&Metal–Organic Frameworks

Recent Advances in Micro-/Nanostructured Metal–OrganicFrameworks towards Photonic and Electronic Applications

Xiaogang Yang,[a] Xianqing Lin,[b] Yong Sheng Zhao,*[b] and Dongpeng Yan*[a]

Chem. Eur. J. 2018, 24, 6484 – 6493 T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim6484

MinireviewDOI: 10.1002/chem.201704650

Abstract: Micro- and nanometer-sized metal–organic frame-

works (MOFs) materials have attracted great attention dueto their unique properties and various potential applications

in photonics, electronics, high-density storage, chemo-, andbiosensors. The study of these materials supplies insight intohow the crystal structure, molecular components, andmicro-/nanoscale effects can influence the performance of

inorganic–organic hybrid materials. In this Minireview article,we introduce recent breakthroughs in the controlled synthe-sis of MOF micro-/nanomaterials with specific structures and

compositions, the tunable photonic and electronic proper-

ties of which would provide a novel platform for multifunc-

tional applications. Firstly, the design strategies for MOFsbased on self-assembly and crystal engineering principles

are introduced. Attention is then focused on the methods offabrication of low-dimensional MOF micro-/nanostructures.Their new applications including two-photon excited fluores-cence, multi-photon pumped lasing, optical waveguides,

nonlinear optical (NLO), and field-effect transistors are alsooutlined. Finally, we briefly discuss perspectives on the fur-ther development of these hybrid crystalline micro-/nanoma-

terials.

1. Introduction

The development of information and optoelectronic technolo-

gies is one of central scientific issues in the modern world.Smaller and faster integrated chips are always being pursued

for their essential role in both the technical frontier and ourdaily life. As the optical analog of electronics, photonics share

the logic of miniaturization that drives research in semiconduc-

tor and information technology.[1] Currently, the imminent limi-tations of electronic integrated circuits, affected by heat pro-

duction and quantum tunneling effect, are stimulating intenseactivity in the area of photonics for the development of on-

chip optical components.[2–4] In this regard, photons couldoffer new supplements for electrons as information carriers, in-

cluding speed, bandwidth, capacity, etc. , which provide a

promising solution to the current bottlenecks that limit the fur-ther improvement of modern electronics.[5] Manipulation of

photons at the microscale and/or nanoscale is essential forclassical and quantum communicating and computing. To

date, a variety of materials have been applied to enrich photo-functionalities in a broad frequency range. Thanks to the pio-

neering work of chemists and materials scientists, large num-

bers of inorganic semiconductor materials (such as ZnO, CdS,GaAs) have been developed, which show high-performanceoptical properties.[6–12] As well, along with the advantages ofhigh photoluminescence quantum efficiencies, color tunabili-

ties, and size-dependent optical properties, organic nanomate-rials have captured increasing interest in fabricating light-emit-

ting devices.[13–19] Besides the composition, the morphology,size, and dimension control also strongly influence the opticalbehaviors by reflecting, refracting, diffracting, and so on.[20]

Therefore, control over photon behavior at the wavelength

scale remains a great challenge.[21]

Recently, with the development of nanoscience and nano-

technology, low-dimensional nano-/micro-sized systems (e.g. ,1D wires, tubes, rods, and belts, as well as 2D sheets) have

captured broad attention because of their novel physical andchemical properties, and their wide range of potential applica-

tions in nano- and microdevices.[22–25] Metal-organic frame-

works (MOFs), also known as porous coordination polymers,represent an interesting type of solid crystalline materials that

can be straightforwardly self-assembled through the coordina-tion of metal ions/clusters with organic linkers.[26] Owing to the

modular nature, predictable structures, versatile host–guestchemistries, responsiveness to physical and chemical stimuli,

and their fascinating physical/chemical properties such as high

surface areas, uniform nanoscale cavities, controlled pore sizes,tailorable molecular structures, and catalysis activity,[27] MOFs

have demonstrated extensive applications in gas storage andseparations,[28] energy storage,[29] nonlinear optics,[30] electronic

devices,[31] catalysis,[32] and drug delivery.[33] Moreover, lumines-cent MOFs have also been extensively studied owing to theirfacile synthesis routes and applications in chemical sensing,[34]

light-emitting devices,[35] anti-counterfeiting tags,[36] and opticalfiber lasers.[37] The luminescent behavior of MOF materials can

be modulated by the richness of metal ions/clusters, organiccompounds as linkers, and encapsulation of guest species such

as cations, anions, vapors, dyes and so on.[38–41] These uniqueadvantages enable MOFs to be used as a highly versatile and

tunable platform for exploring multifunctional photonic mate-rials.

However, despite considerable attention has been paid onthe design of bulk MOFs with fascinating structures and func-tions, it has been a challenge to precisely control the size,

shape, and morphology in a technically simple and easy han-dling way. In addition, the exploration of excitonic/photonic

and electronic behaviors in MOF materials still remains a signif-icant challenge. In this review, we will outline recent develop-ment of the micro- and nano-sized MOF materials as building

blocks for several kinds of integrated photonic (such as multi-photon excited fluorescence, multi-photon pumped lasing, op-

tical waveguides, and nonlinear optical (NLO)) and electronicapplications (such as field-effect transistors). The goal of this

[a] X. Yang, Prof. D. YanBeijing Key Laboratory of Energy Conversion and Storage MaterialsCollege of Chemistry, Beijing Normal UniversityNo. 19, XinJieKouWai St. , HaiDian District (P. R. China)E-mail : yandp@bnu.edu.cn

[b] X. Lin, Prof. Y. S. ZhaoCAS key Laboratory of Photochemistry, Institute of ChemistryChinese Academy of Science, Beijing 100190 (P. R. China)E-mail : yszhao@iccas.ac.cn

The ORCID identification number for the author of this article can be foundunder https ://doi.org/10.1002/chem.201704650.

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review article is to provide an early overview of recent micro-/nano-scaled MOF-based photonic and optoelectronic materi-

als/devices with an emphasis on functional tunability, underthe backdrop of a briskly evolving realm of MOF-based integral

component of the actual device structure.

