Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene

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Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene Dots Immobilized on Zinc Oxide Nanorods Dibyendu Ghosh, Sutanu Kapri, and Sayan Bhattacharyya* Department of Chemical Sciences and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur - 741246, India S Supporting Information *

ABSTRACT: A combination of dimensionally reduced graphene quantum dots (GQDs) having edge effects and the vertically aligned ZnO nanorods shows highly selective visible-blind ultraviolet (UV) sensing. The GQD immobilized ZnO nanorod heterostructure shows remarkable responsivity of ∼6.62 × 104 A/ W and detectivity of ∼1.78 × 1015 Jones under 365 nm (10 μW) incident light and 2 V bias potential with high stability of at least 5 cycles, fast response time of 2.14 s, and recovery time of 0.91 s. The grain boundary assisted electron transport across GQDs was calculated from the normalized absorption below bandgap. The highest UV responsivity and detectivity were found to be proportional to the lowest trap state density at the grain boundaries (Qt) and minimum grain boundary potential (Eb). For the best GQD, Qt and Eb were found to be ∼4 × 1013 cm−2 and 0.4 meV, respectively. The phenomenal performance of ZnO-GQD heterostructure is attributed to the efficient immobilization of GQDs on ZnO nanorods and the idea of employing GQDs as photosensitizers than solely as electron transporting medium. The efficiency of GQDs is superior to carbon quantum dots (CQDs) containing minimal graphitic domains, and graphene oxide (GO) or reduced graphene oxide (rGO) having larger dimensions preventing their immobilization on ZnO nanorods. KEYWORDS: graphene dots, ZnO nanorod, photoresponsivity, UV detection, grain boundary low at 180 A/W.6,7 There are very rare instances where both responsivity and detectivity are high, however at the cost of a high applied bias. Few-layer black phosphorus-based photodetectors require 80 V gate bias to demonstrate enhanced photoresponsivity of ∼9 × 104 A/W and specific detectivity of ∼3 × 1013 Jones.8 In this context, UV detectors based on ZnO have tremendous potential due to their wide bandgap (Eg) of 3.4 eV, large exciton binding energy of 60 meV, high carrier mobility,9,10 and the ease of synthesizing various morphologies of ZnO.11−15 High surface-to-volume ratio introduces a huge number of surface trap states in one-dimensional ZnO which helps to attain higher responsivity and photoconductivity.16−18 The interfacial trap controlled charge injection in the ZnOpolymer heterostructure maintains a responsivity of ∼721− 1001 A/W at −9 V,19,20 whereas ZnO/Spiro-MeOTAD heterojunction detector had a low responsivity of 0.8 mA/W although displaying a high detectivity of 4.2 × 109 Jones.21,22 Also, the piezo-phototronic effect of type-II ZnO/ZnS heterostructure displays a responsivity of ∼2.5 A/W at 1.5 V bias and 0.4 kg compressive load.4 In order to boost the performance of ZnO devices, graphene, graphene oxide (GO), or reduced graphene oxide (rGO) is generally supplemented to ZnO.23,24 With a high electron mobility of 250 000 cm2/(V s) at room temperature,25,26

1. INTRODUCTION UV photodetectors have shown great promise in communications, environmental monitoring, satellite-based missile plume detection, defense, and medical tools.1,2 The photodetection can be enhanced either by improving the device architecture, where research is mainly focused on Schottky junction,3 and piezo-phototronics,4 or by designing next-generation materials for enhanced photoresponsivity and detectivity. The performance of UV detectors is evaluated based on these two figures of merit where responsivity (R) is defined as the ratio of photocurrent density (Jph) to incident-light intensity (Llight); and specific detectivity (D*) is the smallest detectable signal and is a measure of noise in the photodetector. Therefore, R can be written as2 R=

Jph L light

(1)

and D* is defined as2 D* =

Jph R = (2qJd )1/2 L light × (2qJd )1/2

(2)

where q is the absolute value of electron charge (1.6 × 10−19 Coulombs) and Jd is the dark current. Besides the traditional Si and GaN or their heterostructures,5 photodetectors based on the emerging perovskite methylammonium lead halide have shown detectivity of ∼109 Jones whereas responsivity remains © 2016 American Chemical Society

Received: October 13, 2016 Accepted: December 2, 2016 Published: December 2, 2016 35496

