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Broadband High Sensitivity ZnO Colloidal Quantum Dots / Selfassembled Au Nano-antennas Heterostructures Photodetectors SiSi Liu, Ming-Yu Li, Dong Su, Muni Yu, Hao Kan, Huan Liu, Xian Wang, and Shenglin Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09442 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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ACS Applied Materials & Interfaces

Broadband High Sensitivity ZnO Colloidal Quantum Dots / Selfassembled Au Nano-antennas Heterostructures Photodetectors

Sisi Liua, Ming-Yu Li a*, Dong Sua, Muni Yua, Hao Kanb, c, Huan Liua, Xian Wanga, Shenglin Jiang a*

a School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China. b Shenzhen Key Laboratory of Advanced Thin Films and Application, College of Physics and Energy, Shenzhen University, 518060, Shenzhen, China c Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China * Corresponding author.

E-mail address: [email protected] [email protected]

ABSTRACT: Tunable plasmonic resonance induced by the collective oscillation of the electrons on metallic nanostructures can excellently enhance the light response of the ZnO films, which provide an effective way to break through the limitation of the performance of the ZnO photodetectors. Here, broadband high-performance ZnO/Au heterostructures photodetectors with various morphologies of the self-assembled Au nano-antennas are fabricated via a facile approach under the spin-coated ZnO colloidal quantum dots films. With a systematic control on growth condition, the self-assembled Au nano-antennas undergo a drastic evolution from the corrugated nano-mounds to the island-like nanostructures, and the light absorption of the 1

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resulting ZnO/Au heterostructures correspondingly exhibits a strongly morohological dependence on the Au nano-antennas. Meanwhile, the photoresponse of the ZnO-based photodetectors is significantly improved throughout a wide spectrum between UV and visible regions owing to the enhanced light absorption induced by the localized surface plasmon resonance. As a result, the optimal switch ratio of the ZnO/Au heterostructures photodetector increases by one order to ~1.13×105 than that of pristine ZnO one because of the obviously increased photocurrent (Iph) and comparable dark current, thus leading to ~9.1 and ~4.9 times increases in the photoresponsivity and the normalized detectivity. Meanwhile, the significant increases in the Iph of ~5.2 and ~9.7 times are likewise observed with the ZnO/Au heterostructures under 530 nm and white light illumination. This work can offer a handy and effective approach for the frabrication of ultra-sensitive ZnO-based photodetectors within a broadband wavelength by utilizing the Au plasmonic nanostructures.

KEYWORDS: ZnO/Au heterostructures photodetectors, Plasmonic enhancement, Broadband photoresponse, ZnO colloidal quantum dots, Self-assembled Au nano-antennes

1.INTRODUCTION Over the past decades, with successful synthesis of wide bandgap semiconductors (i.e. GaN, SiC, ZnO, and ZnS

1-4

), ultraviolet (UV) photodetectors have attracted extensive

attentions owing to their wide applications in commercial and military fields, such as missile launch detection, space and astronomical researches, fire monitoring, UV radiation calibration and monitoring, optical communication, and so on 1, 5-8. Among these materials, ZnO has been regarded as a competitive candidate due to its direct bandgap at 3.37 eV, large exciton binding energy of 60 meV, low cost, and low environmental impact

9-12

. Particularly, ZnO

nanostructures with a high surface-to-volume ratio could guarantee a high responsivity and photoconductivity gain for the photodetectors, which can be caused by formation of a relatively larger low-conductivity depletion layer as a result of abundant oxygen molecules 2


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absorbed on the enlarged surface area of the nanostructures

13, 14

. However, nanostructures

commonly suffers from a narrowed response bandwidth than the bulk materials due to quantum confinement effect

13

. Thus, several efforts have been tried to extend the response

spectra of the ZnO photodetetctors by atomic doping 15, 16 and synthesis of the composites 17-19. For example, Kouklin has intentionally introduced Cu into ZnO nanowires photodetector to achive a broadband photoresponse up to the visible region

20

. Zhang et al. developed a

photodetector based on ZnO-CdS core-shell micro/nanowire to broaden the spectral range from 372 to 548 nm

21

. However, these methods unfortunately gave rise to unsatisfactory

device performance 22, 23 or need to consider the complex lattice matching 21. Therefore, it has become a great challenge to find an effective way for the fabrication of the high performance ZnO photodetectors within broadband spectra. Noble metal nanoparticles (NPs) can intensely interact with the incident light to form a near surface electric filed by collective oscillations of the electron on the metal NPs, in turn, provide an effective approach to enhance light absorption of the photodetectors at various wavelengths via localized surface plasmon resonance (LSPR) metallic NPs including Au

