Articles

2019

41. Atomistic modeling of the nucleation and growth of pure and hybrid nanoparticles by cluster beam deposition (invited review)
P. Grammatikopoulos
Current Opinion in Chemical Engineering 23 (2019) 164-173

One great challenge of nanotechnology concerns the design and synthesis of multi-component nanoparticles that beat the properties of their elemental counterparts. Cluster beam deposition (CBD) provides a solvent- and effluent-free method to achieve nanomaterials with tailored characteristics. However, CBD is hindered by low cluster yield and, often, by limited structural control. In gas-phase methods one cannot inhibit or enhance growth by adding or removing chemical precursors or surfactants; cluster formation happens in-flight, and the only viable strategy for superior control is by understanding the kinetics and thermodynamics of nucleation & growth. To this end, atomistic computer modelling is an invaluable tool, complementing experimental fabrication, as demonstrated by numerous case studies based on various setups of CBD sources

40. Site-specific wetting of iron nanocubes by gold atoms in gas-phase synthesis
J. Vernieres, S. Steinhauer, J. Zhao, P. Grammatikopoulos, R. Ferrando, K. Nordlund, F. Djurabekova, and M. Sowwan
Advanced Science 6 (2019) 1900447 (8pp)

A key challenge in nanotechnology is the rational design of multicomponent materials that beat the properties of their elemental counterparts. At the same time, when considering the material composition of such hybrid nanostructures and the fabrication process to obtain them, one should favor the use of nontoxic, abundant elements in view of the limited availability of critical metals and sustainability. Cluster beam deposition offers a solventand, therefore, effluent-free physical synthesis method to achieve nanomaterials with tailored characteristics. However, the simultaneous control of size, shape, and elemental distribution within a single nanoparticle in a small-size regime (sub-10 nm) is still a major challenge, equally limiting physical and chemical approaches. Here, a single-step nanoparticle fabrication method based on magnetron-sputtering inert-gas condensation is reported, which relies on selective wetting of specific surface sites on precondensed iron nanocubes by gold atoms. Using a newly developed Fe–Au interatomic potential, the growth mechanism is decomposed into a multistage model implemented in a molecular dynamics simulation framework. The importance of growth kinetics is emphasized through differences between structures obtained either experimentally or computationally, and thermodynamically favorable configurations determined via global optimization techniques. These results provide a roadmap for engineering complex nanoalloys toward targeted applications.

39. Computational modeling of nanoparticle coalescence (invited review)
P. Grammatikopoulos, M. Sowwan, and J. Kioseoglou
Advanced Theory and Simulations 2(6) (2019) 1900013 (inside front cover)

The coalescence of nanoclusters fabricated in the gas phase is a fundamental growth mechanism determining cluster shapes, sizes, compositions, and structures, with resultant effects on practically all of their physical and chemical properties. Furthermore, coalescence can affect properties of larger structures that consist of nanoparticles as their elementary building blocks, such as the fractal dimension of cluster aggregates and the porosity and conductance of thin films. Therefore, it comes as no surprise that a great body of research, both experimental and theoretical, has focused on nanoparticle coalescence over the course of the past few decades. This review attempts to summarize the most important recent results from computational studies on nanoparticle coalescence and draw parallels between theoretical and experimental findings. The approach used here aspires to explain nanoparticle coalescence within the framework of a single intuitive narrative by integrating previous results obtained using various methods by the authors and others. Simultaneously, it is discussed where understanding and controlling (i.e., enhancing or inhibiting) nanoparticle coalescence can have great technological interest.

38. Gas-phase synthesis of trimetallic nanoparticles
J.-G. Mattei, P. Grammatikopoulos, J. Zhao, V. Singh, J. Vernieres, S. Steinhauer, A. Porkovich, E. Danielson, K. Nordlund, F. Djurabekova, and M. Sowwan
Chemistry of Materials 31(6) (2019) 2151-2163

To this day, engineering nanoalloys beyond bimetallic compositions has scarcely been within the scope of physical deposition methods due to the complex, non-equilibrium processes they entail. Here, we report a gas-phase synthesis strategy for the growth of multi-metallic nanoparticles: magnetron-sputtering inert-gas condensation from neighboring mono-elemental targets provides the necessary compositional flexibility, while in-depth atomistic computer simulations elucidate the fast kinetics of nucleation and growth that determine the resultant structures. We fabricated consistently trimetallic Au-Pt-Pd nanoparticles, a system of major importance for heterogeneous catalysis applications. Using high-resolution transmission electron microscopy we established their physical and chemical ordering: Au/Pt-rich core@Pd-shell atomic arrangements were identified for particles containing substantial amounts of all elements. Decomposing the growth process into basic steps by molecular dynamics simulations we identified a fundamental difference between Au/Pt and Pd growth dynamics: Au/Pt electronic arrangements favor the formation of dimer nuclei instead of larger-size clusters, thus significantly slowing down their growth rate. Consequently, larger Pd particles formed considerably faster and incorporated small Au and Pt clusters by means of in-flight decoration and coalescence. A broad range of icosahedral, truncated-octahedral, and spheroidal FCC trimetallic nanoparticles were reproduced in simulation, in good agreement with experimental particles. Comparing them with their expected equilibrium structures obtained by Monte Carlo simulations, the particles were identified as metastable, due to out-of-equilibrium growth conditions. We aspire that our in-depth study will constitute a significant advance towards establishing gas-phase aggregation as a standard method for the fabrication of complex nanoparticles by design.

37. Nanoassemblies of ultrasmall clusters with remarkable activity in carbon dioxide conversion into C1 fuels
A. Halder, J. Kioseoglou, B. Yang, K.L. Kolipaka, S. Seifert, J. Ilavsky, M. Pellin, M. Sowwan, P. Grammatikopoulos, and S. Vajda
Nanoscale 11 (2019) 4683-4687 (inside back cover)

Cu nanoassemblies formed transiently during reaction from size-selected subnanometer Cu4 clusters supported on amorphous OH-terminated alumina convert CO2 into methanol and hydrocarbons under near-atmospheric pressure at rates considerably higher than those of individually standing Cu4 clusters. An in situ characterization reveals that the clusters self-assemble into 2D nanoassemblies at higher temperatures which then disintegrate upon cooling down to room temperature. DFT calculations postulate a formation mechanism of these nanoassemblies by hydrogen-bond bridges between the clusters and H2O molecules, which keep the building blocks together while preventing their coalescence.

