Interface Interaction Controlled Transport of CdTe Nanoparticles in the

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Interface Interaction Controlled Transport of CdTe Nanoparticles in the Microcontact Printing Process Xiaochun Wu,*,†,‡ Steven Lenhert,†,§ Lifeng Chi,*,† and Harald Fuchs† Physikalisches Institut, Westfaelisch Wilhelms-UniVersitaet, Wilhelm-Klemm-Strasse 10, 48149, Mu¨nster, Germany, and National Center for Nanoscience and Technology, 100080, Beijing, People’s Republic of China ReceiVed March 6, 2006. In Final Form: May 31, 2006 High-quality CdTe nanoparticles stabilized with thioglycolic acid (TGA) are patterned on SiO2/Si surfaces using microcontact printing (µCP). Due to the weak interaction of the nanoparticles with the stamp surface, tailoring of gas flow rate during the inking process as well as the type and scale of the patterns on the stamp are used to control the distribution of the nanoparticles on the structured stamp surface. This distribution is then transferred the printed regions. Either edge printing or homogeneous printing can be achieved under optimized conditions. In addition, new structures such as nanowires form under certain conditions.

Introduction Microcontact printing (µCP) is a simple and convenient method of obtaining patterned structures of various functional materials.1,2 Often, a structured elastomeric stamp molded from a master is first inked with a solution of the desired material (ink) and dried under a stream of compressed gas (inking step). The stamp is then briefly placed in contact with a solid substrate of interest and peeled off; thus a desired pattern of the ink on the substrate is obtained (printing step). Since the creation of this technique, many different materials have been successfully printed.3 Generally speaking, a stronger interaction between the ink and the substrate is necessary in order to achieve a better delivery of the ink from the stamp to the substrate. Up to now, various interactions have been studied and utilized to initiate the transport of the ink.1,4-7 Among them, covalent interactions (very strong), such as that between alkanethiols and gold8-10 and other metals11,12 and that between alkyl silanes and hydroxyl-terminated silicon,13-16 are best studied. Apart from this, printing of * Corresponding author. E-mail: [email protected] (X.W.); chi@ uni-muenster.de (L.C.). † Westfaelisch Wilhelms-Universitaet. ‡ National Center for Nanoscience and Technology. § Present address: Forschungszentrum Karlsruhe GmbH, Institut fuer NanoTechnologie, 76344, Hermann-von-Helmholtz-Platz 1, EggensteinLeopoldshafen, Germany. (1) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (2) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (3) Geissler, M.; Xia, Y. AdV. Mater. 2004, 16, 1249. (4) Hovis, J. S.; Boxer, S. G. Langmuir 2000, 16, 6773. (5) Park, J.; Hammond, P. T. AdV. Mater. 2004, 16, 520. (6) Bernard, A.; Delamarche, E.; Schmid, H.; Michel, B.; Bosshard, H. R.; Biebuyck, H. Langmuir 1998, 14, 2225. (7) Wu, X. C.; Bittner, A. M.; Kern, K. Langmuir 2002, 18, 4984. (8) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324. (9) Delamarche, E.; Michel, B.; Biebuyck, H.; Gerber, C. AdV. Mater. 1996, 8, 719. (10) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (11) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070. (12) Xia, Y.; Kim, E.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. (13) Jeon, N. L.; Lin, W. B.; Erhardt, M. K.; Girolami, G. S.; Nuzzo, R. G. Langmuir 1997, 13, 3833. (14) Wang, D. W.; Thomas, S. G.; Wang, K. L.; Xia, Y.; Whitesides, G. M. Appl. Phys. Lett. 1997, 70, 1593. (15) St. John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022. (16) Pompe, T.; Fery, A.; Herminghaus, S.; Kriele, A.; Lorenz, H.; Kotthaus, J. P. Langmuir 1999, 15, 2398.

