Local Thickness Fluctuations Research Articles

A density-functional theory study of the Al/AlOx/Al tunnel junction

The aluminum oxide tunnel junction is a key component of the majority of superconducting quantum devices. For high-quality, reproducible, and scalably manufacturable qubits, the ability to fabricate Josephson junctions (JJs) with a targeted critical current and high uniformity is essential. We use first-principles modeling to assess fundamental aspects of the atomic structure of both amorphous and crystalline aluminum oxide tunnel junctions and relate the structure to predicted performance metrics. We use modified ab initio molecular dynamics to develop realistic models of the tunnel junction, from which interface roughness and local thickness fluctuations are analyzed in an unbiased manner by training a neural network to identify the boundary between metal and oxide. We show that the effective thickness of the insulating part of the junction can be different from the apparent physical thickness. We calculate the rate of Cooper pair tunneling for the atomically resolved electrostatic potential using direct numerical solution in 3D, which shows a channeling effect that impacts the junction critical current. The predicted critical current is a useful JJ design parameter that can be accessed from the ab initio calculations without fitting parameters. To assess the limits of uniformity and fabrication choices (e.g., oxidation vs epitaxy), we compare the amorphous junctions to crystalline models, which show order of magnitude more efficient tunneling compared to the amorphous case, underlining the connection between atomistic structure and Cooper pair tunneling efficiency. Further, this work provides a foundation for ab initio materials design and evaluation to help accelerate future development of improved tunnel junctions.

A New Computational Method for Membrane Compressibility: Bilayer Mechanical Thickness Revisited

Because lipid bilayers can bend and stretch in ways similar to thin elastic sheets, physical models of bilayer deformation have utilized mechanical constants such as the moduli for bending rigidity (Kc) and area compressibility (Ka). However, the use of these models to quantify the energetics of membrane deformation associated with protein-membrane interactions and the membrane response to stress is often hampered by the shortage of experimental data suitable for the estimation of the mechanical constants of various lipid mixtures. While computational tools such as Molecular Dynamics (MD) simulations can provide alternative means to estimate Ka values, current approaches suffer significant technical limitations. Here, we present a novel computational framework that allows for a direct estimation of Ka values for individual bilayer leaflets. The theory is based on the concept of elasticity and derives Ka from real-space analysis of local thickness fluctuations sampled in MD simulations. We explore and validate the model on a large set of single and multicomponent bilayers of different lipid composition and sizes, simulated at different temperatures. The calculated bilayer compressibility moduli agree with values estimated previously from experiments and those obtained from a standard computational method based on a series of constrained tension simulations. We further validate our framework in a comparison with an existing polymer brush model (PBM) and confirm the PBM's predicted linear relationship with proportionality coefficient of 24 using elastic parameters calculated from the simulation trajectories. The robustness of the results that emerge from the new method allows us to revisit the origins of the bilayer mechanical (compressible) thickness and in particular, its dependence on acyl chain unsaturation and the presence of cholesterol.

Towards behavior by design: A case study on corrugated architectures

A design strategy, referred to as behavior by design, was introduced to develop novel architectured materials starting from their expected stress-strain response. Target behaviors in this strategy have unusual shapes that provide new functions to the material. Here, a numerical toolbox was employed to predict the geometry of metal tensile samples with a corrugated gauge section, given the expected characteristics of their stress-strain response. A multiscale approach, based on a finite element model, was used to construct characteristic points and indices on the macroscopic stress-strain curves to select the relevant input geometrical parameters. Additive manufacturing (electron beam melting) was employed to build several predicted geometries in Ti-6Al-4V titanium alloy. Mechanical testing revealed a good agreement between the experimental and predicted behaviors with limited difference in strain (0.8%) and stress (50 MPa). Shape variations such as local thickness fluctuations were identified using X-ray tomography as a source of mismatch between simulations and experiments. The ability to control the whole shape of unusual stress-strain curves is expected to bring new exciting functionalities to architectured materials.

