At the level of individual molecules, familiar concepts of heat transport no longer apply. When large amounts of heat are transported through a molecule, a crucial process in molecular electronic devices, energy is carried by discrete molecular vibrational excitations. We studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse. When the heat reached the methyl groups at the chain ends, a nonlinear coherent vibrational spectroscopy technique detected the resulting thermally induced disorder. The flow of heat into the chains was limited by the interface conductance. The leading edge of the heat burst traveled ballistically along the chains at a velocity of 1 kilometer per second. The molecular conductance per chain was 50 picowatts per kelvin.H eat transport is central to the operation of mechanical and electronic machinery, but at the level of individual molecules, the familiar concepts of heat diffusion by phonons in bulk materials no longer apply. Heat is transported through a molecule by discrete molecular vibrations. An emerging area in which vibrational energy transfer becomes crucial is the field of molecular electronics, where longchain molecules attached to tiny electrodes are used to transport and switch electrons. When an electron is transported through a molecule, a portion of the electron's kinetic energy can be lost, appearing as molecular vibrational energy (1). In studies such as this one, in which molecular energy levels are not individually resolved, it is conventional to call such processes "heat dissipation" or "nanoscale thermal transport" (2), even though an equilibrium Boltzmann distribution is not necessarily achieved. Nitzan and co-workers (3) have estimated that 10 to 50% of the electron energies could be converted to heat, so that a power of 10 11 eV/s may be dissipated on a molecular electronic bridge carrying 10 nA under a bias of 1 eV. Using classical and quantum mechanical methods, they and others (1) have calculated steadystate temperatures resulting from such dissipation. Steady-state calculations, however, do not entirely capture the essence of this phenomenon. The energy lost when electrons are transported through a molecular wire in a fraction of a picosecond appears as staccato bursts, up to 1 eV per burst. On a 10-carbon alkane molecule, for instance, 1eV is enough energy to produce a transient temperature jump DT ≈ 225 K. At the temperatures associated with these ultrafast energy bursts, Nitzan and coworkers (3) suggest that, instead of the usual phonon mechanisms prevalent in ordinary thermal conduction processes (1), much of the heat is carried by higher-energy molecular vibrations such as carbon-carbon bending and stretching and carbon-hydrogen bending, which are delocalized over a few carbon segments (3).To study molecular energy transport in the regime of short distances, short time intervals, and large temperature bursts, we have used an ultr...

Suppression of the nonresonant background in vibrational sum-frequency generation (SFG) in the broadband multiplex configuration is achieved using a time-asymmetric pulse, created by passing a femtosecond pulse through a Fabry-Perot e ´talon, to temporally discriminate between the faster nonresonant and slower resonant contributions. A mixed time and frequency domain explanation of the SFG process is presented, and spectra with high time resolutions and high degrees of nonresonant background suppression are obtained using selfassembled alkanethiolate monolayers on Au.

No abstract

Solid-state quantum emitters are in high demand for emerging technologies such as advanced sensing and quantum information processing. Generally, these emitters are not sufficiently bright for practical applications, and a promising solution consists in coupling them to plasmonic nanostructures. Plasmonic nanostructures support broadband modes, making it possible to speed up the fluorescence emission in room-temperature emitters by several orders of magnitude. However, one has not yet achieved such a fluorescence lifetime shortening without a substantial loss in emission efficiency, largely because of strong absorption in metals and emitter bleaching. Here, we demonstrate ultrabright single-photon emission from photostable nitrogen-vacancy (NV) centers in nanodiamonds coupled to plasmonic nanocavities made of low-loss single-crystalline silver. We observe a 70-fold difference between the average fluorescence lifetimes and a 90-fold increase in the average detected saturated intensity. The nanocavity-coupled NVs produce up to 35 million photon counts per second, several times more than the previously reported rates from room-temperature quantum emitters.

No abstract

The potential-dependent (2x2)-3CO-->(radical19x radical19)R23.4 degrees-13CO adlayer phase transition on Pt(111) with 0.1M H(2)SO(4) electrolyte was studied using femtosecond broadband multiplex sum frequency generation (SFG) spectroscopy combined with linear scan voltammetry. Across the phase boundary the SFG atop intensity jumps, and at the same time the SFG spectrum of threefold CO sites is transformed into a bridge site spectrum with a small decrease in integrated SFG intensity. The SFG atop intensity jump and three fold-to-bridge intensity drop are noticeably different from what would be expected for these structures on the basis of coverage alone. This occurs because the SFG signal is sensitive to both the coverage and changes in the local field that result from a changing adlayer structure. We derive an equation that allows us to correct the SFG intensities for these effects using information derived from infrared absorption-reflection spectroscopy (IRAS) and second-harmonic generation (SHG) measurements. With this correction, the SFG results agree well with what would be expected for a transition between perfect adlattices. A small (approximately 20%) discrepancy in the SFG determination of atop coverage is attributed to either a small amount of surface disorder or uncertainties in the SFG, SHG, and IRAS measurements. SFG is also used to examine the reversibility hysteresis and kinetics of the phase transition and its dependence on electrolyte composition. The phase transition is reversible with an approximately 150 mV anodic overpotential and the forward (2x2)-->(radical19x radical19) transition is slower than the reverse. Repeated cycles of phase transition indicate that the 25 microm electrolyte layer used here does not appreciably distort the potential-coverage relationships.

Femtosecond laser-driven approximately 1 GPa shock waves are used to compress monolayers of hydrocarbon chains. Vibrational sum-frequency generation spectroscopy probes the orientation of the terminal methyl groups. With an odd number (15) of carbon atoms, shock compression is an elastic process that causes the methyl groups to tilt. With an even number (18) of carbon atoms, shock compression is viscoelastic, creating single and double gauche defects. When the shock unloads, single gauche defects remain while double defects relax in 30 ps to single-defect states with more upright methyl groups.

No abstract