Enhanced fluorescence of CsPbBr3/ZnO heterojunction enabled by titanium nitride nanoparticles

We prepared CsPbBr3/ZnO heterojunctions by self-assembling colloidal CsPbBr3 quantum dots (QDs) on the surface of the ZnO film. The fluorescence of CsPbBr3/ZnO heterojunctions was modulated by titanium nitride nanoparticles (TiN NPs) to obtain highly photoluminescent CsPbBr3/TiN/ ZnO heterojunctions. The results showed that when the TiN thickness was 51 nm, the fluorescence of the CsPbBr3/TiN/ZnO heterojunction was enhanced by 3.2 times compared to that of the CsPbBr3/ZnO heterojunction. TiN NPs combined most of the photo-generated electrons with the holes on the surface of the TiN/CsPbBr3 heterojunction, which increased the electron transfer rate and reduced non-radiative recombination. This method of enhancing heterojunction fluorescence could provide a new pathway for photovoltaic, light-emitting diode (LED), photodetector, light sensor, and image sensor applications.

However, there are many defects and impurities on the surface of metal oxide semiconductors, which can carry traps on the heterogeneous interface, increase non-radiative recombination, and reduce light radiation [14][15][16], resulting in the suppression of heterojunction fluorescence. Much effort has been devoted to improving the luminous performance of the heterojunctions. Placing either nanoparticles or a thin film on the surface of a metal oxide semiconductor material may help overcome the limitations derived from the intrinsic bandgap of the material [17,18]. Bai et al [19] synthesized a CsPbBr 3 /Au/Al 2 O 3 heterojunction using a self-assembly method. Compared with the CsPbBr 3 /Al 2 O 3 heterojunction, the fluorescence was enhanced by 2.8 times. The surface plasmon resonance of Au NPs can stimulate the production of hot carriers, which promotes the utilization of photo-generated carriers in the heterojunction of perovskite quantum dots (PQDs) thus enhancing the heterojunction fluorescence [19]. Au NPs are preferred for thermo-electronic applications because of their high carrier concentration. However, its high cost does not lend itself to its mass production. In contrast, another class of highly conductive materials, such as titanium nitride (TiN), zirconium nitride (ZrN), and tantalum nitride (TaN), have an energy band structure similar to that of noble metals [20]. These materials are inexpensive and require a simple preparation method. Therefore, they are promising candidates for effective excitation of hot electrons in the visible region of the spectrum [21][22][23]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
TiN NPs are among the most widely used nanomaterials in optoelectronic devices [24][25][26]. Because the electronic structure of TiN is composed of ionic bonds, covalent bonds, and metal bands, the p-orbital energy level of nitrogen is lower than the Fermi energy level, resulting in movement similar to that of the free electrons of precious metals such as Au and Ag [27,28]. Ishii et al [29] prepared planar TiN/ZnO/TiN trailers. Compared with the Au/ZnO/Au structure, the photocurrent prepared with TiN was much larger than that prepared with the Au structure. In addition, TiN is also a quasi-metallic material, and its high conductivity enables it to replace noble metal materials such as Au [30][31][32] and facilitates carrier transport and collection. The TiN film can also act as a remover that selectively contacts the electro-conductive and hole-blocking effects, promoting carrier separation and extraction [33].
In this work, we prepared TiN/ZnO heterojunctions by depositing TiN NPs on the surface of ZnO thin films using the centrifugal casting method. A CsPbBr 3 /TiN/ZnO heterojunction was fabricated on a TiN/ZnO surface by colloidal self-assembly of CsPbBr 3 QDs. Compared to the CsPbBr 3 /ZnO heterojunction, the fluorescence of the CsPbBr 3 /TiN/ZnO heterojunction was enhanced. The role of TiN NPs was to control the direction of electron transfer in the heterojunction to enhance fluorescence. Furthermore, this method does not require complex experimental conditions or chemical processing. This easily scalable and highly reproducible strategy may provide new avenues for the application of large-scale photosensors and photodetectors.

