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“TEM sample preparation by FIB for carbon nanotube interconnects”. Ke X, Bals S, Romo Negreira A, Hantschel T, Bender H, Van Tendeloo G, Ultramicroscopy 109, 1353 (2009). http://doi.org/10.1016/j.ultramic.2009.06.011
Abstract: A powerful method to study carbon nanotubes (CNTs) grown in patterned substrates for potential interconnects applications is transmission electron microscopy (TEM). However, high-quality TEM samples are necessary for such a study. Here, TEM specimen preparation by focused ion beam (FIB) has been used to obtain lamellae of patterned samples containing CNTs grown inside contact holes. A dual-cap Pt protection layer and an extensive 5 kV cleaning procedure are applied in order to preserve the CNTs and avoid deterioration during milling. TEM results show that the inner shell structure of the carbon nanotubes has been preserved, which proves that focused ion beam is a useful technique to prepare TEM samples of CNT interconnects.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.843
Times cited: 21
DOI: 10.1016/j.ultramic.2009.06.011
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“Atomic oxygen functionalization of vertically aligned carbon nanotubes”. Bittencourt C, Navio C, Nicolay A, Ruelle B, Godfroid T, Snyders R, Colomer J-F, Lagos MJ, Ke X, Van Tendeloo G, Suarez-Martinez I, Ewels CP, The journal of physical chemistry: C : nanomaterials and interfaces 115, 20412 (2011). http://doi.org/10.1021/jp2057699
Abstract: Vertically aligned multiwalled carbon nanotubes (v-MWCNTs) are functionalized using atomic oxygen generated in a microwave plasma. X-ray photoelectron spectroscopy depth profile analysis shows that the plasma treatment effectively grafts oxygen exclusively at the v-MWCNT tips. Electron microscopy shows that neither the vertical alignment nor the structure of v-MWCNTs were affected by the plasma treatment. Density functional calculations suggest assignment of XPS C 1s peaks at 286.6 and 287.5 eV, to epoxy and carbonyl functional groups, respectively.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 4.536
Times cited: 31
DOI: 10.1021/jp2057699
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“New insights into the early stages of nanoparticle electrodeposition”. Ustarroz J, Ke X, Hubin A, Bals S, Terryn H, The journal of physical chemistry: C : nanomaterials and interfaces 116, 2322 (2012). http://doi.org/10.1021/jp210276z
Abstract: Electrodeposition is an increasingly important method to synthesize supported nanoparticles, yet the early stages of electrochemical nanoparticle formation are not perfectly understood. In this paper, the early stages of silver nanoparticle electrodeposition on carbon substrates have been studied by aberration-corrected TEM, using carbon-coated TEM grids as electrochemical electrodes. In this manner we have access to as-deposited nanoparticle size distribution and structural characterization at the atomic scale combined with electrochemical measurements, which represents a breakthrough in a full understanding of the nanoparticle electrodeposition mechanisms. Whereas classical models, based upon characterization at the nanoscale, assume that electrochemical growth is only driven by direct attachment, the results reported hereafter indicate that early nanoparticle growth is mostly driven by nanocluster surface movement and aggregation. Hence, we conclude that electrochemical nulceation and growth models should be revised and that an electrochemical aggregative growth mechanism should be considered in the early stages of nanoparticle electrodeposition.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 4.536
Times cited: 104
DOI: 10.1021/jp210276z
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“Ultrasonication induces oxygenated species and defects onto exfoliated graphene”. Skaltsas T, Ke X, Bittencourt C, Tagmatarchis N, The journal of physical chemistry: C : nanomaterials and interfaces 117, 23272 (2013). http://doi.org/10.1021/jp4057048
Abstract: The effect of ultrasonication parameters, such as time and power applied, to exfoliate graphite in o-dichlorobenzene (o-DCB) and N-methyl-1,2-pyrrolidone (NMP) was examined. It was found that the concentration of graphene was higher in o-DCB, while its dispersibility was increased when sonication was applied for a longer period and/or at higher power. However, spectroscopic examination by X-ray photoelectron spectroscopy (XPS) revealed that ultrasonication causes defects and induces oxygen functional groups in the form of carboxylic acids and ethers/epoxides onto the graphene lattice. Additional proof for the latter arose from Raman, IR, and thermogravimetry studies. The carboxylic acids and ethers/epoxides onto exfoliated graphene were derived from air during ultrasonication and found independent of the solvent used for the exfoliation and the power and/or time ultrasonication applied. Quantitative evaluation of the amount of oxygenated species present on exfoliated graphene as performed by high-resolution XPS revealed that the relative oxygen percentage was higher when exfoliation was performed in NMP. Finally, the sonication time and/or power affected the oxygen content on exfoliated graphene, since extended ultrasonication resulted in a decrease in the oxygen content on exfoliated graphene, with a simultaneous increase of defected sp(3) carbon atoms.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 4.536
Times cited: 65
DOI: 10.1021/jp4057048
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“Dynamic motion of Ru-polyoxometalate ions (POMs) on functionalized few-layer graphene”. Ke X, Turner S, Quintana M, Hadad C, Montellano-López A, Carraro M, Sartorel A, Bonchio M, Prato M, Bittencourt C, Van Tendeloo G;, Small 9, 3922 (2013). http://doi.org/10.1002/smll.201300378