noncovalently binding molecules (e.g., lipid bilayers5 and proteins6 on glass) has also attracted a lot of attention and greatly expanded the scope of printable materials. Some new phenomena may appear for noncovalent interactions, depending on the difference in interaction strength between the ink and the substrate. For example, Workman et al. observed fingering instabilities of the printed molecules when printing octadecanol, docosanol, stearylamine, and stearic acid on mica surfaces at moderate relative humidity.17 A capillary-condensed meniscus of water at the edge of a contact region is believed to guide the transport of ink molecules along the air-water interface to the mica surface and to be responsible for the appearance of fingering instability. They concluded that, due to the weaker interaction between these molecules and the mica surface, environmental factors such as temperature and humidity play an important role in printing. In general, the interaction between the ink and the substrate is very important to the fidelity of the transferred pattern and has therefore been extensively investigated. In comparison, the interaction between the ink and the stamp during the inking step has been relatively unexplored.18 Here we have studied microcontact printing of CdTe nanoparticles stabilized with a shortchain stabilizer (thioglycolic acid, TGA) on SiO2/Si surfaces. The interactions of the nanoparticles with both the stamp surface and the substrate are noncovalent and relatively weak. Some interesting features are found in the inking step that lead to a controlled distribution of the nanoparticles on the stamp. This distribution was subsequently transferred to the substrate in the printing step. Either edge printing or homogeneous printing can be achieved under optimized conditions. Microcontact printing has been used to obtain nanoparticle patterns by an indirect approach where printed molecular monolayers on the substrate are used as templates to guide the selective adsorption of the nanoparticles from solution.19-23 Direct (17) Workman, R. K.; Manne, S. Langmuir 2004, 20, 805. (18) Cherniavskaya, O.; Adzic, A.; Knutson, C.; Gross, B. J.; Zang, L.; Liu, R.; Adams, D. M. Langmuir 2002, 18, 7029. (19) Vossmeyer, T.; Jia, S.; Delono, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (20) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (21) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. F. Langmuir 2000, 16, 4409. (22) Lu, N.; Chen, X.; Molenda, D.; Naber, A.; Fuchs, H.; Talapin, D. V.; Weller, H.; Muller, J.; Lupton, J. M.; Feldmann, J.; Rogach, A. L.; Chi, L. F. Nano Lett. 2004, 4, 885.

10.1021/la060615v CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006

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Figure 1. Topographic SFM images of (a) DPPC stripes on SiO2/Si surface, (b) molded structure in polystyrene, and (c) molded structure in PDMS from a polystyrene master.

printing of nanoparticles has also been demonstrated, albeit less frequently.24 In both approaches, the nanoparticles distribute randomly over the whole pattern. Recently, high-quality, closepacked monolayers (multilayers) of nanoparticles have also been successfully printed.25,26 In contrast, printing of nanoparticles (and other materials) into much smaller patterns (less than 100 nm) is still facing a great challenge due to the softness of the standard elastomer Sylgard 184. Some edge transfer methods have been suggested in order to achieve sub-100-nm feature sizes by using Sylgard 184 with micrometer pattern sizes.18,27-32 They are however more complex, either requiring sophisticated instrumentation27,28 or involving more steps30-32 than the method we present here.

Printing of the Nanoparticles. A freshly activated stamp was covered with one drop (25 µL) of CdTe/TGA ink solution for 1 min, and then blown dry with N2 (1 bar, 1 mm orifice). It was immediately put in a conformal contact with a clean Si(100) substrate for 25 s and subsequently peeled from the substrate. Adsorption of the Nanoparticles. A clean Si(100) substrate or a freshly activated stamp was immersed in the CdTe/TGA ink solution, then rinsed several times with Millipore water, and finally blown dry with N2. Scanning Force Microscopy (SFM). SFM measurements were performed with a commercial instrument (Digital Instruments, Nanoscope IIIa, Dimension 3000, Santa Barbara, CA) operating in tapping mode using silicon cantilevers (Nanosensors) of spring constant 250-350 kHz.

