Research Highlights

Quantum Tomography of a Molecular Bond in Ice

I. U. Goldschleger, M. van Staveren, V. A. Apkarian, J. Chem. Phys. 139, 034201 (2013); DOI: 10.1063/1.4813437

Selected frames of the moving bond captured in phase space, at 5 fs per frame. The abscissa (q) is the bond length of Bromine (0.1 Å full scale), and ordinate (p) is the momentum of the moving bond. The images are the tomographically reconstructed Wigner distribution function (WDF), which provides the complete uncertainty limited quantum mechanical description of a dynamical system. The white line is the zero-contour, which shows the strictly quantum effect of a negative volume in the WDF, The graph is the time dependent zero-volume, which decays as the system relaxes to v = 0. (The complete video will appear on line, linked through the manuscript.)

The measurements are carried out in a sliver of dirty ice: Bromine doped ice, at T = 120 K. The molecule creates a cage around itself (clathrate) as show in the first image. Despite the environment, the molecular motion is fully quantum mechanical, which is certified by the negative hole in the WDF and the observed t-dependent wavefunctions (middle). The negative hole arises only from quantum interference, and such states are referred to as “cat” states.

A molecular GPS, AC/DC voltmeter, and a rectifying antenna consisting of a chemically engineered nano-dumbbell

M. Banik, P. Z. El-Khoury, A. Nag, A. Rodriguez-Perez, N. Guarrottxena, G. C. Bazan, and V. A. Apkarian

Project Outcome:
This research demonstrates a “global positioning system” for tracking single molecules in 3D space by equipping them with nano-dumbbell antennae; and the use of Raman spectra as a voltmeter to characterize AC and DC fields on nanometric scale. When a small asymmetric molecule is trapped in the junction of metallic nano-spheres, the dumbbell acts as a rectifying antenna (rectenna).

Impact & Benefits:
The ability to remotely track single molecules in 3D space is an essential step toward chemistry of individual molecules. Rectennas fulfill ideal design requirements of a photocell, with enhanced spectral response and conversion efficiency over existing cells. The demonstrated principles should advance the state-of-the-art in harnessing the energy of the sun.

Background & Explanations:
In an approach analogous to GPS, by equipping molecules with nano-antennae and taking advantage of the tensor nature of Raman scattering, it is demonstrated that fluctuations in surface enhanced Raman spectra (SERS) of single molecules can be used to locate them in 3D space, as a function of time. Relying on the vibrational Stark shift of CO, it’s SERS spectrum serves as a molecular voltmeter, which is used to characterize the plasmonic fields at the junction of silver nano-spheres. The observed DC field of 1 V/Å, characterizes the dumbbell as a rectenna with a spectral response that covers the visible to the IR, as a function of junction gap.

Publications Relevant to Project Outcome:
" Surface-Enhanced Raman Trajectories on a Nano-Dumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse" ACS Nano, DOI:10.1021/nn304277n, (2012).

Spatially and Spectroscopically Resolved Molecules on a Surface

N. Jiang, E. Foley, J. Klingsporn, M. Sonntag, N. Valley, J. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne

Project Outcome:
Professor Van Duyne’s group has combined scanning tunneling microscopy (STM) with tip-enhanced Raman spectroscopy (TERS) to both spatial image at sub-nm resolution and resolve chemical bonding information of large polyatomic molecular adsorbates on a metal surface. Theoretical electronic structure calculations and simulated Raman spectra were carried out and showed strong agreement between theory and experiment of the vibrational modes of the absorbate.

Impact & Benefits:
The ability to resolve multiple vibrational peaks while simultaneously imaging at the sub-nm molecular levels allows the chemical relationship between polyatomic adsorbates and solid surfaces to be quantitatively interrogated. This technique presents new opportunities for the study of catalysis and other molecule-surface related phenomena necessary for the understanding of important innovations. For example, the adsorbate studied, copper phthalocyanine (CuPc), has potential applications in organic light-emitting diodes, thin-film transistors, as well as other optical and electronic devices.

Background & Explanations:
Tip-enhanced Raman spectroscopy (TERS) combined with surface imaging techniques, such as STM, can provide quantitative information on surface morphology, binding sites, and the vibrational fingerprint of adsorbates on a solid surface. The Van Duyne’s group experimental setup allows for unprecedented spatial and spectroscopic resolutions, with the full potential of this technique not yet realized.

Publications Relevant to Project Outcome:
"Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy" N. Jiang, E. Foley, J. Klingsporn, M. Sonntag, N. Valley, J. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, Nano Lett., ASAP, DOI: 10.1021/nl2039925. (2012).

> View Article From American Chemical Society

Spatially and Spectroscopically Resolved Molecules on a Surface” Caption: Schematic (center) of the experimental set up using both scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS). The left image is the molecule, copper phthalocyanine (CuPc), on a metal surface, Ag (111), taken at a resolution below the nanometer length scale. The Raman spectra (right) shows the vibrational bonding modes of the same molecule.

