Decomposable s-tetrazine based polymers for single walled carbon nanotube applications

Abstract

A process for purifying semiconducting single-walled carbon nanotubes (sc-SWCNTs) extracted with a conjugated polymer, the process comprising exchanging the conjugated polymer with an s-tetrazine based polymer in a processed sc-SWCNT dispersion that comprises the conjugated polymer associated with the sc-SWCNTs. The process can be used for production of thin film transistors. In addition, disclosed herein is use of an s-tetrazine based polymer for purification of semiconducting single-walled carbon nanotubes (sc-SWCNTs).

Claims

1. A process for purifying semiconducting single-walled carbon nanotubes (sc-SWCNTs) extracted with a conjugated polymer, the process comprising exchanging the conjugated polymer with an s-tetrazine based polymer in a processed sc-SWCNT dispersion that comprises the conjugated polymer associated with the sc-SWCNTs.

2. The process of claim 1, further comprising decomposing the s-tetrazine based polymer by photo irradiation or thermal treatment; followed by removal of the decomposition products.

3. The process of claim 1, wherein the s-tetrazine based polymer has the following structure: ##STR00011## where A is O, S, Se or CC; n is an integer from 1 to 4; R.sub.1 is independently H, F, CN or a C.sub.1-C.sub.20 linear or branched aliphatic group; Ar is one or more substituted or unsubstituted aromatic units; and, m is an integer 5 or greater.

4. The process of claim 1, wherein the s-tetrazine based polymer is PBDTFTz: ##STR00012##

5. The process of claim 1, wherein the s-tetrazine based polymer is PDTSTTz: ##STR00013##

6. The process of claim 1, wherein the s-tetrazine based polymer is: ##STR00014## and wherein: R.sub.1 and R.sub.2=2-ethylhexyl; or R.sub.1=2-ethylhexyl and R.sub.2=hexyl; or R.sub.1=hexyl and R.sub.2=2-ethylhexyl; or R.sub.1 and R.sub.2=hexyl; or R.sub.1=methyl and R.sub.2=2-ethylhexyl.

7. The process of claim 1, wherein the s-tetrazine based polymer is PCPDTFTz: ##STR00015## in which Ar=cyclopenta[2,1-b; 3.4-b]dithiophene.

8. The process of claim 2, wherein the decomposition products are removed by rinsing or evaporation.

9. The process of claim 1, wherein the conjugated polymer comprises a polyfluorene.

10. The process of claim 1, wherein the conjugated polymer comprises a polythiophene.

11. The process according to claim 1, wherein the conjugated polymer is poly(9,9-di-n-dodecylfluorene) (PFDD).

12. The process according to claim 1, wherein the weight ratio of the conjugated polymer to the sc-SWCNTs has a maximum value of 2.

13. The process according to claim 1, wherein the weight ratio of the s-tetrazine based polymer to the sc-SWCNTs has a maximum value of 4.

14. The process according to claim 1, wherein the weight ratio of the s-tetrazine based polymer to the sc-SWCNTs is between 1 and 4.

15. A method for producing thin film transistors, the method comprising: a) exchanging a conjugated polymer with an s-tetrazine based polymer in a processed sc-SWCNT dispersion that comprises the conjugated polymer associated with the sc-SWCNTs, resulting in an associated complex of s-tetrazine/SWCNTs (15); b) removing the displaced conjugated polymer; c) applying the resulting dispersion to a substrate; d) applying heat and/or UV light to decompose the s-tetrazine based polymer; and e) removing the resulting decomposition products.

16. The method according to claim 15, wherein the s-tetrazine based polymer has the following structure: ##STR00016## where A is O, S, Se or CC; n is an integer from 1 to 4; R.sub.1 is independently H, F, CN or a C.sub.1-C.sub.20 linear or branched aliphatic group; Ar is one or more substituted or unsubstituted aromatic units; and, m is an integer 5 or greater.

