Combined experimental and computational study of the photoabsorption of the monodoped and nondoped nanoclusters Au24Pt(SR)18, Ag24Pt(SR)18, and Ag25(SR)18.

Assessing the accuracy of first-principles computational approaches is instrumental to predict electronic excitations in metal nanoclusters with quantitative confidence. Here we describe a validation study on the optical response of a set of monolayer-protected clusters (MPC). The photoabsorption spectra of Ag25(DMBT)18-, Ag24Pt(DMBT)182- and Au24Pt(SC4H9)18, where DMBT is 2,4-dimethylbenzenethiolate and SC4H9 is n-butylthiolate, have been obtained at low temperature and compared with accurate TDDFT calculations. An excellent match between theory and experiment, with typical deviations of less than 0.1 eV, was obtained, thereby validating the accuracy and reliability of the proposed computational framework. Moreover, an analysis of the TDDFT simulations allowed us to ascribe all relevant spectral features to specific transitions between occupied/virtual orbital pairs. The doping effect of Pt on the optical response of these ultrasmall MPC systems was identified and discussed.

High performance multi-purpose nanostructured thin films by inkjet printing: Au micro-electrodes and SERS substrates.

Nanostructured thin metal films are exploited in a wide range of applications, spanning from electrical to optical transducers and sensors. Inkjet printing has become a compliant technique for sustainable, solution-processed, and cost-effective thin films fabrication. Inspired by the principles of green chemistry, here we show two novel formulations of Au nanoparticle-based inks for manufacturing nanostructured and conductive thin films by using inkjet printing. This approach showed the feasibility to minimize the use of two limiting factors, namely stabilizers and sintering. The extensive morphological and structural characterization provides pieces of evidence about how the nanotextures lead to high electrical and optical performances. Our conductive films (sheet resistance equal to 10.8 ± 4.1 Ω per square) are a few hundred nanometres thick and feature remarkable optical properties in terms of SERS activity with enhancement factors as high as 107 averaged on the mm2 scale. Our proof-of-concept succeeded in simultaneously combining electrochemistry and SERS by means of real-time tracking of the specific signal of mercaptobenzoic acid cast on our nanostructured electrode.

Copper and silver nanowires for CO2 electroreduction.

Copper and silver nanowires have been extensively investigated as the next generation of transparent conductive electrodes (TCEs) due to their ability to form percolating networks. Recently, they have been exploited as electrocatalysts for CO2 reduction. In this review, we present the most recent advances in this field summarizing different strategies used for the synthesis and functionalization/activation of copper and silver nanowires, as well as, the state of the art of their electrochemical performance with particular emphasis on the effect of the nanowire morphology. Novel perspectives for the development of highly efficient, selective, and stable electrocatalysts for CO2 reduction arise from the translation of NW-based TCEs in this challenging field.

Isolation of the Au145(SR)60X compound (R = n-butyl, n-pentyl; X = Br, Cl): novel gold nanoclusters that exhibit properties subtly distinct from the ubiquitous icosahedral Au144(SR)60 compound.

We report the identification and quantitative isolation of Au145(SR)60X (R = n-butyl, n-pentyl; X = halide) along with elucidation of key properties as compared to the corresponding ubiquitous chiral-icosahedral Au144(SR)60 cluster known to have a central vacancy. The stoichiometries were assessed by electrospray mass spectrometry (ESI-MS) at isotopic resolution, and induced dissociation patterns indicate the 'extra' (Au,Br) atoms are strongly bound components of these structures. Voltammetric and spectroscopic characterization reveals Au145(SR)60X behaviors that are qualitatively similar to yet fascinatingly distinct from those of Au144(SR)60. (1H,13C)-NMR spectra clearly show how both Au145(SR)60X and Au144(SR)60 are capped by 12 distinct ligand types of 5-fold equivalence, as was recently established for Au144(SR)60 capped by shorter ligands, demonstrating that this novel cluster shares the same chiral-icosahedral motif. Intriguingly, Au145(SR)60X is strongly near-IR luminescent, whereas under comparable conditions Au144(SR)60 barely emits. The photoluminescence pattern of Au145(SR)60X is very similar to that observed for Au25(SR)18, which contains the Au13 core. The combined results are interpreted as consistent with neutral Au145(SR)60X as a diamagnetic species, electronically and structurally similar to the corresponding Au144(SR)60 compounds.

