Structural color filters with compensated angle-dependent shifts

Structural color filters use nano-sized elements to selectively transmit incident light, offering a scalable, economical, and environmentally friendly alternative to traditional pigment- and dye-based color filters. However, their structural nature makes their optical response prone to spectral shifts whenever the angle of incidence varies. We address this issue by introducing a conformal VO2 layer onto bare aluminum structural color filters. The insulator-metal transition of VO2 compensated the spectral shift of the filter's transmission at a 15° tilt with 80% efficiency. Unlike solutions that require adjustment of the filter's geometry, this method is versatile and suitable also for existing structural filters. Our findings also establish tunable materials in general as a possible solution for angle-dependent spectral shifts.

DFT study of water on graphene: Synergistic effect of multilayer p-doping

Recent experiments related to a study concerning the adsorption of water on graphene have demonstrated the p-doping of graphene, although most of the ab initio calculations predict nearly zero doping. To shed more light on this problem, we have carried out van der Waals density functional theory calculations of water on graphene for both individual water molecules and continuous water layers with coverage ranging from one to eight monolayers. Furthermore, we have paid attention to the influence of the water molecule orientation toward graphene on its doping properties. In this article, we present the results of the band structure and the Bader charge analysis, showing the p-doping of graphene can be synergistically enhanced by putting 4-8 layers of an ice-like water structure on graphene having the water molecules oriented with oxygen atoms toward graphene.

Chiral Nanoparticle Chains on Inorganic Nanotube Templates

Fabrication of chiral assemblies of plasmonic nanoparticles is a highly attractive and challenging task, with promising applications in light emission, detection, and sensing. So far, primarily organic chiral templates have been used for chirality inscription. Despite recent progress in using chiral ionic liquids in synthesis, the use of organic templates significantly limits the variety of nanoparticle preparation techniques. Here, we demonstrate the utilization of seemingly achiral inorganic nanotubes as templates for the chiral assembly of nanoparticles. We show that both metallic and dielectric nanoparticles can be attached to scroll-like chiral edges propagating on the surfaces of WS₂ nanotubes. Such assembly can be performed at temperatures as high as 550 °C. This large temperature range significantly widens the portfolio of nanoparticle fabrication techniques, allowing us to demonstrate a variety of chiral nanoparticle assemblies, ranging from metals (Au, Ga), semiconductors (Ge), and compound semiconductors (GaAs) to oxides (WO₃).

Correlative Imaging of Individual CsPbBr3 Nanocrystals: Role of Isolated Grains in Photoluminescence of Perovskite Polycrystalline Thin Films

We report on the optical properties of a CsPbBr₃ polycrystalline thin film on a single grain level. A sample composed of isolated nanocrystals (NCs) mimicking the properties of the polycrystalline thin film grains that can be individually probed by photoluminescence spectroscopy was prepared. These NCs were analyzed using correlative microscopy allowing the examination of structural, chemical, and optical properties from identical sites. Our results show that the stoichiometry of the CsPbBr₃ NCs is uniform and independent of the NCs' morphology. The photoluminescence (PL) peak emission wavelength is slightly dependent on the dimensions of NCs, with a blue shift up to 9 nm for the smallest analyzed NCs. The magnitude of the blueshift is smaller than the emission line width, thus detectable only by high-resolution PL mapping. By comparing the emission energies obtained from the experiment and a rigorous effective mass model, we can fully attribute the observed variations to the size-dependent quantum confinement effect.

Plasmonic Properties of Individual Gallium Nanoparticles

Gallium is a plasmonic material offering ultraviolet to near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In this work, we experimentally demonstrate the link between the shape and size of individual gallium nanoparticles and their optical properties. To this end, we utilize scanning transmission electron microscopy combined with electron energy loss spectroscopy. Lens-shaped gallium nanoparticles with a diameter between 10 and 200 nm were grown directly on a silicon nitride membrane using an effusion cell developed in house that was operated under ultra-high-vacuum conditions. We have experimentally proven that they support localized surface plasmon resonances and their dipole mode can be tuned through their size from the ultraviolet to near-infrared spectral region. The measurements are supported by numerical simulations using realistic particle shapes and sizes. Our results pave the way for future applications of gallium nanoparticles such as hyperspectral absorption of sunlight in energy harvesting or plasmon-enhanced luminescence of ultraviolet emitters.

