Freestanding palladium nanosheets with plasmonic and catalytic properties

Two-dimensional engineering of Pd nanosheets as advanced electrocatalysts toward formic acid oxidation

Materials that consist of just one or a few layers of atoms could have a range of useful applications. Computer simulations now show that the element tellurium might form three such phases, and that they have potentially useful properties. Materials that consist of just one or a few layers of atoms could have a range of useful applications. Computer simulations now show that the element tellurium might form three such phases, and that they have potentially useful properties.

Transparent conductive electrodes, as transmission windows of photons and electrons, play important roles in high‐performance organic optoelectronic devices. The replacement of widely used indium tin oxide (ITO) electrodes has been attempted due to the increasing cost and intrinsically brittle characteristics of ITO. Ultrathin metal films, with excellent optoelectrical features, high flexibility, and sufficient mechanical stability, have been considered a potential candidate for the use as transparent conductive electrodes. However, ultrathin metal films follow the Volmer–Weber mechanism, resulting in a rough and discontinuous morphology with poor optoelectrical properties due to the bad adhesion to substrates. This review summarizes the progress in strategies for preparing ultrathin and ultrasmooth metal films with superior transmittance and conductivity by successfully suppressing the Volmer–Weber mechanism. The electrical and optical performances of the ultrathin metal films based on improved nucleation processes, as well as applications in ITO‐free organic optoelectronic devices, are also described and discussed in detail.

Plasmonic materials and metamaterials have been widely utilized to achieve spectral transmission, reflection and absorption filters based on localized or delocalized resonances arising from the interaction of photons with nanostructured materials. Realization of visible-frequency, high-performance, large-area, optical filters based on nanoplasmonic materials is rather challenging due to nanofabrication related problems (cost, fabrication imperfection, surface roughness) and optical losses of metals. Here, we propose and demonstrate large-area perfect absorbers and transmission filters that overcome difficulties associated with the nanofabrication using a lithography-free approach. We also utilize and benefit from the optical losses in metals in our optical filter designs. Our resonant optical filter design is based on a modified, asymmetric metal-insulator-metal (MIM) based Fabry-Perot cavity with plasmonic, lossy ultra-thin (~30 nm) metallic films used as the top metallic layer. We demonstrated a narrow bandwidth (~17 nm) super absorber with 97% maximum absorption with a performance comparable to nanostructure/nanoparticle-based super absorbers. We also investigated transmission (color) filters using ultra-thin metallic films, in which different colors can be obtained by controlling the dielectric spacer thickness. With performance parameters of transmission peak intensity reaching 60% and a narrow-band of ~ 40 nm, our color filters exceed the performance of widely studied plasmonic nanohole array based color filters. Proposed asymmetric Fabry-Perot cavities using ultra-thin metallic films could find applications in spectrally selective optical (color and absorber) filters, optoelectronic devices with controlled bandwidth such as narrow-band photodetectors, and light-emitting devices.

Interfacial adhesion between metallic thin films and polymers is a critical performance metric for a number of microelectronics and packaging applications. Delamination of metal-polymer interfaces is a frequent failure mode for many multilayer structures, like those used for electronics packaging. Such a failure is even more likely when electronic packages are operated under extreme conditions like high-power, high-temperature, and/or high-humidity operation. Roughening or direct chemical modification of the few layers of atoms that make up the interface is often used to promote adhesion at these interfaces. Here, the authors investigate a new process, vapor phase infiltration, that infiltrates inorganic constituents into the bulk of the polymer, creating an interpenetrating network within the subsurface of the polymer that further enhances interfacial adhesion. For the authors’ model system of copper films on a benzocyclobutene polymer, they are able to increase the interfacial adhesion strength by as much as 3×, resulting in cohesive rather than adhesive failure. The authors attribute this increased interfacial adhesion to physicochemical interlocking of the organic and inorganic phases within the subsurface of the polymer, generating a “root system” that impedes interfacial delamination.

Chapter 1 Magnetism in ultrathin transition metal films

Silicon is the most widely used material for visible photodetection, with extensive applications in both consumer and industrial products. Further, its excellent optoelectronic properties and natural abundance have made it nearly ideal for microelectronic devices and solar cells. However, its lack of absorption in the infrared precludes its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications. Here we show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal films (<20 nm). Through appropriate optical design, a zeroth-order Fabry–Perot resonance is achieved, enabling ∼80% light absorption below the bandgap of the semiconductor. Absorption within the metal film results in excitation and injection of hot carriers through a Schottky junction into the Si. We experimentally demonstrate this phenomenon with four ultrathin planar metal films (Pt, Fe, Cr, and Ti), chosen to satisfy the resonant condition over a wide rang...