We illustrate the electric field lines (red/yellow) and equipotential lines (blue) of the fundamental plasmon mode (left) and an excited mode (right) of a rectangular metallic nanowire. Note the polarization charges at the corner, and how the fields localized to the surfaces for the highly excited mode.

We show the square of the electric field, normalized to the incident field as one moves around the periphery of a gold nanowire excited at the frequency of the fundamental plasmon resonance. Note the strong enhancement at the corners where the polarization charges reside.

Resolving Individual Molecules Using Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy

Renee R. Frontiera, Anne-Isabelle Henry, Natalie L. Gruenke and Richard Van Duyne
March 2011

Project Outcome:
Professor Van Duyne's group, in collaboration with coworkers at UC Irvine, has succeeded in integrating extremely fast time-resolved spectroscopy to dynamically probe nanomaterials. Using the novel technique of Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy (SE-FSRS), we are able to detect extremely small quantities of molecules bound to nanoparticles. In the bottom right image, we show the first ever SE-FSRS spectra; these spectra of bipyridine molecules bound to gold nanoparticles are highly resolved. We are thus able to examine molecules on the billionth of a meter length scale with time resolution of up to a million billionth of a second, achieving the ability to examine chemistry at the limits of space and time.

Impact & Benefits:
This research is aimed at understanding the molecular processes that occur in novel nanomaterials that are becoming important players in solving the global energy crisis. New solar cell technologies that use these novel materials, such as dye-sensitized solar cells, are very promising due to the use of inexpensive materials and solution-based processing. By examining the motions of small number of molecules after they absorb light and convert it to energy, we can determine the optimal chemistry and molecular orientations that are needed to achieve a high-energy conversion efficiency. This fundamental understanding can be used to rationally design new materials, molecules, and solar cell device architectures.

Relevant Publications:
"Surface Enhanced-Femtosecond Stimulated Raman Spectroscopy" R. R. Frontiera, A.-I. Henry, N. L. Gruenke, and R. P. Van Duyne Journal of Phys. Chem. Lett. 2 (10), 1199-1203 (2011)

> View Article From The Journal of Physical Chemistry Letter
 

Experimental setup showing methods for creating molecular movies at the natural length (nanometers) and time scales (femtoseconds) of individual molecules.

Left: CaSTL researchers developing new techniques for following chemical reactions in progress. Right: Spectra from new techniques pioneered in the Van Duyne lab.

Lightning Without Thunder

Joonhee Lee, Shawn M. Perdue, Desiré Whitmore, and V. Ara Apkarian
November 2010

Project Outcome:
The Apkarian group has demonstrated unprecedented thermal management of an STM tip under irradiation with femtosecond laser pulses, keeping its expansion stable to less that 1/100 of an atom, while modulating the output current. They achieved this through a novel "double beat" method that maintains a constant heat load on the tip, yet still gives a signal modulation for increased sensitivity. In this method, a laser pulse is split in two, with one copy being frequency shifted from the other by an acousto-optic modulator (AOM). When the two pulses are recombined in an interferometer, they almost cancel out, but leave that frequency shift as the first beat. This new frequency then beats with the repetition rate of the laser for the second beat. The use of a double beat brings the signal modulation into the base-band, where it can be amplified by a high gain preamplifier.

The key to eliminating the thermal shaking, or thunder, is in cross-polarizing the two pulses. In this way, there is no interference between them until they impinge on the STM tip itself. The tip then sees the pulse trains individually, which come too fast for it to respond thermally. However, the tunneling electrons, or lightning, do see the interference in the electric field, and are thus modulated themselves. Lightning without thunder!

Impact & Benefits:
The double beat method opens the possibility of ultrafast triggering of STM, and consequently, videography of single chemical events. Although this is the primary goal of this research group, the applications of this new method extend much further. Light plays a key role in many chemical processes, and the ability to optically stimulate a sample while imaging it on an atomic level enables access to a new level of the experimental study of single molecules.

Background & Explanations:
To explore chemistry at the measurable limits of space and time, Ångstroms and femtoseconds, the major challenge has been in combining ultrashort laser pulses with scanning tunneling microscopy (STM). Specifically, the thermal modulation of the gap between the STM tip and the sample has been difficult to control. To be useful in microscopy, the gap must be stable to within 0.01 Å. That's more than 30 times smaller than a hydrogen atom! Even a 1 mW laser, like a laser pointer, will cause a typical tip to expand by 10,000 times this limit. The double beat method simultaneously enables gap stability and signal modulation.

Relevant Publications:
Lee, Joonhee, Shawn M. Perdue, Desiré Whitmore, and V. Ara Apkarian. "Laser-induced scanning tunneling microscopy: Linear excitation of the junction plasmon." Journal of Chemical Physics 133 (2010): 104706.

Highlighted By:
Virtual Journal of Nanoscale Science & Technology, September 20, 2010
Virtual Journal of Ultrafast Science, October 2010

> View Article From The Journal Chemical Physics

A scanning tunneling microscope (STM) tip above a silver atom on a NiAl surface. The tip is being excited by a pair of laser pulses that are cross-polarized.

STM image of a single silver atom on a NiAl surface.