Au Implantation Research Articles

Nano-Crystal Formation and Growth from High-Fluence Ion Implantation of Au, Ag or Cu in Silica

The linear and non-linear optical properties of silica may be tailored by the introduction of a random distribution of nanocrystallites of an immiscible metal within a near-surface region. The size, size distribution, and spatial distribution of these crystallites must be controllable in order to optimize the functional properties for device applications. In this paper, we present a novel fabrication technique that offers such control. Energetic metal ions are implanted in silica at room temperature. Subsequent heat treatment leads to diffusion of the implanted atoms, nucleation and growth of metal crystallites, and Ostwald ripening of the resulting clusters. We have observed the kinetics and effective activation energies describing the multiple processes involved, for the cases of Au, Ag or Cu implanted at MeV energies, at various fluences, and then annealed at fixed temperatures in the range 500°C-1000°C. Effective activation energies found for nanocrystal nucleation and growth at temperatures below 800°C (e.g. 64 meV for Ag) are replaced above this temperature range by much higher activation energies (e.g. 400 meV for Ag). We may attribute this to the depletion of un-attached mobile metal atoms (so that ripening of clusters will be limited by energy barriers for escape of such mobile atoms from small crystallites), and/or the annealing of implant-caused stress in the silica structure at high temperatures, that creates new channels for thermal diffusion of metal atoms within the silica host.

Acceptor-Compensated Charge Transport and Surface Chemical Reactions in Au-Implanted SnO2 Nanowires

A new deep acceptor state is identified by density functional theory calculations, and physically activated by an Au ion implantation technique to overcome the high energy barriers. And an acceptor-compensated charge transport mechanism that controls the chemical sensing performance of Au-implanted SnO2 nanowires is established. Subsequently, an equation of electrical resistance is set up as a function of the thermal vibrations, structural defects (Au implantation), surface chemistry (1 ppm NO2), and solute concentration. We show that the electrical resistivity is affected predominantly not by the thermal vibrations, structural defects, or solid solution, but the surface chemistry, which is the source of the improved chemical sensing. The response and recovery time of chemical sensing is respectively interpreted from the transport behaviors of major and minor semiconductor carriers. This acceptor-compensated charge transport mechanism provides novel insights not only for sensor development but also for research in charge and chemical dynamics of nano-semiconductors.

Synergistic Modification of Electronic and Photocatalytic Properties of TiO2 Nanotubes by Implantation of Au and N Atoms

The structural and electronic properties of N-doped, Au-adsorbed, and Au/N co-implanted TiO2 nanotubes (NTs) were investigated by performing first-principle density functional theory (DFT) calculations. For all the possible implanted configurations, the radius and bond length do not change significantly relative to the clean NTs. Our results indicate that the introduction of N into NTs is in favor of implantation of Au, and Au pre-adsorption on the NTs can also enhance the N concentration in NTs. The synergistic stability can be mainly attributed to charge transfer between Au and N atoms. In co-implanted configurations, the empty N 2p states in the band gap are occupied by one electron; denoted by Au 5s states. Thus, the associated electron transition among the valence band, the conduction band and the gap states results in redshift of the light absorption. In addition, the disappearance of N 2p empty states can effectively decrease the photogenerated carrier combination. Therefore, the Au/N implanted NTs should be regarded as a promising photocatalytic material under the visible light region.

Grazing-incidence X-ray diffraction of single GaAs nanowires at locations defined by focused ion beams

Grazing-incidence X-ray diffraction measurements on single GaAs nanowires (NWs) grown on a (111)-oriented GaAs substrate by molecular beam epitaxy are reported. The positions of the NWs are intentionally determined by a direct implantation of Au with focused ion beams. This controlled arrangement in combination with a nanofocused X-ray beam allows the in-plane lattice parameter of single NWs to be probed, which is not possible for randomly grown NWs. Reciprocal space maps were collected at different heights along the NW to investigate the crystal structure. Simultaneously, substrate areas with different distances from the Au-implantation spots below the NWs were probed. Around the NWs, the data revealed a 0.4% decrease in the lattice spacing in the substrate compared with the expected unstrained value. This suggests the presence of a compressed region due to Au implantation.

In-Plane Epitaxial Growth of Self-Assembled Ge Nanowires on Si Substrates Patterned by a Focused Ion Beam

We report a novel method for obtaining ordered arrays of self-assembled Ge nanowires (NWs) using Au seed catalysts, with the latter deposited using a focused ion beam (FIB). For this purpose we apply a three-step process involving first FIB nanopatterning, a second step of AuSi seed formation during UHV annealing, and third the nucleation and growth of Ge NWs by combining molecular-beam epitaxy (MBE) and the vapor–liquid–solid (VLS) process. We show that FIB allows for the local implantation of Au in the areas impacted by the ion beam; the implanted Au evolves during annealing into AuSi clusters, serving as nucleation seeds for the nucleation and growth of Ge NWs. We thus prove that FIB with gold ions is a successful method to obtain gold-catalyzed self-assembled nanowires. We obtain Ge NWs of homogeneous dimensions and oriented in-plane along [110] directions, as a consequence of a strain-driven process. Wire kinking is governed by surface morphological features. Based on our experimental results, we ela...

