Ultrafast Shock Compression of Self-Assembled Monolayers: A Molecular Picture

Laser pulses into bullets: tabletop shock experiments

This article discusses tabletop high-throughput laser experiments on shock waves in solids and liquids, where the more usual laser pump pulse is replaced by a 0.5 mm diameter laser-launched bullet, a thin metal disk called a flyer plate. The hypervelocity flyer (up to 6 km s^-1 or Mach 18) can have kinetic energy (∼1 J) to briefly produce extreme conditions of temperature and pressure, thousands of K and tens of GPa (1 GPa = 10 000 bar) in a small volume with a rise time <2 ns. The experiments are performed using a "shock compression microscope", a microscope fitted with the laser flyer launcher plus an optical velocimeter, a high-speed laser interferometer that measures the motion of the flyer plate or the sample material after impact. This makes it possible to generate extreme conditions at the push of a button in an intrinsically safe environment, and probe with any of the diagnostics used in microscope experiments, such as high-speed video, optical emission, nonlinear coherent spectroscopies and so on. The barrier to entering this field is relatively low since many laser laboratories already possess much of the needed instrumentation. A brief introduction to shock waves and instrumentation is presented. Then several examples of recent applications are described, including shocked water, the photophysics of fluorescent molecules under extreme conditions, shocked protein solutions, shocked metal-organic frameworks (MOFs), shocked explosives, chemical catalysis in a shocked liquid, and molecules at shocked interfaces. Since one can shoot a bullet at practically anything, there are many emerging opportunities in chemistry, biophysics, materials science, physics and hypervelocity aerodynamics.

New Developments in the Physical Chemistry of Shock Compression

This review discusses new developments in shock compression science with a focus on molecular media. Some basic features of shock and detonation waves, nonlinear excitations that can produce extreme states of high temperature and high pressure, are described. Methods of generating and detecting shock waves are reviewed, especially those using tabletop lasers that can be interfaced with advanced molecular diagnostics. Newer compression methods such as shockless compression and precompression shock that generate states of cold dense molecular matter are discussed. Shock compression creates a metallic form of hydrogen, melts diamond, and makes water a superionic liquid with unique catalytic properties. Our understanding of detonations at the molecular level has improved a great deal as a result of advanced nonequilibrium molecular simulations. Experimental measurements of detailed molecular behavior behind a detonation front might be available soon using femtosecond lasers to produce nanoscale simulated detonation fronts.

Ultrafast flash thermal conductance of molecular chains

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.

Nanolithographic write, read, and erase via reversible nanotemplated nanostructure electrodeposition on alkanethiol-modified Au(111) in an aqueous solution

A write, read, and erase nanolithographic method, combining in situ electrodeposition of metal nanostructures with atomic force microscopy (AFM) nanoshaving of a 1-hexadecanethiol (HDT) self-assembled monolayer (SAM) on Au(111) in an aqueous solution, is reported. The AFM tip defines the local positioning of nanotemplates via the irreversible removal of HDT molecules. Nanotemplates with lateral dimensions as narrow as 25 nm are created. The electroactive nanotemplates determine the size, shape, and position of the metal nanostructures. The potential applied to the substrate controls the amount of metal deposited and the kinetics of the deposition. Metal nanostructures can be reversibly and repeatedly electrodeposited and stripped out of the nanotemplates by applying appropriate potentials.

Ultrafast Dynamics of Self-Assembled Monolayers under Shock Compression: Effects of Molecular and Substrate Structure

Laser-driven approximately 1 GPa shock waves are used to dynamically compress self-assembled monolayers (SAMs) consisting of octadecanethiol (ODT) on Au and Ag, and pentanedecanethiol (PDT) and benzyl mercaptan (BMT) on Au. The SAM response to <4 ps shock loading and approximately 25 ps shock unloading is monitored by vibrational sum-frequency generation spectroscopy (SFG), which is sensitive to the instantaneous tilt angle of the SAM terminal group relative to the surface normal. Arrival of the shock front causes SFG signal loss in all SAMs with a material time constant <3.5 ps. Thermal desorption and shock recovery experiments show that SAMs remain adsorbed on the substrate, so signal loss is attributed to shock tilting of the methyl or phenyl groups to angles near 90 degrees. When the shock unloads, PDT/Au returns elastically to its native structure whereas ODT/Au does not. ODT evidences a complicated viscoelastic response that arises from at least two conformers, one that remains kinetically trapped in a large-tilt-angle conformation for times >250 ps and one that relaxes in approximately 30 ps to a nearly upright conformation. Although the shock responses of PDT/Au, ODT/Ag, and BMT/Au are primarily elastic, a small portion of the molecules, 10-20%, evidence viscoelastic response, either becoming kinetically trapped in large-tilt states or by relaxing in approximately 30 ps back to the native structure. The implications of the observed large-amplitude monolayer dynamics for lubrication under extreme conditions of high strain rates are discussed briefly.