2018

36. In-situ open cell TEM/STEM environmental study of iron oxides nanoparticles and sample-beam interaction in O2 gas
L. Lari, S. Steinhauer, and M. Sowwan and V.K. Lazarov
Microscopy & Microanalysis 24(S1) (2018) 260-261

In-situ environmental (scanning) transmission electron microscopy E-(S)TEM gives a new opportunity of studying oxidation and reduction of nanoparticles under controlled atmosphere and temperature at the nanoscale. This technique provides the possibility of imaging NPs and to follow their structural transformations as a function of temperature and gas pressure. In addition to structural information, electron diffraction, energy dispersive x-ray analysis as well as electron energy loss spectroscopy can provide chemical information at the same time.

2017

35. Tuning the onset of ferromagnetism in heterogeneous bimetallic nanoparticles by gas phase doping
M. Bohra, P. Grammatikopoulos, V. Singh, J. Zhao, E. Toulkeridou, S. Steinhauer, J. Kioseoglou, J.-F. Bobo, K. Nordlund, F. Djurabekova, and M. Sowwan
Physical Review Materials 1 (2017) 066001 (12pp)

In the nanoregime, chemical species can reorganize in ways not predicted by their equilibrium bulk behavior. Here, we engineer Ni-Cr nanoalloys at the magnetic end of their compositional range (i.e., 0–15 at. % Cr), and we investigate the effect of Cr incorporation on their structural stability and resultant magnetic ordering. To ensure their stoichiometric compositions, the nanoalloys are grown by cluster beam deposition, a method that allows one-step, chemical-free fabrication of bimetallic nanoparticles. While full Cr segregation toward nanoparticle surfaces is thermodynamically expected for low Cr concentrations, metastability occurs as the Cr dopant level increases in the form of residual Cr in the core region, yielding desirable magnetic properties in a compensatory manner. Using nudged elastic band calculations, residual Cr in the core is explained based on modifications in the local environment of individual Cr atoms. The resultant competition between ferromagnetic and antiferromagnetic ordering gives rise to a wide assortment of interesting phenomena, such as a cluster-glass ground state at very low temperatures and an increase in Curie temperature values. We emphasize the importance of obtaining the commonly elusive magnetic nanophase diagram for M-Cr (M=Fe, Co, and Ni) nanoalloys, and we propose an efficient single-parameter method of tuning the Curie temperature for various technological applications.

34. New approaches to the lithiation kinetics in reaction-limited battery electrodes through electrochemical impedance spectroscopy
N. Vicente, M. Haro, and G. Garcia-Belmonte
Chemical Communications 54 (2018) 1025-1040

Electrochemical impedance spectroscopy is a widely employed technique probing kinetic limitations in the charging of battery electrodes. Hindrance mechanisms locate at the interfaces between the active material and the electrolyte, and in the bulk of the reacting compound. Rate-limiting mechanisms are viewed as resistive circuit elements and can be extracted using standard impedance analyzers. Classical impedance models consider charge transport, mainly ion diffusion as slower carrier, as the principal kinetic limitation impeding full electrode charging. This is indeed the case for many technologically relevant battery compounds. In other instances, instead of being diffusion-limited, electrodes may undergo charging limitation caused by the kinetics of the reduction reaction itself. Specific impedance models for reactionlimited mechanisms are summarized here and proved for relevant electrode compounds, in particular for conversion or alloying electrodes in which Li+ intake produces a full rearrangement of the lattice structure with significant atomic displacement.

33. Hydrogen flux through size selected Pd nanoparticles into underlying Mg nanofilms
S. Kumar, T. Pavloudis, V. Singh, H. Nguyen, S. Steinhauer, C. Pursell, B. Clemens, J. Kioseoglou, P. Grammatikopoulos, and M. Sowwan
Advanced Energy Materials 8 (2018) 1701326 (14pp) (inside front cover)

The application of Mg for hydrogen storage is hindered due to the slow absorption of hydrogen in Mg films. Herein, the hydrogenation process is explored theoretically using density functional theory calculations, and energy barriers are compared for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e., Pd(H)/Mg(O). Decomposing the mechanism into basic steps, it is shown that Pd undergoes a strain-induced crystallographic phase transformation near the interface, and indicated that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, it is explained why hydrogen diffusion is practically prohibited through native Mg oxide and seriously suppressed through existing hydride domains. Hydrogen flux is experimentally investigated through the nanoportals in Pd-nanoparticle decorated Mg films by pressure-composition isotherm measurements. An r ≈ t1/3 relationship is theoretically calculated for the radial growth of hemispherical hydride domains, and this relationship is confirmed by atomic force microscopy. The diffusion constant of hydrogen in Mg films is estimated as DHfilm ≈ 8 × 10−18 m2 s−1, based on transmission electron microscopy characterization. The unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of hydrogenation in any material.

32. Nanoscale heterogeneity of multilayered Si anodes with embedded nanoparticle scaffolds for Li-ion batteries
M. Haro, V. Singh, S. Steinhauer, E. Toulkeridou, P. Grammatikopoulos, and M. Sowwan
Advanced Science 4 (2017) 1700180 (10pp) (inside front cover)

A new approach on the synthesis of Si anodes for Li-ion batteries is reported, combining advantages of both nanoparticulated and continuous Si films. A multilayered configuration prototype is proposed, comprising amorphous Si arranged in nanostructured, mechanically heterogeneous films, interspersed with Ta nanoparticle scaffolds. Particular structural features such as increased surface roughness, nanogranularity, and porosity are dictated by the nanoparticle scaffolds, boosting the lithiation process due to fast Li diffusion and low electrode polarization. Consequently, a remarkable charge/discharge speed is reached with the proposed anode, in the order of minutes (1200 mAh g−1 at 10 C). Moreover, nanomechanical heterogeneity self-limits the capacity at intermediate charge/discharge rates; as a consequence, exceptional cycleability is observed at 0.5 C, with 100% retention over 200 cycles with 700 mAh g−1. Higher capacity can be obtained when the first cycles are performed at 0.2 C, due to the formation of microislands, which facilitate the swelling of the active Si. This study indicates a method to tune the mechanical, morphological, and electrochemical properties of Si electrodes via engineering nanoparticle scaffolds, paving the way for a novel design of nanostructured Si electrodes for high-performance energy storage devices.

