3D Confinement Research Articles

Janus nanoplates, -bowls, and -cups: controlling size and curvature via terpolymer/homopolymer blending in 3D confinement.

The synthesis and properties of Janus nanoparticles with spherical, cylindrical, and disk-like shapes are nowadays rather well understood. Other topologies such as nanorings and bowl-shaped Janus nanoparticles are believed to show distinctly different solution behavior and interaction with interfaces, but limitations in their synthesis currently prevents a proper investigation of these properties. Especially the combination of shape- and surface-anisotropy of bowl-shaped Janus nanoparticles could result in enhanced selectivity in uptake of cargo and enhanced directional diffusion. We here produce bowl-shaped Janus nanoparticles without noticeable side products through evaporation-induced confinement assembly (EICA) of triblock terpolymers blended with high molecular weight homopolymer. The triblock terpolymer phase separates from the homopolymer into spherical domes, where the terpolymer adopts a hemispherical lamella-lamella morphology (ll). Selective cross-linking, removal of the homopolymer, and disassembly of the microparticles releases the bowl-shaped Janus nanoparticles. The amount of blended homopolymer determines the size of the spherical dome, allowing to control particle curvature into flat Janus nanoplates, hemispherical Janus nanobowls, and deep Janus nanocups. The use of Shirasu Porous Glass (SPG) membranes with pore sizes in the range of dpore = 0.2-2.0 μm further provides control of particle diameter. Size and shape were analyzed with electron microscopy and the Janus character through selective surface decoration. The diffusion behavior of bowl-shaped Janus nanoparticles was investigated depending on particle curvature and anisotropy using angle-dependent dynamic light scattering.

Ultra-thin magnetic film with giant phonon-drag for heat to spin current conversion.

A tightly confined 2D electron gas with good carrier mobility and large spin-polarization is an essential ingredient for the implementation of spin-caloritronic conversion device technology. Here we give evidence that the SrTiO3/EuTiO3/LaAlO3 heterostructure is a prototype material for this purpose. The presence of Eu induces strong spin-polarization in the 2D electron gas spontaneously formed at the interface and ferromagnetic order at low temperature. Furthermore, tight 2D confinement and spin-polarization can be highly enhanced upon charge depletion, in turn generating huge thermopower associated with the phonon-drag mechanism. Most importantly, the remarkable difference in the population of the two spin channels results in the giant spin-polarized Seebeck effect and in turn, giant spin voltages of mV K-1 order at the two ends of an applied thermal gradient. Our results represent a strong assessment to the capabilities of this interface for low-temperature spin-caloritronic applications.

Trion confinement in monolayer MoSe2 by carbon nanotube local gating

We have successfully confined trions into a one-dimensional restricted space of a MoSe2 device with carbon nanotube (CNT) gate electrodes. The dry transfer process, including deterministic dry transfer of aligned CNTs, has led to an hBN-encapsulated MoSe2 device with CNT back gate electrodes. In contrast to a location without CNT gate electrodes, applying voltage via CNT gate electrodes significantly alters PL spectra at a location with CNT gate electrodes. PL imaging has revealed that image contrast from trions is linear along the CNT electrode underneath, consistent with 1D confinement of trions in response to the CNT local gating. The confinement width obtained from the PL image is 5.5 × 102 nm, consistent with nanoscale 1D confined trions with the diffraction limit broadening. This work has demonstrated electrical control of excitonic states at the nanoscale, leading to novel optoelectronic properties and exciton devices in the future.

Anisotropic 3D confinement of MCF-7 cells induces directed cell-migration and viscoelastic anisotropy of cell-membrane

