bIIIranging from 71° to 74° (~72° on average)
(Fig. 3F). By contrast, region II was barely
affected, andbIIremained at ~70° (Fig. 3F).
However,bIIincreased instantly to ~72° as the
5-FT formed at 80.6 s (Fig. 3F) and remained
at ~72°, sharing the angle misfit (7.35°) with the
other twin units. Molecular dynamics (MD)
simulations confirmed a similar oscillation
(Fig. 3G), showing comparable energies be-
tween them (Fig. 3H). MD results also con-
sistently demonstrated the relatively minor
shearstraininregionIwithstackingfaults
(Fig. 3G,a~ 132.4°) and an increase of shear
strain near theS27 boundary without stacking
faults (Fig. 3G,a~ 146.4°).
As described above, region I was highly
strained without stacking faults, withbIof
~74°; the shear strain was concentrated near
thetwinpolealongS27-(111)IV(Fig.4,AandE),
where at 80.6 s, three layers of {111} planes
deformed into {200} planes (Fig. 4B and fig.
S8). This deformation did not affect the neigh-
boring lattice structures, indicating nucleation
of a ZST. A ZST is typically formed by atoms
deforming from {111} into {200} planes through
partial dislocation slipping of three consecutive
{111} planes (Fig. 4C) with Burgers vectors (b)
ofb 1 ¼ 61 ½ 21 1 ,b 2 ¼^16 ½ 112 , andb 3 ¼ 61 ½ 1 2 1 ,
resulting in a totalbof zero (Fig. 4C) ( 33 , 34 ).
We found with MD simulation that the shear
strain concentration on theS27-{111}IV(Fig.
4E) induces nucleation of ZST (Fig. 4, D to G
and I; figs. S9 and S10; and movies S2 and S3).
The ZST grows laterallyalong the three layers
of the {111}Iplanes in region I (Fig. 4, F and G,
yellow arrows), resulting in strain relaxation
on the unstableS27 (Fig. 4G) and formation
ofS 35 in region I (Fig. 4G). The continuous
nucleation and growth of ZST (movies S2 and
S3)leadtothecompletedecompositionofS27-
(111)IVintoS 34 andS 35 ,formingthe5-FT(Fig.
4H)withalargeenergydecrease(Fig.4J).
Residual layers of {111}Iplanes (less than three
layers) near the NP surface can be twinned
through partial dislocation slipping (Fig. 2G).
Fully decomposingS27 through {111} partial
dislocation slipping nucleated on the surface
of the NP is unfavorable because of the angle
mismatch between {111} planes of regions I
and IV (fig. S11), although it has been observed
to form a few layer twin units. There was a
substantial configuration adjustment of the
NP during theS27 oscillatory evolution pro-
cess (Fig. 2, D to H, and fig. S12), indicating
surface adjustment of the NP. During this pe-
riod, a nucleation attempt of the ZST at 71.0 s
occurred, but it disappeared in ~0.3 s (fig. S13),
indicating a failure to overcome the energy
barrier to nucleate and grow.
Mechanism 2: Formation of 5-FT through
partial dislocation slipping
At the resulting ~150° concave surface (Fig. 5,
A to C), the surface is relatively more stable
than the 94° concave surface, and the atoms
seldom migrated to the concave surface area.
Under e-beam irradiation, one layer of twin
structures formed at the concave area through
GB decomposition and partial dislocation slip-
ping (Fig. 5, C, D, and G; and fig. S14). Twin
interfaces subsequently migrated from the
periphery of the particle toward the center
and formedS 33 ,S 34 ,andS 35 (Fig. 5, D and E).
On the basis of our eight observations of this
process, twin boundaries only migrate one to
five atomic layers, resulting in the unstable
asymmetric 5-FT (Fig. 5, D and E, and fig. S15).
The asymmetric 5-FT underwent detwinning
through partial dislocation slipping, as dem-
onstrated by the varying number of {111} layers
in each twin unit (Fig. 5, E and F), to decrease
the twin interface and volume strain ener-
gies ( 36 ). During detwinning processes, twin
boundary migration and pole splitting were
observed (more examples are provided in figs.
S16 and S17), similar to what we reported pre-
viously, the detwinning of Ag 5-FT particles
in solution ( 36 ).
In addition, the asymmetric 5-FT can also
grow into a stable structure through particle
aggregation (OA or non-OA) (fig. S18, A to E).Similarly, a symmetrical 5-FT can evolve into
an asymmetrical structure through particle
aggregation (fig. S18, F to J). On the basis of
15 observations of 5-FT formation, the prob-
ability of these two twinning mechanisms oc-
curring is almost equal (7:8) because of the
random selectivity of the attached {111} sur-
faces during OA. We collected a number of
other examples (figs. S15 and S17) of these two
types of twinning processes (fig. S19).Formation of 5-FT through OA in other
metal systems
We observed the two fivefold twinning mech-
anisms not only in Au NPs with a low twin-
fault energy (~26 mJ/m^2 )( 37 )butalsoinother
metal NP systems with high twin-fault ener-
gies, palladium (Pd) NPs (~106 mJ/m^2 )(Fig.6,A
to H) ( 37 )andplatinum(Pt)NPs(~179mJ/m^2 )
(Fig.6,ItoP)( 37 ). For example, a twinned
NP (Fig. 6A) underwent OA with another NP
(Fig. 6B), inducing the secondS 32 and a ~85°
concave surface; after atomic surface dif-
fusion, a high-energyS9GBformed(Fig.6,
A and B). Subsequently,S9 decomposed into
another high-energy GBS27 and the third
S 33 (Fig. 6, B and C). Last,S27 evolved intoSonget al.,Science 367 ,40–45 (2020) 3 January 2020 4of6
Fig. 5. Formation mechanism 2 through OA and partial dislocation slipping.(AandB) Formation of
S 32 and a concave surface after the OA process. (C) Formation ofS9 after diffusion of atoms onto
the concave surface. (D) Formation of 5-FT induced through GB decomposition and partial dislocation slipping.
(EandF) Migration of twin boundaries. (G) Schematic illustration of the decomposition ofS9intothreeS 3
GBs via partial dislocation slipping.RESEARCH | RESEARCH ARTICLE