arXiv subjects

G. Palasantzas

Publications and source records attributed to G. Palasantzas.

At least 19 recordsLinked to original sources

arXiv2026

Tuning the memristive response of TaO$_x$-based devices with Ag Nanoparticles

Defect engineering is a key strategy to control resistive switching (RS) in oxide-based memristive devices, where oxygen vacancy (OV) dynamics governs filament formation and rupture. We investigate the effect of Ag nanoparticles (AgNPs) embedded in the top electrode of Pt/Ta2O5/TaO2/Pt memristors and analyze their RS behavior and statistical stability. Devices without AgNPs exhibit two hysteresis switching loops (HSLs) with opposite chiralities, originating from the participation of the Pt/Ta2O5 top interface and the Ta2O5/TaO2 bottom interface. Incorporating AgNPs reduces the overall device resistance and selectively suppresses one loop, yielding a single, well-defined switching mode. Moreover, devices incorporating Ag-NPs show markedly reduced cycle-to-cycle variability of the high-resistance state, as confirmed by Weibull analysis, indicating improved endurance and switching reproducibility. Within a filamentary RS framework, we attribute this behavior to local metallization of the top interface by AgNPs, which partially inhibit OV transport and confines the RS dynamics to the bottom interface. Numerical simulations with the Oxygen Vacancy Resistive Network (OVRN) model succesfully reproduce the experimental HSLs, statistical trends, and tunable ON/OFF ratios with AgNPs coverage. These findings demonstrate that targeted interface metallization via metallic nanoparticles provides an effective route to control multi-interface RS dynamics and improve switching stability in without modifying the oxide architecture.

cond-mat.mes-hall↗

arXiv2026

Inverse design for Casimir-Lifshitz force near heterogeneous gapped metal surface

The Casimir-Lifshitz force is calculated between a heterogeneous gapped metal surface and a silica sphere attached to an AFM cantilever tip. We demonstrate that heterogeneous surface patches with different off-stoichiometry surface properties lead to changes in the predicted distances for a specific force. This can incorrectly be interpreted as occurrences of surface roughness.

cond-mat.mtrl-sci↗

arXiv2020

A way to measure the dispersion forces near the van der Waals-Casimir transition

Forces induced by quantum fluctuations of electromagnetic field control adhesion phenomena between rough solids when the bodies are separated by distances ~10nm. However, this distance range remains largely unexplored experimentally in contrast with the shorter (van der Waals forces) or the longer (Casimir forces) separations. The reason for this is the pull-in instability of the systems with the elastic suspension that poses a formidable limitation. In this paper we propose a genuine experimental configuration that does not suffer from the short distance instability. The method is based on adhered cantilever, whose shape is sensitive to the forces acting near the adhered end. The general principle of the method, its possible realization and feasibility are extensively discussed. The dimensions of the cantilever are determined by the maximum sensitivity to the forces. If the adhesion is defined by strong capillary or chemical interactions, the method loses its sensitivity. Special discussion is presented for the determination of the minimum distance between the rough solids upon contact, and for the compensation of the residual electrostatic contribution. The proposed method can be applied to any kind of solids (metals, semiconductors, dielectrics) and to any intervening medium (gas or liquid).

cond-mat.mes-hall↗

arXiv2019

Comparison of Casimir forces and electrostatics from conductive SiC-Si/C and Ru surfaces

