Localization of lattice dynamics in low-angle twisted bilayer graphene.

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.

Engineering of robust topological quantum phases in graphene nanoribbons.

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.

Spin-polarized electronic properties of naphthylene-based carbon nanostructures.

The 2D naphthylene-ß structure is a theoretically proposed sp2 nanocarbon allotrope based on the assembly of naphthalene-based molecular building blocks, which features metallic properties. We report that 2D naphthylene-ß structures host a spin-polarized configuration which turns the system into a semiconductor. We analyze this electronic state in terms of the bipartition of the lattice. In addition, we study the electronic properties of nanotubes obtained from the rolling up of 2D naphthylene-ß. We show that they inherit the properties of the parent 2D nanostructure, such as the emergence of spin-polarized configurations. We further rationalize the results in terms of a zone-folding scheme. We also show that the electronic properties can be modulated using an external transverse electric field, including a semiconducting-to-metallic transition for sufficiently large field strength.

Electron-Phonon Coupling in a Magic-Angle Twisted-Bilayer Graphene Device from Gate-Dependent Raman Spectroscopy and Atomistic Modeling.

The importance of phonons in the strong correlation phenomena observed in twisted-bilayer graphene (TBG) at the so-called magic-angle is under debate. Here we apply gate-dependent micro-Raman spectroscopy to monitor the G band line width in TBG devices of twist angles θ = 0° (Bernal), ∼1.1° (magic-angle), and ∼7° (large-angle). The results show a broad and p-/n-asymmetric doping behavior at the magic angle, in clear contrast to the behavior observed in twist angles above and below this point. Atomistic modeling reproduces the experimental observations in close connection with the joint density of electronic states in the electron-phonon scattering process, revealing how the unique electronic structure of magic-angle TBGs influences the electron-phonon coupling and, consequently, the G band line width. Overall, the value of the G band line width in magic-angle TBG is larger when compared to that of the other samples, in qualitative agreement with our calculations.

Growth Optimization and Device Integration of Narrow-Bandgap Graphene Nanoribbons.

The electronic, optical, and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom-up fabrication based on molecular precursors. This approach offers a unique platform for all-carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for successful device integration, with GNR length being the most critical parameter. In this work, the growth, characterization, and device integration of 5-atom wide armchair GNRs (5-AGNRs) are studied, which are expected to have an optimal bandgap as active material in switching devices. 5-AGNRs are obtained via on-surface synthesis under ultrahigh vacuum conditions from Br- and I-substituted precursors. It is shown that the use of I-substituted precursors and the optimization of the initial precursor coverage quintupled the average 5-AGNR length. This significant length increase allowed the integration of 5-AGNRs into devices and the realization of the first field-effect transistor based on narrow bandgap AGNRs that shows switching behavior at room temperature. The study highlights that the optimized growth protocols can successfully bridge between the sub-nanometer scale, where atomic precision is needed to control the electronic properties, and the scale of tens of nanometers relevant for successful device integration of GNRs.

Electronic properties of 2D and 1D carbon allotropes based on a triphenylene structural unit.

Several bottom-up chemical routes have been developed in the last few years to find ways to grow new forms of nanocarbon by devising a strategical selection of molecular precursors. Here, theoretical calculations are performed on 2D nanocarbon allotropes obtained from the fusion of triphenylene-like units through tetragonal rings. This 2D triphenylene structure has a metallic character in a closed shell configuration, but it also features a spin-polarized semiconducting state. The behavior of the electronic properties of the system is investigated when the structure is cast into nanoribbon forms. It is found that to be metallic in the nonpolarized case, the ribbons must be sufficiently wide while narrow 1D systems are semiconducting. A lower threshold width is also needed for the emergence of a spin-polarized semiconducting configuration in these nanoribbons. These behaviors are robust as they do not depend on edge geometry and chirality, thus offering opportunities for their possible applications in nanoscale devices.

Electronic properties of N-rich graphene nano-chevrons.

Theoretical analysis based on density functional theory describes the microscopic origins of emerging electronic and magnetic properties in quasi-1D nitrogen-rich graphene nanoribbon structures with chevron-like (or wiggle-edged) configurations. The study focuses on systems with structural units composed of hexagonal graphitic units featuring one and two nitrogen atoms substituted in the graphitic structure, in positions contrasting with the more commonly considered pyridinic configurations. This type of substitution introduces nitrogen levels close to the Fermi level which in turn induce spin polarization depending on a number of structural features. We demonstrate that these systems present a broader set of electronic and magnetic behaviors relative to their pure hydrocarbon counterparts, with the possibility of engineering the electronic band gap strategically using different spin configurations and positions of the substituting nitrogen atoms.

Reversible Pressure-Induced Partial Phase Transition in Few-Layer Black Phosphorus.

The experimental identification of structural transitions in layered black phosphorus (BP) under mechanical stress is essential to extend its application in microelectromechanical (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 Ag2) 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.

The effects of substitutional Fe-doping on magnetism in MoS2and WS2monolayers.

