Ultrathin Silver Layers Research Articles

Early-stage silver growth during sputter deposition on SiO2 and polystyrene – Comparison of biased DC magnetron sputtering, high-power impulse magnetron sputtering (HiPIMS) and bipolar HiPIMS

The integration of silver thin films into optoelectronic devices has gained much interest due to their exceptional properties in terms of conductivity and compatibility with flexible substrates. For this type of application, ultra-thin layers are desirable, because of their optical transparency. Standard direct current magnetron sputtering (DCMS) is known to lead to undesirable formation of islands at low effective film thicknesses on typical substrates like SiO2 or polystyrene (PS). Therefore, in this study, we explore high-power impulse magnetron sputtering (HiPIMS) with optional further acceleration of metal ions by biasing the substrate or an additional positive pulse (bipolar HiPIMS) for the fabrication of ultra-thin silver layers. The morphology and electrical properties of ultra-thin silver layers with selected effective thicknesses are characterized on SiO2 and PS substrates. The growth evolution of characteristic parameters is further investigated by in-situ grazing-incidence small-angle X-ray scattering (GISAXS). The results show that HiPIMS deposition yields films with a higher density of clusters than DCMS leading to a percolation threshold at lower effective film thicknesses. This effect is amplified by further ion acceleration. Thus, we suggest HiPIMS as a promising technique for fabricating ultra-thin, conductive layers on organic and oxide substrates.

Surface segregation of rigid polyarylene ether amidoxime on polyurethane nanofiber into hierarchical membranes as substrate of flexible SERS nanosensor for sulfamethoxazole detection

Electrospun polymeric nanofibrous membranes are emerging as the promising substrates for preparation of flexible SERS nanosensors due to their intrinsic nanoscale surface roughness, easy scalability as well as rich surface reactivity. Although the nanofiber membranes prepared from high performance thermoplastics exhibit good mechanical stability, the SERS nanosensors based on these substrates normally have lower signal-to-noise ratio because of the interference from background Raman signals of aromatic moieties. Herein, we synthesized an optically transparent polyurethane (PU) and rigid polyarylene ether amidoxime (PEA), which were electrospun into core-shell nanofibers membranes with a “beads-on-web” morphology. Furthermore, the PU-PEA membranes were coated with ultra-thin silver layer and thermally annealed to prepare the flexible SERS nanosensor without any background noises. In addition, the Raman enhancement of SERS nanosensor can be readily improved by tuning of PU-PEA composition, silver thickness as well as thermal annealing temperature. Finally, the optimized SERS nanosensor enables label-free detection of sulfamethoxazole as low as 0.1 nM with a good reproducibility and detection performance in real water sample. Meanwhile, the optimized SERS nanosensor shows long term anti-biofouling capacity. Thanks to its facile fabrication, competitive analytical performance and resistance to biofouling, the current work basically open new way for design of flexible SERS nanosensors for biomedical applications.

Scalable fabrication of silver covered polyurethane nanofibers as flexible SERS nanosensors for aflatoxin detection

Flexible surface enhanced Raman spectroscopy (SERS) nanosensors, constructed by integration of plasmonic nanostructures with polymeric substrates, have received increasing research interests for recent decades. When compared to abundant works on optimization of the plasmonic nanostructures, the research involving the influence of polymeric substrates on analytical performance of resultant flexible SESR nanosensors is unexpectedly limited. Herein, the ultra-thin silver layer has been deposited on the electrospun polyurethane (ePU) nanofibrous membranes via vacuum evaporation to prepare the flexible SRES nanosensors. Interestingly, we find that the molecular weight and polydispersion index of synthesized PU play important roles in regulating the fine morphology of electrospun nanofibers, which in turns determine the Raman enhancement of resultant flexible SERS nanosensors. Specifically, the optimized SERS nanosensor, obtained by evaporating 10 nm silver layer on top of nanofibers derived from electrospinning of PU with a weight-average molecular weight of 140,354 and polydispersion index of 1.26, enables label-free detection of the carcinogen of aflatoxin down to 0.1 nM. Thanks to its scalable fabrication and good sensitivity, the current work opens new way for design of cost-effective flexible SERS nanosensors for environmental monitoring and food security applications.

