Archive for the ‘Research’ Category

The title is taken from a recent review posted to the arXiv.

When we speak of quantum mechanical systems, what usually comes to mind are optical, electrical, and magnetic phenomena associated with atoms, molecules or solids. Rarely do we think of mechanical systems per se.

The review discusses progress in cooling nanoscale mechanical systems to the ground state, and then measuring position and momentum. Remembering the peculiarities of quantum mechanics, it is evident that measuring location of such an object, for example, disturbs the very location being measured. Nevertheless, the ground state motion of nanomechanical systems has been observed.

Interesting reading.

Electron tomography is now able to resolve atomic scale structure in three dimensions. The novelty of yesterday’s preprint lies in absence of a requirement for a priori information about the structure. Further, while conventional electron tomography achieves ~ 1 nm resolution, this paper demonstrates 2.4 Å resolution.

In March I wrote about Point Projection Microscopy (PPM), in which electrons are emitted from a single-atom tip and accelerated to (and through) a target with very high coherence. The spatial resolution is quite high. I found that the same research group had earlier calculated that PPM should be able to distinguish atoms of differing atomic number one from another. This would imply analytical applications of the technique in addition to image formation. See arXiv:1101.5135.

Mutus et al. at the University of Alberta have recently examined a suspended graphene film by low energy electron point projection microscopy (LEEPPM). [arXiv:1102.1758] Even though the electron energy was only 100-200 eV, approximately 75% of the electrons get through the film. With an effective source size of < 5Å, imaging resolution is quite good. The authors claim that the low beam energy does not result in sample contamination in the way higher voltage techniques such as STEM do. (I find this claim puzzling, since it is the secondary electrons which are largely responsible for chemical reactions induced by high voltage e-beams.) They speculate that a graphene film could be used as the ultimate microscope slide for imaging thin objects by PPM.

In EUV lithography, carbon deposition on the optics seriously degrades performance. A nice experiment at the University of Hyogo has shown that bleeding oxygen or ozone into the optics while irradiating with EUV can remove the deposits. [J. Vac. Sci. Technol. B 29, 011030 (2011); doi:10.1116/1.3533945]
An old invention by Somekh comes to mind; he discovered the same trick for electron beam systems in 1999. [US 6394109]

The Lyding group at the University of Illinois at Urbana—Champaign reports the writing of < 5 nm metallic structures by EBL. The lithography was carried out by electron beam induced deposition of hafnium diboride in a UHV scanning tunneling microscope.

Find the publication at DOI: 10.1021/nn1018522 .

In electron beam lithography, we usually attempt to squeeze as much current into the beam as possible. What about the other extreme? Can we arrange to get one electron at a time in the beam, and what use might that have?

The Baum group at the Max Planck Institute of Quantum Optics has done just this. Reporting in PNAS [doi: 10.1073/pnas.1010165107], they demonstrate single-electron pulses generated by photoemission. The photoemission in turn is driven by tuned, femtosecond UV pulses. Tuning of the UV wavelength turns out to be important for the bandwidth, coherence and duration of the resulting electron pulse. The experiment results in a transverse (electron) coherence of 2.5 nm, quite adequate for diffraction studies.

Now, with a coherence length of 2.5 nm and a duration of 100 fs or less, one can study atomic-scale dynamics in condensed matter and molecules by so-called four-dimensional imaging. Many interesting phenomena become visible; see the first twelve or so references in the paper.

For several years the Quantum Chemistry section (Kvankemiska avdelningen) at the University of Uppsala has held a mini-symposium honoring Per-Olov Löwdin, the institute’s founder. This year, being the 50th year anniversary of the institute, the symposium was a grand affair. The papers were all authored by graduates of Kvantkem, many of whom have now gone on to pursue careers well outside of quantum chemistry.

What has quantum chemistry to do with nanolithography? As we engineer truly nano-scale devices, we see more and more quantum effects. The theory developed in quantum chemistry may be brought to bear. Already, the community uses software like the Atomistix ToolKit to study nano-scale systems. I anticipate that quantum chemical methods will be used (and abused) as we engineer ever smaller systems. I will briefly summarize a few of the papers in what follows.

Several authors, led off by Prof. Yngve Öhrn, emphasized that our usual intuitive picture of more or less stationary nuclei embedded in a sea of electrons is but a theoretical fiction. If the system we study is in the ground state and there are no degeneracies among the electron states, then this so-called Born-Oppenheimer approximation is valid. Otherwise, it most certainly is not so. Prof. Öhrn has devoted many years of his career to develop computational methods to attack such systems.

Prof. Karl-Fredrik Berggren gave a fascinating talk on current flow in quantum dots. His talk was a summary of a recently published paper appearing in the New Journal of Physics. In my opinion, this is excellent physics; Prof. Berggren makes judicious approximations which tease out images by which we can comprehend complex phenomena.

I was intrigued by Prof. David Micha‘s talk on methods to compute spectra and dissipative dynamics in photoelectric materials. In this rather heavily technical overview, he also discussed photo-absorption by surface-bound clusters. The agreement between experiment and theory is not exact (due in part to experimental noise and in part to the state of the theory) but sufficient that we can probably start using computational methods to supplement experiments to improve photovoltaic performance.

In all, quantum chemistry and nano-engineering have each advanced to the point that we begin to see significant opportunities for the one to inform the other. Attending the 2010 Löwdinföreläsningar was time well spent.