2. Design Principles for Construction of Micro-/Nanoscale MOF-based Photonic and ElectronicMaterials

2.1 Photofunctional MOFs

Photofunctional systems are an important sub-category of

MOFs, in which photo-related properties arise following theabsorption of radiative excitation energy. For example, photo-luminescence usually contains two basic forms, fluorescenceand phosphorescence, depending on multiple spin statesduring the radiative relaxation process. Fluorescence is a spin-

allowed radiative transition from the lowest singlet excitedstate S1 of the fluorophore to its singlet ground state S0, and

the process generally lasts no more than about 10 ns; while

phosphorescence refers to the spin-forbidden radiative transi-tion from the triplet state T1 to ground state S0, which lasts a

microsecond to seconds. The complex MOF systems withstructural diversity potentially include multiple types of ligand

molecules, inorganic ions, and clusters, as well as guest mole-cules or ions, and thus the photoluminescence can arise from

a variety of mechanisms. The luminescence in MOFs are gener-

ally generated from organic ligand excitation, metal-centeredemission, ligand-to-metal charge transfer (LMCT), and metal-to-

ligand charge transfer (MLCT). Besides, the guest moleculeswithin the MOFs can also result in photoemission. It is worthy

to note that the organic chromophores are highly ordered in aMOF structure, so that the nature of their intermolecular com-

munication can be altered, which results in photoemissions

that are different from their free form. The more detailed ex-amples of luminescent MOFs used in tunable emission and

sensor applications have been described in several previous re-views.[34, 42–45]

Light-responsive performance also stands for another typeof photofunctionality in MOF families. The modification of vari-

ous light-responsive groups onto the organic linkers will signif-icantly enrich the photoactivity of MOFs. Several different

light-responsive building blocks (such as azobenzene, diaryle-thene, aryl azide, nitrobenzyl moieties, and metalloporphyrincenters) can be incorporated into MOFs for the purpose of

constructing photosensitive materials with enhanced per-formance.[28, 46, 47] For example, such MOFs can be developed by

directly introducing light-responsive motifs into the nanochan-nels, or by chemically grafting light-responsive motifs on the

organic ligands of the MOFs (Scheme 1). It is notable that the

modification of such materials still maintains controllable size,shape, and high uniformity.

Alternatively, the advantages of highly regular channel struc-tures, large surface areas, and controllable pore sizes make

MOFs appropriate hosts for active phases with various applica-tions. Significant efforts have been directed towards position-

ing functional units within MOF crystals. Controlling the prop-erties and the position of functional units inside MOF crystals

allows for the fabrication of a novel class of host–guest materi-als, which can take mutual advantages to elicit certain proper-

Xiaogang Yang received his Master’s degreefrom Yan’an University (P. R. China) in 2009.Then he worked at the Shaanxi Yanchang Pe-troleum (Group) Corp. Ltd. He is currently aPh.D. student under the supervision of Prof.Dongpeng Yan at Beijing Normal University.His research interest is centered on the designand preparation of metal–organic framework.for host–guest optoelectronic functional ma-terials.

Xianqing Lin received his B.Sc. degree fromCentral South University (CSU, P. R. China) in2012. He is currently a Ph.D. student underthe supervision of Prof. Yong Sheng Zhao inthe Institute of Chemistry, Chinese Academyof Sciences (ICCAS). His research interests arefocused on the optoelectronic properties of1D and 2D semiconductor nanomaterials.

Yong Sheng Zhao received his Ph.D. degree in2006 at the Institute of Chemistry, ChineseAcademy of Sciences (ICCAS, P. R. China).After that, he joined the University of Califor-nia at Los Angeles (UCLA) and NorthwesternUniversity (USA) as a postdoctoral fellow. In2009, he returned to ICCAS as a Professor ofChemistry and Materials Science. His researchinterests include the controllable synthesis oflow-dimensional organic materials, photo-physical and photochemical processes, as wellas the fabrication and performance optimiza-tion of photonic/optoelectronic devices.

Dongpeng Yan obtained his Ph.D. degree atBeijing University of Chemical Technology(BUCT, P. R. China) in 2012. Then, he becamean associate professor at BUCT. In 2014, hemoved to Beijing Normal University (P. R.China) as a full professor. In 2011 and 2013,as a visiting student/scholar, he studied at theDepartment of Chemistry, University of Cam-bridge (UK) and School of Pharmacy, Universi-ty College London (UK). His research topicsare functional molecular materials and host–guest chemistry. He has received an AwardNomination of Annual Figure among ChineseUniversity Students (2011), a “Talent Model”Award in the Universities in Beijing (2010), the Tang Aoqing Chemical Scholar-ship (2011), and an Outstanding Doctoral Dissertation Prize in Beijing (2013)amongst others.

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ties that are difficult to achieve by using the pure guests orthe host framework alone. The encapsulation of nanosized

guests, such as photoluminescent quantum dots (QDs), dyemolecules, metallorganic molecules, as well as conjugated

polymers, metal nanoparticles (NPs), polyoxometalates, and

metal–organic polyhedra, into MOFs has led to the develop-ment of new photofunctional materials (Scheme 2).[48–51]

The integration of multiple functional sites into a singlesystem can also allow them to work cooperatively and syner-gistically. Owing to the high crystallinity, MOFs consist of uni-

form cavities distributed throughout their 3D structure withlong-range order. Therefore, the use of MOFs as hosts enablesthe ordered arrangement of discrete guest molecules through-out the MOF lattice and thus facilitates the investigation of in-teractions between host MOFs and trapped guests by crystallo-

graphic methods (e.g. , X-ray crystallography). For instance, inhost–guest photofunctional MOF systems, by incorporating

photoemissive dyes as guests into MOFs with defined poreconfinement, the highly oriented assembly of dye moleculeswithin the MOFs can minimize the serious aggregation-caused

quenching and twisted intramolecular charge transfer losses oforganic dyes by weakening the intermolecular interactions and

intramolecular rotations. Meanwhile, the high-quality resonantchannels of MOFs will provide an opportunity to construct

low-threshold miniaturized lasers.[52] Strategies for positioningfunctional units within MOFs have involved ship-in-bottle (as-

sembly of guests inside MOFs), bottle-around-ship (assemblyof MOFs around guests), sandwich assembly (embedding NPs

between MOF layers), and in situ encapsulation (simultaneoussynthesis of MOF and guest nanoentities) (Scheme 3).[51, 53, 54]

