Structural Characterization of Atomic Layer Deposited Vanadium

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Structural Characterization of Atomic Layer Deposited Vanadium Dioxide Alexander C Kozen, Howie Joress, Marc Currie, Virginia R Anderson, Charles R Eddy, and Virginia D Wheeler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04682 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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The Journal of Physical Chemistry

Structural Characterization of Atomic Layer Deposited Vanadium Dioxide Alexander C. Kozen,1 Howie Joress,2 Marc Currie,3 Virginia R. Anderson,1 Charles R. Eddy, Jr.,3 Virginia D. Wheeler3,* 1: American Society for Engineering Education, 1818 N Street NW, Washington, DC 20018 (Residing at the U.S. Naval Research Laboratory) 2: Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14853, USA 3: U.S. Naval Research Laboratory, Washington, DC 20375 *[email protected], Phone: (202) 404-4450, Address: 4555 Overlook Ave SW, Washington DC, 20375

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ABSTRACT: Vanadium dioxide (VO2) is a promising smart material particularly appealing for added functionality in both electronic and optical applications. We investigate the structure of VO2 films fabricated using Atomic layer deposition (ALD) on c-Al2O3 substrates, and the structure of the same VO2 films after an optimized anneal for 2 hours at 585 ˚C in 10-5 Torr O2. Synchrotron-based grazing incidence x-ray diffraction (GIXRD) measurements revealed that the as-deposited ALD VO2 contains small crystalline inclusions of both V2O5 and VO. Ex-situ annealing of the ALD VO2 film results in formation of mixed phase VO2 (predominantly M1 with indications of a small amount of M2), with a twinned monoclinic structure that is highly oriented and lattice matched to the c-Al2O3 substrate. Using in-situ temperature dependent measurements, the M1 and M2 components were shown to transition across different temperature ranges, which could account for the hysteretic behavior seen in these VO2 thin films. Additionally, these high-quality crystalline VO2 films exhibit a 50% transmission modulation at a wavelength of 2 µm, with only a 35 nm thick VO2 film.


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INTRODUCTION Vanadium dioxide (VO2) is a promising phase change material, particularly appealing for applications such as thermal sensors, electro-optical devices, and passive thermal management for spacecraft. VO2 is of particular interest for many applications since it undergoes a first order metal-insulator phase transition (MIT) near room temperature at Tc = 68 ˚C, which results in significant changes in electronic and optical properties. This MIT occurs through the reorganization of the VO2 crystal lattice from a low temperature monoclinic structure to a high temperature tetragonal structure.1 The MIT is coupled with a drastic increase in the free carrier density, which results in changes in electronic and optical behavior, especially in the infrared. Traditional thin film VO2 growth methods include sputtering2, molecular beam epitaxy (MBE),3 pulsed laser deposition (PLD),4 and solution methods.5,6 While high quality VO2 films can be fabricated using these methods, there are a number of drawbacks. Many of these deposition methods require substrate temperatures above 150 ˚C, which limit possible substrates for deposition. Moreover, most of these are lineof-sight deposition techniques that cannot produce conformal films on high aspect ratios often required for nanostructured devices. Independent of deposition method, the VO2 film produced must be of high crystalline quality and chemical purity to attain a quality thermochromic transition.7 Atomic layer deposition (ALD) is unique from traditional thin-film growth methods in that it allows deposition of highly conformal VO2 at low temperatures over large areas with 3D features; however, the resulting VO2 material properties and behavior are poorly understood.8-10 As-deposited ALD VO2 is amorphous11 and unsuitable for many 3 ACS Paragon Plus Environment


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of the desired applications due to the lack of significant change in optical and electronic properties over the MIT region. However, post-deposition annealing can produce VO2 films with desirable optical and electronic properties.10 Additionally, it has been demonstrated that the substrate can also impact the structural quality of the deposited VO2, and thus the properties of the MIT. Specifically, differing transition behavior has been seen among m-, c-, and r- plane sapphire substrates.4,12,13

