Limits...
Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy.

Pawlak R, Marot L, Sadeghi A, Kawai S, Glatzel T, Reimann P, Goedecker S, Güntherodt HJ, Meyer E - Sci Rep (2015)

Bottom Line: The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland.

ABSTRACT
The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

No MeSH data available.


Related in: MedlinePlus

Real-space observation of short- and medium-range order.(a) STM overview of the Ni40Ta60 sample revealing the lack of long-range order, (200 × 200) nm2. (b) The close-up view of the surface shows that mainly small clusters are observed with few embedded and randomly oriented chain-like structures (dashed lines). (c) STM image of an amorphous area composed of small clusters and where a medium-range (MR) order is locally observed. (d) Derivative STM image showing that short chain-like structures (≤10 nm) are running parallel favoring the MR order. However, their short lengths, between 2 to 10 nm, avoid arrangements over more than few tens of nms thus keeping the disordered nature of the MG, (I = 10 pA, Vt = 50 mV). (e,f) Topographic STM profiles taken along such chain-like structures revealing an atomic periodicity of ≈0.38 nm. The comparison between (e) and (f) shows that this periodicity locally varies by ≈0.17 nm due to the structural deformability of the chains. Both profiles have been background substracted and taken along the dashed lines of their insets. For clarity, both inset pictures are derivatives of the corresponding STM images, the scale bars are equal to 1 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4542518&req=5

f2: Real-space observation of short- and medium-range order.(a) STM overview of the Ni40Ta60 sample revealing the lack of long-range order, (200 × 200) nm2. (b) The close-up view of the surface shows that mainly small clusters are observed with few embedded and randomly oriented chain-like structures (dashed lines). (c) STM image of an amorphous area composed of small clusters and where a medium-range (MR) order is locally observed. (d) Derivative STM image showing that short chain-like structures (≤10 nm) are running parallel favoring the MR order. However, their short lengths, between 2 to 10 nm, avoid arrangements over more than few tens of nms thus keeping the disordered nature of the MG, (I = 10 pA, Vt = 50 mV). (e,f) Topographic STM profiles taken along such chain-like structures revealing an atomic periodicity of ≈0.38 nm. The comparison between (e) and (f) shows that this periodicity locally varies by ≈0.17 nm due to the structural deformability of the chains. Both profiles have been background substracted and taken along the dashed lines of their insets. For clarity, both inset pictures are derivatives of the corresponding STM images, the scale bars are equal to 1 nm.

Mentions: To reveal the metallic glass structure in the real-space, we systematically performed STM at various sample areas to characterize the typical surface topologies. Figure 2 shows a collection of STM images of the sample surface obtained at 4 K. At scans larger than 50 × 50 nm2 and independently on the scanning position, no crystalline structures were observed confirming the lack of long-range order and the absence of re-crystallization after preparations (Fig. 2a). At first glance, close-up STM views (Fig. 2b,c) show that the MG surface consists of small and randomly distributed clusters. Chain-like structures, which the structure varies between 2 to 10 nm, are often found embedded in the field of clusters as marked with dark lines in Fig. 2b. The inset of Fig. 2f is a derivative STM image showing that such chain (dashed line) is surrounded by a few clusters (white dashed circle). The cluster sizes are about 0.6 nm with a sharp distribution around this value (≈0.2 nm). We attribute these clusters to icosahedral-like structures as recently reported in the literature1415. Along the chain, the inter-unit periodicity is ≈0.38 nm (profile of Fig. 2e) from the STM data which is much smaller than the cluster size. It thus corresponds to an atomic chain rather than an assembly of icosahedrons16. Moreover, the distance between units has an important site-dependent modulation of the bond length (up to ≈150 pm) and the bond angle (≈30°) suggesting the important deformability of the chains. The observation of two embedded structure elements, i.e. icosahedrons and short wires, agrees with the experimental results obtained by nano-beam diffraction reporting mixed fcc and icosahedral structures in metallic glasses and supports recent theoretical works121415161718. Importantly, both structural motifs must have sizes in the nano-meter range which foster their entanglement and a close-packing. Since there is no long-range order in any directions of the sample, these structures cannot be detected by XRD which is in analogy to amorphous polymers showing strong disorder by XRD pattern although the fundamental molecular motif is well defined.


Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy.

Pawlak R, Marot L, Sadeghi A, Kawai S, Glatzel T, Reimann P, Goedecker S, Güntherodt HJ, Meyer E - Sci Rep (2015)

Real-space observation of short- and medium-range order.(a) STM overview of the Ni40Ta60 sample revealing the lack of long-range order, (200 × 200) nm2. (b) The close-up view of the surface shows that mainly small clusters are observed with few embedded and randomly oriented chain-like structures (dashed lines). (c) STM image of an amorphous area composed of small clusters and where a medium-range (MR) order is locally observed. (d) Derivative STM image showing that short chain-like structures (≤10 nm) are running parallel favoring the MR order. However, their short lengths, between 2 to 10 nm, avoid arrangements over more than few tens of nms thus keeping the disordered nature of the MG, (I = 10 pA, Vt = 50 mV). (e,f) Topographic STM profiles taken along such chain-like structures revealing an atomic periodicity of ≈0.38 nm. The comparison between (e) and (f) shows that this periodicity locally varies by ≈0.17 nm due to the structural deformability of the chains. Both profiles have been background substracted and taken along the dashed lines of their insets. For clarity, both inset pictures are derivatives of the corresponding STM images, the scale bars are equal to 1 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4542518&req=5

f2: Real-space observation of short- and medium-range order.(a) STM overview of the Ni40Ta60 sample revealing the lack of long-range order, (200 × 200) nm2. (b) The close-up view of the surface shows that mainly small clusters are observed with few embedded and randomly oriented chain-like structures (dashed lines). (c) STM image of an amorphous area composed of small clusters and where a medium-range (MR) order is locally observed. (d) Derivative STM image showing that short chain-like structures (≤10 nm) are running parallel favoring the MR order. However, their short lengths, between 2 to 10 nm, avoid arrangements over more than few tens of nms thus keeping the disordered nature of the MG, (I = 10 pA, Vt = 50 mV). (e,f) Topographic STM profiles taken along such chain-like structures revealing an atomic periodicity of ≈0.38 nm. The comparison between (e) and (f) shows that this periodicity locally varies by ≈0.17 nm due to the structural deformability of the chains. Both profiles have been background substracted and taken along the dashed lines of their insets. For clarity, both inset pictures are derivatives of the corresponding STM images, the scale bars are equal to 1 nm.
Mentions: To reveal the metallic glass structure in the real-space, we systematically performed STM at various sample areas to characterize the typical surface topologies. Figure 2 shows a collection of STM images of the sample surface obtained at 4 K. At scans larger than 50 × 50 nm2 and independently on the scanning position, no crystalline structures were observed confirming the lack of long-range order and the absence of re-crystallization after preparations (Fig. 2a). At first glance, close-up STM views (Fig. 2b,c) show that the MG surface consists of small and randomly distributed clusters. Chain-like structures, which the structure varies between 2 to 10 nm, are often found embedded in the field of clusters as marked with dark lines in Fig. 2b. The inset of Fig. 2f is a derivative STM image showing that such chain (dashed line) is surrounded by a few clusters (white dashed circle). The cluster sizes are about 0.6 nm with a sharp distribution around this value (≈0.2 nm). We attribute these clusters to icosahedral-like structures as recently reported in the literature1415. Along the chain, the inter-unit periodicity is ≈0.38 nm (profile of Fig. 2e) from the STM data which is much smaller than the cluster size. It thus corresponds to an atomic chain rather than an assembly of icosahedrons16. Moreover, the distance between units has an important site-dependent modulation of the bond length (up to ≈150 pm) and the bond angle (≈30°) suggesting the important deformability of the chains. The observation of two embedded structure elements, i.e. icosahedrons and short wires, agrees with the experimental results obtained by nano-beam diffraction reporting mixed fcc and icosahedral structures in metallic glasses and supports recent theoretical works121415161718. Importantly, both structural motifs must have sizes in the nano-meter range which foster their entanglement and a close-packing. Since there is no long-range order in any directions of the sample, these structures cannot be detected by XRD which is in analogy to amorphous polymers showing strong disorder by XRD pattern although the fundamental molecular motif is well defined.

Bottom Line: The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging.In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable.The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland.

ABSTRACT
The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

No MeSH data available.


Related in: MedlinePlus