Predictive optical photoabsorption of Ag24Au(DMBT)18 - via efficient TDDFT simulations.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

Atomically Precise Metal Nanoclusters: Novel Building Blocks for Hierarchical Structures.

Atomically precise ligand-protected nanoclusters (NCs) constitute an important class of compounds that exhibit well-defined structures and, when sufficiently small, evident molecular properties. NCs provide versatile building blocks to fabricate hierarchical superstructures. The assembly of NCs indeed offers opportunities to devise new materials with given structures and able to carry out specific functions. In this Concept article, we highlight the possibilities offered by NCs in which the physicochemical properties are controlled by the introduction of foreign metal atoms and/or modification of the composition of the capping monolayer with functional ligands. Different approaches to assemble NCs into dimers and higher hierarchy structures and the corresponding changes in physicochemical properties are also described.

Graphene transistors for real-time monitoring molecular self-assembly dynamics.

Mastering the dynamics of molecular assembly on surfaces enables the engineering of predictable structural motifs to bestow programmable properties upon target substrates. Yet, monitoring self-assembly in real time on technologically relevant interfaces between a substrate and a solution is challenging, due to experimental complexity of disentangling interfacial from bulk phenomena. Here, we show that graphene devices can be used as highly sensitive detectors to read out the dynamics of molecular self-assembly at the solid/liquid interface in-situ. Irradiation of a photochromic molecule is used to trigger the formation of a metastable self-assembled adlayer on graphene and the dynamics of this process are monitored by tracking the current in the device over time. In perspective, the electrical readout in graphene devices is a diagnostic and highly sensitive means to resolve molecular ensemble dynamics occurring down to the nanosecond time scale, thereby providing a practical and powerful tool to investigate molecular self-organization in 2D.

X-Ray-Induced Growth Dynamics of Luminescent Silver Clusters in Zeolites.

Herein, AlKα X-rays are used to drive the growth of luminescent silver clusters in zeolites. The growth of the silver species is tracked using Auger spectroscopy and fluorescence microscopy, by monitoring the evolution from their ions to luminescent clusters and then metallic, dark nanoparticles. It is shown that the growth rate in different zeolites is determined by the mobility of the silver ions in the framework and that the growth dynamics in calcined samples obeys the Hill-Langmuir equation for noncooperative binding. Comparison of the optical properties of X-ray-grown silver clusters with silver clusters formed by standard heat treatment indicates that the latter have a higher specificity toward the formation of luminescent clusters of a specific (small) nuclearity, whereas the former produce a wide distribution of cluster species as well as larger nanoparticles.

Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers.

Singlet oxygen, 1O2, can be generated by molecules that upon photoexcitation enable the 3O2 → 1O2 transition. We used a series of atomically precise Au24M(SR)18 clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, 1O2 was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the 1O2 lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate 1O2 by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer 1O2 lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of 1O2 by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in 1O2 photosensitization is instrumental to controlling their performance.

Glutathionylation primes soluble glyceraldehyde-3-phosphate dehydrogenase for late collapse into insoluble aggregates.

Protein aggregation is a complex physiological process, primarily determined by stress-related factors revealing the hidden aggregation propensity of proteins that otherwise are fully soluble. Here we report a mechanism by which glycolytic glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana (AtGAPC1) is primed to form insoluble aggregates by the glutathionylation of its catalytic cysteine (Cys149). Following a lag phase, glutathionylated AtGAPC1 initiates a self-aggregation process resulting in the formation of branched chains of globular particles made of partially misfolded and totally inactive proteins. GSH molecules within AtGAPC1 active sites are suggested to provide the initial destabilizing signal. The following removal of glutathione by the formation of an intramolecular disulfide bond between Cys149 and Cys153 reinforces the aggregation process. Physiological reductases, thioredoxins and glutaredoxins, could not dissolve AtGAPC1 aggregates but could efficiently contrast their growth. Besides acting as a protective mechanism against overoxidation, S-glutathionylation of AtGAPC1 triggers an unexpected aggregation pathway with completely different and still unexplored physiological implications.

Boosting and Balancing Electron and Hole Mobility in Single- and Bilayer WSe2 Devices via Tailored Molecular Functionalization.

WSe2 is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe2 typically exhibit unipolar transport and poor electrical performance when conventional SiO2 dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe2 surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO2 dielectric substrates and top Au electrodes, we measure unipolar mobility as high as µh = 150 cm2 V-1 s-1 and µe = 17.9 cm2 V-1 s-1 in WSe2 single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe2, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe2 mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials.

