Justin D. Holmes

Trinity College Dublin, Dublin, Leinster, Ireland

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Publications (207)778.95 Total impact

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    ABSTRACT: Many applications of germanium (Ge) are underpinned by effective oxide removal and surface passivation. This important surface treatment step often requires H-X (X = Cl, Br, I), or HF etchants. Here, we show that aqueous citric acid solutions are effective for the removal of GeOx. The stability of citric acid treated Ge(100) is compared with HF and HCl and analyzed by X-ray photoelectron spectroscopy. Further Ge surface passivation was investigated by thiolation using alkane mono and dithiols. The organic passivation layers show good stability with no oxide regrowth observed after 3 days of ambient exposure.
    Langmuir 12/2014; 30:14123−14127. · 4.38 Impact Factor
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    ABSTRACT: We report an inexpensive nanoscale patterning process for epitaxial lateral overgrowth (ELOG) in AlN layers grown by metal organic vapour phase epitaxy (MOVPE) on sapphire. The pattern was produced by an inductively coupled plasma etch using a self-assembled monolayer of silica spheres on AlN as the lithographic mask. The resulting uniform 2 µm length rod structure across a wafer provided an almost ‘threading dislocation (TD) free’ AlN template for subsequent growth when annealed at 1100 C. Overgrowing homoepitaxial AlN on top of the nanorods, at a temperature of 1100 C, produced a coalesced film at around 800 nm, which is formed at a much lower temperature compared to 1400°C typically required for microscale ELOG. The improved crystal quality, in terms of TD reduction, of the AlN above the rods was determined by detailed weak beam (WB) electron microscopy studies and showed that the threading dislocation density (TDD) was greatly reduced, by two orders of magnitude in the case for edge-type dislocations. In-situ reflectance measurements during the overgrowth allowed for thickness coalescence to be determined along with wafer curvature changes. The in-situ measurements also confirmed that tensile strain built up at a much slower rate in the ELOG AlN layer compared to the same thickness of AlN prepared directly on sapphire.
    J. Mater. Chem. C. 11/2014;
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    ABSTRACT: To maintain semiconductor device scaling, in recent years industry has been forced to move from planar to non-planar device architectures. This alone has created the need to develop a radically new, non-destructive method for doping. Doping alters the electrical properties of a semiconductor, related to the access resistance. Low access resistance is necessary for high performance technology and reduced power consumption. In this work the authors reduced access resistance in top–down patterned Ge nanowires and Ge substrates by a non-destructive dopant in-diffusion process. Furthermore, an innovative electrical characterisation methodology is developed for nanowire and fin-based test structures to extract important parameters that are related to access resistance such as nanowire resistivity, sheet resistance, and active doping levels. Phosphine or arsine was flowed in a Metalorganic Vapour Phase Epitaxy reactor over heated Ge samples in the range of 650–700 °C. Dopants were incorporated and activated in this single step. No Ge growth accompanied this process. Active doping levels were determined by electrochemical capacitance–voltage free carrier profiling to be in the range of 1019 cm−3. The nanowires were patterned in an array of widths from 20–1000 nm. Cross-sectional Transmission Electron Microscopy of the doped nanowires showed minimal crystal damage. Electrical characterisation of the Ge nanowires was performed to contrast doping activation in thin-body structures with that in bulk substrates. Despite the high As dose incorporation on unpatterned samples, the nanowire analysis determined that the P-based process was the better choice for scaled features.
    J. Mater. Chem. C. 10/2014;
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    ABSTRACT: We investigate the impact of using a carbon based gas diffusion layer (GDL) as the current collector for Li-O2 batteries. It is shown that the GDL actively participates in ORR during discharge conditions and, if its mass is not accounted for, can lead to inflated discharge capacity figures compared to inert cathode supports. SEM and XRD analyses show that Li2O2 discharge products form on cathodes composed of as-received GDL in a similar manner to that observed for carbon on stainless steel (SS) current collectors (at applied currents of 100 μA cm −2 or less). The relative activity of the GDL, carbon on GDL and carbon-on-stainless steel current collectors from voltammetric measurements confirmed ORR and OER processes to be similar at all carbon-based surfaces. When heated above 300 • C, degradation of the binder in the GDL and associated loss of carbon from the substrate surface leads to reduced discharge times compared to the pristine GDL substrates. The data highlight the importance of the contribution to ORR/OER in carbon-based active current collector substrates when determining gravimetric capacities of Li-O2 batteries.