Transparent electrodes (TEs) are the essential elements of many optoelectronic devices such as solar cells, touch screens, organic LEDs, and LCDs. Consequently demand for TEs is growing very steeply and the market value presently stands at 8 billion USDs. The state-of-art indium tin oxide (ITO) has an excellent trade-off between optical transparency and electrical sheet resistance but suffers from several drawbacks, mainly the increasing cost due to indium shortage, and inadequate flexibility due to poor mechanical ductility. This thesis presents the development of a new class of TEs based on ultrathin metal films (UTMFs). The work started from understanding the fundamental aspects of UTMF growth and properties, and then focused on different UTMF based geometries, composition, and combination for potential applications in different optoelectronic applications. Single component ultrathin Ni and Cr films were shown to possess significantly high transparency in the ultraviolet (175–400 nm) and mid-infrared (2.5–25 μm) regions making them viable TE for devices such as UV photodiodes, and IR pyroelectric detectors. The natural oxidation process, which is a major concern for metal films, has been exploited to achieve stable metallic films by inducing a protective oxide layer. In another proposed novel design, incorporating an ad hoc conductive grid, the sheet resistance of UTMFs can be reduced by more than two orders of magnitude with negligible loss in transparency, which in turn eliminates the inverse trade-off relationship between optical transparency and electrical conductivity of continuous metal based TEs. A TE structure based on the ultrathin conductive Cu films with an application specific functionalized capping layer of Ti or Ni layer has been demonstrated. The properties of the TE can be tuned accordingly and show excellent stability against temperature, and oxidation. The suitability of Ag-Cu alloy films as TE as an alternative to ITO has been also investigated. The optical spectrum of such alloy films follows the average optical behavior of single component Cu and Ag layers, thus resulting in a much flatter optical response in the visible region. UTMFs combined with Al doped ZnO (AZO), which is possible ITO replacement, has also been demonstrated to show the possibility of hybridizing the two technologies. A bilayer Ag/AZO has been developed which can overcome the high reflection of metals and retain their good electrical behavior, while maintaining a minimum total film thickness. In another structure, UTMF capping layer were used to improve the stability of AZO. It was found that an ultrathin oxidized Ni capping layer with a thickness at percolation threshold greatly enhances the stability of AZO layer in harsh environment without affecting the electro-optical properties

Ultrathin metallic films are important functional materials for optical and microelectronic devices. Dedicated characterization with high spatial resolution and sufficient field of view is key to the understanding of the relation between microstructure and optical and electrical properties of such thin films. Here, we have applied on-axis transmission Kikuchi diffraction (TKD) and scanning precession electron diffraction (SPED) to study the microstructure of 10 nm thick polycrystalline gold films. The study compares the results obtained from the same specimen region by the two techniques and provides insights on the limits of each diffraction technique. We compare the physical spatial resolution of on-axis TKD and SPED and discuss challenges due to the larger probe size in scanning electron microscopy (SEM). Moreover, we present an improvement for the physical spatial resolution (PSR) of on-axis TKD through acquisition in immersion mode. We show how this method extends the capabilities of SEM-based microstructure characterization of ultrathin films and achieve PSR comparable to semi-automated SPED. • State of the art on-axis TKD SEM system comparison with SPED TEM system. • High resolution orientation mapping of ultrathin nano-crystalline metal films. • Improving TKD physical spatial resolution maps by immersion mode.

We predict spin Hall angles up to 80% for ultrathin noble metal films with substitutional Bi impurities. The colossal spin Hall effect is caused by enhancement of the spin Hall conductivity in reduced sample dimension and a strong reduction of the charge conductivity by resonant impurity scattering. These findings can be exploited to create materials with high efficiency of charge to spin current conversion by strain engineering.

In order to investigate quantum size effects in ultrathin metal films, tunneling spectroscopy measurements have been performed in the epitaxial CoSi2/Si system, with a metal thickness ranging from 1000 A down to 35 A, i.e. a few de Broglie wavelengths of electrons in CoSi 2. The resulting spectra show extremely rich sets of features, the origin of which are investigated. The peaks observed at low energy (-100 meV, +100 meV) are thickness independent and attributed to phonon emission by hot electrons. The peaks observed at higher energy (up to 600 meV) are thickness dependent but their physical origin is not yet fully ascertained. The absence of unambiguous electron quantization effects in these epitaxial films is discussed and tentatively attributed to small thickness fluctuations (of the order of a few monolayers), which tend to blur the quantization of the electronic energies.

The temperature coefficient of resistance (TCR) of ultrathin (40–120 Å) metal films is investigated. The effects of temperature, island size, interisland separation, and thickness are incorporated into an analytic expression for the TCR based upon an activated tunneling model. Experimental data are presented to verify the relationships. Anomalous effects, including experimental data, are discussed. An analytic treatment of this situation results in the inclusion of an additional negative term in the TCR expression when compared to the activated tunneling case. The possibilities of positive and negative TCR are indicated.

Near-bulk resistivity of sub-10nm Au films by breaking free from thickness downscaling effects

Understanding energy transfer via near-field thermal radiation is essential for applications such as near-field imaging, thermophotovoltaics and thermal circuit devices. Evanescent waves and photon tunneling are responsible for the near-field energy transfer. In bulk noble metals, however, surface plasmons do not contribute efficiently to the near-field energy transfer because of the mismatch of wavelength. In this paper, a giant near-field radiative heat transfer rate that is orders-of-magnitude greater than the blackbody limit between two ultrathin metallic films is demonstrated at nanoscale separations. Moreover, different physical origins for near-field thermal radiation transfer for thick and thin metallic films are clarified, and the radiative heat transfer enhancement in ultrathin metallic films is proved to come from the excitation of surface plasmons. Meanwhile, because of the inevitable high sheet resistance of ultrathin metal films, the heat transfer coefficient is 4600 times greater than the Planckian limit for the separation of 10 nm in ultrathin metallic films, which is the same order or even greater than that in other 2D materials with low carrier density. Our work shows that ultrathin metallic films are excellent materials for radiative heat transfer, which may find promising applications in thermal nano-devices and thermal engineering.

A three-stage engineered deposition rate profile in evaporation processes for ultra-thin metallic transparent conductors