Experimental Section

Results and Discussion

Materials. Silicon substrates (WaferNet Co., Type N, 0.5 mm thick, orientation (100), resistivity 2-5 Ω cm) were cut and ultrasonicated successively in acetone (p.a.), chloroform (p.a.), 2-propanol (p.a.), and water for 10 min each. They were then cleaned with the standard RCA procedure: 15 min immersion into a 70 °C 1:1:5 mixture of 25% NH4OH (Fluka, p.a.), 31% H2O2 (Fluka, p.a.), and water (Millipore, 18.2 MΩ cm); 15 min immersion into a 70 °C 1:1:5 mixture of 37% HCl (Aldrich, ACS reagent), 31% H2O2 (Fluka, p.a), and water (Millipore, 18.2 MΩ cm). They were finally rinsed with water and then dried under a stream of nitrogen. The static water contact angles for these substrates were less than 10°. Polystyrene masters with stripe structures were prepared as follows: l-R-dipalmitoylphosphatidycholine (DPPC) molecules form stripe structures on SiO2/Si(100) surfaces via a Langmuir-Blodgett (LB) method.33 OTS (octadecyltrichlorsilane, 90+%, Aldrich) selfassembled monolayers were formed on the bare SiO2 parts by immersion of the sample into a 1 mM solution of OTS in hexadecane (anhydrous, Aldrich) for 10 min. The sample was then sonicated in chloroform to remove the DPPC. Etching was carried out by immersion of the sample for 10-18 h at room temperature in a 20 mM glycine buffer with the pH adjusted to 9 ( 0.5 with KOH. The etched structures were transferred to polystyrene by placing the sample in an oven at 190 °C for 30 min under a pressure of 1 kg cm-2.34 Poly(dimethysiloxane) (PDMS) stamps (Sylgard 184, Dow Corning) were formed on polystyrene masters from DPPC stripe structures or on a patterned Si wafer master (IMS Stuttgart). The Si master was rendered hydrophobic with fluoroalkyl-trichlorosilane vapor. The stamps were activated by plasma oxidation in 1 mbar of O2 at 300 W for 20 s (Templa System 100-E plasma system). The static water contact angle for the activated flat stamp was also less than 10°, similar to that of a freshly cleaned SiO2/Si (100) surface. Thioglycolic acid capped CdTe (CdTe/TGA) nanocrystals (∼4 nm in size with a band edge photoluminescence maximum at 590 nm, 6% room-temperature quantum efficiency) dispersed in water were kindly provided by Dr. A. L. Rogach from Photonics and Optoelectronics Group, Physics Department and CeNS, LudwigMaximilians Universita¨t Mu¨nchen.

DPPC molecules can form nanostructured striped patterns over large areas on mica35 and Si(100) surfaces via a LB method.33 A typical SFM topographical image of the DPPC stripes is presented in Figure 1a. The brighter areas with holes are the DPPC monolayer stripes, separated by lower regions of the SiO2/ Si substrate. The periodicity of this stripe pattern is 700 nm with 500 nm widths of the DPPC stripes. The periodicity can be tuned through control of the experimental parameters during the LB transfer. The exposed SiO2/Si regions of the substrate can be covalently modified with OTS. After the removal of the physisorbed DPPC, the resulting OTS stripes can be used as an etch resist.34 We first molded the channel structures to polystyrene using nanoimprinting. A molded structure in polystyrene is shown in Figure 1b. The average depth of the channels is ∼100 nm. The polystyrene with channel structures can be used as the master for the µCP process. The topography is then once again molded from the polystyrene master into PDMS. Figure 1c is an example of the molded PDMS. (23) Bae, S.; Lim, D. K.; Park, J.; Lee, W.; Cheon, J.; Kim, S. J. Phys. Chem. B 2004, 108, 2575. (24) Wu, X. C.; Bittner, A. M.; Kern, K. AdV. Mater. 2004, 16, 413. (25) Gu, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630. (26) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 41. (27) Rogers, J. A.; Paul, K. E.; Jackman, R. J.; Whitesides, G. M. J. Vac. Sci. Technol., B 1998, 16, 59. (28) Rogers, J. A.; Paul, K. E.; Jackman, R. J.; Whitesides, G. M. Appl. Phys. Lett. 1997, 70, 2658. (29) Black, A. J.; Paul, K. E.; Aizenberg, J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 8356. (30) Messer, B.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2000, 122, 10232. (31) McLellan, J. M.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10830. (32) Geissler, M.; McLellan, J. M.; Xia, Y. Nano Lett. 2005, 5, 31. (33) Lu, N.; Gleiche, M.; Zheng, J.; Lenhert, S.; Xu, B.; Chi, L. F.; Fuchs, H. AdV. Mater. 2002, 14, 1812. (34) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G.; Fuchs, H.; Chi, L. F. AdV. Mater. 2004, 16, 619. (35) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173.

