Article pubs.acs.org/JACS
Intramolecular Insight into Adsorbate−Substrate Interactions via Low-Temperature, Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy Jordan M. Klingsporn,†,⊥ Nan Jiang,†,⊥ Eric A. Pozzi,† Matthew D. Sonntag,† Dhabih Chulhai,‡ Tamar Seideman,† Lasse Jensen,‡ Mark C. Hersam,*,†,§ and Richard P. Van Duyne*,†,∥ †
Northwestern University, Department of Chemistry, 2145 Sheridan Road, Evanston, IL 60208, United States The Pennsylvania State University, Department of Chemistry, University Park, PA, 16802, United States § Northwestern University, Department of Materials Science and Engineering, 2220 Campus Drive, Evanston, IL 60208, United States ∥ Northwestern University, Department of Biomedical Engineering, 2145 Sheridan Road, Evanston, IL 60208, United States ‡
S Supporting Information *
ABSTRACT: Tip-enhanced Raman spectroscopy (TERS) provides chemical information for adsorbates with nanoscale spatial resolution, single-molecule sensitivity, and, when combined with scanning tunneling microscopy (STM), Ångstrom-scale topographic resolution. Performing TERS under ultrahigh-vacuum conditions allows pristine and atomically smooth surfaces to be maintained, while liquid He cooling minimizes surface diffusion of adsorbates across the solid surface, allowing direct STM imaging. Low-temperature TER (LT-TER) spectra differ from room-temperature TER (RTTER), RT surface-enhanced Raman (SER), and LT-SER spectra because the vibrational lines are narrowed and shifted, revealing additional chemical information about adsorbate− substrate interactions. As an example, we present LT-TER spectra for the rhodamine 6G (R6G)/Ag(111) system that exhibit such unique spectral shifts. The high spectral resolution of LT-TERS provides intramolecular insight in that the shifted modes are associated with the ethylamine moiety of R6G. LT-TERS is a promising approach for unraveling the intricacies of adsorbate− substrate interactions that are inaccessible by other means. magnetic (EM) field to provide nanoscale spatial resolution. The high EM field intensity at the tip−sample junction provides enhancements of the Raman signals that are sufficient to provide ultrahigh sensitivity down to the SM level.1−5 Most of the work in the TERS field has concentrated on studies performed under ambient conditions.6−10 Performing TERS in ultrahigh vacuum (UHV) can minimize contamination and maximize STM spatial resolution. The first TERS experiment performed in UHV2 showed SMTER spectra that were dominated by a single vibrational mode for brilliant cresyl blue (BCB) dye molecules deposited from solution on Au(111). The first multivibrational mode UHV-TERS experiment, along with complete UHV sample preparation,11 was performed for copper phthalocyanine (CuPc) on Ag(111). The recent report of UHV-TERS at 80 K for the meso-tetrakis(3,5di-tert-butylphenyl)porphyrin (H2TBPP)/Ag(111) system has further energized this field with its demonstration of sub-
1. INTRODUCTION Understanding the nature of molecular adsorption geometries and the attendant adsorbate−surface interactions is of fundamental importance in the development of technologies such as dye-sensitized solar cells, organic photovoltaics, heterogeneous catalysts, and molecular electronics. Adsorbate−surface interactions are inherently heterogeneous because of the number of crystallographically distinct surface sites available as well as the various adsorbate−adsorbate interactions possible. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) provide unprecedented detail about these interactions at the single-molecule (SM) and single-site level for “flat” molecules. Such direct high-resolution imaging is compromised for molecules that have complex 3D adsorption geometries. Tip-enhanced Raman spectroscopy (TERS) is an established tool for surface science that can probe the details of adsorbate−surface interactions for individual molecules adsorbed at individual surface sites. TERS, unlike surface-enhanced Raman spectroscopy (SERS), utilizes a controllable hot spot formed at the tip−sample junction. This hot spot allows nanofocusing of the electro© 2014 American Chemical Society
Received: November 27, 2013 Published: February 18, 2014 3881
dx.doi.org/10.1021/ja411899k | J. Am. Chem. Soc. 2014, 136, 3881−3887
Journal of the American Chemical Society
Article
Figure 1. (a) STM topograph of sub-monolayer R6G molecules on a Ag(111) surface measured with a Ag tip. The tip was parked at the lower left corner of the image during the TERS collection (imaging conditions: 2.0 V, 10 pA, 50 nm × 50 nm). (b) Zoom-in STM topograph of individual R6G molecules and clusters (imaging conditions: 2.0 V, 50 pA, 15 nm × 15 nm). (c) Molecular model showing the dimensions of R6G. (d) Line profile across a single R6G molecule. (e) LT-SERS of R6G on an AgFON: λex = 532 nm, Pacq = 47 μW, tacq = 0.1 s × 100. (f) Engaged and retracted RT-TERS of R6G on Ag(111): λex = 532 nm, Pacq = 500 μW, tacq = 10 s × 30. (g) Engaged and retracted LT-TERS of R6G on Ag(111): λex = 532 nm, Pacq = 2 mW, tacq = 60 s.
nanometer spatial resolution.5 However, the effect of temperature on the TERS signal has not yet been quantified. Here we present low-temperature (LT) (19 K), UHV (99%) was introduced into the UHV chamber in an alumina-coated tungsten filament sublimation module. The powder was degassed prior to deposition via thermal sublimation at 430 K onto Ag(111), which was kept at room temperature (292 K). LT-TERS and STM were performed using liquid He cooling. With the sample and probe in the STM cryostat, liquid He was delivered to the cryostat via a cold finger. Initially, cryostat shutters covering the sapphire windows on either side of the cryostat were kept closed while the sample was brought down to 8 K. The shutters were then opened to allow optical access to the tip− sample junction, leading to a temperature increase due to thermal radiation, and the microscope was allowed to stabilize at 19 K. Illumination of the tip−sample junction using typical experimental parameters showed no measurable increase in the temperature of the sample. STM voltages were applied to the sample with respect to the grounded tip, and images were acquired in constant-current mode. Tip Preparation. Ag probes were electrochemically etched from 250 μm silver wire (99.9%, Alfa Aesar) under potentiostatic control using the method previously described.11 Etched probes were rinsed with warm deionized water (Millipore) followed by ethanol and immediately introduced into the UHV chamber for use without further processing. Spectroscopy. Optics external to the UHV chamber were used for laser (Spectra-Physics, 532 nm) excitation and collection of scattered light (Supporting Information S1). A glass viewport and two sapphire windows afforded optical access to the tip−sample junction. The laser beam was expanded to approximately 25 mm in diameter and focused at the tip−sample junction. Excitation and collection were accomplished through opposing viewports with excitation and collection both centered at approximately 75° with respect to the surface normal. Laser excitation was polarized along the axis of the tip and stabilized (Brockton Electro-Optics Corp.) to minimize intensity fluctuations. A high-resolution witness camera (Uniq, UM-300) was
2. EXPERIMENTAL SECTION Sample Preparation. Spectroscopy and imaging were performed with a home-built, cryogenic variable-temperature UHV-STM instrument at a base pressure in the low 10−11 Torr range.11 Clean Ag(111) (Princeton Scientific Corp., one side polished
