Justin D. Holmes

Tyndall National Institute, Corcaigh, Munster, Ireland

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Publications (194)711.92 Total impact

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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; · 5.01 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 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 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
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    ABSTRACT: This article reports the fabrication of sub-50 nm field effect transistor (FET)-type silicon (Si) nanowire (Si NW) chemical and biological sensing devices with a junctionless architecture, as well as on the initial characterisation of their electrical and sensing performance. The devices were fabricated using a fully complementary metal-oxide-semiconductor (CMOS)-compatible top-down process on silicon-on-insulator (SOI) wafers. The fabrication process was mainly based on high-resolution electron beam lithography (EBL) and reactive ion etching (RIE) but also included photolithography (mix-and-match lithography), thin film deposition by electron beam evaporation, lift-off, thermal annealing and wet etching. The sensing performance of a matrix of nanowire devices, i.e. containing 1, 3 and 20 NWs with lengths of 0.5, 1 and 10 μm was examined. Each element of the matrix also contained five devices with different NW widths: 10, 20, 30, and 50 nm and 5 μm (a Si belt reference device). Electrical characterisation of the devices showed excellent performance as backgated junctionless nanowire transistors (JNTs): high on-currents in the range of 1–10 μA and high ratios between the on-state and off-state currents (Ion/Ioff) of 6–7 orders of magnitude. In addition, the results of ionic strength sensing experiments demonstrate the very good sensing capabilities of these devices. To the best of our knowledge, these nanowire sensors are among the smallest top-down fabricated Si NW devices reported to date.
    Microelectronic Engineering 01/2014; · 1.22 Impact Factor
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    ABSTRACT: Using an alkoxide based precursor, a strategy for producing highly uniform thin films and multilayers of V2O5 is demonstrated using dip-coating. Defect free and smooth films of V2O5 on different surfaces can be deposited from liquid precursors. We show how pinholes are formed due to heterogeneous nucleation during hydrolysis as the precursor forms a nanofluid. Using knowledge of instability formation often found in composite nanofluid films, and the influence of cluster formation on the stability of these films, we show how polymer-precursor mixtures provide optimum uniformity and very low surface roughness in amorphous V2O5 and also orthorhombic V2O5 after crystallization by heating. Pinhole and roughness instability formation during the liquid stage of the nanofluid on gold and ITO substrates is suppressed giving a uniform coating. Practically, understanding evolution pathways that involve dewetting processes, nucleation, decomposition or hydrolysis in complex nanofluids provides a route for improved uniformity of thin films. The method could be extended to improve the consistency in sequential or iterative multilayer deposits of a range of liquid precursors for functional materials and coatings.
    ACS Applied Materials & Interfaces 01/2014; · 5.01 Impact Factor
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    ABSTRACT: We report the size-controlled growth of Ge nanowires from Au-Ag-Ge ternary alloy catalysts. Significantly, Au-Ag-Ge layered thin films enabled, for the first time, the synthesis of high aspect ratio Ge nanowires by simultaneously manipulating both the solute concentration (C) and equilibrium concentration (Ceq.) of Ge in the catalysts, thereby increasing the Ge supersaturation during vapour-liquid-solid (VLS) growth. Simultaneous manipulation of C and Ceq. to enhance nanowire growth rates was also achieved using colloidal Au0.75-Ag0.25 nanoparticles deposited on a Ge film. These nanoparticles produced Ge nanowires with more uniform diameter distributions than those obtained from the thin films. The manifestation of the Gibbs-Thomson effect, resulting in a diameter dependent growth rate, was observed for all nanowires grown from Au0.75-Ag0.25 nanoparticles. In situ TEM heating experiments performed on the as-grown Ge nanowires enabled direct determination of the Ge equilibrium concentrations in the Au-Ag-Ge ternary alloys.
    Journal of Materials Chemistry C. 01/2014; 2(23):4597-4605.
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    ABSTRACT: Assertions that a new material may offer particularly advantageous properties should always be subjected to careful critical evaluation, especially when those properties can be affected by the presence of inclusions at trace level. This is particularly important for claims relating to new multiferroic compounds, which can easily be confounded by unobserved second phase magnetic inclusions. We demonstrate an original methodology for the detection, localization and quantification of second phase inclusions in thin Aurivillius type films. Additionally, we develop a dedicated statistical model and demonstrate its application to the analysis of Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) thin films, that makes it possible to put a high, defined confidence level (e.g. 99.5%) to the statement of 'new single phase multiferroic materials'. While our methodology has been specifically developed for magnetic inclusions, it can easily be adapted to any other material system that can be affected by low level inclusions.
