Metallic Nanoislands Research Articles

Ferrimagnetic Decorated Ternary Metallic Nanoislands with Hexagonal and Square Structures

We attempt to study the mixed-spin Ising on two decorated ferrimagnetic ternary metallic nanoislands possessing hexagonal and square structures, with a mixture of spin numbers 3/2, 1, and 1/2. Via the finite cluster approximation, the critical and compensation characteristics have been explored by implementing the influences of decorating and interface exchange couplings ([Formula: see text], [Formula: see text]), along with the crystal fields ([Formula: see text], [Formula: see text]) on the phase diagrams and magnetizations. In the absence of crystal fields, the hexagonal structure exhibits only the second-order transition with one or two compensation points; meanwhile, the square structure undergoes the multiple transitions behavior (two or three transition temperatures) with a first-order transition at low temperatures. The inclusion of the crystal fields demonstrates the elevation of the ferrimagnetic region of order. Interestingly, we have uncovered under only the effect of [Formula: see text] acting on spin-1 sublattice the occurrence of the first-order with reentrance indicating two and three transitions for the hexagonal and square structures, respectively. Finally, under a uniform crystal field ([Formula: see text]), the first-order keeps existing only for the square nanoisland.

Improving the switching behavior of TaOx/HfO2-based non-volatile memristors by embedded Ti and Pt nano-islands

To investigate the effects of embedded metal nano-island (NI) arrays on reducing the variations of the device switching parameters, we fabricated Pt/HfO2/TiN, Pt/TaOx/HfO2/TiN devices, as well as devices with embedded NI arrays, including Pt/TaOx/HfO2/Pt NI/TiN and Pt/TaOx/HfO2/Ti NI/TiN devices. Compared to devices without nano-islands, the Pt/TaOx/HfO2/Pt NI/TiN devices with abrupt resistive switching behavior exhibit more stable resistance states (coefficient of variations (CVs) of RHRS and RLRS are 11.9 % and 13.0 %, respectively), and a large on/off ratio (>103), representing a high performance level among the HfO2-based non-volatile memristors reported to date, but the devices require a Forming process. The Pt/TaOx/HfO2/Ti NI/TiN devices with analog resistive switching behavior show low operating voltage (≤1 V), low resistance variations (CVs of RHRS, and RLRS are 15.8 %, and 17.6 %, respectively), but their on/off ratio (<10) is significantly reduced. The results indicate that embedding metal nano-island arrays decreases the fluctuations in device switching parameters, the different properties of nano-island materials significantly affect the abrupt/analog switching behavior, on/off ratio and the Forming of the non-volatile memristors. The methods employed in this study can be extended or evolved for other memristors to enhance their performance and applicability.

Coulomb blockade and Coulomb staircases in CoBi nanoislands on SrTiO3 (001)

We successfully fabricated two-dimensional metallic CoBi nanoislands on SrTiO3 (001) substrate by molecular beam epitaxy, and systematically investigated their electronic structures by scanning tunneling microscopy and spectroscopy in situ at 4.2 K. Coulomb blockade and Coulomb staircases with discrete and well-separated levels are observed for the individual nanoisland, which is attributed to single-electron tunneling via two tunnel junction barriers. They are in excellent agreement with the simulations based on orthodox theory. Furthermore, we demonstrated that the Coulomb blockade becomes weaker with increasing temperature and almost disappears at ∼22 K in our variable temperature experiment, and its full-width at half-maximum of dI/dV peaks with temperature is ∼6 mV. Our results provide a new platform for designing single-electron transistors that have potential applications in future microelectronics.

An approach for patterned molecular adsorption on ferromagnets, achieved via Moiré superstructures.

We have used a scanning tunneling microscope operated under ultrahigh vacuum conditions to investigate an oxo-vanadium-salen complex V(O)salen, that has potential applications as qubits in future quantum-based technologies. The adsorption and self-assembly of V(O)salen on a range of single crystal metal surfaces and nanoislands and the influence of substrate morphology and reactivity has been measured. On the close-packed flat Ag(111) and Cu(111) surfaces, the molecules adsorb isolated or form small clusters arranged randomly on the surface, whereas structured adsorption occurs on two types of Co nanoislands; Co grown on Ag(111) and Ag capped Co islands grown on Cu(111), both forming a Moiré pattern at the surface. The adsorption configuration can by scanning tunneling spectroscopy be linked to the geometric and electronic properties of the substrates and traced back to a Co d-related surface state, illustrating how the modulated reactivity can be used to engineer a pattern of adsorbed molecules on the nanoscale.