Doping of two-dimensional (2D) semiconductors has been intensively studied toward modulating their electrical, optical, and magnetic properties. While ferromagnetic 2D semiconductors hold promise for future spintronics and valleytronics, the origin of ferromagnetism in 2D materials remains unclear. Here, we show that substitutional Fe-doping of MoS2and WS2monolayers induce different magnetic properties. The Fe-doped monolayers are directly synthesized via chemical vapor deposition. In both cases, Fe substitutional doping is successfully achieved, as confirmed using scanning transmission electron microscopy. While both Fe:MoS2and Fe:WS2show PL quenching and n-type doping, Fe dopants in WS2monolayers are found to assume deep-level trap states, in contrast to the case of Fe:MoS2, where the states are found to be shallow. Usingµm- and mm-precision local NV-magnetometry and superconducting quantum interference device, we discover that, unlike MoS2monolayers, WS2monolayers do not show a magnetic phase transition to ferromagnetism upon Fe-doping. The absence of ferromagnetism in Fe:WS2is corroborated using density functional theory calculations.

Tripentaphenes: two-dimensional acepentalene-based nanocarbon allotropes.

Tripentaphenes are 2D nanocarbon lattices conceptually obtained from the assembly of acepentalene units. In this work, density functional theory is used to investigate their structural, electronic, and vibrational properties. Their bonding configuration is rationalized with a resonance mechanism, which is unique to each of the 2D assemblies. Their formation energies are found to lie within the range of other previously synthesized carbon nanostructures and phonon calculations indicate their dynamical stability. In addition, all studied tripentaphenes are metallic and display different features (e.g., Dirac cone) depending on the details of the atomic structure. The resonance structure also plays an important role in determining the electronic properties as it leads to delocalized electronic states, further highlighting the potential of the structures in nanoelectronics.

Magnetic Proximity Coupling of Quantum Emitters in WSe2 to van der Waals Ferromagnets.

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.

Direct Observation of Symmetry-Dependent Electron-Phonon Coupling in Black Phosphorus.

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.

On-Surface Synthesis and Characterization of Acene-Based Nanoribbons Incorporating Four-Membered Rings.

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.

Shell model extension to the valence force field: application to single-layer black phosphorus.

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.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Quantum-Confined Stark Effect of Individual Defects in a van der Waals Heterostructure.

The optical properties of atomically thin semiconductor materials have been widely studied because of the isolation of monolayer transition metal dichalcogenides (TMDCs). They have rich optoelectronic properties owing to their large direct bandgap, the interplay between the spin and the valley degree of freedom of charge carriers, and the recently discovered localized excitonic states giving rise to single photon emission. In this Letter, we study the quantum-confined Stark effect of these localized emitters present near the edges of monolayer tungsten diselenide (WSe2). By carefully designing sequences of metallic (graphene), insulating (hexagonal boron nitride), and semiconducting (WSe2) two-dimensional materials, we fabricate a van der Waals heterostructure field effect device with WSe2 hosting quantum emitters that is responsive to external static electric field applied to the device. A very efficient spectral tunability up to 21 meV is demonstrated. Further, evaluation of the spectral shift in the photoluminescence signal as a function of the applied voltage enables us to extract the polarizability volume (up to 2000 Å3) as well as information on the dipole moment of an individual emitter. The Stark shift can be further modulated on application of an external magnetic field, where we observe a flip in the sign of dipole moment possibly due to rearrangement of the position of electron and hole wave functions within the emitter.

Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons.

Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing a limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts.

Quantum Dots in Graphene Nanoribbons.

Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics, and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over 1 order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices.

Revealing the Electronic Structure of Silicon Intercalated Armchair Graphene Nanoribbons by Scanning Tunneling Spectroscopy.

The electronic properties of graphene nanoribbons grown on metal substrates are significantly masked by the ones of the supporting metal surface. Here, we introduce a novel approach to access the frontier states of armchair graphene nanoribbons (AGNRs). The in situ intercalation of Si at the AGNR/Au(111) interface through surface alloying suppresses the strong contribution of the Au(111) surface state and allows for an unambiguous determination of the frontier electronic states of both wide and narrow band gap AGNRs. First-principles calculations provide insight into substrate induced screening effects, which result in a width-dependent band gap reduction for substrate-supported AGNRs. The strategy reported here provides a unique opportunity to elucidate the electronic properties of various kinds of graphene nanomaterials supported on metal substrates.

On-Surface Cyclization of ortho-Dihalotetracenes to Four- and Six-Membered Rings.

We report on the surface-catalyzed formal [2+2] and [2+2+2] cycloadditions of ortho-activated tetracene species on a Ag(111) substrate under ultrahigh vacuum conditions. Three different products are obtained: tetracene dimers, trimers, and tetramers. The former results from the formation of a four-membered ring while the other two arise from cyclization into six-membered rings. These on-surface reactions have been monitored by scanning tunneling microscopy and rationalized by density functional theory calculations. Our approach, based on the reaction of ortho-dihalo precursor monomers via formal cycloadditions, establishes an additional method for the highly active field of on-surface synthesis and enables the development of novel 1D and 2D covalent carbon nanostructures.