The highest quality ultrathin VO2 films were grown epitaxially on

lattice-matched TiO2 substrates.14,15 Other groups have used synchrotron radiation to study both the morphology and the electronic structure of thick VO2 films.16 Results of these studies elucidate the structural contribution of two coexisting monoclinic VO2 phases known as M1 and M2, with M1 being the primary monoclinic phase responsible for the thermochromic phase transition. Previous diffraction experiments on ALD VO2,8 due to their out-of-plane geometry, are unable to determine thin film texture and anisotropy, which is known to be critical to VO2 fundamental properties and thermochromic transition behavior.17 Rampelberg et.al. used glancing incidence XRD to monitor the intensity of the in-plane peaks when sweeping VO2 thin films through the transition temperature (Tc).10 However, they used silicon substrates for their work and, as discussed above, substrate selection plays a critical role in VO2 microstructure and thermochromic behavior. This paper serves to correlate the structure of as-deposited and annealed ALD VO2 films on c-Al2O3 substrates with their optical and physical properties both above and below the thermochromic Tc. We show that the as-deposited ALD VO2 films do in


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fact contain evidence of crystallinity, even though this structure in the as-deposited films is not active during the MIT.

EXPERIMENTAL ALD VO2 films were deposited onto both c-Al2O3 and Si(100) substrates using an Ultratech Savannah S200 ALD reactor employing Tetrakis(ethylmethylamino)vanadium (TEMAV) and O3 as precursors.

The growth temperature was 150 ˚C, while the

pulse/purge times were .03 s / 30 s and .05 s / 30 s for TEMAV and O3 precursors, respectively. The O3 generator, supplied with the ALD system, was fed with O2 gas (Airgas, 99.9999%) at a feed pressure of 6 psi and a flow rate of 0.3 slm, operating at an approximate O3 concentration of 125 mg/L. The TEMAV was kept at 105 ˚C during depositions to ensure sufficient vapor pressure. ALD films used in this study were deposited for 450 cycles, resulting in 35 ± 1 nm thick films and a growth rate of 0.77 Å/cycle. The thickness of these films was determined on the Si(100) witness samples by ex-situ spectroscopic ellipsometry using a Cauchy optical model with an Urbach extinction coefficient to account for the ultraviolet absorption in VO2. Samples were annealed for 2 hours at 585 ˚C in a custom ultra-high vacuum chamber with UHP O2 (Airgas, 99.999%) flowing at an absolute pressure of 1 x 10-5 Torr.

All annealed

samples were compared against non-annealed samples from the same ALD growth and substrate. Sample morphology was determined using a Bruker FastScan AFM in tapping mode, while sample chemistry and stoichiometry was determined using a Thermo KAlpha XPS with a monochromated Al Kα x-ray source (1486.7 eV). Raman data was 5 ACS Paragon Plus Environment


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collected using a Thermo DXRi mapping system with a 100 X optical objective and a 532 nm excitation laser at 4 mW with a spot size of 0.58 µm. Optical transmittance and reflectance measurements were collected from 750 nm – 2.2 µm using a Cary UMA spectrophotometer. The sample temperature was varied by attaching the samples with metal spring fingers to a thermoelectric heated copper surface with a 3 mm hole in the copper for transmittance measurements. The temperature was monitored using a thermistor mounted within the copper block as well as a thermocouple attached to the copper surface. Grazing incidence x-ray diffraction (GIXRD) was performed at the G2 end station at the Cornell High Energy Synchrotron Source (CHESS) using a 6-axis Kappa goniometer setup.18 The experimental geometry is defined as follows: ߟ is the incident angle of the incoming x-ray beam, ߜ is the out-of-plane angle of the detector from the sample surface, ߥ is the in-plane angle of the detector away from the direct x-ray beam, and ߶ is the rotation around sample normal. A schematic of the experimental setup can be found in literature.18 We operated at a monochromated photon energy of 11.31 or 11.51 KeV and a photon flux of 1012 photons/sec. Diffraction data was collected using a 640 pixel 1D diode array (Siddons) detector with pixel size of 125 µm with the array axis along the out-of-plane direction.19 The detector was 0.38 m away from the sample and corresponding to a 0.01889° angular range for each pixel. A Soller slit with a 0.1° acceptance angle between the sample and the detector defines the resolution in ߥ, the in-plane 2θ angle. A fixed out-of-plane incident angle of ߟ = 0.2° was used; this angle was just above the critical angle.