Preparation of ZnO Sol-gel and ZnO thin film
To prepare the precursor solution, zinc acetate dihydrate (0.5 g, 99.9%, Sigma -Aldrich) and ethanolamine (137.5 μl, 99.5%, Sigma-Aldrich) were dissolved in 2-methoxy ethanol (5 ml, 99.0%, Sigma-Aldrich). The rubber-sealing paste was stirred for 24 h at room temperature. Using a 1.5 cm ×1.5 cm ×0.5 mm polished silicon wafer as a substrate, The Si substrate was treated before cleaning. The substrate was cleaned with a 2:1 volume ratio of H 2 SO 4 : H 2 O 2 , etched with a HF acid solution (1:10 volume ratio of HF:H 2 O), and washed with deionized water (DI).The substrate was ultrasonically cleaned with acetone, isopropanol, and deionized water for 10 min dried with nitrogen, and treated with plasma for 15 min The sol-gel was spin-coated on the substrate at 1500 rpm for 15 s and then annealed at 150°C, 200°C, 250°C and 300°C.

Fabrication of CsPbBr 3 /ZnO heterojunction
Raw colloidal CsPbBr 3 QDs were prepared using the heat injection method (in the supplementary information (available online at stacks.iop.org/MRX/9/026406/mmedia)). S1 shows a flow chart of the synthesis of colloidal CsPbBr 3 QDs. The average size of the single CsPbBr 3 QDs was 9.55 nm in S2 (in the supplementary Information). The CsPbBr 3 QDs stock solution was diluted with n-hexane at a ratio of 1:10 to the CsPbBr 3 QDs solution and pasted the ZnO film on the inner side of the cuvette with double-sided tape. The CsPbBr 3 QDs solution was slowly added and the concave surface of the liquid was sufficient to cover the entire film. As the solvent continued to evaporate, CsPbBr 3 QDs self-assembled on the surface of the ZnO film. Took it out gently, put it into the vacuum drying oven at 80°C to dry and ready for use.

Fabrication of CsPbBr 3 /TiN/ZnO heterojunction
The process for preparing the CsPbBr 3 /TiN/ZnO heterojunction was shown in figure 1. The ZnO film substrate was placed in a 50 ml centrifuge tube and tilted slightly, followed by the addition of TiN (particle size of approximately 20 nm, 99.0%, Aladdin) solution (TiN NPs were dispersed in deionized water by sonication at concentrations of 1, 3, and 10 μg ml −1 ). This tube was then centrifuged at 8,000 rpm for 20 min, the clear supernatant was discarded, the substrate was gently removed from the tube, and uniform deposition of TiN NPs on the surface of the ZnO film was observed. The films were further dried in a vacuum drying oven at 80°C to allow complete evaporation of the deionized water from the films. The CsPbBr 3 QD stock solution and n-hexane were diluted at a ratio of 1:10, and the TiN/ZnO film was pasted on the inner side of the cuvette with doublesided tape. The diluted CsPbBr 3 QD solution was slowly added and the concave surface of the liquid surface was sufficient to cover the entire film. As the solvent continued to evaporate, CsPbBr 3 QDs self-assembled on the surface of the TiN/ZnO film; Took it out gently, put it into a vacuum drying oven at 80°C for drying and ready for use.

Characterizations
The optical quality of the film was characterized by a custom PL system, which included a continuous 365 nm wavelength laser (LLS365, Ocean Optics) as the excitation source and a fiber optic spectrometer (QE6500, Ocean Optics). The crystal structure of the CsPbBr 3 QDs and ZnO films were characterized using an x-ray diffractometer (XRD, Smart Lab XG, Rigaku). The morphology of CsPbBr 3 was observed using transmission electron microscopy (TEM, HT7800, Hitachi). The surface morphology of the TiN/ZnO was observed using a scanning electron microscope (SEM, Regulus 8100, Hitachi). The surface morphologys of the CsPbBr 3 /ZnO and CsPbBr 3 /TiN/ZnO heterojunctions were characterized by atomic force microscopy (AFM, CSPM5500, Ben Yuan). A high-speed desktop centrifuge (TG20KR, Shanghai Jipu Electronic Technology) was used to deposit the TiN NPs on the ZnO film. The thickness of the TiN NPs films was measured using a step tester (ET150, Surfcorder). ) (see inset). The root mean square values of the surface roughness of the ZnO films annealed at different temperatures were 11, 2.59, 2.57 and 3.39 nm, respectively. The surface roughness indicated that better flatness of ZnO film could be obtained when the annealing temperature was 250°C.