31. Thermal oxidation of size-selected Pd nanoparticles supported on CuO nanowires: the role of the CuO-Pd interface
S. Steinhauer, J. Zhao, V. Singh, T. Pavloudis, J. Kioseoglou, K. Nordlund, F. Djurabekova, P. Grammatikopoulos, and M. Sowwan
Chemistry of Materials 29(14) (2017) 6153-6160

The structure of heterogeneous nanocatalysts supported on metal oxide materials and their morphological changes during oxidation/reduction processes play a crucial role in determining the resulting catalytic activity. Herein, we study the thermal oxidation mechanism of Pd nanoparticles supported on CuO nanowires by combining in situ environmental transmission electron microscopy (TEM), ex situ experiments and ab initio density functional theory (DFT) calculations. High resolution TEM imaging assisted by geometric phase analysis enabled the analysis of partially oxidized, fully oxidized and distinct onion-like Pd nanoparticles with sub-surface dislocations. Furthermore, preferential crystalline orientations between PdO nanoparticles and the CuO nanowire support have been found. Hence, the CuO-Pd interface is crucial for the thermal oxidation of Pd nanoparticles, as corroborated by electron energy loss spectroscopy and DFT calculations. The latter revealed a considerably lower energy barrier for oxygen penetration into the Pd lattice at the CuO-Pd interface, promoting nanoparticle oxidation. The obtained results are compared with literature reports on different material systems and potential implications for catalysis and chemoresistive sensing applications are discussed.

30. In situ chemoresistive sensing in the environmental TEM: probing functional devices and their nanoscale morphology
S. Steinhauer, J. Vernieres, J. Krainer, A. Köck, P. Grammatikopoulos, and M. Sowwan
Nanoscale 9 (2017) 7380-7384 (inside back cover)

In situ transmission electron microscopy provides exciting opportunities to address fundamental questions and technological aspects related to functional nanomaterials, including the structure–property relationships of miniaturized electronic devices. Herein, we report the in situ chemoresistive sensing in the environmental transmission electron microscope (TEM) with a single SnO2 nanowire device, studying the impact of surface functionalization with heterogeneous nanocatalysts. By detecting toxic carbon monoxide (CO) gas at ppm-level concentrations inside the microscope column, the sensing properties of a single SnO2 nanowire were characterized before and after decoration with hybrid Fe–Pd nanocubes. The structural changes of the supported nanoparticles induced by sensor operation were revealed, enabling direct correlation with CO sensing properties. Our novel approach is applicable for a broad range of functional nanomaterials and paves the way for future studies on the relationship between chemoresistive properties and nanoscale morphology.

29. Probing electron beam effects with chemoresistive nanosensors during in situ environmental transmission electron microscopy
S. Steinhauer, Z. Wang, Z. Zhou, J. Krainer, A. Köck, K. Nordlund, F. Djurabekova, P. Grammatikopoulos, and M. Sowwan
Applied Physics Letters 110 (2017) 094103 (4pp)

We report in situ and ex situ fabrication approaches to construct p-type (CuO) and n-type (SnO2) metal oxide nanowire devices for operation inside an environmental transmission electron microscope (TEM). By taking advantage of their chemoresistive properties, the nanowire devices were employed as sensitive probes for detecting reactive species induced by the interactions of high-energy electrons with surrounding gas molecules, in particular, for the case of O2 gas pressures up to 20 mbar. In order to rationalize our experimental findings, a computational model based on the particle-in-cell method was implemented to calculate the spatial distributions of scattered electrons and ionized oxygen species in the environmental TEM. Our approach enables the a priori identification and qualitative measurement of undesirable beam effects, paving the way for future developments related to their mitigation.

28. Gas phase synthesis of multifunctional Fe-based nanocubes
J. Vernieres, S. Steinhauer, J. Zhao, A. Chapelle, P. Menini, N. Dufour, R.E. Diaz, K. Nordlund, F. Djurabekova, P. Grammatikopoulos, and M. Sowwan
Advanced Functional Materials 27 (2017) 1605328 (12pp) (frontispiece)

Magnetron-sputtering inert-gas condensation is an emerging technique offering single-step, chemical-free synthesis of nanoparticles with well-defined morphologies optimized for specific applications. In this study, the authors report a flexible approach to produce Fe nanocubes as building blocks for high-performance NO2 gas sensor devices, and hybrid FeAu nanocubes with magneto-plasmonic properties. Considering that nucleation happens within a short distance from the sputtering target, the authors utilize the high-permeability and resultant screening effect induced by magnetic Fe targets of various thicknesses to manipulate the magnetic field configuration and plasma confinement. The authors thus readily switch from bimodal to single-Gaussian size distributions of Fe nanocubes by modifying their primordial thermal environments, as explained by a combination of modeling methods. Simultaneously, the authors obtain a material yield increase of more than one order of magnitude compared to experiments using postgrowth mass filtration. The effectiveness of the method is demonstrated by the deposition of Fe nanocubes on microhotplate devices, leading to unprecedented NO2 detection performance for Fe-based chemoresistive gas sensors. The exceedingly low detection limit down to 3 ppb is attributed to a morphological change in operando from Fe/Fe-oxide core/shell to specific hollow-nanocube structures, as revealed by in situ environmental transmission electron microscopy.