Enhanced MOKE Via Tunable Plasmon Resonance Field in the Otto Configuration

Here, we demonstrate the enhancement of the transverse magneto-optical Kerr effect (TMOKE) signal, due to surface plasmon resonance in the Otto configuration, where the low index dielectric has a variable thickness. This constitutes a demonstration that, in principle, a separation of the magnetic sample from the plasmonic device and the modulation of the plasmon resonance with an enhancement of the MOKE signal is possible. We have achieved this by using air as a low index dielectric where the evanescent wave extends, preceding to excitation of surface plasmons. The magnetic sample under consideration is a thin layer of cobalt coated by an ultrathin silver layer, on a silicon substrate (Ag/Co/Si). The sample is brought close enough to the prism/air interface, allowing surface plasmon excitation in the air/Ag interface. This leads to an increase of the TMOKE signal up to ~ 2 ‰ with respect to the incident light. This is about 7 times the traditional MOKE signal in the absence of plasmons. This is comparable with previous works using the Kretschmann-Raether configuration. Furthermore, the fact that the plasmon field generated at the metal-air interface substitutes the laser light used in traditional MOKE allows new functionalities such as controlling the penetration depth of the plasmonic field into the sample. This should find applications in magnetometry and related technology.

Probing the stability of perovskite solar cell under working condition through an ultra-thin silver electrode: Beyond the halide ion diffusion and metal diffusion

Perovskite solar cells (PSCs) with an excellent optoelectronic performance have intrigued mushrooming research interests, and the undesirable intrinsic stability of halide perovskite materials still remains a severe constraint for their practical application. Fortunately, the ambiguous and complicated incentives for the degradation process of PSCs under working condition can be directly reflected on the corrosion of silver electrode by halogens. Here, a new perspective for evaluating the long-term stability of PSCs is presented by the time-dependent transverse resistance of the ultrathin silver electrode under different working conditions. Being anode or cathode, the stability of ultra-thin silver layer has been systematically investigated through adjusting the external operating conditions of devices, i.e. light illumination and bias voltage. Experimental results indicate that the gradual resistance increases of silver film can be attributed to the oxidation of I− existing on the top surface of perovskite layer by non-equilibrium holes generated from light illumination or electrical injection for producing corrosive I2 gas, which will diffuse through the carrier transporting layer and attack the thin silver layer. The interaction probability of I− and non-equilibrium holes at the interface plays a critical role on the generation of I2 gas and resistance variation of the top thin silver electrode. The stability study of electrode indicator will shed light on the ambiguous degradation mechanisms of PSC under working condition, paving a path for conquering the fatal problems of the practical application.

Ultrathin sputtered silver films protected by ALD alumina: Comparison of in-situ investigation with ex-situ resistance and ellipsometric measurements

We compare in-situ transmittance and plasma emission spectra with ex-situ resistance and ellipsometric measurements performed on ultrathin silver films with thicknesses varying from 3 nm through 40 nm and on relatively thick layers up to 100 nm. We show that the transmittance broadband monitoring (BBM) of the growing silver layer allows for in-situ reliable estimation of the minimum thickness when the film becomes continuous. Moreover, a proper selection of the monitoring wavelength(s) renders the coalescence and percolation stages of the film growth clearly distinguishable. For our films deposited by magnetron sputtering on fused silica substrates, the minimum thickness when a film becomes continuous was found to be approximately 12 nm. The percolation threshold was determined as approximately 6 nm. The method of selective-wavelength transmittance BBM is practical, intuitive and hence may represent a useful tool enabling high-precision growth of ultrathin silver layers. Furthermore, we show here for the first time that ellipsometry is sensitive to the metal coalescence development. In particular, the ellipsometric analysis suggests that the percolation of silver sputtered films begins at the top of the growing layer and hence propagates towards the film bottom.

