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Abstract
Two-dimensional (2D) metal-organic frameworks (MOFs) have attracted considerable attention owing to their fascinating properties, including ordered crystalline structures, large surface areas, and related unique 2D properties. Moreover, 2D MOFs have been widely used in energy, catalysis, and optoelectronic applications. However, researchers have performed fewer investigations on photonic applications. To remedy this gap in knowledge, recent progress in the development of 2D MOFs for photonic applications was investigated. First, the background and motivation of this review are introduced. Then, the synthesis method and properties are presented, followed by an introduction to their photonic device applications. Finally, future research prospects and challenges in 2D MOFs for photonic applications are proposed.
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1. Introduction
Two-dimensional (2D) materials have attracted extensive attention since the discovery of single-layer graphene in 2004 [1]. In sharp contrast to conventional bulk semiconductors, 2D materials possess superior properties, such as high room temperature carrier mobility, tunable bandgaps, high speed photoresponse, strong light–matter interaction, and nanoscale integrated degree [2–9]. Combined with several advanced techniques, such as doping, and the use of plasmon and heterojunctions, the properties of 2D materials can be further enhanced [10–17]. These properties enable 2D materials to be widely employed in photonic and optoelectronic applications, including modulators [18,19], photodetectors [20,21], integrated chips [22,23], light emission devices [24,25] and others [4,26–29]. Owing to rapid developments in the semiconductor industry over the past several decades, a considerable number of novel 2D materials have been developed, including MXene [30,31], graphdiyne [32,33], hybrid perovskites [34,35], tellurene [36,37], and bismuth chalcogenides [38], which provide extra opportunities for realizing high-performance photonic and optoelectronic devices. Metal-organic frameworks (MOFs) possess porous crystalline structures, consist of organic linkers and transition metal joints that construct nanometer-sized spaces, and exhibit ordered crystalline structures and large surface areas [39–42]. According to calculations, almost 2000 types of MOFs have been developed and more than one thousand of these were reported to feature featured a 2D layered structure, which indicates that 2D MOFs possess excellent properties related to 2D materials [43]. Thus, this discovery paves a way to realizing high-performance photonic [42,44], optoelectronic [45,46], energy [47,48], sensing, [49,50] and catalysis [51,52] applications. Additionally, combined with various extrinsic functional materials, multi-function 2D MOFs can be achieved, which further extends their application and enhances 2D MOF-based device performance [53–56]. Although 2D MOFs feature advantageous properties related to 2D materials, several distinct differences or gaps still exist between 2D materials and 2D MOFs. In terms of 2D materials, the strong chemical activity can be attributed to the combination of the state and edge effect [3,4]. Additionally, owing to their porous nature and large atomic scale flexibility, 2D materials exhibit great potential for high-speed and wearable device applications [6,7]. In contrast, 2D MOFs feature typical ordered porous crystalline structures, which provide different properties than that of 2D materials, such as intrinsic porosity, tunable crystalline structures, and well defined and accessible active or binding sites [39,43]. Owing to these superior properties, considerable attention has been directed at 2D MOF-based electronic, energy, and catalysis applications. Motivated by this, a comprehensive and detail review of 2D MOFs in photonic applications is presented, which is favorable for further development of 2D MOFs.
In this paper, recent progress in 2D MOF-based photonic applications is summarized. First, the synthesis method and properties of 2D MOFs are briefly intorduced. Then, several representative photonic applications based on 2D MOFs are highlighted, including modulators, photodetectors, light emission devices, and luminescence sensors. A prospective on future research and the challenges of 2D MOFs for photonic applications are proposed at the end of this paper.
2. Synthesis
As mentioned, 2D MOFs play a key role in various applications owing to their excellent performance. In the past few years, numerous investigations have been performed on the synthesis of 2D MOFs [57,58], and these methods can be divided into two categories: top-down and down-top. To obtain a better insight into these fascinating materials, recent progress in the synthesis of 2D MOFs is summarized in this section.
2.1 Top-down means
The top-down synthesis method is widely used to fabricate 2D MOFs owing to its simple process; in particular, ultrasound, shaking, and ball milling processes are commonly used to generate nanoflakes from its bulk condition [59]. Moreover, to achieve layered structures within these processes, these bulk materials interact with weak forces, such as hydrogen bonds or π-π interactions [60]. Although its simple fabrication process enables the top-down method to be widely employed, it suffers several severe defects owing to the shear moduli and low bulk of 2D MOFs, including undesired fragmentation and morphological damage, uncontrolled structural deterioration, a low yield efficiency, and non-protected nanoflakes that strongly tend to agglomeration [59,60].
Despite considerable efforts to synthesize 2D MOFs, versatile 2D MOFs are mainly required for energy storage and catalysis applications [45,47,51]. In this section, several representative progresses in 2D MOF synthesis related to photonic applications are highlighted. First, two investigations are presented on 2D MOFs based on 1,3,5-triphenylbenzene corelayers and Zn-BTA in fluorescent applications [61,62], followed by a brief introduction on Zn- and Ni-based 2D MOFs in luminescence-sensing applications [63,64]. Moreover, these investigations also present the controllable and efficient synthesis of 2D MOFs, indicating that top-down synthesized 2D MOFs possess great potential for diverse high-performance photonic applications.
