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Heat Assisted Inkjet Printing of Tungsten Oxide for High-Performance Ultraviolet Photodetectors Brent Cook, Qingfeng Liu, Jackson Butler, Keifer Smith, Karen Shi, Dan Ewing, Matthew Casper, Alex Stramel, Alan Elliot, and Judy Z. Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15391 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Heat Assisted Inkjet Printing of Tungsten Oxide for High-Performance Ultraviolet Photodetectors Brent Cook,*,† Qingfeng Liu,† Jackson Butler, Keifer Smith, Karen Shi, Dan Ewing,‡ Matthew Casper,‡ Alex Stramel,‡ Alan Elliot, ‡ and Judy Wu*,† †

Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States

‡

Department of Energy's National Security Campus, Kansas City, MO, 64147, USA

ABSTRACT: An ammonium metatungstate precursor ink (WO3Pr) was printed for tungsten oxide (WO3) ultraviolet (UV) detectors on SiO2/Si wafers with prefabricated Au electrodes. A systematic study was carried out on the printing parameters including substrate temperature in the range of 22-80 oC, WO3Pr molar concentrations of 0.01, 0.02 and 0.03 M, and printing scan number up to 7 to understand their effects on the resulted WO3 film morphology and optoelectronic properties. It has been found that the printing parameters can sensitively affect the WO3 film morphology, which in turn impacts the WO3 photodetector performance. In particular, the printed films experienced a systematic change from discontinuous droplets at below 40 oC, to continuous films at 40-60 oC of the substrate temperature. At higher temperatures, the excessive heat from the substrate not only caused drastic evaporation of the printed ink, resulting in highly non-uniform films, but also detrimental heating of the ink in the printer nozzle in proximity of the substrate, preventing continuous printing operation. An optimal printing window of the substrate temperature of 45-55 oC at molar concentration of 0.02 M of ammonium metatungstate and three printing scans was obtained for the best UV detector performance. A large on/off ratio of 3538 and high responsivity up to 2.70 A/W at 5V bias (0.54 A/W∙V) represent a significant improvement over the best report of ~0.28 μA/W∙V on WOX photodetectors, which indicates the printed WO3 films are promising for various applications of optoelectronics and sensors. Keywords: inkjet printing, tungsten oxide, heated printing, photodetector, sensor.

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Introduction Inkjet printing offers advantages of mass scalability, cost reduction, low-waste and direct deposition on targeted regions.1,2

Most importantly, it offers a pathway to integrate high-

performance electronic, photonic and optoelectronic components based on functional nanomaterials and quantum nanostructures with complementary metal-oxide semiconductor (CMOS) circuits.3 Extensive efforts have been put in research of inkjet printing with exciting progress made in the last decade or so.2-5 However, many issues remain in controllable inkjet printing, among them is controlling the ink drop spreading for the desired morphology and uniformity of the printed thin films, which is important for device performance and yield.6 Without such a control, the printed devices typically have lower performance than their counterparts made using other methods such as spin-coating, chemical vapor deposition, physical vapor deposition, etc.7 For a well-controlled ink drop, Weber, Reynolds and Ohnesorge numbers must be adjusted to optimal ranges.4,8-10 The Ohnesorge number is the square-root of the Weber number divided by the Reynolds number. The Weber and Reynolds number are defined as, NW =v2ρα/γ

and 1/2

NR=vρα/η

respectively,

which

results

in

an

Ohnesorge

number

of

1/2

NO=(NW) /NR=η/(ργα) , where v, ρ, η, γ, and α are the ink’s average travel velocity, density, viscosity, surface tension, and the printer head’s nozzle diameter.8-10 The inverse of the Ohnesorge number is the Z parameter which offers the best indicator for printable inks. A Z parameter between 1 – 10 is desirable. If Z < 1, the ink is too viscous, and at Z > 10 it is too fluid.4,8,9 Lastly, the substrate must be considered because a hydrophobic or hydrophilic surface will either result in strong droplet formation or droplet spreading respectively.1 Materials that have been studied for printing include inorganic7,9,11-13 and organic1,3 solutions, and suspensions of well-dispersed nanostructures.3,14-16 Among the materials studied, tungsten oxide (WO3)11-13 is a particularly interesting sensor material. WO3 is a n-type semiconductor with an energy bandgap around ~3.2 eV and has been extensively studied for applications in gas sensors and electrochromic displays.17 WO3 is capable of detecting gases such as NH3,18 H2,19 NO2,19-21 and Cl2,22,23 which are gases that are produced in chemical processes and industrial plants. WO3 also exhibits strong electrochromism being able to change color and modulate transmission up to 83% of wavelengths in the visible range of 400-800 nm.13 The electrochromic properties of WO3 make it very capable for applications in screen displays11,13 and solar cells.17,24 2 ACS Paragon Plus Environment

