Mixed Stacks Research Articles

Remote epitaxy of single-crystal rhombohedral WS2 bilayers

Compared to transition metal dichalcogenide (TMD) monolayers, rhombohedral-stacked (R-stacked) TMD bilayers exhibit remarkable electrical performance, enhanced nonlinear optical response, giant piezo-photovoltaic effect and intrinsic interfacial ferroelectricity. However, from a thermodynamics perspective, the formation energies of R-stacked and hexagonal-stacked (H-stacked) TMD bilayers are nearly identical, leading to mixed stacking of both H- and R-stacked bilayers in epitaxial films. Here, we report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates. The bilayer growth is realized by a high flux feeding of the tungsten source at high temperature on substrates. The R-stacked configuration is achieved by the symmetry breaking in a-plane sapphire, where the influence of atomic steps passes through the lower TMD layer and controls the R-stacking of the upper layer. The as-grown R-stacked bilayers show up-to-30-fold enhancements in carrier mobility (34 cm2V−1s−1), nearly doubled circular helicity (61%) and interfacial ferroelectricity, in contrast to monolayer films. Our work reveals a growth mechanism to obtain stacking-controlled bilayer TMD single crystals, and promotes large-scale applications of R-stacked TMD.

Smoke exposure, hemoglobin levels and the prevalence of anemia: a cross-sectional study in urban informal settlement in Southern Ghana

BackgroundIn sub-Saharan African cities, more than half of the population lives in informal settlements. These settlements are close to smoky dumpsites, industrial plants, and polluted roads. Furthermore, polluting fuels remain their primary sources of energy for cooking and heating. Despite evidence linking smoke and its components to anaemia, none of these studies were conducted on populations living in urban informal settlements. This study investigated the risks of anemia/mean Haemoglobin (HB) levels in an informal settlement in Accra, Ghana. Exposure to smoke was examined across various sources, encompassing residences, neighborhoods, and workplaces.MethodsThe study was a facility-based cross-sectional design among residents at Chorkor, an informal settlement in the Greater Accra region of Ghana. A questionnaire was administered at a community hospital during an interview to gather data on sources of smoke exposure in the household, in the neighbourhood, and in the workplace. A phlebotomist collected blood samples from the participants after the interview to assess their anaemia status.ResultsThe population (n = 320) had a high prevalence of anemia, with 49.1% of people fitting the WHO’s definition of anemia, while the average HB level was 12.6 ± 2.1 g/dL. Anemia was associated with the number of different types of waste burnt simultaneously [(1 or 2: prevalence ratio (PR): 95% confidence interval (CI), 1.14, 0.99–1.28: 3+: 1.16, 1.01–1.63, p-for-trend = 0.0082)], fuel stacking [(mixed stacking: 1.27, 1.07–1.20: dirty stacking:1.65, 1.19–2.25, p-for-trend = 0.0062)], and involvement in fish smoking (1.22, 0.99–1.06). However, the lower limit of the CIs for number of different forms of garbage burned simultaneously and engagement in fish smoking included unity. Reduced mean HB levels were associated with the number of different types of waste burnt simultaneously [(1 or 2: regression coefficient (β): 95% confidence interval (CI), -0.01, -0.97- -0.99: 3+: -0.14, -0.77- -0.05)], current smoker [(yes, almost daily: -1.40, -2.01- -0.79: yes, at least once a month: -1.14, -1.79- -0.48)], Second-Hand-Smoking (SHS) (yes, almost daily: -0.77, -1.30- -0.21), fuel stacking [(mixed stacking-0.93, -1.33–0.21: dirty stacking-1.04, -1.60- -0.48)], any smoke exposure indicator in the neighbourhood (-0.84, -1.43- -0.25), living close to a major road (-0.62, -1.09- -0.49), and fish smoking (-0.41,-0.93- -0.12).ConclusionAlthough the cross-sectional design precludes causality, smoke exposure was associated with mean HB levels and anaemia among populations living in informal settlements.

Pressure-Induced Neutral to Ionic Phase Transition in TTF-Fluoranil, DimethylTTF-Fluoranil and DimethylTTF-Chloranil: A Comparative THz Raman Study

The Neutral to Ionic phase Transition (NIT) that occurs in few mixed stack charge transfer cocrystals at high pressure or low temperature is a charge instability combined with a structural instability. The lattice contraction, which increases the 3D Coulomb interactions, favors a higher degree of charge transfer. Due to Peierls instability, this leads to the dimerization of the stack, breaking its inversion symmetry. The 3D interactions also determine the arrangement of the adjacent dimerized polar stacks, making the ionic phase ferroelectric or antiferroelectric. The role of these parameters that modulate the NIT has been widely studied in Tetrathiafulvalene-haloquinone cocrystals. Here, we compare the high-pressure behavior of three of them: the newly synthesized TTF-FA and DMTTF-FA with the known DMTTF-CA and isostructural DMTTF-FA. We followed the evolution of the lattice phonons via THz Raman spectroscopy, assessing the pressure-dependent structural changes. While the FA-based crystals undergo strong first-order NIT, DMTTF-CA shows a continuous transition. The high-pressure behavior of each crystal is also compared with the low-temperature behavior.

