Nanocavity Formation Research Articles

Nanocavity tuning and formation controlled by the phase change of sub-micron-square GST patterns on Si photonic crystals.

It has been well established that photonic crystal nanocavities with wavelength sized mode volume enable various integrable photonic devices with extremely small consumption energy and small footprint. In this study, we explore the possibility of non-volatile functionalities employing photonic crystal nanocavities and phase change material, Ge2Sb2Te5 (GST). Recently, non-volatile photonic devices based on GST have attracted significant interest and are expected to enable energy-efficient photonic processing, especially for optical computing. However, the device size and the area of GST in previous studies have been rather large. Here, we propose and fabricate Si photonic crystal nanocavities on which submicron-square GST patterns are selectively loaded. Because of the strong light confinement, extremely small area of GST is sufficient to manipulate the cavity mode. We have succeeded to fabricate 30-nm-thick and several-100nm-square GST blocks patterned at the center of photonic crystal cavity with a high alignment accuracy. We confirmed that the resonant wavelength and Q-factor of cavity modes are controlled by the phase change of GST. Moreover, cavity formation controlled by submicron-sized GST is also demonstrated by GST-loaded photonic-crystal line-defect waveguides. Our approach in which we place sub-micron-sized GST inside a photonic crystal nanocavity is promising for realizing extremely energy-efficient non-volatile integrable photonic devices, such as switches, modulators, memories, and reconfigurable novel devices.

Early-stage structure and evolution mechanism of hydrogen supersaturated surface layers on tungsten under low-energy plasma exposure

Trapping at irradiation-induced defects significantly enhances hydrogen-isotope retention in metals with low hydrogen solubility. Here, we present the first microscopic observation of nanocavity formation on tungsten surfaces exposed to low-dose hydrogen plasma at room temperature. The nanocavities exhibit very high aspect ratios between their lateral size L and thickness ω (L/ω: 60-90) and represent the predominant microstructural defects in the early development stage of the hydrogen supersaturated surface layer on hydrogen plasma-irradiated tungsten (see Nucl Fusion57 (2017) 016026, Acta Mater201 (2020) 55). We also quantify the vacancy yield in tungsten under hydrogen irradiation with ion energies below the threshold to create stable Frenkel pairs. Such defect production at sub-threshold energy is conventionally deemed impossible, but we consider that the exposed surface acts as a defect sink with strong absorption bias for mobile self-interstitial atoms from temporarily created Frenkel pairs. This results in a high excess of immobile vacancies in the nanometer-thin surface layer and their agglomeration causes the evolution of large, disc-like nanocavities.

Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures

Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures

Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures.

We propose and numerically demonstrate that a reconfigurable nanocavity can be created in a graphene-loaded Si photonic crystal waveguide. The cavity formation is caused by the local mode-gap modulation induced by electrostatic gate-tuning of graphene. Although most recent graphene photonic devices are based on a change in the imaginary part of the refractive index, here we make use of a change in the real part of the refractive index for gated graphene. We clarify that nanocavities can be formed in two different cases, red-shifted and blue-shifted tunings. These novel formation mechanisms enable us to create and annihilate a nanocavity in a reconfigurable way by varying the gate voltage, which is promising for novel control in photonic processing.

Nanoscale Structuring in Confined Geometries using Atomic Layer Deposition: Conformal Coating and Nanocavity Formation

Abstract Nanoscale structuring in confined geometries using atomic layer deposition (ALD) is demonstrated for surfaces of nanochannels in track-etched polymer membranes and in mesoporous silica (SBA-15). Suitable process conditions for conformal ALD coating of polymer membranes and SBA-15 with inorganic oxides (SiO2, TiO2, Al2O3) were developed. On the basis of the oxide-coated layers, nanochannels were further structured by a molecular-templated ALD approach, where calixarene macromolecules are covalently attached to the surface and then embedded into an Al2O3 layer. The removal of calixarene by ozone treatment results in 1–2 nm wide surface nanocavities. Surfaces exposed to different process steps are analyzed by small angle X-ray scattering (SAXS) as well as by X-ray photoelectron and infrared spectroscopy. The proposed nanostructuring process increases the overall surface area, allows controlling the hydrophilicity of the channel surface, and is of interest for studying water and ion transport in confinement.