Dual-Mode, Anisotropy-Encoded, Ratiometric Fluorescent Nanosensors: Towards Multiplexed Detection.

A nanosensor with dual-mode fluorescence response to pH and an encoded identification signal, was developed by exploiting excitation energy transfer and tailored control of molecular organization in core-shell nanoparticles. Multiple signals were acquired in a simple single-excitation dual-emission channels set-up.

Publisher Correction: Collective molecular switching in hybrid superlattices for light-modulated two-dimensional electronics.

The original version of this article incorrectly listed an affiliation of Sara Bonacchi as 'Present address: Institut National de la Recherche Scientifique (INRS), EMT Center, Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, 1650, Canada', instead of the correct 'Present address: Department of Chemical Sciences - University of Padua - Via Francesco Marzolo 1 - 35131 Padova - Italy'. And an affiliation of Emanuele Orgiu was incorrectly listed as 'Present address: Department of Chemical Sciences, University of Padua, Via Francesco Marzolo 1, Padova, 35131, Italy', instead of the correct 'Present address: Institut National de la Recherche Scientifique (INRS), EMT Center, Boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, 1650, Canada'. This has been corrected in both the PDF and HTML versions of the article.

Collective molecular switching in hybrid superlattices for light-modulated two-dimensional electronics.

Molecular switches enable the fabrication of multifunctional devices in which an electrical output can be modulated by external stimuli. The working mechanism of these devices is often hard to prove, since the molecular switching events are only indirectly confirmed through electrical characterization, without real-space visualization. Here, we show how photochromic molecules self-assembled on graphene and MoS2 generate atomically precise superlattices in which a light-induced structural reorganization enables precise control over local charge carrier density in high-performance devices. By combining different experimental and theoretical approaches, we achieve exquisite control over events taking place from the molecular level to the device scale. Unique device functionalities are demonstrated, including the use of spatially confined light irradiation to define reversible lateral heterojunctions between areas possessing different doping levels. Molecular assembly and light-induced doping are analogous for graphene and MoS2, demonstrating the generality of our approach to optically manipulate the electrical output of multi-responsive hybrid devices.

Hybrid Copper-Nanowire-Reduced-Graphene-Oxide Coatings: A "Green Solution" Toward Highly Transparent, Highly Conductive, and Flexible Electrodes for (Opto)Electronics.

This study reports a novel green chemistry approach to assemble copper-nanowires/reduced-graphene-oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances (<30 Ω sq-1 ) and transparencies in the visible region (transmittance > 70%) that are rivalling those of indium-tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet-chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof-of-concept, this study demonstrates the successful use of such coatings as electrodes in high-performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light-emitting diodes, and displays.

Reversible, Fast, and Wide-Range Oxygen Sensor Based on Nanostructured Organometal Halide Perovskite.

Nanostructured materials characterized by high surface-volume ratio hold the promise to constitute the active materials for next-generation sensors. Solution-processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self-assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O2 has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000-fold) in the resistance of perovskite-based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating from an O2 -mediated iodine vacancies filling. A variation as small as 70 ppm in the oxygen concentration can be detected. The effect is fast (<400 ms) and fully reversible, making organohalide perovskites ideal active materials for oxygen sensing. The effect of oxygen on the electrical characteristics of organohalide perovskites must be taken into deep consideration for the design and optimization of any other perovskite-based (opto-) electronic device working in ambient conditions.

Engineering Chemically Active Defects in Monolayer MoS2 Transistors via Ion-Beam Irradiation and Their Healing via Vapor Deposition of Alkanethiols.

Irradiation of 2D sheets of transition metal dichalcogenides with ion beams has emerged as an effective approach to engineer chemically active defects in 2D materials. In this context, argon-ion bombardment has been utilized to introduce sulfur vacancies in monolayer molybdenum disulfide (MoS2 ). However, a detailed understanding of the effects of generated defects on the functional properties of 2D MoS2 is still lacking. In this work, the correlation between critical electronic device parameters and the density of sulfur vacancies is systematically investigated through the fabrication and characterization of back-gated monolayer MoS2 field-effect transistors (FETs) exposed to a variable fluence of low-energy argon ions. The electrical properties of pristine and ion-irradiated FETs can be largely improved/recovered by exposing the devices to vapors of short linear thiolated molecules. Such a solvent-free chemical treatment-carried out strictly under inert atmosphere-rules out secondary healing effects induced by oxygen or oxygen-containing molecules. The results provide a guideline to design monolayer MoS2 optoelectronic devices with a controlled density of sulfur vacancies, which can be further exploited to introduce ad hoc molecular functionalities by means of thiol chemistry approaches.

Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene.

The rise of 2D materials made it possible to form heterostructures held together by weak interplanar van der Waals interactions. Within such van der Waals heterostructures, the occurrence of 2D periodic potentials significantly modifies the electronic structure of single sheets within the stack, therefore modulating the material properties. However, these periodic potentials are determined by the mechanical alignment of adjacent 2D materials, which is cumbersome and time-consuming. Here we show that programmable 1D periodic potentials extending over areas exceeding 104 nm2 and stable at ambient conditions arise when graphene is covered by a self-assembled supramolecular lattice. The amplitude and sign of the potential can be modified without altering its periodicity by employing photoreactive molecules or their reaction products. In this regard, the supramolecular lattice/graphene bilayer represents the hybrid analogue of fully inorganic van der Waals heterostructures, highlighting the rich prospects that molecular design offers to create ad hoc materials.

High, Anisotropic, and Substrate-Independent Mobility in Polymer Field-Effect Transistors Based on Preassembled Semiconducting Nanofibrils.

Achieving nanoscale control over the crystalline structure and morphology of electroactive polymer films and the possibility to transfer them onto any solid substrate are important tasks for the fabrication of high-performance organic/polymeric field-effect transistors (FETs). In this work, we demonstrate that ultrathin active layers preassembled at the water/air interface can possess high, anisotropic, and substrate-independent mobility in polymer FETs. By exploiting a modified approach to the Langmuir-Schaeffer technique, we self-assemble conjugated polymers in fibrillar structures possessing controlled thickness, nanoscale structure, and morphology; these highly ordered nanofibrils can be transferred unaltered onto any arbitrary substrate. We show that FETs based on these films possess high and anisotropic hole mobility approaching 1 cm2 V-1 s-1 along the nanofibrils, being over 1 order of magnitude beyond the state-of-the-art for Langmuir-Schaefer polymer FETs. Significantly, we demonstrate that the FET performances are independent of the chemical nature and dielectric permittivity of the substrate, overcoming a critical limit in the field of polymer FETs. Our method allows the fabrication of ultrathin films for low-cost, high-performance, transparent, and flexible devices supported on any dielectric substrate.

Variable Doping Induces Mechanism Swapping in Electrogenerated Chemiluminescence of Ru(bpy)32+ Core-Shell Silica Nanoparticles.

The impact of nanotechnology on analytical science is hardly overlooked. In the search for ever-increasing sensitivity in biomedical sensors, nanoparticles have been playing a unique role as, for instance, ultrabright labels, and unravelling the intimate mechanisms which govern their functioning is mandatory for the design of ultrasentitive devices. Herein, we investigated the mechanism of electrogenerated chemiluminescence (ECL) in a family of core-shell silica-PEG nanoparticles (DDSNs), variously doped with a Ru(bpy)32+ triethoxysilane derivative, and displaying homogeneous morphological, hydrodynamic, and photophysical properties. ECL experiments, performed in the presence of 2-(dibutylamino)ethanol (DBAE) as coreactant, showed two parallel mechanisms of ECL generation: one mechanism (I) which involves exclusively the radicals deriving from the coreactant oxidation and a second one (II) involving also the direct anodic oxidation of the Ru(II) moieties. The latter mechanism includes electron (hole) hopping between neighboring redox centers as evidenced in our previous studies and supported by a theoretical model we have recently proposed. Quite unexpectedly, however, we found that the efficiency of the two mechanisms varies in opposite directions within the DDSNs series, with mechanism I or mechanism II prevailing at low and high doping levels, respectively. Since mechanism II has an intrinsically lower efficiency, the ECL emission intensity was also found to grow linearly with doping only at relatively low doping levels while it deviates negatively at higher ones. As the ζ-potential of DDSNs increases with the doping level from negative to slightly positive values, as a likely consequence of the accumulating cationic charge within the silica core, we attributed the observed change in the ECL generation mechanism along the DDSN series to a modulation of the electrostatic and hydrophobic/hydrophilic interactions between the DDSNs and the radical cationic species involved in the ECL generation. The results we report therefore show that the ECL intensity of a nanosized system cannot be merely incremented acting on doping, since other parameters come into play. We think that these results could serve as valuable indications to design more efficient ECL nano- and microsized labels for ultrasensitive bioanalysis.