Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea.
Acc Chem Res. 2010 Jan 19;43(1):111-20. doi: 10.1021/ar900156u.
With the advance of nanotechnology, a variety of molecules, from single atoms to large-scale structures such as graphene or carbon nanotubes, have been investigated for possible use as molecular devices. Molecular orbitals (MOs) are a key ingredient in determining the transport properties of molecules, because they contain all the quantum mechanical information of molecular electronic structures and offer spatial conduction channels for electron transport. Therefore, the delicate modulation of the MOs enables us to tune the performance of electron transport through the molecule. Electric and magnetic fields are powerful and readily accessible means for that purpose. In this Account, we describe the effects of external fields on molecular electronic and spintronic devices. Quantum transport through a molecule that connects source and drain electrodes depends strongly on the alignment of molecular energy levels with respect to the chemical potentials at both electrodes. This dependence results from the energy levels being exploited in resonant tunneling processes when the molecule is weakly coupled to the electrodes in the molecular junction. Molecular energy levels can be shifted by the Stark effect of an external electric field. For a molecule with no permanent dipole moment, the polarizability is the primary factor determining the energy shift of each MO, according to the second-order Stark effect; more polarizable MOs undergo a larger energy shift. Interestingly, even a small shift may lead to a completely nontrivial result. For example, we show a magnetic on-off switching phenomenon of a molecule controlled by an electric field. If a molecule has a nonmagnetic ground state but a highly polarizable magnetic excited state with an energy slightly above the ground state, the magnetic excited state can have lower energy than the ground state under a sufficiently strong electric field. A magnetic field is normally used to control spin orientation in a ferromagnetic system. Here we show that the magnetic field can also be used to control MOs. A graphene nanoribbon with zig-zag-shaped edges (ZGNR) has a ferromagnetic spin ordering along the edges, and the spin states have unique orbital symmetries. Both spin polarizations and orbital symmetries can simultaneously be controlled by means of an external magnetic field. The ZGNR spin-valve devices incorporating this effect are predicted to show an extreme enhancement (compared with conventional devices) of magnetoresistance due to the double spin-filtering process. In such a system, spins are filtered not only by spin matching-mismatching between both electrodes as in normal spin-valve devices, but also by the orbital symmetry matching-mismatching. Thus, a new type of magnetoresistance, and with extremely large values, so-called super-magnetoresistance (distinct from the conventional tunneling or giant magnetoresistance), is available with this method. MOs are at the heart of understanding and tuning transport properties in molecular systems. Therefore, investigating the effects of external fields on MOs is important not only for understanding fundamental quantum phenomena in molecular devices but also for practical applications in the development of interactive devices.
随着纳米技术的发展，各种分子，从单原子到石墨烯或碳纳米管等大规模结构，都被研究用于可能作为分子器件。分子轨道（MOs）是确定分子输运性质的关键成分，因为它们包含分子电子结构的所有量子力学信息，并为电子输运提供空间传导通道。因此，MOs 的精细调制使我们能够调整通过分子的电子输运性能。电场和磁场是实现这一目的的强大且易于获得的手段。在本专题介绍中，我们描述了外场对分子电子和自旋电子器件的影响。连接源极和漏极的分子中的量子输运强烈依赖于分子能级相对于两个电极处化学势的对齐。当分子在分子结中与电极弱耦合时，这种依赖性是由于能级在共振隧穿过程中被利用的结果。分子能级可以通过外电场的斯塔克效应移动。对于没有永久偶极矩的分子，根据二阶斯塔克效应，极化率是决定每个 MO 能量位移的主要因素；更具极化率的 MO 经历更大的能量位移。有趣的是，即使是很小的位移也可能导致完全非平凡的结果。例如，我们展示了一种由电场控制的分子的磁开关现象。如果分子具有非磁性基态但具有能量略高于基态的高极化率磁性激发态，则在足够强的电场下，磁性激发态的能量可能低于基态。磁场通常用于控制铁磁系统中的自旋取向。在这里，我们表明磁场也可以用于控制 MOs。具有锯齿形边缘的石墨烯纳米带（ZGNR）具有沿边缘的铁磁自旋有序，并且自旋态具有独特的轨道对称性。通过外加磁场可以同时控制两种自旋极化和轨道对称性。由于双自旋过滤过程，包含这种效应的 ZGNR 自旋阀器件预计会表现出磁电阻的极大增强（与传统器件相比）。在这样的系统中，自旋不仅通过两个电极之间的自旋匹配不匹配（如在常规自旋阀器件中），而且通过轨道对称性匹配不匹配来过滤。因此，通过这种方法可以获得一种新型的磁电阻，并且具有非常大的值，即所谓的超磁电阻（与传统的隧穿或巨磁电阻不同）。MOs 是理解和调整分子系统输运性质的核心。因此，研究外场对 MOs 的影响不仅对于理解分子器件中的基本量子现象很重要，而且对于交互式器件开发中的实际应用也很重要。
