This work was supported by core programs at the U.S. Naval Research Laboratory.

Caution: Consult all relevant material safety data sheets (MSDS) before use, and follow all appropriate safety practices and procedures. The atomic layer deposition growth of vanadium dioxide uses an ALD reactor. The precursors used for the ALD growth are tetrakis(ethylmethylamido)vanadium(IV) (TEMAV) and ozone (generated from ultra-high purity, UHP, 99.999% oxygen gas at 0.3 slm flow and 5 psi backing pressure). In addition, UHP (99.999%) nitrogen gas is used for purging the reactor chamber. For the subsequent vacuum anneal, UHP oxygen gas is used during annealing and UHP nitrogen for venting. TEMAV is flammable and should only be used with appropriate engineering controls. Compressed oxygen gas is a hazard and should only be used with appropriate engineering controls. Compressed nitrogen gas is a hazard and should only be used with appropriate engineering controls. All gases (TEMAV, oxygen, ozone and nitrogen) are connected to the ALD reactor using appropriate engineering safety controls. Stainless steel tubing connects the ozone generator to the ALD reactor, since it is cleaner and more reliable then plastic tubing. Separate UHP oxygen and nitrogen sources are connected to the vacuum annealing chamber using appropriate engineering safety controls before beginning the procedure. Acetone and 2-propanol are irritants and should only be used with appropriate personal protective equipment and safety procedures (e.g., gloves, fume hood, etc.)
1. Atomic Layer Deposition of Vanadium Dioxide on Sapphire Substrates
<ol>
	<li>Clean a c-Al2O3 (sapphire) substrate as follows: Solvent clean the substrate in acetone at 40 &#176;C in sonicator for 5 min, then transfer it directly (no rinse) to 2-propanol at 40 &#176;C and sonicate for 5 min. Rinse the substrate in running deionized water for 2 min, and dry it with nitrogen gas.</li>
	<li>Ensure that ALD reactor chamber is at 150 &#176;C and vent the ALD reactor with nitrogen gas.</li>
	<li>Load cleaned sapphire substrate into the reactor, close the reactor, and pump to &#60;17 Pa vacuum. Wait at least 300 s to ensure that the sample reaches 150 &#176;C.</li>
	<li>Prepare the ALD chamber by flowing 20 sccm UHP nitrogen into the chamber (the base pressure should not exceed 36 Pa), and then pulse ozone for 15 saturated cycles, where one cycle is a 0.05-s pulse followed by a 15 s purge.</li>
	<li>To grow vanadium dioxide, pulse TEMAV for 0.03 s followed by a 30 s purge, then pulse ozone for 0.075 s followed by a 30 s purge. Repeat this pulse and purge cycle until desired growth is reached. 
	NOTE: This process is nearly linear with a growth rate of 0.7-0.9 &#197; per cycle.</li>
	<li>Remove the sample from the ALD reactor by first venting the ALD reactor chamber with UHP nitrogen gas. Place the sample on a metal plate (heat sink) to cool it to room temperature. Close the ALD reactor and pump to &#60;17 Pa vacuum. The sample now contains an amorphous vanadium oxide film on sapphire substrate. 
	Caution: Remove the sample carefully, since the sample is heated to 150 &#176;C.</li>
</ol>
2. Annealing
NOTE: VO2 films grown by the ALD technique in step 1 produce amorphous VO2. To create oriented polycrystalline VO2 films, the samples are annealed in a custom ultra-high vacuum annealing chamber with a six-way cross. To keep the annealing chamber clean, a load lock is created to insert and remove samples. A 3&#34; diameter oxygen resistant heater is composed of a custom platinum wire heater. This heater provides radiative heating of an oxidized Inconel sled, to which the samples are mounted. The sled has high emissivity for good heat transfer from the heater to the samples.
<ol>
	<li>Ensure that the sled is in load lock, then vent the load lock with UHP nitrogen gas, and open the load lock. Place the sample on the sled in load lock and close load lock chamber.</li>
	<li>Pump the load lock to ~0.1 Pa using roughing pump. Next, switch to turbo pump and pump load lock to &#60;10-4 Pa. Open the gate valve and transfer the sled to the annealing chamber, and pump the annealing chamber to &#60;10-5 Pa.</li>
	<li>Flow 1.5 sccm ultra-high purity (UHP, 99.999%) oxygen into the annealing chamber. 
	NOTE: Run oxygen through 5 sccm mass flow controller to ensure a low flow rate. The pressure must be between 1x10-4 and 7x10-4 Pa. This pressure must be achieved before the sample reaches 150 &#176;C.</li>
	<li>Heat the sled to 560 &#176;C (measure with pyrometer and thermocouple) using a heating ramp rate of ~20 &#176;C/min. Hold the sled at 560 &#176;C for 2 h (for 300 &#197; film). 
	NOTE: The annealing time is thickness dependent. Empirical data suggests annealing 1 h for samples &#60;250 &#197; thick, 2 h for samples &#62;250 &#197; but &#60;500 &#197; thick, and 3 h for samples &#62;500 &#197; thick.</li>
	<li>Quench the sample by turning off the heater and removing the sled from the heater assembly (toward load lock).</li>
	<li>Keep the sample in the oxygen environment until the sample temperature becomes less than 150 &#176;C (e.g., use pyrometer in load lock to measure sample temperature). 
