Dimensional Transition Research Articles

The PELskin project: part IV\u2014control of bluff body wakes using hairy filaments

The passive control of bluff body wakes using a sparse layer of elastic hairy filaments has been investigated via a series of numerical simulations and compared to selected experiments under well-controlled boundary conditions. It has been found that a distribution of filaments spaced half of the dominant three dimensional instability and resonating with the main shedding frequency can drastically delay the three dimensional transition of the wake behind a circular cylinder. It will also be shown that when using a pair of rows of filaments symmetrically spaced by an azimuthal angle, the wake topology can be deeply affected as well as the value of the integral force coefficients of the cylinder. In the most favourable case, a coupled three dimensional transition delay and strongly reduced values of the drag and of the lift fluctuation can be simultaneously achieved. These results hold also for higher Reynolds-number flows as shown in experiments on a cylinder with hairy flaps attached to the aft part. The lock-in effect of structural vibration of the flaps with the vortex shedding is assumed to be the reason for a sudden change in the shedding cycle as soon as the motion amplitude is high enough to modify the wake. In line with this hypothesis, it has been demonstrated that a long elastic filament pinned on the centerline of a forced spatially developing mixing layer can interact with the vortex dynamics delaying the pairing process-leading to a reduced thickness of the layer. These findings show that a properly designed fluid structure interaction can indeed lead to technological benefits in terms of wake control: drag reduction, vibration control and possibly palliation of aeroacoustic emissions.

Single and Few Layered WS2 Nanoflowers: Synthesis, Characterization and Their Piezoresponce

Two Dimensional (2D) transition metal dichalcogenides (TMDS) plays an emerging key potential applications in optoelectronics, electronic devices, sensors, batteries, supercapacitors, biomedicine and electrocatalytic and photocatalysis owing its wide range of chemical, electrical and mechanical properties. Until now, a wide range of synthesis approaches have been developed for the preparation of single or few-layered TMD hybrid nanostructures, which can be synthesized directly from both physical and chemical vapor depositions. Here we demonstrate the single and few layered tungsten disulfide (WS2) as synthesized by solvothermal method. The morphologies of WS2 were recorded by field emission scanning electron microscopy (SEM), the size of the Nanoflowers ranges from 400-600nm. The XRD pattern shows a crystalline hexagonal structure. All the diffraction peaks could be indexed to the p63/mmc space group, JCPDS no. 08-0237. To further revel the layered charactertistics, HRTEM and XPS spectra was collected. The piezoresponse force microscopy (PFM) image reveals that the WS2 nanoflowers exhibits a significantly piezoelectric potential up to 12mV. The WS2 exhibited significantly piezoresponse, which may result from the single and fewlayers due to its noncentrosymmetric structure. The unique piezoelectric property of the crystalline WS2 nanoflowers is opening up the new field applications such as low-power consumption of logic-circuit switchers, and ultrasensitive biological sensors, and piezoelectric nanogenerators.

(Invited) Growth of Transfer-Free 2D Materials: From Controllable Growth to Material Characterizations and Device Applications

Novel condensed matter systems can be understood as new compositions of elements or old materials in new forms. According to the definition, various new condensed matter systems have been developed or are under development in the recent years. 2D layered materials, including graphene, transition metal dichalcogenides (TMDs) allow the scaling down to atomically thin thicknesses and possess unique physical properties under dimensionality confinement. Chemical vapor deposition (CVD) process is the most popular approach for all kind of 2D materials due to its high yield and quality. Nevertheless, the need of high temperature and the relative long process time within each cycle hinders for commercial development in terms of production cost. However, the transfer procedure has become one of the major limitations of the overall performance. In the first part of my talk, I will present several approaches, including ultra-fast microwave heating, plasma selective reaction, controlled segregation process by laser irradiation and metal vapers-assisted growth processes developed in my lab these years to directly grow graphene with controllable thicknesses on arbitrary substrates without any extra transfer process. In the 2nd part of my talk, I will also introduce development of various methods, including laser irradiation assisted-selenization (LIAS) process, fast microwave annealing and plasms enhance selenization processes on different two dimensional transition metal dichalcogenides materials in my lab. The detailed formation mechanisms, microstructures and physical performances as well as its applications on field emission and gas sensors based on these two 2D materials were reported and investigated.

