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Growth and characterization of anodic Films on InP in KOH and (NH4)2S
(Electrochemical Society, 2001-03) Harvey, E.; O'Dwyer, Colm; Melly, T.; Buckley, D. Noel; Cunnane, V. J.; Sutton, David; Newcomb, Simon B.
The current-voltage characteristics of InP were investigated in (NH4)2S and KOH electrolytes. In both solutions, the observation of current peaks in the cyclic voltammetric curves was attributed to the growth of passivating films. The relationship between the peak currents and the scan rates suggests that the film formation process is diffusion controlled in both cases. The film thickness required to inhibit current flow was found to be much lower on samples anodized in the sulphide solution. Focused ion beam (FIB) secondary electron images of the surface films show that film cracking of the type reported previously for films grown in (NH4)2S is also observed for films grown in KOH. X-ray and electron diffraction measurements indicate the presence of In2O3 and InPO4 in films grown in KOH and In2S3 in films grown in (NH4)2S.
Electrochemical pore formation on InP in alkaline solutions
(Electrochemical Society, 2001-09) Harvey, E.; O'Dwyer, Colm; Melly, T.; Buckley, D. Noel; Cunnane, V. J.; Sutton, David; Newcomb, Simon B.; Chu, S. N. G.
The surface properties of InP electrodes were examined following anodization in (NH4)2S and KOH electrolytes. In both solutions, the observation of current peaks in the cyclic voltammetric curves was attributed to selective etching of the substrate and a film formation process. AFM images of samples anodized in the sulfide solution, revealed surface pitting and TEM micrographs revealed the porous nature of the film formed on top of the pitted substrate. After anodization in the KOH electrolyte, TEM images revealed that a porous layer extending 500 nm into the substrate had been formed. Analysis of the composition of the anodic products indicates the presence of In2S3 in films grown in (NH4)2S and an In2O3 phase within the porous network formed in KOH.
Formation and characterization of porous InP layers in KOH Solutions
(Electrochemical Society, 2002-10) O'Dwyer, Colm; Buckley, D. Noel; Cunnane, V. J.; Sutton, David; Serantoni, M.; Newcomb, Simon B.
Porous InP layers were formed electrochemically on (100) oriented n-InP substrates in various concentrations of aqueous KOH under dark conditions. In KOH concentrations from 2 mol dm-3 to 5 mol dm-3, a porous layer is obtained underneath a dense near-surface layer. The pores within the porous layer appear to propagate from holes through the near-surface layer. Transmission electron microscopy studies of the porous layers formed under both potentiodynamic and potentiostatic conditions show that both the thickness of the porous layer and the mean pore diameter decrease with increasing KOH concentration. The degree of porosity, estimated to be 65%, was found to remain relatively constant for all the porous layers studied.
Anodic oxidation of InP in KOH electrolytes
(Electrochemical Society, 2002-10) O'Dwyer, Colm; Melly, T.; Harvey, E.; Buckley, D. Noel; Cunnane, V. J.; Sutton, David; Serantoni, M.; Newcomb, Simon B.
The anodic behavior of InP in 1 mol dm-3 KOH was investigated and compared with its behavior at higher concentrations of KOH. At concentrations of 2 mol dm-3 KOH or greater, selective etching of InP occurs leading to thick porous InP layers near the surface of the sustrate. In contrast, in 1 mol dm-3 KOH, no such porous layers are formed but a thin surface film is formed at potentials in the range 0.6 V to 1.3 V. The thickness of this film was determined by spectroscopic ellipsometry as a function of the upper potential and the measured film thickness corresponds to the charge passed up to a potential of 1.0 V. Anodization to potentials above 1.5 V in 1 mol dm- 3 KOH results in the growth of thick, porous oxide films (~ 1.2 µm). These films are observed to crack, ex-situ, due to shrinkage after drying in ambient air. Comparisons between the charge density and film thickness measurements indicate a porosity of approximately 77% for such films.
