Synergistic Effect of Au Interband Transition on Graphene Oxide/ZnO Heterostructure: Experimental Analysis with FDTD Simulation.

We furnish a comprehensive study on light-induced carrier generation due to the synergistic contribution of Au interband transition and graphene oxide (GO)/ZnO heterostructure. Plasmonic gold nanoparticles (Au_nps) are incorporated as a substructure sandwiched between GO and ZnO, assisting in additional photo-induced charge carrier generation. GO is prepared by a single-step plasma-enhanced chemical vapor deposition process. The GO/ZnO heterostructure having an active working area of 0.25 cm2 is created to unleash the pyroelectric property of ZnO, and subsequently, Au_np is introduced at the interface of GO/ZnO. Here, the interband transition of Au_np and its capability for charge carrier generation combined with the excitonic charge carrier generation of the highly crystalline non-centrosymmetric hexagonal wurtzite ZnO enhances the photoresponse. Furthermore, the interaction of Au_np with ZnO and its spatial electric field intensity distribution is demonstrated by finite difference time domain simulation which indicate toward an efficient carrier generation at the interface of Au_np and ZnO. The fabricated heterostructure has an active working wavelength in the UV-A region with the highest responsivity at 375 nm of the electromagnetic spectrum. The ultrafast response time (∼29 μs) of the device is due to the pyro-phototronic effect of the GO/ZnO heterostructure enhanced by the interband transition of Au. In the comparative study of the Au_np-enriched GO/ZnO heterostructure device with a GO/ZnO device, the former shows better performance. Both the devices work in the self-powered mode as well as the photoconductive mode, but with a higher on-off current ratio in the photoconductive mode. Hence, this work helps in properly understanding photo-induced charge generation in a Au interband transition enriched GO/ZnO heterostructure.

Introduction

Graphene is among the few 2D materials that are quite stable in one atomic layer thick.[1,2] Graphene-based materials are unique as their structures are formed by carbon in a unique arrangement due to its catenation property. These materials have a hexagonal honeycomb structure with a carbon–carbon bond in sp2 hybridization and the remaining valence electrons are bonded with hydrogen or other functional groups. The proportions of functional groups such as carbonyl, carboxyl, hydroxyl, and so forth determine the properties of graphene-based materials. Unlike pristine graphene, graphene oxide (GO) contains a mixture of sp2 and sp3 hybridized carbon–carbon bonds whose proportions determine the variations in the quality of GO. GO has many versatile applications in the field of optoelectronics,[3,4] transistors,[5,6] solar cells,[7] battery,[8,9] supercapacitor,[10,11] photo-catalysis,[12] biomedical applications[5,13,14] and so forth. It is well known that graphene is a transparent material[15] with semi-metallic behavior. In the band structure of graphene, a small overlap of the valence and conduction band occurs. Also, because the main structure of GO is graphene,[15] it inherits similar properties as that of graphene, but due to the functional groups present in GO, the band gap opening occurs, which can be tuned by controlling the synthesis process[16] or by the chemical functionalization of graphene.[17] Hence, GO possesses semiconducting property whose application as a p-type semiconductor has already been reported.[18] Thus, GO has advantages where the semiconducting property is preferable, for example, microelectronics, optoelectronics, photovoltaics,[4] sensors and detectors,[16] and so forth. Therefore, GO can be explored for utilization as a transparent electrode as well as a semiconductor material simultaneously.

To explore and utilize the dual properties of GO for harvesting the UV portion of the electromagnetic spectrum, a wide band gap n-type semiconducting material such as ZnO has been selected for this work. It is well known that the c-axis-oriented hexagonal wurtzite ZnO exhibits a significant and extraordinary property[19]—the pyroelectric property due to its non-centrosymmetric arrangement of the lattice points. It is a wide band gap n-type semiconductor[20] with a large excitonic binding energy of 60 MeV.[21] The absorption spectrum of ZnO follows a similar trend to the absorption band of GO, and the GO/ZnO composite inhibits the electron–hole combination and enhances UV detection ability.[22]

Moreover, both GO and ZnO are transparent materials, and they can endure high temperature ranges, as the synthesis process of GO furnishes the evidence[3] whereas many experimentalists[20,23,24] have investigated ZnO for the same. The beauty of ZnO is its intrinsic pyroelectric property which is essentially crucial for its ultrafast photoresponse. Due to the non-centrosymmetric lattice arrangement in ZnO, whenever the light of a suitable wavelength falls on it, an impulse of current is generated due to induced polarization. This transient current is so fast that the overall response time of the device is tremendously reduced.