Facile hydrothermal synthesis of hexapod-like two dimensional dichalcogenide NiSe2 for supercapacitor

Hexapod-like two dimensional transition metal dichalcogenide nickel diselenide (NiSe2) structures was synthesized by a facile hydrothermal approach and investigated its electrochemical performance as a supercapacitor electrode for the first time. The obtained material was characterized by XRD, XPS and TEM analysis. The results confirmed the presence of orthorhombic phase of NiSe2 in the synthesized hexapod-like NiSe2 structures. The fabricated NiSe2 electrode on graphite foil exhibited a specific capacitance of 75Fg−1 with good cyclic retention of 94% at a current density of 1mAcm−2 after 5000 charge-discharge cycles. Our results indicate that the synthesized hexapod-like NiSe2 structures hold as a promising candidate for the supercapacitor applications.

Enhanced Air Stability in REPb3 (RE = Rare Earths) by Dimensional Reduction Mediated Valence Transition.

We conceptually selected the compounds REPb3 (RE = Eu, Yb), which are unstable in air, and converted them to the stable materials in ambient conditions by the chemical processes of "nanoparticle formation" and "dimensional reduction". The nanoparticles and the bulk counterparts were synthesized by the solvothermal and high-frequency induction furnace heating methods, respectively. The reduction of the particle size led to the valence transition of the rare earth atom, which was monitored through magnetic susceptibility and X-ray absorption near edge spectroscopy (XANES) measurements. The stability was checked by X-ray diffraction and thermogravimetric analysis over a period of seven months in oxygen and argon atmospheres and confirmed by XANES. The nanoparticles showed outstanding stability toward aerial oxidation over a period of seven months compared to the bulk counterpart, as the latter one is more prone to the oxidation within a few days.

Metallic 1T phase MoS2 nanosheets for high-performance thermoelectric energy harvesting

Thermoelectric materials that can generate electricity from waste heat will play an important role in the global sustainable energy solution. Low dimensional materials open new routes to high performance thermoelectric properties due to their unique density of states with confined electrons and holes. Here, we report that the phase engineered two dimensional transition metal dichalcogenides represent a new class of high performance thermoelectric materials. The organolithium chemically exfoliated nanosheets of MoS2 containing a high concentration of metallic 1T phase MoS2 show superior thermoelectric properties, with a room temperature power factor of 73.1μWm−1K−2, which is much higher than the pristine graphene or single wall carbon nanotubes can yield. Our first principle calculations on monolayer 1T phase MoS2 provide physical insight of their metallic and hole conductive nature, which happens to be important origins of their advantageous thermoelectric properties. These metallic 1T phase MoS2 nanosheets also reveal interesting moisture sensitive thermoelectric properties, indicating their potential diverse applications not only as thermoelectric energy harvesting devices but also as self-powered gas sensors.

Thickness-Dependent and Magnetic-Field-Driven Suppression of Antiferromagnetic Order in Thin V5S8 Single Crystals

With materials approaching the 2D limit yielding many exciting systems with intriguing physical properties and promising technological functionalities, understanding and engineering magnetic order in nanoscale, layered materials is generating keen interest. One such material is V5S8, a metal with an antiferromagnetic ground state below the Néel temperature TN ∼ 32 K and a prominent spin-flop signature in the magnetoresistance (MR) when H∥c ∼ 4.2 T. Here we study nanoscale-thickness single crystals of V5S8, focusing on temperatures close to TN and the evolution of material properties in response to systematic reduction in crystal thickness. Transport measurements just below TN reveal magnetic hysteresis that we ascribe to a metamagnetic transition, the first-order magnetic-field-driven breakdown of the ordered state. The reduction of crystal thickness to ∼10 nm coincides with systematic changes in the magnetic response: TN falls, implying that antiferromagnetism is suppressed; and while the spin-flop signature remains, the hysteresis disappears, implying that the metamagnetic transition becomes second order as the thickness approaches the 2D limit. This work demonstrates that single crystals of magnetic materials with nanometer thicknesses are promising systems for future studies of magnetism in reduced dimensionality and quantum phase transitions.

(Invited) 2D Materials for Electrochemical Applications

Two dimensional (2D) transition metal dichalcogenides (TMDs) are being studied for devices that utilize electrochemistry. Examples include hydrogen evolution reaction catalysis and energy storage. Typical semiconducting 2D TMDs are not well suited for catalysis or energy storage because they are not sufficiently electrically conducting. To circumvent this issue, metallic edges of TMDs have been used to increase the catalytic performance. Increasing strain in 2D materials generally leads to an increase in the density of states at the Fermi level and therefore the electrical conductivity. In addition to increasing the fraction of edges and strain, it is also possible to introduce semiconduction to metallic phase transformation in 2D TMDs by chemistry. The metallic TMDs exhibit exceptional electrochemical properties as catalysts for evolving hydrogen and storing charge in supercapacitors. In this talk, I will highlight the fundamental structural properties of 2D TMDs and correlate them to mechanisms responsible for the electrochemical behavior of 2D TMDs.

