Mechanical and optical properties of ultralarge flakes of a metal–organic framework with molecular thickness† †Electronic supplementary information (ESI) available: Extended experimental details, additional Figures and theoretical calculations. See DOI: 10.1039/c4sc03115f Click here for additional data file.

The red emission on isolated 2d-mof flakes with areas of square microns and molecular thicknesses (from single up to ca. 50 layers) has been characterized. Free-standing flakes have also been produced and their mechanical and optical properties studied.


Introduction
Two-dimensional (2D) materials have attracted increasing attention in the last few years due to their potential expected applications. 1 Graphene, a single layer of graphite, represents the rst material of this kind. 2 The fascinating physical properties 3 and potential applications of graphene 4,5 have stimulated the development of a number of 2D related materials including graphene oxide, 6,7 BN, 8 MoS 2 , 9 and clays. 10 However all of them show a rather limited or no chemical design and functionalities. Recently, it has been pointed out that covalent polymers, 11 and layered covalent organic frameworks (COFs) 1,12,13 or metalorganic frameworks (MOFs) 14 could bringà la carte 2D materials with a variety of architectures, pre-designed cavities, and chemical functionalities. 15 Even more importantly, suitable selection of the initial building blocks should enable preparation of multifunctional materials with interesting physical and chemical properties. In particular, MOFs are crystalline porous materials formed by linking organic molecules with metal fragments. They show a large number of properties and potential applications ranging from gas storage and separation, molecular sieves and catalysis to sensing. 16,17 Additionally they have been used as a source of nanomaterials for the production, for instance, of nanoparticles 18 and nanowires. 19 The expected potential of metal-organic sheets to provide novel 2D-materials has triggered global research interest. 1,13 Indeed, intensive attempts to generate 2D-layers of MOFs have been reported, applying different interesting strategies to this purpose. Thus, bottom-up procedures based on on-surface synthesis have been used to produce layers of MOFs but, unfortunately, the lateral dimensions of these structures are, so far, too small to expect relevant sheet-like properties; 20,21 furthermore, they cannot yet be isolated and manipulated. Additionally, layer formation at the air-water interphase and subsequent deposition on surfaces has been proved as an alternative for production of MOFs exceeding micron lateral dimensions. [22][23][24] However, although free-standing layers on TEM grids have demonstrated neither mechanical nor any other physical properties, they have been reported.
Alternatively, the top-down approach based on liquid phase exfoliation (LPE) of crystals of layered MOFs assisted by ultrasound has been developed as an alternative procedure. 25 Some examples following this top-down approach have been recently published, but in all of them the lateral dimensions of the layers have precluded both free-standing isolation and physical characterization. [26][27][28] Along this line we have designed a new laminar MOF of formula [Cu(m-pym 2 S 2 )(m-Cl)] n (pymS 2 ¼ dipyrimidindisulde), showing an interlayer interaction so weak that it can be delaminated just by interaction with an excess of solvent. 29 Despite the fact that the bulk material showed intense red emission, the physical characterization of the layers of [Cu(m-pym 2 S 2 )(m-Cl)] n was not possible due to their reduced lateral size. In this work, we have been able to set up a method to produce ultra-large layers with control over the thickness that has allowed us to obtain free-standing few-layers akes of this MOF and characterize their mechanical properties and its emission in 2D-materials ranging from single to a few layers. The results show the high potential of several chemically designed layered materials, such as MOFs, to produce novel 2D layered functional polymers.