Abstract: The interaction and stability of Ru4POM on few layer graphene via functional groups is investigated by time-dependent imaging using aberration-corrected transmission electron microscopy. The Ru4POM demonstrates dynamic motion on the graphene surface with its frequency and amplitude of rotation related to the nature of the functional group used. The stability of the Ru4POMgraphene hybrid corroborates its long-term robustness when applied to multielectronic catalytic processes.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 8.643
Times cited: 16
DOI: 10.1002/smll.201300378
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“Prussian blue analogues of reduced dimensionality”. Gengler RYN, Toma LM, Pardo E, Lloret F, Ke X, Van Tendeloo G, Gournis D, Rudolf P, Small 8, 2532 (2012). http://doi.org/10.1002/smll.201200517
Abstract: Mixed-valence polycyanides (Prussian Blue analogues) possess a rich palette of properties spanning from room-temperature ferromagnetism to zero thermal expansion, which can be tuned by chemical modifications or the application of external stimuli (temperature, pressure, light irradiation). While molecule-based materials can combine physical and chemical properties associated with molecular-scale building blocks, their successful integration into real devices depends primarily on higher-order properties such as crystal size, shape, morphology, and organization. Herein a study of a new reduced-dimensionality system based on Prussian Blue analogues (PBAs) is presented. The system is built up by means of a modified Langmuir-Blodgett technique, where the PBA is synthesized from precursors in a self-limited reaction on a clay mineral surface. The focus of this work is understanding the magnetic properties of the PBAs in different periodic, low-dimensional arrangements, and the influence of the “on surface” synthesis on the final properties and dimensionality of the system.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 8.643
Times cited: 17
DOI: 10.1002/smll.201200517
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“High-mobility Bi2Se3 nanoplates manifesting quantum oscillations of surface states in the sidewalls”. Yan Y, Wang L-X, Ke X, Van Tendeloo G, Wu X-S, Yu D-P, Liao Z-M, Scientific reports 4, 3817 (2014). http://doi.org/10.1038/srep03817
Abstract: Magnetotransport measurements of topological insulators are very important to reveal the exotic topological surface states for spintronic applications. However, the novel properties related to the surface Dirac fermions are usually accompanied by a large linear magnetoresistance under perpendicular magnetic field, which makes the identification of the surface states obscure. Here, we report prominent Shubnikov-de Haas (SdH) oscillations under an in-plane magnetic field, which are identified to originate from the surface states in the sidewalls of topological insulator Bi2Se3 nanoplates. Importantly, the SdH oscillations appear with a dramatically weakened magnetoresistance background, offering an easy path to probe the surface states directly when the coexistence of surface states and bulk conduction is inevitable. Moreover, under a perpendicular magnetic field, the oscillations in Hall conductivity have peak-to-valley amplitudes of 2 e(2)/h, giving confidence to achieve a quantum Hall effect in this system. A cross-section view of the nanoplate shows that the sidewall is (015) facet dominant and therefore forms a 586 angle with regard to the top/ bottom surface instead of being perpendicular; this gives credit to the surface states' behavior as two-dimensional transport.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 4.259
Times cited: 31
DOI: 10.1038/srep03817
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“Nanoscale spectroscopy with polarized X-rays by NEXAFS-TXM”. Guttmann P, Bittencourt C, Rehbein S, Umek P, Ke X, Van Tendeloo G, Ewels CP, Schneider G, Nature photonics 6, 25 (2012). http://doi.org/10.1038/NPHOTON.2011.268
Abstract: Near-edge X-ray absorption spectroscopy (NEXAFS)1 is an essential analytical tool in material science. Combining NEXAFS with scanning transmission X-ray microscopy (STXM) adds spatial resolution and the possibility to study individual nanostructures2, 3. Here, we describe a full-field transmission X-ray microscope (TXM) that generates high-resolution, large-area NEXAFS data with a collection rate two orders of magnitude faster than is possible with STXM. The TXM optical design combines a spectral resolution of E/ΔE = 1 × 104 with a spatial resolution of 25 nm in a field of view of 1520 µm and a data acquisition time of ~1 s. As an example, we present image stacks and polarization-dependent NEXAFS spectra from individual anisotropic sodium and protonated titanate nanoribbons. Our NEXAFS-TXM technique has the advantage that one image stack visualizes a large number of nanostructures and therefore already contains statistical information. This new high-resolution NEXAFS-TXM technique opens the way to advanced nanoscale science studies.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 37.852
Times cited: 76
DOI: 10.1038/NPHOTON.2011.268
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“Local destruction of superconductivity by non-magnetic impurities in mesoscopic iron-based superconductors”. Li J, Ji M, Schwarz T, Ke X, Van Tendeloo G, Yuan J, Pereira PJ, Huang Y, Zhang G, Feng HL, Yuan YH, Hatano T, Kleiner R, Koelle D, Chibotaru LF, Yamaura K, Wang HB, Wu PH, Takayama-Muromachi E, Vanacken J, Moshchalkov VV;, Nature communications 6, 7614 (2015). http://doi.org/10.1038/ncomms8614