(Invited) Critical Review of CMP Requirements in the Future

With the introduction of tri-gate at 22nm technology node by Intel, other logic chip manufacturing technology developers have steadily migrated from planar to 3 dimensional FinFET technology at the fron-end-of-line. Historically, at 0.8μm technology node, there was no CMP although 2-3 metal levels were used. At 0.35μm technology node, a CMP step was introduced. CMP became mainstream with multiple steps requiring CMP at 90nm technology node which continued until 2x nm technology node. However, at 1x (e.g. 14nm) technology node, CMP began to transition from a ‘process simplifier’ to a ‘process enabler’ step. Expanding applications of CMP have made possible the integration of FinFET, RMG (Replacement Metal Gate) and TSV (Through-Silicon-Via)[1]. In sub 1x logic fabrication, the total number of CMP steps may reach 20 or more. Use of FinFET technology in 1x and sub 1x provides much better performance at the same power budget or equal performance at a much lower power budget compared to planar mode[2]. Further, III-V compounds are expected to be integrated at 5nm technology node. III-V materials enable Vdd scaling to address power without sacrificing performance due to the very high electron injection velocity at virtual source compared to Si[3]. These III-V films will require CMP with special care due to the probability of toxic gas release (AsH3/PH3). Beyond 5nm, it is widely assumed that FinFET technology will not be able to provide the small scale features and therefore, gate-all-around technology will be used. To implement gate-all-around, one potential pathway may be to use 2D TMD (2 dimensional transition metal dichalogenide) films. 2D TMD gate-all-around film allows for increasing direct S/D (source/drain) tunneling[4]. With new types of films getting integrated in smaller features, it is necessary therefore to understand the type and nature of various films being used to fabricate devices using sub 1x nm features. In this paper, various US patent applications by leading device manufacturers within the past year will be reviewed to explore the most important CMP steps needed to fabricate new devices. Further, new slurry formulation efforts to enable new CMP requirements for the sub 1x nm devices will be reviewed for the same timeline.

An analytical solution for quantum size effects on Seebeck coefficient

There are numerous experimental and numerical studies about quantum size effects on Seebeck coefficient. In contrast, in this study, we obtain analytical expressions for Seebeck coefficient under quantum size effects. Seebeck coefficient of a Fermi gas confined in a rectangular domain is considered. Analytical expressions, which represent the size dependency of Seebeck coefficient explicitly, are derived in terms of confinement parameters. A fundamental form of Seebeck coefficient based on infinite summations is used under relaxation time approximation. To obtain analytical results, summations are calculated using the first two terms of Poisson summation formula. It is shown that they are in good agreement with the exact results based on direct calculation of summations as long as confinement parameters are less than unity. The analytical results are also in good agreement with experimental and numerical ones in literature. Maximum relative errors of analytical expressions are less than 3% and 4% for 2D and 1D cases, respectively. Dimensional transitions of Seebeck coefficient are also examined. Furthermore, a detailed physical explanation for the oscillations in Seebeck coefficient is proposed by considering the relative standard deviation of total variance of particle number in Fermi shell.

Thermodynamics of the classical spin-ice model with nearest neighbour interactions using the Wang-Landau algorithm

In this article we study the classical nearest-neighbour spin-ice model (nnSI) by means of Monte Carlo simulations, using the Wang-Landau algorithm. The nnSI describes several of the salient features of the spin-ice materials. Despite its simplicity it exhibits a remarkably rich behaviour. The model has been studied using a variety of techniques, thus it serves as an ideal benchmark to test the capabilities of the Wang Landau algorithm in magnetically frustrated systems. We study in detail the residual entropy of the nnSI and, by introducing an applied magnetic field in two different crystallographic directions ([111] and [100],) we explore the physics of the kagome-ice phase, the transition to full polarisation, and the three dimensional Kasteleyn transition. In the latter case, we discuss how additional constraints can be added to the Hamiltonian, by taking into account a selective choice of states in the partition function and, then, show how this choice leads to the realization of the ideal Kasteleyn transition in the system.

Novel Quantum Criticality in Two Dimensional Topological Phase transitions.

