2D Electronic Structure Research Articles

Atomic scale depletion region at one dimensional MoSe2-WSe2 heterointerface

Lateral heterojunctions based on two dimensional (2D) transition metal dichalcogenides (TMDCs) potentially realize monolayer devices exploiting 2D electronic structures and the functions introduced by the presence of 1D heterointerfaces. Electronic structures of a lateral MoSe2-WSe2 junction have been unveiled using scanning tunneling microscopy and spectroscopy. A smooth and narrow depletion region exists despite a defect-rich heterointerface deviating from the preferred zigzag orientations of the TMDC lattice. From the characteristics of the depletion region, a high carrier concentration and high internal electric fields are inferred, offering to benefit designs of lateral TMDC devices.

Asymmetric 3d Electronic Structure for Enhanced Oxygen Evolution Catalysis.

The oxygen evolution reaction (OER) is an essential process for renewable energy, and designing a bifunctional oxygen electrocatalyst with high catalytic performance plays a significant role. In this work, FeS, Ni3S2, Fe5Ni4S8, and N, O, S-doped meshy carbon base were successfully synthesized. The sample containing Fe5Ni4S8 exhibited excellent OER performance. The density functional theory calculations indicate that the partial density of states for 3d electrons (3d-PDOS) of Fe and Ni atoms are changed from monometallic sulfide to bimetallic sulfide at the sulfur vacancy. The asymmetric 3d electronic structure optimizes the 3d-PDOS of Fe and Ni atoms, and leads to an enhanced OER activity. This work provides a new strategy to prepare a low-cost electrocatalyst for oxygen evolution with high-efficiency.

(Invited) New Concepts in Biosensing Using Single Walled Carbon Nanotubes and Graphene

Our lab at MIT has been interested in how the 1D and 2D electronic structures of carbon nanotubes and graphene respectively can be utilized to advance new concepts in molecular detection. We introduce CoPhMoRe or corona phase molecular recognition1 as a method of discovering synthetic antibodies, or nanotube-templated recognition sites from a heteropolymer library. We show that certain synthetic heteropolymers, once constrained onto a single-walled carbon nanotube by chemical adsorption, also form a new corona phase that exhibits highly selective recognition for specific molecules. To prove the generality of this phenomenon, we report three examples of heteropolymers–nanotube recognition complexes for riboflavin, L-thyroxine and estradiol. The platform opens new opportunities to create synthetic recognition sites for molecular detection. We have also extended this molecular recognition technique to neurotransmitters, producing the first fluorescent sensor for dopamine. Another area of advancement in biosensor development is the use of near infrared fluorescent carbon nanotube sensors for in-vivo detection2. Here, we show that PEG-ligated d(AAAT)7 DNA wrapped SWNT are selective for nitric oxide, a vasodilator of blood vessels, and can be tail vein injected into mice and localized within the viable mouse liver. We use an SJL mouse model to study liver inflammation in vivo using the spatially and spectrally resolved nIR signature of the localized SWNT sensors. Lastly, we discuss graphene as an interfacial optical biosensor, showing that it possesses two pKa values in alkaline and basic ranges. We use this response to measure dopamine in real time, spatially resolved at the interface with living PC12 cells which efflux dopamine, indicating graphene’s promise as an interfacial sensor in biology. Zhang, JQ et. al. Molecular recognition using corona phase complexes made of synthetic polymers adsorbed on carbon nanotubes. Nature Nanotechnology, 8, 12, 2013, 959-968Iverson, NM, et. al. In vivo biosensing via tissue-localizable near-infrared-fluorescent single-walled carbon nanotubes. Nature Nanotechnology, 8, 11, 2013, 873-880

Large Charge-Transfer Energy in LiFePO4 Revealed by Full-Multiplet Calculation for the Fe L3 -edge Soft X-ray Emission Spectra.

