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1.
Sci Rep ; 9(1): 8889, 2019 Jun 20.
Artigo em Inglês | MEDLINE | ID: mdl-31222124

RESUMO

Low-energy electrons offer a unique possibility for long exposure imaging of individual biomolecules without significant radiation damage. In addition, low-energy electrons exhibit high sensitivity to local potentials and thus can be employed for imaging charges as small as a fraction of one elementary charge. The combination of these properties makes low-energy electrons an exciting tool for imaging charge transport in individual biomolecules. Here we demonstrate the imaging of individual deoxyribonucleic acid (DNA) molecules at the resolution of about 1 nm with simultaneous imaging of the charging of the DNA molecules that is of the order of less than one elementary charge per nanometer. The cross-correlation analysis performed on different sections of the DNA network reveals that the charge redistribution between the two regions is correlated. Thus, low-energy electron microscopy is capable to provide simultaneous imaging of macromolecular structure and its charge distribution which can be beneficial for imaging and constructing nano-bio-sensors.

2.
Ultramicroscopy ; 197: 46-52, 2019 02.
Artigo em Inglês | MEDLINE | ID: mdl-30496888

RESUMO

We investigate imaging of moiré structures in free-standing twisted bilayer graphene (TBG) carried out by transmission electron microscopy (TEM) in diffraction and in-line Gabor holography modes. Electron diffraction patterns of TBG acquired at typical TEM electron energies of 80-300 keV exhibit the diffraction peaks caused by diffraction on individual layers. However, diffraction peaks at the scattering angles related to the periodicity of the moiré structure have not been observed in such diffraction patterns. We show that diffraction on moiré structure can create intense diffraction peaks if the energy of the probing electrons is very low, in the range of a few tens of eV. Experimental diffraction patterns of TBG acquired with low-energy electrons of 236 eV exhibiting peaks attributed to the moiré structure periodicity are shown. In holography mode, the intensity of the wave transmitted through the sample and measured in the far-field can be enhanced or decreased depending on the atomic arrangement, as for example AA or AB stacking. Thus, a decrease of intensity in the far-field must not necessarily be associated with some absorption inside the sample but can simply be a result of a particular atomic arrangement. We believe that our findings can be important for exploiting graphene as a support in electron imaging.

3.
Opt Express ; 26(23): 30991-31017, 2018 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-30469988

RESUMO

Coherent diffraction imaging (CDI) allows the retrieval of an isolated object's structure, such as a macromolecule, from its diffraction pattern. CDI requires the fulfillment of two conditions: the imaging radiation must be coherent and the object must be isolated. We discuss that it is possible to directly retrieve the molecular structure from its diffraction pattern, which was acquired neither with coherent radiation nor from an individual molecule. This is provided that the molecule exhibits periodicity in one direction, as in the case of fiber diffraction. We demonstrate that, when we apply iterative phase retrieval methods to a fiber diffraction pattern, the repeating unit; that is, the molecule structure, can directly be reconstructed without any prior modeling. For example, we recover the the DNA double helix's structure in three-dimensions from its two-dimensional X-ray fiber diffraction pattern, Photograph (Photo) 51, which was acquired in Raymond Gosling and Rosalind Franklin's famous experiment at a resolution of 3.4 Å.


Assuntos
DNA/química , Imagem Tridimensional/métodos , Difração de Raios X/métodos , Algoritmos , Animais , Humanos , Modelos Moleculares
4.
Nano Lett ; 18(6): 3421-3427, 2018 06 13.
Artigo em Inglês | MEDLINE | ID: mdl-29733660

RESUMO

The interaction of metals with carbon materials (and specifically with graphene) is of importance for various technological applications. In particular, the intercalation of alkali metals is believed to provide a means for tuning the electronic properties of graphene for device applications. While the macroscopic effects of such intercalation events can readily be studied, following the related processes at an atomic scale in detail and under well-defined experimental conditions constitutes a challenge. Here, we investigate in situ the adsorption and nucleation of the alkali metals K, Cs, and Li on free-standing graphene by means of low-energy electron point source microscopy. We find that alkali metals readily intercalate between the layers of bilayer graphene. In fact, the equilibrium distribution of K and Cs favors a much-higher particle density between the layers than on the single-layer graphene. We obtain a quantitative value for the difference of the free energy of the binding between these two domains. Our study is completed with a control experiment introducing Pd as a representative of the nonalkali metals. Now, we observe cluster formation in equal measure on both single-layer and bilayer graphene; however, there was no intercalation. Our investigations thus constitute the first in situ study of metal-atom sorption of different specificity on free-standing graphene.