Comprehensive knowledge of Casimir forces and associated electrostatics from conductive SiC and Ru surfaces can be essential in diverse areas ranging from micro/nanodevice operation in harsh environments to multilayer coatings in advanced lithography technologies. Hence, the Casimir force was measured between an Au-coated microsphere and N-doped SiC samples with Si- and C-terminated faces, and the results were compared with the measurements using the same microsphere and a metallic Ruthenium surface. Electrostatic calibration showed that the Si- and C-faces behave differently with a nearly ~0.6-0.7 V difference in the contact potentials V0Si/C. We attribute this to a higher incorporation of N on the C-terminated face in the near surface region resulting in the formation of NOx and an increased work function compared to the Si-terminated surface which is in agreement with x-ray photoelectron spectroscopy data. Notably, the contact potential of the SiC-C face was closer to the metallic Ru-Au system. However, the measured optical properties of the SiC-Si/C terminated surfaces with ellipsometry did not show any substantial differences indicating that the effective depth of the Si/C terminating surface layers are significantly smaller than the photon penetration depth not leading to any differences in the calculated forces via Lifshitz theory. Nonetheless, the measured Casimir forces, after compensation of the electrostatics contributions, showed differences between the Si/C faces, whereas the comparison with the Lifshitz theory prediction shows better agreement for the SiC-Si face. Finally, comparison of the Casimir forces below 40 nm separations between the SiC-Si/C and Ru surfaces indicated that the short-range roughness effects on the Casimir force increase in magnitude with increasing metallic behavior of the plate surface.

physics.app-ph↗

arXiv2018

Dependence of chaotic behavior on optical properties and electrostatic effects in double beam torsional Casimir actuation

We investigate the influence of Casimir and electrostatic torques on double beam torsional microelectromechanical systems with materials covering a broad range of conductivities of more than three orders of magnitude. For the frictionless autonomous systems, bifurcation and phase space analysis shows that there is a significant difference between stable and unstable operating regimes for equal and unequal applied voltages on both sides of the double torsional system giving rise to heteroclinic and homoclinic orbits, respectively. For equal applied voltages, only the position of a symmetric unstable saddle equilibrium point is dependent on the material optical properties and electrostatic effects, while in any other case there are stable and unstable equilibrium points are dependent on both factors. For the periodically driven system, a Melnikov function approach is used to show the presence of chaotic motion rendering predictions of whether stiction or stable actuation will take place over long times impossible. Chaotic behavior introduces significant risk for stiction, and it is more prominent to occur for the more conductive systems that experience stronger Casimir forces and torques. Indeed, when unequal voltages are applied, the sensitive dependence of chaotic motion on electrostatics is more pronounced for the highest conductivity systems.

physics.app-ph↗

arXiv2017

Global consequences of a local Casimir force: Adhered cantilever

Although stiction is a cumbersome problem for microsystems, it stimulates investigations of surface adhesion. In fact, the shape of an adhered cantilever carries information of the adhesion energy that locks one end to the substrate. We demonstrate here that the system is also sensitive to the dispersion forces that are operative very close to the point of contact, but their contribution to the shape is maximum at about one third of the unadhered length. When the force exceeds a critical value the cantilever does not lose stability but it settles at smaller unadhered length, whose relation to adhesion energy is only slightly affected by the force. Our calculations suggest to use adhered cantilevers to measure the dispersion forces at short separations, where other methods suffer from jump-to-contact instability. Simultaneous measurement of the force and adhesion energy allows the separation of the dispersion contribution to the surface adhesion.

cond-mat.mes-hall↗

arXiv2014

Graphene-on-silicon near-field thermophotovoltaic cell

A graphene layer on top of a dielectric can dramatically influence ability of the material to radiative heat transfer. This property of graphene is used to improve the performance and reduce costs of near-field thermophotovoltaic cells. Instead of low bandgap semiconductors it is proposed to use graphene-on-silicon Schottky photovoltaic cells. One layer of graphene absorbs around 90% of incoming radiation and increases the heat transfer. This is due to excitation of plasmons in graphene, which are automatically tuned in resonance with the emitted light in the mid infrared range. The absorbed radiation excites electron-hole pairs in graphene, which are separated by the surface field induced by the Schottky barrier. For a quasi-monochromatic source the generated power is one order of magnitude larger and efficiency is on the same level as for semiconductor photovoltaic cells.

cond-mat.mes-hall↗

arXiv2014

Universal correlators and distributions as experimental signatures of 2+1 Kardar-Parisi-Zhang growth

