Partially Ionized Beam Growth of Tungsten Oxide Nanowires by

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Partially Ionized Beam Growth of Tungsten Oxide Nanowires by Oblique Angle Deposition Dexian Ye Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Crystal Growth & Design

Partially Ionized Beam Growth of Tungsten Oxide Nanowires by Oblique Angle Deposition Dexian Ye Department of Physics, Virginia Commonwealth University, Richmond, VA 23284

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Tungsten oxide flux from an electron-beam evaporation source was partially ionized through electron bombardment. The positive ions travel through an electric field established by a pair of electrodes with applied voltage

and are accelerated towards the substrates positioned at an oblique

incident angle of 85° with respect to substrate normal. Uniform and well-aligned nanowires were formed with a tilt angle of 50° measured from the substrate normal as observed in scanning electron microscope images. The lengths and heights of nanowires shrink with the introduction of energetic ions in the deposition vapor, which are accelerated towards the substrate by a voltage higher than 2 kV. X-ray diffraction (XRD) patterns of the nanowire samples revealed that the crystal growth was promoted by the partially ionized tungsten oxides. The main XRD peak can be related to the [022] crystallographic planes of the -phase tungsten oxide (-WO3) in the orthorhombic structure. The intensity of the [022] diffraction peak increases with the increased

up to 3 kV. At

kV, the peak disappears,

indicating the amorphous state of tungsten oxide nanowires, which is confirmed by the selected area electron diffraction patterns in transmission electron microscope. The combination of partially ionized beam and oblique angle deposition provides a powerful method for controlling the morphology and crystal growth of metal oxide nanostructures.

Keywords: tungsten oxide nanowires, oblique angle deposition, partially ionized beam, X-ray diffraction


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INTRODUCTION Transition metal oxide nanowires have broad applications in electronics, memories, optics, sensors, catalysts, supercapacitors, and batteries due to their unique functions and physical and chemical properties [1-15]. In metal oxides, the metallic ions have partially filled d-orbitals or f-orbitals where the on-site Coulomb repulsion between electrons are strong, resulting unique electric, optical, and chemical properties [6, 16]. Nanowires have large surface areas, large area-to-volume ratios, and size effect at nanoscales, which can enhance the physical and chemical properties of the bulk materials. Therefore, transition metal oxides in the form of nanowire are particularly suitable for sensors, catalysts, and energy harvesting and storage devices [6]. A large variety of nanofabrication techniques have been exploited to synthesize transition metal oxide nanowires, which are roughly categorized into vapor phase methods and solution phase methods. Liquid phase methods include techniques such as sol-gel, hydrothermal, solvothermal, and electrodeposition [17]. Nanowires produced by chemical reaction of reagents in the liquid phase are randomly oriented, which usually require further filtration, purification, and assembly in nanodevice applications. Furthermore, the shapes, sizes, and crystalline structures of transition metal oxides nanowires are sensitively dependent on the selection of precursor chemistry and reaction conditions in liquid phase methods, thus limits the uniformity and mass production of nanostructures. Physical vapor deposition (PVD), vapor-liquid-solid epitaxy, and chemical vapor deposition are among the vapor phase methods, which utilize the surface condensation and assembly of constituting atoms or gas phase chemical reaction of precursors to form nanowires. Pre-fabricated templates or catalysts are usually required to guide the growth of nanowires in vapor phase methods [17]. The substrate is usually heated to a sufficient high temperature to initiate the physical and chemical processes to grow nanowires in most of the vapor phase methods. Recently, oblique angle deposition (OAD) method, a novel vapor phase nanofabrication method based on PVD, was developed to overcome the drawback of conventional physical vapor deposition of 3 ACS Paragon Plus Environment


