Browsing by Author "Jiang, H."
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- Catalyst-free growth of In(As)P nanowires on silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2006) Mattila, M.; Hakkarainen, T.; Lipsanen, Harri; Jiang, H.; Kauppinen, Esko I.The catalyst-free metal organic vapor phase epitaxialgrowth of In(As)P nanowires on silicon substrates is investigated using in situ deposited In droplets as seeds for nanowiregrowth. The thin substrate native oxide is found to play a crucial role in the nanowire formation. The structure of the nanowires is characterized by photoluminescence and electron microscopy measurements. The crystal structure of the InPnanowires is wurtzite with its c axis perpendicular to the nanowire axis. Adding arsenic precursor to the gas phase during growth results in a bimodal photoluminescence spectrum exhibiting peak at the InAsP and InP band gap energies. - CVD synthesis of hierarchical 3D MWCNT/carbon fiber nanostructures
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2008) Susi, T.; Nasibulin, A.G; Jiang, H.; Kauppinen, E.IMultiwalled carbon nanotubes (MWCNTs) were synthesized by CVD on industrially manufactured highly crystalline vapor-grown carbon fibers (VGCFs). Two catalyst metals (Ni and Fe) and carbon precursor gases (C2H2 and CO) were studied. The catalysts were deposited on the fibers by sputtering and experiments carried out in two different reactors. Samples were characterized by electron microscopy (SEM and TEM). Iron was completely inactive as catalyst with both C2H2 and CO for reasons discussed in the paper. The combination of Ni and C2H2 was very active for secondary CNT synthesis, without any pretreatment of the fibers. The optimal temperature for CNT synthesis was 750◦C, with total gas flow of 650 cm3min−1 of C2H2, H2, and Ar in 1.0:6.7:30 ratio. - Determination of helicities in unidirectional assemblies of graphitic or graphiticlike tubular structures
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2008) Jiang, H.; Brown, D. P.; Nikolaev, P.; Nasibulin, Albert G.; Kauppinen, Esko I.Here we propose a universal method for the determination of all helicities present in unidirectional assemblies of hexagon-based graphitic or graphiticlike tubular structures, e.g., multiwalled or bundled carbon nanotubes(CNTs) or boron-nitride nanotubes and their structural analogs. A critical dimension characteristic of a fundamental structural property, i.e., the atomic bond length, is discerned from electron diffraction patterns by which all helicities present in the assemblies are identified. Using this method, we determine the helicity population in a single-walled CNT sample produced by laser ablation technique. - Enhanced luminescence from catalyst-free grown InP nanowires
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2007) Mattila, M.; Hakkarainen, T.; Lipsanen, Harri; Jiang, H.; Kauppinen, Esko I.The surface effects in the optical properties of catalyst-free grownInPnanowires are investigated. Both as-grown nanowires and nanowires treated with hydrofluoric acid are studied using low- and room-temperature continuous-wave and time-resolved photoluminescence measurements and transmission electron microscopy. It is shown that the room-temperature photoluminescence intensity is increased by two orders of magnitude after the surface treatment, and that there is also a significant increase in the double-exponential photoluminescence decay time. - Gas phase synthesis of non-bundled, small diameter single-walled carbon nanotubes with near-armchair chiralities
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015) Mustonen, K.; Laiho, P.; Kaskela, A.; Zhu, Z.; Reynaud, O.; Houbenov, N.; Tian, Y.; Susi, Toma; Jiang, H.; Nasibulin, Albert G.; Kauppinen, Esko I.We present a floating catalyst synthesis route for individual, i.e., non-bundled, small diameter single-walled carbon nanotubes (SWCNTs) with a narrow chiral angle distribution peaking at high chiralities near the armchair species. An ex situ spark discharge generator was used to form iron particles with geometric number mean diameters of 3–4 nm and fed into a laminar flow chemical vapour deposition reactor for the continuous synthesis of long and high-quality SWCNTs from ambient pressure carbon monoxide. The intensity ratio of G/D peaks in Raman spectra up to 48 and mean tube lengths up to 4 μm were observed. The chiral distributions, as directly determined by electron diffraction in the transmission electron microscope, clustered around the (n,m) indices (7,6), (8,6), (8,7), and (9,6), with up to 70% of tubes having chiral angles over 20°. The mean diameter of SWCNTs was reduced from 1.10 to 1.04 nm by decreasing the growth temperature from 880 to 750 °C, which simultaneously increased the fraction of semiconducting tubes from 67% to 80%. Limiting the nanotube gas phase number concentration to ∼10 exp 5 cm exp −3 prevented nanotube bundle formation that is due to collisions induced by Brownian diffusion. Up to 80% of 500 as-deposited tubes observed by atomic force and transmission electron microscopy were individual. Transparent conducting films deposited from these SWCNTs exhibited record low sheet resistances of 63 Ω/□ at 90% transparency for 550 nm light. - Measurement of optical second-harmonic generation from an individual single-walled carbon nanotube
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Huttunen, M. J.; Herranen, O.; Johansson, A.; Jiang, H.; Mudimela, P. R.; Myllyperkiö, P.; Bautista, G.; Nasibulin, Albert G.; Kauppinen, Esko I.; Ahlskog, M.; Kauranen, M.; Pettersson, M.We show that optical second-harmonic generation (SHG) can be observed from individual single-walled carbon nanotubes (SWCNTs) and, furthermore, allows imaging of individual tubes. Detailed analysis of our results suggests that the structural non-centrosymmetry, as required for SHG, arises from the non-zero chiral angle of the SWCNT. SHG thus has potential as a fast, non-destructive and simple method for imaging of individual nanomolecules and for probing their chiral properties. - Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Chávez-Ángel, E.; Reparaz, J. S.; Gomis-Bresco, J.; Wagner, M. R.; Cuffe, J.; Graczykowski, B.; Shchepetov, A.; Jiang, H.; Prunnila, M.; Ahopelto, J.; Alzina, F.; Sotomayor Torres, C. M.We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality. - Ultra-thin free-standing single crystalline silicon membranes with strain control
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Shchepetov, A.; Prunnila, M.; Alzina, F.; Schneider, L.; Cuffe, J.; Jiang, H.; Kauppinen, Esko I.; Sotomayor Torres, C. M.; Ahopelto, J.We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.