	NOTE: Better samples are achieved by waiting until even lower temperatures. Once the sample is &#60;150 &#176;C, turn off the oxygen flow and close the gate valve.</li>
	<li>Vent with UHP nitrogen gas. Remove sample at &#60;50 &#176;C and place the sample on metal plate (heat sink) to cool to room temperature. Close load lock with the empty sled and pump to 0.1 Pa using roughing pump. Switch to turbo pump and pump load lock to &#60;10-4 Pa.</li>
</ol>
3. Characterization
<ol>
	<li>Examine the sample using Raman spectroscopy with a 532-nm laser excitation source.
	<ol>
		<li>Load the sample into a microscope and bring it into focus. Verify the sample focus of the camera image in the software. Set the scan Laser Power to 4 mW, Exposure Time to 0.125 s, Number of Scans to 10, and preview size to 40 &#181;m.</li>
		<li>Click Live Spectrum to observe Raman spectrum. Optimize focus, laser power, exposure time, and number of scans to maximize signal to noise ratio. Click Save spectrum to save the data. 
		Open Spectrum in OMNIC. Click the &#34;Find Pks&#34; button to identify peaks. Determine crystallinity, phase (VO2 vs. VO, V2O3, V2O5, etc.), and strain by comparing peaks to reference data for vanadium oxides12,13. 
		NOTE: Narrow peaks indicate high crystalline quality, while hardening of Raman phonon modes 193, 222, and 612 cm-1 and/or softening of the 389 cm-1 mode are indicators of tensile strain in the VO2 crystal.</li>
	</ol>
	</li>
	<li>Determine orientation, crystallinity, and phase by X-ray diffraction (XRD). 
	NOTE: The appearance of peaks in the XRD spectra indicates the nature of the crystalline structure, specifically the crystal structure and orientation. Determine the orientation of VO2 by matching the 2&#920; angle of the XRD data peak to the Inorganic Crystal Structure Database (ICSD) cards for different planes of VO2. Determine the phase by matching data peaks to cards of different vanadium oxide phases. Manual comparison of experimental peaks to the standard database was performed in this work. A single VO2 peak at 39.9 degrees verifies the VO2 crystal quality and demonstrates the monoclinic (020) orientation.</li>
	<li>Determine stoichiometry and impurity levels by X-ray photoelectron spectroscopy (XPS)
	<ol>
		<li>Load the sample onto the sample holder. Open software and click vent load lock. Insert sample holder into load lock, and click pump down. Wait until pressure &#60;4x10-5 Pa. Click the transfer button to move the sample holder into the chamber. Verify the chamber pressure is &#60;7x10-6 Pa.</li>
		<li>Input measurement parameters into the experiment tree. Turn on X-ray gun with 400 &#181;m spot size and then turn on flood gun. Add a point for survey measurement, and then add points for high resolution scans of precursor elements (C and N) as well as the film elements (V and O). For each measurement, add pass energy (200 eV for survey and 20 eV for high-resolution scans) and number of scans (2 or more for survey and 15 for high resolution).</li>
		<li>Place crosshairs for point measurement at the desired location on the sample. Click and highlight the experiment tree title and then click &#34;Experimental Run&#34; on the menu bar to execute all measurement scans.</li>
		<li>Execute the survey ID procedure to identify and analyze the elements in the film. Select Manual Peak Add followed by Peak Fit buttons to analyze bonding in high-resolution data scans. Determine stoichiometry by taking the ratio of the integrated peak intensities according to the XPS Handbook14.</li>
		<li>When finished, shutdown the guns, then click the transfer button to move the sample holder into the load lock. Once the sample is in the load lock, click vent load lock and unload the sample. 
		NOTE: This identifies the presence of all elements in survey, thereby identifying impurities. XPS peaks at specific binding energies show the valences of vanadium oxides, as shown in Figure 1. For example, these results show two representative XPS measurements obtained by depth profiling with a cluster ion gun etching the as-deposited sample. The XPS measurement of the surface shows a peak at ~518 eV demonstrating the presence of V2O5, while the XPS measurement of the etched sample (in the film&#39;s bulk) shows a peak at ~516 eV demonstrating the presence of VO2. The full-width-at-half-max indicates the bonding uniformity of the vanadium in the films (in other words, the phase purity). In addition, the analysis of the other energy peaks enables identification of contaminates (&#62;1 %).</li>
	</ol>
	</li>
	<li>Determine morphology by using atomic force microscopy (AFM)
	<ol>
		<li>Turn on the computer and AFM electronics. Start the Nanoscope program and select Tapping Mode, then select load Experiment. Initialize Stage.</li>
		<li>Follow the experimental order on the left side of the screen and click Setup Menu. Use focus controls to focus optics on the cantilever. Align the laser on the probe by clicking Optimize Laser Position. Click Autoalign detector button, and then click the Autotune cantilever.</li>
		<li>Load sample and turn on vacuum.</li>