Fertilizer management methods affect bacterial community structure and diversity in the maize rhizosphere soil of a coal mine reclamation area

BackgroundThe filling or mixed stack mode is a frequently used coal mine reclamation engineering technique that results in changes in soil microbial community structure and nutrient content, which lead to considerable deviations from the characteristics of restored coal mine reclamation areas that can be used for farming. Fertilization is an effective strategy for improving soil fertility in such areas; however, the response of soil bacterial communities, especially in the crop rhizosphere soil, to different fertilization techniques in such soils remains unclear. Therefore, we investigated the effects of different fertilization management methods, including no fertilizer, farmers' practice, inorganic fertilizer, organic fertilizer, and organic–inorganic fertilizer, on maize yield, rhizosphere soil bacterial community and diversity, soil physicochemical properties, and nitrogen cycle-related gene abundance (nifH, AOB, and nirS).ResultsThe results showed that organic–inorganic fertilizer treatment significantly improved maize yield. The relative abundance of the dominant phyla did not significantly differ between the treatment groups. However, the Chao 1 and Shannon indices of the bacterial community significantly changed between the no fertilizer and organic–inorganic fertilizer treatments. Notably, organic–inorganic fertilizer application significantly increased the copy numbers of nifH and nirS. Further, moisture, bulk density, and available phosphorus content were identified as the major driving factors responsible for the changes in bacterial community structure, diversity, and copy numbers of nifH, nirS, and AOB.ConclusionsThe results of this study revealed that organic–inorganic fertilizer application improved soil bacterial diversity and the copy numbers of nifH and nirS in maize rhizosphere soil. Therefore, we concluded that organic–inorganic fertilizer is an effective strategy for the restoration of maize rhizosphere soil properties and bacterial communities in coal mine reclamation areas.

Visible White‐Light Emission and Carrier Dynamics Observed in Full‐Series GaSe1‐xSx (0≤x≤1) Multilayers

AbstractNear‐band‐edge emissions ranging from red (≈630 to 602 nm) to green (≈550 to 534 nm) and blue (≈494 to 480 nm) colors have been detected in full‐series GaSe1‐xSx (0≤x≤1) multilayers using micro‐photoluminescence (µPL) measurements from 4 to 300 K. The multilayered chalcogenides crystallize in a hexagonal structure, with observed mixed stacking phases of ε and β polymorphs through X‐ray diffraction and Raman measurements. The µPL results, along with experimental band‐edge characterization through thermoreflectance, identify the ε‐stacked phase as the crucial phase responsible for direct recombination and the emission of free excitons within the visible range. A mixed‐color white light is created by PL, which is emitted from the layered GaSe1‐xSx series of different colors, and is positioned at the center of the CIE coordination plot (i.e., white color). To assess the emission properties of both band edges and defects in the layered compounds, time‐resolved photoluminescence (TRPL) is employed, using an area mapping function. The photoluminescence decay lifetime increases as the sulfur content is increased, owing to the greater occurrence of mixed‐phase stacking faults near the β‐GaS end within the GaSe1‐xSx (0≤x≤1) series. The multilayer GaSe1‐xSx (0≤x≤1) represents a distinct 2D chalcogenide series well‐suited for emitting full‐color visible light.

Thermochromism of rapid responses to temperature changes versus mechanochromism in a Ni-dithiolene complex salt.

A thermochromic or mechanochromic material can switch between at least two stable states in response to changes in temperature or static pressure/strain. In this study, we investigated a Ni-dithiolene dianion salt, 1,1'-diheptyl-4,4'-bipyridinium bis(maleonitriledithiolato)nickelate (1), and found that its cations and anions stack alternately to form a uniform mixed stack. These mixed stacks then combine to form a molecular solid through Coulomb and van der Waals interactions. Upon heating, 1 undergoes a reversible phase transition at around 340/320 K during the first heating/cooling cycle, resulting in rapid thermochromism with a color change from green (stable state) to red (metastable state) within a few seconds. This is the first report of a crystal of bis(maleonitriledithiolato)nickelate(II) salt with green color. Additionally, 1 exhibits irreversible mechanochromism, intense near-IR absorbance, and a dielectric anomaly. The structural phase transition is responsible for these properties, as it induces alterations in the π-orbital overlap between the anion and cation within a mixed stack. The intense near-IR absorbance arises from the ion-pair charge transfer transition from [Ni(mnt)2]2- to 4,4'-bipyridinium.