Real-Time Study of Surface-Guided Nanowire Growth by In Situ Scanning Electron Microscopy

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

Low temperature 2D GaN growth on Si(111) 7 × 7 assisted by hyperthermal nitrogen ions

As the characteristic dimensions of modern top-down devices are getting smaller, such devices reach their operational limits imposed by quantum mechanics. Thus, two-dimensional (2D) structures appear to be one of the best solutions to meet the ultimate challenges of modern optoelectronic and spintronic applications. The representative of III-V semiconductors, gallium nitride (GaN), is a great candidate for UV and high-power applications at a nanoscale level. We propose a new way of fabrication of 2D GaN on the Si(111) 7 × 7 surface using post-nitridation of Ga droplets by hyperthermal (E = 50 eV) nitrogen ions at low substrate temperatures (T < 220 °C). The deposition of Ga droplets and their post-nitridation are carried out using an effusion cell and a special atom/ion beam source developed by our group, respectively. This low-temperature droplet epitaxy (LTDE) approach provides well-defined ultra-high vacuum growth conditions during the whole fabrication process resulting in unique 2D GaN nanostructures. A sharp interface between the GaN nanostructures and the silicon substrate together with a suitable elemental composition of nanostructures was confirmed by TEM. In addition, SEM, X-ray photoelectron spectroscopy (XPS), AFM and Auger microanalysis were successful in enabling a detailed characterization of the fabricated GaN nanostructures.

Approaching the Topological Low-Energy Physics of the F Model in a Two-Dimensional Magnetic Lattice

We demonstrate that the physics of the F model can be approached very closely in a two-dimensional artificial magnetic system. Faraday lines spanning across the lattice and carrying a net polarization, together with chiral Faraday loops characterized by a zero magnetic susceptibility, are imaged in real space using magnetic force microscopy. Our measurements reveal the proliferation of Faraday lines and Faraday loops as the system is brought from low- to high-energy magnetic configurations. They also reveal a link between the Faraday loop density and icelike spin-spin correlations in the magnetic structure factor. Key for this Letter, the density of topological defects remains small, on the order of 1% or less, and negligible compared to the density of Faraday loops. This is made possible by replacing the spin degree of freedom used in conventional lattices of interacting nanomagnets by a micromagnetic knob, which can be finely tuned to adjust the vertex energy directly, rather than modifying the two-body interactions.

Plasmon-induced trap filling at grain boundaries in perovskite solar cells

The deep-level traps induced by charged defects at the grain boundaries (GBs) of polycrystalline organic-inorganic halide perovskite (OIHP) films serve as major recombination centres, which limit the device performance. Herein, we incorporate specially designed poly(3-aminothiophenol)-coated gold (Au@PAT) nanoparticles into the perovskite absorber, in order to examine the influence of plasmonic resonance on carrier dynamics in perovskite solar cells. Local changes in the photophysical properties of the OIHP films reveal that plasmon excitation could fill trap sites at the GB region through photo-brightening, whereas transient absorption spectroscopy and density functional theory calculations correlate this photo-brightening of trap states with plasmon-induced interfacial processes. As a result, the device achieved the best efficiency of 22.0% with robust operational stability. Our work provides unambiguous evidence for plasmon-induced trap occupation in OIHP and reveals that plasmonic nanostructures may be one type of efficient additives to overcome the recombination losses in perovskite solar cells and thin-film solar cells in general.

Single-Shot Three-Dimensional Orientation Imaging of Nanorods Using Spin to Orbital Angular Momentum Conversion

The key information about any nanoscale system relates to the orientations and conformations of its parts. Unfortunately, these details are often hidden below the diffraction limit, and elaborate techniques must be used to optically probe them. Here we present imaging of the 3D rotation motion of metal nanorods, restoring the distinct nanorod orientations in the full extent of azimuthal and polar angles. The nanorods imprint their 3D orientation onto the geometric phase and space-variant polarization of the light they scatter. We manipulate the light angular momentum and generate optical vortices that create self-interference images providing the nanorods' angles via digital processing. After calibration by scanning electron microscopy, we demonstrated time-resolved 3D orientation imaging of sub-100 nm nanorods under Brownian motion (frame rate up to 500 fps). We also succeeded in imaging nanorods as nanoprobes in live-cell imaging and reconstructed their 3D rotational movement during interaction with the cell membrane (100 fps).

Plasmonic Metasurface Resonators to Enhance Terahertz Magnetic Fields for High-Frequency Electron Paramagnetic Resonance

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high-resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back-reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30-fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.

