Shortcuts to Equilibrium with a Levitated Particle in the Underdamped Regime.

We report on speeding-up equilibrium recovery in the previously unexplored general case of the underdamped regime using an optically levitated particle. We accelerate the convergence toward equilibrium by an order of magnitude compared to the natural relaxation time. We then discuss the efficiency of the studied protocols, especially for a multidimensional system. These results pave the way for optimizing realistic nanomachines with application to sensing and developing efficient nanoheat engines.

Interplay of structure and photophysics of individualized rod-shaped graphene quantum dots with up to 132 sp² carbon atoms.

Nanographene materials are promising building blocks for the growing field of low-dimensional materials for optics, electronics and biophotonics applications. In particular, bottom-up synthesized 0D graphene quantum dots show great potential as single quantum emitters. To fully exploit their exciting properties, the graphene quantum dots must be of high purity; the key parameter for efficient purification being the solubility of the starting materials. Here, we report the synthesis of a family of highly soluble and easily processable rod-shaped graphene quantum dots with fluorescence quantum yields up to 94%. This is uncommon for a red emission. The high solubility is directly related to the design of the structure, allowing for an accurate description of the photophysical properties of the graphene quantum dots both in solution and at the single molecule level. These photophysical properties were fully predicted by quantum-chemical calculations.

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.

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.

Solution-Processed Graphene-Nanographene van der Waals Heterostructures for Photodetectors with Efficient and Ultralong Charge Separation.

Sensitization of graphene with inorganic semiconducting nanostructures has been demonstrated as a powerful strategy to boost its optoelectronic performance. However, the limited tunability of optical properties and toxicity of metal cations in the inorganic sensitizers prohibits their widespread applications, and the in-depth understanding of the essential interfacial charge-transfer process within such hybrid systems remains elusive. Here, we design and develop high-quality nanographene (NG) dispersions with a large-scale production using high-shear mixing exfoliation. The physisorption of these NG molecules onto graphene gives rise to the formation of graphene-NG van der Waals heterostructures (VDWHs), characterized by strong interlayer coupling through π-π interactions. As a proof of concept, photodetectors fabricated on the basis of such VDWHs show ultrahigh responsivity up to 4.5 × 107 A/W and a specific detectivity reaching 4.6 × 1013 Jones, being competitive with the highest values obtained for graphene-based photodetectors. The outstanding device characteristics are attributed to the efficient transfer of photogenerated holes from NGs to graphene and the long-lived charge separation at graphene-NG interfaces (beyond 1 ns), as elucidated by ultrafast terahertz (THz) spectroscopy. These results demonstrate the great potential of such graphene-NG VDWHs as prototypical building blocks for high-performance, low-toxicity optoelectronics.

Spin-Mechanics with Nitrogen-Vacancy Centers and Trapped Particles.

Controlling the motion of macroscopic oscillators in the quantum regime has been the subject of intense research in recent decades. In this direction, opto-mechanical systems, where the motion of micro-objects is strongly coupled with laser light radiation pressure, have had tremendous success. In particular, the motion of levitating objects can be manipulated at the quantum level thanks to their very high isolation from the environment under ultra-low vacuum conditions. To enter the quantum regime, schemes using single long-lived atomic spins, such as the electronic spin of nitrogen-vacancy (NV) centers in diamond, coupled with levitating mechanical oscillators have been proposed. At the single spin level, they offer the formidable prospect of transferring the spins' inherent quantum nature to the oscillators, with foreseeable far-reaching implications in quantum sensing and tests of quantum mechanics. Adding the spin degrees of freedom to the experimentalists' toolbox would enable access to a very rich playground at the crossroads between condensed matter and atomic physics. We review recent experimental work in the field of spin-mechanics that employ the interaction between trapped particles and electronic spins in the solid state and discuss the challenges ahead. Our focus is on the theoretical background close to the current experiments, as well as on the experimental limits, that, once overcome, will enable these systems to unleash their full potential.

Negatively Curved Nanographene with Heptagonal and [5]Helicene Units.

