Composition-Tuned Co_<i>n</i>Si Nanowires: Location-Selective Simultaneous Growth along Temperature Gradient

Seo, Kwanyong,Lee, Sunghun,Yoon, Hana,In, Juneho,Varadwaj, Kumar S. K.,Jo, Younghun,Jung, Myung-Hwa,Kim, Jinhee,Kim, Bongsoo American Chemical Society 2009 ACS NANO Vol.3 No.5

<P>We report the simultaneous and selective synthesis of single-crystalline Co(n)Si NWs (n = 1-3) and their corresponding crystal structures--simple cubic (CoSi), orthorhombic (Co(2)Si), and face-centered cubic (Co(3)Si)--following a composition change. Co(n)Si NWs were synthesized by placing the sapphire substrates along a temperature gradient. The synthetic process is a successful demonstration of tuning the chemical composition in Co(n)Si NWs. The synthesis and detailed crystal structure of single-crystalline Co(2)Si and Co(3)Si are reported for the first time including the bulk and the nanostructure phases. The electrical and magnetic properties of Co(2)Si NWs are investigated and compared with those of CoSi NWs.</P>

Diffusion-Driven Crystal Structure Transformation: Synthesis of Heusler Alloy Fe₃Si Nanowires

Seo, Kwanyong,Bagkar, Nitin,Kim, Si-in,In, Juneho,Yoon, Hana,Jo, Younghun,Kim, Bongsoo American Chemical Society 2010 NANO LETTERS Vol.10 No.9

<P>We report fabrication of Heusler alloy Fe<SUB>3</SUB>Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe<SUB>3</SUB>Si nanowire ensemble show high-temperature ferromagnetic properties with <I>T</I><SUB>c</SUB> ≫ 370 K. This methodology is also successfully applied to Co<SUB>2</SUB>Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2010/nalefd.2010.10.issue-9/nl102093e/production/images/medium/nl-2010-02093e_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl102093e'>ACS Electronic Supporting Info</A></P>

Itinerant Helimagnetic Single-Crystalline MnSi Nanowires

Seo, Kwanyong,Yoon, Hana,Ryu, Seong-Wan,Lee, Sunghun,Jo, Younghun,Jung, Myung-Hwa,Kim, Jinhee,Choi, Yang-Kyu,Kim, Bongsoo American Chemical Society 2010 ACS NANO Vol.4 No.5

<P>We report the synthesis of free-standing MnSi nanowires <I>via</I> a vapor transport method with no catalyst and measurements of their electrical and magnetic properties for the first time. The single-crystalline MnSi nanowire ensemble with a simple cubic (B20) crystal structure shows itinerant helimagnetic properties with a <I>T</I><SUB>c</SUB> of about 30 K. A single MnSi nanowire device was fabricated by a new method using photolithography and a nanomanipulator that produces good ohmic contacts. The single-nanowire device measurements provide large (20%) negative magnetoresistance and very low electrical resistivity of 544 μΩcm for the MnSi nanowire.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2010/ancac3.2010.4.issue-5/nn901653q/production/images/medium/nn-2009-01653q_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn901653q'>ACS Electronic Supporting Info</A></P>

Vertical Epitaxial Co₅Ge₇ Nanowire and Nanobelt Arrays on a Thin Graphitic Layer for Flexible Field Emission Displays

Yoon, Hana,Seo, Kwanyong,Bagkar, Nitin,In, Juneho,Park, Jeunghee,Kim, Jaemyung,Kim, Bongsoo WILEY‐VCH Verlag 2009 Advanced Materials Vol.21 No.48

<P><B>Vertically aligned single‐crystalline Co<SUB>5</SUB>Ge<SUB>7</SUB> nanowire (NW) and nanobelt arrays</B> are grown on a very thin graphite layer as well as a curved graphite layer with a good epitaxial lattice match. Co<SUB>5</SUB>Ge<SUB>7</SUB> NW arrays, thus grown, show very efficient field emission properties comparable to those of carbon nanotubes and may be used for flexible field emission displays in the future. </P>

Steering Epitaxial Alignment of Au, Pd, and AuPd Nanowire Arrays by Atom Flux Change

Yoo, Youngdong,Seo, Kwanyong,Han, Sol,Varadwaj, Kumar S. K.,Kim, Hyun You,Ryu, Ji Hoon,Lee, Hyuck Mo,Ahn, Jae Pyoung,Ihee, Hyotcherl,Kim, Bongsoo American Chemical Society 2010 Nano letters Vol.10 No.2

<P>We have synthesized epitaxial Au, Pd, and AuPd nanowire arrays in vertical or horizontal alignment on a c-cut sapphire substrate. We show that the vertical and horizontal nanowire arrays grow from half-octahedral seeds by the correlations of the geometry and orientation of seed crystals with those of as-grown nanowires. The alignment of nanowires can be steered by changing the atom flux. At low atom deposition flux vertical nanowires grow, while at high atom flux horizontal nanowires grow. Similar vertical/horizontal epitaxial growth is also demonstrated on SrTiO<SUB>3</SUB> substrates. This orientation-steering mechanism is visualized by molecular dynamics simulations.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2010/nalefd.2010.10.issue-2/nl903002x/production/images/medium/nl-2009-03002x_0001.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl903002x'>ACS Electronic Supporting Info</A></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl903002x'>ACS Electronic Supporting Info</A></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl903002x'>ACS Electronic Supporting Info</A></P>