A New Computational Method for Membrane Compressibility: Bilayer Mechanical Thickness Revisited

Because lipid bilayers can bend and stretch in ways similar to thin elastic sheets, physical models of bilayer deformation have utilized mechanical constants such as the moduli for bending rigidity (κC) and area compressibility (KA). However, the use of these models to quantify the energetics of membrane deformation associated with protein-membrane interactions, and the membrane response to stress is often hampered by the shortage of experimental data suitable for the estimation of the mechanical constants of various lipid mixtures. Although computational tools such as molecular dynamics simulations can provide alternative means to estimate KA values, current approaches suffer significant technical limitations. Here, we present a novel, to our knowledge, computational framework that allows for a direct estimation of KA values for individual bilayer leaflets. The theory is based on the concept of elasticity and derives KA from real-space analysis of local thickness fluctuations sampled in molecular dynamics simulations. We explore and validate the model on a large set of single and multicomponent bilayers of different lipid compositions and sizes, simulated at different temperatures. The calculated bilayer compressibility moduli agree with values estimated previously from experiments and those obtained from a standard computational method based on a series of constrained tension simulations. We further validate our framework in a comparison with an existing polymer brush model and confirm the polymer brush model’s predicted linear relationship with proportionality coefficient of 24, using elastic parameters calculated from the simulation trajectories. The robustness of the results that emerge from the method allows us to revisit the origins of the bilayer mechanical (compressible) thickness and in particular its dependence on acyl-chain unsaturation and the presence of cholesterol.

(Invited) Layer-Controlled, Wafer-Scale Fabrication of 2D Semiconductor Materials

The rapid cadence of MOSFET scaling is stimulating the development of new technologies and accelerating the introduction of new semiconducting materials as silicon alternative. In this context, transition metal dichalcogenides (TMDs) with a unique layered structure have attracted tremendous interest in recent years mainly motivated by the characteristic 2D nature together with distinctive and tunable optoelectronic properties which make them appealing for a wide variety of applications. Their ultra-thin body nature, in particular, is expected to provide superior immunity to short channel effects therefore extending the potential to scale transistors down to the few-nanometer-scale. [1-4] Another key feature of 2D materials is the absence of surface dangling bonds. The latter property has the potential to eliminate lattice mismatch constraints thus paving the way for hybrid integration of TMDs into artificial heterostructures with sharp interfaces and designed band alignment. These ingredients, as recently demonstrated, make TMDs ideal building blocks for the fabrication of tunnel field effect transistors with very steep sub-threshold slope which is mandatory for low voltage device operation. [5] Based on these premises, there are some practical issues that need to tackled in order to enable a TMD-based technology. Given their 2D nature, the electronic properties of TMDs critically depend on the physico-chemical characteristics of the interfaces. Therefore, interface/surface engineering represents a logical route to control the electrical performance of TMD-based devices. Surface quality control is mainly a material growth-related issue. From that standpoint, the development of scalable synthesis techniques is obviously a fundamental step towards the development of a manufacturable technology. However, another important step still needs to be achieved: the ability to precisely control the number of layers and surface uniformity at the nano-to micro-length scale over the entire wafer surface to obtain TMD films with atomically flat, self-passivated surfaces. This challenge is further complicated by the fact that powder vaporization techniques and CVD processes used to grow TMD films do not exhibit a self-limiting character. Because of that, local thickness fluctuations and layer discontinuities are commonly observed in synthetic TMD films. In this work, we will discuss fundamental aspects of film formation and provide a pathway towards layer-controlled, wafer-scale synthesis of TMD films. The high temperatures involved in the synthesis process as well as the use of growth templates require the development of a reliable wafer-scale transfer technology. Such a heterogeneous integration, on the one hand, is useful to overcome the incompatibility of the growth process with processing requirements for the BEOL interconnect structures. On the other hand, it reduces the level of control over the interface between the 2D layer and the dielectric substrate. It is not surprising that electronic transport in transferred TMD films is strongly affected by hydrocarbon residues, non-homogeneities (e.g. uncontrolled strain and contaminated regions), dipole formation, and charge transfer effects. Monitoring environmental effects in ultra-thin TMD films and minimizing the impact of layer transfer on the electrostatic potential distribution across the interface is of utmost importance. A wafer-scale transfer process and related experimental challenges and opportunities will be also presented. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nature Nanotechnol., 6, 147 (2011).R. Ganatra and Q. Zhang, ACS Nano, 8, 4074 (2014).A. Nourbakhsh, A. Zubair, R. N. Sajjad, A. Tavakkoli K. G., W. Chen, S. Fang, X. Ling, J. Kong, M. S. Dresselhaus, E. Kaxiras, K. K. Berggren, D. Antoniadis, and T. Palacios, Nano Lett., 16 7798 (2016).A. Nourbakhsh, A. Zubair, S. Joglekar, M. Dresselhaus, and T. Palacios, Nanoscale, 18, 6122 (2017).D. Sarkar, X. Xie, W. Liu, W. Cao, J. Kang, Y. Gong, S. Kraemer, P. M. Ajayan and K. Banerjee, Nature, 526, 91 (2015).