We analyzed the Fe 3d electronic structure in LiFePO4 /FePO4 (LFP/FP) nanowire with a high cyclability by using soft X-ray emission spectroscopy (XES) combined with configuration-interaction full-multiplet (CIFM) calculation. The ex situ Fe L2,3 -edge resonant XES (RXES) spectra for LFP and FP are ascribed to oxidation states of Fe2+ and Fe3+ , respectively. CIFM calculations for Fe2+ and Fe3+ states reproduced the Fe L3 RXES spectra for LFP and FP, respectively. In the calculations for both states, the charge-transfer energy was considerably larger than those for typical iron oxides, indicating very little electron transfer from the O 2p to Fe 3d orbitals and a weak hybridization on the Fe-O bond during the charge-discharge reactions.

Imaging Individual Molecular-Like Orbitals of a Non-Planar Naphthalene Diimide on Pt(111): A Combined STM and DFT Study

Functionalized naphthalene diimides (NDIs) are attractive candidates as small molecule acceptors for various molecular electronic applications due to their reversible two-step one-electron reductions at relatively low potentials. Here, we utilize low-temperature scanning tunneling microscopy (STM) to study the spatial extent and the electronic structure of 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI) adsorbed on the Pt(111) surface. We succeeded to map in real space the electronic structure of this three-dimensional (3D) molecule with orbital resolution, and thus were able to image an in-plane π-nodal plane located at the benzyl side arms. Furthermore, on the basis of the comparison of voltage dependent STM images and ab initio density functional theory simulations, we are able to explain the STM features of BNTCDI in terms of a convolution between its 3D shape and electronic structure. Importantly, for this weakly coupled molecule on the Pt(111) substrate, the intramolecular N···H–C h...

Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure.

Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.

Ar Plasma Exfoliated Nickel Iron Layered Double Hydroxide Nanosheets into Ultrathin Nanosheets As Highly-Efficient Electrocatalysts for Water Oxidition

Designing electrocatalysts for oxygen evolution reaction with low cost, high activity and stable are crucial extremely to water splitting. Layered double hydroxides (LDHs) as a electrocatalyst exhibits a good oxygen evolution reaction (OER) performance because of its unique 2D lameller and electronic structure. However, the bulk NiFe LDHs nanosheets with large lateral size and bulk thickness hindering the exposition active sites reduce the electrocatalytic activity. To solve this problem, herein, the bulk NiFe LDHs nanosheets were successfully exfoliated into 2D ultrathin NiFe LDHs nanosheets by one-step Ar plasma. This electrocatalyst with a structure of ultrathin nanosheet and exposed more edge sites was synthesized by the Ar plasma treatment and was used to catalyze the oxidation of water. This unique structure of NiFe LDH increases the surface area and the exposure of active sites which extremely facilitate to improve the electrocatalytic performance for oxygen evolution reaction. In additions, Ar plasma also futher etched the basal planes of 2D crystals leading to production of more edge sites. Enormous studies have confirmed that positions in edge have much higher catalytic activity than positions in the basal planes. Therefore, this ultrathin NiFe LDHs nanosheets with rich edge sites as electrocatalyst showed a good OER activity with a low overpotential of 278 mV at 10 mA/cm2. Figure 1

Nontrivial Berry phase in magnetic BaMnSb2 semimetal

The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material.

DNA cleavage properties and synthesis of metallophthalocyanines with 5-methyl-[1, 2, 4] triazolo [1, 5-a] pyrimidin-7-oxy substituents

ABSTRACTThe aim of the present study is to perform synthesis of novel metallophthalocyanines (pcs) with high solubility. The synthesis and characterization of 5-methyl-[1, 2, 4] triazolo [1, 5-a] pyrimidin-7-yloxy-substituted zinc, magnesium, and cobalt pcs are reported. These compounds have been characterized using electronic absorption, nuclear magnetic resonance spectroscopy, infrared, elemental analysis and mass spectra. The aggregation investigations carried out in different concentrations indicate that 5-methyl-[1, 2, 4] triazolo [1, 5-a] pyrimidin-7-yloxy-substituted pc complexes do not have any aggregation behavior for the concentration range of 1 × 10−5−1 × 10−6 M in THF. DPPH radical scavenging activity, metal chelating activity, and reducing power of the compounds were evaluated. The DNA gel electrophoresis studies revealed that new phthalonitrile and its metallophthalocyanine compounds cleavaged plasmid DNA (pBR322). Additionally, the ground-state geometries of the complexes were optimized using density functional theory methods at B3LYP/6-31G (d, p) level in order to obtain information about the 3D geometries and electronic structure.