Strain-induced stacking transition in bilayer graphene

Strain, both naturally occurring and deliberately engineered, can have a considerable effect on the structural and electronic properties of 2D and layered materials. Uniaxial or biaxial heterostrain modifies the stacking arrangement of bilayer graphene (BLG) which subsequently influences the electronic structure of the bilayer. Here, we use density functional theory (DFT) calculations to investigate the interplay between an external applied heterostrain and the resulting stacking in BLG. We determine how a strain applied to one layer is transferred to a second, ‘free’ layer and at what critical strain the ground-state AB-stacking is disrupted. To overcome limitations introduced by periodic boundary conditions, we consider an approximate system consisting of an infinite graphene sheet and an armchair graphene nanoribbon. We find that above a critical strain of , it is energetically favourable for the free layer to be unstrained, indicating a transition between uniform AB-stacking and non-uniform mixed stacking. This is in agreement with a simple model estimate based on the individual energy contributions of strain and stacking effects. Our findings suggest that small levels of strain provide a platform to reversibly engineer stacking order and Moiré features in bilayers, providing a viable alternative to twistronics to engineer topological and exotic physical phenomena in such systems.

A hybrid of tetrazolium and pentazolate: An energetic salt with ultrahigh nitrogen content and energy

As a full nitrogen energetic anion, pentazolate (cyclo-N5ˉ) holds great promise in the fields of propellants and explosives. Nowadays, nonmetallic pentazolate salts have received extensive attention as excellent nitrogen-rich energetic materials for their high enthalpies of formation, good oxygen balance, and eco-friendly decomposition products. In this study, a 1,4,5-triaminotetrazolium-based pentazolate salt (TATe+N5ˉ, 8) with a nitrogen content of up to 90.30% was designed and synthesized. Its crystal structure indicates that a large number of hydrogen bonds form a hydrogen-bonded network, and the crystal has a mixed stacking pattern. TATe+N5ˉ, which has a relatively high density (1.64 ​g·cm−3), high heat of formation (861.9 ​kJ·mol−1), and excellent detonation performances (detonation velocity: 9487 ​m·s−1, detonation pressure: 32.5 ​GPa), provides new insights into the stability of ultrahigh-nitrogen compounds.

Video See-Through Mixed Reality with Focus Cues.

This work introduces the first approach to video see-through mixed reality with full support for focus cues. By combining the flexibility to adjust the focus distance found in varifocal designs with the robustness to eye-tracking error found in multifocal designs, our novel display architecture reliably delivers focus cues over a large workspace. In particular, we introduce gaze-contingent layered displays and mixed reality focal stacks, an efficient representation of mixed reality content that lends itself to fast processing for driving layered displays in real time. We thoroughly evaluate this approach by building a complete end-to-end pipeline for capture, render, and display of focus cues in video see-through displays that uses only off-the-shelf hardware and compute components.

Temperature-induced neutral-to-ionic phase transition of the charge-transfer crystal tetrathiafulvalene-fluoranil

The Temperature Induced Neutral to Ionic phase Transition (TI-NIT) is a rare phenomenon occurring in mixed stack charge transfer (CT) crystals made up of alternating $\pi$-electron Donor (D) and Acceptor (A) molecules. We were able to grow crystals of Tetrathiafulvalene-Fluoranil (TTF-FA), and to show that it undergoes TI-NIT like the prototype CT crystal TTF-Chloranil. We characterized both room and low T phases through IR and Raman spectroscopy and XRD, demonstrating that while TTF-FA is quasi-neutral at room T, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the Neutral-Ionic borderline. The transition, occuring around 150K, is first order, with large thermal hysteresis and accompanied by crystal cracking. In the high T phase D and A molecules lie on inversion center, i.e. the stacks are regular, whereas the low T phase is characterized by the loss of the inversion symmetry along the stack as the stacks are strongly dimerized and by the doubling of the unit cell.

