Ultrathin Au Films Research Articles

A study of multiple solid-state dewetting of sputtered Au ultra-thin films for chip-based LSPR sensor applications

This article proposes a lithography-free technique to fabricate Au NPs on glass substrates using multiple solid-state dewetting (SSD) of sputtered Au ultra-thin films for LSPR sensor chip applications. We studied the influence of initial film thickness and the number of repeated process cycles on the morphology and LSPR sensing performance. This fabrication process allowed control over particle size, gap spacing, and density of the Au NPs, which influenced the LSPR peak position as observed using field emission scanning electron microscopy (FE-SEM) and UV–Vis–NIR spectrophotometry. To demonstrate LSPR sensing performance, the refractive index (RI) sensitivity was evaluated by measuring the wavelength shift of the LSPR peak in a series of glycerol/phosphate-buffered saline (PBS) mixtures, varying the refractive index from 1.33909 to 1.37409. The results showed that RI sensitivity and the figure of merit (FOM) for all prepared samples ranged from 37.191 ± 12.26–73.592 ± 9.70 and 0.35 ± 0.12–0.75 ± 0.02, respectively. An increase in repeated process cycles tended to decrease RI sensitivity and FOM. The best LSPR performance was achieved with an 8 nm initial film thickness after the first cycle, with an RI sensitivity of 70.937 ± 2.60 and an FOM of 0.75 ± 0.02, attributed to optimal Au size and density. Additionally, the binding efficiency response to human IgG with high regeneration cycles was demonstrated, highlighting the potential for biosensor applications.

Scalable hot carrier–assisted silicon photodetector array based on ultrathin gold film

Abstract Silicon (Si) offers cost-effective production and convenient on-chip integration for photodetection due to its well-established CMOS technology. However, the indirect bandgap of Si inherently limits its detection efficiency in the near-infrared (NIR) regime. Here, we propose a strategy to achieve high NIR photoresponse in Si by introducing a strong light-absorbing ultrathin gold (Au) film to generate hot carriers. Using a 4.6 nm thick-Au film deposited on Si, we achieved photoresponsivity of 1.6 mA/W at 1310 nm under zero-bias conditions, and rapid temporal responses of 7.5 and 8 μs for rise and fall times, respectively, comparable to germanium (Ge) photodiodes. By utilizing an ultrathin (<6 nm) Au film as the light-detecting layer and thicker (>100 nm) Au film as electrodes, we introduce a unique approach to design a photodiode array based on a single metal (Au) platform. Comparative analysis with a commercial beam profiler image validates the performance of our designed array. This work presents an efficient strategy for manufacturing cost-effective and scalable NIR photodetector arrays, which eliminates the need for additional insulator layers.

Effect of High-Temperature Annealing on Raman Characteristics of Silicon Nanowire Arrays

We demonstrate two distinct experimental processes involving the large-area growth of ordered and disordered silicon nanowire arrays (SiNWs) on a p-type silicon substrate using the metal-assisted chemical etching method. The two processes are based on the etching of monocrystalline silicon wafers by randomly distributed Ag films and ultra-thin Au films with ordered nano-mesh arrays, respectively, wherein the growth of SiNWs is implemented using a specific proportion of a HF-containing solution at room temperature. In this study, the microstructural change mechanisms for the two morphologically different arrays before and after annealing were investigated using Raman spectra. The effects of various mechanisms on the observed Raman scattering peak’s deviation from symmetry, redshift and broadening were analyzed. The evolution of the unstable amorphous structures of nanoscale materials during the high-temperature annealing process was observed via high-resolution scanning electron microscope (SEM) observations. The scattering peak parameters determined from the Raman spectra led to conclusions concerning the various mechanisms by which high-temperature annealing influences the microstructures of the two morphologically different SiNWs fabricated on the p-type silicon substrate. Therefore, the deviation of SiNWs from the monocrystalline silicon scattering peak at 520.05 cm−1 when changing the diameter of the nanowire columns was calculated to further analyze the effect of thermal annealing on Raman characteristics.