Two-Step Approach for Conformal Chemical Vapor-Phase Deposition of Ultra-Thin Conductive Silver Films.

Conductive ultra-thin silver films are commonly fabricated by physical vapor deposition methods such as evaporation or sputtering. The line-of-sight geometry of these techniques impedes the conformal growth on substrates with complex morphology. In order to overcome this issue, volume deposition technologies such as chemical vapor deposition or atomic layer deposition are usually preferred. However, the silver films fabricated using these methods are generally non-electrically conductive for thicknesses below 20-50 nm due to island formation. Here, we demonstrate a novel approach for producing ultra-thin conductive silver layers on complex substrates. Relying on chemical vapor-phase deposition and plasma post-treatment, this two-step technique allows the synthesis of highly conductive and uniform silver films with a critical thickness lower than 15 nm and a sheet resistance of 1.6 Ω/□ for a 40 nm-thin film, corresponding to a resistivity of 6.4 μΩ·cm. The high infrared reflectance further demonstrates the optical quality of the films, despite a still large root-mean-square roughness of 8.9 nm. We successfully demonstrate the highly conformal deposition in lateral structures with an aspect ratio of up to 100. This two-step deposition method could be extended to other metals and open new opportunities for depositing electrically conductive films in complex 3D structures.

Record-High Transparent Electromagnetic Interference Shielding Achieved by Simultaneous Microwave Fabry-Pérot Interference and Optical Antireflection.

As a potential risk to human and environmental health, radio frequency (RF) radiation should be studied due to the higher frequencies and larger bandwidths that may be employed. Electromagnetic interference (EMI) shielding materials can prevent exposure to RF radiation, but most of them are visibly opaque. In this work, we propose and fabricate visibly transparent EMI shielding materials using an ultrathin silver layer sandwiched by oxides (SLSO) as building blocks. The samples with a double-sided SLSO (D-SLSO) structure exhibit the highest EMI shielding effectiveness (SE) of 70 dB at 27.6 GHz (>62 dB on average at 4-40 GHz) and a transmittance close to 90% at a visible wavelength of 550 nm, which is comparable with those of polyethylene terephthalate (PET) and glass substrates. The D-SLSO structure plays a dual role: it suppresses optical reflections as antireflection coatings and enhances EMI shielding via Fabry-Pérot interference. In addition, we discuss the origin of the extraordinary frequency dependence of SE, which monotonically increases, contrary to that of conventional metallic mesh. This report describes SLSO-based transparent EMI shielding materials with record-high SE and visible transmittance that provide optoelectronic applications with robust safety and reliability under RF radiation with high and broad frequencies.

Inkjet Process for Conductive Patterning on Textiles: Maintaining Inherent Stretchability and Breathability in Knit Structures

AbstractIn this work, a novel technique of inkjet printing e‐textiles with particle free reactive silver inks on knit structures is developed. The inkjet‐printed e‐textiles are highly conductive, with a sheet resistance of 0.09 Ω sq‐1, by means of controlling the number of print passes, annealing process, and textile structures. It is notable that the inkjet process allows textiles to maintain its inherent properties, including stretchability, flexibility, breathability, and fabric hand after printing process. This is achieved by formation of ultrathin silver layers surrounding individual fibers. The silver layers coated on fibers range from 250 nm to 2.5 µm, maintaining the size of interstices and flexibility of fibers. The annealing process, structure of fibers, and printed layers significantly influence the electrical conductivity of the patterned structures on textiles. Outstanding electrical conductivity and durability are demonstrated by optimizing print passes, controlling textile structures, and incorporating an in situ annealing process. The electrical resistance dependence on the strain rate of the textiles is examined, showing the ability to maintain electrical conductivity to retain light‐emitting diode use, stable more than 500 consecutive strain cycles. Most importantly, inkjet‐printed e‐textiles maintain their characteristic washability, breathability, and fabric hands for applications in wearable technology.

