Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy.

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy with the single-molecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy. Here, we use these techniques in combination with theory to reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules.

Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy.

Molecule-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, organic photovoltaics, and nanoelectronics, yet they are rarely well understood at the molecular level. Given the inhomogeneous nature of surfaces, molecular properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resolution comparable to the size of surface sites under study, analytical power capable of examining chemical properties, and single-molecule sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for molecular-resolution imaging, low-temperature operation, minimized tip and molecular degradation, and improved stability in the presence of ultrafast irradiation. The aim of this review is to give an overview of TERS experiments performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-molecule properties, reactions, and dynamics with spatial resolution below 1 nm.

Conformational Contrast of Surface-Mediated Molecular Switches Yields Ångstrom-Scale Spatial Resolution in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy.

Tip-enhanced Raman spectroscopy (TERS) combines the ability of scanning probe microscopy (SPM) to resolve atomic-scale surface features with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). Here, we report additional insights into the nature of the conformational dynamics of a free-base porphyrin at room temperature adsorbed on a metal surface. We have interrogated the conformational switch between two metastable surface-mediated isomers of meso-tetrakis(3,5-ditertiarybutylphenyl)-porphyrin (H2TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H2TBPP-Cu(111) system is easily overcome, and a 2.6 Å lateral resolution by simultaneous TERS and STM analysis is achieved under ultrahigh vacuum (UHV) conditions. This work demonstrates the first UHV-TERS on Cu(111) and shows TERS can unambiguously distinguish the conformational differences between neighboring molecules with Ångstrom-scale spatial resolution, thereby establishing it as a leading method for the study of metal-adsorbate interactions.

Operational Regimes in Picosecond and Femtosecond Pulse-Excited Ultrahigh Vacuum SERS.

We report a systematic study performed in ultrahigh vacuum designed to identify the laser excitation regimes in which plasmonically enhanced ultrashort pulses may be used to nondestructively probe surface-bound molecules. A nondestructive, continuous-wave spectroscopic probe is used to monitor the effects of four different femtosecond- and picosecond-pulsed beams on the SER signals emanating from molecular analytes residing within plasmonically enhanced fields. We identify the roles of plasmonic amplification and alignment with a molecular electronic transition on the observed changes in the SER signals. Our results indicate that overlap of the laser wavelength with the plasmon resonance is the dominant contributor to signal degradation. In addition, signal loss for a given irradiation condition is observed only for molecules residing in hot spots above a threshold enhancement. Identification of suitable laser energy density ranges demonstrates the importance of considering these parameters when implementing SERS in the presence of pulsed irradiation.

Nanoscale Chemical Imaging of a Dynamic Molecular Phase Boundary with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy.

Nanoscale chemical imaging of a dynamic molecular phase boundary has broad implications for a range of problems in catalysis, surface science, and molecular electronics. While scanning probe microscopy (SPM) is commonly used to study molecular phase boundaries, its information content can be severely compromised by surface diffusion, irregular packing, or three-dimensional adsorbate geometry. Here, we demonstrate the simultaneous chemical and structural analysis of N-N'-bis(2,6-diisopropylphenyl)-1,7-(4'-t-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI) molecules by UHV tip-enhanced Raman spectroscopy. Both condensed and diffusing domains of PPDI coexist on Ag(100) at room temperature. Through comparison with time-dependent density functional theory simulations, we unravel the orientation of PPDI molecules at the dynamic molecular domain boundary with unprecedented ∼4 nm spatial resolution.

Selective Crystal Growth and Structural, Optical, and Electronic Studies of Mn3Ta2O8.

Mn3Ta2O8, a stable targeted material with an unusual and complex cation topology in the complicated Mn-Ta-O phase space, has been grown as a ≈3-cm-long single crystal via the optical floating-zone technique. Single-crystal absorbance studies determine the band gap as 1.89 eV, which agrees with the value obtained from density functional theory electronic-band-structure calculations. The valence band consists of the hybridized Mn d-O p states, whereas the bottom of the conduction band is formed by the Ta d states. Furthermore, out of the three crystallographically distinct Mn atoms that are four-, seven-, or eight-coordinate, only the former two contribute their states near the top of the valence band and hence govern the electronic transitions across the band gap.