    Journal of The Electrochemical Society 09/2014; 161:1964-1968. · 2.86 Impact Factor
  • Roisin A Kelly, Justin D Holmes, Nikolay Petkov
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    ABSTRACT: In this article we detail the application of electron microscopy to visualise discrete structural transitions incurring in single crystalline Ge nanowires upon Ga-ion irradiation and subsequent thermal annealing. Sequences of images for nanowires of varying diameters subjected to an incremental increase of the Ga-ion dose were obtained. Intricate transformations dictated by a nanowire's geometry indicate unusual distribution of the cascade recoils in the nanowire volume, in comparison to planar substrates. Following irradiation, the same nanowires were annealed in the TEM and corresponding crystal recovery followed in situ. Visualising the recrystallisation process, we establish that full recovery of defect-free nanowires is difficult to obtain due to defect nucleation and growth. Our findings will have large implications in designing ion beam doping of Ge nanowires for electronic devices but also for other devices that use single crystalline nanostructured Ge materials such as thin membranes, nanoparticles and nanorods.
    Nanoscale 09/2014; · 6.73 Impact Factor
  • Roisin A. Kelly, Justin D. Holmes, Nikolay Petkov
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    ABSTRACT: In this article we detail the application of electron microscopy to visualise discrete structural transitions incurring in single crystalline Ge nanowires upon Ga-ion irradiation and subsequent thermal annealing. Sequences of images for nanowires of varying diameters subjected to an incremental increase of the Ga-ion dose were obtained. Intricate transformations dictated by a nanowire's geometry indicate unusual distribution of the cascade recoils in the nanowire volume, in comparison to planar substrates. Following irradiation, the same nanowires were annealed in the TEM and corresponding crystal recovery followed in situ. Visualising the recrystallisation process, we establish that full recovery of defect-free nanowires is difficult to obtain due to defect nucleation and growth. Our findings will have large implications in designing ion beam doping of Ge nanowires for electronic devices but also for other devices that use single crystalline nanostructured Ge materials such as thin membranes, nanoparticles and nanorods.
    Nanoscale 09/2014; · 6.73 Impact Factor
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    ABSTRACT: A facile bottom-up approach for the synthesis of inorganic/organic bioconjugated nanoprobes based on iron oxide nanocubes as core with a nanometric silica shell is demonstrated. Surface coating and functionalization protocols developed in this work offered a good control over the shell thickness (8 - 80 nm) and enabled bio-vectorization of SiO2@Fe3O4 core-shell structures by covalent attachment of folic acid (FA) as a targeting unit for cellular uptake. The successful immobilization of folic acid was investigated both quantitatively (TGA, EA, XPS) and qualitatively (AT-IR, UV-Vis, Zeta potential). Additionally the magnetic behavior of the nanocomposites was monitored after each functionalization step. Cell viability studies confirmed low cytotoxicity of FA@SiO2@Fe3O4 conjugates, which in conjunction with their high dispersibility and stability in cell medium makes them promising nanoprobes for targeted internalization by cells and their imaging.
    ACS Applied Materials & Interfaces 09/2014; · 5.90 Impact Factor
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    ABSTRACT: Vapour-liquid-solid (VLS) techniques are popular routes for the scalable synthesis of semiconductor nanowires. In this article, in-situ electron microscopy is used to correlate the equilibrium content of ternary (Au0.75Ag0.25–Ge and Au0.65Ag0.35–Ge) metastable alloys with the kinetics, thermodynamics and diameter of Ge nanowires grown via a VLS mechanism. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires varies as a function of nanowire diameter and eutectic alloy composition. The behaviour of the faceted heterogeneous liquid–solid interface correlates with the growth kinetics of the nanowires, where the main growth facet at the solid nanowire–liquid catalyst drop contact line lengthens for faster nanowire growth kinetics. Pronounced diameter dependent growth kinetics, as inferred from liquid–solid interfacial behaviour, is apparent for the synthesised nanowires. Direct in-situ microscopy observations facilitates the comparison between the nanowire growth behaviour from ternary (Au–Ag–Ge) and binary (Au–Ge) eutectic systems.
