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SUMMARY:Mapping Carrier Dynamics on Semiconductor Material Surfaces and at
  Interfaces using Laser Spectroscopy and 4D Electron Microscopy (LACUS)
DTSTART:20170622T163000
DTEND:20170622T173000
DTSTAMP:20260407T020832Z
UID:91190d37d9ee2905ac374deb5727bae8011317dedbf9da74851ae3a9
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Omar Mohammed\nKAUST \nSaudi Arabia\nInteractions at m
 aterial surfaces and interfaces have been among the most active areas of r
 esearch in the last decade [1\, 2]. This activity can be attributed to the
  important role that surface reactions play in a wide range of technologic
 al applications\, including interfacial chemistry\, Nanotechnology and pho
 tovoltaics. However\, the ability to access carrier dynamics selectively o
 n material surfaces with high spatial and temporal control in a photo-indu
 ced reaction is a particularly challenging task that can only be achieved 
 by applying four-dimensional ultrafast electron microscopy (4D UEM). For t
 his purpose\, we established and developed the second generation of four-d
 imensional scanning ultrafast electron microscopy (4D S-UEM) at KAUST and 
 demonstrate the ability to take time-resolved secondary electrons images (
 snapshots) of material surfaces with 650 fs and  5 nm temporal and spat
 ial resolutions\, respectively. In this method\, the surface of the photoa
 ctive materials is excited by a clocking optical pulse and the photo-induc
 ed changes will be imaged using a pulsed primary electron beam as a probe 
 pulse\, generating secondary electrons\, which are emitted from the surfac
 e of the specimen in a manner that is sensitive to the local electron/hole
  density. This method provides direct and controllable dynamical informati
 on regarding surface dynamics. For instance\, we clearly demonstrate how t
 he surface morphology\, grains\, defects and nanostructured features can s
 ignificantly impact the overall dynamical processes on the surface of phot
 oactive-materials [3]. Moreover\, we show that the energy loss and carrier
  spreading on the surfaces of InGaN nanowires can be achieved now in real 
 space [4]. We have also mapped the charge-carrier recombination selectivel
 y on the surface of these nanowires before and after surface passivation w
 ith octadecylthiol (ODT). The time-resolved images (snapshots) clearly dem
 onstrate that carrier recombination on the nanowires surface is significan
 tly slowed after surface treatment\, providing clear evidence of the minim
 ization of the surface defects upon passivation\, explaining clearly why t
 he performance of optoelectronic device based on these materials is much b
 etter after surface passivation [5]. In another interesting work describes
  a breakthrough in mapping charge carrier dynamics on the surface of quate
 rnary copper indium gallium selenide (CIGSe) nanocrystals (commonly used i
 n solar and optoelectronic devices) using S-UEM. The time-resolved images 
 provided by S-UEM clearly demonstrate how surface treatment with high band
  gap materials such as ZnS can control the overall carrier relaxation proc
 ess on the surfaces of these materials [6]. More specifically\, SE images 
 at different time delays of treated nanocrystal surfaces clearly indicate 
 that the carrier dynamics is slowed down by at least factor of two\, indic
 ating that the density of the trap states is significantly reduced after s
 urface treatment with ZnS inorganic layer.\nReferences\n1- A. O. El-Ballou
 li\, E. Alarousu\, M. Bernardi\, S. M. Aly\, A. P. Lagrow\, O. M. Bakr\, O
 . F. Mohammed.\, J. Am. Chem. Soc.136\, 6952 (2014).\n2- O. F. Mohammed\, 
 D.-S. Yang\, S. Pal\, A. H. Zewail\, J. Am. Chem. Soc. 133\, 7708 (2011).\
 n3- J. Sun\, V. A. Melnikov\, J. I. Khan\, O. F. Mohammed\, J. Phys. Chem.
  Lett.\, 6\, 3884 (2015).\n4- R. Bose\, J. Sun\, J. I. Khan\, B. S. Shahee
 n\, A. Adhikari\, T. K. Ng\, V. M. Burlakov\, M. P. Parida\, D. Priante\, 
 A. Goriely\, B. S. Ooi\, O. M. Bakr\, O. F. Mohammed\, Adv. Mater. 28\, 51
 06 (2016).\n5- J. I. Khan\, A. Adhikari\, J. Sun\, D. Priante\, R. Bose\, 
 B. S. Shaheen\, T. K. Ng\, O. M. Bakr\, B. S. Ooi\, O. F. Mohammed\, Small
 . 12\, 2313 (2016).\n6- R. Bose\, A. Bera\, M. R. Parida\, A. Adhikari\, B
 . S. Shaheen\, E. Alarousu\, J. Sun\, T. Wu\, O. M. Bakr\, O. F. Mohammed\
 , Nano Lett. 16\, 4417 (2016).
LOCATION:BCH 2201 https://plan.epfl.ch/?room==BCH%202201
STATUS:CONFIRMED
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