5.
Opt Express ; 25(17): 20109-20124, 2017 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-29041695

RESUMO

Mechanical vibrations of components of the optical system is one of the sources of blurring of interference pattern in coherent imaging systems. The problem is especially important in holography where the resolution of the reconstructed objects depends on the effective size of the hologram, which is on the extent of the interference pattern, and on the contrast of the interference fringes. We discuss the mathematical relation between the vibrations, the hologram contrast and the reconstructed object. We show how vibrations can be post-filtered out from the hologram or from the reconstructed object assuming a Gaussian distribution of the vibrations. We also provide a numerical example of compensation for directional motion blur. We demonstrate our approach for light optical and electron holograms, acquired with both, plane- as well as spherical-waves. As a result of such hologram deblurring, the resolution of the reconstructed objects is enhanced by almost a factor of 2. We believe that our approach opens up a new venue of post-experimental resolution enhancement in in-line holography by adapting the rich database/catalogue of motion deblurring algorithms developed for photography and image restoration applications.

6.
Ultramicroscopy ; 182: 276-282, 2017 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-28780143

RESUMO

While imaging individual atoms can routinely be achieved in high resolution transmission electron microscopy, visualizing the potential distribution of individually charged adsorbates leading to a phase shift of the probing electron wave is still a challenging task. Low-energy electrons (30 - 250 eV) are sensitive to localized potential gradients. We employed low-energy electron holography to acquire in-line holograms of individual charged impurities on free-standing graphene. By applying an iterative phase retrieval reconstruction routine we recover the potential distribution of the localized charged impurities present on free-standing graphene.

7.
Proc Natl Acad Sci U S A ; 114(7): 1474-1479, 2017 02 14.
Artigo em Inglês | MEDLINE | ID: mdl-28087691

RESUMO

Imaging single proteins has been a long-standing ambition for advancing various fields in natural science, as for instance structural biology, biophysics, and molecular nanotechnology. In particular, revealing the distinct conformations of an individual protein is of utmost importance. Here, we show the imaging of individual proteins and protein complexes by low-energy electron holography. Samples of individual proteins and protein complexes on ultraclean freestanding graphene were prepared by soft-landing electrospray ion beam deposition, which allows chemical- and conformational-specific selection and gentle deposition. Low-energy electrons do not induce radiation damage, which enables acquiring subnanometer resolution images of individual proteins (cytochrome C and BSA) as well as of protein complexes (hemoglobin), which are not the result of an averaging process.


Assuntos
Holografia/métodos , Proteínas/ultraestrutura , Imagem Individual de Molécula/métodos , Animais , Bovinos , Citocromos c/ultraestrutura , Elétrons , Grafite , Hemoglobinas/ultraestrutura , Holografia/instrumentação , Soroalbumina Bovina/ultraestrutura , Imagem Individual de Molécula/instrumentação , Espectrometria de Massas por Ionização por Electrospray/métodos , Eletricidade Estática , Vácuo
8.
Appl Opt ; 55(22): 6095-101, 2016 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-27505393

RESUMO

We present a detailed study of two novel methods for shaping the light optical wavefront by employing a transmissive spatial light modulator (SLM). Conventionally, optical Airy beams are created by employing SLMs in the so-called all-phase mode. In the first method, a numerically simulated lens phase distribution is loaded directly onto the SLM, together with the cubic phase distribution. An Airy beam is generated at the focal plane of the numerical lens. We provide for the first time, to the best of our knowledge, quantitative properties of the formed Airy beam. We derive the formula for deflection of the intensity maximum of the so-formed Airy beam, which is different from the quadratic deflection typical of Airy beams. We cross-validate the derived formula by both simulations and experiment. The second method is based on the fact that a system consisting of a transmissive SLM sandwiched between two polarizers can create a transmission function with negative values. This observation alone has the potential for various other wavefront modulations where the transmission function requires negative values. As an example for this method, we demonstrate that a wavefront can be modulated by passing the SLM system with transmission function with negative values by loading an Airy function distribution directly onto the SLM. Since the Airy function is a real-valued function but also with negative values, an Airy beam can be generated by direct transfer of the Airy function distribution onto such an SLM system. In this way, an Airy beam is generated immediately behind the SLM. As both new methods do not employ a physical lens, the two setups are more compact than conventional setups for creating Airy beams. We compare the performance of the two novel methods and the properties of the created Airy beams.

