AVS2012 Session SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM: Probe-Sample Interactions, Nano-Manipulation and Fabrication
Time Period WeM Sessions | Abstract Timeline | Topic SP Sessions | Time Periods | Topics | AVS2012 Schedule
| Start | Invited? | Item |
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| 8:20 AM | Invited |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-2 Controlled Coupling of Silicon Atomic Quantum Dots at Room Temperature: A Basis for Atomic Electronics?
Robert Wolkow (University of Alberta and The National Institute for Nanotechnology, Canada); Jason Pitters (The National Institute for Nanotechnology, Canada); Gino DiLabio, Marco Taucer, Paul Piva, Lucian Livadaru (University of Alberta and The National Institute for Nanotechnology, Canada) Quantum dots are small entities, typically consisting of just a few thousands atoms, that in some ways act like a single atom. The constituent atoms in a dot coalesce their electronic properties to exhibit fairly simple and potentially very useful properties. It turns out that collectives of dots exhibit joint electronic properties of yet more interest. Unfortunately, though extremely small, the finite size of typical quantum dots puts a limit on how close multiple dots can be placed, and that in turn limits how strong the coupling between dots can be. Because inter-dot coupling is weak, properties of interest are only manifest at very low temperatures (milliKelvin). In this work the ultimate small quantum dot is described – we replace an “artificial atom” with a true atom - with great benefit.
It is demonstrated that the zero-dimensional character of the silicon atom dangling bond (DB) state allows controlled formation and occupation of a new form of quantum dot assemblies - at room temperature. Coulomb repulsion causes DBs separated by less than ~2 nm to experience reduced localized charge. The unoccupied states so created allow a previously unobserved electron tunnel-coupling of DBs, evidenced by a pronounced change in the time-averaged view recorded by scanning tunneling microscopy. It is shown that fabrication geometry determines net electron occupation and tunnel-coupling strength within multi-DB ensembles and moreover that electrostatic separation of degenerate states allows controlled electron occupation within an ensemble.
Some speculation on the viability of a new “atomic electronics” based upon these results will be offered. |
| 9:00 AM |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-4 Atomic Forces and Energy Dissipation of a Bi-Stable Molecular Junction
Christian Lotze (Freie Universtiät Berlin, Germany); Martina Corso, Katharina Franke, Felixvon Oppen, Jose Ignacio Pascual (Freie Universität Berlin, Germany) Tuning Fork based dynamic STM/AFM is a well established method combining the advantages of scanning tunneling and dynamic force microscopy. Using tuning forks with high stiffness, stable measurements with small amplitudes, below 1 Å can be performed. In this way, conductance and frequency shift measurements of molecular junction can be obtained simultaneously [1] with intramolecular resolution [2]. One of the most intriguing aspects of molecular junctions relates to the effect of structural bi-stabilities to the properties of the junction. These lead, for example, to conductance fluctuations, telegraph noise and the possibility to switch the electrical transport through the junction. In this presentation, we characterize a model bi-stable molecular system using dynamic force spectroscopy. The effect of current-induced stochastic fluctuations of conductance are correlated with fluctuations in force. In our experiment we identified the last from both, frequency shifts and energy dissipation measurements, picturing a regime in which electrical transport and mechanical motion are coupled. [1] N. Fournier et. al, PhysRevB 84, 035435 (2011), [2] L. Gross et. al, Science 324, 1428 (2009) |
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| 9:20 AM |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-5 Acetylene on Cu(111): Imaging a Molecular Pattern with a Constantly Rearranging Tip
Yeming Zhu, Jonathan Wyrick, Kamelia Cohen, Katie Marie Magnone, Connor Holzke, Daniel Salib, Quan Ma, Dezheng Sun, Ludwig Bartels (University of California Riverside) Abstract: Using variable temperature STM and DFT simulation, we identify the phases of acetylene adsorbed on the Cu(111) surface. Depending on the coverage, a diffraction-derived surface pattern of acetylene on Cu(111) is validated by STM. The modification of the STM image transfer function through the adsorption of an acetylene molecule onto the tip apex is taken into account. In this case, the images of acetylene patterns on Cu(111) also include direct evidence of the rotational orientation and dynamics of the acetylene species attached to the tip apex. DFT modeling of acetylene/Cu(111) reveals that the molecular orientation and separation is governed by a balance of repulsive interactions associated with stress induced in the top surface layer and attractive interactions mediated by the electronic structure of the substrate. Computationally modeling of the substrate with 3 layers obtains the periodicity of the intermolecular interaction that provides a theoretical underpinning for the experimentally observed molecular arrangement. |