Tumor-associated collagen signature-3 (TACS-3) is a prognostic indicator for breast cancer survival. It is characterized by highly organized, parallel bundles of collagen fibers oriented perpendicular to the tumor boundary, serving as directional, confining channels for cancer cell invasion. Here we design a TACS-3-mimetic anisotropic, confined collagen I matrix and examine the relation between anisotropy of matrix, directed cellular migration, and anisotropy of cell membrane-the first direct contact between TACS-3 and cell-using Michigan Cancer Foundation-7 (MCF-7) cells as cancer-model. Using unidirectional freezing, we generated ∼50 μm-wide channels filled with collagen I. Optical tweezer (OT) microrheology shows that anisotropic confinement increases collagen viscoelasticity by two orders of magnitude, and the elastic modulus is significantly greater along the direction of anisotropic confinement compared to that along the orthogonal direction, thus establishing matrix anisotropy. Furthermore, MCF-7 cells embedded in anisotropic collagen I, exhibit directionality in cellular morphology and migration. Finally, using customized OT to trap polystyrene probes bound to cell-membrane (and not to ECM) of either free cells or cells under anisotropic confinement, we quantified the effect of matrix anisotropy on membrane viscoelasticity, both in-plane and out-of-plane, vis-à-vis the membrane. Both bulk and viscous modulus of cell-membrane of MCF-7 cells exhibit significant anisotropy under anisotropic confinement. Moreover, the cell membrane of MCF-7 cells under anisotropic confinement is significantly softer (both in-plane and out-of-plane moduli) despite their local environment being five times stiffer than free cells. In order to test if the coupling between anisotropy of extracellular matrix and anisotropy of cell-membrane is regulated by cell-cytoskeleton, actin cytoskeleton was depolymerized for both free and confined cells. Results show that cell membrane viscoelasticity of confined MCF-7 cells is unaffected by actin de-polymerization, in contrast to free cells. Together, these findings suggest that anisotropy of ECM induces directed migration and correlates with anisotropy of cell-membrane viscoelasticity of the MCF-7 cells in an actin-independent manner.

Microgel dynamics within the 3D porous structure of transparent PEG hydrogels

We report an investigation on the effects of the confinement imposed by application-relevant poly(ethylene glycol) (PEG) hydrogel matrices with controlled porosity on the dynamics of soft microgels. Through a detailed characterization of the internal structure of the hydrogels at the nano and microscale, we were able to link the microgel dynamics, measured by particle tracking, to the 3D geometrical confinement imposed by the porous matrices. PEG hydrogels with a high degree of transparency and tunable pore sizes and volume fractions were obtained using freeze-thawing. We found that the porosity of the hydrogel networks is characterized by elongated channels having asymmetric sections, with the average size decreasing from about 7 to about 2 particle diameters, and the size distribution becoming narrower with increasing PEG content in the pre-reaction mixture. The microgel dynamics slowdown and change from diffusive to sub-diffusive as a result of the increasing confinement. The observed decrease in diffusivity is consistent with models of diffusion in cylindrical pores and can be attributed to hydrodynamic and steric effects in addition to geometrical constriction. A dependence of the effective diffusion coefficient on the pore volume fraction, which is unusually pronounced, suggests the presence of microgel-hydrogel interactions. Our results demonstrate that a detailed characterization of the 3D geometry of the porous network is of primary importance for the understanding of transport properties in complex, random porous media.

Effect of temperature on CO oxidation over Pt(111) in two-dimensional confinement.

Confined catalysis between a two-dimensional (2D) cover and metal surfaces has provided a unique environment with enhanced activity compared to uncovered metal surfaces. Within this 2D confinement, weakened adsorption and lowered activation energies were observed using surface science experiments and density functional theory (DFT) calculations. Computationally, the role of electronic and mechanical factors responsible for the improved activity was deduced only from static DFT calculations. This demands a detailed investigation on the dynamics of reactions under 2D confinement, including temperature effects. In this work, we study CO oxidation on a 2D graphene covered Pt(111) surface at 90 and 593K using DFT-based ab initio molecular dynamics simulations starting from the transition state configuration. We show that CO oxidation in the presence of a graphene cover is substantially enhanced (2.3 times) at 90K. Our findings suggest that 2D confined spaces can be used to enhance the activity of chemical reactions, especially at low temperatures.

Surface Diffusion Aided by a Chirality Change of Self‐Assembled Oligomers under 2D Confinement

AbstractChirality switching of self‐assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self‐assembled oligomers using variable‐temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self‐assemble into chiral wheel‐shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self‐assembled structure. Each chirality change is accompanied by a slight center‐of‐mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self‐assembled supramolecular structures.

Modeling of 1D confinement in FD-SOI trigate nanowires at deep cryogenic temperatures

Multi-gate FD-SOI qubit MOS devices were electrically characterized. At deep cryogenic temperatures and in linear regime, oscillations in both the current and the transconductance were observed above threshold. These are likely related to the formation of one-dimensional sub-bands in the silicon active region. A compact model taking into account a 1D density of states (DOS) has been developed in order to fathom how the 1D confinement influences both the inversion charge and the transconductance. The impact of temperature was shown to be crucial in order to distinguish the sub-bands contribution to the transconductance oscillations. Furthermore, the impact of drain voltage was also investigated, showing that the oscillations were smoothed out at high drain biases.

