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1.
Nano Lett ; 2020 Jul 08.
Artigo em Inglês | MEDLINE | ID: mdl-32639741

RESUMO

The experimental identification of structural transitions in layered black phosphorus (BP) under mechanical stress is essential to extend its application in micro-electro\-mechanical (MEMS) devices under harsh conditions. High-pressure Raman spectroscopic analysis of BP flakes suggests a transition pressure at ~4.2 GPa, where the BP's crystal structure progressively transforms from an orthorhombic to a rhombohedral symmetry (blue Phosphorus, bP). The phase transition has been identified by observing a transition from blueshift to redshift of the in-plane characteristic Raman modes (B2g and A2g) with increasing pressure. Recovery of the vibrational frequencies for all three characteristic Raman modes confirms the reversibility of the structural phase transition. First-principles calculations provide insight into the behavior of the Raman modes of BP under high pressure and reveal the mechanism responsible for the partial phase transition from BP to bP, corresponding to a metastable equilibrium state where both phases coexist.

2.
ACS Nano ; 2020 Apr 29.
Artigo em Inglês | MEDLINE | ID: mdl-32330006

RESUMO

Hotspot engineering has the potential to transform the field of surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive and reproducible detection of analytes. However, the ability to controllably generate SERS hotspots, with desired location and geometry, over large-area substrates, has remained elusive. In this study, we sculpt artificial edges in monolayer molybdenum disulfide (MoS2) by low-power focused laser-cutting. We find that when gold nanoparticles (AuNPs) are deposited on MoS2 by drop-casting, the AuNPs tend to accumulate predominantly along the artificial edges. First-principles density functional theory (DFT) calculations indicate strong binding of AuNPs with the artificial edges due to dangling bonds that are ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation of AuNPs along the artificial edges intensifies plasmonic effects in these regions, creating hotspots exclusively along the artificial edges. DFT further indicates that adsorption of AuNPs along the artificial edges prompts a transition from semiconducting to metallic behavior, which can further intensify the plasmonic effect along the artificial edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2 surface (away from the cut edges) or along the natural (passivated) edges of the MoS2 sheet. To demonstrate the practical utility of this concept, we use our substrate to detect Rhodamine B (RhB) with a large SERS enhancement (∼104) at the hotspots for RhB concentrations as low as ∼10-10 M. The single-step laser-etching process reported here can be used to controllably generate arrays of SERS hotspots. As such, this concept offers several advantages over previously reported SERS substrates that rely on electrochemical deposition, e-beam lithography, nanoimprinting, or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety of 2D material platforms.

3.
Adv Mater ; : e1906054, 2020 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-32048409

RESUMO

Graphene nanoribbons (GNRs) have attracted much interest due to their largely modifiable electronic properties. Manifestation of these properties requires atomically precise GNRs which can be achieved through a bottom-up synthesis approach. This has recently been applied to the synthesis of width-modulated GNRs hosting topological electronic quantum phases, with valence electronic properties that are well captured by the Su-Schrieffer-Heeger (SSH) model describing a 1D chain of interacting dimers. Here, ultralow bandgap GNRs with charge carriers behaving as massive Dirac fermions can be realized when their valence electrons represent an SSH chain close to the topological phase boundary, i.e., when the intra- and interdimer coupling become approximately equal. Such a system has been achieved via on-surface synthesis based on readily available pyrene-based precursors and the resulting GNRs are characterized by scanning probe methods. The pyrene-based GNRs (pGNRs) can be processed under ambient conditions and incorporated as the active material in a field effect transistor. A quasi-metallic transport behavior is observed at room temperature, whereas at low temperature, the pGNRs behave as quantum dots showing single-electron tunneling and Coulomb blockade. This study may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.

