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1.
J Synchrotron Radiat ; 27(Pt 1): 217-221, 2020 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-31868755

RESUMO

Time-resolved X-ray excited optical luminescence (TR-XEOL) was developed successfully for the 23A X-ray nanoprobe beamline located at the Taiwan Photon Source (TPS). The advantages of the TR-XEOL facility include (i) a nano-focused X-ray beam (<60 nm) with excellent spatial resolution and (ii) a streak camera that can simultaneously record the XEOL spectrum and decay time. Three time spans, including normal (30 ps to 2 ns), hybrid (30 ps to 310 ns) and single (30 ps to 1.72 µs) bunch modes, are available at the TPS, which can fulfil different experimental conditions involving samples with various lifetimes. It is anticipated that TR-XEOL at the TPS X-ray nanoprobe could provide great characterization capabilities for investigating the dynamics of photonic materials.

2.
Sci Rep ; 9(1): 207, 2019 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-30659221

RESUMO

The multifunctional hard X-ray nanoprobe at Taiwan Photon Source (TPS) exhibits the excellent ability to simultaneously characterize the X-ray absorption, X-ray excited optical luminescence (XEOL) as well as the dynamics of XEOL of materials. Combining the scanning electron microscope (SEM) into the TPS 23A end-station, we can easily and quickly measure the optical properties to map out the morphology of a ZnO microrod. A special phenomenon has been observed that the oscillations in the XEOL associated with the confinement of the optical photons in the single ZnO microrod shows dramatical increase while the X-ray excitation energy is set across the Zn K-edge. Besides having the nano-scale spatial resolution, the synchrotron source also gives a good temporal domain measurement to investigate the luminescence dynamic process. The decay lifetimes of different emission wavelengths and can be simultaneously obtained from the streak image. Besides, SEM can provide the cathodoluminescence (CL) to be a complementary method to analyze the emission properties of materials, we anticipate that the X-ray nanoprobe will open new avenues with great characterization ability for developing nano/microsized optoelectronic devices.

3.
Opt Express ; 26(3): 2731-2739, 2018 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-29401809

RESUMO

Polarization-dependent hard X-ray excited optical luminescence (XEOL) was used to study not only the optical properties but also the crystallographic orientations of a non-polar a-plane ZnO wafer. In addition to a positive-edge jump and extra oscillations in the near-band-edge (NBE) XEOL yield, we observed a blue shift of the NBE emission peak that follows the polarization-dependent X-ray absorption near-edge structure (XANES) as the X-ray energy is tuned across the Zn K-edge. This NBE blue shift is caused by the larger X-ray absorption, generating higher free carriers to reduce the exciton-LO phonon coupling, which causes a decrease in the exciton activation energy. The extra oscillations in XANES and XEOL as the polarization is set parallel to the c-axis is attributed to simultaneous excitations of the Zn 4p - O 2pπ -bond along the c-axis and the bilayer σ-bond, whereas only the σ-bond is excited when the polarization is perpendicular to the c-axis. The polarization-dependent XEOL spectra can be used to determine the crystallographic orientations.

4.
Opt Express ; 23(17): 21719-29, 2015 Aug 24.
Artigo em Inglês | MEDLINE | ID: mdl-26368150

RESUMO

We propose an innovative method to extend the utilization of the phase space downstream of a synchrotron light source for X-ray transmission microscopy. Based on the dynamical theory of X-ray diffraction, asymmetrically cut perfect crystals are applied to reshape the position-angle-wavelength space of the light source, by which the usable phase space of the source can be magnified by over one hundred times, thereby "phase-space-matching" the source with the objective lens of the microscope. The method's validity is confirmed using SHADOW code simulations, and aberration through an optical lens such as a Fresnel zone plate is examined via matrix optics for nano-resolution X-ray images.

5.
Chemistry ; 19(3): 905-15, 2013 Jan 14.
Artigo em Inglês | MEDLINE | ID: mdl-23197430

RESUMO

A controlled composition-based method--that is, the microwave-assisted ethylene glycol (MEG) method--was successfully developed to prepare bimetallic Pt(x)Ru(100-x)/C nanoparticles (NPs) with different alloy compositions. This study highlights the impact of the variation in alloy composition of Pt(x)Ru(100-x)/C NPs on their alloying extent (structure) and subsequently their catalytic activity towards the methanol oxidation reaction (MOR). The alloying extent of these Pt(x)Ru(100-x)/C NPs has a strong influence on their Pt d-band vacancy and Pt electroactive surface area (Pt ECSA); this relationship was systematically evaluated by using X-ray absorption (XAS), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), density functional theory (DFT) calculations, and electrochemical analyses. The MOR activity depends on two effects that act in cooperation, namely, the number of active Pt sites and their activity. Here the number of active Pt sites is associated with the Pt ECSA value, whereas the Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. Among the Pt(x)Ru(100-x)/C NPs with various Pt:Ru atomic ratios (x = 25, 50, and 75), the Pt(75)Ru(25)/C NPs were shown to be superior in MOR activity on account of their favorable alloying extent, Pt d-band vacancy, and Pt ECSA. This short study brings new insight into probing the synergistic effect on the surface reactivity of the Pt(x)Ru(100-x)/C NPs, and possibly other bimetallic Pt-based alloy NPs.


