Dr Shengfu Yang's Research Interests

Probing molecules using helium nanodroplets

Nanodroplets of superfluid liquid helium show some extraordinary physical properties, including extremely low temperatures (0.4 K for 4He) and the potential for rapid cooling of molecules embedded in their interior. For chemists, this offers a unique and exciting environment for exploring a variety of phenomena, such as the formation of metastable molecular complexes. The rapid cooling rate is the key to producing these unusual complexes, because molecules are almost continuously cooled as they are drawn together by intermolecular forces. This process can result in species being trapped in shallow, long-range minima on the potential energy surface.

    Working with Dr Andrew Ellis, we recently constructed a pulsed helium nanodroplet source, one of only two in the world.  It was found to be possible to continuously tune the helium nanodroplet sizes by variation of the stagnation pressure and nozzle temperature. The key factor for this tuneability lies in the careful design of the nozzleis a channel section of a conical nozzle (see Figure 1 below). With this helium droplet source we have begun a wide range of studies of molecules and molecular clusters.  For example, we are exploring the possibility of using helium droplets as an ultrasoft ‘cushion’ during electron impact ionization, allowing molecular ions, derived from molecules inside the droplets, to survive intact in the gas phase.  Other work is exploring the behaviour of molecules inside the droplets using laser spectroscopy.

Dr Shenfu Yang in Lab

New frontiers in nanochemistry: the formation of novel core-shell nanoparticles using liquid helium droplets

Liquid helium droplets are capable of picking up atoms and molecules whenever they are encountered on the droplet journey.  With very large helium droplets, many atoms and/or molecules can be captured, and will enter, each droplet.  This, coupled with the exceptionally low temperature, brings about condensation inside the helium droplet.  Not only is it possible to grow clusters of any type of material, but different materials can be added in sequence, allowing the synthesis of multilayer, or so-called core-shell, nanoparticles.

Figure 2 (below) shows the principle of the experiment.  To grow very large helium droplets (containing up to 1010 helium atoms/droplet), helium is liquefied prior to entering the vacuum chamber by cooling to ~ 1 K.  The liquid then enters the vacuum chamber through a 2 mm nozzle, where it forms a narrow liquid microjet.  After a short distance, this microjet will break up into micron-sized droplets.  After picking up the desired atoms/molecules, the helium can be stripped away by collision with a deposition plate.  This soft-landing will allow the nanoparticles to remain intact.

Formation of core shell

Figure 2. Scheme for formation of core-shell nanoparticles using liquid helium droplets.

This technique can potentially be made to make a vast array of new types of nanoparticles, including some extraordinary nano-objects.  These are being explored in a joint project between the Chemistry (Yang, Ellis) and Physics (Binns, von Haeften) departments at Leicester.  Applications are being sought in a number of areas, including nanoelectronics, electro-optics, and heterogeneous catalysis.

Link to research group web pages

Recent Publications

  1. F. Ferreira da Silva, S. Jaksch, G. Martins, H. M. Dang, M. Dampc,S. Denifl, T. D. Mark, P. Limao-Vieira, J. Liu, S. Yang, A. M. Ellis and P. Scheier, "Electron attachment and electron ionization of acetic acid clusters embedded in helium nanodroplets", Phys. Chem. Chem. Phys. (2009) (in press)
  2. Shengfu Yang and Andrew M. Ellis, "Selecting the size of helium nanodroplets using time-resolved probing of a pulsed helium droplet beam", Rev. Sci. Instrum. 79 (2008) 016106.
  3. Andrew M. Ellis and Shengfu Yang*, "Modelling the charge transfer probability in helium nanodroplets following electron impact ionization", Phys. Rev. A 76 (2007) 032714.
  4. Shengfu Yang, Scott M. Brereton, Satvinder Nandhra, Andrew M. Ellis, Bo Shang, Lanfeng Yuan and Jinlong Yang, "Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)+n clusters", J. Chem. Phys. 127 (2007) 134303.
  5. Shengfu Yang, Scott M. Brereton and Andrew M. Ellis, "Electron impact ionization mass spectrometry of aliphatic alcohol clusters in helium nanodroplets", Intl. J. Mass. Spectro. 253 (2006) 79-86.
  6. Shengfu Yang, Scott M. Brereton, Martyn D. Wheeler and Andrew M. Ellis, "Electron impact ionization of haloalkanes in helium nanodroplets", J. Phys. Chem. A 110 (2006) 1791-1797.

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Contact details

Dr S. Yang
Department of Chemistry
University of Leicester

tel: 0116 252 2127

email: sfy1@le.ac.uk