4.
J Am Chem Soc ; 141(48): 18994-19001, 2019 Dec 04.
Artigo em Inglês | MEDLINE | ID: mdl-31689101

RESUMO

Electron-phonon coupling in two-dimensional nanomaterials plays a fundamental role in determining their physical properties. Such interplay is particularly intriguing in semiconducting black phosphorus (BP) due to the highly anisotropic nature of its electronic structure and phonon dispersions. Here we report the direct observation of symmetry-dependent electron-phonon coupling in BP by performing the polarization-selective resonance Raman measurement in the visible and ultraviolet regimes, focusing on the out-of-plane Ag1 and in-plane Ag2 phonon modes. Their intrinsic resonance Raman excitation profiles (REPs) were extracted and quantitatively compared. The in-plane Ag2 mode exhibits remarkably strong resonance enhancement across the excitation wavelengths when the excitation polarization is parallel to the armchair (Ag2//AC) direction. In contrast, a dramatically weak resonance effect was observed for the same mode with the polarization parallel to zigzag (Ag2//ZZ) direction and for the out-of-plane Ag1 mode (Ag1//AC and Ag1//ZZ). Analysis on quantum perturbation theory and first-principles calculations on the anisotropic electron distributions in BP demonstrated that electron-phonon coupling considering the symmetry of the involved excited states and phonon vibration patterns is responsible for this phenomenon. Further analysis of the polarization-dependent REPs for Ag phonons allows us to resolve the existing controversies on the physical origin of Raman anomaly in BP and its dependence on excitation energy, sample thickness, phonon modes, and crystalline orientation. Our study gives deep insights into the underlying interplay between electrons and phonons in BP and paves the way for manipulating the electron-phonon coupling in anisotropic nanomaterials for future device applications.

5.
ACS Nano ; 13(11): 13083-13091, 2019 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-31573799

RESUMO

Graphene nanoribbons (GNRs) have attracted considerable interest, as their atomically tunable structure makes them promising candidates for future electronic devices. However, obtaining detailed information about the length of GNRs has been challenging and typically relies on low-temperature scanning tunneling microscopy. Such methods are ill-suited for practical device application and characterization. In contrast, Raman spectroscopy is a sensitive method for the characterization of GNRs, in particular for investigating their width and structure. Here, we report on a length-dependent, Raman-active low-energy vibrational mode that is present in atomically precise, bottom-up-synthesized armchair graphene nanoribbons (AGNRs). Our Raman study demonstrates that this mode is present in all families of AGNRs and provides information on their length. Our spectroscopic findings are corroborated by scanning tunneling microscopy images and supported by first-principles calculations that allow us to attribute this mode to a longitudinal acoustic phonon. Finally, we show that this mode is a sensitive probe for the overall structural integrity of the ribbons and their interaction with technologically relevant substrates.

6.
Nano Lett ; 19(10): 7301-7308, 2019 10 09.
Artigo em Inglês | MEDLINE | ID: mdl-31550164

RESUMO

The realization of on-chip quantum networks requires tunable quantum states to encode information carriers on them. We show that Cr2Ge2Te6 (CGT) as a van der Waals ferromagnet can enable magnetic proximity coupling to site-controlled quantum emitters in WSe2, giving rise to ultrahigh exciton g factors up to 20 ± 1. By comparing the same site-controlled quantum emitter before and after ferromagnetic proximity coupling, we also demonstrate a technique to directly measure the resulting magnetic exchange field (MEF) strength. Experimentally determined values of MEF up to 1.2 ± 0.2 meV in the saturation regime approach the theoretical limit of 2.1 meV that was determined from density functional theory calculations of the CGT/WSe2 heterostructure. Our work extends the on-chip control of magneto-optical properties of excitons via van der Waals heterostructures to solid-state quantum emitters.

7.
ACS Nano ; 13(9): 10456-10468, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31436958

RESUMO

We report a temperature-dependent Raman spectroscopy study of few-layer black phosphorus (BP) with varied incident polarization and sample thickness. The Raman-active modes Ag1, B2g, and Ag2 exhibit a frequency downshift, while their line width tends to increase with increasing temperature. To understand the details of these phenomena, we perform first-principles density functional theory calculations on freestanding monolayer BP. The effect of thermal expansion is included by constraining the temperature-dependent lattice constant. The study of the temperature-induced shift of the phonon frequencies is carried out using ab initio molecular dynamics simulations. The normal-mode frequencies are calculated by identifying the peak positions from the magnitude of the Fourier transform of the total velocity autocorrelation. Anharmonicity induces a frequency shift for each individual mode, and the three- and four-phonon process coefficients are extracted. These results are compared with those obtained from many-body perturbation theory, giving access to phonon lifetimes and lattice thermal conductivity. We establish that the frequency downshift is primarily due to phonon-phonon scattering while thermal expansion only contributes indirectly by renormalizing the phonon-phonon scattering. Overall, the theoretical results are in excellent agreement with experiment, thus showing that controlling phonon scattering in BP could result in better thermoelectric devices or transistors that dissipate heat more effectively when confined to the nanoscale.

