Research ArticleNANOTECHNOLOGY

Approaching the ideal elastic strain limit in silicon nanowires

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Science Advances  17 Aug 2016:
Vol. 2, no. 8, e1501382
DOI: 10.1126/sciadv.1501382
  • Fig. 1 Sample and experimental configuration.

    (A) VLS-grown Si nanowire sample with a uniform diameter of ~100 nm. Inset: Selected area electron diffraction (SAED) pattern indicates that Si nanowire is a single-crystal cubic diamond structure grown along the <110> orientation, which has been confirmed by the corresponding high-resolution transmission electron microscopy (HRTEM) image. (B) Lattice spacing of ~0.19 nm with respect to the <110> plane of Si. (C) HRTEM side view of a Si nanowire showing the atomically smooth surface. (D) In situ scanning electron microscopy (SEM) tensile testing of a single nanowire based on a push-to-pull MMD actuated by an external quantitative nanoindenter. (E to G) Zoom-in views (G) of the yellow frame in (D) are presented in (E) and (F), showing the detailed clamping configuration of a single nanowire sample at a lower voltage of 2 kV (E) and a regular working voltage of 20 kV (F). The tensile gauge length is indicated by the red bar in (E), whereas the yellow arrows in (F) indicate the uniaxial tensile loading direction. (H) Typical load-versus-displacement curve read from the nanoindenter for a monotonic tensile test under the displacement control mode. The abrupt force drop indicates the failure of the nanowire sample.

  • Fig. 2 In situ SEM tensile tests and postmortem TEM analysis.

    (A to F) Elongation of a single Si nanowire (diameter, ~86 nm). (A) Original status before test. (B to E) Extracted frames show gradual elongation of Si nanowire under tensile straining, with a maximum strain of 13% just before fracture (E). (F) Most of the nanowires flew away right after fracture, except the clamped portion. (G) The corresponding stress-versus-strain curve is nearly linear, with a fracture stress of ~16 GPa. (H and I) Loading-unloading test with increasing tensile strain amplitude and full unloading in each cycle. (H) Si nanowire before test (diameter, ~120 nm). (I) Loading–fully unloading process, in which the nanowire fully recovers its original length after strain values of ~5.8, ~8.1, ~9.7, and 13.2% are experienced. The nanowire finally broke at the fifth cycle with a strain value of ~13.5%, with one piece of broken nanowire remaining on the stage (as marked in the white frame); note that there is a thin, nonuniform layer of glue coating the nanowire [the red dashed lines in (H) indicate the true nanowire boundary]. (J and K) Bright-field TEM images showing a typical brittle fracture surface morphology, in which the nanowire remained in the single-crystalline structure [inset SAED pattern in (K)] with a uniform diameter [highlighted by red dashed lines in (J), with a small amount of conductive epoxy glue over the surface] and a flat fracture surface. All the images in (J) and (K) were taken in <1-11> zone axis. (L) The HRTEM image of the front end of the nanowire fracture surface [the red rectangular area in (K)] shows the single-crystalline structure with flat fracture surface (highlighted by red dashed lines); during the sample transfer and posttesting TEM analysis, a thin layer (~5 to 6 nm) of amorphous silicon oxide was formed at the fracture surface (L) with no visible sign of plastic deformation.

  • Fig. 3 Tensile test in ambient environment under an optical microscope.

    (A) Loading-unloading tensile test of a Si nanowire with increasing tensile strain amplitude and full unloading in each cycle. Again, the nanowire recovered its original length after strain values of ~5, ~7.3, and ~10% were experienced in each cycle and eventually fractured at the fourth cycle with strain value of ~11.7% (top to bottom: the last three cycles), where most of the broken nanowire flew away, as shown in the last frame. Note that the contrast in the optical images was slightly enhanced for clarity. (B) Corresponding stress-versus-strain curves of the multicycle loading–fully unloading test, using different colors to better illustrate the data from each cycle.

  • Fig. 4 Comparison between VLS-grown Si nanowires and top-down etched Si nanowires.

    (A) Bright-field TEM showing a top-down etched single-crystal Si nanowire (also in <110> orientation) with a uniform diameter but a relatively rough surface. (B) Deformation process of a top-down etched Si nanowire upon tensile straining (diameter, ~140 nm) with the corresponding stress-versus-strain curve. (C) Si nanowire broken in an elastic manner with a fracture strain of ~3.7% (fracture stress, ~5 GPa; calculated Young’s modulus, ~135 GPa). (D) Summary of the comparison for the fracture strain versus nanowire (NW) diameter between the VLS-grown (red dots) and top-down etched (blue triangles) Si nanowires. The shaded bottom area indicates the range of previously reported tensile strain values for Si nanowires with diameters of >20 nm (2527). (E) Summary of the fracture strains of VLS-grown Si nanowires versus their strain/loading rates, indicating that the elastic limit is insensitive to strain rate in this range.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/8/e1501382/DC1

    fig. S1. Monotonic tensile straining of a VLS-grown Si nanowire under an optical microscope.

    fig. S2. Additional details about sample clamping and gauge length/strain measurement.

    movie S1. Monotonic tensile test of a VLS-grown Si nanowire inside a scanning electron microscope (as shown in Fig. 2, A to G); video speed is played at ~3× speed.

    movie S2. Loading–partially unloading tensile test of a VLS-grown Si nanowire with increased strain values until finally fractured at a stress of ~20 GPa; video speed is played at ~10× speed.

    movie S3. Loading–fully unloading tensile test of a VLS-grown Si nanowire inside a scanning electron microscope (as shown in Fig. 2, H and I); video speed is played at ~9× speed.

    movie S4. Loading–fully unloading tensile test of a VLS-grown Si nanowire in ambient environment under an optical microscope (as shown in Fig. 3); video speed is played at ~8× speed.

    movie S5. Monotonic tensile test of a top-down etched Si nanowire (as shown in Fig. 4, B and C); video speed is played at ~2× speed.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Monotonic tensile straining of a VLS-grown Si nanowire under an optical microscope.
    • fig. S2. Additional details about sample clamping and gauge length/strain measurement.
    • Legends for movies S1 to S5

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Monotonic tensile test of a VLS-grown Si nanowire inside a scanning electron microscope (as shown in Fig. 2, A to G); video speed is played at ~3× speed.
    • movie S2 (.mp4 format). Loading–partially unloading tensile test of a VLS-grown Si nanowire with increased strain values until finally fractured at a stress of ~20 GPa; video speed is played at ~10× speed.
    • movie S3 (.mp4 format). Loading–fully unloading tensile test of a VLS-grown Si nanowire inside a scanning electron microscope (as shown in Fig. 2, H and I); video speed is played at ~9× speed.
    • movie S4 (.mp4 format). Loading–fully unloading tensile test of a VLS-grown Si nanowire in ambient environment under an optical microscope (as shown in Fig. 3); video speed is played at ~8× speed.
    • movie S5 (.mp4 format). Monotonic tensile test of a top-down etcheed Si nanowire (as shown in Fig. 4, B and C); video speed is played at ~2× speed.

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