Interface Nanojunction Engineering of Electron-Depleted Tungsten

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Interface Nanojunction Engineering of Electron-Depleted Tungsten Oxide Nanoparticles for High-Performance Ultraviolet Photodetection Qingfeng Liu, Brent Cook, Karen Shi, Jackson Butler, Keifer Smith, Maogang Gong, Dan Ewing, Matthew Casper, Alex Stramel, Alan Elliot, and Judy Z. Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00217 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Nano Materials

Interface Nanojunction Engineering of Electron-Depleted Tungsten Oxide Nanoparticles for High-Performance Ultraviolet Photodetection †

†

†

†

†

†

Qingfeng Liu,* Brent Cook, Karen Shi, Jackson Butler, Keifer Smith, Maogang Gong,* , Dan Ewing,

‡

Matthew Casper,‡ Alex Stramel,‡ Alan Elliot, ‡ and Judy Wu*† †

Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA

‡

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

ABSTRACT: This work reports a general and facile route, i.e., thermal decomposition of precursor followed by ultrafast thermal annealing (TDP-UTA), to in situ fabrication of nanojunction-interlinked tungsten oxide (WO3) nanoparticle (NP) networks for extraordinary ultraviolet (UV) photodetection. TDP leads to a spin-coated ammonium metatungstate thin layer to in situ self-assemble into a highly crystalline WO3-NP mesoporous film on SiO2/Si substrates with prepatterned electrodes. The as-syntheized WO3-NPs have dimension comparble to the Debye length (≈43 nm), which is critical to the optimal electron depletion effect for high gain in photodetection. UTA creats the NP-NP interface nanojunctions between neighboring WO3-NPs, which is the key to high-efficiency electron transport with minimized charge recombination in optoelectronic processes. The photodetectors based on such nanojunction-interlinked WO3-NP networks exhibit photocurrent to dark current ratio of 5600, the highest value for any WOx-based photodetectors ever reported. Moreover, the obtaned photoresponsivity is up to 139 A/W (or 27.8 A/W-V) upon 360-nm illumination, which is over one order of magnitude higher than that of any previously-reported WOx-nanostructure film photodetectors. These results demonstrate the TDPUTA route is a low-cost, robust and scalable pathway to in situ fabrication of interlinked semiconducting-nanostructure networks for high-performance optoelectronics and sensors. KEYWORDS: Tungsten oxide, in situ fabrication, nanojunction, thermal decomposition, ultrafast thermal annealing.

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■ INTRODUCTION Owing to the high-crystalline microstructure, increased surface to volume ratio and quantum confinement effects, semiconducting nanostructures can enhance the device performance of the functional material and provide with unique properties that do not exist in its bulk form.1-3 In the family of semiconductors, nanostructured tungsten oxide (WOx) with n-type conductivity has received increasingly attention due to its unique characteristics and versatile functionalities such as structural flexibility, switchable optical properties, catalytic behavior, electrochromic properties, and gas-sensing properties.4-6 For example, nanostructured WOx is the most known electrochromic and photochromic material used in today’s ‘smart’ windows due to its exceptional ability to change color easily,7,8 and also acts as a popular and well-studied material for the development of various gas sensors.9,10 However, for ultraviolet (UV) photodetection research, nanostructured WOx has received relatively less attention when compared to other ntype semiconducting metal oxide materials such as ZnO and TiO2. Actually, the application of WOx in photodetection should be encouraged, since it offers comparable structural and functional properties to ZnO and TiO2 such as wide band gap and intrinsic oxygen-deficient defects. Despite exciting progress made in recent years,11-21 achieving high-performance WOxbased photodetectors is still challenging up to now. For example, most photodetectors built on individual single-crystalline WOx-nanostructures such as nanoparticles (NPs), nanowires, nanobelts and nanosheets exhibit a low photocurrent (IPh)-to-dark current (IDark) ratio below 2000.11-16 Other photodetectors built on WOx-nanostructure-constructed thin films have low photoresponsivity (R) typical below 0.25 A/W-V.17-21 Such photoresponse performance suggests that the WOx-nanostructure film photodetectors suffer from a serious charge recombination mainly due to the poor nanostructure-nanostructure connections. This presents a major challenge in the development of high-performance optoelectronics based on WOx-nanostructure thin films. Very recently, we have successfully developed a simple and general route, i.e., thermal decomposition of precursor followed by ultrafast thermal annealing (TDP-UTA), to in situ fabrication of the nanojunction-interlinked ZnO-NP networks for extraordinary photoresponse performance.22 The nanojunction-interlinked ZnO-NP networks exhibit rise/decay time of 9.4 s/13.5 s, IPh/IDark ratio of 3.1×105 and photoresponsivity of 15.9 A/W-V, which is the highest overall performance for any ZnO-nanostructure film photodetectors ever reported.22 Such


