Growth and characterization of germanium telluride nanowires via vapor–liquid–solid mechanism

Phase-change materials (PCMs), which can transition reversibly between crystalline and amorphous phases, have shown great promise for next-generation memory devices due to their nonvolatility, rapid switching periods, and random-access capability. Several groups have investigated phase-change nanowires for memory applications in recent years. The ability to regulate the scale of nanostructures remains one of the most significant obstacles in nanoscience. Herein, we describe the growth and characterization of germanium telluride (GeTe) nanowires, which are essential for phase-change memory devices. GeTe nanowires were produced by combining thermal evaporation and vapor–liquid–solid (VLS) techniques, using 8 nm Au nanoparticles as the metal catalyst. The influence of various growth parameters, including inert gas flow rate, working pressure, growth temperature, growth duration, and growth substrate, was examined. Ar gas flow rate of 30 sccm and working pressure of 75 Torr produced the narrowest GeTe nanowires horizontally grown on a Si substrate. Using scanning electron microscopy, the dimensions, and morphology of GeTe nanowires were analyzed. Transmission electron microscopy and energy-dispersive x-ray spectroscopy were utilized to conduct structural and chemical analyses. Using a SiO2/Si substrate produced GeTe nanowires that were thicker and lengthier. The current–voltage characteristics of GeTe nanowires were investigated, confirming the amorphous nature of GeTe nanowires using conductive atomic force microscopy. In addition, the effects of the VLS mechanism and the Gibbs–Thomson effect were analyzed, which enables the optimization of nanowires for numerous applications, such as memory and reservoir computing.


Introduction
Recently, phase-change materials (PCMs)-based random access memory (PRAMs) have attracted significant interest as a viable replacement for flash memory technology [1][2][3].The scalability of PCMs is driving this class of materials to be the next successor in the memory device market.Flash memories adopt floating-gate structures that restrict their scalability due to the number of electrons available per bit as the device is scaled down.In contrast, the storage mechanism of PRAMs' depends on the resistance contrast and not the charge, indicating that scalability is practically feasible for obtaining highdensity memory devices.As the size of the material was scaled down to a quasi-one-dimensional (quasi-1D) nanowire structure, a reduction in the writing current was observed, which is essential for obtaining high-density memory devices [2].However, with the aggressive scaling down of materials, their properties differ from those of their bulk counterparts [4,5].Therefore, to realize the high-density memory devices envisioned for this developing material, a profound development effort must be undertaken to discern the relationship between the fundamental PCM nanowire properties, the growth mechanism, and the resulting device properties.PCMs can repeatedly switch back and forth between amorphous (highresistance state) and crystalline (low-resistance state) phases with an adequate heat budget [6].Several studies have demonstrated the synthesis and characterization of PCM nanowires such as GeSbTe alloys [7][8][9][10], GeSb [11], Sb 2 Te 3 [12,13], and GeTe [1,[14][15][16][17][18][19][20][21][22][23], focusing on increasing the phase-switching speed and lowering the power consumption [2,24,25].GeTe is a viable PCM in this context.It has attracted significant attention in PCM-based applications due to its ability to permit defect-free scaling down in the fabrication of low-power memory devices [15,26].Several studies have reported the preparation and characterization of GeTe nanowires.For instance, Sun et al [23] fabricated single-crystal GeTe nanowires via metal-catalyzed vapor-liquid-solid (VLS) growth with 40-80 nm diameters using 20 nm colloidal Au nanoparticles.Tian et al [22] synthesized GeTe nanowires with a wide diameter range of 80-150 nm using a colloidal Au catalyst-assisted chemical vapor deposition technique.Meister et al [16] synthesized straight and helical single-crystalline GeTe nanowires via the VLS mechanism.Yu et al [15] reported the synthesis of ∼65 ± 20 nm and 135 ± 30 nm single-crystalline GeTe nanowires and nano-helices, respectively, using colloidal approximately ∼5-10 nm Au nanoparticles on SiO 2 substrate.Commonly, catalytic Au nanoparticles used to nucleate semiconductor nanowires are prepared using Au colloidal or annealed Au islands with diameters >10 nm, resulting in nanowires with diameters >20 nm [15].The smallest reported GeTe nanowire diameter to date was 20 nm by Lee et al [14].
The fabrication of GeTe nanowires with precisely controlled diameter distributions is essential to study the phase change mechanisms within GeTe and leverage their potential for memory applications.Controlling the nanowire size by VLS is quite challenging and is typically accomplished by controlling the catalyst size, which is the starting point for nanowire growth.However, all previously reported syntheses of GeTe nanowires using the VLS mechanism have shown poor control over their size distribution, including various inessential nano/microstructures from side reactions, regardless of the size of the starting colloidal Au nanoparticles.Two main concerns exist when using colloidal Au nanoparticles to grow GeTe nanowires: (i) reproducibility because Au nanoparticles can vary in size and shape, yielding poor control over their size distribution and morphology.(ii) Contamination can originate from the synthesis process or the handling and storage of the colloidal Au nanoparticles.
This work investigates the fabrication of GeTe nanowires via a bottom-up catalyst-mediated VLS process using monodispersed 8 nm Au nanoparticles using direct-current magnetron sputtering technique and size-selected via quadrupole mass filter.
This approach ensures both contamination-free conditions and precise control over the catalyst size [27].The effect of various growth parameters, namely growth pressure (P), inert gas flow rate ( f ), growth time, growth temperature, and type of growth substrate, on the morphology of GeTe nanowire is also examined.The morphology and size distribution of the grown nanowires were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively.TEM characterized the nanowire structure and composition.Utilizing conductive atomic force microscopy (C-AFM), the currentvoltage (I-V ) characteristics of GeTe nanowires were analyzed.

