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Structural defects, such as heteroatoms or atomic vacancies, are always present in materials and significantly affect their physical properties, in both positive or unwanted ways. Interestingly, defects generate an impressive range of functionalities in many materials, such as catalysis, electrical and thermal conductivity tuning, thermoelectricity, enhanced ion storage, magnetism, and others. These properties enable the use of defective materials in a great variety of technological applications. Here we review the principal properties generated by atomic vacancies in 2D compounds and thin films of transition metal dichalcogenides and the most consolidated methods for their formation and engineering. Eventually, we critically analysed the most important advantages, the limits and the current open challenges. [ABSTRACT FROM AUTHOR]

Abstract Spin crossover compounds are multifunctional switching materials that change their spin state and many other physical properties, such as colour, magnetic susceptibility, electric conductivity, dielectric constant and mechanical properties, upon external stimuli. Spin crossover materials have been proposed for a variety of technological applications that require the elaboration of highly controlled thin films and patterns. Here, we present a brief overview of the most diffused approaches for thin film growth and patterning, showing both conventional and unconventional approaches and the most recent advancement in their applications, highlighting the most promising cases and the most critical problems. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogenated nanocrystalline silicon (nc-Si:H) is a very promising material for photovoltaic applications. Notwithstanding its wide application as instrinsic layer in solar cells, many issues regarding its electronic and optical properties are not completely understood. The influence of crystal defects and impurities on the physical properties of nc-Si:H is not completely explored yet. The present contribution aims to report on defective states associated with crystal defects and impurities in nc-Si:H films grown by low-energy plasma enhanced chemical vapor deposition (LEPECVD) technique. Electronic levels associated to defect states were investigated by surface photovoltage spectroscopy (SPS) and the role of defects and impurities on the conduction mechanisms at a microscopic level were investigated by conductive atomic force microscopy (C-AFM). The results will be compared with structural analyses [Copyright &y& Elsevier]

When a local mechanical perturbation is applied to the surface of a thin film of a mechanically interlocked molecule (a rotaxane), the molecules self-organize into periodic arrays of discrete dots or lines. The dimensionality of the nanostructures depends on whether the mechanical stimulus acts along a 1D line or over a 2D area. The size (50–500 nm) and periodicity (100–600 nm) of the patterns are controlled solely by the film thickness. This self- organization at the mesoscopic scale occurs via a nucleation-ripening mechanism eased by the relatively low energy barriers of the intramolecular rearrangement introduced by the mechanical bond. The phenomenon can be exploited as a bottom-up nano-fabrication method. [ABSTRACT FROM AUTHOR]

Charge transport properties in organic semiconductors depend strongly on molecular order. Here we demonstrate field-effect transistors where drain current flows through a precisely defined array of nanostripes made of crystalline and highly ordered molecules. The molecular stripes are fabricated across the channel of the transistor by a stamp-assisted deposition of the molecular semiconductors from a solution. As the solvent evaporates, the capillary forces drive the solution to form menisci under the stamp protrusions. The solute precipitates only in the regions where the solution is confined by the menisci once the critical concentration is reached and self-organizes into molecularly ordered stripes 100−200 nm wide and a few monolayers high. The charge mobility measured along the stripes is 2 orders of magnitude larger than the values measured for spin-coated thin films. [ABSTRACT FROM AUTHOR]

Hydrogenated nanocrystalline silicon (nc-Si:H) is a multiphase, heterogeneous material, composed of Si nanocrystals embedded in an amorphous matrix. It has been intensively studied recently due to very promising photovoltaic and optoelectronics applications. The present paper aims to study electronic transitions in nc-Si:H grown by Low Energy Plasma Enhanced Chemical Vapor Deposition by Surface Photovoltage Spectroscopy. The study of several thin films grown at different conditions and the comparison with recently published computational studies allowed us to advance a hypothesis on the defect structure responsible for electronic transitions in nc-Si:H. [ABSTRACT FROM AUTHOR]

