Li Q, Walter E C, van der Veer W E, Murray B J, Newberg J T, Bohannan E W, Switzer J A, Hemminger J C, Penner R M
Department of Chemistry, University of California, Irvine, California 92679-2025, USA.
J Phys Chem B. 2005 Mar 3;109(8):3169-82. doi: 10.1021/jp045032d.
Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoO(x) wires (a mixture of MoO(2) and MoO(3)) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoO(x) precursor wires were converted to MoS(2) by exposure to H(2)S either at 500-700 degrees C, producing "low-temperature" or LT MoS(2) nanowires that were predominantly 2H phase, or above 800 degrees C producing "high-temperature" or HT MoS(2) ribbons that were predominantly 3R phase. The majority of these MoS(2) wires and ribbons were more than 50 microm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS(2) nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV-visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS(2) nanowires were structurally distinct: LT MoS(2) wires were hemicylindrical in shape and nearly identical in diameter to the MoO(x) precursor wires from which they were derived. LT MoS(2) wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS(2); HT MoS(2) ribbons were 50-800 nm in width and 3-100 nm thick, composed of planar crystallites of 3R-MoS(2). These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS(2) unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS(2) wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS(2) nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS(2). These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS(2) layers). The energy shifts were proportional to 1/ L( parallel)(2), and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS(2) nanostructures were probed as a function of temperature from -33 degrees C to 47 degrees C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.
通过两步电化学/化学合成方法合成了二硫化钼纳米线和纳米带。第一步，通过电化学台阶边缘修饰在高度取向热解石墨（HOPG）表面尺寸选择性地电沉积MoO(x)线（MoO₂和MoO₃的混合物）。然后，通过在500 - 700℃暴露于H₂S将MoO(x)前驱体线转化为MoS₂，生成主要为2H相的“低温”或LT MoS₂纳米线，或在800℃以上生成主要为3R相的“高温”或HT MoS₂带。这些MoS₂线和带中的大多数长度超过50微米，并排列成包含数百根线或带的平行阵列。通过X射线光电子能谱、扫描和透射电子显微镜、选区电子衍射、X射线衍射、紫外 - 可见吸收光谱和拉曼光谱对MoS₂纳米结构进行了表征。HT和LT MoS₂纳米线在结构上不同：LT MoS₂线呈半圆柱形，直径与从中衍生的MoO(x)前驱体线几乎相同。LT MoS₂线是多晶的，内部结构由许多相互交织的多层MoS₂股组成；HT MoS₂带宽度为50 - 800纳米，厚度为3 - 100纳米，由3R - MoS₂的平面微晶组成。这些层以范德华接触方式生长在HOPG表面，使得3R - MoS₂晶胞的c轴垂直于石墨表面平面取向。MoS₂线和带的阵列可以从HOPG表面干净地分离并转移到玻璃上进行电学和光学表征。HT MoS₂纳米带的光吸收测量揭示了在1.95 eV附近的直接带隙以及两个激子峰A1和B1，这是3R - MoS₂的特征。当线厚度减小到7纳米（十一层MoS₂）时，这些激子峰向更高能量移动高达80 meV。能量移动与1/L(平行)²成比例，并计算了有效质量。对LT和HT MoS₂纳米结构的电流 - 电压曲线在 - 33℃至47℃的温度范围内进行了探测。传导是欧姆性的，主要由沿导线存在的晶界控制。发现热激活势垒与微晶的有序度有关，并且可以从LT纳米线的126 meV调整到HT纳米带的26 meV。