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photoresponse performance is mainly due to a multiscale microstructure control: (1) atomic-scale surface optimization of the ZnO-NPs with desired dimension comparable to the Debye length, which is critical to taking full advantage of the optimal electron depletion effect to achieve high gain; (2) interlink of crystalline ZnO-NPs via interface nanojunctions, which is beneficial to the minimized charge recombination; and (3) macroscopic device scale interlinked ZnO-NP networks, which facilitate high-efficiency electron transport in optoelectronic processes.22 In order to address the challenge in photodetection application of nanostructured WOx, we further extend this TDP-UTA route to in situ fabrication of WOx-based UV photodetectors. After TDP at 550 oC, a spin-coated ammonium metatungstate precursor thin layer on SiO2/Si substrates with prepatterned electrodes self-assembles into a highly crystalline WO3-NP mesoporous film. The as-produced WO3-NPs have desired dimension comparable to the Debye length (≈43 nm), which is critical to achieve the optimal surface electron depletion effect for high gain in optoelectronic processes. UTA at 700 oC was subsequently employed to enable the formation of NP-NP interface nanojunctions between neighbouring WO3-NPs, which is beneficial to high-efficiency electron transport with minimized charge recombination in optoelectronic processes. The photodetectors based on such nanojunction-interlinked WO3-NP networks exhibit extraordinary performance with photoresponsivity of up to ~139 A/W, detetivity of 4.2×1014 Jones, rise/decay time of 25 s/45 s and IPh/IDark ratio of ~5600 so far reported. Our results therefore demonstrate the TDP-UTA route paves the way for large-scale fabrication of the WO3-NP (or in general WO3 nanostructure) devices with high performance in optoelectronics and sensors.

■ EXPERIMENTAL SECTION The ammonium metatungstate precursor was dissolved in DMF/water (volume ratio of 7:1) with a concentration of 0.02 M to make a clear solution by sonication. A thin precursor films was deposited on the SiO2/Si substrate with prepatterned Au/Ti (40/5 nm) electrodes (channel length =0.3 mm) by spin coating at a rotation speed of 3000 rpm for 1 min, followed by curing at 180 o

C for 10 min. This process was repeated five times to form a uniform precursor layer in


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thickness of ~100 nm. Finally, the substrate was annealed at 550 °C for 60 min in air with a controlled ramp rate of 2 oC/min, followed by UTA at 700 oC for a short time of 2 s. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using FEI Tecnai F20 XT Field Emission TEM. Atomic force microscope (AFM) images were obtained by using an Agilent 5500 AFM (Agilent Technologies, Tempe, AZ). XPS spectra were measured by PHI 5000 Versa Probe ll Scanning XPS Microprobe. The transmittance spectra were obtained with Varian 50 Bio UV-Visible Spectrophotometer. The current-voltage (I-V) characteristics of the WO3-NP film photodetectors was measured by using a CHI660D electrochemical workstation. Spectral responses at different wavelengths were recorded by using an Oriel Apex Monochromator illuminator. ■ RESULTS AND DISCUSSION The TDP-UTA route to in situ fabrication process of nanojunction-interlinked WO3-NP networks is schematically shown in Figure 1a. The decompostion of ammonium metatungstate started at near 140 oC and fully completed at near 500 oC.23 The curing temperature was selected at 180 oC in order to initiate the formation WO3 nuclei throughout the multilply spin-coated precursor film from the initial decomposition of ammonium metatungstate. This is the key to the formation of uniform mesoporous NP thin film during the TDP process, as demonstrated by our previous report.22 During TDP at 550 oC for 1h over the full decompostion temperature (~500 o

C), the WO3-NPs further initiated and grew up from these nuclei, and ultimately in situ

assembled into a thin mesoporous WO3-NP film with uniforom particle size (Figure S1a, Supporting Information). This confirms the critical role of the layer-by-layer coating and curing process of ammonium metatungstate precursor in controlling the WO3-NP nucleation and evolution. In the TDP process, the temperature ramp rate plays an improtant role in the formation of continuous WO3-NP thin film. Fast temperature ramp rate over 5 oC/min led to many cracks of the WO3-NP thin film (Figure S1b, Supporting Information). Meanwhile, higher contration solution of ammonium metatungstate precursor also easily resulted in many cracks of the WO3NP thin film (Figure S1c, Supporting Information) during the TDP process. UTA at 700 oC for 2s leads to the formation of the interlinked WO3-NP networks by fusing neighboring WO3-NPs through NP-NP nanojunctions, as shonw in Figure 1b.


