arXiv subjects

Roman Krahne

Publications and source records attributed to Roman Krahne.

At least 37 records · Page 2Linked to original sources

arXiv2024

Tailored Fabrication of 3D Nanopores with Dielectric Oxides for Multiple Nanoscale Applications

Nanopore sensing is a key technology for single-molecule detection and analysis. Solid-state nanopores have emerged as a versatile platform, since their fabrication allows to engineer their properties by controlling size, shape, and chemical functionalization. However, lithography-based fabrication approaches for non-planar nanopores-on-chip rely on polymers that have limits with respect to hard- and robustness, durability, and refractive index. In this respect, nanopores made of metal oxides with high dielectric constant would be much more favourable and have the potential to extend the suitability of solid-state nanopores towards optoelectronic technologies. Here, we present a versatile method to fabricate three-dimensional nanopores of different dielectric oxides with controlled shapes. Our approach uses photoresist only as a template in the focused-ion-beam lithography to define the nanopore shape, which is subsequently coated with different oxides (SiO2, Al2O3, TiO2 and HfO2) by atomic-layer deposition. Then the photoresist is fully removed by chemo-physical treatment, resulting in nanopores entirely made from dielectric oxides on a thin solid-state membrane. Our methodology allows straightforward fabrication of convex, straight, and concave nanopore shapes that can be employed in various technologies and applications. We explored their performance as ionic nanochannels and investigated the dependence of the ionic current rectification on the nanopore geometry. We found hysteresis in the ionic conductance that enables potential applications of the nanopores in memristors. We also investigated the dielectric oxide nanopores for DNA sensing by measuring both cis-trans and trans-cis translocations and support our data with numerical simulations based on the Poisson-Nernst-Planck model.

physics.app-ph↗

arXiv2024

Porous aluminium decorated with rhodium nanoparticles, preparation and use as platform for UV plasmonics

There is a high current interest for novel plasmonic platforms and materials able to extend their applicability into the ultraviolet (UV) region of the electromagnetic spectrum. In the UV it is possible to explore spectral properties of biomolecules with small cross section in the visible spectral range. However, most used metals in plasmonics have their resonances at wavelengths > 350 nm. Aluminum and rhodium are two interesting candidate materials for UV plasmonics, and in this work we developed a simple and low-cost preparation of functional substrates based on nanoporous aluminum decorated with rhodium nanoparticles. We demonstrate that these functionalized nanoporous metal films can be exploited as plasmonic materials suitable for enhanced UV Raman spectroscopy

physics.app-ph↗

arXiv2024

Ionic Current Rectification in Nanopores: Effects of Nanopore Material, Electrolyte and Surface Treatment

Ionic Current Rectification (ICR) can appear in nanopores, causing a diode-like behavior that originates from different efficiency of ion transport through the pore channel with respect to the applied voltage bias polarity. This effect is particularly interesting for nanopores with a short channel length, that is smaller than 500 nm, because then the dependence of the ion current on the direction along which the ions pass through the channel is determined by the geometry and material of the nanopore, and by the ion concentration in the electrolyte. Surface charges can play an important role, and therefore nanopores consisting of multiple materials can induce ICR because of different charge distributions at the nanopore-electrolyte interface for the different material sections. In this work, we study four solid-state nanopore designs considering different geometries of charge distributions along the inner and outer surfaces of nanopores, and evaluate their impact on ICR with six different electrolytes. The direct comparison between experimental data and modeling enables to understand how the different surface charge configurations impact the ICR.