Assuntos
Carbono/química , Metanol/química , Nanopartículas/química , Platina/química , Rutênio/química , Catálise , Técnicas Eletroquímicas , Estrutura Molecular , Oxirredução , Tamanho da Partícula , Teoria Quântica , Propriedades de Superfície
6.
ACS Nano ; 5(12): 9370-81, 2011 Dec 27.
Artigo em Inglês | MEDLINE | ID: mdl-22047129

RESUMO

Although bimetallic core@shell structured nanoparticles (NPs) are achieving prominence due to their multifunctionalities and exceptional catalytic, magnetic, thermal, and optical properties, the rationale underlying their design remains unclear. Here we report a kinetically controlled autocatalytic chemical process, adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, in which a sacrificial Cu ultrathin layer is autocatalytically deposited on a dimensionally stable noble-metal core under kinetically controlled conditions, which is then displaced to form an active ultrathin metal-layered shell by redox-transmetalation. Unlike thermodynamically controlled under-potential deposition processes, this general strategy allows for the scaling-up of production of high-quality core-shell structured NPs, without the need for any additional reducing agents and/or electrochemical treatments, some examples being Pd@Pt, Pt@Pd, Ir@Pt, and Ir@Pd. Having immediate and obvious commercial potential, Pd@Pt NPs have been systematically characterized by in situ X-ray absorption, electrochemical-FTIR, transmission electron microscopy, and electrochemical techniques, both during synthesis and subsequently during testing in one particularly important catalytic reaction, namely, the oxygen reduction reaction, which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts.


Assuntos
Galvanoplastia/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Paládio/química , Platina/química , Catálise , Cristalização/métodos , Cinética , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Porosidade , Propriedades de Superfície
7.
Chemistry ; 17(38): 10724-35, 2011 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-21837730

RESUMO

Two methods were used to prepare bimetallic Pt(3)Cr(1)/C nanocatalysts with similar composition but different alloying extent (structure). We investigated how these differences in alloying extent affect the catalytic activity, stability and selectivity in the oxygen reduction reaction (ORR). One method, based on slow thermal decomposition of the Cr precursor at a rate that matches that of chemical reduction of the Pt precursor, allows fine control of the composition of the Pt(3)Cr(1)/C alloy, whereas the second approach, using the ethylene glycol method, results in considerable deviation (>25 %) from the projected composition. Consequently, these two methods lead to variations in the alloying extent that strongly influence the Pt d-band vacancy and the Pt electroactive surface area (Pt ESCA). This relationship was systematically evaluated by transmission electron microscopy, X-ray absorption near edge structure spectroscopy, and electrochemical analysis. The ORR activity depends on two effects that nullify each other, namely, the number of active Pt sites and their activity. The Pt-site activity is more dominant in governing the ORR activity. The selectivity of the nanocatalyst towards the ORR and the competitive methanol oxidation reaction (MOR) depend on these two effects acting in cooperation to give enhanced ORR activity with suppressed MOR. The number of active Pt sites is associated with the Pt ESCA value, while Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. The presence of Cr atoms in Pt(3)Cr(1)/C enhances stability during electrochemical treatment. Overall, the Pt(3)Cr(1)/C catalyst prepared by controlled-composition synthesis was shown to be superior in ORR activity, selectivity and stability owing to its favorable alloying extent, Pt d-band vacancy, and Pt ESCA.


Assuntos
Carbono/química , Cromo/química , Nanopartículas Metálicas/química , Oxigênio/química , Platina/química , Ligas/química , Catálise , Metanol/química , Oxirredução
8.
J Phys Condens Matter ; 22(3): 036003, 2010 Jan 27.
Artigo em Inglês | MEDLINE | ID: mdl-21386301

RESUMO

We report the observation of a modulated structure and a ferromagnetic insulating state in a high quality single crystal of a nine-layer BaRuO(3). Using x-ray scattering, the modulated satellites were observed to double the unit cell along the c-axis at low temperature. The ferromagnetic insulating state is confirmed by magnetic and resistivity measurements. Analyzing the peak profiles from the modulation and host structure respectively, showed a lattice distortion at T∼55 K. These findings elucidate the intimate relationship between ferromagnetism and lattice distortion in a nine-layer BaRuO(3).