Now, this is a peculiar coincidence that plasmonic nanomaterials have strong absorbance of light at higher energy than that of plasmon resonance due to interband transition.[25] In plasmonic nanomaterials, the photo-induced transitions by localized surface plasmon resonance (LSPR) occur due to transition within the partially filled sp-band resulting in the collective oscillation of free electrons. In contrast, the interband transition occurs when the transition takes place from the inner filled bands to the conduction band, which is due to bound electrons.[26,27] Au is a well-established plasmonic material whose plasmonic peak usually occurs at ∼520–530 nm. Its plasmonic properties are well studied in the visible and near-infrared regions. Although incorporating plasmon-generated carriers with excitonic devices is well established and can be found elsewhere, incorporating the interband transition of the plasmonic material with excitonic transition is relatively new in the field of optoelectronics.[28,29] The interband transition of the gold nanoparticles (Au_nps) lies in the same range of the excitonic transition of ZnO nanoparticles at the UV-A region of the electromagnetic spectrum. It is a comparatively less studied and underestimated transition for the application in the field of optoelectronics. Therefore, the combination of the GO/Au_np/ZnO heterostructure is an open field of research to unclutter a new insight. Hence, leveraging the potential of the Au_np interband transition is requisite. Furthermore, the absorption spectrum trend of active materials, that is, GO, Au_np, and ZnO, shows absorption maxima in the UV region. Thus, we study the electrical, optoelectronic, and photovoltaic properties of the Au interband transition enriched GO/ZnO heterostructure to gain insights.

GO is synthesized by plasma enhanced chemical vapor deposition (PECVD) without any catalyst and truncated the complicated pre-deposition and post-treatment process,[3] which is advantageous to the chemical process or mere CVD process, considering its direct technological importance.[30] The interaction of Au_np and ZnO is reviewed by simulation using the finite difference time domain (FDTD) method. After that, two devices are fabricated using the synthesized GO, where device-1 has the architectural layout of GO/ZnO/Au_film, and device-2 has an architectural layout of GO/Au_np/ZnO/Au_film for a healthy comparison. Here, GO acts both as a transparent electrode as well as an active semiconductor material and Au_film plays the role of a counter electrode in both the devices. The synthesis process of each functional material is vividly explained in the experimental setup and procedure section. The comparative analysis of fabricated devices is done through photoelectrical characterizations, viz., I–V characteristics and I–t characteristics.

Results and Discussion

Material Analysis

Raman spectroscopy is an amiable and non-destructive technique for analyzing graphene-based materials. The Raman spectrum of the as-grown GO is shown in Figure a, which shows typical peaks for graphene-based materials, that is D and G at 1354 and 1592 cm–1, respectively.[31] The D and G peaks are the fingerprints of GO, and they can be observed in any carbon-based materials with the basic honeycomb structure of graphene. The D peak corresponds to the breathing mode of k-point phonons of A1g symmetry due to linear dispersion,[32] and the peak at 1580 cm–1 corresponds to the E2g mode, which is usually observed for graphite. It is related to the vibration of the sp2 bonded carbon atoms, which is non-dispersive in nature.[3,32,33] The observed D peak occurs due to the possible disorder in the graphene stacks and incorporated defects causing structural imperfections, which is absent in single-layer graphene. The graphene structure can be confirmed from the G band and 2D band at 1584 and 2697 cm–1 respectively.[34] The number of layers in the graphene structure can be determined by the 2D band and is very useful for the few-layer graphene. It is a single sharp peak for the monolayer graphene, but due to interlayer interaction in multilayer graphene-based materials, the 2D peak appears as a broad peak.[34,35] The other second-order bands, that is D + G and 2G bands are observed at 2932 and 3208 cm–1, respectively. All the second-order Raman shifts are essential to ascertain the quality of the possible graphene structure. These second-order peaks measure the quantity and quality of the defects in the graphene structure. Besides peak positions of D, G, and 2D, their ratio of area intensity, that is, ID/IG and I2D/IG are equally crucial as it is used to determine the degree of disorder and quantification of the defects. In our case, the ratio of the area intensity of D to G peak, that is, ID/IG is 1.76, indicating the formation of carbon–carbon sp3 bond.[36] The crystallite size is calculated using the ratio ID/IG as depicted by Cancado et al.,[37] which is inversely related to the disorder/defective intensity ratio. It is calculated to be 9.5 nm (calculation is shown in Supporting Information section-1). The 2D to G peak area intensity, that is, I2D/IG is measured to be 0.37, indicating a multilayer formation.[38] From the Raman spectra, we observed that the second-order peaks in our case are not spike peaks in particular but a band of peaks with shoulders. Therefore, from the overall understanding of the Raman spectrum, it can be deduced that the synthesized material has a graphene stack with few tens of layers and indicates the formation of GO.

Figure 1

(a) Raman spectra of GO, (b) XPS survey spectrum of GO, (c) high-resolution deconvoluted C 1s peak, (d) high-resolution deconvoluted O 1s peak, (e) UV–vis–NIR absorption spectra of Au_np, ZnO, GO, Au_np/ZnO, and GO/Au_np/ZnO, and (f) XRD pattern of Au_np and ZnO.

