Au Thin Films Research Articles

Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS2 and fabrication of top-edge hybrid contacts for field-effect transistors

Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS2 flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS2 flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS2 were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl2 plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS2-based FETs on SiO2/p +-Si with Ti/Au contacts were fabricated using a Cl2 etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾106, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance.

Laser writing of CsPbBr3 nanocrystals mediated by closely-packed Au nanoislands

Metal halide perovskites have attracted great interest in recent years owing to their excellent luminescence properties. The low formation energy makes it possible to synthesize perovskites of different types via direct laser writing. Here, we report on the formation of metal halide perovskite (CsPbBr3) nanocrystals coupled to the surface plasmon resonances of gold (Au) nanoparticles by irradiating a polymer film doped with the precursors (CsBr and PbBr3) and coated on a thin Au film. The Au film, which is composed of closely-packed Au nanoislands deposited on a silica substrate, act as not only the absorber of 800-nm femtosecond laser pulses but also the heat source for synthesizing and annealing of CsPbBr3 nanocrystals. The formation of CsPbBr3 nanocrystals in the area of laser irradiation was confirmed by Raman scattering spectra measurements. In addition, closely-packed Au nanoislands are transformed into isolated Au nanoparticles which exhibit surface plasmon resonances at ~530 nm. The CsPbBr3 nanocrystals coupled to isolated Au nanoparticles emit two-photon-induced luminescence at ~513 nm, which is enhanced by the surface plasmon resonances through the Purcell effect. An optimal laser power for direct laser writing was determined and the decomposition of CsPbBr3 nanocrystals was observed if a large laser power was employed. Our results indicate the possibility for the formation and annealing of perovskite nanocrystals coupled to surface plasmon resonances by one-step laser writing and open new horizon for their practical applications in color display and optical memory.

Fabrication and characteristics of highly langle {110} rangle -oriented nanotwinned Au films

Fine grained and nanotwinned Au has many excellent properties and is widely used in electronic devices. We have fabricated langle {110} rangle preferred-oriented Au thin films by DC plating at 5 mA/cm2. Microstructure analysis of the films show a unique fine grain structure with a twin formation. Hardness tests performed on electroplated langle {110} rangle Au films show a hardness 47% greater than random and untwinned Au. We then achieved direct bonding between two Au langle {110} rangle surfaces operating at 200 °C for an hour in a vacuum oven. The highly-oriented langle {110} rangle nanotwinned Au films could be an ideal material in many gold products.

Au–Pd core–shell nanoparticle film for optical detection of hydrogen gas

Hydrogen use, as a clean and almost infinite energy source, has an economic impact in many industries. The problem is that this gas cannot be used like any gas because of its explosiveness at 4% in the air, hence the need to know its concentration any time for security reasons. The permanent detection of hydrogen leaks is essential to monitor and to control the hydrogen concentration to prevent any possible risk. In our current research, we have developed hydrogen ultrasensitive sensors by depositing a thin film of Au–Pd core–shell nanoparticles (NPs) on a transparent glass substrate in order to detect hydrogen in its gaseous form. The colloidal Au–Pd core–shell NPs were synthesized according to a multi-reduction step method. The structural characterizations, the nature, and the density of Au–Pd core–shell NPs have been characterized by scanning electron microscopy and transmission electron microscopy. The morphology, size, and structure of Au–Pd core–shell NPs can be controlled under synthesis conditions. The size of the core–shell studied in this work is 13 nm for gold NP diameter and 0 nm–2.3 nm for palladium thicknesses. The physical properties of NPs, such as the optical absorbance response under hydrogen, strongly depend on the nature of the shell and the ratio between the core and the shell. At different hydrogen concentrations ranging from 1% to 4%, the optical response changes in the position of the surface plasmon resonance peak on the absorbance spectrum after the first loading/unloading hydrogen cycle.

Flexible Au micro-array electrode with atomic-scale Au thin film for enhanced ethanol oxidation reaction

The catalysis of Au thin film could be improved by fabrication of array structures in large area. In this work, nanoimprint lithography has been developed to fabricate flexible Au micro-array (MA) electrodes with ∼ 100% coverage. Advanced electron microscopy characterisations have directly visualised the atomic-scale three-dimensional (3D) nanostructures with a maximum depth of 6 atomic layers. In-situ observation unveils the crystal growth in the form of twinning. High double layer capacitance brings about large number of active sites on the Au thin film and has a logarithmic relationship with mesh grade. Electrochemistry testing shows that the Au MAs perform much better ethanol oxidation reaction than the planar sample; MAs with higher mesh grade have a greater active site utilisation ratio (ASUR), which is important to build electrochemical double layer for efficient charge transfer. Further improvement on ASUR is expected for greater electrocatalytic performance and potential application in direct ethanol fuel cell.