DOI: 10.1021/acsami.6b13037 ACS Appl. Mater. Interfaces 2016, 8, 35496−35504

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematics showing the formation of GQDs. (b−e) TEM images, (f) UV−vis absorption spectra, (g) PL spectra, (h) XRD patterns, (i) FTIR spectra, and (j) Raman spectra of GQDs and CQDs. Inset of (b) shows the lattice fringes of G1.

unprecedented thermal conductivity (5000 W/m K),27 superior mechanical properties,28 and bandgap generation and tunability by functionalization,29,30 graphene, and its derivatives are employed in different research areas such as semiconductor, biomedical, single molecule gas detection, transparent conducting electrodes, optoelectronics, energy storage, and conversion.31−35 However, the weak light absorption of graphene restricts its application in optoelectronics, and therefore the photoresponsivity remains low at ∼0.5 mA/W with a high gate bias of 80 V.36 In the ZnO-graphene heterostructured UV detectors, graphene is usually considered as an electron transporting medium without taking advantage of its photosensitization property. This approach particularly limits the injection efficiency or responsivity of these heterostructures. For example, ZnO nanorod/graphene heterostructure demonstrates photoresponsivity of only ∼22.7 A/W at 20 V bias,24 and free-standing ZnO nanorod array with transparent monolayer graphene film could manage a photoresponsivity of ∼113 A/W at 5 V bias.10 Herein highly efficient GQDs are used as photosensitizers and due to its favorable band position for absorbing UV light, GQDs can promote electron transfer to ZnO nanorods. GQDs are zero-dimensional small pieces of graphene with properties such as high photostability against photobleaching and blinking, high water dispersion, and excellent biocompatibility.37 Recent attempt to fabricate UV detectors by spin coating GQDs on ZnO nanorods could achieve neither high responsivity nor detectivity,38 whereas GQD sensitized ZnO nanorods/polymer Schottky junction based UV detector showed a responsivity of only 36 A/W.39 We have prepared low cost colloidal GQDs decorated ZnO nanorods and achieved simultaneously a high responsivity ∼6.9 × 104 A/W and detectivity ∼1.78 × 1015

Jones at a low applied bias of 2 V. The advantages of our approach are efficient immobilization of GQDs on vertically aligned ZnO nanorods, efficient electron transport across grain boundaries, and employment of GQDs as photosensitizers. The UV responsivity and detectivity of ZnO-GQD heterostructure is compared with CQD supplements having stronger light absorption and the conventional GO and rGO. The role of trap state density and potential energy barrier at the grain boundary of GQDs in promoting inter-GQD electron transport is investigated.

2. RESULTS AND DISCUSSION 2.1. Characterization of GQDs and CQDs. GQDs (G1 and G2) were synthesized by high pressure microwave irradiation of citric acid as the building block and ethanolamine as the capping agent (Figure 1a). The CQDs (C1 and C2) were synthesized from sucrose, oxalic acid, and polyvinylpyrrolidone (PVP) by microwave irradiation under ambient pressure. The notations 1 and 2 are for the samples prepared at different microwave reaction times. The diameters of the GQDs remain in the 3−5 nm range (Figure 1b,c), whereas the diameters of CQDs increase from ∼3 nm for C1 to ∼4 nm for C2 with an increase in reaction time from 5 to 30 min (Figure 1d,e). High resolution transmission electron microscope (TEM) image of G1 (Figure 1b inset) shows lattice fringes corresponding to (101) planes. The UV−vis absorption spectra for C1 and C2 (Figure 1f) reveal prominent UV absorption with an absorption maximum at ∼260 nm attributed to π−π* transition of sp2 domains. The GQDs show comparatively weaker absorption where the maxima at ∼380 nm is due to n−π* transition of oxygen containing functional groups.40 It is worth paying attention that both CQDs and GQDs absorb light mainly in the 35497


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Figure 2. (a) FESEM image of vertically aligned ZnO nanorod arrays. Inset shows the hexagonal cross section. (b) XRD pattern of ZnO nanorods on FTO substrate along with the substrate reflections. (c) TEM image of GQD decorated on the surface of ZnO nanorod with the regions (1) and (2) magnified in separate panels. Inset shows part of a ZnO-GQD nanorod. Current density versus voltage plots for (d) GQDs and CQDs decorated ZnO nanorods under dark and 10 μW UV light (365 nm) illumination, and (e) ZnO-G1 under different intensities of 365 nm UV light. Inset shows a schematic of the device. (f) Selectivity of ZnO and ZnO-G1 at different wavelengths. Inset shows the schematic of electron transfer from GQD to ZnO nanorods. (g) CV plots of GQDs and CQDs. (h) Band positions calculated from the CV plots.