To further ascertain, X-ray photoelectron spectroscopy (XPS) is among the most reliable techniques to determine the functional groups present in GO. The XPS survey spectrum of GO shown in Figure b reveals prominent peaks of carbon and oxygen at 284.8 and 532.1 eV, respectively. The quantification of the XPS survey spectrum reveals that the surface composition of GO for carbon and oxygen is 83.55 and 16.45%, respectively. The presence of the nitrogen group is not vivid from the survey spectrum. From the high-resolution deconvolution peak of C 1s as shown in Figure c, it is found that the C=C sp2 bond appears at 284.7 eV, which arises from the graphene structure present in GO or graphitic materials.[39,40] The predominant peak of the sp2 carbon–carbon bond is the signature of the graphene structure in GO. Another deconvoluted peak at 285.1 eV, which arises from the C–C sp3 bond, is observed.[40] The proportion of sp2 hybridized bond is found to be more in graphene and graphene-based materials, whereas the sp3 hybridized bond are more likely in diamond-like carbon. The other deconvoluted peaks of the C 1s are at 286.6 and 289 eV, representing the functional groups C–O[39] and COO,[39,41] respectively.

The high-resolution deconvoluted O 1s peak is shown in Figure d. The component peaks are identified as the functional groups of carbon and oxygen. The peak at 530 eV stands for O=C–OH,[41] 531.7 eV for COO–,[40] 532.5 eV for C=O,[40] 533.9 eV for C–OH,[40] and 536.6 eV for CO.[42]

Now, it is evident to study the UV–visible spectra of all the constituent layers of the device, which is shown in Figure e. The GO absorbance spectra show the continuum band with the highest peak/edge at ∼305 nm with a gradual decrease in intensity to a higher wavelength of electromagnetic spectrum indicated by the blue line in the spectra. The GO absorbance peak/edge at 305 nm may be due to n–π* transition.[33,43] The transmittance spectra (Figure S1 in Supporting Information) of GO show that the GO sample is 83% transparent at 1400 nm wavelength. It is observed that the transparency gradually decreases from 1400 to 340 nm, with the lowest transmittance of 60.2% at 340 nm. This result complies with the absorbance spectra and is consistent with it, which is commendable because GO also assists UV absorption. ZnO is the dominant layer for UV absorption in both device-1 and device-2. Its strong absorbance peak is observed from 300 to 375 nm in the spectrum due to excitonic transition, which falls in this region, as it is a wide band gap semiconductor. The band gap of the synthesized ZnO is calculated using the Tauc plot[44] through the UV–vis spectrum, and it is found to be 3.25 eV[45] (calculations and graph are shown in Supporting Information section-2b, Figure S2).

The Au_nps show unique optical properties than their bulk counterparts, which show the LSPR peak in the visible region of the electromagnetic spectrum and the interband transition in the UV region. The interband transition of the Au_np is due to the electronic transition from the d-band to the sp-band, which is confirmed by the peak shown in the inset of Figure e, that is the absorbance graph.[28,46] In our case, both the peaks, viz., LSPR and interband transition, are observed at ∼665 and ∼365 nm wavelengths, respectively. Also, we know that the optical properties of nanostructures have a high dependency on the shape and size of the nanostructures. For spherical Au_nps, this LSPR peak is around 520 nm. In our case, the LSPR peak is red-shifted, which may be either due to deviation from the regular spherical shape or an increase in the size of nanoparticles.[46] When the particles are aggregated, the conduction electrons near the surface of each nanoparticle are delocalized and are shared among neighboring particles, so the plasmon peak shifts to lower energies, due to which the absorption and scattering get red-shifted to a longer wavelength. Also, the shift in absorption may have other reasons such as the change in refractive indices of the surrounding materials/layers through which the light passes.[28]

The absorbance spectrum of the combined layer of Au_np and ZnO is also taken, and it is observed that after the incorporation of Au_np, the UV absorbance of ZnO is enhanced significantly. The absorbance spectrum of the combination of three active layers, that is, GO/Au_np/ZnO, shows further enhancement in UV absorption because GO has its absorbance in the UV region and in the proximity of ZnO absorption. This indicates that the low transmittance of GO at ∼340 nm is not a bane but a boon for the UV response because the absorbance peak shows enhancement upon ZnO and Au_np, which means it can facilitate the charge carrier generation and enhance the performance of the device. Hence, all the layers, that is, GO, Au_np, and ZnO, are active participants and have a tremendous contribution to the UV response of the device.