Controlled Transport of CdTe Nanoparticles in µCP

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Figure 2. SFM topographical images of printed nanoparticles on SiO2/Si substrates using grooved masters with different periodicities (a, b, c, f), an inked stamp (d), and the phase image of a stamp after printing (e). Ink concentration from (a) to (e) was 1 mM CdTe/TGA in water, while it was 17 mM in (f). The scale bar in the inset of (c) is 250 nm.

Two interesting phenomena occur when using such stamps, which result in the nanostructures shown in the SFM images of the printed nanoparticles on SiO2/Si substrates (Figure 2). One is that the nanoparticles are printed mainly at edges (Figure 2a,b,f). We refer to this as edge printing. The other is that, in addition to isolated nanoparticles at the edges of patterns, some nanowires also form mainly at the edges. The inset in Figure 2c shows the topography and phase images of a single nanowire. It appears to be a continuous wire with a height of ca. 3 nm, which is the expected height of a single CdTe nanoparticle. Of course, highresolution transmission electron microscopic measurement is needed to unambiguously determine the structure of the nanowires. In addition, low-density isolated nanoparticles are also sometimes present at the central part of the printed region (Figure 2c). We believe that these two phenomena are closely related to the structure of the master and to the weak interaction between the nanoparticles and the stamp. First, the pattern edges of the master are quite rough. This is determined by the initial DPPC stripe pattern (Figure 1a) and by the etching process (Figure 1b). Figure 2d shows an SFM topographical image of a stamp inked with 1 mM nanoparticles in water. There are many nanoparticles around the edges of the stamp. Due to the weak adhesion of the nanoparticles to the stamp surface, they moved to the pattern edges during drying under the N2 stream, stayed there, and later were printed from the edges of the pattern. This leads to the edge printing. Figure 2e presents an SFM phase image of a stamp after printing. The edges of the stamp are quite rough. Some nanoparticles remain on the edges of the stamp features, whereas there is no apparent appearance of the nanoparticles on the protruding regions of the stamp. Figure 2f shows the case with a higher ink concentration (17 mM). At this concentration, mainly nanoparticle aggregates are printed at the edges of the pattern. In addition, no formation of nanowires is observed at higher ink concentrations. Therefore, we choose the ink concentration of 1 mM for further experiments.