Molecular-Resolution Interrogation of a Porphyrin Monolayer by Ultrahigh Vacuum Tip-Enhanced Raman and Fluorescence Spectroscopy.

Tip-enhanced Raman scattering (TERS) and optically excited tip-enhanced fluorescence (TEF) of a self-assembled porphyrin monolayer on Ag(111) are studied using an ultrahigh vacuum scanning tunneling microscope (UHV-STM). Through selectively exciting different Q-bands of meso-tetrakis- (3,5-ditertiarybutylphenyl)-porphyrin (H2TBPP), chemical information regarding different vibronic excited states is revealed by a combination of theory and experiment; namely, TERS and time-dependent density functional theory (TDDFT) simulations. The observed TEF spectra suggest a weak coupling of H2TBPP to the substrate due to the bulky t-butyl groups and a possible alternative excited state decay path. This work demonstrates the potential of combining TERS and TEF for studying surface-mounted porphyins on substrates, thus providing insight into porphyrin-sensitized solar cells and catalysis.

The [U2(µ-S2)2Cl8](4-) anion: synthesis and characterization of the uranium double salt Cs5[U2(µ-S2)2Cl8]I.

Red plates of Cs5[U2(µ-S2)2Cl8]I were obtained in good yield from the reaction at 1173 K of U, GeI2 or SnI4, and S, with CsCl flux. The compound crystallizes in space group D2h25-Immm of the orthorhombic system in the Cs5[Nb2(µ-S2)2Cl8]Cl structure type. The centrosymmetric [U2(µ-S2)2Cl8]4­ anion in the structure has mmm symmetry with the two U4+ atoms separated by 3.747(1) Å. Each U atom is coordinated to four Cl atoms and four S atoms from two S22­ groups in a square-antiprismatic arrangement. The polarized absorbance spectra of Cs5[U2(µ-S2)2Cl8]I display prominent optical anisotropy. Magnetic measurements are consistent with the modified Curie­Weiss law at high temperatures. The low-temperature behavior may arise from antiferromagnetic coupling of the U4+ ions within the anion.

Positional flexibility: syntheses and characterization of six uranium chalcogenides related to the 2H hexagonal perovskite family.

Six new uranium chalcogenides, Ba4USe6, Ba3FeUSe6, Ba3MnUSe6, Ba3MnUS6, Ba3.3Rb0.7US6, and Ba3.2K0.8US6, related to the 2H hexagonal perovskite family have been synthesized by solid-state methods at 1173 K. These isostructural compounds crystallize in the K4CdCl6 structure type in space group D3d6­R3̅c of the trigonal system with six formula units per cell. This structure type is remarkably flexible. The structures of Ba3FeUSe6, Ba3MnUSe6, and Ba3MnUS6 consist of infinite ∞1[MUQ66­] chains (M = Fe or Mn; Q = S or Se) oriented along the c axis that are separated by Ba atoms. These chains are composed of alternating M-centered octahedra and U-centered trigonal prisms sharing triangular faces; in contrast, in the structures of Ba4USe6, Ba3.3Rb0.7US6, and Ba3.2K0.8US6, there are U-centered octahedra alternating with Ba-, Rb-, or K-centered trigonal prisms. Moreover, the Ba4USe6, Ba3FeUSe6, Ba3MnUSe6, and Ba3MnUS6 compounds contain U4+, whereas Ba3.3Rb0.7US6 and Ba3.2K0.8US6 are mixed U4+/5+ compounds. Resistivity and µ-Raman spectroscopic measurements and DFT calculations provide additional insight into these interesting subtle structural variations.

Intramolecular insight into adsorbate-substrate interactions via low-temperature, ultrahigh-vacuum tip-enhanced Raman spectroscopy.

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 (RT-TER), 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.

Polar alignment of Λ-shaped basic building units within transition metal oxide fluoride materials.