Design, stereoselective synthesis and computational calculations of novel hybrid compounds via Pauson-Khand reactions

A series of novel chiral hybrid compounds between benzofuran and bicyclic cyclopentenone and also benzothiophene and bicyclic cyclopentenone have been designed and synthesized. Chiral enynes derived from enantiomerically enriched homoallyl and homopropargyl alcohols were converted into bicyclic cyclopentenone structures by intramolecular Pauson-Khand reactions. This strategy provides a facile access to various bicyclic cyclopentenones substituted with benzofuran or benzothiophene ring systems in good yields. In addition to the experimental work, the ground state geometries of the hybrid compounds were optimized using Density Functional Theory applications at the B3LYP/6-31G(d,p) level in order to obtain information about the 3D geometries and electronic structure.

Surface State Tunneling Signatures in the Two-Component Superconductor UPt_{3}.

Quasiparticle interference (QPI) imaging of Bogoliubov excitations in quasi-two-dimensional unconventional superconductors has become a powerful technique for measuring the superconducting gap and its symmetry. Here, we present the extension of this method to three-dimensional superconductors and analyze the expected QPI spectrum for the two-component heavy-fermion superconductor UPt_{3} whose gap structure is still controversial. Starting from a 3D electronic structure and the three proposed chiral gap models E_{1g,u} or E_{2u}, we perform a slab calculation that simultaneously gives extended bulk states and topologically protected in-gap dispersionless surface states. We show that the number of Weyl arcs and their hybridization with the line node provides a fingerprint that may finally determine the true nodal structure of the UPt_{3} superconductor.

Low-Cost and Sustainable Organic Thermoelectrics Based on Low-Dimensional Molecular Metals.

Thermoelectric generator composed of crystalline radical ion salts: The unipolar charge transport along the molecular stacks facilitates complementary p- and n-type organic thermoelectric materials of high electrical conductivity and of 1D electronic structure. The specific power output of 5 mW cm-2 and the zT > 0.15 below 40 K demonstrate a new field of low-temperature thermoelectric applications unlocked by organic metals.

Influence of surface symmetry breaking on the magnetism, collapsing, and three-dimensional dispersion of the Co pnictidesACo2As2(A=Ba, Sr, Ca)

We use angle-resolved photoemission to study the three dimensional (3D) electronic structure of Co pnictides ACo2As2 with A=Ba, Sr, Ca or a mixture of Sr and Ca. These compounds are isostructural to Fe based superconductors, but have one more electron in the Co $3d$ orbitals. Going from Ba to Ca, they become more and more 3D, eventually forming a 'collapsed' tetragonal phase, where the distance between CoAs layers is markedly reduced. The observed periodicity of the 3D electronic structure matches in each case that expected from the distance between the planes in the bulk. However, the electronic structure is better fitted by a calculation corresponding to a slab with 2 CoAs layers than to the bulk structure. We attribute this to subtle modifications of the 2D electronic structure induced by the truncation of the 3D dispersion at the surface in the ARPES measurement. We further study how this affects the electronic properties. We show that, despite this distortion, the electronic structure of CaCo2As2 is essentially that expected for a collapsed phase. Electronic correlations produce a renormalization of the electronic structure by a factor 1.4, which is not affected by the transition to the collapsed state. On the other hand, a small shift of the Fermi level reduces the density of states in the eg bands and suppresses the magnetic transition expected in CaCo2As2. Our study evidences that observing the bulk periodicity is not sufficient to ensure bulk sensitivity. It further gives direct information on the role of 3D interactions, mostly governed by Co-As hybridization, among eg and t2g orbitals. It is also useful to better understand the electronic structure of Fe superconductors and the range of validity of ARPES measurements.

