November 21st, 2013
The Nouchi group has discovered two methods for inducing oxidation in graphene. In the first, they noted that graphene oxidized while performing Raman spectroscopy. The presence of water is required. They further established that, while exposure to water alone did not change the characteristics of the graphene film, exposure to water simultaneously with UV irradiation did change the electron transport properties of the film.
In the second experiments, the group explored the reaction between graphene and oxygen caused by UV irradiation, and the physisorption of oxygen molecules when a gate voltage is applied to graphene field effect transistors. Irradiation of positively biased graphene gates results in oxygen desorption, while similar irradiation of negatively biased gates results in etching of the graphene. This etching proceeds predominately from the edges.
N Mitoma, R Nouchi, and K Tanagaki, “Photo-oxidation of graphene in the presence of water”, arXiv:1311.3447.
N Mitoma and R Nouchi, “Gate-controlled ultraviolet photo-etching of graphene edges”, arXiv:1311.3452.
November 8th, 2013
Field ion microscopy has a long history, going back to Mueller’s 1951 paper in Z. Phys.. A major advance leading to practical sub-nanometer application came in 2006 with the appearance of the ALIS paper in JVST B. This paper revealed the tri-atomic ionization source, which permits high brightness and very small source size.
Hlawacek et al. at Twente posted a review of helium ion microscopy to the arXiv yesterday. Despite its brevity (16 pages) this review covers many theoretical and practical aspects. With 136 references, it provides a jumping-off point for anyone wanting a serious introduction to this technology.
E. W. Müller, “Das Feldionenmikroskop,” Zeitschrift für Phys. 131, 136–142 (1951)
B. W. Ward, J. A. Notte, and N. P. Economou, “Helium ion microscope: A new tool for nanoscale microscopy and metrology,” J. Vac. Sci. Technol. B 24, 2871 (2006).
G Hlawacek, V Veligura, R van Gastel, and B. Poelsema, “Helium Ion Microscopy”, arXiv:1311.1711v1
November 1st, 2013
It is by now routine to achieve sub-atomic resolution (a few 10′s of pm) by aberration-corrected TEM and by STEM. However, reducing the signal to noise ratio sufficient for the highest resolution requires exposing the sample to fairly high electron dose. The electron irradiation causes sample damage while the image is being acquired. Several mechanisms may contribute: electron knock-on (in which an imaging electron transfers enough momentum to an atom to displace the atom from its original position), radiolysis (in which chemical bonds are rearranged, leading the target material to change into something else), heating (including sample melting), and Coulomb explosion (due to sample charging).
In viewing two-dimensional materials, one cannot average the imaging over depth; a two-dimensional material is by definition all surface. So any damage immediately and irreversibly degrades the image.
Two groups, one at Manchester, the other at Ulm, have independently discovered that it is possible to stablize MoS2 during (S)TEM imaging. This is done making a sandwich of graphene/MoS2/graphene. The Ulm group imaged with aberration-corrected TEM, while at Manchester, they used STEM. Additions to the image from the graphene layers, being periodic, can be subtracted out, leaving only the image of the study material, MoS2. It is an interesting coincidence that the two groups compared identical material stacks.
“The pristine atomic structure of MoS2 monolayer protected from electron radiation
damage by graphene”, G Algara-Siller, et al. [arXiv:1310.2411]
“Control of Radiation Damage in MoS2 by Graphene Encapsulation”, R Zan et al. [ACS Nano (DOI: 10.1021/nn4044035), see also arXiv:1310.4012]
October 25th, 2013
Professor Jun-Bo Yoon’s group at KAIST has recently demonstrated bulk production of 200 mm long nanowires. The process begins with a silicon master template, which is used to fabricate many (thousands?) of disposable plastic templates. One then applies the nanowire material to the plastic template by PVD. It would appear that any material which can be deposited by PVD onto the plastic can be used, viz., metals, oxides and nitrides are demonstrated in the paper. Shadowing during PVD leads to the deposition being thinner in the trenches than on the peaks; a clean-up etch removes the trench material and leaves isolated wires atop each peak.