    Small 09/2014; · 7.82 Impact Factor
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    ABSTRACT: Microwave annealing is an emerging technique for achieving ordered patterns of block copolymer films on substrates. Little is understood about the mechanisms of microphase separation during the microwave annealing process and how it promotes the microphase separation of the blocks. Here, we use controlled power microwave irradiation in the presence of tetrahydrofuran (THF) solvent, to achieve lateral microphase separation in high- lamellar forming poly(styrene-b-lactic acid) PS-b-PLA. A highly ordered line pattern was formed within seconds on a silicon substrate. In-situ temperature measurement of the silicon substrate coupled to condition changes during "solvo-microwave" annealing allowed understanding of the processes to be attained. Our results suggest that the substrate has little effect on the ordering process and is essentially microwave transparent but rather, it is direct heating of the polar THF molecules that causes microphase separation. It is postulated that the rapid interaction of THF with microwaves and the resultant temperature increase to 55 ºC within seconds causes an increase of the vapor pressure of the solvent from 19.8 kPa to 70 kPa. This enriched vapor environment increases the plasticity of both PS and PLA chains and leads to the fast self-assembly kinetics. Comparing the patterns formed on silicon, germanium and silicon on insulator (SOI) and also an in-situ temperature measurement of silicon in the oven confirms the significance of the solvent over the role of substrate heating during "solvo-microwave" annealing. Besides the short annealing time which has technological importance, the coherence length is on a micron scale and dewetting is not observed after annealing. The etched pattern (PLA was removed by an Ar/O2 reactive ion etch) was transferred to the underlying silicon substrate fabricating sub-20 nm silicon nanowires over large areas demonstrating that the morphology is consistent both across and through the film.
    Langmuir : the ACS journal of surfaces and colloids. 08/2014;
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    ABSTRACT: The structure−property relationship of palla-dium (Pd) catalysts in Suzuki−Miyaura cross-coupling reactions was investigated using Pd nanocrystals of uniform size and shape. Superior catalytic reactivity was observed for Pd nanoparticles with high-index {730} surface facets compared to low-index {100} facets. Although the nanocrystal morphologies were maintained during the reaction, the presence of Pd clusters, identified by high-resolution trans-mission electron microscopy (TEM), indicates a leaching mechanism. The nature of the surface facets on the nanoparticles was observed to influence the rate of Pd leaching during the Suzuki coupling reaction. The enhanced reactivity observed for the high-index facet catalysts stems from the greater number of leachable atoms of low abstraction energy available on high-index planes. KEYWORDS: palladium nanocrystals, shape control nanoparticles, Suzuki coupling, leaching ■ INTRODUCTION Noble metal nanocrystals (NCs) with high-index surface facets have attracted much interest due to their potential for enhanced catalytic performance. 1 High index facets are denoted by a set of Miller indices {hkl}, where one index is greater than 1. Unlike low-index planes characterized by {111} and {100} facets, which are relatively smooth, the surface atomic structure of high-index facets is characterized by a high-density of step, terrace, and kink sites. 2 Such surfaces are well-known to improve catalytic rates for many reactions. 3 The physical origins of structure sensitivity are complex and generally ascribed to electronic and geometrical effects that influence adsorption energies and reaction pathways. 4 Chemisorption of reaction species can be preferential on step and kink sites due to their lower co-ordination numbers (6−7) or allow more energeti-cally favorable transition states compared to close-packed surfaces. 5
    ACS Catalysis 07/2014; 4:3105−3111. · 5.27 Impact Factor
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    ABSTRACT: The need for materials for high energy storage has led to very significant research in supercapacitor systems. These can exhibit electrical double layer phenomena and capacitances up to hundreds of F/g. Here, we demonstrate a new supercapacitor fabrication methodology based around the microphase separation of PS-b-PMMA which has been used to prepare copper nanoelectrodes of dimension -13 nm. These structures provide excellent capacitive performance with a maximum specific capacitance of -836 F/g for a current density of 8.06 A/g at a discharge current as high as 75 mA. The excellent performance is due to a high surface area: volume ratio. We suggest that this highly novel, easily fabricated structure might have a number of important applications.
    Journal of Nanoscience and Nanotechnology 07/2014; 14(7):5221-7. · 1.15 Impact Factor
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    Microelectronic Engineering 07/2014; 123:126-130. · 1.22 Impact Factor
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    ABSTRACT: The increasing use of gold nanoparticles in medical diagnosis and treatment has raised the concern over their blood compatibility. The interactions of nanoparticles with blood components may lead to platelet aggregation and endothelial dysfunction. Therefore, medical applications of gold nanoparticles call for increased nanoparticle stability and biocompatibility. Functionalisation of nanoparticles with polythelene glycol (PEGylation) is known to modulate cell-particle interactions. Therefore, the aim of the current study was to investigate the effects of PEGylated-gold nanoparticles on human platelet function and endothelial cells in vitro. Gold nanoparticles, 15 nm in diameter, were synthesised in water using sodium citrate as a reducing and stabilising agent. Functionalised polyethylene glycol-based thiol polymers were used to coat and stabilise pre-synthesised gold nanoparticles. The interaction of gold nanoparticles-citrate and PEGylated-gold nanoparticles with human platelets was measured by Quartz Crystal Microbalance with Dissipation. Platelet-nanoparticles interaction was imaged using phase-contrast, scanning and transmission electron microscopy. The inflammatory effects of gold nanoparticles-citrate and PEGylated-gold nanoparticles in endothelial cells were measured by quantitative real time polymerase chain reaction. PEGylated-gold nanoparticles were stable under physiological conditions and PEGylated-gold nanoparticles-5400 and PEGylated-gold nanoparticles-10800 did not affect platelet aggregation as measured by Quartz Crystal Microbalance with Dissipation. In addition, PEGylated-gold nanoparticles did not induce an inflammatory response when incubated with endothelial cells. Therefore, this study shows that PEGylated-gold nanoparticles with a higher molecular weight of the polymer chain are both platelet- and endothelium-compatible making them attractive candidates for biomedical applications.