		<li>Click Navigate button and use the trackball to place the sample under the head. Click Tip Reflection and lower the head to the sample surface using Trackball, while holding the focus button down until the tip is in focus. Click the Sample button and close the AFM hood.</li>
		<li>Click the Check Parameter Menu. Ensure the scan size is &#60;1 &#181;m and set the samples/line to 512. Click the Engage Menu button. Wait 20 s.</li>
		<li>Set the Scan size for 3 &#181;m and leave the scan speed at 3.92 Hz. Optimize the image, if necessary, by changing parameters: drive amplitude, amplitude set point, integral and proportional gains.</li>
		<li>Capture the desired image by clicking the Frame Down button followed by the Capture Button. Following the scan, click the Withdraw button.</li>
		<li>Double click on the desired image to open it in analysis software. To determine morphology, click the Flatten button and then click execute. Extract statistical parameters by clicking Roughness to calculate surface roughness and click Particle Analysis to calculate the depth histogram and mean grain size.</li>
		<li>Click the Navigate Menu and click the Sample Load Position. Turn off the vacuum and unload the sample. 
		NOTE: For consistency from sample to sample, keep the same AFM scan parameters.</li>
	</ol>
	</li>
	<li>Determine optical transmittance and reflectance.
	<ol>
		<li>Use a tunable optical source and measure optical transmittance and reflectance at (or near) normal incidence in the near-infrared region. Calibrate the system without sample for 100% transmittance and with a gold mirror for near 100% reflectance.</li>
		<li>Load the VO2 sample onto the temperature controlled stage. Stabilize the sample at the desired temperature. Measure transmittance and reflectance over the desired spectral range, for example, throughout the near infrared region. 
		NOTE: For optimal results, the temperature should be varied at least 20 &#176;C above and below the transition temperature of VO2 (usually, 68 &#176;C).</li>
	</ol>
	</li>
</ol>
4. Modelling Optical Constants (Permittivity and Refractive Index)
<ol>
	<li>Model the complex dielectric permittivity, &#949;, as a function of photon energy, E, using the following equation (where &#949;&#8734; is the high frequency permittivity followed by the sum of n oscillators where An is the oscillator amplitude, En the oscillator energy, and Bn the oscillator damping): 
	<img alt="Equation 1" src="/files/ftp_upload/57103/57103eq1.jpg" /> 
	NOTE: Custom Matlab programs analyzed and modelled the data.</li>
	<li>Enter known parameters for sapphire substrate in step 4.1, and use these to calculate the dielectric permittivity as a function of optical wavelength. Then calculate the refractive index (n = &#8730;&#949;), and using the refractive index, calculate the reflectance and transmittance of the substrate.</li>
	<li>Compare the results of step 4.2 with the measured data, and then employ an optimization technique (such as Nelder Mead15) to update the input parameters of step 4.2 in order to reduce the error between the measured and calculated results, thereby optimizing the parameters for the sapphire substrate.</li>
	<li>Estimate parameters for VO2 in the equation in step 4.1, and use these to calculate the dielectric permittivity. Then calculate the refractive index (n = &#8730;&#949;) and use that, along with the VO2 thickness, substrate thickness, refractive index (from step 4.3), optical wavelength, optical polarization, and angle of incidence as inputs to the transfer16 to obtain the reflectance and transmittance of the VO2 coated substrate. Employ an optimization technique (such as Nelder Mead15) to update the VO2 input parameters and reduce the error between the measured and calculated results, thereby optimizing the parameters for the VO2.</li>
	<li>Perform steps 4.2 to 4.4 at several temperatures spanning the insulating and metallic states of vanadium dioxide (temperatures from 30 to 90 &#176;C). Model the temperature dependence of the refractive index of VO2 as follows: 
	<img alt="Equation 3" src="/files/ftp_upload/57103/57103eq3.jpg" /> 
	where &#949;VO2(T) is the refractive index of VO2 as a function of temperature, &#949;ins and &#949;metal are the dielectric permittivity of the insulating and metallic phases, and f(T) is a temperature-dependent function governing the distribution of insulating and metallic optical properties. Use a Fermi-Dirac-like function for f(T) to govern the transition between insulating and metallic state, similar to effective-medium approximations17,18,19, given by: 
	<img alt="Equation 4" src="/files/ftp_upload/57103/57103eq4.jpg" /> 
	where Tt is the transition temperature and W controls the width of the transition.</li>
	<li>Employ an optimization technique to update the parameters (W and Tt) in the equation for f(T) to reduce the error between the measured and calculated results, thereby optimizing the transition temperature and width. This results in a temperature and wavelength dependent model for the dielectric permittivity and refractive index of VO2.</li>
</ol>

