Letter pubs.acs.org/NanoLett
Molten Au/Ge Alloy Migration in Ge Nanowires Qian Liu,† Rujia Zou,†,§ Jianghong Wu,† Kaibing Xu,† Aijiang Lu,*,⊥ Yoshio Bando,∥ Dmitri Golberg,∥ and Junqing Hu*,† †
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China § Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China ⊥ Department of Physics, Donghua University, Shanghai 201620, China ∥ International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *
ABSTRACT: Herein, we report time-resolved in situ transmission electron microscopy observation of Au particle melting at a Ge nanowire tip, subsequent forming of Au/Ge alloy liquid, and its migrating within the Ge nanowire. The migration direction and position of the Au/Ge liquid can be controlled by the applied voltage and the migration speed shows a linear deceleration in the nanowire. In a migration model proposed, the relevant dynamic mechanisms (electromigration, thermodiffusion, and viscous force, etc.) are discussed in detail. This work associated with the liquid mass transport in the solid nanowires should provide new insights into the crystal growth, interface engineering, and fabrication of the heterogeneous nanostructure-based devices. KEYWORDS: Ge nanowires, Au/Ge alloy melting, in situ manipulation, solid−liquid system, dynamics
T
simple electronic device based on individual nanostructures could be in situ constructed using a piezo-driven TEM-STM (scanning tunneling microscope) holder, it becomes possible to induce a controllable electric field on the nanowires and monitor the migration process of the liquid metal/alloy in the solid nanowires and the associated electrical performances in a real time. Thus, detailed studies on these fundamentally interesting and practically important effects are required. Herein, we report a time-resolved, high-resolution in situ TEM observation of Au particle melting at a Ge nanowire tip, subsequent forming of an Au/Ge alloy liquid and its migration within the solid Ge nanowire. The adjustment of the applied voltage could control the migration direction and position of the Au/Ge liquid within the nanowire. Under a constant voltage, the stopping position was found to be inversely proportional to the squared length-to-diameter ratio. We proposed a model for the regarded process and discussed the relevant dynamic mechanisms (such as electromigration, thermodiffusion, and viscous force, etc.). These studies should broaden our understanding of the liquid mass transport in multicomponent nanomaterials and are envisaged to provide
ransmission electron microscopy (TEM) has been demonstrated to be a very useful tool to discover new physical transformations associated with one-dimensional nanomaterials. So far, various kinds of physical phenomena occurring at the nanoscale, including melting,1−3 crystallization,4 growth,5 and mass transport6−10 have been studied in real time using TEM. This provided new insights into the crystal growth kinetics. Among the effects observed, the physical transformations within the two-component systems, for instance, the formation and expansion of metal-rich liquid at the tips of semiconductor nanowires,1−3 the thermomigration of Pt/Si alloy droplets on the Si surface,11 and so forth, have received increasing attention due to their potential applications in nanowire machining,1−3 microfluidics,10−13 pipetting,4 and semiconducting crystal doping.14 An interesting migration of the liquid phase within a solid (rather than on a surface) has been demonstrated as a valuable technique in several areas of technology.15,16 For example, during the crystal growth, the temperature gradient zone melting is based on the migration of a semiconductor−metal molten zone driven by a temperature gradient. However, such liquid zone migration within the solid has never been in situ observed in the experiments, which is largely due to the existing technical limits.15−20 In addition, the temperature gradient fields in which these physical transformations took place were significantly different from actual environments of the practical devices.14,21,22 Considering that a © XXXX American Chemical Society
Received: October 26, 2014
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DOI: 10.1021/acs.nanolett.5b01144 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
(Nanofactory Instruments). The detailed experimental procedure was described in our previous reports.28,29 Figure 1c is a TEM image showing the Ge nanowire stretched between the fixed Au tip (top, cathode) and the movable Au wire (bottom, the anode is out of view). The fixed Ge nanowire has a diameter of ∼110 nm and a length of ∼1.8 μm and is coated by a ∼ 15 nm oxide layer (see Supporting Information Figure S2). A voltage of 13 V was applied between the two Au electrodes, and drove a current flow through the device; the electrical properties were in situ measured for evaluating the electrical performance of the device. An initial current of 0, both the directions of J concent and J electro point to the cathode, which keep the Au/Ge liquid flowing in the direction of the electron flow (Figure 5b(i)). Once the left end of the Au/Ge liquid passed through the middle point (i.e., x0 < 0), the ⇀