17. The method of claim 16, wherein the wherein the s-tetrazine based polymer is PBDTFTz: ##STR00017##

18. The method of claim 16 wherein the conjugated polymer is poly(9,9-di-n-dodecylfluorene) (PFDD).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) For clearer understanding, preferred embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates UV absorption spectra of PBDTFTz film on glass slides before and after being heated at 300 C. for 10 s. Inset shows the pictures of the solution or film before (left) and after (right) decomposition.

(3) FIG. 2 illustrates UV absorption spectra of PBDTFTz in toluene solution before and after irradiated with UV light for 10 or 20 min.

(4) FIG. 3 illustrates UV absorption spectra of polymer/SWCNT dispersions in toluene of an original PFDD/SWCNT dispersion followed by polymer exchanges with PBDTFTz.

(5) FIG. 4 illustrates the application of decomposable s-tetrazine based polymer for preparation of SWCNT thin film transistors with enhanced contact.

(6) FIG. 5 illustrates Raman spectra of a PBDTFTz/SWCNT film on a silicon substrate during laser irradiation.

(7) FIG. 6 illustrates the transfer curve of TFTs from a PBDTFTz/SWCNT dispersion at different irradiation time under 405 nm laser.

(8) FIGS. 7a and 7b are SEM images of the tube network on SiO.sub.2 substrate prepared from polymer (a) PBDTFTz/SWCNT and (b) PFDD/SWCNT dispersion.

(9) FIGS. 8a-8f illustrate a comparison of properties at various channel lengths, between TFTs prepared with PBDTFTz/SWCNT (a, c and, e) or PFDD/SWCNT (b, d and f) dispersions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) The following s-tetrazine based polymers can be used for SWCNT purification, dispersion and device fabrication:

(11) ##STR00005##
where each A is O, S, Se or CC; each n is an integer from 1 to 4; each R1 is independently H, F, CN or a C1-C20 linear or branched aliphatic group; Ar is one or more substituted or unsubstituted aromatic units; and, m is an integer 5 or greater.

(12) Examples of s-tetrazine based polymers include poly[2,6-(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole)-alt-5,5-(3,6-bis[4-(2-ethylhexyl)thienyl-2-yl]-s-tetrazine)], also identified with the acronym PDTSTTz:

(13) ##STR00006##
The synthesis, characterization and photovltaic applications of PDTSTTz are disclosed by J. Ding et al. in Chem. Commun., 2010, 45, 8668-8670, the contents of which are incorporated herein by reference.

(14) Another class of s-tetrazine based polymers include the following five, which are disclosed by Z. Li et al. in Chem. Mater. 2011, 23, 1977-1984, the contents of which are incorporate herein by reference:

(15) ##STR00007##
In particular, P4, also known as PCPDTTTz, is used in the production of efficient solar cells, as disclosed by Z. Li et al. in J. Am. Chem. Soc., 2010, 132, 13160-13161, the contents of which are incorporate herein by reference.

(16) Another example includes PCPDTFTz, the synthesis, characterization and photovoltaic properties of which are disclosed by Z. Li et al. in Macromol. Chem. Phys. 2011, 212, 2260-2267, the contents of which are incorporate herein by reference:

(17) ##STR00008##

(18) In one embodiment, the following s-tetrazine based polymer (PBDTFTz), which contains alternating bisfuran-s-tetrazine and benzo [1,2-b:4,b-b]dithiophene units, can be used for SWCNT purification, dispersion and device fabrication:

(19) ##STR00009##
Decomposition of S-Tetrazine Based Polymers

(20) Differential scanning calorimetry (DSC) curves demonstrate that s-tetrazine polymer can be decomposed thermally at around 250 C.

(21) This is illustrated in FIG. 1, in which a PBDTFTz film on a glass slide was heated at 300 C. for 10 s. The polymer PBDTFTz was synthesized as disclosed by Z. Li et al., Macromol. Chem. Phys., 2011, 212, 2260. Broad UV absorption bands at 511 nm and 552 nm decrease with the appearance of new peaks around 380 nm, following thermal degradation. The solid line shows the UV spectra of the product after thermal decomposition in toluene solution. Inset shows the pictures of the solution or film before (left) and after (right) decomposition, in which the purple color of the initial polymer film/solution decays to yellow. GPC analysis of the decomposed product from thermal degradation confirms a dramatically decreased molecular weight.

