Session G62: Nanostructures and Metamaterials V

Soft porous polymer materials, a new range of acoustic metasurfaces

In 2011 a new range of 2D materials, widely known as metasurfaces, has emerged. These planar metamaterials of subwavelength thicknesses, capable of wave front shaping, consist of a single or several layers of artificial structure¹.

In general, metasurfaces are engineered by assembling arrays of deep sub wavelength building blocks. Here, we present a bottom-up approach which is based on soft-porous material: the dependence of the refractive index of soft porous polymer materials with their porosity enable us to fabricate acoustic gradient-index (GRIN) metasurfaces¹. Yet, their fabrication requires photo-polymerization of the thin film, which is made difficult by the presence of porogenic agents that scatter light. We will present a rational approach to understand the coupling between photo-chemistry and light scattering (in scattering media), and we will illustrate how the use of grayscale lithography facilitates the generation of this high precision microstructures.

(1) Jin, Y.; Kumar, R.; Poncelet, O.; Mondain-Monval, O.; Brunet, T. Flat Acoustics with Soft Gradient-Index Metasurfaces. Nat. Commun. 2019, 10 (1).   

Self-Assembly of Poly(ethylene glycol)-Functionalized Gold Nanorods at the Vapor/Liquid Interface

Using surface sensitive synchrotron X-ray diffraction, we report on the self-assembly of gold nanorods (AuNRs) into 2D films at the vapor/liquid interface. The films are facilitated by grafting the AuNRs with poly(ethylene glycol) (PEG). Grazing incidence small angle X-ray scattering (GISAXS) and specular X-ray reflectivity (XRR), show that PEG-AuNRs in aqueous suspensions migrate to the vapor/liquid interface in the presence of salt, forming a uniform monolayer with planar-to-surface orientation. Furthermore, the 2D assembled PEG functionalized AuNRs exhibit short range order into rectangular symmetry with side-by-side and tail-to-tail nearest-neighbor packing. The effect of PEG chain length and salt concentration on the 2D assembly are also reported.   

Temperature-induced self-assembly and crystallization of gold nanorods

We report on temperature-induced and improved assembly and crystallization of gold nanorods (AuNRs) that are grafted with polye(ethylene glycol) (PEG) in electrolytic suspensions. Small angle X-ray scattering (SAXS) reveals that elevating the temperature of dispersed PEG-AuNRs suspensions in the presence of electrolytes or poly-electrolytes spontaneously induces the assembly of the AuNRs into hexagonal superlattices. Surprisingly, we find that the lattice constant of the crystalline AuNRs significantly decreases and crystal quality improves by increasing the temperature of the suspensions. Implications of the results for engineering new nanomaterials are discussed.   

Laser-written nanograting deep inside silicon

Functional optical elements fabricated on the surface of silicon (Si) constitute fundamental building blocks of Si-photonics. For fabrication of these elements, generally conventional lithography and etching techniques are used. We recently demonstrated a laser-writing method that enables direct creation of buried or “in-chip” optical elements inside the wafer [1]. This direct laser-writing method had enabled 3D optical elements at 1-µm resolution [1]. Here, we expand the technique for demonstrating the first in-chip nanogratings crated in Si, realized without damaging the wafer surface. In order to achieve this, we exploit the ‘non-diffracting’ nature of Bessel beams, and infrared laser pulses of ~5 ns, 1.55 µm where Si is transparent. The crystal structure is modified in the form of periodic rod-like structures with structure-widths 250 nm to 1-micrometer, resulting in grating efficiencies up to 45%. These in-chip optical elements constitute the first examples of a larger class of emerging in-chip nano-photonic components, that can potentially lead to novel infrared elements at the nanoscale.

[1] Tokel et al, “In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon,” Nature Photonics, 11, 639, 2017.   

Large Area Superhydrophilic Silicon Surface Texturing using Nanosecond Laser Pulses

We report experimental investigations of large area laser micro/nano texturing of silicon (100) targets by Nd: YAG laser pulses (λ=532 nm, τ =7 ns) addressing the wetting behavior of processed silicon surface. In particular, ns laser surface processing is used to develop hierarchical surface structures by the line scanning method to create parallel micro-channels with proper overlap between the lines at different laser pulse fluence ranging from 2 J/cm² to 4 J/cm². We observe that the bottom of craters formed due to single and multipulse laser irradiations are rather flat, but some concentric nano-ripple features are present along the rim of the craters, resulting in the formation of multiscale surface morphology. The topography of the samples is investigated using AFM and SEM, whereas the wetting property is measured through sessile drop contact angle measurements. The combination of microscale channels written by parallel line scan, with self-organized surface patterns and random nanoparticles decoration, formed on the surface allow developing highly hydrophilic silicon surfaces with contact angle values reaching around 5°, presenting potential interest for superwetting applications.
  

