Hexagonal Transition Research Articles

(Invited) Two-Dimensional Materials for Electronic Devices: Transition-Metal Dichalcogenides and Topological Insulators

As scaling of electronic devices has reduced dimensions into the nanoscale regime, investigations towards intrinsically two-dimensional (2D) materials are being pursued. Examples of 2D materials under investigation are monolayers of graphite, transition-metal dichalocogenides (TMDs), tellurium, and 2D topological insulators (TIs). To enable practical applications however, 2D materials must outperform currently employed three-dimensional materials. Properties of interest are mobilities, device drive current, and dielectric response but these properties are not yet fully known and understood in 2D materials. We present a theoretical investigation of the dielectric response in transition-metal dichalcogenides. We determine a method to define and extract, from first principles, the dielectric constant of a two-dimensional material. We apply this method to calculate the low-frequency and high-frequency dielectric constant of semiconducting TMDs, both those in the hexagonal phase, e.g. MoS2, WSe2, as well as those in the tetragonal phase, e.g. ZrS2 and HfSe2. We find that the in-plane static dielectric constant for the TMDs in the tetragonal phase is very large, which will give rise to large polar-optical phonon scattering and low mobilities in tetragonal TMDs. [1] We also present a new strategy to deal with both the high concentration of defects found in 2D materials as well as the difficulty of making perfectly straight edges. The strategy proposes to make transistors out of 2D TIs since 2D TI materials host topologically protected edge states that are robust against perturbations. In fact, we show how 2D TI field-effect transistor performance is actually improved in the presence of defects. The high on-current, related to the high electron velocity in these devices gives 2D field-effect transistors a predicted performance that exceeds that of conventional field-effect transistors. [2, 3] [1] A. Laturia, M. L. Van de Put, and W. G. Vandenberghe, "Dielectric properties of hexagonal boron nitride and transition metal dichalcogenides: from monolayer to bulk," npj 2D Materials and Applications, vol. 2, no. 1, p. 6, 2018/03/08 2018. [2] W. G. Vandenberghe and M. V. Fischetti, "Imperfect two-dimensional topological insulator field-effect transistors," Nature Communications, vol. 8, 2017. [3] S. Tiwari, M. L. Van de Put, B. Sorée, and W. G. Vandenberghe, "Carrier Transport in a Two-Dimensional Topological Insulator Nanoribbon in the Presence of Vacancy Defects," in 2018 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD), 2018, pp. 92-96: IEEE.

Freestanding carbon nanomembranes and graphene monolayers nanopatterned via EUV interference lithography

Two-dimensional materials, such as graphene, molecular nanosheets, hexagonal boron nitride or transition metal dichalcogenides have recently attracted substantial interest due to their potential use in electronics, chemical and biological sensors, nanooptics, and catalysis. For most of these applications, the functional nanostructures have to be prepared lithographically. In this respect, extreme ultraviolet interference lithography provides both high-resolution patterning with an ultimate limit in the sub-10 nm range and high throughput capability. Here we present the preparation of nanopatterned 1 nm thick freestanding molecular nanosheets—carbon nanomembranes (CNMs)—and single layer graphene employing this method and their characterization with a helium ion microscope. We demonstrate periodic arrays of suspended nanostructures of CNMs and graphene including nanoribbons with width between 20 nm and 500 nm and nanomeshes with openings between 140 nm and 300 nm and mesh lines down to 90 nm.

Direct atomic identification of cation migration induced gradual cubic-to-hexagonal phase transition in Ge2Sb2Te5

GeTe-Sb2Te3 pseudobinary system, especially Ge2Sb2Te5 alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge2Sb2Te5 during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge2Sb2Te5 alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below: -Te-Sbx/Gey-Te-Gex/Sby-Te-Gex/Sby-Te-Sbx/Gey-Te- (x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge2Sb2Te5 alloy.

Valley selective optical control of excitons in 2D semiconductors using a chiral metasurface [Invited

Recent advances in condensed matter physics have shown that the valley degree of freedom of electrons in 2D materials with hexagonal symmetry, such as graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs), can be efficiently exploited, leading to the emergent field of valleytronics, which offers unique opportunities for efficient data transfer, computing and storage. The ability to couple the valley degree of freedom of electrons with light can further expand the ways one manipulates this degree of freedom, thus envisioning a new class of solid-state-photonic interfaces and devices. Besides this expansion of control of valley by light-waves, coupling of photons with valley-polarized electrons can dramatically expand the landscape of available optical responses, which may bring new means of controlling light in photonic devices. In this work we design such hybrid solid-state photonic metasurface integrating 2D TMD and photonic all-dielectric metasurface. While TMD is naturally endowed with the property of valley to optical-polarization coupling, the photonic metasurface is designed to produce chiral field which selectively couples to the valley degree of freedom of solid-state TMD component. We experimentally demonstrate that such coupling leads to controlled valley polarization due to the coupling of 2D materials with the chiral photonic metasurface. The measured emission from valley excitons in this hybrid system yields the preferential emission of specific helicity.