27

24-26

. Up to date, various

and Ag 28 was employed on the surface of ZnO to optimize the

performance of the photodetectors at a certain wavelength owing to the their configuration dependent response. For instance, Wang et al. have demonstrated an enhanced photoresponsivity for the ZnO photodetector at ~ 380 nm with Ag NPs decorated on its surface via magnetron sputtering technique

29

. However, given that metallic NPs were

commonly achieved with a fixed configuration by defining the as-deposited metallic layers via sputtering through a complicated process, the resulting metallic NPs can only respond at a fixed wavelength with mechanical instability due to the relatively week bindings between NPs and substrates, which is not suitable for industrial applications even regardless of the tremendous costs. Herein, we demonstrate a novel approach to prepare ZnO/Au heterostructures photodetectors with the self-assembled Au nano-antennas of high mechanical stability under 3



ZnO colloidal quantum dots (CQDs) films via solid-state dewetting for the first time, aiming to improve the films absorption over a broadening response spectra range. Depending on the morphological evolution of the resulting Au nano-antennas, the photoresponse of the photodetetctors correspondingly develop due to the enhanced light absorption by the configuration tunable LSPR. After the incorporation of the island-like Au nano-antennas, the photoresponsivity and the normalized detectivity are significantly improved by ~9.1 and ~4.9 times respectively when compared with the pristine ZnO photodetector and the optimal switch ratio (Iph/Id) reaches at ~1.48×105. Moreover, the enhancement of photoresponse for the ZnO/Au heterostructures photodetectors is successfully extended to the visible regions as result of the broadband response of the Au nano-antennas.

2 METHODS 2.1 ZnO CQDs Synthesis ZnO CQDs were synthesized by the solvothermal method according to our previous work

30

. In brief, 4.46 mmol zinc acetate dihydrate (Zn(CH3COO)2, 99.99%, aladdin) was

dissolved into 42 mL methanol and heated to 60 °C. 7.22 mmol potassium hydroxide (KOH, 85%, aladdin) was dissolved into 23 mL methanol, and gradually dropped into the zinc acetate methanol solution within 4 ~ 6 min. The mixed solution maintained at 60 °C for 2.25 h with stirring, and was subsequently washed by the precipitation method for twice. The nearspherical ZnO CQDs were synthesized with a diameter of ~9 nm as evidenced with the transmission electron microscope (TEM) image in Figure 1(a). As shown with high resolution TEM (HRTEM) image in Figure 1(b), the lattice fringes with interplanar spacing of 0.26 nm and 0.24 nm corresponding to the (002) and (101) crystal planes of wurtzite ZnO. The six diffraction rings were clearly observed with the selected area electron diffraction (SAED) pattern in Figure 1(c), suggesting a high degree crystallinity. As shown in Figure 1(d), the characteristic diffraction peaks of ZnO CQDs was detected with X-ray diffraction (XRD) spectrum, verifying the hexagonal wurtzite structure 5. Finally, the obtained product was 4


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dispersed in a mixed solvent of chloroform and methanol (6 mL, 2:1 by volume) by ultrasonic for 5 min, and the absorbance spectrum is shown in Figure 1(e). 2.2 Au nano-antennas fabrication In this work, the self-assembled Au nano-antennas were fabricated on glass substrates under various growth conditions. Prior to the fabrication, the glass substrates were successively treated with ultrasonication cleaning in acetone and ethanol for 10 min to remove the contaminants on the surface, and were degassed at 500 °C for 30 min under a vacuum below 1×10-4 Torr to desorb water and other chemicals on the surface. To subsequent, an identical amount of Au was deposited on the substrate with ionization current of 65 mA for 60 s in an ion-coater. After the deposition, samples were systematically annealed at a various temperatures for 900 s with a ramping rate of 10 °C/s in a rapid thermal annealing furnace (OTF-1200-4-RTP, He Fei Ke Jing Materials Technology Co., Ltd., China), and directly quenched down to the room temperature. To investigate the effect of background gas and annealing temperature, samples were annealed at 650 °C under N2 (Au-1) and vacuum below 1×10-4 Torr (Au-2), and at 700 °C under N2 (Au-3) and vacuum below 1×10-4 Torr (Au-4), respectively. 2.3 Au nano-antennas/ZnO CQDs photodetector fabrication As shown in Figure 1(f), ZnO CQDs films were deposited by layer by layer spincoating at 2000 rpm for 30 s on the glass substrates with self-assembled Au nano-antennas. To subsequent, the samples were annealed at 250 °C for 1 h in a tube furnace. Finally, Au electrode were deposited on the surface of samples by thermal evaporation utilizing a shadow mask. According to the types of Au nano-antennas, the devices were marked as ZnO/Au-1, ZnO/Au-2, ZnO/Au-3, ZnO/Au-4, respectively. Each ZnO thin films was with a comparable thickness of ~420 nm as witnessed by the scanning electron microscope (SEM) in Figure S1.