Growth Kinetics of Metal Nanowires and Nanoplates

Metal nanocrystals have gained tremendous attention due to their superior performance in various applications, ranging from selective catalysis to electronic devices to plasmonic applications. For these and many other applications, the properties of the nanocrystals are highly sensitive to their shape. However, it remains a challenge to achieve high shape selectivity in solution-phase syntheses. I will discuss our efforts to understand the growth of Cu and Ag nanocubes, nanowires and nanoplates, which is facilitated by the inclusion of halide ions (chloride, bromide, and iodide) in the synthesis protocol. We use ab initio thermodynamics based on quantum density-functional theory (DFT) to demonstrate how these halides create thermodynamic driving forces for cubes, plates and wires, by selectively altering surface energies. Using a combination of DFT and the theory of absorbing Markov chains, we demonstrate how halides affect surface diffusion and interfacet transport to drive the growth of particular nanoshapes in the presence of halide. Our results agree with experiment and indicate a promising way to exact control over nanocrystal synthesis.   

Molecular dynamics simulation of silicon nanowires with oxide layers under uniaxial tension

The size-dependent brittle-to-ductile transition (BDT) of silicon is an important scientific topic, but its underlying mechanism is still unclear. Silicon nanostructures such as nanowires have been widely used in electronics and optoelectronics applications and have also been used to study the BDT phenomenon of silicon in both experiments and simulations. While silicon is spontaneously oxidized when exposed to air, most of the previous simulation studies have neglected the effect of oxide layers on BDT. In this work, we employ the molecular dynamics (MD) simulation method to study the influence of oxide layers on the BDT of silicon nanowires under uniaxial tension loading. The oxide layers are created by inserting oxygen atoms into the Si-Si bonds from the nanowire surface and several key factors such as nanowire diameter, thickness of oxide layers, temperature, and strain rate are considered. The MD simulation results reveal the effect of oxide layers on the plastic deformation of silicon nanowires and elucidate its atomic-scale mechanisms.   

Porous Silica Cluster Collision Molecular Dynamics

Silica nanoparticles are theorized to exist in the interstellar medium, where collision velocities range over several orders of magnitude. We report molecular dynamics simulation studies of the coalescence of small (approx. 2 nm) porous silica clusters with each other for a range of velocities and initial cluster porosities. Our use of the ReaxFF bond-order potential allows us to account for bond dynamics during the collision. We vary cluster porosity through the random removal of silica base units during the cluster formation stage and monitor post-collision structural dynamics. We compare both the pair correlation function and structure factors of prepared porous clusters as verification of our preparation method. We use simulation data to study the evolution of post-collision cluster morphology characterized in terms of final porosities and fractal dimensions as a function of the initial range of porosities and velocities. We compare our results with similar studies using different potentials during collision processes.   

Granular aluminum meandered superinductors for quantum circuits

We report on the optimization, fabrication, and characterization of superinductors made of meandered nanowires of strongly disordered (granular) Aluminum [1]. Superinductors are essential for numerous superconducting quantum circuits operating at millikelvin temperatures. We succeeded in increasing the self-resonance frequency of superinductors by the optimization of its in-plane dimensions. These compact superinductors are beneficial for a wide range of applications, from superconducting circuits for quantum computing to microwave elements of cryogenic parametric amplifiers and kinetic-inductance photon detectors. We have also demonstrated that the superinductors based on granular Aluminum films can be integrated in “hybrid” superconducting circuits containing conventional Al-Al2O3-Al Josephson junctions.

[1] P. Kamenov, W.-S. Lu, K. Kalashnikov, T. DiNapoli, M. T. Bell, and M. E. Gershenson, “Granular aluminum meandered superinductors for quantum circuits,” arXiv:1910.00996 (2019).   

Characterization of Spin-Thermal Environment Interaction Leading to Nuclear Quadrupolar Spin Relaxation

The control of spin dynamics at the nanoscale is crucial for quantum technologies based on spin qubits. Quadrupolar nuclei with spin > ½ facilitates creation of N-qubit systems, the representation of many qubits with one nuclear species. Additionally, nuclei with strong quadrupole moments can be used to perform QIP with NQR. However, the strong quadrupole couplings lead to faster spin relaxation and stronger dissipation due to spin-environment interaction. It is crucial to analyze the effect of local environment fluctuations on spin-environment relaxation (SER) to correctly predict the evolution of the qubits implemented by the quadrupolar spins. The prediction of quadrupolar relaxation time (T1) in alkali halides improved considerably due to the inclusion of realistic phonon density of states, over approximate phonon dispersions. However, the predicted T1 is still off by an order of magnitude compared to measured data. Here, we extend the formulation by incorporating the full dispersion relations of thermal phonons to describe T1. Our study provides a framework to characterize the thermal noise affecting the nuclei spin qubits. We anticipate that this work will lead to tailoring the relaxation time by controlling phononic environment of the spin qubits.   

