Fabrication and Characterization of Au Nanoparticle-aggregated Nanowires by Using Nanomeniscus-induced Colloidal Stacking Method
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We fabricate and characterize Au nanoparticle-aggregated nanowires by using the nano meniscus-induced colloidal stacking method. The Au nanoparticle solution ejects with guidance of nanopipette/quartz tuning fork-based atomic force microscope in ambient conditions, and the stacking particles form Au nanoparticle-aggregated nanowire while the nozzle retracts from the surface. Their mechanical properties with relatively low elastic modulus are in situ investigated by using the same apparatus.
KeywordsAu nanoparticle-aggregated nanowire Nanomeniscus-induced colloidal stacking method Atomic force microscope Liquid–solid coexistence phase
The characteristics of the nanowires (NWs), which are one of the critical bridging elements in nanoscience and technology, have been widely studied. In particular, special attention has been focused on various applications, such as biomedical sensing, nano-optoelectronics, and photovoltaic devices due to their advanced electrical, optical, mechanical, and geometrical properties [1, 2, 3, 4, 5]. Since the vapor–liquid–solid fabrication method of NW was first invented , based on the crystal seed-based growth method, the research of NW has been established as a convergence field [7, 8, 9]. Recently, various materials of NW composite have been investigated for versatile functionalization in specific systems and devices [10, 11]. These fabricated NWs are mostly crystalline solid phase and thus have important features of high-speed operation and reproducible response suitable for the nanodevices and related technologies. However, unlike the solid-state applications, nanoscale biological systems are involved with objects that usually exhibit soft matter properties, such as the liquid–solid coexistence (LSC) phase which can be defined by the volume fraction of the constituent particles in the liquid [12, 13]. Thus, the fabrication and characterization of soft NWs gives a key for understanding complicated biological systems. Moreover, investigation of their physical properties consequentially should be performed for the one dimensional applications of bioscience and technology. Here, we introduce a direct non-template fabrication and characterization of the Au nanoparticle-aggregated (ANA) NWs which shows soft matter properties using nanomeniscus-induced colloidal stacking method in ambient conditions. We used a non-contact, small lateral oscillation mode, nanopipette  combined with a quartz tuning fork-atomic force microscope (QTF-AFM)  for Au nanoparticle solution delivery, which is one of tool for scanning probe lithography . And we in situ investigated the mechanical properties of ANA-NWs which show a relatively low elastic modulus by using the same apparatus facilitated with QTF sensor for small force measurement.
3 Results and Discussion
3.1 Control of Stacked Colloidal Density
3.2 Physical Properties of ANA-NWs
3.3 In Situ Measured Shear Modulus of ANA-NWs
Figure 4b shows the results of the fast oscillation experiment 1 for the phase-3 NW with ~15 μm length and ~100 nm in diameter. The oscillation frequency of the NW is same as the resonance frequency of the QTF (~32 kHz), while each lateral displacement ΔL is half of the oscillation amplitude a of the QTF (100 nm–1.3 μm), determined by the stroboscope images of QTF sensor . Figure 4b-i presents the elliptic hysteresis curves of the stress and strain, which reveal the time delay Δt between stress and strain, that is, the viscoelasticity information of the NWs. As a (or ΔL) is increased, the normalized hysteresis curves exhibit increase of the enclosed area of the ellipsoid, which indicates the strain energy per unit volume that is released as internal heat in each cycle. For large oscillation (a = 1.3 μm), the measured Δt is about 5 μs, whereas only a slight delay occurs for small oscillation (a = 100 nm), indicating the dependency of viscoelasticity of the phase-3 NW on the oscillation velocity. Figure 4b-ii plots the shear stress versus the strain for three different phases of the NW and the slopes represent the shear moduli of the phase-1, -2, and -3 NWs given by ~100, 220, and 400 MPa, respectively. The modulus of phase-1 of the NW is about 4 times smaller than the results of phase-3, namely, the shear modulus of the ANA-NW increases, as the density of the particles increases.
Figure 4c studies the lateral movement (experiment 2) of the same 15 μm-long NW, while the PZT provides slow (at ~50 nm/s) unilateral displacement ΔL, where the QTF is now used as a force sensor. Figure 4c-i presents the QTF sensor responses versus the tip-sample distance during approach and retraction of the sample, which shows that the NWs can be fabricated at an arbitrary length, as marked by three arrows (a, b, and c) (here, liquid ejection starts at the zero-distance position). Figure 4c-ii plots the dependence of the NW’s length on the stress–strain curves. As the length increases from a (5 μm) to c (15 μm), the fracture occurs at the increased shear strain, whereas the shear stress at the fracture slightly decreases by ~20 MPa. The fluctuating behavior of the shorter NWs (5 and 10 μm) is attributed to rearrangement of the constituent nanoparticles as the NW is laterally stressed, which is averaged out in the longer 15 μm NW over its length probably due to its lesser sensitivity to movement of individual nanoparticles. In Fig. 4c-iii, the differing shear stress–strain curves are presented for the three LSC phases, where each slope provides the shear modulus of about 80, 180, and 400 MPa, respectively. The good agreement of the values with those of experiment 1 (Fig. 4b-ii) indicates that the oscillation motion at 32 kHz is still slow enough to be considered as static. Note that the measured shear modulus of the phase-3 NW shows similar values with the protein crystal (100–1,000 MPa) or ANA-polymer (~500 MPa) . Figure 4c-iv shows the repetitive stress–strain measurements for the phase-3 NW during stepwise increase of the shear strain followed by the subsequent relaxation. Up to ~25 % strain (black curve, region I), there is no significant hysteresis between the forward and backward displacements. However, at above ~25 % strain, permanent deformation (or thinning) of the NW occurs, accompanied by a sudden slight drop of the shear stress and recovery of zero strain (red curve, region II). Then, a second deformation takes place at ~30 % strain, followed by another hysteresis and subsequent ultimate fracture.
We have fabricated and characterized the ANA-NW with LSC phase by using colloidal stacking method with a guidance of nanopipette/QTF-AFM which operates in a non-contact, small lateral oscillation mode in ambient conditions. One can progress scientific improvements dealt with vital phenomena of low dimensional biological media using this fabricated LSC phase ANA-NW, 3D nanoscale structures of particle-aggregated system with various materials (inks) for electrical/biological/chemical engineering, or development of molecular electronics.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No. 200983512), Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013R1A6A3A03063900), and the Brain Korea 21.
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