Multi-Functional Applications of H-Glass Embedded with Stable Plasmonic Gold Nanoislands.

Metal nanoparticles (MNPs) are synthesized using various techniques on diverse substrates that significantly impact their properties. However, among the substrate materials investigated, the major challenge is the stability of MNPs due to their poor adhesion to the substrate. Herein,it isdemonstrated how a newly developed H-glass can concurrently stabilize plasmonic gold nanoislands (GNIs) and offer multifunctional applications. The GNIs on the H-glassaresynthesized using a simple yet, robust thermal dewetting process. The H-glass embedded with GNIs demonstrates versatility in its applications, such as i) acting as a room temperature chemiresistive gas sensor (70% response for NO2 gas); ii) serving as substrates for surface-enhanced Raman spectroscopy for the identifications of Nile blue (dye) and picric acid (explosive) analytes down to nanomolar concentrations with enhancement factors of 4.8 × 106 and 6.1 × 105 , respectively; and iii) functioning as a nonlinear optical saturable absorber with a saturation intensity of 18.36 × 1015 Wm-2 at 600nm, and the performance characteristics are on par with those of materials reported in the existing literature. This work establishes a facile strategy to develop advanced materials by depositing metal nanoislands on glass for various functional applications.

Metal–Insulator–Metal Nanoislands with Enhanced Local Fields for SERS-Based Detection

Metal–Insulator–Metal Nanoislands with Enhanced Local Fields for SERS-Based Detection

Impact of coherent coupling of plasmonic dipole modes in far-field scattering of templated metallic nanoislands

Impact of coherent coupling of plasmonic dipole modes in far-field scattering of templated metallic nanoislands

Deterministic Conductive Filament Formation and Evolution for Improved Switching Uniformity in Embedded Metal-Oxide-Based Memristors─A Phase-Field Study

The extreme device-to-device variation of switching performance is one of the major obstacles preventing the applications of metal-oxide-based memristors in large-scale memory storage and resistive neural networks. Recent experimental works have reported that embedding metal nano-islands (NIs) in metal oxides can effectively improve the uniformity of the memristors, but the underlying role of the NIs is not fully understood. Here, to address this specific problem, we develop a physical model to understand the origin of the variability and how the embedded NIs can improve the performance and uniformity of memristors. We find that due to the dimension confinement effect, embedding metal NIs can modulate the electric field distribution and lead to a more deterministic formation of the conductive filament (CF) from their vicinity, in contrast to the random growth of CFs without embedded NIs. This deterministic CF formation, via vacancy nucleation, further reduces the forming, reset, and set voltages and enhances the uniformity of the operation voltages and current ON/OFF ratios. We further demonstrate that modifying the shapes of the metal NIs can modulate the field strengths/distributions around the NIs and that choosing the NI metal composition and shape that chemically facilitate vacancy formations can further optimize the CF morphology, reduce the operation voltages, and improve the switching performance. Our work thus provides a fundamental understanding of how embedded metal NIs improve the resistive switching performance in oxide-based memristors and could potentially guide the selection of embedded NIs to realize a more uniform memristor that also operates at a higher efficiency than present materials.

Charge-State-Enhanced Ion Sputtering of Metallic Gold Nanoislands.

Experimental results on the charge-state-dependent sputtering of metallic gold nanoislands are presented. Irradiations with slow highly charged ions of metallic targets werepreviously considered to show no charge state dependent effects on ion-induced material modification, since these materials possess enough free electrons to dissipate the deposited potential energy before electron-phonon coupling can set in. By reducing the size of the target material down to the nanometer regime and thus enabling a geometric energy confinement, a possibility is demonstrated to erode metallic surfaces by charge state related effects in contrast to regular kineticsputtering.

Plasmonic Effect of Ag/Au Composite Structures on the Material Transition

Noble metal nanostructures can produce the surface plasmon resonance under appropriate photoexcitation, which can be used to promote or facilitate chemical reactions, as well as photocatalytic materials, due to their strong plasmon resonance in the visible light region. In the current work, Ag/Au nanoislands (NIs) and Ag NIs/Au film composite systems were designed, and their thermocatalysis performance was investigated using luminescence of Eu3+ as a probe. Compared with Ag NIs, the catalytic efficiency and stability of surface plasmons of Ag/Au NIs and Ag NIs/Au film composite systems were greatly improved. It was found that the metal NIs can also generate strong localized heat at low temperature environment, enabling the transition of NaYF4:Eu3+ to Y2O3: Eu3+, and anti-oxidation was realized by depositing gold on the surface of silver, resulting in the relative stability of the constructed complex.