Spin transport in ferromagnet-InSb nanowire quantum devices

Semiconductor nanowires (NWs) such as InSb are a leading platform for realizing future Majorana zero modes based on quantum computing¹. However, the appliance of external magnetic fields can suppress the superconductivity and place geometric restrictions on the device. These challenges can be circumvented by integrating magnetic elements with the NWs. In our experiments, by fabrication ferromagnetic contacts, we investigate spin transport across InSb NWs in a quasi-1D ballistic regime². Hysteretic magnetoconductance can be observed proving spin-polarized transport across the NWs. Moreover, we show that electrostatic gating tunes the observed hysteretic signal and also reveals a transport regime where the device acts as a spin filter.

1. Mourik, V. et al. Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices. Science 336, 1003–1007 (2012).
2. Yang, Z. et al. Spin transport in ferromagnet-InSb nanowire quantum devices, arXiv:1909.07431 (2019).   

Optical Properties of Monolayer Bismuthene under Electric Field based on First-principles Calculations

Monolayer bismuthene has extraordinary optoelectronic, catalytic, and biocompatible properties, and potential as a 2D topological insulator. When monolayer bismuthene deposited on the surface of the object to form sufficiently thin, there is a stable form of a low buckling hexagonal structure. If the thickness of the crucible becomes thinner than the Fermi wavelength, there may be a transition from a semimetal to a semiconductor due to the quantum confinement effect. So this could be a promising low-dimensional thermoelectric material. The monolayer bismuthene is a p-type semiconductor, but the hole concentration arising from the intrinsic defects is very low and hard to control. It is found that the optical properties can be changed dramatically by applying external electric field. In this work, the energy band structure, density of states, and optical constants of bismuthene have been calculated using the first-principles calculations based on density functional theory (DFT). With applying an electric field, the optical properties of bismuthene are determined and compared to those calculated from the tight-binding model. The controlled optical properties of monolayer bismuthene may have some applications in optoelectronics, either combined with other 2D or topological materials   

Shadowing of epitaxial superconductors on selectively grown in-plane semiconductor wires for advanced quantum devices

In superconductor-semiconductor systems, a clean interface between superconductor and semiconductor is crucial for quantum transport studies. Advances in epitaxial Al growth on low-dimensional semiconductors have resulted in enhanced features of superconducting proximity effect. In addition, development of clean junctions by in-situ shadowing has led to further improvement of superconducting features in 1D nanowire-based systems. Here we demonstrate a shadowing scheme of epitaxial superconductors on in-plane quantum wires by selective-area growth. Shadow wall structures are pre-fabricated next to wire-shape trenches. After epitaxial growth of semiconductor wires on the trenches, Al deposition is carried out at an angle between 40 and 60 degrees from the normal direction of the substrate. The shadowing of Al results in clean Al-semiconductor junctions based on the shape of the shadow wall structures. This shadow scheme can be applied to any selective-area growth wires by various techniques such as molecular beam epitaxy, chemical beam epitaxy, and metalorganic vaporphase epitaxy. We further discuss transport studies in devices prepared by this scheme as well as deposition of another superconductor with a higher superconducting transition temperature.   

Synthetic Weyl Points and Chiral Anomaly in Majorana Devices

We demonstrate how to design various nonstandard types of Andreev-bound-state (ABS) dispersions, via a composite construction relying on Majorana bound states (MBSs). Here, the MBSs appear at the interface of a Josephson junction consisting of two topological superconductors (TSCs). Each TSC harbors multiple MBSs per edge by virtue of a chiral or unitary symmetry. We find that, while the ABS dispersions are 2π periodic, they still contain multiple crossings which are protected by the conservation of fermion parity. A single junction with four interface MBSs and all MBS couplings fully controllable, or networks of such coupled junctions with partial coupling tunability, open the door for topological band structures with Weyl points or nodes in synthetic dimensions, which in turn allow for fermion-parity (FP) pumping with a cycle set by the ABS-dispersion details. In fact, in the case of nodes, the FP pumping is a manifestation of chiral anomaly in 2D synthetic spacetime. The possible experimental demonstration of ABS engineering in these devices further promises to unveil new paths for the detection of MBSs and higher-dimensional chiral anomaly. References: P. Kotetes, M. T. Mercaldo, and M. Cuoco PRL (2019) and M. T. Mercaldo, P. Kotetes and M. Cuoco PRB (2019).