physics.app-ph↗

arXiv2023

Dry synthesis of bi-layer nanoporous metal films as plasmonic metamaterial

Nanoporous metals are a class of nanostructured materials finding extensive applications in multiple fields thanks to their unique properties attributed to their high surface area and interconnected nanoscale ligaments. They can be pre-pared following different strategies, but the deposition of an arbitrary pure porous metal is still challenging. Recently, a dry synthesis of nanoporous films based on the plasma treat-ment of metal thin layers deposited by physical vapour deposition has been demonstrated, as a general route to form pure nanoporous films from a large set of metals. An interest-ing aspect related to this approach is the possibility to apply the same methodology to deposit the porous films as a multilayer. In this way, it is possible to explore the properties of different porous metals in close contact. As demonstrated in this paper, interesting plasmonic properties emerge in a nanoporous Au-Ag bi-layer. The versatility of the method coupled with the possibility to include many different metals, provides an opportunity to tailor their optical resonances and to exploit the chemical and mechanical properties of compo-nents, which is of great interest to applications ranging from sensing, to photochemistry and photocatalysis.

physics.app-ph↗

arXiv2023

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in the different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured have significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This manuscript reviews the wide range of available nanoporous metals, mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

physics.app-ph↗

arXiv2023

Plasmonic Photochemistry as a Tool to Prepare Metallic Nanopores with Controlled Diameter

We show that plasmonic solid-state nanopores with tunable hole diameter can be prepared via a photocatalytic effect resulting from the enhanced electromagnetic field inside a metallic ring prepared on top of a dielectric nanotube. Under white light illumination, the maximum field intensity in these nanorings induces a site selective metal nucleation and growth. We used this approach to prepare bare Au and bimetallic Au-Ag nanorings and demonstrate the reduction of the initial inner diameter of the nanopore down to 4 nanometers. This process can be applied over large arrays with good reproducibility and good control on the nanopore diameter. The tunability of the nanopore diameter can be used to enable optimized detection of single entities with different size, such as single nanoparticles or biomolecules. As proof-of-concept, we demonstrate the versatility of the platform to perform single object detection of dsDNA, and Au nanoparticles with a diameter of 15 nm and 30 nm. We support our experimental findings with numerical simulations that provide insights into the electromagnetic field intensity distribution, showing that a field intensity enhancement of up to 104 can be achieved inside the nanopores. This strong field confinement inside the final nanopore can be used to perform enhanced optical measurements, and to generate local heating, thereby modifying the ionic conductance of the nanopore

physics.optics↗

arXiv2022

Excitons and their Fine Structure in Lead Halide Perovskite Nanocrystals from Atomistic GW/BSE Calculations

Atomistically detailed computational studies of nanocrystals, such as those derived from the promising lead-halide perovskites, are challenging due to the large number of atoms and lack of symmetries to exploit. Here, focusing on methylammonium lead iodide nanocrystals, we combine a real-space tight binding model with the GW approximation to the self-energy and obtain exciton wavefunctions and absorption spectra via solutions of the associated Bethe-Salpeter equation. We find that the size dependence of carrier confinement, dielectric contrast, electron-hole exchange, and exciton binding energies has a strong impact on the lowest excitation energy, which can be tuned by almost 1 eV over the diameter range of 2-6 nm. Our calculated excitation energies are about 0.2 eV higher than experimentally measured photoluminescence, and they display the same qualitative size dependence. Focusing on the fine structure of the band-edge excitons, we find that the lowest-lying exciton is spectroscopically dark and about 20-30 meV lower in energy than the higher-lying triplet of bright states, whose degeneracy is slightly broken by crystal field effects.

cond-mat.mtrl-sci↗

arXiv2020

Graphene Plasmonic Fractal Metamaterials for Broadband Photodetectors

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of $10^{11}$ Jones and a gain up to $10^{6}$, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of $|E/E_{0}|\approx20$ for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors.

physics.app-ph↗

arXiv2020

Temperature Driven Transformation of CsPbBr$_3$ Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light emitting applications. In addition, they can be used as components to create a variety of materials through physical and chemical transformations. Recent studies focused on nanoplatelets that are able to self-assemble and transform on solid substrates. Yet, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present the transformation of self-assembled stacks of CsPbBr$_3$ nanoplatelets in solution, capturing the different stages of the process by keeping the solutions at room temperature and monitoring the nanocrystal morphology over a period of a few months. Using ex-situ transmission electron microscopy and surface analysis, we demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: i) desorption of the ligands from the particles surfaces, causing the merging of nanoplatelet stacks, which first form nanobelts; ii) merging of neighboring nanobelts that form more extended nanoplates; and iii) attachment of nanobelts and nanoplates, which create objects with an atomic structure that resemble a mosaic made of broken nanotiles. We reveal that the starting nanoplatelets merge seamlessly and defect-free on an atomic scale in small and thin nanobelts. However, aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr. In this case, the atomic columns of neighboring perovskite lattices shift by a half-unit-cell, forming Ruddlesden-Popper planar faults.