Surface roughness variation and microstructural evolution of Au thin films in rapid annealing by microwave and electric furnace heating

Microstructural evolution of Au thin film in post-annealing by microwave and by electric furnace heating was compared. Wave number spectra of film surface roughness were analyzed to characterize their roughness variation by annealing. Coarsening of film microstructure occurred consistently as the increase of the annealing temperature in both methods. Labyrinth-shaped morphologies consisting of bump-ridge crystals were formed by annealing, which might be originated from the fine-grained colonies existed in the as-deposited state. Through coalescence and growth of the grains within the colony, the bump-ridge crystals increased their height as the increase of annealing temperature. Eventually, microstructures and crystal morphologies became similar by both annealing methods above 800–850 °C.However, there are following differences observed in the low temperature annealing cases: 1. While a temporal variation of the film microstructure was observed in furnace annealing, microwave annealing exhibited smaller tendency of this feature. 2. Fine roughness (higher wave number range) of the films was reduced more in the temperature below 720 °C by microwave annealing than by furnace annealing. 3. Increase of bump-ridge height was pronounced more by microwave annealing between 600 and 800 °C, corresponding to the roughness increase in the lower wave number range. On the other hand, film coverage decreased. The reasons for the difference was discussed by consideration of the competition among the capillary driven surface diffusion, contribution of electro-migration and the thermodynamic equilibrium shapes of the film grain structures.

Anti-fouling SERS-based immunosensor for point-of-care detection of the B7–H6 tumor biomarker in cervical cancer patient serum

Herein, we report the development of sandwich type Surface Enhanced Raman Spectroscopy (SERS) immunosensor modified to be zwitterionic for the detection of soluble B7–H6 biomarker in blood serum from cervical cancer patients. Anti-fouling capture SERS substrate of biosensor based on gold (Au) thin film was modified with a self-assembled monolayer of zwitterionic l-cysteine to combat serum fouling and was then conjugated with NKp30 receptor protein to capture the B7–H6 biomarker in blood serum. The SERS nanoprobe based on spiky gold nanoparticles (AuNPs) was functionalized with ATP reporter molecule, that is stable at a wide range of pH, making the SERS signal reliable in complex media. Then, it was conjugated with anti-B7-H6 antibody forming the complex anti-B7-H6@ATP@AuNPs (i.e., SERS nanoprobe). The proposed immunosensor demonstrated high reproducibility for the quantitative detection of soluble tumor biomarker B7–H6 within the range of 10−10 M to 10−14 M with limit of detection (LOD) of 10−14 M or 10.8 fg mL−1, in the cancer patient serum, greatly exceeding (100 fold) the LOD of commercially available ELISA kits. Such low LOD is partially the result of zwitterionic modification which reduces the serum fouling by 55% compared to traditionally used BSA blocked capture substrates (i.e., control). Notably, this immunosensors demonstrated higher accuracy for detecting the B7–H6 biomarker in undiluted blood serum samples from cervical cancer patients and outperforms the currently available analytical techniques, making it reliable for point of care (POC) testing.

Phosphine Electrochemical Sensor Using Gold-Deposited Polytetrafluoroethylene Membrane.

Phosphine gas is a toxic and flammable gas, which are useful dopant in the semiconducting industry. OSAH (Occupational Safety and Health Administration) regulates the threshold limit value as 0.3 ppm. To detect phosphine gas, an electrochemical sensor cell using gold as a working electrode is available. In this paper, we prepared Au thin film electrode on porous PTFE (polytetrafluoroethylene) membrane by using d.c. sputtering machine. Au thin film deposited on PTFE membrane for 300 s showed the thickness about 220 nm and electrical resistance between 5 and 10 ohm. We adopted this Au thin film as a working electrode and reference electrode. The sensitivity of the electrochemical cell is about 3000 nA/ppm for phosphine gas. Using Cottrell equation and COMSOL program, we simulated the diffusion coefficient of phosphine gas within the electrochemical cell, which showed the value of 1.21×10-7 m²/s.