(22) The product contains 90% of dicyano compound (1): It has much shorter conjugation length than PBDTFTz so the absorption spectrum is blue shifted and contains well-resolved peaks. The decomposition scheme is shown as follows:

(23) ##STR00010##

(24) Furthermore, s-tetrazine polymer is sensitive to strong UV light. This is illustrated in FIG. 2, in which the broad absorption bands at 511 nm and 552 nm decrease after a solution of PBDTFTz in toluene solution is irradiated with UV light for 10 or 20 min. As with thermal decomposition, the purple color of the initial polymer solution decays to yellow. GPC analysis of the decomposed product from photolytic degradation confirms that the degradation product is primarily 90% of dicyano compound (1) shown above.

(25) Displacement of PFDD with S-Tetrazine Based Polymers

(26) The interaction between s-tetrazine based polymers and SWCNT is quite strong, but not strong enough to disrupt the SWCNT structure. Other polymers, such as those of the polyfluorene class (PFDD), can be easily displaced by treating the PFDD dispersion with s-tetrazine polymer solution.

(27) In one embodiment, a simple polymer exchange process can be used to replace poly(9,9-di-n-dodecylfluorene) (PFDD) on SWCNTs with PBDTFTz by a simple polymer exchange.

(28) The polymer PBDTFTz was synthesized as disclosed in Z. Li et al., Macromol. Chem. Phys., 2011, 212, 2260. High purity PFDD/sc-SWCNT solution was prepared as disclosed by Ding, Z et al. Nanoscale, 2014, 6, 2328, with a polymer/tube ratio of 1.3 and tube concentration at 165 mg/L. A PBDTFTz solution (1 g at 0.87 mg/mL) and toluene (3 g) was added to above solution (1 g), and the mixture was bath sonicated for 30 min. Then the solution was filtered on a Teflon membrane with pore size of 200 nm and washed with toluene (10 mL). The filter cake was then dispersed in toluene (4 g) and labeled as the product after first exchange. This process was repeated to obtain the product from a second polymer exchange. The polymer/tube ratio and solution concentration can be easily adjusted by filtration, dilution or addition of polymer. The final PBDTFTz/SWCNT dispersion has tube concentration at 25.5 mg/L and polymer/tube ratio at 4/1.

(29) FIG. 3 illustrates a UV absorption spectra of the aforementioned polymer/SWCNT dispersions in toluene: original PFDD/SWCNT (dotted line), after 1st (solid line) and 2nd (dashed line) polymer exchange with PBDTFTz. In the first exchange, after bath sonication, filtration and rinsing, more than half of PFDD on the SWCNT surface was replaced by PBDTFTz. After the second ligand exchange, the PFDD peak at 380 nm in the UV spectra totally disappeared, indicating the complete replacement of PFDD by PBDTFTz, whose absorption peaks are located at 511 and 552 nm. In the meantime, the shape and resolution of S11 (1500 to 1900 nm) and S22 (700-1100 nm) bands of the SWCNTs stay untouched, indicating the formation of a stable PBDTFTz/SWCNT dispersion. It was also found that the ratio of PBDTFTz/SWCNT can be reduced to 2.6/1 in the dispersion even after thorough dilution and rinsing step, agreeing well with the stronger interaction of PBDTFTz with SWCNTs than that of PFDD, whose ratio to SWCNT can be reduced to 1.2/1 under similar condition.

(30) Clean SWCNT Networks

(31) As discussed above, s-tetrazine based polymers can be decomposed by photo irradiation or heating. After decomposition, the resulting small molecules can be washed away in solution or evaporated under laser irradiation or heating under vacuum if it is in the solid state. In this manner, clean SWCNT networks can be obtained, which is desirable for electrical devices application, such as thin film transistors (TFTs) or sensors. This is discussed further below in reference to FIG. 4.

(32) Use of PBDTFTz/SWCNT Dispersions for Preparation of TFT

(33) PBDTFTz/SWCNT dispersions can be used to prepare electronic devices.

