Flat Si Research Articles

(Invited) Interactions of Block Co-Polymer Ionomers on Metal Surfaces for Tuned Electrocatalysis

In acidic fuel cells, the ionomer and catalyst layer were developed empirically, and only recently studies have begun investigating the success of the heterogeneous microstructure and triple-phase boundary. Currently, the catalyst layer is not well understood due to its complexity; however, model interfaces between ionomers and catalyst particles have been used to provide valuable insight on ionomer-catalyst interfacial interactions. It is known from studies with Nafion® that restructuring of the polymer morphology can occur when it exists as an ionomer thin film at a catalyst interface. The behavior of the thin film ionomer changes compared to its bulk characteristics, which ultimately affects electrode kinetics by altering ionic and water transport networks. In the field of alkaline fuel cells, there is limited research published on ionomer developments in general or on specific interactions between ionomers and catalysts. The few fuel cell studies that compare the effects of ionomer chemistry have conflicting conclusions, which likely arise because the ionomer chemistries interact with Pt in different ways. Overall, this provides the motivation for our work to study idealized structures in the electrode, specifically to elucidate and modify interactions occurring at the interface between ionomers and non-PGM catalysts. This insight will enable the rational and systematic development of ionomer chemistry with the ultimate goal of improving electrode kinetics for a variety of important electrochemical reactions.We have synthesized a large number of model cationic block co-polymers, diblocks, triblocks and pentablocks (AB, ABA, ABABA). Where the hydrophobic block is either polyisoprene or polycyclooctadiene, or their hydrogenated analogues, polymethylbutylene or polyethylene. The other block is polychloromethylstyrene that can be quaternized with a variety of amines to vary the interaction with the surface, inmost of work we use the relatively simple benzyltrimethylammonium (TMA) cation or the reportedly more stable and more bulky benzylmethylpyrolidinium (MPRD) cation. Our initial studies using commercial Ag nano powders showed that the olefinic groups of the polymers interacted wit the Ag and that the interaction of the cations with the Ag surface could completely remove the crystallinity from the entire sample in the case of the polyethylene block. Probing the effects of these interactions on electrocatalysis of the oxen reduction reaction is on going and we will report our progress in this paper.To further constrain the polymers we have also studied them as thin films on flat Si wafers and the same substrate coated with thin metallic films and studied by GISAXS. When block co-polymers are spin coated on plain silicon wafers we almost always see no scattering as there is no alignment of the polymer on the silicon surface. When a thin layer of silver is coated onto the silicon, discrete alignment of features perpendicular to the surface are observed for many of the polymers we have synthesized on the nanoscale. The size of these features in the thin films is not always correlated with the block sizes observed in the bulk films by SAXS. AFM imaging of the polymer air interface suggests that these vertically aligned features persist throughout the film, a property presumably needed for transport, but that the size can vary. The data shown below is typical of aligned vertical triblock ABA polymers on Ag. We have also investigated these thin films as a function of temperature and hummidity and observed transients of the changes in the morphology of these materials. In this paper we will show how these interactions can be utilized for the beneficial development of electrocatalysts. Figure 1

Improved photovoltaic performance of graphene-based solar cells on textured silicon substrate

Abstract Graphene has attracted much interest as an active layer in heterojunction solar cells due to its outstanding properties such as flexibility, transparency, mechanical strength and elevated carrier mobility. In this research, a new technique was presented in order to enhance the efficiency of graphene–based heterojunction solar cells by employing a textured silicon (Si) substrate. Here, two sets of devices were fabricated based on flat and pyramidal structure of Si and the photovoltaic properties of graphene/Si heterojunction solar cells were compared. Selective chemical dissolution of Si wafers was carried out in order to produce pyramidal skeleton. Reduced graphene oxide (rGO) was then transferred on pyramidal Si through electrophoretic deposition (EPD) technique. The evidence of graphene layers on Si substrates was studied using Raman spectroscopy, X–ray diffractometry (XRD) and atomic force microscopy (AFM) analysis. The morphology of samples indicated an enhancement in rGO/Si interface area when the pyramidal structure is applied. Moreover, the enhanced surface area of this sample which is due to elevated roughness of pyramidal structure and wrinkles of graphene layers promotes its antireflective behavior which was proven using reflectance spectroscopy. The average reflectance of the graphene layer on the textured Si was ∼14% in the wavelength range of 400–800 nm which is lower than that of rGO on flat Si. The improved optical properties of graphene on pyramidal silicon can broaden its potential applications in optoelectrical devices such as high-efficiency solar cells. In order to study the photovoltaic properties of rGO/Si samples, a passive layer was formed on Si substrate and a square frame of Ag was coated on it which was acted as a top contact. The current–voltage characteristics showed that the efficiency of rGO/Si heterojunction solar cells was improved when textured silicon was applied.