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Furthermore, WO3 can take many different morphologies such as nanobelts, nanowires, nanosquares, and monolayer films which have been tested for UV photodetection.25-29 However, these different morphologies were fabricated using atomic layer deposition,25 hydrothermal,26,27 electrospinning28 and chemical vapor deposition,29 which can be costly and lack targeted deposition. Alternatively, inkjet printing offers a robust and low-cost method for direct deposition of WO3-based devices. Currently, most printing of WO3 involves prefabricated WO3 nanoparticles (NPs) in solution.11,12 The difficulties in printing a WO3 film from the WO3 NP ink include: 1) the inter-NP junctions are weak links for electric current flow and can significantly reduce the sensitivity, 2) it would be time consuming to obtain a WO3 film with a thickness much beyond the diameter of the WO3 NPs, and opting for a thinner film (tens of nanometers) will result in reduced effective sensor material; 3) larger NP size could resolve these issues but may be problematic for dispersing the NPs in the ink. An alternative is to directly print tungsten oxide precursor ink (WO3Pr) solution followed with post heat treatment in air or oxygen. This approach has been applied successfully for printing ZnO using zinc acetate as the ink.9,30 In the case of WO3, however, the water content of the precursor solution makes ink spreading difficult, leading to formation of discontinuous droplets, especially on hydrophobic surfaces such as on SiO2/Si wafers. The high fluidity causes WO3Pr ink to easily aggregate and form droplets on the surface of SiO2 wafers, this is likely because of the large Z values of 53 (DMF) and 63 (H2O). In the case for our precursor ink, there tends to be spraying and satellite droplets upon ejection from the tip likely do to the high Z values of the solvents. To limit the spraying and satellite droplets the printing nozzle is placed approximately 20 µm from the surface of the substrate so that a liquid bridge is formed from the nozzle tip to the surface. A low dispensing voltage is used of approximately 2.5 V on the piezoelectric to maintain the ink bridge formed between the nozzle and surface as the nozzle prints the pattern. In combination with a heated surface we can achieve a film. The printability of an ammonium metatungstate precursor ink for WO3 film has been investigated in this work by varying the printing parameters such as substrate temperature (22 – 80 oC), WO3Pr molar concentrations of 0.01, 0.02 and 0.03 M, and printing scan number up to 7 to extract the correlation between the WO3 film morphology and its performance in ultraviolet (UV) photodetection. The substrate temperature adds an additional tuning to the ink drop spreading when the ink is incapable of forming a film due to the substrate having a hydrophobic



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surface. An optimal temperature window of 45-55 °C was obtained for the printed WO3 films with desired morphology and thickness together with high performance for UV detection. Experimental Section The ammonium metatungstate ((NH3)6H2W12O40) and a Dimethylformamide (DMF)\water mixture was used as the tungsten oxide precursor (WO3Pr) ink, with a Z value most likely greater than 10 given that the WO3Pr tends to have spraying and satellite droplet formation. The Z values for the solvent were calculated assuming the ink solution is composed mainly of Dimethylformamide (DMF) and water in a 7:1 ratio. DMF is shown to have a density, viscosity and surface tension of 0.944 g/cm3, 0.794 mPa∙s and 37.1 mN/m respectively.24,