Low-dose-rate Interstitial Brachytherapy Preserves Good Quality of Life in Buccal Mucosa Cancer Patients

To determine the results and long-term changes in radiation toxicity of stage I-II buccal mucosa cancer patients treated by low-dose-rate (LDR) brachytherapy with (198)Au grains. A total of 133 stage I-II buccal mucosa carcinomas patients received (198)Au grain implantation brachytherapy between January 1982 and July 2005: 75 of them were treated by (198)Au grain implantation alone and 58 were treated by (198)Au implantation in combination with external irradiation. The average (198)Au-grain dose was 70 Gy in 7 days. Gross tumor areas ranged from 2.4 cm(2) to 9 cm(2), and the clinical target areas ranged from 6 cm(2) to 15 cm(2). The follow-up periods ranged from 3 months to 20 years (mean: 5 years 11 months and median: 5 years 1 months). Failure at the site of the primary lesion occurred in 17 patients. Post-treatment mucosal ulceration developed in 15 patients, and all were cured within 25 months by conservative treatment. Osteoradionecrosis was diagnosed in 8 patients, but only one patient required surgical treatment. No severe complications or aggravation of complications developed more than 10 years after treatment. The results of low-dose-rate (LDR)-brachytherapy (BT) alone and LDR-BT in combination with external irradiation at a total dose of 25 Gy were acceptable from the standpoint of cure rate and QOL.

Raman scattering study of Si nanoclusters formed in Si through a double Au implantation

ABSTRACTA sequential two step 32 keV Au implantation and 1.5 MeV Au irradiation technique has been used to synthesize Si nanoclusters (NCs) in Si. The low energy amorphising implantation has been carried out over a fluence range of (1 – 100) × 1015 cm−2 while the high energy recrystallizing irradiation fluence was fixed at 1 × 1015 cm−2. Samples were further annealed in air at temperatures between 500° to 950° C for a fixed annealing time of 1 hr and were characterized using Raman scattering at an excitation wavelength of 514.5 nm. Results on as-implanted and irradiated samples indicate formation of strained NCs in the top amorphised layer. Annealing around 500°C has been found to result in strain relief after which the data could be well explained using a phonon confinement model with an extremely narrow size distribution.

Study of annealing induced redistribution of implanted Au in Si: Fluence dependence

One hour 500 °C air annealing induced movement of implanted Au in Si have been studied for 32 keV Au implantation in Si, in the fluence range of 1 × 10 15 – 1 × 10 17 ions cm - 2 . Samples were characterized using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy. The results indicate that, depending on the initial state of Au in the matrix, there is a clear difference in the diffusion behaviour of Au in Si. When Au is precipitated as gold-silicide nanoclusters inside the Si matrix, annealing is found to cause diffusion of Au into the bulk Si. Compared to this, for a random atomic distribution of Au in an amorphous Si matrix, annealing is found to result in out-diffusion of Au towards the surface.

Au implantation into various types of silicate glasses

The implantation of gold ions into three types of silicate glass was studied. The energies of the implanted Au+ ions were 1701keV, and the fluences of the ions were 1×1014, 1×1015, 3×1015 and 1×1016cm−2. The as-implanted samples were annealed in air at two temperatures (400 and 600°C). The Au concentration depth profiles were investigated using Rutherford Backscattering Spectrometry (RBS) and compared to simulated profiles from the SRIM. The structural changes were studied by UV–vis absorption spectroscopy. The obtained mono-mode waveguides were characterised using Dark Mode Spectroscopy at 671nm to yield information on the refractive index changes. The results showed interesting differences depending on the type of glass and the post-implantation treatment. The obtained data were evaluated on the basis of the structure of the glass matrix, and the relations between the structural changes, waveguide properties and absorption, which are important for photonics applications, were formulated.