2016

27. Kinetic trapping through coalescence and the formation of patterned Ag-Cu nanoparticles
P. Grammatikopoulos, J. Kioseoglou, A. Galea, J. Vernieres, M. Benelmekki, R.E. Diaz, and M. Sowwan
Nanoscale 8 (2016) 9780-9790

In recent years, due to its inherent flexibility, magnetron-sputtering has been widely used to synthesise bi-metallic nanoparticles (NPs) via subsequent inert-gas cooling and gas-phase condensation of the sputtered atomic vapour. Utilising two separate sputter targets allows for good control over composition. Simultaneously, it involves fast kinetics and non-equilibrium processes, which can trap the nascent NPs into metastable configurations. In this study, we observed such configurations in immiscible, bi-metallic Ag–Cu NPs by scanning transmission electron microscopy (S/TEM) and electron energy-loss spectroscopy (EELS), and noticed a marked difference in the shape of NPs belonging to Ag- and Cu-rich samples. We explained the formation of Janus or Ag@Cu core/shell metastable structures on the grounds of in-flight mixed NP coalescence. We utilised molecular dynamics (MD) and Monte Carlo (MC) computer simulations to demonstrate that such configurations cannot occur as a result of nanoalloy segregation. Instead, sintering at relatively low temperatures can give rise to metastable structures, which eventually can be stabilised by subsequent quenching. Furthermore, we compared the heteroepitaxial diffusivities along various surfaces of both Ag and Cu NPs, and emphasised the differences between the sintering mechanisms of Ag- and Cu-rich NP compositions: small Cu NPs deform as coherent objects on large Ag NPs, whereas small Ag NPs dissolve into large Cu NPs, with their atoms diffusing along specific directions. Taking advantage of this observation, we propose controlled NP coalescence as a method to engineer mixed NPs of a unique, patterned core@partial-shell structure, which we refer to as a “glass-float” (ukidama) structure.

26. Local CuO nanowire growth on microhotplates: in situ electrical measurements and gas sensing application
S. Steinhauer, A. Chapelle, P. Menini, and M. Sowwan
ACS Sensors 1(5) (2016) 503-507

We report on local CuO nanowire growth on microhotplates combined with in situ measurement of the electrical resistance for well-controlled integration into conductometric gas sensing devices. Discrete current steps were observed during the CuO nanowire synthesis process, which is attributed to individual nanowire connections being formed. The high gas sensitivity of the CuO nanowire devices was confirmed by detection of carbon monoxide CO in the low-ppm-level concentration range. Furthermore, we demonstrate that CuO nanowire growth inside a gas measurement setup allows studies on gas sensor poisoning/deactivation processes. A significant decrease of CO response was found after controlled exposure to humidity, which suggests sensor deactivation by surface hydroxylation. Thus, our approach could be a novel and simple way for revealing new insights in various gas sensor degradation mechanisms in the future and might also be adapted for different metal oxide nanomaterials.

25. Formation mechanism of Fe nanocubes by magnetron sputtering inert gas condensation
J. Zhao, E. Baibuz, J. Vernieres, P. Grammatikopoulos, V. Jansson, M. Nagel, S. Steinhauer, M. Sowwan, A. Kuronen, K. Nordlund, and F. Djurabekova
ACS Nano 10(4) (2016) 4684-4694

In this work, we study the formation mechanisms of iron nanoparticles (Fe NPs) grown by magnetron sputtering inert gas condensation and emphasize the decisive kinetics effects that give rise specifically to cubic morphologies. Our experimental results, as well as computer simulations carried out by two different methods, indicate that the cubic shape of Fe NPs is explained by basic differences in the kinetic growth modes of {100} and {110} surfaces rather than surface formation energetics. Both our experimental and theoretical investigations show that the final shape is defined by the combination of the condensation temperature and the rate of atomic deposition onto the growing nanocluster. We, thus, construct a comprehensive deposition rate–temperature diagram of Fe NP shapes and develop an analytical model that predicts the temporal evolution of these properties. Combining the shape diagram and the analytical model, morphological control of Fe NPs during formation is feasible; as such, our method proposes a roadmap for experimentalists to engineer NPs of desired shapes for targeted applications.

24. Hydrogenation of Mg nanofilms catalyzed by size-selected Pd nanoparticles: Observation of localized MgH2 nanodomains
S. Kumar, V. Singh, C. Cassidy, C. Pursell, C. Nivargi, B. Clemens, and M. Sowwan
Journal of Catalysis 337 (2016) 14-25

We utilized gas-phase condensation to deposit size-selected Pd nanoparticles (NPs) on Mg nanofilms and systematically studied the catalytic conversion to localized MgH2 nanodomains upon exposure to hydrogen. Atomic force microscopy (AFM), aberration corrected transmission electron microscopy (TEM), and electron energy-loss spectroscopy (EELS) experiments were applied to map localized embryonic hydride nanodomains protruding from the Mg surface as a function of hydrogenation time, NP surface coverage, applied hydrogen pressure, and NP size. The results show that Pd NPs dissociate hydrogen and create atomic hydrogen pathways for hydrogenating the Mg nanofilm. The Pd NPs also inhibit oxidation of the underlying Mg nanofilm. Interestingly, the Mg nanofilm could be fully hydrogenated with a small quantity of Pd NPs at room temperature and modest hydrogen pressures. The localized hydrogenation enables improved control over the spatial distribution of hydride nanodomains making this configuration promising for future on-board hydrogen storage applications.

23. Nanoparticle design by gas-phase synthesis (Invited Review)
P. Grammatikopoulos, S. Steinhauer, J. Vernieres, V. Singh, and M. Sowwan
Advances in Physics: X 1(1) (2016) 81-100

Gas-phase synthesis characterizes a class of bottom-up methods for producing multifunctional nanoparticles (NPs) from individual atoms or molecules. This review aims to summarize recent achievements using this approach, and compare its potential to other physical or chemical NP fabrication techniques. More specifically, emphasis is given to magnetron-sputter gas-phase condensation, since it allows for flexible growth of complex, sophisticated NPs, owing to the fast kinetics and non-equilibrium processes it entails. Nanoparticle synthesis is decomposed into four stages, i.e. aggregation, shell-coating, mass-filtration, and deposition. We present the formation of NPs of various functionalities for different applications, such as magnetic, plasmonic, catalytic and, gas-sensing, emphasizing on the primary dependence of each type on a different stage of the fabrication process, and their resultant physical and chemical properties.