Nanowires growth
GeTe nanowires were grown by a combination of thermal evaporation and VLS techniques, using 8 nm Au nanoparticles as the metal catalyst.The fabrication process is illustrated in figure 1 and accomplished in the following four steps: Step 1. Substrates cleaning Si (100) and SiO 2 /Si substrates (1 cm × 2 cm) were degreased by consecutive 5 min sonication in acetone, isopropanol (IPA), and deionized (DI) water and then blown dry with compressed nitrogen.
Step II.Deposition of Au nanoparticles The substrates were then loaded into a Nanogen-50 deposition system (Mantis Deposition Ltd, Oxfordshire, U. K) for direct-current (DC) magnetron sputtering of ∼8 nm Au-nanoparticles using a 2 inch Au sputtering target with 99.99% purity (Testbourne Ltd, Basingstoke, UK).The Au nanoparticles' size was selected using a quadrupole mass filter attached to the deposition system, yielding contamination-free size-controlled monodispersed Au nanoparticles, as described in our previous study [27].The size of the Au nanoparticles and concentration were fixed for all samples in this work.
Step III.Annealing of Au nanoparticles The samples (Au nanoparticles) were annealed at 700 °C for 20 min to remove any organic contamination before nanowire growth, which improved the quality of the grown GeTe nanowires (figure S1, Supporting Information).
Step IV.GeTe nanowires synthesis The annealed Au nanoparticles samples were then placed upside down at the center of a 2 inch OFT-1200X tube furnace (MTI Corporation, Richmond, CA) at approximately 1.3 cm away from a GeTe source powder (0.1 g) with a purity of 99.99% (Testbourne Ltd, Basingstoke, UK), as shown in figure 1. Notably, the evaporation of the GeTe source powder and GeTe nanowire growth occurred concurrently at the same place.Argon gas was selected as the carrier gas to transport excess GeTe vapor away from the tube, controlled using a mass-flow controller.The tube furnace was flushed with 100 sccm of Ar gas for 20 min before the growth of the nanowires to remove any residual oxygen in the furnace.The tube furnace temperature was then ramped to 450 °C at a heating rate of 6 °C min −1 while maintaining a P of 75 Torr.All the samples were subjected to a fixed growth time of 4 h.Subsequently, the samples were cooled to room temperature (25 °C) in an Ar environment before they were removed.The effect of f on the produced GeTe nanowires was investigated at 10, 30, 60, and 120 sccm flow rates.