We examine height-height correlations in the transient growth regime of the 2+1 Kardar-Parisi-Zhang (KPZ) universality class, with a particular focus on the {\it spatial covariance} of the underlying two-point statistics, higher-dimensional analog of the 1+1 KPZ Class Airy$_1$ process. Making comparison to AFM kinetic roughening data in 2d organic thin films, we use our universal 2+1 KPZ spatial covariance to extract key scaling parameters for this experimental system. Additionally, we explore the i) height, ii) local roughness, and iii) extreme value distributions characteristic of these oligomer films, finding compelling agreement in all instances with our numerical integration of the KPZ equation itself. Finally, investigating nonequilibrium relaxation phenomena exhibited by 2+1 KPZ Class models, we have unearthed a universal KPZ ageing kinetics. In experiments with ample data in the time domain, our 2+1 KPZ Euler {\it temporal covariance} will allow a quick, independent estimate of the central KPZ scaling parameter.

cond-mat.stat-mech↗

arXiv2013

Sensitivity of micromechanical actuation on amorphous to crystalline phase transformations under the influence of Casimir forces

Amorphous to crystalline phase transitions in phase change materials (PCM) can have strong influence on the actuation of microelectromechanical systems under the influence of Casimir forces. Indeed, the bifurcation curves of the stationary equilibrium points and the corresponding phase portraits of the actuation dynamics between gold and AIST PCM, where an increase of the Casimir force of up ~25% has been measured upon crystallization, show strong sensitivity to changes of the Casimir force as the stiffness of the actuating component decreases and/or the effective interaction area of the Casimir force increases, which can also lead to stiction. However, introduction of intrinsic energy dissipation (associated with a finite quality factor of the actuating system) can prevent stiction by driving the system to attenuated motion towards stable equilibrium depending on the PCM state and the system quality factor.

cond-mat.mtrl-sci↗

arXiv2011

Characterization of optical properties and surface roughness profiles: The Casimir force between real materials

The Lifshitz theory provides a method to calculate the Casimir force between two flat plates if the frequency dependent dielectric function of the plates is known. In reality any plate is rough and its optical properties are known only to some degree. For high precision experiments the plates must be carefully characterized otherwise the experimental result cannot be compared with the theory or with other experiments. In this chapter we explain why optical properties of interacting materials are important for the Casimir force, how they can be measured, and how one can calculate the force using these properties. The surface roughness can be characterized, for example, with the atomic force microscope images. We introduce the main characteristics of a rough surface that can be extracted from these images, and explain how one can use them to calculate the roughness correction to the force. At small separations this correction becomes large as our experiments show. Finally we discuss the distance upon contact separating two rough surfaces, and explain the importance of this parameter for determination of the absolute separation between bodies.}

cond-mat.other↗

arXiv2010

Switching Casimir forces with Phase Change Materials

We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were achieved upon crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro- or nanoswitches by stimulating the phase change transition via localized heat sources.

cond-mat.mtrl-sci↗

arXiv2010

Repulsive Casimir forces between solid materials with high refractive index intervening liquids

In order to explore repulsive Casimir/van der Waals forces between solid materials with liquid as the intervening medium, we analyze dielectric data for a wide range of materials as for example PTFE, polystyrene, silica and more than twenty liquids. Although significant variation in the dielectric data from different sources exist, we provide a scheme based on measured static dielectric constants, refractive indices, and applying Kramers Kronig (KK) consistency to dielectric data to create accurate dielectric functions at imaginary frequencies. The latter is necessary for more accurate force calculations via the Lifshitz theory allowing reliable predictions of repulsive Casimir forces.

cond-mat.mtrl-sci↗

arXiv2010

Influence of random roughness on cantilever curvature sensitivity

In this work we explore the influence of random surface roughness on the cantilever sensitivity to respond to curvature changes induced by changes in surface stress. The roughness is characterized by the out-of-plane roughness amplitude w, the lateral correlation length x, and the roughness or Hurst exponent H (0<H<1). The cantilever sensitivity is found to decrease with increasing roughness (decreasing H and/or increasing ratio w/x) or equivalently increasing local surface slope. Finally, analytic expressions of the cantilever sensitivity as a function of the parameters w, x, and H are derived in order to allow direct implementation in sensing systems.