CdTe/TGA nanoparticles are quite stable in water. Tang et al. found that CdTe nanowires with lengths of several hundred nanometers formed when excess TGA molecules were removed from a CdTe/TGA nanoparticle solution by precipitating the nanoparticles with methanol and then redispersing the nanoparticle precipitates in water and further aging in darkness at room temperature for several days.36 They believe that the strong dipole-dipole interactions between the CdTe nanoparticles are responsible for the formation of the nanowires. In our case, excess TGA molecules might be removed from the stamp together with the bulk water during the drying process and the irregular pattern edges may supply some localized regions with a high density of the nanoparticles. At these regions, strong dipole-dipole interactions between the nanoparticles can lead to the formation of the nanowires. Due to the random distribution of these regions, the distribution of nanowires is also inhomogeneous. When the CdTe nanoparticles are not close to each other, the interactions between them are not strong enough to induce the formation of the nanowires and they retain their original spacings. This situation corresponds to the isolated nanoparticles that appear at edges of the pattern. In addition, the surface of the stamp is also rough (Figure 2e). This means that some nanoparticles adhere more strongly to the stamp surface (will not be pushed to the stamp edges during the drying process) and are printed on the surface in the next step. This explains the appearance of some nanoparticles at the central part of the printed regions (Figure 2c). At the moment, due to the poor control over the edge quality of the DPPC masters, we cannot predict which master will give edge printing and which master will show the formation of nanowires. The majority of masters used from the DPPC stripes give edge printing while only a few show the formation of nanowires, indicating the difficulty in obtaining high-density nanoparticles at the correct positions. (36) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237.

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Figure 3. SFM topographical images of printed nanoparticles on SiO2/Si substrates using a standard master with ink concentrations of 1 mM. Arrows indicate the blowing directions.

To gain more insight, we changed to a standard master that provides more complex patterns but with sharp pattern edges and smooth surfaces. By using a standard master, the following printing results are observed: an inhomogeneous distribution of the nanoparticles on the printed regions with more nanoparticles at pattern edges (Figure 3a); nanoparticles homogeneously distributed on the printed regions (Figure 3b); edge printing on some regions (Figure 3c, upper part) and homogeneous printing on other regions (Figure 3c, lower part). Figure 3a shows the influence of blowing with N2 on the distribution of nanoparticles. In Figure 3a, there are more nanoparticles at the regions near the contact line of the right edges than at the left edges of the pattern, indicating that the blowing direction is from left to right. In this process, nanoparticles moved from the left side to the right side. This results in a lower density of nanoparticles in the middle part and a higher density at the edges. Due to the larger pattern sizes, nanoparticles were not totally pushed to the right edge of the pattern. By decreasing pattern sizes, edge printing can be realized as shown in the last pattern of Figure 3a and as we often observed in the case of using DPPC master (with protrusion sizes less than 1 µm). In Figure 3c, the blowing direction is indicated by the arrow (from top to bottom). Edge printing is observed at the upper part of Figure 3c. A transition region is observed in the middle part, where nanoparticles from the upper part accumulate due to the direction of the N2 stream. Homogeneous printing is observed in the lower part of Figure 3c, indicating that the influence of blowing on larger pattern sizes is greatly reduced. A general trend is that smaller feature sizes result in easier edge printing while larger feature sizes result in more homogeneous printing. In addition, when patterns are far from the source of the N2 stream (i.e., lower gas velocity), the influence of blowing is less apparent. Homogeneous printing can be obtained at these

Figure 4. Topographic SFM images of nanoparticles after (a) overnight adsorption and (b) blown dry after 1 min immersion (ink concentrations of 1 mM) on SiO2/Si substrates, and (c) 3 h adsorption and (d) blown dry after 1 min immersion (ink concentrations of 17 mM) on flat hydrophilic PDMS surfaces.

regions (Figure 3b). All of these results are due to the weak interaction of the nanoparticles with the surface of the stamp. Figure 4 verifies this. Figure 4a presents a topographic SFM image of a SiO2/Si surface after overnight immersion in a 1 mM nanoparticle aqueous dispersion. There are only a few nanoparticles on a 16 µm2 image. Although the nanoparticles show