Solution-processed, fullerene-free organic ultraviolet photodetectors using thermally activated delayed fluorescence materials as electron acceptor

Solution-processed organic ultraviolet photodetectors (UV-PDs) have been investigated for many years due to their mechanical flexibility and the potential for large-scale roll-to-roll processability. Fullerene derivatives are the most widely used electron acceptors in solution-processed photodetectors. However, they are not suitable for selective UV detection due to their wide absorption spreading the whole visible range. Here, three thermally activated delayed fluorescence (TADF) materials of 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), 3,4,5,6-tetrakis(carbazol-9-yl)-1,2-dicyanobenzene (4CzPN) and 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) with strong ultraviolet absorption are first utilized as the electron acceptors in solution-processed UV-PDs. Operating at room temperature, all UV-PDs display comparatively photo-detective ability, and the device composed of poly (N-vinyl carbazole) (PVK):4CzPN behaves the best comprehensive performance with a high detectivity of 1.09 × 1012 Jones. Additionally, we find that the variations in energy level, film morphology and carrier mobility of the photoactive layer affect the UV-PDs performance, particularly with regards to the dark current and thus the detectivity. By using ultra-thin silver layer as the electrode, visibly semitransparent UV-PDs with an average transmittance of 43% are obtained, and the optical simulations indicate that the light harvest and exciton generation properties of active layers have a strong influence on the performance of the semitransparent devices.

Integrated structural color array enabled by ultrathin silver film via cavity-enhanced absorption

In this work, we experimentally demonstrate enhanced absorption resonance enabled by the deconstructive interference of the reflected light in a sandwiched nanostructures composing of the ultrathin silver layer, transparent SiO2 cavity, and the opaque metallic substrate (e.g., 300-nm Ag). Such cavity-enhanced absorption is tunable via modulating the thickness of the absorptive ultrathin silver layer or the embedded transparent SiO2 cavity, giving rise to tunable structural coloration. Through five deposition processes, a 32-channel colorful sample is successfully fabricated. We believe the concepts and results make it evident that such ultrathin absorptive film offers us unprecedented opportunities on developing various photonic applications such as labeling, visual arts, and optical filters.

Multispectral surface plasmon resonance approach for ultra-thin silver layer characterization: Application to top-emitting OLED cathode

While dielectric/metal/dielectric (DMD) multilayer thin films have raised considerable interest as transparent and conductive electrodes in various optoelectronic devices, the knowledge of optical characteristics of thin metallic layers integrated in such structures is still rather approximate. The multispectral surface plasmon resonance characterization approach described in this work precisely aims at providing a rigorous methodology able to accurately determine the optical constants of ultra-thin metallic films. As a practical example, the refractive index and extinction dispersion curves of 8 to 25 nm-thick silver layers have been investigated. As a result, their extreme dependence on the layer thickness is highlighted, in particular in a thickness range close to the critical threshold value (∼10 nm) where the silver film becomes continuous and its electrical conductance/optical transmittance ratio particularly interesting. To check the validity of the revisited Ag layers constant dispersion curves deduced from this study, they were introduced into a commercial optical model software to simulate the behavior of various optoelectronic building blocks from the simplest ones (DMD electrodes) to much more complex structures [full organic light emitting device (OLED) stacks]. As a result, a much better prediction of the emission spectrum profile as well as the angular emission pattern of top-emitting OLEDs is obtained. On this basis, it is also shown how a redesign of the top encapsulation thin film of OLEDs is necessary to better take benefit from the advanced DMD electrode. These results should particularly interest the micro-OLED display field where bright and directive single color pixel emission is required.

A novel PLED architecture containing biologically synthesized gold nanoparticles and ultra thin silver layer

The influences of biologically synthesized gold nanoparticles (bio-GNPs) and ultra thin silver layer (UTSL) on the polymer light emitting diodes (PLEDs) are investigated for the first time. The performance of the fabricated PLEDs is enhanced by embedding of bio-GNPs into the hole transport layer (HTL). Furthermore, the tailored device architecture containing UTSL increases the electron transport through lightning rod effect. Bio-GNPs are successfully produced as spherical shape with size of circa 10.4nm. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and [poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate); (PEDOT:PSS)/bio-GNPs blends are used as an active layer and HTL, respectively. Novel PLEDs fabricated with 0.125wt% bio-GNPs/PEDOT:PSS and 0.5nm UTSL exhibit nearly 2.5-fold enhancement in the device efficiency.