22. Control of surface segregation in bimetallic NiCr nanoalloys immersed in Ag matrix​
M. Bohra,V. Singh, P. Grammatikopoulos, E. Toulkeridou, R.E. Diaz, J.-F. Bobo, and M. Sowwan
Scientific Reports  (2016) 19153 (8pp)

Cr-surface segregation is a main roadblock encumbering many magneto-biomedical applications of bimetallic M-Cr nanoalloys (where M = Fe, Co and Ni). To overcome this problem, we developed Ni95Cr5:Ag nanocomposite as a model system, consisting of non-interacting Ni95Cr5 nanoalloys (5 ± 1 nm) immersed in non-magnetic Ag matrix by controlled simultaneous co-sputtering of Ni95Cr5 and Ag. We employed Curie temperature (TC) as an indicator of phase purity check of these nanocomposites, which is estimated to be around the bulk Ni95Cr5 value of 320 K. This confirms prevention of Cr-segregation and also entails effective control of surface oxidation. Compared to Cr-segregated Ni95Cr5 nanoalloy films and nanoclusters, we did not observe any unwanted magnetic effects such as presence Cr-antiferromagnetic transition, large non-saturation, exchange bias behavior (if any) or uncompensated higher TC values. These nanocomposites films also lose their unique magnetic properties only at elevated temperatures beyond application requirements (≥800 K), either by showing Ni-type behavior or by a complete conversion into Ni/Cr-oxides in vacuum and air environment, respectively.

21. Identifying the multiplicity of crystallographically equivalent variants generated by iterative phase transformations in Ti​
P. Grammatikopoulos, and R.C. Pond
Acta Crystallographica B72 (2016) 67-74

This work describes phase transformations in Ti from a purely crystallographic perspective. Iterative heating and cooling above and below 1155 K induce phase transitions between a low-temperature h.c.p. (hexagonal close packed) ({6 \over m}mm) and a high-temperature b.c.c. (body centred cubic) (m\bar 3m) structure. The crystallography of the two phases has been found to be related by the Burgers Orientation Relationship (Burgers OR). The transitions are accompanied by changes in texture, as an ever-increasing number of crystallographically equivalent variants occur with every cycle. Identifying their multiplicity is important to relate the textures before and after the transformation, in order to predict the resultant one and refine its microstructure. The four-dimensional Frank space was utilized to describe both h.c.p. and b.c.c. structures within the same orthogonal framework, and thus allow for their easy numerical manipulation through matrix algebra. Crystallographic group decomposition showed that the common symmetry maintained in both groups was that of group 2/m; therefore, the symmetry operations that generated the variants were of groups 3m and 23 for cubic and hexagonal generations, respectively. The number of all potential variants was determined for the first three variant generations, and degeneracy was indeed detected, reducing the number of variants from 72 to 57 and from 432 to 180 for the second and third generations, respectively. Degeneracy was attributed on some special alignments of symmetry operators, as a result of the Burgers OR connecting the relative orientation of the two structures.

2015

20. Engineering high-performance Pd core-MgO porous shell nanocatalysts via heterogeneous gas-phase synthesis
V. Singh,  C. Cassidy, F.A. Pedersen, J.-H. Kim, K. Aranishi, S. Kumar, C. Lal, C. Gspan, W. Grogger, and M. Sowwan
Nanoscale 7 (2015) 13387-13392

We report on the design and synthesis of high performance catalytic nanoparticles with a robust geometry via magnetron-sputter inert-gas condensation. Sputtering of Pd and Mg from two independent neighbouring targets enabled heterogeneous condensation and growth of nanoparticles with controlled Pd core–MgO porous shell structure. The thickness of the shell and the number of cores within each nanoparticle could be tailored by adjusting the respective sputtering powers. The nanoparticles were directly deposited on glassy carbon electrodes, and their catalytic activity towards methanol oxidation was examined by cyclic voltammetry. The measurements indicated that the catalytic activity was superior to conventional bare Pd nanoparticles. As confirmed by electron microscopy imaging and supported by density-functional theory (DFT) calculations, we attribute the improved catalytic performance primarily to inhibition of Pd core sintering during the catalytic process by the metal-oxide shell.

19. Surface segregation in Cr-doped NiCr alloy nanoparticles and its effect on their magnetic behavior
M. Bohra, P. Grammatikopoulos, R.E. Diaz, V. Singh, J. Zhao, J.-F. Bobo, A. Kuronen, F. Djurabekova, K. Nordlund, and M. Sowwan
Chemistry of Materials 27 (2015) 3216-3225

Surface segregation designates the phenomenon of variation in chemical composition between the surface and the bulk of an alloy, which can have a beneficial or detrimental effect on its physical and chemical properties. This is even more pronounced in nanoalloys, i.e., alloy systems comprised of nanoparticles, with significant surface-to-volume ratios. In this case study we demonstrate the element-specific Cr segregation in Ni-rich NiCr alloy nanoparticles and nanogranular films grown by gas-phase synthesis methods. In situ annealing measurements (300–800 K), performed under vacuum using aberration-corrected environmental transmission electron microscopy (E-TEM), and vibrating sample magnetometry (VSM) revealed progressive Cr segregation with annealing temperature and subsequent complete transformation into core–satellite structures at 700 K. Simultaneously, atomistic computer simulations (molecular dynamics (MD) and Metropolis Monte Carlo (MMC)) elucidated the resultant structures, explaining the driving force behind segregation energetically. Most importantly, we emphasize the significant effects of Cr segregation on magnetic properties, namely, (i) the highly nonsaturated MH loops (below the Néel temperature of antiferromagnetic Cr) with reduced coercivities and (ii) the uncompensated high Curie temperatures, TC, compared to the NiCr bulk, which approach bulk Ni values upon annealing. Both are clear evidence that the distribution of Cr in the nearest-neighbor shells of Ni atoms differs from that of the bulk NiCr alloy, reconfirming our structural findings.

18. Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere
S. Steinhauer, V. Singh, C. Cassidy, C. Gspan, W. Grogger, M. Sowwan, and A. Köck
Nanotechnology 26 (2015) 175502 (6pp)

We report on conductometric gas sensors based on single CuO nanowires and compare the carbon monoxide (CO) sensing properties of pristine as well as Pd nanoparticle decorated devices in humid atmosphere. Magnetron sputter inert gas aggregation combined with a quadrupole mass filter for cluster size selection was used for single-step Pd nanoparticle deposition in the soft landing regime. Uniformly dispersed, crystalline Pd nanoparticles with size-selected diameters around 5 nm were deposited on single CuO nanowire devices in a four point configuration. During gas sensing experiments in humid synthetic air, significantly enhanced CO response for CuO nanowires decorated with Pd nanoparticles was observed, which validates that magnetron sputter gas aggregation is very well suited for the realization of nanoparticle-functionalized sensors with improved performance.