9.
J Synchrotron Radiat ; 16(Pt 1): 97-104, 2009 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-19096180

RESUMO

At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5 GeV storage ring, a dedicated small-angle X-ray scattering (SAXS) beamline has been installed with an in-achromat superconducting wiggler insertion device of peak magnetic field 3.1 T. The vertical beam divergence from the X-ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (DeltaE/E approximately 2 x 10(-4)) in the energy range 5-23 keV, or by a double Mo/B4C multilayer monochromator for 10-30 times higher flux ( approximately 10(11) photons s(-1)) in the 6-15 keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of approximately 0.9 mm x 0.3 mm (horizontal x vertical) at the focus point located 26.5 m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing-incidence SAXS (GISAXS) from liquid surfaces. Two online beam-position monitors separated by 8 m provide an efficient feedback control for an overall beam-position stability in the 10 microm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray-tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core-shell quantum dots) and GISAXS from liquid surfaces.

10.
J Synchrotron Radiat ; 14(Pt 4): 320-5, 2007 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-17587656

RESUMO

Using a superconducting-wavelength-shifter X-ray source with a photon flux density of 10(11)-10(13) photons s(-1) mrad(-1) (0.1% bandwidth)(-1) (200 mA)(-1) in the energy range 5-35 keV, three hard X-ray beamlines, BL01A, BL01B and BL01C, have been designed and constructed at the 1.5 GeV storage ring of the National Synchrotron Radiation Research Center (NSRRC). These have been designed for structure-related research using X-ray imaging, absorption, scattering and diffraction. The branch beamline BL01A, which has an unmonochromatized beam, is suitable for phase-contrast X-ray imaging with a spatial resolution of 1 microm and an imaging efficiency of one frame per 10 ms. The main beamline BL01B has 1:1 beam focusing and a medium energy resolution of approximately 10(-3). It has been designed for small-angle X-ray scattering and transmission X-ray microscopy, used, respectively, in anomalous scattering and nanophase-contrast imaging with 30 nm spatial resolution. Finally, the branch beamline BL01C, which features collimating and focusing mirrors and a double-crystal monochromator for a high energy resolution of approximately 10(-4), has been designed for X-ray absorption spectroscopy and high-resolution powder X-ray diffraction. These instruments, providing complementary tools for studying multiphase structures, have opened up a new research trend of integrated structural study at the NSRRC, especially in biology and materials. Examples illustrating the performances of the beamlines and the instruments installed are presented.

11.
Langmuir ; 23(10): 5802-9, 2007 May 08.
Artigo em Inglês | MEDLINE | ID: mdl-17425346

RESUMO

The chemical state and formation mechanism of Pt-Ru nanoparticles (NPs) synthesized by using ethylene glycol (EG) as a reducing agent and their stability have been examined by in situ X-ray absorption spectroscopy (XAS) at the Pt LIII and Ru K edges. It appears that the reduction of Pt(IV) and Ru(III) precursor salts by EG is not a straightforward reaction but involves different intermediate steps. The pH control of the reaction mixture containing Pt(IV) and Ru(III) precursor salts in EG to 11 led to the reduction of Pt(IV) to Pt(II) corresponding to [PtCl4](2-) whereas Ru(III)Cl3 is changed to the [Ru(OH)6](3-) species. Refluxing the mixture containing [PtCl4](2-) and [Ru(OH)6](3-) species at 160 degrees C for 0.5 h produces Pt-Ru NPs as indicated by the presence of Pt and Ru in the first coordination shell of the respective metals. No change in XAS structural parameters is found when the reaction time is further increased, indicating that the Pt-Ru NPs formed are extremely stable and less prone to aggregation. XAS structural parameters suggest a Pt-rich core and a Ru-rich shell structure for the final Pt-Ru NPs. Due to the inherent advantages of the EG reduction method, the atomic distribution and alloying extent of Pt and Ru in the Pt-Ru NPs synthesized by the EG method are higher than those of the Pt-Ru/C NPs synthesized by a modified Watanabe method.

