Unconventional Magnetism of Nanographite and Molecular Devices

纳米石墨烯（单层石墨）是一种有趣的基于分子的纳米磁性体系，其特定的电子结构表现出各种独特的磁性特征。采用活性炭纤维（ACF）和热处理纳米金刚石颗粒制备的纳米石墨，研究了纳米石墨烯和网状纳米石墨的磁性能。由纳米金刚石颗粒得到的纳米石墨形成一个内部中空的多面体，其中每个面由3-6个平均尺寸为7nm的纳米石墨烯片堆叠而成。ACFs具有纳米石墨畴的三维网络，其中每个畴由3-4块石墨烯片堆叠而成，平均尺寸为2-3nm。ACFs的电子特性表现为库仑间隙变跳变状态下的安德森绝缘子。在ACFs中的三维纳米石墨网络中，每个纳米石墨畴的自旋为ca.1，其磁性能受边缘态spi的支配，更多的ns服从居里-魏斯定律，负魏斯温度较小，为-2-4K。当客体物质（如水或乙醇）被物理吸附到纳米石墨畴周围的纳米空间（微孔）时，客体物质的阈值会导致边缘自旋磁矩的不连续下降。从容纳在微孔中的客体挤压纳米石墨烯，使得石墨烯层间距离缩小来看，这可以解释为内压诱导纳米石墨烯之间的交换相互作用增强。热处理在1300℃左右引起绝缘体-金属转变，在1300℃以上的磁性可以用泡利顺磁性来描述。在这里，无限渗透路径网络的发展是导致金属态的原因。在绝缘体-金属跃迁附近，在ca.7K以下出现了一种类似自旋玻璃态的新型无序磁性，这是由于非键合π边态自旋之间的交换相互作用强度的随机性，这种相互作用是由传导π电子介导的。上述实验结果揭示了纳米石墨基分子磁体的新型纳米磁性。我们还可以通过电泳处理后的纳米金刚石颗粒的热处理，在石墨衬底上成功地制备出单个纳米石墨烯片。层相对于衬底倾斜的纳米石墨显示出沿倾斜方向周期性变化的电子衍射图样。少

Nano-sized graphene (mono-layer graphite) is an interesting molecule-based nanoscopic magnetic system, in which a variety of unique magnetic features appear in relation to its secific electronic structure. We investigated magnetic properties of nano-graphene and networked nano-graphites using activated carbon fibers (ACF) and nano-graphite prepared by heat-treatment of nano-diamond particles. The nano-graphite obtained from nano-diamond particle forms a polyhedron with a hollow inside, where each facet comprises a stacking of 3-6 nano-graphene sheets with the mean size of 7nm. ACFs are featured with a 3D network of nano-grsphite domains, where each domain is formed with a stacking of 3-4 graphene sheets with the mean size of 2-3nm. The electronic features of ACFs are characterized as Anderson insulator in the Coulomb gap variable hopping regime. In the 3D nano-graphite network in ACFs, where each nano-graphite domain has ca.1 spin, the magnetic properties governed by the edge-state spi … More ns obey the Curie-Weiss law with a small negative Weiss temperature of -2-4K. The physisorption of guest species such as water or ethanol into the nano-space (micropore) surrounding nano-graphite domains brings about a discontinuous drop in the magnetic moment of the edge spin at a threshold amount of guest species. Judging from the fact that guests accommodated in micropores squeeze nano-graphites, which makes the inter-graphene layer distance shrunk, this can be explained with the internal-pressure-induced enhancement of the exchange interaction between nano-graphenes. Heat-treatment induces an insulator-metal transition around 1300℃, above which the magnetism can be described in terms of Pauli paramagnetism. Here, the development of an infinite percolation path network is responsible for the metallic state. In the vicinity of the insulator-metal transition, a novel disordered magnetism similar to spin-glass state appears below ca.7K due to the randomness in the strengths of exchange interactions between non-bonding π edge-state spins, which are mediated by the conduction π-electrons. The experimental findings presented above reveal novel nanoscopic magnetism of nano-graphite-based molecular magnets. We could also successfully prepare a single nano-graphene sheet on a graphite substrate by heat-treatment of a nanodiamond particle after electrophoretic treatment. A nano-graphite with its layer tilted with respect to the substrate show an electron diffraction pattern with the periodicity varying along the tilting direction. Less