Surface Diffusion Aided by a Chirality Change of Self-Assembled Oligomers under 2D Confinement.

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures.

Fast-Response Photodetector Based on Hybrid Bi2Te3/PbS Colloidal Quantum Dots.

Colloidal quantum dots (CQDs) as photodetector materials have attracted much attention in recent years due to their tunable energy bands, low cost, and solution processability. However, their intrinsically low carrier mobility and three-dimensional (3D) confinement of charges are unsuitable for use in fast-response and highly sensitive photodetectors, hence greatly restricting their application in many fields. Currently, 3D topological insulators, such as bismuth telluride (Bi2Te3), have been employed in high-speed broadband photodetectors due to their narrow bulk bandgap, high carrier mobility, and strong light absorption. In this work, the advantages of topological insulators and CQDs were realized by developing a hybrid Bi2Te3/PbS CQDs photodetector that exhibited a maximum responsivity and detectivity of 18 A/W and 2.1 × 1011 Jones, respectively, with a rise time of 128 μs at 660 nm light illumination. The results indicate that such a photodetector has potential application in the field of fast-response and large-scale integrated optoelectronic devices.

Universal Hopping Motion Protected by Structural Topology.

A scaling law elucidates the universality in nature, presiding over many physical phenomena which seem unrelated. Thus, exploring the universality class of scaling law in a particular system enlightens its physical nature in relevance to other systems and sometimes unearths an unprecedented new dynamic phase. Here, the dynamics of weakly driven magnetic skyrmions are investigated, and its scaling law is compared with the motion of a magnetic domain wall (DW) creep. This study finds that the skyrmion does not follow the scaling law of the DW creep in 2D space but instead shows a hopping behavior similar to that of the particle-like DW in 1D confinement. In addition, the hopping law satisfies even when a topological charge of the skyrmion is removed. Therefore, the distinct scaling behavior between the magnetic skyrmion and the DW stems from a general principle beyond the topological charge. This study demonstrates that the hopping behavior of skyrmions originates from the bottleneck process induced by DW segments with diverging collective lengths, which is inevitable in any closed-shape spin structure in 2D. This work reveals that the structural topology of magnetic texture determines the universality class of its weakly driven motion, which is distinguished from the universality class of magnetic DW creep.

Early-age strains and stresses in bi-axially restrained expansive concrete elements

Practical application of expansive concretes in reinforced concrete structures needs the rational design model for assessment the early-age stress-strain parameters of self-stressing concrete elements under any restraint conditions during the expansion stage. Various strategies are used to calculate the restrained expansion strains and self-stresses but, in most cases, disregarding the elastic-plastic behaviour of expansive concrete at early ages. It’s noticeable that in case of bi-axial restraint conditions both the elastic-plastic parameters of expansive concrete and the influence of restraint in orthogonal directions should be taken into consideration. To achieve a reliable solution a design model for estimation the restrained expansion strains and self-stresses in self-stressed structural elements under bi-axial restraint conditions (2D MSDM) at expansion stage has been developed. The proposed design model is based on the MSDM for the case of uniaxial restraint arrangement. The validation of 2D MSDM was confirmed by the experimental results of expansive concrete elements under 2D confinement carried out by authors as well as obtained by other researchers.

Regioselective Friedel-Crafts Acylation Reaction Using Single Crystalline and Ultrathin Nanosheet Assembly of Scrutinyite-SnO2.

Peculiar physicochemical properties of two-dimensional (2D) nanomaterials have attracted research interest in developing new synthetic technology and exploring their potential applications in the field of catalysis. Moreover, ultrathin metal oxide nanosheets with atomic thickness exhibit abnormal surficial properties because of the unique 2D confinement effect. In this work, we present a facile and general approach for the synthesis of single crystalline and ultrathin 2D nanosheets assembly of scrutinyite-SnO2 through a simple solvothermal method. The structural and compositional characterization using X-ray diffraction (Rietveld refinement analysis), high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and so on reveal that the as-synthesized 2D nanosheets are ultrathin and single crystallized in the scrutinyite-SnO2 phase with high purity. The ultrathin SnO2 nanosheets show predominant growth in the [011] direction on the main surface having a thickness of ca. 1.3 nm. The SnO2 nanosheets are further employed for the regioselective Friedel–Crafts acylation to synthesize aromatic ketones that have potential significance in chemical industry as synthetic intermediates of pharmaceuticals and fine chemicals. A series of aromatic substrates acylated over the SnO2 nanosheets have afforded the corresponding aromatic ketones with up to 92% yield under solvent-free conditions. Comprehensive catalytic investigations display the SnO2 nanosheet assembly as a better catalytic material compared to the heterogeneous metal oxide catalysts used so far in the view of its activity and reusability in solvent-free reaction conditions.

