Graphene, the 2D form of carbon based material existing as a single layer of atoms arranged in a honeycomb lattice, has set the science and technology sectors alight with interest in the last decade in view of its astounding electrical and thermal properties, combined with its mechanical stiffness, strength and elasticity. Two distinct strategies have been undertaken for graphene production, i.e. the bottom-up and the top-down. The former relies on the generation of graphene from suitably designed molecular building blocks undergoing chemical reaction to form covalently linked 2D networks. The latter occurs via exfoliation of graphite into graphene. Bottom-up techniques, based on the organic syntheses starting from small molecular modules, when performed in liquid media, are both size limited, because macromolecules become more and more insoluble with increasing size, and suffer from the occurrence of side reactions with increasing molecular weight. Because of these reasons such a synthesis has been performed more and more on a solid (ideally catalytically active) surface. Substrate-based growth of single layers can be done also by chemical vapor deposition (CVD) or via reduction of silicon carbide, which unfortunately relies on the ability to follow a narrow thermodynamic path. Top-down approaches can be accomplished under different environmental conditions. Alongside the mechanical cleavage based on the scotch tape approach, liquid-phase exfoliation (LPE) methods are becoming more and more interesting because they are extremely versatile, potentially up-scalable, and can be used to deposit graphene in a variety of environments and on different substrates not available using mechanical cleavage or growth methods. Interestingly, LPE can be applied to produce different layered systems exhibiting different compositions such as BN, MoS2, WS2, NbSe2, and TaS2, thereby enabling the tuning of numerous physico-chemical properties of the material. Furthermore, LPE can be employed to produce graphene-based composites or films, which are key components for many applications, such as thin-film transistors, conductive transparent electrodes for indium tin oxide replacement, e.g. in light-emitting diodes, or photovoltaics. In this review, we highlight the recent progress that has led to successful production of high quality graphene by means of LPE of graphite. In particular, we discuss the mechanisms of exfoliation and methods that are employed for graphene characterization. We then describe a variety of successful liquid-phase exfoliation methods by categorizing them into two major classes, i.e. surfactant-free and surfactant-assisted LPE. Furthermore, exfoliation in aqueous and organic solutions is presented and discussed separately.

Organic semiconductors have generated considerable interest for their potential for creating inexpensive and flexible devices easily processed on a large scale [1][2][3][4][5][6][7][8][9][10][11]. However technological applications are currently limited by the low mobility of the charge carriers associated with the disorder in these materials [5][6][7][8]. Much effort over the past decades has therefore been focused on optimizing the organisation of the material or the devices to improve carrier mobility. Here we take a radically different path to solving this problem, namely by injecting carriers into states that are hybridized to the vacuum electromagnetic field. These are coherent states that can extend over as many as 10 5 molecules and should thereby favour conductivity in such materials. To test this idea, organic semiconductors were strongly coupled to the vacuum electromagnetic field on plasmonic structures to form polaritonic states with large Rabi splittings ∼ 0.7 eV. Conductivity experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility as revealed when the structure is gated in a transistor configuration. A theoretical quantum model is presented that confirms the delocalization of the wave-functions of the hybridized states and the consequences on the conductivity. While this is a proof-of-principle study, in practice conductivity mediated by light-matter hybridized states is easy to implement and we therefore expect that it will be used to improve organic devices. More broadly our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.Light and matter can enter into the strong coupling regime by exchanging photons faster than any competing dissipation processes. This is normally achieved by placing the material in a confined electromagnetic environment, such as a Fabry-Perot (FP) cavity composed of two parallel mirrors that is resonant with an electronic transition in the material. Alternatively, one can use surface plasmon resonances as in this study. Strong coupling leads to the formation of two hybridized light-matter polaritonic states, P+ and P-, separated by the so-called Rabi splitting, as illustrated in Figure 1a. According to quantum electrodynamics, in the absence of dissipation, the Rabi splitting for a single molecule is given bywhere ω is the cavity resonance or transition energy ( the reduced Planck constant), 0 the vacuum permittivity, v the mode volume, d the transition dipole moment of the material and n ph the number of photons present in the system. The last term implies that, even in the dark, the Rabi splitting has a finite value which is due to the interaction with the vacuum electromagnetic field. This vacuum Rabi splitting can be further increased by coupling a large number N of oscillators to the electromagnetic mode since Ω N R ∝ √ N [12]. In this ensemble coupling, in addition to P+ an...

Control of intermolecular interactions is crucial to the exploitation of molecular semiconductors for both organic electronics and the viable manipulation and incorporation of single molecules into nano-engineered devices. Here we explore the properties of a class of materials that are engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4'-diphenylene vinylene) or polyfluorene through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects that red-shift and partially quench the luminescence. Our approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport. We used the polymers to prepare single-layer light-emitting diodes with Ca and Al cathodes, and observed blue and green emission. The reduced tendency for polymer chains to aggregate allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning force microscopy.