Comparison of oscillatory behavior on InP electrodes in KOH solutions
(Electrochemical Society, 2002-10) O'Dwyer, Colm; Melly, T.; Harvey, E.; Buckley, D. Noel; Cunnane, V. J.; Sutton, David; Newcomb, Simon B.
The observation of current oscillations under potential sweep conditions when an n-InP electrode is anodized in a KOH electrolyte is reported and compared to the oscillatory behavior noted during anodization in an (NH4)2S electrolyte. In both cases oscillations are observed above 1.7 V (SCE). The charge per cycle was found to increase linearly with potential for the InP/KOH system but was observed to be independent of potential for the InP/(NH4)2S system. The period of the oscillations in the InP/KOH was found to increase with applied potential. In this case the oscillations are asymmetrical and the rising and falling segments have a different dependence on potential. Although the exact mechanism is not yet know for either system, transmission electron microscopy studies show that in both cases, the electrode is covered by a thick porous film in the oscillatory region.
A mechanistic study of anodic formation of porous InP
(The Electrochemical Society, 2003-01) O'Dwyer, Colm; Buckley, D. Noel; Sutton, David; Newcomb, Simon B.; Serantoni, M.
When porous InP is anodically formed in KOH electrolytes, a thin layer ~40 nm in thickness, close to the surface, appears to be unmodified. We have investigated the earlier stages of the anodic formation of porous InP in 5 mol dm-3 KOH. TEM clearly shows individual porous domains which appear triangular in cross-section and square in plan view. The crosssections also show that the domains are separated from the surface by a ~40 nm thick, dense InP layer. It is concluded that the porous domains have a square-based pyramidal shape and that each one develops from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the <100> directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain, and these domains eventually form a continuous porous layer. This implies that the development of porous domains beneath the surface is also progressive in nature. Evidence for this was seen in plan view TEM images. Merging of domains continues to occur at potentials more anodic than the peak potential, where the current is observed to decrease. When the domains grow, the current density increases correspondingly. Eventually, domains meet, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Quantitative models of this process are being developed.
A study of anodic films on n-InP by spectroscopic ellipsometry and atomic force microscopy
(The Electrochemical Society, 2003-01) Buckley, D. Noel; O'Dwyer, Colm; Melly, T.; Serantoni, M.; Sutton, David; Newcomb, Simon B.
The growth of anodic films on n-InP in 1 mol dm-1 KOH is investigated under potential sweep conditions. At lower potentials a thin surface film is formed and a peak is observed on the current-voltage curve. Ellipsometric measurements show that this film increases in thickness with increasing potential but the observed thickness values are significantly less than the corresponding coulometrically estimated values. This indicates that much of the charge passed is not involved in the formation of a surface film but presumably in the formation of soluble anodic reaction products. Cyclic voltammograms show that a current peak is also observed on the reverse sweep and ellipsometric measurements show that the anodic film thickness also increases during the reverse sweep until the peak potential is reached. Atomic force microscopy (AFM) shows that the surface becomes smoother as the potential is increased. We attribute this to the formation of nuclei at lower potentials, which coalesce as the layer becomes thicker. Electron diffraction and x-ray photoelectron spectroscopy (XPS) analysis show that the surface film is predominantly In2O3 with no evidence of InPO4.
Anodic behavior of InP: film growth, porous structures and current oscillations.
(Electrochemical Society, 2003-04) Buckley, D. Noel; O'Dwyer, Colm; Harvey, E.; Melly, T.; Sutton, David; Newcomb, Simon B.