Substrate-free, stretchable electrolyte gated transistors

Abstract Improving device deformability under mechanical stress is an essential requirement for the realization of next-generation wearable electronic applications. Herein, we report a new concept of stretchable thin-film transistors (TFTs) that can be fabricated without substrate components. Instead of using plastic substrates, we employ an elastic (and also free-standing) solid-state electrolyte as a gate insulator. The strategy not only realizes stretchable TFTs, but also enhances the device performance (e.g. mobility, operation voltage) by the formation of high-capacitance electric double layers. As source-drain electrodes, semi-circular shaped gold (Au) thin-film was employed to enhance the device stretchability. By the combination of a solid electrolyte (gate insulator), metal film (electrodes), and polymer semiconductor (channel layer), the device is stretchable up to around 10%, and shows typical p-channel switching performance with low operation voltage of 1 V and TFT mobility of about 0.81 cm2/V s. The devices were processed by a simple solution process and thermal evaporation under low temperature (

Highly sensitive surface plasmon resonance optical detection of ferric ion using CTAB/hydroxylated graphene quantum dots thin film

The development of optical sensors for heavy metal ion detection has been rapidly growing; unfortunately, the current methods suffer limitations that led to the emergence of an outstanding technique called surface plasmon resonance (SPR) spectroscopy. In this paper, the performance of the SPR optical sensor in detecting ferric ions (Fe3+) was successfully enhanced by depositing novel cetyltrimethylammonium bromide modified hydroxylated graphene quantum dots (CTAB/HGQDs) onto a gold (Au) thin film using the spin coating technique. Upon exposure to Fe3+, the SPR responses of both CTAB/HGQDs thin film and bare Au thin film were compared and studied, emphasizing the sensitivity, binding affinity, full width at half maxima, signal-to-noise ratio, and data accuracy. The CTAB/HGQDs thin film achieved a high sensitivity value of 29.886° ppm−1 for Fe3+ up to 0.1 ppm. The strong binding affinity was confirmed using the Langmuir isotherm model calculation. To the end, the CTAB/HGQDs thin film was characterized using atomic force microscopy for morphological study, confirming its interaction with Fe3+.

Young’s modulus and vibrational frequency dependence on shape and size in nanomaterials

In the present work, the dependence of thermodynamic properties on shape, size and dimension of the nanomaterials is studied. The theoretical model proposed by Lu et al. is used in the present work and the model is extended to calculate the variation in Young’s modulus and vibrational frequency with size of the nanomaterial for spherical, hexahedral and tetrahedral nanomaterials along with nanowires and thin films of metals Au, Ag and Cu. The present calculated results are found in good agreement with the available experimental data which proves the validity of the theoretical model used.

Cascaded wavelength multiplexed refractive index sensors in optical fibers based on surface plasmon resonances

Surface plasmon resonance (SPR) based cascaded wavelength multiplexed fiber-optic sensors provide immense scope for multi-channel and multi-analyte detection benefiting from all the advantages prerogative to single-channel fiber-optic sensors. This work describes the numerical simulations carried out for performance analysis of SPR based refractive index (RI) sensor implemented in cascaded dual- and triple-channel fiber-optic configurations using thin films of Ag, Cu, and Au as plasmonic metals and ZnO and Si as high RI overlayers. A dual-channel sensor is simulated using Ag and Cu/ZnO layers in two well-separated sensing channels on a multimode optical fiber, while a triple-channel sensor is studied using Ag, Cu/ZnO, and Au/Si in the three sensing channels. Due to different SPR resonance wavelengths of Ag, Cu, and Au, distinct sets of SPR spectra are obtained when the sensor response is analyzed by varying analyte RIs circumscribing different sensing regions of the cascaded fiber-optic probe. This feature furnishes an efficient option for easy tailoring of operating range of the sensor desired for different applications. The reported results provide promising perspectives for in-line sensing applications to analyze different constituents of the mixtures of biomolecules, clinical samples, petroleum products, and others.

Layer-by-layer growth of molecular self-assembled monolayers /sputtered gold thin films/graphene oxide on glass substrate

In the present study, 1-dodecanethiol self-assembled monolayers (SAMs) on graphene oxide (GO) –Au (Gold) thin films as molecular templates for the selective deposition of multilayer films was performed. Thus, heterostructured GO/Au/DDT and DDT/Au/GO thin film are fabricated by sputtering Au target onto modified GO structure or self-assembled 1-dodecanethiol (DDT) arrays. It has been shown that a new device concept can be made by completing the glass substrate with the surface heterostructures; i.e. layer GO sheet or DDT with Au nanoparticles (Au NPs). In the light of the performed study, the layer-by-layer assembling of DDT on the GO sheet-Au thin films template has been proved for the distinct conjugation. Also, Optical and morphological characterizations are coherent with the instantaneous nucleation and growth model over the modified DDT-Au NPs-GO or GO-Au NPs-DDT thin films in the aspect of surface roughness, the phonon transport, and other multilayer films properties. As a consequence, the multilayer films of DDT-capped Au thin films and GO sheets have shown a better hybrid nanostructure with higher consistency, hierarchy and surface area. In addition, this work will present a new perspective for template preparation based on nanostructured thin films due to its unique properties resulting from nanoscale properties.