Characterization of nanowires
Morphological analysis of the GeTe nanowires was conducted using Nova NanoSEM 650 and Helios NanoLab 650 (Thermo Fisher Scientific Inc., Waltham, Massachusett, USA).ImageJ software was used to estimate the diameter and length of the nanowires.
The I-V characteristics of the GeTe nanowires were analyzed by C-AFM.The backside of the sample was mounted on a circular metallic AFM holder using silver paint.The electrical conductivity of the GeTe nanowires was investigated in a dark box under ambient conditions using an Asylum Research MFP-3D atomic force microscope (AFM) with a probe holder (908.036).The system has a sensitivity of 2 nA V −1 and a gain R of 500 MΩ, enabling it to measure currents from ∼1 pA to 50 nA.The I-V characteristics were measured using a Ti-Ir-coated Si cantilever with a force constant of 2.8 N m −1 and resonance frequency of 75 kHz.C-AFM analysis is performed in contact mode, where the conductive tip is in continuous contact with the sample.
An aberration-corrected Titan 80-300 ST transmission electron microscope (TEM) (Thermo Fisher Scientific Inc., Waltham, Massachusetts, USA) was used to conduct Brightfield TEM imaging.Imaging data were obtained by applying an accelerating voltage of 300 kV.The chemical composition of the nanowires was analyzed by energy dispersive x-ray spectroscopy (EDS) in scanning TEM (STEM) mode.2(a)) implies that the nanowires were formed via the VLS growth mechanism.The Au nanoparticle catalyst is imperative for the growth of GeTe nanowires, as the absence of these nanoparticles prevents the detection of GeTe nanowires on the Si substrate.The increase in the Au nanoparticles size after VLS could be ascribed to two possible reasons: (1) lateral growth of GeTe nanowires as more nanoparticles adhere to the initial nanoparticle [3], and/or (2) increase in volume due to the formation of the Au-GeTe alloy, which could elucidate the broad distribution of the diameter of the Au tip in the resultant GeTe nanowires [22].

Results
The narrow dispersion of the GeTe nanowire diameter distribution shown in figure 2(b) is an advantage of using our well-controlled monodispersed Au nanoparticles compared to colloidal Au nanoparticles or annealed Au thin films, which are conventionally used.Furthermore, the size-controlled Au nanoparticles used in this study produced Au nanoparticles with well-controlled diameters and well-dispersed on the substrate without clustering and agglomeration, which is imperative for controlling the diameter of the GeTe nanowires' diameter for device applications.

Impact of growth duration
The morphology and size of the resultant GeTe nanowires after doubling the duration of the nanowire growth are illustrated in figure 3. The density of the GeTe nanowires (i.e. the number of nanowires) increased when the growth time was doubled.GeTe nanowires grown for 8 h covered the entire substrate with tingled nanowires.Consequently, the diameter and length of the GeTe nanowires increased by 34% and 40%, respectively.
Imaging the GeTe nanowires using SEM was challenging owing to the sample charging and visible electron beam damage of the nanowires; therefore, we coated the nanowires with a ∼7 nm Au thin film to study the impact of different growth parameters on the synthesized nanowires through SEM.value of 75 Torr was considered to further investigate other growth parameters.

Impact of Ar gas flow rate
Figure 5 shows the impact of different f values on the growth of GeTe nanowires.The formation of large microparticles with secondary branches over the entire substrate was observed using f = 10 sccm.The average length of secondary branches is 4.96 ± 1.4 μm, as shown in figure 5(a).Nanowires are grown as secondary branches, and the average length of a nanowire is 0.84 ± 0.36 μm.The highest yield of well-defined GeTe nanowires is obtained using f = 30 sccm (figure 5(b)).Higher f values of 60 and 120 sccm would result in fewer nanowires with a large number of terminated nucleation sites, as shown in figures 5(c) and (d), respectively, which is probably due to the vapor depletion effect as it swiftly gets swept with the Ar gas out of the tube.A moderate f value is required to produce GeTe nanowires with ideal diameters and lengths.Therefore, a value of 30 sccm was chosen because it yielded the optimal nanowires, as shown in figure 5(b).

Impact of growth temperature
The impact of the growth temperature was also investigated.GeTe nanowires were grown using optimized conditions at 650 °C.Unlike the nanowires obtained at 450 °C (figure 2), SEM images of the nanowires synthesized at 650 °C revealed cross-linked rope-like elemental Ge nanowires on top of the Ge layer (figure 6(a)).The EDS elemental mapping shown in figure 6(b) reveals that the sample grown at 650 °C comprised Ge alone.The high growth temperature resulted in faster evaporation of the Te source powder, leaving only the Ge powder behind (figure S2, Supporting Information).Moreover, the evaporation of Te from the sample should also be considered.Therefore, 450 °C was the optimal temperature for the growth of GeTe nanowires.