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Figure 1. (a) Schematic of the TDP-UTA route to in situ fabrication of the mesoporous nanojunction-interlinked WO3-NP photodetectors. (b) SEM image of the surface morphology of the resulting WO3-NP networks. (c) TEM and (d) HRTEM images of the as-synthesized WO3NPs. (e) HRTEM image of the NP-NP interface nanojunction between the neighboring WO3NPs.

Based on the TEM studies (Figure 1c-d), the diameter of the majority of the interlinked WO3NPs with clean surface is mainly in the range of 15-30 nm, which is comparable to the Debye length of WO3-NPs (~43 nm).24 For TEM studies, the WO3 nanoparticles were scraped off from the substrate and then sonicated in ethanol for 30 min. Then, a droplet was applied on a TEM grid for TEM observation. Such preparation method of TEM specimen is not expected to affect the sample’s original particles size, as illustrated in the comparison of the SEM in Figure 1b and TEM images in Figure 1c-d. Figure 1e reveals that the neighboring WO3-NPs are highly crystalline and interlinked by crystalline interface nanojunctions after UTA. The measured spacing between two neighbored lattice fringes is ~0.38 nm, conresponding to the lattice spacing of the (200) plane of monoclinic WO3.14 It should be noted exposuring WO3-NPs to excessive heat for a short period of 2 s not only maintains their original dimension and morphology, but also avoids the damage to the electrodes and circuits.22 Clearly, this TDP-UTA route pushes


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forward the in situ preparation of WOx-based photodetectors. In stark contrast, the previous exsitu fabrication methods of WOx-based photodetectors have to involve tedious WOxnanostructure synthesis, cumbersome separation and dispersion, and meticulous assembly on substrates with prepatterned electrodes.11-21 For example, Shukla et al reported an aqueous solution reaction process to synthesize the WO3-NPs after a long time of 3 days.24

Figure 2. (a-b) XPS spectra of (a) O1s and (b) W4f of the nanojunction-interlinked WO3-NP networks by the TDP-UTA route. (c) UV-visible transmittance of the WO3-NP networks with (red) and without (black) UTA treatment.

In order to clarify the chemical structure and surrounding of the as-synthesized WO3-NPs, XPS measurement was performed on the resulting nanojunction-interlinked WO3-NP networks, as shown in Figure S2 (Supporting Information). The XPS wide survey spectrum exhibits that only O and W were detected with the atomic ratio of 3 in the sample. Figure 2a-b shows the high resolution XPS spectra of the O1s and W4f core-level peaks, respectively. The O1s peak at ~530.5 eV corresponds to the lattice oxygen in WO3, while W4f spectrum exhibits two dominant peaks of W4f5/2 (37.9 eV) and W4f7/2 (35.8 eV), confirming the representative of W state with an oxidation state +6. These measured binding energies of O and W are consistent with those of stoichiometric WO3 crystals.25 Figure 2c compares the optical transmittance of the WO3-NP sample fabricated on fused silica before and after the UTA treatment. It illustrates high transparency to visible light with negligible absorption at 380-420 nm (the plateau on the right of a broad shoulder at around 380-420 nm) or longer wavelength. The much enhanced absorption at


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short wavelengths on the left of the shoulder is due to the bandgap of WO3 at ~3.3 eV.5 This result suggests high crystallinity of WO3-NPs in addition to the unique mesoporous film morphology and the non-scattering nature of the WO3-NPs.

Figure 3. Schematic of the energy band diagram (top) and the photoresponse mechanism (bottom) of the nanojunction-interlinked WO3-NPs based on the adsorption/desorption of oxygen molecules on the WO3-NP surface (a) in dark and (b) on UV illumination. Green and yellow dots are electrons and holes, respectively.

It is known that WOx is of n-type conductivity with intrinsic oxygen vacancies as shallow donors as ZnO. The photoresponse mechanism of the WO3-NPs can be explained by the schematic model shown in Figure 3. In dark, oxygen molecules are adsorbed onto the WO3-NP surface via capturing free electrons [O2(g) + e− → O2–(ad)] from its conduction band to form oxygen ions (O2-), thereby creating band bending and a low-conductivity electron depletion layer on the WO3-NP surface (Figure 3a). When the WO3-NPs are exposed to light illumination with photon energy larger than the semiconductor band gap, electron-hole pairs are photogenerated [hv → e− + h+]. Due to the potential slope produced by band bending, the holes migrate to the WO3-NP surface and are trapped by the negatively charged oxygen ions [h+ + O2–(ad) → O2(g)], resulting in the desorption of oxygen from WO3-NP surface (Figure 3b). Therefore, light illumination decreases the depletion layer thickness and increases the free carrier concentration of the nanojunction-interlinked WO3-NP networks. When compared to physically contacting WO3-NPs, the interface nanojunctions of the interlinked WO3-NP networks can significantly