Charge-transfer complexes of arylthiotetrathiafulvalenes and TCNQF4: their structural diversity and electronic states

A series of charge-transfer (CT) complexes have been prepared via diffusion and evaporation methods comprising arylthio-substituted tetrathiafulvalene (Ar-S-TTF) derivatives 1–5 and TCNQF4. Crystallographic studies revealed the diverse crystal packing of the CT complexes, which is related to their flexible structure and the planar central cores of the Ar-S-TTF. The complexes could be divided into three systems according to the packing similarities and dissimilarities of Ar-S-TTF (donor, D) and TCNQF4 (acceptor, A), namely Class 1 (D on D and A on A segregated stacking), Class 2 ((–A–D–)n mixed stacking) and Class 3 (–D–D–D–D– and A, A stacks with each other in edge-to-face interactions between cations and anions). The presence of multiple intermolecular interactions promotes the formation of various stacking structures. There are two factors affecting the packing structures and compositions of the CT complexes: (1) the aryl groups thereon of Ar-S-TTF; and (2) the expanded π-conjugated system.

Unravelling the semiconductor properties of mixed stack donor acceptor cocrystals of pyrene derivatives and TCNQ: effect of crystal packing versus super-exchange mechanism

The role of crystal packing and super-exchange mechanism to dictate the semiconductor properties of two mixed DDADDA stack charge transfer cocrystals is elucidated via theoretical study.

Procena i višekriterijumska optimizacija hrapavosti površine, odstupanja od tačnosti dimenzija i greške zaobljenosti u bušenju CFRP/Ti6Al4 naslaga

In this study, machinability tests were carried out to investigate the effects of control factors (cutting tool geometry, cutting speed, and feed rate) on the surface roughness (Ra), deviation from dimensional accuracy (Da_dev), roundness error (Re) in drilling CFRP/Ti6Al4V mixed metallic stack and to determine the optimum levels of drilling parameters. The effects of each control factor and their interactions on three quality characteristics were analyzed, and their levels were single-objectively optimized for each component material by the Taguchi method. The material has components (CFRP and Ti6Al4V) with essentially different properties (mechanical, physical, machinability). Single-objective optimization has limited usability as the drilling must be performed in one through both layers. Therefore, in an additional step, the optimum levels of the control factors were determined by optimizing multi-objective with the Additive Ratio Assessment (ARAS) method. Higher Ra, Da_dev, and Re values were obtained on the CFRP component compared to the Ti6Al4V component. The CFRP/Ti6Al4V stack should be drilled with a nano fire coated carbide drill (T3) at medium cutting speed and high feed rate to achieve minimum Ra, Da_dev, and Re values in one go.

Enhanced luminescence of single-benzene fluorescent molecules through halogen bond cocrystals

The formation of mixed stacking mode in the halogen cocrystals weakens the π–π interactions between DMCAT molecules and suppresses the aggregation-induced quenching effect, which in turn leads to enhanced emission.

Hierarchical supramolecular co-assembly formation employing multi-component light-harvesting charge transfer interactions giving rise to long-wavelength emitting luminescent microspheres.

Charge transfer (CT) interaction induced formation of a hierarchical supramolecular assembly has attracted attention due to its wide diversity of structural and functional characteristics. In the present work, we report the generation of green luminescent microspheres from the charge transfer interaction induced co-assembly of a bis-naphthyl dipicolinic amide (DPA) derivative with tetracyanobenzene (TCNB) for the first time. The properties of these self-assemblies were studied both in solution and the solid-state using spectroscopic and a variety of microscopy techniques. The X-ray crystal structure analysis showed a mixed stack arrangement of DPA and TCNB. The molecular orbital and energy level calculations confirm the charge transfer complex formation between DPA and TCNB. Furthermore, energy transfer was observed from the green luminescent CT complex to a red-emitting dye, pyronin Y, in the microsphere matrix, leading to the formation of a light-harvesting tri-component self-assembly.

Unravelling supramolecular features and opto-electronic properties of a binary charge transfer cocrystal of a blue fluorescent di-carbazole and TFT

Strong emission in a mixed stack charge transfer cocrystal comprising a bis-carbazole donor and TFT acceptor is elucidated by crystallographic, fluorescence and electronic property studies.

Comprehensively understanding the natures of crystal structure and magnetic behavior of a radical salt with regular mixed anion-cation stacks

In this study, the ionic crystals of [TTF][Au(mnt)2] (1; mnt2− ​= ​maleonitriledithiolate) have been grown using solvent diffusion method, and characterized by microanalysis, IR and Raman spectra, TG and powder X-ray diffraction (PXRD) techniques. Crystal of 1 belongs to triclinic space P−1 with Z ​= ​1 and both planar TTF+ and [Au(mnt)2]− locate at an inversion center, respectively. Cation (C) and anion (A) form uniform … CACA … mixed stack along [1 0 0] direction, and the charge-assisted H-bonds form between neighboring mixed stacks. 1 features an S ​= ​½ Heisenberg uniform antiferromagnetic (AFM) chain with J/kB ​= ​−2.08(1) K with g ​= ​2.0085 fixed, and absent spin-Peierls transition above 1.9 ​K, however, with long-range AFM ordering below 1.9 ​K. Additionally, 1 displays different style of mixed stack from [TTF][M(mnt)2] (M ​= ​Pd or Pt; … C2A2C2A2 … manner mixed stack) owing to strong AFM couplings (−J/kB ​> ​1000 ​K) in a [M(mnt)2]22− dimer.