See the abstract (and select images) at ACS NanoLetters. The full paper can be found on Prof. Yoon’s website.
October 4th, 2013
We have seen many publications on the topic of surface plasmon polaritons (SPP) in semiconductors. The patterns taken by the SPPs have invariably been static either in space, in time or in both. Experimentalists in The Netherlands recently used optical pumping to generate localized SPPs dynamically. This was achieved by inserting spatial light modulation (SLM) in the optical pump path. For the sake of clean, definitive experiments, the authors reported [arXiv:1309.7190] THz wave extinction by pseudo-randomly spaced dipoles of fixed lengths. In their conclusions, they point to exciting possible extensions (e.g., spatial and temporal control of resonant frequencies and local field enhancements).
September 6th, 2013
IMS and JEOL are now partners in developing a multi-beam mask writer, according to the press release at Businesswire.
August 30th, 2013
Not surprisingly, properties of a two-dimensional material, being all surface, may be exquisitely susceptible to contamination. (See “Experimenting with monolayers” and links therein.) This general observation has been recently confirmed in MoS2 as well.
On the other hand, this susceptibility can be exploited for band engineering, doping, etc. Two recent preprints on the arXiv make interesting reading. See arXiv:1308.4924, discussing organometallic complexes of graphene, as well as arXiv:1308.1645, discussing the reversible grafting of naphthylmethyl radicals to graphene.
August 30th, 2013
Related to the automated search which I mentioned in my previous post Beyond graphene, AFLOWLIB.ORG have posted a description and announcement of an extensive automated “software framework for high-throughput calculation of crystal structure properties of alloys, intermetallics and inorganic compounds.” See the publication “AFLOW: an automatic framework for high-throughput materials discovery“, as well as their website.
August 2nd, 2013
I have posted several times on the topic of graphene. There is considerable interest in other two-dimensional (2D) materials. I would like to point out two recent papers.
S Butler, et al., ACS Nano, 2013, 7 (4), pp 2898–2926
“Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene”
This is a big review (29 pages, 340 references) with an experimental emphasis. It encompasses four major sections: known syntheses and preparation strategies, sample characterization techniques, bulk vs. single/few layer electronic behavior, and applications. It is quite readable by the non-specialist.
(The paper can be found on-line.)
S Lebègue, et al., Phys. Rev. X 3, 031002 (2013)
“Two-Dimensional Materials from Data Filtering and Ab Initio Calculations”
The emphasis of this theoretical paper is to identify previously unknown 2D materials which may have interesting properties. Running through a database of crystollographic structures, the authors select bulk materials according to their unit cell packing ratios and the gap between adjacent c-planes. The geometries of the corresponding single layer structures are optimized, driving the forces on the constituent atoms toward zero. As a last step, the band structure and density of states are determined for each candidate structure. This process resulted in 92 2D candidate compounds.
July 30th, 2013
Careful experiments from CEOS Corrected Electron Optical Systems GmbH in Heidelberg have shown that stochastic magnetic field noise limits the ultimate resolution of transmission electron microscopes. This magnetic field noise is due to unavoidable thermal currents in the conductors of which the TEM is constructed. Best summarized in the authors’ words, “[E]xperiments have revealed a hitherto unknown fundamental performance limitation for electron microscopy due to the stochastic beam deflection caused by the noise fields.” The authors estimate that observed image blur of 15-25 pm can be explained by this phenomenon.
Reference: Stephan Uhlemann, Heiko Müller, Peter Hartel, Joachim Zach, and Max. Haider, “Thermal Magnetic Field Noise Limits Resolution in Transmission Electron Microscopy”, Phys. Rev. Lett. 111, 046101 (2013)
See also: “Viewpoint: What Are the Resolution Limits in Electron Microscopes?“