Letter Cite This: Nano Lett. 2018, 18, 6597−6603
pubs.acs.org/NanoLett
In Situ TEM Observation of Crystal Structure Transformation in InAs Nanowires on Atomic Scale Zhi Zhang,† Nishuang Liu,† Luying Li,† Jun Su,† Ping-Ping Chen,‡ Wei Lu,‡ Yihua Gao,*,† and Jin Zou*,§
Nano Lett. 2018.18:6597-6603. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/26/18. For personal use only.
†
Center for Nanoscale Characterization & Devices (CNCD), School of Physics & Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Luoyu Road 1037, Wuhan 430074, P. R. China ‡ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yu-Tian Road, Shanghai 200083, China § Materials Engineering & Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Queensland 4072, Australia S Supporting Information *
ABSTRACT: In situ transmission electron microscopy investigation of structural transformation in III−V nanowires is essential for providing direct insight into the structural stability of III−V nanowires under elevated temperature. In this study, through in situ heating investigation in a transmission electron microscope, the detailed structural transformation of InAs nanowires from wurtzite structure to zincblende structure at the catalyst/nanowire interface is witnessed on the atomic level. Through detailed structural and dynamic analysis, it was found that the nucleation site of each new layer of InAs and catalyst surface energy play a decisive role in the growth of the zinc-blende structure. This study provides new insights into the growth mechanism of zinc-blende-structured III−V nanowires.
KEYWORDS: In situ, TEM, nanowire, structure transformation
O
growth temperature, the formation of WZ structure in III−V nanowires is preferred. In addition, it has been well documented that the band structure and electrical, optical, and physical properties are different for ZB-structured and WZ-structured III−V nanowires.15−17 To achieve these superior properties of III−V nanowires with different crystal structures, axial and radial heterostructures have been grown with highly lattice mismatched materials with different band structures along the length of the nanowires or in their radial plane as core/shellstructured nanowires.18−21 This has led to the realization of single-electron transistors,22 field-effect transistors,23 and photodetectors.24 On the contrary, it is scientifically interesting and technically necessary to achieve the growth of axial heterostructures with different crystal structures and hence different band structures17 but with the same material such as InAs. Therefore, crystal structure selection and manipulation is one of the most fundamental issues in the applications of semiconductor nanostructures. Several strategies have been implemented to tune nanowire crystal structures by tuning growth parameters, such as the V/III ratio or growth temperature.12,25 Recently, in situ growth of GaAs nanowires
ne-dimensional nanowires made from III−V compound semiconductors have attracted significant research interests in the recent decade due to their distinct physical and chemical properties that can potentially lead to a wide range of applications in nanoelectronics and optoelectronics.1−3 The most typical approach for the growth of III−V nanowires is the vapor−liquid−solid (VLS) mechanism,4 in which reactant atoms from ambient vapor are incorporated into foreign metallic catalyst nanoparticles to form liquid droplets, and the supersaturation of reactant atoms in the liquid catalysts leads to anisotropic nanowire growth underneath the liquid droplets. On the contrary, group-III liquid droplets have also been adopted to achieve the self-catalyzed growth of III−V nanowires.5−8 Although the thermodynamic stable phase of III−V compound is zinc-blende (ZB) structure with the stacking sequence of ABCABC along the [1̅1̅1̅] direction, III−V nanowires can easily adopt wurtzite (WZ) structure9,10 with the stacking sequence of ABABAB along the [0001̅] direction as well as the ZB structure,11,12 as shown in Figure S1. The mechanism for the formation of WZ structure has been demonstrated by the nucleation-based model during the goldcatalyzed growth of III−V nanowires.13,14 It was proposed that when the nucleation barrier for the formation of a WZ nucleus is smaller than that of a ZB nucleus, which can be easily achieved by tuning the growth conditions such as the V/III ratio or © 2018 American Chemical Society
Received: August 7, 2018 Revised: September 10, 2018 Published: September 20, 2018 6597
DOI: 10.1021/acs.nanolett.8b03231 Nano Lett. 2018, 18, 6597−6603
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Nano Letters
transformation at the atomic level. To determine the temperature range of structural transformation, TEM specimens were initially heated from 25 to 450 °C with a temperature increment of 10 °C/s and were maintained for 2 min at each increment to identify the important crystal structure transformation temperature, from which we found that the structural transformation took place in the temperature range between 300 and 350 °C. Accordingly, we verify the heating procedure as follows. First, the TEM specimens were heated from 25 °C to 300 °C using an increment of 10 °C/s. From 200 °C, the stage was maintained for 2 min at each increment. From 300 °C, the heating rate was reduced to 0.1 °C/s, and the stage was maintained for 5 min with the temperature increased for each 10 °C, until the structural transformation was observed. To minimize the electron irradiation effect on the nanowires, the electron beam was shut