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reduce the inter-NP potential barrier and hence decrease charge recombination through transport process, resuling in high photocurrent and responsivity (corresponding results shown below). Figure 4a shows the typical I-V curves of the nanojunction-interlinked WO3-NP photodetector in dark and upon 360-nm illumination of 0.15 mW/cm2. The photodetector is very resistive in dark, with a resistance of ~1.3 GΩ at room temperature. Remarkably, the illuminated current (ILight) increased over 3 orders of magnitude upon 360-nm illumination, indicating a highly UVsensitive photoconduction. Meanwhile, the nanojunction-interlinked WO3-NP mesoporous networks exhibit IPh/Idark ratio of 5600, which is the highest for any previously-reported WOxbased photodetectors.11-21 Figure 4b shows the time-resolved photocurrent of the nanojunction-interlinked WO3-NP photodetector in response to the on and off of 360-nm UV illumination. The reproducible response of the photocurrent suggests the excellent stability of the device. Rise (decay) time was calculated from the time required for the current from 10 % to increase to 90 % (decrease from 90 % to 10 %) of the maximum photocurrent value. The photodetector exhibits rise time of 25 s and decay time of 45 s (Figure 4c), both of which are faster than those of the previously-reported WOx-nanostructure film photodetectors.18,19 This suggests the TDP-UTA process developed in this work yields high crystalline WO3-NPs with negligible defects as deep charge traps. For a photodetector, photoresponsivity (R) is the ratio of the photocurrent to the incident light power on the effective area of a photoconductor, which can be expressed as follows:22 R = IPh/[PPh(1-T)]

(1)

where IPh (IPh=ILight-IDark) is the photocurrent, PPh is the incident illumination power on the effective detection area of the photodetector, and T is the transmission of the light through the WO3-NP film.


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Figure 4. (a) I-V characteristics of the nanojunction-interlinked WO3-NP photodetector in dark and upon 360-nm illumination of 0.15 mW/cm2. (b) Dynamic photoresponse and (c) an enlarged dynamic photoresponse circle of the photodetector to the 360-nm UV illumination of 0.15 mW/cm2. (d) Spectral photoresponsivity of the nanojunction-interlinked WO3-NP photodetector. (e) Dynamic photoresponse at different incident 360-nm UV illumination powers. (f) Photocurrent and photoresponsivity as function of the 360-nm UV illumination power density.

Figure 4d shows the spectral photoresponsivity of the photodetector under the light excitation with wavelengths ranging from 300 to 500 nm. It can be seen that the response spectrum is nearly coincident with the transmittance spectrum both in the absorption edge and in the long wavelength direction, reflecting the high selectivity and the energy band structure of the nanojunction-interlinked WO3-NP networks. For the incident light with wavelengths longer than 460 nm, negligible photoresponse was observed in the nanojunction-interlinked WO3-NP networks, because light has not enough photon energy to excite electrons from the valence band to the conduction band and thus contributes little to the photocurrent. With decreasing light wavelengths from 460 nm, the photoresponsivity of the photodetector exhibits a sharp increase by about four orders of magnitude, reaching its maximal value at ~360 nm, and then a gradual


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decrease from ~360 nm. Such photoresponse characteristics were also coincident with the previous

observations

in

the

WOx-based

photodetectors.12-14,26-28

The

decrease

of

photoresponsivity on the shorter wavelengths from ~360 nm is attributed to the enhanced absorption of higher energy photons near the surface region of the semiconducting WO3-NPs, where the photogenerated electron-hole pairs typically have a lifetime shorter than those in the bulk, contributing less to the photoconductance.26

Table 1. Comparison of the characteristic parameters of the WOx-based photodetectors ever reported. Photodetectors WO3-NP thin film

Wavelength Bias nm V 360 5

IPh/IDark ratio 5600

Rise/decay time Responsivity Reference s A/W (A/W-V) 25/45 139 (27.8) This work

WO3 thin film

370

-

-

48/48

WO3 nanoshale film

335

20

400/>400

0.0268 (-)

[18]

365

1

60

-

-

[17]

WO3 monolayer

320

1

-

0.04/0.04

0.329 (0.329)

[16]

Single WO3 nanosheet

365

5

2000

0.04/0.08

293 (58.6)

[15]

Single WO3 nanobelt

405

5

1000

-

Single WO3 square

365-577

5

20

Single WO3 nanowire

375

10

Single WO3 nanowire

312

1

2.6×10

5

[20]

[14]

4

0.05/0.05

(5.2×10 ) -

[13]

Interface Nanojunction Engineering of Electron-Depleted Tungsten

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