5



Figure 1. (a) - (c) Transmission electron microscope (TEM), high resolution TEM (HRTEM) and selected area electron diffraction (SAED) images of the synthesized ZnO CQDs. (d) - (e) X-ray diffraction (XRD) and UV-visible absorption spectra of the ZnO CQDs. (f) Scheme of the fabrication process for the ZnO/Au heterostructures photodetector.

2.4 Characterization The crystallographic information of ZnO CQDs was determined by using an X-ray diractometer (XRD, 7000S/L, Shimadzu Corp., Japan). The morphology of ZnO CQDs was characterized by using a high-resolution transmission electron microscope (HRTEM, JME2100(HR)). Atomic force microscopy (AFM) (Demension EDGE, USA) was employed for the characterization of the resulting Au nano-antennas’ surface morphology under a non6


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contact (tapping) mode. The absorption and transmittance spectra of the Au nano-antennas and Au nano-antennas/ZnO CQDs heterostructures were recorded with a UV-VIS-NIR spectrophotometer (UV 3600 Plus, Japan) over the wavelength range of 310 ~ 700 nm. Photoluminescence of the Au nano-antennas/ZnO CQDs heterostructures were carried out by a Raman microscope with an excitation laser of 325 nm (LabRAM HR800, Horiba JobinYvon Corp., France). Scanning electron microscope (SEM) was employed for characterization of the thickness of the ZnO thin films (GeminiSEM 300, Carl Zeiss Microscopy GmbH, Corp., Germany). The elements analysis of the Au nano-antennas/ZnO CQDs heterostructures were performed by energy dispersive spectroscopy (EDS) (GeminiSEM 300, Carl Zeiss Microscopy GmbH, Corp., Germany). Photodetector device performance was measured by semiconductor device analyzer (Agilent techhnologies B1500A, America) inside an optically and electrically sealed box. Lighting was generated through a functional generator (Agilent 33210A) controlled light-emitting diode.

3. RESULTS AND DISCUSSION Figure 2 shows the surface mophological evolution of the self-assembled Au nanoantennas under various growth conditions. The surface morphologies of the samples fabricated at various condition are revealed with the AFM side-views in Figure 2(a) - 2(d), and the corresponding size distribution of the islands-like Au nano-antennas is summarized in Figure 2(b-1) and 2(d-1), respectively. The localized topologies of the resulting samples are shown with AFM side-views and top-views of 5 × 5 µm2 in Figure S2(a) – S2(d) and S2(a-1) – S2(d-1), and the cross-sectional line-profiles of the samples are correspondingly provided in Figure S2(a-2) – S2(d-2). In general, the aggregation of the self-assembled Au nano-antennas sensitively responded to the annealing temperature and background gas, which in turn resulted in development of the configuration and distribution of the Au nano-antennas. Specifically, at an identical annealing temperature of 650 °C, the island-like Au nanoantennas evenly formed under vacuum as shown in Figure 2(b), nevertheless, the Au adatoms 7



Page 8 of 37

agglomerated into corrugated nano-mounds under N2 as shown in Figure 2(a). As illustrated in Figure 2(e), in a thermodynamic system, the Au adatoms were activated to randomly nucleate at lower energy sites with thermal energy supply, and the surface diffusion length of Au adatoms (LD) can be given with 31 LD =DS t

(1)

where DS is the surface diffusion coefficient and t is the residence time of adatoms on the substrate. The DS can be expressed with 32

Figure 2. Atomic force microscope (AFM) side-views (10×10 µm2) of Au nano-antennes fabricated at 650 °C (a) under N2 (Au-1) and (b) under vacuum (Au-2), at 700 °C (c) under N2 (Au-3) and (d) under vacuum (Au-4). (e) Schematics of the formation of the self-assembled Au nano-antennas. (f) Root-mean-squared roughness of the samples.

8


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DS ∝ exp (- EAu ⁄kT )

(2)

, where EAu is the activation energy, k is the Boltzmann contant, and T is the annealing temperature. Given that the EAu is a function of the pressure

33

, the Au adatoms tended to

require more thermal energy for diffusion under N2, which resulted in the formation of the corrugated nano-mounds due to the limited diffusion area as shown in Figure S2. With the increased T, the Au adatoms obtained a broadened LD, leading to the separation of the Au corrugated nano-mounds owing the Rayleigh instability

34, 35

as shown in Figure 2(c).