Formation of Au nanoparticles in sapphire by using Ar ion implantation and thermal annealing

In this paper, we present results of the synthesis of gold nanoclusters in sapphire, using Ar ion implantation and annealing in air. Unlike the conventional method of Au implantation followed by thermal annealing, Au was deposited on the surface of m- and a- cut sapphire single crystal samples including those pre-implanted with Ar ions. Au atoms were brought into the substrate by subsequent implantation of Ar ions to form Au nanoparticles. Samples were finally annealed stepwisely in air at temperatures ranging from 400 to 800 °C and then studied using UV–vis absorption spectrometry, transmission electron microscopy and Rutherford backscattered spectrometry. Evidence of the formation Au nanoparticles in the sapphire can be obtained from the characteristic surface plasmon resonance (SPR) absorption band in the optical absorption spectra or directly from the transmission electron microscopy. The results of optical absorption spectra indicate that the specimen orientations and pre-implantation also influence the size and the volume fraction of Au nanoparticles formed. Theoretical calculations using Maxwell–Garnett effective medium theory supply a good interpretation of the optical absorption results.

MeV Au irradiation induced nanoparticle formation and recrystallization in a low energyAu implanted Si layer

Au implantation at 32 keV into Si(100), in a fluence range of1 × 1015–1 × 1017 cm−2, has been used to produce a gold-rich damaged Si layer of thickness around 30 nm. Localrecrystallization of this layer, induced by 1.5 MeV Au irradiation, to a fluence of1 × 1015 cm−2, has been studied using Raman scattering, photoluminescence (PL) and x-rayphotoemission spectroscopy (XPS). For a sample with a low energy Au fluence of5 × 1015 cm−2, the MeV Au irradiation has been found to result in Si nanocrystal (NC) formation. Thesize of the NCs, as estimated from the PL data, has been found to be about 4 nm, whichagrees well with the result of a thermal spike model calculation. Annealing of the sample at500 °C resulted in an enhanced PL signal, without any significant shift in peak position, indicating anincrease in the local concentration of the NCs. In the case of samples with an initial Au fluence above1 × 1016 cm−2, the MeV Au irradiation has been found to result in better overall recrystallization of theamorphous layer, with silicide formation as observed by XPS. However, there was no PLsignal, indicating the absence of Si NCs in the system. The results suggest that the initialamorphizing Au fluence plays a crucial role in Si NC formation induced by MeV ionirradiation.

Formation and characterization of nanoparticles formed by sequential ion implantation of Au and Co into SiO2

Nanoparticles (NPs) were formed by sequential ion implantation of Au and Co into thin SiO2. After Au implantation and annealing, Co implantations were carried out at room temperature (RT) and 400°C, respectively, with no subsequent annealing. The NPs were investigated by means of Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). TEM shows the formation of Co–Au core-shell NPs for the Co implantation at 400°C. EXAFS measurements indicate significant strain in the NPs and a bond-length expansion of the Co–Co bonds in the NP core with a concomitant contraction of the Au–Au bonds in the Au shells. NPs are also observed by TEM for the Co implantation performed at RT, however, a lack of crystallinity is apparent from electron diffraction and EXAFS measurements.

Temperature response of C13 atoms in amorphized 6H–SiC

Implantation of C2+13 ions was employed to produce a concentration profile in 6H–SiC at 140K. In situ study of C13-implanted species was performed using the resonant reaction of C13(p,γ)N14 at Ep=1.748MeV. Significant C13 diffusion in the amorphized SiC does not occur up to 1130K. The presence of Au implants (1.9at.%) does not affect the C13 behavior. High-energy H+ irradiation also does not promote the C13 diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important in understanding the amorphization and recrystallization processes in SiC.

Direct Comparison of Au 3+ and C 60+ Cluster Projectiles in SIMS Molecular Depth Profiling

The sputtering properties of two representative cluster ion beams in secondary ion mass spectrometry (SIMS), C(60)(+) and Au(3)(+), have been directly compared. Organic thin films consisting of trehalose and dipalmitoylphosphatidylcholine (DPPC) are employed as prototypical targets. The strategy is to make direct comparison of the response of a molecular solid to each type of the bombarding cluster by overlapping the two ion beams onto the same area of the sample surface. The ion beams alternately erode the sample while keeping the same projectile for spectral acquisition. The results from these experiments are important to further optimize the use of cluster projectiles for SIMS molecular depth profiling experiments. For example, Au(3)(+) bombardment is found to induce more chemical damage as well as Au implantation when compared with C(60)(+). Moreover, C(60)(+) is found to be able to remove the damage and the implanted Au effectively. Discussions are also presented on strategies of enhancing sensitivity for imaging applications with cluster SIMS.

Co–Au core-shell nanocrystals formed by sequential ion implantation into SiO2

Co–Au core-shell nanocrystals (NCs) were formed by sequential ion implantation of Au and Co into thin SiO2. The NCs were investigated by means of transmission electron microscopy and extended x-ray absorption fine structure spectroscopy. The latter reveals a bond length expansion in the Co core compared to monatomic Co NCs. Concomitantly, a significant contraction of the bond length and a significant reduction of the effective Au–Au coordination number were observed in the Au shells. Increased Debye-Waller factors indicate significant strain in the NCs. These experimental results verify recent theoretical predictions.

