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Structural damage reduction in protected gold clusters by electron diffraction methods

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ABSTRACT

The present work explores electron diffraction methods for studying the structure of metallic clusters stabilized with thiol groups, which are susceptible to structural damage caused by electron beam irradiation. There is a compromise between the electron dose used and the size of the clusters since they have small interaction volume with electrons and as a consequence weak reflections in the diffraction patterns. The common approach of recording individual clusters using nanobeam diffraction has the problem of an increased current density. Dosage can be reduced with the use of a smaller condenser aperture and a higher condenser lens excitation, but even with those set ups collection times tend to be high. For that reason, the methods reported herein collects in a faster way diffraction patterns through the scanning across the clusters under nanobeam diffraction mode. In this way, we are able to collect a map of diffraction patterns, in areas with dispersed clusters, with short exposure times (milliseconds) using a high sensitive CMOS camera. When these maps are compared with their theoretical counterparts, oscillations of the clusters can be observed. The stability of the patterns acquired demonstrates that our methods provide a systematic and precise way to unveil the structure of atomic clusters without extensive detrimental damage of their crystallinity.

Electronic supplementary material: The online version of this article (doi:10.1186/s40679-016-0026-x) contains supplementary material, which is available to authorized users.

No MeSH data available.


Effects and variations of electron dosage in metallic nanoparticles. Frame shot sequence of Au144(SCH2CH2Ph)60 on amorphous carbon. The insets show the FFT for the framed region, the structure of the particle is modified by the irradiation: a fcc-like orientation, b fivefold orientation, c and d other two different orientations. TEM-Nanobeam-diffraction irradiated areas at different magnifications, the dose rates calculated within the screen of the microscope are: e 15  Å−2 s−1, f 80  Å−2 s−1, g 400  Å−2 s−1 and h 13,500  Å−2 s−1
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Fig3: Effects and variations of electron dosage in metallic nanoparticles. Frame shot sequence of Au144(SCH2CH2Ph)60 on amorphous carbon. The insets show the FFT for the framed region, the structure of the particle is modified by the irradiation: a fcc-like orientation, b fivefold orientation, c and d other two different orientations. TEM-Nanobeam-diffraction irradiated areas at different magnifications, the dose rates calculated within the screen of the microscope are: e 15  Å−2 s−1, f 80  Å−2 s−1, g 400  Å−2 s−1 and h 13,500  Å−2 s−1

Mentions: Conventional high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) are typically acquired with a large condenser aperture (30–40 μm) which yields density currents, recorded on the phosphorous plate, of around 100 pA cm−2. If we consider radiolysis as the primary radiation damage in the studied clusters, then for a HRTEM image taken at M = 500 K the electron dose exerted on the sample will be around 12,500 Å−2, this dose rate is capable of producing changes in the structure of the metallic clusters because the thiol groups are more sensitive and are located in the surface of the metallic core atoms. The images shown in Fig. 3a–d indicate the structural transformations from an individual Au144(SR)60 cluster, the structure has been compared with the relaxed structure calculated in Ref. [20] (see the full video in Additional file 1). Experimental HRTEM observations of gold nanoparticles show the occurrence of structural instabilities, such as quasi melting as reported in the literature [25], this process is produced by existence of multiple structural configurations separated by low energy barriers. Hence, to diminish the radiolysis effect, the nanoprobe conditions are set up in the microscope with a small condenser lens aperture (5 μm) and a large demagnification of the condenser lens in the JEOL 2010F microscope. Under nanoprobe mode a sample can be irradiated in a quasi-parallel illumination reducing the current density in about two orders of magnitudes. The images shown in Fig. 3e–h represent a sequence of the irradiated area in Au144(SR)60 clusters. In these series, the current density increases as the irradiated area decreases. At low magnifications, the structure transformations are drastically reduced but are still present after few seconds in which the beam is positioned in the cluster as we demonstrated in a previous article using nanobeam diffraction in STEM mode [21]. In order to register proper diffraction patterns with a reduced noise-signal ratio, the patterns need to be registered with the minimum probe size, which increases the electron dose in almost three orders of magnitude as depicted when comparing Fig. 3e, h. To protect these clusters from radiolysis effects the approach then relies in a reduction of the acquisition time using a fast scanning and fast detection experimental set ups.Fig. 3