Abstract: The determination of the pairing symmetry is one of the most crucial issues for the iron-based superconductors, for which various scenarios are discussed controversially. Non-magnetic impurity substitution is one of the most promising approaches to address the issue, because the pair-breaking mechanism from the non-magnetic impurities should be different for various models. Previous substitution experiments demonstrated that the non-magnetic zinc can suppress the superconductivity of various iron-based superconductors. Here we demonstrate the local destruction of superconductivity by non-magnetic zinc impurities in Ba0.5K0.5Fe2As2 by exploring phase-slip phenomena in a mesoscopic structure with 119 × 102 nm2 cross-section. The impurities suppress superconductivity in a three-dimensional Swiss cheese-like pattern with in-plane and out-of-plane characteristic lengths slightly below ~1.34 nm. This causes the superconducting order parameter to vary along abundant narrow channels with effective cross-section of a few square nanometres. The local destruction of superconductivity can be related to Cooper pair breaking by non-magnetic impurities.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.124
Times cited: 12
DOI: 10.1038/ncomms8614
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“A nanoscale shape memory oxide”. Zhang J, Ke X, Gou G, Seidel J, Xiang B, Yu P, Liang WI, Minor AM, Chu Yh, Van Tendeloo G, Ren X, Ramesh R;, Nature communications 4, 2768 (2013). http://doi.org/10.1038/ncomms3768
Abstract: Stimulus-responsive shape-memory materials have attracted tremendous research interests recently, with much effort focused on improving their mechanical actuation. Driven by the needs of nanoelectromechanical devices, materials with large mechanical strain, particularly at nanoscale level, are therefore desired. Here we report on the discovery of a large shape-memory effect in bismuth ferrite at the nanoscale. A maximum strain of up to ~14% and a large volumetric work density of ~600±90 J cm−3 can be achieved in association with a martensitic-like phase transformation. With a single step, control of the phase transformation by thermal activation or electric field has been reversibly achieved without the assistance of external recovery stress. Although aspects such as hysteresis, microcracking and so on have to be taken into consideration for real devices, the large shape-memory effect in this oxide surpasses most alloys and, therefore, demonstrates itself as an extraordinary material for potential use in state-of-art nanosystems.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.124
Times cited: 67
DOI: 10.1038/ncomms3768
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“Ferroelastic switching in a layered-perovskite thin film”. Wang C, Ke X, Wang J, Liang R, Luo Z, Tian Y, Yi D, Zhang Q, Wang J, Han X-F, Van Tendeloo G, Chen L-Q, Nan C-W, Ramesh R, Zhang J, Nature communications 7, 10636 (2016). http://doi.org/10.1038/ncomms10636
Abstract: A controllable ferroelastic switching in ferroelectric/multiferroic oxides is highly desirable due to the non-volatile strain and possible coupling between lattice and other order parameter in heterostructures. However, a substrate clamping usually inhibits their elastic deformation in thin films without micro/nano-patterned structure so that the integration of the non-volatile strain with thin film devices is challenging. Here, we report that reversible in-plane elastic switching with a non-volatile strain of approximately 0.4% can be achieved in layered-perovskite Bi2WO6 thin films, where the ferroelectric polarization rotates by 90 degrees within four in-plane preferred orientations. Phase-field simulation indicates that the energy barrier of ferroelastic switching in orthorhombic Bi2WO6 film is ten times lower than the one in PbTiO3 films, revealing the origin of the switching with negligible substrate constraint. The reversible control of the in-plane strain in this layered-perovskite thin film demonstrates a new pathway to integrate mechanical deformation with nanoscale electronic and/or magnetoelectronic applications.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.124
Times cited: 40
DOI: 10.1038/ncomms10636
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“Nematic superconducting state in iron pnictide superconductors”. Li J, Pereira PJ, Yuan J, Lv Y-Y, Jiang M-P, Lu D, Lin Z-Q, Liu Y-J, Wang J-F, Li L, Ke X, Van Tendeloo G, Li M-Y, Feng H-L, Hatano T, Wang H-B, Wu P-H, Yamaura K, Takayama-Muromachi E, Vanacken J, Chibotaru LF, Moshchalkov VV, Nature communications 8, 1880 (2017). http://doi.org/10.1038/s41467-017-02016-y
Abstract: Nematic order often breaks the tetragonal symmetry of iron-based superconductors. It arises from regular structural transition or electronic instability in the normal phase. Here, we report the observation of a nematic superconducting state, by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity of Ba 0.5 K 0.5 Fe 2 As 2 single crystals. We find large twofold oscillations in the vicinity of the superconducting transition, when the direction of applied magnetic field is rotated within the basal plane. To avoid the influences from sample geometry or current flow direction, the sample was designed as Corbino-shape for in-plane and mesa-shape for out-of-plane measurements. Theoretical analysis shows that the nematic superconductivity arises from the weak mixture of the quasi-degenerate s-wave and d-wave components of the superconducting condensate, most probably induced by a weak anisotropy of stresses inherent to single crystals.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.124
Times cited: 8
DOI: 10.1038/s41467-017-02016-y
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“Carbon nanotube TiO2 hybrid films for detecting traces of O2”. Llobet E, Espinosa EH, Sotter E, Ionescu R, Vilanova X, Torres J, Felten A, Pireaux JJ, Ke X, Van Tendeloo G, Renaux F, Paint Y, Hecq M, Bittencourt C;, Nanotechnology 19, 375501 (2008). http://doi.org/10.1088/0957-4484/19/37/375501