For example, by a facile one-pot method, He et al. reportedthat individual Au nanoparticles can be encapsulated within asingle MOF-5 particle.[55] Moreover, with a similar approach,

ZIF-8 crystals containing luminescent quantum dots and lan-thanide-doped NaYF4 nanoparticles have also been preparedsuccessfully.[56]

2.2 Micro-/nanoscale processing of MOFs

Recently, the preparation of micro-/nano-scaled MOFs crystalswith controlled size, shape, and morphology has become an

emerging topic to fulfill the specific requirements for low-di-mensional photonic, electronic, and optoelectronic applica-tions.[57] Generally, two strategies have been developed for the

synthesis of micro-/nano-scaled MOF materials, the top-downand the bottom-up strategies. As for the direct synthesis ofMOF micro-/nano-particles by using the bottom-up strategy,several operationally simple, inexpensive, rapid, and even com-

mercially viable synthesis approaches including nanoscale pre-cipitation, hydro-/solvothermal, reverse microemulsion, mecha-

nochemical (solid-state grinding and liquid-assisted grinding),sonochemical, electrochemistry, microwave-assisted syntheses,macrostructural template (hard template) methods, and molec-

ular template (soft template) methods have been established(Scheme 4), which are well summarized by several previous

review articles.[58–64] There have been successful examples tosynthesize MOF micro/nanostructures (e.g. , ZIF-8, MIL-88,

HKUST-1, and Tb-BDC) under ambient conditions.[51, 65] Mean-

while, a top-down processing strategy has also been devel-oped, in which the pre-synthesized MOF crystals are partially

dissolved and are transformed into low-dimensional architec-tures. Typically, by a method of sonication exfoliation, single-

or few-layered MOF nanosheets can be isolated from layeredbulk 2D MOF crystals constructed by stacking each layer along

Scheme 1. Schematic representation of light-responsive MOF with azoben-zene groups as photoisomerizable pendants.

Scheme 2. Summary of MOFs functioning as versatile hosts for the encapsu-lation of nano-sized guests.

Scheme 3. Schematic representation of main methodologies used to pre-pare guest@MOF composites. a) Ship-in-bottle ; b) bottle-around-ship.

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the vertical direction through weak interactions (van der Waalsforces and/or hydrogen bonding).[24]

3. Optoelectronic Properties for Micro-/Nano-scale MOFs

3.1 3D data storage and photon upconversion properties

Multiple-photon responsive materials have received tremen-dous attention because of their potential in various applica-

tions, such as up-converted lasing, high resolution imaging,optical power limiting, ultrafast micro- and nanofabrication,

and so on.[66, 67] From a viewpoint of design, the introduction of

organic linkers modified by various light-responsive groupscan significantly enhance the light-responsive performance of

MOFs. For example, under mild condition of light irradiation,the chemical and structural transformation of MOFs can be ef-

ficiently read, serving as three-dimensional optical data-storagematerials.

In 2015, by introducing the zwitterionic pyridinium light-re-sponsive building block 2,5-bis(3,5-dicarboxyphenyl)-1-methyl-

pyridinium hydroxide into a MOF, through a multivariate strat-egy, Qian and co-workers first reported a two-photon respon-

sive MOF (ZJU-56-0.20) with two-photon excited fluorescencechange.[68] As expected, based on the photochemical reactionof the zwitterionic organic linkers, the single crystals of ZJU-56-

0.20 show significant fluorescence emission color transforma-tion from blue to yellow after UV (365 nm) and 710 nm laserlight irradiation. Due to the unique two-photon response prop-erty, the obtained fluorescent pattern features high resolution(1 V 1 V 5 mm) and stability, serving as photofunctional datastorage microdevices in MOFs (Figure 1).

The rational selection of ligands in MOFs allows efficient

energy transfer between the organic struts, which could mimicthe function of energy harvesting proteins in photosynthesis,

with long-distance directional transport of excitons. Suchenergy transfer fashion has been demonstrated as an effective

way to decrease the threshold of photon upconversion in MOFthin films.[69] By utilizing a layer-by-layer technique, a series of

ordered heterostructures stacked by emitter and sensitizer sur-

face-anchored MOF (SURMOF) with different thicknesses havebeen reported by Oldenburg et al. (Figure 2).[70] In the hetero-

structures, the sensitizer layer (Zn-(Pd-DCP) MOF, DCP = 5,15-di-phenyl-10,20-di(4-carboxyphenyl) porphyrin) absorbs photons

and creates triplet excited states, while the emitter layer (Zn-ADB MOF, ADB = 4,4’-(anthracene-9,10-diyl)dibenzoate) accepts

these triplet states upon triplet–triplet annihilation (TTA), emit-

ting a photon with higher energy than the original one. Dextertransfer of triplet excitons over the SURMOF demonstrates that

the heterojunctions are of sufficient quality to allow triplettransfer from sensitizer to emitter layers. Furthermore, by con-

trolling the thickness of sensitizer and emitter layers, a thresh-

Scheme 4. Schematics showing methods for the synthesis of micro-/nano-scale MOFs by using the bottom-up strategy.

Figure 1. a) Zwitterionic and neutral tetracarboxylate linkers for the construction of two-photon responsive MOFs. b) Fluorescent micrographs of a crystalZJU-56–50.20 upon UV light irradiation. c) Schematic illustration of the assembly of L1 incorporated MOF ZJU-56–0.20 through a multivariate approach andfemtosecond laser writing inside the MOF single crystal through a two-photon process. d) Top view of two-photon excited fluorescent image of a 2D codestack. Scale bar, 25 mm. e) Reconstructed lateral image along the indicated line in panel d (left), and lateral view imaged by using one photon fluorescence(right). f) 3D reconstructed image of the stacked 2D code pattern. g) Intensity profiles of the fluorescent codes along lines 1 and 2 in panels b and c, respec-tively. Reproduced with permission from reference [68] Copyright 2015, American Chemical Society.