Observation of local thickness fluctuations in surfactant membranes using neutron spin echo

Experimental evidence of local thickness fluctuations of a surfactant membrane, as observed by neutron scattering, is reported. A swollen lamellar structure consisting of nonionic surfactant, water, and oil was investigated by neutron spin echo spectroscopy. Different dynamical processes are recognized at three different length scales. At length scales larger than the membrane thickness, the bending motion is observed, which follows the Zilman and Granek theory [Phys. Rev. Lett. 77, 4788 (1996)]. At the length scale corresponding to the membrane thickness, a clear excess dynamics in addition to the bending motion is observed. This mode is interpreted as the local thickness fluctuations. At even shorter length scales, smaller than the membrane thickness, intramembrane dynamics such as protrusion motions may have been observed.

The Theoretical Challenge Posed by Low‐Voltage Membrane Electroporation, Viewed from the Perspective of Continuum and Molecular‐Level Models

AbstractThe effect of applied voltage (V) or a membrane‐bound charge (q) on local thickness fluctuations (LTFs) in lipid bilayers is examined using the continuum approach. Ponderomotive forces, calculated from the linearized Poisson‐Boltzmann equation, reduce the electroelastic energy of LTFs quadratically with V and q. Combined with a significant increase in the apex field gradient and perturbation of lipid packing at large amplitude LTFs, this creates a driving force for instability resulting in water penetration, formation of pores, and membrane rupture. The estimated critical voltages for membrane electroporation are compared with the classical Crowley prediction and recent molecular dynamics (MD) simulations. Unsolved questions impeding a full understanding of low‐voltage breakdown are critically examined. Possible connections between MD results and the issue of negative capacitance and related instabilities at electrified interfaces are outlined.

Wetting and dynamics of structured liquid films

AbstractThe stability of a sufficiently thin, supported, homopolymer film against the development of local thickness fluctuations which can become amplified, eventually leading to structural destabilization of the film, is typically determined by long and short‐range intermolecular forces. In A‐B diblock copolymers, the connectivity between the blocks, the preferential attraction of one block to an external interface, combined with an incompatibility between the A‐B segments, the situation is very different. Two cases, largely dictated by χN, wher χ is the Flory‐Huggins interaction parameter and N is the degree of polymerization, can arise in thin copolyme films. When χN is large, thin films exhibit comparatively stable topographical structures, where the dimensions of the topographies normal to the substrate reflect a natural length‐scale associated with phase separation in the material. In the other situation, where χN is sufficiently small, the copolymer bulk structure is homogeneous. An ordered structure can be induced into the otherwise compositionally homogeneous structure in the vicinity of a substrate. Here, depending on film thickness, a series of transient and stable topographies can develop. Wetting, early stage structural destabilization dynamics leading to the formation of droplets, and late stage coarsening of the droplets are discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2219–2235, 2003