Wafer-level fabrication of 3D nanoparticles assembled nanopillars and click chemistry modification for sensitive SERS detection of trace carbonyl compounds

In this work, we develop a new method for fabricating wafer-level gold nanoparticles covered silicon nanopillars (SNPs) combined with surface chemical modification to detect trace level carbonyl compounds based on surface-enhanced Raman scattering (SERS) technique. The SNPs are fabricated with an etching process using nano masks synthesized in oxygen-plasma bombardment of photoresist, and further deposited with gold nanoparticles on the surface, thus forming a 3D ‘particles on pillars’ nanostructure for sensitive SERS detection. The enhancement factor (EF) of the devices for R6G detection can achieve 1.56 × 106 times compared with a flat Si substrate. We also developed an oximation click chemistry reaction procedure by chemically modifying the nanostructures with aminooxy dodecane thiol (ADT) self-assemble modification. The chip is further integrated with a polydimethylsiloxane (PDMS) microfluidic chamber, which allows fast and convenient detection of trace carbonyl compounds in liquid samples. The SERS detection capability was demonstrated by the dropwise addition of fluorescent carbonyl compounds before and after elution. Furthermore, the device was proved with high surface consistency(<70%) for repeated measurement, which has the potential for ppb(parts per billion) level concentration of carbonyl compounds detection.

Magnetic Property of Thin Film of Co-Tb Alloys Deposited on the Barrier Layer of Ordered Anodic Alumina Templates

Recently, search for novel materials for bit patterned media (BPM) has been very intense due to their potential application in ultra-high-density hard disk drive. In a quest of finding new materials for BPM, we have taken up to study ordered array of nanostructured Co-Tb alloy. Thin films of Ta (1 nm)/Co1−xTbx (24 nm)/Ta (4 nm) (x = 0.10, 0.15, 0.18 and 0.33) alloys were deposited on the barrier layer of auto-assembled anodic alumina template using dc magnetron sputtering to form a nanostructured array, the so-called nanobump. Similar array of compounds on a flat Si substrate was also deposited for comparison (reference sample). The measurement of room temperature magnetization (M) versus field (H) curve indicates that the coercivity and the squareness [Mr/Ms] of nanobumps as well as reference samples were found to increase with increase of Tb content. However, the values of HC and Mr/MS for nanobumps were found to be higher than those of reference samples. The measurements of in-plane and out-plane M-H curves reveal that Co1−xTbx (x = 0.15, 0.20 and 0.33) nanobumps exhibit magnetic isotropy behaviour. The measurements of temperature variation of magnetization demonstrate that these films exhibit ferrimagnetism with a compensation temperature much above the room temperature.

Strain-induced phase selection in epitaxial Ge2Sb2Te5 thin films

Controlling strain in hetero-epitaxial material systems can be used to optimize properties and functionalities for specific applications. For this purpose, a lattice mismatch between substrate and epitaxial layer is typically exploited. Beyond that, vicinal substrate surfaces can be used to enhance strain-induced effects. In both cases, a profound understanding of the impact of strain on the growth mode and the resulting structure and morphology of the epitaxial layer is of great importance in order to achieve the desired modifications. In this study, the impact of vicinal substrate surfaces on the phase, structure as well as on surface pattern formation in epitaxial phase-change material thin films is investigated. It is found that in the case of the prototypical phase-change material $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$, the change from nominally flat Si(111) substrates to substrates with a miscut of 6\ifmmode^\circ\else\textdegree\fi{} and terrace widths lying between 2 and 5 nm enhances compressive strain and a transition from 2D island nucleation to step-flow growth. This results in a different $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ phase evolution (change from metastable to stable phase) and surface nanostructuring by regularly arranged terraces that resemble the surface topography of the substrates. This work therefore provides deeper insights into different regimes of growth of epitaxial $\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ thin films and paves the way for strain-induced modification possibilities such as band-gap tuning, optimization of the switching energy or the figure of merit.

