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Researches Use Gold Coating on Nanowire to Control Luminescence

Published on 2017-03-20. Author : SpecialChem

Metal coatings to change the properties of nanowire semiconductors
UC graduate student Fatemesadat Mohammadi, right, said
future research might examine how different metal
coatings change the properties of nanowire semiconductors.
Physicists at the University of Cincinnati are working to harness the power of nanowires, microscopic wires that have the potential to improve solar cells or revolutionize fiber optics.

Hans-Peter Wagner, associate professor of physics, and doctoral student Fatemesadat Mohammadi are looking at ways to transmit data with the speed of fiber optics but at a significantly smaller scale.

Study of Plasmonics

Wagner and lead author Mohammadi are studying this field, called plasmonics, with researchers from three other universities. For the novel experiment, they built nanowire semiconductors with organic material, fired laser pulses at the sample and measured the way light traveled across the metal; technically, the excitations of plasmon waves.

So, if we succeed in getting a better understanding about the coupling between the excitations in semiconductor nanowires and metal films, it could open up a lot of new perspectives,” Wagner said.

The successful harnessing of this phenomenon -- called plasmon waveguiding -- could allow researchers to transmit data with light at the nano level.

How the Experiment Works

  • UC graduate student Fatemesadat Mohammadi and Associate Physics Professor Hans-Peter Wagner fire laser pulses at semiconductor nanowires to excite electrons that potentially serve as an energy pump to guide plasmon waves over a coated metal film just a few nanometers thick without losing power, a nettlesome physical property called resistivity
  • They measure the resulting luminescence of the nanowire to observe how light couples to the metal film
  • By sending light over a metal film, a process called plasmonic waveguiding, researchers one day could transmit data with light at the nano level

UC researchers fire laser pulses
UC researchers fire laser pulses at a complicated array
of mirrors and beam splitters for their nanorod experiment.
The luminescence is our interest. So we coat them and see: How does the photoluminescence characteristic change?” Mohammadi said.

To make the semiconductor, they use a technique called high-vacuum organic molecular beam deposition to spread organic and metal layers on gallium-nitride nanorods.

The use of organic film is unique to the UC experiment, Wagner said. The film works as a spacer to control the energy flow between excitons in the nanowire and the oscillation of metal electrons called plasmons.

Coating Nanorods with Gold

The organic material has the added benefit of also containing excitons that, arranged properly, could support the energy flow in a semiconductor, he said.

  • Coating the nanorods with gold significantly shortens the lifetime of the exciton emission resulting in what’s called a quenched photoluminescence
  • By using organic spacers between the nanorod and the gold film, the researchers are able to extend the emission lifetime to nearly the equivalent of nanorods without a coating
  • Once the gold-coated sample is prepared, they take it to an adjacent lab room and subject it to pulses of laser light
  • Mohammadi said it took days of painstaking work to arrange the small city of mirrors and beam splitters bolted at precise angles to a workbench for the experiment 

The reactions in the nanowire take just 10 picoseconds (which is a trillionth of a second.) And the laser pulses are faster still — 20 femtoseconds (a figure that has 15 zeros following it or a quadrillionth of a second. The UC project used a gold coating so that experiments could be replicated at a later date without risk of oxidation. Traditional coatings such as silver, Mohammadi said, hold even more promise

About University of Cincinnati

The University of Cincinnati (commonly referred to as UC or Cincinnati) is a comprehensive public research university in Cincinnati, in the U.S. state of Ohio, and a part of the University System of Ohio. Founded in 1819 as Cincinnati College, it is the oldest institution of higher education in Cincinnati and has an annual enrollment of over 44,000 students, making it the second largest university in Ohio

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Source: University of Cincinnati
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