Two types of scans were performed: ߶-ߥ rotate ߶

about the sample normal half of ߥ such the scattering vector stays along a certain


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crystal direction. For these measurements multiple scans were performed with the detector at different out-of-plane angles to cover a larger range of reciprocal space. ߶ scans were also performed to determine texture in the plane of the sample.

For

temperature dependent measurements, the sample temperature was tuned in-situ using a temperature controlled water block stage from 20 ˚C to 100 ˚C with a ramp rate of 2 ˚C per minute.

RESULTS & DISCUSSION Figure 1 shows the AFM surface morphology of both the as-deposited (Figure 1a) and annealed (Figure 1b) 35 nm thick ALD VO2. Annealing the VO2 causes an increase in the RMS roughness of the films from 2 nm to 3.1 nm, presumably due to faceting from film crystallization. This small increase in RMS roughness is consistent with a small average grain size increase of the films from 5 nm to 7 nm after annealing. The expected VO2 stoichiometry is corroborated by XPS data collected both before and after annealing these films (Figure 2a). The O1s photoelectron peak for both as-deposited and annealed films is at 530.2 eV, the nominal position for V(IV) oxide.20,21 For the annealed film, the V2p3/2 peak is at 516.1 eV, also the expected position for V(IV) oxide.20,21 The V2p3/2 peak of the as-deposited film is located at 517.0 eV, closer to V(V) oxide, indicating the existence of V5+ in the film, likely at the surface due to carboxylate formation from atmospheric exposure.20,21

However, the

approximate ratio of the O1s and V2p3/2 peaks of the annealed film is ~ 1:2, again consistent with VO2.

20,21

We also find a V2p3/2 to V2p1/2 splitting of 7.33 eV for the

annealed VO2, consistent with values from the literature.20,21 We are unable to obtain 7 ACS Paragon Plus Environment


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accurate depth profile stoichiometry or oxidation information due to the propensity of V reduction during Ar ion bombardment resulting in preferential etching and a mixed valence V oxide.22

Figure 1: AFM images of the a) the as-deposited ALD VO2 and b) the annealed ALD VO2. The RMS roughness of the as deposited and annealed films are 2 nm and 3.1 nm, respectively. Scale bar = 600 nm.

Raman spectroscopy can be used to identify the presence of various crystalline vanadium oxide phases.23

Figure 2b shows room temperature Raman spectra

collected from both as-deposited and annealed VO2 films. The as-deposited films show only c-plane sapphire peaks and contain no peaks associated with any polymorph of vanadium oxide, indicating the material does not contain a sufficient volume of crystalline VO2 to produce a Raman signal. After annealing, the room temperature Raman signature shows eight sharp peaks associated with high quality crystalline monoclinic VO2.24 However, it is difficult to distinguish between M1 and M2 monoclinic


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VO2 phases using Raman spectroscopy, and Raman is generally not useful for quantitative determination of crystal size or preferred orientation.4

Figure 2: a) XPS high resolution spectra showing both the O1s and V2p photoelectron peaks for as-deposited (black) and annealed (red) films; b) Raman spectra of 35 nmthick as-deposited (black) and annealed (red) ALD VO2 on c-Al2O3. Raman peaks associated with monoclinic VO2 are labelled on the annealed spectrum. Substrate peaks are marked with a *.

Near-IR transmission data for the as-deposited and annealed VO2 films are shown in Figure 3, both below and above the VO2 Tc (68°C). As-deposited, the VO2 films exhibit minimal optical modulation, indicating that the VO2 does not have a high enough degree of crystallinity to undergo the anticipated phase transition. However, after annealing sweeping through the VO2 Tc results in a decrease of the optical transmission by 50% at a wavelength of 2 µm.