2.2. Photosensitivity and Detectivity of ZnO and GQD/CQD Heterostructures. UV detectors were fabricated using the heterostructures of GQDs/CQDs intricately attached to the vertically aligned ZnO nanorods on fluorine doped tin oxide (FTO) substrate. The 2−2.2-μm-long nanorods have a hexagonal cross section with diameter ∼500 nm (Figure 2a). The indexed reflections in the XRD pattern correspond to the wurtzite crystal structure of ZnO according to JCPDS 05−0664 (Figure 2b). TEM image of a representative ZnO-G1 heterostructure shows a near-perfect homogeneous decoration of G1 over the nanorods (Figure 2c). The magnified regions of Figure 2c indicate minimal agglomeration as a result of immobilization. The lattice fringes corresponding to (002) crystallographic plane demonstrate high crystallinity of the ZnO nanorods. Figure 2d shows the I−V characteristics of ZnO-GQD and ZnO-CQD heterostructures in the dark and 365 nm (10 μW) of UV light illumination. The pre-optimized concentration of 2 mg/mL of GQDs and CQDs was fixed throughout the work unless mentioned (see Experimental Section for details). The photocurrent rectification is the highest for ZnO-G1 sample (Table 1) which is ∼100 times more than the dark current measured at 1 V forward bias. All the heterostructures show better activities than pristine ZnO nanorod. Nanorods have a high density of surface hole-trap states which governs the

UV region and remain transparent in the visible region, a property suitable for applying them as front contact in optoelectronics devices. The photoluminescence (PL) spectra (Figure 1g) show blue emission when excited with 365 nm light and a slight red shift in the PL maxima with increasing size of the QDs. The blue emission of GQDs is analogous to the reported blue emission of GO with an origin from the isolated sp2 clusters within the sp3 C−O matrix.41 The X-ray diffraction (XRD) patterns show a broad (002) graphitic reflection at 2θ ≈ 25° (Figure 1h) for the GQDs and CQDs. The reflection maximum of G2 is shifted to a higher angle by Δ2θ ≈ 0.3° as compared to G1 indicating more compact interplanar spacing in G2 than G1.42 On the contrary G1 shows a sharper peak in analogy to better crystallinity. The reflections of C1 and C2 remain broader due to the presence of aromatic molecules and surface active groups.30 The GQDs and CQDs have various functional groups as evidenced by Fourier transform infrared (FT-IR) spectroscopy (Figure 1i). The broad frequency band at 3200−3500 cm−1 is due to −OH stretching vibration from the carboxyl (−COOH) and hydroxyl (−OH) surface groups. The absorption bands at 2950, 1636, 1415, 1054, and 624 cm−1 can be correlated to C−H stretching, CO stretching in −COOH, C−H bending, C−O stretching, and C−H bending vibrations, respectively.40,43,44 Raman spectra of GQDs show three bands at 1365, 1580, and 2670 cm−1 corresponding to the D, G, and 2D bands, respectively (Figure 1j). The A1g vibrational mode from the vibration of dangling bonds within the disordered domains of G1 and G2 gave rise to the D-band.45 The G-band arises from the E2g vibrational mode of C−C stretching in the graphitic domains. The intensity ratio (ID/IG) is 0.97 for G1 and 1.01 for G2 implying more disorder in G2. The 2D-mode comes from second-order double resonance process in the Brillouin zone involving transverse optical (TO) derived two zero-boundary phonons.46 The intensity ratio (I2D/IG) is ∼1.01 for G1 higher than ∼0.98 for G2 suggesting better formation of the graphene structure in G1.10 The Raman spectra of CQDs have low signal-to-noise ratio which is primarily due to the presence of various functional aliphatic and aromatic groups, antioxidants, and minimal graphitic domains.30

Table 1. UV Detector Parameters for ZnO-GQDsa samples ZnO ZnOG1 ZnOG2 ZnOC1 ZnOC2

dark current density (mA/cm2)

photocurrent density (mA/cm2)

responsivity (A/W) × 104

detectivity (Jones) × 1014

2.10 2.24

34.8 320.3

0.34 3.20

1.34 11.9

2.28

203.5

2.03

7.53

2.16

54.8

0.54

2.08

2.21

78.4

0.78

2.94

a

The illumination wavelength is 365 nm and the applied potential is 1 V.

