Advanced 1D heterostructures based on nanotube templates and molecules.

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities.

Luminescence Properties of Closely Packed Organic Color Centers Grafted on a Carbon Nanotube.

We report on the photoluminescence of pairs of organic color centers in single-wall carbon nanotubes grafted with 3,5-dichlorobenzene. Using various techniques such as intensity correlations, superlocalization microscopy, and luminescence excitation spectroscopy, we distinguish two pairs of color centers grafted on the same nanotube; the distance between the pairs is on the order of several hundreds of nanometers. In contrast, by studying the strong temporal correlations in the spectral diffusion in the framework of the photoinduced Stark effect, we can estimate the distance within each pair to be on the order of a few nanometers. Finally, the electronic population dynamics is investigated using time-resolved luminescence and saturation measurements, showing a biexponential decay with a fast overall recombination (compatible with a fast population transfer between the color centers within a pair) and a weak delayed repopulation of the traps, possibly due to the diffusion of excitons along the tube axis.

Impact of Bright-Dark Exciton Thermal Population Mixing on the Brightness of CsPbBr3 Nanocrystals.

Understanding the interplay between bright and dark exciton states is crucial for deciphering the luminescence properties of low-dimensional materials. The origin of the outstanding brightness of lead halide perovskites remains elusive. Here, we analyze temperature-dependent time-resolved photoluminescence to investigate the population mixing between bright and dark exciton sublevels in individual CsPbBr3 nanocrystals in the intermediate confinement regime. We extract bright and dark exciton decay rates and show quantitatively that the decay dynamics can only be reproduced with second-order phonon transitions. Furthermore, we find that any exciton sublevel ordering is compatible with the most likely population transfer mechanism. The remarkable brightness of lead halide perovskite nanocrystals rather stems from a reduced asymmetry between bright-to-dark and dark-to-bright conversion originating from the peculiar second-order phonon-assisted transitions that freeze bright-dark conversion at low temperatures together with the very fast radiative recombination and favorable degeneracy of the bright exciton state.

Spectral Fingerprint of Quantum Confinement in Single CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals are promising materials for classical and quantum light emission. To understand these outstanding properties, a thorough analysis of the band-edge exciton emission is needed, which is not reachable in ensemble and room-temperature studies because of broadening effects. Here, we report on a cryogenic-temperature study of the photoluminescence of single CsPbBr3 nanocrystals in the intermediate quantum confinement regime. We reveal the size-dependence of the spectral features observed: the bright triplet exciton energy splittings, the trion and biexciton binding energies, and the optical phonon replica spectrum. In addition, we show that bright triplet energy splittings are consistent with a pure exchange model and that the variety of polarization properties and spectra recorded can be rationalized simply by considering the orientation of the emitting dipoles and the populations of the emitting states.

Vibronic fingerprints in the luminescence of graphene quantum dots at cryogenic temperature.

Atomically precise graphene quantum dots synthesized by bottom-up chemistry are promising versatile single emitters with potential applications for quantum photonic technologies. Toward a better understanding and control of graphene quantum dot (GQD) optical properties, we report on single-molecule spectroscopy at cryogenic temperature. We investigate the effect of temperature on the GQDs' spectral linewidth and vibronic replica, which we interpret building on density functional theory calculations. Finally, we highlight that the vibronic signatures are specific to the GQD geometry and can be used as a fingerprint for identification purposes.

Davydov Splitting and Self-Organization in a Porphyrin Layer Noncovalently Attached to Single Wall Carbon Nanotubes.

We study the ability of porphyrin molecules to cooperate upon adsorption on the sp2 curved surface of carbon nanotube. We discuss the role of the phenyl substituents in the cooperativity of the functionalization reaction. Moreover, a specific spatial organization of the molecules around the nanotube is unveiled through polarization sensitive experiments. Furthermore, we observe an increase of the energy splitting of the porphyrin main transition upon the adsorption on the nanotube. This effect, interpreted as a Davydov splitting, is analyzed quantitatively using a dipole-dipole coupling model. This study demonstrates the ability of porphyrin molecules to create an organized self-assembled layer at the surface of the nanotubes where molecules are electronically coupled together.