Meanwhile, for the island-shaped Au nano-antennas, the bigger one can gradually grow bigger at an expense of the smaller ones to minimize the Gibbs energy of the thermal dynamic system 34 as clearly witnessed in Figure 2(d). As a result, the isolated Au islands with slightly decreased size evolved from the corrugated nano-mounds owing the enhanced surface diffusion as shown in Figure S2(a-2) and S2(c-2). With sufficient thermal energy, the size expension of the island-like Au nano-antennas can be obviously witnessed with the crosssectional line-profiles in Figure S2(b-2) and S2(d-2), and the average height (AH) and diameter (AD) increased by ~19.4% and ~38.2% as shown in Figure 2(b-1) and 2(d-1). To compensate the size expansion, the average density decreased from ~1.68×108 cm-2 to ~0.8 ×108 cm-2 as summarized in Table S1. Correspondingly, depending on the thermal energy supply, the morphological evolution underwent two phases: With insufficient thermal energy, the Au nano-antennas had a tendency to separate into smaller ones with the increased temperatures due to the limited surface diffusion, resulting in the 39.5% decrease in RMS roughness as shown in Figure 2(f). Providing with sufficient thermal energy, the RMS roughness increased from 37 to 42.1 as a function of temperature because of the aggregation of the Au nano-antennas. Figure 3 shows the energy dispersive spectroscopy (EDS) element analysis of ZnO/Au-4 heterostructures with the well-defined Au nano-antennas under the ZnO CQDs layers. As shown with the scanning electron microscope (SEM) image in Figure 3(a), the white spots evenly distributed, suggesting a homogenous distribution of the resulting Au 9



nano-antennas under the ZnO CQDs layers. Regardless of the Au Mα1 (at 2.12 keV) and Au Lα1 (at 9.60 keV) peaks, more noticeable signals were acquired from the area with the nanostructure (area A) when compared with the area without the nanostructure (area B), indicating that the Au adatoms uniformly nucleated for the nano-antennas rather than stuck on the surface as evidenced in Figure 3(b) and 3(c). The quantitative element analysis for the area A and B are provided in Figure 3(d) and 3(e). In general, the Lα (at 1.02 keV) and Kα

Figure 3. Energy dispersive spectrometer (EDS) elemental characterization of the ZnO/Au-4 heterostructures fabricated at 700 °C under vaccum. (a) SEM image of the ZnO/Au heterostructures with a size of 3.2(x) × 2.3(y) µm2. EDS spectra within ranges of (b) 1.9 – 2.5 keV and (c) 9 – 10.5 keV. The full-range EDS spectra for the ZnO/Au heterostructures in the area (d) with Au nano-antennas and (d) without Au nano-antennas. EDS maps of (f) Zn, (j) O, and (h) Au. 10


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(at 8.63 keV) peaks of Zn and the Kα (at 0.53 keV) peak of O were similarly witnessed in the area A and B, which can be caused by the identical thickness of ZnO as evidenced with the contents of Zn and O. Correspondingly, the uniform distribution of Zn (fuchasia) and O (yellow) were observed with EDS maps throughout the whole area as shown in Figure 3(f) and 3(j). Meanwhile, as shown with tables in Figure 3(d) and 3(e), the Au content in area A was 3.73 at.%, suggesting a high reaction between X-rays and Au. However, the Au content neglectably was 0.96 at.%, which can be affected by the signals from the surrounded Au nano-attennnas. As a result, the Au was mainly detected in the area with the nanostructures as shown with the map in Figure 3(h). Figure 4 shows the UV photoresponse performance of the pristine ZnO and ZnO/Au heterostructures photodetectors fabricated at 650 °C, and the corresponding values are summarized in Table S2. The photodetectors were constructed by evaporating a pair of 90 nm Au electrodes with a spacing gap of 200 µm and a channel length of 4 mm as illustrated in Figure S3(a). The photoresponse sensitively varied depending on the surface morphology of the Au nano-antennas fabricated at 650 °C as revealed in Figure 4. In the dark, the current (Id) of all devices increased linearly as a function of the applied voltage as shown in the inset of Figure 4(a), indicating an ohmic contact between ZnO films and the Au electrodes. Meanwhile, the dark current at each bias voltage for the devices with Au nano-antennas was comparatively similar compared with the device of the pristine ZnO. However, photocurrent (Iph) at each bias voltage was enhanced with the existence of the Au nano-antennas, and the current significantly increased for the device with island-like Au nano-antennas (sample ZnO/Au-2) throughout the voltage range as clearly observed in Figure 4(a). As shown in Figure 4(b), the Iph of the sample ZnO/Au-2 stably exhibited a noticeable enhancement by ~8.2 times than the pristine one at 10 V bias under the 6.9 mW/cm2 pulse UV light illumination, and the Iph of the device with the corrugated nano-mounds shaped Au nanoantennas (sample ZnO/Au-1) was also consistently increased by ~1.7 times under an identical condition. At an identical bias voltage of 10 V, a constant increase was similarly witnessed 11



