Microscopic mechanisms for initiation and evolution of femtosecond laser-induced columnar structures above the surface level of aluminum.

A better understanding of the formation of femtosecond (fs) laser-induced surface structures is key to the control of their morphological profiles for desired surface functionalities on metals. In this work, with fs laser pulse irradiation, the two stages of formation mechanisms of the columnar structures (CSs) grown above the surface level are investigated on pure Al plates in ambient air. Here, we find that the redeposition of ablated microscale clusters following fs laser pulses of irradiation acts as the nucleation sites of CS formation, which strongly affects their location and density within the laser spot.

Multi-Angular Colorimetric Responses of Uni- and Omni-Directional Femtosecond Laser-Induced Periodic Surface Structures on Metals.

We investigated the colorimetric behaviors of metal surfaces with unidirectional low-spatial-frequency laser-induced periodic surface structures (UD-LSFLs) and omnidirectional LSFLs (OD-LSFLs) fabricated using femtosecond laser pulse irradiation. With the CIE standard illuminant D65, incident at -45°, we show that UD-LSFLs on metals transform polished metals to gonio-apparent materials with a unique behavior of colorimetric responses, depending on both the detection and rotation angles, whereas OD-LSFLs have nearly uniform gonio-apparent colors at each detection angle, regardless of their rotation. These colorimetric behaviors can be observed not only at the angles of diffraction but also near the angle of reflection, and we find that the power redistribution due to Rayleigh anomalies also plays an important role in the colorimetric responses of UD- and OD-LSFLs, in addition to diffraction.

Observation of dynamical crater-shaped charge distribution in the space-time imaging of monolayer graphene.

A better understanding of charge carrier dynamics in graphene is key to improvement of its electronic performance. Here, we present direct space-time visualization of carrier relaxation and diffusion in monolayer graphene using time-resolved scanning electron microscopy techniques. We observed striking fluence-dependent dynamic images, changing from a Gaussian shape to a novel crater-shaped pattern with increasing laser fluence.

Effects of Cracking Test Conditions on Estimation Uncertainty for Weibull Parameters Considering Time-Dependent Censoring Interval.

It is extremely difficult to predict the initiation time of cracking due to a large time spread in most cracking experiments. Thus, probabilistic models, such as the Weibull distribution, are usually employed to model the initiation time of cracking. Therefore, the parameters of the Weibull distribution are estimated from data collected from a cracking test.

Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy.

Four-dimensional scanning ultrafast electron microscopy is used to investigate doping- and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured time-resolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration.

Structural and electronic decoupling of C₆₀ from epitaxial graphene on SiC.

We have investigated the initial stages of growth and the electronic structure of C(60) molecules on graphene grown epitaxially on SiC(0001) at the single-molecule level using cryogenic ultrahigh vacuum scanning tunneling microscopy and spectroscopy. We observe that the first layer of C(60) molecules self-assembles into a well-ordered, close-packed arrangement on graphene upon molecular deposition at room temperature while exhibiting a subtle C(60) superlattice. We measure a highest occupied molecular orbital-lowest unoccupied molecular orbital gap of ∼3.

Atomic-scale investigation of graphene grown on Cu foil and the effects of thermal annealing.

We have investigated the effects of thermal annealing on ex-situ chemically vapor deposited submonolayer graphene islands on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum scanning tunneling microscopy. Low-temperature annealed graphene islands on Cu foil (at ∼430 °C) exhibit predominantly striped Moiré patterns, indicating a relatively weak interaction between graphene and the underlying polycrystalline Cu foil. Rapid high-temperature annealing of the sample (at 700-800 °C) gives rise to the removal of Cu oxide and the recovery of crystallographic features of the copper that surrounds the intact graphene.

Single-molecule-resolved structural changes induced by temperature and light in surface-bound organometallic molecules designed for energy storage.

We have used scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory calculations to investigate thermal and photoinduced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules (designed for light energy storage) on a Au(111) surface. We find that both the parent complex and the photoisomer exhibit striking thermally induced structural phase changes on Au(111), which we attribute to the loss of carbonyl ligands from the organometallic molecules. Density functional theory calculations support this conclusion.

Functionalization, self-assembly, and photoswitching quenching for azobenzene derivatives adsorbed on Au(111).

We have used scanning tunneling microscopy to investigate the structure and photoswitching behavior of azobenzene molecules functionalized with bulky spacer groups and adsorbed onto Au(111). We find that positioning tert-butyl "legs" in a canted arrangement on the azobenzene phenyl rings quenches photoisomerizability of the molecule on Au(111). Addition of cyano groups at the para positions changes the molecular self-assembly significantly, but does not alter the quenched photoisomerizability.

Surface anchoring and dynamics of thiolated azobenzene molecules on Au(111).

We have investigated the temperature-dependent behavior of thiolated azobenzene molecules on Au(111) using scanning tunneling microscopy. The addition of a thiol functional group to azobenzene molecules leads to increased surface anchoring of single azobenzene molecules to gold. Thiolated azobenzene shows diverse surface morphology and does not form well-ordered structures at low coverage.

Self-patterned molecular photoswitching in nanoscale surface assemblies.

Photomechanical switching (photoisomerization) of molecules at a surface is found to strongly depend on molecule-molecule interactions and molecule-surface orientation. Scanning tunneling microscopy was used to image photoswitching behavior in the single-molecule limit of tetra-tert-butyl-azobenzene molecules adsorbed onto Au(111) at 30 K. Photoswitching behavior varied strongly with surface molecular island structure, and self-patterned stripes of switching and nonswitching regions were observed having approximately 10 nm pitch.

Reversible photomechanical switching of individual engineered molecules at a metallic surface.

We have observed reversible light-induced mechanical switching for individual organic molecules bound to a metal surface. Scanning tunneling microscopy (STM) was used to image the features of individual azobenzene molecules on Au(111) before and after reversibly cycling their mechanical structure between trans and cis states using light. Azobenzene molecules were engineered to increase their surface photomechanical activity by attaching varying numbers of tert-butyl (TB) ligands ("legs") to the azobenzene phenyl rings.