Modulation of epsilon-near-zero wavelength and enhancement of third-order optical nonlinearity in ITO/Au multilayer films

We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

Phonon‐Dominated Energy Transport in Purely Metallic Heterostructures

AbstractUltrafast X‐ray diffraction is used to quantify the transport of energy in laser‐excited nanoscale gold–nickel (Au–Ni) bilayers. Electron transport and efficient electron–phonon coupling in Ni convert the laser‐deposited energy in the conduction electrons within a few picoseconds into a strong non‐equilibrium between hot Ni and cold Au phonons at the bilayer interface. Modeling of the subsequent equilibration dynamics within various two‐temperature models confirms that for ultrathin Au films, the thermal transport is dominated by phonons instead of conduction electrons because of the weak electron–phonon coupling in Au.

Dual functional anode for organic light-emitting devices by directly imprinted nanostructured ultrathin Au film

The ultrathin metal film with excellent electrical conductivity and high visible-band transmittance has attracted considerable attention as a transparent electrode for the organic light-emitting devices (OLEDs). However, the deficient surface morphology and poor continuity of low thickness evaporated metal films and the surface plasmon-polaritons (SPPs) mode induced energy loss still seriously limit the actual efficiency of OLEDs. In this work, the thermal nanoimprint lithography has been further modified and directly applied to fabricate nanostructures onto the ultrathin Au film. During the imprinting process, the surface smoothness and conductivity of the Au film are obviously optimized with the formation of nanostructures. After integrating proper nanostructures into OLEDs, the photons that trapped by SPPs mode within the device were effectively out-coupled. The current efficiency and luminance have been enhanced 45.3% and 18.1%, respectively. Furthermore, the emitting properties of the OLEDs were also modified by the nanostructures, and the directional transmission capability of output light was significantly improved.

Strategy for Fabricating Ultrathin Au Film Electrodes with Ultralow Optoelectrical Losses and High Stability.

A vital objective in the wetting of Au deposited on chemically heterogeneous oxides is to synthesize a completely continuous, highly crystalline, ultrathin-layered geometry with minimized electrical and optical losses. However, no effective solution has been proposed for synthesizing an ideal Au-layered structure. This study presents evidence for the effectiveness of atomic oxygen-mediated growth of such an ideal Au layer by improving Au wetting on ZnO substrates with a substantial reduction in free energy. The unexpected outcome of the atomic oxygen-mediated Au growth can be attributed to the unconventional segregation and incorporation of atomic oxygen along the outermost boundaries of Au nanostructures evolving in the clustering and layering stages. Moreover, the experimental and numerical investigations revealed the spontaneous migration of atomic oxygen from an interstitial oxygen surplus ZnO bulk to the Au-ZnO interface, as well as the segregation (float-out) of the atomic oxygen toward the top Au surfaces. Thus, the implementation of a 4-nm-thick, two-dimensional, quasi-single-crystalline Au layer with a nearly complete crystalline realignment at a mild temperature (570 K) enabled exceptional optoelectrical performance with record-low resistivity (<7.5 × 10-8 Ω·m) and minimal optical loss (∼3.5%) at a wavelength of 700 nm.

Temperature-Dependent Growth and Evolution of Silicene on Au Ultrathin Films-LEEM and LEED Studies.

The formation and evolution of silicene on ultrathin Au films have been investigated with low energy electron microscopy and diffraction. Careful control of the annealing rate and temperature of Au films epitaxially grown on the Si(111) surface allows for the preparation of a large scale, of the order of cm2, silicene sheets. Depending on the final temperature, three stages of silicene evolution can be distinguished: (i) the growth of the low buckled phase, (ii) the formation of a layered heterostructure of the low buckled and planar phases of silicene and (iii) the gradual destruction of the silicene. Each stage is characterized by its unique surface morphology and characteristic diffraction patterns. The present study gives an overview of structures formed on the surface of ultrathin Au films and morphology changes between room temperature and the temperature at which the formation of Au droplets on the Si(111) surface occurs.