Monitoring the Process of Nanocavity Formation on a Monomolecular Level

Abstract Controlling the synthesis of nanostructured surfaces is essential to tailor the properties of functional materials such as catalysts. We report on the synthesis of nanocavities of 1–2 nm dimension on planar Si-wafers by sacrificial nanotemplating and atomic layer deposition (ALD). It is shown that the process of nanocavity formation can be directly monitored on a monomolecular level through imaging with an atomic force microscope (AFM). In particular, by employing the AFM peak force tapping mode the simultaneous mapping of surface topography and tip-surface adhesion forces is accessible, which is useful for the assignment of topographical features and determining the orientation of the template molecules on the wafer surface. Detailed analysis based on the three-dimensional AFM topography allows for a quantification of the template and nanocavity surface coverage. The results are of importance for a detailed understanding of the processes underlying template-based nanocavity formation on oxide surfaces.

Fabrication of Suspended III-V Nanofoils by Inverse Metal-Assisted Chemical Etching of In0.49Ga0.51P/GaAs Heteroepitaxial Films.

Metal-assisted chemical etching (MacEtch) has been established as a low-cost, benchtop, and versatile method for large-scale fabrication of semiconductor nanostructures and has been heralded as an alternative to conventional top-down approaches such as reactive-ion etching. However, extension of this technique to ternary III-V compound semiconductor alloys and heteroepitaxial systems has remained relatively unexplored. Here, Au-assisted and inverse-progression MacEtch (I-MacEtch) of the heteroepitaxial In0.49Ga0.51P/GaAs material system is demonstrated, along with a method for fabricating suspended InGaP nanofoils of tunable thickness in solutions of hydrofluoric acid (HF) and hydrogen peroxide (H2O2). A comparison between Au- and Cr-patterned samples is used to demonstrate the catalytic role of Au in the observed etching behavior. Vertical etch rates for nominally undoped, p-type, and n-type InGaP are determined to be ∼9.7, ∼8.7, and ∼8.8 nm/min, respectively. The evolution of I-MacEtch in the InGaP/GaAs system is tracked, leading to the formation of nanocavities located at the center of off-metal windows. Upon nanocavity formation, additional localized mass-transport pathways to the underlying GaAs substrate permit its rapid dissolution. Differential etch rates between the epilayer and substrate are exploited in the fabrication of InGaP nanofoils that are suspended over micro-trenches formed in the GaAs substrate. A model is provided for the observed I-MacEtch mechanism, based on an overlap of neighboring injected hole distribution profiles. The nanofabrication methodology shown here can be applied to various heteroepitaxial III-V systems and can directly impact the conventional processing of device applications in photonics, optoelectronics, photovoltaics, and nanoelectronics.

Effects of surface oxide layer on nanocavity formation and silver gettering in hydrogen ion implanted silicon

We have made an investigation of the surface oxide effects on nanocavity formation in hydrogen implanted silicon and the influence of resultant nanocavities on diffusion and gettering of implanted silver atoms. A wafer with a 200-nm SiO2 surface layer was implanted with 22.5 keV H ions to a dose of 1 × 1017 cm−2, yielding the concentration peak of implanted H ions at ∼140 nm below the SiO2/Si interface. Subsequently, two sets of Si samples were prepared, depending on whether the oxide layer was etched off before (Group-A) or after (Group-B) post-H-implantation annealing. As evidenced by transmission electron microscopy, Group-A samples exhibited an array of large-sized nanocavities in hexagon-like shape, extending from the surface to the depth ∼140 nm, whereas a narrow band of sphere-shaped nanocavities of small size was present around 140 nm below the surface in Group-B samples. These Si samples with pre-existing nanocavities were further implanted with Ag ions in the surface region (∼40 nm projected range) and post-Ag-implantation annealing was conducted in the temperature range between 600 and 900 °C. Measurements based on Rutherford backscattering spectroscopy revealed much different behaviors for Ag redistribution and defect accumulation in these two sets of samples. Compared to the case for Group-B Si, Group-A Si exhibited a lower concentration of residual defects and a slower kinetics in Ag diffusion as well. We discuss the role of thick surface oxide in point defect generation and recombination, and the consequence on nanocavity formation and defect retention in Si. The properties of nanocavities, e.g., their depth distribution, size, and even shape, are believed to be responsible for the observed disparities between these samples, including an interesting contrast of surface vs. bulk diffusion phenomena for implanted Ag atoms.