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The authors have nothing to disclose.

The growth methods described here provide reproducible results with regards to uniformity, chemistry, structure, and morphology. The vanadium precursor is critical to producing the correct stoichiometry of as-deposited ALD films. This particular precursor promotes the +4 vanadium valence state, unlike many of the others listed in the literature that promote the more common +5 valence state. Additionally, this particular precursor has a fairly low vapor pressure and requires heating to provide a sufficient dose to saturate under the conditions given. Since this precursor starts to degrade around 175 &#176;C, this sets an upper temperature limit to both heating of the precursor and ALD growth. Another critical aspect to achieving correct stoichiometry is the ozone concentration (~125 mg/L here) during dosing. Often the concentration of ozone produced by a generator under particular conditions degrades or drifts over time. If this happens, the ozone pulse and purge durations will have to be adjusted to maintain stoichiometry, morphology, and wafer uniformity. What is described here is how to grow ALD VO2 on c-plane sapphire substrates, which includes in-situ ozone pre-treatment. The steps prior to growth for cleaning and nucleation are dependent on the substrate; however, the process described here works for most substrates (inert, oxides, metals, etc.) To determine the best termination cleaning and preparation for VO2 growth, one should consider reactivity between species termination and the vanadium precursor while minimizing any native oxide on the substrate. Finally, this process has been demonstrated on high aspect ratio substrates (up to ~100) but for extreme cases, one should consider an exposure or static ALD method to enhance conformality further.
The ability to achieve high quality, crystalline ALD VO2 films is quite dependent on the post-deposition annealing parameters. The most critical aspect is the pressure, specifically the partial pressure of oxygen. High oxygen pressures lead to faceting and grain growth, eventually causing nanowire formation, as well as results in the V2O5 phase. If the oxygen pressure is too low, oxygen is annealed out of the films resulting in V2O3 phase. Thus, to maintain the correct phase and minimize film roughness, the oxygen pressure should be maintained in the range of 1x10-4 to 7x10-4 Pa. Similarly, the temperature is critical to both being able to crystallize the film, maintain stoichiometry, and minimize roughening of the film. While the temperature of the VO2 film is difficult to measure, empirical findings suggest that crystallization requires stage temperatures greater than 500 &#176;C. At higher temperatures, it is harder to maintain the correct stoichiometry and phase and produce pinhole free films. There is also a trade-off between temperature and anneal time, specifically higher temperatures can reduce the anneal time. Additionally, the anneal duration is directly tied to the thickness of the film. Thicker films require longer times to achieve maximum crystallization. Thus, the oxygen pressure, anneal temperature, and anneal time described in the methods above were optimized to produce high quality VO2 films that exhibit the largest change in optical properties at a nearly ideal transition temperature. Finally, the ramping and cooling rates during the oxygen anneal have an effect on roughness and morphology; the slower these are, the smoother the films.
ALD deposition and subsequent anneal of VO2 produces oriented polycrystalline films with large area uniformity. ALD offers conformally grown films on three-dimensional nanoscale morphologies of almost any substrate. This enables VO2 integration into novel applications, and is especially well suited for optical devices.
Following growth and optical measurements, a model is created which provides a good fit to the data for both the transmittance and reflectance of VO2 in its metallic and insulating phases in the near infrared spectral region (R2 = 0.96-0.99). The reflectance of the infrared insulating phase is the most challenging process in creating this model. Additional oscillator terms were added, but this increased model complexity, only marginally improve the fit in this region. It should be noted that in this model, the superposition of Lorentz oscillators is a common optical model and do not necessarily correspond to specific electronic transitions. Initially, the models included a Drude term, however, after mathematical optimization, the Drude term was essentially eliminated. For this reason, several minimization techniques were examined. However, these different techniques converged upon similar solutions that did not involve a Drude term. The absence of a Drude term in the ALD VO2 could be due to a number of factors, such as 1) doped-semiconductor-like resistivity, or 2) a plasma frequency shift to lower energies and/or large collision rate (damping term), in agreement with the metallic properties of these films.
In the insulating phase, T&#60;60 &#176;C, the permittivity and refractive index of the ALD VO2 agree well with the other fabrication methods (sputtered4,20,21 and pulsed-laser deposition22,23). In the metallic state, T&#62;70 &#176;C, these ALD films exhibit lower loss than the VO2 fabricated by other methods. It is important to note that while different fabrication methods produce somewhat different values for the permittivity and refractive index of VO2, all films show similar trends.
The model in this paper of the temperature and wavelength dependence of the optical permittivity and refractive index agrees well with the experimentally measured data. This model&#39;s ability to produce a good quality fit to the measured optical data demonstrates it can reliably predict the optical properties of VO2 as the phase changes from an insulator to a metal. Using these models, the optical properties of VO2 can be predictably tuned by temperature, thickness, and wavelength to design optical systems that achieve static and dynamic goals. These models enable the design and development of optical systems using VO2 in passive and active systems by modifying the film&#39;s thickness as well as temperature.