Impact of the growth substrate
GeTe nanowires grown on SiO 2 /Si substrate showed similar morphologies to those grown on a (100) Si substrate as depicted in figure 7(a).A striking feature of GeTe nanowires grown on SiO 2 compared to those grown on Si is that the nanowires do not have a uniform diameter along their length.Unlike nanowires grown on Si substrate, the diameter of the GeTe nanowires increases as they get closer to the Au nanoparticle bead, indicating that more material is precipitating as the nanowire grows longer.Furthermore, the average diameters and lengths of the GeTe nanowires are 25.08 ± 0.53 nm (figure 7(b)) and 0.57 ± 0.01 μm (figure 7(c)), respectively, slightly larger than those grown on Si substrate using the same conditions.Moreover, the average size of the Au tip is 46.77 ± 1.12 nm (figure 7(d)), which is more than twice that of the Si substrate, which explains the increase in the diameter and length of GeTe nanowires.This increase in the average Au tip size compared with that of the Si substrate can be attributed to the different chemistry of Au nanoparticles on the SiO 2 substrate than the Si substrate.The electrical properties of the GeTe nanowires grown using the optimized parameters were examined using C-AFM.Figure 8(a) shows a schematic for probing the electrical characteristics of the GeTe nanowires.The sample and the tip were subjected to a slight DC bias voltage ranging from −2.0 to 2.0 V, while the substrate was grounded.The bias voltage range was kept at a minimum to avoid short-circuiting the AFM and melting the GeTe nanowires.As the probe raster scanned along the surface, the current flow was measured, thus generating a current map.Simultaneously, a topographic image was also generated from the same area of the sample, thereby facilitating the identification of features on the surface, where current is conducted.The typical scan rate for the current measurement was 1 Hz.A feedback loop was used during the scan to maintain the cantilever deflection constant through the z-piezo motion.Figures 8(b) and (c) show the AFM 3D topography image of the GeTe nanowires and the I-V characteristics of the selected GeTe nanowires, respectively.The C-AFM tip approached the substrate gradually until the GeTe nanowires were imaged, ensuring that neither the GeTe nanowires nor the probe tip was damaged.The I-V characteristics of three different GeTe nanowires show   typical Schottky contact behavior between the Ti/Ir-coated AFM tip and the GeTe nanowires.Schottky behavior can be attributed to the amorphous nature of the GeTe nanowires, which is a plausible explanation for this phenomenon.
3.6.2.TEM analysis.Figure 9(a) shows the bright-field TEM image of a GeTe nanowire, confirming the amorphous nature of the grown GeTe nanowires.Figure 9(b) shows that only the Au tip is crystalline, exhibiting lattice fringes of 3.1 Å.We could not match this spacing with that of Au or GeTe phases.However, this crystalline phase was composed of Au, Ge, and Te, as confirmed by the EDS spectra in figure 9(c).