(34) The use of decomposable s-tetrazine based polymer for producing SWCNT thin film transistors with enhanced contact, is summarized in FIG. 4. A dispersion of PFDD (20) and PFDD/SWCNT (10) undergoes a polymer exchange in which an s-tetrazine based polymer (25) displaces the PFDD, resulting in an associated complex of s-tetrazine/SWCNTs (15). Once the PFDD (20) is removed, the resulting dispersion is then applied to a substrate (30). Following the application of heat and/or UV light, the s-tetrazine based polymer decomposes, with the resulting decomposition products removed, leaving behind clean SWCNT networks (35).

(35) In-situ transistor characterization under laser reveals the decomposition of PBDTFTz and evaporation of the small molecule compounds formed. Further investigation of the resistance from different channel length devices demonstrates dramatically improved contact between tubes due to removal of wrapping polymers. This fully exposed tube network can be particularly attractive for sensor applications, and results in improved contact.

(36) TFT devices were fabricated using prefabricated devices with a 230 nm thick thermal oxide layer. The chip has pre-patterned Au electrodes with 44 TFT devices at channel lengths of 20, 10, 5, 2.5 m and a channel width of 2,000 m respectively. The chip was soaked in a 5% Hellmanex solution for 20 min at 60 C. before rinsed with water and isopropanol, blow-dried with nitrogen. The polymer/tube dispersion (0.1 mL) was then spread on the chip surface and the chip was soaked for 10 min under toluene vapor. The chip was then rinsed with toluene (5 mL) and blow dried with nitrogen before annealed at 140 C. for 10 min in air.

(37) As an example, the PBDTFTz/SWCNT dispersion prepared above was used to prepare thin film transistors (TFT) on a freshly cleaned and pre-patterned SiO.sub.2 substrate according a procedure disclosed by Z. Li, J. Ding et al., in Org. Electron. 2015, 26, 15. The resulting TFT devices have a bottom contact and common bottom gate configuration. For comparison, devices prepared from a PFDD/SWCNT dispersion were also fabricated at the same concentration and polymer/tube ratio.

(38) The degradation of the PBDTFTz on the SWCNT network was monitored by resonance Raman spectroscopy as shown in FIG. 5. Under 514 nm laser irradiation, the intensities of Raman shift at 1430 and 1530 cm.sup.1 from PBDTFTz gradually decreased, while the D and G bands from SWCNTs began to dominate. After 30 minutes exposure of the network to a 405 nm laser, all of the signals from PBDTFTz disappeared and only a clean spectrum of the SWCNTs remained.

(39) The TFT transistor was also characterized simultaneously under 405 nm laser irradiation. FIG. 6 illustrates the transfer curve of TFTs from a PBDTFTz/SWCNT dispersion at different irradiation time under 405 nm laser. V.sub.SD=1V and active channel length and width are 20 and 2000 m respectively. Arrows show the sweeping direction.

(40) In the first 2 min, the on-current (at V.sub.g=10V) of the TFT increased gradually from 70 to 170 A while the off-current at V.sub.g=10V increased more dramatically by several orders of magnitude, which resulted to very poor on/off ratio. The hysteresis of the transfer curve also became more severe. However, this change reached plateau at 2 min and then slowly moved back.

(41) This phenomenon can be explained by the degradation of PBDTFTz. Under 405 nm laser irradiation, PBDTFTz begins to decompose with the formation of dicyano (compound (1)) and release of nitrogen gas according to the decomposition scheme above. Compound (1) contains two cyano groups in each molecule and is a very strong p-doping agent for SWCNT. During PBDTFTz degradation, compound (1) that is formed will adhere on tubes first, and this will cause more p-doping effect (in addition to oxygen from the air) and shift the threshold voltage towards a positive direction. Longer time laser irradiation will further evaporate compound (1) that is formed and this p-doping effect will then alleviate.