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old of photon upconversion lower than 1 mW cm@2 can beachieved.

3.2 Multi-photon pumped lasing in MOFs microcrystals

Benefitting from the advantages of good directionality, highbrightness, monochromaticity, and coherence, lasers have

been widely applied in various fields. To date, a variety of lasermaterials such as semiconductor and rare-earth doped crystals

have been developed. However, the exploration and develop-

ment of novel laser materials with high stability, wide applica-bility, low fabrication cost, and environmentally friendly synthe-

sis continue to be topics of intense interest. A laser device usu-ally contains three key components : gain medium, pump

source, and resonant cavity. As discussed in previous sections,modulated by light-responsive organic linkers in the frame-

work or encapsulated with photoluminescent nanoentities(dye molecules, for example) in regular channel structures,

MOFs can potentially serve as ideal models to develop new

micro-laser devices. The orderly arrangement of organic gainmaterials in MOFs structure would minimize the serious aggre-

gation-caused quenching (ACQ) and twisted intramolecularcharge transfer (TICT) losses. Meanwhile, the regular external

morphologies and highly oriented assembly of molecules inthe framework provide an opportunity to achieve high-qualityresonant cavities for micro-lasers.

Recently, Zhao et al.[71] reported a DASP+@MOF hybrid mate-rial with low threshold and wavelength-tunable lasing. By uti-

lizing a facile cationic exchange process, the intramolecularcharge transfer (ICT) cationic dye (4-p-(dimethylamino)styryl)-1-

methylpyridinium (DASP+) was encapsulated in the anionicmesoporous bio-MOF-100 host matrix (Figure 3). In the DASP+

@MOF host–guest system, the pore confinement effect ofMOFs can minimize the ACQ of the DASP+ cationic dye, which

Figure 2. a) SEM cross-section of a three-layer emitter-sensitizer-emitter (A-B-A) SURMOF heterostructure on a Si substrate. b) Schematic diagram of triplet–triplet annihilation upconversion (TTA-UC). c) Schematic illustration of different building blocks. d) The absorption spectrum of sensitizer (B) layer is shown inred and the wavelength of the 532 nm excitation laser is shown in green. The observed upconverted emission is shown in blue. The observation of upcon-verted emission provides direct optical evidence that the SURMOF-SURMOF heterojunction is of sufficient quality to allow triplet, and therefore necessarilyalso electron, transfer between the SURMOF layers. e) The upconversion thresholds for all nine B-A samples. Reproduced with permission from reference [70]Copyright 2016, Wiley-VCH.

Figure 3. a) Schematic of the introduction of the cationic dye DASP+ via anion-exchange process. b) Schematic view presenting the solvation of MOFpores and dye molecules. c) Histogram of emission peaks of DASP+@MOFafter adsorbing different guest solvents. d) Energy diagram for the deactiva-tion from the LE and ICT states of DASP+@MOF loaded with different sol-vents. e) Blue shifting of the lasing peaks by decreasing the polarity of thesolvent. Reproduced with permission from reference [71] Copyright 2016,Wiley-VCH.

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effectively reduces the non-radiative decay from the TICT stateand thus significantly enhances the radiative process of locally

excited (LE) and ICT states. In addition, by controlling the pop-ulation distribution in LE and ICT states with the introduction

of guest solvents with different polarities, the wavelength ofDASP+@MOF micro-lasers can be tuned over a wide range.

This work not only supplies a facile way to construct stableand tunable micro-lasers in confined systems, but also pro-vides a strategy to develop miniaturized lasers with high per-

formance.

3.3 Nonlinear optics (NLO) applications

Nonlinear optical (NLO) materials play a unique and crucial

role in electro-optical switching, optical communication tech-nologies, and biomedical researches. Among various reported

materials, solid-state hybrid MOFs are recognized as one of the

most promising candidates to exploit prominent NLO per-formance, owing to their tailorability and relative stability. The

versatile metal–organic framework ZIF-8, featuring high ther-mal and chemical stabilities, ease of processing, transparency,

and biocompatibility, has been widely investigated in manyfields such as gas storage, energy conversion, drug delivery,

catalysis, and so on. However, its outstanding properties are

not limited to this. For example, ZIF-8 also exhibits strongsecond-order nonlinear optical response.

Cleuvenbergen and co-workers recently reported the mor-phology-dependent NLO activity of ZIF-8 nano- and microcrys-

tals.[72] Originated from the character of non-centrosymmetricspace group I4̄3m, the NLO activity of ZIF-8 possesses strong

second-harmonic generation (SHG) and two-photon fluores-

cence (TPF) as shown in Figure 4. The results demonstrate that

the synthesis procedures play an important influence on theNLO activity of ZIF-8. In this work, the size and morphology of

ZIF-8 materials can be controlled by different synthesis ap-proaches. The fast synthesis at room temperature (RT-synthe-

sis) give rise to ZIF-8 materials in nanoscale, displaying lowsecond-order NLO coefficient (hdeffi= 0.05 pm V@1) but the

highest TPF. Microscale ZIF-8 materials, isolated under solvo-thermal microwave synthesis (MW-synthesis) condition, exhibita larger hdeffi coefficient (0.16 pm V@1). When delaying the crys-

tallization time by using a conventional oven (O-synthesis),high crystallinity micrometer sized ZIF-8 materials were ob-tained, showing the largest hdeffi coefficient (0.25 pm V@1). It isnoteworthy that all of these values are larger than those of

commercial potassium dihydrogen phosphate (KDP) crystals.Experimental and theoretical studies reveal that the nucleation

rate leads to the crystal defects which can influence the reor-

ganization of 2-methylimidazole linkers. The orientation of or-ganic linkers can introduce a center of inversion in the crystalli-

zation process of ZIF-8, resulting in no SHG activity.