Nanocavity formation and hardness increase by dual ion beam irradiation of oxide dispersion strengthened FeCrAl alloy

Open volume defects generated by ion implantation into oxide dispersion strengthened (ODS) alloy and the related hardness were investigated by positron annihilation spectroscopy and nanoindentation measurements, respectively. Synchronized dual beam implantation of Fe and He ions was performed at room temperature and at moderately enhanced temperature of 300°C. For room temperature implantation a significant hardness increase after irradiation is observed which is more distinctive in heat treated than in as-received ODS alloy. There is also a difference between the simultaneous and sequential implantation mode as the hardening effect for the simultaneously implanted ODS alloy is stronger than for sequential implantation.The comparison of hardness profiles and of the corresponding open volume profiles shows a qualitative agreement between the open volume defects generated on the nanoscopic scale and the macroscopic hardness characteristics.Open volume defects are drastically reduced for performing the simultaneous dual beam irradiation at 300°C which is a more realistic temperature under application aspects. Few remaining defects are clusters of 3–4 vacancies in connection with Y oxide nanoparticles. These defects completely disappear in a shallow layer at the surface. The results are in agreement with hardness measurements showing little hardness increase after irradiation at 300°C.Suitable characteristics of ODS alloy for nuclear applications and the close correlation between He-related open volume defects and the hardness characteristics are verified.

Polydentate disulfides for enhanced stability of AuNPs and facile nanocavity formation

Stabilization of gold nanoparticles (AuNPs) on a unique polydentate disulfide ligand, poly(DSDMA) synthesized by selective polymerization of bis(methacryloyl hydroxyethyl)disulfide (DSDMA), results in 2.2 nm monodisperse AuNPs, which exhibit Coulomb blockade at room temperature. A higher graft density (1.8 chains nm−2) of the polydentate ligand results in additional stability in DMF up to one year and competitive exchange against DTT (100 mM) up to 72 h. Crosslinking of the polymer remarkably enhances the thermal stability up to 140 °C for 4 h and also during selective etching by methyl iodide which results in nanocapsules as confirmed by TEM and AFM.

Temperature-induced evolution of subsurface nanocavities in argon-implanted copper

The evolution of argon-filled nanocavities in a copper crystal under annealing is studied experimentally and theoretically. The subsurface argon-filled nanocavities are formed after a short annealing at a temperature \ensuremath{\sim}1000 K by coalescence of subsurface defects initially created by argon implantation. The further prolonged annealing at a temperature above 1075 K leads to decomposition of the nanocavities and diffusion of implanted argon out of the sample. According to a simple analysis, the mechanism of the nanocavity formation is governed not only by the migration of simplest defects, such as vacancies and argon and copper interstitials, but also to a large extent, by diffusion and interaction of the complexes of these simplest defects. The experimental studies with x-ray photoelectron spectroscopy and scanning tunneling microscopy and spectroscopy provide valuable data sets of the density of nanocavities and their size and depth distribution. Based on the experimental results, a theoretical model is developed. The calculation with the model proves that the growth of the nanocavities is mainly determined by the temperature-induced migration of vacancy-argon complexes. By combining the experimental data with the simulation results, the migration energy of these kinds of complexes is estimated \ensuremath{\sim}2.55--2.75 eV. Moreover, the calculation with our model provides the estimate of the dissociation energy of a multiple complex, consisting of two vacancies and two argon atoms, as 1.10--1.18 eV. These parameters, reported in this article, play a key role in the description of the kinetics of the growth and decomposition of nanocavities.