17. A magnetic anti-cancer compound for magnet-guided delivery and magnetic resonance imaging
H. Eguchi, M. Umemura, R. Kurotani, H. Fukumura, I. Sato, J.-H. Kim, Y. Hoshino, J. Lee, N. Amemiya, M. Sato, K. Hirata, D. Singh, T. Masuda, M. Yamamoto, T. Urano, K. Yoshida, K. Tanigaki, M. Yamamoto, M. Sato, S. Inoue, I. Aoki, and Y. Ishikawa
Scientific Reports 5 (2015) 9194 (14pp)

Research on controlled drug delivery for cancer chemotherapy has focused mainly on ways to deliver existing anti-cancer drug compounds to specified targets, e.g., by conjugating them with magnetic particles or encapsulating them in micelles. Here, we show that an iron-salen, i.e., μ-oxo N,N'- bis(salicylidene)ethylenediamine iron (Fe(Salen)), but not other metal salen derivatives, intrinsically exhibits both magnetic character and anti-cancer activity. X-Ray crystallographic analysis and first principles calculations based on the measured structure support this. It promoted apoptosis of various cancer cell lines, likely, via production of reactive oxygen species. In mouse leg tumor and tail melanoma models, Fe(Salen) delivery with magnet caused a robust decrease in tumor size, and the accumulation of Fe(Salen) was visualized by magnetic resonance imaging. Fe(Salen) is an anti-cancer compound with magnetic property, which is suitable for drug delivery and imaging. We believe such magnetic anti-cancer drugs have the potential to greatly advance cancer chemotherapy for new theranostics and drug-delivery strategies.

16. Crystallization of silicon nanoclusters with inert gas temperature control   
J. Zhao, C. Cassidy,  P. Grammatikopoulos, V. Singh, K. Aranishi, M. Sowwan,  K. Nordlund, and F. Djurabekova  
Physical Review B 91 (2015) 035419 (12pp)

We analyze the fundamental process of crystallization of silicon nanoclusters by means of molecular dynamics simulations, complemented by magnetron-sputter inert gas condensation, which was used to synthesize polycrystalline silicon nanoclusters with good size control. We utilize two well-established Si interatomic potentials: the Stillinger-Weber and the Tersoff III. Both the simulations and experiments show that upon cooling down by an Ar gas thermal bath, initially liquid, free-standing Si nanocluster can grow multiple crystal nuclei, which drive their transition into polycrystalline solid nanoclusters. The simulations allow detailed analysis of the mechanism, and show that the crystallization temperature is size-dependent and that the probability of crystalline phase nucleation depends on the highest temperature the cluster reaches during the initial condensation and the cooling rate after it.

2014

15. Endotaxially stabilized B2-FeSi nanodots in Si(100) via ion beam co-sputtering
C. Cassidy, J. Kioseoglou, V. Singh, P. Grammatikopoulos, and M. Sowwan
Applied Physics Letters 104 (2014) 161903 (5pp)

We report on the formation of embedded B2-FeSi nanodots in [100]-oriented Si substrates, and investigate the crystallographic mechanism underlying the stabilization of this uncommon, bulk-unstable, phase. The nanodots were approximately 10 nm in size, and were formed by iron thin film deposition and subsequent annealing. Cross-sectional transmission electron microscopy, energy loss spectroscopy mapping, and quantitative image simulation and analysis were utilized to identify the phase, strain, and orientational relationship of the nanodots to the host silicon lattice. X-ray photoelectron spectroscopy was utilized to analyze the surface composition and local bonding. Elasticity calculations yielded a nanodot residual strain value of −18%. Geometrical phase analysis graphically pinpointed the positions of misfit dislocations, and clearly showed the presence of pinned (1-1-1)Si || (100)FeSi, and unpinned (-242)Si || (010)FeSi interfaces. This partial endotaxy in the host silicon lattice was the mechanism that stabilized the B2-FeSi phase.

14. Assembly of tantalum porous films with graded-oxidation profile with size-selected nanoparticles
V. Singh, P. Grammatikopoulos, C. Cassidy, M. Benelmekki, M. Bohra, Z. Hawash, K. Baughman, and M. Sowwan
Journal of Nanoparticle Research 16 (2014) 2373 (10pp)

Functionally graded materials offer a way to improve the physical and chemical properties of thin films and coatings for different applications in the nanotechnology and biomedical fields. In this work, design and assembly of nanoporous tantalum films with a graded oxidation profile perpendicular to the substrate surface are reported. These nanoporous films are composed of size-selected, amorphous tantalum nanoparticles, deposited using a gas-aggregated magnetron sputtering system, and oxidized after coalescence, as samples evolve from mono- to multi-layered structures. Molecular dynamics computer simulations shed light on atomistic mechanisms of nanoparticle coalescence, which govern the films porosity. Aberration-corrected (S) TEM, GIXRD, AFM, SEM, and XPS were employed to study the morphology, phase and oxidation profiles of the tantalum nanoparticles, and the resultant films.

13. A facile single-step synthesis of ternary multicore magneto-plasmonic nanoparticles
M. Benelmekki, M. Bohra, J.-H. Kim, R.E. Diaz, J. Vernieres, P. Grammatikopoulos, and M. Sowwan
Nanoscale 6 (2014) 3532-3535

We report a facile single-step synthesis of ternary hybrid nanoparticles (NPs) composed of multiple dumbbell-like iron–silver (FeAg) cores encapsulated by a silicon (Si) shell using a versatile co-sputter gas-condensation technique. In comparison to previously reported binary magneto-plasmonic NPs, the advantage conferred by a Si shell is to bind the multiple magneto-plasmonic (FeAg) cores together and prevent them from aggregation at the same time. Further, we demonstrate that the size of the NPs and number of cores in each NP can be modulated over a wide range by tuning the experimental parameters.

