Au Nanoislands Research Articles

Self-limiting growth and bimodal size distribution of Au nanoislands on InAs(111)B surface

Gold nanoparticles are widely used for catalytic growth of different semiconductor nanowires. It is important therefore to have a full appreciation of the factors influencing the size distribution of such particles on semiconductor substrates. Here, we describe some interesting phenomena observed during low-temperature deposition of Au onto InAs(111)B substrates where Au islands are solid. In particular, we find a bimodal character of the size distribution of these islands. We then develop a nucleation model which is based on a specific form of the island formation energy. The model yields a self-limiting growth behavior in which the growth rate of large islands is suppressed e.g. due to dislocations. We show that the regime of self-regulated nucleation with a few pulses is observed for short growth times, leading to a bimodal size distribution or even a more complex shapes. For longer growth times, the system acquires a steady-state regime with a time-independent supersaturation and hence the pulses disappear.

The influence of cyclic deposition and anneal on growth of isolated and well-dispersed Au nanoisland arrays

Isolated and well-dispersed Au nanoisland arrays were prepared with sputter deposition and post-deposition anneal in a cyclic fashion. Scanning electron microscopy (SEM) characterization and associated image analysis in aspects of nanoisland size, coverage, circularity, and density were performed to investigate the influence of cyclic deposition and anneal (DAA) on the morphology of as-prepared Au nanoisland arrays. This study revealed that, given a fixed amount of Au deposition per cycle, increasing the number of DAA cycles drove Au nanoislands’ gradual growth that was interpreted with the increase of their size and coverage and decrease of their density and circularity. In addition, given a fixed amount of total Au deposition, dividing the growth process into more cycles was shown to lower the growth rate of Au nanoislands. Moreover, the as-prepared Au nanoisland arrays were demonstrated to be a viable option as surface-enhanced Raman spectroscopy (SERS) sensing platform whose SERS enhancement was found to be highly dependent upon the size of nanoislands.

The disorder-induced Raman scattering in Au/MoS2 heterostructures

The Raman scattering has been studied in heterostructures composed of a thin MoS2 flake and a 1-1.5 nm layer of thermally evaporated gold (Au). There have been Au nanoislands detected in the heterostructure. It has been found that their surface density and the average size depend on the MoS2 thickness. The Raman scattering spectrum in the heterostructure with a few monolayer MoS2 only weakly depends on the excitation (resonant vs. non-resonant) mode. The overall Raman spectrum corresponds to the total density of phonon states, which is characteristic for disordered systems. The disorder in the MoS2 layer is related to the mechanical strain induced in the MoS2 layer by the Au nanoislands. The strain results in the localization of phonon modes, which leads to the relaxation of the momentum conservation rule in the scattering process. The relaxation allows phonons from the whole MoS2 Brillouin zone to interact with electronic excitations. Our results show that the Au nanoislands resulted from thermal evaporation of a thin metal layer introduce substantial disorder into the crystalline structure of the thin MoS2 layers.

Surface roughness-correlated SERS effect on Au island-deposited substrate

Evaporation and sputtering-deposition of noble metal films on substrates are two most popular and easiest methods for creating stable surface-enhanced Raman scattering (SERS)-active metal nanoparticles (NPs). However, intrinsically low SERS effect and unclearly inherent relationship between SERS effect and surface roughness of created Au NPs make SERS-related applications are limited. Here, we present the enhanced SERS effect observed on Au nanoislands (AuNIs)-deposited substrates prepared by thermal evaporation and followed by post-annealing treatment (PAT). Experimental results indicate that the SERS intensity of Rhodamine 6G (R6G) adsorbed on AuNIs, corresponding to average particle diameter of ca. 220nm and to mean roughness (Ra) of 50nm, can be increased by one order of magnitude after PAT at 700°C. Meanwhile, the AuNIs (annealed at 700°C)-deposited silicon substrate demonstrates a large enhancement factor (EF) of 2.2×107. The relative standard deviation is ca. 7% in SERS measurements. In addition, detailed relationships of Ra derived from analysis of atomic force microscopy (AFM) and SERS effect are correlated for metal particles smaller and larger than 300nm.