During the last decade, two-dimensional materials (2DMs) have attracted great attention due to their unique chemical and physical properties, which make them appealing platforms for diverse applications in opto-electronic devices, energy generation and storage, and sensing. Among their various extraordinary properties, 2DMs possess high surface area-to-volume ratios and ultra-high surface sensitivity to the environment, which are key characteristics for applications in chemical sensing. Furthermore, 2DMs' superior electrical and optical properties, combined with their excellent mechanical characteristics such as robustness and flexibility, make these materials ideal components for the fabrication of a new generation of high-performance chemical sensors. Depending on the specific device, 2DMs can be tailored to interact with various chemical species at the non-covalent level, making them powerful platforms for fabricating devices exhibiting a high sensitivity towards detection of various analytes including gases, ions and small biomolecules. Here, we will review the most enlightening recent advances in the field of chemical sensors based on atomically-thin 2DMs and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and sensing devices.

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that the surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of ad hoc molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light on the theoretical and experimental studies, we provide a guide for improving the electronic performance of 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.

Photochromic systems can convert light energy into mechanical energy, thus they can be used as building blocks for the fabrication of prototypes of molecular devices that are based on the photomechanical effect. Hitherto a controlled photochromic switch on surfaces has been achieved either on isolated chromophores or within assemblies of randomly arranged molecules. Here we show by scanning tunneling microscopy imaging the photochemical switching of a new terminally thiolated azobiphenyl rigid rod molecule. Interestingly, the switching of entire molecular 2D crystalline domains is observed, which is ruled by the interactions between nearest neighbors. This observation of azobenzenebased systems displaying collective switching might be of interest for applications in high-density data storage.scanning tunnel microscopy ͉ molecular switches ͉ photochromic system ͉ data storage

：Room temperature strong coupling of WS 2 monolayer exciton transitions to metallic Fabry-Perot and plasmonic optical cavities is demonstrated. A Rabi splitting of 101 meV is observed for the Fabry-Perot cavity, more than double those reported to date in other 2D materials. The enhanced magnitude and visibility of WS 2 monolayer strong coupling is attributed to the larger absorption coefficient, the narrower linewidth of the A exciton transition, and greater spin-orbit coupling. For WS 2 coupled to plasmonic arrays, the Rabi splitting still reaches 60 meV despite the less favorable coupling conditions, and displays interesting photoluminescence features. The unambiguous signature of WS 2 monolayer strong coupling in easily fabricated metallic resonators at room temperature suggests many possibilities for combining light-matter hybridization with spin and valleytronics. induces the splitting of the excitonic transition by ca. 150 meV such that both the so-called A and B exciton transitions (see Figure 1b) can simultaneously interact with cavity modes complicating the studies of such systems. 27 The WS 2 monolayer has the advantage that it presents a much sharper isolated absorption band as can be seen in Figure 1b. In addition it displays an intense photoluminescence (PL) peak at 2.016 eV (Figure 1c). Hence WS 2 constitutes a natural choice for light-matter strong coupling. In this letter we demonstrate that by coupling WS 2 monolayers to metallic resonators, the magnitude and visibility of light-monolayer TMD strong coupling at room temperature is substantially enhanced with a Rabi splitting of 101 meV in Fabry-Perot cavities and 60 meV on plasmonic arrays. The energy-momentum dispersion properties of the monolayer WS 2 excitonpolaritons are explored by transmission, reflection and photoluminescence (PL) spectroscopy. In particular Rabi splittings in TE and TM dispersion curves give rise to unusual PL behavior. The results are discussed in terms of the potential of coherent light-matter interactions using WS 2 monolayers.To ensure high quality samples and to avoid environmental contamination, the TMD monolayers were exfoliated from bulk single crystals and then dry-transferred onto substrates as the holes possibly due to two factors: firstly, the plasmonic field has a maximum above the holes, enhancing the photonic mode density at this point, thereby increasing the excitonic radiative rate, 32 and secondly the increased dielectric screening where the monolayer is suspended over the hole rather than in Van der Waals contact with the substrate could enhance the emission. 10Angle-resolved transmission spectra of the FP cavity with WS 2 monolayer are shown in Figure 3a for TE polarization. The progressive dispersion of the cavity mode through the energy of the A exciton is accompanied by a clear anti-crossing, which is mapped out in terms of spectral maxima in Figure 3b. After fitting the energy of the two peaks as a function of in-plane momentum k // using the coupled oscillator model (described in the Met...

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write-erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.