nanostructures [18-23]. In the conventional PVD process, atoms arrive on the substrate along the surface normal direction, and within first few monolayers nucleate into nanoclusters and nanoparticles due to limited surface diffusion. At ambient temperature, the growth of those nanoparticles can merge and form a continuous thin film with enough material deposited. A shadow mask may be used in conventional PVD to confine the growth of nanoparticles laterally in order to form nanowires. On the contrary, the OAD technique arranges the substrate at the off-normal direction to allow a large deposition angle in a PVD system. In this arrangement, the depositing atoms approach the surface at an oblique angle with regard to the surface normal. At large oblique angles, the shadowing effect is significant enough to prevent the merging of nanoparticles deposited on the surface, yielding separate and well-aligned nanowires at room temperature. The nanowires have a major axis tilting to the incoming flux direction due to the shadowing effect and limited surface diffusion. In general, the tilt angle of the axis is smaller than the incident angle of the vapor. The substrate can be rotated in several ways as in the glancing angle deposition (GLAD) techniques, providing the flexibility of creating complex three-dimensional nanostructures that cannot be fabricated by other techniques [21-25]. In GLAD, the substrate rotates around its normal axis while the substrate is held steady in conventional OAD. OAD and GLAD have been employed to fabricate semiconducting transition metal oxide nanowires with energy band gaps less than 3.0 eV, such as iron oxide and tungsten oxide, which have important applications in water splitting using visible light [26-29]. Tungsten oxide is also applied in electrochromic devices as in smart windows for varying the adsorption of sunlight, as well as gas sensors [30-32]. The fascinating properties of tungsten oxide depend on the crystal phases and good crystalline structures of the nanostructures. Although OAD and GLAD can fabricate tungsten oxide nanowires, they are usually amorphous at low substrate temperatures due to low surface diffusivity. Post-growth annealing at sufficient high temperatures is necessary to convert the amorphous phase to crystalline phase [28]. The high temperature annealing process can limit the choice of substrates, change the composition, and modify the morphology of the nanowires.


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Energetic ion beam bombardment has been exploited to enhance surface processes of atoms in order to achieve crystal growth and modify the microstructure at relatively low temperatures in physical vapor deposition of thin films [33, 34]. There are two types of energetic ion beam sources developed in thin film deposition: (1) external ion source that is introduced to the substrate separately from the deposition source, as in the ion-beam assisted deposition method, and (2) partially ionized beam (PIB) that consists ions generated from the depositing particles, as in the ionized cluster beam method. Recently, external argon ion source has been used in GLAD to modify the morphology of nanostructures [35, 36]. Yet, the PIB has not been applied in OAD and GLAD. In this report, we developed a PIB-OAD method in an electron-beam evaporation system and used it to grow tungsten oxide nanowires.

EXPERIMENTAL METHODS The PIB system is based on a custom-built electron beam evaporator with a 10 kW power supply and a four-pocket copper hearth, as shown in Figure 1. A stepper motor is fixed to the goniometer head of a UHV compatible precision gearbox (Thermionics, Port Townsend, WA) to position and rotate the substrate in OAD and GLAD mode. The deposition angle can be continuously varied from 0 to 90° through the gearbox. In this report, the incident angle of the deposition beam was set to 85° with respect to the substrate normal in OAD mode and the GLAD mode was not used in preparing the samples. The distance from the source to the center of the substrate holder is 45 cm. The main vacuum chamber is a cube of side 60 cm, which is pumped by a molecular turbo pump to achieve a base pressure around 2.0 × 10-6 Torr. The partially ionized beam was obtained by the electron impact ionization method similar to the ones published in literature [37]. Briefly, a tungsten filament in the ionization zone as shown in Figure 1 was heated by a 24 V AC power supply to emit electrons. The electrons were then



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accelerated by a DC power supply. The ionization voltage Vi was 60 V. Thus, the electrons with a kinetic energy 60 eV collided with evaporated molecules and ionized them. The generated positive ions were then accelerated in the acceleration zone above the ionization zone by the electric field established by a set of electrodes. The electrode near the ionization zone was grounded and the electrode above the substrate holder was kept at negative potentials

. A DC power supply (Keithley 248, Keithley Instruments Inc., Cleveland, OH)

was used to provide the acceleration voltages. The acceleration voltages

were set to 0, 1000,

2000, 3000, and 4000 V, respectively, in this experiment. The samples were labeled as 0 V, 1 kV, 2 kV, 3 kV, and 4 kV accordingly. Tungsten oxide pieces (99.99%, 3 – 10 mm) were purchased from Kurt J. Lesker Company and used as the deposition source without further purification. In each deposition, about 5 grams of tungsten oxide was added to a graphite crucible and slowly heated under vacuum for 1 hour for outgassing purpose in the electron beam evaporation system. In this process, the emission current of the electron gun was about 1 mA to avoid the evaporation of the source. Then the electron beam emission current was increased slowly by 2 mA in every 5 minutes with a 10 kV acceleration potential until the evaporation rate of 3 Å/s was measured by a quartz crystal microbalance (QCM) monitor positioned near the substrate and facing towards the source, as shown in Figure 1. Meanwhile, the ionization filament was turned on and the ionization voltage was set to 60 V, which was a constant in this set of experiments. The deposition was performed on double-side polished silicon (100) substrates of the size 3 cm × 3 cm when the pressure in the vacuum chamber was about 1×10-5 Torr. The substrate were mounted on the substrate holder with an 85° oblique angle between the substrate normal and the incident flux direction. The nominal thickness of the deposition was measured by the QCM. The deposition was stopped at 6 ACS Paragon Plus Environment

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about 1000 nm nominal thickness. The samples were characterized by a scanning electron microscope (SEM, SU-70, Hitachi), a transition electron microscope (TEM, Libra 120, Zeiss), and an X-ray diffractometer (XRD, Panalytical X’pert pro).