Determination of thickness-dependent damping constant and plasma frequency for ultrathin Ag and Au films: nanoscale dielectric function.

This paper is devoted to determine an analytical expression for the thickness dependent complex dielectric function for the case of Ag and Au thin films. Free and bound electron contributions are dealt with independently. Using Drude model for the former and taking experimental refractive index values for Ag and Au thin films, we apply a previously developed method to determine for the first time damping constant and plasma frequency parameters for specific film thicknesses. Fitting separately each one of these parameters allowed us to find an analytical expression for their dependence on arbitrary film thickness and consequently for the free electron contribution. Concerning bound electrons, its contribution for small wavelengths is the same for all analyzed thicknesses and may be set equal to the bulk bound contribution. Taking these facts into account, the complex dielectric function is rewritten analytically, in terms of bulk dielectric function plus corrective film thickness dependent terms. From the fitting process for the damping constant we determine that electron scattering at the film boundary is mainly inelastic for both silver and gold thin films. It is also shown that, in accordance with theoretical studies, plasma frequency shows a red shift as the film thickness decreases.

Planar narrowband Tamm plasmon-based hot-electron photodetectors with double distributed Bragg reflectors

Hot-electron photodetectors (HE PDs) are attracting a great deal of attention from plasmonic community. Many efficient HE PDs with various plasmonic nanostructures have been demonstrated, but their preparations usually rely on complicated and costly fabrication techniques. Planar HE PDs are viewed as potential candidates of cost-effective and large-area applications, but they likely fail in the simultaneous achievement of outstanding optical absorption and hot-electron collection. To reconcile the contradiction between optical and electrical requirements, herein, we propose a planar HE PD based on optical Tamm plasmons (TPs) consisted of an ultrathin gold film (10 nm) sandwiched between two distributed Bragg reflectors (DBRs). Simulated results show that strong optical absorption (>0.95) in the ultrathin Au film is realized. Electrical calculations show that the predicted peak photo-responsivity of proposed HE PD with double DBRs is over two times larger than that of conventional single-DBR HE PD. Moreover, the planar dual-DBR HE PDs exhibit a narrowband photodetection functionality and sustained performance under oblique incidences. The optical nature associated with TP resonance is elaborated.

Pseudoequilibrium between Etching and Selective Grain Growth: Chemical Conversion of a Randomly Oriented Au Film into a (111)-Oriented Ultrathin Au Film

Metal thin films with a specific orientation play vital roles in electronics, catalysts, and epitaxial templates. Although oriented metal films have been produced in the recent years, ultrathin oriented metal films (<10 nm) have not been achieved owing to the interfacial instability of the ultrathin films during the thermal annealing process. This study investigates chemical conversion of randomly oriented multigrain Au ultrathin films into (111)-oriented Au ultrathin films. A novel chemical process, termed pseudoequilibrium of etching and selective grain growth, is presented for the chemical conversion by using a quaternary ammonium halide. The reaction variables (reaction time, reaction temperature, species of halide ions) for the chemical conversion process are systematically investigated. This study reveals the in-plane rotational degeneracy in the Au(111) thin film epitaxially grown on a Si(111) substrate. The chemical process can be applied to a broad range of thicknesses from 9 to 100 nm.