12.
ACS Nano ; 1(2): 114-25, 2007 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-19206527

RESUMO

In this study, we demonstrate the unique application of X-ray absorption spectroscopy (XAS) as a fundamental characterization tool to help in designing and controlling the architecture of Pd-Au bimetallic nanoparticles within a water-in-oil microemulsion system of water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane. Structural insights obtained from the in situ XAS measurements recorded at each step during the formation process revealed that Pd-Au bimetallic clusters with various Pd-Au atomic stackings are formed by properly performing hydrazine reduction and redox transmetalation reactions sequentially within water-in-oil microemulsions. A structural model is provided to explain reasonably each reaction step and to give detailed insight into the nucleation and growth mechanism of Pd-Au bimetallic clusters. The combination of in situ XAS analysis at both the Pd K-edge and the Au L(III)-edge and UV-vis absorption spectral features confirms that the formation of Pd-Au bimetallic clusters follows a (Pd(nuclei)-Au(stack))-Pd(surf) stacking. This result further implies that the thickness of Au(stack) and Pd(surf) layers may be modulated by varying the dosage of the Au precursor and hydrazine, respectively. In addition, a bimetallic (Pd-Au)(alloy) nanocluster with a (Pd(nuclei)-Au(stack))-(Pd-Au(alloy))(surf) stacking was also designed and synthesized in order to check the feasibility of Pd(surf) layer modification. The result reveals that the Pd(surf) layer of the stacked (Pd(nuclei)-Au)(stack) bimetallic clusters can be successfully modified to form a (Au-Pd alloy)(surf) layer by a co-reduction of Pd and Au ions by hydrazine. Further, we demonstrate the alloying extent or atomic distribution of Pd and Au in Pd-Au bimetallic nanoparticles from the derived XAS structural parameters. The complete XAS-based methodology, demonstrated here on the Pd-Au bimetallic system, can easily be extended to design and control the alloying extent or atomic distribution, atomic stacking, and electronic structure to construct many other types of bimetallic systems for interesting applications.

13.
J Phys Chem B ; 110(21): 10287-95, 2006 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-16722731

RESUMO

We have investigated systematically the mechanistic aspects of the Ag-Pd bimetallic cluster formation within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles by using in-situ X-ray absorption spectroscopy (XAS). A two-step sequential reduction method is employed for the synthesis of Ag-Pd bimetallic clusters. The first step involves preparation of Ag nanoclusters, by mixing the Ag+ ions containing the AOT microemulsion system with a reducing agent hydrazine (N2H4) containing the AOT microemulsion system. In the second step, the addition of Pd2+ ions to Ag nanoclusters led to the formation of Ag-Pd bimetallic clusters via the reaction between Ag nanoclusters and Pd2+ ions in AOT reverse micelles. The reduction of silver ions and the formation of corresponding Ag nanoclusters are monitored as a function of the dosage of the reducing agent, hydrazine. In-situ XAS allowed probing of the reaction between Ag nanoclusters and Pd2+ ions during the formation of Ag-Pd bimetallic clusters. Analysis of Ag and Pd K-edge XAS spectra reveals that in the final stage Ag-Pd clusters, in which "Ag" atoms prefer to be surrounded by "Pd" and "Pd" atoms prefer to be surrounded by "Pd", were formed. On the basis of XAS results presented here, we are able to propose a structural model for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of Ag-Pd bimetallic clusters. We also discussed the atomic distribution of Ag and Pd atoms in Ag-Pd bimetallic clusters based on the calculated XAS structural parameters.

14.
J Phys Chem B ; 110(13): 6475-82, 2006 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-16570944

RESUMO

The understanding of the formation mechanism of nanoparticles is essential for the successful particle design and scaling-up process. This paper reports findings of an X-ray absorption spectroscopy (XAS) investigation, comprised of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions, to understand the mechanism of the carbon-supported Pt-Ru nanoparticles (NPs) formation process. We have utilized Watanabe's colloidal reduction method to synthesize Pt-Ru/C NPs. We slightly modified the Watanabe method by introducing a mixing and heat treatment step of Pt and Ru oxidic species at 100 degrees C for 8 h with a view to enhance the mixing efficiency of the precursor species, thereby one can achieve improved homogeneity and atomic distribution in the resultant Pt-Ru/C NPs. During the reduction process, in situ XAS measurements allowed us to follow the evolution of Pt and Ru environments and their chemical states. The Pt LIII-edge XAS indicates that when H2PtCl6 is treated with NaHSO3, the platinum compound is found to be reduced to a Pt(II) form corresponding to the anionic complex [Pt(SO3)4]6-. Further oxidation of this anionic complex with hydrogen peroxide forms dispersed [Pt(OH)6]2- species. Analysis of Ru K-edge XAS results confirms the reduction of RuIIICl3 to [RuII(OH)4]2- species upon addition of NaHSO3. Addition of hydrogen peroxide to [RuII(OH)4]2- causes dehydrogenation and forms RuOx species. Mixing of [Pt(OH)6]2- and RuOx species and heat treatment at 100 degrees C for 8 h produced a colloidal sol containing both Pt and Ru metallic as well as ionic contributions. The reduction of this colloidal mixture at 300 degrees C in hydrogen atmosphere for 2 h forms Pt-Ru nanoparticles as indicated by the presence of Pt and Ru atoms in the first coordination shell. Determination of the alloying extent or atomic distribution of Pt and Ru atoms in the resulting Pt-Ru/C NPs reveals that the alloying extent of Ru (JRu) is greater than that of the alloying extent of Pt (JPt). The XAS results support the Pt-rich core and Ru-rich shell structure with a considerable amount of segregation in the Pt region and with less segregation in the Ru region for the obtained Pt-Ru/C NPs.

