Chemically Induced Magnetism in Atomically Precise Gold Clusters
Chemically Induced Magnetism in Atomically Precise Gold Clusters
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DOI：
10.1002/smll.201302393
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发表时间：
2014-03-01
期刊：
影响因子：
13.3
通讯作者：
Kumar, Challa S. S. R.
中科院分区：
文献类型：
--
作者：
Krishna, Katla Sai;Tarakeshwar, Pilarisetty;Kumar, Challa S. S. R.
最近的研究表明，各种纳米级材料（金属纳米颗粒、金属氧化物纳米颗粒、纳米晶体薄膜）都具有意想不到且不寻常的磁性行为，而这些材料在体相中是抗磁性（即非磁性）的。 [1]其中最突出的是配体稳定的铁磁贵金属纳米颗粒，例如 Au、Ag、Cu 和 Pt。[2, 3] 已经研究了许多由不同配体稳定的金纳米颗粒的磁性行为，现在有充分证据表明磁性是化学诱导的并且强烈依赖于尺寸。[4]这些磁性本质上与其电子结构有关，电子结构受到纳米颗粒尺寸和装饰其的配体性质的影响。 [5]虽然强结合的直径为 1.4 nm 的十二烷硫醇封端的金纳米粒子表现出铁磁性，但弱结合的四辛基铵封端的 1.5 nm 金纳米粒子却具有抗磁性。 [6]我们最近还证明了肽帽金纳米壳（约 0.5 nm 厚）的铁磁性，以及通过稳定配体的逐步功能化来调节其磁性行为的可能性。 [7]通常观察到，金纳米颗粒在室温下的铁磁性在硫醇稳定的金纳米颗粒中占主导地位。 [8]盖帽金纳米粒子磁化开始涉及的关键电子事件是与参与配体化学键的金原子的 5 d 和 6 s 电子相关的自旋对称性破缺，这反过来又改变了费米能处的相对自旋密度，从而产生非零磁矩和相应的磁化强度。 Miyake 和同事最近报道了十二烷硫醇封端的金纳米粒子的铁磁性的直径依赖性（尺寸效应）。 [9]表 S1（支持信息）总结了先前报道的有关在配体稳定的金纳米颗粒中观察到的尺寸依赖性磁性的文献。从之前发表的结果中可以得出金纳米粒子的尺寸和磁性行为之间的简单相关性。在一些情况下，尺寸约为 2 至 3 nm 的硫醇封端的金纳米颗粒主要表现出铁磁行为。当金属纳米颗粒进入量子尺寸范围（通常< 2 nm）时，它们的特性对颗粒尺寸极其敏感。例如，由于能谱的离散化，主导较大金纳米颗粒光谱的特征表面等离子体共振（SPR）带被阶梯式多带取代。 [10, 11] 过去几年，从最初的 Brust 方法 [12] 开始，在纳米颗粒合成中实现原子水平控制方面取得了令人瞩目的进展，随后由 Jin 和同事 [13] 开发了尺寸聚焦方法，并由其他研究人员进一步扩展。 [14]尽管在研究配体稳定的金纳米颗粒的尺寸控制光学和磁性行为方面取得了所有这些重大进展，但它们在尺寸和原子精度方面仍然或多或少存在异质性。令人惊讶的是，目前还没有发表的报告关注小于 2 nm 的原子精确金簇 (APGC) 的实验磁性行为。这个方向的研究极其重要，因为配体稳定的 APGC 提供了一种化学开启和调整其磁性的方法，从而提供了定制原子级精确纳米磁体的机会。以原子精度探测磁性的能力迄今为止是一个未知的研究领域。作为潜力的一个例子......
Recent investigations demonstrate unexpected and unusual magnetic behavior in a wide range of nanoscale materials–metal nanoparticles, metal oxide nanoparticles, nanocrystalline films-which are otherwise diamagnetic (ie nonmagnetic) in their bulk phase.[1] Most prominent among these are ligand stabilized ferromagnetic noble metal nanoparticles such as Au, Ag, Cu and Pt.[2, 3] A number of gold nanoparticles stabilized by different ligands have been investigated for their magnetic behavior and it is now well documented that the magnetism is chemically-induced and strongly sizedependent.[4] These magnetic properties are intrinsically related to their electronic structure, which is influenced both by the size of the nanoparticle and the nature of the ligand decorating it.[5] While strongly binding dodecanethiol-capped Au nanoparticles of 1.4 nm in diameter exhibit ferromagnetism, a weakly binding tetraoctylammonium capped∼ 1.5 nm Au nanoparticle is diamagnetic.[6] We have also recently demonstrated ferromagnetism in peptide-capped gold nanoshells (∼ 0.5 nm thick) and the possibility to modulate their magnetic behavior by step-wise functionalization of stabilizing ligands.[7] It is generally observed that ferromagnetism at room temperature in gold nanoparticles prevails in thiol-stabilized Au nanoparticles.[8] The crucial electronic event involved on the onset of magnetization in capped gold nanoparticles is a spin symmetry breaking associated with the 5 d and 6 s electrons of the Au atoms involved in the chemical bond with the ligands, which in turn modifies the relative spin densities at the Fermi energy thus creating a non-zero magnetic moment and a corresponding magnetization. Miyake and coworkers have recently reported diameter dependence (size effect) on ferromagnetism of dodecanethiol-capped gold nanoparticles.[9] Table S1 (Supporting Information) summarizes the previously reported literature on size-dependent magnetic properties observed in ligand-stabilized gold nanoparticles. A simple correlation between size and magnetic behavior in gold nanoparticles from the previously published results can be drawn. In several instances, thiol-capped gold nanoparticles of size around 2 to 3 nm have predominantly exhibited ferromagnetic behavior. When metal nanoparticles enter the quantum size regime (typically< 2 nm), their properties are extremely sensitive to the particle size. For instance, the characteristic surface Plasmon resonance (SPR) band which dominates the optical spectrum of larger gold nanoparticles is replaced by steplike multiband, due to the discretization of the energy spectrum.[10, 11] Impressive progress has been made in the past few years in bringing atomic level control in the synthesis of nanoparticles starting from the original Brust's method [12] followed by size focusing methodology developed by Jin and coworkers [13] and expanded further by other researchers.[14] Despite all this significant progress in studying the size-controlled optical and magnetic behavior in ligand-stabilized gold nanoparticles, they are still more or less heterogeneous in terms of size with respect to atomic precision. Surprisingly, there are no published reports focusing on the experimental magnetic behavior of atomically precise gold clusters (APGCs) which are< 2 nm. Investigations in this direction are extremely important as ligand stabilized APGCs provide a means to chemically turn-on and tune-in their magnetism and thereby providing an opportunity to tailor-make atomically precise nanomagnets. The ability to probe magnetism with atomic precision is hitherto an unchartered area of investigation. As an example of the potential of …