This magnitude of transmission



modulation is comparable to a 100 nm thick pulse laser deposited VO2 film,25 suggesting excellent VO2 crystalline quality.

a) 100 90

As-deposited ALD VO2

b)

100

80

Annealed ALD VO2

90 80

99 C

70 60

20 C

50 40 30

Transmission (%)

Transmission (%)

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60 50

30 20

10

10 1000

1250 1500 1750 2000 Wavelength (nm)

90 C

40

20

0 750

20 C

70

0 750

1000

1250 1500 1750 2000 Wavelength (nm)

Figure 3: Infrared transmission as a function of wavelength at temperatures below (black) and above (red) the thermochromic Tc at 68 °C for a 35 nm thick VO2 film a) as-deposited and b) annealed.

Transmission electron microscope (TEM) bright field images of the as-deposited and annealed ALD VO2 thin films are shown in Figure 4a and Figure 4b, respectively. The as-deposited films show an abrupt interface with the sapphire substrate and areas of mottled contrast within the film, possibly due to density differences in the VO2 bulk. These density differences could be due to areas of differing stoichiometry within the bulk of the film or strain between crystalline and amorphous regions of the film. The morphology is similar to amorphous ALD VO2 films grown using H2O as the oxidation precursor.26 While the growth temperatures are not high enough to form fully crystalline material, there are pockets of crystallinity seen throughout the bulk of the film and 10 ACS Paragon Plus Environment

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verified by spots within amorphous rings in fast Fourier transform (FFT) pattern (inset of Figure 4a). The regions of crystallinity seen in these films have a small domain size and/or of poor quality such that the signal-to-noise ratio is below the detection limit of Raman spectroscopy or conventional laboratory XRD measurements. Figure 4b shows the TEM image of the annealed VO2 film, which exhibits excellent crystallinity composed of monoclinic VO2, confirmed by the inset FFT pattern. Strain contrast fringes are visible in the VO2 lattice, though due to the small interrogation volume evidence of the M2 monoclinic phase is not seen.

Figure 4: TEM micrographs of the a) as-deposited and b) annealed ALD VO2 on cAl2O3, along with inset FFT patterns. Scale bar = 10 nm.

To further investigate the crystallinity of both the as-deposited and annealed ALD VO2 films, we collected GIXRD measurements using high-flux synchrotron radiation. Raw GIXRD ߶-ߥ scan data, equivalent to in-plane θ-2θ scan data, were converted from 11 ACS Paragon Plus Environment


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angular reciprocal space maps to χ-d plots, which plot the diffraction vector tilt angle from sample normal as a function of d-spacing (߯ = 90 - tan-1(Q⊥ ⁄ Q∥; d=Q/2π). These plots are advantageous for the breadth of lattice information and the crystal orientation, and taking a horizontal slice across the plot will show the intensity as a function of dspacing at a fixed χ angle, simplifying unknown peak identification and matching to known powder diffraction patterns from the International Crystallographic Diffraction Database (ICDD). Figure 5a shows matching of the ICDD pattern (ICDD# 00-054-0513) of monoclinic V2O5 to a limited trace of the diffraction peaks collected from χ=89˚ to χ=90˚(red box in Figure 5b). Peaks at d = 1.78 Å, d = 2.36 Å, d = 2.57 Å, d = 2.87 Å and d = 3.43 Å which are likely matches to the (420), (020), (002), (400), and (111) respectively. The diffraction intensity of these peaks increase as the diffraction vector is tilted from χ=60˚ to χ=90˚ indicating a slightly preferred in-plane orientation, but are smeared out in χ due to mosaicity. Integrating the out-of-plane intensity peaks (Figure 5c) from χ=23˚ to χ=24˚ (blue box) shows a different intensity trace, with peaks at d = 1.65 Å, d = 2.24 Å, d = 2.78 Å, and d = 3.73 Å which are possible matches to the (224), (111), (202), and (002) peaks from the rutile VO phase (ICDD# 01-070-2716). No other patterns from the ICDD were identified as matching these peaks; GIXRD shows no evidence of crystalline VO2. The observation of crystalline XRD peaks corroborate the spots in the FFT pattern indicating regions of crystallinity for the as-deposited film in Figure 4a. Previous reports from literature have concluded that as-deposited ALD VO2 films are completely amorphous, however our results here indicate otherwise.