31

In

combination with a tip diameter of α ≈ 50 μm the Z values is approximately 53, meaning the ink has high fluidity. Similarly, using 1 g/cm3, 0.954 mPa∙s, and 72.7 mN/m for the density, viscosity and surface tension of water a Z ≈ 63 is obtained, these values can be summarized in Table 1.32-34 Ultrasonication for 30 min. or longer was employed to generate a uniform ink. The DMF to water ratio was 7.0 to 1.0 and the molecular weight of ammonium metatungstate of total solution was selected at 0.01, 0.02 and 0.03 M. With the imperfect Z value of the selected ink and hydrophobic surface of SiO2/Si substrate, additional control in the inkjet printing is necessary. A homemade heater was therefore employed as a sample platform for a SonoPlot Microplotter inkjet printer to control the substrate temperature (monitored by a K-type thermocouple) during printing. For this experiment, the platform temperature was varied from 22 °C to 80 °C. A glass capillary is used to store the ink and attached to it is an ultrasonicating piezoelectric device that undergoes a continuous sinusoidal waveform was used to dispense the ink during the inkjet printing. Depending on the glass tip length, size, and amount of ink in the glass capillaries, the piezoelectric dispensing voltage varied typically in the range of 1-3 volts, usually 2.5 V is used with a frequency in range of 350 – 450 kHz, with the nozzle a distance of 20 µm from the surface. The electrodes with a channel length of 0.3 mm were fabricated using standard photolithography followed with e-beam evaporation of Au (40 nm)/Ti (5 nm) on SiO2 (500 nm)/Si wafers before printing. The wafers were then cleaned with a spray of DI water, acetone and isopropanol and air dried with N2 gas then treated with UV light for 30 min. prior to printing. In addition to the printing temperature and molar concentration of the ink, the number of printing scans was also varied in the range of 3 – 7 for optimal sensor thickness control. All 4 ACS Paragon Plus Environment

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printed samples were cured on a hotplate for 10 min. at 180 oC, followed with annealing at 500 o

C in air for 2 hours. Optical microscopy and scanning electron microscopy (SEM, JEOL JSM-

6380) images were taken for the analysis of the sample morphology. A KLA Tencor P16 profiler was used to obtain the printed WO3 film thickness. Raman spectroscopy was employed to confirm the crystallinity of WO3 and optical transmittance spectra were taken to confirm the band gap. For optoelectronic characterization, a CHI660D electrochemical station together with a Newport Oriel Apex Monochromator with an Oriel Cornerstone 130 1/8m monochromator filter were used to measure the current-voltage characteristic in dark and under illumination, dynamic and spectral photoresponse. Table 1. Solvent Component Properties of the WO3Pr Ink Solvent Density [g/cm3] Viscosity [mPa·s] Surface Tension [mN/m] DMF 0.944 0.794 37.1 H2O 1 0.954 72.7

Z value ~53 ~63

Results and Discussion

Figure 1. (a) WO3Pr ink forms droplets at room temperature due to hydrophobicity of SiO2/Si substrates. (b) Heated SiO2/Si substrate facilitates spreading of WO3Pr ink on SiO2/Si. (c)



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Overheated substrate results in clogging and non-uniform film. (d) Diagram of printer with arms that allow the nozzle to move in the x, y, and z-axis. Figure 1 illustrates schematically the importance of the heated printing the substrate when printing WO3Pr thin films. When the SiO2/Si substrate was maintained at 22 °C, the ink drops cannot spread effectively; instead, droplets form as shown in Figure 1a. At elevated substrate temperatures of 40-80 °C, the ink droplets spread momentarily on the substrate surface to produce a continuous film as illustrated in Figure 1b. This means the surface tension of the ink drops was reduced considerably on the heated SiO2/Si substrates. Qualitatively, the morphology of the printed film is sensitive to the substrate temperature, and if the temperature is increased significantly the printing behavior becomes unpredictable, usually resulting in spraying and clogging of the ink. In addition, there are very prominent cracks that form on the peaks and frequently in the troughs between the peaks (Figure 1c). This is likely the result of stress and strain caused by a very uneven film due to a strong coffee ring effect. In Figure 1d is depicted a diagram of the printer, which can move in the x-y-z directions along the three arms.

Figure 2. Printing three scans of the WO3Pr ink at (a) room-temperature (b) 40 oC (c) 50 oC (d) 60 oC (e) 70 oC and (f) 80 oC. The ink concentration was 0.02 M. 6 ACS Paragon Plus Environment

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Figure 2 compares surface morphology of six printed samples using the same interdigitated printing pattern at substrate temperatures of 22, 40, 50, 60, 70 and 80 oC, respectively. The ink concentration of 0.02 M and 3.0 scans were applied to all samples. At 22 oC, the WO3Pr ink could not spread well on the SiO2/Si, resulting in isolated droplets of the printed ink as shown in Figure 2a. With further increased substrate temperatures, improved ink spreading is clearly seen up to 50 °C (Figure 2b-d). However, a more prominent coffee ring effect was observed at higher temperatures.6,10 In addition, ink evaporation increased which prevented continuous films to be obtained since a significant amount of ink evaporated before even spreading (Figure 2e) and resulted also in spraying (Figure 2f). In addition, the high substrate temperature was found to also cause drying, boiling, and clogging of the capillary tip at a close proximity of the substrate. Therefore, the optimal substrate temperature window is 40-60 oC. Based on this, the rest of the paper will be focused on printing at 50 oC unless it is otherwise indicated.