Results and discussion
The surface morphology of the ZnO film annealed at 150°C showed a striped structure (as shown in figure 2(a)), due to the greater activity of the ZnO NPs in the film, there was a greater chance of collisions between the particles, causing them to agglomerate [34]. When the annealing temperature was increased, the striped structures of the clusters disappeared. The gradual decrease in ZnO surface activity was attributed to the decrease in organic matter in the ZnO film. However, when the annealing temperature exceeded 250°C, the organic matter on the surface of ZnO was completely evaporated, and the roughness of the ZnO film was increased with the boosting temperature [35]. The XRD patterns of ZnO film annealed at different temperatures were shown figure 3(a). Obviously, the characteristic peaks at 31.76°, 34.42°, and 36.25°correspond to the (100), (002), and (101) diffractions of the ZnO film, respectively, and match well with PDF# 36-1451 [36]. When the annealing temperature was set at 250°C, the primary peaks became stronger and sharper, indicating that they had the better crystal quality. The Raman spectra of ZnO film annealed at different temperatures were shown in figure 3(b). The scattering peaks located at 330 cm −1 , 378 cm −1 , and 435 cm −1 originate from the second-order phonon modes and E 2H phonon modes, respectively [37].

Surface characteristics of TiN/ZnO heterojunction
As shown in figure 4(a), TiN NPs were deposited on the surface of ZnO under the action of centrifugal force, and the positive charges caused by the oxygen vacancies on the surface of the ZnO film could adsorb the negatively charged TiN NPs. The surface topography of the TiN NPs sample was obtained by SEM, and it can be clearly seen from figure 4(b) that a large number of almost spherical TiN NPs were deposited uniformly on the substrate. [38] From the particle size distribution, the average particle size was calculated and found to be approximately 21.06 nm, which was basically consistent with the original particles in figure 4(c). The same method was used to deposit different concentrations of TiN NPs on the surface of ZnO, which were 1, 3, and 10 μg ml −1 , respectively. The SEM images of different concentrations of TiN NPs were shown in S3 (supplementary information). In the inset, it was observed that the particle size of TiN NPs hardly changed. The thickness of TiN NPs deposited at different concentrations. It can be seen that the thickness of the TiN NPs increased with the increase of concentration. We measured the thicknesses of the TiN NPs deposited under the same conditions, as shown in figure 4(d), with corresponding film thicknesses of 51, 96, and 151 nm, for TiN NPs concentrations of 1, 3, and 10 μg ml −1 , respectively.

Surface characteristics of CsPbBr 3 /ZnO heterojunction and CsPbBr 3 /TiN/ZnO heterojunction
The SEM image of the surface morphology of the CsPbBr 3 /ZnO heterojunction was shown in figure 5(a). The CsPbBr 3 QDs and their clusters were clearly observed in figure 5(b). The size of a single CsPbBr 3 QD was vertically 11.5 nm [39], which was in agreement with the raw material of CsPbBr 3    been caused by the incomplete coverage of TiN NPs by CsPbBr 3 QDs. Compared to figure 4(b), it can be clearly observed that the CsPbBr 3 QDs were covered around the TiN NPs.