In the top-down method, liquid-phase exfoliation (LPE) is a commonly used approach for achieveing high-quality or high-performance 2D MOFs. Typically, with the help of solvent molecules, ultrasonic treatment is employed to delaminate the target material from bulk to layered conditions. In 2018, Moorthy et al. demonstrated the synthesis of 1,3,5-triphenylbenzene corelayered 2D MOFs via ultrasonic assisted LPE [61]. As shown in Fig. 1(a), the synthesis process can be divided into three steps: First, a honeycomb 2D network structure of the 1,3,5-triphenylbenzene core can be achieved through a self-assembly process; second, honeycomb MOFs are obtained in an autogenous solvothermal environment; third, following the heating and cooling down processes combined with an ultrasonic and LPE treatment, 1,3,5-triphenylbenzene core 2D layered MOFs are produced from the bulk condition. In 2019, Maka et al. presented 2D MOFs based on a benzoquinone/hydroquinone redox couple (Fig. 1(b)) [62]. Utilizing a two-step process, H4BAT was synthesized. Subsequently, H4BAT was dissolved in a DMF/DMSO solution to form a mixed solution. Following the heating and cooling down processes, block-shaped Zn-BTA was achieved, and combined with the ultrasonic LPE method, layered 2D Zn-BTA MOFs were eventually obtained from its bulk state. To pursue a higher exfoliation rate in 2D MOFs, in 2020, Liu et al. illustrated a high-efficiency and environmental friendly synthesis of Zn2(bim)4 MOFs upto 47%, as shown in Fig. 1(c) [63]. Accordingly, through a hydrothermal transformation, Zn2(bim)4 precursors were achieved. Then, combined with dissolved (10 mL water and ionic liquid solution), ultrasonic, and centrifugation (3000 rpm for 20 min) processes, 2D Zn2(bim)4 MOFs were eventually achieved. Noticeably, after the exfoliation process, the size of the obtained free standing 2D Zn2(bim)4 MOFs presented in the micrometer scale, which indicates the ultrathin nature of 2D Zn2(bim)4 MOFs. To realize the controllable synthesis of 2D MOFs, numerous investigations have been performed. In 2021, Zhang et al. presented the controllable synthesis of 2D Ni(pyz)2Cl2/Ni(pyz)2Br2 MOFs via halogen interactions [64]. First, Ni(pyz)2Cl2 and Ni(pyz)2Br2 were synthesized. Then, these two prepared samples were dispersed in de-ionized water and ethanol, respectively. Via sonication, stirring, and standing treatments, the target 2D Ni(pyz)2Cl2/Ni(pyz)2Br2 MOFs were obtained, as shown in Fig. 1(d). Remarkably, controllable synthesis was achieved by tuning the polarity of the solvent and ultrasound time and employing various halogen compositions during the synthesis process.
Fig. 1. (a) The designed two-dimensional (2D) metal-organic framework (MOF) nanosheets based on the 1,3,5-triphenylbenzene core [61], reproduced with permission from [61] (copyright 2018, The Royal Society of Chemistry). (b) 2D MOFs based on 2,3,5,6-tetrakis(pcarboxyphenyl) hydroquinone H4BTA with Zn(NO3)2 [62], reproduced with permission from [62] (copyright 2019, MDPI). (c) Transmission electron microscopy (TEM) images of 2D MOFs based on bulk and layered Zn2(bim)4, respectively [63], reproduced with permission from [63] (copyright 2020, American Chemical Society). (d) TEM images of 2D MOFs based on bulk and layered Ni(pyz)2Br2, respectively [64], reproduced with permission from [64] (copyright 2021, Elsevier Ltd).
2.2 Down-top method
In the down-top method, 2D MOFs can be grown from metal ions and organic linkers directly onto target substrates. Beneficial from non-limited lateral dimensions, the synthesized 2D MOFs are not based on precursor structures. Additionally, through the surfactant-assisted down-top approach, the thickness of anisotropically-grown 2D MOFs demonstrates that they can be confined to an ultrathin scale. To ensure a stable synthesis process, the growth of 2D MOFs is selectively prevented in one direction without affecting the growth of MOFs in bulk and while avoiding the reprocessing of 2D nanoflakes during the synthesis process.
In sharp contrast to the top-down approach, 2D MOFs are synthesized directly through the down-top method, including the ligand modulation [65,66], interface [67] and surfactant [64] assisted synthesis methods. Moreover, without applied external forces, 2D MOFs synthesized via the down-top method exhibit better qualities, such as fewer structural defects, a large scale, and high efficiency [68,69]. Motivated by these advantages, in 2018, Li et al. illustrated a simple growth process for highly-oriented ultrathin 2D ZnO-based MOFs [70]. The ZnO was coated on the substrate via a dip-coating technique. Following the heating and cooling down processes, using a simple ammonia-assisted self-conversion method, typical 2D MOFs based on ZnO were synthesized, as shown in Fig. 2(a). Remarkably, the obtained MOFs exhibited a highly oriented [002] direction, indicating that this growth process has great potential for large-scale molecular sieving applications [70]. To meet the efficient and simple synthesis of 2D MOFs, a one-step synthesis process is in high demand. Consequently, in 2019, Suginome et al., developed a one-step synthesis method for adaptive 2D nano-graphene-based MOFs (Fig. 2(b)) [71]. The prepared hexa-peri-hexabenzocoronene-based ligand (HBCLH2) was reacted with Zn(NO3)2·6H2OinN,N-diethylformamide (DEF) solution for 72 hours at 25 °C to achieve yellow rhombic HBCMOFs. Subsequently, in 2019, Wang et al. demonstrated the synthesis of highly stable 2D MOFs based on Zr-BTB (BTB =1,3,5-tris(4-carboxyphenyl)benzene) nanoflakes via a one-step process [72]. As shown in Fig. 2(c), Zr-BTB nanoflakes were obtained by linking Zr6 clusters with the tritopic Zr-BTB linker to form a (3,6)-connected network. Noticeably, the thickness of the Zr-BTB nanoflakes can be efficiently modified by adding water during the synthesis process, which can partially replace the terminal carboxylate ligands and have a positive contribution to thickness modulation of the target 2D MOFs. Furthermore, compared with that of conventional solvothermal synthesis processes, this simple and efficient one-step synthesis method can prevent close packing of the Zr-BTB bulk materials caused by strong interlayer interactions. To further improve the synthesis efficiency and simplify the synthesis process, in 2021, Yuan et al. demonstrated a surfactant-free, one-pot, scalable synthesis of highly stable 2D MOFs based on NUS-8 nanoflakes [73]. During the synthesis process, formic acid was employed as the capping agent to act as selective coordination to form 2D MOFs. Through a drop-casting process, the synthesized NUS-8 sample was transferred onto the cleaned target substrate, as illustrated in Fig. 2(d). Remarkably, the synthesized 2D MOFs based on NUS-8 nanoflakes exhibited an average area greater than 15000 µm2, indicating the great potential of this one-step method in the synthesis of large scale 2D MOFs. Regarding commercial application, the cost and quality of 2D MOFs are two key parameters. To significantly suppress the cost and enhance the quality of 2D MOFs, extensive investigations have been performed. In 2020, Xu et al. presented low-cost and high-quality synthesis of 2D MXene-based MOFs [74]; 2D atomically thin MXene was used to form the 2D MOFs. Accordingly, MXene and tetrakis(4carboxyphenyl)porphyrin (TCPP) were dissolved in 20 mL DMF to form a mixed solution. Then, the mixture was transferred to a Teflon autoclave for the heating process (150 °C for 4 h), and centrifugation, transfer, and spin-coating processes were carried out. During the heating process in a vacuum environment, 2D MXene-based MOF films were produced (Fig. 2(d)).