Structural Evolution of AuPt and AuPd Nanoparticles Fabricated by Microwave Assisted Synthesis: A Comparative Study

ABSTRACTBimetallic nanoparticles (NPs), particularly Au/Pd and Au/Pt, have attracted extensive attention due to their wide-spread application in catalysis, optoelectronics and energy recuperation.[1] Here we have attempted the fabrication of Au/Pt and Au/Pd bimetallic NPs by an energy-efficient eco-friendly microwave methodology. The microwave-assisted reactions enable considerably large product yields over conventional colloidal methods due to (a) almost two-fold increased reaction kinetics, (b) localized superheating at reaction sites and rapid rise of initial temperature.[2] Au NPs (sizes 20 ± 3 nm) are fabricated in the first step followed by the reduction of [PdCl2(NH3)2] or [K2PtCl6]in tetraethylene glycol at 180 ºC for 2 min. Controlling and understanding the atomic structure and elemental distributions of these NPs are crucial for their optimized performances. So, we address the fundamental question of the most likely arrangement of Au and Pd or Pt atoms in these bimetallic NPs prepared under similar conditions by complementary characterizations using UV-Vis spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The UV-Vis spectroscopy reveals the formation of an alloy shell. The extent of depression of the plasmon peak of Au and its blue-shift reveals substantial deposition of Pd atoms on an Au core and significant alloying in comparison to Au/Pt NPs. XRD reveals the gradual shift of the diffraction peak from the position of Au to the position of Pd or Pt with change in composition. XRD supports the formation of a thick alloy shell in these NPs. However, the TEM images reveal a very interesting result. With increase in Pt concentration, the size of the dispersed NPs decreases from 20 ± 3 nm to about 16 nm (± 1 nm) and there is evolution of a bimodal particle size distribution with small particles about 1-2 nm diameters. On the contrary, with increasing Pd concentration, the particle size of the dispersed particles increases to about 32 nm (± 1 nm). This discrepancy of particle size evolution for the two systems arises due to the differences in surface energies (Pt > Pd > Au atoms). Pt atoms tend to diffuse towards the core with the formation of Au nano-islands which eventually segregates leading to a reduction in particle size and bimodal distribution. At higher concentration of Pt, Pt and Au atoms tend to nucleate separately also contribute to the bimodal distribution. While for Au/Pd NPs, we have an Au core with an alloyed shell having higher Pd concentration. This is further supported by experimental evidence by selective etching and dissolution of Au by potassium-iodide solution. Furthermore, the Au/Pd bimetallic NPs are found to possess better catalytic activities in the reduction of 4-nitrophenol to 4-aminophenol than Au/Pt and monometallic NPs.

Random sized plasmonic nanoantennas on Silicon for low-cost broad-band near-infrared photodetection.

In this work, we propose Silicon based broad-band near infrared Schottky barrier photodetectors. The devices operate beyond 1200 nm wavelength and exhibit photoresponsivity values as high as 3.5 mA/W with a low dark current density of about 50 pA/µm2. We make use of Au nanoislands on Silicon surface formed by rapid thermal annealing of a thin Au layer. Surface plasmons are excited on Au nanoislands and this field localization results in efficient absorption of sub-bandgap photons. Absorbed photons excite the electrons of the metal to higher energy levels (hot electron generation) and the collection of these hot electrons to the semiconductor results in photocurrent (internal photoemission). Simple and scalable fabrication makes these devices suitable for ultra-low-cost NIR detection applications.