physics.app-ph↗

arXiv2020

Giant-Shell CdSe/CdS Nanocrystals: Exciton Coupling to Shell Phonons Investigated by Resonant Raman Spectroscopy

The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton2 phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to get insight into resonance effects of the CdS LO phonon with high energy excitonic transitions. The isotropic spherical giant-shell nanocrystals show consistently stronger exciton-phonon coupling as compared to the anisotropic rod-shaped dot-in-rod (DiR) architecture, and the 2LO/LO intensity ratio decreases for excitation wavelengths approaching the CdS band edge. The strong exciton-phonon coupling in the spherical giant-shell nanocrystals can be related to the delocalization of the electronic wave functions. Furthermore, we observe the radial breathing modes of the GS nanocrystals and their overtones by ultralow frequency Raman spectroscopy with nonresonant excitation, using laser energies well below the band gap of the heteronanocrystals, and highlight the differences between higher order

physics.chem-ph↗

arXiv2020

Enhancing the Performance of CdSe/CdS Dot-in- Rod Light Emitting Diodes via Surface Ligand Modification

The surface ligands on colloidal nanocrystals (NCs) play an important role in the performance of NCs based optoelectronic devices such as photovoltaic cells, photodetectors and light emitting diodes (LEDs). On one hand, the NC emission depends critically on the passivation of the surface to minimize trap states that can provide non-radiative recombination channels. On the other hand, the electrical properties of NC films are dominated by the ligands that constitute the barriers for charge transport from one NC to its neighbor. Therefore, surface modifications via ligandexchange have been employed to improve the conductance of NC films. However, in light emitting devices, such surface modifications are more critical due to their possible detrimental effects on the emission properties. In this work, we study the role of surface ligand modifications on the optical and electrical properties of CdSe/CdS dot-in-rods (DiRs) in films, and investigate their performance in all-solution processed LEDs. The DiR films maintain high PLQY, around 40-50%, and their electroluminescence in the LED preserves the excellent color purity of the PL. In the LEDs, the ligand-exchange boosted the luminance, reaching a four-fold increase from 2200 cd/m2 for native surfactants to 8500 cd/m2 for the exchanged aminoethanethiol ligands. Moreover, the efficiency roll-off, operational stability, and shelf life are significantly improved, and the external quantum efficiency is modestly increased from 5.1 % to 5.4 %. We relate these improvements to the increased conductivity of the emissive layer, and to the better charge balance of the electrically injected carriers. In this respect, we performed ultraviolet photoelectron spectroscopy (UPS) to obtain deeper insight in the band alignment of the LED structure.

physics.chem-ph↗

arXiv2020

Robust and Bright Photoluminescence from Colloidal Nanocrystal / Al$_2$O$_3$ Composite Films fabricated by Atomic Layer Deposition

Colloidal nanocrystals are a promising fluorescent class of materials, whose spontaneous emission features can be tuned over a broad spectral range via their composition, geometry and size. However, towards embedding nanocrystals films in elaborated device geometries, one significant drawback is the sensitivity of their emission properties on further fabrication processes like lithography, metal or oxide deposition, etc. In this work, we demonstrate how bright emitting and robust thin films can be obtained by combining nanocrystal deposition from solutions via spin2 coating with subsequent atomic layer deposition of alumina. For the resulting composite films, the layer thickness can be controlled on the nanoscale, and their refractive index can be finely tuned by the amount of deposited alumina. Ellipsometry is used to measure the real and imaginary part of the dielectric permittivity, which gives direct access to the wavelength dependent refractive index absorbance of the film. Detailed analysis of the photophysics of thin films of core-shell nanocrystal with different shape and different shell thickness allow to correlate the behavior of the photoluminescence and of the decay life time to the changes in the non-radiative rate that are induced by the alumina deposition. We show that the photoemission properties of such composite films are stable in wavelength and intensity over several months, and that the photoluminescence completely recovers from heating processes up to 240$^\circ$C. The latter is particularly interesting, since it demonstrates robustness to the typical heat treatment that is needed in several process steps like resist-based lithography and deposition by thermal or electron beam evaporation of metals or oxides.

