniformly rotated, (ii) uniformly aligned, or (iii) nonuniformly rotated relative to the nucleation lattice. We rationalized our experimental data using a mathematical model, which examines all the alignment possibilities between the nucleation centers and the ideal hexagonal assembly. Our finding provides a new approach for the à la carte obtainment of various nanoscale structures unachievable by natural self-assembly and opens a route for the fabrication of numerous functional nanodevices and nanosystems that could not be realized so far by the standard bottom-up approach.

Light Extraction Enhancement Techniques for Inorganic Scintillators

Scintillators play a key role in the detection chain of several applications which rely on the use of ionizing radiation, and it is often mandatory to extract and detect the generated scintillation light as efficiently as possible. Typical inorganic scintillators do however feature a high index of refraction, which impacts light extraction efficiency in a negative way. Furthermore, several applications such as preclinical Positron Emission Tomography (PET) rely on pixelated scintillators with small pitch. In this case, applying reflectors on the crystal pixel surface, as done conventionally, can have a dramatic impact of the packing fraction and thus the overall system sensitivity. This paper presents a study on light extraction techniques, as well as combinations thereof, for two of the most used inorganic scintillators (LYSO and BGO). Novel approaches, employing Distributed Bragg Reflectors (DBRs), metal coatings, and a modified Photonic Crystal (PhC) structure, are described in detail and compared with commonly used techniques. The nanostructure of the PhC is surrounded by a hybrid organic/inorganic silica sol-gel buffer layer which ensures robustness while maintaining its performance unchanged. We observed in particular a maximum light gain of about 41% on light extraction and 21% on energy resolution for BGO, a scintillator which has gained interest in the recent past due to its prompt Cherenkov component and lower cost.

Multifunctional Structured Platforms: From Patterning of Polymer-Based Films to Their Subsequent Filling with Various Nanomaterials

There is an astonishing number of optoelectronic, photonic, biological, sensing, or storage media devices, just to name a few, that rely on a variety of extraordinary periodic surface relief miniaturized patterns fabricated on polymer-covered rigid or flexible substrates. Even more extraordinary is that these surface relief patterns can be further filled, in a more or less ordered fashion, with various functional nanomaterials and thus can lead to the realization of more complex structured architectures. These architectures can serve as multifunctional platforms for the design and the development of a multitude of novel, better performing nanotechnological applications. In this work, we aim to provide an extensive overview on how multifunctional structured platforms can be fabricated by outlining not only the main polymer patterning methodologies but also by emphasizing various deposition methods that can guide different structures of functional nanomaterials into periodic surface relief patterns. Our aim is to provide the readers with a toolbox of the most suitable patterning and deposition methodologies that could be easily identified and further combined when the fabrication of novel structured platforms exhibiting interesting properties is targeted.

Pattern detection in colloidal assembly: A mosaic of analysis techniques

Characterization of the morphology, identification of patterns and quantification of order encountered in colloidal assemblies is essential for several reasons. First of all, it is useful to compare different self-assembly methods and assess the influence of different process parameters on the final colloidal pattern. In addition, casting light on the structures formed by colloidal particles can help to get better insight into colloidal interactions and understand phase transitions. Finally, the growing interest in colloidal assemblies in materials science for practical applications going from optoelectronics to biosensing imposes a thorough characterization of the morphology of colloidal assemblies because of the intimate relationship between morphology and physical properties (e.g. optical and mechanical) of a material. Several image analysis techniques developed to investigate images (acquired via scanning electron microscopy, digital video microscopy and other imaging methods) provide variegated and complementary information on the colloidal structures under scrutiny. However, understanding how to use such image analysis tools to get information on the characteristics of the colloidal assemblies may represent a non-trivial task, because it requires the combination of approaches drawn from diverse disciplines such as image processing, computational geometry and computational topology and their application to a primarily physico-chemical process. Moreover, the lack of a systematic description of such analysis tools makes it difficult to select the ones more suitable for the features of the colloidal assembly under examination. In this review we provide a methodical and extensive description of real-space image analysis tools by explaining their principles and their application to the investigation of two-dimensional colloidal assemblies with different morphological characteristics.

Nanophotonics: Energy Transfer towards Enhanced Luminescent Chemosensing

We discuss a recently proposed novel photonic approach for enhancing the fluorescence of extremely thin chemosensing polymer layers. We present theoretical and experimental results demonstrating the concept of gain-assisted waveguided energy transfer (G-WET) on a very thin polymer nanolayer spincoated on an active ZnO thin film. The G-WET approach is shown to result in an 8-fold increase in polymer fluorescence. We then extend the G-WET concept to nanostructured media. The benefits of using active nanostructured substrates on the sensitivity and fluorescence of chemosensing polymers are discussed. Preliminary theoretical results on enlarged sensing surface and photonic band-gap are presented.

Template-controlled mineralization: Determining film granularity and structure by surface functionality patterns

We present a promising first example towards controlling the properties of a self-assembling mineral film by means of the functionality and polarity of a substrate template. In the presented case, a zinc oxide film is deposited by chemical bath deposition on a nearly topography-free template structure composed of a pattern of two self-assembled monolayers with different chemical functionality. We demonstrate the template-modulated morphological properties of the growing film, as the surface functionality dictates the granularity of the growing film. This, in turn, is a key property influencing other film properties such as conductivity, piezoelectric activity and the mechanical properties. A very pronounced contrast is observed between areas with an underlying fluorinated, low energy template surface, showing a much more (almost two orders of magnitude) coarse-grained film with a typical agglomerate size of around 75 nm. In contrast, amino-functionalized surface areas induce the growth of a very smooth, fine-grained surface with a roughness of around 1 nm. The observed influence of the template on the resulting clear contrast in morphology of the growing film could be explained by a contrast in surface adhesion energies and surface diffusion rates of the nanoparticles, which nucleate in solution and subsequently deposit on the functionalized substrate.