Abstract: Hybrid titania films have been prepared using an adapted sol-gel method for obtaining well-dispersed hydrogen plasma-treated multiwall carbon nanotubes in either pure titania or Nb-doped titania. The drop-coating method has been used to fabricate resistive oxygen sensors based on titania or on titania and carbon nanotube hybrids. Morphology and composition studies have revealed that the dispersion of low amounts of carbon nanotubes within the titania matrix does not significantly alter its crystallization behaviour. The gas sensitivity studies performed on the different samples have shown that the hybrid layers based on titania and carbon nanotubes possess an unprecedented responsiveness towards oxygen (i.e. more than four times higher than that shown by optimized Nb-doped TiO(2) films). Furthermore, hybrid sensors containing carbon nanotubes respond at significantly lower operating temperatures than their non-hybrid counterparts. These new hybrid sensors show a strong potential for monitoring traces of oxygen (i.e. <= 10 ppm) in a flow of CO(2), which is of interest for the beverage industry.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 3.44
Times cited: 48
DOI: 10.1088/0957-4484/19/37/375501
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“Electrical tomography using atomic force microscopy and its application towards carbon nanotube-based interconnects”. Schulze A, Hantschel T, Dathe A, Eyben P, Ke X, Vandervorst W, Nanotechnology 23, 305707 (2012). http://doi.org/10.1088/0957-4484/23/30/305707
Abstract: The fabrication and integration of low-resistance carbon nanotubes (CNTs) for interconnects in future integrated circuits requires characterization techniques providing structural and electrical information at the nanometer scale. In this paper we present a slice-and-view approach based on electrical atomic force microscopy. Material removal achieved by successive scanning using doped ultra-sharp full-diamond probes, manufactured in-house, enables us to acquire two-dimensional (2D) resistance maps originating from different depths (equivalently different CNT lengths) on CNT-based interconnects. Stacking and interpolating these 2D resistance maps results in a three-dimensional (3D) representation (tomogram). This allows insight from a structural (e.g. size, density, distribution, straightness) and electrical point of view simultaneously. By extracting the resistance evolution over the length of an individual CNT we derive quantitative information about the resistivity and the contact resistance between the CNT and bottom electrode.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 3.44
Times cited: 29
DOI: 10.1088/0957-4484/23/30/305707
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“Magnetotransport across the metal-graphene hybrid interface and its modulation by gate voltage”. Chen J-J, Ke X, Van Tendeloo G, Meng J, Zhou Y-B, Liao Z-M, Yu D-P, Nanoscale 7, 5516 (2015). http://doi.org/10.1039/c5nr00223k
Abstract: The graphene-metal contact is very important for optimizing the performance of graphene based electronic devices. However, it is difficult to probe the properties of the graphene/metal interface directly via transport measurements in traditional graphene lateral devices, because the dominated transport channel is graphene, not the interface. Here, we employ the Au/graphene/Au vertical and lateral hybrid structure to unveil the metal-graphene interface properties, where the transport is dominated by the charge carriers across the interface. The magnetoresistance (MR) of Au/monolayer graphene/Au and Au/stacked two-layered graphene/Au devices is measured and modulated by gate voltage, demonstrating that the interface is a device. The gate-tunable MR is identified from the graphene lying on the SiO2 substrate and underneath the top metal electrode. Our unique structures couple the in-plane and out-of-plane transport and display linear MR with small amplitude oscillations at low temperatures. Under a magnetic field, the electronic coupling between the graphene edge states and the electrode leads to the appearance of quantum oscillations. Our results not only provide a new pathway to explore the intrinsic transport mechanism at the graphene/metal interface but also open up new vistas of magnetoelectronics.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 7.367
Times cited: 3
DOI: 10.1039/c5nr00223k
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“Two-in-one solution using insect wings to produce graphene-graphite films for efficient electrocatalysis”. Li H, Zhang L, Li L, Wu C, Huo Y, Chen Y, Liu X, Ke X, Luo J, Van Tendeloo G, Nano Research 12, 33 (2019). http://doi.org/10.1007/S12274-018-2172-Z
Abstract: Natural organisms contain rich elements and naturally optimized smart structures, both of which have inspired various innovative concepts and designs in human society. In particular, several natural organisms have been used as element sources to synthesize low-cost and environmentally friendly electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries, which are clean energy devices. However, to date, no naturally optimized smart structures have been employed in the synthesis of ORR catalysts, including graphene-based materials. Here, we demonstrate a novel strategy to synthesize graphene-graphite films (GGFs) by heating butterfly wings coated with FeCl3 in N-2, in which the full power of natural organisms is utilized. The wings work not only as an element source for GGF generation but also as a porous supporting structure for effective nitrogen doping, two-dimensional spreading, and double-face exposure of the GGFs. These GGFs exhibit a half-wave potential of 0.942 V and a H2O2 yield of < 0.07% for ORR electrocatalysis; these values are comparable to those for the best commercial Pt/C and all previously reported ORR catalysts in alkaline media. This two-in-one strategy is also successful with cicada and dragonfly wings, indicating that it is a universal, green, and cost-effective method for developing high-performance graphene-based materials.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 7.354