Page 12 of 37

for each sample along with the elevated light intensity from 1.35 to 5.91 mW/cm2 as shown in Figure 4(c) – 4(e). At each light intensity, the sample ZnO/Au-2 constantly exhibited a much more remarkable increase than the sample ZnO/Au-1, which suggested a strong morphological dependence of the localized surface plasma resonance (LSPR)

36, 37

of the Au

nano-antennas as shown in Figure 4(f). Accordingly, the Iph increased by ~2.1 and ~10.4 times at 1.35 mW/cm2 for the samples ZnO/Au-1 and ZnO/Au-2, respectively. As a result, an exponential increase in Iph/Id was constantly observed for the sample ZnO/Au-2 as a function of the light intensity, and the optimal Iph/Id ratio reached at ~1.48×105 under the light intensity of 6.9 mW/cm2 as shown in Figure 4(f). Table 1 lists the recent works on the investigation of the performance of the Au decorated ZnO UV photodetectors

27, 38-43

. As shown in Table 1,

the Iph/Id ratio of our devices was two orders higher than Au NPs/p-ZnO NSs/n-ZnO 40 and Au NPs/ZnO nanosheets 41 photodetectors. To further evaluate the photoresponse performance of the detectors, the photoresponsivity (Rs) (solid) and the normalized detectivity (D*) (dashed) under various light intensity were calculated as shown in Figure 4(g). The Rs suggests the ratio of the generated photodurrent to the illumination power on the devices, which can be expressed as 14: Rs =

(Iph -Id ) P0 A

(3)

where P0 is the light intensity, and A is the active area between two electrodes. With the Au nano-antennas, an obvious enhancement of the Rs was similarly witnessed at each light intensity owing to the radically increased Iph as shown in Figure 4(g). As a result, the Rs of the sample ZnO/Au-2 increased by ~721% than that of the bare ZnO detector with an identical P0 of 6.9 mW/cm2 as summarized in Table S2. Meanwhile, as the other critical performance indicator for a photodetector, the normalized detectivity (D*) can be given by 44 D* = 2qI Rs A

(4)

d

, where q is an elementary charge. Likewise, the D* was significantly enhanced by introducing with the Au nano-antennes as a function of the increased P0 as shown in Figure 12


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4(g), and the optimal D* was ~2.83×1012 Jones for the sample ZnO/Au-2 with the P0 of 6.9 mW/cm2 as summarized in Table S2.

Figure 4. The photoresponse performance of the pristine ZnO, ZnO/Au-1, and ZnO/Au-2 devices. (a) I-V curve of each devices under 365 nm UV light illumination (6.9 mW/cm2) and in the dark (inset). (b) Time-resolved UV photoresponse of each devices under 365 nm UV light illumination (6.9 mW/cm2) at a bias of 10 V. Light intensity dependent photoresponse of (c) ZnO, (d) ZnO/Au-1 and (e) ZnO/Au-2 under 365 nm light illumination. (f) Photocurrent (solid with left scale) and on/off ratio (Iph/Id) (dashed with right scale) of each devices as a function of the light intensity. (g) Photoresponsivity (solid with left scale) and detectivity D*(dashed with right scale) of each devices as a function of the light intensity.

Figure 5 shows the UV photoresponse performance of the ZnO/Au heterostructures photodetectors fabricated at a higher temperature of 700 °C. Similar to the samples fabricated 13



at 650 °C, the Id for each sample was comparable with the pristine ZnO photodetector, while, the Iph was much more sensitive to the bias voltage for the samples with the Au nano-antennas utilized at 700 °C as shown in Figure 5(a), resulting in a ~9.1 times increase in the Iph than that of pristine ZnO device at 11 V bias. At 10 V bias, the obvious increase in the Iph was

Figure 5. The photoresponse performance of the pristine ZnO, ZnO/Au-3, and ZnO/Au-4 devices. (a) I-V curve of each devices under 365 nm UV light illumination (6.9 mW/cm2) and in the dark (inset). (b) Time-resolved UV photoresponse of each devices under 365 nm UV light illumination (6.9 mW/cm2) at a bias of 10 V. The time-resolved UV photoresponse in a period of the (c) pristine ZnO and (d) ZnO/Au-4 devices under 365 nm UV light illumination. (e) Photocurrent (solid with left scale) and Iph/Id (dashed with right scale) of each devices. (f) Photoresponsivity (solid with left scale) and D*(dashed with right scale) of each devices. 14