down during the heating procedure. To understand the structural transformation dynamics, we first investigated the structural information on nanowires under different heating temperatures. Figure 2 shows the highresolution TEM images taken from a typical InAs nanowire heated at various temperatures. All TEM images were taken along the zone axis. As can be seen, the InAs nanowire has pure WZ structure with the [0001̅] growth direction (Figure 2a), and no ZB-structured necking region is seen,13,31 indicating that there is no ZB structure in the nanowire before the heating experiment. When the heating temperature is increased to 300 °C, Figure 2b shows that the nanowire structure is still maintained as WZ structure, although the catalyst morphology has changed slightly. With further increasing the heating temperature to 350 °C, two interesting phenomena can be noted: (1) catalyst diameter has increased and the catalyst morphology has changed and (2) underneath the catalyst, the nanowire section shows a crystal structure different from the WZ structure in the original nanowire section, and the interface between these two sections is sharp. The inset in Figure 2c shows an enlarged high-resolution TEM image, which clearly shows different crystal structures between these two sections. The newly formed section above the WZ-structured nanowire section is found to adopt the ZB structure. In addition, it is of interest to note that the ZB section on the left side of the nanowire is in contact with the catalyst rather than in contact with vapor, as marked by the yellow dotted line. This suggests that the nucleation of ZB structure is not at the TPL. As shown in the high-resolution TEM image in Figure 2d, when the heating temperature is as high as ∼420 °C, the catalyst dissolved part of the nanowire sidewall and flowed randomly on the nanowire sidewall. In addition, the sharp interface between the ZB structure and WZ structure can be clearly seen as well. The insets in Figure 2d show the fast Fourier transformation (FFT) images of the marked green and red areas, which clearly indicate different crystal structures of these two sections. Detailed crystal structure analysis further confirmed the formation of ZB structure underneath the catalyst. In addition, the growth direction of the newly formed ZB section can be determined as [1̅1̅1̅], comparable to the [0001̅] growth direction of the WZstructured nanowire. If the heating temperature keeps increasing, then the entire nanowire is decomposed gradually (refer to the Supporting Information Movie M1). The fact that the decomposition of nanowire always starts from the nanowire tip on which the catalyst sits (shown in Figure S2) suggests that the catalyst can promote the decomposition of nanowire. To understand the composition and elemental distribution of the nanowire after structural transformation, we performed
in a transmission electron microscope (TEM) suggested that the catalyst volume and the contact angle determine the crystal structure of GaAs nanowires.26 In addition, theoretical works proposed that the triple-phase line (TPL) that separates the vapor, liquid, and solid phases plays a decisive role. Nucleation at the TPL favors the formation of WZ phase, whereas nucleation outside the TPL favors the formation of ZB phase.27,28 However, explicit experimental evidence to support this hypothesis has not been available. Technical advances in TEM with high spatial and temporal resolution allow the observation of real-time growth processes and dynamic behaviors of low-dimensional nanomaterials at an atomic level.29,30 In this study, through in situ heating WZ-structured InAs nanowires in a TEM, their structural transformation from WZ structure to ZB structure was directly revealed on the atomic scale. Through detailed structural and nucleation dynamic analyses, it was found that the nucleation site of each new layer of InAs and catalyst surface energy play a decisive role in the growth of ZB structure. A nucleation model is proposed to understand the formation of the ZB-structured sections. In this study, the in situ heating experiment was performed on a DENSsolutions double-tilted platform using a wildfire D6 heating nanochip (with fast heating ability and with highly accurate temperature control of the TEM specimen) in an atomic-resolution TEM (FEI Titan G2 60−300 equipped with X-ray energy-dispersive spectroscopy (EDS) for compositional analysis). The setup of the in situ heating experiment is illustrated in Figure 1. Figure 1a is a TEM image showing the overview of the
Figure 1. Configuration of the heating experiment setup. (a) TEM image of the heating nanochip. (b) Schematic of the side view of the heating nanochip.
heating nanochip, with the white rectangular area being the silicon nitride (SiNx) supporting film on the chip. Figure 1b shows a schematic side view of the in situ TEM heating stage. The as-grown InAs nanowires induced by the Au catalysts were dispersed on the SiNx supporting film. The design of this nanochip assures the high stability of the TEM specimens, even at a very high temperature, and, in turn, guarantees the atomic resolution during in situ TEM investigations of structural 6598
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Figure 2. Evolution of crystal structure of nanowire under different heating temperatures: (a) 25, (b) 300, (c) 350, and (d) 420 °C. All scales represent 10 nm. The inset in panel c shows an enlarged high-resolution TEM image. The insets in panel d show the FFT images of the green and red areas.