Raman Scattering Study of Amino Acids Adsorbed on a Silver Nanoisland Film.

We studied the surface-enhanced Raman spectra of amino acids D-alanine and DL-serine and their mixture on silver nanoisland films (SNF) immersed in phosphate-buffered saline (PBS) solution at millimolar amino acid concentrations. It is shown that the spectra from the amino acid solutions differ from the reference spectra for microcrystallites due to the electrostatic orientation of amino acid zwitterions by the metal nanoisland film. Moreover, non-additive peaks are observed in the spectrum of the mixture of amino acids adsorbed on SNF, which means that intermolecular interactions between adsorbed amino acids are very significant. The results indicate the need for a thorough analysis of the Raman spectra from amino acid solutions, particularly, in PBS, in the presence of a nanostructured silver surface, and may also be of interest for studying molecular properties and intermolecular interactions.

Formation of Island SERS Films on Surfaces of Track Membranes and Silicon Substrates

It is shown that metallic nano-island SERS-active layers can be created on the surfaces of porous track membranes made of polyethylene terephthalate and silicon substrates optimized for exciting laser radiation with wavelengths of 532 and 785 nm. Characteristic patterns of changes in the optical properties of SERS structures upon altering the morphology of metal nanoparticles on a substrate’s surface are established. Such structures can be used as SERS chips and highly sensitive filtering SERS elements in optical biosensors.

Tuning the feature size of nanoimprinting stamps: A method to enhance the flexibility of nanoimprint lithography

In the field of nanoimprinting lithography, fabricating large-area imprinting stamps is often the most time- and resource-consuming step. Specifically in research, it is often not reasonable to produce a new imprinting stamp for each new experimental configuration. Therefore, the lack of flexibility in feature sizes makes prototyping and tailoring the feature sizes according to their application challenging. To overcome these restrictions, we developed an imprinting stamp reproduction and tuning method which enables the size of the features of existing imprinting stamps to be tuned within nanometer precision. For replication, we first fabricate a chromium nanoisland array on silicon dioxide using the to-be tuned imprinting stamp. Then, the silicon dioxide is anisotropically etched in a reactive ion etching process with chromium as a hard mask. The formed replica of the imprinting stamp is subsequently tuned in an isotropic etching step with hydrofluoric acid. The method enables us to tune the size of the features of our nanoimprinting stamps within nanometer precision without influencing their shape with a yield above 96%. The tuned stamps are then used to fabricate metal nanoisland arrays with the respective tuned sizes. To evaluate the influence of the feature sizes, we exemplarily study the plasmonic resonance of gold nanoisland arrays fabricated using stamps with different feature diameters. Here, we see a good agreement between measured and simulated plasmonic resonance wavelengths of the samples. Hence, with the tuning method, we can tailor specific size-dependent properties of our nanoisland arrays according to individual experiments and applications.

Label-free detection of single nanoparticles with disordered nanoisland surface plasmon sensor

We report sensing of single nanoparticles using disordered metallic nanoisland substrates supporting surface plasmon polaritons (SPPs). Speckle patterns arising from leakage radiation of elastically scattered SPPs provide a unique fingerprint of the scattering microstructure at the sensor surface. Experimental measurements of the speckle decorrelation are presented and shown to enable detection of sorption of individual gold nanoparticles and polystyrene beads. Our approach is verified through bright-field and fluorescence imaging of particles adhering to the nanoisland substrate.

An Investigation of Surface-Enhanced Raman Scattering of Different Analytes Adsorbed on Gold Nanoislands

In this study, metallic nanoislands were prepared by thermal annealing of gold thin film produced by vacuum evaporation on a glass substrate to investigate the surface-enhanced Raman scattering (SERS) effect on them. The influence of the analyte on the enhancement factor of SERS was studied with riboflavin and rhodamine 6G dye. Two laser excitation sources at 532 and 633 nm wavelengths were used for SERS measurements. We found that the enhancement factors of the gold nanoisland SERS substrates were influenced by the analytes’ adsorption tendency onto their surfaces. The SERS amplification was also found to be dependent on the electronic structure of the molecules; higher enhancement factors were obtained for rhodamine 6G with 532 nm excitation, while for riboflavin the 633 nm source performed better.

