Rao C N R, Kalyanikutty K P
Chemistry and Physics of Materials Unit, DST Nanoscience Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India.
Acc Chem Res. 2008 Apr;41(4):489-99. doi: 10.1021/ar700192d. Epub 2008 Mar 12.
Unlike the air-water interface, the organic-aqueous (liquid-liquid) interface has not been exploited sufficiently for materials synthesis. In this Account, we demonstrate how ultrathin nanocrystalline films of metals such as gold and silver as well as of inorganic materials such as semiconducting metal chalcogenides (e.g., CdS, CuS, CdSe) and oxides are readily generated at the liquid-liquid interface. What is particularly noteworthy is that single-crystalline films of certain metal chalcogenides are also obtained by this method. The as-prepared gold films at the toluene-water interface comprise fairly monodisperse nanocrystals that are closely packed, the nature and properties of the films being influenced by various reaction parameters such as reaction temperature, time, reactant concentrations, mechanical vibrations, and the viscosity of the medium. The surface plasmon band of gold is markedly red-shifted in the films due to electronic coupling between the particles. The shift of the surface plasmon band of the Au film toward higher wavelengths with an accompanying increase in intensity as a function of reaction time marks the growth of the film. Depending on the reaction temperature, the Au films show interesting electrical transport properties. Films of metals such as gold are disintegrated by the addition of alkanethiols, the effectiveness depending on the alkane chain length, clearly evidenced by shifts of the surface plasmon bands. A time evolution study of the polycrystalline Au and CdS films as well as the single-crystalline CuS films is carried out by employing atomic force microscopy. X-ray reflectivity studies reveal the formation of a monolayer of capped clusters having 13 gold atoms each, arranged in a hexagonal manner at the toluene-water interface. The measurements also reveal an extremely small value of the interfacial tension. Besides describing features of such nanocrystalline films and their mode of formation, their rheological properties have been examined. Interfacial rheological studies show that the nanocrystalline film of Ag nanoparticles, the single-crystalline CuS film, and the multilayered CdS film exhibit a viscoelastic behavior strongly reminiscent of soft-glassy systems. Though both CuS and CdS films exhibit a finite yield stress under steady shear, the CdS films are found to rupture at high shear rates. An important advantage of the study of materials formed at the liquid-liquid interface is that it provides a means to investigate the interface itself. In addition, it enables one to obtain substrate-free single-crystalline films of materials.
与气-水界面不同，有机-水（液-液）界面在材料合成方面尚未得到充分利用。在本综述中，我们展示了如何在液-液界面轻松生成金、银等金属以及半导体金属硫族化物（如CdS、CuS、CdSe）和氧化物等无机材料的超薄纳米晶薄膜。特别值得注意的是，通过这种方法还能获得某些金属硫族化物的单晶薄膜。在甲苯-水界面制备的金薄膜由紧密堆积的相当单分散的纳米晶体组成，薄膜的性质和特性受各种反应参数影响，如反应温度、时间、反应物浓度、机械振动和介质粘度。由于颗粒间的电子耦合，金的表面等离子体带在薄膜中明显红移。随着反应时间的增加，金薄膜的表面等离子体带向更高波长移动并伴随强度增加，这标志着薄膜的生长。根据反应温度，金薄膜表现出有趣的电输运性质。添加链烷硫醇会使金等金属薄膜分解，其效果取决于烷烃链长度，表面等离子体带的移动清楚地证明了这一点。通过原子力显微镜对多晶金和硫化镉薄膜以及单晶硫化铜薄膜进行了时间演化研究。X射线反射率研究表明，在甲苯-水界面形成了由每个包含13个金原子的封端簇组成的单层，这些簇以六边形方式排列。测量还揭示了界面张力的极小值。除了描述此类纳米晶薄膜的特征及其形成方式外，还研究了它们的流变性质。界面流变学研究表明，银纳米颗粒的纳米晶薄膜、单晶硫化铜薄膜和多层硫化镉薄膜表现出强烈类似于软玻璃体系的粘弹性行为。虽然硫化铜和硫化镉薄膜在稳定剪切下都表现出有限的屈服应力，但发现硫化镉薄膜在高剪切速率下会破裂。研究在液-液界面形成的材料的一个重要优点是，它提供了一种研究界面本身的方法。此外，它使人们能够获得无基底的材料单晶薄膜。