RESULTS AND DISCUSSION The SEM images exhibited in Figure 2 show the morphology of the tungsten oxide nanowires fabricated by using OAD without PIB (a and b) and with PIB at different acceleration voltages (c – j). The top-view images were arranged in the left column while the cross-sectional images were aligned to the right column in Figure 2. The flux direction is from left to right in the top-view images. The white bar in each image represents a scale of 100 nm. Using the scale bar, the lengths and heights of tungsten oxide nanowires were measured from the cross-sectional images by using the ImageJ software. The tilt angles of the nanowires with respect to the substrate normal were also measured from the same cross-sectional images. At least ten nanowires were randomly selected for the measurement and the results were averaged as listed in Table S1 in Supporting Information and depicted in Figure 3. It is observed that the tilt angle decreases slightly by about 1° when the PIB is applied in the deposition. Moreover, the tilt angles do not change with the increase of acceleration voltage in the experiment with PIB, as shown in Figure 3(a). The averaged diameters of the nanowires measured from the cross-sectional SEM images are plotted as Figure 3(a). The length of the nanowires was measured along the major axis of the nanowires in the SEM cross-sectional images as shown in Figure 2 by using the ImageJ software. The height of the nanowire was measured from the top of the nanowire to the surface of the substrate along the normal direction using the similar approach. The averaged length and height of the nanowires decrease



dramatically when the acceleration voltage

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≥ 2 kV, as shown in Table S1 in Supporting

Information and depicted in Figure 3(b). The nominal deposition thickness was kept as a constant around 1000 nm for most of the samples except for the 2 kV ones, which has a nominal deposition thickness of about 920 nm. Nevertheless, the reduction of the length and height of nanowires from that of the 1 kV sample are more than 100 nm, which is much larger than the 80 nm difference of the nominal thickness. The measured height of nanowires from each sample can be normalized by its nominal thickness and plotted in the graph shown in Figure S3 in Supporting Information. The trend of the change is similar to that shown in Figure 3 (b). The diameters increase from 20 nm to 25 nm when

≥ 2 kV. Therefore, the reduction of nanowire

lengths from about 800 nm to about 500 nm when

2 kV indicates the increased solid

density of the nanowires. The increase of density of individual nanowires is attributed to the effect of PIB, as the high energy ions can transfer their kinetic energies to the atoms near the landing sites of the ions and enhance the mobility of atoms [38]. The mobility of surface vacancies can be increased to a comparable level of diffusivity of adatoms by the bombardment of energetic ions as well [39]. The increased mobility of vacancy also contributes to the increasing of the solid density of nanowires. From the top-view images, the number density of nanowires on the substrate surface apparently does not change. This result is consistent with previously reported observations in the deposition of thin film with energetic ionized beams [33, 34, 38, 40]. XRD is used to analyze the crystallinity of the tungsten oxide samples in this report. Figure 4 shows the XRD patterns of the tungsten oxide nanowires prepared with and without PIB in the range of 2 angles between 20 and 60°. From the X-ray diffractograms shown in Figure 4(a), the tungsten oxide nanowires are amorphous when they were prepared at room temperature by OAD 8 ACS Paragon Plus Environment

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method without PIB, i.e.

0 V. A Bragg diffraction peak emerges near 2 = 33.0° in the

diffractograms when the PIB was included and accelerated with a voltage intensity of this peak increases with the increase of acceleration voltage

1 kV. The

until 3 kV, where it

reaches the highest intensity. When the voltage increased to 4 kV, the Bragg peak disappears in Figure 4(a), indicating the amorphization of tungsten oxide nanowires by ions with high kinetic energies. The amorphization caused by energetic ion bombardment was reported previously in thin film deposition [41]. The XRD peak profiles around 2 = 33.0° were illustrated in Figure 4(b). This graph reveals the doublet peaks features that may be due to the characteristic copper K (

1.5406 Å) and K

(

1.5443 Å) radiation in the X-ray source. K

is stronger than K

. Using the higher intensity peak at Bragg angle 2 = 33.0° due to K , the d-spacing of the tungsten oxide crystal planes was calculated as