Negatively curved nanographene (NG) 4, having two heptagons and a [5]helicene, was unexpectedly obtained by aryl rearrangement and stepwise cyclodehydrogenations. X-ray crystallography confirmed the saddle-shaped structures of intermediate 3 and NG 4. The favorability of rearrangement over helicene formation following radical cation or arenium cation mechanisms is supported by theoretical calculations. NG 4 demonstrates a reversible mechanochromic color change and solid-state emission, presumably benefiting from its loose crystal packing. After resolution by chiral high-performance liquid chromatography, the circular dichroism spectra of enantiomers 4-(P) and 4-(M) were measured and showed moderate Cotton effects at 350 nm (|Δε| = 148 M-1 cm-1).

Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers.

Pressure can be used to tune the interplay among structural, electronic, and magnetic interactions in materials. High pressures are usually applied in the diamond anvil cell, making it difficult to study the magnetic properties of a micrometer-sized sample. We report a method for spatially resolved optical magnetometry based on imaging a layer of nitrogen-vacancy (NV) centers created at the surface of a diamond anvil. We illustrate the method using two sets of measurements realized at room temperature and low temperature, respectively: the pressure evolution of the magnetization of an iron bead up to 30 gigapascals showing the iron ferromagnetic collapse and the detection of the superconducting transition of magnesium dibromide at 7 gigapascals.

Optical Magnetometry of Single Biocompatible Micromagnets for Quantitative Magnetogenetic and Magnetomechanical Assays.

The mechanical manipulation of magnetic nanoparticles is a powerful approach to probing and actuating biological processes in living systems. Implementing this technique in high-throughput assays can be achieved using biocompatible micromagnet arrays. However, the magnetic properties of these arrays are usually indirectly inferred from simulations or Stokes drag measurements, leaving unresolved questions about the actual profile of the magnetic fields at the micrometer scale and the exact magnetic forces that are applied. Here, we exploit the magnetic field sensitivity of nitrogen-vacancy color centers in diamond to map the 3D stray magnetic field produced by a single soft ferromagnetic microstructure. By combining this wide-field optical magnetometry technique with magneto-optic Kerr effect microscopy, we fully analyze the properties of the micromagnets, including their magnetization saturation and their size-dependent magnetic susceptibility. We further show that the high magnetic field gradients produced by the micromagnets, greater than 104 T·m-1 under an applied magnetic field of about 100 mT, enables the manipulation of magnetic nanoparticles smaller than 10 nm inside living cells. This work paves the way for quantitative and parallelized experiments in magnetogenetics and magnetomechanics in cell biology.

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.

Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion.

Among organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption, and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, nonplanarity can be an additional parameter to modulate their properties without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with "cove"-type edges, having an identical aromatic core but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the "cove"-type edges can "bend" the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and nonplanar GNRs through UV-vis-near-infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.

Direct measurement of Kramers turnover with a levitated nanoparticle.

Understanding the thermally activated escape from a metastable state is at the heart of important phenomena such as the folding dynamics of proteins, the kinetics of chemical reactions or the stability of mechanical systems. In 1940, Kramers calculated escape rates both in the high damping and low damping regimes, and suggested that the rate must have a maximum for intermediate damping. This phenomenon, today known as the Kramers turnover, has triggered important theoretical and numerical studies. However, as yet, there is no direct and quantitative experimental verification of this turnover. Using a nanoparticle trapped in a bistable optical potential, we experimentally measure the nanoparticle's transition rates for variable damping and directly resolve the Kramers turnover. Our measurements are in agreement with an analytical model that is free of adjustable parameters. The levitated nanoparticle presented here is a versatile experimental platform for studying and simulating a wide range of stochastic processes and testing theoretical models and predictions.

Cancellation of non-conservative scattering forces in optical traps by counter-propagating beams: erratum.

In a previous Letter [Opt. Lett.40, 1900 (2015)OPLEDP0146-959210.1364/OL.40.001900], we asserted that two counter-propagating beams must be polarized with opposite handedness to cancel scattering forces. A more careful calculation shows that this is not the case. The correct condition is achieved by beams with the same handedness, as derived in this Erratum.

Cancellation of non-conservative scattering forces in optical traps by counter-propagating beams.

Non-conservative forces in optical tweezers generate undesirable behavior, such as particle loss due to radiation pressure and the preclusion of the thermodynamic equilibrium. Here, we rigorously derive criteria for the elimination of non-conservative forces, and describe how these criteria can be met by a large class of counter-propagating, focused optical beams.