We review our recent work on the anodization of InP in KOH electrolytes. The anodic oxidation processes are shown to be remarkably different in different concentrations of KOH. Anodization in 2 - 5 mol dm-3 KOH electrolytes results in the formation of porous InP layers but, under similar conditions in a 1 mol dm-3 KOH, no porous structure is evident. Rather, the InP electrode is covered with a thin, compact surface film at lower potentials and, at higher potentials, a highly porous surface film is formed which cracks on drying. Anodization of electrodes in 2 - 5 mol dm-3 KOH results in the formation of porous InP under both potential sweep and constant potential conditions. The porosity is estimated at ~65%. A thin layer (~ 30 nm) close to the surface appears to be unmodified. It is observed that this dense, near-surface layer is penetrated by a low density of pores which appear to connected it to the electrolyte. Well-defined oscillations are observed when InP is anodized in both the KOH and (NH4)2S. The charge per cycle remains constant at 0.32 C cm-2 in (NH4)2S but increases linearly with potential in KOH. Although the characteristics of the oscillations in the two systems differ, both show reproducible and well-behaved values of charge per cycle.
Pitting and porous layer formation on n-InP anodes
(Electrochemical Society, 2003-10) O'Dwyer, Colm; Buckley, D. Noel; Serantoni, M.; Sutton, David; Newcomb, Simon B.
Surface pitting occurs when InP electrodes are anodized in KOH electrolytes at concentrations in the range 2 - 5 mol dm-3. The process has been investigated using atomic force microscopy (AFM) and the results correlated with cross-sectional transmission electron microscopy (TEM) and electroanalytical measurements. AFM measurements show that pitting of the surface occurs and the density of pits is observed to increase with time under both potentiodynamic and potentiostatic conditions. This indicates a progressive pit nucleation process and implies that the development of porous domains beneath the surface is also progressive in nature. Evidence for this is seen in plan view TEM images in which individual domains are seen to be at different stages of development. Analysis of the cyclic voltammograms of InP electrodes in 5 mol dm-3 KOH indicates that, above a critical potential for pit formation, the anodic current is predominantly time dependent and there is little differential dependence of the current on potential. Thus, pores continue to grow with time when the potential is high enough to maintain depletion layer breakdown conditions.
Numerical simulation of the anodic formation of nanoporous InP
(The Electrochemical Society, 2004-01) Lynch, Robert P.; O'Dwyer, Colm; Clancy, Ian; Corcoran, David; Buckley, D. Noel
Anodic etching of n-type InP in KOH electrolytes under suitable conditions leads to the formation of a nanoporous region beneath a ~40 nm dense near-surface layer [1]. The early stages of the process involve the formation of square-based pyramidal porous domains [2] and a mechanism is proposed based on directional selectivity of pore growth along the <100> directions. A numerical model of this mechanism is described in this paper. In the algorithm used the growth is limited to the <100> directions and the probability of growth at any pore tip is controlled by the potential and the concentration of electrolyte at the pore tip as well as the suitability of the pore tip to support further growth. The simulated porous structures and their corresponding current versus time curves are in good agreement with experimental data. The results of the simulation also suggest that, after an initial increase in current caused by the spreading out of the porous domains from their origins, growth is limited by the diffusion rate of electrolyte along the pores with the final fall-off in current being caused by irreversible processes such as the formation of a passivating film at the tips or some other modification of the state of the pore tip.
Sub-100 nm Feature Definition Optimization using Cold Cs Beam Exposed Self-Assembled Monolayers on Au
(The Electrochemical Society, 2004-01) O'Dwyer, Colm
The results of a study into the dependency of SAM coverage, subsequent post-etch pattern definition and minimum feature size on the quality of the Au substrate used in both physical mask and optical mask atomic nanolithographic experiments are presented in this paper. In comparison, sputtered Au substrates yield much smoother surfaces and a higher density of {111} oriented grains than evaporated Au surfaces. Phase imaging with an atomic force microscope shows that the quality and percentage coverage of uniform alkanethiol monolayer adsorption was much greater for sputtered Au surfaces. Exposure of the monolayer with a laser-cooled Cs beam allowed determination of the minimum Cs dose (2 monolayers) to expose the SAM with lateral force microscopy. Suitable wet-etching, with etch rates of 2.2 nm min-1, results in optimized pattern definition. Utilizing these optimizations, features as small as 50 nm were achieved using both a sub-100 nm physical mask and optical standing wave mask.
Formation of nanoporous InP by electrochemical anodization
(The Electrochemical Society, 2004-01) Buckley, D. Noel; O'Dwyer, Colm; Lynch, Robert P.; Newcomb, Simon B.