Vanadium dioxide undergoes a crystalline phase transition near 68 &#176;C. This produces a structural crystal change from monoclinic to tetragonal. The origin of this transition remains controversial1, however recent research is helping develop an understanding of the processes that produce this transition2,3,4 . Irrespective of the origin, the phase transition changes the optical properties of VO2 from an insulator (transmitting light) at room temperature to a more metallic material (reflecting and absorbing light) above the transition temperature2.
A variety of methods have been used to fabricate VO2 in the past (sputtering, physical vapor deposition, chemical vapor deposition, molecular beam epitaxy, solution, etc.)5. The properties of VO2 largely depend on the technology used to fabricate the films6, which has produced significant variability between different growth techniques and subsequent anneal and led to varying crystallinity and film properties. This work investigates the optical properties of atomic layer deposited (ALD) grown films, however, the approach is applicable to modeling all types of VO2 films.
Recently, groups are constructing optical devices by incorporating thin films of VO2 onto optical substrates. As a rapidly growing new deposition method, ALD can assist in fabricating these optical devices and has several advantages over alternative techniques, such as large-area uniformity, angstrom level thickness control, and conformal film coverage7,8,9. ALD is the preferred technique for applications requiring a self-limiting layer-by-layer deposition approach, fabrication on a wide variety of substrate materials (e.g., for heterogeneous integration), or conformal coating of 3D structures10. Finally, the conformal coating of 3D structures of ALD process is particularly useful in optical applications.
For the experiments in this paper, ultrathin, amorphous ALD films were grown on double-side-polished, c-plane sapphire substrates at low temperatures and annealed in an oxygen environment to produce high-quality crystalline films. Using the experimental measurements, a model is created for temperature and wavelength dependent optical changes in VO2 to enable its use as a tunable refractive index material11.