Figure 3. SEM images of the printed films on substrates heated to (a) 40 oC, (b) 50 oC, and (c) 60 oC, respectively.



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The SEM images of the three samples printed at 40 oC, 50 oC, and 60 oC substrate temperature, respectively, are shown in Figure 3a-c after the post annealing treatment to form crystalline WO3. At the printing temperature of 40 oC, the resulted WO3 film is rather porous, rough and cracked in some places (Figure 3a). At higher printing temperatures of 50-60 oC, the film becomes less porous, more uniform, and instead just rough and rigid with what seem to be microscopic cracking at the surface (Figure 3b-c). The rough and rigid film comes about from the small non-uniformity of the surface of the printed film. The inkjet printing at higher temperatures does not produce uniform films due to the coffee ring effect. The coffee ring effect is caused by fluid drying at the edges of the printed line faster than the center of the printed line.6,10 This results in an early nucleation and accumulation of the ink at the edges of printed line and hence a non-uniform film. Furthermore, the non-uniform film is likely the cause of cracking at the surface while annealing. The annealing results in nucleation and reactions to occur on the outer surface of the film, this causes thicker parts of the film to harden on the outside before the center of the film producing stress and strain in the film.

Figure 4. SEM images of WO3 films and optical images of their corresponding printed WO3Pr films of (a) and (b) at 0.01M; (c) and (d) at 0.02M, and (e) and (f) at 0.03M. All samples have three printing scans and the 50 °C substrate temperature. 8 ACS Paragon Plus Environment

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Changing the ammonium metatungstate molar concentration from 0.01, 0.02, and 0.03 M is then performed to understand the concentration effect on the morphology of the printed WO3Pr and the WO3 films after the annealing. The films varied from rough and rigid films (Figure 4a-d) to being cracked (Figure 4e-f). This is likely caused by the thickness of the film. Thinner films have the better opportunity to anneal and nucleate evenly throughout the film than thicker uneven films. Looking at the thickness profiles of the different concentrations, the 0.01 M and 0.02 M show little difference in thickness after three printing scans, varying in thickness between 100-200 nm respectively (Figure S1a, b), however the three scans at 0.03 M shows an increase in thickness to approximately 400 nm (Figure S1c). To further investigate the thickness effect on the film morphology 0.02 M was chosen and printed at 3, 5, and 7 printing scans. The resulting films have film thickness of 150, 350, 500 nm for 3, 5, and 7 printing scans respectively. To test the difference in performance of the different film thickness the photoresponsivity, on-off ratio, and rise and fall times were measured on these samples of different film thicknesses. Increasing the film thickness shows an increase in photoresponsivity and decreasing on/off ratio with thickness (Table 2), while the rise time remains comparable and the fall time increases. The increasing fall time as the film thickness increases can be attributed to an increase in defects in the film. The source of defects could be the cracking of the film which is indicated in the SEM images, which show cracking of the film with increasing film thickness (Figure S2). As a result, three scans (Figure 2Sa) is chosen to be the number of printing scans to be used, since it shows to have the lease amount of cracking compared to the 5 or 7 scans (Figures S2b, c).

Table 2. Summary of Printed WO3 UV Photodetector Performance with Varying Thickness Thickness [nm] 150 350 500

Responsivity [A/W] On/Off Ratio 0.024 0.170 0.245

3538 11 4

Rise Time [s] Fall Time [s] 24 37 35

94 173 168



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Figure 5. The current-voltage and dynamic characteristic curves for the 50 oC are shown in (a) and (b) respectively along with the responsivity as a function of power and wavelength in (c) and (d) respectively. Note that in (a) the dark current is scaled by 1000 for clarity. The photovoltaic properties of the WO3 printed film is taken at a wavelength of 360 nm, since the bandgap for WO3 is ~3.2 eV, which is around 380 nm, a slightly smaller wavelength is chosen to account for binding and kinetic energy of the excitons. In Figure 5a is the currentvoltage characteristic curve (IV curve) and depicts the linearity of the IV curves, which means the film follows Ohms law and has a constant resistance, indicating a stable film under light with a power of 4.93 µW. The on/off ratio of the IV curve depicted Figure 5a is the photocurrent (Iph) divided by the dark current (Idark), where Iph is defined as the difference between the current through the device under light (Ilight) and the dark (Idark). Based on the data in Figure 5a, the Iph/Idark on/off ratio obtained is about 3538, which is remarkable for practical applications requiring a high signal-to-noise ratio. Figure 5b depicts the dynamic photoresponse under UV light of 360 nm at a power of 4.93 μW. A rise time of 24 s and fall time of 94 s can be observed. Typically, the range of the rise time is 25-30 s and that for the fall time is 90-120 s on the printed WO3 devices. The power of the light was varied and the photoresponsivity was calculated for the