Optoelectronic properties of multiple quantum barriers nano-scale avalanche photo diodes

The important optoelectronic properties like spectral response, excess noise characteristics, time and frequency response of multiple quantum barrier (MQB) nano-scale avalanche photodiodes (APDs) based on Si~3C-SiC material system have been studied. A self-consistent simulation method based on quantum drift-diffusion model has been presented. Simulation results show that the Si~3C-SiC MQB nano-APDs are capable of detecting significantly longer wavelengths as compared to infrared flat Si APDs. The multiplication gain and excess noise factor (ENF) of the MQB APDs have been calculated by varying the number of quantum barriers (QBs). The numerically calculated ENF values of MQB APDs have been compared with the ENF of Si flat conventional APDs of similar dimensions and it is observed that the use of QBs leads to significant reduction in ENF of the APDs. Simulation results also show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the Si counterparts.

Development and Optical Properties of ZnO Nanoflowers on Porous Silicon for Photovoltaic Applications

In the present work, a comparison between zinc oxide (ZnO) nanoflowers and nanorods for photovoltaic applications is presented. Using chemical bath deposition technique, ZnO nanoflowers were grown on porous silicon (PS) while ZnO nanorods were deposited on flat Si substrate. The morphology of ZnO/PS sample indicated the formation of nanoflowers by accumulation of ZnO nanorods on PS walls. The structural studies indicated that ZnO nanoflowers experienced a stress relief compared to ZnO nanorods which was due to the role of porous substrate for accommodating the lattice strain in order to obtain the subsequent ZnO nanoflowers with reduced strain. The optical results obtained from ZnO nanoflowers showed more intense photoluminescence and Raman peaks compared with nanorods. It was due to the higher specific surface area of nanoflowers which led to a higher absorption coefficient and increased the generation of electron–hole pairs in this sample. Due to their elevated specific surface area, ZnO nanoflowers can capture the incident light and reduce the reflection coefficient of silicon substrates. Thus, they can be considered as an effective antireflective layer to improve the efficiency of solar cells. The optoelectrical results showed an improvement in the efficiency of fabricated solar cells by use of ZnO nanoflowers on PS structures when compared to the conventional ones possessing ZnO nanorods on flat silicon substrates. Development of ZnO nanoflowers on PS substrates can further extend the applications of ZnO nanostructures in photovoltaic devices.

Multiple Quantum Barrier Nano-avalanche Photodiodes - Part I: Spectral Response

Background: The spectral response of Multiple Quantum Barrier (MQB) nano-scale avalanche photodiodes (APDs) based on Si~3C-SiC material system shows considerable responsivity of the device within a very wide wavelength range which includes some portion of Ultra- Violet (UV) spectrum (200- 90 nm), visible spectrum (390-770 nm), near-infrared (700-1400 nm), short-wavelength infrared (1400-3000 nm) and mid-infrared (3000-4000 nm) wavelengths. It has already been concluded from preceding studies that Si~3C-SiC MQB APDs shows better spectral response and excess noise characteristics as compared to equivalent conventional APDs based on Si. Moreover, the superiority of the illumination through p+-side (ITPS) structure has been observed among two probable optical illumination configurations such as illumination through n+- side (ITNS) and illumination through p+-side (ITPS) structures. Methods: In this paper, the time and frequency responses of Si~3C-SiC MQB APDs have been investigated. A very narrow rectangular pulse of pulse-width of 0.4 ps has been used as the input optical pulse having 850 nm wavelength incident on the p+-side of the MQB APD structures (i.e. ITPS is considered here) and corresponding current responses have been calculated by using a rigorous simulation method developed by the authors; finally the frequency responses of the devices are obtained via the Fourier transform of the corresponding pulse current responses in time domain. Results: The width of the current responses are limited to 4.7 and 3.1 ps in Si nano-APD and Si~3C-SiC MQB (consisting of five quantum barriers) nano-APD respectively for the input optical pulse of width 0.4 ps of 850 nm wavelength. On the other hand, the 3 dB upper cut-off frequencies of the above-mentioned diodes are obtained to be 68.63 and 82.64 GHz respectively. Conclusion: Simulation results show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the flat Si nano-APDs under similar operating conditions.