8.
J Phys Condens Matter ; 31(45): 455402, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31342917

RESUMO

Density-functional theory was used to investigate the effect of atomic impurities on the structural and vibrational properties of zircon (tetragonal ZrSiO4). Atomic impurities considered include radioactive elements U and Th, as well as Hf, Sn, and Ti, substituted on the Zr-site. Using the supercell approach to model a range of substitutional concentrations, impurities were found to cause changes in the volume of the host lattice. This effect was shown to be partially equivalent to the application of a lattice strain. This quantum-based finding is in excellent agreement with the heuristic lattice-strain model traditionally employed in the geosciences to account for the compatibility of impurities in host lattices. Vibrational properties of substituted zircon were also investigated in order to provide a quantum mechanical understanding of Raman spectroscopy measurements on natural zircon. The computational analysis reproduces existing experimental data reported for uranium-substituted zircon and provides general predictive trends for other impurities including Th, Hf, Sn, and Ti. The insights gained by this study regarding the Raman signature of the presence of substitutional impurities set the groundwork for future study of the more substantial lattice disruptions that characterize radiation damage due to alpha decay in zircon.

9.
Chemistry ; 25(52): 12074-12082, 2019 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-31190412

RESUMO

A bottom up method for the synthesis of unique tetracene-based nanoribbons, which incorporate cyclobutadiene moieties as linkers between the acene segments, is reported. These structures were achieved through the formal [2+2] cycloaddition reaction of ortho-functionalized tetracene precursor monomers. The formation mechanism and the electronic and magnetic properties of these nanoribbons were comprehensively studied by means of a multitechnique approach. Ultra-high vacuum scanning tunneling microscopy showed the occurrence of metal-coordinated nanostructures at room temperature and their evolution into nanoribbons through formal [2+2] cycloaddition at 475 K. Frequency-shift non-contact atomic force microscopy images clearly proved the presence of bridging cyclobutadiene moieties upon covalent coupling of activated tetracene molecules. Insight into the electronic and vibrational properties of the so-formed ribbons was obtained by scanning tunneling microscopy, Raman spectroscopy, and theoretical calculations. Magnetic properties were addressed from a computational point of view, allowing us to propose promising candidates to magnetic acene-based ribbons incorporating four-membered rings. The reported findings will increase the understanding and availability of new graphene-based nanoribbons with high potential in future spintronics.

10.
Nanoscale ; 11(16): 7682-7689, 2019 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-30946426

RESUMO

Ullmann coupling or, more generally, dehalogenative aryl-aryl coupling, is one of the most widely exploited chemical reactions to obtain one- and two-dimensional polymers on metal surfaces. It is generally described as a two-step reaction: (i) dehalogenation, resulting in the formation of a stable intermediate organometallic phase and subsequent (ii) C-C coupling. The topology of the resulting polymer depends on the number and positions of the halogen atoms in the haloaromatic precursor, although its orientation and order are determined by the structure of the intermediate phase. Hitherto, only one intermediate structure, identified as an organometallic (OM) phase, has been reported for such a reaction. Here we demonstrate the formation of two distinct OM phases during the temperature-induced growth of poly(para-phenylene) from 1,4-dibromobenzene precursors on Cu(110). Beyond the already known linear-OM chains, we show that a phase reorganization to a chessboard-like 2D-OM can be activated in a well-defined temperature range. This new intermediate phase, revealed only when the reaction is carried out at low molecular coverages, was characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, and modeled by density functional theory calculations. Our data show that the 2D-OM remains stable after cooling down the sample and is stabilized by four-Cu clusters at each node. The observation of such unexpected intermediate phase shows the complexity of the mechanisms underlying on-surface synthesis and broadens the understanding of Ullmann coupling, which continues to be astonishing despite its extensive use.