Topological quantum phase transitions intrinsically intertwine self-similarity and topology of many-electron wave-functions, and divining them is one of the most significant ways to advance understanding in condensed matter physics. Our focus is to investigate an unconventional class of the transitions between insulators and Dirac semimetals whose description is beyond conventional pseudo relativistic Dirac Hamiltonian. At the transition without the long-range Coulomb interaction, the electronic energy dispersion along one direction behaves like a relativistic particle, linear in momentum, but along the other direction it behaves like a non-relativistic particle, quadratic in momentum. Various physical systems ranging from TiO2-VO2 heterostructure to organic material α-(BEDT-TTF)2I3 under pressure have been proposed to have such anisotropic dispersion relation. Here, we discover a novel quantum criticality at the phase transition by incorporating the long range Coulomb interaction. Unique interplay between the Coulomb interaction and electronic critical modes enforces not only the anisotropic renormalization of the Coulomb interaction but also marginally modified electronic excitation. In connection with experiments, we investigate several striking effects in physical observables of our novel criticality.

Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides.

This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

Computational Studies on the Mo-Doped Gold Nanoclusters Au n Mo(n = 1–10): Structures, Stabilities and Magnetic Properties

The geometric structures, relative stabilities, magnetic properties of Mo-doped gold clusters Au n Mo(n = 1–10) have been investigated at the PBE1PBE/def2TZVP level of theory. The results show that molybdenum doping has a significant effect on the geometric structures and electronic properties of Au n Mo(n = 1–10) clusters. For the lowest energy structures of Au n Mo(n = 1–10), the two dimensional to three dimensional transition occurs at cluster size n ≥ 8, and their relative stabilities exhibit odd–even oscillation with the change of Au atom number. It is found that charge in corresponding Au n Mo clusters transfers from Mo atom to Au n host in the size range n = 1–7, whereas the charge in opposition direction in the size range n = 8–10. In addition, the magnetic properties of Au n Mo clusters are enhanced after doping single Mo atom into the corresponding gold clusters. Our results are valuable for the design of magnetic material.

Evolutionarily conserved mechanisms for the selection and maintenance of behavioural activity.

Survival and reproduction entail the selection of adaptive behavioural repertoires. This selection manifests as phylogenetically acquired activities that depend on evolved nervous system circuitries. Lorenz and Tinbergen already postulated that heritable behaviours and their reliable performance are specified by genetically determined programs. Here we compare the functional anatomy of the insect central complex and vertebrate basal ganglia to illustrate their role in mediating selection and maintenance of adaptive behaviours. Comparative analyses reveal that central complex and basal ganglia circuitries share comparable lineage relationships within clusters of functionally integrated neurons. These clusters are specified by genetic mechanisms that link birth time and order to their neuronal identities and functions. Their subsequent connections and associated functions are characterized by similar mechanisms that implement dimensionality reduction and transition through attractor states, whereby spatially organized parallel-projecting loops integrate and convey sensorimotor representations that select and maintain behavioural activity. In both taxa, these neural systems are modulated by dopamine signalling that also mediates memory-like processes. The multiplicity of similarities between central complex and basal ganglia suggests evolutionarily conserved computational mechanisms for action selection. We speculate that these may have originated from ancestral ground pattern circuitries present in the brain of the last common ancestor of insects and vertebrates.

Graphene on transition-metal dichalcogenides: A platform for proximity spin-orbit physics and optospintronics

Hybrids of graphene and two dimensional transition metal dichalcogenides (TMDC) have the potential to bring graphene spintronics to the next level. As we show here by performing first-principles calculations of graphene on monolayer MoS$_2$, there are several advantages of such hybrids over pristine graphene. First, Dirac electrons in graphene exhibit a giant global proximity spin-orbit coupling, without compromising the semimetallic character of the whole system at zero field. Remarkably, these spin-orbit effects can be very accurately described by a simple effective Hamiltonian. Second, the Fermi level can be tuned by a transverse electric field to cross the MoS$_2$ conduction band, creating a system of coupled massive and massles electron gases. Both charge and spin transport in such systems should be unique. Finally, we propose to use graphene/TMDC structures as a platform for optospintronics, in particular for optical spin injection into graphene and for studying spin transfer between TMDC and graphene.

Unusual Enhancement in Intrinsic Thermal Conductivity of Multilayer Graphene by Tensile Strains.

Using the Boltzmann-Peierls equation for phonon transport approach with the inputs of interatomic force constants from the self-consistent charge density functional tight binding method, we calculate the room-temperature in-plane lattice thermal conductivities k of multilayer graphene (up to four layers) and graphite under different isotropic tensile strains. The calculated in-plane k of graphite, finite monolayer graphene and 3-layer graphene agree well with previous experiments. For unstrained graphene systems, both the intrinsic k and the extent of the diffusive transport regime present a drastic dimensional transition in going from monolayer to 2-layer graphene and thereafter a gradual transition to the graphite limit. We find a peak enhancement of intrinsic k for multilayer graphene and graphite with increasing strain with the largest enhancement amplitude ∼40%. Competition between the decreased mode heat capacities and the increased lifetimes of flexural phonons with increasing strain contribute to this k behavior. Similar k behavior is observed for 2-layer hexagonal boron nitride systems. This study provides insights into engineering k of multilayer graphene and boron nitride by strain and into the nature of thermal transport in quasi-two-dimensional and highly anisotropic systems.

