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Discrete plasticity in sub-10-nm-sized gold crystals.

Zheng H, Cao A, Weinberger CR, Huang JY, Du K, Wang J, Ma Y, Xia Y, Mao SX - Nat Commun (2010)

Bottom Line: Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking.Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure.These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering & Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.

ABSTRACT
Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

No MeSH data available.


Related in: MedlinePlus

Surface stress-induced phase transformation.(a) The moment before nanocrystal fractures. (b) The contraction (relaxation) of the bottom part of Au crystal after the crystal fractures. The scale bar in each figure represents 3 nm. (c, d) Enlarged HRTEM images of the white-boxed area in a and b, respectively, accompanied with the corresponding crystallographic orientation. (e) Simulated HRTEM image of FCC Au along [110] zone axis with lattice parameter of a=4.078 Å. (f) Simulated image of BCT Au along [100] zone axis with lattice parameter of a=b=3.34 Å and c=2.86 Å. The simulations were conducted by applying the parameters including the acceleration voltage of 300 kV, spherical aberration coefficient of the 1.2 mm, the specimen thickness of 3 nm and the focus value of −43 nm. (g) Martensitic transition (FCC–BCT) due to lattice distortion along [001] direction in FCC crystal on the basis of the Bain path. The unit cells of FCC and BCT crystals are outlined with black and blue lines, respectively.
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f3: Surface stress-induced phase transformation.(a) The moment before nanocrystal fractures. (b) The contraction (relaxation) of the bottom part of Au crystal after the crystal fractures. The scale bar in each figure represents 3 nm. (c, d) Enlarged HRTEM images of the white-boxed area in a and b, respectively, accompanied with the corresponding crystallographic orientation. (e) Simulated HRTEM image of FCC Au along [110] zone axis with lattice parameter of a=4.078 Å. (f) Simulated image of BCT Au along [100] zone axis with lattice parameter of a=b=3.34 Å and c=2.86 Å. The simulations were conducted by applying the parameters including the acceleration voltage of 300 kV, spherical aberration coefficient of the 1.2 mm, the specimen thickness of 3 nm and the focus value of −43 nm. (g) Martensitic transition (FCC–BCT) due to lattice distortion along [001] direction in FCC crystal on the basis of the Bain path. The unit cells of FCC and BCT crystals are outlined with black and blue lines, respectively.

Mentions: After fracture, the crystal unloads and, in some cases, undergoes a phase transformation from a FCC to body-centered tetragonal (BCT) structure (Fig. 3), which is predicted in recent theoretical work by Diao et al.23 It is theorized that the transformation occurs when compressive stresses caused by the tensile surface stress components along the 〈001〉 direction exceed the stress required to transform bulk gold (Au; FCC) to its higher-energy crystal structure (BCT). Our experimental observation provides the first direct evidence of such kind of phase transformation. As can be clearly seen in Figure 3a,b (see also Supplementary Movie 3), the bottom half of the nanocrystal contracts after fracture. The HRTEM images indicating the transformation from FCC to BCT is shown in Figure 3c,d. The lattice constants of FCC are ao=bo=co=4.07 Å, whereas those of BCT phase are determined to be a=b=3.34 Å and c=2.86 Å (the error estimation is presented in the Methods section, see also Supplementary Fig. S7). Correspondingly, a lattice contraction of 30% along the length direction (〈001〉 direction) is obtained, in accordance with the theoretical prediction23. In addition, the phase transition can be well explained by the Bain model24, which suggests that on biaxial expansion of the lattice constant in the {001} plane, a FCC solid may transform spontaneously into a body-centered cubic or BCT phase through relaxation of the interlayer spacing along the perpendicular 〈001〉 direction (Fig. 3g). It is worth noting that the theory has also predicted that Au nanocrystals may re-orient to 〈110〉 FCC crystals rather than transforming to BCT25. However, the lattice image in Figure 3d cannot be interpreted according to a FCC Au structure, which rules out the possibility of reorientation mechanism that was proposed25. Additionally, the simulated images of a FCC Au crystal along [110] zone axis (Fig. 3e) and a BCT Au crystal along the [100] zone axis (Fig. 3f) agree with the experimental observed HRTEM images shown in Figure 3c,d, respectively. In our experiment, the phase transformation is only observed in the fractured, or relaxed, nanocrystal after 〈001〉 tensile loading, consistent with the MD simulation results. As the compressive stresses induced in the core of the nanocrystal by the surface stress scale inversely with the size of the crystal, phase transformation is a size-dependent phenomenon. Our experiments show that the top half remains FCC (Fig. 3b), which may result from the crystal being larger or different surface stresses as a result of faceting. From MD predictions, the transformation was observed in 2.65 nm×2.65 nm NW with cross-sectional area of 7 nm2 (at room temperature 300 K)23, which we believe is quite close to that of the nanocrystal observed in the current experiment (Fig. 3b).


