Moiré Lattice of Twisted Bilayer Graphene as Template for Non-Covalent Functionalization.

We present a novel approach to achieve spatial variations in the degree of non-covalent functionalization of twisted bilayer graphene (tBLG). The tBLG with twist angles varying between ~5° and 7° was non-covalently functionalized with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) molecules. Our results show a correlation between the degree of functionalization and the twist angle of tBLG.

Production of Ultrathin and High-Quality Nanosheet Networks via Layer-by-Layer Assembly at Liquid-Liquid Interfaces.

Solution-processable 2D materials are promising candidates for a range of printed electronics applications. Yet maximizing their potential requires solution-phase processing of nanosheets into high-quality networks with carrier mobility (μ) as close as possible to that of individual nanosheets (μ). In practice, the presence of internanosheet junctions generally limits electronic conduction, such that the ratio of junction resistance () to nanosheet resistance (), determines the network mobility via μ/μ ≈ / + 1.

TEM-processed defect densities in single-layer TMDCs and their substrate-dependent signature in PL and Raman spectroscopy.

The optical properties of the direct-bandgap transition metal dichalcogenides (TMDCs) MoSand WSare heavily influenced by their atomic defect structure and substrate interaction. In this work we use low-voltage chromatic and spherical aberration (C/C)-corrected high-resolution transmission electron microscopy to simultaneously create and image chalcogen vacancies in TMDCs. However, correlating the defect structure, produced and analyzed using transmission electron microscopy (TEM), with optical spectroscopy often presents challenges because of very different fields of view and sample platforms involved.

Hybrid Moiré Excitons and Trions in Twisted MoTe-MoSe Heterobilayers.

We report experimental and theoretical studies of MoTe-MoSe heterobilayers with rigid moiré superlattices controlled by the twist angle. Using an effective continuum model that combines resonant interlayer electron tunneling with stacking-dependent moiré potentials, we identify the nature of moiré excitons and the dependence of their energies, oscillator strengths, and Landé -factors on the twist angle. Within the same framework, we interpret distinct signatures of bound complexes among electrons and moiré excitons in nearly collinear heterostacks.

Family behavior and Dirac bands in armchair nanoribbons with 4-8 defect lines.

Bottom-up synthesis from molecular precursors is a powerful route for the creation of novel synthetic carbon-based low-dimensional materials, such as planar carbon lattices. The wealth of conceivable precursor molecules introduces a significant number of degrees-of-freedom for the design of materials with defined physical properties. In this context,knowledge of the electronic, vibrational and optical properties provided by modernsimulation methods can act as a valuable guide for the design of novel synthetic carbon-based building blocks.

Simultaneous Inside and Outside Functionalization of Single-Walled Carbon Nanotubes.

Functionalizing single-walled carbon nanotubes (SWCNTs) in a robust way that does not affect the sp carbon framework is a considerable research challenge. Here we describe how triiodide salts of positively charged macrocycles can be used not only to functionalize SWCNTs from the outside, but simultaneously from the inside. We employed disulfide exchange in aqueous solvent to maximize the solvophobic effect and therefore achieve a high degree of macrocycle immobilization.

Cove-Edged Chiral Graphene Nanoribbons with Chirality-Dependent Bandgap and Carrier Mobility.

Graphene nanoribbons (GNRs) have garnered significant interest due to their highly customizable physicochemical properties and potential utility in nanoelectronics. Besides controlling widths and edge structures, the inclusion of chirality in GNRs brings another dimension for fine-tuning their optoelectronic properties, but related studies remain elusive owing to the absence of feasible synthetic strategies. Here, we demonstrate a novel class of cove-edged chiral GNRs () with a tunable chiral vector (,).

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe under inert conditions.

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties' associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions.

Click-Functionalization of Silanized Carbon Nanotubes: From Inorganic Heterostructures to Biosensing Nanohybrids.

Here we present an approach to functionalize silanized single-walled carbon nanotubes (SWNTs) through copper-free click chemistry for the assembly of inorganic and biological nanohybrids. The nanotube functionalization route involves silanization and strain-promoted azide-alkyne cycloaddition reactions (SPACC). This was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Fourier transform infra-red spectroscopy.

Full-Spectrum InP-Based Quantum Dots with Near-Unity Photoluminescence Quantum Efficiency.

Photoluminescent color conversion by quantum dots (QDs) makes possible the formation of spectrum-on-demand light sources by combining blue LEDs with the light generated by a specific blend of QDs. Such applications, however, require a near-unity photoluminescence quantum efficiency since self-absorption magnifies disproportionally the impact of photon losses on the overall conversion efficiency. Here, we present a synthesis protocol for forming InP-based QDs with +90% quantum efficiency across the full visible spectrum from blue/cyan to red.

Covalent Patterning of 2D MoS.

The development of an efficient method to patterning 2D MoS into a desired topographic structure is of particular importance to bridge the way towards the ultimate device. Herein, we demonstrate a patterning strategy by combining the electron beam lithography with the surface covalent functionalization. This strategy allows us to generate delicate MoS ribbon patterns with a minimum feature size of 2 μm in a high throughput rate.

Covalent Bisfunctionalization of Two-Dimensional Molybdenum Disulfide.

