Morton Seth M, Jensen Lasse
The Pennsylvania State University, Department of Chemistry, 104 Chemistry Building, University Park, Pennsylvania 16802, USA.
J Am Chem Soc. 2009 Mar 25;131(11):4090-8. doi: 10.1021/ja809143c.
The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized using time-dependent density functional theory. This has been achieved with a systematical study of the chemical enhancement of meta- and para-substituted pyridines interacting with a small silver cluster (Ag(20)). Changing the functional groups on pyridine enabled us to modulate the direct chemical interactions between the pyridine ring and the metal cluster. Surprisingly, we find that the enhancement does not increase as more charge is transferred from the pyridine ring to the cluster. Instead, we find that the magnitude of chemical enhancement is governed to a large extent by the energy difference between the highest occupied energy level (HOMO) of the metal and the lowest unoccupied energy level (LUMO) of the molecule. The enhancement scales roughly as (omega(X)/omega(e))(4), where omega(e) is an average excitation energy between the HOMO of the metal and the LUMO of the molecule and omega(X) is the HOMO-LUMO gap of the free molecule. The trend was verified by considering substituted benzenethiols, small molecules, and silver clusters of varying sizes. The results imply that molecules that show significant stabilization of the HOMO-LUMO gaps (such as those that readily accept pi-backbonding) would be likely to have strong chemical enhancement. The findings presented here provide the framework for designing new molecules which exhibit high chemical enhancements. However, it remains a challenge to accurately describe the magnitude of the Raman enhancements using electronic structure methods, especially density functional theory, because they often underestimate the energy gap.
利用含时密度泛函理论对表面增强拉曼散射（SERS）中分子 - 表面化学耦合引起的增强机制进行了表征。通过对间位和对位取代吡啶与小银簇（Ag(20)）相互作用的化学增强进行系统研究实现了这一点。改变吡啶上的官能团使我们能够调节吡啶环与金属簇之间的直接化学相互作用。令人惊讶的是，我们发现随着更多电荷从吡啶环转移到簇上，增强并没有增加。相反，我们发现化学增强的幅度在很大程度上由金属的最高占据能级（HOMO）与分子的最低未占据能级（LUMO）之间的能量差决定。增强大致按(ω(X)/ω(e))(4)缩放，其中ω(e)是金属的HOMO与分子的LUMO之间的平均激发能，ω(X)是自由分子的HOMO - LUMO能隙。通过考虑取代苯硫醇、小分子和不同尺寸的银簇验证了这一趋势。结果表明，那些显示出HOMO - LUMO能隙显著稳定的分子（例如那些容易接受π反馈键合的分子）可能具有很强的化学增强。这里提出的研究结果为设计具有高化学增强的新分子提供了框架。然而，使用电子结构方法，特别是密度泛函理论准确描述拉曼增强的幅度仍然是一个挑战，因为它们常常低估能隙。