Large active area inverted tandem polymer solar cell with high performance via insertion of subnano-scale silver layer

We propose the deposition of ultra-thin silver layer (in subnano-scale) between bottom subcell and the hole-transport sublayer in the interlayer (IL), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), in the inverted tandem polymer solar cell (t-PSC) for improving the wettability of aqueous hole-transport interlayer on hydrophobic active layer so that the performance is significantly improved and larger active area is allowed. The inverted t-PSC is composed of bottom subcell (regioregular poly(3-hexylthiophene) (P3HT): indene-C60-bisadduct (ICBA)) and the top subcell (thieno[3,4-b]thiophene/benzodithiophene (PTB7) with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM)) along with the IL, PEDOT:PSS/ZnO nanoparticles. For the small-area (0.03cm2) t-PSC, the power conversion efficiency (PCE) can be improved from 7.06% without Ag layer to 7.81% with depositing 0.5nm Ag layer, and the device production yield from 40% (4 out of 10 devices) to near 100% (10 out of 10 devices), which can be ascribed to more efficient hole collection and reducing leakage current for the bottom subcell due to improvement of film forming of PEDOT:PSS. As the active area is increased to 1cm2, a PCE of over 6% (6.11%) can also be achieved by this approach, which is much higher than that without such Ag layer (2.19%) by a factor of 2.78. The reported active area of inverted t-PSC is in the range 0.02–0.15cm2; while the present method allows an inverted t-PSC to enlarge its active area to 1cm2, yet the PCE can maintain over 6%.

Nucleation of ultrathin silver layer by magnetron sputtering in Ar/N2 plasma

We focus our attention on the study of the initial stage of the silver growth. The silver layers were deposited by RF magnetron sputtering in pure argon or its mixture with nitrogen. The initial stage of the nucleation and the layer growth was studied by means of the optical and electrical monitoring, which are based on a principle of in-situ spectral ellipsometry. The measured data were fitted to a mathematical model. The nano-granular character of the grown layer is revealed by resonant features in the extinction coefficient spectra in visible spectral range. Thus, the mode of the layer growth (Volmer–Weber or Stranski–Krastanov) was clearly distinguished in the monitored data. The microstructure of the completed layer was further analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM). In this work, we show that the growth mode of the silver can be controlled by the deposition condition. We also present a method for preparation of an ultra-thin, continuous and smooth silver layer. The nucleation process is significantly influenced by presence of the nitrogen in the sputtering gas.

Oxide Sandwiched Metal Thin‐Film Electrodes for Long‐Term Stable Organic Solar Cells

AbstractOxide/silver/oxide multilayers as semitransparent top electrode for small molecule organic solar cells (OSCs) are presented. It is shown that two oxide layers sandwiching a central metal layer greatly improve the stability and lifetime of the organic solar cell. Thermally evaporated MoO3, WO3, or V2O5 layers are employed as an interlayer for subsequent silver deposition and significantly change the morphology of the ultrathin silver layer, improving charge extraction and electrodes series resistance. The transmittance of the electrode is increased by introducing oxide or oxide and organic multilayers as capping layer, which leads to higher photocurrent generation in the absorber layer. Application of 1 nm MoO3/11 nm Ag/10 nm MoO3/50 nm Alq3 multilayer electrodes in OSCs lead to an efficiency of 2.6% for a standard ZnPc:C60 cell, showing superior performance compared to devices with pure silver top contacts. The device lifetime is also strongly increased. MoO3 layers can saturate and stabilize the inner and outer metal surface, passivating it against most of the degradation mechanisms. With such an oxide/silver/oxide multilayer electrode, the time until the glass encapsulated OSC is degraded to 80% of its starting efficiency is enhanced from 86 h to approximately 4500 h compared to an OSC without an oxide interlayer.