Calculation of 2D electronic band structure using matrix mechanics

We extend previous work, applying elementary matrix mechanics to one-dimensional periodic arrays (to generate energy bands), to two-dimensional arrays. We generate band structures for the square-lattice “2D Kronig-Penney model” (square wells), the “muffin-tin” potential (circular wells), and Gaussian wells. We then apply the method to periodic arrays of more than one atomic site in a unit cell, specifically to the case of materials with hexagonal lattices like graphene. These straightforward extensions of undergraduate-level calculations allow students to readily determine band structures of current research interest.

Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2.

Layered compounds AMnBi2 (A = Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm2V−1S−1) and a π Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120 K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits a G-type antiferromagnetic order below 283 K. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.

Molecular Orbital Simulations of Metal 1s2p Resonant Inelastic X-ray Scattering.

For first-row transition metals, high-resolution 3d electronic structure information can be obtained using resonant inelastic X-ray scattering (RIXS). In the hard X-ray region, a K pre-edge (1s→3d) excitation can be followed by monitoring the dipole-allowed Kα (2p→1s) or Kβ (3p→1s) emission, processes labeled 1s2p or 1s3p RIXS. Here the restricted active space (RAS) approach, which is a molecular orbital method, is used for the first time to study hard X-ray RIXS processes. This is achieved by including the two sets of core orbitals in different partitions of the active space. Transition intensities are calculated using both first- and second-order expansions of the wave vector, including, but not limited to, electric dipoles and quadrupoles. The accuracy of the approach is tested for 1s2p RIXS of iron hexacyanides [Fe(CN)6](n-) in ferrous and ferric oxidation states. RAS simulations accurately describe the multiplet structures and the role of 2p and 3d spin-orbit coupling on energies and selection rules. Compared to experiment, relative energies of the two [Fe(CN)6](3-) resonances deviate by 0.2 eV in both incident energy and energy transfer directions, and multiplet splittings in [Fe(CN)6](4-) are reproduced within 0.1 eV. These values are similar to what can be expected for valence excitations. The development opens the modeling of hard X-ray scattering processes for both solution catalysts and enzymatic systems.

Large-scale chemical assembly of atomically thin transistors and circuits.

Next-generation electronics calls for new materials beyond silicon, aiming at increased functionality, performance and scaling in integrated circuits. In this respect, two-dimensional gapless graphene and semiconducting transition-metal dichalcogenides have emerged as promising candidates due to their atomic thickness and chemical stability. However, difficulties with precise spatial control during their assembly currently impede actual integration into devices. Here, we report on the large-scale, spatially controlled synthesis of heterostructures made of single-layer semiconducting molybdenum disulfide contacting conductive graphene. Transmission electron microscopy studies reveal that the single-layer molybdenum disulfide nucleates at the graphene edges. We demonstrate that such chemically assembled atomic transistors exhibit high transconductance (10 µS), on-off ratio (∼106) and mobility (∼17 cm2 V-1 s-1). The precise site selectivity from atomically thin conducting and semiconducting crystals enables us to exploit these heterostructures to assemble two-dimensional logic circuits, such as an NMOS inverter with high voltage gain (up to 70).

Unoccupied electronic states of Ru(0001)

This report presents a combined theoretical and experimental description of the unoccupied electronic states of Ru(0001), along the Γ̅M̅ high symmetry direction of the Brillouin zone. A direct comparison between angle-resolved inverse photoemission spectroscopy and ab initio calculations of the 3-dimensional (3D) electronic structure of Ru(0001) have been used to determine the energy dispersion and the identification of different states and surface resonances. Both, measurements and calculations, complement previous reports regarding the electronic structure of Ru.