In situ TEM and half cell investigation of sodium storage in hexagonal FeSe nanoparticles.

A hexagonal FeSe nanoparticle anode with a novel reaction mechanism and mechanical stability may fully facilitate the desirable rate capability and cycling performance in sodium-ion batteries. In situ TEM reveals that hexagonal FeSe nanoparticle transition to the Fe and Na2Se phase during sodiation, while the products transform to the tetragonal FeSe phase after desodiation.

Dirac–Weyl semimetal phase in noncentrosymmetric transition metal monochalcogenides MoTe and WTe

We investigated the topological properties of hexagonal transition metal monochalcogenides (TMMs) MoTe and WTe by combining first-principles calculations, the Wannier-based tight-binding method and the low energy k·p effective model.

Electronic structure theory of strained two-dimensional materials with hexagonal symmetry

We derive electronic tight-binding Hamiltonians for strained graphene, hexagonal boron nitride and transition metal dichalcogenides based on Wannier transformation of {\it ab initio} density functional theory calculations. Our microscopic models include strain effects to leading order that respect the hexagonal crystal symmetry and local crystal configuration, and are beyond the central force approximation which assumes only pair-wise distance dependence. Based on these models, we also derive and analyze the effective low-energy Hamiltonians. Our {\it ab initio} approaches complement the symmetry group representation construction for such effective low-energy Hamiltonians and provide the values of the coefficients for each symmetry-allowed term. These models are relevant for the design of electronic device applications, since they provide the framework for describing the coupling of electrons to other degrees of freedom including phonons, spin and the electromagnetic field. The models can also serve as the basis for exploring the physics of many-body systems of interesting quantum phases.

Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations.

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag1- xCuS and AgCu1- xS samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag+/Cu+) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag1- xCuS and AgCu1- xS samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.

Quasi-Two-Dimensional β-Ga2O3 based Hetero-Structure Transistors

β-Ga2O3 is a wide band gap (4.8 eV) semiconducting material with outstanding chemical and thermal stabilities. As the breakdown voltage of β-Ga2O3 is estimated to be as high as 8 MV/cm, it can be applied to high-power electronics. After β-Ga2O3 thin layers were successfully exfoliated from bulk single crystals by mechanical exfoliation method, the study of high power 2D nano-electronics using β-Ga2O3 received a considerable attention. Though β-Ga2O3 is not a two-dimensional Van der Waals material like graphene and MoS2, a bulk β-Ga2O3 crystal can be easily cleaved into thin nano-flakes in the (100) direction; it has monoclinic structure and the lattice constant is larger along the cleaved plane. Moreover, this ultrathin β-Ga2O3 flakes can be integrated with other 2D materials such as hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs) to make down-scaled, flexible devices. We demonstrated two types of heterostructure devices (MISFETs and JFETs) based on β-Ga2O3 flakes and 2D materials using PDMS stamp method and photolithography. β-Ga2O3/h-BN hetero-structure based quasi-two-dimensional metal insulator semiconductor field-effect transistors(MISFETs) are fabricated. The h-BN dielectric, which has an extraordinarily flat and clean surface, provides a minimal density of charged impurities at the interface between β-Ga2O3 and h-BN, resulting in excellent device performances. The fabricated MISFETs showed superior drain current modulation with high on/off ratio and low gate leakage current. The double-gating of the fabricated device was demonstrated by biasing both top and bottom gates, achieving the modulation of the threshold voltage. N-channel β-Ga2O3/WSe2 heterostructure junction field effect transistors (JFETs) are demonstrated with good gate modulation and negligible hysteresis. Tungsten diselenide (WSe2) is mostly known as a stable p-type 2D layered material. The fabricated heterostructure JFETs also showed excellent ambient stability. The successful JFET operation is attributed to Van der Waals junction interface between β-Ga2O3 and WSe2. These heterostructured wide-band-gap nanodevices show a new route toward stable and high-power nanoelectronic devices. The details of the fabrication procedure and results will be presented.

Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries.