physics.chem-ph↗

arXiv2020

Ultrafast all-optical switching enabled by epsilon-near-zero modes in metal-insulator nanocavities

Ultrafast control of light-matter interactions constitutes a crucial feature in view of new technological frontiers of information processing. However, conventional optical elements are either static or feature switching speeds that are extremely low with respect to the timescales at which it is possible to control light. Here, we exploit high-quality-factor engineered epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to realize an all-optical ultrafast modulation of the reflectance of light at a tailored wavelength. Our approach is based on the presence of the two, spectrally separated, ENZ absorption resonances of the cavity. Optical pumping of the system at its high energy ENZ mode leads to a strong red-shift of the low energy mode because of the transient increase of the local dielectric function, which leads to a sub-3-ps control of the reflectance at a specific wavelength with a relative modulation depth approaching 120%.

physics.optics↗

arXiv2020

Solution-processed silver sulphide nanocrystal film for resistive switching memories

Resistive switching memories allow electrical control of the conductivity of a material, by inducing a high resistance (OFF) or a low resistance (ON) state, using electrochemical and ion transport processes. As alternative to high temperature and vacuum-based physical sulphurization methods of silver (Ag), here we propose, as resistive switching medium, a layer built from colloidal Ag$_{2-x}$S nanocrystals -compatible with solution-processed approaches. The effect of the electrode size (from macro- to micro-scale), composition (Ag, Ti and Pt) and geometry on the device performance together with the electrochemical mechanisms involved are evaluated. We achieved an optimized Ag/Ti bowtie proof-of-concept configuration by e-beam lithography, which fulfils the general requirements for ReRAM devices in terms of low power consumption and reliable $I_{ON}/I_{OFF}$ ratio. This configuration demonstrates reproducible switching between ON and OFF states with data endurance of at least 20 cycles; and an $I_{ON}/I_{OFF}$ ratio up to 10$^3$ at low power consumption (0.1V readout), which outperforms previous results in literature for devices with resistive layers fabricated from silver chalcogenide nanoparticles.

physics.app-ph↗

arXiv2020

Angle and Polarization Selective Spontaneous Emission in Dye-doped Metal/Insulator/Metal Nanocavities

Directing and polarizing the emission of a fluorophore is of fundamental importance in the perspective of novel photonic sources based on emerging nano-emitter technologies. These two tasks are usually accomplished by a sophisticated and demanding structuring of the optical environment in which the emitter is immersed, or by non-trivial chemical engineering of its geometry and/or band structure. In this paper, the wavelength and polarization selective spontaneous emission from a dye-embedded in a Metal/Insulator/Metal (d-MIM) nanocavity is demonstrated. A push-pull chromophore with large Stokes shift is embedded in a MIM cavity whose resonances are tuned with the spectral emission band of the chromophore. Angular and polarization resolved spectroscopy experiments reveal that the radiated field is reshaped according to the angular dispersion of the nanocavity, and that its spectrum manifests two bands with different polarization corresponding to the p- and s-polarized resonances of the cavity. The d-MIM cavities are a highly versatile system for polarization and wavelength division multiplexing applications at the nanoscale, as well as for near-field focused emission and nanolenses.

physics.app-ph↗

arXiv2020

A Semi-Classical View on Epsilon-Near-Zero Resonant Tunneling Modes in Metal/Insulator/Metal Nanocavities

