Nanomolding of Crystalline Metals: The Smaller the Easier

We report on a thermomechanical nanomolding method for crystalline metals. Quantified by the aspect ratio, this process becomes easier with decreasing mold diameter. As the responsible underlying diffusion mechanism is present in all metals and alloys, the discovered nanomolding process provides a toolbox to shape essentially any metal and alloy into a nanofeature. Technologically, this highly versatile and practical thermomechanical nanomolding technique offers a method to fabricate high-surface-area metallic nanostructures which are impactful in diverse fields of applications including catalysts, sensors, photovoltaics, microelectronics, and plasmonics.

Figure 1

Scaling of thermomechanical molding with crystalline metals. (a) Schematics of the experimental setup of TMNM with CMs. (b) Thermomechanical molding, taking Ag as an example, with millimeter, micrometer, and nanometer size molds, using molding pressure, temperature, and time of 100 MPa, $620°\mathrm{C}$, and 100 s, respectively. Examples of prepared Ag rods with diameters of 0.57 mm, $10\mu \mathrm{m}$, 375 nm, and 36 nm are shown in (i)–(iv). (c) Aspect ratio of the molded Ag rods as the function of mold diameter ($d$) (the dashed-red line is given as a guide for the eye). (d) The various dominant deformation mechanisms at play during thermomechanical molding at various length scales. Plastic deformation of metals is generally associated with dislocations from grain boundary sources and intragranular sources such as grain sliding and rotating. These processes become decreasingly and rapidly less effective with decreasing mold size. Hence, diffusion becomes the dominant deformation mechanism on the nanoscale.

Figure 2

Deformation mechanism of TMNM with CMs compared to other nanomolding mechanisms. (a) Moldability, quantified by $L/d$, scales qualitatively differently for bulk metallic glasses (black solid squares; the data are from Ref. [4]), and for crystalline metals (red solid pentagons; the dashed-red line is a guide for the eye). The processing of Au nanorods was carried out at $622°\mathrm{C}$ under $\sim 800\mathrm{MPa}$ for 100 s. The Hagen-Poiseuille law (H-P law) with capillary force where $\gamma =1\mathrm{N}/\mathrm{m}$ and $\theta =120°$ [10] (magenta line) and without capillary force (blue line) are calculated. The molding time and loading pressure are 180 s and $\sim 350\mathrm{MPa}$, respectively, taken from Ref. [4]. The discrepancy between theory (magenta line) and experiments (black solid squares) is attributed to an enhanced viscosity on the nanoscale [4]. (b) Calculated growth rates for the diffusion driven growth of Au nanorods at $500°\mathrm{C}$ [Eq. (2), red and purple lines]. Viscous flow based mechanisms with and without the capillary effect for nanomolding of Pt-BMG are also plotted for comparison [Eq. (1), black and dashed-black lines). The loading pressure and aspect ratio are 500 MPa and $\lambda =5$, respectively. The other parameters are taken from the literature and are listed in Table S1 of the Supplemental Material [17]. (c) Growth of Au nanorods at $500°\mathrm{C}$ and $\sim 500\mathrm{MPa}$ versus molding time. The dots represent the experimental data and the solid line is fitted according to Eq. (3).

Figure 3

Atomic growth mechanism of thermomechanical nanomolding with crystalline metals. (a)–(c). TEM images from a diameter of 50 nm Au nanorod fabricated by TMNM. (a) A bright field image of an Au nanorod. (Inset) Its corresponding selected area electron diffraction pattern reveals the growth direction as [110]. The scale bar is $0.41/\AA$. (b),(c) Enlarged images of the tip region [(b) and (c) are captured from yellow and orange rectangles in (a) and (b), respectively]. Twins are revealed with twin planes of $\{111\}$ close to the tip, and $\{111\}$ facets at the tip. $\{111\}$ planes are marked by red-dashed lines in (b) and (c). $⟨112⟩$ directions are indicated as blue arrows in (c) to highlight mirrorlike symmetrical characteristics on the twin boundaries. Twins can be generated to release local energy or misfit upon growing, which is commonly observed in face-centered-cubic Au. (d) A TEM image from a $\sim 100\mathrm{nm}$ Au nanorod showing multiple grains with different crystallographic orientations at the root of the nanorod. (e)–(g) Schematics of TMNM with CMs indicating atomistic processes as a sequence of growth of a metal nanorod and corresponding pressure profiles along its axis. Blue arrows represent $⟨112⟩$ direction vectors. (e) A feedstock metal (here for fcc) is positioned on the mold at the molding temperature, and a molding pressure is applied. (f) The applied pressure and temperature result in diffusion down the pressure gradient, which causes a vacancy gradient. As the initial orientation of the feedstock metal consists of multiple and randomly oriented grains, growth originally starts along a nonpreferred orientation. This causes nucleation of new grains which progress toward a more preferred orientation (for fcc closer to [110]). There are two possible and size dependent diffusion mechanisms: (i) interface diffusion along the mold-metal interface (path 1) and (ii) lattice diffusion through the rod (path 2). (g) A rod at a later stage of molding which contains small crystals on the root, a long region of a perfect single crystal which can exceed $L/d>100$, and twins and faceted edges at the tip.

Figure 4

Examples of fabricated nanorods arrays through TMNM with CMs. (a) Au nanorods of 5–13 nm in diameter, formed under a pressure of $\sim 500\mathrm{MPa}$ at $500°\mathrm{C}$ for 100 s. (b) TMNM with Ni (fcc) at 600 MPa and at $600°\mathrm{C}$ for 240 s. (c) TMNM with V (bcc) at 437 MPa and at $650°\mathrm{C}$ for 100 s. (d) TMNM with Fe (bcc) at 400 MPa and at $600°\mathrm{C}$ for 240 s. (e) TMNM with ${\mathrm{Ni}}_{60}{\mathrm{Ti}}_{40}$ at $650°\mathrm{C}$, under the forming pressure of 800 MPa for 180 s. (f),(g) TMNM with ${\mathrm{Ag}}_{75}{\mathrm{Ge}}_{25}$ and ${\mathrm{Cu}}_{34.7}{\mathrm{Zn}}_{3.0}{\mathrm{Sn}}_{62.3}$ at $500°\mathrm{C}$, loaded to $\sim 350\mathrm{MPa}$ under a constant loading rate of $1.8\mathrm{mm}/\min$. (h) TMNM with PdCuNi at $695°\mathrm{C}$, under the forming pressure of 800 MPa for 180 s. (i) TMNM with high entropy alloy (PdCuNiPtRhIr) at $650°\mathrm{C}$, under the forming pressure of $\sim 1000\mathrm{MPa}$ for 180 s.