9.
Nano Lett ; 16(9): 5469-74, 2016 09 14.
Artigo em Inglês | MEDLINE | ID: mdl-27536886

RESUMO

Visualizing individual charges confined to molecules and observing their dynamics with high spatial resolution is a challenge for advancing various fields in science, ranging from mesoscopic physics to electron transfer events in biological molecules. We show here that the high sensitivity of low-energy electrons to local electric fields can be employed to directly visualize individual charged adsorbates and to study their behavior in a quantitative way. This makes electron holography a unique probing tool for directly visualizing charge distributions with a sensitivity of a fraction of an elementary charge. Moreover, spatial resolution in the nanometer range and fast data acquisition inherent to lens-less low-energy electron holography allows for direct visual inspection of charge transfer processes.

10.
Sci Rep ; 6: 26312, 2016 05 20.
Artigo em Inglês | MEDLINE | ID: mdl-27199254

RESUMO

It is well known that by modifying the wavefront in a certain manner, the light intensity can be turned into a certain shape. However, all known light modulation techniques allow for limited light modifications only: focusing within a restricted region in space, shaping into a certain class of parametric curves along the optical axis or bending described by a quadratic-dependent deflection as in the case of Airy beams. We show a general case of classical light wavefront shaping that allows for intensity and phase redistribution into an arbitrary profile including pre-determined switching-off of the intensity. To create an arbitrary three-dimensional path of intensity, we represent the path as a sequence of closely packed individual point-like absorbers and simulate the in-line hologram of the created object set; when such a hologram is contrast inverted, thus giving rise to a diffractor, it creates the pre-determined three-dimensional path of intensity behind the diffractor under illumination. The crucial parameter for a smooth optical path is the sampling of the predetermined curves, which is given by the lateral and axial resolution of the optical system. We provide both, simulated and experimental results to demonstrate the power of this novel method.

11.
Ultramicroscopy ; 160: 74-79, 2016 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-26458026

RESUMO

We have designed, fabricated and tested a micron-sized electron column with an overall length of about 700 microns comprising two electron lenses; a micro-lens with a minimal bore of 1 micron followed by a second lens with a bore of up to 50 microns in diameter to shape a coherent low-energy electron wave front. The design criteria follow the notion of scaling down source size, lens-dimensions and kinetic electron energy for minimizing spherical aberrations to ensure a parallel coherent electron wave front. All lens apertures have been milled employing a focused ion beam and could thus be precisely aligned within a tolerance of about 300 nm from the optical axis. Experimentally, the final column shapes a quasi-planar wave front with a minimal full divergence angle of 4 mrad and electron energies as low as 100 eV.

12.
Appl Opt ; 54(9): 2424-34, 2015 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-25968531

RESUMO

Here we present practical methods for simulation and reconstruction of in-line digital holograms recorded with plane and spherical waves. The algorithms described here are applicable to holographic imaging of an object exhibiting absorption as well as phase-shifting properties. Optimal parameters, related to distances, sampling rate, and other factors for successful simulation and reconstruction of holograms are evaluated and criteria for the achievable resolution are worked out. Moreover, we show that the numerical procedures for the reconstruction of holograms recorded with plane and spherical waves are identical under certain conditions. Experimental examples of holograms and their reconstructions are also discussed.