Å. This result is consistent with the

[022] crystallographic planes of the -phase tungsten oxide (-WO3) in the orthorhombic structure with the lattice constants

Å,

Å, and

Å [42]. Similar

result was obtained using the data reported by Howard et al. with the lattice constants Å,

Å, and

Å, which is identified as the Pcnb space group in the

orthorhombic -WO3 structure family [43]. Tungsten oxide nanowires grown with

= 3 and 4 kV were selected for the TEM

investigations as shown in Figure 5. TEM bright field image and the selected area electron diffraction (SAED) patterns of the 3 kV sample prepared with PIB are arranged in Figure 5(a), while that of the 4 kV sample are in Figure 5(b). The length of the nanowires from 3 kV sample is about 500 nm, which is consistent with the measurement obtained from SEM images. The SAED pattern in Figure 5(a) shows a clear sharp ring, which is broken into arcs with a dot



visible at the center of each arc. The sharp ring with the dots demonstrated the good crystallinity of tungsten nanowires prepared with PIB at 3 kV, which may be due to the SAED pattern is formed by multiple scattering of electrons by the bundle of nanowires as seen in the bright field image. The d-spacing of the tungsten oxide crystal planes corresponding to the ring is calculated to be 1.93 ± 0.01Å, which is the (004) crystal orientation of the orthorhombic -WO3 phase as mentioned above. From this result, it is confirmed that the tungsten oxide nanowires grow in the [001] direction along the main axis of the nanowire with the (022) out-of-plane texture indicated by XRD data. The angle between and texture axes of a orthorhombic crystal structure is about 46°, which is consistent with the tilt angle measured for the nanowires from SEM images. On the other hand, as in Figure 5(b), the SAED pattern of the 4 kV sample shows a broad and blur ring, which confirms the amorphous phase of the tungsten oxide nanowires prepared with PIB at 4 kV. Tungsten oxide has several different crystal structures in monoclinic, orthorhombic, and hexagonal phases at different temperatures and preparation methods [43]. Tungsten oxide is monoclinic at room temperature and changes to orthorhombic phase at the transition temperature around 350°C. At higher temperatures, it changes to hexagonal phase around 800°C [43]. Crystalline tungsten oxide nanowires in triclinic/monoclinic phases were prepared by GLAD previously with either elevated substrate temperatures or post-deposition annealing, which are polycrystalline with [020] planes as the preferred crystal orientation [44, 45]. In this report, only one out-of-plane preferred crystallographic direction was observed in XRD, indicating wellaligned crystal orientation along the substrate normal. The observed (022) crystal orientation in this report usually appears as a minor peak in the XRD patterns reported in literature. The minor out-of-plane crystal orientation in a thin film changing into a major orientation in OAD is 10 ACS Paragon Plus Environment

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possible due to the shadowing effect and the tilted growth direction of nanowires from the surface normal. It is shown that the out-of-plane orientations of a growing crystal can be altered by the off-normal incident direction of the deposition flux, forcing the rotation of the fast growing crystal direction toward the direction of the flux [25, 46]. The energetic ion bombardment also has impact on the selection of the crystal growth direction. The ions transfer kinetic energy to the surface atoms and enhance their mobility, thus a particular preferred growth direction can be achieved and some other directions may be suppressed, which depends on material, processing parameters, and ion’s energy [47]. The energetic ions have dual effects on the behavior of crystal growth at low substrate temperatures when the adatom’s mobility is limited. On one hand, they promote the crystal growth by increasing the mobility. On the other hand, high-energy ions can destroy the preferred orientations of the crystallites, resulting random oriented nanocrystallites in the deposited material. The competition between these two effects of ions depends on the ion energy. Therefore, when the ion energy increases, the XRD reflection intensity of the preferred orientation increases. The intensity decreases with further increased ion energy if the ions have enough energy to cause the random orientation of nanocrystallites. In this report, tungsten oxide ions with 4 keV have such kinetic energy to destroy the preferred crystal growth direction of tungsten oxides, resulted an amorphous like XRD pattern without dramatically changing the morphology of nanowires. In this report, it is concluded that the acceleration voltage of 3 kV is ideal for well-aligned crystal growth under our experimental conditions for tungsten oxide nanowires. One parameter we did not explore in this work is the ionization voltage