3.4 Optical waveguides MOF microrods

The shape and dimension of the photonic materials have a

strong impact on the confinement effect of light flow. Thus,manipulating the flow of photons at the microscale/nanoscale

is essential for the development of integrated optical circuits.For example, the use of nano- to micro-sized waveguides has

attracted considerable interest for their possible application as

basic components in optical communication systems. In thenext-generation photonic integrated circuits, waveguide mate-

rials are supposed to connect the light-emitting and light-de-tecting elements, and at the same time can tune the emission

spectra in the guiding process. Up to now, research on wave-guides has focused on inorganic semiconductors, organic chro-

mophores, and polymers. However, few MOFs had been used

to construct the optical waveguides, probably due to the lackof suitable low-dimensional MOF micro/ nanostructures. Lan-

thanide-based MOFs (Ln-MOFs) have attracted great attentionbecause of their unique crystal structures and fascinating pho-

tophysical properties (such as sharp emission band and highquantum yields). Ln-MOFs can be potential models to develop

new optical waveguide systems due to their high emissionquantum yields and tunable emission colors. Moreover, the

well-organized and ordered assembly of ligands and lantha-

nide cations could decrease the optical loss. Quite recently, wehave developed Tb-/Eu-based MOFs with the antenna phos-

phor 1,3,5-benzenetricarboxylic acid (BTC) as the ligand; three1D MOF microrod materials have been obtained using a facile

solvothermal method (Figure 5).[73] Based on the effective ion-doping and energy transfer in these Ln-MOFs, color tunable

optical waveguides (green, orange, and red) can be achieved,

with low waveguide loss (0.012~0.033 dB m@1) and high photo-luminescent quantum yield (PLQY). In addition, these micro-

rods present high linear and chiral polarization anisotropy be-cause of the orderly orientation and helical distribution of the

photoactive Ln3 + . By virtue of both photonic transport and po-larized luminescence within the 1D microrod, the self-assembly

Figure 4. Scanning electron microscopy (SEM), second harmonic generation(SHG), and two-photon fluorescence (TPF) images of ZIF-8 crystals under dif-ferent synthesis approaches (RT: room temperature synthesis, MW: solvo-thermal microwave synthesis, O: oven synthesis). The scale bar is set to25 mm for all images. Reproduced with permission from reference [72] Copy-right 2016, American Chemical Society.

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of orderly Ln-MOFs microstructures has potential applications

in optical communication and photofunctional microdevices.

3.5 MOF-based field-effect transistors

The versatile features of MOFs, as discussed above, endow

them as ideal candidates for various potential photonic and

photofunctional applications. In contrast, applications of MOFmicro/nanostructures in the field of electronic devices are still

rare, in spite of much efforts made in recent years.[31, 74–76] Thisis mainly due to the lack of electronic conductivity in the MOFstructures. The absence of conductivity limits the use of MOFmaterials in a number of desirable applications, such as super-

capacitors, batteries, porous electrodes, fuel cells, etc. Fortu-nately, by carefully selecting the metal ions/clusters and design

of organic building block and/or functional guests, high con-

ductivity of MOF materials can be further explored. For exam-ple, by assembling of electroactive CuII ion and 2,3-pyrazinedi-

thiolate (pdt) organic linker through Cu@S and Cu@N bonds, Ki-tagawa et al. reported the first example of a conductive Cu-

[Cu(pdt)2] MOF showing relatively high electrical conductivityof 6 V 10@4 S cm@1 at 300 K.[77] In 2016, Kitagawa et al. also dem-

onstrated a series of conductive porous composites by encap-

sulating conductive polymerization of 3,4-ethylenedioxythio-phene (EDOT) in the cavities of MIL-101(Cr). By controlling the

loading amount of EDOT, the host–guest can simultaneouslypossess reasonable electronic conductivity and maintain high

porosity. Further application in chemiresistive sensors for thedetection of NO2 can also be realized.[78]

Recently, the emergence of conductive MOF based field-

effect transistors (FETs) has also attracted great attention. Xu

and co-workers developed ultrasmooth and compact Ni3(HITP)2

(HITP = 2,3,6,7,10,11-hexaiminotriphenyl-enesemiquinonate)

membranes for FET devices by an air-liquid interfacial growthmethod (Figure 6).[79] In the structure of Ni3(HITP)2, the square-

planar Ni2 + centers are linked by the tritopic HITP moietiesgenerating a 2D graphene-like honeycomb porous layer withABAB stacking mode, affording a 3D porous network with 1D

Figure 5. a) Ball-and-stick/polyhedral representations of the 3D framework structure of Ln-BTC. b) PXRD patterns of Ln-BTC. c) Photoluminescence (PL) imagesobtained from a single Ln-BTC microrods. d) Spatially resolved PL spectra from the tip of the Ln-BTC microrod for different distances between the excitationspot and tip of the rod shown in the PL images 1–8 in (c). Inserts show the optical-loss coefficient (R). Reproduced with permission from reference [73] Copy-right 2017, Wiley-VCH.

Figure 6. a) Space-fill representations of Ni3(HITP)2 MOF. b) Schematic viewof the fabrication of MOF-based field-effect transistors (FETs). Output curves(c) and transfer curves (d) of MOF-based FETs. Reproduced with permissionfrom reference [79] Copyright 2017, American Chemical Society.

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open channels. The high-quality, freestanding conductive MOFmembrane and porous FETs were further successively fabricat-

ed by an air-liquid interfacial growth method and depositionof patterned Au electrodes. The results show that the conduc-

tive MOF-based FET exhibits excellent field-effect hole mobili-ties as high as 48.6 cm2 V@1 s@1, which can be ascribed to the ul-

traflat, compact, and uniform surface of the Ni3(HITP)2 mem-brane. Further in-depth analysis indicates that the assemblingof Ni2 + and radical o-semiquinonate HITP gives rise to an ex-tended 2D graphene-like charge-delocalized layer. Meanwhile,the interval of the conjugated layer overlap along the c-axis ofthe structure is short enough to create sufficient orbital over-lap between adjacent layers through p–p interactions. This af-fords convenience for charge transport along both the hori-zontal and vertical direction in the 3D structure of Ni3(HITP)2,

which may synergistically enhance the conductive per-

formance of the membrane. The fabrication for both conduc-tive MOF membranes and MOF-based FET devices could

expand and promote the applications of micro/nano-sizedMOF materials in future electronic devices.