12. Heterogeneous gas-phase synthesis and molecular dynamics modeling of Janus and core-satellite Si-Ag nanoparticles
V. Singh, C. Cassidy, P. Grammatikopoulos, F. Djurabekova, K. Nordlund, and M. Sowwan
Journal of Physical Chemistry C 118 (2014)
13869-13875

Heterogeneous gas-phase condensation is a promising method of producing hybrid multifunctional nanoparticles with tailored composition and microstructure but also intrinsically introduces greater complexity to the nucleation process and growth kinetics. Herein, we report on the synthesis and growth modeling of silicon–silver (Si–Ag) hybrid nanoparticles using gas-aggregated cosputtering from elemental Si and Ag source targets. The final Si–Ag ensemble size was manipulated in the range 5–15 nm by appropriate tuning of the deposition parameters, while variations in the Si–Ag sputtering power ratio, from 1.8 to 2.25, allowed distinctive Janus and core–satellite structures, respectively, to be produced. Molecular dynamics simulations indicate that the individual species first form independent clusters of Si and Ag without significant intermixing. Collisions between unlike species are unstable in the early stages of growth (<100 ns), with large temperature differences resulting in rapid energy exchange and separation. Upon further cooling and depletion of isolated Si and Ag atoms through collection by parent clusters (>100 ns), Si–Ag cluster collisions ultimately result in stable hybrid structures.

11. Coalescence-induced crystallisation wave in Pd nanoparticles
P. Grammatikopoulos, C. Cassidy, V. Singh, and M. Sowwan
Scientific Reports 4 (2014) 5779 (9pp)

Palladium nanoparticles offer an attractive alternative to bulk palladium for catalysis, hydrogen storage and gas sensing applications. Their performance depends strongly on surface structure; therefore, nanoparticle coalescence can play an important role, as it determines the resultant structure of the active sites where reactions (e.g. catalysis) actually take place, i.e. facets, edges, vertices or protrusions. With this in mind, we performed classical molecular dynamics (MD) simulations and magnetron-sputtering inert gas condensation depositions of palladium nanoparticles, supported by high-resolution transmission electron microscopy (HRTEM), to study the mechanisms that govern their coalescence. Surface energy minimisation drove the interactions initially, leading to the formation of an interface/neck, as expected. Intriguingly, at a later stage, atomic rearrangements triggered a crystallisation wave propagating through the amorphous nanoparticles, leading to mono- or polycrystalline fcc structures. In the case of crystalline nanoparticles, almost-epitaxial alignment occurred and the formation of twins and surface protrusions were observed.

10. In-situ scanning transmission electron microscopy annealing studies of Ni(1-x)Cr(x) nanocluster and correlation with magnetic properties
R.E. Diaz, M. Bohra, V. Singh, and M. Sowwan
Microscopy & Microanalysis 20 (2014) 1664-1665

Metallic nanoparticles show unique physical and chemical properties compared to their bulk counterparts due to their high surface area to volume ratio. In this work, we report the correlation between structural and magnetic properties of Ni(1-x)Cr(x) nanoclusters that were prepared by modified magnetron sputtering system.

9. On the formation of ternary metallic-dielectric multicore-shell  nanoparticles
M. Benelmekki, J. Vernieres, J.-H. Kim, R.E. Diaz, P. Grammatikopoulos, and M. Sowwan
Materials Chemistry & Physics 151 (2015) 275-281

Magneto-plasmonic hybrid nanoparticles (HNPs) are promising for a large number for dual magneto-optical bioapplications. Gas-phase techniques offer a good alternative to chemical routes for the generation of tailored HNPs. Here, we present a novel method to synthesize ternary HNPs composed of multiple dumbbell-like FeAg cores encapsulated by an amorphous Si shell. The method involves a simultaneous sputtering of Fe, Ag and Si targets under controlled conditions. We demonstrate that the morphology and the size of the HNPs can be modulated by tuning experimental parameters such as the energy and the cooling rate, or the collision and coalescence processes experienced by the HNPs during their formation. We find that by increasing the residence time of the HNPs in the aggregation zone, we increase both the size of the HNPs, and the thickness of the Si shell. HNPs exhibit ferromagnetic behavior and show an enhanced, red-shifted, light absorption band due to the strong near-field coupling between the Ag cores and the Si shell. A mechanism of formation of these HNPs is suggested, combining the physico-chemical properties of the materials (Fe, Ag, Si) with the experimental conditions.

8. Simple analytical model of nanocluster coalescence for porous thin film design
P. Grammatikopoulos, E. Toulkeridou, K. Nordlund, and M. Sowwan
Modelling & Simulation in Materials Science & Engineering  23 (2014) 015008 (15pp)

We propose an approach to coalescence studies that encompasses the random nature of nanoparticle deposition, which results in a statistical cancellation of individual sintering mechanisms. We present a rigorous, yet simple and intuitive, analytical method that describes the average coalescence behaviour of nanoparticles, regardless of constituent element or crystallinity, emphasizing only the predominant coalescence dependencies on temperature and size-dependent nanoparticle melting points. We assessed our model using molecular dynamics (MD) computer simulations of dissimilar systems, and found remarkable agreement between its predictions and the MD results. Its simplicity makes our model a suitable starting point for the development of a meso-scale simulation technique that can describe the growth of porous films and allow for their bespoke design.

7. Influence of packaging on the surface oxidation and magnetic properties of cobalt nanocrystals
M. Bohra, V. Singh, M. Sowwan, J.-F. Bobo, C.-J. Chung, and B. Clemens
Journal of Physics D: Applied Physics 47 (2014) 305002 (6pp)

One frequently encountered obstacle during both ambient fabrication and the use of metal nanoclusters is spontaneous oxidation, which hampers many technological applications. In this work, we studied the influence of packaging material on the surface oxidation and magnetic properties of Co nanocrystals. We demonstrate that 'epoxy-electronic-varnish' capping (60–80 nm) effectively preserves the magnetic properties of pristine Co nanocrystals (~14 nm); without showing any exchange bias coupling over a period of >720 h, slower temperature variation of coercivity reminiscent of a Co single crystal and intercluster interaction dominated high blocking temperatures >300 K. Packaging by a silver (Ag) capping layer of similar thickness facilitates pronounced exchange bias compared to the even air exposed Co nanocrystals. Numerical fits M(H) = M()[1 − (H*/H)1/2] to the high field part (10–50 kOe) of MH loops yield a sharp rise in H* values at cryogenic temperatures (5–25 K) only in Ag capped Co nanocrystals, indicating a large induced anisotropy due to altered Co/Ag interfacial spin structures.