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and optoelectronic devices. The silk replication of patterned substrates with features down to tens of nanometers is exploited to realize highly transparent, mechanically stable, and free-standing structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave the way to the development of an optically active biocompatible technological platform based on silk. [ABSTRACT FROM AUTHOR]

Abstract: Here we address the problem of time and temperature instability of the electrical current related to parasitic doping effects in discotic liquid crystals (DLCs). We investigated the electrical transport in thin films of a phthalocyanine-based DLC in-dark and upon laser irradiation during long thermal cycles. Measuring the electrical transport between two electrodes we observed that in-dark the current is mostly due to doping levels while, upon laser irradiation, the photocurrent reflects the diffusion length of the photogenerated charge carriers and it is limited by the coherence length of a column. We prove here that the two contributions are completely independent. Our result experimentally supports the theoretical studies performed at the molecular level on discotic liquid crystals. [Copyright &y& Elsevier]

We describe a setup suitable for simultaneously measuring optical and electrical properties of a liquid crystal mesophase upon temperature variation, and the difference in the order parameters between the bulk and the interface with the substrate. It integrates high-resolution polarized light optical microscopy, temperature regulation, and electrical measurements in a controlled atmosphere with a software kernel that controls the instruments and synchronizes the data streams. A user-friendly interface allows us to program multistep experiments controlling all the instruments and data acquisition by a specifically designed scheduler. We tested our system on a thin film of alkoxy-substituted phthalocyanines deposited on a test pattern with interdigitated electrodes. We studied the optical and electrical behavior in the proximity of the bulk phase transition to isotropic liquid, identifying a few ordered monolayers anchored to the substrate above the transition temperature. [ABSTRACT FROM AUTHOR]

Photocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin-film transistors (OFETs). We have applied PC analyses to the study of the electronic density of states (DOS) distribution of the OFETs with crystalline-mixed films of two derivatives of a conjugated molecule [ α-sexithiophene (6T), and its alkylated analogue α, ω-dihexylsexithiophene (DH6T)] as the active semiconductor. We have investigated the modifications in the DOS distribution associated to variations in the carrier density in the OFET channel and we have detected the formation of deep electrically active states in the below-band-gap region, associated to polaron states induced by a prolonged exposure of the device to atmosphere. A clear correlation between the PC results and the electrical characteristics of the corresponding FET devices has been observed. [ABSTRACT FROM AUTHOR]

We show that ultrathin films of a semiconductive discotic liquid crystal, viz. phthalocyanines, can be organized to form a conductive channel tens of microns long between Au electrodes with thickness control over a single monolayer. Our approach exploits the electromigration of the isotropic phase formed starting from the pretransitional region of the columnar-isotropic phase transition. Dewetted isotropic material accumulates to the negative electrode by applying a longitudinal electric field of about 1 V/µm. Dewetting and electromigration expose an ultrathin film, a few monolayers thick, exhibiting columnar liquid crystal order. The layers of this ultrathin film melt progressively above Tc and can be individually exfoliated by electromigration, starting from the ninth down to the first monolayer. The analysis of the current flowing through the junction as a function of the temperature, together with the comparative imaging of the evolution of morphology, yields a detailed picture of the changes in the dimensionality of the conductive phthalocyanine film and allows us to extract the behavior of the order parameter. The phenomenon of electromigration opens interesting questions on the technological control of individual monolayers on device patterns. [ABSTRACT FROM AUTHOR]

Abstract: La0.7Sr0.3MnO3 (LSMO) manganite thin films were grown by pulsed plasma deposition on silicon (Si) and gallium arsenide (GaAs) substrates covered by an amorphous oxide. Manganite films are characterized by polycrystalline structure. Ferromagnetic transition is above room temperature and for 50nm thick film the Curie temperature was as high as 325K and 305K for LSMO/SiO x /Si and LSMO/AlO x /GaAs, respectively. [Copyright &y& Elsevier]