Understanding the role of annealing temperature and ion energy in the growth of Au clusters

Many of the established processes in the fabrication of thin films use plasma or ion beam assisted techniques to control film composition and structure. To improve the understanding of the role of low energy ions we investigate the influence of temperature (273–873K) on the growth of Au clusters, which are deposited on amorphous carbon substrates. The use of a mass selected ion beam facility allowed to control the ion energies and afforded a narrow energy distribution. The Au+ energy was adjusted between 320 and 20eV, a substantial structural modification of the carbon layer surface due to irradiation or annealing can be excluded. The films were characterized with transmission electron microscopy (TEM), and photoelectron spectroscopy (PES) and subjected to equivalent annealing cycles in situ. In the core level PES a dynamic final state effect which leads to a cluster size dependent peak shift was exploited to determine cluster sizes in the subnanometer regime. TEM and PES are established successfully as complementary methods and allow to access the complete size range and implantation depth of the Au ions. Annealing leads to a substantial increase in the number of small clusters, a cluster ripening is only observed towards the end of the annealing cycle. At the highest ion energies of 200 and 320eV, the initial distribution contains only very small clusters (a few atoms) and the growth proceeds faster for 200eV. A qualitative model, which considers the particle fluxes within the system, is used to analyze these results and confirms the critical role of the subsurface Au reservoir. The initial ion energy determines the Au implantation depth and controls the Au concentration in the reservoir. The Au concentration then determines the diffusive flux of atoms to the surface and thus drives the nucleation of new, small clusters and controls the growth rate. Only the presence of such a reservoir allows the nucleation of new Au clusters as the annealing temperature is increased. These results indicate pathways to the control of cluster size and impact on the interpretation and control of thin film deposition.

Patterned microstructures formed with MeV Au implantation in Si(1 0 0)

Energetic (MeV) Au implantation in Si(100) (n-type) through masked micropatterns has been used to create layers resistant to KOH wet etching. Microscale patterns were produced in PMMA and SU(8) resist coatings on the silicon substrates using P-beam writing and developed. The silicon substrates were subsequently exposed using 1.5MeV Au3+ ions with fluences as high as 1×1016ions/cm2 and additional patterns were exposed using copper scanning electron microscope calibration grids as masks on the silicon substrates. When wet etched with KOH microstructures were created in the silicon due to the resistance to KOH etching cause by the Au implantation. The process of combining the fabrication of masked patterns with P-beam writing with broad beam Au implantation through the masks can be a promising, cost-effective process for nanostructure engineering with Si.

Structure and thermal stability of Au–Fe alloy nanoclusters formed by sequential ion implantation in silica

Sequential ion implantation of Au and Fe ions has been performed in silica. Despite the fact that the two species are not miscible in the bulk, structural characterizations show that the nanoparticles produced are a Au–Fe alloy. The crystalline structure of the nanoparticles is fcc with a lattice parameter of 0.395nm. The nanocomposite is ferromagnetic at 3K, with a magnetic moment per Fe atom equal to 1.4μB and it shows a hysteresis loop with a coercive field of 24mTorr. Due to alloying, the optical absorption spectrum does not exhibit the typical surface plasmon resonance of gold nanoparticles. The sample exhibits a change of its optical and structural properties when is annealed at 600°C.

Local structure of gold impurities in silicon determined by EXAFS

The incorporation of Au atoms within the silicon lattice was determined from X-ray absorption fine structure (XAFS). A detection limit of about 10 14 cm - 2 doses equivalent Au atoms in silicon was achieved by grazing incidence of the X-rays and fluorescence detection. Our results are (i) after Au implantation (as-implanted state) single Au atoms occupy regular high symmetric substitutional lattice sites in silicon, (ii) after thermal treatments some of the Au-atoms remain substitutional other diffuse to the sample surface. For the Au atoms near the surface very similar short range parameters as for metallic gold are detected and X-ray reflectrometry gives evidence for a near-surface segregation of gold atoms.

High-energy Au implantation of GaAs at 16 K

GaAs was irradiated with 10MeV or 17.5MeV Au ions at 16K and subsequently measured at the same temperature. Under these conditions ion-beam induced processes can be studied whilst thermal effects are widely excluded. It is found that the measured defect concentration scales with the number of displacements per lattice atom (representing the nuclear energy deposition) independent of the ion energy and of the depth. This result demonstrates that, for the chosen ion energies, the electronic energy deposition itself does not influence the damage production in GaAs. Contrary in GaAs implanted with high-energy ions (10MeV range) and analysed at room temperature, the defect concentration close to the surface was found to be lower than expected from theoretical simulations. The explanation for this previously observed effect is a preferred annealing of lightly damaged areas during warming the samples to room temperature for measurement.