Publications in 2011

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Variations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory
D. Ziegler, P.Gava, J. Guttinger, F. Molitor, L. Wirtz, M. Lazzeri, A.M. Saitta, A. Stemmer, F. Mauri, C. Stampfer
Physical Review B 83 (23) Article Number 235434, 2011
DOI:10.1103/PhysRevB.83.235434

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO(2) substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single-and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene.

Force gradient sensitive detection in lift-mode Kelvin probe force microscopy
D. Ziegler, A. Stemmer
Nanotechnology 22 (7), Article Number: 075501, 2011
DOI:10.1088/0957-4484/22/7/075501

We demonstrate frequency modulation Kelvin probe force microscopy operated in lift-mode under ambient conditions. Frequency modulation detection is sensitive to force gradients rather than forces as in the commonly used amplitude modulation technique. As a result there is less influence from electric fields originating from the tip's cone and cantilever, and the recorded surface potential does not suffer from the large lateral averaging observed in amplitude modulated Kelvin probe force microscopy. The frequency modulation technique further shows a reduced dependence on the lift-height and the frequency shift can be used to map the second order derivative of the tip-sample capacitance which gives high resolution material contrast of dielectric sample properties. The sequential nature of the lift-mode technique overcomes various problems of single-scan techniques, where crosstalk between the Kelvin probe and topography feedbacks often impair the correct interpretation of the recorded data in terms of quantitative electric surface potentials.

Fully Automated Microinjection System for Xenopus laevis Oocytes With Integrated Sorting and Collection
S.F. Graf, T. Madigou, R.Y. Li, C. Chesne, A. Stemmer, H.F. Knapp
JALA 16 (3) 186-196, 2011
DOI:10.1016/j.jala.2011.03.006

Microinjection is the most flexible transfection method in terms of choice of reagents to inject into cells. But this method lacks the high throughput to compete with less flexible methods like chemical- or viral-based approaches. Various approaches have been pursued to increase the throughput by automating the microinjection process. However, these approaches focused solely on the microinjection itself and disregarded the tasks before and after the injection, which also belong to the critical time path of the whole process, that is, sorting out viable cells from a cell suspension, placing the cell for injection, and collecting the cell after the injection. In the approach with our XenoFactor, we demonstrate a system capable of running the whole process automatically. By optimizing the XenoFactor for Xenopus laevis oocytes, we could demonstrate the successful automated injection. Starting from a suspension with a mixture of defolliculated oocytes at different stages and quality levels, the manual approach requires 1 day in total for the preparation of 400 microinjected oocytes. The XenoFactor takes only 4 h for the same amount and delivers injected oocytes of reproducible quality and without the fatigue symptoms experienced during the manual approach.

Image-Based Fluidic Sorting System for Automated Zebrafish Egg Sorting into Multiwell Plates
S.F. Graf, S. Hotzel, U. Liebel, A. Stemmer, H.F. Knapp
JALA 16 (2) 105-111, 2011
DOI:10.1016/j.jala.2010.11.002

The global demand for the reduction of animal testing has led to the emergence of Zebrafish eggs/larvae as model organisms to replace current adult animal testing in, for example, toxicity testing. Because of the egg size (diameter 1.6 mm) and the relatively easy maintenance of Zebrafish farms the eggs also offer high-throughput screening (HTS). However, the current bottleneck for HTS is the cost-efficient placing of individual organisms into single wells of a multiwell plate (MWP). The system presented here is capable of storing, sorting, and placing individual organisms in a highly reproducible manner. In about 11 min a complete 96-MWP is filled, which corresponds to about 8 sec per egg. The survival rate of fertilized transgenic and wild-type eggs was comparable to the one of the control (control 6.7%, system 7.6%). Furthermore, it was also possible to place dechorionated eggs into individual wells. The results demonstrate that the cost efficient system works gentle and reliable enough to disburden scientists from the exhausting and monotonous job of placing single eggs into single wells, such that they can concentrate on the scientific aspects of their experiments and create results with a higher statistical relevance.

A high frequency sensor for optical beam deflection atomic force microscopy
R. Enning, D. Ziegler, A. Nievergelt, R. Friedlos, K. Venkataramani, A. Stemmer
Review of Scientific Instruments 82 (4) Article Number: 043705, 2011
DOI:10.1063/1.3575322

We demonstrate a novel electronic readout for quadrant photodiode based optical beam deflection setups. In our readout, the signals used to calculate the deflections remain as currents, instead of undergoing an immediate conversion to voltages. Bipolar current mirrors are used to perform all mathematical operations at the transistor level, including the signal normalizing division. This method has numerous advantages, leading to significantly simpler designs that avoid large voltage swings and parasitic capacitances. The bandwidth of our readout is solely limited by the capacitance of the quadrant photodiode junctions, making the effective bandwidth a function of the intensity of photocurrents and thus the applied power of the beam deflection laser. Using commercially available components and laser intensities of 1-4 mW we achieved a 3 dB bandwidth of 20 MHz with deflection sensitivities of up to 0.5-1 V/nm and deflection noise levels below 4.5 fm/root Hz. Atomic resolution imaging of muscovite mica using FM-AFM in water demonstrates the sensitivity of this novel readout.