13.
Ultramicroscopy ; 159 Pt 2: 395-402, 2015 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-25687733

RESUMO

The current state of the art in structural biology is led by NMR, X-ray crystallography and TEM investigations. These powerful tools however all rely on averaging over a large ensemble of molecules. Here, we present an alternative concept aiming at structural analysis at the single molecule level. We show that by combining electron holography and coherent diffraction imaging estimations concerning the phase of the scattered wave become needless as the phase information is extracted from the data directly and unambiguously. Performed with low-energy electrons the resolution of this lens-less microscope is just limited by the De Broglie wavelength of the electron wave and the numerical aperture, given by detector geometry. In imaging freestanding graphene, a resolution of 2Å has been achieved revealing the 660.000 unit cells of the graphene sheet from a single data set. Once applied to individual biomolecules the method shall ultimately allow for non-destructive imaging and imports the potential to distinguish between different conformations of proteins with atomic resolution.


Assuntos
Holografia/métodos , Elétrons , Desenho de Equipamento , Grafite/química , Holografia/instrumentação , Microscopia Eletrônica/instrumentação , Microscopia Eletrônica/métodos , Estrutura Molecular , Proteínas/química , Espalhamento de Radiação
14.
Opt Express ; 22(17): 20994-1003, 2014 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-25321300

RESUMO

Holographic particle image velocimetry allows tracking particle trajectories in time and space by means of holography. However, the drawback of the technique is that in the three-dimensional particle distribution reconstructed from a hologram, the individual particles can hardly be resolved due to the superimposed out-of-focus signal from neighboring particles. We demonstrate here a three-dimensional volumetric deconvolution applied to the reconstructed wavefront which results in resolving all particles simultaneously in three-dimensions. Moreover, we apply the three-dimensional volumetric deconvolution to reconstructions of a time-dependent sequence of holograms of an ensemble of polystyrene spheres moving in water. From each hologram we simultaneously resolve all particles in the ensemble in three dimensions and from the sequence of holograms we obtain the time-resolved trajectories of individual polystyrene spheres.

15.
Ultramicroscopy ; 145: 80-4, 2014 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-24503115

RESUMO

An ideal support for an electron microscopy should be as thin as possible and be able to interact as little as possible with the primary electrons. Since graphene is atomically thin and made up of carbon atoms arranged in a honeycomb lattice, the potential to use graphene as a substrate in electron microscopy is enormous. Until now graphene has hardly ever been used for this purpose because the cleanliness of freestanding graphene before or after deposition of the objects of interest was insufficient. We demonstrate here by means of low-energy electron holographic imaging that freestanding graphene prepared with a platinum-metal catalysis method remains ultraclean even after re-exposure to ambient conditions and deposition of gold nanorods from the liquid phase. In the holographic reconstruction of gold particles the organic shell surrounding the objects is apparent while it is not detectable in SEM images of the very same sample, demonstrating the tremendous potential of low-energy electron holography for imaging of graphene-supported single biomolecules.

16.
Ultramicroscopy ; 145: 22-7, 2014 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-24331233

RESUMO

Low-energy electrons (30-250eV) have been successfully employed for imaging individual biomolecules. The most simple and elegant design of a low-energy electron microscope for imaging biomolecules is a lensless setup that operates in the holographic mode. In this work we address the problem associated with the reconstruction from the recorded holograms. We discuss the twin image problem intrinsic to inline holography and the problem of the so-called biprism-like effect specific to low-energy electrons. We demonstrate how the presence of the biprism-like effect can be efficiently identified and circumvented. The presented sideband filtering reconstruction method eliminates the twin image and allows for reconstruction despite the biprism-like effect, which we demonstrate on both, simulated and experimental examples.


Assuntos
Holografia/métodos , Artefatos , Elétrons , Holografia/estatística & dados numéricos , Processamento de Imagem Assistida por Computador/métodos , Processamento de Imagem Assistida por Computador/estatística & dados numéricos
17.
Phys Rev Lett ; 110(25): 255501, 2013 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-23829743

RESUMO

We have imaged a freestanding graphene sheet of 210 nm in diameter with 2 Å resolution by combining coherent diffraction and holography with low-energy electrons. The entire sheet is reconstructed from a single diffraction pattern displaying the arrangement of 660.000 individual graphene unit cells at once. Given the fact that electrons with kinetic energies of the order of 100 eV do not damage biological molecules, it will now be a matter of developing methods for depositing individual proteins onto such graphene sheets.