Novel Bimodal Silver Nanowire Network as Top Electrodes for Reproducible and High‐Efficiency Semitransparent Organic Photovoltaics

Semitransparent organic photovoltaics (ST‐OPVs) provide a potentially facile route for some applications in building integrated photovoltaics. One of the challenges in developing large‐scale, printable ST‐OPVs is to address the need for high‐performance and fully solution‐processed top electrodes, allowing the replacement of the evaporated thin metallic films (Ag, Au, and Al). Silver nanowire (AgNW) is considered a promising candidate for the substitution due to its excellent transparency, conductivity, and solution processability. Herein, a novel bimodal AgNW (AgNW‐BM) electrode is reported, comprising AgNWs of two different aspect ratios. It is shown that the AgNW‐BM film achieves lower sheet resistance and higher visible transmittance than each monodisperse AgNW film, respectively. Furthermore, ST‐OPVs based on PTB7‐Th:IEICO‐4F with AgNW‐BM top electrodes are fabricated, which can obtain a maximum power conversion efficiency (PCE) of 7.49% with an average visible transmittance (AVT) of 33%. The ST‐devices also demonstrate an enhanced reproducibility and excellent color‐rendering index of 90. In addition, the bimodal top electrode is successfully implemented in the PM6:Y6 system with a higher PCE of 9.79% and with an AVT of 23%, demonstrating the universality for various semiconductor systems. Our work provides a simple strategy to realize fully solution‐processed, highly efficient ST‐OPVs.

Facile electrochemical synthesis of dilute AuCu alloy nanostructures for selective and long-term stable CO2 electrolysis.

Electrochemical CO production from CO2 electrolysis has been considered the most economically viable approach among various candidate products. AuCu bimetallic alloys are currently receiving attention for their potential to tailor catalytic activity. Here, we synthesized a dilute AuCu alloy nanostructure with an AuCu atomic composition ratio of 3% by using a simple electrochemical treatment method on a 200 nm-thick Au thin film. The dilute AuCu alloy catalyst shows an exceptional CO2 reduction activity in terms of selectivity and overpotential for CO production. In addition, the stability property is more significantly enhanced as compared to pure Au nanostructures. In addition, we describe an in situ tailoring method of catalytic activity for Au nanostructures by repeating an electrochemical treatment process that is performed for forming the Au nanostructure. This approach will be a promising and facile strategy not only for reactive Au catalysts but also to increase the stability activity simultaneously by utilizing Cu impurities existing in an aqueous electrolyte for CO2 reduction.

Metallization of High Purity Al2O3 Substrate with Autocatalytic NiP Thin Films for Au Interconnections in MCM packaging technology.

A Multichip Module (MCM) is a structure consisting of several ICs (typically bare chips) interconnected on a common supporting substrate and packaged as a single device. In this packaging technology, gold (Au) thin films are used as interconnection tracks terminated by wire bonding process to the chips. Thus, the good quality of theses Au films (for interconnects purpose) is essential. The present work proposes a metallization sequence of high purity (99.9%) and polished Al2O3 substrates, with an autocatalytic (electroless) NiP thin film follow by an electrolytic Au film in order to improve the MCM interconnections quality. The results show NiP and Au films with good adhesion, low roughness, good thickness distribution and optimal electrical properties, which allows us to establish a methodology that guarantees the reproducibility and quality of the Au interconnections in MCM devices.