Discussion
Based on the Au-GeTe quasi-binary phase diagram (No. 1200939) [28], when the ratio of its Au:GeTe constituents is 45:55 [12], Au nanoparticles can form Au-GeTe eutectic droplets at temperatures at approximately 480 °C and adsorb Ge and Te vapor species, as GeTe can form a solid solution with Au concentrations up to 100% at temperatures lower than 480 °C until they reach a state of supersaturation in Au.In this study, GeTe nanowires were grown at a temperature slightly lower than 480 °C, as the nanosized Au-GeTe droplets might have much lower eutectic points than the bulk Au-GeTe system [23].Nanoparticles are clustered together in droplets with spherical or semi-spherical shapes to reduce their surface energy.This results in an increase in the surface area-to-volume ratio, precipitating Ge and Te atoms from the supersaturated Au-GeTe droplet at the liquid-alloy/solid-Si substrate interface as the melting point of GeTe is substantially higher (725 °C [11]) than that of the eutectic alloy (480 °C [12] ).As shown in figure 2, Ge and Te atoms precipitate laterally on the surface, resulting in continuous quasi-1D formation of the GeTe nanowire.
The droplet size of a liquid alloy catalyst dictates the VLS-grown nanowire size.Using small Au nanoparticles yields GeTe nanowires with a small diameter.While the growth rate of the GeTe nanowire depends on its diameter ( ) d GeTe under the liquid droplet, the smaller the diameter, the slower the nanowire growth [22].The driving force of GeTe nanowire growth is the supersaturation (∆ ) m of the Au droplet, which is defined as the difference in the chemical potential of GeTe atoms in the vapor and solid phases, as expressed by the Gibbs-Thomson effect [22]: where ∆m 0 is the bulk supersaturation (i.e.d GeTe → ¥), Ω is the atomic volume of GeTe, a VS is the average surface energy density of the surface facets of the GeTe nanowires.Moreover, the GeTe concentration (C GeTe ) in the Au-GeTe alloy droplet is dependent on the diameter of the Au-GeTe droplet as expressed by the following equations, C e where 3 GeTe d kT where k is the Boltzmann's constant, T is the growth temperature, a is a temperature-dependent constant, C 0 is the equilibrium concentration in the bulk Au-Ge-Te alloy, γ is the surface energy density of the Au liquid droplet, V m is the molar volume of liquid Au, R is a constant, and d Au is the diameter of the catalytic Au nanoparticles used.
The amorphous nature of the GeTe nanowires is attributed to the small size of the Au nanoparticles used, which results in high supersaturation of the Ge and Te vapor constituents in the Au-GeTe liquid droplet.This causes rapid precipitation and nucleation of Ge and Te vapors, which are randomly deposited on the nanowire surface leading to a short-range order of atoms, forming an amorphous phase.A larger liquid catalyst provides a larger surface area for the Ge and Te vapors to deposit, allowing for additional duration for the atoms in the nanowire to rearrange into a crystalline structure.Moreover, because the surface-area-to-volume ratio is high for the small Au nanoparticles used in this study, more rapid cooling of the growing nanowire can occur.This can prevent the atoms in the nanowires from arranging into a crystalline structure, leading to the formation of amorphous GeTe nanowires.
To test this hypothesis, we used larger Au nanoparticles fabricated by depositing a ∼12 nm Au thin film and subsequently annealed at 720 °C for 20 min leading to agglomeration of Au nanoparticles to grow GeTe nanowires, as shown in figure 10(a).The Au agglomerates had a wide diameter dispersion, large concentration, and were in very close proximity to each other.Using our optimized growth parameters for 4 h, GeTe nanowires with a diameter of 54.6 ± 1.7 nm (figure 10(b)) and a length of 1.44 ± 0.06 μm (figure 10(c)) were synthesized.The wide dispersion of the diameter is due to poor control over the size of Au nanoparticles produced by the agglomeration of the Au thin film as shown in figure 10(d).Single-crystalline GeTe nanowires were produced, as shown in figure 10(e), which is consistent with the above discussion.The corresponding fast Fourier transform (FFT) pattern (inset in figure 10(e)) of the dashed area shows the crystalline nature of the GeTe nanowires produced using larger Au nanoparticles.The high-resolution image of the GeTe nanowire in figure 10(f) exhibits lattice fringes spaced at 4.34 Å, corresponding to the (101) plane of orthorhombic GeTe (Space group: Pnma, table No. 62, ICSD file number: 45572).Additionally, the area between the nanowire and Au tip was amorphous, which might be attributed to the difference in growth rates between the nanowire and Au-GeTe droplet.
In the GeTe nanowires grown in this study, the relationship between the diameter and length of the nanowire is proportional.A growth time of 4 h (figure 2(a)) produced the thinnest GeTe nanowires reported thus far with a diameter of 13.06 ± 0.1 nm.By doubling the growth time, the diameter and length of the GeTe nanowires were increased by 34% and 40%, respectively.As the size of the Au nanoparticles increased, the nanowires lengthened.As d GeTe decreases, the driving force for nanowire growth decreases; therefore, shorter nanowires are produced, as shown in equation (1).This observation has also been reported by Lee et al [14].Finally, when using a high P value, most of the GeTe nanowire growth was terminated or inhibited.As P increases, the concentration of Ge and Te vapor reacting with the Au-GeTe catalyst decreases which reduces the reaction rate, resulting in the termination or inhibition of GeTe nanowires.

Conclusions and future work
The synthesis of germanium telluride nanowires via a VLS mechanism using size-controlled Au nanoparticles as the metal catalyst is reported.The impact of different growth parameters, such as the growth temperature, growth time, growth pressure, inert gas flow rate, and growth substrate, was thoroughly investigated.Using growth times of 4 and 8 h, GeTe nanowires with diameters of 13.06 ± 0.1 nm and 17.44 ± 0.26 nm and lengths of 0.48 ± 0.01 and 0.7 ± 0.01 μm were produced, respectively.A moderate Ar flow rate of 30 sccm was required to obtain the GeTe nanowires without any secondary growth.The optimal growth temperature for the GeTe nanowires was 450 °C, which is close to the Au-GeTe eutectic temperature.TEM analysis revealed that the GeTe nanowires from 8 nm Au catalyst on the (100) Si substrate were amorphous.Further, we demonstrated that the size of the Au nanoparticles used to catalyze the VLS mechanism is essential in determining the morphology, size, and crystallinity of GeTe nanowires.It is noteworthy to acknowledge the potential applicability of GeTe nanowires in energy storage applications.This necessitates a more comprehensive examination, encompassing aspects such as thermal characteristics and other pertinent parameters, to gain a deeper insight into its viability for this purpose.It is important to underscore that this investigation remains designated for future work.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Figure 1 .
Figure 1.Illustration depicting the sequential stages involved in the production of GeTe nanowires.