Structural damage reduction in protected gold clusters by electron diffraction methods
Effects and variations of electron dosage in metallic nanoparticles. Frame shot sequence of Au144(SCH2CH2Ph)60 on amorphous carbon. The insets show the FFT for the framed region, the structure of the particle is modified by the irradiation: a fcc-like orientation, b fivefold orientation, c and d other two different orientations. TEM-Nanobeam-diffraction irradiated areas at different magnifications, the dose rates calculated within the screen of the microscope are: e 15  Å−2 s−1, f 80  Å−2 s−1, g 400  Å−2 s−1 and h 13,500  Å−2 s−1
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Fig3: Effects and variations of electron dosage in metallic nanoparticles. Frame shot sequence of Au144(SCH2CH2Ph)60 on amorphous carbon. The insets show the FFT for the framed region, the structure of the particle is modified by the irradiation: a fcc-like orientation, b fivefold orientation, c and d other two different orientations. TEM-Nanobeam-diffraction irradiated areas at different magnifications, the dose rates calculated within the screen of the microscope are: e 15  Å−2 s−1, f 80  Å−2 s−1, g 400  Å−2 s−1 and h 13,500  Å−2 s−1
Mentions: Conventional high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) are typically acquired with a large condenser aperture (30–40 μm) which yields density currents, recorded on the phosphorous plate, of around 100 pA cm−2. If we consider radiolysis as the primary radiation damage in the studied clusters, then for a HRTEM image taken at M = 500 K the electron dose exerted on the sample will be around 12,500 Å−2, this dose rate is capable of producing changes in the structure of the metallic clusters because the thiol groups are more sensitive and are located in the surface of the metallic core atoms. The images shown in Fig. 3a–d indicate the structural transformations from an individual Au144(SR)60 cluster, the structure has been compared with the relaxed structure calculated in Ref. [20] (see the full video in Additional file 1). Experimental HRTEM observations of gold nanoparticles show the occurrence of structural instabilities, such as quasi melting as reported in the literature [25], this process is produced by existence of multiple structural configurations separated by low energy barriers. Hence, to diminish the radiolysis effect, the nanoprobe conditions are set up in the microscope with a small condenser lens aperture (5 μm) and a large demagnification of the condenser lens in the JEOL 2010F microscope. Under nanoprobe mode a sample can be irradiated in a quasi-parallel illumination reducing the current density in about two orders of magnitudes. The images shown in Fig. 3e–h represent a sequence of the irradiated area in Au144(SR)60 clusters. In these series, the current density increases as the irradiated area decreases. At low magnifications, the structure transformations are drastically reduced but are still present after few seconds in which the beam is positioned in the cluster as we demonstrated in a previous article using nanobeam diffraction in STEM mode [21]. In order to register proper diffraction patterns with a reduced noise-signal ratio, the patterns need to be registered with the minimum probe size, which increases the electron dose in almost three orders of magnitude as depicted when comparing Fig. 3e, h. To protect these clusters from radiolysis effects the approach then relies in a reduction of the acquisition time using a fast scanning and fast detection experimental set ups.Fig. 3

View Article: PubMed Central - PubMed

ABSTRACT

The present work explores electron diffraction methods for studying the structure of metallic clusters stabilized with thiol groups, which are susceptible to structural damage caused by electron beam irradiation. There is a compromise between the electron dose used and the size of the clusters since they have small interaction volume with electrons and as a consequence weak reflections in the diffraction patterns. The common approach of recording individual clusters using nanobeam diffraction has the problem of an increased current density. Dosage can be reduced with the use of a smaller condenser aperture and a higher condenser lens excitation, but even with those set ups collection times tend to be high. For that reason, the methods reported herein collects in a faster way diffraction patterns through the scanning across the clusters under nanobeam diffraction mode. In this way, we are able to collect a map of diffraction patterns, in areas with dispersed clusters, with short exposure times (milliseconds) using a high sensitive CMOS camera. When these maps are compared with their theoretical counterparts, oscillations of the clusters can be observed. The stability of the patterns acquired demonstrates that our methods provide a systematic and precise way to unveil the structure of atomic clusters without extensive detrimental damage of their crystallinity.

Electronic supplementary material: The online version of this article (doi:10.1186/s40679-016-0026-x) contains supplementary material, which is available to authorized users.

No MeSH data available.