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Figure 3. (a) Schematic representation of the grain boundary potential between GQDs. x denotes distance. (b) Normalized absorption fitted with the experimental data for the representative G1 sample. (c) Trap state density (Qt) and (d) barrier height (Eb) at the grain boundary of GQDs and CQDs derived from the fitting parameters.

photoconduction mechanism, whereby when illuminated by light, the holes of the photogenerated excitons are trapped at the surface and the electrons are left behind.17 According to an established mechanism,23 the “hot” electrons are captured by the adsorbed oxygen on the n-type ZnO nanorod surface, and under UV light illumination, the photogenerated holes are trapped, photodesorbing the adsorbed oxygen according to the consecutive steps: O2(g) + e− → O2− (ad) and h+ + O2− (ad) → O2(g). In ZnO-GQD or ZnO-CQD heterostructures electron−hole pairs are generated in both ZnO nanorods and GQDs/CQDs. The holes migrate to the surface which facilitates the photodesorption process of adsorbed oxygen ions. The unpaired electrons, left behind after migration of photogenerated holes, enhance the free carrier concentration in the heterostructures and increase the photocurrent. As a result post decoration of the ZnO nanorods with GQDs/CQDs, both responsivity and detectivity increase as compared to pristine ZnO nanorods. In the best performing ZnO-G1 heterostructure, responsivity is 3.2 × 104 A/W with 365 nm light at 1 V bias, which is ∼10 times higher than pristine ZnO nanorods (Table 1). ZnO-G1 also shows the highest detectivity of 1.19 × 1015 Jones. It is important to note that the performance of GQDs is higher than CQDs because of the more graphitic nature of GQDs (Figure 1j), and irrespective of higher UV light absorption by the CQDs (Figure 1f). The extent of photogeneration of electron−hole pairs is directly proportional to the intensity of incident UV light. A large number of high energy UV photons create more electron−hole pairs, which in turn increases the current density (Figure 2e). Figure 2e inset shows a schematic of the photodetector, and its cross-sectional FESEM image is shown

in Figure S1. The representative spectral selectivity curves of ZnO and ZnO-G1 shown in Figure 2f demonstrate the responsivity as a function of different incident light wavelengths. Because of the prominent detection of UV light, the responsivity starts to increase from below 400 and 440 nm for ZnO and ZnO-G1, respectively, and maximizes at 365−375 nm. The slight red shift of the selectivity curve for ZnO-G1 (Figure 2f) and ZnO-G2 (Figure S2) clearly suggests the influence of GQDs in the overall UV detection. The ZnO-CQD samples show similar selectivity in the UV region (Figure S2). Electron transfer occurs from the lowest unoccupied molecular orbital (LUMO) of GQD to the conduction band of ZnO (Figure 2f inset), whereas the holes are involved to release the adsorbed oxygen. The band positions of the GQDs and CQDs were measured by cyclic voltammetry (CV) which shows the redox peaks (Figure 2g). The highest occupied molecular orbital (HOMO) and LUMO positions were estimated from the onset potential of oxidation and reduction peaks according to the relations: EHOMO = −e(Eox + 4.14) and ELUMO = −e(Ered + 4.14),47 and shown schematically in Figure 2h. The LUMO positions of GQDs and CQDs are above the conduction band of ZnO indicating favorable electron transfer to the latter. The asymmetric nature of the I−V characteristics in Figure 4d under forward and reverse biased conditions is due to this favorable band position. Electron transfer is facilitated from GQDs to ZnO nanorods under forward bias whereas it is restricted in reverse bias. Although both G1 and G2 have similar graphitic content and favorable band positions for electron transfer, the UV responsivity of G1 is higher. The comparative responsivity can be explained by the electron transfer across grain boundaries between two GQDs attached to the ZnO nanorod 35499


