3D Shape Reconstruction of Ge Nanowires during Vapor-Liquid-Solid Growth under Modulating Electric Field.

Bottom-up growth offers precise control over the structure and geometry of semiconductor nanowires (NWs), enabling a wide range of possible shapes and seamless heterostructures for applications in nanophotonics and electronics. The most common vapor-liquid-solid (VLS) growth method features a complex interaction between the liquid metal catalyst droplet and the anisotropic structure of the crystalline NW, and the growth is mainly orchestrated by the triple-phase line (TPL). Despite the intrinsic mismatch between the droplet and the NW symmetries, its discussion has been largely avoided because of its complexity, which has led to the situation when multiple observed phenomena such as NW axial asymmetry or the oscillating truncation at the TPL still lack detailed explanation. The introduction of an electric field control of the droplet has opened even more questions, which cannot be answered without properly addressing three-dimensional (3D) structure and morphology of the NW and the droplet. This work describes the details of electric-field-controlled VLS growth of germanium (Ge) NWs using environmental transmission electron microscopy (ETEM). We perform TEM tomography of the droplet-NW system during an unperturbed growth, then track its evolution while modulating the bias potential. Using 3D finite element method (FEM) modeling and crystallographic considerations, we provide a detailed and consistent mechanism for VLS growth, which naturally explains the observed asymmetries and features of a growing NW based on its crystal structure. Our findings provide a solid framework for the fabrication of complex 3D semiconductor nanostructures with ultimate control over their morphology.

Nanoscale Elastocapillary Effect Induced by Thin-Liquid-Film Instability.

Dense arrays of high-aspect-ratio (HAR) vertical nanostructures are essential elements of microelectronic components, photovoltaics, nanoelectromechanical, and energy storage devices. One of the critical challenges in manufacturing the HAR nanostructures is to prevent their capillary-induced aggregation during solution-based nanofabrication processes. Despite the importance of controlling capillary effects, the detailed mechanisms of how a solution interacts with nanostructures are not well understood. Using in situ liquid cell transmission electron microscopy (TEM), we track the dynamics of nanoscale drying process of HAR silicon (Si) nanopillars in real-time and identify a new mechanism responsible for pattern collapse and nanostructure aggregation. During drying, deflection and aggregation of nanopillars are driven by thin-liquid-film instability, which results in much stronger capillary interactions between the nanopillars than the commonly proposed lateral meniscus interaction forces. The importance of thin-film instability in dewetting has been overlooked in prevalent theories on elastocapillary aggregation. The new dynamic mechanism revealed by in situ visualization is essential for the development of robust nanofabrication processes.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

Selective Wet Etching of Silicon Germanium in Composite Vertical Nanowires.

Silicon germanium (SixGe1-x or SiGe) is an important semiconductor material for the fabrication of nanowire-based gate-all-around transistors in the next-generation logic and memory devices. During the fabrication process, SiGe can be used either as a sacrificial layer to form suspended horizontal Si nanowires or, because of its higher carrier mobility, as a possible channel material that replaces Si in both horizontal and vertical nanowires. In both cases, there is a pressing need to understand and develop nanoscale etching processes that enable controlled and selective removal of SiGe with respect to Si. Here, we developed and tested solution-based selective etching processes for SiGe in composite (SiNx/Si0.75Ge0.25/Si) vertical nanowires. The etching solutions were formed by mixing acetic acid (CH3COOH), hydrogen peroxide (H2O2), and hydrofluoric acid (HF). Here, CH3COOH and H2O2 react to form highly oxidizing peracetic acid (PAA or CH3 CO3H). The hydrofluoric acid serves both as a catalyst for PAA formation and as an etchant for oxidized SiGe. Our study shows that an increase in any of the two oxidizer (H2O2 and PAA) concentrations increases the etch rate, and the fastest etch rate of SiGe is associated with the highest PAA concentration. Moreover, using in situ liquid-phase TEM imaging, we tested the stability of nanowires during wet etching and identified the SiGe/Si interface to be the weakest plane; we found that once the diameter of the 160-nm-tall Si0.75Ge0.25 nanowire reaches ∼15 nm during the etching, the nanowire breaks at or very close to this interface. Our study provides important insight into the details of the nanoscale wet etching of SiGe and some of the associated failure modes that are becoming extremely relevant for the fabrication processes as the size of the transistors shrink with every new device generation.