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| 9:40 AM |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-6 Atomic Scale Imaging and Electronic Structure of Trimethylaluminum Deposition on III-V Semiconductor (110) Surfaces
Tyler Kent, Mary Edmonds, Evgueni Chagarov, Andrew Kummel (University of California San Diego) Silicon based metal oxide semiconductor field effect transistors (Si-MOSFETs) are quickly approaching their theoretical performance limits, as a result many semiconductors are being explored as an alternative channel material for use in MOSFETs. III-V semiconductors are an appealing alternative to Si because of their higher electron mobilities. The limiting factor in III-V based MOSFET performance is defect states which prevent effective modulation of the Fermi level. The InGaAs (001) As-rich (2x4) surface contains two types of unit cells: ideal unit cells with double As-dimers and defect unit cells with single As-Dimers. The missing As-dimer unit cells, which comprise ~50% of the surface, are believed to cause electronic defect states at the semiconductor-oxide interface, specifically at the conduction band edge of the semiconductor. In-situ scanning tunneling microscopy and spectroscopy (STM/STS) and density function theory (DFT) modeling show that TMA readily passivates the As-As dimers in the ideal unit cell but the missing InGaAs(001)-2x4 may not be fully passivated by TMA. To improve the electronic structure of the interface, the sidewalls of the finFETs on InGaAs(001) can be fabricated along the (110) direction. The (110) surface contains only buckled III-V heterodimers in which the lower group III atom is sp2 hybridized with an empty dangling bond and the upper group V atom is sp3 hybridized with a full dangling bond. This results in an electrically unpinned surface.
To investigate the benefits of using a (110) surface as a channel material, the atomic and electronic structure of the ALD precursor trimethylaluminum (TMA) monolayer deposited on III-V (110) surfaces has been studied using in-situ STM and STS. Both GaAs and InGaAs samples were studied. GaAs wafers were obtained from Wafertech with a Si doping concentration of 4x1018/cm3. The (001) samples were cleaved in-situ to expose the (110) surface. Samples were transferred to the STM chamber (base pressure 1x10-11 torr) where the atomic bonding structure of the precursor monolayer unit cell was determined. STS, which probes the local density of states (LDOS), was used to determine Fermi level pinning. A model of TMA chemisorption was developed in which TMA chemisorbs between adjacent As atoms on the surface, giving a highly ordered monolayer with a high nucleation density which could allow for aggressive effective oxide thickness (EOT) scaling. |
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| 10:00 AM | BREAK - Complimentary Coffee in Exhibit Hall | |
| 10:40 AM |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-9 A New Experimental Method to Determine the Torsional Spring Constants of Microcantilevers
Georg Haehner, John Parkin (University of St Andrews, UK) Cantilever based technologies have seen an ever increasing level of interest since the atomic force microscope (AFM) was introduced more than two decades ago. Recent developments employ microcantilevers as stand-alone sensors by exploiting the dependence of their oscillating properties on external parameters such as adsorbed mass [1], or the density and the viscosity of a liquid environment [2,3]. They are also a key part in many microelectromechanical systems (MEMS) [4]. In order to quantify measurements performed with microcantilevers their stiffness or spring constants have to be known. Following calibration of the spring constants a change in oscillation behavior can be quantitatively related to physical parameters that are probed. The torsional modes of oscillation have attracted significant attention due to their high sensitivity towards lateral and friction forces, and recent developments in torsional-tapping AFM technology [5]. However, the methods available to determine the torsional spring constants experimentally are in general not simple, not very reliable, or risk damage to the cantilever [6].