Porous InP layers can be formed electrochemically on (100) oriented n- InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains which appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the <100> directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually, the domains meet forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Numerical models of this process have been developed. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value.
Adsorption of alkanethiol self-assembled monolayers on sputtered gold substrates for atomic nanolithography applications
(The Electrochemical Society, 2004-01) O'Dwyer, Colm
A detailed study of the self-assembly and coverage by 1-nonanethiol of sputtered Au surfaces using molecular resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au surface composed predominantly of {111} oriented grains. The domains of the alkanethiol monolayer are observed with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. STM imaging shows that the (4 × 2) superlattice structure is observed as a (3 × 2√3) structure when imaged under noncontact AFM conditions. The 1-nonanethiol molecules reside in the threefold hollow sites of the Au{111} lattice and aligned along its [112] lattice vectors. The self-assembled monolayer (SAM) contains many nonuniformities such as pinholes, domain boundaries, and monatomic depressions which are present in the Au surface prior to SAM adsorption. The detailed observations demonstrate limitations to the application of 1-nonanethiol as a resist in atomic nanolithography experiments to feature sizes of ~20 nm.
Nanoporous InP: anodic formation and growth mechanism in aqueous electrolytes
(Electrochemical Society, 2005-01) O'Dwyer, Colm
Porous layers can be formed electrochemically on (100) oriented n-InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains that appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the <100> directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually the domains meet, forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value. Porous layers are also observed in highly doped InP but are not observed in wafers with doping densities below ~5 × 1017 cm-3. Numerical models of this process have been developed invoking a mechanism of directional selectivity of pore growth preferentially along the <100> lattice directions. Manipulation of the parameters controlling these curves shows that the fall-off in current is controlled by the rate of diffusion of electrolyte through the pore structure with the final decline in current being caused by the termination of growth at the pore tips through the formation of passivating films or some other irreversible modification of the pore tips.
STM observation of sulfur dimerization in alkanethiol self-assembled monolayers on Au{111}
(The Electrochemical Society, 2005-01) O'Dwyer, Colm
We present for the first time, direct microscopical observation by STM of sulfur dimer formation on alkanethiol self-assembled monolayers (SAM) on sputtered Au substrates. The sulfur dimers are observed when imaging at a bias where the tip-molecule interaction occurs, and are formed by displacement of sulfur atoms from their normal three-fold hollow site residence of the (4 × 2) superlattice to nearest-neighbor bridge-site residence between two Au atoms. The displacement is believed to occur due to defects induced in the alkyl chain of the monolayer due to the proximity of the STM tip. Only one of the sulfur atoms forming the dimer is bound to the surface and they are commensurate with the Au{111} adlattice along its [112] directions.
Characterization of resistivity of Sb2S3 semiconductor nanowires by conductive AFM and in-situ methods
(Trans Tech Publications, 2011-04) Bukins, J.; Kunakova, Gunta; Birjukovs, P.; Prikulis, Juris; Varghese, Justin M.; Holmes, Justin D.; Erts, Donats; Medvids, Arturs
Conductive AFM and in situ methods were used to determine contact resistance and resistivity of individual Sb2S3 nanowires. Nanowires were deposited on oxidized Si surface for in situ measurements and on Si surface with macroelectrodes for conductive AFM (C-AFM) measurements. Contact resistance was determined by measurement of I(V) characteristics at different distances from the nanowire contact with the macroelectrode and resistivity of nanowires was determined. Sb2S3 is a soft material with low adhesion force to the surface and therefore special precautions were taken during measurements.
Nanochemistry in the new leaving certificate chemistry syllabus
(Chemistry Education Research Group, University of Limerick, 2012) Holmes, Justin D.
AuxAg1-x alloy seeds: A way to control growth, morphology and defect formation in Ge nanowires
(2012-06) Biswas, Subhajit; Holmes, Justin D.