After optical properties, the crystallographic information, such as crystallinity, phase detection, plane identification, and so forth of the synthesized materials, viz., Au_np and ZnO, are studied by XRD analysis. The XRD pattern of ZnO is shown in red and Au_np in blue colored lines in Figure f. The XRD pattern of ZnO clearly shows high-quality wurtzite crystal oriented along the c-axis, which is confirmed by JCPDS 00-003-0888 and belongs to the space group of P63mc (186) with lattice parameters of a = 3.2427 and c = 5.1948. This peak at 34.4° corresponding to the (002) plane is only distinct in the XRD pattern. Thus, we may infer that the synthesized ZnO is a single crystal as the other peaks are not observable in the graph. The crystallite size of ZnO is calculated by the Debye–Scherer formula, which is ∼18 nm[47] (calculation is shown in Supporting Information section-3, Figure S3). The XRD graph in the inset of Figure f of Au_np shows that the Au_np synthesized by magnetron sputtering is highly crystalline with a peak at 38.04°, which corresponds to the (111) plane of the FCC structure. Though the (111) plane is dominant in the structure, another peak is seen at 44.27°, corresponding to the (200) plane. The synthesized Au_np belongs to the space group of Fm-3m (225). Hence, the XRD analysis manifests that the magnetron sputtering synthesized Au_np is highly crystalline.

A sophisticated microscopic analysis technique is adopted to elucidate the above results further. The surface morphology of GO is studied by FESEM, as shown in Figure a. Here, we observe a uniform nanostructure formation throughout the sample. The average size of the GO nanostructure is measured to be ∼23 nm. The HRTEM image of GO (Figure b) provides the information of layer stacks and folding, which measures the interplanar spacing of ∼0.34 nm. From the HRTEM micrograph of Au_np as shown in Figure c, we observed that some Au_nps are aggregated and finally transformed into distorted spheres or elongated shapes, which is why there is broadening and red-shifting of the LSPR peak in the UV–visible spectrum, as observed in the inset of Figure e. This nucleation process and nanoparticles’ final size depend on the sputtering time and other optimized parameters.[28,46] The average size of Au_np is measured and found to be ∼4.5 nm with an interparticle distance of 2–3 nm. Also, the d-spacing of the Au_np crystal is measured to be ∼0.20 nm, which corresponds to the (200) plane, which confirms the XRD analysis results. It is also observed that Au_nps are uniformly sprinkled over the entire substrate, as shown in Figure c. The FESEM micrograph of ZnO nanoparticles as shown in Figure e also appears to be uniform over the entire substrate. The particles of ZnO are more closer and compact with no measurable interparticle distance. Also, the average size of ZnO nanoparticles is measured to be ∼30 nm (approx), from the size distribution of ZnO nanoparticles, as shown in Supporting Information section-4, Figure S4. The high-resolution HRTEM image of ZnO in Figure f shows the (002) plane with a measurable d-spacing of ∼26 nm, confirming the XRD findings.

Figure 2

(a) FESEM image of GO, (b) HRTEM image of GO showing interplanar spacing, (c) HRTEM image of Au_np, (d) HRTEM image of Au_np with crystal plane, (e) FESEM image of ZnO, and (f) HRTEM image of ZnO with crystal plane.

Photoelectrical Analysis

The excellent and concomitant optical behaviors of the synthesized materials, as shown in Figure e, indicate the device’s ability to respond in the UV region. Also, because all of them can generate photo-induced charge carriers, we have fabricated two devices—one having the configuration of GO/ZnO/Au film (device-1) and the other device having the configuration of GO/Au_np/ZnO/Au film (device-2) in which Au_nps are introduced. Au_nps are incorporated to enhance the photoelectrical performance of the device. Figure a,b shows the layout of the fabricated devices, that is, device-1 and device-2, respectively. ZnO is primarily responsible for photocurrent generation in device-1, whereas the combination of ZnO and Au_np works for the same in device-2. Moreover, apart from performing the role of a transparent electrode, GO also contributes significantly to enhancing the device performance by improving light absorption in the UV region, as shown in the UV–visible spectroscopic analysis.

Figure 3

(a) Architectural structure of device-1, (b) architectural structure of device-2, (c) current–voltage (I–V) characteristic curve of device-1 in the dark and under the light (365 nm wavelength and 2 mW intensity), (d) current–voltage (I–V) characteristic curve of device-2 in the dark and under the light (365 nm wavelength and 2 mW intensity), (e) on–off response of device-1 w.r.t. light (wavelength 365 nm, 2 mW at 0.1 Hz frequency) in the self-powered mode, and (f) on–off response of device-2 w.r.t. light (wavelength 365 nm, 2 mW at 0.1 Hz frequency) in the self-powered mode.

To analyze the photoelectrical performance of the fabricated devices and to realize the contribution of Au_np for photocurrent generation, a comparative study between the performances of the devices is carried out and presented in the following sections.