Molecular self-assembly and fluorescence change of donor-acceptor cocrystal based on naphthalimide derivatives

<p indent="0mm">Organic cocrystals formed by two or more components through π-π interactions, charge transfer, and hydrogen/halogen bondings have attracted extensive attention because of their applications in the field of organic optoelectronics. Here, we prepared three charge-transfer cocrystals by solution-process self-assembly method which used Coronene and pyrene as donors, <italic>N</italic>,<italic>N</italic>′-dimethyl-1,4,5,8-naphthalenetetracarboxylic diimide (NDI-1) and <italic>N</italic>,<italic>N</italic>′-bis(ethyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NDI-2) as acceptors. Detailed structural characterizations clarified the mixed stacking mode in the charge-transfer cocrystals, and the supramolecular network was stabilized by C–H···O hydrogen bonds and π<italic>-</italic>π interactions. Interestingly, compared with the original donors, three cocrystals show different fluorescence emissions, which is related to the formation of charge-transfer states between donors and acceptors. This cocrystal strategy provides a new opportunity for the design of new organic light-emitting charge-transfer complexes.

Tetramethylbenzidine–TetrafluoroTCNQ (TMB–TCNQF4): A Narrow-Gap Semiconducting Salt with Room-Temperature Relaxor Ferroelectric Behavior

We present an extension and revision of the spectroscopic and structural data of the mixed stack charge transfer (CT) crystal 3,3$^\prime$,5,5$^\prime$-tetramethylbenzidine--tetrafluoro-tetracyanoquinodimethane (TMB-TCNQF4), associated with new electric and dielectric measurements. Refinement of syncrotron structural data at low temperature has led to revise the previously reported [Phys. Rev. Mat. 2, 024602 (2018)] $C2/m$ structure. The revised structure is $P2_1/m$, with two dimerized stacks per unit cell, and is consistent with the vibrational data. However, polarized Raman data in the low-frequency region also indicate that by increasing temperature above 200 K the structure presents an increasing degree of disorder mainly along the stack axis. X-ray diffraction data at room temperature have confirmed that the correct structure is $P2_1/m$ -- no phase transitions -- but did not allow to definitely substantiate the presence of disorder. On the other hand, dielectric measurement have evidenced a typical relaxor ferroelectric behavior already at room temperature, with a peak in real part of dielectric constant $\epsilon'(T,\nu)$ around 200 K and 0.1 Hz. The relaxor behavior is explained in terms of the presence of spin solitons separating domains of opposite polarity that yield to ferroelectric nanodomains. TMB-TCNQF4 is confirmed to be a narrow gap band semiconductor ($E_a \sim 0.3$ eV) with room temperature conductivity of $\sim 10^{-4}~ \Omega^{-1}$ cm$^{-1}$.

Strain-induced abnormal Ge/Si inter-diffusion during hetero-epitaxy process

In this work, strain-induced abnormal inter-diffusion at Ge/Si interface during the hetero-epitaxy process is reported. By mediating the residual strain in Ge epitaxial layer, we reveal an anisotropic inter-diffusion phenomenon during conventional two-step growth process of Ge film. The diffusion length of Ge in Si substrate is found to greatly exceed the theoretical value derived from generalized diffusion model, indicating that the inter-diffusion at the interface is considerably enhanced by the residual strain at growth temperature of 600 °C. The diffusion profiles are {111}-faceted, and a rather abrupt diffusion-front is pictured implying that the Fick's law becomes invalid during the inter-diffusion process. Extensive 60° mixed dislocation and stacking faults (SFs) are identified at the diffusion front signifying that the slip of {111} lattice plane is favored at the diffusion front. The net increase of inter-diffusivity at the Ge/Si interface is ascribed to the slip of {111} lattice plane and the preferential diffusion along the dislocation segments, both of which allow the Ge(Si) atoms to migrate across the diffusion front more easily. The findings in this study may cast light on the formation of micro-roughness at interface of Ge/Si hetero-structures during the epitaxy process.