    Journal of Biomedical Nanotechnology 06/2014; 10(6):1004-15. · 7.58 Impact Factor
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    ABSTRACT: Self-assembled thin films of a lamellar forming polystyrene-block-poly(d,l)lactide (PS-b-PLA) block copolymer (BCP) contain a “reactive” block that can be readily removed to provide a template for substrate pattern formation. Various methods of PLA removal were studied here with a view to develop the system as an on-chip etch mask for substrate patterning. Solvo-microwave annealing was used to induce microphase separation in PS-b-PLA BCP with a periodicity of 34 nm (Lo) on silicon and silicon on insulator (SOI) substrates. Wet etches based on alkaline and enzymatic solutions were studied in depth. Fourier transform-infrared (FT-IR) analysis showed that basic hydrolysis using sodium hydroxide (NaOH) or ammonium hydroxide (NH4OH) solutions resulted in greater PLA removal in comparison to an enzymatic approach using Proteinase K in a Tris-HCl buffer solution. However, in the enzymatic approach, the characteristic self-assembled fingerprint patterns were retained with less damage. Comparison to a dry etch procedure using a reactive ion etch (RIE) technique was made. A detailed study of the etch rate of PS and PLA homopolymer and PS-b-PLA shows depending on DC bias, the etch selectivity of PLA and PS can be almost doubled from 1.7 at DC bias 145 V to 3 at DC bias 270 V. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40798.
    Journal of Applied Polymer Science 04/2014; · 1.40 Impact Factor
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    ABSTRACT: The nanometer range structure produced by thin films of diblock copolymers makes them a great of interest as templates for the microelectronics industry. We investigated the effect of annealing solvents and/or mixture of the solvents in case of symmetric Poly (styrene-block-4vinylpyridine) (PS-b-P4VP) diblock copolymer to get the desired line patterns. In this paper, we used different molecular weights PS-b-P4VP to demonstrate the scalability of such high χ BCP system which requires precise fine-tuning of interfacial energies achieved by surface treatment and that improves the wetting property, ordering, and minimizes defect densities. Bare Silicon Substrates were also modified with polystyrene brush and ethylene glycol self-assembled monolayer in a simple quick reproducible way. Also, a novel and simple in situ hard mask technique was used to generate sub-7nm Iron oxide nanowires with a high aspect ratio on Silicon substrate, which can be used to develop silicon nanowires post pattern transfer.
    SPIE Advanced Lithography; 03/2014
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    ABSTRACT: We report a simple technique to fabricate horizontal, uniform Si nanowire arrays with controlled orientation and density at spatially well defined locations on substrate based on insitu hard mask pattern formation approach by microphase separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin films. The methodology may be applicable to large scale production. Ordered microphase separated patterns of the BCP were defined by solvent annealing and the orientation was controlled by film thickness and annealing time. Films of PEO cylinders with parallel orientation (to the surface plane) were applied to create ‘frames’ for the generation of inorganic oxide nanowire arrays. These PEO cylinders were subject to selective metal ion inclusion and subsequent processing was used to create iron oxide nanowire arrays. The oxide nanowires were isolated, of uniform diameter and their structure a mimic of the original BCP nanopatterns. The phase purity, crystallinity and thermal stability of the nanowires coupled to the ease of large scale production may make them useful in technological applications. Here, we demonstrate that the oxide nanowire arrays could be used as a resist mask to fabricate densely packed, identical ordered, good fidelity silicon nanowire arrays on the substrate. The techniques may have significant application in the manufacture of transistor circuitry.