The electrochemical atomic layer epitaxy (ECALE) methodology is a valid and low-cost approach for thin film formation on a metallic substrate. This paper reports on a morphological and photoelectrochemical investigation carried out on CdS samples formed with various numbers of deposition cycles. Ex-situ atomic force microscopy (AFM) measurements showed the attainment of a well-defined morphology, with an almost constant size and density of clusters. The layer-by-layer growth mechanism that is the aim of ECALE methodology, and that is strongly suggested by the electrochemical characterisation, seems to be confirmed. The photoresponse of the material, evaluated in alkaline polysulphide medium, shows a spectral dependence in very good agreement with literature reports for CdS single crystals. [Copyright &y& Elsevier]

AlInN/AlN/GaN heterostructures were characterized by atomic force microscopy. V-defects and channels were observed. In phase-contrast mode, these features were found related to inhomogeneities associated with In-segregation (and/or In-diffusion) and Al-rich surface reconstruction. The electrical characterization via conductive atomic force microscopy showed enhanced conductivity regions related to In-rich traces within channels and V-defects. [ABSTRACT FROM AUTHOR]

Abstract: We report on the growth of magnetite films directly on thin layers of organic semiconductors by means of an electron beam ablation method. The deposition was performed at room temperature in a reactive plasma atmosphere. Thin films show ferromagnetic (FM) hysteresis loops and coercive fields of hundreds of Oersted. Micro Raman analysis indicates no presence of spurious phases. The morphology of the magnetite film is strongly influenced by the morphology of the underlayer of the organic semiconductor. These results open the way for the application of magnetite thin films in the field of organic spintronics. [Copyright &y& Elsevier]

In this work the authors report on photocurrent spectroscopy analyses on pentacene thin film transistors that have been bias stressed and exposed to atmosphere over a long period of time (up to 80 days). They have studied the charge transport properties and the electronic excited state energy and distribution of these organic thin film devices. They also have identified the major excitonic and band gap transition energies and have investigated their quite different behavior following exposure to atmosphere or to bias stress. Different charge carrier distributions and trapping phenomena are proposed to describe the observed effects. [ABSTRACT FROM AUTHOR]

Abstract: A study on the self-assembly of regiochemically differentiated methyl substituted quaterthiophenes is presented. Hierarchical self-assembly of 5,4′,3″,5‴-tetramethyl-2,2′:5′,2″:5″,2‴-quaterthiophene molecule, 7 leads to unprecedented fluorescent superstructures on glass and mica surfaces made by reiteration of a peculiar V-shaped recurring motif that extended at least for three generations. [Copyright &y& Elsevier]

Abstract: The attainment of ultrathin films of well ordered II–VI and III–V compound semiconductors is an active research area of the electrochemistry group at the University of Florence. The method employed is the electrochemical atomic layer epitaxy (ECALE) which is based on self limiting reactions such as underpotential deposition (UPD). Recently, the possibility of combining the electrodeposition at nanometer scale and the self assembly phenomena has been investigated. In particular, the ECALE method was used to deposit CdS on Ag(111) covered by a patterned hexadecanthiol (HDT) self assembled monolayer (SAM). In fact, the deposition of CdS only takes place on the uncovered silver substrate, whereas it is not possible on the thiols monolayers. The deposits have been characterized by electrochemical techniques and AFM measurements. [Copyright &y& Elsevier]

Abstract: La0.7Sr0.3MnO3 polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO x amorphous native oxide. Curie temperatures of about 325K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed. [Copyright &y& Elsevier]