Figure 3. High-resolution TEM investigations in the initial stage of in situ heating process. High-resolution TEM images taken at (a) 25 and (b) 310 °C. (c,d) High-resolution TEM images taken at different time with the heating temperature of 320 °C.
Because the structural transformation took place between 300 and 350 °C, we repeat the in situ heating experiment with much slower heating procedure, especially above 300 °C. Figure 3a is a high-resolution TEM image taken from another nanowire that has the WZ structure before heating, and the original catalyst/ nanowire interface is very sharp, as indicated by the red arrow. When the heating temperature is slowly increased to 310 °C, the catalyst morphology starts to change, as indicated by the yellow arrow, as shown in Figure 3b. In addition, it can be noted that the entire catalyst/nanowire interface is no longer atomically flat, as marked by the dashed yellow lines, with the interface on right side being lower than that on left side, which is still the original catalyst/nanowire interface. This suggests that the InAs nanowire was slowly dissolved into the catalyst from one side to the other, and this dissolving process was recorded
detailed EDS mapping and point analysis at the tip of the nanowire. Figure S3a is a scanning transmission electron microscopy−high-angle annular dark field (STEM-HAADF) image and shows the brighter catalyst and relative darker nanowire due to the fact that the catalyst contains heavier Au. Figure S3b−d shows, respectively, EDS elemental maps of Au, In, and As, which show that the nanowire is indeed InAs, with In and As homogeneously distributed in the nanowire, whereas Au and In are distributed in the catalyst, which was confirmed by the point EDS analysis (Figure S4). These elemental analyses indicate that only the crystal structure of nanowire has changed from WZ to ZB, whereas the composition of nanowire is still maintained as InAs. In addition, it can be noted that the Au catalyst is still sitting on the top of nanowire. 6599
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Figure 4. Evolution of nanowire crystal structure at 330 °C. (a−i) High-resolution TEM images captured at different heating time. The dotted yellow line indicates the original catalyst/nanowire interface, and the dashed red line indicates the moving interface between the catalyst and newly formed ZB structure.
(Supporting Information Movie M2). It is expected that In was absorbed into the catalyst, whereas As was evaporated due to the extremely low solubility of As in Au according to the Au−As phase diagram.26 When the temperature is further increased to 320 °C, a new and interesting feature can be observed at the left corner of the catalyst/nanowire interface; that is, the left side of the catalyst is no longer in contact with the nanowire side facet, as shown in Figure 3c. The new catalyst/nanowire interface moves toward the nanowire side when compared with the original interface marked by the red arrow. In addition, the nonflat catalyst/nanowire interface becomes more obvious. With continuing heating, the interface keeps moving toward the nanowire side from the right to the left side, and Figure 3d shows such a typical captured high-resolution TEM image. It can be observed that the new interface is even lower than the original interface marked by the red arrow. The entire catalyst/nanowire interface moving process can be clearly observed in the captured video (Supporting Information Movie M2). To clarify the structural transformation process, the heating temperature was further increased to 330 °C and maintained at this temperature. Figure 4a shows a captured high-resolution TEM of the same nanowire shown in Figure 3 at the beginning of 330 °C heating temperature. It is of interest to note that the catalyst is not in contact with the nanowire side facet on both sides of the nanowire. Because TEM images present 2D projections of a 3D object, our TEM observation suggests that the catalyst is sitting in the middle of nanowire top surface. With prolonging the time, a small section of the ZB structure quickly forms at the left corner of catalyst/nanowire interface, as shown in Figure 4b. During the formation of the ZB section, As species in the local environment would diffuse to the catalyst/nanowire interface to react with the In species that precipitate from the catalyst to form the InAs compound at the catalyst/nanowire interface. The nonflat interface between the catalyst and
nanowire can be observed, with the newly formed interface between the catalyst and the ZB-structured section being higher than the original catalyst/nanowire interface. This is a typical feature for the formation of the ZB-structured nanowire section, which is different from the layer-by-layer nucleation mode of WZ-structured nanowires, as demonstrated by the in situ growth of GaAs nanowires inside a TEM.26 Soon after (8 s later, as shown in Figure 4c), the catalyst dissolves the newly formed ZBstructured nanowire section and some WZ-structured nanowire section, leading to a new and relatively flat catalyst/nanowire interface. At this heating temperature, the catalyst can dissolve the nanowire and absorbs more In into the catalyst, as indicated by the increased catalyst height (h2 > h1) (comparison of catalyst height