Discrete plasticity in sub-10-nm-sized gold crystals.

Zheng H, Cao A, Weinberger CR, Huang JY, Du K, Wang J, Ma Y, Xia Y, Mao SX - Nat Commun (2010)

Surface stress-induced phase transformation.(a) The moment before nanocrystal fractures. (b) The contraction (relaxation) of the bottom part of Au crystal after the crystal fractures. The scale bar in each figure represents 3 nm. (c, d) Enlarged HRTEM images of the white-boxed area in a and b, respectively, accompanied with the corresponding crystallographic orientation. (e) Simulated HRTEM image of FCC Au along [110] zone axis with lattice parameter of a=4.078 Å. (f) Simulated image of BCT Au along [100] zone axis with lattice parameter of a=b=3.34 Å and c=2.86 Å. The simulations were conducted by applying the parameters including the acceleration voltage of 300 kV, spherical aberration coefficient of the 1.2 mm, the specimen thickness of 3 nm and the focus value of −43 nm. (g) Martensitic transition (FCC–BCT) due to lattice distortion along [001] direction in FCC crystal on the basis of the Bain path. The unit cells of FCC and BCT crystals are outlined with black and blue lines, respectively.
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Related In: Results  -  Collection

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f3: Surface stress-induced phase transformation.(a) The moment before nanocrystal fractures. (b) The contraction (relaxation) of the bottom part of Au crystal after the crystal fractures. The scale bar in each figure represents 3 nm. (c, d) Enlarged HRTEM images of the white-boxed area in a and b, respectively, accompanied with the corresponding crystallographic orientation. (e) Simulated HRTEM image of FCC Au along [110] zone axis with lattice parameter of a=4.078 Å. (f) Simulated image of BCT Au along [100] zone axis with lattice parameter of a=b=3.34 Å and c=2.86 Å. The simulations were conducted by applying the parameters including the acceleration voltage of 300 kV, spherical aberration coefficient of the 1.2 mm, the specimen thickness of 3 nm and the focus value of −43 nm. (g) Martensitic transition (FCC–BCT) due to lattice distortion along [001] direction in FCC crystal on the basis of the Bain path. The unit cells of FCC and BCT crystals are outlined with black and blue lines, respectively.
Mentions: After fracture, the crystal unloads and, in some cases, undergoes a phase transformation from a FCC to body-centered tetragonal (BCT) structure (Fig. 3), which is predicted in recent theoretical work by Diao et al.23 It is theorized that the transformation occurs when compressive stresses caused by the tensile surface stress components along the 〈001〉 direction exceed the stress required to transform bulk gold (Au; FCC) to its higher-energy crystal structure (BCT). Our experimental observation provides the first direct evidence of such kind of phase transformation. As can be clearly seen in Figure 3a,b (see also Supplementary Movie 3), the bottom half of the nanocrystal contracts after fracture. The HRTEM images indicating the transformation from FCC to BCT is shown in Figure 3c,d. The lattice constants of FCC are ao=bo=co=4.07 Å, whereas those of BCT phase are determined to be a=b=3.34 Å and c=2.86 Å (the error estimation is presented in the Methods section, see also Supplementary Fig. S7). Correspondingly, a lattice contraction of 30% along the length direction (〈001〉 direction) is obtained, in accordance with the theoretical prediction23. In addition, the phase transition can be well explained by the Bain model24, which suggests that on biaxial expansion of the lattice constant in the {001} plane, a FCC solid may transform spontaneously into a body-centered cubic or BCT phase through relaxation of the interlayer spacing along the perpendicular 〈001〉 direction (Fig. 3g). It is worth noting that the theory has also predicted that Au nanocrystals may re-orient to 〈110〉 FCC crystals rather than transforming to BCT25. However, the lattice image in Figure 3d cannot be interpreted according to a FCC Au structure, which rules out the possibility of reorientation mechanism that was proposed25. Additionally, the simulated images of a FCC Au crystal along [110] zone axis (Fig. 3e) and a BCT Au crystal along the [100] zone axis (Fig. 3f) agree with the experimental observed HRTEM images shown in Figure 3c,d, respectively. In our experiment, the phase transformation is only observed in the fractured, or relaxed, nanocrystal after 〈001〉 tensile loading, consistent with the MD simulation results. As the compressive stresses induced in the core of the nanocrystal by the surface stress scale inversely with the size of the crystal, phase transformation is a size-dependent phenomenon. Our experiments show that the top half remains FCC (Fig. 3b), which may result from the crystal being larger or different surface stresses as a result of faceting. From MD predictions, the transformation was observed in 2.65 nm×2.65 nm NW with cross-sectional area of 7 nm2 (at room temperature 300 K)23, which we believe is quite close to that of the nanocrystal observed in the current experiment (Fig. 3b).

Bottom Line: Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking.Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure.These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering & Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.

ABSTRACT
Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

No MeSH data available.


Related in: MedlinePlus