11.
Nat Commun ; 10(1): 1764, 2019 04 16.
Artigo em Inglês | MEDLINE | ID: mdl-30992432

RESUMO

Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g-1 @200 mA g-1 current density) and rate capability (~70 mAh g-1 @1000 mA g-1), while achieving capacity retention close to 100% after 400 charge-discharge steps.

12.
Carbohydr Polym ; 213: 147-158, 2019 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-30879654

RESUMO

The rheological behavior of blends of a fractionated maltopolymer (Mw = 1.4⋅104 Da) and the disaccharide maltose is investigated as a function of water content and temperature, with emphasis on the viscosity and molecular relaxations in the approach to the glass transition. Shear rheology is combined with dynamic mechanical thermal analysis to probe viscosities between 1 mPa s and 1012 Pa s. Differential scanning calorimetry is used to determine glass transition and enthalpy relaxation of the carbohydrate blends. The rheology data are fitted with a modified version of Williams-Landel-Ferry equation (Williams et al., 1955). The fragility of the blends is quantified using Angell's fragility parameters m and F1/2 (Angell, 1991) and Roos' strength parameter S (Roos, 1995b). The increase in fragility of the maltopolymer systems with increasing water and maltose contents is interpreted as a reduction of the entanglement density and an interference of water molecules with the hydrogen bonding between the carbohydrate chains.


Assuntos
Maltose/química , Polímeros/química , Reologia , Água/química , Temperatura
13.
J Phys Chem B ; 123(10): 2342-2353, 2019 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-30768898

RESUMO

Solid-state nanopores (SSN) made of two-dimensional materials such as molybdenum disulfide (MoS2) have emerged as candidate devices for biomolecules sequencing. SSN sequencing is based on measuring the variations in ionic conductance as charged biomolecules translocate through nanometer-sized channels, in response to an external voltage applied across the membrane. Although several experiments on DNA translocation through SSNs have been performed in the past decade, translocation of proteins has been less studied, partly due to small protein size and detection limits. Moreover, the threading of proteins through nanopore channels is challenging, because proteins can exhibit neutral global charge and not be sensitive to the electric field. In this paper, we investigate the translocation of lysine residues and a model protein with polylysine tags through MoS2 nanoporous membranes using molecular dynamics simulations. Adding lysine tags to biological peptides is the method proposed here to promote the entrance of proteins through SSN. Specifically, we study the relationship existing between the translocation events and the ionic conductance signal drops. We show that individual lysine residues translocate easily through MoS2 nanopores, but the translocation speed is extremely fast, which leads to indiscernible ionic conductance drops. To reduce the translocation speed, we demonstrate that increasing the thickness of the membrane from single-layer to bilayer MoS2 reveals a stepwise process of translocation with discernible conductance drops that could be measured experimentally. Finally, a study of the threading of proteins with polylysine tags through MoS2 nanopores is presented. The addition of the positively charged tag to the neutral protein allows the threading and full translocation of the protein through the pore (at least two lysine residues are necessary in this case to observe translocation) and a similar sequence of translocation events is detected, independently of the tag length.

14.
Annu Rev Food Sci Technol ; 10: 457-478, 2019 03 25.
Artigo em Inglês | MEDLINE | ID: mdl-30633567

RESUMO

Water is ubiquitous in the environment and is present to varying degrees even within dry powder products and most ingredients. Water migration between the environment and a solid, or between different components of a product, may lead to detrimental physical and chemical changes. In efforts to optimize the quality of dry products, as well as the efficiency of production practices, it is crucial to understand the cause-effect relationships of water interactions with different solids. Therefore, this review addresses the basis of moisture migration in dry products, and the modes of water vapor interactions with crystalline and amorphous solids (e.g., adsorption, capillary condensation, deliquescence, crystal hydrate formation, absorption into amorphous solids) and related moisture-induced phase and state changes, and provides examples of how these moisture-induced changes affect the quality of the dry products.