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steadily witnessed with the formation of the self-assembled Au nano-antennas as shown in Figure 5(b). For each photodetector, the current sensitively responded to the UV illumination, which can be evaluated with the rise time (t90) and decay time (t10), indicating the time for the photocurrent at 90 % of its maximum value during increasing and 10 % of the maximum value during decreasing

45

, respectively. Meanwhile, the t90 and t10 slightly increased along

with the morphological evolution of the Au nano-antennas as shown in Figure 5(c) - 5(d) and S3(b) – S3(d), which can because that the enlarged interface area between Au nano-antennas and ZnO CQDs prolonged the electron collection paths to the electrodes

38, 46

. Consequently,

the t90 and t10 for the sample ZnO/Au-4 increased to ~3.55 s and ~1.49 s, and those of pristine sample was ~1.26 s and ~0.18 s as revealed in Figure 5(c) and 5(d). However, the response speed was still quite fast in comparison with that of previous reported devices as shown in Table 1. Figure S4 shows the light intensity dependence of the resulting photodetectors with various Au nano-antennas fabricated at 700 °C, and the Iph/Id, Rs and D* for the corresponding samples ZnO/Au-3 and ZnO/Au-4 are plotted in Figure 5(e) – 5(f). Compared with samples fabricated at 650 °C, the enhancement in the photoresponse for the samples fabricated at 700 °C became more drastic along with the increased light intensity as shown in Figure 5(e) – 5(f), suggesting a much more sensitive response of the sample with relatively larger dimension of Au nano-antennas for the incident light. Thus, under the 6.9 mW/cm2 UV illumination, the Iph/Id increased by one order to ~1.13×105 with the ~9.1 times higher Rs of ~231.5 mA/W and ~4.9 times higher D* of ~3.44×1012 Jones than pristine sample as shown in Figure 5(f) and Table S2. Meanwhile, the external quantum effciency (EQE) for the acquired ZnO/Au-4 heterostructures showed a relatively high value of ~78.8%, as compared with the previous reports in Table 1. The EQE can be expressed by 4 EQE =

hc R qλ s

(5)

where h is the Planck’s constant, c is the velocity of the light, and λ is the the exciting wavelength. In addition, the resulting photodectors were of good stability and repeatability as

15



Page 16 of 37

evidenced with the normalized time-resolved photocurrent in multiple on/off cycles for ZnO/Au-4 and the pristine ZnO photodetectors (Figure S5).

Table 1 Comparison of the characteristic photoresponse parameters of the state-of-the-art Au nanoparticles based photodetectors. Rs Materials

Light source

Iph/Id

trise

tdecay

EQE

(s)

(s)

(%)

(mA/W)

P-ZnO-Au 38

245 nm

9478

--

24

15

--

Au NPs/CdMoO4 microplates/ZnO film39

350 nm, 450 W

25816

70.4

32.6

1.8

--

Au NPs/p-ZnO NSs/nZnO 40

365 nm, 6.0 mW/cm2

1168

--

∼70

∼150

--

Au NPs/ZnO nanosheets41

UV

~ 1000

~ 60

--

--

--

Au NPs/ZnO nanowires27

350 nm, 1.3 mW/cm2

5×106

--

25

10

--

Au NPs/TiO2/ZnO:Y NWs 42

365 nm,26.6mW

1786

14.8

50 nm) 36, 50, 51

, which could possiblly intensify the photon absorption of the ZnO CQDs. Therefore,

ZnO/Au-4 heterostructures showed the strongest light absobrance, corresponding the optimal photoresponse performance. Noticeably, as shown in Figure S6(a), given that the absorption spectra for the self-assembled Au nano-antennas possessed a broadband absorption range without obvious peaks due to the plasmon coupling among Au nano-antennas 52, the enhanced absorbance can be similarly observed in both of the UV and visible regions as shown in Figure 6(a), which can be also evidenced with the simulated absorption spectra of the ZnO/Au-2 and ZnO/Au-4 heterostructures in Figure S6(b). Additionally, the pristine ZnO CQDs device showed a relatively weak green and white light photoresponse above the band 17



Figure 6. The absorbance spectra (a) and the photoluminescence (PL) emission (b) for all samples. The inset of (a) shows the absorbance intensity of all the samples at 365 nm. (c) - (d) Electric field distribution of the infinite difference time domain (FDTD) simulation for the samples ZnO/Au-2 and ZnO/Au-4. (e) The band diagrams of ZnO/Au photodetectors in the dark. (f) Schematic of the interaction between incident light and ZnO/Au heterostructures. The band diagrams of ZnO/Au photodetectors under (g) UV and (h) green/white light illumination. 18


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gap at ~3.37 eV due to the intermediate states, which could be induced in the band structure by the native point and structural detects in ZnO lattice including oxygen vacancy and Zinc vacancy as well as interstitial zinc