Times cited: 7
DOI: 10.1007/S12274-018-2172-Z
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“Topological surface state enhanced photothermoelectric effect in Bi2Se3 nanoribbons”. Yan Y, Liao ZM, Ke X, Van Tendeloo G, Wang Q, Sun D, Yao W, Zhou S, Zhang L, Wu HC, Yu DP;, Nano letters 14, 4389 (2014). http://doi.org/10.1021/nl501276e
Abstract: The photothermoelectric effect in topological insulator Bi2Se3 nanoribbons is studied. The topological surface states are excited to be spin-polarized by circularly polarized light. Because the direction of the electron spin is locked to its momentum for the spin-helical surface states, the photothermoelectric effect is significantly enhanced as the oriented motions of the polarized spins are accelerated by the temperature gradient. The results are explained based on the microscopic mechanisms of a photon induced spin transition from the surface Dirac cone to the bulk conduction band. The as-reported enhanced photothermoelectric effect is expected to have potential applications in a spin-polarized power source.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.712
Times cited: 51
DOI: 10.1021/nl501276e
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“Asymmetric Modulation on Exchange Field in a Graphene/BiFeO3Heterostructure by External Magnetic Field”. Song H-D, Wu Y-F, Yang X, Ren Z, Ke X, Kurttepeli M, Tendeloo GV, Liu D, Wu H-C, Yan B, Wu X, Duan C-G, Han G, Liao Z-M, Yu D, Nano letters 18, 2435 (2018). http://doi.org/10.1021/acs.nanolett.7b05480
Abstract: Graphene, having all atoms on its surface, is favorable to extend the functions by introducing the spin–orbit coupling and magnetism through proximity effect. Here, we report the tunable interfacial exchange field produced by proximity coupling in graphene/BiFeO3 heterostructures. The exchange field has a notable dependence with external magnetic field, and it is much larger under negative magnetic field than that under positive magnetic field. For negative external magnetic field, interfacial exchange coupling gives rise to evident spin splitting for N ≠ 0 Landau levels and a quantum Hall metal state for N = 0 Landau level. Our findings suggest graphene/BiFeO3 heterostructures are promising for spintronics.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.712
Times cited: 9
DOI: 10.1021/acs.nanolett.7b05480
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“One particle@one cell : highly monodispersed PtPd bimetallic nanoparticles for enhanced oxygen reduction reaction”. Ying J, Yang X-Y, Hu Z-Y, Mu S-C, Janiak C, Geng W, Pan M, Ke X, Van Tendeloo G, Su B-L, Nano energy 8, 214 (2014). http://doi.org/10.1016/j.nanoen.2014.06.010
Abstract: Highly monodispersed platinum-based nanoalloys are the best-known catalysts for the oxygen reduction reaction. Although certainly promising, the durability and stability are among the main requirements for commercializing fuel cell electrocatalysts in practical applications. Herein, we synthesize highly stable, durable and catalytic active monodispersed PtPd nano-particles encapsulated in a unique one particle@one cell structure by adjusting the viscosity of solvents using mesocellular foam. PtPd nanoparticles in mesocellular carbon foam exhibit an excellent electrocatalytic activity (over 4 times mass and specific activities than the commercial Pt/C catalyst). Most importantly, this nanocatalyst shows no obvious change of structure and only a 29.5% loss in electrochemically active surface area after 5000 potential sweeps between 0.6 and 1.1 V versus reversible hydrogen electrode cycles. (C) 2014 Elsevier Ltd. All rights reserved.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 12.343
Times cited: 40
DOI: 10.1016/j.nanoen.2014.06.010
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“Three-dimensional analysis of carbon nanotube networks in interconnects by electron tomography without missing wedge artifacts”. Ke X, Bals S, Cott D, Hantschel T, Bender H, Van Tendeloo G, Microscopy and microanalysis 16, 210 (2010). http://doi.org/10.1017/S1431927609991371
Abstract: The three-dimensional (3D) distribution of carbon nanotubes (CNTs) grown inside semiconductor contact holes is studied by electron tomography. The use of a specialized tomography holder results in an angular tilt range of ±90°, which means that the so-called missing wedge is absent. The transmission electron microscopy (TEM) sample for this purpose consists of a micropillar that is prepared by a dedicated procedure using the focused ion beam (FIB) but keeping the CNTs intact. The 3D results are combined with energy dispersive X-ray spectroscopy (EDS) to study the relation between the CNTs and the catalyst particles used during their growth. The reconstruction, based on the full range of tilt angles, is compared with a reconstruction where a missing wedge is present. This clearly illustates that the missing wedge will lead to an unreliable interpretation and will limit quantitative studies
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.891
Times cited: 42
DOI: 10.1017/S1431927609991371
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“Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies”. Ribbens S, Meynen V, Van Tendeloo G, Ke X, Mertens M, Maes BUW, Cool P, Vansant EF, Microporous and mesoporous materials: zeolites, clays, carbons and related materials 114, 401 (2008). http://doi.org/10.1016/j.micromeso.2008.01.028