Multiple Quantum Barrier Nano-avalanche Photodiodes - Part III: Time and Frequency Responses

Background: The time and frequency responses of Multiple Quantum Barrier (MQB) nano-scale Avalanche Photodiodes (APDs) based on Si~3C-SiC material system have been investigated in this final part. Methods: A very narrow rectangular pulse of pulse-width of 0.4 ps has been used as the input optical pulse having 850 nm wavelength incidents on the p+-side of the MQB APD structures and corresponding current responses have been calculated by using a simulation method developed by the authors. Results: Finally the frequency responses of the devices are obtained via the Fourier transform of the corresponding pulse current responses in time domain. Conclusion: Simulation results show that MQB nano-APDs possess significantly faster time response and wider frequency response as compared to the flat Si nano-APDs under similar operating conditions.

A Study of the Wettability of Single-Walled Carbon Nanotube Films on Flat and Textured Si Substrates.

Contact angle measurements are investigated on the surface of single-walled carbon nanotube (SWNT) films directly formed on flat and textured Si substrates using a thermal chemical vapor deposition method. The SWNT films on the textured Si consist of a multiscale structure composed of nanoscale SWNTs and a microscale textured Si. They show superhydrophobic properties in which the water contact angle was around 161°. A direct surface treatment to them increase the contact angle to 174°. The reversible wettability of the SWNT films formed on the textured Si substrates is confirmed through the oxidation process using an acid mixture of nitric and sulfuric acids and a successive reduction procedure via heating treatment in an NH₃ environment.

Improved coverage of rGO film on Si inverted pyramidal microstructures for enhancing the photovoltaic of rGO/Si heterojunction solar cell

Solar cell fabricated by reduced graphene oxide (rGO)/textured silicon (T-Si) structure is feasible due to the capability to form a continuous film of graphene oxide (GO) on Si surface by direct drop-casting or spin coating. In this article, we have fabricated a uniform array of Si inverted pyramidal microstructures by anisotropic etching using a mixture of tetramethyl-ammonium hydroxide and isopropyl alcohol. Low average reflectance of 10.4% is obtained by T-Si structure as compared to that of 53.5% obtained by flat Si (F-Si) structure in UV–visible range. As a result, the front surface area of solar cell has been increased up to 43.2%. The dispersion of GO on Si surface textured with inverted pyramidal microstructures is performed by direct drop-casting of GO droplet or a combination of spin-coating and drop-casting on. The reduction of GO to form rGO is achieved by annealing in N2 at 400 °C. Direct drop-casting of GO droplet on T-Si results to the formation of suspended GO film, covering only the top of Si inverted pyramidal array. Due to the minimum contact area of rGO and T-Si after the reduction, higher diode ideality factor, η = 4.02 is obtained as compared to the reference rGO/F-Si diode with η = 3.99. A combination of spin-coating and drop-casting of GO droplet on T-Si helps in filling the GO sheets into the pyramidal structures, thus creating a large contact area with sidewalls of Si inverted pyramidal structures. As a result, the η value has been significantly reduced to 3.50. The power conversion efficiency also shows an increment due to the improvement of both short circuit current (Jsc) and open circuit voltage (Voc). Jsc and Voc have been increased from 0.010 mA/cm2 up to 0.081 mA/cm2, and from 96 mV up to 304 mV, respectively. Hence, the power conversion efficiency has been improved in several orders due to the improvement in both Jsc and Voc, which is from 2.0 × 10−4% up to 5.2 × 10−3%.

Vacuum deposition of CsPbI3 layers on textured Si for Perovskite/Si tandem solar cells

We attempt the conformal deposition of CsPbI3 layers on pyramidal-shaped textured crystalline Si (c-Si) surfaces, aiming at application to Perovskite/Si tandem solar cells. CsPbI3 layers are deposited through vacuum evaporation on textured c-Si surfaces with various pyramid sizes. Conformal CsPbI3 layers are formed on the textured c-Si, which is, in principle, difficult to be realized by conventional solution processes. We also confirmed a reduction in the optical reflectance of CsPbI3/Si structures by ∼10% (absolute) by using textured c-Si compared to the case of flat Si. A co-evaporation method can prevent the aggregation of CsI particles, which are seen when the films are formed by sequential evaporation. These results indicate the feasibility of Perovskite/c-Si tandem cells with textured c-Si, leading to low optical reflectivity and a high photocurrent.