Covalent functionalization of two-dimensional molybdenum disulfide (2D MoS ) holds great promise in developing robust organic-MoS hybrid structures. Herein, for the first time, we demonstrate an approach to building up a bisfunctionalized MoS hybrid structure through successively reacting activated MoS with alkyl iodide and aryl diazonium salts. This approach can be utilized to modify both colloidal and substrate supported MoS nanosheets.

Hybridized intervalley moiré excitons and flat bands in twisted WSe bilayers.

The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe2 bilayers.

Twist-tailoring Coulomb correlations in van der Waals homobilayers.

The recent discovery of artificial phase transitions induced by stacking monolayer materials at magic twist angles represents a paradigm shift for solid state physics. Twist-induced changes of the single-particle band structure have been studied extensively, yet a precise understanding of the underlying Coulomb correlations has remained challenging. Here we reveal in experiment and theory, how the twist angle alone affects the Coulomb-induced internal structure and mutual interactions of excitons.

Infrared Interlayer Exciton Emission in MoS_{2}/WSe_{2} Heterostructures.

We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system.

Strain in InP/ZnSe, S core/shell quantum dots from lattice mismatch and shell thickness-Material stiffness influence.

We investigate the buildup of strain in InP quantum dots with the addition of shells of the lower-lattice constant materials ZnSe and ZnS by Raman spectroscopy. Both materials induce compressive strain in the core, which increases with increasing shell volume. We observe a difference in the shell behavior between the two materials: the thickness-dependence points toward an influence of the material stiffness.

Reductive diazotation of carbon nanotubes: an experimental and theoretical selectivity study.

The reaction of neutral single-walled carbon nanotubes (SWCNTs) with diazonium salts proceeds with a high selectivity towards metallic carbon nanotube species; this reaction is well-understood and the mechanism has been elucidated. In the present joint theoretical and experimental study, we investigate the reaction of negatively charged SWCNTs - carbon nanotubides - with diazonium salts. Our density functional theory calculations predict a stronger binding of the aryl diazonium cations to charged metallic SWCNTs species and therefore lead to a preferential addend binding in the course of the reaction.

Resonance Profiles of Valley Polarization in Single-Layer MoS_{2} and MoSe_{2}.

In this Letter, we present photoluminescence measurements with different excitation energies on single-layer MoS_{2} and MoSe_{2} in order to examine the resonance behavior of the conservation of circular polarization in these transition metal dichalcogenides. We find that the circular polarization of the emitted light is conserved to 100% in MoS_{2} and 84%/79% (A/A^{-} peaks) in MoSe_{2} close to resonance. The values for MoSe_{2} surpass any previously reported value.

Synthesis and Characterization of Nanotubes from Misfit (LnS) TaS (Ln=Pr, Sm, Gd, Yb) Compounds.

The synthesis and characterization of nanotubes from misfit layered compounds (MLCs) of the type (LnS) TaS (denoted here as LnS-TaS ; Ln=Pr, Sm, Gd, and Yb), not reported before, are described (the bulk compound YbS-LaS was not previously documented). Transmission electron microscopy and selected area electron diffraction showed that the interlayer spacing along the c axis decreased with an increase in the atomic number of the lanthanide atom, which suggested tighter interaction between the LnS layer and TaS for the late lanthanides. The Raman spectra of the tubules were studied and compared to those of the bulk MLC compounds.

Breakdown of Far-Field Raman Selection Rules by Light-Plasmon Coupling Demonstrated by Tip-Enhanced Raman Scattering.

We present an experimental study on the near-field light-matter interaction by tip-enhanced Raman scattering (TERS) with polarized light in three different materials: germanium-doped gallium nitride (GaN), graphene, and carbon nanotubes. We investigate the dependence of the TERS signal on the incoming light polarization and on the sample carrier concentration, as well as the Raman selection rules in the near-field. We explain the experimental data with a tentative quantum mechanical interpretation, which takes into account the role of plasmon polaritons, and the associated evanescent field.

From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase.

The electronic properties of sp/sp diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.

Fundamental Insights into the Degradation and Stabilization of Thin Layer Black Phosphorus.

Herein, we have developed a systematic study on the oxidation and passivation of mechanically exfoliated black phosphorus (BP). We analyzed the strong anisotropic behavior of BP by scanning Raman microscopy providing an accurate method for monitoring the oxidation of BP via statistical Raman spectroscopy. Furthermore, different factors influencing the environmental instability of the BP, i.

Interface formation during silica encapsulation of colloidal CdSe/CdS quantum dots observed by in situ Raman spectroscopy.

We investigate the encapsulation of CdSe/CdS quantum dots (QDs) in a silica shell by in situ Raman spectroscopy and find a distinct shift of the CdS Raman signal during the first hours of the synthesis. This shift does not depend on the final silica shell thickness but on the properties of the initial core-shell QD. We find a correlation between the Raman shift rate and the speed of the silica formation and attribute this to the changing configuration of the outermost layers of the QD shell, where an interface to the newly formed silica is created.

Degree of functionalisation dependence of individual Raman intensities in covalent graphene derivatives.

Covalent functionalisation of graphene is a continuously progressing field of research. The optical properties of such derivatives attract particular attention. In virtually all optical responses, however, an enhancement in peak intensity with increase of sp carbon content, and a vanishing of the peak position shift in monolayer compared to few-layer systems, is observed.