    SPIE Advanced Lithography; 03/2014
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    ABSTRACT: The shape sensitivity of Pd catalysts in Suzuki-Miyaura coupling reactions is studied using nanocrystals enclosed by well-defined surface facets. The catalytic performance of Pd nanocrystals with cubic, cuboctahedral and octahedral morphologies are compared. Superior catalytic reactivity is observed for Pd NCs with {100} surface facets compared to {111} facets. The origin of the enhanced reactivity associated with a cubic morphology is related to the leaching susceptibility of the nanocrystals. Molecular oxygen plays a key role in facilitating the leaching of Pd atoms from the surface of the nanocrystals. The interaction of O2 with Pd is itself facet-dependent, which in turn gives rise to more efficient leaching from {100} facets, compared to {111} facets under the reaction conditions.
    Angewandte Chemie International Edition 03/2014; · 11.34 Impact Factor
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    ABSTRACT: In block copolymer (BCP) nanolithography, microphase separated polystyrene- block-polydimethylsiloxane (PS-b-PDMS) thin films are particularly attractive as they can form small features and the two blocks can be readily differentiated during pattern trans- fer. However, PS-b-PDMS is challenging because the chemical differences in the blocks can result in poor surface-wetting, poor pattern orientation control and structural insta- bilities. Usually the interfacial energies at substrate surface are engineered with the use of a hydroxyl-terminated polydimethylsiloxane (PDMS-OH) homopolymer brush. Herein, we report a facile, rapid and tuneable molecular functionalization approach using hexam- ethyldisilazane (HMDS). The work is applied to both planar and topographically patterned substrates and investigation of graphoepitaxial methods for directed self-assembly and long-range translational alignment of BCP domains is reported. The hexagonally arranged in-plane and out-of-plane PDMS cylinders structures formed by microphase separation were successfully used as on-chip etch masks for pattern transfer to the underlying silicon substrate. The molecular approach developed here affords significant advantages when compared to the more usual PDMS-OH brushes used.
    Advanced Materials Interfaces 02/2014;
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    ABSTRACT: We present the design considerations of an autonomous wireless sensor and discuss the fabrication and testing of the various components including the energy harvester, the active sensing devices and the power management and sensor interface circuits. A common materials platform, namely, nanowires, enables us to fabricate state-of-the-art components at reduced volume and show chemical sensing within the available energy budget. We demonstrate a photovoltaic mini-module made of silicon nanowire solar cells, each of 0.5 mm2 area, which delivers a power of 260 μW and an open circuit voltage of 2 V at one sun illumination. Using nanowire platforms two sensing applications are presented. Combining functionalised suspended Si nanowires with a novel microfluidic fluid delivery system, fully integrated microfluidic–sensor devices are examined as sensors for streptavidin and pH, whereas, using a microchip modified with Pd nanowires provides a power efficient and fast early hydrogen gas detection method. Finally, an ultra-low power, efficient solar energy harvesting and sensing microsystem augmented with a 6 mAh rechargeable battery allows for less than 20 μW power consumption and 425 h sensor operation even without energy harvesting.
    Microsystem Technologies 02/2014; · 0.83 Impact Factor
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    ABSTRACT: The performance of the lithium-ion cell is heavily dependent on the ability of the host electrodes to accommodate and release Li+ ions from the local structure. While the choice of electrode materials may define parameters such as cell potential and capacity, the process of intercalation may be physically limited by the rate of solid-state Li+ diffusion. Increased diffusion rates in lithium-ion electrodes may be achieved through a reduction in the diffusion path, accomplished by a scaling of the respective electrode dimensions. In addition, some electrodes may undergo large volume changes associated with charging and discharging, the strain of which, may be better accommodated through nanostructuring. Failure of the host to accommodate such volume changes may lead to pulverisation of the local structure and a rapid loss of capacity. In this review article, we seek to highlight a number of significant gains in the development of nanostructured lithium-ion battery architectures (both anode and cathode), as drivers of potential next-generation electrochemical energy storage devices.
    Nano Research 01/2014; 7(1):1-62. · 7.39 Impact Factor

Publication Stats

997 Citations
778.95 Total Impact Points

Institutions

  • 2008–2014
    • Trinity College Dublin
      • Centre for Research on Adaptive Nanostructures and Nanodevices
      Dublin, Leinster, Ireland
    • Tyndall National Institute
      • Materials Chemistry and Analysis Group
      Corcaigh, Munster, Ireland
  • 2004–2014
    • University College Cork
      • Department of Chemistry
      Corcaigh, Munster, Ireland
  • 2009
    • University of Latvia
      • Institute of Chemical Physics
      Riga, Riga, Latvia
  • 1998–2008
    • University of East Anglia
      • School of Biological Sciences
      Norwich, ENG, United Kingdom
  • 2003
    • Chalmers University of Technology
      Goeteborg, Västra Götaland, Sweden
  • 2000
    • University of Texas at Austin
      • Department of Chemical Engineering
      Texas City, TX, United States