Density functional study of gallium clusters on graphene: electronic doping and diffusion

Motivated by experimental results on transport properties of graphene covered by gallium atoms, the density functional theory study of clustering of gallium atoms on graphene (up to a size of 8 atoms) is presented. The paper explains a rapid initial increase of graphene electron doping by individual Ga atoms with Ga coverage, which is continually reduced to zero, when bigger multiple-atom clusters have been formed. According to density functional theory calculations with and without the van der Waals correction, gallium atoms start to form a three-dimensional cluster from five and three atoms, respectively. The results also explain an easy diffusion of Ga atoms while forming clusters caused by a small diffusion barrier of 0.11 eV. Moreover, the calculations show this barrier can be additionally reduced by the application of an external electric field, which was simulated by the ionization of graphene. This effect offers a unique possibility to control the cluster size in experiments only by applying a gate-voltage to the graphene in a field-effect transistor geometry and thereby without growth temperature assistance.

Structural and optical properties of monocrystalline and polycrystalline gold plasmonic nanorods

The quality of lithographically prepared structures is intimately related to the properties of the metal film from which they are fabricated. Here we compare two kinds of thin gold films on a silicon nitride membrane: a conventional polycrystalline thin film deposited by magnetron sputtering and monocrystalline gold microplates that were chemically synthesised directly on the membrane's surface for the first time. Both pristine metals were used to fabricate plasmonic nanorods using focused ion beam lithography. The structural and optical properties of the nanorods were characterized by analytical transmission electron microscopy including electron energy loss spectroscopy. The dimensions of the nanorods in both substrates reproduced well the designed size of 240×80 nm² with the deviations up to 20 nm in both length and width. The shape reproducibility was considerably improved among monocrystalline nanorods fabricated from the same microplate. Interestingly, monocrystalline nanorods featured inclined boundaries while the boundaries of the polycrystalline nanorods were upright. Q factors and peak loss probabilities of the modes in both structures are within the experimental uncertainty identical. We demonstrate that the optical response of the plasmonic nanorods is not deteriorated when the polycrystalline metal is used instead of the monocrystalline metal.

Mechanism and Suppression of Physisorbed-Water-Caused Hysteresis in Graphene FET Sensors

Hysteresis is a problem in field-effect transistors (FETs) often caused by defects and charge traps inside a gate isolating (e.g., SiO₂) layer. This work shows that graphene-based FETs also exhibit hysteresis due to water physisorbed on top of graphene determined by the relative humidity level, which naturally happens in biosensors and ambient operating sensors. The hysteresis effect is explained by trapping of electrons by physisorbed water, and it is shown that this hysteresis can be suppressed using short pulses of alternating gate voltages.

Collector Droplet Behavior during Formation of Nanowire Junctions

Formation of nanowire networks is an appealing strategy for demonstrating novel phenomena at the nanoscale, e.g., detection of Majorana Fermions, as well as an essential step in realizing complex nanowire-based architectures. However, a detailed description of mechanisms taking place during growth of such complex structures is lacking. Here, the experimental observations of gold-catalyzed germanium nanowire junction formation are explained utilizing phase field modeling corroborated with real-time in situ scanning electron microscopy. When the two nanowires collide head on during the growth, we observe two scenarios. (i) Two catalytic droplets merge into one, and the growth continues as a single nanowire. (ii) The droplets merge and subsequently split again, giving rise to the growth of two daughter nanowires. Both the experiments and modeling indicate the critical importance of the liquid-solid growth interface anisotropy and the growth kinetics in facilitating the structural transition during the nanowire merging process.

Bypassing Dynamical Freezing in Artificial Kagome Ice

Spin liquids are correlated, disordered states of matter that fluctuate even at low temperatures. Experimentally, the extensive degeneracy characterizing their low-energy manifold is expected to be lifted, for example, because of dipolar interactions, leading to an ordered ground state at absolute zero. However, this is not what is usually observed, and many systems, whether they are chemically synthesized or nanofabricated, dynamically freeze before magnetic ordering sets in. In artificial realizations of highly frustrated magnets, ground state configurations, and even low-energy manifolds, thus remain out of reach for practical reasons. Here, we show how dynamical freezing can be bypassed in an artificial kagome ice. We illustrate the efficiency of our method by demonstrating that the a priori dynamically inaccessible ordered ground state and fragmented spin liquid configurations can be obtained reproducibly, imaged in real space at room temperature, and studied conveniently. We then identify the mechanism by which dynamical freezing occurs in the dipolar kagome ice.