cond-mat.mes-hall↗

arXiv2009

The distance upon contact: Determination from roughness profile

The point at which two random rough surfaces make contact takes place at the contact of the highest asperities. The distance upon contact d_0 in the limit of zero load has crucial importance for determination of dispersive forces. Using gold films as an example we demonstrate that for two parallel plates d_0 is a function of the nominal size of the contact area L and give a simple expression for d_0(L) via the surface roughness characteristics. In the case of a sphere of fixed radius R and a plate the scale dependence manifests itself as an additional uncertainty δd(L) in the separation, where the scale L is related with the separation d via the effective area of interaction L^2\simπRd. This uncertainty depends on the roughness of interacting bodies and disappears in the limit L\to \infty.

cond-mat.mes-hall↗

arXiv2009

Influence of water adsorbed on gold on van der Waals/Casimir forces

In this paper we investigate the influence of ultra thin water layer (1-1.5 nm) on the van der Waals/Casimir force between gold surfaces. Adsorbed water is inevitably present on gold surfaces at ambient conditions as jump-up-to contact during adhesion experiments demonstrate. Calculations based on the Lifshitz theory give very good agreement with the experiment in absence of any water layer for surface separations d>10 nm. However, a layer of thickness h<1.5 nm is allowed by the error margin in force measurements. At shorter separations, d<10 nm, the water layer can have a strong influence as calculations show for flat surfaces. Nonetheless, in reality the influence of surface roughness must also be considered, and it can overshadow any water layer influence at separations comparable to the total sphere-plate rms roughness w_{shp}+w.

cond-mat.mes-hall↗

arXiv2009

The influence of dielectric properties on van der Waals/Casimir forces in solid-liquid systems

In this article we present calculations of van der Waals/Casimir forces, described by Lifshitz theory, for the solid-liquid-solid system using measured dielectric functions of all involved materials for the wavelength range from millimeters down to subnanometers. It is shown that even if the dielectric function is known over all relevant frequency ranges, the scatter in the dielectric data, can lead to very large scatter in the calculated van der Waals/Casimir forces. Especially when the liquid dielectric function becomes comparable in magnitude to the dielectric function of one of the interacting solids, the associated variation in the force can be up to a factor of two for plate-plate separations 5-500 nm. This corresponds to an uncertainty up to 100% in the theory prediction for a specific system. As a result accuracy testing of the Lifshitz theory under these circumstances is rather questionable. Finally we discuss predictions of Lifshitz theory regarding multiple repulsive-attractive transitions with separation distance, as well as nontrivial scaling of the van der Waals/Casimir force with distance.

cond-mat.soft↗

arXiv2009

Weak dispersive forces between glass-gold macroscopic surfaces in alcohols

In this work we concentrate on an experimental validation of the Lifshitz theory for van der Waals and Casimir forces in gold-alcohol-glass systems. From this theory weak dispersive forces are predicted when the dielectric properties of the intervening medium become comparable to one of the interacting surfaces. Using inverse colloid probe atomic force microscopy dispersive forces were measured occasionally and under controlled conditions by addition of salt to screen the electrostatic double layer force if present. The dispersive force was found to be attractive, and an order of magnitude weaker than that in air. Although the theoretical description of the forces becomes less precise for these systems even with full knowledge of the dielectric properties, we find still our results in reasonable agreement with Lifshitz theory.

cond-mat.soft↗

arXiv2008

Transition from Casimir to van der Waals force between macroscopic bodies

The transition of van der Waals to Casimir forces between macroscopic gold surfaces is investigated by Atomic Force Microscopy in the plane-sphere geometry. It was found that the transition appears to take place at separations about 10 % the plasma wavelength for evaporated gold surfaces, which compares to theoretical predictions by incorporation of experimental optical data and roughness corrections. Moreover, the force data allow estimation of the Hamaker constant AH in the van der Waals regime, which is in good agreement with the Lifshitz theory predictions (even if roughness corrections are taken into account) and former surface force apparatus measurements.

cond-mat.mtrl-sci↗