Flakes production and characterization
Prismatic crystals [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH (pymS 2 ¼ dipyrimidindisulde) were synthesized by slow evaporation of a dipyrimidinedisulde MeOH : MeCN solution into a methanolic solution of CuCl 2 $2H 2 O (ESI † for details). Fig. 1 shows schematic views of the structure of the MOF [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH in which the presence of a layered structure with cavities containing solvent molecules is remarkable. Large orange crystals of [Cu(m-pym 2 S 2 )-(m-Cl)] n $nMeOH were suspended in water, then sonicated and centrifuged to obtain a homogeneous suspension of larger surface area akes. The integrity of the material obtained aer sonication was conrmed by X-ray diffraction analysis, Fig. S5. † The productions of these large lateral dimensions are a consequence of the size of the starting crystals (dimensions ca. 125-60 Â 45-80 Â 35-70 mm 3 ) which are signicantly bigger than those previously used. 29 These improvements in the dimensions of the laminar crystals combined with control of the exfoliation and centrifugation parameters allowed the production of large MOF-layers with controlled thickness (Fig. 2). The akes were adsorbed by dip-coating at room temperature on Si/SiO 2 (300 nm) (ESI † for experimental details). Fig. 2 shows AFM vs. optical images of MOF-layers with thicknesses ranging from 2 to 30 nm. XPS characterization of these akes deposited on highly oriented pyrolytic graphite (HOPG) agrees with the data previously published by us 29 conrming their composition. The control over the thickness of the MOF layers is achieved by adjusting the sonication time in the exfoliation procedure (ESI † for experimental details). The statistical analysis of the thickness of the MOF-layers shows the excellent control of the exfoliation within the sonication time and the production of homogeneous materials (Fig. S1 †).
Free-standing akes of [Cu(m-pym 2 S 2 )(m-Cl)] n are obtained by dip-coating with a water suspension of the polymer at 55 C on a Si/SiO 2 (300 nm) substrate with predened circular wells (diameters ranging from 0.5 to 3 mm and 400 nm depth; Fig. S2 in ESI †). The increase of temperature of the suspension helps to reduce the surface tension of water minimizing the formation of the meniscus in the wells and makes the solution slightly more volatile improving the ratio of free-standing vs. collapsed MOF layers.
The adjustment of withdrawal speed and solution concentration seems to be critical to obtain free-standing akes during dip-coating. Inspection by optical microscopy revealed polymer akes covering several holes of the substrate (Fig. 3b). Importantly, the existence of suspended layers implies that we are not facing the deposition of random material, or some kind of precipitation process, we are really delaminating the crystals.
Atomic force microscopy (AFM) images 30 ( Fig. 3a and c) conrm this observation and provide a narrow height distribution for the akes that ranges between 4-8 nm corresponding, to 5 to 10 layers of [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH, with lateral dimensions ca. 300-3600 mm 2 . Although in the ideal case such sheets consist of single monolayers, they are oen manifested as incompletely exfoliated akes comprising a small number (<10) of stacked monolayers.
Additionally the AFM images reveal holes where the ake is perfectly suspended and others where it has collapsed, probably as a consequence of the capillary forces introduced by the solvent during evaporation. The high ratio of holes with suspended layers indicates a strong tendency of this large polymer  akes to withstand these forces. As a matter of fact, if a water suspension of graphene oxide akes is deposited by dropcasting on a holed surface, they also exhibit an even higher tendency to collapse as a consequence of the capillary forces. 31 These results show these akes are mechanically stable to be spanned over the holes and give qualitative insights into the robustness of the sheets that further will be tested by their mechanical characterization.