The Prospect of the Electroless Atomic Layer Deposition (ALD) for Applications in Semiconductor Technology

The metallic thin films are widely used for in semiconductor technology. Surface morphology, thickness and purity of the film are the key attributes determining its functionality 1 , 2. Recent developments indicate that Cu/liner/TaN structures where liner could be either Co, Ru or Rh and TaN serves as barrier are of particular interest. The electrochemical doping of Zn with liner material to develop barrier-less Cu structures has been demonstrated3. In this talk we demonstrate a protocol to deposit conformal Cu thin film on Co by replacement of Zn overlayer without etching Co. EDTA and BTA are used for corrosion inhibition of Co film once all Zn capping layer is consumed in galvanic displacement with Cu. TEM, XPS and EDS indicate conformal Cu layer deposition Co layer underneath being intact. The second part of the talk focuses on electroless ALD application for deposition of Pt and Rh layers on Cu, Au or Ru substrates. 4 , 5. Surface reflectivity changes during electroless ALD serves as an indicator of surface roughness evolution after each cycle. Surface morphology after 20 electroless ALD cycles is studied by STM and AFM. Cyclic voltammetry illustrates catalytically active Pt and Rh surface after deposition. All electroless ALDs are performed at room temperature in glove box with inert atmosphere. Surface selective, self-terminating deposition and precise control over the film thickness are achieved demonstrating potential of this deposition method for scale up and industrial applications. Ahmadi, K., Wu, D., Dole, N., R. Monteiro, O. & R. Brankovic, S. Tuning Surface Chemoresistivity of Au Ultrathin Films Using Metal Deposition via Surface-Limited Redox Replacement of the Underpotentially Deposited Pb Monolayer. ACS Sensors 4, 2442–2449 (2019).Wu, D. et al. Pb Monolayer Mediated Thin Film Growth of Cu and Co: Exploring Different Concepts. Journal of The Electrochemical Society 166, D3013–D3021 (2019).Kailash Venkatraman, Yezdi Dordi, A. J. Electrochemical doping of thin metal layers employing underpotential deposition and thermal treatment. 1 (2019).Wu, D. et al. Electroless Deposition of Pb Monolayer: A New Process and Application to Surface Selective Atomic Layer Deposition. Langmuir 34, 11384–11394 (2018).Wu, D. et al. Underpotential and Electroless Pb Monolayer Deposition on Ru(0001). Journal of The Electrochemical Society 166, D359–D365 (2019). Figure 1

Growth evolution and infrared response of thermally dewetted Au nano-structures for bolometric applications

This paper discusses the growth and characterization of thermally dewetted gold nanostructures for enhanced infrared absorption in bolometer applications. The Au nanostructures are fabricated by solid-state dewetting of ultrathin (18 nm) Au film on Si substrate. The duration of dewetting is varied from 1 min to 30 min. The Au film exhibited the highest X-ray diffraction (XRD) peak intensity along the (111) plane; while the Au nanostructure showed the rising XRD peak along the (220) plane with the dewetting duration. The dewetting process also yielded grain growth and recrystallization of Au nanostructure. The effect of the dewetting on surface morphologies of the Au nanostructure is studied. The Au nanostructures exhibited broadband infrared absorption in the 500–6500 cm−1 range. The sample dewetted for 10 min exhibited up to 200% enhanced absorbance (with respect to the silicon substrate) in the IR range due to the well-suited dimensions of the nanostructures. The hypothesis is also substantiated by examining the plasmonic response of the nanostructure by simulation.

28.3%-efficiency perovskite/silicon tandem solar cell by optimal transparent electrode for high efficient semitransparent top cell