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Figure 4. (a) FESEM image of rGO deposited on ZnO nanorod array. (b) Current density versus voltage characteristics under 365 nm UV light. (c) Responsivity. (d) Detectivity. (e) Time response curve under periodic chopped UV light. (f) Stability test for five successive on/off states for ZnO, ZnO-G1, ZnO-GO, and ZnO-rGO.

according to the Dow-Redfield model48 of defect oriented optical absorption below Eg, the gap between HOMO and LUMO. 2.3. Grain Boundary Properties. Similar to that observed in the absorption spectra of GQDs and CQDs (Figure 1f), the band tailing in polycrystalline semiconductors is assigned to two major types of absorption below Eg; one is due to defects or impurities in the semiconductor and the other is produced by the Franz-Keldysh effect. The Franz-Keldysh effect is a phonon assisted tunneling and describes the probability of finding an electron in the forbidden gap in the presence of an electric field. This electron absorbs a photon with energy less than Eg and therefore transits to the conduction band. Dow and Redfield first derived the expression for absorption coefficient (α) considering the probability of the above transitions, which was subsequently modified by Bujatti, Marcelja, and Bugnet.48−50 Assuming spherical probability distribution P(R) of grains having radius (R) from which the potential barriers at grain boundaries can be obtained, the normalized total absorption (A) is expressed as49 ∞

is the lattice temperature, k is the Boltzmann constant, and ND is the free carrier concentration. Therefore, the total normalized absorption can be written as53 (α /αo) = (α E /α E o) + (α M /α M o)

where αE/αE0 is the electronic contribution and αM/αM0 is the mechanical contribution to the below-band-edge optical absorption. Figure 3a shows the schematic representation of the formation of a potential barrier (Eb) at the grain boundary of two identical GQDs of radius R. For nanocrystals like GQDs with high surface to volume ratio, the number of acceptor-like surface states is larger than the total number of donor states and therefore the Fermi level will be pinned to the surface states and the electric field will be then proportional to R.49 The experimental normalized absorption of GQDs and CQDs deposited on quartz substrates below their fundamental Eg (Figure S3) is fitted with the theoretically derived normalized absorption according to eq 5 using Qt, Eb, lattice constant and deposition temperature as input parameters. The experimental data of G1 was fitted to the theoretical model and is shown in Figure 3b, and those of G2, C1 and C2 are shown in Figure S4. Minimizing the trap states and potential energy barrier at the grain boundaries of GQDs and CQDs, enhances the electron transfer to the conduction band of ZnO nanorods. In fact Qt is found to be lower for the GQDs than the CQDs (Figure 3c). Qt is in the order of ∼1013cm−2 indicating the presence of large number of trap states at the grain boundaries in comparison to 1010−1012 cm−2 for larger grains.51,52 Also Eb (in meV) is lower for the GQDs than CQDs (Figure 3d). The highest UV responsivity and detectivity of G1 can be understood from its lowest Qt and Eb, facilitating smooth flow of charge carriers in between the GQDs for their transport to ZnO. 2.4. Comparison with GO and rGO. The UV responsivity and detectivity of the best performing ZnO-G1 is compared with the conventional heterostructures of ZnO-GO and ZnOrGO. Although GO and rGO are structurally different, both contain a significant fraction of disordered carbon (Figures S5 and S6). The UV absorption of GO and rGO is in the range

R

∫ P(R ) dR ∫0 α E(ν , F )r 2 dr A = 0∞ R A0 ∫0 P(R ) dR ∫0 α0 E(F )r 2 dr

(3)

where subscript zero indicates the quantities at Eg, r is the radius distribution, and αE is the absorption coefficient and a function of an internal electric field (F). Along with the electric field contribution, the mechanical stress at the grain boundaries also influence the electronic structure and absorption tail in the below-band-edge absorption region.51 Considering the combined effect of the density of trap states at the grain boundaries (Qt) and grain boundary potential (Eb) on the below-bandedge absorption (αM).52 Eb is related to the average internal electric field (Fav) by the relation E b = eFavλD

(5)

(4)

where e is the electronic charge, Fav = eQt/ε, and λD = (4πεεokT/NDe2)1/2 is the Debye screening length, ε is the low frequency dielectric constant and εo is free space permittivity, T 35500


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essential for UV detectors. In addition, after five on/off cycles, the current density does not degrade and the dark current remains the same as before UV illumination, highlighting excellent stability of the device (Figure 4f).

