SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Thin films

166

The coating of thin films is applied in numerous fields and many methods are employed for the deposition of these films. Some coating techniques may deposit films at high speed; for example, ordinary printing paper is coated with micrometre-thick layers of clay at a speed of tens of meters per second. However, to coat nanometre thin films at high speed, vacuum techniques are typically required, which increases the complexity of the process. Here, we report a simple wet chemical method for the high-speed coating of films with thicknesses at the nanometre level. This soap-film coating technique is based on forcing a substrate through a soap film that contains nanomaterials. Molecules and nanomaterials can be deposited at a thickness ranging from less than a monolayer to several layers at speeds up to meters per second. We believe that the soap-film coating method is potentially important for industrial-scale nanotechnology.

Concepts: Light, Coating, Thin film, Speed, Deposit account, Thin films, Miles per hour, Speed of light

29

In this article, we report only 10 atomic layer thick, high mobility, transparent thin film transistors (TFTs) with ambipolar device characteristics fabricated on both a conventional silicon platform as well as on a flexible substrate. Monolayer graphene was used as metal electrodes, 3-4 atomic layers of h-BN were used as the gate dielectric, and finally bilayers of WSe2 were used as the semiconducting channel material for the TFTs. The field effect carrier mobility was extracted to be 45 cm(2)/(V s), which exceeds the mobility values of state of the art amorphous silicon based TFTs by ∼100 times. The active device stack of WSe2-hBN-graphene was found to be more than 88% transparent over the entire visible spectrum and the device characteristics were unaltered for in-plane mechanical strain of up to 2%. The device demonstrated remarkable temperature stability over 77-400 K. Low contact resistance value of 1.4 kΩ-μm, subthreshold slope of 90 mv/decade, current ON-OFF ratio of 10(7), and presence of both electron and hole conduction were observed in our all two-dimensional (2D) TFTs, which are extremely desirable but rarely reported characteristics of most of the organic and inorganic TFTs. To the best of our knowledge, this is the first report of all 2D transparent TFT fabricated on flexible substrate along with the highest mobility and current ON-OFF ratio.

Concepts: Semiconductor, Silicon, Transistor, Field-effect transistor, Thin film, Thin-film transistor, Amorphous silicon, Thin films

9

The emerging field of plasmonic metamaterials has introduced new degree of freedom to manipulate optical field from nano to macroscopic scale, offering an attractive platform for sensing applications. So far, metamaterial sensor concepts, however, have focused on hot-spot engineering to improve the near-field enhancement, rather than fully exploiting tailored material properties. Here, we present a novel spectroscopic technique based on the metamaterial infrared (IR) absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement. Specifically, we experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer. The metamaterial consisting of an array of Au/MgF2/Au structures exhibits an anomalous absorption at ~3000 cm(-1), which spectrally overlaps with C-H stretching vibrational modes. Symmetric/asymmetric C-H stretching modes of a 16-Mercaptohexadecanoic acid monolayer are clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. Spectral analysis using Fano line-shape fitting reveals the underlying resonant interference in plasmon-molecular coupled systems. Our metamaterial approach achieves the attomole sensitivity with a large signal-to-noise ratio in the far-field measurement, thus may open up new avenues for realizing ultrasensitive IR inspection technologies.

Concepts: Spectroscopy, Optics, Electromagnetic radiation, Nanomaterials, Materials science, Infrared, Self-assembled monolayer, Thin films

3

Despite the need for molecularly smooth self-assembled monolayers (SAMs) on silicon dioxide surfaces (the most common dielectric surface), current techniques are limited to non-ideal silane grafting. Here, we show unique bio-inspired zwitterionic molecules forming a molecularly smooth and uniformly thin SAM in “water” in <1 min on various dielectric surfaces, which enables a dip-coating process that is essential for organic electronics to become reality. This monomolecular layer leads to high mobility of organic field-effect transistors (OFETs) based on various organic semiconductors and source/drain electrodes. A combination of experimental and computational techniques confirms strong adsorption (Wad>20 mJ m-2), uniform thickness (~0.5 or ~1 nm) and orientation (all catechol head groups facing the oxide surface) of the “monomolecular” layers. This robust (strong adsorption), rapid, and green SAM represents a promising advancement towards the next generation of nanofabrication compared to the current non-uniform and inconsistent polysiloxane-based SAM involving toxic chemicals, long processing time (>10 h), and/or heat (>80°C).

Concepts: Oxygen, Silicon, Transistor, Germanium, Self-assembled monolayer, Thin films, Monolayer, Organic semiconductor

3

The conversion of a biphenylthiol self-assembled monolayer (BPT SAM) on copper into graphene via electron irradiation and annealing is described by Andrey Turchanin and co-workers on page 4146. This 2D solid-state reaction can be tuned by temperature, allowing the crystallinity of the graphene layers to be adjusted. It is feasible to create graphene structures of any 3D shape as molecular self-assembly can be conducted on nonplanar suraces. Scanning tunneling microscopy of the PT SAM on Cu(111) is described.