4. Conclusions and Perspectives

In this review, we have attempted to introduce recent progresson crystalline MOF-based micro-/nanostructures for emerging

photonic and electronic applications. Micro/nano-sized MOFmaterials have several advantages of modular nature, easy

processability, predictable structures, and fascinating chemical

properties (such as uniform nanoscale cavities, controlled poresizes, tailorable molecular structures, and high surface areas),

which endow such organic–inorganic hybrid materials as idealmodels to develop new applications in luminescence, optical,

and optoelectronic devices.The ever-growing demand for multi-functional photonic and

electronic devices at the micro-/nanoscale greatly promotes

the development of novel low-dimensional materials. The di-verse micro-/nanoscale structures of MOFs offer excellent func-

tionality and textural features for potential applications inmultifunctional photonic and electronic devices. To date, sever-

al different strategies have been developed to enhance thestructure and functionality of MOFs such as linker modification

and functional hybridization by encapsulating photolumines-cent quantum dots (QDs), dye molecules, metallorganic mole-cules, as well as conductive polymers, metal nanoparticles

(NPs), polyoxometalates, and metal–organic polyhedra. The in-tegration of multiple functional sites into a single system can

allow them to work cooperatively and synergistically, which isdifficult to achieve in their individual parts.

Up to now, lots of efforts have been made to understandstructure-dependent photonic and electronic properties ofthese MOF micro/nanocrystals. However, it is still relatively dif-

ficult to predict the crystal structure, modulate the solid-statemorphology, and manipulate the optical and electronic per-

formances of MOFs systems at the micro-/nanometer scale.Moreover, in contrast with inorganic nanomaterials, micro-/nanoscale processability of MOFs is still in its early stage. Inthis regard, a continuous investigation on bottom-up assembly

is highly desirable to make improvements in reproducibility,uniformity, controllable morphology and dimensions. More-

over, the development of new computational methods forcrystal structure and morphology predictions is highly desir-

able, since this could allow the structure, morphology, andproperties of MOFs to be known to some extent before the

hybrid is fabricated.From the application perspective, these crystalline MOF ma-

terials have already emerged as good candidates for applica-

tions in photofunctional nano- and microdevices. However,most of the present photonic devices primarily take advantage

of the linear optical properties only, especially light emission. Itis difficult to realize some sophisticated applications solely

with the linear response, such as an ultrafast switcher or asignal amplifier. MOF nanostructures with large nonlinear opti-

cal coefficients would be appreciated alternatives in such

areas. In the electronic field, although much effort has beenmade, it is still desired to further develop conductive MOF ma-

terials or devices with high mobility and conductivity. The fab-rication of these low-dimensional MOFs crystals calls for the

development of new and effective mechanisms for modulatingorganic–inorganic hybrid materials.

Acknowledgements

D.Y. and Y.Z. are grateful for the invitation from the Editorial

Office. This work was supported by the 973 Program (GrantNo. 2014CB932103), the National Natural Science Foundation

of China (Grant No. 21771021 and 21473013), the Beijing Mu-nicipal Natural Science Foundation (Grant No. 2152016), the

Fundamental Research Funds for the Central Universities.

Conflict of interest

The authors declare no conflict of interest.

Keywords: electronics · luminescence · metal–organic

frameworks · micro-/nanomaterials · photonics

[1] M. Law, D. J. Sirbuly, C. Johnson, J. Goldberger, R. J. Saykally, P. Yang, Sci-ence 2004, 305, 1269 – 1273.

[2] R. Kirchain, L. Kimerling, Nat. Photonics 2007, 1, 303 – 305.[3] H. J. Caulfield, S. Dolev, Nat. Photonics 2010, 4, 261 – 263.[4] B. Piccione, C.-H. Cho, L. K. van Vugt, R. Agarwal, Nat. Nanotechnol.

2012, 7, 640 – 645.[5] Y. Yan, Y. S. Zhao, Chem. Soc. Rev. 2014, 43, 4325 – 4340.[6] X. Zhou, L. Gan, W. Tian, Q. Zhang, S. Jin, H. Li, Y. Bando, D. Golberg, T.

Zhai, Adv. Mater. 2015, 27, 8035 – 8041.[7] F. Qian, Y. Li, S. Gradecak, H. G. Park, Y. Dong, Y. Ding, Z. L. Wang, C. M.

Lieber, Nat. Mater. 2008, 7, 701 – 706.[8] M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo,

P. Yang, Science 2001, 292, 1897 – 1899.[9] S. Yu, X. Wu, Y. Wang, X. Guo, L. Tong, Adv. Mater. 2017, 29, 1606128.

[10] C. J. Barrelet, A. B. Greytak, C. M. Lieber, Nano Lett. 2004, 4, 1981 – 1985.[11] X. Duan, Y. Huang, R. Agarwal, C. M. Lieber, Nature 2003, 421, 241 – 245.[12] R. Liu, Z.-A. Li, C. Zhang, X. Wang, M. A. Kamran, M. Farle, B. Zou, Nano

Lett. 2013, 13, 2997 – 3001.[13] D. Yan, Chem. Eur. J. 2015, 21, 4880 – 4896.[14] Q. H. Cui, Y. S. Zhao, J. Yao, Adv. Mater. 2014, 26, 6852 – 6870.