240−260 nm with an additional hump ∼320 nm in GO (Figure S7) due to presence of oxygen containing functional groups.40 The two-dimensional rGO sheet is found to envelope the ZnO nanorods (Figure 4a) and a thorough immobilization could not be established, unlike GQDs (Figure 2c). The HOMO and LUMO positions of GO and rGO are also calculated from the onset potential of oxidation and reduction peak of CV plots (Figure S8) by the method previously applied for GQDs and CQDs. The HOMO and LUMO levels of GO and rGO also have favorable band alignment for electron transfer to ZnO. When illuminated with a 365 nm UV light, ZnO-rGO shows better response than ZnO-GO (Figure 4b−d) due to the minimal presence of electron transport hindering oxygen containing functional groups in rGO.10,23 The I−V characteristics of ZnO-GO and ZnO-rGO based devices in the dark are shown in Figure S9. Under UV light illumination, the current density increases by 80−90 times (Figure 4b). As compared to ZnO-GO and ZnO-rGO, ZnO-G1 demonstrates the highest photoresponsivity and detectivity (Table 2). The selectivity plots for ZnO-GO and ZnO-rGO

3. CONCLUSIONS In summary, we demonstrated the fabrication of a highly efficient hybrid UV photodetector consisting of GQD decorated ZnO nanorods displaying simultaneously high photoresponsivity and detectivity at low applied potential. The selectivity of detector is very prominent having high response in UV region and nearly zero response in the rest of the spectrum. The best photoresponsivity and detectivity observed are ∼6.92 × 104 A/W and ∼1.78 × 1015 Jones under relatively high illumination 10 μW and low bias of 2 V. Calculations based on the Franz-Keldysh effect show that the best performing GQD has the lowest density of trap states and minimum potential energy barrier at the grain boundaries for providing excellent conduction path for facile inter-GQD electron transport and hence better transfer to the conduction band of ZnO nanorods. Experimentally it is proven that higher graphitic content in the GQDs plays a vital role in the transport of charge carriers and therefore the CQDs with higher absorbance albeit lower graphitization show lower performance. Additionally, decreasing the size of GO/rGO sheets to GQDs helps in achieving efficient immobilization on the ZnO nanorods, which further improves the device efficiency in terms of enhanced UV photoresponsivity, detectivity, and faster response. Moreover, the detector shows continuous performance without any degradation up to several cycles. Our approach may open a new direction to adapt green strategies for designing next-generation optoelectronic devices with high efficiency yet low cost.

Table 2. Comparison of UV Detectivity of ZnO-G1 with ZnO-GO and ZnO-rGO, Measured Under 365 nm UV Light and 2 V Bias Potential samples ZnO ZnOG1 ZnOGO ZnOrGO

response time, τr (s)

decay time, τd (s)

responsivity (A/W) × 104

detectivity (Jones) × 1015

2.7 2.14

1.05 0.91

1.05 6.92

0.27 1.78

2.63

1.01

4.64

1.18

2.50

0.95

5.49

1.41

(Figure S10) show a specific wavelength range 300−380 nm, similar to the characteristics shown in Figure 2f. The reason behind the superior activity of ZnO-G1 is that the zerodimensional G1 can be easily decorated on ZnO nanorod array having intimate contact for charge transfer to the nanorods. On the contrary, the micron-sized lateral dimensions of rGO and GO keep them on top of the nanorods which restricts their area of contact for efficient charge transfer (Figure 4a). It is noteworthy that the UV photodetection parameters of ZnO-G1 under 2 V bias potential are superior among the available literature reports of the best performing UV detector materials (Table 3).6,8,10,19,24,39,54,55 The response time curves (Figure 4e and Figure S11) show faster response for ZnO-G1 (Table 2 and Table S1) with response time (τr) ∼ 2.14 s and fast recovery time (τd) ∼ 0.91 s which reflects faster recombination,

