General Nanomolding of Ordered Phases

Large-scale, controlled fabrication of ordered phases is challenging at the nanoscale, yet highly demanded as their well-ordered structure and chemistry is the key for advanced functionality. Here, we demonstrate a general nanomolding process of ordered phases based on atomic diffusion. Resulting nanowires are single crystals and maintain their composition and structure throughout their length, which we explain by a self-ordering process originating from their narrow Gibbs free energy. The versatility, control, and precision of this thermomechanical nanomolding method of ordered phases provides new insights into single crystal growth and suggest itself as a technology to enable wide spread usage for nanoscale and quantum devices.

Figure 1

Thermomechanical nanomolding (TMNM) of ordered phases. (a) For TMNM, (i) a mold composed of nanocavities (e.g., anodic aluminum oxide) and an ordered phase feedstock (typically 0.5 mm thick and 5 mm wide) are prepared. (ii) The mold is pressed against the feedstock at 10–500 MPa and a temperature of $\sim 0.5T_m$ over a time between 1 sec and 1 h. [(ii) top] A cross-section view of the ordered phase nanowires filling the nanocavities of the mold (scale bar: 500 nm). (iii) Subsequently, for large aspect ratios, the mold is chemically etched, leaving behind only the nanowire array of the ordered phase (scale bar: $1\mu \mathrm{m}$). (iv) Nanowires can be separated from the array by sonication (scale bar: $1\mu \mathrm{m}$). (b)–(e) ${\mathrm{Au}}_2\mathrm{Al}$ nanowires with different diameters: 250, 120, 10, and 40 nm (scale bars: $1\mu \mathrm{m}$). (e)–(h) ${\mathrm{Au}}_2\mathrm{Al}$ nanowires with different lengths: 300 nm, 1, 5, and $20\mu \mathrm{m}$. The length is controlled by processing temperature, time, or pressure (scale bars: 100 nm, 200 nm, $1\mu \mathrm{m}$, and $1\mu \mathrm{m}$).

Figure 2

Ordered phase nanowires as a single crystal, fabricated by TMNM. (a) A SnTe nanowire fabricated by TMNM (aspect ratio $\sim 25$, 80 nm diameter and $2\mu \mathrm{m}$ length, scale bar: 200 nm). (b) TEM images from selected regions in (a), showing atomic planes with the same orientation along the entire (scale bar: 2.5 nm). (c) A typical electron diffraction pattern from molded SnTe nanowires, revealing the crystal structure of SnTe viewed from [100] direction (scale bar: $5{\mathrm{nm}}^{-1}$). (d) ${\mathrm{Au}}_2\mathrm{Al}$ nanowire (aspect ratio $>250$, 40 nm diameter and $10\mu \mathrm{m}$ length, scale bar: $1\mu \mathrm{m}$). Diffractograms from high-resolution TEM images (see Supplemental Material, Fig. S7 [27]) from selected regions reveal ${\mathrm{Au}}_2\mathrm{Al}$ crystal structure with the same orientation along the nanowire. $Y$ axis of FFT are normalized to growth direction of nanowire. (e),(f) EDS mapping and EDS spectra from the selected regions of the ${\mathrm{Au}}_2\mathrm{Al}$ nanowire (diameter 80 nm) reveal the uniform composition along the nanowire, which is identical to the composition of the feedstock (scale bar: 200 nm).

Figure 3

Atomic mechanism for TMNM of ordered phases. (a) Molding length and molding time exhibit a square root relation, suggesting atomic diffusion as the underlying TMNM mechanism (see Supplemental Material for details [27]). (b) The competition between the lowering of chemical potential due to the reduction of the pressure gradient ${\mu }_p$ and the energetics associated with the growth of the phase $\mu$ determines if a specific composition can be nanomolded. Specifically, if ${\mu }_p<\mu$, nanomolding of such a composition is prohibited and if ${\mu }_p>\mu$, nanomolding is possible. For solid solutions, the growth condition ${\mu }_p>\mu$ is typically fulfilled over a wide composition range. Therefore, a change in composition of the growing nanorod occurs according to the typically different diffusivities of the alloy’s constituents; the small size specie (green sphere) diffuse down the potential gradient faster and hence change the composition of the nanowire. For complete solubility and different diffusivity among the constituents, this can result in a change in composition to entirely that of the faster diffusing constituent [for example in the formation of Cu nanowires when using ${\mathrm{Au}}_{50}{\mathrm{Cu}}_{50}$ as feedstock (d)]. For ordered phases, however, the condition ${\mu }_p>\mu$ is satisfied only in a very narrow composition range due to their narrow Gibbs free energy curve. Hence, the faster diffusing specie (red sphere) can not occupy the sublattice for the slower specie (blue sphere), as it would increase the chemical potential so that ${\mu }_p>\mu$ which would prohibit growth thermodynamically (iii). Therefore, TMNM of ordered phases is a self-regulating process to occur only at the specific stoichiometry of the ordered phase. (c)–(e) Composition distribution in nanowires of typical ordered phases SnTe (c), with red sphere representing Te atoms and blue spheres Sn atoms and solid solution (d) ${\mathrm{Au}}_{50}{\mathrm{Cu}}_{50}$, Au orange, Cu green, and (e) ${\mathrm{Au}}_{50}{\mathrm{Ag}}_{50}$, Au orange, Ag white. (c) Ordered phase nanowires (SnTe) with narrow chemical potential show uniform composition, identical to the feedstock material. (d) TMNM of ${\mathrm{Au}}_{50}{\mathrm{Cu}}_{50}$ solid solution with different size components and broad chemical potential result in composition gradient along the nanowire which eventually forms a pure Cu nanowire. (e) However, solid solutions with components of similar diffusivity and broad chemical potential can hold similar composition as feedstock material along the nanowire, for example in the case of ${\mathrm{Au}}_{50}{\mathrm{Ag}}_{50}$.

Figure 4

TMNM map and versatility. (a) TMNM map as a function of diffusivity. Achievable aspect ratios $L/d$, are calculated according to $L/d=\sqrt{({L_0}^2/d^2)+(2p\mathrm{\Omega }Dt/d^{2}T)}$, where $L_0$ is the length of nanowires after the pressure ramping, $d$ is the cavity diameter, $p$ is processing pressure, $t$ is time, $T$ is temperature, and $D$ is the diffusivity, assuming practical fabrication parameters (pressure of 400 MPa and processing time of 1 hr, see Supplemental Material for more details [27]). (b) Fabrication of nanowires of ordered phases suggested by the TMNM map. TMNM was successfully applied to fabricate nanowires of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST), FeSe, ${\mathrm{Au}}_2\mathrm{Al}$, SnTe, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, ${\mathrm{Cu}}_7{\mathrm{In}}_3$, ${\mathrm{In}}_2\mathrm{Bi}$, BiSb, ${\mathrm{CuAl}}_2$, ${\mathrm{AuAl}}_2$, ${\mathrm{In}}_{75}{\mathrm{Sn}}_{25}$, AuSn, ${\mathrm{AuIn}}_2$, InBi, and InSb with high aspect ratios. The demonstrated ordered phases cover many functional materials including semiconductors (InSb), topological insulators (SnTe, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, BiSb), superconductors (FeSe, ${\mathrm{AuIn}}_2$), phase change material (${\mathrm{Sb}}_2{\mathrm{Te}}_3$, GST) and colorful material (${\mathrm{AuAl}}_2$). Scale bars in (b) are $1\mu \mathrm{m}$.