Chem. Eur. J. 2018, 24, 6484 – 6493 www.chemeurj.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim6492

Minireview

[15] D. Yan, D. G. Evans, Mater. Horiz. 2014, 1, 46 – 57.[16] J. Clark, G. Lanzani, Nat. Photonics 2010, 4, 438 – 446.[17] Y. Zhou, Nature 2015, 528, S170 – S173.[18] F. Di Benedetto, A. Camposeo, S. Pagliara, E. Mele, L. Persano, R. Stabile,

R. Cingolani, D. Pisignano, Nat. Nanotechnol. 2008, 3, 614.[19] C. Zhang, C.-L. Zou, Y. Zhao, C.-H. Dong, C. Wei, H. Wang, Y. Liu, G.-C.

Guo, J. Yao, Y. S. Zhao, Sci. Adv. 2015, 1, e1500257.[20] Y. S. Zhao, J. Wu, J. Huang, J. Am. Chem. Soc. 2009, 131, 3158 – 3159.[21] C. Zhang, Y. Yan, Y. S. Zhao, J. Yao, Acc. Chem. Res. 2014, 47, 3448 – 3458.[22] T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda, S. Kitagawa,

Angew. Chem. Int. Ed. 2009, 48, 4739 – 4743; Angew. Chem. 2009, 121,4833 – 4837.

[23] D. Zacher, R. Schmid, C. Wçll, R. A. Fischer, Angew. Chem. Int. Ed. 2011,50, 176 – 199; Angew. Chem. 2011, 123, 184 – 208.

[24] M. Zhao, Q. Lu, Q. Ma, H. Zhang, Small Methods 2017, 1, 1600030.[25] X. Duan, C. Wang, J. C. Shaw, R. Cheng, Y. Chen, H. Li, X. Wu, Y. Tang, Q.

Zhang, A. Pan, J. Jiang, R. Yu, Y. Huang, X. Duan, Nat. Nanotechnol.2014, 9, 1024 – 1030.

[26] A. Schoedel, M. Li, D. Li, M. O’Keeffe, O. M. Yaghi, Chem. Rev. 2016, 116,12466 – 12535.

[27] H.-C. J. Zhou, S. Kitagawa, Chem. Soc. Rev. 2014, 43, 5415 – 5418.[28] H. Li, M. R. Hill, Acc. Chem. Res. 2017, 50, 778 – 786.[29] L. Wang, Y. Han, X. Feng, J. Zhou, P. Qi, B. Wang, Coord. Chem. Rev.

2016, 307, 361 – 381.[30] L. R. Mingabudinova, V. V. Vinogradov, V. A. Milichko, E. Hey-Hawkins,

A. V. Vinogradov, Chem. Soc. Rev. 2016, 45, 5408 – 5431.[31] I. Stassen, N. Burtch, A. Talin, P. Falcaro, M. Allendorf, R. Ameloot, Chem.

Soc. Rev. 2017, 46, 3185 – 3241.[32] Y.-B. Huang, J. Liang, X.-S. Wang, R. Cao, Chem. Soc. Rev. 2017, 46, 126 –

157.[33] M.-X. Wu, Y.-W. Yang, Adv. Mater. 2017, 29, 1606134.[34] W. P. Lustig, S. Mukherjee, N. D. Rudd, A. V. Desai, J. Li, S. K. Ghosh,

Chem. Soc. Rev. 2017, 46, 3242 – 3285.[35] W. Xie, J.-S. Qin, W.-W. He, K.-Z. Shao, Z.-M. Su, D.-Y. Du, S.-L. Li, Y.-Q.

Lan, Inorg. Chem. Front. 2017, 4, 547 – 552.[36] O. Guillou, C. Daiguebonne, G. Calvez, K. Bernot, Acc. Chem. Res. 2016,

49, 844 – 856.[37] J. D. B. Bradley, M. Pollnau, Laser Photonics Rev. 2011, 5, 368 – 403.[38] B. Li, H.-M. Wen, Y. Cui, W. Zhou, G. Qian, B. Chen, Adv. Mater. 2016, 28,

8819 – 8860.[39] a) X. Yang, D. Yan, Chem. Sci. 2016, 7, 4519 – 4526; b) X. Yang D. Yan,

Adv. Opt. Mater. 2016, 4, 897 – 905; c) Y. Yang, K. Wang, D. Yan, ACS Appl.Mater. Interfaces 2016, 8, 15489 – 15496; d) Y. Yang, K. Wang, D. Yan, ACSAppl. Mater. Interfaces 2017, 9, 17400 – 17408.

[40] a) X. Yang, D. Yan, Chem. Commun. 2017, 53, 1801 – 1804; b) Y. Yang, K.Wang, D. Yan, Chem. Commun. 2017, 53, 7752 – 7755.

[41] a) D. Yan, Y. Tang, H. Lin, D. Wang, Sci. Rep. 2014, 4, 4337; b) X. Yang, D.Yan, J. Mater. Chem. C 2017, 5, 7898 – 7903.

[42] M. D. Allendorf, C. A. Bauer, R. K. Bhakta, R. J. T. Houk, Chem. Soc. Rev.2009, 38, 1330 – 1352.

[43] Y. Cui, Y. Yue, G. Qian, B. Chen, Chem. Rev. 2012, 112, 1126 – 1162.[44] L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T.

Hupp, Chem. Rev. 2012, 112, 1105 – 1125.[45] Z. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev. 2014, 43, 5815 – 5840.[46] J. Park, D. Yuan, K. T. Pham, J. R. Li, A. Yakovenko, H. C. Zhou, J. Am.

Chem. Soc. 2012, 134, 99 – 102.[47] H. Huang, H. Sato, T. Aida, J. Am. Chem. Soc. 2017, 139, 8784 – 8787.[48] a) D. Yan, G. O. Lloyd, A. Delori, W. Jones, X. Duan, ChemPlusChem 2012,

77, 1112; b) C.-Y. Sun, X.-L. Wang, X. Zhang, C. Qin, P. Li, Z.-M. Su, D.-X.Zhu, G.-G. Shan, K.-Z. Shao, H. Wu, J. Li, Nat. Commun. 2013, 4, 2717;c) Y. Tang, W. He, Y. Lu, J. Fielden, X. Xiang, D. Yan, J. Phys. Chem. C2014, 118, 25365 – 25373.