4. EXPERIMENTAL DETAILS 4.1. Materials. F:SnO2 (FTO)-coated glass (TCO 22−7, Solaronix), zinc acetate dihydrate (Merck India), zinc nitrate hexahydrate (Sigma-Aldrich, >98%), hexamethylenetetramine (Sigma-Aldrich, >98%), sucrose, glucose (pure grade, Merck India), citric acid (Sigma-Aldrich, ACS reagent, >99.5%), oxalic acid dehydrate (Purified, Merck India), ethylenediamine (synthesis grade, Merck India), ethanolamine (Sigma-Aldrich, >99.5%), PVP (Loba Chemie, Mol. Wt. 40 000), acetonitrile, and tetrabutylammonium hexafluorophosphate (TBAPF6, Sigma-Aldrich) were used without further purification 4.2. Methods. 4.2.1. Synthesis of GQDs. In a typical synthesis, 0.5 g of citric acid was dissolved in 10 mL of distilled water and then mixed with 1.45 mL of ethanolamine under vigorous stirring until a clear transparent solution was obtained. This solution was taken in a 50 mL round-bottom flask and treated by microwave irradiation at 180 °C in Anton Paar Monowave 300 microwave reactor under constant

Table 3. Comparison of Responsivity and Detectivity of ZnO-G1 with the Reported Parameters materials

operating voltage (V)

intensity of illumination (μW/cm2)

responsivity (A/W)

detectivity (Jones)

ref

ZnO Nanowire ZnO nanorod/graphene ZnO/PVK or P3HT GaN/Ag Graphene/ZnO Nanorod array Few layer black phosphorus Perovskite/Graphene ZnO-GQD-Polymer ZnO nanorods ZnO-G1

−5 20 −9 5 −1 −80 0.1 −1 2 2

0.75 0.1305 1.25 -100 25 × 103 1000 70 10 10

4.5 × 104 22.7 721−1001 4.2 113 9 × 104 180 36 1.05 × 104 6.9 × 104

--3.43 × 1015 --3 × 1013 109 1.3 × 1012 2.7 × 1014 1.78 × 1015

54 24 19 55 10 8 6 39 This work This work

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Research Article

ACS Applied Materials & Interfaces pressure for different time periods. After completion of the reaction, the obtained solution was allowed to cool to room temperature for several minutes. 5 and 10 min reactions gave greenish-yellow G5 and yellow G10 colloidal solutions. 4.2.2. Synthesis of CQDs. A homogeneous mixture of 0.005 g of PVP and aqueous solution of 10 mL 0.5 M sucrose and 5 mL of 0.5 M oxalic acid was prepared under stirring. The solution was subjected to microwave irradiation at 100 °C under constant stirring in a Sineo MASII-1000W commercial microwave oven, with well-equipped refluxing system under constant stirring at 900 W. The reactions were performed for 5 and 30 min to obtain light brown C1 and blackish brown C2, respectively. 4.2.3. Separation and Purification. The uniformly sized particles were separated and the samples were purified to remove the impurities and byproducts. The crude colloidal suspensions of GQDs and CQDs were half-diluted with 2:1 ethanol−water mixture and centrifuged for 15 min at 13 000 rpm followed by collection of clear supernatant. The whole exercise was repeated 3 times for each sample. For purification the dispersions were introduced on a Fisher scientific dialysis tubing membrane (with a molecular weight cutoff 3500 Da) and extensively dialyzed against deionized water for 36 h. The as-received suspensions were solidified using a rotary evaporator. All the samples were demoisturized under high vacuum. 4.2.4. Synthesis of ZnO Nanorods. Vertically aligned ZnO nanorod arrays were grown using a reported two-step process with slight modification.56 In the first step, ZnO seed layer (50−100 nm) was prepared from a solution of zinc acetate dihydrate (0.01m) in methanol onto a precleaned FTO coated glass substrate by spin coating method at 3000 rpm and annealed at 300 °C for 1 h to ensure particle adhesion to the FTO surface. The ZnO seed layer coated FTO substrate was then placed upside-down in Teflon-lined autoclave containing aqueous solution of 25 mM zinc nitrate hexahydrate and 25 mM hexamethylenetetraamine for hydrothermal growth of ZnO nanorods. The reaction was performed for 3−4 h at 92 °C. Finally the sample was rinsed with deionized water, dried at 80 °C, and then calcined at 500 °C for 1 h in a Thermo Linderberg/Blue M brand box furnace under atmospheric conditions. 4.2.5. Synthesis of GO and rGO. GO was synthesized from graphite according to a reported protocol albeit slight modifications.57 Briefly, 20 mL concentrated H2SO4/H3PO4 (9:1) mixture was added to 1.0 g, 1 wt equivalent graphite flakes and sonicated for 15 min. 6.0 g KMnO4 (6 wt equiv) was mixed with the above solution and heated at 50 °C under stirring for 12 h. The cooled solution was poured into ice and 2 mL of 30% H2O2 was added dropwise with vigorous stirring. For purification, additional 50 mL water was added and the solution was centrifuged at 7000 rpm for 30 min. After decanting the supernatant, the solid was dispersed in 100 mL 30% HCl and washed repeatedly with water and ethanol until pH ∼ 7. The solid product (GO) was dried in oven for further use. For the synthesis of rGO, 0.5 mg of GO was dispersed in 20 mL water and sonicated for 15 min. 15 μL of hydrazine hydrate was added into the dispersion of GO with stirring at 90 °C for 2 h. The solution was washed with ethanol and water thrice. Finally, the product was dried in inert atmosphere for further use. 4.2.6. Device Fabrication. The UV detectors were prepared by depositing GQDs and CQDs on vertically aligned ZnO nanorods, coated on FTO. The deposition was performed by spin coating the aqueous solution of GQDs and CQDs at 3000 rpm for 5 min followed by drying at 95 °C. Initially the concentration of the GQDs was varied from 0.5 to 4 mg/mL, whereby a concentration of 2 mg/mL was optimized based on the best performance of the device. This concentration of 2 mg/mL was fixed to fabricate the photodetectors with CQDs, rGO, and GO. The spin coating process was repeated 3 times to obtain the desired QD thickness over ZnO nanorods. GO and rGO were also deposited by the similar protocol of spin-casting. The top contact was made by thermally evaporating silver over a masked area of 0.15 cm2 to fix the number of participating ZnO nanorods in the UV detection process. 4.2.7. Characterization. The FESEM images were recorded by Carl Zeiss SUPRA 55VP FESEM. TEM images were obtained with a UHR-