Fig. 2. (a) The synthesis process of two-dimensional (2D) ZnO-based metal-organic frameworks (MOFs) [70], reproduced with permission from [70] (copyright 2018, Tsinghua University Press and Springer-Verlag GmbH Germany). (b) The synthesis process of 2D HBC-based MOFs [71], reproduced with permission from [71] (copyright 2019, American Chemical Society). (c) The one-step synthesis of PCN-134-3D-based 2D MOFs [72], reproduced with permission from [72] (copyright 2019, WILEY-VCH). (d) A one-pot, surfactant-free, and scalable synthesis of highly stable 2D MOFs [73], reproduced with permission from [73] (copyright 2021, WILEY-VCH). (e) The synthesis process of 2D V2CTx Mxene-based MOFs [74], reproduced with permission from [74] (copyright 2020, American Chemical Society).
To facilitate a better understanding of the various 2D MOF synthesis methods, a comparison of their key features is presented in Table 1.
Table 1. The key features of two-dimensioanl (2D) metal-organic frameworks (MOFs) from various synthesis techniques
2.3. Properties
Owing to the great potential of 2D MOFs in various applications, particularly photonic applications, it is important to obtain a better insight into their properties, including electrical and optical properties, which enhance 2D MOF-based photonic device performance. Thus, recent progress in the understanding of their physical properties is briefly summarized in this section.
2.3.1. Electrical properties
As previously mentioned, 2D MOFs possess numerous advantageous properties and are considered a promising candidate for various photonic applications. The electronic band structure of 2D MOFs has attracted many studies using density-functional theory (DFT) calculations. In 2018, Kumar et al. theoretically investigated the band structure of 2D DCBP3Co2 and DCA3Co2 MOFs [75]. The calculated band structures of these two MOFs are illustrated in Fig. 3(a) and (b); owing to the ligand states, the occupied states possessed a stronger metal character than that of the unoccupied states. Moreover, the lowest unoccupied molecular orbital exhibited a shift behavior to the Fermi level, suggesting an electrostatic shift in the orbital. In terms of electromagnetic wave absorption applications, dielectric loss is thought to be a determine parameter that has considerable influence over the device performance. In this regard, in 2020, Wang et al. demonstrated extremely low dielectric loss electromagnetic wave absorption based on MXene@RGO MOFs [76]. The conductivity of three different concentration MXene@RGO MOFs is presented in Fig. 3(c). The calculated conductivity increased monotonously as the applied filler loading increased, which can be attributed to electron hopping on the Ti3C2Tx nanosheets and RGO networks. Meanwhile, the MXene@RGO MOFs with higher concentrations exhibited higher conductivities than those of lower concentration. This is mainly because the unique conductive network structure is more sufficiently formed at higher concentration. Figure 3(d) illustrates a typical Tauc plot for 2D MOFs based on Fe3(THT)2(NH4)3 nanofilms with a thickness of 1.7 µm; the sample had a direct bandgap with a value of 0. 45 eV, indicating that 2D Fe3(THT)2(NH4)3-based MOFs can be used to fabricate infrared photonic and optoelectronic devices [77].
Fig. 3. (a and b) Calculated band structure and total PDOS of DCBP3Co2 and DCA3Co2 metal-organic frameworks (MOFs), respectively [75] (copyright 2018, American Chemical Society). (c) The conductivity of Mxene@RGO composites [76], reproduced with permission from [76] (copyright 2020, Elsevier Ltd). (d) Tauc plot of the MOF [77], reproduced with permission from [77] (copyright 2020, WILEY-VCH).