(42) Since the resulting TFTs always show a low on/off ratio, this suggests that compound (1) may not be completely removed from tube surface by simple laser irradiation. However, this decomposition reaction can be accelerated at a higher temperature; compound (1) can be completely removed at 300 C. under vacuum. The TFTs from PFDD/SWCNT was also characterized under laser irradiation, only slightly decreasing of on-current was observed, which can be attributed to the decreased p-doping of O.sub.2 under laser light as all measurements were carried out in ambient conditions.

(43) Comparison of Networks Based on PFDD/SWCNT and PBDTFTz/SWCNT

(44) In general, the TFTs from PFDD/SWCNT have higher current and mobility, although their SEM images show quite similar tube density to those of PBDTFTz/SWCNT (see FIGS. 7a and 7b). The weight ratio of polymer/SWCNT is 4:1, while the concentration of SWCNT is 25.5 mg/L. The scale bar on the right bottom corner of the image is 1 m.

(45) Further examination reveals a higher degree of bundles and curved tube conformation in the PBDTFTz/SWCNT network, which may limit the contact between tubes.

(46) FIGS. 8a-8f illustrate a comparison of properties at various channel lengths, between TFTs prepared with PBDTFTz/SWCNT (a, c and, e) or PFDD/SWCNT (b, d and f) dispersions. In particular, FIGS. 8(a) and (b) measure mobility; 8(c) and (d) measure On/off ratio; and 8(e) and (f) measure resistance of TFTs prepared from PBDTFTz/SWCNT (a, c, e) or PFDD/SWCNT (b, d, f) dispersions at various channel length. The channel width is 2,000 m. Polymer/tube weight ratios are 4/1 and concentration of SWCNT is 25.5 mg/L.

(47) More detailed characterization of the TFTs with different channel length from PBDTFTz/SWCNT before, and after, decomposition is shown in FIG. 8. For PBDTFTz/SWCNT TFTs, after removal of PBDTFTz, the mobility almost doubled due to better contact between tubes, whereas the mobility from PFDD/SWCNT slightly decreased due to decreased p-doping level (from O.sub.2 and moisture) after heating and vacuum treatment. The on/off ratio of PBDTFTz/SWCNT TFTs decreased slightly after decomposition due to the raised off-current. The sheet resistance (R.sub.) and contact resistance (R.sub.c) could be extracted from the data of the devices with various channel length.

(48) For PBDTFTz/SWCNT TFTs, after degradation, R.sub. decreased from 0.481 to 0.296 M while R.sub.c decreased more dramatically from 0.960 to 0.242 K m. It is interesting that the removal of insulating polymer layer on tubes has more effect on R.sub.c than R.sub.. This also demonstrates the significance of removal of the insulation polymer layer on the tube surface within a network.

(49) For PFDD/SWCNT TFTs, after similar treatment, on the contrary, R.sub. increased from 0.117 to 0.164 M, while R.sub.c remained almost unchanged, which can be attributed to decreased p-doping level after vacuum and thermal treatment.

(50) The complete and easy removal of dispersant from the tube surface not only improves the device performance of transistors, but also benefits the sensitivity of the devices. This kind of wholly exposed tube surfaces is highly desired for sensor applications.

(51) In the aforementioned TFT device characterization: I-V curves were collected on a probe station at ambient condition and the mobility was calculated from the I.sub.sd-V.sub.g transfer curve in the linear regime based on a parallel plate model. Due to high channel width/length ratio (100), the contribution arising from tubes outside the defined channel area can be ignored. For TFT testing under laser irradiation, a 405 nm LDCU laser control unit was used and the laser beam was reflected onto the active channel as shown in FIG. 5. Raman spectra were acquired with an InVia Raman microscope (Renishaw) on finished devices, using 514 nm laser excitation source and 100 magnification objective lens. SEM images were obtained using Hitachi SU-5000 operated at 1 kV (charge contrast imaging mode on SiO.sub.2/Si substrate). UV absorption spectra were collected on a UV-Vis NIR spectrophotometer (Cary 5000, Varian) over a wavelength range from 300 to 2100 nm.

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(53) The novel features will become apparent to those of skill in the art upon examination of the description. It should be understood, however, that the scope of the claims should not be limited by the embodiments, but should be given the broadest interpretation consistent with the wording of the claims and the specification as a whole.