Perovskites with tuned bandgap provide an attractive opportunity to realize perovskite/silicon tandem solar cells that offer tremendous potential in exceeding the intrinsic power conversion efficiency (PCE) of each subcell. The transparency and conductivity of the top electrode for top cell are the main keys in tandem solar cell. Here, we demonstrate a continuous, ultrathin Au film as the top electrode by introducing a Cr seed layer, which improves the efficiency of semitransparent perovskite devices and tandem solar cells. The Cr seed layer has large surface energy, which induces Au growth according to the Frank-van der Merwe principle to form a continuous, ultrathin film with exciting transmittance and conductivity. As a result, the highest PCE of 19.8% for semitransparent perovskite cells is achieved. The simulated optical field distribution by COMSOL and experimental results reveal that the semitransparent perovskite device has high transmittance in the near-infrared range, demonstrating that it is optimal top cell in a tandem solar cell with a silicon heterojunction device. The tandem solar cell, with the semitransparent perovskite top cell mechanically stacked on the silicon heterojunction bottom cell, demonstrates a PCE of 28.3%. This breakthrough in the design of the tandem cell architecture based on a transparent electrode offers an efficient route towards the transition of perovskite and tandem solar cells. The efficiency of 28.3% for 4T perovskite/silicon tandem solar cell is obtained using semitransparent perovskite top cell with the highest efficiency of 19.8% as a window, whose transparent electrode employs the ultrathin gold film possessed excellent conductivity and outstanding transmittance in near-infrared region. • The ultrathin hybrid metal transparent electrode has been developed to fabricate semitransparent perovskite solar cell. • A COMSOL Multiphysics software is employed to simulate the optical field distribution in semitransparent device. • The highest efficiency of 19.8% for semitransparent perovskite cell and 28.3%-efficiency are achieved.

Pulsed laser deposition of Au nanoparticles on ZnO nanostructures

Au nanoparticles and ultrathin gold films were obtained on the surface of thin (100 nm) ZnO films on Si (001) by pulsed laser deposition (PLD) at high argon pressure by sputtering a pure gold target with a CL3100 pulsed excimer laser (λ = 248 nm) at room temperature. The dependence of the size and distribution of nanoparticles on the argon pressure in the vacuum chamber (PAr), the power density of laser radiation (j), and the number of laser pulses (N) was investigated. Also, to obtain nanostructures, axial and non-axial deposition was used, where the substrate was located perpendicular and parallel to the plasma torch, respectively. Stable modes of obtaining gold nanoparticles with high uniformity and average size from 4 to 10 nm were demonstrated. The modes of stable and reproducible deposition of ultrathin Au films (23-42 nm) and percolation structures (18-20 nm) were also demonstrated. Thus, PLD is a reliable and flexible tool for obtaining nanoparticles and ultrathin Au films the average size and thickness of which can be predictably controlled by varying the deposition parameters. This technique is well suited for coating with gold nanoparticles the surface of nanostructured materials based on chemically active substances that are particularly sensitive to surface cleanliness, which cannot be coated by standard methods.

Enhanced efficiency of organic light-emitting devices by using a directly imprinted nanopillared ultrathin metallic electrode.

An ultrathin metal film with high transmittance and conductivity has been demonstrated to be a promising transparent electrode for organic light-emitting devices (OLEDs). However, mediocre surface morphology and continuity of evaporated metal films and the surface plasmon-polaritons (SPPs) energy loss between the metal electrode and organic layer still limit the external quantum efficiency (EQE) of OLEDs. Here, nanoimprint lithography has been directly applied on the ultrathin Au film with underlying uncured photopolymer to fabricate the nanopillared anode. Both the conductivity and transmittance of the nanopillared ultrathin Au film have been improved due to the improvement of continuity and surface smoothness. As we expected, the SPPs mode has been coupled into photons and further extracted from OLEDs by using the nanopillared Au film anode. Finally, 19.2% and 70.1% enhancement of current efficiency were achieved compared to the planar device with ultrathin Au anode and ITO anode, respectively.

Highly Deformable Transparent Au Film Electrodes and Their Uses in Deformable Displays.