    Scientific reports. 01/2014; 4:5712.
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    ABSTRACT: One-dimensional semiconductor nanostructures have been studied in great depth over the past number of decades as potential building blocks in electronic, thermoelectric, optoelectronic, photovoltaic and battery devices. Silicon has been the material of choice in several industries, in particular the semiconductor industry, for the last few decades due to its stable oxide and well documented properties. Recently however, Ge has been proposed as a candidate to replace Si in microelectronic devices due to its high charge carrier mobilities. A number of various ‘bottom-up’ synthetic methodologies have been employed to grow Ge nanowires, including chemical vapour deposition, thermal evaporation, template methods, supercritical fluid synthesis, molecular beam epitaxy and solution phase synthesis. These bottom-up methods afford the opportunity to produce commercial scale quantities of nanowires with controllable lengths, diameters and crystal structure. An understanding of the vapour–liquid–solid (VLS) and vapour–solid–solid (VSS) mechanism by which most Ge nanowires are produced, is key to controlling their growth rate, aspect ratio and morphology. This article highlights the various bottom-up growth methods that have been used to synthesise Ge nanowires over the past 5–6 years, with particular emphasis on the Au/Ge eutectic system and the VLS mechanism. Thermodynamic and kinetic models used to describe Ge nanowire growth and morphology control will also be discussed in detail.
    J. Mater. Chem. C. 11/2013; 2(1).
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    ABSTRACT: A simple technique is demonstrated to fabricate horizontal, uniform, hexagonally arranged Si nanowire arrays with controlled orientation and density at spatially well defined locations on a substrate based on in situ hard mask pattern formation approach by microphase separated block copolymer thin films. The techniques may have significant application in the manufacture of transistor circuitry.
    Advanced Materials 11/2013; · 14.83 Impact Factor
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    ABSTRACT: Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, …, 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce(4+) ions by Eu(3+) ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1-1 atom% due to the reduction of Ce(4+) to Ce(3+) and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7-8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S-W plot showed a kink-like feature at the particle sizes where quantum confinement effects began.
    Nanoscale 11/2013; · 6.23 Impact Factor
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    ABSTRACT: We present the facile synthesis of crystalline V2O5 nanorods and V2O5/TiO2 nanocomposites structures by a carbon nanocage (CNC)-assisted growth process, using vanadium triisopropoxide oxide and titanium isopropoxide precursors in air at 500 °C. The diameters of the resultant V2O5 nanorods ranged between 10 and 70 nm, while the crystalline V2O5/TiO2 nanocomposite structures adopted a unique morphology, due to both crystallisation and templating processes, with V2O5 adopting small-diameter nanowire and nanorod morphologies surrounded by sub-30 nm TiO2 nanoparticles. The V2O5 nanorods and V2O5/TiO2 nanocomposites were characterised by electron microscopy and X-ray diffraction techniques and subsequently reviewed as positive Li-ion electrodes. The phase-pure V2O5 nanorod structures exhibited appreciable Li+ storage properties over the potential range of 2.0–4.0 V vs. Li/Li+, displaying capacities of up to 288 mA h g−1 with appreciable cyclic behaviour at test rates of up to 1 C. The crystalline V2O5/TiO2 nanocomposite structures displayed similar Li+ storage properties, however, increasing molar fractions of TiO2 led to a decline in the overall capacity versus the single-phase V2O5 counterparts. Interestingly, the Li+ insertion behaviour of the V2O5/TiO2 nanocomposite displayed character more-typical of amorphous V2O5, which was ascribed to a structural buffering effect of the inactive TiO2 phase.
    J. Mater. Chem. A. 09/2013; 1(40).
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    ABSTRACT: A unique two-source controlled nanoelectromechanical switch has been assembled from individual, single-clamped Ge nanowires. The switching behaviour was achieved by superimposing the control signals of specific frequencies to the electrostatic potential of the output terminals, eliminating the need for an additional gate electrode. Using an in situ manipulation technique inside a scanning electron microscope, we demonstrate that the pull-out force required to overcome adhesion at the contact can be significantly reduced by exciting mechanical resonant modes within the nanowire.
    Journal of Materials Chemistry 09/2013; · 5.97 Impact Factor

Publication Stats

920 Citations
711.92 Total Impact Points

Institutions

  • 2008–2014
    • Tyndall National Institute
      • Materials Chemistry and Analysis Group
      Corcaigh, Munster, Ireland
  • 2004–2014
    • University College Cork
      • Department of Chemistry
      Corcaigh, Munster, Ireland
  • 2008–2013
    • Trinity College Dublin
      • Centre for Research on Adaptive Nanostructures and Nanodevices
      Dublin, L, 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