Ge mass flux J ′concent caused by the concentration gradient on the segment of [x0, 0] pointed to the anode, which is in the ⇀
direction opposite to J
concent
on the segment of [0, x1] (Figure ⇀
5b(ii)). With the gradual increase of J ′concent, the shape of the Au/Ge liquid changed significantly and even broke into several fragments, which is largely due to the unbalance of the mass flux in the liquid zone (Figure 5b(iii)). During the whole process, viscous force always manifested as a resistance force ⇀
until the liquid stopped. Upon the competition of J ′concent,
⇀
⇀
⇀
J concent, J electro and F η, the migration speed v of the Au/Ge alloy liquid in this process could be expressed by an eq 1 (see Supporting Information Method 4) ⎛ ∂C L V 2 ηl ⎞ ⎛ L⎞ + ⎟⎜x 0 − ⎟ v = v0 + ⎜D0 m ⎠⎝ 2⎠ ⎝ CS∂T ARLκ
(1)
where D0 is the diffusion coefficient of Ge in the Au/Ge liquid system, CL is the concentration of Ge atoms in the Au/Ge liquid, CS is the atomic concentration of Ge in the nanowire, V is the applied bias, A is the cross-sectional area of the Ge nanowire, L is the length of the Ge nanowire, κ is the thermal conductivity, R is the resistance, η is the viscous coefficient, l is the length of the fluid, m is the mass of the Au/Ge alloy liquid, v0 is the initial speed once the liquid moved into the cylindrical body of the Ge nanowire, and x0 is the front position of the Au/ Ge liquid (in the coordinate system with its origin at the middle point of the Ge nanowire). Assuming a constant voltage for a given Ge nanowire, the migration speed v should be a linear function of x0 (−L/2 < x0 < L/2). Our measured migration speeds (v) with respect to x0 of the Au/Ge liquid (see Supporting Information Movie 3) are consistent with a linear relation, as shown in Figure 5c. Therefore, it is reasonable to infer that the Au/Ge alloy liquid moves with a decelerated speed into the Ge nanowire body (with an initial speed of v0) so as to stop at some point (xE) or completely pass through the Ge nanowire and reach the other end. Here, xE is defined as the coordinate position at the left end of the stopped Au/Ge liquid. Considering that the coordinate system has its origin at the middle point of the Ge nanowire with x axis parallel to its axial direction, the magnitude of xE is directly affected by the length of the Ge nanowire (L). Here, we define β as a stopping factor (β = 1/2 − xE/L) (see Supporting Information Method 5): β=
C L D0eZ * V CS kBT0 ∂C
V2
S
0
D0 C ∂LT ρ κ +
4η ρπ
2
( Ld )
(2)
where Z* is the effective valence for the direct force mediated process, ρ0 is the electrical resistivity, and ρ is the density of the Au/Ge liquid. Because the magnitude of xE is directly affected by the length of the Ge nanowires (−L/2 < xE < L/2), thus the stopping factor β (β = 1/2 − xE/L, 0 < β < 1) is dimensionless and indicates the proportion of the Au/Ge liquid migrating from the tip into the Ge nanowires under the present conditions. According to eq 2, assuming a definite voltage, constant thermal conductivity, and resistivity, the factor β F
DOI: 10.1021/acs.nanolett.5b01144 Nano Lett. XXXX, XXX, XXX−XXX
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holder. At the beginning of each TEM session, the computer controlling the two-probe setup was synchronized with the image acquisition workstation. Mechanically cut Au wires and Au tips, 0.25 mm in diameter, were used as electrodes.
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ASSOCIATED CONTENT
S Supporting Information *
Six movies (Movies S1, S2, S3, S4, S5, and S6), a table, five methods about the synthesis of the Au-tipped Ge nanowires and theoretical analyses of the migration mechanism, additional HRTEM images, STEM-EDS chemical maps and XPS, in situ heating experiment, Au/Ge phase diagram, electron beam irradiation, and further experimental evidence of migration process of Au/Ge liquid are included. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J.H.). *E-mail:
[email protected] (A.L.). Author Contributions
These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Q.L. and R.Z. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21171035, 51302035, 51472049, and 11204030), the Ph.D. Programs Foundation of the Ministry of Education of China (Grants 20110075110008 and 20130075120001), the National 863 Program of China (Grant 2013AA031903), and the Fundamental Research Funds for the Central Universities. The Key Grant Project of Chinese Ministry of Education (Grant 313015), the Science and Technology Commission of Shanghai Municipality (Grant 13ZR1451200), the Program Innovative Research Team in University (Grant IRT1221), the Hong Kong Scholars Program, the Shanghai Leading Academic Discipline Project (Grant B603), and the Program of Introducing Talents of Discipline to Universities (Grant 111-2-04).
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