Realization of ultrathin silver layers in highly conductive and transparent zinc tin oxide/silver/zinc tin oxide multilayer electrodes deposited at room temperature for transparent organic devices

We report on transparent and highly conductive multilayer electrodes prepared at room temperature by RF sputtering of zinc tin oxide (ZTO) and thermal evaporation of ultrathin silver (Ag) as top contact for transparent organic light emitting diodes (TOLED). Specifically, we study the morphological, electrical and optical properties of the multilayer structure in particular of the thin Ag film. The tendency of Ag to form agglomerates over time on top of ZTO is shown by atomic force microscopy. From the optical constants derived from ellipsometric measurements we evidenced a bulk like behavior of an Ag film with a thickness of 8nm embedded in ZTO leading to a low sheet resistance of 9Ω/sq. Furthermore we verify the optical constants by simulation of an optimized ZTO/Ag/ZTO structure. As an application we present a highly efficient TOLED providing a device transmittance of >82% in the visible part of the spectrum. The TOLED shows no damage caused by sputtering on a lighting area of 80mm2 and exhibits efficiencies of 43cd/A and 36lm/W.

Preparation of nanostructured ultrathin silver layer

Silver is widely used for a fabrication of plasmonic devices due to its unique optical constants. Nanostructured Ag layer can exhibit strong localized surface plasmon resonance, which mainly affects its optical behavior in visible and near infrared spectra. The nanostructure of the Ag layer is mainly influenced during the initial stage of the silver nucleation. Therefore we focused our attention on the study of this stage of the silver growth. The nanostructured ultra-thin silver layers were prepared by means of the magnetron sputtering. The nucleation mode and the resulting nanostructure was controlled by the deposition conditions. The initial stage of the nucleation and the layer growth was studied by means of an optical monitoring, which is based on a principle of spectrophotometric measurement of sample reflectivity. The measured data were fitted to a model of layered structure. The non-continual (Volmer-Weber) mode of the layer nucleation was clearly distinguished in the monitored data. Thus we were able to estimate the point of the non-continual layer coalescence as well as the subsequent evolution of the surface roughness. The prepared nanostructured Ag layers were analyzed by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Optical properties were studied by spectroscopic ellipsometry and spectrophotometry. The aim of this work was to study the formation of nanostructured Ag layer and its correlation to the optical behavior.

Influence of the ultra-thin Ag layer on Si(1 1 1)7 × 7 on directional elastic peak electron spectroscopy profiles

The directional elastic peak electron spectroscopy (DEPES) polar profiles for the clean Si(1 1 1)7 × 7 surface and the Si(1 1 1)√ 3 × √3 R30°–Ag system are presented. The results were obtained for the [ 1 1 2 ¯ ] – [ 1 ¯ 1 ¯ 2 ] and [ 0 1 1 ¯ ] – [ 0 1 ¯ 1 ] azimuths of the substrate for primary electron energies from the range 0.5–2 keV. A simple qualitative analysis of the observed profiles revealed the influence of the ultra-thin silver layer on the shape of the measured DEPES polar profiles, i.e. both on their background level and on the height of some intensity maxima. Thus, the information on the position of silver atoms in the investigated structure and other ultra-thin layers on crystalline substrates seems to be obtainable by the analysis of the DEPES profiles. The presence of numerous maxima in the measured profiles imply the application of a more advanced method in qualitative and quantitative interpretation of the DEPES profiles.

Optical bistability of planar metal/dielectric nonlinear nanostructures

AbstractWe study theoretically a nonlinear response of the planar metal/dielectric nanostructures constituted from periodical array of ultra thin silver layers and the layers of Kerr-like nonlinear dielectric. We predict hysteresis-type dependences of the components of the tensor of effective dielectric permittivity on the field intensity allowing the change in material transmission properties from transparent to opaque and back at extremely low intensities of the light. It makes possible to control the light by light in all-optical nanoscale devices and circuits.