Atomic Structures of Single-Layer Nanoislands of Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au Supported on Au(111) from Density Functional Theory Calculations

We have used density functional theory calculations to study the atomic structure of single-layer nanoislands of metal M (M=Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au) supported on M(111) and Au(111) surfaces. Nanoislands of Cu, Pd, Ag, Pt, and Au have planar structures on Au(111), while nanoislands of Ni, Rh, and Ir are nonplanar. The calculations also show that nanoislands of Cu, Pd, Pt, and Au on Au(111) with a diameter below 3 nm can have one of several atomic structures. Two of these structures have atoms at the edges of the nanoislands located near bridge sites on Au(111), and the other structures have atoms at the edges and center of the nanoislands located near bridge sites. The relative stability of these atomic structures depends on the size and nature of the Au-supported nanoparticles. Our findings provided computational support for the work of Liao and Ya [J. Phys. Chem. C. 121 (2017) 19218–19225] reporting the formation of two phases of Pt nanoislands on Au(111). These findings also reveal the rich and complex atomic structures of small single-layer metal nanoislands supported on metal surfaces.

From regular arrays of liquid metal nano-islands to single crystalline biatomic-layer gallium film: Molecular dynamics and first principle study

The two-dimensional (2D) materials provide an excellent platform for the study of the dimensional effect. The richer the types of 2D materials, the broader the unknown field we can explore. However, among the large number of 2D materials manufactured by humans, true single-crystalline (SC) atomically thin 2D metals are rare. The instability of SC 2D metal materials puts high demands on its fabrication process. By implementing molecular dynamics (MD) simulations, we proved that the SC biatomic-layer (BL) gallium film can be formed at the interface between two graphene layers. The Ga atoms deposited on the surface of the graphene on the copper substrate will spontaneously evolve into independent liquid nano-islands, and then cover the nano-island with a monolayer graphene. When the Ga nano-islands confined under the graphene layer are heated to 500 °C, they will expand into a BL Ga film, and finally, the entire system is cooled to room temperature to obtain the SCBL Ga film. It is found that these nano-islands are in the liquid state at ∼400 °C, but they undergo a phase transition and evolve into the solid state at ∼500°C. At the same time, the nano-islands also drop from 3D to 2D. In addition, the vertical heterostructure with moiré superstructure is formed between the SCBL Ga and the top layer graphene. The calculations of the electronic properties show that the Dirac conical point of the graphene in the heterostructure is shifted below the Fermi level, which proves that SCBL Ga is able to induce semimetallic to metallic conversion in graphene, indicating SCBL Ga can be used for metal contacts in 2D devices.

Epidermal Graphene Sensors and Machine Learning for Estimating Swallowed Volume

Assessment of liquid intake is necessary to obtain a complete picture of an individual’s hydration status. Measurements using state-of-the-art wearable devices have been demonstrated, but none of these devices have combined high sensitivity, unobtrusiveness, and automated estimation of volume, i.e., using machine learning. Such a capability would have immense value in a variety of medical contexts, such as monitoring patients with dysphagia or the performance of athletes. Here, an epidermal sensor platform is combined with machine learning to measure swallowed liquid volume based on signals obtained from the surface of the skin. The key component of the device is a composite piezoresistive sensor consisting of single-layer graphene decorated with metallic nanoislands and coated with a highly plasticized form of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesuflonate) (PEDOT:PSS). Surface electromyography (sEMG) signals obtained with conventional electrodes are used in concert with the strain measurements. The use of strain and sEMG measurements together both (1) improve the accuracy of estimated volumes and (2) permit the differentiation of swallowing from motion artifacts. In a cohort consisting of 11 participants, the combined measurements of strain and sEMG—processed by the machine learning algorithm—were able to estimate unknown swallowed volumes cumulatively between 5 and 30 mL of water with greater than 92% accuracy. Ultimately, this system holds promise for numerous applications in sports medicine, rehabilitation, and the detection of nascent dysfunction in swallowing.

Structural changes in metallic nanoislands on 2D materials during in situ annealing in ultra-high vacuum TEM

Structural changes in metallic nanoislands on 2D materials during in situ annealing in ultra-high vacuum TEM

Controlling the Nucleation of Metal Nanoislands on Two-dimensional Materials via Focused Ion Beam Patterning

Controlling the Nucleation of Metal Nanoislands on Two-dimensional Materials via Focused Ion Beam Patterning