Abstract: Titanate nanotubes were prepared via a hydrothermal treatment of TiO2 powders (Riedel De Haen) in a basic solution. Morphology and structure of the prepared samples were characterized by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), XRD, FT-Raman spectroscopy, nitrogen sorption and DSC. The photocatalytic activity was evaluated by photocatalytic oxidation of rhodamine 6G. Trititanate nanotubes (TTNT) with inner pore diameters between 4 and 4.2 nm and surface areas up till 360 m(2)/g could be synthesized. The synthesis route was modified by introduction of a calcination step, by applying a lower hydrothermal temperature and microwave irradiation in order to increase the photocatalytic activity of the porous photoactive nanotubular materials. Calcination and a softer hydrothermal treatment led to the formation of anatase without affecting the surface area and nanotubular shape of the samples. In this way, the photocatalytic activity of the original trititanate nanotubes could be significantly increased. By making use of microwave assisted synthesis, the photocatalytic activity call also be increased due to the presence of anatase. However, by applying microwave synthesis, a different structure was obtained, nanoribbons (NR) instead of nanotubcs, resulting in a decrease in surface area and porosity.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Laboratory of adsorption and catalysis (LADCA); Organic synthesis (ORSY)
Impact Factor: 3.615
Times cited: 47
DOI: 10.1016/j.micromeso.2008.01.028
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“Graphitic nanocrystals inside the pores of mesoporous silica : synthesis, characterization and an adsorption study”. de Clippel F, Harkiolakis A, Vosch T, Ke X, Giebeler L, Oswald S, Houthoofd K, Jammaer J, Van Tendeloo G, Martens JA, Jacobs PA, Baron GV, Sels BF, Denayer JFM, Microporous and mesoporous materials: zeolites, clays, carbons and related materials 144, 120 (2011). http://doi.org/10.1016/j.micromeso.2011.04.003
Abstract: This work presents a new carbonsilica hybrid material, denoted as CSM, with remarkable sorption properties. It consists of intraporous graphitic nanocrystals grown in the pores of mesoporous silica. CSM is obtained by a subtle incipient wetness impregnation of Al-containing mesoporous silica with furfuryl alcohol (FA)/hemelitol solutions. Both the volume match of the impregnation solution with that of the silica template pore volume, and the presence of Al3+ in the silica, are crucial to polymerize FA selectively inside the mesopores. Carbonization of the intraporous polymer was then performed by pyrolysis under He up to 1273 K. The resulting CSMs were examined by SEM, HRTEM, 27Al MAS NMR, N2 adsorption, XRD, TGA, TPD, XPS, pycnometry and Raman spectroscopy. Mildly oxidized graphitic-like carbon nanoblocks, consisting of a few graphene-like sheets, were thus identified inside the template mesopores. Random stacking of these carbon crystallites generates microporosity resulting in biporous materials at low carbon content and microporous materials at high carbon loadings. Very narrow pore distributions were obtained when pyrolysis was carried out under slow heating rate, viz. 1 K min−1. Adsorption and shape selective properties of the carbon filled mesoporous silica were studied by performing pulse chromatography and breakthrough experiments, and by measuring adsorption isotherms of linear and branched alkanes. Whereas the parent mesoporous silica shows unselective adsorption, their CSM analogues preferentially adsorb linear alkanes. The sorption capacity and selectivity can be adjusted by changing the pore size of the template or by varying the synthesis conditions. A relation between the carbon crystallites size and the shape selective behaviour of the corresponding CSM for instance is demonstrated. Most interestingly, CSM shows separation factors for linear and branched alkanes up to values comparable to those of zeolitic molecular sieves.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.615
Times cited: 15
DOI: 10.1016/j.micromeso.2011.04.003
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“Systematic evaluation of thermal and mechanical stability of different commercial and synthetic photocatalysts in relation to their photocatalytic activity”. Ribbens S, Beyers E, Schellens K, Mertens M, Ke X, Bals S, Van Tendeloo G, Meynen V, Cool P, Microporous and mesoporous materials: zeolites, clays, carbons and related materials 156, 62 (2012). http://doi.org/10.1016/j.micromeso.2012.01.036
Abstract: The effect of thermal treatment and mechanical stress on the structural and photocatalytic properties of eight different (synthetic and commercial) photocatalysts has been thoroughly investigated. Different mesoporous Ti-based materials were prepared via surfactant based synthesis routes (e.g. Pluronic 123, CTMABr = Cetyltrimethylammonium bromide) or via template-free synthesis routes (e.g. trititanate nanotubes). Also, the stabilizing effect of the NaOH/NH4OH post-treatment on the templated mesoporous materials and their photocatalytic activity was investigated. Furthermore, the thermal and mechanical properties of commercially available titanium dioxides such as P25 Evonik® and Millenium PC500® were studied. The various photocatalysts were analyzed with N2-sorption, X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) to obtain information concerning the specific surface area, pore volume, crystal structure, morphology, phase transitions, etc. In general, results show that the NaOH post-treatment leads to an increased control of the crystallization process during calcination resulting in a higher thermal stability, but at the same time diminishes the photocatalytic activity. Mesoporous