Structure and localized surface plasmon tuning of sputtered Au nano-islands through thermal annealing

Localized surface plasmon resonance of noble metallic nanoparticles has been widely used in the fabrication of various sensors. Turning the nanostructure of localized surface plasmon resonance is significant because the resonance induced localized surface plasmon resonance shift is found to be strongly dependent on the structural characteristics, and thus the performance of the sensors. A simple sputtering, annealing, re-sputtering, and re-annealing process was proposed to tune the structural and optical characteristics of Au nano-islands deposited on the glass substrate. It was found that the size and inter-particle distance of nano-islands depend on annealing time and temperature. High temperature annealing tended to increase the size and inter-islands distance of Au islands. Re-sputtering and re-annealing under different conditions made size and inter-particle distance further tuning possible. Investigations on the optical characteristics of Au nano-islands demonstrated that the surface plasmon resonance peak and the spectral bandwidth of islands were tunable from 510 nm to 620 nm and from 50 nm to approximately 400 nm, respectively. The refractive index sensitivity of Au nano-islands determined by the surface plasmon band position change in different surrounding medium was compared. 500 °C 5 h annealing increased the value of refractive index sensitivity to approximately 58 nm/RIU from 26 nm/RIU under 100 °C 5 h annealing. Besides, Au nano-islands with the same re-sputtering condition but different re-annealing conditions showed the maximum value when the re-annealing temperature is at 500 °C for 5 h. In addition, the refractive index sensitivity, surface plasmon band position, and figure of merit were dependent with each other. These results suggest that the scheme “sputtering, annealing, re-sputtering, and re-annealing” is an effective method to adjust the structure and increase the refractive index sensitivity of sputtered Au islands.

Photoluminescence quenching in gold - MoS2 hybrid nanoflakes.

Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing. Here we consider pristine MoS2 and study its interaction with Au nanoislands, resulting in local variations of photoluminescence (PL) in Au-MoS2 hybrid structures. By depositing monolayers of Au on MoS2, we investigate the electronic structure of the resulting hybrid systems. We present strong evidence of PL quenching of MoS2 as a result of charge transfer from MoS2 to Au: p-doping of MoS2. The results suggest new avenues for 2D nanoelectronics, active control of transport or catalytic properties.

Preparation and testing of silicon nanowires

Here we present two ways of preparing lateral (in plane) silicon nanowires with the help of gold nanoislands catalysed plasma enhanced chemical vapour deposition. The role of the applied potential and eventual consecutive hydrogen plasma treatment is tested together with the thickness of the thin Au layer used for self-organised preparation of Au nanoislands.

Improvement of Plasmon-Enhanced Photocurrent Generation by Interference of TiO2 Thin Film

In this study, we demonstrated an improvement in the plasmon-enhanced photocurrent generation due to Fabry–Perot interference of titanium dioxide (TiO2) thin-film electrodes loaded with Au nanoislands (Au-NIs). TiO2 thin films with thicknesses of 215–274 nm, which show obvious Fabry–Perot interference, were deposited via atomic layer deposition (ALD) onto silica glass and were modified by the deposition of Au-NIs onto their surface. Anodic plasmon-enhanced photocurrent generation was observed over the Au-NIs-loaded TiO2 thin-film photoelectrodes. The incident photon to current efficiency (IPCE) action spectra correlated strongly with the Au-NIs plasmon resonance and exhibited a strong dependence on the thickness of the TiO2 thin film. The photocurrent conversion efficiency increased when the transmission constructive interference wavelength overlapped with the Au-NIs plasmon resonance band. This work provides a simple and applicable approach for the further design of low-cost and lightweight plasmon-enhan...

Deep etched porous Si decorated with Au nanoparticles for surface-enhanced Raman spectroscopy (SERS)

Au nanoparticles decorated porous Si was fabricated following a two-step process for SERS applications. First, a Si wafer coated with predefined Au nanoisland arrays (∼15nm in average island size and ∼15nm in average inter-island spacing) was etched into porous structure by metal-assisted wet etching with etching time ranging from 0.5 to 40min. Second, Au with thickness ranging from 30 to 300nm was sputtered onto porous Si to activate the SERS effects. By testing 1,2-benzenedithiol with these SERS-active substrates, the influences of etching time and sputtered Au coating thickness on SERS enhancement factor were systematically investigated. Our analysis indicated that deep etching (etching time ≥ 10min) raised SERS enhancement factor by an order of magnitude over shallow etching (etching time ≤ 2min), while the thickness of sputtered Au coating, in the range of 30–300nm, only changed SERS enhancement factor by a factor of 3. The optimal parameters for etching time and sputtered Au coating thickness were experimentally determined to be 40min and 200nm, respectively. At the end of the study, the rationale of using predefined Au nanoisland arrays, instead of plain sputtered Au, to achieve better SERS performance was also experimentally justified.