Microstructure, roughness, and stress properties of silicon coatings for shape correction of Kirkpatrick–Baez mirrors

Coating techniques are investigated for accurate shape correction of Kirkpatrick–Baez (KB) mirrors. Au and Pt have been used as coating materials to obtain elliptical KB mirrors from flat or cylindrical Si substrates. However, due to large differences in the thermal expansion coefficients between Au (or Pt) and Si, thermal stress can be induced not only when coating the substrate, but also during use when focusing synchrotron beams. Si is a promising alternative for profile coating because not only the same material as substrate can induce much smaller thermal stress, but also Si is generally smooth. The microstructure and roughness of 1000- and 2000-nm-thick Si coatings deposited at two different pressures (0.133 and 0.266 Pa) are explored. When the thickness increases from 50 to 2000 nm, the film microstructure and surface morphology change and root mean square roughness increases from 0.164 to 0.232 nm. The increase in Si coating thickness contributes the most to power spectral density curves when the frequency is between 1 and 20 μm − 1. No obvious changes resulting from intrinsic stress are observed among the samples when the Si coating thickness is increased from 50 to 2000 nm.

(S)TEM methods contributions to improve the fabrication of InGaN thin films on Si, and InN nanostructures on flat Si and rough InGaN

The main results of a complete study by Transmission and Scanning-Transmission Electron Microscopies ((S)TEM) are described for (i) InGaN/Si (111) heterostructures in the whole compositional range of the alloys, and (ii) InN quantum dots (InN QDs) directly grown on Si wafers or on relatively rough InGaN/Si (111) templates. The combination of many (S)TEM-based techniques allowed to evaluate different characteristics of the systems under study: (InN QD/) InGaN/Si and InN QD/Si interfaces and crystal qualities, structural and chemical imperfections and other important features. InxGa1-xN thin films are often identified as single-crystalline, very homogeneous in composition, and mostly wurtzite-type, remarkably at any value of x. Also, (S)TEM techniques revealed that the InN nanostructures were hexagonal single crystals, mostly epitaxial to the supporting lattice. The InN crystals also exhibited partially cubic arrangements when allocated onto In-rich InxGa1-xN (i.e. x > 0.7).

Spatially Ordered Poly(3‐hexylthiophene) Fibril Nanostructures via Controlled Evaporative Self‐Assembly

AbstractHierarchically structured, gradient concentric rings of regioregular conjugated polymer, rr‐poly(3‐hexylthiophene) (rr‐P3HT) with randomly or directionally crystallized nanofibrils over large areas are obtained by regulating consecutive “stick‐slip” motions of the three‐phase contact line in a confined geometry consisting of an upper curved sphere surface and a lower flat Si substrate (i.e., sphere‐on‐flat geometry), which leads to an axially symmetric capillary‐bridge (bound solution). Self‐organized concentric P3HT ring patterns result from mediating intermolecular interactions between the polymer molecules, and varying the solvent concentration. More interestingly, from the lower solution concentration “shish‐kebab” like morphologies are observed in between adjacent rings. These unusual, directionally crystallized structures are formed spontaneously parallel to the moving direction of the contact line on solvent evaporation. Subsequently, I–V characteristics of the directionally crystallized and randomly distributed P3HT nanofibrils formed across the electrodes are investigated and compared.

Fluorocarbon-Free Dual-Action Textile Finishes Based on Covalently Attached Thermoresponsive Block Copolymer Brush Coatings.

We present fluorocarbon-free block copolymer brushes as potential systems for dual-action, i.e., soil-repellent and soil-releasing textile finishes. Polymer brushes were prepared by employing specifically engineered triblock copolymers consisting of a hydrophobic, a hydrophilic, and either a central or a terminal anchor block bearing several anchoring groups for sustainable immobilization using the "grafting to" approach on both flat Si wafers and rough cotton fabrics. The switching characteristics of both types of block copolymer brushes were investigated by exposing the brushes to conditions and stimuli that are similar to those applied during laundering in a washing machine and drying in a laundry dryer, respectively. Contact angle measurements were performed to evaluate the polarity and wettability of the block copolymer brushes after treatment in hot water and in air, or in a vacuum at elevated temperatures simulating the washing and the drying procedure of a textile fabric. While the block copolymer brush with the terminal anchor showed only minor changes in terms of the wetting characteristics and the brush morphology upon the applied stimuli, the block copolymer brush with the central anchoring block exhibited a significant change from a hydrophilic (soil-releasing) to a hydrophobic (stain-repellent) surface. This switching behavior was reversible and could be achieved on both, flat Si wafers, and much more pronounced on rough cotton fabrics. Atomic force microscopy and angle-resolved X-ray photoelectron spectroscopy investigations further indicated a complete rearrangement of the brush morphology. Accordingly, we regard this type of block copolymer brushes as a system that fully meets the basic requirements for an application as a dual-action textile finish, which can be reversibly switched with respect to water repulsion.