Influence of experimental conditions on localized surface plasmon resonances measurement by electron energy loss spectroscopy

Scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) has become a standard technique to map localized surface plasmon resonances with a nanometer spatial and a sufficient energy resolution over the last 15 years. However, no experimental work discussing the influence of experimental conditions during the measurement has been published up to now. We present an experimental study of the influence of the primary beam energy and the collection semi-angle on the plasmon resonances measurement by STEM-EELS. To explore the influence of these two experimental parameters we study a series of gold rods and gold bow-tie and diabolo antennas. We discuss the impact on experimental characteristics which are important for successful detection of the plasmon peak in EELS, namely: the intensity of plasmonic signal, the signal to background ratio, and the signal to zero-loss peak ratio. We found that the primary beam energy should be high enough to suppress the scattering in the sample and at the same time should be low enough to avoid the appearance of relativistic effects. Consequently, the best results are obtained using a medium primary beam energy, in our case 120 keV, and an arbitrary collection semi-angle, as it is not a critical parameter at this primary beam energy. Our instructive overview will help microscopists in the field of plasmonics to arrange their experiments.

In Situ Variable-Temperature Scanning Tunneling Microscopy Studies of Graphene Growth Using Benzene on Pd(111)

Using a combination of in situ ultrahigh-vacuum variable-temperature scanning tunneling microscopy, ex situ Raman spectroscopy, and scanning electron microscopy, we investigated the growth of graphene using benzene on Pd(111) at temperatures up to 1100 K. Benzene adsorbs readily on Pd(111) at room temperature and forms an ordered superstructure upon annealing at 473 K. Exposure to benzene at 673 K enhances Pd step motion and yields primarily amorphous carbon upon cooling to room temperature. Monolayer graphene domains, 10-30 nm in size, appear during annealing this sample at 873 K. Dosing benzene at 1100 K results in graphene domains with varying degrees of crystallinity, while post-deposition annealing at 1100 K for 1200 s yields monolayer graphene domains larger than 150 × 150 nm². Our results, which indicate that graphene growth on Pd(111) using benzene requires deposition/annealing temperatures higher than 673 K, are in striking contrast with the reported growth of graphene using benzene at temperatures as low as 373 K on relatively inert Cu surfaces.

Vacuum Rabi splitting of a dark plasmonic cavity mode revealed by fast electrons

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the periphery of the bowtie gaps, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes.

Dark plasmonic modes fare better in some applications due to longer lifetimes but, being subradiant, are difficult to probe. The authors apply electron energy loss spectroscopy to demonstrate that a dark mode of a plasmonic cavity can couple with a few quantum emitters to exhibit vacuum Rabi splitting.

Limits of Babinet's principle for solid and hollow plasmonic antennas

We present an experimental and theoretical study of Babinet’s principle of complementarity in plasmonics. We have used spatially-resolved electron energy loss spectroscopy and cathodoluminescence to investigate electromagnetic response of elementary plasmonic antenna: gold discs and complementary disc-shaped apertures in a gold layer. We have also calculated their response to the plane wave illumination. While the qualitative validity of Babinet’s principle has been confirmed, quantitative differences have been found related to the energy and quality factor of the resonances and the magnitude of related near fields. In particular, apertures were found to exhibit stronger interaction with the electron beam than solid antennas, which makes them a remarkable alternative of the usual plasmonic-antennas design. We also examine the possibility of magnetic near field imaging based on the Babinet’s principle.

High-Resolution Quantitative Phase Imaging of Plasmonic Metasurfaces with Sensitivity down to a Single Nanoantenna

Optical metasurfaces have emerged as a new generation of building blocks for multifunctional optics. Design and realization of metasurface elements place ever-increasing demands on accurate assessment of phase alterations introduced by complex nanoantenna arrays, a process referred to as quantitative phase imaging. Despite considerable effort, the widefield (nonscanning) phase imaging that would approach resolution limits of optical microscopy and indicate the response of a single nanoantenna still remains a challenge. Here, we report on a new strategy in incoherent holographic imaging of metasurfaces, in which unprecedented spatial resolution and light sensitivity are achieved by taking full advantage of the polarization selective control of light through the geometric (Pancharatnam-Berry) phase. The measurement is carried out in an inherently stable common-path setup composed of a standard optical microscope and an add-on imaging module. Phase information is acquired from the mutual coherence function attainable in records created in broadband spatially incoherent light by the self-interference of scattered and leakage light coming from the metasurface. In calibration measurements, the phase was mapped with the precision and spatial background noise better than 0.01 and 0.05 rad, respectively. The imaging excels at the high spatial resolution that was demonstrated experimentally by the precise amplitude and phase restoration of vortex metalenses and a metasurface grating with 833 lines/mm. Thanks to superior light sensitivity of the method, we demonstrated for the first time to our knowledge the widefield measurement of the phase altered by a single nanoantenna while maintaining the precision well below 0.15 rad.