materials in which pre-synthesized nanoparticles are used as titania source have the best mechanical stability whereas the mechanical stability of the nanotubes is the most limited. At increased temperatures and pressures, the tested commercial titanium dioxides lose their superior photocatalytic activity caused by a decreased accessibility of the active sites. The observed changes in adsorption capacities and photocatalytic activities cannot be assigned to one single phenomenon. In this respect, it shows the need to define a general/standard method to compare different photocatalysts. Furthermore, it is shown that the photocatalytic properties do not necessarily deteriorate under thermal stress, but can be improved due to crystallization, even though the initial material is (partially) destroyed. It is shown that the usefulness of a specific type of photocatalyst strongly depends on the application and the temperature/pressure to which it needs to resist.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT); Laboratory of adsorption and catalysis (LADCA)
Impact Factor: 3.615
Times cited: 8
DOI: 10.1016/j.micromeso.2012.01.036
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“Tuning metal sites of DABCO MOF for gas purification at ambient conditions”. Chemchuen S, Zhou K, Kabir NA, Chen Y, Ke X, Van Tendeloo G, Verpoort F, Microporous and mesoporous materials: zeolites, clays, carbons and related materials 201, 277 (2015). http://doi.org/10.1016/j.micromeso.2014.09.038
Abstract: Metalorganic frameworks (MOFs) have emerged as new porous materials for capture and separation of binary gas mixtures. Tuning the metal sites in MOF structures has an impact on properties, which enhance affinity of gas adsorption and selectivity (e.g., surface area, cavity, electric field, etc.). The synthesis and characterization of a M-DABCO series (M = Ni, Co, Cu, Zn) of MOFs are described in this study. The experiments were conducted using multicomponent gas mixtures and the Ideal Adsorbed Solution Theory (IAST) was applied to determine the CO2/CH4 selectivity. Experimental adsorption isotherms were fitted with a model equation to evaluate the characteristic adsorption energy (Isosteric, Qst) of this series. The Ni metal in the M-DABCO series reveals the best performance concerning CO2 adsorption and CH4/CO2 selectivity at ambient conditions based on IAST calculations. The combination of characterizations, calculations and adsorption experiments were used to discuss the metal impact on the adsorption sites in the M-DABCO series at ambient conditions.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 3.615
Times cited: 38
DOI: 10.1016/j.micromeso.2014.09.038
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“Gold clusters on WO3 nanoneedles grown via AACVD : XPS and TEM studies”. Navío C, Vallejos S, Stoycheva T, Llobet E, Correig X, Snyders R, Blackman C, Umek P, Ke X, Van Tendeloo G, Bittencourt C;, Materials chemistry and physics 134, 809 (2012). http://doi.org/10.1016/j.matchemphys.2012.03.073
Abstract: We have prepared tungsten oxide films decorated with gold particles on Si substrates by aerosol assisted chemical vapor deposition (AACVD) and characterized them using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). SEM shows that the films are composed of needle-like structures and TEM shows that both the needles and the gold particles are crystalline. XPS indicates the presence of oxygen vacancies, i.e. the films are WO3−x, and hence the deposited material is composed of semiconducting nanostructures and that the interaction between the gold particles and the WO3 needles surface is weak. The synthesis of semiconducting tungsten oxide nanostructures decorated with metal particles represents an important step towards the development of sensing devices with optimal properties.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 2.084
Times cited: 52
DOI: 10.1016/j.matchemphys.2012.03.073
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“Probing the electron beam-induced structural evolution of halide perovskite thin films by scanning transmission electron microscopy”. Zhou X-G, Yang C-Q, Sang X, Li W, Wang L, Yin Z-W, Han J-R, Li Y, Ke X, Hu Z-Y, Cheng Y-B, Van Tendeloo G, Journal Of Physical Chemistry C 125, 10786 (2021). http://doi.org/10.1021/ACS.JPCC.1C02156
Abstract: A deep understanding of the fine structure at the atomic scale of halide perovskite materials has been limited by their sensitivity to the electron beam that is widely used for structural characterization. The sensitivity of a gamma-CsPbIBr2 perovskite thin film under electron beam irradiation is revealed by scanning transmission electron microscopy (STEM) through a universal large-range electron dose measurement, which is based on discrete single-electron events in the STEM mode. Our research indicates that the gamma-CsPbIBr2 thin film undergoes structural changes with increasing electron overall dose (e(-).A(-2)) rather than dose rate (e(-).A(-2).s(-1)), which suggests that overall dose is the key operative parameter. The electron beam-induced structural evolution of gamma-CsPbIBr2 is monitored by fine control of the electron beam dose, together with the analysis of high-resolution (S)TEM, diffraction, and energy-dispersive X-ray spectroscopy. Our results show that the gamma-CsPbIBr2 phase first forms an intermediate phase [e.g., CsPb(1-x)(IBr)((3-y))] with a superstructure of ordered vacancies in the pristine unit cell, while a fraction of Pb2+ is reduced to Pb-0. As the electron dose increases, Pb nanoparticles precipitate, while the remaining framework forms the Cs2IBr phase, accompanied by some amorphization. This work provides guidelines to minimize electron beam irradiation artifacts for atomic-resolution imaging on CsPbIBr2 thin films.