Electrochemical Properties of Ordered, Two-Dimensional, Double Transition Metals Carbides (MXenes)

Two dimensional (2D) transition metal carbides (MXenes) are a new family of nanomaterials with a very high potential as electrode materials for batteries and electrochemical capacitors (ECs). These materials are labeled MXene because they are obtained by selectively etching of the A-layers from their 3D layered, parent compounds the Mn+1AXn, or MAX, phases, where M is an early transition metal, A is an A-group element, such as Al or Si, X is carbon and/or nitrogen and n is 1 to 3. The excellent performance of MXenes as electrode materials for ECs was recently reported in the case of Ti3C2.1,2 Here, we report on the synthesis and electrochemical properties of the ordered MXenes, Mo2TiC2 and Mo2TiC3, wherein the Mo atoms are on the surface and the Ti atoms are between the Mo. The volumetric capacitance of freestanding electrodes fabricated by vacuum filtration of 2D Mo2TiC2 sheets, was about 400 F/cm3. (1) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science (80-. ). 2013, 341, 1502–1505. (2) Ghidiu, M.; Lukatskaya, M. R.; Zhao, M.-Q.; Gogotsi, Y.; Barsoum, M. W. Conductive two-dimensional titanium carbide “clay” with high volumetric capacitance. Nature 2014, 516, 78–81.

Liquid Phase Exfoliation of Two Dimensional Transition Metal Oxides for Electrochemical Applications

Transition metal oxides (TMOs) represent an attractive family of materials for their electrochemical charge storage properties. The theory is mainly based on the two dimensional layered structure of these materials providing increased surface area for charge storage alongside decreased diffusion paths for lithium and other intercalation ions. Much work is done into the exfoliation of two dimensional materials such as V2O5 and anatase TiO2, however many methods require multistep chemical processes which can be lengthy and introduce the possibility of unwanted by-products. Liquid phase exfoliation by sonolysis provide a simple cost effective method to exfoliate these two dimensional TMOs to produce relatively thin nanoflakes with impressive charge storage capacities, cycle lifetime and efficiency. This work aims to show the charge storage capabilities of V2O5 and anatase TiO2 which both exceed their theoretical capacities without the use of polymer binders or conductive additives. Sonication of bulk material in common solvents for a relatively short time scale produces desirable nanoflakes while ultrasonic spraying produces uniform electrodes which are tested electrochemically vs. Li/Li+. In conclusion, this project, using simple methodology, produces electrochemically impressive two dimensional materials. Figure 1

Age-based analysis of pediatric upper airway dimensions using computed tomography imaging.

Recent studies have challenged the historically accepted fact that the larynx is cone-shaped in infants and children. The present study used computed tomography (CT)-based measurements to evaluate airway dimensions. The purpose of this investigation was to determine the dimensional transition between the subglottic area and the cricoid ring in children. This is a retrospective review of 220 CT scans of children aged 1 month to 10 years undergoing radiological evaluation unrelated to airways symptomatology. The CT scans were evaluated in children either sleeping naturally or sedated throughout the study period. Anteroposterior (AP) and Transverse (T) diameters were measured at the subglottic level and at the cricoid ring. The mean (±SD) age was 47.4 ± 33.1 months. The mean AP and transverse diameters were 9.2 ± 1.9 and 7.5 ± 1.6 mm at the subglottic area and 8.5 ± 1.7 and 8.3 ± 1.5 mm at the cricoid. AP dimension showed a decrease from the subglottis to the cricoid ring. A more rapid enlargement of the airway from the subglottis to cricoid ring is observed in the transverse dimension (P < 0.05). A linear progression in the size of airway dimensions between both levels was observed with age (r > 0.7). The narrower transverse dimension compared to the AP diameter suggests that the airway is elliptical immediately below the vocal cords. The present study demonstrates that the airway characteristics in children between the subglottic area and the cricoid change from an elliptical to a round (circular) shape. The cone-shaped airway characteristic, which has been historically proposed, was not observed. Given that subglottic transverse diameter is the smallest area dimension, one must assume this is the most likely area of resistance to the passage of an endotracheal tube rather than only the cricoid.