Germanium (Ge) nanowires are of current research interest for high speed nanoelectronic devices due to the lower band gap and high carrier mobility compatible with high K-dielectrics and larger excitonic Bohr radius ensuing a more pronounced quantum confinement effect [1-6]. A general way for the growth of Ge nanowires is to use liquid or a solid growth promoters in a bottom-up approach which allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, operating pressure, precursor flow rate etc [3, 7-11]. The Solid-phase seeding is preferred for more control processing of the nanomaterials and potential suppression of the unintentional incorporation of high dopant concentrations in semiconductor nanowires and unrequired compositional tailing of the seed-nanowire interface [2, 5, 9, 12]. There are therefore distinct features of the solid phase seeding mechanism that potentially offer opportunities for the controlled processing of nanomaterials with new physical properties. A superior control over the growth kinetics of nanowires could be achieved by controlling the inherent growth constraints instead of external parameters which always account for instrumental inaccuracy. The high dopant concentrations in semiconductor nanowires can result from unintentional incorporation of atoms from the metal seed material, as described for the Al catalyzed VLS growth of Si nanowires [13] which can in turn be depressed by solid-phase seeding. In addition, the creation of very sharp interfaces between group IV semiconductor segments has been achieved by solid seeds [14], whereas the traditionally used liquid Au particles often leads to compositional tailing of the interface [15] . Korgel et al. also described the superior size retention of metal seeds in a SFSS nanowire growth process, when compared to a SFLS process using Au colloids [12]. Here in this work we have used silver and alloy seed particle with different compositions to manipulate the growth of nanowires in sub-eutectic regime. The solid seeding approach also gives an opportunity to influence the crystallinity of the nanowires independent of the substrate. Taking advantage of the readily formation of stacking faults in metal nanoparticles, lamellar twins in nanowires could be formed.
Low resistivity Pt interconnects developed by electron beam assisted deposition using novel gas injector system
(IOP Publishing, 2012-07-02) Dias, R. J.; O'Regan, Colm; Thrompenaars, P.; Romano-Rodriguez, A.; Holmes, Justin D.; Mulder, J. J. L.; Petkov, Nikolay; Vol. 371; Science Foundation Ireland
Electron beam-induced deposition (EBID) is a direct write process where an electron beam locally decomposes a precursor gas leaving behind non-volatile deposits. It is a fast and relatively in-expensive method designed to develop conductive (metal) or isolating (oxide) nanostructures. Unfortunately the EBID process results in deposition of metal nanostructures with relatively high resistivity because the gas precursors employed are hydrocarbon based. We have developed deposition protocols using novel gas-injector system (GIS) with a carbon free Pt precursor. Interconnect type structures were deposited on preformed metal architectures. The obtained structures were analysed by cross-sectional TEM and their electrical properties were analysed ex-situ using four point probe electrical tests. The results suggest that both the structural and electrical characteristics differ significantly from those of Pt interconnects deposited by conventional hydrocarbon based precursors, and show great promise for the development of low resistivity electrical contacts.
Highly stable PEGylated gold nanoparticles in water: applications in biology and catalysis
(NSTI-Nanotech, 2012-08) Rahme, Kamil; Nolan, Marie-Therese; Doody, Timothy; McGlacken, Gerard P.; O'Driscoll, Caitríona M.; Holmes, Justin D.
Gold nanoparticles (Au NPs) with diameters ranging between 5-60 nm have been synthesised in water, and further stabilized with polyethylene glycol-based thiol polymers (mPEG-SH). Successful PEGylation of the Au NPs was confirmed by Dynamic Light scattering (DLS) and Zeta potential measurements. PEG coating of the Au NPs is the key of their colloidal stabilty, and its successful applications. Catalytic efficiency testing of the PEG-AuNPs were carried out on homocoupling of boronic acid. PEG-Au NPs with AuNps diameter < 30 nm were useful as catalyst in water. Finally, the PEG-Au NPs were also shown to be stable in biological fluid and not cytotoxic on B16.F10 cell line, making them attractive for further studies.

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