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SUMMARY:Mechanism and New Applications of Large and Persistent Photoconduc
 tivity
DTSTART:20181001T131500
DTEND:20181001T141500
DTSTAMP:20260916T032117Z
UID:69b5c8ab8187d8c3ab7d9231892ed74bd50a7705ddb884e9028e17df
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Rafael Jaramillo\, MIT USA\nLarge and persistent photoco
 nductivity (LPPC) in semiconductors is due to the trapping of photo-genera
 ted minority carriers at crystal defects. Theory has suggested that anion 
 vacancies in II-VI semiconductors are responsible for LPPC due to negative
 -U behavior\, whereby two minority carriers become kinetically trapped by 
 lattice relaxation following photo-excitation [1-2]. By performing a detai
 led analysis of photoconductivity in CdS\, we provide experimental support
  for this negative-U model of LPPC [3]. We also show that LPPC is correlat
 ed with sulfur deficiency. We use this understanding to vary the photocond
 uctivity of CdS films over nine orders of magnitude\, and vary the LPPC ch
 aracteristic decay time from seconds to 10\,000 seconds\, by controlling t
 he activities of Cd2+ and S2- ions during chemical bath deposition. We sug
 gest a screening method to identify other materials with long-lived\, non-
 equilibrium\, photo-excited states based on the results of ground-state ca
 lculations of atomic rearrangements following defect redox reactions\, wit
 h a conceptual connection to polarons and organic dyes.\nWe then apply our
  knowledge of defect physics in CdS to design selectors for resistive proc
 essing units\, and aqueous chemical sensors. We make two-terminal selector
  devices using hole injection layers to control the charge state of sulfur
  vacancies\, and thereby modify the film conductivity. We make chemical se
 nsors that take advantage of the sensitive dependence of photoconductivity
  on electron-hole recombination at interfaces. The resulting photoconducti
 vity chemical sensors offer a path to low-cost chemical sensing that doesn
 ’t rely on electrochemical charge transfer.\n[1] S. B. Zhang\, S.-H. Wei
  & A. Zunger\, Phys. Rev. B 63\, 075205 (2001).\n[2] S. Lany & A. Zuner\, 
 Phys. Rev. B 72\, 035215 (2005).\n[3] H. Yin\, A. Akey & R. Jaramillo\, ar
 Xiv 1806.01894 (2018).\nBio: Rafael Jaramillo is an assistant professor in
  the Department of Materials Science and Engineering at MIT. His research 
 sits in the big\, fun space between materials science\, solid state physic
 s\, and opto-electronic technologies. His current interests can be charact
 erized as defect and phase engineering of chalcogenide semiconductors\, wi
 th emphasis on developing processing methods to control sulfide and seleni
 de thin films. Previously he worked as a postdoc at Harvard and at MIT on 
 topics in oxide electronic materials and chalcogenide thin film solar cell
 s. He earned his PhD from The University of Chicago for work on quantum ph
 ase transitions in antiferromagnets. His thesis work included the discover
 y of the pressure-driven magnetic quantum phase transition in antiferromag
 netic chromium. Dr. Jaramillo is the recipient of numerous awards includin
 g the Rosalind Franklin Young Investigator Award from the Advanced Photon 
 Source at Argonne National Laboratory\, the Department of Energy SunShot P
 otdoctoral Fellowship\, and the National Science Foundation Faculty Early 
 Career Development Award (CAREER). He lives in Cambridge\, MA with his wif
 e and two young kids.\n\n 
LOCATION:MXF 1 https://plan.epfl.ch/?room==MXF%201
STATUS:CONFIRMED
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