, which was kept at

60 V. With this voltage, the ion percentage was estimated to be around 0.1%. The optimization of our experimental setup and the ionization conditions should be able to increase the percentage 11 ACS Paragon Plus Environment


of ions in the deposition flux. In the future, the effects of the ionization voltages together with the acceleration voltages and the incident angles on the crystal growth will be explored, as well as the study of the in-plane crystal orientations of nanowires grown in PIB using XRD pole figure technique. CONCLUSION In this work, we have investigated the effects of the partially ionized tungsten oxide beam in oblique angle deposition. Tungsten oxide nanowires were deposited at 85° oblique angle on planar silicon surface. The nanowires tilt toward the electron beam evaporation source with an angle of about 50° from the surface normal, which does not change with the increase of acceleration voltages in PIB. Furthermore, it is observed in SEM cross-sectional images that the length and height of the nanowires are reduced when the acceleration voltages are above 2 kV. By changing the acceleration voltage, the deposited tungsten oxide nanowires evolve from amorphous to crystalline phase with the (022) orientation in XRD patterns at the substrate temperature near room temperature. The intensity of the (022) diffraction peak increases with the increase of ion energy. If the acceleration voltage is above 4 kV, the crystalline structure of tungsten oxide nanowires is destroyed, as such the diffraction peak disappears in XRD diffractograms. Samples grown with 3 kV acceleration voltage show crystalline structures in the selected area electron diffraction in TEM, while the samples grown with 4 kV acceleration voltage demonstrate amorphous diffraction pattern. From XRD and TEM analysis, the -phase tungsten oxide nanowires in the orthorhombic structure were formed in oblique angle deposition with partially ionized beam. We observed that the morphology of the nanowires are not changed significantly by the introduction of the partially ionized beam. Our work extends the capability


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of OAD and GLAD to grow single crystal nanowires without substrate heating or postdeposition annealing. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images and data of the lengths, diameters, tilted angles, and vertical heights of tungsten oxide nanowires measured from SEM images. AUTHOR INFORMATION Corresponding Author *Correspondence should be addressed to the email address: [email protected]. ACKNOWLEDGMENTS We thank Mr. Rezaul (Reza) K. Khan for the assistance of obtaining XRD data and Dr. Carl Mayer for the TEM characterization.



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FIGURE CAPTIONS Figure 1. Schematic diagram of oblique angle deposition system with partially ionized beam in a conventional electron-beam evaporation system. Figure 2. Scanning electron microscope images of tungsten oxide nanowires grown without (a and b) and with PIB at different acceleration voltages

(c – j). The images of the same sample

are arranged in the same row. The top-view images are in left column and the cross-sectional images are in the right column. The incident angle is 85°. The direction of the flux is from left to right on the top-view images. (a) and (b) (f) with PIB at

0 V; (c) and (d) with PIB and

2 kV; (g) and (h) with PIB and

1 kV; (e) and

3 kV; (i) and (j) are for PIB with

4

kV. Figure 3. The shape parameters of tungsten oxide nanowires obtained from SEM cross-sectional images. (a) The tilt angle (to the left axis) and diameter (to the right axis) of the nanowires as a function of PIB acceleration voltages. (b) The length and height of the nanowires as a function of . Figure 4. X-ray diffraction patterns of tungsten oxide nanowires grown by PIB at 85° oblique angle. (a) The  – 2 diffraction patterns recorded from 20 to 60°. (b) The peak profile of orthorhombic tungsten oxide (022) around

33.0°.

Figure 5. Transmission electron microscope bright field images and selected area electron diffraction patterns of tungsten oxide nanowires prepared with PIB at (a)

3 kV and (b)

4 kV. The bright field images are arranged in the left column, while the diffraction patterns are in the right column.



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Fig. 1


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(b)

(a) 0V

100 nm

100 nm

(d)

(c) 1 kV

100 nm

(e)

100 nm

(f)

2 kV

100 nm

100 nm

(g)

(h)

3 kV

100 nm

100 nm

(j)

(i) 4 kV) 100 nm

100 nm


Fig. 2


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(a)

(b) Fig. 3


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(a)

(b) Fig. 4



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(a)

(b)

Fig. 5

For Table of Contents Use Only 22 ACS Paragon Plus Environment

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Partially Ionized Beam Growth of Tungsten Oxide Nanowires by Oblique Angle Deposition

Dexian Ye

TOC Graphic

100 nm

100 nm

Synopsis

Partially ionized tungsten oxide vapor is generated from an electron-beam evaporation source by electron impact ionization method. Tungsten oxide nanowires are grown on silicon substrates by the oblique angle deposition approach. Crystal growth is promoted by the ionized tungsten oxide particles with high kinetic energy.


Partially Ionized Beam Growth of Tungsten Oxide Nanowires by

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