Near-field digital holography: a tool for plasmon phase imaging

The knowledge of the phase distribution of the near electromagnetic field has become very important for many applications. However, its experimental observation is still technologically a very demanding task. In this work, we propose a novel method for the measurement of the phase distribution of the near electric field based on the principles of phase-shifting digital holography. In contrast to previous methods the holographic interference occurs already in the near field and the phase distribution can be determined purely from the scanning near-field optical microscopy measurements without the need for additional far-field interferometric methods. This opens a way towards on-chip phase imaging. We demonstrate the capabilities of the proposed method by reconstruction of the phase difference between interfering surface plasmon waves and by imaging the phase of a single surface plasmon wave. We also demonstrate a selectivity of the method towards individual components of the field.

Ambipolar remote graphene doping by low-energy electron beam irradiation

We employ low-energy electron beam irradiation to induce both n- and p-doping in a graphene layer. Depending on the applied gate voltage during the irradiation, either n- or p-doping can be achieved, and by setting an appropriate irradiation protocol, any desired doping levels can be achieved.

Comparative study of plasmonic antennas fabricated by electron beam and focused ion beam lithography

We present a comparative study of plasmonic antennas fabricated by electron beam lithography and direct focused ion beam milling. We have investigated optical and structural properties and chemical composition of gold disc-shaped plasmonic antennas on a silicon nitride membrane fabricated by both methods to identify their advantages and disadvantages. Plasmonic antennas were characterized using transmission electron microscopy including electron energy loss spectroscopy and energy dispersive X-ray spectroscopy, and atomic force microscopy. We have found stronger plasmonic response with better field confinement in the antennas fabricated by electron beam lithography, which is attributed to their better structural quality, homogeneous thickness, and only moderate contamination mostly of organic nature. Plasmonic antennas fabricated by focused ion beam lithography feature weaker plasmonic response, lower structural quality with pronounced thickness fluctuations, and strong contamination, both organic and inorganic, including implanted ions from the focused beam. While both techniques are suitable for the fabrication of plasmonic antennas, electron beam lithography shall be prioritized over focused ion beam lithography due to better quality and performance of its products.

Catalyst-substrate interaction and growth delay in vapor-liquid-solid nanowire growth

Understanding of the initial stage of nanowire growth on a bulk substrate is crucial for the rational design of nanowire building blocks in future electronic and optoelectronic devices. Here, we provide in situ scanning electron microscopy and Auger microscopy analysis of the initial stage of Au-catalyzed Ge nanowire growth on different substrates. Real-time microscopy imaging and elementally resolved spectroscopy clearly show that the catalyst dissolves the underlying substrate if held above a certain temperature. If the substrate dissolution is blocked (or in the case of heteroepitaxy) the catalyst needs to be filled with nanowire material from the external supply, which significantly increases the initial growth delay. The experiments presented here reveal the important role of the substrate in metal-catalyzed nanowire growth and pave the way for different growth delay mitigation strategies.

Kelvin Probe Force Microscopy and Calculation of Charge Transport in a Graphene/Silicon Dioxide System at Different Relative Humidity

The article shows how the dynamic mapping of surface potential (SP) measured by Kelvin probe force microscopy (KPFM) in combination with calculation by a diffusion-like equation and the theory based on the Brunauer-Emmett-Teller (BET) model of water condensation and electron hopping can provide the information concerning the resistivity of low conductive surfaces and their water coverage. This is enabled by a study of charge transport between isolated and grounded graphene sheets on a silicon dioxide surface at different relative humidity (RH) with regard to the use of graphene in ambient electronic circuits and especially in sensors. In the experimental part, the chemical vapor-deposited graphene is precisely patterned by the mechanical atomic force microscopy (AFM) lithography and the charge transport is studied through a surface potential evolution measured by KPFM. In the computational part, a quantitative model based on solving the diffusion-like equation for the charge transport is used to fit the experimental data and thus to find the SiO₂ surface resistivity ranging from 10⁷ to 10¹⁰ Ω and exponentially decreasing with the RH increase. Such a behavior is explained using the formation of water layers predicted by the BET adsorption theory and electron-hopping theory that for the SiO₂ surface patterned by AFM predicts a high water coverage even at low RHs.