15.
ACS Nano ; 13(2): 2481-2489, 2019 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-30673215

RESUMO

Isotopes represent a degree of freedom that might be exploited to tune the physical properties of materials while preserving their chemical behaviors. Here, we demonstrate that the thermal properties of two-dimensional (2D) transition-metal dichalcogenides can be tailored through isotope engineering. Monolayer crystals of MoS2 were synthesized with isotopically pure 100Mo and 92Mo by chemical vapor deposition employing isotopically enriched molybdenum oxide precursors. The in-plane thermal conductivity of the 100MoS2 monolayers, measured using a non-destructive, optothermal Raman technique, is found to be enhanced by ∼50% compared with the MoS2 synthesized using mixed Mo isotopes from naturally occurring molybdenum oxide. The boost of thermal conductivity in isotopically pure MoS2 monolayers is attributed to the combined effects of reduced isotopic disorder and a reduction in defect-related scattering, consistent with observed stronger photoluminescence and longer exciton lifetime. These results shed light on the fundamentals of 2D nanoscale thermal transport important for the optimization of 2D electronic devices.

16.
Phys Chem Chem Phys ; 21(1): 322-328, 2018 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-30520896

RESUMO

We propose an extension of the traditional valence force field model to allow for the effect of electronic polarization to be included in the inter-atomic potential. Using density functional theory as a reference, this model is parameterized for the specific case of single-layer black phosphorus by fitting the phonon dispersion relation over the entire Brillouin zone. The model is designed to account for the effect of induced dipole interaction on the long-wavelength (|q[combining right harpoon above]| → 0) modes for the case of homopolar covalent crystals. We demonstrate that the near Γ-point frequencies of the IR-active modes are substantially damped by the inclusion of the induced dipole interaction, in agreement with experiment. The fitting procedure outlined here allows for this model to be adapted to other materials, including but not limited to two-dimensional crystals.

17.
Appl Phys Rev ; 5(1)2018 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-30397419

RESUMO

We review the concept of stochasticity-i.e., unpredictable or uncontrolled fluctuations in structure, chemistry, or kinetic processes-in materials. We first define six broad classes of stochasticity: equilibrium (thermodynamic) fluctuations; structural/compositional fluctuations; kinetic fluctuations; frustration and degeneracy; imprecision in measurements; and stochasticity in modeling and simulation. In this review, we focus on the first four classes that are inherent to materials phenomena. We next develop a mathematical framework for describing materials stochasticity and then show how it can be broadly applied to these four materials-related stochastic classes. In subsequent sections, we describe structural and compositional fluctuations at small length scales that modify material properties and behavior at larger length scales; systems with engineered fluctuations, concentrating primarily on composite materials; systems in which stochasticity is developed through nucleation and kinetic phenomena; and configurations in which constraints in a given system prevent it from attaining its ground state and cause it to attain several, equally likely (degenerate) states. We next describe how stochasticity in these processes results in variations in physical properties and how these variations are then accentuated by-or amplify-stochasticity in processing and manufacturing procedures. In summary, the origins of materials stochasticity, the degree to which it can be predicted and/or controlled, and the possibility of using stochastic descriptions of materials structure, properties, and processing as a new degree of freedom in materials design are described.

