PacSurf2014 Session NM-TuE: Nanomaterials Characterization & Reactivity II

Jing G Y et al. Surface effects on elastic properties of silver nanowires: contact atomic-force microscopy[J]. Physical Review B, 2006, 73(23): 235409.

2] Han X B et al. Electronic and mechanical coupling in bent ZnO nanowires. Advanced Materials, 2009, 21(48): 4937-4941.

3] Han X B et al. Strain-Gradient Effect on Energy Bands in Bent ZnO Microwires. Advanced Materials, 2012, 24(34): 4707-4711.

4] Fu X W et al. Exciton Drift in Semiconductors under Uniform Strain Gradients: Application to Bent ZnO Microwires. ACS nano 8.4 (2014): 3412-3420.

Luminescence from the systems consisting of metal nanostructures (NSs) and adsorbed molecules can be strongly influenced by quantum many-body effects which arise from the interplay between dielectric response of metal NSs and intra-molecular electronic/vibrational excitations. In light emission induced by the tunneling current of a scanning tunneling microscope (STM) from molecule-covered metal surfaces, interface plasmons localized near the tip-substrate gap region play important roles in electronic excitations and radiative decays of the molecule. Recent experimental results have also suggested that the dynamics of molecules (e.g., luminescence and energy absorption) have an influence on the luminescence-spectral profiles of interface plasmons [1]. Since the dynamics of molecules and interface plasmons have influence on each other, quantum many-body effects resulting from interplay between these dynamics are expected to occur. To unveil these effects from a microscopic point of view, there is a need to investigate the dynamics of the molecule and interface plasmons within the framework of quantum many-body theory. In this study, we develop the effective model of the system and investigate the effects of coupling between molecular exciton and interface plasmon (exciton-plasmon coupling) on the luminescence properties using the nonequilibrium Green’s function method [2-5]. It is found that in addition to the dynamics of the molecule, the dynamics of interface plasmons plays an essential role in determining the luminescence spectral profiles of interface plasmons. Prominent peak and dip structure observed in recent experiments are interpreted by the developed theory. The details of exciton-plasmon coupling on the luminescence properties will be discussed.

References

[1] N. L. Schneider and R. Berndt, Phys. Rev. B 86 (2012) 035445.

[2] K. Miwa, M. Sakaue, and H. Kasai, J. Phys. Soc. Jpn. 82 (2013) 063745.

[3] K. Miwa, M. Sakaue, and H. Kasai, J. Phys. Soc. Jpn. 82 (2013) 124707.

[4] K. Miwa, M. Sakaue, and H. Kasai, Nano. Res. Lett. 8 (2013) 204.

[5] K. Miwa, M. Sakaue, B. Gumhalter and H. Kasai, J. Phys.: Condens. Matter 26 (2014) 222001.

Submolecular imaging using atomic force microscopy (AFM) has recently been established as a stunning technique to reveal the chemical structure of unknown molecules, to characterize intramolecular charge distributions, and bond ordering, as well as to study chemical transformations and intermolecular interactions. So far, most of these feats were achieved on planar molecular systems because high-resolution imaging of three-dimensional (3D) surface structures with AFM remains challenging. Here we present a method for high-resolution imaging of non-planar molecules and 3D surface systems using silicon cantilever based AFM. We demonstrate this method by resolving the step-edges of the (101) anatase surface at the atomic scale, by simultaneously visualizing the structure of a pentacene molecule together with the atomic positions of the substrate, and by resolving the contour and tip-surface force field on a C₆₀ molecule with intramolecular resolution. The method holds substantial promise for the study of 3D surface structures such as nanotubes, clusters, nanoparticles, polymers, and biomolecules using AFM with unprecedented resolution.

The quest for novel semiconductor materials with improved optoelectronic performance has triggered intense research activities to exploit the great diversity of effects offered by low dimensional systems. In this work, we demonstrate that the recombination dynamics of excitons in ZnO nanowires can be well understood within the concept of optical nanocavities. We investigate the spatial distribution of the lifetimes of the near-band-edge and bound-exciton emissions in single ZnO nanowires with different dimensions by means of temperature dependent and time-resolved spectroscopy. We demonstrate that the lifetime of the excitons is systematically reduced by 30% at the tips of the nanowires with respect to their maximum value at the center, which originates from the combined effect of the cavity-like properties of these nanostructures with the Purcell effect. In addition, show that the model of Rashba and Gurgenishvili is valid even at the nanoscale, i.e. the lifetime of the bound excitons is proportional to the localization energy (Eloc) to the power of 3/2. This result provides a means to understand the spatial dependence of the lifetimes of the near-band-edge emission (NBE), which is not intuitive due to their spatially extended nature. Finally, the temperature dependence of the photoluminescence and lifetimes of the excitons in single nanowires is also briefly discussed in comparison to bulk ZnO samples.

Electron Microscopy methods to characterize Thin films and interfaces have advanced very substantially during the last decade.In particular two methods are some of the most significant : Aberration corrected TEM-STEM images and Precession Electron Diffraction.In this paper we describe this methods and apply them to the characterization of gold thin films. It is possible to obtain atomic images of the interfaces using STEM-HAADF which yield realiable information about the atomic positions .When we combine this with single grain diffraction we can obtain a very complete description of the grain structure.

We present the case of polycrystalline Gold thin films grown at different temperatures.We discuss the distribution of most likely boundaries present and its frecuency as a function of the temperature .In addition by using STEM-HAADF it is possible to obtain the surface topography evolution as function of the temperature.

Atomically-thin transition metal dichalcogenide (TMD) semiconductors have generated great interest recently due to their remarkable physical properties. Reduced screening in 2D has been predicted to result in dramatically enhanced Coulomb interactions that should cause giant bandgap renormalization and exotic excitonic effects in single-layer TMD semiconductors. (1, 2). Here we present a direct experimental observation of extraordinarily high exciton binding energy and bandgap renormalization in a single-layer of semiconducting TMD (3). We determined the binding energy of correlated electron-hole excitations in monolayer MoSe2 grown on bilayer graphene (BLG) using high-resolution scanning tunneling spectroscopy (STS) and photoluminescence spectroscopy. We have measured both the quasiparticle electronic bandgap and the optical transitions of monolayer MoSe2/BLG, enabling us to obtain an exciton binding energy of 0.55 eV for this system, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. We have also studied the influence of external dielectric screening by repeating the measurements on MoSe2 on HOPG graphite. These results are of fundamental importance for room-temperature optoelectronic devices involving 2D semiconducting TMDs, as well as more complex layered heterostructures.

1. H. P. Komsa, A. V. Krasheninnikov, Physical Review B. 86, 241201 (2012).

2. D. Y. Qiu, et al., Physical Review Letters. 111, 216805 (2013).

3. Miguel M. Ugeda, et al., Nature Materials (2014) - accepted.

In this work, a model to study the coupling between a semiconductor qubit and two time-dependent electric fields is developed. By using it in the resonantly monochromatic double dressing regime, control of the local density of optical states is theoretically and numerically demonstrated for a strongly confined exciton.

As a main result, tailored manipulation of the optical density of states in semiconductor quantum dots is proved. It is shown that by coupling a nanostructured qubit simultaneously to two distinguishable lasers whose frequencies match the exciton transition, a discrete eigenstate turns into an energy subband in a process closely analogous to band formation in solid state physics.

Such strong changes in the local density of optical states, controllable through the ratio between the driving laser intensities, open new possibilities for on-demand photon emission from artificial atoms.

The presented results are in remarkable qualitative and quantitative agreement with experimental measurements.