With emerging interest in foldable and stretchable displays, the need to develop transparent deformable electrode and interconnection is increasing. Even though metal films have been standard electrodes in conventional electronic devices due to their high conductivity and well-established process, they have never been used for transparent deformable electrodes. We present highly conductive transparent deformable Au film electrodes and use them to fabricate a foldable perovskite light-emitting diode (PeLED) and a biaxially stretchable alternating current electroluminescence (ACEL) display. We exhibit the formation of an ultrathin (6 nm) continuous Au film on an anisotropic conductive ultrathin film (ACUF) of amorphous carbon. The ultrathin Au film was first formed on an ACUF-coated Si wafer (4 in. scale) through metal evaporation and transferred to the polymer substrates by a simple and effective water-assisted delamination process. Then, a hybrid electrode (ACUF/ACUF/Au) was produced as the transparent deformable electrode. Complicated interconnections could be created by metal deposition through a mask. The electrical conductance of the hybrid electrode was not affected by the crack formation in the Au film during electrode folding, crumpling, and stretching. We reveal the reason why the hybrid electrode can maintain such excellent electrical stability under deformation.

Self-Organized Nanogratings for Large-Area Surface Plasmon Polariton Excitation and Surface-Enhanced Raman Spectroscopy Sensing

Surface plasmon polaritons (SPP) are exploited due to their intriguing properties for the fabrication and miniaturization of photonic circuits, for surface-enhanced spectroscopy and imaging beyond the diffraction limit. However, excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in the literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nanolithographic processes. In this work, we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted ion-beam sputtering can be exploited to fabricate large-area (cm2 scale) nanorippled soda lime templates, which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-one-dimensional (1D) gratings characterized by a remarkably high spatial order, which properly matches the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength that can be tuned by modifying the grating period, photon incidence angle, or, potentially, the choice of the thin-film conductive material. Surface-enhanced Raman scattering experiments show promising gains in the range of 103, which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or biosensing.

Photonic Spin Hall Effect Modified by Ultrathin Au Films and Monolayer Transition Metal Dichalcogenides in One-Dimensional Photonic Crystal

When the metal film becomes far thinner than the electron mean free path (EMFP) in their bulk counterpart, many exotic optical properties accompanied with numbers of novel applications may emerge. Herein, the photonic spin Hall effect (PSHE) occurring on the surface of one-dimensional photonic crystal consisting of ultrathin Au films, monolayer transition metal dichalcogenides (TMDCs), and one defective layer is investigated. The Au film thickness ranges from 2.4 to 40 nm which covers the thickness range below the EMFP (37.7 nm) in bulk Au. Results show that ultrathin Au films are more competent to enhance the PSHE than bulk Au film. In particular, the maximum spin-dependent transverse displacement (37.94 times of the incident wavelength) at the Au film thickness of 4.4 nm is more than 12-fold higher than that at the bulk Au film with thickness of 40 nm. Besides it may be due to the coupling between the short-range surface plasmon polaritons and the B excitonic resonance in monolayer WS2, among four monolayer TMDCs (i.e., MoS2, MoSe2, WS2, WSe2), the photonic crystal with monolayer WS2 gives the strongest PSHE. Additionally, evolutions of photonic spin Hall shifts with the refractive index of defective layer, incident wavelength, and incident angle are also studied in detail. These findings shed light on the superiority of ultrathin metal films for light manipulation in spinoptics and may provide new routes to tailor the optical helicity in nanophotonic devices.

NaCl substrates for high temperature processing and transfer of ultrathin materials

Ultrathin materials often require high temperatures for growth and processing, which cannot be withstood by the substrate underneath. For example, polymers are widely used as a supporting layer but unfortunately have low strain-point temperatures. This is the case of polyethylene terephthalate (PET) which has glass transition and melting temperatures of 76 and 250 °C, respectively. In this paper we propose to use polished salt, a material that can withstand high temperatures during fabrication and, at the same time, can be sacrificed during the transfer onto the final substrates. More specifically, we demonstrate thermal dewetting of Au ultrathin metal films and growth of MoS2 on NaCl at 750 and 650 °C, respectively, and subsequent transfer onto PET films, after which the salt is easily dissolved by water. We believe that the proposed technique can be extended to fabrication of other ultrathin materials, e.g. graphene, as well as final substrates for a wide range of applications, including flexible electronic and optoelectronic devices.