<table border="0" cellpadding="0" cellspacing="0" width="731">
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			<td height="21" style="height:21px;width:343px;">c-Al2O3</td>
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			<td height="21" style="height:21px;width:343px;">UHP Oxygen</td>
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			<td></td>
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			<td height="21" style="height:21px;width:343px;">UHP Nitrogen</td>
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			<td></td>
		</tr>
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			<td height="21" style="height:21px;">Tetrakis(ethylmethylamido)vanadium(IV) (TEMAV)&#160;</td>
			<td style="width:111px;">Air Liquide</td>
			<td style="width:111px;"></td>
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			<td height="42" style="height:42px;width:343px;">Acetone</td>
			<td style="width:111px;">Fischer Scientific&#160;</td>
			<td style="width:111px;">A18-4</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:343px;">2-propanol</td>
			<td style="width:111px;">Fischer Scientific&#160;</td>
			<td style="width:111px;">A416P-4</td>
			<td></td>
		</tr>
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			<td height="42" style="height:42px;">Savannah S200-G2&#160;</td>
			<td>Veeco - CNT</td>
			<td style="width:111px;">Savannah S200-G2&#160;</td>
			<td></td>
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			<td height="42" style="height:42px;width:343px;">ozone generator&#160;</td>
			<td style="width:111px;">Veeco - CNT</td>
			<td style="width:111px;">&#160;ozone generator&#160;</td>
			<td></td>
		</tr>
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			<td height="21" style="height:21px;width:343px;">Platinum wire heater</td>
			<td style="width:111px;">HeatWave Labs</td>
			<td style="width:111px;">custom</td>
			<td></td>
		</tr>
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atomic layer deposition of vanadium dioxide and a temperature-dependent optical model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (&#8804;150 &#176;C) growth of ultrathin films (100-1000 &#197;) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films&#39; optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