[49] Y. Cui, B. Li, H. He, W. Zhou, B. Chen, G. Qian, Acc. Chem. Res. 2016, 49,483 – 493.

[50] P. Falcaro, R. Ricco, C. M. Doherty, K. Liang, A. J. Hill, M. J. Styles, Chem.Soc. Rev. 2014, 43, 5513 – 5560.

[51] L. Chen, R. Luque, Y. Li, Chem. Soc. Rev. 2017, 46, 4614 – 4630.[52] J. Yu, Y. Cui, H. Xu, Y. Yang, Z. Wang, B. Chen, G. Qian, Nat. Commun.

2013, 4, 2719.[53] C. Doherty, D. Buso, A. J. Hill, S. Furukawa, S. Kitagawa, P. Falcaro, Acc.

Chem. Res. 2014, 47, 396 – 405.[54] Y. V. Kaneti, S. Dutta, M. S. A. Hossain, M. J. A. Shiddiky, K.-L. Tung, F.-K.

Shieh, C.-K. Tsung, K. C.-W. Wu, Y. Yamauchi, Adv. Mater. 2017, 29,1700213.

[55] L. He, Y. Liu, J. Liu, Y. Xiong, J. Zheng, Y. Liu, Z. Tang, Angew. Chem. Int.Ed. 2013, 52, 3741 – 3745; Angew. Chem. 2013, 125, 3829 – 3833.

[56] G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S.Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. C. J.Loo, W. D. Wei, Y. Yang, J. T. Hupp, F. Huo, Nat. Chem. 2012, 4, 310 – 316.

[57] A. Carn8, C. Carbonell, I. Imaz, D. Maspoch, Chem. Soc. Rev. 2011, 40,291 – 305.

[58] S. Furukawa, J. Reboul, S. Diring, K. Sumida, S. Kitagawa, Chem. Soc. Rev.2014, 43, 5700 – 5734.

[59] N. Stock, S. Biswas, Chem. Rev. 2012, 112, 933 – 969.[60] V. Valtchev, L. Tosheva, Chem. Rev. 2013, 113, 6734 – 6760.[61] V. Safarifard, A. Morsali, Coord. Chem. Rev. 2015, 292, 1 – 14.[62] M. Sindoro, N. Yanai, A.-Y. Jee, S. Granick, Acc. Chem. Res. 2014, 47, 459 –

469.[63] D. Yan, R. Gao, M. Wei, S. Li, J. Lu, D. G. Evans, X. Duan, J. Mater. Chem.

C 2013, 1, 997.[64] D. Rodr&guez-San-Miguel, P. Amo-Ochoa, F. Zamora, Chem. Commun.

2016, 52, 4113 – 4127.[65] C. Avci, I. Imaz, A. Carn8-S#nchez, J. A. Pariente, N. Tasios, J. P8rez-Carva-

jal, M. I. Alonso, A. Blanco, M. Dijkstra, C. Ljpez, D. Maspoch, Nat. Chem.2018, 10, 78 – 84.

[66] M. Pawlicki, H. A. Collins, R. G. Denning, H. L. Anderson, Angew. Chem.Int. Ed. 2009, 48, 3244 – 3266; Angew. Chem. 2009, 121, 3292 – 3316.

[67] B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L.Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J.Qin, H. Rockel, M. Rumi, X.-L. Wu, S. R. Marder, J. W. Perry, Nature 1999,398, 51 – 54.

[68] J. Yu, Y. Cui, C.-D. Wu, Y. Yang, B. Chen, G. Qian, J. Am. Chem. Soc. 2015,137, 4026 – 4029.

[69] P. Mahato, N. Yanai, M. Sindoro, S. Granick, N. Kimizuka, J. Am. Chem.Soc. 2016, 138, 6541 – 6549.

[70] M. Oldenburg, A. Turshatov, D. Busko, S. Wollgarten, M. Adams, N.Baroni, A. Welle, E. Redel, C. Wçll, B. S. Richards, I. A. Howard, Adv.Mater. 2016, 28, 8477 – 8482.

[71] Y. Wei, H. Dong, C. Wei, W. Zhang, Y. Yan, Y. S. Zhao, Adv. Mater. 2016,28, 7424 – 7429.

[72] S. Van Cleuvenbergen, I. Stassen, E. Gobechiya, Y. Zhang, K. Markey,D. E. De Vos, C. Kirschhock, B. Champagne, T. Verbiest, M. A. vander Veen, Chem. Mater. 2016, 28, 3203 – 3209.

[73] X. Yang, X. Lin, Y. Zhao, Y. S. Zhao, D. Yan, Angew. Chem. Int. Ed. 2017,56, 7853 – 7857; Angew. Chem. 2017, 129, 7961 – 7965.

[74] L. Sun, M. G. Campbell, M. Dinca, Angew. Chem. Int. Ed. 2016, 55, 3566 –3579; Angew. Chem. 2016, 128, 3628 – 3642.

[75] V. Stavila, A. A. Talin, M. D. Allendorf, Chem. Soc. Rev. 2014, 43, 5994 –6010.

[76] G. Givaja, P. Amo-Ochoa, C. J. Gjmez-Garc&a, F. Zamora, Chem. Soc. Rev.2012, 41, 115 – 147.

[77] S. Takaishi, M. Hosoda, T. Kajiwara, H. Miyasaka, M. Yamashita, Y. Naka-nishi, Y. Kitagawa, K. Yamaguchi, A. Kobayashi, H. Kitagawa, Inorg. Chem.2009, 48, 9048 – 9050.

[78] B. Le Ouay, M. Boudot, T. Kitao, T. Yanagida, S. Kitagawa, T. Uemura, J.Am. Chem. Soc. 2016, 138, 10088 – 10091.

[79] G. Wu, J. Huang, Y. Zang, J. He, G. Xu, J. Am. Chem. Soc. 2017, 139,1360 – 1363.

Manuscript received: October 1, 2017Version of record online: January 16, 2018

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