Abstract: π-Conjugated polymers are central ingredients in the development of organic electronics and it is well-known that the degree of order in thin films of those materials is a major factor governing the performances of the devices (field effect transistors, light-emitting diodes, photovoltaic cells). In this review, we present some approaches for the control of the molecular assembly of conjugated polymers into well-defined nanostructures. We consider self-assembly leading to thin deposits from compounds molecularly dispersed in a solution and we focus on the relationship between the supramolecular interactions between the individual π-conjugated molecules/polymers and the microscopic morphology appearing upon aggregation. We describe approaches to control the self-assembly of conjugated systems in the solid-state via (i) the interactions with the substrate surface; (ii) the design of a well-defined molecular structure; and (iii) the use of block-copolymers comprising conjugated and non-conjugated blocks. To understand how supramolecular interactions influence the microscopic morphology, we developed a joint experimental–theoretical approach, combining scanning probe microscopy characterization of thin deposits and force-field molecular modeling of supramolecular aggregates and adsorbates. With this methodology, a strong correlation is established between the polymer molecular structure, the degree of supramolecular order, and the solid-state photoluminescence properties (for light-emitting applications), or the charge transport properties (for the design of field-effect transistors). Since the construction of small-size devices requires that the spatial growth of the thin deposits be perfectly controlled, we illustrate in the last section of the paper how soft lithographic methods and nanorubbing can be used to orient the formation of the self-assembled conjugated nanostructures in a confined space. [Copyright &y& Elsevier]

The self-organization of rotaxane thin films into spatially correlated nanostructures is shown to occur upon a thermal stimulus. The mechanism of formation of nanostructures and their organization has been investigated using atomic force microscopy, bright field transmission electron microscopy, selected area electron diffraction, and molecular mechanics simulations. The evolution of the nanostructures follows a complex pathway, where a rotaxane thin film first dewets from the substrate to form nanosized droplets. Droplets coalesce by ripening, generating spatially correlated motifs. In a later stage, the larger droplets change shape, nucleate, and coalesce to yield crystallites that grow into larger crystals by incorporating the surrounding droplets. The results show the following: (i) the nanostructures represent a metastable state of a crystallization process; (ii) spatial correlations emerge during ripening, but they are destroyed as stable nuclei are formed and crystallization proceeds to completion; iii) crystallization, either on graphite or amorphous carbon films, leads to a precise basal plane, viz. (010), which has minimum surface energy. The inherent degrees of freedom permitted in the rotaxane architecture favors the re-organization and nucleation of the film in the solid state. Low-energy trajectories leading to crystallites with stable surfaces and minimum energy contact plane are found to occur via concerted, small amplitude, internal motions without disruption of packing and intermolecular contacts. [ABSTRACT FROM AUTHOR]

Bottom contact FET devices were realized, using vacuum evaporated thin films of unsubstituted quinquethiophene and two modified derivatives deposited at different substrate temperatures. X-ray diffraction (XRD) and atomic force microscopy (AFM) data on thin films deposited in the same conditions are reported and related to the electrical characteristics of the devices. [Copyright &y& Elsevier]

Thin films made of organic semiconductors (α-sexithiophene, PDAS and PBAS) have been printed and the impact on morphology studied by optical, atomic force and electron microscopy. Surfaces in contact with the stamp during printing undergo a change towards smoother and more ordered material at the macromolecular scale. Interdigitated nanoelectrodes to be used as source and drain in TFTs have been made and printed down to 100 nm. PDAS and PBAS can be printed at room temperature and preserve their printed feature provided they are cross-linked afterwards. [Copyright &y& Elsevier]

Abstract: Magnetic and morphological properties of Cobalt thin films grown by RF sputtering on organic Alq3 layers were investigated by magneto-optical Kerr effect (MOKE) technique and atomic force microscopy (AFM). The AFM images indicate a template growth of Co layers on top of Alq3, the magnetic film “decorates” the surface of organic material. This peculiar morphology induces a strong uniaxial magnetic anisotropy in the Co films, as detected by MOKE measurements. Results are important for the operation of a new class of devices-vertical organic spin valves. [Copyright &y& Elsevier]

Abstract: Ultrathin ferromagnetic films of La0.7Sr0.3MnO3 were deposited by electron-beam ablation on standard SrTiO3(100) and NdGaO3(110) substrates. Even 4nm films are metallic and develop spontaneous magnetization at room temperature: similar hysteresis cycles were measured by magneto optical Kerr effect (MOKE) in the longitudinal configuaration and by SQUID magnetometry. To the best of our knowledge the 4nm thickness corresponds so far to the thinnest manganite film showing room-temperature ferromagnetism. [Copyright &y& Elsevier]