18.
J Food Sci ; 83(11): 2827-2839, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30320406

RESUMO

Amorphous sucrose is a component of many food products but is prone to crystallize over time, thereby altering product quality and limiting shelf-life. A systematic investigation was conducted to determine the effects of two monosaccharides (glucose and fructose), five disaccharides (lactose, maltose, trehalose, isomaltulose, and cellobiose), and two trisaccharides (maltotriose and raffinose) on the stability of amorphous sucrose in lyophilized two-component sucrose-saccharide blends exposed to different relative humidity (RH) and temperature environmental conditions relevant for food product storage. Analyses included X-ray diffraction, differential scanning calorimetry, microscopy, and moisture content determination, as well as crystal structure overlays. All lyophiles were initially amorphous, but during storage the presence of an additional saccharide tended to delay sucrose crystallization. All samples remained amorphous when stored at 11% and 23% RH at 22 °C, but increasing the RH to 33% RH and/or increasing the temperature to 40 °C resulted in variations in crystallization onset times. Monosaccharide additives were less effective sucrose crystallization inhibitors relative to di- and tri-saccharides. Within the group of di- and tri-saccharides, effectiveness depended on the specific saccharide added, and no clear trends were observed with saccharide molecular weight and other commonly studied factors such as system glass transition temperature. Molecular level interactions, as evident in crystal structure overlays of the added saccharides and sucrose and morphological differences in crystals formed, appeared to contribute to the effectiveness of a di- or tri-saccharide in delaying sucrose crystallization. In conclusion, several di- and tri-saccharides show promise for use as additives to delay the crystallization kinetics of amorphous sucrose during storage at moderate temperatures and low RH conditions. PRACTICAL APPLICATION: Amorphous sucrose is desirable in a variety of food products, wherein crystallization can be problematic for texture and shelf-life. This study documents how different mono-, di-, and tri-saccharides influence the crystallization of sucrose. Monosaccharide additives were less effective sucrose crystallization inhibitors relative to di- and tri-saccharides. These findings increase the understanding of how different mono-, di-, and tri-saccharide structures and their solid-state properties influence the crystallization of amorphous sucrose and show that several di- and tri-saccharides have potential for use as sucrose crystallization inhibitors.


Assuntos
Polissacarídeos/química , Sacarose/química , Varredura Diferencial de Calorimetria , Celobiose/química , Cristalização , Análise de Alimentos , Isomaltose/análogos & derivados , Isomaltose/química , Lactose/química , Maltose/química , Microscopia Eletrônica de Varredura , Estrutura Molecular , Rafinose/química , Temperatura de Transição , Trealose/química , Trissacarídeos/química , Viscosidade , Difração de Raios X
19.
Nature ; 560(7717): 209-213, 2018 08.
Artigo em Inglês | MEDLINE | ID: mdl-30089919

RESUMO

Boundaries between distinct topological phases of matter support robust, yet exotic quantum states such as spin-momentum locked transport channels or Majorana fermions1-3. The idea of using such states in spintronic devices or as qubits in quantum information technology is a strong driver of current research in condensed matter physics4-6. The topological properties of quantum states have helped to explain the conductivity of doped trans-polyacetylene in terms of dispersionless soliton states7-9. In their seminal paper, Su, Schrieffer and Heeger (SSH) described these exotic quantum states using a one-dimensional tight-binding model10,11. Because the SSH model describes chiral topological insulators, charge fractionalization and spin-charge separation in one dimension, numerous efforts have been made to realize the SSH Hamiltonian in cold-atom, photonic and acoustic experimental configurations12-14. It is, however, desirable to rationally engineer topological electronic phases into stable and processable materials to exploit the corresponding quantum states. Here we present a flexible strategy based on atomically precise graphene nanoribbons to design robust nanomaterials exhibiting the valence electronic structures described by the SSH Hamiltonian15-17. We demonstrate the controlled periodic coupling of topological boundary states18 at junctions of graphene nanoribbons with armchair edges to create quasi-one-dimensional trivial and non-trivial electronic quantum phases. This strategy has the potential to tune the bandwidth of the topological electronic bands close to the energy scale of proximity-induced spin-orbit coupling19 or superconductivity20, and may allow the realization of Kitaev-like Hamiltonians3 and Majorana-type end states21.

20.
Nanoscale ; 10(17): 7912-7917, 2018 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-29666851

RESUMO

Two-dimensional (2D) junction devices have recently attracted considerable attention. Here, we show that most 2D junction structures, whether vertical or lateral, act as a lateral monolayer-bilayer-monolayer junction in their operation. In particular, a vertical structure cannot function as a vertical junction as having been widely believed in the literature. Due to a larger electrostatic screening, the bilayer region in the junction always has a smaller bandgap than its monolayer counterpart. As a result, a potential well, aside from the usual potential barrier, will form universally in the bilayer region to affect the hole or electron quantum transport in the form of transmission or reflection. Taking black phosphorus as an example, our calculations using a non-equilibrium Green function combined with density functional theory show a distinct oscillation in the transmission coefficient in a two-electrode prototypical device, and the results can be qualitatively understood using a simple quantum well model.

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