Electronic transport properties of graphene doped by gallium

In this work we present the effect of low dose gallium (Ga) deposition (<4 ML) performed in UHV (10^-7 Pa) on the electronic doping and charge carrier scattering in graphene grown by chemical vapor deposition. In situ graphene transport measurements performed with a graphene field-effect transistor structure show that at low Ga coverages a graphene layer tends to be strongly n-doped with an efficiency of 0.64 electrons per one Ga atom, while the further deposition and Ga cluster formation results in removing electrons from graphene (less n-doping). The experimental results are supported by the density functional theory calculations and explained as a consequence of distinct interaction between graphene and Ga atoms in case of individual atoms, layers, or clusters.

Boundary element method for 2D materials and thin films

2D materials emerge as a viable platform for the control of light at the nanoscale. In this context the need has arisen for a fast and reliable tool capable of capturing their strictly 2D nature in 3D light scattering simulations. So far, 2D materials and their patterned structures (ribbons, discs, etc.) have been mostly treated as very thin films of subnanometer thickness with an effective dielectric function derived from their 2D optical conductivity. In this study an extension to the existing framework of the boundary element method (BEM) with 2D materials treated as a conductive interface between two media is presented. The testing of our enhanced method on problems with known analytical solutions reveals that for certain types of tasks the new modification is faster than the original BEM algorithm. Furthermore, the representation of 2D materials as an interface allows us to simulate problems in which their optical properties depend on spatial coordinates. Such spatial dependence can occur naturally or can be tailored artificially to attain new functional properties.

Imaging of near-field interference patterns by aperture-type SNOM - influence of illumination wavelength and polarization state

Scanning near-field optical microscopy (SNOM) in combination with interference structures is a powerful tool for imaging and analysis of surface plasmon polaritons (SPPs). However, the correct interpretation of SNOM images requires profound understanding of principles behind their formation. To study fundamental principles of SNOM imaging in detail, we performed spectroscopic measurements by an aperture-type SNOM setup equipped with a supercontinuum laser and a polarizer, which gave us all the degrees of freedom necessary for our investigation. The series of wavelength- and polarization-resolved measurements, together with results of numerical simulations, then allowed us to identify the role of individual near-field components in formation of SNOM images, and to show that the out-of-plane component generally dominates within a broad range of parameters explored in our study. Our results challenge the widespread notion that this component does not couple to the aperture-type SNOM probe and indicate that the issue of SNOM probe sensitivity towards the in-plane and out-of-plane near-field components - one of the most challenging tasks of near field interference SNOM measurements - is not yet fully resolved.

Nanometer-Sized Water Bridge and Pull-Off Force in AFM at Different Relative Humidities: Reproducibility Measurement and Model Based on Surface Tension Change

This article deals with the analysis of the relationship between the pull-off force measured by atomic force microscopy and the dimensions of water bridge condensed between a hydrophilic silicon oxide tip and a silicon oxide surface under ambient conditions. Our experiments have shown that the pull-off force increases linearly with the radius of the tip and nonmonotonically with the relative humidity (RH). The latter dependence generally consists of an initial constant part changing to a convex-concave-like increase of the pull-off force and finally followed by a concave-like decrease of this force. The reproducibility tests have demonstrated that the precision limits have to be taken into account for comparing these measurements carried out under atmospheric conditions. The results were fitted by a classical thermodynamic model based on water-bridge envelope calculations using the numerical solution of the Kelvin equation in the form of axisymmetric differential equations and consequent calculation of adhesive forces. To describe the measured data more precisely, a decrease of the water surface tension for low RH was incorporated into the calculation. Such a decrease can be expected as a consequence of the high surface curvature in the nanometer-sized water bridge between the tip and the surface.

Electron beam directed etching of hexagonal boron nitride

Hexagonal boron nitride (hBN) is a wide bandgap van der Waals material with unique optical properties that make it attractive for two dimensional (2D) photonic and optoelectronic devices. However, broad deployment and exploitation of hBN is limited by alack of suitable material and device processing and nano prototyping techniques. Here we present a high resolution, single step electron beam technique for chemical dry etching of hBN. Etching is achieved using H2O as a precursor gas, at both room temperature and elevated hBN temperatures. The technique enables damage-free, nano scale, iterative patterning of supported and suspended 2D hBN, thus opening the door to facile fabrication of hBN-based 2D heterostructures and devices.