Hofstadter Butterfly and Many-Body Effects in Epitaxial Graphene Superlattice.

Graphene placed on hexagonal boron nitride (h-BN) has received a wide range of interest due to the improved electrical performance and rich physics from the interface, especially the emergence of superlattice Dirac points as well as Hofstadter butterfly in high magnetic field. Instead of transferring graphene onto h-BN, epitaxial growth of graphene directly on a single-crystal h-BN provides an alternative and promising way to study these interesting superlattice effects due to their precise lattice alignment. Here we report an electrical transport study on epitaxial graphene superlattice on h-BN with a period of ∼15.6 nm. The epitaxial graphene superlattice is clean, intrinsic, and of high quality with a carrier mobility of ∼27 000 cm(2) V(-1) s(-1), which enables the observation of Hofstadter butterfly features originated from the superlattice at a magnetic field as low as 6.4 T. A metal-insulator transition and magnetic field dependent Fermi velocity were also observed, suggesting prominent electron-electron interaction-induced many-body effects.

Measuring the photon coalescence time window in the continuous-wave regime for resonantly driven semiconductor quantum dots.

We revisit Mandel's notion that the degree of coherence equals the degree of indistinguishability by performing Hong-Ou-Mandel- (HOM-)type interferometry with single photons elastically scattered by a cw resonantly driven excitonic transition of an InAs/GaAs epitaxial quantum dot. We present a comprehensive study of the temporal profile of the photon coalescence phenomenon which shows that photon indistinguishability can be tuned by the excitation laser source, in the same way as their coherence time. A new figure of merit, the coalescence time window, is introduced to quantify the delay below which two photons are indistinguishable. This criterion sheds new light on the interpretation of HOM experiments under cw excitation, particularly when photon coherence times are longer than the temporal resolution of the detectors. The photon indistinguishability is extended over unprecedented time scales beyond the detectors' response time, thus opening new perspectives to conducting quantum optics with single photons and conventional detectors.

Complex diffusion behavior of oxygen in nanocrystalline BaTiO3 ceramics.

(18)O/(16)O exchange annealing and subsequent Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis is used to investigate oxygen transport in dense, nanocrystalline (average grain size d ≈ 300 nm) ceramics of nominally un-doped BaTiO3. Isotope penetration profiles are obtained as a function of temperature, 973 < T/K < 1173, at an oxygen activity aO2 = 0.20 and as a function of oxygen activity, 0.002 < aO2 < 0.20, at T = 1073 K. All isotope profiles show the same unusual shape: a flattened profile over the first ∼10(2) nm, followed by a short, conventional diffusion profile. We demonstrate that the entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation based on an increase in the local oxygen diffusion coefficient close to the surface. This position-dependent increase is attributed to additional oxygen vacancies that are generated by diffusion of chlorine impurities out of the ceramics. The presence of chlorine derives from the chemical route necessary to produce nanometric powders: it thus indicates a new manner in which nanocrystalline ceramics may differ from their microcrystalline counterparts.

Chirality dependence of the absorption cross section of carbon nanotubes.

The variation of the optical absorption of carbon nanotubes with their geometry has been a long-standing question at the heart of both metrological and applicative issues, in particular because optical spectroscopy is one of the primary tools for the assessment of the chiral species abundance of samples. Here, we tackle the chirality dependence of the optical absorption with an original method involving ultraefficient energy transfer in porphyrin-nanotube compounds that allows uniform photoexcitation of all chiral species. We measure the absolute absorption cross section of a wide range of semiconducting nanotubes at their S22 transition and show that it varies by up to a factor of 2.2 with the chiral angle, with type I nanotubes showing a larger absorption. In contrast, the luminescence quantum yield remains almost constant.

Vibronic effect and influence of aggregation on the photophysics of graphene quantum dots.

Graphene quantum dots, atomically precise nanopieces of graphene, are promising nano-objects with potential applications in various domains such as photovoltaics, quantum light emitters and bio-imaging. Despite their interesting prospects, precise reports on their photophysical properties remain scarce. Here, we report on a study of the photophysics of C96H24(C12H25) graphene quantum dots. A combination of optical studies down to the single molecule level with advanced molecular modelling demonstrates the importance of coupling to vibrations in the emission process. Optical fingerprints for H-like aggregates are identified. Our combined experimental-theoretical investigations provide a comprehensive description of the light absorption and emission properties of nanographenes, which not only represents an essential step towards precise control of sample production but also paves the way for new exciting physics focused on twisted graphenoids.