2.3.2. Optical properties
The optical properties of 2D MOFs play a key role in their photonic applications. To obtain a better insight and full understanding of these optical properties, extensive investigations have been performed. In 2018, Jiang et al. systemically examined the nonlinear optical properties (the interaction of light with materials whose light scattering and refraction are nonlinear functions of the applied optical or electric fields) of 2D Ni-MOFs [78]. Through Fourier transform infrared spectroscopy (FTIR) measurements, the molecular vibration modes of 2D Ni-MOFs were investigated. As shown in Fig. 4(a), two peaks at 1507 cm-/1 and 3583 cm-/1 were observed and assigned to the stretching vibrations of the para-aromatic C–H and OH− groups. Additionally, due to symmetric stretching and the asymmetry of the coordinated−COO− groups, two peaks located at 1374 cm−1 and 1573 cm−1 were also observed. Owing to the H2O molecules in the 2D Ni-MOFs, three peaks (3049, 3336, and 3421 cm−1), corresponding to the stretching vibration modes of H2O molecules were observed. The transmission spectrum of three different samples is illustrated in Fig. 4(b); a clear absorption edge is observed at approximately 350 nm, which matches well with the bandgap of 2D Ni-MOFs (3.12 eV). To investigate the nonlinear optical properties of 2D Ni-MOFs, the Z-scan technical method was performed. The dependence of the nonlinear absorption coefficient and the third-order nonlinear optical susceptibility to various characterization wavelengths is shown in Fig. 4(c); the effective absorption coefficient and third-order nonlinear optical susceptibility was obtained as −3 × 10−2 cm GW−1 and −3 × 10−14 esu, respectively. The measured negative absorption coefficient suggests that the transmittance increased monotonously as the incident laser intensity increased, a behavior known as saturable absorption. Moreover, the nonlinear transmission spectrum as a function of various incident laser intensity was investigated. The measured outcomes are similar to that of other commonly used 2D materials, such as graphene and black phosphorus (BP), indicating that 2D Ni-MOFs have great potential for saturable absorption applications. Furthermore, 2D MOFs are applied as efficient photocatalysts owing to their functionality using the clever selection of organic linkers and inorganic nodes. Motivated by this, in 2019, Wang et al. designed a porphyrin-based MOF and investigated its optical properties [72]. The designed porphyrin-based MOF was used to monitor the 1O2 generation of 1,3-diphenylisobenzofuran (DPBF). As shown in Fig. 4(e), the absorption spectra of DPBF exhibited a distinct peak at approximately 410 nm and decreased monotonously as the wavelength further increased; this is attributed to the generation of 1O2. Remarkably, the measured degradation rate of DPBF presented its highest value when the PCN-134-2D MOFs were employed as photocatalysts, which is more rapid than that of PCN-134-3D and blank conditions, suggesting the superior catalytic activity of PCN-134-2D MOFs (as shown in Fig. 4(f)). This is mainly a result of the small particle size and large pore apertures of PCN-134-2D, which can efficiently accelerate the diffusion speed of DPBF into the MOF particles, leading to rapid reaction kinetics. To evaluate the microwave absorption performance of 2D MOFs, in 2020, Han et al. designed 2D MXene@Co-CZIF and MXene@Ni-CZIF-based MOFs for microwave absorption applications [79]. Raman spectra measurements were performed to characterize the optical properties of these two 2D MOFs, as shown in Fig. 4 g; two clear peaks can be observed (D 1350 cm−1 and G 1580 cm−1), which can be classified as the carbon atomic lattice defects and in-plane sp2 hybridization stretching vibration mode, respectively. Additionally, the ratio of ID and IG were 1.07 for 2D MXene@Co-CZIF MOFs and 1.08 for 2D MXene@Ni-CZIF MOFs, respectively, indicating that these two 2D MOFs possess a higher graphitization degree. To explore further novel 2D MOFs, in 2021, Li et al. demonstrated 2D Pb2+ ions and 2-nitroimidazole MOFs for the first time and systematically investigated their optical properties [80]. The luminescent spectra of these two novel 2D MOFs are presented in Fig. 4 h under an illumination of 300 nm and 423 nm. Two emission peaks at 470 and 485 nm can be observed for both types of 2D MOFs, which were related to the π → π* or n → π* transitions and the intralig and charge transfer of nIm ligand, respectively, suggesting these two novel 2D MOFs are promising for light emission device applications.
Fig. 4. (a and b) Fourier transform infrared spectroscopyand the transmission spectrum of Ni-MOFs, respectively. (c) The nonlinear absorption coefficient and third-order nonlinear optical susceptibility of Ni-MOFs as a function of various characterization wavelengths. (d) The dependence of the transmittance to the various incident laser intensities [78], reproduced with permission from [78] (copyright 2018, WILEY-VCH). (e) The absorption spectra of 2D PCN-134-based MOFs. (f) The degradation of 2D/3D PCN-134-based MOFs using the absorbance decay at 410 nm [72], reproduced with permission from [72] (copyright 2019, WILEY-VCH). (g) The Raman spectra of MXene@Co-CZIF and MXene@Ni-CZIF [79], reproduced with permission from [79] (copyright 2020, American Chemical Society). (h) The emission spectrum of 2D nIm and [Pb5(OAc)7(nIm)3]n-based MOFs [80], reproduced with permission from [80] (copyright 2021, Springer).
3. 2D MOFs for photonic applications
Owing to their advantageous properties and ultrathin layered structures, including a tunable bandgap, relatively high optical transparency, the broadband emission spectrum, 2D MOFs exhibit great potential for intensive photonic and optoelectronic applications, such as modulators, photodetectors, light emission devices, and luminescence sensors. As a result, recent progress in 2D MOF-based photonic applications is briefly summarized in this section.
