Sercan Keskin

Direct observation of collective modes coupled to molecular orbital-driven charge transfer

The making of a molecular movie Phase transitions familiar from everyday life, such as boiling or melting, are caused by changing the temperature. In the laboratory, however, researchers can also change the phase of a material by shining intense light on it. During such transitions, changes occur in both the electronic and lattice structure of the material. Ishikawa et al. used ultrafast optical and electron diffraction probes to monitor both types of change simultaneously during a photo-induced phase transition in a molecular crystal. The resulting molecular movies showed expansion of the intermolecular distance, flattening of the molecules, and tilting of molecular dimers. Science, this issue p. 1501 Ultrafast spectroscopy and electron diffraction are used to create molecular movies of a phase transition in Me4P[Pt(dmit)2]2. Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Visualization of Multimerization and Self-Assembly of DNA-Functionalized Gold Nanoparticles Using In-Liquid Transmission Electron Microscopy

Base-pairing stability in DNA-gold nanoparticle (DNA-AuNP) multimers along with their dynamics under different electron beam intensities was investigated with in-liquid transmission electron microscopy (in-liquid TEM). Multimer formation was triggered by hybridization of DNA oligonucleotides to another DNA strand (Hyb-DNA) related to the concept of DNA origami. We analyzed the degree of multimer formation for a number of samples and a series of control samples to determine the specificity of the multimerization during the TEM imaging. DNA-AuNPs with Hyb-DNA showed an interactive motion and assembly into 1D structures once the electron beam intensity exceeds a threshold value. This behavior was in contrast with control studies with noncomplementary DNA linkers where statistically significantly reduced multimerization was observed and for suspensions of citrate-stabilized AuNPs without DNA, where we did not observe any significant motion or aggregation. These findings indicate that DNA base-pairing interactions are the driving force for multimerization and suggest a high stability of the DNA base pairing even under electron exposure.

Visualization of Cellular Components in a Mammalian Cell with Liquid-Cell Transmission Electron Microscopy

We present liquid-cell transmission electron microscopy (liquid-cell TEM) imaging of fixed and non-fixed prostate cancer cells (PC3 and LNCaP) with high resolution in a custom developed silicon nitride liquid cell. Fixed PC3 cells were imaged for 90-120 min without any discernable damage. High contrast on the cellular structures was obtained even at low electron doses (~2.5 e-/nm2 per image). The images show distinct structures of cell compartments (nuclei and nucleoli) and cell boundaries without any further sample embedding, dehydration, or staining. Furthermore, we observed dynamics of vesicles trafficking from the cell membrane in consecutive still frames in a non-fixed cell. Our findings show that liquid-cell TEM, operated at low electron dose, is an excellent tool to investigate dynamic events in non-fixed cells with enough spatial resolution (few nm) and natural amplitude contrast to follow key intracellular processes.

Ultrafast dissolution and creation of bonds in IrTe2 induced by photodoping

Photoinduced bonding instability of IrTe2 studied with ultrafast electron diffraction and density functional theory. The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe2 using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding dxy orbital. Bond length changes of 20% in IrTe2 are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner.

Photochromism of Amphiphilic Dithienylethenes as Langmuir–Schaefer Films

Surface pressure-area isotherms were recorded under different irradiation conditions for single-component Langmuir films of three photochromic amphiphilic dithienylethenes. Nonirradiated films of these photochromic amphiphiles were mechanically stable. In addition, a shift of the isotherms to larger mean molecular areas was observed for films prepared from UV-light-irradiated dithienylethenes. Unexpectedly, a significant expansion was observed for a film prepared from visible-light-irradiated dithienylethene incorporating large branched alkyl chains. Upon further study, atomic force microscopy and transmission electron microscopy images of Langmuir-Schaefer films revealed that this pronged dialkyl derivative undergoes a photoinduced change in morphology, as circular aggregates coalesce into larger continuous aggregated structures. Nevertheless, its photoisomerization was completely reversible as single-component multilayer thin films upon direct UV or visible light irradiation.

Bandgap modulation in photoexcited topological insulator Bi2Te3 via atomic displacements

The atomic and electronic dynamics in the topological insulator (TI) Bi2Te3 under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi2Te3 trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results.