Seminar by Prof. Omar F. Mohammed on Mapping Charge Carrier Dynamics in Real-space and Time on Semiconductor Material Surfaces and Interfaces using Ultrafast Spectroscopy and Four-dimensional Electron Microscopy

21 May 2018

Date: 21 May 2018 (Monday)

Time: 10:00 am

Venue: B5-309, Yeung Kin Man Academic Building

Understanding light-triggered charge carrier dynamics on photovoltaic-material surfaces and at interfaces has been a key element and one of the major challenges for the development of real-world energy devices [1-4]. We achieved the challenging task of accessing carrier dynamics selectively on material surfaces with high spatial and temporal control in a photo-induced reaction by applying four-dimensional ultrafast electron microscopy (4D UEM) along with time-resolved laser spectroscopy. The time-resolved secondary electrons provide images (snapshots) of material surfaces with 650 fs and ~4 nm temporal and spatial resolutions, respectively. In this method, the surface of the photoactive materials is excited by a clocking optical pulse and the photo-induced changes are imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons, which are emitted from the surface of the specimen in a manner that is sensitive to the local electron/hole density. Using this method, we obtained controllable dynamical information on surface dynamics. For instance, we clearly demonstrate how the surface morphology, grains, defects and nanostructured features can significantly impact the overall dynamical processes on the surface of photoactive-materials [5]. Moreover, we show that the energy loss and carrier spreading on the surfaces of InGaN nanowires can be achieved now in real space [6]. The time-resolved images (snapshots) clearly demonstrate that carrier recombination on the nanowires surface is significantly slowed after surface treatment, providing clear evidence of the minimization of the surface defects upon passivation, explaining clearly why the performance of optoelectronic device based on these materials is much better after surface passivation [7]. In another interesting work on quaternary copper indium gallium selenide (CIGSe) nanocrystals (commonly used in solar and optoelectronic devices), the time-resolved images from S-UEM provided by S-UEM clearly demonstrate how surface treatment with high band gap materials such as ZnS can control the overall carrier relaxation process on the surfaces of these materials [8]. Charge carrier dynamics in semiconductor quantum dots and perovskite single crystal will be also presented and discussed.

References

1- O. M. Bakr, O. F. Mohammed., Science 355, 1260 (2017).

2- A. O. El-Ballouli, E. Alarousu, M. Bernardi, S. M. Aly, A. P. Lagrow, O. M. Bakr, O. F.�� Mohammed., J. Am. Chem. Soc. 136, 6952 (2014).

3- R. Begum, M. R. Parida, A. L. Abdelhady, B. Murali, N. Alyami, G. H. Ahmed, M. N. Hedhili, O. M. Bakr, and O. F. Mohammed.,J. Am. Chem. Soc. 139, 731 (2017).

4- O. F. Mohammed, D.-S. Yang, S. Pal, A. H. Zewail, J. Am. Chem. Soc. 133, 7708 (2011).

5- J. Sun, V. A. Melnikov, J. I. Khan, O. F. Mohammed, J. Phys. Chem. Lett. 6, 3884 (2015).

6- R. Bose, J. Sun, J. I. Khan, B. S. Shaheen, 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, 5106 (2016).

7- 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).

8- 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).

Short Biography:

Prof. Omar F. Mohammed

KAUST Solar Center, Division of Physical Sciences and Engineering, KAUST, Thuwal 23955-6900, KSA

�Professor Mohammed is the principal investigator of ultrafast laser spectroscopy and four-dimensional electron imaging laboratory and he is affiliated with Solar and Photovoltaics Engineering Research Center at KAUST, and at present, his research activities are focused on the development of highly efficient solar cells with the aid of cutting-edge nanotechnology, laser spectroscopy, and ultrafast electron imaging. Prior to joining KAUST in December 2012, Dr. Mohammed was a senior research associate in Professor Ahmed Zewail�s group at Caltech, USA. While there, Dr. Mohammed joined the research group of Professor Zewail and worked on the development of new laser spectroscopic and time-resolved electron imaging techniques. Prior to his arrival in Pasadena, California, Dr. Mohammed spent more than a decade in Germany, Switzerland and Japan, embarking on the development of new laser spectroscopic techniques for direct observation of many chemical, physical and biological processes in real time. The accomplishments of Dr. Mohammed have resulted in more than 200 articles, invited talks and conference proceedings; and a large number of these papers are published in very high impact scientific journals including Science, Nature Materials, Advanced Materials, JACS, Nano Letters, Nature Communications, Advanced Energy Materials, Angewandte Chemie, PNAS, Chemical Communications, Advanced Functional Materials, Journal of Physical Chemistry Letters, Small and many others.

AWARDS

2002-2006: Long-term fellowship (4 years), Max-Born Institute, Berlin, Germany

2006-2007: Post-doctoral Fellowship, University of Geneva, Switzerland

2007-2008: Post-doctoral Fellowship (JSPS fellowship) & Special post-doctoral researcher (RIKEN��fellowship), RIKEN, Japan

2009-2010: Distinguished scholar award from Arab Fund for Economic and Social Development

2010-2011: The State Prize in Basic Sciences, Egypt

PUBLICATION HIGHLIGHTS

� 116 peer-reviewed journal articles: 80 as an independent faculty, 64 as corresponding author.

� 12 Thomson Reuters� Highly Cited Papers

� 5 Thomson Reuters� Hot Papers

� 14 Front covers of the journals

� 2 ACS Editors' Choice Articles

� 49 articles in Nature Index� journals, including: 3 Science, 3 Nature Communications, 1 Nature Materials, 4 Advanced Materials, 8 Journal of the American Chemical Society, 8 Angewandte Chemie International Edition, 3 Nano Letters, 14 Journal of Physical Chemistry Letters, 2 Chemical Communications, 2 Applied Physics Letters and 1 Chemical Science).

Enquiry:

Department of Materials Science and Engineering

Email: mse@cityu.edu.hk

Tel: 3442 2985