3.1 Modulator
The superior properties of 2D MOFs, including strong light–material interactions [76,78–80], broadband third-order nonlinear optical properties [81,82], and a tunable bandgap [83,84], enable 2D MOFs to be wildly used to fabricate high-performance nonlinear optical devices, particularly modulators. In this regard, in 2019, Sun et al. designed a single-frequency fiber laser with 2D Ni MOFs used as a saturable absorber, as shown in Fig. 5(a) [85]. The typical 3 dB line width of the fiber laser was measured to be 3.2 kHz (Fig. 5(c)), which is comparable to that of other 2D material-based single-frequency fiber lasers. Moreover, the signal-to-noise ratio was measured to further evaluate the performance of this fabricated device, as illustrate in Fig. 5(d); a high signal-to-noise ratio (larger than 52 dB) was achieved, which can be ascribed to the sufficient suppression of most longitudinal modes and their corresponding losses. The operation stability is another key parameter. As presented in Fig. 5(c), the fluctuation of the output power was less than 1.3%, which is almost one time smaller than that of other 2D material-based single-frequency fiber lasers. Subsequently, in 2018, Jiang et al. demonstrated a broadband modulator based on 2D Ni MOFs, the experiment setup of which is present in Fig. 5(b) [78]. The measured time interval of two adjacent pulses was approximately 104.5 ns, and the corresponding repetition rate was 9.57 MHz (Fig. 5(f)), which is comparable to that of other 2D material-based modulators. Additionally, the operation stability is presented in the inset of Fig. 5(f), which proves the long-term stability of this fabricated device. To evaluate the further performance of the device, the optical spectrum and line width measurements were obtained, as shown in Fig. 5 g and h. The output beam was concentrated at 1062 nm with a bandwidth of 2.23 nm at 3 dB, and the pulse duration was estimated to be 240 ps. Remarkably, the signal-to-noise ratio was higher than 52 dB, which is mainly caused by the absence of parasitic side peaks. These outcomes confirm that 2D MOFs have great potential for high performance modulator applications.
Fig. 5. (a) The experiment setup of a 2D Ni MOF-based fiber laser [85], reproduced with permission from [85] (copyright 2019, The Royal Society of Chemistry). (b) The experiment setup of a 2D Ni MOF-based fiber laser [78], reproduced with permission from [78] (copyright 2018, WILEY-VCH). (c) The 3 dB line width of a 2D Ni MOF-based fiber laser. (d and e) The measured signal-to-noise ratio and stability of a 2D Ni MOF-based fiber laser, respectively [78], reproduced with permission from [78] (copyright 2018, WILEY-VCH). (f) The measured pulse trains of a 2D Ni MOF-based fiber laser. (g and h) The measured mode-locked optical spectrum and pulse duration, respectively [85], reproduced with permission from [85] (copyright 2019, The Royal Society of Chemistry).
3.2 Photodetector
The photodetector, defined as the device that converts light signals into electrical signals [86,87], has been applied in intensive fundamental research and practical applications. The excellent light–matter interactions, broadband optical response, and large scale highlight 2D MOFs as a promising candidate channel material for high-performance photodetector applications [77]. Typically, several metrics, such as the response time (describes changes in the photodetector velocity with changing light signal frequencies, including rising and falling times (τr and τf), which represent the times required for the photocurrents to rise from 10 to 90% and fall from 90 to 10%, respectively), noise equivalent power (NEP: refers to the minimum radiation power that photodetectors can detect or distinguish from noise) [88], responsivity (defined as the photocurrent Ip or photovoltage Vp generated by the optical power and can be expressed as $ R_{v}=V_{p / P} $ or $R_{I}=I_{p} / P$, where P is the power of the incident light.) [21], specific detectivity (given by D* = (AΔf)1/2/NEP, where A is the effective device area and Δf is the bandwidth. D* is expressed in cm·Hz1/2·W−1 or Jones—named in honor of Robert Clark Jones, who originally defined the unit) [14], and external quantum efficiency (EQE: the ratio of the number of photogenerated electron–hole pairs to the number of incident photons. It is given by EQE = Rhν/e, where e is the elementary charge, h is the Planck constant, ν is the incident light frequency, and R is the photoresponsivity) [89] are commonly used to evaluate the performance of a photodetector. In this section, the performance of a photodetector based on 2D MOFs is discussed using these metrics.
With the rapid development of the semiconductor industry, there is an urgent demand for photodetectors with ultra-sensitive and broadband response detection. Therefore, intensive investigations have been performed. Owing to the highly tailorable electrification and mobility of 2D MOFs, they have attracted considerable attention for high performance photodetector applications. In 2020, Arora et al. demonstrated a 2D Fe3(THT)2(NH4)3 MOF-based broadband photodetector; the schematic of this device is shown in Fig. 6(a) [77]. A photosensitivity measurement was performed to quantify the optically generated charge carrier process. As shown in Fig. 6(b), as the temperature decreased, the measured photosensitivity increased monotonously for four different incident laser power densities, which can be attributed to the higher photocurrent generation. To further evaluate the device performance, NEP, responsivity, and specific detectivity measurements were performed, as illustrated in Fig. 6(c)–(d). The temperature-dependent responsivity exhibited its highest value of 2.5 kV W−1 at 77 K, and as the temperature increased from 77 K to 300 K, the responsivity decreased to 0.07 kV W−1; this is mainly attributed to the efficient suppression of thermally activated charge carriers at lower temperatures. Furthermore, the NEP, which denotes the detection limit of the photodetector, was measured to estimate the device performance. As shown in Fig. 6(c), the NEP possessed its lowest value at 77 K and increased as the temperature increased. For instance,under this experimental condition, both the generation-recombination noise and Johnson noise were significant effects to consider during the NEP calculation process. Following this, the temperature-dependent specific detectivity was investigated to evaluate the device performance; similar to theNEP, the specific detectivity decreased monotonously as the temperature increased from 77 K to 300 K for six various incident beam power densities. Remarkably, the measured highest value of specific detectivity was 7 × 108 cm Hz1/2 W−1 at 77 K, which is comparable to that of other 2D material-based photodetectors. Subsequently, the photo-switching performance, particularly the response time, was measured to evaluate the detected speed of the device. As shown in Fig. 6(e), the rise and decay times were 2.3 s and 2.15 s at 77 K, respectively, which are mainly affected by the various defect densities and types introduced during the device fabrication processes. To test the broadband response, the responsivity was measured as a function of various incident wavelength laser beams, as illustrated in Fig. 6(f). As the efficient absorption of 2D Fe3(THT)2(NH4)3 MOFs decreased from 400 to 1575 nm, the measured responsivity of the device exhibited no significant fluctuation tendency, indicating that 2D Fe3(THT)2(NH4)3 MOFs are promising for high performance broadband photodetector applications.