Figure c,d shows the comparative current–voltage (I–V) characteristics of both devices. I–V characteristic curves are recorded in the dark mode and with illumination of the device with UV light of wavelength 365 nm. From these figures, it is very much clear that the devices generate a significant amount of photocurrent with UV illumination. UV-365 nm light is utilized to record the current–voltage data because this is the region where both ZnO and Au strongly absorb the incoming photon. Consequently, both generate efficient charge carriers. The insets of both the figures show the photovoltaic nature of the devices. The open-circuit voltage (Voc) and short-circuit current (Isc) of device-1 is found to be 5 mV and 544 nA, respectively, whereas the value of these quantities is 6 mV and 1.52 μA, respectively, in the case of device-2. Therefore, the comparative study confers ample proof that device-2 is more efficient in generating photocurrent than device-1. This enhanced performance in device-2 is attributed to the Au_np, which generates additional charge carriers, that is, electrons and holes by the interband transition from d-band to sp-band of Au_np, whose energy ranges from zero to plasmon energy as compared to the energy of Fermi level.[25,48] This is in agreement with the UV–visible absorption results.

Now, to check the stability of the devices against time-varying light signals and to calculate the response speed, temporal evolution, that is I–t characteristics of both the devices, are analyzed at a constant frequency. Figure e,f shows the comparative temporal variation, viz., I–t responses of both the devices in the self-powered mode. UV-365 nm light source with on–off frequency of 0.1 Hz and intensity of 2 mW/cm2 is used for recording the I–t responses. Both the figures signify the excellent stability of the fabricated devices. The superior performance of device-2 compared to device-1 is also reflected in this analysis. The on–off current difference of device-1 is around 452 nA, whereas this current ratio is approximately 816 nA in device-2.

The sharp spikes observed in the I–t characteristic curves result from the pyroelectric current induced in ZnO. Because the XRD analysis of the synthesized ZnO shows its non-centrosymmetric hexagonal structure, due to this non-centrosymmetricity, the pyroelectric nature is induced in ZnO. When the illuminating light turns on or off, the device encounters a significantly rapid and high current enhancement with transient temperature change induced by light absorption, which is the origin of the pyroelectric current. Thus, besides photovoltaic current, pyroelectric current also contributes to photocurrent generation, making these devices ideal pyro-phototronic ones.[22]

From the comparative I–t characteristic curves, it is also clear that the pyroelectric current is much higher in device-2. This higher pyro current is generated due to the additional local heating in the Au_nps.[28] Therefore, apart from producing charge carriers through interband transition, Au_np also helps in enhancing the pyroelectric current, improving the device’s performance tremendously.

To elucidate the charge transfer mechanism of the devices to shred the ambiguity, the ultraviolet photoelectron spectroscopy (UPS) of GO is carried out. After the estimation of work function[26,49,50] as ∼5.84 eV for GO[51] (calculations and UPS spectrum is shown in Supporting Information section-6 with Figure S7), the energy level diagrams[45] of both the devices in the self-powered mode are set forth and are shown in Figure e,f. In device-1, the light-induced excitonic charge carriers (i.e., electrons and holes) are generated in ZnO and transported in two different paths, as shown in Figure e. The path of the charge carriers in the vertically sandwiched layers are determined by the difference in the energy levels among the layers and the p-type or n-type materials of the adjacent layer, which favors a built-in electric field. Because the band gap of ZnO is calculated to be 3.25 eV (shown in Supporting Information section-2b, Figure S2) and the conduction band and valence band are taken to be 4.4 and 7.7 eV, respectively,[45] and the work function of Au_film is 5.22 eV. Therefore, the energy difference between the conduction band of ZnO and the work function of Au_film is achievable as the transportation of electrons toward Au_film from the conduction band of ZnO takes place, as shown in Figure e. However, some of the electrons may not have enough energy to cross the energy difference of 0.82 eV, so the recombination process may be inevitable, which deteriorate the overall performance of device-1. However, as soon as the Au_np is incorporated in the junction between GO and ZnO, as in device-2 (shown in Figure f), the energy level gets reorganized due to additional energy levels of Au_np. This Au_np not only reorganizes the energy levels but also steers the flow of electrons and holes along with the generation of its own charge carriers through interband transition when a suitable UV light is incident on it. Because the energy levels of Au_np[52,53] and ZnO[45] can be paired to form a Schottky junction,[54] the electrons generated by an interband transition in Au_np feel free to move toward ZnO due to the built-in electric field at the interface and pile-up with the electrons generated by the excitonic transition of ZnO. These built-in electric fields allow the electrons to drift from the Au_film to the external circuit, which gives rise to the photovoltaic effect.[28] Similarly, the holes in the valence band of ZnO are drifted by the same built-in electric field toward Au_np and move past the external circuit through GO. Apart from modifying the energy levels that help enhance the performance of device-2, the Au_np also inhibits the recombination of the charge carriers by transporting the electrons sooner than the charge carrier lifetime.

Figure 4

(a) On–off response of device-1 w.r.t. light (wavelength 365 nm, 2 mW at 0.1 Hz frequency) at 5 mV bias voltage Voc, (b) on–off response of device-2 w.r.t. light (wavelength 365 nm, 2 mW at 0.1 Hz frequency) at 6 mV bias voltage Voc, (c) on–off response showing exponential fitting to measure the response time of device-1, (d) on–off response showing exponential fitting to measure the response time of device-2, (e) charge transfer mechanism with energy diagram for device-1, and (f) charge transfer mechanism with energy diagram for device-2.