Mechanical properties of free-standing akes
Mechanical stability is a relevant topic for 2D materials. The mechanical properties are sensitive to defects and thus they can be used as an indicator for the structural integrity and stability of the layers towards their application potential and device fabrication. 35 Mechanical characterization of the suspended akes was performed using a standard nanoindentation set up with AFM. 32 Fig. 4a displays a scheme of the experimental set up. The mechanical load applied by the AFM tip can be considered to a good approximation as a punctual force F that produces an indentation d given by: Where F is the loading force, d is the indentation at the central point, T is the pretension accumulated in the sheet during the preparation procedure, q y 1, h is the layer thickness and a is the drumhead radius. E is the Young's modulus, a fundamental parameter that characterizes the stiffness of a material. Fitting eqn (1) to curves measured in up to 7 different drumheads in 8 layer akes yielded values of E and T of 5 AE 0.5 GPa and 0.12 AE 0.09 N m À1 , respectively. Eqn (1) is a good approximation since the tip radius (z25 nm) is much smaller than the hole radius (z1000 nm) used to suspend the akes in the experiments. The same experimental set up can also be used to estimate the breaking force F 0 and breaking strength given by s* ¼ [F 0 E/ (hpR tip )] 1/2 where R tip is the tip radius (we have used two different AFM tips with radii 25 and 15 nm). 32 We measured breaking forces of about 40 nN yielding s* ¼ 1 AE 0.4 GPa. Notice that the expression for the breaking force is for a linear material that tends to overestimate this gure.
Density functional calculations were carried out on [Cu(mpym 2 S 2 )(m-Cl)] n $nMeOH (ESI † for details) yielding E ranging from 3.4 GPa for the polymer single layer without solvent molecules, up to 4.1 GPa when methanol molecules are considered. 36 Both gures are in good agreement with the experimental results. Table 1 summarizes the results. The Young's modulus measured for the MOF studied here is the lowest reported so far, being approximately 200 times lower than the one measured for pristine graphene. The breaking strength follows a similar   tendency being 150 times lower than the one for graphene, yet it was still possible to suspend very thin layers of this compound from solution, where the capillary forces tend to collapse the membranes. Therefore, contrary to what one might expect, the weak strength bonds are enough to retain the 2D-layer structure as mechanically coherent entities.

Luminescence studies
Studies of additional physical properties are of the greatest interest and never reported before for layers of MOFs of nanometer thickness. Therefore, since optical properties of single crystals of [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH were previously reported by us, 29 we decided to study how these optical properties persist to isolated layers. To this end, samples of [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH were imaged in reection to locate large 'akes' and suitable candidates for spectroscopy were identied by their apparent colour in the optical microscope, which is observed due to interference effects in very thin few layers akes on the Si/SiO 2 substrate. These akes were subsequently characterized by AFM in order to determine their thickness and thereby to calculate the number of layers in each akethe individual akes are easily recognizable by their individual shapes and orientations relative to each other. Aer locating a ake, confocal spectral images were obtained by rastering the laser focus across the selected area of the sample. Each image corresponds to 10 4 individual spectra and the colour-scale is determined by integration of the spectra over a certain range of Raman shi. Fig. 5c shows an image of elastically-scattered light (<100 cm À1 Raman shi from the laser line 488 nm). Several large akes are clearly visible and the identical akes can also be identied in the optical and AFM images of Fig. 5a and b, respectively. Height analysis of the AFM images revealed that these akes correspond to a single layer (height ca. 2 nm). 29 Fig . 5d shows the Raman/luminescence spectra of the sample from Fig. 5a-c. It is clear that there are similarities as well as differences between the ake spectra and the bulk spectrum. Three bands (indicated by grey vertical lines) are present in the bulk and ake spectra near 580 nm, 615 nm and 650 nm. We assign these features to PL because the same emission wavelength was observed with a different excitation wavelength of 531 nm (in ESI Fig. S9 † feature (iii)); they are blue-shied in the ake spectra compared to the bulk spectrum by about 5-10 nm. In Fig. 5d we also observe bands centred at 526 nm (1470 cm À1 Raman shi) and 569 nm (2920 cm À1 Raman shi). Similar features appear in Fig. S9 (ESI †), although more details are visible because the longer wavelength excites PL to a lesser extent. Because these bands appear at xed energy with respect to the laser, we assign them to Raman processes described by C-H bending and C-H stretching modes of the 2D-MOF and associated solvent (MeOH) molecules.
Density functional calculations (B3LYP/6-31G(d)) (ESI † for details) of the vibration modes of the ligand conrm that there are groups of vibrations near 1470 cm À1 (Fig. S9 in ESI †) which are associated with normal modes that are combinations of sp 2 C-H bending and either C-C or C-N modes of the ring. These features are sharp in the bulk spectrum, but are broad and weak for akes, which can be understood in terms of the partial loss of translational symmetry in the thin layers. In addition, it should be noted that solvent exchange effects MeOH/H 2 O are known for this compound 29 and these may have a strong inuence on the Raman features in this region because solvent molecules present in the bulk crystal are less easily exchanged than those present near the surface, which will exchange even in air. These solvent effects could explain the differences in behaviour of [Cu(m-pym 2 S 2 )(m-Cl)] n compared to graphite 37 and MoS 2 (ref. 38) with respect to the evolution of Raman spectra with sample thickness.
Other properties appear less sensitive to solvent effects. The luminescence properties of akes on silicon oxide with thickness ranging from single, few to many layers, shi to the red, but otherwise do not signicantly change as the number of layers increases (see also Fig. S8 and S9 †).
Finally, we also measured Raman spectra of akes suspended over circular holes ( Fig. 6 and S11 †). It is observed that the spectrum of free-standing akes are qualitatively the same, though with a slightly larger intensity, to those lying on the )(m-Cl)] n $nMeOH as a bulk sample on an Si/SiO 2 substrate and as 'flakes' comprising single layers on the Si/SiO 2 substrate. The x-axis is shown as both the Raman shift with respect to the incident light (l ex ¼ 488 nm) and as absolute wavelength because different features are due to Raman bands (shift with the laser wavelength) and PL (independent of laser wavelength). The individual spectra have been scaled and offset in order to display all the spectra on plot. The layer spectra were obtained using a confocal microscope (Witec CRM200); the bulk spectrum was recorded on a 48000s (T-Optics) spectrofluorometer from SLM-Aminco (l ex ¼ 531 nm).
Si/SiO 2 substrates previously analysed and do not arise from ake-substrate interactions.