We demonstrate a new method to determine the spring constants of the torsional modes of microcantilevers experimentally with high accuracy and precision. The method is fast, non-destructive and non-invasive. It is based on measuring the change in the resonance frequencies of the torsional modes as a function of the fluid flow escaping from a microchannel. Results for rectangular cantilevers will be presented and compared to results obtained with other methods [7].
[1] J. D. Parkin and G. Hähner, Rev. Sci. Instrum. 82 (3), 035108 (2011).
[2] N. McLoughlin, S. L. Lee, and G. Hähner, Appl. Phys. Lett. 89 (18), 184106 (2006).
[3] N. McLoughlin, S. L. Lee, and G. Hähner, Lab Chip , 1057 (2007).
[4] S. Beeby, G. Ensell, N. Kraft, and N. White, MEMS Mechanical Sensors. (Artech House London, 2004).
[5] O. Sahin and N. Erina, Nanotechnology 19 (44), 445717 (2008).
[6] M. Munz, Journal of Physics D-Applied Physics 43 (6), 063001 (2010).
[7] C. P. Green, H. Lioe, J. P. Cleveland, R. Proksch, P. Mulvaney, and J. E. Sader, Rev. Sci. Instrum. 75 (6), 1988 (2004). |
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| 11:00 AM |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-10 A Torsional Device for Easy, Accurate and Traceable Force Calibration of AFM Cantilevers
Jose Portoles, Peter Cumpson (Newcastle University, UK) Accurate measurement of biologically-relevant forces in the range of pN to μN is an important problem in nanoscience. A number of force probe techniques have been applied in recent years. The most popular is the Atomic Force Microscope (AFM). Accuracy of force measurement relies on calibration of the probe stiffness which has led to the development of many calibration methods[1], particularly for AFM microcantilevers. However these methods typically exhibit uncertainties of at best 15% to 20% and are often very time consuming. Dependency on material properties and cantilever geometry further complicate their application and take extra operator time. In contrast, one rapid and straightforward method involves the use of reference cantilevers (the "cantilever-on-cantilever" method) or MEMS reference devices. This approach requires that a calibrated reference device is available, but it has been shown to be effective in providing measurement traceability[2]. The main remaining difficulty of this approach for typical users is the positional uncertainty of the tip on the reference device, which can introduce calibration uncertainties of up to around 6%. Here we present a new reference device based on a torsional spring of relatively large dimensions compared to the typical AFM cantilever and demonstrate how it is calibrated. This method has the potential to calibrate the reference device traceably[3] to the SI with a 1% accuracy by applying techniques typically used for the characterisation of micromechanical devices. The large dimensions of the device reduce the positional uncertainty below 1% and simultaneously allow the use of the device as an effective reference array with different reference stiffnesses at different positions ranging from 0.090 N/m to 4.5 N/m [1] P J Cumpson, C A Clifford, J F Portolés, J E Johnstone, M Munz Cantilever Spring-Constant Calibration in Atomic Force Microscopy, pp289-314 in Volume VIII of Applied Scanning Probe Methods, Ed. B Bhushan and H Fuchs (Springer, New York, 2009) [2] P J Cumpson PJ, J Hedley, Nanotechnology 14 (2003) pp. 1279-1288 [3] J F Portolés, P J Cumpson, J Hedley, S Allen, P M Williams & S J B Tendler, Journal of Experimental Nanoscience 1 (2006) pp51-62. |
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| 11:20 AM | Invited |
SP+AS+BI+ET+MI+NM+NS+SS+TF-WeM-11 Nanoscale Surface Assembly by Single-Molecule Cut-and-Paste
Hermann Gaub (Ludwig-Maximilians Universitat, Germany) Bottom up assembly of functional molecular ensembles with novel properties emerging from |