shown in Figure 4c,d). With further prolonging the heating time, a small ZB section with ABCABC stacking sequence is formed again, as shown in Figure 4e. As shown in Figure 4f, with the time going on, ZB section is grown at the catalyst/nanowire interface. Soon after, the catalyst dissolves both ZB and WZ nanowire sections, as indicated by the even larger catalyst height (h3 > h2) (comparison of catalyst height shown in Figure 4d,g). Again, the ZB-structured nanowire section is formed at the catalyst/nanowire interface, as shown in Figure 4h and its inset. Under such a “come-and-go” manner, a larger ZB section is eventually formed at the catalyst/nanowire interface with the heating time. A detailed structural transformation process can be observed in the captured video (Supporting Information Movie M3). After the formation of the ZB-structured section, if the heating temperature goes to as high as ∼420 °C, then the catalyst reacts with the nanowire significantly and even becomes uncontrollable to some extent. In this situation, the “come-and-go” growth manner of ZBstructured section disappears. Therefore, the catalyst would randomly dissolve part of the already formed ZB-structured section and would flow on the nanowire sidewall to decompose 6600
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shown in Figure S5. The ratio between the nucleation barriers (ΔGWZ and ΔGZB) for the formation of WZ and ZB structures can be expressed as13,14,33
part of the nanowire sidewall (as shown in Figure 2d). Eventually, the catalyst decomposes the nanowire, as demonstrated in Figure S2 and Movie M1. To understand the mechanism of the structural transformation from WZ structure to ZB structure in InAs nanowire with in situ heating, we consider the nucleation process during the VLS nanowire formation. Several theories have proposed that nucleation at the TPL favors the formation of a WZ phase, as schematically illustrated in Figure 5a, which is commonly
ξ=
ΔμLS η2 ΔG WZ = ΔGZB ΔμLS − ψWZ
(1)
where ΔμLS is the liquid−solid supersaturation, ψWZ is the extra cohesive energy associated with WZ formation, and η is the ratio between the change in effective surface energies due to the formation of a WZ and a ZB nucleus (ΓWZ and ΓZB), as expressed below η=
(1 − x)ΥLS − x ΥLV sin β + xτ ΥSV(ZB) ΓWZ = ZB (1 − x)ΥLS − x ΥLV sin β + x ΥSV(ZB) Γ
(2)
where ΥLS, ΥLV, and ΥSV are the liquid−solid, liquid−vapor, and solid−vapor surface energies, respectively, x is the share of nucleus perimeter in contact with the vapor, β is the contact angle, and τ = ΥSV(WZ)/ΥSV(ZB) is the ratio between the lateral solid−vapor surface energies of WZ and ZB nuclei, which is expected to be slightly less than 1. According to eq 1, both ΔμLS and η can affect the nanowire crystal structure; namely, higher η and lower ΔμLS favor the formation of ZB structure. Otherwise, the formation of WZ structure will be preferred. We now consider the effect of η first. On the basis of eqs 1 and 2, the newly formed monolayers will adopt the ZB structure for higher η value. Accordingly, we anticipate that a lower ΥLV, or more precisely an increase in the ΥLS/ΥLV ratio, will increase η and, in turn, increase the probability of ZB structure nucleation. In our in situ heating process, it can be noted that the In concentration in the Au−In catalyst increases with prolonging the heating process. It has been well documented that the liquid surface energy, ΥLV, of the catalyst droplet is a function of the In concentration in the catalyst.34 As shown in ref 35, we can linearly interpolate ΥLV between the value for pure Au (1.25 J/m2) and pure In (0.55 J/ m2) at the given temperature of 330 °C.36 In this regard, we expect that ΥLV decreases with increasing the In concentration in the catalyst. On this basis, as the heating process goes on, the catalyst keeps dissolving InAs nanowire and absorbs more In into the catalyst, decreases the catalyst surface energy ΥLV, increases η, and, in turn, promotes the formation of ZB nucleus. When considering the effect of liquid−solid supersaturation, ΔμLS, Bakkers proved that ΥLV (and hence η) is much more sensitive than ΔμLS in determining the crystal structure of nanowire.37 This effect can also be understood by the eq 1, in which the power of η is 2, whereas the power of ΔμLS is 1. In this regard, a small increase in ΔμLS due to the increased In concentration in the catalyst is more than compensated for by an increased η due to the decreased ΥLV.14 Therefore, η is the most critical factor that determines the nucleation kinetics. With the further heating of nanowire, more In is included in the catalyst, the catalyst surface is decreased, which leads to the increase in η, and then the growth of ZB structure is preferred, as shown in our in situ heating study. In addition, our detailed EDS investigations on the nanowires after crystal structure transformation show that the In concentration in the catalyst is ∼50 at %, which suggests that the In concentration could be even higher before the formation of the ZB-structured nanowire section. This value is much higher than the original In concentration (∼30 at %) in the catalyst on the top of WZ-structured InAs nanowire. In this
Figure 5. . Schematic of the catalyst/nanowire interface for crystal structure selection of WZ and ZB structures.