Photoswitching of fulgimides in different environments on silicon surfaces

Reversible light-modulation of fulgimide based monolayers on Si(1 1 1) and Si(1 0 0) was investigated using ATR-FTIR spectroscopy. Fulgimide monolayers were prepared from neat COOH-terminated SAMs on Si(1 1 1) obtained from methyl undec-10-enoate, (1:1)-diluted COOH-terminated monolayers on Si(1 1 1), and GPTMS monolayers on Si(1 0 0). The epoxy-terminated monolayer on oxidized Si(1 0 0) was characterized with ellipsometry, XPS, as well as contact angle measurements, and ATR-FTIR spectroscopy revealed a strong influence of toluene water content on reproducible high-quality monolayer formation. The results of this study show that environmental polarity has a strong influence on fulgimide imide IR band locations and read-out options for the two photostationary states PSS(365 nm), containing E/Z- and C-isomers, and PSS(545 nm), with solely the E/Z-isomers. Neat COOH-terminated monolayers on flat Si(1 1 1) have the advantage of high functional group concentration, orientation and stability, and an upright arrangement of fulgimide head groups.

Effect of nanostructures on anchoring stem cell-derived neural tissue to artificial surfaces

Objective. Chronic application of brain implants monitoring or modulating neuronal activity are hindered by the foreign body response of the tissue. Topographical modification of implant surfaces may reduce negative tissue response by imitating the structure of the extracellular matrix and therefore affecting the attachment and behavior of neural cells. Approach. In our in vitro study, the effect of nanostructuring was investigated on two commercially used neural implant materials: silicon and platinum. The adhesion, survival and arrangement of neural stem cells (NE4C) and microglial cells (BV2) were investigated and compared to nanostructured and flat Si and Pt surfaces using cell viability studies and fluorescent microscopy image analysis. Main results. Our data indicated that neural cells established strong adhesive couplings with each other, instead of binding to the artificial surfaces. Significance. The phenomena resemble some features of in vivo separation of living tissue from the implanted artificial material, providing an in vitro model for studying immune response.

All-Dielectric Metasurfaces with High-Fluorescence-Enhancing Capability

All-dielectric metasurfaces are an emerging subfield in photonics. Light-wave manipulation has been extensively explored in these metasurfaces. Although light–matter interaction has also been investigated in these metasurfaces, only a limited number of studies have been reported to date. Here, we employ Si-rod-array metasurfaces to examine their fluorescence-enhancing capability. They were designed to have prominent resonances at the working wavelengths of fluorescent molecules. As a result, we experimentally observed significant fluorescence intensity enhancement, exceeding 1000-fold for a reference substrate that was a non-enhancing, flat Si wafer. Thus, we conclude that the all-dielectric metasurfaces can potentially serve as highly fluorescence-enhancing platforms. Their performance is comparable to the best performance reported for metallic metasurfaces. These results strongly suggest that all-dielectric metasurfaces can contribute to fluorescence-sensing of diverse molecules, including biomolecules.

A comparative study of defect formation in GaAs nanocrystals selectively grown on nanopatterned and flat Si(001) substrates

Crystal defects present in GaAs nanocrystals ∼15–50 nm in diameter and grown by metal organic vapor phase epitaxy on top of two different nanopatterned Si(001) substrates (nanopillars and nanotips with ∼40–80 nm openings embedded in a SiO2 matrix) and on a planar substrate, have been investigated by means of atomic-resolution aberration-corrected scanning transmission electron microscopy. Conditions of their formation are discussed. The defect analysis of the three GaAs/Si systems reveals a higher defect density in the GaAs crystals grown on nanopillars as compared to those grown on nanotips and the planar substrate, possibly concomitant to the atomic-scale irregularities identified at the patterned Si(001) nanopillars. It is concluded that the misfit strain in the GaAs nanocrystals is fully plastically relaxed while no noticeable substrate compliance effects are observed on any of the studied substrates.