Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V26: Chemical Physics at the Edges IIFocus
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Sponsoring Units: DCP Chair: Eric Potma, University of California - Irvine Room: 289 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V26.00001: Ultrafast Microscopy of Energy and Charge Transport Invited Speaker: Libai Huang The frontier in solar energy research now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. Advancing the field requires transformative experimental tools that probe energy transfer processes from the nano to the meso lengthscales. To address this challenge, we aim to understand multi-scale energy transport across both multiple length and time scales, coupling simultaneous high spatial, structural, and temporal resolution. In my talk, I will focus on our recent progress on visualization of exciton and charge transport in solar energy harvesting materials from the nano to mesoscale employing ultrafast optical nanoscopy. With approaches that combine spatial and temporal resolutions, we have recently revealed a new singlet-mediated triplet transport mechanism in certain singlet fission materials. This work demonstrates a new triplet exciton transport mechanism leading to favorable long-range triplet exciton diffusion on the picosecond and nanosecond timescales for solar cell applications. We have also performed a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and 50 nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of 220nm in 2 ns for solution-processed polycrystalline CH3NH3PbI3 thin films. The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:42PM |
V26.00002: Resolving ultrafast exciton migration in organic solids at the nanoscale Invited Speaker: Naomi Ginsberg The migration of Frenkel excitons, tightly-bound electron-hole pairs, in photosynthesis and in organic semiconducting films is critical to the efficiency of natural and artificial light harvesting. While these materials exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton migration lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to uncover the correlations between the structural and energetic landscapes that the excitons explore. By combining the ultrafast super-resolved measurements with exciton hopping simulations we furthermore specify the nature (in addition to the extent) of exciton migration as a function of the intrinsic and ensemble chromophore energy scales that determine a spatio-energetic landscape for migration.\\ \\In collaboration with: Samuel Penwell, Lucas Ginsberg, University of California, Berkeley and Rodrigo Noriega University of Utah. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 4:18PM |
V26.00003: Visualization of Transport Dynamics in Nanostructures with Pump-Probe Microscopy. Invited Speaker: John Papanikolas A detailed understanding of the factors that govern the transport of mobile charge carriers, acoustic phonons and thermal energy in nanostructures is critical to many emerging nanotechnologies in electronics, optoelectronics and solar energy conversion. We have combined ultrafast pump-probe spectroscopy with optical microscopy to directly image the transport phenomena in individual Si nanowires (NWs) with both spatial and temporal resolution. In these experiments, an individual NW is excited by a 425 nm femtosecond pump pulse that has been focused to a diffraction limited spot (350 nm) by a microscope objective, exciting a localized region of the structure. After a well-defined delay, pump-induced changes to the transmission of an 850 nm probe pulse are detected, providing the time evolution of the photoexcitation at a specific point within the structure. By correlating optical images with scanning electron microscopy images obtained from the same structures, we are able to correlate recombination behavior with specific structural features. Motion of the photogenerated carriers, propagation of acoustic modes and thermal transport are observed using a spatially-separated pump-probe configuration, in which carriers are created in one location and detected in another, allowing direct imaging of charge carriers and phonons as they move away from the excitation spot. In this configuration the pump beam is held fixed and the position of the probe beam is scanned by varying the angle of the probe beam as it enters the objective, resulting in a spatial map of the photoinduced transparency at a specified pump-probe delay. Images collected at a series of delays shows the spatial-temporal evolution of the excitation, providing a direct visualization of carrier diffusion, acoustic mode propagation and thermal transport in semiconducting NWs. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V26.00004: Ultrafast spectroscopy at the nanoscale using photo-induced force microscopy. Bongsu Kim, Ryan Khan, Sung Park, Eric Potma Photo-induced force microscopy (PiFM) is an emerging nano-imaging method based on detecting the photo-induced force between the sample and a sharp atomic tip. Because the photo-induced force is a local and near-field effect, this technique enables imaging with a very high spatial resolution, down to 5 nm at ambient conditions. When combined with ultrafast illumination, PiFM makes it possible to examine the nonlinear optical properties of materials at the nanoscale. In this contribution, we present the latest advances in ultrafast spectroscopy with PiFM. We will discuss ultrafast pump-probe measurements as well as highlight experiments in which femtosecond stimulated Raman transitions are detected with PiFM. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V26.00005: Significant Variation of Surface Quality in VLS grown Silicon Nanowires Observed by Pump Probe Microscopy Emma Cating, Christopher Pinion, Caleb Christie, Erika Van Goethem, James Cahoon, John Papanikolas Free carrier recombination in nanowires (NWs) is impacted by surface quality (i.e. surface trap density). In NWs, surface quality is described by the surface recombination velocity (SRV) which relates NW diameter (d) with carrier lifetime ($\tau )$ according to SRV $=$ d/4$\tau $. It is often assumed that all NWs in an ensemble grown at same time have similar surface properties, and so SRV is usually found through either ensemble averaged techniques, or device-based measurements which only examine a small number (less than 10) of NWs. Neither technique provides a means of examining the distribution of SRVs between wires, so the question remains: how much variation in SRV is there between NWs grown at the same time? We determine SRV in nearly 300 individual silicon NWs using pump-probe microscopy to measure the lifetime of the free carrier population at a specific location in the NW, and SEM to measure NW diameter at the same position. In this way we determine SRV on a point-by-point basis for each individual wire. We find that SRV, and therefore surface quality, varies along an individual NW by as much as a factor of 10, and by up to two orders of magnitude between NWs grown at the same time. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V26.00006: Imaging Phonon Propagation Dynamics in Germanium Nanowires using Ultrafast Pump-Probe Microscopy Erika Van Goethem, Christopher Pinion, Emma Cating, James Cahoon, John Papanikolas We have directly imaged the phonon dynamics in individual germanium nanowires (Ge NWs) using ultrafast pump-probe microscopy with high spatial and temporal resolution to observe excited state transient decay kinetics. A femtosecond laser pulse impulsively excites the lattice, launching acoustic waves which appear in suspended Ge NWs as coherences in the transient decays. We measured the coherence period for a range of NW diameters (25-300 nm) and noticed a linear increase in coherence period with increasing NW diameter. Comparing this trend to an elastic isotropic cylinder we conclude that Ge NWs display a fundamental radial breathing mode (RBM). We observed the RBM propagating along the NW by using spatially-separated pump-probe. In this mode of operation a pump pulse excites the NW at one spot and a probe pulse detects the arrival of phonons at another location. By varying the spatial separation between the pump and probe, we see the RBM propagate at least 2.5 um along the NW in 2 ns. We also observe the appearance of a low frequency longitudinal mode at large spatial separations (2-4 um). This mode propagates at least 4 um along the NW at 6700 m/s (nearly the speed of sound in Ge), while the RBM spreads approximately 6 times slower than the longitudinal mode. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V26.00007: Super-Diffusion of Excited Carriers in Semiconductors Marco Bernardi, Ebrahim Najafi, Vsevolod Ivanov, Ahmed Zewail We characterize the spatiotemporal dynamics of excited carriers in silicon using scanning ultrafast electron microscopy (SUEM), a technique that combines the nanometer spatial resolution of electron microscopy and the femtosecond time resolution of ultrafast lasers. Following excitation with a short laser pulse, our experiments show direct evidence of a transient super-diffusive regime in which electrons and holes exhibit a diffusivity up to 1,000 times higher than the room temperature diffusivity, $D_0 \!\approx\! 30$ cm$^2$/s. The diffusivity then decreases rapidly for delay times longer than 200 ps, reaching a steady-state value of $D_0$ roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behavior to the rapid expansion of the hot carrier gas generated by the laser pulse, which decays to equilibrium with a time scale of 100$-$150 ps. This interpretation is supported by numerical solution of the diffusion equation with an exponentially decaying diffusivity, as well as ab initio calculations of the initial temperature of the hot carrier gas. Our findings open new avenues for investigating the ultrafast spatial dynamics of excited carriers in materials. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V26.00008: Photo-excited carrier dynamics of single defects on TiO$_{\mathrm{2}}$(110) probed by a laser-combined scanning tunneling microscope Ying Jiang, Chaoyu Guo, Xiangzhi Meng, Huixia Fu, Sheng Meng Titanium dioxide (TiO$_{\mathrm{2}})$ is well-known as one of the most widely used materials in photocatalysis and solar energy conversion. Although it is well accepted that the surface and near-surface defects play crucial roles as active sites in the photocatalytic and photoconcersion process of TiO$_{\mathrm{2}}$, the atomic-scale information of photo-excited carrier dynamics of those defects is still lacking. Here, we addressed this important issue using a home-made laser-combined scanning tunneling microscope (STM). Surface and subsurface oxygen vacancies of rutile TiO$_{\mathrm{2}}$(110) led to prominent in-gap states below the Fermi level (E$_{\mathrm{F}})$. Upon the light illumination, those gap states exhibited significant energetic shift. Interestingly, the subsurface defects showed two distinct photo response: redshift and blueshift, while the surface defects only showed blueshift. Based on density functional theory calculations (DFT), the redshift/blueshift of the gap states were ascribed to the photo-excited charge transfer between the gap states and valance/conduction band, which changed the charge states of the oxygen vacancy. Time-resolved experiments suggested that the lifetime of the photo-excited hot electrons/holes can be in the order of nanoseconds. Our work highlights the importance of atomic environment in the photoactivity of the defects and may help to improve the photocatalytic efficiency by engineering the defect types properly\textbf{.} [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V26.00009: Probing the Correlated Triplet Pair in TIPS-Pentacene Using Transient Absorption Microscopy Brendan D. Folie, Naomi S. Ginsberg Singlet fission, the process by which a singlet exciton splits into two triplet excitons, has been shown to increase the efficiency of photovoltaics made from organic semiconductors. Fission is believed to occur via a correlated triplet pair intermediate, but direct measurements of this state remain scant. We use polarization-resolved white light transient absorption microscopy to observe the correlated triplet pair in TIPS-Pentacene, a common model system. We are able to measure the binding energy of the triplet pair, and find that this interaction tends to diminish the triplet absorbance spectrum. Our results shed light on the kinetics and electronic structure of the correlated triplet pair, which have important implications for the creation of singlet fission based photovoltaic devices. [Preview Abstract] |
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