Combining the first-principles density functional method and crystal structure prediction techniques, we report a series of hexagonal two-dimensional transition metal borides including Sc2B2, Ti2B2, V2B2, Cr2B2, Y2B2, Zr2B2, and Mo2B2. Their dynamic and thermal stabilities are testified by phonon and molecular dynamics simulations. We investigate the potential of the two-dimensional Ti2B2 monolayer as an anode material for Li-ion and Na-ion batteries. The Ti2B2 monolayer possesses high theoretical specific capacities of 456 and 342 mA h g-1 for Li and Na, respectively. The very high Li/Na diffusivity with an ultralow energy barrier of 0.017/0.008 eV indicates an excellent charge-discharge capability. In addition, good electronic conductivity during the whole lithiation process is found by electronic structure calculations. The very small change in volume after the adsorption of one, two, and three layers of Li and Na ions indicates that the Ti2B2 monolayer is robust. These results highlight the suitability of Ti2B2 monolayer as well as the other two-dimensional transition metal borides as excellent anode materials for both Li-ion and Na-ion batteries.

Magnetism and superconductivity in the layered hexagonal transition metal pnictides

We investigate the electronic and magnetic structures of the 122 (AM$_2$B$_2$) hexagonal transition-metal pnictides with A=(Sr, Ca), M=(Cr, Mn, Fe, Co, Ni) and B=(As, P, Sb). It is found that the family of materials share critical similarities with those of tetragonal structures that include the famous iron-based high temperature superconductors. In both families, the next nearest neighbor(NNN) effective antiferromagnetic(AFM) exchange couplings reach the maximum value in the iron-based materials. While the NNN couplings in the latter are known to be responsible for the C-type AFM state and to result in the extended s-wave superconducting state upon doping, they cause the former to be extremely frustrated magnetic systems and can lead to an time reversal symmetry broken $d+id$ superconducting state upon doping. The iron-based compounds with the hexagonal structure, thus if synthesized, can help us to determine the origin of high temperature superconductivity.

Role of indium tin oxide electrode on the microstructure of self-assembled WO3-BiVO4 hetero nanostructures

Self-assembled WO3-BiVO4 nanostructured thin films were grown on a (001) yttrium stabilized zirconia (YSZ) substrate by the pulsed laser deposition method with and without the indium tin oxide (ITO) bottom electrode. Their microstructures including surface morphologies, crystalline phases, epitaxial relationships, interface structures, and composition distributions were investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray energy dispersive spectroscopy. In both samples, WO3 formed nanopillars embedded into the monoclinic BiVO4 matrix with specific orientation relationships. In the sample with the ITO bottom electrode, an atomically sharp BiVO4/ITO interface was formed and the orthorhombic WO3 nanopillars were grown on a relaxed BiVO4 buffer layer with a mixed orthorhombic and hexagonal WO3 transition layer. In contrast, a thin amorphous layer appears at the interfaces between the thin film and the YSZ substrate in the sample without the ITO electrode. In addition, orthorhombic Bi2WO6 lamellar nanopillars were formed between WO3 and BiVO4 due to interdiffusion. Such a WO3-Bi2WO6-BiVO4 double heterojunction photoanode may promote the photo-generated charge separation and further improve the photoelectrochemical water splitting properties.

Near-field edge fringes at sharp material boundaries.

We have studied the formation of near-field fringes when sharp edges of materials are imaged using scattering-type scanning near-field optical microscope (s-SNOM). The materials we have investigated include dielectrics, metals, a near-perfect conductor, and those that possess anisotropic permittivity and hyperbolic dispersion. For our theoretical analysis, we use a technique that combines full-wave numerical simulations of tip-sample near-field interaction and signal demodulation at higher orders akin to what is done in typical s-SNOM experiments. Unlike previous tip-sample interaction near-field models, our advanced technique allows simulation of the realistic tip and sample structure. Our analysis clarifies edge imaging of recently emerged layered materials such as hexagonal boron nitride and transition metal dichalcogenides (in particular, molybdenum disulfide), as well as traditional plasmonic materials such as gold. Hexagonal boron nitride is studied at several wavelengths, including the wavelength where it possesses excitation of phonon-polaritons and hyperbolic dispersion. Based on our results of s-SNOM imaging in different demodulation orders, we specify resonant and non-resonant types of edges and describe the edge fringes for each case. We clarify near-field edge-fringe formation at material sharp boundaries, both outside bright fringes and the low-contrast region at the edge, and elaborate on the necessity of separating them from propagating waves on the surface of polaritonic materials.

Theoretical calculations of hardness and metallicity for multibond hexagonal 5d transition metal diborides with ReB2 structure

The hardness, electronic, and elastic properties of 5d transition metal diborides with ReB2 structure are studied theoretically by using the first principles calculations. The calculated results are in good agreement with the previous experimental and theoretical results. Empirical formulas for estimating the hardness and partial number of effective free electrons for each bond in multibond compounds with metallicity are presented. Based on the formulas, IrB2 has the largest hardness of 21.8 GPa, followed by OsB2 (21.0 GPa) and ReB2 (19.7 GPa), indicating that they are good candidates as hard materials.