Intermediate Structures of Pt-Ni Nanoparticles during Selective Chemical and Electrochemical Etching.

Both chemical and electrochemical etching are effective methods for tailoring the surface composition of Pt-based catalytic bimetallic nanoparticles (NPs). However, the detailed nanoscale etching mechanisms, which are needed for achieving fine control over the etch processes, are still not understood. Here, we study selective chemical and electrochemical Ni etching of Pt-Ni rhombic dodecahedron NPs using in situ liquid-phase transmission electron microscopy. Our real-time observations show that the intermediate NP structures evolve differently in the two cases. Chemical etching of Ni starts from localized pits on the NP surface, in contrast to the uniform dissolution of Ni during the electrochemical etching. Our study reveals how oxidative etching participates in the removal of a non-noble metal and the subsequent formation of noble-metal-rich NPs. The mechanistic insights reported here highlight the role of a native surface oxide layer on the etching behavior, which is important for the design of NPs with specific surface composition for applications in electrocatalysis.

Growth Dynamics of Gallium Nanodroplets Driven by Thermally Activated Surface Diffusion.

The growth of catalytic liquid-metal nanodroplets on flat substrates is essential for many technological applications. However, the detailed nucleation and growth dynamics of these nanodroplets remain unclear. Here, using in situ transmission electron microscopy (TEM) imaging, we track in real time the growth of individual Ga nanodroplets from a beam of Ga vapor. We show that the nucleation and growth are driven by thermally activated surface diffusion of Ga adatoms, with the diffusion activation energy of ED = 95 ± 10 meV on a SiNx surface. More importantly, our analysis shows that Ga dimers serve as the critical nucleation clusters and that the nanodroplet growth follows a power-law of the form R(t) ∝ e-ED/kBT(t - t0)1/2. These insights into the growth dynamics of metallic nanodroplets are essential for tailoring their size and density for their application in self-catalyzed growth of nanomaterials.

Interface-mediated Kirkendall effect and nanoscale void migration in bimetallic nanoparticles during interdiffusion.

At elevated temperatures, bimetallic nanomaterials change their morphologies because of the interdiffusion of atomic species, which also alters their properties. The Kirkendall effect (KE) is a well-known phenomenon associated with such interdiffusion. Here, we show how KE can manifest in bimetallic nanoparticles (NPs) by following core-shell NPs of Au and Pd during heat treatment with in situ transmission electron microscopy. Unlike monometallic NPs, these core-shell NPs did not evolve into hollow core NPs. Instead, nanoscale voids formed at the bimetallic interface and then, migrated to the NP surface. Our results show that: (1) the direction of vacancy flow during interdiffusion reverses due to the higher vacancy formation energy of Pd compared to Au, and (2) nanoscale voids migrate during heating, contrary to conventional assumptions of immobile voids and void shrinkage through vacancy emission. Our results illustrate how void behavior in bimetallic NPs can differ from an idealized picture based on atomic fluxes and have important implications for the design of these materials for high-temperature applications.

Direct observation of the nanoscale Kirkendall effect during galvanic replacement reactions.

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature.

Dynamics of Templated Assembly of Nanoparticle Filaments within Nanochannels.

Nanoparticles (NPs) can self-assemble into complex, organized superstructures on patterned surfaces through fluid-mediated interactions. However, the detailed mechanisms for such NP assemblies are largely unknown. Here, using in situ transmission electron microscopy, the stepwise self-assembly dynamics of hydrophobic gold NPs into long filaments formed on the surfaces of water-filled patterned nanochannel templates is observed. First, the formation of a meniscus between the nanochannel walls, during the slow drying of water, causes accumulation of the NPs in the middle of the nanochannels. Second, owing to the strong van der Waals attraction between the NP ligands, the NPs condense into filaments along the centers of the nanochannels. Filaments with highly fluctuating longitudinal NP densities are also observed to fragment into separated structures. Understanding the intermediate stages of fluid-mediated NP self-assembly on patterned surfaces will have important implications for the controlled formation of templated NP assemblies with numerous applications.