General methods
All chemicals were of reagent grade and were used as commercially obtained. The reactions were carried out under dry argon atmosphere using Schlenk techniques and vacuumline systems. The dipyrimidinedisulde (pym 2 S 2 ) ligand was prepared according to the published procedure. 39 [Cu(m-pym 2 S 2 )-(m-Cl)] n $nMeOH was prepared with a slight modication of the procedure already published. 29 Luminescence/Raman measurements The spectral images were recorded on a300RA and CRM200 confocal Raman microscopes (Witec GmbH, Ulm, Germany). The 532 and 488 nm lines, respectively, of an Nd-YAG and an argon ion laser provided the excitation light and the emitted and/or scattered light passed through a Raman edge lter to remove elastically scattered light. The ltered light was collected by a multimode optical bre that also served as the confocal pinhole. The objective was a (100Â) lens with a numeric aperture of 0.95 and a 50 mm single mode bre was used to supply the excitation light. Luminescence excitation and emission spectra of the solid [Cu(m-pym 2 S 2 )(m-Cl)] n $nMeOH were performed at 25 C on a 48000s (T-Optics) spectrouorometer from SLM-Aminco.

AFM measurements
Atomic Force Microscope (AFM) techniques were used in dynamic mode using a Nanotec Electronica system operating at room temperature in ambient conditions. The images were processed using WSxM. 30 For AFM measurements, commercial Olympus Si/N and Ti/Pt cantilevers were used with a nominal force constant of 0.75 N m À1 and 2 N m À1 , respectively (ESI † for details). Mechanical characterization of the free-standing akes was performed by indenting an AFM tip at the centre of the suspended area. Only membranes showing a at and homogeneous surface were selected for the measurements. Curves acquired on the Si/SiO 2 (300 nm) non-deforming substrate were used as a reference for calculating the applied force and the resulting deection of the layers (indentation d).

Conclusions
In summary, we report the use of liquid phase exfoliation and dip-coating as a simple an efficient top-down method for the production of akes of a metal-organic framework with lateral dimensions of hundreds of square microns and with an excellent control of the thickness. The isolated layers on SiO 2 have been characterized by AFM and Raman, they show red emission, and this observation remains for isolated free-standing akes. The mechanical characterization conrms the stability of these layers. To the best of our knowledge, this is rst time that the physical properties of layers of a free-standing MOF are reported. Mechanical stability of layers based on MOFs 15 will be a requirement for device fabrication. Herein we have shown a proof-of-concept on the feasibility of materials based on MOFlayers. Obviously, polymers such as covalent organic fragmentworks (COFs) will be a source of 2D-materials in the near future.