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respective light power at a wavelength of 360 nm, the results are shown in Figure 5c. The photoresponsivity was calculated by taking Iph and dividing by the power (P) of the light on the channel, so R = Iph/P. The photoresponsivity decreases inversely as the power increases since the number of charges generated in the crystal is reaching saturation and Iph reaches a constant value, resulting in a high quantum efficiency at lower light intensities as observed in other photodetecors.30,35 The highest photoresponsivity obtained in the printed WO3 UV detector is 2.70 A/W at 5 V bias (~0.54 A/W∙V) at a power of 0.27 μW, which is the best so far achieved on WO3 UV detectors. This photoresponsivity is better than recent reported results of WO3 films of 0.17 μA/W (~0.28 μA/W∙V),36 and ~0.29 A/W (~0.056 A/W∙V).37 Figure 5d shows the spectral responsivity on which the band edge begins around 375 nm which is right around the expected band edge of 380 nm for WO3 meaning the crystal obtained is indeed tungsten oxide. Inset in Figure 5d is the optical transmittance spectra of a uniform WO3-NP film on fused silica. A low absorption to visible light was observed with a sharp cut-off below ≈450 nm, corresponding to the intrinsic band gap of WO3 at ≈2.8 eV which is in agreement with the spectral response in Figure 5d.38

Conclusion A mixed solution of ammonium metatungstate ((NH3)6H2W12O40), Dimethylformamide (DMF) and water was employed as a tungsten oxide precursor ink for inkjet printing of WO3 thin film UV detectors. In order to address the issues stemming from the non-ideal viscosity of the ink and hydrophobic surface of SiO2/Si, a systematic study of substrate temperature during printing, molar concentration of ammonium metatungstate and number of printing scans was conducted to identify the optimal parameter window for printing high-performance WO3 thin film UV detectors. We have found that substrate heating in the temperature range of 40-60 oC allows fairly uniform spreading of the WO3Pr ink on SiO2/Si. At lower temperatures, unconnected ink droplets formed on the substrates while at higher temperatures, the accelerated solvent evaporation prevents uniform spreading of the ink and also causes the inkjet nozzle clogging. In addition, the precursor concentration of 0.02 M allows the best control on the microstructure and film thickness. In the concentration range of 0.01-0.03 M, the printed film thickness increases



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with the precursor concentration and the number of printing scans. While smaller specific thickness (per scan at 0.01 M) is less efficient to achieve optimal thickness in the range of 200400 nm, a larger one at 0.03 M or with higher number of scans beyond 3 scans could lead to cracking. At a wavelength of 360 nm and a power of 4.93 μW an on/off ratio of 3538 and rise and fall times of 24 s and 94 s are found respectively. The highest photoresponsivity obtain is 2.70 A/W at a UV power of 0.27 μW. This is considerably better than that reported previously on WO3 thin films detectors. Aside from photodetection further work can now be done to implement the WO3 printed films in gas detection and electrochromic devices. This work therefore lays the foundation for multi-sensorial printed WO3 films which can be integrated onto Si-based microelectronics.

ASSOCIATED CONTENT Supporting Information The supporting information contains profiles of the thicknesses at different ammonium metatungstate concentrations, and the SEM of different printing scans. In addition there is AFM images and responsivity as a function of thickness is included as well. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (B.C.). *E-mail: [email protected] (J.W.).

Acknowledgements This research was supported by Plant Directed Research and Development funds from the Department of Energy’s National 400 Security Campus, operated and managed by Honeywell Federal Manufacturing and Technologies, LLC under Contract No. DE-NA0002839. J.W. acknowledges support in part by ARO Contract No. ARO-W911NF-16-1-0029 and NSF Contract Nos. NSF-DMR-1337737 and NSF-DMR-1508494. 12 ACS Paragon Plus Environment

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