observed in the catalyst-induced growth of III−V nanowires. However, two different growth behaviors can be noted during the in situ heating process. (1) In the initial heating stage before structural transformation, the catalyst dissolves the WZstructured nanowire, adjusts its morphology, and then is not in contact with the nanowire side facet anymore, receding from the nanowire side facet, as shown in Figure 4a. Then, the nucleation of the ZB section begins, as shown in Figure 4b. This can be explained by the theoretical calculations,27,32 in which the nanowire nucleus favors the formation of the ZB-structured nucleus if the TPL recedes from the nanowire side facet, which is well consistent with our observation. As the catalyst recedes from the nanowire side facet, the ZB-structured nanowire section nucleates at the catalyst/nanowire interface, as schematically shown in Figure 5b. (2) On the contrary, with prolonging the heating, the catalyst dissolves more InAs nanowire. In this regard, the In concentration in the catalyst becomes higher and the height of catalyst becomes larger, as shown in Figure 4d and schematically shown in Figure 5c. To understand the formation of ZB structure in the second case, as observed in Figure 4d−i, we consider the nucleation kinetics in Au-catalyzed nanowire growth, as schematically 6601
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regard, such a growth of ZB structure can be understood as the transition from Au-catalyzed growth mode to pseudogroup-III element-catalyzed growth mode, as demonstrated by the theoretical prediction38 and experimental investigation39 in GaAs nanowires. On the basis of our detailed in situ TEM investigations and discussions, it can be concluded that although the WZ-structured InAs nanowires were grown by MBE, they can be transformed to ZB structure. These results suggest that as long as certain growth conditions are reached, Au-catalyzed InAs nanowires with ZB structure can be achieved in MBE. Therefore, this study may provide an insight into the growth of ZB-structured III−V nanowires in MBE. In conclusion, through in situ heating investigations in the high-resolution TEM, the structural transformation of InAs nanowires from WZ structure to ZB structure was directly revealed. Through detailed structural and nucleation dynamic analyses, it was found that the nucleation site of each new layer of InAs and catalyst surface energy play a decisive role in the growth of ZB-structured nanowires. This study provides new insights into the growth of ZB-structured III−V nanowires and crystal-structure-controlled growth of III−V nanowires.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b03231. Schematic of the atomic structure and band structure of InAs nanowire, EDS analysis of the nanowire catalyst after heating, and the schematic of nucleation model (PDF) Movie M1. Video of nanowire decomposition (AVI) Movie M2. Video of the catalyst/nanowire interface moving process during heating (AVI) Movie M3. Video showing the crystal structure transformation process (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*Y.G.: E-mail:
[email protected]. *J.Z.: E-mail:
[email protected]. ORCID
Zhi Zhang: 0000-0001-7794-1763 Nishuang Liu: 0000-0002-2507-7229 Yihua Gao: 0000-0003-1905-9531 Jin Zou: 0000-0001-9435-8043 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is financially supported by the National Science Foundation of China (Grant No. 11674113, No. 11634009) and the Australian Research Council. We thank Dr. Wei Lu from the Hong Kong Polytechnic University for the technical support.
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REFERENCES
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Letter
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