Direct bandgap dependence of bismuth films on their thickness

Thin bismuth films of various thicknesses between 5 and 32 nm grown by molecular beam epitaxy on Si (111) substrates were investigated. The samples were characterized by the x-ray diffraction method, which allowed us to identify two types of Bi crystallographic structures—α and β bismuth. Terahertz radiation pulses emitted from the samples after their illumination by femtosecond optical pulses with different wavelengths were characterized. The main THz emission features were similar for both types of Bi layers. Due to 2D confinement, the electron energy band structure depends on the thickness. With the terahertz excitation spectroscopy method, direct bandgaps were determined to be in the range from 0.25 to 0.5 eV—much greater than the indirect bandgaps of the layers. A simple model was used to describe the nature of THz emission from these films, which is the cause of uncompensated lateral photocurrents occurring because of the diffusive electron scattering at the Bi/Si interface.

Two-dimensional quantum motion of a levitated nanosphere

We report on the two-dimensional (2D) dynamics of a levitated nanoparticle in an optical cavity. The motion of the nanosphere is strongly coupled to the cavity field by coherent scattering and heavily cooled in the plane orthogonal to the tweezer axis. Due to the characteristics of the 2D motion and the strong optomechanical coupling, the motional sideband asymmetry that reveals the quantum nature of the dynamics is not limited to mere scale factors between Stokes and anti-Stokes peaks, as customary in quantum optomechanics, but assumes a peculiar spectral dependence. We introduce and discuss an effective thermal occupancy that quantifies how close the system is to a minimum uncertainty state and allows us to consistently characterize the particle motion. By rotating the polarization angle of the tweezer beam we tune the system from a 1D cooling regime, where we achieve a best thermal occupancy of $0.51\ifmmode\pm\else\textpm\fi{}0.05$, to a regime in which the fully 2D dynamics of the particle exhibit strong nonclassical properties. We achieve a strong 2D confinement with thermal occupancy of $3.4\ifmmode\pm\else\textpm\fi{}0.4$ along the warmest direction and around one in the orthogonal one. These results represent a major improvement over previous experiments considering both the 1D and the 2D motion and pave the way toward the preparation of tripartite optomechanical entangled states and novel applications to directional force and displacement quantum sensing.

Wettability of graphite under 2D confinement

The thermodynamics of solid/liquid interfaces under nanoconfinement has tremendous implications for liquid transport properties. Here using molecular dynamics, we investigate graphite nanoslits and study how the water/graphite interfacial tension changes with the degree of confinement. We found that, for nanochannel heights between 0.7 nm and 2.6 nm, graphite becomes more hydrophobic than in bulk, and that the value of the surface tension oscillates before eventually converging towards a constant value for larger slits. The value of the surface tension is correlated with the slip length of the fluid and explained in terms of the effective and interfacial density, hydration pressure and friction coefficient. The study clearly indicates that there is a critical channel height of 0.9 nm (achievable experimentally [1]) at which the surface tension reaches its highest value, but the water diffusion across the channel is at its minimum. The structural analysis shows that for this pore size a transition between a 2D and 3D hydrogen bond network is accompanied by an abrupt increase in configurational entropy. Our results show that the wettability of solid surfaces can change under nanoconfinement and the data can be used to interpret the experimental permeability data.

Van der Waals epitaxial growth of two-dimensional PbSe and its high-performance heterostructure devices

Inspired by the great success of ultrathin two-dimensional (2D) layered crystals, more and more attention is being paid to preparing 2D nanostructures from non-layered materials. They can significantly enrich the 2D materials and 2D heterostructures family, extend their application prospects, and bring us distinct properties from their bulk counterparts due to the strong 2D confinement effect. However, the realization of 2D non-layered semiconductors with strong light-harvesting capability and the ability to construct high-performance 2D heterostructures is still a critical challenge. Herein, we successfully synthesized 2D PbSe semiconductors with a large lateral dimension and ultrathin thickness via van der Waals epitaxy. The fabricated 2D PbSe device exhibits good electrical conductivity and superior multi-wavelength photoresponse performance with high responsivity (∼103 A/W) and impressive detectivity (∼2 × 1011 Jones). Furthermore, we demonstrate that 2D PbSe nanosheets can serve as component units for constructing high-performance heterostructure devices. With our strategy, ultrahigh current on/off ratio (∼108) and rectification ratio (∼106), as well as high responsivity (∼3 × 103 A/W) and detectivity (∼7 × 1012 Jones), can be achieved in PbSe/MoS2 back-gated transistors. These results indicate that 2D PbSe nanosheets and their heterostructures have tremendous applications potential in electrical and optoelectronic devices.

