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<p class="MsoNormal"><span style="font-size:15.0pt">There will be a <b><span style="color:red">virtual</span></b><span style="color:red">
</span>Modern Optics and Spectroscopy Seminar held <b>tomorrow, December 14 at 12pm.
</b>A short Q&A segment will immediately follow the conclusion of the seminar.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><b><i><span style="font-size:15.0pt">Zoom link:<span style="color:red">
<a href="https://mit.zoom.us/j/96365754552?pwd=S2hSdmFhdk5UY0VVelRibnRtaFByZz09">
<span style="color:red">https://mit.zoom.us/j/96365754552?pwd=S2hSdmFhdk5UY0VVelRibnRtaFByZz09</span></a></span></span></i></b><o:p></o:p></p>
<p class="MsoNormal"><b><span style="font-size:16.0pt">Password: <span style="color:red">
045380</span></span></b><o:p></o:p></p>
<p class="MsoNormal"><b><span style="font-size:15.0pt">__________________________</span></b><o:p></o:p></p>
<p class="MsoNormal"><b><span style="font-size:18.0pt"><br>
Julia Stähler<br>
</span></b><b><span style="font-size:17.0pt">Humboldt University of Berlin (Germany)</span></b><i><span style="font-size:14.0pt"><br>
<br>
</span></i><b><i><span style="font-size:16.0pt">“ZnO: Ultrafast generation and decay of a surface metal”</span></i></b><o:p></o:p></p>
<p class="MsoNormal"><i><br>
Band bending (BB) at semiconductor surfaces or interfaces plays a pivotal role in technology, ranging from field effect transistors to nanoscale devices for quantum technologies. The control of BB via chemical doping or electric fields can create metallic surfaces
with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of metallic surfaces via BB on ultrafast timescales would facilitate a drastic manipulation of the conduction, magnetic and optical properties
of semiconductors for novel high-speed electronics. We demonstrate the ultrafast (20 fs) generation of a metal at the (10</i><i><span style="font-family:"Cambria Math",serif">‑</span>10) surface of ZnO upon photoexcitation. This semiconductor is widely used
in optoelectronics due to its transparency for visible light and its ease of nanostructuring. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions (SMTs) that occur in the bulk of inorganic semiconductors, the SMT at the ZnO
surface is launched by 3-4 orders of magnitude lower photon fluxes; also, the back-transition to the semiconducting state is at least one order of magnitude faster than in previous studies of other materials. Using time- and angle-resolved photoelectron spectroscopy,
we show that the SMT is caused by the photoexcitation of deep surface defects. The resulting positive surface charges lead to downward BB toward the surface. Above a critical excitation density, a metallic band below the equilibrium Fermi level is formed.
This process is in analogy to chemical doping of semiconductor surfaces. Hence, it is not material-specific and presents a general route for controlling metallicity confined to semiconductor interfaces on ultrafast timescales.</i><o:p></o:p></p>
<p class="MsoNormal"><i> </i><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;background:white">
<i><span lang="DE" style="font-size:10.0pt;color:black">L. Gierster<sup>1</sup>, S. Vempati<sup>1,2</sup>, and <u>J. Stähler</u><sup>1,3</sup></span></i><o:p></o:p></p>
<p class="MsoNormal"><i><sup><span lang="DE" style="font-size:10.0pt;color:black;background:white">1</span></sup></i><i><span lang="DE" style="font-size:10.0pt;color:black;background:white">Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abt. Physikalische
Chemie, Faradayweg 4-6, 14195 Berlin, Germany</span></i><i><span style="font-size:10.0pt;color:black;background:white"><br>
<sup>2</sup>Present address: Department of Physics, Indian Institute of Technology Bhilai, Raipur-492015, India<br>
</span></i><i><sup><span lang="DE" style="font-size:10.0pt;color:black;background:white">3</span></sup></i><i><span lang="DE" style="font-size:10.0pt;color:black;background:white">Humbolt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Str. </span></i><i><span style="font-size:10.0pt;color:black;background:white">2,
12489 Berlin, Germany</span></i><i><span style="font-size:10.0pt;font-family:-webkit-standard;color:black;background:white"> </span></i><o:p></o:p></p>
<p class="MsoNormal"><i><br>
_____________________</i><i><span style="font-size:11.0pt"><br>
</span></i> <o:p></o:p></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt">Christine Brooks</span></b><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Administrative Assistant</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Massachusetts Institute of Technology</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Department of Chemistry</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">77 Massachusetts Ave, 6-333</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Cambridge, MA 02139</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">p: 617.253.7239</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">e: <a href="mailto:cbrooks@mit.edu">cbrooks@mit.edu</a></span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
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