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Sep 2007

Volume 78, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

Rev. Sci. Instrum. 78, 091301 (2007); http://dx.doi.org/10.1063/1.2783112 (11 pages)

Jean-Claude Labiche, Olivier Mathon, Sakura Pascarelli, Mark A. Newton, Gemma Guilera Ferre, Caroline Curfs, Gavin Vaughan, Alejandro Homs, and David Fernandez Carreiras
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Novel approach to Abel inversion

R. K. Paul

Rev. Sci. Instrum. 78, 093701 (2007); http://dx.doi.org/10.1063/1.2777159 (5 pages) | Cited 3 times

Online Publication Date: 4 September 2007

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Simple yet versatile, physically valid emissivity functions for peaked and hollow profiles with only two determinable parameters are proposed for performing Abel inversion. The advantages of the proposed functions have been explored. The inversion is very fast, accurate, convenient, and viable, in contrast to the existing methods. The validation of these functions has been confirmed by using simulated data under various conditions. The error in the process has been computed and found to depend on the functional form of the model emissivity. A comprehensive comparison has been drawn with the existing method and it has been found to offer a definite advantage over the existing technique in some respects, especially for real time applications. Limitation of this technique has also been discussed. The soft x-ray and visible light emissivity profile of SINP tokamak has been successfully obtained by using this method.
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52.70.-m Plasma diagnostic techniques and instrumentation
52.55.Fa Tokamaks, spherical tokamaks
02.30.Rz Integral equations

Comparison of different methods to calibrate torsional spring constant and photodetector for atomic force microscopy friction measurements in air and liquid

Torbjörn Pettersson, Niklas Nordgren, Mark W. Rutland, and Adam Feiler

Rev. Sci. Instrum. 78, 093702 (2007); http://dx.doi.org/10.1063/1.2779215 (8 pages) | Cited 24 times

Online Publication Date: 5 September 2007

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A number of atomic force microscopy cantilevers have been exhaustively calibrated by a number of techniques to obtain both normal and frictional force constants to evaluate the relative accuracy of the different methods. These were of either direct or indirect character—the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967 (1999) ] and Cleveland [Rev. Sci. Instrum. 64, 403 (1993) ] techniques are compared for the normal force constant calibration and while agreement was good, a systematic difference was observed. For the torsional force constants, all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calculated. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liquid as well as air, and a conversion factor was obtained showing that quantitative friction measurements in liquid are equally feasible provided the correct calibration is performed.
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07.79.Lh Atomic force microscopes
07.10.Pz Instruments for strain, force, and torque
06.20.fb Standards and calibration

Simple fiber-optic confocal microscopy with nanoscale depth resolution beyond the diffraction barrier

Ilko Ilev, Ronald Waynant, Israel Gannot, and Amir Gandjbakhche

Rev. Sci. Instrum. 78, 093703 (2007); http://dx.doi.org/10.1063/1.2777173 (4 pages) | Cited 6 times

Online Publication Date: 19 September 2007

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A novel fiber-optic confocal approach for ultrahigh depth-resolution ( ⩽ 2 nm) microscopy beyond the diffraction barrier in the subwavelength nanometric range below 200 nm is presented. The key idea is based on a simple fiber-optic confocal microscope approach that is compatible with a differential confocal microscope technique. To improve the dynamic range of the resolving laser power and to achieve a high resolution in the nanometric range, we have designed a simple apertureless reflection confocal microscope with a highly sensitive single-mode-fiber confocal output. The fiber-optic design is an effective alternative to conventional pinhole-based confocal systems and offers a number of advantages in terms of spatial resolution, flexibility, miniaturization, and scanning potential. Furthermore, the design is compatible with the differential confocal pinhole microscope based on the use of the sharp diffraction-free slope of the axial confocal response curve rather than the area around the maximum of that curve. Combining the advantages of ultrahigh-resolution fiber-optic confocal microscopy, we can work beyond the diffraction barrier in the subwavelength (below 200 nm) nanometric range exploiting confocal nanobioimaging of single cell and intracellular analytes.
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42.81.Wg Other fiber-optical devices
07.60.Pb Conventional optical microscopes
42.15.Eq Optical system design
87.64.mk Confocal
87.80.-y Biophysical techniques (research methods)
87.17.-d Cell processes

Atomic force microscopy of protein films and crystals

Eugenia Pechkova, Marco Sartore, Luca Giacomelli, and Claudio Nicolini

Rev. Sci. Instrum. 78, 093704 (2007); http://dx.doi.org/10.1063/1.2785032 (7 pages) | Cited 1 time