Because one of the main advantages of the device is its ability to work in the photoconductive mode apart from the photovoltaic mode, after an extensive study of the photovoltaic mode, it is evident to analyze the photoconductive mode of the devices. Hence, to visualize the bias voltage dependence, I–t characteristic curves are recorded with different applied bias voltages, as shown in Supporting Information section-5, Figures S5 and S6. Here also a comparative study is carried out between device-1 and device-2. Both the devices show significant enhancement in photocurrent with the increment of applied bias voltage. However, following the earlier trend, device-2 displays better performance due to the contribution coming from Au_np. The applied external bias minimizes the recombination processes and helps the smooth movement of charge carriers in the devices, which can also be inferred from the energy level diagrams.[28] It is noteworthy to mention here that the pyroelectric current becomes more prominent when the applied bias voltage is equal to the open-circuit voltage (Voc), which is shown in Figure a,b, however, above or below the Voc, photocurrent dominates over the pyroelectric current.[3,28]

Because the I–t characteristics are also helpful for determining the response speed of the device, here, we have recorded single cycle on–off switching of both the devices with the help of the LabView measurement system, which records ultrafast response of up to 10 μs. The single-cycle on–off switching with response to UV light of both the devices is shown in Figure c,d. The cycles are then fitted with the required exponential functions to calculate the response speed. Device-1 shows a response speed of about 36 μs, whereas the response speed of device-2 is measured to be 29 μs. The pyroelectric effect induced in ZnO is the origin of such an ultrafast response. The incorporation of Au_nps helps in achieving better response time in device-2. As discussed earlier, the Au_nps also help to improve the pyroelectric current apart from the photocurrent.

FDTD Simulation and Photoresponse Study

For in-depth insights into the Au_np/ZnO interaction, the charge transfer mechanism and electric field enhancement in device-2, the FDTD simulation method is explored. A commercial software “Ansys Lumerical” is used for the simulation to look into the optical cross section and spatial electric field intensity distribution associated with Au, ZnO, and Au/ZnO heterostructure[55] (All the simulation information is provided in the Supporting Information section-7). Spherical geometry is considered for simulating nine Au nano-spherical particles, and hexagonal geometry is considered for ZnO. The range of incident wavelength is chosen from 300 to 800 nm because it encompasses both the LSPR and interband transition of Au_np as well as the excitonic transition of ZnO nanoparticles. Initially, the size of the nanoparticle is tuned from 4 to 14 nm, and the cross-sectional absorption spectrum is simulated in each case as shown in Supporting Information section-7, Figure S9. (c). The absorption cross section of Au_np in Figure a shows interband and LSPR peaks at around 375 and 526 nm, respectively, which is apparent for Au_np. From the simulated size distribution of Au_np of different sizes ranging from 4 to 14 nm, a set of 4 nm sizes are considered, and its interaction with ZnO is simulated. The extinction coefficient of the Au_np of the 4 nm nanosphere for the range of 300 to 420 nm shows a similar trend as the absorption cross sections. Then, the entire simulation is repeated to get the absorption spectrum of the combined nanostructure (i.e., Au_np/ZnO). Furthermore, the individual absorption spectrum of ZnO in the region is also obtained. The spectra of ZnO, Figure a, shows an increase in the absorption peak at around 375 nm due to minimum scattering and maximum absorption corresponding to the band gap energy,[56] and these results merge with our experimental findings (Figure e). This fact is further justified from the photoresponsivity and external quantum efficiency (EQE) studies. Photoresponsivity is the ability of a device to convert optical power into an electrical signal. The EQE signifies the amount of photocurrent generated from the incident photon. Figure b,c shows the photoresponsivity and EQE, respectively, of device-2 in the self-powered mode and the applied bias voltage as a function of wavelength. Both these quantities show maxima at around 375 nm. The photoresponsivity of both devices increases as the bias voltage increases. It increases linearly with respect to bias voltages whose values for device-2 are 2.52 mA/W @ 0 V, 3.83 mA/W @ 0.5 V, 5.14 mA/W @ 1 V, and 6.53 mA/W @ 1.5 V and is shown in Figure b. The nature of both the photoresponsivity and EQE graphs follows the UV–vis absorption spectrum of the active material of the devices, as shown in Figure e. We also compared the photoresponsivity of device-1 and device-2, as shown in Figure d, at an applied bias voltage of 1.5 V, which yields 0.39 and 6.53 mA/W for device-1 and device-2, respectively. From the comparison, it is clear that Au_np plays a vital role in enhancing the device’s performance. This indicates photo-induced charge carrier transfer between Au_np and ZnO.[54] Therefore, we can conclude that the concomitant interband transitions in both ZnO and Au_np are the source of the generated photocurrent in device-2. Hence, further validation is done by calculating the responsivity enhancement factor of device-2 compared to device-1, which furnishes the enhancement of performance after the incorporation of Au_np, as shown in Figure e. It shows that the Au_np enhances the device performance by a factor of 80.