PL spectrum analysis of CsPbBr 3 /ZnO heterojunction and CsPbBr 3 /TiN/ZnO heterojunction
We measured the PL spectra of CsPbBr 3 , ZnO film, CsPbBr 3 /ZnO and CsPbBr 3 /TiN/ZnO heterojunctions using a customized PL system at room temperature, as shown in figure 6(a). The PL peak of the CsPbBr 3 film exhibited a single-peak distribution, with a peak at 510 nm. The PL spectrum of the ZnO film consists of two emission peaks, one being the excitonic emission peak at 372 nm and the other a broad peak caused by the deeplevel emission of surface oxygen vacancies in the visible range of 426-700 nm [40]. Similarly, the CsPbBr 3 /ZnO heterojunction also exhibited two PL spectra: one was the excitonic emission peak at 372 nm, and the other was continuously distributed in the visible region from 463nm to 675 nm, which was due to the self-assembly of CsPbBr 3 QDs on the surface of the ZnO film to form a CsPbBr 3 /ZnO heterojunction. Most of the energy from the colloidal CsPbBr 3 QDs was transferred to the ZnO film, and some of the photo-carriers were quenched on the ZnO surface [41]. Another part of the photo-generated carriers underwent electron-hole reconstruction at the ZnO defects, which enhanced the defect fluorescence of ZnO [42]. However, the enhancement of the interface fluorescence was relatively small. The photo-generated carriers were significantly lost during the transportation process, which reduced the carrier utilization.
To suppress the defect fluorescence of the CsPbBr 3 /ZnO heterojunction, TiN NPs were added between the CsPbBr 3 QD film and ZnO film as a storage medium for photo-generated carriers [43]. The PL spectra of the CsPbBr 3 /TiN/ZnO heterojunction were significantly stronger than those of the CsPbBr 3 /ZnO heterojunction. This further confirms that TiN NPs promoted the utilization of photo-generated carriers.
To clearly observe the enhanced fluorescence of differential heterojunctions, we drew a bar diagram at the peak fluorescence of ZnO, CsPbBr 3 /ZnO, and CsPbBr 3 /TiN/ZnO heterojunctions sand the bar diagram, as shown in figure 6(b). Compared to the ZnO and CsPbBr 3 /ZnO heterojunctions, the fluorescence of the CsPbBr 3 /TiN/ZnO heterojunctions was enhanced. Owing to the induction of TiN NPs. The photo-generated carriers of CsPbBr 3 QDs were transferred to the ZnO film to increase the peak fluorescence and interfacial fluorescence of the two heterojunctions. TiN NPs played a more important role in promoting the transfer of photo-generated carriers.
As shown in figure 6(c), the PL spectra of the CsPbBr 3 /TiN/ZnO heterojunction were the most intense when the thickness of the TiN NPs was 51 nm, and the PL spectral intensity of the CsPbBr 3 /TiN/ZnO heterojunction varied with the thickness of the deposited TiN NPs. The PL spectra first increased and then decreased with an increase in the deposited thickness of TiN NPs. To compare the PL enhancement effect on the surface of the CsPbBr 3 /TiN/ZnO heterojunction, the enhancement coefficient plot shown in figure 6(d) shows that the maximum enhanced fluorescence was 3.2 times higher than that of the CsPbBr 3 /ZnO heterojunction.
Compared with the fluorescence of the CsPbBr 3 film in figure 6(a), the fluorescence peak of the TiN/CsPbBr 3 interface radiation was obviously red-shifted, because the Fermi level of TiN was lower than that of the CsPbBr 3 film, and the electrons in the conduction band of CsPbBr 3 moved toward TiN/CsPbBr 3 interface transfer, and the interface electron-hole recombination occurred below the conduction band of the CsPbBr 3 film to radiate fluorescence. Comparing the fluorescence peak intensities of CsPbBr 3 /TiN/ZnO heterojunctions with different deposition thicknesses, the fluorescence peaks gradually weakened with the increasing of deposition thickness. The analysis shows that with an increase in the TiN NPs concentration, more TiN NPs were deposited on the surface of ZnO, and the TiN film become thicker, which blocked the transport of photo-generated carriers between the CsPbBr 3 QDs film and the ZnO film. The carriers were more easily consumed during the transport process, resulting in a decrease in the number of electron-hole recombination at the heterojunction interface.  those of ZnO defects, the excited hot electrons and E C electrons were easily transported to the E C of ZnO defects through diffusion effects and built-in electric fields, while holes also accumulate at their interfaces. (2) E C electrons and holes recombined to emit fluorescence. The higher the photo-carrier density, the more fluorescent  photons were radiated at the ZnO interface. Owing to the electron transfer of CsPbBr 3 QDs, the fluorescence intensity of the ZnO film was enhanced, whereas that of the CsPbBr 3 QDs was reduced. (3) Some of the electrons that were not transferred to the interface fall back into their E , V recombine with holes, and emit intrinsic fluorescence. Therefore, exciton fluorescence of CsPbBr 3 QDs can also be observed.