Fig. 6. (a) A schematic diagram of a 2D Fe3(THT)2(NH4)3 MOF-based photodetector. (b) The photosensitivity of the device as a function of the incident laser intensity with a bias of -1 V. (c) The responsivity and NEP of the device as a function of temperature with a bias of -1 V and a power density of 0.14 W cm-2. (d) The specific detectivity of the device as a function of power density with a bias of -1 V. (e) The measured response time of the device under an illumination of 785 nm with a bias of -1 V and a power density of 0.14 W cm-2. (f) The measured responsivity of the device as a function of photon energy with a power density of 0.14 W cm-2 at 100 K [77]. Reproduced with permission from [77] (copyright 2020, WILEY-VCH).
3.3 Light emission devices
Currently, multicolor, particularly white color, emission materials have attracted intensive scrutiny owing to their great potential in light emission devices (LEDs) [42,90–92]. Since graphene was discovered in 2004, 2D materials have been widely applied in LEDs due to their superior properties, such as the quantum confinement effect, large exciton binding energy, and tunable bandgap [93–96]. Recently, 2D MOFs have also been proven to have great potential for LEDs because of their high photoluminescence efficiency, long luminescence lifetimes, and intense and sharp emission peaks. Motivated by this, recent progress in the use of 2D MOFs in LEDs is briefly introduced in this section.
As one of the most important applications in LEDs, white light emission devices have attracted considerable attention owing to their wide application in visible-light communication, wireless communication, and Bluetooth [97,98]. 2D MOFs are a promising candidate for high-performance white light emission devices because of their aforementioned superior properties. Therefore, in 2017, Hu et al. fabricated a white light emission device based on 2D Zr-TCBPE MOFs (TCBPE = tetraphenylethylene-based tetracarboxylate) [99]. After illumination at 450 nm, the generated broad emission spectrum was concentrated at 560 nm with a full width half maximum of 100 nm and a 50% quantum yield, as illustrated in Fig. 7(a). Additionally, combined with a blue-light emission device, the emitted spectra spanned the entire visible range. The fluorescence lifetime was measured to be 2.6 ns (Fig. 7(b)), which can provide an on/off frequency of over 60 MHz; this is considerably higher than several commercial devices. These outcomes indicate the great potential of 2D Zr-TCBPE MOFs in high performance, specifically, efficient, broadband, and high-speed, white light emission device applications. To meet the demands of practical applications, such as lower costs, environmental friendliness and safety, and lower energy consumption, 2D MOFs based on non-rare earth metals must be explored. In 2018, Mondal et al. demonstrated a white light emission device based on 2D Zn MOFs; the schematic of the device is shown in Fig. 7(c) [100]. Typically, the device is fabricated through spin-coating, deposition, transfer, and evaporation processes. Voltage-dependent electroluminescence measurements were performed to evaluate the device performance. As shown in Fig. 7(d), the generated emission contained a broad peak at 537 nm for 6 V, which can be ascribed to the electron–hole recombination process. As the bias voltage increased to 7 V and 8 V, the emission presented a broad peak at 537 nm and a sharp peak at 445 nm and spanned the entire visible band, which is mainly caused by the accumulation of charged particles in the HOMO and LUMO energy levels. Apart from visible light, near infrared (NIR) emission is widely applied in both civilian and military fields. Thus, it is of great significance to explore novel 2D MOFs that can efficiently generate NIR light emission. In 2020, Chang et al. fabricated a flexible and efficient NIR emission device based on 2D Ln(Ln = Nd, Yb, Er, or Gd) MOFs [101]. Upon a 341-nm and 350-nm illumination, the generated NIR emissions ranged from 888 to 1532 nm (Fig. 7(e)), which can be attributed to the π−π* transitions of 1,4-Naphthalenedicarboxylic acid. Meanwhile, 3D Ln MOFs were synthesized to evaluate the performance of the 2D Ln MOF-based NIR emission device. As shown in Fig. 7(f), upon a-357 nm and 360-nm illumination, the intensity and spectra range of the generated NIR emissions were smaller than that of 2D Ln MOFs. Moreover, benefiting from the high transparency and excellent flexibility and mechanical properties, 2D Ln MOFs exhibit great potential for light-softening, NIR light emission device applications. Besides noble metals, earth abundant metals, in particular, Cu, Mn, Cd, Zn, and Co, have been used to synthesize 2D MOFs. In 2020, Steuber et al. investigated the photo-physical property of five novel 2D M MOFs (M = Cu, Mn, Cd, Zn, and Co) [102]. Upon a 405-nm photo excitation, the generated emissions of five different 2D MOFs exhibited similar behaviors, with a central peak at approximately 728 nm, as shown in Fig. 7 g. Among these five 2D MOFs, the 2D Zn MOFs possessed the highest quantum yield (nearly six times larger than that of 2D Cu MOFs). Additionally, time dependence spectra measurements were performed to evaluate the performance of these five 2D MOFs. As illustrated in Fig. 7 h, 2D Co MOFs possessed the shortest average lifetime (3.0 ns), which is nearly nine times smaller than that of 2D Cu MOFs; this can be attributed to the fast photo-induced electron transfer process. Two-photon absorption up-converted emission is another emission mechanism that is strongly affected by the cross-section of two-photon absorption. To significantly improve the cross-section, in 2021, Hu et al. designed 2D metal−organic layers (MOLs) from 2D H3TCBPA (tri(4-carboxylic-biphenyl)-4′-amine) MOFs [103]. The two-photon absorption cross-section was improved to 13 000 ± 2000 GM, which is seven times larger than that of pristine H3TCBPA. Accordingly, the intensity of the generated emission of 2D H3TCBPA MOLs was much larger, as shown in Fig. 7(i). These exciting findings provide an extra opportunity to realize high-performance two-photon absorption up-converted emission devices.