Figure 5

(a) FDTD simulated UV–vis absorption crosssection spectra of the Au_np, ZnO, and Au_np/ZnO, (b) photoresponsivity of device-2 at different bias voltages, (c) EQE of device-2 at different bias voltages, (d) photoresponsivity comparison of device-1 and device-2 (e) responsivity enhancement factor of device-2 as compared to device-1, and (f) internal quantum efficiency of device-2 at different bias voltages.

In addition to that, the spatial distribution of the electric field intensity (|E|2) of Au_np and combined (Au_np/ZnO) nanostructure is also simulated (all the parameters are listed in Tables S1 and S2 in Supporting Information section-7). The electric field intensity of the Au_np and Au_np/ZnO combination is simulated in the range of 300 to 800 nm for the same absorption cross section range and is shown in Figure a–h. The absorption cross section of the ZnO nanoparticle has size dependency, which blue-shifts with a decrease in the particle size because it depends on the optical thickness of the hexagonal nanoparticle, which is a function of the number of constituent particles ( N) and wavelength (λ).[56,57] The electric field intensity in (V/m)2 of Au_np (shown in Figure a–d) is highest at the region of the interband transition of the Au_np which is 375 nm (Figure c) and LSPR region is ∼526 nm (shown in Supporting Information section-7, Figure S9a). The electric field intensity of Au_np/ZnO (Figure e–h) is estimated to be the highest at around 375 nm (Figure g), which means that maximum electron–hole pair is generated at this particular wavelength because both Au_np and ZnO possess absorption peaks at that region (shown in Figure a), due to the interband transition of Au_np and excitonic transition of ZnO. This very finding is verified by the internal quantum efficiency (IQE), which is the measure of the charge carrier generated as per the absorbed photon. The maximum IQE in the 375 nm wavelength, as shown in Figure f, justifies the theoretical field enhancement at that wavelength. IQE follows a similar trend of photoresponsivity and EQE at the applied bias voltage, as shown in Figure f,b,c. Therefore, the final overall results of the simulation comply with the experimental results.

Figure 6

(a–d) Spatial distribution of the electric field intensity of Au_np of 4 nm size at 300, 350, 375, and 420 nm wavelengths, respectively, and (e–h) spatial distribution of the electric field intensity of Au_np/ZnO at 300, 350, 375, and 420 nm wavelengths, respectively.

Hence, we can infer from our study that the utilization of plasmonic Au_nps paves the way for improving the performance of GO/ZnO heterostructure devices through their highly efficient interband transitions.

Conclusions

GO is a versatile material for harnessing its potential for application as a transparent electrode as well as an active UV-A absorbing material. It is anticipated by plasmonic Au_np, which is synergized along with ZnO. The caliber of the materials for photoresponse is well-established in this work by fabricating the GO/ZnO heterostructure and Au_np interband transition enriched GO/ZnO heterostructure. Au_np facilitates additional photogenerated carriers due to its interband transition for the GO/Au_np/ZnO heterostructure and exemplifies that its performance is enhanced than its solely GO/ZnO heterostructure counterpart. Highly crystalline non-centrosymmetric ZnO plays a predominant role in the pyroelectric current generation. Also, the interband transition of Au_np at 375 nm wavelength complies with the excitonic transition of highly crystalline wurtzite ZnO to promote charge carrier generation and boost the overall photoresponse. Nevertheless, the reorganized energy level to steer the flow of the charge carriers due to the incorporation of plasmonic Au_np is consequential. Moreover, the spatial electric field intensity (|E|2) and the absorption cross section study provides insights into the Au_np and ZnO interaction from FDTD simulation, whose peak appears at 375 nm in the UV region. The fabricated heterostructure devices perform efficiently both as a self-powered and as well as a photoconductive pyro-phototronic device in the UV-A region. The photo-induced pyroelectricity of ZnO makes the device ultrafast responsive to UV-A light with the response time of ∼29 and ∼36 μs for the devices with and without Au_np, respectively. Hence, this work provides a very good understanding of the combined contribution of GO, ZnO, and Au interband transition in photo-induced charge carrier generation and ultrafast photoresponse of devices.

