Importance of Correctly Describing Charge-Transfer Excitations for Understanding the Chemical Effect in SERS Justin E. Moore, Seth M. Morton, and Lasse Jensen* Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, United States * S Supporting Information ABSTRACT: The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) is governed to a large extent by the energy difference between the highest occupied molecular orbital (HOMO) of the metal and the lowest unoccupied molecular orbital (LUMO) of the molecule. Here, we investigate the importance of correctly describing charge-transfer excitations, using time- dependent density functional theory (TDDFT), when calculating the chemical coupling in SERS. It is well-known that TDDFT, using traditional functionals, underestimates the position of charge-transfer excitations. Here, we show that this leads to a significant overestimation of the chemical coupling mechanism in SERS. Significantly smaller enhancements are found using long-range corrected (LC) functionals as compared with a traditional generalized gradient approximation (GGA) and hybrid functionals. Enhancement factors are found to be smaller than 530 and typically less than 50. Our results show that it is essential to correctly describe charge-transfer excitations for predicting the chemical enhancement in SERS. SECTION: Plasmonics, Optical Materials, and Hard Matter M etal nanoparticles that can support plasmon excitations are known to generate large local fields at the surface of the particles. This strong local field can greatly affect the optical properties of molecules situated near the metal surface and has led to a whole range of surface-enhanced spectroscopic techniques, with surface-enhanced Raman scattering (SERS) 1-4 being the best known. These techniques all rely on the strong local field to enhance the optical properties of molecules near the surface of the metal nanoparticle. Using SERS, it is possible to detect and identify a single molecule due to the large enhancements. 5-9 The fundamental enhancement mechanisms for SERS have been extensively studied over the last 4 decades, and it is generally accepted that there are two mechanisms contributing to SERS, 1-4,10-13 (1) the electromagnetic mechanism (EM) due to the strong local field and (2) the chemical mechanism (CM). The largest enhancements in SERS stem from EM, which is generally considered to be independent of the molecule. In contrast, the CM depends strongly on the specific molecule and the local environment of the metal surface because it results from the overlap between the wave functions of the molecule and the metal nanoparticle. This overlap results in a renormalization of the molecular orbitals as well as the introduction of new mixed charge-transfer states. Both of these effects will contribute to the CM enhancement of the Raman signal and can be classified as the nonresonant chemical mechanism (CHEM) and a resonant charge-transfer chemical mechanism (CT), respectively. 2,13,14 In the following, we will only consider the nonresonant mechanism that is expected to be important when the incident light is not resonant with a molecular or charge-transfer excitation of the system. Using a two-state model, Morton and Jensen 14 found that CHEM is related to orbital interactions between the metal and molecule complex and scales as (ω X /ω e ) 4 , where ω X is the HOMO-LUMO excitation energy of the free molecule and ω e is lowest charge-transfer excitation energy of the metal- molecule complex. Similar expressions have recently been shown to describe the chemical enhancement in surface- enhanced hyper-Raman scattering 15 and coherent anti-Stokes Raman scattering. 16 This model has also been extended to describe the enhancement of individual normal modes by considering the deformation potential. 17 To correctly describe the chemical enhancement in SERS, it is important to correctly describe the CT excitations. Due to the relatively large size of the systems needed for describing the chemical mechanism in SERS, electronic structure studies have been predominately performed using time-dependent density functional theory (TDDFT). 2,13 Although TDDFT has been demonstrated to be accurate for many molecular systems, it is well-known that conventional exchange-correlation (XC) functionals have certain categorical failures, such as describing CT excitations between weakly interacting systems and in condensed-phase systems. 18-21 Recently, progress has been made with the introduction of Received: April 21, 2012 Accepted: August 18, 2012 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 2470 dx.doi.org/10.1021/jz300492p | J. Phys. Chem. Lett. 2012, 3, 2470-2475