The Fermi level of TiN on oxides was 4.2∼4.3 eV, [50] and the work function was 4.2-4.6 eV [33,51]. It was clear from the schematic in figure7(b) that a small Schottky barrier was formed between TiN and ZnO. Under appropriate UV irradiation, both TiN and ZnO generate charge carriers through their respective inter-band transition processes. Then, the charge carriers from TiN can be easily injected into the E C of ZnO, thereby enhancing the hole recombination of carriers in the ZnO E .
V [50] The energy transfer process of the CsPbBr 3 TiNZnO heterojunction can be understood as: (1) A 365 nm laser simultaneously excites the ZnO surface and CsPbBr 3 QDs to facilitate the separation of their electron-hole pairs. Electrons were then pumped to their corresponding E , C leaving holes in their E . V Hot electrons above its E C may be transferred to its interface and fall directly from its E C to emit fluorescence. A reduced recombination rate was considered an effective way to enhance interfacial radiation. (2) The organic molecules on the surface of TiN NPs and colloidal CsPbBr 3 QDs can store electrons and holes, [52] reduce the direct recombination of electron-hole pairs on the corresponding E C and E , V and increase the number of different interface carriers. The electron trap effect of TiN NPs can capture more hot electrons, E C electrons, and defect electrons on the heterojunction to migrate to the CsPbBr 3 QDs interface and the interface of the ZnO film, and achieve potential equilibrium through electrons. (3) The up-and-down transfer of electrons in the s-p band to the Fermi energy was beneficial to the energy utilization of fluorescence and increased the number of interface electrons. The d-band electrons from TiN NPs can absorb the fluorescence energy of CsPbBr 3 QDs, and ZnO film were pumped to their s-p bands to enhance the interfacial radiation. (4) With an increase in the number of surface electrons in TiN NPs, the interfacial recombination of electrons and holes leads to enhanced interfacial radiation. [53,54] We observed fluorescence enhancement at 376 nm for the ZnO film and at 519 nm for the CsPbBr 3 QDs. The enhanced fluorescence was due to the transfer of photo-generated carriers through carrier diffusion, as well as the trap and electron conductor effects of TiN. Owing to the existence of surface plasmons in the surface state of the TiN NPs, [29,55] the hot electrons of the higher E C of the CsPbBr 3 QDs with higher E C were directly swept and transferred to the E C in the defect state of the ZnO film. Only a small fraction of the electrons fell and recombined with holes. Therefore, the defect fluorescence was weakened due to the inhibitory effect of TiN NPs.

Conclusion
In summary, CsPbBr 3 /ZnO and CsPbBr 3 /TiN/ZnO heterojunctions were prepared using colloidal selfassembly and centrifugal casting methods, and the PL effect of TiN NPs on the CsPbBr 3 /ZnO heterojunction was fully studied. Compared with the CsPbBr 3 /ZnO heterojunction, the fluorescence of the CsPbBr 3 /TiN/ZnO heterojunction was enhanced by 3.2 times with a thickness of TiN film of 51 nm. This was attributed to the surface plasmon state of TiN NPs serving as an intermediate energy level between CsPbBr 3 QDs and ZnO, which improved the electron transfer rate at the heterojunction interface and reduced non-radiative reconstruction. However, as the deposition thickness of the TiN NPs was increased, the photo-generated carriers of CsPbBr 3 QDs were more easily lost during the transfer process, resulting in the wreaking of the radiation fluorescence of the heterojunction CsPbBr 3 /TiN/ZnO being wreaked. This method of fluorescence enhancement and energy transfer can be widely used in photovoltaics, photodetectors, LEDs, light sensing, and photocatalysis.