The Synergic Effect of Atomic Hydrogen Adsorption and Catalyst Spreading on Ge Nanowire Growth Orientation and Kinking

Hydride precursors are commonly used for semiconductor nanowire growth from the vapor phase and hydrogen is quite often used as a carrier gas. Here, we used in situ scanning electron microscopy and spatially resolved Auger spectroscopy to reveal the essential role of atomic hydrogen in determining the growth direction of Ge nanowires with an Au catalyst. With hydrogen passivating nanowire sidewalls the formation of inclined facets is suppressed, which stabilizes the growth in the ⟨111⟩ direction. By contrast, without hydrogen gold diffuses out of the catalyst and decorates the nanowire sidewalls, which strongly affects the surface free energy of the system and results in the ⟨110⟩ oriented growth. The experiments with intentional nanowire kinking reveal the existence of an energetic barrier, which originates from the kinetic force needed to drive the droplet out of its optimum configuration on top of a nanowire. Our results stress the role of the catalyst material and surface chemistry in determining the nanowire growth direction and provide additional insights into a kinking mechanism, thus allowing to inhibit or to intentionally initiate spontaneous kinking.

High-resolution fully vectorial scanning Kerr magnetometer

We report on the development of a high-resolution scanning magnetometer, which fully exploits the vectorial nature of the magneto-optical Kerr effect. The three-dimensional nature of magnetization is at the basis of many micromagnetic phenomena and from these data, we can fully characterize magnetization processes of nanostructures in static and dynamic regimes. Our scanning Kerr magnetometer uses a high numerical aperture microscope objective where the incident light beam can be deterministically deviated from the objective symmetry axis, therefore, both in-plane (via the longitudinal Kerr effect) and out-of-plane (via the polar Kerr effect) components of the magnetization vector may be detected. These components are then separated by exploiting the symmetries of the polar and longitudinal Kerr effects. From four consecutive measurements, we are able to directly obtain the three orthogonal components of the magnetization vector with a resolution of 600 nm. Performance of the apparatus is demonstrated by a measurement of 3D magnetization vector maps showing out-of-plane domains and in-plane domain walls in an yttrium-iron-garnet film and on a study of magnetization reversal in a 4-μm-wide magnetic disk.

Silicon oxide nanowire growth mechanisms revealed by real-time electron microscopy

Growth of one-dimensional materials is possible through numerous mechanisms that affect the nanowire structure and morphology. Here, we explain why a wide range of morphologies is observed when silicon oxide nanowires are grown on silicon substrates using liquid gallium catalyst droplets. We show that a gallium oxide overlayer is needed for nanowire nucleation at typical growth temperatures, and that it can decompose during growth and, hence, dramatically alter the nanowire morphology. Gallium oxide decomposition is attributed to etching caused by hydrogen that can be supplied by thermal dissociation of H2O (a common impurity). We show that H2O dissociation is catalyzed by silicon substrates at temperatures as low as 320 °C, identify the material supply pathways and processes that rate-limit nanowire growth under dry and wet atmospheres, and present a detailed growth model that explains contradictory results reported in prior studies. We also show that under wet atmospheres the Ga droplets can be mobile and promote nanowire growth as they traverse the silicon substrate.

Decolorization of organic dyes by gold nanoflowers prepared on reduced graphene oxide by tea polyphenols

The green approaches for chemical syntheses are becoming important in various fields comprising chemical synthesis.

Spatially resolved electron energy loss spectroscopy of crescent-shaped plasmonic antennas

We present a study of the optical properties of gold crescent-shaped antennas by means of electron energy loss spectroscopy. These structures exhibit particularly large field enhancement near their sharp features, support two non-degenerate dipolar (i.e., optically active) localised surface plasmon resonances, and are widely tunable by a choice of their shape and dimensions. Depending on the volume and shape, we resolved up to four plasmon resonances in metallic structures under study in the energy range of 0.8 - 2.4 eV: two dipolar and quadrupolar mode and a multimodal assembly. The boundary-element-method calculations reproduced the observed spectra and helped to identify the character of the resonances. The two lowest modes are of particular importance owing to their dipolar nature. Remarkably, they are both concentrated near the tips of the crescent, spectrally well resolved and their energies can be tuned between 0.8 - 1.5 eV and 1.2 - 2.0 eV, respectively. As the lower spectral range covers the telecommunication wavelengths 1.30 and 1.55 μm, we envisage the possible use of such nanostructures in infrared communication technology.