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Figure 5: GIXRD of the as-deposited ALD sample showing a) integrated intensity profile of the in-plane intensity with peaks associated with monoclinic V2O5 (ICDD# 00-054-0513) for the highlighted integrated area in the red box of the χ-d plot in b); b) χ-d map of the as-deposited ALD VO2 film showing highlighted areas used for peak integration in a) and c); c) out-of-plane peaks matching the rutile VO phase (ICDD# 01-070-2716) for the highlighted integrated area in the blue box of b).

Figure 6 shows the χ-d plots of the ALD VO2 film after annealing for 2 hours at 585 ˚C in 1x10-5 Torr O2. Annealing procedures were empirically optimized in order to produce films with the greatest magnitude of optical transmission modulation. Figure 6a shows the 2D trace from the in-plane peaks as a function of d-spacing (highlighted in the yellow box of Figure 6b), along with indicators matching the peaks to known dspacing of diffraction planes in M1 VO2 (ICDD# 01-070-2716). However, due to the highly anisotropic texture of the polycrystalline films, other diffraction peaks with different d-spacings appear throughout the entire film at various χ angles tilted from the



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sample normal shown in Figure 6b. We match these peaks predominantly to those of the M1 phase of VO2, and we identify one peak associated with the M2 VO2 phase (Figure 6c). We are unable to determine an index-match to the peak at d = 2.9 Å in Figure 6c. While one diffraction peak is not conclusive evidence of the M2 phase, the combination of coexisting phases is unsurprising, as others have asserted that monoclinic VO2 exists as a solid solution of M1 and M2 phases below Tc. 27 These data lead us to conclude based on peak indexing that the M1 VO2 phase has a highly preferred (010)M1 out of plane orientation, with in-plane (011)M1 and (-102)M1 components, and minor inclusions of M2 VO2 that may have an epitaxial relationship to the M1 crystallites.

Figure 6: a) Integrated intensity trace showing in-plane peak matching to monoclinic M1 VO2 (pdf# 00-009-0142) from the selected area in the χ-d map in b); b) Entire χ-d map for the annealed VO2 film showing highlighted areas integrated in a) and c); c) Integrated intensity trace integrated over the entire film showing peak matching (for all values of χ) to the monoclinic M1 VO2 (pdf# 00-009-0142) and monoclinic M2 VO2 (pdf# 00-033-1441).


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Figure 7 shows a ߶ scan around the high intensity VO2 (-211)M1 reflection from Figure 6. This scan shows the 6-fold symmetry of the M1 VO2, indicating lattice matching between the VO2 and the c-Al2O3 substrate. The offset angle from the twinned peaks is 4.2˚, indicating that the β angle for M1 monoclinic VO2 is β=122.1˚, consistent with previously published value of β=122.16˚ for VO2 lattice deposited on c-Al2O3.7

Figure 7: a) Normalized ߶ scan around the (-211)M1 VO2 peak. The heat map is the original data, while the overlay line trace is the normalized intensity of the diffraction peaks. b) Schematic of the M1 VO2 lattice matching to the c-Al2O3 substrate to help visualize symmetry.

The role of microstructure is fundamentally important to the MIT behavior of VO2 films,28 and temperature modulated synchrotron GIXRD can provide a method to understand the dynamical relationship between crystallography and MIT behavior of VO2 thin films. This experiment monitors two VO2 diffraction peaks concurrently: the (211)M1 peak and the (221)M2 peak. Simultaneous investigation of both of these peaks allows a way to more clearly understand the role of both M1 and M2 components in the MIT process. 15 ACS Paragon Plus Environment