15.
J Am Chem Soc ; 127(31): 11140-5, 2005 Aug 10.
Artigo em Inglês | MEDLINE | ID: mdl-16076222

RESUMO

In this report, we describe a general methodology to determine the extent of alloying or atomic distribution quantitatively in bimetallic nanoparticles (NPs) by X-ray absorption spectroscopy (XAS). The structural parameters determined in these studies serve as a quantitative index and provide a general route to determine the structural aspects of the bimetallic NPs. We have derived various types of possible structural models based on the extent of alloying and coordination number parameters of bimetallic NPs. We also discussed the nature of homo- and heterometallic interactions in bimetallic NPs based on the extent of alloying. Herein, we use carbon-supported platinum-ruthenium bimetallic nanoparticles to demonstrate the proposed methodology, and this can be extended further to get more insights into the alloying extent or atomic distribution of other bimetallic systems. The results demonstrated in this paper open up methods to determine the atomic distribution of bimetallic NPs, which is an extremely important parameter that strongly influences the physicochemical properties of NPs and their applications.

16.
J Phys Chem B ; 109(46): 21566-75, 2005 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-16853800

RESUMO

We report in situ X-ray absorption spectroscopy (XAS) investigations on the formation of palladium-platinum (Pd/Pt) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/AOT/n-heptane. The reduction of palladium and platinum ions and the formation of corresponding clusters are monitored as a function of dosage of reducing agent, hydrazine (N(2)H(5)OH). Upon successive addition of the reducing agent, hydrazine (N(2)H(5)OH), five distinguishable steps are observed in the formation process of Pd/Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis for both the Pd K-edge and Pt L(III)-edge revealed the formation of Pd/Pt bimetallic clusters. A corresponding structural model is proposed for each step to provide a detailed insight into the nucleation and growth mechanism of Pd/Pt bimetallic clusters. We also discussed the atomic distribution of Pd and Pt atoms in Pd/Pt bimetallic clusters based on the calculated XAS structural parameters.


Assuntos
Ácido Dioctil Sulfossuccínico/química , Micelas , Compostos Organometálicos/síntese química , Paládio/química , Platina/química , Compostos Organometálicos/química , Sensibilidade e Especificidade , Análise Espectral/métodos , Raios X
17.
Acta Crystallogr A ; 60(Pt 3): 209-13, 2004 May.
Artigo em Inglês | MEDLINE | ID: mdl-15103163

RESUMO

The first observation of the X-ray multiple-wave interaction in an incommensurate charge-density-wave (CDW) modulated structure at low temperatures is reported for an example of a quasi-two-dimensional material, NbSe(2)-2H. Via the coherent interaction between the X-ray waves propagating in the CDW-modulated structure and the host structure, the phase-dependent intensity variations of a CDW reflection were detected. In accord with a centrosymmetric structure, the phases of the structure-factor triplets of two CDW reflections and a Bragg reflection of the host structure were determined to be either 0 or 180 degrees, and not to vary with temperature. Relative phase differences of the two CDW reflections are also deduced.


Assuntos
Nióbio/química , Compostos de Selênio/química , Cristalografia por Raios X , Modelos Químicos , Análise de Sistemas , Temperatura Ambiente
18.
Acta Crystallogr A ; 60(Pt 1): 64-74, 2004 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-14691329

RESUMO

The iterative Born approximation is derived for three-wave dynamical X-ray diffraction. Dependence of the three-wave diffraction profiles of the diffracted wave on the polarization state of a linearly polarized incident wave is theoretically and experimentally investigated. General conditions of the phase sensitivity as well as the asymmetry of diffraction profiles are obtained from this approximation and compared with direct dynamical calculations. Reasonable qualitative agreement between the results obtained from this iterative approach and the exact dynamical calculation is shown. A new feature of reversing asymmetry of an intensity profile with respect to phase change is theoretically predicted.

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