Calibration for medium resolution off-axis electron holography using a flexible dual-lens imaging system in a JEOL ARM 200F microscope
Introduction
About two decades have passed since the introduction of aberration-corrected electron microscopy [1]. The ease of use of the improved atomic resolution became almost instantly a must for a large number of discoveries in nanotechnology, and thus the use of aberration corrected microscopy and related publications in nanotechnology have risen in parallel. As an initially unnoticed side effect, an equivalent technology jump has occurred in electron holography. The quality of electron holograms and the corresponding attainable reconstructed phase resolution is mainly improved by a high coherence of beam importantly improved in the field emission gun (FEG) instruments [2], [3]. However, scientific contributions in the area of electron holography have not shown a corresponding consequential growth. Instead, the number of publications over the past two decades has fluctuated but overall remains constant. One of the key reasons for the lack of an increase in the use of the holographic technique is the absence of dedicated electron holography instruments (i.e.: a machine in which magnification of the sample can be varied without having to re-align the microscope). Additionally the necessary modifications to the optical settings of any conventional transmission electron microscope for holography are manual and time consuming and are incompatible with its standard optical settings. Thus the optical state for a holography setting remains difficult to reproduce and even uncalibrated in the sense that the effective magnification remains to be determined by the user on each occasion.
In this work we report and describe a reproducible routine for variable magnifications in off-axis electron holography using the flexible dual-lens system in a JEOL JEM ARM 200F. The method to be described allows a calibrated setup for predefine FOV ranging from 35 nm to 2500 nm, ideal for the characterization of materials in nanotechnology from where we can extract quantitative information such as, e.g., electrostatic fields [4], [5], [6], [7], [8], [9], magnetic fields [10], [11], [12], [13], [14], [15], [16], non-stained biological samples [17], [18], determination of the thickness and surface morphology in nanostructured materials [19], [20], [21], dopant profiles and strain measurement in semiconductor technology [22], [23], [24], [25], [26], [27], [28], [29]. New developments in improving the reconstructed phase and to avoid Frensel fringes can be obtained by novel configurations as the double or triple biprims in the microscope as well as a modified Lorentz conditions by controlling the diffraction lens in the microscope [30], [31], [32].
Section snippets
Electron optics in off-axis electron holography
Transmission Electron Microscopy (TEM) can be best explained in terms of wave optics due to the wave-like behavior of electrons. An incident plane wave coming from the highly-coherent field emission electron source interacts with the sample. The transmitted electrons though the specimen are considered as the object exit wave, which contains key physical information about the sample. During the image formation process the intensity of the image is recorded (I=|ψ|2), the projected image contains
Experimental procedure
In previous work we have calibrated the dual-lens imaging system for one specific FOV, fixing the OL at 5.5 V, in order to extract morphological information such as surface discontinuities and irregularities in gold decahedral nanoparticle surface [21]. Now, we are reporting the calibration of the whole range of operation for this instrument under dual-lens mode. The conventional setup for off-axis electron holography can be thought of as consisting of three parts, an illumination system, an
Results
The whole FOV range from 1 V to 7 V in OL was investigated and the images are available in the supplementary information (SI) described as follow: In order to have sufficient flexibility in the dual-lens system it is necessary to work using the low magnification mode. In this way, values of the OL can be changed systematically to calibrate the settings of the microscope. The first step to calibrate the electron holography system is to setup for different magnifications which are determined by the
Conclusion
The reconstruction phase of the metallic nanoparticles included in this work shows the optimized parameters for off-axis electron holography in a JEOL ARM200F microscope using dual-lens mode, included in the SI. The calibration provides the optimal conditions for a wide range of field of view using different objective lens excitation and biprism bias voltages. The calibration of the parameters were achieved by using a gold nanoparticle of known size, in this way we report the interference
Acknowledgments
This project was supported by grants from the National Center for Research Resources (5 G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health. The authors would also like to acknowledge the NSF PREM # DMR 0934218. A special recognition to Holowerks LLC., and Dr. Edgar Voelkl for their guidance and support. Also, would like to acknowledge Dr. Masahiro Kawasaki for his help in the microscope configuration.
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On Sabbatical leave from Centro de Investigaciones en Optica, A.C., León, Guanajuato, México