Spin Ordering Induced Broadband Photodetection Based on Two-Dimensional Magnetic Semiconductor α-MnSe.

Two‐dimensional (2D) magnetic semiconductors are considered to have great application prospects in spintronic logic devices, memory devices, and photodetectors, due to their unique structures and outstanding physical properties in 2D confinement. Understanding the influence of magnetism on optical/optoelectronic properties of 2D magnetic semiconductors is a significant issue for constructing multifunctional electronic devices and implementing sophisticated functions. Herein, the influence of spin ordering and magnons on the optical/optoelectronic properties of 2D magnetic semiconductor α‐MnSe synthesized by space‐confined chemical vapor deposition (CVD) is explored systematically. The spin‐ordering‐induced magnetic phase transition triggers temperature‐dependent photoluminescence spectra to produce a huge transition at Néel temperature (T N ≈ 160 K). The magnons‐ and defects‐induced emissions are the primary luminescence path below T N and direct internal 4 aT1g→6A1g transition‐induced emissions are the main luminescence path above T N . Additionally, the magnons and defect structures endow 2D α‐MnSe with a broadband luminescence from 550 to 880 nm, and an ultraviolet–near‐infrared photoresponse from 365 to 808 nm. Moreover, the device also demonstrates improved photodetection performance at 80 K, possibly influenced by spin ordering and trap states associated with defects. These above findings indicate that 2D magnetic semiconductor α‐MnSe provides an excellent platform for magneto‐optical and magneto‐optoelectronic research.

Electro‐Thermo Controlled Water Valve Based on 2D Graphene–Cellulose Hydrogels

AbstractNovel 2D hydrogels with coupled electro‐thermoregulation for water transport are constructed via self‐assembly of hydroxypropyl cellulose (HPC) with graphene oxide (GO) and reduced GO (rGO), where the capability of HPC to switch between hydrophobic and hydrophilic states is exploited. Amphiphilic GO surface is used to guide the ordering and alignment of liquid crystalline HPC domains. The changes in conformation and alignment of HPC serve as a switch for optical properties and water transport. A composite consisting of electrically conductive rGO and 2D hydrogel is developed to produce thermal stimulus and thus construct an electro‐thermo controlled valve for regulated water transfer. The prepared 2D hydrogel has a large swelling rate, outstanding mechanical properties (Young's modulus is 2.5 GPa), and superior electrical conductivity (176 S cm−1). The ability of HPC domains to change conformation in 2D confinement, when Joule heating is applied, can function as a 2D low footprint water switcher with optical control in smart membranes. The proposed sustainable approach to self‐assembly of HPC in 2D confinement of GO and rGO is applicable to the whole family of lower critical solution temperature polymers. Thus, a ubiquitous and sustainable synthesis of novel low dimensional robust multifunctional hydrogels is demonstrated.

Actin Turnover Required for Adhesion-Independent Bleb Migration

Cell migration is critical for many vital processes, such as wound healing, as well as harmful processes, such as cancer metastasis. Experiments have highlighted the diversity in migration strategies employed by cells in physiologically relevant environments. In 3D fibrous matrices and confinement between two surfaces, some cells migrate using round membrane protrusions, called blebs. In bleb-based migration, the role of substrate adhesion is thought to be minimal, and it remains unclear if a cell can migrate without any adhesion complexes. We present a 2D computational fluid-structure model of a cell using cycles of bleb expansion and retraction in a channel with several geometries. The cell model consists of a plasma membrane, an underlying actin cortex, and viscous cytoplasm. Cellular structures are immersed in viscous fluid which permeates them, and the fluid equations are solved using the method of regularized Stokeslets. Simulations show that the cell cannot effectively migrate when the actin cortex is modeled as a purely elastic material. We find that cells do migrate in rigid channels if actin turnover is included with a viscoelastic description for the cortex. Our study highlights the non-trivial relationship between cell rheology and its external environment during migration with cytoplasmic streaming.