Designer molecular probes for phosphonium ionic liquids

Investigations into the extent of structuring present in phosphonium based ionic liquids (ILs) have been carried out using photochromic molecular probes. Three spiropyran derivatives containing hydroxyl (BSP-1), carboxylic acid (BSP-2) and aliphatic chain (C(14)H(29)) (BSP-3) functional groups have been analysed in a range of phosphonium based ionic liquids and their subsequent physico-chemical interactions were reported. It is believed that the functional groups locate the probe molecules into specific regions based upon the interaction of the functional groups with particular and defined regions of the ionic liquid. This structuring results in thermodynamic, kinetic and solvatochromic parameters that are not predictable from classical solvent models. BSP-1 and BSP-2 exhibit generally negative entropies of activation ranging from -50 J K(-1) mol(-1) to -90 J K(-1) mol(-1) implying relatively low solvent-solute interactions and possible anion interactions with IL polar functional groups. Higher than expected activation energies of 60 kJ mol(-1) to 100 kJ mol(-1) obtained for polar probes maybe be due to IL functional groups competing with the charged sites of the merocyanine (MC) isomer thus reducing MC stabilisation effects. Differences in thermal relaxation rate constants (2.5 x 10(-3) s(-1) in BSP-1 and 3 x 10(-4) s(-1) in BSP-2 in [P(6,6,6,14)][dbsa]) imply that while the polar probe systems are primarily located in polar/charged regions, each probe experiences slightly differing polar domains. BSP-3 entropies of activation are positive and between 30 J K(-1) mol(-1) to 66 J K(-1) mol(-1). The association of the non-polar functional group is believed to locate the spiropyran moiety in the interfacial polar and non-polar regions. The thermal relaxation of the MC form causes solvent reorientation to accommodate the molecule as it reverts to its closed form. Slow thermal relaxation rate constants were obserevd in contrast to high activation energies (5 x 10(-4) s(-1) and 111.91 kJ mol(-1) respectively, for BSP-3 in [P(6,6,6,14)][dbsa]). This may be due to steric effects arising from proposed nano-cavity formation by the alkyl chains in phosphonium based ILs.

The heat of vaporization and nanocavity formation

Two distinct approaches are used to derive a unique and exact analytical expression for the heat of vaporization with respect to the nanocavity energy formation and the molecular hard-core diameter. This expression provides a new method to compare different models of cavity formation in the liquids as well as to predict the effective molecular hard-core diameter at different temperatures and pressures. The effective hard-core diameters and cavity formation energies of liquid Ar, Xe and CH4 are calculated at various temperatures and pressures using different cavity formation models and they are compared. It is found that the effective hard-core diameter generally decreases with increase of temperature as expected and it increases with increase of pressure.

Trapping of Pd, Au, and Cu by implantation-induced nanocavities and dislocations in Si

The gettering of metallic impurities by nanocavities formed in Si is a topic of both scientific importance and technological significance. Metallic precipitates observed in the regions where nanocavities were formed have been considered the result of the metal filling the nanocavities, either as elemental metal or a silicide phase. However, our transmission electron microscopy observations demonstrate that many of these precipitates are concentrated along dislocations, rather than randomly distributed as expected for precipitates formed by the filling of nanocavities. Consequently, the gettering contribution of dislocations in the lattice caused by nanocavity formation must be considered. For Pd, dislocations are the preferred sites for the precipitation of the metal silicide. We compare results of gettering by nanocavities and dislocations for Pd, Au, and Cu to determine which structure is the dominant influence for the formation of precipitates of these metals and/or their silicides.

On the interpretation of some electrical aging and relaxation phenomena in solid dielectrics

Electrical aging and relaxation in solid dielectrics are phenomena studied for decades but still largely poorly understood. The lack of understanding, or sometimes the misunderstanding, are partly due to inadequate or deficient theories and partly due to wrong interpretation of theories or concepts developed for other materials than dielectrics. In addition, a serious analysis of some of the empirical and approximate relationships often used to interpret polymer relaxations reveals that they are not only limited and restrictive but also that they are based on very dubious assumptions. Dry or wet electrical aging is much more complex and often includes many phenomena with synergetic effects such as thermal aging, electrochemical reactions, space charge effects, etc. In this lecture, we intend to show that some of the theories often cited in the literature are wrong or inappropriate. We show why they cannot describe the phenomena at play and we present some alternatives for future work in the domains of relaxation and electrical aging. The relations between aging, space charges and polarization of polyethylene (PE) under the influence of high electrical fields (above 20 kV/mm) are discussed in light of our electrical aging model. It is our contention that strong charge injection occurs only after nanocavity formation, i.e. above the critical field. The amorphous phase is then significantly deformed and weak van der Waals attraction bonds are broken, leading to another faster aging regime. The possible relation between the nanocavity formation at moderate fields and bonds breaking at higher fields proposed in our aging model and various polarization measurements is discussed. One objective of this paper is to encourage the development of more complex and complete theories specific to dielectrics. Some experimental work needed to achieve this goal is pointed out.