38, 53

. As shown in Figure S8, the photoresponse was

gradually increased as a function of the morphologies of the Au nano-antennas, leading to ~5.2 times and ~9.7 times increases in the photocurrent for the sample ZnO/Au-4 under 530 nm and white light than that of pristine ZnO CQDs device, respectively. Moreover, the photoresponsivity spectra of all the photodetectors was measured under illumination with UV-vis light at a 10 V bais, as shown in Figure S9, which further confirmed the enhanced photoresponsivity of ZnO/Au heterostructures in the broadband wavelength compared to that of pristine ZnO device. Figure 6(b) shows the photoluminescence (PL) spectra for all the samples under 325 nm laser excitation. The near band edge (NBE) excitonic emission peak at 370 nm resulted from the radiative recombination of electrons in conduction band (CB) and holes in valence band (VB) was drastically increased with the existence of the Au nano-antennas, which can be mainly attributed to the resonant coupling between the Au nano-antennas and ZnO. Namely, the additional electron-hole pairs can be generated from the ZnO thin films by the increased scattered light beams from Au nano-antennas, resulting in the intensified recombination for the PL emission

54.

Besides, the “hot electrons” excited in Au nano-

antennas by the 325 nm incident photon can transfer from Au nano-antennas to the conduction band of ZnO, which could also further enhance the NBE PL emission

51

.

Meanwhile, the peak intensities of the samples ZnO/Au-2 and ZnO/Au-4 were obviously lower than that of the samples ZnO/Au-1 and ZnO/Au-3, which can because the carrier recombination was restrained by the enhanced surface electric field

55

. As a result, the PL

intensities of the samples ZnO/Au-2 and ZnO/Au-4 increased by ~36 times (the enhanced factor) higher than that of the pristine ZnO sample, while, the enhanced factors of the samples ZnO/Au-1 and ZnO/Au-3 were ~50 as shown in Table S3. In addition, the peak in the visible region can be regarded as the defect related emission (DLE), and the DLE peaks were also 19



clearly increased for the samples with self-assembled Au nano-antennas, which differed from the Au nanoparticles associated ZnO photodetectors 56, 57. It can be ascribe to two reasons: (I) The increased incident light absorption by defects due to the enhanced surface electric fields as a result of the broadband response Au nano-antennas as mentioned before, and (II) “hot electrons” injection from the Au nano-antennas 24, 56. Figures 6(e) - 6(h) illustrate the band diagrams of ZnO and Au nano-antennas in dark and under light illumination at various wavelengths. Generally, the absorbed oxygen species on the surface of ZnO tend to grab the electrons from ZnO to form O2¯, resulting in a depletion region on the surface of ZnO

14

. ZnO nanostructures were reported to process a

comparatively higher work function of 5.2 ~ 5.3 eV

58

, which was higher than that of Au

(~5.1 eV). Therefore, as depicted in Figure 6(e), the electrons will flow from Au nanoantennas to ZnO CQDs to establish a equalized Femi levels (EF) with a formation of downdard band bending, and thus, the conductivity of the films can in turn slightly increase with a decreased depletion layer, leading to a slight increase in the Id as observed with the ZnO/Au samples. Under the UV light illumination, the Au/ZnO heterostructures can react with incident lights more radically owing the confinement of the incident lights by the enhanced surface electric fields on Au nano-antennas because of collective oscillation of surface electrons 41, 59 as shown in Figure 6(f)-6(g). Under the visible light illumination, more electrons from the defect levels (ED) can be excited to the CB because of the increased incident light density induced by the LSPR, meanwhile, the Au nano-antennas can also generate high-energy electrons to fill the CB of ZnO, spontaneously. Consequently, the photodetector based Au/ZnO heterostructures exhibited an improved photoresponse within a wide range between UV and visible regions.

4.CONCLUSIONS In summary, we have demonstrated a masterly approach to fabricate broadband ultrasensitive ZnO/Au heterostructures photodetectors associated with various morphologies self20


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assembled Au nano-antennas under the spin-coated ZnO CQDs films. The photoresponse of the photodetectors sensitively responded to the evolution of the resulting Au nano-antennas from the corrugated nano-mounds to the island-like nanostructures based on the soliddewetting mechanism, nevertheless, each ZnO/Au heterostructures photodetector similarly exihibited a noticeable enhancement in the photoresponse within a broadband wavelength up to the visible regions. Consequently, under UV light illumiation, the photoresponsivity and normalized detectivity of the detector with the relatively larger island-like Au nano-antennas fabricated at 700 °C under vacuum were enhanced by up to ~813% and ~389% when compared with those of the pristine ZnO CQDs photodetector. Likewise, the switch ratio of the detector increased by one order to ~1.13×105 with a comparable dark current due to a drastic increase in photocurrent. The mechanism of the absorption enhancement of the ZnO/Au heterostructures were systematically investigated and discussed with the PL spectra and FDTD stimulations, which can be resulted from a combination of the enhanced near surface electric field by the intense LSPR and the “Hot electron” injection process. In addiation, owing to the broadband response of the resulting Au nano-antennas, the photoresponse of ZnO/Au heterostructures photodetectors was effectively extended to the visible range with a remarkable improved photoresponse. This work has important implications for the understanding of the coupling between Au and ZnO nanostructures, and in turn provide an effective method to achieve high-performance broadband ZnO-based photodetectors.