Metal/Insulator/Metal nanocavities (MIMs) are highly versatile systems for nanometric light confinement and waveguiding, and their optical properties are mostly interpreted in terms of surface plasmon polaritons. Although classic electromagnetic theory accurately describes their behavior, it often lacks physical insight, letting some fundamental aspects of light interaction with these structures unexplored. In this work, we elaborate a quantum mechanical description of the MIM cavity as a double barrier quantum well. We identify the square of the imaginary part of the refractive index of the metal as the optical potential, and find that MIM cavity resonances are suppressed if the ratio n/\k{appa} exceeds a certain limit, which shows that low n and high \k{appa} are desired for strong and sharp cavity resonances. Interestingly, the spectral regions of cavity mode suppression correspond to the interband transitions of the metals, where the optical processes are intrinsically non-Hermitian. The quantum treatment allows to describe the tunnel effect for photons, and reveals that the MIM cavity resonances can be excited by resonant tunneling via illumination through the metal, without the need of momentum matching techniques such as prisms or grating couplers. By combining this analysis with spectroscopic ellipsometry on experimental MIM structures, and by developing a simple harmonic oscillator model of the MIM for the calculation of its effective permittivity, we show that the cavity eigenmodes coincide with low-loss zeros of the effective permittivity.

physics.app-ph↗

arXiv2020

Planar Aperiodic Arrays as Metasurfaces for Optical Near-Field Patterning

Plasmonic metasurfaces have spawned the field of flat optics using nanostructured planar metallic or dielectric surfaces that can replace bulky optical elements and enhance the capabilities of traditional far-field optics. Furthermore, the potential of flat optics can go far beyond far-field modulation, and can be exploited for functionality in the near-field itself. Here, we design metasurfaces based on aperiodic arrays of plasmonic Au nanostructures for tailoring the optical near-field in the visible and near-infrared spectral range. The basic element of the arrays is a rhomboid that is modulated in size, orientation and position to achieve the desired functionality of the micron-size metasurface structure. Using two-photon-photoluminescence as a tool to probethe near-field profiles in the plane of the metasurfaces, we demonstrate the molding of light into different near-field intensity patterns and active pattern control via the far-field illumination. Finite element method simulations reveal that the near-field modulation occurs via a combination of the plasmonic resonances of the rhomboids and field enhancement in the nanoscale gaps in between the elements. This approach enables optical elements that can switch the near-field distribution across the metasurface via wavelength and polarization of the incident far-field light, and provides pathways for light matter interaction in integrated devices.

physics.app-ph↗

arXiv2020

Hybridization of Epsilon-Near-Zero Modes via Resonant Tunneling in Layered Metal/Insulator Double Nanocavities

The coupling between multiple nanocavities in close vicinity leads to hybridization of their modes. Stacked Metal/Insulator/Metal (MIM) nanocavities constitute a highly versatile and very interesting model system to study and engineer such mode coupling, since they can be realized by lithography-free fabrication methods with fine control on the optical and geometrical parameters. The resonant modes of such MIM cavities are epsilon-near-zero (ENZ) resonances, which are appealing for non-linear photophysics and a variety of applications. Here we study the hybridization of ENZ resonances in MIMIM nanocavities, obtaining a very large mode splitting reaching 0.477 eV, Q factors of the order of 40 in the visible spectral range, and fine control on the resonance wavelength and mode linewidth by tuning the thickness of the dielectric and metallic layers. A semi-classical approach that analyses the MIMIM structure as a double quantum well system allows to derive the exact analytical dispersion relation of the ENZ resonances, achieving perfect agreement with numerical simulations and experiments. Interestingly, the asymmetry of the mode splitting in a symmetric MIMIM cavity is not reflected in the classical model of coupled oscillators, which can be directly related to quantum mechanical tunneling for the coupling of the two cavities. Interpreting the cavity resonances as resonant tunneling modes elucidates that can be excited without momentum matching techniques. The broad tunability of high-quality ENZ resonances together with their strong coupling efficiency makes such MIMIM cavities an ideal platform for exploring light-matter interaction, for example, by integration of quantum emitters in the dielectric layers.

physics.app-ph↗