Gettering of Pd and Cu by nanocavities and dislocations in Si

Previous workers have assumed that the metallic precipitates observed in the regions where nanocavities are found were the result of the metal filling the nanocavities, either as bulk metal or as a silicide phase. Recent TEM observations have demonstrated that many of these precipitates appear to be concentrated along lattice dislocations, rather than randomly distributed as would be expected for precipitates formed by the filling of nanocavities. Consequently, the gettering contribution of dislocations in the lattice that are caused by nanocavity formation must be considered. In some cases, such as that of Pd, the dislocations are the preferred sites for the bulk precipitation of metals, rather than nanocavities. Our investigations compare the results of gettering by preformed nanocavities and dislocations for Pd and Cu in order to determine which structure is the dominant influence for the formation of bulk precipitates of these metals.

A Non-Covalent Approach for Depositing Spatially Selective Materials on Surfaces

We describe a new method for depositing patterned materials, based on non-covalent trapping of ligands in solvent-templated nanocavities created in aromatic, self-assembled monolayer or polymer films. A model has been developed and tested to describe nanocavity formation and the ligand adsorption process, which occurs via ligand exclusion from ambient, aqueous solution into the hydrophobic nanocavities. Ligand adsorption rates and ligand adsorbate reactivity with solution species are governed by ligand size/geometry design factors identified using the model. Spatial control of adsorption is achieved via film photochemical changes that inhibit subsequent ligand adsorption/accessibility (UV or X-ray) or displacement of entrapped ligands (50 keV electron-beam) during film patterning. The reactivity of the adsorbed ligand is illustrated by the selective binding of PdII species that catalyze electroless metal deposition. Fabrication of high-resolution (≈ 50 nm), positive-tone patterns in nickel with acceptable feature-edge acuity and critical dimension control (≈ 5 %) is demonstrated.

Diffusion-controlled annihilation and aggregation of F-centers in thermochemically reduced MgO crystals

The dynamics of F-center (an oxygen vacancy which has trapped two electrons) aggregation in thermochemically reduced MgO single crystals with an exceptionally high F-center concentration (6×10 18 cm −3) is discussed. A theory of the Mg nanocavity formation process is developed based on diffusion-controlled aggregation of elastically interacting F centers and their annihilation at traps. We show that in contrast to the generally accepted viewpoint, the F centers in the bulk are not annealed out at the external sample surface but at internal defects, such as dislocations, subgrain boundaries and impurities. The mutual attraction of the F centers is a key factor controlling the aggregation process.

Kinetics of nanocavity formation based onF-center aggregation in thermochemically reduced MgO single crystals

The dynamics of interacting F centers resulting in F aggregates and nanocavities is modeled in thermochemically reduced MgO single crystals. We have recently shown that thermal annealing of thermochemically reduced MgO with an exceptionally high F-center concentration $(6\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ induces a brown coloration in the crystals, which was attributed to scattering from nanosize cavities with their walls plated with magnesium metal [Phys. Rev. B 62, 9299 (2000)]. In the present paper, a theory of the nanocavity formation process is developed based on diffusion-controlled aggregation of elastically interacting F centers and their annihilation at traps. We show that in contrast to the generally accepted viewpoint, the F centers in the bulk are not annealed out at the external sample surface but at internal defects, such as dislocations, subgrain boundaries, and impurities. The mutual attraction of the F centers is a key factor controlling the aggregation process.

A TEM study of the effect of oxygen on nanocavity formation and precipitation in rapid - solidified Fe-16Ni-9Cr stainless steels

Vacancies are introduced into the crystal phase during quenching of rapid solidified materials. Cavity formation occurs because of the coalescence of the vacancies into a cluster. However, because of the high mobility of vacancies at high temperature, most of them will diffuse back into the liquid phase, and some will be lost to defects such as dislocations. Oxygen is known to stabilize cavities by decreasing the surface energy through a chemisorption process. These stabilized cavities, furthermore, act as effective nucleation sites for precipitates to form during aging. Four different types of powders with different oxygen contents were prepared by gas atomization processing. The atomized powders were then consolidated by hot extrusion at 900 °C with an extrusion ratio 10,5:1. After consolidation, specimens were heat treated at 1000 °C for 1 hr followed by water quenching. Finally, the specimens were aged at 600 °C for about 800 hrs. TEM samples were prepared from the gripends of tensile specimens of both unaged and aged alloys.