18.
Opt Express ; 21(6): 7726-33, 2013 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-23546153

RESUMO

It is generally believed that the resolution in digital holography is limited by the size of the captured holographic record. Here, we present a method to circumvent this limit by self-extrapolating experimental holograms beyond the area that is actually captured. This is done by first padding the surroundings of the hologram and then conducting an iterative reconstruction procedure. The wavefront beyond the experimentally detected area is thus retrieved and the hologram reconstruction shows enhanced resolution. To demonstrate the power of this concept, we apply it to simulated as well as experimental holograms.


Assuntos
Holografia/instrumentação , Aumento da Imagem/instrumentação , Processamento de Sinais Assistido por Computador/instrumentação , Projeto Auxiliado por Computador , Desenho de Equipamento , Análise de Falha de Equipamento
19.
Opt Express ; 20(27): 28871-92, 2012 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-23263128

RESUMO

The phase problem is inherent to crystallographic, astronomical and optical imaging where only the intensity of the scattered signal is detected and the phase information is lost and must somehow be recovered to reconstruct the object's structure. Modern imaging techniques at the molecular scale rely on utilizing novel coherent light sources like X-ray free electron lasers for the ultimate goal of visualizing such objects as individual biomolecules rather than crystals. Here, unlike in the case of crystals where structures can be solved by model building and phase refinement, the phase distribution of the wave scattered by an individual molecule must directly be recovered. There are two well-known solutions to the phase problem: holography and coherent diffraction imaging (CDI). Both techniques have their pros and cons. In holography, the reconstruction of the scattered complex-valued object wave is directly provided by a well-defined reference wave that must cover the entire detector area which often is an experimental challenge. CDI provides the highest possible, only wavelength limited, resolution, but the phase recovery is an iterative process which requires some pre-defined information about the object and whose outcome is not always uniquely-defined. Moreover, the diffraction patterns must be recorded under oversampling conditions, a pre-requisite to be able to solve the phase problem. Here, we report how holography and CDI can be merged into one superior technique: holographic coherent diffraction imaging (HCDI). An inline hologram can be recorded by employing a modified CDI experimental scheme. We demonstrate that the amplitude of the Fourier transform of an inline hologram is related to the complex-valued visibility, thus providing information on both, the amplitude and the phase of the scattered wave in the plane of the diffraction pattern. With the phase information available, the condition of oversampling the diffraction patterns can be relaxed, and the phase problem can be solved in a fast and unambiguous manner. We demonstrate the reconstruction of various diffraction patterns of objects recorded with visible light as well as with low-energy electrons. Although we have demonstrated our HCDI method using laser light and low-energy electrons, it can also be applied to any other coherent radiation such as X-rays or high-energy electrons.


Assuntos
Holografia/instrumentação , Aumento da Imagem/instrumentação , Imagem Tridimensional/instrumentação , Refratometria/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento
20.
Opt Express ; 19(20): 19330-9, 2011 Sep 26.
Artigo em Inglês | MEDLINE | ID: mdl-21996873

RESUMO

Coherent diffraction imaging (CDI) for visualizing objects at atomic resolution has been realized as a promising tool for imaging single molecules. Drawbacks of CDI are associated with the difficulty of the numerical phase retrieval from experimental diffraction patterns; a fact which stimulated search for better numerical methods and alternative experimental techniques. Common phase retrieval methods are based on iterative procedures which propagate the complex-valued wave between object and detector plane. Constraints in both, the object and the detector plane are applied. While the constraint in the detector plane employed in most phase retrieval methods requires the amplitude of the complex wave to be equal to the squared root of the measured intensity, we propose a novel Fourier-domain constraint, based on an analogy to holography. Our method allows achieving a low-resolution reconstruction already in the first step followed by a high-resolution reconstruction after further steps. In comparison to conventional schemes this Fourier-domain constraint results in a fast and reliable convergence of the iterative reconstruction process.


Assuntos
Análise de Fourier , Holografia/métodos , Processamento de Imagem Assistida por Computador , Modelos Teóricos , Difração de Raios X/métodos , Algoritmos
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