Plasmon assisted thermal modulation in nanoparticles

Single-particle interactions hold the promise of nanometer-scale devices in areas such as data communications and storage, nanolithography, waveguides, renewable energy and therapeutics. We propose that the collective electronic properties possessed by noble metal nanoparticles may be exploited for device actuation via the unapparent mechanism of plasmon-assisted heat generation and flux. The temperature dependence of the dielectric function and the thermal transport properties of the particles play the central role in the feasibility of the thermally-actuated system, however the behavior of these thermoplasmonic processes is unclear. We experimentally and computationally analyzed modulation via thermoplasmonic processes on a test system of gold (Au) nano-islands. Modulation and energy transport in discontinuous domains exhibited quantitatively different characteristics compared to thin films. The results have implications for all surface plasmon based nano-devices where inevitable small-scale thermal processes are present.

STEMによる材料界面の原子レベル観察

Aberration-corrected scanning transmission electron microscopy (STEM) is becoming a very powerful tool to directly characterize defect structures in materials and devices at atomic dimensions. Here, we review our recent researches on material interface characterization using STEM. First example is that the direct imaging of individual dopant atoms within a buried alumina interface. The second example is the atomic-scale characterization of Au nanoislands on TiO2 substrates. The ability to directly probe individual atoms within buried interfaces at very high-resolution will significantly assists our understanding of interface structures and related properties in many advanced materials and devices.

Geometrically Planar Ion-Implant Patterned Magnetic Recording Media Using Block Copolymer Aided Gold Nanoisland Masks

We have developed patterned media via ion implantation using Au nano mask approach for local control of coercivity of magnetically hard [Co/Pd] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> multilayer film. Au nano-islands produced through a di-block copolymer templated technique is used as the mask for ion implantation. The sputter deposited [Co 0.3 nm/Pd 0.8 nm] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> /Pd 3 nm/Ta 3 nm multilayer film having vertical magnetic anisotropy is coated with a diblock copolymer layer, two phase decomposed into vertically pored nanostructure, then chemically processed to nucleate gold nanoislands corresponding to the diblock copolymer nanostructures. Subsequent nitrogen (N) ion implantation, using these Au islands as implantation-blocking masks, allows a patterned penetration of implanted ions into unmasked portion of the [Co/Pd] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> multilayer film, thus creating invisible but magnetically isolated bit island geometry while maintaining the overall flat configuration of the patterned media.

The role of contact size on the formation of Schottky barriers and ohmic contacts at nanoscale metal-semiconductor interfaces

We have measured the electronic structure at Au nanoisland--niobium doped SrTiO3 interfaces over a range of contact diameters. Electron transport processes at the interface transition from thermionic emission dominated to tunneling dominated, leading to ohmic behavior at small sizes. The transition increases at a much higher rate than is generally expected, emphasizing the need for precise control of nanoscale dimensions for reproducible effects in nanoscale electronic devices.