Experimental Setup and Procedure

Material Synthesis

GO thin films are synthesized by PECVD.[3] It is equipped with a stage and a heater integrated with proportional integral derivative controller assembly for facilitating substrate heating. Although the required substrate heating in PECVD is less (∼750 °C) than CVD (∼1200 °C), its contribution to the growth process is indispensable.[3,58,59] The complete synthesis process of GO is carried out without using any catalyst. The glass substrate is cleaned with isopropanol, sonicated for 30 min, and dried in a furnace at 50 °C. Prior to the initiation of the growth process, the PECVD chamber is brought to a vacuum level of 3 × 10–2 mbar with the help of a rotary pump and then the gradual heating of the substrate is started. When the temperature reaches 500 °C, the helium and hydrogen gases are fed with flow rates of 100 and 20 sccm, respectively. The flow rates of the gases are controlled through mass flow controllers. The radio frequency (RF) power supply of 13.56 MHz is used to generate plasma, and its power is set to 30 W for substrate treatment. The plasma treatment of the substrate is done in this condition for 15 min. Meanwhile, the temperature reaches 750 °C and then the synthesis of GO is initiated when acetylene (C2H2) is fed to the PECVD chamber at a flow rate of 3 sccm. The working pressure of the chamber is maintained at 4.4 × 10–1 mbar throughout the growth duration. The RF power is set to 15 W during the growth process. The total growth duration of GO is 60 min. The RF plasma plays a pivotal role in ionizing the gas precursor and decomposition of the hydrocarbon gases and forming active carbon species, known as dehydrogenation in PECVD. The dissociation of the hydrocarbon gas involves both physical and chemical processes, that is, the impact of energetic electrons and the dehydrogenation effect of hydrogen.[38] After the growth period is over, the substrate heater proportional integral derivative controller is turned off and gradual cooling is allowed. The plasma synthesis is a competing process of growth and etching of carbon species in the plasma, where the amorphous carbon and other radicals which do not take part in the main structure of GO or are weakly bonded gets removed and if the equilibrium is maintained, good quality carbon-based materials such as graphene, GO, CNT, and so forth can be synthesized. Hydrogen gas plays a crucial role in this regard, and the complex reaction takes place between hydrocarbon and hydrogen gases, affecting the quality of the synthesized material.[60] Hence, the plasma process stands out among all other synthesis processes.

In this work, the sole motive for synthesizing GO is to utilize it as a transparent electrode as well as an active semiconducting material by incorporating it separately with ZnO and Au_np/ZnO for UV light detection. The single-crystal hexagonal wurtzite ZnO is sputter deposited on the top of GO in device-1, and in the case of device-2, Au_np is sputter deposited on GO followed by ZnO in a separate deposition chamber by reactive magnetron sputtering. For ZnO synthesis, a highly pure Zn target (99.95% purity) is placed at an optimum distance of 7 cm from the substrate holder. Initially, the chamber is brought down to ∼5 × 10–5 mbar pressure. Then, argon and oxygen gases are injected into the chamber at the flow rates of 15.7 and 9 sccm, respectively, so that the working pressure of the chamber is maintained at 5 × 10–2 mbar. A pulse DC power supply is used with a power of 150 W to generate plasma for 5 min of deposition time. For the deposition of Au_np, the magnetron sputtering technique with Au (purity 99.99%) target is used. Argon is used as a sputtering gas with the flow rate of 53 sccm, which builds up 1.33 × 10–1 mbar of working pressure at a controlled pumping rate. The pulse DC power supply at the power of 45 W is used to generate plasma for the deposition time of 30 s. A highly smooth sputtering synthesized Au_film acts as the counter electrode for the devices. Each layer is separately grown/deposited on a glass substrate for morphological, structural, and optical characterization. After that, the materials are fabricated in device configuration, in steps, and their photoelectrical characterization as well as comparative analysis is performed.

Material Characterization Techniques

Characterization and analysis of all the active materials are done by non-invasive techniques. The identification of the graphene structure and defects in the structure of GO are studied by Raman spectroscopy (HORIBA Jobin Yyon, model LabRAM HR) using a laser of wavelength 514 nm (E514 nm = 2.41 eV). The identification of the functional groups and their quantification are done by XPS and UPS (ESCALAB Xi+, Thermo Fisher) and the analysis of the acquired data is done by CasaXPS and XPSpeak 4.1 software. Field emission scanning electron microscopy (Sigma VP ZEISS, Germany) and high-resolution transmission electron microscopy (Jeol, USA) are employed to study the crystallinity and morphology of the active layers, and ImageJ software is used to analyze the obtained image. UV–vis spectroscopy (UV-2600, Shimadzu, Japan) is used to study the transmittance and absorption spectrum of the materials. The active materials such as ZnO and Au are characterized by various techniques such as XRD (D8 Advance, Bruker AXS, Germany) using Cu Kα radiation (k = 0.15406 nm) for crystallinity, structural information, and crystal plane identification. Electrical measurement such as the on–off response with respect to light and I–V characteristics of the device is recorded using a SourceMeter (Keithley 2634B), whereas the responsivity of the device is measured using a Bentham PVE 300 photovoltaic characterization system. The response time of both the devices are measured by LabView (LabView 8.6 National Instruments with DAQ card no. CB-68LP), which has the caliber to detect the response time as fast as 10 μs.