Metal Thio- and Selenophosphates as Multifunctional van der Waals Layered Materials.

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow-on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase-transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate- to wide-bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought-after properties of complex oxides with electronic functions of 2D and quasi-2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.

Transformation behavior and magnetocaloric effect in Mn1−xCrxCoGe (x = 0.04 and 0.11) melt-spun ribbons tailored by heat treatment

Recently, MnNiGe and MnCoGe intermetallic compounds have received a considerable attention due to the coupled tunable martensitic and magnetic transitions as well as giant magnetocaloric effect (MCE) they exhibit. In this work, by doping with Cr and varying of the annealing regime, a merged magnetostructural transition between a ferromagnetic orthorhombic and a paramagnetic hexagonal structure was received near room temperature in Mn1−xCrxCoGe melt-spun ribbons with x=0.04 and 0.11. Structural and magnetic characterizations have been performed by X-ray diffraction, differential scanning calorimetry and magnetization measurements. It is found that, in ribbons with x=0.04 (0.11) annealed at 1148K, the peak value of the magnetic entropy change and the refrigerant capacity through the orthorhombic to hexagonal transition strongly increase with the annealing time from 7.3 (9.4) Jkg−1K−1 to 10.6 (13.7) Jkg−1K−1 and from 182 (235) Jkg−1 to 214 (267) Jkg−1, respectively, at 5T for the time increase from 4h to 8h. Compared to bulk alloys of similar composition, the fabricated melt-spun ribbons show much broader working temperature range and a higher cooling efficiency.

The α–ω phase transition in shock-loaded titanium

We present a series of experiments probing the martensitic α–ω (hexagonal close-packed to simple hexagonal) transition in titanium under shock-loading to peak stresses around 15 GPa. Gas-gun plate impact techniques were used to locate the α–ω transition stress with a laser-based velocimetry diagnostic. A change in the shock-wave profile at 10.1 GPa suggests the transition begins at this stress. A second experiment shock-loaded and then soft-recovered a similar titanium sample. This recovered material was then analyzed with electron-backscatter diffraction methods, revealing on average approximately 65% retained ω phase. Furthermore, based on careful analysis of the microstructure, we propose that the titanium never reached a full ω state, and that there was no observed phase-reversion from ω to α. Texture analysis suggests that any α titanium found in the recovered sample is the original α. At these stresses, the data show that both the α and ω phases are stable and can coexist even though the shock-wave presents as steady-state.

Band structure engineered layered metals for low-loss plasmonics

Plasmonics currently faces the problem of seemingly inevitable optical losses occurring in the metallic components that challenges the implementation of essentially any application. In this work, we show that Ohmic losses are reduced in certain layered metals, such as the transition metal dichalcogenide TaS2, due to an extraordinarily small density of states for scattering in the near-IR originating from their special electronic band structure. On the basis of this observation, we propose a new class of band structure engineered van der Waals layered metals composed of hexagonal transition metal chalcogenide-halide layers with greatly suppressed intrinsic losses. Using first-principles calculations, we show that the suppression of optical losses lead to improved performance for thin-film waveguiding and transformation optics.

Tracing the coupled atomic shear and shuffle for a cubic to a hexagonal crystal transition

Tracing the rearranged atoms in the first-order phase transformation is unrealistic due to the discrete structure change. Here we report that, by tuning a nano-scale decomposition in a titanium alloy, the bcc crystal distorts successively toward the hcp crystal by keeping an orthorhombic symmetry. Thus, the shear-shuffle relationship is traced experimentally to enrich the well-known Burgers mechanism. Our results reveal also that the successive tuning on crystal structure at the atomic level leads to some novel properties which are unexpected from the discrete phase transformations.

Thermodynamic study of a plastic columnar discotic material 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene dispersed with gold nanoparticles under elevated pressure

We report a systematic work on the effect of pressure on thermodynamic properties of a plastic columnar discotic liquid crystal (DLC), namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene (HAT4) dispersed with gold nanoparticles up to 1000 MPa by using a high-pressure differential scanning calorimeter. The goal is to determine how elevated pressure can be used to modify the physical properties of plastic columnar DLC and their nanocomposites via changes in inter- and intra-molecular distances of discotics. We find that the columnar hexagonal plastic to isotropic liquid (Colhp–IL) phase transition temperatures and enthalpy decrease with increasing pressure. However, crystal to plastic columnar hexagonal (Cr–Colhp) transition temperature remains almost unchanged for pure and their nanocomposites. The effect of pressure on the phase behaviour of DLC has been studied to elucidate the nature of the mesophase behaviour.