Concepts: Electron, Nanotechnology, Self-organization, Microscopy, Scanning tunneling microscope, Scanning probe microscopy, Self-assembled monolayer, Thin films

1

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is the interface losses in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials, without compromising efficiency, stability or scalability of perovskite solar cells. Tantalum doped tungsten oxide (Ta-WO x )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. Using a simple regular planar architecture device, Ta-WO x doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and combined with over 1000 hours of light stability based on a self-assembled monolayer. By eliminating additional ionic dopants, these findings open up the whole class of organics as scalable hole-transporting materials for perovskite solar cells.

Concepts: Solar cell, Photovoltaics, Dye-sensitized solar cell, Thin film solar cell, Germanium, Thin films, P-n junction, Anwell Technologies Limited

1

The fabrication of porous coordination frameworks in thin-film forms has been investigated intensively with a view to using their structural response to external stimuli and guests for potential nanotechnological applications, for example as membranes for gas separation. Here we report a coordination framework that exhibits a dynamic guest-sorption behaviour in a nanometre-sized thin-film form (16 nm thick), yet shows no guest uptake in the bulk. Highly oriented crystalline thin films of this coordination framework-which consists of interdigitated two-dimensional layers of {Fe(py)2[Pt(CN)4]} (py, pyridine)-were fabricated through liquid-phase layer-by-layer synthesis. The resulting thin film exhibited a clear guest uptake with a structural transformation of the gate-opening type as characterized by in situ X-ray diffraction. Increasing the film’s thickness markedly suppressed this behaviour. We envisage that such a crystal-downsizing effect may be observed with other coordination frameworks, and may be of use to develop functional materials, for example, for switching or sensing devices.

Concepts: Diffraction, X-ray, Semiconductor, Chemical vapor deposition, Thin film, Thin films

1

Multilayer thin films have garnered intense scientific interest due to their potential application in diverse fields such as catalysis, optics, energy, membranes, and biomedicine. Here we review the current technologies for multilayer thin-film deposition using layer-by-layer assembly, and we discuss the different properties and applications arising from the technologies. We highlight five distinct routes of assembly—immersive, spin, spray, electromagnetic, and fluidic assembly—each of which offers material and processing advantages for assembling layer-by-layer films. Each technology encompasses numerous innovations for automating and improving layering, which is important for research and industrial applications. Furthermore, we discuss how judicious choice of the assembly technology enables the engineering of thin films with tailor-made physicochemical properties, such as distinct-layer stratification, controlled roughness, and highly ordered packing.

Concepts: Engineering, Technology, Thin film, Thin films

0

The reaction mechanism of area-selective atomic layer deposition (AS-ALD) of Al2O3 thin films using self-assembled monolayers (SAMs) was systematically investigated by theoretical and experimental studies. Trimethylaluminum (TMA) and H2O were used as the precursor and oxidant, respectively, with octadecylphosphonic acid (ODPA) as an SAM to block Al2O3 film formation. However, Al2O3 layers began to form on the ODPA SAMs after several cycles, despite reports that CH3-terminated SAMs cannot react with TMA. We showed that TMA does not react chemically with the SAM, but is physically adsorbed, acting as a nucleation site for Al2O3 film growth. Moreover, the amount of physisorbed TMA was affected by the partial pressure. By controlling it, we developed a new AS-ALD Al2O3 process with high selectivity, which produces films of ~60 nm thickness over 370 cycles. The successful deposition of Al2O3 thin film patterns using this process is a breakthrough technique in the field of nanotechnology.

Concepts: Oxygen, Physics, Chemistry, Layer, Chemical vapor deposition, Thin film, Chemical engineering, Thin films

0

Recently two dimensional (2D) organic-inorganic perovskites emerge as an alternative material for their 3D counterparts in photovoltaic applications with improved moisture resistance. Here, we report a stable, high-gain phototransistor consisting of a monolayer graphene on hexagonal boron nitride (hBN) covered by a 2D multiphase perovskite heterostructure, which was realized using a newly developed two-step ligand exchange method. In this phototransistor, the multiple phases with varying bandgap in 2D perovskite thin films are aligned for the efficient electron-hole pair separation, leading to a high responsivity of ~10(5) A W(-1) at 532 nm. Moreover, the designed phase alignment method aggregates more hydrophobic butylammonium cations close to the upper surface of the 2D perovskite thin film, preventing the permeation of moisture and enhancing the device stability dramatically. In addition, faster photoresponse and smaller 1/f noise observed in the 2D perovskite phototransistors indicate a smaller density of deep hole traps in the 2D perovskite thin film compared with their 3D counterparts. These desirable properties not only improve the performance of the phototransistor, but also provide a new direction for the future enhancement of the efficiency of 2D perovskite photovoltaics.

Concepts: Condensed matter physics, Semiconductor, Solar cell, Superconductivity, Noise, 2D computer graphics, Thin films, Boron nitride