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Figure 8a and Figure 8b show linear-log stacked plots of ߶-ߥ scans (equivalent to in-plane ߠ-2ߠ scans) of the (-211)M1 and the (221)M2 peak region respectively, as the temperature is stepped through the VO2 Tc. Figure 8c plots the normalized intensities of the peaks defined in Figure 8a and Figure 8b after background subtraction. Here, structural components of the VO2 transition over two distinct temperature ranges, associated with the M1 and M2 phases. The first transition, shown in Figure 8a, begins with a decrease of intensity of the (-211)M1 peak around 51 ˚C, and is fully converted to (111)R phase by 62 ˚C. The second transition related to the (221)M2 peak also starts to decrease in intensity at 51 ˚C, but is fully extinguished by 74 ˚C. As the temperature is swept above Tc, the intensity of the (111)R peak continues to slightly increase until 100 ˚C. These data suggest that the phase transition in the films is a two-stage process, where both phases change concurrently at slightly different transition rates. M1 and M2 phases are known to occur together in solid solution when under tensile strain above the M1+M2+R triple point,27 consistent with the data presented here. Previously it has been suggested that multiple simultaneous phase transitions can occur when transitioning across this triple point, such as a transition from M1 to R phase directly or alternatively a transition from M1 to M2 to R phase.27 This subtle effect could broaden the ensemble Tc, manifesting as the hysteresis of the transition temperature during heating and cooling. Some component of the Tc shift could also be due to the effect of grain size on the transition temperature, a well-known effect of nanocrystalline VO2.29 The temperature where the transitions are complete of the M1 and M2 phases is 60 ˚C


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and 68 ˚C respectively, comparable to the optical hysteresis (8 °C) seen in these films when swept above and then below the transition temperature.

Figure 8: Stacked line traces of ߶-ࣇ scans (in-plane θ-2θ) around the location of a) the (-211)M1 peak and (111)R peak and; b) the (221)M2 peak as the temperature is swept upward from 20 ˚C through the thermochromic transition to 100 ˚C; c) Normalized intensities of the peaks associated with the (-211)M1, (111)R, and (221)M2 peaks from a) and b) above as a function of temperature. Solid lines are a sigmoidal fit to the data to help visualize the phase transition regions.

CONCLUSIONS While ALD is a promising route to fabrication of functional thin film VO2 for numerous applications, our films require post deposition annealing in order to become fully crystalline and exhibit optimal phase-transition behavior. Using a combination of ex-situ characterization techniques and synchrotron GIXRD confirmed that as-deposited ALD VO2 films, originally thought to be fully amorphous, show partial crystallinity identified as monoclinic V2O5 and tetragonal VO when deposited onto c-Al2O3 substrates at 150 °C. 17 ACS Paragon Plus Environment


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These crystalline regions result from small crystalline domains in the bulk of the amorphous VO2 film, which can be seen in high-resolution TEM images, but are both too small and of the wrong phase to result in a thermochromic phase transition. Annealing the ALD VO2 results in a crystalline film consisting of predominantly twinned M1 VO2 phase oriented to the c-Al2O3 substrate with inclusions of M2 VO2. Temperature modulated GIXRD indicates that the ALD VO2 thin films undergo two concurrent phase transitions associated with the transition of both constituent M1 and M2 monoclinic phases that proceed though the MIT at different rates. These high-quality crystalline 35 nm films exhibit a 50% transmission modulation at 2 µm. Correlation between the optical and morphological transition properties will allow tailoring of the materials properties to improve their transition behavior for future applications.

Acknowledgements Funding for this research was provided by the Office of Naval Research. This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. ACK and VRA acknowledge support from the American Society for Engineering Education Postdoctoral Fellowship. TEM images were procured through Evans Analytical Group. Special thanks to Sonal Dey for assistance with data analysis scripts.


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TOC IMAGE


As-deposited ALD VO2

Annealed ALD VO2

1 2 3 4

3

3

2.5

2.5

Q (Å-1)

Q (Å-1)

Page The 25 Journal of 25 of Physical Chemistry 2 1.5 1

2 1.5

ACS Paragon Plus Environment 1

0.5

0.5 1

1.5

2

Q|| (Å-1)

2.5

3

1

1.5

2

Q|| (Å-1)

2.5

3

Structural Characterization of Atomic Layer Deposited Vanadium

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