Molecular-Scale Surface Chemistry of a Common Metal Nanoparticle Capping Agent: Triphenylphosphine on Au(111)

Phosphine-stabilized Au clusters have been extensively studied and are used in various applications due to their unique structural, catalytic, and electronic properties. Triphenylphosphine (PPh(3)) is a key stabilizing ligand in the synthesis of Au nanoclusters. Despite its intense use in nanoparticle synthesis protocols, little is known regarding its surface chemistry, monolayer structure, density, and packing arrangement, all of which are important descriptors of functionality. Here, in contrast to sparse earlier investigations, we report that PPh(3) forms very ordered structures on Au(111). Atomic-scale imaging reveals that monolayer formation is accompanied by a partial lifting of the Au(111) surface reconstruction and ejection of extra Au atoms in the surface layer. Interestingly, these atoms are trapped and stabilized as two-dimensional Au nanoislands within the molecular layer. This behavior is in contrast to thiols, also common capping agents, which tend to remove Au atoms beyond those extra atoms present in the native reconstruction and form vacancy islands on the surface. Our data illustrate PPh(3)'s milder reactivity and reveal a new picture of its packing structure. These results shed new light on the surface chemistry of this important ligand for organic, organometallic, and nanoparticle synthesis.

Metal Organic Chemical Vapor Deposition of Phase Change Ge1Sb2Te4 Nanowires

The self-assembly of Ge(1)Sb(2)Te(4) nanowires (NWs) for phase change memories application was achieved by metal organic chemical vapor deposition, catalyzed by Au nanoislands in a narrow range of temperatures and deposition pressures. In the optimized conditions of 400 °C, 50 mbar, the NWs are Ge(1)Sb(2)Te(4) single hexagonal crystals. Phase change memory switching was reversibly induced by nanosecond current pulses through metal-contacted NWs with threshold voltage of about 1.35 V.

Underlayer Growth of a Nanoporous Single Atomic Layer of Gold

We have grown a single-layer-thick porous gold film with pore size of ∼1 nm. The film is created by deposition of gold atoms onto a C60 covered Au(111) surface. By controlling the sample temperature at 120–150 K, the deposited gold atoms are able to penetrate the molecular layer and reach the C60–Au(111) interface. At the interface, the gold atoms squeeze beneath some C60 molecules forming single-layer-high Au islands with lateral dimensions of the order of 1 nm. C60 molecules sitting directly above such a Au nanoisland are hence lifted up from the initial Au(111) substrate. The above process self-replicates leading to the formation of a single atomic layer porous gold film with pore size of ∼1 nm.

Anomalous Oxidation Resistance of “Core-Only” Copper Nanoparticles Electrochemically Grown on Gold Nanoislands Prefunctionalized by 1,4-phenylene Diisocyanide

Copper was electrodeposited on bare and pre-functionalized Au nanoislands with 1,4-phenylene diisocyanide (PDI, −C≡N+–C6H4– N+≡C−), on a Si(100) substrate. The electrodeposited Cu nanoparticles on bare Au nanoislands are found to consist of a Cu core and a Cu2O shell with a CuO overlayer. For Cu nanoparticles electrodeposited on Au nanoislands pre-functionalized with PDI, the metallic Cu core is covered by a barely detectable Cu2O overlayer, without any evidence of the CuO shell. PDI-functionalization of the Au nanoislands could be used to suppress the Cu oxidation and significantly alter the interfacial electronic nature of Cu on PDI-functionalized Au nanoislands. © 2012 The Electrochemical Society. [DOI: 10.1149/2.006205esl] All rights reserved.

Photo-induced suppression of plasmonic emission enhancement of CdSe/ZnS quantum dots

Emission of semiconductor quantum dots can be increased via two fundamentally differentprocesses: (i) surface plasmon resonances (plasmonic emission enhancement) and(ii) irradiation with light (photo-induced fluorescence enhancement). In this paper wetheoretically and experimentally study the mutual impacts of these processes on each otherin quantum dot solids. We show that when thin films of colloidal quantum dots are placedin the vicinity of Au nano-islands, the plasmonic enhancement of the radiative decay ratesof quantum dots and Forster energy transfer can hinder the photo-induced fluorescenceenhancement of these films. This in turn leads to significant suppression of theirplasmonic emission enhancement when they are irradiated with a laser beam. Weinvestigate the impact of the sizes and shapes of the metallic nanoparticles in thisprocess and theoretically analyze how plasmons and energy transfer can hinder theelectrostatic barrier responsible for photo-induced fluorescence enhancement.