Fig. 7. (a) The emission spectra of 2D Zr-TCBPE MOFs. (b) The time dependence spectra of 2D Zr-TCBPE MOFs [99], reproduced with permission from [99] (copyright 2017, MDPI). (c) A schematic diagram of 2D Zn MOF-based LEDs. (d) The electroluminescence spectra of ZnipaPy2 as a function of voltage [100], reproduced with permission from [100] (copyright 2018, The Royal Society of Chemistry). (e and f) The excitation and emission spectra of solid 2D Ln MOFs and Ln nano-papers, respectively [101], reproduced with permission from [101] (copyright 2020, American Chemical Society). (g and h) The emission and time dependence spectra of 2D M MOFs (M = Mn, Co, Cu, Zn, and Cd) [102], reproduced with permission from [102] (copyright 2020, American Chemical Society). (i) The excitation spectrum of 2D H3TCBPA and Hf-TCBPA MOFs [103], reproduced with permission from [103] (copyright 2021, American Chemical Society).
3.4 Luminescent Sensor
Currently, human health is one of the most popular global topics and attracts considerable attention. Typically, the selective sensing of metal ions (in particular, Fe2+ and Fe3+) and small organic molecules is of great significance in the monitoring of human health. Thus, luminescence or fluorescence sensors have been developed to realize high-sensitivity detection [104,105]. Owing to the aforementioned superior properties of 2D MOFs, they have been considered a promising candidate for the fabrication of high-performance sensor applications. Motivated by this, in 2016, Zhao et al. demonstrated novel organic ligands with chromophores in 2D ([Zn2(TPC4A)(DMF)(H2O)4]3H2O MOFs and 2D [(CH3)2NH2]2[Zn(TNC4A)]·4H2O) MOF-based luminescence sensors for sensitivity and the selectivity detection of Fe2+ and Fe3+ ions [106]. The anti-interference sensing ability of the fabricated sensors was demonstrated to evaluate the sensitivity and selectivity detection performance. As shown in Fig. 8(a) and (b), no significant changes in emission intensity were observed for both sensors in the presence of mixed metal ions, which indicates that the sensitivity and selectivity detection of Fe2+ and Fe3+ ions are not affected by other metal ions, and the device exhibits a remarkable ability to sense Fe2+ and Fe3+ ions. In 2018, Zhang et al. designed five novel 2D Ln MOF-based luminescence sensors to selective sense Fe3+ ions [107]. The quenching efficiency of the Fe3+ ions was measured to be 97.8%, indicating that the fabricated sensor has great potential for recognizing Fe3+ ions.
Fig. 8. (a and b) The emission intensities of two types of sensors with various metal ions [106], reproduced with permission from [106] (copyright 2016, American Chemical Society). (c) The PL spectra of 2D 4-DMF MOFs as a function of various Fe3+ ion concentrations. Inset: the nonlinear Stern-Volmer curve at low Fe3+ ion concentrations [107], reproduced with permission from [107] (copyright 2018, The Royal Society of Chemistry). (d) The luminescence intensity of 2D Co-Zn MOFs as a function of Fe3+ ion concentrations [108], reproduced with permission from [108] (copyright 2020, John Wiley & Sons, Ltd). (e and f) The Stern-Volmer curves of two types of 2D Cd(II) MOF-based sensors as a function of Fe3+ ion concentration [109], reproduced with permission from [109] (copyright 2021, The Royal Society of Chemistry). (g to i) The emission spectra of three different types of 2D MOFs as a function of various ACE concentrations [110], reproduced with permission from [110] (copyright 2018, Elsevier Ltd).
To further evaluate the sensitivity and selectivity detection abilities of Fe3+ ions, the emission spectra intensity was measured as a function of various Fe3+ ion concentrations. As illustrated in Fig, 8c, a detectable concentration limit of 1.0 × 10−5 M was achieved. Meanwhile, the spectra intensity quenching constant was measured to be 2.1 × 10−5 M-1 in the low-Fe3+ ion concentration range. The recyclability of the sensor was also measured to estimate the device performance. No significance fluctuation in the fluorescence intensity was observed after five cycles. These outcomes indicate that 2D Ln MOF-based luminescence sensors can be considered a promising candidate for high-sensitivity and selectivity Fe3+ ion detection. Besides the previously mentioned 2D MOFs, 2D Co-Zn MOFs also have great potential for high-performance Fe3+ ion luminescence sensors. In 2020, Chen et al. demonstrated a 2D Co-Zn MOF-based Fe3+ ion sensor in an aqueous solution for the first time [108]. Fluorescence titration measurements were performed to evaluate the detected sensitivities of the 2D Co-Zn MOF-based sensor toward Fe3+ ions in aqueous solutions. The measured spectra intensity as a function of various concentrations of Fe3+ ions in aqueous solutions is illustrated in Fig. 8(d). The emission intensities clearly decreased monotonously and varied sharply as the Fe3+ ion concentration increased from 0 to 60 µL, which indicates excellent sensor detection selectivities and sensitivities toward Fe3+ ions. To further improve the detection limits and efficiency, in 2021, Fu et al. proposed two types of 2D Cd(II) ([Cd(L)0.5(tdc)·H2O]n) and ([Cd(L)0.5(btec)0.5]n) MOF-based sensors with high-selectivities and sensitivities to Fe3 + ions detection [109]. Similarly, Fe3+ ion concentration dependence measurements were performed for both types of sensors, and quenching efficiencies of 98.7% and 98.2% were obtained. Additionally, a Stern-Volmer curve was plotted to further evaluate the device performance. As shown in Fig. 8(e) and (f), at a low Fe3+ ion concentration range (0–0.06 mM), a linear relationship was observed. As the Fe3+ ion concentration further increased, a nonlinear relationship appeared, which is mainly caused by the energy transfer process. Remarkably, the calculated detection limits of two types of sensors were 1.08 and 1.74 mM, respectively, which are considerably smaller than those of similar sensors.