6. Single-step gas phase synthesis of iron aluminide nanoparticles with soft magnetic properties
J. Vernieres, M. Benelmekki, J.-H. Kim, P. Grammatikopoulos, J.-F. Bobo, R.E. Diaz, and M. Sowwan
APL Materials (2014) 116105 (6pp)

Soft magnetic alloys at the nanoscale level have long generated a vivid interest as candidate materials for technological and biomedical purposes. Consequently, controlling the structure of bimetallic nanoparticles in order to optimize their magnetic properties, such as high magnetization and low coercivity, can significantly boost their potential for related applications. However, traditional synthesis methods stumble upon the long standing challenge of developing true nanoalloys with effective control over morphology and stability against oxidation. Herein, we report on a single-step approach to the gas phase synthesis of soft magnetic bimetallic iron aluminide nanoparticles, using a versatile co-sputter inert gas condensation technique. This method allowed for precise morphological control of the particles; they consisted of an alloy iron aluminide crystalline core (DO3 phase) and an alumina shell, which reduced inter-particle interactions and also prevented further oxidation and segregation of the bimetallic core. Remarkably, the as-deposited alloy nanoparticles show interesting soft magnetic properties, in that they combine a high saturation magnetization (170 emu/g) and low coercivity (less than 20 Oe) at room temperature. Additional functionality is tenable by modifying the surface of the particles with a polymer, to ensure their good colloidal dispersion in aqueous environments.

5. Smart composite nanosheets with adaptive optical properties
J.-H. Kim, M. Bohra, V. Singh, C. Cassidy, and M. Sowwan
ACS Applied Materials and Interfaces (2014) 13339-13343

We report efficient design and facile synthesis of size-tunable organic/inorganic nanosheets, via a straightforward liquid exfoliation-adsorption process, of a near percolating gold (Au) thin film deposited onto a branched polyethylenimine (bPEI) matrix. The nanosheets are stiff enough to sustain their two-dimensional (2D) nature in acidic conditions, yet flexible enough to undergo a perfect reversible shape transformation to 1D nanoscrolls in alkaline conditions. The shape transformations, and associated optical property changes, at different protonation states are monitored by transmission electron microscopy (TEM), atomic force microscopy (AFM), UV–visible spectroscopy and zeta potential measurements. Because of their large surface area, both nanosheets and nanoscrolls could be used as capturing substrates for surface-enhanced Raman scattering (SERS) applications.

2013

4. Coalescence behaviour of amorphous and crystalline tantalum nanoparticles: a molecular dynamics study
P. Grammatikopoulos, C. Cassidy, V. Singh, M. Benelmekki, and M. Sowwan
Journal of Materials Science 49(11) (2014) 3890-3897

Porous films of tantalum (Ta) and its oxides exhibit numerous properties suitable for high surface area applications, mainly in the semiconductor and bio-implant industries. Such films can be developed by Ta nanoparticle deposition using DC magnetron sputtering with gas aggregation. In order to engineer films of desirable properties, accurate control and in-depth understanding of the processes and parameters of nanoparticle growth, deposition and coalescence are crucial. Of utmost importance is to control the film’s porosity, since it determines many of the other physical properties. To this end, we performed a number of classical Molecular Dynamics simulations to study the coalescence of two or more Ta nanoparticles. Temperature, relative size and crystallographic orientation, defect content, degree of crystallinity and deposition rate effects were taken into account, and a mapping of the sintering processes was acquired. A broad range of possible interaction mechanisms were observed, from simple nanoparticle reorientation in order to achieve epitaxial configuration, to atomic adsorption, neck formation, twinning within the nanoparticles and full consolidation into a single, larger nanoparticle. The parameters studied are directly linked to experimental deposition parameters; therefore, fitting them accordingly can lead to growth of films with bespoke properties.

3. Inoculation of silicon nanoparticles with silver atoms
C. Cassidy, V. Singh, P. Grammatikopoulos, F. Djurabekova, K. Nordlund, and M. Sowwan
Scientific Reports 3 (2013) 3083 (7pp)

Silicon (Si) nanoparticles were coated inflight with silver (Ag) atoms using a novel method to prepare multicomponent heterostructured metal-semiconductor nanoparticles. Molecular dynamics (MD) computer simulations were employed, supported by high-resolution bright field (BF) transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM) with a resolution ≤0.1 nm in high angle annular dark field (HAADF) mode. These studies revealed that the alloying behavior and phase dynamics during the coating process are more complex than when attaching hetero-atoms to preformed nanoparticles. According to the MD simulations, Ag atoms condense, nucleate and diffuse into the liquid Si nanoparticles in a process that we term “inoculation”, and a phase transition begins. Subsequent solidification involves an intermediate alloying stage that enabled us to control the microstructure and crystallinity of the solidified hybrid heterostructured nanoparticles.

2. Surface morphology of films grown by size-selected Ta nanoparticles
V. Singh, C. Cassidy, M. Bohra, A. Galea, Z. Hawash, and M. Sowwan
Advanced Materials Research 647  (2013) 732-737

Tantalum nanoparticle (NP) films have been deposited on silicon substrates, using sputter deposition with gas aggregation. The resultant NP films have been characterized using high resolution atomic force microscopy and X-ray fluorescence spectroscopy. The films remain stable and the NPs maintain a spherical structure on annealing up to 600 °C. In addition to characterization, these NP films have been locally patterned by atomic force microscope scanning of the surface in contact mode.

1. Size-controlled deposition of Ag and Si nanoparticle structures with gas-aggregated sputtering
C. Cassidy, V. Singh, Z. Hawash, M. Bohra, J.-H. Kim, and M. Sowwan
MRS Proceedings 106  (2013)  1546 (7pp)

Physical vapor deposition, in combination with gas-aggregation (PVD-GA), is a controllable method for creation of diverse nanoparticle structures. Given the size effects that dominate the physics of nanoparticles, a particular advantage of the PVD-GA technique is the compatibility with in situ mass filtering of the nanocluster beam. In the current work, PVD-GA has been utilized to deposit Ag and Si nanoparticles. Nanoparticles were analyzed using in situ quadrupole mass spectrometry (charge/mass ratio), atomic force microscopy (nanoparticle height), and transmission electron microscopy (nanocluster diameter & crystallinity). The results for particle size distribution were cross-correlated, with excellent agreement. Different growth methods & conditions were explored, resulting in controlled differences in the measured particle size distributions and surface coverage. A novel growth configuration utilizing a conventional sputter source in combination with a linear magnetron allowed a significant (fivefold) increase in Ag cluster yield.