To identify the quality of the ALD grown vanadium oxide, X-ray photoelectron spectroscopy (XPS) was performed on the as-deposited, primarily amorphous VO2 films (Figure 1) as well as annealed crystalline VO2 films (not shown). X-ray diffraction (XRD) was performed on the annealed VO2 films (Figure 2). In addition, to help quantify the vertical profile of the chemistry within the film, depth profiling was performed with a cluster ion source to minimize preferential etching of cation/anion species. Two representative traces are shown in Figure 1, one at the surface and one in the bulk. The depth profile and subsequent XPS measurements show that the top 1-nm of the as-deposited film is not VO2 due to excess environmental (adventitious) oxygen and carbon, but after a more controlled annealing procedure in low-pressure oxygen even the surface stabilizes to VO2. X-ray diffraction measurements were performed with a Cu K-alpha X-ray energy source and show, in Figure 2, a single VO2 peak at 39.9&#730;. The signature of this peak verifies the quality of the ALD-grown VO2 as well as that the (020) crystal orientation aligns with the sapphire substrate&#39;s peak.
To analyze the crystallinity, phase, and strain, Raman spectroscopy was performed using a 532-nm laser for excitation. Figure 3 shows a Raman spectrum of the VO2 film and shows narrow peaks which indicate high crystalline quality. In addition, the increased energy in the vanadium-vanadium low-frequency phonons (193 and 222 cm-1) and the 612 cm-1 mode, as well as the decreased energy of the 389 cm-1 mode, suggest tensile strain in these films12,13.
The morphology was observed by atomic force microscopy (AFM). Figure 4 shows crystal grain sizes on the order of 20-40 nm and a root-mean-square (RMS) roughness of 1.4 nm for as-deposited films (Figure 4A) and an RMS roughness of 2.6 nm for annealed films (Figure 4B).
Optical transmittance and reflectance data were obtained using a white light source with a scanning monochromator and a photodetector, which provided coverage in the visible and near infrared region. Figure 5 shows the temperature dependence of the film as it transitions from an insulator to a metal, demonstrating a transition temperature of 61 &#176;C. Analyzing the experimental data enables the modeling of the temperature and wavelength dependent permittivity of the VO2 as it transitions from insulator to metal. Figure 5 shows how the model accurately predicts the optical behavior when using the parameters in Table 1.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57103/57103fig1.jpg" /> 
Figure 1: Representative XPS measurements of 35-nm thick VO2 on c-Al2O3. XPS shows that the bulk of the film is VO2 while the surface, which contains C and O contaminates, is shifted more towards V2O5. Stoichiometry suggests VO2. <a href="/files/ftp_upload/57103/57103fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57103/57103fig2v3.jpg" /> 
Figure 2: XRD measurements of 35-nm thick VO2 on c-Al2O3. This XRD measurement shows a single VO2 peak at 39.9&#730; that independently verifies the crystal quality and demonstrates the monoclinic (020) orientation is aligned with the underlying sapphire peak. <a href="/files/ftp_upload/57103/57103fig2v3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57103/57103fig3.jpg" /> 
Figure 3: Raman spectra of VO2 on c-Al2O3. This Raman spectrum has narrow peaks, indicating high crystalline quality, and shows a slight tensile strain. <a href="/files/ftp_upload/57103/57103fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57103/57103fig4.jpg" /> 
Figure 4: Morphology of VO2 on c-Al2O3. The AFM images show uniform, continuous films with grain sizes on the order of 20-40 nm and RMS roughnesses of (A) 1.4 nm for the as-grown film and (B) 2.6 nm for the annealed film. <a href="/files/ftp_upload/57103/57103fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57103/57103fig5.jpg" /> 
Figure 5: Near-infrared optical transmittance and reflectance of 35-nm thick VO2 on c-Al2O3. The temperature-dependent behavior of the optical transmittance and reflectance of vanadium dioxide film are show at 40, 60, 70, and 90 &#176;C. The open circles in the plot are the measured transmittance, reflectance, and calculated absorptance of the VO2 on sapphire structure at various temperatures, while the solid lines are the predicted values from the two-dimensional temperature- and wavelength-dependent model of VO2. <a href="/files/ftp_upload/57103/57103fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>&#949;&#8734;</td>
			<td></td>
			<td>osc. 1</td>
			<td>osc. 2</td>
		</tr>
		<tr>
			<td>Insulator</td>
			<td colspan="4"></td>
		</tr>
		<tr>
			<td rowspan="3"></td>
			<td rowspan="3">3.4</td>
			<td>En</td>
			<td>3.8</td>
			<td>1.2</td>
		</tr>
		<tr>
			<td>An</td>
			<td>33</td>
			<td>2.1</td>
		</tr>
		<tr>
			<td>Bn</td>
			<td>1.4</td>
			<td>1.3</td>
		</tr>
		<tr>
			<td>Metal</td>
			<td colspan="4"></td>
		</tr>
		<tr>
			<td rowspan="3"></td>
			<td rowspan="3">4.5</td>
			<td>En</td>
			<td>3.2</td>
			<td>0.6</td>
		</tr>
		<tr>
			<td>An</td>
			<td>13</td>
			<td>5.3</td>
		</tr>
		<tr>
			<td>Bn</td>
			<td>1.1</td>
			<td>1</td>
		</tr>
	</tbody>
</table>
Table 1: Representative model parameters for VO2. These parameters are representative of those used in an oscillator model to estimate the permittivity of VO2 in its metallic and insulating phases.

Watch this Scientific Journal Video about Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model at JoVE.com

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Thin films (100-1000 &#197;) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...