Controlled Transport of CdTe Nanoparticles in µCP

very weak adsorption on hydrophilic OH-terminated Si substrates, enough nanoparticles are left by first putting 25 µL of 1 mM nanoparticle dispersion on it (1 cm2 size) and then blowing it dry with a N2 flow (Figure 4b). The majority of the nanoparticles are isolated and distribute randomly over the whole surface. In addition, some larger aggregates also form. At an ink concentration of 17 mM the nanoparticles mainly exist in the form of aggregates. The density of aggregates on the surface increases a little, but is still very low (Figure 4c). In contrast, a high density of the aggregates is left on the surface upon blowing (Figure 4d). The formation of the aggregates on the stamp induces the edge printing of the aggregates in Figure 2f. For such weakly adsorbed nanoparticles on small pattern sizes, blowing leads to a particle distribution mainly at the stamp edges. This induces edge printing. For large pattern sizes, the influence of blowing is greatly reduced, and therefore homogeneous printing can be realized. The dependence of edge printing on pattern sizes was also observed by Cherniavskaya et al. on their edge transfer lithography (ETL), where discontinuous dewetting behaviors of ink solutions selectively apply ink only within the recesses of the stamp and initiate the transfer of the ink along the edges. They found that stamp feature sizes of greater than 2 µm significantly reduce the effectiveness of the ETL to form edge structures.18 Stronger capillary forces present in smaller structures are claimed to allow for trapping and retaining more ink solution in the recesses.18 In our case, the net force (pushing force from the fluid motion minus the frictional force between the particles and the surface) acting on the particles plays a key role in forming edge structures. In terms of the Cherniavskaya explanation, one could say that the N2 stream enhances the dewetting (i.e., direction and velocity of the contact line motion). Microscopic amounts of solution would still tend to be left in the recesses due to capillary effects, but their amounts and distribution would depend on the gas velocity. Therefore, through the tailoring of patterns (size, distance, and complexity of the pattern) and the blowing flux, we can acquire either edge printing or homogeneous printing. For the standard master, we do not observe the obvious formation of nanowires, although larger spherical aggregates are observed around pattern edges due to higher particle density there (Figure 3d). This might also be due to the sharp edge of the standard master that does not supply the localized regions with high density of nanoparticles. In our case, variations in ambient humidity (30-50%) and temperature (1530 °C) seem to have no observable influence on the distribution of the nanoparticles on the stamp surface. Normally, by direct

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inking of a structured stamp, more ink molecules accumulate at structure edges, and thus destroy the homogeneous printing. To obtain homogeneous printing, a low concentration of ink solution or a flat stamp as an “inker pad”16 are often employed to reduce the accumulation of ink molecules at pattern edges. In our case, we actually benefit from this effect. Due to accumulation at the pattern edges, the nanoparticles are not totally pushed into the recesses of the stamp which leads to the edge printing.

Conclusions The edge printing that we show here only needs one step to fabricate patterned nanoparticles at a resolution in the range of several tens of nanometers. In principle, the eventual resolution can reach the size of the individual nanoparticle (for example, several nanometers for the CdTe nanoparticles used here). We believe that, with better control of the blowing force and optimization of pattern design, we could achieve much better edge printing for masters fabricated using standard methods. In addition, since the synthesis method of CdTe nanoparticles is well-developed, a similar protocol can be adopted for the syntheses of other II-VI nanoparticles37 or for the posttreatment of nanoparticle surfaces.38 The patterning method that we present here might therefore represent a general and easy way for controlled patterning of those nanoparticles. Furthermore, edge printing will not be limited to the above-mentioned materials. If we can achieve a weak adhesion of the transferred materials on the stamp, they will be pushed to the edges of the pattern during the inking step. Edge printing can then be realized in the subsequent printing step. Of course, pattern sizes and structures need to be taken into account. Considering the large versatilities in changing the interfacial chemistry of both the ink and the stamp, we can intentionally tune the interfacial chemistry of either the transferred materials or the stamp or both to create a proper weak adhesion of the former on the latter. The results we show here therefore point out an easy and effective way to obtain edge printing by control of the interfacial chemistry of the stamp and the transferred materials. Last, we want to say that exploring the interaction between the ink and stamp will surely enrich the µCP technique. LA060615V (37) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (38) Babayan, Y.; Barton, J. E.; Greyson, E. C.; Odom, T. W. AdV. Mater. 2004, 16, 1341.