ASSOCIATED CONTENT Supporting Information Figure S1: Evolution of the self-assembled Au nano-antennes under various growth condition. Atomic force microscope (AFM) side-views (5×5 µm2) of Au nano-antennes fabrciated at 650 °C (a) under N2 (Au-1) and (b) under vacuum (Au-2), at 700 °C (c) under N2 (Au-3) and (d) under vacuum (Au-4). (a-1) - (d-1) Corresponding AFM top-views. (a-2) - (d21



2) Corresponding cross-sectional line profiles acquired from the white lines drawn area in the AFM top-views; Figure S2: (a) The schematic of configuration of the ZnO/Au heterostructures photodetector. The time-resolved UV photoresponse in a period of the (b) ZnO/Au-1, (c) ZnO/Au-2, and (d) ZnO/Au-3 photodetectors under 365 nm UV light illumination; Figure S3: The light intensity dependent photoresponse of the (a) ZnO/Au-3 and (b) ZnO/Au-4 under 365 nm light illumination; Figure S4: (a) The absorbance spectra of the Au nano-antennas with various topologies. (b) The simulation absorbance spectra of the sample ZnO/Au-2 and ZnO/Au-4; Figure S5: The electric field distribution of the sample (a) Au-2 and (b) Au-4 via the infinite difference time domain (FDTD) simulation. The corresponding FDTD simulation (c) scattering spectra and (d) extinction spectra of the samples; Figure S6: (a) The photoresponse of the ZnO/Au heterostructures photodetectors under 530 nm green light illumination at a bias of 10 V. (b) The photoresponse of the ZnO/Au heterostructures photodetectors under white light illumination at a bias of 10 V. (a-1) - (b-1) The histogram of corresponding photocurrent for each ZnO/Au heterostructures photodetector; Table S1: The average diameter, average diameter, and density of the sample Au-2 and Au-4; Table S2: The photocurrent (Iph), on/off ratio (Iph/Id), responsivity (Rs), and normalized detectivity (D*) of the pristine ZnO and ZnO/Au heterostructures photodetectors; Table S3: Comparison of the characteristic photoresponse parameters of the state-of-the-art Au nanoparticles based photodetectors; Table S4: The enhancement factor of the NBE peaks for the pristine ZnO and ZnO/Au heterostructures photodetectors. (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected] [email protected] Author Contributions

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SL, M-Y L and SJ participated in the experiment design and carried out the experiments. SL, M-Y L, DS, MY, HK, HL and SJ participated in the analysis of date. SL, M-Y L, DS, MY, XW and SJ designed the experiments and testing methods. SL and M-Y L carried out the writing of the manuscript. All authors helped in the drafting and read and approved the final manuscript. Notes The authors declare no conflicts of interest.

ACKNOWLEGEMENTS We acknowledge the National Science Foundation of China Grant (Nos. 61705070, U1532146 and 61675076), the China Postdoctoral science Foundation (Grant Nos. 2017M612449 and 2017T200545), the National Key Research and Development Plan (2016YFB0402705), the Fundamental Research Funds for the Central Universities (2015TS047), Basic Science and Technology Project (JSZL2016212C001). We would also like to acknowledge the Analytical and Testing Center of Huazhong University of Science and Technology.

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REFERENCES (1) Lu, J.; Xu, C.; Dai, J.; Li, J.; Wang, Y.; Lin, Y.; Li, P. Improved UV Photoresponse of ZnO Nanorod Arrays by Resonant Coupling with Surface Plasmons of Al Nanoparticles. Nanoscale 2015, 7, 3396–3403. (2) Liu, L.; Yang, C.; Patanè, A.; Yu, Z.; Yan, F.; Wang, K.; Lu, H.; Li, J.; Zhao, L. Highdetectivity Ultraviolet Photodetectors Based on Laterally Mesoporous GaN. Nanoscale 2017, 9, 8142–8148. (3) Zheng, W.; Huang, F.; Zheng, R.; Wu,

H. Low-Dimensional Structure Vacuum-

Ultraviolet-Sensitive (λ

Self

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