A series of pseudosymmetrical structures of formula K10(M2OnF11-n)3X (M = V and Nb, n = 2, X = (F2Cl)1/3, Br, Br4/2,I4/2; M = Mo, n = 4, X = Cl, Br4/2, I4/2) illustrates generation of polar structures with the use of Λ-shaped basic building units (BBUs). For a compound to belong to a polar space group, dipole moments of individual species must be partially aligned. Incorporation of d(0) early transition metal polyhedral BBUs into structures is a common method to create polar structures, owing to the second-order Jahn-Teller distortion these polyhedra contain. Less attention has been spent examining how to align the polar moments of BBUs. To address alignment, we present a study on previously reported bimetallic BBUs and synthesized compounds K10(M2OnF11-n)3X. These materials differ in their (non)centrosymmetry despite chemical and structural similarities. The vanadium compounds are centrosymmetric (space groups P3m1 or C2/m) while the niobium and molybdenum heterotypes are noncentrosymmetric (Pmn21). The difference in symmetry occurs owing to the presence of linear, bimetallic BBUs or Λ-shaped bimetallic BBUs and related packing effects. These Λ-shaped BBUs form as a consequence of the coordination environment around the bridging anion of the metal oxide fluoride BBUs.

Recent Advances in Tip-Enhanced Raman Spectroscopy.

Tip-enhanced Raman spectroscopy (TERS) has experienced tremendous growth in the last 5 years. Specifically, TER imaging has provided invaluable insight into the spatial distribution and properties of chemical species on a surface with spatial resolution that is otherwise unattainable by any other analytical method. Additionally, there has been further development in coupling ultrafast spectroscopy with TERS in the hope of obtaining both ultrafast temporal and nanometer-scale spatial resolution. In this Perspective, we discuss several recent advances in TERS, specifically highlighting those in the areas of TER imaging and integrating ultrafast spectroscopy with TERS.

Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy with Picosecond Excitation.

Tip-enhanced Raman spectroscopy (TERS) provides chemical information about adsorbates with nanoscale spatial resolution, but developments are still required in order to incorporate ultrafast temporal resolution. In this Letter, we demonstrate that a reliable TER signal of rhodamine 6G (R6G) using picosecond (ps)-pulsed excitation can be obtained in ultrahigh vacuum (UHV). In contrast to our previous observation of irreversible signal loss in ambient TERS ( Klingsporn , J. M. ; Sonntag , M. D. ; Seideman , T. ; Van Duyne , R. P. J. Phys. Chem. Lett. 2014 , 5 , 106 - 110 ), we demonstrate that the UHV environment decreases irreversible signal degradation. As a complement to the TERS experiments, we examined the rate of surface-enhanced Raman (SER) signal decay under picosecond irradiation and found that it is also slowed in UHV compared to that in ambient. Signal decay kinetics suggest that the predominant mechanism responsible for signal loss in ps SERS of R6G is surface diffusion. Both diffusive and reactive phenomena can lead to pulsed excitation TER signal loss, and a UHV environment is advantageous in either scenario.

Tip-enhanced Raman imaging: an emergent tool for probing biology at the nanoscale.

Typically limited by the diffraction of light, most optical spectroscopy methods cannot provide the spatial resolution necessary to characterize specimens at the nanoscale. An emerging exception to this rule is tip-enhanced Raman spectroscopy (TERS), which overcomes the diffraction limit through electromagnetic field localization at the end of a sharp metallic tip. As demonstrated by the Zenobi group in this issue of ACS Nano, TER imaging is an analytical technique capable of providing high-resolution chemical maps of biological samples. In this Perspective, we highlight recent advances and future applications of TER imaging as a technique for interrogating biology at the nanoscale.

Pressure-induced changes in the fluorescence behavior of red fluorescent proteins.

We present an experimental study on the fluorescence behavior of the red fluorescent proteins TagRFP-S, TagRFP-T, mCherry, mOrange2, mStrawberry, and mKO as a function of pressure up to several GPa. TagRFP-S, TagRFP-T, mOrange2, and mStrawberry show an initial increase in fluorescence intensity upon application of pressure above ambient conditions. At higher pressures, the fluorescence intensity decreases dramatically for all proteins under study, probably due to denaturing of the proteins. Small blue shifts in the fluorescence spectra with increasing pressure were seen in all proteins under study, hinting at increased rigidity of the chromophore environment. In addition, mOrange2 and mStrawberry exhibit strong and abrupt changes in their fluorescence spectra at certain pressures. These changes are likely due to structural modifications of the hydrogen bonding environment of the chromophore. The strong differences in behavior between proteins with identical or very similar chromophores highlight how the chromophore environment contributes to pressure-induced behavior of the fluorescence performance.