Figure 2 (
Figure 2(a) shows an SEM micrograph of the as-grown GeTe nanowires synthesized using f = 30 sccm and P = 75 Torr on a p-type Si (100) substrate at 450 °C, demonstrating highyield horizontal GeTe nanowire growth.The produced GeTe nanowires have an average diameter of 13.06 ± 0.1 nm and (figure 2(b)) an average length of 0.48 ± 0.01 μm (figure 2(c)).The grown GeTe nanowires have a uniform diameter along their entire length, with Au nanoparticle beads of an average size of 17.76 ± 0.5 nm (figure 2(d)) located at the tip of the nanowire.The presence of Au catalyst beads at the end of the nanowires (shown in the magnified image in figure2(a)) implies that the nanowires were formed via the VLS growth mechanism.The Au nanoparticle catalyst is imperative for the growth of GeTe nanowires, as the absence of these nanoparticles prevents the detection of GeTe nanowires on the Si substrate.The increase in the Au nanoparticles size after VLS could be ascribed to two possible reasons: (1) lateral growth of GeTe nanowires as more nanoparticles adhere to the initial nanoparticle[3], and/or (2) increase in volume due to the formation of the Au-GeTe alloy, which could elucidate the broad distribution of the diameter of the Au tip in the resultant GeTe nanowires[22].The narrow dispersion of the GeTe nanowire diameter distribution shown in figure 2(b) is an advantage of using our

Figure 2 .
Figure 2. (a) SEM micrograph of GeTe nanowires grown at 450 °C using f = 30 sccm at P = 75 Torr on a (100) Si substrate for 4 h.Inset figure shows a magnified image of the nanowires.Histograms of GeTe nanowires: (b) average diameter, (c) length, and (d) tip diameter of Au nanoparticles.

Figure 3 .
Figure 3. (a) SEM micrograph of GeTe nanowires grown at 450 °C using f = 30 sccm at P = 75 Torr on a (100) Si substrate for 8 h.Inset figure shows a magnified image of the nanowires.Histograms of GeTe nanowires: (b) average diameter, (c) length, and (d) tip diameter of Au nanoparticles.

Figure 6 .
Figure 6.(a) SEM micrograph of GeTe nanowires grown at 650 °C using f = 30 sccm, at P = 75 Torr using (100) Si substrate for 4 h.(b) EDS elemental mapping of the sample, revealing the growth of Ge rope-like nanowires.

Figure 7 .
Figure 7. (a) SEM micrographs of GeTe nanowires grown at 450 °C for 4 h using an Ar flow rate of 30 sccm at a working pressure of 75 Torr on SiO 2 /Si substrate at different magnifications.The inset figure shows a magnified image of the nanowires.Histograms of GeTe nanowires: (b) average diameter, (c) length, and (d) tip diameter of Au nanoparticles, grown on SiO 2 /Si substrate.

Figure 8 .
Figure 8.(a) Schematic of the electrical measurement using C-AFM on GeTe nanowires grown at 450 °C for 4 h using an Ar flow rate of 30 sccm at a working pressure of 75 Torr on (100) Si substrate.(b) 3D AFM topography and (c) I-V characteristics of GeTe nanowires.

Figure 9 .
Figure 9. (a) Bright-field TEM image of a GeTe nanowire grown at 450 °C using f = 30 sccm at P = 75 Torr on (100) Si substrate for 4 h.(b) Magnified image of the tip of the nanowire.(c) EDS point spectrum of the middle of the tip.

Figure 10 .
Figure 10.(a) SEM micrograph of GeTe nanowires grown using 12 nm Au thin film annealed at 720 °C for 20 min as the catalyst and synthesized using our optimized conductions on Si substrate.Histograms of synthesized GeTe nanowires: (b) average diameter, (c) length, and (d) tip diameter of Au nanoparticles.(e) Bright-field TEM image of an individual GeTe nanowire (Inset: FFT of the dashed area).(f) Magnified image of the squared area.