Online Publication Date: 21 September 2007

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A customized atomic force microscopy (AFM) instrument optimized for imaging protein crystals in solution is described. The device was tested on crystals and Langmuir-Blodgett (LB) films of two proteins with quite different molecular weights. This approach enables the periodicity and morphology of crystals to be studied in their mother liquid, thereby preserving the native periodic protein crystal structure, which is typically destroyed by drying. Moreover, the instrument appears to distinguish protein crystals from salt crystals, which under the optical microscope are frequently quite similar, the difference between them often being revealed only during x-ray analysis. AFM estimates of the packing, order, and morphology of the given single proteins appear quite similar in the LB thin film and in the crystals, which means that routine crystal measurements can be performed at high resolution. The AFM consists of a custom-built measuring head and a homemade flexible SPM controller which can drive the head for contact, noncontact and spectroscopy modes, thus providing the user with a high degree of customization for crystal measurement.
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07.79.Lh Atomic force microscopes
68.37.Ps Atomic force microscopy (AFM)
87.15.B- Structure of biomolecules
87.14.E- Proteins
36.20.Hb Configuration (bonds, dimensions)
68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces

Spring constant calibration of atomic force microscopy cantilevers with a piezosensor transfer standard

E. D. Langlois, G. A. Shaw, J. A. Kramar, J. R. Pratt, and D. C. Hurley

Rev. Sci. Instrum. 78, 093705 (2007); http://dx.doi.org/10.1063/1.2785413 (10 pages) | Cited 21 times

Online Publication Date: 24 September 2007

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We describe a method to calibrate the spring constants of cantilevers for atomic force microscopy (AFM). The method makes use of a “piezosensor” composed of a piezoresistive cantilever and accompanying electronics. The piezosensor was calibrated before use with an absolute force standard, the NIST electrostatic force balance (EFB). In this way, the piezosensor acts as a force transfer standard traceable to the International System of Units. Seven single-crystal silicon cantilevers with rectangular geometries and nominal spring constants from 0.2 to 40 N/m were measured with the piezosensor method. The values obtained for the spring constant were compared to measurements by four other techniques: the thermal noise method, the Sader method, force loading by a calibrated nanoindentation load cell, and direct calibration by force loading with the EFB. Results from different methods for the same cantilever were generally in agreement, but differed by up to 300% from nominal values. When used properly, the piezosensor approach provides spring-constant values that are accurate to ±10% or better. Methods such as this will improve the ability to extract quantitative information from AFM methods.
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89.20.Kk Engineering
06.20.fb Standards and calibration
46.32.+x Static buckling and instability

Control of tip-to-sample distance in atomic force microscopy: A dual-actuator tip-motion control scheme

Younkoo Jeong, G. R. Jayanth, and Chia-Hsiang Menq

Rev. Sci. Instrum. 78, 093706 (2007); http://dx.doi.org/10.1063/1.2785158 (7 pages) | Cited 6 times

Online Publication Date: 26 September 2007

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The control of tip-to-sample distance in atomic force microscopy (AFM) is achieved through controlling the vertical tip position of the AFM cantilever. In the vertical tip-position control, the required z motion is commanded by laser reading of the vertical tip position in real time and might contain high frequency components depending on the lateral scanning rate and topographical variations of the sample. This paper presents a dual-actuator tip-motion control scheme that enables the AFM tip to track abrupt topographical variations. In the dual-actuator scheme, an additional magnetic mode actuator is employed to achieve high bandwidth tip-motion control while the regular z scanner provides the necessary motion range. This added actuator serves to make the entire cantilever bandwidth available for tip positioning, and thus controls the tip-to-sample distance. A fast programmable electronics board was employed to realize the proposed dual-actuator control scheme, in which model cancellation algorithms were implemented to enlarge the bandwidth of the magnetic actuation and to compensate the lightly damped dynamics of the cantilever. Experiments were conducted to illustrate the capabilities of the proposed dual-actuator tip-motion control in terms of response speed and travel range. It was shown that while the bandwidth of the regular z scanner was merely a small fraction of the cantilever’s bandwidth, the dual-actuator control scheme led to a tip-motion control system, the bandwidth of which was comparable to that of the cantilever, where the dynamics overdamped, and the motion range comparable to that of the z scanner.
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07.79.Lh Atomic force microscopes

Inspection of refractive x-ray lenses using high-resolution differential phase contrast imaging with a microfocus x-ray source

Martin Engelhardt, Joachim Baumann, Manfred Schuster, Christian Kottler, Franz Pfeiffer, Oliver Bunk, and Christian David

Rev. Sci. Instrum. 78, 093707 (2007); http://dx.doi.org/10.1063/1.2786273 (5 pages) | Cited 7 times

Online Publication Date: 26 September 2007

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A refractive x-ray lens was characterized using a magnifying cone beam setup for differential phase contrast imaging in combination with a microfocus x-ray tube. Thereby, the differential and the total phase shift of x rays transmitted through the lens were determined. Lens aberrations have been characterized based on these refractive properties.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
42.87.-d Optical testing techniques
42.79.Bh Lenses, prisms and mirrors
42.15.Fr Aberrations
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