Gold-induced nanowires on the Ge(100) surface yield a 2D and not a 1D electronic structure

Atomic nanowires on semiconductor surfaces induced by the adsorption of metallic atoms have attracted a lot of attention as possible hosts of the elusive, Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the subject of controversy as to whether the Au-induced nanowires do indeed host exotic, 1D metallic states. We report on a thorough study of the electronic properties of high quality nanowires formed at the Au/Ge(100) surface. High resolution ARPES data show the low-lying Au-induced electronic states to possess a dispersion relation that depends on two orthogonal directions in k-space. Comparison of the E(k$_x$,k$_y$) surface measured using ARPES to tight-binding calculations yields hopping parameters in the two different directions that differ by a factor of two. We find that the larger of the two hopping parameters corresponds, in fact, to the direction perpendicular to the nanowires (t$_{\perp}$). This, the topology of the $E$=$E_F$ contour in k$_{\||}$, and the fact that $t_{\||}$/$t_{\perp}\sim 0.5$ proves that the Au-induced electron pockets possess a 2D, closed Fermi surface, this firmly places the Au/Ge(100) nanowire system outside being a potential hosts of a Tomonaga-Luttinger liquid. We combine these ARPES data with STS measurements of the spatially-resolved electronic structure and find that the spatially straight conduction channels observed up to energies of order one electron volt below the Fermi level do not originate from the Au-induced states seen in the ARPES data. The former are more likely to be associated with bulk Ge states that are localized to the subsurface region. Despite our proof of the 2D nature of the Au-induced nanowire and sub-surface Ge-related states, an anomalous suppression of the density of states at the Fermi level is observed in both the STS and ARPES data, this phenomenon is discussed in the light of the effects of disorder.

(Invited) Interaction of Molecules with Single-Walled Carbon Nanotube Probed By Photoluminescence and Raman Spectroscopy

The interaction of molecules and material surfaces is one of the fundamental topics in surface science. Also, understanding the behavior of molecules in nano-space is increasingly important in nano- and biotechnologies. A single-walled carbon nanotube (SWNT) is a monolayered material composed of all surface atoms, which provide a simple van der Waals potential originated from a single layer both for the outer surface (quasi 2D) and inner nano-space (quasi 1D). Furthermore, the quasi 1D electronic structure of SWNT allows resonant optical transitions, which are sensitively affected by the surrounding environment of SWNT. SWNTs are thus an ideal material for examining the behaviors of molecules on the surface or in the nano-space by means of optical spectroscopy. We use a singly suspended SWNT between mesa structures to minimize the interactions with the substrate and other nanotubes, and expose it directly to the ambient gas. Semiconducting SWNTs suspended in space exhibit resonant photoluminescence (PL) and Raman scattering, and their optical responses depend on the state of molecules adsorbed on the SWNT surface or encapsulated in the tube [1-3]. Therefore, gas molecule adsorption/desorption on the SWNT surface or inside of the tube can be probed by PL and Raman spectroscopy. We have found that water molecules form an adsorption layer of two-monolayer thickness on the hydrophobic SWNT surface as a result of hydrogen-bonding network of water molecules [2]. The water adsorption layer has a sizable effect on the radial breathing mode frequency in Raman scattering [4]. The phase of water molecules can also affect those properties of SWNTs. The phase diagrams of the water both outside and inside of SWNT will be constructed based on the results of spectroscopy. Furthermore, PL peak energies from hybrids of single-stranded DNA and SWNT were found to depend on the DNA base type, namely thymine, adenine, cytosine, and guanine [5]. The base type dependence was attributed to the adsorption energy of the DNA bases on the SWNT surface. [1] S. Chiashi, et al. Nano Lett. 8, 3097 (2008). [2] Y. Homma, et al. Phys. Rev. Lett. 110, 157402 (2013). [3] S. Chiashi, et al. J. Phys. Chem. Lett. 5, 408 (2014). [4] S. Chiashi, et al. Phys. Rev. B 91, 155415 (2015). [5] M. Ito, et al. J. Phys. Chem. C 119, 21141 (2015).