Microchemical Investigation of Long-Term Buried Gilded and Silvered Artifacts From Ancient Peru

A large number of metal artefacts with exceptional artistic value of the Moche culture have been found in the tombs of the Lords of Sipan (Lambayeque, Peru) and of the Lady of Cao (El Brujo, Peru) characterised by different burial conditions. Some of the objects, dated around 300-400 AD, are constituted by substrates of Cu- or Ag-based alloys coated by uniformly distributed thin films of precious metal (1 - 4 microns) that create also polymetallic bicoloured surfaces with “gold” and “silver” areas. In order to investigate the corrosion product structure and composition as well as to identify the techniques used to give the gold or silver appearance, an integrated analytical approach has been adopted. The selected complementary methodologies were scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and optical microscopy (OM). The findings reveal that the substrates are mainly composed by Cu-Ag-Au alloys that at the site of Sipan have been almost completely corroded during the burial. Furthermore, the results show that the main aggressive agent is Cl- coming from the soil and that the degradation phenomena were likely enhanced by the galvanic coupling between the precious metal layer and the less noble substrate. The degradation products have formed mainly layered structures containing chloroargyrite (AgCl), cuprite (Cu2O), nantokite (CuCl), and atacamite (CuCl23Cu(OH)2) polymorphs. These latter species warn that dangerous copper cyclic corrosion is occurring, a harmful phenomenon, commonly defined as “bronze disease”, which must be be firmly mitigated. Finally, the findings reveal that the Moche metal workers used the depletion gilding to selectively modify the surface chemical composition of the artefacts to produce the Ag or Au thin films. According to this subtractive method, the surface of the Cu-Au-Ag alloys was enriched with a layer of precious metal by means of cycles of thermal treatments and removal of Cu or both Cu and Ag from the outermost region by using pickling solutions.

Pulsed laser induced dewetting of Au thin films on Ta2O5 substrates

Au nanoparticle arrays are formed by pulsed laser-induced dewetting from chemically polished and anodically formed templated (dimpled) Ta. Here we demonstrate clear evidence that pulsed laser-induced dewetting (PLiD) of thin films of Au on Ta 2 O 5 /Ta follows the spinodal dewetting mechanism, which was further found to be dependent on the surface topology of the substrate. We examine two different Ta 2 O 5 -coated Ta substrates (dimpled Ta and chemically polished Ta), and find that for Au nanoparticles (NPs) formed on polished Ta, the diameter, spacing and NP number density were related to the initial Au film thickness ( h ) by h 5/3 , h 2 and h −4 , respectively. The diameter, spacing and number density of NPs formed on chemically polished Ta were found to not vary with either laser fluence (260–300 mJ·cm −2 ) or irradiation time (0.1–60 s). On dimpled/patterned Ta substrates, thicker (>4 nm) Au films resulted in Au NPs that exhibit a more uniform diameter and spacing that mirrors the dimpled Ta template, and the dewetting is thought to proceed by an alternative mechanism.

Laser-printed hollow nanostructures for nonlinear plasmonics

Ultrafast deposition of laser energy to a thin substrate-supported Au film enables formation of unique 3D surface morphologies, hollow nanobumps, possessing prospective linear optical properties useful for advanced plasmonic biosensors and nanoscale light sources. Here, we study the nonlinear optical response of such nanostructures by measuring second harmonic generation (SHG) from the individual nanobumps and their ordered arrays supporting local- and lattice-type plasmons at near-infrared frequencies. The results reveal the maximal SHG yield for the isolated nanobump correlated with its geometry-defined electromagnetic near-field enhancement at fundamental frequency. Additional (up to 110-fold) SHG enhancement with respect to the signal from a smooth Au film is achieved via a positive feedback provided through proper arrangement of the resonant nanobumps into the array supporting surface plasmon polariton resonant excitation near the fundamental frequency. Our results substantiate the laser-printed nanobumps as an inexpensive and flexible platform for nanoscale infrared-to-visible light conversion that can be applied for nonlinear plasmonics.

Facile synthesis of size- and shape-controlled freestanding Au nanohole arrays by sputter deposition using anodic porous alumina templates

To fabricate a freestanding through-hole Au membrane using an anodic porous alumina template, Au was deposited on the outermost surface of an anodic film followed by the removal of the template. Alumina templates with different dimensions (e.g. diameters and number of pores) were prepared by two-step anodization in the range of 40–80 V and pore-widening. The Au thin films were deposited onto alumina templates with well-controlled surface morphologies by sputter deposition using a commercially available ion sputter coater. After the removal of the alumina template, a variety of Au membranes with nanoholes, nanotubes, or branched pores were obtained, which reflect the morphology of the alumina template. When the sputtered Au penetrates the pores of the alumina film, Au nanotube arrays with an aspect ratio of ∼3 can be fabricated. The present method is much simpler than the traditional template process involving multi-step replication because there is no need to separate the alumina template from the aluminum substrate before Au deposition.