Keywords: A1 Journal article; Engineering sciences. Technology; Electron microscopy for materials research (EMAT)
Impact Factor: 4.536
DOI: 10.1021/ACS.JPCC.1C02156
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“Homopolymers as nanocarriers for the loading of block copolymer micelles with metal salts : a facile way to large-scale ordered arrays of transition-metal nanoparticles”. Shan L, Punniyakoti S, Van Bael MJ, Temst K, Van Bael MK, Ke X, Bals S, Van Tendeloo G, D'Olieslaeger M, Wagner P, Haenen K, Boyen HG;, Journal of materials chemistry C : materials for optical and electronic devices 2, 701 (2014). http://doi.org/10.1039/c3tc31333f
Abstract: A new and facile approach is presented for generating quasi-regular patterns of transition metal-based nanoparticles on flat substrates exploiting polystyrene-block-poly2vinyl pyridine (PS-b-P2VP) micelles as intermediate templates. Direct loading of such micellar nanoreactors by polar transition metal salts in solution usually results in nanoparticle ensembles exhibiting only short range order accompanied by broad distributions of particle size and inter-particle distance. Here, we demonstrate that the use of P2VP homopolymers of appropriate length as molecular carriers to transport precursor salts into the micellar cores can significantly increase the degree of lateral order within the final nanoparticle arrays combined with a decrease in spreading in particle size. Thus, a significantly extended range of materials is now available which can be exploited to study fundamental properties at the transition from clusters to solids by means of well-organized, well-separated, size-selected metal and metal oxide nanostructures.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 5.256
Times cited: 5
DOI: 10.1039/c3tc31333f
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“From polyester grafting onto POSS nanocage by ring-opening polymerization to high performance polyester/POSS nanocomposites”. Goffin A-L, Duquesne E, Raquez J-M, Miltner HE, Ke X, Alexandre M, Van Tendeloo G, van Mele B, Dubois P, Journal of materials chemistry 20, 9415 (2010). http://doi.org/10.1039/c0jm00283f
Abstract: Polyester-grafted polyhedral oligomeric silsesquioxane (POSS) nanohybrids selectively produced by ring-opening polymerization of ε-caprolactone and L,L-lactide (A.-L. Goffin, E. Duquesne, S. Moins, M. Alexandre, Ph. Dubois, Eur. Polym. Journal, 2007, 43, 4103) were studied as masterbatches by melt-blending within their corresponding commercial polymeric matrices, i.e., poly(ε-caprolactone) (PCL) and poly(L,L-lactide) (PLA). For the sake of comparison, neat POSS nanoparticles were also dispersed in PCL and PLA. The objective was to prepare aliphatic polyester-based nanocomposites with enhanced crystallization behavior, and therefore, enhanced thermo-mechanical properties. Wide-angle X-ray scattering and transmission electron microscopy attested for the dispersion of individualized POSS nanoparticles in the resulting nanocomposite materials only when the polyester-grafted POSS nanohybrid was used as a masterbatch. The large impact of such finely dispersed (grafted) nanoparticles on the crystallization behavior for the corresponding polyester matrices was noticed, as evidenced by differential scanning calorimetry analysis. Indeed, well-dispersed POSS nanoparticles acted as efficient nucleating sites, significantly increasing the crystallinity degree of both PCL and PLA matrices. As a result, a positive impact on thermo-mechanical properties was highlighted by dynamic mechanical thermal analysis.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Times cited: 42
DOI: 10.1039/c0jm00283f
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“Wafer-level electrical evaluation of vertical carbon nanotube bundles as a function of growth temperature”. Vereecke B, van der Veen MH, Sugiura M, Kashiwagi Y, Ke X, Cott DJ, Hantschel T, Huyghebaert C, Tökei Z, Japanese journal of applied physics 52, 04cn02 (2013). http://doi.org/10.7567/JJAP.52.04CN02
Abstract: We have evaluated the resistance of carbon nanotubes (CNTs) grown at a CMOS-compatible temperature using a realistic integration scheme. The structural analysis of the CNTs by transmission electron microscopy (TEM) showed that the degree of graphitization decreased significantly when the growth temperature was decreased from 540 to 400 °C. The CNTs were integrated to form 150-nm-diameter vertical interconnects between a TiN layer and Cu metal trenches on 200 mm full wafers. Wafers with CNTs grown at low temperature were found to have a lower single-contact resistance than those produced at high temperatures. Thickness measurements showed that the low contact resistance is a result of small contact height. This height dependence is masking the impact of CNT graphitization quality on resistance. When benchmarking our results with data from the literature, a relationship between resistivity and growth temperature cannot be found for CNT-based vertical interconnects.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 1.384
Times cited: 5
DOI: 10.7567/JJAP.52.04CN02
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“The role of oxygen at the interface between titanium and carbon nanotubes”. Felten A, Suarez-Martinez I, Ke X, Van Tendeloo G, Ghijsen J, Pireaux J-J, Drube W, Bittencourt C, Ewels CP, ChemPhysChem : a European journal of chemical physics and physical chemistry 10, 1799 (2009). http://doi.org/10.1002/cphc.200900193
Abstract: We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti-CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pre-treatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti-nanotube contact formation.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT)
Impact Factor: 3.075
Times cited: 38
DOI: 10.1002/cphc.200900193
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