2D MOF-based sensors can not only sense Fe3+ ions for human health-related investigations but also be employed to sense harmful pollutants and toxic organic small molecules, which consequently have great significance for the environment and human health. In this regard, in 2018, Wang et al. fabricated three types 2D MOF-based luminescence sensors ([Yb (L1)1.5(DMF)]n, [Yb(L2)0.5(NO3)(DMF)2]n, and [Yb(L3)0.5(L3)0.5(NO3)(DMF)(H2O)]n) to sense acetone [110]. The acetone concentration dependence on the emission spectra intensity is shown in Fig. 8(g)–(e). In terms of a 5.00 vol % acetone concentration, the emission spectra for the first and third sensors disappeared. For the second sensor, the detection limit was 2.50 vol %, indicating that the fluorescent quenching process of the first and third sensors was diffusion controlled by the acetone. These findings also suggest that the three types of sensors have great potential for the selective and sensitive sensing of acetone molecules.
4. Summary and perspectives
The superior properties of 2D MOFs, including their ordered crystalline structures, large surface areas, and related unique 2D properties, provide these materials with great potential in photonic applications. In this paper, the synthesis methods, properties, and various photonic applications of 2D MOFs were summarized.
As a versatile material, 2D MOFs have been widely applied for various photonic applications and have already gained several milestone achievements. Regarding 2D MOF-based modulator applications, the typical 3 dB line width of the fiber laser was measured to be 3.2 kHz, which is comparable to that of other 2D material-based single-frequency fiber lasers. Meanwhile, the operation stability was less than 1.3%, which is almost one time smaller than that of other 2D material-based single-frequency fiber lasers [78,85]. In terms of photodetector applications based on 2D MOFs, the NEP and detective spectra range were 7 × 108 cm Hz1/2 W−1 and from 400 to 1575 nm, which are comparable to those of other 2D material-based photodetectors [77]. Regarding 2D MOF-based light emission device applications, the quantum yield efficiency was remarkably over 50% and the on/off frequency over 60 MHz, which are higher than several commercial devices [99–103]. Regarding 2D MOF-based luminescence or fluorescence sensor applications, various types of sensors have already been successfully fabricated. The lowest detection limit was approximately 1.08 mM, which is considerably smaller than that of similar sensors, indicating the ultra-high detection sensitivity of 2D MOF-based sensors [105–109]. Furthermore, the high-detection selectivity of 2D MOF-based sensors have been proven [110].
However, challenges and opportunities remain for researchers. In terms of synthesis processes, the controllable synthesis process still requires further development to obtain 2D MOFs with desired morphologies. In addition, the eco-friendly and low-cost synthesis of 2D MOFs must be developed.
Regarding modulators applications, long-term operational stability and modulation depth remain a significant challenge. Furthermore, few investigations have been performed to evaluate the modulation performance of 2D MOFs. More research is required to comprehensively explore the performances and mechanisms of these modulators. To meet the demand of next-generation modulators, the response spectra range, modulation speed, depth, and integration degree must be significantly improved.
In terms of the photodetector, the device performance at room temperature was considerably worse than that at 77 K. To meet the demand of practical applications, high-performance room temperature 2D MOF-based photodetectors should be developed. Meanwhile, to satisfy the versatile requirements of different applications, self-powered and flexible photodetectors based on 2D MOFs should also be developed.
For light emission device applications, the external quantum efficiency is considerably lower than that of commercial devices. Furthermore, current investigations mainly focus on the visible band; hence, the emission spectra range should be further extended to the optical communication band. For 2D MOF-based two-photon absorption up-converted emission devices, the emission efficiency is strictly limited by the cross-section; therefore, novel 2D MOFs with large emission cross-sections should be further explored.
In terms of luminescence or fluorescence sensors, current investigations mainly focus on Fe3+ ion sensing. The realization of simultaneous multi-metal ion sensing would significantly promote the development of 2D MOF-based sensors within the human health field. Meanwhile, the selective and sensitive sensing ability must be further improved to meet the requirements of practical applications.
In conclusion, 2D MOFs are fascinating materials owing to their excellent properties and great potential in both academic and industrial applications. With continuing research on photonic applications based on 2D MOFs, several milestone investigations will emerge in the near future, which will efficiently promote the development of 2D MOF-based photonic applications
Funding
Natural Science Foundation of Jiangsu Province (BK20170247, BK20210076); National Natural Science Foundation of China (62105209, 62105211); Foundation and applied foundation research fund of Guangdong province (2019A1515111060); China Postdoctoral Science Foundation (2021M702237); Deanship of Scientific Research (DSR) at King Abdulaziz University (KEP-MSc-70-130-42); State Key Laboratory Open Fund of Millimeter Waves (K202105).
Acknowledgements
This research was partially supported by the Natural Science Foundation of Jiangsu Province (Grants No. BK20210076, BK20170247), the National Natural Science Foundation of China (Grants No. 62105211, 62105209), Foundation and Applied Foundation Research Fund of Guangdong province (2019A1515111060), and the China Postdoctoral Science Foundation (2021M702237). Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. (KEP-MSc-70-130-42). State Key Laboratory Open Fund of Millimeter Waves (K202105), Dongdong Liu is supported by the Jiangsu Qinglan project. The authors, therefore, acknowledge with thanks DSR for technical and financial support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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