FEG TEM system (JEOL, Model JEM 2100 F) using a 200 kV electron source. The XRD measurements were carried out with a Rigaku (Mini Flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation. UV−vis absorbance spectra were recorded using a Jasco Model V-670 spectrophotometer equipped with an integrating sphere. PL spectra were measured with Horiba ScientificFluoromax-4 spectrofluorometer using a Xe lamp as the excitation source and excitation at 309 nm. FTIR studies were performed with a PerkinElmer spectrum RX1 with KBr pellets. A LABRAM HR800 Raman spectrometer was employed using the 633 nm line of a He−Ne ion laser as the excitation source to analyze the QDs. UV detector performances (J−V curves) were measured using an electrochemical workstation CH Instruments (Model CHI604D), and a Newport Apex monochromatic illuminator. The CV studies were performed with a conventional three electrode cell in a CHI604D electrochemical workstation. The three electrode cell was assembled by using Pt wire as the counter electrode, Ag/0.01 M AgNO3 + 0.1 M TBAPF6 in acetonitrile as the reference electrode, and glassy carbon as working electrode. The scanning rate was maintained constant at 10 mV/s. 4.2.8. Theoretical Calculations. Theoretical calculations for the normalized absorption below bandgap were performed using Wolfram Mathematica 8.0 software. Equation 5 was solved for different Qt and Eb values to fit the experimental curve keeping the other parameter constant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13037. Cross-sectional FESEM image of the device; selectivity curves for ZnO-G2 and ZnO-CQDs; UV−vis absorption spectra of GQDs and CQDs; Normalized absorption fitted with the theoretical data for G2, C1, and C2; XRD, Raman and UV−vis absorption spectra of GO and rGO; CV plot and band positions of GO and rGO with respect to ZnO nanorods; dark current and selectivity for ZnOGO and ZnO-rGO device; time response plot for ZnOCQDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. ORCID

Sayan Bhattacharyya: 0000-0001-8074-965X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.G. acknowledges Science and Engineering Research Board (SERB), Department of Science and Technology (DST) for his fellowship under NPDF scheme. S.K. thanks Council of Scientific and Industrial Research (CSIR), New Delhi for his fellowship. The DST Indo-Israeli S&T Programme of Cooperation is duly acknowledged for the financial support, under Sanction No. DST/INT/ISR/P-11/2014. S.B. thanks IISER Kolkata for the academic and research funding.

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REFERENCES

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Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene

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