Controlling the Light Absorption in a Photodetector Via Nanowire Waveguide Resonances for Multispectral and Color Imaging

Crozier, Kenneth B.,Seo, Kwanyong,Park, Hyunsung,Solanki, Amit,Li, Shi-Qiang IEEE 2018 IEEE journal on selected topics in quantum electro Vol.24 No.6

<P>The responsivity spectrum of a photodetector is one of its key specifications. It ultimately originates from the combination of the absorption spectrum of the photosensitive region and the internal quantum efficiency. Many applications of photodetectors would benefit from an improved ability to tailor the responsivity spectrum. This is particularly true for color and multispectral imaging. The absorption spectrum of a bulk (unstructured) semiconductor is fixed however, being determined by its complex refractive index. Here, we review recent work that demonstrates that the absorption spectrum of a photodetector can be controlled via waveguide resonances in semiconductor nanowires. We discuss the physical interpretation for this phenomenon. We review work in which p-i-n photodiodes were incorporated into vertically oriented silicon nanowires, and then used for color imaging. We review work in which tandem-style photodetectors were demonstrated, with a p-i-n silicon nanowire photodiode formed above an n-i-p planar silicon photodiode. We review work in which narrowband photodetection across the visible-to-infrared was demonstrated using germanium nanowires. Finally, we describe related work in which silicon nanowires have been explored for other applications, namely solar cells.</P>

Filter-Free Image Sensor Pixels Comprising Silicon Nanowires with Selective Color Absorption

Park, Hyunsung,Dan, Yaping,Seo, Kwanyong,Yu, Young J.,Duane, Peter K.,Wober, Munib,Crozier, Kenneth B. American Chemical Society 2014 NANO LETTERS Vol.14 No.4

<P>The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-4/nl404379w/production/images/medium/nl-2013-04379w_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl404379w'>ACS Electronic Supporting Info</A></P>

A diameter-dependent separation of semiconducting from metallic single-wall carbon nanotubes by using nitronium ions

안계혁,Cheol-Min Yang,Kwanyong Seo,박경아,이영희 한국물리학회 2006 Current Applied Physics Vol.6 No.1l

We have reported the density-functional calculations of NO2 that NO2 adsorption is strongly electronic structure- and strain-dependent. The NO2 adsorption on metallic nanotubes (m-SWCNTs)was energetically more favorable than that on semiconducting nanotubes (s-SWCNTs) and furthermore the adsorption became less sta-ble with increasing diameters of nanotubes. The adsorption barrier height shows similar dependence on the electronic structure anddiameter to the adsorption energy. On the base of our theoretical model, we have found a method for a diameter-selective removaltetramethylene sulfone (TMS)/chloroform solution with nitronium hexauoroantimonate (NO2SbF6: NHFA) and nitronium tetrauo-roborate (NO2BF4: NTFB). Positively charged nitronium ions were intercalated into nanotube bundles, where the intercalation was pro-moted also by the counter ions. Nitronium ions selectively attacked the sidewall of the m-SWCNTs due to the abundant presence ofelectron density at the Fermi level, thus yielding stronger binding energy compared to the counterpart s-SWCNTs. The s-SWCNTs wereremoving m-SWCNTs was conrmed by the resonant Raman spectra and absorption spectra.

In Situ TEM Observation of Heterogeneous Phase Transition of a Constrained Single-Crystalline Ag₂Te Nanowire

In, Juneho,Yoo, Youngdong,Kim, Jin-Gyu,Seo, Kwanyong,Kim, Hyunju,Ihee, Hyotchel,Oh, Sang Ho,Kim, Bongsoo American Chemical Society 2010 Nano letters Vol.10 No.11

<P>Laterally epitaxial single crystalline Ag<SUB>2</SUB>Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag<SUB>2</SUB>Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2010/nalefd.2010.10.issue-11/nl102350j/production/images/medium/nl-2010-02350j_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl102350j'>ACS Electronic Supporting Info</A></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl102350j'>ACS Electronic Supporting Info</A></P>

Truncated Tetrahedron Seed Crystals Initiating Stereoaligned Growth of FeSi Nanowires

Kim, Si-in,Yoon, Hana,Seo, Kwanyong,Yoo, Youngdong,Lee, Sungyul,Kim, Bongsoo American Chemical Society 2012 ACS NANO Vol.6 No.10

<P>We have synthesized epitaxially grown freestanding FeSi nanowires (NWs) on an <I>m</I>-Al<SUB>2</SUB>O<SUB>3</SUB> substrate by using a catalyst-free chemical vapor transport method. FeSi NW growth is initiated from FeSi nanocrystals, formed on a substrate in a characteristic shape with a specific orientation. Cross-section TEM analysis of seed crystals reveals the crystallographic structure and hidden geometry of the seeds. Close correlation of geometrical shapes and orientations of the observed nanocrystals with those of as-grown NWs indicates that directional growth of NWs is initiated from the epitaxially formed seed crystals. The diameter of NWs can be controlled by adjusting the composition of Si in a Si/C mixture. The epitaxial growth method for FeSi NWs <I>via</I> seed crystals could be employed to heteroepitaxial growth of other compound NWs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2012/ancac3.2012.6.issue-10/nn302141w/production/images/medium/nn-2012-02141w_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn302141w'>ACS Electronic Supporting Info</A></P>