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11.8K Views.  Naval Research Laboratory. Thin films (100-1000 Å) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Video: Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Construction and Testing of Coin Cells of Lithium Ion Batteries

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime. Lithium ion batteries have also been considered to be used in electric and hybrid vehicles1 or even electrical grid stabilization systems2. All these applications simulate a dramatic increase in the research and development of battery materials3-7, including new materials3,8, doping9, nanostructuring10-13, coatings or surface modifications14-17 and novel binders18. Consequently, an increasing number of physicists, chemists and materials scientists have recently ventured into this area. Coin cells are widely used in research laboratories to test new battery materials; even for the research and development that target large-scale and high-power applications, small coin cells are often used to test the capacities and rate capabilities of new materials in the initial stage. 
 In 2010, we started a National Science Foundation (NSF) sponsored research project to investigate the surface adsorption and disordering in battery materials (grant no. DMR-1006515). In the initial stage of this project, we have struggled to learn the techniques of assembling and testing coin cells, which cannot be achieved without numerous help of other researchers in other universities (through frequent calls, email exchanges and two site visits). Thus, we feel that it is beneficial to document, by both text and video, a protocol of assembling and testing a coin cell, which will help other new researchers in this field. This effort represents the "Broader Impact" activities of our NSF project, and it will also help to educate and inspire students.
In this video article, we document a protocol to assemble a CR2032 coin cell with a LiCoO2 working electrode, a Li counter electrode, and (the mostly commonly used) polyvinylidene fluoride (PVDF) binder. To ensure new learners to readily repeat the protocol, we keep the protocol as specific and explicit as we can. However, it is important to note that in specific research and development work, many parameters adopted here can be varied. First, one can make coin cells of different sizes and test the working electrode against a counter electrode other than Li. Second, the amounts of C black and binder added into the working electrodes are often varied to suit the particular purpose of research; for example, large amounts of C black or even inert powder were added to the working electrode to test the "intrinsic" performance of cathode materials14. Third, better binders (other than PVDF) have also developed and used18. Finally, other types of electrolytes (instead of LiPF6) can also be used; in fact, certain high-voltage electrode materials will require the uses of special electrolytes7.

A protocol to construct and test coin cells of lithium ion batteries is described. The specific procedures of making a working electrode, preparing a counter electrode, assembling a cell inside a glovebox and testing the cell are presented.

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime.  Lithium ion ...

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Atomic Force Microscopy of Red-Light Photoreceptors Using PeakForce Quantitative Nanomechanical Property Mapping

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be measured to ~10 pN by a laser and sectored detector, which can be converted to image topography. Amplitude modulation or &#8220;tapping mode&#8221; AFM involves the probe making intermittent contact with the surface while oscillating at its resonant frequency to produce an image. Used in conjunction with a fluid cell, tapping-mode AFM enables the imaging of biological macromolecules such as proteins in physiologically relevant conditions. Tapping-mode AFM requires manual tuning of the probe and frequent adjustments of a multitude of scanning parameters which can be challenging for inexperienced users. To obtain high-quality images, these adjustments are the most time consuming.
PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) produces an image by measuring a force response curve for every point of contact with the sample. With ScanAsyst software, PF-QNM can be automated. This software adjusts the set-point, drive frequency, scan rate, gains, and other important scanning parameters automatically for a given sample. Not only does this process protect both fragile probes and samples, it significantly reduces the time required to obtain high resolution images. PF-QNM is compatible for AFM imaging in fluid; therefore, it has extensive application for imaging biologically relevant materials.
The method presented in this paper describes the application of PF-QNM to obtain images of a bacterial red-light photoreceptor, RpBphP3 (P3), from photosynthetic R. palustris in its light-adapted state. Using this method, individual protein dimers of P3 and aggregates of dimers have been observed on a mica surface in the presence of an imaging buffer. With appropriate adjustments to surface and/or solution concentration, this method may be generally applied to other biologically relevant macromolecules and soft materials.

A method for investigating the structure of a protein photoreceptor using atomic force microscopy (AFM) is described in this paper. PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) reveals intact protein dimers on a mica surface.

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Fabrication and Operation of a Nano-Optical Conveyor Belt

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and&#160;limits the&#160;resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological ...

Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 &#181;m thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm2 and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 &#176;C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 &#730;C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO2 nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm2 and an efficiency of 3.5%.

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less ...

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 &#176;C and 200 &#176;C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

A method of uniform thickness solution-derived chalcogenide glass film deposition is demonstrated using computer numerical controlled motion of a single-nozzle electrospray.