Spin-dependent phenomena mediated by silicon nanocrystal assemblies

为了鼓励化学反应继续进行，并对其进行控制，一个有希望的策略是为参与反应的分子增加能量，使它们处于更活跃的状态。增加这种能量的一种便捷方式是通过光束。不幸的是，由于基本量子力学的障碍，利用光将能量直接注入分子中往往是不可能的；分子可能根本无法吸收光。避免这一问题的一种方法是将能够吸收光线的纳米颗粒(称为供体)聚集在一起，然后将能量转移到相关分子。这一建议是基于我们的发现，即硅(Si)纳米结构是能量转移到O2和各种有机分子的理想的供体候选者。我们最近在发展独立式球形硅纳米晶方面的进展也支持了这一点。理想的施主纳米颗粒应该具有许多关键特性，而纳米硅满足了所有这些特性。它具有极长的间接激子寿命(从而有效地存储能量)、激子能量可调(1.1-2.5 eV)和较大的比表面积(有利于转移过程)。这种独特的因素组合意味着，例如，即使在室温下，O2分子激发到单线态的效率也是~90%。这个过程伴随着接受能量的分子的自旋翻转激发(通过直接的电子交换机制)，并且应该导致各种自旋允许的光化学过程。因此，含硅纳米晶的光激发体系可以看作是单重态-三重态分裂能低于2.5 eV的分子和团簇的通用自旋翻转激活剂。对于能量转移，相互作用物种的相互自旋取向是至关重要的。我们建议通过磁场和微波实验控制参与的自旋态来研究这一点。值得注意的是，一个很小的磁能(~1 meV)可以通过排列相互作用物种的自旋来有效地控制EV尺度上的能量交换过程。能量传递也可以通过改变硅纳米晶和接受物质之间的间距，或者改变表面势垒的高度和宽度来影响。对这些的研究将使我们能够实现对能量转移过程的完全控制。多孔硅的一个关键特性是它的孔洞几乎完全可以被接受能量的分子填充。因此，光激发和自旋交换过程显著改变了复合纳米硅材料的磁性状态(从顺磁性到抗磁性或反之亦然)。外加磁场应该导致自旋排列，从而改变材料的磁性状态。这将产生法拉第旋转和磁光克尔效应，我们将对此进行研究。单态有机分子和三态O2分子之间的化学反应形成新的单态有机分子是自旋选择规则所禁止的。因此，O2分子的三重态是大多数O2与有机物质在室温下不发生反应的原因。硅纳米晶既可以介导O2分子的单重态激发，也可以介导有机分子的三重态激发。因此，如果只改变一种物质的自旋状态，就可以克服氧化反应的自旋选择规则。将使用光漂白实验和红外吸收光谱来研究被激发的O2或有机分子的光诱导化学反应能力，以监测有机分子的氧化并识别反应物和产物。由于可规模化生产硅纳米晶体组件是可行的，这些纳米硅基复合材料系统对最终的“绿色化学”工业发展具有真正的潜力。

To encourage chemical reactions to proceed, and to exercise control over them, a promising strategy is to add energy to the molecules involved in the reaction, putting them in a more reactive state. A convenient way to add this energy would be via a beam of light. Unfortunately, it is often impossible to pump energy directly into a molecule using light because of obstacles due to fundamental quantum mechanics; the molecule may simply not be able to absorb the light. A way of avoiding this problem is to bring together a nanoparticle (termed a donor) that is able to absorb the light and then transfer that energy to the molecule in question. This proposal is based on our discovery that silicon (Si) nanostructures are ideal candidates to donors for energy transfer to, e.g, O2 and a variety of organic molecules. It is also supported by our recent advances in the development of freestanding spherical Si nanocrystals. The ideal donor nanoparticle should have many key characteristics and nanosilicon satisfies all of them. It has an extremely long indirect exciton lifetime (thus storing the energy effectively), tunable energy of excitons (1.1-2.5 eV) and a large surface area (facilitating the transfer process). This unique combination of factors means that, for instance, the efficiency of excitation of O2 molecules to singlet states is found to be ~ 90 % even at room temperature. This process is accompanied by a spin-flip excitation of energy-accepting molecules (via a direct electron exchange mechanism) and should result in a variety of spin allowed photo-chemical processes. Therefore photoexcited systems containing Si nanocrystals can be viewed as universal spin-flip activators for molecules and clusters having singlet-triplet splitting energies below 2.5 eV. For energy transfer, the mutual spin orientation of interacting species is crucial. We propose to investigate this through control of the participating spin states by magnetic field and microwave experiments. Remarkably, a very small magnetic energy (~1 meV) should efficiently control the energy exchange processes at the scale of eV by aligning the spins of the interacting species. Energy transfer can also be affected via variation of spacing between Si nanocrystals and accepting substances, or by variation of the surface potential barrier height and width. Studies of these will allow us to achieve full control over energy transfer process. One key property of porous Si is that its pores can be almost completely be filled by energy accepting molecules. Optical excitation followed by the spin-exchange process thus significantly modifies the magnetic state of composite nanosilicon materials (from para- to diamagnetic or vice versa). An applied magnetic field should result in spin alignment and thus modify the magnetic state of the material. This will produce Faraday rotation and magneto-optic Kerr effects, which we will study. Chemical reactions between singlet organic molecules and triplet O2 molecules forming new singlet organic molecules are forbidden by spin selection rules. Thus, the triplet multiplicity of O2 molecules is the reason why most reactions do not occur between O2 and organic substances at room temperature. Si nanocrystals can mediate either the excitation of O2 molecules into singlet states or of organic molecules into triplet states. Thus spin selection rules for oxidation reactions can be overcome if the spin state of only one substance is changed. The light-induced chemical reactivity of excited O2 or organic molecules will be studied using photo-bleaching experiments and infrared absorption spectroscopy to monitor the oxidation of the organic molecules and to identify reactants and products. Because scalable production of Si nanocrystal assemblies is feasible, these nanosilicon-based composites systems have real potential for eventual "green chemistry" industrial development.