Pi-stacking functionalization of carbon nanotubes through micelle swelling.

We report on a new, original and efficient method for pi-stacking functionalization of single-wall carbon nanotubes. This method is applied to the synthesis of a high-yield light-harvesting system combining single-wall carbon nanotubes and porphyrin molecules. We developed a micelle-swelling technique that leads to controlled and stable complexes presenting an efficient energy transfer. We demonstrate the key role of the organic solvent in the functionalization mechanism. By swelling the micelles, the solvent helps the non-water-soluble porphyrins to reach the micelle core and allows a strong enhancement of the interaction between porphyrins and nanotubes. This technique opens new avenues for the functionalization of carbon nanostructures.

A graphene Zener-Klein transistor cooled by a hyperbolic substrate.

The engineering of cooling mechanisms is a bottleneck in nanoelectronics. Thermal exchanges in diffusive graphene are mostly driven by defect-assisted acoustic phonon scattering, but the case of high-mobility graphene on hexagonal boron nitride (hBN) is radically different, with a prominent contribution of remote phonons from the substrate. Bilayer graphene on a hBN transistor with a local gate is driven in a regime where almost perfect current saturation is achieved by compensation of the decrease in the carrier density and Zener-Klein tunnelling (ZKT) at high bias. Using noise thermometry, we show that the ZKT triggers a new cooling pathway due to the emission of hyperbolic phonon polaritons in hBN by out-of-equilibrium electron-hole pairs beyond the super-Planckian regime. The combination of ZKT transport and hyperbolic phonon polariton cooling renders graphene on BN transistors a valuable nanotechnology for power devices and RF electronics.

Single photon emission from graphene quantum dots at room temperature.

Graphene being a zero-gap material, considerable efforts have been made to develop semiconductors whose structure is compatible with its hexagonal lattice. Size reduction is a promising way to achieve this objective. The reduction of both dimensions of graphene leads to graphene quantum dots. Here, we report on a single-emitter study that directly addresses the intrinsic emission properties of graphene quantum dots. In particular, we show that they are efficient and stable single-photon emitters at room temperature and that their emission wavelength can be modified through the functionalization of their edges. Finally, the investigation of the intersystem crossing shows that the short triplet lifetime and the low crossing yield are in agreement with the high brightness of these quantum emitters. These results represent a step-forward in performing chemistry engineering for the design of quantum emitters.

Controlling the kinetics of the non-covalent functionalization of carbon nanotubes using sub-cmc dilutions in a co-surfactant environment.

We investigate the origin of the slow kinetics of functionalization processes in micellar environments. We show that the ionic nature of the surfactants used to solubilize small molecules and nano-objects plays a central role in the slowness of the kinetics. In order to solve this issue, we have developed an innovative method that we apply to the hybrid compound porphyrin molecule/carbon nanotube. We use two ionic surfactants to solubilize the molecules and the nanotubes respectively. Passing the molecule suspension below the cmc allows circumventing the stability of the ionic surfactant while keeping the benefit of working with highly concentrated solutions. This method allows fine control of the functionalization reaction and tuning of the kinetics characteristic time over more than two orders of magnitude.

Local field effects in the energy transfer between a chromophore and a carbon nanotube: a single-nanocompound investigation.

Energy transfer in noncovalently bound porphyrin/carbon nanotube compounds is investigated at the single-nanocompound scale. Excitation spectroscopy of the luminescence of the nanotube shows two resonances arising from intrinsic excitation of the nanotube and from energy transfer from the porphyrin. Polarization diagrams show that both resonances are highly anisotropic, with a preferred direction along the tube axis. The energy transfer is thus strongly anisotropic despite the almost isotropic absorption of porphyrins. We account for this result by local field effects induced by the large optical polarizability of nanotubes. We show that the local field correction extends over several nanometers outside the nanotubes and drives the overall optical response of functionalized nanotubes.