Real-time observation of self-limiting SiO2/Si decomposition catalysed by gold silicide droplets

Gold enhances the decomposition rate of thin SiO₂ layers on Si(001) in a self-limiting fashion.

Optimization of ion-atomic beam source for deposition of GaN ultrathin films

We describe the optimization and application of an ion-atomic beam source for ion-beam-assisted deposition of ultrathin films in ultrahigh vacuum. The device combines an effusion cell and electron-impact ion beam source to produce ultra-low energy (20-200 eV) ion beams and thermal atomic beams simultaneously. The source was equipped with a focusing system of electrostatic electrodes increasing the maximum nitrogen ion current density in the beam of a diameter of ≈15 mm by one order of magnitude (j ≈ 1000 nA/cm(2)). Hence, a successful growth of GaN ultrathin films on Si(111) 7 × 7 substrate surfaces at reasonable times and temperatures significantly lower (RT, 300 °C) than in conventional metalorganic chemical vapor deposition technologies (≈1000 °C) was achieved. The chemical composition of these films was characterized in situ by X-ray Photoelectron Spectroscopy and morphology ex situ using Scanning Electron Microscopy. It has been shown that the morphology of GaN layers strongly depends on the relative Ga-N bond concentration in the layers.

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

Real-time observation of collector droplet oscillations during growth of straight nanowires

A liquid droplet sitting on top of a pillar is crucially important for semiconductor nanowire growth via a vapor-liquid-solid (VLS) mechanism. For the growth of long and straight nanowires, it has been assumed so far that the droplet is pinned to the nanowire top and any instability in the droplet position leads to nanowire kinking. Here, using real-time in situ scanning electron microscopy during germanium nanowire growth, we show that the increase or decrease in the droplet wetting angle and subsequent droplet unpinning from the growth interface may also result in the growth of straight nanowires. Because our argumentation is based on terms and parameters common for VLS-grown nanowires, such as the geometry of the droplet and the growth interface, these conclusions are likely to be relevant to other nanowire systems.

Control and near-field detection of surface plasmon interference patterns

The tailoring of electromagnetic near-field properties is the central task in the field of nanophotonics. In addition to 2D optics for optical nanocircuits, confined and enhanced electric fields are utilized in detection and sensing, photovoltaics, spatially localized spectroscopy (nanoimaging), as well as in nanolithography and nanomanipulation. For practical purposes, it is necessary to develop easy-to-use methods for controlling the electromagnetic near-field distribution. By imaging optical near-fields using a scanning near-field optical microscope, we demonstrate that surface plasmon polaritons propagating from slits along the metal-dielectric interface form tunable interference patterns. We present a simple way how to control the resulting interference patterns both by variation of the angle between two slits and, for a fixed slit geometry, by a proper combination of laser beam polarization and inhomogeneous far-field illumination of the structure. Thus the modulation period of interference patterns has become adjustable and new variable patterns consisting of stripelike and dotlike motifs have been achieved, respectively.

Guided assembly of gold colloidal nanoparticles on silicon substrates prepatterned by charged particle beams

Colloidal gold nanoparticles represent technological building blocks which are easy to fabricate while keeping full control of their shape and dimensions. Here, we report on a simple two-step maskless process to assemble gold nanoparticles from a water colloidal solution at specific sites of a silicon surface. First, the silicon substrate covered by native oxide is exposed to a charged particle beam (ions or electrons) and then immersed in a HF-modified solution of colloidal nanoparticles. The irradiation of the native oxide layer by a low-fluence charged particle beam causes changes in the type of surface-terminating groups, while the large fluences induce even more profound modification of surface composition. Hence, by a proper selection of the initial substrate termination, solution pH, and beam fluence, either positive or negative deposition of the colloidal nanoparticles can be achieved.

Response of plasmonic resonant nanorods: an analytical approach to optical antennas

An analytical model of the response of a free-electron gas within the nanorod to the incident electromagnetic wave is developed to investigate the optical antenna problem. Examining longitudinal oscillations of the free-electron gas along the antenna nanorod a simple formula for antenna resonance wavelengths proving a linear scaling is derived. Then the nanorod polarizability and scattered fields are evaluated. Particularly, the near-field amplitudes are expressed in a closed analytical form and the shift between near-field and far-field intensity peaks is deduced.